<figure><img src="image_1.png" /></figure>

HAL
open science

# Gas sensing using porous materials for automotive applications

Dominic J. Wales, Julien Grand, Valeska P. Ting, Richard D. Burke, Karen J.

Edler, Chris R. Bowen, Svetlana Mintova, Andrew D. Burrows

► To cite this version:

Dominic J. Wales, Julien Grand, Valeska P. Ting, Richard D. Burke, Karen J. Edler, et al. Gas sensing using porous materials for automotive applications. Chemical Society Reviews, 2015, 44 (13), pp.4290-4321. 10.1039/C5CS00040H. hal-01840206

HAL Id: hal-01840206

https://hal.science/hal-01840206

Submitted on 23 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Chem Soc Rev

Chemical Society Reviews

www.rsc.org/chemsocrev

<figure><img src="image_1.png" /></figure>

Themed issue: Sensor targets

ISSN 0306-0012

<figure><img src="image_2.png" /></figure>

REVIEW ARTICLE

Andrew D. Burrows et al.

Gas sensing using porous materials for automotive applications
# Chem Soc Rev

Chemical Society Reviews

www.rsc.org/chemsocrev

<figure><img src="image_3.png" /></figure>

## Themed issue: Sensor targets

ISSN 0306-0012

<figure><img src="image_4.png" /></figure>

### REVIEW ARTICLE

Andrew D. Burrows et al.

Gas sensing using porous materials for automotive applications
REVIEW ARTICLE

View Article Online

View Journal | View Issue

<figure><img src="image_3.png" /></figure>

CrossMark

← click for updates

Cite this: *Chem. Soc. Rev.*, 2015, 44, 4290

Received 16th January 2015

DOI: 10.1039/c5cs00040h

www.rsc.org/chemsocrev

# Gas sensing using porous materials for automotive applications

Dominic J. Wales,ᵃ Julien Grand,ᵇ Valeska P. Ting,ᵃ Richard D. Burke,ᶜ Karen J. Edler,ᵈ Chris R. Bowen,ᶜ Svetlana Mintovaᵇ and Andrew D. Burrows*ᵈ

Improvements in the efficiency of combustion within a vehicle can lead to reductions in the emission of harmful pollutants and increased fuel efficiency. Gas sensors have a role to play in this process, since they can provide real time feedback to vehicular fuel and emissions management systems as well as reducing the discrepancy between emissions observed in factory tests and 'real world' scenarios. In this review we survey the current state-of-the-art in using porous materials for sensing the gases relevant to automotive emissions. Two broad classes of porous material – zeolites and metal-organic frameworks (MOFs) – are introduced, and their potential for gas sensing is discussed. The adsorptive, spectroscopic and electronic techniques for sensing gases using porous materials are summarised. Examples of the use of zeolites and MOFs in the sensing of water vapour, oxygen, NOₓ, carbon monoxide and carbon dioxide, hydrocarbons and volatile organic compounds, ammonia, hydrogen sulfide, sulfur dioxide and hydrogen are then detailed. Both types of porous material (zeolites and MOFs) reveal great promise for the fabrication of sensors for exhaust gases and vapours due to high selectivity and sensitivity. The size and shape selectivity of the zeolite and MOF materials are controlled by variation of pore dimensions, chemical composition (hydrophilicity/hydrophobicity), crystal size and orientation, thus enabling detection and differentiation between different gases and vapours.

## 1. Introduction

Climate change and the need to combat its associated negative effects constitutes one of the key global challenges of the modern age. It is generally accepted that anthropogenic climate change is largely the result of the production and release of unprecedented levels of greenhouse gases, including carbon dioxide, since the industrial revolution as a result of the combustion of fossil-based fuels such as oil, coal and natural gas.

<figure><img src="image_4.png" /><figcaption>Dominic J. Wales</figcaption></figure>

Dominic Wales is a Research Officer in the Department of Chemical Engineering at the University of Bath. His project involves the design and application of metal-organic frameworks and zeolites for exhaust gas sensing applications.

<figure><img src="image_5.png" /><figcaption>Julien Grand</figcaption></figure>

Julien Grand is a Research Engineer in the Laboratoire Catalyse et Spectrochimie (LCS, CNRS), Caen. His project involves the synthesis of new types of zeolites and the development of selective zeolite sensors for automotive exhaust gases.

<figure><img src="image_6.png" /></figure>

<footnote>ᵃDepartment of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK</footnote>

<footnote>ᵇLaboratoire Catalyse & Spectrochimie, ENSICAEN, 6 Boulevard Maréchal JUIN, 14000 Caen, France</footnote>

<footnote>ᶜDepartment of Mechanical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK</footnote>

<footnote>ᵈDepartment of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK
E-mail: a.d.burrows@bath.ac.uk</footnote>
REVIEW ARTICLE

View Article Online

View Journal | View Issue

<figure><img src="image_5.png" /></figure>

CrossMark

<figure><img src="image_6.png" /></figure>

Cite this: *Chem. Soc. Rev.*, 2015, 44, 4290

Received 16th January 2015

DOI: 10.1039/c5cs00040h

www.rsc.org/chemsocrev

# Gas sensing using porous materials for automotive applications

Dominic J. Wales,ª Julien Grand,ᵇ Valeska P. Ting,ª Richard D. Burke,ᶜ

Karen J. Edler,ᵈ Chris R. Bowen,ᶜ Svetlana Mintovaᵇ and Andrew D. Burrows*ᵈ

Improvements in the efficiency of combustion within a vehicle can lead to reductions in the emission of harmful pollutants and increased fuel efficiency. Gas sensors have a role to play in this process, since they can provide real time feedback to vehicular fuel and emissions management systems as well as reducing the discrepancy between emissions observed in factory tests and 'real world' scenarios. In this review we survey the current state-of-the-art in using porous materials for sensing the gases relevant to automotive emissions. Two broad classes of porous material – zeolites and metal-organic frameworks (MOFs) – are introduced, and their potential for gas sensing is discussed. The adsorptive, spectroscopic and electronic techniques for sensing gases using porous materials are summarised. Examples of the use of zeolites and MOFs in the sensing of water vapour, oxygen, NOₓ, carbon monoxide and carbon dioxide, hydrocarbons and volatile organic compounds, ammonia, hydrogen sulfide, sulfur dioxide and hydrogen are then detailed. Both types of porous material (zeolites and MOFs) reveal great promise for the fabrication of sensors for exhaust gases and vapours due to high selectivity and sensitivity. The size and shape selectivity of the zeolite and MOF materials are controlled by variation of pore dimensions, chemical composition (hydrophilicity/hydrophobicity), crystal size and orientation, thus enabling detection and differentiation between different gases and vapours.

## 1. Introduction

Climate change and the need to combat its associated negative effects constitutes one of the key global challenges of the modern age. It is generally accepted that anthropogenic climate change is largely the result of the production and release of unprecedented levels of greenhouse gases, including carbon dioxide, since the industrial revolution as a result of the combustion of fossil-based fuels such as oil, coal and natural gas.

<figure><img src="image_7.png" /><figcaption>Dominic J. Wales</figcaption></figure>

Dominic Wales is a Research Officer in the Department of Chemical Engineering at the University of Bath. His project involves the design and application of metal-organic frameworks and zeolites for exhaust gas sensing applications.

<figure><img src="image_8.png" /><figcaption>Julien Grand</figcaption></figure>

Julien Grand is a Research Engineer in the Laboratoire Catalyse et Spectrochimie (LCS, CNRS), Caen. His project involves the synthesis of new types of zeolites and the development of selective zeolite sensors for automotive exhaust gases.

<figure><img src="image_9.png" /></figure>

<footnote>ª Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK</footnote>

<footnote>ᵇ Laboratoire Catalyse & Spectrochimie, ENSICAEN, 6 Boulevard Maréchal JUIN, 14000 Caen, France</footnote>

<footnote>ᶜ Department of Mechanical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK</footnote>

<footnote>ᵈ Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK
E-mail: a.d.burrows@bath.ac.uk</footnote>
Review Article

Under conditions that allow for complete combustion, carbon dioxide is produced in large quantities. However, under conditions that do not allow for complete combustion, undesired and toxic by-products such as oxides of nitrogen (NOx), carbon monoxide, sulfur dioxide and various unburnt hydrocarbons are also emitted in smaller quantities, some from impurities present in the fuel.¹ The release of these gases into the atmosphere has resulted in diminished air quality in major cities and built up areas.²⁻⁴ Consequently an increasing amount of research has been directed at the development of technological and materials-based mitigation strategies, for example, the capture and sequestration of CO₂

and methane from the atmosphere. Porous materials, such as activated carbons, zeolites and most recently, metal-organic frameworks (MOFs) have been intensely studied for CO₂ sequestration applications over the last two decades, as their high surface areas and large void volumes make them uniquely suited for adsorptive separation and storage of gases. However, rather than developing these materials for separating and capturing gases, such as CO₂, from dilute mixtures in air (<1% CO₂) or even from more concentrated flue gases (~14% CO₂), a more realistic approach could be to employ these materials to reduce the amount of gas emitted at the point of generation.

<figure><img src="image_7.png" /><figcaption>Valeska P. Ting</figcaption></figure>

Valeska Ting is a Lecturer in the Department of Chemical Engineering and the University of Bath's Prize Fellow in Smart Nanomaterials.

<figure><img src="image_8.png" /><figcaption>Richard D. Burke</figcaption></figure>

Richard Burke is a Prize Fellow in Automotive Powertrain Systems in the Department of Mechanical Engineering at Bath.

<figure><img src="image_9.png" /><figcaption>Karen J. Edler</figcaption></figure>

Karen Edler is Professor of Soft Matter at the University of Bath. Her research group works on self-assembly in systems from inorganic surfactant-templated materials, to polyelectrolyte films, nanofibre gels and lipid-polymer particles.

<figure><img src="image_10.png" /><figcaption>Chris R. Bowen</figcaption></figure>

Chris Bowen is Professor of Materials at the University of Bath and ERC Advanced Investigator on Novel Energy Materials, Engineering Science and Integrated Systems (NEMESIS).

<figure><img src="image_11.png" /><figcaption>Svetlana Mintova</figcaption></figure>

Svetlana Mintova is Director of Research, 1st class at CNRS, Laboratoire de Catalyse et Spectrochimie, ENSICAEN and University of Caen, in Basse Normandy, France.

<figure><img src="image_12.png" /><figcaption>Andrew D. Burrows</figcaption></figure>

Andrew Burrows is a Reader in Inorganic Chemistry at the University of Bath. His research interests lie in functionalised materials with extended structures such as metal-organic frameworks (MOFs).

<figure><img src="image_13.png" /></figure>
Review Article

Under conditions that allow for complete combustion, carbon dioxide is produced in large quantities. However, under conditions that do not allow for complete combustion, undesired and toxic by-products such as oxides of nitrogen (NOx), carbon monoxide, sulfur dioxide and various unburnt hydrocarbons are also emitted in smaller quantities, some from impurities present in the fuel.¹ The release of these gases into the atmosphere has resulted in diminished air quality in major cities and built up areas.²⁻⁴ Consequently an increasing amount of research has been directed at the development of technological and materials-based mitigation strategies, for example, the capture and sequestration of CO₂

and methane from the atmosphere. Porous materials, such as activated carbons, zeolites and most recently, metal-organic frameworks (MOFs) have been intensely studied for CO₂ sequestration applications over the last two decades, as their high surface areas and large void volumes make them uniquely suited for adsorptive separation and storage of gases. However, rather than developing these materials for separating and capturing gases, such as CO₂, from dilute mixtures in air (<1% CO₂) or even from more concentrated flue gases (~14% CO₂), a more realistic approach could be to employ these materials to reduce the amount of gas emitted at the point of generation.

<figure><img src="image_10.png" /><figcaption>Valeska P. Ting</figcaption></figure>

Valeska Ting is a Lecturer in the Department of Chemical Engineering and the University of Bath's Prize Fellow in Smart Nanomaterials.

<figure><img src="image_11.png" /><figcaption>Richard D. Burke</figcaption></figure>

Richard Burke is a Prize Fellow in Automotive Powertrain Systems in the Department of Mechanical Engineering at Bath.

<figure><img src="image_12.png" /><figcaption>Karen J. Edler</figcaption></figure>

Karen Edler is Professor of Soft Matter at the University of Bath. Her research group works on self-assembly in systems from inorganic surfactant-templated materials, to polyelectrolyte films, nanofibre gels and lipid-polymer particles.

<figure><img src="image_13.png" /><figcaption>Chris R. Bowen</figcaption></figure>

Chris Bowen is Professor of Materials at the University of Bath and ERC Advanced Investigator on Novel Energy Materials, Engineering Science and Integrated Systems (NEMESIS).

<figure><img src="image_14.png" /><figcaption>Svetlana Mintova</figcaption></figure>

Svetlana Mintova is Director of Research, 1st class at CNRS, Laboratoire de Catalyse et Spectrochimie, ENSICAEN and University of Caen, in Basse Normandy, France.

<figure><img src="image_15.png" /><figcaption>Andrew D. Burrows</figcaption></figure>

Andrew Burrows is a Reader in Inorganic Chemistry at the University of Bath. His research interests lie in functionalised materials with extended structures such as metal-organic frameworks (MOFs).

<figure><img src="image_16.png" /></figure>
<figure><img src="image_14.png" /></figure>

One of the largest contributors to greenhouse gas emissions is the transportation sector, which currently accounts for over a quarter of the total global energy demand worldwide.⁵ Between 1990 and 2012, transportation was the second largest source of emitted greenhouse gases in the USA and accounted for 24% of CO₂ emissions in the UK, according to an IMechE report from 2010, with the majority originating from road transport.³,⁶ While promising alternative energy technologies (e.g. battery and fuel-cell powered vehicles) are being developed in an effort to reduce future carbon emissions and remove the reliance on fossil fuels, there are many infrastructural and technological hurdles to overcome before these technologies become widespread. Similarly, while mid-term solutions such as the development of biofuels (including bioethanol and biodiesel) offer a lower-carbon alternative to traditional fossil fuels they can still lead to unwanted emissions. The outlook for the next 40 years is that the transportation industry will remain reliant on the internal combustion engine and hydrocarbon-based fuels.⁷,⁸ Hence there is an urgent need for concerted efforts to reduce emissions from transport sources, as well as ensuring that the most efficient use is made of the limited fossil fuel reserves remaining.⁹

## 1.1 Use of sensors in the automotive industry

The major motivations for the use of sensors to monitor automotive emissions are the legislative requirements to reduce harmful emissions that have negative effects on health and air quality and contribute to global climate change. However, the potential economic benefits from increasing fuel efficiency through reductions in unburnt fuels and increased fuel energy density is also a factor. The greatest gains in developing new multi-functional sensors based on porous materials for the automotive industry are in two main areas: for the design and manufacture of road vehicles, and for use in production vehicles for real-time fuel and emissions management.

### 1.1.1 Sensors for vehicle design and manufacture.
Vehicle design and manufacture is subject to stringent legislative targets in terms of fuel consumption and CO₂ emissions with legislation stipulating a fleet average of 136 g km⁻¹ in 2011 with a proposed target of reducing emissions to a 95 g km⁻¹ fleet average by 2020.¹⁰ The advent of turbocharging and supercharging technologies, which reduce the engine size and increase the specific power of diesel engines, have also resulted in an increase in the peak cylinder temperatures. This, in turn, results in higher emission levels of NOₓ and particulate matter.¹¹ Carslaw *et al*. showed that NOₓ emissions for modern diesel engines can be twice those of older models, which they attribute to the reduction in engine size.¹²

There is also a need to reduce the discrepancy between emission levels in diagnostic tests used to provide emissions certification in a factory setting (for example on rolling roads using the New European Drive Cycle (NEDC)) and real world emissions, which has become more significant over recent years. As an example, Carslaw *et al*. used road-side remote sensing equipment to measure real world vehicle NOₓ emissions from a large number of vehicles.¹² The results suggested that whilst legislation imposed on factory-tested NOₓ emissions

had achieved a reduction from 0.5 g km⁻¹ to 0.2 g km⁻¹ since the year 2000 (with all new vehicles sold meeting this standard), real world emissions had stagnated at around 1 g km⁻¹.¹² Another review of real world fuel consumption by Mock *et al*. summarising customer reviews of observed mileage, on-board emissions measurement and laboratory tests, showed that whilst in 2002 the average vehicle had 7% higher fuel consumption on the road compared to the NEDC, in 2012 the difference had increased to 25%.¹³ Such studies are motivating the automotive industry to move away from current legislative testing on idealised drive cycles and replace this with more dynamic and representative test cycles.¹⁴ This change presents significant opportunities for emissions sensing. Firstly, replacement drive cycles must be proven to be representative of real world driving and to establish the emissions associated with a real world drive cycle, real data must be captured under representative conditions. This is particularly challenging since current high-accuracy lab-based emissions sampling tends to be a large, intrusive mechanism which cannot be practically deployed on a significantly large scale. Secondly, engine emissions control is primarily achieved indirectly by modelling emissions empirically in order to determine optimum control of different engine features. Without a direct measure of emissions for use as feedback, less accurate indirect control with significant calibration and on-board diagnostics effort is required.¹⁵

### 1.1.2 Sensors for on-board fuel and emissions management.

For both petrol- and diesel-powered vehicles, the continued tightening of legislated emission limits is increasing the complexity of emissions control systems on engines.¹¹ In particular, active devices for NOₓ and particulate matter rely on an estimation of emissions levels under highly transient conditions and would benefit from a direct, fast and accurate measurement of individual emissions species in real time. Specific active emissions control technologies can already be found on commercially available vehicles and include the use of the following:¹⁵

(i) Catalytic converters:¹⁶ these reduce emissions of carbon monoxide, unburnt hydrocarbons and NOₓ gases. Optimal operation of catalytic converters requires precise control of the concentration of oxygen by the engine management system, in order to tailor the intake air:fuel ratio, known as $\lambda$.¹⁷ However, under cold start conditions, in order to raise the temperature of the catalytic convertor, the engine is run in a less fuel-efficient mode, involving higher speed idling, exhaust throttling and over-fuelling.

(ii) Selective catalytic reduction (SCR) catalysts: these reduce emissions of NOₓ from diesel engines. Ammonia, as a urea-water solution, is injected into the exhaust stream for the reduction of NOₓ into nitrogen and water in the presence of oxygen on the catalyst. The amount of injected urea must be carefully controlled to match the NOₓ concentration to ensure full reduction and avoid NOₓ emission due to underdosing or ammonia emission from overdosing.

(iii) Particulate filters: exhaust particulates are captured in a ceramic filter, but must be routinely purged by raising the exhaust temperature either by overfuelling or throttling the intake air flow to reduce the particulates.
<figure><img src="image_17.png" /></figure>

One of the largest contributors to greenhouse gas emissions is the transportation sector, which currently accounts for over a quarter of the total global energy demand worldwide.⁵ Between 1990 and 2012, transportation was the second largest source of emitted greenhouse gases in the USA and accounted for 24% of CO₂ emissions in the UK, according to an IMechE report from 2010, with the majority originating from road transport.³,⁶ While promising alternative energy technologies (e.g. battery and fuel-cell powered vehicles) are being developed in an effort to reduce future carbon emissions and remove the reliance on fossil fuels, there are many infrastructural and technological hurdles to overcome before these technologies become widespread. Similarly, while mid-term solutions such as the development of biofuels (including bioethanol and biodiesel) offer a lower-carbon alternative to traditional fossil fuels they can still lead to unwanted emissions. The outlook for the next 40 years is that the transportation industry will remain reliant on the internal combustion engine and hydrocarbon-based fuels.⁷,⁸ Hence there is an urgent need for concerted efforts to reduce emissions from transport sources, as well as ensuring that the most efficient use is made of the limited fossil fuel reserves remaining.⁹

## 1.1 Use of sensors in the automotive industry

The major motivations for the use of sensors to monitor automotive emissions are the legislative requirements to reduce harmful emissions that have negative effects on health and air quality and contribute to global climate change. However, the potential economic benefits from increasing fuel efficiency through reductions in unburnt fuels and increased fuel energy density is also a factor. The greatest gains in developing new multi-functional sensors based on porous materials for the automotive industry are in two main areas: for the design and manufacture of road vehicles, and for use in production vehicles for real-time fuel and emissions management.

### 1.1.1 Sensors for vehicle design and manufacture.
Vehicle design and manufacture is subject to stringent legislative targets in terms of fuel consumption and CO₂ emissions with legislation stipulating a fleet average of 136 g km⁻¹ in 2011 with a proposed target of reducing emissions to a 95 g km⁻¹ fleet average by 2020.¹⁰ The advent of turbocharging and supercharging technologies, which reduce the engine size and increase the specific power of diesel engines, have also resulted in an increase in the peak cylinder temperatures. This, in turn, results in higher emission levels of NOₓ and particulate matter.¹¹ Carslaw *et al.* showed that NOₓ emissions for modern diesel engines can be twice those of older models, which they attribute to the reduction in engine size.¹²

There is also a need to reduce the discrepancy between emission levels in diagnostic tests used to provide emissions certification in a factory setting (for example on rolling roads using the New European Drive Cycle (NEDC)) and real world emissions, which has become more significant over recent years. As an example, Carslaw *et al.* used road-side remote sensing equipment to measure real world vehicle NOₓ emissions from a large number of vehicles.¹² The results suggested that whilst legislation imposed on factory-tested NOₓ emissions

had achieved a reduction from 0.5 g km⁻¹ to 0.2 g km⁻¹ since the year 2000 (with all new vehicles sold meeting this standard), real world emissions had stagnated at around 1 g km⁻¹.¹² Another review of real world fuel consumption by Mock *et al.* summarising customer reviews of observed mileage, on-board emissions measurement and laboratory tests, showed that whilst in 2002 the average vehicle had 7% higher fuel consumption on the road compared to the NEDC, in 2012 the difference had increased to 25%.¹³ Such studies are motivating the automotive industry to move away from current legislative testing on idealised drive cycles and replace this with more dynamic and representative test cycles.¹⁴ This change presents significant opportunities for emissions sensing. Firstly, replacement drive cycles must be proven to be representative of real world driving and to establish the emissions associated with a real world drive cycle, real data must be captured under representative conditions. This is particularly challenging since current high-accuracy lab-based emissions sampling tends to be a large, intrusive mechanism which cannot be practically deployed on a significantly large scale. Secondly, engine emissions control is primarily achieved indirectly by modelling emissions empirically in order to determine optimum control of different engine features. Without a direct measure of emissions for use as feedback, less accurate indirect control with significant calibration and on-board diagnostics effort is required.¹⁵

### 1.1.2 Sensors for on-board fuel and emissions management.

For both petrol- and diesel-powered vehicles, the continued tightening of legislated emission limits is increasing the complexity of emissions control systems on engines.¹¹ In particular, active devices for NOₓ and particulate matter rely on an estimation of emissions levels under highly transient conditions and would benefit from a direct, fast and accurate measurement of individual emissions species in real time. Specific active emissions control technologies can already be found on commercially available vehicles and include the use of the following:¹⁵

(i) Catalytic converters:¹⁶ these reduce emissions of carbon monoxide, unburnt hydrocarbons and NOₓ gases. Optimal operation of catalytic converters requires precise control of the concentration of oxygen by the engine management system, in order to tailor the intake air:fuel ratio, known as $\lambda$.¹⁷ However, under cold start conditions, in order to raise the temperature of the catalytic convertor, the engine is run in a less fuel-efficient mode, involving higher speed idling, exhaust throttling and over-fuelling.

(ii) Selective catalytic reduction (SCR) catalysts: these reduce emissions of NOₓ from diesel engines. Ammonia, as a urea-water solution, is injected into the exhaust stream for the reduction of NOₓ into nitrogen and water in the presence of oxygen on the catalyst. The amount of injected urea must be carefully controlled to match the NOₓ concentration to ensure full reduction and avoid NOₓ emission due to underdosing or ammonia emission from overdosing.

(iii) Particulate filters: exhaust particulates are captured in a ceramic filter, but must be routinely purged by raising the exhaust temperature either by overfuelling or throttling the intake air flow to reduce the particulates.
<figure><img src="image_15.png" /></figure>

(iv) Lean NOₓ traps: NOₓ emissions bind onto barium nitrate coated surfaces when the engine is operating in a lean mode with excess oxygen in the exhaust. When the storage catalyst is full, the engine must induce an oxygen-deprived environment by throttling the engine intakes and operating with a close to stoichiometric air:fuel ratio, to reduce the NO₂ into N₂ and CO₂.

Typically the use of the above technologies will have a detrimental effect on fuel economy either by requiring the injection of additional fuel or the throttling of the engine to control oxygen concentrations and exhaust temperatures. Many modern vehicles powered by an internal combustion engine are fitted with a solid state Zr and Pt-based oxygen sensor and/or a NOₓ sensor, which have response times in the order of a few tens of milliseconds.¹⁸,¹⁹ However, if the engine management system had access to sensors that could provide real-time information on the concentrations of a larger diversity of products in the exhaust stream, the engine management system could more finely tune the active emissions control, which would be greatly advantageous for increasing engine efficiency.

Thus, there is a need for emissions sensors across a range of applications from high cost, high accuracy research grade analysers for lab-based testing to robust, low cost production-level devices capable of differentiating between a large variety of gaseous species for on-board emissions monitoring. In both of these cases, the different applications will require sensors with different characteristics in terms of the intrusiveness of instrumentation (size, ability to install on vehicles), absolute measurement accuracy, sensor/measurement chain dynamics, feedback and cost.

## 2. Porous materials for sensing in the automotive industry

There is much potential for the use of porous materials as sensing elements for monitoring and feedback control of the wide variety of gases emitted in vehicle exhausts. The design and tailoring of porous materials for high selectivity allows more sensitive tracking and real-time response to emissions. The large-scale processing of porous materials is established and the production of miniaturised porous-materials sensors is eminently scalable. The vast array of different materials that could be incorporated lends itself to the optimisation of low cost and robust sensors that reduce the need for incorporation of expensive precious metals. In addition, the ability for porous materials to be made into layers of permeable thin films of differently selective materials can potentially be exploited to introduce additional multifunctionality to the final product, allowing a single sensor to detect a wide spectrum of sensing targets.

This review will focus on two major classes of porous materials for sensing applications:

(i) Zeolites, which represent a well-known and industrially relevant class of porous aluminosilicate materials. Zeolites are thermally and chemically robust and made from readily-available and abundant starting materials, which make them attractive for low cost and robust sensors.

(ii) Metal-organic frameworks (MOFs), which constitute a relatively new class of hybrid framework material, consisting of metal nodes connected by organic linkers. These materials have incredible structural diversity and the combination of tailoring metal nodes and organic components to achieve desired interactions is an attractive possibility.

Other classes of porous materials have been used for gas sensing, such as mesoporous silica and metal oxides,²⁰ carbon nanomaterials²¹ and porous silicon.²² These materials are, however, beyond the scope of this review.

There are several advantages to the use of porous materials in gas sensing applications. Porous materials offer flexibility and versatility in terms of structure and design. Size and shape selectivity for adsorbed species can be imparted to zeolites and MOFs through control of the dimensions of the pores enabling detection and differentiation between the multitude of combustion products in exhaust emissions.²³ In addition the structural flexibility of some frameworks can add a further dynamic dimension to gas selectivity.²⁴ Chemical selectivity and sensitivity can also be controlled by the incorporation of selected dopants into the zeolite and MOF frameworks in addition to the wide choice of organic linkers and metal nodes that can be incorporated into MOFs.²⁵⁻²⁸ Furthermore, MOFs and zeolites can exhibit high temperature stability, i.e. ~500 °C for some MOF-related zeolitic imidazolate framework materials and ~1200 °C for some zeolites.²⁹,³⁰ Indeed, MOFs and zeolites have been utilised as the active sensing element or as a gas selective filter/pre-concentrator in gas sensors which has been noted in several reviews on their applications.²⁵,²⁶,²⁸,³¹⁻³⁵ However, these reviews do not focus on the gases and vapours found in engine exhaust gases; some include examples of dissolved gases and none critically review zeolite- and MOF-based sensors in terms of thermal and selectivity suitability for sensing of gases in exhaust streams.

This review aims to provide an insight into the current state of the art of zeolites and MOFs for the sensing of gases found in vehicle exhausts along with a discussion of the associated sensor technologies allowing them to be employed. The review will also compare and contrast the performance of exemplar porous materials for such applications, to provide a perspective on the future prospects of zeolite- or MOF-based exhaust gas sensors in transportation. Examples of zeolite- and MOF-based sensors for the following important exhaust gases will be examined: water vapour (H₂O), oxygen (O₂), nitrogen oxides (NOₓ), carbon monoxide (CO) and carbon dioxide (CO₂) together, hydrocarbons (in particular methanol, ethanol, aldehydes and alkanes), ammonia (NH₃, the reagent in SCR catalysts), hydrogen sulfide (H₂S), sulfur dioxide (SO₂) and finally hydrogen (H₂). Hydrogen is included due to the potential for it to be employed as an alternative low-carbon fuel for future vehicles. MOFs have been reported as sensing materials for nitroaromatic and nitroalkane vapours. Since these gases are not found in exhaust streams, sensors for these species are not included in this review. However, an excellent review by Banerjee *et al*. on the sensing of these species was published recently.³⁶
(iv) Lean NOₓ traps: NOₓ emissions bind onto barium nitrate coated surfaces when the engine is operating in a lean mode with excess oxygen in the exhaust. When the storage catalyst is full, the engine must induce an oxygen-deprived environment by throttling the engine intakes and operating with a close to stoichiometric air:fuel ratio, to reduce the NO₂ into N₂ and CO₂.

Typically the use of the above technologies will have a detrimental effect on fuel economy either by requiring the injection of additional fuel or the throttling of the engine to control oxygen concentrations and exhaust temperatures. Many modern vehicles powered by an internal combustion engine are fitted with a solid state Zr and Pt-based oxygen sensor and/or a NOₓ sensor, which have response times in the order of a few tens of milliseconds.¹⁸,¹⁹ However, if the engine management system had access to sensors that could provide real-time information on the concentrations of a larger diversity of products in the exhaust stream, the engine management system could more finely tune the active emissions control, which would be greatly advantageous for increasing engine efficiency.

Thus, there is a need for emissions sensors across a range of applications from high cost, high accuracy research grade analysers for lab-based testing to robust, low cost production-level devices capable of differentiating between a large variety of gaseous species for on-board emissions monitoring. In both of these cases, the different applications will require sensors with different characteristics in terms of the intrusiveness of instrumentation (size, ability to install on vehicles), absolute measurement accuracy, sensor/measurement chain dynamics, feedback and cost.

## 2. Porous materials for sensing in the automotive industry

There is much potential for the use of porous materials as sensing elements for monitoring and feedback control of the wide variety of gases emitted in vehicle exhausts. The design and tailoring of porous materials for high selectivity allows more sensitive tracking and real-time response to emissions. The large-scale processing of porous materials is established and the production of miniaturised porous-materials sensors is eminently scalable. The vast array of different materials that could be incorporated lends itself to the optimisation of low cost and robust sensors that reduce the need for incorporation of expensive precious metals. In addition, the ability for porous materials to be made into layers of permeable thin films of differently selective materials can potentially be exploited to introduce additional multifunctionality to the final product, allowing a single sensor to detect a wide spectrum of sensing targets.

This review will focus on two major classes of porous materials for sensing applications:

(i) Zeolites, which represent a well-known and industrially relevant class of porous aluminosilicate materials. Zeolites are thermally and chemically robust and made from readily-available and abundant starting materials, which make them attractive for low cost and robust sensors.

(ii) Metal-organic frameworks (MOFs), which constitute a relatively new class of hybrid framework material, consisting of metal nodes connected by organic linkers. These materials have incredible structural diversity and the combination of tailoring metal nodes and organic components to achieve desired interactions is an attractive possibility.

Other classes of porous materials have been used for gas sensing, such as mesoporous silica and metal oxides,²⁰ carbon nanomaterials²¹ and porous silicon.²² These materials are, however, beyond the scope of this review.

There are several advantages to the use of porous materials in gas sensing applications. Porous materials offer flexibility and versatility in terms of structure and design. Size and shape selectivity for adsorbed species can be imparted to zeolites and MOFs through control of the dimensions of the pores enabling detection and differentiation between the multitude of combustion products in exhaust emissions.²³ In addition the structural flexibility of some frameworks can add a further dynamic dimension to gas selectivity.²⁴ Chemical selectivity and sensitivity can also be controlled by the incorporation of selected dopants into the zeolite and MOF frameworks in addition to the wide choice of organic linkers and metal nodes that can be incorporated into MOFs.²⁵⁻²⁸ Furthermore, MOFs and zeolites can exhibit high temperature stability, i.e. ~500 °C for some MOF-related zeolitic imidazolate framework materials and ~1200 °C for some zeolites.²⁹,³⁰ Indeed, MOFs and zeolites have been utilised as the active sensing element or as a gas selective filter/pre-concentrator in gas sensors which has been noted in several reviews on their applications.²⁵,²⁶,²⁸,³¹⁻³⁵ However, these reviews do not focus on the gases and vapours found in engine exhaust gases; some include examples of dissolved gases and none critically review zeolite- and MOF-based sensors in terms of thermal and selectivity suitability for sensing of gases in exhaust streams.

This review aims to provide an insight into the current state of the art of zeolites and MOFs for the sensing of gases found in vehicle exhausts along with a discussion of the associated sensor technologies allowing them to be employed. The review will also compare and contrast the performance of exemplar porous materials for such applications, to provide a perspective on the future prospects of zeolite- or MOF-based exhaust gas sensors in transportation. Examples of zeolite- and MOF-based sensors for the following important exhaust gases will be examined: water vapour (H₂O), oxygen (O₂), nitrogen oxides (NOₓ), carbon monoxide (CO) and carbon dioxide (CO₂) together, hydrocarbons (in particular methanol, ethanol, aldehydes and alkanes), ammonia (NH₃, the reagent in SCR catalysts), hydrogen sulfide (H₂S), sulfur dioxide (SO₂) and finally hydrogen (H₂). Hydrogen is included due to the potential for it to be employed as an alternative low-carbon fuel for future vehicles. MOFs have been reported as sensing materials for nitroaromatic and nitroalkane vapours. Since these gases are not found in exhaust streams, sensors for these species are not included in this review. However, an excellent review by Banerjee *et al*. on the sensing of these species was published recently.³⁶

<figure><img src="image_18.png" /></figure>
<figure><img src="image_16.png" /></figure>

## 2.1 Zeolites for sensing

Zeolites are defined as crystalline nano- and mesoporous materials with 3D framework structures that form regular and uniform pores and channels. They can be synthesised with different chemical compositions and frameworks which allows for a wide variety of materials; in fact more than 200 known synthetic zeolites have been reported, in addition to 40 natural ones.³⁷,³⁸ Zeolites consist of tetrahedral units of T (where T is typically Si, Al but can also be another element e.g. B, Ga, Ge, or P) bonded by oxygen atoms resulting in a framework containing cages and channels of distinct sizes and shapes. The internal microporosity of zeolites provides high surface areas and active sites for adsorption. In terms of designing zeolites for specific applications, zeolite crystallisation is an important process to control.³⁹ The nature of precursors, the molar ratios, the type of solvents, the presence or absence of templates and additives or seeds all highly influence the final arrangement of the SiO₄ and AlO₄ tetrahedra. In addition, post-synthesis treatment can be used to tune the obtained properties. Some examples are ion-exchange reactions, surface modification reactions to increase or decrease the hydrophilic character of the zeolite, dealumination reactions to modify the Si/Al ratio of the zeolite framework, tailor the hydrophilic/hydrophobic character and to control the number of cations within the zeolite, and porosity alteration to obtain a hierarchical zeolite which exhibits additional porosity in the mesopore size region.⁴⁰

Molecular sieves are well known for application as adsorption media for various gases, and they can selectively adsorb molecules that are smaller than the pore sizes of the zeolites. Crystalline microporous materials, in particular zeolites, have significant impact in different applications ranging from petroleum refining and petrochemical processes to air separation and nuclear waste management. The completely controlled porous structure of zeolitic materials makes these materials true shape-selective molecular sieves. The presence of charge-compensating cations such as alkali metal, alkaline earth metal and protons within the inorganic frameworks allows fine-tuning of the ion exchange and catalytic properties. The hydrophobic nature of high-silica zeolites or the hydrophilic nature of aluminium-rich zeolites make these solids useful as specific adsorbents of organic molecules or water in the gas or liquid phase. Zeolites are therefore versatile materials in terms of potential applications, and are used as heterogeneous catalysts in industrial processes, ion-exchangers for water purification and water softening, as separating membranes and also as catalyst supports.⁴¹⁻⁴⁴

Furthermore, a number of novel applications depend not only on the control of pore structure and intrazeolite chemistry, but also on the ability to control the morphology and pre-shaping, for example the growth of thin zeolite films for separation and sensing purposes. A number of novel applications of zeolites depend on the ability to create thin films. For numerous synthetic strategies, the main goal is to obtain adhesive layers of zeolites on various substrates such as noble and non-noble metals, glass, ceramics, silicon, and even three-dimensional substrates.⁴⁵ The approach employed for a particular zeolite film preparation

<figure><img src="image_17.png" /><figcaption>Fig. 1 An overview of the different functions of zeolites in gas sensor applications.</figcaption></figure>

depends on the final application. The porous films can be deposited on the gas-sensitive element in a continuous and crack-free manner. The following three approaches are applied for deposition of zeolite crystals:

(a) spin (dip) coating (thickness in the range of 200–1000 nm)

(b) hydrothermal growth of seeded zeolite layers (thickness of 500–5000 nm)

(c) screen printing based on self-bonded zeolite crystals with/without additives (thickness in the range of 2000–10 000 nm).

In sensing applications, zeolites are used in two main ways, as shown in Fig. 1. Firstly as a functional element with an active role (e.g. incorporating a physical response to adsorbed gases) or secondly as an auxiliary element with a supporting role, e.g. encapsulating the sensing agent, thereby increasing the concentration of an analyte via adsorption or molecular filtering of others gases to improve selectivity or sensitivity of the active sensing agent.

Nine types of zeolite have been reported for sensing in the literature (Table 1) and will be summarised in this review.

In summary, creation of materials with high surface area might allow the selective sensing of gases from exhausts. A porous structure with open accessible volume, high contact surface and strong interactions with only one or more desired analyte would be very important for the design of the required material.

## 2.2 Metal-organic frameworks for sensing

Metal-organic frameworks (MOFs) are nano- and mesoporous hybrid materials with open 2D or 3D framework structures that contain potential voids. MOFs are composed of metal cations or metal cation aggregates linked by multitopic organic linkers which ultimately assemble into crystalline networks. They can be synthesised from the combination of a variety of di-, tri- and tetravalent cations and many different linkers, and this allows for a huge variety of MOF structures.¹³² In addition this large degree of choice of the components in MOFs allows for the design of selective sensing interactions with analyte molecules, based on size or shape exclusion or relative strength of intermolecular interactions. Indeed many types of MOFs have been reported for sensing in the literature.²⁸,³²
## 2.1 Zeolites for sensing

Zeolites are defined as crystalline nano- and mesoporous materials with 3D framework structures that form regular and uniform pores and channels. They can be synthesised with different chemical compositions and frameworks which allows for a wide variety of materials; in fact more than 200 known synthetic zeolites have been reported, in addition to 40 natural ones.³⁷,³⁸ Zeolites consist of tetrahedral units of T (where T is typically Si, Al but can also be another element e.g. B, Ga, Ge, or P) bonded by oxygen atoms resulting in a framework containing cages and channels of distinct sizes and shapes. The internal microporosity of zeolites provides high surface areas and active sites for adsorption. In terms of designing zeolites for specific applications, zeolite crystallisation is an important process to control.³⁹ The nature of precursors, the molar ratios, the type of solvents, the presence or absence of templates and additives or seeds all highly influence the final arrangement of the SiO₄ and AlO₄ tetrahedra. In addition, post-synthesis treatment can be used to tune the obtained properties. Some examples are ion-exchange reactions, surface modification reactions to increase or decrease the hydrophilic character of the zeolite, dealumination reactions to modify the Si/Al ratio of the zeolite framework, tailor the hydrophilic/hydrophobic character and to control the number of cations within the zeolite, and porosity alteration to obtain a hierarchical zeolite which exhibits additional porosity in the mesopore size region.⁴⁰

Molecular sieves are well known for application as adsorption media for various gases, and they can selectively adsorb molecules that are smaller than the pore sizes of the zeolites. Crystalline microporous materials, in particular zeolites, have significant impact in different applications ranging from petroleum refining and petrochemical processes to air separation and nuclear waste management. The completely controlled porous structure of zeolitic materials makes these materials true shape-selective molecular sieves. The presence of charge-compensating cations such as alkali metal, alkaline earth metal and protons within the inorganic frameworks allows fine-tuning of the ion exchange and catalytic properties. The hydrophobic nature of high-silica zeolites or the hydrophilic nature of aluminium-rich zeolites make these solids useful as specific adsorbents of organic molecules or water in the gas or liquid phase. Zeolites are therefore versatile materials in terms of potential applications, and are used as heterogeneous catalysts in industrial processes, ion-exchangers for water purification and water softening, as separating membranes and also as catalyst supports.⁴¹⁻⁴⁴

Furthermore, a number of novel applications depend not only on the control of pore structure and intrazeolite chemistry, but also on the ability to control the morphology and pre-shaping, for example the growth of thin zeolite films for separation and sensing purposes. A number of novel applications of zeolites depend on the ability to create thin films. For numerous synthetic strategies, the main goal is to obtain adhesive layers of zeolites on various substrates such as noble and non-noble metals, glass, ceramics, silicon, and even three-dimensional substrates.⁴⁵ The approach employed for a particular zeolite film preparation

<figure><img src="image_19.png" /><figcaption>Fig. 1 An overview of the different functions of zeolites in gas sensor applications.</figcaption></figure>

depends on the final application. The porous films can be deposited on the gas-sensitive element in a continuous and crack-free manner. The following three approaches are applied for deposition of zeolite crystals:

(a) spin (dip) coating (thickness in the range of 200–1000 nm)

(b) hydrothermal growth of seeded zeolite layers (thickness of 500–5000 nm)

(c) screen printing based on self-bonded zeolite crystals with/without additives (thickness in the range of 2000–10 000 nm).

In sensing applications, zeolites are used in two main ways, as shown in Fig. 1. Firstly as a functional element with an active role (e.g. incorporating a physical response to adsorbed gases) or secondly as an auxiliary element with a supporting role, e.g. encapsulating the sensing agent, thereby increasing the concentration of an analyte via adsorption or molecular filtering of others gases to improve selectivity or sensitivity of the active sensing agent.

Nine types of zeolite have been reported for sensing in the literature (Table 1) and will be summarised in this review.

In summary, creation of materials with high surface area might allow the selective sensing of gases from exhausts. A porous structure with open accessible volume, high contact surface and strong interactions with only one or more desired analyte would be very important for the design of the required material.

## 2.2 Metal-organic frameworks for sensing

Metal-organic frameworks (MOFs) are nano- and mesoporous hybrid materials with open 2D or 3D framework structures that contain potential voids. MOFs are composed of metal cations or metal cation aggregates linked by multitopic organic linkers which ultimately assemble into crystalline networks. They can be synthesised from the combination of a variety of di-, tri- and tetravalent cations and many different linkers, and this allows for a huge variety of MOF structures.¹³² In addition this large degree of choice of the components in MOFs allows for the design of selective sensing interactions with analyte molecules, based on size or shape exclusion or relative strength of intermolecular interactions. Indeed many types of MOFs have been reported for sensing in the literature.²⁸,³²

<figure><img src="image_20.png" /></figure>
<table><caption>**Table 1** The nine zeolite types reported for gas/vapour sensing. The name and International Zeolite Association three-letter code for each zeolite type is presented. In addition, the representative pore dimensions within the structures are given,¹³¹ along with descriptions of the sensor types and the concentration ranges that were investigated</caption><thead><tr><th>Framework type</th><th>Zeolite</th><th>Pore openings (Å)</th><th>Analyte/s, sensor type (range)</th><th>Ref.</th></tr></thead><tbody><tr><td>FER</td><td>Ferrierite</td><td>4.2 × 5.4;<br>3.5 × 4.8</td><td>Carbon monoxide (CO) – conductivity (ppm range)</td><td>46</td></tr><tr><td>MOR</td><td>Mordenite</td><td>6.5 × 7.0;<br>2.6 × 5.7</td><td>Water vapour (H₂O) – UV-vis (9–92% RH), nitrogen dioxide (NO₂) – conductivity (ppb range), ethanol vapour (C₂H₅OH) – conductivity (200 ppm) & ammonia (NH₃) – conductivity (ppm range)</td><td>47–54</td></tr><tr><td>LTA</td><td>Zeolite A</td><td>4.1 × 4.1</td><td>Water vapour (H₂O) – QCM (150 ppm), nitrogen dioxide (NO₂) – QCM (50 ppm), nitric oxide (NO) – QCM (50 ppm), carbon monoxide (CO) conductivity (ppm range), ethanol vapour (C₂H₅OH) – cantilever (25 ppmv), hydrocarbons (HCs) – conductivity (1250 ppm), Metglas (6 ppm) & sulfur dioxide (SO₂) – QCM (50 ppm)</td><td>55–70</td></tr><tr><td>MFI</td><td>ZSM-5<br>Silicalite-1</td><td>5.3 × 5.6;<br>5.1 × 5.5</td><td>Water vapour (H₂O) – cantilever (ng), QCM (250–750 ppm), UV vis (2.8% RH), conductivity (300 ppm), nitrogen dioxide (NO₂) – QCM (200 ppm), carbon monoxide (CO) – impedance (350 ppm), carbon dioxide (CO₂) – calorimetry (ppm range), ethanol vapour (C₂H₅OH) – QCM (50 ppm), cantilever (25 ppmv), IR (16.5 ppm), hydrocarbons (HCs) – conductivity (1250 ppm), IR (22 ppb), Metglas (180 ppm), ammonia (NH₃) – conductivity (5 ppm; 612 ppmv) & hydrogen (H₂) – fibre optic (0.14 nm kPa⁻¹)</td><td>50–52, 57–61, 66 and 71–93</td></tr><tr><td>BEA</td><td>Beta</td><td>6.6 × 6.7;<br>5.6 × 5.6</td><td>Water vapour (H₂O) – QCM (150 ppm), nitrogen dioxide (NO₂) – conductivity (ppb range), carbon dioxide (CO) – IR (2–100 ppm), ethanol vapour (C₂H₅OH) – conductivity (200 ppm) & ammonia (NH₃) – impedance (25 ppm)</td><td>49, 51, 52, 55, 56, 58, 60, 61, 64 and 94–96</td></tr><tr><td>FAU</td><td>Zeolite X<br>Zeolite Y</td><td>7.4 × 7.4<br>7.1 × 7.1</td><td>Water vapour (H₂O) – UV vis (200 ppm), conductivity (40% RH), oxygen (O₂) – UV vis, conductivity (100 ppm), nitrogen dioxide (NO₂) – conductivity (204 ppb), potentiometry (200 ppb), amperometry (ppm range), carbon dioxide (CO) – impedance (1000 ppm), magnetic (0.33%), ethanol vapour (C₂H₅OH) – cantilever (1.4 ppmv), hydrocarbons (HCs) – UV vis (0.06–31.2 µg mL⁻¹), impedance (10 ppm), cantilever (0.7 ppmv), ammonia (NH₃) – conductivity (ppm range) & sulfur dioxide (SO₂) – conductivity (100 ppm)</td><td>49–52, 54, 60, 61, 66 and 97–127</td></tr><tr><td>STI</td><td>Stilbite</td><td>4.7 × 5.0;<br>2.7 × 5.6</td><td>Water vapour (H₂O) – conductivity (63% RH), methanol vapour (CH₃OH) & hydrocarbons (HCs) – conductivity (ppm range)</td><td>128 and 129</td></tr><tr><td>LTL</td><td>Perlialite</td><td>7.1 × 7.1</td><td>Carbon monoxide (CO) – IR (2–100 ppm)</td><td>94</td></tr><tr><td>AFI</td><td>AlPO-5</td><td>7.3 × 7.3</td><td>Carbon monoxide (CO) – microcalorimetry (ppm range)</td><td>130</td></tr></tbody></table>

In a similar way to zeolite materials, the internal micro-porosity of MOFs provides high surface areas and active sites for adsorption in addition to selectivity provided by the dimensions of the pores. Thus, sensing elements comprised of MOFs can also be used for direct adsorption of and interaction with the analyte of interest or as a support to improve selectivity or sensitivity of the active sensing agent (Fig. 2).

Many different metal-organic frameworks have been reported for sensing in the literature (Table 2) and these will be summarised in this review.

<figure><img src="image_18.png" /><figcaption>**Fig. 2** An overview of the different functions of metal-organic frameworks in gas sensor applications.</figcaption></figure>

## 2.3 Techniques for monitoring gas interactions

Chemical sensors are miniaturised devices that can deliver real-time and on-line information on the presence of specific compounds or ions in complex samples.¹⁷⁵ In some cases MOFs and zeolites have been used to simply interact with gases and vapours in an irreversible manner and for the purpose of this review this will be referred to as gas detection rather than gas sensing.

When a zeolite or a MOF is used as a functional sensing element with an active role, the real-time gas sensing response can be monitored through (i) mass/mechanical changes, for example *via* a quartz crystal microbalance (QCM), microcantilevers and/or microresonator, (ii) measuring optical property changes (*e.g.* IR spectroscopic measurements, refractive index measurements and/or luminescence spectroscopic measurements), (iii) variation in electric properties (*e.g.* impedance, capacitance and/or resistance), (iv) heat liberation (calorimetry) or even (v) by visible colour change.

Similarly, when the porous material is used as an auxiliary filtering element, the monitoring of the gas absorbance on corresponding sensing agents, such as metal oxides or polymers, can be achieved using the same techniques. The use of the porous materials as hosts for gasochromic sensing agents, such as reactive dyes, allows for visual monitoring in addition to conventional measurements. These monitoring techniques are briefly described below.

<figure><img src="image_19.png" /></figure>
<table><caption>**Table 1** The nine zeolite types reported for gas/vapour sensing. The name and International Zeolite Association three-letter code for each zeolite type is presented. In addition, the representative pore dimensions within the structures are given,¹³¹ along with descriptions of the sensor types and the concentration ranges that were investigated</caption><thead><tr><th>Framework type</th><th>Zeolite</th><th>Pore openings (Å)</th><th>Analyte/s, sensor type (range)</th><th>Ref.</th></tr></thead><tbody><tr><td>FER</td><td>Ferrierite</td><td>4.2 × 5.4;<br>3.5 × 4.8</td><td>Carbon monoxide (CO) – conductivity (ppm range)</td><td>46</td></tr><tr><td>MOR</td><td>Mordenite</td><td>6.5 × 7.0;<br>2.6 × 5.7</td><td>Water vapour (H₂O) – UV-vis (9–92% RH), nitrogen dioxide (NO₂) – conductivity (ppb range), ethanol vapour (C₂H₅OH) – conductivity (200 ppm) & ammonia (NH₃) – conductivity (ppm range)</td><td>47–54</td></tr><tr><td>LTA</td><td>Zeolite A</td><td>4.1 × 4.1</td><td>Water vapour (H₂O) – QCM (150 ppm), nitrogen dioxide (NO₂) – QCM (50 ppm), nitric oxide (NO) – QCM (50 ppm), carbon monoxide (CO) conductivity (ppm range), ethanol vapour (C₂H₅OH) – cantilever (25 ppmv), hydrocarbons (HCs) – conductivity (1250 ppm), Metglas (6 ppm) & sulfur dioxide (SO₂) – QCM (50 ppm)</td><td>55–70</td></tr><tr><td>MFI</td><td>ZSM-5<br>Silicalite-1</td><td>5.3 × 5.6;<br>5.1 × 5.5</td><td>Water vapour (H₂O) – cantilever (ng), QCM (250–750 ppm), UV vis (2.8% RH), conductivity (300 ppm), nitrogen dioxide (NO₂) – QCM (200 ppm), carbon monoxide (CO) – impedance (350 ppm), carbon dioxide (CO₂) – calorimetry (ppm range), ethanol vapour (C₂H₅OH) – QCM (50 ppm), cantilever (25 ppmv), IR (16.5 ppm), hydrocarbons (HCs) – conductivity (1250 ppm), IR (22 ppb), Metglas (180 ppm), ammonia (NH₃) – conductivity (5 ppm; 612 ppmv) & hydrogen (H₂) – fibre optic (0.14 nm kPa⁻¹)</td><td>50–52, 57–61, 66 and 71–93</td></tr><tr><td>BEA</td><td>Beta</td><td>6.6 × 6.7;<br>5.6 × 5.6</td><td>Water vapour (H₂O) – QCM (150 ppm), nitrogen dioxide (NO₂) – conductivity (ppb range), carbon dioxide (CO) – IR (2–100 ppm), ethanol vapour (C₂H₅OH) – conductivity (200 ppm) & ammonia (NH₃) – impedance (25 ppm)</td><td>49, 51, 52, 55, 56, 58, 60, 61, 64 and 94–96</td></tr><tr><td>FAU</td><td>Zeolite X<br>Zeolite Y</td><td>7.4 × 7.4<br>7.1 × 7.1</td><td>Water vapour (H₂O) – UV vis (200 ppm), conductivity (40% RH), oxygen (O₂) – UV vis, conductivity (100 ppm), nitrogen dioxide (NO₂) – conductivity (204 ppb), potentiometry (200 ppb), amperometry (ppm range), carbon dioxide (CO) – impedance (1000 ppm), magnetic (0.33%), ethanol vapour (C₂H₅OH) – cantilever (1.4 ppmv), hydrocarbons (HCs) – UV vis (0.06–31.2 µg mL⁻¹), impedance (10 ppm), cantilever (0.7 ppmv), ammonia (NH₃) – conductivity (ppm range) & sulfur dioxide (SO₂) – conductivity (100 ppm)</td><td>49–52, 54, 60, 61, 66 and 97–127</td></tr><tr><td>STI</td><td>Stilbite</td><td>4.7 × 5.0;<br>2.7 × 5.6</td><td>Water vapour (H₂O) – conductivity (63% RH), methanol vapour (CH₃OH) & hydrocarbons (HCs) – conductivity (ppm range)</td><td>128 and 129</td></tr><tr><td>LTL</td><td>Perlialite</td><td>7.1 × 7.1</td><td>Carbon monoxide (CO) – IR (2–100 ppm)</td><td>94</td></tr><tr><td>AFI</td><td>AlPO-5</td><td>7.3 × 7.3</td><td>Carbon monoxide (CO) – microcalorimetry (ppm range)</td><td>130</td></tr></tbody></table>

In a similar way to zeolite materials, the internal micro-porosity of MOFs provides high surface areas and active sites for adsorption in addition to selectivity provided by the dimensions of the pores. Thus, sensing elements comprised of MOFs can also be used for direct adsorption of and interaction with the analyte of interest or as a support to improve selectivity or sensitivity of the active sensing agent (Fig. 2).

Many different metal-organic frameworks have been reported for sensing in the literature (Table 2) and these will be summarised in this review.

<figure><img src="image_21.png" /><figcaption>**Fig. 2** An overview of the different functions of metal-organic frameworks in gas sensor applications.</figcaption></figure>

## 2.3 Techniques for monitoring gas interactions

Chemical sensors are miniaturised devices that can deliver real-time and on-line information on the presence of specific compounds or ions in complex samples.¹⁷⁵ In some cases MOFs and zeolites have been used to simply interact with gases and vapours in an irreversible manner and for the purpose of this review this will be referred to as gas detection rather than gas sensing.

When a zeolite or a MOF is used as a functional sensing element with an active role, the real-time gas sensing response can be monitored through (i) mass/mechanical changes, for example *via* a quartz crystal microbalance (QCM), microcantilevers and/or microresonator, (ii) measuring optical property changes (*e.g.* IR spectroscopic measurements, refractive index measurements and/or luminescence spectroscopic measurements), (iii) variation in electric properties (*e.g.* impedance, capacitance and/or resistance), (iv) heat liberation (calorimetry) or even (v) by visible colour change.

Similarly, when the porous material is used as an auxiliary filtering element, the monitoring of the gas absorbance on corresponding sensing agents, such as metal oxides or polymers, can be achieved using the same techniques. The use of the porous materials as hosts for gasochromic sensing agents, such as reactive dyes, allows for visual monitoring in addition to conventional measurements. These monitoring techniques are briefly described below.

<figure><img src="image_22.png" /></figure>
<figure><img src="image_20.png" /></figure>

<table><caption>**Table 2** The metal–organic frameworks reported for sensing of exhaust gases with descriptions of the sensor types and the concentration ranges that were investigated</caption><thead><tr><th>Metal–organic framework<sup>a</sup></th><th>Analyte/s</th><th>Detector/sensor type</th><th>Dynamic range (where stated)</th><th>Ref.</th></tr></thead><tbody><tr><td>[Eu(L<sup>1</sup>)[oxalate]<sub>0.5</sub>(H<sub>2</sub>O)]·2H<sub>2</sub>O</td><td>Water vapour (H<sub>2</sub>O)</td><td>Luminescence quenching</td><td>75.8–85% RH</td><td>133</td></tr><tr><td>[Tb(L<sup>1</sup>)[oxalate]<sub>0.5</sub>(H<sub>2</sub>O)]·2H<sub>2</sub>O</td><td>Water vapour (H<sub>2</sub>O)</td><td>Luminescence quenching</td><td>75.8–85% RH</td><td>133</td></tr><tr><td>[Tb(cepto)[H<sub>2</sub>O]]·2DMF·H<sub>2</sub>O (PCM-15)</td><td>Water vapour (H<sub>2</sub>O) &amp; ammonia (NH<sub>3</sub>)</td><td>Luminescence quenching</td><td>NH<sub>3</sub>: 4.5–13.4 µmol</td><td>134 and 135</td></tr><tr><td>[Zn(dpe)[bdc]]·4H<sub>2</sub>O (ZndB)</td><td>Water vapour (H<sub>2</sub>O)</td><td>Luminescence quenching</td><td>—</td><td>136</td></tr><tr><td>[In(OH)<sub>3</sub>][bdc]</td><td>Water vapour (H<sub>2</sub>O) &amp; ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Luminescence spectroscopy</td><td>Saturated atmosphere</td><td>137</td></tr><tr><td>[Zn<sub>4</sub>O(bdc)<sub>3</sub>] (MOF-5)</td><td>Water vapour (H<sub>2</sub>O) &amp; ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Luminescence spectroscopy</td><td>Saturated atmosphere</td><td>137</td></tr><tr><td>[Cu<sub>3</sub>(btc)<sub>2</sub>] (HKUST-1)</td><td>Water vapour (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH), methanol vapour (CH<sub>3</sub>OH), hydrocarbons (HCs), ammonia (NH<sub>3</sub>), hydrogen sulfide (H<sub>2</sub>S) &amp; hydrogen (H<sub>2</sub>)</td><td>Piezoresistive microcantilever sensor (PRMS), colloidal crystal Bragg reflector (CCBR), microcantilever sensor (MCS), Kelvin probe (KP), quartz crystal microbalance (QCM), localised surface polarized resonance (LSPR), surface polarized resonance (SPR) &amp; filter layer on optical sensor (FL)</td><td>H<sub>2</sub>O: 277–1330 ppm (PRMS), 0–8000 ppm (CCBR), 0.2–1.6% (MCS) CO<sub>2</sub>: measurement at 100 ppm (CCBR), 8–70% (MCS), measurement at 10% (in N<sub>2</sub>) (QCM), 0–100% (LSPR) C<sub>2</sub>H<sub>5</sub>OH: 0–6000 ppm (CCBR), 0.6–2.8% (MCS), 1–70% (SPR), 2–50 ppm (QCM), 2–50 ppm (KP) CH<sub>3</sub>OH: 1368–6580 ppm (PRMS), 1.2–6.0% (MCS), 1–70% SPR, 25–50 ppm (KP), 10–50 ppm (QCM) HCs: C<sub>2</sub>H<sub>6</sub> – measurement at 100 ppm (CCBR), pentanal 1–15 ppm (KP) NH<sub>3</sub>: measurement at 5 ppm (KP) H<sub>2</sub>S: measurement at 2.5 ppm (KP) H<sub>2</sub>: (FL)</td><td>138–147</td></tr><tr><td>[Fe<sub>3</sub>(btc)<sub>2</sub>] (Basolite<sup>b</sup> F300)</td><td>Water vapour (H<sub>2</sub>O), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) &amp; methanol vapour (CH<sub>3</sub>OH)</td><td>Impedimetric gas sensors</td><td>H<sub>2</sub>O: 0–2.5% vol C<sub>2</sub>H<sub>5</sub>OH: 7–18% vol</td><td>148</td></tr><tr><td>[Zn<sub>4</sub>O(benztb)(btc)<sub>2/3</sub>] (DUT-25)</td><td>Water vapour (H<sub>2</sub>O), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) &amp; methanol vapour (CH<sub>3</sub>OH)</td><td>Solvatochromism</td><td>—</td><td>149</td></tr><tr><td>[Fe<sub>3</sub>O(abdc)<sub>3</sub>X] (where X = Cl, OH) (NH<sub>2</sub>-MIL-88B)</td><td>Water vapour (H<sub>2</sub>O) &amp; ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH), Oxygen (O<sub>2</sub>)</td><td>UV-visible reflection spectroscopy</td><td>—</td><td>150</td></tr><tr><td>[Ti<sub>3</sub>O<sub>8</sub>(OH)<sub>4</sub>(abdc)<sub>6</sub> NH<sub>2</sub>-MIL-125(Ti)</td><td>Water vapour (H<sub>2</sub>O)</td><td>Impedimetric gas sensor</td><td>11–95% RH</td><td>151</td></tr><tr><td>Yb<sup>3+</sup>@bio-MOF-1, where bio-MOF-1 = (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>[Zn<sub>8</sub>O(ad)<sub>4</sub>(bpdc)<sub>6</sub>]·8DMF·11H<sub>2</sub>O</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>Measurements at 0% and 100%</td><td>152</td></tr><tr><td>[Zn<sub>4</sub>O(L<sup>2</sup>)<sub>2</sub>·6DMF·H<sub>2</sub>O</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–1.00 atm P<sub>O<sub>2</sub></sub></td><td>153</td></tr><tr><td>[Cd(L<sup>3</sup>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>·3DMF·6H<sub>2</sub>O</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>Lower limit 0.50%</td><td>154</td></tr><tr><td>[Zn<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(bpdc)<sub>5.971</sub>(I<sup>4</sup>)<sub>0.029</sub> (derivative of UiO-67)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–0.80 atm P<sub>O<sub>2</sub></sub></td><td>155</td></tr><tr><td>[Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(bpdc)<sub>5.942</sub>(L<sup>5</sup>)<sub>0.058</sub> (derivative of UiO-67)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–0.80 atm P<sub>O<sub>2</sub></sub></td><td>155</td></tr><tr><td>[Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(bpdc)<sub>5.821</sub>(L<sup>6</sup>)<sub>0.179</sub> (derivative of UiO-67)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–0.80 atm P<sub>O<sub>2</sub></sub></td><td>155</td></tr><tr><td>[Ru<sub>7</sub>Zn<sub>7</sub>-x ip<sub>12</sub>](OH)<sub>2</sub> (x = 0.10–0.16) (ruthenium doped MAF-34 MOFs)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.03–1.013 bar P<sub>O<sub>2</sub></sub></td><td>156</td></tr><tr><td>[Zn<sub>4</sub>O(bpz)<sub>2</sub>(abdc)] (MAF-X11)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–1.00 bar P<sub>O<sub>2</sub></sub></td><td>157</td></tr><tr><td>[[CH<sub>3</sub>NH<sub>2</sub>][In<sub>3</sub>O(btc)<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub>][In<sub>3</sub>(btc)<sub>4</sub>]</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>[CPM-5]</td><td></td><td>Luminescence quenching</td><td></td><td></td></tr><tr><td>[In<sub>3</sub>O(OH)(H<sub>2</sub>O)<sub>2</sub>(btc)<sub>2</sub>] MIL-100(In)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>Cu-Y(btc), copper doped Y(btc) (JUC-32-Y)</td><td>Carbon monoxide (CO)</td><td>In situ IR spectroscopy</td><td>0–100 ppm</td><td>159</td></tr><tr><td>[Zn(ebM)<sub>1</sub>(nIM)] (ZIF-69)</td><td>Carbon dioxide (CO<sub>2</sub>)</td><td>Microscale resonator</td><td>1300–5200 ppm</td><td>160</td></tr><tr><td>MV[Mn<sub>2</sub>Cu<sub>3</sub>(mpba)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>·20H<sub>2</sub>O</td><td>Carbon dioxide (CO<sub>2</sub>)</td><td>Luminescence spectroscopy</td><td>Measurement at 1 bar</td><td>161</td></tr></tbody></table>
<figure><img src="image_23.png" /><figcaption>**Table 2** The metal–organic frameworks reported for sensing of exhaust gases with descriptions of the sensor types and the concentration ranges that were investigated</figcaption></figure>

<table><thead><tr><th>Metal–organic framework<sup>a</sup></th><th>Analyte/s</th><th>Detector/sensor type</th><th>Dynamic range (where stated)</th><th>Ref.</th></tr></thead><tbody><tr><td>[Eu(L<sup>1</sup>)[oxalate]<sub>0.5</sub>(H<sub>2</sub>O)]·2H<sub>2</sub>O</td><td>Water vapour (H<sub>2</sub>O)</td><td>Luminescence quenching</td><td>75.8–85% RH</td><td>133</td></tr><tr><td>[Tb(L<sup>1</sup>)[oxalate]<sub>0.5</sub>(H<sub>2</sub>O)]·2H<sub>2</sub>O</td><td>Water vapour (H<sub>2</sub>O)</td><td>Luminescence quenching</td><td>75.8–85% RH</td><td>133</td></tr><tr><td>[Tb(cepto)[H<sub>2</sub>O]]·2DMF·H<sub>2</sub>O (PCM-15)</td><td>Water vapour (H<sub>2</sub>O) &amp; ammonia (NH<sub>3</sub>)</td><td>Luminescence quenching</td><td>NH<sub>3</sub>: 4.5–13.4 µmol</td><td>134 and 135</td></tr><tr><td>[Zn(dpe)[bdc]]·4H<sub>2</sub>O (ZndB)</td><td>Water vapour (H<sub>2</sub>O)</td><td>Luminescence quenching</td><td>—</td><td>136</td></tr><tr><td>[In(OH)[bdc]]</td><td>Water vapour (H<sub>2</sub>O) &amp; ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Luminescence spectroscopy</td><td>Saturated atmosphere</td><td>137</td></tr><tr><td>[Zn<sub>4</sub>O(bdc)<sub>3</sub>] (MOF-5)</td><td>Water vapour (H<sub>2</sub>O) &amp; ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Luminescence spectroscopy</td><td>Saturated atmosphere</td><td>137</td></tr><tr><td>[Cu<sub>3</sub>(btc)<sub>2</sub>] (HKUST-1)</td><td>Water vapour (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH), methanol vapour (CH<sub>3</sub>OH), hydrocarbons (HCs), ammonia (NH<sub>3</sub>), hydrogen sulfide (H<sub>2</sub>S) &amp; hydrogen (H<sub>2</sub>)</td><td>Piezoresistive microcantilever sensor (PRMS), colloidal crystal Bragg reflector (CCBR), microcantilever sensor (MCS), Kelvin probe (KP), quartz crystal microbalance (QCM), localised surface polarized resonance (LSPR), surface polarized resonance (SPR) &amp; filter layer on optical sensor (FL)</td><td>H<sub>2</sub>O: 277–1330 ppm (PRMS), 0–8000 ppm (CCBR), 0.2–1.6% (MCS) CO<sub>2</sub>: measurement at 100 ppm (CCBR), 8–70% (MCS), measurement at 10% (in N<sub>2</sub>) (QCM), 0–100% (LSPR) C<sub>2</sub>H<sub>5</sub>OH: 0–6000 ppm (CCBR), 0.6–2.8% (MCS), 1–70% (SPR), 2–50 ppm (QCM), 2–50 ppm (KP) CH<sub>3</sub>OH: 1368–6580 ppm (PRMS), 1.2–6.0% (MCS), 1–70% SPR, 25–50 ppm (KP), 10–50 ppm (QCM) HCs: C<sub>2</sub>H<sub>6</sub> – measurement at 100 ppm (CCBR), pentanal 1–15 ppm (KP) NH<sub>3</sub>: measurement at 5 ppm (KP) H<sub>2</sub>S: measurement at 2.5 ppm (KP) H<sub>2</sub>: (FL)</td><td>138–147</td></tr><tr><td>[Fe<sub>3</sub>(btc)<sub>2</sub>] (Basolite<sup>b</sup> F300)</td><td>Water vapour (H<sub>2</sub>O), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) &amp; methanol vapour (CH<sub>3</sub>OH)</td><td>Impedimetric gas sensors</td><td>H<sub>2</sub>O: 0–2.5% vol C<sub>2</sub>H<sub>5</sub>OH: 7–18% vol</td><td>148</td></tr><tr><td>[Zn<sub>4</sub>O(benztb)(btc)<sub>2/3</sub>] (DUT-25)</td><td>Water vapour (H<sub>2</sub>O), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) &amp; methanol vapour (CH<sub>3</sub>OH)</td><td>Solvatochromism</td><td>—</td><td>149</td></tr><tr><td>[Fe<sub>3</sub>O(abdc)<sub>3</sub>X]<sub>1</sub> (where X = Cl, OH) (NH<sub>2</sub>-MIL-88B)</td><td>Water vapour (H<sub>2</sub>O) &amp; ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH), Oxygen (O<sub>2</sub>)</td><td>UV-visible reflection spectroscopy</td><td>—</td><td>150</td></tr><tr><td>[Ti<sub>3</sub>O<sub>8</sub>(OH)<sub>4</sub>(abdc)<sub>6</sub> NH<sub>2</sub>-MIL-125(Ti)</td><td>Water vapour (H<sub>2</sub>O)</td><td>Impedimetric gas sensor</td><td>11–95% RH</td><td>151</td></tr><tr><td>Yb<sup>3+</sup>@bio-MOF-1, where bio-MOF-1 = (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>[Zn<sub>8</sub>O(ad)<sub>4</sub>(bpdc)<sub>6</sub>]·8DMF·11H<sub>2</sub>O</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>Measurements at 0% and 100%</td><td>152</td></tr><tr><td>[Zn<sub>4</sub>O(L<sup>2</sup>)<sub>2</sub>·6DMF·H<sub>2</sub>O</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–1.00 atm P<sub>O<sub>2</sub></sub></td><td>153</td></tr><tr><td>[Cd(L<sup>3</sup>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>·3DMF·6H<sub>2</sub>O</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>Lower limit 0.50%</td><td>154</td></tr><tr><td>[Zn<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(bpdc)<sub>5.971</sub>(I<sup>4</sup>)<sub>0.029</sub>] (derivative of UiO-67)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–0.80 atm P<sub>O<sub>2</sub></sub></td><td>155</td></tr><tr><td>[Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(bpdc)<sub>5.942</sub>(L<sup>5</sup>)<sub>0.058</sub>] (derivative of Oxygen (O<sub>2</sub>)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–0.80 atm P<sub>O<sub>2</sub></sub></td><td>155</td></tr><tr><td>UiO-67</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–0.80 atm P<sub>O<sub>2</sub></sub></td><td>155</td></tr><tr><td>[Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(bpdc)<sub>5.821</sub>(L<sup>6</sup>)<sub>0.179</sub>] (derivative of Oxygen (O<sub>2</sub>)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.03–1.013 bar P<sub>O<sub>2</sub></sub></td><td>156</td></tr><tr><td>UiO-67</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.05–1.00 bar P<sub>O<sub>2</sub></sub></td><td>157</td></tr><tr><td>[Ru<sub>7</sub>Zn<sub>7</sub>-x ip<sub>12</sub>](OH)<sub>2</sub> (x = 0.10–0.16) (ruthenium doped MAF-34 MOFs)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>[Zn<sub>4</sub>O(bpz)<sub>2</sub>(abdc)] (MAF-X11)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>[[CH<sub>3</sub>NH<sub>2</sub>][In<sub>3</sub>O(btc)<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub>][In<sub>3</sub>(btc)<sub>4</sub>]</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>[CPM-5]</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>[In<sub>3</sub>O(OH)(H<sub>2</sub>O)<sub>2</sub>(btc)<sub>2</sub>] MIL-100(In)</td><td>Oxygen (O<sub>2</sub>)</td><td>Luminescence quenching</td><td>0.0–1.0 atm P<sub>O<sub>2</sub></sub></td><td>158</td></tr><tr><td>Cu-Y(btc), copper doped Y(btc) (JUC-32-Y)</td><td>Carbon monoxide (CO)</td><td>In situ IR spectroscopy</td><td>0–100 ppm</td><td>159</td></tr><tr><td>[Zn(ebM)(nIM)] (ZIF-69)</td><td>Carbon dioxide (CO<sub>2</sub>)</td><td>Microscale resonator</td><td>1300–5200 ppm</td><td>160</td></tr><tr><td>MV[Mn<sub>2</sub>Cu<sub>3</sub>(mpba)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>·20H<sub>2</sub>O</td><td>Carbon dioxide (CO<sub>2</sub>)</td><td>Luminescence spectroscopy</td><td>Measurement at 1 bar</td><td>161</td></tr></tbody></table>
<figure><img src="image_21.png" /></figure>

<table><caption>Table 2 (continued)</caption><thead><tr><th>Metal-organic framework<sup>a</sup></th><th>Analyte/s</th><th>Detector/sensor type</th><th>Dynamic range (where stated)</th><th>Ref.</th></tr></thead><tbody><tr><td>[Zn(mIM)<sub>2</sub>] (ZIF-8)</td><td>Carbon dioxide (CO<sub>2</sub>), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH), hydrocarbons (HCs) & hydrogen (H<sub>2</sub>)</td><td>Fabry-Pérot device (FPD), filter layer (FL)</td><td>C<sub>2</sub>H<sub>5</sub>OH: 0-40% (FPD)<br>HCs: C<sub>3</sub>H<sub>8</sub>: 0-100% (FPD)<br>H<sub>2</sub>: (FL)</td><td>162 and<br>163</td></tr><tr><td>[Co(mIM)<sub>2</sub>] (ZIF-67)</td><td>Formaldehyde (HCHO) & methanol vapour (CH<sub>3</sub>OH)</td><td>Interdigitated electrode resistance sensor (IERS)</td><td>HCHO: 5-500 ppm CH<sub>3</sub>OH: measurement at 100 ppm</td><td>164</td></tr><tr><td>[Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>]<sub>6</sub> (UiO-66)</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>UV-visible reflectance spectroscopy</td><td>CH<sub>3</sub>OH: 10 µL (vapour) C<sub>2</sub>H<sub>5</sub>OH: 10 µL (vapour)</td><td>165</td></tr><tr><td>[Cu<sub>2</sub>(OH)<sub>2</sub>]<sub>2</sub>·2bipy<sub>2</sub>]<sub>2</sub>(bdc)<sub>6</sub>·2H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>Quartz crystal microbalance</td><td>64-160 ppm</td><td>166</td></tr><tr><td>[Co(4,4'-bipy][m-bdc]</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>Quartz crystal microbalance</td><td>2-16 ppm</td><td>166</td></tr><tr><td>[Al<sub>12</sub>O(OH)<sub>18</sub>(H<sub>2</sub>O)<sub>3</sub>(Al<sub>2</sub>(OH)<sub>4</sub>)]<sub>6</sub>·24H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>CH<sub>3</sub>OH: 100 ppm (lower limit)</td><td>167, 168</td></tr><tr><td>[Co<sub>3</sub>(btc)<sub>2</sub>]·H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>CH<sub>3</sub>OH: 20-50 ppm</td><td>167, 168</td></tr><tr><td>[Ni<sub>3</sub>(btc)<sub>2</sub>]·H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>C<sub>2</sub>H<sub>5</sub>OH: 10-50 ppm</td><td>167, 168</td></tr><tr><td>[Cd<sub>3</sub>(btc)<sub>2</sub>]·H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>C<sub>2</sub>H<sub>5</sub>OH: 10-50 ppm</td><td>167, 168</td></tr><tr><td>[Zn<sub>2</sub>(ndc)<sub>2</sub>]<sub>2</sub>(dpe)]·2.5DMF·0.25H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) & hydrocarbons (HCs)</td><td>Luminescence quenching (LQ)</td><td>Concentration of all analytes was vapour pressure of each analytes at room temperature.</td><td>169</td></tr><tr><td>[Zn<sub>2</sub>(ndc)<sub>2</sub>]<sub>2</sub>(bpee)]·2.25DMF·0.5H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) & hydrocarbons (HCs)</td><td>Luminescence quenching (LQ)</td><td>Concentration of all analytes was vapour pressure of each analytes at room temperature.</td><td>169</td></tr><tr><td>[Zn<sub>2</sub>(bpdc)<sub>2</sub>]<sub>2</sub>(bpee)]·2DMF (RPM3)</td><td>Hydrocarbons (HCs)</td><td>Luminescence spectroscopy</td><td>—</td><td>170</td></tr><tr><td>[Zn(NO<sub>2</sub>-ip)<sub>2</sub>·(4,4'-bipy)] (Zn-CID-5)</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>Quartz crystal microbalance</td><td>Measurement at 40%</td><td>171</td></tr><tr><td>[Co<sub>3</sub>(btc)<sub>2</sub>(DMF)<sub>2</sub>]·4DMF</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>UV-visible spectroscopy and visible colour change</td><td>Vapour pressure of methanol at room temperature.</td><td>172</td></tr><tr><td>[Zn<sub>2</sub>(tpce)]</td><td>Ammonia (NH<sub>3</sub>)</td><td>Luminescence spectroscopy</td><td>1% ammonia (NH<sub>3</sub>) in nitrogen carrier gas</td><td>173</td></tr><tr><td>[Mg<sub>3</sub>(dobdc)]</td><td>Ammonia (NH<sub>3</sub>)</td><td>Luminescence spectroscopy</td><td>1% ammonia (NH<sub>3</sub>) in nitrogen carrier gas</td><td>173</td></tr><tr><td>[Cu(att)<sub>1,5</sub>]</td><td>Hydrogen sulfide (H<sub>2</sub>S)</td><td>Luminescence spectroscopy</td><td>0-650 ppm</td><td>174</td></tr></tbody></table>

a L¹ = pyridyl-4,5-imidazole dicarboxylate; tctpo = tris(p-carboxylate)triphenylphosphine oxide; dpe = 1,2-bis(4-pyridyl)ethane; bdc = 1,4-benzenedicarboxylate; btc = 1,3,5-benzenetricarboxylate; benztb = N,N,N',N'-benzidinetetrabenzoate; abdc = 2-amino-1,4-benzenedicarboxylate; ad-adeninate; bpcd = 4,4'-biphenyldicarboxylate; L² = [Ir(3-(2-pyridyl)]benzoate)³]; L³ = [Ir(2-(4-carboxylato-2-pyridyl)pyridine-4-carboxylate)(2-phenylpyridine)²]; L⁴ = [Ir(6-(4-carboxylatophenyl)pyridine-3-carboxylate)(2-phenylpyridine)²]; L⁵ = [Ir(6-(5-carboxylato-2-pyridyl)pyridine-3-carboxylate)(2-phenylpyridine)²]; L⁶ = [Ru(6-(5-carboxylato-2-pyridyl)pyridine-3-carboxylate)(2,2'-bipyridine)²]; ip = imidazo[4,5-f][1,10]phenanthrolin-1-ide; bpz = 4-(3,5-dimethylpyrazol-1-ide)-3,5-dimethyl-pyrazol-1-ide; cbIM = 5-chlorobenzoimidazol-1-ate; nIM = 2-nitroimidazolate; MV = methylviologen dication; mpba = N,N'-1,3-phenylenebis(oxamate); mM = 2-methylimidazolate, 2,2'-bipy = 4,4'-bipyridine; m-bdc = 1,3-benzenedicarboxylate; ndc = 2,6-naphthalenedicarboxylate; bpee = 1,2-bis(4-pyridyl)ethylene; NO₂-ip = 5-nitro-1,3-benzenedicarboxylate; tbc = 4-[3,5-bis[4-carboxylatophenoxyethyl]-2,4,6-trimethyl-phenyl]methoxybenzoate; tpc = tetrakis(phenylcarboxylate)ethylene; dodbc = 2,5-dioxo-1,4-benzenedicarboxylate; att = 3-amino-1,2,4-triazole-5-thiol.
<figure><img src="image_24.png" /></figure>

<table><caption>Table 2 (continued)</caption><thead><tr><th>Metal-organic framework<sup>a</sup></th><th>Analyte/s</th><th>Detector/sensor type</th><th>Dynamic range (where stated)</th><th>Ref.</th></tr></thead><tbody><tr><td>[Zn(mIM)<sub>2</sub>] (ZIF-8)</td><td>Carbon dioxide (CO<sub>2</sub>), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH), hydrocarbons (HCS) & hydrogen (H<sub>2</sub>)</td><td>Fabry-Pérot device (FPD), filter layer (FL)</td><td>C<sub>2</sub>H<sub>5</sub>OH: 0-40% (FPD)<br>HCs: C<sub>3</sub>H<sub>8</sub>: 0-100% (FPD)<br>H<sub>2</sub>: (FL)</td><td>162 and<br>163</td></tr><tr><td>[Co(mIM)<sub>2</sub>] (ZIF-67)</td><td>Formaldehyde (HCHO) & methanol vapour (CH<sub>3</sub>OH)</td><td>Interdigitated electrode resistance sensor (IERS)</td><td>HCHO: 5-500 ppm CH<sub>3</sub>OH: measurement at 100 ppm</td><td>164</td></tr><tr><td>[Zr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>]<sub>6</sub> (UiO-66)</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>UV-visible reflectance spectroscopy</td><td>CH<sub>3</sub>OH: 10 µL (vapour) C<sub>2</sub>H<sub>5</sub>OH: 10 µL (vapour)</td><td>165</td></tr><tr><td>[Cu<sub>2</sub>(OH)<sub>2</sub>]<sub>2</sub>·2bipy<sub>2</sub>]<sub>2</sub>(bdc)<sub>6</sub>·2H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>Quartz crystal microbalance</td><td>64-160 ppm</td><td>166</td></tr><tr><td>[Co(4,4'-bipy)(m-bdc)]<sub>6</sub></td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>Quartz crystal microbalance</td><td>2-16 ppm</td><td>166</td></tr><tr><td>[Al<sub>12</sub>O(OH)<sub>18</sub>(H<sub>2</sub>O)<sub>3</sub>(Al<sub>2</sub>(OH)<sub>4</sub>)]<sub>6</sub>·24H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>CH<sub>3</sub>OH: 100 ppm (lower limit)</td><td>167, 168</td></tr><tr><td>[Co<sub>3</sub>(btc)<sub>2</sub>]·H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>CH<sub>3</sub>OH: 20-50 ppm</td><td>167, 168</td></tr><tr><td>[Ni<sub>3</sub>(btc)<sub>2</sub>]·H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>C<sub>2</sub>H<sub>5</sub>OH: 10-50 ppm</td><td>167, 168</td></tr><tr><td>[Cd<sub>3</sub>(btc)<sub>2</sub>]·H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH) & ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH)</td><td>Kelvin probe</td><td>C<sub>2</sub>H<sub>5</sub>OH: 10-50 ppm</td><td>167, 168</td></tr><tr><td>[Zn<sub>2</sub>(ndc)<sub>2</sub>]<sub>2</sub>(dpe)]·2.5DMF·0.25H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) & hydrocarbons (HCS)</td><td>Luminescence quenching (LQ)</td><td>Concentration of all analytes was vapour pressure of each analytes at room temperature.</td><td>169</td></tr><tr><td>[Zn<sub>2</sub>(ndc)<sub>2</sub>]<sub>2</sub>(bpee)]·2.25DMF·0.5H<sub>2</sub>O</td><td>Methanol vapour (CH<sub>3</sub>OH), ethanol vapour (C<sub>2</sub>H<sub>5</sub>OH) & hydrocarbons (HCS)</td><td>Luminescence quenching (LQ)</td><td>Concentration of all analytes was vapour pressure of each analytes at room temperature.</td><td>169</td></tr><tr><td>[Zn<sub>2</sub>(bpdc)<sub>2</sub>]<sub>2</sub>(bpee)]·2DMF (RPM3)</td><td>Hydrocarbons (HCS)</td><td>Luminescence spectroscopy</td><td>—</td><td>170</td></tr><tr><td>[Zn(NO<sub>2</sub>-ip)<sub>2</sub>·(4-4'-bipy)] (Zn-CID-5)</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>Quartz crystal microbalance</td><td>Measurement at 40%</td><td>171</td></tr><tr><td>[Co<sub>3</sub>(btc)<sub>2</sub>(DMF)<sub>2</sub>]·4DMF</td><td>Methanol vapour (CH<sub>3</sub>OH)</td><td>UV-visible spectroscopy and visible colour change</td><td>Vapour pressure of methanol at room temperature.</td><td>172</td></tr><tr><td>[Zn<sub>2</sub>(tpce)]</td><td>Ammonia (NH<sub>3</sub>)</td><td>Luminescence spectroscopy</td><td>1% ammonia (NH<sub>3</sub>) in nitrogen carrier gas</td><td>173</td></tr><tr><td>[Mg<sub>3</sub>(dobdc)]</td><td>Ammonia (NH<sub>3</sub>)</td><td>Luminescence spectroscopy</td><td>1% ammonia (NH<sub>3</sub>) in nitrogen carrier gas</td><td>173</td></tr><tr><td>[Cu(att)<sub>1,5</sub>]</td><td>Hydrogen sulfide (H<sub>2</sub>S)</td><td>Luminescence spectroscopy</td><td>0-650 ppm</td><td>174</td></tr></tbody></table>

a L¹ = pyridyl-4,5-imidazole dicarboxylate; tctpo = tris(p-carboxylate)triphenylphosphine oxide; dpe = 1,2-bis(4-pyridyl)ethane; bdc = 1,4-benzenedicarboxylate; btc = 1,3,5-benzenetricarboxylate; benztb = N,N,N',N'-benzidinetetrabenzoate; abdc = 2-amino-1,4-benzenedicarboxylate; ad-adeninate; bpcd = 4,4'-biphenyldicarboxylate; L² = [Ir(3-(2-pyridyl)]benzoate)³]; L³ = [Ir(2-(4-carboxylato-2-pyridyl)pyridine-4-carboxylate)(2-phenylpyridine)²]; L⁴ = [Ir(6-(4-carboxylatophenyl)pyridine-3-carboxylate)(2-phenylpyridine)²]; L⁵ = [Ir(6-(5-carboxylato-2-pyridyl)pyridine-3-carboxylate)(2-phenylpyridine)²]; L⁶ = [Ru(6-(5-carboxylato-2-pyridyl)pyridine-3-carboxylate)(2,2'-bipyridine)²]; ip = imidazo[4,5-f][1,10]phenanthrolin-1-ide; bpz = 4-(3,5-dimethylpyrazol-1-ide)-3,5-dimethyl-pyrazol-1-ide; cbIM = 5-chlorobenzoimidazol-1-ate; nIM = 2-nitroimidazolate; MV = methylviologen dication; mpba = N,N'-1,3-phenylenebis(oxamate); mM = 2-methylimidazolate, 2,2'-bipy = 4,4'-bipyridine; m-bdc = 1,3-benzenedicarboxylate; ndc = 2,6-naphthalenedicarboxylate; bpee = 1,2-bis(4-pyridyl)ethylene; NO₂-ip = 5-nitro-1,3-benzenedicarboxylate; tbc = 4-[3,5-bis[4-carboxylatophenoxyethyl]methyl]-2,4,6-trimethyl-phenylmethoxybenzoate; tpc = tetrakis(phenylcarboxylate)ethylene; dodbc = 2,5-dioxo-1,4-benzenedicarboxylate; att = 3-amino-1,2,4-triazole-5-thiol.
<figure><img src="image_22.png" /></figure>

**2.3.1 Detection of mechanical and resonant changes upon adsorption.** A quartz crystal microbalance (QCM) transducer consists of a thin slice of a piezoelectric quartz crystal that is used as a sensitive microbalance. The thin section of quartz crystal is electroded and when an AC potential difference is applied to the quartz crystal it oscillates at a specific resonant frequency. Upon adsorption of analyte molecules, the mass increase on the quartz surface is directly related to a reduction in the resonant frequency of the oscillating crystal. Thus, this change in QCM frequency corresponds to the mass of analyte adsorbed, usually on the order of ~$10^{-9}$ g cm$^{-2}$. Selective adsorption of specific analytes is achieved by application of a discriminatory thin film on the surface of the quartz. Further information is available in a review on the use of QCMs for sensing by Vashist and Vashist.¹⁷⁶

A piezoelectric microcantilever-based transducer uses a similar approach to a QCM transducer but consists of a cantilever paddle that is anchored at one end to a substrate with the other end modified with a sensing material and free to vibrate. When excited by an AC potential difference, the paddle vibrates at its resonance frequency. Upon adsorption of an analyte within or on the sensing layer this specific frequency changes due to mass gain and thus the frequency variation is related to the mass of analyte adsorbed. Further information is available in a review on the use of microcantilevers for chemical sensing.¹⁷⁷ The closely related microresonator-based chemical sensors also operate by adsorption of an analyte leading to a change in mass which in turn changes the resonant frequency of the structure. An excellent paper describing microresonator theory was published by Schmidt and Howe.¹⁷⁸

A magnetoelastic transducer usually consists of an amorphous ferromagnetic material, in a ribbon form, covered by a sensing film. When an alternating magnetic field is applied, the magnetoelastic material resonates at a specific frequency which changes due to adsorption of an analyte within or on the sensing film. Further information is available in a review on the use of magnetoelastic transducers for sensing.¹⁷⁹

**2.3.2 Spectroscopic sensing techniques.** Infrared (IR) spectroscopy for analyte detection can be performed in transmission or reflection mode. In transmission mode, a beam of infrared light is incident to and transmitted by the sensing material. The transmitted intensity vs. frequency spectrum of the sensing material is collected periodically and changes in the infrared spectrum corresponding to adsorption of an analyte can be quantified. In contrast, in reflection mode, the sensing material is deposited on a substrate which consists of an optical material with a higher relative refractive index, such as a silicon wafer. A beam of infrared light interacts with the sample due to reflecting times between the bare external surface of the silicon substrate and its internal surface which is in contact with the sensing material. At each reflection an evanescent wave extends into the sample. The monitoring of the transmitted light or the evanescent wave reveals how much energy was absorbed at each frequency. This is described as attenuated total reflection (ATR). In both cases, the properties of the obtained IR spectrum (i.e. wavenumber of absorption bands and shape and intensity

of bands) is related to the nature and concentration of the analyte interacting with the sensing material.

Optical fibre Bragg gratings can either be long-period gratings or short-period gratings. For both types of optical fibre Bragg grating, the period describes the distance between a fringe of low or high refractive index and the sequential fringe of the same refractive index. Long-period gratings typically have periods on the sub-millimeter length scale and work by coupling forward propagating light of a certain wavelength into forward coupling cladding modes. The coupled wavelength is dependent on the period of the Bragg grating. The power of the forward coupling modes is typically attenuated through absorption and scattering losses. If the cladding of the optical fibre is covered with a sensing material then interaction with analyte species will alter the absorbance, or change the magnitude of scattering losses within the cladding. This can result in a change in the magnitude of the received power at the coupled wavelength at the detector or even a wavelength shift of the transmission peak features. Further information is available in a review on the use of long period Bragg gratings for sensing.¹⁸⁰

Luminescence-based detector or sensor systems measure changes in the fluorescence or phosphorescence emission spectrum of a luminescent material due to interaction with analyte molecules. Certain molecules, such as dioxygen $O_2$, quench or “turn-off” the luminescence of the sensing material; the degree of change in luminescent intensity or wavelength shifting is related to the concentration of the quenching analyte.

Optical sensing can be employed if encapsulated reactive dyes are contained inside the zeolite cages. Colour changes caused by the presence of an analyte can be monitored with UV-visible spectroscopy and/or the naked eye.

**2.3.3 Electronic sensing.** Electrical property-based sensors can be used to monitor the adsorption of analytes within the porous sensing material by measuring changes in conductivity, impedance or resistance of the sensing material upon interaction with an analyte.¹⁸¹ The change in electrical properties of sensing materials that contain semiconducting metal oxide guests upon exposure to analytes can be monitored in the same manner as the electrical property-based sensors described above. For example, the imaginary permittivity of the sensing material can be correlated to the resonant impedance spectrum or to conductivity.¹⁸²

The different types of sensing and detection techniques used in zeolite and MOF sensors are summarised in Fig. 3.

# 3. Exhaust gas sensing using porous materials

## 3.1 Humidity sensors

**3.1.1 Zeolite-based $H_2O$ sensors.** Tracking of water vapour concentration is important for determining the extent of combustion. Early examples of humidity sensors reported the advantages of zeolite-based sensing devices combined with mass detectors. In one of the first examples, ZSM-5 type zeolite single crystals were aligned on a high sensitivity micromechanical
<figure><img src="image_25.png" /></figure>

**2.3.1 Detection of mechanical and resonant changes upon adsorption.** A quartz crystal microbalance (QCM) transducer consists of a thin slice of a piezoelectric quartz crystal that is used as a sensitive microbalance. The thin section of quartz crystal is electroded and when an AC potential difference is applied to the quartz crystal it oscillates at a specific resonant frequency. Upon adsorption of analyte molecules, the mass increase on the quartz surface is directly related to a reduction in the resonant frequency of the oscillating crystal. Thus, this change in QCM frequency corresponds to the mass of analyte adsorbed, usually on the order of ~$10^{-9}$ g cm$^{-2}$. Selective adsorption of specific analytes is achieved by application of a discriminatory thin film on the surface of the quartz. Further information is available in a review on the use of QCMs for sensing by Vashist and Vashist.¹⁷⁶

A piezoelectric microcantilever-based transducer uses a similar approach to a QCM transducer but consists of a cantilever paddle that is anchored at one end to a substrate with the other end modified with a sensing material and free to vibrate. When excited by an AC potential difference, the paddle vibrates at its resonance frequency. Upon adsorption of an analyte within or on the sensing layer this specific frequency changes due to mass gain and thus the frequency variation is related to the mass of analyte adsorbed. Further information is available in a review on the use of microcantilevers for chemical sensing.¹⁷⁷ The closely related microresonator-based chemical sensors also operate by adsorption of an analyte leading to a change in mass which in turn changes the resonant frequency of the structure. An excellent paper describing microresonator theory was published by Schmidt and Howe.¹⁷⁸

A magnetoelastic transducer usually consists of an amorphous ferromagnetic material, in a ribbon form, covered by a sensing film. When an alternating magnetic field is applied, the magnetoelastic material resonates at a specific frequency which changes due to adsorption of an analyte within or on the sensing film. Further information is available in a review on the use of magnetoelastic transducers for sensing.¹⁷⁹

**2.3.2 Spectroscopic sensing techniques.** Infrared (IR) spectroscopy for analyte detection can be performed in transmission or reflection mode. In transmission mode, a beam of infrared light is incident to and transmitted by the sensing material. The transmitted intensity vs. frequency spectrum of the sensing material is collected periodically and changes in the infrared spectrum corresponding to adsorption of an analyte can be quantified. In contrast, in reflection mode, the sensing material is deposited on a substrate which consists of an optical material with a higher relative refractive index, such as a silicon wafer. A beam of infrared light interacts with the sample due to reflecting times between the bare external surface of the silicon substrate and its internal surface which is in contact with the sensing material. At each reflection an evanescent wave extends into the sample. The monitoring of the transmitted light or the evanescent wave reveals how much energy was absorbed at each frequency. This is described as attenuated total reflection (ATR). In both cases, the properties of the obtained IR spectrum (i.e. wavenumber of absorption bands and shape and intensity

of bands) is related to the nature and concentration of the analyte interacting with the sensing material.

Optical fibre Bragg gratings can either be long-period gratings or short-period gratings. For both types of optical fibre Bragg grating, the period describes the distance between a fringe of low or high refractive index and the sequential fringe of the same refractive index. Long-period gratings typically have periods on the sub-millimeter length scale and work by coupling forward propagating light of a certain wavelength into forward coupling cladding modes. The coupled wavelength is dependent on the period of the Bragg grating. The power of the forward coupling modes is typically attenuated through absorption and scattering losses. If the cladding of the optical fibre is covered with a sensing material then interaction with analyte species will alter the absorbance, or change the magnitude of scattering losses within the cladding. This can result in a change in the magnitude of the received power at the coupled wavelength at the detector or even a wavelength shift of the transmission peak features. Further information is available in a review on the use of long period Bragg gratings for sensing.¹⁸⁰

Luminescence-based detector or sensor systems measure changes in the fluorescence or phosphorescence emission spectrum of a luminescent material due to interaction with analyte molecules. Certain molecules, such as dioxygen $O_2$, quench or “turn-off” the luminescence of the sensing material; the degree of change in luminescent intensity or wavelength shifting is related to the concentration of the quenching analyte.

Optical sensing can be employed if encapsulated reactive dyes are contained inside the zeolite cages. Colour changes caused by the presence of an analyte can be monitored with UV-visible spectroscopy and/or the naked eye.

**2.3.3 Electronic sensing.** Electrical property-based sensors can be used to monitor the adsorption of analytes within the porous sensing material by measuring changes in conductivity, impedance or resistance of the sensing material upon interaction with an analyte.¹⁸¹ The change in electrical properties of sensing materials that contain semiconducting metal oxide guests upon exposure to analytes can be monitored in the same manner as the electrical property-based sensors described above. For example, the imaginary permittivity of the sensing material can be correlated to the resonant impedance spectrum or to conductivity.¹⁸²

The different types of sensing and detection techniques used in zeolite and MOF sensors are summarised in Fig. 3.

# 3. Exhaust gas sensing using porous materials

## 3.1 Humidity sensors

**3.1.1 Zeolite-based $H_2O$ sensors.** Tracking of water vapour concentration is important for determining the extent of combustion. Early examples of humidity sensors reported the advantages of zeolite-based sensing devices combined with mass detectors. In one of the first examples, ZSM-5 type zeolite single crystals were aligned on a high sensitivity micromechanical
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /><figcaption>Fig. 3 A comparison of the different types of sensing/detection techniques cited in this review.</figcaption></figure>

cantilever to fabricate a chemical sensor for humidity sensing in the nanogram range.⁷¹ In 2001, other QCM sensors were prepared by modification with either zeolite A or beta polymorph A and used as humidity sensors.⁵⁵,⁵⁶ Due to the large pore size channels (6.7 Å), beta polymorph A-based sensors exhibited more interference from alkanes compared to zeolite A-based sensors. Larger resonant frequency shifts were measured with the zeolite A coated QCM sensor, e.g. 2000 Hz after 150 ppm water exposure for zeolite A, compared to 400 Hz for beta polymorph A. However, the response and recovery times were faster for the beta polymorph A zeolite sensor, which was rationalised on the basis of its larger pore channel diameter and higher hydrophobicity due to a higher Si/Al ratio. Other QCM oscillator-based sensors have been developed for humidity sensing, based on zeolite A, silicalite-1 and sodalite deposited onto QCMs, and showed similar results when monitoring from 250 to 750 ppm of water in He at 170 °C.⁵⁷ The sodalite-coated QCM was found to be promising as a discriminatory humidity sensor as it showed poor responses towards NO and SO₂.

Host–guest chemistry using zeolites as hosts for a dye or conductive material has also been employed to construct optical or electrochemical detectors and sensors for humidity, respectively. Spectroscopic changes in the nature of the dye loaded into

the zeolite cages indicated the presence or disappearance of water. Protonation/deprotonation of methylene blue held in mordenite-type zeolite showed spectral changes in a range of 9 to 92% of relative humidity with good reversibility and fast responses of approximately 2 min for the adsorption step and 4 min for the desorption step.⁴⁷,⁴⁸ Two absorption bands (the monomeric methylene blue and its protonated form at 650 and 745 nm, respectively) were monitored to obtain these results. Another example of an optical humidity detector utilised solvatochromic Nile Red dye encapsulated in a NaY zeolite.¹⁰² This device had a detection limit of 200 ppm humidity, a response time of around 4 min and discrimination against hexane was also demonstrated. One further example of an optical detector involved impregnation of silver on ZSM-5 type zeolite⁷² where the authors showed that Ag⁺ species in the Ag-ZSM-5 zeolite exhibited stable optical properties under high-temperature treatment. The behaviour of the 37 700 cm⁻¹ band was followed with *in situ* UV-vis spectroscopy. Above 150 °C, this band was found to be sensitive to the presence of water vapour in the gas stream (2.8% H₂O in He), disappearing in the presence of water vapour while full intensity could be recovered under water-free conditions.

As an example of electrochemical sensing, Yu *et al.* employed a porous zeolite support to improve the stability and durability of LiCl at high humidity. Indeed, when the humidity exceeds 40% relative humidity (RH), the conductivity response of LiCl is no longer linear and thus it becomes difficult to use the material as a humidity sensor.¹⁰³ The LiCl was dispersed into a NaY zeolite to obtain a linear response to up to 75% RH. The authors claimed that the well-defined cavities of NaY-type zeolites are more suitable as host materials for preparation of humidity-sensitive composite materials than those with channel structure, such as ZSM-5 or mordenite zeolites. Zou *et al.* similarly reported that the loading of LiCl into stilbite resulted in an electrically conductive material exhibiting very high humidity sensitivities with a linear response of 10⁴ in conductivity with a humidity change from 0 to 63% and fast response times for adsorption and desorption.¹²⁸

Changes in zeolite conductivity due to extra-framework cations contained in the framework have also been employed for the concept of humidity sensors with measurements based on changes in either impedance or capacitance. The impedance of the H-form of ZSM-5 was studied as a function of water concentration (0–300 ppm) in H₂.⁷³,⁷⁴ ZSM-5 zeolite was chosen for its small pore size (5.5 Å), which improved water selectivity, and also for its acidic properties, due to the acid–base chemistry involved in impedance measurements. In addition, the high thermal stability of the zeolite in a reducing environment allowed sensing experiments in H₂ up to 600 °C, measurements which are not possible with humidity sensors based on metal oxides or polymers.

To allow sensing under milder ambient experimental conditions, the capacitance of a zeolite can be measured instead of impedance. For this approach, electrodes coated with various zeolite films (A-, Y-, beta polymorph A- and silicalite-type) with different Si/Al ratios were studied.⁵⁸ It was shown that the hydrophilic nature of zeolite A provided the most sensitive film,

<figure><img src="image_25.png" /></figure>
Review Article

<figure><img src="image_26.png" /><figcaption>Fig. 3 A comparison of the different types of sensing/detection techniques cited in this review.</figcaption></figure>

cantilever to fabricate a chemical sensor for humidity sensing in the nanogram range.⁷¹ In 2001, other QCM sensors were prepared by modification with either zeolite A or beta polymorph A and used as humidity sensors.⁵⁵,⁵⁶ Due to the large pore size channels (6.7 Å), beta polymorph A-based sensors exhibited more interference from alkanes compared to zeolite A-based sensors. Larger resonant frequency shifts were measured with the zeolite A coated QCM sensor, e.g. 2000 Hz after 150 ppm water exposure for zeolite A, compared to 400 Hz for beta polymorph A. However, the response and recovery times were faster for the beta polymorph A zeolite sensor, which was rationalised on the basis of its larger pore channel diameter and higher hydrophobicity due to a higher Si/Al ratio. Other QCM oscillator-based sensors have been developed for humidity sensing, based on zeolite A, silicalite-1 and sodalite deposited onto QCMs, and showed similar results when monitoring from 250 to 750 ppm of water in He at 170 °C.⁵⁷ The sodalite-coated QCM was found to be promising as a discriminatory humidity sensor as it showed poor responses towards NO and SO₂.

Host–guest chemistry using zeolites as hosts for a dye or conductive material has also been employed to construct optical or electrochemical detectors and sensors for humidity, respectively. Spectroscopic changes in the nature of the dye loaded into

the zeolite cages indicated the presence or disappearance of water. Protonation/deprotonation of methylene blue held in mordenite-type zeolite showed spectral changes in a range of 9 to 92% of relative humidity with good reversibility and fast responses of approximately 2 min for the adsorption step and 4 min for the desorption step.⁴⁷,⁴⁸ Two absorption bands (the monomeric methylene blue and its protonated form at 650 and 745 nm, respectively) were monitored to obtain these results. Another example of an optical humidity detector utilised solvatochromic Nile Red dye encapsulated in a NaY zeolite.¹⁰² This device had a detection limit of 200 ppm humidity, a response time of around 4 min and discrimination against hexane was also demonstrated. One further example of an optical detector involved impregnation of silver on ZSM-5 type zeolite⁷² where the authors showed that Ag⁺ species in the Ag-ZSM-5 zeolite exhibited stable optical properties under high-temperature treatment. The behaviour of the 37 700 cm⁻¹ band was followed with *in situ* UV-vis spectroscopy. Above 150 °C, this band was found to be sensitive to the presence of water vapour in the gas stream (2.8% H₂O in He), disappearing in the presence of water vapour while full intensity could be recovered under water-free conditions.

As an example of electrochemical sensing, Yu *et al.* employed a porous zeolite support to improve the stability and durability of LiCl at high humidity. Indeed, when the humidity exceeds 40% relative humidity (RH), the conductivity response of LiCl is no longer linear and thus it becomes difficult to use the material as a humidity sensor.¹⁰³ The LiCl was dispersed into a NaY zeolite to obtain a linear response to up to 75% RH. The authors claimed that the well-defined cavities of NaY-type zeolites are more suitable as host materials for preparation of humidity-sensitive composite materials than those with channel structure, such as ZSM-5 or mordenite zeolites. Zou *et al.* similarly reported that the loading of LiCl into stilbite resulted in an electrically conductive material exhibiting very high humidity sensitivities with a linear response of 10⁴ in conductivity with a humidity change from 0 to 63% and fast response times for adsorption and desorption.¹²⁸

Changes in zeolite conductivity due to extra-framework cations contained in the framework have also been employed for the concept of humidity sensors with measurements based on changes in either impedance or capacitance. The impedance of the H-form of ZSM-5 was studied as a function of water concentration (0–300 ppm) in H₂.⁷³,⁷⁴ ZSM-5 zeolite was chosen for its small pore size (5.5 Å), which improved water selectivity, and also for its acidic properties, due to the acid–base chemistry involved in impedance measurements. In addition, the high thermal stability of the zeolite in a reducing environment allowed sensing experiments in H₂ up to 600 °C, measurements which are not possible with humidity sensors based on metal oxides or polymers.

To allow sensing under milder ambient experimental conditions, the capacitance of a zeolite can be measured instead of impedance. For this approach, electrodes coated with various zeolite films (A-, Y-, beta polymorph A- and silicalite-type) with different Si/Al ratios were studied.⁵⁸ It was shown that the hydrophilic nature of zeolite A provided the most sensitive film,

<figure><img src="image_27.png" /></figure>
<figure><img src="image_26.png" /></figure>

with a seven times higher response under a 350 ppm water concentrated atmosphere compared with the other films.

Metal oxide based sensors can be further modified with zeolite films to improve their response to gas analytes compared with uncoated metal oxides. A zeolite layer can be used as an adsorbent filter to enhance the selectivity of the metal oxide by retaining analytes and/or interfering species. An example was reported using SnO₂ as the oxide and zeolite A and ZSM-5 zeolites as filters.⁵⁹ Results showed that zeolite A improved the selectivity towards H₂O by the elimination of the response to propane, hydrogen and methane while continuing to respond significantly to ethanol and CO. On the other hand, the ZSM-5-type silicalite-coated sensor gave enhanced response to both H₂O and H₂.

**3.1.2 MOF-based H₂O sensors.** There have been a large number of reports of different MOF-based humidity detectors and sensors employing a range of analysis techniques due to the variety of interactions of water with the organic moieties or metal centres. For sensing water vapour levels, several research groups have utilised lanthanide-based luminescence, which is sensitive to the chemical environment of the lanthanide ions. Dong *et al.* developed lanthanide ion MOFs using a mixture of two different ligands: oxalate (O₂C-CO₂⁻, ox) and pyridyl-4,5-imidazole dicarboxylate (L¹), and either terbium nitrate or europium nitrate to form [M(L¹)(ox)₀.₅(H₂O)]·2H₂O (M = Eu, Tb).¹³³ Upon dehydration, both MOFs showed a decrease in luminescence emission intensity and upon rehydration the intensity of the luminescence emissions increased again; the MOFs therefore functioned as probes for humidity which could be easily interrogated with luminescent spectroscopic methods. The terbium MOF showed the greatest change in luminescence intensity when the dehydrated form was rehydrated upon exposure to known quantities of water vapour in air, i.e. different relative humidity levels. However, the response time of the terbium MOF probe was slow even in high relative humidity environments. For recovery of the luminescence emission to an intensity equivalent to the luminescence emission intensity of the hydrated form, the dehydrated form had to be exposed to 85% relative humidity (%RH) for 9 h.

Humphrey *et al.* synthesised a terbium(III) MOF, ([Tb(tctpo)-(H₂O)]·2DMF·H₂O) (DMF = N,N-dimethylformamide), known as PCM-15, with tris(p-carboxylate)triphenylphosphine oxide (tctpo) as the linker (Fig. 4).¹³⁵ The resulting MOF is comprised of a 5,5-connected net structure, with each Tb³⁺ centre octa-coordinated, forming seven bonds to tctpo linkers and one to a water molecule. Upon activation of the MOF the intensity of the

photoluminescence due to the terbium ion was found to double. Upon treatment with moisture-saturated N₂, the photoluminescence intensity returned to that for the as-synthesised MOF. Thus, PCM-15 can act as a luminescence-based detector for humidity, though neither the response time nor the luminescence response at different % RH levels were reported. Another MOF which acted as a detector for water vapour was demonstrated by Chou *et al.*¹³⁶ As with the examples above, changes in the luminescence intensity of the MOF upon dehydration and rehydration were used as the measurable output. However this MOF, [Zn(dpe)(bdc)]·4H₂O (dpe = 1,2-bis(4-pyridyl)ethane, bdc = 1,4-benzenedicarboxylate), labelled as ZndB, utilises ligand-based luminescence rather than luminescent lanthanide metal ions. The MOF contains 2D hydrogen-bonded rings of sixteen water molecules and showed reversible on(dry)/off(hydrated) luminescent response to humidity upon cycling between room temperature and 85 °C. The mechanism behind the increase in fluorescence intensity upon heating at 85 °C was thought to be removal of the intra-channel water molecules resulting in a shortening of the inter-channel distance between two dpe moieties. This enhances the π-π stacking between the two interchain dpe moieties and thus increases dpe(π*)-dpe(π) excimer formation. An exposure time of >100 min of the MOF to the ambient % RH level at room temperature was required for the fluorescence intensity to return to as-prepared levels.

Lee *et al.* also reported two MOFs that can be used to detect humidity by quantifying the differences in the luminescence emission spectra of a MOF when 'wet' and the same MOF when dry.¹³⁷ Upon excitation at 270 nm, it was found that for dry [In(OH)(bdc)] emission λmax = 326 nm and for dry [Zn₄O(bdc)₃], MOF-5, emission λmax = 366 nm.¹⁸³ Exposure to water vapour at 40 °C resulted in an emission wavelength redshift of 64 nm for [In(OH)(bdc)] (λmax water = 390 nm) and an emission wavelength blueshift of 27 nm (λmax water = 339 nm) for MOF-5. Therefore, both MOFs act as fluorescent probes for humidity. No reversibility experiments or experiments detailing a fluorescence change upon exposure to different concentrations of water vapour were reported for these MOFs, but they do show reasonable thermal stability, decomposing above 450 °C for [In(OH)(bdc)] and above 400 °C for MOF-5. Such thermal stability would be necessary for a MOF active sensing element for an exhaust gas sensor.¹⁸⁴

The copper(II) MOF [Cu₃(btc)₂] (btc = 1,3,5-benzenetri-carboxylate), HKUST-1, has also been used in the development of a piezoelectric cantilever sensor for both humidity and hydrocarbon vapours.¹³⁸ Robinson *et al.* fabricated N-doped piezoelectric microcantilever sensors, ~230 µm long and ~80 µm wide, that were coated with a layer of HKUST-1. The microcantilever sensor device was then activated prior to the start of a sensing experiment by passage of dry nitrogen gas over the device for 15 min at 40 °C. Higher temperatures, required to completely remove adsorbed water, were not possible as the integrity of the device was found to be compromised at temperatures >50 °C at 1 atm pressure. Therefore, the authors waited until a steady state was achieved before sensing experiments were performed. The sensor device was exposed to different concentrations of water in nitrogen gas at 23 °C and at 1 atm pressure. Exposure to

<figure><img src="image_27.png" /><figcaption>**Fig. 4** The tris(p-carboxylate)triphenylphosphine oxide (tctpo) ligand in the Tb³⁺ luminescent PCM-15 MOF prepared by Humphrey *et al.*¹³⁵</figcaption></figure>
<figure><img src="image_28.png" /></figure>

with a seven times higher response under a 350 ppm water concentrated atmosphere compared with the other films.

Metal oxide based sensors can be further modified with zeolite films to improve their response to gas analytes compared with uncoated metal oxides. A zeolite layer can be used as an adsorbent filter to enhance the selectivity of the metal oxide by retaining analytes and/or interfering species. An example was reported using SnO₂ as the oxide and zeolite A and ZSM-5 zeolites as filters.⁵⁹ Results showed that zeolite A improved the selectivity towards H₂O by the elimination of the response to propane, hydrogen and methane while continuing to respond significantly to ethanol and CO. On the other hand, the ZSM-5-type silicalite-coated sensor gave enhanced response to both H₂O and H₂.

**3.1.2 MOF-based H₂O sensors.** There have been a large number of reports of different MOF-based humidity detectors and sensors employing a range of analysis techniques due to the variety of interactions of water with the organic moieties or metal centres. For sensing water vapour levels, several research groups have utilised lanthanide-based luminescence, which is sensitive to the chemical environment of the lanthanide ions. Dong *et al.* developed lanthanide ion MOFs using a mixture of two different ligands: oxalate (O₂C-CO₂⁻, ox) and pyridyl-4,5-imidazole dicarboxylate (L¹), and either terbium nitrate or europium nitrate to form [M(L¹)(ox)₀.₅(H₂O)]·2H₂O (M = Eu, Tb).¹³³ Upon dehydration, both MOFs showed a decrease in luminescence emission intensity and upon rehydration the intensity of the luminescence emissions increased again; the MOFs therefore functioned as probes for humidity which could be easily interrogated with luminescent spectroscopic methods. The terbium MOF showed the greatest change in luminescence intensity when the dehydrated form was rehydrated upon exposure to known quantities of water vapour in air, i.e. different relative humidity levels. However, the response time of the terbium MOF probe was slow even in high relative humidity environments. For recovery of the luminescence emission to an intensity equivalent to the luminescence emission intensity of the hydrated form, the dehydrated form had to be exposed to 85% relative humidity (%RH) for 9 h.

Humphrey *et al.* synthesised a terbium(III) MOF, ([Tb(tctpo)-(H₂O)]·2DMF·H₂O) (DMF = N,N-dimethylformamide), known as PCM-15, with tris(*p*-carboxylate)triphenylphosphine oxide (tctpo) as the linker (Fig. 4).¹³⁵ The resulting MOF is comprised of a 5,5-connected net structure, with each Tb³⁺ centre octa-coordinated, forming seven bonds to tctpo linkers and one to a water molecule. Upon activation of the MOF the intensity of the

photoluminescence due to the terbium ion was found to double. Upon treatment with moisture-saturated N₂, the photoluminescence intensity returned to that for the as-synthesised MOF. Thus, PCM-15 can act as a luminescence-based detector for humidity, though neither the response time nor the luminescence response at different % RH levels were reported. Another MOF which acted as a detector for water vapour was demonstrated by Chou *et al.*¹³⁶ As with the examples above, changes in the luminescence intensity of the MOF upon dehydration and rehydration were used as the measurable output. However this MOF, [Zn(dpe)(bdc)]·4H₂O (dpe = 1,2-bis(4-pyridyl)ethane, bdc = 1,4-benzenedicarboxylate), labelled as ZndB, utilises ligand-based luminescence rather than luminescent lanthanide metal ions. The MOF contains 2D hydrogen-bonded rings of sixteen water molecules and showed reversible on(dry)/off(hydrated) luminescent response to humidity upon cycling between room temperature and 85 °C. The mechanism behind the increase in fluorescence intensity upon heating at 85 °C was thought to be removal of the intra-channel water molecules resulting in a shortening of the inter-channel distance between two dpe moieties. This enhances the π-π stacking between the two interchain dpe moieties and thus increases dpe(π*)-dpe(π) excimer formation. An exposure time of >100 min of the MOF to the ambient % RH level at room temperature was required for the fluorescence intensity to return to as-prepared levels.

Lee *et al.* also reported two MOFs that can be used to detect humidity by quantifying the differences in the luminescence emission spectra of a MOF when 'wet' and the same MOF when dry.¹³⁷ Upon excitation at 270 nm, it was found that for dry [In(OH)(bdc)] emission λmax = 326 nm and for dry [Zn₄O(bdc)₃], MOF-5, emission λmax = 366 nm.¹⁸³ Exposure to water vapour at 40 °C resulted in an emission wavelength redshift of 64 nm for [In(OH)(bdc)] (λmax water = 390 nm) and an emission wavelength blueshift of 27 nm (λmax water = 339 nm) for MOF-5. Therefore, both MOFs act as fluorescent probes for humidity. No reversibility experiments or experiments detailing a fluorescence change upon exposure to different concentrations of water vapour were reported for these MOFs, but they do show reasonable thermal stability, decomposing above 450 °C for [In(OH)(bdc)] and above 400 °C for MOF-5. Such thermal stability would be necessary for a MOF active sensing element for an exhaust gas sensor.¹⁸⁴

The copper(II) MOF [Cu₃(btc)₂] (btc = 1,3,5-benzenetri-carboxylate), HKUST-1, has also been used in the development of a piezoelectric cantilever sensor for both humidity and hydrocarbon vapours.¹³⁸ Robinson *et al.* fabricated N-doped piezoelectric microcantilever sensors, ~230 µm long and ~80 µm wide, that were coated with a layer of HKUST-1. The microcantilever sensor device was then activated prior to the start of a sensing experiment by passage of dry nitrogen gas over the device for 15 min at 40 °C. Higher temperatures, required to completely remove adsorbed water, were not possible as the integrity of the device was found to be compromised at temperatures >50 °C at 1 atm pressure. Therefore, the authors waited until a steady state was achieved before sensing experiments were performed. The sensor device was exposed to different concentrations of water in nitrogen gas at 23 °C and at 1 atm pressure. Exposure to

<figure><img src="image_29.png" /><figcaption>**Fig. 4** The tris(*p*-carboxylate)triphenylphosphine oxide (tctpo) ligand in the Tb³⁺ luminescent PCM-15 MOF prepared by Humphrey *et al.*¹³⁵</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>Fig. 5 (a) Schematic of the planar interdigital electrode impedance sensor and (c) photographs of the actual planar interdigital electrode impedance sensors covered with thick films of different MOFs by Moos et al.¹⁴⁸ (b) Schematic of the pellet-type metal-disc electrode impedance sensor and (d) photograph of the realised pellet-type metal-disc electrode impedance sensor, by Moos et al., containing a pellet of a MOF.¹⁴⁸ Reproduced under the terms of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/) from ref. 148 with copyright belonging to the authors of ref. 148.</figcaption></figure>

water vapour resulted in water being adsorbed on the HKUST-1 layer and a change in the resonant frequency of the microcantilever. The device demonstrated sensitivity between ~300 ppm and ~3000 ppm of water in nitrogen carrier gas and showed sensing reversibility. However, the magnitude of resonant frequency changes that were measured at equilibrium for different concentrations of water vapour in nitrogen carrier gas were identical to those measured upon exposure to different concentrations of methanol vapour. Therefore, based on measurable resonant frequency changes, the sensor device cannot differentiate between water and methanol in nitrogen. However, if the response times of the device are taken into account, i.e. the adsorption response time and the desorption response time, then analyte differentiation is possible. The adsorption and desorption response times for water were both <30 s, but this was only calculated at one water concentration. Robinson et al. defined the response time as the time taken to reach 63.2% of the equilibrium value either during desorption or adsorption as opposed to the definition of gas sensing response time $t_{90}$, (the time taken to reach 90% of the equilibrium value during adsorption or desorption) that is used more routinely in the literature.¹⁸⁵,¹⁸⁶ No sensing results for mixed analyte gas streams were reported. The major disadvantage of this gas sensor in terms of application as a sensor for exhaust gases is the poor thermal instability of the microcantilever.

Impedance humidity sensors that utilise MOFs as the active sensing materials have been developed by Moos et al.¹⁴⁸

The authors investigated two types of impedance humidity sensors; a planar interdigital electrode impedance sensor and a pellet-type metal-disc electrode impedance sensor, both of which are shown in Fig. 5.

For the pellet-type sensor, a pellet of a MOF was placed between the two disc electrodes. For the planar interdigital electrode sensor a thick film of a MOF was deposited on top of the interdigitated electrodes. The authors investigated several MOFs, but found that Fe-btc (Basolite® F300), the iron derivative of HKUST-1, afforded the highest magnitude measured changes in absolute impedance $|Z|$, and capacitance $C$, upon exposure to humidity in a range of 0–3 vol% water in nitrogen carrier gas. Further experiments revealed that the best sensor response in terms of largest magnitude changes in measurable parameters and reversibility was achieved with the planar interdigital electrode impedance sensor format at the frequency of 1 Hz and with a thick film of Fe-btc MOF as the active sensing material.

At an operating temperature of 120 °C, there was a linear response between $|Z|$ and water concentration (in vol%), an overall impedance range of 1.2 GΩ and a sensitivity of 590 MΩ/vol% water, in the range of 0–2.5 vol% water in nitrogen. In addition, the sensor response was reproducible with ~100% recovery to pre-exposure impedance values after each exposure to water vapour. Therefore this sensor shows promise as a water sensor in exhaust emission sensing applications especially as Fe-btc is thermally stable up to ~250 °C. However, a disadvantage of

<figure><img src="image_29.png" /></figure>
<figure><img src="image_30.png" /><figcaption>Fig. 5 (a) Schematic of the planar interdigital electrode impedance sensor and (c) photographs of the actual planar interdigital electrode impedance sensors covered with thick films of different MOFs by Moos et al.<sup>148</sup> (b) Schematic of the pellet-type metal-disc electrode impedance sensor and (d) photograph of the realised pellet-type metal-disc electrode impedance sensor, by Moos et al., containing a pellet of a MOF.<sup>148</sup> Reproduced under the terms of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/) from ref. 148 with copyright belonging to the authors of ref. 148.</figcaption></figure>

water vapour resulted in water being adsorbed on the HKUST-1 layer and a change in the resonant frequency of the microcantilever. The device demonstrated sensitivity between ~300 ppm and ~3000 ppm of water in nitrogen carrier gas and showed sensing reversibility. However, the magnitude of resonant frequency changes that were measured at equilibrium for different concentrations of water vapour in nitrogen carrier gas were identical to those measured upon exposure to different concentrations of methanol vapour. Therefore, based on measurable resonant frequency changes, the sensor device cannot differentiate between water and methanol in nitrogen. However, if the response times of the device are taken into account, i.e. the adsorption response time and the desorption response time, then analyte differentiation is possible. The adsorption and desorption response times for water were both <30 s, but this was only calculated at one water concentration. Robinson et al. defined the response time as the time taken to reach 63.2% of the equilibrium value either during desorption or adsorption as opposed to the definition of gas sensing response time $t_{90}$, (the time taken to reach 90% of the equilibrium value during adsorption or desorption) that is used more routinely in the literature.<sup>185,186</sup> No sensing results for mixed analyte gas streams were reported. The major disadvantage of this gas sensor in terms of application as a sensor for exhaust gases is the poor thermal instability of the microcantilever.

Impedance humidity sensors that utilise MOFs as the active sensing materials have been developed by Moos et al.<sup>148</sup>

The authors investigated two types of impedance humidity sensors; a planar interdigital electrode impedance sensor and a pellet-type metal-disc electrode impedance sensor, both of which are shown in Fig. 5.

For the pellet-type sensor, a pellet of a MOF was placed between the two disc electrodes. For the planar interdigital electrode sensor a thick film of a MOF was deposited on top of the interdigitated electrodes. The authors investigated several MOFs, but found that Fe-btc (Basolite® F300), the iron derivative of HKUST-1, afforded the highest magnitude measured changes in absolute impedance $|Z|$, and capacitance $C$, upon exposure to humidity in a range of 0–3 vol% water in nitrogen carrier gas. Further experiments revealed that the best sensor response in terms of largest magnitude changes in measurable parameters and reversibility was achieved with the planar interdigital electrode impedance sensor format at the frequency of 1 Hz and with a thick film of Fe-btc MOF as the active sensing material.

At an operating temperature of 120 °C, there was a linear response between $|Z|$ and water concentration (in vol%), an overall impedance range of 1.2 GΩ and a sensitivity of 590 MΩ/vol% water, in the range of 0–2.5 vol% water in nitrogen. In addition, the sensor response was reproducible with ~100% recovery to pre-exposure impedance values after each exposure to water vapour. Therefore this sensor shows promise as a water sensor in exhaust emission sensing applications especially as Fe-btc is thermally stable up to ~250 °C. However, a disadvantage of
this sensor is that the sensing response becomes non-linear and tends towards exponential at temperatures higher than 120 °C; this limits potential use in hot exhaust gas sensing applications.

As stated above, the large surface areas and high porosity of MOFs allows for the uptake of gaseous analytes and thus favours MOFs as active sensing materials. In addition it is expected that the effective refractive index of the MOF will be sensitive to the uptake of analyte molecules in the pores. Therefore Hupp et al. fabricated a refractometric optical-based detector which utilised HKUST-1 as the detection material; adsorption of gaseous analytes in the pores within HKUST-1 changed the effective refractive index of the MOF.¹³⁹ The refractometric optical detector consisted of an ordered hexagonal-packed array of sub-micrometer-sized silica microspheres coated in HKUST-1; this is known as a colloidal crystal. A colloidal crystal reflects light at particular wavelengths due to the periodic variation of refractive index within the structure. The reflected wavelength is known as the ‘stop band’. At normal incidence the ‘stop band’ $\lambda_{SB}$, in the [111] direction of a colloidal crystal is defined by:¹³⁹

$$ \lambda_{SB} = \frac{2\sqrt{2}}{\sqrt{3}}nD \quad (1) $$

$n$ is the effective refractive index of the colloidal crystal (unitless) and $D$ is the average diameter of the microspheres that make up the colloidal crystal (units = nm). As can be seen in eqn (1), there is a direct relationship between $\lambda_{SB}$ and $n$ as $D$ remains unchanged; if $n$ increases $\lambda_{SB}$ also increases and thus redshifts. When Hupp et al. exposed the colloidal crystal, modified with HKUST-1 to water vapour at concentrations ranging from 100–12 000 ppm in nitrogen carrier gas, ‘stop band’ redshifts were measured. The relationship between water concentration and measured ‘stop band’ wavelength shift was approximately linear between 100–1000 ppm and then plateaued after ~4000 ppm. The authors estimated that the lowest change in concentration of water that could be measured with the device was 2.6 ppm, based on the wavelength resolution of 0.015 nm of the measuring equipment. The detection response was also reversible. However a major limitation of this proof-of-concept device was the MOF was not selective; many different vapours and gases could adsorb onto the MOF which resulted in a change in the effective refractive index of the colloidal crystal. For example, some concentrations of water, methanol and ethanol vapours in nitrogen carrier gas resulted in identical magnitude ‘stop band’ shifts. One advantage of this device is that it is potentially thermally robust for exhaust gas sensing applications; the silica microspheres were sintered at 600 °C during fabrication of the device and HKUST-1 is thermally stable up to ~250 °C. However, a disadvantage of this current iteration of the detection device is that a light source and a high resolution optical spectrometer are needed to achieve high precision measurements of water vapour. The response of this device to other vapours and gases is discussed in the appropriate sections below.

HKUST-1 was also the chosen active sensing material for piezoelectric microcantilever humidity sensors demonstrated by Hesketh et al.¹⁴⁰ The reason for their choice was that small distortions of the HKUST-1 framework occur upon adsorption

of analytes in the pores. If the MOF is in the form of a thin film then the distortion within the framework creates a measurable stress on the piezoelectric microcantilever. The sensor device consisted of a 10-microcantilever array in which each microcantilever incorporated a piezoresistive element. This enabled measurement of changes in stress due to distortion of the framework of the MOF thin film caused by adsorption of analytes. For deposition of the HKUST-1 thin film (~100 nm thick) some of the microcantilevers were coated with gold, which was then modified with a self-assembled monolayer of a thiol; the thiol facilitated the formation of the HKUST-1 thin film on those cantilevers when immersed in the reactant solutions. Some microcantilevers were masked to prevent gold deposition and thus were not subsequently coated with HKUST-1 in order to act as reference microcantilevers. The authors tested the response of the device to water vapour in nitrogen carrier gas with HKUST-1 in either the hydrated as-synthesised state or the ‘activated’ dehydrated state. The dehydrated state was achieved by passing current through the piezoresistive elements so that temperature reached 50 °C whilst under a flow of dry nitrogen gas for 2 h. It was found that the sensing response to water vapour did not significantly differ whether the HKUST-1 was hydrated or dehydrated. Measurements were performed by exposing the sensor to a flow of water vapour at various concentrations (0–1%) in nitrogen carrier gas at room temperature and pressure. The sensor response was non-linear within the range of water vapour concentrations investigated. The authors reported a measurable change in resistance for adsorption after 0.5 s upon exposure to moist nitrogen gas, though this was the shortest measurement interval used. The time constant for desorption was ~10 s. Fitting of the water resistance curve to a Langmuir isotherm resulted in reasonable agreement with a 0–3 mbar isotherm of water adsorption on HKUST-1, which Hesketh et al. state corresponds to a resistance of 7.0 Ω for saturated HKUST-1.¹⁸⁷ The main disadvantage of the sensor was that the HKUST-1 MOF layer did not demonstrate selectivity; methanol and ethanol vapour also adsorbed which resulted in a change in stress. Some concentrations of water, methanol and ethanol vapours in nitrogen carrier gas resulted in identical magnitude measured resistance values. The response of this device to other vapours and gases is discussed in the appropriate sections below.

Fleischer et al. also utilised HKUST-1 as the active sensing material for water and other analytes.¹⁴¹ The device was a Kelvin probe work function-based sensor that consisted of an alumina substrate coated on the top side with a ~2 µm TiN back electrode and a screen printed platinum resistive heater element on the bottom side. On top of the TiN back electrode was a drop cast film of HKUST-1 as the sensing layer (Fig. 6).

Using this Kelvin probe setup, the measured signal represented the difference in the work function of the oscillating gold paddle and the sensing layer. As the gold paddle is not sensitive to the gases used, the signal represents the changes of the electronic structure of the sample under investigation. Humidity sensing measurements were performed in synthetic air (20% $O_2$ and 80% $N_2$) at a flow rate of 1 L min$^{-1}$ at 25 °C, and the humidity was studied in the range 0–50%RH. It was

<figure><img src="image_30.png" /></figure>
this sensor is that the sensing response becomes non-linear and tends towards exponential at temperatures higher than 120 °C; this limits potential use in hot exhaust gas sensing applications.

As stated above, the large surface areas and high porosity of MOFs allows for the uptake of gaseous analytes and thus favours MOFs as active sensing materials. In addition it is expected that the effective refractive index of the MOF will be sensitive to the uptake of analyte molecules in the pores. Therefore Hupp et al. fabricated a refractometric optical-based detector which utilised HKUST-1 as the detection material; adsorption of gaseous analytes in the pores within HKUST-1 changed the effective refractive index of the MOF.¹³⁹ The refractometric optical detector consisted of an ordered hexagonal-packed array of sub-micrometer-sized silica microspheres coated in HKUST-1; this is known as a colloidal crystal. A colloidal crystal reflects light at particular wavelengths due to the periodic variation of refractive index within the structure. The reflected wavelength is known as the ‘stop band’. At normal incidence the ‘stop band’ $λ_{SB}$, in the [111] direction of a colloidal crystal is defined by:¹³⁹

$$ \lambda_{SB} = \frac{2\sqrt{2}}{\sqrt{3}}nD \qquad (1) $$

$n$ is the effective refractive index of the colloidal crystal (unitless) and $D$ is the average diameter of the microspheres that make up the colloidal crystal (units = nm). As can be seen in eqn (1), there is a direct relationship between $λ_{SB}$ and $n$ as $D$ remains unchanged; if $n$ increases $λ_{SB}$ also increases and thus redshifts. When Hupp et al. exposed the colloidal crystal, modified with HKUST-1 to water vapour at concentrations ranging from 100–12 000 ppm in nitrogen carrier gas, ‘stop band’ redshifts were measured. The relationship between water concentration and measured ‘stop band’ wavelength shift was approximately linear between 100–1000 ppm and then plateaued after ~4000 ppm. The authors estimated that the lowest change in concentration of water that could be measured with the device was 2.6 ppm, based on the wavelength resolution of 0.015 nm of the measuring equipment. The detection response was also reversible. However a major limitation of this proof-of-concept device was the MOF was not selective; many different vapours and gases could adsorb onto the MOF which resulted in a change in the effective refractive index of the colloidal crystal. For example, some concentrations of water, methanol and ethanol vapours in nitrogen carrier gas resulted in identical magnitude ‘stop band’ shifts. One advantage of this device is that it is potentially thermally robust for exhaust gas sensing applications; the silica microspheres were sintered at 600 °C during fabrication of the device and HKUST-1 is thermally stable up to ~250 °C. However, a disadvantage of this current iteration of the detection device is that a light source and a high resolution optical spectrometer are needed to achieve high precision measurements of water vapour. The response of this device to other vapours and gases is discussed in the appropriate sections below.

HKUST-1 was also the chosen active sensing material for piezoelectric microcantilever humidity sensors demonstrated by Hesketh et al.¹⁴⁰ The reason for their choice was that small distortions of the HKUST-1 framework occur upon adsorption

of analytes in the pores. If the MOF is in the form of a thin film then the distortion within the framework creates a measurable stress on the piezoelectric microcantilever. The sensor device consisted of a 10-microcantilever array in which each microcantilever incorporated a piezoresistive element. This enabled measurement of changes in stress due to distortion of the framework of the MOF thin film caused by adsorption of analytes. For deposition of the HKUST-1 thin film (~100 nm thick) some of the microcantilevers were coated with gold, which was then modified with a self-assembled monolayer of a thiol; the thiol facilitated the formation of the HKUST-1 thin film on those cantilevers when immersed in the reactant solutions. Some microcantilevers were masked to prevent gold deposition and thus were not subsequently coated with HKUST-1 in order to act as reference microcantilevers. The authors tested the response of the device to water vapour in nitrogen carrier gas with HKUST-1 in either the hydrated as-synthesised state or the ‘activated’ dehydrated state. The dehydrated state was achieved by passing current through the piezoresistive elements so that temperature reached 50 °C whilst under a flow of dry nitrogen gas for 2 h. It was found that the sensing response to water vapour did not significantly differ whether the HKUST-1 was hydrated or dehydrated. Measurements were performed by exposing the sensor to a flow of water vapour at various concentrations (0–1%) in nitrogen carrier gas at room temperature and pressure. The sensor response was non-linear within the range of water vapour concentrations investigated. The authors reported a measurable change in resistance for adsorption after 0.5 s upon exposure to moist nitrogen gas, though this was the shortest measurement interval used. The time constant for desorption was ~10 s. Fitting of the water resistance curve to a Langmuir isotherm resulted in reasonable agreement with a 0–3 mbar isotherm of water adsorption on HKUST-1, which Hesketh et al. state corresponds to a resistance of 7.0 Ω for saturated HKUST-1.¹⁸⁷ The main disadvantage of the sensor was that the HKUST-1 MOF layer did not demonstrate selectivity; methanol and ethanol vapour also adsorbed which resulted in a change in stress. Some concentrations of water, methanol and ethanol vapours in nitrogen carrier gas resulted in identical magnitude measured resistance values. The response of this device to other vapours and gases is discussed in the appropriate sections below.

Fleischer et al. also utilised HKUST-1 as the active sensing material for water and other analytes.¹⁴¹ The device was a Kelvin probe work function-based sensor that consisted of an alumina substrate coated on the top side with a ~2 µm TiN back electrode and a screen printed platinum resistive heater element on the bottom side. On top of the TiN back electrode was a drop cast film of HKUST-1 as the sensing layer (Fig. 6).

Using this Kelvin probe setup, the measured signal represented the difference in the work function of the oscillating gold paddle and the sensing layer. As the gold paddle is not sensitive to the gases used, the signal represents the changes of the electronic structure of the sample under investigation. Humidity sensing measurements were performed in synthetic air (20% $O_2$ and 80% $N_2$) at a flow rate of 1 L min$^{-1}$ at 25 °C, and the humidity was studied in the range 0–50%RH. It was

<figure><img src="image_31.png" /></figure>
Review Article

<figure><img src="image_31.png" /><figcaption>Fig. 6 (a) Photograph of the Kelvin probe type sensor fabricated by Fleischer et al. and (b) schematic of the Kelvin probe highlighting the different layers that comprise the device.¹⁴¹ Reprinted from Sensors and Actuators B: Chemical, 187, Fleischer et al., Work function based gas sensing with Cu-BTC metal-organic framework for selective aldehyde detection, 142–146, Copyright 2012, with permission from Elsevier.</figcaption></figure>

found that the sensing response was reversible and stable with a decrease in work function (Δφ) of 5.5 mV per percent increase in relative humidity. In addition it was found that there was only a small influence of variation in humidity on the work function response of the TiN sample; the HKUST-1 layer imparted sensitivity to water vapour. However, the authors also demonstrated the sensitivity of the device to aldehydes and thus the device suffers from cross sensitivity. No response time was given for sensing of water and the thermal stability of the sensor device was also not indicated.

Kaskel et al. reported a mesoporous MOF that acted as a humidity detector.¹⁴⁹ The MOF was the first in a class of MOFs based on the N,N,N',N'-benzidinetetrabenzoate ligand (benztb) (Fig. 7) with auxiliary 1,3,5-benzenetricarboxylate (btc) ligands to strengthen the mesoporous framework.

The particular MOF used in this work was [Zn₄O(benztb)(btc)₂/3] (DUT-25) in which two types of pore were found to exist: one mesopore with an internal diameter of 32 × 20 Å and the other with internal dimensions of 18 × 8 Å. The BET specific surface area of the MOF was found to be 4670 m² g⁻¹. This coupled with the large mesopore means that DUT-25 is suitable for doping with a gasochromic species to enable visual response detecting of gases and vapours. Kaskel et al. loaded the porous structure with the solvatochromic dye Nile Blue and demonstrated that the solvatochromism of the dye was intact despite encapsulation. Upon exposure to water vapour the colour changed from dark blue (due to the presence of Nile Blue in the 'dry' state) to light blue and thus the MOF is a visible indicator material for humidity. The highly porous material could be of interest to developers of exhaust gas sensors based on MOFs as DUT-25 was determined to be thermally stable up to ~400 °C in air.

<figure><img src="image_32.png" /><figcaption>Fig. 7 The N,N,N',N'-benzidinetetrabenzoate (benztb) ligand.</figcaption></figure>

Wang et al. have reported a photonic film of a MOF which was used to detect water vapour.¹⁵⁰ Nanoparticles of the iron-based MOF [Fe₃O(abdc)₃X], NH₂-MIL-88B, (X = Cl, OH, abdc = 2-amino-1,4-benzenedicarboxylate) were deposited by spin deposition onto a silicon wafer. The coated silicon wafer was then placed into a quartz cuvette which contained a small volume of water, however the wafer did not come into contact with the water. The UV-visible reflection spectrum of the MOF film was then recorded every 2 min at 20 °C until the reflection spectra became stable. Adsorption of water into the pores of NH₂-MIL-88B replaced air and thus the effective refractive index of the MOF increased. This increase in refractive index resulted in a redshift of the interference peaks in the reflection spectrum by 50 nm. However, greater redshifts were measured upon exposure to hydrocarbon species. These responses are described in Section 3.5.2.

Finally, Ruan et al. have reported an impedance-based sensor for humidity that utilises the MOF [Ti₈O₈(OH)₄(abdc)₆] (NH₂-MIL-125(Ti)).¹⁵¹ A film of NH₂-MIL-125(Ti) was deposited onto five pairs of Ag-Pd interdigitated electrodes on a ceramic substrate. Upon exposure to relative humidity ranging from 11–95%RH, the largest magnitude change in impedance was measured at 100 Hz. Within this humidity range the impedance decreased from 4.5 MΩ at 11% RH to ~124 Ω at 95% RH in a non-linear manner. The t₉₀ response time of the sensor, upon increasing concentration of water vapour, was 45 s and the t₉₀ recovery time was ~50 s.

## 3.2 Oxygen sensors

### 3.2.1 Zeolites for O₂ sensing.
For O₂ detection and sensing, zeolite-based devices mainly rely on optical host-guest chemistry. For optical oxygen detectors, zeolites have been used as hosts for dyes. Dutta et al. developed a Ru(II) bipyridyl complex as a visual probe which showed a fluorescence (red) in the presence of oxygen contained in macrophage cells.⁹⁷,⁹⁸ They used the supercages of faujasite as holders for the Ru complex to improve its long-term stability and sensing could be carried out under 0 to 1 atmospheres. The faujasite-type zeolite is hydrophobic due to its highly siliceous framework, which enhanced the oxygen transfer from water to zeolite and thus allowed the detection of dissolved oxygen in water with the same Ru complex/ zeolite Y sensor.¹⁰⁴

The supercages of a faujasite-type zeolite have been used to host the dye 2-hydroxymethylanthraquinone for oxygen detection.⁹⁹ After photo-activation, the anthraquinone is transformed into oxygen-reactive dihydroxyanthracene, which in the presence of oxygen turns into a coloured anthraquinone allowing the reaction to be followed quantitatively using UV-visible spectroscopy. However, the utility of this detector is limited to one-off oxygen contamination detection by the fact that the reactive dihydroxyanthracene is irreversibly turned into the anthraquinone.

For potentiometric/amperometric O₂ sensing, thin films of zeolites were used to cover conducting sensing electrodes to selectively limit CO₂ exposure of the sensing surface.¹⁰⁵ As the diffusion of oxygen through the zeolite film is preferential to that of CO₂, the coated sensor showed a marked drop in response towards CO₂ along with slightly diminished but still discernable

<figure><img src="image_33.png" /></figure>
Review Article

<figure><img src="image_32.png" /><figcaption>Fig. 6 (a) Photograph of the Kelvin probe type sensor fabricated by Fleischer et al. and (b) schematic of the Kelvin probe highlighting the different layers that comprise the device.¹⁴¹ Reprinted from Sensors and Actuators B: Chemical, 187, Fleischer et al., Work function based gas sensing with Cu-BTC metal-organic framework for selective aldehyde detection, 142–146, Copyright 2012, with permission from Elsevier.</figcaption></figure>

found that the sensing response was reversible and stable with a decrease in work function (Δφ) of 5.5 mV per percent increase in relative humidity. In addition it was found that there was only a small influence of variation in humidity on the work function response of the TiN sample; the HKUST-1 layer imparted sensitivity to water vapour. However, the authors also demonstrated the sensitivity of the device to aldehydes and thus the device suffers from cross sensitivity. No response time was given for sensing of water and the thermal stability of the sensor device was also not indicated.

Kaskel et al. reported a mesoporous MOF that acted as a humidity detector.¹⁴⁹ The MOF was the first in a class of MOFs based on the N,N,N',N'-benzidinetetrabenzoate ligand (benztb) (Fig. 7) with auxiliary 1,3,5-benzenetricarboxylate (btc) ligands to strengthen the mesoporous framework.

The particular MOF used in this work was [Zn₄O(benztb)(btc)₂/3] (DUT-25) in which two types of pore were found to exist: one mesopore with an internal diameter of 32 × 20 Å and the other with internal dimensions of 18 × 8 Å. The BET specific surface area of the MOF was found to be 4670 m² g⁻¹. This coupled with the large mesopore means that DUT-25 is suitable for doping with a gasochromic species to enable visual response detecting of gases and vapours. Kaskel et al. loaded the porous structure with the solvatochromic dye Nile Blue and demonstrated that the solvatochromism of the dye was intact despite encapsulation. Upon exposure to water vapour the colour changed from dark blue (due to the presence of Nile Blue in the 'dry' state) to light blue and thus the MOF is a visible indicator material for humidity. The highly porous material could be of interest to developers of exhaust gas sensors based on MOFs as DUT-25 was determined to be thermally stable up to ~400 °C in air.

<figure><img src="image_33.png" /><figcaption>Fig. 7 The N,N,N',N'-benzidinetetrabenzoate (benztb) ligand.</figcaption></figure>

Wang et al. have reported a photonic film of a MOF which was used to detect water vapour.¹⁵⁰ Nanoparticles of the iron-based MOF [Fe₃O(abdc)₃X], NH₂-MIL-88B, (X = Cl, OH, abdc = 2-amino-1,4-benzenedicarboxylate) were deposited by spin deposition onto a silicon wafer. The coated silicon wafer was then placed into a quartz cuvette which contained a small volume of water, however the wafer did not come into contact with the water. The UV-visible reflection spectrum of the MOF film was then recorded every 2 min at 20 °C until the reflection spectra became stable. Adsorption of water into the pores of NH₂-MIL-88B replaced air and thus the effective refractive index of the MOF increased. This increase in refractive index resulted in a redshift of the interference peaks in the reflection spectrum by 50 nm. However, greater redshifts were measured upon exposure to hydrocarbon species. These responses are described in Section 3.5.2.

Finally, Ruan et al. have reported an impedance-based sensor for humidity that utilises the MOF [Ti₈O₈(OH)₄(abdc)₆] (NH₂-MIL-125(Ti)).¹⁵¹ A film of NH₂-MIL-125(Ti) was deposited onto five pairs of Ag-Pd interdigitated electrodes on a ceramic substrate. Upon exposure to relative humidity ranging from 11–95%RH, the largest magnitude change in impedance was measured at 100 Hz. Within this humidity range the impedance decreased from 4.5 MΩ at 11% RH to ~124 Ω at 95% RH in a non-linear manner. The t₉₀ response time of the sensor, upon increasing concentration of water vapour, was 45 s and the t₉₀ recovery time was ~50 s.

## 3.2 Oxygen sensors

### 3.2.1 Zeolites for O₂ sensing.
For O₂ detection and sensing, zeolite-based devices mainly rely on optical host-guest chemistry. For optical oxygen detectors, zeolites have been used as hosts for dyes. Dutta et al. developed a Ru(II) bipyridyl complex as a visual probe which showed a fluorescence (red) in the presence of oxygen contained in macrophage cells.⁹⁷,⁹⁸ They used the supercages of faujasite as holders for the Ru complex to improve its long-term stability and sensing could be carried out under 0 to 1 atmospheres. The faujasite-type zeolite is hydrophobic due to its highly siliceous framework, which enhanced the oxygen transfer from water to zeolite and thus allowed the detection of dissolved oxygen in water with the same Ru complex/ zeolite Y sensor.¹⁰⁴

The supercages of a faujasite-type zeolite have been used to host the dye 2-hydroxymethylanthraquinone for oxygen detection.⁹⁹ After photo-activation, the anthraquinone is transformed into oxygen-reactive dihydroxyanthracene, which in the presence of oxygen turns into a coloured anthraquinone allowing the reaction to be followed quantitatively using UV-visible spectroscopy. However, the utility of this detector is limited to one-off oxygen contamination detection by the fact that the reactive dihydroxyanthracene is irreversibly turned into the anthraquinone.

For potentiometric/amperometric O₂ sensing, thin films of zeolites were used to cover conducting sensing electrodes to selectively limit CO₂ exposure of the sensing surface.¹⁰⁵ As the diffusion of oxygen through the zeolite film is preferential to that of CO₂, the coated sensor showed a marked drop in response towards CO₂ along with slightly diminished but still discernable

<figure><img src="image_34.png" /></figure>
<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /><figcaption>Fig. 8 The adeninate ligand in the $Yb^{3+}@bio-MOF-1$ reported by Rosi et al. as a potential near-infrared luminescent sensor for oxygen gas.¹⁵²</figcaption></figure>

response towards $O_2$. A further amperometric and potentiometric bifunctional sensor was developed by Dutta et al. which could measure $O_2$ and $NO_x$ simultaneously in ppm concentrations.¹⁰⁶ The $O_2$ and $NO_x$ work by Dutta et al. is discussed further in Section 3.3.1.¹⁰⁶

### 3.2.2 MOFs for $O_2$ sensing.
Rosi et al. reported a MOF for $O_2$ detection that comprised of zinc metal centres and adeninate ligands which encapsulated and sensitised lanthanide cations in solution. The MOF, $(Me_2NH_2)_2[Zn_8O(ad)_4(bpdc)_6]·8DMF·11H_2O$, bio-MOF-1 (ad = adeninate, Fig. 8, bpdc = 4,4'-biphenyldicarboxylate) was used to encapsulate lanthanide cations to form $Ln^{3+}@bio-MOF-1$. The ytterbium-containing species $Yb^{3+}@bio-MOF-1$ was then used as a probe for gaseous $O_2$.¹⁵² The response of $Yb^{3+}@bio-MOF-1$ to $O_2$ gas was monitored by measuring the change in intensity of the ytterbium near-infrared luminescence signal at 970 nm, upon excitation with 340 nm light. An approximate 40% decrease in luminescence intensity was observed within the first 5 min of the introduction of $O_2$ gas to the purged chamber under ambient pressure. After 5 min, the luminescent system reached equilibrium and the intensity of the luminescence remained unchanged during 1 h exposure. Purging the chamber with $N_2$ resulted in the restoration of the ytterbium near-infrared luminescence to its original intensity even after several cycles of exposure to $O_2$ and $N_2$.

Ir(ppy)$_3$ complexes (ppy = 2-phenylpyridine) possess highly efficient phosphorescence owing to the facile intersystem crossing to populate the ³MLCT state. This ³MLCT phosphorescence can be readily quenched by molecules with a triplet ground state such as $O_2$. Lin et al. devised a phosphorescent MOF that consisted of Zn-based secondary building units and ligands that are derivatives of Ir(ppy)$_3$.¹⁵³ The linker was a cyclometalated Ir complex based on the 2-phenylpyridine carboxylate derivative $L^2$ (Fig. 9).

The porous 2D MOF $[Zn_4O(L^2)_2]·6DMF·H_2O$ was ground and pressed onto the surface of a KBr pellet, which in turn was placed into a quartz cuvette that was equipped with an inlet/outlet port for $O_2$ ingress or vacuum. Upon excitation at 385 nm, and after evacuation for 2 h under dynamic vacuum, the luminescence

<figure><img src="image_36.png" /><figcaption>Fig. 9 The iridium(III) metalloligand $L^2$ used by Lin et al. to fabricate a luminescent MOF for oxygen gas detection.¹⁵³</figcaption></figure>

<figure><img src="image_37.png" /><figcaption>Fig. 10 The iridium(III) metalloligand $[Ir(ppy)_2(dcbpy)]^-$, $L^3$, used in the oxygen sensing MOFs synthesised by Sheu et al.¹⁵⁴</figcaption></figure>

intensity centred at 538 nm decreased upon exposure to $O_2$ with a sensitivity down to 0.05 atm and quenching efficiency of 59% at 1 atm of $O_2$. The MOF also demonstrated reversible quenching by $O_2$ with <5% of the original luminescence intensity lost after 8 cycles with an adsorption response time of ~30 s and a desorption recovery time of ~120 s. The MOF was thermally stable up to ~300 °C.

Sheu et al. also used an Ir(III) complex as a ligand to create luminescent MOFs for which the luminescence was quenched in the presence of oxygen.¹⁵⁴ The Ir(III) complex $[Ir(ppy)_2(dcbpy)]^-$ ($L^3$) contains two bidentate 2-phenylpyridine (ppy) ligands and 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) as the third bidentate ligand (Fig. 10). Four MOFs were synthesised by stirring $H_2L^3$ with the perchlorate salts of zinc(II), cadmium(II), cobalt(II) and nickel(II) in aqueous DMF solution. Of these compounds, the cadmium MOF $[Cd(L^3)_2(H_2O)_2]·3DMF·6H_2O$ demonstrated the greatest oxygen detection characteristics with a quenching efficiency of 74% at 1 atm $O_2$, a linear relationship on the Stern-Volmer plot (ratio of initial emission intensity $I_o$, to measured emission intensity $I_i$, against mole fraction of oxygen $\chi_{O_2}$; $I_o/I_i$ vs. $\chi_{O_2}$) in the $\chi_{O_2}$ range 0-1, and the highest Stern-Volmer quenching constant, $K_{SV}$, at both 1 atm $O_2$ and at $O_2$ concentration <0.2 mole fraction. In addition this material had favourable detection kinetics; a 95% response time of 70 s and a 95% recovery time of 30 s. The MOF also showed good reproducibility and the detection response was reversible. Despite the MOF being stable up to ~300 °C, the detection response rapidly deteriorated at temperatures >50 °C. This was ascribed to the loss of co-ordinated guest/free solvent molecules that are involved in the oxygen-detection mechanism.

Lin et al. further investigated the use of metal complexes as ligands in three-dimensional MOFs for luminescence-based detection of oxygen.¹⁵⁵ By incorporating the iridium and ruthenium complex ligands $L^4$, $L^5$ and $L^6$ (Fig. 11) into the zirconium-based MOF $[Zr_6O_4(OH)_4(bpdc)_6]$, UiO-67, via a ligand substitution strategy, three phosphorescent MOFs were fabricated.

Nitrogen adsorption/desorption type 1 isotherms at 77 K were obtained for all three of the MOFs and $[Zr_6O_4(OH)_4·(bpdc)_{5,971}(L^4)_{0.029}]$, $[Zr_6O_4(OH)_4(bpdc)_{5,942}(L^5)_{0.058}]$ and $[Zr_6O_4(OH)_4·(bpdc)_{5,821}(L^6)_{0.179}]$ all had BET surface areas >1000 m² g⁻¹ indicating microporosity. The MOFs were ground and pressed onto the surface of KBr pellets, which in turn were placed into quartz cuvettes that were equipped with an inlet/outlet port for $O_2$ ingress or vacuum. Upon excitation at 420 nm, changes in the emission intensity at 600 nm were measured upon stepwise
<figure><img src="image_35.png" /><figcaption>Fig. 8 The adeninate ligand in the $Yb^{3+}$@bio-MOF-1 reported by Rosi et al. as a potential near-infrared luminescent sensor for oxygen gas.¹⁵²</figcaption></figure>

response towards $O_2$. A further amperometric and potentiometric bifunctional sensor was developed by Dutta et al. which could measure $O_2$ and $NO_x$ simultaneously in ppm concentrations.¹⁰⁶ The $O_2$ and $NO_x$ work by Dutta et al. is discussed further in Section 3.3.1.¹⁰⁶

### 3.2.2 MOFs for $O_2$ sensing.
Rosi et al. reported a MOF for $O_2$ detection that comprised of zinc metal centres and adeninate ligands which encapsulated and sensitised lanthanide cations in solution. The MOF, (Me₂NH₂)₂[Zn₈O(ad)₄(bpdc)₆]·8DMF·11H₂O, bio-MOF-1 (ad = adeninate, Fig. 8, bpdc = 4,4''-biphenyldicarboxylate) was used to encapsulate lanthanide cations to form Ln³⁺@bio-MOF-1. The ytterbium-containing species $Yb^{3+}$@bio-MOF-1 was then used as a probe for gaseous $O_2$.¹⁵² The response of $Yb^{3+}$@bio-MOF-1 to $O_2$ gas was monitored by measuring the change in intensity of the ytterbium near-infrared luminescence signal at 970 nm, upon excitation with 340 nm light. An approximate 40% decrease in luminescence intensity was observed within the first 5 min of the introduction of $O_2$ gas to the purged chamber under ambient pressure. After 5 min, the luminescent system reached equilibrium and the intensity of the luminescence remained unchanged during 1 h exposure. Purging the chamber with $N_2$ resulted in the restoration of the ytterbium near-infrared luminescence to its original intensity even after several cycles of exposure to $O_2$ and $N_2$.

Ir(ppy)₃ complexes (ppy = 2-phenylpyridine) possess highly efficient phosphorescence owing to the facile intersystem crossing to populate the ³MLCT state. This ³MLCT phosphorescence can be readily quenched by molecules with a triplet ground state such as $O_2$. Lin et al. devised a phosphorescent MOF that consisted of Zn-based secondary building units and ligands that are derivatives of Ir(ppy)₃.¹⁵³ The linker was a cyclometalated Ir complex based on the 2-phenylpyridine carboxylate derivative $L²$ (Fig. 9).

The porous 2D MOF [Zn₄O(L²)₂]·6DMF·H₂O was ground and pressed onto the surface of a KBr pellet, which in turn was placed into a quartz cuvette that was equipped with an inlet/outlet port for $O_2$ ingress or vacuum. Upon excitation at 385 nm, and after evacuation for 2 h under dynamic vacuum, the luminescence

<figure><img src="image_36.png" /><figcaption>Fig. 9 The iridium(III) metalloligand $L²$ used by Lin et al. to fabricate a luminescent MOF for oxygen gas detection.¹⁵³</figcaption></figure>

<figure><img src="image_37.png" /><figcaption>Fig. 10 The iridium(III) metalloligand [Ir(ppy)₂(dcbpy)]⁻, $L³$, used in the oxygen sensing MOFs synthesised by Sheu et al.¹⁵⁴</figcaption></figure>

intensity centred at 538 nm decreased upon exposure to $O_2$ with a sensitivity down to 0.05 atm and quenching efficiency of 59% at 1 atm of $O_2$. The MOF also demonstrated reversible quenching by $O_2$ with <5% of the original luminescence intensity lost after 8 cycles with an adsorption response time of ~30 s and a desorption recovery time of ~120 s. The MOF was thermally stable up to ~300 °C.

Sheu et al. also used an Ir(III) complex as a ligand to create luminescent MOFs for which the luminescence was quenched in the presence of oxygen.¹⁵⁴ The Ir(III) complex [Ir(ppy)₂(dcbpy)]⁻($L³$) contains two bidentate 2-phenylpyridine (ppy) ligands and 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) as the third bidentate ligand (Fig. 10). Four MOFs were synthesised by stirring $H_2L³$ with the perchlorate salts of zinc(II), cadmium(II), cobalt(II) and nickel(II) in aqueous DMF solution. Of these compounds, the cadmium MOF [Cd(L³)₂(H₂O)₂]·3DMF·6H₂O demonstrated the greatest oxygen detection characteristics with a quenching efficiency of 74% at 1 atm $O_2$, a linear relationship on the Stern-Volmer plot (ratio of initial emission intensity $I_o$, to measured emission intensity $I_i$, against mole fraction of oxygen $\chi_{O_2}$; $I_o/I_i$ vs. $\chi_{O_2}$) in the $\chi_{O_2}$ range 0-1, and the highest Stern-Volmer quenching constant, $K_{SV}$, at both 1 atm $O_2$ and at $O_2$ concentration <0.2 mole fraction. In addition this material had favourable detection kinetics; a 95% response time of 70 s and a 95% recovery time of 30 s. The MOF also showed good reproducibility and the detection response was reversible. Despite the MOF being stable up to ~300 °C, the detection response rapidly deteriorated at temperatures >50 °C. This was ascribed to the loss of co-ordinated guest/free solvent molecules that are involved in the oxygen-detection mechanism.

Lin et al. further investigated the use of metal complexes as ligands in three-dimensional MOFs for luminescence-based detection of oxygen.¹⁵⁵ By incorporating the iridium and ruthenium complex ligands $L⁴$, $L⁵$ and $L⁶$ (Fig. 11) into the zirconium-based MOF [Zr₆O₄(OH)₄(bpdc)₆], UiO-67, via a ligand substitution strategy, three phosphorescent MOFs were fabricated.

Nitrogen adsorption/desorption type 1 isotherms at 77 K were obtained for all three of the MOFs and [Zr₆O₄(OH)₄·(bpdc)₅.971(L⁴)₀.029], [Zr₆O₄(OH)₄(bpdc)₅.942(L⁵)₀.058] and [Zr₆O₄(OH)₄·(bpdc)₅.821(L⁶)₀.179] all had BET surface areas >1000 m² g⁻¹ indicating microporosity. The MOFs were ground and pressed onto the surface of KBr pellets, which in turn were placed into quartz cuvettes that were equipped with an inlet/outlet port for $O_2$ ingress or vacuum. Upon excitation at 420 nm, changes in the emission intensity at 600 nm were measured upon stepwise

<figure><img src="image_38.png" /></figure>
<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /><figcaption>Fig. 11 The phosphorescent metalloligands L⁴–L⁶ incorporated into UiO-67 by Lin et al. for oxygen sensing application.¹⁵⁵</figcaption></figure>

increase of O₂ pressure starting from vacuum. Decreasing emission intensity with increasing oxygen concentration was observed for the MOFs between 0.001–0.839 atm O₂. The phosphorescence emission of the L⁵-based MOF was most efficiently quenched by oxygen with a quenching efficiency of 65% at 0.8 atm O₂. It was found for all three MOFs that the luminescence intensity decreased instantly after initial O₂ concentrations and then much smaller extents of luminescence intensity reduction were observed for the subsequent stepwise O₂ pressure increases. Thus all three MOFs displayed a non-linear response to increasing oxygen concentration which was further demonstrated by the non-linear Stern–Volmer curves. This suggested that the phosphorescent sites were distributed in an inhomogeneous manner within the MOF frameworks. The MOFs displayed reversible, reproducible sensing responses that returned to ~1–3% of the initial intensities even after >5 oxygen sensing cycles.

Chen et al. also used the approach of doping a parent MOF framework with phosphorescent ligands to afford a phosphorescent oxygen-detecting MOF.¹⁵⁶ Chen et al. chose MAF-34 [Zn₇(ip)₁₂](OH)₂ (ip = imidazo[4,5-f][1,10]phenanthrolin-1-ide, Fig. 12) as the parent MOF and doped this within varying amounts of ruthenium to form four phosphorescent MOFs of the formula [RuₓZn₇⁻ₓ(ip)₁₂](OH)₂ where x = 0.10–0.16.

The low concentration of the phosphorescent {Ru(ip)₃}⁻ units in the MOF meant that self-quenching was avoided. Upon excitation at 480 nm and exposure to increasing oxygen pressure the intensity of the phosphorescent emission at ~600 nm decreased and blueshifted ~5 nm with a very fast response. At 1 atm of O₂ the emission intensities of the MOFs were quenched by between 75 and 88%, the authors attributing the high quenching efficiencies to the ultramicroporous channels within the MOFs and low ruthenium content which leads to more oxygen adsorbed per ruthenium phosphorescent centre. Indeed, high O₂ uptake of 3.2 cm³ g⁻¹ (0.20 mol L⁻¹) was measured at 1 atm and 25 °C.

<figure><img src="image_40.png" /><figcaption>Fig. 12 The imidazo[4,5-f][1,10]phenanthrolin-1-ide ligand in the ruthenium-doped MAF-34 MOFs of Chen et al.¹⁵⁶</figcaption></figure>

Time dependent luminescence emission profiles, in the absence of oxygen, show biexponential decay characteristics which suggests that the ruthenium phosphorescent centres are in two different environments within the MOFs. This was further confirmed by the non-linear Stern–Volmer plots recorded for all the MOFs, which indicate heterogeneous distribution of the luminescent species. However, a linear response to changing oxygen concentration is measured if a Freundlich adsorption isotherm treatment is applied to the data.

Chen et al. fabricated a composite thin film of [Ru₀.₁₆Zn₆.₈₄⁻(ip)₁₂](OH)₂ in silicon rubber on a glass substrate. The thin film demonstrated very fast responses to changing oxygen concentration, good reversibility, reproducibility and stability. In addition, they demonstrated that this composite thin film could be applied to the outside of a commercial blue-light LED which enabled easy identification of changing oxygen concentration with a colour-changing device. The MOF-modified LED emitted magenta light in the presence of N₂, due to the blue emission of the LED combined with the reddish-orange emission of the MOF. In the presence of O₂ the intensity of the reddish-orange emission of the MOF decreased significantly so that the emitted light from the device was violet.

Chen et al. have also developed a fluorescent MOF which did not contain any precious metals.¹⁵⁷ [Zn₄O(bpz)₂(abdc)] (bpz = 4-(3,5-dimethylpyrazol-1-id-4-yl)-3,5-dimethyl-pyrazol-1-ide, Fig. 13), MAF-X11, exhibited fluorescent emission at ~470 nm upon excitation at 345 nm, the intensity of which decreased with increasing oxygen pressure. At 1 bar of O₂, the fluorescence was quenched by 96.5% which is comparable to quenching intensities reported for luminescent hybrid materials containing precious metal complexes. The ratio of initial fluorescent intensity to measured luminescent intensity linearly related to changing pressure of O₂ with a Stern–Volmer constant KₛV = 27.1 bar⁻¹. Upon repeated cycling between 1 bar O₂ and vacuum the quenching response was found to be rapid, reversible and highly stable.

<figure><img src="image_41.png" /><figcaption>Fig. 13 The 4-(3,5-dimethylpyrazol-1-id-4-yl)-3,5-dimethyl-pyrazol-1-ide ligand in the oxygen sensing fluorescent MOF MAF-X11, reported by Chen et al.¹⁵⁷</figcaption></figure>
<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /><figcaption>Fig. 11 The phosphorescent metalloligands L⁴–L⁶ incorporated into UiO-67 by Lin et al. for oxygen sensing application.¹⁵⁵</figcaption></figure>

increase of O₂ pressure starting from vacuum. Decreasing emission intensity with increasing oxygen concentration was observed for the MOFs between 0.001–0.839 atm O₂. The phosphorescence emission of the L⁵-based MOF was most efficiently quenched by oxygen with a quenching efficiency of 65% at 0.8 atm O₂. It was found for all three MOFs that the luminescence intensity decreased instantly after initial O₂ concentrations and then much smaller extents of luminescence intensity reduction were observed for the subsequent stepwise O₂ pressure increases. Thus all three MOFs displayed a non-linear response to increasing oxygen concentration which was further demonstrated by the non-linear Stern–Volmer curves. This suggested that the phosphorescent sites were distributed in an inhomogeneous manner within the MOF frameworks. The MOFs displayed reversible, reproducible sensing responses that returned to ~1–3% of the initial intensities even after >5 oxygen sensing cycles.

Chen et al. also used the approach of doping a parent MOF framework with phosphorescent ligands to afford a phosphorescent oxygen-detecting MOF.¹⁵⁶ Chen et al. chose MAF-34 [Zn₇(ip)₁₂](OH)₂ (ip = imidazo[4,5-f][1,10]phenanthrolin-1-ide, Fig. 12) as the parent MOF and doped this within varying amounts of ruthenium to form four phosphorescent MOFs of the formula [RuₓZn₇⁻ₓ(ip)₁₂](OH)₂ where x = 0.10–0.16.

The low concentration of the phosphorescent {Ru(ip)₃}⁻ units in the MOF meant that self-quenching was avoided. Upon excitation at 480 nm and exposure to increasing oxygen pressure the intensity of the phosphorescent emission at ~600 nm decreased and blueshifted ~5 nm with a very fast response. At 1 atm of O₂ the emission intensities of the MOFs were quenched by between 75 and 88%, the authors attributing the high quenching efficiencies to the ultramicroporous channels within the MOFs and low ruthenium content which leads to more oxygen adsorbed per ruthenium phosphorescent centre. Indeed, high O₂ uptake of 3.2 cm³ g⁻¹ (0.20 mol L⁻¹) was measured at 1 atm and 25 °C.

<figure><img src="image_41.png" /><figcaption>Fig. 12 The imidazo[4,5-f][1,10]phenanthrolin-1-ide ligand in the ruthenium-doped MAF-34 MOFs of Chen et al.¹⁵⁶</figcaption></figure>

Time dependent luminescence emission profiles, in the absence of oxygen, show biexponential decay characteristics which suggests that the ruthenium phosphorescent centres are in two different environments within the MOFs. This was further confirmed by the non-linear Stern–Volmer plots recorded for all the MOFs, which indicate heterogeneous distribution of the luminescent species. However, a linear response to changing oxygen concentration is measured if a Freundlich adsorption isotherm treatment is applied to the data.

Chen et al. fabricated a composite thin film of [Ru₀.₁₆Zn₆.₈₄⁻(ip)₁₂](OH)₂ in silicon rubber on a glass substrate. The thin film demonstrated very fast responses to changing oxygen concentration, good reversibility, reproducibility and stability. In addition, they demonstrated that this composite thin film could be applied to the outside of a commercial blue-light LED which enabled easy identification of changing oxygen concentration with a colour-changing device. The MOF-modified LED emitted magenta light in the presence of N₂, due to the blue emission of the LED combined with the reddish-orange emission of the MOF. In the presence of O₂ the intensity of the reddish-orange emission of the MOF decreased significantly so that the emitted light from the device was violet.

Chen et al. have also developed a fluorescent MOF which did not contain any precious metals.¹⁵⁷ [Zn₄O(bpz)₂(abdc)] (bpz = 4-(3,5-dimethylpyrazol-1-id-4-yl)-3,5-dimethyl-pyrazol-1-ide, Fig. 13), MAF-X11, exhibited fluorescent emission at ~470 nm upon excitation at 345 nm, the intensity of which decreased with increasing oxygen pressure. At 1 bar of O₂, the fluorescence was quenched by 96.5% which is comparable to quenching intensities reported for luminescent hybrid materials containing precious metal complexes. The ratio of initial fluorescent intensity to measured luminescent intensity linearly related to changing pressure of O₂ with a Stern–Volmer constant KₛV = 27.1 bar⁻¹. Upon repeated cycling between 1 bar O₂ and vacuum the quenching response was found to be rapid, reversible and highly stable.

<figure><img src="image_42.png" /><figcaption>Fig. 13 The 4-(3,5-dimethylpyrazol-1-id-4-yl)-3,5-dimethyl-pyrazol-1-ide ligand in the oxygen sensing fluorescent MOF MAF-X11, reported by Chen et al.¹⁵⁷</figcaption></figure>
Furthermore it was demonstrated that the quenching efficiency of the MOF in air was equivalent to the quenching efficiency at an O₂ pressure of 0.21 bar, thus indicating that the MOF was not cross-sensitive to other gases in air. In addition the MOF was found to be thermally stable up to >450 °C.

Ligand-to-metal energy transfer (EnT) is responsible for the characteristic luminescence of lanthanide MOFs. It has been proposed that the quenching of the luminescence of lanthanide MOFs upon exposure to oxygen gas is due to oxygen deactivating the triplet-state of the organic ligands whilst having very little interaction with the lanthanide ions. For efficient oxygen sensing with lanthanide MOFs, the MOF needs to possess an effective EnT and an appropriate thermally activated energy back transfer (BenT) that will prolong the triplet-state lifetime of the ligands. With this in mind, Qian *et al.* proposed doping [(CH₃)₂NH₂][In₃O(btc)₂(H₂O)₃]₂[In₃(btc)₄], CPM-5, and [In₃O(OH)- (H₂O)₂(btc)₂], MIL-100(In), with Tb³⁺ as they realised that the first triplet-state energy of the btc ligand is ~23 200 cm⁻¹, which is close to the ⁵D₄ excited state of Tb³⁺ (~20 400 cm⁻¹).¹⁵⁸ They proposed that this would result in an effective EnT and appropriate BenT, both of which would be beneficial to oxygen detection. Thin films (~2.5 µm thick) of CPM-5 and MIL-100(In) were grown solvothermally on indium tin oxide glass and then doped with Tb³⁺ by a post-fabrication immersion treatment into an aqueous DMF solution of terbium(III) nitrate to form Tb³⁺-doped CPM-5 and MIL-100(In) known as CPM-5>Tb³⁺ and MIL-100(In)>Tb³⁺ respectively. When the luminescent properties of CPM-5>Tb³⁺ and MIL-100(In)>Tb³⁺ were investigated, it was found that the quantum yield of MIL-100(In)>Tb³⁺ was 16.8% which was much larger than the quantum yield of CPM-5>Tb³⁺ (1.1%).

Upon exposure to O₂ in a N₂ carrier gas the respective luminescent emissions, at λem = 544 nm for CPM-5>Tb³⁺ and λem = 546 nm for MIL-100(In)>Tb³⁺, were quenched with a quenching efficiency of 47% at 1 atm O₂ for CPM-5>Tb³⁺ and 88% at 1 atm for MIL-100(In)>Tb³⁺. Stern–Volmer plots for the two MOFs showed good linearity suggesting the Tb³⁺ was distributed homogeneously within the MOFs. The Stern–Volmer quenching constant (Ksv) for MIL-100(In)>Tb³⁺ was an order of magnitude greater than Ksv for CPM-5>Tb³⁺. This was ascribed to the greater efficiency of the intramolecular energy transfer within MIL-100(In)>Tb³⁺ as opposed to the intermolecular energy transfer within CPM-5>Tb³⁺. Overall the MIL-100(In)>Tb³⁺ MOF possessed more desirable properties for O₂ sensing as exemplified by a relatively faster sensing time. It took 6 s to achieve 95% of the maximum reduction in luminescent intensity upon changing from a 100% N₂ atmosphere to a 100% O₂ atmosphere for MIL-100(In)>Tb³⁺ and 90 s for CPM-5>Tb³⁺. Upon changing from a 100% O₂ atmosphere to a 100% N₂ it took 53 s for 95% of the maximum recovery in luminescent intensity to be achieved for MIL-100(In)>Tb³⁺ and 60 s for CPM-5>Tb³⁺. Furthermore, MIL-100(In)>Tb³⁺ exhibited good reversible oxygen quenching and nitrogen recovering properties.

### 3.3 NOₓ sensors

#### 3.3.1 Zeolites for NOₓ sensing.
Conductivity measurements are the most developed method for efficient NOₓ gas sensing,

usually with conductive metal oxides improved by a zeolite layer working as a filter or induced catalyst. Concerning the use of zeolites as filters, semiconducting WO₃ covered with four different types of zeolite (H-forms of zeolite A, ZSM-5, beta polymorph A and zeolite Y) was investigated as a NO₂ sensor.⁶⁰ The best result was obtained with zeolite Y as the covering layer. It exhibited a sensing response to 204 ppb of NO₂ with a response time half that of the bare WO₃ sensor. However, upon exposure to water vapour, the response time increased. More recently, similar results were obtained using ZnO inks as the metal oxide element covered with either zeolite beta polymorph A, zeolite Y or mordenite.⁴⁹ Compared to unmodified ZnO sensors, zeolite modification was found to increase the sensitivity of the sensor down to ppb concentrations of NO₂.

In the examples discussed below, the zeolite layer acts as a catalyst prior to sensing in order to maintain a constant composition of the NOₓ mixture thus enabling measurement of the equilibration between NO and NO₂ at different temperatures by potentiometry between working and reference electrodes. A NOₓ sensor with minimal interferences from CO and O₂ comprised yttria-stabilised zirconia (YSZ) pellets and a Pt-loaded zeolite Y layer.¹⁰⁷ These sensors contain catalytic filters based on YSZ working at temperatures in the range of 300–700 °C. The development of a NO sensor capable of operating in automotive exhaust streams was reported from the same group based on a Pt-yttria-stabilised zirconia (Pt-YSZ) covered by zeolite. The sensors were tested at temperatures between 500 and 700 °C, and the zeolite was permeable to oxygen, thus, was minimising the interference from O₂. The sensor shows interference from CO and NO₂, however these tests indicate that the device is able to detect NOₓ in engine exhausts.¹⁰⁹ Using Pt-doped zeolite Y on a yttria-stabilised zirconia electrolyte, the NOₓ concentration could be measured using a sensing electrode.¹⁰⁸ This sensor device was then optimised to reduce interference from other gases. The addition of a WO₃ electrode provided the highest reactivity difference with the Pt-doped zeolite Y and interference from 2000 ppm CO, 800 ppm propane, 10 ppm NH₃, as well as the effects of 1–13% O₂, CO₂, and H₂O were minimised.¹¹⁰,¹¹¹ A human breath analysis was performed with 20 of those sensors connected in series for the diagnosis of asthma since NO in breath increases from 5–10 ppb (normal state) to 100 ppb in an asthmatic state.¹¹²

The imbalance in equilibration between NO₂ and NO could also be managed by a voltage change, instead of temperature, to build an amperometric sensor. An example has been reported using the same material, a Pt-doped zeolite Y on a yttria-stabilised zirconia, connected to a potentiostat.¹¹³ It demonstrated a ppm detection of NOₓ with no interference from CO and hydrocarbons. Both amperometric and potentiometric monitoring were combined to build a bifunctional sensor which could measure NO₂ and O₂ simultaneously in ppm concentration with almost no interference between each other.¹⁰⁶

A QCM oscillator-based sensor was also developed for NO sensing that took advantage of the adsorption of NO on zeolites. Zeolite A, silicalite-1 and sodalite were deposited on a QCM and NO adsorption was followed. The zeolite-A doped QCM was
<figure><img src="image_43.png" /></figure>

Furthermore it was demonstrated that the quenching efficiency of the MOF in air was equivalent to the quenching efficiency at an O₂ pressure of 0.21 bar, thus indicating that the MOF was not cross-sensitive to other gases in air. In addition the MOF was found to be thermally stable up to >450 °C.

Ligand-to-metal energy transfer (EnT) is responsible for the characteristic luminescence of lanthanide MOFs. It has been proposed that the quenching of the luminescence of lanthanide MOFs upon exposure to oxygen gas is due to oxygen deactivating the triplet-state of the organic ligands whilst having very little interaction with the lanthanide ions. For efficient oxygen sensing with lanthanide MOFs, the MOF needs to possess an effective EnT and an appropriate thermally activated energy back transfer (BenT) that will prolong the triplet-state lifetime of the ligands. With this in mind, Qian *et al.* proposed doping [(CH₃)₂NH₂][In₃O(btc)₂(H₂O)₃]₂[In₃(btc)₄], CPM-5, and [In₃O(OH)- (H₂O)₂(btc)₂], MIL-100(In), with Tb³⁺ as they realised that the first triplet-state energy of the btc ligand is ~23 200 cm⁻¹, which is close to the ⁵D₄ excited state of Tb³⁺ (~20 400 cm⁻¹).¹⁵⁸ They proposed that this would result in an effective EnT and appropriate BenT, both of which would be beneficial to oxygen detection. Thin films (~2.5 µm thick) of CPM-5 and MIL-100(In) were grown solvothermally on indium tin oxide glass and then doped with Tb³⁺ by a post-fabrication immersion treatment into an aqueous DMF solution of terbium(III) nitrate to form Tb³⁺-doped CPM-5 and MIL-100(In) known as CPM-5>Tb³⁺ and MIL-100(In)>Tb³⁺ respectively. When the luminescent properties of CPM-5>Tb³⁺ and MIL-100(In)>Tb³⁺ were investigated, it was found that the quantum yield of MIL-100(In)>Tb³⁺ was 16.8% which was much larger than the quantum yield of CPM-5>Tb³⁺ (1.1%).

Upon exposure to O₂ in a N₂ carrier gas the respective luminescent emissions, at λem = 544 nm for CPM-5>Tb³⁺ and λem = 546 nm for MIL-100(In)>Tb³⁺, were quenched with a quenching efficiency of 47% at 1 atm O₂ for CPM-5>Tb³⁺ and 88% at 1 atm for MIL-100(In)>Tb³⁺. Stern-Volmer plots for the two MOFs showed good linearity suggesting the Tb³⁺ was distributed homogeneously within the MOFs. The Stern-Volmer quenching constant (Ksv) for MIL-100(In)>Tb³⁺ was an order of magnitude greater than Ksv for CPM-5>Tb³⁺. This was ascribed to the greater efficiency of the intramolecular energy transfer within MIL-100(In)>Tb³⁺ as opposed to the intermolecular energy transfer within CPM-5>Tb³⁺. Overall the MIL-100(In)>Tb³⁺ MOF possessed more desirable properties for O₂ sensing as exemplified by a relatively faster sensing time. It took 6 s to achieve 95% of the maximum reduction in luminescent intensity upon changing from a 100% N₂ atmosphere to a 100% O₂ atmosphere for MIL-100(In)>Tb³⁺ and 90 s for CPM-5>Tb³⁺. Upon changing from a 100% O₂ atmosphere to a 100% N₂ it took 53 s for 95% of the maximum recovery in luminescent intensity to be achieved for MIL-100(In)>Tb³⁺ and 60 s for CPM-5>Tb³⁺. Furthermore, MIL-100(In)>Tb³⁺ exhibited good reversible oxygen quenching and nitrogen recovering properties.

### 3.3 NOₓ sensors

#### 3.3.1 Zeolites for NOₓ sensing.
Conductivity measurements are the most developed method for efficient NOₓ gas sensing,

usually with conductive metal oxides improved by a zeolite layer working as a filter or induced catalyst. Concerning the use of zeolites as filters, semiconducting WO₃ covered with four different types of zeolite (H-forms of zeolite A, ZSM-5, beta polymorph A and zeolite Y) was investigated as a NO₂ sensor.⁶⁰ The best result was obtained with zeolite Y as the covering layer. It exhibited a sensing response to 204 ppb of NO₂ with a response time half that of the bare WO₃ sensor. However, upon exposure to water vapour, the response time increased. More recently, similar results were obtained using ZnO inks as the metal oxide element covered with either zeolite beta polymorph A, zeolite Y or mordenite.⁴⁹ Compared to unmodified ZnO sensors, zeolite modification was found to increase the sensitivity of the sensor down to ppb concentrations of NO₂.

In the examples discussed below, the zeolite layer acts as a catalyst prior to sensing in order to maintain a constant composition of the NOₓ mixture thus enabling measurement of the equilibration between NO and NO₂ at different temperatures by potentiometry between working and reference electrodes. A NOₓ sensor with minimal interferences from CO and O₂ comprised yttria-stabilised zirconia (YSZ) pellets and a Pt-loaded zeolite Y layer.¹⁰⁷ These sensors contain catalytic filters based on YSZ working at temperatures in the range of 300–700 °C. The development of a NO sensor capable of operating in automotive exhaust streams was reported from the same group based on a Pt-yttria-stabilised zirconia (Pt-YSZ) covered by zeolite. The sensors were tested at temperatures between 500 and 700 °C, and the zeolite was permeable to oxygen, thus, was minimising the interference from O₂. The sensor shows interference from CO and NO₂, however these tests indicate that the device is able to detect NOₓ in engine exhausts.¹⁰⁹ Using Pt-doped zeolite Y on a yttria-stabilised zirconia electrolyte, the NOₓ concentration could be measured using a sensing electrode.¹⁰⁸ This sensor device was then optimised to reduce interference from other gases. The addition of a WO₃ electrode provided the highest reactivity difference with the Pt-doped zeolite Y and interference from 2000 ppm CO, 800 ppm propane, 10 ppm NH₃, as well as the effects of 1–13% O₂, CO₂, and H₂O were minimised.¹¹⁰,¹¹¹ A human breath analysis was performed with 20 of those sensors connected in series for the diagnosis of asthma since NO in breath increases from 5–10 ppb (normal state) to 100 ppb in an asthmatic state.¹¹²

The imbalance in equilibration between NO₂ and NO could also be managed by a voltage change, instead of temperature, to build an amperometric sensor. An example has been reported using the same material, a Pt-doped zeolite Y on a yttria-stabilised zirconia, connected to a potentiostat.¹¹³ It demonstrated a ppm detection of NOₓ with no interference from CO and hydrocarbons. Both amperometric and potentiometric monitoring were combined to build a bifunctional sensor which could measure NO₂ and O₂ simultaneously in ppm concentration with almost no interference between each other.¹⁰⁶

A QCM oscillator-based sensor was also developed for NO sensing that took advantage of the adsorption of NO on zeolites. Zeolite A, silicalite-1 and sodalite were deposited on a QCM and NO adsorption was followed. The zeolite-A doped QCM was