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

HAL
open science

# Recent progress on MOF-based optical sensors for VOC sensing

Yuwei Shen, Antoine Tissot, Christian Serre

► To cite this version:

Yuwei Shen, Antoine Tissot, Christian Serre. Recent progress on MOF-based optical sensors for VOC sensing. Chemical Science, 2022, 13 (47), pp.13978-14007. 10.1039/d2sc04314a. hal-04057459

HAL Id: hal-04057459

https://hal.science/hal-04057459

Submitted on 4 Apr 2023

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
Recent progresses on MOF-based optical sensors for VOCs sensing

Yuwei Shen,ª Antoine Tissot ª and Christian Serre ª

ªInstitut des Matériaux Poreux de Paris, Ecole Normale Supérieure, ESPCI Paris, CNRS, PSL University, 75005 Paris, France

**Abstract**

The raising apprehension of volatile organic compounds (VOCs) exposures urges the exploration of advanced monitoring platforms. Metal-organic frameworks (MOFs) provide many attractive features including tailorable porosity, high surface areas, good chemical/thermal stability, and various host-guest interactions, making them appealing candidates for VOCs capture and sensing. To comprehensively exploit the potential of MOFs as sensing material, great efforts have been dedicated to the shaping and patterning of MOFs for next-level device integration. Among different types of sensors (chemiresistive sensors, gravimetric sensors, optical sensors, etc) MOFs coped with optical sensors feature distinctive strength. This review summarized the latest advancements of MOF-based optical sensors with a particular focus on VOCs sensing. The subject is discussed by different mechanisms: colorimetry, luminescence, and sensors based on optical index modulations. Critical analysis for each system highlighting practical aspects is also deliberated.

**1. Introduction**

Air pollution becomes a growing concern worldwide. Especially owing to the recent pandemic, the time people spend indoors escalates, which comes with a higher risk of indoor pollutant exposure. Volatile organic compounds (VOCs) are an entire set of vapour-phase atmospheric organics (excluding CO and CO2) associated with relatively high volatility, according to their high vapour pressures, and low boiling points at ambient temperature and pressure. Common species of VOCs include (apolar) alkanes, alkenes, aromatic hydrocarbons, and heteroatom-containing (polar) organic compounds. VOCs exposure can cause acute and long-term health issues depending on exposure durations and doses. A direct example is the sick building syndromes, where people in a building experience acute symptoms such as eye, nose, or throat irritations, headaches, fatigue, etc. that disappear after exiting the building. Besides, odours generated by VOCs are possible to induce psychological negative effects. Long-term chronic exposure to certain VOCs can also cause adverse health impacts including cancers and liver, kidney or central nervous system damages.¹ Compounds of major concern include benzene, toluene, xylenes, styrene, acetaldehyde, formaldehyde and naphthalene, which are classified by the European Commission as the priority pollutants to be regulated. Specifically, benzene, formaldehyde, acetaldehyde, and naphthalene are considered carcinogenic.² Given the possible health risk caused by VOCs exposures, it is imperative to monitor the ambient concentrations of VOCs.

Identification of individual components of VOCs in air samples can be achieved in laboratories via analytical methods using gas chromatography (GC) coupled with flame ionization detection (FID), electron capture detection or mass spectrometry (MS).³ However, these laboratory-based instruments are typically bulky in size, time and power-consuming, complex to operate and expensive. Many other types of sensors have therefore become commercially available or established in research for VOC detection to address the aforementioned limitations. Specifically, commercially available techniques include photo-ionization detectors (PID), electrochemical cells, metal-oxides-based chemiresistive sensors (MOs), etc. These sensors are generally with miniature size, sensitive to a range of VOCs and have short response time. However, the main concern for these sensors is cross-sensitivity for multiple gases/vapours simultaneously leading to a lack of selectivity and the fact that their response can be largely perturbed by the interfering species. More advanced sensing platforms are therefore emerging. One solution for circumventing lack of sensitivity is to adopt multiplexing approaches, for example, sensor arrays, also known as e-nose. They combine multiple sensors with smart pattern recognition software for improved sensitivity and selectivity. In some cases, employing sensor arrays can realize individual discrimination of each component in vapour mixtures.⁴ Coupling MOs sensor with porous solids (e.g. zeolites) can also enhance the sensing performance by employing the intrinsic chemical affinity from the porous solids for better selectivity and/or pre-concentrating the selected VOCs for improved sensitivity.⁵

Metal-Organic Frameworks (MOFs) are an extensive class of crystalline micro- or meso-porous hybrid materials that has been widely developed and investigated over the past 20 years.⁶ MOFs are built with metal cations based nodes
and organic linkers linked via coordination bonds. Great variety of chemical combinations between metal secondary
building units (SBU) and organic linkers gives rise to diverse topologies and extended physical and chemical properties.
These remarkable properties often include tailorable porosity, high surface area, chemical/thermal stability, etc.<sup>7,8</sup>
MOFs are therefore of great interest for multiple applications, such as gas separation<sup>9–12</sup>, gas storage<sup>13,14</sup>, catalysis<sup>15–17</sup>
and bio-applications<sup>18–20</sup>, among others.

MOFs are also appealing candidates for sensing applications owing to the following reasons: 1) their intrinsic porosity can favour the diffusion, absorption, and/or interactions with analyte molecules; 2) they can provide various active sites, for instance open metal coordination sites as well as possible intermolecular coupling between the analytes and the ligands (hydrogen bonds, π-π interaction, etc.). These sites can consequently favour sensitive and selective binding with the target gas molecules; 3) their tunable pore size can enable molecular sieving phenomenon, which can also improve the selectivity; 4) the host-guest interactions can lead to detectable changes in chemo-physical properties (resistance, mass, magnetism, colour, etc.), in favour of further signal transduction and quantification; 5) the reversible physisorption process assures the reusability of MOFs; in the case of chemisorption, irreversible responses may appear yet induce higher sensitivity and selectivity.<sup>21–25</sup> Abundant research have been conducted applying MOFs for the design of various types of gas/vapour sensors, including chemiresistive sensors, capacitive sensors, gravimetric sensors, and optical sensors.<sup>23,26–33</sup> Among these categories, colorimetric and optical sensors are greatly attractive due to their apparent advantages such as immunity to electromagnetic interferences, capabilities of remote sensing, facile operations, and direct naked-eye detection.

The sensing process generally involves selective adsorption of targeted molecules. MOFs feature attractive aspects for gas adsorption and capture, especially for VOCs. Functionalization of organic ligands is a powerful approach to not only systematically tailor the pore size and aperture of the MOFs structure³⁴, but also to modify the chemical properties of MOFs including hydrophilicity and hydrophobicity, leading to desired sorption properties. It is also possible to combine multiple species of organic linkers in one framework, namely multivariate (MTV)-MOFs. Such structure is proved to provide a more complex pore environment and exhibit better sorption and separation performance.³⁵ Besides, defects in MOFs are a unique element to tune the adsorption properties. Missing nodes and missing linkers are the two defects that are mainly observed. The formation of defects in MOFs can change the pore size, rigidity, flexibility and density of the framework. Especially, missing linker defects leads to more open metal sites, which can favour sorption processes.³⁶ However, increasing the amount of defects in MOF crystals can possibly comprise their chemical stability.

This review summarizes the recent progress of MOF-based optical sensors, with specific highlights for VOCs sensing. We will first outline the state of the art in the patterning techniques of MOFs that facilitate the integration of MOFs into different platforms. The next parts focus on the latest advancements of MOF-based optical sensors, which are categorized by sensing mechanisms: colorimetric sensing, luminescent sensing and sensing based on optical index modulation. In particular, the section on optical-index-based sensors describes the development of MOFs incorporated with Fabry-Perot interferometers, Bragg stacks, diffraction grafting and colloidal film engineering, optical fibres, micro-ring resonators, surface resonance and surface-enhanced Raman scattering. In the last, we conclude by highlighting the opportunities and challenges of MOF-based optical sensors with critical insight for their further practical applications.

## 2. Patterning techniques of MOF thin films

In order to exploit the great potential of MOFs for sensing applications, it is vital to integrate MOFs onto substrates for further real device construction. To date, diverse techniques have been addressed to fabricate MOFs films with various substrates, including in-situ direct growth from hydro/solvothermal synthesis³⁷,³⁸, electrochemical deposition³⁹, colloidal deposition via spin/dip coating⁴⁰,⁴¹, atomic layer deposition⁴²⁻⁴⁴ and layer-by-layer epitaxy⁴⁵⁻⁴⁷. Each method has its attribute for producing films with different thicknesses, preferred orientation, morphology and mechanical properties. Numerous reviews have been written on this topic, covering various aspects from general fundamentals to advanced practical implementations of MOFs films.⁴⁸⁻⁵⁸ In the following, we describe the most investigated methods for MOFs film preparation.
## 2.1 In-situ growth

In-situ direct growth is one of the most straightforward methods where the substrate is placed in the reacting solution and film growth happens simultaneously during the synthesis. Choice of substrate is critical for this approach despite its ostensible simplicity. Indeed, the surface structure and chemical composition of the substrate influences the nucleation, anchoring and adhesion process of the MOF particles. For example, some metal oxide surfaces with basicity can form strong covalent bonds with the complexing groups from the acidic linkers (carboxylates, phosphonates...), accordingly anchoring the MOF. On the opposite, naturally acidic surfaces like silica ($SiO_2$) do not favour the growth of most carboxylate MOFs.⁵⁹ Metal pieces³⁷,⁶⁰,⁶¹ and polymers⁶² have also been employed as substrates for successful MOFs film installation. The thickness of these films is typically related to the reaction time and fall within the range of a few micrometers.⁴⁸ However, controlling of polymorphism can be complicated and conditions may vary for different MOFs for in-situ grown films.⁶³,⁶⁴

Self-assembled monolayers (SAMs) are active molecules attached to the surface of substrates.⁶⁵ Tail functional groups (e.g. carboxylic acid) on the SAM can be used to coordinate the metal centres or SBUs from the mother solution, therefore promoting the nucleation and growth of MOFs.⁶⁶ Deploying SAMs can also lead to patterning MOF films via micro-patterning techniques⁶⁷, which will be discussed in the next section. Various MOF films have been developed on SAMs functionalized substrates (Au, $SiO_2$, Si, FTO, etc.).³⁸,⁵⁹,⁶⁸⁻⁷¹ Noticeably, exchanging functionality of the SAM layer can consequently in some cases impact the nature of the MOF that is formed⁶¹ and/or tune the orientation of formed crystals.⁵¹,⁵⁹,⁷² One major disadvantage of this approach is the compatibility between the high reaction temperature for hydro/solvothermal MOF synthesis with temperature-sensitive substrates. However, some MOF films can be grown at room temperature under certain conditions.⁷¹⁻⁷³ In addition, the complex preparation process and limited control over morphology and defects of the films can still be problematic.⁴⁸

## 2.2 Liquid phase epitaxy

Unlike one-pot direct growth of MOF films, liquid phase epitaxy (LPE) alternatively grows MOF film layer by layer through immersing substrates into separated solutions of the metal cation precursor and the ligand in a stepwise.⁵⁷ Epitaxially grown MOF films are also defined as surface-mounted MOFs (SURMOFs), which consist of highly-oriented ultrathin MOF multilayers instead of crystalline grains. The SURMOF growth is generally conducted from room temperature to 60°C in ethanol, which are milder conditions than the usual ones used for in-situ growth.⁴⁵ These films tend to be very smooth with a roughness of molecular level. The thickness and crystallite domain size can be precisely controlled considering an ideal layer-by-layer growth mechanism.⁴⁸ Despite unlimited choices of organic linkers for SURMOF construction, the metal nodes are to date mainly limited to divalent metals, as mild synthetic conditions are required for the SURMOFs growth.⁷⁴

SAMs substrates are broadly employed in this approach to initiate the nucleation of highly oriented SURMOFs. Similar to in-situ growth, different functional groups on SAMs also affect the growth orientation of SURMOFs. Likewise, tuning the density of functional groups alters the preferential orientations.⁷⁴ Arranging SAM functional groups with certain patterns prior to SURMOF growth can strategically template well-defined, patterned films. Li et al. reported ZIF-8 films patterning by arrangement of 1-octadecanethiol(ODT)/16-mercaptohexadecanoic acid (MHA) groups with micro-contact printing on gold substrates.⁷⁵ The selective growth of ZIF-8 on ODT SAM surface leads to a dot patterning of the film. By switching the solvent from water to methanol, ZIF-8 only nucleated on MHA surface and produced a negative pattern. Several research groups have also demonstrated combining LPE methods with microprint contact technique⁷⁵, lithography⁷⁶ and spray coating⁷⁷ to pattern the films.⁷⁴

It is also possible to couple Langmuir-Blodgett (LB) technique with layer-by-layer deposition for fabricating MOFs with a paddle-wheel unit, where 2D nanosheets are firstly synthesized via air-liquid interfacial synthesis, followed by introducing the pillar linkers for interlayer connection (Figure 1a).⁷⁸ Extensions of this layer-by-layer strategy also include forming films of multivariate MOFs (MTV-MOFs) and MOF on MOF structures.⁷⁹ Another similar approach named substrate-seeded heteroepitaxy uses metal substrates as direct metal sources matching unit-cells for a perfect growth. Falcaro et al. fabricated an oriented polycrystalline $Cu_2(BDC)_2$ thin film on $Cu(OH)_2$ nanotubes using this strategy.⁸⁰ However, it should be noted that only limited MOFs with certain topology (e.g. paddle-wheel structure) can be constructed by the aforementioned approach. The lengthy synthesis and time-consuming step-by-step process
should also be considered. In addition, some MOFs synthesis requires harsh conditions including acids, heat or specific
solvents, which cannot be used for standard LPE or LB procedures.<sup>48,50,74</sup> A simpler LB route starting from pre-
synthesized MOF nanoparticles for film deposition will be discussed in the following section dealing with colloidal
deposition.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Schematic illustration of the fabrication of NAFS-1 with bottom-up modular assembly by combining a layer-by-layer growth technique with the Langmuir–Blodgett method. Reproduced from ref. 78. (b) Vapour-phase deposition of ZIF-8 thin films via ALD ZnO film. The procedure consists of a metal oxide vapour deposition (Step 1) and a consecutive vapour-solid reaction (Step 2). Metal, oxygen and ligand sources are labelled as M, O and L, respectively. Metal oxide deposition can be achieved by atomic layer deposition (M, diethylzinc; O, oxygen/water) or by reactive sputtering (M, zinc; O, oxygen plasma). Atom colours: zinc (grey) oxygen (red), nitrogen (light blue) and carbon (dark blue); hydrogen atoms are omitted for clarity. Illustration reproduced from Ref 78.</figcaption></figure>

**2.3 Atomic/Molecular layer deposition**

Vapour-based processes avoid the use of solvents, which can intrinsically overcome the limitations of liquid-based
strategies such as toxicity, difficulty in scale-up, surface-tension involved issues, etc.⁸² One of the well-established
techniques is atomic layer deposition (ALD). It has been widely investigated and applied in producing inorganic
nanomembranes (e.g. oxides, nitrides, elementary metals).⁸³ Combining ALD with molecular layer deposition (MLD),
which is often used with organic molecules can lead to the fabrication of crystalline MOF thin films.⁴²,⁴³,⁸⁴ Typical
ALD/MLD process consists of layer-by-layer deposition of a metal precursor and an organic linker vapour, with purges
of inert gases in between depositing the upcoming layer. The construction of the MOF films occurs through vapour-
solid reaction, where the choice of metal precursor, the linker exposure time, temperature and humidity require
precise control.⁸²,⁸⁴ Film thickness can be carefully controlled down to nanoscale. However, the directly deposited film
is amorphous in most cases, but it can be crystallized by post-treatments, for instance, annealing under humid
condition⁴³, or subsequent exposure to acid.⁴⁴,⁸⁵ This post-crystallization process consequently changes the
morphology and roughness of the film, which counteracts the targeted precise thickness control.

To avoid post-deposition treatments, Ameloot et al. fabricated a uniform and compact ZIF-8 film through a vapour-phase transformation from ALD ZnO films to ZIF-8, as shown in Figure 1b.⁸¹ A recent review by Su et al.⁸² is suggested for further insight regarding this strategy. The solvent-free strategy makes ALD/MLD appealing for the microelectronics industry,⁸⁶ as it can lead to a meticulous tuning of thickness and possess a good potential for scale-up. However, this approach is employable almost only for ZIF materials that are easy to produce but possess limited chemical stability. In addition, the large differences in the unit cell parameters between the initial oxide substrates and the MOF might
induce the formation of polymorphs and affects the quality of the films. Besides, the necessity of post-synthetic treatments, requirement of high processing temperature and high cost may impede the practical implementation of this method.⁸⁷

## 2.4 Colloidal deposition

Another easier option for MOF film fabrication is to use colloidal deposition of pre-formed MOF nanoparticles on a substrate. This strategy requires colloidal-stable suspensions of MOFs nanocrystals prior to the deposition, where MOFs are dispersed in volatile dispersants. Commonly employed techniques include spin coating, dip coating, spray coating and Langmuir Blodgett deposition.⁸⁶ The choice of MOF candidates that can be adopted via this approach are with less limitations, even including high-valence metal based MOFs such as MIL-101(Cr), MIL-96(Al) and UiO-66.⁸⁸⁻⁹² The quality of the resulting films highly depends on the homogeneity of the MOF suspension, where the choice of dispersant and surfactant can be crucial.⁸⁹,⁹⁰ In this strategy, the MOF suspensions are deposited onto flat substrates, after which solvent evaporates leading to uniform and polycrystalline films. In a recent study, Rauf et al. reported a homogenous coating of MIL-96(Al) monolayer thin film onto conductive and insulating polymer threads, evidencing the applicability of this approach to a wide choice of substrates.⁹¹ The deposited films possess the hierarchical microporosity from the MOF and additional meso- to macro-porosity from assembled MOF particles.⁴⁰ Moreover, the film thickness is often controllable by tuning the particle size of deployed MOFs, the concentration and viscosity of MOF suspension, as well as deposition parameters (e.g. withdraw speed in case of dip coating).²¹

This approach is easy to operate and cost-efficient. It can be adapted to a broad range of MOFs, as well as combined with other film fabricating processes (e.g. CVD)⁹². The roughness and orientation of MOF crystals yet remain uncontrolled and depends on the polydispersity and size of the starting nanoparticles. The density of the film also depends on the kinetics of deposition that controls the packing density of the nanoparticles on the substrate. In addition, the adhesion strength between substrates and MOF colloidal films can be weak, leading to insubstantial mechanical strength.⁴⁰ A feasible strategy to reinforce the mechanical strength of MOF colloidal films is incorporating additives, such as inorganic binders or polymers⁹³,⁹⁴, or via functionalizing the substrates⁸⁷.

# 3. MOFs-based optical sensors for VOCs sensing

Optical sensors are driven by the light-matter interaction in the forms of light absorption, emission, reflection, refraction, scattering or transmission. They are usually composed of a light source, a sensing material that can interact with the analytes and a light detector. The spectrum of commonly employed light sources covers from UV to visible region. Depending on specific techniques, detectable parameters include refractive indexes, scattering, absorbance and luminescence, which are typically guest molecule dependent.⁹⁵ The sensitivity and limit of detection (LoD), therefore, can be interpreted and quantified by the change in the detected parameters induced by analyte exposure. To be specific, the response of colorimetric sensors is defined as changes in color, while luminescent sensing materials are evaluated according to their luminescence intensity or position change upon stimulation. For optical-index-dependent sensors, their sensitivity is defined by the changes in the monitored indices, e.g. peak shift in reflectance/transmission spectra. This section will focus on the different platforms that enable MOF-based optical sensing, especially highlighting their application for VOCs detection. Table 1 summarizes the recent MOF-based sensors and their performance based on different sensing mechanisms for VOCs sensing.

## 3.1 MOF-based colorimetric sensing

Colorimetry is one of the oldest analytical techniques for its straightforward visualization of results⁹⁵ that is broadly applied in current life, e.g. pH scales. The phenomenon of chromism originates from interactions between a responsive centre and a triggering analyte. The range involving intermolecular interactions can go from the weakest van der Waals interactions to bond formation and ligand coordination. The main mechanisms of MOF-based chromism sensing include coordination geometry of transition metals triggered colour change, ligand-based charge transfer and functional MOF-composite based sensing.
<table><caption>Table 1. MOF-based sensing for VOCs categorized by different mechanisms</caption><thead><tr><th>Sensing mechanism</th><th>MOF</th><th>Sensing parameters</th><th>VOCs</th><th>Response time</th><th>LOD</th><th>Ref</th></tr></thead><tbody><tr><td rowspan="10">Colorimetric</td><td>Co<sub>3</sub>[Co(CN)<sub>6</sub>]<sub>2</sub></td><td>Co<sup>2+</sup></td><td>Ethanol</td><td>1 min</td><td>50 to 500 ppm</td><td>96</td></tr><tr><td>{[Ni(pzt)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>](H<sub>2</sub>O)(DMF)}<sub>n</sub></td><td>Ni<sup>2+</sup></td><td>Primary amine and methanol</td><td>20 min to 50 min</td><td>-</td><td>97</td></tr><tr><td>[Cu(HL)(DMSO)·(MeOH)]<sub>n</sub></td><td>Cu<sup>2+</sup> and phosphonate group</td><td>DMSO Methanol</td><td>39 h</td><td>-</td><td>98</td></tr><tr><td>Cu<sup>1</sup>-1-benzimidazolyl-3,5-bis(4-pyridyl)benzene</td><td>Ligand-involved charge transfer process</td><td>Aliphatic VOCs</td><td>20 min to 2.5 h</td><td>1 ppm (alcohols) 10 ppm (CH<sub>2</sub>Cl<sub>2</sub>, CHCl<sub>3</sub>)</td><td>99</td></tr><tr><td>UiO-67-dmbpy</td><td>Ligand-involved charge transfer process</td><td>Alkylamines</td><td>Immediate response</td><td>-</td><td>100</td></tr><tr><td>UiO-67-bpy-A</td><td>Covalent condensation with amino groups</td><td>Formaldehyde</td><td>3h</td><td>3 ppm</td><td>101</td></tr><tr><td>[Zn<sub>7</sub>(bpybc)<sub>3</sub>(o-BDC)<sub>6</sub>]·2NO<sub>3</sub>·6H<sub>2</sub>O</td><td>Radical generation of the viologen ligand</td><td>Dimethyamine (DEA)<br>Ethylamine (EA); Propylamine (PA)<br>Butylamine; Benzylamine</td><td>Immediate response<br>Immediate response<br>~10 s</td><td>30% DEA aqueous solution<br>-<br>-</td><td>102</td></tr><tr><td>[Cd<sub>2</sub>(ipbp)<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>]·2DMF</td><td>Synergistic effects of host-guest interactions</td><td>Dimethyamine (DEA)</td><td>2 min</td><td>400 ppm</td><td>103</td></tr><tr><td>TMU-34</td><td>Ligand oxidation</td><td>Chloroform</td><td>10 s</td><td>3 ppm</td><td>104</td></tr><tr><td>{[Zn(3-DPMNI)<sub>0.5</sub>(NDC)]·3DMF}</td><td>Host-guest electron transfer</td><td>n-butylamine</td><td>2 s</td><td>300 ppm</td><td>105</td></tr><tr><td>[Fe<sup>II</sup>(BPI)(HBPI)(ClO<sub>4</sub>)<sub>2</sub>] ⊆ MFU-4/(Zn)</td><td>Spin-crossover complex</td><td>alcohols</td><td>-</td><td>-</td><td>106</td></tr><tr><td rowspan="3">Luminescence</td><td>NKU-128</td><td>Chromophore grafting</td><td>Toluene<br>THF<br>DMF<br>DMSO</td><td>24h</td><td>-</td><td>107</td></tr><tr><td rowspan="2">[Cu<sub>4</sub>I<sub>4</sub>(Py<sub>3</sub>P)<sub>2</sub>]<sub>n</sub><br>(Py<sub>3</sub>P=tris(2-pyridyl)phosphine)</td><td rowspan="2">Guest-lock-induced luminescence</td><td>CH<sub>2</sub>Cl<sub>2</sub></td><td>&lt;1 s</td><td rowspan="2">Saturated vapours</td><td rowspan="2">108</td></tr><tr><td>Chlorobenzene</td><td>455 s</td></tr><tr><td rowspan="2">NR~ccZIF-8 film (NR = Nil red)</td><td rowspan="2">Fluorescent dye</td><td>Acetone</td><td>5 min</td><td>60 ppm</td><td rowspan="2">109</td></tr><tr><td>Toluene</td><td>5 min</td><td>95 ppm</td></tr><tr><td rowspan="2">Fabry-Perot interferometers</td><td>ZIF-8</td><td>Reflectance</td><td>Acetone</td><td>&lt; 2 s</td><td>1000 ppm</td><td>110</td></tr><tr><td>HKUST-1>TiO<sub>2</sub></td><td>Reflectance</td><td>Acetone</td><td>12.2 s</td><td>11.27 ppm</td><td>111</td></tr></tbody></table>
<table><tbody><tr><td></td><td>Polyhedral ZIF-8</td><td>Reflectance</td><td>n-heptane</td><td>~250 s</td><td>500 ppm</td><td>112</td></tr><tr><td rowspan="3">Bragg stacks</td><td>TiO<sub>2</sub>/ZIF-8</td><td rowspan="3">Reflectance</td><td rowspan="3">Ethanol</td><td>4.5 s</td><td rowspan="3">10% dosage in 0.1g/h flow</td><td rowspan="3">113</td></tr><tr><td>TiO<sub>2</sub>/HKUST-1</td><td>7.7 s</td></tr><tr><td>TiO<sub>2</sub>/CAU-1-NH<sub>2</sub></td><td>10.1 s</td></tr><tr><td rowspan="2">Diffraction grafting</td><td>TiO<sub>2</sub>/CAU-1</td><td>Reflectance</td><td>Methanol</td><td>18 s</td><td>-</td><td>114</td></tr><tr><td>colloidal ZIF-8</td><td>Refractive index</td><td>Heptane</td><td>~5 s</td><td>-</td><td>115</td></tr><tr><td></td><td>ZIF-8/TiO<sub>2</sub></td><td>Colour luminance</td><td>Styrene</td><td>-</td><td>57 ppm</td><td>116</td></tr><tr><td></td><td>MIL-100(Cr)</td><td>First order diffraction intensity</td><td>Toluene</td><td>-</td><td>200 ppm</td><td>117</td></tr><tr><td rowspan="2">Optical fibres</td><td>ZIF-8/SiO<sub>2</sub> waveguide</td><td>Transmission spectra</td><td>Ethanol</td><td>-</td><td>1.6 ppm</td><td>118</td></tr><tr><td>ZIF-8/graphene oxide</td><td>Transmission spectra</td><td>Ethanol</td><td>118 ms</td><td>5.26 ppm</td><td>119</td></tr><tr><td>Long-period grafted (LPG) optical fibre</td><td>ZIF-8</td><td>Transmission spectra</td><td>Ethanol</td><td>-</td><td>9.8 ppm</td><td>119</td></tr><tr><td rowspan="5">Micro-ring resonators</td><td rowspan="5">ZIF-8</td><td rowspan="5">Transmission spectra</td><td>Methanol</td><td rowspan="5">~30 min</td><td>58 ppb</td><td rowspan="5">29</td></tr><tr><td>Propylene</td><td>29 ppb</td></tr><tr><td>Benzene</td><td>35 ppb</td></tr><tr><td>Toluene</td><td>76 ppb</td></tr><tr><td>Styrene</td><td>99 ppb</td></tr><tr><td rowspan="2">Surface plasmon resonance</td><td>ZIF-8</td><td rowspan="2">Refractive index</td><td>Methanol</td><td>30 s</td><td>2.5 ppm</td><td rowspan="2">120</td></tr><tr><td>ZIF-93</td><td>n-BuOH</td><td></td><td>73 ppm</td></tr><tr><td rowspan="2">Surface plasmon (SPP) resonance</td><td>ZIF-8</td><td rowspan="2">Surface plasmon polariton shift</td><td>Methanol</td><td>-</td><td>0.5 ppm in dry N<sub>2</sub> 8 ppm in 40% humidity</td><td rowspan="2">121</td></tr><tr><td></td><td>Toluene</td><td></td><td>90-100 ppb</td></tr><tr><td rowspan="4">Surface-enhanced Raman scattering</td><td>Ag nanotubes@ZIF-8</td><td>SERS spectra</td><td>Toluene</td><td>80 s</td><td>200 ppm</td><td>122</td></tr><tr><td>Au@Ag@ZIF-8</td><td>SERS spectra</td><td>Dimethyl methylphosphonate (DMMP)</td><td>30 s</td><td>0.2 ppbV</td><td>123</td></tr><tr><td>Ag@ZIF-8</td><td>SERS spectra</td><td>4-methylbenzenethiol (4-MBT)</td><td>10 s</td><td>50 ppb</td><td>124</td></tr><tr><td>MIL-100(Fe)</td><td>SERS spectra</td><td>Toluene</td><td>-</td><td>0.48 ppb</td><td>27</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Figure 2. Illustration of chromism-based sensing mechanisms of MOFs, adopted from ref 24.</figcaption></figure>

### 3.1.1 Transition metals

MOFs composed of transition metal ions (Fe, Co, Cu, etc.) can change colour upon the sorption of guest molecules. Particularly, guest molecules interacting with unsaturated metal sites modify the coordination geometry of the metal ions, thus changing its electronic properties, which eventually afford a detectable change in the optical absorption spectrum of the MOF.<sup>24,125</sup> For example, Chen et al. reported a reversible structure transformation of a cobalt-based MOF accompanied by colour change upon H<sub>2</sub>O or NH<sub>3</sub> sorption in liquid phase. The presence of H<sub>2</sub>O/NH<sub>3</sub> molecules changes the coordination number of Co<sup>2+</sup> ions from 4 to 6 and consequently the geometry from tetrahedral to octahedral, leading to a colour change from blue to red.<sup>125</sup>

<figure><img src="image_5.png" /><figcaption>Figure 3. (a) Coordination environment of Co(II), packing of {[Co(3,4pba)<sub>2</sub>-(H<sub>2</sub>O)]·1/2DMF·H<sub>2</sub>O}<sub>n</sub>; and pictures of powered samples after exposure to different solvents. Figures adapted from ref 126. (b) Structure of Co<sub>3</sub>[Co(CN)<sub>6</sub>]<sub>2</sub>, and TEM of the nanoparticles; The colour of Co<sub>3</sub>[Co(CN)<sub>6</sub>]<sub>2</sub> films changes from pink to purple gradually with an increasing ethanol concentrations gradient. Adapted with permission from ref 96.</figcaption></figure>

This principle can also be applied for VOCs sensing. Dzesse et. al have reported preliminary results of solvatochromic behaviour with a transition metal-based MOF by soaking it in different solvents. The MOF, built with Co<sup>2+</sup> and 3-(4-pyridyl)benzoate, presents solvatochromism upon sorption of various VOCs including polar protic ethanol and methanol, polar aprotic acetone, DMF, DMA, DMSO and nonpolar aprotic dioxane (colours are shown in the Figure 3a).<sup>126</sup> In this case, the solvatochromic
response is accompanied by a phase transition. In addition, a Mn-MOF showing solvatochromic behaviour toward ketone molecules was investigated by Li et.al, where the introduced ketone molecules distorts the parent framework and brought changes in d-d transition into the visible region.<sup>127</sup>

Vapour-phase VOCs induced changes in the metal centres have also been achieved. Wang and Chen fabricated a film of Co₃[Co(CN)₆]₂ nanoparticles exhibiting a colorimetric and selective detection of ethanol vapour.⁹⁶ The prepared Co₃[Co(CN)₆]₂ film undergoes a gradual colour transition from pink to purple when subjected to different concentrations of ethanol vapour (Figure 3b). Such colour change was not observed with other solvents (H₂O, methanol, formaldehyde, etc.), evidencing the selectivity of the sensor. The detection range of the sensor is from 0.05 mg/mL to 0.5 mg/mL of ethanol, with a response time around 1 min and recovery within a few seconds. The vapochromic behaviour can be explained by the conversion of Co²⁺ ion coordination geometry from octahedral to tetrahedral after ethanol molecules entered the pores and coordinated to Co²⁺ ions. Consequently, the resulting tetrahedral geometry induces a reduction in the d-orbital splitting energy resulting in a redshift in the adsorption spectrum and the colour change of Co₃[Co(CN)₆]₂. In another example, Tunsrichon and co-workers introduced a Ni-viologen based 2D MOF {[Ni(pzt)₂(H₂O)₂](H₂O)(DMF)}n (Hpzt = 5-(3-pyridyl)-1,3,4-oxadiazole-2-thiol) with a naked-eye chromism in presence of primary amines and methanol vapours.⁹⁷ Upon exposure of NH₃, methylamine, ethylamine and ethylenediamine, the intrinsic blue colour changed by replacing coordinated H₂O molecules with a strong-field amine through forming Ni-N bonds. Bulkier amine vapours did not induce a chromic response due to higher steric hindrance. However, the vapochromism towards amines is irreversible due to the formation of a more stable coordination compound and to the poor chemical stability of this low valence based MOFs. This material also exhibited a selective detection of methanol vapour over other alcohols with a reversible colour change from blue to pale green. The incorporation of MeOH molecules into the MOF distorts the octahedral geometry of Ni¹¹ and consequently induces chromism. Vapochromism was also revealed in a single-crystal-to-single-crystal (SC-SC) transformation of a Cu-based MOF.⁹⁸ For example, the 3D-MOF [Cu(HL)(DMSO)·(MeOH)]n (L= triphosphaazatriangulene) exhibits a structural transformation into a 1D-columnar assembly H₃L·0.5[Cu₂(OH)₄·6H₂O]·4H₂O under humid conditions accompanied by a yellow to blue-green colour change of the crystals. The reverse 1D-to-3D transformation structure also occurred upon the sorption of DMSO/MeOH vapours.

Crystals containing Pt(II) complexes can also be coloured due to metal-metal-to-ligand charge-transfer (MMLCT) from the Pt-Pt orbital to the π* orbital of the organic ligand. Slight modifications of the complex structure upon external stimuli can induce a significant change in the transition energy of MMLCT, resulting in a colour change. For example, Kato *et al.* demonstrated novel Pt(II) diamine based crystals with vapochromic behaviour towards H₂O and alcohol vapours by switching the structures between an amorphous compound and a porous crystalline network.<sup>128,129</sup>

### 3.1.2 Redox-active ligands

As in the aforementioned (expensive) Pt-based complexes, MOFs linkers can also participate as responsive centres towards targeted analytes. Here, the interaction between the guest molecules and the ligands triggers charge transfer processes, accordingly changing the perceived colour of MOFs. For example, a porous Cu¹⁻-MOF reported by Yu et al. shows naked-eye chromic detection of polar aliphatic VOCs.⁹⁹ The response time of the MOF upon saturated VOCs vapours at ambient temperature ranges from 20 min to 2.5 h. The colorimetric detection limit for alcohols and ketones is about 5 ppm. Single crystal X-ray diffraction analysis evidenced that the guest species are located inside the pores in hydrogen-bonding interactions with the ligand instead of coordinated to the Cu ions. The UV-visible diffuse reflectance spectra of VOCs-enclosed MOF displayed a gradually broadened absorption band in the visible region. The colour change can be attributed to an intermolecular electron-transfer transition between the ligand and the encapsulated guests. A follow-up study showed that this Cu¹⁻-MOF can be a highly selective visual sensor for recognizing C₂H₂, which was also associated to an intermolecular electron-transfer between the linker and the encapsulated guests.¹³⁰
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 4. (a) Color change and response time of [Zn(3-DPMNI)₀.₅(NDC)] (denoted as 1 in the figure and following text), (H₂NDC = 2,6-naphthalenedicarboxylic acid, 3-DPMNI = N,N'-bis(3-pyridylmethyl)-1,4,5,8-naphthalene diimide) in the presence of different amine vapors. (b) UV-vis absorption spectra of 1 in the presence of different organic amines. Figures adapted from ref 105. (c) Structure of N-methylated UiO-67-bpy and its reversible colour change when deposited UiO-67-dmbpy was exposed to amine vapours. n-BUA(in a)/BA (in c) = butylamine, DEA = diethylamine, AH = hydrazine, DIP(in a)/DiPA (in c) = diisopropylamine, TEA = triethylamine, TEOA = triethanolamine, AN = aniline, EA = ethylamine. Figures adapted from ref 100.</figcaption></figure>

MOFs composed of redox-active ligands such as viologens¹⁰⁰⁻¹⁰³,¹³¹⁻¹³³, tetrazine derivatives¹⁰⁴ and naphthalene diimide¹⁰⁵ have shown great potential in colorimetric sensing through charge transfer processes. Viologens are 4,4'-bipyridinium derivatives with particularly electron-deficient nature. Upon interaction with electron-rich molecules, e.g. NH₃ and amines, viologen ligands received electrons from the donors and produced radicals, leading to colour changes.²⁴,¹³⁴,¹³⁵ For example, Gao et al. synthesized a naphthalene-diimide-based Zn MOF exhibiting photochromic behavior in the presence of various amine vapors (Figure 4a&b).¹⁰⁵ The nature of its photochromism is from photo-induced electron transfer between the electron-rich guest molecules and electron-deficient MOF. It showed varied color and response times towards different amine vapors. Especially for n-butylamine (n-BUA), the MOF changed its color from yellow to red in 2s. The similar color induced by n-BUA and hydrazine (AH) can be further discriminated by their UV-vis absorption spectra. Yang et al. modified the 2,2'-bipyridyl-5,5'-dicarboxylate ligand (bpydc) of the large pores Zr-MOF UiO-67-bpy into N,N'-dimethyl-2,2'-bipyridinium (dmbpy) through post synthetic N-methylation (Figure 4c).¹⁰⁰ Such modification provides charge-transfer interacting sites for selectively capturing and sensing alkylamines. In this example, the CT interaction between the electron-deficient 2,2'-bipyridinium site and the electron-rich analytes is the driving force for selective molecular recognition and adsorption, as it induces a reversible vapochromism and luminescence quenching. A follow-up study on the same Zr-bpy MOF with N-amination modification showed similar chromism behaviour in presence of amines.¹⁰¹ Owing to imine condensation, the N-aminated MOF also has a unique colorimetric response to formaldehyde. Another chromic Zn-MOF constructed from a tris-pyridium ligand was reported by the same group in 2020.¹³¹ The material showed direct colour change towards primary amines (methylamine, ethylamine, etc.) and can also be responsive to methanol and ethanol. The mechanism is proposed to be radical formation through spontaneous/photo-induced guest-to-host electron transfer. In addition, the small and interconnected voids due to the 2-fold interpenetration of the MOF give rise to a size and shape selectivity of the chromic process.

Interpenetrated structures of metal-viologen frameworks can stabilize the reduced radicals viologen ligands and also create multi-bridges for electron transfers, which eventually brings stable and fast responses to multiple stimuli.¹⁰²,¹³⁶ For example, a 7-fold interpenetrated Anderson-like cluster-based Zn(II) MOF [Zn₇(bpybc)₃(o-BDC)₆]·2NO₃·6H₂O, reported by Li et al, exhibited visible response to different types of stimuli including soft X-ray, UV-light, heat, electricity and in presence of some organic amines.¹⁰² When exposed to vapours of dimethylamine (DEA), ethylamine (EA), propylamine (PA), an immediate colour change from the original yellow was observed (pale blue for DEA and purple for EA and PA). Bulkier amine like butylamine and benzylamine had prolonged response time of ca. 10s and in these cases, the colour became brown and green, respectively. Solid-state UV-vis and EPR spectra confirmed the generation of viologen radicals after exposure to amines. Such vapochromism was also observed in MOF-PMMA polymer films and is reversible by introducing HCl vapour on the amine-exposed samples.

Tetrazine derivatives are another type of ligand that can undergo reversible colorimetric responses through redox reactions. Razavi and co-workers developed a dihydrotetrazine-functionalized pillared MOF [Zn(OBA)(H₂DPT)₀.₅]·DMF (TMU-34) for selective and colorimetric detection of chloroform (CHCl₃), where H₂DPT is 3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine is
the pillar spacer and H₂OBA is 4,4'-oxybis(benzoic acid).¹⁰⁴ In the presence of CHCl₃, a colour change from yellow to pink was observed by naked eye within 10s, at the lowest concentration 2.5 × 10⁻⁵ M in both liquid and vapour phases and was regenerated by washing the compound in DMF. In the compound, CHCl₃ oxidizes the dihyrotetrazine (H₂DPT) into tetrazine (DPT), therefore inducing changes in the geometry and functionality of the pillar spacer, leading to a colour change.

## MOF composites

Given the fact that most MOFs are colourless or do not change significantly of colour upon sorption of guests, a rational strategy is to load functional (optically active) guests within the host MOF to achieve visual detection of analytes. Common functional guests include organic dyes (rhodamine, spiropyrans, etc.)¹³⁷⁻¹³⁹ and metal nanoclusters¹⁴⁰⁻¹⁴². These compounds are either grafted on the metal oxoclusters of the MOFs, or loaded in their pores through direct adsorption or covalent grafting.¹³⁴,¹⁴³ For example, Koning and co-workers anchored two carboxylates group of DTNB (5,5'-di-thio-bis-2-nitrobenzoic acid), a bidentate reactive dye, on the unsaturated inorganic node of the mesoporous Zr tetracarboxylate NU-1000 for degradation and visually detection of the nerve agent VX.¹⁴³ The octahedral Zr₆ oxoclusters catalyze the hydrolysis of VX, and the DTNB reacts with the thiol group from degraded products providing a bright yellow colour (Figure 5a). This dual-functional composite also brings higher detection efficiency compared to the separated DTNB/NU-1000 system. A follow-up study reported the grafting of DTNB onto the large pores Zr trimesate MOF-808 immobilized on cotton forming a MOF/fabric composite enabling practical application of VX degradation and detection.¹⁴⁴ The cloth is capable of visualizing amounts of VX down to 0.16 µg/cm², which was more sensitive compared to DTNB@NU-1000 (0.8 µg/cm²). It also displayed an improvement in catalytic performance with rapid conversion within 5 min (Figure 5a-b). However, most functional loaded compounds are photochromic or luminescent, which requires extra light source to be applied in naked-eye detections. Limited research reported guest chromophores induced colorimetric sensing, and the majority focused on liquid phase detection of heavy metal ions and organic pollutants.¹⁴¹,¹⁴⁵,¹⁴⁶

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

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Figure 5. (a) Illustration of DTNB@NU-1000 for degradation and visual detection of the nerve agent VX.¹⁴³ (b) Two anchoring modes of DTNB in MOF-808 and the proposed mechanism of the colour change of DTNB@MOF-808 upon VX. DTNB@MOF-808 degradation kinetics of VX compared to DTNB@NU-1000. Photographs of the MOF-textile composite upon contact with VX.¹⁴⁴ (c) Sequential installation strategy for anchoring a SCO complex in a MOF and UV-Vis spectra at room temperature of the MOF-SCO composites dispersed in acetone (orange) and EtOH (red) for MIL-100, and dispersed in acetone (orange) and MeOH (red) for MFU-4I.¹⁰⁶</figcaption></figure>

Another strategy to achieve colorimetric sensing towards VOCs is through encapsulation of spin-crossover (SCO) complexes into colourless large pores MOFs. As mentioned previously in **section 2**, spin-crossover complexes are responsive to external stimuli
that are accompanied by a colour change. For example, an Fe(II) complex, $[Fe(HB(pz)_3)_2]$ was confined in a diamagnetic and optically-transparent $NH_2$-MIL-100(Al) and showed a colour change induced by solvent adsorption.<sup>147</sup> Despite that a range of polar to non-polar solvents can induce spin switching accompanied by off-white to red colour change, it is a proof-of-concept that SCO-MOF composites have a great potential for sensing applications. Some of us have successfully introduced an Fe(III) SCO compound, $Fe(sal_2trien)NO_3$, into MIL-100(Al) through an *in situ* synthesis.<sup>148</sup> A spin state switching from the inserted Fe<sup>III</sup>(sal<sub>2</sub>trien) cation was observed upon water sorption. Another SCO-MOF system designed by our group was built with $[Fe<sup>II</sup>(BPI)(HBPI)(ClO<sub>4</sub>)<sub>2</sub>]<sup>-</sup>$ anions anchored in MIL-100(Al) and MFU-4I(Zn), which displayed a selective solvatochromism towards alcohols.<sup>106</sup> In this work, the Fe<sup>II</sup> SCO complex was anchored on the SBUs of the MOFs via the sequential installation strategy. The coordination environment surrounding Fe(II) can be modulated upon alcohols sorption, which leads to a reversible colour change from orange to red (Figure 5c). This strategy is currently at the very early stage of research but presents a great potential for VOCs sensing.

## 3.2 MOF-based Luminescent Sensing

Luminescent metal-organic frameworks (LMOFs) have been actively explored and investigated for their potential in biological and chemical sensing applications. We will focus in this part on their application towards VOCs sensing.

### 3.2.1 Origins of luminescence in LMOFs

The luminescence in LMOFs is commonly originated from the following sources: (1) photo-responsive metal SBUs (e.g. lanthanide metal ions, $d^{10}$ transition metal ions), especially trivalent lanthanide ions display sharp emission spectra with wide emission wavelengths from ultraviolet (UV) to near-infrared (NIR) ranges; (2) ligand-centred emission, where the featured ligand typically have extended $\pi$-conjugation. Briefly speaking, upon photoexcitation, fluorescence occurs from the emission of a photon from the lowest singlet state ($S_1$) to the ground state ($S_0$), showing a typical lifetime in the nanosecond range. With a suitable triplet state ($T_1$), an intersystem crossing can take place to populate the $T_1$ state. Subsequently, phosphorescence with a longer lifetime (microsecond to second range) can occur from the photon emission to the ground state (See Figure 6b) (3) Charge-transfer (CT) based luminescence, which corresponds to the transition from the charge-transfer excited state to the ground state, includes metal-to-metal charge transfer (MMCT), ligand-to-ligand charge transfer (LLCT), metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT). (4) Guest-induced emission, by encapsulating luminescent guests such as lanthanide ions, quantum dots, organic dyes, etc. in the pores of MOFs. Several books and reviews have described the origin of luminescence in a detailed aspect, providing a more comprehensive understanding and interpretation of luminescence in MOFs. For example, Cui et al. described thoroughly the design and construction strategies for luminescent MOFs categorized by the three elementary units: SBUs, linkers, and guests.<sup>149</sup> Recently, Wang et al. carefully depicted the energy transfer mechanisms in LMOFs with the main focus on the most recent research development of LMOFs.<sup>150</sup> Gutierrez et al. focused on the confinement of luminescent guests in MOFs, where they gave a comprehensive overview on synthetic methodologies, a systematic discussion of characterization methods, followed by a brief tutorial on the host-guest phenomena and potential applications for such hybrid materials.<sup>141</sup>

<figure><img src="image_14.png" /><figcaption>Figure 6. (a) Schematic diagram of various origins of luminescence in LMOFs. (b) Illustration of photon emission in ligand-centred luminescence. Adopted from ref 151.</figcaption></figure>
### 3.2.2 MOF-based luminescent sensing for VOCs

LMOFs combine the general advantages of MOFs (porosity, tailorability, etc.) with their inherent luminescent properties, making them interesting for sensing applications. The luminescent sensing is essentially based on the host-guest interactions between the analytes molecules and the LMOFs, which gives rise to variations in the luminescent intensity (e.g. turn-on and turn-off effects) or peak positions. Classic sensing mechanisms are photo-induced electron transfer (PET), intermolecular charge transfer (ICT) and Forster energy transfer (FRET).<sup>24,152</sup> Detailed description of every sensing mechanism in LMOFs can be found in the reviews by Yan<sup>153</sup>, while Cui et al. specifically focused on sensing strategy with potential in practical applications, which include the use of functional sites for selective sensing, turn-on luminescent sensing and self-referencing sensing based on dual-emitting MOFs.<sup>149</sup> It is noteworthy that LMOFs also bear limitations such as relatively low sensitivity compared to organic based system when the sensitivity comes from ligand-to-metal-transfer.

LMOFs-based gas/vapour sensing has been extensively investigated and comprehensively reviewed recently.<sup>141,149,151,152,154-159</sup> Similar to chromism-based sensing, finely tuning the physical and chemical properties of LMOFs, such as introducing functional groups, tuning the hydrophobicity/hydrophilicity, the Lewis acidity/basicity, or playing with steric hindrance effect, can improve selectivity.<sup>24,160</sup> Li et al. provided their comprehensive insights on the rational designs of LMOFs as functional chemosensors, along with notable considerations of practical parameters for seeking real-time applicability. They also extensively reviewed the progress of LMOFs as explosive sensors, humidity sensors, gas sensors, ion sensors and VOCs sensors.<sup>151</sup> Another review published later by the same group highlighted the detection of different types of VOCs, chemical warfare agents and hazardous gases.<sup>158</sup> Sensing of environmental pollutants including pesticides, antibiotics, heavy-metal-ions, explosives and VOCs by LMOFs have also been highlighted.<sup>152,159</sup> Reviews from Lin et al.<sup>156</sup> and Khatua et al.<sup>157</sup> also highlighted the advantages of using flexible LMOFs in gas/vapour sensing applications. Recently, Yao et al. grafted a non-planar organic chromophore, 3-TPPA (tri(4-(pyridine-3-yl)phenyl)amine (3-TPPA)), into a 2D flexible MOF, MOF-105, via one-pot synthesis and realized a wide range emission colour tuning<sup>107</sup> (Figure 7a). The bulk crystal of the free-standing 3-TPPA chromophore exhibited only emission intensity changes under different temperatures. However, the resulting hybrid material emission can be modulated by both temperature and pressure stimulus as well as VOCs sorption via positive (solvato-)fluoro-chromism interactions.

With the rapid growth of LMOFs-based sensing, new mechanisms and platforms are also emerging. In 2020, Liu et al. designed and synthesized a rigid Cu(I)-based MOF $[Cu_4I_4(Py_3P)_2]_n$ ($Py_3P$ = tris(2-pyridyl)phosphine) that displayed a guest-induced luminescence enhancement mechanism.<sup>108</sup> Upon exposure of chlorinated VOCs vapours ($CH_2Cl_2$, $CHCl_3$ and $C_2H_4Cl_2$), a turn-on detection mode was observed with an obvious enhancement of luminescence intensity, compared to other VOCs vapour exposure such as benzene, $CH_3OH$, acetone and DMF. The weak host-guest interactions between chloroalkanes molecules and the host framework decrease the molecular vibrations inside the MOF, resulting in luminescence enhancement (Figure 7b). In the same year, the first report on enhanced fluorescence of self-assembled colloidal crystal film was achieved by Olorunyomi et al. They incorporated a fluorescent dye Nil red (NR) into ZIF-8 colloidal crystals, followed by self-assembly of dye-functionalized MOF particles into colloidal crystal films via dip-coating.<sup>109</sup> The periodically ordered assembly of NR~ZIF-8 colloidal crystals (denoted as NR~ccZIF-8 film) enhanced the fluorescence by 186 folds compared to disordered samples. Furthermore, the NR~ccZIF-8 films respond to different organic vapours (acetone, methanol, toluene and xylenes) under various concentrations via fluorescent quenching. The highest response was obtained with toluene vapours, whereas the lowest occurs from xylene isomers exposure. The selectivity is evoked from molecular size and shape exclusion, which indicates the importance of porosity in colloidal crystal assemblies for fluorescent sensing.
<figure><img src="image_15.png" /><figcaption>Figure 7. (a) Illustration of the organic chromophore 3-TPPA grafted in the dynamic coordination space of a flexible MOF. (b) Wide-range emission colour tuning of 3-TPPA grafted MOF-105 material.¹⁰⁷ (c) Illustration of the guest-induced luminescence enhancement mechanism in the presence of VOCs. (d) Emission spectra of [Cu₄I₄(Py₃P)₂]ₘ after incubation for 30 min in various solvent vapours.¹⁰⁸</figcaption></figure>

Despite advancements in LMOFs for sensing applications, further improvements are still required for the practical implementations of LMOFs into sensors: (1) In the aspect of sensing VOCs, the majority of the reported results was examined in the liquid phase, yet the response might differ when the material encounters vapour-phase analytes. (2) The dominant construction strategy of LMOFs relies on either lanthanides metal ions or bulky π-conjugated ligands, which generally are costly and produced on small scales. The typical synthesis of LMOFs highly depends on solvothermal reactions. Thus, scale-up synthesis and stability of LMOFs on a longer time scale still await further examination. (3) The luminescent responses are, mostly, changes in emission intensity and positions, which currently are measurable through laboratory instruments. The signal transduction can therefore not be trivial, and the material needs to be firstly coupled with electronics with proper shaping processes. (4) Shaping of LMOFs is also an important step toward practical applications, calls for further attention, as it is another concern for fabricating a luminescent sensing device. In conclusion, luminescent MOFs for gas/vapour sensing application is still a young and active field with many promising results and several opportunities remain to move LMOFs from lab-scale to real-world applications.

## 3.3 Sensors based on optical index modulation

### 3.3.1 Fabry-Perot Interferometers

Interferometry is a technique based on the interaction between waves.¹⁶¹ Fabry-Perot interferometers, also known as etalons, typically utilize the reflections and transmissions between two closely-seated parallel reflective surfaces. When the incident light travels in the cavity, multiple reflections occur and the path-length difference between subsequent transmissions results in an interference pattern, as shown in Figure 8.¹⁶² The intensity of the output interference pattern is highly dependent on the distance between the mirrors l. When the distance l equals half of the wavelength of the incident light λ or one of its multiple, a standing wave with the longitudinal mode is created and a maximum transmittance is observed at the detector. The following equation can express this correlation:

$$ \lambda = \frac{2nl}{m} $$

where m is the number of half wavelength inside the cavity, and n is the refractive index of the active medium.
<figure><img src="image_16.png" /><figcaption>Figure 8. Illustration of the working principle of a Fabry-Perot interferometer and a typical Fabry-Perot interference pattern.</figcaption></figure>

MOF films can function as the responsive medium of the Fabry-Perot (F-P) cavity. The first proof of concept was described by Lu and Hupp in 2010 for selective sensing of chemical vapours.<sup>163</sup> For example, a transparent thin film of ZIF-8 was deposited on an appropriate substrate (glass or silicon) and films with different thicknesses exhibited various colours, which can be attributed to the thickness-dependent optical interference in the visible region. Given that the cavity of ZIF-8 is evacuated and under vacuum ($n_{vac}=1$), absorption of guest molecules modifies the bulk refractive index of the MOF, and red-shifts the interference peaks accordingly. In 2016, Chen et al. presented a high-performance etalon composed of hollow ZIF-8 nanoshell arrays and a thin layer of Pt coating, named hollow MOF nanoshell-based etalon (HMNSE).<sup>110</sup> The hollow ZIF-8 arrays function as the dielectric medium in between the reflective Si wafer substrate and the metal layer. The fabrication of the device requires several steps including repetitive *in-situ* growth of ZIF-8 on a monolayer of polystyrene (PS) nanospheres, removal of the PS cores to form the hollow structure of the MOF, followed by sputtering of Pt (Figure 9a). The interference peak wavelength and accordingly the colour of HMNSE was tuned by varying the size of ZIF-8 shell. Such construction allows large optical response and distinct colour output for selective detection of alcohols, ketones, nitriles, cyclohexane/benzene and xylene isomers in the vapour phase, as well as good repeatability and recyclability. Figure 9 presents the sensor optical shift upon exposure to saturated acetonitrile vapour (b) and different alcohols (c). This device also exhibits a wide detection range from 1000 to 20000 ppm for acetone vapours with a good correlation between the wavelength shift and the vapour concentration. Zhang et al. have also reported a Fabry-Perot optical sensor based on self-assembled UiO-66 nanocrystals for VOCs vapour sensing. They demonstrated that controlling the size and the missing-linker amount in UiO-66 crystals can tune the optical properties of the film, as well as enhance the sensitivity and recovery performance of the sensor.<sup>164</sup> Recently, Li et al. encapsulated TiO<sub>2</sub> nanoparticles in HKUST-1 thin film in order to improve the sensitivity and response time of the sensor.<sup>111</sup> The HKUST-1>TiO<sub>2</sub> composite film was fabricated through the layer-by-layer deposition of spray-coated MOF precursor solutions and pre-synthesized TiO<sub>2</sub> NPs solution alternatively (Figure 9d). The authors demonstrated that the incorporation of TiO<sub>2</sub> NPs increases the optical quality of the film, with narrower reflection bands and more intense reflectance compared to a bare MOF film of similar thickness. Furthermore, TiO<sub>2</sub> NPs brought a higher affinity to isopropanol, ethanol and methanol to the HKUST-1-based F-P cavity, which was revealed by a larger redshift of the reflection bands upon vapour exposure. A shorter response time (27.6 s for HKUST-1 vs. 12.6 s for HKUST-1>TiO<sub>2</sub> to acetone) was also observed with the HKUST-1>TiO<sub>2</sub> composite due to enhanced H-bonding between TiO<sub>2</sub> NPs and the tested oxygen-containing molecules.
<figure><img src="image_17.png" /><figcaption>Figure 9. (a) Illustration of the experimental procedure used to fabricate HMNSE. (b) HMNSE with 2 nm Pt coating before and after exposure to a saturated acetonitrile vapour at 293 K. Insets are corresponding optical microphotographs. (c) Optical response of HMNSE to different alcohols, including methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, and tert-pentanol.<sup>110</sup> (d) Schematic of the liquid phase epitaxy (LPE) method for sequential TiO<sub>2</sub>-loading in MOF films.<sup>111</sup></figcaption></figure>

Colloidal crystals are typically submicron-sized spheres of silica or polymer in an ordered arrangement. The periodic variation of the refractive index in the crystals enables them to reflect light at a specific wavelength, also known as stopband. Adsorption of guest molecules can shift the stopband due to the changes in their effective refractive indexes and/or lattice spacing.<sup>165</sup> Based on this strategy, Hupp et al. have shown that coating a thin layer of ZIF-8 on colloidal crystals (CCs) can give rise to a device capable of VOCs sensing.<sup>165</sup> Li et al. also managed to deposit an ultrathin layer (~ 80 nm) of ZIF-8 on monolayer colloidal crystals (MCCs) via the LBL method.<sup>166</sup> This sensor can discriminate between different alcohols via the colorimetric response together
with shifts in the reflectance peak. Later on, Chen et al. fabricated another optical sensor based on the self-assembly of polyhedral ZIF-8 into MCCs.<sup>112</sup> Three polyhedral ZIF-8 particles with different exposed facets were assembled, namely truncated cubic (TC) ZIF-8, truncated rhombic dodecahedral (TRD) ZIF-8, and rhombic dodecahedral (RD) ZIF-8 (Figure 10 a-d). A thin layer of Pt was sputtered on the MOF layer to enhance the output signal, leading to narrower full width at maximums (FWHMs), higher reflectance peaks to valley ratios (p-v ratios) and more vivid colour (Figure 10e-f). Compared to polycrystalline ZIF-8, RD ZIF-8 MCCs lead to a more controlled crystal orientation and elimination of grain boundaries, resulting in a refined selectivity towards linear olefins (e.g. 1-hexane) over other BTEX species (benzene, toluene, ethylbenzene and xylene). Furthermore, the authors demonstrated that different exposed facets of ZIF-8 can regulate the sensitivity of the sensor via differentiated diffusion (Figure 10h). Especially, exposing (110) facet of ZIF-8 allows a more completed exposure of the six-membered window and consequently realizes the detection of C1-C8 linear alcohols, as well as the colorimetric discrimination of C5-C8 alkanes (Figure 10g).

<figure><img src="image_18.png" /><figcaption>Figure 10. (a–c) SEM images of ZIF-8 MCCs of different crystal morphologies: TC ZIF-8 (a), TRD ZIF-8 (b) and RD ZIF-8 (c). Insets show schematic representations of corresponding crystal morphologies. (d) PXRD patterns of ZIF-8 MCCs of different crystal morphologies showing preferential orientations. Inset show schematic representations of corresponding arrangement of ZIF-8 nanoparticles. From top to bottom: TC ZIF-8, TRD ZIF-8 and RD ZIF-8. (e and f) Reflectance spectra of TC ZIF-8 MCCs (e) and TRD ZIF-8 MCCs (f). Insets show corresponding optical micrographs. (g) Schematics showing the adsorption of C5–C8 linear alcohols on ZIF-8 through (110) and (100) planes. (h) $\Delta\lambda_{max}$ of ZIF-8 MCCs of different crystal morphologies in response to C1–C8 linear alcohols: RD ZIF-8 (pink), TRD ZIF-8 (cyan) and TC ZIF-8 (yellow).<sup>112</sup></figcaption></figure>

### 3.3.2 Bragg stacks

Bragg stacks (BSs), also known as Bragg mirrors or one-dimensional photonic crystals (1D PCs), typically consist of multiple dielectric layers with alternating high- and low refractive indexes (RI). The periodic structure enhances the reflections of certain wavelengths because of the diffraction and interferences at every interface.<sup>113,167–169</sup> The optical response can often be
intrinsically tuned by modifying the incident angles¹⁷⁰ and the stack thickness¹¹³,¹¹⁴. Similar to F-P interferometers, the position of the diffraction maxima $\lambda_{max}$ is at normal incidence given by:¹¹³,¹¹⁴

$$ \lambda_{max} = \frac{2(n_H l_H + n_L l_L)}{m} $$

where the indices $H$ and $L$ represent the high- and low-RI layer, respectively. In MOF-based BSs, MOFs typically function as the low-RI layers and $TiO_2$¹¹³,¹¹⁴,¹⁷⁰ or polymers¹⁶⁸ are often employed as the high-RI layers. Adsorption of guest molecules varies the refractive index of the MOF layers ($n_L$) and accordingly shifts the reflectance peaks. Selectivity can be achieved via size sieving, pore accessibility and/or host-guest interactions between MOF and the analytes.

The first MOF-based Bragg stack based on ZIF-8 and $TiO_2$ for ethanol sensing was introduced by Lotsch et al., where ZIF-8 functions as the low-RI layer and $TiO_2$ possesses high RI.¹⁶⁹ Patterning of ZIF/Pt BSs¹⁷¹ and 3D-ordered MOF photonic films¹⁷² for sensing application have also been achieved. BSs were also prepared with $NH_2$-MIL-88 for selective sensing of EtOH vapour, taking profit from the selective breathing behaviour of MIL-88.¹⁷⁰ Lotsch et al. also assembled an optical sensor array from three $TiO_2$/MOFs BSs ($TiO_2$/ZIF-8, $TiO_2$/HKUST-1, and $TiO_2$/CAU-1-NH$_2$).¹¹³ Combining responses from multiple MOF elements with appropriate signal analysis methods (colour image analysis and principal component analysis-PCA) generated analyte-specific fingerprints, which facilitated the single component discrimination of mixed vapours. In another study from the same group, post-synthetic modification (PSM) on either the SBU or the linker of CAU-1(Al) directly on an assembled BS of $TiO_2$/CAU-1 was achieved to precisely tune the sensitivity and selectivity of the active MOF layers.¹¹⁴ De-methoxylation of the SBU increased the pore accessibility and improved the possibility to form hydrogen bonds, while amidification of the linker with hexanoic acid brought hydrophobicity. Both modulations allowed a better distinction among water, alcohols (methanol, ethanol and isopropanol) and n-heptane vapours. The first Bragg stack composed of $NH_2$-MIL-88B nanoparticles and a functional polymer poly(styrene-acrylic acid) was reported in 2020.¹⁶⁸ The polymer and the flexible MOF can both undergo swelling upon BTEX sorption and the associated volume change induces modifications in the thickness and RI of both components. Consequently, the sensitivity of the sensor is boosted with contributions from both parts. The brown-purple film also exhibited a colour variation correlated to the concentration of exposed BTEX vapours. Selective sensing of chlorobenzene has also been achieved using two other strategies: combining MOFs and graphene oxide as the sensing layer¹⁷³ and MOF metal doping.¹⁷⁴

### 3.3.3 Diffraction grafting and colloidal film engineering

Engineering colloidal films of MOFs can create photonic structures for sensing applications. For example, Faustini et al. have described a soft-lithographic technique for the fabrication of sub-micrometer 2D photonic structures based on colloidal ZIF-8 or ZIF-8/$TiO_2$ nanocomposites.¹¹⁵ The sensor presented a selective detection of styrene under the interference of humidity, with a LoD of 57 ppm. The signal was transduced by the change in colour luminance, which originated from the MOF refractive index variation upon adsorption of the targeted analytes and can be detected by integrated cameras in smartphones. The same group also demonstrated an evaporation-directed crack patterning of MIL-101(Cr) colloidal films.¹¹⁶ The crack pattern was promoted during dip-coating deposition, where controlling the evaporation front and the withdrawal speed guide the crack propagation and periodicity differently. The crack arrays acted as diffraction grafting and allowed the detection of toluene down to 200 ppm. Bai et al. introduced a responsive amorphous photonic structure (APS) constructed by the colloidal assembly of spherical/polyhedral ZIF-8 particles on a liquid-permeable substrate.¹⁷⁵ The infiltration-assisted Mayer rod coating can customize the colour of the film by controlling the wetting process (see Figure 11). This photonic structure exhibited a vapour-responsive colour variation, where distinctions among ethanol, methanol, acetone, toluene and DMF can be realized.
<figure><img src="image_19.png" /><figcaption>Figure 11. Illustration of Mayer rod coating of MOF amorphous photonic structure (APS). (a) Selective wetting of the hydrophilic area of the substrate during coating by spherical/polyhedral MOF inks; b) Formation of APS patterns of spherical or polyhedral MOF particles after the removal of water; c) Vapour-responsive colour changes of the MOF APS patterns.¹⁷⁵</figcaption></figure>

### 3.3.4 Optical fibres

Optical fibres are often made of silica or plastic for the core, covered by a cladding layer with a lower refractive index. This structure allows light to propagate along the fibre by total internal reflections (TIR), where the fibre acts as a waveguide. In such manner, light can be transmitted over long distances with low loss and immunity to electromagnetic interferences. Optical fibres are commonly employed in telecommunication, but they have also been explored as chemical sensors. Fibre optic interferometry is one of the chemical sensing approaches that make use of optical fibres that is known for its high sensitivity, but lacks chemical specificity.¹⁶² One solution is to combine MOFs nanocrystals with optical fibres for improved selectivity. Several MOF-coated optical fibres have been reported to be able to selectively detect CO₂ and H₂.¹⁷⁶⁻¹⁷⁹ Some studies have also explored their potential for VOCs sensing. For example, Zhu and coworkers coupled a single crystal of HKUST-1 to the end-face of an optical fibre using epoxy glue for quantitative detection of nitrobenzene (Figure 12a).¹⁸⁰ The sensing mechanism is based on the variation of the refractive index induced by the adsorption of nitrobenzene within the MOF crystal. The quantification was achieved by analysing the shifts of the interference spectra derived from the fibre optic interferometry (Figure 12b). Cao et al. also developed a methane-sensitive cladding layer by integrating ZIF-8 into a UV-curable polymer.¹⁸¹ The detection limit of 1% methane in nitrogen was achieved by employing the ZIF-8/polymer coating in either single-mode or multi-mode fibres. In another work, Chen et al. realized the on-chip integration of ZIF-8 on the surface of a SiO₂ waveguide.¹¹⁷ The fabricated sensor exhibited high sensitivity towards ethanol with a large detection range of 0-1000 ppm and a low detection limit of 1.6 ppm. Recently, Huang et al. fabricated an optical microfibre interferometric VOC sensor based on evanescent field interaction with ZIF-8/graphene oxide nanocoating.¹¹⁸ The surface of the optical fibre was pre-modified with graphene oxide (GO), where multilayers of ZIF-8 were grown subsequently via the in situ synthesis (Figure 12c). The GO layer enhanced the adhesion and
homogeneity of ZIF-8 layers due to the large π-conjugated network of GO. The sensor presented an ultrafast response of 118 ms towards ethanol with the LoD of 5.26 ppm, which was attributed to the porous structure of the ZIF-8/GO composite.

<figure><img src="image_20.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>(c)</figcaption></figure>

<caption>(c)</caption>

<figure><img src="image_23.png" /><figcaption>Figure 12. (a) Schematic diagram of the HKUST-1 coated optical fibre sensor and an optical microscope image of the sensor. (b) Illustration of a section of the original (blue) and shifted (red) interferograms.¹⁸⁰ (c) Schematics of the modification step at the interface of the optical microfibre with ZIF-8/GO functional layer.¹¹⁸</figcaption></figure>

Long-period grafting (LPG) optical fibres are another well-known type of sensor that relies on the modulation of refractive indexes. Typically, a periodic perturbation of the refractive index is present on an LPG fibre, which results in a series of attenuation bands in the transmission spectrum. Each band corresponds to different couplings between the core mode and a particular cladding mode. With such, the exposure of guest vapours induced changes in the refractive index of the cladding that shifted the attenuation band.¹⁶²,¹⁸²⁻¹⁸⁴ Hromadka et al. have reported a LPG-based sensor working at phase-matching turning point presenting a high sensitivity for ethanol and acetone.¹⁸⁴,¹⁸⁵ In another study, Chiang et al. developed a fibre interferometer with pair of cascaded LPGs, where ZIF-8 was coated on the surface of the cladding.¹¹⁹ The LPGs enable a co-propagating mode, where the light in the core and the cladding interfere with each other and result in a sharp interference fringe as shown in Figure 13. Ethanol and methanol showed the largest interfering fringe shifts and greatest RI changes from 1.216 to 1.499 and 1.476, respectively. DFT calculations attributed the distinctive response toward ethanol and methanol to their different desorption energy from ZIF-8. In a follow-up study, Chiang et al. utilized the same configuration for real-time monitoring of bulkier molecules (e.g. toluene, pyridine) and proved that the adsorption dynamics is greatly linked to molecular geometries and kinetic diameters of the guest molecules.¹⁸³ Adsorption of toluene on ZIF-8 shifted the outcome interference spectrum by 4.3 nm after 40 min exposure. However, due to the slow adsorption dynamics and difficulty in desorption, the device is rather suitable for toluene capture than for the sensing application.
<figure><img src="image_24.png" /><figcaption>Figure 13. (a) Optical power distribution of the fundamental mode (LP₀₁, core mode) and (b) higher-order mode (LP₀₈, cladding mode); (c) near-field image of the LP₀₈ mode pattern obtained with an infrared camera; (d) schematic diagram of the proposed fibre interferometer with cascaded LPFGs. ASE: amplified spontaneous emission light source; OSA: optical spectrum analyzer.¹¹⁹</figcaption></figure>

### 3.3.5 Micro-ring Resonators

Similar to optical-fibre-based sensors, photonic micro-ring resonators detect the analytes based on evanescent field interaction and consequent changes in refractive index. However, the waveguides, in that case, are bendable and connected in a closed loop with a light source and a detector. Light in micro-ring resonators is propagated in the fashion of a circulating waveguide mode, which is derived from the total internal reflection of an arced boundary between a high and a low refractive index medium. As a result, the interaction between the sensitive layer and the analytes give rises to a shift in the resonant wavelength.²¹,²⁹ Tao et al. exemplified a micro-ring resonator with ZIF-8 coating for detection of multiple VOCs at ppb level including styrene, toluene, benzene, propylene and methanol.²⁹ The coating of ZIF-8 was achieved by in situ growth followed by chemical etching. With the presence of ZIF-8, the sensitivity of the resonator was enhanced by a factor up to 1000 for propylene compared to a bare resonator. This can be attributed to the preconcentration of the analytes in ZIF-8. In addition, the hydrophobicity of ZIF-8 also made the device much less sensitive to humidity. The LoD ranged from 29 to 99 ppb with a response time of 30 min, which can be suitable for applications requiring high-accuracy.

### 3.3.6 Surface plasmon resonance

Surface plasmon resonance (SPR) occurs when incident light stimulates the resonant oscillation of plasmon at the interface between two media with opposite permittivity. When plasmonic metal nanoparticles are involved, the nanoscale dimension confines the oscillation on the particle surface, which is known as localized surface plasmon resonance. Plasmonic nanoparticles, typically Ag, Au and Cu, can interact with light in the visible and near-infrared regions. The presence of analytes at their surface can modify the local RI and results in a shift in the resonance wavelength. MOFs incorporating SPR platforms can amplify the sensor response, therefore extending its applications from biomolecule detection to VOCs sensing.¹⁸⁶,¹⁸⁷

Vanderzande et al. presented a ppm-level detection of alcohol vapours using MOF-functionalized SPR sensors.¹²⁰ A layer of ZIF-8 or ZIF-93 was deposited as a recognition layer on a gold surface of the fibre-optic-based SPR sensors (see Figure 14a). Upon exposure to several alcohol vapours (MeOH, n-BuOH, etc.), the bare SPR sensor did not exhibit any detectable SPR activity due to the low intrinsic RI of the alcohols. With the assistance of MOF coating, vapours with ppm concentration were detected. A LoD of 2.5 ppm for methanol was realized by the ZIF-8 coated sensor. ZIF-93 coating presented the highest sensitivity and selectivity towards MeOH at low partial pressure due to its hydrophilicity, whereas ZIF-8 showed high sensitivity to MeOH at
high partial pressure because of the molecular clustering effect. Such difference in recognition behaviour can be used for qualitative distinction of the vapour composition. Lately, Ameloot et al. explored surface plasmon polariton (SPP) resonance by incorporating a ZIF-8 thin film via CVD approach onto an AlOx/Ag layered surface on a glass substrate for methanol detection.¹²¹ The sample configuration is illustrated in Figure 14b. The signal of VOC uptake was transduced into SPP shifts by applying spectroscopic total internal reflection ellipsometry (TIRE). Upon exposure to MeOH vapour, the sensor responded up to 160 nm in the SPP shifts with a LoD lower than 1 ppm (Figure 14c). A resolution of 10⁻⁵ in refractive index units was achieved by optical modelling of measured TIRE/SPP spectra, which outperformed conventional ellipsometric porosimetry.

<figure><img src="image_25.png" /><figcaption>Figure 14. (a) Schematic representation of an MOF-FO-SPR probe (not in real scale). The probe consists of an optical fibre with a diameter of 400 µm with the inner TECS cladding and the outer protective cladding removed from the end. A gold layer is covered with a MOF which functions as a recognition layer. The atoms in the floating methanol molecules are oxygen (red), carbon (black), and hydrogen (white).¹²⁰ (b) TIRE/SPP principle and TIRE measurement geometry with the sample layer stacks. (c) The measured SPP peak redshift under Methanol in dry N₂.¹²¹</figcaption></figure>

### 3.3.7 Surface-enhanced Raman scattering

Surface-enhanced Raman scattering is an ultrasensitive vibrational spectroscopic analytical technique. It is based on the working principle of traditional Raman spectroscopy that identifies and quantifies the analytes with their characteristic vibrational fingerprints. Enhancement of Raman signals is achievable by adopting SERS-active substrates such as plasmonic particles, for instance Au and Ag, which can generate an enhanced electromagnetic field on the surface upon excitations and accordingly amplify the signals. The poor affinity between the plasmonic surface and analytes is often addressed as a problem of SERS platforms. Incorporating MOFs with SERS essentially facilitates the selectivity and sensitivity by preconcentrating the analytes and introducing intermolecular interactions between the framework and guest molecules.¹⁸⁸⁻¹⁹⁰

Koh et al. reported a plasmonic nose by encapsulating Ag nanotubes in ZIF-8 (Ag@ZIF-8) for molecular-level recognition of VOCs.¹²² The combination of Ag and ZIF-8 leads to pre-concentration of analyte molecules and enhancement of the electromagnetic (EM) field at the nanoscale. The ultra-trace SERS detection of VOCs was facilitated by optimizing the thickness
of the ZIF-film for maximizing the volume of preconcentrated vapour and tailoring the plasmonic coupling between
neighbouring Ag nanotubes to intensify the EM hotspots. The Ag@ZIF-8 plasmonic nose exhibited SERS response towards
multiple vapours including toluene and chloroform. Especially, it presented unique vibrational features for toluene down to 200
ppm and a linear quantification of the toluene in the range from 200 to 2000 ppm. Osterrieth et al. fabricated a core-shell
structure of Au nanorods (Au NRs) and Zr-MOFs NU-901 for SERS detection of thiolated analytes.<sup>191</sup> NU-901 is composed of
eight-connected Zr<sub>6</sub> oxoclusters and tetratopic 1,3,5,8- (p-benzoate)pyrene linkers (H4TBAPy). The 1D diamond-shaped channel
of NU-901 has the dimension ca. 12 × 26 Å which allows fast diffusion of relatively large analytes. Au NRs as seeds facilitated the
MOF growth at room temperature with core-shell structure and a yield higher than 99% under optimal conditions. The
composite maintained the SERS capability of the Au NRs, allowing the selective adsorption and detection of thiolated
polystyrene with SERS spectra. In other examples, Li et al. constructed a core-shell HKUST-1@Ag NPs via in situ
electrodeposition for SERS detection of polycyclic aromatic hydrocarbons<sup>192</sup> and Chen et al. achieved SERS detection for toluene,
ethylbenzene, and chlorobenzene by constructing a similar core-shell structure of Au@ZIF-8.<sup>193</sup> Another platform of Au@Ag
nanorods encapsulated in ZIF-8 was fabricated by Lafuente et al. for chemical warfare agents detection in ambient air.<sup>123</sup> Yang et
al. also demonstrated a site-selective deposition of ZIF-8 on Au nanobipyramids for SERS detection of toluene and aniline
vapours.<sup>194</sup> ZIF-8 was selectively grown either at the ends, the waist, or forming a core-shell structure on the bipyramids. The
formation of the selectively deposited ZIF-8 was confirmed by SEM and TEM images shown in Figure 15a-b. All three
nanostructures presented distinctive Raman peaks of toluene and aniline under the vapour exposure, indicating the occurrence
of analyte interactions with the MOF. The end-deposited Au NBP@ZIF-8 presented the strongest Raman signals, which can be
explained by the large local electric field at the sharp ends of the Au NBPs.

<figure><img src="image_26.png" /><figcaption>Figure 15. (a) Schematics of the routes for selective deposition of ZIF-8 on the Au NBPs. (b) TEM images of the Au NBPs, NBP/end-ZIF, NBP/waist-ZIF, and NBP@ZIF nanostructures, respectively.¹⁹⁴ (c) Illustration of the setup for stand-off SERS sensing. (d) Stand-off multiplex spectra obtained in outdoor condition with natural light, for two different mixtures of naphthalene and toluene, with reference to individual naphthalene and toluene SERS spectra.</figcaption></figure>
Phan-Quang et al. also realized a real-time stand-off SERS spectroscopy with the integration of Ag@ZIF-8 for remote and multiplex detection of airborne molecules.<sup>124</sup> The platform was fabricated via the drop-casting multiple layers of the solution of pre-synthesized Ag@ZIF-8 core-shell composite. The formed 3D structure of Ag@ZIF-8 has a thickness of ca. 1.3 µm after 10-15 layers with an interparticle distance of Ag cores of ca. 10 nm, which allows strong plasmonic coupling for intense SERS hotspots. Upon exposure to vapours of the probe molecule 4-methylbenzenethiol (4-MBT), it presented a rapid response of 10 s under 500 ppm and the signals were 6-fold more intense compared to the control platforms (the MOF-coated Ag platform and bare Ag nanocubes). Notably, the multilayered structure also brought an appropriate hotspot focal depth to be able to cooperate with the stand-off Raman system. The response of the sensor can therefore be recorded with the detector placed 2-10 m apart with 8-fold less laser power employed (≤ 55 mW, see Figure 15c). The Ag@ZIF-8 combined stand-off Raman detection can respond to a minimum of 50 ppb aerosolized 4-MBT and a linear calibration curve can be constructed within the range of 500 ppm to 50 ppb. Outdoor sensing of aerosolized mixtures of (poly)cyclic aromatic hydrocarbons (PAHs), naphthalene and toluene was also realized. The reported platform can elucidate the characteristic vibrational fingerprints of each species within 10 s under the interference of a daylight background (Figure 15d).

Min et al. also explored a SERS-active sensory array based on MIL-100(Fe) for ultrasensitive and multiplex detection of VOCs.<sup>27</sup> The LoD for toluene was 2.5 ppm on solely MIL-100(Fe) substrate, and depositing gold nanoparticles over MIL-100 greatly amplified the signal with the enhancement factor up to 10<sup>10</sup>, which consequently reduced the LoD for toluene to 0.48 ppb. The high-SERS activity of MIL-100(Fe) can be accounted for a charge transfer enhancement mechanism between the framework and the analytes, which was confirmed by the specific adsorption energy and DFT calculation. Especially, the aromatic ligand and metal active sites are preferably combined by different molecules (e.g. toluene adsorption via π-π interaction and acetone via Fe-O coordination), providing a sensor array property for multiplex VOCs detection. Finally, MOF-integrated plasmonic surface-enhanced infrared adsorption platform was explored by Mu et al., where ZIF-8 was coated on Au antenna to realize on-chip sensing of greenhouse gases (CH<sub>4</sub> and CO<sub>2</sub>).<sup>195</sup>

## 4. Conclusions and perspectives

This review summarizes recent progresses on MOF-based colorimetric sensors with particular highlights on VOCs sensing. The versatility of MOFs features their unique chemical and physical properties, drawing extensive research interest on them, especially as chemical sensing materials. The advancement of integrated circuits and miniaturization of electronic devices, combined with the development of MOF film fabrication, recently allowed the preliminary laboratory demonstrations to practical VOCs sensing application of MOFs.

Compared to MOF-based electronic sensors, research on MOF-based colorimetric sensors is still in the early stage. Chromism sensing exhibits straightforward output with often naked-eye observable colour change. The colorimetric responses highly rely on the design of MOF structures including modifying the coordination geometry of the metal sites in MOFs, using chromophoric ligands and encapsulating functional chromophores. Although many studies have demonstrated sensitive and selective sensing of VOCs using chromic MOFs, the delivered selectivity targets groups of VOCs and often lack refined selectivity towards individual compounds as well as missing the examination of the materials under environmental conditions. The combination of multiple MOFs to create a sensor array can take advantage of the differentiated affinity of each MOF component and thus, an improved selectivity might be achieved. Besides, the shaping of colorimetric MOFs requires further attention, as it can also strongly influence the sensing performances. The reproducibility of nanoMOF-integrated sensing devices also needs critical recognition as it highly relies on the purity of MOFs, their activation status, and water-stability for consistent long-term performance. Luminescent sensing shares similar characteristics, where the intensity of luminescence is modified by the targeting analytes. The various choices of building blocks and diverse guest-recognition mechanisms have made luminescent MOFs rapidly evolving for sensing applications. The prominent challenge of luminescent MOFs is, similar to colorimetric MOFs, a lack of processability for practical implementations. Specifically, the construction of MOFs as thin films or membranes is starting, yet comprehensive evaluations of their performances including sensitivity, selectivity, response time, stability and recyclability still await further efforts. In addition, evaluating the luminescence change often requires specific instruments, which may pose additional obstacles to their practical applications. Overall, there is still a long journey ahead for chromism- and luminescent-based MOFs to achieve ultra-sensitive VOC sensing, especially for real-world sensing installation. Their limitation of sensitivity comes intrinsically from their sensing mechanisms, thus colour-based sensors can hardly compete with other techniques like
SERS. Nonetheless, colour-based MOFs for VOC sensing possess their own advantages as they are easy to operate and interpret, not even to mention the customizable selectivity provided by desired MOFs. Yet, one needs to note that the unique selective behaviour of MOFs is not a confirmation of outstanding sensing ability. It is also worth to investigate and examine promising materials under different circumstances. To summarize, despite the advantage of naked-eye observation, limited studies examined chromic and luminescent MOFs under the presence of VOCs vapors and sensing performance was often not systematically investigated. The reported LoD of some chromic or luminescent MOFs falls in the ppm levels and can be varied greatly depending on chosen MOFs. It is worth to mention that their sensitivity is sometimes limited by the sensing mechanisms, especially for the ones relying on ligand-to-metal transfer, which are often less sensitive than the organic-based system. Furthermore, considering that the shaping process are not often developed with these compounds, considerable efforts are still required in order to use these type of MOFs into real devices.

Other optical sensors are based on the change of optical index induced by the absorbance of analytes on the MOF sensing layer. For such purpose, multiple techniques have integrated MOFs in the form of thin films or photonic crystals into sensing devices. They present great potential in VOC vapour sensing with sensitivity from ppb to ppm level. Particularly, Fabry-Perot interferometers can efficiently transduce adsorption-induced refractive index change, with sensing performances including sensitivity, selectivity and response time that are highly associated with the sorption features of integrated MOF layers. MOFs sorption ability not only depends on their microstructure and intrinsic physico-chemical properties but is also related to the attributes of MOF-film engineering. Reducing the thickness of MOF films can accelerate the mass-transport process of analytes, but at the same time, can damage the optical properties of the sensor by diminishing the magnitude of RI, leading to undermined performance. Furthermore, the orientation of MOF particles can also impact the optical properties of the film, as highly oriented films can enhance both the selectivity¹¹² and signal intensity for potential colorimetric sensing. Another possible strategy to optimize the signal transduction is the hybridization of MOFs with oxide nanoparticles. One way is to integrate MOF photonic crystal into Bragg stacks with contrast oxide NPs, which requires the size and morphology control during MOF synthesis. The other approach is by encapsulating oxide NPs into MOF films, however, obtaining a uniform and homogenous distribution of NPs is still challenging.

Optical fibres based on RI changes can also encode sensing events into spectra. They are known for immunity to electromagnetic interference and are generally light-weighted with small probe heads and high flexibility, which allows highly sensitive and multiplex detection in remote or harsh environments. The MOF coatings in optical fibres act as concentrators to boost the sensitivity (LoD under 10 ppm) and also potentially as sieving layer for enhanced selectivity. However, it is, like the aforementioned others, a technique under preliminary investigation, as only ZIF-8 was coated on the fibres and ethanol was the only tested vapor. Thus, the selectivity of this method must still be explored. The mechanical property of the MOF coating is a key factor for the practical integration of MOFs into optical fibres. In-situ growth of MOF films is widely adopted for optical fibre coating. Despite the convenience of this method, it is challenging to precisely control the morphology and defects of the film, as well as not easily scalable. Besides, mismatching MOFs with weakly adhered optical fibres can cause damage to the MOF coating (e.g. cracks and shedding), thus hindering the performance of the sensor. MOF-polymer-based mixed-matrix membranes might be an alternative solution to tackle such drawbacks, yet a systematic investigation is still required.

SERS is recognized as a highly sensitive and selective technique for quantitative detection of chemicals at trace levels. MOFs have been identified as Raman active substrates and, with appropriate choices of candidates, their enhancement factor can be competitive with classic SERS substrates including noble metal NPs. These MOF-based SERS systems have been actively applied in vapour-phase sensing with promising processes recently. Configuration of core-shell structures between MOFs and plasmonic noble metal NPs is also a popular strategy to improve sensing performances via enhanced substrate-analyte affinity or molecular trapping.

As multiple sensing techniques are involved in this review, it is also important to para-compare among them for their performance and potential in practical applications. The detection and monitoring of VOCs in real-world require sufficient sensitivity of sensors considering the toxicity and exposure limits of VOCs, especially carcinogenic ones including benzene, formaldehyde, toluene, etc. For example, the exposure limit of formaldehyde regulated by the EU is 300 ppb for 8h average exposure, and 600 ppb for short term expoure (data from GESTIS database). The LoD of MOF-incorporated colorimetric and luminescent sensing is grouped around ppm level, as they are mostly preliminary research. Micro-ring resonators, SPP
resonance, and SERS can handle ppb-level detection. However, micro-ring resonators with MOFs coating and SPP resonance are currently at the proof-of-concept level. Their performance under practical conditions including interfering gases/vapors and humidity needs further examination. At the current stage, SERS can be considered as the platform closest to actual application for its refined LoD (lowest report value 0.48 ppb) and fast response time (several seconds). The cost of MOFs is a generally concerned topic when speaking about their real application due to harsh synthetic conditions limiting the production scale and its price. Optimizing MOF synthesis and large-scale production are the two contributing factors that can benefit MOFs' practical use.

The key challenge for MOF-based SERS, as well as for other MOF-integrated proof-of-concept techniques, is to expand the spectrum of MOF candidates. The majority of research on sensing applications employed limited benchmark MOFs such as ZIF-8 or HKUST-1, due to their well-developed facile synthesis and diverse shaping approaches. However, they have drawbacks including poor chemical stability (HKUST-1), insufficient water stability and the narrow pore sizes of ZIF-8(4 Å) that limit their potential for practical usage, especially in large-molecule sensing. Besides, difficulty in functionalization further obstacles them by restricted tailorability for desired analytes. Conventional one-pot MOF synthesis generally acquires harsh conditions (e.g. high pressure and temperature), especially for water-stable high-valence MOFs, which makes it complicated to further incorporate with nanoparticles. Thus, developments on the mild-condition synthesis of MOFs expect to promote a further diverse application, where the potential of MOFs can be fully explored.

## Author contributions

Y. S. wrote the manuscript. A. T. and C. S. revised and supervised the manuscript.

## Conflicts of interest

There are no conflicts of interest to declare.

## Acknowledgements

YS thanks the UptoParis European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 754387.

## References

(1) Joshi, S. The Sick Building Syndrome. *Indian J. Occup. Environ. Med.* **2008**, *12* (2), 61. https://doi.org/10.4103/0019-5278.43262.

(2) Sarigiannis, D. A.; Karakitsios, S. P.; Gotti, A.; Liakos, I. L.; Katsoyiannis, A. Exposure to Major Volatile Organic Compounds and Carbonyls in European Indoor Environments and Associated Health Risk. *Environ. Int.* **2011**, *37* (4), 743–765.
https://doi.org/10.1016/j.envint.2011.01.005.

(3) Ras, M. R.; Borrull, F.; Marcé, R. M. Sampling and Preconcentration Techniques for Determination of Volatile Organic Compounds in Air Samples. *TrAC - Trends Anal. Chem.* **2009**, *28* (3), 347–361. https://doi.org/10.1016/j.trac.2008.10.009.

(4) Zhang, Y.; Zhao, J.; Du, T.; Zhu, Z.; Zhang, J.; Liu, Q. A Gas Sensor Array for the Simultaneous Detection of Multiple VOCs. *Sci. Reports* **2017** *71* **2017**, *7* (1), 1–8. https://doi.org/10.1038/s41598-017-02150-z.

(5) Park, J.; Tabata, H. Gas Sensor Array Using a Hybrid Structure Based on Zeolite and Oxide Semiconductors for Multiple Bio-Gas Detection. *ACS Omega* **2021**, *6* (33), 21284–21293.
https://doi.org/10.1021/ACSOMEGA.1C01435/ASSET/IMAGES/LARGE/AO1C01435_0009.JPEG.

(6) Yaghi, O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular Synthesis and the Design of New Materials. *Nat. 2003* **423** **6941** **2003**, *423* (6941), 705–714. https://doi.org/10.1038/NATURE01650.

(7) Howarth, A. J.; Liu, Y.; Li, P.; Li, Z.; Wang, T. C.; Hupp, J. T.; Farha, O. K. Chemical, Thermal and Mechanical Stabilities of Metal-Organic Frameworks. *Nat. Rev. Mater.* **2016**, *1* (15018), 1–15. https://doi.org/10.1038/natrevmats.2015.18.

(8) Bigdeli, F.; Lollar, C. T.; Morsali, A.; Zhou, H. C. Switching in Metal–Organic Frameworks. *Angew. Chemie - Int. Ed.* **2020**, *59* (12), 4652–4669. https://doi.org/10.1002/ANIE.201900666.

(9) Niu, Z.; Cui, X.; Pham, T.; Verma, G.; Lan, P. C.; Shan, C.; Xing, H.; Forrest, K. A.; Suepaul, S.; Space, B.; Nafady, A.; Al-Enizi, A. M.; Ma, S. A MOF-Based Ultra-Strong Acetylene Nano-Trap for Highly Efficient C2H2/CO2 Separation. *Angew. Chemie* **2021**, *133* (10), 5343–5348. https://doi.org/10.1002/ANGE.202016225.

(10) Fan, H.; Peng, M.; Strauss, I.; Mundstock, A.; Meng, H.; Caro, J. MOF-in-COF Molecular Sieving Membrane for Selective Hydrogen Separation. *Nat. Commun.* **2021** *121* **2021**, *12* (1), 1–10. https://doi.org/10.1038/s41467-020-20298-7.

(11) Li, Y.; Liu, H.; Wang, H.; Qiu, J.; Zhang, X. GO-Guided Direct Growth of Highly Oriented Metal–Organic Framework Nanosheet Membranes for H2/CO2 Separation. *Chem. Sci.* **2018**, *9* (17), 4132–4141. https://doi.org/10.1039/C7SC04815G.
(12) Jaramillo, D. E.; Jaffe, A.; Snyder, B. E. R.; Smith, A.; Taw, E.; Rohde, R. C.; Dods, M. N.; DeSnoo, W.; Meihaus, K. R.; Harris, T. D.; Neaton, J. B.; Long, J. R. Metal–Organic Frameworks as O2-Selective Adsorbents for Air Separations. *Chem. Sci.* **2022**, *13* (35), 10216–10237.
https://doi.org/10.1039/D2SC03577D.

(13) Bonneau, M.; Lavenn, C.; Ginet, P.; Otake, K. I.; Kitagawa, S. Upscale Synthesis of a Binary Pillared Layered MOF for Hydrocarbon Gas Storage and Separation. *Green Chem.* **2020**, *22* (3), 718–724. https://doi.org/10.1039/C9GC03561C.

(14) Farha, O. K.; Yazaydin, A. Ö.; Eryazici, I.; Malliakas, C. D.; Hauser, B. G.; Kanatzidis, M. G.; Nguyen, S. T.; Snurr, R. Q.; Hupp, J. T. De Novo Synthesis of a Metal–Organic Framework Material Featuring Ultrahigh Surface Area and Gas Storage Capacities. *Nat. Chem.* **2010** *211* **2010**, 944–948. https://doi.org/10.1038/nchem.834.

(15) Wang, T.; Gao, L.; Hou, J.; Herou, S. J. A.; Griffiths, J. T.; Li, W.; Dong, J.; Gao, S.; Titirici, M. M.; Kumar, R. V.; Cheetham, A. K.; Bao, X.; Fu, Q.; Smoukov, S. K. Rational Approach to Guest Confinement inside MOF Cavities for Low-Temperature Catalysis. *Nat. Commun.* **2019** *101* **2019**, *10* (1), 1–9. https://doi.org/10.1038/s41467-019-08972-x.

(16) Dai, S.; Ngoc, K. P.; Grimaud, L.; Zhang, S.; Tissot, A.; Serre, C. Impact of Capping Agent Removal from Au NPs@MOF Core–Shell Nanoparticle Heterogeneous Catalysts. *J. Mater. Chem. A* **2022**, *10* (6), 3201–3205. https://doi.org/10.1039/D1TA09108E.

(17) Heidary, N.; Chartrand, D.; Guiet, A.; Kornienko, N. Rational Incorporation of Defects within Metal–Organic Frameworks Generates Highly Active Electrocatalytic Sites. *Chem. Sci.* **2021**, *12* (21), 7324–7333. https://doi.org/10.1039/D1SC00573A.

(18) Suresh, K.; Matzger, A. J. Enhanced Drug Delivery by Dissolution of Amorphous Drug Encapsulated in a Water Unstable Metal–Organic Framework (MOF). *Angew. Chemie Int. Ed.* **2019**, *58* (47), 16790–16794. https://doi.org/10.1002/ANIE.201907652.

(19) Gkaniatsou, E.; Sicard, C.; Ricoux, Rø.; Benahmed, L.; Bourdreux, F.; Zhang, Q.; Serre, C.; ean-Pierre Mahy, J.; Nathalie Steunou, and; Gkaniatsou, E.; Sicard, C.; Benahmed, L.; Bourdreux, F.; Zhang, D.; Steunou, N.; Ricoux, D.; Mahy, J.; Serre, D. Enzyme Encapsulation in Mesoporous Metal–Organic Frameworks for Selective Biodegradation of Harmful Dye Molecules. *Angew. Chemie* **2018**, *130* (49), 16373–16378. https://doi.org/10.1002/ANGE.201811327.

(20) Pinto, R. V.; Wang, S.; Tavares, S. R.; Pires, J.; Antunes, F.; Vimont, A.; Clet, G.; Daturi, M.; Maurin, G.; Serre, C.; Pinto, M. L. Tuning Cellular Biological Functions Through the Controlled Release of NO from a Porous Ti-MOF. *Angew. Chemie Int. Ed.* **2020**, *59* (13), 5135–5143. https://doi.org/10.1002/ANIE.201913135.

(21) Yuan, H.; Li, N.; Fan, W.; Cai, H.; Zhao, D.; Yuan, H.; Fan, W.; Zhao, D.; Li, N.; Cai, H. Metal-Organic Framework Based Gas Sensors. *Adv. Sci.* **2021**, 2104374. https://doi.org/10.1002/ADVS.202104374.

(22) Meng, Z.; Stolz, R. M.; Mendecki, L.; Mirica, K. A. Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials. *Chem. Rev.* **2019**, *119* (1), 478–598. https://doi.org/10.1021/ACS.CHEMREV.8B00311/SUPPL_FILE/CR8B00311_S1_004.XLSX.

(23) Zhang, L.-T.; Zhou, Y.; Han, S.-T. The Role of Metal–Organic Frameworks in Electronic Sensors. *Angew. Chemie Int. Ed.* **2021**, *60* (28), 15192–15212. https://doi.org/10.1002/ANIE.202006402.

(24) Li, H.-Y.; Zhao, S.-N.; Zang, S.-Q.; Li, J. Functional Metal-Organic Frameworks as Effective Sensors of Gases and Volatile Compounds. *Chem. Soc. Rev* **2020**, *49*, 6364. https://doi.org/10.1039/c9cs00778d.

(25) Humby, J. D.; Benson, O.; Smith, G. L.; Argent, S. P.; Da Silva, I.; Cheng, Y.; Rudić, S.; Manuel, P.; Frogley, M. D.; Cinque, G.; Saunders, L. K.; Vitórica-Yrezábal, I. J.; Whitehead, G. F. S.; Easun, T. L.; Lewis, W.; Blake, A. J.; Ramirez-Cuesta, A. J.; Yang, S.; Schröder, M. Host–Guest Selectivity in a Series of Isoreticular Metal–Organic Frameworks: Observation of Acetylene-to-Alkyne and Carbon Dioxide-to-Amide Interactions. *Chem. Sci.* **2019**, *10* (4), 1098–1106. https://doi.org/10.1039/C8SC03622E.

(26) Ma, Z.; Yuan, T.; Fan, Y.; Wang, L.; Duan, Z.; Du, W.; Zhang, D.; Xu, J. A Benzene Vapor Sensor Based on a Metal-Organic Framework-Modified Quartz Crystal Microbalance. *Sensors Actuators, B Chem.* **2020**, *311*, 127365. https://doi.org/10.1016/j.snb.2019.127365.

(27) Fu, J.-H.; Zhong, Z.; Xie, D.; Guo, Y.-J.; Kong, D.-X.; Zhao, Z.-X.; Zhao, Z.-X.; Li, M. SERS-Active MIL-100(Fe) Sensory Array for Ultrasensitive and Multiplex Detection of VOCs. *Angew. Chemie Int. Ed.* **2020**, *59* (46), 20489–20498.
https://doi.org/10.1002/ANIE.202002720.

(28) Tchalala, M. R.; Bhatt, P. M.; Chappanda, K. N.; Tavares, S. R.; Adil, K.; Belmabkhout, Y.; Shkurenko, A.; Cadiau, A.; Heymans, N.; De Weireld, G.; Maurin, G.; Salama, K. N.; Eddaoudi, M. Fluorinated MOF Platform for Selective Removal and Sensing of SO2 from Flue Gas and Air. *Nat. Commun.* **2019**, *10* (1), 1328. https://doi.org/10.1038/s41467-019-09157-2.

(29) Tao, J.; Wang, X.; Sun, T.; Cai, H.; Wang, Y.; Lin, T.; Fu, D.; Lee, L.; Ting, Y.; Gu, Y.; Zhao, D. Hybrid Photonic Cavity with Metal-Organic Framework Coatings for the Ultra-Sensitive Detection of Volatile Organic Compounds with High Immunity to Humidity OPEN. *Nat. Publ. Gr.* **2017**. https://doi.org/10.1038/srep41640.

(30) Yeung, H. H.-M.; Yoshikawa, G.; Minami, K.; Shiba, K. Strain-Based Chemical Sensing Using Metal–Organic Framework Nanoparticles. *J. Mater. Chem. A* **2020**, *8* (35), 18007–18014. https://doi.org/10.1039/D0TA07248F.

(31) Mü, P.; Wisser, F. M.; Freund, P.; Bon, V.; Senkovska, I.; Kaskel, S. Optical Sensors Using Solvatochromic Metal–Organic Frameworks. *Inorg. Chem* **2017**, *56* (22), 14164–14169. https://doi.org/10.1021/acs.inorgchem.7b02241.

(32) Tissot, A.; Kesse, X.; Giannopoulou, S.; Stenger, I.; Binet, L.; Rivière, E.; Serre, C. A Spin Crossover Porous Hybrid Architecture for Potential Sensing Applications. *Chem. Commun.* **2019**, *55* (2), 194–197. https://doi.org/10.1039/C8CC07573E.

(33) Qin, P.; Okur, S.; Li, C.; Chandresh, A.; Mutruc, D.; Hecht, S.; Heinke, L. A Photoprogrammable Electronic Nose with Switchable
Selectivity for VOCs Using MOF Films. *Chem. Sci.* **2021**, *12* (47), 15700–15709. https://doi.org/10.1039/D1SC05249G.

(34) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. *Science* **2002**, *295* (5554), 469–472.
https://doi.org/10.1126/SCIENCE.1067208.

(35) Deng, H.; Doonan, C. J.; Furukawa, H.; Ferreira, R. B.; Towne, J.; Knobler, C. B.; Wang, B.; Yaghi, O. M. Multiple Functional Groups of Varying Ratios in Metal-Organic Frameworks. *Science* **2010**, *327* (5967), 846–850. https://doi.org/10.1126/SCIENCE.1181761.

(36) Thornton, A. W.; Babarao, R.; Jain, A.; Trousselet, F.; Coudert, F. X. Defects in Metal–Organic Frameworks: A Compromise between Adsorption and Stability? *Dalt. Trans.* **2016**, *45* (10), 4352–4359. https://doi.org/10.1039/C5DT04330A.

(37) Yuan, H.; Tao, J.; Li, N.; Karmakar, A.; Tang, C.; Cai, H.; Pennycook, S. J.; Singh, N.; Zhao, D. On-Chip Tailorability of Capacitive Gas Sensors Integrated with Metal–Organic Framework Films. *Angew. Chemie - Int. Ed.* **2019**, *58* (40), 14089–14094.
https://doi.org/10.1002/anie.201906222.

(38) Lee, C. T.; Shin, M. W. Solvothermal Growth of Mg-MOF-74 Films on Carboxylic Functionalized Silicon Substrate Using Acrylic Acid. *Surfaces and Interfaces* **2021**, *22*, 100845. https://doi.org/10.1016/J.SURFIN.2020.100845.

(39) Zhang, X.; Wan, K.; Subramanian, P.; Xu, M.; Luo, J.; Fransaer, J. Electrochemical Deposition of Metal–Organic Framework Films and Their Applications. *J. Mater. Chem. A* **2020**, *8* (16), 7569–7587. https://doi.org/10.1039/D0TA00406E.

(40) Jiang, D.; Burrows, A. D.; Xiong, Y.; Edler, K. J. Facile Synthesis of Crack-Free Metal–Organic Framework Films on Alumina by a Dip-Coating Route in the Presence of Polyethylenimine. *J. Mater. Chem. A* **2013**, *1* (18), 5497–5500. https://doi.org/10.1039/C3TA10766C.

(41) Horcajada, P.; Serre, C.; Grosso, D.; Boissiè, C.; Perruchas, S.; Sanchez, C.; Férey, G.; Sanchez, C.; Boissiè, C.; Férey, G.; Horcajada, P.; Serre, C.; Perruchas, S. Colloidal Route for Preparing Optical Thin Films of Nanoporous Metal–Organic Frameworks. *Adv. Mater.* **2009**, *21* (19), 1931–1935. https://doi.org/10.1002/ADMA.200801851.

(42) Ahvenniemi, E.; Karppinen, M. Atomic/Molecular Layer Deposition: A Direct Gas-Phase Route to Crystalline Metal–Organic Framework Thin Films. *Chem. Commun.* **2016**, *52* (6), 1139–1142. https://doi.org/10.1039/C5CC08538A.

(43) Salmi, L. D.; Heikkilä, M. J.; Puukilainen, E.; Sajavaara, T.; Grosso, D.; Ritala, M. Studies on Atomic Layer Deposition of MOF-5 Thin Films. *Microporous Mesoporous Mater.* **2013**, *182*, 147–154. https://doi.org/10.1016/J.MICROMESO.2013.08.024.

(44) Lausund, K. B.; Nilsen, O. All-Gas-Phase Synthesis of UiO-66 through Modulated Atomic Layer Deposition. *Nat. Commun.* **2016** **7** 101–109. https://doi.org/10.1038/ncomms13578.

(45) Yu, X.-J.; Xian, Y.-M.; Wang, C.; Mao, H.-L.; Kind, M.; Abu-Husein, T.; Chen, Z.; Zhu, S.-B.; Ren, B.; Terfort, A.; Zhuang, J.-L. Liquid-Phase Epitaxial Growth of Highly Oriented and Multivariate Surface-Attached Metal–Organic Frameworks. *J. Am. Chem. Soc* **2019**, *141*, 21.
https://doi.org/10.1021/jacs.9b08169.

(46) Summerfield, A.; Cebula, I.; Schröder, M.; Beton, P. H. Nucleation and Early Stages of Layer-by-Layer Growth of Metal Organic Frameworks on Surfaces. *J. Phys. Chem. C* **2015**, *119*, 2022. https://doi.org/10.1021/acs.jpcc.5b07133.

(47) Liu, J.; Lukose, B.; Shekhah, O.; Arslan, H. K.; Weidler, P.; Gliemann, H.; Bräse, S.; Grosjean, S.; Godt, A.; Feng, X.; Müllen, K.; Magdau, I. B.; Heine, T.; Wöll, C. A Novel Series of Isoreticular Metal Organic Frameworks: Realizing Metastable Structures by Liquid Phase Epitaxy. *Sci. Reports* **2012** **21** 2012, 1 (1), 1–5. https://doi.org/10.1038/srep00921.

(48) Bétard, A.; Fischer, R. A. Metal-Organic Framework Thin Films: From Fundamentals to Applications. *Chem. Rev.* **2012**, *112* (2), 1055–1083. https://doi.org/10.1021/cr200167v.

(49) Falcaro, P.; Buso, D.; Hill, A. J.; Doherty, C. M.; Falcaro, P.; Buso, D.; Hill, A. J.; Doherty, C. M. Patterning Techniques for Metal Organic Frameworks. *Adv. Mater.* **2012**, *24* (24), 3153–3168. https://doi.org/10.1002/ADMA.201200485.

(50) Falcaro, P.; Ricco, R.; Doherty, C. M.; Liang, K.; Hill, A. J.; Styles, M. J. MOF Positioning Technology and Device Fabrication. *Chem. Soc. Rev.* **2014**, *43* (16), 5513–5560. https://doi.org/10.1039/C4CS00089G.

(51) Falcaro, P.; Ricco, R.; Doherty, C. M.; Liang, K.; Hill, A. J.; Styles, M. J. MOF Positioning Technology and Device Fabrication. *Chem. Soc. Rev.* **2014**, *43* (16), 5513–5560. https://doi.org/10.1039/C4CS00089G.

(52) Dong, R.; Zhang, T.; Feng, X. Interface-Assisted Synthesis of 2D Materials: Trend and Challenges. *Chem. Rev.* **2018**, *118* (13), 6189–6325.
https://doi.org/10.1021/ACS.CHEMREV.8B00056.

(53) Stassen, I.; Burtch, N.; Talin, A.; Falcaro, P.; Allendorf, M.; Ameloot, R. An Updated Roadmap for the Integration of Metal–Organic Frameworks with Electronic Devices and Chemical Sensors. *Chem. Soc. Rev.* **2017**, *46* (11), 3185–3241.
https://doi.org/10.1039/C7CS00122C.

(54) Chakraborty, G.; Park, I. H.; Medishetty, R.; Vittal, J. J. Two-Dimensional Metal-Organic Framework Materials: Synthesis, Structures, Properties and Applications. *Chem. Rev.* **2021**, *121* (7), 3751–3891. https://doi.org/10.1021/ACS.CHEMREV.0C01049.

(55) Allendorf, M. D.; Dong, R.; Feng, X.; Kaskel, S.; Matoga, D.; Stavila, V. Electronic Devices Using Open Framework Materials. *Chemical Reviews.* **2020**, pp 8581–8640. https://doi.org/10.1021/acs.chemrev.0c00033.

(56) Ren, J.; Jen, T. C. Atomic Layer Deposition (ALD) Assisting the Visibility of Metal-Organic Frameworks (MOFs) Technologies. *Coord. Chem. Rev.* **2021**, *430*. https://doi.org/10.1016/J.CCR.2020.213734.

(57) Sanchez, C.; Shea, K. J.; Kitagawa, S.; Shekhah, O.; Liu, J.; Fischer, R. A.; Wöll, C. Hybrid Materials Themed Issue MOF Thin Films:
Existing and Future Applications. **Chem. Soc. Rev. 2011**, 4, 1081–1106. https://doi.org/10.1039/c0cs00147c.

(58) Wang, Z.; Liu, L.; Li, Z.; Goyal, N.; Du, T.; He, J.; Li, G. K. Shaping of Metal-Organic Frameworks: A Review. *Energy and Fuels* **2021**. https://doi.org/10.1021/acs.energyfuels.1c03426.

(59) Zacher, D.; Baunemann, A.; Hermes, S.; Fischer, R. A. Deposition of Microcrystalline [Cu3(Btc)2] and [Zn2(Bdc)2(Dabco)] at Alumina and Silica Surfaces Modified with Patterned Self Assembled Organic Monolayers: Evidence of Surface Selective and Oriented Growth. *J. Mater. Chem.* **2007**, 17 (27), 2785–2792. https://doi.org/10.1039/b703098c.

(60) Liu, J.; Sun, F.; Zhang, F.; Wang, Z.; Zhang, R.; Wang, C.; Qiu, S. In Situ Growth of Continuous Thin Metal–Organic Framework Film for Capacitive Humidity Sensing. *J. Mater. Chem.* **2011**, 21 (11), 3775–3778. https://doi.org/10.1039/C0JM03123B.

(61) Ji, H.; Hwang, S.; Kim, K.; Kim, C.; Jeong, N. C. Direct in Situ Conversion of Metals into Metal-Organic Frameworks: A Strategy for the Rapid Growth of MOF Films on Metal Substrates. *ACS Appl. Mater. Interfaces* **2016**, 8 (47), 32414–32420. https://doi.org/10.1021/ACSAMI.6B12755/SUPPL_FILE/AM6B12755 SI_001.PDF.

(62) Centrone, A.; Yang, Y.; Speakman, S.; Bromberg, L.; Rutledge, G. C.; Hatton, T. A. Growth of Metal-Organic Frameworks on Polymer Surfaces. *J. Am. Chem. Soc.* **2010**, 132 (44), 15687–15691. https://doi.org/10.1021/JA106381X/SUPPL_FILE/JA106381X SI_001.PDF.

(63) Verma, P. K.; Huelsenbeck, L.; Nichols, A. W.; Islamoglu, T.; Heinrich, H.; Machan, C. W.; Giri, G. Controlling Polymorphism and Orientation of NU-901/NU-1000 Metal-Organic Framework Thin Films. *Chem. Mater.* **2020**, 32 (24), 10556–10565. https://doi.org/10.1021/ACS.CHEMMATER.0C03539/ASSET/IMAGES/LARGE/CM0C03539_0008.JPEG.

(64) Yuan, H.; Fu, W.; Soulmi, N.; Serre, C.; Steunou, N.; Rosso, M.; Henry de Villeneuve, C. Growth of Fe-BDC Metal-Organic Frameworks onto Functionalized Si (111) Surfaces. *Chem. – An Asian J.* **2022**, 17 (13), e202200129. https://doi.org/10.1002/ASIA.202200129.

(65) Ulman, A. Formation and Structure of Self-Assembled Monolayers. *Chem. Rev.* **1996**, 96 (4), 1533–1554. https://doi.org/10.1021/CR9502357.

(66) Yuan, H. Growth of Metal Organic Frameworks (MOFs) Layers on Functionalized Surfaces, 2019.

(67) Carbonell, C.; Imaz, I.; Maspoch, D. Single-Crystal Metal-Organic Framework Arrays. *J. Am. Chem. Soc.* **2011**, 133 (7), 2144–2147. https://doi.org/10.1021/JA2002428/SUPPL_FILE/JA2002428 SI_001.PDF.

(68) Hermes, S.; Schröder, F.; Chelmowski, R.; Wöll, C.; Fischer, R. A. Selective Nucleation and Growth of Metal-Organic Open Framework Thin Films on Patterned COOH/CF3-Terminated Self-Assembled Monolayers on Au(111). *J. Am. Chem. Soc.* **2005**, 127 (40), 13744–13745. https://doi.org/10.1021/JA053523L.

(69) Yang, H.; Fei, H. A Generic and Facile Strategy to Fabricate Metal–Organic Framework Films on TiO2 Substrates for Photocatalysis. *Dalt. Trans.* **2017**, 46 (9), 2751–2755. https://doi.org/10.1039/C7DT00082K.

(70) Kung, C. W.; Chang, T. H.; Chou, L. Y.; Hupp, J. T.; Farha, O. K.; Ho, K. C. Post Metalation of Solvothermally Grown Electroactive Porphyrin Metal–Organic Framework Thin Films. *Chem. Commun.* **2015**, 51 (12), 2414–2417. https://doi.org/10.1039/C4CC09272D.

(71) Scherb, C.; Schödel, A.; Bein, T. Directing the Structure of Metal-Organic Frameworks by Oriented Surface Growth on an Organic Monolayer. *Angew. Chemie – Int. Ed.* **2008**, 47 (31), 5777–5779. https://doi.org/10.1002/ANIE.200704034.

(72) Rosenhahn, A.; Schilp, S.; Jürgen Kreuzer, H.; Grunze, M.; Egger, D. A.; Rissner, F.; Rangger, G. M.; Hofmann, O. T.; Wittwer, L.; Heimel, G.; Zojer, E.; Wang, L.; Ma, Z.; Li, Q.; Shuai, Z.; Chem, P.; Zhang, C.; Grandner, J.; Liu, R.; Bok Lee, S.; Eichhorn, B. W.; Duay, J.; Lane, T.; Baumann, R.; Schranz, M.; Hampp, N.; Chem Chem, P.; Thompson, L. B.; Mack, N. H.; Nuzzo, R. G.; Kuhn, S.; Baisch, B.; Jung, U.; Johannsen, T.; Kubitschke, J.; Herges, R.; Magnussen, O.; Mahanandia, P.; Schneider, J. J.; Khaneft, M.; Stühn, B.; Peixoto, T. P.; Drossel, B.; Liang, H.; Li, Z.; Yang, J.; Bonnet, N.; Hähner, G.; Dabirian, R.; Feng, X.; Ortolani, L.; Liscio, A.; Morandi, V.; Müllen, K.; Samorì, P.; Palermo, V.; Duan, P.; Liu, M. Oriented Growth of the Functionalized Metal–Organic Framework CAU-1 on –OH- and –COOH-Terminated Self-Assembled Monolayers. *Phys. Chem. Chem. Phys.* **2010**, 12 (17), 4515–4520. https://doi.org/10.1039/B924657F.

(73) Biemmi, E.; Scherb, C.; Bein, T. Oriented Growth of the Metal Organic Framework Cu3(BTC) 2(H2O)3·xH2O Tunable with Functionalized Self-Assembled Monolayers. *J. Am. Chem. Soc.* **2007**, 129 (26), 8054–8055. https://doi.org/10.1021/JA0701208/SUPPL_FILE/JA0701208SI20070504_125904.PDF.

(74) Liu, J.; Wöll, C. Surface-Supported Metal–Organic Framework Thin Films: Fabrication Methods, Applications, and Challenges. *Chem. Soc. Rev.* **2017**, 46 (19), 5730–5770. https://doi.org/10.1039/C7CS00315C.

(75) Li, S.; Shi, W.; Lu, G.; Li, S.; Loo, S. C. J.; Huo, F. Unconventional Nucleation and Oriented Growth of ZIF-8 Crystals on Non-Polar Surface. *Adv. Mater.* **2012**, 24 (44), 5954–5958. https://doi.org/10.1002/ADMA.201201996.

(76) Ladnorg, T.; Welle, A.; Heißler, S.; Wöll, C.; Gliemann, H. Site-Selective Growth of Surface-Anchored Metal-Organic Frameworks on Self-Assembled Monolayer Patterns Prepared by AFM Nanografting. *Beilstein J. Nanotechnol.* **2013**, 4 (1), 638–648. https://doi.org/10.3762/BJNANO.4.71.

(77) Arslan, H. K.; Shekhah, O.; Wohlgemuth, J.; Franzreb, M.; Fischer, R. A.; Wöll, C. High-Throughput Fabrication of Uniform and Homogenous MOF Coatings. *Adv. Funct. Mater.* **2011**, 21 (22), 4228–4231. https://doi.org/10.1002/ADFM.201101592.

(78) Makiura, R.; Motoyama, S.; Umemura, Y.; Yamanaka, H.; Sakata, O.; Kitagawa, H. Surface Nano-Architecture of a Metal–Organic Framework. *Nat. Mater.* **2010**, 9 (7), 565–571. https://doi.org/10.1038/nmat2769.

(79) Zhuang, J. L.; Terfort, A.; Wöll, C. Formation of Oriented and Patterned Films of Metal–Organic Frameworks by Liquid Phase Epitaxy: A
Review. *Coord. Chem. Rev.* **2016**, *307*, 391–424. https://doi.org/10.1016/J.CCR.2015.09.013.

(80) Falcaro, P.; Okada, K.; Hara, T.; Ikigaki, K.; Tokudome, Y.; Thornton, A. W.; Hill, A. J.; Williams, T.; Doonan, C.; Takahashi, M. Centimetre-Scale Micropore Alignment in Oriented Polycrystalline Metal-Organic Framework Films via Heteroepitaxial Growth. *Nat. Mater.* **2017**, *16* (3), 342–348. https://doi.org/10.1038/NMAT4815.

(81) Stassen, I.; Styles, M.; Grenci, G.; Van Gorp, H.; Vanderlinden, W.; De Feyter, S.; Falcaro, P.; De Vos, D.; Vereecken, P.; Ameloot, R. Chemical Vapour Deposition of Zeolitic Imidazolate Framework Thin Films. *Nat. Mater.* **2016** *153* **2015**, *15* (3), 304–310.
https://doi.org/10.1038/nmat4509.

(82) Su, P.; Tu, M.; Ameloot, R.; Li, W. Vapor-Phase Processing of Metal-Organic Frameworks. *Acc. Chem. Res.* **2022**, *55* (2), 186–196.
https://doi.org/10.1021/acs.accounts.1c00600.

(83) Zhang, Z.; Zhao, Y.; Zhao, Z.; Huang, G.; Mei, Y. Atomic Layer Deposition-Derived Nanomaterials: Oxides, Transition Metal Dichalcogenides, and Metal–Organic Frameworks. **2020**. https://doi.org/10.1021/acs.chemmater.9b04414.

(84) Tanskanen, A.; Karppinen, M. Iron-Terephthalate Coordination Network Thin Films Through In-Situ Atomic/Molecular Layer Deposition. *Sci. Reports* **2018** *81* **2018**, *8* (1), 1–8. https://doi.org/10.1038/s41598-018-27124-7.

(85) Virmani, E.; Rotter, J. M.; Mähringer, A.; Von Zons, T.; Godt, A.; Bein, T.; Wuttke, S.; Medina, D. D. On-Surface Synthesis of Highly Oriented Thin Metal-Organic Framework Films through Vapor-Assisted Conversion. *J. Am. Chem. Soc.* **2018**, *140* (14), 4812–4819.
https://doi.org/10.1021/JACS.7B08174/ASSET/IMAGES/LARGE/JA-2017-08174V_0006.JPEG.

(86) Crivello, C.; Sevim, S.; Graniel, O.; Franco, C.; Pané, S.; Puigmartí-Luis, J.; Muñoz-Rojas, D. Advanced Technologies for the Fabrication of MOF Thin Films. *Mater. Horizons* **2021**, *8*. https://doi.org/10.3929/ethz-b-000466506.

(87) Jia, S.; Ji, D.; Wang, L.; Qin, X.; Ramakrishna, S. Metal–Organic Framework Membranes: Advances, Fabrication, and Applications. *Small Struct.* **2022**, *3* (4), 2100222. https://doi.org/10.1002/SSTR.202100222.

(88) Benito, J.; Sorribas, S.; Lucas, I.; Coronas, J.; Gascon, I. Langmuir-Blodgett Films of the Metal-Organic Framework MIL-101(Cr): Preparation, Characterization, and CO2 Adsorption Study Using a QCM-Based Setup. *ACS Appl. Mater. Interfaces* **2016**, *8* (25), 16486–16492. https://doi.org/10.1021/ACSAMI.6B04272/ASSET/IMAGES/LARGE/AM-2016-04272X_0008.JPEG.

(89) Andrés, M. A.; Sicard, C.; Serre, C.; Roubeau, O.; Gascón, I. Ultrathin Hydrophobic Films Based on the Metal Organic Framework UiO-66-COOH(Zr). *Beilstein J. Nanotechnol* **2019**, *10*, 654–665. https://doi.org/10.3762/bjnano.10.65.

(90) Andrés, M. A.; Benzaqui, M.; Serre, C.; Steunou, N.; Gascón, I. Fabrication of Ultrathin MIL-96(Al) Films and Study of CO2 Adsorption/Desorption Processes Using Quartz Crystal Microbalance. *J. Colloid Interface Sci.* **2018**, *519*, 88–96.
https://doi.org/10.1016/J.JCIS.2018.02.058.

(91) Rauf, S.; Andrés, M. A.; Roubeau, O.; Gascón, I.; Serre, C.; Eddaoudi, M.; Salama, K. N. Coating of Conducting and Insulating Threads with Porous MOF Particles through Langmuir-Blodgett Technique. *Nanomater.* **2021**, *Vol. 11*, *Page 160* **2021**, *11* (1), 160.
https://doi.org/10.3390/NANO11010160.

(92) Andrés, M. A.; Vijjapu, M. T.; Surya, S. G.; Shekhah, O.; Salama, K. N.; Serre, C.; Eddaoudi, M.; Roubeau, O.; Gascón, I. Methanol and Humidity Capacitive Sensors Based on Thin Films of MOF Nanoparticles. *ACS Appl. Mater. Interfaces* **2020**, *12* (3), 4155–4162.
https://doi.org/10.1021/ACSAMI.9B20763/ASSET/IMAGES/LARGE/AM9B20763_0004.JPEG.

(93) Burtch, N. C.; Heinen, J.; Bennett, T. D.; Dubbeldam, D.; Allendorf, M. D. Mechanical Properties in Metal–Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications. *Adv. Mater.* **2018**, *30* (37), 1704124.
https://doi.org/10.1002/ADMA.201704124.

(94) Denny, M. S.; Kalaj, M.; Bentz, K. C.; Cohen, S. M. Multicomponent Metal–Organic Framework Membranes for Advanced Functional Composites. *Chem. Sci.* **2018**, *9* (47), 8842–8849. https://doi.org/10.1039/C8SC02356E.

(95) Askim, J. R.; Mahmoudi, M.; Suslick, K. S. Optical Sensor Arrays for Chemical Sensing: The Optoelectronic Nose Chemical Society Reviews Optical Sensor Arrays for Chemical Sensing: The Optoelectronic Nose. *Chem. Soc. Rev* **2013**, *42*, 8649.
https://doi.org/10.1039/c3cs60179j.

(96) Wang, Z.; Chen, Q. Vapochromic Behavior of MOF for Selective Sensing of Ethanol. *Spectrochim. Acta Part A Mol. Biomol. Spectrosc.* **2018**, *194*, 158–162. https://doi.org/10.1016/J.SAA.2017.12.072.

(97) Tunsrichon, S.; Youngme, S.; Boonmak, J. Coexistence of Naked-Eye Mechanochromism, Vapochromism, and Thermochromism in a Soft Crystalline Layered Nickel(II) Coordination Polymer. *Inorg. Chem.* **2021**, *60* (23), 18242–18250.
https://doi.org/10.1021/ACS.INORGCHEM.1C02875/SUPPL_FILE/IC1C02875_SI_001.PDF.

(98) Nakatsuka, S.; Watanabe, Y.; Kamakura, Y.; Horike, S.; Tanaka, D.; Hatakeyama, T. Solvent-Vapor-Induced Reversible Single-Crystal-to-Single-Crystal Transformation of a Triphosphaazatriangulene-Based Metal–Organic Framework. *Angew. Chemie Int. Ed.* **2020**, *59* (4), 1435–1439. https://doi.org/10.1002/ANIE.201912195.

(99) Yu, Y.; Ma, J. P.; Zhao, C. W.; Yang, J.; Zhang, X. M.; Liu, Q. K.; Dong, Y. Bin. Copper(I) Metal-Organic Framework: Visual Sensor for Detecting Small Polar Aliphatic Volatile Organic Compounds. *Inorg. Chem.* **2015**, *54* (24), 11590–11592.
https://doi.org/10.1021/ACS.INORGCHEM.5B02150/SUPPL_FILE/IC5B02150_SI_007.CIF.

(100) Yang, N. N.; Sun, W.; Xi, F. G.; Sui, Q.; Chen, L. J.; Gao, E. Q. Postsynthetic N-Methylation Making a Metal–Organic Framework
Responsive to Alkylamines. *Chem. Commun.* **2017**, *53* (10), 1747–1750. https://doi.org/10.1039/C6CC10278F.

(101) Liu, X. Y.; Yin, X. M.; Yang, S. L.; Zhang, L.; Bu, R.; Gao, E. Q. Chromic and Fluorescence-Responsive Metal-Organic Frameworks Afforded by N-Amination Modification. *ACS Appl. Mater. Interfaces* **2021**, *13* (17), 20380–20387.
https://doi.org/10.1021/ACSAMI.1C03937/SUPPL_FILE/AM1C03937 SI 001.PDF.

(102) Li, S. L.; Han, M.; Zhang, Y.; Li, G. P.; Li, M.; He, G.; Zhang, X. M. X-Ray and UV Dual Photochromism, Thermochromism, Electrochromism, and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster-Based 7-Fold Interpenetrated Framework. *J. Am. Chem. Soc.* **2019**, *141* (32), 12663–12672. https://doi.org/10.1021/JACS.9B04930/SUPPL_FILE/JA9B04930 SI 002.CIF.

(103) Zhang, C.; Sun, L.; Yan, Y.; Liu, Y.; Liang, Z.; Liu, Y.; Li, J. Metal–Organic Frameworks Based on Bipyridinium Carboxylate: Photochromism and Selective Vapochromism. *J. Mater. Chem. C* **2017**, *5* (8), 2084–2089. https://doi.org/10.1039/C6TC04724F.

(104) Razavi, S. A. A.; Masoomi, M. Y.; Morsali, A. Stimuli-Responsive Metal–Organic Framework (MOF) with Chemo-Switchable Properties for Colorimetric Detection of CHCl3. *Chem. – A Eur. J.* **2017**, *23* (51), 12559–12564. https://doi.org/10.1002/CHEM.201702127.

(105) Gao, B.; Hao, P.; Li, G. P.; Shen, J.; Fu, Y. An Electron-Deficient Naphthalene Diimide-Based Metal–Organic Framework for Detecting Electron-Rich Molecules through Photo-/Chemo-Induced Chromism. *Dalt. Trans.* **2021**, *50* (39), 13993–14000.
https://doi.org/10.1039/D1DT02555D.

(106) Moreau, F.; Marrot, J.; Banse, F.; Serre, C.; Tissot, A. Sequential Installation of Fe(II) Complexes in MOFs: Towards the Design of Solvatochromic Porous Solids. *J. Mater. Chem. C* **2020**, *8* (47), 16826–16833. https://doi.org/10.1039/D0TC03756G.

(107) Yao, Z. Q.; Wang, K.; Liu, R.; Yuan, Y. J.; Pang, J. J.; Li, Q. W.; Shao, T. Y.; Li, Z. G.; Feng, R.; Zou, B.; Li, W.; Xu, J.; Bu, X. H. Dynamic Full-Color Tuning of Organic Chromophore in a Multi-Stimuli-Responsive 2D Flexible MOF. *Angew. Chemie Int. Ed.* **2022**, *61* (17), e202202073. https://doi.org/10.1002/ANIE.202202073.

(108) Liu, C. Y.; Chen, X. R.; Chen, H. X.; Niu, Z.; Hirao, H.; Braunstein, P.; Lang, J. P. Ultrafast Luminescent Light-Up Guest Detection Based on the Lock of the Host Molecular Vibration. *J. Am. Chem. Soc.* **2020**, *142* (14), 6690–6697.
https://doi.org/10.1021/JACS.0C00368/SUPPL_FILE/JA0C00368 SI 002.CIF.

(109) Olorunyomi, J. F.; Sadiq, M. M.; Batten, M.; Konstas, K.; Chen, D.; Doherty, C. M.; Caruso, R. A. Advancing Metal-Organic Frameworks toward Smart Sensing: Enhanced Fluorescence by a Photonic Metal-Organic Framework for Organic Vapor Sensing. *Adv. Opt. Mater.* **2020**, *8* (19), 2000961. https://doi.org/10.1002/ADOM.202000961.

(110) Li, C.; Li, L.; Yu, S.; Jiao, X.; Chen, D. High Performance Hollow Metal–Organic Framework Nanoshell-Based Etalons for Volatile Organic Compounds Detection. *Adv. Mater. Technol.* **2016**, *1* (7), 1600127. https://doi.org/10.1002/ADMT.201600127.

(111) Li, Z.; Liu, J.; Yi, X.; Wu, W.; Li, F.; Zhu, Z.; Li, H.; Shi, J.; Xu, Y.; Zhou, F.; Liu, W. Metal–Organic Frameworks-Based Fabry–Pérot Cavity Encapsulated TiO2 Nanoparticles for Selective Chemical Sensing. *Adv. Funct. Mater.* **2022**, *32* (9), 2109541.
https://doi.org/10.1002/ADFM.202109541.

(112) Yu, S.; Wang, X.; Jiao, X.; Li, C.; Chen, D. Polyhedral Metal–Organic Framework Monolayer Colloidal Crystals with Sharpened and Crystal Facet-Dependent Selectivity for Organic Vapor Sensing. *J. Mater. Chem. C* **2021**, *9* (16), 5379–5386.
https://doi.org/10.1039/D1TC00810B.

(113) Ranft, A.; Niekiel, F.; Pavlichenko, I.; Stock, N.; Lotsch, B. V. Tandem MOF-Based Photonic Crystals for Enhanced Analyte-Specific Optical Detection. *Chem. Mater.* **2015**, *27* (6), 1961–1970. https://doi.org/10.1021/CM503640C/SUPPL_FILE/CM503640C SI 001.PDF.

(114) Von Mankowski, A.; Szendrei-Temesi, K.; Koschnick, C.; Lotsch, B. V. Improving Analyte Selectivity by Post-Assembly Modification of Metal–Organic Framework Based Photonic Crystal Sensors. *Nanoscale Horizons* **2018**, *3* (4), 383–390.
https://doi.org/10.1039/C7NH00209B.

(115) Dalstein, O.; Ceratti, D. R.; Boissière, C.; Grosso, D.; Cattoni, A.; Faustini, M. Nanoimprinted, Submicrometric, MOF-Based 2D Photonic Structures: Toward Easy Selective Vapors Sensing by a Smartphone Camera. *Adv. Funct. Mater.* **2016**, *26* (1), 81–90.
https://doi.org/10.1002/ADFM.201503016.

(116) Dalstein, O.; Gkaniatsou, E.; Sicard, C.; Sel, O.; Perrot, H.; Serre, C.; Boissière, C.; Faustini, M. Evaporation-Directed Crack-Patterning of Metal-Organic Framework Colloidal Films and Their Application as Photonic Sensors Evaporation-Directed Crack-Patterning of Metal-Organic Framework Colloidal Films and Their Ap-lication as Photonic Sensors. *Angew. Chemie Int. Ed.* **2017**, *56* (45).
https://doi.org/10.1002/anie.201706745i.

(117) Ma, X.; Wu, J.; Jiang, L.; Wang, M.; Deng, G.; Qu, S.; Chen, K. On-Chip Integration of a Metal–Organic Framework Nanomaterial on a SiO2 Waveguide for Sensitive VOC Sensing. *Lab Chip* **2021**, *21* (17), 3298–3306. https://doi.org/10.1039/D1LC00503K.

(118) Huang, Y.; Lin, W.; Huang, T.; Li, Z.; Zhang, Z.; Xiao, R.; Yang, X.; Lian, S.; Pan, J.; Ma, J.; Wang, W.; Sun, L.-P.; Li, J.; Guan, B.-O.; Huang, Y.; Lin, W. F.; Huang, T. S.; Li, Z. R.; Zhang, Z. R.; Xiao, R. T.; Yang, X.; Lian, S. T.; Ma, J.; Wang, W.; Sun, L.-P.; Li, J.; Guan, B.-O.; Pan, J. S. Ultrafast Response Optical Microfiber Interferometric VOC Sensor Based on Evanescent Field Interaction with ZIF-8/Graphene Oxide Nanocoating. *Adv. Opt. Mater.* **2022**, *10* (3), 2101561. https://doi.org/10.1002/ADOM.202101561.

(119) Wu, J.; Zhang, W.; Wang, Y.; Li, B.; Hao, T.; Zheng, Y.; Jiang, L.; Chen, K.; Chiang, K. S. Nanoscale Light–Matter Interactions in Metal–Organic Frameworks Cladding Optical Fibers. *Nanoscale* **2020**, *12* (18), 9991–10000. https://doi.org/10.1039/C9NR09061D.

(120) Vandezande, W.; Janssen, K. P. F.; Delport, F.; Ameloot, R.; De Vos, D. E.; Lammertyn, J.; Roeffaers, M. B. J. Parts per Million Detection
of Alcohol Vapors via Metal Organic Framework Functionalized Surface Plasmon Resonance Sensors. Anal. Chem. **2017**, *89* (8), 4480–4487. https://doi.org/10.1021/ACS.ANALCHEM.6B04510/ASSET/IMAGES/LARGE/AC-2016-04510E_0008.JPEG.

(121) Tietze, M. L.; Obst, M.; Arnauts, G.; Wauteraerts, N.; Rodríguez-Hermida, S.; Ameloot, R. Parts-per-Million Detection of Volatile Organic Compounds via Surface Plasmon Polaritons and Nanometer-Thick Metal–Organic Framework Films. ACS Appl. Nano Mater. **2022**, *5* (4), 5006–5016. https://doi.org/10.1021/ACSANM.2C00012/ASSET/IMAGES/MEDIUM/AN2C00012_M005.GIF.

(122) Sher, C.; Koh, L.; Lee, H. K.; Han, X.; Yi, H.; Sim, F.; Ling, X. Y.; Li, R., Plasmonic Nose: Integrating the MOF-Enabled Molecular Preconcentration Effect with a Plasmonic Array for Recognition of Molecular-Level Volatile Organic Compounds. Chem. Commun. **2018**, *54* (20), 2546–2549. https://doi.org/10.1039/C8CC00564H.

(123) Lafuente, M.; De Marchi, S.; Urbiztondo, M.; Pastoriza-Santos, I.; Pérez-Juste, I.; Santamaría, J.; Mallada, R.; Pina, M. Plasmonic MOF Thin Films with Raman Internal Standard for Fast and Ultrasensitive SERS Detection of Chemical Warfare Agents in Ambient Air. ACS Sensors **2021**, *6* (6), 2241–2251. https://doi.org/10.1021/ACSSENSORS.1C00178/ASSET/IMAGES/LARGE/SE1C00178_0007.JPEG.

(124) Phan-Quang, G. C.; Yang, N.; Lee, H. K.; Sim, H. Y. F.; Koh, C. S. L.; Kao, Y. C.; Wong, Z. C.; Tan, E. K. M.; Miao, Y. E.; Fan, W.; Liu, T.; Phang, I. Y.; Ling, X. Y. Tracking Airborne Molecules from Afar: Three-Dimensional Metal–Organic Framework–Surface-Enhanced Raman Scattering Platform for Stand-off and Real-Time Atmospheric Monitoring. ACS Nano **2019**, *13* (10), 12090–12099. https://doi.org/10.1021/ACSNANO.9B06486/SUPPL_FILE/NN9B06486 SI_003.MOV.

(125) Chen, Q.; Chang, Z.; Song, W. C.; Song, H.; Song, H. Bin; Hu, T. L.; Bu, X. H. A Controllable Gate Effect in Cobalt(II) Organic Frameworks by Reversible Structure Transformations. Angew. Chemie Int. Ed. **2013**, *52* (44), 11550–11553. https://doi.org/10.1002/ANIE.201306304.

(126) Dzesse T, C. N.; Nfor, E. N.; Bourne, S. A. Vapor Sorption and Solvatochromism in a Metal–Organic Framework of an Asymmetric Pyridylcarboxylate. Cryst. Growth Des. **2018**, *18* (1), 416–423. https://doi.org/10.1021/ACS.CGD.7B01417/SUPPL_FILE/CG7B01417 SI_001.PDF.

(127) Li, B.; Chen, X.; Hu, P.; Kirchon, A.; Zhao, Y. M.; Pang, J.; Zhang, T.; Zhou, H. C. Facile Fabrication of a Multifunctional Metal–Organic Framework-Based Sensor Exhibiting Exclusive Solvochromic Behaviors toward Ketone Molecules. ACS Appl. Mater. Interfaces **2019**, *11* (8), 8227–8233. https://doi.org/10.1021/ACSAMI.8B19815/SUPPL_FILE/AM8B19815 SI_002.CIF.

(128) Shigeta, Y.; Kobayashi, A.; Ohba, T.; Yoshida, M.; Matsumoto, T.; Chang, H. C.; Kato, M. Shape-Memory Platinum(II) Complexes: Intelligent Vapor-History Sensor with ON-OFF Switching Function. Chem. – A Eur. J. **2016**, *22* (8), 2682–2690. https://doi.org/10.1002/CHEM.201503247.

(129) Shigeta, Y.; Kobayashi, A.; Yoshida, M.; Kato, M. Crystal Engineering of Vapochromic Porous Crystals Composed of Pt(II)-Diimine Luminophores for Vapor-History Sensors. Cryst. Growth Des. **2018**, *18* (6), 3419–3427. https://doi.org/10.1021/acs.cgd.8b00130.

(130) Jin, G. X.; Wang, J.; Liu, J. Y.; Ma, J. P.; Dong, Y. Bin. Visual Recognition and Removal of C2H2 from C2H4/C2H2 Mixtures by a Cul-MOF. Inorg. Chem. **2018**, *57* (11), 6218–6221. https://doi.org/10.1021/ACS.INORGCHEM.8B00971/SUPPL_FILE/IC8B00971 SI_001.PDF.

(131) Li, P.; Sui, Q.; Guo, M. Y.; Yang, S. L.; Bu, R.; Gao, E. Q. Selective Chemochromic and Chemically-Induced Photochromic Response of a Metal–Organic Framework. Chem. Commun. **2020**, *56* (44), 5929–5932. https://doi.org/10.1039/D0CC01627F.

(132) Li, L.; Hua, Y.; Guo, Y.; Wang, H. Y.; Li, X. N.; Zhang, H. Bifunctional Photo- and Vapochromic Behaviors of a Novel Porous Zwitterionic Metal–Organic Framework. New J. Chem. **2019**, *43* (8), 3428–3431. https://doi.org/10.1039/C8NJ04905J.

(133) Guo, M. Y.; Li, P.; Yang, S. L.; Bu, R.; Piao, X. Q.; Gao, E. Q. Distinct and Selective Amine- And Anion-Responsive Behaviors of an Electron-Deficient and Anion-Exchangeable Metal–organic Framework. ACS Appl. Mater. Interfaces **2020**, *12* (39), 43958–43966. https://doi.org/10.1021/ACSAMI.0C14648/SUPPL_FILE/AM0C14648 SI_002.CIF.

(134) Feng, Y.; Wang, Y.; Ying, Y. Structural Design of Metal–Organic Frameworks with Tunable Colorimetric Responses for Visual Sensing Applications. Coord. Chem. Rev. **2021**, *446*, 214102. https://doi.org/10.1016/J.CCR.2021.214102.

(135) Yang, X. D.; Zhu, R.; Yin, J. P.; Sun, L.; Guo, R. Y.; Zhang, J. Bipyridinium-Bearing Multi-Stimuli Responsive Chromic Material with High Stability. Cryst. Growth Des. **2018**, *18* (5), 3236–3243. https://doi.org/10.1021/ACS.CGD.8B00390/SUPPL_FILE/CG8B00390 SI_001.PDF.

(136) Hu, S.; Zhang, J.; Chen, S.; Dai, J.; Fu, Z. Efficient Ultraviolet Light Detector Based on a Crystalline Viologen-Based Metal–Organic Framework with Rapid Visible Color Change under Irradiation. **2017**. https://doi.org/10.1021/acsami.7b13367.

(137) Zhang, Y.; Gutiérrezgutiérrez, M.; Chaudhari, A. K.; Tan, J.-C. Dye-Encapsulated Zeolitic Imidazolate Framework (ZIF-71) for Fluorochromic Sensing of Pressure, Temperature, and Volatile Solvents. ACS Appl. Mater. Interfaces **2020**, *12*, 37488. https://doi.org/10.1021/acsami.0c10257.

(138) Liu, X. Y.; Xing, K.; Li, Y.; Tsung, C. K.; Li, J. Three Models to Encapsulate Multicomponent Dyes into Nanocrystal Pores: A New Strategy for Generating High-Quality White Light. J. Am. Chem. Soc. **2019**, *141* (37), 14807–14813. https://doi.org/10.1021/JACS.9B07236/ASSET/IMAGES/LARGE/JA9B07236_0004.JPEG.

(139) Schwartz, H. A.; Olthof, S.; Schaniel, D.; Meerholz, K.; Ruschewitz, U. Solution-Like Behavior of Photoswitchable Spiropyrans Embedded in Metal–Organic Frameworks. *Inorg. Chem.* **2017**, *56* (21), 13100–13110. https://doi.org/10.1021/ACS.INORGCHEM.7B01908/ASSET/IMAGES/LARGE/IC-2017-019085_0008.JPEG.

(140) Gao, Y.; Hilbers, M.; Zhang, H.; Tanase, S. Designed Synthesis of Multiluminescent Materials Using Lanthanide Metal–Organic
Framework and Carbon Dots as Building-Blocks. *Eur. J. Inorg. Chem.* **2019**, *2019* (35), 3925–3932.
https://doi.org/10.1002/EJIC.201900876.

(141) Gutiérrez, M.; Zhang, Y.; Tan, J.-C. Confinement of Luminescent Guests in Metal–Organic Frameworks: Understanding Pathways from Synthesis and Multimodal Characterization to Potential Applications of LG@MOF Systems. *Chem. Rev.* **2022**, acs.chemrev.1c00980.
https://doi.org/10.1021/ACS.CHEMREV.1C00980.

(142) Su, J.; Yuan, S.; Wang, T.; Lollar, C. T.; Zuo, J. L.; Zhang, J.; Zhou, H. C. Zirconium Metal–Organic Frameworks Incorporating Tetrathiafulvalene Linkers: Robust and Redox-Active Matrices for in Situ Confinement of Metal Nanoparticles. *Chem. Sci.* **2020**, *11* (7), 1918–1925. https://doi.org/10.1039/C9SC06009J.

(143) De Koning, M. C.; Peterson, G. W.; Van Grol, M.; Iordanov, I.; McEntee, M. Degradation and Detection of the Nerve Agent VX by a Chromophore-Functionalized Zirconium MOF. *Chem. Mater.* **2019**, *31* (18), 7417–7424.
https://doi.org/10.1021/ACS.CHEMMATER.9B02073/ASSET/IMAGES/LARGE/CM9B02073_0007.JPEG.

(144) De Koning, M. C.; Ma, K.; Van Grol, M.; Iordanov, I.; Kruijne, M. J. L.; Idrees, K. B.; Xie, H.; Islamoglu, T.; Bross, R. P. T.; Farha, O. K. Development of a Metal-Organic Framework/Textile Composite for the Rapid Degradation and Sensitive Detection of the Nerve Agent VX. *Chem. Mater.* **2022**, *34* (3), 1269–1277. https://doi.org/10.1021/ACS.CHEMMATER.1C03895/SUPPL_FILE/CM1C03895_SI_007.CIF.

(145) Gao, Y.; Qi, Y.; Zhao, K.; Wen, Q.; Shen, J.; Qiu, L.; Mou, W. An Optical Sensing Platform for the Dual Channel Detection of Picric Acid: The Combination of Rhodamine and Metal-Organic Frameworks. *Sensors Actuators B Chem.* **2018**, *257*, 553–560.
https://doi.org/10.1016/J.SNB.2017.11.007.

(146) Shu, Y.; Ye, Q.; Dai, T.; Xu, Q.; Hu, X. Encapsulation of Luminescent Guests to Construct Luminescent Metal-Organic Frameworks for Chemical Sensing. *ACS Sensors* **2021**, *6* (3), 641–658.
https://doi.org/10.1021/ACSSENSORS.0C02562/ASSET/IMAGES/ACSSENSORS.0C02562.SOCIAL.JPEG_V03.

(147) Zhao, T.; Boldog, I.; Spasojevic, V.; Rotaru, A.; Garcia, Y.; Janiak, C. Solvent-Triggered Relaxative Spin State Switching of [Fe(HB(Pz)3)2] in a Closed Nano-Confinement of NH2-MIL-101(Al). *J. Mater. Chem. C* **2016**, *4* (27), 6588–6601. https://doi.org/10.1039/c6tc01297c.

(148) Tissot, A.; Kesse, X.; Giannopoulou, S.; Stenger, I.; Binet, L.; Rivière, E.; Serre, C., A Spin Crossover Porous Hybrid Architecture for Potential Sensing Applications *Chem. Commun* **2019**, *55*, 194–197. https://doi.org/10.1039/c8cc07573e.

(149) Cui, Y.; Zhang, J.; He, H.; Qian, G. Photonic Functional Metal-Organic Frameworks. *Chemical Society Reviews*. 2018, pp 5740–5785.
https://doi.org/10.1039/c7cs00879a.

(150) Wang, J.-X.; Yin, J.; Shekhah, O.; Bakr, O. M.; Eddaoudi, M.; Mohammed, O. F. Energy Transfer in Metal–Organic Frameworks for Fluorescence Sensing. *ACS Appl. Mater. Interfaces* **2022**, *14* (8), 9970–9986. https://doi.org/10.1021/ACSAMI.1C24759.

(151) Lustig, W. P.; Mukherjee, S.; Rudd, N. D.; Desai, A. V.; Li, J.; Ghosh, S. K. Metal–Organic Frameworks: Functional Luminescent and Photonic Materials for Sensing Applications. *Chem. Soc. Rev.* **2017**, *46* (11), 3242–3285. https://doi.org/10.1039/C6CS00930A.

(152) Yang, G. L.; Jiang, X. L.; Xu, H.; Zhao, B. Applications of MOFs as Luminescent Sensors for Environmental Pollutants. *Small* **2021**, *17* (22), 1–19. https://doi.org/10.1002/smll.202005327.

(153) Yan, B. Luminescence Response Mode and Chemical Sensing Mechanism for Lanthanide-Functionalized Metal–Organic Framework Hybrids. *Inorg. Chem. Front.* **2021**, *8* (1), 201–233. https://doi.org/10.1039/D0QI01153C.

(154) Leith, G. A.; Martin, C. R.; Mayers, J. M.; Kittikhunnatham, P.; Larsen, R. W.; Shustova, N. B. Confinement-Guided Photophysics in MOFs, COFs, and Cages. *Chem. Soc. Rev.* **2021**, *50* (7), 4382–4410. https://doi.org/10.1039/D0CS01519A.

(155) Haldar, R.; Heinke, L.; Wöll, C.; Haldar, R.; Heinke, L.; Wöll, C. Advanced Photoresponsive Materials Using the Metal–Organic Framework Approach. *Adv. Mater.* **2020**, *32* (20), 1905227. https://doi.org/10.1002/ADMA.201905227.

(156) Lin, R.-B.; Liu, S.-Y.; Ye, J.-W.; Li, X.-Y.; Zhang, J.-P.; Lin, R.-B.; Liu, S.-Y.; Ye, J.-W.; Li, X.-Y.; Zhang, J.-P. Photoluminescent Metal–Organic Frameworks for Gas Sensing. *Adv. Sci.* **2016**, *3* (7), 1500434. https://doi.org/10.1002/ADVS.201500434.

(157) Khatua, S.; Krishnaraj, C.; Baruah, D. C.; Van Der Voort, P.; Jena, H. S. Flexible Luminescent Non-Lanthanide Metal–Organic Frameworks as Small Molecules Sensors. *Dalt. Trans.* **2021**, *50* (41), 14513–14531. https://doi.org/10.1039/D1DT03175A.

(158) Wang, H.; Lustig, W. P.; Li, J. Sensing and Capture of Toxic and Hazardous Gases and Vapors by Metal–Organic Frameworks. *Chem. Soc. Rev.* **2018**, *47* (13), 4729–4756. https://doi.org/10.1039/C7CS00885F.

(159) Rasheed, T.; Nabeel, F. Luminescent Metal-Organic Frameworks as Potential Sensory Materials for Various Environmental Toxic Agents. *Coord. Chem. Rev.* **2019**, *401*. https://doi.org/10.1016/j.ccr.2019.213065.

(160) Zhang, Y.; Yuan, S.; Day, G.; Wang, X.; Yang, X.; Zhou, H.-C.; Zhou, H.-C. Luminescent Sensors Based on Metal-Organic Frameworks. *Coord. Chem. Rev.* **2018**, *354*, 28–45. https://doi.org/10.1016/j.ccr.2017.06.007.

(161) Bunch, B. H.; Hellemans, A. *The History of Science and Technology: A Browser's Guide to the Great Discoveries, Inventions, And; Scientific Publishing, 2004*.

(162) Zhu, C.; Gerald, R. E.; Huang, J. Metal-Organic Framework Materials Coupled to Optical Fibers for Chemical Sensing: A Review. *IEEE Sens. J.* **2021**, *21* (18), 19647–19661. https://doi.org/10.1109/JSEN.2021.3094092.

(163) Lu, G.; Hupp, J. T. Metal-Organic Frameworks as Sensors: A ZIF-8 Based Fabry-Pérot Device as a Selective Sensor for Chemical Vapors and Gases. *J. Am. Chem. Soc.* **2010**, *132* (23), 7832–7833. https://doi.org/10.1021/JA101415B/SUPPL_FILE/JA101415B SI 002.PDF.
(164) Zhang, R.; Zhang, D.; Yao, Y.; Zhang, Q.; Xu, Y.; Wu, Y.; Yu, H.; Lu, G. Metal–Organic Framework Crystal-Assembled Optical Sensors for Chemical Vapors: Effects of Crystal Sizes and Missing-Linker Defects on Sensing Performances. *ACS Appl. Mater. Interfaces* **2019**, *11*, 28. https://doi.org/10.1021/acsami.9b05933.

(165) Lu, G.; Farha, O. K.; Kreno, L. E.; Schoenecker, P. M.; Walton, K. S.; Van Duyne, R. P.; Hupp, J. T. Fabrication of Metal-Organic Framework-Containing Silica-Colloidal Crystals for Vapor Sensing. *Adv. Mater.* **2011**, *23* (38), 4449–4452.
https://doi.org/10.1002/ADMA.201102116.

(166) Li, L.; Jiao, X.; Chen, D.; Lotsch, B. V.; Li, C. Facile Fabrication of Ultrathin Metal-Organic Framework-Coated Monolayer Colloidal Crystals for Highly Efficient Vapor Sensing. *Chem. Mater.* **2015**, *27* (22), 7601–7609.
https://doi.org/10.1021/ACS.CHEMMATER.5B02476/ASSET/IMAGES/LARGE/CM-2015-02476J_0007.JPEG.

(167) Wang, Z.; Zhang, J.; Xie, J.; Li, C.; Li, Y.; Liang, S.; Tian, Z.; Wang, T.; Zhang, H.; Li, H.; Xu, W.; Yang, B. Bioinspired Water-Vapor-Responsive Organic/Inorganic Hybrid One-Dimensional Photonic Crystals with Tunable Full-Color Stop Band. *Adv. Funct. Mater.* **2010**, *20* (21), 3784–3790. https://doi.org/10.1002/ADFM.201001195.

(168) Kou, D.; Ma, W.; Zhang, S.; Li, R.; Zhang, Y. BTEX Vapor Detection with a Flexible MOF and Functional Polymer by Means of a Composite Photonic Crystal. *ACS Appl. Mater. Interfaces* **2020**, *12* (10), 11955–11964.
https://doi.org/10.1021/ACSAMI.9B22033/SUPPL_FILE/AM9B22033 SI 001.PDF.

(169) Hinterholzinger, F. M.; Ranft, A.; Feckl, J. M.; Rühle, B.; Bein, T.; Lotsch, B. V. One-Dimensional Metal–Organic Framework Photonic Crystals Used as Platforms for Vapor Sorption. *J. Mater. Chem.* **2012**, *22* (20), 10356–10362. https://doi.org/10.1039/C2JM15685G.

(170) Hu, Z.; Tao, C. A.; Wang, F.; Zou, X.; Wang, J. Flexible Metal–Organic Framework-Based One-Dimensional Photonic Crystals. *J. Mater. Chem. C* **2014**, *3* (1), 211–216. https://doi.org/10.1039/C4TC01501K.

(171) Lu, G.; Farha, O. K.; Zhang, W.; Huo, F.; Hupp, J. T. Engineering ZIF-8 Thin Films for Hybrid MOF-Based Devices. *Adv. Mater.* **2012**, *24* (29), 3970–3974. https://doi.org/10.1002/ADMA.201202116.

(172) Wu, Y.-N.; Li, F.; Zhu, W.; Cui, J.; Tao, C.-A.; Lin, C.; Hannam, P. M.; Li, G.; Wu, Y.; Li, F.; Hannam, P. M.; Wu, Y.; Zhu, W.; Cui, J.; Tao, C.; Lin, C.; Li, G. Metal–Organic Frameworks with a Three-Dimensional Ordered Macroporous Structure: Dynamic Photonic Materials. *Angew. Chemie Int. Ed.* **2011**, *50* (52), 12518–12522. https://doi.org/10.1002/ANIE.201104597.

(173) Wang, Z.; Zhan, K.; Zhu, Y.; Yan, J.; Liu, B.; Chen, Y. Label-Free One-Dimension Photonics Crystals Sensors Assembled by UiO-66 and Graphene Oxide: A Platform to Quick and Efficiently Detect Chlorobenzene Vapors. *J. Environ. Chem. Eng.* **2021**, *9* (4), 105445.
https://doi.org/10.1016/J.JECE.2021.105445.

(174) Xing, Y.; Wang, Z.; Zhu, Y.; Yan, J.; Chen, Y. High-Performance Chlorobenzene Sensor of Bimetallic ZIF-8(Zn@Co) Based One-Dimensional Photonic Crystals. *Phys. Lett. A* **2021**, *400*, 127301. https://doi.org/10.1016/J.PHYSLETA.2021.127301.

(175) Bai, L.; He, Y.; Zhou, J.; Lim, Y.; Mai, V. C.; Chen, Y.; Hou, S.; Zhao, Y.; Zhang, J.; Duan, H. Responsive Amorphous Photonic Structures of Spherical/Polyhedral Colloidal Metal–Organic Frameworks. *Adv. Opt. Mater.* **2019**, *7* (13), 1–8.
https://doi.org/10.1002/adom.201900522.

(176) Kim, K. J.; Lu, P.; Culp, J. T.; Ohodnicki, P. R. Metal-Organic Framework Thin Film Coated Optical Fiber Sensors: A Novel Waveguide-Based Chemical Sensing Platform. *ACS Sensors* **2018**, *3* (2), 386–394.
https://doi.org/10.1021/ACSSENSORS.7B00808/ASSET/IMAGES/LARGE/SE-2017-008088_0005.JPEG.

(177) Miliutina, E.; Guselnikova, O.; Chufistova, S.; Kolska, Z.; Elashnikov, R.; Burtsev, V.; Postnikov, P.; Svorcik, V.; Lyutakov, O. Fast and All-Optical Hydrogen Sensor Based on Gold-Coated Optical Fiber Functionalized with Metal-Organic Framework Layer. *ACS Sensors* **2019**, *4* (12), 3133–3140. https://doi.org/10.1021/ACSSENSORS.9B01074/ASSET/IMAGES/LARGE/SE9B01074_0002.JPEG.

(178) Kim, K. J.; Culp, J. T.; Ohodnicki, P. R.; Cvetic, P. C.; Sanguinito, S.; Goodman, A. L.; Kwon, H. T. Alkylamine-Integrated Metal-Organic Framework-Based Waveguide Sensors for Efficient Detection of Carbon Dioxide from Humid Gas Streams. *ACS Appl. Mater. Interfaces* **2019**, *11* (36), 33489–33496. https://doi.org/10.1021/ACSAMI.9B12052/ASSET/IMAGES/LARGE/AM9B12052_0005.JPEG.

(179) Kim, H.-T.; Hwang, W.; Liu, Y.; Yu, A. M. Ultracompact Gas Sensor with Metal-Organic-Framework-Based Differential Fiber-Optic Fabry-Perot Nanocavities. *Opt. Express*, *Vol. 28*, *Issue 20*, pp. 29937-29947 **2020**, *28* (20), 29937–29947. https://doi.org/10.1364/OE.396146.

(180) Zhu, C.; Perman, J. A.; Gerald, R. E.; Ma, S.; Huang, J. Chemical Detection Using a Metal-Organic Framework Single Crystal Coupled to an Optical Fiber. *ACS Appl. Mater. Interfaces* **2019**, *11* (4), 4393–4398.
https://doi.org/10.1021/ACSAMI.8B19775/ASSET/IMAGES/LARGE/AM-2018-19775A_0002.JPEG.

(181) Cao, R.; Ding, H.; Kim, K. J.; Peng, Z.; Wu, J.; Culp, J. T.; Ohodnicki, P. R.; Beckman, E.; Chen, K. P. Metal-Organic Framework Functionalized Polymer Coating for Fiber Optical Methane Sensors. *Sensors Actuators B Chem.* **2020**, *324*, 128627.
https://doi.org/10.1016/J.SNB.2020.128627.

(182) Martan, T.; Aubrecht, J.; Podrazký, O.; Matějec, V.; Kašík, I. Detection of Hydrocarbons Using Suspended Core Microstructured Optical Fiber. *Sensors Actuators B Chem.* **2014**, *202*, 123–128. https://doi.org/10.1016/J.SNB.2014.04.103.

(183) Wu, J.; Tang, C.; Zhang, W.; Ma, X.; Qu, S.; Chen, K.; Hao, T.; Chiang, K. S. Lab on Optical Fiber: Surface Nano-Functionalization for Real-Time Monitoring of VOC Adsorption/Desorption in Metal-Organic Frameworks. *Nanophotonics* **2021**, *10* (10), 2705–2716.
https://doi.org/10.1515/NANOPH-2021-0192/ASSET/GRAPHIC/J_NANOPH-2021-0192_FIG_009.JPEG.
(184) Hromadka, J.; Tokay, B.; James, S.; Tatam, R. P.; Korposh, S. Optical Fibre Long Period Grating Gas Sensor Modified with Metal Organic Framework Thin Films. *Sensors Actuators B Chem.* **2015**, *221*, 891–899. https://doi.org/10.1016/J.SNB.2015.07.027.

(185) Hromadka, J.; Tokay, B.; Correia, R.; Morgan, S. P.; Korposh, S. Highly Sensitive Volatile Organic Compounds Vapour Measurements Using a Long Period Grating Optical Fibre Sensor Coated with Metal Organic Framework ZIF-8. *Sensors Actuators B Chem.* **2018**, *260*, 685–692. https://doi.org/10.1016/J.SNB.2018.01.015.

(186) Kreno, L. E.; Hupp, J. T.; Van Duyne, R. P. Metal-Organic Framework Thin Film for Enhanced Localized Surface Plasmon Resonance Gas Sensing. *Anal. Chem.* **2010**, *82* (19), 8042–8046. https://doi.org/10.1021/AC102127P/SUPPL_FILE/AC102127P_SI_001.PDF.

(187) Koh, C. S. L.; Sim, H. Y. F.; Leong, S. X.; Boong, S. K.; Chong, C.; Ling, X. Y. Plasmonic Nanoparticle-Metal-Organic Framework (NP-MOF) Nanohybrid Platforms for Emerging Plasmonic Applications. *ACS Mater. Lett.* **2021**, *3* (5), 557–573.
https://doi.org/10.1021/acsmaterialslett.1c00047.

(188) Huang, C.; Li, A.; Chen, X.; Wang, T.; Huang, C. H.; Li, A. L.; Chen, X. Y.; Wang, T. Understanding the Role of Metal–Organic Frameworks in Surface-Enhanced Raman Scattering Application. *Small* **2020**, *16* (43), 2004802. https://doi.org/10.1002/SMLL.202004802.

(189) Lai, H.; Li, G.; Xu, F.; Zhang, Z. Metal–Organic Frameworks: Opportunities and Challenges for Surface-Enhanced Raman Scattering – a Review. *J. Mater. Chem. C* **2020**, *8* (9), 2952–2963. https://doi.org/10.1039/D0TC00040J.

(190) Langer, J.; de Aberasturi, D. J.; Aizpurua, J.; Alvarez-Puebla, R. A.; Auguié, B.; Baumberg, J. J.; Bazan, G. C.; Bell, S. E. J.; Boisen, A.; Brolo, A. G.; Choo, J.; Cialla-May, D.; Deckert, V.; Fabris, L.; Faulds, K.; Javier García de Abajo, F.; Goodacre, R.; Graham, D.; Haes, A. J.; Haynes, C. L.; Huck, C.; Itoh, T.; Käll, M.; Kneipp, J.; Kotov, N. A.; Kuang, H.; Le Ru, E. C.; Lee, H. K.; Li, J. F.; Ling, X. Y.; Maier, S. A.; Mayerhöfer, T.; Moskovits, M.; Murakoshi, K.; Nam, J. M.; Nie, S.; Ozaki, Y.; Pastoriza-Santos, I.; Perez-Juste, J.; Popp, J.; Pucci, A.; Reich, S.; Ren, B.; Schatz, G. C.; Shegai, T.; Schlücker, S.; Tay, L. L.; George Thomas, K.; Tian, Z. Q.; van Duyne, R. P.; Vo-Dinh, T.; Wang, Y.; Willets, K. A.; Xu, C.; Xu, H.; Xu, Y.; Yamamoto, Y. S.; Zhao, B.; Liz-Marzán, L. M. Present and Future of Surface-Enhanced Raman Scattering. *ACS Nano* **2020**, *14* (1), 28–117. https://doi.org/10.1021/ACSNANO.9B04224/ASSET/IMAGES/MEDIUM/NN9B04224_0043.GIF.

(191) Osterrieth, J. W. M.; Wright, D.; Noh, H.; Kung, C. W.; Vulpe, D.; Li, A.; Park, J. E.; Van Duyne, R. P.; Moghadam, P. Z.; Baumberg, J. J.; Farha, O. K.; Fairen-Jimenez, D. Core-Shell Gold Nanorod@Zirconium-Based Metal-Organic Framework Composites as in Situ Size-Selective Raman Probes. *J. Am. Chem. Soc.* **2019**, *141* (9), 3893–3900.
https://doi.org/10.1021/JACS.8B11300/SUPPL_FILE/JA8B11300 SI 004.MPG.

(192) Li, D.; Cao, X.; Zhang, Q.; Ren, X.; Jiang, L.; Li, D.; Deng, W.; Liu, H. Facile in Situ Synthesis of Core–Shell MOF@Ag Nanoparticle Composites on Screen-Printed Electrodes for Ultrasensitive SERS Detection of Polycyclic Aromatic Hydrocarbons. *J. Mater. Chem. A* **2019**, *7* (23), 14108–14117. https://doi.org/10.1039/C9TA03690C.

(193) Chen, Q. Q.; Hou, R. N.; Zhu, Y. Z.; Wang, X. T.; Zhang, H.; Zhang, Y. J.; Zhang, L.; Tian, Z. Q.; Li, J. F. Au@ZIF-8 Core-Shell Nanoparticles as a SERS Substrate for Volatile Organic Compound Gas Detection. *Anal. Chem.* **2021**, *93* (19), 7188–7195.
https://doi.org/10.1021/ACS.ANALCHEM.0C05432/ASSET/IMAGES/LARGE/AC0C05432_0007.JPEG.

(194) Yang, X.; Liu, Y.; Lam, S. H.; Wang, J.; Wen, S.; Yam, C.; Shao, L.; Wang, J. Site-Selective Deposition of Metal-Organic Frameworks on Gold Nanobipyramids for Surface-Enhanced Raman Scattering. *Nano Lett.* **2021**, *21* (19), 8205–8212.
https://doi.org/10.1021/ACS.NANOLETT.1C02649/ASSET/IMAGES/LARGE/NL1C02649_0004.JPEG.

(195) Zhou, H.; Hui, X.; Li, D.; Hu, D.; Chen, X.; He, X.; Gao, L.; Huang, H.; Lee, C.; Mu, X. Metal–Organic Framework-Surface-Enhanced Infrared Absorption Platform Enables Simultaneous On-Chip Sensing of Greenhouse Gases. *Adv. Sci.* **2020**, *7* (20), 2001173.
https://doi.org/10.1002/ADVS.202001173.