shown to be suitable for monitoring 50 ppm of NO in He at 443 K.⁵⁷

Another approach involves preparation of sulfur-resistant bimetallic noble metal layers, where Cr and W improve the sulfur resistance of Pd–Y zeolite (FAU-type framework structure). On the other hand, La, Mn, Mo and Ag make the sulfur resistance worse and the second metal had no evident influence on product selectivity and the acidic properties of the catalysts. In all of these cases the metals were introduced by ion exchange, and the metal clusters tend to agglomerate with time and migrate to the surface of the sensing layers.¹⁸⁸

Sensor protection in a destructive thermal and chemical environment was ensured by depositing a porous absorption material around the gas-sensitive section. A zeolite-type material or boehmite was introduced in the sensor substrate with additives and binders and subjected to thermal treatment up to 700 °C. The protective layers were stable at the described working conditions.¹⁸⁹ Protective hydrophilic layers were prepared to prevent water from interfering with the operating sensors. These layers were based on highly hydrophilic materials such as zeolites A, X, and Y.⁶⁸

It is notable that no examples of MOFs for NOₓ sensing could be found in the literature at the time of writing. However, NO adsorption in MOFs has been reported by Morris and co-workers for biological applications,¹⁹⁰⁻¹⁹³ suggesting MOFs that possess suitable stability for automotive exhaust gas sensing hold promise as potential sensing materials for NO.

## 3.4 CO and CO₂ sensors

### 3.4.1 Zeolites for CO/CO₂ sensing.
The detection of CO and CO₂ is a major target for vehicular emissions sensing. CO adsorption is usually assessed by conductivity measurements, either from metal oxides encapsulated within the zeolite or in a polymer–zeolite composite. For CO₂, examples include polymer–zeolite composites or zeolites directly grown or deposited on the sensing element. One of the first examples of a metal oxide in a zeolite for sensing was the combination of a sensing layer of La₂O₃-Au/SnO₂ ceramic and a layer of siliceous ferrierite-type zeolite which acted as an ethanol catalytic filter.⁴⁶ Indeed, the acidic nature of the zeolite converted ethanol into ethane to reduce the sensitivity of the ceramic towards ethanol and thus improve sensitivity to CO. The ferrierite coating afforded a ten times enhanced sensitivity towards CO at 300 °C. Semiconducting tungsten oxide and chromium titanium oxide covered with zeolite have also been found to be sensitive to CO, but have been reported as more efficient materials for NO₂ and ethanol sensing, respectively, as described herein.⁶⁰,⁶¹

With respect to conductive polymer and zeolites for CO sensing, two composites have been investigated. Firstly, a polyaniline/zeolite A-type zeolite sensor has been developed which exhibited high selectivity towards CO, especially with the Ca²⁺ form of the zeolite.⁶² Indeed, Ca-zeolite A possesses larger pores than the K⁺ and Na⁺ analogues and thus does not greatly influence the diffusion of CO through the conductive polymer.⁶⁹ The second example reported the utilisation of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid and ZSM-5.⁷⁵ The sensitivity of the

composite increased when the Si/Al mole ratio of the zeolite decreased. However, this composite only produced irreversible responses.

Under a CO reductive atmosphere, the reversible redox behaviour of TiO₂ clusters within Y-type zeolite pores was followed by *in situ* UV-vis spectroscopy providing an efficient CO detector.¹¹⁴ The zeolite matrix permitted good stability of the clusters towards several oxidation–reduction cycles. In comparison with bulk TiO₂, the TiO₂–Y composite showed 10 times faster response times, of about 1 s at 800 °C.

As an alternative to conductivity measurements, a calorimetric sensor has been developed that measures the heat liberated during the catalytic reaction occurring in Pd-doped zeolites.⁶³ Combined with a thin-film calorimeter, Pd-zeolite A exhibited a selective response towards CO over alkanes such as *n*- and i-butane, cyclo- and *n*-hexane.

Recently, Pd-containing perllalite and beta polymorph A zeolite films showed high molecular recognition towards CO through *in situ* IR spectroscopy.⁹⁴ Fast detection of 2 to 100 ppm concentration of CO in the presence of highly concentrated vapours of methanol or pentane (400–4000 ppm) has been demonstrated. Similar results were obtained using a Pt-containing beta polymorph A zeolite film for CO detection in the presence of a highly concentrated methanol atmosphere.⁹⁵

A convenient gas sensor combining a solid electrolyte NASICON, lithium carbonate and a NaY-type zeolite as filter has been reported for air quality control through CO₂ sensing.¹¹⁵ The fabricated sensor was found to be stable over two years and could follow the CO₂ concentration *via* impedance measurements. For example, when CO₂ exposure increased from 350 to 1000 ppm, the response time was about 2 min. More recently, the NASICON was replaced with sodium conductive ZSM-5 zeolite which provided higher selectivity towards CO₂.⁸⁸ The zeolite layer works as both a catalyst and filter due to its sodium cation conductive activity and its well-defined small pore sizes. Conductive-based CO₂ sensing has also been demonstrated using a polymer poly(aniline)–zeolite Y composite.¹⁹⁴ This sensor showed good ability for CO₂ sensing at room temperature.

A composite made of a zeolite Y film grown on a Metglas magnetoelastic strip has also been developed for CO₂ sensing in a N₂ atmosphere.¹⁹⁵ Indeed, when an alternating current magnetic field is applied, the specific resonant frequency showed a discernable decrease down to a concentration of 0.33% CO₂ in N₂. However, a dry atmosphere is required to avoid interference with water as 2% relative humidity affected measurements. A similar device was subsequently reported using silicalite-1 thus permitting the sensing of CO₂ in air instead of a N₂ atmosphere.⁷⁶

Microcalorimetric devices can now be made using lithographic techniques. One of the two sensitive areas of such a device (where evolved heat can be measured) was coated with a thin film of CoAlPO₄-5, the other was kept open as a reference.¹³⁰ The additional benefit of a zeolite with catalytic activity for such a device is the molecular sieving effect that can be combined in the response of the sensor (a molecule too big to enter the catalytically active interior of the zeolite should only show a weak response). The change in temperature was measured with

<figure><img src="image_42.png" /></figure>
shown to be suitable for monitoring 50 ppm of NO in He at 443 K.⁵⁷

Another approach involves preparation of sulfur-resistant bimetallic noble metal layers, where Cr and W improve the sulfur resistance of Pd–Y zeolite (FAU-type framework structure). On the other hand, La, Mn, Mo and Ag make the sulfur resistance worse and the second metal had no evident influence on product selectivity and the acidic properties of the catalysts. In all of these cases the metals were introduced by ion exchange, and the metal clusters tend to agglomerate with time and migrate to the surface of the sensing layers.¹⁸⁸

Sensor protection in a destructive thermal and chemical environment was ensured by depositing a porous absorption material around the gas-sensitive section. A zeolite-type material or boehmite was introduced in the sensor substrate with additives and binders and subjected to thermal treatment up to 700 °C. The protective layers were stable at the described working conditions.¹⁸⁹ Protective hydrophilic layers were prepared to prevent water from interfering with the operating sensors. These layers were based on highly hydrophilic materials such as zeolites A, X, and Y.⁶⁸

It is notable that no examples of MOFs for NOₓ sensing could be found in the literature at the time of writing. However, NO adsorption in MOFs has been reported by Morris and co-workers for biological applications,¹⁹⁰⁻¹⁹³ suggesting MOFs that possess suitable stability for automotive exhaust gas sensing hold promise as potential sensing materials for NO.

## 3.4 CO and CO₂ sensors

### 3.4.1 Zeolites for CO/CO₂ sensing.
The detection of CO and CO₂ is a major target for vehicular emissions sensing. CO adsorption is usually assessed by conductivity measurements, either from metal oxides encapsulated within the zeolite or in a polymer-zeolite composite. For CO₂, examples include polymer-zeolite composites or zeolites directly grown or deposited on the sensing element. One of the first examples of a metal oxide in a zeolite for sensing was the combination of a sensing layer of La₂O₃-Au/SnO₂ ceramic and a layer of siliceous ferrierite-type zeolite which acted as an ethanol catalytic filter.⁴⁶ Indeed, the acidic nature of the zeolite converted ethanol into ethane to reduce the sensitivity of the ceramic towards ethanol and thus improve sensitivity to CO. The ferrierite coating afforded a ten times enhanced sensitivity towards CO at 300 °C. Semiconducting tungsten oxide and chromium titanium oxide covered with zeolite have also been found to be sensitive to CO, but have been reported as more efficient materials for NO₂ and ethanol sensing, respectively, as described herein.⁶⁰,⁶¹

With respect to conductive polymer and zeolites for CO sensing, two composites have been investigated. Firstly, a polyaniline/zeolite A-type zeolite sensor has been developed which exhibited high selectivity towards CO, especially with the Ca²⁺ form of the zeolite.⁶² Indeed, Ca-zeolite A possesses larger pores than the K⁺ and Na⁺ analogues and thus does not greatly influence the diffusion of CO through the conductive polymer.⁶⁹ The second example reported the utilisation of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid and ZSM-5.⁷⁵ The sensitivity of the

composite increased when the Si/Al mole ratio of the zeolite decreased. However, this composite only produced irreversible responses.

Under a CO reductive atmosphere, the reversible redox behaviour of TiO₂ clusters within Y-type zeolite pores was followed by *in situ* UV-vis spectroscopy providing an efficient CO detector.¹¹⁴ The zeolite matrix permitted good stability of the clusters towards several oxidation-reduction cycles. In comparison with bulk TiO₂, the TiO₂-Y composite showed 10 times faster response times, of about 1 s at 800 °C.

As an alternative to conductivity measurements, a calorimetric sensor has been developed that measures the heat liberated during the catalytic reaction occurring in Pd-doped zeolites.⁶³ Combined with a thin-film calorimeter, Pd-zeolite A exhibited a selective response towards CO over alkanes such as *n*- and i-butane, cyclo- and *n*-hexane.

Recently, Pd-containing perllalite and beta polymorph A zeolite films showed high molecular recognition towards CO through *in situ* IR spectroscopy.⁹⁴ Fast detection of 2 to 100 ppm concentration of CO in the presence of highly concentrated vapours of methanol or pentane (400–4000 ppm) has been demonstrated. Similar results were obtained using a Pt-containing beta polymorph A zeolite film for CO detection in the presence of a highly concentrated methanol atmosphere.⁹⁵

A convenient gas sensor combining a solid electrolyte NASICON, lithium carbonate and a NaY-type zeolite as filter has been reported for air quality control through CO₂ sensing.¹¹⁵ The fabricated sensor was found to be stable over two years and could follow the CO₂ concentration *via* impedance measurements. For example, when CO₂ exposure increased from 350 to 1000 ppm, the response time was about 2 min. More recently, the NASICON was replaced with sodium conductive ZSM-5 zeolite which provided higher selectivity towards CO₂.⁸⁸ The zeolite layer works as both a catalyst and filter due to its sodium cation conductive activity and its well-defined small pore sizes. Conductive-based CO₂ sensing has also been demonstrated using a polymer poly(aniline)-zeolite Y composite.¹⁹⁴ This sensor showed good ability for CO₂ sensing at room temperature.

A composite made of a zeolite Y film grown on a Metglas magnetoelastic strip has also been developed for CO₂ sensing in a N₂ atmosphere.¹⁹⁵ Indeed, when an alternating current magnetic field is applied, the specific resonant frequency showed a discernable decrease down to a concentration of 0.33% CO₂ in N₂. However, a dry atmosphere is required to avoid interference with water as 2% relative humidity affected measurements. A similar device was subsequently reported using silicalite-1 thus permitting the sensing of CO₂ in air instead of a N₂ atmosphere.⁷⁶

Microcalorimetric devices can now be made using lithographic techniques. One of the two sensitive areas of such a device (where evolved heat can be measured) was coated with a thin film of CoAlPO₄-5, the other was kept open as a reference.¹³⁰ The additional benefit of a zeolite with catalytic activity for such a device is the molecular sieving effect that can be combined in the response of the sensor (a molecule too big to enter the catalytically active interior of the zeolite should only show a weak response). The change in temperature was measured with

<figure><img src="image_44.png" /></figure>
<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /><figcaption>**Fig. 14** Schematic of the layout of the *in situ* IR reactor cell and IR *operando* gas detection experimental set-up used by Mintova *et al.* for detection of carbon monoxide.¹⁵⁹ Reprinted with permission from X. Zou, J.-M. Goupil, S. Thomas, F. Zhang, G. Zhu, V. Valtchev and S. Mintova, *J. Phys. Chem. C*, 2012, **116**, 16593–16600. Copyright 2012 American Chemical Society.</figcaption></figure>

a meandering Pt-wire resistor. This device was examined for the detection of CO and cyclohexane, and sensitivity and selectivity in the low ppm range was observed.

### 3.4.2 MOFs for CO/CO₂ sensing.
Mintova *et al.* described a Cu-Y(btc) MOF that was selective to carbon monoxide over water and some hydrocarbons.¹⁵⁹ The yttrium-based MOF [Y(btc)], JUC-32-Y, was doped with copper clusters to afford sensitivity to carbon monoxide. Before sensing of gaseous analytes, nanocrystals of Cu-Y(btc) were deposited onto silicon wafer substrates using a dip coating technique. Before detection of carbon monoxide, the thin films were treated *in situ* within the IR spectrometer gas detection set-up (outlined in Fig. 14) with 5% hydrogen in argon at 300 °C for 9 h to reduce the Cu²⁺ ions to Cu⁰ and some Cu⁺ₙ species, which was monitored with UV-visible spectroscopy and confirmed by X-ray diffraction.

The IR spectral changes of the thin films of Cu-Y(btc) due to exposure to a range of concentrations of CO (1–100 ppm) were monitored *in operando* at room temperature. The adsorption of CO on the Cu⁰ metal clusters resulted in the appearance of an IR absorption peak centred at 2114 cm⁻¹; the area of this peak could be quantified and related to the concentration of CO present in the Ar carrier gas. Remarkably a concentration as low as 1 ppm of CO in Ar can be detected by measuring the change in absorption at 2114 cm⁻¹ and the detection was reversible, albeit requiring a purge of Ar gas for 4 h at room temperature. Additionally the MOF is thermally stable up to ~500 °C and selectively detected CO over water, which was always present at a concentration of ~100 ppm in the Ar carrier gas, and to vapours of chloroform and 2-ethylthiophene that were deliberately introduced. The MOF also showed reproducibility with no diminished response after 10 sensing cycles.

The piezoelectric microcantilever humidity sensor, modified with HKUST-1, demonstrated by Hesketh *et al.* (Section 3.1.2) also displayed sensitivity to carbon dioxide.¹⁴⁰,¹⁴² The authors

tested the response of the device to CO₂ in nitrogen carrier gas at room temperature and pressure with HKUST-1 in the dehydrated state. The sensor response was non-linear within the range of water vapour concentrations investigated, ~10%–~70% concentration. However, when HKUST-1 was in the hydrated state no response to carbon dioxide was observed. The authors postulated that the removal of water molecules coordinated to the copper centres was required for carbon dioxide to bind.

The refractometric optical-based detector which utilised HKUST-1 as the sensing material, fabricated by Hupp *et al.* (Section 3.1.2), displayed limited sensitivity to carbon dioxide.¹³⁹ Upon exposure to carbon dioxide the measured ‘stop band’ redshift was ~3 nm relative to N₂. The device also displayed sensitivity to other analytes other than water vapour, thus isolation of carbon dioxide from other exhaust gases and vapours would be required.

Candler *et al.* utilised zeolitic imidazolite framework (ZIF) nanoparticle-couple microresonators to sense carbon dioxide.¹⁶⁰ The silicon microresonators were coated with particles of [Zn(cbIM)(nIM)] (cbIM = 5-chlorobenzimidazol-1-ate, nIM = 2-nitroimidazolate, Fig. 15), ZIF-69, by dielectrophoresis. The resonator structure was then exposed to various concentrations of CO₂ at room temperature and the change in resonant frequency of the microresonator relative to the anchor was measured with a laser Doppler vibrometer set-up. A linear response was observed between the change in relative frequency and concentration of CO₂ in the range 0–4000 ppm. At 5200 ppm of CO₂ it was determined that 72 times more CO₂ was adsorbed onto the ZIF-69 coated microresonator than on the bare silicon microresonator, thus highlighting the value of increasing the surface area for gas adsorption and thus enhancing the sensitivity of gas sensors. The sensor was cross-sensitive to isopropanol vapour, but differentiation was possible when the decay constant of the adsorption times were compared. The lowest mass change that was measureable and thus the lower detection limit of the sensor set-up was calculated to be ~26 fg.

Duyne *et al.* utilised a localised surface plasmon resonance (LSPR) spectroscopic approach to detect the interaction of carbon dioxide with triangular silver nanoparticles that had been coated with HKUST-1.¹⁴³ Upon exposure to CO₂ a redshift in the localised surface plasmon resonance wavelength was measured due to adsorption of CO₂ within the MOF and thus increase of the refractive index of the MOF-coated silver nanoparticles. The measured redshift in the LSPR wavelength for the device coated with HKUST-1 coated silver nanoparticles was 14 times greater than for the device coated with bare silver nanoparticles. The sensor device displayed a dynamic range in the concentration range 0–100% CO₂ in N₂ carrier gas and the sensing responses were reversible and reproducible over 5 cycles.

<figure><img src="image_45.png" /><figcaption>**Fig. 15** The 5-chlorobenzimidazol-1-ate (cbIM) and 2-nitroimidazol-3-ate (nIM) ligands in ZIF-69.</figcaption></figure>
<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /><figcaption>**Fig. 14** Schematic of the layout of the *in situ* IR reactor cell and IR *operando* gas detection experimental set-up used by Mintova *et al.* for detection of carbon monoxide.¹⁵⁹ Reprinted with permission from X. Zou, J.-M. Goupil, S. Thomas, F. Zhang, G. Zhu, V. Valtchev and S. Mintova, *J. Phys. Chem. C*, 2012, **116**, 16593–16600. Copyright 2012 American Chemical Society.</figcaption></figure>

a meandering Pt-wire resistor. This device was examined for the detection of CO and cyclohexane, and sensitivity and selectivity in the low ppm range was observed.

### 3.4.2 MOFs for CO/CO₂ sensing.
Mintova *et al.* described a Cu-Y(btc) MOF that was selective to carbon monoxide over water and some hydrocarbons.¹⁵⁹ The yttrium-based MOF [Y(btc)], JUC-32-Y, was doped with copper clusters to afford sensitivity to carbon monoxide. Before sensing of gaseous analytes, nanocrystals of Cu-Y(btc) were deposited onto silicon wafer substrates using a dip coating technique. Before detection of carbon monoxide, the thin films were treated *in situ* within the IR spectrometer gas detection set-up (outlined in Fig. 14) with 5% hydrogen in argon at 300 °C for 9 h to reduce the Cu²⁺ ions to Cu⁰ and some Cu⁺ₙ species, which was monitored with UV-visible spectroscopy and confirmed by X-ray diffraction.

The IR spectral changes of the thin films of Cu-Y(btc) due to exposure to a range of concentrations of CO (1–100 ppm) were monitored *in operando* at room temperature. The adsorption of CO on the Cu⁰ metal clusters resulted in the appearance of an IR absorption peak centred at 2114 cm⁻¹; the area of this peak could be quantified and related to the concentration of CO present in the Ar carrier gas. Remarkably a concentration as low as 1 ppm of CO in Ar can be detected by measuring the change in absorption at 2114 cm⁻¹ and the detection was reversible, albeit requiring a purge of Ar gas for 4 h at room temperature. Additionally the MOF is thermally stable up to ~500 °C and selectively detected CO over water, which was always present at a concentration of ~100 ppm in the Ar carrier gas, and to vapours of chloroform and 2-ethylthiophene that were deliberately introduced. The MOF also showed reproducibility with no diminished response after 10 sensing cycles.

The piezoelectric microcantilever humidity sensor, modified with HKUST-1, demonstrated by Hesketh *et al.* (Section 3.1.2) also displayed sensitivity to carbon dioxide.¹⁴⁰,¹⁴² The authors

tested the response of the device to CO₂ in nitrogen carrier gas at room temperature and pressure with HKUST-1 in the dehydrated state. The sensor response was non-linear within the range of water vapour concentrations investigated, ~10%–~70% concentration. However, when HKUST-1 was in the hydrated state no response to carbon dioxide was observed. The authors postulated that the removal of water molecules coordinated to the copper centres was required for carbon dioxide to bind.

The refractometric optical-based detector which utilised HKUST-1 as the sensing material, fabricated by Hupp *et al.* (Section 3.1.2), displayed limited sensitivity to carbon dioxide.¹³⁹ Upon exposure to carbon dioxide the measured ‘stop band’ redshift was ~3 nm relative to N₂. The device also displayed sensitivity to other analytes other than water vapour, thus isolation of carbon dioxide from other exhaust gases and vapours would be required.

Candler *et al.* utilised zeolitic imidazolite framework (ZIF) nanoparticle-couple microresonators to sense carbon dioxide.¹⁶⁰ The silicon microresonators were coated with particles of [Zn(cbIM)(nIM)] (cbIM = 5-chlorobenzimidazol-1-ate, nIM = 2-nitroimidazolate, Fig. 15), ZIF-69, by dielectrophoresis. The resonator structure was then exposed to various concentrations of CO₂ at room temperature and the change in resonant frequency of the microresonator relative to the anchor was measured with a laser Doppler vibrometer set-up. A linear response was observed between the change in relative frequency and concentration of CO₂ in the range 0–4000 ppm. At 5200 ppm of CO₂ it was determined that 72 times more CO₂ was adsorbed onto the ZIF-69 coated microresonator than on the bare silicon microresonator, thus highlighting the value of increasing the surface area for gas adsorption and thus enhancing the sensitivity of gas sensors. The sensor was cross-sensitive to isopropanol vapour, but differentiation was possible when the decay constant of the adsorption times were compared. The lowest mass change that was measureable and thus the lower detection limit of the sensor set-up was calculated to be ~26 fg.

Duyne *et al.* utilised a localised surface plasmon resonance (LSPR) spectroscopic approach to detect the interaction of carbon dioxide with triangular silver nanoparticles that had been coated with HKUST-1.¹⁴³ Upon exposure to CO₂ a redshift in the localised surface plasmon resonance wavelength was measured due to adsorption of CO₂ within the MOF and thus increase of the refractive index of the MOF-coated silver nanoparticles. The measured redshift in the LSPR wavelength for the device coated with HKUST-1 coated silver nanoparticles was 14 times greater than for the device coated with bare silver nanoparticles. The sensor device displayed a dynamic range in the concentration range 0–100% CO₂ in N₂ carrier gas and the sensing responses were reversible and reproducible over 5 cycles.

<figure><img src="image_47.png" /><figcaption>**Fig. 15** The 5-chlorobenzimidazol-1-ate (cbIM) and 2-nitroimidazol-3-ate (nIM) ligands in ZIF-69.</figcaption></figure>
<figure><img src="image_46.png" /><figcaption>Fig. 16 (left) N,N'-1,3-phenylenebis(oxamate) tetraanion and (right) methylviologen dication.¹⁶¹</figcaption></figure>

Pardo *et al.* demonstrated a detector for carbon dioxide based on the two-dimensional MOF MV[Mn₂Cu₃(mpba)₃(H₂O)₃]·20H₂O (MV = methylviologen dication and mpba = N,N'-1,3-phenylenebis- (oxamate), Fig. 16), which is both magnetic and luminescent.¹⁶¹ Upon excitation at 400 nm, exposure to 1 bar carbon dioxide or a 1.5 bar 1:1 mixture of carbon dioxide and methane, a luminescent wavelength redshift of 17 nm was measured indicating an interaction of CO₂ but not methane with the MOF. It was shown that the MOF was sensitive to both water and methanol in toluene. The authors predict that differentiation of carbon dioxide from both water and methanol would be possible as methanol and water exposure resulted in an emission wavelength blueshift whereas an emission wavelength redshift was measured upon exposure to carbon dioxide.

## 3.5 Hydrocarbon and volatile organic compound sensors

### 3.5.1 Zeolites for sensing hydrocarbons and volatile organic compounds.
Tracking of unburnt hydrocarbons and volatile organic compounds (VOCs) is important for determining the extent of combustion. The first example of VOCs and hydrocarbon sensing used a silicalite-1 film on a QCM for ethanol sensing. Monitoring the ethanol adsorption showed a lower limit response at 50 ppm.¹⁹⁶ However, since this work in 1992 no further exploitation of the QCM sensor format has been reported. Indeed, the most frequent zeolite-based hydrocarbon sensors rely on conductivity measurements. An early example of Pt-doped faujasite-type zeolite deposited as a thin film on an inter-digital capacitor was used as a butane sensing material by measuring its impedance response to alternating current excitation.¹⁰⁰ At about 350 °C, a 10 ppm concentration of butane gas could be detected with a time response of 1 s.

As described in Section 3.1.1, sensors have been prepared using SnO₂ with zeolite A- and ZSM-5-type zeolites as filters.⁵⁹ Results showed that the SnO₂/zeolite A sensor responded significantly to ethanol vapour along with improved selectivity towards H₂O by the elimination of the response to hydrocarbons (propane and methane). Pd-doping of the SnO₂/zeolite A sensor improved the selectivity against hydrocarbons as only 1250 ppm of propane provided any response.⁵³,⁷⁰ The water adsorbed by the hydrophilic zeolite A blocked the ingress of propane and any propane that did ingress was subsequently oxidised on Pd.

Another example reported the use of silicalite-1 on SnO₂ for the sensing of ethylene in the presence of water, where the zeolite layer acted as a filter to improve selectivity towards hydrocarbons.⁸⁹ The [010] oriented zeolite layer gave improved response and recovery time (14 s, 144 s) compared to a randomly oriented zeolite layer (25 s, 208 s). This was rationalised on the basis of vertical channels of the silicalite-1 permitting the concentration of ethylene faster than with randomly orientated silicalite-1.

Recently, a Si cantilever was used to detect toluene and ethanol vapours down to ~25 ppmv in air.⁵⁴ The adsorbent layer consisted of either organophilic silicalite or hydrophilic zeolites A and Y. To remove the adsorbed species from the nanoporous structure and thus improve performance, the hydrophilic zeolite-coated cantilevers were degassed at high temperature. Therefore, zeolite Y cantilevers with integrated heaters have been developed and could detect 1.4 ppmv of ethanol and 0.7 ppmv of toluene in air.

Semiconducting chromium titanium oxide covered with different types of zeolite (H-forms of zeolite A, ZSM-5, beta polymorph A and zeolite Y) have also been investigated as ethanol sensors.⁶¹ The highest sensitivity was obtained with the zeolite Y-modified sensor; the sensing response was doubled compared to a bare sensor. Computational studies showed that the improved sensitivity was due to a catalytic reaction and diffusion behaviour within the zeolite. Two semiconducting metal oxides (tungsten and zinc) covered with zeolite have also been described as ethanol sensors, but it has been reported that the cross-sensitivity to NO₂ was significant (Section 3.3.1).⁴⁹

Using two semiconducting tungsten and chromium titanium oxide sensors covered with the acidic form of zeolite A, it was reported that ethanol could be discriminated from isopropanol.⁶⁴ Computational studies indicated that this discrimination was due to size and shape selectivity afforded by the zeolite A layer. Earlier, an electrical impedance study took advantage of the shape-selectivity of zeolite channels using natural stilbite zeolite for the discrimination of methanol, 2-propanol and 3-pentanol against water and 2,2-dimethylpropanol.¹²⁹ Additionally, a study on the electrical conductivity response of three different zeolites (ZSM-5, mordenite and zeolite Y) towards ethanol was reported.⁵⁰ The effects on sensing response of the framework structure, the nature of the charge balancing cation (NH₄⁺ or H⁺ forms) and the hydrophilic/hydrophobic character due to Si/Al ratio were investigated. The sensitivity decreased with the increase of the Si/Al ratio and ammonium forms of zeolites gave negative responses whereas all H⁺ forms gave positive responses. The H⁺-zeolite Y with a Si/Al ratio of 30 provided the highest electrical conductivity sensitivity to ethanol vapour.

Another possibility for alcohol and hydrocarbon sensing is the use of a conductive polymer combined with zeolites. Two polymers combined with the K⁺ form of zeolite A were investigated. Both polypyrrole and polypyrrole-polyamide-6 composite sensors exhibited sensitivity towards acetone, methylether ketone, methanol and toluene.⁶⁵ Interestingly, this study showed that the preparation of composites influenced the results, as pellets and films were found to be vapour sensitive whereas electrospun fibre bundles were insensitive. However, in the case of methanol sensing, the role of the zeolite remained unclear as the sensitivity could also be explained by the interaction between methanol and polyamide-6.

A silicalite-1-Metglas composite originally developed for CO₂ sensing in air (see Section 3.4.1) was found to efficiently discriminate linear and branched hydrocarbons.⁷⁶ It selectively detected n-butane while it did not respond to the presence of i-butane. Recently, a new study concerning zeolite-Metglas

<figure><img src="image_47.png" /></figure>
<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /><figcaption>Fig. 16 (left) N,N'-1,3-phenylenebis(oxamate) tetraanion and (right) methylviologen dication.¹⁶¹</figcaption></figure>

Pardo *et al.* demonstrated a detector for carbon dioxide based on the two-dimensional MOF MV[Mn₂Cu₃(mpba)₃(H₂O)₃]·20H₂O (MV = methylviologen dication and mpba = N,N'-1,3-phenylenebis-(oxamate), Fig. 16), which is both magnetic and luminescent.¹⁶¹ Upon excitation at 400 nm, exposure to 1 bar carbon dioxide or a 1.5 bar 1:1 mixture of carbon dioxide and methane, a luminescent wavelength redshift of 17 nm was measured indicating an interaction of CO₂ but not methane with the MOF. It was shown that the MOF was sensitive to both water and methanol in toluene. The authors predict that differentiation of carbon dioxide from both water and methanol would be possible as methanol and water exposure resulted in an emission wavelength blueshift whereas an emission wavelength redshift was measured upon exposure to carbon dioxide.

## 3.5 Hydrocarbon and volatile organic compound sensors

### 3.5.1 Zeolites for sensing hydrocarbons and volatile organic compounds.
Tracking of unburnt hydrocarbons and volatile organic compounds (VOCs) is important for determining the extent of combustion. The first example of VOCs and hydrocarbon sensing used a silicalite-1 film on a QCM for ethanol sensing. Monitoring the ethanol adsorption showed a lower limit response at 50 ppm.¹⁹⁶ However, since this work in 1992 no further exploitation of the QCM sensor format has been reported. Indeed, the most frequent zeolite-based hydrocarbon sensors rely on conductivity measurements. An early example of Pt-doped faujasite-type zeolite deposited as a thin film on an inter-digital capacitor was used as a butane sensing material by measuring its impedance response to alternating current excitation.¹⁰⁰ At about 350 °C, a 10 ppm concentration of butane gas could be detected with a time response of 1 s.

As described in Section 3.1.1, sensors have been prepared using SnO₂ with zeolite A- and ZSM-5-type zeolites as filters.⁵⁹ Results showed that the SnO₂/zeolite A sensor responded significantly to ethanol vapour along with improved selectivity towards H₂O by the elimination of the response to hydrocarbons (propane and methane). Pd-doping of the SnO₂/zeolite A sensor improved the selectivity against hydrocarbons as only 1250 ppm of propane provided any response.⁵³,⁷⁰ The water adsorbed by the hydrophilic zeolite A blocked the ingress of propane and any propane that did ingress was subsequently oxidised on Pd.

Another example reported the use of silicalite-1 on SnO₂ for the sensing of ethylene in the presence of water, where the zeolite layer acted as a filter to improve selectivity towards hydrocarbons.⁸⁹ The [010] oriented zeolite layer gave improved response and recovery time (14 s, 144 s) compared to a randomly oriented zeolite layer (25 s, 208 s). This was rationalised on the basis of vertical channels of the silicalite-1 permitting the concentration of ethylene faster than with randomly orientated silicalite-1.

Recently, a Si cantilever was used to detect toluene and ethanol vapours down to ~25 ppmv in air.⁵⁴ The adsorbent layer consisted of either organophilic silicalite or hydrophilic zeolites A and Y. To remove the adsorbed species from the nanoporous structure and thus improve performance, the hydrophilic zeolite-coated cantilevers were degassed at high temperature. Therefore, zeolite Y cantilevers with integrated heaters have been developed and could detect 1.4 ppmv of ethanol and 0.7 ppmv of toluene in air.

Semiconducting chromium titanium oxide covered with different types of zeolite (H-forms of zeolite A, ZSM-5, beta polymorph A and zeolite Y) have also been investigated as ethanol sensors.⁶¹ The highest sensitivity was obtained with the zeolite Y-modified sensor; the sensing response was doubled compared to a bare sensor. Computational studies showed that the improved sensitivity was due to a catalytic reaction and diffusion behaviour within the zeolite. Two semiconducting metal oxides (tungsten and zinc) covered with zeolite have also been described as ethanol sensors, but it has been reported that the cross-sensitivity to NO₂ was significant (Section 3.3.1).⁴⁹

Using two semiconducting tungsten and chromium titanium oxide sensors covered with the acidic form of zeolite A, it was reported that ethanol could be discriminated from isopropanol.⁶⁴ Computational studies indicated that this discrimination was due to size and shape selectivity afforded by the zeolite A layer. Earlier, an electrical impedance study took advantage of the shape-selectivity of zeolite channels using natural stilbite zeolite for the discrimination of methanol, 2-propanol and 3-pentanol against water and 2,2-dimethylpropanol.¹²⁹ Additionally, a study on the electrical conductivity response of three different zeolites (ZSM-5, mordenite and zeolite Y) towards ethanol was reported.⁵⁰ The effects on sensing response of the framework structure, the nature of the charge balancing cation (NH₄⁺ or H⁺ forms) and the hydrophilic/hydrophobic character due to Si/Al ratio were investigated. The sensitivity decreased with the increase of the Si/Al ratio and ammonium forms of zeolites gave negative responses whereas all H⁺ forms gave positive responses. The H⁺-zeolite Y with a Si/Al ratio of 30 provided the highest electrical conductivity sensitivity to ethanol vapour.

Another possibility for alcohol and hydrocarbon sensing is the use of a conductive polymer combined with zeolites. Two polymers combined with the K⁺ form of zeolite A were investigated. Both polypyrrole and polypyrrole-polyamide-6 composite sensors exhibited sensitivity towards acetone, methylether ketone, methanol and toluene.⁶⁵ Interestingly, this study showed that the preparation of composites influenced the results, as pellets and films were found to be vapour sensitive whereas electrospun fibre bundles were insensitive. However, in the case of methanol sensing, the role of the zeolite remained unclear as the sensitivity could also be explained by the interaction between methanol and polyamide-6.

A silicalite-1-Metglas composite originally developed for CO₂ sensing in air (see Section 3.4.1) was found to efficiently discriminate linear and branched hydrocarbons.⁷⁶ It selectively detected n-butane while it did not respond to the presence of i-butane. Recently, a new study concerning zeolite-Metglas
<figure><img src="image_48.png" /></figure>

composites was reported for hydrocarbon sensing.⁶⁶ It was found that a faujasite-based composite exhibited the lowest detection limits for o-xylene at just 6 ppm but suffered of a lack of selectivity with cross-sensitivity to p-xylene. On the other hand, a zeolite A-based sensor showed high selectivity between xylene isomers. Concerning n-hexane sensing, the best sensitivity (180 ppm) was obtained using the randomly-oriented ZSM-5-zeolite/Metglas sensor.

A composite comprised of zeolite Y cages as hosts for C₆₀ has also been developed.¹¹⁶ Two types of composites, C₆₀/zeolite Y and K-C₆₀/zeolite Y, were deposited onto tantalum comb-type electrodes providing semiconductor sensors. In the presence of ethylene, the composites exhibited measurable changes in conductivity. Similarly, the zeolite ZSM-5 was used as a host for Pt clusters on two electrodes, Au and Cr, in order to build a propane-sensitive detector in the presence of O₂/CO₂/N₂.⁷⁷ The catalytic oxidation of propane in the presence of Pt and O₂ resulted in a reversible drop in potential.

The conductivity induced by the mobility of extra-framework cations can also be measured by impedance or potentiometry methods. Impedance response towards hydrocarbons has been reported using a Cr₂O₃ layer on top of an ion-conducting Pt-doped ZSM-5.⁷⁸ The device showed low interference to CO and H₂, but no selectivity with NH₃. Several studies by the same group were carried out to elucidate the mechanism of the sensing. It was found that the interfacial effect between Cr₂O₃ and Pt-doped ZSM-5 on the electrodes led to the selectivity of propane and propylene and that the impedance response was influenced by the presence of hydrocarbons at low frequencies.⁷⁹ Two explanations were proposed to explain the impedance variation. On one hand, the presence of hydrocarbons might act as a charge density carrier with the Cr₂O₃ layer providing the impedance changes.¹⁹⁷ On the other hand, the sorption of hydrocarbons could lead to the insertion of Na⁺ from the zeolite to the Cr₂O₃ layer which then led to the formation of sodium chromate at the interface providing the impedance changes.¹⁹⁸

Potentiometric measurements between a reference Au/Na₂CO₃ electrode and a measuring electrode, comprised of BaCO₃/ZSM-5/Au, have been studied for propane sensing at 400 °C.⁸⁰,⁸¹ The differential between Na⁺ activity in the reference and Na⁺ activity in the ZSM-5 zeolite was monitored.⁸² The highest sensitivity was obtained using a high Si/Al ratio within the zeolite and time responses were improved by the use of films instead of pellets.⁶⁷ More recently, the design of a planar potentiometric gas sensor based on Pt-doped zeolite films was envisaged by the same group.¹⁹⁹

Catalytic reactions inside zeolite cages have also been developed for detection of hydrocarbons. As many zeolites possess acid sites, they can easily promote cracking reactions of C₇–C₁₀ linear alkanes. This property was exploited using Cr-zeolite Y and Mo-zeolite Y coated on semiconducting chromium titanium oxide.¹¹⁷,¹¹⁸ Using Cr-zeolite Y, nonane could be discriminated from heptane, octane and decane by producing a distinct response due to its catalytic transformation into methylpropanol and water. Using Mo-zeolite Y, linear alkanes provided mainly acetaldehyde along

with water and ketones. Even if the discrimination and/or sensitivity was not high, an interesting point is that the sensing was monitored by the products of the catalytic reaction instead of by the analytes themselves.

A catalytic reaction was also used to increase selectivity to propane over propylene. A Pt-doped ZSM-5 layer covering semiconducting SrTi₀.₈Fe₀.₂O₃ suppressed the propylene response while the propane was selectively detected.⁸³,⁸⁴ However, the mechanism of the suppression remains unclear. Pt was also used to dope zeolite Y which was then mixed with semiconducting TiO₂.¹¹⁹ The obtained Pt-doped zeolite Y/TiO₂ composite showed selectivity towards propane over CO. It was suggested that the propane oxidation catalysed by Pt-zeolite Y produced water which then interacted with TiO₂ providing a resistance change which was monitored.

Optical measurements can also be performed. Adsorption within the zeolite increases the analyte concentration and thus enhances the sensitivity of the optical detector device. One example exploited the adsorption of *n*-hexane in silicalite-1 to provide a 180 times enhanced signal compared with an uncoated ATR element.⁹⁰ The refractive index can also change due to the presence of the analyte. For example, when silicalite-1 was coated onto a long period fibre grafting detection limits of 16.5 ppm of isopropanol and 22 ppb of toluene were measured.⁹¹ However, the response time was faster for isopropanol than for toluene due to its smaller size; isopropanol undergoes a faster equilibration within the zeolite structure.

A different approach utilised the light emission obtained by the reaction of acetaldehyde with oxygen atoms in the zeolite cages.¹²⁰ Followed by cataluminescence, the signal showed a linear response for 0.06–31.2 µg mL⁻¹ acetaldehyde vapour. In addition, acetaldehyde could be discriminated from other aldehydes due to structural constraints within the zeolite; no response was measured with methanol, ethanol, isopropanol, toluene, chloroform, dichloromethane and acetonitrile.

An optical detector based on light emission upon detection of linear alkanes such as *n*-hexane which used a CsNaY zeolite has been developed.¹²¹ The catalytic activity has been explained by zeolite basicity which might enhance the formation of a four-membered ring transition state involving a carbonium ion and oxygen atom. The detection of linear alkanes was demonstrated down to a 0.55 µg mL⁻¹ concentration limit with minor interference from alcohols and aromatics. In addition, linear alkanes could be discriminated from branched alkanes due to structural constraints within the zeolite under more concentrated atmospheres (3.88 µg mL⁻¹).

### 3.5.2 MOFs for sensing hydrocarbons and volatile organic compounds.
The refractometric optical-based detector which utilised HKUST-1 as the active sensing material, fabricated by Hupp *et al.* (Section 3.1.2), also displayed sensitivity to vapours of ethanol, ethane and ethylene.¹³⁹ Upon exposure to ethanol vapour at concentrations ranging from 100–12 000 ppm in nitrogen carrier gas, ‘stop band’ redshifts were measured. The relationship between ethanol vapour concentration and measured ‘stop band’ wavelength shift was approximately linear between 100–1000 ppm and then plateaued after ~4000 ppm. It was
<figure><img src="image_50.png" /></figure>

composites was reported for hydrocarbon sensing.⁶⁶ It was found that a faujasite-based composite exhibited the lowest detection limits for o-xylene at just 6 ppm but suffered of a lack of selectivity with cross-sensitivity to p-xylene. On the other hand, a zeolite A-based sensor showed high selectivity between xylene isomers. Concerning n-hexane sensing, the best sensitivity (180 ppm) was obtained using the randomly-oriented ZSM-5-zeolite/Metglas sensor.

A composite comprised of zeolite Y cages as hosts for C₆₀ has also been developed.¹¹⁶ Two types of composites, C₆₀/zeolite Y and K-C₆₀/zeolite Y, were deposited onto tantalum comb-type electrodes providing semiconductor sensors. In the presence of ethylene, the composites exhibited measurable changes in conductivity. Similarly, the zeolite ZSM-5 was used as a host for Pt clusters on two electrodes, Au and Cr, in order to build a propane-sensitive detector in the presence of O₂/CO₂/N₂.⁷⁷ The catalytic oxidation of propane in the presence of Pt and O₂ resulted in a reversible drop in potential.

The conductivity induced by the mobility of extra-framework cations can also be measured by impedance or potentiometry methods. Impedance response towards hydrocarbons has been reported using a Cr₂O₃ layer on top of an ion-conducting Pt-doped ZSM-5.⁷⁸ The device showed low interference to CO and H₂, but no selectivity with NH₃. Several studies by the same group were carried out to elucidate the mechanism of the sensing. It was found that the interfacial effect between Cr₂O₃ and Pt-doped ZSM-5 on the electrodes led to the selectivity of propane and propylene and that the impedance response was influenced by the presence of hydrocarbons at low frequencies.⁷⁹ Two explanations were proposed to explain the impedance variation. On one hand, the presence of hydrocarbons might act as a charge density carrier with the Cr₂O₃ layer providing the impedance changes.¹⁹⁷ On the other hand, the sorption of hydrocarbons could lead to the insertion of Na⁺ from the zeolite to the Cr₂O₃ layer which then led to the formation of sodium chromate at the interface providing the impedance changes.¹⁹⁸

Potentiometric measurements between a reference Au/Na₂CO₃ electrode and a measuring electrode, comprised of BaCO₃/ZSM-5/Au, have been studied for propane sensing at 400 °C.⁸⁰,⁸¹ The differential between Na⁺ activity in the reference and Na⁺ activity in the ZSM-5 zeolite was monitored.⁸² The highest sensitivity was obtained using a high Si/Al ratio within the zeolite and time responses were improved by the use of films instead of pellets.⁶⁷ More recently, the design of a planar potentiometric gas sensor based on Pt-doped zeolite films was envisaged by the same group.¹⁹⁹

Catalytic reactions inside zeolite cages have also been developed for detection of hydrocarbons. As many zeolites possess acid sites, they can easily promote cracking reactions of C₇–C₁₀ linear alkanes. This property was exploited using Cr-zeolite Y and Mo-zeolite Y coated on semiconducting chromium titanium oxide.¹¹⁷,¹¹⁸ Using Cr-zeolite Y, nonane could be discriminated from heptane, octane and decane by producing a distinct response due to its catalytic transformation into methylpropanol and water. Using Mo-zeolite Y, linear alkanes provided mainly acetaldehyde along

with water and ketones. Even if the discrimination and/or sensitivity was not high, an interesting point is that the sensing was monitored by the products of the catalytic reaction instead of by the analytes themselves.

A catalytic reaction was also used to increase selectivity to propane over propylene. A Pt-doped ZSM-5 layer covering semiconducting SrTi₀.₈Fe₀.₂O₃ suppressed the propylene response while the propane was selectively detected.⁸³,⁸⁴ However, the mechanism of the suppression remains unclear. Pt was also used to dope zeolite Y which was then mixed with semiconducting TiO₂.¹¹⁹ The obtained Pt-doped zeolite Y/TiO₂ composite showed selectivity towards propane over CO. It was suggested that the propane oxidation catalysed by Pt-zeolite Y produced water which then interacted with TiO₂ providing a resistance change which was monitored.

Optical measurements can also be performed. Adsorption within the zeolite increases the analyte concentration and thus enhances the sensitivity of the optical detector device. One example exploited the adsorption of *n*-hexane in silicalite-1 to provide a 180 times enhanced signal compared with an uncoated ATR element.⁹⁰ The refractive index can also change due to the presence of the analyte. For example, when silicalite-1 was coated onto a long period fibre grafting detection limits of 16.5 ppm of isopropanol and 22 ppb of toluene were measured.⁹¹ However, the response time was faster for isopropanol than for toluene due to its smaller size; isopropanol undergoes a faster equilibration within the zeolite structure.

A different approach utilised the light emission obtained by the reaction of acetaldehyde with oxygen atoms in the zeolite cages.¹²⁰ Followed by cataluminescence, the signal showed a linear response for 0.06–31.2 µg mL⁻¹ acetaldehyde vapour. In addition, acetaldehyde could be discriminated from other aldehydes due to structural constraints within the zeolite; no response was measured with methanol, ethanol, isopropanol, toluene, chloroform, dichloromethane and acetonitrile.

An optical detector based on light emission upon detection of linear alkanes such as *n*-hexane which used a CsNaY zeolite has been developed.¹²¹ The catalytic activity has been explained by zeolite basicity which might enhance the formation of a four-membered ring transition state involving a carbonium ion and oxygen atom. The detection of linear alkanes was demonstrated down to a 0.55 µg mL⁻¹ concentration limit with minor interference from alcohols and aromatics. In addition, linear alkanes could be discriminated from branched alkanes due to structural constraints within the zeolite under more concentrated atmospheres (3.88 µg mL⁻¹).

### 3.5.2 MOFs for sensing hydrocarbons and volatile organic compounds.
The refractometric optical-based detector which utilised HKUST-1 as the active sensing material, fabricated by Hupp *et al.* (Section 3.1.2), also displayed sensitivity to vapours of ethanol, ethane and ethylene.¹³⁹ Upon exposure to ethanol vapour at concentrations ranging from 100–12 000 ppm in nitrogen carrier gas, ‘stop band’ redshifts were measured. The relationship between ethanol vapour concentration and measured ‘stop band’ wavelength shift was approximately linear between 100–1000 ppm and then plateaued after ~4000 ppm. It was
<figure><img src="image_49.png" /></figure>

estimated that the lowest change in concentration of ethanol vapour that could be measured with the device was 0.3 ppm. The sensitivity limit to ethanol vapour was lower than that for water vapour due to the higher relative refractive index of ethanol. However, a major limitation of this proof-of-concept device was the MOF was not selective; different concentrations of water and ethanol vapours in nitrogen carrier gas resulted in identical magnitude ‘stop band’ shifts. The device demonstrated limited sensitivity to ethane and ethylene gases in nitrogen carrier gas. The measured ‘stop band’ redshifts were ~5 nm and ~6 nm relative to the ‘stop band’ wavelength in N₂ for ethane and ethylene respectively.

Hupp et al. also reported a ZIF-8 based Fabry-Pérot device as a selective sensor for propane and ethanol vapours.¹⁶³ Adsorption of ethanol or propane into the pores of [Zn(mIM)₂] (mIM = 2-methylimidazolate), ZIF-8, replaced nitrogen and thus the effective refractive index of the MOF increased. The increase in refractive index due to exposure to either ethanol or propane vapour resulted in a redshift of the interference fringes in the transmission spectrum. The response of the system to propane in nitrogen gas in a concentration range 0–100% was linear with a maximum interference fringe redshift of ~50 nm measured in 100% propane. The device was unresponsive to sterically bulkier cyclohexane. In addition, the response of the system to ethanol vapour in nitrogen gas in a concentration range 0–100% was non-linear with a maximum interference fringe redshift of ~60 nm measured in 40% ethanol with no further interference redshift occurring at concentration of ethanol vapour >40%.

The Kelvin probe sensor setup demonstrated by Fleischer et al. (Section 3.1.2) was also sensitive to aldehydes.¹⁴¹ Aldehyde vapour sensing measurements were performed in synthetic air (20% O₂ and 80% N₂) at a flow rate of 1 L min⁻¹ at 25 °C, and at 40% RH. The sensing response was reversible and stable with changes in work function (Δφ) measured upon exposure to the vapours of individual aldehydes. The change in work function upon exposure to 10 ppm of acetaldehyde and propanal were –1 mV and +1.4 mV respectively whereas the device was more sensitive to pentanal with a change in work function of –5 mV upon exposure to just 2 ppm. The device suffered from limited cross sensitivity with differences in the measured work function upon exposure to just pentanal, pentanal with acetaldehyde and pentanal with propanal.

Lee et al. reported two MOFs that can also be used to detect ethanol by quantifying the differences in the luminescence emission spectra before and after exposure to ethanol vapour.¹³⁷ Upon excitation at 270 nm, it was found that for dry [In(OH)(bdc)] emission λ<sub>max</sub> = 326 nm and for dry MOF-5 emission λ<sub>max</sub> = 366 nm. Upon exposure to ethanol vapour the profile of the emission peak of [In(OH)(bdc)] changed from a single peak to a triple peak emission feature with λ<sub>max</sub><sub>EtOH</sub> = 389 nm. However in the case of MOF-5 the emission wavelength blueshifted 27 nm to λ<sub>max</sub><sub>EtOH</sub> = 339 nm, which was an identical to the λ<sub>max</sub> water value. Therefore MOF-5 was not able to differentiate between water and ethanol vapours.¹⁸⁴

Robinson et al. fabricated N-doped piezoelectric micro-
cantilever sensors, ~230 µm long and ~80 µm wide, that were

coated with a layer of HKUST-1 (Section 3.1.2).¹³⁸ The sensor
device was exposed to different concentrations of methanol
vapour in nitrogen gas at 23 °C and at 1 atm pressure. Exposure
to methanol vapour resulted in methanol being adsorbed onto
the HKUST-1 layer, changing the resonant frequency of the
microcantilever. The device demonstrated sensitivity between
~1000 ppm and ~13 000 ppm of methanol vapour in nitrogen
carrier gas and demonstrated a reversible sensing response.
However, the magnitude of changes in voltage that were measured
at equilibrium for different concentrations of methanol vapour in
nitrogen carrier gas were identical to those measured upon expo-
sure to different concentrations of water vapour. The sensor device
was however able to differentiate between water and methanol
vapour by taking into account response times.

The piezoelectric microcantilever humidity sensor modified with HKUST-1, demonstrated by Hesketh et al. (Section 3.1.2), also displayed sensitivity to vapours of ethanol and methanol.¹⁴⁰ The authors tested the response of the device to methanol and ethanol vapours in nitrogen carrier gas, at room temperature and pressure, with the HKUST-1 in both the dehydrated and hydrated state. The sensor response to both ethanol and methanol vapours was non-linear within the range of ethanol vapour concentrations (up to ~2%) and methanol vapour concentrations (up to ~8%) investigated. Whether the HKUST-1 was in the hydrated or dehydrated state made no discernible difference to the response for both alcohols.

The mesoporous MOF [Zn₄O(benztb)(btc)₂/₃] loaded with Nile Blue, reported by Kaskel et al., also was a detector for vapours of ethanol and methanol.¹⁴⁹ Upon exposure to ethanol and methanol vapours the colour changed from dark blue (due to the presence of Nile Blue in the 'dry' state) to light blue.

The planar interdigital electrode impedance sensor coated with a thick film of Fe-btc MOF and operating at a frequency of 1 Hz, reported by Moos et al. as a humidity sensor (Section 3.1.2), was also sensitive to methanol and ethanol vapours.¹⁴⁸ At an operating temperature of 120 °C, there was a linear response between absolute impedance |Z| and concentration of either methanol or ethanol in the range of 0–35 vol% methanol vapour in nitrogen and 0–18 vol% ethanol vapour in nitrogen. However, after exposure to either methanol or ethanol vapour, a longer desorption time was required for the sensor to return to the pre-exposure impedance value relative to water vapour. In addition, a drifting impedance value was observed upon repeated exposures to ethanol vapour.

The photonic film of NH₂-MIL-88B MOF on a silicon wafer was also sensitive to ethanol vapour.¹⁵⁰ Adsorption of ethanol into the pores of NH₂-MIL-88B replaced air and thus the effective refractive index of the MOF increased. This resulted in a redshift of the interference peaks in the reflection spectrum by ~300 nm.

Zhang et al. reported that the MOF [Co(mIM)₂], ZIF-67, acted as a selective and highly sensitive sensing material for formaldehyde at 150 °C.¹⁶⁴ ZIF-67 was coated onto silver–palladium interdigitated electrodes and the resistance of the MOF was measured during exposure to various analytes at 150 °C. The highest response was observed upon exposure to formaldehyde,
<figure><img src="image_51.png" /></figure>

estimated that the lowest change in concentration of ethanol vapour that could be measured with the device was 0.3 ppm. The sensitivity limit to ethanol vapour was lower than that for water vapour due to the higher relative refractive index of ethanol. However, a major limitation of this proof-of-concept device was the MOF was not selective; different concentrations of water and ethanol vapours in nitrogen carrier gas resulted in identical magnitude ‘stop band’ shifts. The device demonstrated limited sensitivity to ethane and ethylene gases in nitrogen carrier gas. The measured ‘stop band’ redshifts were ~5 nm and ~6 nm relative to the ‘stop band’ wavelength in N₂ for ethane and ethylene respectively.

Hupp et al. also reported a ZIF-8 based Fabry-Pérot device as a selective sensor for propane and ethanol vapours.¹⁶³ Adsorption of ethanol or propane into the pores of [Zn(mIM)₂] (mIM = 2-methylimidazolate), ZIF-8, replaced nitrogen and thus the effective refractive index of the MOF increased. The increase in refractive index due to exposure to either ethanol or propane vapour resulted in a redshift of the interference fringes in the transmission spectrum. The response of the system to propane in nitrogen gas in a concentration range 0–100% was linear with a maximum interference fringe redshift of ~50 nm measured in 100% propane. The device was unresponsive to sterically bulkier cyclohexane. In addition, the response of the system to ethanol vapour in nitrogen gas in a concentration range 0–100% was non-linear with a maximum interference fringe redshift of ~60 nm measured in 40% ethanol with no further interference redshift occurring at concentration of ethanol vapour >40%.

The Kelvin probe sensor setup demonstrated by Fleischer et al. (Section 3.1.2) was also sensitive to aldehydes.¹⁴¹ Aldehyde vapour sensing measurements were performed in synthetic air (20% O₂ and 80% N₂) at a flow rate of 1 L min⁻¹ at 25 °C, and at 40% RH. The sensing response was reversible and stable with changes in work function (Δφ) measured upon exposure to the vapours of individual aldehydes. The change in work function upon exposure to 10 ppm of acetaldehyde and propanal were –1 mV and +1.4 mV respectively whereas the device was more sensitive to pentanal with a change in work function of –5 mV upon exposure to just 2 ppm. The device suffered from limited cross sensitivity with differences in the measured work function upon exposure to just pentanal, pentanal with acetaldehyde and pentanal with propanal.

Lee et al. reported two MOFs that can also be used to detect ethanol by quantifying the differences in the luminescence emission spectra before and after exposure to ethanol vapour.¹³⁷ Upon excitation at 270 nm, it was found that for dry [In(OH)(bdc)] emission λ<sub>max</sub> = 326 nm and for dry MOF-5 emission λ<sub>max</sub> = 366 nm. Upon exposure to ethanol vapour the profile of the emission peak of [In(OH)(bdc)] changed from a single peak to a triple peak emission feature with λ<sub>max ETOH</sub> = 389 nm. However in the case of MOF-5 the emission wavelength blueshifted 27 nm to λ<sub>max ETOH</sub> = 339 nm, which was an identical to the λ<sub>max water</sub> value. Therefore MOF-5 was not able to differentiate between water and ethanol vapours.¹⁸⁴

Robinson et al. fabricated N-doped piezoelectric micro-
cantilever sensors, ~230 µm long and ~80 µm wide, that were

coated with a layer of HKUST-1 (Section 3.1.2).¹³⁸ The sensor
device was exposed to different concentrations of methanol
vapour in nitrogen gas at 23 °C and at 1 atm pressure. Exposure
to methanol vapour resulted in methanol being adsorbed onto
the HKUST-1 layer, changing the resonant frequency of the
microcantilever. The device demonstrated sensitivity between
~1000 ppm and ~13 000 ppm of methanol vapour in nitrogen
carrier gas and demonstrated a reversible sensing response.
However, the magnitude of changes in voltage that were measured
at equilibrium for different concentrations of methanol vapour in
nitrogen carrier gas were identical to those measured upon expo-
sure to different concentrations of water vapour. The sensor device
was however able to differentiate between water and methanol
vapour by taking into account response times.

The piezoelectric microcantilever humidity sensor modified with HKUST-1, demonstrated by Hesketh et al. (Section 3.1.2), also displayed sensitivity to vapours of ethanol and methanol.¹⁴⁰ The authors tested the response of the device to methanol and ethanol vapours in nitrogen carrier gas, at room temperature and pressure, with the HKUST-1 in both the dehydrated and hydrated state. The sensor response to both ethanol and methanol vapours was non-linear within the range of ethanol vapour concentrations (up to ~2%) and methanol vapour concentrations (up to ~8%) investigated. Whether the HKUST-1 was in the hydrated or dehydrated state made no discernible difference to the response for both alcohols.

The mesoporous MOF [Zn₄O(benztb)(btc)₂/₃] loaded with Nile Blue, reported by Kaskel et al., also was a detector for vapours of ethanol and methanol.¹⁴⁹ Upon exposure to ethanol and methanol vapours the colour changed from dark blue (due to the presence of Nile Blue in the 'dry' state) to light blue.

The planar interdigital electrode impedance sensor coated with a thick film of Fe-btc MOF and operating at a frequency of 1 Hz, reported by Moos et al. as a humidity sensor (Section 3.1.2), was also sensitive to methanol and ethanol vapours.¹⁴⁸ At an operating temperature of 120 °C, there was a linear response between absolute impedance |Z| and concentration of either methanol or ethanol in the range of 0–35 vol% methanol vapour in nitrogen and 0–18 vol% ethanol vapour in nitrogen. However, after exposure to either methanol or ethanol vapour, a longer desorption time was required for the sensor to return to the pre-exposure impedance value relative to water vapour. In addition, a drifting impedance value was observed upon repeated exposures to ethanol vapour.

The photonic film of NH₂-MIL-88B MOF on a silicon wafer was also sensitive to ethanol vapour.¹⁵⁰ Adsorption of ethanol into the pores of NH₂-MIL-88B replaced air and thus the effective refractive index of the MOF increased. This resulted in a redshift of the interference peaks in the reflection spectrum by ~300 nm.

Zhang et al. reported that the MOF [Co(mIM)₂], ZIF-67, acted as a selective and highly sensitive sensing material for formaldehyde at 150 °C.¹⁶⁴ ZIF-67 was coated onto silver–palladium interdigitated electrodes and the resistance of the MOF was measured during exposure to various analytes at 150 °C. The highest response was observed upon exposure to formaldehyde,
<figure><img src="image_50.png" /></figure>

but there was slight cross-sensitivity to methanol and acetone vapours. The response of the sensor to ammonia vapour and methane was negligible. The sensor was shown to be sensitive to formaldehyde in the concentration range 5–500 ppm with a linear sensing response between 5–50 ppm of formaldehyde. ZIF-67 is stable up to 250 °C in air.

Huo et al. reported that self-assembled crystals of a MOF can be used for optical detection of methanol and ethanol vapours.¹⁶⁵ Particles of [Zr₆O₄(OH)₄(bdc)₆], UiO-66, were modified with poly-vinylpyrrolidone and then films on the surface of a surfactant solution were formed using the Langmuir–Blodgett technique. These films were then transferred to a silica substrate using a dip coating technique. The thin films of self-assembled crystals of UiO-66 were then put into a quartz cuvette that was placed into a UV-visible spectrometer. Adsorption of ethanol or methanol into the pores of UiO-66 replaced nitrogen and the refractive index of the thin film changed. This caused the colour of the photonic crystal thin films to change. A redshift of the absorption peak by 30 nm occurred upon exposure to methanol vapour. This change in colour due to exposure to methanol vapour was also visible to the naked eye.

Zhao et al. reported two MOFs, [Cu₂(OH)(2,2'-bipy)₂(btc)]·2H₂O and [Co(4,4'-bipy)(m-bdc)] (2,2'-bipy = 2,2'-bipyridine; 4,4'-bipy = 4,4'-bipyridine; m-bdc = 1,3-benzenedicarboxylate), which were highly sensitive to methanol vapour at room temperature when deposited as films on QCM sensors.¹⁶⁶ When the QCM sensor coated with [Cu₂(OH)(2,2'-bipy)₂(btc)]·2H₂O was exposed to methanol vapour it exhibited an approximately linear response with larger magnitude negative frequency shifts with higher concentrations of methanol vapour. The response time was approximately on the order of 1–2 min and the responses were reversible. The QCM sensor coated with [Co(4,4'-bipy)(m-bdc)] was also exposed to methanol vapour and it exhibited a non-linear response with larger magnitude negative frequency shifts with higher concentrations of methanol. Both MOFs had relatively poor thermal stability, degrading at 170 °C or below.

The techniques of surface plasmon resonance sensing and imaging, using HKUST-1 as the active sensing element, for the detection of straight chain alcohols was reported by Chen et al.¹⁴⁴ Crystals of HKUST-1 were self-assembled from solutions of the reactants using a stepwise-immersion protocol onto a layer of gold, the surface of which had been modified so as to have terminal alcohol groups present. The polarisation of the surface plasmon mode was found to have a linear relationship with concentration of both ethanol and methanol vapour in the range 1% to 70% (v/v), although the sensor was overall more sensitive to methanol than ethanol. The response of the sensor was reversible and highly reproducible.

Urban et al. also used MOFs based on the bdc ligand as the active sensing elements on a work function based sensor.¹⁶⁷,¹⁶⁸ The design and operation of the sensor was similar to the water vapour sensor reported by Hesketh et al. (Section 3.1.2) and the MOFs investigated were [Al₁₂O(OH)₁₈(H₂O)₃{Al₂(OH)₄}(bdc)₆]·24H₂O, [Co₃(btc)₂]·H₂O, [Ni₃(btc)₂]·H₂O and [Cd₃(btc)₂]·H₂O. Upon exposure to methanol or ethanol vapour at different concentrations in synthetic air carrier gas, at 25 °C and at 40% RH, the work

function was found to decrease insignificantly for methanol and only marginally for ethanol vapour. The largest response was measured with the work function sensor modified with the layer of [Cd₃(btc)₂]·H₂O. However, upon repeating the experiment in dry synthetic air (i.e. 0% RH), the work function decreased more significantly for both methanol and ethanol. In both cases the largest sensor response was measured upon exposure to ethanol vapour, however differentiation between the two alcohols in a mixed vapour stream would be difficult as the magnitude change in work function upon exposure to methanol at ~50 ppm was identical to that upon exposure to ethanol at ~10 ppm. Additionally, the sensor device showed cross sensitivity to other alcohols such as 2-propanol and n-butanol. Upon exposure to straight chain alkanes slight changes in measured work function were measured, but only in dry conditions. In all cases the sensing responses were reversible.

Work by Pohle et al. compared the response of this work function-based sensor to the response of a QCM sensor upon exposure to vapours of alcohols.¹⁴⁵ Both sensors were modified with films of Cu-btc and exposed to different concentrations of methanol and ethanol vapours under different conditions. For both sensor types it was found that operation in dry conditions resulted in the largest measureable responses to methanol and ethanol vapour. In addition, it was found that the largest changes in frequency response from the QCM-based sensor were measured upon exposure to methanol vapour rather than ethanol vapour. In contrast, methanol vapour caused the smallest work function changes measured with the work function-based sensor compared to ethanol vapour. Therefore, if both sensors were operated at the same time, then two-dimensional discrimination of the nature of the analyte could be achieved.

Li et al. measured the change in fluorescence intensity and the wavelength shift of the fluorescence peak for two MOFs upon exposure to the vapours of many different organic compounds and then plotted these data onto a two-dimensional detection map which allowed differentiation between all of the analytes.¹⁶⁹ The MOFs employed were [Zn₂(ndc)₂(dpe)]·2.5DMF·0.25H₂O and [Zn₂(ndc)₂(bpee)]·2.25DMF·0.5H₂O (ndc = 2,6-naphthalenedicarboxylate; bpee = 1,2-bis(4-pyridyl)ethylene). [Zn₂(ndc)₂(bpee)]·2.25DMF·0.5H₂O displayed the larger responses to analytes in terms of fluorescence intensity change and fluorescence peak wavelength shift after exposure to the vapour of the analyte at room temperature for 5 min. The two-dimensional detection response map allowed for facile differentiation between methanol and ethanol. In addition it was found that both MOFs were stable up to ~400 °C in a nitrogen atmosphere.

An orientated film of a fluorescent MOF on a glass substrate has been reported as a luminescence-based detector of alkenes by Balkus et al.¹⁷⁰ [Zn₂(bpdc)₂(bpee)]·2DMF, RPM3, was grown solvothermally on a glass substrate, the surface of which had been seeded with zinc oxide particles. The resulting film of RPM3 was ~3 µm thick and had grown preferentially in the crystallographic c-axis direction. Interestingly the thin film format of the MOF seemed to introduce stability to relative humidity compared to RPM3 powder. The MOF thin film was then doped with silver cations. The presence of Ag⁺ within the
<figure><img src="image_52.png" /></figure>

but there was slight cross-sensitivity to methanol and acetone vapours. The response of the sensor to ammonia vapour and methane was negligible. The sensor was shown to be sensitive to formaldehyde in the concentration range 5–500 ppm with a linear sensing response between 5–50 ppm of formaldehyde. ZIF-67 is stable up to 250 °C in air.

Huo et al. reported that self-assembled crystals of a MOF can be used for optical detection of methanol and ethanol vapours.¹⁶⁵ Particles of [Zr₆O₄(OH)₄(bdc)₆], UiO-66, were modified with poly-vinylpyrrolidone and then films on the surface of a surfactant solution were formed using the Langmuir–Blodgett technique. These films were then transferred to a silica substrate using a dip coating technique. The thin films of self-assembled crystals of UiO-66 were then put into a quartz cuvette that was placed into a UV-visible spectrometer. Adsorption of ethanol or methanol into the pores of UiO-66 replaced nitrogen and the refractive index of the thin film changed. This caused the colour of the photonic crystal thin films to change. A redshift of the absorption peak by 30 nm occurred upon exposure to methanol vapour. This change in colour due to exposure to methanol vapour was also visible to the naked eye.

Zhao et al. reported two MOFs, [Cu₂(OH)(2,2'-bipy)₂(btc)]·2H₂O and [Co(4,4'-bipy)(m-bdc)] (2,2'-bipy = 2,2'-bipyridine; 4,4'-bipy = 4,4'-bipyridine; m-bdc = 1,3-benzenedicarboxylate), which were highly sensitive to methanol vapour at room temperature when deposited as films on QCM sensors.¹⁶⁶ When the QCM sensor coated with [Cu₂(OH)(2,2'-bipy)₂(btc)]·2H₂O was exposed to methanol vapour it exhibited an approximately linear response with larger magnitude negative frequency shifts with higher concentrations of methanol vapour. The response time was approximately on the order of 1–2 min and the responses were reversible. The QCM sensor coated with [Co(4,4'-bipy)(m-bdc)] was also exposed to methanol vapour and it exhibited a non-linear response with larger magnitude negative frequency shifts with higher concentrations of methanol. Both MOFs had relatively poor thermal stability, degrading at 170 °C or below.

The techniques of surface plasmon resonance sensing and imaging, using HKUST-1 as the active sensing element, for the detection of straight chain alcohols was reported by Chen et al.¹⁴⁴ Crystals of HKUST-1 were self-assembled from solutions of the reactants using a stepwise-immersion protocol onto a layer of gold, the surface of which had been modified so as to have terminal alcohol groups present. The polarisation of the surface plasmon mode was found to have a linear relationship with concentration of both ethanol and methanol vapour in the range 1% to 70% (v/v), although the sensor was overall more sensitive to methanol than ethanol. The response of the sensor was reversible and highly reproducible.

Urban et al. also used MOFs based on the bdc ligand as the active sensing elements on a work function based sensor.¹⁶⁷,¹⁶⁸ The design and operation of the sensor was similar to the water vapour sensor reported by Hesketh et al. (Section 3.1.2) and the MOFs investigated were [Al₁₂O(OH)₁₈(H₂O)₃{Al₂(OH)₄}(bdc)₆]·24H₂O, [Co₃(btc)₂]·H₂O, [Ni₃(btc)₂]·H₂O and [Cd₃(btc)₂]·H₂O. Upon exposure to methanol or ethanol vapour at different concentrations in synthetic air carrier gas, at 25 °C and at 40% RH, the work

function was found to decrease insignificantly for methanol and only marginally for ethanol vapour. The largest response was measured with the work function sensor modified with the layer of [Cd₃(btc)₂]·H₂O. However, upon repeating the experiment in dry synthetic air (i.e. 0% RH), the work function decreased more significantly for both methanol and ethanol. In both cases the largest sensor response was measured upon exposure to ethanol vapour, however differentiation between the two alcohols in a mixed vapour stream would be difficult as the magnitude change in work function upon exposure to methanol at ~50 ppm was identical to that upon exposure to ethanol at ~10 ppm. Additionally, the sensor device showed cross sensitivity to other alcohols such as 2-propanol and n-butanol. Upon exposure to straight chain alkanes slight changes in measured work function were measured, but only in dry conditions. In all cases the sensing responses were reversible.

Work by Pohle et al. compared the response of this work function-based sensor to the response of a QCM sensor upon exposure to vapours of alcohols.¹⁴⁵ Both sensors were modified with films of Cu-btc and exposed to different concentrations of methanol and ethanol vapours under different conditions. For both sensor types it was found that operation in dry conditions resulted in the largest measureable responses to methanol and ethanol vapour. In addition, it was found that the largest changes in frequency response from the QCM-based sensor were measured upon exposure to methanol vapour rather than ethanol vapour. In contrast, methanol vapour caused the smallest work function changes measured with the work function-based sensor compared to ethanol vapour. Therefore, if both sensors were operated at the same time, then two-dimensional discrimination of the nature of the analyte could be achieved.

Li et al. measured the change in fluorescence intensity and the wavelength shift of the fluorescence peak for two MOFs upon exposure to the vapours of many different organic compounds and then plotted these data onto a two-dimensional detection map which allowed differentiation between all of the analytes.¹⁶⁹ The MOFs employed were [Zn₂(ndc)₂(dpe)]·2.5DMF·0.25H₂O and [Zn₂(ndc)₂(bpee)]·2.25DMF·0.5H₂O (ndc = 2,6-naphthalenedicarboxylate; bpee = 1,2-bis(4-pyridyl)ethylene). [Zn₂(ndc)₂(bpee)]·2.25DMF·0.5H₂O displayed the larger responses to analytes in terms of fluorescence intensity change and fluorescence peak wavelength shift after exposure to the vapour of the analyte at room temperature for 5 min. The two-dimensional detection response map allowed for facile differentiation between methanol and ethanol. In addition it was found that both MOFs were stable up to ~400 °C in a nitrogen atmosphere.

An orientated film of a fluorescent MOF on a glass substrate has been reported as a luminescence-based detector of alkenes by Balkus et al.¹⁷⁰ [Zn₂(bpdc)₂(bpee)]·2DMF, RPM3, was grown solvothermally on a glass substrate, the surface of which had been seeded with zinc oxide particles. The resulting film of RPM3 was ~3 µm thick and had grown preferentially in the crystallographic c-axis direction. Interestingly the thin film format of the MOF seemed to introduce stability to relative humidity compared to RPM3 powder. The MOF thin film was then doped with silver cations. The presence of Ag⁺ within the
<figure><img src="image_51.png" /></figure>

MOF partially quenched the fluorescence intensity at ~422 nm due to the π-cation interaction with the bpee ligand. Upon exposure to propylene gas in dry nitrogen carrier gas the fluorescence intensity was enhanced by ~43%, but partially quenched by 12% when exposed to propane gas in dry nitrogen. Thus this system can differentiate between alkanes and alkenes. The enhancement in fluorescence intensity upon exposure to propylene was proposed to be due to the formation of a π-cation interaction between an alkene analyte and the Ag⁺ cation thus weakening the π-cation interaction between the Ag⁺ cation and the bpee ligand. The sensing response to propylene was reversible, but heating at 60 °C under vacuum followed by nitrogen purging was required. The response to 1-hexene was also tested. Upon exposure to 1-hexene vapour a fluorescence intensity enhancement of ~43% was measured which suggests that differentiation between different alkenes may not be possible with this MOF system.

Furukawa et al. investigated how the crystal orientation of a MOF impacted on its adsorption kinetics.¹⁷¹ The MOF used was [Zn(NO₂-ip)(4,4''-bipy)] (NO₂-ip = 5-nitroisophthalate), Zn-CID-5, and thin films of the MOF were grown solvothermally on QCM sensors, one of which had been modified with 16-mercapto-hexadecanoic acid and the other remained bare. The presence of 16-mercaptohexadecanoic acid on the surface of the QCM favoured the orientation of the Zn-CID-5 crystals in the [1-1-1] direction whereas the preferred crystal direction of the Zn-CID-5 crystals on the bare QCM sensor was [100]. Zn-CID-5 crystals were also solvothermally grown on the surface of a bare QCM sensor in the presence of 4-phenylpyridine, which is a crystal size modulator. This led to a preferred orientation of the Zn-CID-5 crystals along the [010] direction. From adsorption kinetic studies it was found that Zn-CID-5 orientated along the [010] direction had the fastest adsorption kinetics and thus was the most suitable for sensing. The sensor showed the largest sensing responses to methanol compared to hexane as the methanol induced a 'gate-opening' phenomenon whereas hexane did to a far lesser extent. The sensing response to methanol was reversible, but the time response was over 100 s.

Finally Cao et al. reported a colorimetric detector for methanol vapour that was comprised of a solvothermally grown film of the MOF [Co₃(tbtc)₂(DMF)₂]·4DMF (tbtc = 4-[[3,5-bis[(4-carboxylato-phenoxy)methyl]-2,4,6-trimethyl-phenyl]methoxy]benzoate, Fig. 17) on an alumina support.¹⁷² Upon exposure to air saturated with methanol vapour the colour of the thin film changed from blue to pink over several hours of exposure. However, the colour could only be restored by exposing the MOF thin film to DMF vapour.

## 3.6 Ammonia sensors

### 3.6.1 Zeolites for sensing NH₃.
The detection of residual ammonia in the exhaust stream of diesel engines is necessary for efficient use of selective catalytic reduction (SCR) catalysts. Many zeolite-based ammonia sensing techniques exploit the proton conductive properties of zeolites. The earliest example reported the effect of NH₃ (up to 25 ppm concentration) on the electrical properties of the Na⁺- and H⁺-forms of beta polymorph A-type zeolites using impedance spectroscopy.⁹⁶ In the presence

of ammonia the ionic conductivity of Na-beta polymorph A is unaffected, while it promotes proton conduction in H-beta polymorph A, opening the way to conductive proton zeolite sensing. Other zeolite types have also been found to be efficient proton conductive materials.

Pt-doped zeolite Y assembled in films was promising, but showed a strong cross-sensitivity to NO.⁹² In contrast, the zeolite H-ZSM-5 had very small cross sensitivity to NO, CO, CO₂ and hydrocarbons as no metal elements were incorporated in the zeolite structure. It provided a response time of seconds and a detection limit down to 5 ppm concentration of ammonia in synthetic air.⁸⁵ To improve the sensitivity of a Fe-doped poly(*p*-phenylene) polymer (Fe-dPPP) sensor towards NH₃, several forms of ZSM-5 zeolites (Na⁺, K⁺, NH₄⁺, H⁺) were added into the conductive polymer matrix.⁸⁶ The highest electrical conductivity sensitivity was obtained with the composite Fe-dPPP-H-ZSM-5. This was explained by the high acidity, pore volume and surface area of the H-form of ZSM-5 zeolite, as those properties favour both conductive polymer interaction and NH₃ adsorption. In addition, the FTIR spectrum showed that the NH₃-polymer interaction is irreversible while the NH₃-zeolite interaction is reversible. The irreversibility of the NH₃-polymer interaction was recently resolved by the same research group. Indeed, they reported that a composite comprised of HClO₄-doped poly(3-thiopheneacetic acid) as polymer and a zeolite Y gave similar results with a fully reversible NH₃-polymer interaction.¹²²

The interaction of ammonia with Ag⁺-exchanged zeolite Y was recently exploited to demonstrate potential for NH₃ detection.¹²³ As no measurable self-reduction of Ag⁺, and therefore, no proton-mediated change of impedance could be recorded in the presence of NH₃, the impedance measurements were based directly on the mobility change of the Ag⁺ in the presence of NH₃. No cross-sensitivity was observed for O₂, CO, CO₂ and propane, though there was some interference from NO and water.

Sensors for ammonia based on non-proton conductive zeolite properties have also been developed. An H-ZSM-5 zeolite thin film was grown on a long-period fibre grating whereupon the adsorption-induced shift of the resonant wavelength of the fibre grating was monitored during exposure to ammonia.⁸⁷ This sensor had a 2 min response time for adsorption-desorption of NH₃ in 612 ppmv ammonia in nitrogen atmosphere. Recently, a

<figure><img src="image_52.png" /><figcaption>Fig. 17 The 4-[[3,5-bis[(4-carboxylatophenoxy)methyl]-2,4,6-trimethyl-phenyl]methoxy]benzoate (tbtc) ligand.¹⁷²</figcaption></figure>
<figure><img src="image_53.png" /></figure>

MOF partially quenched the fluorescence intensity at ~422 nm due to the π-cation interaction with the bpee ligand. Upon exposure to propylene gas in dry nitrogen carrier gas the fluorescence intensity was enhanced by ~43%, but partially quenched by 12% when exposed to propane gas in dry nitrogen. Thus this system can differentiate between alkanes and alkenes. The enhancement in fluorescence intensity upon exposure to propylene was proposed to be due to the formation of a π-cation interaction between an alkene analyte and the Ag⁺ cation thus weakening the π-cation interaction between the Ag⁺ cation and the bpee ligand. The sensing response to propylene was reversible, but heating at 60 °C under vacuum followed by nitrogen purging was required. The response to 1-hexene was also tested. Upon exposure to 1-hexene vapour a fluorescence intensity enhancement of ~43% was measured which suggests that differentiation between different alkenes may not be possible with this MOF system.

Furukawa et al. investigated how the crystal orientation of a MOF impacted on its adsorption kinetics.¹⁷¹ The MOF used was [Zn(NO₂-ip)(4,4''-bipy)] (NO₂-ip = 5-nitroisophthalate), Zn-CID-5, and thin films of the MOF were grown solvothermally on QCM sensors, one of which had been modified with 16-mercapto-hexadecanoic acid and the other remained bare. The presence of 16-mercaptohexadecanoic acid on the surface of the QCM favoured the orientation of the Zn-CID-5 crystals in the [1-1-1] direction whereas the preferred crystal direction of the Zn-CID-5 crystals on the bare QCM sensor was [100]. Zn-CID-5 crystals were also solvothermally grown on the surface of a bare QCM sensor in the presence of 4-phenylpyridine, which is a crystal size modulator. This led to a preferred orientation of the Zn-CID-5 crystals along the [010] direction. From adsorption kinetic studies it was found that Zn-CID-5 orientated along the [010] direction had the fastest adsorption kinetics and thus was the most suitable for sensing. The sensor showed the largest sensing responses to methanol compared to hexane as the methanol induced a 'gate-opening' phenomenon whereas hexane did to a far lesser extent. The sensing response to methanol was reversible, but the time response was over 100 s.

Finally Cao et al. reported a colorimetric detector for methanol vapour that was comprised of a solvothermally grown film of the MOF [Co₃(tbtc)₂(DMF)₂]·4DMF (tbtc = 4-[[(3,5-bis[(4-carboxylato-phenoxy)methyl]-2,4,6-trimethyl-phenyl]methoxy]benzoate, Fig. 17) on an alumina support.¹⁷² Upon exposure to air saturated with methanol vapour the colour of the thin film changed from blue to pink over several hours of exposure. However, the colour could only be restored by exposing the MOF thin film to DMF vapour.

## 3.6 Ammonia sensors

### 3.6.1 Zeolites for sensing NH₃.
The detection of residual ammonia in the exhaust stream of diesel engines is necessary for efficient use of selective catalytic reduction (SCR) catalysts. Many zeolite-based ammonia sensing techniques exploit the proton conductive properties of zeolites. The earliest example reported the effect of NH₃ (up to 25 ppm concentration) on the electrical properties of the Na⁺- and H⁺-forms of beta polymorph A-type zeolites using impedance spectroscopy.⁹⁶ In the presence

of ammonia the ionic conductivity of Na-beta polymorph A is unaffected, while it promotes proton conduction in H-beta polymorph A, opening the way to conductive proton zeolite sensing. Other zeolite types have also been found to be efficient proton conductive materials.

Pt-doped zeolite Y assembled in films was promising, but showed a strong cross-sensitivity to NO.⁹² In contrast, the zeolite H-ZSM-5 had very small cross sensitivity to NO, CO, CO₂ and hydrocarbons as no metal elements were incorporated in the zeolite structure. It provided a response time of seconds and a detection limit down to 5 ppm concentration of ammonia in synthetic air.⁸⁵ To improve the sensitivity of a Fe-doped poly(*p*-phenylene) polymer (Fe-dPPP) sensor towards NH₃, several forms of ZSM-5 zeolites (Na⁺, K⁺, NH₄⁺, H⁺) were added into the conductive polymer matrix.⁸⁶ The highest electrical conductivity sensitivity was obtained with the composite Fe-dPPP-H-ZSM-5. This was explained by the high acidity, pore volume and surface area of the H-form of ZSM-5 zeolite, as those properties favour both conductive polymer interaction and NH₃ adsorption. In addition, the FTIR spectrum showed that the NH₃-polymer interaction is irreversible while the NH₃-zeolite interaction is reversible. The irreversibility of the NH₃-polymer interaction was recently resolved by the same research group. Indeed, they reported that a composite comprised of HClO₄-doped poly(3-thiopheneacetic acid) as polymer and a zeolite Y gave similar results with a fully reversible NH₃-polymer interaction.¹²²

The interaction of ammonia with Ag⁺-exchanged zeolite Y was recently exploited to demonstrate potential for NH₃ detection.¹²³ As no measurable self-reduction of Ag⁺, and therefore, no proton-mediated change of impedance could be recorded in the presence of NH₃, the impedance measurements were based directly on the mobility change of the Ag⁺ in the presence of NH₃. No cross-sensitivity was observed for O₂, CO, CO₂ and propane, though there was some interference from NO and water.

Sensors for ammonia based on non-proton conductive zeolite properties have also been developed. An H-ZSM-5 zeolite thin film was grown on a long-period fibre grating whereupon the adsorption-induced shift of the resonant wavelength of the fibre grating was monitored during exposure to ammonia.⁸⁷ This sensor had a 2 min response time for adsorption-desorption of NH₃ in 612 ppm ammonia in nitrogen atmosphere. Recently, a

<figure><img src="image_54.png" /><figcaption>Fig. 17 The 4-[[(3,5-bis[(4-carboxylatophenoxy)methyl]-2,4,6-trimethyl-phenyl]methoxy]benzoate (tbtc) ligand.¹⁷²</figcaption></figure>
<figure><img src="image_53.png" /></figure>

sensor based on the conductivity of metal oxide was reported. Indeed, semiconducting zinc oxide covered with zeolite (Y, mordenite or beta polymorph A) showed sensitivity down to a few ppm concentration of NH₃.⁴⁹

Another gas detector containing a protective layer of a zeolite with a thickness of approximately 10 µm was applied as ammonia sensor in the control loop of a NOₓ storage catalyst for diesel engines.⁵¹ It was reported that ammonia could be detected in oxygen-rich gases containing nitrogen oxides such as exhaust gases or flue gases, using a condenser based on a hydrophobic, noble metal-free zeolite of low acidity having an ordered crystalline structure such as zeolite Y, mordenite or ZSM-5.⁵²

**3.6.2 MOFs for sensing NH₃.** The floating gate field effect transistor (FGFET) gas sensor consisting of a gold electrode modified with HKUST-1, developed by Fleischer *et al.* and described in Section 3.1.2, also demonstrated sensitivity to 5 ppm of ammonia.¹⁴⁶ The authors reasoned that the small kinetic diameter of ammonia allowed for it to pass through the pores of the HKUST-1 overlayer and interact with the gold electrode, thus causing a measurable change in work function. However, the magnitude of the change in the work function was similar to that measured upon exposure to 3 ppm of hydrogen sulfide.

Humphrey *et al.* have developed the PCM-15 MOF previously reported as a detector material for humidity (Section 3.1.2) for the detection of other gases and vapours.¹³⁴,¹³⁵ In particular, ammonia was found to be a more efficient quencher of the luminescence of the terbium(III) ion than water. Upon activation of the MOF at 150 °C under vacuum for 1 h, the intensity of the photoluminescence due to the terbium ion was found to double. Then upon treatment with 1 atm of ammonia gas, the photoluminescence intensity decreased 1.4 times relative to the photoluminescence intensity measured when the MOF was exposed to water vapour. PCM-15 exhibited selectivity to ammonia over hydrogen when exposed to low concentrations of ammonia gas in hydrogen carrier gas. This shows promise in detection of low concentrations of impurity gases or vapours in feedstock gases, but the major disadvantage with this system is that the detection of ammonia is irreversible.

Dincă *et al.* reported two MOFs that function as selective detectors for ammonia at 100 °C, but not at room temperature.¹⁷³ [Zn₂.tpce)] (tpce = tetrakis(phenylcarboxylate)ethylene, Fig. 18) and [Mg₂(dobdc)] (dobdc = 2,5-dioxobenzene-1,4-dicarboxylate, Fig. 18), both displayed fluorescence “turn-on” upon exposure to ammonia as opposed to the typical fluorescence “turn-off” mechanism, otherwise known as quenching. Fluorescence “turn-on” describes the interaction between an analyte and a “dark” fluorophore resulting in the “switch on” of the “dark” species so that it luminesces. In addition the maximum emission wavelengths of both MOFs shifted upon interaction with ammonia at 100 °C. [Zn₂.tpce]] exhibited a emission wavelength redshift from 487 nm to 511 nm when exposed to ammonia at 100 °C. In addition [Zn₂.tpce]] was thermally stable up to ~400 °C. Exposure to ammonia led to an irreversible phase change in [Zn₂.tpce]] to an unidentified crystalline form, so the detection of ammonia is irreversible. However, [Mg₂(dobdc)], in which Mg²⁺ ions are coordinated by dobdc ions, exhibited a

reversible “turn-on” fluorescence response upon exposure to ammonia at 100 °C, after activation at 180 °C for 12 h. This MOF also exhibited selectivity over methanol and ethanol and was thermally stable up to ~250 °C.

## 3.7 Hydrogen sulfide sensors

**3.7.1 MOFs for sensing H₂S.** While there have been no reports of zeolites for sensing H₂S in the literature, there have been reports of MOF materials having sensitivity for H₂S. For example, Lu *et al.* reported a nanoparticulate copper(I) MOF, with a ligand based on 3-amino-1,2,4-triazole-5-thiol (Fig. 19), that exhibited a decrease in fluorescence intensity and a visible colour change upon exposure to hydrogen sulfide gas.¹⁷⁴

The material demonstrated a remarkable fluorescence Stokes shift of large magnitude upon excitation at 365 nm when dispersed in DMF. The resulting red fluorescence emission, λmax = 620 nm, was quenched by as little as 2 ppm of H₂S in nitrogen carrier gas, but not by other gases or vapours such as CO₂, ethanol or even air. In addition an accompanying colour change from yellow to brown could also act as an indicator of hydrogen sulfide exposure. The proposed mechanism for fluorescence quenching involves a reaction between the MOF and H₂S to form a copper sulfide, Cu₉S₈ suggesting that the detection mechanism is irreversible.

The FGFET gas sensor modified with HKUST-1 reported by Fleischer *et al.*, and described in Section 3.1.2, also demonstrated sensitivity to 3 ppm of hydrogen sulfide.¹⁴⁶ The authors reasoned that the small kinetic diameter of hydrogen sulfide allowed for it to pass through the pores of the HKUST-1 overlayer and interact with the gold electrode. However, the magnitude of the change in the work function was similar to 5 ppm of ammonia.

## 3.8 Sulfur dioxide sensors

**3.8.1 Zeolites for sensing SO₂.** Sulfur dioxide contributes to acid rain and decreased air quality and is subject to legislated emissions limits. The sensors currently used for controlling the

<figure><img src="image_54.png" /><figcaption>Fig. 19 The 3-amino-1,2,4-triazole-5-thiol ligand used by Lu *et al.* to synthesise MOF nanoparticles which were fluorescently quenched in the presence of hydrogen sulfide gas.¹⁷⁴</figcaption></figure>

<figure><img src="image_55.png" /><figcaption>Fig. 18 The tetrakis(phenylcarboxylate)ethylene (tpce) ligand (left) and 2,5-dioxobenzene-1,4-dicarboxylate (dobdc) ligand (right) in the MOFs reported by Dinč *et al.* for detection of ammonia.¹⁷⁵</figcaption></figure>
<figure><img src="image_55.png" /></figure>

sensor based on the conductivity of metal oxide was reported. Indeed, semiconducting zinc oxide covered with zeolite (Y, mordenite or beta polymorph A) showed sensitivity down to a few ppm concentration of NH₃.⁴⁹

Another gas detector containing a protective layer of a zeolite with a thickness of approximately 10 µm was applied as ammonia sensor in the control loop of a NOₓ storage catalyst for diesel engines.⁵¹ It was reported that ammonia could be detected in oxygen-rich gases containing nitrogen oxides such as exhaust gases or flue gases, using a condenser based on a hydrophobic, noble metal-free zeolite of low acidity having an ordered crystalline structure such as zeolite Y, mordenite or ZSM-5.⁵²

**3.6.2 MOFs for sensing NH₃.** The floating gate field effect transistor (FGFET) gas sensor consisting of a gold electrode modified with HKUST-1, developed by Fleischer *et al.* and described in Section 3.1.2, also demonstrated sensitivity to 5 ppm of ammonia.¹⁴⁶ The authors reasoned that the small kinetic diameter of ammonia allowed for it to pass through the pores of the HKUST-1 overlayer and interact with the gold electrode, thus causing a measurable change in work function. However, the magnitude of the change in the work function was similar to that measured upon exposure to 3 ppm of hydrogen sulfide.

Humphrey *et al.* have developed the PCM-15 MOF previously reported as a detector material for humidity (Section 3.1.2) for the detection of other gases and vapours.¹³⁴,¹³⁵ In particular, ammonia was found to be a more efficient quencher of the luminescence of the terbium(III) ion than water. Upon activation of the MOF at 150 °C under vacuum for 1 h, the intensity of the photoluminescence due to the terbium ion was found to double. Then upon treatment with 1 atm of ammonia gas, the photoluminescence intensity decreased 1.4 times relative to the photoluminescence intensity measured when the MOF was exposed to water vapour. PCM-15 exhibited selectivity to ammonia over hydrogen when exposed to low concentrations of ammonia gas in hydrogen carrier gas. This shows promise in detection of low concentrations of impurity gases or vapours in feedstock gases, but the major disadvantage with this system is that the detection of ammonia is irreversible.

Dincă *et al.* reported two MOFs that function as selective detectors for ammonia at 100 °C, but not at room temperature.¹⁷³ [Zn₂.tpce)] (tpce = tetrakis(phenylcarboxylate)ethylene, Fig. 18) and [Mg₂(dobdc)] (dobdc = 2,5-dioxobenzene-1,4-dicarboxylate, Fig. 18), both displayed fluorescence “turn-on” upon exposure to ammonia as opposed to the typical fluorescence “turn-off” mechanism, otherwise known as quenching. Fluorescence “turn-on” describes the interaction between an analyte and a “dark” fluorophore resulting in the “switch on” of the “dark” species so that it luminesces. In addition the maximum emission wavelengths of both MOFs shifted upon interaction with ammonia at 100 °C. [Zn₂.tpce]] exhibited a emission wavelength redshift from 487 nm to 511 nm when exposed to ammonia at 100 °C. In addition [Zn₂.tpce]] was thermally stable up to ~400 °C. Exposure to ammonia led to an irreversible phase change in [Zn₂.tpce]] to an unidentified crystalline form, so the detection of ammonia is irreversible. However, [Mg₂(dobdc)], in which Mg²⁺ ions are coordinated by dobdc ions, exhibited a

reversible “turn-on” fluorescence response upon exposure to ammonia at 100 °C, after activation at 180 °C for 12 h. This MOF also exhibited selectivity over methanol and ethanol and was thermally stable up to ~250 °C.

## 3.7 Hydrogen sulfide sensors

**3.7.1 MOFs for sensing H₂S.** While there have been no reports of zeolites for sensing H₂S in the literature, there have been reports of MOF materials having sensitivity for H₂S. For example, Lu *et al.* reported a nanoparticulate copper(I) MOF, with a ligand based on 3-amino-1,2,4-triazole-5-thiol (Fig. 19), that exhibited a decrease in fluorescence intensity and a visible colour change upon exposure to hydrogen sulfide gas.¹⁷⁴

The material demonstrated a remarkable fluorescence Stokes shift of large magnitude upon excitation at 365 nm when dispersed in DMF. The resulting red fluorescence emission, λmax = 620 nm, was quenched by as little as 2 ppm of H₂S in nitrogen carrier gas, but not by other gases or vapours such as CO₂, ethanol or even air. In addition an accompanying colour change from yellow to brown could also act as an indicator of hydrogen sulfide exposure. The proposed mechanism for fluorescence quenching involves a reaction between the MOF and H₂S to form a copper sulfide, Cu₉S₈ suggesting that the detection mechanism is irreversible.

The FGFET gas sensor modified with HKUST-1 reported by Fleischer *et al.*, and described in Section 3.1.2, also demonstrated sensitivity to 3 ppm of hydrogen sulfide.¹⁴⁶ The authors reasoned that the small kinetic diameter of hydrogen sulfide allowed for it to pass through the pores of the HKUST-1 overlayer and interact with the gold electrode. However, the magnitude of the change in the work function was similar to 5 ppm of ammonia.

## 3.8 Sulfur dioxide sensors

**3.8.1 Zeolites for sensing SO₂.** Sulfur dioxide contributes to acid rain and decreased air quality and is subject to legislated emissions limits. The sensors currently used for controlling the

<figure><img src="image_56.png" /><figcaption>Fig. 19 The 3-amino-1,2,4-triazole-5-thiol ligand used by Lu *et al.* to synthesise MOF nanoparticles which were fluorescently quenched in the presence of hydrogen sulfide gas.¹⁷⁴</figcaption></figure>

<figure><img src="image_57.png" /><figcaption>Fig. 18 The tetrakis(phenylcarboxylate)ethylene (tpce) ligand (left) and 2,5-dioxobenzene-1,4-dicarboxylate (dobdc) ligand (right) in the MOFs reported by Dinč *et al.* for detection of ammonia.¹⁷⁵</figcaption></figure>
combustion process in track, car and jet engines suffer from poisoning by sulfur oxides. A gas-absorbing layer for trapping SO₂ prior to sensing the other gases at high-temperature is still a challenging assignment.

The history of sulfur dioxide removal from gas streams by using zeolite adsorbents is lengthy. A comparative study including natural and synthetic zeolites revealed that the SO₂ capacity of the latter is higher, which is due to higher purity and the controlled characteristics of the synthetic materials.²⁰⁰ In the 1970s, synthetic mordenite- and faujasite (FAU)-type zeolites were employed for SO₂ adsorption because of their stability under acidic conditions. A general understanding is that the sulfur dioxide is adsorbed by hydrogen bonding to one or two conveniently positioned surface hydroxyl groups of the zeolites. It was found that the stronger the acidity, the higher the electron deficiency, the better the sulfur resistance. It was also found that the adsorption of sulfur on Pd-containing FAU zeolite is reversible.¹²⁴,¹²⁵ The investigation of SO₂ adsorption on silicalite-1 and de-aluminated zeolite Y showed the supremacy of all-silica pentasil-type molecular sieve towards SO₂.

Faujasite-type zeolites were reported to be useful for sensing SO₂ when combined with metal species. In a first example, a layer of faujasite-type zeolite deposited on QCM oscillators, with an electrode of gold, is active as the increased mass due to absorbed SO₂ reduces the frequency of the QCM.¹⁰¹ In further work zeolite A, was deposited on the QCM. The zeolite-A doped QCM could monitor 50 ppm of SO₂ in He at 170 °C.⁵⁷

More recently, the effect of various transition metal cations (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) exchanged into hydrated Y-type zeolites for SO₂ sensing were investigated by electrical conductivity.¹²⁶ When exposed to a SO₂ atmosphere, all poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid)/metal doped zeolite Y composites, and especially the Mn²⁺ doped zeolite, showed higher sensitivity than a pristine conductive polymer matrix used as reference though at the expense of longer response times. The authors attributed those results to the electronegativity of the metal cations which strongly improved the adsorption properties of the zeolite Y, and hence the electrical sensitivity of the composite.

Often sensing of different gases by zeolite molecular sieves faces problems due to catalytic side reactions. For instance, the sodium form of zeolite Y demonstrated impressive adsorptive and catalytic properties in NOₓ elimination from exhaust gases of diesel and lean burn engines. Physisorbed SO₂, HSO₃⁻ and S₂O₅²⁻ species in the Na-Y adsorbent were observed, and the poisoning of N₂O₃ adsorption sites by SO₂ was found to be a function of sodium hydrogen sulfite formed in the channels. A possibility of overcoming the poisoning of the N₂O₃ adsorption capacity of Na-Y zeolite by SO₂ was based on short adsorption-desorption cycles, high temperature and low oxygen concentrations during the desorption process. Sultana *et al*.¹²⁷ studied the adsorptive separation of NOₓ in the presence of SOₓ on Na-Y zeolite. They found that when Na-Y zeolite operates in a pressure swing process for NOₓ removal from engine exhausts, it is fairly resistant to SO₂. However, poisoning by SO₂ could be suppressed by using short adsorption-desorption cycles, by limiting the

oxygen concentration in the regeneration gas and by working at temperatures higher than 255 °C.

The elimination of SO₂ by microporous zeolite-type materials is based on either low or high silica zeolites. The SO₂ molecule reacts with the metals situated in the pores of the zeolite structure leading to metal sulfate formation. This leads to channel blocking and a substantial decrease of transport is observed. Another disadvantage of low silica zeolite materials is their hydrophilic nature. Thus, the gas adsorption capacity is dramatically reduced by concurrent water adsorption. Furthermore, low-silica zeolites are not adapted for SO₂ retention since their chemical stability in acidic media is limited. The increase of silicon content in zeolites leads to a decrease of their hydrophilicity, which can reach a level when the water molecules are rejected by the microporous structure. Such types of zeolitic materials are often employed in selective adsorption of SO₂ and other sulfur compounds. The main advantage of these materials is their higher stability than other absorbers towards SO₂ and possible regeneration for multiple usages due to their high thermal stability (900 °C) and high stability in an acidic media. In summary, the high silica zeolite films will allow discrimination of H₂O molecules and selective adsorption of SO₂. The latter is particularly important to retain the high capacity of the zeolite sorbent. SO₂ detection on high silica zeolites is essentially unaffected by the presence of CO₂ and other gases.

No examples of the use of MOFs for SO₂ sensing were found in the literature. However, it should be noted that sulfur dioxide (SO₂) adsorption in MOFs has been reported,²⁰¹⁻²⁰⁵ thus MOFs that possess suitable stability for automotive exhaust gas sensing hold promise as potential sensing materials for SO₂.

## 3.9 Hydrogen sensors

### 3.9.1 Zeolites for sensing of H₂
Metal oxides impregnated into zeolite pores have been used to achieve sensitive H₂ sensing. One example sensor by Wark *et al*. was described in Section 3.4.1 as a CO sensor. The detector consisted of titanium dioxide clusters immobilised in the pores of zeolite NaY and it showed similar stability and response times towards H₂ as it did to CO.¹¹⁴ Other reports described the use of tin oxide impregnated on the external surface of zeolite Y.²⁰⁶,²⁰⁷ The composite material obtained showed a linear resistance response to the adsorption of H₂.

Silicalite zeolite can also be used as a filter. As described for humidity sensors, Santamaria *et al*. showed that the silicalite coating on SnO₂ gave enhanced response to both H₂O and H₂.⁵⁸ Recently, a fibre optic sensor consisting of a proton-conducting SrCe₀.₈Zr₀.₁Y₀.₁O₂.₉₅ thin film overcoated by silicalite zeolite layer was reported.⁹³ Pores of the zeolitic filter were found to be small enough to permit H₂ diffusion but restricted the diffusion of other gases and fine particles from dusty coal or biomass gasification processes. When the silicalite film was less than 2.5 µm thick, a high sensitivity of 0.14 nm kPa⁻¹·H₂ was observed.

### 3.9.2 MOFs for sensing of H₂
As described above, there are two uses of MOFs in gas sensors; as a functional active sensing element or as an auxiliary element. For sensing gaseous hydrogen, H₂, MOFs have been used as auxiliary filter layer elements, enhancing the selectivity of the sensor device to hydrogen.
combustion process in track, car and jet engines suffer from poisoning by sulfur oxides. A gas-absorbing layer for trapping SO₂ prior to sensing the other gases at high-temperature is still a challenging assignment.

The history of sulfur dioxide removal from gas streams by using zeolite adsorbents is lengthy. A comparative study including natural and synthetic zeolites revealed that the SO₂ capacity of the latter is higher, which is due to higher purity and the controlled characteristics of the synthetic materials.²⁰⁰ In the 1970s, synthetic mordenite- and faujasite (FAU)-type zeolites were employed for SO₂ adsorption because of their stability under acidic conditions. A general understanding is that the sulfur dioxide is adsorbed by hydrogen bonding to one or two conveniently positioned surface hydroxyl groups of the zeolites. It was found that the stronger the acidity, the higher the electron deficiency, the better the sulfur resistance. It was also found that the adsorption of sulfur on Pd-containing FAU zeolite is reversible.¹²⁴,¹²⁵ The investigation of SO₂ adsorption on silicalite-1 and de-aluminated zeolite Y showed the supremacy of all-silica pentasil-type molecular sieve towards SO₂.

Faujasite-type zeolites were reported to be useful for sensing SO₂ when combined with metal species. In a first example, a layer of faujasite-type zeolite deposited on QCM oscillators, with an electrode of gold, is active as the increased mass due to absorbed SO₂ reduces the frequency of the QCM.¹⁰¹ In further work zeolite A, was deposited on the QCM. The zeolite-A doped QCM could monitor 50 ppm of SO₂ in He at 170 °C.⁵⁷

More recently, the effect of various transition metal cations (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) exchanged into hydrated Y-type zeolites for SO₂ sensing were investigated by electrical conductivity.¹²⁶ When exposed to a SO₂ atmosphere, all poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid)/metal doped zeolite Y composites, and especially the Mn²⁺ doped zeolite, showed higher sensitivity than a pristine conductive polymer matrix used as reference though at the expense of longer response times. The authors attributed those results to the electronegativity of the metal cations which strongly improved the adsorption properties of the zeolite Y, and hence the electrical sensitivity of the composite.

Often sensing of different gases by zeolite molecular sieves faces problems due to catalytic side reactions. For instance, the sodium form of zeolite Y demonstrated impressive adsorptive and catalytic properties in NOₓ elimination from exhaust gases of diesel and lean burn engines. Physisorbed SO₂, HSO₃⁻ and S₂O₅²⁻ species in the Na-Y adsorbent were observed, and the poisoning of N₂O₃ adsorption sites by SO₂ was found to be a function of sodium hydrogen sulfite formed in the channels. A possibility of overcoming the poisoning of the N₂O₃ adsorption capacity of Na-Y zeolite by SO₂ was based on short adsorption-desorption cycles, high temperature and low oxygen concentrations during the desorption process. Sultana *et al*.¹²⁷ studied the adsorptive separation of NOₓ in the presence of SOₓ on Na-Y zeolite. They found that when Na-Y zeolite operates in a pressure swing process for NOₓ removal from engine exhausts, it is fairly resistant to SO₂. However, poisoning by SO₂ could be suppressed by using short adsorption-desorption cycles, by limiting the

oxygen concentration in the regeneration gas and by working at temperatures higher than 255 °C.

The elimination of SO₂ by microporous zeolite-type materials is based on either low or high silica zeolites. The SO₂ molecule reacts with the metals situated in the pores of the zeolite structure leading to metal sulfate formation. This leads to channel blocking and a substantial decrease of transport is observed. Another disadvantage of low silica zeolite materials is their hydrophilic nature. Thus, the gas adsorption capacity is dramatically reduced by concurrent water adsorption. Furthermore, low-silica zeolites are not adapted for SO₂ retention since their chemical stability in acidic media is limited. The increase of silicon content in zeolites leads to a decrease of their hydrophilicity, which can reach a level when the water molecules are rejected by the microporous structure. Such types of zeolitic materials are often employed in selective adsorption of SO₂ and other sulfur compounds. The main advantage of these materials is their higher stability than other absorbers towards SO₂ and possible regeneration for multiple usages due to their high thermal stability (900 °C) and high stability in an acidic media. In summary, the high silica zeolite films will allow discrimination of H₂O molecules and selective adsorption of SO₂. The latter is particularly important to retain the high capacity of the zeolite sorbent. SO₂ detection on high silica zeolites is essentially unaffected by the presence of CO₂ and other gases.

No examples of the use of MOFs for SO₂ sensing were found in the literature. However, it should be noted that sulfur dioxide (SO₂) adsorption in MOFs has been reported,²⁰¹⁻²⁰⁵ thus MOFs that possess suitable stability for automotive exhaust gas sensing hold promise as potential sensing materials for SO₂.

## 3.9 Hydrogen sensors

### 3.9.1 Zeolites for sensing of H₂.
Metal oxides impregnated into zeolite pores have been used to achieve sensitive H₂ sensing. One example sensor by Wark *et al*. was described in Section 3.4.1 as a CO sensor. The detector consisted of titanium dioxide clusters immobilised in the pores of zeolite NaY and it showed similar stability and response times towards H₂ as it did to CO.¹¹⁴ Other reports described the use of tin oxide impregnated on the external surface of zeolite Y.²⁰⁶,²⁰⁷ The composite material obtained showed a linear resistance response to the adsorption of H₂.

Silicalite zeolite can also be used as a filter. As described for humidity sensors, Santamaria *et al*. showed that the silicalite coating on SnO₂ gave enhanced response to both H₂O and H₂.⁵⁸ Recently, a fibre optic sensor consisting of a proton-conducting SrCe₀.₈Zr₀.₁Y₀.₁O₂.₉₅ thin film overcoated by silicalite zeolite layer was reported.⁹³ Pores of the zeolitic filter were found to be small enough to permit H₂ diffusion but restricted the diffusion of other gases and fine particles from dusty coal or biomass gasification processes. When the silicalite film was less than 2.5 µm thick, a high sensitivity of 0.14 nm kPa⁻¹·H₂ was observed.

### 3.9.2 MOFs for sensing of H₂.
As described above, there are two uses of MOFs in gas sensors; as a functional active sensing element or as an auxiliary element. For sensing gaseous hydrogen, H₂, MOFs have been used as auxiliary filter layer elements, enhancing the selectivity of the sensor device to hydrogen.
<figure><img src="image_56.png" /></figure>

Hupp et al. developed an optical sensor device that utilised Bragg grating stacks made from subsequential layers of ZIF-8 and palladium. A Bragg grating stack operates as a wavelength-selective mirror; some wavelengths are reflected whereas other wavelengths are transmitted. The wavelength reflected depends on the refractive index of the materials in the Bragg stack. If the refractive index of some of the layers within the Bragg grating stack changes then the wavelengths that are reflected also change. Hupp et al.'s Bragg grating stack comprised of two sets of a bilayer structure of Pd (~10 nm)/ZIF-8 (~300 nm) that operate at visible-light wavelengths, and the authors predicted, that upon exposure to certain analyte gases, the refractive index of the ZIF-8 layers would change due to adsorption of the analyte gases.¹⁶² Upon exposure to vapours of short chain length alkanes, which all have kinetic diameters >4 Å,²⁰⁸ the refractive index of the ZIF-8 layers increased due to adsorption of these compounds. However, the sensor device did not offer differentiation between the alkane analytes. In contrast, when the sensor was exposed to increasing concentrations of H₂ in N₂ carrier gas, from 0% up to 5%, the wavelengths reflected did not change but the transmission intensity increased. The authors therefore postulated that the H₂ was insignificantly adsorbed within or on the ZIF-8 layers so as not to change the refractive index of the Bragg grating stack, but was reversibly adsorbed onto the palladium layer and forming a palladium hydride that is more transparent in the visible wavelengths than the ~10 nm thick palladium layer. Therefore, by measuring the transmission intensity of the device during exposure to analytes, it was observed that the device possessed a selective affinity for hydrogen gas molecules over short alkyl chain vapour molecules which is due to difference in kinetic diameters; the kinetic diameter of H₂ is 2.89 Å and the pore window size of ZIF-8 is 3.40 Å.²⁰⁹ Therefore the ZIF-8 layers acted as size selective filter layers. Sensing experiments with carbon dioxide, CO₂, which has a kinetic diameter of 3.30 Å²¹⁰ further confirmed that ZIF-8 acted as a size selective filter layer. Upon exposure to CO₂ an increase in transmission intensity was measured; carbon dioxide is smaller than the pore size window of ZIF-8 and thus passes through the ZIF-8 pores and interacts with the palladium layer.

Dam et al. also utilised a MOF layer as a filter to achieve selective optical detection of hydrogen gas.¹⁴⁷ The authors chose a copperbtc material as the gas selective filter layer as this had been shown to possess increased selectivity towards H₂ when in a thin film. Dam et al. deposited the MOF thin film through a layer-by-layer deposition process onto a palladium substrate that had been initially covered with a self-assembled monolayer of glycine. With this technique, the MOF thin film was found to be mostly amorphous though the authors argued that the chemical environment within this film was similar to that within HKUST-1 based on FTIR spectral data. The resulting sensor was interrogated with optical transmission spectroscopy whilst exposed to a H₂ environment. Increases in the optical transmission were measured upon exposure to hydrogen gas at various concentrations at room temperature and 100 °C, but concentration differentiation was greatest when measurements

were taken at 100 °C. The device showed stable repeatable and reversible responses, but the response time was on the order of 1000 s. Selectivity to hydrogen gas over O₂, CO₂ and CH₄ in gas mixtures was demonstrated, but the kinetics of adsorption of hydrogen were significantly decreased when the device was exposed to H₂–CO and H₂–H₂O mixtures. This could be due to the affinity of the copper MOF for H₂O and CO; these gases preferentially adsorb and thus hinder the ingress of H₂.²¹¹,²¹²

## 4. Conclusions and future perspectives

It is clear that porous materials, exemplified by zeolites and MOFs, offer tremendous flexibility and versatility in terms of selectivity and sensitivity and thus demonstrate great promise for the fabrication of sensors for exhaust gases and vapours. Both size and shape selectivity have been demonstrated with zeolite- and MOF-based detectors *via* control of the dimensions of the pores, and in some cases the crystal orientation when in ordered films, thus enabling detection and differentiation between gases and vapours. Chemical selectivity and sensitivity have been controlled by the incorporation of select dopants into the frameworks so as to elicit desirable optical and or electrochemical responses upon interaction with analytes.

In this review, we summarised 170 different published sensors including around a third based on MOFs and two-thirds based on zeolites (Fig. 20). Among those publications, hydrocarbon sensors are currently the most common for both zeolites and MOFs. Zeolite and MOF sensors were developed in similar numbers for automotive exhaust vapours, though no NOx and SO₂ sensors based on MOFs were reported whereas only MOF-based sensors have been reported to detect H₂S.

The greater chemical and thermal stability of zeolites, coupled with the ability to be more easily integrated into smaller electrical

<figure><img src="image_57.png" /><figcaption>Fig. 20 Reported sensors based on zeolites or MOFs along with the type of sensors.</figcaption></figure>
<figure><img src="image_58.png" /></figure>

Hupp et al. developed an optical sensor device that utilised Bragg grating stacks made from subsequential layers of ZIF-8 and palladium. A Bragg grating stack operates as a wavelength-selective mirror; some wavelengths are reflected whereas other wavelengths are transmitted. The wavelength reflected depends on the refractive index of the materials in the Bragg stack. If the refractive index of some of the layers within the Bragg grating stack changes then the wavelengths that are reflected also change. Hupp et al.'s Bragg grating stack comprised of two sets of a bilayer structure of Pd (~10 nm)/ZIF-8 (~300 nm) that operate at visible-light wavelengths, and the authors predicted, that upon exposure to certain analyte gases, the refractive index of the ZIF-8 layers would change due to adsorption of the analyte gases.¹⁶² Upon exposure to vapours of short chain length alkanes, which all have kinetic diameters >4 Å,²⁰⁸ the refractive index of the ZIF-8 layers increased due to adsorption of these compounds. However, the sensor device did not offer differentiation between the alkane analytes. In contrast, when the sensor was exposed to increasing concentrations of H₂ in N₂ carrier gas, from 0% up to 5%, the wavelengths reflected did not change but the transmission intensity increased. The authors therefore postulated that the H₂ was insignificantly adsorbed within or on the ZIF-8 layers so as not to change the refractive index of the Bragg grating stack, but was reversibly adsorbed onto the palladium layer and forming a palladium hydride that is more transparent in the visible wavelengths than the ~10 nm thick palladium layer. Therefore, by measuring the transmission intensity of the device during exposure to analytes, it was observed that the device possessed a selective affinity for hydrogen gas molecules over short alkyl chain vapour molecules which is due to difference in kinetic diameters; the kinetic diameter of H₂ is 2.89 Å and the pore window size of ZIF-8 is 3.40 Å.²⁰⁹ Therefore the ZIF-8 layers acted as size selective filter layers. Sensing experiments with carbon dioxide, CO₂, which has a kinetic diameter of 3.30 Å²¹⁰ further confirmed that ZIF-8 acted as a size selective filter layer. Upon exposure to CO₂ an increase in transmission intensity was measured; carbon dioxide is smaller than the pore size window of ZIF-8 and thus passes through the ZIF-8 pores and interacts with the palladium layer.

Dam et al. also utilised a MOF layer as a filter to achieve selective optical detection of hydrogen gas.¹⁴⁷ The authors chose a copperbtc material as the gas selective filter layer as this had been shown to possess increased selectivity towards H₂ when in a thin film. Dam et al. deposited the MOF thin film through a layer-by-layer deposition process onto a palladium substrate that had been initially covered with a self-assembled monolayer of glycine. With this technique, the MOF thin film was found to be mostly amorphous though the authors argued that the chemical environment within this film was similar to that within HKUST-1 based on FTIR spectral data. The resulting sensor was interrogated with optical transmission spectroscopy whilst exposed to a H₂ environment. Increases in the optical transmission were measured upon exposure to hydrogen gas at various concentrations at room temperature and 100 °C, but concentration differentiation was greatest when measurements

were taken at 100 °C. The device showed stable repeatable and reversible responses, but the response time was on the order of 1000 s. Selectivity to hydrogen gas over O₂, CO₂ and CH₄ in gas mixtures was demonstrated, but the kinetics of adsorption of hydrogen were significantly decreased when the device was exposed to H₂–CO and H₂–H₂O mixtures. This could be due to the affinity of the copper MOF for H₂O and CO; these gases preferentially adsorb and thus hinder the ingress of H₂.²¹¹,²¹²

## 4. Conclusions and future perspectives

It is clear that porous materials, exemplified by zeolites and MOFs, offer tremendous flexibility and versatility in terms of selectivity and sensitivity and thus demonstrate great promise for the fabrication of sensors for exhaust gases and vapours. Both size and shape selectivity have been demonstrated with zeolite- and MOF-based detectors *via* control of the dimensions of the pores, and in some cases the crystal orientation when in ordered films, thus enabling detection and differentiation between gases and vapours. Chemical selectivity and sensitivity have been controlled by the incorporation of select dopants into the frameworks so as to elicit desirable optical and or electrochemical responses upon interaction with analytes.

In this review, we summarised 170 different published sensors including around a third based on MOFs and two-thirds based on zeolites (Fig. 20). Among those publications, hydrocarbon sensors are currently the most common for both zeolites and MOFs. Zeolite and MOF sensors were developed in similar numbers for automotive exhaust vapours, though no NOx and SO₂ sensors based on MOFs were reported whereas only MOF-based sensors have been reported to detect H₂S.

The greater chemical and thermal stability of zeolites, coupled with the ability to be more easily integrated into smaller electrical

<figure><img src="image_59.png" /><figcaption>Fig. 20 Reported sensors based on zeolites or MOFs along with the type of sensors.</figcaption></figure>
<figure><img src="image_58.png" /></figure>

sensors, means zeolites may be the most suitable sensing materials for use in prospective on-board exhaust gas sensors. The lower chemical and thermal stability of MOFs suggests that these materials are more suited to fabricate sensors for exhaust gas emissions during commissioning of new vehicles in production; location in a production setting will allow for appropriate gas cooling and clean-up before sensing which would be impractical on-board vehicles.

The wide variety of organic linkers and metal nodes that can be incorporated into MOFs has allowed for increased sensitivity and selectivity based on control of the interactions between adsorbed analytes. Finally, it needs to be acknowledged that the field of MOFs for sensing is young in comparison to the research into the use of zeolites as sensor materials. While in most cases at the current time, zeolite-based sensors are more advanced than MOF-based analogues, it is expected that advances in discriminatory sensing, sensitivity and thermal stability will enable this gap to be closed in the future.

Despite their promise, few of these systems have to date been incorporated into devices that allow for real-time and on-line data collection and thus most only exist as detectors or probes under specific conditions. In addition, few examples of these materials have been shown to exhibit successful sensing of specific species when exposed to varying temperatures, pressures and in the presence of contaminants.

The sensitivity and selectivity that can be explicitly controlled by sensor designers when using MOFs and zeolites and the great need economically and environmentally for energy efficient transport indicates that the development of porous materials for automotive gas sensing will continue to be an active research field in the future.

## Acknowledgements

Financial support from Materials for Energy Efficiency in Transport (MEET) funded by the European Regional Development Fund (ERDF) INTERREG IV programme is acknowledged.

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<figure><img src="image_60.png" /></figure>

sensors, means zeolites may be the most suitable sensing materials for use in prospective on-board exhaust gas sensors. The lower chemical and thermal stability of MOFs suggests that these materials are more suited to fabricate sensors for exhaust gas emissions during commissioning of new vehicles in production; location in a production setting will allow for appropriate gas cooling and clean-up before sensing which would be impractical on-board vehicles.

The wide variety of organic linkers and metal nodes that can be incorporated into MOFs has allowed for increased sensitivity and selectivity based on control of the interactions between adsorbed analytes. Finally, it needs to be acknowledged that the field of MOFs for sensing is young in comparison to the research into the use of zeolites as sensor materials. While in most cases at the current time, zeolite-based sensors are more advanced than MOF-based analogues, it is expected that advances in discriminatory sensing, sensitivity and thermal stability will enable this gap to be closed in the future.

Despite their promise, few of these systems have to date been incorporated into devices that allow for real-time and on-line data collection and thus most only exist as detectors or probes under specific conditions. In addition, few examples of these materials have been shown to exhibit successful sensing of specific species when exposed to varying temperatures, pressures and in the presence of contaminants.

The sensitivity and selectivity that can be explicitly controlled by sensor designers when using MOFs and zeolites and the great need economically and environmentally for energy efficient transport indicates that the development of porous materials for automotive gas sensing will continue to be an active research field in the future.

## Acknowledgements

Financial support from Materials for Energy Efficiency in Transport (MEET) funded by the European Regional Development Fund (ERDF) INTERREG IV programme is acknowledged.

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<figure><img src="image_62.png" /></figure>

Review Article

Chem Soc Rev

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203 T. G. Glover, G. W. Peterson, B. J. Schindler, D. Britt and O. Yaghi, *Chem. Eng. Sci.*, 2011, **66**, 163.

204 S. Yang, J. Sun, A. J. Ramirez-Cuesta, S. K. Callear, W. I. F. David, D. P. Anderson, R. Newby, A. J. Blake, J. E. Parker, C. C. Tang and M. Schröder, *Nat. Chem.*, 2012, **4**, 887.

205 K. Tan, P. Canepa, Q. Gong, J. Liu, D. H. Johnson, A. Dyevoich, P. K. Thallapally, T. Thonhauser, J. Li and Y. J. Chabal, *Chem. Mater.*, 2013, **25**, 4653.

206 X. Xu, J. Wang and Y. Long, *Microporous Mesoporous Mater.*, 2005, **83**, 60.

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208 S. J. Geier, J. A. Mason, E. D. Bloch, W. L. Queen, M. R. Hudson, C. M. Brown and J. R. Long, *Chem. Sci.*, 2013, **4**, 2054.

209 J. Caro and M. Noack, in *Advances in Nanoporous Materials*, ed. S. Ernst, Elsevier, Oxford, 2010, vol. 1, p. 1.

210 N. Mehio, S. Dai and D. Jiang, *J. Phys. Chem. A*, 2014, **118**, 1150.

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