Ind. Eng. Chem. Res. 2007, 46, 2335-2341
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An Optochemical Humidity Sensor Based on Immobilized Nile Red in Y Zeolite Ismael Pellejero,† Miguel Urbiztondo,† David Izquierdo,‡ Silvia Irusta,*,† In˜ igo Salinas,‡ and Marı´a P. Pina† Departamento de Ingenierı´a Quı´mica y Tecnologı´as del Medio Ambiente, Instituto UniVersitario de Nanociencia en Aragon, Plaza San Francisco, UniVersity of Zaragoza, Pedro Cerbuna 12, 50009, Zaragoza, Spain, C/ Marı´a de Luna, 1, 50018, Zaragoza, Spain, and Grupo de Tecnologı´as Foto´ nicas, Instituto de InVestigacio´ n en Ingenierı´a de Arago´ n, UniVersidad de Zaragoza
A simple and low-cost cost optical reflectance sensor has been developed based on the solvatochromic Nile Red immobilized within NaY zeolite supercages. The sensor performance has been tested with water and n-hexane vapors. The Nile Red-Y composite exhibits excellent properties as humidity sensor in terms of sensitivity (much lower than 200 ppm), response time (around 4 min), and dye stability toward migration upon high light exposures. Besides, the zeolite host provides an enhanced selectivity to moisture in mixtures where organic compounds are also present. The benefits of the dye encapsulation inside the zeolitic pores in terms of signal-to-noise ratio and baseline stability have also been demonstrated. Introduction The development of stand-alone devices for monitoring the concentration of selected chemical species in complex samples (or in different environments) has been a major goal in analytical science for several decades. Indeed, chemical microsensors play an increasing role in the areas of environmental monitoring and industrial processing. The application of nanomaterials to the design of chemical sensors is nowadays one of the most active research fields because of their high performance and small size.1 Particularly, the well-defined porous structure of zeolitic materials combined with their tunable adsorption properties and ion-exchange capability make them true shape-selective molecular sieves useful as specific adsorbents toward certain analytes.2-5 Among nonreactive systems, zeolite thin films located on the active areas of piezoelectric sensor devices (QCMs and microcantilevers) have been found to effectively control molecular access to the device allowing it to sense ethanol, humidity,6-8 or to conduct complex analysis of vapors.9-10 Catalytic and/or adsorbing properties of zeolites have also been used by the group of Plog and co-workers11-14 to improve interdigital capacitors (IDCs) performance using temperatures high enough to where chemical reactions may also occur (above 200 °C) and no water condensation occurs in the zeolite-pore system. Over the last years, more attention is being paid to sensors based on the optical properties of nanomaterials because of several advantages over conventional sensors: resistivity to electromagnetic noise, fire resistance, larger potential for miniaturization, and the capability to remote control and information transfer through an optical fiber network. Over electronic-based systems, optical sensing provides a large amount of information, which can be readily multiplexed, almost instantly. In a typical optical chemical sensor, an analytesensitive indicator, whose optical properties vary in the presence of such species, is employed for detecting chemicals. However, * To whom correspondence should be addressed. E-mail: sirusta@ unizar.es. † Departamento de Ingenierı´a Quı´mica y Tecnologı´as del Medio Ambiente. ‡ Grupo de Tecnologı´as Foto´nicas.
optical based sensors have the nagging problem of photostability or decomposition on extended exposure to light.15 This issue is exarcebated by the fact that the response depends on exciting the sample. Moreover, as many of the sensors are designed for use in fiber optic systems, the sensor is small and relatively few molecules must handle the high light exposures necessary for adequate signal-to-noise ratio. The high thermal and mechanical stability as well as optical transparency in the visible region and fastness toward ultraviolet radiation are some of the main features of zeolites for being used as organic chromophores host. Indeed, molecular-sieve-encapsulated dye molecules are currently attracting increasing attention with respect to new photonic devices and optical sensor applications16,17 due to an increased stability against chemical attack, light durability, migration stability, photobleaching, and thermal decomposition (up to 2 orders of magnitude) with respect to their organic counterparts. Moreover, the structural peculiarities of zeolite materials enable the incorporation of optically active guest molecules in crystallographically defined positions or highly organized arrangements resulting in peculiar host-guest interactions.18 A large variety of synthetic methods for the inclusion of dyes in molecular sieves, including dye synthesis in nanopores and encapsulation during the hydrothermal synthesis, have been already reported in the literature.16 For instance, the encapsulation of the solvatochromic dye Nile red inside the pores of dealuminated zeolite Y by “ship in a bottle” synthesis and dye adsorption has been investigated by Meinershagen and Bein19 for optical sensing of acetone and ethanol using spectral absorbance in the UV-vis range. The sensing principle is based on the chromophore spectral change as a function of the polarity/ dielectric constant of a solvent20. Nile red (NR), a hydrophobic fluorescent and solvatochromic dye of molecular formula C20H18-N2O2, with a length of 10 Å and a width of ∼6 Å, is one of the most interesting molecules highly sensitive to the polarity of the microenvironment 21. It has been described as displaying positive solvatochromism leading to a bathochromic shift of 128 nm upon measuring the UV-vis spectra in hexane (a nonpolar solvent) and water (a polar solvent)2 This large shift is due to large changes in the dipole moment of the first excited singlet state .23 The structure of FAU-type zeolite, with a unit cell of which consists of 8 supercages 11 Å in diameter and 16
10.1021/ie061025v CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006
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Table 1. Main Properties of the Prepared Samples dye content chemical analysis sample
color
(wt %)
(dye/SC)a
TGA (wt %)
temperature DTGAb
NaY NRY1 NRY2 NRYMM
white dark blue light blue Violet
2.3 1.5 2.0
0.11 0.07 0.09
7.5 1.1 1.9
723 K 883 K 723 K
a
Dye/supercage ratio evaluated according to the published literature.25
windows 7.4 Å in diameter, allows diffusion of NR molecules through the free diameter of the channels. When dye molecules are included in the FAU-type zeolite supercages, the solvent loading in the nanoscale cages in combination with the molecular sieving properties as well as hydrophilic-hydrophobic interactions, control the spectral signature of the dye. In this work, the same concept has been used for continuous water and hexane detection with a robust and low-cost configuration based on standard plastic optical fiber. In our case, the reflection of the light on the surface of the zeolite layer deposited onto a glass substrate placed at the end of an optical fiber has been continuously measured as a function of the atmosphere composition. As the analyte molecules are adsorbed on the dyezeolite composite, the reflectance spectrum of the embedded NR changes. The magnitude of these variations is related to the vapor concentration, which makes feasible the design of an optochemical sensor. A similar configuration with a vapochromic material as sensing layer and standard optical fiber working at telecommunication wavelengths (1310 nm) has been used for single volatile organic compound detection24 exhibiting recovery times of up to 20 min. The aim of this investigation is to demonstrate the potentialities of a simple and low-cost optical sensor using NR embedded in Y zeolite in terms of sensibility, selectivity, recovery-response time and photostability for hexane and water vapor detection. Experimental Commercial NaY zeolite (Si/Al ratio equal to 3) from Zeolyst International has been used as host. For the inclusion of Nile Red (standard Fluka) by adsorption, 0.20 g of dry zeolite were dispersed under vigorous stirring in a solution of the dye (1.3 mM in hexane) for 12 h at room temperature19 followed by washing. At this stage, dye remains over the internal and external surface of the dark-blue filtrated solid, which has been denoted as NRY1. To asses about the dye migration stability improvement on extended exposure to light attained in NR-zeolite composites, the above procedure was used to impregnate bare quartz (0.4 m2/g of external surface) obtaining a pink solid, denoted as NRQ. After filtration, some samples, denoted as NRY2, have been Soxhlet extracted with ethanol until the solvent remains colorless. After leaching, the dye-zeolite ensembles changes to sky blue and remain in atmospheric conditions for several days before any color changes were observed. Determination of dye content was performed by thermogravimetric analysis carried out in a Mettler Toledo TGA/SDTA (Model 851) system. The solids (usually 10 mg) were heated at 10 K min-1 to 1173 K under flowing air (90 mL min-1). Carbon elemental analyses in a Carlo Erba 1108 instrument have also been performed for comparison purposes. In order to ascertain about the deep incorporation of NR inside the zeolite host, a mechanical mixture 2 wt %, denoted as NRYMM, has also been prepared by addition of the required amounts of both
b
BET (m2/g)
micropore volume (cm3/g)
863.8 ( 1.8 313.9 ( 4.9 838.7 ( 0.7
0.307 0.140 0.290
Maximum of the DTGA peak associated to NR removal.
chemicals. The position of the maximum DTGA peak associated to NR removal provides us information on the host-guest interaction. N2 physisorption analysis have been performed using Micromeritics ASAP 2020 V1 device at 77 K to characterize the available specific area of the dye-zeolite composites for adsorption. Prior to adsorption experiment samples were degassed under vacuum (0.27 mbar) for 10 h at 110 °C. The micropore volume contribution was calculated using the t-plot method. All these results are summarized in Table 1. Water and n-hexane adsorption measurements have also been carried out at room temperature to assess about the adsorption capability toward the specific analytes of the as prepared samples. For such purpose, a dye/zeolite crystal suspension was dispersed over the surface of commercial quartz crystal microbalances (Fortiming Corporation, AT cut 10 MHz) by microdrop coating. The coating of the glass plates used for optical sensing has been carried out by the same procedure. The experimental setup used has been described elsewhere.8 Basically, the coated piezoelectric devices were placed in a 200mL chamber to which analytes at the desired concentration were fed. Frequency measurements, conducted with a Philips frequency measuring device (Programmable Timer/Counter, PM 6666), allowed continuous monitoring of the oscillation frequency of the QCM sensor, from which the mass adsorbed could be easily determined using the Sauerbrey equation.8 This optochemical sensor is based on the measurement of the diffuse reflectance of the dye encapsulated in Y-type zeolite. Plastic optical fiber is an adequate choice for this kind of measurement, as its relatively large core diameter (1 mm) and numerical aperture result in efficient capture of the reflected light, while the power density of guided light is enough to allow the use of a light-emitting diode (LED), cheaper and more stable than a laser diode, as the optical source. The optical setup is shown in Figure 1A. Light from the LED is injected via an optical fiber into a gas chamber (see Figure 1B) containing a layer of dye-zeolite deposited onto a glass plate and exposed to a controlled atmosphere. Another plastic optical fiber is placed at an angle of 45° in order to collect only diffuse light, avoiding the specular reflection. The spectrum of this light is analyzed using an Ocean Optics USB2000 spectrophotometer. The vapor concentration in the gas phase in contact with the sensing material was achieved by saturation of a mass flow controlled stream (N2) at 273 K with distilled water or n-hexane (Aldrich, 99+%) and mixed with a second mass flow controlled N2 stream to give the desired values (from 200 to 7000 ppmV). Before optical measurements, the dye-zeolite ensemble was dried at 100 °C in N2 flow during 2 h. Preliminary measurements were performed using a white LED as optical source, in order to detect the optimal wavelengths, those where the presence of analyte caused the most important reflectance variation. For subsequent measurements, this optical source has been substituted by a LED centered in those optimal wavelengths.
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Figure 1. (A) Schematic experimental setup for vapor detection. (B) Detailed images for the gas chamber and sample holder.
Results and Discussion It has been already demonstrated by Meinershagen and Bein19 that NR can penetrate into the supercages of siliceous zeolite with faujasite structure. For a Si/Al ratio ca. 30, these authors incorporate 0.03 dye molecules per supercage after Soxhlet extraction during 24 h. In this work (see Table 1), the estimated dye contents by chemical analysis for a Y-type zeolite with a Si/Al ratio of 3 was approximately twice after 3 days of continuous leaching with ethanol (i.e., 0.07 dye molecules per supercage in NRY2 sample). This Soxhlet extraction allows reducing the dye loading in a 55% with respect NRY1 sample (i.e., from 2.3 wt % to 1.5 wt %). In parallel, the chromophore contents have been also estimated by thermogravimetric analysis carried out immediately after NR incorporation. These results are clearly different for the non-extracted NRY1 sample probably due to the gradual dye loss by sublimation under atmospheric conditions. Even though Meinershagen and Bein19 sustain that the dye remained very close to the surface when adsorbed; several of our results indicate the inclusion of the Nile Red into the zeolite pores. First, the water adsorption uptake at room temperature, which decreases from 150 mg/g for NaY sample to 70 and 90 mg/g for NRY1 and NRY2, respectively (see Figure 2.A), is in agreement with dye loadings. This would be due to the fact that the dye dispersion over the external and/or internal crystalline surface diminishes the available area for analytes adsorption. Similarly, Figure 2B shows the n-hexane isotherms obtained in quartz crystal microbalances coated with NaY and NRY2 crystals. As a consequence of NR encapsulation, the n-hexane uptake decreases by 60% (from 85 mg/g in NaY to 34 mg/g in NRY2). As it could be expected, the hydrophilic nature of Y zeolite encapsulating NR is responsible of the adsorption capability differences observed for both analytes with
Figure 2. (A) Adsorption isotherms at 298 K for water on QCM sensors coated with zeolite Y and NR-zeolite Y crystals. (B) Adsorption isotherms at 298 K for n-hexane on QCM sensors coated with zeolite Y and NRzeolite Y crystals.
a clearly distinguished polar character (i.e., polarity index of 9.0 and 0.1 for water and n-hexane, respectively). However, the aim of this study is focused on the development of a selective humidity sensor. In principle, both water and hexane, as single vapors, are quantitatively adsorbed onto the zeolite surface rendering in a modification of the NR microenvironment susceptible of being used as sensing parameter. On the other hand, when binary mixtures are tested, a competitive adsorption with water prevalence takes place whatever the relative concentration employed as it will be shown below. Unlike it was expected, the Brunauer-Emmett-Teller (BET) surface analyses indicate that the specific surface area available for N2 adsorption in the NRY2 is the same as it is in the original zeolite; this is probably due to the low concentration of dye in this sample. On the other hand, NRY1 shows an important decrease in both the surface area and the pore volume; this change could be attributed to the fact that NR is mostly on the external surface of the zeolite occluding the entrance of the pores. The spectra of the diffuse reflectance of the as prepared samples, i.e., NRY1, NRY2, NRMM, exposed to atmospheric conditions (see Figure 3) have been used as an additional tool to provide us with valuable information to ascertain about the inclusion of NR inside the zeolitic pores. The NRMM presents a broadband in the range 450-750 nm with two distinct band maxima around 520 and 610 nm. When the NR is incorporated into the zeolite framework (NRY2), the spectra also have a
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Figure 3. Reflectance spectra of the as prepared samples exposed to air.
Figure 4. Evolution of reflectance spectra in NaY- and RNY2-coated glass plates upon introduction of 1000 ppmV of water in nitrogen. The 0-dB level belongs to a white paper reference sample.
broadband in the 450-700-nm range, but in these cases the maxima of the peak appears around 610 nm. In the NRY1 sample the peak is broader suggesting that two peaks could be involved in such feature. It is known that the geometry of the restricted environments plays a crucial role to control the photophysical properties and dynamics of excited NR molecules;26 for that reason the shifts observed in the maxima of the diffuse reflectance spectra of the extracted any nonextracted samples would be related to the interaction between the NR and the zeolite matrix. Similar spectra have been already published in the literature19 for dye/zeolite Y ensembles prepared by inclusion synthesis. Preliminary humidity sensing experiments were performed using a white LED as optical source, in order to detect the optimal wavelengths, those where the presence of water caused the most important reflectance variation. The analysis of the spectrum of the diffuse reflectance of the NRY2 sample (see Figure 4) shows significant changes at the 580-670-nm zone with H2O concentration after 4 min of analyte introduction into the gas chamber, variations which are not present for bare NaY type zeolite, except for small variations accounting to the different reflectance of the hydrated zeolite. From these results, and in order to improve the signal-to-noise ratio for low reflectance measurements, white LED was replaced by a 645nm LED (red). In subsequent experiments, although all the LED
Figure 5. Reflectance response to different concentrations of water vapor in nitrogen of a NRY2-coated glass plate.
spectrum was recorded, the diffuse reflectance at 644 nm was chosen as dependent parameter of the relative humidity presence. Figure 5 shows the evolution of the optical power reflected by NRY2 sample at room temperature exposed to water vapor concentration steps ranging from 200 (0.46% relative humidity) to 2000 ppmV (4.6% relative humidity). Even though there was a previous evaluation of an optical humidity sensor based on zeolite27 with similar response time, the usable range reported by the authors is 1 order of magnitude higher than our results. A sharp initial increase in reflectance can be observed upon the introduction of 200 ppm of water. Further increases in water concentration lead to further increases of the reflectance response (dB) until a maximum is observed at 2000 ppmV in accordance with the shape of the water adsorption isotherm shown in Figure 2A. However, as water concentration is decreased, the reflectance response (dB) decreases again, although it can be observed that at every concentration level the response stabilizes at a higher reflectance, indicating that some excess water remains adsorbed on the zeolite coating, an expected result given the hydrophilic character of zeolite Y. Moreover, the final hysteresis observed is due to the slow kinetics for water desorption at these concentration levels (i.e., 200 ppm) where water molecules are strongly attached to the surface and the mass transfer driving force is extremely low. To assess about sensor repetitivity, several steps of 1050 ppm of water vapor 15 min in duration have been introduced to the gas chamber, and the reflectance response has been plotted in Figure 6A. A raise of 1.34 ( 0.02 dB was observed after the introduction of each water pulse. Moreover, a response time of 4.25 min (defined for a 95% signal variation) could be extracted from the data analysis. On the other hand, the recovery time estimated from the water desorption branch is about 9 times higher (37 min). The Y zeolite, used as host for NR encapsulation, plays as a sorbent phase, retaining and concentrating analytes, modifying the dye microenvironment, and consequently, a sensibility improvement must be attained. Therefore, if NR is dispersed onto a nonporous solid, a minor effect on reflectance response should be detected in presence of water vapor. To corroborate such a hypothesis, a similar experiment to the one shown in Figure 6A has been performed with NRQ crystals (see Figure 6B). As it would be expected the quartz sample leads to an increase in the reflectance of less than 0.2 db, i.e., seven times lower than NRY2 response, demonstrating the sensibility enhancement imposed by the host. It is worthwhile to emphasize that the signal-to-noise ratio and the baseline stability have been notably improved in dye-zeolite ensembles.
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Figure 6. (A) Evolution of reflectance response in NRY2 coated glass plates with water vapor pulses of constant concentration in nitrogen. (B) Evolution of reflectance response in NRQ coated glass plates with water vapor pulses of constant concentration in nitrogen.
Another important feature in sensors containing dyes is the stability provided by the host. It is known that NR is not stable in aqueous solution and decomposes with a half-life of ∼20 min.26 However, in this work during 8 h of being exposed to water vapor, no decrease of the optical power was observed in NRY2 sample, indicating the stabilization of the NR inside the zeolite supercages. Besides, the dye/zeolite composite is stable in air, and color change due to a very slow dye elution takes several days. In order to test the influence on dye location on the sensor performance, an experiment of over 700 min was carried out with NRY1 and NRY2 samples in the QCM device. Figure 7A shows (at room temperature) the frequency response to increasing (and then decreasing) concentrations of water in N2. When 1250 ppmV of water are introduced to the measuring chamber, a sharp response of the sensor follows, with a steep decrease of frequency (more pronounced for NRY2 sample) corresponding to the increase in sensor mass. The most important feature observed in Figure 7A is that the level of frequency exhibited by sample NRY1 at the end of the sequence is above the starting stabilized value indicating a loss of mass attributed to dye elution from the external surface of the zeolite crystals in contrast with the NRY2 pattern. Extended experiments carried out in the optochemical chamber, under dry atmosphere reveals a linear optical power decay with time on stream due to dye migration by high light exposures. The reflected power decrease observed in NRY2 after 13 h of high light exposition is around 16% of its initial value, whereas the NRY1 sample is about 36.5% (i.e., a 52% of signal losses using NRY2 as reference). This fact reinforces again the benefits of deep dye incorporation inside the zeolite host.
Figure 7. (A) Evolution of frecuency response in NRY1 and NRY2 coated QCM sensors upon introduction of increasing and decreasing concentrations of water. (B) Evolution of the reflected optical power for NRY1 coated glass plate exposed at dry atmosphere with time on stream. NRY2 sample has been used as reference.
Figure 8. Evolution of reflectance response in NRY2-coated glass plates with n-hexane pulses of constant concentration in nitrogen.
Given the promising sensor performance for single water vapor detection, it was decided to carry out specific experiments in order to asses the capability of the sensor to detect water in the presence of n-hexane. First of all, single organic detection experiments at constant concentration were performed to analyze sensibility toward the linear alcane (see Figure 8). When 1550 ppmV of the organic compound were introduced in the gas chamber, the diffuse reflectance increased around 0.3 dB. From data analysis, response and recovery times of around 2 and 26 min have been calculated. These values are lower than those
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Conclusions NR-Y zeolite composites have been successfully demonstrated as humidity sensors using a robust and low-cost optical configuration. In further developments, the experimental setup will be simplified with the substitution of the spectrophotometer by a silicon photodiode and improved with the inclusion of an optical coupler to allow reference measurements. To the best of our knowledge, this is the first dye-zeolite optochemical sensor reported for continuous monitoring of water and organic vapors. The extracted dye-zeolite samples exhibit improved properties in terms of water sensitivity (much lower than 200 ppm), response time (around 4 min), and chromophore stability toward migration upon high light exposures. In presence of water, the dye-zeolite composites developed are almost insensitive to n-hexane presence whatever the concentration used due to the selective imposed by the zeolite host. Moreover, a deep encapsulation of NR inside the zeolite supercages has been revealed as crucial to ensure the baseline stability and an adequate signal-to-noise ratio. However, due to the hydrophilic nature of the zeolite employed as a host, the recovery times registered are around 35 min. For practical applications, a heating device is being considered in further developments to ensure a quick signal recovery and also enough water uptake capability, a critical issue for sensing humidity at very low concentration levels (less than 200 ppm). Acknowledgment
Figure 9. (A) Evolution of reflectance response in NRY2-coated glass plates in the presence of n-hexane in nitrogen upon the introduction of a water concentration step. (B) Evolution of reflectance response in NRY2coated glass plates in the presence of water vapor in nitrogen upon the introduction of increasing concentrations of n-hexane.
reported in the literature for similar analytes using thin films of a vapochromic material.24 As it could be expected, the NRY2 sensibility to water and n-hexane single vapors is in accordance with the adsorption properties exhibited by the zeolite host. On the other hand, the selectivity imposed by the zeolite matrix should be verified with binary mixtures sensing experiments (see parts A and B of Figure 9). For such purpose, n-hexane steps of 6200 ppm were introduced in the measuring chamber leading to a reflectance enhance of around 0.4 dB. Upon the introduction of the second n-hexane step, and once the reflectance signal was stabilized, 580 ppm of water were co-fed with a sharp increase of the optical response (up to 1.42 dB). Even in the presence of a large concentration of the organic, the sensor responded quickly to the addition of a much lower concentration of water (see Figure 9A). Moreover, when water was removed from the mixture, the optical response recovered the corresponding value to n-hexane indicating that the presence of organic vapors allowed a complete desorption of water from the surface in 15 min due to its lower adsorption strength onto the surface sites induced by the high concentration levels of hexane in the restricted environment of the zeolitic host. The second experiment with mixtures (see Figure 9.B) consisted of introduction of increasing n-hexane concentration steps (from 930 to 3100 ppmV) keeping constant water concentration (730 ppmV). Once water was co-fed to the chamber, the signal remained unchanged whatever the n-hexane concentration introduced even when the organic was completely removed from the mixture. Therefore, in presence of water, the dye-zeolite composite is almost insensitive to n-hexane.
Financial support from M.E.C. (Spain) and D.G.A. (Aragon, Spain) for this work is gratefully acknowledged. S.I. acknowledges the “Ramo´n y Cajal” program. Literature Cited (1) Shi, J.; Zhu, Y.; Zhang, X.; Baeyens, W. R. G.; Garcı´a-Campan˜a, A. M. Recent developments in nanomaterial optical sensors. Trends Anal. Chem. 2004, 23 (5), 351. (2) Granda, Valde´s, M.; Pe´rez-Cordoves, A. I.; Dı´az-Garcı´a, M. E. Zeolites and zeolite-based materials in analytical chemistry. Trends Anal. Chem. 2006, 25 (1), 24. (3) Fukui, K.; Nishida, S. CO gas sensor based on Au-La2O3 added SnO2 ceramics with siliceous zeolite coat. Sens. Actuators, B 1997, 45 (2), 101. (4) Szabo, N. F.; Du, H.; Akbar, S. A.; Soliman A.; Dutta P. K. Microporous zeolite modified yttria stabilized zirconia (YSZ) sensors for nitric oxide (NO) determination in harsh environments. Sens. Actuators, B 2002, 82 (2-3), 142. (5) Vilaseca, M.; Coronas, J.; Cirera, A.; Cornet, A.; Morante, J. R.; Santamarı´a, J. Use of zeolite films to improve the selectivity of reactive gas sensors. Catal. Today 2003, 82 (1-4), 179. (6) Mintova, S.; Bein, T. Nanosized zeolite films for vapor-sensing applications. Microporous Mesoporous Mater. 2001, 50 (2-3), 159. (7) Mintova, S.; Mo, S.; Bein, T. Humidity sensing with ultratin LTAtype molecular sieve films grown on piezoelectric devices. Chem. Mater. 2001, 13, 901. (8) Vilaseca, M.; Yagu¨e, C.; Coronas, J.; Santamarı´a, J. Development of QCM sensors modified by AlPO4-18 films. Sens. Actuators, B 2006, 117 (1), 143. (9) Scandella, L.; Binder, G.; Mezzacasa, T.; Gobrecht, J.; Berber, R.; Lang, H. P.; Gerber, C. H.; Gimsewski, J. K.; Koegler, J. H.; Hansen, J. C. Combination of single crystal zeolites and microfabrication: Two applications towards zeolite nanodevices. Microporous Mesoporous Mater. 1998, 21 (4-6), 403. (10) Zhou, J.; Li, P.; Zhang, S.; Long, Y.; Zhou, F.; Huang, Y.; Yang, P.; Bao, M. Zeolite-modified microcantilever gas sensor for indoor air quality. Sens. Actuators, B 2003, 94 (3), 337. (11) Plog, C.; Kurzweil, P.; Maunz, W. Combustion gas sensitivity of zeolite layers on thin-film capacitors. Sens. Actuators, B 1995, 25 (1-3), 403. (12) Plog, C.; Maunz, W.; Kurzweil, P.; Obermeier, E.; Scheibe, C. Impedance of zeolite-based gas sensors. Sens. Actuators, B 1995, 25 (13), 653.
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ReceiVed for reView August 4, 2006 ReVised manuscript receiVed October 17, 2006 Accepted October 18, 2006 IE061025V
