Anal. Chem. 2006, 78, 1034-1041
Chemical Gas Sensor Application of Open-Pore Mesoporous Thin Films Based on Integrated Optical Polarimetric Interferometry Zhi-mei Qi,† Itaru Honma,‡ and Haoshen Zhou*,†,‡
Light and Control Research Group, PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan, and Energy Technology Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan
Chemical gas sensors that employ integrated optical polarimetric interferometry were fabricated by the solgel synthesis of transparent mesoporous thin films of TiO2-P2O5 nanocomposite on tapered layers of TiO2 sputtered on tin-diffused glass waveguides. Atomic force microscopy images of the mesoporous thin film clearly show the open pore mouths on the film surface that favor rapid diffusion and adsorption of gas-phase analytes within the entire film. Adsorption of gas and vapor induces changes (∆n) in the refractive index of the mesoporous thin film that lead to shifts in the phase difference between the fundamental transverse electric and magnetic modes simultaneously excited in the glass waveguide via singlebeam incidence. Upon exposure to NH3 gas at concentrations as low as 100 ppb in dry air at room temperature, the sensor exhibits a reversible change in the phase difference with the response and recovery times of less than 60 and 90 s, respectively. It is unexpected that the sensor is unresponsive to either NO2 or C6H6 vapor, leading to a somewhat selective sensitivity to NH3. Determination of ∆n was carried out with a combination of the experimental results and the theoretical calculations. The sensor design represents a novel, effective, and easily accessible approach to mesoporous thin-film-based integrated optical chemical sensors. Sol-gel surfactant templated mesoporous materials are amorphous or nanocrystalline inorganic frameworks with periodically arranged pores 2-50 nm in diameter. They are thermally stable, possess high surface area and low refractive index, and enable the pore wall to interact with molecules that diffuse into the pores and the capillary condensation of gas and vapor within the pores. These distinctive properties render mesoporous materials useful in the fields of optics, catalysis, sorption, separation, and chemical/ biochemical sensings. Since the discovery of mesoporous MCM41 silica in 1992,1 much effort has been devoted to the sol-gel templating synthesis of mesoporous nonsiliceous materials,2-9 leading to considerable success with the fabrication of high-quality * To whom correspondence should be addressed. E-mail:
[email protected]. † Japan Science and Technology Agency. ‡ National Institute of Advanced Industrial Science and Technology. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712.
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optical thin films of mesoporous metal oxides.7-9 This offers an exciting opportunity to exploit the strong potential for creating mesoporous thin-film-based chemical sensors by integrated optical technology. To date, the application of porous thin films for optical chemical sensors has been extensively studied based on a variety of techniques such as absorption,10 fluorescence,11 surface photovoltage,12 Febry-Perot interferometry,13 and spectroscopic ellipsometry14 measurements. The use of bulk light beams to directly interact with the porous sensing layers makes these existing sensors excluded from the family of integrated optical chemical sensors. Chemical sensors based on integrated optical technology are of considerable current interest, which generally employ an evanescent wave as probe to interact with the sensing layer, thereby sometimes called evanescent-wave sensors.15 In general, evanescent-wave sensors are very sensitive because they allow for extending the path of interaction between the evanescent wave and the sensing layer up to several centimeters long. Conventional evanescent-wave sensors with dense and lowrefractive-index thin films as the chemical-sensing layers are limited to surface detection.15,16 In contrast, mesoporous thin films (2) Tian, B.; Liu, X.; Tu, B.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stuky, G. D.; Zhao, D. Nat. Mater. 2003, 2, 159-163. (3) Li, D.; Zhou, H.; Honma, I. Nat. Mater. 2004, 3, 65-72. (4) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stuky, G. D. Nature 1998, 396, 152-155. (5) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56-77. (6) Brezesinski, T.; Erpen, C.; Iimura, K.; Smarsly, B. Chem. Mater. 2005, 17, 1683-1690. (7) Yun, H.-S.; Miyazawa, K.; Zhou, H.; Honma, I.; Kuwabara, M. Adv. Mater. 2001, 13, 1377-1380. (8) Crepaldi, E. L.; Grosso, D.; Soler-Illia, G. J.; Albouy, P.-A.; Amenitsch, H.; Sanchez, C. Chem. Mater. 2002, 14, 3316-3325. (9) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A. Adv. Func. Mater. 2004, 14, 335-344. (10) Fiorilli, S.; Onida, B.; Macquarrie, D.; Garrone, E. Sens. Actuators, B 2004, 100, 103-106. (11) Wirnsberger, G.; Scott, B. J.; Stucky, G. D. Chem. Commun. 2001, 119120. (12) Yamada, T.; Zhou, H.; Uchida, H.; Tomita, M.; Ueno, Y.; Ichio, T.; Honma, I.; Asai, K.; Katsube, T. Adv. Mater. 2002, 14, 812-815. (13) Lin, V. S.-Y.; Motesharei, K.; Dancil, K.-P. S.; Sailor, M. J.; Ghadirl, M. R. Science 1997, 278, 840-843. Gao, J.; Gao, T.; Sailor, M. J. Appl. Phys. Lett. 2000, 77, 901-903. (14) Bjorklund, R. B.; Zangooie, S.; Arwin, H. Appl. Phys. Lett. 1996, 69, 30013003. Zangooie, S.; Bjorklund, R.; Arwin, H. Sens. Actuators, B 1997, 43, 168-174. (15) Helmers, H.; Greco, P.; Rustad, R.; Kherrat, R.; Bouvier, G.; Benech, P. Appl. Opt. 1996, 35, 676-680. 10.1021/ac051380f CCC: $33.50
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serving as the chemical-sensing layers enable the evanescent-wave sensors to detect molecular interaction occurring at both the outer and inner surfaces of the film. It is thereby anticipated that mesoporous thin-film-based evanescent-wave sensors are much more sensitive than conventional counterparts. Trace detection could be easy to do using mesoporous thin-film-based evanescentwave sensors. In this study, we describe a transparent mesoporous thin-filmbased chemical gas sensor that utilizes integrated optical polarimetric interferometry, a quite simple yet highly sensitive technique.17,18 With the polarimetric interferometry, there is no need to incorporate functional solvatochromic or fluorescent dyes into the mesoporous thin films for absorption or fluorescence measurements. Consequently, the surface area reduction for the mesoporous thin film and the complication in fabrication of sensors were avoided. The present sensor is a planar optical waveguide with three dielectric layers (Figure 1A): the top layer is a crack-free, 60-nm-thick mesoporous TiO2-P2O5 (TPO) nanocomposite film with open pore mouths on the film surface; the middle layer is a sputtered thin TiO2 film with two tapers for adiabatic mode propagation in the inhomogeneous waveguide; and the bottom layer is a tin-diffused glass layer that is intrinsic to conventional float glass substrates. Float glass substrates with tindiffused layers have proved to be simple, inexpensive slab optical waveguides.18-20 The present sensor exhibits a fully reversible and somewhat selective response at room temperature to NH3 gas at concentrations between 100 ppb and 10 ppm and has the response and recovery times of less than 60 and 90 s, respectively. By combining the theoretical calculations with the experimental results, we determined the changes in the refractive index of the mesoporous sensing layer and confirmed that the sensor sensitivity arises mainly from adsorption of NH3 molecules on the inner rather than the outer surface of the mesoporous thin film. EXPLANATION OF THE POLARIMETRIC INTERFEROMETER SENSOR In a previous work, we have demonstrated that tin-diffused glass slab waveguides locally coated with tapered layers of TiO2 can be used as a highly sensitive polarimetric interferometer for detection of both refractive index of liquid and protein submonolayer adsorbed.18 Compared with biomolecules such as protein and DNA, gaseous molecules, e. g., NH3 and NO2, are too small to be detected in the case of submonolayer adsorption. To enable the polarimetric interferometer to detect gas-phase analytes at low concentrations, the mesoporous thin films were used in this study to form an analyte-rich region on top of the tapered layer of TiO2. The polarimetric interferometer works on the principle of the locally induced polarization mode dispersion in a nonbirefringent single-mode glass waveguide. In brief, the fundamental transverse electric (TE0) and magnetic (TM0) modes excited by a singlebeam coupling in the uncovered region of the tin-diffused glass
Figure 1. (A) Schematic diagram of a polarimetric interferometerbased gas sensor. Layers 1-3 correspond, respectively, to a tindiffused glass waveguiding layer with thickness T ) 3 µm and refractive index n ) 1.530, a tapered layer of TiO2 with T ) 18 nm and n ) 2.30, and a transparent mesoporous TPO nanocomposite film with T ) 60 nm and n ) 1.592. (B) Calculated intensity profiles for the TE0 and TM0 modes confined to the multilayer waveguide illustrated in (A) (calculations were carried out using λ ) 632.8 nm, n ) 1.525 for the float glass substrate and 1 for the air substrate). The vertical dotted lines indicate the interfaces between two neighboring layers. The inset clearly shows the difference in intensity distribution in the mesoporous film between both modes. (C) Calculated changes in the effective refractive indexes of the TE0 (NTE) and TM0 (NTM) modes and the difference between ∆NTE and ∆NTM as a function of changes in the refractive index (n) of the mesoporous thin film (n changes from 1.592 to 1.597).
(16) Klein, R.; Voges E., Sens. Actuators, B 1993, 11, 221-225. (17) Qi, Z.; Itoh, K.; Murabayashi, M.; Yanagi, H. J. Lightwave Technol. 2000, 18, 1106-1110. (18) Qi, Z.; Honma, I.; Zhou, H. Anal. Chem. 2005, 77, 1163-1166. (19) Kakarantzas, G.; Glavas, E.; Townsend, P. D. Electron. Lett. 1989, 25, 102104. (20) Yang, B.; Townsend, P. D.; Holgate, S. A. J. Phys. D: Appl. Phys. 1994, 27, 1757-1762.
waveguide adiabatically propagate through the tapered thin TiO2 layer-covered region, where they are spatially separated from each other: the TE0 mode shifts toward the film surface relative to the TM0 one. This upward shift makes the TE0 mode a very strong evanescent field in the covered region in contrast with the TM0 one. Therefore, physical and chemical changes occurring within Analytical Chemistry, Vol. 78, No. 4, February 15, 2006
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the penetration depth of evanescent field over the tapered layer of TiO2 can lead to a large variation in the effective refractive index of the TE0 mode (NTE) relative to that of the TM0 one (NTM). The difference between two changes, ∆NTE and ∆NTM, is related to the shift (∆φ) in the phase difference between both modes by eq 1. Information on the surface changes can be achieved by determination of ∆φ from eq 2.
∆φ )
∫ (∆N
2π λ
L
0
TE
- ∆NTM) dz
I ) I0[1 + γ cos(φ0 + ∆φ)]
(1) (2)
where λ is the wavelength of light, L is the length of the tapered layer of TiO2 along the mode-propagating direction z, I is the light intensity detected, γ is the fringe contrast, and φ0 is the initial phase difference. In the case of covering the tapered layer of TiO2 with a uniform mesoporous thin film, the optical fields penetrating into the top layer are different between the TE0 and TM0 modes. Calculations below indicate that with the TE0 mode the electromagnetic field in the top layer is much stronger than that with the TM0 mode. This suggests that the polarimetric interferometer could have a high sensitivity to molecular adsorption within the mesoporous thin film. Using Maxwell’s equations and boundary conditions, we calculated the intensity profiles at 633-nm wavelength for the TE0 (I ) Ey2) and TM0 (I ) Ex2 + Ez2) modes confined to a multilayer waveguide fabricated in the Experimental Section, which consists of glass substrate (n ) 1.525), the tin-diffused layer (n ) 1.530, T ) 3 µm),20 the sputtered TiO2 layer (n ) 2.30, T ) 18 nm), the mesoporous TPO nanocomposite film (n ) 1.592, T ) 60 nm), and air cladding (n ) 1). Figure 1B shows the calculated intensity profiles that were normalized for a clear comparison. Both profiles are obviously separated from each other although their intensity maximums are present in the same layer (i, e., the tin-diffused layer). At a vertical position in the range x ) 3.018-3.078 µm (corresponding to the mesoporous film), the intensity for the TE0 mode is at least 10 times greater than that for the TM0 mode (see the inset). Therefore, the interaction of the mesoporous film with the TE0 mode is much stronger than that with the TM0 one. Figure 1B also shows that the evanescent tail in air of the intensity profile for the TE0 mode is larger than that for the TM0 mode. Both NTE and NTM were calculated to linearly increase with increasing refractive index (n) of the mesoporous film in a small range (from 1.592 to 1.597) while ∆NTE is much larger than ∆NTM for a positive ∆n (see Figure 1C). The difference, ∆NTE - ∆NTM, is also linearly dependent on ∆n. The slope, (∆NTE - ∆NTM)/∆n, is equal to 8.208 × 10-3. Since two tapered regions of the sputtered TiO2 layer are thin (
