Design and Sensing Properties of an Integrated Optical Gas Sensor

Oct 27, 2008 - Laboratoire Polymères, Colloïdes, Interfaces, UMR-CNRS 6120, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans, France, ...
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Anal. Chem. 2008, 80, 9188–9194

Design and Sensing Properties of an Integrated Optical Gas Sensor Based on a Multilayer Structure Aissam Airoudj,†,‡ Dominique Debarnot,*,† Bruno Beˆche,§ and Fabienne Poncin-Epaillard† Laboratoire Polyme`res, Colloı¨des, Interfaces, UMR-CNRS 6120, Universite´ du Maine, Avenue Olivier Messiaen, 72085 Le Mans, France, Laboratoire d’Acoustique de l’Universite´ du Maine, UMR-CNRS 6613, Avenue Olivier Messiaen, 72085 Le Mans, France, and Institut de Physique de Rennes, Universite´ de Rennes I, IPR UMR CNRS 6251, 35042 Rennes, France In this paper, a new multilayer integrated optical sensor (MIOS) for ammonia detection at room temperature is proposed and characterized. The sensor is integrated on a single-mode TE0-TM0 planar polymer waveguide and based on polyaniline (PANI) sensitive material. A polymethyl methacrylate (PMMA) passive layer is deposited between the waveguide core and PANI sensitive layer in order to decrease optical losses induced by evanescent wave/sensitive material coupling. The design of this new sensor is discussed. Moreover, in order to investigate the feasibility of this sensor, the sensing properties to ammonia at room temperature are studied. A significant change is observed in the guided light output power after the sensor is exposed to ammonia gas, due to PANI absorption coefficient variation. This new ammonia sensor shows fast response and recovery times, good reversibility and repeatability. The metrological parameters (sensitivity, response time and recovery time) of the sensor are strongly influenced by the interaction length (length of sensing region) and the PANI forms (doped and dedoped). The sensor has a logarithmic linear optical response within the ammonia concentration range between 92 to 4618 ppm. These experimental results demonstrate that the MIOS structure presents a potential innovation to elaborate integrated optical sensor based on non transparent (opaque) sensitive material. In the past few years, optical methods for gas detection have become attractive due to their insensitivity to electromagnetic interference, fast response time, and possible use in a dangerous environment.1 These methods are based on absorption1 or reflectance2 changes, variation of luminescence intensity,3 surface * Author to whom correspondence should be addressed. Tel: +33 2 43 83 39 82. Fax: +33 2 43 83 35 58. E-mail: [email protected]. † Laboratoire Polyme`res, Colloı¨des, Interfaces, UMR-CNRS 6120, Universite´ du Maine. ‡ Laboratoire d’Acoustique de l’Universite´ du Maine, UMR-CNRS 6613. § Institut de Physique de Rennes, Universite´ de Rennes I, IPR UMR CNRS 6251. (1) Ando, M.; Kobayashi, T.; Haruta, M. Sens. Actuators, B 1995, 24-25, 851– 853. (2) Gu, Z.; Liang, P. Opt. Laser Technol. 2004, 36, 211–217. (3) Shinbo, K.; Minagawa, M.; Takasaka, H.; Kato, K.; Kanek, F.; Kawakami, T. Colloids Surf., A. 2002, 198-200, 905–909.

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plasmon resonance (SPR),4,5 interferometric system5 or evanescent wave analysis.6 Recently, waveguide-based chemical sensors with thin polymer films (i.e., optical fibers and planar waveguides) have been widely studied due to the advantages of such configurations in many applications. Among them, planar waveguide structures have been examined for gas detection.7,8 Planar waveguides present advantages over the conventional transmission measurements for gas detection such as higher sensitivity.9-16 Indeed, the sensitivity depends on the light propagation distance which is higher for the planar waveguide sensor due to the multiple reflections of the guided light at the core/cladding (sensitive layer) interface. However, in the planar waveguide configuration, the sensitive material must be transparent with a refractive index lower than that of the waveguide core. To solve these limitations, we have recently proposed an integrated optical sensor based on composite sensitive material.17 Our results indicate that the optical quality of the sensitive material can be improved by the dispersion of sensitive material into transparent polymer matrix. Herein, we propose a novel concept valid whatever the sensitive material is: a thin passive film is deposited between the sensitive layer and the waveguide core. The thin passive layer must be transparent with a refractive index lower than that of the waveguide core to satisfy the propagation (4) Homola, J.; Yee, S. S.; Auglitz, G. G. Sens. Actuators, B 1999, 41, 3–15. (5) Agbor, N. E.; Cresswell, J. P.; Petty, M. C.; Monkman, A. P. Sens. Actuators, B 1997, 41, 137–141. (6) Yimit, A.; Itoh, K.; Murabayashi, M. Sens. Actuators, B 1999, 54, 239– 245. (7) Lavers, C. R.; Itoh, K.; Wu, S. C.; Murabayashi, M.; Mauchline, I.; Stewart, G.; Stout, T. Sens. Actuators, B 2000, 69, 85–95. (8) Lukosz, W. Sens. Actuators, B 1995, 29, 37–50. (9) Lee, Y.-S.; Joo, B.-S.; Choi, N.-J.; Lim, J.-O.; Huh, J.-S.; Lee, D.-D. Sens. Actuators, B 2003, 93, 148–152. (10) Jin, Z.; Su, Y.; Duan, Y. Sens. Actuators, B 2001, 72, 75–79. (11) Tsunoda, K.; Itabashi, H.; Akaiwa, H. Anal. Chim. Acta 1995, 299, 327– 332. (12) Alava-Moreno, F.; Garcia, R. P.; Garcia, M. E. D.; Medel, A. S. Sens. Actuators 1993, 11, 413–419. (13) Kang, S. W.; Sasaki, K.; Minamitani, H. Appl. Opt. 1993, 32, 3544–3549. (14) Choquette, S.; Locascio-Brown, L.; Durst, R. A. Anal. Chem. 1992, 64, 55– 60. (15) Walczak, I. M.; Love, W. F.; Cook, T. A.; Slovacek, R. E. Biosens. Bioelectron. 1992, 7, 39–48. (16) DeGrandpre, M. D.; Burgess, L. W.; White, P. L.; Goldman, D. S. Anal. Chem. 1990, 62, 2012–2017. (17) Airoudj, A.; Debarnot, D.; Beˆche, B.; Poncin-Epaillard, F. Talanta 2008, 76, 314–319. 10.1021/ac801320g CCC: $40.75  2008 American Chemical Society Published on Web 10/28/2008

Figure 1. Schematic structures of the two types of waveguide sensors: (a) conventional waveguide sensor; (b) multilayer integrated waveguide sensor developed in this study.

conditions. In this concept, the role of the passive layer is to allow light propagation but also to decrease optical losses due to evanescent wave/sensitive material coupling. We have called this new integrated waveguide “multilayer integrated optical sensor” (MIOS). By using such a multilayer integrated optical sensor, sensitive material with high refractive index and absorption coefficient (opaque sensitive polymer) can be used. To our knowledge, this is the first study of multilayer integrated waveguide used for the measurement of chemical substances. Figure 1 shows a comparison of the conventional waveguide sensor and the multilayer integrated optical sensor. In the MIOS structure (Figure 1(b)), the passive layer is deposited on a small section of the planar waveguide, known as interaction length (LI), and then, the sensitive material is coated. Thus, in this configuration, the cladding layer is constituted of two layers: passive and sensitive ones. In this paper, we describe gas (ammonia) detection thanks to this new MIOS based on polyaniline (PANI) as a sensitive material. PANI is one of the most promising conducting polymers because of its good combination of easy synthesis, low cost and easy conductivity control.18-20 Its interests for gas detection are its sensitivity at room temperature and its selectivity to ammonia. The interaction between the conductive form of PANI, the emeraldine salt (ES), and the ammonia gas results in a change of the doping state of PANI leading then to variation of its electrical and optical properties. More precisely, NH3 adsorption and desorption induce deprotonation (dedoping) and protonation (doping) of polyaniline respectively.18,21,22 The planar waveguide core of this new MIOS is composed of glycidyl ether of bisphenol (18) Nicolas-Debarnot, D.; Poncin-Epaillard, F. Anal. Chim. Acta 2003, 475, 1–15. (19) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91–97. (20) Pud, A. A. Synth. Met. 1994, 66, 1–18. (21) Chabukswar, V. V.; Pethkar, S.; Athawale, A. A. Sens. Actuators, B 2001, 77, 657–663. (22) Kukla, A. L.; Shirshov, Y. M.; Piletsky, S. A. Sens. Actuators, B 1996, 37, 135–140. (23) Pelletier, N.; Beˆche, B.; Gaviot, E.; Camberlein, L.; Grossard, N.; Polet, F.; Zyss, J. IEEE Sens. J. 2006, 6, 565–570. (24) Beˆche, B.; Pelletier, N.; Gaviot, E.; Zyss, J. Opt. Commun. 2004, 230, 91– 94. (25) Kim, J. S.; Kang, J. W.; Kim, J. J. Jpn. J. Appl. Phys 2003, 42, 1277–1279. (26) Zhang, J.; Tan, K. L.; Gongh, Q. Polym. Test. 2000, 20, 1693–1701. (27) Beˆche, B.; Papet, P.; Debarnot, D.; Gaviot, E.; Zyss, J.; Poncin-Epaillard, F. Opt. Commun. 2005, 246, 25–28. (28) Pelletier, N.; Beˆche, B.; Tahani, N.; Zyss, J.; Camberlein, L.; Gaviot, E. Sens. Actuators, A 2007, 135, 179–184. (29) Airoudj, A.; Debarnot, D.; Beˆche, B.; Boulard, B.; Poncin-Epaillard, F. Plasma Processes Polym. 2008, 5, 275–288.

A (SU-8). SU-8 is a good candidate for the simple fabrication of optical waveguides due to its excellent optical transparency in the telecommunication wavelengths and its easy processing.23-29 Finally, the choice of the passive layer with appropriate refractive index must be judicious to satisfy the total internal reflection condition. In this case, polymethyl methacrylate (PMMA) has been chosen due to its excellent optical transparency and its refractive index lower than that of SU-8. This article describes the design of this new sensor and investigates its sensing properties to ammonia at room temperature. PRINCIPLE OF THE SENSOR The principle of the sensor is based on the interaction of the evanescent wave of the guided mode (orthogonally polarized TE0 and TM0 modes) with the sensitive material. Then, the penetration depth of the evanescent field must be higher than the passive layer thickness. The penetration depth dp is defined as8,30 dp ) (λ ⁄ 2π) [neff2- nc2]1⁄2

(1)

where neff is the effective refractive index of the guided mode, nc is the refractive index of the cladding layer and λ is the wavelength. Moreover, the evanescent field decreases proportionally to exp(-x/dp) with the distance x from the core-cladding interface. The absorption coefficient and/or refractive index of the sensitive material change when the material is exposed to different vapors or gases, thus intensity and distribution of evanescent wave change near the waveguide surface. Therefore, change in the effective refractive index of the guided mode is induced leading then to alteration of the output transmitted light intensity of the waveguide. EXPERIMENTAL SECTION Elaboration of Planar Optical Waveguides on SU-8 Polymer. To obtain single-mode TE0-TM0 planar waveguides on SU-8 polymer (SU-8 as core, silicon dioxide layer as lower cladding layer and air as superstrate) with a good confinement of both optical modes, all optogeometric parameters of planar waveguides had to be defined. To this end, a numerical method based on semivectorial finite difference (SVFD) simulation was used.31 The (30) Li, K.; Meichsner, J. J. Phys. D: Appl. Phys. 2001, 34, 1318–1325. (31) Beˆche, B.; Jouin, J. F.; Grossard, N.; Gaviot, E.; Toussaere, E.; Zyss, J. Sens. Actuators, A 2004, 114-1, 59–64.

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Figure 2. TE0 optical mode, calculated by SVFD, on Si/SiO2/SU8/PMMA planar waveguides. Film thicknesses: SiO2 ) 1.2 µm, SU-8 ) 1 µm, PMMA ) 0.5 µm.

single-mode planar waveguide structure was realized on a (100) silicon substrate. At first, the lower cladding, the SiO2 layer, was obtained by thermal oxidation of the silicon wafer yielding a thickness of 1.20 ± 0.12 µm with an index value nSiO2 ) 1.45 at 980 nm. Then, the guiding layer of SU-8 polymer (MicroChem) with nSU-8 ) 1.58 at 980 nm, was deposited by spin-coating yielding a thickness of 1.0 ± 0.1 µm. After deposition, the SU-8 was dried according to different steps of temperature.24 In such planar structures, the index of the superstrate above the SU-8 planar waveguide is the unity (air). Deposition of PMMA Passive Layer. The PMMA film was deposited on the surface of the SU-8 waveguide by using spincoating process. PMMA (Aldrich) was first weighed and dissolved in chloroform (Aldrich). The solution was then spin-coated onto SU-8 planar waveguide to form PMMA film. The PMMA layer thickness depends on the concentration of PMMA in the coating solution and increases linearly from 70 to 500 nm when PMMA concentration varies from 0.25 wt % to 3 wt %. The thickness of the thin film has been measured by a Veeco profilometer (Dektak 8 model). The thickness values were the average of at least five measurements taken at different locations of the polymer film. The influence of the PMMA passive layer thickness on the light propagation is discussed in the following sections. The index value of PMMA is equal to 1.48 at 980 nm. The guiding properties of this integrated optical waveguide structure (Si/SiO2/SU-8/PMMA) have also been studied with the SVFD method, in order to demonstrate the confinement of both optical modes TE0 and TM0 in the SU-8 waveguide. The thickness of PMMA for the simulation has been chosen equal to 0.50 ± 0.05 µm. In this work, optical refractive indexes of the polymer films were measured using a Metricon 2010 prism coupler at the Laboratoire des Oxydes et Fluorures (Universite´ du Maine, France). As an example, Figure 2 represents the single-mode TE0 optical distribution for a specific Si/SiO2/SU-8/PMMA planar waveguide with an effective index value evaluated by way of the SVFD method to neff(TE0) ) 1.54. Deposition of PANI Sensitive Layers. PANI films were deposited following the method described in refs 32, 33 with a slight modification. One milliliter of aniline monomer (99.8%, Aldrich) was added dropwise into 100 mL of 1 M hydrochloric acid (HCl, Aldrich) aqueous solution under stirring at room temperature. Then, SU-8 waveguide was immersed in the mono(32) Zheng, W.; Min, Y.; MacDiarmid, A. G.; Angelopoulos, M.; Liao, Y.-H.; Epstein, A. J. Synth. Met. 1997, 84, 63–64. (33) Sapurina, I.; Riede, A.; Stejskal, J. Synth. Met. 2001, 123, 503–507.

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Figure 3. UV-visible spectra of PANI-EB (dedoped) and PANI-ES (doped) films.

mer solution. After the yellow drops of aniline dissolved completely in the acidic solution, 5 mL of 0.1 M ammonium persulfate ((NH4)2S2O8, Aldrich) aqueous solution was added into the monomer solution to initiate the polymerization of the aniline monomer. Polymerization begins immediately with an evident color change after 3-5 min, indicating polymer formation. The thickness of the green film deposited on the waveguide depends on the polymerization time and can vary between 50 to 130 nm. The PANI deposit was then thoroughly washed with 0.1 M HCl aqueous solution, and immersed into another aniline solution (1 mL of aniline dissolved in 100 mL of 1 M HCl) for 30 min. This step is necessary to completely convert the polypernigraniline form into the polyemeraldine salt or doped form, PANI-ES.34 Finally, the thin film was dried in air at room temperature. In order to obtain the emeraldine base or dedoped form (PANIEB), the final film was treated with 0.1 M hydroxide ammonium (NH4OH, Aldrich) for 2 h. After drying in air at room temperature, the blue film of the EB form was obtained. The difference between both forms of polyaniline (ES and EB) appears clearly from their UV-visible absorption spectra (Figure 3). The UV-vis spectrum of dedoped PANI film shows two absorption bands. The first one with maximum at ca. 325 nm is associated to π-π* transition of the conjugated ring systems and the second band (at 640 nm) is assigned to the quinoid excitonic transition.35,36 The UV-vis spectrum of doped PANI film shows an absorption band at 325 nm associated with π-π* transition of the conjugated ring systems and two new absorption bands: at ∼410 nm and between 820 and 900 nm, assigned to polaron band transition.35,37 Optical Bench and Ammonia Sensing System for Sensor Characterization. The MIOS was characterized by measuring the optical intensity variation when it has been exposed to different concentrations of ammonia diluted in nitrogen. Then, a specific micro-optical bench was designed and allowed to realize effective light injection, after having cleaved both input and output faces (34) Yuan, J.; El-Sherif, M. A.; MacDiarmid, A. G.; Jones, W. E., Jr SPIE Proc. 2001, 4205, 170–179. (35) Nicho, M. E.; Trejo, M.; Garcia-Valenzuela, A.; Saniger, J. M.; Palacios, J.; Hu, H. Sens. Actuators, B 2001, 76, 18–24. (36) Dhawan, S. K.; Kumar, D.; Ram, M. K.; Chandra, S.; Trivedi, D. C. Sens. Actuators, B 1997, 40, 99–103. (37) Araujo, P. L. B.; Araujo, E. S.; Santos, R. F. S.; Pacheco, A. P. L. Microelectron. J. 2005, 36, 1055–1057.

of the waveguide. This micro-optical injection bench, presented in ref 17, consisted of a laser source operating at 980 nm, fitted with an enhanced control in temperature (Opton Laser International) and associated with objectives and polarizers (Newport Inc.). Then, the light was focused onto the cleaved input face of the planar waveguide leading to the excitation of the fundamental mode of the structure. The extremity of this optical bench was fitted with a highly sensitive powermeter (Ophir Optronics Inc.; photodiode allowing a power range until 3 W with supplied filter, resolution 1 nW, response time 0.2 s, automatic background subtraction) and a video system with CCD camera to visualize and quantify the setup output signal (Jai Inc.; 625 lines, 25 frames/ s, resolution 752 × 582 pixels, sensitivity 0.02 lx). The gas dilution system was composed of a sealed measuring chamber associated to the optical bench. This cell was equipped of two optical glass windows (transparent and plane) allowing the light injection onto the cleaved input face of the planar waveguide without perturbation. The optical bench and measuring cell were linked to mass flow meters and a computer for data collection and analysis. Nitrogen gas (99.99%, Air Liquide) was used as dilution gas. The concentration and flow of the ammonia gas and nitrogen gas were precisely controlled by two flow meters, which were plugged in a mass flow controller (MFC). The concentration of NH3 gas in the measuring chamber was varied by mixing different flows of NH3 and 500 sccm of N2. The concentration of NH3 (ppm) was defined as the ratio of the flow rate of NH3 gas to the total flow rate of NH3 and N2 gases. In this work, the ammonia concentration was varied between 92 ppm and 4618 ppm. The ammonia concentrations lower than 92 ppm were not obtained due to experimental setup limitation. After inserting the PANI sensor inside the measuring chamber, a certain amount of dried ammonia gas, diluted in nitrogen gas, was introduced into the measuring chamber. The interaction between the NH3 gas and the PANI film led to an optical power variation. When the optical power variation attained a constant value depending on time, the NH3 gas introduction was turned off and a stream of pure N2 gas was passed through the sensor to purge completely the NH3 molecules in the measuring chamber and to regenerate the integrated optical sensor. During all the sensing experiments, the 980 nm laser was used as the incident beam. All the experiments were performed at room temperature. The sensitivity (S) of the integrated optical sensor was calculated as (P - P0)/P0 ratio, where P0 is the initial transmitted light power of the sensor and P the transmitted light power of the sensor when exposed to ammonia gas. Finally, an SU-8 waveguide (without passive and sensitive layers) was placed in the measuring chamber. The injection of NH3 into the chamber did not lead to any change of the transmitted light power, indicating that there is no interaction between the NH3 gas and the SU-8 polymer. RESULTS AND DISCUSSION Design of the Sensor. Choice of the Passive Layer. As a reference, we have tested the sensor composed of chemical PANI directly deposited on a small section of the SU-8 waveguide, without passive layer (conventional waveguide sensor). Different interaction lengths (0.5

to 5 mm) and PANI thicknesses (50 to 130 nm) have been tested. Then, the guiding of light through the Si/SiO2/SU-8/PANI structure was verified at λ ) 980 nm. No optical signal was detected at the output of the waveguide, whatever interaction lengths and PANI thicknesses are. The absence of transmitted light at the exit of this conventional waveguide sensor has been explained by high absorption coefficient of chemical PANI which has been determined equal to about 105 cm-1 at λ ) 980 nm. It is then necessary to decrease the absorption coefficient of the sensitive material or, as we propose here, to introduce a passive layer between sensitive layer and waveguide core. One of the main criteria for the choice of the passive layer is the low absorption coefficient at the working wavelength. Then, we have chosen PMMA as passive layer. Indeed, PMMA transmits well from the visible to near-infrared region with a low absorption coefficient. Moreover, the refractive index of PMMA used in this study is estimated as 1.483 at 980 nm allowing then to satisfy propagation conditions. Furthermore, no obvious refractive index change of PMMA before and after sensing process was observed. For these reasons and also its easy deposition as thin film, PMMA is suitable as passive material for the multilayer waveguide sensor. Determination of the Thickness of the Passive Layer. The sensitivity of the MIOS depends on the sensing region distance (interaction length) and the penetration depth of the evanescent field into the sensitive layer. Consequently, the control of the thickness of the passive layer is important for the sensitivity of the sensor. Theoretical Study The important criterion for this MIOS is the penetration depth (dp) of the evanescent wave so that it can interact with the molecules/species of sensitive layer. In order to estimate this penetration depth with eq 1, the effective refractive index of a guided wave (mode) in the planar waveguide is considered. If the refractive index of the passive layer (n3) is lower than that of the waveguide core (n2), the model of our waveguide corresponds to a three-layer structure. The dispersion relation is derived from the given boundary conditions, and the eigenvalue equation of the three-layer structure at transverse electric wave (TE) and transverse magnetic wave (TM) modes is

[ pq ] - arctan[η pq ] ) mπ

hq - arctan η12

13

(m ) mode order ) 0, 1, 2, 3...)(2)

with η12 ) (n1/n2)2 and η13 ) (n1/n3)2 for TM mode, η12 ) η13 ) 1 for TE mode, q ) √k02n12 - β2, p ) √β2 - k02n22, r ) √β2 - k02n32 where β ) neffk0 is the propagation constant in the waveguide, k0 the wavenumber in the vacuum ()2π/λ), neff the effective refractive index of the waveguide, n1 the refractive index of the lower cladding layer and h the waveguide thickness. The effective indexes of the fundamental modes (TE0 and TM0) and the penetration depth of the evanescent wave at wavelength of 632.8 and 980 nm are presented in Table 1 for the two different structures: Si/SiO2/SU-8/PMMA and Si/SiO2/SU-8/air. These results show that the increase of the refractive index of the top Analytical Chemistry, Vol. 80, No. 23, December 1, 2008

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Table 1. Effective Refractive Indexes of Guided Modes (TE0, TM0) and Penetration Depth of the Evanescent Wave for Si/SiO2/SU-8/PMMA and Si/SiO2/SU-8/Air Structures (λ ) 632.8 and 980 nm) λ (nm) 632.8 980

632.8 980

top layer

n3

n2

neff

dp (nm)

air PMMA air PMMA

TE0 Mode 1 1.591 1.489 1.591 1 1.580 1.483 1.580

1.570 1.573 1.541 1.549

83 199 133 348

air PMMA air PMMA

TM0 Mode 1 1.588 1.488 1.588 1 1.578 1.482 1.578

1.565 1.569 1.532 1.544

84 203 134 360

layer increases the penetration depth of the evanescent field. Moreover, the penetration depth increases with increasing the wavelength according to eq 1. Then, this theoretical study shows that the thickness of the passive layer must be lower than 199 and 348 nm at λ equals to 632.8 and 980 nm respectively for the evanescent wave to interact with the sensitive layer. Experimental Study To elaborate this new sensor, we have chosen a thickness of the sensitive layer (PANI) of 130 nm which is the highest thickness that can be deposited. Indeed, this important thickness will allow obtaining the best sensitivity with the highest quantity of adsorption sites. Then, we have tested the light guiding properties of the MIOS as a function of the PMMA thickness (from 70 to 500 nm) and the interaction length between evanescent wave and PANI layer (2 and 5 mm) for two wavelengths λ ) 632.8 and 980 nm. The results show that when the PMMA thickness is lower than 90 nm, no optical signal is detected at the exit of the sensor, whatever interaction lengths and wavelengths are. In these conditions, the evanescent field is probably totally absorbed by the PANI sensitive layer. If the PMMA thickness is about 90 nm, the guided light transmission is only obtained for λ ) 632.8 nm. At λ ) 980 nm (dp ) 348 nm), the evanescent wave is totally absorbed by the PANI layer. In addition, the absorption coefficient of doped PANI at 632.8 nm is lower than that of doped PANI at 980 nm. When the PMMA thickness is equal or higher than 175 nm, the output light of MIOS is detected, whatever interaction lengths and wavelengths are. These results can be attributed to the low interaction between evanescent wave and PANI layer, and when the PMMA thickness is about 500 nm, the evanescent wave only propagates through transparent PMMA (cf. dp in Table 1). In conclusion, we have chosen the PMMA layer thickness equal to 175 nm at the wavelength λ ) 980 nm. This value of PMMA thickness satisfies the propagation conditions and gives the most sensitive MIOS to ammonia gas. Moreover, 980 nm has been chosen as the working wavelength because the absorbance of doped PANI is the highest around this wavelength (Figure 3) which must lead to the highest sensitivity of the sensor. Sensing Properties of MIOS. Figure 4(a) shows the variation of the transmitted light power versus NH3 concentration at the working wavelength 980 nm for Si/SiO2/SU-8/PMMA (175 nm)/ doped PANI (130 nm) sensor. This figure shows that the transmitted power increases after NH3 exposure. The variation 9192

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Figure 4. TE0 transmitted light power variations of Si/SiO2/SU-8/ PMMA/PANI exposed to different NH3 concentrations: (a) doped PANI; (b) dedoped PANI. PANI film thickness: 130 nm. Interaction length: 2 mm.

of the output power after NH3 introduction in the measuring cell can be attributed to the interaction between the NH3 molecules and the reactive sites of PANI sensitive layer. When the ammonia injection is on, the charge carrier (polaron) distribution and the surface conductivity of the sensitive layer are modified and, consequently, the absorption coefficient and refractive index of PANI change. Indeed, when the doped PANI is exposed to ammonia gas, the polaron density decreases leading to dedoped PANI, thus the PANI absorption decreases around 980 nm as shown on Figure 3. The evanescent wave interacts with this modified PANI, which modifies then the propagation conditions of the guiding structure and leads in this case to an increase in the total transmitted intensity of the SU-8 waveguide. When the equilibrium between ammonia molecules in the gas phase and ammonia molecules adsorbed on PANI layer is obtained, no more variation of the absorption coefficient is observed and then the output power is stabilized (tends to a constant value with time). Finally, when the injection of NH3 is turned off, the regeneration of the sensor starts. Then, the ammonia molecule desorption begins to restore the equilibrium of the concentrations and as a result, the output power of the sensor decreases. Indeed, the regeneration of the sensitive layer increases the charge carrier concentration, thus increasing the PANI absorption coefficient around the working wavelength 980 nm. Response Time and Recovery Time. The response and recovery times of the MIOS at room temperature can be investigated from Figure 4(a). The recovery time seems to increase with the increase of ammonia concentration, which causes the apparent increase of the baseline in Figure 4(a), meaning that the NH3 molecules are slowly desorbed from the PANI sensitive layer at high ammonia concentrations. For example, at 92 ppm, the sensor is completely regenerated after only 8 min of regeneration. However, after exposition to 3237 ppm of NH3, only 80% of NH3 molecules desorbed from the sensor after 20 min of regeneration. The increase of NH3 concentration increases the quantity of adsorbed ammonia molecules on the PANI surface to obtain the equilibrium of NH3 concentrations, then increasing probably the time for the complete desorption of the ammonia molecules from the PANI

layer. From the same figure, it is observed that the response time of the sensor is also depending on the ammonia concentration. Based on the definition of response time determined as the time interval between 10% and 90% of the stationary value,38 the response time of the MIOS to ammonia is about 5 and 3 min at 92 ppm and 3237 ppm respectively. These results suggest that the NH3 absorption process is faster compared to the NH3 desorption phenomenon and that the response time decreases when the ammonia concentration increases. In general, the absorption and desorption processes of ammonia molecules are slow for the integrated sensor compared to transmission measurements. It could be due to the evanescent sensing mechanism. When the ammonia gas is introduced in the measuring cell, the ammonia molecules must diffuse into the sensitive layer where the polarons can absorb the evanescent field. Thus, this type of sensors usually has relatively longer response and recovery times. Similar results concerning response and recovery times were also reported for optical sensors based on the evanescent field sensing method.34,39-42 However, the improvement of these metrological parameters can be obtained through optimization of the thickness of sensitive film and passive layer. When the sensitive material thickness is close to the penetration depth dp of the evanescent wave expressed in eq 1, the variation of the polaron density, due to the interaction between ammonia molecules and polaron lattice, immediately occurs at the surface where the evanescent wave of the guided mode interacts with the polarons. Thus the sensor has the optimized response and recovery times. However, if the cladding (sensitive layer) thickness decreases further, the sensor sensitivity will decrease due to the low interaction of evanescent field with the polarons of the sensitive material. In addition, the gas desorption process can be improved by increasing the temperature.22,43 On the other hand, PANI can be fully regenerated by exposition to the doping agents such as hydrochloric acid at room temperature.10 Influence of the Interaction Length. In order to study the effect of the interaction length (LI) between the PANI sensitive layer of the multilayer integrated optical sensor and the evanescent field of the guided mode on the sensitivity of the sensor, two interaction lengths have been studied: 2 mm and 5 mm. The sensitivity of the sensors as a function of ammonia concentration and interaction length is illustrated in Figure 5. The sensitivity increases rapidly with the NH3 concentration until 3000 ppm, then the sensitivity tends to be constant and approaches a plateau value at high concentrations. This behavior suggests that the interaction of ammonia molecules with polyaniline sensitive material is a nonlinear phenomenon and probably controlled by the gas diffusion in the material. The nonlinear sensing response of the detected gas is a well-known phenomenon in gas detection and (38) D’Amico, A.; Di Natale, C.; Taroni, A. Proceedings of the First European School on Sensors (ESS’94), Castro Marina: Lee, Italy, 1994; pp 3-13. (39) Scorsone, E.; Christie, S.; Persaud, K. C.; Simon, P.; Kvasnik, F. Sens. Actuators, B 2003, 90, 37–45. (40) Kumar, P. S.; Vallabhan, P. G.; Nampoori, V. P. N.; Radhakrishnan, P. Proc. SPIE-Int. Soc. Opt. Eng. 2004, 5280, 617–621. (41) Scorsone, E.; Christie, S.; Persaud, K.; Simon, P.; Kvasnik, F. Proc. SPIEInt. Soc. Opt. Eng. 2003, 4829, 978–979. (42) Christie, S.; Scorsone, E.; Persaud, K.; Kvasnik, F. Sens. Actuators, B 2003, 90, 163–169. (43) Cao, W.; Duan, Y. Sens. Actuators, B 2005, 110, 252–259.

Figure 5. Sensitivity of the Si/SiO2/SU-8/PMMA/PANI sensor as a function of ammonia concentration and interaction length (LI). PANI film thickness: 130 nm.

was also observed for several PANI-based ammonia sensors.10,35,44 Moreover, the results show that the interaction length significantly affects the sensitivity of this type of sensor. Indeed, the increase of the interaction length considerably increases the sensitivity of the MIOS. As an example, the sensitivity at 4618 ppm of NH3 is about 17% for LI ) 5 mm and 6% for LI ) 2 mm. The increase of the interaction length increases the number of absorption sites which participate in gas/sensitive material interaction process, thus enhancing the sensitivity of the sensor. Influence of the PANI Doping. In order to study the influence of the PANI doping on the metrological parameters (sensitivity, response time, recovery time) of the multilayer integrated optical sensor, the PANI layer was dedoped by NH4OH (cf. Experimental Section). Figure 4(b) presents the optical response of the MIOS based on dedoped PANI with an interaction length of 2 mm, in contact with different concentrations of NH3. First of all, this figure shows that the initial output power of the sensor is higher than that of the sensor based on doped PANI (Figure 4(a)). Indeed, the deprotonation process decreases the polaron density of PANI, thus the absorption of the evanescent field by the PANI sensitive layer decreases around the working wavelength 980 nm. Moreover, contrary to the doped PANI, the interaction between ammonia and dedoped PANI layer immediately leads to the decrease of the transmitted optical power. The interaction between dedoped PANI layer and NH3 molecules seems to increase the PANI layer absorption. This increase is probably due to the variation of charge distribution in the PANI layer. As in the case of doped PANI, the response time, recovery time and sensitivity of the MIOS at room temperature can be investigated from Figure 4(b). The recovery time corresponding to the NH3 molecules desorption from the dedoped PANI seems lower than that of the doped PANI. For example, the recovery times are about 1 and 2 min after exposition of 92 and 3237 ppm of NH3 respectively. Moreover, the same observations can be done for the response time which is lower than 1 min whatever ammonia concentration is. Furthermore, the reproducibility of the sensor response is confirmed for 3237 ppm of NH3. We think that (44) Madou, M. J.; Morrison, S. R. Chemical Sensing with Solid State Devices; Academic Press: Boston, 1989.

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the better characteristics (recovery and response times) of this sensor are attributed to the physicochemical properties of dedoped PANI. Contrary to the dedoped PANI, the doped PANI is insoluble in most of solvents, thus doped PANI has probably a cross-linked structure. The gas diffusion occurs more easily in linear structures, and the reaction between gas molecules and adsorption sites occurs then easily. So, the NH3 molecule absorption and desorption occur more easily in the case of dedoped PANI. The sensitivity of the MIOS based on dedoped PANI (LI ) 2 mm) is represented on Figure 5. The sensitivity of this sensor increases with increasing NH3 concentration. The nonlinear behavior of the optical response to ammonia is also observed for this sensor. Moreover, it appears that the dedoped PANI is less sensitive to ammonia compared to the doped PANI. Indeed, the sensitivity at 4618 ppm of NH3 is about 2.5% for the MIOS based on dedoped PANI and 6% for the MIOS based on doped PANI. Indeed, the structure of doped PANI has a significant number of reactive sites (polarons) for the detection of the NH3 molecules. Sensor Calibration and Detection Limit. Since the transmitted light variation is controlled by the ammonia diffusion into the film, the relationship between output power and ammonia concentration (N) can be expressed by the following equation:10 P ) P0 exp[(RN)γ]

(3)

where P is the transmitted light power at ammonia concentration equal to N, P0 is the initial transmitted light power at N ) 0 ppm, and R and γ are constants. From eq 3, log ln(P/P0) is proportional to log N. The calibration curve obtained from eq 3 for the multilayer waveguide sensor based on doped PANI shows a logarithmic linear response with the concentration of ammonia from 92 ppm to 4618 ppm (cf. Figure 6). The logarithmic linear calibration curve was also observed for the sensor based on dedoped PANI. In our case, the response of the sensor (power variation) with ammonia concentration is nonlinear, thus the estimation of the detection limit is not possible. Due to experimental setup, the lowest obtained ammonia concentration is 92 ppm. However, according to Figure 5 showing that the sensitivity of the sensor at 92 ppm is about 5%, we can conclude that this new device has probably a very low detection limit (