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Nafion Film/K+-Exchanged Glass Optical Waveguide sensor for BTX Detection Hayrensa Ablat,† Abliz Yimit,*,† Mamtimin Mahmut,† and Kiminori Itoh‡ College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, P. R. China, and Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan An optical waveguide (OWG) sensor for the detection of BTX gases is reported. The highly sensitive element of this sensor was made by coating the copper Nafion film over a single-mode potassium ion exchanged glass OWG. We used the OWG sensor to detect toluene gas as a typical example BTX gas. The sensor exhibits a linear response to toluene in the range of 0.25-4250 ppm with response and recovery times less than 25 s. The sensor has a short response time, high sensitivity, and good reversibility. Benzene, toluene, and xylenes (BTX) are volatile organic compounds (VOCs) of great social and environmental significance, are widely used in industry, and can present serious medical, environmental, and explosion dangers.1 The main sources of BTX emissions (more than 80%) are automobile exhaust and other traffic-related processes. Because they are toxic even at partsper-billion concentrations, it is important to know their concentration in the air.2 BTX is classified as a human carcinogen and is a risk factor for leukemia and lymphomas. The regulated standard concentration of benzene is 3 µg/m3 (1.0 ppb) in Japan. The guidelines for the upper indoor concentration limits of toluene and xylenes are 260 (0.07 ppm) and 870 µg/m3 (0.20 ppm), respectively.3 The development of chemical sensors capable of monitoring gaseous analytes at trace concentrations continues to attract widespread interest.4 In this arena of chemical sensors, a number of groups have developed BTX sensors. For example, BTX was measured by using piezoelectric crystal detector early;5 it was also detected by electrochemical sensors,6-8 Mach-Zehnder interferometer,9 and optical redox chemical sensor.10 Additionally, optical fiber chemical sensors have been proposed for * To whom correspondence should be addressed. E-mail:
[email protected]. † Xinjiang University. ‡ Yokohama National University. (1) Patel, S. V.; Mlsna, T. E.; Fruhberger, B.; Klaassen, E.; Cemalovic, S.; Basel, D. R. Sens. Actuators, B 2003, 96, 541-543. (2) Ueno, Y.; Horiuchi, T.; Morimoto, T.; Niwa, O. Anal. Chem. 2001, 73, 4688– 4690. (3) Ueno, Y.; Horiuchi, T.; Niwa, O.; Zhou, H. S. Sens. Actuators, B 2003, 95, 282–287. (4) Yang, L.; Saavedra, S. S.; Armstrong, N. R. Anal. Chem. 1996, 68, 1834. (5) Mat, H. Ho; Guilbault, G. G.; Rietz, B. Anal. Chem. 1980, 52, 1489–1492. (6) Cox, J. A.; Alber, K. S.; Brockway, C. A.; Tess, M. E.; Gorski, W. Anal. Chem. 1995, 67, 993–998. (7) Wang, J.; Chen, Q.; Cepria, G. Talanta 1996, 43, 1387–1391. (8) Jereschewski, P.; Steuckart, C.; Kuhl, M. Anal. Chem. 1996, 68, 4351– 4357. (9) Elling, B.; Danz, R. Mater., Sci. Eng., C 1999, 8 (9), 401–405.
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the determination of aromatic hydrocarbons in contaminated waters.11-16 Recently, optical sensors for gaseous analytes have been described.17 Among them, the optical waveguide (OWG) sensor18,19 has attracted considerable attention for monitoring environmental pollution,20 industrial processes,21 and biological fields,22 particularly, for the detection of gaseous elements in trace concentrations. OWG methods for sensing have several merits over other types of sensors, such as the potentials for high sensitivity, fast response time, remote controllability, system compactness, and intrinsically safe detection. Furthermore, they suffer little or no interence in the waveguide element of the sensor and can be made at a very low cost. Nafion is special material that possesses widespread applications in the area of electrochemistry analysis, chemical sensor, and nanomaterials.23,24 In this paper, Nafion was selected for the BTX vapor sensor. Nafion is a good cation exchanger with the following advantages of thermal stability, chemical inertness, and mechanical strength. As a membrane, the material Nafion has often been employed to achieve selective transport within devices such as fuel cells and chemical sensors.25The microstructure of Nafion consists of three regions: the hydrophobic fluorocarbon backbone, hydrophilic ionic clusters of sulfonic acid groups, and an interfacial region.26 The superselectivity of Nafion and its (10) Damien, T.; Newcombe, T.; Cardwell, J.; et al. Anal. Chem Acta 1999, 401, 137–144. (11) Blair, D. S.; Burguess, L. W.; Brodsky, A. M. Anal. Chem. 1997, 69, 2238– 2246. (12) Buerck, J.; Roth, S.; Kraemer, K.; et al. J. Hazard. Mater. 2001, 83, 11–28. (13) Tobiska, P.; Chomat, M.; Matejec, V.; et al. Sens. Actuators, B 1998, 51, 152–158. (14) Jakusch, M.; Mizaikoff, B.; Kellner, R.; Katzir, A. Sens. Actuators, B 1997, 38, 83–87. (15) Schwotzer, G.; Latka, I.; Lehmann, H.; Willsch, R. Sens. Actuators, B 1997, 38, 150–153. (16) Gobel, R.; Seitz, R. W.; Tomellini, S. A.; Krska, R.; Kellner, R. Vib. Spectrosc. 1995, 8, 141–149. (17) Lev, O. Analusis 1992, 20, 543–553. (18) Yimit, A.; Itoh, K.; Murabayashi, M.; Sens, T. Actuators, B 2003, 88, 239– 245. (19) Yimit, A.; Rossberg, A. G.; Amemiya, T.; Itoh, K. Talanta 2005, 65, 1102– 1109. (20) Yimit, A.; Talip, D.; Tursun, E.; Itoh, K.; Chin, J. Anal. Chem. 2005, 33, 1663. (21) Abdukayum, A.; Yimit, A.; Mahmut, M.; Itoh, K. Sens. Lett. 2007, 5, 395– 397. (22) Yimit, A.; Huang, X.; Xu, Y.; Amemiya, T.; Itoh, K. Chem. Lett. 2003, 32, 86. (23) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 2001, 105, 3350–3352. (24) Lee, J.; Li, Z.; Hodgson, M. Curr. Appl. Phys. 2004, 4 (2-4), 398–401. (25) Basnayake, R.; Wever, W.; Korzeniewski, C. Electrochim. Acta 2007, 53, 1259. 10.1021/ac800815g CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
Figure 1. Sensor element of Nafion film/K+-exchange glass OWG.
chemical and thermal stability are ascribed to the structure.27According to our survey, no optical waveguide sensor that detects the BTX gases using Nafion has been reported. In this paper, we begin by describing how to fabricate a singlemode potassium ion (K+) exchanged glass OWG coated with Nafion film as BTX gases sensor. We then report several characteristics of the device; finally, we provide results for the detection of parts-per-million levels of toluene gas as an example of BTX gases detection. EXPERIMENTAL SECTION The planar waveguide sensing device consisted of three major components: a substrate, a thin top layer (waveguide layer) with a refractive index greater than that of the substrate, and the covering material (usually air).28 The K+-exchanged glass OWG device is fabricated on a soda-lime glass substrate (76 mm × 26 mm × 1 mm), which contains different oxides such as SiO2, Na2O, and CaO, which were immersed in molten KNO3 salt at 400 °C for 40 min. The net index variation depends on the ionic polarizability, the molar volume, and the stress state created by the substitution. In the case of K+-Na+ exchange, since the polarizability of K+ is considerably greater than that of Na+ on glass, K+ can be easily incorporated in glass, and the accompanying index change is substantially smaller (n e 0.01) along with a smaller diffusion rate.29 After the ion-exchange process, the substrate was taken out of the molten KNO3, cooled at room temperature, and then washed with distilled water in order to remove the solid KNO3 on its surface. The sensitive element was synthesized by the following approach: Nafion solution (5 wt %) was purchased from ShangHai Hesen Co. Ltd.; it was immobilized directly onto the surface of K+-exchanged glass OWG by spin coating at 2500 rpm for 20 s. After the coating process, the coated waveguide was then baked for 60 min at 60 °C. Nafion is of particular interest because it can be processed into thin membranes that would be well suited to an optical sensor. These materials were evaluated on the quality of the resulting films when cast on a glass surface. The refractive index of Nafion film refraction index; n ) 1.37, is lower than that of the K+-exchanged layer (n ) 1.52). So light is transmitting in the K+-exchanged layer. Figure 1 shows the schematic diagram of the Nafion film/K+ ion-exchange glass OWG. The BTX gases testing apparatus (Figure 2a) was constructed from compressed air sources, a flowmeter, a diffusion tube, and gas that contained BTX gases. A gas mixing manifold was used (26) (27) (28) (29)
Yeo, S. C.; Eisenberg, A. J. Appl. Polym. Sci. 1977, 21, 875. Zook, L. A.; Leddy, J. Anal. Chem. 1996, 68, 3793. Lukosz, W. Sens. Actuators, B 1995, 29, 37–50. Ramaswamy, R. V.; Srivastava, R. J. Lightwave Technol. 1988, 6, 984.
to mix the air stream that contained BTX gases with a stream of pure air and to introduce the mixture into the flow cell, which enclosed the waveguide sensor. The Nafion film/K+-exchanged glass OWG gas sensor device in a flow cell (2 cm × 1 cm × 1 cm) was mounted on a rotational stage equipped with X-Y-Z translation. A He-Ne laser (λ ) 632.8 nm) beam was introduced into the OWG using a prism coupler (glass prism, n ) 1.78; and a matching liquid, diiodomethane, n ) 1.74), and it emerged from another prism coupler. The distance between the two prism couplers was 15 mm. The intensity of the output light was monitored by using a radiometer, and the signal was recorded by a computer. In every measurement, a new syringe was used to inject 10 mL of the toluene gas sample into the flow chamber and then out from the vent (Figure 2b). Pure air that functioned as a carrier and dilution gas flowed through the cell at a constant rate of 50 mL/min in order to transfer the toluene gas to the sensor. The transmission of the charge-transfer complex formed between toluene and Nafion film was measured using an ultraviolet spectrophotometer. All measurements were made at room temperature. Standard toluene gas was obtained by vaporizing a given amount of 99.5% toluene solution inside a 600-cm3 standard vessel filled with pure nitrogen gas. The concentration of the toluene gas was confirmed by a commercial toluene gas detection tube (Gastec). Different amounts of standard toluene gas were diluted to obtain the desired concentrations with pure air in a second standard vessel (600 cm3). Using this standard vessel dilution method, very low concentrations of toluene (in the ppm range) were obtained. A new glass syringe was used to inject the same volume of toluene sample into the flow cell. RESULT AND DISCUSSION Waveguides coated with Nafion films were exposed to toluene vapors, and the transmission at 632.8 nm was monitored. These results are presented in Figure 3. The test was performed using an ultraviolet spectrophotometer (UV-2450 Japan). Nafion film’s transmission is nearly 100% when the toluene steam is not injected. When a toluene steam exists, the transmission decreases because of chemical adsorption between toluene gas and Nafion film; thus, it causes the output light intensity to reduce. There is a strong linear relationship between the light intensity change value and concentration of toluene gas. For the OWG sensor, any change in the optical properties of the thin film that translates into a change in light intensity reaching the light detector will affect the analytical signal. The typical reversible response of the Nafion film/K+-exchanged glass optical waveguide sensor to toluene vapor is given in Figure 4. As the Nafion film was exposed to toluene vapor, the sensor baseline steadily decreased. Upon exposure to toluene vapor, the sensor response returning to dry air results in partial recovery of sensor signal. The extent of irreversible signal decrease seems to increase as the toluene vapor concentration increases and constitutes a dosimetric response. The response and recovery times of the planar OWG toluene sensor were fast. As can be seen from Figure 4, the intensity of output light decreased with the increase in the concentration of toluene in the range of 0.25-4250 ppm and decreased when the concentration of toluene was increased. The recovery time was found to increase with an increase in toluene gas concentration, by means Analytical Chemistry, Vol. 80, No. 20, October 15, 2008
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Figure 2. Schematic view of BTX gases sensor system (a) and amplificatory diagram of detection cell (b). The measurement procedure is described in the text.
Figure 3. Transmission changes of the Nafion film.
of dry air. The recovery time of the sensor was longer than its response time because the oxidant (oxygen) concentration was lower and a longer time was required for a sufficient amount of oxidant to reach the indicator in the flow chamber. The response and recovery times of the planar OWG toluene sensor were fast and less than 3 and 25 s, and highest output light intensity of this vapor is 0.25 ppm. Evanescent wave is produced when the light waves travel through the waveguide layer. As Figure 5 shows, sensitive film was immobilized onto the waveguide layer surface, as the light 7680
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traveling through waveguide layer evanescent wave came into the sensitive film (under the condition that the refractive index of sensitive film is smaller than the refractive index of waveguide layer). When the sensitive layer was exposed to the detected gas, the optical properties of the sensitive layer changed. Thus, the decrease or the increase of absorption of evanescent wave induces the change in the intensity of the output light. In this paper, when Nafion film was exposed to toluene gas, the decrease in transmission (T %) induced the changes of intensity of output light.
Figure 4. Typical response of Nafion film/K+-exchanged glass OWG sensor when exposed to toluene vapor in air.
Substituting (2) and (3) into (1), we found that 1 Pabs ) ChνB(nf2 + nc2)ε0Ey(0)2 2
(4)
The relation between T (transmission) and nf is given by31
Figure 5. Guide light travel in the Nafion film/K+-exchanged glass OWG sensor.
The change of response curve can be explained by using the following formulas: Pabs (molecular absorption power) is given by30
Pabs ) ChvBFEsurf
(1)
where C is the concentration of sensitive layer, hv is the Planck constant, B is theEinstein constant, and FEsurf is the surface electric field energy density. Where FEsurf is given by
1 2 FEsurf ) nsurf ε0Ey(0)2 2
(2)
In this calculation, ε0 is the dielectric constant and Ey(0) is the intensity of the electric field on the surface of optical waveguide. nsurf was the average surface refractive index defined by
2 nsurf ) nf2 + nc2
(3)
The refractive index of thin film is given as nf; that of the cladding layer nc (usually air, 1.0). (30) Itoh, K.; Murabayashi, M. Trends Phys. Chem. Res. Trends, India 1991, 1, 179–187.
T)
4nf (nf + 1)2
(5)
Combining (4) and (5) yields Pabs ∝
1 T
(6)
Function 6 means the light power of molecular absorption increases as the transmission T decreases, and then the intensity of the output light decreases, as shown in Figure 4. A calibration curve of absorbance measured as a function of toluene vapor concentration is plotted in Figure 6. These curves were obtained by plotting the signal (A) of the sensor against the concentration of toluene gas. On the ordinate, logA ) log[log(Iair/ IToluene)] is plotted, where Iair is the initial output light intensity and Itoluene is the lowest point of the output light intensity corresponding to before and after the injection of toluene gas into the flow chamber, respectively. The relationship between the signal and the increasing concentration of toluene was found to be linear. From the data, it can be observed that the sensor response is strongly dependent on the toluene gas concentration. The response was linear between toluene concentration of 0.25 and 265 ppm (Log A ) (-1.61 ± 0.015) + (0.25 ± 0.011) log [toluene], A[Toluene](0.25+0.011). R ) 0.99). Several OWG sensors were fabricated and tested for response to gaseous toluene. The repetitive responses of typical sensor exposed alternately to pure air and 4 ppm toluene vapor is shown in Figure 7. At this concentration, the relative standard deviation of output light intensity is ±1.73%. The response time was rapid (31) Chen, K.; Wu, W.; Zhen, S. Laser Technol. [J] 2001, 63, 209–213.
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Figure 6. Relationship between the signal of the sensor and the concentration of toluene gas. The experiment results using different devices and identical methods: (b)first; ([) third; Y ) (-1.61 ( 0.015) + (0.25 ( 0.011)X.
Figure 7. Repetitive response curve of OWG sensor to 4 ppm toluene gas.
Figure 8. Response stereogram of OWG sensor to various gases
(5 s) and fully reversible (16 s) after three times consecutive injected 4 ppm toluene vapor. These results demonstrated that the OWG sensor was fully reversible and reproducible for toluene gas sensing. 7682
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For applications in environmental monitoring, a selective fast response to Nafion film/K+-exchanged glass OWG in the presence of some interference like benzene, xylene, methanol, ethanol, ammonia, and hydrochloric acid is descriable. The OWG sensor
was used and the experimental result shows that the OWG sensor has not any considerable response to methanol, ethanol, ammonia, and hydrochloric acid, even at concentrations higher than 2 × 104 ppm. Figure 8 shows the response of the OWG sensor to 200 ppm benzene, 200 ppm toluene and xylene, 1 × 105 ppm methanol, and 5 × 104 ppm ethanol, saturated ammonia, and hydrochloric acid. Among benzene, toluene, and xylene of the same concentration, the sensor has a higher response to toluene. Besides, the response of low consistence of toluene gas is remarkably better than the response of high consistence of methanol and ethanol, and the response to saturated ammonia and hydrochloric acid is negligible. Detailed studies on the OWG sensor’s response to benzene and xylene gases are still in progress and will be investigated further. CONCLUSION A novel optical waveguide sensor for measuring gaseous BTX wasrapidly developed. We measured the detection limit of the device under operating conditions that are determined according to the investigations described above. We succeeded in detecting parts-per-million toluene gas, which is a typical example of BTX vapor; it exhibits general sensitive and reversible response to the
toluene vapor. The detection limit for toluene vapor is
