Yttrium-doped LiFePO4 powder was synthesized using the hydrothermal method in one step and was used as a sensing material. An optical waveguide (OWG) sensor based on Yttrium-doped LiFePO4 has been developed by spin coating a thin film of LiFe0.99Y0.01PO4 onto a single-mode Tin-diffused glass optical waveguide. Light was coupled into and out of glass OWG employed by a pair of prisms. The guided wave transmits in waveguide layer and passes through the film as an evanescent wave. The sensing film is stable in air, but when exposed to target gas at room temperature, its optical properties such as transmittance (T) and refractive index ( ) were changed; thus, the transmitted light intensity was changed. The LiFe0.99Y0.01PO4 thin film OWG exhibits reversible response to xylene gas in the range of 0.1–1000？ppm. When the concentration of BTX gases was lower than 1ppm, other substances caused a little interference with the detection of xylene vapor. Compared to pure LiFePO4 thin film OWG, this sensor exhibited higher sensitivity to BTXs. 1. Introduction Benzene, toluene, and xylene (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]. BTX is also classified as a human carcinogen and is a risk factor for leukemia and lymphomas. The regulated standard concentration of benzene is 1.0？ppb (3？μg/m3) in Japan. The guidelines for the upper indoor concentration limits of toluene and xylene are 70？ppb (260？μg/m3) and 200？ppb (870？μg/m3), respectively [2]. Because of BTX’s acute toxicities, there has been an increasing need for highly sensitive, rapidly responding, portable devices for monitoring trace levels of them in various environmental and industrial applications. Many works have been done on sensitivity to BTX such as electric noses [3, 4], chromatography [5], and electrochemical sensor [6], and these detectors are accurate, yet bulky and expensive, and require higher operating temperature. In comparison, the optical waveguide (OWG) sensors [7–9] are small in size, of high sensitivity, of fast response time, monitored at room temperature, and of intrinsically safe detection. Furthermore, they suffer little or no interference in the waveguide element of the sensor and can be made at a very low cost. A simple planar OWG consists of a substrate, a thin top layer (waveguide layer) with a refractive index greater than that of the substrate and the covering material (usually air) [8]. Single-mode Tin-diffused glass waveguide has a
References
[1]
S. V. Patel, T. E. Mlsna, B. Fruhberger, E. Klaassen, S. Cemalovic, and D. R. Baselt, “Chemicapacitive microsensors for volatile organic compound detection,” Sensors and Actuators B, vol. 96, no. 3, pp. 541–553, 2003.
[2]
Y. Ueno, T. Horiuchi, O. Niwa, H. S. Zhou, T. Yamada, and I. Honma, “Portable automatic BTX measurement system with microfluidic device using mesoporous silicate adsorbent with nano-sized pores,” Sensors and Actuators B, vol. 95, no. 1–3, pp. 282–286, 2003.
[3]
F. R？ck, N. Barsan, and U. Weimar, “Electronic nosescurrent status and future trends,” Chemical Reviews, vol. 108, no. 2, pp. 705–725, 2008.
[4]
C. P. Melo, B. B. Neto, E. G. Lima, L. F. B. Lir, and J. E. G. Souza, “Use of conducting polypyrrole blends as gas sensors,” Sensors and Actuators B, vol. 109, pp. 348–354, 2005.
[5]
J. M. Juarez-Galan and I. Valor, “New universal, portable and cryogenic sampler for time weighted average monitoring of H2S, NH3, benzene, toluene, ethylbenzene, xylenes and dimethylethylamine,” Journal of Chromatography A, vol. 1216, no. 15, pp. 3003–3011, 2009.
[6]
C. Sun, X. Su, F. Xiao, C. Niu, and J. Wang, “Synthesis of nearly monodisperse Co3O4 nanocubes via a microwave-assisted solvothermal process and their gas sensing properties,” Sensors and Actuators B, vol. 157, no. 2, pp. 681–685, 2011.
[7]
H. Ablat, A. Yimit, M. Mahmut, and K. Itoh, “Nation film/K+-exchanged glass optical waveguide sensor for BTX detection,” Analytical Chemistry, vol. 80, no. 20, pp. 7678–7683, 2008.
[8]
A. Yimit, A. G. Rossberg, T. Amemiya, and K. Itoh, “Thin film composite optical waveguides for sensor applications: a review,” Talanta, vol. 65, no. 5, pp. 1102–1109, 2005.
[9]
A. Yimit, K. Itoh, and M. Murabayashi, “Detection of ammonia in the ppt range based on a composite optical waveguide pH sensor,” Sensors and Actuators B, vol. 88, no. 3, pp. 239–245, 2003.
[10]
Z. M. Qi, I. Honma, and H. Zhou, “Chemical gas sensor application of open-pore mesoporous thin films based on integrated optical polarimetric interferometry,” Analytical Chemistry, vol. 78, no. 4, pp. 1034–1041, 2006.
[11]
Y. D. Cho, G. T. K. Fey, and H. M. Kao, “Physical and electrochemical properties of La-doped LiFePO4/C composites as cathode materials for lithium-ion batteries,” Journal of Solid State Electrochemistry, vol. 12, no. 7-8, pp. 815–823, 2008.
[12]
C. S. Li and S. Y. Zhang, “Porous LiFePO4/NiP composite nanospheres as the cathode materials in rechargeable lithium ion batteries,” Journal of Nanoparticle Research, vol. 1, pp. 242–248, 2008.
[13]
K. Zaghib, A. Mauger, F. Gendron, and C. M. Julien, “Relationship between local structure and electrochemical performance of LiFePO4 in Li-ion batteries,” Ionics, vol. 14, no. 4, pp. 271–278, 2008.
[14]
C. Legrand, L. Dupont, K. Tang, H. Li, X. J. Huang, and E. Baudrin, “Structural and textural characterization of LiFePO4 thin films prepared by pulsed laser deposition on Si substrates,” Thin Solid Films, vol. 518, no. 19, pp. 5447–5451, 2010.
[15]
K. S. Park, J. T. Son, H. T. Chung et al., “Surface modification by silver coating for improving electrochemical properties of LiFePO4,” Solid State Communications, vol. 129, no. 5, pp. 311–314, 2004.
[16]
H. Liu and J. Xie, “Synthesis and characterization of LiFe0.9Mg0.1PO4/nano-carbon webs composite cathode,” Journal of Materials Processing Technology, vol. 209, no. 1, pp. 477–481, 2009.
[17]
F. Sauvage, J. M. Tarascon, and E. Baudrin, “Insights into the potentiometric response behaviour vs. Li+ of LiFePO4 thin films in aqueous medium,” Analytica Chimica Acta, vol. 622, no. 1-2, pp. 163–168, 2008.
[18]
D. G. Zhuang, X. B. Zhao, G. S. Cao, C. H. Mi, J. Tu, and J. P. Tu, “Morphology and reaction mechanism of LiFePO4 prepared by hydrothermal synthesis,” Chinese Journal of Nonferrous Metals, vol. 15, no. 12, pp. 2034–2039, 2005.
[19]
H. Nishihara, M. Haruna, and T. Suhara, Integrated Optics, Science Press, Beijing, China, 1st edition, 2004.
[20]
S. Kondo, T. Ishikawa, and I. Abe, Adsorb Science, Chemical Industry Press, Beijing, China, 1st edition, 2006.
[21]
Z. Gui Lv and L. Xue Jin, “Optical waveguide gas sensor,” Chinese Journal of Instrument Technique and Sensor, vol. 5, pp. 17–19, 1990.
