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Fabrication of an Integrated Zeolite-fiber Optical Chemical Sensor for Detection of Mercury (II) Ion in Water [Sensors & Transducers (Canada)](Sensors & Transducers (Canada) Via Acquire Media NewsEdge) Abstract: This paper describes a process for fabricating an integrated zeolite-captured silver fiber optical chemical sensor. b-oriented zeolite thin film was first coated onto the end surface of an optical fiber. Then Ag+ ions were loaded onto the zeolite crystal via ion exchange. The exchanged Ag+ ions were reduced in the zeolite channel by formaldehyde. The resulting Ag-embedded zeolite-fiber optical sensor was characterized with a number of techniques including scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. By monitoring the reflection intensity change, this sensor can be used for in-situ Hg2+ detection. Copyright © 2011 IFSA. Keywords: Zeolite, Ion exchange, Optical chemical sensor, Mercury. 1. Introduction Chemical sensors for real-time online monitoring have proven useful in many areas such as emission control, environmental pollutant monitoring, and industral control processes. In particular, optical-fiber based sensors have attracted more interest because of their outstanding advantages such as small size, immunity to electromagnetic interference, passivity and intrinsic safety, remote operation, and robustness under hostile environments [1-3]. These advantages have promoted worldwide research activity on optical fiber sensors for many applications [4-6]. Optical fiber chemical sensors have been developed based on various mechanisms including spectroscopy [7, 8], evanescent wave absorption [9], fluorescence [10] and surface plasmon resonance [H]. Despite the great effort that has been made in the development of optical chemical sensors, in-situ and real-time monitoring of the vast number of chemicals used today, especially under harsh conditions, still remains a great challenge. Coating/immobilizing different functional materials onto the fiber surface to fabricate an integrated optical-fiber chemical sensor is a promising strategy for the development of different types of fiber optic sensor. Arregui reported an ionic self-assembly monolayer process for depositing ultrathin films on the end surface of the optical fiber [12]. A nanometer-scale Fabry-Perot interferometer sensor was developed with an easily controlled coating film thickness. Kim et al. studied immobilization of immunoglobulin G (antigen) onto a long period fibre grating (LPFG) surface [13]. By monitoring the reflection spectra, they found that the sensor's fringe position shifted as the coated sensor tip was immersed in the antibody solution. They demonstrated that this sensor could be used for various bio/chemical applications. More recently, Watanabe et al. investigated the deposition of tungsten trioxide (WO3) and platinum (Pt) thin films onto the core of an optical fiber with an electron beam [14]. The WO3 layer can easily react with hydrogen atoms and the Pt layer serves as a catalyst to stimulate the reaction. By measuring the evanescent wave absorption, the sensor thus fabricated can detect hydrogen in few seconds and also can recovery quickly in air. Our earlier research demonstrated an integrated zeolite-fiber optic sensor, which can detect different dissolved organics in water [15]. The sensor comprises a dense silicalite thin film grown on the straight-cut endface of a standard telecommunication optical fiber. Silicalite is an all-silica MFI zeolite with effective pore size of 5.5 Â. It is highly hydrophobic, adsorbing organics with appropriate molecular sizes. The sensor operated by measuring the optical reflectivity of the coated zeolite film, which changed reversibly in response to the adsorption and desorption of organic molecules into its crystalline structure. The sensor exhibited a rapid response time and the capacity for quantitatively measuring dissolved organics in water. In this paper, we focus on the preparation of a fiber optic sensor for Hg2+ ion detection, ò-oriented Na-ZSM-5 zeolite thin film is first coated onto the endface of an optical fiber. Then Ag+ ions are loaded into the zeolite frame by ion exchange. The exchanged Ag+ ions are reduced to silve in the zeolite channel and an Ag-embedded zeolite-coated fiber optic sensor is fabricated. The sensor is then tested for Hg2+ detection. 2. Experimental 2.1. Synthesis of a b-oriented Zeolite-coated Fiber Sensor In order to prepare the zeolite-coated fiber optic sensor, a single mode optical fiber was cleaved at a right angle to its axis and cleaned with isopropanol in an ultrasonic bath. The synthesis solution was prepared by mixing 40 ml deionized (DI) water, 5.0 ml TPAOH solution (tetrapropyl ammonium hydroxide, IM, Aldrich), 4.0 ml TEOS (tetraethyl orthosilicate, 98 %, Acros), and 0.029 g sodium aluminate (> 99 %, Sigma). The mixture was stirred vigorously at 50 0C for 10 hr to obtain a completely clear solution. Finally, 5.0 ml of the final synthesis solution was transferred into the synthesis reactor. Hydrothermal synthesis was used to fabricate the zeolite-coated optical fiber. The experiment was performed in a lab-designed autoclave as shown in Fig. 1 . The Teflon-lined stainless steel reactor had a chamber volume of -10 cm3 and an inner diameter of 18 mm. The cleaved fiber endface, facing downwards, was placed 12-15 mm beneath the liquid surface. The synthesis reactor was then moved into an oven preheated to 180 0C. In-situ hydrothermal synthesis was conducted at 180 0C for three hours. After synthesis, the zeolite-coated fiber end was rinsed with DI water and further cleaned in an ultrasonic bath for 5 min. After drying at 80 0C for 10 hr, the zeolite film was activated by firing at 500 0C in air for 3 hr with a heating and cooling rate of 1 .0 °C/min. Ag+ ions were loaded into the zeolite-coated thin film by ion-exchanging with 0.1M AgN03. After the zeolite-coated thin film was activated, the fiber tip was equilibrated with 10.0 ml 0.1 M AgN03 in the dark at room temperature for 24 hr. Then the fiber end was ultrasonically washed with deionized (DI) water three times to remove the surface-adsorbed Ag+ ions. Reduction of Ag+zeolite was carried out by immersing the zeolite-coated fiber end into a mixture of NaOH and formaldehyde. After the Ag+ reduction, the fiber end was extensively washed with DI water to remove any unreacted formaldehyde and dried at 70 0C. 2.2. Experimental Setup The zeolite-coated optical fiber was tested as a chemical sensor using the system shown schematically in Fig. 2. The sensor operation is based on the measurement of the reflectivity changes. A laser beam was launched from the laser source and split into two paths through a 3 dB fiber coupler. One path was spliced to the zeolite-coated sensor head as shown in Fig. 2. The other path was directly connected to compensate for the power variation during the experiment. The reflection intensity from the zeolite-fiber interface was measured by the optical power sensor shown in Fig. 2. All of the reported experiments were conducted at room temperature. The sensor output was recorded by a computer at a time interval of 0. 1 s. 3. Results and Discussion 3.1. b-oriented Zeolite Thin Film Coated onto the Endface of an Optical Fiber Fig. 3 shows the scanning electron microscopic (SEM) images of the optical fiber endfaces with and without a zeolite film. A single-mode optical fiber with 125 µp?-dia. cladding and 9 µ??-dia. core was cleaved in a right angle to its axis and cleaned with isopropanol (Fig. 3a). After zeolite crystal deposition, the whole end surface was completely covered by the zeolite thin film. Fig. 3b displays the SEM image of optical fiber endface coated with zeolite thin film, in which almost all the crystals are ò-oriented with the crystal size of 3.1 µp? in width and 4.6µ?? in length. The formed boriented zeolite film can decrease unnecessary reflection and improve the reflection signal [15]. The zeolite coating can readily form a Fabry-Perot interferometer in which the reflection from the fiberzeolite and zeolite-ambient interfaces produce an interference signal. The intensity of the reflection is known to be related to the refactive index of optical fiber, zeolite thim film, and the medium [16]. 3.2. Ag+ Exchanged into Zeolite Crystal The b-oriented zeolite thin film synthesized contains a three-dimensional, interconnected fivemember ring linked together through oxygen bridges. The Al3+ doped in the synthesis could replace the Si4+ in the zeolite framework. To maintain an overall neutral charge, an additional positive charge is required in the zeolite crystal to compensate for the deficient positive charge due to the substitution. The positive charge could be proton or sodium ions, which could then be substituted by other ions such as Ag+ through ion exchange. Fig. 4 shows the reflection intensity change during Ag+ diffusion into the zeolite channel. The test was performed at room temperature with the experimental apparatus. During the test, the calcined, zeolite-coated optical fiber tip was first immersed in 20 ml DI water and then moved into 20 ml 0.1M AgN03. Fig. 4 shows that the reflection intensity was stable at -54.4 dBm in DI water. After placing the sensor tip in 0.1 M AgN03, Ag+ ions began to diffuse in the crystal channel and exchanged the existing Na+ ions, resulting in an increase in reflection intensity from -54.4 dBm to -53.4 dBm. The increase in reflection intensity can be explained by the refractive index change of the zeolite-coated thin film. As the Ag+ ions exchanged the existing Na+ ions, the composition of the zeolite thin film was changed, leading to a change in the refractive index of the zeolite. Since the refractive indexes of the optical fiber and AgN03 solution were supposed to remain constant during the test, the refractive index change of the zeolite thin film due to Ag+ ions replacing the existing Na+ resulted in the reflection intensity increase. We also performed the energy-dispersive X-ray spectroscopy (EDS) test with the same synthesized zeolite crystals and the same process to confirm that Ag+ exchanged in the crystal channel. Fig. 5 shows the EDS result of Ag+-exchanged ZSM-5 crystals. It clearly indicated that silver was successfully loaded in the crystals. After around 18 hr, the reflection intensity leveled at -53.4 dBm, which indicated that Ag+ diffusion reached equilibrium. The reference intensity for monitoring the laser power was -14.5dBm during the whole process, indicating the power source is stable and confirming that the reflection intensity change is from Ag+ diffusion in the zeolite channel. The results also revealed that the zeolite-coated fiber optic sensor had a very high sensitivity to the change in the zeolite refractive index, in which the zeolite crystal was used to enhance sensor sensitivity. Ag+ Ions Reduced in a Zeolite Channel After Ag+ ions were exchanged into the zeolite channel, the zeolite coated fiber tip was cleaned ultrasonically three times in DI water to remove the adsorbed ions on the surface. Then the sensor tip was immersed in 10 ml DI water for equilibrium. A drop of formaldehyde and sodium hydroxide mixture solution was then added. The concentrations of formaldehyde and sodium hydroxide were 3.0 mM and 1.2 mM, respectively. Fig. 6 shows the changes in reflection and reference intensities during the the process. The reference intensity in Fig. 6 was -14.5d Bm; no change was observed during the process. The reflection intensity was -53.8 dBm before the formaldehyde and sodium hydroxide mixture was added. After that, the reflection intensity remained at -53.8 dBm for around 30 minutes and then suddenly increased. The final reflection intensity leveled at -44.IdBm. As the zeolite-coated sensor tip was immersed in the formaldehyde solution, the small HCHO molecule could be adsorbed onto the crystal surface and easily penetrate into the crystal channel, where it reduced the exchanged Ag+ ions to metal Ag. The whole process resulted in the increase in reflection intensity in the zeolite film from -54.IdBm to -44.IdBm. To further explore the status change of the exchanged Ag+ ions and provide more information to confirm our conclusion, x-ray photoelectron spectroscopy (XPS) spectra of Ag+-ZSM-5 and Ag-ZSM-5 were performed to provide the local environment and oxidation state of silver in ZSM-5 crystals. It is known that the Ag+ 3d bonding energy peak appears at 368. 8eV, whereas that of metallic silver appears at 368. 2eV. The experimental peak for Ag+-ZSM-5 was observed at 369.3 eV (refer to Fig. 7). The slight difference could be explained by the Agn5+ cluster formed in the zeolite channel [17, 18]. A small peak observed at 368. 5eV was attributed to partial reduction of the silver ions by water in the zeolite channel.[18] After Ag+ reduction to metallic silver, one peak observed at 368.5eV was identified as the metallic silver 3d peak. The other peak, appearing at 374.5eV, is the silver 2p peak due to spin orbital coupling. The results of XPS spectra in Fig. 7 demonstrated that all the exchanged Ag+ ions were completely reduced by formaldehyde. 3.4. Monitoring Hg2+ in Solution It is known that Ag nanoparticles can selectively react with Hg ions in solutions with high sensitivity [19]. In our study, nano- or subnano- silver particles were homogeneously dispersed along the whole zeolite membrane. In particular, almost all the silver particles located inside the zeolite crystal channel. The small pore size of the zeolite crystal could hinder some large molecular into the channel and react with Ag particles. So our zeolite-captured Ag could imporve the sensor's selectivity. Fig. 8 shows the reflection intensity change in 20 ppm Hg2+ solution. The fiber sensor was first equilibrated in DI water and Hg2+ was added to the solution. The reflection intensity from the zeolite-coated fiber surface decreased from -37.9 dBm to -53.3 dBm after Hg2+ was added. As the sensor tip was immersed into a solution contained Hg2+ ions, Hg2+ ions could be immediately adsorbed onto the membrane surface and diffuse into the crystal channel. The crystal's refractive index could be changed when the Hg2+ ions react with the zeolte-captured Ag particles. Meanwhile, the reflection light intensity will also change as light is reflected on the crystal surface. After the Hg2+ ions reached equilibrium with the captured silver, the crystal's refractive index did not change any more and the reflection light intensity leveled at -53.3 dBm. Considering its small size and capacity for long-distance remote measurements, the sensor, by monitoring the refection intensity change, can be used for real-time Hg2+ ion monitoring. 4. Conclusion 6-oriented zeolite thin film was successfully synthesized onto the end surface of an optical fiber. Ag+ ions were loaded into the crystal by ion exchange. Exchanging Na+ with Ag+ in the crystal resulted in the zeolite crystal's refractive index change, which was demonstrated by monitoring the reflection light from the zeolite membrane surface change. The exchanged Ag+ ions were then reduced in the zeolite channel by formaldehyde. XPS results revealed almost all the exchanged Ag+ ions were reduced to silver and the reduction process was monitored by the refection light intensity change. The zeolite-captured Ag fiber optical sensor thus prepared can monitor Hg ions in-situ in solution. Acknowledgements The authors gratefully acknowledge support from the Water Innovation Fund of the State of New Mexico (CHE-0632071) and New Mexico Petroleum Recovery Research Center (PRRC) for this research. The authors would also like to thank Ms. Liz Bustamante for assistance in preparing the manuscript. References [1]. M. Misushio, K. Miyashita, M. Higo, Sensor properties and surface characterization of the metaldeposited SPR optical fiber sensros with Au, Ag, Cu, and Al, Sensors and Actuators A: Physical, Vol. 125, Issue 2, 2006, pp. 296-303. [2]. Y. Tseng, Y. Chuang, Y. Wu, C. Yang, M. Wang, F. Tseng, A gold-nanoparticle-enhanced immune sensor based on fiber optic interferometry, Nanotechnology, Vol. 19, 2008, p. 345501. [3]. G. Orellana, D. Haigh, New trends in fiber-optic chemical and biological sensors, Current Analytical Chemistry Vol. 4, 2008, pp. 273-295. [4]. B. Soller, Design of intravascular fiber optic blood gas sensors, IEEE engineering in medicine and biology, Vol. 13, Issue 3, 1994, pp. 327-335. [5]. D. Ulieru, The applications of fibre optic sensors at the waste water management from microelectronics fab, in Proceedings of the SPIE Conference on Fiber Optic Sensor Technology and Applications, Vol.3821, 1999, pp. 309-318. [6]. R. Schroeder, R. Ramos, T. Yamate, Fiber optic sensors for oilfield services, in Proceedings of the SPIE Conference on Fiber Optic Sensor Technology and Applications, Vol. 3860, 1999, pp. 12-22. [7]. R. Wolthuis, D. McCrae, E. Saaski, J. Hartl, G. Mitchell, Development of a medical fiberoptic pH sensor based on optical absorption, IEEE Transactions on Biomedical Engineering Vol. 39, Issue 5, 1992, pp. 531-537. [8]. R. A. Wolthuis, D. McCrae, J. C. Hartl, E. Saaski, G. L. Mitchell, K. Garcin, R. Willard, Development of a medical fiber-optic oxygen sensor based on optical absorption change, IEEE Transactions on Biomedical Engineering Vol. 39, Issue 2, 1992, pp. 185-193. [9]. R. A. Potyrailo, S. E. Hobbs, G. M. Hieftje, Near-Ultraviolet Evanescent- Wave AbsorptionSensor Based on a Multimode Optical Fiber, Anal. Chem., Vol. 70, 1998, pp. 1639-1645. [10]J. H. Watterson, P. A. E. Piunno, C. C. Wust, U. J. Krull, Controlling the density of nucleic acid oligomers on fiber optic sensors for enhancement of selectivity and sensitivity, Sensors and Actuators, B: Chemical, Vol. 74, Issue 1-3, 2001, pp. 27-36. [11]. O. Esteban, A. Gonzalez-Cano, N. Diaz-Herrera, M. Navarrete, Absorption as a selective mechanism in surface plasmon resonance fiber optic sensors, Optics Letters, Vol. 31, Issue 21, 2006, pp. 3089-3091. [12]. F. J. Arregui, I. R. Matias, Y. Liu, K. M. Lenahan, R. O. Claus, Optical fiber nanometer-scale FabryPerot interferometer formed by the ionic self-assembly monolayer process, Optics Letters, Vol. 24, Issue 9, 1999, pp. 596-598. [13].D. W. Kim, Y. Zhang, K. L. Cooper, A. Wang, Fibre-optic interferometric immuno-sensor using long period grating, Electronics Letters, Vol. 42, Issue 6, 2006, pp. 324-325. [14]. T. Watanabe, S. Okazaki, H. Nakagawa, K. Murata, K. Fukuda, A fiber-optic hydrogen gas sensor with low propagation loss, Sensors and Actuators B; Chemical, Vol. 145, Issue 2, 2010, pp. 781-787. [15]. N. Liu, J. Hui, C. Sun, J. Dong, L. Zhang, H. Xiao, Nanoporous zeolite thin film-based fiber intrinsic Fabry-Perot interferometric sensor for detection of dissolved organics in water, Sensors, Vol. 6, Issue 8, 2006, pp. 835-847. [16]. Hecht, E., Optics, 4th ed., Addison Wesley, 2002. [17]. P. Sazama, H. Jirglova', J. Devdecvek, Ag-ZSM-5 zeolite as high-temperature water-vapor sensor material, Mater. Lett., Vol. 62, Issue 27, 2008, pp. 4239-4241. [18]. G. Ozin, F. Hugues, S. Mattar, D. Mcintosh, Low nuclearity silver clusters in Faujasite-type zeolitesoptical sepectroscopy, photochemistry, and relationship to the photo-dimerization of alkanes, J. Phys. Chem., Vol. 87, Issue 18, 1983, pp. 3445-3450. [19]. Y. Fan, Z. Liu, L. Wang, J. Zhan, Synthesis of starch-stabilized Ag nanoparticles and Hg2+ recognition in aqueous media, Nanos cale Res. Lett., Vol. 4, Issue 10, 2009, pp. 1230-1235. *Ning Liu, Liangxiong Li, Robert Lee Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Tel: 01-575-835-5739, fax: 01-575-835-6031 * E-mail: [email protected] Received: 16 June 2011 /Accepted: 22 August 2011 /Published: 30 August 2011 (c) 2011 International Frequency Sensor Association |