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Zeolite-Fiber Integrated Optical Chemical Sensors for Detection of Dissolved Organics in Water Jian Zhang and Junhang Dong Department of Petroleum and Chemical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801
Ming Luo Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801
Hai Xiao* Department of Electrical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801
Sohail Murad Department of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60607
Randy A. Normann Sandia National Laboratory, Geothermal Research Division, Albuquerque, New Mexico 87185-1033 Received June 6, 2005. In Final Form: August 2, 2005 MFI zeolite coated optical fiber sensors have been developed for in situ detection of dissolved organics in water. The sensors operate by monitoring the optical reflectivity changes caused by the selective adsorption of organic molecules, i.e., 2-propanol or pentanoic acid in this study, from aqueous solutions in the zeolitic pores. Reversible and monotonic sensor signals were observed in response to the variation of 2-propanol concentration in water with fast response. However, the sensor exhibited a much slower response to pentanoic acid than to 2-propanol. It was also found that substitution of Si by Al in the MFI framework increased the adsorption of pentanoic acid that resulted in enhanced sensor responses.
Information on trace level dissolved organics in water with high temporal and spatial resolutions is highly demanded for effective environmental management. However, most of the existing approaches for trace detection of dissolved organics involve use of sophisticated instruments that require costly and time-consuming sample collection/preparation and laboratory analyses. Miniaturized and low-cost chemical sensors suitable for fast and in situ measurement have received tremendous interest in recent years. Electrochemical1 and optical sensors are the two major classes of chemical sensors with potential for in situ detection of various organics. An optical sensor detects changes in optical properties, such as light intensity, absorption/fluorescent spectrum, refractive index, and birefringence, caused by analyte molecules.2-4 Compared to electrochemical sensors, optical sensors * Corresponding author. Phone: (505) 835-5199. Fax: (505) 8355332. E-mail:
[email protected]. (1) Hanrahan, G.; Patil, D. G.; Wang, J. Electrochemical sensors for environmental monitoring: design, development and applications. J. Environ. Monit. 2004, 6, 857-664. (2) Wolfbeis, Fiber-optic chemical sensors and biosensors. Anal. Chem. 2004, 76, 3269-3284. (3) Brolo, A. G.; Gordon, R.; Leathem, B.; Kavanagh, K. L. Surface Plasmon sensor based on the enhanced light transmission through arrys of nanoholes in gold films. Langmuir 2004, 18, 4813-4815. (4) Gao, J.; Gao, T.; Li, Y.; Sailor, M. J. Vapor sensors based on optical interferometry from oxidized microporous silicon films. Langmuir 2002, 18, 2229-2233.
generally offer the advantages of immunity to electromagnetic interference (EMI), large bandwidth, passiveness, and intrinsic safety. Zeolites are microporous aluminosilicate crystals with uniform subnanometer- or nanometer-scale pore systems capable of discriminating molecules by size exclusion or shape selectivity. The nanoporous zeolites can also selectively adsorb molecules depending primarily on the crystal structure, framework Si/Al ratio, and type of extraframework cations. In the last few decades, zeolite membranes and thin films have been synthesized and investigated extensively as a new class of materials for chemical separations5-7 and selectivity enhancement materials for electrochemical sensors.8-10 (5) Bowen, T. C.; Noble, R. D.; Falconer, J. L. Fundamentals and Applications of Pervaporation through Zeolite Membranes. J. Membr. Sci. 2004, 245, 1-33. (6) Caro, J.; Noack, M.; Kolsch, P.; Schafer, R, Zeolite membraness state of their development and perspective. Micropor. Mesopor. Mater. 2000, 38, 3-24. (7) Li, L.; Dong, J.; Nenoff, T. M.; Lee, R. Desalination by Reverse Osmosis Using MFI Zeolite Membranes. J. Membr. Sci. 2004, 243, 401404. (8) Hugon, O.; Sauvan, M.; Benech, P.; Pijolat, C.; Lefebvre, F. Gas separation with a zeolite filter, application to the selectivity enhancement of chemical sensors. Sens. Actuator B-Chem. 2000, 67, 235243. (9) Li, S.; Wang, X.; Beving, D.; Chen, Z.; Yan, Y. Molecular sieving in a nanoporous b-oriented pure-silica-zeolite MFI monocrystal film. J. Am. Chem. Soc. 2004, 126, 4122-4123.
10.1021/la0514967 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/13/2005
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It has been revealed that some optical properties of zeolite crystals are dependent on the type, amount, and state of the molecular species loaded in the zeolitic channels.11 For example, the refractive index of zeolite crystals changes upon loading and unloading the guest molecules in the zeolitic cavities.12-14 The high surfaceto-mass ratio, chemical selectivity, and flexibility for structure and surface chemistry modification make zeolites very promising optical chemical sensor materials.15-17 Recently, we demonstrated a new type of zeolite-enabled optical fiber chemical sensors for detection of organics in gas phases.18 The sensor comprised 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 Å. It is highly hydrophobic and selectively adsorbs organics with appropriate molecular sizes. The sensor device operated through measuring the optical reflectivity of the coated zeolite film which changed reversibly in response to the adsorption and desorption of organic molecules in its crystalline structure. The sensor exhibited a fast response time and the suitability for quantitatively measuring organic vapors in gas phase. In this letter, we present the preliminary investigations on such MFI zeolite-coated chemical sensors for detection of dissolved organics in aqueous solutions and the effect of Si/Al ratio of the zeolite on the sensor behavior.
Letters
Figure 1. SEM image of MFI thin films grown on endface of an optical fiber (inset is the blank fiber tip).
Experimental Section The sensor was made by growing a thin film of MFI zeolite on the cleaved endface of a singlemode optical fiber with 125µm cladding and 9µm core (Corning SMF28). The detailed synthesis procedure was described in a previous paper.18 For synthesis of silicalite-coated sensors, named as “silicalite sensors” hereafter, the synthesis solution was prepared by mixing 30 mL of H2O, 5.65 mL (1M) of TPAOH (tetrapropylammonium hydroxide), and 10.2 mL of TEOS (tetraethyl orthosilicate). A controlled quantity of sodium aluminate was dissolved into the solution to synthesize MFI zeolite thin coatings with a Si/Al atomic ratio of 50:1 in the framework, referred to as ZSM-5 sensors hereafter. Elemental composition of the zeolite was confirmed by X-ray fluorescence (XRF) tests for the zeolite crystals collected from the residual synthesis solutions. Figure 1 shows the scanning electron microscopic (SEM) images of the optical fiber endfaces with and without a MFI zeolite film. The effective thickness of the (10) Walcarius, A. Zeolite-modified electrodes in electroanalytical chemistry. Anal. Chim. Acta 1999, 384, 1-16. (11) Hoffmann, K.; Marlow, F. Molecular Sieve-Based Materials for Photonic Applications. In Handbook of Zeolite Science and Technology; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003. (12) Striebel, Ch.; Hoffmann, K.; Marlow, F. The microcrystal prism method for refractive index measurements on zeolite-based nanocomposites. Micropor. Mater. 1997, 9, 43-50. (13) Marlow, F.; Hoffmann, K. Switching of optical properties in zeolite nanocomposites. J. Phys. Chem. 1997, 101, 1731-1734. (14) Bjorklund, R. B.; Hedlund, J.; Sterte, J.; Arwin, H. Vapor Adsorption in Thin Silicalite-1 Films Studied by Spectroscopic Ellipsometry. J. Phys. Chem. B 1998, 102, 2245-2250. (15) Meinershagen, J. L.; Bein, T. Optical sensing in nanopores: Encapsulation of the solvatochromic dye Nile red in zeolites. J. Am. Chem. Soc. 1999, 121, 448-449. (16) Meier, B.; Werner, T.; Klimant, I.; Wolfbeis, O. S. Novel oxygen sensor material based on a ruthenium bipyridyl complex encapsulated in zeolite Y: dramatic differences in the efficiency of luminescence quenching by oxygen on going from surface-adsorbed to zeoliteencapsulated fluorophores. Sens. Actuator B-Chem. 1995, 29, 240245. (17) Remillard, J. T.; Jones, J. R.; Poindexter, B. D.; Narula, C. K.; Weber, W. H. Demonstration of a high-temperature fiber-optic gas sensor made with a sol-gel process to incorporate a fluorescent indicator. Appl. Opt. 1999, 38, 5306-5309. (18) Xiao, H.; Zhang, J.; Dong, J.; Luo, M.; Lee, R.; Romero, V. Synthesis of MFI zeolite films on optical fibers for detection of chemical vapors. Opt. Lett. 2005, 30, 1270-1272.
Figure 2. Schematic illustration of the sensor principle and experimental setup. intergrown zeolite layer was estimated to be ∼4 µm in our previous work.18 Operating Principle and Experimental Setup. The zeolite-coated optical fibers were tested as chemical sensors using a system shown schematically in Figure 2. The sensor operation is based on the measurement of adsorption-induced reflectivity changes. The light from a stabilized laser diode (Agilent 81651A, 1539.5 nm, 1mW) was launched into a singlemode optical fiber and split into two paths through a 3dB fiber coupler. One path was angle-cleaved to eliminate the back reflection. The other path was spliced to the zeolite-coated sensor head as illustrated in the enlarged view. An optical power meter (Agilent 8163A) was connected to the exit port of the 3dB coupler to monitor the reflectivity change of the zeolite-coated fiber endface. The experimental setup for sensor test is also illustrated in Figure 2. The fiber sensor head was placed in the center of the dionized (DI) water contained in a flask. The concentration of organic analyte in water was varied by injecting an organic solution of known concentration under stirring. The flask was sealed to avoid evaporation of the solution during the experiment. All of the reported experiments were conducted at room temperature (21 °C) in a laboratory fume hood. The sensor output was recorded by a computer at a time interval of 0.1 s.
Results and Discussions The silicalite sensor was tested for detection of 2-propanol in water. Figure 3 shows the sensor output in response to the variation of 2-propanol concentration in DI water from 9.99 ppmv to 16915 ppmv. The reflected power from the sensor head changed from -34.666 dBm (DI water) to -38.673 dBm (16915 ppmv). The silicalite sensor responded to the variation of 2-propanol concentration in water reversibly and monotonically with a clear correlativity for quantitative measurement. The inset of Figure 3 shows the recorded temporal sensor signal
Letters
Figure 3. 2-Propanol concentration dependence of zeolite-fiber sensor. Inset: Temporal sensor signal corresponding to the progression of increasing 2-propanol concentration by 10 ppmv in DI water.
Figure 4. Relationship between the sensor output and PAc concentration.
corresponding to the progression of increasing 2-propanol concentration by 10 ppmv. The sensor output was stabilized in 10-12 s after the injection of 2-propanol. The good signal-to-noise ratio at 10 ppmv level suggests that the detection limits can be well below 10 ppmv. It is worth noting that the silicalite sensor had a much shorter response time (
