© Copyright 1998 American Chemical Society
MARCH 31, 1998 VOLUME 14, NUMBER 7
Letters Growth of Ultrathin Covalently Attached Polymer Films: Uniform Thin Films for Chemical Microsensors Xiaoguang Yang,* Jingxuan Shi, Sabina Johnson, and Basil Swanson Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received January 5, 1998 A novel approach to covalently attached polymer films by surface functionalization and hydrosilylation reaction with reactive polymers on the surface is reported. Stable, uniform, and highly sensitive thin film sensors can thus be fabricated. The covalent attachment of polymer through a silane linker results in films with high uniformity, stability, and reasonable reproducibility as far as film thickness is concerned. Incorporation of molecular recognition reagents into the sensing film increases the sensor sensitivity dramatically, which is critical for detecting vapors such as certain explosives at parts per thousand or parts per billion levels. The sensors fabricated can detect o-nitrotoluene, a TNT simulant, at parts per billion concentrations.
In addition to sensitive and selective polymer coatings, fabrication of organic polymer films on solid supports with defined structures and properties such as uniformity, stability, and reproducibility is crucial to the development of thin-film chemical microsensors.1 Since the synthesis of interfaces between organic polymers and solid substrates is of importance in various industriesse.g., microelectronics, aerospace, automotive, food packaging, and building materials, and in some biomedical and biotechnological applications such as biocompatability of artificial organs, drug delivery, and chromatographys significant research has been focused on polymer films and progress has been made in the preparation of polymer thin films on surfaces over the past decade. Advanced techniques such as polyelectrolyte deposition,2 diblock copolymer adsorption,3 plasma deposition,4 in-situ polymerization,5 and polymerization within a LangmuirBlodgett (LB) film have been developed.6 However, it is (1) Swalen, J. D.; Allara, D. L. Andrade, J. D.; Chandross, E. A.; Garoff, S., Israelachvili, J.; Mccarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 930. (2) Decher, G. Science 1997, 277, 1232. (3) Milner, S. T. Science 1991, 251, 905. (4) Bonnar, M. P.; Burnside, B. M.; Little, A.; Reuben, R. L.; Wilson, J. I. B. Chem. Vap. Deposition 1997, 3, 201.
Scheme 1
still difficult to fabricate ultrathin polymer films that are uniform, continuously defect-free, and stable.7 Moreover, controlled growth of stable polymer films at the nanoscale level, which is important in many applications, remains a challenge. Currently, polymer thin film microsensors are mostly fabricated with techniques such as spray coating, solution casting, brushing, and dip-coating, which are not desirable for fabricating nanometer thick thin polymer films and have minimum control of film properties such as film thickness, uniformity, stability, and thus (5) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25. (6) Aoki, A.; Miyashita, T. Adv. Mater. 1997, 9, 361. (7) (a) Stange, T. G.; Mathew, R.; Evans, D. F. Langmuir 1992, 8, 920. (b) Stange, T. G.; Evans, D. F. Langmuir 1997, 13, 4459.
S0743-7463(98)00001-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/27/1998
1506 Langmuir, Vol. 14, No. 7, 1998
Letters Scheme 2
reproducibility.8 In this report, we describe a molecular self-assembly and reactive polymer deposition approach to covalently attached polymer films and its application for fabricating thin-film microsensors. The general scheme consists of surface functionalization and then surface reaction with reactive polymers to form covalent linkages between the surface and the polymer (Scheme 1). A similar approach has been reported for covalent grafting of polymers with isocyanate pendent group onto an amine-functionalized surface.9 There are also reported industrial procedures to enhance polymersolid interfacial properties by using adhesion promoters, coupling reagents, functionalized polymers, and other surface treatment reagents, exploiting surface reactions to promote polymer cross-linking and covalent bond formation between polymers and the solid surfaces.10 In our polymer film growth approach, we employed a well-known catalytic hydrosilylation reaction to covalently attach poly(methylhydrosiloxane) (PMHS) polymer or its derivatives onto the solid surface.11 First, using welldeveloped silane surface chemistry,12 a self-assembled (8) Grate, J. W.; McGill, R. A. Anal. Chem. 1995, 67, 4015. (9) Beyer, D.; Bohanon, T. M.; Knoll, W.; Ringsdorf, H.; Elender, D.; Sackmann, E. Langmuir 1996, 12, 2514. (10) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1991. (11) The addition of a Si-H bond to a CdC group has been explored intensively as a route to alkylsilanes, and the reaction is used in the manufacture of silicone polymers. It is known that there is a strong steric effect that the silicon almost always attaches to the less-crowded end to CdC bond. The fact that no byproduct is formed makes this type of reaction particularly advantageous for the film growth. (12) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991; pp 245-256.
Figure 1. Transmission FT-IR spectra of (a) the linker monolayer formed from 5-hexenyltrichlorosilane, (b) the film from 1 and the linker monolayer, (c) the film from the reaction of PMHS film with 2, and (d) the film from 3 and the linker monolayer on wedged silcon substrates.
monolayer with vinyl termini is formed on the oxide surface (SiO2). The monolayer from 5-hexenyltrichlorsilane has a film thickness of 10-11 Å as measured by ellipsometry, which is consistent with monolayer thickness as reported in the literature,13 indicating that the solid surface is well covered. Fourier transform infrared (FT-IR) spectrometry (Figure 1a) indicates that the monolayer is liquid-like with CH2 stretching at 2925 and 2856 cm-1, respectively.14 The reaction of the polymer (1 or 3)15 with the vinyl-terminated (13) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
Letters
self-assembled monolayer (SAM) in dry toluene was carried out under an inert atmosphere in the presence of a catalytic amount of chloroplatinic acid at 100 °C (Scheme 2). The formed polymer films were washed extensively with organic solvents and sonicated in chloroform. The film thickness is in the range of 100-110 Å, as measured with ellipsometry. We also used the mass loading on surface acoustic wave (SAW) device to estimate the film thickness, and the results are consistent with those obtained with ellipsometry measurements. Figure 1 shows IR spectra of the films made sequentially according to Scheme 2.16 The catalytic hydrosilylation of the vinyl termini introduces covalent bond formation of Si-C between the siloxane polymers and the SAM. In the case of PMHS polymer film, there are unreacted Si-H bonds present after the first layer formation, as shown in Figure 1b. After the reaction of PMHS film with 2, the Si-H vibration (2167 cm-1) disappears, the peak intensities at 2925 and 2856 cm-1 (CH2 stretching) increase due to the derivatized cyclodextrin tails, and a new peak at 2879 cm-1 due to the cyclodextrin glucose C-H vibrations appears (Figure 1c). The resulting film from PMHS/ cyclodextrin (3) has an almost identical IR spectrum (Figure 1d) as those grown stepwise from 1 and 2 (Figure 1c). Due to the strong absorption of siloxane at 11301000 cm-1, characteristic C-O vibrations of cyclodextrins could not be distinguished. The film topology was studied with atomic force microscopy and the film is found to be relatively smooth, uniform, and defect-free. The relative roughness of the film is about 10% of the film thickness, and no pinhole or island formation was observed. Thin film SAW sensors fabricated from the above polymer film growth approach were studied for sensing organic vapors. It is known that the flexible siloxane skeleton allows greater orientational freedom to pendent groups which can be exposed toward the interfacing environment to their best effect.17 Incorporation of molecular hosts such as cyclodextrins, calixarenes, and cavitands into the thin films is known to increase sensitivity and selectivity.18-21 SAW devices (14) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (15) Poly(methylhydrosiloxane) (1) (Mw ) 1700-3000) was purchased from Aldrich and used without further purification. 2 and 3 were prepared according to the literature: Schurig, V.; Schmaizing, D.; Muhleck, U.; Jung, M.; Schleimer, M.; Mussche, P.; Duvekot, C.; Buyten, J. C. J. High Resolut. Chromatogr. 1990, 13, 713. (16) Films were fabricated on a wedged silicon wafer (from Harrick) for infrared transmission measurement. (17) Morra, M.; Occhiello, E.; Garbassi, F.; Marola, R.; Humphrey, P. J. Colloid Interface Sci. 1990, 137, 11. (18) (a) Moore, L. W.; Springer, K. N.; Shi, J. X.; Yang, X. G.; Swanson, B. I.; Li, D. Q. Adv. Mater. 1995, 7, 729. (19) Yang, X.; Shi, J.; Johnson, S.; Swanson, B. Sens. Actuators, B 1997, 45, 79. (20) Yang, X.; Johnson, S.; Shi, J.; Swanson, B. Sens. Actuators, B 1997, 45, 87.
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Figure 2. Real time SAW response (baseline-corrected) toward o-nitrotoluene vapor over (a) PMHS (1) coated 250 MHz SAW device and (b) 3 coated 250 MHz SAW device at o-nitrotoluene concentrations of 6 ppm (I) and 600 ppb (II).
coated with 3 were studied for sensing volatile organic and semivolatile organic species. Figure 2 shows a typical time course of frequency changes of the SAW device responding to o-nitrotoluene at various concentrations. The device shows high sensitivity toward o-nitrotoluene, which is used as a TNT surrogate. In comparison, a device coated with PMHS (1) did not respond to o-nitrotoluene at a concentration of 600 ppb (Figure 2a). The device coated with 3 can detect o-nitrotoluene at 600 ppt with a frequency shift of 10 Hz, which demonstrates the importance of cyclodextrin molecules and molecular recognition22 in sensor development. In addition, the sensors coated with covalently attached polymers are stable and showed no signal decay after 6 months. In summary, a novel approach to covalently attached polymer films has been developed. Stable, uniform, and highly sensitive thin film sensors can be fabricated. The covalent attachment of polymer through a silane linker results in films with high uniformity, stability, and reasonable reproducibility. Incorporation of molecular recognition reagents into the sensing film increases the sensor sensitivity dramatically, which is critical for detecting explosive vapors at parts per thousand or parts per billion levels. Acknowledgment. This work was supported by the DARPA program and LDRD funding. We also thank Atul Parikh and Ariane Eberhardt for helping obtain IR and atomic force microscopy data. LA980001T (21) Schierbaum, K. D.; Weiss, T.; Thoden, E. U.; Velzen, T.; Engbersen, J. F. J.; Reinhoudt, D. N.; Go¨pel, W. Science 1994 265, 1413. (22) Lipkowitz, K. B.; Pearl, G.; Coner, B.; Peterson, M. A. J. Am. Chem. Soc. 1997, 119, 600.
