Langmuir 1993,9, 3341-3344
3341
Surface Acoustic Wave Thin-Film Chemical Microsensors
Based on Covalently Bound c60 Derivatives: A Molecular Self-Assembly Approach DeQuan Li’ and Basil I. Swanson Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 Received August 9, 1 9 9 9 This communication describes a novel approach for developing ultrathin-film chemical microsensor materials by anchoring a covalently bound, self-assembled Cw multilayer on the surface acoustic wave (SAW) resonator via a siloxane bond linkage. The multilayer structure has been characterized by optical absorption, surface acoustic wave mass transduction, and polarized variable-angle internal attenuated total-reflection infrared spectroscopy (PVAI-ATR-IR). On a 200-MHz SAW device, typical mass loading from saturated organicvapors at room temperature causes a rapid reversible frequencyshiftof approximately 10 kHz. The Cw-based microsensors are most selective to those organic vapors such as decahydronaphthalene, perchloroethylene, and toluene, which is consistent with the fact that thete are good organic solventa for buckyballs. Results from studies of Langmuir-Blodgettl and polymeric2thin-film approaches to chemical microsensors and their potential applications in environmental monitoring have been encouraging. Nonetheless, long term stability: poor adhesion to substrates, polymer swelling, and uniformity are significant problems in the use of these materials. Covalently bound, self-assembly or spontaneous organization into monolayer or multilayer thin films offers a new and exciting alternative approach for overcoming these problems because the formation of covalently bound self-assembled monolayers is a process of materials synthesis as well as molecular-level materials engineering. For instance, covalently bound self-assembly can be exposed to vigorous sonications for a prolonged period of time in various organic solvents without noticeable loss of surface coverage,whereas filmsresulting from noncovalent bonding techniques can be easily rinsed off with organic solvents or damaged by sonications. In recent reports, self-assembled monolayer (SAM) formation on surface acoustic wave (SAW) devices4and “molecular recognition” using electrochemistry and SAM methods are described.6 We discuss here the construction of covalently bound, selfassembled, Cm multilayer supermolecular lattices and the surface modification of a SAW resonator that employs the covalent bonding of Cm derivatives to its SiO, passivating layer. The latter concept is significant because it demonstrates the feasibility of using self-assembled e Abstract published in Advance ACS Abstracts, November 15, 1993. (1) (a) Wohltjen, H.; Barger, W. R.; Snow, A. W.; Jarvie, N. L. ZEEE Trans. Electron Deuices 1985, ED-32 (7), 1170-1174. (b) Aizawa, M.; Matsuzawa, M.; Shinohara, H. Thin Solid Films 1988,160,477-481. (c) Nagase, S.; Kataoka, M.; Naganawa, R.; Komatsu, R.; Odashima, K.; Umezawa, Y.Anal. Chem. 1990, 62, 1252-1259. (d) Sriyudthsak, M.; Yamagishi, H.; Moriizumi, T. Thin Solid Films 1988,160,463-469. (e) Robert,G. G.; Petty, M. C.; Baker, S.;Fowler, M. T.; Thomas, N. J. Thin Solid Films 1985,132,113-123. (2) (a) Dong, S.; Sun,Z.; Lu, Z. J. Chem. Soc., Chem. Commun. 1988, 993-996. (b) Blyler, L. L., Jr.; Lieberman, R. A.; Cohen, L. G.; Ferrara, J. A.;Macchesney, J. B. Polym. Eng. Sci. 1989,29 (17), 1215-1218. (3) (a) Sauer, Th.; Caseri, W.; Wegner, G.; Vogel, A.; Hoffmann, B. J. Phys. D Appl. Phys. 1990, 23, 79-84. (b) Moriizumi, T. Thin Solid Films 1988,160,413-429. (4) (a) Sun,L.; Thomas, R. C.; Crooks, R. M. J. Am. Chem. SOC.1991, 113, 8550-8552. (b) Thomas, R. C.; Sun,L.; Crooks, R. M. Langmuir 1991, 7, 620-622. (c) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992,64, 3191-3193. (5) (a) Rubinstein,I.;Steinberg,S.;Tor,Y.;Shanzer,A.;Sagiv, J.Nature 1988,332,426-429. (b) Rubinetain, I.;Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, 5.J.Am. Chem. SOC.1990,112,6135-6136. (c)Steinberg, S.; Rubinstein, I. Langmuir, 1992,8, 1183-1187.
multilayer thin films to fabricate chemical microsensors for toxic organic vapor detections. Buckminsterfullerene (Cm) is a unique molecule and its solid-state packing structure has unavoidable free space due to the large diameter of these spherical buckyballs. This void space is more pronounced when the Cm is functionalized into “hairy” buckyballs, and these lattice free volumes in the self-assembled thin films can be used to accommodate organic vapor molecules and hence detect organic toxins. The general synthetic strategy used in this paper was based on the established silicon molecular self-assembly technology of the hydrolysis of methoxy silanes by silanol moieties on the oxide surfaces to form siloxane linkagesas Buckminsterfullerene Cm was functionalized with a nucleophilic addition of (3-aminopropy1)trimethoxysilane (APTS) to the Cm double bonds yielding Cm[NHzCH2CH&H2Si(OCH3)31n, Le., Cm[APTSI, by dissolving Cm in APTS at room temperature for 2 days. After the reaction, the APTS was removed by high vacuum (5 X le7Torr), and the resulting product contained no measurable unreacted Cm (TLC, silica gel, CHCh). For the model compound CW[NH~CH~CH~CH~I,, as described by Wudl,7the average number of propylamines added was n = 6. Proton lH NMR suggests the formation of the adduct, Cm[APTS],, with 1H resonances a t 3.57 ppm (8, 9H, OCH3), 3.47-2.22 ppm (m, 3H, CH2, CHI, 1.58 ppm (septet, 2H, CHd, 1.224.89 ppm (m, H, NH), and 0.66 ppm (t, 2H, CH& Laser desorption mass spectra show that Cm was the most prominent high mass ion (mlz = 360) and this result indicates that the Cm structure is maintained in the functionalization reaction. The Cm adduct polymerizes in toluene solution to form high molecular weight oligomers, especially in the presence of water, and this polymerization rate is slowed down by about 3 orders of magnitude in dry toluene (stirred over P2O6, vacuum distilled). By utilization of this polymerization reaction, a multilayer of Cw[APTSI, was anchored (6) (a) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, ~~~
G. K. J. Am. Chem. SOC.1990,112,7389-7390. (b) Li, D.; Marks, T. J.; Zhang,C.; Yang, J.; Wang, G. K. SPZE Proceedings, Nonlinear Optical
Properties ofOrganicMaterioleZZZl990,1337,341-347.(c) Li,D.;Marks, T. J.; Zhang, C.; Wang, G. K. Synthetic Metals 1991,4143,3157-3162. (d) Chen, K.; Caldwell,B.; Mirkin, C. J.Am. Chem. SOC.1993,115,11931194. (7) (a) Wudl, F.; H h c h , A.; Khemani, K. C.; Suzuki, T.; Allemand, P. M.; Koch, A.; Eckert, H.; Srdanov, G.; Webb, H. M. In Fullerenes: Synthesis, Properties, and Chemistry; pp 161-175. (b) H m h , A.; Li, Q.; Wudl, F. Angew. Chem., Znt. Ed. Engt. 1991,30 (lo), 13W10.
0743-7463/93/2409-3341$04.00/00 1993 American Chemical Society
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3342 Langmuir, Vol. 9, No. 12, 1993
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Figure 1. Idealized schematic structure of a covalently bound, self-assembledCa multilayer thin film (top). Schematic diagram of a 200-MMz surface acoustic wave resonator (bottom).
on the surface of 200-MHz SAW resonators by covalently bonding to the SiO, oxide (Figure 1). The self-assembled C a multilayers were formed by immersing the corresponding substrates in a toluene solution of C6o[APTS], at room temperature for 1h. The substrates were then cleaned by four successive 2-min sonications in CHCl3 and dried under air flow for complete removal of organic solvent as monitored by SAW frequency shift and postverified by surface IR. We monitored the attachment of Cm(APTS), multilayers to various surfaces by observing the absorbance increase in the electronic as well as infrared spectra, the decrease in hydrophilicity as derived from water contact angle measurements, and the mass loading as indicated by the shift of the SAW resonating frequency. The self-assembled c60 multilayers were first analyzed by the polarized variable-angle internal total-reflection infrared (PVAI-ATR-IR)technique? SurfacePVAI-ATRIR spectra as illustrated in Figure 2a were collected from the internal reflection at 39" incident angle within a ZnSe hemisphere crystal. Identical results were obtained with a Ge hemisphere crystal at 45' incident angle. The covalently bound multilayers of Cm derivatives, (C~O[APTS],),n, show several major features in the c60Skeleton ring vibration region at 1651,1554,and 1450 cm-l as well as the APTS CH2 bands at 2936 cm-l (shoulder at 28422883 cm-l). The intensities of these PVAI-ATR-IR absorptions support the film thickness deduced from the SAW measurements (vide infra). The same IR vibration bands are observed in the polymeric c60 derivatives (8) For a discumion of the PVAI-ATR-IR technique, the reader is referred to: (a) Li, D.; Swanson, B. I.; Robinson,J. M.; Hoffbauer, M. A. J. Am. Chem. SOC.1993, 115, 69754980. (b) Li, D.;Swanson, B. I.; Robinrron, J. M.; Hoffbauer, M. A. SPIE Proceedings, Nonlinear Optics III, 1992,1626,426-30. (c) Li, D.; Moore, L.; Swanson, B. I. Manuscript in preparation.
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Figure 2. FTIR-ATR spectra of (a) the Ca[APTS], covalently bound to the native oxide surface of Si(100) with a ZnSe hemisphere crystal and p-polarized light incident at 39O, 1024 scans, 8 cm-l resolution; (b) polymer {Ca[APTS],,j,, KBr pellet; and (c)monomer Ca[APTS],on NaCl crystal disks. The baseline was corrected to zero absorbance and offset for display purposes.
prepared by hydrolysis of the silane monomer in a toluene solution containing trace amounts of water (Figure 2b). The C a monomer shows clear signatures as a silicon methoxy compound by its characteristic vibrations at 2839 cm-l (CH3, sharp), 1192 cm-l (Si-OCHs), and 1081 cm-l (Si-OCH3) as shown in Figure 2c. After polymerization, these characteristic bands are replaced by new siloxane vibrations with two broader overlapping bands at 1046 and 1104 c d , which are signatures of polymeric and branched Si-0-Si network^.^ A surface acoustic wave is launched by applying an rf potential to the source metal interdigital transducer; it traverses the surface of the piezoelectric ST-quartz and is converted back to an electrical signal at the pickup metal interdigital transducer.1° The SAW resonators consist of a pair of metalized interdigital transducers which are placed centrally on the planar ST-quartz device inside a resonant cavity defined by microfabricated reflectors (Figure 1, bottom). The resonance frequency and amplitude of this acoustic wave are sensitive functions of the chemical, physical, mechanical, and electrical properties of any contacting materials, such as chemical vapor adsorption (mass loading). The formation of Cm multilayer supermolecular lattices was accomplished by five (9) (a) Anderson, D.R. In Analysis of Silicones; Smith, A. L., Ed.; Wiley-Interscience: New York, 1974; Chapter 10. (b) Bellamy, L. J. In The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975; Chapter 20. (c) Smith, A. L. Spectrochim. Acta 1960, 16, 87.
(10) For ti general description of SAW, see (a) Campbell, C. Surface Acoustic Wave Devices and Their Signal Processing Applications; Academic Press, Inc.: Boston, MA, 1989. (b) Feldmann, M.; Henaff, J. Surface Acoustic Wave for Signal Processing; Artech House: Boston, MA, 1989.
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Langmuir, Vol. 9,No. 12, 1993 3343 30
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Figure 3. Volatile organic vapor response of a 200-MHz SAW resonator microsensor coated with covalently bound, self-assembled Cw derivatized multilayers exposed to a number of analyte streams: (a, left) halogenated; (b, center) aromatics and hydrocarbons; (c, right) oxygen-containing organics. The organic vapors were initiated (upward arrows) into and purged from (downward arrows) the SAW system at the times indicated by arrows, with a constant dry Nz flow rate of 0.12 L/min. The spectra were offset for display
purposes.
successive dips in a dry toluene solution of Cm[APTS]n followed by hydrolysis of methoxysilane with water. The successive frequency shifts after each immersion were -24.5, -30.6, -48.8, -27.7, and -100.9 kHz. These SAW frequency shifts are not necessary linear due to slow polymerization of the Cm derivatives in toluene solution. The hydrolysis of methoxysilane followingeach of the first three dips produced a corresponding increase of SAW resonance frequency (4.8,6.6, and 19.2 kHz, respectively) due to the loss of MeOH, which was consistent with the increase in hydrophilicity.11 The SAW resonating frequency can be also affected by an increase in the thin-film elastic modulus due to siloxane cross-links. The latter is likely a small effect because six flexible CH2 spacers between Cm should decouple the rigidity in the films. Indeed, siloxane polymers are known to be soft flexible materials with extremely low glass transition temperature Tg.The total mass loading of Cm[APTSlnresulting from the formation of covalently bound multilayers is given by12
A f = K F Am aA where K = -1.3 X lo4 s.cm2/g, Am = mass change, A = surface area, F = resonating frequency, and total frequency shift Af = 232.5 kHz. The surface coverage calculated from eq 1is 4.47 X 10-‘jg/cm2. The estimated film thickness is approximately 200-300 A when a graphite density13of 1.48-2.23 g/cm3 is used for the Cm based thin film. Dry nitrogen saturated with the organic vapor of interest at room temperature was passed over the SAW resonator at a flow rate of 0.12 L/min and the vapor partial pressures (11) The hydrophilicity (wetting property)wasdetermined by dropping 25 mg of DI water on the newly prepared surfaces and measure the water bead diameter, d before hydrolysis d = 5.5 mm; after hydrolysis d = 8.0 mm. (12) (a) Grate, J. W.; Kluaty, M. Anal. Chem. 1951,63 (17), 1719-27. (b) Grate, J. W.; Snow, A,; Ballantine, D. S., Jr.; Wohltjen, H.; Abraham, M. H.; McGill, R.A.; Sawon, P. Awl. Chem. 1988,60 (17), 869-75. (13) For the density of the carbon element, see footnote loa. For the size of Ceo, see footnote lob. (a) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergmon Press: Oxford, 1989. (b) Chandrasekaran, S. Indian J. Chem. 1992,31 A&B, F36F41.
were calibrated gravimetrically using a LN2 trap. Upon exposure to various organic vapors, the &-coated SAW resonator responded quickly ( seconds) and reversibly (Figure 3) to the analytes. The measured vapor partial pressures of 0.230,0.0939,0.148,0.126, and 0.0241 atm for CHC13, CsHs, CHsOH, CCk, and CsH&Hs, respectively, were in agreement with values reported in the 1iterat~re.l~ The shape of the SAW frequency can be basically divided into three different categorieu-those with a steady “plateau” or those “plateaus” with a continuous increase or decrease (Figure 3). The steady response indicates that the organic vapor is at an equilibrium with organic molecules in the solid-state thin film and therefore the net mass exchange is zero. The gradual increase “plateau” of the SAW resonating frequency suggests that the organic vapor has a greater affinity to the Cm sensing layer and mass loading continues while the film is exposed to the organic species. The gradual decease “plateau”of the SAW frequency implies the exact opposite and the initial high frequency is caused by surface wetting of the solid-state thin film with the organic vapor. A clean SAW resonator without covalently bound CSOcoating responded to the same organic vapors with a frequency shift of 0.9-1.6 kHz except decahydronaphthalene, DMF, and DMSO which caused a rather small frequency change of 0.4,0.2, and 0.1 kHz, respectively. Sensor selectivity depends on optimum chemical or physical interactions between the analyte and the sensing layer such as mutual matching of polarity, size, and structural properties. The present Cm-coated SAW sensors show greater affinity to decahydronaphthalene, perchloroethylene, and toluene as illustrated in Figure 4 although the free volume in these thin films can be considered as irregular. This result is expected because these organics are good solvents for Cm;16the Cm sensing layer can be solvated by these organic vapors, thus yielding N
(14) For typical organic solvents such as CHCh, C& CHaOH, CCL, and CsH6CH3 at room temperature, the literature vapor pressures are 0.226, 0.0845, 0.142, 0.130, and 0.0323 atm, respectively. Also see (a) Dean, J. A. Lange’sHandbook of Chemistry, 13thed.;McGraw-HillBook Co.: New York, 1985. (b) Lide, David R. Handbook of Chemistry and Physics, 72nd ed.; CRC Press: Boston, MA, 1991.
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3344 Langmuir, Vol. 9, No. 12, 1993 600
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of the CSOthin-film-coated SAW sensor to toluene which is another targeted organic toxin is also approximately an order of magnitude higher than other organic vapors. Maximum SAW drift under our laboratory conditions is 500 Hz due to extreme temperture difference between day and night. The baseline noise12on our experimental time scale, 1h, is only 1-2 Hz when taking into account the drift due to temperature. Based on this baseline noise, and a typical SAW frequency response of -10 kHz, the estimated detection limit for current microsensors is at least 3 orders of magnitude smaller than saturated vapor pressure at room temperature.16 The 6-15 kHz frequency shift to most organic vapors (shown in Figure 3), which corresponds to 1-2 monolayers of material, suggests that the organic vapors only interact with the extreme outer layers on the surface of the covalently bound {Cm[APTSI,Jm multilayers. To summarize,we have demonstrated the formation of covalently bound, self-assembled, multilayers of Cm derivatives on the surfaces of Si wafers, quartz, and SiO, of SAW resonators. Surface acoustic wave microsensors provide a selective real-time analysis, a rapid response (-seconds), and a reversible sensing for volatile organics. We are currently exploring the relationship between SAW frequency shift and organic vapor concentrations, especially aromatic or halogenated vapors.
o CFM CTC DCM PCE BEN TOL HXN MTL ATN THF DHN
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Figure 4. Comparisons of the responses of the self-assembled Cm-coated200-MHz SAW resonator microsensors to a variety of analytes at unit organic vapor concentration (kHz/atm): chloroform (CFM), carbon tetrachloride (CTC), dichloromethane (DCM), perchloroethylene (PCE), benzene (BEN), toluene (TOL), hexanes (HXN), methanol (MTL), acetone (ATN), tetrahydrofuran (THF), and decahydronaphthalene(DHN); (a) measured and calculated, (b) calculated. Note the &-coated thin film is very selective to DHN, PCE, and TOL.
higher mass loading. As one of the best solvents for Ceo, decahydronaphthalene yields a SAW frequency shift per unit vapor pressure of a factor >lo0 over poor solvents such as dichloromethane, hexanes, tetrahydrofuran, and acetone. Similarly, a good Cm solvent perchloroethylene which is a primary target as an organic toxin in environmental restoration shows a SAW frequency shift per unit vapor pressure of about a factor of 20 over dichloromethane,hexanes, and acetone. Moreover,the response (15) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993,97,3379-3383.
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Acknowledgment. This work was performed under the auspices of the DOE. The authors acknowledge the support of TTP-OTD. We thank Kendall Springer for helpful discussions. (16)Estimation of the current detection limit is based on the assumption of a linear relationship between SAW frequency shift and mass loading. For example, decahydronaphthalenehas a vapor pressure of P(t=23'C) = 8.9 X 1V atm, and ita SAW response is -2.5 kHz. The detection limit would be 8.9 X 10-7 atm at room temperature. This estimated detection limit is not valid if the sensing layer is saturated.
