Articles Anal. Chem. 1995, 67,519-521
Vapor Detection Using Resonating T. Thundat,* G. Y. Chen, R. J. Wannack, D. P. Allison, and E. A. Wachter Health Sciences Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Changes in the resonance frequency of microcantilevers due to adsorption of analyte vapor on exposed surfaces is shown to provide a novel means for detection of the analyte. Frequency changes can be due to mass loading or adsorption-inducedchanges in cantilever spring constant. Sensitization to water vapor is demonstrated by coating cantilever surfaces with hygroscopic materials, such as phosphoric acid. Cantilevers coated with a thin gelatin film exhibit high sensitivity and a linear response with changes in relative humidity, apparently due to changes in the spring constant of the coated cantilever. In addition to frequency response, static cantilever deflection also changes with vapor adsorption. Both phenomena can be used to detect adsorbed vapors with picogram mass resolution. New microinstrumentation for measuring trace gases is important to the advancementof detector technology, and a variety of sensors based on electrical, electrochemical,spectroscopic, and chromatographic responses have been developed.' One simple but sensitive method for determining minute quantities of gases employs mass loading of resonant surfaces. Detectors using resonating piezoelectric crystals (such as the quartz crystal microbalance, QCM) have been shown to offer low detection limits, wide operating ranges, and continuous operation and can be made selective by coating with appropriate compound-specific ads or bent^.^-^ Micromachined cantilevers also utilize the resonant surface approach and offer a number of interesting possibilities for use as microchemical sensors. Such devices are simple, extremely small, and potentially have very high sensitivity. Cleveland et al. have reported the use of cantilever resonance frequency to detect nanogram changes in mass loading when small particles are deposited onto atomic force microscopy (AFM) probe tips.5 Binh et al. have proposed the use of a novel microcantilever design with megahertz resonance frequency for achieving 10-ls g mass (1) Janata, J. Pnnciples of Chemical Senson; Plenum Press: New York, 1989. (2) Warner, A W. In Ultra Micro-Weight Determination in Controlled Enuironments Wolsky, S. P., Zdanuk, E. J., Eds.; Interscience: New York, 1969. (3) King,W. H., Jr. Anal. Chem. 1964,36, 1735. (4) Guilbaut, G. G. In Methods and Phenomena; Lu, C., Czadema, A W., Eds.; Elsevier: Amsterdam, 1984; Vol. 7. (5) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K Rev. Sci. Instrum. 1993,64,403. 0003-2700/95/0367-0519$9.00/0 0 1995 American Chemical Society
resolution for adsorbed chemical species? And Gimzewski et al. have used static cantilever bending to detect chemical reactions with very high ~ensitivity.~Recently we have shown that the resonance frequency as well as static bending of microcantilevers can be influenced by ambient conditions, such as moisture adsorption, and that deflection of metal-coated cantilevers can be further influenced by thermal effects (bimetallic effect) .* In this report we describe a novel micromechanicalsensor that makes use of surfacemodified AFM cantilevers. Surface modification with phosphoric acid or gelatin is used in this example to demonstrate how sensitivity can be augmented by coating with materials having a high affinity for the analyte. This approach can also be used to increase selectivity of response and provides the basis for development of a new family of extremely compact sensors offering both high sensitivity and specificity. EXPERIMENTAL SECTION
Commercially available, V-shaped silicon cantilevers (180 pm long, 0.09 and 0.05 N/m Ultralevers, Park Scientific Instruments, Inc.) were used in this study. Surface modification was performed by coating one or both surfaces of the cantilever. One set of cantilevers was coated using a dilute solution (0.5 M) of phosphoric acid, while the second set of cantilevers was coated with 0.1%gelatin in distilled water (bovine skin gelatin, Sigma). In both cases this was achieved by placing a drop of solution on a glass slide and sliding the cantilever into the solution until one side of the cantilever was completely wet. The cantilever was then pulled out and dried in a desiccator for 2 days. The thickness of the coating was estimated by determining the change in resonance frequency for the coated cantilever. Note that gelatin modifcation of QCMs for improved water adsorption has been reported previously by Lee et Cantilevers were exposed under controlled conditions in a chamber continuously purged with humidified nitrogen gas.l0 Resonance frequency was unaffected by flow of this humid gas, (6) Bmh, V. T.; Garcia, N.; Levanuk, A L.; Surf: Sci. Lett. 1994,301,L224228. (7) Gimzewski, J. K; Gerber, Ch.; Meyer, E.; Schlittler, R R Chem. Phys. Lett. 1994,217, 589. (8) Thundat, T.; Warmack, R J.; Chen, G. Y.; Allison, D. P. Appl. Phys. Lett. 1994,64,2894. (9) Lee, C. W.; Fung, Y. S.; FUilg, IC W. Anal. Chim. Acta 1982,135,277. (10) Thundat, T.;Warmack, R J. Chen, G. Y.; Allison, D. P. Appl. Phys. Lett. 1994,64,2894.
Analytical Chemistry, Vol. 67, No. 3, February 1 , 7995 519
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Figure I. Typical variation in resonance frequency as a function of relative humidity for a silicon cantilever (K = 0.05 N/m) coated with
phosphoric acid (both sides). Due to the extremely small variation of adsorbed mass, a nearly linear curve is expected from eq 1. Thickness of coating estimated at 30 nm.
and at a constant humidity the resonance frequency was stable (within 1 Hz). Simultaneous detection of cantilever deflection (error voltage) and resonance frequency was achieved optically using a laser focused at the end of the cantilever and a positionsensitive detector." For convenience, we used existing electronics of an atomic force microscope (Multi-Mode Nanoscope 111, Digital Instruments, Inc.) to drive the cantilever. Since these cantilevers were not coated with a metal overlayer, parasitic deflection due to the bimetallic effect was negligible.12-15 RESULTS AND DISCUSSION
Response of Phosphoric Acid Coated Cantilevers. Most commercially available, unmodified silicon cantilevers show only slight variation in resonance frequency for changes in relative humidity in a range from 10 to 60%. This response can be amplified by coating the cantilever surface with hygroscopic materials such as phosphoric acid (Figure 1). Prior to coating, this particular cantilever showed a frequency change of only 30 Hz when the relative humidity (RH) was changed from 5 to 60%. Following coating, the magnitude of response increased by an order of magnitude. Note that below 5% and above 90%RH our reference hygrometer was not reliable, so no tests were conducted outside this range. Also, it was difficult to observe the resonance peak for a wide range due to extensive cantilever bending (a discussion of this effect is given later). Although any starting point for RH measurement could be chosen, the window over which RH could be measured without resetting the AFM detection system was limited by cantilever bending. The curves in Figures 1and 2 were obtained by raising the humidity in the chamber to around 60% and then reducing the RH to lower values. The sensitivity (or relative frequency shift) obtained at different RH windows was almost identical. The resonance frequency, v, of a cantilever can be expressed as (11) Sarid, D.Atomic Force Microscow, Oxford University Press: New York, 1991. (12) Umeda, N.; Ish&, S.; Uwai, H.]. Vuc. Sci. Technol. 1991,E9,1318. (13) Allegrini, M.; Ascoli, C.; Baschieri, P.; Dinelli, F.; Frediani, C.; Lio, A; Mariani, T.Ultramicroscopy 1992,42-44, 371. (14)Marti, 0.; Ruf, A; Hipp, M.; Bielefeldt, B.; Colchero, J.; Mlynek, J. Ultramicroscopy 1992,42-44, 345. (15) Mertz, J.; Marti, 0.;Mylnek, J. Appl. Phys. Left. 1993,62, 2344.
520 Analytical Chemistry, Vol. 67,No. 3, February 1, 1995
where the effective mass of a triangular cantilever, m*, is O.18mb,l6 where mb is the mass of the beam, and K is the spring constant. Therefore, changes observed in v can be due to changes in m or K or both. The negative slope of frequency response in Figure 1 is consistent with mass loading due to water adsorption. The nonlinearity observed in this &re may be due to the nonlinear nature of moisture adsorption on the cantilever surface. Assuming that changes in Kwith RH are negligible and that water adsorption is uniform on the cantilever surface, the mass change due to adsorption of water vapor can be estimated to be 200 x g. The thickness of the acid coating was estimated to be a p proximately 30 nm based on change of frequency before and after coating. Sensitivity of this combination of cantilever and coating is 5 &/%RH. This could be improved by using a more efficient coating and by improving the geometrical design of the cantilever. Response of Gelatin-Coated Cantilevers. Equation 1 implies that changes in v can also take place due to changes in K. This has been observed for cantilevers coated with a thin layer of gelatin. Figure 2 shows the resonance frequency response of a gelatincoated cantilever to changes in RH from 40 to 65%. The small range of RH shown is due to hitations of the current optical detection system (discussed below), which was unable to measure resonance outside this range due to extreme bending of the cantilever. This modification approach showed an improved sensitivity to moisture adsorption (50 Hz/%RH) compared with phosphoric acid modifcation. In contrast with phosphoric acid coating, gelatincoated cantilevers demonstrated a linear response with positive slope, suggesting that change in K was the dominant effect. Response was completely reversible and reproducible and virtually instantaneous (within seconds). Film thickness for the cantilever used in Figure 2 was calculated to be 23 nm, which interestingly is 4 orders of magnitude smaller than that previously reported for sensor applications.' Numerous experiments demonstrated that frequency response to changes in RH was critically dependent on the gelatin film thickness. Thin uniform coatings (applied using a single coat) always produced cantilevers with high dynamic range, minimal hysteresis, and linear frequency response relative to cantilevers with thick coatings (multiple coating). Cantilever response was linear only when a single side of the cantilever was coated with gelatin. The positive slope of gelatincoated cantilevers cannot be explained simply as a result of mass loading but rather appears to be due almost entirely to changes in K resulting from adsorption-induced differential stress on the cantilever. Cantilevers will bend if stress on the two sides are unequal.17 This was evidenced by large changes in cantilever bending (approximately 1.2 pm for the case shown in Figure 2). A small change in K (AK=0.006 N/m, AK/K = 0.112) can explain the linear behavior observed. Bending measurements showed that the cantilevers bent up (toward gelatin surface) as RH was decreased, consistent with the notion that a gelatin film shrinks with decreased RH. Hence change in stress as the film shrinks or expands should affect resonance frequency due to changes in K. Also, cantilevers coated with gelatin showed large changes in static deflection (16) Landau, L D.; Lifshitz, E. M. Fluid Mechanics; Pergamon Press: New York, 1959. (17) Stoney, G. G. Proc. R. SOC.London 1909,A82, 172.
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Relative Humidity (%) Figure 2. Resonance frequency for a gelatin-coated (single side) silicon cantilever (K = 0.05 N/m) as a function of relative humidity. The positive slope and linearity are consistent with small adsorptioninduced variations in K (AKIK * 0.006 N/m). Coating thickness approximately 23 nm. Cantilevers coated on both sides exhibit nonlinear response.
compared to cantilevers coated with phosphoric acid, supporting the conclusion that the latter were dominated by changes in m rather than K. Comparisonof Cantilever Responses. Based on the negative slope of response in Figure 1, it appears that resonance frequency response of phosphoric acidcoated cantilevers is due to mass loading. Since the maximum amount of water absorbed by the coating is limited by the small size of the cantilever, the dynamic range for detection can be limited. On the other hand, the positive slope of response for gelatincoated cantilevers appears to be entiely due to variations in K. This indicates a volume-based response (due to absorption of analyte) which should have a much higher capacity and dynamic range. The response time of gelatincoated cantilevers (less than a second) was much faster than that of the phosphoric acid-coated cantilevers (tens of seconds). By designing cantilevers where adsorption is restricted to the area at the apex of the cantilever, it is possible to eliminate effects due to changes in K.18 Also, by simultaneously measuring cantilever deflection and resonance frequency change, it should be possible to decouple the effects of mass loading and variations in K.19 Instrumental me&. As mentioned earlier, both resonance frequency and cantilever bending can change due to moisture adsorption. With the detection scheme used in this work, severe bending of the cantilever can hamper optical detection by deflecting the cantilever so far that the laser beam may not reflect into the position-sensitive detector, resulting in loss of signal. This limited the range over which RH could be detected, especially for the gelatincoated cantilevers. Rectangular silicon cantilevers having a large K (42 N/m) exhibit only minor bending, and by using these we were able to increase the operating range for gelatin to 5-90% RH. Sensitivity was lower, however, since the relative change in K was much smaller. Signal loss encountered using optical detection could be avoided by use of capacitive or piezoresistive detection methods, which have the additional advantage of avoiding problems that may arise due to laser heating of the cantilever. (18)Chen, G. Y.; Warmack, R J. Thundat, T.; Allison,D. P.Rev. Sci. Instrum. 1994, 65,2532. (19)Chen, G. Y.; Thundat, T.; Warmack, R J.; Wachter, E. A, to be published.
Resonance frequencies in these experiments were measured by sweeping the cantilever drive frequency across the peak of the resonance curve. The precision of this approach is around 1 Hz due to the relatively low Q values of triangular cantilevers. This could be improved by several orders of magnitude if rectangular cantileverswith a high Q factor were paired with phase detection methods. The sensitivity could also be improved by using cantilevers with a higher resonance frequency or by using higher order resonance modes. It is important to consider possible interferences that might cause resonance frequency changes independent of analyte adsorption. One potential source is “induced mass”, where the motion of oscillating cantilevers produces an induced mass due to drag from ambient fluid.l’J8 In vapor environments, sigdicant changes in resonance frequency can be observed when very low density gases (such as He) are used or when pressure changes substantially. Hence, this technique may not be suitable for use in an atmosphere where the concentration or pressure of background gases varies during operation, unless some form of reference cantilever is used to compensate for the effects of induced mass. CONCLUSIONS These examples have shown that resonant AFM cantilevers offer excellent potential for development of simple, sensitive microsensors, where both deflection and resonance frequency variations can be used to monitor analyte exposure. In the specific cases examined, changes in resonance frequency provided a means for determining the amount of water vapor adsorbed with picogram mass resolution. In general, this approach provides a basis for development of new chemical sensors using surface coatings that adsorb, absorb, or react with specific analytes. Sensitivity and dynamic range can be improved by careful design of the cantilevers and by use of better resonance detection methods. It should be possible to fabricate complete sensors (cantilever, detection system, and control electronics) on a single hybrid microchip measuring less than 1 x 1 mm, so very inexpensive microsensors are conceivable. Furthermore, if an array of cantileverswere used having a variety of coatings, speci6c chemical fingerprinting of mixed or unknown analytes should be possible based on pattern recognition from the differential response of the individual cantilevers. ACKNOWLEDGMENT We would like to thank B. K. Annis and W. G. Oliver for useful discussions. This research was sponsored by the US. Department of Energy under Contract No. DEAC05840R21400 with Martin Marietta Energy Systems, Inc. Support from the DOE Office of Health and Environmental Research and from the ORNL Seed Money program is gratefully acknowledged. Received for review August 9, 1994. Accepted October
25, 1994.@ AC940789U @Abstractpublished in Advance ACS Abstmcts, December 15, 1994.
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