Chemical Sensing With Resistive Microcantilevers
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Chemical Sensing With Resistive Microcantilevers G. Muralidharan1, A. Wig1, L. A. Pinnaduwage1, D. L. Hedden1, P. G. Datskos1, T. Thundat1, and R. T. Lareau2 1 Oak Ridge National Laboratory, Oak Ridge, TN-37831-6123. 2 Federal Aviation Administration, Atlantic City, NJ 08405.
ABSTRACT MEMS-based microcantilevers have been proposed for a variety of biological and chemical sensing applications. Measuring the magnitude of microcantilever deflection due to adsorption-induced bending, and following the variation in the resonant frequency of the microcantilevers due to the adsorbed mass are two techniques commonly employed for sensing analytes. Apart from possessing a high level of sensitivity to small changes in mass, microcantilevers are also very sensitive to small changes in temperature and hence the flow of heat. One way of achieving high sensitivity in thermal measurements is by using a bimaterial microcantilever and measuring its deflection as a result of thermal fluctuations. Commercially available piezoresistive microcantilevers are an example of bimaterial cantilevers and in this study, we propose the use of such cantilevers for sensing explosives. We show that sensing can be accomplished by following the differences in the thermal response of the cantilevers introduced by the presence of explosives adsorbed from the vapor phase onto the surface of the cantilever. We discuss the issues involved in determining the sensitivity of detection and selectivity of detection. INTRODUCTION Real-time detection of explosives is important for practical applications ranging from passenger baggage-screening to the disarming of landmines. With their compactness and potential low cost, detection techniques based on Silicon-based Micro Electro-Mechanical Systems (MEMS) provide a path for the development of miniaturized sensors. One method for detecting explosives is through the detection of vapors in equilibrium with the solid explosive [1]. To design devices that would detect explosive chemicals such as TNT through detecting the presence of their vapors, a detailed understanding of the physical chemistry parameters involved in vapor transport, knowledge of the diffusion coefficients, molecular sticking coefficients and vapor pressures over non-ideal solid solutions are all of importance[2]. In addition, detailed knowledge of the interaction of such explosive vapors with exposed surfaces is of interest. For example, it would be of interest to know how much vapor would adsorb and stick to the sensing surface over a certain period of time under typical conditions at which the sensor would be used. Although data are available on the long-term adsorption and desorption of vapors such as TNT from surfaces of soils [3], very little data is available on the short-term behavior related to surfaces that would be common in MEMS. This information along with the known minimum detectable quantity would determine the time necessary for detection. In addition, knowledge of the relative affinities of different surfaces to these vapors would hel
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