Microcantilevers, such as those used in atomic force microscopy, undergo Brownian motion due to mechanical thermal noise. The root mean square amplitude of the Brownian motion of a cantilever typically ranges from 0.01–0.1 nm, which limits its use in practical applications. Here we describe a technique by which the Brownian amplitude and the Q factor in air and water can be amplified by three and two orders of magnitude, respectively. This technique is similar to a positive feedback oscillator, wherein the Brownian motion of the vibrating cantilever controls the frequency output of the oscillator. This technique can be exploited to improve sensitivity of microcantilever-based chemical and biological sensors, especially for sensors in liquid environments.
We describe in detail the detection of deflagration of trinitrotoluene (TNT) deposited on a piezoresistive microcantilever and point out its possible use for explosive-vapor detection. The deflagration of TNT causes the cantilever to bend (due to released heat) and its resonance frequency to shift (due to mass unloading). Explosive vapors provide unique responses that are absent for “interferences” such as water or alcohol vapors. The proposed sensor makes possible a sensitive, miniature explosives detection device that may be deployed in large numbers. The minimum amount of TNT detected on the cantilever depends on the cantilever dimensions and was ≈50 pg for the batch of cantilevers used.
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