Comparison of self-sensing techniques for mercury vapor detection using piezoelectric microcantilevers

Self-sensing piezoelectric microcantilevers do not require alignment of external optics, use very little power and are well suited to array applications. We demonstrate a commercially available, piezoelectric cantilever as a mercury vapor detector. The cantilever is an atomic force microscopy cantilever with self-sensing and self-actuating capabilities. In the first detection technique, adsorption-induced bending, caused by mercury adsorption onto gold on the cantilever, is measured by bringing the resonating cantilever into intermittent contact with a reference surface. Cantilever bending causes changes in its oscillatory amplitude, detected using the voltage output of the piezoelectric film, and a piezotube in feedback is adjusted to compensate. A 50 ppb mercury concentration in nitrogen gas was detected. The second technique capitalizes on the stiffening effect adsorbed gold has on the cantilever; natural frequency changes are detected with the piezoelectric cantilever in conjunction with a bridge circuit and amplifier. A mercury concentration of 93 ppb in nitrogen is detected. These techniques are discussed and compared. Additionally, consideration is given to the effect of the location and size of the gold-adsorption pad on sensitivity.