All-optical micromechanical chemical sensors

We describe experimental results from micromechanical resonators coated with chemoselective polymers that detect chemical vapors from volatile organic compounds or explosives using all-optical interrogation. The shift in the resonant frequency of a gold microbeam is read-out using photothermal actuation and microcavity interferometry. For detection of toluene vapor, response times of less than 5 seconds are achieved for vapor concentrations as low as 60 ppm. For detection of TNT vapor, concentrations as low as 10 ppb are detected in 100 seconds. An analysis of the measured frequency noise in these sensors shows that it is dominated by thermal-mechanical fluctuations at the fundamental flexural mode. Our measurements thus indicate that thermal-mechanical frequency noise is the primary intrinsic detection limit for typical resonant-frequency MEMS biosensors or chemical vapor sensors.