A sensor and a method for sensing a change in mass of a reflective microbeam in a sensor, the sensor having a reflective layer disposed on a substrate and spaced apart from the reflective microbeam. The microbeam receives amplitude modulated laser energy at a first wavelength and is photothermally excited into resonance at the frequency of amplitude modulation, the reflective microbridge and the reflective layer receive optical energy at a second wavelength and reflect the optical energy toward a receiver. A change in reflectivity of the microbeam at different frequencies is determined. A change in reflectivity indicates a change in resonant frequency of the microbeam. Mass of the microbeam changes when a chemoselective material on the microbeam sorbs a target chemical.