A sensor includes an optical resonator, a trap, and a measurement system. The trap is positioned to hold captured contaminant within an evanescent field of the optical resonator. The measurement system can detect the effect of the captured contaminant on the optical resonator. The sensor can includes a photonic crystal that contains defects forming the optical resonator and a waveguide to which the optical resonator is coupled. Yet another embodiment is a sensing method that exposes a trap that is adjacent to an optical resonator to an environment that may contain a contaminant that the trap can capture. Light having a wavelength that interacts with the contaminant can then be coupled into the optical resonator, and the effect of any captured contaminant on the optical resonator can be measured.