A sensor includes traps that are adjacent to a waveguide and capable of holding a contaminant for an interaction with an evanescent field surrounding the waveguide. When held in a trap, a particle of the contaminant, which may be an atom, a molecule, a virus, or a microbe, scatters light from the waveguide, and the scattered light can be measured to detect the presence or concentration of the contaminant. Holding of the particles permits sensing of the contaminant in a gas where movement of the particles might otherwise be too fast to permit measurement of the interaction with the evanescent field. The waveguide, a lighting system for the waveguide, a photosensor, and a communications interface can all be fabricated on a semiconductor die to permit fabrication of an autonomous nanosensor capable of suspension in the air or a gas being sensed.