The strong evanescent fields inherent in air-clad or polymer-clad sub-micron waveguides make these systems intriguing candidates for chemical sensing based on infrared absorption, fluorescence, or Raman scattering from adsorbed analyte species. We have recently shown that by coating ultra-thin silicon nitride waveguides (150 nm – 175 nm) with functionalized sorbent polymers, we can spectroscopically detect trace gases (such as simulants for CW agents) at concentrations as low as 30 ppb. Our measurements are presently based on near-infrared absorption spectroscopy at an overtone of the OH molecular resonance in sorbent materials. These weak absorption features can be probed with high fidelity using resonant waveguide cavities such as Fabry-Perot and microring resonators. Spectral features in the infrared absorption can be used to identify the toxicity of the analyte molecule, and perhaps even identify the molecule itself.