The surface plasmon resonance (SPR) response of Au nanoparticles to adsorption of NO2, C6H5NO2, and 2,4-dinitrotoluene (DNT) is probed using the cavity ring-down technique. A uniquely sensitive first-layer response is observed for nanoparticles having a mean diameter of 4.5 nm, which appears characteristic of the -NO2 group. Detection limits of 1.2 nmol/L (29 ppb), 7.6 nmol/L (184 ppb). and 0.17 nmol/L (4.1 ppb), are estimated for NO2, C6H5NO2, and DNT, respectively. (c) 2005 Published by Elsevier B.V.