Surface-enhanced Raman spectroscopy characterization of chemical warfare agents adsorbed from the gas phase on silver film over nanosphere surfaces

Development of a robust, rapid, and sensitive detector for gases is an important analytical goal. The timely and accurate detection of hazardous chemicals is a pervasive challenge to environmentalists, industry, first responders, and the military. A chemical warfare agent (CWA) gas detector based on surface-enhanced Raman spectroscopy (SERS) using robust nanostructured substrates is a promising alternative to existing modalities.;Herein, it is demonstrated that CWA simulant, 2-chloroethyl ethyl sulfide (half-mustard, CEES) gas can be successfully analyzed using SERS by virtue of a silver film over nanosphere (AgFON) surface and a AgFON functionalized with a self-assembled monolayer (SAM). A gas dosing apparatus was constructed to simulate chemical gas exposure and provide a platform for quantitative analysis of SERS gas detection. As a first step towards characterizing SERS detection limits from the gas phase, benzenethiol (BT) was chosen as the test analyte. BT adsorption onto a AgFON substrate is monitored by SERS and expressed as a detection limit time (DLt). The SERS DLt for BT gas on a AgFON at 358 K is found to be 6 ppm-s (30 mg-s m-3) with a data acquisition time (tacq) of 1 s. The DLt for this sensor is fundamentally determined by the sticking probability of BT on the AgFON, found to be ∼2 x 10-5 at 358 K. Although the DLt determined for BT is in the low ppm-s, a theoretical analysis of SERS detection indicates DLt's below 1 ppb-s with tacq = 1 s are, in fact, achievable using existing portable Raman instrumentation and AgFON like surfaces. Achieving this goal requires the sticking probability be increased three orders of magnitude illuminating the importance of appropriate surface functionalization for achieving lower DLt's.