Surface-enhanced Raman spectroscopy of benzenethiol adsorbed from the gas phase onto silver film over nanosphere surfaces: determination of the sticking probability and detection limit time.

A chemical warfare agent (CWA) gas detector based on surface-enhanced Raman spectroscopy (SERS) using robust nanostructured substrates and a portable Raman spectrometer is a promising alternative to existing modalities. A gas-dosing apparatus was constructed to simulate chemical gas exposure and provide a platform for quantitative analysis of SERS detection. As a first step toward characterizing SERS detection from the gas phase, benzenethiol (BT) has been chosen as the test analyte. SERS spectra were monitored during BT adsorption onto a silver film over a nanosphere (AgFON) substrate. The SERS detection limit time (DLt) for BT on a AgFON at 356 K is found to be 6 ppm-s (30 mg-s m(-3)) for a data acquisition time (t(acq)) of 1 s. The DLt for this kinetically controlled sensor is fundamentally determined by the low sticking probability of BT on AgFONs which is determined to be approximately 2 x 10(-5) at 356 K. The sticking probability increases with increasing temperature consistent with an adsorption activation barrier of approximately 13 kJ mol(-1). Although the DLts found in the present study for BT are in the low ppm-s, a theoretical model of SERS detection indicates DLts below 1 ppb s(-1) for t(acq)= 1 s are, in fact, achievable using existing portable Raman instrumentation and AgFON surfaces. Achieving this goal requires the sticking probability be increased 3 orders of magnitude, illuminating the importance of appropriate surface functionalization.