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

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Abstract
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 (tacq) 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 ∼2 × 10−5 at 356 K. The sticking probability increases with increasing temperature consistent with an adsorption activation barrier of ∼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 tacq= 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.

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