Analysing volatile organic compounds (VOCs) with SERS by bubbling into colloidal nanoparticles

Surface-enhanced Raman scattering (SERS) enables real-time analysis of volatile organic compounds (VOCs) and other gaseous molecules using portable Raman devices, offering faster measurements compared to GC-MS. To facilitate this, numerous solid SERS substrates have been developed and applied across multiple fields. However, they often require complex fabrication due to poor adsorption of gases onto solid surfaces leading to high production costs and inconsistencies in sensitivity and reproducibility. In contrast, colloidal nanoparticles are less expensive to produce and established facile synthesis methods help their broader implementation. In this study a simple approach has been conceptualised whereby gaseous analytes are bubbled directly into colloidal nanoparticles. This was achieved by pumping air into a flask containing the volatile analyte, which in turn was bubbled into a vial containing colloidal nanoparticles and measured with a portable Raman spectrometer. Initially, 3-mercapto-hexanol (3MH) was successfully measured using this method. 3MH was chosen due to the method originally being developed to measure thiols associated to underarm odour. Subsequently, 3MH was used to optimize the air flow rate by testing at eight different flow rates. The results demonstrated that flow rate influenced the timing of analyte spectral appearance as seen by the rate of change in peak height at 634 cm^-1 for 3MH, with the most reproducible results observed at 50 mL min^-1. The technique was further evaluated with two other analytes: 2-methyl-3-mercapto-pentanol (2M3MP) and indole. The latter needed the addition of an aggregating agent (NaCl) to the nanoparticles. These findings indicate that this approach offers a promising alternative for detecting gaseous analytes using colloidal nanoparticles.