Optimization of SAM-based multilayer SERS substrates for intracellular analyses: the effect of terminating functional groups

Abstract

Surface-enhanced Raman scattering (SERS) has become an attractive analytical tool for intracellular analyses due to its minimally invasive nature and molecular specificity. However, highly reproducible and optimized SERS substrates have been seen as a key to developing SERS as a reliable analytical methodology. This research focuses on optimizing self assembled monolayer (SAM)-based multilayer SERS substrates for a wide range of applications, including ultratrace detection of biomolecules within individual living cells. Multilayer SERS substrates are comprised of alternating layers of metal film and dielectric spacer cast on a monolayer of nanostructures. Using these substrates, varying degrees of SERS enhancement factors (EF) have been achieved, some as large as 10-fold relative to optimized single film over nanostructures substrates. To gain a mechanistic understanding of multilayered SERS enhancements, SAMs have been used to systematically vary spacer thickness. The results revealed spacer-dependent SERS EFs. To further the understanding of multilayer SERS enhancement, this work discusses the use of terminating functional groups in the optimization of SAM multilayer SERS substrates. SAMs having various functional groups were used as dielectric spacers to systematically vary the dielectric constant. To investigate the effect of the pH on the uniformity of the SAMs and their multilayer SERS enhancement, SAMs were formed in alkylthiol solutions of different pH and the subsequent SERS enhancement were evaluated. It was found that using alklythiol SAMs with appropriate terminating functional groups the SAM multilayer can achieve SERS EFs ranging between 10⁸ and10¹⁰ and the substrates yielded highly reproducible SERS signals. The effect of the pH on the SERS enhancement is selective on the type of the terminating functional group of the alkylthiol used for SAM formation.