Biosynthetically grown dendritic silver nanostructures for visible Surface Enhanced Resonance Raman Spectroscopy (v-SERRS)

Abstract

A simple approach to achieve high SERS enhancement for bio-analyte detection at visible wavelength through a resonance Raman (RR) effect has been proposed in this study. Usually, SERS accompanies a significant drop in background fluorescence signals leading to a dramatic increase in signal strength. Under resonance conditions, however, the SERS increment is limited by the loss of electromagnetic (EM) enhancement. The extent of loss in signal could be as high as 100-fold, which is typically significant when sampling low concentrations of molecules with small cross-sections such as biomolecules. EM loss could be compensated either by tuning the optical properties of the substrate or by finding alternative enhancement pathways. In order to achieve this, a one-step reduction protocol for synthesizing highly stable SERS-active dendritic silver nanostructures was optimised, which took into account the optical tuning under visible light due to their hot-spot-rich morphology. Adopting multiple roles, metabolite-enriched green tea used as a bio reductant triggered silver salt reduction, stabilized the nanostructure through capping and also functionalized the surface, leaving numerous active chemical groups on the nanostructure surface. The surface groups identified as amides and glycosides work as anchorage points allowing interaction with the analytes, facilitating inter-charge transfers. A 10¹³ order enhancement factor (EF) has been achieved through using the substrate. The substrate shows excellent reproducibility and acceptable relative standard deviation (RSD) values (<10%). Notably, the RR spectra were characterised by sharp and significantly high not totally symmetric (b₂) vibrations, confirming the effective chemical enhancement contribution to the achieved SERS enhancement. For a test analyte, S. aureus, the RR spectrum exhibited selective vibrational enhancements only, which are plausibly ascribed to signal acquisition below the conventional ensemble averaging regime and are also due to specific cell–nanostructure interaction on account of indigenous surface functionalization achieved using green tea extract.