Self-Assembled Gold Nano-Bipyramids for Solution-Based Surface-Enhanced Raman Spectroscopy Detection

In solution-based surface-enhanced Raman spectroscopy (SERS) detection, plasmonic metal nanoparticles’ controllable assembly and aggregation into highly enhanced electromagnetic coupling nanostructures remains a challenge. In this study, we demonstrate a facile and rapid strategy to control the assembly of poly­(styrene sulfonate) functionalized gold nano-bipyramids (PSS-Au BPs) as highly sensitive SERS substrates for solution-based SERS detection. The assembly process of the PSS-Au BPs was controlled by adding halide ions as aggregation agents under various concentrations in the presence of analytic molecules. In comparison, the citrate-capped spherical gold nanoparticles (Cit-Au NPs) were used to investigate the influence of the shape of NPs and their assembly state on SERS performance. Both in situ ultraviolet–visible (UV–vis) and Raman spectroscopy characterizations revealed that the shape of the NPs and aggregation state are critical to achieving a highly enhanced SERS property. The PSS-Au BPs, owing to the formation of tip-to-tip dimers, have a better electromagnetic enhancement capacity than Cit-Au NP aggregation states. Thus, the PSS-Au BPs display an improved lowest recorded concentration downs to the level of 5 × 10^–11 M for crystal violet (CV) molecules, which is 2 or 3 orders of magnitude lower than those of the Cit-Au NPs.