Tailored Colloidal Nanostars for Surface-Enhanced Raman Spectroscopy: Optimization of Formulation Components and Study of the Stabilizer–Nanoparticle Interactions

While the effects of the morphology and composition of plasmonic substrates in Surface-Enhanced Raman Spectroscopy (SERS) are widely studied, surface chemistry and, more specifically, the role of preadsorbed species on colloidal substrates (i.e., stabilizers and synthesis byproducts) are typically less explored. In this paper, a surfactant-free synthesis of sparingly capped bimetallic colloidal AuAg nanostars was selected as a basis to (1) examine the effect of varying stabilizers and (2) systematically assess the impact of the resulting surface environment on SERS intensity. The latter entailed the characterization of the colloidal formulations in terms of optical reproducibility, suitability for analytical applications, long-term colloidal stability, and SERS performance. Emphasis was given to the elucidation of the stabilizer–metal interactions, which were studied by electrophoretic light scattering and infrared spectroscopy. It was found that the capping process is the result of chemisorption to an essentially neutral alloy and that the capping environment has effects on the SERS response that can overtop those caused by nanoparticle morphology. The model stabilizer, citrate, was found to weakly chemisorb (−4.36 ± 0.08 and −4.58 ± 0.05 kJ/mol at 10 and 20 °C, respectively) to the bimetallic surface in a positively cooperative fashion (n_Hill > 1) via the unidentate mode.