A surface chemistry perspective on SERS: revisiting the basics to push the field forward

Surfaces are well known to be complex entities that are extremely difficult to study, and any phenomenon that is related to them is consequently challenging to approach. Moving from the bulk to the nanoscale adds a further layer of complexity to the problem. Because SERS relies on surfaces at the nanoscale, a rigorous understanding of the chemical phenomena that concur in the observation of the SERS signal is still limited or disorganized at best. Specifically, the lack of understanding of the chemical properties of nanoparticle surfaces has direct consequences on the development of SERS-based devices, causing a widespread belief that SERS is an inherently unreliable and fundamentally irreproducible analytical technique. Herein, we discuss old and new literature from SERS and related fields to accompany the reader through a journey that explores the chemical nature and architecture of colloidal plasmonic nanoparticles as the most popular SERS-active surfaces. By examining the chemistry of the surface landscape of the most common SERS colloids and the thermodynamic equilibria that characterize it, we aim to paint a chemically realistic picture of what a SERS analyst deals with on a daily basis. Thus, our goal for this review is to provide a centralized compilation of key, state-of-the-art surface chemistry information that can guide the rational development of analytical protocols and contribute an additional path through which our community can continue to advance SERS as a reliable and robust analytical tool.