Self-assembled monolayers for bio-analytical SERS applications: from single to mixed component SAMs

Self-assembled monolayers (SAMs) represent an elegant and powerful strategy for achieving precise control over the surface chemistry of plasmonic nanomaterials. This capability has significantly advanced both fundamental and applied research in surface-enhanced Raman spectroscopy (SERS), a technique whose performance is critically dependent on the chemical nature of the enhancing substrate. While single-component SAMs have served as a cornerstone in foundational SERS studies, their functionality is inherently limited, rendering them inadequate for complex real-world applications. Mixed-component SAMs offer a compelling solution by enabling the integration of multiple functional ligands on a single plasmonic interface, thereby overcoming these limitations. Nevertheless, the routine and reliable deployment of mixed component SAMs in SERS faces fundamental challenges, particularly in their controlled synthesis and precise characterization. As a result, despite their widespread empirical use, the field of mixed-component SAMs remains underdeveloped and insufficiently reviewed. This article aims to address this gap by first outlining the fundamentals of single-component SAMs, then delving into the synthesis strategies and characterization techniques specific to mixed component SAMs. We emphasize the unique role of SERS as a powerful in situ tool for probing key parameters of mixed monolayers. Using biological applications as a primary example, we illustrate how the multifunctionality afforded by mixed component SAMs leads to enhanced sensitivity, selectivity, stability, and accuracy in quantitative SERS analysis. Finally, we discuss the persisting challenges in the rational design of mixed component SAMs, highlighting the need for further mechanistic research and the integration of artificial intelligence.