Surface Chemistry and Adsorption Thermodynamics as Rational Enablers of SERS as a Robust and Reliable Analytical Tool

In fifteen words or less, explain the significance of this contribution (Novel Aspect).: A surface chemistry perspective on SERS allows for new physical insight enabling complex spectral elucidation.

Abstract Text:

Although SERS relies on nanoscale surfaces, a rigorous understanding of the surface chemical phenomena that concur in the observation of the SERS signal is still limited and disorganized at best. Such a lack of understanding has direct consequences on the transition of SERS from academia to routine applications, causing the widespread belief that SERS is an inherently unreliable and irreproducible analytical technique, in which even the smallest change in sample preparation, such as the order of species addition, can produce uncontrollable and inexplicable effects. This presentation will illustrate how surface chemistry and surface adsorption thermodynamics can be leveraged as foundational pillars of SERS method development, providing additional ways to control SERS signal production and systematize the understanding of complex SERS outputs, thus contributing to the demonstration that SERS can in fact be reliable and reproducible.

This approach to method development and SERS output elucidation will be exemplified by our recent work on the direct detection of nucleic acids (NAs) by SERS in the presence of spermine—a biogenic polyamine that is traditionally introduced in NA sample preparation, either as a nanoparticle capping species or as an aggregating agent. This new methodological approach was enabled by a unified nanoparticle platform, providing a spermine-free and a spermine-bearing system using the same nanoparticle synthesis. By maintaining the synthesis constant, the confounding bias related to the baseline nanoparticle surface chemistry is eliminated. The elimination of such a bias poses, for the first time, fair grounds to “isolate” and compare the different pairwise interactions that are established, as a function of sample preparation order, in the SERS sample, where the thermodynamic equilibria dictate the adsorbate landscape, and thus, the SERS spectrum. The obtained results allowed for a reframing of the mechanistic hypotheses on spermine-containing SERS samples: The spermine-NA interaction is thermodynamically dominant, and the equilibria that are established at the nanoscale surface in the presence of spermine are independent of sample preparation order.

The presented findings support the introduction of surface chemistry and adsorption thermodynamics as additional (i.e., beyond plasmonics) rational enablers of SERS as a reliable and robust analytical tool.