Surface Enhanced Raman Spectroscopy (SERS) as a Nanoscale Adsorption Phenomenon: Development of Tailored Nanomaterials for Applications in Drug Detection

Surface Enhanced Raman Spectroscopy (SERS) is an analytical technique in which nanostructured substrates amplify the inherently weak Raman signal of an adsorbed species by several orders of magnitude, enabling the detection of trace compounds, up to the single molecule level. While this may be an exceptional tool for any analytical scientist, SERS is at present relegated to the role of academic sensation, and is underutilized in everyday analytical practice. The SERS community is increasingly attributing this setback to a poor understanding of nanoscale surfaces and their chemical environment; since molecular adsorption at the nanostructured surface enables SERS detection, uncertainty about what happens at the surface makes SERS experiments convoluted and often inaccessible. Therefore, there is a pressing need to further nanoscale surface chemistry studies: they are the key to effect the transition of SERS from academic sensation to benchmark technique for routine diagnostics. The present research takes this call by developing a library of SERS-active bimetallic nanostars, and utilizing them to systematically study the interplay between colloidal stability and SERS performance. Particular emphasis is given to elucidating the adsorption process of stabilizers to the nanoparticle surface, which was studied utilizing a vi multi-analytical approach. In addition, DFT calculations were performed in support of mechanistic and structural hypotheses. A population of structures for a simplified all-gold cluster system were obtained at the B3LYP/LANL2DZ level of theory, using both explicit solvent molecules and the continuum solvent model SMD. The experimental results suggest that stabilizers such as citrate are weakly chemisorbed to the bimetallic surface, and have Kad that make them easily displaceable by drug analytes. The DFT results provided a population of possible gold-citrate structures, which show potential significance of gold-ligated water in the adsorption of carboxylates. Preliminary results on a bimetallic system suggest a similar trend as well. This multi-analytical, theory-assisted approach to colloid development allowed for the formulation of a set of well characterized SERS-active colloidal surfaces, capable of providing a high level of surface control during SERS measurements. This ultimately allows for straightforward protocol development, which was demonstrated in the detection of forensically relevant analytes, such as opioids.