interfaces. In techniques such as surface-enhanced Raman scattering (SERS), molecules must interact with the metal surface at short distances of 2 nm or less. For non-thiolated molecules, these interactions are energetically unfavourable because of ion stabilization, surface solvation, and/or competition with other molecules in the system. These effects often lead to unsuccessful detection using SERS. Herein, the surface chemistry of HEPES synthesized and stabilized gold nanostars are evaluated as a function of pH using both computation and spectroscopic methodologies. The binding affinity and structure of HEPES on gold nanostars is largely influenced by solution pH and the surface pK<sub>a</sub> of HEPES on gold relative to solution. This impacts the morphology, surface potential, and surface activity of the nanostars in SERS. These effects are monitored using time-dependent LSPR spectra and TEM analysis, which confirm that gold nanostars restructure upon acidification. Spectroscopic evidence supports that both the sulfonate and amines groups in the piperazine ring of HEPES can bind to gold but affinity of all of these interactions weaken upon acidification. DFT calculations support this binding interaction. Upon weakening of the surface stabilizing agent, gold nanostars can be used to detect small non-thiolated molecules. Notably, benzene, a molecule that interacts to gold through London dispersion forces only, successfully adsorbs to gold. The thermodynamics of benzene adsorption is monitored through SERS. Intermolecular interactions between both benzene and gold as well as benzene and benzene are observed and interactions quantified. As a result, we expect these studies will broaden the scope of SERS and plasmonic-based assays as small molecules with weak affinity to metal nanostructures will be more readily detected.<br/>

Competing role of surface chemistry on nanostar stability and SERS activity

2019 Spring National Meeting

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