Effects of Cosolvent\non the Intermolecular Interactions\nbetween an Analyte and a Gold Nanostar Surface Studied Using SERS

For surface-enhanced Raman scattering (SERS), reproducible\nsolution-phase\nresults are typically obtained using nanoparticles functionalized\nwith surface-stabilizing molecules that can prevent the adsorption\nof analyte molecules with surface affinities lower than those of the\nstabilizing agent. Herein, we investigate the effects of intermolecular\ninteractions between a nonthiolated analyte and cosolvent to facilitate\nand modulate analyte adsorption to gold nanostars. SERS, extinction\nspectroscopy, transmission electron microscopy (TEM), and density\nfunctional theory (DFT) calculations are employed in this regard.\nTetrahydrofuran (THF) is utilized as a cosolvent in water to facilitate\nthe detection of acetylsalicylic acid (aspirin) on anisotropic gold\nnanoparticles. Intermolecular interactions between the analyte, solvent,\nand surface are modulated by changing the solution composition to\nunderstand how THF facilitates the SERS detection of aspirin in THF–water\ncosolvents. SERS signals for 5 mM aspirin arise only in the presence\nof THF at and above 60 mM, while no signal with or without THF below\n60 mM is observed. SERS detection of aspirin is hypothesized to depend\non THF forming a hydrogen-bonded complex with aspirin that reduces\naspirin hydrophobicity, thus stabilizing the acid form of the molecule\nand allowing it to weakly interact with the gold nanoparticles. The\naspirin–THF complex adsorbs to the gold surface through π-orbital\noverlap between the aromatic ring and gold, where additional THF weakens\norbital overlap. Understanding the mechanism by which organic cosolvents\nfacilitate the SERS detection of nonthiolated analytes such as aspirin,\nin aqueous media, allows other cosolvents, nonthiolated analytes,\nand other surfaces to obtain a SERS signal in a variety of systems.