Impacts Of Ph And Intermolecular Interactions On Surface-Enhanced Raman Scattering Chemical Enhancements

Surface-enhanced Raman scattering (SERS) is a surface sensitive technique that reveals information regarding molecular adsorption driving forces at nanoparticles surfaces. While the plasmonic properties of SERS substrates provide the largest signal enhancements, chemical enhancement mechanisms are more sensitive to molecular adsorption and intermolecular interactions. Herein, gold coated silver nano particles that are stabilized inside microporous silica membranes are used for monitoring short-range chemical enhancement effects. First, the silica membrane provides plasmonic stability while 2 also facilitating kinetic measurements so that impacts of molecular protonation, molecule-molecule interactions, molecule-silica interactions, and molecule-Au interactions can be identified. To do this, the vibrational frequencies of 4-mercaptobenzoic acid (4-MBA) are monitored as a function of time and pH. Applying Fick's second law to time-dependent responses reveals that molecular flux decreases with increasing pH. SERS spectra suggest that the kinetics of this phenomenon depend on the protonation state of 4-MBA and, hence, the energy required for the molecules to pass through the negatively charged silica membrane. Namely, repulsive electrostatic interactions between deprotonated molecules (R-COO-) and the silica shell increase the energy required for transport, which subsequently decreases the flux of molecules through the silica shell and subsequent adsorption to the metal surface. As pH approaches neutral conditions, the fraction of deprotonated 4-MBA increases. These molecules, which have a higher electron density in the aromatic rings versus protonated ones, favor selective chemical enhancement of the asymmetric versus symmetric C-C stretching modes. In addition, increasing intermolecular interactions between adsorbed molecules promote electron delocalization from aromatic rings to the carboxylate groups of 4-MBA. This response causes the pK(a) of the carboxylate to gradually increase from 4.8 (in solution) to 7.7 (on nanoparticle surfaces). Consequently, SERS signals for this molecule can be understood with respect to molecular protonation state, flux, and intermolecular interactions using these electromagnetically stable plasmonic nanostructures.