Molecular dynamics characterization of silver colloidal interfaces for SERS applications. Gallic acid test

Abstract

One of the most useful applications of silver colloidal solutions is in surface-enhanced Raman spectroscopy (SERS), because the amplification factor of about 10(6) allows the vibrational study and detection of highly diluted species in aqueous environment, and more recently in early diagnosis of cancer and imaging. A useful colloid for SERS is that reported by Leopold and Lendl (Colloid1). However, SERS response from anions or rich electron density molecules has been difficult to obtain in this colloid. Recently, a minor modification of the surface charge density (Colloid 2) allowed to observe reproducible SERS spectrum from gallate anion (GA(-)). In this work, the structure of both solid and solution interfaces were characterized using molecular dynamics. Experimental values of -potentials were reproduced by simulations, and the chemical potential of GA(-) approaching both interfaces was calculated using umbrella sampling and the weighted histograms analisys methodology (WHAM). The calculated barrier to approach the interface of Colloid 1 is 2.8kJmol(-1) greater than in Colloid 2, and the stability of GA(-) with Colloid 2 at the minimum is 3.5kJmol(-1) more stable than with Colloid 1. Finally, the calculated average orientation of GA(-) adsorbed onto the colloidal surface is in excellent agreement with the experimental SERS observations.