Determination of the Binding Affinity, Packing, and Conformation of Thiolate and Thione Ligands on Gold Nanoparticles

Determination of the ligand conformation on gold nanoparticle (AuNP) is of fundamental importance in nanoparticle research and applications. Using a combination of surface-enhanced Raman spectroscopy (SERS), density function calculation, and normal Raman spectroscopy, the pH dependence of mercaptobenzimadazole (MBI) adsorption onto AuNP was systematically studied. Structures and conformations of MBI adsorbates on AuNP were determined together with their binding constants, and saturation packing densities were determined at three different pHs (1.4, 7.9, and 12.5). While MBI thione is the predominant tautomer in solution with a pH value lower than 10.3, MBI thiolate is the main adsorbate on AuNP surface in solution with pH > 2. MBI thiones dominate the AuNP surface only in solutions with pH < 2. While MBI thione has a higher saturation packing density (similar to 632 pmol/cm(2)) than MBI thiolate (similar to 540 pmol/cm(2)), its binding constant (2.14 x 10(6) M-1) is about five times smaller than that for MBI thiolate (10.12 x 10(6) M-1). Using the MBI footprint deduced from its saturation packing density on AuNP, the conformation of MBI was determined. While the MBI thione binds monodentately to the AuNP with a perfectly upright orientation, MBI thiolate binds bidentately to AuNP with a tilt angle that allows interaction of AuNP with both the sulfur and the nitrogen atoms in MBI thiolate. In addition to the new insights provided on MBI binding onto gold nanoparticle, the methodology employed in this study can be particularly useful for studying AuNP interactions with other imidazole-thiol compounds, a class of heterocylic compounds that can exist in different tautomeric forms.