Gold nanoparticles (Au NPs) are commonly utilized as sensors due to the sensitivity of their characteristic LSPR peak, but literature related to their use as catalysts is rapidly increasing. The LSPR peak is influenced by the microenvironment of the NPs, which can have an effect on catalytic activity as well. Colloidal Au NPs require a stabilizing ligand shell which can consist of a broad range of molecules from simple organic ligands to macromolecules. Most of the literature surrounding ligand functionalization has shown inhibitory effects, but the extent and mechanism of these effects require more analysis.  Studies are further complicated by the fact that many researchers chose to focus on a single class of ligands, avoid a direct comparison to truly bare NPs, and often used different synthetic methods to achieve various ligand shells leading to variation in NP core size and shape. Our current work focuses on the effect of ligands on the catalytic reduction of 4-nitrophenol, a commonly studied model reaction to test Au NP activity. Au NPs partially-embedded in glass slides, allowing for complete removal of surface ligands, were fabricated and modified with a wide range of ligands. Small molecule ligands showed little to no inhibition of the reaction, with activity decreasing as the size of the ligands increased. Positively-charged ligands, even long chain polymers, showed increased turnover numbers (TONs) in comparison to uncoated Au NP, a phenomenon which we are further exploring. The increase is potentially caused by favorable electrostatic interactions between the positively-charge ligand shells and the negatively-charged 4-nitrophenolate and borohydride substrates. While increased activity is a significant finding, we aim to expand our focus to selectivity for a broader scope of reactions. Modification of the surface of the Au NPs is one route to changing the microenvironment of the particles, while modification of the support on which the particles are immobilized could prove effective as well, maintaining the open active sites on the surface of the gold.

Effect of microenvironmental changes on catalytic activity of gold nanostructures

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ACS Fall 2022

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