Au nanoparticles (NPs) are widely used catalysts, but colloidal NPs are coated with ligands, which often inhibit the activity. Ligand effects have been studied for isolated cases, but cross-class comparisons are needed, as well a direct comparison with uncoated Au surfaces, to establish the absolute effect of ligands. We accomplished such a comparison using gold nanoisland (Au NI) slides in which Au NP were partially embedded in glass slides via thermal annealing, using them to catalyze the reduction of 4-nitrophenol by sodium borohydride. We observed both inhibitory and enhancing effects, as a function of the ligands coating the NIs. Most small molecule ligands showed similar turnover numbers to bare gold slides, due to removal of ligands by BH₄^-, as confirmed by XPS measurements. Increasing the length of alkyl thiol ligands resulted in longer ligand removal times and lower catalytic activity. We attribute this effect to strong hydrophobic interactions between the chains, creating a denser shell. When interactions are weakened by addition of a terminal carboxylic acid (in 11-mercaptoundecanoic acid), or replacing the alkyl with a tetraethyleneglycol, higher turnover was observed. Positively-charged ligand shells produced higher average turnover numbers (TONs) than the bare gold slides, likely due to electrostatic attraction between the negatively-charged 4-nitrophenolate substrate and the positively-charged surface. An exploration into positively-charged polymer ligands is needed to determine the extent of this effect. The use of gold NIs allowed for a direct comparison to bare gold, as well as facile ligand functionalization in various solvents without the risk of particle aggregation.

Extinction of reaction solutions before (black) and after catalysis by Au NI slides functionalized with alkylthiol ligands of varying chain length, monitored at 400 nm.

Ofer Kedem

Assistant Professor, Marquette University

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