Impact of Gold Nanoparticle Stabilizing Ligands on the Colloidal Catalytic Reduction of 4-Nitrophenol

Gold nanoparticles (AuNPs) have received considerable interest owing to their unique properties and applications in catalysis. One of the major challenges for colloidal nanoparticles in catalysis is the limited stability and resulting aggregation. Nanoparticle functionalization with ligands or polymers is a common strategy to improve the colloidal stability, which in turn blocks the reactive surface sites and eliminates catalytic activity. Here, we investigate thiolated polyethylene glycol (HS-PEG) as a stabilizing ligand during AuNP catalytic reduction of 4-nitrophenol. We show a direct relationship between the chain length and packing density of HS-PEG with respect to AuNP catalytic activity. High surface coverage of low molecular weight HS-PEG (1 kDa) completely inhibited the catalytic activity of AuNPs. Increasing HS-PEG molecular weight and decreasing surface coverage was found to correlate directly with increasing rate constants and decreasing induction time. Time-resolved UV-vis absorbance spectroscopy of 2-mercaptobenzimidazole (2-MIB) adsorption on AuNPs was used to study the ligand adsorption kinetics and to quantify the free active sites available for catalysis as a function of HS-PEG molecular weight and packing density. HS-PEG packing density and estimation of free active sites, coupled with the kinetics of 2-MBI adsorption onto AuNP ruled out the possibility of an educt diffusion barrier as the main cause of reduced catalytic activity and induction time for HS-PEG functionalized AuNPs (molecular weight >= 1 kDa). Instead, selective blocking of more active sites by adsorbed thiol functionality is attributed to the induction period and reduced catalytic activity. It is also noticed that H- induced desorption/mobility of thiols regenerates the catalytic activity.