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

Abstract

Gold nanoparticles (AuNPs) have received\nconsiderable interest\nowing to their unique properties and applications in catalysis. One\nof the major challenges for colloidal nanoparticles in catalysis is\nthe limited stability and resulting aggregation. Nanoparticle functionalization\nwith ligands or polymers is a common strategy to improve the colloidal\nstability, which in turn blocks the reactive surface sites and eliminates\ncatalytic activity. Here, we investigate thiolated polyethylene glycol\n(HS-PEG) as a stabilizing ligand during AuNP catalytic reduction of\n4-nitrophenol. We show a direct relationship between the chain length\nand packing density of HS-PEG with respect to AuNP catalytic activity.\nHigh surface coverage of low molecular weight HS-PEG (1 kDa) completely\ninhibited the catalytic activity of AuNPs. Increasing HS-PEG molecular\nweight and decreasing surface coverage was found to correlate directly\nwith increasing rate constants and decreasing induction time. Time-resolved\nUV–vis absorbance spectroscopy of 2-mercapto­benz­imidazole\n(2-MIB) adsorption on AuNPs was used to study the ligand adsorption\nkinetics and to quantify the free active sites available for catalysis\nas a function of HS-PEG molecular weight and packing density. HS-PEG\npacking density and estimation of free active sites, coupled with\nthe kinetics of 2-MBI adsorption onto AuNP ruled out the possibility\nof an educt diffusion barrier as the main cause of reduced catalytic\nactivity and induction time for HS-PEG functionalized AuNPs (molecular\nweight ≥1 kDa). Instead, selective blocking of more active\nsites by adsorbed thiol functionality is attributed to the induction\nperiod and reduced catalytic activity. It is also noticed that H^– induced desorption/mobility of thiols regenerates\nthe catalytic activity.