Modifying Ligand Chemistry To Enhance Reusability of pH-Responsive Colloidal Gold Nanoparticle Catalyst

Thiolated pH-responsive poly(acrylic acid) functionalized gold nanoparticles (AuNP-SPAA) were investigated as recoverable and reusable catalysts in catalyzing the 4-nitrophenol reduction reaction. The thiolated PAA (PAA-SH) functionality enabled successful catalyst recovery via two different methods: (1) pH-triggered aggregation/redispersion and (2) pH-triggered phase transfer to organic solvent from aqueous media via a phase transfer agent. Confirmed by small angle neutron scattering, both small and large molecular weight polymers with randomly distributed thiol groups exhibited a compact train–loop–tail conformation on the AuNP surface (unlike “brush” or “mushroom” conformations in cases of end-functionalized thiolated polymers), providing multiple anchor sites on the AuNP surface for a single polymer chain. AuNP surface accessibility by reactants, active site blockage, and catalyst recoverability were greatly affected by the polymer thiol content. Polymer functionalized AuNP catalysts with low packing density (low thiol content) demonstrated enhanced catalytic rate, however at the cost of catalyst reusability. Increasing the number of thiol moieties on the polymer backbone (anchor sites/train segments) greatly increased catalyst robustness under harsh reducing environment. Catalytic activity loss was attributed to reducing agent induced ligand desorption from the AuNP surface leading to permanent catalyst aggregation.