Catalysis by Au@pNIPAM Nanocomposites: Effect of the Cross-Linking Density

Gold nanoparticles encapsulated in a thermoresponsive microgel (pNIPAM) were used as catalysts in the electron-transfer reaction between hexacyanoferrate(III) and borohydride ions. The thermosensitive pNIPAM network can act as a "nanogate" that can be opened or closed to a certain extent, thereby controlling the diffusion of reactants toward the catalytic core. Interestingly, the crosslinker density plays an important role, because it defines the thermal response of the Au@pNIPAM system and, in turn, the extent of the volume change and therefore the polymeric density. The catalytic activity of the encapsulated gold nanoparticles is thus affected both by temperature and by the composition of the shell. A mathematical model reproducing the key features of the temperature-controlled catalysis by our thermosensitive nanoparticles confirms the effect of diffusion rate through the shell on the actual reaction rate.