Structure and activity of apoferritin-stabilized gold nanoparticles

A simple method for synthesizing gold nanoparticles stabilized by horse spleen apoferritin (HSAF) is reported using NaBH4 or 3-(N  -morpholino)propanesulfonic acid (MOPS) as the reducing agent. AuCl4- reduction by NaBH4 was complete within a few seconds, whereas reduction by MOPS was much slower; in all cases, protein was required during reduction to keep the gold particles in aqueous solution. Transmission electron microscopy (TEM) showed that the gold nanoparticles were associated with the outer surface of the protein. The average particle diameters were 3.6 and 15.4 nm for NaBH4-reduced and MOPS-reduced Au-HSAF, respectively. A 5-nm difference in the UV–Vis absorption maximum was observed for NaBH4-reduced (530 nm) and MOPS-reduced Au-HSAF (535 nm), which was attributed to the greater size and aggregation of the MOPS-reduced gold sample. NaBH4-reduced Au-HSAF was much more effective than MOPS-reduced Au-HSAF in catalyzing the reduction of 4-nitrophenol by NaBH4, based on the greater accessibility of the NaBH4-reduced gold particle to the substrate. Rapid reduction of AuCl4- by NaBH4 was determined to result in less surface passivation by the protein. Methods for studying ferritin–gold nanoparticle assemblies may be readily applied to other protein–metal colloid systems.