Au nanoparticles on citrate-functionalized graphene nanosheets with a high peroxidase-like performance

In this paper, Au nanoparticles (AuNPs) have been homogeneously deposited on citrate-functionalized graphene nanosheets (Cit-GNs) by a simple one-pot reducing method. The morphology and composition of the thus-prepared AuNPs/Cit-GNs were characterized by transmission electron microscopy (TEM), high resolution TEM, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The results showed that the AuNPs with a uniform size are well dispersed on the surface of the Cit-GNs. Significantly, the as-prepared AuNPs/Cit-GNs possess intrinsic peroxidase-like activity, which can catalyze the oxidation of the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) by hydrogen peroxide (H2O2) to develop a blue color in aqueous solution. The catalysis was in accordance with Michaelis-Menten kinetics and the AuNPs/Cit-GNs showed a strong affinity for both H2O2 and TMB. Moreover, by comparing with Cit-AuNPs, AuNPs/GNs and AuNPs/PVP-GNs, the AuNPs/Cit-GNs composite exhibits a higher catalytic ability with a lower Michaelis constant (Km) value, suggesting that the GNs with a large surface area and the citrate ions with more carboxyl groups around the AuNPs can greatly enhance the peroxidase-like activity of AuNPs/Cit-GNs. Taking the advantages of the high catalytic activity, the good stability and the low cost, the novel AuNPs/Cit-GNs represent a promising candidate as an enzyme mimic and may find a wide range of new applications in biochemistry and biotechnology.