Improving peroxidase activity of gold nanorod nanozymes by dragging substrates to the catalysis sites via cysteine modification

Surface chemistry control is a key means to improve substrate selectivity and enhance catalytic activity of nanozymes, a kind of novel artificial enzymes. Herein, we demonstrated that apart from chemical properties of functional groups, their spatial distance to the catalytic sites is also very important to improve the catalytic performance of nanozymes. Using cetyltrimethylammonium bromide (CTAB) coated gold nanorods (AuNR) as the example, we showed that cysteine (Cys) surface modification can greatly enhance the peroxidase activity of AuNR for the oxidation of substrate 3,3',5,5'-tetramethylbenzidine (TMB). By using cysteine derivatives, the key role of the carboxylic group in cysteine is revealed in improving substrate binding and activity enhancement. The electrostatic interactions of carboxylic groups from adsorbed cysteine molecules with protonated amino groups of TMB bring TMB molecules to the surface Au active sites and thus markedly increase catalytic activity. In contrast, despite having two carboxylic groups, glutathione (GSH) surface modification only leads to quite limited improvement of catalytic activity. We speculated that due to large molecular size of GSH, the spatial distance between TMB-GSH and Au is larger than that between TMB-Cys and Au. Furthermore, Raman characterization indicated that at high Cys coverage, they form patches on rod surface via zwitterionic interactions, which may give additional benefits by decreasing the steric hindrance of TMB diffusion to surface Au atom sites. In all, our study highlights the importance of fine surface tuning in the design of nanozymes.