Effect of surface modification on the peroxidase-like behaviors of carbon dots

Carbon dots (CDs) have drawn much attention in recent decades due to their outstanding biocompatibility and environmental friendliness. In this work, the surface modifications of peroxidase-like carbon dots were carried out to show the influence of surface structure on the catalytic activity. Poly(ethyleneimine) and citric acid modified CDs, i.e. PEI-CDs and CA-CDs, were obtained and used in the catalytic oxidation of two different substrates 3,3',5,5'-tetramethylbenzidine (TMB) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The catalytic activity of CA-CDs for TMB is greater than that of unmodified CDs while the catalytic activity of PEI-CDs for ABTS is greater than that of unmodified CDs. These results may be ascribed to that TMB shows stronger affinity to negatively charged CA-CDs and ABTS shows stronger affinity to positively charged PEI-CDs. These results clearly suggested that the surface charge plays an important role in the catalytic activity of nanoenzyme. Furthermore, the mechanism of peroxidase-like activity of all CDs was investigated and both the generation of hydroxyl radical and enhanced electron transfer were involved in the catalytic process.