Fig. 1 UV-Vis absorption spectra and SEM (A), X-ray diffraction (B), FTIR spectra (C) and silver ion release concentration (concentration of AgNPs is 10 mg/L) (D) of AgNPs.
Peroxidase-like activity of AgNPs
The peroxidase-like activity of the AgNPs nanozyme was assessed by monitoring the oxidation of TMB by H2O2. As shown in Fig. 2, several solutions containing different components were subjected to using UV/visible absorption spectroscopy measurements. The solutions containing TMB (a), TMB+ R12-AgNPs (b) and TMB+ C-AgNPs (c) showed no peak at 652 nm associated with ox-TMB absorption. In the case of both groups of TMB + C-AgNPs + H2O2 (d) and TMB + H2O2 (e), the TMB oxidation products appeared slightly blue with weak adsorption bonds at 652 nm, signifying the limited catalytic potential for C-AgNPs and a slow oxidation of TMB in the presence of H2O2, respectively. In contrast, the combinations of R12-AgNPs + TMB + H2O2 (f) exhibited a strong blue color with the highest absorbance at 652 nm (Fig. 2), which indicated the peroxidase-like catalytic activity of R12-AgNPs (Eq. 3). However, the catalytic mechanism remains unclear. Sun et al. deactivated the hydroxy groups, carboxylic groups, or ketonic carbonyl groups on purpose and assessed the catalytic activities of these derivatives of graphene quantum dots (GQD). Their findings suggested that the –C=O groups served as the catalytically active sites, while the O=C–O– groups acted as substrate-binding sites [42]. It can therefore be inferred that the peroxidase-like activity of R12-AgNPs may be attributed to the presence of the additional functional groups on their surface (Fig. 1C).
H2O2+TMBH2O+ox-TMB (3)