To investigate the POD-like activity of d-NC nanozymes, 3, 3′, 5, 5′-tetramethylbenzidine (TMB) as the chromogenic molecule can be oxidized by the proposed nanozymes in the presence of H
2O
2 (
Fig. 3a). The resultant characteristic absorption peak at 652 nm can quantitatively reflect the POD-like activity of d-NC nanozymes. As can be seen from
Fig. 3b,
d-NC nanozymes show much higher POD-like activity than NC nanozymes. In detail, the specific activity (SA) of d-NC and NC nanozymes is 3.76 U/mg and 0.40 U/mg, respectively (
Fig. 3c). Meanwhile, it was observed that the catalytic activity of the two nanozymes reaches the maximum at pH 3.0 (Fig. S3 in Supporting information). Compared with natural enzymes horseradish peroxidase (HRP), d-NC nanozymes exhibited excellent tolerance at strong acid/basic conditions and high temperatures (Fig. S4 in Supporting information). In addition, when the annealing temperature is 900 ℃ and the amount of NH
4Cl is 5 g, d-NC nanozymes own the best POD-like activity (Fig. S5 in Supporting information). The steady-state kinetics of these nanozymes were investigated to acquire the enzymatic kinetics constants including Michaelis–Menten constant (
Km) and maximal reaction velocity (
Vmax) (Fig. S6 in Supporting information) [
45]. The
Vmax of d-NC about TMB and H
2O
2 is 7.56-fold and 8.38-fold higher than those of NC (Table S4 in Supporting information), demonstrating d-NC nanozymes possess excellent POD-like activity. To further understand the catalytic mechanism, the active intermediates were explored. Generally, it is established that hydroxyl radical (
·OH) or adsorbed oxygen is considered to be the active intermediate in the reaction catalyzed by carbon nanozymes [
20,
30]. However, as shown in
Fig. 3d and Fig. S7 (Supporting information), the special peak of
·OH was not detected by using electron paramagnetic resonance (EPR) and
p-phthalic acid (PTA) probe, demonstrating that
·OH is not the active intermediate. Moreover, other reactive oxygen species (ROS) including superoxide anion (O
2·−) and singlet oxygen (
1O
2) were not detected in the catalytic processes (Fig. S8 in Supporting information), demonstrating that ROS may not the active intermediates for the POD-like activity of d-NC nanozymes. In consequence, we speculate that adsorbed oxygen species as the active intermediates are responsible for the POD-like activity of d-NC nanozymes. Generally, the carbon atoms next to nitrogen (N-C) are considered to be the active sites of carbon materials in the catalytic progress [
46]. On the one hand, the electron structure of active carbon atoms attached to pyridine N could be further optimized, which would facilitate the desorption of H
2O in the catalytic process and thus improve the catalytic activity of carbon nanozymes [
42]. The increase in the content of pyridine N can significantly enhance the POD-like activity of d-NC nanozymes. On the other hand, edge defects play an important role in enhancing the catalytic activity of d-NC nanozymes. The rich edge defects can be capable of tuning the charge distribution of carbon atoms and affect the adsorption of intermediate products in catalytic progress [
40], which are favorable for the POD-like catalytic process. Moreover, the porous structure facilitates the exposure of active sites and the mass transfer capacity, thus boosting the POD-like activity of d-NC nanozymes.