# Carbon Nanodots as Peroxidase Nanozymes for Biosensing
## 6 GQDs/C-Dots Conjugates and/or Nanocomposites as POD Nanozymes
In another work, [PERSON] and co-workers [55] employed positively charged NiAl-layered double hydroxide (NiAl-LDH) nanoplates for electrostatic self-assembly with negatively charged C-dots. Compared to the bare C-dots and NiAl-LDH, self-assembled C-dots/NiAl-LDH exhibited a POD-like activity, producing a color reaction of H\({}_{2}\)O\({}_{2}\)-TMB system. When exposed to different conditions (solvents, temperature), C-dots/NiAl-LDH showed better stability than native HRP and could be used for colorimetric detection of H\({}_{2}\)O\({}_{2}\) with a detection limit of 0.11 uM. Furthermore, the practical utility of this assay was established for the determination of H\({}_{2}\)O\({}_{2}\) in milk samples. Intrigued by their early work [53], [PERSON], [PERSON] and co-workers integrated GQDs with Fe\({}_{3}\)O\({}_{4}\) NPs via a one-step co-precipitation approach [56]. The as-prepared GQDs-Fe\({}_{3}\)O\({}_{4}\) composites exhibited excellent POD-like activities than those of individual GQDs, individual Fe\({}_{3}\)O\({}_{4}\) NPs, and the composites  
Figure 6: **(A)** Neat carbon nanodots (C-dots) did not display any catalytic activity, as evidenced by the lack of green color in **(Ai)**; while native cytochrome \(c\) (cyt _c_) was found to exhibit weak peroxidase activity **(Aii)**; When C-dots were combined with cyt \(c\), the peroxidase activity was dramatically enhanced, indicated by the rapid formation of the oxidized form of ABTS (i.e., appearance of intense green color), as shown in **(Aiii)**; **(B)** Different orientations of the electrostatic calculations performed on cyt \(c\) show that a majority of the protein surface is positively charged (blue areas), with the bulk of the positive charge residing in the vicinity of the heme group. Reproduced from [54]. of micrometer-sized GO and Fe\({}_{3}\)O\({}_{4}\) NPs. The excellent POD-like activities of GQDs-Fe\({}_{3}\)O\({}_{4}\) composites were attributed to the unique properties of GQDs and the synergistic interactions between the GQDs and Fe\({}_{3}\)O\({}_{4}\) NPs. Compared to native HRP, the GQDs-Fe\({}_{3}\)O\({}_{4}\) composite as a catalyst showed better or comparable removal efficiencies for some phenolic compounds from aqueous solutions, with higher stability and reusability. Owing to its superior physical properties in conjunction with catalytic activity, GQDs-Fe\({}_{3}\)O\({}_{4}\) nanocomposite holds great promise for industrial wastewater treatment. [PERSON] and co-workers [57] synthesized a ZnFe\({}_{2}\)O\({}_{4}\)-GQDs nanocomposite through a photo-Fenton reaction and demonstrated its POD-like activity in conjunction with HRP, graphene, CNTs, hemigraphene hybrid, and graphene supported ferric porphyrin. The ZnFe\({}_{2}\)O\({}_{4}\)-GQDs nanocomposite was found to possess higher POD-like activity than each individual, which was attributed to the synergistic effect of the GQDs and ZnFe\({}_{2}\)O\({}_{4}\). Using the ZnFe\({}_{2}\)O\({}_{4}\)-GQDs as a trace, an electrochemical sensor was fabricated to detect DNA. Under the optimal conditions, the approach provided a wide linear range from 10\({}^{-16}\) to 5 \(\times\) 10\({}^{-9}\) M and low detection limit of 6.2 \(\times\) 10\({}^{-17}\) M. As a novel trace label mimic for electrochemical biosensing, ZnFe\({}_{2}\)O\({}_{4}\)-GQDs nanocomposite can be widely used to replace conventional POD-based assays for the identification of other target molecules. Pt-C-dots nanocomposite constitute the another example of the effective POD mimic based on the synergistic effects between C-dots and Pt NPs [58]. The catalytic efficiency of this composite was found nine and five times higher than those of bare C-dots and Pt NPs, respectively. Mechanistically, the oxidation of TMB was contributed to the production of active oxygen species, in particular, hydroxyl radical by H\({}_{2}\)O\({}_{2}\) decomposition.