# Carbon Nanodots as Peroxidase Nanozymes for Biosensing
## 4 Graphene Quantum Dots (GQDs) as POD Nanozymes  
The infinite graphene lattice is a non-luminescent zero-bandgap material with limited practical applications. However, due to their outstanding quantum confinement and edge effect, graphene quantum dots (GQDs) or graphene dots (edge-bound graphene pieces) exhibit bright PL with high photostability and low cytotoxicity, making them an ideal candidate for biochemical sensing and imaging [45]. [PERSON], [PERSON] and co-workers [46] synthesized GQDs by hydrothermal treatment of carbon black (Vulcan XC-72) in the presence of nitric acid. The as-prepared GQDs could effectively catalyze the TMB + H\({}_{2}\)O\({}_{2}\) system and produced blue coloration in acetate buffer (pH 4.0). By measuring the absorbance changes or monitoring the generation/consumption of H\({}_{2}\)O\({}_{2}\), the intrinsic POD-like \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Nanomaterial(s)** & **Synthesis Method** & **Substrate(s)** & **Application(s)** & **LOD** & **Reference** \\ \hline C-dots b & \multirow{2}{*}{Oxidation with HNO\({}_{3}\)} & TMB, OPD, & H\({}_{2}\)O\({}_{2}\) detection & 0.2 uM & [36] \\  & & and THB & Glucose detection & 0.4 uM & [37] \\ \hline \multirow{2}{*}{CNPs c} & \multirow{2}{*}{Oxidation with HNO\({}_{3}\)} & TMB and & H\({}_{2}\)O\({}_{2}\) detection & 1.0 uM & [37] \\  & & DAB & Glucose detection & 20.0 uM & [38] \\ \hline C-dots & Electrochemical & – & Degradation of MO azo dye & – & [38] \\ \hline CDs d & MW-IL method & – & Degradation of MO and MR azo dyes & – & [39] \\ \hline CDs d & Hydrothermal & TMB & GSH detection & 0.3 uM & [40] \\ \hline CDs d & Hydrothermal & TMB & H\({}_{2}\)\({}^{\text{a}}\) detection & 23 nM & [41] \\ \hline \multirow{2}{*}{CDs d} & \multirow{2}{*}{Hydrothermal} & TMB & H\({}_{2}\)O\({}_{2}\) detection & 0.6 uM & [42] \\  & & Glucose detection & 5.2 uM & [43] \\ \hline \multirow{2}{*}{CDs d} & \multirow{2}{*}{Calcination followed by oxidation with HNO\({}_{3}\)} & \multirow{2}{*}{TMB} & H\({}_{2}\)O\({}_{2}\) detection & 1.0 uM & \\  & & oxidation with HNO\({}_{3}\) & & Fe\({}^{3+}\) detection & 0.8 uM & [43] \\  & & Ag\({}^{+}\) detection & 0.5 uM & \\ \hline r-CDs d & Oxidation with HNO\({}_{3}\) & TMB & Glucose detection & 2.0 uM & [44] \\ \hline \end{tabular}  
* C-dots, Carbon nanodots; LOD, Limit of detection; TMB, 3,3,5,5,5-tetramethylbenzidine; OPD, \(\alpha\)-phenylenediamine; THB, 1,2,3-trihydroxybenzene; CNPs, Carbon nanoparticles; DAB, Diazoaminobenzene; MW-IL, Microwave-assisted ionic liquid; GSH, Glutathione; b Also exhibit catalase-mimicking activity; c SNPs represent C-dots as per ref. [37]; d CDs represent C-dots as per refs. [39; 40; 41; 42; 43; 44].  
\end{table} 
Table 1: POD-like activity of carbon nanodots. a activity of as-prepared GQDs was used in the detection of H\({}_{2}\)O\({}_{2}\), glucose, and GSH with detection limits of 10 nM, 0.5 uM, and 0.5 uM, respectively. Furthermore, GQDs exhibited high selectivity and was found capable of sensing in complicated biological samples such as cell lysate. The relatively high catalytic activity of GQDs than that of large-sized GO can be attributed to their high diffusion rate and excellent ability to combine with biomolecules. [PERSON] and co-workers [47] have reported a one-step wet chemical method to generate GQDs directly from graphite. Using TMB as a substrate, the POD-like activity of as-synthesized GQDs was used to detect free cholesterol in human serum with a detection limit of 6 uM (Scheme 2). In an elegant work, [PERSON] and co-workers [48] have recently provided new insights into the high enzymatic activity of GQDs that may allow one to add critical details to existing catalytic mechanisms with higher confidence. In particular, three GQD derivatives were synthesized through chemical reactions between GQDs and phenyl hydrazine, benzoic anhydride, and 2-bromo-1-phenylethanoneone as the selective titrants or deactivation agents to respectively react with the ketonic carbonyl, hydroxyl, and carboxylic groups on the GQD's surface.