Carbon dots as artificial peroxidases for analytical applications

Abstract

Nanozymes have become attractive in analytical and biomedical fields, mainly because of their low cost, long shelf life, and less environmental sensitivity. Particularly, nanozymes formed from nanomaterials having high surface area and rich active sites are interesting since their activities can be tuned through carefully controlling their size, morphology, and surface properties. This review article focuses on preparation of carbon dots (C dots) possessing peroxidase-like activity and their analytical applications. We highlight the important roles of the oxidation states and surface residues of C dots and their nanocomposites with metal, metal oxides, or metal sulfides playing on determining their specificity and sensitivity toward H₂O₂. Examples of C dot nanozymes (CDzymes) for developing sensitive and selective absorption, fluorescence, and electrochemical sensing systems in the presence of substrates are presented to show their potential in analytical applications. For example, CDzymes couple with glucose oxidase and cholesterol oxidase are specific and sensitive for quantitation of glucose and cholesterol, separately, when using 3,3',5,5'-tetrame-thylbenzidine as the signal probe. This review article concludes with possible strategies for enhancing and tuning the catalytic activity of CDzymes.

Fig. 1

(A) Schematic illustration of oxidation color reaction of TMB with H₂O₂ catalyzed by C dots. (B) Time-dependent absorbance changes at 652 nm of TMB in different reaction systems: (a) C dots + TMB, (b) TMB + H₂O₂ and (c) TMB + C dots + H₂O₂ in a pH 3.5 NaAc buffer (0.2 M) at 35°C. C dots are presented in C-Dots in the figure. Reproduced from Ref. [83] with permission from The Royal Society of Chemistry.

Fig. 2

Schematic presentation of the colorimetric method for glucose detection. GOx (glucose oxidase), O₂ (oxygen), H₂O₂ (hydrogen peroxide), HNO₃ (nitric acid), C dots, KI (potassium iodide), I₂ (iodine). C dots are presented in CQD in the figure. Reproduced from Ref. [86] with permission from Springer Nature.

Fig. 3

Schematic illustration of oxidation color reaction of TMB with H₂O₂ catalyzed by C dots. C dots are presented in GQDs in the figure. Reproduced from Ref. [89] with permission from Elsevier.

Fig. 4

Schematic representation of oxidation of TMB by N-doped C dots and colorimetric detection of AA in a real sample. C dots are presented in N-CQDs in the figure. Reprinted with permission from Ref. [99]. Copyright (2019) American Chemical Society.

Fig. 5

Schematic representation of a colorimetric turn-on assay for mercury ion detection. Reproduced from Ref. [101] with permission from Elsevier.

Fig. 6

Schematic illustration of a chemiluminescence sensor for detection of total cholesterol. C dots are presented in graphene quantum dots in the figure. Reproduced from Ref. [110] with permission from Elsevier.

Fig. 7

Sensing mechanism for the C dots/CuO nanocomposites–H₂O₂–TMB system. C dots are presented in GQDs in the figure. Reproduced from Ref. [113] with permission from Elsevier.

Fig. 8

Schematic illustration of ZnFe₂O₄/C dots as a mimicking trace label for electrochemical detection of DNA. C dots are presented in GQDs in the figure. Reproduced from Ref. [115] with permission from Elsevier.