Carbon Dots in Enantioselective Sensing

Abstract

Chirality has a crucial effect on clinical, chemical and biological research since most bioactive compounds are chiral in the natural world. It is thus important to evaluate the enantiomeric ratio (or the enantiopurity) of the selected chiral analytes. To this purpose, fluorescence and electrochemical sensors, in which a chiral modifier is present, are reported in the literature. In this review, fluorescence and electrochemical sensors for enantiorecognition, in which chiral carbon dots (CDs) are used, are reported. Chiral CDs are a novel zero-dimensional carbon-based nanomaterial with a graphitic or amorphous carbon core and a chiral surface. They are nanoparticles with a high surface-to-volume ratio and good conductivity. Moreover, they have the advantages of good biocompatibility, multi-color emission, good conductivity and easy surface functionalization. Their exploitation in enantioselective sensing is the object of this review, in which several examples of fluorescent and electrochemical sensors, containing chiral CDs, are analyzed and discussed. A brief introduction to the most common synthetic procedures of chiral CDs is also reported, evidencing strengths and weaknesses. Finally, consideration concerning the potential challenges and future opportunities for the application of chiral CDs to the enantioselective sensing world are outlined.

Keywords: chiral carbon dots; electrochemical sensors; enantiorecognition; fluorescence sensors.

Figure 1

Schematic illustration of carbon quantum dots (CQDs) preparation via bottom-up and top-down approaches. Reproduced with permission from ref. [28]. Copyright 2017, Royal Society of Chemistry.

Figure 2

Schematic illustration of chiral carbon quantum dots (CQDs) preparation by hydrothermal method. (Left): using only one starting material. (Right): using two starting materials. Reproduced with permission from ref. [30]. Copyright 2016, Royal Society of Chemistry.

Figure 3

Schematic illustration of post-functionalization of carbon dots. Reproduced with permission from ref. [31]. Copyright 2023, Wiley-VCH GmbH.

Figure 4

Schematic illustration of chiral carbon quantum dots (CQDs) preparation by chiral post-functionalization of electrochemically synthesized CDs from ethanol. Reproduced with permission from ref. [32]. Copyright 2022, MDPI.

Figure 5

Fluorescent (A) and colorimetric (B) chiral recognition ability of TCDs. Reproduced with permission from ref. [33]. Copyright 2023, American Chemical Society.

Figure 6

Gd-doped carbon dots as fluorescent/colorimetric dual-mode sensor for D-Glu recognition. Reproduced with permission from ref. [34]. Copyright 2023, Elsevier.

Figure 7

Fluorescence response of Gd-doped CDs to various amino acids (c) and different relationship concentration/fluorescence emission for D/L-Glu (d). Reproduced with permission from ref. [34]. Copyright 2023, Elsevier.

Figure 8

Fluorescence response of NADES-derived CDs to various amino acids. Reproduced with permission from ref. [35]. Copyright 2022, Royal Society of Chemistry.

Figure 9

Proposed mechanism for the enantioselective determination of D/L-Leu. Reproduced with permission from ref. [36]. Copyright 2021, Elsevier.

Figure 10

Schematic illustration of chiral CDs fabrication used for L-Lys assay in the presence of Sn²⁺. Reproduced with permission from ref. [39]. Copyright 2020, Elsevier.

Figure 11

Different fluorescence response of chiral CDs to amino alcohols enantiomers. Reproduced with permission from ref. [40]. Copyright 2020, Royal Society of Chemistry.

Figure 12

Schematic illustration of enantioselective quenching due to S-ABO. Reproduced with permission from ref. [40]. Copyright 2020, Royal Society of Chemistry.

Figure 13

Schematic illustration of chiral CDs synthesis and enantioselective fluorescent emission enhancement in the presence of L-Lys. Reproduced with permission from ref. [41]. Copyright 2019, Elsevier.

Figure 14

(Left): linear relationship of CDs fluorescent emission with different concentrations of D- and L-folic acid. (Right): CDs fluorescence intensity ratio I_D/I_L with the addition of various chiral substrates. Reproduced with permission from ref. [43]. Copyright 2023, American Chemical Society.

Figure 15

(Left): images of optical tongue before and after exposure to amino acids (a) and corresponding colorimetric difference maps (b). (Right): 2D principal component analysis (PCA), linear discriminant analysis (LDA) (a) and hierarchical cluster analysis (HCA) plot (b) for L-, D-, racemic amino acids. Reproduced with permission from ref. [45]. Copyright 2023, Royal Society of Chemistry.

Figure 16

Schematic illustration for fabrication of a chiral sensor for tryptophan. Reproduced with permission from ref. [47]. Copyright 2015, Elsevier.

Figure 17

(Left): (a) CDs on GCE, (b) CS on GCE, (c) noncovalent CDs and CS on GCE, (d) covalent CDs–CS on GCE results for Trp enantiorecognition; (e) covalent CDs–CS on GCE results for Phe enantiorecognition; (f) covalent CDs–CS on GCE results for mandelic acid enantiorecognition; (g) covalent CDs–CS on GCE results for His enantiorecognition. (Right): CDs–CS on GCE performance on Trp enantiorecognition: (a) peak current vs. L-Trp concentration, (b) peak current vs. D-Trp concentration, (c) I_L/I_D current ratio at different concentrations, (d) linearity range. Reproduced with permission from ref. [48]. Copyright 2023, Wiley.

Figure 18

Photoelectrochemical responses for solutions of thyroxine. Reproduced with permission from ref. [49]. Copyright 2015, Elsevier.

Figure 19

(a) CVs of Tyr enantiomers at different enantiomeric excesses using CDs on MOF and (b) linear relationship between current and L-Tyr percentage. Reproduced with permission from ref. [50]. Copyright 2020, Royal Society of Chemistry.

Figure 20

Schematic illustration of (A) chiral carbon quantum dots (CDs) preparation and (B) electrode assembly for Trp enantiorecognition. Reproduced with permission from ref. [51]. Copyright 2016, MDPI.

Figure 21

Synthesis of β-cyclodextrin functionalized CDs. Reproduced with permission from ref. [52]. Copyright 2021, Elsevier.

Figure 22

DPV of Trp enantiomers solutions on (a) GCE, (b) GCE functionalized with CDs, (c) GCE functionalized with β-cyclodextrin, (d) GCE functionalized with CDs covalently bonded to β-cyclodextrin and (e) histogram of the results. Reproduced with permission from ref. [52]. Copyright 2021, Elsevier.

Figure 23

CVs of Tyr enantiomers on a GCE functionalized with β-cyclodextrin covalently bonded to graphene-derived CDs. (a): D-Tyr, (b): L-Tyr. Reproduced with permission from ref. [54]. Copyright 2017, Elsevier.

Figure 24

Linear diagram of the concentration of L-Tyr (A) and D-Tyr (B) in solution and the oxidation peak current of CV of a GCE functionalized with β-cyclodextrin covalently bonded to graphene-derived CDs. Reproduced with permission from ref. [54]. Copyright 2017, Elsevier.