Tetrabutylammonium-chloride-glycerol of deep eutectic solvent functionalized MnO: a novel mimic enzyme for the quantitative and qualitative colorimetric detection of L-cysteine.

L-Cysteine is a common amino acid that plays an important role in human livelihood and production. Therefore, a novel method for the simultaneous quantitative and qualitative determination of L-cysteine by a colorimetric detection system is proposed. As a viable oxidase mimic, [N]Cl-G/MnO, which consisted of MnO nanosheets functionalized by a tetrabutylammonium chloride-glycerol ([N]Cl-G) based deep eutectic solvent (DES) was fabricated. Owing to the oxidation of MnO nanosheets, [N]Cl-G/MnO could oxidize the colorless 3,3',5,5'-tetramethylbenzidine (TMB) into a blue product (oxTMB) with the characteristic UV-vis spectrum absorbance at 652 nm. The oxidation of TMB by DES/MnO was inhibited when L-cysteine was introduced, and the absorbance decreased proportionally with the increase in L-cysteine concentration. Due to this inhibition effect, a colorimetric detection system ([N]Cl-G/MnO-TMB) was developed for the quantitative determination of L-cysteine. Under optimal conditions, the assay showed good linearity over the concentration range of 0.125-2.00 μg mL with a low detection limit of 5.96 ng mL. A study of the inhibition mechanism demonstrated that the sulfhydryl group of L-cysteine could decompose [N]Cl-G/MnO into Mn, thus limiting the conversion of TMB to oxTMB. In addition, the [N]Cl-G/MnO-TMB system was used in test strips for the visual qualitative detection of L-cysteine. The selectivity and test strip results demonstrated the high selectivity, simple operation, and rapid response of the [N]Cl-G/MnO-TMB system for the qualitative detection of L-cysteine. Given the satisfying performance of the detection strategy, colorimetric sensing based on the [N]Cl-G/MnO-TMB system is considered to have prospective application value in the quantitative and qualitative detection of L-cysteine.