Herein, we report an electrochemical sensor based on gold nanoparticle (AuNPs)-modified, Cu-assisted, chloride-terminated MXene for the selective detection of L-cysteine (L-Cys). Nb4C3Cl2 was synthesized via a fluorine-free molten salt etching approach using CuCl, avoiding the use of hazardous hydrofluoric acid. Structural characterization using X-ray diffraction (XRD) confirmed the successful aluminum removal from Nb4AlC3 and the formation of layered chloride-terminated MXene with residual Cu. AuNPs were modified onto Cu-Nb4C3Cl2 via an ultrasound-assisted method. TEM and SEM confirmed the morphological transition from highly stacked MAX phase layers to well-separated MXene layers. Electrochemical characterization using DPV, CV, and EIS demonstrated significantly enhanced electrochemical performance of the modified electrode. The Au@Cu-Nb4C3Cl2/SPE exhibited a linear response toward L-Cys over 0¿100 nM, with a detection limit of 0.51 nM and sensitivity of 7.82× 10¿8 A·nM¿1. The proposed sensor demonstrated minimal interference from common biomolecules, including uric acid, glucose, ascorbic acid and glutathione. Practical applicability was validated in diluted fetal bovine serum and undiluted human urine samples, yielding recovery values of 100.20 ± 0.6% and 99.24 ± 7.79%, respectively, with endogenous L-Cys concentrations consistent with reported physiological levels. These results establish Au@Cu-Nb4C3Cl2 as a robust and environmentally benign platform for selective electrochemical detection of L-cysteine in biological samples.
