Fe2O3–MgO–CuO@MWCNT Nanocomposite for Highly Sensitive Electrochemical Detection of l -Cysteine at Low Potential in Biological Samples

A Fe2O3–MgO–CuO composite integrated with multiwalled carbon nanotubes (MWCNTs) was employed to develop a platform for the indirect electrochemical detection of l-cysteine (l-CYS). The material was characterized by XRD, FTIR, SEM, EDS, and EIS, confirming the formation of a hybrid nanostructure. After optimization of the experimental parameters, the modified glassy carbon electrode exhibited a well-defined cathodic response at −0.25 V vs Ag/AgCl (saturated KCl). Although l-CYS undergoes electrochemical oxidation at positive potentials (approximately +0.25 V), quantitative analysis was based on the effect of the presence of l-CYS on the cathodic response of the modified electrode. The determination of the analyte was performed using linear sweep voltammetry, yielding two linear response ranges of 0.5–100 μmol L–1 (R2 = 0.992) and 100–1000 μmol L–1 (R2 = 0.996). The lowest detectable concentration of l-CYS with the Fe2O3–MgO–CuO@MWCNT/GCE electrode was 0.5 μmol L–1. The sensor exhibited high selectivity, excellent repeatability (RSD = 1.58%), and reproducibility (RSD = 1.82%). The sensor was evaluated for l-CYS determination in urine and breast milk samples, providing satisfactory recovery values.