Enhanced peroxidase-like activity of L-glutamic acid-coated Fe3O4 quantum dots for colorimetric detection of L-cysteine in serum and milk

A novel colorimetric sensing platform was developed for rapid detection of L-cysteine (L-Cys) in human serum and milk samples, utilizing L-glutamic acid-coated Fe 3 O 4 quantum dots (Glu-Fe 3 O 4 QDs) with enhanced peroxidase-like activity. The Glu-Fe 3 O 4 QDs were synthesized via a two-step hydrothermal method, where L-glutamic acid modification significantly improved both catalytic performance and stability compared to unmodified Fe 3 O 4 nanoparticles. Characterization by TEM, XPS, and FTIR confirmed the ultrasmall size (2.57 ± 0.78 nm) and successful surface functionalization with carboxyl/amino groups. The nanozymes catalyzed the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H 2 O 2 , generating a blue color change measurable at 652 nm. L-Cys suppressed this reaction by competitively consuming hydroxyl radicals, enabling quantitative detection with a linear range of 5–50 µM (R 2  = 0.997) and a low detection limit of 0.73 µM. The method exhibited high selectivity against interfering amino acids and ions, and achieved recoveries of 96.99–107.59% in spiked serum and milk samples with RSDs < 3.86%. This work provides a simple, cost-effective tool for monitoring L-Cys in complex biological matrices, with potential applications in food safety and clinical diagnostics. Graphical abstract The alternative text for this image may have been generated using AI. Ultrasmall L-glutamic acid-coated Fe3O4 quantum dots (Glu-Fe3O4 QDs) were synthesized, exhibiting enhanced peroxidase-like activity that catalyzes TMB-H2O2 chromogenic reaction. The system enables rapid colorimetric detection of L-cysteine in human serum and milk with high selectivity and a detection limit of 0.73 μM