Electrospun FeN@CNT single-atom nanozyme for triple-mode ROS-mediated colorimetric and fluorometric detection of vitamin C

Accurate vitamin C quantification is essential for evaluating nutritional status and supporting clinical assessment of deficiency risk, oxidative stress, and supplementation outcomes, while also enabling quality control of foods and pharmaceuticals by verifying label claims and monitoring vitamin C degradation during processing and storage. Here, we report a highly sensitive dual-mode (colorimetric/fluorometric) sensing platform based on iron–nitrogen co-doped carbon nanotubes (FeN@CNTs) prepared via electrospinning, hydrothermal treatment, and carbonization. The resulting single-atom nanozyme contains atomically dispersed Fe–Nₓ catalytic sites that emulate peroxidase-like activity and deliver strong catalytic performance. The assay exploits reactive oxygen species (ROS)-driven oxidation of a chromogenic reporter (TMB) and a fluorogenic reporter (Amplex Red), while vitamin C is quantified through its antioxidant/ROS-scavenging effect that suppresses signal formation in a concentration-dependent manner. Under optimized conditions, the platform achieves ultralow detection limits of 0.042 µM (colorimetric) and 0.003 µM (fluorometric), broad linearity up to 360 µM, and high analytical precision (RSD < 5%). A smartphone-assisted RGB readout further enables rapid, portable quantification suitable for on-site screening. The FeN@CNTs sensor demonstrates strong operational stability, high selectivity, and good tolerance toward common interferents, including biothiols and polyphenols. Practical feasibility was confirmed in pharmaceutical tablets, fruit juices, and urine, affording recoveries of 94.5–107.7% and results statistically consistent with a reference HPLC method. Collectively, these outcomes position FeN@CNTs as a robust single-atom nanozyme platform for real-time vitamin C analysis with potential impact in food safety surveillance, clinical testing, and point-of-care sensing.