Graphitic Carbon Nitride-Copper Oxide Nanocomposites with Enhanced Peroxidase-Mimicking and Electrocatalytic Activity for Vitamin B₁₂ Detection

Vitamin B12 (VB12) is essential for a variety of metabolic processes. Its lack and excess can both cause serious disorders. Given the increased reliance on VB12 supplements, creating a precise and quick point-of-care VB12 sensor is critical. In this investigation, graphitic carbon nitride and copper oxide composite (gCN.CuO) is used to develop a dual-mode sensor for the colorimetric and voltammetric detection of VB12. First, gCN.CuO is employed as a peroxidase-mimetic nanozyme to catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB). The addition of VB12 is shown to quench the UV–vis band corresponding to oxTMB at 652 nm due to specific interactions between VB12 and gCN.CuO. Based on the same principle, the indirect colorimetric sensing system for VB12 has been developed. Furthermore, a direct electrochemical sensor based on a gCN.CuO-modified screen-printed electrode (gCN.CuO|SPE) for chemical-free detection of VB12 is proposed. The gCN.CuO|SPE demonstrated remarkable electrocatalytic activity toward VB12, showcasing a 3.5x higher peak current with ∼25 mV negative potential shift. With a 50 μL sample requirement, the electrochemical sensor exerted excellent sensitivity with an extensive linear range (1–400 μM) and an LOD of 159.5 nM. The gCN.CuO|SPE selectively detected VB12 in the presence of various potential interferents and real samples demonstrating potential real-life applicability.