A label-free impedimetric immunosensor based on nitrogen-doped graphene acid for sensitive detection of vitamin D₃

A simple metal-free electrochemical immunosensor based on nitrogen-doped graphene acid (NGA) for the detection of 25-hydroxy vitamin D₃ (25-OH D₃) is presented. The use of NGA, with its well-defined chemical composition, high density of carboxyl functional groups, and excellent biocompatibility, provided a stable and efficient platform for biomolecule immobilization without the need for complex surface modifications. The proposed immunosensor demonstrated high sensitivity (1.97 kΩ ng^‒1 mL cm^‒2) and a working range of 3.96 to 48.83 ng mL^‒1, aligning with clinically relevant vitamin D levels. Moreover, the developed immunosensor exhibited excellent specificity, with minimal cross-reactivity and strong resistance to non-specific adsorption. Furthermore, it maintained stable electrochemical performance for up to 30 days, demonstrating its suitability for practical applications. Compared to existing vitamin D sensors, our platform introduces a streamlined fabrication process that avoids heavy metals, improves reproducibility, and enhances accessibility for point-of-care diagnostics.