Nitrogen-Doped GrapheneQuantum Dots (N-GQDs): A PromisingMaterial for the Development of Electrochemical Immunosensors

Electrochemical immunosensors have emerged as alternatives for the early diagnosis of diseases. The performance of such devices can be significantly improved by incorporating quantum dot materials, which enhance electron transfer and biomolecule immobilization. In this study, nitrogen-doped graphene quantum dots (N-GQDs) were synthesized, characterized, and applied to the assembly of label-free electrochemical immunosensors for the detection of antibodies against the receptor-binding domain (RBD) of the SARS-CoV-2 virus. The N-GQDs consisted of spherical-shaped particles, with a relatively narrow size distribution between 12.1 and 16.4 nm. Material characterization also showed the presence of oxygen groups as well as the presence of nitrogen heteroatoms. The N-GQDs were electrodeposited on screen-printed carbon electrodes, and the recognition site (RBD) of the SARS-CoV-2 S-protein was immobilized on them. Devices were applied to the determination of antibodies against SARS-CoV-2 RBD protein (Ab-RBD), and enabled analyte determination for the concentration level as low as 500 ng mL^–1 in the presence of a commercial serum matrix and a linear detection range up to 4.0 μg mL^–1. Additionally, the immunosensor was selective toward the presence of antibodies against the SARS-CoV-2 nucleocapsid protein (Ab-N) and enabled the differentiation of the response from negative and positive serum samples. The assembled device provided a stable analytical response for Ab-RBD detection when stored for up to 28 days. Therefore, the N-GQD material was obtained, and its performance as a modifying material for immunosensor assembly was successfully demonstrated, representing an alternative screening method to be employed in endemic and pandemic scenarios and to verify the efficiency of vaccines and humoral immunity.