Electrochemical Detection of SARS‐CoV‐2 in Saliva Using ZnO Nanorods Functionalized with Gold‐Conjugated Antireceptor‐Binding Domain Antibodies

The COVID-19 pandemic highlights the urgent need for rapid, accessible, and cost-effective diagnostic technologies. Traditional diagnostic methods, although effective, often require high-cost equipment and lengthy processing times. Herein, the development of an electrochemical immunosensor based on fluorine-doped tin oxide (FTO) electrodes modified with zinc oxide nanorods (ZnONRs) for detecting the receptor-binding domain (RBD) of SARS-CoV-2 in saliva is presented. ZnONRs offer a favorable platform due to their large surface area, low production cost, and efficient electron transport properties. To improve selectivity and sensitivity, ZnONRs are functionalized with monoclonal antibodies (mAbs) conjugated to gold nanoparticles (AuNPs). Four murine anti-RBD mAbs (2B9F9, 3E5G8, 4B1D3, and 4H4A2) are evaluated by ELISA and electrochemical methods. While all mAbs demonstrate recognition of the RBD in ELISA, only the 4B1D3 mAb produces a measurable electrochemical signal, achieving a detection limit of 1.7 μg mL−1 and exhibiting recognition of both the original Wuhan-Hu-1 strain and the Omicron variant. The immunosensor demonstrates excellent performance in tests with real human saliva samples, reinforcing its potential as a noninvasive, rapid, and scalable platform for point-of-care viral diagnostics.