Advanced fluorescence biosensor utilizing nitrogen/oxygen-doped carbon quantum dots in a modified carboxy cellulose matrix for quantitative detection of SARS-CoV-2 spike protein.

The persistent need for fast and sensitive point-of-care (POC) diagnostics for SARS-CoV-2 requires alternatives to time-consuming RT-PCR. To address this challenge, we designed and fabricated a novel fluorescence biosensor based on nitrogen/oxygen-doped carbon quantum dots embedded in an epichlorohydrin-modified carboxymethyl cellulose thin film. This robust and easily synthesized platform enabled the immobilization of spike protein-specific monoclonal antibodies, facilitating detection via fluorescence quenching and recovery at Ex/Em 320 nm/440 nm. The biosensor was characterized using Fourier-transform infrared spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and fluorescence spectroscopy, confirming successful doping, nanoscale morphology, and stable optical properties. The biosensor achieved a remarkably low limit of detection of 0.0323 pg/mL across a wide dynamic range of 3.1-700 pg/mL. When validated with clinical nasal swab samples, it demonstrated high analytical performance, with a sensitivity of 95.13% and specificity of 93.1%, along with excellent reproducibility and room-temperature stability. This rapid, cost-effective fluorescence biosensor represents a significant advancement for POC SARS-CoV-2 testing, offering a practical solution for clinical and decentralized diagnosis, especially in resource-limited settings.