Eco-Friendly Engineered Ternary Nanocomposite for Label-Free Fluorescent SARS-CoV‑2 N‑Gene DNA Detection

The COVID-19 pandemic brought to light the demand for nucleic-acid sensing platforms that are rapid, resilient, and scalable for integration at the point-of-care. In this study, we described a green-engineered fluorescence biosensing platform using a Ag-TiO₂-CeO₂ ternary nanocomposite fabricated with Morinda citrifolia leaf extract. The biosynthetic approach facilitated the production of a nanocomposite that is biocompatible, exhibits high surface area, and contains oxygen vacancy related to surface sites that promote favorable interactions with nucleic acids. The nanocomposite was functionalized with thiol-terminated antisense probes targeting the SARS-CoV-2 N-gene and employed in enzyme-free hybridization assays, using synthetic DNA as a model target and human genomic DNA as a model for nonspecific hybridization. The platform showed a significant shift in fluorescence after 30 min of hybridization and was capable of detecting target unlabeled DNA at a concentration of 3 pM. The interaction of the nanocomposite with short DNA oligonucleotides and long double-stranded genomic DNA showed the potential of the surface chemistry of the nanocomposite for nucleic acid sensing. This study provides a nanocomposite fluorescence biosensing platform at the material proof level and paves the way for future extensions and viral diagnostics in complex biological mixtures.