Surface-Enhanced Raman Spectroscopy for Viral Diagnostics: Principles, Strategies, Clinical Challenges, and Future Directions.

Viral outbreaks such as SARS-CoV, MERS-CoV, and COVID-19 underscore the urgent need for rapid, sensitive, and scalable diagnostic technologies. Current standard methods, including nucleic acid amplification tests and immunoassays, offer complementary strengths but face limitations in cost, turnaround time, and early-stage detection. Surface-enhanced Raman spectroscopy (SERS) has emerged as a promising alternative, leveraging plasmonic nanostructures to amplify weak Raman signals and provide molecular "fingerprints" of viral components with single-molecule sensitivity. This review provides a comprehensive overview of SERS-based virus detection, highlighting the fundamental principles of Raman enhancement, the role of substrates and hotspots, and analyte-specific challenges. We categorize sensing strategies into direct detection of intact viruses and components, affinity-based approaches using antibodies, aptamers, or viral receptors, and labeled methods that amplify specificity and multiplexing. Advances in nanofabrication, receptor engineering, and machine learning have significantly improved sensitivity, reproducibility, and classification accuracy, with detection limits reaching down to a few viral particles per milliliter. Despite these advances, challenges remain in handling biological complexity, ensuring reproducibility, and translating assays into clinical practice. We conclude by outlining opportunities for integrating SERS with portable devices, standardized spectral libraries, and artificial intelligence, paving the way toward rapid, robust, and deployable viral diagnostics for future pandemic preparedness.