Nanomaterials in Optical Biosensors for Nucleic Acid Detection

This review presents a comprehensive overview of modern nanomaterial-based biosensors for nucleic acid detection, with a particular focus on optical, colorimetric, and surface-enhanced Raman spectroscopy (SERS) methods that operate without enzymes or external labels. The fundamental principle underlying many of these platforms is the specific hybridization between a probe and a complementary target nucleic acid sequence, which triggers a measurable optical signal change. Noble metal nanoparticles (gold and silver), quantum dots, upconversion nanoparticles (UCNPs), graphene oxide, and various nanoclusters serve as the core signal transducers, exploiting phenomena such as fluorescence resonance energy transfer (FRET), localized surface plasmon resonance (LSPR), and nanozyme activity. Fluorescence-based sensors rely on the quenching and restoration of emission from fluorophores (e.g., quantum dots, UCNPs, silver nanoclusters) in the presence of graphene oxide or gold nanoparticles as acceptors, with detection limits reaching femtomolar concentrations. Colorimetric methods offer the distinct advantage of visual readout without instrumentation, leveraging the distance-dependent aggregation of gold or silver nanoparticles, which produces a visible color shift from red to blue or yellow to red upon target hybridization. SERS-based approaches provide ultra-sensitive molecular fingerprinting through electromagnetic field enhancement on plasmonic metal surfaces, enabling direct detection of DNA or RNA via intrinsic nucleobase vibrations. SPR directly detects changes in the refractive index occurring at the sensor surface and is capable of real-time monitoring. This makes it possible not only to confirm the binding but also to observe the kinetics of the process. These biosensors have been successfully applied to detect clinically and environmentally significant pathogens including SARS-CoV-2, HIV, MERS-CoV, Mycobacterium tuberculosis, etc—as well as cancer-associated biomarkers. The feasibility of point-of-care analysis has been demonstrated using paper-based platforms, validation in artificial and real saliva and whole serum, highlighting the practical potential of these technologies.