Gradient nanoplasmonic imaging metasurface for rapid and label-free detection of SARS-CoV-2 sequences.

Compact and user-friendly nucleic acid biosensors play a crucial role in advancing infectious disease research, particularly for coronavirus (COVID-19). While nanophotonic metasurface sensors hold promise for high-performance sensing, they face challenges due to their complexity and bulky readout instruments. In this study, we propose a gradient nanoplasmonic imaging (GNI) metasurface that incorporates the concept of an optical potential well, enabling label-free single-step detection of SARS-CoV-2 sequences. The metasurface sensor consists of nanopillars with continuous variations, forming an optical potential well that results in a centimeter-scale dark ring. This dynamic well exhibits high sensitivity to refractive index changes, recorded by a CCD. To further enhance the visualized sensing performance, plasmonic coupling of gold nanoparticles with the gold nanostructure is employed. Our metasurface-based biosensor achieves rapid single-step detection of SARS-CoV-2 sequences, with a low detection limit of 77.2 pM and a detection range of 0.1-100 nM. This biosensor not only demonstrates exceptional reproducibility and outstanding detection performance, but also maintains remarkable specificity in differentiating SARS-CoV-2 from other diseases with similar symptoms. This simple and spectrometer-free refractometric sensing scheme enables the construction of a compact and cost-efficient prototype. Our imaging-based metasurface biosensing strategy demonstrates valuable merits for rapid, sensitive, and quantitative detection, showcasing its potential as a valuable on-site nucleic acid diagnostic tool.