Viral particles imaging through evanescent wave scattering in a total internal reflection laser microscope

Optical imaging of objects at the nanometric scale is limited by light diffraction, and for this reason several subdiffractive optical microscopy techniques have been developed in the last decades. In this article we present an optical laser microscope specifically designed for detection of nano-materials exploiting the Mie scattering between the evanescent wave generated by a laser beam travelling in a transparent substrate through total internal reflection and the nano-objects deposited on the substrate itself. The setup, including the laser source, can be fully integrated inside an optical microscope requiring only a geometric alignment with the substrate thus reducing the complexity and the cost. We also report two possible applications: quantitative detection of Au nanoparticles in the 20-100 nm size range and semi-quantitative detection of virions immobilized on the substrate through bioreceptor / antigen binding, including, but not limited to, SARS-CoV-2. A minimum virus amount of approximate to 105 in the reaction volume is observed. The results presented here open the route towards unprecedent applications in molecular biology or molecular diagnostics.