Real-time optical detection of single human and bacterial viruses based on dark-field interferometry

► We developed a label-free optical technique for detection of single nanoparticles. ► Combines advantages of heterodyne interferometry and dark-field microscopy. ► Characterizes single human virus and bacteriophage particles down to ∼24 nm size. ► Can distinguish populations in heterogeneous mixtures of biological nanoparticles. ► Resolves distribution of nano-impurities (∼20-30 nm) in clinically relevant media.