A label-free technique has demonstrated the ability to detect freely diffusing viruses smaller than 20 nm. Although existing methods for detecting viruses such as confocal or fluorescence microscopy — which generally involve fixation and fluorescent-dye staining of samples — offer high spatial resolution and high signal-to-background ratios, fluorescence emission lifetime limits the measurement speed, and photobleaching and thermal diffusion limit the duration of measurements. Further, objects smaller than 100 nm often elude conventional characterization methods. Schematic of the nanofluidic optical fiber. Courtesy of Markus A. Schmidt. Now scientists at the Leibniz Institute of Photonic Technologies (IPHT) have developed a label-free technique on a single-mode silica fiber with a core integrating a subwavelength (200-nm-diameter) nanofluidic channel. Within the hole, the test viruses swam in water, and were illuminated using the fiber’s strongly confined optical mode. The diffusing particles in the cylindrical geometry were continuously illuminated inside the collection focal plane. The method was used to track unlabeled dielectric particles as small as 20 nm, as well as individual cowpea chlorotic mottle virus virions that were 26 nm in size and 4.6 megadaltons in mass at rates of over 3 kHz for durations of tens of seconds. The team’s tracking method, based on elastic light scattering, enabled long-duration measurements of nanoparticle dynamics at rates of thousands of frames per second. The fiber are special because the smaller refractive index of the water was bypassed by the surrounding fiber material with a higher refractive index, thereby preventing the coupled light from lateral escape; that is, the light was held in the core by way of total internal reflection, the researchers said. The setup could be easily incorporated into common optical microscopes to extend their detection range to nanometer-scale particles and macromolecules, the researchers said. Potential applications include medical diagnostics and micro total analysis systems, such as for drinking water. The Leibniz team worked in cooperation with Heraeus Quarzglas GmbH & Co. of Kleinostheim, Germany, Harvard University in Cambridge, Mass., and Leiden University in the Netherlands. The research was published in ACS Nano (doi: 10.1021/acsnano.5b05646).