Vinothan N. Manoharan, Harvard University, Cambridge, MA, USA, and colleagues have succeeded in the marker-free and nondestructive detection of viruses with dimensions smaller than 20 nm. They developed a nanohole fiber similar to an internet fiber and made of quartz glass from Heraeus, Hanua, Germany. The technique makes it possible to observe viruses in their natural environment, without manipulation.

The viruses are put into quartz glass – one of the purest technical materials manufactured today. The fiber itself conducts light almost perfectly, without scattering it. As soon as the light encounters a virus, some of it is diffracted. This allows very rapid observations with an extremely limited background. As a result, the movement of a virus can be observed and recorded within a period of several seconds.

The core of the fiber contains a nanohole with a diameter of 200 nm that extends along the entire fiber. Viruses swimming in water are poured into this hole, and light is fed into the fiber core. The size and movement of the viruses can be determined by means of light scattering. The transmission of light in an optical fiber is based on a refractive index that decreases from the core to the cladding. If the light fed into the capillaries encounters a virus, part of the light is scattered. When this scattering is observed through a microscope, the size of the virus can be determined.

The method can track unlabeled dielectric particles as small as 20 nm as well as individual cowpea chlorotic mottle virus (CCMV) virions which are 26 nm in size and 4.6 megadaltons in mass at rates of over 3 kHz for durations of tens of seconds. Fields of application range from medical diagnostics to the analysis of drinking water.

• Fast, Label-Free Tracking of Single Viruses and Weakly Scattering Nanoparticles in a Nanofluidic Optical Fiber,
  Sanli Faez, Yoav Lahini, Stefan Weidlich, Rees F. Garmann, Katrin Wondraczek, Matthias Zeisberger, Markus A. Schmidt, Michel Orrit, Vinothan N. Manoharan,
  ACS Nano 2015, 9 (12), 12349–12357.
  DOI: 10.1021/acsnano.5b05646