A detector that is able to count viruses one by one at a rate of up to 500,000 every second has been constructed by Jean-Luc Fraikin and colleagues (Nature Nanotechnol. doi:10.1038/nnano.2011.24; 2011).

Rapid improvements in materials engineering and imaging technology has brought us face to face with the tiny world of the nanoscale — structures too small to be seen by conventional optical microscopes. Nanoparticles, both man-made and natural, abound and there is a growing need, particularly in medical applications, to analyse them quickly. But present separation techniques, such as a centrifuge, tend to only produce averages over necessarily large samples. Fraikin et al. now show how microfluidics provides a solution that is far more precise.

Credit: J. FRAIKEN ET AL.

Microfluidic systems, in which liquids flow along micrometre-wide channels, are fast developing into an important analysis tool because they operate in a flow regime very different from the one that governs bulk liquids. The rewards are a far higher degree of analyte control and a route towards so-called lab-on-a-chip technology.

The sensor designed by Fraikin et al. comprises two 'fluidic resistors', which resist an ionic electrical current passing along the microfluidic channel. The first resistor has a fairly constant resistance, but the second is a nanoconstriction in the channel (pictured), and its resistance depends heavily on whether there is a particle present or not. A sensing electrode between the two resistors measures the corresponding changes in electrical potential.

This approach is sensitive enough to distinguish between particles of different sizes by recording the length of time each one spends in the constriction. Constriction times down to two or three microseconds are readily detectable, which equates to a maximum count rate of approximately 500,000 particles per second.

Fraikin et al. tested the applicability of their system using an archetypal bacterial virus known as the T7 bacteriophage suspended in both salt solution and blood plasma. Intriguingly, the experiment could detect both a 60-nanometre-diameter singlet version of the virus and a larger dimer version, a distinction not possible using laser-based measurement techniques.

The device is made from low-cost materials using tools that are already available and so it should be cheap and simple to mass produce. Importantly, and unlike many other detection schemes, the particles do not need to be labelled.