Researchers have developed tiny valves that enable individual nanoparticles in liquids to be separated and sorted. The valves can be used for a very broad range of tiny particles, including individual metal and semiconductor nanoparticles, virus particles, liposomes and larger biomolecules such as antibodies. Newly-developed nanovalves allow the flow of individual nanoparticles in liquids to be controlled in tiny channels. This is of interest for lab-on-a-chip applications such as in materials science and biomedicine.

The nanovalves work differently than classic valves, which are used to mechanically close and open flow in pipelines, as in a tap. These mechanical valves can be miniaturised, but not as far as we would need for nanoscale applications. If channels are thinner than a few dozen micrometres, they cannot be mechanically closed and opened with any regularity.

Bottleneck with electrodes

In order to open and close the nanoparticle flow in ultrathin channels, the ETH scientists made use of electric forces. They worked with channels etched into a silicon chip. These had a diameter of just 300 to 500 nanometres less than a hundredth of the diameter of a human hair. 

Nanoparticles in pure water cannot simply pass through the bottleneck; for them, the valve in its basic state is closed. By activating the electrode in particular ways, the electrical field in the bottleneck can be changed. This leads to a force acting on any nanoparticles present, which pushes the particles through the bottleneck this is how the valve is "opened." Nanoparticles in a saline solution, however, behave differently: they can pass through the bottleneck in its basic state for them, the valve is "open." 

Controlling vibrating nanoparticles

"It is fundamentally difficult to examine individual nanoparticles in a liquid because Brownian motion acts on the nanoscale. The tiny particles do not remain still but instead vibrate constantly, with a movement radius that is many times their diameter.  As part of a proof of concept, the scientists prepared an isolation and sorting lock with a junction and three valves on a silicon chip. An individual nanoparticle can be captured and examined at the junction.

The valves can then be controlled so that the particle leaves the system through one of two outlet channels, allowing nanoparticles in a liquid to be sorted into two classes. 

Lab-on-a-chip applications

As the scientists emphasize, it is, in principle, possible to arrange a complex nanochannel system with any number of controllable valves on a silicon chip. By fine-tuning the electrical field at the electrodes, in the future it could be possible to use the valves as a filter, letting particles with particular physical properties pass through while blocking others.

In future work, the author develops the technology together with partners to bring it up to readiness for standard use in research. Since it enables particles on a small chip to be sorted. It may also be possible to use this technique to isolate synthetic or biological particles to examine them microscopically or to analyze them under the influence of pharmaceutical drugs.