Nanoparticles in a light-sensitive medium scatter in the light (left) and aggregate in the dark (right). Image: Weizmann Institute of Science.
Nanoparticles in a light-sensitive medium scatter in the light (left) and aggregate in the dark (right). Image: Weizmann Institute of Science.

Scientists at the Weizmann Institute of Science, led by Rafal Klajn, have demonstrated an innovative method for getting nanoparticles to self-assemble that utilizes the medium in which the particles are suspended.

Existing methods for getting nanoparticles to self-assemble require them to be coated with light-sensitive molecules that alter the particles' state when light is shined on them. As described in a paper in Nature Chemistry, Klajn and his group have come up with a simpler approach, which involves putting regular, uncoated nanoparticles into a light-sensitive medium. The resulting system is more efficient and durable than existing ones, with possible applications ranging from rewritable paper to water decontamination to the controlled delivery of drugs or other substances.

The medium, in this case, is made up of small ‘photo-switchable’ (or ‘photoresponsive’) molecules called spiropyrans. When these molecules absorb light, they switch to a form that is more acidic, altering the acidity of the medium in which the nanoparticles are suspended. This change in acidity will cause any nanoparticles that respond to acid – a much larger group than those that respond to light – to be repelled from each other. The end result is that the nanoparticles will aggregate in the dark and disperse in the light.

This novel method offers the best of both worlds for self-assembly. By continuing to use light to control the agglomeration process, Klajn and his group can precisely govern when and where the nanoparticles aggregate. Doing away with the light-sensitive coating, however, prevents the nanoparticles from degrading over time –a problem that plagues coated nanoparticles.

"We ran one hundred cycles of writing and rewriting with the nanoparticles in a gel-like medium – what we call reversible information storage – and there was no deterioration in the system. So you could use the same system over and over again," says Klajn. "And, although we used gold nanoparticles for our experiments, theoretically one could even use sand, as long as it was sensitive to changes in acidity."

Klajn points out that the photoresponsive molecules used in his method have a long history at the Weizmann Institute. "Two Institute scientists, Ernst Fischer and Yehuda Hirshberg, were the first to demonstrate the light-responsive behavior of spiropyrans in 1952,” he says. “Later on, in the 1980s, Prof. Valeri Krongauz used these molecules to develop a variety of materials including photosensitive coatings for lenses. Now, 63 years after the first demonstration of its light-responsive properties, we are using the same simple molecule for another use entirely."

In addition to durable ‘rewritable paper’, Klajn suggests several other potential applications for this self-assembly method. These include removing pollutants from water – nanoparticles could aggregate around contaminants and release them later on demand – and the controlled delivery of tiny amounts of substances such as drugs that could be released with light.

This story is adapted from material from the Weizmann Institute of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.