Rewritable paper, water decontamination and the controlled delivery of medications and other substances are just some of the possible applications of an innovative Weizmann Institute of Science method for getting nanoparticles to self assemble.Dr. Rafal Klajn of the Rehovot institute’s organic chemistry department and his group have given new meaning to Marshall McLuhan’s maxim “The medium is the message.” The team has now focused on the medium in which nanoparticles are suspended, and these assemblies can be used, among other things, for reversibly writing information.This approach is an elegant alternative to present methods that require nanoparticles to be coated with light-sensitive molecules; these then switch the particles’ state when light is shined on them. According to the group’s research, which recently appeared in the journal Nature Chemistry, putting regular, uncoated nanoparticles into a light-sensitive medium would be simpler, and the resulting system more efficient and durable than existing ones.The medium, in this case, is made up of small “photo-switchable” (or “photoresponsive”) molecules called spiropyrans. In the version of the photoresponsive molecule employed by Klajn and his group, absorbing light switches the molecule to a form that is more acidic. The nanoparticles then react to the change in acidity in their environment.It is this reaction that causes the particles to aggregate in the dark and disperse in the light. This means that any nanoparticles that respond to acid – a much larger group than those that respond to light – can now potentially be manipulated into self-assembly.By using light – a favored means of generating nanoparticle self-assembly – to control the reaction, one can precisely govern when and where the nanoparticles will aggregate. And since nanoparticles tend to have different properties if they are floating freely or clustered together, the possibilities for creating new applications are nearly limitless.Klajn notes out that these molecules 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 an organic chemical called spiropyrans in 1952. 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,” said Klajn. The advantages of the medium-based approach are clear. For one, the particles do not seem to degrade over time – a problem that plagues the coated nanoparticles.“We ran 100 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,” Klajn continued. “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.”In addition to durable “rewritable paper,” Klajn suggests that future applications of this method might include removing pollutants from water – certain nanoparticles can aggregate around contaminants and release them later on demand – as well as the controlled delivery of tiny amounts of substances, for example, medications, that could be released with light.