Newswise — The medium is the message. Dr. Rafal Klajn of the Weizmann Institute’s Department of Organic Chemistry and his group have given new meaning to that maxim, as they have now demonstrated an innovative method for getting nanoparticles to self-assemble that focuses on the medium in which the particles are suspended. These assemblies can be used, among other things, for reversibly writing information.
This approach is an elegant alternative to current 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 Nature Chemistry, putting regular, uncoated nanoparticles into a light-sensitive medium would be simpler, and the resulting system more efficient and durable, than existing methods. The possible applications range 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. In the version of the photoresponsive molecule employed by Dr. 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; in turn, the reaction 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 depending on whether they are floating freely or clustered together, the possibilities for creating new applications are nearly limitless.
Dr. Klajn points 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 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.”
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,” says Dr. 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.”
In addition to durable “rewritable paper,” Dr. 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, such as medicines, that could be released with light.
Dr. Rafal Klajn’s research is supported by the Abramson Family Center for Young Scientists; the Rothschild Caesarea Foundation; the Mel and Joyce Eisenberg-Keefer Fund for New Scientists; the estate of Olga Klein Astrachan; and the European Research Council.
The Weizmann Institute of Science in Rehovot, Israel, is one of the world’s top-ranking multidisciplinary research institutions. The Institute’s 3,800-strong scientific community engages in research addressing crucial problems in medicine and health, energy, technology, agriculture, and the environment. Outstanding young scientists from around the world pursue advanced degrees at the Weizmann Institute’s Feinberg Graduate School. The discoveries and theories of Weizmann Institute scientists have had a major impact on the wider scientific community, as well as on the quality of life of millions of people worldwide.