self-assembling nanoparticles

Dr Rafal Klajn of the Weizmann Institute’s Department of Organic Chemistry has developed a method for coaxing nanoparticles to self-assemble by focusing on the medium in which the particles are suspended.

The existing method of self-assembly requires nanoparticles to be coated with light-sensitive molecules; these then switch the particles’ state when light is shined on them.

However the new research indicates uncoated nanoparticles placed into a light-sensitive medium would be simpler, as the resulting system is more efficient and durable than existing methods. Possible applications range from rewritable paper (paper that does not use ink but instead uses dyes that respond to ultraviolet light), water decontamination, and the controlled delivery of drugs.

Dr Klajn’s medium is made up of small photoresponsive molecules called spiropyrans.

When he used this medium he found that 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. Consequently, 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 to the control the reaction the researchers could precisely govern when and where the nanoparticles will aggregate. Furthermore, since nanoparticles tend to have different properties depending on whether they are floating freely or clustered together, the possibilities for creating new applications are claimed to be “nearly limitless”.

In a statement Dr Klajn said: “Two Institute scientists, Ernst Fischer and Yehuda Hirshberg, were the first to demonstrate the light-responsive behaviour 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.”

Dr Klajn claimed the advantages to the medium-based approach are the particles do not seem to degrade over time, which is a problem that plagues the coated nanoparticles.

Dr Klajn said: “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. 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.