Self assembling nanoparticles could lead to rewritable paper

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.