“Our ‘dynamic nanoflasks’ could be a useful tool for investigating the optical properties of fluorescent dyes, for example, or for looking at the chemical properties (reactivity) of various chemical compounds in confined spaces,” team leader Rafal Klajn of the Weizmann Institute of Science in Rehovot tells nanotechweb.org. “The flasks might also be used as nanoreactors for polymerization reactions and for preparing various classes of inorganic nanomaterials.”

The dynamic nanoflasks are in the fact nanopores that form between spherical nanoparticles that can assemble and disassemble in a reversible fashion, he explains. When small spheres come together and pack closely, interstitial voids are created. “We have shown that we can trap various molecules within these voids while the nanoparticles self assemble. Within the confines of these pores, the chemical reactivity of the trapped molecules can be different from that in bulk solution.”

The researchers used azobenzene-functionalized inorganic nanoparticles of different sizes and compositions as the precursors for their nanoflasks. The particles included 6 nm gold, 11 nm magnetite and 17 nm silica particles. “Depending on the composition of the core, we used different bonding groups to immobilize the azobenzene part – thiols for gold, catechols for magnetite and silanes for silica (see "Making nanoflasks" figure),” says team member and first author of the study, Hui Zhao.

“Solutions of these azobenzene-decorated nanoparticles in non-polar solvents (in our case Toluene) were stable for at least several months under normal light conditions. However, when exposed to UV light with a wavelength of 365 nm, the nanoparticles self-assemble within minutes as they lose their solvation layer and as attractive dipole–dipole interactions between different azobenzene moieties on the nanoparticles come into play.”

The team, which includes researchers from the University of Illinois at Chicago, says that it would now like to produce light-induced self-assembly in water as the solvent. “Having achieved this goal, we could then consider trapping molecules like proteins/enzymes and study their properties in confined spaces,” says Klajn. “Our photoswitchable nanoparticles might also potentially become useful for purifying water.”

The researchers report their work in Nature Nanotechnology doi:10.1038/nnano.2015.256.