A New Route to Nano Self-Assembly

Scientists have discovered a simple, yet powerful way to induce nanoparticles to assemble themselves into complex arrays. By adding specific types of small molecules to mixtures of nanoparticles and polymers, the researches are able to direct the self-assembly of the nanoparticles into arrays of one, two and even three dimensions with no chemical modification of either the nanoparticles or the block copolymers. In addition, the application of external stimuli, such as light and/or heat can be used to further direct the assemblies of nanoparticles for even finer and more complex structural details.

Most approaches that achieve nanoparticle assembly require modification of the nanoparticle surface. Since nanoparticles are so small that they are essentially all surface, any surface modification can profoundly change the particle's properties. This discovery is significant because it demonstrates a simple process for self-assembly of nanoparticles into complex arrays without surface modification.

Scientists achieved this by combining nanoparticles and small molecules with block copolymers. Block copolymers are composed of long sequences, called "blocks," of small molecules. These small molecules are called "monomers." These monomers chemically bond to other monomers to form "polymers." Block copolymers are a specific type of polymer created when the same monomer sequence bonds to sequences of an unlike type. They are important because they can be made to separate to form nanometer-sized structures.

"Block copolymers readily self-assemble into well-defined arrays of nanostructures of macroscopic [measurable or observable to the naked eye] distances," says Ting Zu, a polymer scientist who led this project and holds joint appointments with Berkeley Lab's Materials Sciences Division and the University of California, Berkeley. "They would be an ideal platform for directing the assembly of nanoparticles except that block copolymers and nanoparticles are not particularly compatible with one another from a chemistry standpoint. A mediator is required to bring them together."

Xu and her group found such a "mediator" in the form of small molecules that will join with nanoparticles and then attach themselves and their nanoparticle partners to the surface of a block copolymer. Xu and her group used two different types of small molecules called surfactants (wetting agents). These small molecules can be stimulated by light or heat to sever their connection to the surface of a block copolymer and be repositioned to another location along the polymeric chain. In this manner, the spatial distribution of the small molecule mediators and their nanoparticle partners can be precisely directed with no need to modify either the nanoparticles or the polymers.

"The beauty of this technique is that it involes no sophisticated chemistry," says Xu. "It really is a plug and play technique, in which you simply mix the nanoparticles with the block copolymers and then add whatever small molecules you need."

Due to its simplicity and its practicality for large-scale fabrication of arrays at the macroscale, this technique has far-reaching implications. While Xu and her group worked with light and heat, she says other stimuli, such as pH, could also be used to reposition small molecules and their nanoparticle partners along block copolymer formations. Strategic substitutions of different types of stimulus-responsive small molecules could serve as a mechanism for structural fine-tuning or for incorporating specific functional properties into nanocomposites. Xu and her colleagues are now in the process of adding functionality to their self-assembly technique.