Molecules aid nanoparticle assembly

Researchers in the US claim to have found a simple and robust way to induce nanoparticles to assemble themselves into complex arrays.

The team, from the US Department of Energy’s Lawrence Berkeley National Laboratory, found that by adding specific types of small molecules to mixtures of nanoparticles and polymers, they could direct the self-assembly of the nanoparticles into arrays of one, two and three dimensions with no additional chemical modification.

The team also found that with the application of external stimuli, such as light or heat, they could further direct the assemblies of nanoparticles for even finer and more complex structural details.

It is hoped the discovery will open routes for fabricating nanoparticle-based devices, including highly efficient systems for the generation and storage of solar energy.

Nano-sized particles display highly coveted properties not found in macroscopic materials such as optical, electronic or magnetic.

Scientists hope that exploiting the unique properties of nanoparticles on a commercial scale could lead to sustainable, clean and cheap energy, and the creation on demand of new materials with properties tailored to meet specific needs.

None of this can be fully realised, however, if nanoparticles cannot organise themselves into complex structures and hierarchical patterns, similar to what nature routinely accomplishes with proteins.

Other research groups have tried to modify the surface of nanoparticles using DNA to change their properties in a way that allows them to self-assemble. However, this approach only works well for organising small-sized arrays.

The leader of this most recent project, Ting Xu, a polymer scientist from Berkeley University, claims a better approach is to use block copolymers – long sequences or ‘blocks’ of one type of monomer molecule bound to blocks of another type of monomer molecule.

‘Block copolymers readily self-assemble into well-defined arrays of nanostructures over macroscopic distances,’ Xu said. ‘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 team 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.

For this study, the team used two different types of small molecules, wetting agents dubbed ‘PDP’ and ‘OPAP’. PDP can be stimulated by light and OPAP can be stimulated by heat to sever their connection to the surface of a block copolymer and reposition 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 the nanoparticles or polymers.

Xu and her team added small molecules of PDP or OPAP to various blends of nanoparticles, such as cadmium selenide and lead sulfide, mixed in with a commercial block copolymer.

While the group used light and heat in this study, Xu said other stimuli, such as pH, could also be used to reposition small molecules and their nanoparticle partners along block copolymer formations.

The team believe that strategic substituting with different types of stimulus-responsive small molecules could be used to fine-tune nano-structures and provide nanocomposites with specific functional properties.

‘Bring together the right basic components – nanoparticles, polymers and small molecules – stimulate the mix with a combination of heat, light or some other factors and these components will assemble into sophisticated structures or patterns,’ said Xu. ‘It is not dissimilar from how nature does it.’