Scientists Create Phase-Shifting Forms of Dynamic Nanomaterials

Introducing "reprogramming" DNA strands into an already assembled nanoparticle array triggers a transition from a "mother phase," where particles occupy the corners and center of a cube (left), to a more compact "daughter phase" (right). The change represented in the schematic diagrams is revealed by the associated small-angle x-ray scattering patterns. Such phase-changes could potentially be used to switch a material's properties on demand.

Scientists from the Brookhaven National Laboratory (BNL) have made significant progress in creating dynamic, phase-shifting forms of nanomaterials by altering the forces of attraction and repulsion between DNA-linked particles.

The structure and properties of these nanomaterials could be switched on when required. The research team has described a method for selective rearrangement of nanoparticles in 3D arrays, which would enable production of different types of phases using the same nano-components.

"One of the goals in nanoparticle self-assembly has been to create structures by design," said Oleg Gang, who led the work at Brookhaven's Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility.

"Until now, most of the structures we've built have been static. Now we are trying to achieve an even more ambitious goal: making materials that can transform so we can take advantage of properties that emerge with the particles' rearrangements."

When it becomes possible to control phase changes or particle, researchers would be able to select the required properties and control or switch them when required. The desired properties could be the response of a material to a magnetic field or to light. Materials having phase-changing ability could possibly be used as dynamic optical materials that are responsive or are capable of harvesting energy.

The research team had earlier conducted studies on developing methods to make nanoparticles to self-assemble in the form of complex composite arrays. Tethers made up of complementary synthetic DNA strands were used to link the nanoparticles together. In the current study, the team considered a nanoparticle assembly that was linked together by A, T, G, and C bases as a complementary binding.

The linking was done on single stranded DNA tethers. Following this, in order to modify the interactions taking place between particles, "reprogramming" DNA strands were added by the team.

"We know that properties of materials built from nanoparticles are strongly dependent on their arrangements," said Gang. "Previously, we've even been able to manipulate optical properties by shortening or lengthening the DNA tethers. But that approach does not permit us to achieve a global reorganization of the entire structure once it's already built."

The researchers used assembled nanoparticles that had open binding sites. Reprogramming DNA strands stuck on to these open binding sites. Additional forces are exerted on the linked-up nanoparticles by these reprogramming DNA strands.

"By introducing different types of reprogramming DNA strands, we modify the DNA shells surrounding the nanoparticles," explained CFN postdoctoral fellow Yugang Zhang, the lead author on the paper.

"Altering these shells can selectively shift the particle-particle interactions, either by increasing both attraction and repulsion, or by separately increasing only attraction or only repulsion. These reprogrammed interactions impose new constraints on the particles, forcing them to achieve a new structural organization to satisfy those constraints."

With accurate control, the research team was able to switch the "mother" phase, which is the original nanoparticle array, into numerous daughter phases that differed from each other.

Gang states that, the phase changes that were achieved are different from phase changes that occur due to temperature or pressure conditions. Only sequential or single phase shifts result from such external physical conditions.

"In those cases, to go from phase A to phase C, you first have to shift from A to B and then B to C," said Gang. "Our method allows us to pick which daughter phase we want and go right to that one because the daughter phase is completely determined by the type of DNA reprogramming strands we use."

The team utilized in situ small-angle x-ray scattering technique to study the structural transformations that took place to different daughter phases. This observation was made at Brookhaven Lab’s National Synchrotron Light Source.

The NSLS-II is now being used at BNL instead of the National Synchrotron Light Source. This has the capability to produce 10,000 times brighter x-ray beams.

For calculating the manner in which various kinds of reprogramming strands could influence interparticle interactions, the scientists employed computational modeling. The experimental observations and the theoretical calculations were found to be in agreement.

"The ability to dynamically switch the phase of an entire superlattice array will allow the creation of reprogrammable and switchable materials wherein multiple, different functions can be activated on demand," said Gang.

"Our experimental work and accompanying theoretical analysis confirm that reprogramming DNA-mediated interactions among nanoparticles is a viable way to achieve this goal."

The researchers have published this study in the journal Nature Materials.

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