Nanoscale building blocks and DNA 'glue' shape 3D superlattices
23 April 2015
Scientists construct 3D 'superlattice' multi-component nanoparticle arrays where the arrangement of particles is driven by the shape of the tiny building blocks.
The method, devised by scientists at the US Department of Energy's Brookhaven National Laboratory, uses linker molecules made of complementary strands of DNA to overcome the blocks' tendency to pack together in a way that would separate differently shaped components.
The results, published in Nature Communications, are an important step on the path toward designing predictable composite materials for applications in catalysis, other energy technologies, and medicine.
"If we want to take advantage of the promising properties of nanoparticles, we need to be able to reliably incorporate them into larger-scale composite materials for real-world applications," says lead researcher and Brookhaven physicist, Oleg Gang.
According to the lead author of the Nature Communications paper, Fang Lu, the work describes a new way to fabricate structured composite materials using directional bindings of shaped particles for predictable assembly.
The research builds on the team's experience linking nanoparticles together using strands of synthetic DNA. Like the molecule that carries the genetic code of living things, these synthetic strands have complementary bases known by the genetic code letters G, C, T, and A, which bind to one another in only one way (G to C; T to A).
Gang has previously used complementary DNA tethers attached to nanoparticles to guide the assembly of a range of arrays and structures. The new work explores particle shape as a means of controlling the directionality of these interactions to achieve long-range order in large-scale assemblies and clusters.
Spherical particles, Gang explained, normally pack together to minimize free volume. DNA linkers - using complementary strands to attract particles, or non-complementary strands to keep particles apart - can alter that packing to some degree to achieve different arrangements.
For example, scientists have experimented with placing complementary linker strands in strategic locations on the spheres to get the particles to line up and bind in a particular way. But it's not so easy to make nanospheres with precisely placed linker strands.
"We explored an alternate idea: the introduction of shaped nanoscale 'blocks' decorated with DNA tethers on each facet to control the directional binding of spheres with complementary DNA tethers," says Gang.
When the scientists mixed nanocubes coated with DNA tethers on all six sides with nanospheres of approximately the same size, which had been coated with complementary tethers, these two differently shaped particles did not segregate as would have been expected based on their normal packing behaviour. Instead, the DNA 'glue' prevented the separation by providing an attractive force between the flat facets of the blocks and the tethers on the spheres, as well as a repulsive force between the non-pairing tethers on same-shaped objects.
"The DNA permits us to enforce rules: spheres attract cubes (mutually); spheres do not attract spheres; and cubes do not attract cubes," says Gang. "This breaks the conventional packing tendency and allows for the system to self-assemble into an alternating array of cubes and spheres, where each cube is surrounded by six spheres (one to a face) and each sphere is surrounded by six cubes." Using octahedral blocks instead of cubes achieved a different arrangement, with one sphere binding to each of the blocks' eight triangular facets.
The method required some thermal processing to achieve the most uniform long-range order. And experiments with different types of DNA tethers showed that having flexible DNA strands was essential to accommodate the pairing of differently shaped particles.
"The flexible DNA shells 'soften' the particles, which allows them to fit into arrangements where the shapes do not match geometrically," Lu said. But excessive softness results in unnecessary particle freedom, which can ruin a perfect lattice, she added. Finding the ideal flexibility for the tethers was an essential part of the work.