Several years ago we pointed to work from the collaboration of two Foresight Feynman Prize winners (Chad A. Mirkin, 2002, Experimental and George C. Schatz, 2008, Theoretical) that advanced the concept of using DNA to link together nanoparticles in specific 3D configurations: “Using DNA as bonds to build new materials from nanoparticles“. A news article written by Robert F. Service in a recent issue of Science “DNA makes lifeless materials shapeshift” describes another major advance from Mirkin’s group, taking their 2011 advance to the next level:
Researchers have engineered tiny gold particles that can assemble into a variety of crystalline structures simply by adding a bit of DNA to the solution that surrounds them. Down the road, such reprogrammable particles could be used to make materials that reshape themselves in response to light, or to create novel catalysts that reshape themselves as reactions proceed.
“This paper is very exciting,” says Sharon Glotzer, a chemical engineer at the University of Michigan, Ann Arbor, who calls it “a step towards pluripotent matter.” David Ginger, a chemist at the University of Washington, Seattle, agrees: “This is a proof of concept of something that has been a nanoparticle dream.” Neither Glotzer nor Ginger has ties to the current research. …
The 2011 paper was titled “Nanoparticle Superlattice Engineering with DNA” (abstract, PDF courtesy of Mirkin lab). The current advance: “Transmutable nanoparticles with reconfigurable surface ligands” (abstract). Service explains the nature of this new advance as no longer needing to re-engineer the DNA linkers each time a material with a new crystal orientation is needed. He describes that it works by coating each nanoparticle, not with single DNA strands, but with a DNA hairpin that can be opened or closed by adding additional short DNA strands to the solution, thus changing the configuration of other nanoparticles with which it will bind.
The researchers also added more control over what crystals formed by changing the length of the DNA hairpins, the concentration at which they were assembled around the particles, and the concentration of different types of particles. They report in today’s issue of Science that they used these different knobs to create 10 different crystals. But according to Mirkin, his team already has the ability to cause particles to assemble into more than 500 different crystal forms. “This gives us the ability to make materials by design,” he says. …
The same issue of Science features another approach to engineering superlattices using DNA nanotechnology: “Diamond family of nanoparticle superlattices” [abstract].
The research is described in this Brookhaven National Laboratory news release “Scientists Guide Gold Nanoparticles to Form ‘Diamond’ Superlattices“:
DNA scaffolds cage and coax nanoparticles into position to form crystalline arrangements that mimic the atomic structure of diamond
Using bundled strands of DNA to build Tinkertoy-like tetrahedral cages, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have devised a way to trap and arrange nanoparticles in a way that mimics the crystalline structure of diamond. The achievement of this complex yet elegant arrangement … may open a path to new materials that take advantage of the optical and mechanical properties of this crystalline structure for applications such as optical transistors, color-changing materials, and lightweight yet tough materials.
“We solved a 25-year challenge in building diamond lattices in a rational way via self-assembly,” said Oleg Gang, a physicist who led this research at the Center for Functional Nanomaterials (CFN) at Brookhaven Lab in collaboration with scientists from Stony Brook University, Wesleyan University, and Nagoya University in Japan.
The scientists employed a technique developed by Gang that uses fabricated DNA as a building material to organize nanoparticles into 3D spatial arrangements. They used ropelike bundles of double-helix DNA to create rigid, three-dimensional frames, and added dangling bits of single-stranded DNA to bind particles coated with complementary DNA strands.
“We’re using precisely shaped DNA constructs made as a scaffold and single-stranded DNA tethers as a programmable glue that matches up particles according to the pairing mechanism of the genetic code—A binds with T, G binds with C,” said Wenyan Liu of the CFN, the lead author on the paper. “These molecular constructs are building blocks for creating crystalline lattices made of nanoparticles.”
The difficulty of diamond
As Liu explained, “Building diamond superlattices from nano- and micro-scale particles by means of self-assembly has proven remarkably difficult. It challenges our ability to manipulate matter on small scales.”
The reasons for this difficulty include structural features such as a low packing fraction—meaning that in a diamond lattice, in contrast to many other crystalline structures, particles occupy only a small part of the lattice volume—and strong sensitivity to the way bonds between particles are oriented. “Everything must fit together in just such a way without any shift or rotation of the particles’ positions,” Gang said. “Since the diamond structure is very open, many things can go wrong, leading to disorder.”
“Even to build such structures one-by-one would be challenging,” Liu added, “and we needed to do so by self-assembly because there is no way to manipulate billions of nanoparticles one–by–one.”
Gang’s previous success using DNA to construct a wide range of nanoparticle arrays suggested that a DNA-based approach might work in this instance.
DNA guides assembly
The team first used the ropelike DNA bundles to build tetrahedral “cages”—a 3D object with four triangular faces. They added single-stranded DNA tethers pointing toward the interior of the cages using T,G,C,A sequences that matched up with complementary tethers attached to gold nanoparticles. When mixed in solution, the complementary tethers paired up to “trap” one gold nanoparticle inside each tetrahedron cage.
The arrangement of gold nanoparticles outside the cages was guided by a different set of DNA tethers attached at the vertices of the tetrahedrons. Each set of vertices bound with complementary DNA tethers attached to a second set of gold nanoparticles.
When mixed and annealed, the tetrahedral arrays formed superlattices with long-range order where the positions of the gold nanoparticles mimics the arrangement of carbon atoms in a lattice of diamond, but at a scale about 100 times larger.
“Although this assembly scenario might seem hopelessly unconstrained, we demonstrate experimentally that our approach leads to the desired diamond lattice, drastically streamlining the assembly of such a complex structure,” Gang said.
The proof is in the images. The scientists used cryogenic transmission electron microscopy (cryo-TEM) to verify the formation of tetrahedral frames by reconstructing their 3D shape from multiple images. Then they used in-situ small-angle x-ray scattering (SAXS) at the National Synchrotron Light Source (NSLS), and cryo scanning transmission electron microscopy (cryo-STEM) at the CFN, to image the arrays of nanoparticles in the fully constructed lattice.
“Our approach relies on the self-organization of the triangularly shaped blunt vertices of the tetrahedra (so called ‘footprints’) on isotropic spherical particles. Those triangular footprints bind to spherical particles coated with complementary DNA, which allows the particles to coordinate their arrangement in space relative to one another. However, the footprints can arrange themselves in a variety of patterns on a sphere. It turns that one particular placement is more favorable, and it corresponds to the unique 3D placement of particles that locks the diamond lattice,” Gang said.
The team supported their interpretation of the experimental results using theoretical modeling that provided insight about the main factors driving the successful formation of diamond lattices.
Sparkling implications
“This work brings to the nanoscale the crystallographic complexity seen in atomic systems,” said Gang, who noted that the method can readily be expanded to organize particles of different material compositions. The group has demonstrated previously that DNA-assembly methods can be applied to optical, magnetic, and catalytic nanoparticles as well, and will likely yield the long-sought novel optical and mechanical materials scientists have envisioned.
“We’ve demonstrated a new paradigm for creating complex 3D-ordered structures via self-assembly. If you can build this challenging lattice, the thinking is you can build potentially a variety of desired lattices,” he said.
Additional news coverage of these papers is provided by Matthew Gunther at Chemistry World “DNA-coated nanoparticles take crystal engineering into the diamond league“. With the technology for arranging nanoparticles into predetermined lattices continuing to improve, it will be interesting to see what kinds of materials and device arrays are produced.
—James Lewis, PhD
