news of the week OCTOBE R 17, 201 1 EDITED BY WILLIAM G. SCHULZ & SOPHIA L. CAI
MATERIALS SCIENCE: Guidelines
predict structures formed by nanoparticles and DNA linkers
A
NEW SET OF RULES does for nanoparticle lat-
pal rule “is very simple,” Mirkin says: “The structure that maximizes the number of hybridization links will be the structure that’s thermodynamically favored.” The lattice structures and their properties depend on the overall size of the particles plus the linkers (the total hydrodynamic radius), not on the sizes of the particles or linkers individually. By changing the size of a particle or length of a linker relative to one another, the team can tailor lattice properties such as interparticle distances on length scales of 25 to 150 nm. They
tices what Linus Pauling’s rules did for atomic crystal lattices—it explains which structures will be the most stable. But the design rules developed by Chad A. Mirkin and coworkers at Northwestern University and Argonne National Laboratory go a step further than Pauling’s rules: They allow researchers to tune materials’ properties rather than just predict them (Science, DOI: 10.1126/science.1210493). Such designed materials have many applications, including optics, catalysis, and separations. LATTICE OPTIONS Gold nanoparticles with DNA linkers assemble into lattices that Mirkin and coworkers use DNA-decmaximize hybridization interactions between neighboring particles. The lattices shown orated gold nanoparticles that assemble have the same structures as (from left) Cr3Si, AlB2, CsCl, NaCl, and Cs6C60 crystals. into periodic lattices via hybridization between DNA strands on adjacent particles. The researchers now report that the resulting designed 41 crystals, each of which assembled into one structures are governed by a set of six rules involving of nine lattice structures, and characterized them usnanoparticle size and DNA length and sequence. ing synchrotron-based small-angle X-ray scattering at The six rules actually boil down to one overarching Argonne. rule, from which the others flow naturally. The princi“In normal chemistry, we can’t do this—we can’t change the size of atoms,” Mirkin says. “But with these types of nanoscale ‘atoms,’ we can adjust their size, their recognition properties, their valency, and thereTHE SIX RULES fore the types of structures we can make in a much ◾ When all DNA-nanoparticles (DNA-NPs) in a more straightforward manner.” system have equal hydrodynamic radii, each NP The Northwestern team used their design rules to in the thermodynamic product will maximize the make a variety of structures with gold nanoparticles, number of nearest neighbors to which it can form but the rules should work with any type of nanoparticle DNA connections. to which DNA can be attached, Mirkin says. ◾ Kinetic products can be formed by slowing The one drawback is that the structures currently the dehybridization/rehybridization rate of DNA exist only in solution, Mirkin notes. “They collapse linkers. in the solid state,” he says. “Moving these to the solid ◾ The overall hydrodynamic radius of a DNA-NP state will be a challenge.” dictates its assembly and packing behavior. The Northwestern-Argonne team “has provided a ◾ In binary systems, the particle size ratio and new handbook for the engineering of nanoparticleDNA linker ratio between two particles dictate DNA hybrids,” says Christopher B. Murray, a nanothe thermodynamic structure. materials expert at the University of Pennsylvania. ◾ Systems with the same size ratio and DNA linker “This is the realization of true programmable matter, ratio will form the same thermodynamic product. and Mirkin and coworkers have shared the source ◾ The most stable structure will maximize all code with the rest of the research community.” These possible types of DNA sequence-specific hybriddesign guidelines “advance significantly what was alization interactions. ready a very hot area for soft nanomaterials design,” he notes.—CELIA ARNAUD WWW.CEN-ONLINE.ORG
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OCTOBER 17, 2011
COURTESY OF CHAD MIRK IN
RULES FOR DESIGN
