“Single crystals are the backbone of many things we rely on—diamonds for beauty as well as industrial applications, sapphires for lasers, and silicon for electronics,” said nanoscientist Chad A. Mirkin of Northwestern University. “The precise placement of atoms within a well-defined lattice defines these high-quality crystals.” Now Mirkin’s research group has built near-perfect single crystals out of nanoparticles and DNA, suggesting that DNA hybridization can drive the assembly of nanoparticles by a similar route to the traditional crystallization of atomic species.
His research group developed the “recipe” for using nanomaterials as atoms, DNA as bonds, and a little heat to form tiny crystals. This single-crystal recipe builds on superlattice techniques that Mirkin’s laboratory has been developing for nearly two decades.
In this recent work, reported in the November 27 online edition of Nature (DOI:10.1038/nature12739), Mirkin, an experimentalist, teamed up with Monica Olvera de la Cruz, a theoretician at Northwestern, to evaluate this technique. Given a set of nanoparticles and a specific type of DNA, Olvera de la Cruz showed they can accurately predict the three-dimensional structure, or crystal shape, into which the disordered components will self-assemble.
In the study, strands of complementary DNA act as bonds between disordered gold nanoparticles, transforming them into an orderly crystal. The researchers determined that the ratio of the DNA linker’s length to the size of the nanoparticle is critical.
The ratio affects the energy of the faces of the crystals, which determines the final crystal shape. Ratios that do not follow the recipe lead to large fluctuations in energy and result in a sphere, not a faceted crystal, Olvera de la Cruz said. With the correct ratio, the energies fluctuate less and result in a crystal every time.
To achieve a self-assembled single crystal, the research team took two sets of gold nanoparticles functionalized with complementary DNA linker strands. Working with approximately 1 million nanoparticles in water, they heated the solution to a temperature just above the DNA linkers’ melting point and then slowly cooled the solution to room temperature, over a period of two to three days.
The very slow cooling process encouraged the single-stranded DNA to find its complement, resulting in a high-quality single crystal approximately 3 µm in size.
The researchers determined that the length of DNA connected to each gold nanoparticle cannot be much longer than the size of the nanoparticle. In the study, the gold nanoparticles varied from 5 nm to 20 nm in diameter; for each, the DNA length that led to crystal formation was about 18 base pairs and six single-base “sticky ends.”
“There’s no reason we can’t grow extraordinarily large single crystals in the future using modifications of our technique,” said Mirkin.
