Highly ordered arrays of nanoparticles hold the promise of functionality determined by the periodic arrangement of their constituent nanoscale building blocks. The rational assembly of DNA-functionalized nanoparticles has proven an effective method for generating well-defined crystalline lattices, but is limited in the number of geometries. C.A. Mirkin, E. Auyeung, J.I. Cutler, and their colleagues at Northwestern University have recently improved the capabilities of this method, allowing them to create lattices with previously unobserved symmetries. As reported in the January issue of Nature Nanotechnology (DOI: 10.1038/NNANO.2011.222; p. 24), Mirkin’ s group demonstrates the utility of three-dimensional hollow spacers, or spherical nucleic acid (SNA) nanostructures, that take the place of DNA-functionalized gold nanoparticles, enabling the researchers to expand the library of superlattices they are able to create as well as to make novel structures not previously observed in nature.

The hollow spacers are made using gold nanoparticles as a template and offer the potential of superior control over the superlattice structure. The technique utilizes gold nanoparticles functionalized with alkyne–modified DNA, which can be employed to generate a rigid network by cross-linking the densely packed alkyne units. The gold particle cores within the DNA shells are then dissolved, generating hollow spherical nucleic acids (SNAs) nanoparticle conjugates which are nearly identical in size to their gold nanoparticle counterparts. They also exhibit many of the same unique chemical and physical properties, including the ability to participate in cooperative binding events, which is a necessary requirement for their use in this programmed assembly application. However, the spacers do not scatter x-rays and are observed as blank positions in x-ray scattering experiments.

The research team demonstrates the utility of hollow SNA nanostructures by first changing the molar ratio of the gold nanoparticle units to spacer particles, and secondly by changing the size of the gold and spacer nanoparticles. Initially, a body centered cubic (bcc) system was formed using two sets of gold nanoparticles of equal size (molar ratio 1:1) and with complementary sticky ends (5´-AAGGAA-3´ for the first group and 5´-TTCCTT-3´ for the second group). By replacing one of the gold nanoparticle groups with a hollow spacer group, the researchers were able to form a simple cubic system.

Alternatively, a 2:1 ratio of 20 nm and 10 nm gold nanoparticles was used to create AB₂-type crystal superlattices. By substituting the gold nanoparticles in this system for spacers, the researchers demonstrated simple hexagonal (10 nm spacer) and graphite-like symmetries (20 nm spacer). Finally, AB₆ symmetry was created using a 1:6 ratio of 20 nm to 10 nm gold nanoparticles. When the 20 nm spacer was substituted for 20 nm gold nanoparticles, a completely new symmetry group was observed that the team dubbed “ Lattice X. ”

The researchers said that the improved structural diversity provided by their hollow particle approach will lead to the development of new functional materials that can be used in a wide variety of applications ranging from plasmonics to catalysis.