Stepwise evolution of DNA-programmable nanoparticle superlattices.

Colloidal crystals can be assembled using a variety of entropic, depletion, electrostatic, or biorecognition forces and provide a convenient model system for studying crystal growth. Although superlattices with diverse geometries can be assembled in solution and on surfaces, the incorporation of specific bonding interactions between particle building blocks and a substrate would significantly enhance control over the growth process. Herein, we use a stepwise growth process to systematically study and control the evolution of a body-centered cubic (bcc) crystalline thinfilm comprised of nanoparticle building blocks functionalized with DNA on a complementary DNA substrate. We examine crystal growth as a function of temperature, number of layers, and substrate–particle bonding interactions. Importantly, the judicious choice of DNA interconnects allows one to tune the interfacial energy between various crystal planes and the substrate, and thereby control crystal orientation and size in a stepwise fashion using chemically programmable attractive forces. This is a unique approach since prior studies involving superlattice assembly typically rely on repulsive interactions between particles to dictate structure, and those that rely on attractive forces (e.g. ionic systems) still maintain repulsive particle–substrate interactions. In addition to providing a model for crystallization, the field of particle assembly has garnered considerable interest because materials generated from ordered particle arrays can have novel optical, 13–17] electronic, and magnetic properties. These properties can be sensitive to the composition, symmetry, and distance between nanoparticles, in addition to the number of layers and orientation. DNA-mediated nanoparticle crystallization is particularly attractive for preparing these materials because the nanoparticle building blocks can be considered a type of “programmable atom equivalent” with tailorable size, composition, shape, and bonding interactions. This tunability allows one to access a diverse class of crystal symmetries, tailor lattice parameters with sub-nanometer resolution, and create structures that have no known mineral equivalent. Indeed, to date, 17 unique symmetries have been realized and over 100 unique crystal structures have been synthesized, all of which conform to a key hypothesis: these atom equivalents assemble into structures that maximize the total number of hybridized DNA interconnects between particles. While these structures have enormous potential, their use is limited because they are typically formed in solution as polycrystalline aggregates with little control over crystal size or orientation. Consequently, it is difficult to measure their properties or integrate them with other device elements using existing microfabrication techniques. The development of thin-film superlattices is therefore necessary to fully realize the potential of these structures as metamaterials, photonic crystals, and data storage elements. The growth of DNA-mediated monoand multi-layered nanoparticle structures was first examined by our group and later by Niemeyer and co-workers. However, the use of strong DNA interactions prevented nanoparticle crystallization. Herein, we exploit multiple weak DNA interactions for superlattice growth to examine the development of crystal orientation (texture) and control film thickness. Body-centered cubic colloidal crystals composed of spherical nucleic acid gold nanoparticle conjugates (SNA-AuNPs) were used as a model system since these structures require two complementary particle types and therefore allow the stepwise introduction of each layer. Alternatively, other crystal symmetries such as face-centered cubic (fcc) require