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Unformatted text preview: LETTERS PUBLISHED ONLINE: 30 NOVEMBER 2008 DOI: 10.1038/NMAT2338 Probing interfacial equilibration in microsphere crystals formed by DNA-directed assembly Anthony J. Kim 1 * , Raynaldo Scarlett 1 * , Paul L. Biancaniello 2 , Talid Sinno 1 and John C. Crocker 1 DNA is the premier material for directing nanoscale self-assembly, having been used to produce many complex forms 14 . Recently, DNA has been used to direct colloids 5,6 and nanoparticles 7,8 into novel crystalline structures, providing a potential route to fabricating meta-materials 9 with unique optical properties. Although theory 1012 has sought the crystal phases that minimize total free energy, kinetic barriers 13 remain essentially unstudied. Here we study interfacial equilibration in a DNA-directed microsphere self-assembly system 5,6,14 and carry out corresponding detailed simulations. We introduce a single-nucleotide difference in the DNA strands on two mixed microsphere species, which generates a free-energy penalty 5,15,16 for inserting impurity spheres into a host sphere crystal, resulting in a reproducible segregation coefficient. Comparison with simulation reveals that, under our experimental conditions, particles can equilibrate only with a few nearest neighbours before burial by the growth front, posing a potential impediment to the growth of complex structures. In earlier studies, we showed that micrometre-sized polymer spheres with single-stranded DNA grafted on their surfaces can form large, close-packed colloidal crystals 5,6 when the DNA strands hybridize to bridge them together. Within this interaction system, some local annealing is possible owing to the fact that bridge formation is a dynamic, reversible process. For crystallization to occur, the microspheres must be highly monodisperse ( < 4% standard deviation in diameter) and the DNA-induced sphere sphere binding energy, E b , must be the proper strengthtoo strong and the particles bind strongly to assemble fractal aggregates, too weak and assembled structures dissociate. One feature of DNA-mediated interactions is that the computed binding energy depends exponentially on the free-energy change, 1 G , for bridge formation (see Supplementary Information), as confirmed by direct interaction measurements 5 . Owing to the strong temperature dependence of 1 G for DNA hybridization 15 , the corresponding temperature window for crystal formation 17 is only . 5 C wide. Within this temperature range, crystallites nucleate homogeneously in less than 24 h, and grow to a size of a few thousand microspheres within another 1224 h. To better understand the annealing and equilibration that takes place on the growing crystal interface, we designed a system expected to form a solid-solution alloy (Fig. 1). Specifically, we combined two populations of 0.98- μ m-diameter polymer spheres (carboxylate-modified polystyrene, Seradyn) that are essentially identical in their preparation 18 and physical parameters, but that...
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