1. Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA
2. X-ray Science Division, Advanced Photon Source, Argonne National Laboratory
3. DND-CAT Synchrotron Research Center, Northwestern University, APS/ANL 432-A004, 9700 S. Cass Avenue, Argonne, Illinois 60439, USA
4. These authors contributed equally to this work.
5. Present address: Department of Biostatistics and Computational Biology, University of Rochester, 601 Elmwood Avenue, Rochester, New York 14642, USA.

Correspondence to: Chad A. Mirkin¹ Correspondence and requests for materials should be addressed to C.A.M. (Email: chadnano@northwestern.edu).

It was first shown^{1, 2} more than ten years ago that DNA oligonucleotides can be attached to gold nanoparticles rationally to direct the formation of larger assemblies. Since then, oligonucleotide-functionalized nanoparticles have been developed into powerful diagnostic tools^{3, 4} for nucleic acids and proteins, and into intracellular probes⁵ and gene regulators⁶. In contrast, the conceptually simple yet powerful idea that functionalized nanoparticles might serve as basic building blocks that can be rationally assembled through programmable base-pairing interactions into highly ordered macroscopic materials remains poorly developed. So far, the approach has mainly resulted in polymerization, with modest control over the placement of, the periodicity in, and the distance between particles within the assembled material. That is, most of the materials obtained thus far are best classified as amorphous polymers^{7, 8, 9, 10, 11, 12, 13, 14, 15, 16}, although a few examples of colloidal crystal formation exist^{8, 16}. Here, we demonstrate that DNA can be used to control the crystallization of nanoparticle–oligonucleotide conjugates to the extent that different DNA sequences guide the assembly of the same type of inorganic nanoparticle into different crystalline states. We show that the choice of DNA sequences attached to the nanoparticle building blocks, the DNA linking molecules and the absence or presence of a non-bonding single-base flexor can be adjusted so that gold nanoparticles assemble into micrometre-sized face-centred-cubic or body-centred-cubic crystal structures. Our findings thus clearly demonstrate that synthetically programmable colloidal crystallization is possible, and that a single-component system can be directed to form different structures.

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