Cancer imaging: Lighting up tumours.

dominated by the interactions between the DNA ligands, the surface energies of individual DNA-capped nanoparticles and their connectivity could be designed so as to obtain a particular (macroscopic) crystal shape. That is, from knowledge of the local DNA interactions and thermodynamic considerations (such as the Wulff construction) one should be able to predict both the interactions among the individual nanoparticles and the macroscopic crystallization outcome. This connection between the microscopic interactions and the macroscopic behaviour in DNA-mediated nanoparticle crystallization is consistent with conventional crystal-growth theory; hence, concepts of crystallization theory may be extended from atoms and molecules to ‘nanoparticle atoms’ connected by programmable DNA bonds. What’s more, the analogies between atomic, molecular and DNA-mediated nanoparticle crystallization suggest that one could think of a multidimensional ‘periodic table’ for nanoparticles by drawing from the already massive library of both DNA motifs with varied topologies and nanoparticles with unique sizeand shape-dependent properties9. In fact, nanoparticle atoms and DNA motifs can already be combined to generate unique DNA-based nanoparticle building blocks10. The possibility of applying the theoretical framework of atomic crystallization to these building blocks will enhance the control of the assembly of a wider variety of macroscopic crystal shapes and sizes with programmable properties. In this regard, Mirkin and colleagues’ observations and insights set the stage for more to come. ❐ Shogo Hamada1, Shawn J. Tan2 and Dan Luo1,3 are at 1Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA, 2Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore, 3Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York 14853, USA. e-mail: [email protected]