It’s not hard to be a perfectionist if you have all the time in the world to do a job right. Guided by that thinking, Northwestern University’s Evelyn Auyeung, Monica Olvera de la Cruz, Chad A. Mirkin, and coworkers have shown that metal nanoparticles tagged with strands of DNA can be coaxed into assembling nearly perfect crystals with predictable geometries simply by cooling the system from a little above to a little below its melting point over the course of several days (Nature 2013, DOI: 10.1038/nature12739). That finding may lead to custom-designed crystals for photonics, electronics, and catalysis applications. Atoms form crystals by way of fairly well understood processes. Not so for molecules and large particles. Previous attempts to use DNA base-pair recognition to form nanoparticle crystals led to ill-formed crystals or ones with unexpected geometries that varied with nanoparticle size. In contrast, the slow-cooling method leads to micrometer-sized faceted crystals with a rhombic dodecahedron shape (shown, with inset rendering of DNA-tagged nanoparticles) regardless of nanoparticle size. That outcome is the thermodynamically favored and theoretically predicted one, the team notes.