Image on the left is the cover of Jan. 31 issue of Nature.
Anyone who took high school chemistry has played with “sticks and balls” models of molecules or crystalline atoms. There are magnetic toy sets which allow kids to assemble their own version of crystals.
A similar feat, but on nanoscale, was accomplished by two groups – one at Brookhaven (Nykypanchuk et. al Nature 451, 549-552 (2008)) and another at Northwestern (Park et. al, Nature 451, 553-556 (2008)).
The basic idea behind these two experiments is to graft different strands of DNA molecules onto particles, creating two “species” of particles with complimentary pairs of DNA strands. These two different types of strands can merge together into a double helix at high temperatures, making a strong connection between particles of the opposite species. The result is a cubic structure, similar to ion salts.
The nanoparticle crystals held together by DNA molecules are rather fragile, and most of volume is occupied by water. The typical size of the unit cell is on the order of 35-50 nm, with particle size just 10 nm in size. The nanoparticles therefore occupy only a tiny fraction of the total crystal volume, with density of such “fluffy” crystals less than milk foam in your latte.