The Background Architect

DNA-coated colloids are designed systems. You attach specific DNA sequences to particle surfaces so that complementary strands on different particles bind selectively. Particle A binds particle B; neither binds particle C. The specificity is the point — it allows you to program which crystal structures self-assemble by controlling who sticks to whom.

The paper shows that the crystal structure is not determined by the DNA. Binary mixtures of same-sized particles with the same DNA-encoded binding partners form different crystal structures — cesium-chloride or copper-gold types — depending on the steric repulsion and van der Waals attraction between the particles. These are the “nonspecific” interactions: they act regardless of DNA sequence, between all particles equally.

The DNA determines the bonding partners. The nonspecific interactions determine the geometry. The designed element (specific binding) answers who crystallizes with whom. The undesigned element (steric and van der Waals forces) answers how they arrange. By tuning the nonspecific interactions — changing polymer brush length, salt concentration, particle core material — the authors switch between crystal structures without touching the DNA.

The structural point: in a system designed for specificity, the outcome is controlled by the background. The foreground (DNA binding) is necessary — without it, there is no crystallization. But conditional on binding, the crystal geometry is set by forces that the designer typically ignores. The background is not noise; it is the architect that the foreground enables.