Programming Colloidal Phase Transitions with DNA Strand Displacement

Interactions induced by the transient bridging of complementary DNA strands grafted to colloidal particles can direct the self-assembly of nanoparticle-based materials. Such DNA-grafted nanoparticles have been called 'programmable atom-equivalents': Like atoms, they form three-dimensional crystals, but unlike atoms, the particles themselves carry information (the sequences of the grafted strands), which can be used to 'program' the equilibrium crystal structures.  We present experimental results showing that the programmability of these colloids can be generalized to the full temperature-dependent phase diagram, and not just the structures themselves.  We add additional information to the buffer, in the form of soluble DNA strands designed to compete with the grafted strands through strand displacement.  Employing only two displacement reactions, we program phase behavior not found in atomic systems or other DNA-grafted colloids, including arbitrarily wide gas-solid coexistence, crystals that melt upon cooling, and even reversible transitions between distinct crystal phases. Our findings outline a new direction in self-assembly, in which additional information supplied to the buffer programs equilibrium pathways between many different target structures within a closed system.