The importance of repulsion in the aggregation-dissociation behavior of DNA coated colloids

Coating particles with DNA gives excellent control over the specificity, strength and range of their interactions. In our experiments, a pair of complementary `Watson-Crick' sequences is used to obtain thermoreversible binding of different colloids. Colloids coated with such complementary `sticky' DNA aggregate when they are mixed together, the aggregates dissolve again when they are heated. We investigate the melting behavior of colloids coated with both sticky and non-sticky DNA. By changing the ratio between the sticky and non-sticky DNA, the obtained melting curves have very different melting temperatures, but almost the same sharpness. We show that the sharpness does not change because a high number of bonds bridges the particles, and that the strong shift in melting temperature is due to a repulsive interaction induced by the confinement of the non-sticky DNA when the particles come close together.