PHASE BEHAVIOR OF STAR-SHAPED DNA NANO-STRUCTURES

In this thesis we studied the collective behavior of limited valence DNA particles. By exploiting the selectivity of Watson-Crick pairing, we synthesized star-shaped DNA particles having either three or four arms, each arm terminating in a sticky overhang sequence that provides interactions between individual particles. Each nano-star can thus be viewed as limited valence particle whose valence number f is dictated by the number of star arms. Solutions of such structures are found to exhibit liquid-vapour-like phase separation. Our results show that by reducing the valence of the structures, the coexistence region is greatly shrunk both in temperature and in concentration, thereby confirming for the first time recent theoretical predictions. As the temperature of the system is reduced, and the critical point approached from above, the dynamic behavior slows down and becomes characterized by a two-step relaxation process. The two characteristic times behave differently: the faster one (τf) changes only very mildly while the slower one (τs) slows down by more than three orders of magnitude in an Arrhenius fashion, without any noticeable divergence as Tc is approached. Quite remarkably, τs does not show the power-law divergence expected for critical slowing down. The colloidal system here proposed makes use of DNA not only to introduce mutual interactions between individual particles, but to model their geometry controlling internal interactions at the nanoscale level. This work proves that DNA is a powerful tool to produce particles with directional interactions, and can be used to design complex structures as colloidal molecules at the nanoscale.