Probing Liquid-Liquid Phase Transition by DNA Cubes

Liquid-liquid phase transitions, for example, between low-density and high-density liquids, are rich phenomena, however, it is difficult to study experimentally. Colloidal analogs of liquids provide an opportunity for understanding the relationship between liquid structure and phase transition behavior. We consider in our study low-density liquid can be a single tetrahedral network, and high-density liquid contains two single tetrahedral networks that interpenetrate each other. Based on structure and symmetry analysis, we developed an experimental single-component colloidal system in which DNA cubic frames can mimic the directionality and connectivity of hydrogen bonding of water molecules, and these cubes can self-assemble into colloidal analogs of the single and interpenetrating water phases. We investigated liquid-liquid phase transition by probing the structural transformation between the assembled colloidal networks. To trigger the phase transition, we use a toehold-mediated strand displacement reaction to activate/deactivate bonds. We utilize small-angle x-ray scattering (SAXS) to characterize the structure factor of the two colloidal networks at different phase states and during a phase transition. The results show that the two assembled networks can be differentiated by a characteristic scattering peak, and the degree of a network order depends on the inter-cube bonds and rigidity of the cubes.