Phase behavior and critical activated dynamics of limited-valence DNA nanostars

Abstract

Colloidal particles with directional interactions are key in the realization of new colloidal materials with possibly unconventional phase behaviors. Here we exploit DNA self-assembly to produce bulk quantities of "DNA stars" with three or four sticky terminals, mimicking molecules with controlled limited valence. Solutions of such molecules exhibit a consolution curve with an upper critical point, whose temperature and concentration decrease with the valence. Upon approaching the critical point from high temperature, the intensity of the scattered light diverges with a power law, whereas the intensity time autocorrelation functions show a surprising two-step relaxation, somehow reminiscent of glassy materials. The slow relaxation time exhibits an Arrhenius behavior with no signs of criticality, demonstrating a unique scenario where the critical slowing down of the concentration fluctuations is subordinate to the large lifetime of the DNA bonds, with relevant analogies to critical dynamics in polymer solutions. The combination of equilibrium and dynamic behavior of DNA nanostars demonstrates the potential of DNA molecules in diversifying the pathways toward collective properties and selfassembled materials, beyond the range of phenomena accessible with ordinary molecular fluids.DNA nanotechnology | limited valence colloids | critical behavior I n recent years, a strong effort has been devoted to introduce a new generation of micro-and nanocolloids interacting via strongly anisotropic forces. Anisotropic interactions can simply arise from a nonspherical particle shape or from more sophisticated physical and/or chemical patterning of the particle surface (1-7). An alternative strategy to produce complex nanoparticles is to exploit the self-assembly of DNA oligomers. The rational design of the DNA sequences enables guiding the association of multiple DNA strands into a rich variety of nanosized objects, such as geometrical figures, hollow capsules, and nanomachines, as well as more complex meso-and macroscopic structures (8-13). The selectivity of DNA binding can also be exploited to control the mutual interactions between the structures (14, 15), whereas the spontaneous assembly of DNA sequences enables producing large ensembles of particles. These properties make DNA a powerful tool to explore fundamental phenomena of soft matter and statistical physics, as indicated by previous studies of liquid-crystalline ordering and phase separations in solutions of short DNA oligomers (16-18). Here we exploit DNA self-assembly to experimentally address the phase behavior of particles interacting with specific valence, strength, and selectivity.Colloidal particles with controlled valence are the next step toward the realization of new colloidal materials and phases dependent on the presence of a small number of bonds (1-7). Theoretical and numerical studies (19) predict that a solution of low-valence particles should exhibit phase coexistence-the colloidal analog of the vapor-liquid coexistence in simple liquidsbut only at v...

Accurate phase diagram of tetravalent DNA nanostars

“…The experiments have confirmed previous theoretical and numerical predictions, in that the gas-liquid instability region has been shown to shrink and move to low temperatures and concentrations as the valence decreases [24], giving rise to very low-density, open equilibrium networks, the so-called empty liquids [25][26][27]. These DNA constructs can be considered as a realisation of patchy particles, which, in the last years, have been used as simple model systems to predict the existence and investigate the properties of exotic states of matter such as empty liquids [25], open crystals [28,29], reentrant gels [30] and self-assembling systems [31,32].…”

“…Fig. 11 shows all the investigated state points and the obtained results, the theoretical phase diagram as computed by Wertheim's theory as well as the experimental phase diagram recently measured experimen-tally [24]. The experiments have been carried out at the very low salt concentration 50 mM NaCl, which explains the difference in temperature between the two phase diagrams [55,56].…”

“…The strand sequences designed to self-assemble into tetramers. We use the same sequences used in the experimental work of Biffi et al [24]. Coloured nucleotides form the double-stranded parts of the arms, spacers are in grey and sticky ends are in black.…”

“…We simulate the same DNA sequences used in the experimental study of Ref. [24], offering a microscopic description of the collective phenomena experimentally observed. More specifically, we make use of extensive molecular dynamics simulations on graphic processor units (GPUs) to simulate bulk systems of DNA tetramers at different concentrations and temperatures.…”

“…These sticky ends are self-complementary and allow for intertetramer bonding. These star-shaped constructs with three (DNA trimers) or four (DNA tetramers) arms have been studied to investigate the effect of lowering the valence on the location of the gas-liquid coexistence region of the phase diagram [24].…”