Emulsion imaging of a DNA nanostar condensate phase diagram reveals valence and electrostatic effects

Liquid-liquid phase separation (LLPS) in macromolecular solutions is relevant both to technology, and to the process of mesoscale structure formation in cells. The LLPS process is characterized by a temperature/concentration phase diagram, which must be quantified to predict the system's behavior. Experimentally, this can be difficult due to complications in handling the dense macromolecular phase. Here, we develop a method for accurately quantifying the phase diagram without direct handling: We confine the sample within micron-scale, water-in-oil emulsion droplets, then use precision fluorescent imaging to measure the volume fraction of the condensate within the droplet. We find this volume fraction grows linearly with macromolecule concentration; thus, by applying the lever rule, we can directly extract the dense and dilute phase concentrations. We use this approach to study a model LLPS system of self-assembled, fixed-valence DNA particles termed nanostars (NSs). We find that phase diagrams of NSs display, with certain exceptions, a larger co-existence regime upon increasing salt or NS valence, in line with expectations. Aspects of the measured phase behavior validate recent predictions that account for the role of NS valence in modulating the connectivity of the condensed phase. Generally, our results on NS phase diagrams give fundamental insight into limited-valence phase separation, while the method we have developed will likely be useful in the study of other LLPS systems.