Polymer/nucleotide droplets as bio-inspired functional micro-compartments

Using a range of physical methods, we describe the formation, structure, stability, physical properties and uptake behavior of condensed liquid micro-droplets prepared by electrostatically induced complexation of poly(diallyldimethylammonium) chloride (PDDA) and adenosine triphosphate (ATP) in water. Depending on the PDDA monomer: ATP molar ratio, positively charged or charge-neutral droplets are produced spontaneously by simple mixing. The former are typically 60–600 nm in mean size and stable with respect to sedimentation up to temperatures of 85 °C, whilst the latter grow into droplets several tens of micrometres in diameter that coalesce into a macroscopic coacervate phase. Coacervation is inhibited at pH values less than 3 and at high ionic strength, confirming the importance of charge interactions in droplet formation and stability. The droplet interior is structurally homogeneous with no surrounding membrane, comprises dynamically fluctuating domains of partially desolvated polymer/nucleotide complexes, and has a dielectric constant considerably lower than water. As a consequence, dye molecules, porphyrin macrocycles, inorganic nanoparticles or globular proteins can be sequestered from the external water phase into the droplets to produce PDDA/ATP droplets comprising supramolecular J-aggregate nanostructures, magnetically responsive deformable fluids, or soft compartments with potential storage and release properties.