Control over the electrostatic self-assembly of nanoparticle semiflexible biopolyelectrolyte complexes

The electrostatic complexation between model negatively charged silica nanoparticles (NPs) with radius R similar to 10 nm and chitosan, a natural polyelectrolyte bearing positive charges with a semi-rigid backbone of a persistence length of L-p similar to 9 nm, was studied by a combination of SANS, SAXS, light scattering, and cryo-TEM. In this system, corresponding to L-p/R similar to 1, we observe the formation of (i) randomly branched complexes in the presence of an excess of chitosan chains and (ii) well-defined single-strand nanorods in the presence of an excess of nanoparticles. We also observe no formation of nanorods for NPs with poly-L-lysine, a flexible polyelectrolyte, corresponding to L-p/R similar to 0.1, suggesting a key role played by this ratio L-p/R. In the intermediate range of nanoparticle concentrations, we observe an associative phase separation (complex coacervation) leading to more compact complexes in both supernatant and coacervate phases. This method might open the door to a greater degree of control of nanoparticle self-assembly into larger nanostructures, through molecular structural parameters like L-p/R, combined with the polyelectrolyte/nanoparticle ratio.