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

Abstract

The electrostatic complexation between model negatively charged silica nanoparticles (NPs) with radius R ∼ 10 nm and chitosan, a natural polyelectrolyte bearing positive charges with a semi-rigid backbone of a persistence length of L_p ∼ 9 nm, was studied by a combination of SANS, SAXS, light scattering, and cryo-TEM. In this system, corresponding to L_p/R ∼ 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 ∼ 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.