title: Role of the ratio of biopolyelectrolyte persistence length to nanoparticle size in the structural tuning of electrostatic complexes.
abstract: Aggregation of nanoparticles of given size R induced by addition of a polymer strongly depends on its degree of rigidity. This is shown here on a large variety of silica nanoparticle self-assemblies obtained by electrostatic complexation with carefully selected oppositely charged biopolyelectrolytes of different rigidity. The effective rigidity is quantified by the total persistence length L_{T} representing the sum of the intrinsic (L_{p}) and electrostatic (L_{e}) polyelectrolyte persistence length, which depends on the screening, i.e., on ionic strength due to counterions and external salt concentrations. We experimentally show that the ratio L_{T}/R is the main tuning parameter that controls the fractal dimension D_{f} of the nanoparticles' self-assemblies, which is determined using small-angle neutron scattering: (i) For L_{T}/R<0.3 (obtained with flexible poly-l-lysine in the presence of an excess of salt), chain flexibility promotes easy wrapping around nanoparticles in excess, hence ramified structures with D_{f}∼2. (ii) For 0.3<L_{T}/R≤1 (semiflexible chitosan or hyaluronan complexes), chain stiffness promotes the formation of one-dimensional nanorods (in excess of nanoparticles), in good agreement with computer simulations. (iii) For L_{T}/R>1,L_{e} is strongly increased due to the absence of salt and repulsions between nanoparticles cannot be compensated for by the polyelectrolyte wrapping, which allows a spacing between nanoparticles and the formation of one-dimensional pearl necklace complexes. (iv) Finally, electrostatic screening, i.e., ionic strength, turned out to be a reliable way of controlling D_{f} and the phase diagram behavior. It finely tunes the short-range interparticle potential, resulting in larger fractal dimensions at higher ionic strength.
authors.affiliation: Matière et Systèmes Complexes (MSC) Laboratory, UMR CNRS 7057, University Paris Diderot-Paris 7, Sorbonne Paris Cité, Bâtiment Condorcet, 75205 Paris cedex 13, France.|Laboratoire Léon Brillouin, UMR 12 CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: Li
authors.initials: L
authors.lastname: Shi
authors.affiliation: Matière et Systèmes Complexes (MSC) Laboratory, UMR CNRS 7057, University Paris Diderot-Paris 7, Sorbonne Paris Cité, Bâtiment Condorcet, 75205 Paris cedex 13, France.
authors.forename: Florent
authors.initials: F
authors.lastname: Carn
authors.affiliation: Laboratoire Léon Brillouin, UMR 12 CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France.|GMPA, UMR INRA 782, 1 avenue Lucien Brétignières, 78850 Thiverval-Grignon, France.
authors.forename: François
authors.initials: F
authors.lastname: Boué
authors.affiliation: Matière et Systèmes Complexes (MSC) Laboratory, UMR CNRS 7057, University Paris Diderot-Paris 7, Sorbonne Paris Cité, Bâtiment Condorcet, 75205 Paris cedex 13, France.|Laboratoire Léon Brillouin, UMR 12 CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: Eric
authors.initials: E
authors.lastname: Buhler