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Aggregation of nanoparticles of given size $R$ induced by addition of apolymer strongly depends on its degree of rigidity. This is shown here on alarge variety of silica nanoparticle self-assemblies obtained by electrostaticcomplexation with carefully selected oppositely charged bio-polyelectrolytes ofdifferent rigidity. The effective rigidity is quantified by the totalpersistence length $L_T$ representing the sum of the intrinsic ($L_p$) andelectrostatic ($L_e$) polyelectrolyte persistence length, which depends on thescreening, i.e., on ionic strength due to counter-ions and external saltconcentrations. We experimentally show for the first time that the ratio L T /Ris the main tuning parameter that controls the fractal dimension D f of thenanoparticles self-assemblies, which is determined using small-angle neutronscattering: (i) For $L_T /R<0.3$ (obtained with flexible poly-L-lysine in thepresence of an excess of salt), chain flexibility promotes easy wrapping aroundnanoparticles in excess hence ramified structures with $D_f \sim 2$. (ii) For$0.3<L_T /R\le1$ (semiflexible chitosan or hyaluronan complexes), chainstiffness promotes the formation of one-dimensional nanorods (in excess ofnanoparticles), in good agreement with computer simulations. (iii) For $L_T/R>1$, $L_e$ is strongly increased due to the absence of salt and repulsionsbetween nanoparticles cannot be compensated by the polyelectrolyte wrapping,which allow a spacing between nanoparticles and the formation of onedimensional pearl necklace complexes. (iv) Finally, electrostatic 2 screening,i.e. ionic strength, turned out to be a reliable way of controlling $D_f$ andthe phase diagram behavior. It finely tunes the short-range interparticlepotential, resulting in larger fractal dimensions at higher ionic strength.

Role of the ratio of biopolyelectrolyte persistence length to nanoparticle size in the structural tuning of electrostatic complexes