Interactions between polycationic and polyanionic layers: Changes in rigidity, charge and adsorption kinetics

Abstract

Interactions between poly-electrolytes on solid substrates and the subsequent formation of films with desirable characteristics is a relatively new technology that strongly depends on the mechanical and chemical properties of the new “two-dimensional” material. We show that binding between positively-charged poly( l-lysine) (PLL) and negatively charged poly(styrene sulfonate) (PSS) result in a significantly more rigid aggregate film than that for non-neutralized layers (∼4 times more rigid). Thus, providing the basis for mechanically stabilized 2D films. The value for the adsorption rate constant, k a, estimated from the random sequential adsorption (RSA) model for the first PLL layer (adsorption onto gold) was ∼0.020 cm/min where as that for the second layer (adsorption on preformed PLL–PSS layer) was ∼0.016 cm/min. The k a, value for the first PSS layer was ∼0.018 cm/min and increased to ∼0.021 cm/min for the second adsorption layer. Using a negatively charged tethered probe with exposed carboxylic acid groups (–COO − at pH 7.4), we could easily follow the charge neutralization process of the adsorbed layers with chemical force microscopy. Sequential layer-by-layer assembly exhibits similar changes in rigidity and adhesion force for the two bi-layers suggesting good reproducibility in physico-chemical properties of the multilayer.