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

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, ka, 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 ka, 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.