AFM study of adhesion and interactions between polyelectrolyte bilayers assembly

In this report we present the influence of the first polyelectrolyte layer and the number of subsequent bilayers on physicochemical interactions and adhesion forces in polyelectrolyte multilayers (PEM) composed of poly(ethyleneimine) (PEI) and poly(styrene sulfonate) (PSS) deposited by layer-by-layer (LbL) technique on gold substrate. Morphology, thickness and adhesive properties of polyelectrolyte multilayers were studied by the means of AFM imaging and force spectroscopy methods, in relation to the number of deposited layers and polyelectrolyte deposition order. Thickness growth was quicker for PEI/PSS comparing to PSS/PEI sequence bilayers. Thickness values varied from 4.9 nm to 25.4 nm. The adhesion measurements demonstrate that interaction forces between the probe and PEM varied from 1 nN to 20 nN and increased with time of probe-sample contact. Attractive interactions were observed for systems capped with either PEI or PSS. For PEI capped films multiple snap-off points were observed suggesting a creation of bridge-like structures between the deposited film and sensing probe. PSS capped films showed no such features. It is proposed that PSS is adsorbed on Au by its hydrophobic carbon chain, while hydrophilic sulfonate groups are oriented towards bulk solution when PEI is adsorbed on Au surface via attractive electrostatic interactions between positively charged nitrogen atoms and a negatively charged gold surface.