Effect of precursor-layer surface charge on the layer-by-layer assembly of polyelectrolyte/nanoparticle multilayers.

In this Article, we investigate the effect of a precursor layer, which is composed of four bilayers of polyethyleneimine (PEI) and poly(sodium styrene sulfonate) (PSS), on the subsequent LBL assembly of hybrid films composed of indium tin oxide (ITO) nanoparticles and PSS. A precursor polyelectrolyte layer is usually deposited to minimize interference by the substrate. It is shown here that the "effective" surface charge of the precursor layer can significantly affect the subsequent assembly behavior of [ITO/PSS](9.5) hybrid thin films. Depending on the surface charge of the precursor layer, the subsequent LbL assembly of [ITO/PSS](9.5) hybrid films can exhibit either one or two regimes. When two growth regimes are present, the first one consists of a "recovery regime", and the second is the expected "linear growth regime." The length of the "recovery regime" is dependent on how much positive charge the precursor layer possesses and how fast this surface charge can be compensated. This work reveals for the first time that changes in the surface charge of the precursor layer can have a significant effect on the subsequent LBL assembly process. The surface charge of the precursor layer was investigated using ζ-potential measurements on model silica microspheres. These experiments showed that the surface charge of the precursor layer, [PEI/PSS](4), is dependent on the pH of the solution in which it is immersed, and that it can reverse from a negatively charged surface to a positively charged one, at sufficiently low pH due to the protonation of PEI, despite having the negatively charged PSS layer as the outermost layer.