Ph-promoted exponential layer-by-layer assembly of bicomponent polyelectrolyte/nanoparticle multilayers

Exponential growth of layer-by-layer (LbL) assembled films is desirable because this method considerably increases the growth rate, resulting in much thicker films in a shorter period of time than is the case with normally linearly grown LbL thin films. For the first time, we demonstrate the exponential LbL (e-LbL) growth of poly(ethyleneimine)/SiO₂ nanoparticles (PEI/SiO₂) bicomponent thin films that consist mostly of SiO₂ nanoparticles (over 90 wt % obtained by thermogravimetric analysis). These results are in contrast to earlier e-LbL studies, where the film thickness was made up mostly of the polyelectrolyte, with a very small percentage coming from the inorganic nanoparticles. Here, we show that the LbL growth of the PEI/SiO₂ system significantly depends on the pH of the PEI and the SiO₂ solutions. The e-LbL growth will only occur when the film is deposited with PEI at a high pH and SiO₂ at a low pH. The exponential growth was characterized using a quartz crystal microbalance, atomic force microscopy and scanning electron microscopy imaging, and neutron reflectometry. It is demonstrated that e-LbL films can grow to thicknesses as large as 2-3 μm within just 10 bilayers. The findings reported in this article emphasize new opportunities for the e-LbL growth of organic/inorganic bicomponent composite thin films that may have applications as electrically conducting films, hydrophobic films, and brick-and-mortar biomimetic films.