Multilayers of charged polypeptides as studied by in situ ellipsometry and quartz crystal microbalance with dissipation

The buildup of poly(L-glutamic acid) (PGA) and poly(L-lysine) (PLL) multilayers on silica and titanium surfaces, with and without an initial layer of polyethyleneimine (PEI), was investigated and characterized by means of in situ ellipsometry and quartz crystal microbalance with dissipation. A two-regime buildup was found in all systems, where the length of the first slow-growing regime is dependent on the structure of the initial layers. In the second fast-growing regime, the film thickness grows linearly while the mass increases more than linearly (close to exponentially) with the number of deposited layers. The film refractive indices as well as the water contents indicate that the film density changes as the multilayer film builds up. The change in film density was proposed to be due to polypeptides diffusing into the multilayer film as they attach. Furthermore, the use of PEI as the initial layer was found to induce a difference in the thickness increments for PGA and PLL.