Evidence of different growth regimes coexisting within biomimetic Layer-by-Layer films

Layer-by-Layer (LbL) films are extensively studied as promising (bio)functional coatings. Although many of their structural features are known, the self-assembly mechanism governing their buildup is still under debate. In this work, we describe the initial buildup and structure of biomimetic LbL films comprised of chondroitin sulfate A (ChS) and poly(L-lysine) (PLL), by means of a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), Optical Waveguide Lightmode Spectroscopy (OWLS), Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Atomic Force Microscopy (AFM). While the QCM-D and ATR-FTIR reveal a supralinear, so-called "exponential" growth regime, OWLS surprisingly indicates a linear growth regime. Based on a thorough discussion of the experimental data, we conclude from this apparent contradiction that, shortly after the beginning of the buildup, an inner "dense" zone and an outer "diffuse" zone governed by different growth regimes coexist within the film. This observation constitutes an experimental validation of the structural model assuming a dense, gradually restructuring internal zone, proposed in the literature to explain the exponential-to-linear regime transition of exponentially growing LbL films.