Influence of the Adsorption Time on the Growth of Bovine Serum Albumin–Chondroitin Sulfate Multilayer Films: A Surface Plasmon Resonance Spectroscopy Study

Multilayer films of bovine serum albumin (BSA) and chondroitin sulfate (CS) were assembled on gold substrates at pH 4.2 using the layer-by-layer (LbL) method, and their structural evolution was investigated via surface plasmon resonance (SPR) spectroscopy. To elucidate the role of adsorption kinetics in multilayer buildup, BSA adsorption times of 15, 30, 60, and 120 min (for each BSA adsorption cycle) were examined, while the CS adsorption time was kept constant at 15 min. Analysis of SPR reflectivity curves revealed two distinct growth regimes for all cases, an initial exponential followed by a linear regime. Longer BSA adsorption time promoted deeper chain infiltration, extending the exponential region and shifting the exponential-to-linear transition to higher bilayer numbers. In parallel, evaluation of the average dielectric constant of the polymer layer enabled quantification of the film volume fraction, which initially increased linearly before reaching a saturation plateau. The evolution of the adsorbed mass was inferred from the combined analysis of film thickness and volume fraction, revealing two distinct growth cases depending on the adsorption time. These cases reflect different relationships between the transition from exponential to linear growth and the saturation of the volume fraction. These findings demonstrate that BSA adsorption time is a key kinetic parameter governing chain diffusion, film compaction, and overall multilayer architecture. The results provide insight into the physicochemical mechanisms that dictate biopolymer film growth and offer a route for tuning the structural and optical properties of protein-polysaccharide coatings.