Paper BI-TuP2
The Role of Surface Chemistry in poly-L-lysine Mediated Alginate Adsorption

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Adsorption of polysaccharides to solid substrates, an essential component of biofilm formation, is generally protein-mediated. Lysine-rich proteins, in particular, are known to have significant adhesive properties in these systems. This work uses poly-L-lysine (PLL) as a model protein to explore the role of the underlying substrate in protein-mediated adsorption of alginate, a negatively-charged polysaccharide. We have prepared films of varying PLL coverage on two types of surfaces: hydrophilic, negatively-charged silica and a hydrophobic, electrically neutral alkylsilane monolayer. PLL coverage on the hydrophilic surface is controlled by varying the ionic strength of the adsorption solution, while PLL coverage on the hydrophobic surface is controlled by varying the molecular weight of the PLL. By a combination of techniques--primarily ellipsometry, dynamic contact angle goniometry and atomic force microscopy--we demonstrate that not only the film morphology of the PLL but also its effect on subsequent alginate adsorption is highly sensitive to the surface chemistry of the underlying substrate. For example, PLL films on silica are more discontinuous at low and high ionic strength, and alginate adsorption is highly influenced by this discontinuity, which allows for repulsive electrostatic interactions between the surface and the alginate. In contrast, while there is an increase in PLL film discontinuity as a function of increasing PLL molecular weight, alginate adsorption appears largely insensitive to these changes in PLL film morphology. Through quantitative analysis of the advancing contact angles on both these (chemically heterogeneous) surfaces we demonstrate unequivocally that the PLL/solution interface is highly dependent on both substrate surface chemistry and PLL adsorption conditions.