Lysozyme Monolayers at Polymer Microparticles: Electrokinetic Characteristics and Modeling

Adsorption of hen egg-white lysozyme (HEWL) molecules at negatively charged polystyrene microparticles was studied using dynamic ligh scattering (DLS), electrophoresis (LDV), solution depletion techniques, enzyme-linked immunosorbent assay (ELISA), and atomic force microscopy (AFM). The measurements were carried out at pHs 3.5 and 7.4 and for NaCl/PBS concentration of 10(-2) and 0.15 M. Initially, the dependence of the electrophoretic mobility and zeta potential of microparticles on the bulk concentration of lysozyme was determined. These results were interpreted in terms of the general electrokinetic model. This allowed the authors to devise a formula for precise determination of lysozyme coverage at microparticles under in situ conditions. The maximum coverage of irreversibly adsorbed lysozyme was also determined with the use of the electrokinetic concentration depletion methods and the ELISA assay. At pH 7.4 (PBS buffer) and ionic strength of 0.15 M, the maximum coverage was equal to 0.95 mg m(-2) that agrees with theoretical modeling performed according to the random sequential adsorption approach. The stability of acid base properties of lysozyme monolayers was also determined via the electrophoretic mobility measurements carried out for different ionic strength and pH range of 3-12. These results allowed one to develop a more sensitive method for determining lysozyme bulk concentration than the ELISA assay a robust procedure for preparing its stable monolayers at microparticles of well controlled coverage and acid base properties.