Biofunctional Polyelectrolyte Multilayers and Microcapsules: Control of Non-Specific and Bio-Specific Protein Adsorption

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Layers of the polyelectrolytes poly(allylamine hydrochloride) (PAH, polycationic) and poly(styrene sulfonate) (PSS, polyanionic) are consecutively adsorbed on flat silicon oxide surfaces, forming stable, ultrathin multilayer films. Subsequently, a final monolayer of the polycationic copolymer poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) is adsorbed onto the PSS-terminated multilayer in order to impart protein resistance to the surface. The growth of each of the polyelectrolyte layers and the protein resistance of the resulting [PAH/PPS]n(PLL-g-PEG) multilayer (n = 1–4) are followed quantitatively ex situ using X-ray photoelectron spectroscopy and in situ using real-time optical-waveguide lightmode spectroscopy. In a second approach, the same type of [PAH/PSS]n(PLL-g-PEG) multilayer coatings are successfully formed on the surface of colloidal particles in order to produce surface-functionalized, hollow microcapsules after dissolution of the core materials (melamine formaldehyde (MF) and poly(lactic acid) (PLA; colloid diameters: 1.2–20 μm). Microelectrophoresis and confocal laser scanning microscopy are used to study multilayer formation on the colloids and protein resistance of the final capsule. The quality of the PLL-g-PEG layer on the microcapsules depends on both the type of core material and the dissolution protocols used. The greatest protein resistance is achieved using PLA cores and coating the polyelectrolyte microcapsules with PLL-g-PEG after dissolution of the cores. Protein adsorption from full serum on [PAH/PPS]n(PLL-g-PEG) multilayers (on both flat substrates and microcapsules) decreases by three orders of magnitude in comparison to the standard [PAH/PPS]n layer. Finally, biofunctional capsules of the type [PAH/PPS]n(PLL-g-PEG/PEG-biotin) (top copolymer layer with a fraction of the PEG chains end-functionalized with biotin) are produced which allow for specific recognition and immobilization of controlled amounts of streptavidin at the surface of the capsules. Biofunctional multilayer films and capsules are believed to have a potential for future applications as novel platforms for biotechnological applications such as biosensors and carriers for targeted drug delivery.