Surface Assembly of Catechol-Functionalized Poly(l-lysine)-graft-poly(ethylene glycol) Copolymer on Titanium Exploiting Combined Electrostatically Driven Self-Organization and Biomimetic Strong Adhesion

Nonfouling coatings, based on surface-tethered, hydrophilic polymer chains, have widespread application in areas such as biosensing, medical devices, and biotechnology. Self-organization of polymers is a particularly attractive approach given its simplicity and cost-effectiveness in the application. Here we present a new class of polymers based on the polycationic poly(l-lysine)-graft-poly(ethylene glycol) copolymer (PLL-g-PEG) with a fraction of the amine-terminated lysine side chains covalently conjugated to 3,4-dihydroxyphenylacetic acid (DHPAA). This copolymer is shown to adsorb and self-organize as a confluent monolayer on negatively charged titanium oxide surfaces, driven by long-range electrostatic attraction, while the catechol groups of DHPAA spontaneously engage in strong, coordinative binding to the substrate surface, similar to the biomimetic dihydroxyphenylalanine (DOPA) found in mussel adhesive proteins. The adsorption kinetics and resulting polymer coverage are demonstrated to critically depend on (a) a rational design of the copolymer architecture with a compromise between sufficient positive charges in the PLL backbone and a minimal grafting density of DHPAA groups and (b) optimum choice of ionic strength and temperature of the assembly solution. PLL-graft-(DHPAA; PEG) adlayers exhibit excellent resistance to nonspecific protein (fibrinogen) adsorption. To test the chemical stability of the polymeric layer, coated substrates were exposed to high ionic salt solutions and proved to remain nonfouling thanks to stable catechol−substrate anchorage, in stark contrast to the control PLL-g-PEG copolymer that desorbed under these conditions as a consequence of screening of the (purely) electrostatic surface forces. Furthermore, polymer-coated substrates resisted attachment of the cyanobacterium Lyngbya sp. over a time frame of at least 100 days.