Stable Protein-Repellent Zwitterionic Polymer Brushes Grafted from Silicon Nitride

Zwitterionic poly(sulfobetaine acrylamide) (SBMAA) brushes were grafted from silicon-rich silicon nitride (Si x N4, x > 3) surfaces by atom transfer radical polymerization (ATRP) and studied in protein adsorption experiments. To this aim ATRP initiators were immobilized onto Si x N4 through stable Si−C linkages via three consecutive reactions. A UV-induced reaction of 1,2-epoxy-9-decene with hydrogen-terminated Si x N4 surfaces was followed by conversion of the epoxide with 1,2-ethylenediamine resulting in primary and secondary amine-terminated surfaces. A reaction with 2-bromoisobutyryl bromide led to ATRP initiator-covered surfaces. Zwitterionic polymer brushes of SBMAA were grown from these initiator-coated surfaces (thickness ∼30 nm), and the polymer-coated surfaces were characterized in detail by static water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and an atomic force microscope (AFM). The adsorption of proteins onto zwitterionic polymer coated surfaces was evaluated by in situ reflectometry, using a fibrinogen (FIB) solution of 0.1 g·L−1, and compared to hexadecyl-coated Si x N4 surfaces (C16−Si x N4), uncoated air-based plasma oxidized Si x N4 surfaces (SiO y −Si x N4), and hexa(ethylene oxide)-coated Si x N4 surfaces (EO6−Si x N4). Excellent protein repellence (>99%) was observed for these zwitterionic polymer-coated Si x N4 surfaces during exposure to FIB solution as compared to C16−Si x N4 surfaces. Furthermore, the stability of these zwitterionic polymer-coated Si x N4 surfaces was surveyed by exposing the surfaces for 1 week to phosphate buffered saline (PBS) solution at room temperature. The zwitterionic polymer-coated Si x N4 surfaces before and after exposure to PBS solution were characterized by XPS, AFM, and water contact angle measurements, and their protein-repelling properties were evaluated by reflectometry. After exposure to PBS solution, the zwitterionic polymer coating remained intact, and its thickness was unchanged within experimental error. No hydrolysis was observed for the zwitterionic polymer after 1 week exposure to PBS solution, and the surfaces still repelled 98% FIB as compared to C16−Si x N4 surfaces, demonstrating the long-term efficiency of these easily prepared surface coatings.