Synthesis and characterization of poly(N-isopropylacrylamide) brushes on Si(100) surface by surface initiated nitroxide mediated polymerization method and its use as bioactive surface

In this study, amino, hydroxyl and carboxylic acid-ended poly(N-isopropylacrylamide) brushes were synthesized on Si(100) surfaces by surface initiated bimolecular nitroxide mediated polymerization method. Chain transfer agents were added into polymerization medium to obtain functional-ended polymeric brushes. For synthesis of amino, hydroxyl and carboxylic acid-ended poly(N-isopropylacrylamide) brushes, cysteamine, 2-mercaptoethanol and 3-mercaptopropionic acid were used, respectively. The polymerization was controlled by adding different chain transfer agents that were in different concentrations in polymerization solution, in this way polymeric brushes having different film thicknesses were synthesized onto silicon surfaces. ATR-FTIR spectroscopy was used for structural characterization of polymeric brushes. Especially, the peaks at the spectrums of amino, hydroxyl and carboxylic acid-ended polymeric brushes are assigned to the NH2, OH and CO stretching vibrations showed that we synthesized functional-ended polymeric brushes onto silicon surfaces. The hydrophilic/hydrophobic behaviors of poly(N-isopropylacrylamide) brush and amino, hydroxyl and carboxylic acid-ended poly(N-isopropylacrylamide) brushes were determined by water contact angle measurements. Poly(N-isopropylacrylamide) brush showed hydrophobic character, while functional-ended poly(N-isopropylacrylamide) brushes showed hydrophilic character and increasing of concentration of chain transfer agent, hydrophilic character of polymeric brushes increased. The surface topographies of the polymeric brushes observed by Atomic Force Microscopy (AFM) were considerably different from the hydroxylated silicon surface and they had many additional islands of various heights over the substrate. While polymeric brushes covalently attached to silicon surfaces were synthesized via bimolecular nitroxide mediated polymerization, free polymer chains were also formed in the polymerization solution, simultaneously. As a consequence of increasing of concentration of chain transfer agent, decreasing of polymeric film thicknesses of polymeric brushes were determined by Ellipsometry and decreasing of molar masses of free polymers in solution were determined by Gel Permeation Chromatography. The fluorescence images were obtained by Fluorescence Microscopy in order to determine surface patterns of polymeric brushes. Although any fluorescence signal was not observed on fluorescence image of hydroxylated silicon surface, many fluorescence signals were obtained on fluorescence image of immunoglobulin G immobilized amino-ended polymeric brushes. Increasing of film thicknesses due to the protein adsorption on the amino terminated surfaces were also determined by Ellipsometry.