Grafting of poly(gamma-benzyl-L-glutamate) on chemically modified silicon oxide surfaces

Three approaches for constructing self-assembled poly(gamma-benzyl-L-glutamate) (PBLG) monolayers on silicon(100) native oxide surfaces are investigated. In the first method (scheme A), we deposit PBLG onto the surface by reacting the N-terminus of the PBLG with a preformed surface chloroformate group. In the second method (scheme B), we use (aminopropyl)triethoxysilane (APS) as the initiator for a bulk polymerization, leading to synthesis of PBLG having a triethoxysilane end group, which will react directly with surface silanol moieties. In the third method (scheme C), we initiate polymerization of the N-carboxy anhydride of gamma-benzyl-L-glutamate by a surface-attached APS. Through the comparison of the ellipsometric thicknesses of the PBLG films prepared by these three different methods, we have found that the ''grafting to'' protocols (schemes A and B) are more successful than the ''grafting from'' approach (scheme C) in constructing chemisorbed PBLG films. We have found from FTIR that PBLG retains its original secondary structure in the ''grafting to'' methods; the higher the molecular weight, the larger the portion of alpha-helical structure and the lower the portion of beta-sheet structure. However, the film orientations for schemes A and B are different. In scheme A, the PBLG molecular axis is roughly parallel to the surface. By contrast, in Scheme B, the surface orientation of PBLG is tilted. Finally, chemical and thermal stability of the films are discussed, and several cleaning procedures and a high-temperature treatment are conducted to serve for this purpose.