Surface modification of the channels of poly(dimethylsiloxane) microfluidic chips with polyacrylamide for fast electrophoretic separations of proteins

The electrophoresis of proteins was investigated using poly(dimethylsiloxane) (PDMS) microfluidic chips whose surfaces were modified with polyacrylamide through atom-transfer radical polymerization. PDMS microchips were made using a glass replica to mold channels 10 microm high and 30 microm wide, with a T-intersection. The surface modification of the channels involved surface oxidation, followed by the formation of a self-assembled monolayer of benzyl chloride initiators, and then atom-transfer radical polymerization to grow a thin layer of covalently bonded polyacrylamide. The channels filled spontaneously with aqueous buffer due to the hydrophilicity of the coating. The resistance to protein adsorption was studied by open-channel electrophoresis for bovine serum albumin labeled with fluorophor. A plate height of 30 microm, corresponding to an efficiency of 33 000 plates/m, was obtained for field strengths from 18 to 889 V/cm. The lack of dependence of plate height on field strength indicates that there is no detectable contribution to broadening from adsorption. A 2- to 3-fold larger plate height was obtained for electrophoresis in a 50-cm polyacrylamide-coated silica capillary, and the shape of the electropherogram indicated the efficiency is limited by a distribution of species. The commercial capillary exhibited both reversible and irreversible adsorption of protein, whereas the PDMS microchip exhibited neither. A separation of lysozyme and cytochrome c in 35 s was demonstrated for the PDMS microchip.