Free-flow isoelectric focusing microfluidic device with glass coating by sol–gel methods

A device for microfluidic free-flow isoelectric focusing (μ-FFIEF) was fabricated in a glass-coated PDMS structure. The microfluidic channel was made by a PDMS replica molding and standard soft photolithography technique. For the prefractionation and preconcentration of a protein mixture, we patterned palladium electrodes on the glass substrate and coated with TEOS–sol to enhance the bonding with PDMS. The PDMS microchannel was coated with glass by flushing TEOS–sol and annealing at 100 °C. The optimum focusing condition was obtained by pH indicator with ampholyte solution made with MES. Model proteins, RFP and EGFP, were successfully concentrated around 214 μm and 357 μm from the anode, respectively. The optimum focusing condition in the μ-FFIEF device was an electric potential of 1.5 V/cm and pH 6.0. The focusing time in the channel without glass coating was approximately 30 s, but the breakdown of the electrode by electrolysis started immediately when the electric field was applied. However, the glass-coated device was kept intact under continuous application of voltage giving a stable pH gradient, but with twice the focusing time.