TOWARDS MINIATURIZED ELECTROPHORESIS AND CHEMICAL-ANALYSIS SYSTEMS ON SILICON - AN ALTERNATIVE TO CHEMICAL SENSORS

The concept of a multi-manifold flow system integrated on a silicon substrate, with valveless switching of solvent flow between channels and electro-osmotic pumping of an aqueous solvent, is presented. A device consisting of two intersecting channels has been micromachined in silicon to test the concept of applying 50-1500 V to the channels to effect electro-osmotic pumping of fluid and electrophoretic separation of sample components. The dielectric breakdown voltages of oxide/nitride films in aqueous solution, in combination with the reverse-bias breakdown potential of silicon itself, are determined. The results show devices that withstand potentials of over 1000 V can be fabricated. Sample volumes on the order of 6-300 pl are defined at the intersection of sample and sensing channels using silicon micromachining. With a pH 8.5 boric acid/tris(hydroxymethyl)aminomethane buffer in 50 mum channels of a device a solvent flow rate of about 0.01 cm/s at 31 V/cm is estimated. A migration rate for the fluorescent indicator fluorescein of 0.003 cm/s is observed, in agreement with theoretical predictions. Coupling to a fluorescence detection system gives detection limits of 10(-8) M.