A new type of capacitance-coupled contactless conductivity detection (C(4)D) system with sidewall electrodes was proposed for integration on a silicon-on-isolator-poly(dimethylsiloxane) (SOI-PDMS) hybrid low-voltage-driven electrophoresis microchip. By a microelectromechanical system process, the sidewall electrodes were fabricated precisely at either side of the separation channel. The area of the capacitor electrodes was the maximum value to improve the detection sensitivity with an enhanced capacitance effect. According to the simulation results, the structural parameters of the sidewall electrodes were determined as 550-microm length, 15-microm width, 80-microm separation distance, and 1-microm isolator thickness. The integrated microdevice with the SOI-PDMS hybrid electrophoresis microchip was very compact and the size was only 15 cm x 15 cm x 10 cm (width x length x height), which permitted miniaturization and portability. The detector performance was evaluated by K(+) testing. The detection limit of the conductivity detector was determined to be 10(-9) and 10(-6) M for K(+) in the static and electric-driven modes, respectively. Finally, the C(4)D was applied to low-voltage-driven electrophoresis on a microchip to carry out real-time measurement of the separation of amino acids. The separations of 10(-4) M lysine and phenylalanine in the low-voltage-driven electrophoresis mode were performed with an electric field of 300 V/cm and were completed in less than 15 min with a resolution of 1.3. The separation efficiency was found to be 1.3 x 10(3) and 2.8 x 10(3) plates for lysine and phenylalanine, respectively, with a migration time reproducibility of 2.7 and 3.2%. The conductivity detection limit of amino acids achieved was 10(-6) M. The proposed method for the construction of a novel C(4)D integrated on an SOI-PDMS hybrid low-voltage-driven electrophoresis microchip showed the most extensive integration and miniaturization of a microdevice, which is a further crucial step toward the realization of the "lab-on-a-chip" concept.
