Integrated bio/chemical sensing system with a microfluidic transport module based on electrowetting

An integrated micro analysis system was fabricated using a microfluidic transport system driven by electrowetting and an air-gap ammonia sensor. The basic element in the system was a row of elongated gold working electrodes and a protruding polydimethylsiloxane (PDMS) structure which form an open channel structure. The wettability of the gold electrode was changed by applying a negative potential with respect to a Ag/AgCl electrode, and a solution introduced from an inlet was mobilized through the gap between the working electrode and the protruding structure. Also, a solution could be transported to any desired directions without using any valves. Furthermore, two solutions could be mixed based on the same principle. The open structure of the flow channel facilitated the integration of an air-gap ammonia sensor. Ammonia diffused from the mixing area was detected as the potential change of a pH-indicator electrode. The 90% response time was 45 s for 10 mM ammonia. The relation between the potential of the pH-indicator electrode and the logarithm of ammonia concentration was linear. Furthermore, a biosensing system was constructed by using immobilized urease or creatinine deiminase and the ammonia sensor. The concentration of urea and creatinine could be determined by measuring ammonia produced enzymatically from a sample solution. Linear calibration plot was obtained for urea concentrations down to 100 μM and creatinine concentration down to 50 μM.