On-chip microfluidic transport, mixing, and sensing using electrochemical principles

An integrated micro analysis system was fabricated using a microfluidic transport system driven by electrowetting and an air-gap ammonia gas-sensing electrode. The basic element in the microfluidic system was an elongated gold working electrode and a protruding polydimethylsiloxane (PDMS) structure. The wettability of the gold electrode was changed by applying a negative potential with respect to an Ag/AgCl electrode, and a solution introduced from an inlet was mobilized through the gap between the working electrode and the protruding structure. Also, with a row of working electrodes with a narrow gap between them, the solution could be transported to any desired directions without using any valves. The principle was also used to realize a mixing mechanism. The simple structure and the principle of operation facilitated a higher level of integration. The open structure of the flow channel was advantageously used to form an air gap between a sample solution and the electrolyte solution for the ammonia electrode. Distinct changes in the indicator electrode potential were observed with 90% response time of 45 s (at 10 mM). A linear relationship was observed between the indicator potential and the logarithm of the concentration of ammonia. The lower detection limit was 50 μM.