Automatic electrochemical sequential processing in a microsystem for urea detection

To demonstrate the possibility of automatic serial operation in a microanalysis system, components for valve operation, adjustment of mixing time, pH regulation, and sensing functions were integrated. Electrodes for actuation and sensing were formed on a glass substrate and flow channels and compartments were formed using poly(dimethylsiloxane) (PDMS). When a urea standard solution was filled in half of a mixing area, a mixing valve opened automatically and the solution merged with a urease solution initially filled in the other half. The length of time required for the mixing of components was adjusted by using a delay line, which extended from the electrode used for the mixing valve. After a time delay, an injection valve at the end of the mixing chamber was opened automatically. After the solution was injected into the compartment of the pH regulator using a nonstandard three-electrode system, the solution pH was elevated automatically. Gaseous ammonia produced accompanying the change was detected using an air-gap ammonia sensor. The existence of ammonia was clearly indicated by a change in electrode potential. The dependence of the potential on the logarithm of the concentration was linear, confirming the Nernstian response.