Microfabricated electrochemical nitrate sensor using double-potential-step chronocoulometry

A simple, miniaturized, and yet sensitive electrochemical nitrate sensor has been designed, fabricated and tested. The thin-film silver sensing electrode, silver-oxide reference electrode, and platinum counter electrode are microfabricated on a silicon substrate. A polyimide insulation layer is also patterned to improve reliability of a sensor chip. The sensor chip incorporates microfluidic channels. A fixture is also created to integrate the sensor chip and fluidic connectors for on-line flow-through analysis. The electrolyte is 0.01 M sodium hydroxide. The nitrate concentration is determined by double-potential-step chronocoulometry (DPSC) because it improves signal-to-noise ratio compared to amperometry and minimizes oxygen background current. Interference analysis of DPSC with 10 ions commonly found in groundwater indicates that HPO42−/PO43−, Ca2+Ca2+, and Sr2+Sr2+ cause significant interference (i.e., more than 20% signal distortion). Performance characterization of sensor chips indicates that the limit of detection (LOD) and the upper limit of the linear range have a chip-to-chip variation of 4–75 μM and 500–2000 μM, respectively.