Self-calibrating microfabricated iridium oxide pH electrode array for remote monitoring

The goal of this work is the development of microfabricated electrochemical sensing systems for environmental, industrial, and security applications requiring long-term unattended operation. The specific advantages of the microfabrication approach include the capability not only to miniaturize the size of the sensor platform but also to create an intelligent design including features such as redundant sensing electrodes, on-chip reference and auxiliary electrodes, and in situ electrode regeneration/calibration. The model system targeted here involves continuous pH monitoring in drinking water at solid-state iridium oxide electrodes. The microchips utilized consist of a flow-through silicon platform (1 cm x 1.2 cm) containing patterned gold electrodes onto which iridium oxide has been deposited electrochemically. To simulate drinking water detection scenarios, sensors are integrated into a flow system. Microfabricated designs include as many as 11 equivalent pH electrodes whose performance was evaluated for factors such as electrode-to-electrode reproducibility, long-term drift, and response to expected interfering agents. With on-chip voltage treatment, absolute potentials measured for an electrode array are within +/-4 mV, with identical (+/-1 mV/pH unit) calibration slopes. This performance level is sustainable over weeks of usage.