Microelectrode array (MEA) dissolved-oxygen (DO) sensors were built and electrochemically tested in a solution of potassium ferricyanide. MEAs are becoming more popular as DO sensors because of their small size and capacity for simultaneous measurements with multiple recording sites. The ability to measure DO with multiple recording sites is useful for monitoring systems such as biofilm metabolism, which is an important factor in wastewater treatment. MEAs are beneficial because they are less destructive than individual microelectrodes that need to be moved through a sample to collect multiple measurements.
The MEA used in the present work used gold electrodes, and a separate silver/silver chloride wire was used as the counter/reference electrode. The ferri/ferrocyanide redox couple is a reversible reaction with a well-known electrochemical behavior, making it a good way to test the electrochemical functionality of the MEA before using it to measure DO. Voltage was applied to the gold electrode in potassium ferricyanide solution, which initiated a redox reaction. The movement of electrons increases with the number of redox reactions occurring, meaning that measured current increases proportionally to the concentration of ions that get oxidized or reduced. The experiments were done inside a Faraday cage to minimize noise, and cyclic voltammograms were collected.
There was a good linear response to the potassium ferricyanide, and the data was close to what was predicted by the Randles-Sevcik equation. The shape of the cyclic voltammograms appeared to have the characteristics of a reversible reaction, but the difference in voltage between the two peak currents was higher than expected. This may be due to an extra voltage drop at the counter electrode that is independent of the reactions, or high resistance due to inadequate electrolyte concentrations. Other possible interfering factors include adsorption at the electrodes and solution composition. The linear response of the sensor and its agreement with the Randles-Sevcik equation are promising signs that the sensor can measure differences in concentration, but a better understanding of what factors interfere with measurements is needed before applying the MEA to measuring DO concentrations.