Electrochemical sensors operate based on the electrochemical reactions that occur at the interface between an electrode and an analyte solution.

Firstly, there are usually three main electrodes in an electrochemical sensor: the working electrode, the counter electrode, and the reference electrode. At the working electrode, redox reactions take place. When the target analyte in the solution comes into contact with the working electrode surface, it may either donate or accept electrons. For example, in some cases, if the analyte is a reducing agent, it will be oxidized at the working electrode, losing electrons.

The reference electrode provides a stable potential reference point. This allows for the accurate measurement of the potential difference between the working electrode and the reference electrode, which is crucial for determining the electrochemical behavior of the analyte.

The counter electrode is used to complete the electrical circuit. During the redox process at the working electrode, electrons flow through the external circuit to the counter electrode. There, a complementary reaction occurs to maintain the electrical neutrality within the electrochemical cell.

Moreover, different types of electrochemical sensors have their specific working mechanisms within this general principle. For instance, in amperometric sensors, the current resulting from the redox reaction of the analyte at a fixed potential is measured. In potentiometric sensors, the potential difference related to the analyte's activity is detected. Conductometric sensors, on the other hand, sense the changes in the electrical conductivity of the solution caused by the analyte. Overall, these principles enable electrochemical sensors to detect and analyze a wide range of analytes with high sensitivity and selectivity.