TABLE OF CONTENTS Chemical sensors are devices that provide information about the types, concentrations and chemical states of the species present within a sample. In this section, we focus on electrochemical sensors which represent a very important group of chemical sensors, some of them with properties approaching those of the ideal sensor. Broadly speaking, electrochemical sensors are based on one of three categories of transduction mechanism: amperometric and voltammetric, potentiometric or conductimetric. Amperometry senses current generated (at a fixed voltage) when an analyte is selectively oxidized or reduced resulting in the exchange of electrons. Two-electrode cells are common. In voltammetry, current is again measured, but as a function of the applied potential. The reference electrode's potential is constant, and the working electrode assumes the value of the applied potential. The working electrode is the site where electrolysis occurs and this generates the measured current. Three-electrode cells are usually used. Potentiometry, where there is no current flow, measures the accumulation of charge density (voltage) at the surface. These reactions are spontaneous and two-electrode cells are used. Finally, conductometric methods measure conductivity through the sample between electrodes.
The variety of systems that can now be probed using electrochemical sensors is truly vast, ranging from the secretions of single cells to the inside of reaction vessels run at high temperatures and pressures. Electrochemical measurements can now be performed in the solid, liquid and even gas phases to accurately determine the concentrations of redox reactions active at concentrations as low as parts per trillion.
