Arsenic(III) detection using electrochemical–chemical–chemical redox cycling at bare indium–tin oxide electrodes

Sensitive As(III) detection in ground water is of great importance for evaluating the quality of drinking water. We report a sensitive electrochemical method for As(III) detection based on electrochemical–chemical–chemical (ECC) redox cycling involving Ru(IV) [an oxidized species of Ru^III(NH₃)₅NH₂²⁺], As(III), and tris(3-carboxyethyl)phosphine (TCEP). Electrochemical oxidation of Ru^III(NH₃)₅NH₂²⁺ formed from Ru^III(NH₃)₆³⁺ generates Ru(IV), which quickly oxidizes As(III). This electro-mediated oxidation of As(III) produces As(v), which is reduced back to As(III) by TCEP. Electrochemically generated Ru(IV) then reoxidizes As(III), allowing ECC redox cycling to occur at a high rate on bare indium-tin oxide (ITO) electrodes without modifying the surfaces with electrocatalytic materials. Because most interfering metal ions precipitate in a carbonate buffer, water samples are mixed with carbonate buffers prior to electrochemical measurements, rendering the effects of Cu⁺, Cu²⁺, Fe²⁺, Fe³⁺, and Pb²⁺ insignificant. The detection limit calculated by ECC redox cycling using a chronocoulogram is 1.2 μM, much lower than that obtained using only the electro-mediated oxidation of As(III) (90 μM).