Electrochemical cysteine biosensor based on the selective oxidase–peroxidase activities of copper, zinc superoxide dismutase

A new cysteine (CySH) biosensor was developed based on its oxidase–peroxidase activities with copper, zinc superoxide dismutase (SOD1). The biosensor comprising of SOD1 coimmobilized with horseradish peroxidase (HRP) on polypyrrole (PPy)-platinum (Pt) electrode was characterized by cyclic voltammetry. The bienzymatic electrode exhibited an electrochemical response with cysteine due to the bicarbonate-dependent peroxidase activity stimulated by thiol oxidase activity of SOD1. Among the thiols, viz., CySH, HCY and GSH, CySH only showed a remarkable amplification of current. Thus, the biosensor is CySH selective. The electrode linearly responded to cysteine concentrations up to 500 μM with a detection limit of 10 μM. The CySH biosensor is highly stable, selective and reproducible, making it suitable for analytical purposes. Using this biosensor, the levels of CySH present in commercially available l-CySH dietary supplement tablets, serum and urine were estimated.