Ratiometric electrochemical sensor for selective monitoring of cadmium ions using biomolecular recognition

A selective, accurate, and sensitive method for monitoring of cadmium ions (Cd²⁺) based on a ratiometric electrochemical sensor was developed, by simultaneously modifying with protoporphyrin IX and 6-(ferroceney) hexanethiol (FcHT) on Au particle-deposited glassy carbon electrode. On the basis of high affinity of biomolecular recognition between protoporphyrin IX and Cd²⁺, the functionalized electrode showed high selectivity toward Cd²⁺ over other metal ions such as Cu²⁺, Fe³⁺, Ca²⁺, and so on. Electroactive FcHT played the role as the inner reference element to provide a built-in correction, thus improving the accuracy for determination of Cd²⁺ in the complicated environments. The sensitivity of the electrochemical sensor for Cd²⁺ was enhanced by ∼3-fold through the signal amplification of electrodeposited gold nanoparticles. Accordingly, the present ratiometric method demonstrated high sensitivity, broad linear range from 100 nM to 10 μM, and low detection limit down to 10 nM (2.2 ppb), lower than EPA and WHO guidelines. Finally, the ratiometric electrochemical sensor was successfully applied in the determination of Cd²⁺ in water samples, and the obtained results agreed well with those obtained by the conventional ICP-MS method.