Specific electrochemical sensor for cadmium detection: Comparison between monolayer and multilayer functionalization

Selective and sensitive cadmium sensors were prepared by functionalizing glassy carbon electrodes (GCE) using the electrochemical reduction of p-benzoic acid diazonium salts according to either a multilayer or a monolayer coating process. Such modified electrodes were then post-functionalized with 1,2-bis-[o-aminophenylthio]ethane (APTE) or its monoprotected derivative leading to specific Cd2+ electrochemical sensors. The step by step preparation of the electrodes as well as the ability of the immobilized receptor to complex Cd2+ was investigated by XPS and Raman spectroscopy. The detection and quantification of cadmium was performed in two steps: the preconcentration of Cd2+ ions by complexation with immobilized APTE and the analysis by Linear Sweep Stripping Voltammetry (LSSV). More importantly, multilayer versus monolayer grafting has a strong influence on the sensor performances. Whereas calibration curves typical of an equilibrium process were obtained for sensors realized with multilayer organic films, sensors with a monolayer organic film led to linear calibration curves, in the same concentration range. Finally, the monolayer sensor gave a lower limit of detection (LOD) and highlighted better selectivity than unmodified electrode.