Electrochemical sensing platform for detection of heavy metal ions without electrochemical signal.

Heavy metal pollution has attracted global attention because of its high toxicity, non-biodegradability, and carcinogenicity. Electrochemical sensors are extensively employed for the detection of low concentrations of heavy metal ions (HMIs). However, their applicability is often limited to the detection of ions that exhibit electrochemical signals exclusively in aqueous solutions. In this study, we proposed a multi-responsive detection platform based on the modification of horseradish peroxidase@zeolitic imidazolate frameworks-8/thionine/gold/ionic liquid-reduced graphene oxide (HRP@ZIF-8/THI/Au/IL-rGO). This platform demonstrated its capability to detect various metal ions, including those without conventional electrochemical signals. The Au/IL-rGO composite structure enhanced the specific surface area available for the reaction. Furthermore, the in situ growth of HRP@ZIF-8 not only shielded the THI signal prior to detection but also protected the electrode material. It was important to note that the introduced edetate disodium dihydrate (EDTA) had the ability to complex with various HMIs. When excess EDTA was present, it could cleave ZIF-8 and release HRP. In the presence of hydrogen peroxide (HO), HRP promoted the oxidation of THI previously reduced by the electrode and thus showed excellent sensitivity for HMIs detection. The proposed method overcame the limitation of traditional electrochemical sensors, which solely relied on electrochemical signals for detecting metal ions. This offers a novel approach to enhance electrochemical ion sensing detection.