A nanozyme-catalysis-based ratiometric electrochemical sensor for general detection of Cd²

A new kind of internal reference probe was developed to construct a ratiometric electrochemical sensor for accurate and sensitive Cd²⁺ detection. AuPt nanoclusters (AuPt NCs) were synthesized and then evenly distributed on reduced graphene oxide (rGO) to form a nanocomposite (AuPt-rGO), which showed peroxidase-like activity and catalyzed the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP) in the presence of H₂O₂. It was found that DAP was further reduced on the electrode with a constant current intensity whether Cd²⁺ was added or not, which enabled it to function as an excellent internal reference probe. Thus, the ratio between the reduction currents of Cd²⁺ and DAP (i_Cd/i_DAP) was proposed to monitor Cd²⁺ based on the ratiometric strategy. Additionally, the high surface area and good conductivity of AuPt-rGO endowed the sensor with enhanced sensitivity. A wide linear range and a low limit of detection were measured at 5.0 × 10^-8-1.0 × 10^-4 mol L^-1 and 3.1 × 10^-8 mol L^-1, respectively. Compared with the single signal sensors, this ratiometric sensor presents more accuracy and reliability in the detection of Cd²⁺, facilitating the quantification of Cd²⁺ concentration in different water samples with good recoveries. The novel internal reference probe proposed here shows great possibilities for accurate and sensitive Cd²⁺ detection based on the ratiometric strategy.