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 (iCd2+/iDAP) 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.