Accurate detection of Cd (II) based on His-Pd@Apt/PANI@Au-modified needle electrochemical ratiometric/colorimetric dual-mode sensor combined with a WeChat mini program

Heavy metal cadmium (Cd (II)) contamination in aquatic products poses severe health risks, necessitating rapid, accurate, and portable detection methods. Traditional electrochemical sensors often suffer from background interference and low accuracy, especially when utilizing stainless steel acupuncture needles as electrodes. Single-signal systems are prone to environmental fluctuations, limiting reliability. Thus, there is an urgent need for a sensitive, low-cost, and visual sensor that integrates dual detection modes for on-site analysis of Cd (II). We report a novel ratiometric/colorimetric dual-mode aptamer sensor based on a polyaniline-gold nanocomposite (PANI@Au)-modified acupuncture needle. This platform uses PANI's peak at -0.9 V as an internal reference and His-Pd@aptamer's peak at -0.2 V as a detection signal, enabling ratiometric electrochemical quantification. In parallel, His-Pd nanoparticles catalyze the oxidation of TMB to produce a blue color, allowing visual detection with a limit of 3.10 × 10^-2 ng/L. RGB values are analyzed by a smartphone and WeChat mini-program for rapid result output. The electrochemical and colorimetric modes demonstrated excellent sensitivity (LOD: 3.40 × 10^-2 ng/L electrochemical; 1.06 × 10^-1 ng/L RGB-based) and linear range of 10¹-10⁸ ng/L. The sensor showed strong selectivity, stability, and successful application in detecting Cd (II) in fish, shrimp, and scallop samples with high recovery rates. This is the first report of a needle-based ratiometric/colorimetric aptamer sensor that enables both electrochemical quantification and smartphone-based visual detection of Cd (II). The device supports rapid, on-site, and low-skill operation with sub-100 μL samples, demonstrating its potential for real-time monitoring of heavy metal contamination in aquatic food safety.