High-Sensitivity Flow Detection of Multiple Metal Ions Using a Smartphone-Based Electrochemical System with Single-Walled Carbon Nanotubes (SWCNTs)/Carbon Black (CB)-Modified ITO Electrode

This paper proposes a smartphone-based electrochemical detection instrument, combined with a microfluidic device using Microelectromechanical systems (MEMS) technology integrating a thin-layer flow cell and a modified ITO electrode to successfully construct a portable multiple metal ions detection system. This effectively addresses the issues of poor portability, complex operation, and strong reliance on the laboratory environment of traditional detection equipment. The electrochemical detection instrument realizes the highly sensitive detection of metal ions through the integrated hardware circuit design and the collaborative work of software algorithms. The hardware circuit is based on an STM32 microcontroller as the core, combined with a constant potential circuit and low-pass filtering circuit, which realizes high-precision acquisition of weak current signal and anti-interference processing, and the low-pass filtering makes the signal-to-noise ratio of cyclic voltammetry curves significantly improved. The application (APP) software for smartphones was developed to improve the data quality based on cubic spline interpolation and smoothing algorithm, and the Levenberg–Marquardt nonlinear fitting algorithm was used to optimize the parameters of the model so that the R² value of data fitting was more than 0.99, which improves the analytical accuracy and ensures the reliability of the measurement. Further, a microfluidic device modified by single-walled carbon nanotubes (SWCNTs)/conductive carbon black (CB) was devised and built, combined with a detection instrument with a fluidic control unit to promote sample diffusion and enhance sensitivity. According to the results, the detection limits of the system for the metal ions, Cu, Pb, and Cd were 88 μg/L, 0.73 μg/L and 2.3 μg/L, respectively, which verified its effectiveness and potential application in the immediate detection of metal ions and can meet the needs of the point-of-use (POU).