**3. Electrochemical Techniques**

E-apt sensors have proven to be effective tools for detecting heavy metal ions. They are cheap, portable, and easy to operate, making them popular with researchers. The concentration of the target is detected by collecting the changes in voltage, current, conductivity, impedance, and other electrochemical signals [67,68]. Table 1 summarizes the application of the E-apt sensor in the detection of heavy metal ions in recent years. According to different electrochemical principles, electrochemical characterization and detection methods can be divided into cyclic electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV), square wave voltammetry (SWV), photoelectric electrochemistry (PEC), electrochemical luminescence (ECL), and other methods [69]. When the analyte is around, the target-induced signal is generated and recorded as the corresponding electrochemical signal, which can be detected in a few minutes with relatively high sensitivity. Sensors with different principles use different nanomaterials and sensitizers, as well as different construction methods. Various electrode structures, construction methods, and responses to inspection objects are summarized in Figure 5.

**Figure 5.** Electrochemical sensor technology is based on aptamers and different electrodes.
