# Heavy Metal Ion Detection Platforms Based on a Glutathione Probe: A Mini Review
## 3 Heavy Metal Ion Detection Platforms Using Glutathione
### HMI Detection Combined with Electrochemical Techniques
#### 3.2.2 Working-Electrode-Based Platforms  
Over the years, nanostructures have become a must to obtain good sensitivity. The electrodes used in this method are all modified with nanomaterials on their surface. Based on a graphene-modified electrode, an electrochemical detection of Cd\({}^{2+}\) was performed by square wave anodic stripping voltammetry (SWASV) [56]. A thoroughly cleaned glassy-carbon electrode (GCE) was adopted to be the working device. Then, an rGO/carboxymethyl cellulose (CMC)/glutathione (GSH) film was coated on the polished bare electrode by coating their suspension followed by drying steps. By analyzing the electrochemical behavior, the rGO/CMC/GSH nanocomposite exhibited a good performance of carrier transport. Electrochemical impedance spectroscopy (EIS) was used to characterize the functionalized electrode. The rGO/CMC/GSH/GCE showed a smaller semicircle in the Nyquist plot compared with other electrodes, which indicated low interface resistance and rapid interfacial charge transfer, which can be considered as advantages when using this nanocomposite structure. The functionalized GCE is described in the schematic diagram of Figure 7. Detection of Cd\({}^{2+}\) (2 nM) using bare and modified electrodes was compared under the voltage of (\(-\)1.0\(\sim\)\(-\)0.6 V). The  
Figure 6: Fabrication of a GSH–gold nanoparticle (GNP)-decorated reduced graphene oxide (rGO) electrode sensor. (**a**) Deposition of GO monolayer sheets on the \(\alpha\)-ethyl-tryptamine (AET)-modified electrodes, and thermal reduction of GO to rGO. (**b**) Assembly of GNPs onto the rGO film. (**c**) GSH modification of GNP-decorated rGO electrodes to form a specific receptor to Pb\({}^{2+}\). response peak current of rGO/CMC/GSH/GCE was much sharper and higher than those of control electrodes, such as GO/GCE, rGO/GCE, and rGO/CMC/GCE. Three reasons were attributed to the enhanced peak current. There were a larger surface area and better conductivity of the electrode from GO; more (-OH) and (-COOH) groups provided by CMC, which can chelate with Cd\({}^{2+}\); and eight heterogeneous active sites in GSH for Cd\({}^{2+}\) coordination. To demonstrate the selectivity of Cd\({}^{2+}\), equimolar samples at 10 nM of Cu\({}^{2+}\), Zn\({}^{2+}\), Hg\({}^{2+}\), and Cd\({}^{2+}\) were tested. Except for Cd\({}^{2+}\) and Pb\({}^{2+}\), the other HMIs showed no obvious response. As for Pb\({}^{2+}\), the authors claimed that its current peak was lower than that of Cd\({}^{2+}\). So, there existed specificity for Cd\({}^{2+}\) in this detection method. According to this research, the LOD of Cd\({}^{2+}\) reached 0.05 nM, and the detection time was optimized to be 120 s considering the Cd\({}^{2+}\) deposition process onto the electrode. In another variation of nanomaterial-modified electrodes, a screen-printed carbon nanofiber electrode combined with GSH was developed for metal ions detection using a voltammetric measurement [57]. In this report, the GSH-modified electrode was electrografted onto a screen-printed carbon nanofiber substrate (GSH-SPCNFE). In contrast to the classical screen-printed carbon electrode modified with GSH (GSH-SPCE), this method exhibited much better electrochemical characterization, indicating better GSH immobilization. An electrochemical workstation was used for stripping measurements, and the sensing was performed based on a three-electrode system in acetate buffer (pH 4.5). Ag/AgCl/KCl and Pt wire worked as the reference and auxiliary electrode, respectively, and a GSH-SPCNFE was used as the working electrode. Voltammetry was applied to analyze the target metal ion complexation level after the metal ions had been deposited onto the electrode for 120 s. As for the binding mechanism, the authors believed that the thiol group in the GSH played an important role to complex with HMI in their experiment. Pb\({}^{2+}\) and Cd\({}^{2+}\) could be detected at two different potentials, both with an LOD of about 3.0 \(\mathrm{\SIUnitSymbolMicro g}\)/L.