# 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
#### 3.2.1 Field Effect Transistor-Based Platforms  
An electrochemical sensing platform using a reduced graphene oxide (rGO)/gold nanoparticle (GNP) FET was developed to detect Pb\({}^{2+}\) in water [34]. The rGO/GSH-GNP hybrid structure was formed on the FET channel and the GNPs were equivalent to the gate of the FET. There was an electronic transfer between the rGO and the GNPs. When the carrier concentration in the rGO changed, the external characteristics of the FET changed. Here, the rGO was used as a conducting channel to transport charge carriers. If the analytes were captured by functionalized GNPs, the charge carrier concentration would be changed, which subsequently caused a current change in the channel of FET. The current change was found to correlate with the Pb\({}^{2+}\) concentration in this work, with the Pb\({}^{2+}\) concentration in a range from 10 nM to 10 mM. In this approach, amino-terminated Au electrodes as the source and drain were employed to anchor the graphene oxide. As shown in Figure 6, a monolayer \(\alpha\)-ethyl-tryptamine (AET) film was first chemically anchored on the source and drain electrode surface. Thermal reduction of GO to rGO was performed. Then, rGO monolayer sheets were self-assembled on the AET-modified electrodes. The next step was assembling GNPs onto the rGO film. Lastly, GSH was immobilized onto GNPs on the rGO sheet surface. In this study, GSH linking with GNPs was considered through -SH linkage. In addition to the complexation mechanism introduced previously, it was hypothesized in [34] that the anions of rGO have electrostatic interactions with Pb\({}^{2+}\), improving the specific binding to Pb\({}^{2+}\) by GSH. A Keithley 4200 semiconductor characterization system was used to measure the change in the FET's electrical characteristics before and after adding Pb\({}^{2+}\). The drain current (I\({}_{\text{ds}}\)) acts as the indicator of Pb\({}^{2+}\) concentration, which would be changed as a function of the gate voltage (V\({}_{\text{gs}}\)) and the drain voltage (V\({}_{\text{ds}}\)). Hence, Pb\({}^{2+}\) of different concentrations could be detected. The authors in [34] reported that the response time was as short as several seconds and the LOD of Pb\({}^{2+}\) was 10 nM. Furthermore, tests of Hg\({}^{2+}\), Ag\({}^{+}\), Cd\({}^{2+}\), As\({}^{5+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\), all at 10 uM, the same as the Pb\({}^{2+}\) concentration, were performed as control experiments. The change of I\({}_{\mathrm{ds}}\) was very slight when the control HMIs were tested, which demonstrated the selectivity of Pb\({}^{2+}\) detection. Recently, there was another similar report on Pb\({}^{2+}\) detection using an rGO-modified FET integrated with a microcontroller-based analyzer [19]. The FET consisted of an rGO channel and an Al\({}_{2}\)O\({}_{3}\) thin layer working as a gate. The GNPs capped with GSH were sputtered onto the Al\({}_{2}\)O\({}_{3}\) thin film gate to capture Pb\({}^{2+}\) in water. Then, a microcontroller was applied to realize a capacitance measurement. The specific binding mechanism was not mentioned in this report. However, based on the foregoing discussion in this review, the mechanism of specific detection of Pb\({}^{2+}\) using GSH-GNPs is attributed to the strong affinity between Pb\({}^{2+}\) and (-COOH) as described in Section 2. Interestingly, the drain current was not adopted to indicate the analyte concentration as is commonly done. Instead, a square pulse wave in series with a reference resistor was used to calculate the time constant of the distorted signal across the drain-source interface, apparently extracting the gate capacitance change due to Pb\({}^{2+}\) binding. For specificity, this sensor exhibited a specific response to Pb\({}^{2+}\) against other common HMIs as well as cations in water, such as Zn\({}^{2+}\), Mg\({}^{2+}\), Fe\({}^{3+}\), and HAsO\({}_{4}\)\({}^{2-}\). Here, the Pb\({}^{2+}\) concentration was 2.5 ppb and the others were of a concentration of 10 ppb.