# Heavy Metal Ion Detection Platforms Based on a Glutathione Probe: A Mini Review
## 3 Heavy Metal Ion Detection Platforms Using Glutathione
### Optical Techniques
#### 3.1.2 Semiconductor Quantum-Dot-Based Platforms
Various solutions of metal ions were tested, including ZnSO\({}_{4}\), AlCl\({}_{3}\), Pb(NO\({}_{3}\))\({}_{2}\), CrCl\({}_{3}\), HgCl\({}_{2}\), FeCl\({}_{3}\), Co(NO\({}_{3}\))\({}_{2}\), Cd(NO\({}_{3}\))\({}_{2}\), phenol, and dimethylbenzene. Except for Pb(NO\({}_{3}\))\({}_{2}\), CrCl\({}_{3}\), and HgCl\({}_{2}\), no obvious decrease in luminescence intensity was observed for the other metal ion solutions, demonstrating the selectivity of GSH-coated QD-LDH. The focus of this research was simultaneous detection of Pb\({}^{2+}\), Cr\({}^{2+}\), and Hg\({}^{2+}\). The LOD for mixed HMIs of Pb\({}^{2+}\), Cr\({}^{3+}\), and Hg\({}^{2+}\) was claimed to be 0.93 uM. The luminescence spectra of QD-LDH in aqueous buffer with various metal ions of 1 mM are shown in Figure 5a, and the corresponding intensities are given in Figure 5b. In Figure 5, DMB (Dimethyl aminoethyl benzoate) and RhB (Rhodamine B) as well as phenol are tested as organic interferers. According to Figure 5, only Pb\({}^{2+}\), Cr\({}^{3+}\), and Hg\({}^{2+}\) cause luminescence intensity quenching, while the other HMIs do not cause an obvious luminescence intensity change. In [49], the probable reason why Pb\({}^{2+}\), Cr\({}^{3+}\), and Hg\({}^{2+}\) caused luminescence quenching was hypothesized as follows. It was attributed to the nonradiative recombination caused by cation exchange and competition of ligands. On the other hand, when the surface status of QDs covered with GSH were changed, the luminescence properties would be affected immediately. Even when the binding of GSH to the surfaces was only slightly broken, the luminescence intensity would be notably reduced. CdTe QDs are also extensively studied for biosensor applications. The authors in [54] reported specific detection of Pb\({}^{2+}\) based on electrochemiluminescence (ECL) of CdTe QDs capped with GSH and thioglycolic acid (TGA). The general strategy and process to detect HMI was similar to the aforementioned method based on GSH-capped Mn-ZnS QDs in [53]. The primary function of GSH here was to bind Pb\({}^{2+}\) with free (-COOH) groups and (-NH\({}_{2}\)) groups. Because the ECL intensity of QDs was low when the QDs were capped with GSH alone, TGA was introduced to improve the sensitivity in this method. Both GSH and TGA were working as stabilizers for the QDs, which led to a much higher photoluminescence quantum yield. As stated in [55], a quantum yield of 63% from CdTe QDs capped by GSH and TGA was achieved as opposed to about 40% using a regular synthetic method without TGA. In [54], the ECL signal was measured in the presence of Pb\({}^{2+}\). As expected, the ECL signal was measured in the presence of Pb\({}^{2+}\). As expected, the ECL signal was measured in the presence of Pb\({}^{2+}\). Figure 5: Detection results using a quantum dot and layered double hydroxide (QD–LDH) sensor in the presence of different metal ions: (**a**) Luminescence spectra, (**b**) intensities [53]. signal of GSH-TGA-CdTe QDs was observed to decrease. Because of the high affinity of GSH to Pb\({}^{2+}\) by (-COOH) and (-NH2), the surface property of the QDs changed, leading to an obvious ECL intensity decrease. As a figure of merit, selectivity in HMI detection is always crucial. When different metal ions, such as Ca\({}^{2+}\), Na\({}^{+}\), Hg\({}^{2+}\), Cu\({}^{2+}\), Zn\({}^{2+}\), and Fe\({}^{2+}\), were added, only Hg\({}^{2+}\) induced noticeable ECL quenching, showing an acceptable selectivity. The LOD of this method reached 0.26 nM, and according to the response curves, the detection time was about 15 s.  
As a relatively new medium, semiconductor QDs are much more versatile and easier to use than conventional fluorescence labeling dyes due to their controllable characteristics of luminescence, which are simply dependent on crystal size. Not surprisingly, other types of QDs have also been investigated for HMI detection. Although carbon itself is not a semiconductor, carbon dots with a diameter below 10 nm exhibit typical quantum dot effects.