# 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  
As an important discovery originally found in the 1990s, semiconductor quantum dots (QDs) have attracted intense research interest for their excellent luminescence properties. In contrast to traditional fluorescence techniques, QDs have advantages in excitation spectra, photo-luminescence quantum efficiency, and so on [50; 51]. Reports of new QD-based sensors are on the rise. Most of the reported QD-based HMI detection schemes utilized II-VI semiconductor QDs. As an example, using Mn-doped ZnS QDs, [52] reported a Pb\({}^{2+}\) detection scheme by phosphorescence measurement. Phosphorescence is generated by the energy transfer from the lowest vibrational energy layer of the excited Mn\({}^{2+}\) triplet state to the vibrational energy layer of the ground state. Mn-ZnS QDs are a popular II-VI semiconductor QD for biosensor applications. Because Mn-ZnS QDs can be functionalized without using deoxidants and other inducers, they are convenient to work with. Furthermore, Mn-ZnS QDs have a long-lived doped emission, which can set the signal of Pb\({}^{2+}\) detection apart from interference by autofluorescence and scattering light. GSH was chosen to be the capping agent due to its specific binding with Pb\({}^{2+}\). To investigate the GSH coating mechanism, a Fourier transform infrared (FTIR) spectrometer was used to indicate the existence and change of functional groups or bonds. The disappearance of the S-H stretching vibrational peak was observed in the FTIR spectra after GSH was coated onto the Mn-ZnS QDs, which indicated the binding between GSH and Mn-ZnS QDs via (-SH) groups. This is similar to the immobilization mechanism when using noble metal NPs [35; 36; 37]. Furthermore, the pH was optimized to be 7.5, which was also in agreement with the Pb\({}^{2+}\) detection condition discussed previously [33]. When Pb\({}^{2+}\) ions were added into a solution at a pH of 7.5, the intensity quenching of GSH-Mn-ZnS QDs was observed, which was proportional to the increase in Pb\({}^{2+}\) concentration. Normalized phosphorescence intensities to the blank solution (P/P\({}_{0}\)) at 590 nm were used to quantify the Pb\({}^{2+}\) concentration. The selectivity of this method was investigated by testing other common metal ions in water, including Na\({}^{+}\), Ca\({}^{2+}\), K\({}^{+}\), Mg\({}^{2+}\), Mn\({}^{2+}\), Co\({}^{2+}\), Zn\({}^{2+}\), Fe\({}^{3+}\), Cd\({}^{2+}\), Hg\({}^{2+}\), and Cu\({}^{2+}\). No metal ions yielded significant responses other than Pb\({}^{2+}\), all at a concentration of 100 nmol-L\({}^{-1}\). This demonstrated the selectivity of Pb\({}^{2+}\) detection. The sensor showed a wide linear range of (1.0-100 \(\upmu\)g/L) and an LOD of 0.45 \(\upmu\)g/L. Furthermore, real water samples were tested containing spiked Pb\({}^{2+}\). The detection results were in good agreement with those measured by the standard ICP-MS method, indicating the practical application value of this platform. Recently, on simultaneous HMI detection of Pb\({}^{2+}\), Cr\({}^{3+}\), and Hg\({}^{2+}\) in water, a luminescence probe using layered double hydroxides (LDHs) loaded with Mn-ZnS QDs was reported [53]. In this work, the preparation of QDs on LDH was a key process. GSH-Mn-ZnS QDs nanocrystals were first synthesized. Then, a hydrothermal method was adopted to make Mg-Al-CO\({}_{3}\) LDHs. Lastly, GSH-Mn-ZnS QDs were immobilized on an Mg-Al-CO\({}_{3}\) LDH by diluting the QD solution in deionized water, adding the LDH, centrifuging the slurry, and performing a drying operation. After the solution to be tested was dropped onto the QD-LDH powder, the luminescence spectra were measured. Due to the transition of Mn\({}^{2+}\) in the ZnS host lattice from the triplet state (\({}^{4}\)T\({}_{1}\)) to the ground state (\({}^{6}\)A\({}_{1}\)), Mn-ZnS QDs emit luminescence at the wavelength of 590 nm, which displays as an orange color. When any of Pb\({}^{2+}\), Cr\({}^{3+}\), and Hg\({}^{2+}\) or their mixtures appear in the solution, they quench the luminescence immediately.