# Heavy Metal Ion Detection Platforms Based on a Glutathione Probe: A Mini Review
## 3 Heavy Metal Ion Detection Platforms Using Glutathione
### Optical Techniques
#### 3.1.1 Metallic-Nanoparticle-Based Platforms
The AgNPs used in this work were initially protected by GSH from water, and the AgNP solution was orange in color due to the surface plasmon resonance band. Without GSH, AgNPs would dissolve in water and the orange fluorescence intensity would decrease. Detection of Pb\({}^{2+}\) was realized when the presence of Pb\({}^{2+}\) caused the dissociation of GSH from AgNPs. To synthesize these water-soluble silver NPs, Ag\({}^{+}\) was reduced to Ag\({}^{0}\) using the reducing agent of hydrazine as shown in Figure 2. The aqueous solution of GSH along with NaOH was mixed into AgNO\({}_{3}\) solution, where NaOH was used to keep the solution pH neutral. Then, hydrazine was added as a reductant and the solution was incubated for 8 hours. Orange fluorescent GSH-coated AgNPs were obtained after being washed. In the presence of Pb\({}^{2+}\), GSH would bind with Pb\({}^{2+}\) to form a stable complex, leaving the AgNP surface. AgNPs would then dissolve as schematically shown in Figure 2. Here, the mechanism for complexation between Pb\({}^{2+}\) and GSH was due to the cysteinyl S-donor atom in GSH. As shown in Figure 2, cysteine acts as a tridentate ligand for Pb\({}^{2+}\). In this work, fluorescence spectra and UV-visible absorption were utilized to indicate the concentration of GSH-protected AgNPs. The initial color of the solution containing fluorescent GSH-AgNPs was orange. After adding Pb\({}^{2+}\), AgNPs were partly dissolved, resulting in a decrease in fluorescence intensity corresponding to the Pb\({}^{2+}\) concentration as shown in Figure 3[45]. The fluorescence spectra in the presence of Pb\({}^{2+}\) from 200 ppq to 2000 ppm are represented in Figure 3a, and the fluorescence intensity at 660 nm in the presence of different types of Pb\({}^{2+}\) salts are given in Figure 3b. The assay took less than 20 min from Pb\({}^{2+}\) binding to detection and analysis. This fluorescence assay is highly sensitive to Pb\({}^{2+}\), with a low LOD of 200 parts per quadrillion (ppq). In addition, six kinds of different lead salts, such as PbSO\({}_{4}\) and PbCO\({}_{3}\), were also tested, which exhibited similar responses as shown in Figure 3b. To demonstrate the selectivity, various alkaline or transition heavy metal ions, including Cd\({}^{2+}\), Zn\({}^{2+}\), and Hg\({}^{2+}\), were tested as shown in Figure 3b. All of the non-target metal ions were tested at a high concentration of 100 parts per billion (ppb), with no obvious decrease in the fluorescence intensity of the solutions, demonstrating good selectivity between Pb\({}^{2+}\) and other HMIs. Furthermore, water samples from the Mississippi river, and dissolved plastic  
Figure 3: **(a)** Fluorescence intensity in the presence of Pb\({}^{2+}\) of different concentrations. **(b)** Fluorescence intensity in the presence of different types of Pb\({}^{2+}\) salts [45]. Figure 2: Fluorescence Silver Nanoparticle Synthesis Procedure and Pb\({}^{2+}\) Complexation [45]. toys, batteries, and paints, were tested. Those tests yielded Pb\({}^{2+}\) concentrations from several tens of ppb to several hundreds of parts per million (ppm), which were in good agreement with the results by ICP-MS, demonstrating the practical value of this Pb\({}^{2+}\) detection platform. Different shapes of NPs were adopted in other reports. For example, GSH mediated end-to-end assembled gold nanorod (GNR) chains [41]. After GNRs of a certain aspect ratio and dimension were synthesized using gold seeds, they were functionalized with GSH. Because GSH tends to complex with GNRs at the endpoints of [111] facets through (-SH) groups, the appended zwitterionic of GSH at the GNR endpoints will become coupled by a two-point electrostatic interaction. The GNRs are then aligned to form a chain through the end-to-end linkage. The chain formations of the GNR were found to be broken in the presence of 0.025 mM Pb\({}^{2+}\), and a number of dimers were observed. The mechanism is depicted in Figure 4, including the processes of assembly and disassembly.