# 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
Colorimetry is based on measuring the change in peak absorption wavelength between the dispersed and aggregation modes of NPs, which is caused by the addition of target analytes into a testing solution. Any change in the size, shape, or geometry of NPs will change the local electron confinement, leading to a change in the absorption maxima. Then, the color of the colloidal solution will change. This change between dispersion and aggregation and the subsequent color change is the basis of colorimetric detection when using metallic NPs to detect specific metal ions. A colorimetric method was developed for Pb\({}^{2+}\) detection using GSH-functionalized SNPs (GSH-GNPs) in [43]. Normally, GSH can inhibit the aggregation of GNPs in solutions with a high concentration of salt. A carboxylate anion of GSH capping the GNPs will exhibit strong interparticle electrostatic repulsion between GNPs. When Pb\({}^{2+}\) are added, complexes will form between Pb\({}^{2+}\) and the carboxyl group of GSH. As a result, the stability of GSH-GNPs are reduced, making GNPs aggregate even under the high salt concentration in solution. Because the color of the GNP solution depends on the GNP size or aggregates, Pb\({}^{2+}\) detection can then be achieved by a colorimetric analysis, either using a UV-vis spectrophotometer or naked eyes. In [43], Pb\({}^{2+}\) was detected by the colorimetric response of GSH-capped GNPs, with the color turning from red to purple in the presence of Pb\({}^{2+}\). During testing, NaCl-based aqueous solution was added to the samples to reach a high salt content. The limit of detection (LOD) was found to be 100 nM. After the addition of Zn\({}^{2+}\), Co\({}^{2+}\), Fe\({}^{2+}\), and eight other types of metal ions, all at 50 uM, negligible responses were observed in contrast to that of 20 uM Pb\({}^{2+}\), indicating that this approach is a specific platform to detect Pb\({}^{2+}\). Based on the transient curve of the absorption ratio (A700/520), the response time was optimized to be 25 min. For fluorescence-based detection, a turn-on fluorescent nanoprobe for Pb\({}^{2+}\) with a fast response was reported recently [44]. The working mechanism is aggregation-induced emission (AIE), which is considered to be an effect induced by a ligand-to-metal-metal charge transfer (LMMCT) through Au-S motifs due to aurophilic interactions. In [44], GSH was used as a thiolate ligand to detect Pb\({}^{2+}\). GNPs capped with GSH via an Au-S bond were synthesized first. When Pb\({}^{2+}\) was added into the system, its strong affinity with GSH caused the GSH-GNPs to aggregate. Then, this aggregation induced a strong fluorescence with an emission peak at 595 nm recorded by a fluorescence spectrometer. In this work, the sensitivity of the designed sensor was significantly improved by adding ethanol. The positive role of ethanol was further investigated, which demonstrated its ability to induce dense NP formation. Furthermore, the ratio of ethanol to the sample solution was optimized, which led to maximal sensitivity when detecting Pb\({}^{2+}\). Considering that the added Pb\({}^{2+}\) needed to be mixed with the testing solution and incubated for 1 min, this method required about 1 min of testing time, which showed a faster than average response time when compared to similar optical sensing platforms, with their response time ranging from 15 s to \(\sim\)40 min) [32; 35; 42; 43; 45]. In this work, only the presence of Pb\({}^{2+}\) caused strong luminescence of GSH-GNPs under UV light, while other ions at a 10-fold higher concentration showed no obvious responses, indicating a good selectivity for Pb\({}^{2+}\) detection. The control metal ions included Ca\({}^{2+}\), Cu\({}^{2+}\), Mn\({}^{2+}\), Fe\({}^{3+}\), Fe\({}^{2+}\), and Hg\({}^{2+}\). In another work of fluorescence-based Pb\({}^{2+}\) detection, highly fluorescent silver nanoparticles (AgNPs) were synthesized and used as the probe in this competitive assay [45].