# 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
As discussed in Section 2, when Pb\({}^{2+}\) is added, its higher affinity with the carboxyl group against other HMIs makes it possible to specifically detect Pb\({}^{2+}\). When GNR chains were disassembled due to the addition of Pb\({}^{2+}\), the size of the GNR chains decreased by more than half, i.e., about 230 nm. Dynamic light scattering (DLS) was employed for precise measurement of the hydrodynamic size of nanorods. The nanorod size measured by DLS was used to indicate the Pb\({}^{2+}\) level and the selectivity of Pb\({}^{2+}\) against other ions. The selectivity of this detection method was verified in the presence of various ions, such as Cu\({}^{2+}\), Hg\({}^{2+}\), Zn\({}^{2+}\), Mn\({}^{2+}\), Ni\({}^{2+}\), and NH\({}^{4+}\). The stronger affinity between Pb\({}^{2+}\) and (-COOH) than that of other HMIs makes it possible to specifically detect Pb\({}^{2+}\), as discussed in Section 2. Analytes other than Pb\({}^{2+}\) did not induce disintegration of the assembled chains. The LOD of this method reached 0.025 mM. GSH-based sensing systems often operate with Pb\({}^{2+}\) as their target metal ion, as discussed in the foregoing examples. Other than Pb\({}^{2+}\), arsenic (As\({}^{3+}\)), nickel (Ni\({}^{2+}\)), and cadmium (Cd\({}^{2+}\)) detection have also been reported using GSH-capped metal NPs, which are also common HMIs of significance in environment and food safety monitoring. A study on As\({}^{3+}\) detection was conducted based on GSH/4-mercaptoethanol(4-MPY)-modified silver nanoparticles (AgNPs), and a surface-enhanced Raman scattering (SERS) spectrum was employed to find the As\({}^{3+}\) concentration [46]. Both GSH and 4-MPY were conjugated to AgNPs by Ag-S or Ag-N bonds. The GSH-modified AgNPs can bind with As\({}^{3+}\) through GSH by forming an As-O linkage, and 4-MPY act as the Raman reporters. One As\({}^{3+}\) could link with three AgNPs to produce SERS \"hot spots\". Subsequently, in the absence of As\({}^{3+}\), the AgNP solution adding GSH and 4-MPY remained yellow in color. With the addition of As\({}^{3+}\) into the AgNP colloidal solution, aggregation of the AgNPs occurred, and the solution's color turned to brown due to AgNP aggregation. In SERS spectra, there were some specific peaks related to As\({}^{3+}\) detection, and the intensity around 1018 cm\({}^{-1}\) was chosen for quantification of As\({}^{3+}\). Figure 4: Assembly by GSH and disassembly by Pb\({}^{2+}\) of gold nanorods (GNRs). This sensor reached an LOD at 0.76 ppb (10.2 nM) for As\({}^{3+}\) detection, and the incubation time between the addition of As\({}^{3+}\) sample and the measurement was 2 min for every test. According to [46], the GSH/4-MPY-capped, AgNP-based sensor had an acceptable selectivity against other common metal ions (MIs). When testing 1.34 \(\upmu\)M As\({}^{3+}\) ions against other metal ions (Cd\({}^{2+}\), Cu\({}^{2+}\), Cr\({}^{3+}\), Zn\({}^{2+}\), Ni\({}^{2+}\), Fe\({}^{3+}\), and As\({}^{5+}\) at 1.34 \(\upmu\)M and K\({}^{+}\), Hg\({}^{2+}\), Mg\({}^{2+}\), Pb\({}^{2+}\), Ca\({}^{2+}\), and Mn\({}^{2+}\) at 13.4 \(\upmu\)M), only As\({}^{3+}\) could induce a significant increase of SERS signals. Further, As\({}^{3+}\) detection of drinking water was performed by the standard addition method, demonstrating recovery rates around 98.1-104.1%. While a GSH ligand could bind with many kinds of metal ions, the specificity of this assay could be attributed to the characteristics of SERS sensing, which is dependent on the charge interaction on and between AgNPs and As\({}^{3+}\), as well as the molecular structure and size. Ni\({}^{2+}\) detection by GSH-capped noble metal NPs has also been reported [47; 48], which was also based on the color change between dispersed and aggregated modes of NPs. The work in [47] investigated the conditions for aggregation of GSH-GNPs induced by Ni\({}^{2+}\). For GSH-capped GNPs used in the detection of Pb\({}^{2+}\), those assays were performed in a neutral or slightly acidic environment.