# 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
In [47], the assay was sensitive to Ni\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\) at pH 9.8. The solution's pH value was carefully adjusted to observe the effects on the GSH-GNP aggregation in the presence of Ni\({}^{2+}\). UV-visible spectra were used to obtain the ratios of the absorbance intensity at 610 and 520 nm (E610/520), which indicated the degree of GNP aggregation. E610/520 changed obviously with pH, and reached a maximum when pH was about 10, which is different from the neutral pH condition for Pb\({}^{2+}\) detection. Ni\({}^{2+}\) had little effect on GSH-GNP aggregation when the pH value was lower than 9.0. Using FTIR, the study found that GSH was immobilized on GNPs through its cysteine sulfhydryl group and the carboxylic acid group (-COOH) of the glycine residue. Carboxyl and amine groups of the glutamyl moiety were available to bind with metal ions. At pH 6.4, after adding Ni\({}^{2+}\), only carboxyl, and not amine, groups contributed to the coordination between Ni\({}^{2+}\) and GSH, while at pH 9.8, both carboxyl and amine groups participated in the coordination. The work actually found that this assay was sensitive to many other metal ions. At pH 6.4, GSH-capped GNPs exhibited weak affinity to Ni\({}^{2+}\) while showing strong affinity to Cu\({}^{2+}\) and Fe\({}^{3+}\), metal ions with strong affinity to the carboxyl group. At pH 9.8, GNP aggregates were formed for metal ions (Ni\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\)) with strong affinity to the deprotonated amine group. Pb\({}^{2+}\) and Hg\({}^{2+}\) were not tested in this work. The ratios of the absorbance intensity at 610 and 520 nm demonstrated a proportional increase with the concentration of Ni\({}^{2+}\) from 10 \(\upmu\)M to 80 \(\upmu\)M [47]. In [48], AgNPs were used to specifically detect Ni\({}^{2+}\) in water. GSH was functionalized onto the surface of AgNPs through the cysteine sulfhydryl group of GSH. It is hypothesized that carboxylate groups of the glycine moiety and amine groups from the glutamate moiety are responsible for the crosslinking between GSH-capped AgNPs and Ni\({}^{2+}\). The addition of Ni\({}^{2+}\) would cause a broadening of the surface plasmon absorption band and a color change from yellow to deep orange of the solution. The ratio of the absorbance intensity (A500/A396) was used for quantification of the Ni\({}^{2+}\). The dose response showed a linear correlation with the logarithm of Ni\({}^{2+}\) concentration over the range of 10\({}^{-6}\)-10\({}^{-4}\) mol/L with the linear detection limit of 7.5\(\times\)10\({}^{-5}\) mol/L. For control tests, the assay also showed a non-negligible response to Co\({}^{2+}\). Pb\({}^{2+}\) was not tested. While assay conditions here are not noticeably different to those developed for Pb\({}^{2+}\) detection, it should be noted that AgNPs were used here as opposed to GNPs. The generation of a plasmon resonance may have a different preference for metal ion complexation when different NPs are used. Very recently, a competitive assay using free GSH and unmodified GNP was reported for Cd\({}^{2+}\) detection [49]. Since GSH was in free form in this detection system, it could bind with metal ions or GNPs through any of the functional groups, i.e., sulfhydryl, carboxyl, and amino groups, on GSH, which may contribute to its somewhat different specificity from that of most GSH-GNP assays. The detection was based on the formation of GNP aggregates in a high-salt solution when GSH preferentially conjugated with Cd\({}^{2+}\). First, test samples were added into GSH solution, then unmodified GNPs were added. The solution was then mixed with 1 M NaCl solution and incubatedfor 17 min. Without Cd\({}^{2+}\),SNPs became conjugated with GSH, and were monodispersed in the high-salt solution. When Cd\({}^{2+}\) was added, GSH complexed with Cd\({}^{2+}\) preferentially owing to its high affinity to Cd\({}^{2+}\). The color of aggregated SNPs showed yellow or red in dark-field images, which were observed through a dark field microscope.