# 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  
Metallic nanoparticles (NPs) are between 1-100 nm in size with a high surface charge density, a high surface to volume ratio, and, oftentimes, special optical properties due to a localized surface plasmon resonance (LSPR) effect. For these reasons, metallic NPs have been widely used in diverse biological and chemical applications, especially for biochemical detection. Various types of biochemical molecules can be easily incorporated onto the NPs while retaining their biochemical activity. For HMI detection, most metallic NPs are made of gold and some are silver, with gold NPs (GNPs) being the most widely used material in HMI sensors. Other shapes of NPs, such as nanorods and nanostars, have been adopted for sensing [41; 42]. From the instrumentation point of view, two types of optical methods are commonly reported for Pb\({}^{2+}\) detection: colorimetric and fluorescent detection. Other types of optical transduction techniques, such as an LSPR sensor (detection of molecular interactions near a nanostructured metal surface through shifts in the LSPR spectral peak) and dynamic light scattering (DLS) have also been reported. Fundamentally, the NP-based colorimetric assay is also based on the phenomenon known as LSPR. In assays based on noble metal NPs, as is well-known, there are a large number of free electrons on the metal surface. When incident photons resonate with free electrons on the NP surface, it will cause a collective charge oscillation. This in turn generates surface plasmon waves, leading to strong electromagnetic fields on the NP surface and enhanced light adsorption and scattering. The plasmon resonance frequencies of gold and silver NPs are generally in the 600-750 nm and 400-550 nm spectral ranges, respectively. Both LSPR sensors and colorimetric assays have been adopted in heavy metal ion detection. Molecular interactions between metal ions and the ligands on NPs will result in changes in the local refractive index near the NP surface. The changes in the refractive index of the surrounding environment is approximately linear with the LSPR peak wavelength shift. In practice, an LSPR sensor is mostly a surface-based method, while a colorimetric assay is solution-based. Typically, LSPR sensors are implemented using a surface with metallic nanostructures, which is functionalized with ligands to bind with metal ions. Binding between the ligands and the metal ions leads to the formation of a self-assembled monolayer (SAM) on the sensor surface. To find changes in the refractive index of a sensor's SAM, a spectral analysis is applied on the SAM film to find the shift in the LSPR peak wavelength. Due to the small size of the metal ions that are added to the SAM during testing, an extinction measurement is usually used. Colorimetric detection is based on the size-dependent optical property and aggregation/dispersion of metal NPs in a solution. Because particle aggregates are of a large size, about tens of nm, in large quantities, a spectral absorbance measurement is often used, and sometimes the color change can be observed by the naked eye. An LSPR sensor for Pb\({}^{2+}\) detection was developed using GSH-functionalized SNPs in [33]. In this report, GNPs were immobilized on a transparent indium tin oxide (ITO) glass substrate, then the ITO/GNPs stripes were functionalized with GSH. The ITO/AuNPs/GSH was used as a plasmonic probe. When the binding between GSH and Pb\({}^{2+}\) occurred, the local refractive index at the ITO/AuNPs/GSH surface changed. As a result, a red-shift in the LSPR peak wavelength was observed when Pb\({}^{2+}\) was added into the solution. The Pb\({}^{2+}\) detection was performed at pH 7.5, which was demonstrated to be a suitable pH value to recognize Pb\({}^{2+}\). This platform achieved a limit of detection (LOD) of 50 pM in 15 min, with a good selectivity against other MIs, such as Hg\({}^{2+}\), Cu\({}^{2+}\), Mg\({}^{2+}\), and Na\({}^{+}\).