# Advances on Water Quality Detection by UV-Vis Spectroscopy
## 4 Research Status and Progress of Main Water Quality Parameters
### Detection of Heavy Metal Ions in Water  
In recent years, researchers have found that colorimetry can accurately reflect the concentration of heavy metal ions in water. Colorimetry is a method used to detect the content in substances by comparing the color change of the substance or measuring the color depth of substance solution [45]. It is performed by selecting a suitable color reagent to react with the component to be tested to form a colored substance or to change the color of the solution, and then the generated colored substance is compared with the standard solution, the color of the reaction solution and the standard solution are observed, or the spectrum of its UV-Vis wave band is measured to quantitatively determine the substance [46].  
The change in the surface properties of nanomaterials will cause an obvious change in their apparent color. Many kinds of research have established methods based on nanogold colorimetry to detect the concentration of heavy metal ions. Many metal ions can cause the aggregation and dispersion state transition of the nanogold probe solution such that the absorbance and color of the mixed solution can change with an increase in the metal ion concentration [47; 48]. Figure 5 shows the reaction mechanism of using nanogold to detect heavy metal ions.  
[PERSON] et al. [49] used the Akaike information criterion to select the optimal model. A method based on modeling was proposed to detect the concentration of Zn\({}^{2+}\) and Co\({}^{2+}\) ions by using UV-Vis absorption spectroscopy. In addition, [PERSON] et al. [50] also proposed a method based on the improved Monte Carlo uninformative variable elimination (MC-UVE) method to detect the concentration of multi metal ions, which solved the problem of spectral overlap. [PERSON] et al. [51] used Hg\({}^{2+}\) mediated gold nanoparticles to rapidly detect Hg\({}^{2+}\) ions with high sensitivity and selectivity. [PERSON] et al. [52] prepared a thymine rich DNA single-strand aptamer by using the characteristic that mercury ions can be covalently mismatched with thymine, thus establishing a colorimetric sensor for detecting mercury ion. Based on the adsorption of Hg\({}^{2+}\) and polydopamine, [PERSON] et al. [53] agglomerated gold nanoparticles, which changed the absorption signal of the system and the color of the solution. The UV-Vis spectrophotometry and a colorimetric method of Hg\({}^{2+}\) were established.  
[PERSON] et al. [54] established two novel isomeric colorimetric probes for the simultaneous determination of copper ions. Two new colorimetric probes can produce obvious peaks at 498 nm  
Figure 5: Reaction mechanism of nano gold in detecting heavy metal ions.  
in the UV-Vis absorption spectrum, as well as a rapid color change from colorless to dark yellow. [PERSON] et al. [55] connected the pyridyl group with tetrathiafulvalene by using a double bond bridge, and the pyridyl group acted as a single dentate ligand of metal ions. \(\pi\)-conjugated double bonds were designed to optimize the communication between the receptor and the tetrathiafulvalene core. When exposed to micro molar concentration Pb\({}^{2+}\), its UV-Vis spectrum showed obvious changes. [PERSON] et al. [56] created a portable analytical system for the detection of metal ions in water, using two single-step methods based on gold nanoparticles to detect Pb\({}^{2+}\) and Al\({}^{3+}\) in water with high selectivity and low detection limit. [PERSON] et al. [57] established a highly selective visualization method for the determination of trace Cr\({}^{3+}\) in water based on citric acid-modified gold nanoparticles.