Water is the source of life [1], and one of the most important components of organisms. Human beings need to drink a lot of water every day to maintain the normal metabolism of the body. However, the amount of selenium in water can directly affect human health. Selenium (Se) is a trace element that can enhance the activity of peroxidase, protect biological cell membranes, reduce blood lipids, prevent cardiovascular diseases, and promote the growth of children [2,3]. In addition, an excessive intake of selenium causes poisoning, which may threaten human health [4]. Therefore, based on the dual nature of selenium, it is of great significance to establish an accurate, efficient, and environmentally friendly method to detect Se in water.

Metal–organic−frameworks (MOFs) are different from traditional inorganic and organic materials, which are hybrid materials formed by the combination of metal ions and organic ligands, generally forming a three−dimensional porous crystal structure in space with an adjustable topological structure [5,6] and high porosity [7,8,9]. Compared with traditional nano−porous materials, the specific surface area is larger, the pore size can be adjusted, and it also has good thermal stability and solution dispersion [10,11,12]. In recent years, MOFs have been frequently applied in various adsorption and catalytic experiments at home and abroad [13,14,15,16]. The current enrichment methods for selenium include pre−enrichment on exchange resin [17], solid−phase extraction (SPE) [18], and nano−dispersion micro−extraction [19]. However, these traditional adsorption materials can only be adsorbed under limited conditions, such as that the SPE column can only adsorb part of the specific target, and the adsorption range is not as wide as MOF materials. In addition, MOF materials also have the ability to enrich gas, organic pollutants, metal pollutants, and other substances at the same time. In addition, the reuse performance of traditional adsorption materials is not as good as that of MOF materials. In this experiment, MIL−125−NH₂, which was synthesized simply and quickly by microwave after micro−adjustment according to a previous report [20], belongs to the MILs series and has larger specific surface area and pore size, and stronger stability than some ZIFs series, UIO series, and IRMOF series [21,22]. The pore size of MIL−125−NH₂ was much larger than the size of the Se(IV), which was not more than 3.20 Å [23]. In addition, there were H₂O/−OH groups on its surface, which could bind SeO₃²⁻, HSeO₃⁻, and H₂SeO₃ through hydrogen bonds and coordination bonds. The N atom in the material was alkaline and could be protonated in an acidic solution [24], which was also conducive to the enrichment of SeO₃²⁻ and HSeO₃⁻. Importantly, not only Se(IV) could form Se−O−Ti complexes with Ti, but also the −NH₂ functional group introduced in MIL−125−NH₂ material, which had a stronger coordination ability with Se(IV), thus enhancing the adsorption capacity.

So far, the detection methods for Se are very traditional, including atomic fluorescence spectrometry [25,26], inductively coupled plasma mass spectrometry [27,28], flame atomic absorption spectrometry [29], inductively coupled plasma−optical emission spectrometry [30], etc., but these methods either have low detection limits or matrix interference. In addition, there are also reports of high−performance liquid chromatography−fluorescence detection (HPLC−FLD) and UV spectrophotometry, but a HPLC−FLD detector is more expensive than a diode array detector (DAD), and both methods do not have a qualitative spectrogram, the determination of the results may lead to false positive misjudgments, as UV spectrophotometry without chromatographic separation will have impurities interference. Therefore, in this study, Se was absorbed by MIL−125−NH₂ in water, then Se was involved in a rapid derivative reaction with 4−nitro−o−phenylenediamine (NPDA), and then produced 5−nitro−2,1, 3−benzooleprazole (the latter was called piaselenole), which had strong ultraviolet absorption, which can be detected by UPLC−DAD. Finally, a method for the determination of trace selenium in water by UPLC−DAD after enrichment by MIL−125−NH₂ was established by optimizing various conditions. The method could detect low concentrations of Se in water which was adsorbed by MOF, and the impurity interference would be greatly reduced after the piaselenole was separated by chromatography. It was more accurate to use DAD for quantitative analysis, and it could also be qualitatively analyzed by an ultraviolet and 3D spectrogram. At present, there are almost no reports on the determination of Se by UPLC−DAD. In addition, the enrichment ratio of selenium in water could be increased by using MOF to achieve a lower detection limit.