Boronic acid-decorated metal-organic frameworks modified via a mixed-ligand strategy for the selective enrichment of cis-diol containing nucleosides
Graphical abstract
Introduction
Saccharides, glycoproteins, catecholamines, and nucleosides are typical cis-diol biomolecules that play essential functional or structure roles in living organisms [1,2]. Studies of cis-diol containing biomolecules are of great biomedical and clinical significance [[3], [4], [5]]. Nucleosides, which are excreted in the urine, are considered potential biomarkers of the immune response to oncological processes [[6], [7], [8]]. Their plasma levels are used to evaluate oxidative stress. Liquid chromatography (LC) and mass spectrometry (MS) techniques have developed rapidly, and are widely used for the analysis of cis-diol compounds. They have high sensitivity and identification capacity, but the direct analysis of biological fluids remains difficult owing to the complex matrix of biological fluids [9,10]. Therefore, reliable and efficient sample preparations for the separation and determination of nucleosides and other cis-diol-containing targets in biological fluids are consequently essential. In recent years, boronate-based affinity isolation has gained increasing research interest [[11], [12], [13], [14]]. Boron affinity refers to the formation of stable cyclic esters from boronic acid and cis-diol compounds under alkaline conditions. Various materials decorated with boron affinity functional groups have been developed for the enrichment and separation of cis-diol-containing compounds. These include: mesoporous silica [[15], [16], [17], [18]], monoliths [[19], [20], [21]], polymers [22], and magnetic nano-composites [[23], [24], [25], [26], [27], [28]]. However, multistep synthesis procedures are usually needed to modify the boronic acid groups on the surfaces of the functionalized materials. Moreover, the low modification efficiency of boronic acid functional groups may limit their application. Therefore, the development of novel materials with abundant boronic acid active sites may improve the separation of cis-diol compounds.
Metal–organic frameworks (MOFs) comprise crystalline porous materials that are formed by inorganic metal nodes and organic linkers. In comparison with traditional porous materials, MOFs feature high porosity, abundant pore structure, and a tunable framework composition. These advantages have qualified MOFs for use in various areas, such as gas storage [29], catalysis [30], enzyme immobilization [31], sensors [32], and sample preparation [33,34]. More recently, functional MOFs based on the pre-installation of ligands containing functional groups have attracted considerable attention in biological applications [35,36]. Two ligands, the original ligand and its fragment are co-assembled and crystallized by one pot synthesis into one MOF, which retain the structure of the parent MOF. This mixed-ligand strategy greatly simplifies the synthesis procedure and introduces specific functional groups. Ligands containing boronic acid functional groups have been used to construct MOFs [[37], [38], [39], [40]]. MOFs are highly porous and are formed by adjustable ligand assembly. Therefore, MOFs containing boronic acid functional groups have a large number of active –B(OH)2 sites, which endow them with enhanced separation selectivity and adsorption capacity.
The UiO-66 framework, a kind of Zr-based MOF, would be suitable for the construction of boronic acid functionalized MOFs by a pre-installation strategy, owing to its superior stability, which makes it relatively resistant to attack by water and acidic or alkaline reactants [41,42]. The extraordinary chemical, thermal, and aqueous stability of UiO-66 would enable it to bear the assembly of ligand fragmentation. Furthermore, many MOFs have been successfully combined with magnetic particles and used as sorbent for magnetic extraction [37,[43], [44], [45]]. Such magnetic adsorbents greatly improve treatment efficiency because they facilitate the ready recovery of material from the matrix [[46], [47], [48], [49]]. Magnetic MOFs have both the excellent properties of magnetic cores for rapid and simple separation process and properties of MOF shells for efficient and selective enrichment.
Herein, we report the successful preparation of novel magnetic MOFs designated MNPs@Zr-MOFs-BA (MNPs stands for Fe3O4@SiO2)with active boronic acid sites. The MOFs were assemble from an original ligand with –B(OH)2 fragments as functional groups. The obtained composites have excellent chemical stability over a large pH range and are suitable for use as adsorbents for the enrichment and separation of nucleosides. The adsorbents have great performance with regard to the enrichment of nucleosides, and are potential candidates for the practical separation of cis-diol targets.
Section snippets
Chemicals
FeCl3·6H2O, ZrCl4, 3-carboxyphenylboronic acid (3-CPBA), tetraethyl orthosilicate (TEOS), and HPLC-grade methanol were obtained from J&K Scientific (Beijing, China). Terephthalic acid (TPA) and the four nucleosides were purchased from Tokyo Chemical Industry Co., Ltd. (Shanghai, China). The formic acid solution was from Acros Organics (Belgium). Anhydrous sodium acetate, ethylene glycol, dimethyl formamide (DMF), NH4Cl and NH3·H2O solution were supplied by Beijing Chemical Reagent Co., Ltd.
Synthesis and characterization of MNPs@Zr-MOFs-BA
The MNPs@Zr-MOFs-BA materials were synthesized by modifying the MOFs on the TEOS coated magnetic particles. The magnetic particles were synthesized by solvothermal method. The size of magnetic particles synthesized by solvothermal method is generally much larger than that of magnetic particles synthesized by co-precipitation method. Magnetic particles with large-size were used in our research, mainly because larger-size magnetic particles possess better magnetic responsiveness which can
Conclusions
In summary, a novel boronic acid functional magnetic MOF composite (MNPs@Zr-MOFs-BA) was successfully prepared via a pre-installation strategy in which a 3-carboxyphenylboronic acid ligand was introduced as a functional group. The obtained material exhibites favorable pH tolerance; the crystals remain intact over large pH range. Moreover, the adsorption rate is rapid, and the adsorption equilibrium can be achieved within a few minutes. Owing to its excellent chemical stability, abundant boronic
CRediT authorship contribution statement
Ruiqi Zhang: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Zhen Wang: Methodology, Validation. Tiefeng Wang: Methodology, Investigation. Ping Su: Conceptualization, Writing - original draft. Yi Yang: Conceptualization, Writing - original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant No. 21675008) and the Fundamental Research Funds for the Central Universities (No.XK1802-6).
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