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  • Title: [Preparation of molecularly imprinted polymers based on covalent organic frameworks and their application to selective recognition of trace norfloxacin in milk].
    Author: Xie Y, Zhang Y, Shi H, Wu Z, Yu X, Zhang C, Feng S.
    Journal: Se Pu; 2022 Jan; 40(1):1-9. PubMed ID: 34985210.
    Abstract:
    Norfloxacin (NFX) is an antibiotic that is widely used in animal husbandry. However, the presence of NFX even in trace amounts in animal-derived food may harm human health. Therefore, it is of practical significance to establish a method for monitoring NFX residues in food. Molecularly imprinted polymers (MIPs) imitate interactions established by natural receptors to selectively retain a target molecule, like antibodies or antigens do. MIPs have been widely employed in the selective recognition of specific target molecules from complex samples. Covalent organic frameworks (COFs) are a new type of organic polymer with uniform and ordered crystal structures. COFs form crystal structures by constructing organic units for ordered assembly through reversible chemical reactions. Their porous structure, regular morphology, and easy modification make COFs promising for use as excellent adsorbent carriers. Owing to these advantages of COFs, researchers have attempted to coat one MIP layer on COFs; however, the preparation methods are time-consuming and laborious, and the conditions are harsh. Hence, this study proposes a simple and rapid method for the preparation of novel MIPs with COFs as the support (DP-COF@MIPs) for the selective recognition of NFX. First, a Schiff-base COF (DP-COF) was rapidly synthesized using 3,3'-diaminobenzidine and p-phthalaldehyde with a metal trifluorate as a catalyst at room temperature. Subsequently, a two-step sequence was adopted as the synthesis strategy using NFX as the template, methacrylic acid as a functional monomer, and ethylene glycol dimethacrylate as a crosslinking agent. The entire synthesis was completed within 5 h under mild conditions. The material was then characterized by multiple analytical methods, including field-emission scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and the Brunauer-Emmett-Teller (BET) method (to determine the specific surface area). The experimental results confirmed the successful preparation of DP-COF@MIPs. The DP-COF@MIPs presented a rough and porous surface, with a pore size of approximately 17.79 nm (mesoporous structure). The properties of the material were evaluated by adsorption and regeneration experiments. The kinetic adsorption experiment showed that the DP-COF@MIPs reached adsorption equilibrium in 90 min. Two straight lines were fitted using the pseudo-second-order kinetics model based on the experimental results for thermodynamic adsorption, indicating that the adsorption process was primarily dominated by chemical adsorption. The maximum apparent adsorption capacity was 41.57 mg/g. In the selective and competitive binding test, five drugs, namely ciprofloxacin, dimetridazole, oxytetracycline, sulfadiazine, and chloramphenicol, were selected as the interferents. The experimental results showed that the DP-COF@MIPs possessed good stereoselectivity and competitive recognition ability. The regeneration of DP-COF@MIPs was evaluated by multiple cycles of adsorption-desorption experiments. The loss in the adsorption capacity of the particles was only approximately 4.7% after seven adsorption-desorption cycles. These results from the regeneration experiments show that the DP-COF@MIPs had high stability and reusability in the selective adsorption and separation of NFX. In addition, the employed method could accurately identify trace NFX in milk samples. The average recoveries were in the range of 88.8%-92.9% at three spiked levels (0.03, 0.10, and 0.30 mg/L) with relative standard deviations (RSDs) in the range of 0.6-1.7% (n=3). Notably, the method could successfully determine NFX at contents as low as 0.0020 mg/L in the milk sample with an average recovery of 77.6% and RSD of 6.4% (n=3). This concentration is one-fiftieth of the maximum residue level stipulated by the European Union (EU), and even lower than the limit of detection (LOD) of conventional high performance liquid chromatography (HPLC) methods. The above results confirm that DP-COF@MIPs can be used to determine trace NFX in actual complex samples using HPLC equipment, even when coupled to conventional UV-Vis detectors. This study offers a facile and general method for the preparation of MIPs based on COFs with selective recognition ability. 诺氟沙星(NFX)作为一种常见的喹诺酮类兽药,被广泛应用于畜牧业中,但其会残留在动物体内,进而对人体健康造成危害,为此有许多国家和组织均对NFX残留量进行了严格限制。为实现对复杂体系中痕量NFX残留的准确与可靠分析,该文制备了一种以共价有机框架(COFs)为载体的分子印迹聚合物(MIPs)。首先,在室温条件下,以金属三氟酸盐为催化剂,对苯二甲醛和3,3'-二氨基联苯为原料快速合成了“席夫碱”型共价有机框架(DP-COF)。然后将NFX、甲基丙烯酸、乙二醇二甲基丙烯酸酯与DP-COF混合,利用偶氮二异丁腈引发聚合反应,即可得到DP-COF@MIPs。整个制备过程条件温和,耗时仅5 h。采用场发射扫描电镜、傅里叶红外光谱、X射线衍射仪、BET比表面积测试仪等对其进行了表征。结果证实成功制备出了DP-COF@MIPs,该材料表面粗糙,拥有介孔范围的孔径(17.79 nm)。通过吸附实验、重复使用性实验对材料性能进行评估,结果表明该材料表观吸附容量高达41.57 mg/g,对NFX具有良好的特异性和选择性识别能力,且重复使用率令人满意。结合HPLC-UV-Vis,实现对牛奶样品中痕量NFX的检测。在3个加标水平下(0.03、0.1、0.3 mg/L),平均回收率为88.8%~92.9%,相对标准偏差小于1.7%。结果表明,该方法可以实现在复杂基质中对兽药残留高选择性、高灵敏度及准确性的检测。
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