分子印迹聚合物功能化二氧化硅纳米颗粒的合成及其分离识别马兜铃酸

马兜铃酸是马兜铃科植物中含有硝基菲羧酸基团的一类物质,被广泛应用于各种疾病的治疗,研究表明含有马兜铃酸的植物或植物衍生产品对人体有害,需要监测药物中马兜铃酸的存在。分子印迹聚合物对目标物的高亲和力使其特别适合作为吸附剂从混合物中去除和识别目标物。以SiO₂胶体纳米颗粒为基底,利用表面分子印迹的方法合成了核-壳结构SiO₂表面印迹纳米颗粒(SiO₂@MIP NPs)。采用紫外可见光谱研究了模板分子马兜铃酸Ⅰ和功能单体丙烯酸、甲基丙烯酸、2-乙烯基吡啶、丙烯酰胺及甲基丙烯酰胺的作用,发现2-乙烯基吡啶与马兜铃酸Ⅰ的作用最强,被选为制备印迹聚合物的单体。采用傅立叶变换红外光谱仪(FT-IR)、透射电子显微镜(TEM)、热重分析仪、氮气吸附比表面分析仪对分子印迹聚合物进行了表征。TEM显示印迹纳米颗粒的粒径在270 nm左右,分子印迹层的厚度为35 nm,有利于模板分子的传输。TEM、FT-IR和热重分析仪的结果均证明实验成功合成了分子印迹聚合物。实验进一步研究了印迹聚合物SiO₂@MIP NPs和非印迹聚合物SiO₂@NIP NPs的吸附性能,并结合SiO₂@MIP NPs和SiO₂@NIP NPs的比表面积和孔径测定数据,发现SiO₂@MIP NPs表面的印迹位点是导致二者吸附差异的主要原因。SiO₂@MIP NPs和SiO₂@NIP NPs的动力学吸附表明SiO₂@MIP NPs具有快的吸附平衡时间(120 s),而且SiO₂@MIP NPs的吸附行为符合Langmuir单分子层吸附。SiO₂@MIP NPs的选择性通过印迹因子(IF)和选择性系数(SC)来评价。实验结果表明,SiO₂@MIP NPs具有高的印迹因子(4.9),对模板结构类似物有较好的选择性,选择系数为2.3~6.6。最后将制备的SiO₂@MIP NPs作为吸附剂用于加标中药样品川木通的预处理,用HPLC进行分析测定,方法的回收率为73%~83%,实验结果显示SiO₂@MIP NPs可作为高选择性材料用于中药中马兜铃酸的选择性分离分析。

Preparation of molecularly imprinted polymers-functionalized silica nanoparticles for the separation and recognition of aristolochic acids

Aristolochic acids (AAs), which is commonly found in Aristolochia and Asarum plants, has been widely used in several traditional medicine practices due to their anti-inflammatory, anti-malarial, and anti-hyperglycemic activities. Recently, researchers have found a “decisive link” between liver cancer and aristolochic acid after analyzing a large number of liver cancer samples around the world. Therefore, a highly sensitive and selective method is required for the analysis of AAs in traditional Chinese medicines (TCM). For the determination of AAs in TCM, pretreatment is indispensable because in actual TCM samples, AAs is present in trace amounts and the complex matrix exerts interference. In the past decades, molecularly imprinted polymers (MIPs) have attracted considerable attention as an alternative for the trace analysis in complicated matrices.
In this study, MIP-coated SiO₂ nanoparticles (SiO₂@MIP NPs) was prepared for the determination of aristolochic acid by surface molecular imprinting using aristolochic acid Ⅰ (AAI ) as the template molecule, 2-vinylpyridine (VPY) as the functional monomer, and ethyleneglycol dimethacrylate (EGDMA) as the cross-linking agent. Core-shell-structure SiO₂@MIP NPs were obtained by modifying vinyl groups on the surface of SiO₂ NPs, coating MIPs films onto the silica surface via selective polymerization, and final extraction of template AAI and generation of the recognition site.
To find a suitable functional monomer for the best imprinting effect, the interaction between the template and the functional monomers, including acrylic acid (AA), methyl acrylic acid (MAA), 2-vinyl pyridine (VPY), acrylamide (AM), and methylacrylamide (MAM) was investigated. Electrostatic interaction between AAI and VPY resulted in the maximum decrease in absorbance of AAI at 250 nm. Therefore, VPY was chosen for the preparation of MIP. The morphological and physical properties of the MIPs were characterized by transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis, and N₂ adsorption and desorption surface analysis. TEM images showed that SiO₂ NPs were monodispersed with diameter of about 200 nm. The clear core-shell structure of SiO₂@MIP NPs was observed, and the thickness of MIPs coating was about 35 nm. The FT-IR spectra of SiO₂ NPs, vinyl group modified SiO₂ and SiO₂@MIP NPs revealed that the vinyl group and organic MIP layer were successfully modified at SiO₂ sequentially. The results of thermogravimetric analysis were consistent with the FT-IR data for different SiO₂ NPs. The nitrogen gas adsorption-desorption experiments showed that SiO₂@MIP NPs and non-imprinted polymer (SiO₂@NIP NPs) have the same pore volumes, while the surface area and pore size of MIPs were slightly larger than those of NIPs. Therefore, the difference in adsorption between SiO₂@MIP NPs and SiO₂@NIP NPs resulted from the imprinted sites on the MIP surface, rather than the difference in their surface areas.
The adsorption properties of SiO₂@MIP NPs were demonstrated by kinetic, isothermal, and selective adsorption experiments. The results of these experiments displayed that SiO₂@MIP NPs reached adsorption equilibrium within a short period (120 s) and possessed a much higher rebinding ability than SiO₂@NIP NPs. To verify the selectivity of SiO₂@MIP NPs for AAI, three structural analogues (viz. tanshinone ⅡA, 2-methoxy-5-nitrophenol, and benzoic acid) were selected. The results showed that the binding capacity of SiO₂@MIP NPs was much higher than those of these analogues. SiO₂@MIP NPs have high adsorption capacity (5.74 mg/g), high imprinting factor (4.9), good selectivity coefficient (2.3-6.6) towards the structural analogues. SiO₂@MIP NPs was used as an adsorbent and combined with HPLC for the selective separation of AAI in TCM. The recoveries of Kebia trifoliate samples spiked with three levels of AAI (0.3, 0.5, and 1.0 μg/mL) ranged from 73% to 83%. The results suggested that the proposed SiO₂@MIP NPs could be used for selective enrichment of AAI from real complex TCM samples.