Polyamidoamine dendrimers functionalized magnetic carbon nanotubes as an efficient adsorbent for the separation of flavonoids from plant extraction
Graphical abstract
Introduction
Studying bioactive components as therapeutic agents is increasingly becoming popular [1]. Flavonoids are the major group of phenolic compounds and diverse common secondary metabolites in plants and fungi. Considerable pharmacological research results have revealed that flavonoids possess diverse pharmacological properties (e.g.,antioxidant, antitumor, anti-inflammatory, and anti-cancer), and these have increased research attention in these compounds in recent years [2], [3], [4], [5]. At present, flavonoids are widely used in pharmaceutical, functional foods, and natural cosmetics. Rapid extraction and isolation of flavonoids from plant extracts for expanding the use of flavonoids in the pharmaceutical field are critical, considering the complexity of plant extracts.
Numerous technologies have been reported for the efficient extraction and separation of flavonoids from plants. These commonly used extraction methods mainly include immersion, solid-phase, ultrasound- and microwave-assisted, ionic liquids, and supercritical fluid extraction [6], [7], [8] (Meng et al., 2018). Some reported methods are time consuming, labor intensive, and require substantial organic solvents because the solubility of flavonoids in water is generally low [8]. In addition, the crude extraction solution of flavonoids needs to be further separated and purified via several methods, such as adsorption, chromatography, high-efficiency countercurrent chromatography, and membrane separation techniques, which exhibit complex operating processes to obtain high-purity flavonoids [9]. Among these methods, adsorption is widely used for the separation of some bioactive components owing to its several advantages, such as relatively simple operation and renewable utilization of adsorbent. At present, the commonly used adsorbents are polyamide, silica gel, and dextran [10], [11], [12], [13]. However, the large-scale separation of flavonoids by adsorption remains challenging due to the limited adsorption capacity of conventional adsorbents and the difficulty of separating adsorbents (solid) from the extracts (liquid). Therefore, developing high performance adsorbents for rapid separation of flavonoids is highly important to support the biological studies and clinical use of this group of compounds.
Various nanomaterials have been widely used in the preparation of new adsorbents with the development of nanotechnology [14]. Moreover, new nanomaterials open up a new avenue to extract and purify the bioactive components of natural products. CNTs have recently attracted great interest owing to their several advantages, such as high surface area and mechanical strength, low mass density, and ease of modification with functional groups [15]. These properties allow CNTs to be used in various applications, such as catalysts, medicine, nanoelectronics, photonics, sensors, composites for mechanical applications, and adsorbents for different target adsorbates [16], [17], [18], [19]. CNTs are hollow graphitic materials composed of one or more coaxially-assembled layers of graphene sheets named as single- or multi-walled CNTs [20]. CNTs offer a notably higher surface area-to-volume ratio and short diffusion route compared with ordinary adsorbents, thereby resulting in high extraction capacity and efficiency. In addition, CNTs have great potential as superior adsorbents for separating many kinds of organic molecules and inorganic ions [21], [22]. The high adsorption ability of CNTs relative to other adsorbents is mainly due to the electrostatic interaction between oxygen functional groups and adsorbates, and partly Van der Waals force and π–π stacking interaction between aromatic rings of adsorbates and graphene framework [19], [23]. Therefore, functional groups, such as hydroxyl, carboxyl, and epoxy, on the surface of CNTs are important in the adsorption process. Correspondingly, one of the effective strategies to improve the adsorption ability of CNTs is specific surface modification or functionalization. Many studies have been performed to further increase the abundance of pre-existing functional groups (such as COOH and OH) or graft of other functional groups (such as NH2 and SH) onto the surface of carbon nanomaterials for enhancing their adsorptive capacities [24], [25], [26]. Various functional groups (e.g., hydroxyl, carboxyl, amine, and ligands) have been introduced on the surface of CNTs to enhance the selective and specific adsorption for adsorbates and extend their practical application [27], [28], [29]. Various compounds, such as polydopamine, chitosan, cyclodextrin, and poly (acrylic acid), have been used to modify CNTs and change functional groups, thereby altering the adsorption performance [30], [31], [32]. Thus, using polymers to modify CNTs is a feasible method to improve its adsorption capacity [33]. The PAMAM dendrimers, a kind of biopolymer, are among the most widely studied polymers in many fields, such as in engineering, materials science, chemistry, and biological sciences, because of their highly branched three-dimensional shapes, considerable end (reactive) terminal groups, space availability within their interiors, and uniform molecular weight structures [34], [35], [36], [37]. The amide units in long chains and abundant amino groups enable them to bind with other compounds and potentials in adsorption.These special properties make dendrimers promising and effective adsorbents.Introducing amide groups to adsorbents could enhance the adsorption selectivity toward flavonoids. Hence, PAMAM maybe be used as an excellent surface modifier for CNTs for the adsorption of flavonoids.
For ordinary adsorbents, separating saturated adsorbents from solution by filtration and/or centrifugation process after the adsorption process is time consuming and requires additional cost [38]. Thus, rapid separation of adsorbent from substantial aqueous solutions in large scale in real work is critical [39]. Nanoscale-based adsorbents exhibit high adsorption efficiency in comparison with ordinary absorbents, but their small size limits their separation performance from aquatic or organic phase. Notably, these disadvantages can be solved by magnetic separation technology [40]. The Fe3O4 magnetic nanoparticles have become a commonly used base material for preparing various adsorbents because they can be easily isolated using an external magnetic field placed outside of the extraction container [41], [42]. Combining magnetic properties and polymers onto the CNTs demonstrates several advantages, such as high adsorption capacity of functional CNTs and separation convenience of magnetic materials.
The PAMAM-modified magnetic CNTs was prepared to improve efficiency and obtain cost-effective adsorbents for separating flavonoids, considering the excellent adsorption performance of the CNTs and ease of separation of magnetic nanomaterials. When preparing PAMAM@MCNTs, multiwalled CNTs-COOH were firstly functionalized by PAMAM through a chemical crosslinking method. The functionalization of CNTs-COOH with PAMAM resulted in considerable amino and amidoamine functional groups on the CNTs-COOH surface, thereby providing active sites for further interaction with flavonoids. Afterward, Fe3O4 nanoparticles were anchored on the surface of PAMAM-modified CNTs-COOH (PAMAM@CNTs). Moreover, PAMAM@MCNTs were applied to rutin adsorption (a model molecule of flavonoids) from the solution. The adsorption process parameters were optimized, and the kinetics and isotherms were investigated in detail. Afterward, separation of the rutin loaded PAMAM@MCNTs from the solution occurred in an external magnetic field. The entire preparation procedure is presented in Scheme 1. The proposed PAMAM@MCNTs combined the excellent adsorption ability of CNTs and rapid separation efficiency of magnetic materials compared with ordinary adsorbent. This PAMAM@MCNTs can be used as a magnetic adsorbent to separate flavonoids from large volumes of plant extraction.
Section snippets
Reagents and materials
Carboxylated multiwalled carbon nanotubes (purity>95%, 8–15 nm in diameter, about 50 μm in length) were obtained from Nanjing XFNANO Materials Technology Corporation (Nanjing, China). Rutin, Quercetin and Apigenin were obtained from Sangon Biotech Co., Ltd (Shanghai, China). Polyamidoamine dendrimers of 2.0 G (PAMAM) were purchased from wheihai CY dendrimer technology Co., Ltd (Shandong, China). N-hydroxysuccinimide (NHS) and N-ethyl-N-(3-(dimethylaminopropyl) carbodiimide (EDC) were obtained
Optimization the preparation conditions of PAMAM@MCNTs
We first optimized the dosage ratio of PAMAM and CNTs-COOH because the coating amount of PAMAM will affect the adsorption properties of prepared materials, as shown in Fig. S1. As illustrated in the figure, when the ratio of PAMAM and CNTs-COOH was 0.5:1, the adsorption of PAMAM–CNTs to rutin was obscured, thereby indicating that the surface of 1 g CNT can modify 0.3 g PAMAM at most. Thus, we used 0.3 g PAMAM to modify 1 g CNTs-COOH.
The superior magnetism might be endowed to PAMAM–CNTs after Fe3
Conclusions
In the present work, the PAMAM-functionalized magnetic multi-walled CNT composites were synthesized, characterized, and applied for the separation of flavonoids from plant extraction fluid. The maximum adsorption capacity for rutin (a model molecule of flavonoids) was 56.48 mg/g at room temperature, and was superior to that of conventional adsorbents, such as silicone, polyamide, and adsorption resin. The adsorption can reach equilibrium within 25 min, thereby indicating a fast separation
Acknowledgements
This work was supported by the National Natural Science Foundation of China [No. 31700862] Natural Science Foundation of Shanxi Province of China [NO. 201601D021098], and Fund for Construction Program of Chemical Advantage and Key Discipline of Shanxi Province of China [NO. 912019]. Graduate innovation Fund of Shanxi Normal University [2017SCX042].College students innovation Fund of Shanxi Province [2018233].
Declaration of Competing Interest
The authors declare no competing financial interest.
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