**1. Introduction**

Pharmacologically active natural products have gained unprecedented popularity in recent decades [1]. They have made great contributions historically to drug development, and many of them have had profound effects on our lives. Dencichine (b-Noxalyl-L-a, b-diaminopropionic acid, b-ODAP), isolated from the roots of panax notoginseng, has a high medicinal value [2]. Dencichine has been reported to show beneficial effects against numerous diseases, such as dispersing stasis and hemostasis, improving platelet number, relieving swelling and pain, kidney diseases, neuroprotection, lowering blood glucose [2–5] and rheumatic diseases [6]. Natural ingredients in plants are characterized by low concentration [7], the existence of multi-component mixtures, structural diversity and similarity, which pose great challenges to the separation and purification of natural ingredients [8]. Therefore, it is essential to explore efficient extraction and purification processes for dencichine. According to several literature reviews, various methods have been developed to carry ou the separation, determination and enrichment of dencichine in panax notoginseng, such as colorimetry [9], high-performance liquid

**Citation:** Li, L.; Liu, X.; Li, L.; Wei, S.; Huang, Q. Preparation of Rosin-Based Composite Membranes and Study of Their Dencichine Adsorption Properties. *Polymers* **2022**, *14*, 2161. https://doi.org/10.3390/ polym14112161

Academic Editors: Wei Wu, Hao-Yang Mi, Chongxing Huang, Hui Zhao and Tao Liu

Received: 20 April 2022 Accepted: 11 May 2022 Published: 26 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

chromatography (HPLC) [10], gas chromatography–mass spectrometry (GC/MS) [11], liquid chromatographic–tandem mass spectrometric (LC/MS) [12], ultra-high-performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) [13], water-methanol method [14] and ultrasonic/microwave-assisted method [15]. The instrumental method employed a specific type of specialization and sophistication of the instruments. The separation method is not highly efficient and profitless to mass production. At the same time, the separated products need to be purified to remove harmful solvents such as acetonitrile. Water-methanol extraction is a traditional and easy-to-operate extraction approach, although inefficient. The ultrasonic/microwave-assisted method in the extraction process has a low separation effect on the structural analogs of dencichine. Molecularly imprinted technology is an easy and efficient method for the preparation of tailor-made polymeric materials with molecular recognition abilities [16]. However, the imprinting of dencichine is problematic and easy to cause template molecule waste. An efficient, low-cost material for dencichine extraction and purification is currently lacking.

In recent years, adsorption-based methods have been widely used in the separation and enrichment of bioactive compounds from many natural products. For example, RPM separation of total alkaloids from coptidis [17] and chitosan membrane purification of artemisinin [18]. The adsorption effect depends on the type and quality of the adsorption material. At present, many kinds of adsorption materials have been developed and deployed for the purification and separation of natural products. The commonly used adsorbents are activated carbon, iron oxide, silica gel, starch, adsorption resin, clay minerals and composite membranes. They use the porous matrices [19–21] as an adsorption platform and have good application prospects in the mass transfer and adsorption process. Among them, the composite membranes [22–25] have excellent thermal stability, high adsorption capacities and stable three-dimensional structure. In addition, they are used repeatedly, so they are widely used for isolating and the enrichment of active ingredients from natural products, such as half terpene [26], flavonoids [27], alkaloids [17] and other active compounds. Poly-ethersulfone (PES) [28], poly-sulfone (PSF) [29], poly-vinylidene fluoride (PVDF) [30,31], poly-vinylpyrrolidone (PVP) [32], poly-acrylonitrile (PAN) [33], poly-vinyl alcohol (PVA) [34] and natural product cellulose [35–38] are widely used as the polymeric membranes. With people paying more attention to the environment, porous chitosan, cellulose, rosin and other natural polymers appear in people's field of vision, and the use of natural adsorption materials is becoming more and more prevalent. The rosinbased crosslinking agent has excellent rigidity and contains three double bonds, non-toxic, non-carcinogenic, high abundance, low cost, environmental protection and other advantages. The polymer materials prepared from rosin have the advantages of degradation, high mechanical strength and excellent luster. Rosin-based polymers have been generally implemented not only in traditional fields such as coatings [39] and adhesives [40], but also in emerging fields such as energy [41], environment [42], drug delivery [43] and drug analysis. There are several methods for the preparation of composite membranes, including the casting flow method [44], freeze-drying approach [45] and electrostatic spinning et al. The composite membranes prepared by electrospinning have unique properties such as high specific surface area and uniform nanofiber structure. Electrospinning is currently applied in various applications including electrochemistry, natural product extraction, medicine, the environment and batteries.

In this work, RCMs were developed as excellent adsorbents for the preparation of dencichine for the first time. The preparation of RPMs was carried out using ethylene glycol maleic rosinate acrylate as a crosslinking by precipitation polymerization, and the RCMs were further prepared by electrospinning. Furthermore, we also study the feasibility of RCMs as effective sorptive materials for the dissociation and enrichment of dencichine. The RCMs were characterized by SEM, TGA and FT-IR. Then, the RCMs were evaluated for their sorbent performance of dencichine from notoginseng extracts. The adsorption kinetics of dencichine on RCMs was studied, and the adsorption mechanism was analyzed

in detail. Studies have shown that the membranes have excellent application prospects for the extraction of dencichine.
