“Cobweb locking raindrops” inspired construction of bio-based 3D porous molecularly imprinted membrane with ultrahigh adsorption capacity and selectivity: Effectively avoiding imprinting sites embedment

Abstract

Generally, 2D “thin-film” structural molecularly imprinted membranes (MIMs) prepared by the common phase inversion method suffer low adsorption capacity due to the easy embedment of imprinting sites. Herein, an alternative “delayed phase inversion” strategy was first developed to construct a natural loofah-based 3D porous MIM (LPMIM), in which the as-prepared molecularly imprinted polymers (MIPs) with artesunate (ARU) as dummy template spontaneously interacted with the polyvinylidene fluoride (PVDF) pre-treated loofah framework (LPM), and evenly anchored on loofah fiber surfaces, similar to the cobweb locking raindrops. Interestingly, the natural 3D fiber cross networks of loofah not only provided sufficient capacity and space to support MIPs without causing aggregation and maintains high flux, but also the inherent high mechanical strength of loofah fibers endowed the LPMIM with excellent stability. Under dynamic conditions the ART adsorption capacity of LPMIM could be further remarkably improved by tailoring the flow rate of 1.19 mL·cm⁻²·min⁻¹, up to 334.70 mg/g. As a result, effective enhancement in the artemisinin (ART) adsorption capacity were achieved for the LPMIM, which was about 2.25 times higher than that of the common blend MIM prepared by common phase inversion method (148.30 mg/g). Moreover, the LPMIM possessed high ART selectivity towards its analogue, artemether (ARE), giving an ART/ARE ratio of 2.7. Furthermore, the LPMIM displayed excellent recycling performance. By altering the template of MIPs, the corresponding LPMIM can be expanded to highly selective separation of other substrates along with large absorption capacity. This work highlights a universal strategy to construct novel MIMs with high absorption capacity and selectivity using bio-based fiber cross frameworks.

Introduction

Molecular imprinting technology (MIT) is a comparatively considerable method with unique tailor-made molecular recognition sites for target molecules, which have been applied in many fields, such as separation purification, drug delivery, catalysis and sensors [1], [2], [3], [4], [5]. Compared with powdered molecular imprinting polymers (MIPs) [6], [7], [8], molecularly imprinted membranes (MIMs) have more advantages in separation and purification, such as convenient operation and easy recovery [9], [10], [11]. However, the MIMs frequently confront with low selectivity and adsorption capacity due to the serious embedment of imprinted sites during preparations. For example, the number of effective imprinted sites on MIMs prepared by the surface imprinting method based on atom transfer radical polymerization (ATRP) technology are limited due to incomplete elution [[12], [13]]. Moreover, the recently popular2D thin-film structural blended MIMs prepared by the phase inversion of blending MIPs and casting solution will also suffer low effective MIPs loading and low flux due to the narrow membrane channels and the easy aggregation of MIPs inside the blended membrane [[14], [15]]. Although the recently reported magnetic induction blending method can avoid the problem of MIPs embedment inside the MIMs to a certain extent, serious aggregation of MIPs on the membrane surfaces still exists, causing less effective imprinted sites exposed, and finally affects the adsorption or separation efficiency [16]. Therefore, it is of great significance to develop a method to prepare MIMs with high MIPs loading and dispersion with high flux to achieve high selective separation efficiency.

In nature, it is often seen that raindrops are evenly and orderly covered on the silk of the spider web after the rain. Inspired by this, we hypothesized whether we could fix the MIPs on a membrane skeleton with a structure of spider web-like cross network, mimicking the “cobweb locking raindrops”, the problems of MIPs agglomeration and low membrane flux would be overcome by then. However, a single-layer network structure skeleton has insufficient space extremely limiting the fixation of sufficient MIPs. In this case, if the framework of a material possesses a three-dimensional (3D) cross-network structure, it may be an ideal membrane substrate that can provide sufficient space for MIPs loading, thereby the adsorption capacity could be effectively improved. It is interesting to find that loofah is a natural 3D fiber-based matrix with multilayer spider web-like structure, more importantly possesses extremely strong mechanical properties, which would be an alternative membrane matrix for constructing novel MIMs [17], [18], [19], [20]. However, it is noteworthy that there are few active functional groups on the internal fiber surfaces of loofah that can lock or fix the MIPs. So, a key question is how to make MIPs autonomously scattered on the fiber surface of loofah? Due to its excellent hydrophobicity, chemical and thermal stability, polyvinylidene fluoride (PVDF) has been widely applied as a basic material for the construction of many kinds of composited membranes [21], [22], [23]. Previous works have also shown that for the PVDF based MIMs a number of MIPs can be well and stably immobilized on the channel surfaces of PVDF membrane due to the hydrophobic interaction between PVDF and MIPs through phase inversion process [[24], [25]]. Based on this, it may be beneficial for enhancing the MIPs adhesion on the loofah framework by fabricating PVDF modified loofah surfaces.

Artemisinin (ART) is a very important antimalarial drug along with excellent anticancer activity [26], [27], [28], [29], which is derived from Artemisia annua L. However, the conventional separation methods of ART usually face the problems of large amount of solvent consumption, complicate steps, and low efficiency [30], [31], [32], [33]. To this end, inspired by the cobweb locking raindrops, we developed a novel “delayed phase inversion” strategy to trigger ART-based MIPs spontaneously anchoring on the surfaces of PVDF-modified loofah framework (LP) to construct an alternative 3D porous LP-based molecularly imprinted membrane (LPMIM) for highly efficient and selective separation of ART, as depicted in Fig. 1. It is noteworthy that the preparation of MIPs was based on the dummy template method with its analog artesunate (ARU) as the real template due to the ART molecule lacking active groups (i.e., -OH, -COOH, -NH₂) that can interact with the functional monomers, which possess relatively high ART selectivity with a separation factor of 2.7 towards artemether (ARE). Through the delayed phase inversion process, the MIPs can be spontaneously anchored and well dispersed on the internal surfaces of the PVDF modified loofah matrix to form the LPMIMs. The intrinsic 3D porous framework of loofah not only provide more space for locating more MIPs with good distribution but also endow the LPMIMs with high flux. Consequently, in a dynamic separation system the ART adsorption capacity of LPMIMs can be remarkably enhanced as compared to the static one (reaching 334.70 mg/g). More importantly, this LPMIMs can be expanded to highly efficient and selective separation of other substrates just by simply tailoring the corresponding template molecule of MIPs. This work highlights a facile and effective strategy to construct novel MIMs with extremely high adsorption capacity and separation efficiency.