Daylight-driven rechargeable antibacterial and antiviral nanofibrous membranes for bioprotective applications

Si, Yang; Zhang, Zheng; Wu, Wanrong; Fu, Qiuxia; Huang, Kang; Nitin, Nitin; Ding, Bin; Sun, Gang

We prepared the RNMs based on three criteria: (i) The membranes can effectively intercept pathogen particles via porous nanostructure, (ii) the membranes must become biocidal under daylight irradiation, and (iii) the membranes must be able to store the biocidal activity and readily release it under dark conditions. The first requirement was satisfied by using micro/nanotextured membranes with small pore sizes; ultrafine electrospun nanofibers were used as templates to achieve a breathable interception barrier. To satisfy the other two criteria (the formation of rechargeable biocidal functions), our molecular design is based on benzophenones and polyphenols, which are widely used as photosensitizers in biochemistry and organic synthesis (30) . The biocidal activity came from the generated ROS when they were exposed to light irradiation in the presence of oxygen (30, 31) . Figure 1A describes the synthesized structures. Two classes of benzophenone with different substitutes, 4-benzoylbenzoic acid (BA) and benzophenone tetracarboxylic dianhydride (BD), and a natural polyphenol, chlorogenic acid (CA), were selected as the photobiocides to bring about the biocidal functions. Poly(vinyl alcohol-co-ethylene) (PVA-co-PE) was used as a polymer precursor and hydrogen donor to construct the nanofibrous networks (32) . The fabrication began with a production of PVA-co-PE nanofibrous membranes in an average fiber diameter of 226 nm by electrospinning ( fig. S1 ). Subsequently, grafting reactions were conducted by immersing the membranes in tetrahydrofuran solution with various photosensitizers, and the esterification reactions between the hydroxyl groups on the membrane with carboxyl groups of the agents were catalyzed by carbonyldiimidazole (CDI) (see details in figs. S2 to S5). Thereafter, the resulting membranes were washed with acetone and dried in vacuum to remove any residual solvents. The nanofibrous membranes modified by BA, BD, and CA were abbreviated as BA-RNM, BD-RNM, and CA-RNM, respectively. In anticipation that the combination of benzophenone and polyphenol moieties might bring in a synergistic effect in photoactivity, the BD-RNM was further grafted with CA to obtain BDCA-RNM samples ( fig. S6 ). The representative field-emission scanning electron microscopy (FE-SEM) image of the relevant samples presented in Fig. 1B revealed randomly oriented threedimensional nonwoven morphology with fiber diameters in the range of 200 to 250 nm. Obvious adhesive was observed among nanofibers, which could be attributed to the superficial swell of nanofibers during the grafting reactions. Because of the simplicity of the modification process and the facile availability of electrospun nanofibers, great versatility in controlling the thickness and scaling up the synthesis is feasible. Figure 1C shows a typical BDCA-RNM with a thickness of 20 mm, and other samples with the thickness from 5 to 100 mm were readily prepared ( fig. S7) . A large-scale BDCA-RNM sample with a size of 40 × 40 cm 2 can also be prepared using a multi-needle spinning device ( fig. S8 ). Figure 1D demonstrates the biocidal effect of the RNMs against pathogenic microbes. Once the pathogens are intercepted and in contact with the surface of the nanofibers, the grafted photobiocides could display an intrinsic biocidal activity as free chemicals; that is, in the presence of oxygen, various ROS, including hydroxyl radicals (•OH), superoxide (•O 2− ), and hydrogen peroxide (H 2 O 2 ), are