key: cord-0860443-ab6n2zju
authors: Tian, Congcong; Wu, Fan; Jiao, Wenling; Liu, Xiaoyan; Yin, Xia; Si, Yang; Yu, Jianyong; Ding, Bin
title: Antibacterial and antiviral N-halamine nanofibrous membranes with nanonet structure for bioprotective applications
date: 2021-02-12
journal: nan
DOI: 10.1016/j.coco.2021.100668
sha: 316c3218911120c3c86672612c804cb1c1cd543b
doc_id: 860443
cord_uid: ab6n2zju

The recent outbreak of a coronavirus disease (COVID-19) has posed a great threat to public health and financial system. Most current masks used to prevent the spread of COVID-19 are typically absence of biocidal properties. We designed a novel polymer, polystyrene grafted by 5, 5-dimethylhydantoin and trimethylamine (PSDT), which possesses halamine site and cationic quaternary ammonia salt site. Furthermore, PSDT/PU nanofiber@net membranes (PSDT/PU NNMs) were obtained by electrospinning technology. Our strategy enables inherent N-halamine and quaternary ammonia salt (QAS) group to be covalently integrated into membranes, realizing the efficient and stable biocidal properties. Meanwhile, the introduction of nanonets endows electrospun membranes with prominent air filtration performance. The resulting membranes exhibit integrated properties of high interception of fine particles (96.7%) and low pressure drop (95.4 Pa). Besides, chlorinated PSDT/PU nanofiber@net membranes (with active chlorine content of 0.60 wt% and quaternary ammonia salt content of 2.20 wt%) exhibited superior bactericidal (>99.9999%) and virucidal (>99.999%) efficiency in a short time (2 min), which enables chlorinated PSDT/PU NNMs to be served as the filtration material by providing bacterial interception (99.77%) and contact killing against pathogens. The successful synthesis of PSDT/PU NNMs provide innovative insights for exploring filtration materials in a nanonet and biocidal form.

bond the N-halamine precursor polymer covalently [22] . However, the halamine precursor compound was incapable of bonding with the textiles sufficiently, thus peeling off the textiles easily under harsh condition and leading the sudden decrease of biocidal activity [23] [24] [25] . Therefore, the difficulty lies in developing homogeneous, covalently integrated and stable protective materials with intrinsically biocidal efficacy.

Here, we synthesized a novel polymer PSDT by nucleophilic substitution reaction with the synthetic system composed of chloromethylated Polystyrene (CMPS), 5, 5-dimethylhydantoin and trimethylamine. Subsequently, PSDT/PU NNMs were obtained by electrospinning technology. The material exhibited the following integrated properties: (i) superior biocidal efficacy originating from the synergistic effect of N-halamine, QAS and nanonets, (ii) high filtration efficiency and low resistance owing to the existence of Voronoi-like nanonets.

We designed PSDT/PU NNMs based on three criteria: (i) the N-halamine and QAS must be homogeneously and chemically integrated into polymers backbone, (ii) the electrospun membranes must possess characters of effectively biocidal efficacy, and (iii) the membrane must have the filtration performance of high efficiency and low resistance. The first two requirements were met by a readily available grafting reaction of polymer in organic solvents. To satisfy the last criteria, hybrid PSDT/PU NNMs were prepared by electrospinning technology. What is noteworthy is that the introduction of quaternary ammonium salt had the following advantages: (i) increasing the chlorine content from 790 ppm to 6030 ppm because enhanced hydrophilicity of nanofiber materials facilitated the grafting of N-halamine；(ii) changing conductivity of the polymer solution, thus enabling the polymer to be drawn into multi-level nanometer structure containing nanofiber with a diameter of 199 ± 55nm and nanonets with a diameter of 38 ± 7nm; (iii) prolong the service life of the materials as a supplementary and stable biocidal group. Plentiful previous researches showed that the antimicrobial material containing N-halamine or quaternary ammonia J o u r n a l P r e -p r o o f salt group showed no cytotoxicity [26] [27] [28] .

The preparation and biocidal procedure of the PSDT/PU NNMs were presented in Figure. 1. Polystyrene (PS) was chloromethylated firstly using 1, 4-bis(chloromethoxy) butane. Subsequently a biocidal precursor, PSDT, was then synthesized by nucleophilic substitution reaction with the synthetic system composed of CMPS, 5, 5-dimethylhydantoin (DMH) and trimethylamine (TMA). PSDT/PU NNMs were obtained using electrospinning technology. In the process, tiny charged droplets were sprayed and rapidly evolved into two-dimensional due to the phase separation. Meanwhile, the jets were drawn into typical nanofibers as a scaffold [29] [30] [31] . The obtained membranes with the PSDT/PU mass ratio of x：y were denoted as PSDT/PU NNMs-x:y. For comparison, PS/PU nanofiber membranes (PS/PU NMs) were prepared by a common electrospinning method.

It is widely reported that the N-halamine precursor compounds can possess highly effective biocidal and recyclable activity by halogenating [32, 33] . The post-halogenation method is easier than the pre-halogenation method in electrospinning and the post-processing process, such as collection and storage [34] .

As demonstrated in Figure. l, N-H groups of PSDT could turn into biocidal N-Cl moieties by chlorinating, thus constructing chlorine-recyclable and quaternary ammonia PSDT/PU NNMs (PSDT/PU NNMs-Cl). Once pathogens were intercepted and in contact with the surface of the PSDT/PU NNMs-Cl, they were captured tightly by positively charged nitrogen of membranes due to the existence of QAS [35] .

Subsequently, oxidized chlorine originated from the N-Cl moieties was transferred to appropriate receptors of pathogens to oxidize vital constituents for microorganisms' survival, like proteins or enzymes containing sulfhydryl group. Meanwhile, the N-Cl reverted to the precursor N-H. It was worth noting that N-halamine played a dominant and crucial role in killing pathogen because of the weakness of QAS biocidal activity. Afterward, the biocidal N-Cl groups can be renewed by soaking or rising the materials with sodium hypochlorite solution [36, 37] . [38] . In 1H NMR spectra of PSDT, the signal observed at 4.47 ppm and 2.95 ppm was ascribed to protons in the chloromethyl group and three methyl groups of quaternary ammonia salt respectively [39] . From peak areas at 4.47 ppm and 2.95 ppm of 1H NMR spectra of PSDT and the chlorine content of CMPS, the content of quaternary ammonia salt group was obtained as 2.20%.

The FT-IR spectra of samples were shown in Figure. 2b. Compared with that of PS, C-Cl bending vibration of chloromethyl group at 1420 cm -1 was detected in the FT-IR spectra of CMPS. The peak centered at 1263 cm −1 was ascribed to the bending vibration of the C-H groups in the 1, 4-disubstituted benzene ring, which was strengthened after the 4th position on the benzene ring was substituted by chloromethyl group [40] . An infrared spectrum of PSDT showed distinct bands at 1715 and 1769 cm -1 , demonstrating the existence of the hydantoin moieties. The infrared spectrum of PSDT-Cl showed two bands of 1728 and 1790 cm -1 derived from monochlorinated hydantoin moieties [12] .

The elemental compositions of CMPS, PSDT and PSDT-Cl were identified by XPS in Figure. 2c. To fully elucidate the chemical environment of N and Cl in the sample, N 1s and Cl 2p core-level spectra were shown in Figure. The chlorine content of PSDT-Cl directly reflected the grafting amount of N-halamine and pathogens killing performance [44] . Therefore, chlorine content of PSDT-Cl prepared by different feed ratio of DMH and TMA was investigated. Figure. 2f showed that PSDT-Cl possessed the highest chlorine content of 2195 ppm when the feed ratio of DMH and TMA was 5:5. With increased feed mass of TMA, enhanced chlorine content of PSDT-Cl might be attributed to the hydrophilicity of QAS, which made the chlorination reaction more sufficient [43] . However, when the feed ratio of 

Nanofibers and nanostructured networks have demonstrated great promise in constructing superior filter property due to their desirable attribute with reduced fiber diameters [45] [46] [47] [48] . The effect of PU on the morphology of the Voronoi-like nanonets in the PSDT/PU NNMs was shown in Figure. 3a-c. With the mass ratio of PSDT and PU decreasing from 5:1 to 4:2, the material exhibited that the coverage rate of Voronoi-nets increased from 15% to 57%. The nanonets were more evenly distributed in the nanofiber scaffold, which is attributed to that PU component could decrease charge dissipation of charged fluid, thus causing more droplets to be ejected. However, with further mass ratio decreasing, nanonets tended to disappear and turned into the coverage rate of 0.3% on account that the reduced charge in Taylor cone liquid made charged liquid incapable of exceeding the droplet threshold to spay droplets [49] . The pore size and distribution of the PSDT/PU NFN were analyzed using a capillary flow porometer (CFP) in Figure. The filtration performances and mechanical properties of nanomaterials are critical in practical application [50, 51] . Figure 3e showed that PSDT/PU NNMs-4:2 exhibited the highest removal efficiency of 96.7%, the pressure drop of 95.4 Pa and favorable quality factor of 0.0359 Pa -1 , approximately three times larger than those of the commercial air filter materials [52] . Figure. As shown in Figure. 4b, PSDT/PU NNMs-Cl showed 5 log PFU of E. coliphage killing within 2 min. The biocidal activity was dramatically better than previously reported virus-killing materials based on active chlorine [50] .

The antibacterial performance of the materials with a mass of 10 mg was tested against E. coli and S. aureus. Figure. 

In summary, we present unique strategy for the construction of intrinsically and SnCl 4 (5 mL). When the drip was over, the mixture was stirred for 17 h at 18 ℃.

Then the mixture treated by HCl (1mol L -1 , 240 mL) was washed and filtered repeatedly with EtOH and Deionized water. Subsequently, the as-obtained CMPS (3 g), DMH, Na 2 CO 3 and TMA were dissolved into DMAC (180mL) and stirred for 6 h at 60 ℃. Then PSDT powder was obtained by washing and filtering with NaCl solution (16.7wt%) and Deionized water. PSDT-Cl powder was obtained when the mixture was dripped into NaClO solution (2.5wt%), stirred for 1h and washed with deionized water.

The electrospinning solution was obtained by dissolving PSDT, PU and TTMA in DMAC. The concentrations of PSDT and TTMA were optimized as 12.5wt% and 10wt%, respectively. The as-prepared solutions were powered by direct voltage of 40kV, pumped out at the feed rate of 1 mL h −1 and collected on a metallic cylinder at the environment of 23 ± 2 ℃ and 40 ± 5%RH. Afterwards, PSDT/PU NNMs were J o u r n a l P r e -p r o o f dried at 60 ℃ for 30 min with the presence of acetic acid for cross-linking.

The coverage rate of the Voronoi-like nanonets was obtained using Adobe Photoshop CS6. The pore size and distribution of materials were investigated using CFP (CFP-1100AI, Porous Materials Inc., American). Filtration performance of PSDT/PU NNMs were evaluated by using TSI 8130 automatic filter tester (TSI Inc.).

The PSDT/PU NNMs were sufficiently chlorinated by NaClO solution(2.5wt%) for 1h and completely quenched with superfluous thiosulfate. After a certain number of cycles, active chlorine content was measured by iodometric titration method. 

The chemical structure of the product was characterized by NMR (AVANCE400, Bruker, Germany), FTIR (Thermo Scientific Nicolet is10, USA) and XPS (Escalab 250Xi, United States). The mechanical performance of the membranes was evaluated using a tensile tester (XQ-1C, China). The bacterial filtration efficiency was evaluated by Bacteria filtration efficiency tester (G299, Qinsun Instruments Co., China).

The authors declare no competing financial interest.

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This work was supported by the National Natural Science Foundation of China (Nos. 51925302, 51903036 and 52041301), Fok Ying Tung Education Foundation (No.