Novel N-Br Bond-Containing N-Halamine Nanofibers with

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Novel N-Br Bond-Containing N-Halamine Nanofibers with Antibacterial Activities Rong Bai, Jing Kang, Oudjaniyobi Jacob Simalou, Wenxin Liu, Hui Ren, Tianyi Gao, Yangyang Gao, Wanjun Chen, Alideertu Dong, and Ran Jia ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.7b00996 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Biomaterials Science & Engineering

Novel N-Br Bond-Containing N-Halamine Nanofibers with Antibacterial Activities Rong Bai,†,§ Jing Kang,†,§ Oudjaniyobi Simalou,‡ Wenxin Liu,† Hui Ren,† Tianyi Gao,† Yangyang Gao,† Wanjun Chen,† Alideertu Dong*,† and Ran Jia*,║

†

College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, People’s

Republic of China ‡

Departement de Chimie, Faculte Des Sciences (FDS), Universite de Lome (UL), BP 1515 Lome, Togo

║

Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin

University, Changchun 130023, People’s Republic of China

ACS Paragon Plus Environment

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ABSTRACT: N-Halamine compounds have attracted great attention because they are recognized as promising antibacterial agents to control microbial contamination, however most of the research interests were focused on N-halamines that contain N-Cl bond(s) rather than N-Br bond(s). In this contribution, we report the facile fabrication of N-Br bond-containing N-halamine nanofibers using the electrospinning method for antibacterial usages. The as-produced N-Br bond-containing N-halamine nanofibers (i.e., DBDMH/PAN nanofibers) comprise an antibacterial component of 1,3-dibromo-5,5dimethylhydantoin (DBDMH) and a support component of polyacrylonitrile (PAN). When systematic characterizations were carried out, the as-obtained DBDMH/PAN nanofibers were proven to exhibit well-defined fiber-like morphology and be highly efficient in the killing of the selected model bacteria (Escherichia coli). Their morphology and size could be well governed by tuning the concentration of electrospinning precursor and the mass ratio of PAN to DBDMH. The antibacterial mechanism of the DBDMH/PAN nanofibers and their stabilities under dry, wet, and bacterial conditions were confirmed as well. Facile synthesis and antibacterial activity allow the feasibility of the final N-Br bond-containing N-halamine nanofibers for antibacterial-related clinical applications in practice. Our work highlights the development of the N-Br bond-containing N-halamine nanofibers as promising antibacterial agents for biomedical applications. KEYWORDS: N-Br bond-containing N-halamine, polyacrylonitrile, nanofiber, antibacterial, electrospinning


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INTRODUCTION Undoubtedly bacteria are responsible for many lethal diseases, which has killed more people than any other disease.1 Increasing bacterial resistance to traditional antibacterial agents needs the exploration of new alternatives with different mechanisms of antibacterial action.2-4 Accordingly, new antibacterial agents, such as metallic silver, quaternary ammonium salt, guanidine, peptide, chitosan, Nhalamine, etc., have received great attention in more recent times.5-10 Especially, N-halamines that contain N-X (where X represents Cl, Br, or I) covalent bond(s) in their structure have become quite attractive because they can work as antimicrobial agents to prevent human bodies from bacterial infection. Notely, N-halamines have been recognized as promising antibacterial compounds for their several advantages, such as antibacterial efficacy against most microorganisms, long-term stability both in aqueous solution and in dry storage, lack of corrosion on materials’ surfaces, almost no toxicity to human body, relatively low cost, etc.11 Owing to their advantages over other antibacterial agents, the application of N-halamines ranges across the area of water disinfection, air purification, food packaging and storage, textile products, dyes and paints, medical and healthcare products, silica-based materials, and others.12-18 After Berliner revealed the success of N-halamines on water disinfection as early as in 1931,19 the development of antibacterial N-halamines has led to great advances.20-22 It was proven that N-halamines could generate via a simple halogenation of the N-H bond-containing precursor (such as hydantoin, imidazolidinone, oxazolidinone, barbituric acid, chitosan, etc.), which allow N-halamines to exhibit diversity in their molecular structure.23-28 When the N-X bond is investigated, the halogen(s) in Nhalamines can be chlorine, bromine, or iodine, with chlorine being the most popular.29-36 Throughout the historical development of antibacterial N-halamines, most of the studies that related to antibacterial Nhalamines concentrated mainly on N-Cl bond-containing N-halamines rather than N-Br (or N-I) bondcontaining ones. Despite several reports pointed out the antibacterial effectiveness of N-Br bondcontaining N-halamines, their antibacterial action against bacteria is not quite clear until now.37-39


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Accordingly, it is urgently need to uncover the mystic veil of the N-Cl bond-containing N-halamines. Nowadays, nanomaterials have developed rapidly in past due to their specific properties such as smaller size, larger surface areas, and higher responsiveness.40-44 Accordingly, thanks to their excellent antibacterial properties, some nanomaterials have sprung up as innovative antibacterial agents. Up to now, many sorts of nanostructures, such as silver nanoparticles, zinc oxide nanoflowers, graphene oxide nanosheet, quaternary ammonium salt nanomembrane, chitosan nanofibers, N-halamine nanospheres, have been reported to display antibacterial functions against pathogenic bacteria.45-54 A great number of techniques, such as sol-gel method, hydrothermal synthesis, self assembly, and electrospinning, have been explored to produce antibacterial nanostructures, with electrospinning being the most popular.55 Electrospinning technology has gained a great achievement because it is an economical and relatively simple method to fabricate continuous and uniform nanofibers from almost any synthetic and many natural polymers.55 Also, electrospinning technique appears to be the most promising approach that can be scaled up for industrial productions. Certain recent, we have designed and synthesized a series of antibacterial N-halamine nanomaterials that contain N-Cl covalent bond(s).56-61 These N-halamine nanomaterials synthesized mainly via the chlorination of N-H bond-containing compounds, such as hydantoin,61 barbituric acid,5658

and 4-piperidinol,59,60 were proven in our studies to be effective to fight against pathogens. The N-Cl

bond-containing N-halamines were widely acceptable because of their advantages, such as long-term physicochemical stability, high structural durability, and the regenerability of their functional groups.4661

And the success of the N-Cl bond-containing N-halamines arouse our interest in a systematic study of

alternative N-halamines that have N-Br covalent bond(s) in their structure. Bearing in mind exploration of novel N-halamines, we extended our investigation onto N-Br bond-containing N-halamine nanomaterials. In this study, we facilely fabricated N-Br bond-containing N-halamine nanofibers, i.e., DBDMH/PAN nanofibers, using electrospinning technique with 1,3-dibromo-5,5-dimethylhydantoin


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(DBDMH) as a model N-Br bond-containing N-halamine resource and polyacrylonitrile (PAN) as a polymeric support material (as seen in Figure 1). The morphologies, sizes, and chemical structures of the as-prepared DBDMH/PAN nanofibers were determined using a series of measurements. Then antibacterial performance of the as-produced DBDMH/PAN nanofibers was evaluated systematically using plate counting method and inhibition zone test by selecting Escherichia coli (E. coli) as the typical microorganism. We are sure that our study should be a good beginning and an enlightenment in the future development of N-Br bond-containing antibacterial N-halamines. EXPERIMENTAL SECTION Materials. 1,3-Dibromo-5,5-dimethylhydantoin (DBDMH) were purchased from Aladdin Industrial Inc. Polyacrylonitrile (PAN) and N,N-dimethylformamide (DMF) were bought from Tianjin Chemical Reagent Plant. Deionized water was utilized for all of the experiments. All chemical reagents were used directly as received without further purification. Materials Synthesis. DBDMH/PAN nanofibers were prepared facilely using an electrospinning method according to our previous report.61 Typically, after 0.053 g DBDMH completely dissolved in 6 mL of DMF, about 0.80 g PAN was added into the above mixture to get a ternary mixture with a final concentration of about 13 wt %, and then stirred overnight to obtain an achromatous, transparent, and viscous electrospinning precursor solution. Subsequently, the electrospinning procedure ran on an electrospinning unit with a high voltage of 12 kV at room temperature to obtain DBDMH/PAN nanofibers. As for controllable synthesis, the concentration of electrospinning precursor solution was tuned from 11, to 12, to 13, to 14, then to 15 wt %, during which the mass ratio of PAN to DBDMH was fixed at 15 : 1. In addition, in order to confirm the impact of the feeding ratio, the mass ratio of PAN to DBDMH was changed as 15 : 1, 3 : 1, 3 : 2, and 1 : 1, with a fixed concentration of 13 wt %. Characterization. Scanning electron microscopy (SEM) images were examined on a SEM Shimadzu SSX 550 to clarify the morphologies, sizes, and surface states of the samples. By the assistance of KBr pellet method, FTIR spectra were recorded by using a Thermo Nicolet (Woburn, MA) Avatar 370 FTIR


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spectrometer.

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C NMR spectra were measured on a Bruker AVANCE III-500 NMR spectrometer in

DMSO solution. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI5000CESCA system with Mg Kα radiation. The active bromine content was quantified using the idometric/thiosulfate titration method.13 Antibacterial test. The antibacterial tests were performed by selecting Escherichia coli (E. coli, ATCC 8099, Gram-negative bacteria) and Staphylococcus aureus (S. aureus, ATCC 25923, Grampositive bacteria) as model bacterial strain using both the inhibition zone test and plate counting method. Prior to the antibacterial tests, E. coli and S. aureus were grown in a Luria-Bertani (LB) broth medium at 37 oC for 24 h. The inhibition zone test was carried out according to a previous report.62 In detail, the sample was added into a circular mold with a diameter of 1.0 cm. Under pressure at room temperature using a tablet machine, the sample discs were obtained and then put it onto the surface of LB agar plate that preoverlaid with 500 µL of 108 colony forming units per mL (CFU/mL) of E. coli. After incubation at 37 o

C for 24 h, the inhibition zone was measured. The inhibition zone tests were performed in triplicate. Also, the plate counting method was performed in triplicate based on a previous report.63 Typically,

the bacterial suspensions employed for the test contained 105 CFU/mL. Typically, the sample was cut into square pieces of four different sizes of 0.5×0.5 cm, 1.0×1.0 cm, 2.0×2.0 cm and 4.0×4.0 cm, respectively, and then added separately into a 50 µL of bacteria suspension, and placed in a rotary shaker with a shaking speed of 200 rpm. After shaking for 60 min, the mixture was serially diluted, and 100 µL of each dilution was dispersed onto Luria-Bertani (LB) growth medium. Survival colonies on LB plates were counted after incubation for 24-36 h at 37 °C. The reduction of bacteria was calculated using the following equation: Bacterial reduction (%) = (B-A)/B × 100 % where A is the number of surviving bacterial colonies of the test sample and B is that of the control. RESULTS AND DISCUSSION


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The overall synthetic procedure of DBDMH/PAN nanofibers using a simple electrospinning route is illustrated in Figure 1A. Prior to the electrospinning procedure, DBDMH molecules were dispersed in the intermolecular vacancies of PAN polymer chains with the help of vigorous stirring after mixing DBDMH with PAN. When electrospinning ran onto the mixture, DBDMH/PAN nanofibers formed. Thanks to the presence of PAN polymer matrix, the DBDMH molecules can be stabilized tightly within this complex system. As the active component in the as-synthesized nanofibers, the DBDMH can effectively stabilize and store bromine atoms in N-Br covalent bond(s), in which bromine shows powerful antibacterial activity against bacteria (seen in Figure 1B) owing to its strong oxidative state (+1). Morphological features of the as-produced DBDMH/PAN nanofibers are presented in Figure 2. It is clear that DBDMH/PAN (Figure 2A-C) exhibits randomly oriented, straight, and continuous fiberlike morphology, as well as smooth surfaces. Besides, quite uniform appearances are visible, suggesting that DBDMH molecules well scattered into PAN polymeric matrix without any aggregations. Based on the SEM images, we can conclude that PAN acts as framework materials to fasten fiber morphology, as well as helps DBDMH molecules scatter uniformly into the PAN polymer matrix. In addition, as can be seen in Figure 2D, their sizes show a quite narrow distribution with a average fiber diameter of 220 ± 60 nm, which well proves our assumption of the practicability of controllable synthesis of DBDMH/PAN nanofibers using a simple electrospinning method. In order to determine chemical compositional information, EDX pattern of the as-produced DBDMH/PAN nanofibers were tested. Figure 2E shows five characteristic peaks of Si, C, N, O, and Br element, among which Si peak is assigned to the silicon wafer used to immobilize sample on it. Other four peaks, i.e., C, N, O, and Br, are attributed to either PAN or to DBDMH, demonstrating the successful combination of PAN and DBDMH. In addition to their morphology, size, and element features, chemical structure of the as-prepared nanofibers was captured as well using FTIR spectroscopic analysis. Figure 3 illustrates the FTIR spectrum of DBDMH, PAN, and DBDMH/PAN. Obviously, the characteristic peak assigned to the


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C=O bond appears at around 1732 cm-1 for DBDMH, and PAN spectrum provides the characteristic peak of C≡N bond at around 2245 cm-1.64,65 And the as-synthesized DBDMH/PAN nanofibers provide both two characteristic peaks (the C=O bond and C≡N bond), which acts as markers to confirm the successful combination of DBDMH and PAN using the electrospinning. More significantly, the presence of both the C=O bond and C≡N bond in the DBDMH/PAN spectrum also implies that the electrospinning treatment has almost no destructive effect on the chemical groups of both DBDMH and PAN. The

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C NMR spectrum was also utilized to further confirm the successful preparation of

DBDMH/PAN nanofibers. Figure 4 shows the

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C-NMR spectrum of DBDMH, PAN, and

DBDMH/PAN. Used as characterization solvent, the corresponding carbon peak of DMF centers at ~39 ppm, which is seen for all three samples. As can be seen, DBDMH presents two C=O type carbon signal from position C2 and C4 at ~178 and ~156 ppm, respectively. The carbon at position C5 shows a peak at ~62 ppm, and two substituted -CH3 groups at position C5 give two separated carbon peaks at ~17 and ~23 ppm, respectively. As for PAN, it shows the characteristic carbon peak of C≡N group at ~121 ppm, with two carbon peaks from polymer chains at ~28 and ~33 ppm. When an electrospinning carried out, although some chemical shifts appear in its 13C-NMR spectrum, the as-prepared DBDMH/PAN shows all characteristic carbon peaks that attributed either to DBDMH or to PAN, illustrating the success of the combination between DBDMH and PAN. XPS analysis that is known as an effective tool to characterize the presence of chemical elements in nano-level,66 was also used to further prove the chemical compositions of DBDMH/PAN nanofibers (as shown in Figure 5). For comparison, the XPS spectrum of pristine PAN is given as well. The characterization was performed by immobilizing sample on glass support, thus two elemental signals Si 2s and Si 2p from glass support are clearly seen for both PAN and DBDMH/PAN nanofibers.62 As for pristine PAN, the C 1s and N 1s signals are observed at about 286 and 400 eV, respectively, and the O 1s peak appeared at 533 eV is possibly from the glass support.62 Different from XPS spectrum of


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pristine PAN, the DBDMH/PAN nanofibers supply an additional peak of elemental signal of Br 3d (the magnified peak is visible in Figure 5B) at about 69 eV, which well verifies the presence of DBDMH molecules among the PAN matrix.67 When magnification was carried out both on C 1s and N 1s peaks, more detailed information about chemical bonds can be detected (as seen in Figure 5C and 5D). Clearly, C-C bond and C≡N bond are obtained on pristine PAN, while DBDMH/PAN nanofibers show three additional bonds (including C=O, C-N, and N-Br bond), acting as effective evidences to the existence of DBDMH molecules. Generally, the XPS spectra well agreed with the FTIR and 13C NMR results. After we well affirmed the feasibility of the facile synthesis of DBDMH/PAN nanofibers using a simple electrospinning method, our attention turned onto the controllability of their synthesis. A comparison experiment was designed, with the concentration of the electrospinning precursor solution varied from 11 wt % to 15 wt %. During this set, the mass ratio of PAN to DBDMH was maintained at 15 : 1. The corresponding results were recorded by SEM images as shown in Figure 6. As expected, no marked change is found in their morphologies with the increase of concentration from 11 wt %, to 12 wt %, to 13 wt %, to 14 wt %, then to 15 wt %, showing that all five products have straight fiber-like appearance and quite smooth surfaces. Nonetheless their sizes evolve when the concentration changes. Apparently, the average fiber diameter of the as-produced nanofibers that synthesized at 11 wt %, 12 wt %, 13 wt %, 14 wt %, and 15 wt %, is 149 ± 43, 213 ± 53, 320 ± 60, 271 ± 67, and 362 ± 64 nm, respectively. Therefore, it can be deduced that the morphology of DBDMH/PAN nanofibers could be well controlled on fiber-like appearance, with varied sizes just by changing the concentration of the electrospinning precursor solution. In addition, we tried to clarify the impact of mass ratios of PAN to DBDMH on the morphology of the DBDMH/PAN nanofibers. Our experiment involved DBDMH/PAN that prepared with different mass ratios of PAN to DBDMH ranged from 15 : 1, to 3 : 1, to 3 : 2, then to 1 : 1, while the concentration of the electrospinning precursor solution was fixed at 13 wt %. Figure 7 displays the SEM images of DBDMH/PAN prepared with different mass ratios of PAN to DBDMH. When the mass ratios


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of PAN to DBDMH are 15 : 1 (Figure 6C) and 3 : 1 (Figure 7A-1 and 7A-2), both two DBDMH/PANs show uniform fiber-like morphology and quite smooth surface. In Figure 7B-1 and 7B-2, although the fiber surfaces remain smooth, some adhesions between fibers (yellow regions) are detectable as the mass ratio of PAN to DBDMH decrease to 3 : 2. With the decrease of the mass ratio of PAN to DBDMH to as low as 1 : 1, the adhesion get more serious (yellow regions in Figure 7C-1). In addition, a widespread barbotage (red arrows) is observed in Figure 7C-1, and even a clear fiber fracture (green region) appears in Figure 7C-2. We speculated that the appearance of barbotage and fracture might be attributed to the high loading of DBDMH onto PAN fibers. Accordingly, we drew a conclusion that the mass ratio of PAN to DBDMH plays an important role in governing the morphology of DBDMH/PAN nanofibers. To assay the antibacterial capability of DBDMH/PAN nanofibers, the plate counting method was carried out as well using E. coli as representative bacteria (a bacterial concentration of 105 CFU/mL), with the pristine PAN as a comparative control. As seen in Figure 8A, the cultural plates pictured after antibacterial test show the survival case of E. coli in the absence and presence of the samples. Seen as small white dots in the control test (yellow plate in Figure 8A), E. coli in the absence of the samples shows robust growth. As similar as the control, E. coli shows good survival after an exposure to the pristine PAN (red plate in Figure 8A), confirming that PAN polymer has almost no obvious killing capability to E. coli. In contrast, an aseptic plate is detected in the presence of DBDMH/PAN nanofibers (green plate in Figure 8A), which is an effective evidence for the antibacterial function of DBDMH/PAN nanofibers against E. coli. Most significantly, the result of this comparison tells us that the antibacterial property of DBDMH/PAN nanofibers was provided by DBDMH molecules rather than PAN matrix. In order to clarify the relationship between sample dosage and antibacterial activity, the DBDMH/PAN nanofiberous membrane that obtained after electrospinning was cut into a square shape with a dimension (Length × Width) of 0.5×0.5 cm, 1.0×1.0 cm, 2.0×2.0 cm, and 4.0×4.0 cm,


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respectively, and then their antibacterial activities against E. coli and S. aureus (a bacterial concentration of 105 CFU/mL) were tested with a fixing antibacterial time of 60 min. The corresponding results are shown Figure 8B. As expected, the bacterial survival reduces gradually with the sample’s dimension changed from 0.5×0.5 cm to 4.0×4.0 cm because the higher the dosage of the sample is, the more active N-Br groups they will provide.68 After 60 min, the 0.5×0.5 cm sample kills only 14.3 % E. coli and 11.6 % S. aureus, while complete killings of both two strains are seen for 4.0×4.0 cm sample. Therefore, it can be concluded that the dosage of the DBDMH/PAN nanofibers has a significant impact on their antibacterial function. Additionally, to clarify the relationship between the mass ratio and antibacterial capability, antibacterial assay was performed using DBDMH/PAN nanofibers with different mass ratios of PAN to DBDMH (from 15 : 1, to 3 : 1, to 3 : 2, then to 1 : 1) by choosing E. coli as model bacteria (a bacterial concentration of 105 CFU/mL). As obviously seen from Figure 9A, the reduction of E. coli decreases generally with the mass ration changed from 15 : 1 to 1 : 1. There is no obvious E. coli reduction detected when the mass ratio is 15 : 1, whereas as high as ~97.9 % E. coli reduction was observed after treatment with a mass ratios of 1 : 1. This comparison is an effective evidence for the impact of the mass ratio of PAN to DBDMH on antibacterial activity. Consequently, it can be demonstrated that the DBDMH/PAN nanofibers kill bacteria in a mass ratio-dependent manner. After the confirmation of the antibacterial activity of DBDMH/PAN nanofibers, antibacterial mechanism was investigated using inhibition zone test, with E. coli (a bacterial concentration of 108 CFU/mL) as model strain. In this test, two DBDMH/PAN nanofibers, with mass ratio of PAN to DBDMH of 15 : 1 and 1 : 1 respectively, were examined. As shown in Figure 9B, DBDMH/PAN nanofiber with a mass ratio of PAN to DBDMH of 15 : 1 shows a crowded bacterial colonies without any aseptic area, implying no antibacterial activity of DBDMH/PAN nanofiber in its present mass ratio. When the mass ratio of PAN to DBDMH reaches to 1 : 1 (seen as Figure 9C), an aseptic ring with a diameter of ~16 mm appears around DBDMH/PAN nanofibers, which could be induced by the release ACS Paragon Plus Environment

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of active bromine by the dissociation of the N-Br bond of DBDMH molecules. The result of inhibition zone test well proves that DBDMH/PAN nanofibers can not only store and stabilize the bromine into NBr bonds but also release the active Br+ to display antibacterial function when they encounter with bacteria. It was widely accepted that the appearance of inhibition zone not only reflects the susceptibility of bacteria toward antibiotics but also to some extent evidences the release mechanism of antibacterial components in bacterial environment.60 To further confirm the release mechanism mentioned above, the release of active bromine from asprepared DBDMH/PAN nanofibers under different conditions

were quantified using the

idometric/thiosulfate titration method. As seen from Figure 10A, the as-synthesized DBDMH/PAN nanofibers have almost no serious reduction in the content of active bromine under a dry condition and maintain above 80 % active bromine content even after three months storage, which is an effective evidence for high storage stability of the DBDMH/PAN nanofibers in dry. In contrast, the DBDMH/PAN nanofibers show lower stability in water system. When they were exposed into water, the content of the active bromine could decrease from 100 % to 8 % with the aging time ranged from 0 to 2 h. The release of active bromine induced by the dissociation of N-Br bond is the most reasonable explanation for the obvious reduction in active bromine content of the DBDMH/PAN nanofibers in water. To understand the release mechanism in a real case, the active bromine content of the DBDMH/PAN nanofibers under the bacteria condition was examined as well. Similar as in water, the active bromine content of the DBDMH/PAN nanofibers give obvious reduction, only 6 % active bromine left after 2 h exposure to bacteria. When a comparison between water condition and bacteria condition is done, the DBDMH/PAN nanofibers are be liable to release Br+ ions when they encounter bacteria rather than water. To estimate the long-term antibacterial activity, the as-prepared DBDMH/PAN nanofibers were stored under a dry condition for three months, and then their antibacterial activity was estimated by the antibacterial kinetic test using E. coli as a model. Figure 10B illustrates the bacterial reduction of E. coli


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upon the exposure to DBDMH/PAN nanofibrous membrane with different dimensions (Length × Width) of 0.5×0.5 cm, 1.0×1.0 cm, 2.0×2.0 cm, and 4.0×4.0 cm, respectively. As expected, remarkable population decreases of the bacterial colonies are visible as treating with DBDMH/PAN nanofibrous membrane, especially for 2.0 × 2.0 cm and 4.0 × 4.0 cm samples. So, we are quite sure that DBDMH/PAN nanofibers could maintain their high antibacterial activity even after three month storages in an open system. In a closer view, different bacterial reduction indications emerge for DBDMH/PAN nanofibrous membrane with different dimensions, showing a change from bacteriostatic to bactericidal action with the increasing dimension. The bactericidal activity of samples is fast-acting for both 2.0×2.0 cm and 4.0×4.0 cm dimensions, and their bactericidal endpoint is 30 min and 15 min, respectively, but bacterial colonies treated with 1.0×1.0 cm sample still remain 74 % survival even after 120 min contact. Unlike three samples above (including 1.0×1.0 cm, 2.0×2.0 cm, and 4.0×4.0 cm sample), the 0.5×0.5 cm sample shows a robust growth of bacteria even after 120 min exposure, suggesting its non-effectiveness to fight against bacteria. Therefore, it is quite evident that the sample dosage is a decisive factor to influence the antibacterial action of DBDMH/PAN nanofibers, indicating that a high N-halamine loading in electrospun nanofibers is necessary for their long-term antibacterial activity. CONCLUSIONS In summary, we report an effective and simple approach for the synthesis of a N-Br bond-containing Nhalamine nanofibers with PAN as support material and DBDMH as N-Br bond-containing N-halamine resource using electrospinning method. By the assistance of several different techniques, such as SEM, FTIR, NMR, and XPS, the as-synthesized DBDMH/PAN nanofibers were well characterized, focusing on the identification to their morphologies, sizes, and chemical structures. Additionally, employing the plate counting method, the antibacterial assay was performed through a contact between DBDMH/PAN nanofibers with E. coli to illustrate their good antibacterial activity against E. coli. When the inhibition


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zone test was carried out, the release killing action of the DBDMH/PAN nanofibers against bacteria was proven. The stabilities of the DBDMH/PAN nanofibers under dry, wet, and bacterial conditions were studied as well. The present work promotes the use of N-Br bond-containing N-halamines for antibacterial applications, offering an alternative way to antibacterial materials for a wide range of biomedical usages. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author E-mail: [email protected], Tel.: +86 471 4992982. E-mail: [email protected], Tel.: +86 431 88498964. Author Contributions §

These authors contribute equally to this work.

ORCID Alideertu Dong: 0000-0002-2812-3649 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51663019) and the National Natural Science Foundation of the Inner Mongolia Autonomous Region (2015MS0520). REFERENCES (1) Carney, D. W.; Schmitz, K. R.; Truong, J. V.; Sauer, R. T.; Sello, J. K. Restriction of the Conformational Dynamics of the Cyclic Acyldepsipeptide Antibitics Improves Their Antibacterial Activity. J. Am. Chem. Soc. 2014, 136, 1922. (2) Rodriguez, R. A.; Steed, D. B.; Kawamata, Y.; Su, S.; Smith, P. A.; Steed, T. C.; Romesberg, F. E.; Baran, P. S. Axinellamines as Broad-Spectrum Antibacterial Agents: Scalable Synthesis and


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Nanotechnology 2011, 22, 295602.


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Figures

Figure 1. (A) Synthesis illustration and (B) antibacterial action of DBDMH/PAN nanofibers.


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Figure 2. (A-C) SEM images of DBDMH/PAN nanofibers with different magnification ratios. (D) Size distribution and (E) EDX spectrum of DBDMH/PAN nanofibers.


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Figure 3. FTIR spectra of DBDMH, PAN, and DBDMH/PAN.


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Figure 4. 13C NMR spectra of DBDMH, PAN, and DBDMH/PAN.


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Figure 5. (A) XPS survey scans of PAN and DBDMH/PAN. (B) Br 3d spectrum of DBDMH/PAN. High-resolution C 1s (C) and N 1s (D) spectrum of PAN and DBDMH/PAN.


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Figure 6. SEM images and fiber diameter distribution (the inserts) of DBDMH/PAN nanofibers prepared with different concentrations of the electrospinning precursor solution: (A) 11 wt %, (B) 12 wt %, (C) 13 wt %, (D) 14 wt %, and (E) 15 wt %. AFD is the average fiber diameter.


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Figure 7. SEM images of DBDMH/PAN prepared with different mass ratios of PAN to DBDMH: (A) 3 : 1, (B) 3 : 2, and (C) 1 : 1.


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Figure 8. (A) Photographs for the bacterial culture plates of E. coli upon a 60 min exposure to the control (yellow), PAN (red), and DBDMH/PAN (green), respectively. Bacterial reduction of E. coli (B) and S. aureus (C) upon a 60 min exposure to DBDMH/PAN nanofibrous membrane with different dimensions: 0.5 × 0.5 cm, 1.0 × 1.0 cm, 2.0 × 2.0 cm, and 4.0 × 4.0 cm.


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Figure 9. (A) Bacterial reduction of E. coli upon a 60 min exposure to DBDMH/PAN with different mass ratios of PAN to DBDMH: 15 : 1, 3 : 1, 3 : 2, and 1 : 1. Optical images of the inhibition zone against E. coli for DBDMH/PAN nanofibers with different mass ratios of PAN to DBDMH: (B) 15 : 1 and (C) 1 : 1. The scale bar is 5 mm.


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Figure 10. (A) Active bromine content of DBDMH/PAN nanofibrous under different conditions. (B) Antibacterial kinetic graphs for DBDMH/PAN nanofibrous membrane with different dimensions against E. coli: 0.5 × 0.5 cm, 1.0 × 1.0 cm, 2.0 × 2.0 cm, and 4.0 × 4.0 cm, respectively.


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Table of Contents Graphic


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Novel N-Br Bond-Containing N-Halamine Nanofibers with

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