AgBr Nanoparticles In-Situ Growth on 2D MoS2 Nanosheets for Rapid

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Applications of Polymer, Composite, and Coating Materials

AgBr Nanoparticles In-Situ Growth on 2D MoS2 Nanosheets for Rapid Bacteria-Killing and Photo-disinfection Weidong Zhu, Xiangmei Liu, Lei Tan, Zhenduo Cui, Xianjin Yang, Yanqin Liang, Zhaoyang Li, Shengli Zhu, Kelvin Wai Kwok Yeung, and Shuilin Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12629 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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ACS Applied Materials & Interfaces

AgBr Nanoparticles In-Situ Growth on 2D MoS2 Nanosheets for Rapid Bacteria-Killing and Photo-Disinfection Weidong Zhua, Xiangmei Liu*a, Lei Tana, Zhenduo Cuib, Xianjin Yangb, Yanqin Liangb, Zhaoyang Lib, Shengli Zhub, Kelvin Wai Kwok Yeungc and Shuilin Wu*a,b a

Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science & Engineering, Hubei University, Wuhan 430062, China b

School of Materials Science & Engineering, the Key Laboratory of Advanced

Ceramics and Machining Technology by the Ministry of Education of China, Tianjin University, Tianjin 300072, China c

Department of Orthopaedics & Traumatology, Li KaShing Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong 999077, China

* To whom correspondence should be addressed: E-mail: [email protected];[email protected]; [email protected]

KEYWORDS: antibacterial; heterostructure; photocatalysis; MoS2 nanosheets; water disinfection; photo-degradation

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ABSTRACT In this study, a multifunctional hybrid coating composed of AgBr nanoparticles (AgBrNPs) and 2D molybdenum sulfide (MoS2) nanosheets (AgBr@MoS2) was constructed on Ti implant materials using an in-situ growth method for the first time. With 660 nm light and visible light irradiation, the electrons were rapidly excited from the VB of MoS2 to its CB, at the same time, AgBrNPs was used as a photoelectric receiver, which exhibited an enhanced photocatalytic activity due to the rapid transfer of photoelectrons from MoS2 nanosheets to AgBrNPs and the suppression of the recombination of electron-hole pairs. This contributed to the rapid production of reactive oxygen species (ROS) under 660 nm light irradiation, thus AgBr@MoS2 system killed bacteria and degraded organic matter quickly and efficiently in a short time. Meanwhile AgBr@MoS2 system showed excellent stability due to the strong covalent binding between S and Ag in the system, thus preventing AgBrNPs from being reduced to metal Ag.

1. INTRODUCTION The bacterial infection of artificial implants and biomedical devices is still an intractable challenge.1-4 After bacterial infection, biofilms may form on the surface of artificial materials if not treated promptly. At present, antibiotics are widely used to resist bacteria, but the formation of biofilms will make the treatment of antibiotics more difficult, and the long-term abuse of antibiotics will make many pathogenic bacteria easily develop resistance to a variety of antibiotics.5,6 In addition, an urgent global concern is the safety of drinking water, pathogenic microorganisms (such as bacterial, fungi, tec.,) in drinking water can cause serious diseases through water transmission. Some conventional water disinfection methods, such as chemical

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disinfectants commonly used in the water industry: free chlorine, chloramine and ozone. However, these chemical disinfectants can react with various components in natural water to produce harmful disinfection by-products.7,8 Therefore, highefficiency self-antimicrobial activity is necessary to be endowed with implants, meanwhile, an effective, rapid, safe method of disinfection is necessary to be developed. At present, with the wide application of photocatalysis technology in environmental repair and energy conversion, great progress has been made in the development of high efficiency photocatalysts, including bacterial inactivation, photodecomposition of harmful substances, artificial photosynthesis, solar energy conversion and so on.9,10 2 dimensional molybdenum disulfide (MoS2), a kind of layer type transition-metal dichalcogenide, is widely used in the fields of photoelectron, solar energy conversion, photocatalytic degradation and others because of its high activity, excellent optical and electronic properties, excellent durability and stability.11-16 In addition, MoS2 is sensitive to the visible light, which makes this material produce reactive oxygen species (ROS) under visible light because of the photoinspired electron-hole pairs separation.17-20 Meanwhile, both Mo and S are the essential trace elements of several enzymes in cells, making MoS2 an excellent biomaterial for biological applications.21 However, because of the low efficiency of charge separation and the rapid recombination of electron-hole pairs, pure MoS2 often exhibits low photocatalytic efficiency.22 Silver halide, especially AgBr, is a traditional photosensitive material, exhibiting excellent photocatalytic activity and electronic properties under visible light irradiation.23-25 However, it also faces the inherent limitation of instability, i.e., it can be easily reduced to metallic Ag under visible light exposure. Therefore, in order to

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make full advantage of the photocatalytic activity, AgBr is often loaded by other materials to suppress the reduction.26,27 For example, the photocatalytic system of Ag@AgBr has been reported to exhibit improved photocatalytic performance, because of the surface plasmon resonance (SPR) effect of Ag nanoparticles.28-30 However, the SPR effect often depends on the shape and distribution of Ag nanoparticles, which leads to serious defects in catalytic activity.26,

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At the same

time, the high concentration of released Ag+ often leads to cytotoxicity of cells and toxic effects on environment.32-34

Scheme 1. Schematic drawing of manufacturing process of AgBr@MoS2-Ti, bacterial killing processes through the excellent photocatalytic capacity of AgBr@MoS2-Ti under the 660 nm light and broad-spectrum antibacterial ability of Ag+.

Hence, in this research, a hybrid coating of MoS2 nanosheets modified by AgBrNPs was constructed on Ti plates for the first time, and the excellent 4


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photocatalytic and antibacterial properties were studied under 660 nm light or visible light irradiation. The loaded AgBrNPs prevented rapid recombination of photoinspired electron-hole pairs and consequently improved the photocatalytic ability effectively, thus increased the yields of ROS and the bacteria can be killed quickly and efficiently in a short time, the formation of bacterial biofilm can be effectively prevented, and the photodegradation of organic fuel also can be carried out quickly, efficiently and safely in a short period of time to further prevent water pollution. Meanwhile, the slow release of Ag+ assisted in preventing the bacterial infection for a long time. The manufacturing process and related antibacterial mechanism under 660 nm light irradiation were shown in Scheme 1.

2. MATERIALS AND METHODS 2.1 Preparation of AgBr@MoS2 coating The medical pure Ti plates (2 mm × Φ 6 mm, Shanghai Baosteel Co. Ltd., China) were used as substrates. First, the substrates were mechanically polished with silicon carbide sandpaper (grain sizes from #240 to #1200), then acetone, ethanol and deionized (DI) water was used to clean sequentially to remove contaminants, and finally dried in oven for further use. MoS2 coating was synthesized by hydrothermal method. In a nutshell, added 20 mg of sodium molybdate dihydrate (Na2MoO4·2H2O) and 40 mg of thioacetamide (C2H5NS) to 40 mL distilled water, then stirred for 30 min, a 100 mL Teflon-lined stainless steel autoclave was used to transfer the liquor with Ti plates in the bottom and heated at 200 oC for 24 h in muffle furnace. After cool to room temperature, the collected samples were rinsed with ethanol and DI water. The AgBr@MoS2 hybrid coating was prepared by in-situ growth method. 7 mg

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of silver nitrate (AgNO3) and 7 mg of cetyltrimethylammonium bromide (CTAB) were mixed with 35 mL of deionized water in the beaker contained the above obtained MoS2-Ti plates with continuous stirring for 1 hour in the dark. Then both MoS2-Ti and solution were heated for 1 h at 120 oC in 50 mL Teflon-lined stainless steel autoclave. When naturally cool, washing samples with ethanol and DI water successively. 2.2 Surface characterization The microstructure and surface configuration were examined by TEM (JEM 2100-F) and SEM (SM-6510LV, JEOL). Surface chemical compositions of samples were determined by XPS (ESCALAB 250Xi, Thermo Scientific). The crystallinity of samples was measured by XRD (D8A25, Bruker). UV-visible absorption spectra were obtained by a UV-visible spectrophotometer (UV-3600, Japan). Confocal Raman microspectrometer (inVia Reflex, Renishaw, England) was operated at 532 nm to record Raman spectra of samples. Photoluminescence (PL) spectra were recorded by fluorescence spectrophotometer (LS-55, American PE) with an excitation wavelength of 325 nm. ICP-AES (Optimal 8000, PE) was used to determine the concentration of the released of Ag+ from AgBr@MoS2-Ti in the solution. In order to further illustrate the generation of ROS under irradiation in the samples, DCFH-DA dye was used to further test the production of ROS. Concretely, the samples of Ti, MoS2-Ti, and AgBr@MoS2-Ti were mixed with 200 μL DCFH-DA solution (50 μmol/L), respectively. Then the solutions were irradiated under 660 nm light for 20 min, and the fluorescence change was measured during excitation by 488 nm and the emission of DCFH-DA at 525 nm was recorded by a microplate reader at a fixed time (0, 4, 8, 12, 16 and 20 min). 2.3 ROS detection

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The generation of ROS was detected by electron spin resonance (ESR) spectroscopy using the 2,2,6,6-tetramethyl piperidyl produced from Ti, MoS2-Ti, and AgBr@MoS2-Ti under 660 nm light (180 mW laser power; MRL-III-660Dnm500mW-16090712, China) for 20 min.35 Additionally, 1O2-generating ability of samples can be further evaluated using 9,10-dimethylanthracene (DMA) as a chemical trap for 1O2. Briefly, the samples of Ti, MoS2-Ti, and AgBr@MoS2-Ti were mixed with 200 μL DMA solution (10-4 M in PBS, 1.5 mL), respectively. Then the solutions were irradiated under 660 nm light for 20 min, and the luminous intensity of DMA at 402 nm was recorded by a microplate reader (SpectraMax I3MD USA) at a fixed time (0, 4, 8, 12, 16 and 20 min). 2.4 Release of Ag+ To analyze Ag+ released in AgBr@MoS2-Ti, five samples of AgBr@MoS2-Ti were socked in 50 mL of PBS at 37 oC in darkness for 10 days. 3 mL of the solution were collected at the regular time interval and 3 mL of fresh PBS were supplemented. ICPAES was applied to measure the release concentration of Ag+.32 2.5 Photoelectrochemical measurements The photocurrent of the samples was characterized in the electrochemical workstation with a three-electrode cell, in which platinum electrode was used as the opposite electrode, Ag/AgCl was used as reference electrode and samples were used as the working electrode. Under the condition of fixed bias voltage of 0.1 V versus Ag/AgCl, time-dependent photocurrent test of the samples (light/dark cycles of 10 s) were carried out using 0.5 M Na2SO4 solution as electrolyte. Meanwhile, the electrochemical impedance spectroscopy (EIS) of the samples was tested at -0.5 V in an electrochemical workstation. 2.6 In vitro antibacterial assay

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The antibacterial efficiency of samples was assessed by spread plate method.35 First, 100 μL 107 CFU/mL of bacterial liquid (S. aureus and E. coli) was added into 96-well plates together with Ti, MoS2-Ti and AgBr@MoS2-Ti. Afterward, one group of samples was exposed to 20 min under 660 nm light, and infrared thermal imager (testo 875i, German) was used to record temperature. The other group of samples was cultured in the dark for 20 min. The antibacterial performance of AgBr@MoS2-Ti under visible light (2 kW/m2, a solar simulator, 300W xenon lamp) was tested as described above. 200 μL bacterial liquid together with Ti, MoS2-Ti and AgBr@MoS2Ti were put into 96-well under ice water bath to exclude photothermal factor, then the samples were exposed for 10 min under visible light or under the dark for 10 min. After culturing, 10 μL of bacterial liquid was extracted from 96-well plates and add it to 1 mL Luria-Bertain (LB) medium dilute 100 times. After fully mixing, 20 μL of dilution bacterial liquid was removed from the medium and evenly spread in LB solid medium, after incubating at 37 oC for 24 h. For reusable antibacterial model, the same sample together with 100 μL S. aureus bacterial liquid was first placed in the dark for 20 min and then spread on LB solid medium with the same method as the above. Then the sample was washed with PBS, and 100 μL S. aureus bacterial liquid was added again and exposed to 660 nm light for 20 min, and the same treatment was repeated four times. The antibacterial activity of AgBr@MoS2-Ti via Ag+ release was investigated by culturing at 37 oC for 24 h. 200 μL 105 CFU/mL bacterial liquid (S. aureus and E. coli) together with samples were added into 96-well plates for 24 h at 37 oC. At the end of the training, the 20 μL bacterial liquid was evenly coated on LB solid medium and cultivated at 37 oC for another 24 h. Antibacterial efficiency was counted by the following formula: Antibacterial efficiency (%) = (number of bacterial colonies on Ti − number of

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bacterial colonies on MoS2 or AgBr@MoS2-Ti) / (number of bacterial colonies on Ti) × 100%. To further illustrate the antibacterial effect. First, 100 μL 107 CFU/mL of bacterial liquid (S. aureus and E. coli) were added into 96-well plates together with Ti, MoS2-Ti and AgBr@MoS2-Ti. Afterward, one group of samples were exposed for 20 min under 660 nm light. The other group of samples were cultured in the dark for 20 min. After the incubation time, the samples were collected and the bacterial suspensions were sucked away, 200 μL 2.5% glutaraldehyde was immersed in the samples for 2 h to fix the bacteria. Then PBS was used to wash samples twice, and dehydrated for 15 min with 30, 50, 70, 90, and 100% v/v alcohol sequentially. Finally, SEM was used to observe the bacterial morphologies on the samples. For the live/dead staining, after the incubation time, in a dark environment, the mixed dyes (LIVE/DEAD BacLight bacteria viability kits) were immersed on the surface of the sample, incubated with 15 min, cleaned with PBS, dried and photographed with a fluorescent microscope (IX73, Olympus, Japan). The quantitative analysis of the live/dead staining by using Image J software. 2.7 Photocatalytic degradation experiment Rhodamine B (RhB) solution (5 mg/L) was used to further test photocatalytic activity of AgBr@MoS2-Ti under 660 nm light and visible light. 200 μL RhB solution was put together with the samples in a 96-well plate under ice water bath to eliminate thermal effect, then a 660 nm light or visible light was used to serve as light source. The 100 μL RhB solution was taken out at regular intervals, and the microplate reader (SpectraMax I3MD USA) was used to measure the loss of RhB with characteristic absorption peak at 554 nm. The spectra of intermediate products degraded by RhB were determined by LC-MS (Agilent 6230).

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2.8 In vitro viability assay In order to assess cell viability of samples under 660 nm light and in the dark, cytotoxicity was determined by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) method. 200 μL of NIH cells, MC3T3-E1 cells and hBMSC cells (1 × 104 cells per mL, Tongji hospital, Wuhan) together with the sterile samples of Ti, MoS2-Ti, and AgBr@MoS2-Ti were added into 96-well plates, respectively. For light groups, Ti, MoS2-Ti, and AgBr@MoS2-Ti were exposed to 660 nm light for 20 min. After incubating for 1, 3, and 7 days at 37 oC with an atmosphere of 5% CO2, the culture medium in the hole plate was sucked out and 100 μL 0.5 mg ml-1 MTT solution was added. Then cultured with MTT for 4 h at 37 oC, then removed MTT solution, 100 μL of the dimethyl sulfoxide (DMSO) was added and oscillated for 15 min. Finally, the microplate reader (SpectraMax I3MD USA) was used to measure absorbance of DMSO at 490 nm. The OD value was used to calculate the cell viability (%). 2.9 Hemolytic measurement The hemolytic test was performed using SD rat blood. Dilute 5 mL of blood with 50 mL of PBS solution, then centrifuge at 1000 × g for 6 min to take red blood cells. The red blood cells were washed 3 times with PBS, after which the red blood cells were resuspended in 20 mL of PBS solution. Five groups of PBS (negative control), 1% TritonX-100 (positive control), Ti, MoS2-Ti and AgBr@MoS2-Ti were mixed with the same concentration of red blood cell solution (1 × 104 cells per mL). After 4 h of co-cultivation, collect the cell fluid into a centrifuge tube and centrifuge at 1000 × g for 6 min, and the supernatant was taken out of the 96-well plate. Subsequently, a microplate reader was used to measure the OD value of supernatant at 451 and 405 nm.

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2.10 Statistical analysis Each experiment was evaluated by mean values ± standard deviation of at least three trials. A one-way analysis of variance (ANOVA) program combined with a student ttest was used to evaluate the statistical significance of the variance. P < 0.05, illustrating there was a significant difference between two groups.

3. RESULTS AND DISCUSSION 3.1 Characterization of the AgBr@MoS2 coating Figure 1a showed the microstructure of MoS2 prepared by hydrothermal method, which showed floral microsphere shape of an average diameter of 500 nm with the high signal of Mo and S. After loading with AgBrNPs (Figure 1b), the surface showed almost the same morphology, and Br and Ag were detected. The thickness of prepared coating of AgBr@MoS2 was 26.1 μm (Figure 1c). TEM image of scraped powders from MoS2-Ti showed the flower-like microsphere structure assembled from many crumpled and edge-folded nanosheets (Figure 1d) with a lattice spacing of 0.60 nm (Figure 1e), which was assigned to the (002) plane of MoS2.12 Figure 1f showed TEM of AgBr@MoS2, the nanoparticles (marked by green arrows) were distributed on the surface of MoS2. As shown in Figure 1g, the interplanar spacing of adjacent lattice fringes with 0.60 nm and 0.27 nm represents the (002) and (100) planes of MoS2,36 while the clear interplanar distance of 0.20 nm corresponded to (220) plane of AgBrNPs,37 indicating the AgBrNPs were loaded onto the surface of MoS2 successfully with tight interface contact. The homogeneous distributions of Mo, S, Ag and Br in the AgBr@MoS2-Ti were disclosed by the EDS mapping (Figure 1h). As shown in Figure 1i, the three peaks of (002), (100) and (110) planes are typical peaks of hexagonal MoS2 (JCPDS Card No. 37-1292).13 The diffraction peaks from the

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(111), (200), (220), (222), (400) and (420) crystal faces are indexed to AgBr (JCPDS No. 06-0438).38 These results revealed that AgBrNPs were successfully synthesized on the surface of MoS2 nanosheets.

Figure 1. SEM and EDS images of (a) MoS2-Ti, (b) AgBr@MoS2-Ti. (c) The thickness of AgBr@MoS2 coating on Ti substrate showed by SEM image. TEM images of (d) MoS2, (e) high-resolution TEM image of the MoS2, TEM images of (f) AgBr@MoS2, (g) high-resolution TEM image of the AgBr@MoS2, (h) EDS mapping profile of AgBr@MoS2 showing the element of Mo, S, Ag and Br in white frame of Fig. 1f. (i) XRD patterns of Ti, MoS2-Ti, and AgBr@MoS2-Ti.

In order to further analyze surface chemical compositions of samples, XPS was utilized to determine the elements and valance state. Figure 2a showed that compared with MoS2-Ti, there were obvious Ag, and Br peaks in the spectra obtained from AgBr@MoS2-Ti. The high-resolution spectra (Figure 2b) showed that peaks of Ag 12


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3d5/2 and Ag 3d3/2 were located at 368.2 eV and at 374.2 eV, respectively, indicating the ion state of Ag in the coating. Meanwhile, Figure 2c showed high-resolution Br 3d spectrum of AgBr@MoS2-Ti, two peaks at 68.4 eV and 69.3 eV represented the binding energy of Br 3d5/2 and Br 3d3/2, respectively, which was assigned to Br-.39 The above results disclosed the existence of AgBr in the coating. Figure 2d and S1a showed the high-resolution spectrum of Mo 3d and S 2p obtained from MoS2-Ti. Mo3d5/2 and Mo 3d3/2 were located at 229.0 eV and 232.2 eV, while S 2p3/2 and S 2p1/2 were located at 161.8 eV and 163.0 eV, which corresponded to MoS2,40 indicating the formation of MoS2 coating, which was consistent with XRD results. Mo 3d spectra (Figure 2e) obtained from AgBr@MoS2-Ti also exhibited the same peaks at the same binding energy. However, S 2p peaks (Figure S1b) in AgBr@MoS2-Ti moved slightly to high binding energy side of S 2p3/2 at 161.9 eV and S 2p1/2 at 163.1 eV, which explained the fact that the electron interaction between AgBr and MoS2 occurred at the interface of composites and thus demonstrated the covalent bond between Ag and S.13,41,42 The results agreed well with those of TEM, indicating that prepared coating was made of AgBr and MoS2. Raman spectra (Figure 2f) showed that after loading AgBrNPs, the frequency of E12g peak of MoS2-Ti (376 cm-1) was lower than that of AgBr@MoS2-Ti (378 cm-1) while the frequency of A1g decreased. The shift was mostly due to a surface strain induced by the loaded AgBrNPs on the surface of MoS2 nanosheets, indicating a tight connection between AgBr and MoS2.13,43,44

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Figure 2. XPS spectra of MoS2-Ti, and AgBr@MoS2-Ti: (a) the survey scan, (b) Ag 3d and (c) Br 3d for the AgBr@MoS2-Ti. The high-resolution spectrum of Mo 3d (d) in MoS2-Ti and (e) in AgBr@MoS2-Ti. (f) Raman spectra of the MoS2-Ti and the AgBr@MoS2-Ti with the excitation of 532 nm line.

3.2 The photocatalysis characterization of AgBr@MoS2-Ti Figure 3a showed UV-visible absorption spectra of the Ti, MoS2-Ti, and AgBr@MoS2-Ti. Clearly, the pure Ti had no apparent absorption peak. Comparing with the MoS2-Ti, after loading with AgBrNPs, the obtained AgBr@MoS2-Ti presented a high absorption at a range of 350-700 nm, which enhanced the absorption efficiency of visible light, contributing to improve the photocatalytic activity of the coating. As shown in Figure 3b, under the excitation of 325 nm, the PL intensity of AgBr@MoS2-Ti was obviously lower than that of MoS2-Ti, confirming the effective suppression of the recombination between photoelectrons and holes.45 These results disclosed that after loading with AgBrNPs, the recombination of electron-hole pairs was effectively delayed in the AgBr@MoS2 and prolonged the lifetime of electronhole pairs, thus improving the photocatalytic activity. There were mainly due to the 14


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rapid excitation of electrons (e-) from the valence band (VB) of MoS2 to its conduction band (CB) under 660 nm light, and thus in AgBr@MoS2 composite coating, the photoelectrons in MoS2 were easily transferred to AgBrNPs, which effectively accelerated the electron transfer. Thus, the recombination of electron-hole pairs was delayed, indicating that the photocatalytic performance was improved effectively. As shown in Figure 3c, the pure Ti had no obvious photocurrent. The MoS2-Ti had obvious photocurrent, indicating that electron hole pairs were inspired by light. As a contrast, the AgBr@MoS2-Ti showed the maximum photocurrent. In addition, EIS measurements showed that AgBr@MoS2-Ti had the lowest impedance (Figure 3d). These results further confirmed that AgBrNPs promoted the transfer of photoelectrons and suppressed the recombination of photoelectrons and holes.46 Therefore, AgBr@MoS2-Ti exhibited an enhanced photocatalytic activity. Generation of ROS was further elaborated the photocatalytic properties of the samples. Figure 3e showed the ESR spectra, reflecting the generation of 1O2 from the Ti, MoS2-Ti and AgBr@MoS2-Ti under 660 nm light. It was seen that the intensity of MoS2-Ti, AgBr@MoS2-Ti was obviously larger than that of Ti, and AgBr@MoS2-Ti showed the strongest signal, indicating that more 1O2 was produced when AgBrNPs were loaded on the surface of MoS2.5,29 In order to further evaluate the production of 1O2, DMA was used a chemical trap for 1O2, which will interact with 1O2 to form an internal peroxide (Figure S2a), and induced the reduction of the absorption intensity at 402 nm.47 As shown in Figure S2b, in the Ti group, there was no significant change in the absorption intensity. The absorption intensity of MoS2-Ti and AgBr@MoS2-Ti group decreased at 402 nm with the increase of irradiation time, and the generation of 1O

2

was described. Within 20 min, the reduced absorption strength of the

AgBr@MoS2-Ti group (decreased from 0.568 to 0.198) was significantly greater than

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that of MoS2-Ti (decreased from 0.568 to 0.342), which also indicated that the AgBr@MoS2 system produced more 1O2. In order to further illustrate the generation of ROS under irradiation in the samples, DCFH-DA dye was used to further test the production of ROS, DCFH-DA is a kind of non-fluorescent molecule, but it can be rapidly oxidized to high fluorescent molecule in the presence of ROS (dichlorofluorescein, DCF).48 The production of ROS can be detected by the increase of fluorescence absorbance of the samples irradiated by visible light at 525 nm. As shown in Figure S3, the fluorescence intensity of Ti group did not change obviously after irradiation of 20 min at 660 nm light, indicating that there was no obvious production of ROS. Compared with Ti group, the amount of ROS in MoS2-Ti group was 4.3-fold higher than that in Ti group. After loading AgBrNPs, the amount of ROS in AgBr@MoS2-Ti group was 12.4-fold higher than that in Ti group. In general, the MoS2 was a narrow band gap ≈ 1.62 eV, while AgBr has a band gap ≈ 1.92 eV.13,15,49 The Figure 3f illustrated the CB, VB of MoS2 and AgBr, and the principle of the separation of charge carriers and ROS generation in AgBr@MoS2 hybrid coating. As described above, upon 660 nm visible light illumination, the electrons were rapidly excited from the VB of MoS2 to its CB, and the holes in the VB were left behind. Meanwhile, because the CB of AgBrNPs was in a lower position than that of MoS2, AgBrNPs was used as a photoelectric receiver. So, the photoelectrons were rapidly transferred to AgBrNPs. Simultaneously, surrounding oxygen molecules captured the photogenerated electrons in aqueous solution to form 1O2: O2

+

e−

(1)

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→

1O

2

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This interfacial charge transfer and separation ability effectively delayed the recombination of electron-hole pairs, thus the photoresponsive ability of AgBr@MoS2 coating had been improved, resulting in the improvement of ROS yield.

Figure 3. Photocatalytic performance characterization. (a) UV-vis absorption spectra of the Ti, MoS2-Ti, and AgBr@MoS2-Ti. (b) PL spectra of MoS2-Ti, and AgBr@MoS2-Ti excited at 325 nm. (c) Photocurrent measurement and (d) EIS of photoelectrodes made of Ti, MoS2-Ti, and AgBr@MoS2-Ti under 660 nm visible light. (e) ESR spectra of Ti, MoS2-Ti, and AgBr@MoS2-Ti to measure the generation of ROS. (f) The separation of charge carriers and ROS generation mechanism of AgBr@MoS2 hybrid coating under 660 nm visible light.

The photocatalytic activity of AgBr@MoS2 under 660 nm light irradiation was further studied by photocatalytic degradation experiment for RhB. As shown in Figure 4a, due to the fact that there was small amount of oxidized titanium oxides film on the surface of Ti and dark adsorption, so the Ti group showed a certain degradation effect.50 And under irradiation of 660 nm light, approximately 92% of RhB in AgBr@MoS2 was degraded within 20 min, and compared with MoS2, it revealed 17



superior photodegradation ability. Figure 4b showed the cyclic stability test of AgBr@MoS2, with 84% degradation efficiency retained after four cycles. As shown in Figure S4a, after 20 min of 660 nm light, the RhB solution was obviously decolorized, which further indicated that RhB was degraded. Meanwhile, the RhB solution was bubbled with continuous N2 for 1 h, and then the sample of AgBr@MoS2-Ti was added and irradiated by 660 nm light for 20 min. Figure S4b showed that the RhB was degraded little, which was mainly due to the lack of oxygen in the solution, which prevented the formation of 1O2, resulting in the inability of RhB to be degraded. In order to probe into the degradation pathway of RhB, the spectrum of degradation intermediate products was determined by LC-MS. Figure S4c showed detected intermediate compounds from R1 to R6. And Figure 4c illustrated a potential decomposition route of RhB by [email protected],52

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Figure 4. (a) Photocatalytic degradation of RhB on Ti, MoS2-Ti and AgBr@MoS2-Ti and (b) the cycling operation on AgBr@MoS2-Ti under 660 nm light. (c) A possible degradation pathway of RhB by AgBr@MoS2-Ti

3.3 In vitro antibacterial test of AgBr@MoS2-Ti under 660 nm light The spread plate results were shown in Figure 5a and 5b. The antimicrobial efficacy was seen directly from the number of bacterial colonies. Obviously, pure Ti had no antibacterial effect regardless of with or without light. The bacterial colonies on the surface of all samples had no obvious reduction compared to the control group in the darkness. After exposing to 660 nm light for 20 min, there was a slight decrease in the number of bacterial colonies of MoS2-Ti. After further loading with AgBrNPs, the

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bacterial number on AgBr@MoS2-Ti decreased significantly, showing excellent antibacterial properties. Figure 5c and 5d showed the calculated antibacterial efficacy against S. aureus and E. coli, respectively. The antibacterial efficiency of pure Ti against E. coli and S. aureus was only 6.40% and 3.54%, respectively. This is because pure Ti had no photocatalytic properties exposed to 660 nm visible light, so obvious death of bacteria in a short time was not caused. Although the MoS2 had a certain photocatalytic capacity, its antibacterial efficiency against E. coli and S. aureus was only 38.77% and 30.46% due to the poor photocatalytic property owing to rapid recombination of photo-produced electrons and holes. As a contrast, the corresponding value of AgBr@MoS2-Ti against E. coli and S. aureus was 99.94% and 99.79%, respectively, which was ascribed to the more yields of ROS (Figure 3e) during light irradiation since loaded AgBrNPs accelerated the transfer of electrons and delay recombination of electron-hole pairs.53-56 In addition, photothermal effect of the sample was excited to some extent by 660 nm visible light. As shown in Figure S5a and S5b, the maximum temperature inspired by 20 min light irradiation only reached up to 45 oC, which had little influence on bacteria, it was observed from spread plate results shown in Figure S5c. The corresponding value at 37 oC and 45 oC in the oven for 20 min was 4.60% against E. coli and 5.17% against S. aureus (Figure S5d). Meanwhile, as shown in Figure S6, when the light time was prolonged, the antibacterial efficiency of AgBr@MoS2-Ti against S. aureus increased by 99.99%, which indicated that AgBr@MoS2 could further increase the yield of ROS and achieve much higher antibacterial efficiency with the prolongation of light time. Simultaneously, the penetration depth of visible light in water was controlled by the absorption coefficient of the medium to a large extent, and the penetration depth also varies with the change of wavelength and intensity of the light. The average

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penetration depth of 660 nm light in water is about 2 m.57 As shown in Figure S7, in this work, 660 nm light (180 mW laser power) could illuminate the water-filled pipe with a length of 3 m, indicating the strong penetration of 660 nm light in water (yellow arrow). In addition, Figure 5e showed the behavior of release Ag+ from AgBr@MoS2-Ti. It was seen that AgBr@MoS2-Ti released Ag ions sustainably during a long period, which prevented bacterial infection for a long-term due to the broad-spectrum antibacterial activity of Ag ions.58 Additionally, since the disinfection time of water was very short, the Ag+ release of AgBr@MoS2-Ti was performed for 20 min in this work. Figure S8 showed the Ag+ release of AgBr@MoS2-Ti in 20 min after light or dark, it was seen that under dark conditions, the release concentration of Ag+ in AgBr@MoS2-Ti was 0.034 μg/mL within 20 min. After 20 min irradiation with 660 nm light, the release concentration of Ag+ was 0.048 μg/mL, which was mainly due to photothermal effect. It was noted that the released Ag+ concentration was far lower than the safety level of 0.1 mg/l, provided by the World Health Organization.59 Figure S9 showed that the samples cultured in darkness for 24 h against E. coli and S. aureus, the spread plate of Ti, and MoS2-Ti, had a large number of colonies, indicating that there was no obvious antimicrobial activity. However, there were almost no colonies on AgBr@MoS2-Ti, and the corresponding antibacterial efficiency against S. aureus and E. coli was 99.98% (Figure 5f) and 99.99% (Figure 5g), respectively after culturing in the dark for 24 h, indicating the long-lasting prevention of bacterial infection of AgBr@MoS2-Ti through the slow release of Ag ions.

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Figure 5. In vitro antibacterial assay under 660 nm light. The surface plate of (a) S. aureus and (b) E. coli. The antibacterial efficiency of the (c) S. aureus, and (d) E. coli after 20 min 660 nm light irradiation or in darkness for 20 min. (e) The Ag+ release curves obtained by immersion of AgBr@MoS2-Ti in PBS at 37 oC for 10 days. The antibacterial efficiency of Ti, MoS2-Ti, and AgBr@MoS2-Ti for (f) S. aureus and (g) E. coli for 24 h. (n=3, mean ± SD: **P < 0.01, ***P < 0.001).

In order to assay stability of AgBr@MoS2-Ti, the antibacterial activity of the same AgBr@MoS2-Ti was carried out several times. As shown in Figure 6a, after in the dark for 20 min, a large number of bacteria appeared on the plate. After 660 nm light irradiation for 20 min, even after three times of high concentration of bacteria,

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AgBr@MoS2-Ti still maintained a high antibacterial effect against S. aureus in the fourth cycle. Figure S10 showed that the antibacterial efficiency of AgBr@MoS2-Ti against S. aureus still maintained 98.96% in the fourth challenge, indicating that AgBr@MoS2-Ti had good stability and cyclic antibacterial properties. Additionally, XPS analysis of the content of elements in AgBr@MoS2-Ti unilluminated or after irradiation for 20 min showed that after 20 min irradiation with 660 nm light, the content of Br element did not decrease, and there was no significant change in the content of other elements (Figure S11a and S11b), At the same time, it was seen from Figure S11c that there was no peak of elemental silver in the Ag3d peak after illumination, which further indicated that AgBr@MoS2 had excellent stability. The bacterial morphologies and membrane integrity before and after irradiation can reflect the antibacterial effect visually. Generally, the normal morphologies of both E. coli and S. aureus were smooth and intact. Figure 6b showed that regardless of irradiation or not, bacteria on both Ti and MoS2-Ti showed relatively normal bacterial morphologies. For AgBr@MoS2-Ti, it was seen that the bacterial membrane was crumpled obviously or broken after 20 min illumination with 660 nm light (both S. aureus and E. coli, marked by yellow arrows). Meanwhile, antimicrobial ability was also evaluated by the live and dead staining of bacteria. Figure 6c showed in the absence of light, fluorescence was green, indicating the live bacteria, also indicating that surfaces of the samples were suitable for the bacterial growth in the dark. When exposed to 660 nm light for 20 min, partial red fluorescence pots were observed on MoS2-Ti and red fluorescence pots occurred on AgBr@MoS2-Ti, indicating that AgBr@MoS2-Ti had better antibacterial efficiency than MoS2-Ti under visible light illumination. In contrast, even under light irradiation, the surface of Ti was the same color as the unexposed surface, suggesting that 660 nm light had no obvious effects

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on the survival of bacteria. Meanwhile, Figure S12 shows the quantitative analysis of the live/dead staining, and the ratio of dead bacteria of Ti+light, MoS2-Ti+Light and AgBr@MoS2-Ti+Light were calculated to be 2.90%, 18.89% and 98.61% against S. aureus and 8.37%, 38.27% and 99.25% against E. coli, respectively. The rate of dead bacteria (%) = [(X0-X)/X0] × 100%, X0 is the intensity of green on the Ti, and B is the intensity of green in the experimental groups.

Figure 6. (a) Reusable antibacterial activities of AgBr@MoS2-Ti against S. aureus. (b) The surface morphologies of S. aureus (scale bar = 400 μm) and E. coli (scale bar = 800 μm) after treatment with Ti, MoS2-Ti and AgBr@MoS2-Ti in dark or irradiation for 20 min, damage of cell membranes was represented by yellow arrows represent. (c) Fluorescence images (scale bar = 100 μm), red and green stains signified dead and viable bacteria.

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3.4 Photocatalytic disinfection performance and photocatalytic degradation ability of AgBr@MoS2-Ti under visible light As shown in Figure 7a and Figure 7b, there were a large number of colonies on the samples for both S. aureus and E. coli in the darkness. After 10 min visible light, the bacterial colonies on AgBr@MoS2-Ti decreased obviously, indicating excellent antibacterial activity. In Figure 7c, no matter it was S. aureus and E. coli, the disinfection efficiency of Ti, MoS2-Ti and AgBr@MoS2-Ti were less than 7%. However, under visible light for 10 min, the killing effect of AgBr@MoS2-Ti (99.64% for S. aureus and 99.52% for E. coli) on bacterial was stronger than that of MoS2-Ti (33.18% for S. aureus and 54.67% for E. coli). Simultaneously, as shown in Figure 7d, under visible light, the photodegradation ability of AgBr@MoS2-Ti (97% for RhB) was better than that of MoS2-Ti (78% for RhB) within 20 min. These results further demonstrated that AgBr@MoS2 also showed excellent light response ability and water disinfection ability under visible light. Compared with other recent MoS2-based and AgBr-based photocatalysts reported, AgBr@MoS2 revealed superior antibacterial efficiency (Figure S13).

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Figure 7. (a) In vitro antibacterial assay under visible light. The surface plate of (a) S. aureus and (b) E. coli. (c) The antibacterial efficiency of S. aureus and E. coli under dark for 10 min or under visible light for 10 min. (d) Photocatalytic degradation of RhB on Ti, MoS2-Ti and AgBr@MoS2-Ti under visible light for 20 min. (n=3, mean ± SD: **P < 0.01, ***P < 0.001).

3.5 In vitro cell viability The methyl thiazolyl tetrazolium (MTT) was used to evaluate the cytotoxicity of samples. Figure 8a showed that the NIH cells viability of AgBr@MoS2-Ti on the first day was slightly lower than that of pure Ti group due to the release of Ag+. However, As the prolongation of incubation time, the amount of released Ag+ decreased, the cell survival rate on AgBr@MoS2-Ti increased continuously, indicating that the cytotoxicity of AgBr@MoS2-Ti decreased with the increase of culturing time. Additionally, the MoS2-Ti group showed a high level of cellular activity, indicating 26


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the excellent biocompatibility of MoS2-Ti. Figure 8b showed the NIH cell viability after 660 nm light for 20 min. When incubating for 24 h, cell survival rate on AgBr@MoS2-Ti decreased significantly, with a survival rate of only 30% relative to Ti, mainly due to the production of 1O2 and partial release of Ag+. However, as the extending of culture time, the cell activity of AgBr@MoS2-Ti increased gradually. On the 7th day, the survival rate of AgBr@MoS2-Ti reached 97%, and there was no significant difference between AgBr@MoS2-Ti and pure Ti. This was due to the fact that 1O2 cannot exist for a long time, the effect of light on cell activity was only temporary.60 Meanwhile, Figure 8c and 8e showed the cell viability of MC3T3-E1 cells and hBMSC cells on the samples for 1, 3 and 7 day in the dark. With the prolongation of culture time, the cell survival rate on AgBr@MoS2-Ti samples increased accordingly. Figure 8d and 8f showed the MC3T3-E1 cells and hBMSC cells viability after 660 nm light for 20 min, As shown in above, as the extending of culture time, the cell activity of AgBr@MoS2-Ti increased gradually. It was further shown that AgBr@MoS2-Ti had good biocompatibility. It was seen from the results of hemolysis text (Figure S14), all samples including negative control (PBS) showed a hemolysis ratio of less than 9% compared to the positive control (1% TritionX-100).

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Figure 8. The cell viability on the samples of Ti, MoS2-Ti and AgBr@MoS2-Ti for 1, 3 and 7 day: (a) NIH cells viability in the dark, (b) NIH cells viability via 660 nm light for 20 min. (c) MC3T3-E1 cells and (e) hBMSC cells viability in the dark. (d) MC3T3-E1 cells and (f) hBMSC cells via 660 nm light for 20 min. (n = 3, mean ± SD: *p < 0.05, **p < 0.01, ***p < 0.001).

4. CONCLUSIONS In this work, for the first time, AgBrNPs were successfully prepared by in-situ growth on the surface of two-dimensional MoS2 nanosheets, effectively suppressing the recombination of photoelectrons and holes. Thus, the AgBr@MoS2 composite coating exhibited excellent photocatalytic performance. The results showed that under 660 nm light or visible light irradiation, AgBr@MoS2 coating produced more ROS in 20 min, which quickly killed 99.94% of S. aureus and 99.79% of E. coli, respectively. And it had a good degradation ability, which degraded 92% of RhB within 20 min. In addition, the tight bonding S and Ag at the interface between AgBrNPs and MoS2 favored the gradual and slow release of Ag ions from the coating, thus avoiding the toxicity of precious metals and preventing bacterial infection for a long time, and simultaneously preventing AgBrNPs from being reduced to elemental silver. Moreover, the prepared coating maintained a stable photostability and high antibacterial property. Although the cell survival rate is low in a short time due to the generation of ROS after 660 nm light irradiation and the release of Ag+, but it also shows good biocompatibility as the extending of culture time. and this suggested that cytotoxicity was only temporary. The AgBr@MoS2 was expected to be prepared on other surfaces because the current work was only applied on Ti substrate. This work provides a new and feasible way to prevent artificial implant infection, disinfection of

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biomedical devices and drinking water.

ASSOCIATED CONTENT Supporting Information Supporting figures: The high-resolution spectrum of S 2p; Decay curves of the absorption of DMA; The fluorescence intensity of DCF at 525 nm; Photocatalytic degradation of RhB with N2 or without N2 and LC-MS spectra of degradation products of RhB by AgBr@MoS2-Ti; The temperature change curve and antibacterial efficiency; The penetrating distance of 660 nm light in water; The Ag+ release curves; antimicrobial activities of Ag+; Circulating antibacterial efficiency against S. aureus; XPS analysis of the content of elements in AgBr@MoS2-Ti; Antibacterial efficiency comparison; Hemolysis percentage of the samples;

ACKNOWLEDGEMENTS This work was jointly supported by the Natural Science Fund of Hubei Province No. 2018CFA064, National Natural Science Foundation of China (Nos. 51671081, 51871162, and 51801056), National Key Research and Development Program of China No. 2016YFC1100600 (sub-project 2016YFC1100604), Hong Kong Research Grants Council (RGC) General Research Funds (GRF) Nos. 11301215, 11205617 and 17214516, and RGC/NSFC (N_HKU725-16), Hong Kong ITC (ITS/287/17, GHX/002/14SZ), as well as Health and Medical Research Fund (No. 03142446).

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AgBr Nanoparticles In-Situ Growth on 2D MoS2 Nanosheets for Rapid

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