A photo catalyst of cuprous oxide anchored MXene nanosheet for dramatic enhancement of synergistic antibacterial ability

Journal Pre-proofs A photo catalyst of cuprous oxide anchored MXene nanosheet for dramatic enhancement of synergistic antibacterial ability Wei Wang, Huimeng Feng, Jianguo Liu, Mutian Zhang, Shuan Liu, Chao Feng, Shougang Chen PII: DOI: Reference:

S1385-8947(20)30107-8 https://doi.org/10.1016/j.cej.2020.124116 CEJ 124116

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

28 October 2019 25 December 2019 12 January 2020

Please cite this article as: W. Wang, H. Feng, J. Liu, M. Zhang, S. Liu, C. Feng, S. Chen, A photo catalyst of cuprous oxide anchored MXene nanosheet for dramatic enhancement of synergistic antibacterial ability, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124116

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© 2020 Published by Elsevier B.V.

A photo catalyst of cuprous oxide anchored MXene nanosheet for dramatic enhancement of synergistic antibacterial ability Wei Wang,*a Huimeng Feng,a Jianguo Liu,b Mutian Zhang,a Shuan Liu,c Chao Feng,d and Shougang Chen*a a.

School of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, China.

b.

Shandong Key Laboratory of Oil & Gas Storage and Transportation Safety, China University of Petroleum (East China), Qingdao, 266580, China.

c.

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key

Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technologies and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. d. Key Laboratory of Tobacco Pest Monitoring & Integrated Management, Tobacco Research Institute, Chinese Academy of Agricultural Sciences, Qingdao 266101, China.

*Corresponding author: Wei Wang; Shougang Chen E-mail: [email protected]; [email protected]

Abstract Ti3C2Tx (MXene) as a two-dimensional material has attracted numerous attentions in photocatalysis and antibacterial, and the effectiveness of MXene for photocatalysis triggered antibacterial is in urgent need of development. Herein, we synthesized a novel cuprous oxide (Cu2O) anchored MXene nanosheet. Cu2O nanospheres can be uniformly anchored on the surface of MXene firmly due to the electrostatic effect between singlelayer MXene and Cu2O nanospheres. In comparison with MXene, Cu2O and mixture (MXene and Cu2O), Cu2O/MXene nanosheet exhibits an excellent antibacterial activity against S. aureus and P. aeruginosa, whose bacteriostasis efficiencies sharply increased to 97.04% and 95.59%, respectively. Photoluminescence spectroscopy reveals that the incorporation of MXene and Cu2O can effectively prevent the recombination of electron-hole pairs of Cu2O and present a high photocatalytic disinfection. Single-layer MXene with large surface area accepts electrons from Cu2O, leading to abundant electrons on the surface of MXene, which provide better charge transfer between bacteria and Cu2O/MXene nanosheets. In addition, density functional theory calculations were used to investigate an optimized band structure of Cu2O-MXene as well as its strong electronic interactions, which are essential for photocatalysis and ROS generation. Finite element method further revealed that MXene promoted an enhancement of surface plasmon resonance to generate ROS on the surface of Cu2O/MXene. Furthermore, Cu2O/MXene shows a synergistic antibacterial effect of MXene

accelerating

photoelectron

transportation,

Cu2O

antimicrobial

and

photocatalysis, elevated ROS production ability, and surface plasmon resonance action

on the surface of Cu2O/MXene. KEY WORDS: MXene, photocatalysis, antibacterial, surface plasmon resonance, reactive oxygen species 1 Introduction Currently, MXenes, as a growing family of two-dimensional (2D) materials, have increasingly attracted a great attention for their potential in various applications, including water desalination[1], photocatalysis[2-5], photoresponse function[6], energy storage[7-10],

light-to-heat

conversion[11],

electromagnetic

shields[12,

13],

antibacterial[14], etc. Marine bacterial and biofouling has adverse impacts on marine industries and maritime activities[15], and it also poses a great threat to human health and environmental safety. Therefore, the prevention and marine anti-biofouling strategies rise the research focuses. Ti3C2Tx (MXene), as one member of MXenes, shows an antibacterial efficiency toward both Gram-negative E. coli and Gram-positive B. subtilis[16, 17]. The antimicrobial activities of Ti3C2Tx have been found to be the synergy of both “chemical” and “physical” effects. Direct physical interactions between the sharp edges of the MXene nanosheets and bacteria membrane surfaces play a crucial part in antibacterial properties of the MXene nanosheets[18]. Antibacterial activity of MXene coated membranes against common waterborne bacteria, promotes their potential application as anti-biofouling membrane in water and wastewater treatment processes[19]. MXene nanosheets have been regarded as a kind of ideal supporter for metal and metal oxide

nanoparticles owing to their large surface area and good hydrophilic properties[20-22]. Nevertheless, up to now, there still remains a great challenge in the antibacterial application. On the other hand, metal and metal oxide nanoparticles (e.g., Ag, zinc, and copper) have the antibacterial activities[23, 24]. Antibacterial activity of metal and metal oxide nanoparticles has been associated with production of reactive oxygen species (ROS) and direct contact with bacteria membrane, leading to bacterial cell death. Cuprous oxide (Cu2O), as a potential and comparatively cheap bactericide, has a wide application in antifouling field. Cu2O can effectively kill bacteria by releasing copper ions and producing ROS when contact with bacteria. However, severe photocorrosion shortens the generation time of ROS from Cu2O as an antibiofouling agent, due to the excessive accumulation of photogenerated electrons and holes inside Cu2O crystal[25]. Moreover, the released Cu2+ destroys the Cu2O semiconductor structure, and unique atom arrangement of nanocrystals dramatically causes Cu2O being unable to generate ROS[26]. Therefore, the rational addition of effective catalytic promoters into the Cu2O bactericide system is urgently required. In this work, we report a simple method to synthesize a Cu2O/MXene nanosheet while MXene as conductor and Cu2O as semi-conductor served as part of heterojunction structure, and explored its applications in antibacterial ability. MXene also promoted the separation of photoexcited charge carriers of Cu2O nanoparticles to enhance the oxidative stress reactive. Meanwhile, Cu2O/MXene exhibits a surface plasmon resonance (SPR) performance to enhance the antibacterial performance.

Cu2O/MXene nanosheets display an admirable antibacterial behaviour. 2 Experimental 2.1 Materials and Chemicals Ti3AlC2 powder (≥ 98 wt.%) was purchased by Beijing WangyiTechnology Company. Tetraethyl orthosilicate (TEOS), hexadecyltrimethylammonuun chloride (CTAC), triethanolamine (TEA), sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP), hydrazine hydrate (HHA) and lithium fluoride (LiF) were purchased from Aladdin Co., Ltd. N, N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), sodium chloride (NaCl), hydrochloric acid (HCl), sodium thiosulfate (Na2S2O3), yeast extract and ethanol were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. The water used in all the experiments was produced by a Millipore Milli-Q Plus 185 purification system and has a resistivity level higher than 18.2 MΩ cm. Phosphate-buffered saline (PBS, pH 6.0) solutions were stored at 4 °C after highpressure steam sterilization. Reactive Oxygen Species Assay Kit was purchased from Beyotime (Hainan, China). Live/dead® BacLight TM Bacterial Viability Kit (L13152) was purchased from life technologies (America). It was stored at 4 °C after highpressure steam sterilization. P. aeruginosa (ATCC27853) and S. aureus (ATCC25923) were provided by Rishui Biotech Co. Ltd (Qingdao, China). All other solvents and reagents were of analytical grade and used as received. 2.2 Synthesis of MXene 4 g of Ti3AlC2 powder was slowly put into a teflon beaker containing 50 mL of HF

solution (40%) and then stir with a teflon rotor to avoid overheating. The etching process was conducted at 35℃ under stirring for 4 h. Then the resultant product was harvested by centrifugation and washed by Milli-Q water several times until the pH of the suspension reached 6-7. At last, the resultant Ti3C2Tx powder was dried in vacuum oven at 40 °C for 24 h. In an argon (Ar) protected environment, the Ti3C2Tx powder was then placed in 10 mL of DMSO, and stirred for 18 h. The DMSO was removed by centrifugation at 4000 rpm for 5 min. The intercalated Ti3C2Tx was then dispersed in Milli-Q water (Ti3C2Tx: water = 1:300, mass ratio) and sonicated for 5 h under the protection of Ar. The MXene–water mixture was then centrifuged at 1000 rpm for 2 h, and the supernatant was collected and freeze dried to obtain the delaminated singlelayer or few-layer MXene nanosheets. 2.3 Synthesis of Cu2O/MXene In brief, 5 mg of MXene nanosheets and 0.5 g of PVP was added into 20 mL of 0.01 M copper (II) sulfate solution. Then, the mixed solution was stirred for 30 min to make sure that Cu2+ was absorbed onto MXene nanosheets. Next, 10 μL of HHA solution was added to solution, and then the whole solution was kept stirring for another 10 min at room temperature. The prepared products, defined as Cu2O/MXene nanosheets, was freeze dried for other experiments. 2.4 Characterizations Field emission scanning electron microscope (FE-SEM) images of the samples were acquired with a Sigma500 FE-SEM microscope with STEM probe (Carl Zeiss Co., Ltd). Transmission electron microscopy (TEM) images of the nanostructures were recorded

using a HT7700 TEM microscope (Hitachi Co., Ltd). The infrared spectra of synthesis products were recorded with the Thermo Scientific Nicolet iS10 spectrometer (Thermo Fisher Scientific Inc.) with a FT-IR scanning range of 4000-600 cm-1 and a resolution of 2 cm-1. The FT-IR mapping of the crevice was recorded with Nicolet iN10 FT-IR Microscope. The X-ray photoelectron spectroscopy (XPS) spectra were acquired using a spectrometer (Thermo VG Scientific ESCALAB 250 system) with a microfocus monochromatic Mg Kα Xray source (1253.6 eV). The spectra were referenced using the most intense hydrocarbon (C 1s) at 284.6 eV. XPS spectra of the samples were fitted through the XPSPEAK 4.1 software. The dried powders were characterized by X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer with Cu Ka radiation to determine the crystal structure. The photoluminescence (PL) spectra were analyzed by a fluorescence spectrophotometer (F-4600, Hitachi, Japan). The time-resolved fluorescence measurements were recorded on a DeltaPro (HORIBA Instruments. Co., Ltd). The samples loaded with fluorescent probe 2’, 7’-dichlorofluorescein diacetate (DCFH-DA) were observed with a confocal laser scanning microscope (Carl Zeiss Co., Ltd). Zeta potential was taken with a Zetasizer3000HS laser particle size analyzer (Malvern Panalytical Ltd). The samples of MXene, Cu2O, and Cu2O/MXene were respectively dispersed in Milli-Q water to prepare 0.01 mg/mL solution. 3 mL of each sample was taken by pipette to analyze the zeta potential. The visible light source for photoelectrochemical measurements was a 250 W lamp with emissions in the range of 390–800 nm (Beijing perfectlight technology Co. Ltd). In order to avoid the influence of strong light to bacteria, visible light for the antibacterial experiments was offered by

four T5 energy-saving lamps purchased from Zhejiang Yankon Group Co., Ltd. The wavelength of light ranges from 400 to 700 nm. Microbial cultivation was controlled for 12 hours under the direct light every day. Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES) was used to measure copper content which was carried out on ICAP-6300 Plasma emission spectrometer (Thermo Scientic Co., Ltd). The photocurrent-time curves were tested by the standard three-electrode configuration in a potentiostat PGSTAT302N Metrohm Autolab electrochemical station (Utrecht, Netherlands). The working electrode was prepared by the following method: a slurry containing 10 mg of sample, 10 mL of water and 50 μL of nafion was coated on an ITO substrate with an area of 1 cm2. The working electrode was dried at 60 °C for 2 h to achieve tight adhesion. A saturated calomel electrode (SCE) and a platinum sheet were used as reference electrode and counter electrode, respectively. The photocurrent response was measured in Na2S2O3 solution (0.1 M) under the irradiation of visible light. Electrochemical Impedance Spectroscopy (EIS) measurements were conducted in a 0.1 M NaOH solution at a steady-state open circuit potential disturbed with 10 mV of perturbation. Cyclic Voltammetry (CV) tests were measured at 10 mV/s scan rate within a voltage range of −0.4 to 0.4 V (vs. SCE) in a 0.1 M NaOH solution. 2.5 ROS determination Intracellular ROS generation was detected by reactive oxygen species assay kit. Before experiments, fluid medium was prepared by 1 g of mixing peptone, 0.5 g of yeast extract, 0.5 g of NaCl with 100 mL Milli-Q water. Then, the fluorescent probe DCFH-DA was diluted 104 times with fluid medium. MXene, Cu2O, mixture (mass

ratio of Cu2O and MXene =43:57) and Cu2O/MXene powders were immersed in PBS for 0 day (the samples were not stored and used immediately) and 3 days to investigate the production of ROS in bacterial cell. 50 μL of activated bacteria was distributed into 120 mL of fluid medium containing 4.5 mg three kinds of prepared samples which were stored for 0 day (the samples were not stored and used immediately) and 3 days, respectively. All the samples were shaken one minute to promote the production of ROS before loaded with 10 μM fluorescent probe DCFH-DA. After diffusing into the cells, DCFH-DA can be easily deacetylated by esterase to gain non-fluorescent 2’,7’dichlorofluorescein (DCFH). When DCFH meats with ROS, the formation of fluorescent product DCF could be detected by a confocal laser scanning microscope after 40 min of incubation at 37 oC. 2.6 Antibacterial property of Cu2O/MXene nanosheets The plate colony counting method was used to investigate the antimicrobial ability of MXene and Cu2O/MXene. S. aureus and P. Aeruginosa were chosen as the representative of Gram-positive and Gram-negative bacteria. All stuffs used in the antibacterial activity test needed be sterilized in an autoclave and the experimental operations were conducted in the super clean bench. Briefly, 50 μL strains of bacteria were transferred to 50 mL of fluid medium and incubated in a shaking incubator at 37 °C at a rotating speed of 100 rpm for 20 h. Then, the activated bacteria was distributed into 50 mL of fluid medium containing 2 mg of MXene nanosheets, 2 mg of Cu2O, 2 mg of mixture, 2 mg of Cu2O/MXene nanosheets and nothing as blank sample respectively for another 20 hours cultivation. All kinds of bacterium solutions

were diluted to 104 times and 20 μL diluted MXene, Cu2O, mixture, Cu2O/MXene and blank solutions were uniformly layered over Luria-Bertani (LB) agar plates. The antibacterial activities of these materials could be observed through the number of colony after a period of time. Moreover, the bacterial suspension was chosen as representative and was dyed by a mixture solution of PI (i.e., red fluorescent dye for dead bacteria) and SYTO9 (i.e., green fluorescent dye for live bacteria). The color of bacteria was observed from a fluorescence microscope to distinguish the live and dead bacteria. All tests were repeated at least 3 times. 2.7 Calculations Density functional theory (DFT) calculations were carried out by the CASTEP simulation module of Material Studio software. Generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional were used. The electron–ion interaction was simulated using the ultrasoft pseudopotential method. The CASTEP software package requires that the calculation system must be periodic. At the same time, the calculation requires that our structure has a vacuum layer large enough to accurately determine the energy level at the vacuum. Therefore, according to the calculation method of work function, we added a vacuum layer so that the total length in the Z direction is 40 Å to establish a periodic unit cell. BFGS optimization algorithm was used for geometric optimization. By convergence test, the cut-off energy was set to 400 eV, and the k-point of the Brillouin zone was set to 5 × 5 × 1 to meet the geometric relationship of reciprocity space. A 2D electrodynamics simulations were established to solve the electric field on the

surface of Cu2O/MXene. Finite element method (FEM) calculation was used to perform computational process by COMSOL software. For the geometrical model, the diameter of Cu2O is set as 100 nm. The geometrical size of the MXene is set as a rectangle with 400 nm × 4 nm. Then, the geometrical models were divided by triangle meshes. 3. Results and Discussions 3.1 Morphology and structure MXene

was

synthesized

by

immersing

Ti3AlC2

powders

slowly

in

a

polytetrafluoroethylene beaker contained an aqueous solution of 40 wt.% HF. Then, single-layer MXene nanosheet was prepared simply via sonication intercalation from multi-layer MXenes. The Cu2O/MXene nanocomposites here were synthesized via copper (II) reduction as schematically presented in Figure 1.

Figure 1 Schematic illustration of the synthesis process of Cu2O/MXene.

Figure 2a shows the accordion-like structural evolution from Ti3AlC2 to Ti3C2Tx, demonstrating clearly the extraction of the Al from Ti3AlC2 and formation of 2D MXene in Figure 2b, afterwards the intercalation of HCl in MXene layers. High magnification of a single-layer MXene is characterized by SEM (Figure 2c), exhibiting that there is a smooth surface of a single-layer MXene after the chemical etching of its

Ti3AlC2 precursor. While, TEM image in Figure 2d shows single-layer or few-layer Cu2O/MXene nanosheet with a wrinkled surface. Figure 2e shows a single-layer Cu2O/MXene nanosheet where Cu2O nanospheres uniformly grow up on the surface of the MXene. Figures S1a and S1b show the same area of a few-layer Cu2O/MXene nanosheet characterized by SEM and STEM, respectively. Moreover, Figure 2f shows AFM morphology of MXene nanosheet. Obviously, the heights of the region are marked by white line distribute uniformly in the range of about 2.7−3.3 nm, corresponding to monolayer Ti3C2TX is about 2.7 nm[27]. The increased height is likely due to the presence of surface adsorbates, such as water molecules, that are trapped under the MXene flake[28, 29]. AFM test in Figure 2g characterizes the Cu2O (diameter: ~95 nm) homogeneously distributes on the surface of MXene nanosheet whose morphology has an agreement with Figure 2e. Then, elemental mappings (Figures 2h2l) show the disperse distribution of O and Cu elements in Cu2O/MXene, demonstrating the existence morphology of Cu2O nanosphere and Cu2O nanospheres successively anchored on the surface of MXene nanosheet. Likewise, homogeneous distributions of Ti and C elements demonstrate that MXene nanosheet was the carrier for Cu2O nanospheres. The ratio of Cu2O and MXene in Cu2O/MXene is about 43:57 which was characterized by ICP.

Figure 2 FE-SEM images of (a) cross section of multi-layer MXene, (b) lowmagnification of single-layer MXene, (c) high-magnification of single-layer MXene and (e) Cu2O/MXene; (d) TEM image of single-layer MXene; AFM mappings of MXene nanosheet (f) and Cu2O/MXene (g); Element mapping of (h) TEM, (i) O, (j) Cu, (k) Ti, (l) C.

Figure 3 Zeta potentials (a) and size distributions (b) of MXene, Cu2O and Cu2O/MXene.

Particle size distributions of microcapsules were measured via the laser diffraction method. The D50 parameters of Cu2O, MXene, Cu2O/MXene as shown in Figure 3a are 46 nm, 91 nm, and 79 nm, respectively. Zeta potential data in Figure 3b suggests that MXene possessed strong negative charge of -43.23 mV that made MXene highly stable in aqueous solution. Thus, in the synthesis process of Cu2O/MXene, the surface of MXene is negatively charged that can easily absorb the Cu2+ cation by electrostatic interaction. As Cu2+ cation was reduced to Cu+ valence as Cu2O in HHA solution, Cu2O nanospheres firmly grow on the surface of MXene. In addition, Zeta potential of singlelayer Cu2O/MXene nanosheets shows -3.97 mV. While, Cu2O nanoparticles exhibited opposite charge of 15.54 mV. These results demonstrated that single-layer or few layer Cu2O/MXene nanosheets tended to be electrically neutral after chemical bonding of Cu2O and MXene.

Figure 4 (a) XRD spectrum of Cu2O, MXene and Cu2O/MXene; (b) FT-IR spectrum of Cu2O, MXene, and Cu2O/MXene.

XRD spectrum of the Cu2O nanospheres in Figure 4a shows the diffraction peaks corresponding to (110), (111), (200), (220), (311) and (222) ate that located at 29.55°,

36.42°, 42.30°, 61.34°, 73.53° and 77.32°, respectively (JCPDS Card no. 05-0667). MXene XRD spectrum shows the diffraction peaks corresponding to (002), (004), (006), (008), and (110) ate that located at Ti3C2(OH)2 and TiC[30, 31]. And Cu2O/MXene powders showed the typical peaks 9.10°, 36.42°, 42.30° which corresponded to (002), (111) and (200) ate of Cu2O, (002) ate of MXene, respectively, implying Cu2O grew on the surface of single-layer MXene. The intensity of Cu2O reflections is enhanced as the Cu2O loading on the MXene is increased. The chemical changes in the MXene nanosheets were investigated via FT-IR in Figure 4b. The characteristic absorption peaks of Cu2O are assigned at 648 cm-1, 1292 cm-1 and 1659 cm-1. The chemical bonds of MXene show C-O (1420 cm-1), C=O (1568 cm-1), C-H (2957 cm-1), and -OH (3405 cm-1)[32]. Comparing with the FT-IR analysis of Cu2O, MXene and Cu2O/MXene, it demonstrates that the surface of Cu2O/MXene becomes terminated with Cu2O (1292 cm-1 and 1659 cm-1), Cu-O-H (743 cm-1), C-H (2957 cm-1), -OH (3405 cm-1), C-O (1420 cm-1), C=O (1568 cm-1) and other similar groups that form in the aqueous environment during synthesis. Thus, it could be spectroscopically proved that Cu2O/MXene was responsible for the successful synthesis.

Figure 5 High-resolution XPS spectrum of Cu 2p for Cu2O (a) and Cu2O/MXene (b); O 1s for MXene (c) and Cu2O/MXene (d); Ti 2p for MXene (e) and Cu2O/MXene (f), respectively. The presence of Cu2O functional groups in MXene surfaces is also further confirmed by X-ray photoelectron spectroscopy (XPS). As shown in Figures 5a and 5b, two dominant peaks at 932.2 eV and 952.1 eV correspond to Cu+ 2p3/2 and Cu+ 2p1/2, respectively. In addition, there are two peaks at 934.4 eV and 954.2 eV, which correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively. It indicated that the content of Cu2+ slightly increased in the synthesis process of Cu2O/MXene. For the Cu2O in Figure S2, the small peak centered at 530.9 eV may be attributed at oxygen in the CuO lattice, while the peak at 532 eV corresponds to lattice oxygen O2- and -OH groups in the Cu2O phase[33]. The O 2p XPS spectra of MXene (Figure 5c) unfold O species of intensive signals at 533.5 eV (H2O water molecules associated with -OH terminal groups), 532.6 eV (Al2O3), 531.7 eV (C-Ti-(OH)x), 530.4 eV (C-Ti-Ox), 529.7 eV (TiO2)[34]. Moreover, the O 2p XPS spectra of Cu2O/MXene in Figure 5d could be deconvoluted into five individual component peaks, which originated from different groups and

overlapped one another. The peaks at binding energies of 532.3, 531.7, 531.2, 530.5 and 529.5 eV were assigned to Cu2O, C-Ti-(OH)x, Cu-OH, C-Ti-Ox, and TiO2. The XPS survey spectra shown in Figures 5e and 5f reveal the presence of chemical changes in oxygen and titanium functional groups in the Cu2O/MXene synthesis process. Eight characteristic peaks of Cu2O/MXene corresponding to the Ti 2p1/2 464.68 eV (C-Ti-Fx), 463.78 eV (Ti3+), 462.78 eV (Ti2+), 460.98 eV (Ti-C) and

Ti 2p3/2 458.98 eV (C-Ti-

Fx), 458.28 eV (Ti3+), 457.63 eV (Ti2+), 455.58eV (Ti-C)[34] were observed in Figure 5f. The Ti 2p1/2 peak explicitly shifted from 463.1 eV (C-Ti-Fx) to 464.68 eV (C-Ti-Fx) due to the addition of Cu2O (Figure 5f) as nanospheres in combination with the surface of MXene, thereby implying the formation of a group (Cu-Ti) network between the Cu2O and MXene. Thus, two main peaks of Ti 2p1/2 and Ti 2p3/2 in Cu2O/MXene nanosheets exhibited a positive shift compared to that in MXene, probably suggesting that electrons might transfer from MXene to Cu2O, and there was strong interaction between Cu2O and MXene nanosheet. 3.2 Antibacterial Performance. The antibacterial rate of fresh Ti3C2Tx MXene membranes reaches more than 73% against B. subtilis and 67% against E. coli[19]. And broth microdilution assay[18] was applied to examine inhibitory properties of Ti3C2Tx MXene nanosheets against the growth of B. subtilis and E. coli. A 3-hour treatment of E. coli and B. subtilis with 0.09, 0.35, 0.57, and 4.40 μm nanosheets resulted in the dispersion of ∼50%, 40%, 30%, and 20% of the bacteria populations, respectively. It showed that smaller lateral sizes of the Ti3C2Tx nanosheets led to more damage to the bacteria envelope. To improve the

activity and stability of Cu2O photocatalyst under visible light, a kind of hyper-crosslinked polymer, named as KAPs-B (Knitting Aromatic compound Polymers-Benzene) with high surface area and special benzene rings structure, was loaded on Cu2O to form a composite of KAPs-B/Cu2O[35]. The colony counting method and the fluorescent live/dead stain method was used to measure the antibacterial properties of the MXene, Cu2O, mixture, and Cu2O/MXene nano materials against S. aureus and P. aeruginosa further. The S. aureus and P. aeruginosa bacteria suspension were incubated with the same volume of sterilized physiological saline solution with blank, MXene, Cu2O, mixture, and Cu2O/MXene at 37 °C for 24 h respectively and then were stained with the BacLight live/dead kit assay. The red and green dots represent dead and living bacteria, respectively[36]. MXene against S. aureus and P. aeruginosa exhibits a weak antibacterial performance as shown in Figures 6a2 and 6c2. Direct physical interactions between the sharp edges of the MXene nanosheets and bacteria membrane promote antibacterial properties of the nanosheets[18]. MXene sharp edges damage the bacterial cell wall, resulting in the release of bacteria DNA from the cytosol. In Figures 6a3 and 6c3, Cu2O shows a better antibacterial ability against S. aureus and P. aeruginosa bacteria. Copper(I) and copper(II) ions release from Cu2O exhibit toxicity to bacteria by denaturing the bacterial DNA[37]. Mixture of Cu2O and MXene in Figures 6a4 and 6c4 also shows an antibacterial ability against S. aureus and P. aeruginosa bacteria. While, there are little survival of bacterial colonies in Figures 6a5 and 6c5. As shown in Figures 6b1-b5 and 6d1-d5, the fluorescence images of the Cu2O/MXene are red dots; while other samples

show green dots interspersed with red dots. Therefore, the Cu2O/MXene exhibit a better antibacterial ability than MXene, Cu2O and mixture. The excellent antibacterial property attributed to the synergistic effect between antibacterial ability of Cu2O and bacteriostatic action of MXene.

Figure 6 Antibacterial activities in aqueous suspensions after 3 d: bacterial suspensions in deionized water without any material were used as control (a1, c1). Photographs of agar plates onto which S. aureus and P. aeruginosa bacterial cells were recultivated after treatment with MXene (a2, c2), Cu2O (a3, c3), mixture (a4, c4), Cu2O/MXene (a5, c5), respectively. Fluorescent live/dead stain images of blank (b1, d1), MXene (b2, d2), Cu2O (b3, d3), mixture (b4, d4), Cu2O/MXene (b5, d5) dialysates for S. aureus and P. aeruginosa, respectively.

Figure 7 The conlony number (a) and bacteriostasis rates (b) of MXene, Cu2O, mixture, and Cu2O/MXene against P. aeruginosa and S. aureus, respectively.

In addition, photogenerated ROS from the Cu2O can damage cell membranes and eventually cause cell death by oxidative stress[38]. The number of bacterial colonies added with Cu2O/MXene decreased much more than the introduction of MXene, Cu2O, and mixture in Figure 7a. This suggested Cu2O/MXene nanosheets possessed a better bactericidal property. The bacteriostasis efficiencies of the MXene, Cu2O, mixture, and Cu2O/MXene against S. aureus and P. aeruginosa are calculated by an equation[39]: Bacteriostasis=(A-B)/A×100(%)

(1),

where A is the count of bacteria of the control group, and B is the count of bacteria of the treatment group. Compared to the blank control group, MXene and Cu2O exhibit an antibacterial performance with bacteriostasis efficiencies against P. aeruginosa (41.67%, 84.30%) and S. aureus (55.56%, 84.69%) in Figure 7b, which was compared with bacteriostasis efficiencies of Cu2O against S. aureus (77.86%) and P. aeruginosa (83.49%)[40]. While, in our pervious study, we showed the bacteriostasis efficiencies

of Cu2O against E. coli and S. aureus[25] were 79.85% and 79.13%, respectively. The bacteriostasis efficiencies of the mechanical mixture against P. aeruginosa and S. aureus are 83.21% and 81.56%, respectively. Furthermore, the bacteriostasis efficiencies of Cu2O/MXene against P. aeruginosa and S. aureus are up to 97.04% and 95.59%, respectively, displaying an admirable antibacterial behavior. Thus, the antibacterial activity of the mechanically mixed Cu2O and MXene were lower than that of Cu2O/MXene. It demonstrates that MXene nanosheets significantly enhance the antibacterial ability of Cu2O nanospheres. PL spectroscopy is a feasible tool to express photoexcited carrier separation and recombination efficiency of photocatalysts, as PL intensity deriving from the recombination of photogenerated electrons and holes. Thus, PL spectra were used to obtain the optical properties of Cu2O/MXene nanosheets to analysis the ROS antibacterial mechanism. Figure 8a displays the PL spectra of Cu2O and Cu2O/MXene under 325 nm excitation. The emission peak intensity of the Cu2O/MXene was obviously decreased in comparison with that of Cu2O. The results indicated that the incorporation of MXene and Cu2O can effectively prevent the recombination of electron-hole pairs of Cu2O and present a high photocatalytic disinfection. This lower carrier recombination rate may be due to the heterojunction formed at the Cu2O/MXene semiconductor interface which acts as an electron sink to prevent the electron-hole recombination. The time-resolved PL decay spectra also confirmed the improvement of charge separation from Cu2O/MXene, as obviously slower exponential decay kinetics was observed in Figure 8b. A bi-exponential

function fitting was employed to analyze the PL decay curves[41], and the average lifetimes were calculated (Table S1) for the photo catalysts. It was found that the average PL lifetime of Cu2O/MXene (0.65 ns) was distinctly enhanced as compared to that of Cu2O (0.20 ns). The longer lifetime was attributed to the improved photocarrier transport rate resulted from the decreased electronic transmission resistance in Cu2O/MXene. The photocurrent responses of Cu2O and Cu2O/MXene nanosheet (Figure 8c) were prompted by several on−off cycles of light. The photocurrent intensity of Cu2O/MXene (24.21 μA/cm2) is approximately 2.1 times as high as that of pristine Cu2O (11.37 μA/cm2), which can be ascribed to MXene as a catalyst more effectively capturing photoexcited electrons from Cu2O. In addition, Cu2O/MXene holds the highest separation efficiency of photoinduced electron-hole pairs, which was attributed to the excellent electron conductivity of MXene. The results of transient photocurrent measurements were consistent with the PL results. It is generally known that a typical smaller EIS arc radius reflects a lower charge transfer resistance and higher charge transfer efficiency[42]. As shown in Figure 8d, the arc radius of the EIS spectra of Cu2O is significantly larger than that of Cu2O/MXene. Charge transfer resistance of Cu2O is 1318 ohm which is 12 times more than that of Cu2O/MXene as 109 ohm. This result indicated that the presence of MXene in Cu2O/MXene sharply reduced the charge transfer resistance, corresponding to improved photocatalytic activity. Finally, Cu2O/MXene caused a higher carrier separation efficiency.

Figure 8 (a) PL spectra of Cu2O and Cu2O/MXene at the excitation wavelength of 325 nm. (b) time-resolved transient PL decay, (c) photocurrent responses and (d) EIS of Cu2O and Cu2O/MXene. (e) 500 CV cycles of Cu2O/MXene in 0.1 M NaOH solution. (f) polarization curves for the Cu2O/MXene before and after 500 CV cycles.

Electrochemical experiments could be operated to prove the stability and the advantage of heterostructure. Long-term stability is another important criterion as a photocatalyst. After continuous operation of 500 potential cycles from -0.4 V to 0.4 V (vs. SCE) in Figure 8e, the polarization curve of Cu2O/MXene exhibits negligible decay on current while compared with the initial one, as shown in Fig. 8e, manifesting excellent cycling stability under the operating conditions. The polarization curves were obtained by scanning the working electrode from -100 mV to +100 mV versus open circuit potential (OCP) with a rate of 0.5 mV/s. The experimental polarisation curves of initial and 500 CV cycles of Cu2O/MXene samples showed a reasonable linear Tafel region in both anodic and cathodic branches in Figure 8f. The corrosion current

densities (at OCP) of initial and 500 CV cycles of Cu2O/MXene samples were 1.36× 10-7 and 1.57×10-7A/cm2 which were in the same order of magnitude, manifesting an excellent cycling stability under the operating conditions. SEM morphologies of bacteria as shown in Figure 9 were to determine the antibacterial mechanism of the Cu2O/MXene. P. aeruginosa and S. aureus showed rodshaped (Figure 9a) and sphere-shaped (Figure 9b), respectively. Comparing with the control group, both S. aureus (Figure 9c) and P. aeruginosa (Figure 9d) displayed crumpled and breakage appearance treated by Cu2O/MXene. And some of them were even broken. The incubated lost their integrity, and the bacteria surface was apparently dissolved which suggests that Cu2O/MXene killed the S. aureus and P. aeruginosa by a mechanism of bacteriolysis. The death bacteria were gathered by Cu2O/MXene. This aggregation phenomenon for antibacterial is like the gathering role of lectin attacking cell walls[43, 44].

Figure 9 SEM images of the morphologies of P. aeruginosa control (a) and treated (b) by Cu2O/MXene and S. aureus control (c) and treated (d) by Cu2O/MXene, respectively.

3.3 Measurements of ROS. Cu2O/MXene induced by sunlight could generate ROS that can damage cell membranes and eventually cause cell death by oxidative stress. Figures 10a1-b2 show that the Cu2O/MXene generated abundant ROS, much more than that produced by Cu2O. Thus, the separation efficiency of photoinduced charges of Cu2O was significantly improved after Cu2O combining with MXene nanosheets forming a heterojunction structure.

Figure 10 ROS generation of 87.5 μg/mL Cu2O/MXene to treat P. aeruginosa (a1, a2) and S. aureus (b1, b2) after storage for 0 and 3 days, respectively. (c) Schematic of the charge separation process and ROS antibacterial mechanism of Cu2O/MXene.

The ROS antibacterial mechanism about Cu2O/MXene nanosheets was exhibited in Figure 10c. The photoinduced electron preferentially transferred from Cu2O to MXene, which could effectively suppress the recombination of electron-hole pairs, because the Fermi potential level of MXene (0.71 V vs NHE, pH=7)[45] was significantly lower than the conduction band of Cu2O (-0.703 V vs NHE, pH=7)[46]. Furthermore, MXene

accepts electrons from Cu2O, leading to abundant electrons on the surface of MXene, which provide better charge transfer between bacteria and Cu2O/MXene nanosheets. The accumulated charges of intracellular ROS, including hydrogen peroxide (H2O2), superoxide anions (•O2−) or hydroxyl radicals (•OH), damage the cell membrane leading to the cell death. 3.4 DFT and FEM calculations

Figure 11 DFT calculated result: electrostatic potentials of (a) MXene, (b) MXene (O-), (c) MXene (F), (d) MXene (-OH), and (e) Cu2O/MXene. The blue and red dashed lines denote the vacuum energy level and Fermi level, respectively.

Density functional theory (DFT) calculations were used to investigate the mechanism of photocatalysis and ROS generation. The work function of the surface is an important parameter with which to investigate the interface charge transfer. Herein, the work functions of the studied heterostructures were calculated according to the following

equation: Wf=Evacuum-Ef

(2)

where Evacuum and Ef represent the energy of a stationary electron in the vacuum near the surface and the Fermi energy, respectively. In Figure 11, the work functions of MXene, MXene (-O-), MXene (F), and MXene (-OH) were computed to be 4.41 eV, 5.72 eV, 5.22 eV, and 5.47 eV, respectively. It is generally believed that Ti3C2 MXene is unstable and easily oxidized in oxygen-containing aqueous solution[47]. As zeta potential data of MXene and Cu2O are opposite in Figure 3b, Cu2O nanoparticles can be firmly anchored on the surface of MXene via the electrostatic effect in the preparation of Cu2O/MXene. After the formation of Cu2O/MXene, the MXene modified by Cu2O becomes stable. In Figure 12, work function of Cu2O/MXene is 4.10 eV which is apparently lower than that of MXene, MXene (-O-), MXene (-F), and MXene (-OH) respectively, indicating that Cu2O replaces the functional groups (-O-, F and -OH) on the surface of MXene and MXene enhanced the stability and photocatalytic ability of Cu2O.

Figure 12 DFT calculated results DOS of MXene (a) and Cu2O/MXene (b)

In order to investigate the interfacial electronic structures of the heterostructures, the Density of state (DOS) was calculated and is depicted in Figure 12 for all materials. DOS diagram of isolated MXene in Figure 12a shows that Ti 3d orbit contributes more electrons than C 2p in the MXene. Moreover, numerous electronic states cross the Fermi level, indicating the excellent conductivity of MXene, which is beneficial for the electron transport[45]. While, Cu2O/MXene show that more electrons of DOS pass through Femi level in Figure 12b because of the existance of Cu 3d and Ti 3d orbits, indicating that MXene from the structure of Cu2O/MXene accelerates photoelectron transportation. DOS calculations demonstrate that the combination of Cu2O and MXene enhances the transportation of photoelectron from Cu2O to MXene. Thus, the Cu2O anchor on MXene surface plays an important role in determining the transport properties of Ti3C2 MXene. Moreover, the results show that the energy bands of MXene effectively insert into the energy bands of Cu2O, resulting in the reduced band gaps for Cu2O.

Figure 13 FEM simulated SPR electric field |E| distributions (wavelength of λ=310

nm) of Cu2O/MXene in solution with light direction of 0o (a), 45o (b), and 90o (c), respectively. And |E| distributions (wavelength of λ=310 nm) of two Cu2O nanosphere without MXene in solution with light direction of 0o (d), 45o (e), and 90o (f), respectively.

SPR phenomenon exists on the surface of Cu2O/MXene. Finite element method (FEM) calculation was used to solve the light vector wave equation for the timeharmonic electric field derived from Maxwell’s equations: ∇×

(

𝟏

)

∇ × 𝑬 ― 𝒌𝟐𝟎𝜺𝒓𝑬 = 𝟎 𝝁𝒓

Where μr and εr are the relative permeability and permittivity, E=(Ex, Ey) is the 2D electric field, and k0=2π/λ is the incident wave vector with λ the wavelength of the incident plane wave. Electric field |E| distributions of Cu2O/MXene (two Cu2O nanoparticles) in solution with light direction of 0o, 45o, and 90o at 310 nm were simulated in Figure 13a–13c, respectively, indicating that Cu2O/MXene causes optical diffraction and resonance |E| distributions influenced by light direction. While, Figure 13d–13f show |E| distributions of two Cu2O nanospheres without MXene in solution with visible light direction of 0o (Figure 13d), 45o (Figure 13e), and 90o (Figure 13f), respectively. Compared to Cu2O/MXene in Figures 13a–13c, the red arrows in Figures 13d–13f marked the major different areas of |E| distributions of two Cu2O nanospheres without MXene. It proved that MXene significantly influenced the propagation routes of light wave which changed the diffraction morphology around the Cu2O/MXene and Cu2O. MXene

nanosheet of Cu2O/MXene has large surface that could add the propagation routes of light wave and dramatically enhance the reflex probability of light wave among the Cu2O/MXene nanosheets. In addition, |E| distributions of nano materials vary with visible light of different directions. Thus, geometric configuration of nano materials is one of the most factors influenced the SPR phenomenon. We have considered three different geometries depending on the number and location of Cu2O. Figure 14a reveals that SPR |E| value of some area on the surface of one Cu2O nanoparticle and between Cu2O and MXene reached 2.2 V/m at 310 nm and 360 nm with different light directions. For one Cu2O anchored MXene nanosheet, the interface between MXene and Cu2O with 0o light direction shows a weak SPR. However, the interface between MXene and Cu2O exhibits an apparent SPR enhancement for 45o and 90o light direction, respectively. These results proved that light direction was a main factor affecting the SPR in the interface of between MXene and Cu2O. Figures 14b and 14c show SPR |E| distributions of two connected Cu2O nanoparticles and two separated Cu2O nanoparticles, respectively. FEM calculation demonstrates the SPR properties on the surface of Cu2O/MXene that significantly enhance the separation of electron and hole of Cu2O as heterojunction structure. SPR |E| on the surface of two Cu2O nanoparticles has a symmetric distribution with 90o light direction. The surface area surrounded by two Cu2O nanoparticles and MXene is a relatively closed area that could provide more opportunities for reflection and scattering of light waves. Thus, the surrounded area has a large probability of SPR effect. Furthermore, SPR process accelerates the electron generation and increases the number of free electrons.

Free electrons in Cu2O are excited to higher energy states due to SPR, then those electrons transfer from Cu2O to the MXene through the charge transfer channel between Cu2O to the MXene, when the Cu2O/ MXene is illuminated under visible light.

Figure 14 Simulated electric field |E| distributions of a MXene nanosheet with one Cu2O nanoparticle (a), two connected Cu2O nanoparticles (b), and two separated Cu2O nanoparticles (c) with different light directions and wavelengths. The above results demonstrated that the antibacterial effect of the Cu2O/MXene nanosheets was effectively enhanced by MXene. The Schematic of enhancement mechanism of synergistic antibacterial ability of Cu2O/MXene is shown in Figure 15. Firstly, MXene could improve the separation efficiency of electron-hole pairs of Cu2O to produce more ROS to kill bacteria (Figure 15a). Secondly, there exists SPR

phenomenon in some local area on the surface of Cu2O/MXene nanosheets, which could effectively enhance the electric field |E| to generate ROS to inhibit microbial growth (Figure 15b). Thirdly, Cu2O/MXene nanosheets possessed a larger contact area with bacteria consequently, which was more conducive to kill bacteria. And copper ion (Cu2+) released from Cu2O/MXene could effectively destroy cell wall of bacteria to increase inhibition of bacterial propagation (Figure 15c). Fourthly, the ‘‘blade like edges” of Cu2O/MXene nanosheets[18], like other 2-D layer nano structures, could destroy the bacterial cells (Figure 15d), making the copper ions react with cytoplasmic constituents more conveniently, and finally inactivate the bacteria. Thus, Cu2O/MXene has a synergistic effect for dramatic enhancement of antibacterial ability.

Figure 15 Schematic of enhancement mechanism of synergistic antibacterial ability of Cu2O/MXene.

4 Conclusions In summary, we provided a novel facile strategy to design stable Cu2O anchored

MXene nanosheets utilizing special electronic transition between Cu2O and MXene, presenting formidable antibacterial capacity. The antibacterial tests demonstrated Cu2O/MXene nanosheets against S. aureus and P. aeruginosa presented the extraordinary antibacterial ability. In addition, the results illustrated that the synergistic mechanism of Cu2O/MXene included MXene accelerating photoelectron transportation, Cu2O antimicrobial and photocatalysis, ROS production, SPR mechanism, physical ‘‘blade like edges” of nanosheets, and Cu2O uniform dispersion of MXene nanosheets. Supporting Information Images of FE-SEM and STEM images of Cu2O/MXene nanosheet, high-resolution XPS spectrum of O element for Cu2O, fitted parameters of PL decay curves are provided in Supporting Information. Acknowledgements This work was supported by the National Natural Science Foundation (51572249), National Natural Science Foundation Joint Fund (U1806223), the Fundamental Research Funds for the Central Universities (201965009, 201964009, 841562011, 19CX05007A), and Science Foundation for Young Scholars of the Tobacco Research Institute of the Chinese Academy of Agricultural Sciences (2018B04). References [1] B. Anasori, M.R. Lukatskaya, Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage, Nat. Rev. Mater., 2 (2017) 16098. [2] H. Chang, Z. Shang, Q. Kong, P. Liu, J. Liu, H.a. Luo, α-Fe2O3 nanorods

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Graphical Abstract A photo catalyst of cuprous oxide anchored MXene nanosheet for dramatic enhancement of synergistic antibacterial ability Wei Wang,*a Huimeng Feng,a Jianguo Liu,b Mutian Zhang,a Shuan Liu,c Chao Feng,d and Shougang Chen*a a.

School of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, China.

b.

Shandong Key Laboratory of Oil & Gas Storage and Transportation Safety, China University of Petroleum (East China), Qingdao, 266580, China.

c.

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key

Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technologies and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. d. Key Laboratory of Tobacco Pest Monitoring & Integrated Management, Tobacco Research Institute, Chinese Academy of Agricultural Sciences, Qingdao 266101, China.

Synergistic effect of MXene accelerating photoelectron transportation, Cu2O antimicrobial and photocatalysis, ROS, surface plasmon resonance and uniform dispersion of MXene.

Highlights 1. A Cu2O anchored MXene nanosheet as heterojunction structure was synthesized. 2. Cu2O uniformly anchored on the surface of MXene due to electrostatic effect. 3. Cu2O/MXene nanosheets display an admirable synergistic antibacterial behavior. 4. Cu2O/MXene shows an antibacterial effect of photocatalysis, ROS and SPR.