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Role of Electric Field and Reactive Oxygen Species in Enhancing Antibacterial Activity: A Case Study of 3D Cu Foam Electrode with Branched CuO−ZnO NWs Caiyu Wang,†,§ Longfei Yue,†,§ Shuting Wang,† Yanan Pu,† Xiao Zhang,‡ Xiangping Hao,† Wenhui Wang,† and Shougang Chen*,† †

College of Materials Science and Engineering, Ocean University of China, Qingdao 266100, People’s Republic of China College of Chemical Molecule Engineering, Qingdao University Science and Technology, Qingdao 266042, People’s Republic of China

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 11/13/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Water is critical to public health. An inexpensive, fast, and effective water treatment has the potential to significantly impact human life. In this work, we reported a novel device for highly effective water disinfection. Branched CuO−ZnO nanowires (NWs) on a foam copper substrate were prepared, which possessed excellent antibacterial performance and good stability. The nanostructured assembly achieved a large electric field concentration and improved production of reactive oxygen species (ROS) to inactivate bacteria at low voltages through electroporation and oxidative stress. The nanostructure materials showed high antibacterial activity against Escherichia coli and Staphylococcus aureus at a high concentration (106 CFU mL−1) under an external voltage of less than 10 V and a hydraulic retention time of less than 2 s. Moreover, the application of electric field favored the production of ROS. The structure containing branched CuO−ZnO NWs possessed a better disinfection ability than that with CuO NWs. modified CNT-coated cotton textile15 and copper oxide NWs (CuONW)-modified copper mesh16 for the same purpose. Huo et al. developed a copper oxide nanowire (CuONWs)-modified three-dimensional (3D) copper foam electrode to assemble filtering equipment,17 and superior disinfection performance was achieved at low operation voltages and increased water− CuONWs contact times (>7 s). It is thus seen that a voltage up to 10 V or a proper treating time is required to achieve good bacteria removal. As reported, the main mechanism for bacterial inactivation is electroporation. The reactive oxygen species (ROS) would be generated under the electric filed.18−21 Work has been conducted to develop antibacterial materials by photoinduced ROS to kill bacteria.22−26 These included some semiconductors such as CuO,27 ZnO,28 and TiO2.29 Therefore, both electroporation and oxidative species play significant roles in sterilization. When extra voltage is applied to the filter electrode, a strong electric field is produced around the nanowires due to the special linear nanostructure. Moreover, ROS can produce in the bacteria due to the interference of extracellular electrons.30−33 The bacteria are killed due to the combined action of

1. INTRODUCTION Water resources become polluted due to increased industrialization, urbanization, and human activities.1−4 Generally, chlorination, ultraviolet (UV), and ozone disinfection are commonly used for disinfection of drinking and wastewater,5−7 whereas chlorine-based disinfectants produce undesirable by-products, such as trihalomethanes, chloral hydrate (CH), chloropicrin, cyanogen chloride, etc., that are toxic and potentially carcinogenic to human health, ozone disinfection requires the unsafe and costly operation of equipment. UV disinfection technology is limited due to high-cost and nondurable performance.8−11 Demand for clean water has stimulated efforts to develop advanced water treatment technologies. Electric field sterilization has been paid much attention in disinfection due to high-efficiency in both cancer therapy12 and water treatment.13 In the past few years, it has been demonstrated that electroporation-based disinfection shows a high-efficiency in water disinfection. The Cui group prepared Ag nanowire (NW)-modified carbon nanotubes (CNTs), which were coated on polyurethane sponges as a conducting filter for water disinfection under 10 V of external voltage. The CNTcoated sponge served as a macroscale porous conducting matrix, and the Ag NWs provided numerous sharp nanoscale tips to achieve a strong electric field, which can be up to 105 V cm−1.14 They also developed some other materials such as Ag NW© XXXX American Chemical Society

Received: August 24, 2018 Revised: October 30, 2018 Published: October 31, 2018 A

DOI: 10.1021/acs.jpcc.8b08232 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) Morphological views of the branched CuO−ZnO nanostructure synthetic process. SEM images of (b) CuO NWs and (c) branched CuO−ZnO NWs (scale bar = 10 μm). Each inset is a high-magnification SEM image (scale bar = 2 μm).

electroporation and oxidative stress.14−16,34−37 Generally, a filter for water disinfection by both electroporation and ROS requires a nanostructure. CuO NWs were used as high-performance water filters due to the high surface to volume ratio and physicochemical properties.16,17 Moreover, due to the strong interaction and fast electron transportation between the closely packed nanofeatures, the effect of electroporation and ROS production is further enhanced by combining multiple kinds of nanowires in a hierarchical nanostructure.38,39 For instance, ZnO is one of the environmentally friendly semiconducting materials, which is used as photocatalysts,40,41 bio sensors,42,43 and food packaging.44,45 ZnO-branched nanostructures have been applied in photoelectronics due to the large surface area and enhanced functionality.46−48 In this work, branched CuO− ZnO NWs on a Cu foam substrate were prepared, where CuO NWs were grown on a Cu foam by a wet chemical process and ZnO NWs were grown on CuO NWs by the hydrothermal method. The nanocomposites were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The morphology and distribution of CuO−ZnO NWs were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electron transport performance of the nanocomposites was investigated by electrochemical measurements. Escherichia coli and Staphylococcus aureus were selected as Gram-negative and Grampositive bacteria, respectively. The disinfection performance of the nanostructured assembly was evaluated by the colony counting method.

from Macklin biochemical Co. Ltd. (Shanghai, China). E. coli (ATCC9522) and S. aureus (ATCC25923) were purchased from RiShui Biotech Co. Ltd. (Qingdao, China). 2.2. Preparation and Characterization of CuO and CuO−ZnO NWs. Branched CuO−ZnO NWs were prepared by a two-step process, as shown in Figure 1a. CuO NWs were grown on a Cu foam by the wet chemical process,49 where the Cu foam was dipped in 1.0 M HCl solution for 2 min to remove oxides on the surface. The Cu foam was then rinsed with deionized water and dried in N2. The CuO NWs were prepared in the mixture solution of NaOH and (NH4)2S2O8 for 10 min at 4 °C, and then heated in air at 180 °C for 3 h. To prepare ZnO NWs,50 a thin ZnO seed layer was deposited on the CuO NWs by immersing into 0.01 M Zn(O2CCH3)2 solution for 10 s. This process was repeated several times to obtain a proper density of the ZnO seed layer. It was then dried at 200 °C for 20 min. For hydrothermal growth of ZnO NWs, a 0.02 M solution containing Zn(NO3)2 and (CH2)6N4 was prepared with a mole ratio of 1:1, adding a certain amount of PEI and adjusting the pH to 10.5 with ammonia. The CuO NWs with a ZnO seed layer were moved to a hydrothermal reactor at 90 °C for 5 h. After the hydrothermal reaction, the samples were washed completely with deionized water and dried in N2. The morphology of the 3D CuO−ZnO NWs was characterized by SEM (Nova 450, USA) and TEM (TF20, Joel 2100F). For the preparation of TEM samples, the copper foam containing the CuO−ZnO NWs was ultrasonically cleaned for 5 min in deionized water. The suspension was dripped on the copper mesh with a supporting membrane. X-ray diffraction (XRD, Bruker D8 Advance) measurements were carried out on a Dmax-3β diffractometer with a nickel-filtered Cu Kα radiation (λ = 1.54178 Å). X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI) was also used and the spectra were recorded on an ESCA Lab MKII X-ray photoelectron spectrometer (KAlpha1063). 2.3. Electrochemical Measurements. The electrochemical measurements were conducted on a three-electrode system using an electrochemical workstation (CHI 760e, Chenhua,

2. MATERIALS AND METHODS 2.1. Materials and Chemicals. Copper foams used in this work had a diameter of 20 mm and a thickness of 1 mm, which were supplied by Shanghai Macklin Biotechnology Co., China. Sodium hydroxide (NaOH), ammonium persulfate ((NH4)2S2O8), zinc acetate dihydrate (Zn(O2CCH3)2·2H2O), zinc nitrate hexahydrate (Zn(NO3)2), and hexamethylenetetramine (C6H12N4) were purchased from Shanghai Chemical Reagent Ltd., China. Polyethylenimine (PEI) was purchased B

DOI: 10.1021/acs.jpcc.8b08232 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. TEM and TEM−EDS analysis of the branched CuO−ZnO NWs. (a) TEM image of branched CuO−ZnO NWs (scale bar = 100 nm). (b) High-magnification TEM image of branched CuO−ZnO NWs (scale bar = 50 nm). (c) STEM (scale bar = 500 nm) and (d−i) EDS elemental mapping of the branched CuO−ZnO NWs (scale bar = 200 nm).

calculate the bacterial removal efficiency. To ensure reproducibility, tests were conducted triple times for each group. 2.6. ROS Level Assay. To measure the ROS generated under the electric field stimulation, a reactive oxygen species assay kit (Beyotime, China) was used, which was based on the fluorescent dye 2,7-dichlorodi-hydrofluoresceindiacetate (DCFH-DA), which changes the fluorescence intensity to detect the level of intracellular ROS. Before the flowing test, 10 μM of DCFH-DA was added into the medium containing microorganisms, which then flowed through the CuO−ZnO NWs filters. The change of ROS content under various conditions was measured by a fluorescence spectrophotometer (confocal laser scanning microscope (ZEISS Scope. A1)). At the same time, to explore the potential role of oxidative stress in bacterial inactivation, the bacteria were incubated with 10 mM glutathione (reduced form, GSH) as the influent microorganism to remove the contribution of oxidative stress to bacterial deactivation.51 After flowing test, the bacteria removal efficiency was calculated by the colony counting method. The antibacterial rate of the oxidative stress caused by ROS was calculated by

China) in Luria-Bertani nutrient solution to simulate the environment for bacterial growth. The prepared nanocomposite sample (1 cm2), a platinum wire, and a Ag/AgCl (saturated KCl) electrode served as the working electrode, counter electrode, and reference electrode, respectively, estimating the electrochemical performance of the CuO−ZnO NWs electrode. 2.4. Stability of the CuO−ZnO NWs Electrode. To evaluate the stability of CuO−ZnO NWs growing on the copper foam, the copper ion and zinc ion released in the flowing test was measured (Figure S2). Deionized water replaced with microorganism was flown through the filter device. Filtered samples were taken at different voltages and flow rates were determined for analysis of the released copper ion and zinc ion concentrations, where each sample was under the flowing test for 2 min. The ionic concentrations were analyzed by inductively coupled plasma (ICP, Thermo, ICAP-6300). 2.5. Antibacterial Testing. Antibacterial performance of the prepared nanocomposites was evaluated in environments containing E. coli of Gram-negative species and S. aureus of Gram-positive species, respectively. Before testing, the copper foam with CuO or CuO−ZnO NWs (ϕ 20 mm × 1 mm) was assembled into a filter device with two parallel electrodes (Figure S2c). During testing, the electrodes were in the absence of an applied electric field and the presence of external voltages ranging from 2 to 10 V (the bacterial concentration is about 106 CFU mL−1 and the flow rate is 10 mL min−1). Microorganisms have the concentration ranging from 104 to 108 CFU mL−1 (the external voltage is 10 V and the flow rate is 10 mL min−1) passing the filter. The device flow rate ranged from 2 to 10 mL min−1 (the bacterial concentration is about 106 CFU mL−1, and the external voltage is 10 V) using a peristaltic pump (BT300F, Lead Fluid, China. Pipe No. 14). Colony counting was performed to

E = E1 − E 2

where, E is the antibacterial rate of oxidative stress caused by ROS, E1 is the antibacterial rate of the influent microorganism without GSH, and E2 is the antibacterial rate of the influent microorganism with GSH.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of the Prepared CuO− ZnO NWs. The copper foam with a porous structure was used as a conductive material for the preparation of CuO NWs and branched CuO−ZnO NWs. The mesh size of the copper foam framework ranged from 200 to 300 μm, and the framework of C

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Figure 3. (a) XRD patterns, (b−d) XPS of the prepared CuO−ZnO heterohierarchical nanostructure.

Figure 4. Electrochemical performance. (a) Nyquist plots and (b) J−V curves of the sample electrode with CuO NWs and CuO−ZnO NWs growing on copper foam.

the foam did not change after treatment (Figure S1). The morphology of the prepared CuO NWs and branched CuO− ZnO NWs is shown in Figure 1. It is seen that the CuO NWs with a diameter of about 200 nm were vertically grow on the foam surface (Figure 1b). Moreover, the CuO NWs are covered with ZnO nanowire branches, forming a CuO−ZnO hierarchical nanostructure (Figure 1c). The inset image shows an enlarged view of individual CuO−ZnO NWs, where the ZnO nanowire branches grow continuously along the length direction of the CuO NWs. The intensive sharp, nanoscale tips of CuO− ZnO play an important role in generating a strong electric filed between the nanowires. The identical morphology is also obtained in TEM views, as shown in Figure 2a,b. It is seen that the ZnO NWs cover CuO NWs uniformly. The diameter of the ZnO branches is approximately 30 nm, and their length ranges from 100 to 200 nm, which is much smaller than copper oxide, providing sufficient nanoscale tips. In elemental mapping as shown in

Figure 2c−i, Cu, Zn, and O elements distribute in the whole structure, since the CuO NWs serve as the backbone and ZnO NWs form uniformly in the branched structure. The crystalline degree of the ZnO−CuO heterohierarchical nanostructure could be observed from the XRD spectrum of Figure 3a. It is seen that the CuO and ZnO diffraction peaks are weak compared with that of Cu as CuO and ZnO grown on the copper foam. The strong peaks associated with the Cu substrate are indicative of a pristine Cu, which is used as a reference, and can be assigned to Cu(111), (200), and (220) planes (JCPDS Card no. 65-9743). The XRD spectra for the CuO−ZnO hierarchical structure demonstrate the presence of well-defined CuO (JCPDS Card no. 45-0937) peaks corresponding to (1̅11) planes at 37°. The diffraction peaks corresponding to (100), (002), (110), and (103) at 31.6, 34.3, 56.2, and 61.8°, respectively, are associated with the hexagonal ZnO (JCPDS Card no. 65-3411), confirming the loading of ZnO on the CuO NWs electrode.52 D

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concentration is about 4.3 mM L−1 at a flow rate of 10 mL min−1, and the lowest one is about 3.2 mM L−1 at 2 mL min−1 (Table 2). The concentrations of both ions are very low. The highest contents of Cu and Zn ions are much lower than the WHO drink water standard, which are 31.5 and 45.9 mM L−1, respectively.39,57 The low ion release in this work indicates that the CuO−ZnO NWs growth is stable on the copper foam, and the NWs do not generate harmful by-products in water treatment. 3.4. Antibacterial Performance. For the antibacterial performance testing, the voltage value smaller than 10 V is selected for safety and low energy consumption, which is less than 50 × 10−3 J S−1, as seen in Table S1. The water samples that contain about 104−108 CFU mL−1 (concentration in the influent) E. coli or S. aureus were used, with the flow rates ranging from 2 to 10 mL min−1. The microorganism flows through the filter device by pipe No. 14 (Figure S2b) during the test. The microorganism stays on the foam electrode for about 1.8 s at a flow rate of 10 mL min−1. After being treated, the microorganism concentrations in the effluent were measured, and the bacterial inactivation efficiency is calculated by58,59

The XPS spectra of the branched ZnO−CuO NWs are shown in Figure 3b−d, where C 1s (CC binding energy of 284.78 eV) is used as the charge correction reference. As expected, the main components of the sample are Zn, Cu, and O. As shown in Figure 3b, two typical peaks located at 933.7 and 953.6 eV correspond to Cu 2p3/2 and Cu 2p1/2, respectively, indicating the existence of Cu2+ in the composite. In addition, the appearance of two shakeup satellite peaks on the higher binding energy indicates the formation of CuO.53 The Zn 2p region shown in Figure 3c includes the peaks at 1044.1 and 1021 eV that are attributed to Zn 2p1/2 and Zn 2p3/2, respectively, in a wurtzite ZnO structure. Furthermore, symmetric O 1s spectra showing a peak at 529.9 eV correspond well to the binding energy of O2− ions in the metal oxide sites (i.e., ZnO and CuO).54 3.2. Electrochemical Measurements. Electrochemical impedance spectroscopy (EIS) is a technique to investigate the interfacial charge transfer properties of an electrode. Generally, the charge transfer resistance can be determined from the semicircle diameter of EIS curves.55 It is seen that the semicircle of CuO−ZnO NWs is smaller than that of CuO NWs (Figure 4a), demonstrating that the CuO−ZnO NWs possesses a lower charge transfer resistance. From the J−V curves (Figure 4b), the CuO−ZnO NWs show a higher current than CuO NWs, which is due to the increased surface area for reaction, resulting in an improved electron transfer rate.56 The results demonstrate that the CuO−ZnO NWs electrode possesses a low resistance, and thus allows faster electron transfer. 3.3. Stability Analysis. In this work, ICP is carried out to measure the Cu and Zn ion release from CuO−ZnO NWs electrode during filtration. As shown in Tables 1 and 2, the

E = (Cin − Ceff )/Cin × 100%

where E is the inactivated percentage, Cin is the concentration of the microorganism in the influent, and Ceff is the concentration of the microorganism in the effluent. As seen in Figure 5a, the antibacterial activity of the CuO− ZnO filter with applied different voltages was measured with the bacterial concentration in the influent as 106 CFU mL−1 and the flow rate as 10 mL min−1. We found that the inactivation performance of the prepared sample is not ideal in the absence of applied voltage, which only depended on the physical contact of bacteria with the nanowires, resulting in the death of a certain percentage of bacteria due to damage to the cell wall.60 However, the inactivation performance increases apparently (up to 90%) at an applied voltage of 2 V, and the inactivation efficiency reaches 98 and 99% at 6 and 10 V, respectively. These results indicated that the effect of electric field is critical in the inactivation of bacteria. Due to the hierarchical nanostructure of the prepared materials, the intensity of the electric field is much higher around the nanowires, resulting in an irreversible electroporation inactivation of bacteria. The bacterial removal efficiencies of the prepared filters as a function of the bacterial concentration (Figure 5b) and the fluid flow rate (Figure 5c) are also determined. With the increasing concentration of E. coli and S. aureus, the bacterial removal efficiencies reduce. As the concentration of the influent bacteria increases, there are more bacteria making the contact with the nanowires difficult at the same time. As the concentration of bacteria reaches 106 CFU mL−1, the filter could reach 99% removal efficiency. Whereas, the concentration of bacteria in actual water is always less than this value.61,62 Moreover, the sterilization efficiency under the flow rate ranging from 2 to 10 mL min−1 is almost above 99%. These results show that the filters enriched in CuO−ZnO nanoscale tips exhibit an excellent disinfecting ability and there is a high concentration of microorganisms and a fast flow rate of bacteria under the low electric field intensity. The excellent sterilization performance of the prepared copper foam electrodes containing branched CuO−ZnO NWs is subjected to the availability of the applied electric filed. The number of electron transitions can be effectively reduced to promote the electron transmission and result in the accumulation of sufficient charges on the nanowire surface.14,34 As

Table 1. Cu Ion Concentration in Flow Testa flow rate (mL min−1) voltage (V)

10

8

6

4

2

0 2 4 6 8 10

0.64 0.62 0.69 0.68 0.65 0.72

0.62 0.61 0.63 0.67 0.55 0.62

0.50 0.52 0.62 0.60 0.54 0.47

0.52 0.53 0.58 0.64 0.55 0.53

0.51 0.49 0.47 0.47 0.50 0.49

The units of concentration are mM L−1.

a

Table 2. Zn Ion Concentration in the Flow Testa flow rate (mL min−1) voltage (V)

10

8

6

4

2

0 2 4 6 8 10

4.28 4.40 4.53 4.27 4.34 4.25

3.81 3.87 3.84 4.05 3.77 3.85

3.66 3.63 3.19 3.58 3.48 3.63

3.34 3.41 3.22 3.46 3.35 3.21

3.29 3.24 3.08 3.22 3.31 3.11

The units of concentration are mM L−1.

a

changes of both Cu and Zn ion concentrations are similar. There is no effect of the change in voltage on the ionic concentration, and thus the flow rate. The ionic concentration is the highest at high flow rates, and then gradually diminishes as the flow rate decreases. The highest Cu ion concentration is about 0.7 mL L−1 at a flow rate of 10 mL min−1, and the lowest one is about 0.5 mM L−1 at 2 mL min−1 (Table 1). The highest Zn ion E

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Figure 5. Antibacterial activity of the CuO−ZnO filter with applied different voltages (a), different microorganism concentrations (b), and different flow rates (c). Inactivation efficiency of E. coli and S. aureus treated the CuO and CuO−ZnO as filter electrodes with 0−6 V voltage, respectively (d). SEM images of E. coli (e, f) and S. aureus (g, h) before and after filtration through CuO−ZnO nanowires flow device. Note: scale bar = 1 μm.

death occurs immediately.15 In addition, during electrochemical disinfection, the generation of ROS also increased the death of bacteria by increasing the oxidative stress in the cells and the permeability of cell membranes, resulting in the penetration and disruption of the bacterial cell membranes.36 Therefore, the ROS effect for sterilization efficiency should also be considered based on the present work. 3.5. Measurements of ROS. ROS play a critical signaling molecule role within bacteria. For instance, superoxide anion (O2−) and hydrogen peroxide (H2O2) are usually produced along the respiratory chain when the electrons are steadily donated to O2.64,65 The general scheme is shown in the equation66

shown in Figure 5d, compared with CuO NWs, CuO−ZnO NWs exhibit a better inactivation efficiency for both E. coli and S. aureus at the same voltage. The bactericidal effect of the filter with CuO NWs at 6 V (∼90%) is equivalent to the bactericidal effect of the filter with CuO−ZnO NWs at 2 V. Therefore, the filter material with substantial nanoscale tips is critical to the electric field sterilization. Electrochemical disinfection is largely based on the mechanism of electroporation and oxide stress to make bacteria inactive.14−17,36 After the action of a strong electric field and ROS, cell membranes can be damaged to varied degrees, leading to ion leakage and escape of metabolites.63 The bacteria before and after filtration are imaged by SEM and shown in Figure 5e,f. It is seen that untreated bacteria have complete and smooth cell membranes, while the bacterial membranes are damaged and pores are formed on the treated bacteria. For E. coli (Figure 5e), holes are observed on the cell surface and there is a partial dissolution of the cell wall. For S. aureus (Figure 5f), the bacterial cell walls are smooth and intact before they are inactivated. However, after filtration it is no longer smooth and there is cell inclusion outflow. As studied previously, under the external voltage applied on the filter electrode, an extremely high electric field is generated on the sharp nanoscale tips.14−17,63 When bacteria flow around the tips, cell membranes become damaged, causing a large change in cell permeability and conductivity. Particularly, the cell membrane is severely damaged, and the cell

e−

e−

O2 → O−2 → H 2O2

In fact, steady electron transfer is beneficial to supply energy for cell growth and maintenance, and disturbing electron transfer in bacteria can increase the production of ROS to hinder growth or even inactivation.30 As discussed above, the electron transfer process will occur once bacterial fluid contacts with the electronic collected CuO−ZnO NWs, and electron transport along the respiratory chain is affected to elevate the intracellular ROS. The electronic disturbance is stronger since a higher voltage has a higher current (Table S2) and more electrons, producing more ROS which results in better antibacterial effects. F

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Figure 6. ROS content enhancement performance. Fluorescence microscope images of bacteria without an applied voltage treatment (a) and after a 2 V (b), 4 V (c), 6 V (d), 8 V (e) and 10 V (f) voltage treatment. Note: scale bar = 20 μm.

To indicate that the increase of the antibacterial activity is attributed to oxidative stress due to the production of ROS, the ROS concentrations were measured. According to the manufacturer, the DCFH-DA itself has no fluorescence and is free to cross the cell membrane. After entering the cells, it can be deacetylated by intracellular esterases into nonfluorescent dichlorofluorescein (DCFH). DCFH does not penetrate the cell membrane and stay in bacteria. Intracellular ROS can oxidize nonfluorescent DCFH to produce highly fluorescent 2,7-dichlorofluorescein (DCF). The intensity of fluorescence is proportional to the amount of ROS. Therefore, the relationship of ROS generation and applied voltage is analyzed. As shown in Figure 6. It is seen that there are more blue fluorescence generating at higher voltages. The intensity of the generated blue fluorescence at 2 V is remarkably more than that in the absence of an applied voltage (0 V). This phenomenon could be attributed to the fact that the applied electric field provides sufficient electrons to the material, and the CuO−ZnO NWs provide abundant reactive sites. As a result, when the bacteria pass through the filter device, the electronic disturbance is stronger, producing more ROS, resulting in a better antibacterial effect. For water treatment under an electric field, the inactivation of bacteria is mainly caused by the strong electric field and oxidative stress. In this work, the bactericidal efficiency could be attributed to the oxidative stress of ROS (Figure 7). At an external voltage of 2 V, the total bactericidal rate (E1: the influent microorganism without GSH) is about 20% higher for the branched CuO−ZnO NWs filter than that of CuO NWs. The bactericidal rates of the two filters have the same results due to the ROS effect, and the branched CuO−ZnO NWs filter is nearly 10% higher than the CuO NWs filter. The result shows that the branched CuO−ZnO NWs are effective to enhance the electrochemical disinfection efficiency. For the semiconductors, e.g., CuO and ZnO, the external electric field provides electrons and changes the transition energy between vibrational levels, resulting in an increased speed of the electron−hole motion.37 Moreover, the small size of ZnO NWs can also contribute to more efficient charge separation and collection,67 giving the

Figure 7. Proportion of inactivation efficiency based on ROS at 2 V.

surface of the material a large amount of charge to disturb electron transfer in bacteria, which induces ROS burst to kill the bacteria. Thus, fabrication of fine ZnO NWs on CuO NWs not only increases the density of sharp nanoscale tips, but also increases the reactive sites for fast electron transfer, improving the antibacterial performance through both electroporation and oxidative stress.

4. CONCLUSIONS In summary, a 3D branched CuO−ZnO NWs structure on a copper foam substrate is prepared by a wet chemical process and a hydrothermal approach. The prepared nanocomposite filter achieves an outstanding antibacterial performance (i.e., inactivation of E. coli and S. aureus) under a low voltage, short contact time, and high concentration of bacteria. When the applied voltage is 10 V and the processing time is less than 2 s, the disinfection efficiency exceeds 99% with the initial bacterial concentration of 106 CFU mL−1. In the process of flow disinfection, the bacterial inactivation is not only due to electroporation, but also due to the effect of ROS which oxidize the bacteria to death. For the CuO−ZnO NWs structure, the extensive nanowires provide a large number of active sites for G

DOI: 10.1021/acs.jpcc.8b08232 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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numerous electron collection and fast electron transfer to induce the bacteria to produce ROS. The branched CuO−ZnO NWs structure has more abundant nanoscale tips than CuO NWs, which show a better bactericidal performance. Considering the facile synthesis and excellent disinfection efficiency, the branched CuO−ZnO nanostructure shows great potential in application for water treatment.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08232. SEM images of the bare copper foam framework, CuO NWs and CuO−ZnO NWs grown on a copper foam framework (Figure S1); images of the setup of flowing test (Figure S2); energy consumption of disinfection at different voltages (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiao Zhang: 0000-0003-0165-1561 Shougang Chen: 0000-0002-1893-1968 Author Contributions §

C.W. and L.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (51572249), Natural Science Foundation for Shandong Province (ZR2014EMM021), Key Research Project of Shandong Province (2018GSF118039), and Fundamental Research Funds for the Central Universities (201562011).

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REFERENCES

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