1. Introduction
At present, chemical substances such as nitrous oxide and ground ozone produced by global urban and industrial transportation activities pose a continuous threat to public health, while bacterial pollution has become a superimposed health risk factor [
1]. Fine particles and biological aerosols, which are easily diffused and can remain suspended in the air for long periods of time, contain substantial quantities of pathogenic bacteria [
2]. Therefore, there is an urgent need for low-cost and high-efficiency air-sterilization technology to improve human livable life and health. Thus far, air-purification technologies such as electrostatic precipitation, cold plasma, wet scrubbing, cyclone air filtration, and physical filters have been developed, as described in the literature [
3,
4]. Among these, physical filtration technology has attracted increasing attention due to its simple operation, low cost, and easy removal of particulate matter in air. For example, Zhong et al. [
5] grew one-dimensional (1D) zinc oxide (ZnO) nanorods on a three-dimensional (3D) porous network of expanded polytetrafluoroethylene (PTFE) to obtain a layered fiber-structure air filter with an ultra-high removal rate and a bactericidal efficiency as high as 99.0% after 15 min under indoor lighting conditions. Similarly, Feng et al. [
6] coated silver-embedded ZnO nanorods on a PTFE nanofiber membrane to form a hierarchical structure with an antibacterial efficiency of 99.5% under a wind speed of 2.5 cm∙s
−1. Hu et al. [
7] used a sol-gel method to encapsulate antibacterial ZnO on silicon carbide (SiC) foam. The slow release of zinc ions in the antibacterial ZnO coating on the surface of the SiC inhibited the growth of harmful microorganisms. Thus far, the sterilization mechanism for physical filters mainly depends on direct contact between metal ions or photo-activated reactive oxygen species (ROS) and bacteria. However, the sterilization achieved through such methods is not ideal for practical applications, especially at higher air-flow speeds.
Electrocatalysis technology has attracted increasing research interest in recent years due to its advantages of low pollution, high energy utilization, and strong controllability [[
8], [
9], [
10]]. This technology has wide application prospects in energy conversion and storage, such as supercapacitors [
10], batteries [
11,
12], and electrochemical sensors [
13]. Electrocatalysis materials have also been used in the field of sterilization. For example, Wang et al. [
14] studied the growth of iron(II,III) oxide (Fe
3O
4) nanowires (NWs) on iron mesh. The resulting structure was able to inactivate bacteria in the air upon applying a voltage of 4.5 V. It was found that the applied voltage produced an electron adsorption effect, which was very useful for capturing biological aerosol in the air [
14].
In addition, other studies have suggested that the co-presence of electricity, oxygen, and water can produce ROS, thereby improving the efficiency of photocatalysis sterilization [
15]. Yue et al. [
16] prepared 3D copper foam nanocomposites co-modified by silver nanoparticle (AgNP)-CuO NWs. The composites exhibited a high sterilization rate under low voltage and an ultra-high water flux (> 99.9%), based on an electroporation sterilization mechanism. Wang et al. [
17] prepared a 3D C/Cu
2O-AgNP electrode to filter
Staphylococcus aureus (
S. aureus) and
Escherichia coli (
E. coli) in water. By optimizing the electrode structure, the bactericidal rate of C/Cu
2O-AgNPs was close to 99.6% at a flow rate of 1200 mL∙min
−1. However, to the best of our knowledge, electrocatalytic sterilization technology has mainly been used in water sterilization [[
17], [
18], [
19], [
20]]. Thus, there is an urgent need to apply electrocatalysis technology to air sterilization in order to improve the efficiency of bacteria inactivation and air purification.
Our previous works have shown that the tip discharge effect of 1D NWs can create a strong electric field in the local tip area, resulting in the electroporation and death of bacteria [
21]. However, the local high-strength electric field range of 1D NWs is very small, and bacteria can only be sterilized when they are close to the tip. Hence, to increase their inactivation efficiency, it is necessary to modify the NWs and expand the range of the local high-strength electric field. Recently, two-dimensional (2D) metal-organic framework (MOF) nanosheets have been the subject of research interest due to their large transverse size, large specific surface area, and abundant surface active sites, which enable the adsorption of reactants and the exposure of active sites [
22]. In addition to these advantages, MOF materials exhibit good sterilization ability [
23,
24], as the strong adsorption capacity of a MOF can locally enrich the surrounding oxygen and generate ROS with a bactericidal effect through various catalytic and reduction conditions (e.g., light and electricity) [
25]. To achieve effective bacterial inactivation, it is anticipated that a novel electrocatalytic bactericidal material based on a 2D MOF can be designed for air sterilization. Toward this end, copper (Cu) mesh is an ideal substrate, thanks to its excellent conductivity and good permeability. Previous studies have shown that a MOF with Cu and cobalt (Co) as metal coordination ions has more advantages in catalytic performance in comparison with materials based on other metal coordination ions [
26,
27].
In this work, different structures of C28O12Co4/Cu@Cu (abbreviated as Co-MOF/Cu@Cu) on copper mesh substrates were designed. Through density functional theory (DFT) calculations and electrochemical testing, we determined that the designed material has high water stability and conductivity. Finite element simulation indicated that the range of the local high-intensity electric field was expanded, allowing more ROS to be generated. Electron paramagnetic resonance (EPR) was used to characterize the existence of superoxide anions. The assembled device composed of 3D MOF nanomaterials had an inactivation efficiency for E. coli as high as 99.51% under a voltage of 24 V and an air flow rate of 1.5 m∙s−1 (equivalent to a treatment time of 0.0026 s).
2. Results and discussion
2.1. Material selection
Fig. 1 describes the preparation method and morphology of Co-MOF/Cu@Cu. As is well known, a certain degree of air humidity is needed for air sterilization. Therefore, it was necessary to first estimate the water stability of the prepared materials. Based on the literature [
28], MOF materials with high-valence metal ions have good water stability in general. For Cu-MOF/Co@Cu materials, the water stability can be tuned by adjusting the proportion of copper and cobalt. As shown in Fig. S1 in Appendix A, 0.3Co-MOF/Cu@Cu and 0.5Co-MOF/Cu@Cu achieved better water stability than the other samples. These samples were also tested by electrochemical impedance spectroscopy (EIS), which indicated that 0.3Co-MOF/Cu@Cu had faster reaction kinetics (Fig. S3 in Appendix A). Therefore, 0.3Co-MOF/Cu@Cu was selected for subsequent analysis and for comparison with Cu-MOF/Co@Cu before modification. A deeper explanation is provided in Appendix A.
2.2. Morphology and structural characterization of Cu-MOF/Co@Cu and 0.3Co-MOF/Cu@Cu
Scanning electron microscope (SEM) images of the Cu(OH)
2 NWs are provided in Fig. S4(a) in Appendix A. The chemical etching of copper mesh yielded uniform and dense NW arrays (∼200 nm in diameter and ∼20 μm in length). As shown in Fig. S4(b) in Appendix A, the Cu-MOF/Co@Cu nanorods were coated with scattered nanosheets to form highly oriented nanorods. The 0.3Co-MOF/Cu@Cu nanorods exhibited a wolf-tooth shape (
Fig. 1(b)), wrapped by longer and thinner nanosheets than the Cu-MOF/Co@Cu. As shown in
Fig. 1(c) and Fig. S4(c) in Appendix A, an energy dispersive spectroscope (EDS) analysis of the Cu-MOF/Co@Cu and 0.3Co-MOF/Cu@Cu nanorods was performed, which showed that the elements were evenly distributed on the nanorods. The microstructures of the nanorods were further observed by means of transmission electron microscope (TEM) images (
Fig. 1(c); Fig. S5 in Appendix A).
The composition of the sample was characterized using X-ray diffraction (XRD) (
Figs. 2(a) and (b)), Fourier-transform infrared (FTIR) spectroscopy (Fig. S6 in Appendix A), and X-ray photoelectron spectroscopy (XPS) (
Figs. 2(c)-(f); Fig. S7 in Appendix A), which showed that the MOF was successfully prepared. The XRD results of the samples are shown in
Figs. 2(a) and
(b). The three strong diffraction peaks at 43.3°, 50.4°, and 74.1° can be ascribed to the metallic copper in the copper mesh substrate, corresponding to the crystal planes (111), (200), and (220) (Powder Diffraction File (PDF) #04-0836), respectively. The diffraction peaks of Cu(OH)
2 at 2
θ of 16.7°, 23.8°, 34.1°, 35.9°, 38.1°, 39.7°, and 53.3° correspond to (020), (021), (002), (111), (041), (130), and (150) (PDF #35-0505), and the diffraction peaks at 2
θ of 29.6°, 36.4°, 42.3°, and 61.3° correspond to (110), (111), (200), and (220) for Cu
2+1O (PDF #05-0667), respectively.
Fig. 2(b) shows a partial enlargement of the XRD spectra, with the addition of a simulation pattern based on C
11H
11CuNO
5 (Cambridge Crystallographic Data Centre (CCDC) No. 687690) single-crystal data. The diffraction peaks at 2
θ of 10.3°, 12.1°, 17.3°, 20.6°, and 25.1° correspond to (110), (001), (021), (220), and (131) of Cu-MOF/Co@Cu. Thus, the Cu-MOF/Co@Cu model is basically consistent with the simulation. It can be shown that cobalt ions have partially replaced the copper ions in the MOF framework while retaining the crystal structure of Cu-MOF [
29]. Similarly, a local enlarged image of 0.3Co-MOF/Cu@Cu was basically consistent with the simulated image of the [Co
2(OH)
2(C
8H
4O
4)]∙2H
2O (CCDC No. 153067) single-crystal data. The diffraction peaks at 2
θ of 8.9°, 14.2°, 16.0°, 17.9°, 29.1°, 30.8°, and 33.1° correspond to (200), (001), (20−1), (400), (20−2), (11−1), and (31−1) of 0.3Co-MOF/Cu@Cu. Based on these findings in combination with the EDS element analysis in
Fig. 1(c), it was determined that the structure of 0.3Co-MOF/Cu@Cu is mainly Co-MOF with a small amount of copper doping.
It can be seen from the XPS fitting curve that Cu
+ and Co
3+ are the main oxidation states in Cu-MOF/Co@Cu and Co-MOF/Cu@Cu (
Figs. 2(c)-(f)).
Figs. 2(c) and
(e) show the Cu 2p XPS spectrum of Cu-MOF/Co@Cu and Co-MOF/Cu@Cu, respectively. The coexistence of Cu
2+ and Cu
+ is due to the formation of a mixed material with a non-stoichiometric mixed valence state of Cu
2+ and Cu
+. The peaks at 789.2 and 805.6 eV in
Figs. 2(d) and
(f) are two satellite peaks, which are related to unpaired electron coupling or multi-electron excitation [
30]. From the Co 2p XPS spectrum (
Figs. 2(d) and
(f)), it can be seen that the Co
3+ content of 0.3Co-MOF/Cu@Cu is higher than that of Cu-MOF/Co@Cu, which may be the cause of the former’s good water stability.
2.3. Performance analysis of materials
The stability of Cu-MOF/Co@Cu and Co/Cu-MOF in an aqueous environment was further evaluated by determining the binding strength between the metal ligand and the organic skeleton [
31]. The stability of the materials was also determined through DFT calculation (
Fig. 3(a)). The binding energy of Co was determined to be greater than that of Cu and the system, indicating that the structure of Co-MOF is more stable than that of Cu-MOF. A higher Co content is thus conducive to the formation of a stable Co-MOF structure, which is also the reason for the good water stability of Co-MOF. Detailed analysis methods are provided in Appendix A.
To enable a deep understanding of the catalysis and semiconductor properties of the material, the surface area and pore structure of the samples were measured via nitrogen gas (N
2) adsorption. The results showed that 0.3Co-MOF/Cu@Cu had the largest specific surface area among the designed samples (
Fig. 3(b); Fig. S8 in Appendix A).
The charge transfer kinetics at the electrode-electrolyte boundary and the diffusion of the electrolyte were further analyzed by means of EIS; the Nyquist diagrams are shown in
Fig. 3(c). The size of the semicircle in the high-frequency region corresponds to the charge transfer resistance at the interface between the electrode and electrolyte. The semicircle of Cu
2O is the largest and that of 0.3Co-MOF/Cu@Cu is the smallest, which further indicates the rapid electron transport of 0.3Co-MOF/Cu@Cu [
32]. As shown in
Fig. 3(d) and Fig. S9 in Appendix A, the number of active sites is usually expressed by the electrochemical active surface area (ECSA) in electrochemistry. The ECSA is approximately expressed by the electrochemical double-layer capacitance (
Cdl) [
33]. The highest
Cdl value of 0.3Co-MOF/Cu@Cu was 18.49 mF∙cm
−2, which was greater than those of Cu-MOF/Co@Cu (14.11 mF∙cm
−2) and Cu
2O (0.27 mF∙cm
−2). This result also indicates that the 0.3Co-MOF/Cu@Cu active site had the highest degree of exposure, which is consistent with the change shown in
Fig. 3(c). In addition, under an applied voltage of 10 V, the currents of Cu
2O, Cu-MOF/Co@Cu, and 0.3Co-MOF/Cu@Cu were 37.04, 31.49, and 47.56 mA, respectively (Fig. S10 in Appendix A), further indicating that the 0.3Co-MOF/Cu@Cu had more active sites and higher electron-transport efficiency. Moreover, when the chronopotentiometry was measured at a current density of 10 mA∙cm
−2 for 25 h, the potential drop of 0.3Co-MOF/Cu@Cu was the smallest (
Fig. 3(e); Fig. S11 in Appendix A), indicating that 0.3Co-MOF/Cu@Cu had better stability and durability. These results are consistent with those of the water stability test (Fig. S1).
2.4. Antibacterial properties
Based on its performance and water stability analysis, 0.3Co-MOF/Cu@Cu has obvious advantages in practical applications. Therefore, 0.3Co-MOF/Cu@Cu was selected to study the bactericidal performance at a humidity of 65%. The antibacterial properties of 0.3Co-MOF/Cu@Cu electrodes were studied by simulating actual air. According to the general principles of biological pollution control (International Organization for Standardization (ISO) 14698-1:2003),
E. coli was selected as the test strain, representing Gram-negative bacteria. The disinfection effect of the electrode was tested in an environment containing
E. coli. In order to study the influence of air velocity, the concentration of microorganisms was measured using a flow rate treatment pattern of 0.5 to 3.0 m∙s
−1, and the bacteria inactivation efficiency was calculated as follows [
34]:
${{E}_{\text{in}}}=\frac{{{C}_{\text{in}}}-{{C}_{\text{eff}}}}{{{C}_{\text{in}}}}\times 100%$
where Ein is the percentage of inactivation, ${{C}_{\text{in}}}$ is the bacteria count before filtration, and ${{C}_{\text{eff}}}$ is the bacteria count after filtration.
As shown in
Fig. 4(a), the pure copper mesh and the 0.3Co-MOF/Cu@Cu electrode materials were treated with an alternating current (AC) voltage of 12 V and a flow rate of 0.5 to 3.0 m∙s
−1. It was found that the sterilization rate of the pure copper mesh only reached 70.48% while that of the 0.3Co-MOF/Cu@Cu electrode material reached 92.42% at an air flow rate of 0.5 m∙s
−1. When the air flow rate was gradually increased, the sterilization rate of both materials increased, with the sterilization efficiency reaching the highest value at an air flow rate of 1.5 m∙s
−1, at 78.43% for the pure copper mesh and 98.68% for the 0.3Co-MOF/Cu@Cu electrode material. Since an increase in the air flow rate increases the humidity between the electrodes, the increase in water molecules caused the conductivity of the whole sterilization device to improve, highlighting the role of voltage. It is worth mentioning that the sterilization rate decreased when the air flow rate was increased past that point. This result can be explained as being due to a too-fast air flow velocity, such that the residence time of bacteria between the electrodes is too short to kill all the bacteria. Therefore, an air flow rate of 1.5 m∙s
−1 achieves the best sterilization rate.
Next, an air flow rate of 1.5 m∙s
−1 was selected to carry out the sterilization experiment under different voltages. As shown in
Fig. 4(b), under a voltage of 0 V, the sterilization rate of the pure copper mesh was 40.56% and that of the 0.3Co-MOF/Cu@Cu electrode material was 69.28%. Fluorescent live/dead staining imaging showed green at 0 V, and the bacteria SEM showed the full shape of living bacteria (Figs. S12(f) and (j) in Appendix A), demonstrating that the bacteriostatic rate at a voltage of 0 V was only caused by physical adsorption, with no obvious bactericidal effect. The 0.3Co-MOF/Cu@Cu had a higher adsorption capacity, larger specific surface area, and more obvious barrier effect on bacteria due to its MOF structure. It was observed that, with an increase in the voltage, the sterilization rate of both the pure copper mesh and the 0.3Co-MOF/Cu@Cu monotonically increased. However, as the voltage increased, the sterilization rate of the pure copper wire mesh also gradually increased, reaching the highest sterilization rate of 78.43% at 12 V. When the voltage was increased past this point, the sterilization rate decreased. This result may be due to the reaction of copper mesh at a higher voltage, which leads to rapid burning and deterioration of the material, thereby reducing its conductivity and sterilization performance. However, under an AC voltage of 24 V, the sterilization rate of the 0.3Co-MOF/Cu@Cu electrode material reached 99.51%, due to the increase in voltage and electro-adsorption performance of the material. Thus, MOF materials can be used to effectively kill bacteria via electricity, thanks to their large specific surface area, numerous active sites, and good stability.
Fig. 4(c) shows the contribution ratio of ROS to the sterilization rate. In this work, 0.3Co-MOF/Cu@Cu was applied as filter electrodes, and the sterilization rate of ROS was found to be as high as 38%—about 10% higher than that of currently known filter electrodes [
17,
19,
20]. This result shows that, although electroporation still plays a leading role in the sterilization, the bactericidal effect of ROS is significantly strengthened.
The 0.3Co-MOF/Cu@Cu electrode was demonstrated to have excellent bactericidal performance by means of colony counting and fluorescent live/dead staining (Fig. S12 in Appendix A). As shown in Figs. S12(i)-(m), a bacterial membrane treated with a 0.3Co-MOF/Cu@Cu electrode had obvious perforation. A detailed explanation is provided in Appendix A.
2.5. The bacteriostatic mechanism
The energy band adjustment of the 0.3Co-MOF/Cu@Cu semiconductor has a very important influence on the bacteriostatic mechanism of the material. In order to further explore the charge transfer path at the interface between the NWs (Cu
2O) and nanosheets (Co-MOF), the work functions (
Φ) of the Cu
2O plane (111) and Co-MOF plane (010) were calculated by means of DFT (
Figs. 4(d) and
(e)). The Fermi level of the nanosheets is higher than that of the NWs. When in close contact, electrons can flow from the NWs to the nanosheets through the interface, as shown in
Fig. 4(f). This greatly increases the surface charge of the material, increases the local electric field intensity and the probability of contact between bacteria and the charge, and then interferes with the physiological activities inside the bacteria to produce ROS.
The local electric field intensity plays an important role in the material’s sterilization performance. The 0.3Co-MOF/Cu@Cu electrode material contains a large number of active sites and has improved conductivity, which promotes the electrocatalytic reaction of the electrode material. In order to clarify the difference in the electric field strength for Cu-MOF/Co@Cu and 0.3Co-MOF/Cu@Cu, a finite-element simulation was performed using the measured voltage (
Fig. 5(a)). On the whole, 0.3Co-MOF/Cu@Cu has more active sites due to the large number of nanosheets, which can obviously increase the electric field. In addition, the electric field intensity of a 1D NW is only increased near the tip, whereas the proposed structure has different degrees of electric field enhancement around the tip and nanosheets; therefore, the range of the high-intensity electric field is wider for the 0.3Co-MOF/Cu@Cu, leading to a wider range of bacteria being attacked and eventually leading to the destruction of the bacterial membrane and even electroporation [
35]. According to the number of ROS produced at a voltage of 24 V, as shown in
Fig. 4(a), 0.3Co-MOF/Cu@Cu can produce an increased amount of ROS, indicating that the increasing effect of the electric field also promotes ROS generation [
36]. More details are provided in Appendix A.
Moreover, the higher the voltage, the higher the efficiency of ROS production and bacteriostatic efficiency [
36]. Fig. S13 in Appendix A shows the staining images of ROS after different voltage treatments. The increase in ROS leads to the death of bacteria, explaining the increased sterilization rate in
Fig. 4(c).
It can be concluded from the above research data that the microstructure, composition, and semiconductor properties of the material surface under the applied electric field increase the electric field intensity and ROS concentration around the nanostructure, thus electroporating bacteria and causing an oxidative stress reaction leading to bacterial death. Compared with previously studied sterilization materials, the proportion of ROS in the proposed sterilization method was found to be greater; therefore, the mechanism of ROS production required further investigation.
The presence of oxygen vacancies is very important for the semiconductor performance and the electric field generation. They affect the catalytic performance by changing the physical and chemical properties and the electronic structure [
37]. Oxygen vacancies can act as active sites in some cases to adjust the local electronic structure [
38]. Studies have shown that surface oxygen defects can improve the separation efficiency of electrons and holes and play a crucial role in the mobility and separation efficiency of carriers [
39,
40]. The density of charge carriers is increased by exciting delocalized electrons from oxygen vacancies to conduction bands, improving the conductivity of the semiconductor catalyst [
41]. As shown in
Fig. 5(b), the peak O 1s spectra at 531.0 and 531.4 eV come from the lattice oxygen and oxygen-vacancy-adsorbed oxygen, respectively [
42,
43]. The presence of oxygen defects leads to obvious coordination of oxygen species. The peak intensity of 0.3Co-MOF/Cu@Cu at 531.4 eV (i.e., the oxygen-vacancy-adsorbed oxygen) is significantly higher than that of Cu-MOF/Co@Cu, which confirms that 0.3Co-MOF/Cu@Cu has more oxygen defects. Oxygen vacancies improve the conductivity of the material, which corresponds to the results shown in
Figs. 3(c) and
(d). The oxygen vacancies can adsorb oxygen molecules in the environment and inhibit the recombination of electrons and holes, showing excellent catalytic activity. This characteristic of the material is beneficial to the activation of molecular oxygen and the production of more ROS.
The ROS staining image only shows the oxygen species produced inside the bacteria. In order to show the ROS produced by the material itself, the energy band structure of 0.3Co-MOF/Cu@Cu was studied (
Fig. 5(c); Fig. S14 in Appendix A). When a voltage is applied, the semiconductor is excited by the electric field and the energy greater than the forbidden bandwidth, causing the directional movement of carriers [
39,
44,
45]. The electrons jump to the conduction band; at the same time, the valence band produces electrically excited holes, and an oxidation-reduction reaction takes place on the semiconductor surface [
39, [
46], [
47], [
48]]. It can be seen from the energy band diagram that the conduction band positions of both Cu
2O and Co-MOF/Cu are more negative than the generation potential of a superoxide anion (−0.33 eV), indicating that they have the potential to generate ·O
2-, as was also demonstrated via EPR (
Figs. 5(d) and
(e)). As shown in
Fig. 5(d), four lines with relative intensities of 1:2:2:1 (the characteristic spectrum of spin adduct 5,5-dimethyl-1-pyrroline-
N-oxide (DMPO)/·OH) are not observed in all groups, which means that ·OH is not the main free radical of the 0.3Co-MOF/Cu@Cu electrodes. When using methanol as the specific scavenger of hydroxyl radicals and trapping labeled DMPO, there were obvious signals of ·O
2- on the Cu
2O and 0.3Co-MOF/Cu@Cu electrodes (
Fig. 5(e)). In general, the main free radical of the 0.3Co-MOF/Cu@Cu electrodes is ·O
2-.
The production of superoxide anions is the result of molecular oxygen activation. After a voltage is applied to 0.3Co-MOF/Cu@Cu, directional movement of the carriers occurs when excited by energy greater than the band gap. This causes the molecular oxygen adsorbed by the oxygen vacancies to gain an electron, weakening the double bond between oxygen and oxygen to generate a superoxide anion. The large number of active sites and oxygen vacancies act as bridges for electron transfer, accelerating the transfer of electrons from the catalyst to molecular oxygen and thereby promoting the activation process of molecular oxygen. The specific process is as follows:
${{\text{O}}_{2}}+{{\text{e}}^{-}}\to \cdot {{\text{O}}_{2}}^{-}$
The higher ROS generation in comparison with other materials increases the concentration of superoxide anions, which will increase the concentration of anions in an indoor environment to a certain extent, improving the comfort level for the people in that environment [
49].
Considering the high density of crowds in many indoor gathering places, it is easy for bacteria to spread through the air. Consequently, there is an urgent need to apply the research presented here to indoor environments in order to reduce the risk of infectious diseases.
Fig. 6(a) depicts a conceptual indoor sterilization application. A filter electrode is installed in an air conditioner as a filter layer to kill bacteria.
Fig. 6(b) shows an enlarged view of the inside of the air conditioner. After air carrying bacteria passes through the filter layer, the concentration of bacteria decreases.
The whole sterilization process is shown in
Fig. 6(c). The high-efficiency sterilization ability of the conceptual device is the result of the joint action of a locally enhanced electric field and active oxygen. Under the action of an external electric field, the electric field intensity on the electrode increases, and oxygen is electrochemically reduced to form active oxygen species (i.e., ·O
2-) [
25]. As the electro-filtration operation proceeds, biological aerosols containing bacteria are continuously adsorbed on the electrode, and electrochemical reactions continue to occur to generate ·O
2-, realizing the local enrichment of ·O
2− and generating a large number of exogenous ROS. At the same time, the material presents a strong local electric field after being electrified, which increases the permeability of the cell membrane and even generates electroporation [
35], causing exogenous ROS to enter and attack bacteria.
The 3D nanostructures provide a large surface area, increase the electro-adsorption capacity of the materials, and have more opportunities to come into contact with biological aerosols. At the same time, the exposure of a large number of active sites effectively transfers electrons from the internal NWs to the surrounding MOF nanosheets, resulting in a significant increase in the electron transfer rate [
22, [
50], [
51], [
52]]. The free electron migration on the surface of the material affects the redox potential on bacterial cell membranes, destroying the steady state of the cell membrane and causing the generation of endogenous ROS in the bacteria [
53]. The ROS then destroy the bacterial proteins and DNA, including adenosine triphosphate, electrolytic byproducts (peroxides, oxidative free radicals, etc.), enzyme oxidation, and so forth, affecting the metabolism of the bacteria [
25,
54].
In sum, thanks to the expanded local electric field intensity range and the band structure of the proposed material, the bacteria finally die under the double attack of a greater electroporation effect and more ROS inside and outside the cell.
3. Conclusions
An efficient and stable air-sterilization material is urgently needed to ensure the safety of living environments. In this work, MOF nanosheets were grown in situ on copper hydroxide NWs to produce 0.3Co-MOF/Cu@Cu. This process not only inhibited the corrosion of the 1D NWs in water but also facilitated the charge separation of the composite and improved the catalytic properties of the materials. The combination of NWs and MOF nanosheets is robust, which increases the mechanical stability of the material. At the same time, the increase in the specific surface area improves the adsorption of bio aerosol and increases the number of reactive sites on the material. The MOF grown in situ demonstrated low impedance and good stability in alkaline medium. Under the action of an external electric field, the electric field intensity at the electrode increases; this leads to electroporation and an electrocatalytic reaction, resulting in a large number of ·O2-. In addition, there are many free electrons on the surface of the material, which disturb the physiological activities inside bacteria, resulting in the production of ROS within bacteria cells that then kill the bacteria. Under an applied voltage of 24 V and an air flow rate of 1.5 m∙s−1 (equivalent to a treatment time of 0.0026 s), the sterilization rate of the proposed material reaches 99.51%. These findings provide new ideas and references for the design, preparation, and practical application of functional air-sterilization materials.
CRediT authorship contribution statement
Liting Dong: Data curation, Writing - original draft, Writing - review & editing. Shougang Chen: Resources, Project administration, Supervision, Funding acquisition. Zhipeng Zhao: Formal analysis, Software. Xiao Sun: Writing - review & editing. Gaojian Lv: Formal analysis, Software. Wei Wang: Formal analysis, Software. Chengcheng Ma: Investigation. Chunchao Hou: Formal analysis. Wen Li: Formal analysis, Investigation. Jiakun Wang: Investigation. Jianglin Gou: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51972290 and 22102086), the National Natural Science Foundation Joint Fund, China (U1806223), the Fundamental Research Funds for the Central Universities (202065001), and the Key Technology Research and Development Program of Shandong Province, China (2020CXGC010703).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2025.05.020.
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