Introduction

Groundwater accounts for 99% of all liquid freshwater on earth and provides nearly 50% of the global population’s domestic water and 25% of agricultural irrigation water, which are critical to poverty eradication, food and water security, and socio-economic development1. Previous studies show that groundwater, associated with many outbreaks2, is a major global reservoir of antibiotic-resistant bacteria3. The waterborne diseases are a sustainable issue for the developing country2. Globally, water disinfection relies on chlorination, however, by-products of chlorination is linked with cancer3. There is a great requirement for focusing on disinfectants without chemical residue formation4,5. Reactive oxygen species (ROS) disinfectants, such as superoxide radical (·O2−), can offer broad-spectrum antibacterial activity without chemical residues5. Among different techniques, photocatalysis provides a sustainable approach to produce ·O2− by activating oxygen (O2). However, low ·O2− flux in the reported photocatalytic systems greatly limits the disinfection efficiency. Therefore, enhancing the photocatalytic ability for the ·O2− production is highly desirable in photocatalytic disinfection.

O2 is the most environmental oxidant on earth, and the activation of O2 strengthens its reactivity6. For the spin distribution in the O2 molecule, a pair of half-occupied π* orbitals endow O2 with nonzero spin (s = 1; mz = 2μB), which is the source of spin-forbidden7,8. If π* orbitals obtain an electron, the spin states of O2 will be split and transformed into ·O2−. However, the spin-forbidden of O2 impedes this electron transfer, leading to the low-efficiency generation of ·O2−, and thus low disinfection efficiency. Till now, in low DO water, such as groundwater, how to eliminate the spin-forbidden of O2 is the key to improving the O2 utilization and sterilization efficiency, however, remains a grand challenge.

Herein, we report a class of 2D/2D In2S3/Mo4/3B2−xTz (IS/MB) heterojunction, in which MB as cocatalyst with atomic strain produces the spin polarization for high-efficiently generate the ·O2− for boosting the disinfection efficiency in groundwater. In-situ electron paramagnetic resonance (EPR) and molecular orbital analysis reveal that spin-polarized orbitals hybridization between IS/MB-20 and O2 can eliminate the spin-forbidden of O2 for facilitating photoelectron transfer for efficient ·O2− production. The as-made IS/MB-20 shows a 16.59-fold increase in ·O2− production relative to IS and exhibits unprecedented MRSA disinfection efficiency (8.06-log reduction within 15 min under visible light irradiation). Continuous flow disinfection system equipped with 50 mg IS/MB-20 can produce 37.2 L bacteria-free water and also shows durability over 62 h, which represents state-of-the-art photodisinfection materials for groundwater disinfection. Furthermore, the continuous-flow-disinfection system’s capacity for disinfection in groundwater is 25 times greater than that of commercial NaOCl.

Results

Synthesis and chemical structure of MB

Mo4/3B2−xTz MBene (MB) was prepared through a HF etching and delamination process of (Mo2/3Y1/3)2AlB2 (i-MAB) (Supplementary Figs. 1, 2)9. The average thickness of as-made MB-20 (20 h HF etching) nanosheets is about 5.67 nm (Supplementary Fig. 3). The dramatical reduction in Y 3d and Al 2p X-ray photoelectron spectra (XPS) peaks of MB-20 confirms the removal of Al and Y atoms during the MB synthesis (Supplementary Figs. 4, 5). X-ray diffraction (XRD) patterns indicate that the peak intensities of (Mo2/3Y1/3)2AlB2 precursor are significantly decreased after the HF treatment (Supplementary Fig. 6). As a comparison, the (000l) peak of MB-20 shifts to a lower diffraction angle of 2θ = 7.47 o, suggesting that the MB-20 is composed of stacked 2D sheets, with an increased d-spacing value (d) of 11.83 Å9.

The aberration corrected-spherical transmission electron microscope (AC-STEM) image of MB-20 shows that it has a hexagonal crystal structure (Supplementary Fig. 7). The uneven brightness of atoms in Supplementary Fig. 8a indicates Mo vacancies are formed. The corresponding three-dimensional (3D) atom-overlapping Gaussian-function fitting mappings further demonstrate the formation of Mo in MB-20 formation. Moreover, the simulated STEM image of MB-20 also demonstrates a darker signal of the Mo vacancies (Supplementary Fig. 8b), in consistent with the experimental results. In the Fourier transform of Mo K-edge extended X-ray absorption fine structure (FT-EXAFS) spectra in R space (Supplementary Fig. 9), MB-20 emerges a primary peak at 1.84 Å, confirming the existence of Mo–B coordination (Supplementary Table 1). The coordination number of Mo–B in MB-20 is determined to be 6.31. Furthermore, based on crystal structure information provided by AC-STEM, FT-EXAFS, and previous study9, we built a crystal structure model of MB-20 (Supplementary Fig. 10). The simulated electron diffraction (ED) and simulated X-ray diffraction (XRD) patterns of MB-20 model are in accordance with the experiment results (Supplementary Figs. 6, 7), supporting the validity of the established crystal structure.

Atomic strain-induced spin polarization in MB

Figure 1a shows the atomic structure and GPA result of MB with different etching times (18–21 h). MB-18 (18 h HF etching) exhibits a regular hexagonal close-packed (hcp) atomic arrangement, and GPA data indicates the existence of slight in-plane strain in MB-18 (average strain: +0.53%). As a comparison, MB-19/20/21 samples experience the tensile strain (average strain: +2.84%, +5.37%, and +8.19%, respectively)10. The precise control of the HF etching time is crucial for regulating the strain degree. XPS, inductively coupled plasma optical emission spectrometer (ICP-OES), and XRD results also demonstrate that MB undergoes the Mo vacancy formation and structure relaxation in the etching time of 18–21 h (Supplementary Figs. 11–13).

Fig. 1: Structure characterization and structure–activity relationship between atomic strain and spin polarization.
figure 1

a AC-STEM and GPA images of MB with different HF etching times (18–21 h). Ɛxx is the strain along the horizontal axis. b The EPR  of MB-18/19/20/21. c Structure–activity relationship among Mo vacancy, tensile strain, and spin polarization. All error bars represent the standard deviation of three independent measurements. d HRTEM image of IS/MB-20. e Magnetic hysteresis loop of IS and IS/MB-20. f and g HRTEM images of areas 1 and 2, and corresponding strain mapping images based on GPA. The inset image in Fig. 1f is the SAED image of MB-20.

Full size image

To reveal the effect of strain on MB’s electronic structure, density functional theory (DFT) calculations were performed. Based on the concentration of Mo vacancy obtained from ICP-OES (Supplementary Fig. 12), the crystal structures of various MB with different amounts of Mo vacancy were established (Supplementary Fig. 14). Supplementary Fig. 15 shows the density of states (DOS) of MB-4%/8%/12%/16% (theoretical model representing MB-18/19/20/21). In general, the difference between the spin-up and spin-down integral DOS (∆IDOS = IDOSspin-down−IDOSspin-up) represents the spin polarization11,12. Among these MB models, MB-12%, which simulates the MB-20 with 11.61% Mo vacancies, has a more pronounced spin polarization than other materials according to the largest ∆IDOS = 1.66 (Supplementary Fig. 15). Electron paramagnetic resonance (EPR) results are consistent with calculated spin polarization (Fig. 1b). At room temperature, all the MB materials exhibit paramagnetic character, attributed to unpaired electrons. The intensity of paramagnetic character spectra increases with increasing Mo vacancy concentration from MB-18 to MB-20, similar to other strained transition-metal 2D materials13. However, the magnetization is weakened when the Mo vacancy concentration is too high in MB-21. In summary, the structure–activity relationship of atomic strain and spin polarization in MB is shown in Fig. 1c. The spin-charge density of MB-12% illustrates asymmetric electron spin polarization distribution (Supplementary Fig. 16), and a large area of negative spatial spin polarization in MB-12% indicates a small possibility of spin polarization flip.

To explore the spin polarization mechanism in the strained MB, the wave function analysis was performed. In Supplementary Fig. 17, there is an obvious d–p orbital hybridization between the Mo dxy orbital and the boron six-membered ring in MB. Obviously, strain weakens the orbital hybridization between Mo and B in MB-12%. To verify the rationality of theoretical calculation, we performed the soft X-ray absorption spectroscopy (XAS) analysis for all MB materials (Supplementary Fig. 18). We found that HF etching breaks the crystal structure of MB, which leads to both weakened 5s → σ* and 5s → π* signals, suggesting weaker hybridization between Mo and B orbitals14. Generally, when the strain changes orbitals, the orbital magnetic moment will inevitably lead to the transformation of the electron spin-state, resulting in the spin–orbit coupling13,15. Spin–orbit coupling is the main cause of spin polarization.

Chemical and band structures of IS/MB

The electronic band structures of MB were analyzed based on DFT calculations. Substantial electronic states crossing the Fermi level, and the conductivity test results demonstrate the metallic conductivity of MB-20, implying its exceptional capability to extract electrons from semiconductors (Supplementary Fig. 20). Therefore, MB-20 can be introduced into semiconductors to afford a Schottky heterojunction photocatalyst. In this study, In2S3 (IS) is selected as a model semiconductor to couple with MB-20 for heterojunction construction, and the role of MB-20 in photocatalysis is explored considering its band alignment and 2D expandable region (Supplementary Fig. 21).

IS/MB-20 heterostructure was then fabricated by a facile self-assembly method. TEM and SEM images display the basic nanoflower structure of IS/MB-20 (Supplementary Figs. 21, 22). High-resolution TEM (HRTEM) image of IS/MB-20 shows a dual-phase heterostructure (Fig. 1d). Hysteresis loop results further reflect the spin polarization in IS/MB-20. At room temperature, IS/MB-20 exhibits paramagnetic character, attributed to the introduction of strained MB-20 (Fig. 1e). In Fig. 1f, fast Fourier transform (FFT) data of Area 1 reveals a hexagonal symmetry, which could be indexed to the MB phase. GPA data further demonstrate the lattice tensile strain of Area 1 (+4.98%), which is inherited from the MB-20. As a comparison, IS shows a weak strain (−0.08%) (Fig. 1g). HRTEM results of IS/MB-20 indicate that stained MB-20 was successfully embedded into IS. And GPA data suggest that the MB-20 in IS/MB-20 composite maintains the tensile strain structure. In addition, element mapping, XRD, and XPS data further confirm the successful preparation of IS/MB-20 (Supplementary Figs. 21e, 23, 24).

The band alignments of IS/MB-20 are calculated according to experiment data (Supplementary Figs. 25–27). The different potential between IS and MB-20 induces band bending. Thus, a Schottky barrier is formed at the interface (Supplementary Fig. 27b). The Schottky barrier can prevent the photogenerated charges trapped by the electron acceptor (like MB-20) from flowing back to the semiconductor (like IS).

MB enhancing ·O2 – photocatalytic production

The energy level diagram reveals that IS/MB-20 can produce ·O2– due to its more negative CBM position (−0.67 eV) than that of O2/·O2– (−0.33 eV) (Supplementary Fig. 27b). Under visible light irradiation, in-situ EPR spectra of IS/MB-20 confirm the production of ·O2– with a hyperfine coupling constant of AN = 13 G and AHβ = 9.5 G (Fig. 2a). The ability for ·O2– production follows the order of IS/MB-20 > IS/MB-21 > IS/MB-19 > IS/MB-18. ·O2– production is related to the spin polarization intensity of MB. Moreover, IS/MB-20 exhibits a stronger EPR signal than IS (16.59-fold increase), indicating it has a stronger ability to produce the ·O2– under visible light irradiation.

Fig. 2: Spin polarization enhanced O2 adsorption.
figure 2

a In-situ EPR spectra of·O2– produced by various materials in low DO condition (2.46 mg L−1) under visible light irradiation. b Adsorption energy of O2 on various materials. The inset figures from left to right represent the configurations of O2 adsorption on the surface of In2S3, IS/MB-perfect, and IS/MB-12%. c Energy levels of IS/MB-perfect’s FMOs and O2’s π* orbitals. d Energy levels of IS/MB-12%’s FMOs and O2’s π* orbitals.

Full size image

To verify the promoting effect of MB in photocatalytic ·O2− generation, the photogenerated charge carrier separation and change lifetime were investigated. The ground state bleaching (GSB) signals of IS/MB-20 are stronger than those of IS, indicating more efficient carrier dynamics in IS/MB-20 than in IS (Supplementary Figs. 28, 29). Transient photocurrent responses, surface photovoltage, and fluorescence emission decay spectra results also provide solid evidence for this claim (Supplementary Fig. 30)16,17. The efficient charge separation can be attributed to the Schottky barrier at the hetero-interface, which facilitates the photocatalytic ·O2− production.

The charge dynamics results obtained from in-situ XPS suggest that electrons accumulate on Mo atoms of IS/MB-20 during photocatalysis (Supplementary Fig. 31). Thus, ·O2– is mainly generated on Mo sites after gaining the electrons. Charge density difference and Bader charge analysis results indicate electrons transfer from Mo to O atom and also confirm the activation of O2 on the Mo sites (Supplementary Figs. 32, 33). The adsorption energies of O2 on different materials are calculated and presented in Fig. 2b. O2 is more likely adsorbed on IS/MB-12% configuration (Eads = −2.52 eV) than IS/MB-perfect (Eads = −2.06 eV) and IS (Eads = −0.18 eV) due to the more negative adsorption energy18. These results suggest that spin polarization improves the adsorption of O2 on the material’s surface.

Molecular orbital theory is further employed to analyze the orbital interaction of Mo and O2. Projected density of states (PDOS) in Supplementary Fig. 34 shows that s−s and s−p orbitals are overlapped between Mo and B, thus enabling the formation of σ bonds. Generally, FMOs have higher reactivity than other orbitals19. The FMOs of IS/MB-12% are mainly composed of \({{{\rm{Mo}}}}\,4{d}_{xz}\), \({{{\rm{Mo}}}}\,4{d}_{{z}^{2}}\), and \({{{\rm{Mo}}}}\,4{d}_{yz}\) orbitals (Supplementary Figs. 35–38). In addition, the hexahedral configuration of MB gives it C6v symmetry. Based on the incommensurable representation of the C6v point group and crystal field theory, the orbital interaction between Mo and B is shown in Supplementary Fig. 39. Obviously, the \({{{\rm{Mo}}}}\,4{d}_{xz}\), \({{{\rm{Mo}}}}\,4{d}_{{z}^{2}}\), and \({{{\rm{Mo}}}}\,4{d}_{yz}\) constitute the FMO of Mo, in consistent with the PDOS results in Supplementary Fig. 34.

During O2 adsorption and activation, the \({\pi }_{2{p}_{x}}^{\ast }\) and \({\pi }_{2{p}_{y}}^{\ast }\) bonds of O2 can interact with the FMOs of IS/MB-12%, because these orbitals are not fully occupied (Supplementary Fig. 40). Then PDOS spectra reveal the orbitals hybridization between FMOs of IS/MB and π* orbitals of O2 (Supplementary Fig. 41). In addition, projected crystal orbital Hamilton population (pCOHP) analysis demonstrates the \({{{\rm{Mo}}}}\,4{d}_{yz}\)−O 2py, and \({{{\rm{Mo}}}}\,4{d}_{xz}\)−O 2px orbital hybridization states (Supplementary Fig. 42). Figure 2c, d and Supplementary Fig. 43 summarize the orbital hybridization energy levels between FMOs of IS/MB and π* orbitals of O2 based on PDOS and pCOHP results. Obviously, the spin-polarized FMOs in IS/MB-12% exhibit a strong coupling character to O2’s π* orbitals, thus reducing the O2 adsorption energy19. After O2 is absorbed on IS/MB-perfect, \({{{\rm{Mo}}}}\,4{d}_{xz}\)−π* and \({{{\rm{Mo}}}}\,4{d}_{yz}\)−π* bonds are formed and located at −0.86 eV. As a comparison, after FMOs of IS/MB-12% are hybridized with O2’s π* orbitals, \({{{\rm{Mo}}}}\,4{d}_{xz}\)−π* and \({{{\rm{Mo}}}}\,4{d}_{{z}^{2}}\)−π* bonds are formed with spin energy splitting. Specifically, the spin-down \({{{\rm{Mo}}}}\,4{d}_{xz}\)−π* and \({{{\rm{Mo}}}}\,4{d}_{{z}^{2}}\)−π* bonds remain at the same level (E = −1.72 eV), while the spin-up \({{{\rm{Mo}}}}\,4{d}_{xz}\)−π* and \({{{\rm{Mo}}}}\,4{d}_{{z}^{2}}\)−π* bonds are located at −1.85 and −1.68 eV, respectively. In addition, for IS/MB-perfect, the integrate pCOHP (ICOHP) of spin up/down \({{{\rm{Mo}}}}\,4{d}_{yz}\)−O 2py and \({{{\rm{Mo}}}}\,4{d}_{xz}\)−O 2px bonds are −0.69. In contrast, the ICOHP of \({{{\rm{Mo}}}}\,4{d}_{{z}^{2}}\)−O 2px bond shifts to −0.73 in IS/MB-12%-O2, indicating stronger bonding than IS/MB-perfect. The different energy levels and bonding strengths of Mo–O bonds are mainly attributed to different spin-orbital hybridization during O2 adsorption19, and the spin–orbital hybridization in IS/MB-12%-O2 facilitates spin state transformation of O2. Supplementary Fig. 44 presents the real space orbital wave-functions post-interaction of IS/MB-perfect-O2 and IS/MB-12%-O2 (with O2 adsorbed) configuration, respectively. Generally, the overlapping of orbital wave-functions represents orbital hybridization19. The FMOs exhibit obvious orbital overlapping with O2’s π* orbitals, which is consistent with the energy levels diagram in Fig. 2c and d.

Supplementary Fig. 45 displays the orbits and electron arrangement in ·O2–. The mechanism of ·O2– formation is filling the π* orbitals with an electron, however, the spin-forbidden nature of O2 hinders this electron transfer. To investigate the electron transfer between Mo sites and O2, the EPR measurements on IS/MB-20 in the Ar/Air atmosphere under light irradiation were conducted (Fig. 3a). In the Ar atmosphere, IS/MB-20 exhibits the existence of unpaired electrons. However, in the Air atmosphere, the intensity of unpaired electrons signal of IS/MB-20 is reduced, indicating the number of spins is reduced after O2 adsorption. This suggests that the O2 can withdraw spin electron density from Mo active sites. In addition, a new weak peak with g = 2.013 is observed under the illumination of IS/MB-20, suggesting the generation of ·O2–. Interestingly, the spin-charge density data demonstrate the spin state transformation of O2 after adsorption on IS/MB-12% (Fig. 3b), as the O2 molecule on IS/MB-12% shows less spin-charge density than that on the IS/MB-perfect configuration, indicating the spin degeneracy. This spin state transformation indicates spin electron rearrangement in π* orbitals and eventually leads to the zero spin states (Fig. 3c). As mentioned above, spin-polarized FMOs coupled with O2’s π* orbital will lead to the formation of strong Mo–O bonds (Fig. 2d and Supplementary Fig. 43). The rearrangement of the energy levels can facilitate spin electron redistribution, and result in zero spin states20. Furthermore, the zero-spin state eliminates the spin-forbidden and thus promotes the generation of ·O2– (Fig. 3d).

Fig. 3: Spin polarization improving the molecular orbital activation in ·O2– formation.
figure 3

a EPR spectra of IS/MB-20 in an Ar/Air atmosphere under light irradiation. b Spin charge density of IS/MB-perfect-O2 and IS/MB-12%-O2 configuration. c The molecular orbital energy and spin electron distribution diagram for O2 and zero spin states O2. d Schematic diagram of spin electron transfer from FMOs of IS/MB to π* orbitals of O2.

Full size image

Photocatalytic disinfection activity of IS/MB

Methicillin-resistant Staphylococcus aureus (MRSA) is a common superbug, which causes bacterial infections in healthcare and community environment21,22. Coagulases of MRSA allow it to proliferate as thromboembolic lesions and are hard to kill, causing a serious infection23,24,25,26. In addition, MRSA belongs to Gram-positive bacteria. The cell wall of Gram-positive bacteria is composed of peptidoglycan, which prevents the ROS produced by photocatalysis from penetrating the cell wall to attack bacterial cells. Therefore, MRSA, a common antibiotic-resistant bacterium, has a higher greater threat to humanity. Therefore, we choose MRSA as our target. Figure 4a shows no significant MRSA colony-forming units (CFU) reduction was observed for all materials under dark conditions, indicating a weak disinfection effect. However, under 15 min of visible light irradiation, an 8.06-log reduction of CFU was observed after incubating with IS/MB-20. The higher antibacterial performance of the IS/MB-18/19/20/21 than the IS group, demonstrating that MB cocatalyst can enhance the photocatalytic sterilization property (Fig. 4a and Supplementary Fig. 46). To the best of our knowledge, the developed IS/MB-20 is the most efficient photocatalytic disinfection material against MRSA ever reported (Fig. 4b and Supplementary Table 2).

Fig. 4: Antibacterial activities of IS/MB.
figure 4

a Quantitative analysis of bacterial colonies for various materials against MRSA after 15 min visible light (λ ≥ 420 nm, 110 mW cm−2) irradiation. No catalyst was added to the bacterial solution of the control group. Data are presented as mean ± s.d. (n = 4). The measurements of bacterial colonies were taken from distinct samples. Data are shown as box-and-whisker plots, with the median represented by the central line inside each box, the 25th and 75th percentiles represented by the edges of the box, and the whiskers extending to the most extreme data points. b Activity comparison of IS/MB-20 with reported state-of-art photocatalysts against MRSA. The inset image in Fig. 1b is the ruler of the bubble. c CLSM overlay projections of live (green fluorescent) and dead (red fluorescent) bacteria. The percentages of green and red fluorescent are plotted in the images. Scale bar: 50 µm. d Coagulase assay of MRSA treated with IS/MB-20 at different times. e Clustering heatmap for expression changes of DGEs. Red represents high expression of genes in the sample, and blue represents low expression. f Size of the MRSA-infected wound of mice during treatment with different materials for 10 days. Data are presented as mean ± standard deviations from a representative experiment (n = 3 independent samples). P-values were analyzed by a one-way ANOVA test.

Full size image

Furthermore, the live/dead fluorescence staining was used to examine the bactericidal activity against MRSA (Fig. 4c). After staining, live and dead bacteria with intact and ruptured cell membranes exhibit green and red fluorescence, respectively. The percentage of red fluorescence significantly rose to ~98% in the IS/MB-20 group, suggesting superior antibacterial activity27. As a comparison, a weak bactericidal effect is observed in the group of IS under light illumination, also demonstrating the key role of MB-20 in photocatalytic sterilization.

The ROS scavenging tests suggest that ·O2– is essential for the photocatalytic inactivation of MRSA (Supplementary Fig. 47)2, as almost no MRSA is inactivated in the presence of ·O2– scavenger (1,4-benzoquinone). Disinfection activity of materials is in the order of IS/MB-20 > IS/MB-21 > IS/MB-19 > IS/MB-18 > IS > MB-20, which is similar to the trend of the photocatalytic ·O2– production (Fig. 2a). Then, we tested the intracellular ROS production capacity of IS/MB-20 treated bacteria by fluorescent ROS probe (DCFH-DA). As shown in Supplementary Fig. 48, without light irradiation, MRSA in three groups showed weak green fluorescence, suggesting low ROS levels in cells28. However, compared with MRSA in dark groups, the fluorescence signals of MB-20, IS, and IS/MB-20 group after light treatment were significantly enhanced, indicating that the ROS content in MRSA cells was greatly enhanced under light illumination, which can be ascribed to oxidative stress induced by photocatalytic processes29. Furthermore, the IS/MB-20 group exhibited the most pronounced fluorescence signal following light exposure, suggesting that the ROS levels in MRSA cells were elevated and oxidative stress was markedly evident. Oxidative stress can cause physiological and biochemical dysfunction of MRSA cells and reduce their infectivity, which is consistent with the results of transcriptomic analysis in the next section.

SEM images in Supplementary Fig. 49 show that the bacterial membrane was severely damaged by IS/MB-20 under light irradiation. Thus, ·O2– can destroy the bacterial membrane and affect its permeability (Supplementary Fig. 50). The bacterial membrane potential assay and total protein determination results further prove that ·O2− can destroy cell membranes (Supplementary Figs. 51, 52)30. In addition, comparable antibacterial efficacy was observed in Gram-negative and antibiotic-sensitive Escherichia coli (E. coli). In detail, a significant improvement was observed in the disinfection performance for IS/MB compared to the control, MB-20, and IS groups. In addition, after 15 min visible light, the sterilization activity of IS/MB is in the order of IS/MB-20 > IS/MB-21 > IS/MB-19 > IS/MB-18 > IS > MB-20, which is consistent with the inactivation effect of MRSA. Furthermore, after visible light irradiation, the 99% attenuation of green fluorescence in IS/MB-20 group indicates that IS/MB-20 exhibits a significant inactivation effect on E. coli. These findings elucidate the broad-spectrum antibacterial activity of IS/MB-20 (Supplementary Fig. 53).

Inhibition of MRSA’s infectivity

The schematic diagram in Supplementary Fig. 54 describes that coagulase secreted from MRSA can promote the binding between MRSA cells, which favors and evades the opsonistic phagocytic clearance of host immune cells. Coagulase is the core virulence factor of MRSA in the process of wound infection21,22,31. After photocatalytic sterilization using IS/MB-20, MRSA cannot coagulate lyophilized plasma, indicating that MRSA bacteria cannot produce coagulase (Fig. 4d). This is compelling evidence that IS/MB-20 influences MRSA infection activity. Transcriptomic analysis reveals the apparent effect of IS/MB-20 on the gene expression of MRSA (Supplementary Fig. 55). IS/MB-20 treatment group has 448 differentially expressed genes (DEGs) in total (251 genes are up-regulated and 197 genes are down-regulated) relative to the control group. Figure 4e displays a significant color distinction between the control group and the IS/MB-20 treatment group, indicating the persistence of up- or down-regulated expression of DEGs between different samples in the group. Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis further prove that photocatalytic treatment using IS/MB-20 greatly affects MRSA’s fundamental metabolic process, which will limit its coagulase formation (Supplementary Figs. 56–59)32,33,34.

The aforementioned findings support our theoretical framework regarding IS/MB-20’s capacity to inhibit the viability and infection activity of MRSA (Supplementary Fig. 60). Furthermore, a rat wound infection model with MRSA (1 × 108 CFU mL−1) was constructed to investigate the inhibiting MRSA infection performance of IS/MB-20 (Supplementary Figs. 61–66). IS/MB-20 can heal the MRSA-infected wounds in the established rat model within 10 days (Fig. 4f). In addition, a large amount of MRSA strain lived on the wounds of the control group after 2 days of infection. In comparison, the IS/MB-20 group exhibits negligible viable MRSA strain (Supplementary Fig. 63). These data demonstrate the inhibition of IS/MB-20 on MRSA infection.

Photocatalytic disinfection of groundwater

Due to the low dissolved oxygen content in groundwater, it is difficult to produce ·O2–. The key to improving the disinfection effect is to enhance the O2 adsorption/activation of the bactericidal material. Herein, we tested the disinfection effect of the IS/MB-20 in groundwater. Figure 5a demonstrates that the DO content in groundwater is 1.46–2.21 mg L−1, which is lower than that in tap water (7.54–10.19 mg L−1). In addition, we designed an IS/MB-20-based water disinfection continuous flow system (Fig. 5b, Supplementary Fig. 67a), which can achieve 100% photocatalytic disinfection of bacteria on-site in water from groundwater with low DO (Fig. 5c, Supplementary Fig. 67b). It is important to note that practical water disinfection test was carried in the ambient conditions of (34°18′ N; 107°9′ E). To standardize the tests, the parameters are set by the standard ambient temperature and pressure (SATP), which are 22 °C and 1.013 bar, respectively. The stability of the continuous flow system was certificated over 62 h, and produced 37.2 L bacteria-free water under the condition of the flow rate of water: 10 mL min−1; catalyst mass, 50 mg (Fig. 5d). The results of inductively coupled plasma-mass spectrometry (ICP-MS) detection showed negligible changes of In and Mo concentrations in the water samples before and after photocatalytic disinfection (Supplementary Fig. 68a), suggesting that IS/MB-20 exhibited good stability and would not affect water quality. In addition, after photocatalytic reaction, the IS/MB-20 maintained the morphology of nano-flowers, further confirming its high stability (Supplementary Fig. 68b). This exceptional stability of IS/MB-20 exceeds other top photocatalytic materials reported, which represents state-of-the-art photo-disinfection materials for practical water disinfection (Fig. 5e)35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53.

Fig. 5: Disinfection performance of IS/MB-20 in groundwater with low DO content.
figure 5

a DO content of tap water and groundwater (n = 13). The measurements of DO content were taken from distinct samples. b Photograph of the continuous flow photocatalytic disinfection. c Groundwater disinfection performance (n = 11) of the continuous flow system. The measurements of bacterial colonies were taken from distinct samples. d Stability of the continuous flow system. The inset images are the plating photographs of water samples after 62, 63, and 64 h photocatalytic disinfection. e Stability comparison of IS/MB-20 with other photocatalysts reported. f Plating photographs showing the disinfection effect of different dosages of NaOCl in 37.2 L groundwater. g Dosage of NaOCl and IS/MB-20 required to produce 37.2 L of bacteria-free water. Data are shown as box-and-whisker plots, with the median represented by the central line inside each box, the 25th and 75th percentiles represented by the edges of the box, and the whiskers extending to the most extreme data points.

Full size image

In order to evaluate the practical application potential of the system, we compared it with the bactericidal capacity of commercial NaOCl. Generally, chlorine-derived disinfectants exhibit high disinfection efficiency only over extended contact times. Therefore, we extended the sterilization time to 62 h, consistent with the IS/MB-20 disinfection test. Under the same conditions (37.2 L groundwater, 22 °C and 1.013 bar), 100% sterilization efficiency was achieved until the dosage of NaOCl reached 1250 mg (Fig. 5f). Above all, the disinfection capability of IS/MB-20-based continuous flow system is 25.0 times that of commercial sodium hypochlorite (NaOCl) disinfectant (Fig. 5g).

Discussion

We report a class of strained MB/IS composite photocatalysts for eliminating the spin forbidden of O2 to boost the ·O2− photocatalytic production to achieve efficient water disinfection. Ascribed to its optimal tensile strain, IS/MB-20 exhibits outstanding ability for photocatalytic ·O2− production. We find that the strain-induced spin-polarized FMO can hybridize with π* orbital of O2 to eliminate the spin forbidden of O2, which leads to the unprecedented photocatalytic disinfection performance (15 min, 8.06-log reduction of MRSA). Moreover, an IS/MB-20-based continuous flow system can produce 37.2 L bacteria-free water with high durability of over 62 h. The system is 25 times more capable of sterilizing groundwater than commercial NaOCl. This study highlights the significance of rational tensile strain engineering in MBene cocatalyst to improve the performance of photocatalytic water disinfection in low DO conditions.

Methods

Materials and reagents

All chemicals used are commercially available and were used without any additional purification steps. (Mo2/3Y1/3)2AlB2 (i-MAB, FoShan Xinxi Technology Co. Ltd.), hydrofluoric acid (HF, 40 wt%), tetrabutylammonium hydroxide (TBAOH, 10 wt%), Indium chloride tetrahydrate (InCl3, 99.9%), ethylene glycol (EG, >99%), Thioacetamide (≥99.0%).

Preparation of MB

1 g of (Mo2/3Y1/3)2AlB2 precursor was gently introduced into 20 mL of HF acid, and then magnetically stirred for 18, 19, 20, and 21 h at room temperature (25 °C), respectively. The resultant suspension was then transferred to a 50 mL centrifuge tube, and centrifugally washed (4025×g, 5 min) multiple times with degassed deionized (DI) water to eliminate the leftover acid until the supernatant became neutral. Then, the precipitation obtained by the previous step of centrifugation will be further intercalated and delamination.

Synthesis of IS/MB

10 mL TBAOH was added to the precipitation and shaken for 3 min. The mixture was centrifuged (2795×g, 5 min) to remove the supernatant. Ethanol and water were added to the centrifuge tube to centrifugal wash away residual TBAOH (5000 rpm for 5 min, repeated 3 times), respectively. Finally, 20 mL DI water was added to the precipitation, which was shaken for 20 min. The solution was then centrifuged at 3000 rpm for 5 min to obtain colloidal suspension. The suspension was the colloidal dispersion of MB-18, MB-19, MB-20, and MB-21 sheets, respectively.

1.14 mL of 28.0 wt% ammonia solution was added into 20 mL colloidal dispersion of MB sheets. Then, 0.18 g of ammonium hydrogencarbonate solution (in 2.28 mL DI water) was added dropwise, accompanied with the formation of floccule. Afterward, the MB floccule was freeze-dried (−30 °C, 6 h) and then annealed at 150 °C in Ar atmosphere for 10 h to obtain MB powders54.

Preparation of IS nanoflowers and IS/MB

For the preparation of IS, 0.1769 g InCl3 was dissolved in 30 mL EG, followed by the addition of 0.1202 g thioacetamide. After magnetically stirring for 40 min, the mixture was poured into a 50 mL Teflon-lined reactor, and heated at 180 °C for 24 h. The obtained IS precipitate was washed with DI water and ethanol three times. Then, pure IS was obtained by drying at 80 °C for 12 h in an oven. For the IS/MB synthesis, 200 mg of pure IS was re-dissolved in 30 mL DI water. Then, 1 mL 0.3 M MB sheets colloidal dispersion was added to the IS solution, and stirred for 6 h to get IS and MB self-assembly. The obtained IS/MB precipitate was washed and dried under the same conditions to obtain pure IS/MB powder.

Characterizations

The defined morphology and composition of the materials were obtained using SEM (S-4800 FE-SEM). Elemental composition and oxidation state were obtained on X-ray photoelectron spectroscopy (ThermoFisher Scientific 250Xi, USA). All the binding energies were calibrated by the C 1s peak located at 284.8 V. Similar conditions were used for in-situ irradiation XPS measurements, while light irradiation was introduced. A high-resolution transmission electron microscope (HRTEM) was performed on the JEOL JEM-2100 F transmission electron microscope. The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images were obtained on Titan Cubed Themis G2 200. XAFS tests were performed at the 1W1B station in BSRF (Beijing Synchrotron Radiation Facility, China). Femtosecond transient absorption spectra (fs-TAS) were detected by using a pump-probe system (Femto-TA100). UPS spectra were obtained on a PHI5000 VersaProbe III (Spherical Analyzer). A JEOL JES-FA200 electron spin resonance (EPR) spectrometer was used to detect the signals of the free radicals under 300 W Xenon Arc light.

Geometric phase analysis

Geometric phase analysis is a digital signal processing method for quantifying displacements and strain fields at atomic resolution. In this study, we utilized an FRWR tools plugin from the Humboldt University of Berlin, which can be plugged into the DigitalMircograph (Gatan) software to establish the strain mapping of HRTEM images in Fig. 1. Then, an in-plane strain (εxx) field is obtained to show the strain distribution.

In-situ EPR test

In situ light excitations EPR experiments were recorded on an EMXplus-6/1 EPR spectrometer (Bruker, Germany) equipped with an Xe light system, and the EPR spectrometer was operated at an X band frequency of 9.84 GHz. The spectra were obtained through rigorous monitoring to ensure that signal saturation from the applied microwave power did not occur during the acquisition of the signals. We introduced catalyst powder into a quartz tube without removing the powder sample throughout all ESR measurements, including both dark and light irradiation conditions. Prior to conducting various gas flows for in situ analysis, the quartz tube was evacuated and purged with nitrogen flow for 1 h to eliminate surface-adsorbed molecules. Under this controlled experimental setup, we were able to minimize peak variations caused by differences in the sample tube or the position of light irradiation.

In vitro antibacterial test

A 300 W Xenon Arc light (PEC2000 light Beijing Perfectlight Technology Co., Ltd., China) with a 420 nm UV cut-off filter was placed at a 40 cm distance over the reactor. The light intensity in the reactor’s center was 100 mW cm−2. The in vitro antibacterial system of Escherichia coli ATCC 25922 (E. coli) and Methicillin-resistant Staphylococcus aureus (MRSA) was studied using plate count assays. First, a single colony of E. coli or MRSA was picked up in lysogeny broth (LB) and cultured at 37 °C overnight. The bacteria suspension was washed and diluted with a phosphate-buffered solution (PBS). Its OD-600nm was adjusted so as to obtain a cell density, corresponding to 9 log CFU mL−1, which was confirmed by plating on LB agar plates. Then, 200 μL of 800 mg L−1 different material solutions (the same amount of PBS solution in the control group) were added into 1.8 mL bacterial suspension and fully mixed. Afterward, the suspension was irradiated under the Xenon light. Finally, 100 μL of treated bacterial suspension was spread on a fresh LB agar plate and incubated for 18 h at 37 °C. By dividing the CFU counts of the various treatment groups by that of the control group, the survival rate was calculated.

Fluorescence imaging of generated ROS in MRSA

ROS generation in MRSA was tested using a ROS assay kit (Beyotime, China). MRSA cells (109 CFU mL−1) were loaded with the fluorescence probe (DCFH-DA) at 37 °C for 30 min. Then the bacteria were incubated with 50 μg mL−1 photocatalysts and exposed to visible light for 4 min. Images of fluorescence were captured using the fluorescence microscope (LECIA TCS SP8).

Computational calculation methods

DFT calculations were carried out by the VASP.6.4.2 code. The exchange-correlation interaction was described by the GGA with the PBE functional. The energy cut-off and Monkhorst–Pack k-point mesh were set at 500 eV and 3 × 3 × 1, respectively. During the geometry relaxation, the convergence tolerance was set as 1.0 × 10−5 eV for energy and 0.01 eV/Å for force. The NEDOS and KPOINTS of DOS calculations were set as 800 and 5 × 5 × 1, respectively. The projected density of states (PDOS) was implemented using vaspkit code55. In addition, the NBANDS and NEDOS of crystal orbital Hamilton populations (COHP) calculations were set as 1500 and 1000, respectively. COHP analysis was performed using the LOBSTER 4.1.0 package56.

Statistical analysis

All the quantitative data in each experiment were evaluated and analyzed by one-way analysis of variance and expressed as the mean values ± standard deviations. Values of *P < 0.05, **P < 0.01, and ***P < 0.001 were considered statistically significant.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.