Abstract
Superbugs in groundwater are posing severe health risks through
waterborne pathways. An emerging approach for green disinfection lies
at photocatalysis, which levers the locally generated superoxide
radical (·O[2]^−) for neutralization. However, the spin-forbidden
feature of O[2] hinders the photocatalytic generation of active
·O[2]^−, and thus greatly limited the disinfection efficiency,
especially for real groundwater with a low dissolved oxygen (DO)
concentration. Herein, we report a class of strained Mo[4/3]B[2-x]T[z]
MBene (MB) with enhanced adsorption/activation of molecular O[2] for
photocatalytic disinfection, and find the strain induced spin
polarization of In[2]S[3]/Mo[4/3]B[2-x]T[z] (IS/MB) can facilitate the
spin-orbit hybridization of Mo sites and O[2] to overcome the
spin-forbidden of O[2], which results in a 16.59-fold increase in
·O[2]^− photocatalytic production in low DO condition (2.46 mg L^-1).
In particular, we demonstrate an In[2]S[3]/Mo[4/3]B[2-x]T[z]
(50 mg)-based continuous-flow-disinfection system stably operates over
62 h and collects 37.2 L bacteria-free groundwater, which represents
state-of-the-art photodisinfection materials for groundwater
disinfection. Most importantly, the disinfection capacity of the
continuous-flow-disinfection system is 25 times higher than that of
commercial sodium hypochlorite (NaOCl), suggesting the practical
potential for groundwater purification.
Subject terms: Photocatalysis, Pollution remediation
__________________________________________________________________
Photocatalytic disinfection is a known pollution-free water treatment
strategy. However, its efficiency is limited due to the spin-forbidden
feature of O[2], which hinders photocatalytic ·O[2]^− formation and
limits its disinfection efficiency. Here the authors produce a strained
Mo[4/3]B[2-x]T[z] MBene that can increase ·O[2]^− photocatalytic
production, which results in high levels of photocatalytic
disinfection.
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
development^[58]1. Previous studies show that groundwater, associated
with many outbreaks^[59]2, is a major global reservoir of
antibiotic-resistant bacteria^[60]3. The waterborne diseases are a
sustainable issue for the developing country^[61]2. Globally, water
disinfection relies on chlorination, however, by-products of
chlorination is linked with cancer^[62]3. There is a great requirement
for focusing on disinfectants without chemical residue
formation^[63]4,[64]5. Reactive oxygen species (ROS) disinfectants,
such as superoxide radical (·O[2]^−), can offer broad-spectrum
antibacterial activity without chemical residues^[65]5. Among different
techniques, photocatalysis provides a sustainable approach to produce
·O[2]^− by activating oxygen (O[2]). However, low ·O[2]^− flux in the
reported photocatalytic systems greatly limits the disinfection
efficiency. Therefore, enhancing the photocatalytic ability for the
·O[2]^− production is highly desirable in photocatalytic disinfection.
O[2] is the most environmental oxidant on earth, and the activation of
O[2] strengthens its reactivity^[66]6. For the spin distribution in the
O[2] molecule, a pair of half-occupied π^* orbitals endow O[2] with
nonzero spin (s = 1; m[z] = 2μ[B]), which is the source of
spin-forbidden^[67]7,[68]8. If π^* orbitals obtain an electron, the
spin states of O[2] will be split and transformed into ·O[2]^−.
However, the spin-forbidden of O[2] impedes this electron transfer,
leading to the low-efficiency generation of ·O[2]^−, and thus low
disinfection efficiency. Till now, in low DO water, such as
groundwater, how to eliminate the spin-forbidden of O[2] is the key to
improving the O[2] utilization and sterilization efficiency, however,
remains a grand challenge.
Herein, we report a class of 2D/2D In[2]S[3]/Mo[4/3]B[2−x]T[z] (IS/MB)
heterojunction, in which MB as cocatalyst with atomic strain produces
the spin polarization for high-efficiently generate the ·O[2]^− 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 O[2] can
eliminate the spin-forbidden of O[2] for facilitating photoelectron
transfer for efficient ·O[2]^− production. The as-made IS/MB-20 shows a
16.59-fold increase in ·O[2]^− 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
Mo[4/3]B[2−x]T[z] MBene (MB) was prepared through a HF etching and
delamination process of (Mo[2/3]Y[1/3])[2]AlB[2] (i-MAB) (Supplementary
Figs. [69]1, [70]2)^[71]9. The average thickness of as-made MB-20 (20 h
HF etching) nanosheets is about 5.67 nm (Supplementary Fig. [72]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. [73]4, [74]5). X-ray diffraction
(XRD) patterns indicate that the peak intensities of
(Mo[2/3]Y[1/3])[2]AlB[2] precursor are significantly decreased after
the HF treatment (Supplementary Fig. [75]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 Å^[76]9.
The aberration corrected-spherical transmission electron microscope
(AC-STEM) image of MB-20 shows that it has a hexagonal crystal
structure (Supplementary Fig. [77]7). The uneven brightness of atoms in
Supplementary Fig. [78]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. [79]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. [80]9), MB-20 emerges
a primary peak at 1.84 Å, confirming the existence of Mo–B coordination
(Supplementary Table [81]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 study^[82]9, we
built a crystal structure model of MB-20 (Supplementary Fig. [83]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. [84]6, [85]7), supporting the validity of
the established crystal structure.
Atomic strain-induced spin polarization in MB
Figure [86]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)^[87]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. [88]11–[89]13).
Fig. 1. Structure characterization and structure–activity relationship
between atomic strain and spin polarization.
[90]Fig. 1
[91]Open in a new tab
a AC-STEM and GPA images of MB with different HF etching times
(18–21 h). Ɛ[xx] 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.
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. [92]12), the crystal structures of various MB with different
amounts of Mo vacancy were established (Supplementary Fig. [93]14).
Supplementary Fig. [94]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 = IDOS[spin-down]−IDOS[spin-up]) represents the spin
polarization^[95]11,[96]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. [97]15). Electron paramagnetic
resonance (EPR) results are consistent with calculated spin
polarization (Fig. [98]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 materials^[99]13. 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. [100]1c. The spin-charge density of
MB-12% illustrates asymmetric electron spin polarization distribution
(Supplementary Fig. [101]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. [102]17, there
is an obvious d–p orbital hybridization between the Mo d[xy] 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. [103]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
orbitals^[104]14. 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
coupling^[105]13,[106]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. [107]20). Therefore, MB-20 can be
introduced into semiconductors to afford a Schottky heterojunction
photocatalyst. In this study, In[2]S[3] (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. [108]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. [109]21, [110]22). High-resolution TEM
(HRTEM) image of IS/MB-20 shows a dual-phase heterostructure
(Fig. [111]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. [112]1e). In Fig. [113]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. [114]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. [115]21e, [116]23, [117]24).
The band alignments of IS/MB-20 are calculated according to experiment
data (Supplementary Figs. [118]25–[119]27). The different potential
between IS and MB-20 induces band bending. Thus, a Schottky barrier is
formed at the interface (Supplementary Fig. [120]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 ·O[2]^– photocatalytic production
The energy level diagram reveals that IS/MB-20 can produce ·O[2]^– due
to its more negative CBM position (−0.67 eV) than that of O[2]/·O[2]^–
(−0.33 eV) (Supplementary Fig. [121]27b). Under visible light
irradiation, in-situ EPR spectra of IS/MB-20 confirm the production of
·O[2]^– with a hyperfine coupling constant of A[N] = 13 G and
A[Hβ] = 9.5 G (Fig. [122]2a). The ability for ·O[2]^– production
follows the order of IS/MB-20 > IS/MB-21 > IS/MB-19 > IS/MB-18. ·O[2]^–
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 ·O[2]^–
under visible light irradiation.
Fig. 2. Spin polarization enhanced O[2] adsorption.
[123]Fig. 2
[124]Open in a new tab
a In-situ EPR spectra of·O[2]^– produced by various materials in low DO
condition (2.46 mg L^−1) under visible light irradiation. b Adsorption
energy of O[2] on various materials. The inset figures from left to
right represent the configurations of O[2] adsorption on the surface of
In[2]S[3], IS/MB-perfect, and IS/MB-12%. c Energy levels of
IS/MB-perfect’s FMOs and O[2]’s π^* orbitals. d Energy levels of
IS/MB-12%’s FMOs and O[2]’s π^* orbitals.
To verify the promoting effect of MB in photocatalytic ·O[2]^−
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. [125]28,
[126]29). Transient photocurrent responses, surface photovoltage, and
fluorescence emission decay spectra results also provide solid evidence
for this claim (Supplementary Fig. [127]30)^[128]16,[129]17. The
efficient charge separation can be attributed to the Schottky barrier
at the hetero-interface, which facilitates the photocatalytic ·O[2]^−
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. [130]31). Thus, ·O[2]^– 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 O[2] on the Mo sites (Supplementary
Figs. [131]32, [132]33). The adsorption energies of O[2] on different
materials are calculated and presented in Fig. [133]2b. O[2] is more
likely adsorbed on IS/MB-12% configuration (E[ads] = −2.52 eV) than
IS/MB-perfect (E[ads] = −2.06 eV) and IS (E[ads] = −0.18 eV) due to the
more negative adsorption energy^[134]18. These results suggest that
spin polarization improves the adsorption of O[2] on the material’s
surface.
Molecular orbital theory is further employed to analyze the orbital
interaction of Mo and O[2]. Projected density of states (PDOS) in
Supplementary Fig. [135]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 orbitals^[136]19. The
FMOs of IS/MB-12% are mainly composed of
[MATH: Mo4dxz :MATH]
,
[MATH: Mo4dz2
:MATH]
, and
[MATH: Mo4dyz :MATH]
orbitals (Supplementary Figs. [137]35–[138]38). In addition, the
hexahedral configuration of MB gives it C[6v] symmetry. Based on the
incommensurable representation of the C[6v] point group and crystal
field theory, the orbital interaction between Mo and B is shown in
Supplementary Fig. [139]39. Obviously, the
[MATH: Mo4dxz :MATH]
,
[MATH: Mo4dz2
:MATH]
, and
[MATH: Mo4dyz :MATH]
constitute the FMO of Mo, in consistent with the PDOS results in
Supplementary Fig. [140]34.
During O[2] adsorption and activation, the
[MATH:
π2p<
/mrow>x* :MATH]
and
[MATH:
π2p<
/mrow>y* :MATH]
bonds of O[2] can interact with the FMOs of IS/MB-12%, because these
orbitals are not fully occupied (Supplementary Fig. [141]40). Then PDOS
spectra reveal the orbitals hybridization between FMOs of IS/MB and π^*
orbitals of O[2] (Supplementary Fig. [142]41). In addition, projected
crystal orbital Hamilton population (pCOHP) analysis demonstrates the
[MATH: Mo4dyz :MATH]
−O 2p[y], and
[MATH: Mo4dxz :MATH]
−O 2p[x] orbital hybridization states (Supplementary Fig. [143]42).
Figure [144]2c, [145]d and Supplementary Fig. [146]43 summarize the
orbital hybridization energy levels between FMOs of IS/MB and π^*
orbitals of O[2] based on PDOS and pCOHP results. Obviously, the
spin-polarized FMOs in IS/MB-12% exhibit a strong coupling character to
O[2]’s π^* orbitals, thus reducing the O[2] adsorption energy^[147]19.
After O[2] is absorbed on IS/MB-perfect,
[MATH: Mo4dxz :MATH]
−π^* and
[MATH: Mo4dyz :MATH]
−π^* bonds are formed and located at −0.86 eV. As a comparison, after
FMOs of IS/MB-12% are hybridized with O[2]’s π^* orbitals,
[MATH: Mo4dxz :MATH]
−π^* and
[MATH: Mo4dz2
:MATH]
−π^* bonds are formed with spin energy splitting. Specifically, the
spin-down
[MATH: Mo4dxz :MATH]
−π^* and
[MATH: Mo4dz2
:MATH]
−π^* bonds remain at the same level (E = −1.72 eV), while the spin-up
[MATH: Mo4dxz :MATH]
−π^* and
[MATH: Mo4dz2
:MATH]
−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
[MATH: Mo4dyz :MATH]
−O 2p[y] and
[MATH: Mo4dxz :MATH]
−O 2p[x] bonds are −0.69. In contrast, the ICOHP of
[MATH: Mo4dz2
:MATH]
−O 2p[x] bond shifts to −0.73 in IS/MB-12%-O[2], 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 O[2] adsorption^[148]19, and the spin–orbital
hybridization in IS/MB-12%-O[2] facilitates spin state transformation
of O[2]. Supplementary Fig. [149]44 presents the real space orbital
wave-functions post-interaction of IS/MB-perfect-O[2] and
IS/MB-12%-O[2](with O[2] adsorbed) configuration, respectively.
Generally, the overlapping of orbital wave-functions represents orbital
hybridization^[150]19. The FMOs exhibit obvious orbital overlapping
with O[2]’s π^* orbitals, which is consistent with the energy levels
diagram in Fig. [151]2c and [152]d.
Supplementary Fig. [153]45 displays the orbits and electron arrangement
in ·O[2]^–. The mechanism of ·O[2]^– formation is filling the π^*
orbitals with an electron, however, the spin-forbidden nature of O[2]
hinders this electron transfer. To investigate the electron transfer
between Mo sites and O[2], the EPR measurements on IS/MB-20 in the
Ar/Air atmosphere under light irradiation were conducted
(Fig. [154]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 O[2] adsorption. This suggests that the O[2]
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 ·O[2]^–. Interestingly, the
spin-charge density data demonstrate the spin state transformation of
O[2] after adsorption on IS/MB-12% (Fig. [155]3b), as the O[2] 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. [156]3c).
As mentioned above, spin-polarized FMOs coupled with O[2]’s π^* orbital
will lead to the formation of strong Mo–O bonds (Fig. [157]2d and
Supplementary Fig. [158]43). The rearrangement of the energy levels can
facilitate spin electron redistribution, and result in zero spin
states^[159]20. Furthermore, the zero-spin state eliminates the
spin-forbidden and thus promotes the generation of ·O[2]^–
(Fig. [160]3d).
Fig. 3. Spin polarization improving the molecular orbital activation in
·O[2]^– formation.
[161]Fig. 3
[162]Open in a new tab
a EPR spectra of IS/MB-20 in an Ar/Air atmosphere under light
irradiation. b Spin charge density of IS/MB-perfect-O[2] and
IS/MB-12%-O[2] configuration. c The molecular orbital energy and spin
electron distribution diagram for O[2] and zero spin states O[2]. d
Schematic diagram of spin electron transfer from FMOs of IS/MB to π^*
orbitals of O[2].
Photocatalytic disinfection activity of IS/MB
Methicillin-resistant Staphylococcus aureus (MRSA) is a common
superbug, which causes bacterial infections in healthcare and community
environment^[163]21,[164]22. Coagulases of MRSA allow it to proliferate
as thromboembolic lesions and are hard to kill, causing a serious
infection^[165]23–[166]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 [167]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. [168]4a and Supplementary Fig. [169]46).
To the best of our knowledge, the developed IS/MB-20 is the most
efficient photocatalytic disinfection material against MRSA ever
reported (Fig. [170]4b and Supplementary Table [171]2).
Fig. 4. Antibacterial activities of IS/MB.
[172]Fig. 4
[173]Open in a new tab
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. [174]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.
Furthermore, the live/dead fluorescence staining was used to examine
the bactericidal activity against MRSA (Fig. [175]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 activity^[176]27. 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 ·O[2]^– is essential for the
photocatalytic inactivation of MRSA (Supplementary
Fig. [177]47)^[178]2, as almost no MRSA is inactivated in the presence
of ·O[2]^– 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 ·O[2]^– production
(Fig. [179]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. [180]48, without light
irradiation, MRSA in three groups showed weak green fluorescence,
suggesting low ROS levels in cells^[181]28. 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 processes^[182]29. 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. [183]49 show that the bacterial
membrane was severely damaged by IS/MB-20 under light irradiation.
Thus, ·O[2]^– can destroy the bacterial membrane and affect its
permeability (Supplementary Fig. [184]50). The bacterial membrane
potential assay and total protein determination results further prove
that ·O[2]^− can destroy cell membranes (Supplementary Figs. [185]51,
[186]52)^[187]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. [188]53).
Inhibition of MRSA’s infectivity
The schematic diagram in Supplementary Fig. [189]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 infection^[190]21,[191]22,[192]31. After
photocatalytic sterilization using IS/MB-20, MRSA cannot coagulate
lyophilized plasma, indicating that MRSA bacteria cannot produce
coagulase (Fig. [193]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. [194]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 [195]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. [196]56–[197]59)^[198]32–[199]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. [200]60). Furthermore, a rat wound infection
model with MRSA (1 × 10^8 CFU mL^−1) was constructed to investigate the
inhibiting MRSA infection performance of IS/MB-20 (Supplementary
Figs. [201]61–[202]66). IS/MB-20 can heal the MRSA-infected wounds in
the established rat model within 10 days (Fig. [203]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. [204]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 ·O[2]^–. The key to improving the disinfection effect is to
enhance the O[2] adsorption/activation of the bactericidal material.
Herein, we tested the disinfection effect of the IS/MB-20 in
groundwater. Figure [205]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. [206]5b, Supplementary
Fig. [207]67a), which can achieve 100% photocatalytic disinfection of
bacteria on-site in water from groundwater with low DO (Fig. [208]5c,
Supplementary Fig. [209]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. [210]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. [211]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. [212]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. [213]5e)^[214]35–[215]53.
Fig. 5. Disinfection performance of IS/MB-20 in groundwater with low DO
content.
[216]Fig. 5
[217]Open in a new tab
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.
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. [218]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. [219]5g).
Discussion
We report a class of strained MB/IS composite photocatalysts for
eliminating the spin forbidden of O[2] to boost the ·O[2]^−
photocatalytic production to achieve efficient water disinfection.
Ascribed to its optimal tensile strain, IS/MB-20 exhibits outstanding
ability for photocatalytic ·O[2]^− production. We find that the
strain-induced spin-polarized FMO can hybridize with π^* orbital of
O[2]to eliminate the spin forbidden of O[2], 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. (Mo[2/3]Y[1/3])[2]AlB[2] (i-MAB, FoShan
Xinxi Technology Co. Ltd.), hydrofluoric acid (HF, 40 wt%),
tetrabutylammonium hydroxide (TBAOH, 10 wt%), Indium chloride
tetrahydrate (InCl[3], 99.9%), ethylene glycol (EG, >99%),
Thioacetamide (≥99.0%).
Preparation of MB
1 g of (Mo[2/3]Y[1/3])[2]AlB[2] 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 powders^[220]54.
Preparation of IS nanoflowers and IS/MB
For the preparation of IS, 0.1769 g InCl[3] 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. [221]1. Then, an in-plane strain (ε[xx]) 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 (10^9 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 code^[222]55. 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 package^[223]56.
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 [224]Nature
Portfolio Reporting Summary linked to this article.
Supplementary information
[225]Supplementary Information^ (6.9MB, pdf)
[226]Reporting Summary^ (80.8KB, pdf)
[227]Transparent Peer Review file^ (5.7MB, pdf)
Source data
[228]Source Data^ (316.7KB, xlsx)
Acknowledgements