Photogenerated Charge Carriers in Molybdenum Disulfide Quantum

Subscriber access provided by Iowa State University | Library

Biological and Medical Applications of Materials and Interfaces

Photogenerated Charge Carriers in Molybdenum Disulfide Quantum Dots with Enhanced Antibacterial Activity Xin Tian, Yurong Sun, Sanhong Fan, M.D. Boudreau, Chunying Chen, Cuicui Ge, and Jun-Jie Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19958 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Photogenerated Charge Carriers in Molybdenum Disulfide Quantum Dots with Enhanced Antibacterial Activity Xin Tian,†,# Yurong Sun,‡,# Sanhong Fan,‡ Mary D. Boudreau,┴ Chunying Chen,§ Cuicui Ge,*,† and Jun-Jie Yin‖

†

State Key Laboratory of Radiation Medicine and Protection, School for Radiological and

Interdisciplinary Sciences (RAD-X), Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China ‡ School

┴

for Life Science, Shanxi University, Taiyuan 030006, China

Division of Biochemical Toxicology, National Center for Toxicological Research, U.S. Food

and Drug Administration, Jefferson, Arkansas 72079, USA §

Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for

Nanoscience and Technology of China and Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100190, China ‖

Division of Analytical Chemistry, Office of Regulatory Science, Center for Food Safety and

Applied Nutrition, U.S. Food and Drug Administration, College Park, Maryland 20740, USA

*Corresponding author, E-mail: [email protected]

KEYWORDS: molybdenum disulfide quantum dots, simulated solar light, charge carriers, reactive oxygen species, antibacterial activity

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Molybdenum disulfide (MoS2) nanosheets have received considerable interests due to their superior physicochemical performances to graphene nanosheets. As the lateral size and layer thickness decrease, the formed MoS2 quantum dots (QDs) show more promise as photocatalysts, endowing them with potential antimicrobial applications under environmental conditions. However, studies on the antibacterial photodynamic therapy of MoS2 QDs have rarely been reported. Here, we show that MoS2 QDs more effectively promote the creation and separation of electron-hole pair than MoS2 nanosheets, resulting in the formation of multiple reactive oxygen species (ROS) under simulated solar-light irradiation. As a result, photoexcited MoS2 QDs show remarkably enhanced antibacterial activity, and the ROS-mediated oxidative stress plays a dominant role in the antibacterial mechanism. The in vivo experiments showed that MoS2 QDs are efficacious in wound-healing under simulated solar-light irradiation and exert protective effects on normal tissues, suggesting good biocompatibility properties. Our findings provide a full description of the photochemical behavior of MoS2 QDs and the resulting antibacterial activity, which might advance the development of MoS2-based nanomaterials as photodynamic antibacterial agents under environmental conditions.


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Antibiotic resistance is one of the most urgent threats to public health. Despite numerous efforts to develop innovative antibacterial drugs, antibiotic resistance persists.1 In recent years, nanostructured materials have created a significant impact in the fields of chemistry, electronics, and biological sciences, and the emergence of novel nanomaterials has provided insight into a potential resolution to the bacterial resistance problem.2,3 Several kinds of nanomaterials, such as silver and carbon-based nanostructures, have been studied for their antibacterial properties;4–7 however, because of their intrinsic toxicity and persistence in living tissues, further applications of these nanomaterials are somewhat limited. Therefore, attention is now focused on new and exciting nanomaterials with high efficient antibacterial activity and good biocompatibility. Semiconductor nanomaterials with photocatalytic activity have been applied in many areas, such as their use as antibacterial agents.8 When excited by photons, semiconductor nanomaterials generate electron-hole pairs with strong reductive and oxidative ability, which react with dissolved oxygen and water separately, forming reactive oxygen species (ROS) that destroy biomolecules.9,10 Several investigations into the development of phototherapy agents have focused on metal oxide photocatalysts (e.g., TiO2, ZnO) and graphene quantum dots (QDs).11–14 However, these agents exhibit low efficiencies in photoenergy conversion, likely due to their large bandgap. Hybrid nanostructures have also been proposed to increase the efficiency of photoenergy conversion;12,15 however, their application is hindered by



the complexity of the synthesis process. Other investigations on phototherapy utilize CdTe (QDs) and graphene oxide,16,17 but these nanomaterials have potential toxic effects on biological systems, thereby, limiting their application as antibacterial agents in vivo. Molybdenum disulfide (MoS2), is one of the most promising candidates among semiconducting photocatalysts to combat bacterial infections. As a two-dimensional layered transition metal dichalcogenide, MoS2 has recently attracted much attention for various applications in environmental and biomedical fields since it exhibits unique electrical, optical, and mechanical properties.18–22 By decreasing the layers of MoS2, its bandgap changes from an indirect bandgap (~1.2 eV) to a direct bandgap (~1.8 eV).23,24 The transition of indirect-to-direct bandgap produces very potent photocatalytic behavior for MoS2 nanomaterials. For instance, few-layered MoS2 nanofilms have much higher water disinfection effects under simulated solar-light irradiation than that of bulk MoS2.25 This suggests that decreasing the layer thickness and lateral size can increase the photocatalytic activity of MoS2 nanomaterials. As compared to MoS2 nanosheets, MoS2 QDs have more active edges, higher charge carriers mobility, as well as special surface area, which are beneficial to the photocatalytic activity.24,26 However, to the best of our knowledge, there are no systematic studies on the bactericidal effects of MoS2 QDs, and the mechanism underlying the light-induced bactericidal effects of MoS2 have not been fully elucidated. In this study, we investigated the photochemical behavior of MoS2 QDs under


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


simulated solar-light irradiation and evaluated the mechanism for enhanced generation of multiple ROS. We revealed that ROS-mediated oxidative stress plays a dominate role in the simulated solar-light enhanced antibacterial activity of MoS2 QDs. Finally, we detected the effects of MoS2 QDs on wound disinfection and healing in a wound infection mouse model. 2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of MoS2 QDs. MoS2 QDs were prepared according to the literature with few modifications.24 In detail, 1 g of bulk MoS2 powder was added into 100 mL of N,N-dimethylformamide and sonicated for 8 h to exfoliate MoS2 powder. After that, the suspension was decanted into flask and kept vigorous stirring for 8 h at 140 °C. Then the supernatant was settled overnight and centrifuged for 15 min at 8000 rpm to remove the residual unexfoliated particles and the supernatant was collected for further use. The morphologies and sizes of the as-received samples were determined by transmission electron microscopy (TEM, Tecnai G-20, FEI) operated at 200 kV. An atomic force microscope (AFM, Dimension Icon, Bruker) was employed to analyze the height of the samples. The structure of MoS2 materials was demonstrated by X-ray diffraction (XRD, X’Pert-Pro MPD, Holland Panalytical) and Raman spectrum (LabRAMHR800, HORIBA JOBIN YVON). The hydrodynamic sizes and surface charges of MoS2 materials were determined by a particle analyzer (Zetasizer Nano-ZS, Malvern). 2.2. Dissolution of MoS2 QDs. To investigate the dissolution of MoS2 QDs, MoS2 QDs dispersions (~ 30 ppm of Mo) were incubated in DI water. After 24 h, the MoS2



QDs were separated from the dissolved species using centrifugal ultrafiltration and the total soluble Mo species concentrations were determined by inductively coupled plasma-mass spectrometry (ICP-MS, ELEMENT 2, Thermo). 2.3. Oxidation of Glutathione and Ascorbic Acid. Glutathione (GSH) and ascorbic acid (AA) oxidation were measured with UV-vis spectroscopy (UV3600, Shimadzu). GSH (50 μM) was mixed with bulk MoS2, MoS2 nanosheets or MoS2 QDs (50 μg/mL) and exposed to simulated solar light from 0 to 30 min. The oxidation of GSH was recorded from the absorption of 412 nm after adding DNTB regent. As for AA, the AA (100 μM) was mixed with bulk MoS2, MoS2 nanosheets or MoS2 QDs (50 μg/mL). The oxidation of AA by MoS2 materials under simulated solar-light irradiation was recorded from the absorption of 265 nm. 2.4. Antibacterial Activity. Escherichia coli (E. coli) and Staphylococcus aureus subsp. aureus (S. aureus) were grown in Luria-Bertani and Trypticase Soy Broth to reach the mid-exponential growth phase at 37 °C, respectively. The bacterial suspensions were diluted to obtain cell samples containing 106 CFU/mL (CFU = colony-forming units), and the bacteria were incubated with 50 μg/mL MoS2 QDs without or with simulated solar-light irradiation for 1 h. To compare the antibacterial activity of MoS2 QDs with bulk MoS2 QDs and MoS2 QDs nanosheets, three samples (50 μg/mL) were incubated with bacteria and then exposed to simulated solar light for 1 h. The survival rate of bacterial was determined by counting the number of CFU. 2.5. Morphological Observation of Bacteria. After antibacterial assessment, the bacteria were harvested by centrifugation and fixed with glutaraldehyde. Then,


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bacteria were dehydrated in ethanol gradually. Finally, the dried bacteria were visualized by scanning electron microscope (SEM, S-4700, Hitachi). 2.6. Measurement of Intracellular ROS. The generation of ROS from bacteria was detected using an oxidant-sensitive dye 2′,7′-dichlorodihydrofluorescein diacetate (DCFH-DA). After treatment, bacteria were incubated with DCFH-DA under dark surroundings. The stained bacteria were observed by laser confocal microscope (FV1200, Olympus). 2.7. In Vitro Cytotoxicity Experiments. Human umbilical vein endothelial cells (HUVECs) were used to investigate the cytotoxicity of MoS2 QDs. Cells were seeded into 96-well plates and allowed to grow overnight. Then, the HUVECs were then incubated with MoS2 QDs (50 μg/mL) with or without simulated solar light for 1 h. After irradiation, the cells were further cultured for 24 h. Cell Counting Kit-8 (CCK-8) assay kit was used to determine the cell viability. 2.8. Co-Culture Assay. Co-culture experiments were carried out with S. aureus and HUVECs according to a previously literature.17 HUVECs were seeded into confocal dishes and allowed to grow overnight. Then, cell culture medium was replaced with fresh medium containing approximately 1 × 107 CFU/mL S. aureus and 50 μg/mL MoS2 QDs. Samples were either or not exposed to simulated solar light for 1 h. After treatment, samples were stained with SYTO9 and propidium iodide (PI) for 30 min and then stained with Hoechst 33342 for another 5 min. Finally, the samples were visualized using confocal laser microscope. 2.9. Mice Injury Model. Eight-week-old BALB/c mice were purchased from



Pengsheng Biotechnology company (Nanjing, China) and had ad libitum access to standard rodent chow and water. Animal care and use followed the principles and procedures outlined by the animal welfare committee of Soochow University. BALB/c mice were divided into four groups (n = 5), whose backs were slashed and injected with S. aureus (50 μL, 1 × 109 CFU/mL). After 24 h, the mice in each group were treated as follows: group (1) PBS, group (2) PBS + Light, group (3) MoS2 QDs (100 μL, 100 μg/mL), and group (4) MoS2 QDs + Light. The light irradiation time was 1 h. The wounds were recorded and photographed every other day for 7 days. To check the antibacterial activity, the bacteria were collected from the wound area of treatment group on the 7th day. The bacteria were cultured overnight and then spread on agar plates for CFU enumeration. 2.10. In Vivo Toxicity Study. MoS2 QDs in phosphate buffer saline (PBS) were subcutaneously injected into BALB/c mice (n = 5). The assigned dose was 10 mg/kg in each mouse (100 μg/mL in PBS buffer). Mice were sacrificed by isoflurane anesthetic and angiocatheter exsanguinations on the 7th day of treatment. The blood samples and tissues were obtained under sterile conditions for blood biochemistry and pathological examination, respectively. 3. RESULTS AND DISCUSSION 3.1. Characterization of MoS2 QDs. In this study, MoS2 QDs were prepared by the combination of sonication and solvothermal treatment of bulk MoS2 according to previous reports.24 As schematically illustrated in Figure 1a, the bulk MoS2 can exfoliate into MoS2 nanosheets and MoS2 QDs via a simple exfoliation strategy. The


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TEM image shows the as-exfoliated MoS2 QDs with a diameter around 3.0 nm (Figure 1b). The thickness of the MoS2 QDs measured by AFM shows a height of about 0.7-1.5 nm (Figure 1c,d), which is consistent with 2-3 layers.27 The TEM images of bulk MoS2 and MoS2 nanosheets were shown in Figure S1. The hydrodynamic diameters and surface charge of bulk MoS2, MoS2 nanosheets and MoS2 QDs in DI water were determined by a particle analyzer (Figure S2a,b). Next, XRD was used to obtain the structure information of the MoS2 materials. The (002) diffraction peak of MoS2 QDs was disappeared compared with the XRD pattern of bulk MoS2 (Figure S2c). This is because fewer layered MoS2 QDs are formed. To further investigate the crystal structure and thickness of the MoS2 materials, Raman spectroscopy was carried out. The two Raman peaks were seen at 380.5 and 407.1 cm−1 for bulk MoS2, which was consistent with a previous report (Figure S2d).21 In comparison with the Raman spectrum of bulk MoS2, MoS2 QDs showed a smaller Raman shift difference between E12g and A1g modes, which was also because MoS2 QDs had few layers. In addition, we used ICP-MS to determine the Mo species release from MoS2 QDs in DI water.28 After 24 h incubation, the concentration of soluble Mo species was very low (~0.1% of total Mo in the sample). All these results demonstrate the successful synthesis of MoS2 QDs, and the prepared MoS2 QDs were stable in water.



Page 10 of 26

Figure 1. Preparation and morphology characterization of MoS2 QDs. (a) Schematic illustration of the synthesis process of MoS2 nanosheets and MoS2 QDs. (b) TEM image of MoS2 QDs. Scale bar: 20 nm. (c) AFM image of MoS2 QDs. Scale bar: 200 nm. (d) Corresponding height image.

3.2. Photochemical Properties of MoS2 QDs. We first examined the photochemical properties of photoexcited MoS2 QDs with electron spin resonance (ESR)

spectroscopy.29–31

We

employed

5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) for the detection of hydroxyl radical (•OH) and superoxide (O2•−). As shown in Figure 2a, simulated solar light alone or MoS2 QDs in the dark exhibited no clear ESR signal, while an obvious characteristic ESR signal of BMPO/•OH adduct was detected when MoS2 QDs irradiated by simulated solar light. To examine the generation of superoxide, a hydroxyl radical scavenger DMSO was added during the test. A significant reduction


Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the BMPO/•OH adduct signal was detected upon addition of DMSO (Figure S3). The residual signal is consistent with the production of superoxide (BMPO/•OOH).32 These results suggest hydroxyl radicals and superoxide are generated by photoexcited MoS2 QDs. Moreover, we made a comparison of the promotion effect on hydroxyl radicals and superoxide generation by different MoS2 materials including MoS2 QDs, MoS2 nanosheets and bulk MoS2 under simulated solar-light irradiation (Figure S4). MoS2 QDs most effectively promoted radicals generation, followed by MoS2 nanosheets, while bulk MoS2 were the least effective. This is mainly because MoS2 QDs have more active sites and higher charge carrier mobility than those of bulk MoS2 and MoS2 nanosheets.24

Figure 2. Generation of free radicals by photoexcited MoS2 QDs. ESR spectra of samples containing 25 mM BMPO (a), 10 mM 4-Oxo-TEMP (b), 0.1 mM CPH (c), or



0.02 mM TEMPO (d) and 50 μg/mL MoS2 QDs, with or without simulated solar-light irradiation. Control represents spin probe alone under irradiation.

Next, we selected a spin trap, 4-Oxo-2,2,6,6-tetramethylpiperidine (4-Oxo-TEMP), for investigating the effect of MoS2 QDs on singlet oxygen (1O2) generation. A characteristic ESR signal of 4-Oxo-TEMP/1O2 adduct was observed during irradiation of MoS2 QDs, indicating the generation of singlet oxygen (Figure 2b). In contrast, other samples exhibited no ESR signals. The generation of singlet oxygen by MoS2 QDs was also compared to bulk MoS2 and MoS2 nanosheets, which exhibited a similar trend: MoS2 QDs > MoS2 nanosheets > bulk MoS2 (Figure S4). These suggest that decreasing the layer thickness and size of MoS2 can increase the ROS generation efficiency of MoS2. Previous studies have reported that nanosized MoS2 have a larger bandgap compared to bulk MoS2, which facilitates the creation of electron-hole pairs.33 Moreover, MoS2 QDs have more active edge sites than MoS2 nanosheets, which can promote electron-hole pairs separation. To explore the mechanisms of light-induced ROS generation, we investigated the charge carriers generated on the surface of MoS2 QDs. 1-Hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) is a spin label that can be oxidized to form CP nitroxide radicals with a characteristic triplet ESR spectrum.16 As shown in Figure 2c, MoS2 QDs without irradiation or irradiated alone showed no characteristic ESR signals, indicating that neither MoS2 QDs nor irradiation cause oxidation of the CPH. However, MoS2 QDs exposed to simulated


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solar light result in a characteristic ESR spectrum for CP nitroxide radicals, implying the generation of photogenerated holes. Moreover, we selected the spin label 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) for determining the generation of photoinduced electrons. As shown in Figure 2d, neither MoS2 QDs nor simulated solar-light irradiation alone affected the TEMPO signal intensity. In contrast, MoS2 QDs under irradiation result in a significant reduction of the signal, indicating the generation of photogenerated electrons. All these results demonstrate that photoexcited MoS2 QDs induce the generation of electron-hole pairs which further react with water and oxygen to form hydroxyl radical and superoxide. In addition, we also found that the ability to generate free radicals

and

electron-hole

pairs

by

photoexcited

MoS2

QDs

was

concentration-dependent (Figure S5). Compared with bulk MoS2 (~1.2 eV) and MoS2 nanosheets (~1.5 eV) ， MoS2 QDs have a larger bandgap (~1.8 eV).25 With the increased bandgaps of these MoS2 materials, their redox potentials are also shift. The valence band (VB) position shifts to more positive, and/or the conduction band (CB) position shifts to more negative.12 As a result, MoS2 QDs exhibit stronger oxidative activity of holes in VB and/or reductive ability of electrons in CB compared with bulk MoS2 and MoS2 nanosheets. Therefore, the larger bandgap of MoS2 QDs are the most likely reason for enhancing ROS generation during light irradiation. 3.3. Effect of Photoexcited MoS2 QDs on Anti-Oxidant Defense System. GSH and AA are known as endogenous antioxidants which can prevent ROS-induced cellular damage.34,35 Thus, the effects of MoS2 QDs, MoS2 nanosheets and bulk MoS2



Page 14 of 26

on GSH and AA oxidation under irradiation were detected. After incubation with GSH, we found that photoexcited MoS2 QDs have significantly enhanced the GSH oxidation compared to other treatment groups, with photoexcited MoS2 QDs achieving 53% oxidation at 30 min (Figure 3a & Figure S6a). In contrast, MoS2 nanosheets or bulk MoS2 under irradiation only exhibited a minor oxidation effect on GSH

(Figure

S6a).

Moreover,

the

oxidation

of

AA

was

monitored

spectrophotometrically at 265 nm. As shown in Figure 3b, photoexcited MoS2 QDs obviously accelerated the oxidation of AA. However, MoS2 nanosheets or bulk MoS2 under irradiation showed extremely limited oxidation of AA (Figure S6b). These results verify that photoexcited MoS2 QDs can accelerate the oxidation of important endogenous antioxidants, GSH and AA.

Figure 3. GSH (a) and AA (b) levels after MoS2 QDs (50 μg/mL) treatment with or without simulated solar-light irradiation. Data represented as ± s.e.m., n = 3,

***p

MoS2 nanosheets > bulk MoS2, in accordance with the level of generated ROS (Figure S7).

Figure 4. Antibacterial activity of MoS2 QDs in killing (a) S. aureus and (b) E. coli after treatment with PBS as control, simulated solar light, or MoS2 QDs (50 μg/mL) with or without simulated solar-light irradiation. Data represented as ± s.e.m., n = 3, ***p

< 0.001 is relative to light irradiation group or MoS2 QDs-treated group. (c) SEM



images of bacteria treated with (1) PBS, (2) PBS + Light, (3) MoS2 QDs, and (4) MoS2 QDs + Light. Scale bar: 1 μm.

To further examine the antibacterial activity, SEM was used to observe the morphology of S. aureus and E. coli (Figure 4c). The untreated S. aureus displayed rod-shaped structure and no obvious morphological change was found after single treatment. Importantly, MoS2 QDs combined with simulated solar-light irradiation caused serious damage with loss of the membrane integrity. A similar tendency was found for E. coli: no significant morphological change was found after upon exposure to simulated solar light. The membranes of E. coli treated with MoS2 QDs became slightly irregular and wrinkled. In contrast, MoS2 QDs irradiated by simulated solar light induced cell lysis without integrity. The preceding ESR results showed that photoexcited MoS2 QDs generated multiple ROS. Therefore, the ROS levels in bacterial was further detected. MoS2 QDs or simulated solar light alone led to limited increase in the peroxide levels relative to the control group (Figure 5a). In contrast, MoS2 QDs irradiated by simulated solar light remarkably increased the peroxide levels, as evidenced by bright fluorescence signals. Our results suggest that light-induced ROS may be the dominant mechanism underlying the highly effective antibacterial activity of irradiated MoS2 QDs.


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Photoinduced formation of ROS do not affect HUVECs. (a) ROS formation of S. aureus after treatment with different conditions. Bacteria were treated with (1) PBS, (2) PBS + Light, (3) MoS2 QDs, and (4) MoS2 QDs + Light. Scale bar: 20 μm. (b) Cell viability of HUVECs after treatment. Data represented as ± s.e.m., n = 3. (c) Fluorescence images of HUVECs (blue, Hoechst 33342, nuclear; green, SYTO9, cytoplasmic) and S. aureus (red, PI, dead bacteria; green, SYTO9, live bacteria) exposed to different treatment conditions. Scale bar: 50 μm.

According to the above findings, a clear application of MoS2 QDs might be as a phototherapy agent for bacterial infections. Such applications would be based on the



photoexcited MoS2 QDs being selective for inhibiting bacteria without killing surrounding host cells. Thus, we first detected the cytotoxicity effects of MoS2 QDs and light irradiation on HUVECs. No significant difference in toxicity was observed for any treatment group when compared to the control (Figure 5b). Moreover, we established a co-culture system with S.aureus and HUVECs. The HUVECs were stained with Hoechst 33342 (nuclear stain, blue fluorescent) and SYTO9 (nuclear and cytoplasmic stain, green fluorescent) to observe cell morphology. SYTO9 (green fluorescent) and PI (red fluorescent) were used to stain the live bacteria and dead bacteria, respectively. In the co-culture system, S.aureus and HUVECs did not exhibit any toxicity in the presence of MoS2 QDs or simulated solar light alone (Figure 5c). Bacteria death was significantly increased when exposed to MoS2 QDs and simulated solar light; whereas, the HUVECs were viable (according to the morphology). Although we found that photoexcited MoS2 QDs are nontoxic to HUVECs, other cell types, such as human microvascular endothelial cells, also need further examination of the toxicity effects of photoexcited MoS2 QDs. These results indicate that photoexcited MoS2 QDs selectively kill the bacteria and suggest that they may be suitable as an antibacterial agent in vivo.

3.5. In Vivo Wound Disinfection Effect of Photoexcited MoS2 QDs. A study was conducted to evaluate the antibacterial effects of photoexcited MoS2 QDs on wound healing. The in vivo wound infection model was constructed by injecting S. aureus on the backs of BALB/c mice. Figure 6a showed the representative images of the


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


healing of bacterial infection wounds in different treatment groups. On the 7th day, the infected wounds were almost completely healed in photoexcited MoS2 QDs group whereas the wound boundary were still observed for other groups (Figure 6a,b). Meanwhile, the bactericidal effect was assessed by determining the number of S. aureus in wound area using standard plating methods (Figure 6c). Compared with the control group, the bacterial colonies of light-irradiated MoS2 QDs treatment group were significantly decreased, while other groups exhibited no obvious differences, in accordance with the antibacterial activity of MoS2 QDs. The effects of photoexcited bulk MoS2 and MoS2 nanosheets on wound healing were also determined. The results showed that photoexcited bulk MoS2 and MoS2 nanosheets did not promote wound healing compared with the irradiation group (Figure S8).

Figure 6. In vivo wound disinfection effect of MoS2 QDs. (a) Representative photographs of S. aureus infected wounds after treatment at different times (n = 5). Scale bar: 5 mm. (b) Related wound size in each treatment group. (c) Bacterial number of infected wounds on the 7th day, determined by plate count method. **p

Photogenerated Charge Carriers in Molybdenum Disulfide Quantum

Recommend Documents