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Fluorescent MoS2 Quantum Dots: Ultrasonic Preparation, Up-Conversion and Down-Conversion Bioimaging, and Photodynamic Therapy HaiFeng Dong, Songsong Tang, Yansong Hao, Haizhu Yu, Wenhao Dai, Guifeng Zhao, Yu Cao, Huiting Lu, Xueji Zhang, and Huangxian Ju ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10459 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 15, 2016

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ACS Applied Materials & Interfaces

Fluorescent MoS2 Quantum Dots: Ultrasonic Preparation, Up-Conversion and Down-Conversion Bioimaging, and Photodynamic Therapy Haifeng Dong,*,† Songsong Tang,† Yansong Hao,† Haizhu Yu,† Wenhao Dai,† Guifeng Zhao,† Yu Cao,† Huiting Lu, § Xueji Zhang,*,† and Huangxian Ju ‡ †

Beijing Key Laboratory for Bioengineering and Sensing Technology, Research Center for

Bioengineering and Sensing Technology, School of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, P.R. China §

Department of Environmental Science and Engineering, School of Chemistry and

Environment, Beijing University of Aeronautics & Astronautics, Beijing 100083, P.R. China ‡

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and

Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China

KEYWORDS: MoS2 quantum dots; ultrasonic preparation; up-conversion fluorescence; bioimaging; photodynamic therapy (PDT);

ACS Paragon Plus Environment


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ABSTRACT

Small size molybdenum disulfide (MoS2) quantum dots (QDs) with desired optical properties

were controllably synthesized by using tetrabutylammonium-assisted ultrasonication of

multilayered MoS2 powder via OH- mediated chain-like Mo-S bond cleavage mode. The

tunable up-bottom approach of precise fabrication of MoS2 QDs finally enables detailed

experimental investigations of their optical properties. The synthesized MoS2 QDs present

good down-conversion photoluminescence behaviors and exhibit remarkable up-conversion

photoluminescence for bioimaging. The mechanism of the emerging photoluminescence was investigated. Furthermore, superior 1O2 production ability of MoS2 QDs to commercial

photosensitizer PpIX was demonstrated, which has great potential application for

photodynamic therapy. These early affording results of tunable synthesis of MoS2 QDs with

desired photo properties can lead to application in fields of biomedical and optoelectronics.


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■

INTRODUCTION

Currently, two-dimensional (2D) nano-materials have attracted a great deal of attention because of their intrinsic characteristics, including mechanical, electronic and optical properties.1-3 As a typical 2D nanomaterial, monolayer graphene consisting of honeycomb lattice carbon atoms has exhibited exceptional physical and chemical properties.4-7 MoS2, a structure analog of graphene, has shown its predominant properties in semiconductor industry for optoelectronics and energy harvesting.8-11 While the bulk MoS2 has an indirect band gap of 1.2 eV, the monolayer MoS2 shows a direct band gap of 1.8eV,12-14 which gives rise to exotic physicochemical properties for catalysis,15 energy storage16 and electronics.17 For example, the monolayer MoS2-based phototransistors exhibit good stability and fast switching behavior with the photocurrent generation and annihilation finished within ca. 50 ms.18 The optical properties of semiconductors directly depend on the electronic band structure.11 The transition of indirect-to-direct band gap of MoS2 produces strong photoluminescent (PL) behavior of monolayer MoS2. When bulk MoS2 becomes monolayer MoS2, the absence of interlayer coupling of electron at the Γ point leads to strong emission at 1.8eV.10 Compared to its multilayer counterpart, a giant enhancement (∼104) in PL quantum yield can be observed in monolayer MoS2, which can be attributed to a dramatically slower electronic relaxation.11 Generally, the electronic band structure of semiconductor materials is relatively sensitive to the quantum size effect. For example, the fluorescence spectrum of graphene demonstrates a size-dependent distinct blue shift phenomenon.19 In principle, monolayer MoS2 with small lateral size can produce novel optical properties due to the quantum confinement and is promising for developing novel optoelectronic devices and optical sensors.14 Thus it is significant to controllably synthesize MoS2 with desired size for obtaining the demanded PL



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properties. Although some breakthroughs in the synthesis of monolayer MoS2 have been made in recent years, the effective and high-yield methods with controllable size are continuously being explored.20 The bulk MoS2 possesses multilayered structures with weak van der waals forces between layers and strong S-Mo-S intralayer covalent bonding. The weak van der waals forces allow exfoliation of the layered bulk crystals to top-down prepare monolayer MoS2. Therefore, mechanical cleavage,21 electrochemical intercalation,22 chemical exfoliation,23-24 ultrasonic route and thermolysis process,25-26 have been explored to fabricate the monolayer MoS2. However, these methods also have the deficiencies such as low productivity,21 complicated process22 , time consumption20 and harsh condition.23, 25 Thus it is a critical need to develop a new strategy to controllably synthesize layer MoS2 with desired size to study the consequent emerging optoelectronics properties. Recently, Xu et al et al. proposed an efficient method to synthesize MoS2 QDs with lateral sizes of ~3.3 nm by combining sonication and solvo-thermal treatment of bulk MoS2/WS2 at a mid-temperature for in vitro cell imaging due to their low cytotoxicity and strong fluorescence.27. Huang et al. developed a facile one-pot hydrothermal approach to fabricate uniform water-soluble monolayer MoS2 QDs with lateral sizes of ~2.1 nm by using ammonium molybdate, thiourea, and N-acetyl- L -cysteine (NAC) as precursors and the capping reagent, which showed promising potential for molecules detection.28 Our group developed a simple and efficient sulfuric acid-assisted ultrasonic route to tunably fabricate MoS2 QDs with good fluorescence properties for bioimaging and nanovector for intracellular microRNA detection.29 Efficient and reliable strategies for MoS2 sheets controllable synthesis are still under urgent need, and the potential biomedical application of these MoS2 sheets or MoS2 QDs should be further explored.


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Herein, a facile approach to efficiently fabricate monolayer MoS2 is developed by using a tetrabutylammonium (TBA)-assisted sonication route (Scheme 1). The TBA can intercalate into the interlayer structure of bulk MoS2 to exfoliate the single-layer MoS2 flakes and further cut the MoS2 flakes into MoS2 QDs. By simply controlling the ultrasonic time, MoS2 flakes with lateral size about ~13 nanometers can be generated. Microscopic and spectroscopic experiments demonstrate that the obtained MoS2 QDs have honeycomb lattice atom structure with uniform crystalline shape. Notably, the resulting MoS2 QDs exhibit good down-conversion and emerge up-conversion PL behavior, which show good feasibility for cell bioimaging. Furthermore, it was found that the resulting MoS2 QDs enable to produce 1O2 for photodynamic therapy (PDT) providing the promising biomedical application.

Scheme 1. Schematic present of synthetic method and resultant MoS2 QDs for bioimaging and PDT.

■ RESULTS AND DISCUSSION The scanning electron microscopic (SEM) image of original MoS2 powder revealed the typical multilayered structure with a neat and delicate stepped fracture (Figure S1), which facilitated TBA solvent molecules to intercalate into the interlayer for exfoliation. After



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TBA-assisted ultrasonic for 15 h, the bulk MoS2 could be exfoliated and cut into MoS2 nanosheets with narrow lateral size distribution around 13 nm (inset in Figure 1a). Furthermore, dynamic light scattering (DLS) was performed to measure the hydrodynamic diameter and the size distribution. As shown in Figure S2, it revealed that MoS2 QDs has narrow size distribution with an average hydrodynamic diameter around 14.7 nm, indicating the good monodisperse of MoS2 QDs. The slight increase in diameter compared to the result of the transmission electron microscope (TEM) (Figure 1a) is resulted from the absorption of water molecules on the surface of MoS2 QDs. The typical tapping atomic force microscopic (AFM) image further confirmed the uniform size distribution of resulting MoS2 QDs (Figure 1b). The height of the resulting MoS2 QDs was about 1~2 nm, indicating the resulting MoS2 QDs was 1-3 layers (inset in Figure 1b).16 High resolution TEM (HRTEM) image was employed to characterize the lattice structure of the obtained MoS2 QDs. It showed an obvious crystalline lattice structure (Figure 1c). The fast Fourier transformation (FFT) pattern of a chosen rectangular area revealed the hexagonal lattice structure with a fringe lattice space of 0.27 nm (Figure 1d), which was assigned to a (100) lattice plane, indicating the structure of hexagonal MoS2.30 In comparison with the X-ray diffraction (XRD) pattern of bulk MoS2, the peak located at 14.5 degree corresponding to the (002) plane disappeared in MoS2 QDs (Figure 1e) due to the formation of five or less layered MoS2 QDs and the fact that the preferred orientation of products laid on the silicon substrate, the obvious broadening peaks in the diffraction pattern of MoS2 QDs resulted from the incomplete crystallization of as-prepared small size MoS2 QDs crystallites appearing in different dimensions, consistent with a previous report.16 Raman spectroscopy was utilized to characterize the crystal structure and thickness of MoS2 QDs. As


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shown in Figure 1f, two strong Raman peaks could be observed at 380.5 and 407.1 cm-1 for bulk MoS2, which was assigned to in-plane (E12g) and vertical plane (A1g) vibration of Mo-S bonds in 2H MoS2, respectively.26 The corresponding bands for MoS2 QDs were located at 380.8 and 406.2 cm-1. It is obvious that the MoS2 QDs had a smaller Raman shift difference between E12g and A1g modes compared to the bulk counterpart, which was attributed to the E12g stiffening and A1g softening with decreasing layer thickness.31 This demonstrated the successful synthesis of the MoS2 QDs with 1-2 layers.31

Figure 1. (a) TEM image and particle size distribution of MoS2 QDs (inset). (b) AFM image of MoS2 QDs and AFM height measurement across MoS2 QDs (inset). (c-d) HRTEM images of MoS2 QDs. Inset in c: FFT pattern of chosen area. (e) XRD pattern and (f) Raman spectra of MoS2 QDs and bulk MoS2.

This element composition of MoS2 QDs was evaluated by energy dispersive X-ray



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spectroscopy (EDX, figure S3) and X-ray photoelectron spectroscopy (XPS) analysis (Figure S4). EDX results reveal that the atomic ratio of Mo:S approaches to 1:2, which demonstrates the composition of obtained MoS2 QDs is still in the form of MoS2 QDs. The XPS analyses are shown in Figure 2a. Two characteristic peaks located at 230 and 233.1 eV assigned to Mo4+ 3d5/2 and Mo4+ 3d3/2 in bulk MoS2 are observed. The corresponding peaks of S2- 2p3/2 and 2p1/2 peaks of bulk MoS2 are located at 162.9 and 164.1 eV respectively (Figure 2c), indicating the original bulk MoS2 has good 2H structure.32 Fig. 2b manifests the spectra of Mo orbital with double peaks at 231.5 and 234.1 eV attributed to Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively, suggesting the dominance of Mo4+ in MoS2 QDs. The S2p3/2 and 2p1/2 peaks of MoS2 QDs are observed at 164.1 and 165.6 eV (Figure 2d), and the single doublet with strong S2p3/2 peak at 164.1 eV corresponding to -2 oxidation state for the sulfur.33 It was found that the Mo and S core-level binding energies in MoS2 QDs exhibit a global positive shift with an average of 1.2 eV compared to bulk MoS2. The blue shift indicates shorter and stronger Mo-S chemical bonds with applied compressive strain,34 which is usually observed in small metal clusters.35 In addition, the formation of Mo-O bond also contributed to the blue shift.

Figure 2. Mo 3d high-resolution XPS spectra of (a) bulk MoS2 and (b) MoS2 QDs; S 2p high-resolution XPS spectra of (c) bulk MoS2 and (d) MoS2 QD.


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The photograph of MoS2 QDs supernatant under bright light is yellow in color, while strong blue emission can be observed under UV light irradiation (inset in Figure 3a,). An obvious broad peak located at 268 nm from the UV-vis absorption spectrum was observed (Figure 3a). It is different from the 2D MoS2 counterpart with large lateral dimensions, which shows the known peaks at ~609 and ~668 nm ascribed to B and A excitonic peaks, respectively, arising from the K point of Brillouin zone.10 The strong blue shift in the optical absorption was ascribed to the quantum size effect,36-38 and the broad absorption at 268 nm can be attributed to blue-shifted convoluted Z, C, and D excitonic peaks.38-39 The PLE spectrum recorded with the strongest luminescence displays a strong sharp peak at 460 nm (2.69 eV) and weak sharp peak at 268 nm (4.63 eV). Similarly, the maximum emission peak centered at 530 nm can be seen in PL spectra excited at 460 nm, differing from the two well-known peaks centered at ~610 and ~661 nm in 2D MoS2 flakes with the large lateral dimensions from the direct band gap hot PL of the K point.10 The strongly blue-shifted hot PL from the K point is assigned to the small lateral dimensions of MoS2 QDs owing to quantum size effect.39-40

Figure 3. (a) UV-Vis absorption (ABS), PLE, and PL (at 460 nm excitation) spectra of the MoS2 QDs aqueous solution. Inset (a): photographs of the MoS2 QDs aqueous solution under visible light and UV irradiation (365 nm, 16W). (b) Influence sonication time to the FL intensity at different TBA: ethanol/water volume ratio 1) 1.5:4; 2) 3:4; 3) 0.5:4; 4) 6:4. (c) pH-dependent PL spectra with pH ranging from 3 to 9, the pH of as-prepared MoS2 QDs is 14. (d) Fluorescent lifetime of MoS2 QDs.



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To evaluate the influences of sonication time and TBA concentration, the sonication duration from 5 h to 20 h with different dosages of TBA were tailored. As shown in Figure 3b, it was revealed that the PL intensity increased with sonication time from 5 h to 15 h, and the strongest PL intensity was generated at 15 h with volume ratio of TBA: ethanol/water at 1.5:4. The corresponding PL spectra and AFM imaging analysis were presented as Figure S3-S4. Lateral dimension decreased along with increasing sonication time (Figure S6), consisting with the increase of PL increase along with increasing sonication time (Figure S5). The as-prepared MoS2 QDs shows a pH-dependent PL emission, and a strong PL emission was observed under alkaline conditions, whereas the PL is nearly completely quenched under acidic conditions (Figure 3c). It was concluded that some S moieties have been changed to –OH group during the cleavage process, which can suppress the non-irradiative process and improve integrity of the π conjugated system as an electron donator to enhance the PL intensity.41 The pH-dependent PL emission behavior is consistent with the proposed mechanism. The exited stated lifetime of as-prepared MoS2 QDs was up to approximately 5.3 ns, which also outperformed its graphene QDs counterpart (Figure 3d).42 Using Rhodamine B as reference, the fluorescent quantum yield was calculated to be 19%, which is superior to the previous reported graphene QDs from the hydrothermal route.41, 43-44 To gain preliminary understandings on the cleavage mechanism of MoS2 QDs, we performed Density Functional Theory (DFT) calculations on the modeling single-layer MoS2 materials.45 The amine group of TBA is sterically bulky, and unlikely to participate in the cleavage mechanism, and thus the anionic HO- is expected to be the main reason for cleavage. HO- first randomly locates in the solvent (II), while the S-H-O hydrogen bonds result in the average Mo-O bond distance of about 4.0 Å (Figure 4). From II，the approach of OH- to the


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Mo significantly weakens the adjacent Mo-S bond strengths, and results in the Mo-S bond distance lengthening from ~2.4 Å in II to > 3.5 Å in III. Thereafter, the nucleophilic attack of another OH- on the adjacent Mo atom ruptures the retained Mo-S bond in III, and generates the cleaved fragment IV. From IV, the OH- mediated Mo-S bond cleavage could then occur subsequently, presumably via the chain-like cleavage mode (Figure 4) due to the significantly weakened Mo-S bonds therein. Finally, the flakes could be released from the MoS2 QDs as long as the chain-like cleavage mechanism finished. It demonstrated that the amine group of TBA is sterically bulky, and unlikely to participate in the cleavage mechanism, and thus the anionic HO- is expected to be the main reason for cleavage.

Figure 4. Cleavage mechanism of single-layer MoS2 mediated by OH- in TBA. (I) MoS2 nanosheet; (II) Formation of S-H-O hydrogen bond in MoS2 nanosheet; (III) Rupture of the Mo-S bond; (IV) Chain-like cleavage of MoS2 nanosheet.

Excitation-dependent PL behaviors phenomenon is observed in our MoS2 QDs. As shown in Figure 5a, when the MoS2 QDs were excited at the maximum absorption wavelength of 460 nm, it exhibited a strong peak at 530 nm. The emission wavelength increased with the red shift of excitation wavelengths from 400 to 520 nm. The different surface defects and edge sites introduced during the sonication may explain the excitation-dependent PL behaviors of the



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as-prepared MoS2 QDs.41, 43 Most interestingly, the synthesized MoS2 QDs showed up-conversion properties. As shown in Figure 5b, when the excitation wavelength switches from 775nm to 900nm, the corresponding emission peaks moves from 525nm to 610nm.Remarkably, it shows an almost constant shifting between the energy of up-converted emission light (Em) and excitation light (Ex). Figure 5c presents the linear relationship between Em and Ex, and the function of the fit line is Em = 1.0Ex + δE (R2 = 0.9989) with δE = 0.72 eV. We speculated the up-conversion mechanism of MoS2 QDs is ananti-Stokes PL. The electrons at the highest occupied molecularorbital (HOMO) were excited and transited to lowest unoccupied molecular orbital (LUMO) with high-energy state such as at the K valley site. The excitons were then relaxed back to orbital designation with lower energy state than HOMO, thus producing an up-conversion behavior Figure 5d.

Figure 5. (a) Down-conversion and (b) up-conversion excitation-dependent PL behaviors of the MoS2 QDs. (c) The energy of the excitation light as a function of the emission. (d) Schematic band structure of electron transitions process of MoS2 QDs.


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The good fluorescence properties inspired us to investigate their potential bioimaging application. We first investigated the physiological stability of the MoS2 QDs. The characteristic Uv-vis absorption at 268 nm shows negligible change when keep the MoS2 QDs in water, PBS with high salt solution or cell medium for two weeks (Figure 6a); coinciding with no distinct precipitate was found from the photograph (inset in Figure 6a,). This indicates the good stability of MoS2 QDs. In vitro cytotoxicity of as-prepared MoS2QDs was then evaluated

with

HeLa

cells

as

a

model

by

using

3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay (Figure 6b). No obvious cytotoxicity was observed to MoS2 QDs-transfected HeLa cells when the concentration of MoS2 QDs rangs from 15 to 100 µg/mL. It appeared a slight reduction of cell viability at extremely high concentrations (200µg/mL) of MoS2 QDs, indicating good biocompatibility. The down-conversion cellular imaging of MoS2 QDs was shown in Figure 6c-d. Green fluorescence was observed in cytoplasm, indicating the MoS2 QDs were successfully internalized by the HeLa cells without any other conjugations. Notably, the up-conversion PL of MoS2 QDs renders the opportunity to harvest near-IR light using multi-photon excitation for cell imaging without introducing harmful damaged from the laser. The bright green area inside HeLa cells under near-IR excitation (800 nm) reveals the successful translocation of MoS2 QDs through the cell membrane (Figure 6e-f), which indicating the great promising of the MoS2 QDs for up-conversion bioimaging. Biological compatibility combined with strong and stable up-conversion PL, allows MoS2 QDs to be an emerging biological imaging labeling agent.



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Figure 6. (a) UV-Vis absorption and photograph (inset) of MoS2 QDs dissolved in water, PBS with 1 M NaCl and cell medium. (b) Cytotoxicity of HeLa cells induced by MoS2 QDs. Down-conversion cellular imaging of MoS2 QDs (c) under 460 nm excitation and (d) corresponding merge images with bright field. Up-conversion cellular imaging of MoS2 QDs (e) under 800 nm excitation and (f) corresponding merge images with bright field.

Photodynamic therapy (PDT) is a photo-based therapeutic by producing highly reactive oxygen species, especially singlet oxygen (1O2).46 The clinical applications of current PDT agents often suffers from some deficiencies such as low 1O2 produce, photobleaching or poor biocompatibility. Semiconductor QDs are alternatively emerging as efficient PDT agents with photostability and water dispensability.47 To explore the potential application of MoS2 QDs in PDT, the 1O2 production capability of MoS2 QDs was further investigated. Figure 7a shows


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the comparison of photostability of MoS2 QDs and protoporphyrin IX (PpIX, a classic photosensitizer). After a 60-min irradiation under a 500 w xenon lamp, no apparent decrease is observed in MoS2 QDs, while the PpIX showed an absorption decrease of 22%, which demonstrated the highly photostability of MoS2 QDs. The singlet oxygen production was detected by chemical probe of 1,3 diphenylisobenzofuran (DPBF), whose absorbance intensity at 410 nm would be reduced in the existence of 1O2.48 Figure 7b showed the DPBF absorption in presence of MoS2 QDs as a function of exposure time under a 630 nm laser irradiation. In Figure 7c, The intensity of DPBF absorption exhibited decrease with 10 min for both solution containing MoS2 QDs or PpIX, and MoS2 QDs induced a faster and larger DPBF absorption decrease compared to PpIX, which indicated much stronger 1O2 production ability of MoS2 QDs.

Figure 7. (a) Photostability comparison of MoS2 QDs and PpIX. A0 and A are the absorbance of the sample at 236 nm before and after irradiation. (b-c) Time-dependent generation of singlet oxygen by MoS2 QDs and PpIX detecting by the bleaching of DPBF absorption at 460 nm under 630 nm laser irradiation.

Furthermore, the 1O2 production within cells was also evaluated by a singlet oxygen sensor green (SOSG) reagent. The fluorescence of SOSG is quenched in its intact form and produces strong fluorescence upon reaction with 1O2.49 The detection of 1O2 generation after 635 nm irradiation was performed by evaluating the fluorescence of SOSG at different conditions. As shown in Figure 8, compared to the weak SOGO fluorescence after 635 nm



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irradiation in the MoS2 QDs-unexposed cells(Figure 8c-d), the MoS2 QDs-incubated cells exhibited much stronger fluorescence (Figure 8a-b), which suggested a higher level of 1O2 production in MoS2 QDs-incubated cells.50 These results revealed that the prepared MoS2 QDs possessed good ability of 1O2 production, providing promising application in PDT and biomedicine.

Figure 8. Confocal fluorescence cell images of SOSG with different condition after irradiating at 630 nm: (a-b) MoS2 QDs+ SOSG. (c-d) SOSG only.

■ CONCLUSIONS A facile method for tunable synthesis of MoS2 QDs using a TBA-assisted sonication route is presented. By simply controlling the sonication duration, the lateral dimension of MoS2 nanoflakes can be effectively tuned to generate desired MoS2 QDs with unique properties. The -OH group in TBA is suggested to intercalate and cleave the bulk MoS2 by substituting S moieties. Comprehensive microscopic and spectroscopic tools including TEM, AFM, XPS, XRD, and Raman spectroscopy were used to confirmed morphology and composition of the MoS2 QDs. The resulting MoS2 QDs emerged promising PL properties including good


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down-conversion fluorescent emission, especially remarkable up-conversion behavior. The PL properties combined with good biocompatibility and physiological stability of MoS2 QDs led to good performance in bioimaging. Notably, it showed stronger 1O2 production ability compared to commercial photosensitizer PpIX, which provide great promising application in photodynamic therapy.

■ EXPERIMENT SECTION Synthesis of MoS2 QDs. Layered MoS2 materials (100 mg, Sigma Aldrich) with lateral dimension size of 1.5 nm were dispersed in solution containing 1.5 ml TBA (~40% in water, from Sigma-aldrich) and 45 vol% ethanol/water mixture (4 ml) and sonicated (ultrasonic processor KH5200E) for different durations (5, 10, 15, and 20 h). After that, the suspension was reserved for 12 h at room temperature and the supernatant was collected. Free TBA and reagents were removed by filtration through a 3-kD filter (Millipore) at 11000 rpm for 10 min for three times. The resulting MoS2 QDs were dispersed in 200 mL ultrapure water and stored at 4 oC, with which the MoS2 QDs were prepared for the research of their microstructure and optical properties. Cell Incubation. HeLa cells (1.0×104) were incubated on glass cover slides loaded in 24-well culture plates containing 500 µL Dulbecco's Modified Eagle Media (DMEM) for 12 h. The medium was then replaced with fresh Opti-MEM (500 µL) containing MoS2 QDs (50 µg/mL) and cultivated for 4 h. After that, washing each well twice by phosphate buffered saline (PBS) (0.1 M, pH=7.4) and fresh DMEM medium (500 µL) was added to culture for another 24 h. The MoS2 QDs-transfected HeLa cells were then detected by confocal microscope (CLS, FV1200, Olympus, Japan or at Olympus Fluoview FV1000). Cellular Toxicity. HeLa cells (5.0×104) were cultured first for 12 h in a 96-well plate containing Dulbecco’s modified Eagle’s medium (DMEM) (100 µL) in each well, and for



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another 4 h after the medium was replaced with fresh Opti-MEM alone or medium containing MoS2 QDs. Then, the Opti-MEM substituted the 100 µL fresh DMEM and incubated for 24 h. MTT (20 µL, 5 mg/mL) was then added to every cell well. The media was removed 4 h later, and sodium dodecylsulfate (DMSO, 100 µL) was added to solubilize the formazan dye. After shocking (37 oC, 120 rpm) for 15 min，the absorbance of each well was measured using a ELx 808 enzyme-linked immunoassay instrument (BioTek, USA) at 530 nm. The cytotoxicity of MoS2 QDs was estimated by the percentage of growth inhibition calculated with the formula. Growth inhibition %= (1-Atext/Acontrol) ×100% Singlet Oxygen Detection. 5 µL DPBF (10 mM) solution was add to 300 µL MoS2 QDs (100 µg/mL) solution. The mixture was irradiated under 630 nm laser for 10 min while recording the absorption intensity of DPBF at 460 nm every minute. For control, the absorption of mixture in the present of PpIX and DPBF was also recorded in the same condition. A 1O2 sensor green (SOSG) regent was utilized to investigate the production of 1O2 by confocal analysis. HeLa cells were treated as same as the previous steps. Irradiating by 630 nm for 10 min, the cells were then washed three times with PBS (10 mM, pH 7.4) and stained with 2 mM of SOSG in PBS for 30 min. The cells were imaged using a CLSM. The dye was excited at 504 nm and observed through a 525 nm emission band-pass. Characterization. Their lateral dimension and crystal structure of MoS2 QDs were characterized using HRTEM (JEM-2010F from JEOL, 200 KV). The hydrodynamic diameter was measured by DLS (Nano-ZS90, UK). AFM (NanoscopeIIIa, USA) was utilized to measure the thickness of MoS2 QDs. Then the structure of MoS2 QDs was further demonstrated by XRD (Bruker-AXS X-ray diffractometer with Cu Kα radiation, λ=1.5418 Å) and Raman spectrum (inVia-Reflex Confocal Raman spectrometer, Renishaw, UK). XPS analyses were


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recorded with an ESCALAB 250 spectrometer (Thermo-VG Scientific, USA). The UV-visible (UV-vis) absorption analysis was measured by the UV-1800 spectrophotometer (Shimadzu, Japan) and all fluorescence measurements were carried out using the Hitachi F-4500 fluorescence spectrofluorometer (Tokyo, Japan) at room temperature. The fluorescent lifetime was obtained by FLS920 Instrument (Edinburgh, UK).

■ ASSOCIATED CONTENT SEM Characterization of bulk MoS2, DLS measurement of MoS2 QDs, EDX spectrum of MoS2 QDs, X-ray photoelectron spectroscopy (XPS) analysis of MoS2 QDs, PL spectrum of MoS2 QDs, DFT calculation analysis cleavage mechanism, AFM characterization of MoS2 QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.



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■ ACKNOWLEDGEMENTS

The work was supported by National Natural Science Foundation of China (Grant No.21305008, 21475008, 21275017, 21127007), China Postdoctoral Special Foundation (NO. 11175039) and Ph.D. Programs Foundation of Ministry of Education of China (No.11170197), the Fundamental Research Funds for the Central Universities (NO. FRF-BR-15-020A) and the Chinese 1000 Elites Program and USTB Start-Up Fund; State Key Laboratory of Analytical Chemistry for Life Science SKLACLS1401.

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Scheme 1. Schematic present of synthetic method and resultant MoS2 QDs for bioimaging and PDT.


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Figure 1. (a) TEM image and particle size distribution of MoS2 QDs (inset). (b) AFM image of MoS2 QDs and AFM height measurement across MoS2 QDs (inset). (c-d) HRTEM images of MoS2 QDs. Inset in c: FFT pattern of chosen area. (e) XRD pattern and (f) Raman spectra of MoS2 QDs and bulk MoS2.



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Figure 2. Mo 3d high-resolution XPS spectra of (a) bulk MoS2 and (b) MoS2 QDs; S 2p high-resolution XPS spectra of (c) bulk MoS2 and (d) MoS2 QD.


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Figure 3. (a) UV-Vis absorption (ABS), PLE, and PL (at 460 nm excitation) spectra of the MoS2 QDs aqueous solution. Inset (a): photographs of the MoS2 QDs aqueous solution under visible light and UV irradiation (365 nm, 16W). (b) Influence sonication time to the FL intensity at different TBA: ethanol/water volume ratio 1) 1.5:4; 2) 3:4; 3) 0.5:4; 4) 6:4. (c) pH-dependent PL spectra with pH ranging from 3 to 9, the pH of as-prepared MoS2 QDs is 14. (d) Fluorescent lifetime of MoS2 QDs.



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Figure 4. Cleavage mechanism of single-layer MoS2 mediated by OH- in TBA.


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Figure 5. (a) Down-conversion and (b) up-conversion excitation-dependent PL behaviors of the MoS2 QDs. (c) The energy of the excitation light as a function of the emission. (d) Schematic band structure of electron transitions process of MoS2 QDs.



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Figure 6. (a) UV-Vis absorption and photograph (inset) of MoS2 QDs dissolved in water, PBS with 1 M NaCl and cell medium. (b) Cytotoxicity of HeLa cells induced by MoS2 QDs. Down-conversion cellular imaging of MoS2 QDs (c) under 460 nm excitation and (d) corresponding merge images with bright field. Up-conversion cellular imaging of MoS2 QDs (e) under 800 nm excitation and (f) corresponding merge images with bright field.


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Figure 7. (a) Photostability comparison of MoS2 QDs and protoporphyrin IX (PpIX, a classic photosensitizer). A0 and A are the absorbance of the sample at 236 nm before and after irradiation. (b-c) Time-dependent generation of singlet oxygen by MoS2 QDs and PpIX detecting by the bleaching of DPBF absorption at 460 nm under 630 nm laser irradiation.



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Figure 8. Confocal fluorescence cell images of SOSG with different condition after irradiating at 630 nm: (a-b) MoS2 QDs+ SOSG. (c-d) SOSG only.


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ToC figure:


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