A facile one-step method to produce MoS₂ quantum dots as promising bio-imaging materials

Abstract

A facile one-step, low cost and green method is developed to produce molybdenum disulfide quantum dots (MoS₂ QDs) by solvothermal treatment in the presence of NaOH. The high resolution transmission electron microscope observation indicated the high crystallinity and narrow particle size distribution of the as-prepared MoS₂ QDs. The as-prepared MoS₂ QDs exhibited bright blue luminescence under UV light irradiation. Excitation-dependent photoluminescence emission is observed in the aqueous dispersion of MoS₂ QDs. By being directly applied for bio-imaging of HeLa cells, MoS₂ QDs showed bright luminescence, low toxicity and excellent biocompatibility. Furthermore, this convenient method may provide a potential advance for large-scale production of other layered materials.

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1. Introduction

Since the discovery and extensive investigation of graphene, other two-dimensional (2D) layered materials such as hexagonal boron nitride, transition metal dichalcogenides, and metal halides, due to their semiconducting electronic properties, large surface area, and promising applications have also been attracting great attention in recent times.^1–4 As a typical transition metal dichalcogenides, molybdenum disulfide (MoS₂) is a 2D hexagonal lattice composing of S–Mo–S triple layers bound by weak van der Waals forces. However, it is only very recently that few-layer MoS₂ has attracted a lot of attention. As demonstrated by Wang et al., MoS₂ photoluminescence increases with decreasing layer thickness, and that luminescence from a monolayer is the strongest while it is absent in bulk material.⁵ The results indicate that MoS₂, an indirect band-gap material in its bulk form, becomes a direct band-gap semiconductor when thinned to a monolayer. Therefore, MoS₂ with a tunable band-gap offers new opportunities for various areas, such as electronics, energy storage, sensors, and biomedicine.^6–9

MoS₂ can be exfoliated into single or few-layer MoS₂ nanosheets via external forces, due to the weak van der Waals forces between the MoS₂ layers. Different methods to obtain single- or few-layered MoS₂ nanosheets have been reported.^10–12 However, the obtained MoS₂ nanosheets suffer from more or less drawbacks, such as low quality, insufficient active sites, and introduction of other hetero atoms, which distinctly hinder the widespread use. Compared with MoS₂ nanosheets, zero-dimensional MoS₂ quantum dots (MoS₂ QDs) possess strong quantum confinement and edge effects,^13,14 resulting in numerous unique and outstanding properties. Besides, the characteristic luminescent property of quantum dots might make MoS₂ QDs promising for biomedical and optical imaging.¹⁵ As a result of their excellent properties and ongoing applications, there is great demand for the MoS₂ QDs. There have been many reports focusing on the preparation of MoS₂ nanosheets; however, reports on synthesis of MoS₂ QDs are relatively few and properties of MoS₂ QDs have not been fully optimized and understood. Therefore, it is essential to develop an efficient method for the preparation of MoS₂ QDs with superior properties.

To date, the preparation methods for MoS₂ QDs can be generally classified into two groups: the bottom-up and the top-down method. Bottom-up methods involve the assembly of atoms or molecules into nanostructured MoS₂ QDs. Ren et al. demonstrated the bottom-up method for the preparation monolayer MoS₂ QDs by using sodium molybdate and dibenzyl disulfides as molybdenum and sulfur sources.¹⁶ Top-down approaches refer to the cutting of MoS₂ sheets into MoS₂ QDs, the method consists of solvothermal treatment,¹⁵ electro-Fenton reaction,¹⁷ electrochemical synthesis¹⁸ and liquid exfoliation,¹⁹ etc. Xu et al. reported the synthesis of uniform MoS₂ and WS₂ quantum dots through solvothermal treatment of MoS₂ and WS₂ nanosheets, the as-prepared quantum dots exhibit strong fluorescence, good cell permeability, and low cytotoxicity make them promising and biocompatible probes for in vitro imaging.¹⁵ Li et al. reported a simple, cost-effective, efficient, and controllable method for production of hydroxyl radicals was used to cut bulk MoS₂ based on the electro-Fenton reaction.¹⁷ Size-controlled synthesis of luminescent MoS₂ QDs with a narrow size distribution using a unique electrochemical etching of bulk MoS₂ was demonstrated by Gopalakrishnan, and the as-synthesized MoS₂ QDs exhibited excellent electrocatalytic activity towards hydrogen evolution reaction.¹⁸ A liquid exfoliation technique reported by Gopalakrishnan et al. through bath sonication followed by ultrasound probe sonication has been shown to produce MoS₂ QDs, showed excellent electrocatalytic activity with low overpotential.¹⁹ However, most of these approaches are usually limited by the tedious process, time consumption, harsh conditions or use expensive and hazardous organic solvents. Therefore, develop a simple and environmentally friendly method for the large scale production of high-quality MoS₂ QDs still remains a challenge.

Herein, a facile one-step route to extraction of blue luminescent MoS₂ QDs from bulk MoS₂ powder is demonstrated by solvothermal process in the presence of NaOH. The advantages of this work are as follows. First, the MoS₂ QDs are produced by a facile one-step approach, which is relatively simple and low cost compared with the previous work. Second, the synthetic process occurs in ethanol and neither a strong acid nor surface passivation reagent is needed, which has the advantage of being nontoxic to the human body and environmental friendly. Third, the as-prepared MoS₂ QDs showed good dispersion, low toxicity and bright blue luminescence in aqueous solution and proved to be a promising new candidate for application in bio-imaging.

2. Experimental section

2.1 Materials

Bulk MoS₂ powder was purchased from Sinopharm chemical reagent Co., Ltd., China. Ethanol and NaOH was purchased from Tianjing Guangfu Technology Development Co., Ltd., China. All reagents were of analytical grade and used without further purification. The water used throughout all experiments was deionized (DI) water purified through a Millipore system.

2.2 Preparation of MoS₂ quantum dots

MoS₂ QDs were obtained by solvothermal treatment of bulk MoS₂ powder. In a typical procedure, 30 mg bulk MoS₂ powder and 45 mg NaOH were mixed with 30 mL ethanol in a glass vial. After stirring for 5 min, the mixture was transferred to a Teflon lined autoclave and heated at 180 °C for 12 h. After cooling to room temperature, the black precipitated were filtered out using a mixed cellulose ester membrane with 25 nm pores (Millipore). The yellow supernatant was dialyzed in a 1000 Da dialysis bag against deionized water for 2 days to remove excess ethanol and NaOH. The resultant light yellow solution was freeze-dried, and a light brown powder of MoS₂ QDs was obtained. The as-prepared MoS₂ QDs were redispersed in water for further use.

2.3 Characterization

High-resolution transmission electron microscopy (HRTEM) images were taken with a JEOL JEM 2010 microscope (Japan) at 200 kV. Atomic force microscopy (AFM) image was taken in tapping mode with the SPM Dimension 3100 (Veeco, USA). X-ray photoelectron spectroscopy (XPS) experiments were conducted using an AXIS ULTRA OLD X-ray photoelectron spectrometer (Kratos, Japan). X-ray diffraction (XRD) patterns were obtained from a TD-3500 Automatic X-ray diffractometer System (Dandong, China) with Cu Kα radiation (40 kV, 20 mA, λ = 1.54051 Å). The UV-vis spectra were recorded on a Hitachi U-3900 spectrophotometer (Japan). The fluorescence spectra of the MoS₂ QDs were measured with a Horiba Jobin Yvon Fluoromax-4 fluorescence spectrophotometer. The confocal fluorescence microscope was examined using a Nikon C2 Plus confocal laser scanning microscope (Nikon Corp, Japan).

2.4 MTT experiments

The cellular cytotoxicity of MoS₂ QDs was tested on HeLa cell by MTT assay. Typically, 100 μL suspension of HeLa cells in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum and 1% penicillin/streptomycin were seeds to 96-well plate at the density of 5 × 10³ cells per well and incubated in a 5% CO₂ humidified incubator at 37 °C for 24 h. Then, the MoS₂ QDs with different concentrations (12.5, 25, 50 and 100 μg mL⁻¹) were introduced into the wells and incubated for another 24, 48, 72 h, respectively. The medium was removed and cells were washed with phosphate-buffered saline (PBS) for three times. Then, 20 μL of MTT solution (5 mg mL⁻¹) was added to each well. The 96-well plates were incubated for another 4 h. Finally, the culture medium was removed and preplaced with 200 μL of DMSO and the optical density of the mixtures at 490 nm was measured. Cell viability was expressed as percentage of absorbance relative to control, the control was obtained in the absence of MoS₂ QDs. Experiments were performed in triplicates, with nine replicate wells for each sample and control per assay.

2.5 Fluorescence imaging experiments

HeLa cells were seeded in each well of a confocal dish (coverglass-bottom dish) and cultured at 37 °C for 12 h. An aqueous solution of the MoS₂ QDs (100 μg mL⁻¹) was passed through a 0.2 μm sterile filter membrane. The filtered fluorescent suspension (50 μL) was mixed with the DMEM (200 μL) and then added the confocal dish in which the HeLa cells were grown. After an incubation of 5 h, the cells were washed with fresh DMEM and PBS for three times and kept in PBS for the fluorescence imaging. Fluorescence images of HeLa cells were acquired by confocal microscopy with excitation wavelengths of 405 nm.

3. Results and discussion

MoS₂ QDs were prepared by a facile one-step solvothermal treatment. The interlamellar spacing of MoS₂ was about 6.5 Å, so the ionic radius of the “intercalation reagent” needs to be lower than the above value in aqueous solution, such as sodium (1.78 Å),²⁰ hydroxyl (2.68 Å),²¹ chloride (1.95 Å),²² and so on. Herein, we demonstrate that in the solvothermal process, NaOH can be chosen as corrosion, intercalation and exfoliation reagent all at the same time. The possible mechanism and synthetic procedure to prepare MoS₂ QDs by a simple solvothermal treatment is illustrated in Fig. 1. First, because of the strong hydrophobic properties of bulk MoS₂ powder, MoS₂ disperses in ethanol can improve the dispersion and increase the contact interface between MoS₂ and NaOH solution. Second, in the solvothermal process, NaOH may corrode the edges of MoS₂ and be beneficial for Na⁺ and OH⁻ to insert between the MoS₂ layers. Third, the high temperature and pressure can further enhance the interaction between the MoS₂ and NaOH solution, which provide advantage for the exfoliation of bulk MoS₂. With NaOH insert between the MoS₂ layers, bulk MoS₂ can be exfoliated layer by layer, and finally break into MoS₂ nanosheets and quantum dots.

Fig. 1 Schematic illustration for the preparation of MoS₂ QDs by solvothermal treatment in the presence of NaOH.

Fig. 2a shows a transmission electron microscope (TEM) image of the as-prepared MoS₂ QDs, exhibiting a relatively narrow size distribution, which was evaluated from the TEM image by measuring about 200 individual dots. TEM measurement showed that MoS₂ QDs size between the 3 and 9 nm with an average diameter of 5.5 nm (Fig. 2b). The high resolution TEM (HRTEM) images (Fig. 2c and d) demonstrate the high crystallinity of the as-prepared MoS₂ QDs, with a lattice parameter of 0.27 nm, consistent with the (100) diffraction planes of MoS₂,^23,24 indicating that the MoS₂ QDs kept the similar crystallinity with bulk material. To further investigate the morphology and thickness of the as-prepared MoS₂ QDs, the suspensions of MoS₂ QDs were transferred to the surface of freshly exfoliated mica and characterized by AFM. As shown in Fig. S1 (ESI†), the heights of most of the MoS₂ QDs are below 2 nm, representing the height of MoS₂ monolayer and bilayers.¹⁵

Fig. 2 (a) TEM image of MoS₂ QDs. (b) Size distribution of MoS₂ QDs. (c and d) HRTEM image of MoS₂ QDs.

X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the surface electronic state composition of the as-produced MoS₂ QDs. As shown in Fig. 3a and b, Mo 3d_3/2, Mo 3d_5/2, S 2p_1/2 and S 2p_3/2, peaks can be observed at 234.8 eV, 231.8 eV, 168.5 eV and 167.5 eV, respectively, indicating the dominant 2H MoS₂ phase in the crystal structure.²⁵ Two characteristic peaks located at 231.8 (Mo 3d_5/2) and 234.8 eV (Mo 3d_3/2) are typical values for Mo⁴⁺ in MoS₂ (Fig. 3a), whereas the binding energies of S 2p_1/2 and S 2p_3/2 are located at 168.5 eV and 167.5 eV, respectively, revealing the dominance of S²⁻ in the sample (Fig. 3b).^19,26,27 Detailed compositional analysis results reveal that the atomic ratios of Mo/S is 1 : 2.28, ascribed to the unsaturated sulfur atoms located at the external edge of MoS₂ QDs.¹⁷

Fig. 3 High resolution (a) Mo 3d and (b) S 2p spectra of MoS₂ QDs.

The crystal structure of the as-prepared MoS₂ QDs was systematically characterized using X-ray diffraction (XRD). As shown in Fig. 4a, the peak position of bulk MoS₂ showing a strong diffraction peak at 2θ = 14.4° and two lower peaks at 2θ = 39.6° and 2θ = 49.8°, corresponding to the (002), (103), and (105) planes.¹⁵ Previous study has demonstrated that the characteristic peak at 2θ = 14.4° disappears when MoS₂ powder are exfoliated to monolayer nanosheets.²⁸ Xu et al. have found that if all the materials are monolayer and have no interaction between each other, there would be no signal or peak on XRD pattern.¹⁵ Ren et al. have proved that for most samples, there is none visible signature seen in XRD analyses.¹⁶ As shown in Fig. 4a, there is no peak of MoS₂ QDs on XRD pattern, which is consistent with the previous reports. The disappearance of the characteristic peaks suggests small and thin structure of the as-prepared MoS₂ QDs.

Fig. 4 (a) XRD patterns of MoS₂ QDs. (b) UV-vis spectra of MoS₂ QDs (inset: Tyndall effect of MoS₂ QDs solution and powder of MoS₂ QDs). (c) PL spectra excited by various wavelengths ranging from 280 to 480 nm (inset: photograph of the MoS₂ QDs under 365 nm UV light). (d) Excitation and emission PL spectra of MoS₂ QDs. (e) PL spectra of MoS₂ QDs prepared by solvothermal treatment with or without the addition of NaOH. (f) Photostability of MoS₂ QDs under the excitation of 370 nm.

The optical properties of MoS₂ QDs were studied by using UV-vis absorption and PL spectroscopy. For the UV-visible absorption spectra (Fig. 4b), MoS₂ QDs show an optical absorption in the UV region (λ < 300 nm), with a tail extending into the visible range, assigned to the excitonic features of MoS₂ QDs.^29,30 As shown in the inset picture of Fig. 4b, after being dialyzed, the light yellow solution was freeze-dried to give a light brown powder (MoS₂ QDs). The as-prepared MoS₂ QDs exhibit excellent stability and solubility in aqueous solution, no obvious aggregation could be observed after standing at room temperature for several weeks, and its high stability in water was confirmed after by observing the presence of a Tyndall effect in the form of a typical green line that results from light scattering when a laser beam is passed through the MoS₂ QDs solution. To further explore the optical properties of MoS₂ QDs, a detailed PL study was carried out by using different excitation wavelengths. Fig. 4c shows the normal PL spectra of MoS₂ QDs. The MoS₂ QDs emitted intense blue luminescence under irradiation by a 365 nm UV lamp (Fig. 4b, inset). The as-prepared MoS₂ QDs exhibit excitation-dependent PL behavior, similar to the previous reports.^15,16 As the excitation wavelength ranges from 280 to 480 nm, the PL peak of MoS₂ QDs shifts to longer wavelength from 380 to 520 nm (Fig. 4c). The strongest emission peak is located at 461 nm when excited at 370 nm. The photoluminescence excitation (PLE) spectra of the blue MoS₂ QDs (λ_em = 461 nm) as well as the PL spectra are shown in Fig. 4d. Fig. 4e shows the difference of PL spectra (at 370 nm excitation) of the MoS₂ QDs prepared by solvothermal treatment with or without the addition of NaOH. It is clear revealed that the MoS₂ QDs prepared by solvothermal treatment with the addition of NaOH exhibit stronger PL. This result indicates that the alkaline environment is the key factor for the forming of MoS₂ QDs with photoluminescent property. Fig. 4f shows the photostability of MoS₂ QDs under different irradiation time (λ_ex = 370 nm). Encouragingly, the emission intensities of MoS₂ QDs show no significant changes even after exposed to UV irradiation for 12 h, indicating the as-prepared MoS₂ QDs are more robust the conventional semiconductor quantum dots (QDs) with a typical photobleaching effect.

We studied the effect factors on the formation of MoS₂ QDs by comparison the PL spectra and discussed the formation mechanism of MoS₂ QDs. First, the reaction time on the formation of MoS₂ QDs was investigated by comparison of the PL spectra of MoS₂ QDs prepared at different reaction time. As shown in Fig. S2 (ESI†), MoS₂ QDs prepared with 12 h solvothermal reaction exhibit maximum PL intensity. The PL intensity decreases when the reaction time prolonged to 18 h. Then, the effect of temperature on the PL intensity of MoS₂ QDs was studied. Fig. S3 (ESI†) shows the PL spectra of MoS₂ QDs prepared at different temperatures. It is clear that with the rise of temperature, the PL intensity of MoS₂ QDs increase. Finally, the effect of the concentration of NaOH was investigated by comprising the PL spectra of the products. As shown in Fig. S4 (ESI†), the intensities of PL spectra increase with the increase of NaOH addition when the amount of NaOH is below 45 mg. However, there is a decrease of PL intensity when the amount of NaOH exceeds 45 mg. The above discussion gives the experimental evidence for the formation mechanism of MoS₂ QDs as illustrated in Fig. 1. From the above results, we might be able to further explain the formation mechanism of MoS₂ QDs as follows. At high temperature and pressure, Na⁺ and OH⁻ can insert into the sheet layer and OH⁻ can corrode the edge of MoS₂ sheets. This corrosion and insert effect is related to the reaction time. In a certain reaction time, the intensity of PL spectra of the MoS₂ QDs reaches the maximum value. But if continue to extend the reaction time, some of the smaller MoS₂ QDs may be completely decomposed, leading to the loss of fluorescence. On the other hand, the reaction temperature can accelerate the reaction between MoS₂ and NaOH, the production of MoS₂ QDs increase with the rise of the temperature. In addition, the concentration of NaOH have an impact on the reaction, an appropriate concentration will play the best corrosion and insert effect. Excess NaOH also may lead to the complete decomposition of MoS₂ QDs.

The MoS₂ QDs were introduced into the HeLa cells to show their bio-imaging capabilities using a confocal microscopy with excitation wavelength λ = 405 nm. As shown in Fig. 5a, bright blue luminescence is observed inside the cell, indicating that the MoS₂ QDs had been internalized by the HeLa cells and mainly localized in the cytoplasm region and could be used as efficient bio-imaging probes. Furthermore, the HeLa cells incubated with the MoS₂ QDs do not weaken the cell activity and maintain their normal morphology. We employed a time- and dose-dependent approach to evaluate the toxicity of the samples. HeLa cells were incubated with various concentrations of MoS₂ QDs for 24, 48 and 72 h. The MTT assays of cell viability studies suggested that MoS₂ QDs do not impose a considerable toxicity to HeLa cells compared to the control (Fig. 6). The above results indicate that the as-prepared MoS₂ QDs might be promising probes in cell imaging.

Fig. 5 (a) Confocal fluorescence microphotograph of HeLa cells incubated with MoS₂ QDs for 5 h (λ_ex = 405 nm); (b) bright-field microphotograph of cells; (c) an overlay image of (a) and (b).

Fig. 6 Dose- and time-dependent viability of HeLa cells incubated with different concentration of MoS₂ QDs. Each data point represents the mean value from at three independent experiments.

4. Conclusions

In summary, this work developed a simple and efficient solvothermal method for the preparation of MoS₂ QDs with excellent water solubility. Moreover, the obtained MoS₂ QDs show excellent optical property. The aqueous solution can directly applied to fluorescence imaging in vitro and showed excellent biocompatibility. The results demonstrated that alkaline environment is the key factor for the formation of MoS₂ QDs and a mechanism based on solvothermal induced intercalation and exfoliation of MoS₂ QDs. This convenient process represents a potential advancement for large-scale production. In addition, this work may provide an alternative facile approach to synthesize the quantum dots of transition metal dichalcogenides or other layered materials on a large scale. Coupled with excellent photoluminescence property, MoS₂ QDs provide promising applications for biological imaging, disease diagnosis and biosensors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51372160, 51572184 and 51402207).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supplementary data. See DOI: 10.1039/c6ra00572a

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