Photoluminescence
Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon Hyun Dong Ha, Dong Ju Han, Jong Seob Choi, Minsu Park, and Tae Seok Seo*
Molybdenum disulfide (MoS2) is a well-known transitionmetal dichalcogenide (LTMD) composed of S-Mo-S triple layers which are weakly bonded by van der Waals forces. In particular, the single layered MoS2 demonstrated its excellent semiconducting electronic properties as well as strong photoluminescence (PL) with sensitive photoresponse.[1] The preparation methods for single or a few layered MoS2 sheets from bulk MoS2 included mechanical exfoliation, intercalationassisted exfoliation, and chemical vapor deposition.[2] Using those synthetic methods, relatively large-sized monolayered MoS2 whose lateral dimension ranged from hundreds of nanometers to submicron has been reported. In addition, the transformation of the bulk MoS2 to the large-sized monolayered MoS2 resulted in the strong PL between 600 nm and 700 nm due to the indirect (Γ point)-to-direct (K point) band-gap transition in the Brillouin zone of the MoS2.[3] Polydisperse MoS2 nanoflakes were also synthesized by using molybdenum oxide sulfurization, and direct exfoliation in an organic solvent.[4,5] Besides the effect of layer number on the PL, the nanoscale dimension of the MoS2 also contributed to the PL emission due to the quantum confinement effect.[6] However, all the previous synthetic methods generated MoS2 nanoflakes in an organic solvent with high polydispersity, limiting its potential for biological applications. High uniformity of MoS2 nanosheets in terms of size and thickness would produce an inherent PL feature which could not be achievable from the polydisperse MoS2 nanoflakes. Instead of the excitation dependent PL, homogeneous MoS2 nanosheets will display a distinct PL emission regardless of the excitation wavelength, so that the utilization of the MoS2 as a biological fluorescent probe like an organic dye would be possible. We notice that the starting material of the bulk MoS2 is critical to determine the size and morphology of the thin MoS2 layer. The MoS2 nanoparticles (MNP, 135 nm
H. D. Ha, D. J. Han, J. S. Choi, M. Park, Prof. T. S. Seo Department of Chemical and Biomolecular Engineering (BK21 program) and Institute for the BioCentury Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305–701, Republic of Korea E-mail: [email protected] DOI: 10.1002/smll.201400988 small 2014, DOI: 10.1002/smll.201400988
diameter), which have ultra-small area of the stacked S-Mo-S layer, would be ideal for liquid-phase exfoliation to produce the single layered MoS2 nanosheets with narrow size distribution due to the low binding energy between the interlayers. On the other hand, despite the growing researches on the PL of MoS2, their biological applications are rare except using as a fluorescent quencher.[7] Our rationale in this study is that the homogeneous MoS2 nanosheet showing unique PL property can be utilized as a fluorescent tag, and such a hypothesis should be proven in the FRET system, which has been widely applied for studying the conformational distribution and dynamics of biological molecules, medical diagnostics by bioimaging or biosensor, and single molecule detection.[8] The expansion of the donor and acceptor molecule from the conventional organic dye to the inorganic nanomaterials would make room for improvement in the FRET based chemical and biological analysis. Scheme 1 shows the overall procedure for constructing the FRET pair using the Alexa 430 fluorescent dye and the MoS2 nanosheet. The uniform monolayered MoS2 nanosheets were synthesized from the MNP through a Li intercalation method. The resultant MoS2 solution emitted a distinct blue luminescence. Then, Alexa Fluor 430 labeled double stranded DNAs (dsDNAs) with poly G tails were prepared and attached on the MoS2 surface to produce the Alexa Fluor 430-dsDNA-MoS2 conjugates. The physisorption of guanine compounds and single stranded DNAs (ssDNAs) on the
Scheme 1. Schematics for constructing the FRET pair using the blue luminescent monolayered MoS2 nanosheets and the Alexa Fluor 430 which are separated by the dsDNA.
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Figure 1. a) SEM image of an MNP. b) A digital image of the colloidal MoS2 suspension (left) and Tyndall effect of the colloidal MoS2 suspension (right). c) An HRTEM image of the monolayered MoS2 nanosheet and d) an enlarged view. Inset: electron diffraction pattern of a high crystalline region of MoS2. e) An AFM image of the monolayered MoS2 nanosheets. Height profile along the white dot line is overlaid on the image. f) An enlarged AFM image. g) Statistical analysis of the lateral dimension profile of the as-synthesized MoS2 nanosheets from 100 samples. h) Raman spectra of the monolayer and a bulk MoS2.
basal plane of MoS2 via van der Waals force based physisorption has been theoretically and experimentally studied.[9] In contrast, dsDNAs weakly interact with the MoS2 due to the burial of nucleobases inside of the double helix structure, causing the dsDNA to be separated from the basal plane of MoS2. The distance between the MoS2 nanosheet and the Alexa Fluor 430 was controlled by the number of base pair of dsDNA from 5 to 18 (Table S1, Supporting Information). Systematic FRET studies on the Alexa Fluor 430-dsDNAMoS2 conjugates were performed to explore the donor and acceptor capability of the MoS2 nanosheets. The starting MNP materials were characterized by lowmagnification scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) (Figure 1a and Supporting Information Figure S1). The SEM and HRTEM images of the MNPs show structure with 135 nm diameter and multilayer stacking of MoS2 with extremely high crystallinity. Such a small dimension allows the MNPs to be easily exfoliated by the Li intercalation and
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sonication process in a large scale, resulting in the homogenous single layered MoS2. Figure 1b displays a faint yellowish colloidal MoS2 suspension in water, and its high stability in water was confirmed after 1 month by observing the presence of a Tyndall effect in the form of a typical discernible red line that results from light scattering when a laser beam is passed through the MoS2 solution. The HRTEM data and the corresponding fast Fourier transform (FFT) pattern indicated that high crystalline structure was retained with a lattice fringe spacing of 0.27 nm, which was matched with the (100) lattice plane of a typical MoS2 (Figure 1c,d).[6a,6b] Figure 1e,f represent an AFM data with an enlarged image of the MoS2 nanosheets. The average thickness of MoS2 nanosheets was ∼0.7 nm which is similar to that of the mechanically exfoliated MoS2, demonstrating that the as-synthesized MoS2 nanosheets are monolayer without surface corrugation.[3a] In addition, the lateral dimension analysis of the monolayered MoS2 nanosheets revealed that more than 89% are in the range of 10–20 nm, showing very narrow size distribution (Figure 1g). The Raman spectrum of the monolayer MoS2 displays typical two phonon modes of E12g and A1g at 385.2 cm−1 and 404.9 cm−1, which are much weaker than those of a bulk MoS2 under identical conditions (Figure 1h). Especially, the E12g mode of the monolayer MoS2 nanosheet, corresponding to the in-plane vibrations, was red-shifted by 4 cm−1 compared to that of a bulk MoS2 (380.69 cm−1). This phenomenon was quite analogous to the E2g mode change of a monolayered boron nitride quantum dots which had thin thickness and small lateral dimensions.[10] In addition, frequency difference of 19.7 cm−1 between E12g and A12g reflected that the produced MoS2 was single layered.[2c] These results suggest that the synthesized MoS2 nanosheets from the MNPs were monolayered, highly stable in water, and homogeneous in size with narrow distribution. According to the report from Eda et al., the chemically exfoliated MoS2 by Li intercalation resulted in the phase transition from the semiconducting 2H-MoS2 to the metallic 1T-MoS2 by octahedral coordination of Mo.[2b] The Li-intercalated MoS2 which has a large lateral dimension of 300 nm to 800 nm exhibited no PL due to the partial metallic character. Upon annealing, the transformation back to the initial 2H phase occurred, revealing the red PL peak around 660 nm. In our case, we also investigate the phase state of the MoS2 by the X-ray photoelectron spectroscopy (XPS) (Figure S2, Supporting Information). Compared to the peak position of the 2H-MoS2, the peaks of Mo4+ 3d5/2 and Mo4+ 3d3/2 were shifted to the lower binding energies, and the S 2p1/2 and S 2p3/2 peaks were merged to be shown as a broad one rather than a typical doublet of a 2H-MoS2. This result implies that the as-synthesized MoS2 contains metallic 1T-MoS2 phase together with semiconducting 2H-MoS2 phase similar to the previous report.[2b] However, the as-synthesized MoS2 solution emitted a distinct blue PL at 415 nm under 300 nm UV illumination even without annealing process (Inset of Figure 2a). Despite the coexistence of the metallic portion, the small domain of the MoS2 (lateral dimension of 10–20 nm) may cause the quantum confinement effect, leading to the enhanced band gap opening.[6,11] Thus, the blue luminescence was generated
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201400988
Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon
The unique PL spectrum of the MoS2 quantum dots allows us to evaluate the possibility of using them as a fluorescent tag in the FRET system. We chose the Alexa Fluor 430 as a counterpart, since the absorption wavelength of Alexa Fluor 430 (430 nm) was quite overlapped with the PL emission wavelength of MoS2 quantum dots (415 nm) as shown in Figure 3a. So, the MoS2 and Alexa Fluor 430 could serve as a donor and an acceptor, respectively, in the FRET pair. The distance between the MoS2 surface and the Alexa Fluor 430 was theoretically calculated by using the fact that
Figure 2. a) PL spectra of the monolayer MoS2 quantum dots at different excitation wavelengths. Inset: a digital image of a blue PL MoS2 solution upon the UV illumination at 300 nm. b) Comparison between a UV–Vis absorbance (red line) and PLE at λde = 415 nm (black dot line) of the as-synthesized MoS2.
from the MoS2 quantum dots. We confirmed the size effect on the blue PL by comparing with the PL of the large-sized MoS2 which was prepared by the same Li intercalation reaction using MoS2 powders (1 µm diameter).[2b] As shown in Figure S3 (Supporting Information), the large-sized MoS2 whose dimension was from 300 nm to 800 nm did not reveal any PL at 415 nm. Interestingly, the blue PL at 415 nm was not changed even at various excitation wavelengths from 270 nm to 350 nm (Figure 2a). The previous reports showed that the MoS2 nanoflakes suspended in an N-methylpyrrolidinone (NMP) solution present the excitation dependent PL emission in a wide range from 450 nm to 600 nm due to the polydispersity in the lateral dimension ranging from 2–1000 nm and the thickness (1–7 layers).[6] Also, the use of NMP is not adequate as a solvent for measuring PL, since the NMP by itself could influence the excitation dependent PL emission (Figure S4, Supporting Information). On the contrary to the previous results, the high homogeneity and water solubility of the MoS2 quantum dots in our case generated the distinct PL behavior regardless of the excitation wavelength without solvent interference. Figure 2b displayed the PLE spectrum of the monolayer MoS2 quantum dots at a detection emission wavelength (λde) of 415 nm. The maximum peak was observed at 300 nm (4.13 eV), which exactly corresponds to a significant absorption peak at 291 nm (4.26 eV) of the UV– Vis absorbance. small 2014, DOI: 10.1002/smll.201400988
Figure 3. a) Normalized spectra of the PL of the MoS2 quantum dots at λEx of 300 nm (blue), and the absorption (black) and emission at λEx of 430 nm (green) of the Alexa Fluor 430. b) PL spectra of the MoS2 and the Alexa Fluor 430 in the Alexa Fluor 430-dsDNA-MoS2 conjugates when excited at 300 nm. Inset: the average PL intensities of the MoS2 at 415 nm and the Alexa Fluor 430 at 530 nm depending on the RD-A. c) PL spectra of the Alexa Fluor 430 in the Alexa Fluor 430-dsDNA-MoS2 conjugates with excitation of 430 nm.
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the distance between the adjacent bases in the dsDNA is 0.34 nm. In addition, the Alexa Fluor 430-dsDNA binding on the MoS2 surface via a Poly G tail (Scheme 1) was confirmed by detecting the N1s and P2p peaks on the Alexa Fluor 430-dsDNA-MoS2 conjugates, which were derived from the DNA molecules (Figure S5, Supporting Information). With irradiation of the excitation wavelength of 300 nm on the Alexa Fluor 430-dsDNA-MoS2 conjugates, the PL profiles of the MoS2 quantum dots and Alexa Fluor 430 were monitored depending on the donor–acceptor distance (RD–A) (Figure 3b).[12] When the number of the base pairs of dsDNA increased from 5 to 13, the emission intensity of the MoS2 quantum dots at 415 nm gradually decreased, while that of Alexa Fluor 430 at 530 nm was augmented. In the case of more than 13 base pairs, the emission intensity of MoS2 quantum dots was restored, and accordingly that of Alexa Fluor 430 reduced. In the FRET system, there is an optimal distance between the donor and the acceptor to maximize the energy transfer efficiency. While the short distance between them makes the quenching phenomenon dominant rather than the energy transfer, the long distance between the donor and the acceptor makes the energy transfer inefficient due to low dipole–dipole interaction. In case of the Alexa Fluor 430-dsDNA-MoS2 conjugates, it turns out that the distance with 13 base pairs induces the maximum energy transfer from MoS2 to the Alexa Fluor 430. Efficiency of the energy transfer from the MoS2 to the Alexa Fluor 430 was estimated by the steady-state measurement using the following equation.[13] E = 1 − I DA / I D where I is the relative donor fluorescence intensity in the absence (ID) and presence (IDA) of the acceptor. Table S2 (Supporting Information) shows the FRET efficiency depending on the donor–acceptor distance. When the 13 base pair dsDNA was used as a spacer whose theoretical length was 4.42 nm, the maximum FRET efficiency of 11.73% was obtained (Inset of Figure 3b). These results demonstrated that the blue luminescent MoS2 quantum dots could play a role as a donor to the fluorescent dye, enhancing the PL intensity of the acceptor at the optimal distance due to the FRET phenomenon. To investigate the role of the MoS2 quantum dot as an acceptor in the Alexa Fluor 430-dsDNA-MoS2 conjugates, the fluorescence emission profiles of Alexa Fluor 430 were obtained under the same conditions as above except using an excitation wavelength of 430 nm at which the fluorescence of the Alexa Fluor 430 was well emitted at 530 nm, while that of the MoS2 was not observed. Figure 3c shows the PL spectra of the Alexa Fluor 430 depending on the length of the dsDNA. As a standard, we first measured the PL intensity of the Alexa Fluor 430 labeled ssDNA in a solution phase without conjugation on the MoS2 nanosheets (see the top red profile in Figure 3c). Then, to make no space between the Alexa Fluor 430 donor and the MoS2 acceptor, we prepared the Alexa Fluor 430-ssDNA-MoS2 conjugates, utilizing the fact that the single stranded DNA has a high physisorption affinity on the MoS2 surface. The emission intensity of the Alexa Fluor 430 of the Alexa Fluor 430-ssDNA-MoS2
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conjugates was measured, revealing significant quenching of the Alexa Fluor 430 (see the bottom gray profile in Figure 3c). To tune the distance between the donor and the acceptor, we employed the Alexa Fluor 430-dsDNA-MoS2 conjugates, since the double stranded DNAs have low affinity to the MoS2 surface, enabling the Alexa Fluor 430 detachment from the MoS2. As the length of the double stranded DNA increased, the Alexa Fluor 430 became positioned far away from the MoS2 quencher, thereby restoring the PL intensity of the Alexa Fluor 430 (see the profile color from the orange to the violet in Figure 3c). The quenching efficiency (QE) was calculated by the following equation:[14] QE = 1 − I / I 0 where I is the PL intensity of the Alexa Fluor 430 in the dsDNA in the absence (I0) and presence (I) of the MoS2 quantum dots (Table S3, Supporting Information). While the PL of the Alexa Fluor 430 labeled in the ssDNA was low with 84.98% of quenching yield, the PL of the Alexa Fluor 430 labeled in the dsDNA gradually increased in proportional to the length of the RD–A. When the number of the base pair in the dsDNA was 5, the QE was 64.75%. However, the QE became minimal with more than 13 base pair dsDNA. These results imply that the MoS2 nanosheet serves well as a quencher, but the quenching effect with an interval above 13 bp seems insignificant. Finally, we evaluated the quantum yield of the MoS2 by using anthracene as a standard (See supporting information), and the quantum yield of MoS2 was calculated as ≈1.3%. In conclusion, we successfully syntheiszed the homogenous MoS2 quantum dots from the MNPs. Unique and homogenous morphology (single layer with 10–20 nm in the lateral dimension as well as narrow size distribution) endows the MoS2 with the blue luminescent emission at 415 nm regardless of the excitation wavelength due to the quantum size effect. The capability of the MoS2 quantum dot as a fluorescence tag was systematically investigated in the Alexa Fluor 430-dsDNA-MoS2 FRET system. The MoS2 quantum dots could play dual role in the FRET system, not only serving as a donor with 11.73% of FRET efficiency at 4.42 nm distance, but also working for a strong fluorescent quencher acceptor. These results imply that the distinct PL emitting transition-metal dichalcogenides MoS2 retains high potentials to be utilized as fluorescent biological probes.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by a grant from the Converging Technology Project funded by the Korean Ministry of Environment (M112–00061–0002–0) of Korea, by the CCS R&D Center
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small 2014, DOI: 10.1002/smll.201400988
Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon
(2013M1A8A1040878), and by the Engineering Research Center of Excellence Program of Korea Ministry of Science, ICT & Future Planning(MSIP)/National Research Foundation of Korea(NRF) (Grant NRF-2014-009799).
[7] [8]
[1] a) B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nat. Nanotechnol. 2011, 6, 147; b) Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, H. Zhang, ACS Nano 2012, 6, 74. [2] a) H. Zeng, J. Dai, W. Yao, D. Xiao, X. Cui, Nat. Nanotechnol. 2012, 7, 490; b) G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla, Nano Lett. 2011, 11, 5111; c) Q. Ji, Y. Zhang, T. Gao, Y. Zhang, D. Ma, M. Liu, Y. Chen, X. Qiao, P.-H. Tan, M. Kan, J. Feng, Q. Sun, Z. Liu, Nano Lett. 2013, 13, 3870. [3] a) A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Nano Lett. 2010, 10, 1271; b) K. F. Mak, C. Lee, J. Hone, J. Shan, T. F. Heinz, Phys. Rev. Lett. 2010, 105, 136805. [4] Y.-C. Lin, W. Zhang, J.-K. Huang, K.-K. Liu, Y.-H. Lee, C.-T. Liang, C.-W. Chu, L.-J. Li, Nanoscale 2012, 4, 6637. [5] K.-G. Zhou, N.-N. Mao, H.-X. Wang, Y. Peng, H.-L. Zhang, Angew. Chem. Int. Ed. 2011, 50, 10839. [6] a) Y. Wang, J. Z. Ou, S. Balendhran, A. F. Chrimes, M. Mortazavi, D. D. Yao, M. R. K. Field Latham, V. Bansal, J. R. Friend, S. Zhuiykov, N. V. Medhekar, M. S. Strano, K. Kalantar-zadeh,
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ACS Nano 2013, 7, 10083; b) J. Z. Ou, A. F. Chrimes, Y. Wang, S.-Y. Tang, M. S. Strano, K. Kalantar-zadeh, Nano Lett. 2014, 14, 857; c) K. Zhou, Y. Zhu, X. Yang, J. Zhou, C. Li, ChemPhysChem 2012, 13, 699; d) V. Štengl, J. Henych, Nanoscale 2013, 5, 3387. C. Zhu, Z. Zeng, H. Li, F. Li, C. Fan, H. Zhang, J. Am. Chem. Soc. 2013, 135, 5998. a) M. R. Arkin, E. D. A. Stemp, C. Turro, N. J. Turro, J. K. Barton, J. Am. Chem. Soc. 1996, 118, 2267; b) M. A. Hink, N. V. Visser, J. W. Borst, A. V. Hoek, A. J. W. G. Visser, J. Fluoresc. 2003, 13, 185; c) X. Zhang, Y. Xiao, X. Qian, Angew. Chem. Int. Ed. 2008, 47, 8025; d) L. Chen, S. Lee, M. Lee, C. Lim, J. Choo, J. Y. Park, S. Lee, S.-W. Joo, K.-H. Lee, Y.-W. Choi, Biosens. Bioelectron. 2008, 23, 1878. W. M. Heckl, D. P. E. Smith, G. Binnig, H. Klagges, T. W. Hänsch, J. Maddocks, Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 8003. L. Lin, Y. Xu, S. Zhang, I. M. Ross, A. C. M. Ong, D. A. Allwood, Small 2014, 10, 60. L. Lin, Y. Xu, S. Zhang, I. M. Ross, A. C. M. Ong, D. A. Allwood, ACS Nano 2013, 7, 8214. Y. Piao, F. Liu, T. S. Seo, Chem. Commun. 2011, 47, 12149. J. R. Lakowicz, Principles of Fluorescence Spectroscopy 3rd ed., Springer: New York 2006, 443–460. F. Liu, H. D. Ha, D. J. Han, T. S. Seo, Small 2013, 9, 3410.
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Received: April 10, 2014 Revised: May 28, 2014 Published online:
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Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon Hyun Dong Ha, Dong Ju Han, Jong Seob Choi, Minsu Park, and Tae Seok Seo*
Molybdenum disulfide (MoS2) is a well-known transitionmetal dichalcogenide (LTMD) composed of S-Mo-S triple layers which are weakly bonded by van der Waals forces. In particular, the single layered MoS2 demonstrated its excellent semiconducting electronic properties as well as strong photoluminescence (PL) with sensitive photoresponse.[1] The preparation methods for single or a few layered MoS2 sheets from bulk MoS2 included mechanical exfoliation, intercalationassisted exfoliation, and chemical vapor deposition.[2] Using those synthetic methods, relatively large-sized monolayered MoS2 whose lateral dimension ranged from hundreds of nanometers to submicron has been reported. In addition, the transformation of the bulk MoS2 to the large-sized monolayered MoS2 resulted in the strong PL between 600 nm and 700 nm due to the indirect (Γ point)-to-direct (K point) band-gap transition in the Brillouin zone of the MoS2.[3] Polydisperse MoS2 nanoflakes were also synthesized by using molybdenum oxide sulfurization, and direct exfoliation in an organic solvent.[4,5] Besides the effect of layer number on the PL, the nanoscale dimension of the MoS2 also contributed to the PL emission due to the quantum confinement effect.[6] However, all the previous synthetic methods generated MoS2 nanoflakes in an organic solvent with high polydispersity, limiting its potential for biological applications. High uniformity of MoS2 nanosheets in terms of size and thickness would produce an inherent PL feature which could not be achievable from the polydisperse MoS2 nanoflakes. Instead of the excitation dependent PL, homogeneous MoS2 nanosheets will display a distinct PL emission regardless of the excitation wavelength, so that the utilization of the MoS2 as a biological fluorescent probe like an organic dye would be possible. We notice that the starting material of the bulk MoS2 is critical to determine the size and morphology of the thin MoS2 layer. The MoS2 nanoparticles (MNP, 135 nm
H. D. Ha, D. J. Han, J. S. Choi, M. Park, Prof. T. S. Seo Department of Chemical and Biomolecular Engineering (BK21 program) and Institute for the BioCentury Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305–701, Republic of Korea E-mail: [email protected] DOI: 10.1002/smll.201400988 small 2014, DOI: 10.1002/smll.201400988
diameter), which have ultra-small area of the stacked S-Mo-S layer, would be ideal for liquid-phase exfoliation to produce the single layered MoS2 nanosheets with narrow size distribution due to the low binding energy between the interlayers. On the other hand, despite the growing researches on the PL of MoS2, their biological applications are rare except using as a fluorescent quencher.[7] Our rationale in this study is that the homogeneous MoS2 nanosheet showing unique PL property can be utilized as a fluorescent tag, and such a hypothesis should be proven in the FRET system, which has been widely applied for studying the conformational distribution and dynamics of biological molecules, medical diagnostics by bioimaging or biosensor, and single molecule detection.[8] The expansion of the donor and acceptor molecule from the conventional organic dye to the inorganic nanomaterials would make room for improvement in the FRET based chemical and biological analysis. Scheme 1 shows the overall procedure for constructing the FRET pair using the Alexa 430 fluorescent dye and the MoS2 nanosheet. The uniform monolayered MoS2 nanosheets were synthesized from the MNP through a Li intercalation method. The resultant MoS2 solution emitted a distinct blue luminescence. Then, Alexa Fluor 430 labeled double stranded DNAs (dsDNAs) with poly G tails were prepared and attached on the MoS2 surface to produce the Alexa Fluor 430-dsDNA-MoS2 conjugates. The physisorption of guanine compounds and single stranded DNAs (ssDNAs) on the
Scheme 1. Schematics for constructing the FRET pair using the blue luminescent monolayered MoS2 nanosheets and the Alexa Fluor 430 which are separated by the dsDNA.
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Figure 1. a) SEM image of an MNP. b) A digital image of the colloidal MoS2 suspension (left) and Tyndall effect of the colloidal MoS2 suspension (right). c) An HRTEM image of the monolayered MoS2 nanosheet and d) an enlarged view. Inset: electron diffraction pattern of a high crystalline region of MoS2. e) An AFM image of the monolayered MoS2 nanosheets. Height profile along the white dot line is overlaid on the image. f) An enlarged AFM image. g) Statistical analysis of the lateral dimension profile of the as-synthesized MoS2 nanosheets from 100 samples. h) Raman spectra of the monolayer and a bulk MoS2.
basal plane of MoS2 via van der Waals force based physisorption has been theoretically and experimentally studied.[9] In contrast, dsDNAs weakly interact with the MoS2 due to the burial of nucleobases inside of the double helix structure, causing the dsDNA to be separated from the basal plane of MoS2. The distance between the MoS2 nanosheet and the Alexa Fluor 430 was controlled by the number of base pair of dsDNA from 5 to 18 (Table S1, Supporting Information). Systematic FRET studies on the Alexa Fluor 430-dsDNAMoS2 conjugates were performed to explore the donor and acceptor capability of the MoS2 nanosheets. The starting MNP materials were characterized by lowmagnification scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) (Figure 1a and Supporting Information Figure S1). The SEM and HRTEM images of the MNPs show structure with 135 nm diameter and multilayer stacking of MoS2 with extremely high crystallinity. Such a small dimension allows the MNPs to be easily exfoliated by the Li intercalation and
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sonication process in a large scale, resulting in the homogenous single layered MoS2. Figure 1b displays a faint yellowish colloidal MoS2 suspension in water, and its high stability in water was confirmed after 1 month by observing the presence of a Tyndall effect in the form of a typical discernible red line that results from light scattering when a laser beam is passed through the MoS2 solution. The HRTEM data and the corresponding fast Fourier transform (FFT) pattern indicated that high crystalline structure was retained with a lattice fringe spacing of 0.27 nm, which was matched with the (100) lattice plane of a typical MoS2 (Figure 1c,d).[6a,6b] Figure 1e,f represent an AFM data with an enlarged image of the MoS2 nanosheets. The average thickness of MoS2 nanosheets was ∼0.7 nm which is similar to that of the mechanically exfoliated MoS2, demonstrating that the as-synthesized MoS2 nanosheets are monolayer without surface corrugation.[3a] In addition, the lateral dimension analysis of the monolayered MoS2 nanosheets revealed that more than 89% are in the range of 10–20 nm, showing very narrow size distribution (Figure 1g). The Raman spectrum of the monolayer MoS2 displays typical two phonon modes of E12g and A1g at 385.2 cm−1 and 404.9 cm−1, which are much weaker than those of a bulk MoS2 under identical conditions (Figure 1h). Especially, the E12g mode of the monolayer MoS2 nanosheet, corresponding to the in-plane vibrations, was red-shifted by 4 cm−1 compared to that of a bulk MoS2 (380.69 cm−1). This phenomenon was quite analogous to the E2g mode change of a monolayered boron nitride quantum dots which had thin thickness and small lateral dimensions.[10] In addition, frequency difference of 19.7 cm−1 between E12g and A12g reflected that the produced MoS2 was single layered.[2c] These results suggest that the synthesized MoS2 nanosheets from the MNPs were monolayered, highly stable in water, and homogeneous in size with narrow distribution. According to the report from Eda et al., the chemically exfoliated MoS2 by Li intercalation resulted in the phase transition from the semiconducting 2H-MoS2 to the metallic 1T-MoS2 by octahedral coordination of Mo.[2b] The Li-intercalated MoS2 which has a large lateral dimension of 300 nm to 800 nm exhibited no PL due to the partial metallic character. Upon annealing, the transformation back to the initial 2H phase occurred, revealing the red PL peak around 660 nm. In our case, we also investigate the phase state of the MoS2 by the X-ray photoelectron spectroscopy (XPS) (Figure S2, Supporting Information). Compared to the peak position of the 2H-MoS2, the peaks of Mo4+ 3d5/2 and Mo4+ 3d3/2 were shifted to the lower binding energies, and the S 2p1/2 and S 2p3/2 peaks were merged to be shown as a broad one rather than a typical doublet of a 2H-MoS2. This result implies that the as-synthesized MoS2 contains metallic 1T-MoS2 phase together with semiconducting 2H-MoS2 phase similar to the previous report.[2b] However, the as-synthesized MoS2 solution emitted a distinct blue PL at 415 nm under 300 nm UV illumination even without annealing process (Inset of Figure 2a). Despite the coexistence of the metallic portion, the small domain of the MoS2 (lateral dimension of 10–20 nm) may cause the quantum confinement effect, leading to the enhanced band gap opening.[6,11] Thus, the blue luminescence was generated
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201400988
Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon
The unique PL spectrum of the MoS2 quantum dots allows us to evaluate the possibility of using them as a fluorescent tag in the FRET system. We chose the Alexa Fluor 430 as a counterpart, since the absorption wavelength of Alexa Fluor 430 (430 nm) was quite overlapped with the PL emission wavelength of MoS2 quantum dots (415 nm) as shown in Figure 3a. So, the MoS2 and Alexa Fluor 430 could serve as a donor and an acceptor, respectively, in the FRET pair. The distance between the MoS2 surface and the Alexa Fluor 430 was theoretically calculated by using the fact that
Figure 2. a) PL spectra of the monolayer MoS2 quantum dots at different excitation wavelengths. Inset: a digital image of a blue PL MoS2 solution upon the UV illumination at 300 nm. b) Comparison between a UV–Vis absorbance (red line) and PLE at λde = 415 nm (black dot line) of the as-synthesized MoS2.
from the MoS2 quantum dots. We confirmed the size effect on the blue PL by comparing with the PL of the large-sized MoS2 which was prepared by the same Li intercalation reaction using MoS2 powders (1 µm diameter).[2b] As shown in Figure S3 (Supporting Information), the large-sized MoS2 whose dimension was from 300 nm to 800 nm did not reveal any PL at 415 nm. Interestingly, the blue PL at 415 nm was not changed even at various excitation wavelengths from 270 nm to 350 nm (Figure 2a). The previous reports showed that the MoS2 nanoflakes suspended in an N-methylpyrrolidinone (NMP) solution present the excitation dependent PL emission in a wide range from 450 nm to 600 nm due to the polydispersity in the lateral dimension ranging from 2–1000 nm and the thickness (1–7 layers).[6] Also, the use of NMP is not adequate as a solvent for measuring PL, since the NMP by itself could influence the excitation dependent PL emission (Figure S4, Supporting Information). On the contrary to the previous results, the high homogeneity and water solubility of the MoS2 quantum dots in our case generated the distinct PL behavior regardless of the excitation wavelength without solvent interference. Figure 2b displayed the PLE spectrum of the monolayer MoS2 quantum dots at a detection emission wavelength (λde) of 415 nm. The maximum peak was observed at 300 nm (4.13 eV), which exactly corresponds to a significant absorption peak at 291 nm (4.26 eV) of the UV– Vis absorbance. small 2014, DOI: 10.1002/smll.201400988
Figure 3. a) Normalized spectra of the PL of the MoS2 quantum dots at λEx of 300 nm (blue), and the absorption (black) and emission at λEx of 430 nm (green) of the Alexa Fluor 430. b) PL spectra of the MoS2 and the Alexa Fluor 430 in the Alexa Fluor 430-dsDNA-MoS2 conjugates when excited at 300 nm. Inset: the average PL intensities of the MoS2 at 415 nm and the Alexa Fluor 430 at 530 nm depending on the RD-A. c) PL spectra of the Alexa Fluor 430 in the Alexa Fluor 430-dsDNA-MoS2 conjugates with excitation of 430 nm.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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H. D. Ha et al.
the distance between the adjacent bases in the dsDNA is 0.34 nm. In addition, the Alexa Fluor 430-dsDNA binding on the MoS2 surface via a Poly G tail (Scheme 1) was confirmed by detecting the N1s and P2p peaks on the Alexa Fluor 430-dsDNA-MoS2 conjugates, which were derived from the DNA molecules (Figure S5, Supporting Information). With irradiation of the excitation wavelength of 300 nm on the Alexa Fluor 430-dsDNA-MoS2 conjugates, the PL profiles of the MoS2 quantum dots and Alexa Fluor 430 were monitored depending on the donor–acceptor distance (RD–A) (Figure 3b).[12] When the number of the base pairs of dsDNA increased from 5 to 13, the emission intensity of the MoS2 quantum dots at 415 nm gradually decreased, while that of Alexa Fluor 430 at 530 nm was augmented. In the case of more than 13 base pairs, the emission intensity of MoS2 quantum dots was restored, and accordingly that of Alexa Fluor 430 reduced. In the FRET system, there is an optimal distance between the donor and the acceptor to maximize the energy transfer efficiency. While the short distance between them makes the quenching phenomenon dominant rather than the energy transfer, the long distance between the donor and the acceptor makes the energy transfer inefficient due to low dipole–dipole interaction. In case of the Alexa Fluor 430-dsDNA-MoS2 conjugates, it turns out that the distance with 13 base pairs induces the maximum energy transfer from MoS2 to the Alexa Fluor 430. Efficiency of the energy transfer from the MoS2 to the Alexa Fluor 430 was estimated by the steady-state measurement using the following equation.[13] E = 1 − I DA / I D where I is the relative donor fluorescence intensity in the absence (ID) and presence (IDA) of the acceptor. Table S2 (Supporting Information) shows the FRET efficiency depending on the donor–acceptor distance. When the 13 base pair dsDNA was used as a spacer whose theoretical length was 4.42 nm, the maximum FRET efficiency of 11.73% was obtained (Inset of Figure 3b). These results demonstrated that the blue luminescent MoS2 quantum dots could play a role as a donor to the fluorescent dye, enhancing the PL intensity of the acceptor at the optimal distance due to the FRET phenomenon. To investigate the role of the MoS2 quantum dot as an acceptor in the Alexa Fluor 430-dsDNA-MoS2 conjugates, the fluorescence emission profiles of Alexa Fluor 430 were obtained under the same conditions as above except using an excitation wavelength of 430 nm at which the fluorescence of the Alexa Fluor 430 was well emitted at 530 nm, while that of the MoS2 was not observed. Figure 3c shows the PL spectra of the Alexa Fluor 430 depending on the length of the dsDNA. As a standard, we first measured the PL intensity of the Alexa Fluor 430 labeled ssDNA in a solution phase without conjugation on the MoS2 nanosheets (see the top red profile in Figure 3c). Then, to make no space between the Alexa Fluor 430 donor and the MoS2 acceptor, we prepared the Alexa Fluor 430-ssDNA-MoS2 conjugates, utilizing the fact that the single stranded DNA has a high physisorption affinity on the MoS2 surface. The emission intensity of the Alexa Fluor 430 of the Alexa Fluor 430-ssDNA-MoS2
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conjugates was measured, revealing significant quenching of the Alexa Fluor 430 (see the bottom gray profile in Figure 3c). To tune the distance between the donor and the acceptor, we employed the Alexa Fluor 430-dsDNA-MoS2 conjugates, since the double stranded DNAs have low affinity to the MoS2 surface, enabling the Alexa Fluor 430 detachment from the MoS2. As the length of the double stranded DNA increased, the Alexa Fluor 430 became positioned far away from the MoS2 quencher, thereby restoring the PL intensity of the Alexa Fluor 430 (see the profile color from the orange to the violet in Figure 3c). The quenching efficiency (QE) was calculated by the following equation:[14] QE = 1 − I / I 0 where I is the PL intensity of the Alexa Fluor 430 in the dsDNA in the absence (I0) and presence (I) of the MoS2 quantum dots (Table S3, Supporting Information). While the PL of the Alexa Fluor 430 labeled in the ssDNA was low with 84.98% of quenching yield, the PL of the Alexa Fluor 430 labeled in the dsDNA gradually increased in proportional to the length of the RD–A. When the number of the base pair in the dsDNA was 5, the QE was 64.75%. However, the QE became minimal with more than 13 base pair dsDNA. These results imply that the MoS2 nanosheet serves well as a quencher, but the quenching effect with an interval above 13 bp seems insignificant. Finally, we evaluated the quantum yield of the MoS2 by using anthracene as a standard (See supporting information), and the quantum yield of MoS2 was calculated as ≈1.3%. In conclusion, we successfully syntheiszed the homogenous MoS2 quantum dots from the MNPs. Unique and homogenous morphology (single layer with 10–20 nm in the lateral dimension as well as narrow size distribution) endows the MoS2 with the blue luminescent emission at 415 nm regardless of the excitation wavelength due to the quantum size effect. The capability of the MoS2 quantum dot as a fluorescence tag was systematically investigated in the Alexa Fluor 430-dsDNA-MoS2 FRET system. The MoS2 quantum dots could play dual role in the FRET system, not only serving as a donor with 11.73% of FRET efficiency at 4.42 nm distance, but also working for a strong fluorescent quencher acceptor. These results imply that the distinct PL emitting transition-metal dichalcogenides MoS2 retains high potentials to be utilized as fluorescent biological probes.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by a grant from the Converging Technology Project funded by the Korean Ministry of Environment (M112–00061–0002–0) of Korea, by the CCS R&D Center
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201400988
Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon
(2013M1A8A1040878), and by the Engineering Research Center of Excellence Program of Korea Ministry of Science, ICT & Future Planning(MSIP)/National Research Foundation of Korea(NRF) (Grant NRF-2014-009799).
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: April 10, 2014 Revised: May 28, 2014 Published online:
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