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One-pot solvothermal synthesis of water-soluble boron nitride nanosheets and fluorescent boron nitride quantum dots
Accepted Manuscript One-pot Solvothermal Synthesis of Water-soluble Boron Nitride Nanosheets and Fluorescent Boron Nitride Quantum Dots Qingqing liu, Chaofan Hu, Xiaomin Wang PII: DOI: Reference:
S0167-577X(18)31409-5 https://doi.org/10.1016/j.matlet.2018.09.031 MLBLUE 24899
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
30 May 2018 2 August 2018 6 September 2018
Please cite this article as: Q. liu, C. Hu, X. Wang, One-pot Solvothermal Synthesis of Water-soluble Boron Nitride Nanosheets and Fluorescent Boron Nitride Quantum Dots, Materials Letters (2018), doi: https://doi.org/10.1016/ j.matlet.2018.09.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot Solvothermal Synthesis of Water-soluble Boron Nitride Nanosheets and Fluorescent Boron Nitride Quantum Dots Qingqing liu,a Chaofan Hu b and Xiaomin Wang *a a
College of Materials Science and Engineering, Taiyuan University of Technology,
Taiyuan 030024, China b
College of Materials and Energy, South China Agricultural University, Guangzhou
510642, China Corresponding author. E-mail address: [email protected] Abstract A simple route for the synthesis of boron nitride nanosheets (BNNSs) and boron nitride quantum dots (BNQDs) simultaneously was demonstrated in this study. The synthesis is accomplished by the high temperature solvothermal treatment of bulk BN in aqueous NaOH. The morphology and properties of the formed BNNSs and BNQDs were characterized, and the results suggest that the bulk h-BN can be efficiently exfoliated to BNNSs and BNQDs. The excellent colloidal stablility of BNNSs and PL emission of BNQDs make them promising candidates for biological applications. Key words Boron nitride; Nanosheets; Nanoparticles; Luminescence; Bioimaging; Biomaterials Introduction Hexagonal boron nitride (h-BN), also known as “white graphene”, has been attracting great interest because of its excellent chemical and thermal stability, as well as its unique electronic and optical properties [1]. Recent studies have showed that 2 D boron nitride nanosheets (BNNSs) and 0 D boron nitride quantum dots (BNQDs) have better performance than the bulk counterpart in a series of applications. Therefore, the demand for a method to produce high quality BNNSs and BNQDs is imminently needed.
1
To date, the primary approaches for preparing BNNSs and BNQDs can be generally classified into two methods: bottom-up and top-down methods. Although the bottom-up methods can produce relatively high quality nano-structured BN, the severe synthetic condition, time consuming processes and low yield have limited their scalability [2]. The top-down method refers to the cutting of bulk BN powder into nanosheets and quantum dots, which has the advantages of abundant and inexpensive raw materials and simple process. These advantages make the top-down method more suitable for large scale production of BNNSs and BNQDs. However, the established top-down methods mainly focus on exclusive synthesis of either nanosheets or quantum dots. As far as we know, a simple approach for the preparation of BNNSs and BNQDs simultaneously have not been reported. Herein, we report a simple method to produce BNNSs and BNQDs simultaneously by solvothermal treatment of bulk h-BN in the present of NaOH. This method mainly relies on the insertion of Na+ and OH- to BN layers at high temperature and pressure. The as-prepared BNNSs showed monolayer morphologies and significant improvement of solubility in water compared with bulk h-BN. The as obtained BNQDs exhibited excitation-dependent photoluminescence behavior and excellent photostability, and the performance of BNQDs as bioimaging probe were investigated. Experimental Section BNNSs and BNQDs were preparation by ethanol-thermal treatment of bulk h-BN powder. Typically, 30 mg of bulk h-BN powder was dispersed in 30 ml ethanol, and then 1 ml NaOH concentrated solution (2.5 M) were added into the h-BN dispersion. After stirring for 10 min, the mixture was transferred into a Teflon-sealed autoclave and maintained at 180 °C for 24 h. After it was naturally cooled down to room temperature, the as-obtained suspension was vacuum-filtrated using a 0.22 μm microporous membrane (Millipore) to filter out the white precipitated. Subsequently, the light-yellow suspension was dialyzed in a 1000 Da dialysis bag against ultrapure water to neutral, which can remove out excessive ethanol and NaOH. Finally, the 2
resultant suspension obtained after dialyzing was freeze-dried, and yellow powder of BNQDs was obtained. The white precipitated filtered out by microporous membrane were redispersed in water, and then sonicated for 30 min, followed by centrifugation at 6000 rpm to remove the residual unexfoliated h-BN, then the BNNS aqueous dispersion was prepared. Subsequently, the white suspension was dialyzed in a 3500 Da dialysis bag against ultrapure water to neutral. The as-obtained suspension was freeze-dried and white powder of BNNSs was obtained. Zeta potential measurements were performed using Zetasizer Nano S (Malvern Instruments) spectrometer and carried out at pH 7. Atomic force microscopy (AFM) images were obtained by the SPM Dimension 3100 from Veeco under the tapping mode. Transmission electron microscopy (TEM) analysis was conducted on a JEOL JEM-2100F field emission electron microscope. Scanning electron microscopy (SEM) image was obtained using a JEOL JSM-7100F scanning electron microscope. The X-ray diffraction (XRD) patterns were performed with a XD-2X/M4600 X-ray diffractometer. Raman spectra were measured by a Renishaw inVia microspectrometer equipped with a 514 nm laser. The photoluminescence (PL) spectra were obtained with a Hitachi F-7000 fluorescence spectrometer. The confocal fluorescence microscope was measured with a Zeiss LSM 710 confocal laser scanning microscope. Results and Discussions The possible mechanism and synthetic procedure to prepare BNNSs and BNQDs by a simple solvothermal treatment is illustrated in Fig. 1. Ethanol was selected as the solvent, which can improve the solubility of bulk h-BN and increase the contact interface between h-BN and intercalating regent. In the solvothermal condition, NaOH may corrode the edges of h-BN and be beneficial for Na+ and OH- to insert between the h-BN layers. In addition, the high temperature and pressure would weaken the can der Waals force and further enhance the interaction between h-BN and NaOH. With NaOH insert between the BN layers, bulk h-BN can be exfoliated layer by layer, and finally break into BNNSs and BNQDs. The yield of BNNSs and 3
BNQDs are 18.1% and 2.9% in weight, respectively. As showed in Fig. 1, the the aqueous dispersion of BNNSs (0.1 mg/mL) displays an obvious Tyndall effect and is highly stable over several weeks. As show in Fig. S2, BNNSs can maintaining a stable dispersion when the concentration is as high as 1 mg/mL. The as-prepared BNNSs exhibit a negative potential values of -23 mV (Fig. S3), which consistent with the reported negative values for BNNSs materials due to the -OH on the surface of BNNSs in water [3]. The light yellow aqueous dispersion BNQDs shows bright fluorescence under 365 nm UV lamp excitation.
Fig. 1. Schematic illustration of the exfoliation procedures to prepare BNNSs and BNQDs by solvothermal method in the present of NaOH.
4
Fig. 2. a) Typical tapping-mode AFM image of BNNSs on mica substrate. The inset shows the height profile along the white line. b) SEM image of BNNSs. c) TEM image of BNNSs. d) HRTEM image of BNNSs. e) Raman spectra of bulk h-BN and BNNSs. f) XRD patterns of bulk h-BN and BNNSs.
The morphology of the as-prepared BNNSs was first investigated using AFM. Fig 2a shows an isolated BNNS with a thickness of ≈1 nm and a lateral size of ≈1.2 μm. Previous reports showed that the AFM height of a BNNS monolayer increases to 1 nm [4, 5]. Thus, the AFM measurement suggests that the bulk h-BN was exfoliated to monolayer BNNSs. Then, we used SEM to further probe the morphology of BNNSs. As shown in Fig 2b, the samples comprise flat and folded nanosheets that are almost 5
transparent, demonstrating their ultrathin nature [6]. The TEM image of BNNSs (Fig. 2c) shows that isolated BNNS is transparent film-like structure with large number of wrinkles and scrolls. In addition, some curled edges can be clearly observed at high magnification (Fig. 2d). As shown in Fig. S1, the edge of BNNSs show four and nine parallel fringes, indicating the few layers in the sample. The lateral size of the BNNSs in not uniform and shows relatively wide range of distribution from 0.3~4.6 μm. Typical Raman spectra of bulk h-BN and BNNSs show a dominant at 1367.6 cm-1 and 1367.1 cm-1 (Fig. 2e), which can be attributed to the E2g mode vibration of h-BN [7]. The slightly red shift Raman band of BNNSs imply that the bulk h-BN has been exfoliated to few-layered nanosheets [8]. It is noted that the XRD pattern (Fig. 2f) of as-prepared BNNSs exhibits characteristic (002) peak at 27.6° [7]. The full width at half maximum (FWHM) of the (002) peak is ~0.4° in the bulk h-BN powder. The FWHM of the (002) peak increases to ~1.6° in BNNSs powder, indicating the efficient exfoliation of bulk h-BN to BNNSs [9]. In addition, after the solvothermal treatment, the intensity decreases sharply, indicating the highly-exfoliated structure of BNNSs [3, 4].
Fig. 3. a) HRTEM image of BNQDs. b) Size distribution of BNQDs. c) HRTEM of individual BNQD. Inset shows the FFT pattern of the crystal. d) Stacked monolayered BNQDs. 6
The morphology and crystallography of BNQDs was investigated by HRTEM. Fig. 3a shows a HRTEM image of BNQDs, which exhibiting a relatively narrow size distribution between 1.7 and 10.9 nm with an average diameter of 5.1 nm (Fig. 3b). The inset of Fig.3a indicates the high crystallinity of BNQDs, with a lattice parameter of ≈0.21 nm, (100) lattice fringes of h-BN [10]. Fig. 3c shows HRTEM image and the corresponding fast Fourier transform (FFT) pattern of an isolated BNQD, which further confirms high crystallinity of hexagonal BN structure. Fig. 3d shows the stacked BNQDs and the interlayer separation is measured to be about 0.8 nm, which is consistent of the previous report [11].
Fig. 4. PL spectra of BNQDs at different excitation wavelengths ranging from 280 to 420. The inset shows the normalized PL spectra of BNQDs. b) PLE (Red), and PL spectra of BNQDs. (c) Confocal fluorescence image microphotograph of the Hela cells incubated with BNQDs QDs for 4 h (λex=405 nm). (d) Bright-field microphotographs of cells. (e) An overlay image of (c) and (d).
We carried out a PL study to explore the optical properties of BNQDs. Fig. 4a shows detailed PL and corresponding normalized PL spectra of BNQDs excited at the wavelengths from 280 to 420 nm. The emission peaks of BNQDs exhibited an excitation-dependent PL behavior, which is consistent with the previously reported BNQDs [10, 11]. The strongest emission peak is located at ≈435 nm when excited at 340 nm，similar to the previous works [11, 12]. Fig. 4b shows the photostability test of BNQDs under different UV irradiation time. The fluorescence intensities of BNQDs 7
show no obvious change, indicating their superior photostability compared to conventional organic dyes and semiconductor quantum dots. To demonstrated the potential bioimaging application of BNQDs, HeLa cells were incubated with BNQDs and
detected by a confocal microscopy. As shown in Fig. 4, bright blue fluorescence
can be observed inside the cell and mainly localized in the cytoplasm region, indicating that the BNQDs had been internalized by the HeLa cells and could be used as efficient bioimaging probes. Conclusion In summary, we developed a simple solvothernal method to synthesize water-soluble BNNSs and fluorescent BNQDs simultaneously. The morphologies and structure of the materials were studied by several microscopic and spectroscopic techniques, confirming the efficient exfoliation of the bulk h-BN. This convenient and cheap process presents a potential advancement for the large-scale production of BN nanostructures. The high water solubility of BNNSs and stable fluorescence of BNQDs make them promising candidates for biological application. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51572184, 51372160). Reference [1] J. Yin, J. Li, Y. Hang, J. Yu, G. Tai, X. Li, Z. Zhang, W. Guo, Small 12(22) (2016) 2942-2968. [2] W. Luo, Y. Wang, E. Hitz, Y. Lin, B. Yang, L. Hu, Adv. Funct. Mate. (2017) 1701450. [3] W. Lei, V.N. Mochalin, D. Liu, S. Qin, Y. Gogotsi, Y. Chen, Nature Comm. 6 (2015) 8849. [4] Y. Lin, J.W. Connell, Nanoscale 4(22) (2012) 6908-6939. [5] F. Yuan, W. Jiao, F. Yang, W. Liu, J. Liu, Z. Xu, R. Wang, J.Mater. Chem. C (2017) 6359-6368. [6] X. Li, X. Hao, M. Zhao, Y. Wu, J. Yang, Y. Tian, G. Qian, Adv. Mater. 25(15) (2013) 2200-2204. [7] F. Xiao, S. Naficy, G. Casillas, M.H. Khan, T. Katkus, L. Jiang, H. Liu, H. Li, Z. Huang, Adv. Mate.27(44) (2015) 7196-7203. [8] T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y.K. Gun’ko, J. Coleman, J Am. Chem. Soc. 134(45) (2012) 18758-18771. [9] Z. Rafiei-Sarmazdeh, S.H. Jafari, S.J. Ahmadi, S.M. Zahedi-Dizaji, J Mater Sci 51(6) (2016) 3162-3169. [10] H. Li, R.Y. Tay, S.H. Tsang, X. Zhen, E.H.T. Teo, Small 11(48) (2015) 6491-6499. [11] L. Lin, Y. Xu, S. Zhang, I.M. Ross, A. Ong, D.A. Allwood, Small 10(1) (2014) 60-65. [12] Z. Lei, S. Xu, J. Wan, P. Wu, Nanoscale 7(45) (2015) 18902-18907. 8
Highlights:
Boron nitride nanosheets and quantum dots were synthesized by a solvothermal method Boron nitride nanosheets showed monolayer morphologies and good solubility Bright blue fluorescence was observed on boron quantum dots Boron quantum dots can be used as fluorescent probe for cancer cells
9
S0167-577X(18)31409-5 https://doi.org/10.1016/j.matlet.2018.09.031 MLBLUE 24899
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
30 May 2018 2 August 2018 6 September 2018
Please cite this article as: Q. liu, C. Hu, X. Wang, One-pot Solvothermal Synthesis of Water-soluble Boron Nitride Nanosheets and Fluorescent Boron Nitride Quantum Dots, Materials Letters (2018), doi: https://doi.org/10.1016/ j.matlet.2018.09.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot Solvothermal Synthesis of Water-soluble Boron Nitride Nanosheets and Fluorescent Boron Nitride Quantum Dots Qingqing liu,a Chaofan Hu b and Xiaomin Wang *a a
College of Materials Science and Engineering, Taiyuan University of Technology,
Taiyuan 030024, China b
College of Materials and Energy, South China Agricultural University, Guangzhou
510642, China Corresponding author. E-mail address: [email protected] Abstract A simple route for the synthesis of boron nitride nanosheets (BNNSs) and boron nitride quantum dots (BNQDs) simultaneously was demonstrated in this study. The synthesis is accomplished by the high temperature solvothermal treatment of bulk BN in aqueous NaOH. The morphology and properties of the formed BNNSs and BNQDs were characterized, and the results suggest that the bulk h-BN can be efficiently exfoliated to BNNSs and BNQDs. The excellent colloidal stablility of BNNSs and PL emission of BNQDs make them promising candidates for biological applications. Key words Boron nitride; Nanosheets; Nanoparticles; Luminescence; Bioimaging; Biomaterials Introduction Hexagonal boron nitride (h-BN), also known as “white graphene”, has been attracting great interest because of its excellent chemical and thermal stability, as well as its unique electronic and optical properties [1]. Recent studies have showed that 2 D boron nitride nanosheets (BNNSs) and 0 D boron nitride quantum dots (BNQDs) have better performance than the bulk counterpart in a series of applications. Therefore, the demand for a method to produce high quality BNNSs and BNQDs is imminently needed.
1
To date, the primary approaches for preparing BNNSs and BNQDs can be generally classified into two methods: bottom-up and top-down methods. Although the bottom-up methods can produce relatively high quality nano-structured BN, the severe synthetic condition, time consuming processes and low yield have limited their scalability [2]. The top-down method refers to the cutting of bulk BN powder into nanosheets and quantum dots, which has the advantages of abundant and inexpensive raw materials and simple process. These advantages make the top-down method more suitable for large scale production of BNNSs and BNQDs. However, the established top-down methods mainly focus on exclusive synthesis of either nanosheets or quantum dots. As far as we know, a simple approach for the preparation of BNNSs and BNQDs simultaneously have not been reported. Herein, we report a simple method to produce BNNSs and BNQDs simultaneously by solvothermal treatment of bulk h-BN in the present of NaOH. This method mainly relies on the insertion of Na+ and OH- to BN layers at high temperature and pressure. The as-prepared BNNSs showed monolayer morphologies and significant improvement of solubility in water compared with bulk h-BN. The as obtained BNQDs exhibited excitation-dependent photoluminescence behavior and excellent photostability, and the performance of BNQDs as bioimaging probe were investigated. Experimental Section BNNSs and BNQDs were preparation by ethanol-thermal treatment of bulk h-BN powder. Typically, 30 mg of bulk h-BN powder was dispersed in 30 ml ethanol, and then 1 ml NaOH concentrated solution (2.5 M) were added into the h-BN dispersion. After stirring for 10 min, the mixture was transferred into a Teflon-sealed autoclave and maintained at 180 °C for 24 h. After it was naturally cooled down to room temperature, the as-obtained suspension was vacuum-filtrated using a 0.22 μm microporous membrane (Millipore) to filter out the white precipitated. Subsequently, the light-yellow suspension was dialyzed in a 1000 Da dialysis bag against ultrapure water to neutral, which can remove out excessive ethanol and NaOH. Finally, the 2
resultant suspension obtained after dialyzing was freeze-dried, and yellow powder of BNQDs was obtained. The white precipitated filtered out by microporous membrane were redispersed in water, and then sonicated for 30 min, followed by centrifugation at 6000 rpm to remove the residual unexfoliated h-BN, then the BNNS aqueous dispersion was prepared. Subsequently, the white suspension was dialyzed in a 3500 Da dialysis bag against ultrapure water to neutral. The as-obtained suspension was freeze-dried and white powder of BNNSs was obtained. Zeta potential measurements were performed using Zetasizer Nano S (Malvern Instruments) spectrometer and carried out at pH 7. Atomic force microscopy (AFM) images were obtained by the SPM Dimension 3100 from Veeco under the tapping mode. Transmission electron microscopy (TEM) analysis was conducted on a JEOL JEM-2100F field emission electron microscope. Scanning electron microscopy (SEM) image was obtained using a JEOL JSM-7100F scanning electron microscope. The X-ray diffraction (XRD) patterns were performed with a XD-2X/M4600 X-ray diffractometer. Raman spectra were measured by a Renishaw inVia microspectrometer equipped with a 514 nm laser. The photoluminescence (PL) spectra were obtained with a Hitachi F-7000 fluorescence spectrometer. The confocal fluorescence microscope was measured with a Zeiss LSM 710 confocal laser scanning microscope. Results and Discussions The possible mechanism and synthetic procedure to prepare BNNSs and BNQDs by a simple solvothermal treatment is illustrated in Fig. 1. Ethanol was selected as the solvent, which can improve the solubility of bulk h-BN and increase the contact interface between h-BN and intercalating regent. In the solvothermal condition, NaOH may corrode the edges of h-BN and be beneficial for Na+ and OH- to insert between the h-BN layers. In addition, the high temperature and pressure would weaken the can der Waals force and further enhance the interaction between h-BN and NaOH. With NaOH insert between the BN layers, bulk h-BN can be exfoliated layer by layer, and finally break into BNNSs and BNQDs. The yield of BNNSs and 3
BNQDs are 18.1% and 2.9% in weight, respectively. As showed in Fig. 1, the the aqueous dispersion of BNNSs (0.1 mg/mL) displays an obvious Tyndall effect and is highly stable over several weeks. As show in Fig. S2, BNNSs can maintaining a stable dispersion when the concentration is as high as 1 mg/mL. The as-prepared BNNSs exhibit a negative potential values of -23 mV (Fig. S3), which consistent with the reported negative values for BNNSs materials due to the -OH on the surface of BNNSs in water [3]. The light yellow aqueous dispersion BNQDs shows bright fluorescence under 365 nm UV lamp excitation.
Fig. 1. Schematic illustration of the exfoliation procedures to prepare BNNSs and BNQDs by solvothermal method in the present of NaOH.
4
Fig. 2. a) Typical tapping-mode AFM image of BNNSs on mica substrate. The inset shows the height profile along the white line. b) SEM image of BNNSs. c) TEM image of BNNSs. d) HRTEM image of BNNSs. e) Raman spectra of bulk h-BN and BNNSs. f) XRD patterns of bulk h-BN and BNNSs.
The morphology of the as-prepared BNNSs was first investigated using AFM. Fig 2a shows an isolated BNNS with a thickness of ≈1 nm and a lateral size of ≈1.2 μm. Previous reports showed that the AFM height of a BNNS monolayer increases to 1 nm [4, 5]. Thus, the AFM measurement suggests that the bulk h-BN was exfoliated to monolayer BNNSs. Then, we used SEM to further probe the morphology of BNNSs. As shown in Fig 2b, the samples comprise flat and folded nanosheets that are almost 5
transparent, demonstrating their ultrathin nature [6]. The TEM image of BNNSs (Fig. 2c) shows that isolated BNNS is transparent film-like structure with large number of wrinkles and scrolls. In addition, some curled edges can be clearly observed at high magnification (Fig. 2d). As shown in Fig. S1, the edge of BNNSs show four and nine parallel fringes, indicating the few layers in the sample. The lateral size of the BNNSs in not uniform and shows relatively wide range of distribution from 0.3~4.6 μm. Typical Raman spectra of bulk h-BN and BNNSs show a dominant at 1367.6 cm-1 and 1367.1 cm-1 (Fig. 2e), which can be attributed to the E2g mode vibration of h-BN [7]. The slightly red shift Raman band of BNNSs imply that the bulk h-BN has been exfoliated to few-layered nanosheets [8]. It is noted that the XRD pattern (Fig. 2f) of as-prepared BNNSs exhibits characteristic (002) peak at 27.6° [7]. The full width at half maximum (FWHM) of the (002) peak is ~0.4° in the bulk h-BN powder. The FWHM of the (002) peak increases to ~1.6° in BNNSs powder, indicating the efficient exfoliation of bulk h-BN to BNNSs [9]. In addition, after the solvothermal treatment, the intensity decreases sharply, indicating the highly-exfoliated structure of BNNSs [3, 4].
Fig. 3. a) HRTEM image of BNQDs. b) Size distribution of BNQDs. c) HRTEM of individual BNQD. Inset shows the FFT pattern of the crystal. d) Stacked monolayered BNQDs. 6
The morphology and crystallography of BNQDs was investigated by HRTEM. Fig. 3a shows a HRTEM image of BNQDs, which exhibiting a relatively narrow size distribution between 1.7 and 10.9 nm with an average diameter of 5.1 nm (Fig. 3b). The inset of Fig.3a indicates the high crystallinity of BNQDs, with a lattice parameter of ≈0.21 nm, (100) lattice fringes of h-BN [10]. Fig. 3c shows HRTEM image and the corresponding fast Fourier transform (FFT) pattern of an isolated BNQD, which further confirms high crystallinity of hexagonal BN structure. Fig. 3d shows the stacked BNQDs and the interlayer separation is measured to be about 0.8 nm, which is consistent of the previous report [11].
Fig. 4. PL spectra of BNQDs at different excitation wavelengths ranging from 280 to 420. The inset shows the normalized PL spectra of BNQDs. b) PLE (Red), and PL spectra of BNQDs. (c) Confocal fluorescence image microphotograph of the Hela cells incubated with BNQDs QDs for 4 h (λex=405 nm). (d) Bright-field microphotographs of cells. (e) An overlay image of (c) and (d).
We carried out a PL study to explore the optical properties of BNQDs. Fig. 4a shows detailed PL and corresponding normalized PL spectra of BNQDs excited at the wavelengths from 280 to 420 nm. The emission peaks of BNQDs exhibited an excitation-dependent PL behavior, which is consistent with the previously reported BNQDs [10, 11]. The strongest emission peak is located at ≈435 nm when excited at 340 nm，similar to the previous works [11, 12]. Fig. 4b shows the photostability test of BNQDs under different UV irradiation time. The fluorescence intensities of BNQDs 7
show no obvious change, indicating their superior photostability compared to conventional organic dyes and semiconductor quantum dots. To demonstrated the potential bioimaging application of BNQDs, HeLa cells were incubated with BNQDs and
detected by a confocal microscopy. As shown in Fig. 4, bright blue fluorescence
can be observed inside the cell and mainly localized in the cytoplasm region, indicating that the BNQDs had been internalized by the HeLa cells and could be used as efficient bioimaging probes. Conclusion In summary, we developed a simple solvothernal method to synthesize water-soluble BNNSs and fluorescent BNQDs simultaneously. The morphologies and structure of the materials were studied by several microscopic and spectroscopic techniques, confirming the efficient exfoliation of the bulk h-BN. This convenient and cheap process presents a potential advancement for the large-scale production of BN nanostructures. The high water solubility of BNNSs and stable fluorescence of BNQDs make them promising candidates for biological application. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51572184, 51372160). Reference [1] J. Yin, J. Li, Y. Hang, J. Yu, G. Tai, X. Li, Z. Zhang, W. Guo, Small 12(22) (2016) 2942-2968. [2] W. Luo, Y. Wang, E. Hitz, Y. Lin, B. Yang, L. Hu, Adv. Funct. Mate. (2017) 1701450. [3] W. Lei, V.N. Mochalin, D. Liu, S. Qin, Y. Gogotsi, Y. Chen, Nature Comm. 6 (2015) 8849. [4] Y. Lin, J.W. Connell, Nanoscale 4(22) (2012) 6908-6939. [5] F. Yuan, W. Jiao, F. Yang, W. Liu, J. Liu, Z. Xu, R. Wang, J.Mater. Chem. C (2017) 6359-6368. [6] X. Li, X. Hao, M. Zhao, Y. Wu, J. Yang, Y. Tian, G. Qian, Adv. Mater. 25(15) (2013) 2200-2204. [7] F. Xiao, S. Naficy, G. Casillas, M.H. Khan, T. Katkus, L. Jiang, H. Liu, H. Li, Z. Huang, Adv. Mate.27(44) (2015) 7196-7203. [8] T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y.K. Gun’ko, J. Coleman, J Am. Chem. Soc. 134(45) (2012) 18758-18771. [9] Z. Rafiei-Sarmazdeh, S.H. Jafari, S.J. Ahmadi, S.M. Zahedi-Dizaji, J Mater Sci 51(6) (2016) 3162-3169. [10] H. Li, R.Y. Tay, S.H. Tsang, X. Zhen, E.H.T. Teo, Small 11(48) (2015) 6491-6499. [11] L. Lin, Y. Xu, S. Zhang, I.M. Ross, A. Ong, D.A. Allwood, Small 10(1) (2014) 60-65. [12] Z. Lei, S. Xu, J. Wan, P. Wu, Nanoscale 7(45) (2015) 18902-18907. 8
Highlights:
Boron nitride nanosheets and quantum dots were synthesized by a solvothermal method Boron nitride nanosheets showed monolayer morphologies and good solubility Bright blue fluorescence was observed on boron quantum dots Boron quantum dots can be used as fluorescent probe for cancer cells
9