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Green preparation of lattice phosphorus doped graphene quantum dots with tunable emission wavelength for bio-imaging
Materials Letters 242 (2019) 156–159
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Green preparation of lattice phosphorus doped graphene quantum dots with tunable emission wavelength for bio-imaging Gang Wang a,⇑, Anli Xu b,g, Peng He b,g, Qinglei Guo c, Zhiduo Liu d,g, Ziwen Wang e, Jiurong Li a, Xurui Hu a, Zihao Wang a, Da Chen a,⇑, Yongqiang Wang f, Siwei Yang b,g, Guqiao Ding a,b,⇑ a
Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, PR China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China c Department of Materials Science, Fudan University, Shanghai 200433, PR China d State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, PR China e Tianjin International Center of Nano Particles and Nano Systems, Tianjin University, Tianjin 300072, PR China f Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA g University of Chinese Academy of Sciences, Beijing 100049, PR China b
a r t i c l e
i n f o
Article history: Received 9 November 2018 Received in revised form 8 January 2019 Accepted 24 January 2019 Available online 31 January 2019 Keywords: Carbon materials Lattice doping Biomaterials Green preparation Bio-imaging
a b s t r a c t Lattice phosphorus (P) doping has been demonstrated as an effective method for tuning the fluorescence of graphene quantum dots (GQDs). Due to the many possible oxidation states of P, lattice P-doped GQDs (P-GQDs) are still difficult to synthesize. Here, we report the green preparation of P-GQDs via solvothermal treatment of lecithin with high yield (71 wt%). The resulting P-GQDs show controllable emission wavelength (457–632 nm) and high quantum yield (0.54–0.73), and have demonstrated potential for applications in fluorescent bio-imaging. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction As a new photoluminescent (PL) material, graphene quantum dots (GQDs) have attracted a great deal of attention due to their enormous potential in fluorescence detection, bio-imaging, photovoltaic conversion and photocatalysis [1–4]. At the moment, emission wavelength control (especially the preparation of red-emitting GQDs) is still one of the key unsolved issues [5–6]. Previous reports have demonstrated the electron injection of phosphorus in the lattice (P(-III)) of GQDs, in which an obvious red shift of the PL spectrum was observed [7]. However, due to the highly reactive of P(-III)-containing compounds (such as PH3), the lattice P-doped GQDs (P-GQDs) are still difficult to synthesize. Here, we report the green preparation of P-GQDs with high yield (71 wt%). The obtained P-GQDs show controllable emission wavelength
⇑ Corresponding authors at: Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, PR China (Gang Wang, Da Chen and Guqiao Ding). E-mail addresses: [email protected] (G. Wang), [email protected] (D. Chen), [email protected] (G. Ding). https://doi.org/10.1016/j.matlet.2019.01.139 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
(457–632 nm) and high quantum yield (0.54–0.73), which makes them potentially useful in PL bio-imaging. 2. Experiments 0.1 g of lecithin (98%, derived from soybeans, Aladdin (Shanghai, China)) was dissolved in 20 mL alcohol (5 mg mL 1). The solution was transferred into a 25 mL Teflon-lined autoclave and heated to 120 °C for 240 h. The resulting faint yellow solution was mixed into 50 mL water and subjected to dialysis (100 Da) for 3 days. The yield of P-GQDs was 71 wt% which is much higher than that of most other reported methods [8–15]. The rADSCs culture were obtained from subcutaneous adipose tissue in the inguinal groove of 6-week-old male Sprague Dawley rats (Shanghai Animal Experimental Center, China) and cultured in F12/DMEM (Dulbecco’s Modified Eagle Media: Nutrient Mixture F-12) supplemented with 10% FBS (Invitrogen) and 100 units mL 1 penicillinstreptomycin (Invitrogen) according to our protocol [10]. Transmission electron microscopy (TEM) was carried out on a Hitachi H-8100. X-ray photoelectron spectroscopy (XPS) was performed with a PHI Quantera II system, Ulvac-PHI (INC, Japan) to
G. Wang et al. / Materials Letters 242 (2019) 156–159
determine the surface chemical composition and the chemical states of P-GQDs. Ultraviolet-visible (UV-Vis) absorption properties were characterized with a UV-5800 spectrophotometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were collected with a PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at room temperature. 3. Results and discussion TEM images of P-GQDs are shown in Fig. 1. Homogeneous dots with a lateral size of 1–5 nm can be seen. The average size of P-GQDs is 3.1 nm. The high resolution TEM (HR-TEM) image of a single dot in Fig. 1a (Fig. 1b) shows a 0.24 nm lattice spacing which can be attributed to the (1 1 2 0) diffraction planes of graphene [9]. Fast Fourier transform (FFT, Fig. 1c) of P-GQDs indicates a standard six-fold symmetry, indicating that the products have good crystalline structure [5]. Fig. 1d shows a Raman spectrum of P-GQDs, in which the intensity ratio of D band to G band (ID/IG) is 0.89. This result supports the findings from the FFT analysis, further indicating that the products have good crystallinity. XPS survey spectrum of P-GQDs (Fig. 1e) shows a predominant graphitic C 1s peak at 285.0 eV, an O 1s peak at 531.7 eV and a P 2p peak at 132.9 eV, indicating successful doping of P in P-GQDs. The dosage concentration (Cd, phosphorus content in P-GQDs) is 2.1 at. %. Core level C 1s XPS spectrum of P-GQDs is shown in Fig. 1f. The well fitted peaks located at 289.2, 285.4, 284.5 eV can be attributed to the –COOH and –OH groups and sp2 carbon, respectively [7].
157
Moreover, the peak located at 281.5 eV can be attributed to the (Ph)3-P groups [11]. Therefore, the doped P atoms have been integrated into the lattice of P-GQDs. We hold that the successful lattice doping of GQDs results from the effective in-situ reduction of lecithin. As shown in Fig. 1g, the standard reduction potentials of P under different oxidation states are mainly 2.76– 0.06 V. It is known that some active reducing agents (such as active enols) will generate in the solvent thermal treatment of ethanol [6,16], the lecithin can be reduced easily. Moreover, with the presence of P (-III), the lattice doped structure of (Ph)3-P groups can be obtained. The optical properties of P-GQDs are shown in Fig. 2. The Uv-vis absorption spectrum of P-GQDs (Fig. 2a) shows peaks typical of a p-p* transition model [10] at 250 nm and a n-p* transition model [10] at 350 nm. The optimal excitation wavelength (kex) and emission wavelength (kem) is located at approximately 438 nm and 565 nm, respectively. The P-GQDs emit bright green photoluminescence under ultraviolet light irradiation (inset of Fig. 2a) with a quantum yield (u) of 0.67 and superior photo-stability. As shown in Fig. 2b, no significant decrease on PL intensity is observed under a 140 h long-term UV light irradiation (Xe lamp with a center wavelength of 320 nm). PL decay of P-GQDs was measured with a time-correlated single photon counting technique as shown in Fig. 2c. the data fit with a single-exponential decay model yields the PL lifetime (s) of P-GQDs is 3.1 ns and the radiative rates (jr, jr = u/s) is 2.16 108 s 1. P-GQDs also show excellent antijamming capability. No significant changes of PL were observed
Fig. 1. (a) TEM image with histogram size distributions of P-GQDs as an insert. (b) HR-TEM image of P-GQDs. (c) FFT of P-GQDs. (d) Raman, (e) XPS and (f) C 1s spectrum of PGQDs. (g) Standard reduction potential of P under different oxidation states.
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G. Wang et al. / Materials Letters 242 (2019) 156–159
Fig. 2. (a) Uv-vis, PLE and PL spectra of P-GQDs. The insert shows a 0.5 mg mL-1P-GQDs aqueous solution under a 365 nm UV light irradiation. (b) The PL intensity of P-GQDs aqueous solution (0.5 mg mL 1) under ultraviolet radiation (150 W, 320 nm in wavelength). F and F0 are the PL intensity of the P-GQD aqueous solution at specific times. (c) PL decay curve of P-GQDs. (d) PL intensity change of P-GQDs under different ionic strength. (e) Influence of different ions on the PL intensity of P-GQDs. F and F0 is the PL intensity of P-GQDs with and without different ions or biomolecules, respectively. The concentration of P-GQDs is 0.1 mg mL 1. The concentration of ions is 0.1 M.
when P-GQDs were subjected to different ionic strength (Fig. 2d) or intentionally disturbed with different atomic or molecular ions (Fig. 2e). The optical properties of P-GQDs systemically characterized in Fig. 2 show their outstanding PL performance. Due to the controllability of the solvothermal reaction, P-GQDs with different kem can be obtained. As shown in Table 1, with increased concentration of lecithin (CLecithin, 10–120 mg mL 1), the Cd increases significantly. The Cd is 0.3, 1.1, 1.6, 2.1, 2.7, 3.1 and 3.6 at.% when the CLecithin is 10, 25, 40, 50, 75, 95 and 120 mg mL 1, respectively. Due to the effective electron injection of P atoms [7], the kem of P-GQDs increases significantly with increased Cd. As a result, the kem of the P-GQDs (P-GQDs and P-GQDs16) can be tuned to be 565, 457, 486, 526, 579, 602 or 632 nm, which is determined by the Cd at.% value. Moreover, all these GQDs show high u due to the efficient band gap PL process in the lattice-doped structure [7].
These results demonstrate that the doping concentration strongly affects the PL behavior of P-GQDs. As shown in Fig. 3a, the kem of PL spectra obtained from P-GQDs dramatically shifts with doping concentrations, in particular exhibiting a red-shift behavior with increasing the doping concentration. Finally, we evaluated the potential of P-GQDs with tunable kem in bioimaging. The in-vitro cytotoxicity of P-GQDs and P-GQDs-16 was evaluated by rADSCs cells. As shown in Fig. 3b, the metabolic activity of rADSCs cells show no obvious change when treated with P-GQDs and P-GQDs-16, even at high concentration (500 lg mL 1). Fig. 3c shows a confocal fluorescence microphotograph of rADSCs cells treated with P-GQDs and P-GQDs-16. Colorful P-GQDs and P-GQDs-16 aqueous solutions were incubated respectively to show their bio-imaging ability. PL signal with different colours can be observed inside the cells, indicating that all GQDs have been internalized by cells and can be regarded as a kind of efficient bio-imaging material.
Table 1 Reaction conditions and optical properties of P-GQDs with different Cd. CLecithin (mg mL P-GQDs-1 P-GQDs-2 P-GQDs-3 P-GQDs P-GQDs-4 P-GQDs-5 P-GQDs-6
10 25 40 50 75 95 120
1
)
Temperature (oC)
Reaction time (h)
Cd (at.%)
kex (nm)
kem (nm)
u
s (ns)
120 120 120 120 120 120 120
180 180 200 240 240 300 300
0.3 1.1 1.6 2.1 2.7 3.1 3.6
411 420 430 438 470 525 550
457 486 526 565 579 602 632
0.73 0.71 0.68 0.67 0.69 0.61 0.54
2.2 2.8 2.6 3.1 2.5 3.2 2.8
159
G. Wang et al. / Materials Letters 242 (2019) 156–159
Fig. 3. (a) PL spectra of P-GQDs and P-GQDs-16. (b) Metabolic activity of rADSCs cells treated with P-GQDs and P-GQDs-16 (500 lg mL microphotograph of rADSCs cells incubated with P-GQDs and P-GQDs-16.
1
). (c) Confocal fluorescence
4. Conclusion
References
In summary, we have demonstrated the successful green preparation of P-GQDs with different doping concentrations (0.3–3.6 at. %). The yield of P-GQDs (71 wt%) is higher than that of most reported methods. These obtained P-GQDs show controllable emission wavelength (457–632 nm) and high quantum yield (0.54–0.73), and have demonstrated potential for PL bio-imaging. We expect that this work will pave the way for controlling the optical property of GQDs.
[1] M. Bacon, S.J. Bradley, T. Nann, Part. Part. Syst. Charact. 31 (2014) 415–428. [2] T. Chen, B. Liu, Mater. Lett. 209 (2017) 163–166. [3] C.H. Chuang, P. Roy, R. Ravindranath, A.P. Periasamy, H.T. Chang, C.T. Liang, Mater. Lett. 170 (2016) 110–113. [4] C. Zhu, S.W. Yang, G. Wang, R.W. Mo, P. He, J. Sun, et al., J. Mater. Chem. C. 3 (2015) 8810–8816. [5] X.T. Zheng, A. Ananthanarayanan, K.Q. Luo, P. Chen, Small 11 (2015) 1620– 1636. [6] K. Kumara, T.C. ShekharaShetty, P.S. Patil, S.M. Dharmaprakash, Mater. Lett. 235 (2019) 19–22. [7] J. Wu, S.W. Yang, J.P. Li, Y.C. Yang, G. Wang, X.M. Bu, et al., Adv. Opt. Mater. 4 (2016) 2095–2101. [8] Y. Li, Y. Hu, Y. Zhao, G.Q. Shi, L.E. Deng, Y.B. Hou, et al., Adv. Mater. 23 (2011) 776–780. [9] G. Wang, Q.L. Guo, D. Chen, Z. Liu, X. Zheng, A.L. Xu, et al., ACS Appl. Mater. Interfaces 10 (2018) 5750–5759. [10] J. Sun, S.W. Yang, Z.Y. Wang, H. Shen, T. Xu, L.T. Sun, et al., Part. Part. Syst. Charact. 32 (2015) 434–440. [11] S. Yang, C. Zhu, J. Sun, P. He, N. Yuan, J. Ding, et al., RSC Adv. 5 (2015) 33347– 33350. [12] S. Yang, J. Sun, P. He, X. Deng, Z. Wang, C. Hu, et al., Chem. Mater. 27 (2015) 2004–2011. [13] X. Zheng, A. Arundithi, Q. Kathy, P. Chen, Small 11 (2015) 1620–1636. [14] S.W. Yang, W. Li, C. Ye, G. Wang, H. Tian, C. Zhu, et al., Adv. Mater. (2017), 1605625. [15] J. Li, S. Yang, Y. Deng, P. Chai, Y. Yang, X. He, et al., Adv. Funct. Mater. (2018), 1800881. [16] J. Liu, M. Shao, X. Chen, W. Yu, X. Liu, Y. Qian, J. Am. Chem. Soc. 125 (2003) 8088–8089.
Acknowledgements This work was supported by projects from National Natural Science Foundation of China under Grant (Nos. 11704204, 11804353, 61604084, 11774368, and 51802337), General Financial Grant from China Postdoctoral Science Foundation (Nos. 2017 M621564 and BX201700271). The project was also funded by Shanghai Science and Technology Committee (18511110600). K. C. Wong Magna Fund in Ningbo University and the Natural Science Foundation of Ningbo under Grant (No. 2017A610104). The authors declare no competing financial interest. Declaration of interest The authors declared that they have no conflicts of interest to this work.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Green preparation of lattice phosphorus doped graphene quantum dots with tunable emission wavelength for bio-imaging Gang Wang a,⇑, Anli Xu b,g, Peng He b,g, Qinglei Guo c, Zhiduo Liu d,g, Ziwen Wang e, Jiurong Li a, Xurui Hu a, Zihao Wang a, Da Chen a,⇑, Yongqiang Wang f, Siwei Yang b,g, Guqiao Ding a,b,⇑ a
Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, PR China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China c Department of Materials Science, Fudan University, Shanghai 200433, PR China d State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, PR China e Tianjin International Center of Nano Particles and Nano Systems, Tianjin University, Tianjin 300072, PR China f Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA g University of Chinese Academy of Sciences, Beijing 100049, PR China b
a r t i c l e
i n f o
Article history: Received 9 November 2018 Received in revised form 8 January 2019 Accepted 24 January 2019 Available online 31 January 2019 Keywords: Carbon materials Lattice doping Biomaterials Green preparation Bio-imaging
a b s t r a c t Lattice phosphorus (P) doping has been demonstrated as an effective method for tuning the fluorescence of graphene quantum dots (GQDs). Due to the many possible oxidation states of P, lattice P-doped GQDs (P-GQDs) are still difficult to synthesize. Here, we report the green preparation of P-GQDs via solvothermal treatment of lecithin with high yield (71 wt%). The resulting P-GQDs show controllable emission wavelength (457–632 nm) and high quantum yield (0.54–0.73), and have demonstrated potential for applications in fluorescent bio-imaging. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction As a new photoluminescent (PL) material, graphene quantum dots (GQDs) have attracted a great deal of attention due to their enormous potential in fluorescence detection, bio-imaging, photovoltaic conversion and photocatalysis [1–4]. At the moment, emission wavelength control (especially the preparation of red-emitting GQDs) is still one of the key unsolved issues [5–6]. Previous reports have demonstrated the electron injection of phosphorus in the lattice (P(-III)) of GQDs, in which an obvious red shift of the PL spectrum was observed [7]. However, due to the highly reactive of P(-III)-containing compounds (such as PH3), the lattice P-doped GQDs (P-GQDs) are still difficult to synthesize. Here, we report the green preparation of P-GQDs with high yield (71 wt%). The obtained P-GQDs show controllable emission wavelength
⇑ Corresponding authors at: Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, PR China (Gang Wang, Da Chen and Guqiao Ding). E-mail addresses: [email protected] (G. Wang), [email protected] (D. Chen), [email protected] (G. Ding). https://doi.org/10.1016/j.matlet.2019.01.139 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
(457–632 nm) and high quantum yield (0.54–0.73), which makes them potentially useful in PL bio-imaging. 2. Experiments 0.1 g of lecithin (98%, derived from soybeans, Aladdin (Shanghai, China)) was dissolved in 20 mL alcohol (5 mg mL 1). The solution was transferred into a 25 mL Teflon-lined autoclave and heated to 120 °C for 240 h. The resulting faint yellow solution was mixed into 50 mL water and subjected to dialysis (100 Da) for 3 days. The yield of P-GQDs was 71 wt% which is much higher than that of most other reported methods [8–15]. The rADSCs culture were obtained from subcutaneous adipose tissue in the inguinal groove of 6-week-old male Sprague Dawley rats (Shanghai Animal Experimental Center, China) and cultured in F12/DMEM (Dulbecco’s Modified Eagle Media: Nutrient Mixture F-12) supplemented with 10% FBS (Invitrogen) and 100 units mL 1 penicillinstreptomycin (Invitrogen) according to our protocol [10]. Transmission electron microscopy (TEM) was carried out on a Hitachi H-8100. X-ray photoelectron spectroscopy (XPS) was performed with a PHI Quantera II system, Ulvac-PHI (INC, Japan) to
G. Wang et al. / Materials Letters 242 (2019) 156–159
determine the surface chemical composition and the chemical states of P-GQDs. Ultraviolet-visible (UV-Vis) absorption properties were characterized with a UV-5800 spectrophotometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were collected with a PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at room temperature. 3. Results and discussion TEM images of P-GQDs are shown in Fig. 1. Homogeneous dots with a lateral size of 1–5 nm can be seen. The average size of P-GQDs is 3.1 nm. The high resolution TEM (HR-TEM) image of a single dot in Fig. 1a (Fig. 1b) shows a 0.24 nm lattice spacing which can be attributed to the (1 1 2 0) diffraction planes of graphene [9]. Fast Fourier transform (FFT, Fig. 1c) of P-GQDs indicates a standard six-fold symmetry, indicating that the products have good crystalline structure [5]. Fig. 1d shows a Raman spectrum of P-GQDs, in which the intensity ratio of D band to G band (ID/IG) is 0.89. This result supports the findings from the FFT analysis, further indicating that the products have good crystallinity. XPS survey spectrum of P-GQDs (Fig. 1e) shows a predominant graphitic C 1s peak at 285.0 eV, an O 1s peak at 531.7 eV and a P 2p peak at 132.9 eV, indicating successful doping of P in P-GQDs. The dosage concentration (Cd, phosphorus content in P-GQDs) is 2.1 at. %. Core level C 1s XPS spectrum of P-GQDs is shown in Fig. 1f. The well fitted peaks located at 289.2, 285.4, 284.5 eV can be attributed to the –COOH and –OH groups and sp2 carbon, respectively [7].
157
Moreover, the peak located at 281.5 eV can be attributed to the (Ph)3-P groups [11]. Therefore, the doped P atoms have been integrated into the lattice of P-GQDs. We hold that the successful lattice doping of GQDs results from the effective in-situ reduction of lecithin. As shown in Fig. 1g, the standard reduction potentials of P under different oxidation states are mainly 2.76– 0.06 V. It is known that some active reducing agents (such as active enols) will generate in the solvent thermal treatment of ethanol [6,16], the lecithin can be reduced easily. Moreover, with the presence of P (-III), the lattice doped structure of (Ph)3-P groups can be obtained. The optical properties of P-GQDs are shown in Fig. 2. The Uv-vis absorption spectrum of P-GQDs (Fig. 2a) shows peaks typical of a p-p* transition model [10] at 250 nm and a n-p* transition model [10] at 350 nm. The optimal excitation wavelength (kex) and emission wavelength (kem) is located at approximately 438 nm and 565 nm, respectively. The P-GQDs emit bright green photoluminescence under ultraviolet light irradiation (inset of Fig. 2a) with a quantum yield (u) of 0.67 and superior photo-stability. As shown in Fig. 2b, no significant decrease on PL intensity is observed under a 140 h long-term UV light irradiation (Xe lamp with a center wavelength of 320 nm). PL decay of P-GQDs was measured with a time-correlated single photon counting technique as shown in Fig. 2c. the data fit with a single-exponential decay model yields the PL lifetime (s) of P-GQDs is 3.1 ns and the radiative rates (jr, jr = u/s) is 2.16 108 s 1. P-GQDs also show excellent antijamming capability. No significant changes of PL were observed
Fig. 1. (a) TEM image with histogram size distributions of P-GQDs as an insert. (b) HR-TEM image of P-GQDs. (c) FFT of P-GQDs. (d) Raman, (e) XPS and (f) C 1s spectrum of PGQDs. (g) Standard reduction potential of P under different oxidation states.
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G. Wang et al. / Materials Letters 242 (2019) 156–159
Fig. 2. (a) Uv-vis, PLE and PL spectra of P-GQDs. The insert shows a 0.5 mg mL-1P-GQDs aqueous solution under a 365 nm UV light irradiation. (b) The PL intensity of P-GQDs aqueous solution (0.5 mg mL 1) under ultraviolet radiation (150 W, 320 nm in wavelength). F and F0 are the PL intensity of the P-GQD aqueous solution at specific times. (c) PL decay curve of P-GQDs. (d) PL intensity change of P-GQDs under different ionic strength. (e) Influence of different ions on the PL intensity of P-GQDs. F and F0 is the PL intensity of P-GQDs with and without different ions or biomolecules, respectively. The concentration of P-GQDs is 0.1 mg mL 1. The concentration of ions is 0.1 M.
when P-GQDs were subjected to different ionic strength (Fig. 2d) or intentionally disturbed with different atomic or molecular ions (Fig. 2e). The optical properties of P-GQDs systemically characterized in Fig. 2 show their outstanding PL performance. Due to the controllability of the solvothermal reaction, P-GQDs with different kem can be obtained. As shown in Table 1, with increased concentration of lecithin (CLecithin, 10–120 mg mL 1), the Cd increases significantly. The Cd is 0.3, 1.1, 1.6, 2.1, 2.7, 3.1 and 3.6 at.% when the CLecithin is 10, 25, 40, 50, 75, 95 and 120 mg mL 1, respectively. Due to the effective electron injection of P atoms [7], the kem of P-GQDs increases significantly with increased Cd. As a result, the kem of the P-GQDs (P-GQDs and P-GQDs16) can be tuned to be 565, 457, 486, 526, 579, 602 or 632 nm, which is determined by the Cd at.% value. Moreover, all these GQDs show high u due to the efficient band gap PL process in the lattice-doped structure [7].
These results demonstrate that the doping concentration strongly affects the PL behavior of P-GQDs. As shown in Fig. 3a, the kem of PL spectra obtained from P-GQDs dramatically shifts with doping concentrations, in particular exhibiting a red-shift behavior with increasing the doping concentration. Finally, we evaluated the potential of P-GQDs with tunable kem in bioimaging. The in-vitro cytotoxicity of P-GQDs and P-GQDs-16 was evaluated by rADSCs cells. As shown in Fig. 3b, the metabolic activity of rADSCs cells show no obvious change when treated with P-GQDs and P-GQDs-16, even at high concentration (500 lg mL 1). Fig. 3c shows a confocal fluorescence microphotograph of rADSCs cells treated with P-GQDs and P-GQDs-16. Colorful P-GQDs and P-GQDs-16 aqueous solutions were incubated respectively to show their bio-imaging ability. PL signal with different colours can be observed inside the cells, indicating that all GQDs have been internalized by cells and can be regarded as a kind of efficient bio-imaging material.
Table 1 Reaction conditions and optical properties of P-GQDs with different Cd. CLecithin (mg mL P-GQDs-1 P-GQDs-2 P-GQDs-3 P-GQDs P-GQDs-4 P-GQDs-5 P-GQDs-6
10 25 40 50 75 95 120
1
)
Temperature (oC)
Reaction time (h)
Cd (at.%)
kex (nm)
kem (nm)
u
s (ns)
120 120 120 120 120 120 120
180 180 200 240 240 300 300
0.3 1.1 1.6 2.1 2.7 3.1 3.6
411 420 430 438 470 525 550
457 486 526 565 579 602 632
0.73 0.71 0.68 0.67 0.69 0.61 0.54
2.2 2.8 2.6 3.1 2.5 3.2 2.8
159
G. Wang et al. / Materials Letters 242 (2019) 156–159
Fig. 3. (a) PL spectra of P-GQDs and P-GQDs-16. (b) Metabolic activity of rADSCs cells treated with P-GQDs and P-GQDs-16 (500 lg mL microphotograph of rADSCs cells incubated with P-GQDs and P-GQDs-16.
1
). (c) Confocal fluorescence
4. Conclusion
References
In summary, we have demonstrated the successful green preparation of P-GQDs with different doping concentrations (0.3–3.6 at. %). The yield of P-GQDs (71 wt%) is higher than that of most reported methods. These obtained P-GQDs show controllable emission wavelength (457–632 nm) and high quantum yield (0.54–0.73), and have demonstrated potential for PL bio-imaging. We expect that this work will pave the way for controlling the optical property of GQDs.
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Acknowledgements This work was supported by projects from National Natural Science Foundation of China under Grant (Nos. 11704204, 11804353, 61604084, 11774368, and 51802337), General Financial Grant from China Postdoctoral Science Foundation (Nos. 2017 M621564 and BX201700271). The project was also funded by Shanghai Science and Technology Committee (18511110600). K. C. Wong Magna Fund in Ningbo University and the Natural Science Foundation of Ningbo under Grant (No. 2017A610104). The authors declare no competing financial interest. Declaration of interest The authors declared that they have no conflicts of interest to this work.