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Promising Fast Energy Transfer System Between Graphene Quantum Dots and the Application in Fluorescent Bio-imaging Gang Wang, Peng He, Anli Xu, Qinglei Guo, Jiurong Li, Zihao Wang, Zhiduo Liu, Da Chen, Siwei Yang, and Guqiao Ding Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03739 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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Promising Fast Energy Transfer System Between Graphene Quantum Dots and the Application in Fluorescent Bio-imaging Gang Wang†, *, 1, Peng He&, §, 1, Anli Xu&, §, 1, Qinglei Guo‡, Jiurong Li†, Zihao Wang†, Zhiduo Liu||, Da Chen†, *, Siwei Yang&, §, Guqiao Ding†, &, §, * † Department

of Microelectronic Science and Engineering, Faculty of Science, Ningbo

University, Ningbo 315211, P. R. China. &State

Key Laboratory of Functional Materials for Informatics, Shanghai Institute of

microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China. ‡Department ||State

of Materials Science, Fudan University, Shanghai 200433, P. R. China.

Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors,

Chinese Academy of Sciences, Beijing 100083, P. R. China. §University 1These

of Chinese Academy of Sciences, Beijing 100049, P. R. China.

authors contributed equally.

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Table of Contents (TOC) Graphic

ABSTRACT: Tunable photoluminescence performance of graphene quantum dots (GQDs) is one of the most important topics for the development of GQDs. In this paper, we report lattice doped GQDs (boron doped GQDs, B-GQDs and phosphorus doped GQDs, P-GQDs). Due to the matched band structure, the fast energy transfer between blue emitted B-GQDs (emission wavelength: 460 nm) and orange emitted P-GQDs (emission wavelength: 630 nm) can induce an efficient fluorescence emission in P-GQDs once B-GQDs are excited under the optimal excitation wavelength of 460 nm. Moreover, with the effective energy transfer, the quantum yield of P-GQDs increased to 0.48 which is much higher than that of pure P-GQDs. We also demonstrated the potentials of this system for fluorescent bio-imaging in vitro. KEYWORDS: Graphene quantum dots, Photoluminescence, Energy transfer, Bio-imaging


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INTRODUCTION Graphene quantum dots (GQDs), a new carbon based photoluminescence (PL) semiconductor, have attracted a great deal of attentions since its first discover in 2007.1 Due to the quantum size effect, the band gap of 2D graphene is opened.2 GQDs show high stability,3 low biotoxicity,3-4 unique PL properties,5 and multifunctional edge groups.5-9 By now, GQDs have been widely used in fluorescent sensing,

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fluorescent bio-imaging,11-12 photon detection,13 photocatalysis,14 and photovoltaic conversion.15-16 Yang et al. reported selenium doped GQDs based fluorescent switch for the detection of oxidative hydroxyl radical and reductive glutathione in vitro.17 Li et al. reported folic acid modified nitrogen doped GQDs (FN-CDs),18 which had the ability to activate both the intrinsic and extrinsic apoptotic signaling pathway and to kill tumor cells efficiently.18 Such method achieves therapeutic effects with high performance in 26 types of tumor cell lines. Animal experiments showed that the 30 days survival rate of this therapeutic strategy is much higher than that with traditional drug treatment.18 It's worth noting that, most application fields (such as fluorescent bio-imaging,19 photon detection,20 photocatalysis, 21 and photovoltaic conversion22) of GQDs requires the obviously opened band gap and effective PL control. Therefore, tunable PL performance of GQDs has been emerged as one of the most important topics for the development of GQDs. Previous works have reported diversified approaches to tune the PL performance of GQDs, including size control,14 surface modification23 and doping24-25. The size control is derived from the quantum size effect of nanomaterials.26 However, due to 3 ACS Paragon Plus Environment

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the complicated surface and edge structures of GQDs, the size control is not the ideal method for tuning the PL performance of GQDs, especially for band gap control and emission wavelength tuning.27 Moreover, the quantum yield (φ) of the obtained GQDs is often lower than most fluorescent dyes.28 As a result, surface modification and doping are the attractive methods for preparing GQDs with tunable emission wavelength and with high φ.29-30 Xu et al. demonstrated that light emission properties of sulfur doped GQDs can be effectively tuned by controlling the doping properties in them.31 However, the PL process of the most modified/doped GQDs can be attributed to the transition of defect mode and functional groups.31 This makes it hard for the energy band structure design of GQDs. Thus, it is necessary to develop an effective method for the band gap and PL controls. Herein, we report lattice doped GQDs (boron doped GQDs and phosphorus doped GQDs). With the electron injection effect of P atoms and electron-effect of B atoms, the phosphorus doped GQDs (P-GQDs) and boron doped GQDs (B-GQDs) showed obvious differences in PL behavior. The emission wavelength of B-GQDs and P-GQDs is 460 nm and 630 nm, respectively. Due to the matched band structure, the fast energy transfer between blue emitted B-GQDs and orange emitted P-GQDs can induce an efficient fluorescence emission in P-GQDs once B-GQDs are excited under the optimal excitation wavelength of 460 nm. Moreover, with the effective energy transfer, the φ of P-GQDs increased to about 0.48 which is much higher than that of pure P-GQDs. Based on the superior and tunable properties, enormous potentials for fluorescent bio-imaging in vitro of this system were also demonstrated. 4 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Characterization of B-GQDs and P-GQDs

Figure 1. TEM image of (a) B-GQDs and (b) P-GQDs. Size distribution histogram of (c) B-GQDs and (d) P-GQDs. HR-TEM image of (e) B-GQDs and (f) P-GQDs. FFT image of (g) B-GQDs and (h) P-GQDs

Figures 1(a-b) show the TEM images of B-GQDs and P-GQDs, respectively. The homogeneous dots can be observed. The average sizes of B-GQDs and P-GQDs are about 3.5 nm and 3.4 nm, respectively (Figures 1(c-d)). No obvious size difference can be found between B-GQDs and P-GQDs. Atomic force microscope (AFM) observation (Figure S1) reveals highly dispersed B-GQDs and P-GQDs on SiO2 substrate with a typical topographic height of 0.5-1.5 nm. This indicates that most B-GQDs and P-GQDs consist of ca. 1-3 graphene layers.10 Moreover, both B-GQDs and P-GQDs show good crystallinity (Figures 1(e-f)). High-resolution TEM (HR-TEM) image of B-GQDs and P-GQDs show a 0.24 nm lattice spacing in crystalline structure. This can be attributed to the (110) diffraction planes of 5 ACS Paragon Plus Environment

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graphene.31, 33 Fast Fourier transform (FFT, Figures 1(g-h)) of TEM images obtained from B-GQDs and P-GQDs indicates a standard six-fold symmetry, which demonstrates the good crystalline structure of both B-GQDs and P-GQDs.34 Raman spectra were further carried out for the crystallinity characterization of B-GQDs and P-GQDs. As shown in Figures S2-S3, the intensity ratio of the D (1337-1346 cm-1) band to G (1584-1589 cm-1) band (ID/IG) can be used for evaluating structure and quality of carbon materials.35 The ID/IG of B-GQDs and P-GQDs is calculated to be about 0.91 and 0.92, respectively, which confirms the well crystallized sp2 structure of as-prepared B-GQDs and P-GQDs,36 and is in accordance with the TEM results.

Figure 2. (a) XPS spectra and (b) the magnified C-1s spectra of B-GQDs and P-GQDs. The inset of b shows structure diagrams of B-GQDs and P-GQDs. The structures of B-GQDs and P-GQDs are characterized by XPS (Figure 2). As shown in Figure 2(a), the XPS spectra of both B-GQDs and P-GQDs show a 6 ACS Paragon Plus Environment

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predominant graphitic C-1s peak at 285.0 eV and an O-1s peak at 531.7 eV.37 The O contents of B-GQDs and P-GQDs are about 14.2 at.% and 11.6 at.%, respectively. An obvious peak located at 190.3 eV (B-1s) in the XPS spectrum of B-GQDs can be attributed to the doped B,38-39 which further demonstrates the successful doping of B in B-GQDs. The doping concentration is 2.8 at.%. For the XPS result of P-GQDs, however, an evident peak located at 132.9 eV (P-2p) can be found,40-41 which also demonstrates the successful doping of P in P-GQDs, and the doping concentration is about 1.9 at.%. Core level C-1s XPS spectrum of B-GQDs is shown in Figure 2(b), and the well fitted peaks located at 289.2, 285.4, 284.5 eV can be assigned as the -COOH, -OH groups and sp2 carbon, respectively.37 The results indicate that diverse oxygen-containing groups exist in B-GQDs. Notably, a shoulder peak , resulted from the (Ph)3-B groups, is found locating at 282.7 eV, which further verifies that B atoms are substitutionally doped in the lattice of B-GQDs. Similarly, the peaks located at 289.2, 285.4, 284.5 eV are found in the core level C-1s XPS spectrum of P-GQDs (also shown in Figure 2(b)), also indicating diverse oxygen-containing groups existing in the synthesized P-GQDs. Unlike with B-GQDs, the shoulder peak has a slight shift, locating at 281.5 eV. We attribute this shoulder peak to the (Ph)3-P groups. Therefore, P atoms also exhibit a substitutional doping in the lattice of P-GQDs. Based on the above analysis, schematic diagram of as-synthesized B-GQDs and P-GQDs with abundant oxygen-containing groups are shown in the inset of Figure 2(b). 7 ACS Paragon Plus Environment

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Optical properties of B-GQDs and P-GQDs

Figure 3. (a) Uv-vis spectra, (b) PL spectra and (c) PL decay curves of B-GQDs and P-GQDs aqueous solution. The inset of b shows photograph of 1.0 mg mL-1 B-GQDs and P-GQDs aqueous solution under the excitation of a 365 nm UV light. Uv-vis spectra of B-GQDs and P-GQDs aqueous solutions are shown in Figure 3(a). Because of the π-π* transition model of aromatic sp2 domains42 and n-π* transition model of oxygen-containing groups,23 typical absorption peaks at 220 and 270 nm can be in B-GQDs. Meanwhile, two typical absorption peaks at 255 and 330 nm, induced by the π-π* and n-π* transition models, are also found in the Uv-vis result of P-GQDs. It's worth noting that, compared with B-GQDs, obvious red shift can be observed in both π-π* and n-π* peaks of P-GQDs. Possible reason for this phenomenon is the electron injection effect of P atoms in the lattice of P-GQDs.41 Both B-GQDs and P-GQDs aqueous solutions show a long tail extending into the visible range, suggesting a considerable absorption in the whole visible light range.43 Moreover, the optical band gaps of B-GQDs and P-GQDs also exhibit significant changes after B and P doping. The optical band gap (Eo) of B-GQDs and P-GQDs is ~3.4 eV and ~2.5 eV which can be calculated from the band edge of the Uv-vis spectrua.41 It is concluded that the B-GQDs have much larger Eo (~3.4 eV) than that 8 ACS Paragon Plus Environment

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of P-GQDs (~2.5 eV). Since the band gap of GQDs can be modulated by B or P doping, one can predict that the corresponding PL properties of GQDs is strongly depended on doping. Figure S4 shows the PLE spectrum of B-GQDs, the optimal excitation wavelength (Ex) of B-GQDs is 360 nm, which is consistent with the band gap position of B-GQDs.41 From the PLE spectrum of P-GQD, the Ex moves to around 510 nm (Figure S5). PL spectra of B-GQDs and P-GQDs are shown in Figure 3(b), the emission wavelengths (Em) of B-GQDs and P-GQDs are 460 and 630 nm, respectively. The emission properties of B-GQDs and P-GQDs are intuitively confirmed that under the ultraviolet irradiation, B-GQDs and P-GQDs emit blight blue and orange lights, as shown in the inset of Figure 3(b). The quantum yields (φ) of B-GQDs and P-GQDs are 0.59 and 0.25, respectively. Notably, the synthesized B-GQDs have a much higher quantum yield than that of mostly reported GQDs (Figure S6). PL decay of B-GQDs and P-GQDs are measured by a time-correlated single photon counting technique and fitted well with a single-exponential decay function. As shown in Figure 3(c), The PL lifetime (τ) of B-GQDs and P-GQDs is about 1.6 and 2.4 ns, respectively. Therefore, the radiative rates (κr), expressed as κr = φ/τ,44 of B-GQDs and P-GQDs are calculated to be 3.69×108 s-1 and 1.04×108 s-1, respectively. The optical properties of B-GQDs and P-GQDs are summarized in Table 1.


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Table 1. Optical properties of B-GQDs and P-GQDs Ex (nm)

Em (nm)

φ

τ (ns)

κr (s-1)

Eo (eV)

B-GQDs

360

460

0.59 a

1.6

3.69×108

3.4

P-GQDs

510

630

0.25 b

2.4

1.04×108

2.5

a

Quinine sulfate in 0.05 M H2SO4 (φ=0.55) as standard b Rhodamine B in ethanol (φ=0.69) as standard

Both B-GQDs and P-GQDs show superior photostability and antijamming capability. As shown in Figures S7-S8, no obvious decrease in PL intensity can be observed under the long time UV light irradiation (Xe lamp with the center wavelength of 320 nm, 48 h) or visible light irradiation (450-850 nm, 60 days). Moreover, no obvious quenching behavior is observed with the present of various ions, including Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Al3+, Co2+, Ni2+, Mn2+, Cu2+, Zn2+, Fe2+, Fe3+, Cd2+, Hg2+, Cl-, Br-, OH-, SO42-, NO3- and NO2- (see Figures S9-S10). The excellent photostability and antijamming capability are ascribed to the stable lattice doping structure of B-GQDs and P-GQDs.31

Fast energy transfer system between B-GQDs and P-GQDs


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Figure 4. (a) Band structure diagrams of B-GQDs and P-GQDs. (b, c) Schematic diagrams of the fast energy transfer system between B-GQDs and P-GQDs. (d) PL spectra of B-GQDs (50 μg mL-1) under Ex=360 nm with increased concentration of P-GQDs (0-50 μg mL-1). (e) PL decay curve of B-GQDs (black curve) and the mixture of B-GQDs and P-GQDs (concentrations of B-GQDs and P-GQDs are all 50 μg mL-1).

As above demonstrated, B-GQDs and P-GQDs show distinct emission properties with different Em because of the modulated band structures by B or P doping. In order to further verify this conclusion, cyclic voltammetry (CV) experiments were performed to measure the bandgap of B-GQDs and P-GQDs.45 Linear potential scans were conducted to determine and analyze the valence band (VB) and conduction band (CB) of B-GQDs and P-GQDs by using Pt wire as the counter electrode and Ag|AgCl as the reference electrode. Figures S11-S14 manifest the anodic/cathodic scans of 11 ACS Paragon Plus Environment

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B-GQDs and P-GQDs. the VB values of B-GQDs and P-GQDs are approximately 1.56 eV and 2.58 eV (versus Ag|AgCl), whereas the CB values are -0.87 eV and -0.75 eV (versus Ag|AgCl), respectively (Figure 4(a)). Hence, the bandgaps of B-GQDs and P-GQDs are determined to be about 3.45 eV and 2.41 eV, respectively. It's worth noting that the CB value of B-GQDs is close to that of P-GQDs. Therefore, possible potential energy transfer between the CB of B-GQDs and P-GQDs can be realized when they are close enough. As schematically shown in Figure 4(b), excited electrons of B-GQDs (B-GQDs*) are easy to move to the CB of P-GQDs. Correspondingly, the electron in VB of P-GQDs can be transferred to the VB of B-GQDs, provoking P-GQDs stay at the excited state (P-GQDs*). Finally, the P-GQDs* could emit photon with a much larger wavelength than that of B-GQDs. Moreover, due to the unoccupied 2p orbital of B and lone pair electrons of P, the B-GQDs and P-GQDs also show strong interaction (Figure 4(c))46, which further promotes the fast energy transfer between B-GQDs and P-GQDs. To verity above speculates, PL measurements are systematically performed. As shown in Figure 4(d), B-GQDs aqueous solution (50 μg mL-1) shows a typical PL signal at 460 nm when the Ex is 360 nm. However, when the P-GQDs aqueous solution was added, a new PL signal at 630 nm can be observed. It's worth noting that, the pure P-GQDs aqueous solution show no obvious PL signal when the Ex is 360 nm. In addition, the mixture of B-GQDs and P-GQDs show an increasing PL signal at 630 nm with the increased P-GQDs concentration. When the concentration ratio of B-GQDs to P-GQDs is 1:1, however, no PL signal at 460 nm can be observed. 12 ACS Paragon Plus Environment

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Compare with pure P-GQDs aqueous solution, the mixture of B-GQDs and P-GQDs show a much higher φ at 630 nm, which is about 0.48. We attribute this enhanced quantum yield to the efficient energy transfer between B-GQDs and P-GQDs.47 PL lifetime measurement was also carried out for investigating the physical mechanism. As shown in Figure 4(e), the PL decay curve of the mixture of B-GQDs and P-GQDs (concentrations of B-GQDs and P-GQDs are all 50 μg mL-1) is measured by a time-correlated single photon counting technique and fitted well with a biexponential decay function. The lifetime τ of this mixture is dominated by a long decay (τ1) component of 13.1 ns (97%) with a small contribution from a short decay (τ2) of 2.5 ns (3%). The τ1 can be attributed to the fast energy transfer between B-GQDs and P-GQDs, while the τ2 can be attributed to the radiative transition of P-GQDs.47 Finally, the weighted-average τ is evaluated to be about 12.8 ns. This fast energy transfer system between B-GQDs and P-GQDs shows good photostability which can be due to the good photostability of B-GQDs and P-GQDs. As shown in Figure S15. No obvious decrease in PL intensity can be observed under the long time UV light irradiation (Xe lamp with the center wavelength of 320 nm, 48 h).


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Fluorescent bio-imaging in vitro with fast energy transfer between B-GQDs and P-GQD

Figure 5. (a) Metabolic activity of rADSCs cells in the mixture of B-GQDs and P-GQD with different concentrations (0-500 μg mL-1). (b) Confocal fluorescence microphotograph of rADSCs cells incubated with B-GQD (20 μg mL-1). (c) Confocal fluorescence microphotograph of rADSCs cells incubated with B-GQD after the introduce of P-GQDs (20 μg mL-1).

In order to exploit the potentials and utilize the fast energy transfer feature of B-GQDs and P-GQDs, in vitro cytotoxicity of B-GQDs, P-GQDs and their mixture were evaluated by rADSCs cells, which were further explored for fluorescent bio-imaging. As shown in Figure 5(a) and Figures S16-17, metabolic activity of rADSCs cells was summarized after treated with various concentrations of B-GQDs, P-GQDs and their mixture. No obvious reduction in cell viability can be observed, 14 ACS Paragon Plus Environment

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even treated with highly concentrated mixture (B-GQDs and P-GQD aqueous, up to 500 μg mL-1). In vitro fluorescent bio-imaging is demonstrated by utilizing B-GQDs and P-GQD. As shown in Figure 5(b), bright blue fluorescent can be observed in rADSCs cells after a sufficient incubate in B-GQDs (20 μg mL-1, 2 h). This indicates the B-GQDs have been internalized by the rADSCs cells. Then, P-GQDs aqueous (20 μg mL-1) was added and incubated the above rADSCs cells for 2 hours. Instead, orange fluorescent can be observed in the incubated rADSCs cells when excited with a 360 nm light (Figure 5(c)). Based on the above results, the demonstrated fast energy transfer system between B-GQDs and P-GQDs has great potentials for the fluorescent bio-imaging in vitro.

CONCLUSION

In summary, the lattice doped B-GQDs and P-GQD were synthesized via solvothermal method. The φ of B-GQDs and P-GQDs are 0.59 and 0.25, respectively. Due to the matched band structure, the fast energy transfer between B-GQDs and P-GQDs was demonstrated. As a result, P-GQDs emit the orange PL with a high φ (0.48) when the Ex is 360 nm. Finally, the lattice doped B-GQDs and P-GQD were explored for the application in in fluorescent bio-imaging in vitro. This work may provide guidance for the PL properties control of GQDs, as well as the design principles for photovoltaic conversion and photocatalysis system.


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EXPERIMENTAL SECTION Materials.

Triphenylphosphine

(98%),

triphenyl

borane

(98%),

anhydrous

tetrahydrofuran (THF) and anhydrous ethanol (EtOH) were purchased from Aladdin (Shanghai, China) and used as received without further purification. GQDs were purchased from Xiwang technology co. LTD (Shanghai, China) and used as received without further purification. Water used throughout all experiments was purified by Millipore system. 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 penicillin-streptomycin (Invitrogen) according to our protocol. Synthesis of B-GQDs. B-GQDs were prepared as follows: 2.0 mg GQDs were dispersed in 20 mL anhydrous THF. Then, 24.2 mg, 0.1 mM triphenyl borane was added in the above solution. The mixture was transferred into a 25 mL Teflons-lined autoclave and heated at 80 oC for 240 h. The obtained light yellow solution was added into 50 mL water and dialysis (100 Da) for 3 days. The yield of B-GQDs is 5.3 wt.%. Synthesis of P-GQDs. P-GQDs were prepared as follows: 4.0 mg GQDs were dispersed in 20 mL anhydrous EtOH. Then, 131.0 mg, 0.5 mM triphenylphosphine was added in the above solution. The mixture was transferred into a 25 mL Teflons-lined autoclave and heated at 120 oC for 120 h. The obtained yellow solution was added into 50 mL water and dialysis (100 Da) for 3 days. The yield of P-GQDs is 2.4 wt.%. Characterization methods. Transmission electron microscopy (TEM) was carried out on Hitachi H-8100. X-ray photoelectron spectroscopy (XPS) was utilized on PHI 16 ACS Paragon Plus Environment

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Quantera II system, Ulvac-PHI (INC, Japan) to determine the surface chemical composition, chemical states of B-GQDs and P-GQDs. Ultraviolet-visible (UV-Vis) absorption

properties

were

characterized

on

UV-5800

spectrophotometer.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were collected on a PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at room temperature. The φ measurements: rhodamine B in ethanol (φ=0.69) and quinine sulfate in 0.05 M H2SO4 (φ=0.55) was chosen as a standard. The quantum yield was calculated according to Equation (1): 32

I AR  2   R    I R A  R2

(1)

Where φ is the quantum yield, I is the measured integrated emission intensity, η is the refractive index of the solvent, A is the optical density, and the subscript R refers to the reference standard with a known quantum yield. In order to minimize reabsorption effects, absorbance in the 10 mm fluorescence cuvette was kept under 0.1 at 360 nm excitation wavelength. PL lifetime was measured via the time-correlated single-photon counting (TCSPC) technique (HydraHarp 400, PicoQuant).19 The samples were excited by using a frequency-doubled titanium: sapphire oscillator laser with a pulse duration of ~150 fs with repetition rate of 80 MHz (Chameleon, Coherent).19 PL emission was sent to a spectrometer (iHR550, Horiba Jobin Yvon) with 300 mm-1 grating and then detected with photomultiplier tube.19 ASSOCIATED CONTENT 17 ACS Paragon Plus Environment

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Supporting Information Electronic Supplementary Information (ESI) available:

details of any supplementary

information available should be included here]. Figures (Figures S1-S15). See DOI: AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

*E-mail:

[email protected]

*E-mail:

[email protected]

Notes The authors declare no competing financial interest.

Acknowledgment This work was supported by projects from National Natural Science Foundation of China under Grant (Nos. 11804353, 11704204, 61604084, 11774368, and 51802337), General Financial Grant from China Postdoctoral Science Foundation (Nos. 2017M621564 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.


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Electrochemical and Photophysical Investigations. Inorg. Chem. 2011, 50, 26 ACS Paragon Plus Environment

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8926-8936.


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Figure 1. TEM image of (a) B-GQDs and (b) P-GQDs. Size distribution histogram of (c) B-GQDs and (d) PGQDs. HR-TEM image of (e) B-GQDs and (f) P-GQDs. FFT image of (g) B-GQDs and (h) P-GQDs

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Figure 2. (a) XPS spectra and (b) the magnified C-1s spectra of B-GQDs and P-GQDs. The inset of b shows structure diagrams of B-GQDs and P-GQDs. 316x219mm (300 x 300 DPI)


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Figure 3. (a) Uv-vis spectra, (b) PL spectra and (c) PL decay curves of B-GQDs and P-GQDs aqueous solution. The inset of b shows photograph of 1.0 mg mL-1 B-GQDs and P-GQDs aqueous solution under the excitation of a 365 nm UV light. 258x88mm (300 x 300 DPI)


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Figure 4. (a) Band structure diagrams of B-GQDs and P-GQDs. (b, c) Schematic diagrams of the fast energy transfer system between B-GQDs and P-GQDs. (d) PL spectra of B-GQDs (50 μg mL-1) under Ex=360 nm with increased concentration of P-GQDs (0-50 μg mL-1). (e) PL decay curve of B-GQDs (black curve) and the mixture of B-GQDs and P-GQDs (concentrations of B-GQDs and P-GQDs are all 50 μg mL-1). 245x221mm (300 x 300 DPI)


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Figure 5. (a) Metabolic activity of rADSCs cells in the mixture of B-GQDs and P-GQD with different concentrations (0-500 μg mL-1). (b) Confocal fluorescence microphotograph of rADSCs cells incubated with B-GQD (20 μg mL-1). (c) Confocal fluorescence microphotograph of rADSCs cells incubated with B-GQD after the introduce of P-GQDs (20 μg mL-1). 156x120mm (300 x 300 DPI)


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