Three-minute ultra-rapid microwave-assisted synthesis of bright

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Three-minute ultra-rapid microwave-assisted synthesis of bright fluorescent graphene quantum dots for live cell staining and white LEDs Weitao Li, Ming Li, Yijian Liu, Dengyu Pan, Zhen Li, Liang Wang, and Minghong Wu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00114 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Applied Nano Materials

Three-minute ultra-rapid microwave-assisted synthesis of bright fluorescent graphene quantum dots for live cell staining and white LEDs Weitao Li,† Ming Li,† Yijian Liu,‡ Dengyu Pan,‡ Zhen Li,§ Liang Wang,*,† Minghong Wu*,§ †

Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical

Engineering, Shanghai University, Shanghai 200444, P. R. China ‡

Department of Chemical Engineering, School of Environmental and Chemical

Engineering, Shanghai University, Shanghai 200444, P. R. China §

Shanghai Institute of Applied Radiation, Shanghai University, Shanghai 200444, P.

R. China ABSTRACT: Here we introduce a simple, super-fast and scalable strategy that obtains graphene quantum dots (GQDs) within three minutes under microwave irradiation (MA-GQDs). The MA-GQDs exhibit excellent fluorescence quantum yields up to 35% in the optimum reaction condition. The MA-GQDs with single-crystalline and few-layers structure can reach the visible region with the longest absorption wavelength at 700 nm. Moreover, these ultra-bright fluorescence and stability MA-GQDs as phosphor and fluorescence probe could be efficiently applied in white light-emitting diodes and cell-imaging fields. The developed pathway to GQDs can provide unambiguous and remarkable insights into the design of high-fluorescence and few-defect GQDs, and expedite the applications of GQDs.

1. INTRODUCTION Graphene quantum dots (GQDs), as one of recently studied fluorescent nanomaterials, have attracted increasing number of attention and play a vital role in many fields on account of their excellent optical and eco-friendly properties.1-4 Until now, multiply routes have been applied to synthesis GQDs, which could be divided into two kinds: the one is top-down route including strong-acid oxidation stripping way and

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thermolysis,11,12 hydrothermal fusing13-17 and electron-beam irradiation.18 However, there are quite a few inferiors in these approaches, which limit their applications. The former usually chose a large number of carbon materials as precursors cutting into the small size of the GQDs. Because there were many uncontrollable factors in that process leading to the destruction of the internal molecular structure, the produced GQDs had a lot of defects; Among the latter, hydrothermal fusing was the most common method, they used small molecules such as citric acid as a precursor fused GQDs. Unfortunately, the long-time consuming of the process couldn’t accommodate the needs of the rapid development of modern society. Accordingly, looking for a quick, environmental-friendly and sustainable way to prepare GQDs is extremely urgent for the development of graphene field. Additionally, excellent optical properties of GQDs are required to ensure good performance in the applications of white light emitting diodes (LEDs) and cell-imaging. Consequently, it is potentially promising to explore alternative strategies to synthesize high-quality GQDs. Over other traditional synthetic approaches, the microwave route has the advantages of the super-rapid and uniform process in preparations of nanomaterials such as nanoparticles,19-25 metal-organic frameworks,26-30 graphene oxide31-35 and nanowire36-38 due to its dielectric heating. The benefits of the route contribute to setting aside an army of time and energy source, and providing a low-cost and high-efficient synthesis process. As a direct result of the reduction in reaction time, the controllability of materials’ growth is enhanced, likewise uniformity of microstructure is improved together.39-41 During all the GQD synthesis methods, the microwave-assisted route looks like the most simple, and high efficient way.42-45 Despite some results acquired by these reports above, there remain some drawbacks in the treatment. For instance, there are also low fluorescent quantum yields (QYs) of GQDs; the reaction times are as long as a hydrothermal process. To date, there are few reports on high QYs GQDs within several minutes via microwave. Recently, we used a small molecule of nitro-compound as precursor, with hydrothermal and electron-beam irradiation, to synthesize high-quality GQDs.18 Compared with other ACS Paragon Plus Environment

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small molecular precursors, nitro-compound is more energetic, easy to be functionalized, and usage to synthesize GQDs with excellent fluorescence characteristics. It seems to be an efficient microwave-assisted approach for GQDs using nitro-compound as precursor. Here we first report an ultra-fast preparation strategy of GQDs within 3 minutes by microwave (MA-GQDs). Despite the ultra-rapid way, the water-soluble MA-GQDs still have excellent fluorescence characteristics. The MA-GQDs exhibit excellent fluorescence QYs up to 35% in the optimum reaction condition. The MA-GQDs with single-crystalline and few-layers structure can reach the visible region with the longest absorption wavelength at 700 nm. Furthermore, yellow fluorescence MA-GQDs was used for white LEDs and cell-imaging. 2. EXPERIMENTAL SECTION 2.1. Method of GQDs’ Preparation. The chemicals were purchased directly without any further purification. Hydrazine hydrate, sodium hydroxide (NaOH) and other experimental materials are similar to our published article previously.10

Scheme 1. Schematic illustration of the preparation route for MA-GQDs in 3 minutes. MA-GQDs and MA-NGQDs were synthesized in a microwave equipment (Anton Paar Monowave 400). Scheme 1 illustrates the primary preparation procedure of the MA-GQDs in 3 minutes via microwave irradiation. A certain number of previously reported 1,3,6-trinitropyrene was added to a 10 mL microwave dedicated reactor glass tube with the certain concentration of NaOH under stirring condition.44 Then the dispersion were transferred into the 10 mL glass tube. After a rapid microwave reaction, the products were filtered through a 0.22 µm microporous membrane to remove insoluble carbon product, and further dialyzed in a dialysis bag for two days ACS Paragon Plus Environment


to remove small unfused molecules. MA-NGQDs were obtained under the same conditions, replacing NaOH with hydrazine hydrate. 2.2. Structural Characterization. The instruments for characterized the samples are similar to our published article previously,18 such as atomic force microscopy (AFM), transmission electron microscope (TEM), X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), Raman, X-ray photoelectron spectroscopy and so on. The photoluminescence (PL) QYs of MA-GQDs and MA-NGQDs were determined by comparing the integrated PL intensities (excited at 510 nm and 490 nm respectively) and the absorbency values using Rhodamine B in ethanol as the reference. 2.3. Preparation of White LEDs. The MA-GQDs (1 mL) were added to 1.6 g of ET-821A silica gel and 0.4 g of ET-821B silica gel, and then the mixture was stirred for 15 minutes (50 r·min-1). After that the mixture of MA-GQDs were added dropwise to a blu-ray chip device with emission wavelength at 450 nm, then the device was dried in an oven at 80 oC for 30 minutes. 2.4. Method of Cell Imaging and Cytotoxicity Assay. The method of culturing Hela cells and cytotoxicity assay adopt routine experimental method.18 The cells with a concentration of 20 mg·L-1 of MA-GQDs aqueous solution were examined under a confocal microscope (Leica TCS SP5) using lasers of 405 nm and 488 nm. For proving where the material internalized within cell, the Hela cells were cultured for 0.5 h and washed two times with Tris-HCL to remove excessive MA-GQDs, then 5 µL nuclear-tracker red dye (KGA263 NucView Red Live of KeyGEN BioTECH, China) with the concentration of 1 mM was added and incubated at 37 oC for 0.5 h. After 0.5 h of incubation, the Hela cells were then washed three times with Tris-HCL to remove dye. Then observing the cells under the same confocal microscope employing the lasers of 405 nm and 633 nm. 3. RESULTS AND DISCUSSION


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Figure 1. PL QYs of the MA-GQDs prepared at different time (a), temperature (b), the concentration of NaOH (c) and quality concentration of 1,3,6-trinitropyrene (d). Figure 1 demonstrated that the PL QYs of the MA-GQDs are strongly dependent on preparation conditions such as reaction time, temperature, NaOH concentration and quality concentration of 1,3,6-trinitropyrene. The 1,3,6-trinitropyrene (0.02 g·mL-1 in 5 mL 0.3 M NaOH solution) was irradiated at a temperature of 200 oC, the QYs increased rapidly to 35% with increasing the reaction time to 3 minutes, subsequently, the tendency maintained stability in the course of extending the reaction time. When 1,3,6-trinitropyrene (0.1 g) added to 5 mL 0.0-0.6 M NaOH solution, the reaction time was 3 minutes, and the temperature ranged from 100 oC to 230 oC, the QYs of the MA-GQDs also increased markedly from 15% to 33%, indicating that the reaction temperature and the concentration of NaOH had a significant effect on the reaction. The effect of the concentration of the precursor on the QYs of MA-GQDs was smaller compared to the previous. At last, the MA-GQDs colloid prepared under these optimal factors (3 minutes, 200 oC and 0.02 g·mL-1 in 5 mL 0.3 M NaOH solution) emitted bright yellow fluorescence under the irradiation of ultraviolet light and had a QYs up to 35% in Scheme 1. In addition, to illustrate the superiority of 1,3,6-trinitropyrene, the precursor was replaced by some other precursors, such as commercial ACS Paragon Plus Environment


1-nitropyrene, urea and citric acid, to synthesize the GQDs at the control requirements. The result was that merely 1-nitropyrene can fusion GQDs with low QYs (5%), whereas the others could not synthesis any GQDs, the result is shown in Figure S1. The contrast indicates that compared with other small molecular precursors, 1,3,6-trinitropyrene is more energetic, easy to be functionalized, and usage to synthesize GQDs with excellent fluorescence characteristics.

Figure 2. (a and b) TEM images and lateral size distributions of MA-GQDs (a) and MA-NGQDs (b). (c and d) HRTEM images of MA-GQDs (c) and MA-NGQDs (d) (Inset: fast fourier transform (FFT) patterns). (e and f) The lattice spacing of MA-GQDs (e) and MA-NGQDs (f) measured from the orange rectangle.


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Figure 3. (a and d) AFM images of MA-GQDs (a) and MA-NGQDs (d) (Inset: height distributions and height profile along the white line). (b and e) 3D morphology of height of MA-GQDs (b) and MA-NGQDs (e). (c and f) 3D lines of height of MA-GQDs (c) and MA-NGQDs (f). The particle size distributions of MA-GQDs and MA-NGQDs were shown in the Figures 2a,b with average diameters of 4.12 nm and 3.33 nm, respectively, which is consistent with the hydrodynamic volume (as measured by dynamic light scattering (DLS)) of these GQDs in the aqueous solution (Figure S2 and S3). The HRTEM images (in Figure 2c,d) gave evidence for the single-crystalline structure of GQDs. The crystal planes of MA-GQDs and MA-NGQDs were 0.22 nm and 0.25 nm, respectively, a little larger consistent with the graphene (100) plane (0.21 nm), as shown in Figure 2e,f. That is probably because OH groups are inserted in the MA-GQDs, whereas the MA-NGQDs not only have OH groups but also have NH2 groups, so their lattice spacing is larger. All of the corresponding FFT images of GQDs (inset in Figure 2c,d) had a hexagonal carbon network, respectively. These two FFT graphs can also illustrate that the MA-GQDs has better crystallinity than that of MA-NGQDs. Figure 3a,d using AFM images displayed the dispersivity of MA-GQDs and MA-NGQDs on the mica sheet, in which yellow spots are regarded as GQDs by their sizes. Figures 3b,c,e,f are the corresponding three-dimensional plots of Figure



3a,d, respectively. All of these figures in Figure 3 revealed that these GQDs had merely several layers of graphene. The synthesized materials have a good crystalline, graphene-like planar structure, which could be obtained by HRTEM images and FFT patterns. Meanwhile, the thickness of the majority synthesized materials is only 0.4-2 nm, exhibiting single or few layer of graphene. As a result, it indicates that the synthesized materials are real GQDs. b

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Figure 4. XRD patterns (a) and FT-IR spectra (b) of MA-GQDs and MA-NGQDs. Survey XPS spectra (c), high-resolution C 1s (d), O 1s (e) and N 1s (f) spectra of MA-GQDs and MA-NGQDs The XRD patterns of MA-GQDs and MA-NGQDs were presented in Figure 4a. The nearly same (002) interlayer spacing of the MA-GQDs and MA-NGQDs was 3.44 and 3.47 Å, respectively. The (002) diffraction peak of MA-GQDs became more narrowed that of MA-NGQDs, indicating that MA-GQDs have a better crystallization effect.44 FT-IR spectra (Figure 4b) are used to determine the surface groups on the surface of MA-GQDs and MA-NGQDs. The significant absorption bands at 3400– 3500 cm-1 represent stretching vibrations of O-H. This claims that the surface of the MA-GQDs and MA-NGQDs have a lot of hydroxyl functional groups, which is the reason for interpreting the phenomena that the GQDs have good water solubility. The bands at 1586, 1268 and 835 cm-1 represent the vibrational absorption band of C=C, C-O (alkoxy) and C-H, respectively. The unique bands at 3200 cm-1 on the


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MA-NGQDs describe the amino-functionalized group, which is derived from hydrazine hydrate. In addition, we also carried out XPS characterization of MA-GQDs and MA-NGQDs. The full scan XPS spectrum, as shown in Figure 4c, presents three peaks at 286, 400, and 532 eV, which corresponds to C1s, N1s and O1s, respectively. The high-resolution spectrums of each region revealed further functional group on the surface of MA-GQDs (Figure 4d,e,f). The peaks (284.6 eV, 285.4 eV, and 288.6 eV) for C1s spectrum indicate the covalent bonds of C=C, C-NO2 and C-OH. In the O1s spectrum of the GQDs, the two O element signals, corresponding to O-H, and O-N, are likewise seen. The N1s spectrum of the GQDs can be divided into two peaks locating at 399.9 eV, and 405.9 eV, indicating N-C, and N-O groups. The XPS results illustrated that the NO2 groups of GQDs are not eliminated totally in the microwave reaction process. Whereas, there is an obvious peak at 399.2 eV in Figure 4f, indicating MA-NGQDs contain NH2 groups, which is different from MA-GQDs.

Figure 5. High resolution microscope Raman images MA-GQDs (a) and MA-NGQDs (c) (Notice: the color represents ratio of D and G peak in (a and b).



Typical Raman spectra of the red box area in left. Figure 5 exhibited the MA-GQDs’ Raman spectra under the Raman imaging spectrometer. Figure S4 is a photograph of the GQDs’ powder and Figure 5a is a chromatic image that represents the ratio of G peak at 1577 cm-1, representing the crystalline sp2 hybrid structure, and D peak at 1360 cm-1, representing the disordered sp3 hybrid carbon. The Figure 5b showed that the ratio of IG/ID is mostly 1.2 up to 1.4, indicating that the MA-GQDs have been perfect crystallized, fewer defects, less sp3 hybrid carbon under microwave-assisted conditions, however, the MA-NGQDs are not good crystallized, because their ratio only is 0.95 in Figure 5c,d.

Figure 6. UV-vis absorption, PL and PLE spectra of MA-GQDs (red traces) and MA-NGQDs (cyan traces) (a). Photographs of MA-GQDs (left) and MA-NGQDs (right) taking under visible and UV lights in (b). PL spectra of MA-GQDs (c) and


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MA-NGQDs (d) at different excitation wavelengths. Time-resolved PL spectrums of MA-GQDs (e) and MA-NGQDs (f) Figure 6 shows the optical properties of the GQDs prepared by direct heating the molecules under microwave assistance. Figure 6a displayed the UV-Vis absorption spectra of MA-GQDs and MA-NGQDs in aqueous solution, the fluorescence optical spectra at 510 nm and near 490 nm and the fluorescence emission spectra at 560 nm and 540 nm, respectively. There is a certain red-shift (40 nm) with the previously reported hydrothermal method44, which may be due to the increase of the diameters of MA-GQDs. From the UV absorption spectra, it can be seen that the absorption regions of MA-GQDs and MA-NGQDs can reach the visible region with the longest absorption wavelength at 700 nm. Compared with MA-NGQDs, MA-GQDs showed significant bandgap absorption and there was a good absorption in the visible region at 500 nm and even longer wavelengths. Interestingly, MA-GQDs were able to emit more bright yellow fluorescence when irradiated with 365 nm UV lamp, while MA-NGQDs emit green fluorescence (Figure 6b), indicating that a uniform and large size of the MA-GQDs were well synthesized under microwave assistance, which may be related to the more hydroxyl groups in the reaction. Among the carbon-based fluorescent materials’ characterization, the excitation dependency of the emission wavelength intensity is a conventional and important mean. These properties can not only reflect the uniformity of the sizes in the nanomaterials but also interpret the emission-active sites of the nanomaterials. Some significant optical properties of the two kinds of GQDs are different when they are eluted with a series of single-color shades of various wavelengths (300-500 nm). Figures 6c,d are the fluorescence spectra with the different excitation wavelengths of MA-GQDs and MA-NGQDs, respectively. The fluorescence intensity of MA-GQDs and MA-NGQDs increase with enhance of the excitation wavelength, and then reach the strongest at 510 nm and 490 nm, respectively. Then the intensity start decreasing with strengthen of excitation wavelength. Wherefore the optimal excitation wavelengths for MA-GQDs and MA-NGQDs are 510 nm and 490 nm, respectively, which corresponds to Figure 6a. Furthermore, the maximum emission peak of MA-GQDs depending on the excitation ACS Paragon Plus Environment


wavelength, ranges from 360 nm to 520 nm, is almost no moving, therefore the MA-GQDs have excitation-independent prosperity. In opposite, in a narrower range of excitation wavelength (410-510 nm), MA-NGQDs’ fluorescence emission peak has an obvious movement (Figure 6d), which illustrates a vigorous fluorescence excitation-dependent behaviors. The phenomenon indicates that MA-GQDs are more uniform in size than MA-NGQDs. Moreover, a monoexponential decay feature was observed from the PL decay curve of MA-GQDs and MA-NGQDs (Figure 6e,f). The observed lifetime of the MA-GQDs and MA-NGQDs was 3.4 and 4.8 ns, respectively.

Figure 7. Comparison chart of PL intensity of MA-GQDs and MA-NGQDs before and after drying at 60 oC (a). Insets: Photographs of MA-GQDs and MA-NGQDs taking under UV lights. Dependence of PL intensity of MA-GQDs and MA-NGQDs on storage times (b). Dependence of PL intensity and Zeta potential of MA-GQDs on pH values (c). To further explore the fluorescent stability of MA-GQDs and MA-NGQDs, the solution of MA-GQDs and MA-NGQDs were dried at 60 oC, and subsequently, the solid powder was redissolved with the same volume of deionized water. The fluorescence intensity of them including before and after drying was measured. As shown in Figure 7a. Fluorescent intensity of GQDs show no significant alter, and also emit bright fluorescence under irradiation by a UV light. The long-term fluorescence stability of MA-GQDs and MA-NGQDs were also tested for seven days, the result shown in Figure 7b indicate that the daily fluorescence intensity of MA-GQDs changed slightly whereas that of MA-NGQDs changed hugely. The MA-GQDs are more stability than MA-NGQDs, which may be related to the robust electronegativity of the GQDs solution (Figure S5 and S6). For the reason that MA-GQDs would be


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applied to biological experiments, it is essential to measure the tolerance of pH. In Figure 7c, we found that the fluorescence intensity changed slightly under acidic conditions, and it is stable in both neutral and alkaline. Which is related to the presence of many OH functional groups at the edge of the MA-GQDs. This is also obtained from the Zeta potential of MA-GQDs, which is more negatively charged with increasing the basicity.

Figure 8. (a) Placement of the MA-GQDs emission spectra on the CIE 1931 chromaticity chart. (b) Off and on of the digital photographs of the working device with MA-GQDs. (c) EL spectra of the white LEDs with MA-GQDs applied as phosphors. The MA-GQDs was further applied in white LEDs field for resolving the issue of warm white light due to its property of fluorescent redshift. We finally developed a promising white LEDs via ceaselessly adjusting the strength of the current and the manifold quality of MA-GQDs. And the composition is shown in the Figure 8b and consists of a LED chip and the MA-GQDs. Figure 8c exhibits the EL spectrum of the white LEDs with the current of 30 mA and 0.15 g MA-GQDs, suggesting that MA-GQDs have very favorable potential application of white lighting. It can be seen from the measurement results in Figure 8a that the coordinate of the CIE is (0.3734, 0.3630) in the warm white-light range, which is highly close to the coordinate of the high-purity white light (0.33, 0.33). The color-temperature received is 4098 K, belonging to the warm white, which is due to the redshift of fluorescent of the MA-GQDs with the aid of microwaves. The above data proves that the microwave ACS Paragon Plus Environment


method may be a promising improvement for optoelectronic components. Particularly, the red-shift effect of MA-GQDs will serve as a novel fluorescent nanomaterial that would lay a foundation for enrichment in the future optoelectronics field.

Figure 9. Cell imaging of MA-GQDs using Hela cells. (a and c) Confocal fluorescence image at 405 nm (a) and 488 nm (c) excitation image, bright field image (b) of Hela cells. (d) Cytotoxicity assessment of MA-GQDs at higher doses for incubation time varied from 6 to 48 h using Hela cells. The MA-GQDs and MA-NGQDs as the bioluminescent probe were used to image Hela cells. Both MA-GQDs (Figure 9a-c) and MA-NGQDs (Figure S7) can be excited at 405 nm and 488 nm, and all of them can image the Hela cells clear. To determine the localization of MA-GQDs in the HeLa cells, the z-axis cell imaging of HeLa cells is shown in Figure S8, demonstrating that the MA-GQDs entered into the cytoplasm of the HeLa cells. Meanwhile, the merged co-localization image with a nucleus-tracker dye was shown in Figure S9, indicating that the labeled compartment is the cytoplasm apparatus. Taken together, these results illustrated that MA-GQDs were localized in the cytoplasm apparatus of HeLa cells. At the same time, the


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cytotoxicity of MA-GQDs was tested by using the MTT method. As shown in Figure 9d, if the concentration is less than 60 mg·L-1, the toxicity is almost none even when the cells are cultured for up to 48 h. When the MA-GQDs concentration reached 80-100 mg·L-1, the cells cultured in 6-12 h, the cytotoxicity is also overwhelmingly low. The cell activity was still above 60% when the incubation time and the MA-GQDs concentration are more than 48 h and 100 mg·L-1, respectively, indicating the low toxicity of MA-GQDs. 4. CONCLUSIONS To sum up, the microwave-assisted synthesis of MA-GQDs has been clarified to be simple usage, high efficient, and easy repetition. High QYs MA-GQDs can be obtained in as little as 3 minutes, as opposed to several hours in typical hydrothermal reactions. The dielectric heating of the molecular through the microwave makes the GQDs stability and few defect. It is very significant that microwave-assisted heating produced MA-GQDs with QYs as high as 35%. The MA-GQDs with single-crystalline and few-layers structure can reach the visible region with the longest absorption wavelength at 700 nm. Moreover, these high-fluorescence and stability MA-GQDs as phosphor and fluorescence probe could be efficiently applied in white LEDs and cell-imaging fields. This work supplied an significant novel route for developing future GQD-based phosphor and fluorescence probes, which have multiply potential applications in optoelectronic devices, biosensors, and bio-imaging. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at AUTHOR INFORMATION Corresponding Authors



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*E-mail: [email protected] *E-mail: [email protected] ORCID Liang Wang: 0000-0002-3771-4627 ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (No. 21671129, 21571124, 21671131), the Shanghai Sailing Program (No. 16YF1404400), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT17R71). We thank the Laboratory for Microstructures of Shanghai University. REFERENCES (1) Dong, H.; Dai, W.; Ju, H.; Lu, H.; Wang, S.; Xu, L.; Zhou, S. F.; Zhang, Y.; Zhang, X. Multifunctional Poly(L-lactide)-Polyethylene Glycol-Grafted Graphene Quantum

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