Green, Rapid, and Universal Preparation Approach of Graphene

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Green, rapid, and universal preparation approach of graphene quantum dots under ultraviolet irradiation Jinli Zhu, Yanfeng Tang, Gang Wang, Jiarong Mao, Zhiduo Liu, Tongming Sun, Miao Wang, Da Chen, Yucheng Yang, Jipeng Li, Yuan Deng, and Siwei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11525 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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ACS Applied Materials & Interfaces

Green, Rapid, and Universal Preparation Approach of Graphene Quantum Dots under Ultraviolet Irradiation Jinli Zhu,1, † Yanfeng Tang,1, *, † Gang Wang,1, *, ‡ Jiarong Mao,† Zhiduo Liu,# Tongming Sun,† Miao Wang,† Da Chen, ‡ Yucheng Yang,& Jipeng Li,$ Yuan Deng,$ and Siwei Yang,* , & †

School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, PR China.

‡

Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, PR China. #

State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, 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. $

Department of Ophthalmology, Shanghai Ninth People’s Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, 200011, PR China. KEYWORDS: Green synthesis, Graphene quantum dot, Photoluminescence, High quantum yield, Bio-imaging.

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ABSTRACT: It is of great significance and importance to explore a mild, clean, and highefficient universal approach for the synthesis of graphene quantum dots. Herein, we introduced a new green, rapid, and universal preparation approach for graphene quantum dots via the free radical polymerization of oxygen containing aromatic compounds under ultraviolet irradiation. This approach had a high yield (86 %) and the by-products are only H2O and CO2. The obtained graphene quantum dots were well-crystallized and showed remarkable optical and biological properties. The colorful different sized graphene quantum dots can be used in fluorescent bioimaging in vitro and vivo. This approach is suitable not only for the preparation of graphene quantum dots but also for heteroatom doped graphene quantum dots.

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INTRODUCTION As a typical hot photoluminescence (PL) functional material, graphene quantum dots (GQDs) are particularly important since their outstanding optical properties, biological properties,

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and semiconducting properties.

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catalytic performance,

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The low biotoxicity and high

stability make GQDs an ideal PL functional material in fluorescent bio-imaging. Moreover, based on the diversified and modifiable edge structures, the GQDs show diversified functions which take prominent advantages over traditional fluorescent dyes. 11 Yang et al. reported a new reversible fluorescent switch for monitoring the oxidative hydroxyl radical and reductive glutathione based on selenium doped GQDs.

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This brand-new GQD-based fluorescent switch

shows a rapid response in viability of HeLa cells. Currently, significant efforts have been pursued in preparation of GQDs, including top-down and bottom-up approaches. 12 Top-down approaches involve the cutting and exfoliation of single graphitic crystallite by physical or chemical means.

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The chemical cutting of sp2 carbon

materials is always carried out by using strong oxidants (such as KClO3 and KMnO4) and acids (such as H2SO4). In spite of its high efficiency, the difficulty in removing the by-products (such as inorganic salts and acids) has limited the use of chemical cutting in large scale preparation of GQDs. On the other hand, due to the low efficiency of physical cutting approaches, it is impossible to be applied in large scale preparation. The bottom-up approaches refer to series of reactions in which GQDs are synthesized from polycyclic aromatic compounds or other aromatic structures of molecules. 12,14 This approach precisely controls the properties of the final products. Unfortunately, the crystallinity of GQDs produced by traditional solvothermal bottom-up approaches is unsatisfactory. Due to the poor crystallinity, it is more reasonable to call the products obtained via solvothermal bottom-up approach “carbon dots” rather than “GQDs”. This can be ascribed to the unordered polymerization process in solvothermal bottom-up conditions. What’s more, currently, there is still no universal approach for the preparation of GQDs and 3



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heteroatom doped GQDs. To solve above problems, the developing of a mild, clean and highefficient universal approaches for preparation of GQDs with easy removed by-products is especially necessary. Herein, we reported a universal preparation approach of both GQDs and heteroatom doped GQDs. The oxygen containing aromatic compounds polymerized under ultraviolet irradiation. The yield of GQDs with good crystalline and high quantum yield (0.7-0.8) in our approach was high (86%). The matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS) and free radical capturing experiment demonstrated the proposed free radical polymerization progress. The lateral size of GQDs can be controlled by adjusting the concentration of the precursor. Moreover, due to the good stability and low biotoxicity, we demonstrated the potential applications of multi-color GQDs for fluorescent bio-imaging in vitro and in vivo. Finally, the universality of our approach to the synthesis of heteroatom doped GQDs was revealed.

EXPERIMENTAL SECTION Chemicals Salicylic acid and pyridine-2,6-dicarboxylic acid (Aladdin Reagents Co. Ltd., Shanghai, China) were demarcated pure and dispensed with purification. Ultra-pure water of resistivity 18.2 MΩ prepared on a Milli-Q system was used throughout all experiments. Characterization methods Transmission electron microscope (TEM, FEI Titan 80-300) measurements were carried out on a spherical aberration-corrected TEM at 80 kV. X-ray photoelectron spectra (XPS) were carried out on a PHI Quantera II system. MALDI-TOF MS spectra were recorded on an ABI Voyager De pro spectrometer. Fluorescent emission-excitation spectra were performed on a PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at 25 °C. The 4


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established procedure was utilized to determine the quantum yield (Φf). The UV-vis spectra were collected on a UV5800 Spectrophotometer. Rhodamine 6G solution was chosen as the standard. For the absolute values calculation, the standard reference sample has a fixed and known fluorescence quantum yield value. The absorbances in the cuvette were kept under 0.1 under the excitation wavelength (data are shown in Table S1) to minimize re-absorption effects. The timecorrelated single-photon counting (TCSPC) technique (Hydra Harp 400, Pico Quant) was utilized to determine time-resolved fluorescence behavior. The samples were excited by a frequency-doubled titanium: sapphire oscillator laser with an approximate pulse duration of 150 fs, and a repetition rate of 80 MHz (Chameleon, Coherent). Spectrometer (iHR550, Horiba Jobin Yvon) was used to determine the fluorescence emission. The spectra were recorded with 300 lines/mm gratings and then detected by a photomultiplier tube. The yield calculation of the preparation approach was described by the ratio of actual yield and theoretical yield ((actual yield/theoretical yield)×100%). Cell experiment methods In vitro bio-imaging: The human uveal melanoma (OCM-1) cells were cultured at 37 °C in humid atmosphere with 5% CO2. Before GQDs treatment, cells were cultured in a 96-well plate for 24 h and added into serial dilutions of GQDs. After 24 h incubation, colorimetric 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays was utilized to determine the relative viabilities of cell samples. Cells were lysed by acidulated sodium dodecyl sulfate (SDS) solution. Microplate reader (Bio-Rad 680, USA) was utilized to determine the absorbance. The spectra were recorded at 570 nm. Cell with triplicate were used in at least three independent experiments. In vivo bio-imaging: OCM-1 cells were harvested with trypsinization and washed with phosphate-buffered saline (PBS; Gibco, Carlsbad, CA, USA). They were injected 5



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subcutaneously into the right flank of male nude mice in a PBS volume of 100 µL. After tumor formation by subcutaneous injection of cells one week later, the GQDs aqueous solution (100 µL, 1 mg mL-1) was intratumorally injected into nude mice with intratumoral injection, then the in vivo bio-imaging were assessed using an in vivo vision systems and lumazone imaging system.

RESULTS AND DISCUSSION Synthesis and characterization of GQDs

Figure 1. Synthesis and characterization of GQDs. (a) schematic diagram showing the synthesis progress of GQDs. (b) Plan-view TEM image of GQDs thus obtained, with inset showing lateral size distribution histogram of GQDs. (c) HRTEM image of a single GQD particle in (b). Inset: Fourier analysis pattern of GQDs. (d) XPS survey spectrum, (e) C 1s and (f) O 1s spectrum of GQDs. (g) The Raman spectrum acquired from the GQDs spin-coated on SiO2 substrate.

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Figure 1 illustrates the GQDs synthesis process. Typically, 500 mL, 100 mM salicylic acid aqueous solution was exposed under ultraviolet source (wavelength of ultraviolet source 185-254 nm, 250 W, Guoda Shanghai, China Co., Ltd.) at 50 oC for 2 h and then cooled to 5 oC. The light yellow GQDs aqueous solution was obtained. The GQD yield from salicylic acid was approximately 86%. Compared with other approaches, our approach was rapid, easily operable, and highly efficient. As shown in Table 1, almost all of other cutting approaches had hard-toremove by-products including organic molecules and inorganic salts. Moreover, due to the long time (4-72 h) of strong oxidation process under high reaction temperature (120-250 oC), the yields of those shearing approaches were lower than 40 %. Besides, the solvothermal bottom-up approaches always require high reaction temperature. Beyond that, due to the incomplete polymerization process, the yields of GQDs were also lower than 50 % and the GQDs were of poor crystallinity. By contrast, the yield of our approach reached up to 86%. The mild reaction temperature (50 oC) and rapid preparation process (2 h) were also the significant advantages of our approach. Table 1. A brief summary of GQD preparation approaches. Reaction temperature (oC) Time (h) Yield 50 2 86 % 200 10 5% 200 8 34.5 % 240 12 32 % Cutting 120-180 12-36 15 %

Solvothermal bottom-up approaches

Others a

250 200 170 160 240 180 180 100-120 160 800-1200

4 72 4 4 2 51 24 24 4 72

26 % 45 % NA 38 % 33.6 % 36.5 % 48 % 20 % 15.59 % 63 %

By-products H2O and CO2 Inorganic salts a Inorganic salts a No by-products Organic molecule b, Inorganic salts a Inorganic salts a NA Large sized particles c Large sized particles c Large sized particles c Oligomers Large sized particles c No by-products No by-products No by-products

Ref. This work 15 16 2 6 7 17 18 19 20 21 22 23 24 17

The inorganic salts were sulfate, nitrate and others. b Such as condensed heterocyclic compounds with low molecular weight. c

The large sized particles were carbonized polymers with large size (larger than 50 nm) in solvothermal bottom-up approach. NA: Not mentioned.

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Plan-view TEM image in Figure 1(b) shows the uniform GQDs with a lateral size distribution about 1-5 nm with the average diameter of 3 nm (Figure 1(b) inset). In order to further explore the microscopic structures, the GQDs were characterized by high-resolution TEM (HRTEM). Figure 1(c) depicts the microscopic structures of the synthesized GQDs. Honeycomb lattices are obviously observed from the GQDs. The HRTEM image displays the perfect lattice structure in a range larger than 2 nm and the high-quality of GQDs can be inferred. The Fourier analysis pattern of the GQDs shows only one set of hexagonal diffraction pattern, indicating the single crystalline lattice structure of obtained GQDs, as shown in the inset of Figure 1(c). AFM observations (Figure S1) reveal highly dispersed GQDs on SiO2 substrate with a typical topographic height of approximately 0.4 to 0.6 nm, indicating that most GQDs consist of monolayer. No obvious diffraction peak can be observed in X-ray diffraction (XRD) pattern of GQDs (Figure S2) which indicates the monolayer structure of obtained GQDs was achieved. Moreover, the XRD results didn’t show the amorphous signal of GQDs, which agrees with the TEM results. XPS analysis was undertaken to understand the structure of GQDs thus obtained (Figure 1(d-f)). The signal of carbon (C) and oxygen (O) can be observed at ca. 284.2 and ca. 532 eV, respectively. The intensity ratio of O to C is 9.3%. The low oxygen content provides evidence that the GQDs have good crystallinity and less surface oxygen-containing groups. The well-fitted C 1s XPS spectrum (Figure 1(e)) can be fitted by three peaks. These peaks located at 282.85, 284.80 and 286.15 eV can be corresponded to the signals of C=C, C-O and C=O, respectively.2 The O 1s spectrum (Figure 1(f)) coincides with the result of C 1s spectrum. The peak located at 531.7 and 531.3 eV can be assigned to the signals of C=O and C-OH/C-O-C, respectively.25 FTIR spectrum (Figure S3) of GQDs shows the signal of -COOH (3500, 1730 cm-1), -OH (3500 cm-1), C=O (1730, 1550 cm-1), C-O-C (1150 cm-1) and aromatic nucleus (700-500 cm-1), which is consistent with the XPS results. Figure 1(g) depicts the Raman spectrum acquired from the 8


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GQDs spin-coated on SiO2 substrate showing three primary bands, i.e., the 2D band at around 2710 cm-1, the D band at around 1340 cm-1 and the G band at around 1580 cm-1. 26 The relatively high intensity of the D and G band with a peak intensity ratio of ID/IG ≈ 0.25 suggests the presence of defects in the GQDs. The attenuation in the D band is regarded as the decrease of the GQDs edges. Reaction mechanism under ultraviolet irradiation

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Figure 2. Mechanism of polymerization process. (a) schematic diagram for the reaction mechanism in synthesis process under ultraviolet irradiation. (b) MALDI-TOF MS spectrum of the reactant solution. Concentration of GQDs in reactant solution under different time, (c) without DPPH, (d) with 100 ppm DPPH. (e) Average lateral size of GQDs under different concentration of precursor (6-100 mM). The reaction mechanism in synthesis progress under ultraviolet irradiation was proposed. As shown in Figure 2(a), high-energy photons of wavelength 185-254 nm excited solvent molecules, resulting in a large number of free radicals. The salicylic acid captured free radicals rapidly and produced secondary aromatic free radicals. These aromatic free radicals were then reorganized by coupling process into 2D graphene fragments (GQDs). The MALDI-TOFMS was carried out to confirm above proposed mechanism. The MALDI-TOF-MASS spectrum of GQDs displays the bands at m/z of 1800-3000 (Figure S4), indicating 80-150 aromatic nucleuses in single GQD which matches the TEM results. Figure 2(b) shows the MALDI-TOFMS spectrum of reactant solution. The spectrum with the strongest band located at m/z=139.1 can be assigned to the signal of protonated precursor (salicylic acid). The peak located at m/z=137.2 can be assigned to the signal of produced secondary aromatic free radicals. The repeating unit of m/z=176.2 can be proved to be an anthracene nucleus bonded to the edge of GQDs. Radical trapping experiment was carried out to further confirm the important role of free radicals in our approach. Figure 2(c) shows the concentration of GQDs in reactant solution under different time. The concentration of GQDs shows a linear growth behavior at first 90 mins. The linear growth behavior implies that the reaction process followed zero-order reaction dynamics during first 90 mins. The corresponding rate constant is 4.5×10-4 mol L-1 min-1. In the following 30 mins, the growth rate slowed down and reached a stable concentration of 5.2 mg mL-1 after 120 min. In comparison, when the radical scavenger (100 ppm, 1,1-diphenyl-2-picrylhydrazyl radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl, DPPH) was introduced (Figure 2(d)), the corresponding rate constant at first 90 min is 3.8×10-5 mol L-1 min-1 which is much smaller than the rate constant without radical scavenger. The stable concentration is 0.51 mg mL-1 after 120 10


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min. This indicated that the free radical process is the main polymerization process in our approach. It's worth noting that, the size of GQDs shows no obvious change with the increasing reaction time (Figure S5). However, the size control of GQDs is quite easy to achieve by changing the concentration of precursor. As shown in Figure 2(e), when the concentration of precursor is 6, 10, 20, 30, 40, 50, 60, 80 and 100 mM, the average lateral size of obtained GQDs is 22, 16, 10, 7.1, 5.8, 4.8, 4.2, 3.6 and 3.0 nm, respectively. The lateral size has high sensitivity to the concentration of precursor, which can be due to the large amount of aromatic free radicals in highly concentrated salicylic acid aqueous solution. The highly concentrated aromatic free radicals resulted in the rapid polymerization reaction, and the abundant nucleation center depressed the further growth of GQDs. Optical properties of GQDs

Figure 3. Optical properties of GQDs. (a) Uv-vis absorption, PLE and PL spectrum of GQDs. (b) EIC chromaticity coordinates of GQDs aqueous solution. (c) The PL intensity of GQDs aqueous solution under ultraviolet radiation (150 W mercury lamp with a wavelength of 320 nm). 11



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F and F0 is the PL intensity of GQDs aqueous solution (0.1 mg mL-1) at special time and t=0, respectively. (d) PL intensity of GQDs aqueous solution (0.1 mg mL-1) when diversified ions (0.1 M) were introduce. F and F0 is the PL intensity of GQDs aqueous solution with and without ions, respectively. (e) A summary of φ and emission wavelengths of GQDs. Figure 3 depicts the optical properties of GQDs. As shown in Figure 3(a), the Uv-vis absorption spectrum of GQDs displays a typical absorption band at around 230 nm (which can be attributed to the π-π* transition of aromatic sp2 domains) and a long tail extending into the visible range. The band located at 260 nm can be due to the n-π* transition between the oxygencontaining groups and sp2 domains. For the PL spectroscopy, the maximum excitation and emission wavelength is 355 and 460 nm, respectively. The quantum yield (φ) of GQDs is 0.81. The Stokes shift (∆νSt) and the full-width half-maximum (FWHM) of these GQDs are 105 nm and 100 nm, respectively. These features clearly indicate that the GQDs have a low energy loss and a weak self-absorption effect.26 Figure 3(b) shows the Commission International d’Eclairage (CIE) chromaticity coordinates of GQDs. The CIE chromaticity coordinates of the GQDs are (0.156, 0.175), indicating that the GQDs emit blue PL. The PL decay of GQDs was characterized by a time-correlated single photon counting technique and fitted well with a bi-exponential decay as shown in Figure S6. The PL lifetime (τ) was dominated by a long decay component of 2.4 ns (88%) plus a small dedication from the short decay of 0.6 ns (12%). The weighted-average lifetime was nearly 2.2 ns. To combine the φ with τ by κr=φ/τ, we got the fluorescence radiative rate (κr).27 The κr of GQDs was 3.6×108 s-1. The high κr implied the excellent electronic transition probability in the GQDs, which confirmed the aforementioned n-π* transition model therein.6 It is noteworthy that no excitation wavelength dependent PL behavior was examined. With the increased excitation wavelength (from 300 nm to 400 nm), these GQDs exhibit a maximum emission wavelength shift of 10 nm (Figure S7). The excitation wavelength independence can 12


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be due to the less and simplex surface groups of GQDs. Moreover, the GQDs also show excellent stability and antijamming capability. Figure 3(c) shows the photo-stability of GQDs. The PL intensity decreases less than 10 % after 72 h ultraviolet radiation (150 W mercury lamp with a center wavelength of 320 nm). Meanwhile, the PL intensity of GQDs shows no obvious change when diversified ions were introduced. As shown in Figure 3(d), the introduction of Li+, Na+, K+, Mg2+, Ca2+, Fe3+, Co2+, Ni2+, Mn2+, Cu2+, Zn2+, Al3+, Ag+, Cd2+, Pb2+, Hg2+, Cl-, Br-, SO42- and NO3- (the concentration of the ions is 0.1 M) has no effect on the PL property of GQDs. Moreover, the PL emission wavelengths of GQDs can be easily adjusted by the control of lateral size (Figure S8-10). The emission wavelength is 460, 551, 579, 602 and 651 nm when the lateral size of GQDs is 3.0, 4.2, 5.8, 7.1 and 16 nm (GQD, GQD-1, GQD-2, GQD-3, and GQD4), respectively. The φ of GQD, GQD-1, GQD-2, GQD-3 and GQD-4 is 0.81, 0.77, 0.83, 0.71 and 0.75, respectively, which is higher than most previously reported GQDs (Figure 3(e)).1,2,6,10,21,28-48 Application of GQDs in fluorescent bio-imaging

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Figure 4. Application of GQDs in fluorescent bio-imaging. (a) Metabolic activity of OCM-1 cells hatched with different concentrations of GQDs. (b) TEM observation of OCM-1 cells incubated with GQDs. The red arrows show the GQDs in cells. (c) PL spectra of GQD, GQD-1, GQD-2, GQD-3 and GQD-4 (the excitation wavelength is 355, 440, 480, 550 and 595 nm respectively). Fluorescent microphotograph (scale bar: 5 µm) of OCM-1 cells incubated with (d) GQD, (e) GQD-1, (f) GQD-2, (g) GQD-3 and (h) GQD-4. (i) Bright-field microphotograph of the nude mice tumor. (j) Photoacoustic image of tumor-bearing mice incubated with GQD-4. (k)

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Bright-field and photoacoustic image of tumor-bearing mice incubated with GQD-4. White circles highlight the tumor site. The practical application of these GQDs for fluorescent bio-imaging was further investigated. The OCM-1 cell line was used to verify the in vitro cytotoxicity of GQDs. The metabolic activity of OCM-1 cells treated with different concentrations of GQDs (0-500 µg mL-1, Figure 4(a) and S11) was observed. The GQDs were added in the cells cultured in 96 well-plates and then incubated for 24 h, followed by a standard assay which was proceeded to measure the cell viabilities after the GQDs treatments. No significant reduction in cell viability was observed after even under high GQDs concentrations (500 µg mL-1), demonstrating that the GQDs produced by our approach were not distinctly toxic in vitro. Further TEM observation (Figure 4(b)) shows the GQDs concentrated in cell by endocytosis. Colorful GQD, GQD-1, GQD-2, GQD-3, and GQD-4 aqueous solutions (Figure (4c)) were incubated respectively to show their bio-imaging ability (Figure (4d-4h)). The multi-color PL can be observed inside the cells, indicating all these GQDs have been internalized by the OCM-1 cells and could be regarded as a kind of efficient bio-imaging material. The red emitted GQD-4 was selected for further application evaluate for in vivo fluorescent bio-imaging. The properties of the GQD-4 and its bio-imaging ability in vivo were assessed using an in vivo vision systems. After 1×106 OCM-1 cells were collected and injected into nude mice with subcutaneous injection for one week, 0.05 mg mL-1 GQD-4 aqueous solution was intratumorally injected into nude mice with intratumoral injection. The Figure 4(i-k) shows the bright-field microphotograph of tumor-bearing mice (i), photoacoustic image of tumor-bearing mice treated with GQD-4 (j), and bright-field and photoacoustic emerged image of tumorbearing mice treated with GQD-4 (k). After the intratumoral injection of GQD-4, the bright red PL can be observed. This indicates that the GQDs with high quantum yield, low biological

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toxicity, high stability, and suitable emission wavelength could be used as a kind of efficient invitro and in-vivo bio-imaging material. Universality of our approach in synthesis of heteroatom doped GQDs

Figure 5. Synthesis of heteroatom doped GQDs. Emission wavelength of N-GQDs with different doping concentration. The emission wavelength is 579, 567, 519, 502, 473, 450, 427, 377, 336 and 307 nm when the doping concentration is 0.3, 1.6, 2.1, 2.9, 3.6, 4.2, 5.3, 6.4 and 7.4 at. %, respectively. All of the above results indicate that the polymerization process of salicylic acid under ultraviolet irradiation is an effective and rapid approach to get GQDs with good optical and biological properties. Finally, we demonstrated the universality of our approach to the synthesis of heteroatom doped GQDs. 2,6-Pyridinedicarboxylic acid was selected to synthesis N doped GQDs (XPS results are shown in Figure S12-13). Moreover, with different doping concentrations the N-GQDs can be obtained by the polymerization process in the mixture of 2,6pyridinedicarboxylic acid and salicylic acid under ultraviolet irradiation. When the molar ratio of salicylic acid to 2,6-pyridinedicarboxylic acid was 50:1, 30:1, 25:1, 15:1, 10:1, 6:1, 4:1, 2:1 and 1:1, the doping concentration (atoms percent: at. %) of obtained N-GQDs was 0.3, 1.6, 2.1, 2.9, 16


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3.6, 4.2, 5.3, 6.4 and 7.4 at. %, respectively. Figure 5 displays the emission wavelength of NGQDs with different doping concentration. The emission wavelength is 579, 567, 519, 502, 473, 450, 427, 377, 336 and 307 nm when the doping concentration is 0.3, 1.6, 2.1, 2.9, 3.6, 4.2, 5.3, 6.4 and 7.4 at. %, respectively. The decrease of emission wavelength with the increasing of doping concentration can be due to the electrophilic induction of N atoms.17, 49, 50 Moreover, the emission wavelength shows the linear relation with the doping concentration.

CONCLUSION In this work, we have developed a new green, rapid, and universal preparation approach for GQDs. Due to the free radical polymerization of oxygen containing aromatic compounds under ultraviolet irradiation, well-crystallized GQDs with remarkable optical and biological properties were obtained. The yield of our approach was high up to 86% and the by-products are only H2O and CO2. The MALDI-TOFMS and free radical capturing experiment demonstrated the proposed free radical polymerization progress. The lateral size of GQDs can be tuned by adjusting the concentration of the precursor. Moreover, due to its good stability and low biotoxicity, the multicolor different sized GQDs can be used in vitro and in vivo fluorescent bio-imaging. The universality of our method for synthesis of N-GQDs was also demonstrated, and the results showed that our approach is suitable not only for the preparation of GQDs but also for heteroatom doped GQDs.

ASSOCIATED CONTENT Supporting Information Available Experimental details, MALDI-TOFMS result of GQDs. PL emission spectra of GQDs with different excitation wavelength. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author Prof. Dr. Yanfeng Tang: [email protected] Prof. Dr. Gang Wang: [email protected] Dr. Siwei Yang: [email protected] Notes 1

Jinli Zhu, Yanfeng Tang and Gang Wang contributed equally.

ACKNOWLEDGMENTS This work was supported by projects from the Supported by NSFC (21476117, 61604084).

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Green, Rapid, and Universal Preparation Approach of Graphene

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