Biomass-derived nitrogen doped graphene quantum dots with color-tunable emission for sensing, fluorescence ink and multicolor cell imaging

Biomass-derived nitrogen doped graphene quantum dots with color-tunable emission for sensing, fluorescence ink and multicolor cell imaging

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117671 Contents lists available at ScienceDirect Spectrochimica Acta ...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117671

Contents lists available at ScienceDirect

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Biomass-derived nitrogen doped graphene quantum dots with colortunable emission for sensing, fluorescence ink and multicolor cell imaging Zihao Wang a, Da Chen a, *, Bingli Gu a, Bo Gao a, Ting Wang a, Qinglei Guo b, Gang Wang a, ** a b

Department of Microelectronic Science and Engineering, School of Physical Science and Technology, Ningbo University, Ningbo, 315211, PR China Center of Nanoelectronics and School of Microelectronics, Shandong University, Jinan, 250100, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 September 2019 Accepted 15 October 2019 Available online 19 October 2019

In this paper, a simple, economical, and green strategy is developed for producing nitrogen doped graphene quantum dots (N-GQDs) with multicolor light emission by hydrothermal treatment of Passiflora edulia Sims. The synthesized N-GQDs exhibit ideal ionic stability, hydrophilicity and antiphotobcleaching properties, and the quantum yield reaches up to about 29%. Because of with the fluorescence quenching effect, the achieved N-GQDs allow to detect Agþ in a linear range of 10 nMe160 mM, and the limit of detection is calculated to be 1.2 nM according to the S/N of 3. Noteworthy, N-GQDs with blue, green and yellow light emissions are demonstrated via regulating the reaction time and temperature, implying a promising fluorescence adjustability. Furthermore, the N-GQDs-based fluorescent probe exhibits low cytotoxicity and favorable biocompatibility. Depending on the superior properties, our N-GQDs are applied in fluorescent ink and multicolor cell imaging. Eventually, the developed sensor is highly selective and accurate for Agþ analysis in real water, which demonstrates the promising practical use in environmental determination and/or biomedical engineering. © 2019 Elsevier B.V. All rights reserved.

Keywords: Graphene quantum dots Passiflora edulia sims Agþ sensing Multicolor light emission Bio-imaging

1. Introduction As a rising star in the nanocarbon family, graphene quantum dots (GQDs) have become attracted numerous attentions in recent years [1,2]. Unlike conventional organic fluorescent dyes and semiconductor quantum dots, GQDs possess unusual properties such as desirable photostability, tunable and strong photoluminescence (PL) property, favorable water solubility, customizable surface functionalization, low cytotoxicity, low environmental risk and favorable biocompatibility [3e6]. Promising applications of GQDs have been widely exploited involved of sensing, fluorescence bio-imaging, photocatalysis, optoelectronic devices, supercapacitors and biomedicine [4,7,8]. A variety of approaches, including oxidative cleavage [9], electrochemical method [10], hydrothermal method [11], ultrasonic exfoliation [12], laser

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang).

(D. 1386-1425/© 2019 Elsevier B.V. All rights reserved.


[email protected]

ablation [13], arc discharge [14] and electron beam irradiation [15], have been demonstrated for preparing GQDs. However, previous approaches are suffered from inevitable byproducts but difficult to remove, thus violating with the standard of green chemistry. Therefore, developing a method that enables the preparation of GQDs in a nontoxic, green and economical manner is still urgently demanded. Continuous efforts have been made for producing GQDs by using ecofriendly chemicals or biomass precursors as raw materials. For instance, Han et al. prepared fluorescent N-GQDs by carbonize diethylene triamine pentacetate at relative low temperature [16]. Chen et al. firstly reported a rose-heart radish precursor approach to synthesis fluorescent carbon dots (CDs) with a quantum yield (QY) of 13.6% [17]. Liu et al. described a kind of hair-derived CDs by a one-step pyrolysis treatment [18]. In addition, some natural green precursors have been utilized to successfully prepare fluorescent GQDs, such as garlic [19], black soya beans [20], grass [21], cotton [22], tartrazine [23], Ocimum sanctum [24] and even cow manure [25]. However, the low QY and single emission peak certainly limited their potentials in the nanomolecular sensing and cell imaging. Consequently, it still remains a major problem to synthesis


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biomass-derived GQDs with high QY and tunable emission wavelength by utilizing facile one-step methodologies. Heteroatoms doping of GQDs not solely enhances the photoluminescence efficiency and catalytic properties [26,27], but also provides active sites distributed on GQDs to expand the potentials in the catalytic, biological and sensing applications [28]. Passiflora edulia Sims (P. edulia Sims) is belonging to the passionaceae family and native to southern Brazil through Paraguay and northern Argentina. P. edulia Sims is often regarded as king of vitamins in fruits as it contains tremendous antioxidants, fibers and other vitamins, which can help to enhance the immune system, eliminate inflammation and prevent both diseases and cancer. More than that, abundant glucoxe, fructose, sucrose, prunasin, niacin, riboflavin and thiamine in P. edulia Sims afford carbon and nitrogen source for preparing of heteroatoms doped GQDs and endow more reactive sites differ from pure GQDs for potential applications in bio-sensing. Silver is one of the trace elements in human tissue, a trace amount of silver is harmless to the human body. For diet, drinking water with Agþ concentration of less than 0.1 ppm will not cause adverse effects on the human body according to the latest WHO (World Health Organization) Guidelines for Drinking Water Quality. Overmuch Agþ intake will lead to the accumulation of unsolvable toxicity in skeletal and liver. In addition, the extensive employment lead to a huge amount of silver is released into the environment [29]. Furthermore, Agþ content of human excretion is useful indices of health monitoring [30]. Therefore, the precise determination of trace amount of Agþ concentration is very significant in environmental, industrial and medical samples. Herein, a simple and green method for synthesizing N-GQDs by using P. edulia Sims as raw material through a one-step thermal treatment is reported. The obtained N-GQDs show multicolor light emission and the QY is as high as 29%. Moreover, the N-GQDs can be employed for selective and sensitive detection of Agþ in the manner of fluorescence ‘turn-off’ mechanism. The procedure shown in Scheme 1 illustrates the preparation of N-GQDs derived from P. edulia Sims. Additionally the prepared N-GQDs are utilized for Agþ sensing, fluorescence ink and multicolor cell imaging, demonstrating significant potentials in biomedical fields.

2. Materials and methods 2.1. Materials P. edulia Sims was procured from local fruit shop. Solution of Agþ was prepared from AgNO3. All reagents were of analytical grade and used as received without any further purification. The ultrapure water with a resistivity of 18.2 MU used throughout all experiments was prepared by a Millipore system. 2.2. Characterization methods Transmission electron microscopy (TEM) was carried out on Hitachi H-8100. Atomic Force Microscope (Cypher S, Asylum research) was used to determine the height profiles of N-GQDs. Xray photoelectron spectroscopy (XPS) was utilized on PHI Quantera II system, Ulvac-PHI (INC, Japan) to determine the surface chemical composition, chemical states of N-GQDs. Ultravioletevisible (UVevis) absorption properties were characterized by 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. PL lifetime was measured via the time-correlated single-photon counting (TCSPC) technique (HydraHarp 400, PicoQuant). Raman spectra (HORIBA Jobin Yvon HR800) were measured by using an Arþ laser. Fourier transform infrared spectroscopy (FT-IR) were measured by using a Nicolet 670 spectrograph. 2.3. Synthesis of N-GQDs P. edulia Sims was used as carbon and nitrogen sources for the synthesis of water soluble N-GQDs by hydrothermal treatment. Briefly, P. edulia Sims flesh was crushed and mixed with deionized water in a weight ratio of 1:1. The obtained viscous mixture was filtered by filter paper and 0.22 mm microporous membrane, respectively. Afterwards, 10 ml aqueous dispersion and 10 ml deionized water were transferred into a 50 ml Teflon-lined autoclave, then heated at 180  C for 4 h and cooled to room temperature. Finally, the obtained dark mixture was filtered by 0.22 mm microporous membrane, resulting in a brown transparent solution for further use.

Scheme 1. Schematic illustration of synthesis process and application of N-GQDs.

Z. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117671

3. Results and discussion 3.1. Characterization of N-GQDs The plan-view TEM image presented in Fig. 1(a) illustrates the uniformly distributed N-GQDs with the average particle sizes of about 3.8 nm (the inset of Fig. 1(a)). To further explore the micro structures, high-resolution TEM (HRTEM) analysis is carried out. Fig. 1(b) depicts the micro structure of a randomly selected N-GQDs. The observed honeycomb structure reveals the favorable crystallinity of the N-GQDs [31]. The lattice parameter is extracted as ~0.24 nm through a line-cutting (Figure S1), corresponding to the (100) in-plane lattice of graphene [7]. Fast Fourier transform (FFT) analysis pattern with a standard six-fold symmetry reveals a high crystalline structure of the N-GQDs (the inset of Fig. 1(b)) [32]. As measured by AFM image of N-GQDs (Fig. 1(c)), the thicknesses are among 0.7e1.2 nm. The height chart (Figure S2) reveals that the average thickness is about 0.9 nm, indicating 2e3 layers of graphene. The Raman spectrum (Fig. 1(d)) suggest three different vibrations at near 1350, 1580, and 2700 cm1, which are assigned as defectrelated D band, G band, and 2D band specific to graphene structure, respectively. Notably, the 2D peak characteristic of few layer graphene is strongly suppressed. The ID/IG is calculated to be around 0.25, indicating the presence of defects in the internal or edge of NGQDs [33]. The XRD pattern of the N-GQDs (Figure S3) reveals a dispersed diffraction peak centered at 2q ¼ 24 (d ¼ 0.37 nm) corresponding to the (002) planes of graphitic carbon [10].


The surface elemental composition of the prepared N-GQDs is further investigated by XPS. As shown in Fig. 2(a), the XPS scanning spectrum reveals the existences of C, O and N with atomic concentrations of 62.7%, 31.1% and 4.6%, respectively. In the C 1s spectrum, the binding energy peak at 284.5 eV denotes the C¼C sp2 structure of graphene [34]. The peaks at about 285.2, 286.3 and 287.4 eV could be respectively assigned to the signals of CeN/CeO, C¼N and C¼O (Fig. 2(b)) [33]. The high resolution spectrum of O 1s can be divided into two peaks around at 532.7 and 533.3 eV, which represent the C¼O, and CeOH bands (Fig. 2(c)) [34]. The high resolution N 1s spectrum (Fig. 2(d)) displayed three peaks at 399.6, 399.9 and 400.3 eV, corresponding to NeH, CeNeC and CeN¼N structure, respectively [35]. Above results indicate the successful doping of N atoms in graphene quantum dots. To further identify the functional groups presented on surface of the obtained N-GQDs, the FT-IR spectrum shown in Fig. 2(e) reveals a broad and intense peak located at 3000-3600 cm1 corresponds to eOH and eNH2 groups [36]. The peak located at 1043.4 cm1 can be ascribed to the OeH bond [16]. Moreover, the peak located at 1290.5 and 1741.8 cm1 are consistent with the NeH and C¼C bond [4]. Based on above analysis results, the chemical structure of N-GQDs is estimated in Fig. 2(f). Furthermore, it is concluded that the N-GQDs have been functionalized by chemical groups with oxygen and nitrogen elements on N-GQDs’ surface, leading to capabilities of hydrophilicity and further modifications.

Fig. 1. Characterization of N-GQDs. (a) Plan-view TEM image of the as-prepared N-GQDs. The inset corresponds to the size distribution histogram of N-GQDs. (b) HRTEM image of a typical N-GQDs. The inset corresponds to the FFT analysis pattern of N-GQDs. (c) Contact-mode AFM image of the N-GQDs transferred on Si/SiO2 substrate. The inset shows the thickness of N-GQDs. (e) Raman spectrum of N-GQDs on Si/SiO2 substrate.


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Fig. 2. Chemical structure analysis of N-GQDs. (a) XPS spectrum of N-GQDs. High-resolution XPS spectra of N-GQDs: (b) C 1s, (c) O 1s, and (d) N 1s. (e) FT-IR spectrum of N-GQDs. (f) Schematic diagram showing the structure of N-GQDs.

3.2. Optical properties of N-GQDs Due to the quantum effect and edge effect, N-GQDs exhibit strong fluorescence under ultraviolet light. UVevis, PLE and PL spectra of N-GQDs aqueous solutions are measured to investigate their optical performances (Fig. 3(a)). It is found that typical p-p* transition of aromatic C¼C bands and n-p* transition of C¼O/C¼N absorption bands are centered at around 230 nm and 280 nm, respectively [37,38]. Such absorbance behavior is similar to the carbon dots made from garlic [19]. Noteworthy, the absorption bands locating around 280 nm is quite strong, indicating the high content of C¼O/C¼N and responding to the results of XPS. In addition, the N-GQDs show obvious fluorescence and the maximum optical excitation wavelength and maximum optical emission wavelength are 467 and 543 nm, respectively. The Commission International d’Eclairage (CIE) chromaticity coordinate of maximum PL spectrum are (0.35, 0.55), as shown in Fig. 3(b), indicating a chartreuse light emission. Thence, N-GQDs aqueous solution can emit distinct chartreuse light emission under the irradiation at 365 nm, as shown in Fig. 3(c). Importantly, the quantum yield (4) of the as-prepared N-GQDs is 0.29 which is relatively higher than most previous biomass-derived GQDs reports [19e25,39]. Figure S4 shows an image of Chinese dragon drawing with the N-GQDs ink under visible and UV light, suggesting that the N-GQDs can be used as stable fluorescent inks. Similar to the common N-GQDs, the PL wavelength of N-GQDs (Fig. 3(d)) is substantially depended on the PLE wavelength. It is worth noting that there is almost no shift of the emission wavelength under excitation from 425 nm to 485 nm compared to that under excitation at 465 nm. However, observable shifts of the emission wavelength can be seen when excited at 505 nm. This may be ascribed to multifunctional surface and edge structure, including lattice defects, doped heteroatoms, functional groups and many more. Moreover, the PL decay of N-GQDs is shown in Fig. 3(e), and the average PL lifetime of 2.55 ns can be calculated approximately.

For better application in practice, the fluorescence stability of NGQDs is tested. Fig. 4(a) shows the PL intensity of N-GQDs solution under various pH values. It can be observed that the initial increase of the pH value causes an enhanced PL intensity and it reaches to the maximum at pH value of 7. The continuous increase of pH to 11 bring about a gradually weaken PL intensity, indicating the PL intensity of N-GQDs is strongly dependent on the pH value. Afterwards, the storage stability of N-GQDs is measured. According to the experimental results, the N-GQDs displayed satisfactory stability under nonstop irradiation of visible light for 8 days (Fig. 4(b)) or ultraviolet radiation (Fig. 4(c)) for 16 h. Such antiphotobcleaching property can be attributed to the special surface structure and composition of as-prepared N-GQDs. Fig. 4(d) shows the outstanding photostabilities of N-GQDs against different concentrations of KCl even at 1.0 M. These results indicate the antijamming capability of N-GQDs, suggesting that N-GQDs have expectable potential for sensing applications under physiological and environmental conditions. 3.3. Fluorescence sensing of Agþ based on the N-GQDs Inspired by the superior antijamming capability, a GQDs-based fluorescent sensor is designed. The selectivity and sensitivity of this system is quite significant for evaluating the performance of the system. First, the selectivity is explored. The photographs of NGQDs aqueous solutions under ultraviolet light with various highly concentrated metal ions that may cause fluorescence quenching are shown in Fig. 5(a). It is observed that the N-GQDs solution with Agþ ions quenches obviously. The F/F0 of all N-GQDs solutions are recorded (see Fig. 5(b), F and F0 is the peak fluorescence intensity of N-GQDs with and without metal ions, respectively). It is noteworthy that a strong quenching effect occurred in the presence of Agþ ions, which is consistent with the results presented in Fig. 5(a). However, for other metal ions (Naþ, Kþ, Mg2þ, Ca2þ, Al3þ, Pb2þ, Cu2þ, Ni2þ, Co2þ, Fe3þ, Fe2þ, Cd2þ, Agþ, Zn2þ, Au3þ, Mn2þ, Hg2þ, Cl,

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Fig. 3. Optical properties of N-GQDs. (a) UVevis, PL and PLE spectra of N-GQDs. (b) The CIE chromaticity coordinate for N-GQDs in aqueous solution. (c) Digital image of N-GQDs in aqueous solution under UV light (center wavelength: 365 nm). (d) PL spectra of N-GQDs recorded for the progressively longer excitation wavelength of 20 nm increments (425e505 nm). (e) PL decay curves of N-GQDs measured at room temperature with excitation at 465 nm.

Fig. 4. Fluorescence stability of N-GQDs. The fluorescence intensity of N-GQDs under (a) various pH values, (b) visible light, (c) UV light (150 W Xe lamp with center wavelength at 365 nm), (d) different ionic strengths.

3 Br, SO2 and S2), no significant fluorescence quenching on 4 , NO N-GQDs is found, indicating the superior selectivity of N-GQDs for Agþ sensing.

The effect of pH on fluorescence quenching is further investigated. Fig. 5(c) shows the fluorescence quenching rate upon addition 120 mM Agþ under different pH values. The result suggests that


Z. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117671

Fig. 5. Application exploration of N-GQDs. (a) The photograph of N-GQDs with different metal ions in the aqueous solution under a 365 nm UV light irradiation. (b) The F/F0 of NGQDs in the presence of different metal ions (120 mM). (c) The DF/FpH of N-GQDs under different pH values. (FpH is the fluorescence intensity of N-GQDs under different pH values). (d) Time dependent fluorescence response of N-GQDs with adding 120 mM Agþ.

the optimum pH value for fluorescence quenching is pH ¼ 7. Then, time dependent fluorescence response of N-GQDs under pH ¼ 7 is explored. As shown in Fig. 5(d), the dynamic quenching process indicates that it takes 6 min to complete the quenching process after the addition of ions. Therefore, 6 min is chosen as the reaction equilibrium time between N-GQDs and Agþ for further exploring the performance of N-GQDs-based sensor system. On the basis of the discussion above, N-GQDs is developed as a fluorescent probe for detecting of Agþ ions. Further experiments are carried out to explore the sensitivity of the system. Fig. 6(a) displays the PL spectra of the N-GQDs under different concentrations of Agþ ranging from 0 to 120 mM. Figure S5 shows the optical images of N-GQDs aqueous solution in the present of increasing concentrations of Agþ. Obviously, the fluorescence of N-GQDs gradually weakens and emits blackish fluorescence when the concentration rises to 100 mM. Fig. 6(b) reveals the relationship between F0/F and the concentration of Agþ. It is worth noting that

F0/F increased linearly (with a correlation coefficient of 0.9996) with the increasing concentration of Agþ from 10 nM to 160 mM (inset of Fig. 6(b)). The limit of detection is calculated to be 1.2 nM with the S/N ratio of 3. The relatively low detection limit may be ascribed to the rich functional groups with special response to Agþ derived from antioxidants, fibers and vitamins in P. edulia Sims. Table 1 shows the comparison of GQDs-based fluorescence probes for Agþ detection. The result suggests the higher sensitivity of our sensing system than other previously reported GQDs-based sensing systems in literatures [27,40e45]. The mechanisms of Agþ sensing by N-GQDs are investigated by measuring the UV absorbance and fluorescence lifetimes before and after quenching. As shown in Figure S6, after the addition of Agþ, a strong absorbance peak centered at 280 nm continues to increase. Meanwhile, a strong absorbance peak at about 220 nm appears. These results suggest that Agþ indeed reacts with N-GQDs, resulting in [email protected]þ complex [19,46]. Moreover, the PL

Fig. 6. Performances of N-GQDs probe. (a) Fluorescence spectra of N-GQDs at various concentrations of Agþ ranging from 0 to 120 mM (lex ¼ 465 nm). (b) The F0/F of N-GQDs as a function of Agþ concentration (inset is the linear relationship between F0/F and Agþ concentration from 10 nM to 160 mM).

Z. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117671


Table 1 Comparison of different GQDs-based probes for Agþ detection. Fluorescent probe

Detection wavelength (nm)

Linear range (mM)

Detection limit (mM)

S-GQDs [email protected] TAPI-dots N-CDs CQDs CDs GQDs-OPD N-GQDs

450 476 370 450 440 e 445 543

0.1e130 0.005e0.05 0-1 and 1-8 0.001e1 0e90 0.5e6 0e115.2 0.01e160

0.03 0.0025 0.0103 0.001 0.32 0.13 0.25 0.0012

lifetime of [email protected]þ complex is calculated to be 2.52 s (Figure S7), which is similar to the fluorescence lifetime of N-GQDs. The almost unchanged fluorescence lifetime of N-GQDs with the absence and presence of Agþ indicates no significant excited-state interaction arise between N-GQDs and Agþ [46]. In this case, the quenching effect is possibly attributed to the synergetic effect of static quenching effect and electron transfer.

Ref 27 40 41 42 43 44 45 This work

3.4. Controllable fluorescent properties and bioimaging potentials of N-GQDs For the purpose of realizing N-GQDs with different fluorescence colors, we further explore the factors that affect the emission wavelength of N-GQDs. First, we investigate the effect of reaction time. As shown in Fig. 7(a), the emission wavelength of N-GQDs

Fig. 7. Control of the fluorescence color of N-GQDs. The PL wavelength of N-GQDs obtained under different (a) reaction times, (b) reaction temperatures. (c) PL spectra of GQDs-1, GQDs-2, and GQDs-3 (the emission wavelengths are 472, 509, and 565 nm, respectively). The inset corresponds to optical images of N-GQDs solutions under irradiation of UV light (center wavelength: 365 nm). (d) Metabolic activity of fibroblast cells treated with different concentrations of N-GQDs. Confocal fluorescence microphotograph of fibroblast cells incubated with 100 mg/mL N-GQDs (e) N-GQDs-1, (f) N-GQDs-2, and (g) N-GQDs-3. (lex ¼ 480 nm). Scale bar: 20 mm.


Z. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117671

Table 2 Detection of Agþ in environmental water samples. Water samples

Added Agþ (mM)

Detected Agþ (mM)

Recovery (%)

Tap water

e 10 50 e 10 50

0 9.84 49.51 0.23 10.29 51.57

e 98.4 99.02 e 102.9 103.14

Yongjiang river water

slightly redshifts as the reaction time becomes longer in the initial stage and reached a maximum of 543 nm. Prolonging the reaction time, however, the emission wavelength decreases to around 480 nm at a reaction time of 24 h, and keeps as constant. Small-sized N-GQDs were synthesized in the initial stage of reaction system. However, the sp2 carbon structure is carbonized into intertwined polymers, thus reducing surface functional groups when the reaction time proceeds 24 h [47]. To demonstrate the above discussion, UVevis absorbance spectra of the N-GQDs at different reaction times are measured. As shown in Figure S8, it is observed that the absorption peak around 280 nm reaches a maximum value at the reaction time of 4 h. Prolonging the reaction time to 24 h, the corresponding UV absorption is shown in Figure S9. Compared with the UVevis absorbance of N-GQDs obtained under the reaction time of 4 h, the absorbance peak at around 230 nm disappears, indicating that the aromatic rings are carbonized into amorphous carbon. Moreover, the decrease of the absorption peak around 280 nm is attributed to that the heteroatom in sp2 carbon structure and surface functional group were removed under a long reaction time. Meantime, the increase of polymer nanoparticle size leads to the redshift of absorbance peak [7,47]. Therefore, 4 h is determined as the optimal reaction time to further study the effect of reaction temperature on the emission wavelength. As shown in Fig. 7(b), the emission wavelength slightly increases with the increase of reaction temperature from 100 to 140  C. The peak emission wavelength reaches its maximum value of 565 nm when the reaction temperature increases to 140  C. Continuing increase of the reaction temperature will lead to a decreased emission wavelength, which can be attributed to insufficient high enough temperature to provide sufficient energy to form large-sized N-GQDs and to remove the heteroatom in sp2 carbon structure [48]. Based on the above investigations, N-GQDs with controllable PL color can be prepared by tuning the reaction time and the reaction temperature. Fig. 7(c) exhibits the PL spectra of N-GQDs obtained by different reaction conditions. Obvious blue, green and yellow PL emission colors are found in N-GQDs solutions, as shown in the inset of Fig. 7(c). The obtained N-GQDs have been demonstrated to have ideal ionic stability and anti-photobcleaching properties. Further exploration is carried out for the potential biological imaging. As a precondition, the cytotoxicity of N-GQDs is determined according to the metabolic activity of fibroblast cells. Cells cultured in 96-well plates upon the addition of different concentrations of N-GQDs (0e500 mg/mL) are incubated for 48 h. After that, the metabolic activity of fibroblast cells is recorded and summarized. As shown in Fig. 7(d), ignorable reduction in metabolic activity indicates that the obtained N-GQDs are nontoxic in vitro. As shown in Fig. 7(eeg), the bioimaging ability of N-GQDs is proved by incubating fibroblast cells in N-GQDs-1, N-GQDs-2, and N-GQDs-3 aqueous solutions. Multicolored PL (blue, green, yellow) inside the cells can be observed. Thence, all of prepared N-GQDs can be utilized as a sort of valuable bioimaging materials. 3.5. Detection of Agþ in real water samples To verify the reliability of the N-GQDs in practical application, NGQDs-based sensing system is utilized to detect Agþ in both natural

water of Yongjiang river and tap water. The river water and tap water samples are filtered by 0.22 mm microporous membranes and centrifuged for 10 min at 8000 rpm. Then Agþ solution with different concentrations is added into the water samples. Table 2 shows the experimental results of detecting Agþ ions by using the proposed N-GQDs-based sensing system, indicating the great prospect of N-GQDs in environmental analysis. 4. Conclusions In summary, N-GQDs with multicolor PL color were successfully synthesized via a facile one-pot hydrothermal treatment of P. edulia Sims. The obtained N-GQDs consist of tremendous oxygencontaining and heteroatom functional groups. Noteworthy, the NGQDs display ideal ionic stability and anti-photobcleaching properties with a high QY of about 29%. Moreover, controllable PL properties are demonstrated by tuning the reaction conditions. Besides, such N-GQDs have been further developed as a sensing system for label-free, sensitive detection of Agþ ions with a detection limit of as low as 1.2 nM. This sensing system is also demonstrated for successful analysis of natural water. This work may provide a feasible strategy to environmentally friendly synthesize larger-scale GQDs with tunable light emission properties and ultrahigh Agþ sensitivity, which probably open up a new way to design effective fluorescence probes for detecting metal ions. Acknowledgment This work was supported by projects from National Natural Science Foundation of China under Grant (Nos. 11704204, 61604084, and 51602056). 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. Appendix A. Supplementary data Supplementary data to this article can be found online at References [1] N.T.N. Anh, A.D. Chowdhury, R.-a. Doong, Highly sensitive and selective detection of mercury ions using N, S-codoped graphene quantum dots and its paper strip based sensing application in wastewater, Sens. Actuators B Chem. 252 (2017) 1169e1178. [2] S. Yang, J. Sun, P. He, X. Deng, Z. Wang, C. Hu, G. Ding, X. Xie, Selenium doped graphene quantum dots as anultrasensitive redox fluorescent switch, Chem. Mater. 27 (2015) 2004e2011. [3] M. Zheng, Z. Xie, D. Qu, D. Li, P. Du, X.B. Jing, Z.C. Sun, On-off-on fluorescent carbon dot nanosensor for recognition of chromium(VI) and ascorbic acid based on the inner filter effect, ACS Appl. Mater. Interfaces 5 (2013) 13242e13247. [4] X. Sun, J. He, S. Yang, M. Zheng, Y. Wang, S. Ma, H. Zheng, Green synthesis of carbon dots originated from Lycii Fructus for effective fluorescent sensing of ferric ion and multicolor cell imaging, J. Photochem. Photobiol., B 175 (2017) 219e225. [5] Y. Dong, H. Pang, H. Yang, C. Guo, J. Shao, Y. Chi, C. Li, T. Yu, Carbon-based dots Co-doped with nitrogen and sulfur for high quantum yield and excitationindependent emission, Angew. Chem. Int. Ed. 52 (2013) 7800e7804. [6] S.T. Yang, L. Cao, P.G. Luo, F. Lu, X. Wang, H. Wang, M.J. Meziani, Y. Liu, G. Qi, Y.P. Sun, Carbon dots for optical imaging in vivo, J. Am. Chem. Soc. 131 (2009) 11308e11309. [7] G. Wang, Q. Guo, D. Chen, Z. Liu, X. Zheng, A. Xu, S. Yang, G. Ding, Facile and highly effective synthesis of controllable lattice sulfur-doped graphene quantum dots via hydrothermal treatment of durian, ACS Appl. Mater. Interfaces 10 (2018) 5750e5759. [8] J. Liu, X. Zhang, Z. Cong, Z. Chen, H. Yang, G. Chen, Glutathione-functionalized graphene quantum dots as selective fluorescent probes for phosphatecontaining metabolites, Nanoscale 5 (2013) 1810e1815. [9] Y. Shin, J. Park, D. Hyun, J. Yang, H. Lee, Generation of graphene quantum dots from oxidative cleavage of graphene oxide using oxone oxidant, New J. Chem. 39 (2015) 2425e2428.

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