DOI: 10.1002/ejic.201701080 Full Paper

Green Synthesis of Carbon Nanodots | Very Important Paper |

One-Pot Synthesis of Highly Fluorescent Carbon Dots fromSpinach and Multipurpose ApplicationsGuojuan Ren,[a] Mingyu Tang,[a] Fang Chai,*[a] and Hongbo Wu*[a]

Abstract: Highly fluorescent carbon dots (CDs) have been pre-pared from natural spinach by a hydrothermal method. In thiswork, we have demonstrated that the CDs have a high quantumyield of 53 %, and exhibit special responses to 2-nitrophenol (2-NP) and 4-nitrophenol (4-NP). Solutions of the CDs showedclear fluorescence quenching in the presence of 2-NP and 4-NP;the detection limits of 2-NP and 4-NP were 1.06 and 0.50 μM,

Introduction

During the past few years, carbon dots (CDs) have becomemore popular and are regarded as a rising star of the “lumines-cent material family”.[1–6] In addition, CDs have many advanta-ges, such as low toxicity, high water solubility, high stability,and good biocompatibility, and are easy to functionalize.[7]

Many different methods have been reported for the preparationof fluorescent CDs, including chemical oxidation, laser ablation,electrochemical synthesis, ultrasonic synthesis, microwavepyrolysis, and hydrothermal carbonization.[8] Because of its sim-plicity, most of the synthetic approaches to fluorescent CDs in-volve the facile, green hydrothermal approach.[9] Afkhami andco-workers developed a facile hydrothermal method usinggluconic acid and N-methylethylenediamine as carbon andnitrogen sources, respectively, to synthesize amino-functional-ized carbon quantum dots (N-CQDs) as effective fluorescentprobes for the detection of isotretinoin.[10] Similarly, citric acidhas been utilized as a carbon source to synthesize CDs that areemployed as a probe for the detection of metacycline.[11] Asalternatives to chemical reagents, environmentally friendlymethods for the synthesis of CDs that exploit the use of renew-able, natural carbon precursors are attracting increasing atten-tion. Exploring green sources for the preparation of CDs, search-ing for clean, inexpensive, easily accessible, and nontoxic pre-cursors, is an exciting challenge.[12,13] For example, Wu and co-workers prepared blue emissive CDs by using prawn shells asthe carbonaceous source[14] and Wang et al. prepared CDs by

[a] Key Laboratory of Photochemical Biomaterials and Energy StorageMaterials, Colleges of Heilongjiang Province, College of Chemistry andChemical Engineering, Harbin Normal University,Harbin 150025, P. R. ChinaE-mail: fangchai@gmail.com

wuhongbo@hrbnu.edu.cnhttp://www.hrbnu.edu.cnSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/ejic.201701080.

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respectively. Simultaneously, the color changed from colorlessto yellow, visible to the naked eye. More importantly, spinach,as a carbon source, is much more eco-friendly than other or-ganic dyes and organic solvents used for biosensors. Biologicalstudies indicated that the CDs exhibit high fluorescent contrastand low toxicity in cells. Overall, the CDs prepared in this workcan be used for fluorescent ink, biosensors, and bioimaging.

using cucumber juice as the carbonaceous source (see Table S1in the Supporting Information).[15] However, most of these CDshave low quantum yields, possessing insufficient fluorescent in-tensity for fluorescent detection. Thus, the preparation of CDsexhibiting high quantum yields remains a great challenge. Ithas previously been demonstrated that the doping of CDs withnitrogen groups can significantly enhance their properties, be-cause the nitrogen-doping process can regulate the intrinsicelectronic and surface properties of CDs.[16,17] Highly fluores-cent carbon dots have been formed from spinach as a result ofits high folic acid content. Folic acid possesses several func-tional groups, such as -COOH and -NH2, which are considered asnecessary groups for the production of CDs with high quantumyields.[18] Herein, we report a simple method using spinach asthe only precursor for the synthesis of N-doped, carbon-rich,highly fluorescent CDs.

Fluorescent CDs with excellent fluorescent properties and anapparent absence of toxicity have been used in environmentaland biomedical fields, including for the detection of heavy-metal ions and nitroaromatic derivatives as well as for live cellimaging.[19–21] Nitrophenols are pollutants deriving from proc-essing and manufacturing as well as from pesticide degradationand are proving a threat to ground and surface waters. Becauseof their good solubility, strong stability, high toxicity, andcarcinogenicity, 2-nitrophenol (2-NP) and 4-nitrophenol (4-NP),in particular, are difficult to remove from water. Therefore, thedesign of inexpensive and convenient methods for the rapid,sensitive, and selective detection of 2-NP and 4-NP by a simpleoperation is in ever-increasing demand.[22,23]

Thus, we report herein the synthesis of CDs that exhibit goodspecificity and sensitivity towards 2-NP and 4-NP by a facileone-step hydrothermal method using natural spinach as theprecursor. The fluorimetric (“on/off” fluorescence transforma-tion) and colorimetric (“colorless–yellow” color conversion)dual-readout methods for 4-NP and 2-NP detection were em-ployed in this study. Owing to their excellent optical properties,

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the CDs are suitable for use as invisible ink, which suggests thatthey have potential applications matters relating to confiden-tiality. The highly fluorescent CDs have a large Stokes shift thatbenefit the distinguishing of target and background signals influorescent imaging.[24] As a result, the synthesized CDs havebeen explored for their potential in bioimaging, with favorableimaging quality and low toxicity being observed.

Results and Discussion

Morphology and Composition of the CDs

Blue-emitting fluorescent CDs were prepared directly by thecarbonization of natural spinach in the absence of any additives(Figure 1A). Then, the morphology and structure of the CDswere analyzed by TEM and X-ray photoelectron spectroscopy(XPS). As shown in Figure 1B-a, TEM was used to study themorphology and distribution of the CDs, which can be seento be uniformly dispersed with no apparent aggregation. Theirdiameters are in the range of 1.0 to 5.0 nm (Figure 1B-b andFigure S1 in the Supporting Information). The HRTEM imagesreveal lattice spacings of 0.21 and 0.34 nm, which correspondto the (100) and (002) facets of graphitic carbon, as shown inthe insets of Figure 1B-a.[25,26]

Figure 1. (A) Schematic for the preparation of CDs. (B) TEM image (a) Inset:high resolution TEM image of CDs and size distribution (b) of the CDs.

The functional groups on the surfaces of the CDs were iden-tified by FTIR spectroscopy (Figure 2A). The CDs exhibit peaksat around 3442 cm–1, which correspond to O–H vibrations. Thepeaks at 1392 and 1105 cm–1 were assigned to C–N stretchingand N–H wagging, respectively. Furthermore, a peak at1633 cm–1 is observed, which indicates the presence of a C=Obending vibration.[27] XPS was used for elemental analysis andto determine the surface composition of the nanodots. The re-sults show that the CDs consist of carbon, nitrogen, and oxygenwith contents of 60.26, 10.76, and 28.99 %, respectively (seeFigure S2 in the Supporting Information). The peak in the C1sspectrum at the binding energy of 284.3 eV indicates the exis-tence of a C=C structure in the prepared CDs,[28] and the peaksat 285.7 and 287.1 eV correspond to C–N/C–O and C=O, respec-tively (Figure 2B).[29] The N1s spectrum of the CDs deconvolutesto two peaks at 399.6 and 401.6 eV (Figure 2C), which can be

Eur. J. Inorg. Chem. 2018, 153–158 www.eurjic.org © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim154

attributed to (C)3-N and N–H groups, respectively.[30] The O1sspectrum exhibits two primary bands at 530.7 and 532.2 eV,attributed to -OH and C–O (Figure 2D).[31] The CDs possess highhydrophilicity and dispersity in water due to their various sur-face functional groups, which enables them to be used for thedetection of nitroaromatic derivatives as well as in biologicalenvironments.

Figure 2. (A) FTIR spectrum of the CDs and their (B) C1s, (C) N1s, and (D) O1shigh-resolution XPS spectra.

UV/Vis Absorption and Fluorescence Properties of CDs

The dilute CD solution is colorless in daylight and emits bright-blue fluorescence under UV excitation (365 nm) that can easilybe seen by the naked eye, which indicates that the CDs haveexcellent fluorescent properties (inset in Figure 3A). To furtherexplore the optical properties of the CDs, their fluorescence andabsorption spectra were recorded (Figure 3A). The absorptionspectrum of the CDs exhibits two peaks at 273 and 320 nm,which correspond to the π–π* transition of the C=C bonds andthe n–π* transition of the C=O bonds.[8] The excitation-depend-ent nature of the CDs is shown in the fluorescence spectra,which were excited in the range from 290 to 390 nm. A broademissive peak is observed upon excitation at 290 nm (black

Figure 3. (A) UV/Vis absorption and fluorescence emission spectra of the CDsat different excitation wavelengths. The insets display the corresponding op-tical photographs of the CD solution. (B) Fluorescence decay profile of theCDs.

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line). Previous studies indicated that the emission peak can bedivided into various individual peaks under different ener-gies.[32] That is, the short wavelengths have sufficient energy toexcite all the individual peaks; however, the excitation energyis not enough to cause the high-energy transitions as the exci-tation wavelength increases. The maximum emission peak ap-pears at 405 nm upon excitation at 330 nm. We then studiedthe fluorescence intensity of the CDs over time. The emissionspectra of the CDs show no clear changes after 12 hours (seeFigure S3 in the Supporting Information), which indicates thatthe CDs have high stability. As shown in Figure 3B, the emissiondecay is well fitted by a double-exponential, and the averagelifetime of the CDs is 6.51 ns.

Selectivity of Detection

The ability of the CDs to act as a sensor for the detection of 4-NP and 2-NP in the presence of different relevant nitroaromaticcompounds was investigated. Various nitroaromatic com-pounds [4-NP, 2-NP, nitrobenzene (NB), 2,4,6-trinitrotoluene(TNT), 3-nitrophenol (3-NP), toluene, 3-nitrotoluene (3-NT),2,4,6-trinitrophenol (TNP), and 2,4-dinitrotoluene (DNT)] wereadded to solutions of the CDs and the fluorescence emissionsof the samples were measured under the same conditions. Asshown in Figure 4, 2-NP quenched the fluorescence intensitydramatically and 4-NP induced a significant spectral change; forthe latter, the fluorescence at 405 nm decreased and split intotwo bands located at 380 and 460 nm. 3-NP and DNT showedminor effects on the fluorescence response of the CDs, butthese effects can be ignored compared with those of 2-NP and4-NP. The presence of the other nitroaromatic compounds didnot obviously affect the fluorescence emission of the CDs, eventhough the concentration of the tested nitroaromatic com-pounds was as high as 10–3 M. Figure S4 in the SupportingInformation shows the relative fluorescence intensities (λex =330 nm) of CD solutions containing 10–3 M of various nitro-

Figure 4. (A) Fluorescence emission of the CDs in the presence of differentnitroaromatic compounds and (B) photographs of aqueous solutions of theCDs under 365 nm UV light and in daylight in the presence of differentnitroaromatic compounds (from left to right: Blank, 4-NP, 2-NP, NB, TNT, 3-NP,toluene, 3-NT, TNP, and DNT).

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aromatic compounds, and reflects the intuited quenching ofthe fluorescence emission of the CDs. The photographs of thesamples taken under UV light show that the CD sample solu-tions darken dramatically in the presence of 2-NP and 4-NP,whereas they remain bright in the presence of the other nitroar-omatic compounds. Meanwhile, under daylight conditions, thecolor of the CD solution is apparently transformed from color-less to yellow upon the addition of 2-NP and 4-NP, whereasthe other nitroaromatic compounds caused no change in color(Figure 4B). The UV/Vis absorption spectra of CD solutions withand without the nine different nitroaromatic compounds aredisplayed in Figure S5. When 9 equivalents of the various nitro-aromatic compounds were added to solutions of the CDs, 2-NPand 4-NP induced a significant spectral change with a redshiftof the absorption band. These results demonstrate that the CDshave potential as a “naked-eye” colorimetric detector for 2-NPand 4-NP, but not as a fluorescent probe.[33]

Sensitivity of Detection

As mentioned above, under the optimal conditions, the concen-tration dependence of the turn-off luminescence intensity ofthe CDs in the presence of 2-NP and 4-NP was also investigated.As illustrated in Figure 5A, the addition of increasing concentra-tions of 2-NP caused a gradual quenching of the fluorescentintensity of the CD system. Correspondingly, in daylight, thecolor gradually changed from colorless to yellow with increas-ing concentration of 2-NP (Figure 5C). Figure 5B displays thegood linear relationship between I/I0 and the concentration of2-NP in the range from 0.001 to 1 μM, and the detection limitof 2-NP by the CDs was 1.06 μM. The change in color of theCDs induced by 2-NP was monitored by UV/Vis spectrophotom-etry. The absorption at 274 and 340 nm increased with increas-ing concentration of 2-NP, and the peaks also became wider(see Figure S6A in the Supporting Information).

Similar behavior was observed for 4-NP (Figure 6A). Uponthe gradual addition of 4-NP, the intensity of the fluorescencesteadily reduced, thereby demonstrating an efficient fluores-cence response. When the concentration of 4-NP exceeded10–4 M, the fluorescence emission (excited at 330 nm) graduallysplit into two emission bands, and when 10–3 M 4-NP had beenadded, the CD fluorescence emission had split into two peaks

Figure 5. (A) Influence of different concentrations of 2-NP on the fluorescenceintensity of the CDs, (B) linear plots of I/I0 vs. 2-NP concentration in the range0.001–1 μM, and (C) photographs of aqueous solutions of the CDs containingdifferent concentrations of 2-NP under 365 nm UV light and daylight.

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located at 380 and 460 nm. A good linear relationship betweenthe concentration of 4-NP and the fluorescence intensity is dis-played in the range of 0.001 to 1 μM, and the linear equationwas found to be y = –0.1618x + 0.9638 (R2 = 0.996, Figure 6B).The detection limit of 4-NP by the CDs was 0.50 μM. Theseresults indicate that the CDs are highly sensitive towards 4-NP.Similarly, with increasing concentrations of 4-NP, a new absorb-ance peak can be observed at 402 nm that gradually broadens(see Figure S6B in the Supporting Information). As shown inFigure 6C, the CD solution shows a highly selective colorimetricresponse from colorless to yellow upon the gradual addition of4-NP.

Figure 6. (A) Influence of different concentrations of 4-NP on the fluorescenceintensity of the CDs, (B) linear plots of I/I0 vs. 4-NP concentration in the range0.001–1 μM, and (C) photographs of aqueous solutions of the CDs under365 nm UV light and daylight.

Application as Fluorescent Ink

As is already known, CDs tend to self-aggregate at high concen-tration.[34] As the nanoparticles come closer together, fluores-cence quenching occurs through the formation of nonfluores-cent species with ground-state or transient excited-state inter-actions.[35] Thus, we investigated the optical properties of theCDs at different concentrations. The fluorescent color changedfrom a dark to a lighter blue under a UV lamp as the concentra-tion of the CDs was reduced (Figure 7A). The short-range inter-actions between the particles gradually disappear as the CDsolution is diluted. Upon excitation with 365 nm UV light, forconcentrations lower than 0.08 g/mL, the as-prepared CDsshow strong blue fluorescence. In addition, because of the lowcytotoxicity, good stability, and excellent fluorescence of theCDs, aqueous CD solutions of four different concentrations(0.08, 0.04, 0.0226, and 0.02 g/mL, showing clear blue fluores-cence) were employed as fluorescent ink. Letters were writtenon commercially available filter paper using a cotton bud. Asshown in Figure 7B, the filter paper, having adsorbed differentconcentrations of CDs, shows different fluorescence intensityunder UV light. The pictures displayed in Figure 7Ba–e, whichwere collected every other month for over five months at roomtemperature, illustrate that the CDs exhibit excellent store- andphotostability, thereby indicating that the CDs have potentialas decorative materials.

To take advantage of the high quantum efficiency of theCDs, further possible applications of the CDs as ink for writingwere investigated. A pen was directly injected with an aqueous

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Figure 7. (A) Photographs of CD aqueous solutions at different concentrations(from left to right: 0.4, 0.08, 0.04, 0.0266, 0.02, 0.016, 0.0133, 0.0113, 0.01,0.008, and 0.0066 g/mL) (a) in daylight and (b) under UV light. (B) Letterswritten with CD ink prepared with four different concentrations of CDs (fromtop to bottom: 0.08, 0.04, 0.0226, and 0.02 g/mL) under UV light.

solution of the CDs. The handwriting produced with this penwas not clear in daylight on filter paper (Figure 8A), but thewords could be distinctly observed with intense blue fluores-cence upon irradiation with 365 nm UV light (Figure 8B).[36]

Then, aqueous solutions of CDs and commercial ink were mixed(ratio between the CD solution and ink was 10:1) and injectedinto a pen and used for writing. It should be noted that thewords exhibited strong enough fluorescence under a UV lamp,and the handwriting could be detected in daylight (Figure 8Cand D). A photoimage of the aqueous CD/ink solution (Fig-ure 8D) was taken again after 1 year (see Figure S7 in the Sup-porting Information); the result was similar to the sample inFigure 8, and the photostability of the sample was uniform. Thisindicates that the aqueous solutions of the CDs could be usedas ink under a variety of conditions, which is beneficial for prac-tical applications.[37] In a word, both the light-invisible and theUV/Vis features of the CDs benefit their applications in counter-feit fields.[38]

Figure 8. Photoimages of letters written with CD aqueous solutions (A) indaylight and (B) under 365 UV light and words written by using a mixture ofCDs and ink (C) in daylight and (D) under 365 UV light.

Application of CDs in Living Cells

To evaluate the cytotoxicity of the CDs, a MTT assay in A-549cells was performed with the CDs. Upon exposure to 0, 0.013,0.016, 0.020, 0.027, and 0.040 g/mL CDs for 12 h, the A-549 cellsexhibited good biological viability compared with the control(Figure 9A), which clearly indicates that the CDs essentiallyshow no cytotoxicity towards the cultured cells. To exploit thegood biocompatibility of the water-soluble CDs, we employedthe CDs in HepG2 cells for in vitro bioimaging using confocalmicroscopy. Green fluorescence was observed in the livingHepG2 cells when they were incubated with CDs (0.027 g/mL)for 3 h at 37 °C (Figure 9B), which indicates that these CDs

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permeate cell membranes and could be used for fluorescenceimaging in living cells.

Figure 9. (A) Cytotoxicity assays of the CDs at different concentrations (0,0.013, 0.016, 0.020, 0.027, and 0.040 g/mL) in A-549 cells and (B) fluorescenceimages of the CDs in living HepG2 cells.

ConclusionsWe have described an easy route for the fabrication of CDs bya one-pot hydrothermal method using spinach as the precursor.The as-prepared CDs show not only low cytotoxicity but also ahigh fluorescence quantum yield and superior biocompatibility.Interestingly, the obtained CDs have been developed as dual-readout nanosensors by fluorescent and colorimetric methodsfor precise, rapid, and quantitative sensing of 2-NP and 4-NP insolution. Crucially, this photoswitching system shows excellentperformance as a fluorescent ink. We expect that this methodwill promote the development and application of CDs in bio-imaging and environmental monitoring.

Experimental SectionChemicals and Characterization: The CDs were synthesized by agreen route and the exclusive raw material was spinach, which wasobtained from a common supermarket. All glassware was thor-oughly cleaned with freshly prepared 3:1 HCl/HNO3 (aqua regia).2,4-Dinitrotoluene (DNT, 98 %) and 2,4,6-trinitrotoluene (TNT,≥98 %) were obtained from Aladdin (Shanghai), 2,4,6-trinitrophenol(TNP, AR, 98 %), 4-nitrophenol (4-NP, 98 %), 3-nitrophenol (3-NP, AR,98 %), and 2-nitrophenol (2-NP, AR, 98 %) were purchased fromAladdin (Shanghai), and 3-nitrotoluene (3-NT, 98 %), nitrobenzene(NB, AR, 98 %), and toluene (98 %) were purchased from the BeijingChemical Reagent Company (Beijing, China). Milli-Q water was usedin all experiments (resistivity = 18 MΩ).

Transmission electron microscopy (TEM) was carried out by using aJEOLFETEM-2100 instrument at an accelerating voltage of 200 kV.Fluorescence spectra were recorded with an Eclipse fluorescencespectrophotometer (Varian, USA) and a LS-55 fluorescence spec-trometer (PerkinElmer, American). Absorption spectra were re-corded with a UV-2550 spectrophotometer (SHIMADZU, Japan) atroom temperature. X-ray photoelectron spectra (XPS) were recordedby using an AXIS Ultra DLD spectrometer. FTIR spectra were ob-tained with a JASCOFT/IR-420 spectrometer.

CD Synthesis: Spinach (10 g) and water (25 mL) were placed in a50 mL hydrothermal reactor and heated at 180 °C for 24 h. Aftercooling the resulting solution naturally to room temperature theproduct was purified by filtration through 0.22 mm filter mem-branes to remove large black insoluble carbonaceous particles. Theresulting C dots were then stored at 4 °C.[39]

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Fluorescence Quenching of CDs by 4-NP and 2-NP: The CDs(100 μL) were diluted in deionized water (4 mL) and the fluores-cence intensities were recorded. 4-NP solutions with differentconcentrations were prepared by diluting different volumes of astandard 4-NP solution. Then, the CD solution (150 μL) and 4-NPsolutions (150 μL) of different concentrations were mixed at roomtemperature for 3 min. Then the fluorescence intensities of all thesamples were measured at the maximum emission wavelength ofthe CDs. The same procedure as was followed for 4-NP, was usedfor the analysis of 2-NP.

MTT and Bioimaging: The MTT assay and cell imaging tests wereperformed according to reported methods; the details of the exper-imental process used for the cell imaging are presented in the Sup-porting Information.[40]

AcknowledgmentsThe authors gratefully acknowledge financial support from theHarbin Science and Technology Bureau (project 2014RFQXJ151),the National Hi-Technology Research and Development Pro-gram (863 Program, No. 2013AA032204), the National ScienceFoundation for Post-doctoral Scientists of China (2012M520659,2013T60307), and the National Natural Science Foundation ofChina (NSF of China, 21205024).

Keywords: Carbon dots · Fluorescence · Sensors ·Fluorescent ink · Bioimaging · Green chemistry

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Received: September 10, 2017

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