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A facile synthesis of highly luminescent nitrogendoped graphene quantum dots for the detection of 2,4,6-trinitrophenol in aqueous solution† Liping Lin,a Mingcong Rong,a Sisi Lu,a Xinhong Song,a Yunxin Zhong,c Jiawei Yan,c Yiru Wanga and Xi Chen*a,b A facile bottom-up method for the synthesis of highly fluorescent nitrogen-doped graphene quantum dots (N-GQDs) has been developed via a one-step pyrolysis of citric acid and tris(hydroxymethyl)aminomethane. The obtained N-GQDs emitted strong blue fluorescence under 365 nm UV light excitation with a high quantum yield of 59.2%. They displayed excitation-independent behavior, high resistance to photobleaching and high ionic strength. In addition to the good linear relationship between the fluorescence intensity of the N-GQDs and pH in the range 2–7, the fluorescence intensity of the N-GQDs could be

Received 29th October 2014, Accepted 2nd December 2014

greatly quenched by the addition of a small amount of 2,4,6-trinitrophenol (TNP). A sensitive approach

DOI: 10.1039/c4nr06365a

has been developed for the detection of TNP with a detection limit of 0.30 μM, and a linearity ranging from 1 to 60 μM TNP could be obtained. The approach was highly selective and suitable for TNP analysis

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in natural water samples.

1 Introduction Luminescent graphene quantum dots (GQDs), a rising star amongst luminescent carbon materials, possess a graphene structure inside the dots regardless of the dot size. Similar to many other luminescent nanomaterials like semiconductor quantum dots, noble-metal nanoclusters, upconversion nanomaterials, carbon dots and so on, GQDs display new properties due to quantum confinement, including excellent optical properties, high photostability, resistance against photobleaching and good water-solubility.1,2 Meanwhile, as one kind of metalfree fluorescent nanomaterial, they also possess the advantages of a cheap and wide source, easy operation, mild preparation conditions, low toxicity and biocompatibility.3,4 These unique physicochemical properties make them potential candidates for application in biolabeling, bioimaging, drug delivery, biological and analytical sensing of biological materials and metal ions, as well as light-emitting and photovoltaic devices for energy conversion.5–7 Although much progress has a Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. E-mail: [email protected]; Fax: +86 592 2184530; Tel: +86 592 2184530 b State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005, China c Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr06365a

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been made in the synthesis of GQDs,8 the relatively low quantum yield of GQDs has restricted their applications. Thus, to obtain luminescent GQDs with a high quantum yield has become a major challenge that has to be urgently solved. Previous studies indicate that the photoluminescence phenomenon of GQDs may be derived from electron–hole recombination, quantum-size effect/free zigzag sites with a carbene-like triplet ground state, doping, and surface defects in the functional groups of the GQDs.3,8 As research has developed and in view of the relationship between the photoluminescence mechanism and the quantum yield of GQDs, researchers have found that both surface passivation and doping with heteroatoms can effectively improve the quantum yield of GQDs. On the one hand, surface passivation can efficiently suppress non-radiative recombination of localized electron–hole pairs and/or enhance the integrity of surface π electron networks, resulting in a quantum yield increase of GQDs.9–14 On the other hand, carbon nanomaterials doped with heteroatoms (typically doped by nitrogen atoms) can effectively manipulate the local chemical features and band gaps of graphene structures, leading to changes in their electronic characteristics and optical properties.15–17 This approach provides a new route to obtain amine-functionalized nitrogen-doped GQDs (N-GQDs) with high quantum yield through a facile synthesis method. At present, the methods available for the synthesis of N-GQDs include hydrothermal treatment of carbon sources in the presence of dicyandiamide,18 ammonia19,20 or hydrazine,21 strong acid treatment of

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N-doped graphene,22 the electrochemical method23 and the solution chemistry approach.24 In contrast to other methods, the hydrothermal approach is the most widely adopted for its simple operation and mild reaction conditions. 2,4,6-Trinitrophenol (TNP) is a hazardous nitroaromatic compound with strong electron-withdrawing groups. Although it has attracted less attention than 2,4,6-trinitrotoluene (TNT), in fact TNP possesses a stronger explosive ability than TNT.25,26 In general, TNP is widely used in the dye industry, fireworks, rocket fuels, pharmaceuticals, and chemical laboratories.27–29 However, a TNP with phenolic and nitro functionalities is poorly biodegradable, explosive and toxic, and is associated with human health problems such as skin irritation, anemia, abnormal liver functioning and damage to respiratory organs when people inhale, ingest or touch it.30–32 It has been recognized as an environmental contaminant and a human carcinogen. Therefore, there is an urgent need to develop methods for sensitive and selective detection of TNP. At present, the methods available for the detection of TNP include the use of a surface plasmon resonance-based immunosensor,33 proton transfer reaction-mass spectrometry,34 and liquid–liquid based microextraction techniques.35 In contrast, fluorescence methods show superiority in terms of high sensitivity, good selectivity, ease of operation and real-time detection, easy operation and lower cost of the instruments. Therefore, fluorescence methods have been most widely used so far, and some attractive fluorescence sensors and/or probes for TNP detection have been constructed using organic molecule dyes,36,37 polymer films,38 metal complexes,39,40 and a few carbon nanomaterials.30,41 In our work, highly fluorescent amine-functionalized N-GQDs were obtained using a one-step pyrolysis with citric acid (CA) as a carbon source, and tris(hydroxymethyl)aminomethane (Tris-HMA) as a surface passivation agent, as well as a dopant. The as-prepared N-GQDs emitted strong blue fluorescence under 365 nm UV light with a high quantum yield of 59.2%. Their emission wavelength displayed excitation-independent behavior, while their fluorescence intensity was in a linear relationship with the pH in the range 2–7. Using these N-GQDs, a sensitive and selective sensing approach was proposed for the detection of TNP in aqueous solution. The amino groups on the surface of N-GQDs enhanced the quantum yield of the N-GQDs and established electrostatic interaction between the N-GQDs and TNP, which played an important role in the detection of TNP in aqueous solution. Experimental results showed that the fluorescence intensity of N-GQDs showed a sensitive response to the concentration of TNP varying from 1 to 60 μM with a detection limit of 0.30 μM. The analytical potential of the sensing approach was also demonstrated in the detection of TNP in natural water samples.

2

Experimental

2.1

Reagents and chemicals

CA, methylbenzene, phenol and nitrobenzene (NB) were obtained from the Sinopharm Chemical Reagent Co., Ltd

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(Shanghai, China); Tris-HMA from Sigma-Aldrich (USA); TNP from Xilong Chemical Co., Ltd (China); TNT and 99% 4-nitrotoluene (4-NT) from J&K Scientific Ltd; 2,4-dinitrotoluene (2,4DNT) from Aladdin Reagent Co., Ltd (Shanghai, China); and 2,4-dinitrophenol (2,4-DNP) from Accu Standard, Inc. (USA). Different cation solutions in the study were prepared from their respective chloride or nitrate salts. All the reagents used were of analytical grade and were used as received without further purification. Nanopure deionized distilled water (18.2 MΩ) from a Millipore Autopure WR600A system (Millipore Ltd, USA) was used in the experiments. 2.2

Instruments

UV-Vis absorption spectra and fluorescence spectra were recorded on a UV2550 spectrophotometer (Shimadzu, Japan) and an F-4500 fluorescence spectrophotometer (Hitachi, Japan), respectively. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 380 spectrophotometer. The high resolution transmission microscopy (HRTEM) observations were performed on a TECNAI F-30 electron microscope (Philips-FEI, The Netherlands) at an accelerating voltage of 200 kV. The atomic force microscopy (AFM) images were obtained on an Agilent 5500 SPM in AC mode with a silicon probe (NT-MDT, cantilever force constant 40 N m−1). X-ray diffraction (XRD) patterns were measured on a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (40 kV, 20 mA, λ = 1.54051 Å) (Kuraray Co., Ltd, Japan). A PHI Quantum 2000 X-ray photo-electron spectroscopy system (Physical Electronics, USA) was used to investigate the functional groups present on the surface of the N-GQDs with all the binding energies calibrated using C 1s as the reference energy (EC 1s = 284.6 eV). Dynamic light scattering (DLS) and zeta potential were detected using a Nano-ZS (Malvern Instruments, UK). Raman spectra were recorded using a Nanophoton Raman 11 (excitation wavelength: 532.00 nm; laser current: 100%; excitation power: 153.978654 mW). The fluorescence lifetime was determined on a FluoroMax-4 spectrofluorometer (Horiba Jobin Yvon, France). An EOS 5D Mark II camera (Canon, Japan) was used to record the color under daylight and UV light at 365 nm. 2.3

Synthesis of N-GQDs

In a typical procedure for the preparation of amine-functionalized N-GQDs (Scheme 1), 2 g of CA and 0.5 g of Tris-HMA

Scheme 1 Illustration of the formation process of amine-functionalized N-GQDs via one-step pyrolysis of CA and Tris-HMA and their application to TNP analysis.

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were mixed in a 25 mL round-bottom flask and heated at 225 °C for 15 min. After approximately 3 min heating, transparent molten CA could be obtained. The molten substance color changed from colorless to pale yellow, and then to orange after 8 min heating, indicating the formation of N-GQDs. After cooling to room temperature, the obtained solid product was orange. 5 mL ultrapure water was added to dissolve the solid, and then 1 M NaOH solution was added dropwise to neutralize the solution to pH 7.0. Finally, the solution was subjected to dialysis to completely remove salt ions in a dialysis bag (retained molecular weight: 1000 Da) overnight to obtain pale yellow N-GQDs. The N-GQDs solution was finally diluted to 50 mL for the experiments. 2.4

Detection procedure of 2,4,6-trinitrophenol

Typically, different amounts of TNP were added into a mixture containing 10 μL of N-GQDs and 200 μL of HAc-NaAc (0.2 M, pH 8.0) buffer solution, and the solution was fast mixed and diluted to 1 mL. After reaction for 15 min, the fluorescence spectrum of the mixture solution was recorded at an excitation wavelength of 425 nm.

3

Results and discussion

3.1

Morphology and composites of the N-GQDs

The TEM image (Fig. 1a) shows that the as-prepared N-GQDs were fairly uniform and well dispersed. The diameter distribution of the N-GQDs was ca. 0.5–4.0 nm (top inset of Fig. 1), which is well consistent with those of N-GQDs prepared using the electrochemical approach.42 From the HRTEM image, the

Fig. 1 (a) TEM image of the N-GQDs (top inset: the diameter distribution of the N-GQDs; lower inset: HRTEM image of individual N-GQDs); (b) AFM image of the N-GQDs deposited on a silicon slice (inset: the height profile along the line in the topographic image of the N-GQDs); (c) FTIR spectrum, (d) zeta potential, (e) XRD pattern and (f ) Raman spectrum of the as-prepared N-GQDs.

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lattice space of the N-GQDs was accurately measured to be 0.246 nm (lower inset of Fig. 1a), which is comparable to that of graphite.43 The structures of the N-GQDs were further characterized using AFM. As shown in Fig. 1b and the inset in Fig. 1b, a typical topographic height of ca. 2.0–4.0 nm could be observed, suggesting that most of the N-GQDs consisted of ca. 5–10 graphene layers. Chemical and structural information of the N-GQDs was further obtained from the FTIR spectrum. As shown in Fig. 1c, the stretching vibrations of –OH/–NH and C–H were located at 3439 cm−1 and 2930 cm−1, respectively. The band at around 1450–1650 cm−1 was attributed to the skeletal vibrations of the aromatic rings. The presence of the stretching vibrations of –NH at 3439 cm−1 and C–NH–C at 1384 cm−1 confirmed the successful surface passivation and doping of nitrogen. In addition, the vibration peaks at 1673, 1746, 1196 and 1076 cm−1 could be ascribed to CvO vibration, CvC stretching, C–O stretching and C–H bending, respectively. These carbonyl groups on the surface resulted in the N-GQDs being highly water-soluble with a zeta potential of −22.4 mV (Fig. 1d). The XRD pattern of the N-GQDs (Fig. 1e) revealed that the peak at around 2θ = 23.5° corresponds to the graphitic structure.44 The Raman spectrum of the N-GQDs (Fig. 1f ) demonstrated the intrinsic characteristics of sp2 carbon with disorder.45 There are an obvious disordered D band at around 1344 cm−1 and a crystalline G band at about 1583 cm−1 with an intensity ratio ID/IG of 0.95. Such a large ratio indicated that the intercalation of N atoms into the conjugated carbon backbone introduced numerous defects into the structure of GQDs.46 XPS measurements were carried out in order to explore the chemical composition and the chemical bonding of the N-GQDs. The survey spectrum (Fig. 2a) of the N-GQDs showed three typical peaks of C 1s at 284.6 eV, N 1s at 399.59 eV, and O 1s at 530.79 eV with the N/C atomic ratio of ca. 4.03%, which was comparable with that of the N-GQDs prepared

Fig. 2 (a) Full-scan XPS spectrum of the N-GQDs; (b) C 1s XPS spectrum; (c) O 1s XPS spectrum; (d) N 1s XPS spectrum.

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electrochemically23 (the corresponding content of each element displayed in Fig. 2a). The high-resolution C 1s spectrum of the N-GQDs (Fig. 2b) revealed the presence of C–C (284.5 eV), CvO (286.8 eV), C–O (287.7 eV), O–CvO (288.9 eV) and C–N (285.4 eV) functional groups. Moreover, the O 1s spectrum (Fig. 2c) further confirmed these observations with two characteristic oxygen states of CvO (531.9 eV) and C–OH/C– O–C (532.9 eV). The spectrum of N 1s (Fig. 2d) in the N-GQDs indicated the presence of three relative nitrogen species of N–H (401.8 eV), N–C3 (400.8 eV) and C–N–C (399.7 eV) with the peak area ratio of 0.34 : 0.33 : 0.33, which is consistent with the results from FTIR, indicating the existence of amide groups and the doping of nitrogen atoms during the synthesis process. 3.2

Optical properties of the obtained N-GQDs

The UV-Vis absorption spectrum of the N-GQDs showed that there was a shoulder peak at 235 nm and a well-defined absorption band at 335 nm (Fig. 3a). The peak at 235 nm was attributed to the π–π* transitions of the CvC bonds. The apparent peak at 335 nm indicated the major uniform size of the sp2 clusters in the N-GQDs even though these sp2 clusters were doped in the sp3 matrix.47 The color of the N-GQDs aqueous solution was pale yellow, while the N-GQDs emitted intensely blue luminescence under irradiation from a 365 nm lamp (Fig. 3a, inset). The strong fluorescence could be observed with the naked eye, which promised potential applications in fluorescence imaging. In addition, the emission wavelength of the N-GQDs at 425 nm was nearly excitationindependent since the excitation wavelength changed from 280 nm to 380 nm, and the fluorescence intensity reached the maximum value when the excitation wavelength was set at 335 nm (Fig. 3a). Since CA exhibited its UV absorption below 250 nm only and Tris-HMA showed no absorption in the violet region (Fig. S1†), the fluorescence properties should have been caused by the formation of the N-GQDs, and the excitationindependent behavior of the N-GQDs was attributed to the fact that both the size and the surface state of the sp2 clusters contained in the N-GQDs should be uniform. Although the exact

Fig. 3 (a) UV-Vis absorption (Abs) and fluorescence emission spectra (Ex: excitation) of the N-GQDs with different excitation wavelengths. Inset: photographs of the solution of the N-GQDs taken under visible light (left) and 365 nm UV light (right); (b) the fluorescence intensity of the N-GQDs varying with the pH change (inset: upper: the color change of the N-GQDs at different pH values under 365 nm UV light; lower: the linear relationship between fluorescence intensities of the N-GQDs and pH).

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mechanisms responsible for N-GQDs photoluminescence remain to be elucidated, it is believed that the luminescence relates to the sp3 matrix.47 Previous studies report that the isolated sp2-hybridized clusters with a size of ca. 3 nm within the carbon–oxygen matrix can yield band gaps consistent with blue emission owing to the localization of electron–hole pairs.48 Therefore, in addition to size and surface effects, the oxygen-rich groups and the doping of N atoms with a strong electron-withdrawing ability make an important contribution to the observed blue shift in the photoluminescence emission of N-GQDs. In order to further explore the fluorescence properties of the as-prepared N-GQDs, the quantum yield and the effects of different extraneous factors on the fluorescence intensity of the N-GQDs were investigated. Under the excitation of 335 nm, the quantum yield of the N-GQDs was measured to be 59.2% using quinine sulfate in 0.1 M H2SO4 (QY = 0.58) as a reference. Since the exact mechanism of photoluminescence for N-GQDs is still under study, in our work we accounted for this relatively high quantum yield in two ways: (1) as illustrated by the results of FTIR (Fig. 1c) and XPS (Fig. 2), surface passivation on the N-GQDs surface with amide groups and nitrogen doping during the synthesis process; and (2) Tris-HMA used in the synthesis process provided abundant hydroxyl groups on the N-GQDs surface and reduced the carbonyl groups. On the one hand, compared with C and O atoms, the stronger electron-withdrawing ability of N atoms significantly altered the whole electronic structure in N-GQDs and enhanced the spin– orbital coupling for the process of intersystem crossing emission.16 On the other hand, the existence of the amide groups and hydroxyl groups could effectively suppress non-radiative recombination of localized electron–hole pairs and enhanced the intrinsic state emission.10 As a result, the photoluminescence and the quantum yield of the N-GQDs were sharply enhanced. The N-GQDs exhibited much higher photostability than the traditional organic dye, fluorescein isothiocyanate (FITC), since the fluorescence intensity of the N-GQDs was nearly constant while that of FITC was bleached 20% under continuous illumination at 335 nm for 2 h (Fig. S2†). Furthermore, there was only a scarce fluorescence intensity change of the N-GQDs in different concentrations of NaCl (Fig. S3†), indicating that the N-GQDs were stable and did not aggregate even under high ionic strength conditions. The results indicated the feasibility of applying N-GQDs in fluorescence imaging and sensing under physiological conditions. In addition, the fluorescence intensity of the N-GQDs presented an excellent linear relationship with pH in the range of 2–7 (Fig. 3b) and also the relative color change of the N-GQDs in different pH solutions was observed under 365 UV light (Fig. 3b inset), illustrating that the pH sensing characteristics could be used for quantitative measurements. 3.3

Fluorescence response strategy for the detection of TNP

3.3.1 Effect of pH value. The effect of pH on TNP detection was investigated over the pH range 2–12. As shown in Fig. 4a, the pH effect on the sensing system for TNP detection varied

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Fig. 4 (a) Effect of pH on the fluorescence intensity of N-GQDs responding to TNP (F0 and F1 are the fluorescence intensities of N-GQDs in the absence and in the presence of TNP, respectively); (b) selectivity of the N-GQDs for TNP in the co-existence of different cations (the concentration of TNP and each metal ion was 10 μM); (c) fluorescence quenching response of the N-GQDs for TNP to the increasing addition of different structurally related aromatics; (d) timeresolved fluorescence decay curve of the N-GQDs for the TNP analysis.

with the pH value. The fluorescence intensity change in the pH range 6–9 was nearly close to each other and showed the largest response at pH 8. The possible reason ascribed to this phenomenon was that lower pH values could decrease the deprotonation degree of TNP, while higher pH values could reduce the protonation degree of the amine group on the N-GQDs surface. This result indicated that the detection process could be carried out over a wide pH range in aqueous solution. The pH value of 8 was chosen in the following experiment since this selection provided a possibility for the formation of the zwitterionic spirocyclic Meisenheimer complex after the negatively charged TNP absorbing to the positively charged N-GQDs via strong electrostatic interactions. 3.3.2 Stability of the detection system. To investigate the stability of N-GQDs for the detection of TNP, the response kinetics of the N-GQDs were checked using the fluorescence intensity of the N-GQDs after incubation with TNP for different times. As shown in Fig. S4,† the fluorescence intensity of the N-GQDs was acutely quenched within 5 min upon the addition of TNP and remained constant when the incubation time was prolonged to 35 min. These results indicated that the fluorescence quenching rate was fast and that the fluorescence intensity remained stable for a long time, which provided the possibility for rapid and stable detection of TNP. 3.3.3 Selectivity of the sensing system. Previous studies report that metal ions combine with the hydroxyl, carboxyl and amino groups on the surface of GQDs and N-GQDs, resulting in their fluorescence quenching.49,50 To examine the selectivity of the N-GQDs towards TNP, the influence of environmentally relevant metal ions was selected and taken as the co-existing substances in the detection of TNP. As shown in

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Fig. 4b, most of the metal ions revealed a negligible effect on the detection, while Fe2+, Fe3+ and Hg2+ presented a slight effect, which could be effectively eliminated by the addition of ethylenediaminetetraacetic acid. The quenching titration experiment was further carried out in the presence of structurally related aromatics (Fig. 4c). The experimental results showed that the fluorescence intensity of the N-GQDs was gradually quenched with increasing addition of TNP, while other compounds induced only weak fluorescence quenching of the N-GQDs. Moreover, as shown in Table S1,† the quenching constant value for TNP was much larger than those of structurally related aromatics, signifying the predominant selectivity of the N-GQDs toward TNP. The quenching mechanism process was studied in order to investigate the reasons for its high selectivity. In general, the fluorescence quenching process may occur in three ways: (1) an inner filter effect (IFE), (2) electron transfer, and (3) fluorescent materials converted into non-fluorescent ones. As shown in Fig. S5,† there was spectral overlap between the absorption band of TNP and the excitation spectrum as well as the emission spectrum of the N-GQDs. Since TNP possesses a broad absorption band in the range of 285 to 460 nm, the excitation and emission bands of the N-GQDs centered at 335 and 425 nm opened up the possibility of fluorescence resonance electron transfer. Meanwhile, the fluorescence lifetime spectra (Fig. 4d) and the fluorescence intensity decay were fitted to a three exponential decay function (Table S2†) and showed that the fluorescence lifetime became shorter with the addition of 40 μM TNP (10.50 ns) and 100 μM TNP (9.18 ns) than that with the N-GQDs alone (10.64 ns). The lifetime reduction indicated fluorescence resonance electron transfer between the electronrich amino group of the N-GQDs and the electron-deficient aromatic ring of TNP, which was in agreement with previous reports.51,52 In view of the fact that the fluorescence lifetime of the fluorescent materials would not change in the IFE process, the predominant fluorescence quenching mechanism was attributed to energy transfer but the IFE. Furthermore, the hydrodynamic diameter obtained from DLS (Fig. S6†) shifted from 13.5 nm for the N-GQDs alone to 37.8 nm in the presence of 40 μM TNP and 58.8 nm in the presence of 100 μM TNP, indicating further that TNP absorbed to the N-GQDs surface via strong electrostatic interactions to form the non-fluorescent Meisenheimer complex. Therefore, the unprecedented selectivity to TNP and the quenching mechanism could be related to electron transfer and the formation of the non-fluorescent Meisenheimer complex via strong electrostatic interactions between N-GQDs and TNP. 3.3.4 Sensitivity of TNP detection. As shown in Fig. 5a, the fluorescence intensity of N-GQDs was gradually quenched upon the addition of TNP in the range 3–500 μM, and the color change could be observed by the naked eye under both daylight and UV light (Fig. 5b). Meanwhile, it should be noted that the fluorescence quenching at 425 nm was in a distinct linear relationship with the TNP concentration in the range 1–60 μM, with a linear regression equation of F0/F − 1 = 0.0081 + 0.0111 CTNP, a correlation coefficient of 0.9914 and a detection

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Fig. 5 (a) Emission spectra of the N-GQDs in the presence of various concentrations of TNP in HAc-NaAc (0.2 M, pH 8.0) buffer solution; (b) photographs of the N-GQDs in the presence of various concentrations of TNP under daylight and a 365 nm UV lamp (the TNP concentration from numbers 1 to 20 was: 0, 1, 3, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 100, 150, 200, 250, 300, 400 and 500 μM); (c) the relationship between the fluorescence intensity variation and the TNP concentration; (d) the linear relationship between the fluorescence intensity variation and TNP concentration in the range 1–60 μM.

Table 1 Detection of spiked TNP in water samples (n = 3, pH = 8.0, total organic carbon: 3.34 mg L−1, chemical oxygen demand: 29.7 mg L−1)

Samples

Added (μM)

Found (μM)

Recovery (%)

R.S.D.

1 2 3

10 15 20

9.52 14.59 19.18

95.2 97.2 95.9

1.25 2.82 3.51

zation results indicated the formation of the N-GQDs and the surface passivation and successful doping of nitrogen. The obtained N-GQDs exhibited excellent photoluminescence and excitation independent and pH-sensitive behavior. In addition, the N-GQDs were applied to the detection of TNP owing to the excellent fluorescence characteristics of the N-GQDs. The developed approach was sensitive for TNP analysis with a detection limit of 0.30 μM and a linearity ranging from 1 to 60 μM could be obtained. This approach also displayed a selective response towards TNP over other structurally related compounds. The fluorescence quenching behavior of the N-GQDs to TNP was possibly accounted for by the electron transfer and the formation of the non-fluorescent Meisenheimer complex via strong electrostatic interactions between the N-GQDs and TNP. The proposed approach was successfully used for the detection of TNP in natural water samples with satisfactory results that predicted its practicability. This study provided a strategy for the simple and fast synthesis of highly photoluminescent N-GQDs, and expanded the applications of N-GQDs in the field of analytical chemistry.

Acknowledgements This research work was financially supported by the National Nature Scientific Foundation of China (no. 21175112 and 21375112) and Program of Science and Technology of Xiamen for University Innovation (3502Z20143025), which are gratefully acknowledged. Furthermore, we would like to extend our thanks to Professor John Hodgkiss of the University of Hong Kong for his assistance with English.

Notes and references limit of 0.30 μM. The detection limit was comparable with or superior to those obtained from other sensitive fluorimetric methods (Table S3†). 3.3.5 Analysis of TNP in real water samples. The sensing approach based on N-GQDs fluorescence quenching was applied to TNP analysis in natural water samples. Water samples from the Furong Lake were centrifuged at 12 000 rpm for 20 min, and then filtered using 0.22 μm filter membranes. A recovery experiment was carried out with spiking TNP of different concentrations to the water samples. As shown in Table 1, the found values were consistent with the addition of TNP. The recoveries were in the range 95.2–97.2% and the relative standard deviations (R.S.D.) of three replicate detections for each sample were below 5%. These results proved that the approach had relatively high reproducibility and precision, and confirmed its feasibility in the detection of TNP in water samples.

4 Conclusions In this work, we synthesized highly photoluminescent N-GQDs via one-step pyrolysis of CA and Tris-HMA. Various characteri-

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A facile synthesis of highly luminescent nitrogen-doped graphene quantum dots for the detection of 2,4,6-trinitrophenol in aqueous solution.

A facile bottom-up method for the synthesis of highly fluorescent nitrogen-doped graphene quantum dots (N-GQDs) has been developed via a one-step pyro...
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