Biosensors and Bioelectronics 58 (2014) 219–225
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Synthesis of highly fluorescent nitrogen-doped graphene quantum dots for sensitive, label-free detection of Fe (III) in aqueous media Jian Ju, Wei Chen n State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 November 2013 Received in revised form 24 February 2014 Accepted 25 February 2014 Available online 6 March 2014
Heteroatom doping can drastically alter the electronic characteristics of graphene quantum dots (GQDs), thus resulting in unusual properties and related applications. Herein, we develop a simple and low-cost synthetic strategy to prepare nitrogen-doped GQDs (N-GQDs) through hydrothermal treatment of GQDs with hydrazine. The obtained N-GQDs with oxygen-rich functional groups exhibit a strong blue emission with 23.3% quantum yield (QY). Compared to GQDs, the N-GQDs exhibit enhanced fluorescence with blue-shifted energy. Due to the selective coordination to Fe3 þ , the N-GQDs can be used as a green and facile sensing platform for label-free sensitive and selective detection of Fe (III) ions in aqueous solution and real water samples. The N-GQDs fluorescence probe shows a sensitive response to Fe3 þ in a wide concentration range of 1–1945 μM with a detection limit of 90 nM (s/N ¼3). Interestingly, it is also found that both dynamic and static quenching processes occur for the detection of Fe3 þ by N-GQDs, while the quenching effect of Fe3 þ on the fluorescence of GQDs is achieved by affecting the surface states of GQDs. & 2014 Elsevier B.V. All rights reserved.
Keywords: Graphene Quantum dots Fluorescence Iron ions Chemical sensor
1. Introduction Photosensitive nanomaterials, including semiconductor quantum dots, metal nanorods, metal nanoclusters and carbon materials, have gained a considerable attention in the past decade because of their highly intense and photostable signals and promising applications in sensors (Chen et al., 2005; Ganguly et al., 2013; Ma et al., 2013; Shen et al., 2013; Wei et al., 2011). In recent years, there has been an increasing interest in carbon-based luminescent nanomaterials due to their unique properties, such as low toxicity, strong and tunable photoluminescence, high stability, high electrical and thermal conductivity, easy preparation, etc. (Lee et al., 2011, 2013; Wang et al., 2010). More recently, a novel type of luminescent carbon nanomaterial, zero-dimensional graphene quantum dots (GQDs) has attracted much attention (Pan et al., 2010). GQDs are single atom-thick graphene sheets with size smaller than 10 nm. Similar to graphene nanosheets, GQDs exhibit some excellent characteristics such as high surface area and good surface grafting by using the π–π conjugated network or abundant surface functional groups (Li et al., 2013; Williams et al., 2013; Zhang et al., 2012). Because of the good biocompatibility and low toxicity, GQDs have been used as fluorescence probes for cell imaging and related analytical applications. Yang's group reported a facile one-step
n
Corresponding author. E-mail address: [email protected] (W. Chen).
http://dx.doi.org/10.1016/j.bios.2014.02.061 0956-5663 & 2014 Elsevier B.V. All rights reserved.
method for the synthesis of GQDs with green photoluminescence (Zhu et al., 2011). Recently, a chemical method was developed by Niu's group to prepare GQDs from graphene oxide (GO) and the multicolored fluorescent GQDs were used for TNT detection (Fan et al., 2012). It was proposed that the fluorescence quenching of GQDs by TNT is closely related to the formation of GQDs–TNT complexes at the surface of GQDs (Fan et al., 2012). In another work, a novel green sensing system was reported for the detection of free residual chlorine in water based on the fluorescence quenching of GQDs by chlorine (Dong et al., 2012). However, the quantum yield (QY) of the as-synthesized GQDs is much lower than that of conventional semiconductor quantum dots. Therefore, improving the QY of GQDs is still a big challenge for their wide applications. Up to now, several methods have been developed to improve the QY of the GQDs (Jiang et al., 2013; Li et al., 2012a, 2012d). Among the reported strategies, chemical doping with heteroatoms is an effective route to tune the intrinsic properties of carbon nanomaterials (Gong et al., 2009; Li et al., 2012b; Wohlgemuth et al., 2012; Yang et al., 2012). Considering the quantum confinement and edge effect, the electronic characteristics of GQDs can be finely tuned by doping chemically-bonded N atoms to manipulate their chemical and optical properties. Several routes have been proposed to incorporate nitrogen groups into the GQD framework such as electrochemical treatments and solution chemistry approaches (Li et al., 2013). The preparation of N-doped GQDs was first reported by Qu et al. (Li et al., 2012c). In the synthesis, N-containing tetrabutylammonium perchlorate (TBAP) in acetontrile
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was used as the electrolyte to introduce nitrogen atoms into the GQD in situ. The prepared N-GQDs exhibited blue luminescence as a result of the relatively strong electron-withdrawing ability of N atoms in the N-GQDs. Hu et al. developed a simple method for the synthesis of N-GQDs through hydrothermal treatment of GO in the presence of ammonia (Hu et al., 2013). The obtained N-GQDs are highly blue-luminescent with a maximum QY as high as 24.6%. However, all these above-mentioned methods suffer from more or less drawbacks such as complex and time-consuming processes and high cost which limit their wide applications. Thus, developing simple and low-cost methods for rapid production of N-GQDs with high quantum yield is highly desired. Moreover, it is also necessary to further explore N-GQDs nanomaterials with novel properties and promising applications. In this work, we develop a simple and low-cost synthetic strategy to prepare N-GQDs by hydrothermal treatment of GQDs with hydrazine. The obtained N-GQDs show blue photoluminescence with a high QY. Meanwhile, to the best of our knowledge, although carbon quantum dots have been used to detect metal ions, no attention has been paid to the use of nitrogen-doped GQD as a fluorescent sensing platform for label-free detection of metal ions, especially Fe3 þ . Here we found that the as-prepared N-GQDs can serve as a very effective fluorescent sensing platform for labelfree sensitive and selective detection of Fe (III) ions with detection limit as low as 90 nM. The successful application of the N-GQDs in the detection of Fe3 þ ions in real water samples is also demonstrated. It is expected that the application of N-GQDs as sensing platform can be extended to other biological samples and environmental systems.
2. Experimental section 2.1. Materials Citric acid (CA), N2H4, LiNO3, KCl, MnCl2, FeCl3, CoCl2, Ni(NO3)2, CuCl2, Zn(NO3)2, AgNO3, CdCl2, PbCl2, NaOH and C2H5OH were purchased from Beijing Chemical Reagent Co., China. Ethylenediaminetetraacetic acid (EDTA) and cysteine (Cys) were purchased from Aladin Ltd. (Shanghai, China). All reagents are of analytical grade and used as received. Nanopure water (18.3 MΩ cm) was used throughout the experiments. 2.2. Characterizations Photoluminescence (PL) spectra were recorded on a PerkinElmer LS-55 luminescence spectrometer (Perkin-Elmer Instruments, UK). UV–vis absorption spectra were collected on a CARY 500 UV–vis-near-IR Varian spectrophotometer using a 1 cm path length quartz cell at room temperature. Atomic force microscopy (AFM) images were acquired using a Multimode Nanoscope V scanning probe microscopy system (Bruker, USA). The specimen was prepared by casting the aqueous suspension of N-GQDs on a freshly cleaved mica surface and then dried in air. The highresolution transmission electron microscopy (HRTEM) measurements were carried out by using a FEI TECNAI F20 EM with an accelerating voltage of 200 kV equipped with an energy dispersive spectrometer. XPS measurements were performed on a Thermo ESCALAB VG Scientific 250 spectrometer equipped with monochromatized Al Kα excitation. 2.3. Preparation of GQDs GQDs were synthesized by pyrolyzing citric acid (CA) according to the previously reported procedure (Dong et al., 2012). Briefly, 2 g of CA was put into a 5 mL beaker and heated to 200 1C by
a heating mantle for about 30 min until the citric acid changed to an orange liquid. The liquid was then dissolved in 100 mL of 10 mg mL 1 NaOH with continuous vigorous stirring. The pH of the obtained GQDs solution was adjusted to 8 by using 1 mg mL 1 CAand the sample was stored in a refrigerator. 2.4. Preparation of N-GQDs N-GQDs were prepared by hydrothermal treatment of GQDs with hydrazine (30%). In a typical synthesis, 0.30 mL hydrazine (30%) was added into 20 mL GQDs (20 mg mL 1). The mixture was then transferred into a 50 mL Teflon-lined autoclave and heated at 180 1C for 12 h. The N-GQDs were collected by removing the large dots though centrifugation at 10,000 rpm for 10 min in aqueous ethanol solution (3:1). The N-GQDs were collected by rotary evaporation after being washed with aqueous ethanol solution several times. 2.5. Quantum yields (QYs) measurements QYs of the GQDs and N-GQDs were determined by using quinine sulfate (QY¼ 0.54 in water) as the standard sample and were calculated according to the following equation: Q ¼ QR
I E R n2 I R E n2R
ð1Þ
where Q is the quantum yield, I is the measured integrated emission intensity, n is the refractive index, and E is the extinction. The subscript “R” refers to the standard with known QY. 2.6. Detection of Fe3 þ ions The detection of Fe3 þ ions was performed at room temperature in aqueous solution. In a typical run, 150 μL N-GQDs (40 mg mL 1) dispersion was added to 10 mL of nanopure water. Fe3 þ aqueous solutions of different concentrations together with other metal ion solutions were freshly prepared before use. To evaluate the sensitivity towards Fe3 þ , different concentrations of Fe3 þ were added into the aqueous solution containing the same amount of N-GQDs and the mixed solutions were equilibrated for 4 min before spectral measurements. The PL spectra were recorded by operating the fluorescence spectrophotometer with an excitation wavelength of 360 nm. 2.7. Detection of Fe3 þ in real sample The lake water was obtained from South Lake of Changchun, Jilin Province, China. The lake water was first filtered twice using a 0.22 μm membrane and then centrifuged at 10,000 rpm for 20 min to remove large solids and main impurities. Water samples with various concentrations of Fe3 þ were added to our sensing system and then the fluorescence spectra were collected. 3. Results and discussion 3.1. Characterization of GQDs and N-GQDs The morphologies of the as-prepared GQDs and N-GQDs were first characterized by AFM and TEM. Fig. 1A and B displays the AFM images of the as-prepared, well-monodispersed GQDs and nitrogen-doped GQDs. From the AFM images, it can be seen that uniform quantum dots have been formed through the synthesis processes. The height profiles reveal that the typical topographic height of the GQDs and N-GQDs is in the range of 1–2 nm. Based on the theoretical thickness of a graphene layer of 0.34 nm, the
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Fig. 1. AFM images of the GQDs (A) and N-GQDs (B) on mica substrate with the height profiles along the lines shown in the images. TEM images of the as-synthesized GQDs (C) and N-GQDs (D). Insets in (C) and (D) show the size distribution histograms of the synthesized GQDs and N-GODs. High-resolution TEM (HRTEM) images of the as-synthesized GQDs (E) and N-GQDs (F).
AFM results suggest that most of the GQDs and N-GQDs consist of 3–6 graphene layers, which is very close to that of the previously reported GQDs with 1–5 graphene layers (Li et al., 2012c). Fig. 1C and D shows the TEM images of the GQDs and N-GQDs, respectively. It can be seen that most of the N-GQDs maintain the original morphology of GQDs after the doping process. From the size histograms shown in Fig. 1C and D, the average diameters of the GQDs and N-GQDs are 4.2 and 3.8 nm, as judged from image analyses of 100 individual particles. Fig. 1E and F shows the HRTEM images of the GQDs and N-GQDs which clearly reveals the good crystallinity of both GQDs and N-GQDs with a lattice spacing of 0.23 nm corresponding to the (1 1 2 0) lattice fringes of graphene (Li et al., 2012a). All these structural characterizations indicate that the GQDs and N-GQDs with thickness of 1–2 nm (3–6 graphene layers) have been successfully prepared. The compositions of the produced GQDs and N-GQDs were then characterized by X-ray photoelectron spectroscopy (XPS). From the survey XPS spectra shown in Fig. 2A, the GQDs mainly
contain carbon and oxygen elements. In sharp contrast, the survey spectrum of N-GQDs clearly shows the presence of N besides the original carbon and oxygen. The XPS spectrum of N-GQDs shows three peaks around 284.0, 400.0, and 530.6 eV, which are attributed to C 1s, N 1s, and O 1s, respectively. The peak at 295.9 eV can be attributed to Na1s which is from NaOH (Liu et al., 2012). The N 1s spectrum of N-GQDs (Fig. 2B) shows two peaks at 399.2 and 400.5 eV which can be ascribed to the C–N–C and N–(C)3 bands, respectively (Liu et al., 2012). Moreover, on the basis of the peak intensities of carbon and nitrogen, the doping concentration of nitrogen was calculated to be 15.7%. The two peaks at 284.6 and 288.1 eV in the C 1s photoelectron spectrum (Fig. 2C) can be assigned to the binding energy of carbon in C–C, and C ¼N/C ¼O, respectively (Lu et al., 2013). The two fitted peaks at 531.7 and 533.0 eV in O 1s spectrum (Fig. 2D) are assigned to C ¼O and C–OH/C–O–C groups, respectively (Lu et al., 2012). In addition, the N-GQDs exhibit a high O/C atomic ratio of 46%. The XPS analyses clearly demonstrate that through the hydrothermal treatment of
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GQDs in the presence of hydrazine, N atoms have been successfully doped into the framework of GQDs and some functional groups in pristine GQDs like carboxyl and hydroxyl groups were also well restored. The GQDs and N-GQDs were also indentified by Fourier transform infrared (FTIR) spectroscopy (Fig. S1). The FTIR spectra from both GQDs (line 1) and N-GQDs (line 2) show the absorption of stretching vibration of C–H below 1350 cm 1. In both spectra the C–O stretching peak at 1100 cm 1 and the broad O–H stretching peak at 3402 cm 1 can be observed suggesting that GQDs and N-GQDs contain some incompletely carbonized CA. Compared to the FTIR spectrum of GQDs, new doublet peaks at 1400 cm 1, 2930 cm 1 and 1650 cm 1 can be observed in the spectrum of N-GQDs which correspond to the N–H bending and C–N stretching, respectively (Li et al., 2012d; Zheng et al., 2013). These results again indicate the successful incorporation of nitrogen atoms into the GQDs by the present synthetic process.
Fig. 3 shows the UV–vis absorption (A) and photoluminescent (PL) emission spectra (B) of the aqueous dispersion of GQDs and N-GQDs. It can be seen that the UV–vis spectrum of GQDs exhibits a typical absorption at 360 nm (Pan et al., 2012). However, the N-GQDs shows a broadened absorption band centered at 332 nm which has a blue-shift of 28 nm compared to that of the GQDs. The inset of Fig. 3A shows the photograph of aqueous solutions of GQDs (right) and N-GQDs (left) collected under excitation with a 365 nm laser. The GQDs exhibit green color, whereas the N-GQDs show blue color. Fig. 3B presents the PL emission spectra of the aqueous solutions of GQDs and N-GQDs with excitation at 360 nm; the GQDs and N-GQDs show emissions at 458 and 440 nm, respectively. The 18 nm blue-shift of the PL emission is believed to be from the strong electron affinity of N atoms doped in the N-GQD (Liu et al., 2012). Moreover, the PL intensity of N-GQDs is considerably greater than that of GQDs. The quantum yields (QYs) of GQDs and N-GQDs were calculated to be 4.8% and 23.3%,
O 1s
C 1s N 1s
Na
N-(C)3
C-N-C
N-GQDs
Intensity (a.u.)
GQDs
200
400
600
800
392
396
400
404
C=O
C-OH/C-O-C
408
C-C
C=N/C=O
280
284
288
292
528
532
536
540
Binding Energy (eV) Fig. 2. (A) XPS survey spectra of GQDs and N-GODs; XPS spectra of N 1s (B), C 1s (C) and O 1s (D) spectra of the N-GQDs.
Fig. 3. (A) UV–vis absorption spectra of the GQDs (red line) and N-GQDs (black line). Insets show the photographs of the as-prepared GQDs (right) and N-GQDs (left) aqueous solutions taken under UV light (365 nm). (B) PL spectra of the GQDs (red line) and N-GQDs (black line) aqueous solutions with excitation at 360 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Fluorescent emission spectra of the N-GQDs (A) and GQDs (C) aqueous solutions upon addition of different concentrations of Fe3 þ . Insets in (A) and (C) show the dependence of F0/F on the concentration of Fe3 þ ions within the range of 1–1945 μM and 1–594 μM, respectively. Stern–Volmer plots for the fluorescence quenching of N-GQDs (B) and GQDs (D) by Fe3 þ ions in aqueous solutions.
respectively. The highly efficient PL emission possibly results from the N-doping-induced modulation of the chemical and electronic characteristics of the GQDs (Li et al., 2012c). In addition, the abundant oxygen-rich groups in N-GQDs probably make an essential contribution to their high PL QYs (Hu et al., 2013).
200 oC 30 min
180 oC
for 12 h
The high quantum yield renders N-GQDs a possible type of promising fluorescence probe for sensing metal ions. To study the sensitivity of N-GQDs for Fe3 þ detection, different concentrations of Fe3 þ were added to the aqueous solution of N-GQDs and the fluorescence intensities were measured. Before the sensitive study, the required time for reaction equilibrium between N-GQDs and Fe3 þ was first investigated. Fig. S2 shows the time-dependent PL spectra of N-GQDs mixed with 225 μM Fe3 þ . It can be seen that the emission intensity first decreases with time due to the quenching of Fe3 þ and the PL intensity shows no change after 4 min. The result indicates that 4 min is required to reach reaction equilibrium between the N-GQDs and Fe3 þ . Therefore, in the present study, each emission spectrum was collected after 4 min of mixing N-GQDs and different concentrations of Fe3 þ . Fig. 4A shows the PL spectra of N-GQDs solution in the presence of different concentrations of Fe3 þ ions which indicates that PL intensity of the mixture is sensitive to Fe3 þ concentration and decreases with increase of Fe3 þ concentration. It is believed that the chelation of Fe3 þ with N of N-GQDs can bring them into close proximity (Zhang et al., 2013; Zong et al., 2011). The proposed complex formation process is illustrated in Scheme 1. Inset in Fig. 4A shows the dependence of F/F0 on the concentrations of
Citric acid
N2H4,
3.2. Label-free and highly selective detection of Fe3 þ ions with N-GQDs
GQDs O C N H
Fe3+
Fe3+ detection
N-GQDs
Scheme 1. Schematic representation of the procedure for synthesizing N-GQDs and the detection of Fe3 þ .
Fe3 þ ions, where F0 and F represent the fluorescence intensities of N-GQDs at 440 nm in the absence and presence of Fe3 þ , respectively. Obviously, the addition of Fe3 þ can lead to an obvious decrease of the fluorescence intensity. Good linear correlation was obtained over the concentration range from 1–1945 μM. The detection limit is estimated to be 90 nM at a signal-to-noise ratio of 3. The quenching efficiency can be fitted to the Stern–Volmer equation: F ¼ 1 þ K SV C F0
ð2Þ
The obtained Stern–Volmer plot shown in Fig. 4B fits a linear equation over the concentration range of 1–1105 μM (R2 ¼ 0.997),
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0.4
F0/F-1
0.3
0.2
0.1
0.0
Fe3+ Pb2+Ag+ Zn2+ Na+ Li+ Cu2+ Mn2+ K+ Co2+ Ni2+Cd2+Al3+ Cr3+ Hg2+
0 µM
4 µM
Fig. 5. (A) Selective PL response of aqueous N-GQDs solution towards different metal ions (excitation at 360 nm; [Mn þ ]¼ 100 μM). F0 and F are the fluorescence intensities of N-GQDs at 440 nm in the absence and presence of metal ions, respectively. Inset: the photographs of the fluorescence response of N-GQDs upon addition of different metal ions in aqueous solution under sunlight (a) and UV lamp (b). Samples from 1–15: Fe3 þ , Pb2 þ , Ag þ , Zn2 þ , Na þ , Li þ , Cu2 þ , Mn2 þ , K þ , Co2 þ , Ni2 þ , Cd2 þ , Al3 þ , Cr3 þ , and Hg2 þ ([Mn þ ]¼ 200 μM). (B) PL spectra of N-GQDs dispersion in the presence of different concentrations of Fe3 þ (from top to bottom: 0–4 mM) in lake water. Inset shows the dependence of F0/F-1 on the concentrations of Fe3 þ ions within the range of 0–4 mM. (F0 and F are the fluorescence intensities of N-GQDs aqueous solution at 440 nm in the absence and presence of Fe3 þ ions).
while the plot does not fit a linear equation over the whole concentration range of 1–1945 μM, indicating both dynamic and static quenching processes occur in this sensing system (Lai et al., 2013). The PL properties of the un-doped GQDs in the presence of Fe3 þ were also studied. Fig. 4C shows the PL spectra of the undoped GQDs upon addition of different concentrations of Fe3 þ and the inset shows the dependence of F/F0 on the concentrations of Fe3 þ ions. In this case, the fluorescence intensity decreases with concentration in the range of 1–594 μM. The Stern–Volmer plot (Fig. 4D) exhibits a good linearity over the whole concentration range. Such a result suggests that the quenching of Fe3 þ on the fluorescence of GQDs occurs by affecting their surface states (Dong et al., 2012). The results clearly show that compared to the undoped GQDs, the N-GQDs exhibit a remarkable improvement in fluorescence intensity and show higher sensitivity towards Fe3 þ quenching in a wide concentration range. The improved fluorescence performance for Fe3 þ detection can be ascribed to the N-doping-induced modulation of the chemical and electronic characteristics and the easy formation of complexes between N-GQDs and Fe3 þ (Jiang et al., 2013; Li et al., 2012b). More work
is needed to further understand the quenching mechanisms of GQDs and N-GQDs. To evaluate the selectivity of this sensing system, we examined the PL intensity change in the presence of representative metal ions under the same conditions including Li þ , K þ , Mn2 þ , Co2 þ , Ni2 þ , Cu2 þ , Zn2 þ , Cd2 þ , Pb2 þ , Na þ , Ag þ , Al3 þ , Cr3 þ and Hg2 þ . As shown in Fig. 5A, high PL quenching was observed upon the addition of Fe3 þ . Among the tested ions only Cu2 þ and Hg2 þ at a high concentration may interfere with the Fe3 þ detection. Fortunately, this issue can be circumvented by using EDTA and Cys as chelating agents for Cu2 þ and Hg2 þ ions as shown in Fig. S3. The addition of Cu2 þ and Hg2 þ ions into the N-GQDs–Fe3 þ mixture in the presence of chelating agents has no effect on the detection of Fe3 þ ions. Insets in Fig. 5A show the photographs of N-GQDs aqueous solution mixed with different metal ions ([Mn þ ¼200 μM) under daylight (inset a) and UV light (inset b). Compared to other metal ions, the significantly quenched florescence can be used for the naked-eye detection of Fe3 þ ions indicating the high selectivity of N-GQDs for Fe3 þ over other cations. The high selectivity of these N-GQDs for Fe3 þ is due to the faster chelating process of Fe3 þ ions with N-GQDs through “N” and “O” in comparison with other transition-metal ions (de Silva et al., 1997; Lai et al., 2013; Liu et al., 2012). To evaluate the N-GQDs-based Fe3 þ ions sensor in a real water system, the performance of the present fluorescence sensor for real water sample analysis was also investigated by lake water samples obtained from the South Lake of Changchun, Jilin Province, China. The resultant water samples were spiked with Fe3 þ at different concentration levels and then analyzed with the proposed method. It can be seen that the PL intensity decreases with increase in the concentration of Fe3 þ from 0 to 4 μM, as shown in Fig. 5B. The calibration curve for determining Fe3 þ in the lake water sample was obtained by plotting the values of F/F0 1 versus the concentrations of Fe3 þ (Fig. 5B inset). By applying a standard addition method over a concentration range of 2–1105 μM (y¼1.75xþ0.01; R2 ¼ 0.99), the concentration of Fe3 þ in the lake water sample was determined to be 0.3 mg/L. These results suggest that the prepared N-GQDs exhibit promising applications for monitoring Fe3 þ in real water samples.
4. Conclusion We have developed a simple hydrothermal method for the preparation of N-doped graphene quantum dots with hydrazine as nitrogen source. The as-synthesized N-GQDs exhibited an enhanced fluorescence compared to the un-doped GQDs. Such N-GQDs have been further used as a novel sensing platform for label-free sensitive and selective detection of Fe3 þ ions in aqueous solutions and real water samples. Also we found that both dynamic and static quenching mechanisms are present for the quenching effect of Fe3 þ ions on the fluorescence of N-GQDs. However, a surface process occurs for the Fe3 þ quenching on the fluorescence of un-doped GQDs. The present study provides a simple, low-cost route toward production of N-GQDs for sensing, bioimaging, optical imaging and other applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21275136 and 21043013) and the National Natural Science Foundation of China, Jilin Province, China (No. 201215090).
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Synthesis of highly fluorescent nitrogen-doped graphene quantum dots for sensitive, label-free detection of Fe (III) in aqueous media Jian Ju, Wei Chen n State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 November 2013 Received in revised form 24 February 2014 Accepted 25 February 2014 Available online 6 March 2014
Heteroatom doping can drastically alter the electronic characteristics of graphene quantum dots (GQDs), thus resulting in unusual properties and related applications. Herein, we develop a simple and low-cost synthetic strategy to prepare nitrogen-doped GQDs (N-GQDs) through hydrothermal treatment of GQDs with hydrazine. The obtained N-GQDs with oxygen-rich functional groups exhibit a strong blue emission with 23.3% quantum yield (QY). Compared to GQDs, the N-GQDs exhibit enhanced fluorescence with blue-shifted energy. Due to the selective coordination to Fe3 þ , the N-GQDs can be used as a green and facile sensing platform for label-free sensitive and selective detection of Fe (III) ions in aqueous solution and real water samples. The N-GQDs fluorescence probe shows a sensitive response to Fe3 þ in a wide concentration range of 1–1945 μM with a detection limit of 90 nM (s/N ¼3). Interestingly, it is also found that both dynamic and static quenching processes occur for the detection of Fe3 þ by N-GQDs, while the quenching effect of Fe3 þ on the fluorescence of GQDs is achieved by affecting the surface states of GQDs. & 2014 Elsevier B.V. All rights reserved.
Keywords: Graphene Quantum dots Fluorescence Iron ions Chemical sensor
1. Introduction Photosensitive nanomaterials, including semiconductor quantum dots, metal nanorods, metal nanoclusters and carbon materials, have gained a considerable attention in the past decade because of their highly intense and photostable signals and promising applications in sensors (Chen et al., 2005; Ganguly et al., 2013; Ma et al., 2013; Shen et al., 2013; Wei et al., 2011). In recent years, there has been an increasing interest in carbon-based luminescent nanomaterials due to their unique properties, such as low toxicity, strong and tunable photoluminescence, high stability, high electrical and thermal conductivity, easy preparation, etc. (Lee et al., 2011, 2013; Wang et al., 2010). More recently, a novel type of luminescent carbon nanomaterial, zero-dimensional graphene quantum dots (GQDs) has attracted much attention (Pan et al., 2010). GQDs are single atom-thick graphene sheets with size smaller than 10 nm. Similar to graphene nanosheets, GQDs exhibit some excellent characteristics such as high surface area and good surface grafting by using the π–π conjugated network or abundant surface functional groups (Li et al., 2013; Williams et al., 2013; Zhang et al., 2012). Because of the good biocompatibility and low toxicity, GQDs have been used as fluorescence probes for cell imaging and related analytical applications. Yang's group reported a facile one-step
n
Corresponding author. E-mail address: [email protected] (W. Chen).
http://dx.doi.org/10.1016/j.bios.2014.02.061 0956-5663 & 2014 Elsevier B.V. All rights reserved.
method for the synthesis of GQDs with green photoluminescence (Zhu et al., 2011). Recently, a chemical method was developed by Niu's group to prepare GQDs from graphene oxide (GO) and the multicolored fluorescent GQDs were used for TNT detection (Fan et al., 2012). It was proposed that the fluorescence quenching of GQDs by TNT is closely related to the formation of GQDs–TNT complexes at the surface of GQDs (Fan et al., 2012). In another work, a novel green sensing system was reported for the detection of free residual chlorine in water based on the fluorescence quenching of GQDs by chlorine (Dong et al., 2012). However, the quantum yield (QY) of the as-synthesized GQDs is much lower than that of conventional semiconductor quantum dots. Therefore, improving the QY of GQDs is still a big challenge for their wide applications. Up to now, several methods have been developed to improve the QY of the GQDs (Jiang et al., 2013; Li et al., 2012a, 2012d). Among the reported strategies, chemical doping with heteroatoms is an effective route to tune the intrinsic properties of carbon nanomaterials (Gong et al., 2009; Li et al., 2012b; Wohlgemuth et al., 2012; Yang et al., 2012). Considering the quantum confinement and edge effect, the electronic characteristics of GQDs can be finely tuned by doping chemically-bonded N atoms to manipulate their chemical and optical properties. Several routes have been proposed to incorporate nitrogen groups into the GQD framework such as electrochemical treatments and solution chemistry approaches (Li et al., 2013). The preparation of N-doped GQDs was first reported by Qu et al. (Li et al., 2012c). In the synthesis, N-containing tetrabutylammonium perchlorate (TBAP) in acetontrile
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was used as the electrolyte to introduce nitrogen atoms into the GQD in situ. The prepared N-GQDs exhibited blue luminescence as a result of the relatively strong electron-withdrawing ability of N atoms in the N-GQDs. Hu et al. developed a simple method for the synthesis of N-GQDs through hydrothermal treatment of GO in the presence of ammonia (Hu et al., 2013). The obtained N-GQDs are highly blue-luminescent with a maximum QY as high as 24.6%. However, all these above-mentioned methods suffer from more or less drawbacks such as complex and time-consuming processes and high cost which limit their wide applications. Thus, developing simple and low-cost methods for rapid production of N-GQDs with high quantum yield is highly desired. Moreover, it is also necessary to further explore N-GQDs nanomaterials with novel properties and promising applications. In this work, we develop a simple and low-cost synthetic strategy to prepare N-GQDs by hydrothermal treatment of GQDs with hydrazine. The obtained N-GQDs show blue photoluminescence with a high QY. Meanwhile, to the best of our knowledge, although carbon quantum dots have been used to detect metal ions, no attention has been paid to the use of nitrogen-doped GQD as a fluorescent sensing platform for label-free detection of metal ions, especially Fe3 þ . Here we found that the as-prepared N-GQDs can serve as a very effective fluorescent sensing platform for labelfree sensitive and selective detection of Fe (III) ions with detection limit as low as 90 nM. The successful application of the N-GQDs in the detection of Fe3 þ ions in real water samples is also demonstrated. It is expected that the application of N-GQDs as sensing platform can be extended to other biological samples and environmental systems.
2. Experimental section 2.1. Materials Citric acid (CA), N2H4, LiNO3, KCl, MnCl2, FeCl3, CoCl2, Ni(NO3)2, CuCl2, Zn(NO3)2, AgNO3, CdCl2, PbCl2, NaOH and C2H5OH were purchased from Beijing Chemical Reagent Co., China. Ethylenediaminetetraacetic acid (EDTA) and cysteine (Cys) were purchased from Aladin Ltd. (Shanghai, China). All reagents are of analytical grade and used as received. Nanopure water (18.3 MΩ cm) was used throughout the experiments. 2.2. Characterizations Photoluminescence (PL) spectra were recorded on a PerkinElmer LS-55 luminescence spectrometer (Perkin-Elmer Instruments, UK). UV–vis absorption spectra were collected on a CARY 500 UV–vis-near-IR Varian spectrophotometer using a 1 cm path length quartz cell at room temperature. Atomic force microscopy (AFM) images were acquired using a Multimode Nanoscope V scanning probe microscopy system (Bruker, USA). The specimen was prepared by casting the aqueous suspension of N-GQDs on a freshly cleaved mica surface and then dried in air. The highresolution transmission electron microscopy (HRTEM) measurements were carried out by using a FEI TECNAI F20 EM with an accelerating voltage of 200 kV equipped with an energy dispersive spectrometer. XPS measurements were performed on a Thermo ESCALAB VG Scientific 250 spectrometer equipped with monochromatized Al Kα excitation. 2.3. Preparation of GQDs GQDs were synthesized by pyrolyzing citric acid (CA) according to the previously reported procedure (Dong et al., 2012). Briefly, 2 g of CA was put into a 5 mL beaker and heated to 200 1C by
a heating mantle for about 30 min until the citric acid changed to an orange liquid. The liquid was then dissolved in 100 mL of 10 mg mL 1 NaOH with continuous vigorous stirring. The pH of the obtained GQDs solution was adjusted to 8 by using 1 mg mL 1 CAand the sample was stored in a refrigerator. 2.4. Preparation of N-GQDs N-GQDs were prepared by hydrothermal treatment of GQDs with hydrazine (30%). In a typical synthesis, 0.30 mL hydrazine (30%) was added into 20 mL GQDs (20 mg mL 1). The mixture was then transferred into a 50 mL Teflon-lined autoclave and heated at 180 1C for 12 h. The N-GQDs were collected by removing the large dots though centrifugation at 10,000 rpm for 10 min in aqueous ethanol solution (3:1). The N-GQDs were collected by rotary evaporation after being washed with aqueous ethanol solution several times. 2.5. Quantum yields (QYs) measurements QYs of the GQDs and N-GQDs were determined by using quinine sulfate (QY¼ 0.54 in water) as the standard sample and were calculated according to the following equation: Q ¼ QR
I E R n2 I R E n2R
ð1Þ
where Q is the quantum yield, I is the measured integrated emission intensity, n is the refractive index, and E is the extinction. The subscript “R” refers to the standard with known QY. 2.6. Detection of Fe3 þ ions The detection of Fe3 þ ions was performed at room temperature in aqueous solution. In a typical run, 150 μL N-GQDs (40 mg mL 1) dispersion was added to 10 mL of nanopure water. Fe3 þ aqueous solutions of different concentrations together with other metal ion solutions were freshly prepared before use. To evaluate the sensitivity towards Fe3 þ , different concentrations of Fe3 þ were added into the aqueous solution containing the same amount of N-GQDs and the mixed solutions were equilibrated for 4 min before spectral measurements. The PL spectra were recorded by operating the fluorescence spectrophotometer with an excitation wavelength of 360 nm. 2.7. Detection of Fe3 þ in real sample The lake water was obtained from South Lake of Changchun, Jilin Province, China. The lake water was first filtered twice using a 0.22 μm membrane and then centrifuged at 10,000 rpm for 20 min to remove large solids and main impurities. Water samples with various concentrations of Fe3 þ were added to our sensing system and then the fluorescence spectra were collected. 3. Results and discussion 3.1. Characterization of GQDs and N-GQDs The morphologies of the as-prepared GQDs and N-GQDs were first characterized by AFM and TEM. Fig. 1A and B displays the AFM images of the as-prepared, well-monodispersed GQDs and nitrogen-doped GQDs. From the AFM images, it can be seen that uniform quantum dots have been formed through the synthesis processes. The height profiles reveal that the typical topographic height of the GQDs and N-GQDs is in the range of 1–2 nm. Based on the theoretical thickness of a graphene layer of 0.34 nm, the
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Fig. 1. AFM images of the GQDs (A) and N-GQDs (B) on mica substrate with the height profiles along the lines shown in the images. TEM images of the as-synthesized GQDs (C) and N-GQDs (D). Insets in (C) and (D) show the size distribution histograms of the synthesized GQDs and N-GODs. High-resolution TEM (HRTEM) images of the as-synthesized GQDs (E) and N-GQDs (F).
AFM results suggest that most of the GQDs and N-GQDs consist of 3–6 graphene layers, which is very close to that of the previously reported GQDs with 1–5 graphene layers (Li et al., 2012c). Fig. 1C and D shows the TEM images of the GQDs and N-GQDs, respectively. It can be seen that most of the N-GQDs maintain the original morphology of GQDs after the doping process. From the size histograms shown in Fig. 1C and D, the average diameters of the GQDs and N-GQDs are 4.2 and 3.8 nm, as judged from image analyses of 100 individual particles. Fig. 1E and F shows the HRTEM images of the GQDs and N-GQDs which clearly reveals the good crystallinity of both GQDs and N-GQDs with a lattice spacing of 0.23 nm corresponding to the (1 1 2 0) lattice fringes of graphene (Li et al., 2012a). All these structural characterizations indicate that the GQDs and N-GQDs with thickness of 1–2 nm (3–6 graphene layers) have been successfully prepared. The compositions of the produced GQDs and N-GQDs were then characterized by X-ray photoelectron spectroscopy (XPS). From the survey XPS spectra shown in Fig. 2A, the GQDs mainly
contain carbon and oxygen elements. In sharp contrast, the survey spectrum of N-GQDs clearly shows the presence of N besides the original carbon and oxygen. The XPS spectrum of N-GQDs shows three peaks around 284.0, 400.0, and 530.6 eV, which are attributed to C 1s, N 1s, and O 1s, respectively. The peak at 295.9 eV can be attributed to Na1s which is from NaOH (Liu et al., 2012). The N 1s spectrum of N-GQDs (Fig. 2B) shows two peaks at 399.2 and 400.5 eV which can be ascribed to the C–N–C and N–(C)3 bands, respectively (Liu et al., 2012). Moreover, on the basis of the peak intensities of carbon and nitrogen, the doping concentration of nitrogen was calculated to be 15.7%. The two peaks at 284.6 and 288.1 eV in the C 1s photoelectron spectrum (Fig. 2C) can be assigned to the binding energy of carbon in C–C, and C ¼N/C ¼O, respectively (Lu et al., 2013). The two fitted peaks at 531.7 and 533.0 eV in O 1s spectrum (Fig. 2D) are assigned to C ¼O and C–OH/C–O–C groups, respectively (Lu et al., 2012). In addition, the N-GQDs exhibit a high O/C atomic ratio of 46%. The XPS analyses clearly demonstrate that through the hydrothermal treatment of
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GQDs in the presence of hydrazine, N atoms have been successfully doped into the framework of GQDs and some functional groups in pristine GQDs like carboxyl and hydroxyl groups were also well restored. The GQDs and N-GQDs were also indentified by Fourier transform infrared (FTIR) spectroscopy (Fig. S1). The FTIR spectra from both GQDs (line 1) and N-GQDs (line 2) show the absorption of stretching vibration of C–H below 1350 cm 1. In both spectra the C–O stretching peak at 1100 cm 1 and the broad O–H stretching peak at 3402 cm 1 can be observed suggesting that GQDs and N-GQDs contain some incompletely carbonized CA. Compared to the FTIR spectrum of GQDs, new doublet peaks at 1400 cm 1, 2930 cm 1 and 1650 cm 1 can be observed in the spectrum of N-GQDs which correspond to the N–H bending and C–N stretching, respectively (Li et al., 2012d; Zheng et al., 2013). These results again indicate the successful incorporation of nitrogen atoms into the GQDs by the present synthetic process.
Fig. 3 shows the UV–vis absorption (A) and photoluminescent (PL) emission spectra (B) of the aqueous dispersion of GQDs and N-GQDs. It can be seen that the UV–vis spectrum of GQDs exhibits a typical absorption at 360 nm (Pan et al., 2012). However, the N-GQDs shows a broadened absorption band centered at 332 nm which has a blue-shift of 28 nm compared to that of the GQDs. The inset of Fig. 3A shows the photograph of aqueous solutions of GQDs (right) and N-GQDs (left) collected under excitation with a 365 nm laser. The GQDs exhibit green color, whereas the N-GQDs show blue color. Fig. 3B presents the PL emission spectra of the aqueous solutions of GQDs and N-GQDs with excitation at 360 nm; the GQDs and N-GQDs show emissions at 458 and 440 nm, respectively. The 18 nm blue-shift of the PL emission is believed to be from the strong electron affinity of N atoms doped in the N-GQD (Liu et al., 2012). Moreover, the PL intensity of N-GQDs is considerably greater than that of GQDs. The quantum yields (QYs) of GQDs and N-GQDs were calculated to be 4.8% and 23.3%,
O 1s
C 1s N 1s
Na
N-(C)3
C-N-C
N-GQDs
Intensity (a.u.)
GQDs
200
400
600
800
392
396
400
404
C=O
C-OH/C-O-C
408
C-C
C=N/C=O
280
284
288
292
528
532
536
540
Binding Energy (eV) Fig. 2. (A) XPS survey spectra of GQDs and N-GODs; XPS spectra of N 1s (B), C 1s (C) and O 1s (D) spectra of the N-GQDs.
Fig. 3. (A) UV–vis absorption spectra of the GQDs (red line) and N-GQDs (black line). Insets show the photographs of the as-prepared GQDs (right) and N-GQDs (left) aqueous solutions taken under UV light (365 nm). (B) PL spectra of the GQDs (red line) and N-GQDs (black line) aqueous solutions with excitation at 360 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Fluorescent emission spectra of the N-GQDs (A) and GQDs (C) aqueous solutions upon addition of different concentrations of Fe3 þ . Insets in (A) and (C) show the dependence of F0/F on the concentration of Fe3 þ ions within the range of 1–1945 μM and 1–594 μM, respectively. Stern–Volmer plots for the fluorescence quenching of N-GQDs (B) and GQDs (D) by Fe3 þ ions in aqueous solutions.
respectively. The highly efficient PL emission possibly results from the N-doping-induced modulation of the chemical and electronic characteristics of the GQDs (Li et al., 2012c). In addition, the abundant oxygen-rich groups in N-GQDs probably make an essential contribution to their high PL QYs (Hu et al., 2013).
200 oC 30 min
180 oC
for 12 h
The high quantum yield renders N-GQDs a possible type of promising fluorescence probe for sensing metal ions. To study the sensitivity of N-GQDs for Fe3 þ detection, different concentrations of Fe3 þ were added to the aqueous solution of N-GQDs and the fluorescence intensities were measured. Before the sensitive study, the required time for reaction equilibrium between N-GQDs and Fe3 þ was first investigated. Fig. S2 shows the time-dependent PL spectra of N-GQDs mixed with 225 μM Fe3 þ . It can be seen that the emission intensity first decreases with time due to the quenching of Fe3 þ and the PL intensity shows no change after 4 min. The result indicates that 4 min is required to reach reaction equilibrium between the N-GQDs and Fe3 þ . Therefore, in the present study, each emission spectrum was collected after 4 min of mixing N-GQDs and different concentrations of Fe3 þ . Fig. 4A shows the PL spectra of N-GQDs solution in the presence of different concentrations of Fe3 þ ions which indicates that PL intensity of the mixture is sensitive to Fe3 þ concentration and decreases with increase of Fe3 þ concentration. It is believed that the chelation of Fe3 þ with N of N-GQDs can bring them into close proximity (Zhang et al., 2013; Zong et al., 2011). The proposed complex formation process is illustrated in Scheme 1. Inset in Fig. 4A shows the dependence of F/F0 on the concentrations of
Citric acid
N2H4,
3.2. Label-free and highly selective detection of Fe3 þ ions with N-GQDs
GQDs O C N H
Fe3+
Fe3+ detection
N-GQDs
Scheme 1. Schematic representation of the procedure for synthesizing N-GQDs and the detection of Fe3 þ .
Fe3 þ ions, where F0 and F represent the fluorescence intensities of N-GQDs at 440 nm in the absence and presence of Fe3 þ , respectively. Obviously, the addition of Fe3 þ can lead to an obvious decrease of the fluorescence intensity. Good linear correlation was obtained over the concentration range from 1–1945 μM. The detection limit is estimated to be 90 nM at a signal-to-noise ratio of 3. The quenching efficiency can be fitted to the Stern–Volmer equation: F ¼ 1 þ K SV C F0
ð2Þ
The obtained Stern–Volmer plot shown in Fig. 4B fits a linear equation over the concentration range of 1–1105 μM (R2 ¼ 0.997),
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0.4
F0/F-1
0.3
0.2
0.1
0.0
Fe3+ Pb2+Ag+ Zn2+ Na+ Li+ Cu2+ Mn2+ K+ Co2+ Ni2+Cd2+Al3+ Cr3+ Hg2+
0 µM
4 µM
Fig. 5. (A) Selective PL response of aqueous N-GQDs solution towards different metal ions (excitation at 360 nm; [Mn þ ]¼ 100 μM). F0 and F are the fluorescence intensities of N-GQDs at 440 nm in the absence and presence of metal ions, respectively. Inset: the photographs of the fluorescence response of N-GQDs upon addition of different metal ions in aqueous solution under sunlight (a) and UV lamp (b). Samples from 1–15: Fe3 þ , Pb2 þ , Ag þ , Zn2 þ , Na þ , Li þ , Cu2 þ , Mn2 þ , K þ , Co2 þ , Ni2 þ , Cd2 þ , Al3 þ , Cr3 þ , and Hg2 þ ([Mn þ ]¼ 200 μM). (B) PL spectra of N-GQDs dispersion in the presence of different concentrations of Fe3 þ (from top to bottom: 0–4 mM) in lake water. Inset shows the dependence of F0/F-1 on the concentrations of Fe3 þ ions within the range of 0–4 mM. (F0 and F are the fluorescence intensities of N-GQDs aqueous solution at 440 nm in the absence and presence of Fe3 þ ions).
while the plot does not fit a linear equation over the whole concentration range of 1–1945 μM, indicating both dynamic and static quenching processes occur in this sensing system (Lai et al., 2013). The PL properties of the un-doped GQDs in the presence of Fe3 þ were also studied. Fig. 4C shows the PL spectra of the undoped GQDs upon addition of different concentrations of Fe3 þ and the inset shows the dependence of F/F0 on the concentrations of Fe3 þ ions. In this case, the fluorescence intensity decreases with concentration in the range of 1–594 μM. The Stern–Volmer plot (Fig. 4D) exhibits a good linearity over the whole concentration range. Such a result suggests that the quenching of Fe3 þ on the fluorescence of GQDs occurs by affecting their surface states (Dong et al., 2012). The results clearly show that compared to the undoped GQDs, the N-GQDs exhibit a remarkable improvement in fluorescence intensity and show higher sensitivity towards Fe3 þ quenching in a wide concentration range. The improved fluorescence performance for Fe3 þ detection can be ascribed to the N-doping-induced modulation of the chemical and electronic characteristics and the easy formation of complexes between N-GQDs and Fe3 þ (Jiang et al., 2013; Li et al., 2012b). More work
is needed to further understand the quenching mechanisms of GQDs and N-GQDs. To evaluate the selectivity of this sensing system, we examined the PL intensity change in the presence of representative metal ions under the same conditions including Li þ , K þ , Mn2 þ , Co2 þ , Ni2 þ , Cu2 þ , Zn2 þ , Cd2 þ , Pb2 þ , Na þ , Ag þ , Al3 þ , Cr3 þ and Hg2 þ . As shown in Fig. 5A, high PL quenching was observed upon the addition of Fe3 þ . Among the tested ions only Cu2 þ and Hg2 þ at a high concentration may interfere with the Fe3 þ detection. Fortunately, this issue can be circumvented by using EDTA and Cys as chelating agents for Cu2 þ and Hg2 þ ions as shown in Fig. S3. The addition of Cu2 þ and Hg2 þ ions into the N-GQDs–Fe3 þ mixture in the presence of chelating agents has no effect on the detection of Fe3 þ ions. Insets in Fig. 5A show the photographs of N-GQDs aqueous solution mixed with different metal ions ([Mn þ ¼200 μM) under daylight (inset a) and UV light (inset b). Compared to other metal ions, the significantly quenched florescence can be used for the naked-eye detection of Fe3 þ ions indicating the high selectivity of N-GQDs for Fe3 þ over other cations. The high selectivity of these N-GQDs for Fe3 þ is due to the faster chelating process of Fe3 þ ions with N-GQDs through “N” and “O” in comparison with other transition-metal ions (de Silva et al., 1997; Lai et al., 2013; Liu et al., 2012). To evaluate the N-GQDs-based Fe3 þ ions sensor in a real water system, the performance of the present fluorescence sensor for real water sample analysis was also investigated by lake water samples obtained from the South Lake of Changchun, Jilin Province, China. The resultant water samples were spiked with Fe3 þ at different concentration levels and then analyzed with the proposed method. It can be seen that the PL intensity decreases with increase in the concentration of Fe3 þ from 0 to 4 μM, as shown in Fig. 5B. The calibration curve for determining Fe3 þ in the lake water sample was obtained by plotting the values of F/F0 1 versus the concentrations of Fe3 þ (Fig. 5B inset). By applying a standard addition method over a concentration range of 2–1105 μM (y¼1.75xþ0.01; R2 ¼ 0.99), the concentration of Fe3 þ in the lake water sample was determined to be 0.3 mg/L. These results suggest that the prepared N-GQDs exhibit promising applications for monitoring Fe3 þ in real water samples.
4. Conclusion We have developed a simple hydrothermal method for the preparation of N-doped graphene quantum dots with hydrazine as nitrogen source. The as-synthesized N-GQDs exhibited an enhanced fluorescence compared to the un-doped GQDs. Such N-GQDs have been further used as a novel sensing platform for label-free sensitive and selective detection of Fe3 þ ions in aqueous solutions and real water samples. Also we found that both dynamic and static quenching mechanisms are present for the quenching effect of Fe3 þ ions on the fluorescence of N-GQDs. However, a surface process occurs for the Fe3 þ quenching on the fluorescence of un-doped GQDs. The present study provides a simple, low-cost route toward production of N-GQDs for sensing, bioimaging, optical imaging and other applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21275136 and 21043013) and the National Natural Science Foundation of China, Jilin Province, China (No. 201215090).
J. Ju, W. Chen / Biosensors and Bioelectronics 58 (2014) 219–225
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