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A facile microwave-assisted fabrication of fluorescent carbon nitride quantum dots and their application in the detection of mercury ions
Accepted Manuscript A facile microwave-assisted fabrication of fluorescent carbon nitride quantum dots and their application in the detection of mercury ions Xiaotong Cao, Jie Ma, Yanping Lin, Bixia Yao, Feiming Li, Wen Weng, Xiuchun Lin PII: DOI: Reference:
S1386-1425(15)30083-4 http://dx.doi.org/10.1016/j.saa.2015.07.034 SAA 13933
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
19 February 2015 23 June 2015 7 July 2015
Please cite this article as: X. Cao, J. Ma, Y. Lin, B. Yao, F. Li, W. Weng, X. Lin, A facile microwave-assisted fabrication of fluorescent carbon nitride quantum dots and their application in the detection of mercury ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.07.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A facile microwave-assisted fabrication of fluorescent carbon nitride quantum dots and their application in the detection of mercury ions Xiaotong Caoa,b, Jie Maa,b, Yanping Lina,b, Bixia Yaoa,b, Feiming Lia,b, Wen Weng*a,b, Xiuchun Lin c a
Department of Chemistry and Environmental Science, Minnan Normal University, Zhangzhou, 363000,
China. E-mail: [email protected]; +86 596 2523251 b
Fujian Provincial Key Laboratory of Modern Analytical Science and Separation Technology, Zhangzhou,
363000, China. c
College of Environmental and Engineering, Putian University, Putian 351100, China
AB STRACT A facile microwave-assisted solvothermal method was used to prepare fluorescent carbon nitride quantum dots (CNQDs) using oleic acid as the reaction media at moderate reaction temperature in a short time (5 min). Citric acid monohydrate and urea were used as the precursors. The as-prepared CNQDs were characterized by multiple analytical techniques. The CNQDs exhibited an uncommon excitation wavelength-dependent fluorescence with two maximum emission peaks at 450 and 540 nm. The CNQDs with a quantum yield of 27.1% could serve as an effective fluorescent sensing platform for label-free sensitive detection of Hg2+ ions with a detection limit of 0.14 µM. This method was also applied to the detection of Hg2+ ions in tap water samples.
Keywords: Carbon nitride quantum dots; Fluorescent probe; mercury ions; Citric acid; Urea
___________________________________________________________________________ Introduction Fluorescent nanoparticles have greatly attracted attention recently because of their promising applications in many fields such as bioimaging, biosensing, ion detection, optoelectronic devices and photocatalysis [1-8]. This kind of material includes carbon dots, nanodiamonds, carbon nanotubes, fluorescent graphene, semiconductor quantum dots, etc. Because people have recognized the toxicity of 1
heavy metals-containing quantum dots, seeking for “safe” and effective fluorescent nanoparticles has become an important tendency. A typical representative is carbon dots. Rapid progress has been made since they were proposed in 2004 [9-15]. Carbon nitride is another promising candidate to complement carbon in materials applications [16]. Graphitic carbon nitride (g-C3N4), considered to be the most stable allotrope among various carbon nitrides, has been widely used in water splitting, solar energy transfer, and pollutant degradation [17]. It is usually prepared from nitrogen-rich precursors such as melamine, cyanamide and dicyandiamide by high-temperature pyrolysis process. The resultant carbon nitrides generally have large particle sizes, poor luminescent properties, and poor water-solubility. Recently, the fabrication and application of fluorescent carbon nitride quantum dots or nanosheets with enhanced photoresponsive property were reported by some research groups. Barman reported a microwave mediated method to prepare highly fluorescent graphitic carbon nitride quantum dots from formamide. The carbon nitride quantum dots can play a dual role for selective and sensitive detection of mercury ions as well as iodide ions in aqueous media [18]. Xie et al. reported a pathway to prepare the ultrathin graphitic-phase C3N4 nanosheets for bioimaging by a liquid exfoliation route from bulk g-C3N4 [19]. Chen et al. developed an effective and facile fluorescence sensing approach for the label-free and selective determination of Cr(VI) using graphitic carbon nitride nanosheets [20]. Developing simple methods for synthesizing fluorescent carbon nitride nanoparticles is very meaningful. Microwave heating has been used in syntheses of organic compounds, organometallic compounds, coordination compounds, and polymers. Microwave irradiation offers a number of advantages over conventional heating methods, including using less energy, offering a higher heating rate, and offering the ability to more quickly start and stop heating [21]. In this report, a facile microwave-assisted solvothermal method was used to prepare CNQDs at moderate reaction temperature in a short time. Unlike hydrothermal method, oleic acid was used as the reaction media. The fabrication process did not use any inorganic acid or metal ion. Water generated in the reaction process may act as a “soft-template”, and modulate the size of the formed nanoparticles [22]. The as-prepared CNQDs were applied to the detection of mercury ions.
Experimental Chemicals and Apparatus Citric acid monohydrate (CA), urea and oleic acid were all analytical reagents. Rhodamine 6G was 2
purchased from Aladdin Industrial Corporation. Ultrapure water was prepared with a Milli-Q system (Millipore, Bedford, MA, USA). An Anton Paar Monowave 300 microwave synthesizer (Austria) was used to perform all the preparation experiments. Preparation of carbon nitride quantum dots (CNQDs) The CNQDs were synthesized by microwave-assisted solvothermal method. A typical synthesis process is as follows. Citric acid monohydrate (0.5 g), urea (0.5 g) and oleic acid (10 ml) were placed in a 30 ml reaction tube. Then the mixture was heated at 180 0C for 5 min under 1200 r/min magnetic stirring. The pressure was maintained at 4.6 bar during the process of the reaction. Black solid precipitate at the bottom of reaction tube could be separated directly after the reaction. The precipitate was washed sufficiently with n-hexane, dispersed in ultrapure water, and centrifuged at 6000 rpm for 30 min to remove large particles product. Characterization A UV-2550 spectrophotometer (Shimadzu) was used to record the UV–vis spectra of the CNQDs. The fluorescence spectra were obtained by a Varian Cary Eclipse Fluorescence Spectrophotometer (Varian). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded using FEI Tecnai G2 F20 instruments (FEI). FTIR spectra were recorded using a Magna-IR 750 Fourier transform infrared spectrometer (Nicolet). The X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Thermo ESCALAB 250Xi multifunctional imaging electron spectrometer (Thermo Fisher). The X-ray diffraction (XRD) spectra were recorded using a Bruker DAVINCI D8 ADVANCE diffractometer. An AV600 nuclear magnetic spectrometer (Bruker) was used to record the 13C-NMR and 1
H-NMR spectra.
Quantum yield measurement The quantum yield (QY) of the obtained CNQDs was measured according to the established procedure [23]. Rhodamine 6G dissolved in ultrapure water (literature ΦR=0.95 at an excitation wavelength of 423 nm) was chosen as the standard. Absorbencies in the 10 mm fluorescence cuvette were kept under 0.1 at the excitation wavelengths. The quantum yield of the CNQDs was determined by the following equation:
3
Φ = ΦR ×
I AR η 2 × × I R A η R2
where Φ is the quantum yield of the sample, I is the measured integrated emission intensity, η is the refractive index of the solvent, and A is the optical density. The subscript “R” refers to the standard with known QY.
Results and discussion Structural characterization of the CNQDs Fig. 1. The TEM image (A), size distribution histogram (B), and XRD pattern (C) of the obtained CNQDs. Inset of (A), the high-resolution TEM (HRTEM) image.
CA monohydrate and urea were used as the precursors to prepare the CNQDs in oleic acid media. Fig. 1A shows that the obtained CNQDs were uniform sphere particles. The size distribution histogram (Fig. 1B) shows that the CNQDs had a narrow size distribution in the range of 1 to 5 nm, with an average diameter of about 2.8 nm. The HRTEM image (inset of Fig. 1A) shows that most particles had an obvious lattice fringes. The lattice parameter was measured to be 0.21 nm. The XRD pattern (Fig. 1C) displays two broad peaks centered at 13.15 and 27.15 degree, which are consistent with the previous reports about graphitic carbon nitride [16, 17]. The intensity of these two peaks was relatively low, suggesting that the crystallinity of the CNQDs was not pronounced. The amorphous part of the CNQDs may contain various functional groups such as carboxyl group, hydroxyl group and amino groups. These functional groups endow the product with good water-solubility.
Fig. 2. The XPS spectrum of the obtained CNQDs (A). (B) and (C) are the corresponding C1s and N1s spectrum.
The product was further characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), UV-Vis spectroscopy, 13C-nuclear magnetic resonance (NMR), and elemental analysis. The XPS spectrum shows three peaks at 284.27, 399.0 and 532.04 eV (Fig. 2), which can be attributed to C1s, N1s, and O1s. The C1s spectrum can be deconvoluted into several peaks at 284.55, 4
285.20, 287.53, 288.21, and 289.00 eV, indicating the presence of various types of carbon bonds: sp2 C=C or sp3 C-C, C-H, C−O or C-N, sp2 N-C=N, and C=O. The deconvolution of the N1s spectrum indicated the presence of three types of nitrogen bonds: C-N-C (399.44 ev), N-(C)3 (400.08 ev), and N-H (401.14 ev) [16, 17, 24].
Fig. 3. The solid-state 13 C NMR spectrum and FT-IR spectrum of the obtained CNQDs.
The solid-state 13C-NMR spectrum further confirmed the existence of carbon nitride units. Two distinct peaks centered at 146.8 and 161.3 ppm (Fig. 3A) were related to the formation of a poly(tris-s-triazine) structure characteristic of melem, melon and the final carbon nitride [25]. It is worth mentioning that no distinct peaks appeared around 120 ppm, further suggesting that the forming conjugate units were carbon nitride, rather than graphene. The peaks centered at 169.8 and 179.2 could be assigned to carbons in the carbonyl group. The FT-IR spectrum (Fig. 3B) exhibited several characteristic bands at 1630, 1410 and 1400 cm-1 , which can be assigned to aromatic C=N stretches. The characteristic band at 780 cm-1 in the fingerprint region can be assigned to the breathing mode of s-triazine rings [26]. The intense band at 1670 and 1710 cm-1 can be assigned to asymmetric C=N and C=O stretching vibrations, respectively. The broad peaks centered at 3200 and 3440 cm-1 are attributed to N-H and O-H stretching vibrations. The elemental analysis showed that the product contained 39.7% of carbon, 23.2% of nitrogen, 4.80% of hydrogen, and 32.3% of oxygen (calculated) by weight. It can be seen that the as-prepared CNQDs contain rich nitrogen and oxygen. From this point of view, the as-prepared CNQDs should be referred to as functional carbon nitride nanoparticles more exactly. Those oxygen-containing or nitrogen-containing functional groups may locate on the edge of the graphitic carbon nitride units. Spectral property of the CNQDs The UV-vis spectrum (Fig. 4A) shows that there are three characteristic peaks centered at around 268, 335 and 400 nm. The adsorption peak at 268 nm was conform to other reports about carbon nitride quantum dots [27, 28]. This peak may be ascribed to π-π* electronic transitions of carbon nitrides containing s-triazine rings. The peak centered at 335 nm can be ascribed to the typical adsorption pattern of carbon nitrides semiconductor [16, 17]. The adsorption peak centered at 400 nm should be ascribed to 5
surface/molecular center [29]. The introduction of oxygen atoms originated from citric acid may extend the conjugation systems, and alter the surface state of the CNQDs.
Fig. 4. The UV-vis spectrum (A) and the excitation-dependent emission spectrum (B) of the obtained CNQDs. Inset of A, the photographs under illumination of white (left) and UV (365 nm, right) light.
The fluorescent spectrum for the obtained CNQDs conformed to the UV-vis absorption features. Fig. 4B shows that the emission peaks are broad under excitation wavelengths of 300 and 320 nm. The broad peaks can be seen as the superposition of three emission peaks at around 370, 450 and 540 nm, suggesting that different energy level or different particle size exists. The PL peaks at around 370 and 450 nm can be ascribed to the uncondensed melem and the final carbon nitride units, respectively [16]. The PL peak centered at 540 nm may arise from the surface functional groups or the surface defects. With the increase of excitation wavelength, the intensity of emission peak at 450 nm increased. The maximum emission peak was observed at an excitation wavelength of 360 nm. Then the emission peak had a red shift as the excitation wavelength continued to increase. The intensity of emission peak reached another peak at 540 nm under an excitation wavelength of 420 nm. These two maximum emission peaks further expressed the nature of functional carbon nitride. The quantum yield (QY) of the CNQDs was calculated to be 27.1% using Rhodamine 6G as a reference. The CNQDs emitted strong blue fluorescence under 365 nm UV light (Fig. 4A, inset). The influences of reaction temperature, reaction time, ratios of citric acid monohydrate and urea were also investigated. The QY value showed little change. We then used the CNQDs obtained from 0.5 g of citric acid monohydrate and 0.5 g of urea in the selective detection of metal ions. Selectivity and sensitivity of Hg2+ detection The CNQDs with a final concentration of 2.5 µg mL-1 was used to evaluate the selectivity towards metal ions. Different metal ions were added into the CNQDs solution with a final concentration of 100 µM, and then the FL spectra were recorded under excitation at 423 nm to study the selectively. As shown in Fig. 5A, the FL intensity decreases remarkably with the addition of Hg2+, whereas other ions had negligible or less impact on the intensity. This suggests that the obtained CNQDs have high selectivity towards Hg2+ 6
ions.
Fig. 5. (A) Normalized fluorescence intensity of aqueous CNQDs solution (2.5 µg mL-1) in the presence of 100 µM of various metal ions at 423 nm excitation wavelength. (B) The dependence of FL intensity on the concentration of Hg2+ ions, insets show the linear relationship (0.1-10 µM) between the FL intensity and Hg2+ concentration. (C) Selective PL response of aqueous CNQDs solution towards 4 µM Hg2+ (black bars), and interference of 4 µM other metal ions with 4 µM Hg2+.
The concentration experiments (Fig. 5B) show that the FL intensity decreases with the increase of Hg2+ concentration. The data-fitting result shows that there exists two parts of linear relationship in the concentration range of 0.1-10 and 10-30 µM. The slope of the fitting straight line within 0.1-10 µM was greater than that within 10-30 µM, suggesting that different interaction sites for Hg2+ ions may exist on the surface of CNQDs particles. At low concentrations, Hg2+ ions interact preferentially with the “strong” binding site, and lead to fluorescence quenching quickly. The limit of detection (LOD) was calculated to be 0.14 µM based on three times the standard deviation rule. Similar results could be obtained in a long time interval, and the relative standard deviation (RSD) was calculated to be 0.34%. The sensitivity and selectivity for Hg2+ ions towards quenching of fluorescence of the CNQDs are possibly due to several reasons, such as greater affinity of Hg2+ ions towards nitrogen, larger radius of Hg2+ ions and its ability to form a stable non-fluorescent complex with the CNQDs [18]. Another possible explanation is aggregate-induced quenching [30]. The Hg2+ ion may simultaneously bind to multiple nitrogen atoms of the CNQDs and oxygen-contained groups. This complexation process may induce the formation of aggregates, change the electronic structure of the CNQDs and then lead to non-radiative electron/hole recombination annihilation. Detection of Hg2+ ions in spiked water samples Fig. 6. (A) Fluorescence emission spectra of the CNQDs in trap water solution upon addition of various concentrations of Hg2+ (from top to bottom: 0, 0.3, 0.5, 0.9, 2, 4, 6, 8 and 10 µM), excitation at 423 nm. (B) The dependence of PL intensity on the concentration of Hg2+ ions in the range of 0-10 µM.
As the obtained CNQDs had excellent sensitivity and selectivity to Hg2+ ions, the method was used to 7
assay the concentration of Hg2+ in real water samples. The tap water samples obtained from our lab without any pretreatment were spiked with Hg2+ ions at different concentration levels. Fig. 6A shows the fluorescence response of the CNQDs dissolved in tap water containing different concentrations of Hg2+ ions. It can be seen that the PL intensity gradually decreased with increasing the concentration of Hg2+ ions in tap water from 0 to 10 µM. Good linear relationship between the PL intensity and the concentration of Hg2+ ions was observed in the range of 0-10 µM (Fig. 6B), suggesting that the as-prepared CNQDs have potential application in the detection of Hg2+ ions in real environmental samples. Similar results were obtained for ground water. Good linear relationship existed between (F0–F)/F0 and the concentration of Hg2+ in the range of 0-10 µM (R2=0.998). Several traditional methods including atomic absorption/emission spectroscopy, selective cold vapor atomic fluorescence spectrometry, X-ray fluorescence spectrometry and inductively coupled plasma mass spectrometry (ICPMS) can be used to the detection of mercury ions. These methods are sensitive and selective but they require sophisticated instruments and complicated sample preparation [31]. The application of photoluminescent carbon-based nanoparticles in the determination of Hg2+ ions has attracted growing interest. Some significative progress has been made recently. The obtained LOD for the as-prepared CNQDs was lower or comparable to those reported with other fluorescent probes [30-32]. Stability and fluorescent green beans The photoluminescence stability of the obtained CNQDs was investigated. It should be pointed out that the as-prepared water-soluble CNQDs exhibit high stability at room temperature. There was no observation of any floating or precipitated nanoparticles after being stored for two month. The PL intensity remained basically unchanged under continuous illumination with an Xe lamp for 8 h.
Fig. 7. Green beans cultured with pure water (left) and CNQDs solution (0.3 mg/mL, right) after 100 h of growth. A, under daylight; B, under 365 nm UV beam.
We also perform the growth experiments using green beans as a model. Fig. 7 shows that green beans have good germination even in a CNQDs solution with concentration of 0.3 mg mL-1, preliminarily suggesting low or negligible biotoxicity. The cultured green beans emitted bright blue color on both the stems and leaves under 365 nm UV beam after 100 h of growth, suggesting that the CNQDs could 8
permeate throughout the plant cells with good biocompatibility. On the contrary, bean sprouts would be pathological if cultured with CdTe QDs solution [29]. Other biocompatibility experiments such as cytotoxicity experiment for this kind of fluorescent nanomaterial will be performed further.
Conclusions In summary, we have expediently prepared fluorescent carbon nitride quantum dots (CNQDs) from citric acid and urea by a simple one-pot microwave-assisted solvothermal method in a short time. Compared with other synthesis process, this method has some advantages such as low reaction temperature and pressure, short reaction time, easy processing, high yield, and simple post-processing. The as-prepared CNQDs possessed favorable fluorescence quantum yields (up to 27.1%) without further passivation treatment. The CNQDs displayed an uncommon excitation-dependent photoluminescence feature, and could be used as a promising probe to assay the concentration of mercury ions. The as-prepared water-soluble CNQDs exhibit high fluorescent stability, and preliminarily show negligible biotoxicity using green beans growth model.
Acknowledgements This work was supported by Natural Science Foundation of Fujian Province of China (No. 2012J06005 and 2014J01053), Education Bureau of Fujian Province of China (Nos. JK2011030 and JA13195), and the External Foundation of MOE Key Laboratory of Analysis and Detection for Food Safety (Fuzhou University) (No. FS1307).
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Graphical abstract
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Highlights
• Fluorescent carbon nitride quantum dots (CNQDs) were prepared by a facile method. • The CNQDs exhibited an uncommon fluorescence feature. • The quantum yield of the obtained CNQDs could reach 27%. • The CNQDs were applied to the detection of mercuric ions.
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S1386-1425(15)30083-4 http://dx.doi.org/10.1016/j.saa.2015.07.034 SAA 13933
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
19 February 2015 23 June 2015 7 July 2015
Please cite this article as: X. Cao, J. Ma, Y. Lin, B. Yao, F. Li, W. Weng, X. Lin, A facile microwave-assisted fabrication of fluorescent carbon nitride quantum dots and their application in the detection of mercury ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.07.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A facile microwave-assisted fabrication of fluorescent carbon nitride quantum dots and their application in the detection of mercury ions Xiaotong Caoa,b, Jie Maa,b, Yanping Lina,b, Bixia Yaoa,b, Feiming Lia,b, Wen Weng*a,b, Xiuchun Lin c a
Department of Chemistry and Environmental Science, Minnan Normal University, Zhangzhou, 363000,
China. E-mail: [email protected]; +86 596 2523251 b
Fujian Provincial Key Laboratory of Modern Analytical Science and Separation Technology, Zhangzhou,
363000, China. c
College of Environmental and Engineering, Putian University, Putian 351100, China
AB STRACT A facile microwave-assisted solvothermal method was used to prepare fluorescent carbon nitride quantum dots (CNQDs) using oleic acid as the reaction media at moderate reaction temperature in a short time (5 min). Citric acid monohydrate and urea were used as the precursors. The as-prepared CNQDs were characterized by multiple analytical techniques. The CNQDs exhibited an uncommon excitation wavelength-dependent fluorescence with two maximum emission peaks at 450 and 540 nm. The CNQDs with a quantum yield of 27.1% could serve as an effective fluorescent sensing platform for label-free sensitive detection of Hg2+ ions with a detection limit of 0.14 µM. This method was also applied to the detection of Hg2+ ions in tap water samples.
Keywords: Carbon nitride quantum dots; Fluorescent probe; mercury ions; Citric acid; Urea
___________________________________________________________________________ Introduction Fluorescent nanoparticles have greatly attracted attention recently because of their promising applications in many fields such as bioimaging, biosensing, ion detection, optoelectronic devices and photocatalysis [1-8]. This kind of material includes carbon dots, nanodiamonds, carbon nanotubes, fluorescent graphene, semiconductor quantum dots, etc. Because people have recognized the toxicity of 1
heavy metals-containing quantum dots, seeking for “safe” and effective fluorescent nanoparticles has become an important tendency. A typical representative is carbon dots. Rapid progress has been made since they were proposed in 2004 [9-15]. Carbon nitride is another promising candidate to complement carbon in materials applications [16]. Graphitic carbon nitride (g-C3N4), considered to be the most stable allotrope among various carbon nitrides, has been widely used in water splitting, solar energy transfer, and pollutant degradation [17]. It is usually prepared from nitrogen-rich precursors such as melamine, cyanamide and dicyandiamide by high-temperature pyrolysis process. The resultant carbon nitrides generally have large particle sizes, poor luminescent properties, and poor water-solubility. Recently, the fabrication and application of fluorescent carbon nitride quantum dots or nanosheets with enhanced photoresponsive property were reported by some research groups. Barman reported a microwave mediated method to prepare highly fluorescent graphitic carbon nitride quantum dots from formamide. The carbon nitride quantum dots can play a dual role for selective and sensitive detection of mercury ions as well as iodide ions in aqueous media [18]. Xie et al. reported a pathway to prepare the ultrathin graphitic-phase C3N4 nanosheets for bioimaging by a liquid exfoliation route from bulk g-C3N4 [19]. Chen et al. developed an effective and facile fluorescence sensing approach for the label-free and selective determination of Cr(VI) using graphitic carbon nitride nanosheets [20]. Developing simple methods for synthesizing fluorescent carbon nitride nanoparticles is very meaningful. Microwave heating has been used in syntheses of organic compounds, organometallic compounds, coordination compounds, and polymers. Microwave irradiation offers a number of advantages over conventional heating methods, including using less energy, offering a higher heating rate, and offering the ability to more quickly start and stop heating [21]. In this report, a facile microwave-assisted solvothermal method was used to prepare CNQDs at moderate reaction temperature in a short time. Unlike hydrothermal method, oleic acid was used as the reaction media. The fabrication process did not use any inorganic acid or metal ion. Water generated in the reaction process may act as a “soft-template”, and modulate the size of the formed nanoparticles [22]. The as-prepared CNQDs were applied to the detection of mercury ions.
Experimental Chemicals and Apparatus Citric acid monohydrate (CA), urea and oleic acid were all analytical reagents. Rhodamine 6G was 2
purchased from Aladdin Industrial Corporation. Ultrapure water was prepared with a Milli-Q system (Millipore, Bedford, MA, USA). An Anton Paar Monowave 300 microwave synthesizer (Austria) was used to perform all the preparation experiments. Preparation of carbon nitride quantum dots (CNQDs) The CNQDs were synthesized by microwave-assisted solvothermal method. A typical synthesis process is as follows. Citric acid monohydrate (0.5 g), urea (0.5 g) and oleic acid (10 ml) were placed in a 30 ml reaction tube. Then the mixture was heated at 180 0C for 5 min under 1200 r/min magnetic stirring. The pressure was maintained at 4.6 bar during the process of the reaction. Black solid precipitate at the bottom of reaction tube could be separated directly after the reaction. The precipitate was washed sufficiently with n-hexane, dispersed in ultrapure water, and centrifuged at 6000 rpm for 30 min to remove large particles product. Characterization A UV-2550 spectrophotometer (Shimadzu) was used to record the UV–vis spectra of the CNQDs. The fluorescence spectra were obtained by a Varian Cary Eclipse Fluorescence Spectrophotometer (Varian). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded using FEI Tecnai G2 F20 instruments (FEI). FTIR spectra were recorded using a Magna-IR 750 Fourier transform infrared spectrometer (Nicolet). The X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Thermo ESCALAB 250Xi multifunctional imaging electron spectrometer (Thermo Fisher). The X-ray diffraction (XRD) spectra were recorded using a Bruker DAVINCI D8 ADVANCE diffractometer. An AV600 nuclear magnetic spectrometer (Bruker) was used to record the 13C-NMR and 1
H-NMR spectra.
Quantum yield measurement The quantum yield (QY) of the obtained CNQDs was measured according to the established procedure [23]. Rhodamine 6G dissolved in ultrapure water (literature ΦR=0.95 at an excitation wavelength of 423 nm) was chosen as the standard. Absorbencies in the 10 mm fluorescence cuvette were kept under 0.1 at the excitation wavelengths. The quantum yield of the CNQDs was determined by the following equation:
3
Φ = ΦR ×
I AR η 2 × × I R A η R2
where Φ is the quantum yield of the sample, I is the measured integrated emission intensity, η is the refractive index of the solvent, and A is the optical density. The subscript “R” refers to the standard with known QY.
Results and discussion Structural characterization of the CNQDs Fig. 1. The TEM image (A), size distribution histogram (B), and XRD pattern (C) of the obtained CNQDs. Inset of (A), the high-resolution TEM (HRTEM) image.
CA monohydrate and urea were used as the precursors to prepare the CNQDs in oleic acid media. Fig. 1A shows that the obtained CNQDs were uniform sphere particles. The size distribution histogram (Fig. 1B) shows that the CNQDs had a narrow size distribution in the range of 1 to 5 nm, with an average diameter of about 2.8 nm. The HRTEM image (inset of Fig. 1A) shows that most particles had an obvious lattice fringes. The lattice parameter was measured to be 0.21 nm. The XRD pattern (Fig. 1C) displays two broad peaks centered at 13.15 and 27.15 degree, which are consistent with the previous reports about graphitic carbon nitride [16, 17]. The intensity of these two peaks was relatively low, suggesting that the crystallinity of the CNQDs was not pronounced. The amorphous part of the CNQDs may contain various functional groups such as carboxyl group, hydroxyl group and amino groups. These functional groups endow the product with good water-solubility.
Fig. 2. The XPS spectrum of the obtained CNQDs (A). (B) and (C) are the corresponding C1s and N1s spectrum.
The product was further characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), UV-Vis spectroscopy, 13C-nuclear magnetic resonance (NMR), and elemental analysis. The XPS spectrum shows three peaks at 284.27, 399.0 and 532.04 eV (Fig. 2), which can be attributed to C1s, N1s, and O1s. The C1s spectrum can be deconvoluted into several peaks at 284.55, 4
285.20, 287.53, 288.21, and 289.00 eV, indicating the presence of various types of carbon bonds: sp2 C=C or sp3 C-C, C-H, C−O or C-N, sp2 N-C=N, and C=O. The deconvolution of the N1s spectrum indicated the presence of three types of nitrogen bonds: C-N-C (399.44 ev), N-(C)3 (400.08 ev), and N-H (401.14 ev) [16, 17, 24].
Fig. 3. The solid-state 13 C NMR spectrum and FT-IR spectrum of the obtained CNQDs.
The solid-state 13C-NMR spectrum further confirmed the existence of carbon nitride units. Two distinct peaks centered at 146.8 and 161.3 ppm (Fig. 3A) were related to the formation of a poly(tris-s-triazine) structure characteristic of melem, melon and the final carbon nitride [25]. It is worth mentioning that no distinct peaks appeared around 120 ppm, further suggesting that the forming conjugate units were carbon nitride, rather than graphene. The peaks centered at 169.8 and 179.2 could be assigned to carbons in the carbonyl group. The FT-IR spectrum (Fig. 3B) exhibited several characteristic bands at 1630, 1410 and 1400 cm-1 , which can be assigned to aromatic C=N stretches. The characteristic band at 780 cm-1 in the fingerprint region can be assigned to the breathing mode of s-triazine rings [26]. The intense band at 1670 and 1710 cm-1 can be assigned to asymmetric C=N and C=O stretching vibrations, respectively. The broad peaks centered at 3200 and 3440 cm-1 are attributed to N-H and O-H stretching vibrations. The elemental analysis showed that the product contained 39.7% of carbon, 23.2% of nitrogen, 4.80% of hydrogen, and 32.3% of oxygen (calculated) by weight. It can be seen that the as-prepared CNQDs contain rich nitrogen and oxygen. From this point of view, the as-prepared CNQDs should be referred to as functional carbon nitride nanoparticles more exactly. Those oxygen-containing or nitrogen-containing functional groups may locate on the edge of the graphitic carbon nitride units. Spectral property of the CNQDs The UV-vis spectrum (Fig. 4A) shows that there are three characteristic peaks centered at around 268, 335 and 400 nm. The adsorption peak at 268 nm was conform to other reports about carbon nitride quantum dots [27, 28]. This peak may be ascribed to π-π* electronic transitions of carbon nitrides containing s-triazine rings. The peak centered at 335 nm can be ascribed to the typical adsorption pattern of carbon nitrides semiconductor [16, 17]. The adsorption peak centered at 400 nm should be ascribed to 5
surface/molecular center [29]. The introduction of oxygen atoms originated from citric acid may extend the conjugation systems, and alter the surface state of the CNQDs.
Fig. 4. The UV-vis spectrum (A) and the excitation-dependent emission spectrum (B) of the obtained CNQDs. Inset of A, the photographs under illumination of white (left) and UV (365 nm, right) light.
The fluorescent spectrum for the obtained CNQDs conformed to the UV-vis absorption features. Fig. 4B shows that the emission peaks are broad under excitation wavelengths of 300 and 320 nm. The broad peaks can be seen as the superposition of three emission peaks at around 370, 450 and 540 nm, suggesting that different energy level or different particle size exists. The PL peaks at around 370 and 450 nm can be ascribed to the uncondensed melem and the final carbon nitride units, respectively [16]. The PL peak centered at 540 nm may arise from the surface functional groups or the surface defects. With the increase of excitation wavelength, the intensity of emission peak at 450 nm increased. The maximum emission peak was observed at an excitation wavelength of 360 nm. Then the emission peak had a red shift as the excitation wavelength continued to increase. The intensity of emission peak reached another peak at 540 nm under an excitation wavelength of 420 nm. These two maximum emission peaks further expressed the nature of functional carbon nitride. The quantum yield (QY) of the CNQDs was calculated to be 27.1% using Rhodamine 6G as a reference. The CNQDs emitted strong blue fluorescence under 365 nm UV light (Fig. 4A, inset). The influences of reaction temperature, reaction time, ratios of citric acid monohydrate and urea were also investigated. The QY value showed little change. We then used the CNQDs obtained from 0.5 g of citric acid monohydrate and 0.5 g of urea in the selective detection of metal ions. Selectivity and sensitivity of Hg2+ detection The CNQDs with a final concentration of 2.5 µg mL-1 was used to evaluate the selectivity towards metal ions. Different metal ions were added into the CNQDs solution with a final concentration of 100 µM, and then the FL spectra were recorded under excitation at 423 nm to study the selectively. As shown in Fig. 5A, the FL intensity decreases remarkably with the addition of Hg2+, whereas other ions had negligible or less impact on the intensity. This suggests that the obtained CNQDs have high selectivity towards Hg2+ 6
ions.
Fig. 5. (A) Normalized fluorescence intensity of aqueous CNQDs solution (2.5 µg mL-1) in the presence of 100 µM of various metal ions at 423 nm excitation wavelength. (B) The dependence of FL intensity on the concentration of Hg2+ ions, insets show the linear relationship (0.1-10 µM) between the FL intensity and Hg2+ concentration. (C) Selective PL response of aqueous CNQDs solution towards 4 µM Hg2+ (black bars), and interference of 4 µM other metal ions with 4 µM Hg2+.
The concentration experiments (Fig. 5B) show that the FL intensity decreases with the increase of Hg2+ concentration. The data-fitting result shows that there exists two parts of linear relationship in the concentration range of 0.1-10 and 10-30 µM. The slope of the fitting straight line within 0.1-10 µM was greater than that within 10-30 µM, suggesting that different interaction sites for Hg2+ ions may exist on the surface of CNQDs particles. At low concentrations, Hg2+ ions interact preferentially with the “strong” binding site, and lead to fluorescence quenching quickly. The limit of detection (LOD) was calculated to be 0.14 µM based on three times the standard deviation rule. Similar results could be obtained in a long time interval, and the relative standard deviation (RSD) was calculated to be 0.34%. The sensitivity and selectivity for Hg2+ ions towards quenching of fluorescence of the CNQDs are possibly due to several reasons, such as greater affinity of Hg2+ ions towards nitrogen, larger radius of Hg2+ ions and its ability to form a stable non-fluorescent complex with the CNQDs [18]. Another possible explanation is aggregate-induced quenching [30]. The Hg2+ ion may simultaneously bind to multiple nitrogen atoms of the CNQDs and oxygen-contained groups. This complexation process may induce the formation of aggregates, change the electronic structure of the CNQDs and then lead to non-radiative electron/hole recombination annihilation. Detection of Hg2+ ions in spiked water samples Fig. 6. (A) Fluorescence emission spectra of the CNQDs in trap water solution upon addition of various concentrations of Hg2+ (from top to bottom: 0, 0.3, 0.5, 0.9, 2, 4, 6, 8 and 10 µM), excitation at 423 nm. (B) The dependence of PL intensity on the concentration of Hg2+ ions in the range of 0-10 µM.
As the obtained CNQDs had excellent sensitivity and selectivity to Hg2+ ions, the method was used to 7
assay the concentration of Hg2+ in real water samples. The tap water samples obtained from our lab without any pretreatment were spiked with Hg2+ ions at different concentration levels. Fig. 6A shows the fluorescence response of the CNQDs dissolved in tap water containing different concentrations of Hg2+ ions. It can be seen that the PL intensity gradually decreased with increasing the concentration of Hg2+ ions in tap water from 0 to 10 µM. Good linear relationship between the PL intensity and the concentration of Hg2+ ions was observed in the range of 0-10 µM (Fig. 6B), suggesting that the as-prepared CNQDs have potential application in the detection of Hg2+ ions in real environmental samples. Similar results were obtained for ground water. Good linear relationship existed between (F0–F)/F0 and the concentration of Hg2+ in the range of 0-10 µM (R2=0.998). Several traditional methods including atomic absorption/emission spectroscopy, selective cold vapor atomic fluorescence spectrometry, X-ray fluorescence spectrometry and inductively coupled plasma mass spectrometry (ICPMS) can be used to the detection of mercury ions. These methods are sensitive and selective but they require sophisticated instruments and complicated sample preparation [31]. The application of photoluminescent carbon-based nanoparticles in the determination of Hg2+ ions has attracted growing interest. Some significative progress has been made recently. The obtained LOD for the as-prepared CNQDs was lower or comparable to those reported with other fluorescent probes [30-32]. Stability and fluorescent green beans The photoluminescence stability of the obtained CNQDs was investigated. It should be pointed out that the as-prepared water-soluble CNQDs exhibit high stability at room temperature. There was no observation of any floating or precipitated nanoparticles after being stored for two month. The PL intensity remained basically unchanged under continuous illumination with an Xe lamp for 8 h.
Fig. 7. Green beans cultured with pure water (left) and CNQDs solution (0.3 mg/mL, right) after 100 h of growth. A, under daylight; B, under 365 nm UV beam.
We also perform the growth experiments using green beans as a model. Fig. 7 shows that green beans have good germination even in a CNQDs solution with concentration of 0.3 mg mL-1, preliminarily suggesting low or negligible biotoxicity. The cultured green beans emitted bright blue color on both the stems and leaves under 365 nm UV beam after 100 h of growth, suggesting that the CNQDs could 8
permeate throughout the plant cells with good biocompatibility. On the contrary, bean sprouts would be pathological if cultured with CdTe QDs solution [29]. Other biocompatibility experiments such as cytotoxicity experiment for this kind of fluorescent nanomaterial will be performed further.
Conclusions In summary, we have expediently prepared fluorescent carbon nitride quantum dots (CNQDs) from citric acid and urea by a simple one-pot microwave-assisted solvothermal method in a short time. Compared with other synthesis process, this method has some advantages such as low reaction temperature and pressure, short reaction time, easy processing, high yield, and simple post-processing. The as-prepared CNQDs possessed favorable fluorescence quantum yields (up to 27.1%) without further passivation treatment. The CNQDs displayed an uncommon excitation-dependent photoluminescence feature, and could be used as a promising probe to assay the concentration of mercury ions. The as-prepared water-soluble CNQDs exhibit high fluorescent stability, and preliminarily show negligible biotoxicity using green beans growth model.
Acknowledgements This work was supported by Natural Science Foundation of Fujian Province of China (No. 2012J06005 and 2014J01053), Education Bureau of Fujian Province of China (Nos. JK2011030 and JA13195), and the External Foundation of MOE Key Laboratory of Analysis and Detection for Food Safety (Fuzhou University) (No. FS1307).
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Graphical abstract
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Highlights
• Fluorescent carbon nitride quantum dots (CNQDs) were prepared by a facile method. • The CNQDs exhibited an uncommon fluorescence feature. • The quantum yield of the obtained CNQDs could reach 27%. • The CNQDs were applied to the detection of mercuric ions.
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