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Polymer nanodots of graphitic carbon nitride as effective fluorescent probes for the detection of Fe3+ and Cu2+ ions† Shouwei Zhang,‡ab Jiaxing Li,‡c Meiyi Zeng,b Jinzhang Xu,b Xiangke Wang*ac and Wenping Hu*d A simple and green route was developed for the first time to produce fluorescent graphitic carbon nitride (Fg-C3N4) by hydrothermal treatment of bulk g-C3N4. The produced F-g-C3N4 dots have blue emission and a high quantum yield, and were applied as a very effective fluorescent probe for label-free selective and sensitive detection of Cu2+ and Fe3+ ions; the limits of detection were as low as 0.5 nM and 1.0 nM,
Received 19th December 2013 Accepted 23rd January 2014
respectively. By using sodium hexametaphosphate (SHPP) as a masking agent of Fe3+, Cu2+ was exclusively detected in the presence of Fe3+ ions. Cu2+ and Fe3+ ions in real water samples were also
DOI: 10.1039/c3nr06744k
detected successfully. This exceptional fluorescent performance makes the probes based on F-g-C3N4
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dots attractive for highly sensitive detection of Cu2+ and Fe3+ ions in real water.
Introduction Iron and copper are two important nutrient elements for human health, which rank as the second and third most essential trace metal elements in the human body. However, excessive intake of iron or copper will lead to diseases, such as kidney disease, and disturb the cellular homeostasis resulting in Alzheimer's, Wilson’s and Menkes diseases.1 However, as a result of wide use in agriculture and industry, the potential toxic effects of iron and copper to humans from contaminated rivers, lakes or oceans remain a global challenge.2,3 Therefore, it is particularly important for human health and monitoring of the environment to develop practical and efficient technologies for eld analysis and rapid determination of iron and copper ions with high sensitivity and selectivity. Many traditional techniques have been developed, including atomic absorption/ emission spectroscopy, electrochemical sensors, plasma mass spectrometry and polarography, but all of them have unavoidable defects limiting their practical applications, such as the
a
School for Radiological and Interdisciplinary Sciences, Soochow University, Suzhou 215123, PR China. E-mail: [email protected]; [email protected]; Fax: +86-55165591310; Tel: +86-551-65592788
b
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230031, PR China
c Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Hefei 230031, PR China d
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail: [email protected]; Fax: +86-10-62527295; Tel: +86-10-82615030 † Electronic supplementary 10.1039/c3nr06744k
information
(ESI)
available.
‡ Shouwei Zhang and Jiaxing Li contributed equally to this paper.
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DOI:
need for complex equipment and tedious sample preparation.4 Fluorescent probes have aroused great attention due to their high accuracy, high sensitivity, fast analysis and simplicity.5–9 To date, various uorescent probes including uorescent metal nanoparticles, organic dyes and semiconductor quantum dots (QD) are exceptional examples among these applications.10–14 However, these nanomaterials also suffer from some defects, including easy oxidation, photobleaching of the uorescent dye, expensive cost of metal sources, high toxicity and low photoresponse, which limit their wide preparation and further applications.15–19 Therefore, exploring novel, highly uorescent, green QDs or related nanomaterials with good photo-stability and low toxicity is urgently needed. Recently, graphitic carbon nitride (g-C3N4) has become an attractive candidate as a photocatalyst for hydrogen/oxygen evolution from water splitting and degradation of organic pollutants under visible light irradiation due to its excellent sunlight harvesting capability, superior photochemical stability, plentiful material source and inexpensive synthesis.20,21 Very recently, uorescent g-C3N4 (F-g-C3N4) has attracted more attention due to the easy preparation, high quantum yield, non-toxicity, low cost, good biocompatibility and excellent photostability.22–26 These excellent works have proved F-g-C3N4 to be an effective probe to detect different metal ions, but the facile preparation processes of the probe and the in-depth analysis of the detection of different metal ions are still quite limited. Especially, the detection in real water containing different heavy metal ions is very lacking. So, it is very important to develop a method that could be used to detect more metal ions with high sensitivity, easy fabrication and with in-depth understanding of the detection behavior in real water. To our best knowledge, the production of F-g-C3N4
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dots from bulk g-C3N4 for detection of heavy metal ions has not been reported. Herein, we developed an extremely simple and green hydrothermal treatment of bulk g-C3N4 to form F-g-C3N4 dots with blue emission and high quantum yield (QY), which were applied as a very effective uorescent probe for label-free selective and sensitive detection of Cu2+ and Fe3+ ions with a limit of detection (LOD) as low as 0.5 nM and 1.0 nM, respectively. By using sodium hexametaphosphate (SHPP) as a masking agent of Fe3+, Cu2+ was exclusively detected in the presence of Fe3+ ions. Cu2+ and Fe3+ ions in real water samples were also successfully detected.
Results and discussion Materials characterization The morphologies of bulk g-C3N4 and F-g-C3N4 dots were observed by TEM, as shown in Fig. 1. For bulk g-C3N4, the two dimensional ake-like structure which was crumpled with a size of several micrometers was observed (Fig. 1A), which was the analogue of wrinkled graphene. However, the size of bulk gC3N4 decreased dramatically aer hydrothermal treatment. The morphologies of F-g-C3N4 dots are depicted in Fig. 1B with dots well separated from each other. Fig. 1C shows an image of individual nanoparticles indicating the high crystallinity with a lattice parameter of 0.32 nm, which was in good agreement with the (002) plane of g-C3N4. It was seen from the corresponding particle size distribution histograms (Fig. 1D) that the diameters of these dots range from 5 to 20 nm. To further conrm the formation of F-g-C3N4 dots, XRD patterns were also obtained. As shown in Fig. 2A, two obvious peaks were observed in bulk g-C3N4. The strong peak located at 27.21 (d ¼ 0.326 nm) was a typical interplanar stacking peak in conjugated aromatic systems. The other peak at 13.02 ,
Fig. 1 The TEM images of bulk g-C3N4 (A) and F-g-C3N4 dots (B), high-resolution TEM image of the individual F-g-C3N4 dots (C) and the corresponding dots size distribution histogram (D).
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The XRD patterns (A) and FTIR spectra (B) of the bulk g-C3N4 and the F-g-C3N4 dots. High-resolution C 1s (C) and N 1s (D) spectra of bulk g-C3N4 and F-g-C3N4 dots. Fig. 2
corresponding to d ¼ 0.682 nm, belonged to an in-planar structural packing motif. Both peaks are characteristic peaks of carbon nitride.27 Aer the hydrothermal treatment of bulk gC3N4, the obtained F-g-C3N4 dots gave two similar peaks as bulk g-C3N4, suggesting the same crystal structure for F-g-C3N4 as bulk g-C3N4 (Fig. 2A). The low-angle peak at 13.02 was weak due to the simultaneously decreased planar size of the layers during hydrothermal cutting of bulk g-C3N4. Meanwhile, the (002) peak increased from 27.21 for bulk g-C3N4 to 27.71 for Fg-C3N4 dots, corresponding to a decrease in the interplanar stacking distance from 0.325 to 0.321 nm. This suggested that the gallery distance was decreased in dots, i.e., the stack of F-gC3N4 dots became denser. These ndings were consistent with previous results that the single layers in bulk g-C3N4 might be undulated, and could be further planarized aer further heating treatment, leading to a denser stacking.28 The doped O heteroatoms and the disturbance of graphitic structure were assigned to interpret this observation. Since the introduced O atoms in the framework had higher electronegativity than the substituted N atoms, the interactions between the carbon nitride layers would be strengthened by the stronger attraction, resulting in the shortened interplanar distance. In order to investigate the chemical bonding between the C atoms and N atoms, high-resolution C 1s and N 1s spectra of bulk g-C3N4 and F-g-C3N4 dots were further studied, as shown in Fig. 2C and D. The C 1s spectrum of bulk g-C3N4 could be divided into two peaks, 284.7 eV and 288.4 eV. Compared with bulk g-C3N4, a new peak at 289.0 eV was detected in F-g-C3N4 dots, which was ascribed to the C–O single bond.29 The observation was also consistent with the FTIR analysis (Fig. 2B). Detailed analysis of the FTIR spectra is given in the ESI.†
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For bulk g-C3N4, three different peaks at 398.5, 399.9 and 401.4 eV were assigned as sp2 (C–N–C), sp3 nitrogen (N–[C]3) and primary amine nitrogen atoms (C–NH2), respectively (Fig. 2D).30 However, for F-g-C3N4 dots, the peak of primary amine nitrogen atoms at 401.4 eV disappeared. In addition, the overall intensity ratio of N(sp2)/N(sp3) decreased from 2.81 to 2.32. This observation showed that the dots surface was poor in nitrogen, which indicated that the hydrothermal cutting process preferred the oxidation of nitrogen rather than carbon. The UV-vis spectra (Fig. 3A) indicated that the absorption edge of the F-g-C3N4 dots showed a slightly blue shi as compared with the bulk g-C3N4.31 Moreover, the PL spectrum of the F-g-C3N4 dots also showed a blue shi of 30 nm as compared with that of bulk g-C3N4 (Fig. 3B), which was ascribed to the O atoms on the surface of the F-g-C3N4 dots. This observation was similar to the reported N-doped graphene QDs.32 The blue shi in both the UV-vis and PL spectra might result from the effect of quantum connement with the conduction band (CB) and valence band (VB) shiing in opposite directions. When the dispersion solution was excited at 365 nm, a strong PL emission peak centered at 437 nm was present, indicating that the dots were uorescent. The photograph of the dispersion exhibited a bright blue color under UV light at 365 nm (inset Fig. 3B), which further revealed that the F-g-C3N4 dots had strong blue uorescence. When the excitation wavelength changed from 300 to 400 nm, the PL spectra were almost invariable and showed a strong peak at 437 nm (Fig. 3C). The above results indicated that the F-g-C3N4 dots indeed exhibited an excitation-independent PL behavior. This excitation-independent emission property is different from most reported carbon dots,33,34 which could avoid auto-uorescence during their applications. Generally, the PL spectra of most luminescent carbon dots are dependent
Fig. 3 (A) UV-visible spectra. (B) The PL spectra of bulk g-C3N4 and Fg-C3N4 dots, the inset is the color change of the F-g-C3N4 dots solution under UV light (l ¼ 365 nm) irradiation. (C) PL spectra of the Fg-C3N4 dots at different excitation wavelengths. (D) The pH-dependent PL intensities of the F-g-C3N4 dots solution.
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on excitation wavelength. In other words, the PL peaks shied to longer wavelengths with a maximum intensity as the excitation wavelength is red shied. However, the as-prepared F-gC3N4 dots showed an excitation-independent PL feature. As reported earlier, the various energy levels associated with different ‘surface states’ formed by heavy derivatization made it self-passivated, which was responsible for the excitationdependent-emission phenomenon.35,36 We observed extraordinary PL behavior of F-g-C3N4 over the excitation wavelength range of 300–400 nm, which might be attributed to the size and the relatively uniform surface state.37,38 The F-g-C3N4 dots showed high PL intensity under basic conditions, which dropped as the pH decreased to acidic conditions (Fig. 3D). This pH-dependent PL behaviour might result from the free zigzag sites in the F-g-C3N4 dots, which were similar to the reported graphene QDs.11,39 Besides, the measured PL QY, using quinine sulfate as a reference, was 16.9%, much higher than that of bulk g-C3N4 (5.2%) and the well-known watersoluble graphene QDs (3.8%).16 From the results discussed above, the hydrothermal treatment exerted a strong effect on the formation, structure, and optical properties of the F-g-C3N4 dots. Two models are presented to interpret the mechanism for the hydrothermal process, as discussed in Fig. S1.† Application as uorescent probes Cu2+ and Fe3+ are well-known to be high toxic to organisms, such as certain algae, fungi, and many bacteria and viruses.40–46 The detection of such metals is thus highly desired. Herein, we report the detection methodology for Fe3+ and Cu2+, utilizing non-toxic F-g-C3N4 dots.
Fig. 4 PL intensity changes of F-g-C3N4 dots for different concentrations of Fe3+ (A) and Cu2+ (C). (B) The difference in relative PL intensities of F-g-C3N4 dots between the blank and solutions containing different metal ions (excitation at 360 nm, [Mn+] ¼ 50 mM, I and I0 are PL intensities of F-g-C3N4 dots in the presence and absence of metal ions, respectively). (D) Selective relative PL intensities of F-gC3N4 dots (black bar) after treatment with 50 mM metal ion solutions, and interference of 50 mM of other metal ions with 50 mM Cu2+ (red bar) and 50 mM Fe3+ (blue bar).
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Screening experiments with different metal ions (50 mM) were carried out by investigating the PL intensity changes of the F-g-C3N4 dots, as shown in Fig. 4B and S4.† The PL intensities of the F-g-C3N4 dots were greatly decreased by Fe3+ and Cu2+ while the other ions demonstrated less or no effect, indicating the uorescence of the F-g-C3N4 dots was sensitive to Fe3+ and Cu2+, and the F-g-C3N4 dots can be used as promising uorescence probes for Fe3+ and Cu2+ detection. The high selectivity of F-g-C3N4 for Cu2+ and Fe3+ may be due to the fact that the Cu2+ and Fe3+ ions have a higher binding affinity and faster chelating kinetics with N and O functional groups of F-g-C3N4 than other transition-metal ions, as indicated previously.47,48 Besides, the redox potentials of Cu2+/Cu+ and Fe3+/Fe2+ are between the conduction band (CB) and valence band (VB) of g-C3N4, so photoinduced electrons can transfer from the CB to the complexed Cu2+ and Fe3+, leading to uorescence quenching.23 To evaluate the F-g-C3N4 dots’ sensitivities toward different concentrations of Fe3+ and Cu2+, PL intensities were monitored by the titration of Fe3+ and Cu2+ at a xed time of 10 min. It can be seen from Fig. 4 that the PL intensities of F-g-C3N4 dots at 437 nm gradually decreased with increasing Fe3+ concentration. The LOD of Fe3+ was about 1.0 nM at a signalto-noise ratio of 3, which was much lower than or at least comparable to the previous results.40,41,46,49 As for Cu2+, the PL intensities of the F-g-C3N4 dots decreased gradually with increasing Cu2+ concentration (Fig. 4C). The LOD of Cu2+ was calculated to be as low as 0.5 nM, which was much lower than the maximum allowable level of Cu2+ (20 mM) in drinking water as regulated by the US Environmental Protection Agency (EPA).50 The time-dependent PL spectra are shown in Fig. S3,† and indicate that only 10 min were required to complete the detection between F-g-C3N4 dots and Fe3+/Cu2+. These excellent results suggested that F-g-C3N4 dots can serve as robust uorescent probes to detect Fe3+ and Cu2+ with high accuracy and simplicity. The selectivity toward Fe3+ and Cu2+ was further investigated in the presence of other metal ions, as shown in Fig. 4D. No obvious PL intensity change was observed aer other metal ions were added. However, further addition of Fe3+ or Cu2+ into the above mixtures caused a sharp decrease in PL intensity, which indicated that the detection of Fe3+ or Cu2+ was not interrupted by other metals. The selectivity was also investigated in the presence of several metal ions, as shown in Fig. S4.† To further improve the selectivity, the effect of Fe3+ ions on the detection of Cu2+ was also studied. In order to achieve the unique detection of Cu2+, the efficient Fe3+ chelating reagent sodium hexametaphosphate (SHPP) was added to minimize the interference of Cu2+.51 As shown in Fig. 5A and B, the PL intensities of F-g-C3N4 dots showed a dramatic decrease with increasing Cu2+ concentrations. Indeed, the introduction of Fe3+ into F-g-C3N4 dots–Cu2+ mixtures had no inuence on the detection of Cu2+in the presence of SHPP. So Fe3+ can be successfully masked under these conditions with little effect on the Cu2+ detection, allowing for the sensitive detection of Cu2+. Besides, the prepared F-g-C3N4 dots were also reusable in the detection of Fe3+, which means that the uorescence can be
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Fig. 5 PL intensities of F-g-C3N4 dots for Cu2+ (A) and Fe3+ (C) in the presence of Fe3+ and SHPP (A: process B(e) and C: process D(e)). Relative fluorescence responses for F-g-C3N4 dots to the sensitive single detection of Cu2+ (B) in the presence and absence of SHPP at 437 nm. Reusable probes for detecting Fe3+ (D), turn-on fluorescence can be obtained in the presence and absence of SHPP at 437 nm. B (a, b, c, d) and D (a, b, c, d) indicate pure F-g-C3N4 dots, F-g-C3N4 dots in the presence of 60 mM SHPP, 50 mM Fe3+, 60 mM SHPP added to the solution of B(c), respectively. B(e) and B(f) indicate 50 mM Cu2+ added to the mixed solution of B(d) and 60 mM SHPP added to the mixed solution of B(e), respectively. The results showed that the probe was turn-off. D(e) and D(f) indicate 50 mM Fe3+ added to the mixed solution of D(d) and 60 mM SHPP added to the mixed solution of D(e), respectively. The results showed that the probe was turn-on.
turned-on again. The on–off–on uorescence process is shown in Fig. 5C and D. As can be seen, the uorescence of F-g-C3N4 dots nearly disappeared in the presence of 50 mM Fe3+. However, the uorescence was recovered aer SHPP was added. Because the specic complex ratio of Fe3+ and SHPP was unknown, an excess of SHPP was added to recover the uorescence to the relatively high LOD value (100 nM), which was still lower than the maximum level (5 mM) of Fe3+ in drinking water permitted by the EPA.52 Therefore, the F-g-C3N4 dots have great potential for repeated detection of Fe3+. The F-g-C3N4 dots were also successfully used to detect Fe3+ and Cu2+ ions in natural water obtained from the Dongpu Lake of Hefei, Anhui province, China (Fig. S5†).
Conclusion In conclusion, for the rst time, green and non-toxic F-g-C3N4 dots were synthesized by a facile hydrothermal treatment of bulk g-C3N4. The obtained F-g-C3N4 dots possess high dispersity, excellent optical properties and high QY. The F-g-C3N4 dots further served as novel and green uorescent probes for the label-free sensitive and selective detection of Fe3+ and Cu2+ ions in lab and natural water samples. The addition of SHPP to the detection system can successfully mask Fe3+, allowing the selective detection of Cu2+. Therefore, we present a simple, lowcost route toward green production of F-g-C3N4 dots uorescent probes for fast, highly selective, and sensitive optical detection of Fe3+ and Cu2+ ions.
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Acknowledgements We greatly appreciate the nancial support from National Natural Science Foundation of China (21225730, 91326202, 21207136 and 21272236), the Ministry of Science and Technology of China (2011CB933700), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Hefei Center for Physical Science and Technology (2012FXZY005).
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Polymer nanodots of graphitic carbon nitride as effective fluorescent probes for the detection of Fe3+ and Cu2+ ions† Shouwei Zhang,‡ab Jiaxing Li,‡c Meiyi Zeng,b Jinzhang Xu,b Xiangke Wang*ac and Wenping Hu*d A simple and green route was developed for the first time to produce fluorescent graphitic carbon nitride (Fg-C3N4) by hydrothermal treatment of bulk g-C3N4. The produced F-g-C3N4 dots have blue emission and a high quantum yield, and were applied as a very effective fluorescent probe for label-free selective and sensitive detection of Cu2+ and Fe3+ ions; the limits of detection were as low as 0.5 nM and 1.0 nM,
Received 19th December 2013 Accepted 23rd January 2014
respectively. By using sodium hexametaphosphate (SHPP) as a masking agent of Fe3+, Cu2+ was exclusively detected in the presence of Fe3+ ions. Cu2+ and Fe3+ ions in real water samples were also
DOI: 10.1039/c3nr06744k
detected successfully. This exceptional fluorescent performance makes the probes based on F-g-C3N4
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dots attractive for highly sensitive detection of Cu2+ and Fe3+ ions in real water.
Introduction Iron and copper are two important nutrient elements for human health, which rank as the second and third most essential trace metal elements in the human body. However, excessive intake of iron or copper will lead to diseases, such as kidney disease, and disturb the cellular homeostasis resulting in Alzheimer's, Wilson’s and Menkes diseases.1 However, as a result of wide use in agriculture and industry, the potential toxic effects of iron and copper to humans from contaminated rivers, lakes or oceans remain a global challenge.2,3 Therefore, it is particularly important for human health and monitoring of the environment to develop practical and efficient technologies for eld analysis and rapid determination of iron and copper ions with high sensitivity and selectivity. Many traditional techniques have been developed, including atomic absorption/ emission spectroscopy, electrochemical sensors, plasma mass spectrometry and polarography, but all of them have unavoidable defects limiting their practical applications, such as the
a
School for Radiological and Interdisciplinary Sciences, Soochow University, Suzhou 215123, PR China. E-mail: [email protected]; [email protected]; Fax: +86-55165591310; Tel: +86-551-65592788
b
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230031, PR China
c Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Hefei 230031, PR China d
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail: [email protected]; Fax: +86-10-62527295; Tel: +86-10-82615030 † Electronic supplementary 10.1039/c3nr06744k
information
(ESI)
available.
‡ Shouwei Zhang and Jiaxing Li contributed equally to this paper.
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See
DOI:
need for complex equipment and tedious sample preparation.4 Fluorescent probes have aroused great attention due to their high accuracy, high sensitivity, fast analysis and simplicity.5–9 To date, various uorescent probes including uorescent metal nanoparticles, organic dyes and semiconductor quantum dots (QD) are exceptional examples among these applications.10–14 However, these nanomaterials also suffer from some defects, including easy oxidation, photobleaching of the uorescent dye, expensive cost of metal sources, high toxicity and low photoresponse, which limit their wide preparation and further applications.15–19 Therefore, exploring novel, highly uorescent, green QDs or related nanomaterials with good photo-stability and low toxicity is urgently needed. Recently, graphitic carbon nitride (g-C3N4) has become an attractive candidate as a photocatalyst for hydrogen/oxygen evolution from water splitting and degradation of organic pollutants under visible light irradiation due to its excellent sunlight harvesting capability, superior photochemical stability, plentiful material source and inexpensive synthesis.20,21 Very recently, uorescent g-C3N4 (F-g-C3N4) has attracted more attention due to the easy preparation, high quantum yield, non-toxicity, low cost, good biocompatibility and excellent photostability.22–26 These excellent works have proved F-g-C3N4 to be an effective probe to detect different metal ions, but the facile preparation processes of the probe and the in-depth analysis of the detection of different metal ions are still quite limited. Especially, the detection in real water containing different heavy metal ions is very lacking. So, it is very important to develop a method that could be used to detect more metal ions with high sensitivity, easy fabrication and with in-depth understanding of the detection behavior in real water. To our best knowledge, the production of F-g-C3N4
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dots from bulk g-C3N4 for detection of heavy metal ions has not been reported. Herein, we developed an extremely simple and green hydrothermal treatment of bulk g-C3N4 to form F-g-C3N4 dots with blue emission and high quantum yield (QY), which were applied as a very effective uorescent probe for label-free selective and sensitive detection of Cu2+ and Fe3+ ions with a limit of detection (LOD) as low as 0.5 nM and 1.0 nM, respectively. By using sodium hexametaphosphate (SHPP) as a masking agent of Fe3+, Cu2+ was exclusively detected in the presence of Fe3+ ions. Cu2+ and Fe3+ ions in real water samples were also successfully detected.
Results and discussion Materials characterization The morphologies of bulk g-C3N4 and F-g-C3N4 dots were observed by TEM, as shown in Fig. 1. For bulk g-C3N4, the two dimensional ake-like structure which was crumpled with a size of several micrometers was observed (Fig. 1A), which was the analogue of wrinkled graphene. However, the size of bulk gC3N4 decreased dramatically aer hydrothermal treatment. The morphologies of F-g-C3N4 dots are depicted in Fig. 1B with dots well separated from each other. Fig. 1C shows an image of individual nanoparticles indicating the high crystallinity with a lattice parameter of 0.32 nm, which was in good agreement with the (002) plane of g-C3N4. It was seen from the corresponding particle size distribution histograms (Fig. 1D) that the diameters of these dots range from 5 to 20 nm. To further conrm the formation of F-g-C3N4 dots, XRD patterns were also obtained. As shown in Fig. 2A, two obvious peaks were observed in bulk g-C3N4. The strong peak located at 27.21 (d ¼ 0.326 nm) was a typical interplanar stacking peak in conjugated aromatic systems. The other peak at 13.02 ,
Fig. 1 The TEM images of bulk g-C3N4 (A) and F-g-C3N4 dots (B), high-resolution TEM image of the individual F-g-C3N4 dots (C) and the corresponding dots size distribution histogram (D).
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The XRD patterns (A) and FTIR spectra (B) of the bulk g-C3N4 and the F-g-C3N4 dots. High-resolution C 1s (C) and N 1s (D) spectra of bulk g-C3N4 and F-g-C3N4 dots. Fig. 2
corresponding to d ¼ 0.682 nm, belonged to an in-planar structural packing motif. Both peaks are characteristic peaks of carbon nitride.27 Aer the hydrothermal treatment of bulk gC3N4, the obtained F-g-C3N4 dots gave two similar peaks as bulk g-C3N4, suggesting the same crystal structure for F-g-C3N4 as bulk g-C3N4 (Fig. 2A). The low-angle peak at 13.02 was weak due to the simultaneously decreased planar size of the layers during hydrothermal cutting of bulk g-C3N4. Meanwhile, the (002) peak increased from 27.21 for bulk g-C3N4 to 27.71 for Fg-C3N4 dots, corresponding to a decrease in the interplanar stacking distance from 0.325 to 0.321 nm. This suggested that the gallery distance was decreased in dots, i.e., the stack of F-gC3N4 dots became denser. These ndings were consistent with previous results that the single layers in bulk g-C3N4 might be undulated, and could be further planarized aer further heating treatment, leading to a denser stacking.28 The doped O heteroatoms and the disturbance of graphitic structure were assigned to interpret this observation. Since the introduced O atoms in the framework had higher electronegativity than the substituted N atoms, the interactions between the carbon nitride layers would be strengthened by the stronger attraction, resulting in the shortened interplanar distance. In order to investigate the chemical bonding between the C atoms and N atoms, high-resolution C 1s and N 1s spectra of bulk g-C3N4 and F-g-C3N4 dots were further studied, as shown in Fig. 2C and D. The C 1s spectrum of bulk g-C3N4 could be divided into two peaks, 284.7 eV and 288.4 eV. Compared with bulk g-C3N4, a new peak at 289.0 eV was detected in F-g-C3N4 dots, which was ascribed to the C–O single bond.29 The observation was also consistent with the FTIR analysis (Fig. 2B). Detailed analysis of the FTIR spectra is given in the ESI.†
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For bulk g-C3N4, three different peaks at 398.5, 399.9 and 401.4 eV were assigned as sp2 (C–N–C), sp3 nitrogen (N–[C]3) and primary amine nitrogen atoms (C–NH2), respectively (Fig. 2D).30 However, for F-g-C3N4 dots, the peak of primary amine nitrogen atoms at 401.4 eV disappeared. In addition, the overall intensity ratio of N(sp2)/N(sp3) decreased from 2.81 to 2.32. This observation showed that the dots surface was poor in nitrogen, which indicated that the hydrothermal cutting process preferred the oxidation of nitrogen rather than carbon. The UV-vis spectra (Fig. 3A) indicated that the absorption edge of the F-g-C3N4 dots showed a slightly blue shi as compared with the bulk g-C3N4.31 Moreover, the PL spectrum of the F-g-C3N4 dots also showed a blue shi of 30 nm as compared with that of bulk g-C3N4 (Fig. 3B), which was ascribed to the O atoms on the surface of the F-g-C3N4 dots. This observation was similar to the reported N-doped graphene QDs.32 The blue shi in both the UV-vis and PL spectra might result from the effect of quantum connement with the conduction band (CB) and valence band (VB) shiing in opposite directions. When the dispersion solution was excited at 365 nm, a strong PL emission peak centered at 437 nm was present, indicating that the dots were uorescent. The photograph of the dispersion exhibited a bright blue color under UV light at 365 nm (inset Fig. 3B), which further revealed that the F-g-C3N4 dots had strong blue uorescence. When the excitation wavelength changed from 300 to 400 nm, the PL spectra were almost invariable and showed a strong peak at 437 nm (Fig. 3C). The above results indicated that the F-g-C3N4 dots indeed exhibited an excitation-independent PL behavior. This excitation-independent emission property is different from most reported carbon dots,33,34 which could avoid auto-uorescence during their applications. Generally, the PL spectra of most luminescent carbon dots are dependent
Fig. 3 (A) UV-visible spectra. (B) The PL spectra of bulk g-C3N4 and Fg-C3N4 dots, the inset is the color change of the F-g-C3N4 dots solution under UV light (l ¼ 365 nm) irradiation. (C) PL spectra of the Fg-C3N4 dots at different excitation wavelengths. (D) The pH-dependent PL intensities of the F-g-C3N4 dots solution.
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on excitation wavelength. In other words, the PL peaks shied to longer wavelengths with a maximum intensity as the excitation wavelength is red shied. However, the as-prepared F-gC3N4 dots showed an excitation-independent PL feature. As reported earlier, the various energy levels associated with different ‘surface states’ formed by heavy derivatization made it self-passivated, which was responsible for the excitationdependent-emission phenomenon.35,36 We observed extraordinary PL behavior of F-g-C3N4 over the excitation wavelength range of 300–400 nm, which might be attributed to the size and the relatively uniform surface state.37,38 The F-g-C3N4 dots showed high PL intensity under basic conditions, which dropped as the pH decreased to acidic conditions (Fig. 3D). This pH-dependent PL behaviour might result from the free zigzag sites in the F-g-C3N4 dots, which were similar to the reported graphene QDs.11,39 Besides, the measured PL QY, using quinine sulfate as a reference, was 16.9%, much higher than that of bulk g-C3N4 (5.2%) and the well-known watersoluble graphene QDs (3.8%).16 From the results discussed above, the hydrothermal treatment exerted a strong effect on the formation, structure, and optical properties of the F-g-C3N4 dots. Two models are presented to interpret the mechanism for the hydrothermal process, as discussed in Fig. S1.† Application as uorescent probes Cu2+ and Fe3+ are well-known to be high toxic to organisms, such as certain algae, fungi, and many bacteria and viruses.40–46 The detection of such metals is thus highly desired. Herein, we report the detection methodology for Fe3+ and Cu2+, utilizing non-toxic F-g-C3N4 dots.
Fig. 4 PL intensity changes of F-g-C3N4 dots for different concentrations of Fe3+ (A) and Cu2+ (C). (B) The difference in relative PL intensities of F-g-C3N4 dots between the blank and solutions containing different metal ions (excitation at 360 nm, [Mn+] ¼ 50 mM, I and I0 are PL intensities of F-g-C3N4 dots in the presence and absence of metal ions, respectively). (D) Selective relative PL intensities of F-gC3N4 dots (black bar) after treatment with 50 mM metal ion solutions, and interference of 50 mM of other metal ions with 50 mM Cu2+ (red bar) and 50 mM Fe3+ (blue bar).
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Screening experiments with different metal ions (50 mM) were carried out by investigating the PL intensity changes of the F-g-C3N4 dots, as shown in Fig. 4B and S4.† The PL intensities of the F-g-C3N4 dots were greatly decreased by Fe3+ and Cu2+ while the other ions demonstrated less or no effect, indicating the uorescence of the F-g-C3N4 dots was sensitive to Fe3+ and Cu2+, and the F-g-C3N4 dots can be used as promising uorescence probes for Fe3+ and Cu2+ detection. The high selectivity of F-g-C3N4 for Cu2+ and Fe3+ may be due to the fact that the Cu2+ and Fe3+ ions have a higher binding affinity and faster chelating kinetics with N and O functional groups of F-g-C3N4 than other transition-metal ions, as indicated previously.47,48 Besides, the redox potentials of Cu2+/Cu+ and Fe3+/Fe2+ are between the conduction band (CB) and valence band (VB) of g-C3N4, so photoinduced electrons can transfer from the CB to the complexed Cu2+ and Fe3+, leading to uorescence quenching.23 To evaluate the F-g-C3N4 dots’ sensitivities toward different concentrations of Fe3+ and Cu2+, PL intensities were monitored by the titration of Fe3+ and Cu2+ at a xed time of 10 min. It can be seen from Fig. 4 that the PL intensities of F-g-C3N4 dots at 437 nm gradually decreased with increasing Fe3+ concentration. The LOD of Fe3+ was about 1.0 nM at a signalto-noise ratio of 3, which was much lower than or at least comparable to the previous results.40,41,46,49 As for Cu2+, the PL intensities of the F-g-C3N4 dots decreased gradually with increasing Cu2+ concentration (Fig. 4C). The LOD of Cu2+ was calculated to be as low as 0.5 nM, which was much lower than the maximum allowable level of Cu2+ (20 mM) in drinking water as regulated by the US Environmental Protection Agency (EPA).50 The time-dependent PL spectra are shown in Fig. S3,† and indicate that only 10 min were required to complete the detection between F-g-C3N4 dots and Fe3+/Cu2+. These excellent results suggested that F-g-C3N4 dots can serve as robust uorescent probes to detect Fe3+ and Cu2+ with high accuracy and simplicity. The selectivity toward Fe3+ and Cu2+ was further investigated in the presence of other metal ions, as shown in Fig. 4D. No obvious PL intensity change was observed aer other metal ions were added. However, further addition of Fe3+ or Cu2+ into the above mixtures caused a sharp decrease in PL intensity, which indicated that the detection of Fe3+ or Cu2+ was not interrupted by other metals. The selectivity was also investigated in the presence of several metal ions, as shown in Fig. S4.† To further improve the selectivity, the effect of Fe3+ ions on the detection of Cu2+ was also studied. In order to achieve the unique detection of Cu2+, the efficient Fe3+ chelating reagent sodium hexametaphosphate (SHPP) was added to minimize the interference of Cu2+.51 As shown in Fig. 5A and B, the PL intensities of F-g-C3N4 dots showed a dramatic decrease with increasing Cu2+ concentrations. Indeed, the introduction of Fe3+ into F-g-C3N4 dots–Cu2+ mixtures had no inuence on the detection of Cu2+in the presence of SHPP. So Fe3+ can be successfully masked under these conditions with little effect on the Cu2+ detection, allowing for the sensitive detection of Cu2+. Besides, the prepared F-g-C3N4 dots were also reusable in the detection of Fe3+, which means that the uorescence can be
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Fig. 5 PL intensities of F-g-C3N4 dots for Cu2+ (A) and Fe3+ (C) in the presence of Fe3+ and SHPP (A: process B(e) and C: process D(e)). Relative fluorescence responses for F-g-C3N4 dots to the sensitive single detection of Cu2+ (B) in the presence and absence of SHPP at 437 nm. Reusable probes for detecting Fe3+ (D), turn-on fluorescence can be obtained in the presence and absence of SHPP at 437 nm. B (a, b, c, d) and D (a, b, c, d) indicate pure F-g-C3N4 dots, F-g-C3N4 dots in the presence of 60 mM SHPP, 50 mM Fe3+, 60 mM SHPP added to the solution of B(c), respectively. B(e) and B(f) indicate 50 mM Cu2+ added to the mixed solution of B(d) and 60 mM SHPP added to the mixed solution of B(e), respectively. The results showed that the probe was turn-off. D(e) and D(f) indicate 50 mM Fe3+ added to the mixed solution of D(d) and 60 mM SHPP added to the mixed solution of D(e), respectively. The results showed that the probe was turn-on.
turned-on again. The on–off–on uorescence process is shown in Fig. 5C and D. As can be seen, the uorescence of F-g-C3N4 dots nearly disappeared in the presence of 50 mM Fe3+. However, the uorescence was recovered aer SHPP was added. Because the specic complex ratio of Fe3+ and SHPP was unknown, an excess of SHPP was added to recover the uorescence to the relatively high LOD value (100 nM), which was still lower than the maximum level (5 mM) of Fe3+ in drinking water permitted by the EPA.52 Therefore, the F-g-C3N4 dots have great potential for repeated detection of Fe3+. The F-g-C3N4 dots were also successfully used to detect Fe3+ and Cu2+ ions in natural water obtained from the Dongpu Lake of Hefei, Anhui province, China (Fig. S5†).
Conclusion In conclusion, for the rst time, green and non-toxic F-g-C3N4 dots were synthesized by a facile hydrothermal treatment of bulk g-C3N4. The obtained F-g-C3N4 dots possess high dispersity, excellent optical properties and high QY. The F-g-C3N4 dots further served as novel and green uorescent probes for the label-free sensitive and selective detection of Fe3+ and Cu2+ ions in lab and natural water samples. The addition of SHPP to the detection system can successfully mask Fe3+, allowing the selective detection of Cu2+. Therefore, we present a simple, lowcost route toward green production of F-g-C3N4 dots uorescent probes for fast, highly selective, and sensitive optical detection of Fe3+ and Cu2+ ions.
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Acknowledgements We greatly appreciate the nancial support from National Natural Science Foundation of China (21225730, 91326202, 21207136 and 21272236), the Ministry of Science and Technology of China (2011CB933700), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Hefei Center for Physical Science and Technology (2012FXZY005).
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