Ultrathin Graphitic Carbon Nitride Nanosheet: A Highly Efficient

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Ultrathin Graphitic Carbon Nitride Nanosheet: A Highly Efficient Fluorosensor for Rapid, Ultrasensitive Detection of Cu2+ Jingqi Tian, Qian Liu, Abdullah M. Asiri, Abdulrahman O. Al-Youbi, and Xuping Sun Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac400924j • Publication Date (Web): 07 May 2013 Downloaded from http://pubs.acs.org on May 12, 2013

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Analytical Chemistry

Ultrathin Graphitic Carbon Nitride Nanosheet: A Highly Efficient

Fluorosensor

for

Rapid,

Ultrasensitive

Detection of Cu2+ Jingqi Tian,†, ‡ Qian Liu,† Abdullah M. Asiri, §,║ Abdulrahman O. Al-Youbi§,║ and Xuping Sun†,§,║* †

State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡

Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

§

Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia ║

Center of Excellence for Advanced Materials Research, King Abdulaziz University,

Jeddah 21589, Saudi Arabia *Corresponding author. Tel/Fax: 0086-431-85262065. E-mail: [email protected] (X. Sun).

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ACS Paragon Plus Environment


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Abstract A highly efficient fluorosensor based on ultrathin graphitic carbon nitride (g-C3N4) nanosheets for Cu2+ was developed. In the absence of metal ions, the nanosheets exhibit high fluorescence; the strong coordination of the Lewis basic sites on them to metal ions, however, causes fluorescence quenching via photoinduced electron transfer leading to the qualitative and semiquantitative detection of metal ions. This fluorosensor exhibits high selectivity toward Cu2+. The whole detection process can be completed within 10 min with a detection limit as low as 0.5 nM. The use of test paper enables the naked-eye detection of Cu2+ with a detection limit of 0.1 nmol. The practical use of this sensor for Cu2+ determination in real water samples was also demonstrated.

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Introduction During the past years, the development of sensitive chemosensors capable of selective recognition of cations, especially for metal ions with biological interest, has always been of particular significance due to their potential application in chemistry, biomedicine, and environmental studies.1 Copper is a widely used industrial metal and plays important roles in either environmental or biological systems. It is an essential trace element for humans and other animals. Proteins such as cytochrome oxidase, zinc-copper superoxide dismutase, lysyl oxidase and several transcription factors require copper for their activities. Copper ions (Cu2+) can accumulate in human and animal livers through bioaccumulation. Under overloading conditions, however, copper becomes high toxicity to humans2 and disruption of copper homeostasis in cells may cause oxidative stress and severe disorders such as Menkes syndrome, Wilson's disease, amyotrophic lateral sclerosis and Alzheimer's disease.3 Therefore, simple and rapid detection of Cu2+ is highly important. Cu2+ sensors based on the ion-induced fluorescence changes appear to be particularly attractive because of the simplicity and high sensitivity of fluorescence technique. The fluorescent probes for the determination of Cu2+ can monitor Cu2+ both in solution and in living cells by fluorescent microscopy.4 A number of fluorescent probes have been developed so far, including small organic molecules,5 noble metal nanoclusters,6 and metal-based quantum dots (MQDs)7 etc. The fluorescent probes mentioned above, however, suffer from some drawbacks, such as photobleaching of fluorescent dye, expensive cost of noble metals and toxicity of 3



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MQDs, which limit their widespread applications. More recently, we have prepared nitrogen-doped, carbon-rich, photoluminescent polymer nanodots (PPNDs) by hydrothermal treatment of grass and demonstrated their application as fluorescent probes for Cu2+ sensing.8 Although the preparation is low-cost and green, it still suffers from pretty low yield (0.1%). Graphitic carbon nitride (g-C3N4) is the most stable allotrope of carbon nitride and has recently attracted great interests because of its semiconductor properties.9 g-C3N4 can be prepared on a large scale by bulk condensation of low cost nitrogen-rich precursors, including cyanamide, dicyandiamide, melamine, ammonium thiocyanate, urea, and thiourea etc. Despite showing high fluorescence, excellent biocompatibility and nontoxicity,10 there has been paid less attention to its fluorosensor application. Only until recently have Lee et al. reported on the optical sensing of Cu2+ utilizing cubic mesoporous ordered g-C3N4 (c-mpg-C3N4) as an all-in-one chemosensor.11 However, the synthesis process involves multiple steps and tends to be laborious on one hand, and a long time is required for diffusion of the metal ions to coordination points deeply buried within the three-dimensional chemosensor on the other hand. Given that g-C3N4 nanosheet offers higher surface-area-to-volume ratio and exposes all its coordination points to metal ions, it should be superior to other g-C3N4 based fluorosensors for Cu2+ with improved detection speed and sensitivity. In this Article, for the first time, we demonstrate the use of ultrathin g-C3N4 nanosheets as highly efficient fluorosensor for Cu2+. The nanosheets were directly prepared by ultrasonication-assisted liquid exfoliation of bulk g-C3N4. We further 4


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demonstrate that such g-C3N4 nanosheet can serve as a very effective fluorescent probe for sensitive and selective detection of Cu2+ with a detection limit as low as 0.5 nM, which is much lower than that of previously reported fluorosensors. The sensing principle is based on fluorescence quenching of g-C3N4 nanosheet by Cu2+ via photoinduced electron transfer. Naked-eye detection with a detection limit of 0.1 nmol was achieved using test paper format. This sensor was also applied successfully for Cu2+ determination in lake water samples with a detection limit of 100 nM.

Experimental section Materials. Cd(NO3)2, Co(NO3)2, Cu(NO3)2, Fe(NO3)3, Hg(NO3)2, Mg(NO3)2, Mn(NO3)2, Ni(NO3)2, Pb(NO3)2, and Zn(NO3)2 were purchased from Beijing Chemical Corp. Qualitative filter paper was purchased from Hangzhou Xinhua Paper Industry Co., Ltd. Melamine, Tris, HCl, sodium hexametaphoshpate (SHHP) and ethylenediaminetetraacetic acid (EDTA) were purchased from Aladin Ltd. (Shanghai, China). All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Characterizations. UV-vis spectra were obtained on a UV5800 Spectrophotometer. Atomic force microscopy (AFM) study in the present work was performed by means of MultiMode-V (Veeco Metrology, Inc.). Fluorescent emission spectra were recorded on a RF-5301PC spectrofluorometer (Shimadzu, Japan). Photochemical test was performed on a Xenon lamp (CHFXQ500W, Beijing) with an UV monochromatic filter (λ = 365 nm, input power: 100 mW/cm2). Preparation of ultrathin g-C3N4 nanosheets. The bulk g-C3N4 was prepared by 5



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direct pyrolysis of melamine in the semiclosed system. In a typical synthesis, 20 g melamine was placed in an alumina crucible with a cover and then heated at 600 °C for 2 h with a heating rate of 3 °C min-1, leading to yellow powder. Although highly water-dispersible g-C3N4 nanoflakes particles can be prepared by chemically oxidizing the bulk g-C3N4 with nitric acid,12 a liquid exfoliating method with minor modification was used to obtain ultrathin g-C3N4 nanosheets in the present study.10b In brief, 50 mg of the bulk g-C3N4 powder was dispersed in 50 mL water and the mixture was ultrasounded consecutively for 10 h. The initial formed suspension was then centrifugated at 5000 rpm to remove the residual unexfoliated g-C3N4 before use. The product yield is measured to be 14.5 %. Fluorescence sensing of Cu2+: The fluorescence sensing of Cu2+ was performed at room temperature in tris-HCl (10 mM, pH 7.4) buffer solution. In a typical run, 30 µL of g-C3N4 nanosheets dispersion was added into 270 µL of tris-HCl buffer, followed by the addition of a calculated amount of Cu2+ ions. The photoluminescence (PL) emission spectra were recorded after reaction for 10 min at room temperature. Preparation of the test paper: 10 µL of g-C3N4 nanosheets solution was dripped on the surface of the filter paper, and after the solvent was evaporated sufficiently, 5 µL of different concentration of Cu2+ aqueous solution was dripped in the zones of g-C3N4 nanosheets on the surface of the filter paper. The test paper were placed at room temperature to evaporate the solvent throughly.

Results and Discussion The SEM image of the bulk g-C3N4 shown in Figure 1A indicates they are solid 6


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agglomerates about several micrometers in size. Figure 1B shows the photograph of the dispersion of products obtained after ultrasolication treatment of bulk g-C3N4. The occurrence of the Tyndall effect of the diluted dispersion in water reveals the colloidal nature of the dispersion. Figure 1C presents the AFM image of the resulting colloidal particles, respecively, revealing that they are nanosheets well separated from each other. The thickness of these nanosheets was measured by section analysis to be ~ 1.0 nm (Figure 1D), suggesting they are in forms of three C-N layers. Figure 2 shows the UV-vis absorption and PL emission spectra of the aqueous dispersion of the g-C3N4 nanosheets. The UV-vis spectrum shows a strong peak at 320 nm. Note that the dispersion shows a strong PL emission peak centered at 455 nm when excited at 355 nm, indicating the nanosheets are fluorescent. The photograph of the dispersion under UV light (365 nm) exhibits bright blue color (inset), further revealing that the resultant g-C3N4 nanosheets exhibit blue fluorescence. It is worthwhile mentioning that these g-C3N4 nanosheets can be very stable for several months without the observations of any floating or precipitated nanoparticles. Moreover, the PL intensity of the g-C3N4 nanosheets is very stable with no obvious photobleaching loss after irradiation with a Xe-lamp (λ=365 nm) for even 5 h. We explored the feasibility of using such g-C3N4 nanosheets for Cu2+ detection. Figure 3 shows the PL spectra of the g-C3N4 nanosheets dispersion under various conditions. The introduction of Cu2+ into the dispersion leads to an obvious decrease of fluorescence in intensity, indicating that Cu2+ can effectively quench the fluorescence of g-C3N4 nanosheets. The clelation of Cu2+ with N of the g-C3N4 7



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nanosheets brings them into close proximity with each other.13 Because the redox potential of Cu2+/Cu+ lies between the conduction band (CB) and valence band (VB) of g-C3N4, photoinduced electron transfer (PET) from the CB to the complexed Cu2+ occurs leading to fluorescence quenching (Scheme 1). For sensitivity study, different concentrations of Cu2+ in the range of 0-1×104 nM were investigated. Figure 3 shows a gradual decrease in PL intensity at 455 nm with increased Cu2+ concentration, revealing that the PL intensity of the mixture is sensitive to Cu2+ concentration. Figure 3 inset shows dependence of F0/F on the concentration of Cu2+ ions, where F0 and F are fluorescence intensities at 455 nm in the absence and presence of Cu2+, respectively. The detection limit is estimited to be 0.5 nM at a signal-to-noise ratio of 3. Table 1 compares the sensing performance of different fluorescent probes for Cu2+, showing our sensing system exhibits superior sensitivity to previously reported sensing systems.5-8, 12 The time-dependent PL spectra of a g-C3N4 nanosheets-Cu2+ solution (Figure 4) indicate that only 10 min is required to complete the reaction between g-C3N4 nanosheets and Cu2+ and thus the detection is rapid. Besides sensitivity, selectivity is another important parameter to evaluate the performance of the sensing system. Therefore, to evaluate the selectivity of this sensing system, we examined the PL intensity changes in the presence of representative metal ions under the same conditions. g-C3N4 nanosheets were added to metal nitrate solutions in tris-HCl buffer and mixed for 10 min to form metal iong-C3N4 complexes. Figure 5 shows the PL quenching result of g-C3N4 in the presence of Cd2+, Co2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+ and Zn2+. No tremendous decrease was 8


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observed by addition of these metal ions into the solution of g-C3N4. However, further addition of Cu2+ into above mixtures causes a dramatic decrease in PL intensity. All these observations indicate a small influence of other metal ions on this Cu2+ sensing system. The high selectivity of g-C3N4 for Cu2+ is due to that Cu2+ has higher thermodynamic affinity and faster chelating process with N of g-C3N4 than other transition-metal ions.14 It is observed that the presence of Fe3+ results in dramatic decrease of PL of g-C3N4 in intensity, indicating Fe3+ at a high concentration heavily interferes with Cu2+ detection. Fortunately, this issue can be circumvented by using sodium hexametaphoshpate (SHPP) as a chelating agent for Fe3+. Indeed, the addition of Fe3+ into the g-C3N4-Cu2+ mixture in the presence of SHPP gives no effect on the detection of Cu2+. All these observations indicate that the present fluorosensor exhibits high selectivity for Cu2+. We also fabricated g-C3N4–based test paper for optical Cu2+ sensing. The color of the zones in the test paper with the addition of high concentration of Cu2+ changed to light yellow, and there was fluorescence in the test zones under UV light (365 nm), as shown in Figure 6. The detection limit of such test paper was 5 nmol and 0.1 nmol under sunlight and UV light, respectively. Compared with other test papers reported, the fabrication of our test paper does not involve premodification of different concentrations of target on the surface of the paper.15 An additional advantage of our test paper is the use of common filter paper as the substrate and thus more economical. Moreover, due to the excellent biocompatibility and nontoxicity of the g-C3N4 nanosheets, our test paper is green and environmental friendly compared with the 9



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o-phenylenediamine-based Cu2+ test paper.16 To evaluate the Cu2+ sensor in an artifical system, the performance of the present method for real water sample anaysis was challenged by lake water sampels obtained form the South Lake of Changchun, Jilin province, China. The lake water samples were filtered through a 0.22 µm membrane and then centrifuged at 12000 rpm for 20 min. The resultant water samples were spiked with standard solutions containing different concentrations of Cu2+. It is seen that the PL intensity decreases with increased concentration of Cu2+ from 100 to 400 nM, as shown in Figure 7. The calibration curve for determining Cu2+ in lake water was obtained by plotting the values of F0/F versus the concentrations of Cu2+ (Figure 7, inset). In spite of the interference from numerous minerals and organics existing in lake water, this sensing system can still distinguish between fresh lake water and that spiked with 100 nM Cu2+, satisfying the sensitivity requirement of Cu2+ detection for drinking water (20 µM) defined by US Environmental Protection Agency (EPA).17 These results imply that the Cu2+ sensor is likely to be capable of prectically useful for Cu2+ detection upon further development.

Conclusions In summary, ultrathin g-C3N4 nanosheets comprised of only about three C-N layers have been fabricated by ultrasonication-assisted liquid exfoliation of bulk C3N4 and utilized as a highly efficient fluorosensor for selective Cu2+ detection for the first time. The detection is rapid, taking only 10 min, and ultrasensitive with a very low detection limit of 0.5 nM in buffer system, which can be attributed to that the g-C3N4 10


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nanosheet has high surface-area-to-volume ratio and exposes all its coordination points to metal ions. The test paper format shows a naked-eye detection limit of 0.1 nmol. Finally, this fluorosensor has been applied successfully for real water sample anaysis. Our present study is important because it provides us an economic, stable, and environmental friendly fluorescent probes for rapid, highly selective and sensitive optical detection of Cu2+.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21175129), the National Basic Research Program of China (No. 2011CB935800), and the Scientific and Technological Development Plan Project of Jilin Province (Nos. 20110448).

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17. US EPA. Maximum contaminant level goals and national primary drinking water regulations for lead and copper; final rule. Fed. Reg. 1991, 56, 6460.

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Figure captions Figure 1 (A) Typical SEM image of the bulk C3N4. (B) Tyndall effect exhibited by g-C3N4 nanosheets dispersion passed through with red laser light. (C) AFM image of thus obtained g-C3N4 nanosheets. (D) The corresponding section analysis of two random nanosheets. Figure 2 UV-vis absorption (black line) and PL emission (blue line) spectra of the g-C3N4 nanosheets thus obtained. Inset: the photograph of g-C3N4 suspension under UV light (365 nm). Figure 3 PL spectra of g-C3N4 nanosheets dispersion in the presence of different Cu2+ concentrations (from top to bottom: 0, 0.0005, 0.001, 0.002, 0.005，0.01, 0.02, 0.1, 0.2，0.5, 1, 2, 5 and 10 µM). Inset: The dependent of F0/F on the concentrations of Cu2+ ions within the range of 0-0.4µM (excitation at 355 nm; F0 and F are g-C3N4 nanosheets fluorescence intensities at 455 nm in the absence and presence of Cu2+ ions, respectively). Scheme 1 The sensing principle of the g-C3N4 nanosheets-based fluorosensor for Cu2+. Table 1 Comparison of sensing performace of different fluorescent probes for Cu2+ detection. Figure 4 Fluorescence quenching of g-C3N4 by 3µM Cu2+ in tris-HCl buffer (pH 7.4) as a function of time (λex=365 nm). Figure 5 Selective PL response of g-C3N4 (cyan bar) after treatment of 1 µM metal ion solutions, and interference of 1 µM of other metal ions with 1 µM Cu2+ (purple 15



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bar). Figure 6 The photograph of test paper with different amount of Cu2+ under irradiation of (A) sunlight and (B) UV light (365 nm). Figure 7 PL spectra of the g-C3N4 nanosheets dispersion in the presence of different Cu2+ concentrations (from top to bottom: 0, 0.1, 0.2, 0.3, 0.4 µM) in lake water. Inset: The dependent of F0/F on the concentrations of Cu2+ ions within the range of 0-0.4µM (excitation at 355 nm; F0 and F are g-C3N4 nanosheets fluorescence intensities at 455 nm in the absence and presence of Cu2+ ions, respectively).

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Figure 1

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Figure 2

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Figure 3

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Scheme 1

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fluorescent probes Coumarin derivative molecular 1,3,5-Triphenylbenzene fluorophore DNA-AgNCs CdS QDs 16-MHA capped CdSe QDs PPNDs c-mpg-C3N4 Peptide-coated ZnS QDs Ligand-conjugated inorganic NPs Ultrathin g-C3N4 nanosheets

performance linear range detection limit (nM) (nM) 87 103 500 0-3×103 100 5 5-1×105 1 0-5×104 12.3 10-100 500 0-2.6×104 380 0.5 0-1×104 Table 1

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ref. 5a 5b 6a 7a 7b 8 11 7c 6b This work


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Figure 4

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Figure 6

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Figure 7

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for TOC only

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Ultrathin Graphitic Carbon Nitride Nanosheet: A Highly Efficient

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