Economical, Green Synthesis of Fluorescent Carbon Nanoparticles

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Economical, Green Synthesis of Fluorescent Carbon Nanoparticles and Their Use as Probes for Sensitive and Selective Detection of Mercury(II) Ions Wenbo Lu,† Xiaoyun Qin,† Sen Liu,† Guohui Chang,† Yingwei Zhang,† Yonglan Luo,† 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, P. R. 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 S Supporting Information *

ABSTRACT: The present article reports on a simple, economical, and green preparative strategy toward water-soluble, fluorescent carbon nanoparticles (CPs) with a quantum yield of approximately 6.9% by hydrothermal process using low cost wastes of pomelo peel as a carbon source for the first time. We further explore the use of such CPs as probes for a fluorescent Hg2+ detection application, which is based on Hg2+induced fluorescence quenching of CPs. This sensing system exhibits excellent sensitivity and selectivity toward Hg2+, and a detection limit as low as 0.23 nM is achieved. The practical use of this system for Hg2+ determination in lake water samples is also demonstrated successfully.

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ercury(II) ion (Hg2+) is one of the most dangerous and ubiquitous pollutants, which raises serious environmental and health concerns.1 It is demonstrated that Hg2+ can easily pass through skin, respiratory, and gastrointestinal tissues, leading to DNA damage, mitosis impairment, and permanent damage to the central nervous system.2,3 Therefore, developing effective analytical methods for the sensitive and selective detection of trace amounts of Hg2+ is especially important. Traditional techniques including atomic absorption/emission spectroscopy, Auger-electron spectroscopy, inductively coupled plasma mass spectrometry, and polarography require sophisticated instrumentation and/or sample preparation, which limit their practical applications.4−6 Fluorescence assays have several advantages, such as high sensitivity, fast analysis, and being nonsample-destructing or less cell-damaging, and have been proven to be an alternative method for Hg2+ detection.7 So far, many fluorescent probes including organic molecule, metal nanoclusters, semiconductor quantum dots (QDs), etc. have been developed for fluorescence detection of Hg2+. For example, Gong et al. described a novel rhodamine thiospirolactam derivative as an “off-on” fluorescent probe for the detection of Hg2+ in aqueous samples;7 Lin et al. demonstrated the use of lysozyme type VI-stabilized gold nanoclusters (Lys VI-AuNCs) for the detection of Hg2+ based on fluorescence quenching;8 Long et al. reported trace Hg2+ analysis via quenching the fluorescence of a CdS-encapsulated DNA nanocomposite.9 However, the above fluorescent materials suffer © 2012 American Chemical Society

from complex synthesis routes or the involvement of toxic or expensive regents. Accordingly, the development of a simple, economical, and green preparative strategy toward fluorescent materials is highly desired. Fluorescent carbon nanoparticles (CPs) or nanodots (CDs) constitute a fascinating class of recently discovered nanocarbons with a size below 10 nm and have attracted considerable research interest due to their excellent photostability, favorable biocompatibility, low toxicity, and good water solubility.10,11 As a consequence of their outstanding properties, CPs or CDs form attractive applications. Sun and co-workers reported the first study on CDs passivated with poly(propionylethylenimine-co-ethylenimine) for two-photon luminescence microscopy imaging of human breast cancer cells, which was followed by a great deal of research.12−15 Li et al. demonstrated the design of photocatalysts based on the upconversion luminescence properties of CDs to utilize the full spectrum of sunlight.16 The electron-donor capabilities of photoexcited CDs have been demonstrated by Wang et al. through photoreduction of Ag+ to Ag by photoirradiating CDs in an aqueous solution of AgNO3 at a visible wavelength.17 Gonçalves et al. described an optical fiber sensor for Hg2+ Received: March 27, 2012 Accepted: May 31, 2012 Published: May 31, 2012 5351

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Figure 1. (a) TEM images of the products thus formed. The inset shows the HRTEM image of one nanoparticle, (b) the corresponding particle size distribution histogram.

Characterizations. UV−vis spectra were obtained on a UV5800 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray photoelectron spectrometer using Mg as the exciting source. FT-IR spectrum was performed on an IFS 66 V/S (Bruker) IR spectrometer in the range of 400−4000 cm−1. Transmission electron microscopy (TEM) measurements were made on a HITACHI H-8100 electron microscope (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of colloidal solution on carbon-coated copper grid and dried at room temperature. Fluorescent emission spectra were recorded on a RF-5301PC spectrofluorometer (Shimadzu, Japan). The ζ potential measurement was performed on a Nano-ZS Zetzsozer ZEN3600 (Malvern Instruments Ltd., U.K.). QY was measured according to an established procedure.27 The optical densities measured on UV−vis spectra were obtained on a UV5800 spectrophotometer. Quinine sulfate in 0.1 M H2SO4 (literature quantum yield 0.54 at 360 nm) was chose as a standard. Absolute values are calculated using the standard reference sample that has a fixed and known fluorescence QY value. In order to minimize reabsorption effects, absorbencies in the 10 mm fluorescence cuvette were kept under 0.1 at the excitation wavelength (360 nm). Preparation of Fluorescent CPs. CPs were prepared by hydrothermal treatment of pomelo peel. In a typical synthesis, 1 g of pomelo peel was added into 15 mL of H2O. Then the mixture was transferred into a 25-mL Teflon-lined autoclave and heated at 200 °C for a period of 3 h. The CPs were collected by removing the large dots through centrifugation at 12 000 rpm for 10 min and finally dried under vacuum for 48 h. The CPs was dispersed in distilled water at a concentration of 0.5 mg/mL for further characterization and use. Fluorescence Assay of Hg2+. The detection of Hg2+ was performed at room temperature in PBS (0.2 M, pH 7.0) buffer solution. In a typical run, 3 μL of CPs dispersion was added into 1 mL of PBS buffer, followed by the addition of a calculated amount of Hg2+ ions. The photoluminescence (PL) emission spectra were recorded after reaction for 16 min at room temperature. The sensitivity and selectivity measurements were conducted in triplicate.

in aqueous solution based on sol−gel immobilized CDs functionalized with PEG200 and N-acetyl-L-cysteine.18 Until now, a variety of preparative methods toward CPs have been developed, including arc-discharge, laser ablation, electrochemical oxidation, combustion/thermal, supported synthesis, and microwave heating.11 However, these approaches suffer from disadvantages, such as involvement of complex or posttreatment processes and the use of a large amount of strong acid or expensive raw materials. More recently, carbonization of chitosan, glucose, coffee grounds, ethylenediamine-tetraacetic acid salts, and grass has been successfully employed to prepare fluorescent CPs.19−25 From the point of view of material synthesis, the exploration of new carbon source for simple, economical, and green synthesis of such CPs is highly desirable. In this article, we demonstrate the first use of low cost wastes of pomelo peel as a carbon source for hydrothermal preparation of water-soluble, fluorescent CPs with a quantum yield (QY) of approximately 6.9%. We further demonstrate that such CPs can serve as a very effective fluorescent probe for label-free, sensitive, and selective detection of Hg2+ with a detection limit as low as 0.23 nM, which is much lower than a previously reported CPs-based sensing system.24 The sensing principle is based on fluorescence quenching of CPs by Hg2+ presumably due to facilitating nonradiative electron/hole recombination annihilation through an effective electron or energy transfer process.24,26 The fluorescence of CPs will recover when a stronger Hg2+ chelator, cysteine (Cys), is added due to the formation of a Hg−S bond with Cys that leads to the removal of Hg2+ from CP.24 To demonstrate the practicality of this fluorescence probe, it is further applied to the determination of Hg2+ in lake water.

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EXPERIMENTAL SECTION Materials. AgNO3, CaCl2, CdCl2, CoCl2, CuCl2, FeCl3, Hg(NO3)2, Mg(OAc)2, MnCl2, NiCl2, Pb(NO3)2, and Zn(OAc)2 were purchased from Beijing Chemical Corp. NaH2PO4 Na2HPO4 and Cys were purchased from Aladin Ltd. (Shanghai, China). All chemicals were used as received without further purification. The pomelo was purchased from local supermarket and washed with water for further use. The water used throughout all the experiments was purified through a Millipore system. 5352

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Figure 2. (a) XPS, (b) C1s, (c) O1s, and (d) N1s spectra of the nanoparticles thus obtained.

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RESULTS AND DISCUSSION Figure 1a shows a TEM image of the products thus formed, revealing that they consist of nanoparticles well separated from each other. The corresponding nanoparticle size distribution histogram obtained by counting about 80 particles (Figure 1b) indicates that these nanoparticles have diameters ranging from 2 to 4 nm. The high-resolution TEM (HRTEM) image taken from one nanoparticle (inset in Figure 1a) shows a crystalline structure with lattice spacing of 0.20 nm which may be attributable to the (102) diffraction planes of graphitic (sp2) carbon (in reference to JCPDS cards 26-1076), which is quite similar with previously reported CDs.28,29 The surface composition and elemental analysis for the resultant nanoparticles were characterized by the XPS technique. The three peaks at 285.0, 400.0, and 532.6 eV shown in the XPS spectrum of the these nanoparticles (Figure 2a) can be attributed to C1s, N1s, and O1s, respectively.30 The XPS results indicate that these nanoparticles are mainly composed of C, O, as well as a limited amount of the N element (the atom ratio of C/O/N is 17.56:6.13:1). The C1s spectrum (Figure 2b) shows three peaks at 284.6, 286.0, and 288.0 eV, which are attributed to C−C, C−N, and CN/ CO, respectively.31 The two peaks at 531.7 and 533.0 eV in O1s spectrum (Figure 2c) are attributed to CO and C−OH/C−O−C groups, respectively; 32 while the N 1s spectrum (Figure 2d) shows four peaks at 399.5, 400.4, 401.5, and 406.4 eV, which are attributed to the C−N−C, N− (C)3, N−H, and −NO3 bands, respectively.33,34 Figure 3 shows the FT-IR spectrum of these nanoparticles. The peaks at about 3300 and 1050 cm−1 can be ascribed to the characteristic absorption bands of the −OH stretching vibration mode. The peaks at 1600 and 1400 cm−1 can be attributed to the asymmetric and symmetric stretching vibration of COO−, respec-

Figure 3. FT-IR spectrum of the nanoparticles thus formed.

tively.35 The characteristic absorption band of C−OH stretching at 1272 cm−1 is also observed,36 and the peak at 2923 and 815 cm−1 can be assigned to the C−H stretching mode and C−H out-of-plane bending mode.24,36 The above observations confirm that the synthesized nanoparticles function with hydroxyl and carboxylic/carbonyl moieties which may originate from carbohydrates in the pomelo peel. Figure 4 shows the UV−vis absorption and PL emission spectra of the aqueous dispersion of the nanoparticles. It is seen that the UV−vis spectrum shows a strong peak at 280 nm. Note that the dispersion shows a strong PL emission peak centered at 444 nm when excited at 365 nm, indicating the nanoparticles are fluorescent. The photograph of the dispersion under UV light (365 nm) exhibits a blue color (inset), further revealing that the resultant CPs exhibit blue fluorescence. The ζ 5353

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Figure 4. UV−vis absorption (black line) and PL emission (blue line) spectra of the aqueous dispersion of the nanoparticles. Inset: the photograph of CPs dispersion under UV light (365 nm).

potential of such CPs was measured to be −20.9 mV, which further confirms that the nanoparticle surface functionalized with hydroxyl and carboxylic/carbonyl moieties.24 It is worthwhile mentioning that these CPs can be very stable for several months without the observations of any floating or precipitated nanoparticles, which can be attributed to their small particle size and electrostatic repulsions between them. It is of importance to mention that hydrothermal treatment of another batch of pomelo peels or other fruit peels such as apple, banana, orange, and pineapple, etc. also produces stable fluorescent CPs, and thus the CPs preparation method proposed herein is universal. Figure 5a shows the fluorescence curves of CPs at different pH values. It is seen that an increase of the pH value from 1.0 to 7.0 leads to increased PL intensity, but a further increase to 13.0 results in a gradual decrease of fluorescence intensity, indicating the fluorescence intensity of CPs strongly depends on the pH value. It is found that such observations are similar to those of CPs modified with hydroxyl and carboxylic/ carbonyl moieties.10,37 To confirm the stability of CPs under high ionic strength environments, their PL intensities were measured in a solution of 0.2 M PBS (pH 7.0) containing different concentrations of NaCl. As shown in Figure 5b, only a slight change in the PL intensity is observed, which reveals that CPs are stable even under high ionic strength conditions. This finding suggests that CPs have great potential for sensing applications under physiological conditions. We explored the feasibility of using such CPs for Hg2+ detection. It is seen that CPs solution in the absence of Hg2+ exhibits a strong PL peak at 444 nm (Figure 6, curve a) and its QY is determined to be 6.9%. In contrast, the presence of Hg2+ leads to an obvious decrease of fluorescence in intensity, indicating that Hg2+ can effectively quench the fluorescence of CPs (Figure 6, curve b). This observation can be attributed to that Hg2+ can quench the fluorescence of CPs presumably via electron or energy transfer.24,26 Indeed, when adding a strong Hg2+ chelator, Cys, Hg2+ is removed from the surface of CPs by forming a Hg−S bond with Cys, which results in the fluorescence recovery of CPs (Figure 6, curve c).24 For a sensitivity study, different concentrations of Hg2+ in the range of 0−4 × 104 nM were investigated. Figure 7a shows a gradual decrease in PL intensity at 444 nm with an increased Hg2+ concentration, revealing that the sensing system is sensitive to Hg2+ concentration. The

Figure 5. The effect of (a) the solution pH value and (b) the NaCl concentration (0, 10, 50, 100, 200, 300 mM) on CPs fluorescence.

Figure 6. PL spectra of solutions of CPs (curve a), CPs-Hg2+ mixture (curve b), and CPs-Hg2+-Cys mixture (curve c).

fluorescence quenching data follows the Stern−Volmer equation, via either a dynamic or a static mechanism:9,27 F0/F − 1 = K svc

where Ksv is the Stern−Volmer quenching constant, c is the analyte (Hg2+) concentration, and F0, F are the PL intensities of CPs at 444 nm in the absence and presence of Hg2+. The Stern−Volmer plot shown in Figure 7b does not fit a linear Stern−Volmer equation in the whole concentration range. The correlation coefficients (R2) are 0.9661 and 0.9876 for 5354

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fluorescent probes for Hg2+ detection, suggesting our sensing system exhibits superior sensitivity than previously reported sensing systems.8,9,24,38−41 The time-dependent PL spectra of CPs-Hg2+ solution shown in Figure S1in the Supporting Information indicate that only 16 min is required to complete the reaction between CPs and Hg2+. Besides sensitivity, selectivity is another important parameter to evaluate the performance of the sensing system. Therefore, we examined the PL intensity changes in the presence of representative metal ions under the same conditions, including Ag+, Ca2+, Cd2+, Co2+, Cu2+, Fe3+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+, as shown in Figure S2 in the Supporting Information. It is seen that a much lower PL was observed for CPs upon addition of Hg2+. In contrast, no tremendous decrease was observed by adding other ions into the CPs dispersion. Furthermore, the selectivity of this nanoprobe in the presence of all possible interference ions was evaluated considering the cross reactivity. As demonstrated in Figure 8, obviously, the present method can still detect

Figure 7. (a) PL spectra of CPs dispersion in the presence of different Hg2+ concentrations (from top to bottom: 0, 0.5, 1, 5, 10, 50, 100, 500, 1 × 103, 5 × 103, 1 × 104, 2 × 104, 3 × 104, and 4 × 104 nM). (b) Plots of the values of (F0/F) − 1 at 444 nm versus the concentrations of Hg2+ (excitation at 360 nm; incubation time was 16 min. The error bars represent standard deviations based on three independent measurements.).

Figure 8. The difference in PL intensity of CPs dispersion under various conditions in PBS (pH 7.0; excitation at 360 nm; mixed metal ions including Ag+, Ca2+, Cd2+, Co2+, Cu2+, Fe3+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+; [Mn+] = 5 × 104 nM. The error bars represent standard deviations based on three independent measurements.).

determining Hg2+ over the two linear concentration ranges of 0.5−10 and 500−4 × 104 nM, respectively. The detection limit is estimated to be 0.23 nM at a signal-to-noise ratio of 3, which is much lower than other previously reported values and lower than the maximum level (10 nM, 2 ppb) for mercury in drinking water permitted by the United States Environmental Protection Agency.7 Table 1 shows the comparison of different

Hg2+ in the presence of all possible interference ions (the concentration of each metal ion in the mixture was 5 × 104 nM). The outstanding selectivity and specificity can be probably attributed to that Hg2+ has a stronger affinity toward the carboxylic group on the CPs surface than other metal ions.42,43 The feasibility of CPs for detecting Hg2+ in real water samples was explored by lake water samples obtained from the South Lake of Changchun, Jilin province, China. The lake water samples were filtered through a 0.22 μm membrane and then centrifuged at 12 000 rpm for 20 min. The resultant water samples were spiked with standard solutions containing different concentrations of Hg2+. It is seen that the PL intensity decreases with increased concentration of Hg2+ from 5 to 50 nM, as shown in Figure 9a. The calibration curve for determining Hg2+ in lake water was obtained by plotting the values of (F0/F) − 1 versus the concentrations of Hg2+ (Figure 9b). 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 5 nM Hg2+, satisfying the practical Hg2+ detection in real samples. These results imply that the Hg2+ probe is likely to be capable of practically useful Hg2+ detection upon further development.

Table 1. Comparison of different fluorescent probes for Hg2+ detection performance fluorescent probes Lys VI-AuNCs CdS-encapsulated DNA CDs fluorescent gold nanoparticles fluorescent Ag clusters Au@Ag core−shell nanoparticles CdTe quantum dots CPs

detection limit (nM)

linear range (nM)

0.003 4.3 4.2 5

0.01−5 10−110 0−3 × 103 10−1 × 104

8 9 24 38

10 9

10−5 × 103 10−450

39 40

2−14 0.5−10

41 this work

1.55 0.23

ref

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ACKNOWLEDGMENTS W. Lu and X. Qin contributed equally to this work. This work was supported by the National Natural Science Foundation of China (Grant No. 21175129), the National Basic Research Program of China (Grant No. 2011CB935800), and the Scientific and Technological Development Plan Project of Jilin Province (Grant Nos. 20100534 and 20110448).

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Figure 9. (a) PL spectra of CPs dispersion in the presence of different Hg2+ concentrations (from top to bottom: 0, 5, 10, 20, 30, 40, and 50 nM) in lake water. (b) The dependence of (F0/F) − 1 on the concentrations of Hg2+ within the range of 0−50 nM.

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CONCLUSIONS In summary, hydrothermal treatment of pomelo peel has been proven to be an effective strategy for producing fluorescent CPs for the first time. Furthermore, no further chemical modification of CPs is required, which offers the advantages of simplicity and cost efficiency. Such CPs have been further used as a novel sensing probe for label-free, sensitive detection of Hg2+ ions with a detection limit as low as 0.23 nM. This sensing system also possesses high selectivity toward Hg2+ analysis and has been successfully used for the analysis of a lake water sample. Our present study allows upward scalability in terms of the green production of fluorescent and biocompatible nanocarbons which could be applied in bioimaging and other applications.11−18

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ASSOCIATED CONTENT

S Supporting Information *

Time-dependent PL spectra and the difference in PL intensity of CPs dispersion between the blank and solutions containing different metal ions. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: 0086-431-85262065. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5356

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