Carbon dots with tunable emission, controllable size and their application for sensing hypochlorous acid

Journal of Luminescence 151 (2014) 100–105

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Carbon dots with tunable emission, controllable size and their application for sensing hypochlorous acid Zhaoxia Huang, Feng Lin, Ming Hu, Chunxiang Li, Ting Xu, Chuan Chen, Xiangqun Guo n The Key Laboratory for Chemical Biology of Fujian Province, Department of Chemistry and The Key Laboratory of Analytical Sciences, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 30 August 2013 Received in revised form 25 December 2013 Accepted 8 February 2014 Available online 22 February 2014

Optically tunable carbon dots (CDs) were fabricated through a simple one-step microwave-assisted procedure. These carbonaceous nanoparticles exhibited tunable emission under a single wavelength excitation, controllable size without any tedious separation process and stabilities towards photobleaching and high ionic strength. The effects of size difference and surface property on the fluorescence behaviors of CDs were explored through a post-reduction/oxidation method. Experimental results also demonstrated the fluorescence of CDs could be tuned when exposed to H2O2/AcOH solutions. Moreover, the use of as-synthesized CDs as a chemical sensor for the quantification of hypochlorous acid (HClO) has been preliminarily tested, showing high sensitivity and selectivity towards HClO over other common ions. The superior optical properties would enable the use of CDs in multiplexed optical coding of biomolecules, light-emitting devices and biological applications. & 2014 Elsevier B.V. All rights reserved.

Keywords: Carbon dots Microwave-assisted synthesis Optical tunable fluorescence Hypochlorous acid sensing

1. Introduction The emergence of carbon dots (CDs) has generated enormous excitement due to their great potential in photocatalysis [1], optoelectronic devices [2], bioimaging [3,4], biosensor [5–7] and analytical applications [8–10]. These fluorescent carbonaceous nanoparticles, with a diameter of less than 10 nm, possess fine biocompatibility, high photostability against blinking, excellent upconversion properties and low toxicity, which make them promising alternatives of semiconductor quantum dots (QDs) in chemical and biological analyses. Although the exact origin of luminescence for CDs remains a matter of debate, it is speculated that quantum confinement effect [11], emissive traps [12], aromatic structures [13], and free zig-zag sites [14] contribute to their fluorescence emission. To date, the synthetic chemistry of CDs has been extensively studied. Various approaches, including discharge [15], laser ablation [12,16,17], electrochemical etching [18,19], strong acid oxidation [20,21], hydrothermal treatment [22,23], pyrolysis of carbon precursors [24], reverse-micelle supported strategy [25] and microwave[26–28]/ultrasound-assisted [29–31] method, have been explored to prepare CDs. Compared to other synthetic routes, microwaveassisted method would dramatically reduce the fabrication time

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Corresponding author. Tel./fax: þ 86 592 2188612. E-mail address: [email protected] (X. Guo).

http://dx.doi.org/10.1016/j.jlumin.2014.02.013 0022-2313 & 2014 Elsevier B.V. All rights reserved.

and greatly improve product yields and purities. The microwaveassisted carbonization of different carbohydrates either in the presence or absence of surface passivating agents has been reported to fabricate CDs [32,33]. CDs prepared by most of these methods mentioned above yield λex-dependent emission. Multicolor bioimaging might be obtained by exciting at different wavelengths [27]. Multicolor fluorescent molecular probes which exhibit multi distinguishable emission signals under a single wavelength excitation have shown great promise in vivo optical imaging in biological applications. Strategies towards multiplexed emission include the use of QDs, which provide a controllable emission and wide absorption bands [34], fluorescence resonance energy transfer (FRET)-mediated multicolor fluorescent nanoparticles incorporated two or more energy level-matched fluorescent dyes [35], and postencoding strategy [36]. Kang's group [11] have obtained CDs with tunable emission by electrochemical cutting of graphite sheets. Hydrothermal treatment of sucrose with different additives (such as HCl, NaOH and hexamine) was also used to prepare CDs with different emission wavelengths [37]. Chen's group [38] have produced multicolor fluorescent CDs via pyrolysis of epoxy-enriched polystyrene photonic crystals. Zhang et al. [39] showed that the emission wavelengths of CDs could be tuned through adjusting N-doping concentration. Zhou et al. [40] have demonstrated a low-temperature solid-phase method with urea and sodium citrate to produce CDs with tunable emission. Intrinsically fluorescent CDs with tunable emission derived from hydrothermal treatment of glucose in the presence of monopotassium phosphate,

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were also reported [22]. Nevertheless, challenges still remain with respect to fabrication of multicolor CDs that could be excited by light of single wavelength and to understanding the driving mechanism in fabricating multicolor CDs. In this work, we present a rapid one-step microwave-assisted procedure, or combined post oxidation, for the fabrication of water-soluble CDs that allow for fine tuning of the size and emission wavelengths. The resulting CDs could exhibit blue, green and yellow fluorescence under irradiation of an ultraviolet (UV) lamp. The effects of size distributions and surface property on the luminescence behaviors of CDs were explored. Interestingly, the CDs prepared by this method could also have their emission properties altered when exposed to H2O2/AcOH solutions. All three types of CDs were examined for the detection of hypochlorous acid (HClO), which expanding the potential applications of these fluorescent nanoparticles. 2. Experimental 2.1. Chemicals All the reagents were of analytical reagent grade and used without further purification. Chemicals were supplied by Sinopharm Reagent Co. Ltd. (Shanghai, China). The solutions of sodium hypochlorite (NaClO) and hydrogen peroxide (H2O2) were prepared daily, and their concentrations were determined using their UV absorbance at 292 nm in basic conditions (pH¼12.0, ε292 ¼ 350 M  1 cm  1 for HClO) and 230 nm (ε230 ¼ 81 M  1 cm  1 for H2O2) [41]. 2.2. Synthesis of multicolor CDs Sucrose (1.0 g) was added into different concentration of phosphoric acid to yield 20 mL transparent solution. The solution was heated in a 200 W domestic oven for varying time periods. When cooled down to room temperature, the supernatant was retained after centrifugation at 10,000 rpm for 5 min. The resulting CDs solution was neutralized by sodium hydroxide solution and then further purified by dialyzing against de-ionized water using a membrane with 1000 MWCO for 3 days. 2.3. Synthesis of reduced CDs The reduced CDs were synthesized according to a previously reported method [42]. Sodium borohydride (NaBH4, 0.5 g) was added to 5 mL CDs aqueous solution and stirred gently overnight at room temperature. The product was subjected to dialysis to completely remove the excess reductant. 2.4. Synthesis of oxidized CDs In a typical oxidized process, 100 μL of the as-prepared CDs was mixed with 6 mL of H2O2/AcOH (v/v ¼2:1) solution, and the solution was stirred gently for different hours at room temperature. Excess oxidant was removed by dialysis as mentioned above. 2.5. HClO assays 20 μL CDs was added into 2 mL PBS buffer (pH 5.0). The resulting solution was incubated with different concentrations of HClO for 5 min and their fluorescence spectra were measured. 2.6. Fluorescence measurements Fluorescence emission spectra were recorded on RF-5301PC fluorescence spectrophotometer (Shimadzu). Time-resolved fluorescence

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measurements were carried out on a FluoroMax-4 TCSPC (HORIBA Jobin Yvon) fluorescence spectrophotometer. Decay data analysis was performed using the DAS6 software (HORIBA Jobin Yvon IBH). 2.7. Quantum yields measurements The quantum yields of CDs were measured by the comparative method, according to the following equation:

Φx ¼ Φstd 

Ix I std



Astd n2  2x Ax nstd

where Φ is the quantum yield, I is the integrated area of the corrected emission spectrum, A is the absorbance at the excitation wavelength, n is the refractive index, and the subscripts x and std refer to CDs and the standard, respectively. Quinine sulfate in 0.1 M sulfuric acid (Φ ¼ 0.54) was used as the reference fluorophore for determination of the quantum yield of bCDs. Fluorescein in 0.1 M sodium hydroxide aqueous solution (Φ ¼ 0.95) was selected as the reference fluorophore for determination of the quantum yields of gCDs and yCDs. In order to minimize re-absorption effects, absorbencies in the 10 mm cuvette were kept under 0.05 at the excitation wavelength (excited at 390 nm for bCDs, 410 nm for gCDs and yCDs). 2.8. Materials characterization Absorption spectra were characterized by Hitachi U-3900 UV– vis spectrophotometer (Hitachi Ltd., Japan). Fourier transform infrared (FTIR) spectra were measured on a Nicolet IR330 spectrophotometer (KBr disc). Transmission electron microscopy (TEM) images were performed in a TECNAI F-30 (Netherlands). X-ray photoelectron spectroscopy (XPS) was carried out on a Quantum 2000 Scanning ESCA Microprobe system (Physical Electronics, USA), for determining the composition and chemical bonding configurations.

3. Results and discussion As a member of carbohydrates that widely distributed in living organisms, sucrose has been chosen for carbon source in this work. With assistance of microwave irradiation the thermal pyrolysis finished within several minutes and strongly fluorescent CDs were obtained without post-passivation. What makes this work more significant is that blue, green and yellow emission CDs were obtained in presence of phosphoric acid of various concentrations (Fig. 1). Qu's group [26] has reported a microwave synthesis route for CDs in the presence of several to tens mM of phosphate salt. The results showed that the amount of phosphate salt affected the formation rate and quantum yield but not the photoluminescence characteristic of the prepared CDs. And phosphate salt was thought to act as an inorganic ion-based catalyst in catalyzing the formation of CDs. Other inorganic ions, cations and anions, including monovalent, divalent, and trivalent as well, have also been explored and showed ability to catalyze carbohydrate carbonization and CDs formation. In the present work, we use phosphate acid of various concentrations in the process of carbonation. By increasing the concentration of phosphoric acid from 0.01 M, 6.0 M to 13.0 M, CDs with blue (bCDs), green (gCDs) and yellow (yCDs) emission under a 365 nm UV lamp were obtained (Fig. 1). The as-prepared CDs exhibited a red shift in both the optimal emission and excitation maximum wavelengths with increasing concentration of phosphate acid (Fig. S1). When sulfuric acid instead of phosphate acid was used in the pyrolysis of carbohydrate, maximum emission peaks of the produced CDs

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Fig. 1. Fluorescence spectra of bCDs (a), gCDs (b) and yCDs (c), respectively. The emission spectra were recorded with different excitation wavelengths increasing from 380 nm to 450 nm for bCDs, 410 nm to 470 nm for gCDs and 430 nm to 490 nm for yCDs, with 10 nm increments. Inset: photographs of each kind of luminescence CDs solutions under a 365 nm UV lamp excitation. (d) Normalized emission spectra of bCDs, gCDs and yCDs in aqueous solution (λex ¼390 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

shifted from 447 nm to 530 nm at a low concentration of sulfuric acid, but showed a slow blue shift at a high concentration (Fig. S2). The fluorescence properties of the CDs were also dependent upon the reaction time. As indicated in Fig. S3, with increasing microwave treatment time, the optimal emission peaks of CDs slightly red shifted in the presence of phosphoric acid of certain concentration. For better understanding the role of phosphate acid in the pyrolysis process, the optical characteristics of CDs were systematically explored. All three of the as-prepared CDs exhibited peak absorption bands at around 280 nm in the UV–vis spectra (Fig. S4), representing a typical absorption of an aromatic π system, which was similar to that of polycyclic aromatic hydrocarbons [11]. While the absorption band at 223 nm might arise from the relatively small polyaromatic structures [43]. All three of the as-prepared CDs fluoresced strongly but with different colors when excited at a same excitation wavelength (λex ¼ 390 nm). The bCDs exhibited λex-dependent emission, with maximum emission wavelength (λem) positioned from 450 nm to 512 nm when λex shifted from 380 nm to 450 nm (Fig. 1a, and Fig. S5a), a typical feature of CDs pervious reported. The gCDs also showed λex-dependent emission when λex shifted from 410 nm to 470 nm, but in less degree (Fig. 1b, and Fig. S5b), and a red shifted shoulder band could be observed. The as-prepared yCDs, with an λem at 550 nm, however, exhibited emission independent of λex as the λex shifted from 430 nm to 490 nm (Fig. 1c, and Fig. S5c). We speculated that the size and surface structures of CDs have become more homogeneous in the presence of large amount of phosphate acid. The comparative quantum yields of these CDs were determined to be 12.0%, 14.2% and 16.8%, respectively, which were comparable to those of reported carbonaceous nanoparticles [27,28]. The lifetime of CDs were measured as 3.4 ns, 1.7 ns and 1.8 ns (Fig. S6). The CDs, after dialysis, were near spherical in morphology as shown by their TEM images in Fig. 2. Their size distribution

histograms (Fig. 2d–f) demonstrated that these nanoparticles had a size distribution between 2 nm and 6 nm. Their average diameters were determined to be 2.4770.06 nm for bCDs, 3.2370.10 nm for gCDs and 3.9970.06 nm for yCDs, indicating that the size of CDs was largely dependent on the concentration of phosphoric acid. Since H þ could effectively accelerate the carbonization process [26], and phosphoric acid is an effective catalyst for the formation of CDs [44], a high concentration of phosphoric acid would facilitate the growing of carbon nuclei into large carbon particles. High resolution transmission electron microscopy (HR-TEM) images (Fig. 2a–c) revealed that the individual CDs possess a crystalline structure consisting of parallel crystal planes with a lattice of 0.34 nm, 0.22 nm and 0.22 nm for bCDs, gCDs and yCDs, corresponded to the (0 0 2) and (1 0 0) facets of graphite carbon [45]. FTIR spectra and XPS spectra were used to identify the functional groups presented on CDs. In the FTIR spectra, an apparent absorption peak at about 3435 cm  1 might ascribe to the O–H vibration stretch of carboxylic moiety, and the shoulder at 1642 cm  1 to C¼O vibration [21] (Fig. S7). The presence of carboxylic groups would render CDs with excellent water solubility and the suitability for the subsequent functionlization with various organic, inorganic and biological species [46]. A peak at 1585 cm  1 could be found which would most probably arise from the C¼C stretch of the aromatic rings [22]. In addition, the peaks in the range of 1000–1300 cm  1 might due to the symmetric and asymmetric stretching vibrations of C–O–C [22]. XPS measurements demonstrated a dominant O1s peak and a C1s peak (Fig. S8). The high-resolution spectra of C1s showed four main peaks of C atoms at 284.6 eV (C¼C, C–C), 285.7 eV (C–OH, C–OR), 287.2 eV (C¼O) and 289.0 eV (COOH), respectively [47]. The chemical environment dependency of CDs' fluorescence was investigated in different aqueous solutions, including high ionic strength conditions as well as in the presence of an oxidant (Fig. S9). The fluorescence intensities of CDs dropped slightly when oxidizing species existed but maintained nearly the same

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Fig. 2. TEM images of bCDs (a), gCDs (b) and yCDs (c). The insets show high resolution TEM images correspondingly. The size distributions of bCDs (d), gCDs (e) and yCDs (f) measured by TEM studies.

in high ionic conditions. All types of CDs were sensitive to the change of pH values in acidic solution (Fig. S10). The apparent pKa of these CDs were estimated to be ca. 2, 4 and 5, suggesting the contents of carboxyl or hydroxyl groups to each type of CDs were different. All three of the as-prepared CDs demonstrated high stability towards light (Fig. S11). The optical properties of CDs are known to be dictated by a combination of a couple of factors, i.e., size, shape and surface structures. The size-dependent fluorescence is a common behavior for other quantum dots. The size-dependent fluorescence was also observed in these CDs. When the average diameter of CDs increased from 2.47 70.06 nm to 3.99 70.06 nm, the corresponding emission peak moved from 450 nm to 550 nm at a fixed λex ¼390 nm (Fig. 1d and Fig. 2). We also investigated the relationship between the energy gap and particle size according to a previous report [48] (Fig. 3). It could be seen that the energy gap

decreased gradually with increment of particle size, a similar trend observed in other quantum dots, due to the quantum confinement effect in nanoparticles at size of o10 nm, such as graphene quantum dots [48] and silicon quantum dots [49]. Thus, we expected that the tunable emission features of the as-prepared CDs were closely related to their different diameters. The effects of surface chemistry on the luminescence of CDs have been studied previously. Different surface oxidation degrees would bring different surface defects to CDs, resulting in the intensities or emission wavelengths varying [42,50]. The surface chemistry of the as-prepared CDs was explored by XPS analysis (Table S1). The results showed that although the percentage of oxygen-bonded carbon in three types CDs decreased progressively from bCDs, gCDs to yCDs, the difference was not obvious. To explore whether the quantum confinement effects or the surface effects would be the dominant factor in shifting the emission

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Fig. 3. The relationship between the energy gap and the size of CDs.

Fig. 4. The normalized fluorescence emission spectra of the samples respectively obtained after oxidation for 0 h, 2 h, 6 h, 12 h, and 24 h excited at wavelength of 390 nm.

wavelength of CDs, a reduction or oxidation method was used. NaBH4 was employed to reduce yCDs. The TEM measurement showed that the reduced yCDs (Re-yCDs) had an average diameter of 3.9870.07 nm (Fig. S12), indicating no obvious difference from that of the original yCDs (3.9970.06 nm). Unlike the yCDs, the emission of Re-yCDs, however, revealed λex-dependent features. Both blue-shifted excitation and emission maximum were observed in fluorescence spectra, a phenomenon previously observed by other researcher [42], with a blue shift of excitation from 430 nm of yCDs to 410 nm of Re-yCDs and that of emission from 550 nm of yCDs to 515 nm of Re-yCDs, respectively (Fig. S13). The XPS analysis revealed the percentage of oxygenated C decreased slightly from 45.10% to 43.77% after reduction (Fig. S14 and Table S2). Previous study has shown that NaBH4 could selectively reduce carbonyl and epoxy group on the surface of CDs to hydroxyl group [42,51], and the removal of carbonyl and epoxy groups would effectively eliminate defect state emission, which has been ascribed to the longer wavelength emission [51]. Here the surface property seemed to have a considerable influence on the emission wavelength. Based on the features of the resulted Re-yCDs and the previous published results [42,50], we had expected that oxidation would result a redshifted emission for oxidized yCDs (Ox-yCDs). The Ox-yCDs was prepared by exposure the as-prepared yCDs to H2O2/AcOH solutions. Different products with various degree of oxidation were obtained by controlling the exposure time, at various times (Ox-yCDsx for x hours of exposure). The surface oxidation degree was characterized by XPS (Fig. S15). Decreased area percentage of C ¼C/C–C with extended reaction time could be clearly observed, accompanied by increasing peak intensities of the oxygen functional groups (Table S3). The initial C¼ C/C–C content of yCDs was approximately 54.90% and this fraction gradually dropped to roughly 46.37% after treatment for 24 h. Meanwhile, smaller CDs with an average diameter of 3.2270.10 nm and 2.4470.06 nm were formed after 12 h and 24 h oxidation, respectively (Fig. S16). The corresponding fluorescence spectra of CDs solutions at different reaction time were recorded, showing a clear λex dependence of the emission wavelength and intensity (Fig. S17). Contrary to our expectation, the maximum emission was blue-shifted towards shorter wavelengths, not red-shifted to longer wavelengths as the degree of oxidation increased. The optimal emission wavelength shifted from 550 nm to 469 nm under an excitation wavelength of 390 nm (Fig. 4). Here, it was suggested that the size other than the surface property has played a dominating role. What's more interesting is that all Ox-yCDx showed improved chemical stability in both strong acid and basic conditions due to the change of functional groups on surface (Fig. S18). These results demonstrated that emission tunable CDs could be obtained by simply oxidizing the yCDs.

For practical application, all three kinds of CDs prepared by microwave treatment were tested for their response to HClO. As an antimicrobial agent, HClO is usually used in water treatment and for natural defense in living organisms [41]. The excessive production of HClO within phagocytes would lead to a variety of human diseases [52]. Therefore, the accurate determination of HClO is of great importance. As depicted in Fig. 5a, under optimal conditions, the fluorescence of gCDs was quenched efficiently and nearly decreased by 76% when the concentration of HClO reached 2.2 μM. The time-dependent fluorescence changes upon addition of HClO were monitored, indicating the fluorescence intensity decreased within 2 min and kept constant for 28 min. Besides, no shift of the emission peaks could be observed during the incubation period (Fig. S19). The fluorescence quenching of the gCDs would attribute to an oxidative quenching process by strong oxidants [42,53]. There is a good linear relationship (R2 ¼ 0.994) between (F0  F)/F0 and the concentration of HClO in the range from 0.2 μM to 2.0 μM (Fig. 5b), where F0 and F are the fluorescence intensities in the absence and presence of HClO. And the limit of detection was calculated to be 15 nM, which was lower than other previously reported values [41,52,53]. The other two CDs exhibited much less sensitivity, which might be ascribed to several factors (Fig. 5b and Fig. S20). Some literatures claimed that the particle size and the surface groups would contribute to the quenching efficiency between fluorescent nanoparticles and oxidation species [41,54], but the exact mechanism still remains investigation. We further examined the influence of some common ions including MnO4 , S2O28  , H2O2, ClO4 , ClO3 , NO2 , SO24  , I  , Br  , Cl  , HCO3 , CO23  , and various metal ions (Fig. S21). Most ions showed no obvious quenching effects. Due to the strong oxidizing ability of MnO4 , a slight fluorescence change could be observed in the presence of MnO4 . The high sensitivity and selectivity suggested that gCDs had great potential in monitoring HClO generation in chemical and biological systems.

4. Conclusions In summary, a microwave-assisted approach to preparation of multicolor CDs in large scale has been developed. The fluorescence and size of these CDs could be adjusted by simply changing the concentration of phosphoric acid. The practicality of the asprepared CDs used as a chemical sensor has been preliminarily tested. It was found that gCDs exhibited the highest sensitivity for the detection of HClO. Besides, the synthesized CDs have excellent stability towards light and their fluorescence intensity did not change under high ionic strength, which would make them a promising candidate for a new class of fluorescence probes.

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Fig. 5. (a) Quenching of the fluorescence of gCDs by HClO. (b) The linear relationship between the quenching ratio of all three types CDs and the concentration of HClO.

Further experiments demonstrated the fluorescence of CDs could also be tuned when exposed to H2O2/AcOH solutions for different periods of time. The efforts of adjusting the emission colors of CDs not only facilitate the fundamental studies but also provide a new approach to applications in multiplexed optical coding of biomolecules, drug delivery and therapeutics. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2011CB910403) and the National Science Foundation of China (20835005, 20975086, J1030415). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2014.02.013. References [1] H. Zhang, H. Ming, S. Lian, H. Huang, H. Li, L. Zhang, Y. Liu, Z. Kang, S.T. Lee, Dalton Trans. 40 (2011) 10822. [2] X. Wang, L. Cao, F. Lu, M.J. Meziani, H. Li, G. Qi, B. Zhou, B.A. Harruff, F. Kermarrec, Y.P. Sun, Chem. Commun. (2009) 3774. [3] S.T. Yang, L. Cao, P.G.J. Luo, F.S. Lu, X. Wang, H.F. Wang, M.J. Meziani, Y.F. Liu, G. Qi, Y.P. Sun, J. Am. Chem. Soc. 131 (2009) 11308. [4] H. Tao, K. Yang, Z. Ma, J. Wan, Y. Zhang, Z. Kang, Z. Liu, Small 8 (2012) 281. [5] W.J. Bai, H.Z. Zheng, Y.J. Long, X.J. Mao, M. Gao, L.Y. Zhang, Anal. Sci. 27 (2011) 243. [6] H. Dai, C. Yang, Y. Tong, G. Xu, X. Ma, Y. Lin, G. Chen, Chem. Commun. 48 (2012) 3055. [7] Y. Song, W. Shi, W. Chen, X. Li, H. Ma, J. Mater. Chem. 22 (2012) 12568. [8] J.M. Liu, L. Lin, X.X. Wang, S.Q. Lin, W.L. Cai, L.H. Zhang, Z.Y. Zheng, Analyst 137 (2012) 2637. [9] W. Wei, C. Xu, J. Ren, B. Xu, X. Qu, Chem. Commun. 48 (2012) 1284. [10] Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Anal. Chem. 84 (2012) 6220. [11] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C.H.A. Tsang, X. Yang, S.T. Lee, Angew. Chem. Int. Ed. 49 (2010) 4430. [12] Y.P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, M.J. Meziani, B.A. Harruff, X. Wang, H.F. Wang, P.J.G. Luo, H. Yang, M.E. Kose, B.L. Chen, L.M. Veca, S.Y. Xie, J. Am. Chem. Soc. 128 (2006) 7756. [13] G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen, M. Chhowalla, Adv. Mater. 22 (2010) 505. [14] D.Y. Pan, J.C. Zhang, Z. Li, C. Wu, X.M. Yan, M.H. Wu, Chem. Commun. 46 (2010) 3681. [15] X. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, J. Am. Chem. Soc. 126 (2004) 12736. [16] S.L. Hu, K.Y. Niu, J. Sun, J. Yang, N.Q. Zhao, X.W. Du, J. Mater. Chem. 19 (2009) 484.

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