Fluorescence determination based on graphene oxide

Materials Letters 76 (2012) 247–249

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Fluorescence determination based on graphene oxide Ruijun Li, Xuqiang Liu, Xiaoli Deng, Shengrui Zhang, Qun He, Xijun Chang ⁎ Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou 730000, PR China

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Article history: Received 22 July 2011 Accepted 25 February 2012 Available online 1 March 2012 Keywords: Carbon materials Oxidation Luminescence

a b s t r a c t Graphene oxide (GO) has been prepared and the structure of the prepared GO was characterized by Transmission Electron Microscopy (TEM). And then we demonstrated that GO could quench the fluorescence of Rhodamine 6G (Rh6G) in aqueous solution. According to the UV–vis absorption spectrum of GO and the fluorescence spectrum of Rh6G, we found that GO quenched the fluorescence of Rh6G because of two factors. Firstly, the electrons moved from the Rh6G to the surface GO; secondly, Rh6G adsorbed onto the GO surface. However, trace Th4+ enhanced the fluorescent signal of Rh6G, which was quenched by GO prior to metal ions addition. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Graphene (G), a very recent rising star in material science, with an atomically thin structure that consists of sp 2-hybridized carbons, exhibits remarkable electronic, mechanical and thermal properties [1–4]. Among various methods developed to produce G, chemical method is a facile, low-cost one and has attracted more attention [5]. Graphene oxide (GO), which is a watersoluble derivative of G, has attracted increasing interest because of its unique characteristics such as good water dispersibility, facile surface modification and high mechanical strength [6,7]. Swathi has predicted through theoretical calculations that G was a superquencher with the long-range nanoscale energy transfer property [8]. And some studies of electron or energy transfer have been carried out, such as, fluorescence quenching of dyes by GO and G [9], energy transfer between Quantum Dots and GO [10]. Herein, we have demonstrated that GO could quench the fluorescence of Rh6G with the electron transfer or adsorption efficiency in aqueous solution. However, Th 4+ ions enhanced the fluorescent signal of Rh6G, which was quenched by GO prior to metal ions addition. This method using GO as an effective fluorescence quencher of Rh6G to investigate Th 4+ has not been reported so far. 2. Experimental 2.1. Chemicals and reagents Graphite flakes were obtained from Sinopharm Chemical Reagent Co. Ltd., China. Rh6G was purchased from Wenzhou Dongsheng

Chemical Reagent Factory (Zhejiang, China). Standard stock solutions of metal ions were prepared by dissolving spectral pure grade chemical materials (The First Reagent Factory, Shanghai, China) in deionized water with the addition of hydrochloric acid (The First Reagent Factory, Shanghai, China) and further diluted daily prior to use. 2.2. Instruments and apparatus The morphology of the GO was characterized with a JEM-1200 EX/S transmission electron microscope (Japan). UV–vis measurement was carried out on a WFH-203 UV analyzer (Shanghai, China). Fluorescence spectra were recorded using an RF-5301PC spectrofluorophotometer (Shimadzu, Japan). 2.3. Synthesis of GO GO was prepared by using the improved method [11]. A 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was added to a mixture of 3.0 g graphite flakes and 18.0 g KMnO4. The reaction was heated to 50 °C and stirred for 12 h. The reaction was cooled and poured onto ice (400 mL) followed by addition of H2O2 until the color changed to brilliant yellow. The mixture was filtered and washed with HCl aqueous solution and deionized water. The obtained graphite oxide powder was dialyzed in graphite oxide dispersion for one week. The GO was exfoliated under sonication for about 2 h to ensure that most GO was exfoliated and obtained homogeneous yellow solution. 3. Results and discussion 3.1. TEM of GO

⁎ Corresponding author at: Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China. Tel.: + 86 931 891 2422; fax: + 86 931 891 2582. E-mail address: [email protected] (X. Chang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.118

The morphology and structure of the GO were further observed by TEM. The overall view shown in Fig. 1 clearly provided more detailed

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Wavelength (nm) Fig. 3. Fluorescence spectra of Rh6G (a), GO–Th4+–Rh6G (b) and GO–Rh6G (c). λex = 500 nm, GO: 150 μg mL− 1, Rh6G: 0.05 μM, Th4+: 5 μM.

Fig. 1. TEM image of GO.

morphological information on the resulting GO. It exhibited that the typical wrinkle morphology of GO was exfoliated into single or very thin layers. The crumpled structure exhibited in TEM image may be due to the multiplicity of oxygen functionalities in thin GO layers [12]. 3.2. UV–vis spectrum Fig. 2 showed that the UV–vis spectrum was plotted in the wavelength range from 200 to 800 nm. The UV–vis spectrum of GO exhibited two characteristic peaks, a maximum at 226 nm, corresponding to π → π* transitions of aromatic C\C bonds, and a shoulder at 300 nm was attributed to n → π* transitions of C_O bonds [13]. 3.3. Fluorescence determination procedure Rh6G was titrated with GO to form the GO–Rh6G complex to demonstrate the feasibility of our proposed approach. Finally, 5 μL 1.0 × 10− 4 mol L− 1 Rh6G and 150 μL of a GO stock solution (1 mg/mL) were added to a 10 mL colorimetric tube, and diluted with deionized water. The mixture was allowed to stand for 20 min before a fluorescence measurement was made. Fig. 3 showed that the intensity of GO–Rh6G was much less than that of Rh6G i.e. GO quenched the fluorescence of Rh6G. Since there was no overlap between the fluorescence emission of Rh6G and the absorption spectrum of GO (Figs. 2 and 3), the highly efficient fluorescence quenching of Rh6G may be caused by two factors. The first was the electrons moved from the Rh6G to the surface GO. According to the reported works, GO was a highly effective absorbent of methylene blue (MB) and can be used to remove MB

from aqueous solution, and GO–Fe3O4 hybrid composites had great potential applications in removing organic dyes from polluted water [14,15]. So, the second was Rh6G adsorbed on the surface of GO. We found that the interaction between Rh6G and GO was disturbed in the presence of metal ions, resulting in restoration of the dye fluorescence in various degrees. Remarkably, the additions of Th 4+ can significantly restore the dye fluorescence (Fig. 3, b). Utilizing the metal ions adsorption capacity of GO [16,17], the fluorescence-enhanced mechanism was presumably due to the chelation of Th 4+ with the oxygen atoms of GO, with which Th 4+ formed more stable complex than Rh6G with GO surface. Th 4+ displaced the Rh6G molecules from the surfaces of GO. Therefore, the electron transfer and adsorption efficiency became weaker after trace Th 4+ added, resulting in the restoration of fluorescence signal of Rh6G. We investigated the specificity of the GO–Rh6G nanoswitch toward Th 4+ relative to other metal ions, including Cr 3+, Mn 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Hg 2+ and Pb 2+ added into the GO–Rh6G solutions, respectively. Fig. 4 illustrated the fluorescence intensity changes (IF–IF0) of the GO–Rh6G complex upon the addition of different metal ions, where IF0 and IF are the fluorescence intensities in the absence and presence of metal ions. The fluorescence intensity enhancements of Cr 3+, Cd 2+ and Zn 2+ were up to 70%, and the effects of other metal ions were not obvious. These results clearly demonstrated that the GO–Rh6G complex could be used in determination of Th 4+ in the aqueous solution.

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Fig. 4. Fluorescence intensity changes (IF–IF0) of the GO–Rh6G toward different metal ions (all at a concentration of 5 μM).

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4. Conclusions Herein, we have demonstrated that the GO–Rh6G complex can be used as a reversible fluorescence nanoswitch for inexpensive, labelfree, simple, sensitive detection of Th 4+. This proposed method expanded the application of graphene oxide and could be potentially applied in the lighting industry and environmental health science. Acknowledgment This study was supported by the Fundamental Research Funds for the Central Universities (No. lzujbky-2010-41). References [1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Science 2004;306:666–9. [2] Li D, Kaner RB. Science 2008;320:1170–1.

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