페이지_ 18권 4호 전체 1-7.pdf765.6KB

• 1. Intruduction
• 2. Experimental
• 2.1. Materials
• 2.2. Measurements
• 2.3. Synthesis
• 2.3.1 Synthesis of 1,10-Phenanthroline-5,6-dione (Dipyridobenzoquinone, dpq)
• 2.3.2 Synthesis of 9,11,20,22-Tetraazatetrapyrido pentacene (TATPP)[8].
• 2.3.3 Synthesis of [Eu2(TTA)6(TATPP)]complex[9].
• 3. Results and discussion
• 4. Conclusions
• Acknowledgments

1. Intruduction

Emerging as atomically thin two-dimensional carbon materials, graphene sheets (GSs) have attracted tremendous attention in many potential applications, such as polymer composites, biosensors and drug delivery^[1]. Recently, graphene oxide sheets (GOSs), heavily oxygenated carbon monolayer’s that contain numerous oxygen functional groups on their surfaces, have been extensively studied as promising precursors for the bulk production of GSs^[2]. The functional groups on GOSs can facilitate not only their dispersion in a range of solvents but also their further functionalization^[3,4]. Therefore, in addition to their reduction to GSs, GOSs are also useful platforms for aforementioned applications. Moreover, compared to zero-gap GSs, GOSs can be weakly fluorescent because of a defect-related optical gap, which may expand their application to the display and lighting fields, such as biological labeling and anti-counterfeiting^[5]. However, when used in practice, the luminescence of GOSs needs to be enhanced. For this purpose, several attempts have been made to label GOSs with luminescent markers. Unfortunately, conventional luminescent tags such as dye molecules have to face the challenge that their fluorescence would be quenched by GOSs^[6]. Therefore, it is significant to search for an alternative way to generate GOSs with visible luminescence. 

2. Experimental

2.1. Materials

Graphene oxide sheets (GOS) was purchased from School of Materials Science and Engineering (BK21), Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon, Korea. Unless otherwise indicated, chemicals were obtained from commercial suppliers and used without further purification. Analytical reagent grade chemicals were used along with deionized water to prepare solutions. The chemicals and Solvents used were obtained from Aldrich (Aldrich, Milwaukee, WI) and Europium complexes were prepared according to the literature procedures^[7,8]. 

2.2. Measurements

¹H NMR spectra were recorded on a Varian 300 MHz NMR spectrometer, IR spectra were recorded on a Perkinelmer Paragon 1000 PC FTIR spectrometer, and electronic absorption spectra were obtained on an Agilent 8453 UV-visible spectrophotometer. Emission measurements were made on methanol solution samples at room temperature. Thermo gravimetric analysis was recorded on a Scinco TGA1000 Thermo gravimetric Analysor (TGA). Mass spectrometric results (LCMS) were obtained from the Mass Spectrometry Bio-Venture Town, Hannam University, Daejeon, Korea.

2.3. Synthesis

Synthesis of Europium complexes contain mainly three steps as given bellow: 

2.3.1 Synthesis of 1,10-Phenanthroline-5,6-dione (Dipyridobenzoquinone, dpq)

The following procedure is based on the report by Yamada^[7]. and it yields the product almost quantitatively. An ice cold mixture of concentrated H₂SO₄(40mL) and HNO₃(20mL) was added to 4.0g of 1,10-phenanthroline and 4.0g of KBr. The mixture was heated at reflux for 3h. The hot yellow solution was poured over 500 mL of ice and neutralized carefully with NaOH until neutral to slightly acidic pH. Extraction with CHCl₃ followed by drying with Na₂SO₄ and removal of solvent gave 4.5g (96%) of dpq. This product is quite pure spectroscopically, but it may be purified further by crystallization from ethanol . ¹HNMR (300 MHz, CDCl₃): δ ppm:9.15(d.d2H),8.53(d.d2H),7.62(d.d2H),LC MS: m/z 210.04.

2.3.2 Synthesis of 9,11,20,22-Tetraazatetrapyrido pentacene (TATPP)[8].

A mixture of 1,10-Phenanthroline-5,6-dione (0.50g, 2.4mmol), 1,2,4,5-benzenetetramine tetrahydrochloride (0.338g, 1.19mmol), and potassium carbonate (0.329g, 2.38mmol) was suspended in ethanol (15mL) and refluxed under nitrogen for 12h. After cooling of the reaction to room temperature, the precipitate was filtered out, washed with 15mL of hot water (3×) and 15mL of boiling ethanol (3×), and dried in vacuo at 600℃. Yield:0.41g(70%). Anal. Calcd for C₃₀H₁₄N₈₁.5H₂O: C,70.17;H,3.34;N,21.82. Found: C,69.77;H,2.95;N,21.82. Found: C,69.77;H,2.95;N,21.82. ¹H NMR (300 MHz, CDCl3):δppm:10.20(d.J=8.0Hz,4H), 9.79(s,2H), 8.33(d.J=4.0Hz,4H), 8.36(dd,J1=7.3Hz,J2=4.5Hz,4H), LCMS: m/z 486.7. 

2.3.3 Synthesis of [Eu2(TTA)6(TATPP)]complex[9].

EuCl₃(0.64g, 2.49mmol) and 2-thenoyltrifluoroacetone (TTA) (5, 1.66g, 7.48mmol) in ethanol (25ml) were mixed and the pH value of the mixture was adjusted to 6～7 by adding triethylamine (0.4mL, 2.49mmol). Then, TATPP (4, 0.60g, 1.24mmol,) was added. The mixture was stirred at 50℃ for 3h then cooled to room temperature. The precipitate was filtered, recrystallized from ethanol and washed with water to afford the desired brown product. Yield: 85%. Anal. Calcd for C₄₆H₂₂O₄Eu₂N₈F₆S₂:C%,44.82,H%,1.80,N%,9.09, Eu%,24.65.Found:C%,45.06,H%,1.75,N%,9.03,E u%,24.59.LC MS: m/z 1233.5. We present a simple and effective method to prepare luminescent GOSs by noncovalently functionalizing them with Europium complexes. The Europium complex used in this study is red-luminescent [Eu₂ (TATPP)(TTA)6] (TATPP=9,11,20,22-tetraaza tetrapyrido pentacene, TTA = α-thenoyl trifluoro acetone). The mixing of a GOS aqueous suspension with a chloroform solution of Europium complexes would lead to a phase transfer of GOSs (Figure. 1), which should be due to the noncovalent interaction between the two components. For the preparation of GOS-Europium complex hybrids (Figure. 1), 10mg of graphite oxide powders were suspended in 40mL of tetrahydrofuran (THF), and exfoliated into single-layer GOSs through sonication^[3]. Then, the GOS dispersion was mixed with a THF solution of 30 mg of Europium complexes, and stirred overnight. To remove free, unattached Europium complexes, the collected powders were repeatedly rinsed with THF until no trace of red emission was detected in the filtrate. The washed products, which are dark in color, can also emit visible red light when excited with UV radiation (Figure. 2a and b). The dispersing of these fluorescent products in THF can yield a light brown suspension with bright red luminescence (inset in Figure. 2). As shown in Figure. 2, the luminescence emission spectrum of this suspension exhibits a strong band at 512 nm, which are characteristic emissions of Eu³⁺ luminescence. Obviously, GOSs do not quench the luminescence of Eu³⁺ complexes that attached on their surfaces. As suggested by Armaroli and co-workers, it is due to the peculiar luminescence properties of trivalent Europium ions: electronic transitions involving Europium ions occur between internal 4f orbitals which are well shielded by the peripheral 5s and 5p ones, granting an efficient degree 6f protection towards potential external quenchers^[10]. 

Figure 1. Schematic illustration of GOS-Europium complex hybrids.

Figure 2. Digital pictures of GOS-Europium complex hybrids under (a) daylight and (b) 345 nm UV light. (c) luminescence emission spectra (λex = 345 nm) of GOS-Europium complex hybrids, GOSs and -Europium complexes in their dispersions. Inset in (c) Photographs of the THF dispersion of GOS-Europium complex hybrids under (left) daylight and (right) 345 nm UV light.

3. Results and discussion

The flat sheets of GOS-Europium complex hybrids, which are about 1 μm in lateral size, have an average height of 2.6 nm. Compared with GOSs, whose thickness is 1.4 nm (Figure. 3, matching well with the reported thickness of single-layer GOSs^[3]), sheets of GOS-Europium complex hybrids are much thicker. Considering that the presence of adsorbed Europium complexes may increase the thickness of GOSs, we can expect that the observed sheets are uniform monolayers. Figure. 3a displays the Fourier transform infrared (FTIR) spectra of the samples. Compared with the FTIR trace of GOSs, that of GOS Europium complex hybrids exhibits two distinct bands at 2870 and 2950cm^-1, which correspond to asymmetric C–H stretch of the alkyl chains assigned to TATPP ligands of Europium complexes. As can be seen in Figure. 3a, the UV absorbance trace of GOS-Europium complex hybrids present the characteristic absorption peaks of Europium complexes, indicating the stacking of Europium complexes onto GOS surfaces. Furthermore, after forming the hybrids, the absorptions of Europium complexes illustrate red shift (Figure. 3b). 

Figure 3. (a) FTIR spectra, (b) UV absorbance curves, (c) TGA curves, of (a) GOSs and (b) GOS-Europium complex hybrids

For example, the band of Europium complexes at 300 nm red shifts to 430 nm after hybridized with GOSs, which have been suggested to be due to the p-p interactions between the two components^[11]. Thermogravimetric analysis (TGA) measurements were carried out under air atmosphere and the results are depicted in Figure. 3c. There are three mass loss peaks found in the derivative TGA plot of GOS -Europium complex hybrids. The first one, which appears at 213℃, can also be observed in the GOS DTA curve. It corresponds to the pyrolysis of the oxide functional groups^[4]. The latter two peaks at 292 and 470℃ should be ascribed to the decomposition of Europium complexes. It is interesting to find that the dramatic weight loss of GOSs at about 600 ℃, which corresponds to the oxidation of bulk materials^[4], cannot be observed in the TGA traces of GOS-Europium complex hybrids. It may be due to the coating of in-situ generated EuCl₂ layers on the carbon-based materials, which may retard their oxidation. Based on the TGA data, we can determine that the loading of Europium complexes in GOS -Europium complex hybrids is about 80 wt%, i.e., there are about 4mg of Europium complexes adsorbed onto 1mg of GOSs. Such a high value should be attributed to the high loading capacity of GOSs, which have been suggested by Yang et al.^[11]. 

4. Conclusions

GOSs with bright red luminescence were produced by noncovalently functionalizing them with Europium complexes. The as-prepared luminescent hybrids can be easily exfoliated into single-layer sheets in some common solvents, yielding strongly fluorescent GOS dispersions. Therefore, it can be expected that our synthesized GOS-Europium complex hybrids hold great potential in many fields such as biological labeling and anti-counterfeiting. 

Acknowledgments

This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (2010-0024478) and Hannam University research fund in 2013.