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Carbon nanoparticle ionic liquid hybrids and their photoluminescence properties
Journal of Colloid and Interface Science 358 (2011) 146–150
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Carbon nanoparticle ionic liquid hybrids and their photoluminescence properties Ying Wei a,c, Yang Liu a,b,⇑, Haitao Li a, Xiaodie He a, Qingguo Zhang c, Zhenhui Kang a,b,⇑, Shuit-Tong Lee a,d a Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University and Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, Jiangsu 215123, PR China b Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China c College of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121000, PR China d Center of Super Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, PR China
a r t i c l e
i n f o
Article history: Received 24 January 2011 Accepted 24 February 2011 Available online 1 March 2011 Keywords: Carbon nanoparticles Ionic liquid Hybrid Photoluminescence
a b s t r a c t A fluorescent carbon nanoparticle ionic liquid hybrids (CNPIL) with high conductivity is synthesized by a facile one-step microwave method from ionic liquid 1-butyl-3-methylimidazolium glutamine salt and Glucose. This CNPIL exhibits excellent PL properties: bright and colorful PL covering the entire visible– NIR spectral range, up conversion PL properties, pH dependent and can be controlled by the reaction condition. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Ionic liquid hybrid is one of the most significant topics in electrochemistry, catalysis, and material science [1,2]. This kind of hybrid, which combines a low-melting ionic liquid and various functional materials or nanospecies (such as metals, metal oxides, metal salts, silica, polyanions, carbon and semiconductor nanoparticles), will possess the combined properties of ionic liquid and other functional species [3,4], such as negligible vapor pressure, tunable composition, polar character, low toxicity, high chemical and thermal stabilities, novel magnetic and optical properties, protonic conductivity, or catalytic properties. Thus, ionic liquid hybrid is widely regarded as a promising key to overcome the present challenges (such as high performance medium, electrode materials, and catalyst) in energy storage, fuel cell, green catalysis, and optoelectronics [3,4]. Further, the carbon based nanomaterial ionic liquid hybrid has become a hot topic recently and many research work focused on the synthesis and properties of such hybrid [5–10]. For example, Fukushima et al. reported that the marriage between ionic liquids and carbon nanotubes formed a gel-like composite (‘‘bucky gel’’) which shows great potential applications in capacitors, sensors, and actuators [5]. Lei and co-workers obtained a ultralong carbon nanotubes ionic liquid hybrid with liquid-like behavior and excellent thermal stability [6a]. Han and co-workers reported the synthesis and conductivity properties of high dispersibility and
long-term stable graphite sheets ionic liquid hybrid [7a]. However, there is no report on the optical properties of this kind of hybrid, especially the photoluminescence (PL) property. It is crucial to the optoelectronic devices, biological labeling, and photocatalyst design [8]. We should further point out that the fluorescent carbon based nanoparticles are interesting new comers to the world of nanomaterials. They are of great potential in applications of biomedical imaging, energy storage, electrode materials, optoelectronics, and photocatalyst [11,12]. Combining these fluorescent carbon nanoparticles with ionic liquid (IL), the novel hybrid may possess strong PL, remarkable conductivity, highly photoactivity and stability. Herein we introduce a facile one-step microwave synthetic method for the fabrication of fluorescent carbon nanoparticle ionic liquid hybrid (CNPIL) from ionic liquid 1-butyl-3-methylimidazolium glutamine salt and Glucose. This kind of highly waterdispersed hybrid has carbon cores with sizes less than 4 nm and exhibits bright and colorful PL covering the entire visible–NIR spectral range. Notably, the obtained CNPIL possesses excellent up conversion PL properties. The PL of CNPIL is pH dependent and can be controlled by adjusting the water content of reaction solution. Furthermore, the CNPIL also exhibits excellent conductivity.
2. Materials and methods 2.1. Materials
⇑ Corresponding authors at: Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China. Fax: +86 512 6588 2846. E-mail addresses: [email protected] (Y. Liu), [email protected] (Z. Kang). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.02.061
All chemicals were purchased from Sigma–Aldrich. The IL 1-butyl-3-methylimidazolium glutamine salt (BMIGlu) in this
Y. Wei et al. / Journal of Colloid and Interface Science 358 (2011) 146–150
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study were synthesized by a neutralization method as the literature procedure [3], and the BMIGlu product was dried in vacuum for 2 days at 353 K. 2.2. Methods Glucose (0.5 g) was firstly dissolved into 4 mL distilled water. Under vigorous stirring, 0.2 mL BMIGlu was added to form a clear solution. The solution was heated in a 500 W microwave oven for different time periods. After that, the solution changed from colorless to dark brown, and then was diluted. In a parallel control experiment, Glucose (0.5 g) was dissolved into 4 mL distilled water by stirring to form a clear solution. The solution was heated in a 500 W microwave oven for 3–10 min. In the same manner, pure BMIGlu (0.2 mL) was also dissolved into 4 mL distilled water by stirring to form a clear solution and then heated in a 500 W microwave oven for 3–10 min. TEM image of CNPIL was obtained with a FEI/Philips Techai 12 BioTWIN TEM, while high-resolution HRTEM images with a CM200 FEG TEM. The FTIR spectrum of CQDs was obtained with a Nicolet 360 spectrometer. PL study was carried out on a Fluorolog-TCSPC Luminescence Spectrometer, while UV–visible spectra were obtained with an Aglient 8453 UV–Vis Diode Array Spectrophotometer. Conductivity was determined by AC impedance at variable temperatures using a IM6 electrochemical workstation in the frequency range from 1 Hz to 100 MHz. 3. Results and discussion In the experiment, briefly, a suitable amount of glucose and ionic liquid (1-butyl-3-methylimidazolium glutamine salt, BMIGlu) were added in water under stirring, and then heated in a 500 W microwave oven for 3–10 min. After reaction, the solution changed from colorless to dark brown, from aqueous to viscous (see Fig. 1a and b). Under UV light (365 nm, center), the bright blue PL of the obtained dark brown CNPIL is strong enough to be easily seen with the naked eyes (see Fig. 1c). The quantum yield of the CNPIL with blue emission was estimated to be about 9.2% by calibrating against quinine sulfate (QS) in ethanol (see ESI) [13]. Fig. 1d shows the transmission electron microscopy (TEM) image of the as prepared CNPIL, indicating that spherical carbon nanoparticles with diameter below 4 nm exist in CNPIL. The high resolution TEM image (see Supplementary Fig S1) shows the carbon particles have graphite structure, with the carbon matrix spacing of 0.32 nm. The structure characterization of CNPIL was carried out by FTIR. As shown in Supplementary Fig S2, the peaks located at 2962 cm 1 and 2853 cm 1 correspond to the C–H stretch, while the peaks at 1408 cm 1 and 1575 cm 1 are due to the C–H vibration. The vibration of the carbon skeleton of carbon nanostrucutres appears at 1575 cm 1. The peak at 1660 cm 1 corresponds to the framework vibration of imidazolium, which further indicates that the CNPIL is composed of IL and carbon nanoparticles [9]. The typical UV– vis absorption and PL spectra of as prepared CNPIL are shown in Fig. 2. The absorption band at 290 nm observed in Fig. 2a (curve A) represents the typical absorption of an aromatic pi system, which is similar to that of polycyclic aromatic hydrocarbons [14]. It is 10 nm red shift compared to the previous report of carbogenic nanoparticles, which may be due to the IL was adsorbed on the surface of the carbon nanoparticles by p–p interactions between the imidazolium rings of the IL and carbon nanoparticles [15]. The typical PL spectra (Fig. 2a, curve B) of CNPIL show strong blue1 emission at 420 nm with the excitation at 350 nm. Further, the PL 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
Fig. 1. Photographs of the glucose/ionic liquid solution (a) before and (b) after reaction; (c) optical photograph of CNPIL obtained under excitation at 365 nm; (d) TEM image of CNPIL with diameters within 4 nm.
emission can extend into NIR wavelength range, and the peak is at 800 nm (Fig. 2b). In addition, the emission spectra of CNPIL also show the excitation dependent feature that similar to previous reported carbon nanoparticles (Fig. 2c): increasing the excitation wavelength leads to the longer wavelength emissions [8]. Remarkably, the obtained CNPIL exhibits excellent up conversion PL properties besides their strong luminescence in visible–NIR range. As shown in Fig. 2d, long wavelength excitation ranged from 750 to 950 nm can lead to the up-converted emissions located in the range from 350 nm to 650 nm. The up-converting nanomaterials have unique luminescence properties with long lifetime and superior photostability for biological luminescent labels, such as noninvasive and deep penetration of NIR radiation, the absence of auto-fluorescence of biological tissues, and feasibility of multiple labeling with different emissions under the same excitation [8]. The photoluminescence from these CNPIL may mainly attribute to two following reasons: (1) the presence of quantum-sized carbon cores [8]. (2) the surface energy traps of the carbon core passivated by ILs. The functionalization of carbon core by IL improves the dispersion of the nanoparticles and diminishes the quenching effects due to interparticle interaction [10]. Further PL experiments indicate that the pH value of CNPIL solution can strongly affect the PL properties of the obtained CNPIL. Fig. 3a shows the typical PL spectra of CNPIL with the same excitation (350 nm) under different pH condition (from pH 1–13). When 1 6 pH 6 3, the PL spectra of CNPIL exhibit two peaks located at about 400 nm and 500 nm, respectively. While, there is only one peak (at about 420 nm) is observed in the PL spectra when pH P 5. On the other hand, the PL intensity also have pH dependent feature (Fig. 3b): 1 6 pH 6 5, the PL intensity of CNPIL increases significantly; 5 < pH 6 9, the PL intensity increases not too much; pH > 10, the PL intensity decreases sharply. The detailed PL spectra of CNPIL at typical acidic (pH = 3) or alkalic (pH = 10) environment with different wavelength excitation are shown in Fig. 3c and d, respectively, from which we can see that the emission spectra also show excitation dependent feature. The above pH-dependent PL phenomena may be resulted from the density of surface electron localized states that participate in the radiative transitions increases by the adsorption of OH ions on the surface of CNPIL [16]. Further detailed experiments confirm that, the ionic liquid/ water ratio (different water content) is also a key point affecting the PL spectra of CNPIL. Though the PL peak positions are almost the same, the PL intensity and full-width half-maximum (FWHM) are different. Supplementary Fig S3a shows the PL spectra covering the visible–NIR wavelength range of CNPIL (pH = 7, excitation at 350 nm) obtained with different water content solution (ionic liquid/water ratio). As shown in Supplementary Fig S3b, the intensity and FWHM of PL emissions in NIR wavelength range
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Fig. 2. (a) UV–vis absorption (curve A) and PL spectra (curve B) of CNPIL in aqueous solution with pH = 7 (excitation at 350 nm); (b) PL spectra of CNPIL (pH = 7) in NIR range (excitation at 350 nm); (c) PL spectra of CNPIL (pH = 7) in visible range (recorded for progressively longer excitation wavelengths from 300 to 450 nm in 25 nm increments); (d) Up conversion PL spectra of CNPIL excited by long wavelength light(from 750 to 950 nm) with pH = 7.
Fig. 3. (a) PL spectra of CNPIL under different pH condition (1 6 pH 6 13) with the same excitation (350 nm); (b) the relationship of fluorescence intensity versus the pH value of solution; PL spectra of CNPIL under (c) acidic (pH = 3) and (d) alkalic (pH = 10) condition in visible range (excitation wavelengths from 300 to 500 nm with 50 nm increments).
are also different. Supplementary Figs. S3c and S3d show the relationship between the PL intensity/FWHM and various water content of reaction solution, respectively. With increasing water content (below 60%), the PL intensity of CNPIL increases significantly, and the maximum intensity is obtained at water
content of about 50%. Further increasing the water content (P60%), the PL intensity of CNPIL decreases contrarily. Water content (in the present system, different water content indicates the different concentration of glucose and ionic liquid) dramatically influences the PL properties of CNPIL. When water content
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extension of the present work, we envision the CNPIL may further combine metals, metal oxides, polyoxometalates, and other functional groups to construct multi-functional novel hybrids with excellent catalytic, magnetic, conductive, optical and electronical properties [22]. 4. Conclusions
Fig. 4. The conductivity of IL(N) and CNPIL(j) solution (10 mg L different temperatures (from 293 to 343 K with 10 k increments).
1
, pH = 7) at
is about 50%, the strongest PL emission can be obtained. In addition, the FWHM also decreases when the water content is higher than 60% or lower than 50%, and it was mostly linear against the water content. In the present system, the strong dependence of the CNPIL PL on the water content may be attributed to that water modifies the patterns/structures of ionic liquid self-organization, which plays an important role in the formation of CNPIL and control of their PL properties [17]. We also measured the conductivity of pure IL and CNPIL at different temperature ranging from 293 to 343 K by impedance spectroscopy. The conductivity of both pure IL and CNPIL all show temperature-dependent feature and a monotonous increase with temperature increasing (Fig. 4) [18]. CNPIL shows the high conductivity of 5.1 S m 1 at 343 K, while IL only give 2.6 S m 1 at the same temperature. The conductivity increase of CNPIL may be attributed to the following factors: high electronic transport property of carbon nanoparticles; homogeneous distribution of carbon nanoparticles in the ionic liquid matrix; p–p interaction between the imidazole rings of ionic liquid and carbon nanoparticles [6a], which agrees well with UV–vis absorption. Combining such high conductivity and the excellent PL properties, this kind of CNPIL is expected to have potential applications in solar energy conversion, catalysis, and optoelectronic devices [8]. The present microwave-assisted reaction is very fast and can be finished within 3 min. A series of further parallel control experiments show that, with increasing reaction time, the peak position and intensity observed in the PL spectra of the obtained CNPIL have no obvious shift (see Supplementary Fig S4). Furthermore, in our present synthetic method, both ionic liquid and glucose are indispensable elements. If only pure ionic liquid or glucose was used in the synthetic experiments, the fluorescent CNPIL can not be obtained. Here, we think that the reaction process seems to conform to the LaMer model [19], shown schematically in Supplementary Fig S5. During the reaction, the glucose polymerizes into oligosaccharides by hydrogen bonding under microwave [20]. When the system reaches a critical condition, a short single bursting into nucleation occurred [21]. At the same time, the resulting nuclei combine with ionic liquid by hydrogen bonding, van der Waals forces, p–p stacking interactions or covalent interactions to form the final CNPIL [8]. We can expect that a better carbon sources (i.e. fructose, sucrose, or starch), other additives (i.e. acid, alkali, or different ionic liquids), or more suitable reaction conditions (i.e. ultrasonic power, reaction temperature, hydrothermal treatment) may further improve the present reaction process and the PL property of the CNPIL. On the other hand, as a possible
In summary, we have successfully developed a simple green microwave method for the synthesis of CNPIL with just one step treatment. The obtained CNPIL are stable for several months with unique optical features, high conductivity and excellent dispersibility in organic or aqueous solvents. Furthermore, the PL properties (i.e. intensity and FWHM) could be adjusted by controlling the synthetic conditions. Combining the PL and high electronic transport property, the CNPIL will be extremely versatile and offer new opportunities for their optoelectronic, catalytic, or biological applications. The present synthetic strategy will also put forward a promising way for the fabrication of other IL hybrid system. Acknowledgments This work was supported by the National Basic Research Program (973 Program) (No. 2010CB934500), the National Natural Science Foundation of China (Nos. 21073127, 21071104, 20801010, 20803008, 21003081), a Foundation for the Author of National Excellent Doctoral Dissertation of PR China (FANEDD) (No. 200929), RGC General Research Fund (GRF) (Grant No. 9041538) and City University of Hong Kong (No. 7002439). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.02.061. References [1] A.B. Bourlinos, K. Raman, R. Herrera, Q. Zhang, L.A. Archer, E.P. Giannelis, J. Am. Chem. Soc. 126 (2004) 15358–15359. [2] E.F. Borra1, O. Seddiki1, R. Ange, D. Eisenstein, P. Hickson, K.R. Seddon, S.P. Worden, Nature 447 (2007) 979–981. [3] [a] Y. Wei, Q.G. Zhang, Y. Liu, X.R. Li, S.Y. Lian, Z.H. Kang, J. Chem. Eng. Data 55 (2010) 2616–2619; [b] P. He, H.T. Liu, Z.Y. Li, Y. Liu, X.D. Xu, J.H. Li, Langmuir 20 (2004) 10260– 10267; [c] Y. Liu, J. Li, M. Wang, Z.Y. Li, H. Liu, P. He, X.R. Yang, J.H. Li, Cryst. Growth Des. 5 (2005) 1643–1649. [4] [a] B.H. Wu, D. Hu, Y.J. Kuang, B. Liu, X.H. Zhang, J.H. Chen, Angew. Chem., Int. Ed. 48 (2009) 4751–4754; [b] Y. Liu, M. Wang, Z. Li, H. Liu, P. He, J.H. Li, Langmuir 21 (2005) 1618–1622; [c] Z.H. Wen, J. Liu, J.H. Li, Adv. Mater. 20 (2008) 743–747. [5] [a] T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T. Aida, Science 300 (2003) 2072–2074; [b] T. Fukushima, T. Aida, Chem. Eur. J. 13 (2007) 5048–5058. [6] (a) Y.A. Lei, C.X. Xiong, L.J. Dong, H. Guo, X.H. Su, J.L. Yao, Y.J. You, D.M. Tian, X.M. Shang, Small 3 (2007) 1889–1893; (b) H.S. Park, B.G. Choi, S.H. Yang, W.H. Shin, J.K. Kang, D. Jung, W.H. Hong, Small 5 (2009) 1754–1760. [7] [a] X.S. Zhou, T.B. Wu, K.L. Ding, B.J. Hu, M.Q. Hou, B.X. Han, Chem. Commun. 46 (2010) 386–388; [b] H.F. Yang, C.S. Shan, F.H. Li, D.X. Han, Q.X. Zhang, L. Niu, Chem. Commun. 26 (2009) 3880–3882. [8] H.T. Li, X.D. He, Z.H. Kang, H. Huang, Y. Liu, J.L. Liu, S.Y. Lian, C.H.A. Tsang, X.B. Yang, S.-T. Lee, Angew. Chem., Int. Ed. 49 (2010) 4430–4434. [9] [a] N. Liu, F. Luo, H.X. Wu, Y.H. Liu, C. Zhang, J. Chen, Adv. Funct. Mater. 18 (2008) 1518–1525; [b] K. Baba, T. Kaneko, R. Hatakeyama, K. Motomiyac, K. Tohji, Chem. Commun. 46 (2010) 255–257; [c] Z.H. Kang, E.B. Wang, B.D. Mao, Z.M. Su, L. Chen, L. Xu, Nanotechnology 16 (2005) 1192–1195. [10] [a] J. Lu, J.X. Yang, J.Z. Wang, A. Lim, S. Wang, K.P. Loh, ACS Nano 3 (2009) 2367–2375; [b] Y. Lin, B. Zhou, R.B. Martin, K.B. Henbest, B.A. Harruff, J.E. Riggs, Z.X. Guo, L.F. Allard, Y.P. Sun, J. Phys. Chem. B 109 (2005) 14779–14782.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Carbon nanoparticle ionic liquid hybrids and their photoluminescence properties Ying Wei a,c, Yang Liu a,b,⇑, Haitao Li a, Xiaodie He a, Qingguo Zhang c, Zhenhui Kang a,b,⇑, Shuit-Tong Lee a,d a Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University and Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, Jiangsu 215123, PR China b Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China c College of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121000, PR China d Center of Super Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, PR China
a r t i c l e
i n f o
Article history: Received 24 January 2011 Accepted 24 February 2011 Available online 1 March 2011 Keywords: Carbon nanoparticles Ionic liquid Hybrid Photoluminescence
a b s t r a c t A fluorescent carbon nanoparticle ionic liquid hybrids (CNPIL) with high conductivity is synthesized by a facile one-step microwave method from ionic liquid 1-butyl-3-methylimidazolium glutamine salt and Glucose. This CNPIL exhibits excellent PL properties: bright and colorful PL covering the entire visible– NIR spectral range, up conversion PL properties, pH dependent and can be controlled by the reaction condition. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Ionic liquid hybrid is one of the most significant topics in electrochemistry, catalysis, and material science [1,2]. This kind of hybrid, which combines a low-melting ionic liquid and various functional materials or nanospecies (such as metals, metal oxides, metal salts, silica, polyanions, carbon and semiconductor nanoparticles), will possess the combined properties of ionic liquid and other functional species [3,4], such as negligible vapor pressure, tunable composition, polar character, low toxicity, high chemical and thermal stabilities, novel magnetic and optical properties, protonic conductivity, or catalytic properties. Thus, ionic liquid hybrid is widely regarded as a promising key to overcome the present challenges (such as high performance medium, electrode materials, and catalyst) in energy storage, fuel cell, green catalysis, and optoelectronics [3,4]. Further, the carbon based nanomaterial ionic liquid hybrid has become a hot topic recently and many research work focused on the synthesis and properties of such hybrid [5–10]. For example, Fukushima et al. reported that the marriage between ionic liquids and carbon nanotubes formed a gel-like composite (‘‘bucky gel’’) which shows great potential applications in capacitors, sensors, and actuators [5]. Lei and co-workers obtained a ultralong carbon nanotubes ionic liquid hybrid with liquid-like behavior and excellent thermal stability [6a]. Han and co-workers reported the synthesis and conductivity properties of high dispersibility and
long-term stable graphite sheets ionic liquid hybrid [7a]. However, there is no report on the optical properties of this kind of hybrid, especially the photoluminescence (PL) property. It is crucial to the optoelectronic devices, biological labeling, and photocatalyst design [8]. We should further point out that the fluorescent carbon based nanoparticles are interesting new comers to the world of nanomaterials. They are of great potential in applications of biomedical imaging, energy storage, electrode materials, optoelectronics, and photocatalyst [11,12]. Combining these fluorescent carbon nanoparticles with ionic liquid (IL), the novel hybrid may possess strong PL, remarkable conductivity, highly photoactivity and stability. Herein we introduce a facile one-step microwave synthetic method for the fabrication of fluorescent carbon nanoparticle ionic liquid hybrid (CNPIL) from ionic liquid 1-butyl-3-methylimidazolium glutamine salt and Glucose. This kind of highly waterdispersed hybrid has carbon cores with sizes less than 4 nm and exhibits bright and colorful PL covering the entire visible–NIR spectral range. Notably, the obtained CNPIL possesses excellent up conversion PL properties. The PL of CNPIL is pH dependent and can be controlled by adjusting the water content of reaction solution. Furthermore, the CNPIL also exhibits excellent conductivity.
2. Materials and methods 2.1. Materials
⇑ Corresponding authors at: Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China. Fax: +86 512 6588 2846. E-mail addresses: [email protected] (Y. Liu), [email protected] (Z. Kang). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.02.061
All chemicals were purchased from Sigma–Aldrich. The IL 1-butyl-3-methylimidazolium glutamine salt (BMIGlu) in this
Y. Wei et al. / Journal of Colloid and Interface Science 358 (2011) 146–150
147
study were synthesized by a neutralization method as the literature procedure [3], and the BMIGlu product was dried in vacuum for 2 days at 353 K. 2.2. Methods Glucose (0.5 g) was firstly dissolved into 4 mL distilled water. Under vigorous stirring, 0.2 mL BMIGlu was added to form a clear solution. The solution was heated in a 500 W microwave oven for different time periods. After that, the solution changed from colorless to dark brown, and then was diluted. In a parallel control experiment, Glucose (0.5 g) was dissolved into 4 mL distilled water by stirring to form a clear solution. The solution was heated in a 500 W microwave oven for 3–10 min. In the same manner, pure BMIGlu (0.2 mL) was also dissolved into 4 mL distilled water by stirring to form a clear solution and then heated in a 500 W microwave oven for 3–10 min. TEM image of CNPIL was obtained with a FEI/Philips Techai 12 BioTWIN TEM, while high-resolution HRTEM images with a CM200 FEG TEM. The FTIR spectrum of CQDs was obtained with a Nicolet 360 spectrometer. PL study was carried out on a Fluorolog-TCSPC Luminescence Spectrometer, while UV–visible spectra were obtained with an Aglient 8453 UV–Vis Diode Array Spectrophotometer. Conductivity was determined by AC impedance at variable temperatures using a IM6 electrochemical workstation in the frequency range from 1 Hz to 100 MHz. 3. Results and discussion In the experiment, briefly, a suitable amount of glucose and ionic liquid (1-butyl-3-methylimidazolium glutamine salt, BMIGlu) were added in water under stirring, and then heated in a 500 W microwave oven for 3–10 min. After reaction, the solution changed from colorless to dark brown, from aqueous to viscous (see Fig. 1a and b). Under UV light (365 nm, center), the bright blue PL of the obtained dark brown CNPIL is strong enough to be easily seen with the naked eyes (see Fig. 1c). The quantum yield of the CNPIL with blue emission was estimated to be about 9.2% by calibrating against quinine sulfate (QS) in ethanol (see ESI) [13]. Fig. 1d shows the transmission electron microscopy (TEM) image of the as prepared CNPIL, indicating that spherical carbon nanoparticles with diameter below 4 nm exist in CNPIL. The high resolution TEM image (see Supplementary Fig S1) shows the carbon particles have graphite structure, with the carbon matrix spacing of 0.32 nm. The structure characterization of CNPIL was carried out by FTIR. As shown in Supplementary Fig S2, the peaks located at 2962 cm 1 and 2853 cm 1 correspond to the C–H stretch, while the peaks at 1408 cm 1 and 1575 cm 1 are due to the C–H vibration. The vibration of the carbon skeleton of carbon nanostrucutres appears at 1575 cm 1. The peak at 1660 cm 1 corresponds to the framework vibration of imidazolium, which further indicates that the CNPIL is composed of IL and carbon nanoparticles [9]. The typical UV– vis absorption and PL spectra of as prepared CNPIL are shown in Fig. 2. The absorption band at 290 nm observed in Fig. 2a (curve A) represents the typical absorption of an aromatic pi system, which is similar to that of polycyclic aromatic hydrocarbons [14]. It is 10 nm red shift compared to the previous report of carbogenic nanoparticles, which may be due to the IL was adsorbed on the surface of the carbon nanoparticles by p–p interactions between the imidazolium rings of the IL and carbon nanoparticles [15]. The typical PL spectra (Fig. 2a, curve B) of CNPIL show strong blue1 emission at 420 nm with the excitation at 350 nm. Further, the PL 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
Fig. 1. Photographs of the glucose/ionic liquid solution (a) before and (b) after reaction; (c) optical photograph of CNPIL obtained under excitation at 365 nm; (d) TEM image of CNPIL with diameters within 4 nm.
emission can extend into NIR wavelength range, and the peak is at 800 nm (Fig. 2b). In addition, the emission spectra of CNPIL also show the excitation dependent feature that similar to previous reported carbon nanoparticles (Fig. 2c): increasing the excitation wavelength leads to the longer wavelength emissions [8]. Remarkably, the obtained CNPIL exhibits excellent up conversion PL properties besides their strong luminescence in visible–NIR range. As shown in Fig. 2d, long wavelength excitation ranged from 750 to 950 nm can lead to the up-converted emissions located in the range from 350 nm to 650 nm. The up-converting nanomaterials have unique luminescence properties with long lifetime and superior photostability for biological luminescent labels, such as noninvasive and deep penetration of NIR radiation, the absence of auto-fluorescence of biological tissues, and feasibility of multiple labeling with different emissions under the same excitation [8]. The photoluminescence from these CNPIL may mainly attribute to two following reasons: (1) the presence of quantum-sized carbon cores [8]. (2) the surface energy traps of the carbon core passivated by ILs. The functionalization of carbon core by IL improves the dispersion of the nanoparticles and diminishes the quenching effects due to interparticle interaction [10]. Further PL experiments indicate that the pH value of CNPIL solution can strongly affect the PL properties of the obtained CNPIL. Fig. 3a shows the typical PL spectra of CNPIL with the same excitation (350 nm) under different pH condition (from pH 1–13). When 1 6 pH 6 3, the PL spectra of CNPIL exhibit two peaks located at about 400 nm and 500 nm, respectively. While, there is only one peak (at about 420 nm) is observed in the PL spectra when pH P 5. On the other hand, the PL intensity also have pH dependent feature (Fig. 3b): 1 6 pH 6 5, the PL intensity of CNPIL increases significantly; 5 < pH 6 9, the PL intensity increases not too much; pH > 10, the PL intensity decreases sharply. The detailed PL spectra of CNPIL at typical acidic (pH = 3) or alkalic (pH = 10) environment with different wavelength excitation are shown in Fig. 3c and d, respectively, from which we can see that the emission spectra also show excitation dependent feature. The above pH-dependent PL phenomena may be resulted from the density of surface electron localized states that participate in the radiative transitions increases by the adsorption of OH ions on the surface of CNPIL [16]. Further detailed experiments confirm that, the ionic liquid/ water ratio (different water content) is also a key point affecting the PL spectra of CNPIL. Though the PL peak positions are almost the same, the PL intensity and full-width half-maximum (FWHM) are different. Supplementary Fig S3a shows the PL spectra covering the visible–NIR wavelength range of CNPIL (pH = 7, excitation at 350 nm) obtained with different water content solution (ionic liquid/water ratio). As shown in Supplementary Fig S3b, the intensity and FWHM of PL emissions in NIR wavelength range
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Fig. 2. (a) UV–vis absorption (curve A) and PL spectra (curve B) of CNPIL in aqueous solution with pH = 7 (excitation at 350 nm); (b) PL spectra of CNPIL (pH = 7) in NIR range (excitation at 350 nm); (c) PL spectra of CNPIL (pH = 7) in visible range (recorded for progressively longer excitation wavelengths from 300 to 450 nm in 25 nm increments); (d) Up conversion PL spectra of CNPIL excited by long wavelength light(from 750 to 950 nm) with pH = 7.
Fig. 3. (a) PL spectra of CNPIL under different pH condition (1 6 pH 6 13) with the same excitation (350 nm); (b) the relationship of fluorescence intensity versus the pH value of solution; PL spectra of CNPIL under (c) acidic (pH = 3) and (d) alkalic (pH = 10) condition in visible range (excitation wavelengths from 300 to 500 nm with 50 nm increments).
are also different. Supplementary Figs. S3c and S3d show the relationship between the PL intensity/FWHM and various water content of reaction solution, respectively. With increasing water content (below 60%), the PL intensity of CNPIL increases significantly, and the maximum intensity is obtained at water
content of about 50%. Further increasing the water content (P60%), the PL intensity of CNPIL decreases contrarily. Water content (in the present system, different water content indicates the different concentration of glucose and ionic liquid) dramatically influences the PL properties of CNPIL. When water content
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extension of the present work, we envision the CNPIL may further combine metals, metal oxides, polyoxometalates, and other functional groups to construct multi-functional novel hybrids with excellent catalytic, magnetic, conductive, optical and electronical properties [22]. 4. Conclusions
Fig. 4. The conductivity of IL(N) and CNPIL(j) solution (10 mg L different temperatures (from 293 to 343 K with 10 k increments).
1
, pH = 7) at
is about 50%, the strongest PL emission can be obtained. In addition, the FWHM also decreases when the water content is higher than 60% or lower than 50%, and it was mostly linear against the water content. In the present system, the strong dependence of the CNPIL PL on the water content may be attributed to that water modifies the patterns/structures of ionic liquid self-organization, which plays an important role in the formation of CNPIL and control of their PL properties [17]. We also measured the conductivity of pure IL and CNPIL at different temperature ranging from 293 to 343 K by impedance spectroscopy. The conductivity of both pure IL and CNPIL all show temperature-dependent feature and a monotonous increase with temperature increasing (Fig. 4) [18]. CNPIL shows the high conductivity of 5.1 S m 1 at 343 K, while IL only give 2.6 S m 1 at the same temperature. The conductivity increase of CNPIL may be attributed to the following factors: high electronic transport property of carbon nanoparticles; homogeneous distribution of carbon nanoparticles in the ionic liquid matrix; p–p interaction between the imidazole rings of ionic liquid and carbon nanoparticles [6a], which agrees well with UV–vis absorption. Combining such high conductivity and the excellent PL properties, this kind of CNPIL is expected to have potential applications in solar energy conversion, catalysis, and optoelectronic devices [8]. The present microwave-assisted reaction is very fast and can be finished within 3 min. A series of further parallel control experiments show that, with increasing reaction time, the peak position and intensity observed in the PL spectra of the obtained CNPIL have no obvious shift (see Supplementary Fig S4). Furthermore, in our present synthetic method, both ionic liquid and glucose are indispensable elements. If only pure ionic liquid or glucose was used in the synthetic experiments, the fluorescent CNPIL can not be obtained. Here, we think that the reaction process seems to conform to the LaMer model [19], shown schematically in Supplementary Fig S5. During the reaction, the glucose polymerizes into oligosaccharides by hydrogen bonding under microwave [20]. When the system reaches a critical condition, a short single bursting into nucleation occurred [21]. At the same time, the resulting nuclei combine with ionic liquid by hydrogen bonding, van der Waals forces, p–p stacking interactions or covalent interactions to form the final CNPIL [8]. We can expect that a better carbon sources (i.e. fructose, sucrose, or starch), other additives (i.e. acid, alkali, or different ionic liquids), or more suitable reaction conditions (i.e. ultrasonic power, reaction temperature, hydrothermal treatment) may further improve the present reaction process and the PL property of the CNPIL. On the other hand, as a possible
In summary, we have successfully developed a simple green microwave method for the synthesis of CNPIL with just one step treatment. The obtained CNPIL are stable for several months with unique optical features, high conductivity and excellent dispersibility in organic or aqueous solvents. Furthermore, the PL properties (i.e. intensity and FWHM) could be adjusted by controlling the synthetic conditions. Combining the PL and high electronic transport property, the CNPIL will be extremely versatile and offer new opportunities for their optoelectronic, catalytic, or biological applications. The present synthetic strategy will also put forward a promising way for the fabrication of other IL hybrid system. Acknowledgments This work was supported by the National Basic Research Program (973 Program) (No. 2010CB934500), the National Natural Science Foundation of China (Nos. 21073127, 21071104, 20801010, 20803008, 21003081), a Foundation for the Author of National Excellent Doctoral Dissertation of PR China (FANEDD) (No. 200929), RGC General Research Fund (GRF) (Grant No. 9041538) and City University of Hong Kong (No. 7002439). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.02.061. References [1] A.B. Bourlinos, K. Raman, R. Herrera, Q. Zhang, L.A. Archer, E.P. Giannelis, J. Am. Chem. Soc. 126 (2004) 15358–15359. [2] E.F. Borra1, O. Seddiki1, R. Ange, D. Eisenstein, P. Hickson, K.R. Seddon, S.P. Worden, Nature 447 (2007) 979–981. [3] [a] Y. Wei, Q.G. Zhang, Y. Liu, X.R. Li, S.Y. Lian, Z.H. Kang, J. Chem. Eng. Data 55 (2010) 2616–2619; [b] P. He, H.T. Liu, Z.Y. Li, Y. Liu, X.D. Xu, J.H. Li, Langmuir 20 (2004) 10260– 10267; [c] Y. Liu, J. Li, M. Wang, Z.Y. Li, H. Liu, P. He, X.R. Yang, J.H. Li, Cryst. Growth Des. 5 (2005) 1643–1649. [4] [a] B.H. Wu, D. Hu, Y.J. Kuang, B. Liu, X.H. Zhang, J.H. Chen, Angew. Chem., Int. Ed. 48 (2009) 4751–4754; [b] Y. Liu, M. Wang, Z. Li, H. Liu, P. He, J.H. Li, Langmuir 21 (2005) 1618–1622; [c] Z.H. Wen, J. Liu, J.H. Li, Adv. Mater. 20 (2008) 743–747. [5] [a] T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T. Aida, Science 300 (2003) 2072–2074; [b] T. Fukushima, T. Aida, Chem. Eur. J. 13 (2007) 5048–5058. [6] (a) Y.A. Lei, C.X. Xiong, L.J. Dong, H. Guo, X.H. Su, J.L. Yao, Y.J. You, D.M. Tian, X.M. Shang, Small 3 (2007) 1889–1893; (b) H.S. Park, B.G. Choi, S.H. Yang, W.H. Shin, J.K. Kang, D. Jung, W.H. Hong, Small 5 (2009) 1754–1760. [7] [a] X.S. Zhou, T.B. Wu, K.L. Ding, B.J. Hu, M.Q. Hou, B.X. Han, Chem. Commun. 46 (2010) 386–388; [b] H.F. Yang, C.S. Shan, F.H. Li, D.X. Han, Q.X. Zhang, L. Niu, Chem. Commun. 26 (2009) 3880–3882. [8] H.T. Li, X.D. He, Z.H. Kang, H. Huang, Y. Liu, J.L. Liu, S.Y. Lian, C.H.A. Tsang, X.B. Yang, S.-T. Lee, Angew. Chem., Int. Ed. 49 (2010) 4430–4434. [9] [a] N. Liu, F. Luo, H.X. Wu, Y.H. Liu, C. Zhang, J. Chen, Adv. Funct. Mater. 18 (2008) 1518–1525; [b] K. Baba, T. Kaneko, R. Hatakeyama, K. Motomiyac, K. Tohji, Chem. Commun. 46 (2010) 255–257; [c] Z.H. Kang, E.B. Wang, B.D. Mao, Z.M. Su, L. Chen, L. Xu, Nanotechnology 16 (2005) 1192–1195. [10] [a] J. Lu, J.X. Yang, J.Z. Wang, A. Lim, S. Wang, K.P. Loh, ACS Nano 3 (2009) 2367–2375; [b] Y. Lin, B. Zhou, R.B. Martin, K.B. Henbest, B.A. Harruff, J.E. Riggs, Z.X. Guo, L.F. Allard, Y.P. Sun, J. Phys. Chem. B 109 (2005) 14779–14782.
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