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Incorporating fluorescent quantum dots into water-soluble polymer
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 277–281 www.elsevier.com/locate/jlumin
Incorporating fluorescent quantum dots into water-soluble polymer Yun Leia,b, Haiyang Tangc, Chunjiao Zhoua,b, Tingting Zhanga,b, Meifu Fengc, Bingsuo Zoua,b, a
State Key Laboratory of CBSC, Micro-Nanotechnology Research Center, Hunan University, Changsha 410082, China b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China c Institute of Zoology, Chinese Academy of Sciences, Beijing 100080, China Received 16 January 2007; received in revised form 25 May 2007; accepted 5 July 2007 Available online 27 July 2007
Abstract The fluorescent quantum dot–polymer composites were fabricated by incorporating thioglycolic acid capped CdTe quantum dots into polyacrylamide via cross-linking agents. The CdTe–polyacrylamide composites were characterized by fluorescence spectrophotometer and fluorescence microscope. The result shows that the quantum dot–polymer composites show strong photoluminescence in aqueous solution. The photoluminescence spectrum of quantum dot–polymer composites exhibits a slight blue shift compared to that of initial CdTe quantum dots. The slight shift might be attributed to the covalently bonding between the carboxyl groups of thiolglycolic acid capped on CdTe quantum dots and the amide groups of the polyacrylamide chains. r 2007 Elsevier B.V. All rights reserved. Keywords: Quantum dots; Polyacrylamide; Photoluminescence spectra; Fluorescence images
1. Introduction Fluorescent semiconductor nanocrystals (NCs) or quantum dots (QDs) have attracted a broad range of attention in the past few decades [1,2]. Embedding colloidal semiconductor QDs into polymer is an effective method of enhancing the functions of these materials. Many efforts have been directed toward the fabrication of polymer nanocomposites. Gaponik et al. [3] used thiol-stabilized CdTe or HgTe QDs to embed into polymer matrices by a partial exchange of capping ligands with dodecan-1-thiol. Zhang et al. [4] used a polymerizable surfactant to transfer water-soluble CdTe NCs into organic solution, and then copolymerized CdTe NCs and monomers to achieve nanocrystal–polymer composites, or coated CdTe NCs directly with copolymer in the related route [5,6]. Li and Gao et al. presented an alternative way to incorporate CdTe QDs into temperature-sensitive hydrogel spheres [7]. Similar QDs conjugation has been Corresponding author. State Key Laboratory of CBSC, MicroNanotechnology Research Center, Hunan University, Changsha 410082, China. Tel.: +86 731 6619421; fax: +86 731 8821480. E-mail address: [email protected] (B. Zou).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.07.011
reported with other gels [8,9]. However, it is worth mentioning that ligands exchange or the adding of organic solvents could have negative effects on the surface structures of QDs, which could adversely affect their photoluminescence properties. The physical entrapment of CdTe QDs in hydrogel matrix is confined by the low critical solution temperature of hydrogel spheres. The lack of covalent interactions between QDs and hydrogel spheres may result in the release of trapped QDs from quantum dot–polymer composites. Thus, it is required to explore a facile method to fabricate stable quantum dot–polymer composites without cost of fluorescence intensity. Herein, we directly incorporate thioglycolic acid capped CdTe QDs into water-soluble polyacrylamide (PAM) via 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) and N-hydroxysulfosuccinimide (SulfoNHS) cross-linking agents. There is no need of ligands exchange or phase separation between the CdTe QDs and the PAM polymer. Therefore, the strong photoluminescence is preserved in the CdTe–PAM composites. The covalently conjugated CdTe–PAM composites are biologically compatible, which can be potentially used as fluorescent markers in biological and clinical detection.
ARTICLE IN PRESS 278
Y. Lei et al. / Journal of Luminescence 128 (2008) 277–281
2. Experimental
3. Results and discussion
2.1. Chemicals
The scheme illustration of TGA-capped CdTe QDs and PAM polymer conjugation is shown in Fig. 1. EDC reacts with the carboxyl group of TGA capped on CdTe QDs to form an amine-reactive O-acylisourea intermediate. This intermediate may react with the amine on PAM, yielding the conjugation of CdTe QDs and PAM. However, the intermediate is unstable and susceptible to hydrolysis in aqueous solution. The addition of Sulfo-NHS stabilizes the amine-reactive intermediate by converting it to an aminereactive Sulfo-NHS ester, thus increasing the efficiency of EDC-mediated coupling reactions. Fig. 2 shows the IR spectrum of CdTe–PAM composites. The amide function can be characterized by the complex bands –CO–NH2 and –CO–NH at 1655 and 1558 cm 1, respectively. The most characteristic bands can be found at 1741 cm 1 for the CO stretching vibration, 1655 cm 1 for the band –CO–NH2, 1558 cm 1 for the band –CO–NH, and 1396 cm 1 for the C–N stretching vibration. Thereinto, the –CO–NH mode contains contributions from C–N stretching vibration and N–H bending vibration, which originates from the bonding between the carboxyl groups of thioglycolic acid capped on CdTe QDs and the amide groups of the PAM chains. Therefore, the IR spectrum shown in Fig. 2 confirms the formation of the composite by chemical conjugation between stabilizer molecules and polymer. The sample of CdTe QDs and PAM polymer was characterized by the FTIR spectra. As shown in Fig. 2, the band at 1670 cm 1 is coincident with carbonyl stretching vibration. The peaks at 1655 and 1558 cm 1 are assigned to –CO–NH2 and –CO–NH bands, respectively. The –CO–NH mode contains contributions from C–N stretching vibration and N–H bending vibration, which originate from the bonding between the carboxyl groups of thioglycolic acid capped on CdTe QDs and the amide groups of the PAM chains. The CdTe–PAM composites were studied by the TEM (Fig. 3). The CdTe QDs could be observed on copper grids, but the PAM polymer matrix could not be detected directly. The TEM image indicates a close packing of CdTe QDs with a separation of several nanometers. The CdTe–PAM composites were prepared by incorporating different color CdTe QDs into PAM. The photoluminescence spectra of green CdTe–PAM composites are shown in Fig. 4a. The photoluminescence peak position of green CdTe–PAM composites (line) exhibits a blue shift of 7 nm in comparison to that of initial CdTe QDs (dashed line) in aqueous solution. This shift is also observed in the red CdTe–PAM composites. As shown in Fig. 4b, the photoluminescence peak position of red CdTe–PAM composites exhibits a blue shift of 6 nm. The shift might be attributed to the covalent bonding between the carboxyl groups of thioglycolic acid capped on CdTe QDs and the amide groups of the PAM chains. An interesting result of this work is that the fluorescence intensity of CdTe QDs improves after combining with
Thioglycolic acid (TGA 497%), tellurium powder (99.999%), CdCl2 2.5H2O (99.5%), and sodium borohydride (NaBH4; 99%) were commercially available products and used without further purification. EDC (1-ethyl-3-[3dimethylaminopropyl]carbodiimide hydrochloride) and Sulfo-NHS (N-hydroxysulfosuccinimide) were purchased from Pierce Biotechnology. 2.2. Synthesis 2.2.1. Synthesis of TGA-capped CdTe QDs TGA-stabilized CdTe QDs were prepared according to the following procedure [10]. First, sodium borohydride was used to react with tellurium to produce bivalent telluride. Briefly, 127 mg of tellurium powder was added to a three-necked flask, and then the three-necked flask was de-aerated by argon. After 80 mg sodium borohydride was transferred to the three-necked flask, the reaction system of sodium borohydride and tellurium was cooled by ice. For about 8 h, the resulting bivalent telluride solution in the three-necked flask was taken out and used in further preparation of CdTe QDs. In the second step, argon-saturated CdCl2 solution (0.0025 mol/L) with TGA was prepared at pH 11.0 in a 150 mL threenecked flask. Under stirring, the freshly prepared bivalent telluride solution was added to the 150 mL three-necked flask, and the precursors were converted to CdTe QDs by refluxing the reaction mixture at 100 1C under open-air conditions. 2.2.2. Preparation of fluorescent quantum dot–polymer composites PAM was dissolved in aqueous solution at a concentration of 5 mg/mL. A PAM solution (10 mL) was added to 20 mL of an aqueous CdTe solution (0.0025 mol/L according to Cd2+) with stirring. To integrate TGA-capped CdTe QDs into water-soluble PAM polymer, coupling agents (EDC/NHS) were added to the CdTe–PAM mixture. The mixture of CdTe QDs, polymer and coupling agents was stirred for 4 h at room temperature. 2.3. Characterizations The sample of CdTe–PAM composites was observed on a JEM2010 transmission electron microscope (TEM). The Fourier transform infrared (FTIR) spectrum was measured on a Varian 3100 spectrometer. The photoluminescence spectra were recorded on a PTI-601 fluorescence spectrophotometer, and the fluorescence images were taken on a fluorescence microscope (OLYMPUS IX71) excited by an ultraviolet source.
ARTICLE IN PRESS Y. Lei et al. / Journal of Luminescence 128 (2008) 277–281
279
Fig. 1. Illustration of TGA-capped CdTe QDs and PAM polymer conjugation. The first step involves the reaction of EDC with the carboxyl group of TGA capped on CdTe QDs to form an amine-reactive O-acylisourea intermediate, which was converted to an amine-reactive Sulfo-NHS ester via the addition of Sulfo-NHS. Then, the amine-reactive Sulfo-NHS ester may react with the amine on PAM, yielding a conjugation of the CdTe–PAM composite joined by a stable amide bond.
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% Transmittance
95 90 85 80 75 70 65 4000
1558 3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 2. FTIR spectra of CdTe–PAM composites.
PAM. The fluorescence intensity of green CdTe–PAM composites increases 13% in comparison to that of initial CdTe solution, and the fluorescence intensity of red CdTe–PAM composites increases 11% compared to that
of initial CdTe solution. The photoluminescence enhancement might be ascribed to the change of physicochemical environment around QD as a result of the polymer coating [4]. Coating QDs with the high molecular weight polymer
ARTICLE IN PRESS Y. Lei et al. / Journal of Luminescence 128 (2008) 277–281
280
might prohibit the electron leakage through the surface groups and might contribute to the continuative electronrich environment for the whole QD. It should be mentioned that a drop in photoluminescence intensity was reported in previous work of nanocrystal–polymer composites [11]. In that work the QDs were polymerized with monomers at a relatively high temperature in the presence of a radical initiator. The generated free radical during the polymerization process acted as a quencher in quantum dot–polymer composites, and the phase separation of QDs from the matrix led to luminescence quenching. On the contrary, our approach uses PAM to directly coat negatively charged CdTe QDs at room temperature. There is no need to handle the phase separation process during the conjugation of CdTe QDs
Fig. 3. TEM image of CdTe–PAM composites.
and PAM polymer. Hence, the strong photoluminescence were preserved in the quantum dot–polymer composites. The fluorescent properties of the quantum dot–polymer composites are mainly determined by the initial CdTe QDs in aqueous solution. Incorporating the initial CdTe QDs with different colors into PAM gives rise to numerous combination units, and a series of fluorescent complexes could thus be obtained. The complexes show excellent stability in aqueous solution. The emission spectra of CdTe–PAM composites were measured after the composites were placed for 6 months under open-air conditions. The full-width at half-maximum of photoluminescence spectra remains little changed. It is therefore expected that the PAM network may efficiently protect CdTe QDs against aggregation and decomposition. For the measurement of fluorescence images of the CdTe–PAM composites, casting films were fabricated on microscope glass slides. The fluorescence images of green CdTe–PAM composites were taken on a fluorescence microscope excited by an ultraviolet source (Fig. 5a). The PAM polymers loaded with CdTe QDs self-assemble into dendritic morphologies. Further observation shows that each dendrite consists of a long central backbone and stretched secondary and tertiary branches. The fluorescence color of the CdTe–PAM composites can easily be tuned by embedding red emitting QDs instead of green ones, and the fluorescence images of red CdTe–PAM composites are shown in Fig. 5b. The fluorescence signals can only be detected at the positions of PAM, which suggests that the CdTe QDs have been combined with PAM successfully. In summary, we have fabricated water-soluble quantum dot–polymer composites by incorporating TGA-capped CdTe QDs into PAM. The incorporation of CdTe QDs in transparent PAM did not change the optical properties of CdTe QDs, and the strong photoluminescence of CdTe QDs were preserved in the quantum dot–polymer composites. The slightly blue shift on the emission peak of the CdTe–PAM composites was ascribed to the covalent
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Fig. 4. The photoluminescence spectra of CdTe–PAM composites with green (a) and red (b) emissions. The photoluminescence spectra of simple CdTe QDs (dashed line) and CdTe–PAM composites (line) in aqueous solution.
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281
Fig. 5. The fluorescent images of PAM loaded with green (a) and red (b) CdTe QDs.
bonding between the carboxyl groups of TGA capped on QDs and the amide groups of the PAM chains. The PAM polymers loaded with CdTe QDs showed a self-organized architecture with dendritic characteristics, composed of a long central backbone, stretched secondary and arranged tertiary branches. The introduction of QDs into PAM polymers gives a simple route to design fluorescent quantum dot–polymer composites, which may find many ever-growing applications in the biological labels, detection and many other fields of nanotechnology. Acknowledgments The authors would like to thank the 973 projects of MOST (Grant no. 2002CB713802), Grant nos. 30370383, 90606001, and 90406024 of NAFC of China and the team fund of China Ministry of Education (705040) for financial supports.
References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] A. Eychmu¨ller, J. Phys. Chem. B 104 (2000) 6514. [3] N. Gaponik, D.V. Talapin, A.L. Rogach, A. Eychmu¨ller, H. Weller, Nano Lett. 2 (2002) 803. [4] H. Zhang, Z.C. Cui, Y. Wang, K. Zhang, X.L. Ji, C.L. Lu, B. Yang, M.Y. Gao, Adv. Mater. 15 (2003) 777. [5] H. Zhang, C.L. Wang, M.J. Li, J.H. Zhang, G. Lu, B. Yang, Adv. Mater. 17 (2005) 853. [6] H. Zhang, C.L. Wang, M.J. Li, X.L. Ji, J.H. Zhang, B. Yang, Chem. Mater. 17 (2005) 4783. [7] J. Li, X. Hong, Y. Liu, D. Li, Y.W. Wang, J.H. Li, Y.B. Bai, T.J. Li, Adv. Mater. 17 (2005) 163. [8] M. Kuang, D.Y. Wang, H.B. Bao, M.Y. Gao, H. Mohwald, M. Jiang, Adv. Mater. 17 (2005) 267. [9] Y.J. Gong, M.Y. Gao, D.Y. Wang, H. Mohwald, Chem. Mater. 17 (2005) 2648. [10] H. Zhang, Z. Zhou, B. Yang, M.Y. Gao, J. Phys. Chem. B 107 (2003) 8. [11] J. Lee, V.C. Sundar, J.R. Heine, M.G. Bawendi, K.F. Jensen, Adv. Mater. 12 (2000) 1102.
Journal of Luminescence 128 (2008) 277–281 www.elsevier.com/locate/jlumin
Incorporating fluorescent quantum dots into water-soluble polymer Yun Leia,b, Haiyang Tangc, Chunjiao Zhoua,b, Tingting Zhanga,b, Meifu Fengc, Bingsuo Zoua,b, a
State Key Laboratory of CBSC, Micro-Nanotechnology Research Center, Hunan University, Changsha 410082, China b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China c Institute of Zoology, Chinese Academy of Sciences, Beijing 100080, China Received 16 January 2007; received in revised form 25 May 2007; accepted 5 July 2007 Available online 27 July 2007
Abstract The fluorescent quantum dot–polymer composites were fabricated by incorporating thioglycolic acid capped CdTe quantum dots into polyacrylamide via cross-linking agents. The CdTe–polyacrylamide composites were characterized by fluorescence spectrophotometer and fluorescence microscope. The result shows that the quantum dot–polymer composites show strong photoluminescence in aqueous solution. The photoluminescence spectrum of quantum dot–polymer composites exhibits a slight blue shift compared to that of initial CdTe quantum dots. The slight shift might be attributed to the covalently bonding between the carboxyl groups of thiolglycolic acid capped on CdTe quantum dots and the amide groups of the polyacrylamide chains. r 2007 Elsevier B.V. All rights reserved. Keywords: Quantum dots; Polyacrylamide; Photoluminescence spectra; Fluorescence images
1. Introduction Fluorescent semiconductor nanocrystals (NCs) or quantum dots (QDs) have attracted a broad range of attention in the past few decades [1,2]. Embedding colloidal semiconductor QDs into polymer is an effective method of enhancing the functions of these materials. Many efforts have been directed toward the fabrication of polymer nanocomposites. Gaponik et al. [3] used thiol-stabilized CdTe or HgTe QDs to embed into polymer matrices by a partial exchange of capping ligands with dodecan-1-thiol. Zhang et al. [4] used a polymerizable surfactant to transfer water-soluble CdTe NCs into organic solution, and then copolymerized CdTe NCs and monomers to achieve nanocrystal–polymer composites, or coated CdTe NCs directly with copolymer in the related route [5,6]. Li and Gao et al. presented an alternative way to incorporate CdTe QDs into temperature-sensitive hydrogel spheres [7]. Similar QDs conjugation has been Corresponding author. State Key Laboratory of CBSC, MicroNanotechnology Research Center, Hunan University, Changsha 410082, China. Tel.: +86 731 6619421; fax: +86 731 8821480. E-mail address: [email protected] (B. Zou).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.07.011
reported with other gels [8,9]. However, it is worth mentioning that ligands exchange or the adding of organic solvents could have negative effects on the surface structures of QDs, which could adversely affect their photoluminescence properties. The physical entrapment of CdTe QDs in hydrogel matrix is confined by the low critical solution temperature of hydrogel spheres. The lack of covalent interactions between QDs and hydrogel spheres may result in the release of trapped QDs from quantum dot–polymer composites. Thus, it is required to explore a facile method to fabricate stable quantum dot–polymer composites without cost of fluorescence intensity. Herein, we directly incorporate thioglycolic acid capped CdTe QDs into water-soluble polyacrylamide (PAM) via 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) and N-hydroxysulfosuccinimide (SulfoNHS) cross-linking agents. There is no need of ligands exchange or phase separation between the CdTe QDs and the PAM polymer. Therefore, the strong photoluminescence is preserved in the CdTe–PAM composites. The covalently conjugated CdTe–PAM composites are biologically compatible, which can be potentially used as fluorescent markers in biological and clinical detection.
ARTICLE IN PRESS 278
Y. Lei et al. / Journal of Luminescence 128 (2008) 277–281
2. Experimental
3. Results and discussion
2.1. Chemicals
The scheme illustration of TGA-capped CdTe QDs and PAM polymer conjugation is shown in Fig. 1. EDC reacts with the carboxyl group of TGA capped on CdTe QDs to form an amine-reactive O-acylisourea intermediate. This intermediate may react with the amine on PAM, yielding the conjugation of CdTe QDs and PAM. However, the intermediate is unstable and susceptible to hydrolysis in aqueous solution. The addition of Sulfo-NHS stabilizes the amine-reactive intermediate by converting it to an aminereactive Sulfo-NHS ester, thus increasing the efficiency of EDC-mediated coupling reactions. Fig. 2 shows the IR spectrum of CdTe–PAM composites. The amide function can be characterized by the complex bands –CO–NH2 and –CO–NH at 1655 and 1558 cm 1, respectively. The most characteristic bands can be found at 1741 cm 1 for the CO stretching vibration, 1655 cm 1 for the band –CO–NH2, 1558 cm 1 for the band –CO–NH, and 1396 cm 1 for the C–N stretching vibration. Thereinto, the –CO–NH mode contains contributions from C–N stretching vibration and N–H bending vibration, which originates from the bonding between the carboxyl groups of thioglycolic acid capped on CdTe QDs and the amide groups of the PAM chains. Therefore, the IR spectrum shown in Fig. 2 confirms the formation of the composite by chemical conjugation between stabilizer molecules and polymer. The sample of CdTe QDs and PAM polymer was characterized by the FTIR spectra. As shown in Fig. 2, the band at 1670 cm 1 is coincident with carbonyl stretching vibration. The peaks at 1655 and 1558 cm 1 are assigned to –CO–NH2 and –CO–NH bands, respectively. The –CO–NH mode contains contributions from C–N stretching vibration and N–H bending vibration, which originate from the bonding between the carboxyl groups of thioglycolic acid capped on CdTe QDs and the amide groups of the PAM chains. The CdTe–PAM composites were studied by the TEM (Fig. 3). The CdTe QDs could be observed on copper grids, but the PAM polymer matrix could not be detected directly. The TEM image indicates a close packing of CdTe QDs with a separation of several nanometers. The CdTe–PAM composites were prepared by incorporating different color CdTe QDs into PAM. The photoluminescence spectra of green CdTe–PAM composites are shown in Fig. 4a. The photoluminescence peak position of green CdTe–PAM composites (line) exhibits a blue shift of 7 nm in comparison to that of initial CdTe QDs (dashed line) in aqueous solution. This shift is also observed in the red CdTe–PAM composites. As shown in Fig. 4b, the photoluminescence peak position of red CdTe–PAM composites exhibits a blue shift of 6 nm. The shift might be attributed to the covalent bonding between the carboxyl groups of thioglycolic acid capped on CdTe QDs and the amide groups of the PAM chains. An interesting result of this work is that the fluorescence intensity of CdTe QDs improves after combining with
Thioglycolic acid (TGA 497%), tellurium powder (99.999%), CdCl2 2.5H2O (99.5%), and sodium borohydride (NaBH4; 99%) were commercially available products and used without further purification. EDC (1-ethyl-3-[3dimethylaminopropyl]carbodiimide hydrochloride) and Sulfo-NHS (N-hydroxysulfosuccinimide) were purchased from Pierce Biotechnology. 2.2. Synthesis 2.2.1. Synthesis of TGA-capped CdTe QDs TGA-stabilized CdTe QDs were prepared according to the following procedure [10]. First, sodium borohydride was used to react with tellurium to produce bivalent telluride. Briefly, 127 mg of tellurium powder was added to a three-necked flask, and then the three-necked flask was de-aerated by argon. After 80 mg sodium borohydride was transferred to the three-necked flask, the reaction system of sodium borohydride and tellurium was cooled by ice. For about 8 h, the resulting bivalent telluride solution in the three-necked flask was taken out and used in further preparation of CdTe QDs. In the second step, argon-saturated CdCl2 solution (0.0025 mol/L) with TGA was prepared at pH 11.0 in a 150 mL threenecked flask. Under stirring, the freshly prepared bivalent telluride solution was added to the 150 mL three-necked flask, and the precursors were converted to CdTe QDs by refluxing the reaction mixture at 100 1C under open-air conditions. 2.2.2. Preparation of fluorescent quantum dot–polymer composites PAM was dissolved in aqueous solution at a concentration of 5 mg/mL. A PAM solution (10 mL) was added to 20 mL of an aqueous CdTe solution (0.0025 mol/L according to Cd2+) with stirring. To integrate TGA-capped CdTe QDs into water-soluble PAM polymer, coupling agents (EDC/NHS) were added to the CdTe–PAM mixture. The mixture of CdTe QDs, polymer and coupling agents was stirred for 4 h at room temperature. 2.3. Characterizations The sample of CdTe–PAM composites was observed on a JEM2010 transmission electron microscope (TEM). The Fourier transform infrared (FTIR) spectrum was measured on a Varian 3100 spectrometer. The photoluminescence spectra were recorded on a PTI-601 fluorescence spectrophotometer, and the fluorescence images were taken on a fluorescence microscope (OLYMPUS IX71) excited by an ultraviolet source.
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279
Fig. 1. Illustration of TGA-capped CdTe QDs and PAM polymer conjugation. The first step involves the reaction of EDC with the carboxyl group of TGA capped on CdTe QDs to form an amine-reactive O-acylisourea intermediate, which was converted to an amine-reactive Sulfo-NHS ester via the addition of Sulfo-NHS. Then, the amine-reactive Sulfo-NHS ester may react with the amine on PAM, yielding a conjugation of the CdTe–PAM composite joined by a stable amide bond.
100
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95 90 85 80 75 70 65 4000
1558 3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 2. FTIR spectra of CdTe–PAM composites.
PAM. The fluorescence intensity of green CdTe–PAM composites increases 13% in comparison to that of initial CdTe solution, and the fluorescence intensity of red CdTe–PAM composites increases 11% compared to that
of initial CdTe solution. The photoluminescence enhancement might be ascribed to the change of physicochemical environment around QD as a result of the polymer coating [4]. Coating QDs with the high molecular weight polymer
ARTICLE IN PRESS Y. Lei et al. / Journal of Luminescence 128 (2008) 277–281
280
might prohibit the electron leakage through the surface groups and might contribute to the continuative electronrich environment for the whole QD. It should be mentioned that a drop in photoluminescence intensity was reported in previous work of nanocrystal–polymer composites [11]. In that work the QDs were polymerized with monomers at a relatively high temperature in the presence of a radical initiator. The generated free radical during the polymerization process acted as a quencher in quantum dot–polymer composites, and the phase separation of QDs from the matrix led to luminescence quenching. On the contrary, our approach uses PAM to directly coat negatively charged CdTe QDs at room temperature. There is no need to handle the phase separation process during the conjugation of CdTe QDs
Fig. 3. TEM image of CdTe–PAM composites.
and PAM polymer. Hence, the strong photoluminescence were preserved in the quantum dot–polymer composites. The fluorescent properties of the quantum dot–polymer composites are mainly determined by the initial CdTe QDs in aqueous solution. Incorporating the initial CdTe QDs with different colors into PAM gives rise to numerous combination units, and a series of fluorescent complexes could thus be obtained. The complexes show excellent stability in aqueous solution. The emission spectra of CdTe–PAM composites were measured after the composites were placed for 6 months under open-air conditions. The full-width at half-maximum of photoluminescence spectra remains little changed. It is therefore expected that the PAM network may efficiently protect CdTe QDs against aggregation and decomposition. For the measurement of fluorescence images of the CdTe–PAM composites, casting films were fabricated on microscope glass slides. The fluorescence images of green CdTe–PAM composites were taken on a fluorescence microscope excited by an ultraviolet source (Fig. 5a). The PAM polymers loaded with CdTe QDs self-assemble into dendritic morphologies. Further observation shows that each dendrite consists of a long central backbone and stretched secondary and tertiary branches. The fluorescence color of the CdTe–PAM composites can easily be tuned by embedding red emitting QDs instead of green ones, and the fluorescence images of red CdTe–PAM composites are shown in Fig. 5b. The fluorescence signals can only be detected at the positions of PAM, which suggests that the CdTe QDs have been combined with PAM successfully. In summary, we have fabricated water-soluble quantum dot–polymer composites by incorporating TGA-capped CdTe QDs into PAM. The incorporation of CdTe QDs in transparent PAM did not change the optical properties of CdTe QDs, and the strong photoluminescence of CdTe QDs were preserved in the quantum dot–polymer composites. The slightly blue shift on the emission peak of the CdTe–PAM composites was ascribed to the covalent
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Fig. 4. The photoluminescence spectra of CdTe–PAM composites with green (a) and red (b) emissions. The photoluminescence spectra of simple CdTe QDs (dashed line) and CdTe–PAM composites (line) in aqueous solution.
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Fig. 5. The fluorescent images of PAM loaded with green (a) and red (b) CdTe QDs.
bonding between the carboxyl groups of TGA capped on QDs and the amide groups of the PAM chains. The PAM polymers loaded with CdTe QDs showed a self-organized architecture with dendritic characteristics, composed of a long central backbone, stretched secondary and arranged tertiary branches. The introduction of QDs into PAM polymers gives a simple route to design fluorescent quantum dot–polymer composites, which may find many ever-growing applications in the biological labels, detection and many other fields of nanotechnology. Acknowledgments The authors would like to thank the 973 projects of MOST (Grant no. 2002CB713802), Grant nos. 30370383, 90606001, and 90406024 of NAFC of China and the team fund of China Ministry of Education (705040) for financial supports.
References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] A. Eychmu¨ller, J. Phys. Chem. B 104 (2000) 6514. [3] N. Gaponik, D.V. Talapin, A.L. Rogach, A. Eychmu¨ller, H. Weller, Nano Lett. 2 (2002) 803. [4] H. Zhang, Z.C. Cui, Y. Wang, K. Zhang, X.L. Ji, C.L. Lu, B. Yang, M.Y. Gao, Adv. Mater. 15 (2003) 777. [5] H. Zhang, C.L. Wang, M.J. Li, J.H. Zhang, G. Lu, B. Yang, Adv. Mater. 17 (2005) 853. [6] H. Zhang, C.L. Wang, M.J. Li, X.L. Ji, J.H. Zhang, B. Yang, Chem. Mater. 17 (2005) 4783. [7] J. Li, X. Hong, Y. Liu, D. Li, Y.W. Wang, J.H. Li, Y.B. Bai, T.J. Li, Adv. Mater. 17 (2005) 163. [8] M. Kuang, D.Y. Wang, H.B. Bao, M.Y. Gao, H. Mohwald, M. Jiang, Adv. Mater. 17 (2005) 267. [9] Y.J. Gong, M.Y. Gao, D.Y. Wang, H. Mohwald, Chem. Mater. 17 (2005) 2648. [10] H. Zhang, Z. Zhou, B. Yang, M.Y. Gao, J. Phys. Chem. B 107 (2003) 8. [11] J. Lee, V.C. Sundar, J.R. Heine, M.G. Bawendi, K.F. Jensen, Adv. Mater. 12 (2000) 1102.