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Novel β-cyclodextrin modified quantum dots as fluorescent probes for polycyclic aromatic hydrocarbons (PAHs)
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Chinese Chemical Letters 19 (2008) 215–218 www.elsevier.com/locate/cclet
Novel b-cyclodextrin modified quantum dots as fluorescent probes for polycyclic aromatic hydrocarbons (PAHs) Cui Ping Han, Hai Bing Li * Key Laboratory of Pesticide & Chemical Biology (CCNU), Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, China Received 3 September 2007
Abstract Water-soluble CdSe/ZnS quantum dots (QDs) were prepared via a simple sonochemical procedure using b-cyclodextrin (CD) as surface coating agent. The QDs displayed a sensitive emission enhancement for anthracene over other related polycyclic aromatic hydrocarbons, and the detection limit was around 1.6 108 mol/L. # 2007 Hai Bing Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Quantum dots; Cyclodextrin; PAHs; Detection
Quantum dots (QDs) have been attracting broad range of attention owing to their novel optical, electrical, and catalytic properties [1]. Many studies are focused on the development of new techniques to synthesize high-quality quantum dots with high luminescence quantum yields and photophysical properties in different media [2,3]. Photoluminescence QDs can respond, via emission variation, to the presence of different analytes, affording new methodologies for spectrochemical analysis. Introduction of organic ligand on nanoparticles surface affords not only the stability of these nanoparticles in different solvents but also the desired surface functionality [4,5]. Cyclodextrins (CDs), because of their special molecular structure and gust inclusion ability, have attracted great interest in supramolecular and organic chemistry. Liu and coworkers have found that the surface of CdS and CdSe/CdS QDs could be modified with monothiolated b-CD to give water-soluble QDs [6,7]. However, the synthesis of thiolated CD was not easy and the fluorescence properties of CD-coated QDs for molecule recognition has not been fully explored. PAHs are pollutants of great environmental concern because of their toxic, mutagenic, and carcinogenic properties [8]. The chemical analysis of PAHs was focused on chromatographic methods. However, these techniques usually need complicated sample pretreatment. Nowadays, as a useful analytical technique, fluorescent detection has been extensively employed with high sensitivity and selectivity. To our knowledge, the use of CD-functionalized QDs as selective probes for fluorescent determination of PAHs is almost unexplored. In this paper, we report the synthesis of water-soluble CdSe/ZnS QDs using b-CD as surface coating agent by a very simple sonochemical method and their potential application as selective fluorescent probes for determination of PAHs.
* Corresponding author. E-mail address: [email protected] (H.B. Li). 1001-8417/$ – see front matter # 2007 Hai Bing Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.11.008
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C.P. Han, H.B. Li / Chinese Chemical Letters 19 (2008) 215–218
1. Experimental Trioctylphosphine oxide (TOPO)-capped CdSe/ZnS QDs were synthesized according to the method developed by Peng et al. [9,10]. The water-soluble CD-QDs were prepared by using the following sonochemical method. Briefly, 0.5 mg of TOPO-capped CdSe/ZnS QDs were dispersed in 1 mL of anhydrous ethanol and 5 mmol of CDs powder was added. The mixture was placed in a high-intensity ultrasonic bath (120 W) for 30 min at room temperature and a rosy precipitate was obtained. The precipitate was separated by centrifuging at 10,000 rpm for 5 min and purified further by cycles of centrifuge in water. The resulting precipitate of the CD-QDs was dispersed in 20 mL water and stored at room temperature for further investigation. 2. Results and discussion The water-soluble CD-QDs were characterized by FT-IR spectroscopy, NMR spectroscopy, luminescence spectroscopy, ultraviolet–visible spectroscopy (UV–vis), transmission electron microscopy (TEM), etc. The FT-IR and 31P NMR spectrum [11] of CD-QDs implies that the TOPO–CD complexation at the QD surface and alter the surface to be hydrophilic, facilitating the phase transfer of QDs into aqueous solutions, as shown in Scheme 1. The absorption and emission spectrum is shown in Fig. 1(a). We observed no obvious shift in emission wavelength and the spectral width of the CD-QDs is almost identical to that of TOPO-QDs. Absorption spectra of CD-QDs exhibits no difference in the position or width of the absorbance bands at 575 nm from hydrophobic QDs, which suggests that the modified CD-QDs in water maintain optical properties of original QDs. It was found that the FL intensity of CD-QDs was decreased in comparison with TOPO-capped QDs. The quenching behavior might be driven by collisions between nanoparticles in ultrasonic bath. The TEM image (Fig. 1(b)) demonstrates that the particles are of monodisperse and uniform in water. The quantum yield (QY) of CD-QDs in water was measured in comparison with the value of Rhodamine B as a criterion (QY = 89%, EtOH) at room temperature, which is about 39%. Fig. 2(a) shows that fluorescent response of CD-QDs to 105 mol/L of PAHs (dissolved in the 2:1 volume of water and ethanol). A significant increase of the CD-QDs emission was observed upon anthracene addition (increased by about 95% as compared to the original value), while other tested PAHs did not. It was found that anthracene enhanced the emission intensity of CD-QDs can be described effectively by a Langmuir-type binding isotherm. The equation can be written as C 1 1 ¼ þ C I BI max I max The dependence of C/I as function of C, where C is the anthracene concentration and I is the fluorescence intensity of the CD-QDs at given anthracene concentrations, is shown in Fig. 2(b). A relative linearity is observed throughout the entire tested concentration range. The binding constant B is found to be 0.47. The surface-tagged CD should played an important role in the recognization of the PAHs, due to CD can complex PAHs easily through hydrophobic interactions and host–guest interaction, etc. [12]. Because the cyclodextrin cavity ˚ ), only the compounds with a similar molecular size and appropriate structure can be has a fixed size (6.0–6.5 A
Scheme 1. Schematic illustration of possible structures of CD-QDs and CD-QDs/anthracene.
C.P. Han, H.B. Li / Chinese Chemical Letters 19 (2008) 215–218
217
Fig. 1. (a) The emission spectra of TOPO-coated CdSe/ZnS QDs in chloroform and (b) CD-QDs in water (e.g. 450 nm). The inset shows the absorption spectra of (a) TOPO-coated CdSe/ZnS QDs in chloroform and (b) CD-QDs in water. (b) TEM images of CD-QDs.
Fig. 2. (a) Effect of 105 mol/L relevant PAHs on the fluorescence of CD-QDs (from 0 to 9: control, acenaphthene, anthracene, fluoranthene, phenanthrene, 9,9-diflurofluorene, carbazole, biphenyl, fluorene, pyrene). (b) Langmuir-binding isotherm description of the data showing a linear fit throughout the anthracene concentration range, the correlation coefficient equal to 0.998.
˚ [12], which fit the cavity of b-CD included into the cavity. According to the literature, the width of anthracene is 5 A well. Therefore, it is no strange that anthracene affects the luminescence of CD-QDs more evidently. The observed luminescence emission enhancement in the presence of anthracene may be correlated to the structural changes of the CD-group shell surrounding the QDs core upon CD–anthracene intercalation. It might lead to the generation of a new and efficient radiative path involving the bound anthracene and/or from the suppression of a nonradiative process. When adding anthracene to CD-QDs, the anthracene intercalation restricts the CD conformation, inducing a uniform arrangement. The ordered orientation and/or the enhanced conformational rigidity of the surface subsistent may suppress the quenching path to the medium and thus increase the luminescence intensity [13], as shown in Scheme 1.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20602015), Program for Distinguish Young Scientist of Hubei Province (2007ABB017) and Program for Chenguang Young Scientist for Wuhan (No. 200750731283).
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C.P. Han, H.B. Li / Chinese Chemical Letters 19 (2008) 215–218
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Trindale, P. O’Brien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. D.M. Willard, Anal. Bioanal. Chem. 376 (2003) 284. W.C.W. Chan, S. Nie, Science 281 (1998) 2016. H.B. Li, Y. Zhang, X.Q. Wang, et al. Mater. Lett. 61 (2007) 1474. H.B. Li, F.G. Qu, Chem. Mater. 19 (2007) 4148. K. Palaniappan, S.A. Hackney, J. Liu, Chem. Commun. 23 (2004) 2704. K. Palaniappan, C.H. Xue, G. Arumugam, et al. Chem. Mater. 18 (2006) 1275. J.V. Pothuluri, C.E. Cerniglia, Bioremediation 2 (1998) 461. Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183. Z.A. Peng, X. Peng, J. Am. Chem. Soc. 124 (2002) 3343. In IR spectrum of CD-QD, the characteristic peaks correspond well to those of pure b-CD. For example, ya (C–O–C): 1157 cm1, y (C–C/C– O): 1080 and 1030 cm1, y (O–H): 3400 cm1. However, the IR spectrum of CD-QDs also has subtle differences from the CD can be ascribed to those belonging to TOPO spectra (2852 cm1 for symmetric stretch of CH2 in the alkyl chain of TOPO), and the blue-shift at 1030 cm1 is indicative of an inclusion-type interaction for the CD-QDs complex. In 31P NMR experiments, a 31P signal appears at 36 ppm, which is differ from the 31P signal of free TOPO ligand (47 ppm) and TOPO-capped QDs (25 ppm), corresponds to TOPO–CD complexes on the surface of QDs. [12] X. Wang, M.L. Brusseau, Environ. Sci. Technol. 29 (1995) 2346. [13] N. Herron, A. Suna, Y. Wang, J. Chem. Soc., Dalton Trans. (1992) 2329.
Chinese Chemical Letters 19 (2008) 215–218 www.elsevier.com/locate/cclet
Novel b-cyclodextrin modified quantum dots as fluorescent probes for polycyclic aromatic hydrocarbons (PAHs) Cui Ping Han, Hai Bing Li * Key Laboratory of Pesticide & Chemical Biology (CCNU), Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, China Received 3 September 2007
Abstract Water-soluble CdSe/ZnS quantum dots (QDs) were prepared via a simple sonochemical procedure using b-cyclodextrin (CD) as surface coating agent. The QDs displayed a sensitive emission enhancement for anthracene over other related polycyclic aromatic hydrocarbons, and the detection limit was around 1.6 108 mol/L. # 2007 Hai Bing Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Quantum dots; Cyclodextrin; PAHs; Detection
Quantum dots (QDs) have been attracting broad range of attention owing to their novel optical, electrical, and catalytic properties [1]. Many studies are focused on the development of new techniques to synthesize high-quality quantum dots with high luminescence quantum yields and photophysical properties in different media [2,3]. Photoluminescence QDs can respond, via emission variation, to the presence of different analytes, affording new methodologies for spectrochemical analysis. Introduction of organic ligand on nanoparticles surface affords not only the stability of these nanoparticles in different solvents but also the desired surface functionality [4,5]. Cyclodextrins (CDs), because of their special molecular structure and gust inclusion ability, have attracted great interest in supramolecular and organic chemistry. Liu and coworkers have found that the surface of CdS and CdSe/CdS QDs could be modified with monothiolated b-CD to give water-soluble QDs [6,7]. However, the synthesis of thiolated CD was not easy and the fluorescence properties of CD-coated QDs for molecule recognition has not been fully explored. PAHs are pollutants of great environmental concern because of their toxic, mutagenic, and carcinogenic properties [8]. The chemical analysis of PAHs was focused on chromatographic methods. However, these techniques usually need complicated sample pretreatment. Nowadays, as a useful analytical technique, fluorescent detection has been extensively employed with high sensitivity and selectivity. To our knowledge, the use of CD-functionalized QDs as selective probes for fluorescent determination of PAHs is almost unexplored. In this paper, we report the synthesis of water-soluble CdSe/ZnS QDs using b-CD as surface coating agent by a very simple sonochemical method and their potential application as selective fluorescent probes for determination of PAHs.
* Corresponding author. E-mail address: [email protected] (H.B. Li). 1001-8417/$ – see front matter # 2007 Hai Bing Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.11.008
216
C.P. Han, H.B. Li / Chinese Chemical Letters 19 (2008) 215–218
1. Experimental Trioctylphosphine oxide (TOPO)-capped CdSe/ZnS QDs were synthesized according to the method developed by Peng et al. [9,10]. The water-soluble CD-QDs were prepared by using the following sonochemical method. Briefly, 0.5 mg of TOPO-capped CdSe/ZnS QDs were dispersed in 1 mL of anhydrous ethanol and 5 mmol of CDs powder was added. The mixture was placed in a high-intensity ultrasonic bath (120 W) for 30 min at room temperature and a rosy precipitate was obtained. The precipitate was separated by centrifuging at 10,000 rpm for 5 min and purified further by cycles of centrifuge in water. The resulting precipitate of the CD-QDs was dispersed in 20 mL water and stored at room temperature for further investigation. 2. Results and discussion The water-soluble CD-QDs were characterized by FT-IR spectroscopy, NMR spectroscopy, luminescence spectroscopy, ultraviolet–visible spectroscopy (UV–vis), transmission electron microscopy (TEM), etc. The FT-IR and 31P NMR spectrum [11] of CD-QDs implies that the TOPO–CD complexation at the QD surface and alter the surface to be hydrophilic, facilitating the phase transfer of QDs into aqueous solutions, as shown in Scheme 1. The absorption and emission spectrum is shown in Fig. 1(a). We observed no obvious shift in emission wavelength and the spectral width of the CD-QDs is almost identical to that of TOPO-QDs. Absorption spectra of CD-QDs exhibits no difference in the position or width of the absorbance bands at 575 nm from hydrophobic QDs, which suggests that the modified CD-QDs in water maintain optical properties of original QDs. It was found that the FL intensity of CD-QDs was decreased in comparison with TOPO-capped QDs. The quenching behavior might be driven by collisions between nanoparticles in ultrasonic bath. The TEM image (Fig. 1(b)) demonstrates that the particles are of monodisperse and uniform in water. The quantum yield (QY) of CD-QDs in water was measured in comparison with the value of Rhodamine B as a criterion (QY = 89%, EtOH) at room temperature, which is about 39%. Fig. 2(a) shows that fluorescent response of CD-QDs to 105 mol/L of PAHs (dissolved in the 2:1 volume of water and ethanol). A significant increase of the CD-QDs emission was observed upon anthracene addition (increased by about 95% as compared to the original value), while other tested PAHs did not. It was found that anthracene enhanced the emission intensity of CD-QDs can be described effectively by a Langmuir-type binding isotherm. The equation can be written as C 1 1 ¼ þ C I BI max I max The dependence of C/I as function of C, where C is the anthracene concentration and I is the fluorescence intensity of the CD-QDs at given anthracene concentrations, is shown in Fig. 2(b). A relative linearity is observed throughout the entire tested concentration range. The binding constant B is found to be 0.47. The surface-tagged CD should played an important role in the recognization of the PAHs, due to CD can complex PAHs easily through hydrophobic interactions and host–guest interaction, etc. [12]. Because the cyclodextrin cavity ˚ ), only the compounds with a similar molecular size and appropriate structure can be has a fixed size (6.0–6.5 A
Scheme 1. Schematic illustration of possible structures of CD-QDs and CD-QDs/anthracene.
C.P. Han, H.B. Li / Chinese Chemical Letters 19 (2008) 215–218
217
Fig. 1. (a) The emission spectra of TOPO-coated CdSe/ZnS QDs in chloroform and (b) CD-QDs in water (e.g. 450 nm). The inset shows the absorption spectra of (a) TOPO-coated CdSe/ZnS QDs in chloroform and (b) CD-QDs in water. (b) TEM images of CD-QDs.
Fig. 2. (a) Effect of 105 mol/L relevant PAHs on the fluorescence of CD-QDs (from 0 to 9: control, acenaphthene, anthracene, fluoranthene, phenanthrene, 9,9-diflurofluorene, carbazole, biphenyl, fluorene, pyrene). (b) Langmuir-binding isotherm description of the data showing a linear fit throughout the anthracene concentration range, the correlation coefficient equal to 0.998.
˚ [12], which fit the cavity of b-CD included into the cavity. According to the literature, the width of anthracene is 5 A well. Therefore, it is no strange that anthracene affects the luminescence of CD-QDs more evidently. The observed luminescence emission enhancement in the presence of anthracene may be correlated to the structural changes of the CD-group shell surrounding the QDs core upon CD–anthracene intercalation. It might lead to the generation of a new and efficient radiative path involving the bound anthracene and/or from the suppression of a nonradiative process. When adding anthracene to CD-QDs, the anthracene intercalation restricts the CD conformation, inducing a uniform arrangement. The ordered orientation and/or the enhanced conformational rigidity of the surface subsistent may suppress the quenching path to the medium and thus increase the luminescence intensity [13], as shown in Scheme 1.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20602015), Program for Distinguish Young Scientist of Hubei Province (2007ABB017) and Program for Chenguang Young Scientist for Wuhan (No. 200750731283).
218
C.P. Han, H.B. Li / Chinese Chemical Letters 19 (2008) 215–218
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Trindale, P. O’Brien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. D.M. Willard, Anal. Bioanal. Chem. 376 (2003) 284. W.C.W. Chan, S. Nie, Science 281 (1998) 2016. H.B. Li, Y. Zhang, X.Q. Wang, et al. Mater. Lett. 61 (2007) 1474. H.B. Li, F.G. Qu, Chem. Mater. 19 (2007) 4148. K. Palaniappan, S.A. Hackney, J. Liu, Chem. Commun. 23 (2004) 2704. K. Palaniappan, C.H. Xue, G. Arumugam, et al. Chem. Mater. 18 (2006) 1275. J.V. Pothuluri, C.E. Cerniglia, Bioremediation 2 (1998) 461. Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183. Z.A. Peng, X. Peng, J. Am. Chem. Soc. 124 (2002) 3343. In IR spectrum of CD-QD, the characteristic peaks correspond well to those of pure b-CD. For example, ya (C–O–C): 1157 cm1, y (C–C/C– O): 1080 and 1030 cm1, y (O–H): 3400 cm1. However, the IR spectrum of CD-QDs also has subtle differences from the CD can be ascribed to those belonging to TOPO spectra (2852 cm1 for symmetric stretch of CH2 in the alkyl chain of TOPO), and the blue-shift at 1030 cm1 is indicative of an inclusion-type interaction for the CD-QDs complex. In 31P NMR experiments, a 31P signal appears at 36 ppm, which is differ from the 31P signal of free TOPO ligand (47 ppm) and TOPO-capped QDs (25 ppm), corresponds to TOPO–CD complexes on the surface of QDs. [12] X. Wang, M.L. Brusseau, Environ. Sci. Technol. 29 (1995) 2346. [13] N. Herron, A. Suna, Y. Wang, J. Chem. Soc., Dalton Trans. (1992) 2329.