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Functionalized ME-capped CdSe quantum dots based luminescence probe for detection of Ba2+ ions
Sensors and Actuators B 164 (2012) 76–81
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Functionalized ME-capped CdSe quantum dots based luminescence probe for detection of Ba2+ ions Waleed E. Mahmoud ∗ King Abdulaziz University, Faculty of Science, Physics Department, Jeddah, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 18 November 2011 Received in revised form 14 January 2012 Accepted 31 January 2012 Available online 8 February 2012 Keywords: Nanosensor Mercaptoethanol CdSe QDs Fluorescence Barium ions
a b s t r a c t Water soluble CdSe quantum dots (QDs) have been synthesized using 2-mercaptoethanol (ME) as surface passivation through one step process by using safe and low cost materials at low temperatures 75 ◦ C. CdSe QDs evaluated as a luminescence probe for barium ions in aqueous solution. Experiment results showed that the fluorescence emission from CdSe QDs was enhanced significantly by barium ions, while other metal ions exhibited no significant effect on QDs. Under the optimal conditions, the response was linearly proportional to the concentration of barium ions ranging from 1.0 × 10−7 to 1.2 × 10−6 mol L−1 with detection limit 4.2 × 10−9 mol L−1 . Based on the distinct optical properties of ME-capped CdSe QDs with the barium ions, the high sensitivity, wide linear range and rapid response to metal ions, CdSe QDs can be developed as a potential identified luminescence probe for detection of barium ions in aqueous solution without interference from other foreign metal ions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The demand for developing of selective and highly sensitive chemical sensors to monitor the metal ions is of great interest. A lot of fluorescent organic probes and sensors were designed for the detection of different metal ions. However, most of them tend to have some limitations such as low fluorescence intensity, photo-bleaching, narrow excitation spectra and often exhibit broad emission bands with red tailing [1–4]. Semiconductor quantum dots have been recognized as an alternative fluorescent probes instead of organic dyes due to their unique optical and electronic properties such as broad excitation spectra, narrow symmetric and tunable emission spectra which can span from the ultraviolet to the infrared region [5,6]. There are few reports on chemical sensing of metal ions based on the use of the QDs have been reported in the literature. Chen and Rosenzweig [7] have reported the first example involving Cu2+ and Zn2+ ions analysis in the aquatic media using the CdS QDs capped with different ligands. Gattas-Asfura and Leblanc [8] have reported peptide coated CdS QDs as luminescence probe for Cu(II) and Ag(I). Zhang et al. [9] have reported the l-cysteine
∗ Correspondence address: Suez Canal University, Faculty of Science, Physics Department, Ismailia, Egypt. Tel.: +966 560 86 0583. E-mail address: w e [email protected] 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.01.073
capped CdSe/CdS QDs for the detection of Cu(II) ion in aqueous media. Koneswaran and Narayanaswamy [10] have reported l-cysteine-capped ZnS QDs for Cu(II) ion is higher than that of the l-cysteine-capped CdSe/CdS QDs. Molnar et al. [11] reported that both of K+ and Na+ ions enhancing the fluorescence of colloidal CdSe/CdS and CdSe/ZnS quantum dots. Koneswaran and Narayanaswamy [12] have reported that mercaptoacetic acidcapped CdS QDs can be used as fluorescence probe shot for Hg(II) ion. Soluble barium compounds are poisonous. At low doses, barium acts as a muscle stimulant, whereas higher doses affect the nervous system, causing cardiac irregularities, tremors, weakness, anxiety, dyspnea and paralysis. This may be due to its ability to block potassium ion channels which are critical to the proper function of the nervous system. Until now, as far as we know, there is no researchers reported the detection of barium ions by using 2-mercaptoethanol capped CdSe quantum dots. Herein, we report the synthesis of 2-mercaptoethanol capped CdSe quantum dots as fluorescence sensor for barium ions for the first time. For our knowledge, this is first such report based on the use of 2-mercaptoethanol capped CdSe QDs for chemical sensing. 2-mercaptoethanol is a water soluble compound and used as a capping agent for CdSe QDs. The surface modification of CdSe QDs with 2-mercaptoethanol prevents the aggregation of nanoparticles and makes them available for the interaction with the target materials. Furthermore, it increases the emission quantum yields of QDs and also stabilizes the nanoparticles.
W.E. Mahmoud / Sensors and Actuators B 164 (2012) 76–81
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Fig. 1. (a) XRD and (b) TEM image of the prepared 2-mercaptoethanol capped CdSe quantum dot at temperature T = 75 ◦ C.
2. Experimental 2.1. Materials Cadmium chloride (CdCl2 ·5H2 O), selenium, sodium sulfite (Na2 SO3 ), 2-mercaptoethanol (C2 H6 SO) and tetra-propylammonium hydroxide (C3 H7 )4 NOH were purchased from Sigma–Aldrich and used without further purification. All the chemicals used were of analytical grade and all the solutions were prepared with double distilled deionized water. 2.2. Preparations of 2-mercaptoethanol capped CdSe quantum dots In a typical synthesis as in our previous work [13], a sodium selenosulfate (Na2 SeSO3 ) solution was obtained by combining Se powder and Na2 SO3 powder into 20 ml water (molar ration of Se:Na2 SO3 = 1:4). An aqueous solution was prepared by dissolving cadmium chloride into 250 ml deionized water. 2-mercaptoethanol (ME) was added to cadmium solution at ratio (Cd:ME = 1:2) into three-neck flask. After the addition of the 2-mercaptoethanol, the pH was adjusted by using tetra-propyl-ammonium hydroxide (CH3 CH2 CH2 )4 NOH solution. For the growth of the CdSe QDs, the solution of Na2 SeSO3 was swiftly injected into the cadmium-2mercaptoethanol solution with constant stirring under bubbling with N2 for 30 min. After that, the temperature was increased to 75 ◦ C for the growth of the CdSe QDs. The color of reaction solution changed almost immediately after the injection of the Na2 SeSO3 solution, which indicated fast in nucleation and growth. The CdSe QDs were then precipitated by adding methanol into the toluene dispersion and were separated and purified by centrifugation. Scheme 1 illustrates the mechanism of formation of ME-capped CdSe QDs. 2.3. Characterization of 2-mercaptoethanol capped CdSe quantum dots The X-ray measurements were performed using Philips X’pert ˚ diffractometer supplied with copper X-ray tube (k␣1 = 1.5406 A), nickel filter, graphite crystal monochromator, proportional counter detector, divergence slit 1◦ and 0.1 mm receiving slit. The working conditions were 40 kV and 30 mA for the X ray tube, scan speed 0.05◦ and 2 s measuring time per step. For each measurement, a complete scan was made between 20◦ and 70◦ (2 ◦ ). To calibrate the measured Bragg 2-angles, a standard reference material (SRM 640a) of pure Si powder was used. High-resolution transmission electron microscopy (HR-TEM) of CdSe nanocrystals was carried out using a JEOL 2010 high-resolution transmission electron microscope operated at 200 kV. To prepare the HR-TEM samples, purified
CdSe nanopowders were first dispersed in methanol and diluted, followed by placing a droplet of the solution onto a 400-mesh carbon-coated copper grid. The grid was then dried in desiccators for 1 day before imaging.
2.4. Optical measurements The optical properties of CdSe quantum dots were studied through fluorescence measurements. The fluorescence was measured at room temperature using Perkin Elmer LS 55 Fluorescence Spectrometer. The slit widths used for excitation and emission were 5 nm. These fluorescence measurements were carried out by preparing a suspension solution of 1.2 mg L−1 CdSe quantum dots in pH 9.0 buffer solution and sonicated for 30 min at ambient temperature until homogenous suspension formed. Different concentrations of Alkali and transition metal ions solution were added into CdSe quantum dot solution and mixed thoroughly. These solutions put into quartz cuvette for fluorescence measurements. The fluorescent intensity of the solutions was recorded at excitation wavelength of 365 nm.
3. Results and discussion 3.1. Characterization of 2-mercaptoethanol capped CdSe QDs Fig. 1a depicts the X-ray diffraction for the prepared CdSe quantum dots. The diffraction peaks at angles (2) of 25.2, 41.9 and 49.7◦ correspond to the reflection planes (1 1 1), (2 2 0) and (3 1 1), respectively. The XRD pattern is identical to the zinc blend phase with cubic structure which has unit cell parameter a = 6.09 A˚ [JCPDS card no. 19-0191]. Fig. 1b shows the typical TEM image of the assynthesized CdSe QDs at temperature 75 ◦ C. The TEM measurement indicated that the size distribution of the as-synthesized CdSe QDs was nearly mono-disperse with the average size of 4.1 nm. FT-IR spectra of free 2-mercaptoethanol and functionalized 2mercaptoethanol capped CdSe QDs are shown in Fig. 2. The most pronounced IR absorption bands occurred at 3423 cm−1 ( OH), 2930–2914 cm−1 ( CH2 ), 2550–2670 cm−1 ( S H), 1661 cm−1 ( C C), 1402 cm−1 ( CH2 ), 1176 cm−1 ( C O) for free 2mercaptoethanol and at 3419 cm−1 ( OH), 2921 cm−1 ( CH2 ), 1400 cm−1 ( CH2 ), 1180 cm−1 ( C O) for 2-mercaptoethanol capped CdSe QDs. IR spectra of both 2-mercaptoethanol capped CdSe QDs and free 2-mercaptoethanol showed absorption peaks for hydroxyl and carbonyl groups. This indicates hydroxyl and carbonyl groups coexistence on the surface of the CdSe QDs. While the peak for S H (2538 cm−1 ) was absent in 2-mercaptoethanol capped CdSe QDs. The reason for the disappearance of S H vibration in IR spectra of 2-mercaptoethanol capped CdSe QDs was the
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W.E. Mahmoud / Sensors and Actuators B 164 (2012) 76–81
Scheme 1. The mechanism of formation of ME-capped CdSe QDs.
3.2. Effect of pH on the fluorescence intensity
Fig. 2. FT-IR spectra for free 2-mercaptoethanol and 2-mercaptoethanol capped CdSe QDs.
result of the covalent bonding between thiols and Cd atom on the QDs surface. Functionalized 2-mercaptoethanol capped CdSe QDs are optically characterized by UV–vis absorption spectroscopy and fluorometry. The absorption and fluorescence spectra of 2mercaptoethanol capped CdSe are depicted in Fig. 3. The absorption peak occurs at 546 nm and emission maxima obtained at 569 nm with the excitation of 365 nm. The small stokes shift (23 nm) between absorption and emission peaks confirmed that the 2mercaptoethanol reduced the surface defect of CdSe QDs [13,14].
Fig. 3. Absorption and fluorescence of the prepared 2-mercaptoethanol capped CdSe quantum dot at temperature T = 75 ◦ C.
The influence of pH along the range between 3 and 12 was examined to determine the optimum conditions for the determination of Ba2+ ions with the 2-mercaptoethanol capped CdSe QDs. Fig. 4 shows that the pH of the solution enhanced the emission intensity of the 2-mercaptoethanol capped CdSe QDs with optimum at pH 9. The reason for the increase of luminescence as the pH increases up to 9 may be attributed to more thiol groups became dehydrogenated, which was expected to strengthen the covalent bond between ME and Cd2+ on the surface of QDs. Meanwhile, the higher pH also promoted the negative charge of hydroxyl groups of ME and helped disperse the nanoparticles better. Both these effects would provide better surface protection for the aqueous CdSe QDs and make them emit photons more efficiently with less non-radiative loss. Furthermore, the solubility of CdSe decreases with increasing pH [15], which benefited the nucleation and growth of QDs and therefore generated more nanoparticles. As a result, higher pH could help improve the PL intensity of the aqueous CdSe QDs. Fig. 4 also shows that the fluorescence intensity of CdSe QDs decreased when the pH was greater than 9. This may be due to the formation of hydrated product, which decreases the fluorescence intensity. Furthermore, cadmium ions are precipitated as their hydroxides at higher pH which also may reduce the fluorescence intensity. Therefore, a pH of 9 was chosen as optimum pH in this analytical study. The influence of various buffers and their ionic strengths on the fluorescence intensity of QDs was investigated. The experimental results showed that the maximum and stable fluorescence intensity occurred when the tetra-propyl-ammonium hydroxide (TPAH) buffer was in the concentration of 0.03 M. Hence, a concentration of 0.03 M TPAH was used as a buffer to adjust the pH of aqueous medium.
Fig. 4. The effect of pH on the fluorescence intensity of ME capped CdSe QDs.
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Fig. 5. The effect of ME/Cd molar ratio on the fluorescence intensity the QDs.
3.3. Effect of ME/Cd molar ratio In order to examine the effect of capping agent concentration on the FL properties of CdSe quantum dots, several batches of samples were synthesized with different ME:Cd molar ratios, while the Cd:Se ratio was maintained as 1:1. It is obvious from Fig. 5 that, the ratio of ME:Cd = 2:1 is optimal which has the highest FL intensity. At ME:Cd = 0.5:1 and 1:1, the PL intensity was lower than that of ME:Cd = 2:1, probably because the CdSe QDs were not covered by ME sufficiently. As a result, more clusters of QDs were generated due to the poor dispersion, and the emission intensity was reduced by non-radiative energy transfer. On the other hand, at ME:Cd = 3:1 and 4:1, the high concentration of ME molecules in the suspension resulted in lower FL intensity as well. There are several possible reasons for this effect. It suggested that, for the as-synthesized QDs at higher capping agent concentrations, the thiol groups tend to react with each other and form S2+ ions which act as trap sites for electrons and in turn reduce the emission intensity. On the other hand, higher concentration might promote the formation of QDs clusters. The poor dispersion would introduce more non-radiative energy dissipation and reduce the FL intensity as well [16]. Therefore, a molar ratio of ME/Cd = 2 was adopted in this study. 3.4. Photostability of 2-mercaptoethanol capped CdSe QDs For chemical sensor applications, the influence of temperature and stored time on the photo stability of 2-mercaptoethanol capped CdSe QDs is critical. The 2-mercaptoethanol capped CdSe QD was stored at 4 ◦ C in darkness and the changes of its fluorescence intensities were monitored at ambient temperature for up to 30 days, as indicated in Fig. 6a. It is clear that, the CdSe QDs remained stable over time, with no observable degradation of the fluorescence. Therefore, the present aqueous CdSe QDs were demonstrated to have excellent stability showing their potential applications as a sensor for long term measuring and tracking. It was found that the fluorescence behavior of the aqueous 2-mercaptoethanol capped CdSe QDs were dependent on the temperature during measurement, where the fluorescence intensity decreased with the increase of temperature as shown in Fig. 6b. The 2-mercaptoethanol capped CdSe QD was cooled down and stored at 4 ◦ C overnight, and then was applied with another cycle of temperature change. Interestingly found that the fluorescence dependence on temperature was reversible and repeatable for successive cycles, where the emission intensity followed almost the same temperature-dependent trend for all four cycles. Similar emission intensity decrease with increasing temperature was reported in other QDs systems [17]. The phenomenon was attributed to the enhanced exciton–phonon interaction at higher temperature.
Fig. 6. The influence of (a) storing time and (b) temperature on the fluorescence intensity of the prepared 2-mercaptoethanol capped CdSe quantum dots.
Therefore the energy was more dissipated in a non-radiative mode rather than contributing to fluorescence emission via the electron–hole recombination. 3.5. Detection of barium ion using ME capped CdSe QDs probe The effect of Ba(II) ions on the fluorescence emission of the 2mercaptoethanol capped CdSe QDs is shown in Fig. 7a and b. The fluorescence intensity is significantly enhanced by the addition of Ba(II) ion. The enhancement effect of Ba2+ ion on the fluorescence emission of CdSe QDs is found to be concentration dependence. Therefore, this ensemble can be used for the development of a sensitive and selective method for Ba2+ ion sensor. The increase of FL intensity may be attributed to the host–phosphor interaction between Ba2+ ions and CdSe QDs. As shown in Scheme 2, the barium cation interact with the hydroxyl group of ME and forming Barium oxide on the surface of CdSe QDs. This barium oxide is well known as a good matrix for hosting luminescence materials. Therefore enhance the transition energy transfer which leads the increase of population of inversion symmetry of the QDs [18]. The enhancement of fluorescence intensity of 2mercaptoethanol capped CdSe QDs is well linearly proportional to the Ba2+ ion concentration along the range from 1.0 × 10−7 to 1.2 × 10−7 mol L−1 as indicated in Fig. 8. The following linear regression equation fits well the experimental data I = 1 + KC Io
(2)
where I is the FL intensity of quantum dots in the presence of Ba(II) ion, Io is the FL intensity of quantum dot in the absence of Ba(II) ion, K is constant and C is the concentration of ions. The linear
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W.E. Mahmoud / Sensors and Actuators B 164 (2012) 76–81
Scheme 2. The proposed interaction between ME capped CdSe QD and barium ions.
correlation coefficient is 0.999. The detection limit (3) according to TUPAC was 4.2 × 10−9 mol L−1 , showing that the presented method is much more sensitive than some of the current luminescent approaches for other metal ions detection [19]. 3.6. Influence of foreign metal ions In order to adopt the functionalized CdSe QDs as a sensing probe for the Ba(II) ion, it is important to examine the effect of some common foreign ions that are naturally abundant in the environment. Fig. 9 shows the effect of some metal ions, at fixed concentration (0.5 M), on the FL intensity of 2-mercaptoethanol capped CdSe QDs. It is clear that, Na+ , Ni2+ , Co2+ , Pb2+ , Zn2+ and Cd2+ ions do not show any effect on the FL intensity of the 2-mercaptoethanol capped CdSe QDs. K+ , Ca2+ and Mg2+ ions show small change on
Fig. 8. The linear relationship between the relative fluorescence intensity of the 2-mercaptoethanol capped CdSe quantum dots and Ba2+ concentration.
the FL intensity of 2-mercaptoethanol capped CdSe QDs. As compared with Ba2+ ion solutions, the influence of these metal ions is comparatively weak. The relative standard deviation for ten replicate determinations of Ba2+ ion at a concentration of 1 × 10−6 M was estimated as 2.1%. The presence of foreign ion in 1:100 and 1:1000 ratios produced lower interference than 2.1%. Previous reports [20,21] stated that, ions causing errors more than 3.7% are considered interference.
Fig. 7. The influence of barium ions concentration on the fluorescence intensity of the 2-mercaptoethanol capped CdSe quantum dots.
Fig. 9. Effect of metal ions on fluorescence intensity of 2-mercaptoethanol-capped CdSe QDs, the concentration of 2-mercaptoethanol-capped CdSe QDs: 1.2 mg L−1 and concentration of metal ions: 0.5 M at pH 9.0.
W.E. Mahmoud / Sensors and Actuators B 164 (2012) 76–81
4. Conclusion The functionalized 2-mercaptoethanol-capped CdSe QDs have been developed as a novel fluorescence nanosensor for Ba(II) ion in aqueous medium. The fluorescence sensor for Ba(II) ion is based on the fluorescence enhancement of 2-mercaptoethanol-capped CdSe QDs. The maximum fluorescence intensity was obtained at the 2-mercaptoethanol-capped CdSe QDs concentration of 1.2 mg L−1 and at a pH 9.0. The detection limit for this sensor system was 4.2 × 10−9 M. The results showed that 2-mercaptoethanol-capped CdSe QDs fluorescence was enhanced by Ba2+ ion by 5 times through very low concentration ranging from 0.1 M to 1.2 M. The 2-mercaptoethanol-capped CdSe QDs showed a facile synthesis, low cost, high sensitivity and also the analytical applications of this sensor are very simple. References [1] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Quantum dot bioconjugates for imaging, labelling and sensing, Nat. Mater. 4 (2005) 435–446. [2] M. Gao, S. Kirstein, H. Mohwald, A.L. Rogach, A. Kornowski, A. Eychmuller, H. Weller, Strongly photoluminescent CdTe nanocrystals by proper surface modification, J. Phys. Chem. B 102 (1998) 8360–8363. [3] J. Drbohlavova, V. Adam, R. Kizek, J. Hubalek, Quantum dots—characterization, preparation and usage in biological systems, Int. J. Mol. Sci. 10 (2009) 656–673. [4] G.-Y. Lan, Y.W. Lin, Y.F. Huang, H.T. Chang, Photo-assisted synthesis of highly fluorescent ZnSe(S) quantum dots in aqueous solution, J. Mater. Chem. 17 (2007) 2661–2666. [5] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science 281 (1998) 2013–2016. [6] W.C.W. Chen, S. Nie, Quantum dot bioconjugates for ultrasensitive nonisotopic detection, Science 281 (1998) 2016–2018. [7] Y.F. Chen, Z. Rosenzweig, Luminescent CdS quantum dots as selective ion probes, Anal. Chem. 74 (2002) 5132–5138. [8] K.M. Gattas-Asfura, R.M. Leblanc, Peptide-coated CdS quantum dots for the optical detection of copper(II) and silver(I), Chem. Commun. 268 (2003) 2684–2685. [9] Y.H. Zhang, H.S. Zhang, X.F. Guo, H. Wang, l-Cysteine-coated CdSe/CdS core–shell quantum dots as selective fluorescence probe for copper(II) determination, Microchem. J. 89 (2008) 142–147.
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[10] M. Koneswaran, R. Narayanaswamy, l-Cysteine-capped ZnS quantum dots based fluorescence sensor for Cu2+ ion, Sens. Actuators B 139 (2009) 104–109. [11] M. Molnar, Z.J. Ning, Y. Chen, P. Friberg, L.M. Gan, Y. Fu, Effects of K+ and Na+ ions on the fluorescence of colloidal CdSe/CdS and CdSe/ZnS quantum dots, Sens. Actuators B 155 (2011) 823–830. [12] M. Koneswaran, R. Narayanaswamy, Mercaptoacetic acid capped CdS quantum dots as fluorescence single shot probe for mercury(II), Sens. Actuators B 139 (2009) 91–96. [13] A.M. Al-Amri, S.J. Yaghmour, W.E. Mahmoud, Low temperature growth of metastable cubic CdSe nanocrystals and their photoluminescence properties, J. Cryst. Growth 334 (2011) 76–79. [14] W.E. Mahmoud, Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets, J. Cryst. Growth 312 (2010) 3075–3079. [15] V. Biju, Y. Makita, A. Sonoda, H. Yokoyama, Y. Baba, M. Ishikawa, Temperature sensitive photoluminescence of CdSe quantum dot clusters, J. Phys. Chem. B 109 (2005) 13899–13905. [16] L. Turyanska, A. Patane, M. Henini, B. Hennequin, N.R. Thomas, Temperature dependence of the photoluminescence emission from thiol-capped PbS quantum dots, Appl. Phys. Lett. 90 (2007) 10. [17] A. Narayanaswamy, L.F. Feiner, P.J. van der Zaag, Temperature dependence of the photoluminescence of InP/ZnS quantum dots, J. Phys. Chem. C 112 (2008) 6775–6780. [18] R. Pazik, R.J. Wiglusz, W. Strek, Luminescence properties of BaTiO3 :Eu3+ obtained via microwave stimulated hydrothermal method, Mater. Res. Bull. 44 (2009) 1328–1333. [19] J. Chen, A. Zheng, Y. Gao, C. He, G. Wu, Y. Chen, X. Kai, C. Zhu, Functionalized CdS quantum dots-based luminescence probe for detection of heavy and transition metal ions in aqueous solution, Spectrochimica Acta A 69 (2008) 1044–1052. [20] J. Chen, Y. Gao, Z. Xu, G. Wu, Y. Chen, C. Zhu, A novel fluorescent array for mercury(II) ion in aqueous solution with functionalized cadmium selenide nanoclusters, Anal. Chim. Acta 577 (2006) 77–84. [21] J. Li, F. Mei, W. Li, X. He, Y. Zhang, Study on the fluorescence resonance energy transfer between CdTe QDs and butyl-rhodamine B in the presence of CTMAB and its application on the detection of Hg(II), Spectrochimica Acta A 70 (2008) 811–817.
Biography Waleed E. Mahmoud received his PhD degree in 2006 in the manufacturing and developing of pressure sensitive nanocomposites. He has years of experience in the fields of optoelectronic devices, dye-sensitized solar cells, pressure sensors, chemosensors and gas sensors. Recently, his research interest includes the synthesis and application of colloidal semiconductor quantum dots as a sensor for DNA and cancer tumors.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Functionalized ME-capped CdSe quantum dots based luminescence probe for detection of Ba2+ ions Waleed E. Mahmoud ∗ King Abdulaziz University, Faculty of Science, Physics Department, Jeddah, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 18 November 2011 Received in revised form 14 January 2012 Accepted 31 January 2012 Available online 8 February 2012 Keywords: Nanosensor Mercaptoethanol CdSe QDs Fluorescence Barium ions
a b s t r a c t Water soluble CdSe quantum dots (QDs) have been synthesized using 2-mercaptoethanol (ME) as surface passivation through one step process by using safe and low cost materials at low temperatures 75 ◦ C. CdSe QDs evaluated as a luminescence probe for barium ions in aqueous solution. Experiment results showed that the fluorescence emission from CdSe QDs was enhanced significantly by barium ions, while other metal ions exhibited no significant effect on QDs. Under the optimal conditions, the response was linearly proportional to the concentration of barium ions ranging from 1.0 × 10−7 to 1.2 × 10−6 mol L−1 with detection limit 4.2 × 10−9 mol L−1 . Based on the distinct optical properties of ME-capped CdSe QDs with the barium ions, the high sensitivity, wide linear range and rapid response to metal ions, CdSe QDs can be developed as a potential identified luminescence probe for detection of barium ions in aqueous solution without interference from other foreign metal ions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The demand for developing of selective and highly sensitive chemical sensors to monitor the metal ions is of great interest. A lot of fluorescent organic probes and sensors were designed for the detection of different metal ions. However, most of them tend to have some limitations such as low fluorescence intensity, photo-bleaching, narrow excitation spectra and often exhibit broad emission bands with red tailing [1–4]. Semiconductor quantum dots have been recognized as an alternative fluorescent probes instead of organic dyes due to their unique optical and electronic properties such as broad excitation spectra, narrow symmetric and tunable emission spectra which can span from the ultraviolet to the infrared region [5,6]. There are few reports on chemical sensing of metal ions based on the use of the QDs have been reported in the literature. Chen and Rosenzweig [7] have reported the first example involving Cu2+ and Zn2+ ions analysis in the aquatic media using the CdS QDs capped with different ligands. Gattas-Asfura and Leblanc [8] have reported peptide coated CdS QDs as luminescence probe for Cu(II) and Ag(I). Zhang et al. [9] have reported the l-cysteine
∗ Correspondence address: Suez Canal University, Faculty of Science, Physics Department, Ismailia, Egypt. Tel.: +966 560 86 0583. E-mail address: w e [email protected] 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.01.073
capped CdSe/CdS QDs for the detection of Cu(II) ion in aqueous media. Koneswaran and Narayanaswamy [10] have reported l-cysteine-capped ZnS QDs for Cu(II) ion is higher than that of the l-cysteine-capped CdSe/CdS QDs. Molnar et al. [11] reported that both of K+ and Na+ ions enhancing the fluorescence of colloidal CdSe/CdS and CdSe/ZnS quantum dots. Koneswaran and Narayanaswamy [12] have reported that mercaptoacetic acidcapped CdS QDs can be used as fluorescence probe shot for Hg(II) ion. Soluble barium compounds are poisonous. At low doses, barium acts as a muscle stimulant, whereas higher doses affect the nervous system, causing cardiac irregularities, tremors, weakness, anxiety, dyspnea and paralysis. This may be due to its ability to block potassium ion channels which are critical to the proper function of the nervous system. Until now, as far as we know, there is no researchers reported the detection of barium ions by using 2-mercaptoethanol capped CdSe quantum dots. Herein, we report the synthesis of 2-mercaptoethanol capped CdSe quantum dots as fluorescence sensor for barium ions for the first time. For our knowledge, this is first such report based on the use of 2-mercaptoethanol capped CdSe QDs for chemical sensing. 2-mercaptoethanol is a water soluble compound and used as a capping agent for CdSe QDs. The surface modification of CdSe QDs with 2-mercaptoethanol prevents the aggregation of nanoparticles and makes them available for the interaction with the target materials. Furthermore, it increases the emission quantum yields of QDs and also stabilizes the nanoparticles.
W.E. Mahmoud / Sensors and Actuators B 164 (2012) 76–81
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Fig. 1. (a) XRD and (b) TEM image of the prepared 2-mercaptoethanol capped CdSe quantum dot at temperature T = 75 ◦ C.
2. Experimental 2.1. Materials Cadmium chloride (CdCl2 ·5H2 O), selenium, sodium sulfite (Na2 SO3 ), 2-mercaptoethanol (C2 H6 SO) and tetra-propylammonium hydroxide (C3 H7 )4 NOH were purchased from Sigma–Aldrich and used without further purification. All the chemicals used were of analytical grade and all the solutions were prepared with double distilled deionized water. 2.2. Preparations of 2-mercaptoethanol capped CdSe quantum dots In a typical synthesis as in our previous work [13], a sodium selenosulfate (Na2 SeSO3 ) solution was obtained by combining Se powder and Na2 SO3 powder into 20 ml water (molar ration of Se:Na2 SO3 = 1:4). An aqueous solution was prepared by dissolving cadmium chloride into 250 ml deionized water. 2-mercaptoethanol (ME) was added to cadmium solution at ratio (Cd:ME = 1:2) into three-neck flask. After the addition of the 2-mercaptoethanol, the pH was adjusted by using tetra-propyl-ammonium hydroxide (CH3 CH2 CH2 )4 NOH solution. For the growth of the CdSe QDs, the solution of Na2 SeSO3 was swiftly injected into the cadmium-2mercaptoethanol solution with constant stirring under bubbling with N2 for 30 min. After that, the temperature was increased to 75 ◦ C for the growth of the CdSe QDs. The color of reaction solution changed almost immediately after the injection of the Na2 SeSO3 solution, which indicated fast in nucleation and growth. The CdSe QDs were then precipitated by adding methanol into the toluene dispersion and were separated and purified by centrifugation. Scheme 1 illustrates the mechanism of formation of ME-capped CdSe QDs. 2.3. Characterization of 2-mercaptoethanol capped CdSe quantum dots The X-ray measurements were performed using Philips X’pert ˚ diffractometer supplied with copper X-ray tube (k␣1 = 1.5406 A), nickel filter, graphite crystal monochromator, proportional counter detector, divergence slit 1◦ and 0.1 mm receiving slit. The working conditions were 40 kV and 30 mA for the X ray tube, scan speed 0.05◦ and 2 s measuring time per step. For each measurement, a complete scan was made between 20◦ and 70◦ (2 ◦ ). To calibrate the measured Bragg 2-angles, a standard reference material (SRM 640a) of pure Si powder was used. High-resolution transmission electron microscopy (HR-TEM) of CdSe nanocrystals was carried out using a JEOL 2010 high-resolution transmission electron microscope operated at 200 kV. To prepare the HR-TEM samples, purified
CdSe nanopowders were first dispersed in methanol and diluted, followed by placing a droplet of the solution onto a 400-mesh carbon-coated copper grid. The grid was then dried in desiccators for 1 day before imaging.
2.4. Optical measurements The optical properties of CdSe quantum dots were studied through fluorescence measurements. The fluorescence was measured at room temperature using Perkin Elmer LS 55 Fluorescence Spectrometer. The slit widths used for excitation and emission were 5 nm. These fluorescence measurements were carried out by preparing a suspension solution of 1.2 mg L−1 CdSe quantum dots in pH 9.0 buffer solution and sonicated for 30 min at ambient temperature until homogenous suspension formed. Different concentrations of Alkali and transition metal ions solution were added into CdSe quantum dot solution and mixed thoroughly. These solutions put into quartz cuvette for fluorescence measurements. The fluorescent intensity of the solutions was recorded at excitation wavelength of 365 nm.
3. Results and discussion 3.1. Characterization of 2-mercaptoethanol capped CdSe QDs Fig. 1a depicts the X-ray diffraction for the prepared CdSe quantum dots. The diffraction peaks at angles (2) of 25.2, 41.9 and 49.7◦ correspond to the reflection planes (1 1 1), (2 2 0) and (3 1 1), respectively. The XRD pattern is identical to the zinc blend phase with cubic structure which has unit cell parameter a = 6.09 A˚ [JCPDS card no. 19-0191]. Fig. 1b shows the typical TEM image of the assynthesized CdSe QDs at temperature 75 ◦ C. The TEM measurement indicated that the size distribution of the as-synthesized CdSe QDs was nearly mono-disperse with the average size of 4.1 nm. FT-IR spectra of free 2-mercaptoethanol and functionalized 2mercaptoethanol capped CdSe QDs are shown in Fig. 2. The most pronounced IR absorption bands occurred at 3423 cm−1 ( OH), 2930–2914 cm−1 ( CH2 ), 2550–2670 cm−1 ( S H), 1661 cm−1 ( C C), 1402 cm−1 ( CH2 ), 1176 cm−1 ( C O) for free 2mercaptoethanol and at 3419 cm−1 ( OH), 2921 cm−1 ( CH2 ), 1400 cm−1 ( CH2 ), 1180 cm−1 ( C O) for 2-mercaptoethanol capped CdSe QDs. IR spectra of both 2-mercaptoethanol capped CdSe QDs and free 2-mercaptoethanol showed absorption peaks for hydroxyl and carbonyl groups. This indicates hydroxyl and carbonyl groups coexistence on the surface of the CdSe QDs. While the peak for S H (2538 cm−1 ) was absent in 2-mercaptoethanol capped CdSe QDs. The reason for the disappearance of S H vibration in IR spectra of 2-mercaptoethanol capped CdSe QDs was the
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Scheme 1. The mechanism of formation of ME-capped CdSe QDs.
3.2. Effect of pH on the fluorescence intensity
Fig. 2. FT-IR spectra for free 2-mercaptoethanol and 2-mercaptoethanol capped CdSe QDs.
result of the covalent bonding between thiols and Cd atom on the QDs surface. Functionalized 2-mercaptoethanol capped CdSe QDs are optically characterized by UV–vis absorption spectroscopy and fluorometry. The absorption and fluorescence spectra of 2mercaptoethanol capped CdSe are depicted in Fig. 3. The absorption peak occurs at 546 nm and emission maxima obtained at 569 nm with the excitation of 365 nm. The small stokes shift (23 nm) between absorption and emission peaks confirmed that the 2mercaptoethanol reduced the surface defect of CdSe QDs [13,14].
Fig. 3. Absorption and fluorescence of the prepared 2-mercaptoethanol capped CdSe quantum dot at temperature T = 75 ◦ C.
The influence of pH along the range between 3 and 12 was examined to determine the optimum conditions for the determination of Ba2+ ions with the 2-mercaptoethanol capped CdSe QDs. Fig. 4 shows that the pH of the solution enhanced the emission intensity of the 2-mercaptoethanol capped CdSe QDs with optimum at pH 9. The reason for the increase of luminescence as the pH increases up to 9 may be attributed to more thiol groups became dehydrogenated, which was expected to strengthen the covalent bond between ME and Cd2+ on the surface of QDs. Meanwhile, the higher pH also promoted the negative charge of hydroxyl groups of ME and helped disperse the nanoparticles better. Both these effects would provide better surface protection for the aqueous CdSe QDs and make them emit photons more efficiently with less non-radiative loss. Furthermore, the solubility of CdSe decreases with increasing pH [15], which benefited the nucleation and growth of QDs and therefore generated more nanoparticles. As a result, higher pH could help improve the PL intensity of the aqueous CdSe QDs. Fig. 4 also shows that the fluorescence intensity of CdSe QDs decreased when the pH was greater than 9. This may be due to the formation of hydrated product, which decreases the fluorescence intensity. Furthermore, cadmium ions are precipitated as their hydroxides at higher pH which also may reduce the fluorescence intensity. Therefore, a pH of 9 was chosen as optimum pH in this analytical study. The influence of various buffers and their ionic strengths on the fluorescence intensity of QDs was investigated. The experimental results showed that the maximum and stable fluorescence intensity occurred when the tetra-propyl-ammonium hydroxide (TPAH) buffer was in the concentration of 0.03 M. Hence, a concentration of 0.03 M TPAH was used as a buffer to adjust the pH of aqueous medium.
Fig. 4. The effect of pH on the fluorescence intensity of ME capped CdSe QDs.
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Fig. 5. The effect of ME/Cd molar ratio on the fluorescence intensity the QDs.
3.3. Effect of ME/Cd molar ratio In order to examine the effect of capping agent concentration on the FL properties of CdSe quantum dots, several batches of samples were synthesized with different ME:Cd molar ratios, while the Cd:Se ratio was maintained as 1:1. It is obvious from Fig. 5 that, the ratio of ME:Cd = 2:1 is optimal which has the highest FL intensity. At ME:Cd = 0.5:1 and 1:1, the PL intensity was lower than that of ME:Cd = 2:1, probably because the CdSe QDs were not covered by ME sufficiently. As a result, more clusters of QDs were generated due to the poor dispersion, and the emission intensity was reduced by non-radiative energy transfer. On the other hand, at ME:Cd = 3:1 and 4:1, the high concentration of ME molecules in the suspension resulted in lower FL intensity as well. There are several possible reasons for this effect. It suggested that, for the as-synthesized QDs at higher capping agent concentrations, the thiol groups tend to react with each other and form S2+ ions which act as trap sites for electrons and in turn reduce the emission intensity. On the other hand, higher concentration might promote the formation of QDs clusters. The poor dispersion would introduce more non-radiative energy dissipation and reduce the FL intensity as well [16]. Therefore, a molar ratio of ME/Cd = 2 was adopted in this study. 3.4. Photostability of 2-mercaptoethanol capped CdSe QDs For chemical sensor applications, the influence of temperature and stored time on the photo stability of 2-mercaptoethanol capped CdSe QDs is critical. The 2-mercaptoethanol capped CdSe QD was stored at 4 ◦ C in darkness and the changes of its fluorescence intensities were monitored at ambient temperature for up to 30 days, as indicated in Fig. 6a. It is clear that, the CdSe QDs remained stable over time, with no observable degradation of the fluorescence. Therefore, the present aqueous CdSe QDs were demonstrated to have excellent stability showing their potential applications as a sensor for long term measuring and tracking. It was found that the fluorescence behavior of the aqueous 2-mercaptoethanol capped CdSe QDs were dependent on the temperature during measurement, where the fluorescence intensity decreased with the increase of temperature as shown in Fig. 6b. The 2-mercaptoethanol capped CdSe QD was cooled down and stored at 4 ◦ C overnight, and then was applied with another cycle of temperature change. Interestingly found that the fluorescence dependence on temperature was reversible and repeatable for successive cycles, where the emission intensity followed almost the same temperature-dependent trend for all four cycles. Similar emission intensity decrease with increasing temperature was reported in other QDs systems [17]. The phenomenon was attributed to the enhanced exciton–phonon interaction at higher temperature.
Fig. 6. The influence of (a) storing time and (b) temperature on the fluorescence intensity of the prepared 2-mercaptoethanol capped CdSe quantum dots.
Therefore the energy was more dissipated in a non-radiative mode rather than contributing to fluorescence emission via the electron–hole recombination. 3.5. Detection of barium ion using ME capped CdSe QDs probe The effect of Ba(II) ions on the fluorescence emission of the 2mercaptoethanol capped CdSe QDs is shown in Fig. 7a and b. The fluorescence intensity is significantly enhanced by the addition of Ba(II) ion. The enhancement effect of Ba2+ ion on the fluorescence emission of CdSe QDs is found to be concentration dependence. Therefore, this ensemble can be used for the development of a sensitive and selective method for Ba2+ ion sensor. The increase of FL intensity may be attributed to the host–phosphor interaction between Ba2+ ions and CdSe QDs. As shown in Scheme 2, the barium cation interact with the hydroxyl group of ME and forming Barium oxide on the surface of CdSe QDs. This barium oxide is well known as a good matrix for hosting luminescence materials. Therefore enhance the transition energy transfer which leads the increase of population of inversion symmetry of the QDs [18]. The enhancement of fluorescence intensity of 2mercaptoethanol capped CdSe QDs is well linearly proportional to the Ba2+ ion concentration along the range from 1.0 × 10−7 to 1.2 × 10−7 mol L−1 as indicated in Fig. 8. The following linear regression equation fits well the experimental data I = 1 + KC Io
(2)
where I is the FL intensity of quantum dots in the presence of Ba(II) ion, Io is the FL intensity of quantum dot in the absence of Ba(II) ion, K is constant and C is the concentration of ions. The linear
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Scheme 2. The proposed interaction between ME capped CdSe QD and barium ions.
correlation coefficient is 0.999. The detection limit (3) according to TUPAC was 4.2 × 10−9 mol L−1 , showing that the presented method is much more sensitive than some of the current luminescent approaches for other metal ions detection [19]. 3.6. Influence of foreign metal ions In order to adopt the functionalized CdSe QDs as a sensing probe for the Ba(II) ion, it is important to examine the effect of some common foreign ions that are naturally abundant in the environment. Fig. 9 shows the effect of some metal ions, at fixed concentration (0.5 M), on the FL intensity of 2-mercaptoethanol capped CdSe QDs. It is clear that, Na+ , Ni2+ , Co2+ , Pb2+ , Zn2+ and Cd2+ ions do not show any effect on the FL intensity of the 2-mercaptoethanol capped CdSe QDs. K+ , Ca2+ and Mg2+ ions show small change on
Fig. 8. The linear relationship between the relative fluorescence intensity of the 2-mercaptoethanol capped CdSe quantum dots and Ba2+ concentration.
the FL intensity of 2-mercaptoethanol capped CdSe QDs. As compared with Ba2+ ion solutions, the influence of these metal ions is comparatively weak. The relative standard deviation for ten replicate determinations of Ba2+ ion at a concentration of 1 × 10−6 M was estimated as 2.1%. The presence of foreign ion in 1:100 and 1:1000 ratios produced lower interference than 2.1%. Previous reports [20,21] stated that, ions causing errors more than 3.7% are considered interference.
Fig. 7. The influence of barium ions concentration on the fluorescence intensity of the 2-mercaptoethanol capped CdSe quantum dots.
Fig. 9. Effect of metal ions on fluorescence intensity of 2-mercaptoethanol-capped CdSe QDs, the concentration of 2-mercaptoethanol-capped CdSe QDs: 1.2 mg L−1 and concentration of metal ions: 0.5 M at pH 9.0.
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4. Conclusion The functionalized 2-mercaptoethanol-capped CdSe QDs have been developed as a novel fluorescence nanosensor for Ba(II) ion in aqueous medium. The fluorescence sensor for Ba(II) ion is based on the fluorescence enhancement of 2-mercaptoethanol-capped CdSe QDs. The maximum fluorescence intensity was obtained at the 2-mercaptoethanol-capped CdSe QDs concentration of 1.2 mg L−1 and at a pH 9.0. The detection limit for this sensor system was 4.2 × 10−9 M. The results showed that 2-mercaptoethanol-capped CdSe QDs fluorescence was enhanced by Ba2+ ion by 5 times through very low concentration ranging from 0.1 M to 1.2 M. The 2-mercaptoethanol-capped CdSe QDs showed a facile synthesis, low cost, high sensitivity and also the analytical applications of this sensor are very simple. References [1] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Quantum dot bioconjugates for imaging, labelling and sensing, Nat. Mater. 4 (2005) 435–446. [2] M. Gao, S. Kirstein, H. Mohwald, A.L. Rogach, A. Kornowski, A. Eychmuller, H. Weller, Strongly photoluminescent CdTe nanocrystals by proper surface modification, J. Phys. Chem. B 102 (1998) 8360–8363. [3] J. Drbohlavova, V. Adam, R. Kizek, J. Hubalek, Quantum dots—characterization, preparation and usage in biological systems, Int. J. Mol. Sci. 10 (2009) 656–673. [4] G.-Y. Lan, Y.W. Lin, Y.F. Huang, H.T. Chang, Photo-assisted synthesis of highly fluorescent ZnSe(S) quantum dots in aqueous solution, J. Mater. Chem. 17 (2007) 2661–2666. [5] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science 281 (1998) 2013–2016. [6] W.C.W. Chen, S. Nie, Quantum dot bioconjugates for ultrasensitive nonisotopic detection, Science 281 (1998) 2016–2018. [7] Y.F. Chen, Z. Rosenzweig, Luminescent CdS quantum dots as selective ion probes, Anal. Chem. 74 (2002) 5132–5138. [8] K.M. Gattas-Asfura, R.M. Leblanc, Peptide-coated CdS quantum dots for the optical detection of copper(II) and silver(I), Chem. Commun. 268 (2003) 2684–2685. [9] Y.H. Zhang, H.S. Zhang, X.F. Guo, H. Wang, l-Cysteine-coated CdSe/CdS core–shell quantum dots as selective fluorescence probe for copper(II) determination, Microchem. J. 89 (2008) 142–147.
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Biography Waleed E. Mahmoud received his PhD degree in 2006 in the manufacturing and developing of pressure sensitive nanocomposites. He has years of experience in the fields of optoelectronic devices, dye-sensitized solar cells, pressure sensors, chemosensors and gas sensors. Recently, his research interest includes the synthesis and application of colloidal semiconductor quantum dots as a sensor for DNA and cancer tumors.