Research article Received: 27 August 2014,
Revised: 12 December 2014,
Accepted: 29 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2859
Fluorescent probe for detection of Cu2+ using core-shell CdTe/ZnS quantum dots Wei Bian,a* Fang Wang,b Hao Zhang,b Lin Zhang,c Li Wangc and Shaomin Shuangc ABSTRACT: Core-shell CdTe/ZnS quantum dots capped with 3-mercaptopropionic acid (MPA) were successfully synthesized in aqueous medium by hydrothermal synthesis. These quantum dots have advantages compared to traditional quantum dots with limited biological applications, high toxicity and tendency to aggregate. The concentration of Cu2+ has a significant impact on the fluorescence intensity of quantum dots (QDs), therefore, a rapid sensitive and selective fluorescence probe has been proposed for the detection of Cu2+ in aqueous solution. Under optimal conditions, the fluorescence intensity of CdTe/ZnS QDs was linearly proportional to the concentration of Cu2+ in the range from 2.5 × 10–9 M to 17.5 × 10–7 M with the limit of 1.5 × 10–9 M and relative standard deviation of 0.23%. The quenching mechanism is static quenching with recoveries of 97.30–102.75%. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: Fluorescence probe; CdTe/ZnS QDs; detection; ion
Introduction Toxic heavy metals such as Cr, Cd, Cu, Pb, and Hg in streams and lakes have an adverse effect on animals, plants and humans (1–4). For example, copper, as a contaminant in food, especially in shellfish, causes liver damage and some functional disorder at high concentration especially for infants (5,6). To date, a variety of methods has been applied for the determination of Cu2+ such as atomic/molecular absorption spectroscopy(7), inductively coupled plasma emission/mass spectroscopy(8), electrochemical methods (9,10), and ion chromatography (11). Although these methods are highly sensitive and selective, they are time consuming and of high cost. So a simple and high effective method is desirable for the measurement of Cu2+. Quantum dots (QDs) are a class of nanocrystal medium between single molecules and materials with three dimensions confined to the 1–10 nm length scale (12–15). The unique advantages of broad excitation, narrow emission, and high quantum yield (16) offer the possible application in fluorescent sensing. To date, group B II–VI semiconductor materials have attracted much attention, such as CdSe, CdTe, and CdS. The emission spectral properties of these QDs can be effectively tuned by simply changing particle size. With a single wavelength excitation, the QDs at different sizes showing different colors has rendered more extensive applications in many fields (17–21). There have been many reports on the development of QDbased probes for the detection of ions. Chen and Rosenweig wrote the first report on detecting Cu2+ and Zn2+ by CdS QDs (22). Gattés-Asfura et al. used peptide-coated CdS QDs as fluorescent probes for Cu2+ and Ag+ (23). Isarov et al. found the functionalized CdTe QDs as luminescent nanoprobe for the Cu2+ (24). However, the high toxicity of QDs such as CdTe, CdS by releasing cadmium ions has greatly limited its application in many fields. ZnS with a higher band gap (3.7 eV) is known as a non-toxic material (25,26) and it also effectively protects the release of cadmium ions from the inner side to reduce the toxicity
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of CdTe (27). For example, Oluwasesan Adegoke and Tebello Nyokong developed a glutathione-capped CdTe/ZnS QDs-based fluorescence probe for the detection of bromide ion (28). Oluwasesan Adegoke, Edith Antunes and Tebello Nyokong recognized a superoxide anion using CdTe/ZnS QDs (26). Although 3-mercaptopropionic acid (MPA) is toxic, it has no significant toxicity at the highest concentrations (29). Thus, this paper describes the synthesis of water-soluble CdTe/ZnS QDs coated with MPA with less toxicity and the application as a fluorescence probe for Cu2+ at pH 7.4 phosphate-buffered solution. This probe has been successfully applied for the determination of trace Cu2+ ion in tap and pond water samples. The interaction mechanism between MPA capped CdTe/ZnS QDs and Cu2+ ion was studied in detail.
Experimental Reagents and chemicals 3-Mercaptopropionic acid (MPA) was obtained from Aladdin Chemicals (Shanghai, China). CdCl2·2.5H2O was purchased from * Correspondence to: W. Bian, School of Basic Medical Science, Shanxi Medical University, Taiyuan 030001, People’s Republic of China. E-mail: [email protected] a
School of Basic Medical Science, Shanxi Medical University, Taiyuan 030001, People’s Republic of China
b
College of Pharmacy, Shanxi Medical University, Taiyuan 030001, People’s Republic of China
c
Department of Chemistry and Chemical Engineering, Research Center of Environmental Science and Engineering, Shanxi University, Taiyuan 030006, People’s Republic of China Abbreviations: PBS, phosphate-buffered saline; QD, quantum dots; TEM, transmission electron microscope.
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W. Bian et al. Xi’an Chemical Factory (Xian, China), tellurium powder, NaBH4 were obtained from Tianjin Fuchen Chemical Reagent Factory (Tianjin, China), ZnSO4·7H2O, Na2S·9H2O were purchased from Tianjin Dengfeng Chemical Reagent Factory (Tianjin, China). All of these reagents were of analytical reagent grade and used as received without further purification. Purified water from a Milli-Q-RO4 water purification system (Millipore, Bedford, MA, USA) was used to prepare all solutions.
Apparatus The absorption and fluorescence measurements were performed with a TU-1901 spectrophotometer (Beijing’s general instrument Co. Ltd, Beijing, China) and an F-4500 spectrofluorometer (Hitachi High-Technologies Corporation, Ibarakiken, Japan) respectively. Fluorescence lifetimes were measured on an Edinburgh FLS920 spectrometer (Edinburgh, UK). The pH values were measured using a model pHS-3C pH meter (Shanghai Rex Instrument Factory, Shanghai, China). Morphologies and structures of QDs were characterized with a transmission electron microscope (TEM) on a model JEM-1011 (JEOL, Japan) at 100 kV. Fourier transform infrared (FT-IR) spectra of QDs and free ligands were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrometer (Waltham, MA, USA).
Results and discussion Characterization of MPA-CdTe/ZnS QDs Absorbance and fluorescence spectra of CdTe QDs and CdTe/ZnS QDs. The absorption and fluorescence spectra of the CdTe/ZnS QDs were recorded as shown in Fig. 1. The absorption and emission maxima gradually red shift with the extending reflux time, suggesting that the ZnS shell is slowly growing on the CdTe core. The size and concentration of CdTe/ZnS QDs are 3.4 nm and 4.0 × 10–6 M, determined by the empirical formula as Equations 1 and 2 (32): A ¼ εbc; ε ¼ 10043ðDÞ2:12
(1)
D ¼ 9:8127 10–7 λ3 – 1:7147 10–3 λ2 þ ð1:0064Þ λ – ð194:84Þ
(2)
where A is the absorbance at the peak position for CdTe/ZnS QDs, c is the molar concentration of the same sample, b is the path length (cm) of the radiation beam used for recording the absorption spectrum, D is the diameter of the sample which is to be tested and λ (nm) is the absorption maxima of CdTe/ZnS QDs.
Synthesis of MPA-CdTe/ZnS QDs Water-soluble CdTe/ZnS QDs stabilized by MPA were prepared based on the previously described method with minor modification (30,31). In detail, a CdTe precursor solution was prepared by adding freshly prepared NaHTe solution to a N2-saturated CdCl2•2.5H2O solution (pH ~11) in the presence of the stabilizer MPA. The molar ratio Cd2+/MPA/HTe– was fixed at 1:1.5:0.25. The solution was heated to 100°C for 1 h, then cooled to room temperature gradually under air, and 1 mM ZnSO4 and 1 mM Na2S slowly added drop-wise to 50 mL. The solution was subjected to reflux at 100°C, and it could be tuned in color by prolonging the heating time. After heating for a certain time, MPA capped CdTe/ZnS QDs were obtained. Samples were precipitated by 2-propanol and dried in a vacuum oven for further use.
Measurement procedure Different concentrations of Cu2+ were added into QDs solution and diluted to 10.0 mL with phosphate-buffered saline (PBS) aqueous solution at room temperature (22 ± 1°C). The fluorescence spectra were recorded in the wavelength range of 460–750 nm with excitation at 450 nm. The scan speed was 1200 nm/min and the slit widths of both excitation and emission were set as 5 nm. All experiments were performed in triplicate.
Procedure for water samples pretreatment Tap water and lake water, collected respectively from our laboratory and a small lake at Shanxi University, with large impurities removed through qualitative filter paper and degased by boiling. The water samples were spiked with different concentrations of Cu2+.
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Figure 1. The absorption (A) and corresponding emission (B) spectra of CdTe and CdTe/ZnS QDs at the excitation wavelength (450 nm). 1 stands for spectra of CdTe core QDs 2–9 stand for spectra of CdTe/ZnS QDs obtained for refluxing 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h and 9 h, respectively.
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Fluorescent probe for detection of Cu2+ using CdTe/ZnS QDs FT-IR studies. Fourier transform infrared (FT-IR) spectra are shown in Fig. 2. Pure 3-mercaptopropionic acid (MPA) (Fig. 2A) shows the characteristic stretching vibration of O–H, S–H, C=O, C–O–C at 3036 cm–1, 2669 cm–1, 2575 cm–1, 1703 cm–1, and 1407 cm–1 respectively. As compared with MPA, the stretching vibrations at 2669 cm–1 and 2575 cm–1 disappear in the spectra of CdTe QDs (Fig. 2B) and CdTe/ZnS QDs (Fig. 2C), which is the evidence for Cd–S bond formation on the surface of QDs, and confirmed the formation of CdTe QDs and CdTe/ZnS QDs. TEM characterization. The TEM image of CdTe/ZnS QDs in Fig. 3 displays a good dispersed crystalline structure with an average diameter of 3.4 nm, which is consistent with the result calculated from the empirical formula of the QDs. Fluorescence quantum yields of CdTe/ZnS QDs The fluorescence quantum yield (Φu) was calculated using Equation 3 (33): Φu ¼ Φs ðDu = DsÞ ðAs= Au Þ ðnu = ns Þ2
(3)
where Du and Ds are integrated emission intensities for sample and reference, respectively. Au and As are the absorbance of the
sample and reference, while nu and ns are the refractive indices of solvents. The fluorescence quantum yields of MPA-capped CdTe/ZnS QDs was measured with rhodamine 6G in ethanol (Φs = 0.94) as a reference (34) , and was 55%.
Optimization of the reactions The effect of pH value. Different pH solutions could affect both the sensitivity and selectivity of detection substances (35). Figure 4 depicts the effect of pH 5.0–10.0 on the fluorescence of CdTe/ZnS QDs in the absence and presence of Cu2+. The intensity of MPA-capped CdTe/ZnS QDs (F0) significantly enhanced as the pH changed from 5.0 to 7.4 with a maximum at pH 7.4–8.0, and then decreased when the pH value continuously increased. As such, the maximum quenched intensity (F0–F) in the presence of Cu2+ reached the maximum at pH 7.4. Therefore, pH 7.4 was selected to develop a sensitive and rapid spectrophotometric method for the determination of Cu2+ in this experiment. Effect of the reaction time. The influence of reaction time on the fluorescence intensity was investigated at room temperature and the results are shown in Fig. 5. From the experimental results, the fluorescence intensity of QDs was quenched quickly in the presence of Cu2+ and reached equilibrium within 10 min and the fluorescence signal was stable for at least a further 60 min. Thus, the experiments were carried out after 10 min.
Figure 2. Fourier transform infrared spectra of (A) pure MPA; (B) CdTe QDs; and (C) CdTe/ZnS QDs. Figure 4. Effect of pH on fluorescence intensity of CdTe/ZnS QDs (-■-) and the –8 2+ –6 quenched fluorescence intensity (-▼-) ( [QDs]: 8.0 × 10 M, [Cu ]: 1.0 × 10 M).
Figure 3. TEM image of MPA-capped CdTe/ZnS QDs.
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Figure 5. Time-dependent fluorescence intensity of CdTe/ZnS QDs upon addition 2+ –8 2+ –6 of Cu ([QDs]: 8.0 × 10 M, [Cu ]: 1.0 × 10 M, pH 7.4 PBS).
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W. Bian et al. The selection of the concentration of CdTe/ZnS QDs. As a sensitive probe to Cu2+ ion, the concentration of CdTe/ZnS QDs was optimized in the range of 8.0 × 10–8 M to 12.0 × 10–8 M for the quenching measurement as shown in Fig. 6. The results demonstrated that a higher response sensitivity of QDs to Cu2+ was achieved with the lower concentration of CdTe/ZnS QDs. But when the concentration of QDs was too low, the fluorescence intensity is low and the linear range narrowed down. Consequently, considering the high sensitivity and linear range, the optimal concentration of 8.0 × 10–8 M was recommended and employed throughout our experiment.
Interaction of Cu2+ with MPA capped CdTe/ZnS QDs It was observed that the fluorescence intensity of QDs was gradually decreased in the presence of various concentrations of Cu2 + (Fig. 7). There was a linear relationship between the quenched fluorescence intensity (ΔF) and the concentration of Cu2+ in the range of 2.5 × 10–9 M to 17.5 × 10–7 M with a correlation coefficient (R2) of 0.9921. The relative standard deviation for 11 replicate detections of 3.5 × 10–7 M Cu2+ was 0.23% with the limit
detection of 1.5 × 10–9 M. Some characteristics of the presented method and other methods for Cu2+ detection were summarized in Table 1. It can be found that the proposed method in this paper had higher sensitivity than most of other QDs-based sensors for Cu2+.
Selectivity of Cu2+ using CdTe/ZnS QDs The fluorescence intensity of CdTe/ZnS QDs was quenched significantly in the presence of 1.0 × 10–6 M of Cu2+, whereas some other metal ions led to very slight intensity change at the same concentration, as shown in Fig. 8 (black column). This result indicated that the QDs-based probe showed good selectivity toward Cu2+ over other competitive cations. To validate CdTe/ZnS QDs as a sensitive probe for Cu2+ ion, the impacts of some coexisting cations were also investigated as shown in Fig. 8 red columns, which showed that the coexistent ions had negligible interfering effects on the quenching fluorescence of the QDs by Cu2+ ion. Thus, the selective binding of Cu2+ with CdTe/ZnS QDs could occur in the presence of most of the competitive coexisting metal ions. Table 1. Comparison of the reported methods for Cu2+ using QDs QDs
Figure 6. Effect of the concentration of aqueous CdTe/ZnS QDs (pH 7.4 PBS).
Coating material
CdTe/ZnS 3-Mercaptopropionic acid CdTe/CdS Glyp CdSe 2-Mercaptoethane sulphonic acid CdSe/ZnS Bovine serum albumin CdTe D-Penicillamine CdSe/CdS Polymer CdSe/CdS Diethyldithiocarbamate CdSe Mercaptosuccinic acid
Detection limit (nM)
References
1.5
This work
20 3
(6) (37)
10 0.4 16 4.5 3.4
(38) (39) (40) (41) (42)
2+
Figure 7. Effect of Cu concentration on fluorescence intensity of CdTe/ZnS QDs. 2+ –8 The concentration of Cu (1–10, 10 M) were 0, 0.25, 1.0, 1.5, 2, 2.5, 5.0, 10.0, 50.0, 175.0. Inset graph is a Lineweaver–Burk plot for the interaction of MPA-capped 2+ –8 CdTe/ZnS QDs and Cu ([QDs]: 8.0 × 10 M, pH 7.4 PBS).
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–6
2+
Figure 8. The fluorescence response of CdTe/ZnS and 1.0 × 10 M Cu in the absence (black column) and presence (red column) of another specified metal –8 ion solution of the same concentration. ([QDs]: 8.0 × 10 M, pH 7.4 PBS).
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Fluorescent probe for detection of Cu2+ using CdTe/ZnS QDs Interaction between MPA capped CdTe/ZnS and Cu2+ ion The fluorescence quenching could be explained by dynamic quenching and static quenching processes. The dynamic quenching is due to the collision between the fluorescence substance and the quencher. The formation of a non-fluorescent ground-state complex between the fluorophore and quencher will result in static quenching. The dynamic and static quenching type could be analyzed by Stern–Volmer equation, Equation 4 and Lineweaver–Burk equation, Equation 5 (36): F0 =F ¼ 1 þ KSV CQ
(4)
1=ðF0 FÞ ¼ 1=F0 þ KLB =F0 CQ
(5)
where F0 and F are the fluorescence intensities in the absence and presence of Cu2+ respectively, CQ was the concentration of quencher, KSV is the Stern–Volmer quenching constant, KLB is static quenching constant. The interaction of Cu2+ with MPA-capped CdTe/ZnS QDs is shown in Fig. 7. There is a red shift in the luminescence of QDs in the presence of Cu2+ and the fluorescence quenching fitted well with the Lineweaver–Burk equation (see the inset graph in Fig. 7). A very good linearity was achieved in the range of 2.5 × 10–9 M to 17.5 × 10–7 M, with a linearly dependent coefficient (R2) of 0.9941. The relative standard deviation for 11 replicate detections of 3.5 × 10–7 M Cu2+ was 0. 23% KLB, calculated by linear regression of the plots, was 3.2 × 103 M for Cu2+. This result suggested that static quenching played an important role in the interaction of Cu2+ ion with MPA-capped CdTe/ZnS QDs. To further confirm the static interaction, the fluorescence lifetime was measured as in Fig. 10, and there was no variation in the decay profiles when Cu2+ was added. Therefore, dynamic quenching was not involved. In Fig. 9, the absorption intensity changes together with a small red shift with the increasing concentration of Cu2+suggested the formation of a ground-state complex (36). Figure 10 shows that there was no variation in the decay profiles when Cu2+ was added, which may exclude dynamic mechanism. The Stern–Volmer plots of F0/F versus [Cu2+] at three different temperatures are shown in Fig. 11 and the values of KSV are calculated in Table 2. The KSV value decreased with the increase in temperature. All above evidence supports the static quenching mechanism for the interaction of CdTe/ZnS QDs and Cu2+. As the quenching mechanism was static quenching, which occurred when the molecules form a
Figure 10. Fluorescence intensity decay curves of CdTe/ZnS QDs with increasing 2+ –8 concentrations of Cu (1–5, 10 M): 0, 15, 30, 40, 50.
Figure 11. The Stern–Volmer curves at different temperatures ([CQDs]: 8.0 × 10 M, pH 7.4 PBS).
–8
Table 2. Parameters of Stern–Volmer plots of CdTe/ZnS and Cu2+ Temperature (K)
KSV (× 106 M–1)
Stern–Volmer linear equation
Correlation
300
3.94
0.9982
310
1.7
320
1.16
F0/F = 3.94 × 106 CCu2+ + 1.0368 F0/F = 1.70 × 106 CCu2+ + 1.0047 F0/F = 1.16 × 106 CCu2+ + 0.9727
0.9923 0.9915
complex in the ground state, the interaction of the MPA-capped CdTe/ZnS QDs with Cu2+ is initially described in Scheme 1, which indicates that some ultrasmall particles CuxS (x = 1, 2) were formed and absorbed onto the surface of QDs during the quenching process.
Application to analysis of water samples 2+
Figure 9. Effect of Cu on UV-absorption spectra of CdTe/ZnS QDs. Concentra2+ –8 tion of Cu (1–5, 10 M): 0, 15, 30, 40, 50.
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To investigate the possibility of practical application, the proposed method was performed on tap water and lake water. The results showed that the recoveries of Cu2+ for these samples
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W. Bian et al.
2+
Scheme 1. Schematic of MPA capped CdTe/ZnS QDs and Cu .
Table 3. Analytical results of samples Water samples Tap water
Pond water
Added (10–8 M)
Found (n = 3) (10–8 M)
Recovery (%)
Relative SD (%)
3.00 5.00 20.0 3.00 5.00 20.0
2.92 5.05 19.7 3.02 6.4 20.2
97.30 101.00 98.67 100.5 102.75 101.00
1.78 2.01 2.23 1.90 2.12 2.34
were in the range of 97.30–102.75%, which was satisfactory for application (Table 3). Therefore, the MPA-capped CdTe/ZnS QDs as fluorescence probes were reliable and practical for the detection of Cu2+ in water samples.
Conclusions MPA-capped CdTe/ZnS QDs were successfully synthesized in aqueous medium. A highly sensitive probe for the quantitative and selective determination of Cu2+ was developed. The quenching mechanism between MPA-capped CdTe/ZnS and Cu2+ was static quenching. This proposed model could be viewed as a fundamental strategy for a specific nanosensing system towards heavy metal ions in biomedicine and environmental fields. Acknowledgements This work is supported by National Natural Science Foundation of China (21175086), Shanxi Medical University of Science and Technology Innovation Fund (01201312), and 331 Early Career Research Grant of Basic Medical College in Shanxi Medical University (201417).
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Revised: 12 December 2014,
Accepted: 29 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2859
Fluorescent probe for detection of Cu2+ using core-shell CdTe/ZnS quantum dots Wei Bian,a* Fang Wang,b Hao Zhang,b Lin Zhang,c Li Wangc and Shaomin Shuangc ABSTRACT: Core-shell CdTe/ZnS quantum dots capped with 3-mercaptopropionic acid (MPA) were successfully synthesized in aqueous medium by hydrothermal synthesis. These quantum dots have advantages compared to traditional quantum dots with limited biological applications, high toxicity and tendency to aggregate. The concentration of Cu2+ has a significant impact on the fluorescence intensity of quantum dots (QDs), therefore, a rapid sensitive and selective fluorescence probe has been proposed for the detection of Cu2+ in aqueous solution. Under optimal conditions, the fluorescence intensity of CdTe/ZnS QDs was linearly proportional to the concentration of Cu2+ in the range from 2.5 × 10–9 M to 17.5 × 10–7 M with the limit of 1.5 × 10–9 M and relative standard deviation of 0.23%. The quenching mechanism is static quenching with recoveries of 97.30–102.75%. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: Fluorescence probe; CdTe/ZnS QDs; detection; ion
Introduction Toxic heavy metals such as Cr, Cd, Cu, Pb, and Hg in streams and lakes have an adverse effect on animals, plants and humans (1–4). For example, copper, as a contaminant in food, especially in shellfish, causes liver damage and some functional disorder at high concentration especially for infants (5,6). To date, a variety of methods has been applied for the determination of Cu2+ such as atomic/molecular absorption spectroscopy(7), inductively coupled plasma emission/mass spectroscopy(8), electrochemical methods (9,10), and ion chromatography (11). Although these methods are highly sensitive and selective, they are time consuming and of high cost. So a simple and high effective method is desirable for the measurement of Cu2+. Quantum dots (QDs) are a class of nanocrystal medium between single molecules and materials with three dimensions confined to the 1–10 nm length scale (12–15). The unique advantages of broad excitation, narrow emission, and high quantum yield (16) offer the possible application in fluorescent sensing. To date, group B II–VI semiconductor materials have attracted much attention, such as CdSe, CdTe, and CdS. The emission spectral properties of these QDs can be effectively tuned by simply changing particle size. With a single wavelength excitation, the QDs at different sizes showing different colors has rendered more extensive applications in many fields (17–21). There have been many reports on the development of QDbased probes for the detection of ions. Chen and Rosenweig wrote the first report on detecting Cu2+ and Zn2+ by CdS QDs (22). Gattés-Asfura et al. used peptide-coated CdS QDs as fluorescent probes for Cu2+ and Ag+ (23). Isarov et al. found the functionalized CdTe QDs as luminescent nanoprobe for the Cu2+ (24). However, the high toxicity of QDs such as CdTe, CdS by releasing cadmium ions has greatly limited its application in many fields. ZnS with a higher band gap (3.7 eV) is known as a non-toxic material (25,26) and it also effectively protects the release of cadmium ions from the inner side to reduce the toxicity
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of CdTe (27). For example, Oluwasesan Adegoke and Tebello Nyokong developed a glutathione-capped CdTe/ZnS QDs-based fluorescence probe for the detection of bromide ion (28). Oluwasesan Adegoke, Edith Antunes and Tebello Nyokong recognized a superoxide anion using CdTe/ZnS QDs (26). Although 3-mercaptopropionic acid (MPA) is toxic, it has no significant toxicity at the highest concentrations (29). Thus, this paper describes the synthesis of water-soluble CdTe/ZnS QDs coated with MPA with less toxicity and the application as a fluorescence probe for Cu2+ at pH 7.4 phosphate-buffered solution. This probe has been successfully applied for the determination of trace Cu2+ ion in tap and pond water samples. The interaction mechanism between MPA capped CdTe/ZnS QDs and Cu2+ ion was studied in detail.
Experimental Reagents and chemicals 3-Mercaptopropionic acid (MPA) was obtained from Aladdin Chemicals (Shanghai, China). CdCl2·2.5H2O was purchased from * Correspondence to: W. Bian, School of Basic Medical Science, Shanxi Medical University, Taiyuan 030001, People’s Republic of China. E-mail: [email protected] a
School of Basic Medical Science, Shanxi Medical University, Taiyuan 030001, People’s Republic of China
b
College of Pharmacy, Shanxi Medical University, Taiyuan 030001, People’s Republic of China
c
Department of Chemistry and Chemical Engineering, Research Center of Environmental Science and Engineering, Shanxi University, Taiyuan 030006, People’s Republic of China Abbreviations: PBS, phosphate-buffered saline; QD, quantum dots; TEM, transmission electron microscope.
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W. Bian et al. Xi’an Chemical Factory (Xian, China), tellurium powder, NaBH4 were obtained from Tianjin Fuchen Chemical Reagent Factory (Tianjin, China), ZnSO4·7H2O, Na2S·9H2O were purchased from Tianjin Dengfeng Chemical Reagent Factory (Tianjin, China). All of these reagents were of analytical reagent grade and used as received without further purification. Purified water from a Milli-Q-RO4 water purification system (Millipore, Bedford, MA, USA) was used to prepare all solutions.
Apparatus The absorption and fluorescence measurements were performed with a TU-1901 spectrophotometer (Beijing’s general instrument Co. Ltd, Beijing, China) and an F-4500 spectrofluorometer (Hitachi High-Technologies Corporation, Ibarakiken, Japan) respectively. Fluorescence lifetimes were measured on an Edinburgh FLS920 spectrometer (Edinburgh, UK). The pH values were measured using a model pHS-3C pH meter (Shanghai Rex Instrument Factory, Shanghai, China). Morphologies and structures of QDs were characterized with a transmission electron microscope (TEM) on a model JEM-1011 (JEOL, Japan) at 100 kV. Fourier transform infrared (FT-IR) spectra of QDs and free ligands were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrometer (Waltham, MA, USA).
Results and discussion Characterization of MPA-CdTe/ZnS QDs Absorbance and fluorescence spectra of CdTe QDs and CdTe/ZnS QDs. The absorption and fluorescence spectra of the CdTe/ZnS QDs were recorded as shown in Fig. 1. The absorption and emission maxima gradually red shift with the extending reflux time, suggesting that the ZnS shell is slowly growing on the CdTe core. The size and concentration of CdTe/ZnS QDs are 3.4 nm and 4.0 × 10–6 M, determined by the empirical formula as Equations 1 and 2 (32): A ¼ εbc; ε ¼ 10043ðDÞ2:12
(1)
D ¼ 9:8127 10–7 λ3 – 1:7147 10–3 λ2 þ ð1:0064Þ λ – ð194:84Þ
(2)
where A is the absorbance at the peak position for CdTe/ZnS QDs, c is the molar concentration of the same sample, b is the path length (cm) of the radiation beam used for recording the absorption spectrum, D is the diameter of the sample which is to be tested and λ (nm) is the absorption maxima of CdTe/ZnS QDs.
Synthesis of MPA-CdTe/ZnS QDs Water-soluble CdTe/ZnS QDs stabilized by MPA were prepared based on the previously described method with minor modification (30,31). In detail, a CdTe precursor solution was prepared by adding freshly prepared NaHTe solution to a N2-saturated CdCl2•2.5H2O solution (pH ~11) in the presence of the stabilizer MPA. The molar ratio Cd2+/MPA/HTe– was fixed at 1:1.5:0.25. The solution was heated to 100°C for 1 h, then cooled to room temperature gradually under air, and 1 mM ZnSO4 and 1 mM Na2S slowly added drop-wise to 50 mL. The solution was subjected to reflux at 100°C, and it could be tuned in color by prolonging the heating time. After heating for a certain time, MPA capped CdTe/ZnS QDs were obtained. Samples were precipitated by 2-propanol and dried in a vacuum oven for further use.
Measurement procedure Different concentrations of Cu2+ were added into QDs solution and diluted to 10.0 mL with phosphate-buffered saline (PBS) aqueous solution at room temperature (22 ± 1°C). The fluorescence spectra were recorded in the wavelength range of 460–750 nm with excitation at 450 nm. The scan speed was 1200 nm/min and the slit widths of both excitation and emission were set as 5 nm. All experiments were performed in triplicate.
Procedure for water samples pretreatment Tap water and lake water, collected respectively from our laboratory and a small lake at Shanxi University, with large impurities removed through qualitative filter paper and degased by boiling. The water samples were spiked with different concentrations of Cu2+.
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Figure 1. The absorption (A) and corresponding emission (B) spectra of CdTe and CdTe/ZnS QDs at the excitation wavelength (450 nm). 1 stands for spectra of CdTe core QDs 2–9 stand for spectra of CdTe/ZnS QDs obtained for refluxing 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h and 9 h, respectively.
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Fluorescent probe for detection of Cu2+ using CdTe/ZnS QDs FT-IR studies. Fourier transform infrared (FT-IR) spectra are shown in Fig. 2. Pure 3-mercaptopropionic acid (MPA) (Fig. 2A) shows the characteristic stretching vibration of O–H, S–H, C=O, C–O–C at 3036 cm–1, 2669 cm–1, 2575 cm–1, 1703 cm–1, and 1407 cm–1 respectively. As compared with MPA, the stretching vibrations at 2669 cm–1 and 2575 cm–1 disappear in the spectra of CdTe QDs (Fig. 2B) and CdTe/ZnS QDs (Fig. 2C), which is the evidence for Cd–S bond formation on the surface of QDs, and confirmed the formation of CdTe QDs and CdTe/ZnS QDs. TEM characterization. The TEM image of CdTe/ZnS QDs in Fig. 3 displays a good dispersed crystalline structure with an average diameter of 3.4 nm, which is consistent with the result calculated from the empirical formula of the QDs. Fluorescence quantum yields of CdTe/ZnS QDs The fluorescence quantum yield (Φu) was calculated using Equation 3 (33): Φu ¼ Φs ðDu = DsÞ ðAs= Au Þ ðnu = ns Þ2
(3)
where Du and Ds are integrated emission intensities for sample and reference, respectively. Au and As are the absorbance of the
sample and reference, while nu and ns are the refractive indices of solvents. The fluorescence quantum yields of MPA-capped CdTe/ZnS QDs was measured with rhodamine 6G in ethanol (Φs = 0.94) as a reference (34) , and was 55%.
Optimization of the reactions The effect of pH value. Different pH solutions could affect both the sensitivity and selectivity of detection substances (35). Figure 4 depicts the effect of pH 5.0–10.0 on the fluorescence of CdTe/ZnS QDs in the absence and presence of Cu2+. The intensity of MPA-capped CdTe/ZnS QDs (F0) significantly enhanced as the pH changed from 5.0 to 7.4 with a maximum at pH 7.4–8.0, and then decreased when the pH value continuously increased. As such, the maximum quenched intensity (F0–F) in the presence of Cu2+ reached the maximum at pH 7.4. Therefore, pH 7.4 was selected to develop a sensitive and rapid spectrophotometric method for the determination of Cu2+ in this experiment. Effect of the reaction time. The influence of reaction time on the fluorescence intensity was investigated at room temperature and the results are shown in Fig. 5. From the experimental results, the fluorescence intensity of QDs was quenched quickly in the presence of Cu2+ and reached equilibrium within 10 min and the fluorescence signal was stable for at least a further 60 min. Thus, the experiments were carried out after 10 min.
Figure 2. Fourier transform infrared spectra of (A) pure MPA; (B) CdTe QDs; and (C) CdTe/ZnS QDs. Figure 4. Effect of pH on fluorescence intensity of CdTe/ZnS QDs (-■-) and the –8 2+ –6 quenched fluorescence intensity (-▼-) ( [QDs]: 8.0 × 10 M, [Cu ]: 1.0 × 10 M).
Figure 3. TEM image of MPA-capped CdTe/ZnS QDs.
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Figure 5. Time-dependent fluorescence intensity of CdTe/ZnS QDs upon addition 2+ –8 2+ –6 of Cu ([QDs]: 8.0 × 10 M, [Cu ]: 1.0 × 10 M, pH 7.4 PBS).
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W. Bian et al. The selection of the concentration of CdTe/ZnS QDs. As a sensitive probe to Cu2+ ion, the concentration of CdTe/ZnS QDs was optimized in the range of 8.0 × 10–8 M to 12.0 × 10–8 M for the quenching measurement as shown in Fig. 6. The results demonstrated that a higher response sensitivity of QDs to Cu2+ was achieved with the lower concentration of CdTe/ZnS QDs. But when the concentration of QDs was too low, the fluorescence intensity is low and the linear range narrowed down. Consequently, considering the high sensitivity and linear range, the optimal concentration of 8.0 × 10–8 M was recommended and employed throughout our experiment.
Interaction of Cu2+ with MPA capped CdTe/ZnS QDs It was observed that the fluorescence intensity of QDs was gradually decreased in the presence of various concentrations of Cu2 + (Fig. 7). There was a linear relationship between the quenched fluorescence intensity (ΔF) and the concentration of Cu2+ in the range of 2.5 × 10–9 M to 17.5 × 10–7 M with a correlation coefficient (R2) of 0.9921. The relative standard deviation for 11 replicate detections of 3.5 × 10–7 M Cu2+ was 0.23% with the limit
detection of 1.5 × 10–9 M. Some characteristics of the presented method and other methods for Cu2+ detection were summarized in Table 1. It can be found that the proposed method in this paper had higher sensitivity than most of other QDs-based sensors for Cu2+.
Selectivity of Cu2+ using CdTe/ZnS QDs The fluorescence intensity of CdTe/ZnS QDs was quenched significantly in the presence of 1.0 × 10–6 M of Cu2+, whereas some other metal ions led to very slight intensity change at the same concentration, as shown in Fig. 8 (black column). This result indicated that the QDs-based probe showed good selectivity toward Cu2+ over other competitive cations. To validate CdTe/ZnS QDs as a sensitive probe for Cu2+ ion, the impacts of some coexisting cations were also investigated as shown in Fig. 8 red columns, which showed that the coexistent ions had negligible interfering effects on the quenching fluorescence of the QDs by Cu2+ ion. Thus, the selective binding of Cu2+ with CdTe/ZnS QDs could occur in the presence of most of the competitive coexisting metal ions. Table 1. Comparison of the reported methods for Cu2+ using QDs QDs
Figure 6. Effect of the concentration of aqueous CdTe/ZnS QDs (pH 7.4 PBS).
Coating material
CdTe/ZnS 3-Mercaptopropionic acid CdTe/CdS Glyp CdSe 2-Mercaptoethane sulphonic acid CdSe/ZnS Bovine serum albumin CdTe D-Penicillamine CdSe/CdS Polymer CdSe/CdS Diethyldithiocarbamate CdSe Mercaptosuccinic acid
Detection limit (nM)
References
1.5
This work
20 3
(6) (37)
10 0.4 16 4.5 3.4
(38) (39) (40) (41) (42)
2+
Figure 7. Effect of Cu concentration on fluorescence intensity of CdTe/ZnS QDs. 2+ –8 The concentration of Cu (1–10, 10 M) were 0, 0.25, 1.0, 1.5, 2, 2.5, 5.0, 10.0, 50.0, 175.0. Inset graph is a Lineweaver–Burk plot for the interaction of MPA-capped 2+ –8 CdTe/ZnS QDs and Cu ([QDs]: 8.0 × 10 M, pH 7.4 PBS).
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–6
2+
Figure 8. The fluorescence response of CdTe/ZnS and 1.0 × 10 M Cu in the absence (black column) and presence (red column) of another specified metal –8 ion solution of the same concentration. ([QDs]: 8.0 × 10 M, pH 7.4 PBS).
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Fluorescent probe for detection of Cu2+ using CdTe/ZnS QDs Interaction between MPA capped CdTe/ZnS and Cu2+ ion The fluorescence quenching could be explained by dynamic quenching and static quenching processes. The dynamic quenching is due to the collision between the fluorescence substance and the quencher. The formation of a non-fluorescent ground-state complex between the fluorophore and quencher will result in static quenching. The dynamic and static quenching type could be analyzed by Stern–Volmer equation, Equation 4 and Lineweaver–Burk equation, Equation 5 (36): F0 =F ¼ 1 þ KSV CQ
(4)
1=ðF0 FÞ ¼ 1=F0 þ KLB =F0 CQ
(5)
where F0 and F are the fluorescence intensities in the absence and presence of Cu2+ respectively, CQ was the concentration of quencher, KSV is the Stern–Volmer quenching constant, KLB is static quenching constant. The interaction of Cu2+ with MPA-capped CdTe/ZnS QDs is shown in Fig. 7. There is a red shift in the luminescence of QDs in the presence of Cu2+ and the fluorescence quenching fitted well with the Lineweaver–Burk equation (see the inset graph in Fig. 7). A very good linearity was achieved in the range of 2.5 × 10–9 M to 17.5 × 10–7 M, with a linearly dependent coefficient (R2) of 0.9941. The relative standard deviation for 11 replicate detections of 3.5 × 10–7 M Cu2+ was 0. 23% KLB, calculated by linear regression of the plots, was 3.2 × 103 M for Cu2+. This result suggested that static quenching played an important role in the interaction of Cu2+ ion with MPA-capped CdTe/ZnS QDs. To further confirm the static interaction, the fluorescence lifetime was measured as in Fig. 10, and there was no variation in the decay profiles when Cu2+ was added. Therefore, dynamic quenching was not involved. In Fig. 9, the absorption intensity changes together with a small red shift with the increasing concentration of Cu2+suggested the formation of a ground-state complex (36). Figure 10 shows that there was no variation in the decay profiles when Cu2+ was added, which may exclude dynamic mechanism. The Stern–Volmer plots of F0/F versus [Cu2+] at three different temperatures are shown in Fig. 11 and the values of KSV are calculated in Table 2. The KSV value decreased with the increase in temperature. All above evidence supports the static quenching mechanism for the interaction of CdTe/ZnS QDs and Cu2+. As the quenching mechanism was static quenching, which occurred when the molecules form a
Figure 10. Fluorescence intensity decay curves of CdTe/ZnS QDs with increasing 2+ –8 concentrations of Cu (1–5, 10 M): 0, 15, 30, 40, 50.
Figure 11. The Stern–Volmer curves at different temperatures ([CQDs]: 8.0 × 10 M, pH 7.4 PBS).
–8
Table 2. Parameters of Stern–Volmer plots of CdTe/ZnS and Cu2+ Temperature (K)
KSV (× 106 M–1)
Stern–Volmer linear equation
Correlation
300
3.94
0.9982
310
1.7
320
1.16
F0/F = 3.94 × 106 CCu2+ + 1.0368 F0/F = 1.70 × 106 CCu2+ + 1.0047 F0/F = 1.16 × 106 CCu2+ + 0.9727
0.9923 0.9915
complex in the ground state, the interaction of the MPA-capped CdTe/ZnS QDs with Cu2+ is initially described in Scheme 1, which indicates that some ultrasmall particles CuxS (x = 1, 2) were formed and absorbed onto the surface of QDs during the quenching process.
Application to analysis of water samples 2+
Figure 9. Effect of Cu on UV-absorption spectra of CdTe/ZnS QDs. Concentra2+ –8 tion of Cu (1–5, 10 M): 0, 15, 30, 40, 50.
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To investigate the possibility of practical application, the proposed method was performed on tap water and lake water. The results showed that the recoveries of Cu2+ for these samples
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W. Bian et al.
2+
Scheme 1. Schematic of MPA capped CdTe/ZnS QDs and Cu .
Table 3. Analytical results of samples Water samples Tap water
Pond water
Added (10–8 M)
Found (n = 3) (10–8 M)
Recovery (%)
Relative SD (%)
3.00 5.00 20.0 3.00 5.00 20.0
2.92 5.05 19.7 3.02 6.4 20.2
97.30 101.00 98.67 100.5 102.75 101.00
1.78 2.01 2.23 1.90 2.12 2.34
were in the range of 97.30–102.75%, which was satisfactory for application (Table 3). Therefore, the MPA-capped CdTe/ZnS QDs as fluorescence probes were reliable and practical for the detection of Cu2+ in water samples.
Conclusions MPA-capped CdTe/ZnS QDs were successfully synthesized in aqueous medium. A highly sensitive probe for the quantitative and selective determination of Cu2+ was developed. The quenching mechanism between MPA-capped CdTe/ZnS and Cu2+ was static quenching. This proposed model could be viewed as a fundamental strategy for a specific nanosensing system towards heavy metal ions in biomedicine and environmental fields. Acknowledgements This work is supported by National Natural Science Foundation of China (21175086), Shanxi Medical University of Science and Technology Innovation Fund (01201312), and 331 Early Career Research Grant of Basic Medical College in Shanxi Medical University (201417).
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