Research article Received: 12 May 2014,
Revised: 15 July 2014,
Accepted: 3 August 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2761
A highly selective and simple fluorescent sensor for mercury (II) ion detection based on cysteamine-capped CdTe quantum dots synthesized by the reflux method Xiaojie Ding,a Lingbo Qu,a,b Ran Yang,a* Yuchen Zhoua and Jianjun Lia ABSTRACT: Cysteamine (CA)-capped CdTe quantum dots (QDs) (CA–CdTe QDs) were prepared by the reflux method and utilized as an efficient nano-sized fluorescent sensor to detect mercury (II) ions (Hg2+). Under optimum conditions, the fluorescence quenching effect of CA–CdTe QDs was linear at Hg2+ concentrations in the range of 6.0–450 nmol/L. The detection limit was calculated to be 4.0 nmol/L according to the 3σ IUPAC criteria. The influence of 10-fold Pb2+, Cu2+ and Ag+ on the determination of Hg2+ was < 7% (superior to other reports based on crude QDs). Furthermore, the detection sensitivity and selectivity were much improved relative to a sensor based on the CA–CdTe QDs probe, which was prepared using a one-pot synthetic method. This CA–CdTe QDs sensor system represents a new feasibility to improve the detection performance of a QDs sensor by changing the synthesis method. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: cysteamine; CdTe quantum dots; mercury (II) ion; fluorescent sensor
Introduction The mercury (II) ion (Hg2+), one of the most hazardous environmental toxins, is ubiquitous in the environment. It can accumulate in the vital organs and tissues of animals and result in a wide variety of diseases (1,2). In particular, mercury harmfully affects the function and development of the central nervous system in people (3,4). Mercury pollution has become a serious environmental health problem all over the word. From the viewpoint of environmental protection and health concerns, developing an effective method for the sensitive detection of trace amounts of Hg2+ is very important and has attracted a significant amount of attention. A variety of traditional methods including atomic absorption spectrometry (AAS) (5), atomic fluorescence spectrometry (AFS) (6), inductively coupled plasma mass spectroscopy (ICP-MS) (7), X-ray absorption spectroscopy and electrochemical methods (8) have been widely used for measuring Hg2+. However, these methods are time-consuming and require complicated instruments. There is a need to develop facile, economical and rapid methodologies with favorable sensitivity and selectivity for mercury sensing. Relative to the above methods, the fluorescence method has the advantages of higher sensitivity, simplicity and lower cost for the monitoring of metal ions. Chemical sensing of Hg2+ with all types of fluorescent probes, via Hg2+-induced changes in photoluminescence (PL), has abstracted much attention and has become a very active field of research (9–11). Quantum dots (QDs) are one of the most promising fluorescent probes because of their broad excitation spectra, narrow symmetric and tunable emission spectra (12,13). Many fluorescence sensors based on a QDs probe have been reported for the determination of Hg2+ (14–20) (Table 1). However, most of these reports are focused on the application of thiol-acid capped QDs, which showed poor
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selectivity with Pb2+, Cu2+, Ag+ and some coexisting metal ions. In 2006, Miyake et al. (21) reported that T–T mismatches could selectively capture Hg2+ to form thymine-Hg2+-thymine (T-Hg2+-T) base pairs, as the intrinsic specific interaction between Hg2+ and the N atom of thymine; this strategy might provide high selectivity toward Hg2+ over other related environmental heavy metal ions. A series of Hg2+ biosensors based on T–Hg2+–T were then developed (22–24). To overcome the high cost and sophisticated procedures using DNA, some ligands with an N atom in the structure were used for Hg2+ monitoring. Pei et al. (25) synthesized CA–CdTe QDs via a facile one-pot method and used it as the fluorescence probe. Because of the selective affinity of Hg2+ to the N atom, CA–CdTe QDs exhibited improved selectivity for the interference of Ag+ and Pb2+. However, the interference of Cu2+ can not be ignored. Furthermore, the detection limit of the method was 70 nmol/L, which was far higher than the regulatory requirements of mercury in drinking water (10 nmol/L) permitted by the United States Environmental Protection Agency (US EPA) (26). Establishing simple, selective and sensitive sensors for Hg2+ determination remains challenging. Because of their small size and high surface area-to-volume ratio, the PL of QDs is very sensitive to the surface states. Any factors that change the surface states of QDs can cause
* Correspondence to: R. Yang, The College of Chemistry and Molecular Engineering, Zhengzhou University, Kexue Road, Zhengzhou 450001, People’s Republic of China. E-mail: [email protected] a
The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China
b
School of Chemistry & Chemical Engineering, Henan University of Technology, Zhengzhou 450001, People’s Republic of China
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X. Ding et al. Table 1. Comparison of the detection performance of QDs-based sensing systems for the determination of Hg2+ Sensor
MPA-capped CdTe QDs C[6]/SiO2/CdTe NPs dOB coate CdTe QDs MAA-capped CdS/ZnS MAA-capped CdS QDs EMIDCA passivated TGA-capped CdTe QDs dBSA-coated CdTe QDs
Linear range (nM)
LOD (nM)
8–2,000 2.0–14.0 8–3,000 2.5–280 5–400 230–53,000 12–1,500
2.7 1.55 4.2 2.2 4.2 230 4
Interfering Ion 2+
Pb
X X √ X — X —
2+
Cu
— √ √ √ √ X X
Ref. +
Ag
— √ — √ X C X
14 15 16 17 18 19 20
MAA, mercaptoacetic acid; C[6], calix[6] arene; dOB, denatured ovalbumin; EMIDCA, 1-ethyl-3-methylimidazolium dicyanamide; TGA, thioglycolic acid; dBSA, denatured bovine serum albumin; √, X, — represent the ion affected, did not affect, was not considered to be the effect of the detection of Hg2+, respectively.
significant alteration in the detection performance of the QDs probe. The synthesis method is one such key factor. For example, Duan et al. (14) Gan et al. (27) and Rodrigues et al. (28) synthesized 3-mercaptopropionic acid (MPA)–CdTe QDs using a microwave irradiation method, a reflux method and a hydrothermal method, respectively, and used the obtained MPA–CdTe QDs as a probe for the determination of metal ions. It is surprising that the three types of MPA–CdTe QDs showed very different selectivity and sensitivity for the determination of Hg2+, Ag+ and Fe3+. In this case, we can conclude that changing the synthesis method may be a feasible way to improve the detection performance of the QDs sensor. However, no study has been published regarding the effect of the synthetic method on the detection performance of QDs as a probe. Reflux is the most widely used method for the preparation of water-soluble QDs. In our study, we found that CA–CdTe QDs synthesized using the reflux method showed better selectivity and sensitivity for the determination of Hg2+ than the CA–CdTe QDs synthesized using a facile one-pot synthetic method (25). Based on this, a very sensitive and selective fluorescence method for the determination of Hg2+ was established. This study reports the improvement in the detection performance of a QDs sensor by changing the synthesis method.
Experimental Reagents All chemicals were of analytical grade and were used without further purification and prepared with double-distilled water (DDW). Cysteamine hydrochloride hydrate (CA), sodium borohydride (NaBH4), Cadmium chloride hemipentahydrate (CdCl2.2.5H2O), Mercury chloride (HgCl2), Lead nitrate (Pb(NO3)2), Silver nitrate (AgNO3), Copper dichloride (CuCl2) and other chemicals were purchased from Sigma-Aldrich. Acetic–acetate buffer (10 mmol/L) was prepared using sodium acetate and acetic acid.
Apparatus The absorption spectra of samples were investigated on a Shimadzu UV-2550 s spectrophotometer. All fluorescence measurements were recorded using a Varian Cary Eclipse fluorescence spectrophotometer with both excitation and emission
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slits set at 10.0 nm. Dilute aqueous solutions of QDs were placed in 1 cm quartz cuvettes to scan the spectrum. High-resolution transmission electron microscopy (HRTEM) images of the nanoparticles were acquired on a JEOL JEM-2100 microscope. HRTEM specimens were prepared by depositing an appropriate amount of the CA–CdTe QDs onto the carbon-coated copper grid, and subsequently dried in air before measuring. Fourier transform infrared (FTIR) spectra were recorded using FTIR-8400 Shimadzu Fourier transform spectroscopy. All pH measurements were made with a pHs-3 digital pH-meter (Shanghai Lei Ci Device Works, Shanghai, China). All optical measurements were performed at room temperature and under ambient conditions.
Synthesis of CA-capped CdTe QDs CA–CdTe QDs were prepared in aqueous solution using the method described previously (29). In brief, 4.0 mL of ultrapure water, 0.1276 g of tellurium (Te) powder and 0.1135 g of NaBH4 were added to a clear flask and reacted in an ice-bath. Until the black Te powder completely disappeared, a purple transparent boron hydrogen telluride (NaHTe) solution was obtained. Under nitrogen protection, the fresh NaHTe solution was added to the aqueous CdCl2 in the presence of CA. The molar ratio of Te2-/Cd2+/CA was 1: 2: 4. After refluxing at 95 °C for 3.5 h, a bright orange CA–CdTe QDs solution was achieved. The CA–CdTe QDs were then purified by precipitation with isopropyl alcohol, the volume ratio of CdTe QDs to isopropyl alcohol was maintained at 1: 4. The obtained QDs were dissolved again in DDW and used in the experiment. No precipitation was observed over a two-month period. The concentration of colloidal quantum dots was calculated using the original tellurium source and found to be 0.01 mol/L.
Fluorescence measurement Fluorescence quenching of the CA–CdTe QDs originating from Hg2+ was performed in acetic–acetate buffer solution at pH 5.0. To a 10.0 mL colorimetric tube, 60 μL of 0.01 mol/L CA–CdTe QDs, 70 μL of 10.0 mmol/L acetic–acetate buffer and an appropriate amount of Hg2+ stock solution were sequentially added to give the desired concentrations. The mixture was diluted with DDW to a final volume of 3.00 mL and then incubated at room temperature for 5 min. The fluorescence intensity was measured at λem/λex = 550/350 nm.
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Fluorescent sensor for mercury (III) Effect of pH and buffer concentration
Results and discussion Characterization of CA–CdTe QDs Absorption and emission spectra of the CA–CdTe QDs are shown in Fig. 1(A). It can be seen that the obtained QDs have obvious absorption and fluorescence emission. In addition, the fluorescence emission spectrum was very narrow and symmetric. This suggested that the obtained CA–CdTe QDs have good monodispersity and homogeneity (30). The morphology of the CA–CdTe QDs verified this (Fig. 1B). Using Eqn 1 below (31), the diameter of these nanocrystals was calculated to be 2.89 nm. D ¼ 9:8127 ´ 107 λ3 – 1:7147 103 λ2
(1)
þ ð1:0064Þ λ – 194:84 where λ is the maximum absorption of CdTe QDs. Figure 1(C) shows the IR spectra of CA and CA–CdTe QDs. Compared with the IR spectra of CA, the missing peak at 2491.62 cm-1 indicates the absence of -SH in the CA–CdTe QDs. Moreover, the peaks at 3439.59 cm -1 and 1589.19 cm-1, and the appearance of peaks from 1038 to 1377 cm-1 represent the presence of -NH 2, -CH2- and C–N the in the CA–CdTe QDs, respectively. All these indicted the covalent bonds thiol with the cadmium (Cd) atom of the surface of nanoparticles.
In this work, the capping molecule of CA is a primary amine compound (a weak base), and its existing form varied with the change of pH, which yields in consequence a significant influence on the surface state of CA–CdTe QDs and its corresponding performance as a sensor. So the effect of pH on the detection of Hg2+ over a range of 3.5–10.0 was studied. The experiment was carried out by detecting the fluorescence intensity of CA–CdTe QDs in the absence and presence of Hg2+ in 0.33 mmol/L acetic–acetate buffer (3.5–6.5) and Tris/HCl buffer (7.0–10.0) at different pH values. The change in pH has little influence on the fluorescence of CA–CdTe QDs, but does have an important impact on the interaction of CA–CdTe QDs and Hg2+ (Fig. 2A). Maximum fluorescence quenching occurred at pH 5.0 (inset of Fig. 2A), so pH 5.0 was selected for all the experiments. To maintain pH 5.0, a series of acetic–acetate buffers with different concentrations ranging from 0.10 to 0.83 mmol/L was prepared. The fluorescence intensity of CA–CdTe QDs was investigated in the absence and presence of Hg2+ in the above-prepared buffers (Fig. 2B). The highest fluorescence quenching efficiency was obtained at 0.23 mmol/L. Therefore, 0.23 mmol/L was chosen as the optimal concentration of acetic–acetate buffer.
Effect of CA–CdTe QDs concentration The synthesized particles are not exactly the same size and the larger particles often absorbed the smaller ones. At different
Figure 1. (A) Absorption spectrum (a) and fluorescence emission spectra of the CA–CdTe QDs (b). (B) TEM image of CA–CdTe QDs. (C) IR spectra of cysteamine hydrochloride hydrate and the CA–CdTe QDs.
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X. Ding et al.
2+
Figure 2. (A) Effect of pH on the fluorescence intensity of the CA–CdTe QDs and Hg system: fluorescence intensity of the CA–CdTe QDs in the absence (a) and presence 2+ 2+ (b) of Hg . (Inset) F0/F vs. pH. (B) Effect of concentration of acetic–acetate buffer on the sensitivity of Hg : fluorescence intensity of the CA–CdTe QDs in the absence (a) and 2+ presence (b) of Hg . (Inset) F0/F vs. the volume of acetic–acetate buffer.
concentrations, the absorption effect is very different, which caused a different surface state yielding as a consequence an alteration in their detection performance as a fluorescence probe. To investigate the effect of the concentration of CA–CdTe QDs, the quenching efficiency of Hg2+ on CA–CdTe QDs at three different concentrations (0.10, 0.15 and 0.20 mmol/L) was investigated. The quenching efficiency against the concentration of Hg2+ at three different concentrations of QDs is shown in Fig. 3. A CA-CdTe QDs concentration of 0.20 mmol/L was chosen because at this concentration the detection system showed the greatest sensitivity and the widest linear range. Effect of reaction time
was selected to guarantee the complete reaction between Hg2+ and CA–CdTe QDs. Effect of foreign ions To investigate the selectivity of the CA–CdTe QDs towards Hg2+ ions, the fluorescence of CA–CdTe QDs in 80 nmol/L Hg2+ and 10-fold concentrations of other metal ions were studied (Fig. 5). It can be seen that even with a 10-fold addition of metal ions, the quenching effect on QDs caused by other metal ions was still not above 7%, which was much smaller than that caused by Hg2+. Therefore, it can be concluded that CA–CdTe QDs showed high recognition selectivity to Hg2+.
The reaction time between CA–CdTe QDs and Hg2+ was investigated by recording the fluorescence of CA–CdTe QDs at various time intervals (0–30 min) after adding different concentrations of Hg2+ to the CA–CdTe QDs solution (Fig. 4). The results showed that the reaction between Hg2+ and CA–CdTe QDs occurs rapidly, reaching equilibrium in ~ 5 min at room temperature, and remaining stable for at least 25 min. Thus, a 5-min reaction time
Under optimum conditions, the fluorescence spectra of CA–CdTe QDs with different concentrations of Hg2+ were investigated and recorded, as shown in Fig. 6. The fluorescence quenching efficiency was proportional to the concentration of Hg2+ over the range of 6.0–450 nmol/L. The linear regression equation can be expressed
Figure 3. Effect of CA–CdTe QD concentration on their detection performances. CA–CdTe QD concentrations (in mmol/L) were: (a) 0.20, (b) 0.15 and (c) 0.10.
Figure 4. Fluorescence intensity of CA–CdTe QDs at varying time intervals and 2+ 2+ different Hg concentrations. The concentrations of Hg (in nmol/L) were: (a) 0, (b) 60 and (c) 200.
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Analytical performance of CA-CdTe QDs
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Fluorescent sensor for mercury (III)
Figure 5. Effect of different metal ions on the fluorescence intensity of CA–CdTe 2+ QDs. Hg : 80 nmol/L; other metal ions: 800 nmol/L.
as F0/F = 1.037 + 0.0093C (C: nmol/L) (r = 0.9990). The detection limit (LOD), calculated following the 3σ IUPAC criteria is 4.0 nmol/L, which is lower than the tolerance limit of 10 nmol/L for mercury in drinking water permitted by the US EPA (26). Relative to the reported methods, it can be seen that the proposed method showed the most sensitivity and selectivity (Table 1).
Where, F0 and F are the fluorescence intensities in the absence and presence of the quencher, Q, Kq is the quenching rate constant of the biomolecule, τ 0 is the average lifetime of molecule without the quencher and its value is 10-8 s. Ksv is the Stern– Volmer dynamic quenching constant. Based on the experimental data, Kq was calculated as 9.3 × 1014 L/mol/s, which was much greater than 40,000 times the maximum scatter collision quenching constant of various other quenchers for the biopolymer (2.0 × 1010 L/mol/s) (32). This implies that the fluorescence quenching arises from the static quenching mechanism. Static quenching of the fluorescence of QDs may happen by energy transfer (34), charge diverting (35) and surface absorption (36). To investigate the fluorescence quenching mechanism of CA–CdTe QDs originated from Hg2+, UV/Vis absorption spectra of the interaction between CA–CdTe QDs and Hg2+ were studied (Fig. 7). On addition of Hg2+, the excitation peak became gentle with no obvious blue-shift or red-shift, showing that the CA–CdTe QDs had become asymmetrical (37). In addition, there is no significant change in the fluorescence spectra of CA–CdTe QDs caused by Hg2+ (Fig. 6). According to studies on the mechanisms of fluorescence quenching of QDs and metal ions (38–40), the above process can be attributed to effective electron transfer from the amide groups of CA to the Hg2+ ion, based on the strong affinity of mercury for nitrogen atoms, which correspondingly yields the non-radiative recombination of excited electrons (e–) in the conduction bands and holes (h+) in the valence band on the surface of QDs (Scheme 1)
The mechanism of fluorescence quenching Fluorescence quenching may result from two causes (32). One is dynamic quenching, which results from the collision between the fluorophore and a quencher. The other is static quenching, which results from the ground-state complex formation between the fluorophore and a quencher. To ascertain the cause of the quenching, the Stern–Volmer equation (33) was first applied to model the quenching of CA–CdTe QDs with respect to the Hg2+ concentration, it is described as:
Analysis of samples
(2)
To investigate the applicability of the proposed method for the determination of Hg2+, a water sample from a local lake (the lake at Zhengzhou University) was used for quantitative analysis. The sample was first filtered through qualitative filter paper. The pH of the sample was detected as 6.39, which was not the optimum pH, so an appropriated amount HAc was added to the samples to adjust the pH. No obvious fluorescence quenching was found for the pretreated lake sample. Recovery experiments were performed by measuring the fluorescence intensity for lake water samples to which known concentrations of Hg2+ had been
Figure 6. The fluorescence spectra of CA–CdTe QDs in the presence various concen2+ 2+ trations of Hg . (Inset) F0/F vs. c. The concentrations of Hg (in nmol/L) were: (a) 0, (b) 6.0, (c) 8.0, (d) 10, (e) 20, (f) 40, (g) 60, (h) 80, (i) 150, (j) 250, (k) 350 ansd (l) 450.
Figure 7. Absorption spectrum of CA-CdTe QDs in the absence (a) and presence 2+ (b) of Hg .
F 0 =F ¼ 1 þ K q τ 0 ½Q ¼ 1 þ K sv ½Q
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X. Ding et al.
Scheme 1. Schematic illustration for mercury sensing based on a cysteamine-capped CdTe QD fluorescence system.
Table 2. The recoveries of the detection of Hg2+ in lake water sample (n = 5) Samples Added standard Found total value solution (mol/L) (mol/L) 1 2 3
2.0 × 108 8.0 × 108 4.0 × 107
1.94 × 108 8.51 × 108 4.19 × 107
RSD (%) 0.41 1.02 4.15
Recovery (%) 97.0 106.4 104.8
added. The recoveries for Hg2+ ranged from 97.0 to 106.4% (Table 2). These results clearly indicated the applicability and reliability of the proposed method.
Conclusion In this study, a highly selective and sensitive fluorescent sensor for Hg2+ was proposed based on selective quenching of the fluorescence of cysteamine-capped water-soluble CdTe QDs by Hg2+. Relative to other fluorescence sensors based on crude QDs, the proposed sensor was more sensitive and selective. In particular, it showed better selectivity and sensitivity than the method based on the CA–CdTe QDs synthesized with the one-pot synthetic method and this illustrates the effect of the synthesis method of QDs on their detection performance as a fluorescence probe. The CA–CdTe QDs sensor system in this study represents a new possibility to improve the detection performance of a QDs sensor by changing the synthesis method.
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Revised: 15 July 2014,
Accepted: 3 August 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2761
A highly selective and simple fluorescent sensor for mercury (II) ion detection based on cysteamine-capped CdTe quantum dots synthesized by the reflux method Xiaojie Ding,a Lingbo Qu,a,b Ran Yang,a* Yuchen Zhoua and Jianjun Lia ABSTRACT: Cysteamine (CA)-capped CdTe quantum dots (QDs) (CA–CdTe QDs) were prepared by the reflux method and utilized as an efficient nano-sized fluorescent sensor to detect mercury (II) ions (Hg2+). Under optimum conditions, the fluorescence quenching effect of CA–CdTe QDs was linear at Hg2+ concentrations in the range of 6.0–450 nmol/L. The detection limit was calculated to be 4.0 nmol/L according to the 3σ IUPAC criteria. The influence of 10-fold Pb2+, Cu2+ and Ag+ on the determination of Hg2+ was < 7% (superior to other reports based on crude QDs). Furthermore, the detection sensitivity and selectivity were much improved relative to a sensor based on the CA–CdTe QDs probe, which was prepared using a one-pot synthetic method. This CA–CdTe QDs sensor system represents a new feasibility to improve the detection performance of a QDs sensor by changing the synthesis method. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: cysteamine; CdTe quantum dots; mercury (II) ion; fluorescent sensor
Introduction The mercury (II) ion (Hg2+), one of the most hazardous environmental toxins, is ubiquitous in the environment. It can accumulate in the vital organs and tissues of animals and result in a wide variety of diseases (1,2). In particular, mercury harmfully affects the function and development of the central nervous system in people (3,4). Mercury pollution has become a serious environmental health problem all over the word. From the viewpoint of environmental protection and health concerns, developing an effective method for the sensitive detection of trace amounts of Hg2+ is very important and has attracted a significant amount of attention. A variety of traditional methods including atomic absorption spectrometry (AAS) (5), atomic fluorescence spectrometry (AFS) (6), inductively coupled plasma mass spectroscopy (ICP-MS) (7), X-ray absorption spectroscopy and electrochemical methods (8) have been widely used for measuring Hg2+. However, these methods are time-consuming and require complicated instruments. There is a need to develop facile, economical and rapid methodologies with favorable sensitivity and selectivity for mercury sensing. Relative to the above methods, the fluorescence method has the advantages of higher sensitivity, simplicity and lower cost for the monitoring of metal ions. Chemical sensing of Hg2+ with all types of fluorescent probes, via Hg2+-induced changes in photoluminescence (PL), has abstracted much attention and has become a very active field of research (9–11). Quantum dots (QDs) are one of the most promising fluorescent probes because of their broad excitation spectra, narrow symmetric and tunable emission spectra (12,13). Many fluorescence sensors based on a QDs probe have been reported for the determination of Hg2+ (14–20) (Table 1). However, most of these reports are focused on the application of thiol-acid capped QDs, which showed poor
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selectivity with Pb2+, Cu2+, Ag+ and some coexisting metal ions. In 2006, Miyake et al. (21) reported that T–T mismatches could selectively capture Hg2+ to form thymine-Hg2+-thymine (T-Hg2+-T) base pairs, as the intrinsic specific interaction between Hg2+ and the N atom of thymine; this strategy might provide high selectivity toward Hg2+ over other related environmental heavy metal ions. A series of Hg2+ biosensors based on T–Hg2+–T were then developed (22–24). To overcome the high cost and sophisticated procedures using DNA, some ligands with an N atom in the structure were used for Hg2+ monitoring. Pei et al. (25) synthesized CA–CdTe QDs via a facile one-pot method and used it as the fluorescence probe. Because of the selective affinity of Hg2+ to the N atom, CA–CdTe QDs exhibited improved selectivity for the interference of Ag+ and Pb2+. However, the interference of Cu2+ can not be ignored. Furthermore, the detection limit of the method was 70 nmol/L, which was far higher than the regulatory requirements of mercury in drinking water (10 nmol/L) permitted by the United States Environmental Protection Agency (US EPA) (26). Establishing simple, selective and sensitive sensors for Hg2+ determination remains challenging. Because of their small size and high surface area-to-volume ratio, the PL of QDs is very sensitive to the surface states. Any factors that change the surface states of QDs can cause
* Correspondence to: R. Yang, The College of Chemistry and Molecular Engineering, Zhengzhou University, Kexue Road, Zhengzhou 450001, People’s Republic of China. E-mail: [email protected] a
The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China
b
School of Chemistry & Chemical Engineering, Henan University of Technology, Zhengzhou 450001, People’s Republic of China
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X. Ding et al. Table 1. Comparison of the detection performance of QDs-based sensing systems for the determination of Hg2+ Sensor
MPA-capped CdTe QDs C[6]/SiO2/CdTe NPs dOB coate CdTe QDs MAA-capped CdS/ZnS MAA-capped CdS QDs EMIDCA passivated TGA-capped CdTe QDs dBSA-coated CdTe QDs
Linear range (nM)
LOD (nM)
8–2,000 2.0–14.0 8–3,000 2.5–280 5–400 230–53,000 12–1,500
2.7 1.55 4.2 2.2 4.2 230 4
Interfering Ion 2+
Pb
X X √ X — X —
2+
Cu
— √ √ √ √ X X
Ref. +
Ag
— √ — √ X C X
14 15 16 17 18 19 20
MAA, mercaptoacetic acid; C[6], calix[6] arene; dOB, denatured ovalbumin; EMIDCA, 1-ethyl-3-methylimidazolium dicyanamide; TGA, thioglycolic acid; dBSA, denatured bovine serum albumin; √, X, — represent the ion affected, did not affect, was not considered to be the effect of the detection of Hg2+, respectively.
significant alteration in the detection performance of the QDs probe. The synthesis method is one such key factor. For example, Duan et al. (14) Gan et al. (27) and Rodrigues et al. (28) synthesized 3-mercaptopropionic acid (MPA)–CdTe QDs using a microwave irradiation method, a reflux method and a hydrothermal method, respectively, and used the obtained MPA–CdTe QDs as a probe for the determination of metal ions. It is surprising that the three types of MPA–CdTe QDs showed very different selectivity and sensitivity for the determination of Hg2+, Ag+ and Fe3+. In this case, we can conclude that changing the synthesis method may be a feasible way to improve the detection performance of the QDs sensor. However, no study has been published regarding the effect of the synthetic method on the detection performance of QDs as a probe. Reflux is the most widely used method for the preparation of water-soluble QDs. In our study, we found that CA–CdTe QDs synthesized using the reflux method showed better selectivity and sensitivity for the determination of Hg2+ than the CA–CdTe QDs synthesized using a facile one-pot synthetic method (25). Based on this, a very sensitive and selective fluorescence method for the determination of Hg2+ was established. This study reports the improvement in the detection performance of a QDs sensor by changing the synthesis method.
Experimental Reagents All chemicals were of analytical grade and were used without further purification and prepared with double-distilled water (DDW). Cysteamine hydrochloride hydrate (CA), sodium borohydride (NaBH4), Cadmium chloride hemipentahydrate (CdCl2.2.5H2O), Mercury chloride (HgCl2), Lead nitrate (Pb(NO3)2), Silver nitrate (AgNO3), Copper dichloride (CuCl2) and other chemicals were purchased from Sigma-Aldrich. Acetic–acetate buffer (10 mmol/L) was prepared using sodium acetate and acetic acid.
Apparatus The absorption spectra of samples were investigated on a Shimadzu UV-2550 s spectrophotometer. All fluorescence measurements were recorded using a Varian Cary Eclipse fluorescence spectrophotometer with both excitation and emission
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slits set at 10.0 nm. Dilute aqueous solutions of QDs were placed in 1 cm quartz cuvettes to scan the spectrum. High-resolution transmission electron microscopy (HRTEM) images of the nanoparticles were acquired on a JEOL JEM-2100 microscope. HRTEM specimens were prepared by depositing an appropriate amount of the CA–CdTe QDs onto the carbon-coated copper grid, and subsequently dried in air before measuring. Fourier transform infrared (FTIR) spectra were recorded using FTIR-8400 Shimadzu Fourier transform spectroscopy. All pH measurements were made with a pHs-3 digital pH-meter (Shanghai Lei Ci Device Works, Shanghai, China). All optical measurements were performed at room temperature and under ambient conditions.
Synthesis of CA-capped CdTe QDs CA–CdTe QDs were prepared in aqueous solution using the method described previously (29). In brief, 4.0 mL of ultrapure water, 0.1276 g of tellurium (Te) powder and 0.1135 g of NaBH4 were added to a clear flask and reacted in an ice-bath. Until the black Te powder completely disappeared, a purple transparent boron hydrogen telluride (NaHTe) solution was obtained. Under nitrogen protection, the fresh NaHTe solution was added to the aqueous CdCl2 in the presence of CA. The molar ratio of Te2-/Cd2+/CA was 1: 2: 4. After refluxing at 95 °C for 3.5 h, a bright orange CA–CdTe QDs solution was achieved. The CA–CdTe QDs were then purified by precipitation with isopropyl alcohol, the volume ratio of CdTe QDs to isopropyl alcohol was maintained at 1: 4. The obtained QDs were dissolved again in DDW and used in the experiment. No precipitation was observed over a two-month period. The concentration of colloidal quantum dots was calculated using the original tellurium source and found to be 0.01 mol/L.
Fluorescence measurement Fluorescence quenching of the CA–CdTe QDs originating from Hg2+ was performed in acetic–acetate buffer solution at pH 5.0. To a 10.0 mL colorimetric tube, 60 μL of 0.01 mol/L CA–CdTe QDs, 70 μL of 10.0 mmol/L acetic–acetate buffer and an appropriate amount of Hg2+ stock solution were sequentially added to give the desired concentrations. The mixture was diluted with DDW to a final volume of 3.00 mL and then incubated at room temperature for 5 min. The fluorescence intensity was measured at λem/λex = 550/350 nm.
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Fluorescent sensor for mercury (III) Effect of pH and buffer concentration
Results and discussion Characterization of CA–CdTe QDs Absorption and emission spectra of the CA–CdTe QDs are shown in Fig. 1(A). It can be seen that the obtained QDs have obvious absorption and fluorescence emission. In addition, the fluorescence emission spectrum was very narrow and symmetric. This suggested that the obtained CA–CdTe QDs have good monodispersity and homogeneity (30). The morphology of the CA–CdTe QDs verified this (Fig. 1B). Using Eqn 1 below (31), the diameter of these nanocrystals was calculated to be 2.89 nm. D ¼ 9:8127 ´ 107 λ3 – 1:7147 103 λ2
(1)
þ ð1:0064Þ λ – 194:84 where λ is the maximum absorption of CdTe QDs. Figure 1(C) shows the IR spectra of CA and CA–CdTe QDs. Compared with the IR spectra of CA, the missing peak at 2491.62 cm-1 indicates the absence of -SH in the CA–CdTe QDs. Moreover, the peaks at 3439.59 cm -1 and 1589.19 cm-1, and the appearance of peaks from 1038 to 1377 cm-1 represent the presence of -NH 2, -CH2- and C–N the in the CA–CdTe QDs, respectively. All these indicted the covalent bonds thiol with the cadmium (Cd) atom of the surface of nanoparticles.
In this work, the capping molecule of CA is a primary amine compound (a weak base), and its existing form varied with the change of pH, which yields in consequence a significant influence on the surface state of CA–CdTe QDs and its corresponding performance as a sensor. So the effect of pH on the detection of Hg2+ over a range of 3.5–10.0 was studied. The experiment was carried out by detecting the fluorescence intensity of CA–CdTe QDs in the absence and presence of Hg2+ in 0.33 mmol/L acetic–acetate buffer (3.5–6.5) and Tris/HCl buffer (7.0–10.0) at different pH values. The change in pH has little influence on the fluorescence of CA–CdTe QDs, but does have an important impact on the interaction of CA–CdTe QDs and Hg2+ (Fig. 2A). Maximum fluorescence quenching occurred at pH 5.0 (inset of Fig. 2A), so pH 5.0 was selected for all the experiments. To maintain pH 5.0, a series of acetic–acetate buffers with different concentrations ranging from 0.10 to 0.83 mmol/L was prepared. The fluorescence intensity of CA–CdTe QDs was investigated in the absence and presence of Hg2+ in the above-prepared buffers (Fig. 2B). The highest fluorescence quenching efficiency was obtained at 0.23 mmol/L. Therefore, 0.23 mmol/L was chosen as the optimal concentration of acetic–acetate buffer.
Effect of CA–CdTe QDs concentration The synthesized particles are not exactly the same size and the larger particles often absorbed the smaller ones. At different
Figure 1. (A) Absorption spectrum (a) and fluorescence emission spectra of the CA–CdTe QDs (b). (B) TEM image of CA–CdTe QDs. (C) IR spectra of cysteamine hydrochloride hydrate and the CA–CdTe QDs.
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X. Ding et al.
2+
Figure 2. (A) Effect of pH on the fluorescence intensity of the CA–CdTe QDs and Hg system: fluorescence intensity of the CA–CdTe QDs in the absence (a) and presence 2+ 2+ (b) of Hg . (Inset) F0/F vs. pH. (B) Effect of concentration of acetic–acetate buffer on the sensitivity of Hg : fluorescence intensity of the CA–CdTe QDs in the absence (a) and 2+ presence (b) of Hg . (Inset) F0/F vs. the volume of acetic–acetate buffer.
concentrations, the absorption effect is very different, which caused a different surface state yielding as a consequence an alteration in their detection performance as a fluorescence probe. To investigate the effect of the concentration of CA–CdTe QDs, the quenching efficiency of Hg2+ on CA–CdTe QDs at three different concentrations (0.10, 0.15 and 0.20 mmol/L) was investigated. The quenching efficiency against the concentration of Hg2+ at three different concentrations of QDs is shown in Fig. 3. A CA-CdTe QDs concentration of 0.20 mmol/L was chosen because at this concentration the detection system showed the greatest sensitivity and the widest linear range. Effect of reaction time
was selected to guarantee the complete reaction between Hg2+ and CA–CdTe QDs. Effect of foreign ions To investigate the selectivity of the CA–CdTe QDs towards Hg2+ ions, the fluorescence of CA–CdTe QDs in 80 nmol/L Hg2+ and 10-fold concentrations of other metal ions were studied (Fig. 5). It can be seen that even with a 10-fold addition of metal ions, the quenching effect on QDs caused by other metal ions was still not above 7%, which was much smaller than that caused by Hg2+. Therefore, it can be concluded that CA–CdTe QDs showed high recognition selectivity to Hg2+.
The reaction time between CA–CdTe QDs and Hg2+ was investigated by recording the fluorescence of CA–CdTe QDs at various time intervals (0–30 min) after adding different concentrations of Hg2+ to the CA–CdTe QDs solution (Fig. 4). The results showed that the reaction between Hg2+ and CA–CdTe QDs occurs rapidly, reaching equilibrium in ~ 5 min at room temperature, and remaining stable for at least 25 min. Thus, a 5-min reaction time
Under optimum conditions, the fluorescence spectra of CA–CdTe QDs with different concentrations of Hg2+ were investigated and recorded, as shown in Fig. 6. The fluorescence quenching efficiency was proportional to the concentration of Hg2+ over the range of 6.0–450 nmol/L. The linear regression equation can be expressed
Figure 3. Effect of CA–CdTe QD concentration on their detection performances. CA–CdTe QD concentrations (in mmol/L) were: (a) 0.20, (b) 0.15 and (c) 0.10.
Figure 4. Fluorescence intensity of CA–CdTe QDs at varying time intervals and 2+ 2+ different Hg concentrations. The concentrations of Hg (in nmol/L) were: (a) 0, (b) 60 and (c) 200.
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Analytical performance of CA-CdTe QDs
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Fluorescent sensor for mercury (III)
Figure 5. Effect of different metal ions on the fluorescence intensity of CA–CdTe 2+ QDs. Hg : 80 nmol/L; other metal ions: 800 nmol/L.
as F0/F = 1.037 + 0.0093C (C: nmol/L) (r = 0.9990). The detection limit (LOD), calculated following the 3σ IUPAC criteria is 4.0 nmol/L, which is lower than the tolerance limit of 10 nmol/L for mercury in drinking water permitted by the US EPA (26). Relative to the reported methods, it can be seen that the proposed method showed the most sensitivity and selectivity (Table 1).
Where, F0 and F are the fluorescence intensities in the absence and presence of the quencher, Q, Kq is the quenching rate constant of the biomolecule, τ 0 is the average lifetime of molecule without the quencher and its value is 10-8 s. Ksv is the Stern– Volmer dynamic quenching constant. Based on the experimental data, Kq was calculated as 9.3 × 1014 L/mol/s, which was much greater than 40,000 times the maximum scatter collision quenching constant of various other quenchers for the biopolymer (2.0 × 1010 L/mol/s) (32). This implies that the fluorescence quenching arises from the static quenching mechanism. Static quenching of the fluorescence of QDs may happen by energy transfer (34), charge diverting (35) and surface absorption (36). To investigate the fluorescence quenching mechanism of CA–CdTe QDs originated from Hg2+, UV/Vis absorption spectra of the interaction between CA–CdTe QDs and Hg2+ were studied (Fig. 7). On addition of Hg2+, the excitation peak became gentle with no obvious blue-shift or red-shift, showing that the CA–CdTe QDs had become asymmetrical (37). In addition, there is no significant change in the fluorescence spectra of CA–CdTe QDs caused by Hg2+ (Fig. 6). According to studies on the mechanisms of fluorescence quenching of QDs and metal ions (38–40), the above process can be attributed to effective electron transfer from the amide groups of CA to the Hg2+ ion, based on the strong affinity of mercury for nitrogen atoms, which correspondingly yields the non-radiative recombination of excited electrons (e–) in the conduction bands and holes (h+) in the valence band on the surface of QDs (Scheme 1)
The mechanism of fluorescence quenching Fluorescence quenching may result from two causes (32). One is dynamic quenching, which results from the collision between the fluorophore and a quencher. The other is static quenching, which results from the ground-state complex formation between the fluorophore and a quencher. To ascertain the cause of the quenching, the Stern–Volmer equation (33) was first applied to model the quenching of CA–CdTe QDs with respect to the Hg2+ concentration, it is described as:
Analysis of samples
(2)
To investigate the applicability of the proposed method for the determination of Hg2+, a water sample from a local lake (the lake at Zhengzhou University) was used for quantitative analysis. The sample was first filtered through qualitative filter paper. The pH of the sample was detected as 6.39, which was not the optimum pH, so an appropriated amount HAc was added to the samples to adjust the pH. No obvious fluorescence quenching was found for the pretreated lake sample. Recovery experiments were performed by measuring the fluorescence intensity for lake water samples to which known concentrations of Hg2+ had been
Figure 6. The fluorescence spectra of CA–CdTe QDs in the presence various concen2+ 2+ trations of Hg . (Inset) F0/F vs. c. The concentrations of Hg (in nmol/L) were: (a) 0, (b) 6.0, (c) 8.0, (d) 10, (e) 20, (f) 40, (g) 60, (h) 80, (i) 150, (j) 250, (k) 350 ansd (l) 450.
Figure 7. Absorption spectrum of CA-CdTe QDs in the absence (a) and presence 2+ (b) of Hg .
F 0 =F ¼ 1 þ K q τ 0 ½Q ¼ 1 þ K sv ½Q
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X. Ding et al.
Scheme 1. Schematic illustration for mercury sensing based on a cysteamine-capped CdTe QD fluorescence system.
Table 2. The recoveries of the detection of Hg2+ in lake water sample (n = 5) Samples Added standard Found total value solution (mol/L) (mol/L) 1 2 3
2.0 × 108 8.0 × 108 4.0 × 107
1.94 × 108 8.51 × 108 4.19 × 107
RSD (%) 0.41 1.02 4.15
Recovery (%) 97.0 106.4 104.8
added. The recoveries for Hg2+ ranged from 97.0 to 106.4% (Table 2). These results clearly indicated the applicability and reliability of the proposed method.
Conclusion In this study, a highly selective and sensitive fluorescent sensor for Hg2+ was proposed based on selective quenching of the fluorescence of cysteamine-capped water-soluble CdTe QDs by Hg2+. Relative to other fluorescence sensors based on crude QDs, the proposed sensor was more sensitive and selective. In particular, it showed better selectivity and sensitivity than the method based on the CA–CdTe QDs synthesized with the one-pot synthetic method and this illustrates the effect of the synthesis method of QDs on their detection performance as a fluorescence probe. The CA–CdTe QDs sensor system in this study represents a new possibility to improve the detection performance of a QDs sensor by changing the synthesis method.
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