Picomolar Level Detection of Copper(II) and Mercury(II) Ions Using Dual-Stabilizer-Capped CdTe Quantum Dots

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Abstract

In this paper, dual-stabilizer-capped CdTe quantum dots were used as modulated photoluminescence (PL) sensors for the subpicomolar level detection of copper(II) (Cu2+) and mercury(II) (Hg2+) ions in aqueous solution for the first time. The dual-stabilizer-capped CdTe quantum dots were synthesized using mercaptopropionic acid (MPA) and sodium hexametaphosphate (SHMP) as surface-modified ligands via a convenient hydrothermal process. The researches showed a low interference response of the MPA-SHMP-capped CdTe quantum dots towards other metal ions. The highly efficient PL quenching ability in the presence of Hg2+ or Cu2+ ions due to the formed nonfluorescent metal complexes via robust Hg2+–O interaction with the carboxy oxygen elements of surface ligands of MPA, and on the basis of the competitive binding of the mercapto groups of the MPA between the CdTe quantum dots and the Cu2+ ions, respectively, which allowed the analysis of Hg2+ or Cu2+ ions down to the picomolar levels. Under optimal conditions, the response of the MPA-SHMP-capped CdTe quantum dot PL intensity is linearly proportional to the Cu2+ and Hg2+ ion concentration ranging from 0.1 to 1000 and 0.3 to 1000 nM with a detection limit of 41.6 and 97.0 pM, respectively. The diagnostic capability and potential in practical applications of this method have been demonstrated by detecting Cu2+ and Hg2+ ions in environmental water samples.

Keywords

Dual-stabilizer-capped CdTe quantum dots Photoluminescence Multi-analyte detection Picomolar levels 

1 Introduction

Semiconductor nanocrystals, usually referred to as quantum dots, are promising luminescent candidates for labeling in photoluminescence (PL) sensing since their discovery by Ekimov and co-workers [1, 2, 3] because of their exceptional electrical [4], optical [5] and mechanical properties [6]. In the past few years, much efforts have been contributed to improve synthesis method of colloidal nanocrystals [3] and their application in electrochemiluminescence (ECL) nano-emitters [7, 8]. Although many sensing strategies based on the ECL of nano-emitters were developed [9, 10, 11], it was still a challenge to synthesize semiconductor nanocrystals as efficient PL emitters, to say nothing of the multi-analyte detection. Recently, by utilizing quantum dots capped with different ligands have been extensively explored as PL probes for the quantitative determination of biological molecules and metal ions on the basis of the PL quenching or enhancing effect. For example, the PL chemosensors with the use of single-stabilizer-capped CdTe quantum dots for the detection of biomolecules [12, 13, 14] and metal ions [15] have been reported. Afterwards, surface-modified CdSe quantum dots as PL probes displayed a selective response for cyanide and iodide ions, whereas pure CdSe quantum dots do not display the similar phenomena [16, 17]. Therefore, surface-modified ligands play an important role and the choice of surface ligands is especially important in the picomolar level detection of metal ions and biomolecules. Nevertheless, nearly all the reported single stabilizer-capped quantum dots sensors for heavy metal ions or biomolecules detection are of a type with one readout signal, leading to the great limitation of further applications of the quantum dots in practical detection. In contrary, the PL chemosensors by the dual-stabilizer-capped quantum dots may be a conceivable strategy to overcome the problems toward the picomolar level detection of multiple heavy metal ions.

The development of transition heavy metal ion chemosensors with high sensitivity, selectivity and ultratrace (picomolar) level detection has been receiving considerable attention due to their potential to do great damage to the environment and the human body even at low concentrations [18, 19, 20, 21]. Among the various transition, heavy metal ions, copper(II) (Cu2+) and mercury(II) (Hg2+) ions are significant environmental contamination and two essential trace elements in biological systems [22]. Specifically, Cu2+ and Hg2+ ions with a very high toxicity also causes a variety of permanent damages and long-term adverse effects to human health, such as neurodegenerative and stomach disturbance, liver or kidney damage, loss of cognition in the elderly, individuals with Alzheimer’s, Menkes, or Wilson’s disease and the central nervous systems [23, 24, 25, 26, 27], and people are thus put on high alert to avoid the occurrence of shocking pollutant events. Therefore, the detection Cu2+ and Hg2+ ions of picomolar levels is a necessary thing to be done by societal institutions and governments, which plays a crucial role in environmental protection and food safety. Up until now, numerous availably of PL chemosensors for Cu2+ or Hg2+ ion in aqueous solvent have been reported [18, 28, 29, 30, 31, 32]. However, there have been great achievements in the development of chemosensors singly for Cu2+ or Hg2+ ion, but research in the detection of Cu2+ and Hg2+ ion with one PL nanomaterial is still rare [15]. Therefore, it is still a great challenge to develop PL chemosensors that can selectively recognize multiple analytes and down to the picomolar levels.

Herein, CdTe quantum dots with a high PL performance were prepared in an aqueous solution with mercaptopropionic acid (MPA) and sodium hexametaphosphate (SHMP) as surface ligands [33]. Hereafter, we present highly sensitive PL method for determination of Cu2+ and Hg2+ ions in solution down to subpicomolar concentrations is proposed. Benefiting from the multivalent heavy metal ion-specific chemical binding sites of MPA-SHMP-capped CdTe quantum dots and the PL quenching effect of Cu2+ and Hg2+ ions, this method provides a label-free, rapid, and highly sensitive detection of Cu2+ and Hg2+ ions. In addition, the CdTe quantum dots with the dual stabilizers, MPA and SHMP, not only greatly enhances the PL intensities and monochromaticity by way of effectively removing the nonradiative surface states and deep surface traps but also makes the chemical reaction processes stable between CdTe quantum dots and Cu2+/Hg2+ ions [34]. The principle of the proposed Cu2+ and Hg2+ ion sensing concept is shown in Scheme 1. To the best of our knowledge, this is the first example of the construction of a PL sensing platform for Cu2+ and Hg2+ ions based on the MPA-SHMP-capped CdTe quantum dots.
Scheme 1

Schematic diagram of the mechanism of the detection of Cu2+ or Hg2+ ions by the PL MPA-SHMP-capped CdTe quantum dots

2 Experimental Section

2.1 Chemicals and Apparatus

Cadmium chloride (CdCl2·2.5H2O, > 99.0%) was obtained from Shanghai Jinshan Tingxin Chemical Reagent Co. Ltd. (Shanghai, China). Sodium hexametaphosphate (SHMP), hydrazine hydrate (N2H4·H2O), tris(hydroxymethyl)aminoethane (Tris), and copper sulfate pentahydrate (CuSO4·5H2O, > 99.0%) were obtained from Shanghai Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Mercaptopropionic acid (MPA) was purchased from Sigma-Aldrich (USA). Sodium tellurite (Na2TeO3, > 97.0%) was obtained from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Mercuric nitrate (Hg(NO3)2) was purchased from Taixing Chemical Reagent Co. Ltd. (Jiangsu, China). Arsenite standard solution was purchased from National Institute of Metrology (Beijing, China). Other chemicals and solvents were of analytical grade or the highest purity grade (commercially available) and used without further purification. Double-distilled water (ultrapure water) obtained from Millipore water purification system (specific resistivity ≥ 18 MΩ cm−1, Milli-Q, Millipore) was used throughout the experiment. And all solutions were prepared with double-distilled water (ultrapure water).

UV–Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer (Tokyo, Japan). The photoluminescence (PL) spectra were performed on a Fluoromax-4 fluorescence spectrofluorometer (Horiba, USA) using the excitation and emission slit widths with 5 nm. Transmission electron microscopy (TEM) measurements were conducted on a JEM-2100 transmission electron microscope (JEOL Ltd.). Photographs were obtained from the Apple iPhone 4S.

2.2 One-Pot Preparation of Dual-Stabilizer CdTe Quantum Dots

Dual-stabilizer-capped CdTe quantum dots were synthesized by hydrothermal treatment with a little modification [33, 35]. For a typical synthesis, a solution (50.0 mL) containing 72.5 mg SHMP and 34.6 μL MPA in 100 mL three-necked flask was dissolved under vigorous stirring, and then 0.80 mL of CdCl2 solution (0.20 M) was introduced successively. Next, the solution pH was adjusted to 9.0 with NaOH, and Na2TeO3 solution (20.0 mM, 0.80 mL) was added to the mixture under magnetic stirring. After being refluxed at 100 °C for 10 min, 3.67 mL of N2H4·H2O solution was introduced the in above mixture and refluxed for another 2 h at 100 °C. The resulting solution was purified three times by isopropyl alcohol with centrifugation at 12 000 rpm and stored in the dark at 4 °C. The concentration of MPA-SHMP-capped CdTe quantum dot stock solution was estimated to be 6.90 μM with an empirical equation [36].

2.3 Optimizing Experimental Conditions

To obtain a highly sensitive response for the detection of Cu2+ and Hg2+ ions, the optimization of the different pH values of Tris–HCl buffer were carried out in our experiment. In a typical experiment, 10 μL of MPA-SHMP-capped CdTe quantum dots (6.90 μM) and 40 μL of Cu2+ or Hg2+ ions (0.01 mM) were incubated for 10 min in different pH value of 50 μL of Tris–HCl buffer (50 mM), respectively, then the final volume of the mixture was adjusted to 500 µL with double-distilled water. The resulting solutions were studied by PL spectra at room temperature with excitation at 340 nm.

2.4 Photoluminescence Assay of Cu2+ and Hg2+ ions

A typical metal ion detection procedure was conducted as follows. In a typical run, 10 μL of MPA-SHMP-capped CdTe quantum dots (6.90 μM) was added to 50 μL of Tris–HCl buffer solution (50 mM, pH 7.4), followed by the addition of different concentrations of Cu2+ or Hg2+ ions, and then, the volume of the mixtures was adjusted to 500 µL with double-distilled water. Finally, the final mixture solution was completely mixed with Vortex mixer at room temperature for a few seconds to accelerate the chelation reaction. The mixtures were equilibrated at room temperature for 10 min before the PL spectra measurements were recorded. The resulting solutions were studied by PL spectra at room temperature with excitation at 340 nm.

2.5 Sensor Selectivity Investigation

In the selectivity experiment, a series of competitive metal ions, including Ba2+, Co2+, Cd2+, Cr3+, Fe2+, Fe3+, Mg2+, Mn2+, Ni2+, Ag+, Pb2+, Sn2+, Sn4+, Sr2+, Zn2+ and As3+ (arsenite) ions were mixed with 10 μL of MPA-SHMP-capped CdTe quantum dots (6.90 μM) in a 50 μL Tris–HCl buffer solution (50 mM, pH 7.4) by Vortex mixer under the same conditions for 30 s, respectively. The final volume of the mixture was adjusted to 500 μL with double-distilled water. The mixtures were equilibrated at room temperature for 10 min before the PL spectra measurements were recorded. The concentration of Cu2+ or Hg2+ ions was 1.0 μM, respectively; the concentrations of other interference ions were 5.0 μM, respectively. The resulting solutions were studied by PL spectroscopy at room temperature with excitation at 340 nm.

2.6 Real Samples

The tap water and lake water (collected from Artificial Lake in Yancheng Institute of Technology) were first centrifuged for 15 min at 12,000 rpm, and then were filtered twice using a syringe filter (0.45 µm) to remove the impurities. Then standard solutions of varying Cu2+ and Hg2+ ion concentrations (250 or 500 nM) were prepared from a concentrated stock solution of Cu2+ and Hg2+ ions (0.01 mM) and were artificially added to the tap water and lake water samples. Before 5-min incubation, pH of these MPA-SHMP-capped CdTe quantum dot (6.90 μM) solution were adjusted to 7.4 by Tris–HCl buffer (50 mM). In the test, 10 μL of MPA-SHMP-capped CdTe quantum dot (6.90 μM) solution was added into 100 μL of water samples followed by PL spectra recording after 30 min incubation.

3 Results and Discussion

3.1 Characterization of Photoluminescence Dual-Stabilizer-Capped CdTe Quantum Dots

The detailed synthetic steps are described in Sect. 2. To observe the morphology of the prepared MPA-SHMP-capped CdTe quantum dots, TEM was performed. TEM image of the PL CdTe quantum dots supported on quantum dots synthesized using MPA and SHMP as dual stabilizers in the precursor solution is shown in Fig. 1a. It clearly shows that CdTe quantum dots are nearly spherical particles with excellent monodispersity, and the average sizes of CdTe quantum dots is estimated to be about 3.0 nm (inset of Fig. 1a). The optical property research for the prepared MPA-SHMP-capped CdTe quantum dots was characterized by the UV–Vis and PL spectrum technique. Normalized UV–Vis absorption and PL spectra of the synthesized MPA-SHMP-capped CdTe quantum dots are presented in Fig. 1b. It is seen that the MPA-SHMP-capped CdTe quantum dots display a well-resolved first electronic transition peaks in the absorbance spectra and a narrow full width at half maximum (FWHM) in the PL spectra, which indicates the narrow size distribution of the as-prepared CdTe quantum dots. On the basis of the sizing curve for aqueous synthesized thiol-capped CdTe quantum dots [37], the diameter of the quantum dots in Fig. 1b was determined to be around 2.9 nm, which are in good accordance with the TEM. The khaki aqueous MPA-SHMP-capped CdTe quantum dot solution emits very intense bright yellow luminescence under UV light (365 nm) even at a very low concentration (0.5 μM), which can be clearly seen in the inset of Fig. 1b.
Fig. 1

a TEM of the MPA-SHMP-capped CdTe quantum dots. Inset: the diameter distribution of MPA-SHMP-capped CdTe quantum dots. b UV–Vis absorption (λ ex = 340 nm) and emission (λ em = 560 nm) of the MPA-SHMP-capped CdTe quantum dots. Inset (from left to right): photographs of an aqueous solution of the MPA-SHMP-capped CdTe quantum dots taken under visible light and 365 nm UV light, respectively

The PL intensity of the MPA-SHMP-capped CdTe quantum dots was found to be excitation-independent emission, and the results are shown in Fig. 2. Upon excitation at 340 nm, the PL spectrum of the CdTe quantum dots shows a strong emission peak at 560 nm with a Stokes shift of 220 nm (Fig. 2). When the CdTe quantum dots were excited at varying wavelengths from 300 to 380 nm, the emission spectra were always centered at 560 nm. Meanwhile, as shown in Fig. 2, the emission intensity increased in the excitation range of 300–340 nm and then decreased gradually. Thus, the as-prepared CdTe quantum dots are of independent with the excitation wavelength.
Fig. 2

PL spectrum of the MPA-SHMP-capped CdTe QDs at different excitation wavelengths from 300 to 380 nm; both the excitation and emission slit widths were 5 nm

3.2 Optimizing Experimental Conditions

Figure 3a shows the PL intensities of CdTe quantum dots at different pH values. It is seen that the PL of the pure CdTe quantum dots is strong and stable over a wide range of pH values (4–11); whereas at pH values lower than 4 or higher than 11, the PL intensities decrease gradually. The effect of pH can be understood in terms of the change in surface charge owing to the intramolecular and intermolecular protonation/deprotonation and thus resulting in a great decrease in the PL intensity, which could be attributed to the presence of –COOH units on the surface of CdTe quantum dots. Meanwhile, to obtain the optimal experimental conditions for the interaction between CdTe quantum dots and the Cu2+ or Hg2+ ions, here, fluctuating pH values in the range of 6.0–9.0 (Tris–HCl buffer, 50 mM) were investigated. As shown in Fig. 3b, only a slight change in the PL intensity of CdTe quantum dots is obtained in the pH ranging from 6.0 to 9.0. Highly acidic or alkaline surroundings would negligibly affect the protonation–deprotonation of the CdTe quantum dots due to lots of –COOH groups of the surface of CdTe quantum dots, leading to the relative stabilization of the forming CdTe quantum dots–Hg2+ or MPA–Cu2+ complexes in different pH values (pH 6.0–9.0). On the other hand, Cu2+/Hg2+ ions can complex with OH to form the insoluble hydrated oxide Cu(OH)2 or Hg(OH)2 under highly alkaline conditions, preventing coordination of Cu2+/Hg2+ ions to the CdTe quantum dot carboxyl or thiol groups, leading to incomplete PL quenching due to the suppression of the CdTe quantum dots–Hg2+ or MPA–Cu2+ complexes formation. Considering the protonation/deprotonation of the CdTe quantum dots and better quenching efficiency between the CdTe quantum dots and metal ions, pH value of 7.4 in Tris–HCl buffer was employed throughout.
Fig. 3

a PL intensity at 560 nm (excitation at 340 nm) of the pure CdTe quantum dots as a function of solution pH value; b Influence of pH on the PL recovering in the pH range of 6.0–9.0. The black curve stands for the MPA-SHMP-capped CdTe quantum dots alone, while the red one for CdTe quantum dots/Cu2+, and the blue one for CdTe quantum dots/Hg2+. Concentration: CdTe quantum dots, 0.14 μM; Cu2+ ions, 0.80 μM; Tris–HCl buffer, 50 mM. λ ex, 340.0 nm; λ em, 560.0 nm. All data were collected at 560.0 nm

3.3 Mechanism of Cu2+ and Hg2+ Ion Detection by the Dual-Stabilizer-Capped CdTe Quantum Dots

We first explored the feasibility of using such MPA-SHMP-capped CdTe quantum dots for Cu2+ and Hg2+ ion detection. Figure 4 shows the PL spectra of CdTe quantum dot dispersion under different conditions. It is seen that the CdTe quantum dot solution in the absence of Cu2+ and Hg2+ ions exhibits a strong PL peak at 560 nm (Fig. 4, curve a). In contrast, the presence of Cu2+ or Hg2+ ions leads to an obvious decrease of PL in intensity (Fig. 4, curve b, c), indicating that Cu2+ or Hg2+ ions can effectively quench the PL of CdTe quantum dots. For Cu2+ ions, the thiol capping agent of the quantum dots could greatly affect PL intensity of quantum dots [15], therefore, the quenching mechanism of the CdTe quantum dot PL might be attributed to Cu2+ ion binding on the surface of the MPA-CdTe quantum dots. That is, Cu2+ ions could have strong and specific interactions with the mercapto groups of surface stabilizer MPA due to the very low solubility product (K sp) value of Cu–S bond, namely 6.3 × 10−36 M2, which is much lower than that of Cd–S bond (K sp, 8.0 × 10−27 M2). And it is supposed that the thiol group coating from the MPA, which is one of the two capping agents of CdTe quantum dots, was preferentially displaced with the binding of Cu2+ ions. The displacement of the thiol groups may be causing imperfections on the quantum dot surface, and resulted in PL quenching. As quenching mechanism of PL intensity in the presence of Hg2+ ions, the reason maybe attributed to the specific and robust affinity between Hg2+ ions and the carboxy oxygen elements of surface ligands of MPA, as a result of forming the more stable metal complexes between Hg2+ ion and O atoms in MPA-SHMP-CdTe quantum dots [38]. And SHMP, as one of the stabilizer, to protect quantum dots in the whole reaction process. Thus, these observations demonstrate that CdTe quantum dots together with heavy metal ions can be utilized as a novel and simple probe for the analysis of Cu2+ or Hg2+ ions.
Fig. 4

PL emission spectrum (λ ex = 340 nm) of free CdTe quantum dots (a) and CdTe quantum dots in the presence of Cu2+ ions (b), Hg2+ ions (c). Inset: Photographs under UV light (365 nm). The final concentrations of CdTe quantum dots, Cu2+ and Hg2+ ions are 0.14, 2 and 2 μM, respectively

3.4 Cu2+/Hg2+ Ion Detection Using the Dual-Stabilizer-Capped CdTe Quantum Dots as Photoluminescence Probes

To evaluate the MPA-SHMP-capped CdTe quantum dots’ sensitivities toward different concentrations of Cu2+ and Hg2+ ions, PL intensities were monitored by the titration of Cu2+ and Hg2+ ions at a fixed time of 5 min. Figure 5a shows the PL intensity of MPA-SHMP-capped CdTe quantum dots in the presence of different Cu2+ ion concentrations in Tris–HCl buffer (pH 7.4). It is obvious that the PL intensity of the mixture is very sensitive to Cu2+ ions and gradually decreases with the increase of Cu2+ ion concentration. Figure 5b shows dependence of (PL0−PL)/PL0 on the concentrations of Cu2+ ions within the range of 0.1–1000 nM, where PL0 and PL are CdTe quantum dot PL intensities at 560 nm in the absence and presence of Cu2+ ions, respectively. The detection limit is estimated to be 41.6 pM at a signal-to-noise ratio of 3, which is much lower than that of previous PL-based Cu2+ ion sensors [15, 19, 20, 29, 31]. As for Hg2+ ions, the relative PL quenching of CdTe quantum dots in the presence of Hg2+ ions is presented in Fig. 5c, d, when Hg2+ ions are added to the MPA-SHMP-capped CdTe quantum dot solution in pH 7.4 Tris–HCl buffer, the PL intensity of the CdTe quantum dots linearly increases with the Hg2+ ion concentration in the range of 0.3–1000 nM with a detection limit of 97 pM (Fig. 5c, d), which is comparable to or better than that of previous PL-based Hg2+ ion sensors [21, 32, 39, 40, 41]. The above results suggest our sensing system exhibits superior sensitivity for Cu2+ and Hg2+ ion detection than previously reported PL sensor systems.
Fig. 5

a PL emission spectrum of CdTe quantum dots (0.14 μM) in the presence of different concentrations of Cu2+ ions. b Plot of the enhanced PL signals [(PL0−PL)/PL0] versus Cu2+ ion concentration. c PL emission spectrum of CdTe quantum dots (0.14 μM) in the presence of different concentrations of Hg2 + ions. d Plot of the enhanced PL signals [(PL0−PL)/PL0] versus Hg2+ ion concentration. λ ex, 340.0 nm; λ em, 560.0 nm. Both the excitation and emission slit widths were 5 nm

3.5 Selectivity of the Dual-Stabilizer-Capped CdTe Quantum Dot Probe

Considering the promise of the MPA-SHMP-capped CdTe quantum dot sensor system for application in intricately environmental fields, the selectivity of the PL sensor for Cu2+ and Hg2+ ions was investigated. Therefore, we examined the PL intensity changes in the presence of representative metal ions under the same conditions, including Ba2+, Co2+, Cd2+, Cr3+, Fe2+, Fe3+, Mg2+, Mn2+, Ni2+, Ag+, Pb2+, Sn2+, Sn4+, Sr2+, Zn2+ and As3+ (arsenite) ions, as shown in Fig. 6. It is seen that a much lower PL intensity was observed for CdTe quantum dots upon addition of Cu2+ and Hg2+ ions. In contrast, no tremendous decrease was observed by adding other metal ions into the CdTe quantum dot dispersion. On the one hand, this high specificity could be attributed to the specific and robust affinity between Hg2+ ions and the carboxy oxygen elements of surface ligands of MPA, and on the other hand, the high selectivity of these CdTe quantum dots for Cu2+ ions is due to that mercapto groups of stabilizer MPA has higher thermodynamic affinity and faster chelating process with Cu2+ ion than cadmium of surface of CdTe quantum dots. Meanwhile, the Cu2+ ion could show a large interference on the sensor for Hg2+ ion detection due to the strong reaction between Cu2+ ions and MPA. Further investigations demonstrate that pyrophosphate could be as Cu2+ ion chelators were able to capture Cu2+ ions to form the metal chelates [42]. These observations suggest that the proposed method is capable of discriminating between Cu2+ and Hg2+ ions and the interference ions.
Fig. 6

Selectivity of the CdTe quantum dot-based detection system. The concentrations of Cu2+ and Hg2+ are 1000 nM, while those of the other ions are 5000 nM. The final concentration of the CdTe quantum dots is 0.14 μM

3.6 Determination of Cu2+ and Hg2+ Ions in Real Sample Testing

The excellent specificity combined with high sensitivity and fast response of the MPA-SHMP-capped CdTe quantum dots to Cu2+ and Hg2+ ions suggest that our method might be directly applied to detecting Cu2+ and Hg2+ ions in real samples. The Cu2+ and Hg2+ ion concentrations in water samples from environmental water samples including tap water and local lake water were determined by a calibration method. The analytical results are shown in Table 1. The recovery of the added known amount of Cu2+ and Hg2+ ions to the three different solutions for each sample was in the range of 96.60–105.80%, and that the RSD were less than 5.76%. And the results reveal that the CdTe quantum dot PL probe offers advantages of simplicity, precision, and rapidity for determining the concentration of Cu2+ and Hg2+ ions in complex real samples.
Table 1

Analytical results of the detection of Cu2+ and Hg2+ ions in environmental samples

Sample

Metal ion

Added (nM)

Sum (nM)

Recovery (%)

RSD (n = 5), %

Tap water

Cu2+

250

241.5

96.60

3.78

Hg2+

500

529.0

105.80

4.97

Lake water

Cu2+

250

259.35

103.67

4.65

Hg2+

500

485.57

97.10

5.76

4 Conclusion

In summary, a novel, rapid and simple method has been developed to detect Cu2+ and Hg2+ ions with very high selectivity and sensitivity of subpicomolar levels using PL MPA-SHMP-capped CdTe quantum dots in aqueous media. The as-prepared CdTe quantum dots with a high quantum yield possessed narrow PL FWHM features as well as the advantages of environment friendliness, low cost and simple operation. Under optimal conditions, using the CdTe quantum dots as PL probes, the method can sensitively measure Cu2+ and Hg2+ ions with detection limits of ~50 pM in both cases, which are superior to most current approaches for metal ion analysis. Therefore, this work provides a good example of a simple and cost-effective system of sensing Cu2+ and Hg2+ ions with broad detection ranges and down to the picomolar levels using the CdTe quantum dots. For the results of detection of Cu2+ and Hg2+ ions in environmental samples, we believe that our luminescent probe represents a promising candidate for applications in biological assay and environmental protection.

Notes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21575022, 21535003), the National High Technology Research and Development Program (“863” Program) of China (2015AA020502), the Fundamental Research Funds for the Central Universities, Qing Lan Project and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. And we greatly appreciate the support of the National Natural Science Foundation of China (21575123, 21675139, 21705140) and the Natural Science Foundation of Jiangsu Province (BK20170474), and the Industry-University-Research Cooperative Innovation Foundation of Jiangsu Province (BY2015057-17).

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Copyright information

© The Nonferrous Metals Society of China and Springer Nature Singapore Pte Ltd. 2017

Authors and Affiliations

  1. 1.School of Chemistry and Chemical EngineeringSoutheast UniversityNanjingChina
  2. 2.School of Chemistry and Chemical EngineeringYancheng Institute of TechnologyYanchengChina

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