Picomolar Level Detection of Copper(II) and Mercury(II) Ions Using Dual-Stabilizer-Capped CdTe Quantum Dots
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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.
KeywordsDual-stabilizer-capped CdTe quantum dots Photoluminescence Multi-analyte detection Picomolar levels
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 , optical  and mechanical properties . In the past few years, much efforts have been contributed to improve synthesis method of colloidal nanocrystals  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  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 . 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 . Therefore, it is still a great challenge to develop PL chemosensors that can selectively recognize multiple analytes and down to the picomolar levels.
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 .
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
3.2 Optimizing Experimental Conditions
3.3 Mechanism of Cu2+ and Hg2+ Ion Detection by the Dual-Stabilizer-Capped CdTe Quantum Dots
3.4 Cu2+/Hg2+ Ion Detection Using the Dual-Stabilizer-Capped CdTe Quantum Dots as Photoluminescence Probes
3.5 Selectivity of the Dual-Stabilizer-Capped CdTe Quantum Dot Probe
3.6 Determination of Cu2+ and Hg2+ Ions in Real Sample Testing
Analytical results of the detection of Cu2+ and Hg2+ ions in environmental samples
RSD (n = 5), %
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.
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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