J. Chil. Chem. Soc, 54, N° 3 (2009), págs.; 274-277

 

A FLUORESCENCE QUENCHING METHOD FOR DETERMINATION OF COPPER IONS WITH CDTE QUANTUM DOTS

 

YILIN WANG*, JIANPING LU, ZHANGFA TONG, HAIFENG HUANG

College of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China. *e-mail address: ylinwang2002@yahoo.com.cn

---

ABSTRACT

CdTe quantum dots (QDs) were synthesized in aqueous medium by employing L-cysteine as a stabilizer. They exhibited high stability and moderate fluorescence quantum yield (12.5%), and were characterized by UV-vis absorption, fluorescence spectra, Fourier transform infrared spectrometry. Based upon the fact that the fluorescence of the QDs could be quenched by Cu²⁺, a simple and rapid method for Cu²⁺ detection was proposed using L-cysteine-capped CdTe QDs as fluorescent probes. Under optimum conditions, the response showed linear proportion to the concentration of Cu²⁺ between 20 to 300µg·L^-1. The detection limit was 9.3µg·L^-1. This method was successfully applied to the determination of trace copper in real samples.

Keywords: CdTe, Quantum dots, Fluorescence quenching, Copper ions

---

INTRODUCTION

Luminescent semiconductor quantum dots (QDs) is a novel fluorescence nano-material, which have gained increasing attention in the past decades. Compared with conventional organic dyes, QDs have some unique optical and electronic properties, such as broad excitation spectra, narrow, tunable and symmetric emission spectra, and highly stability against photobleaching ^1,2. The studies on QDs mainly focused on developing new techniques to synthesize high-quality QDs ^3-5 and expanding the application areas, such as in material, biological and medical fields^6-8. The effect of ionic species onthe luminescence of QDs was also studied. For example, Henglein⁹ reported that cadmium ions increased the luminescence quantum yield of CdS nanoparticles by about 50% in an alkaline solution. Kotov et al¹⁰ found that MoS₄^- bound to the surface of CdS nanoparticles induced enhancement of their excitome emission. Weller et al¹¹ showed that the emission intensity of CdTe nanocrystals stabilized by thioglycolic acid increased as pH decreased. In contrast, some ionic species quench the luminescence of semiconductor nanoparticles. Copper ions quenching the emission of thioglycerol-capped CdS QDs was first reported by Chen¹². Since then, the use of QDs as fluorescence probes for ions sensing has received considerable attention, a few reports based on QDs fluorescence quenching for the determination of heavy metal ions have been published. Among them, CdS QDs as luminescence probes was widely studied. For example, Kerim et al ¹³ described the optical detection of Cu²⁺ and Ag⁺ with peptide-coated CdS QDs. Yan and Zhu's groups^14,15 presented the optical detection of Hg²⁺ and Ag⁺ with L-cysteine-capped CdS QDs, respectively. Recently, CdTe QDs as luminescence probes for determination of Pb²⁺ and Hg²⁺ were also reported ^16,17.

Copper is one of essential elements for all living organisms. Many analytical methods, such as atomic absorption spectrometry, spectrophotometry, inductively coupled plasma atomic emission spectrometry, and electrochemical methods, have been applied to copper determination. Of them, spectrofluorimetry with fluorescent probes has now obtained the attention of many researchers. QDs are one of the fluorescent probes. Specially, the three QDs, CdSe/ZnS modified with bovine serum albumin(BSA)¹⁸, CdSe modified with 2-mercaptoethane sulphonic acid(MESA)¹⁹ and CdSe/CdS core-shell coated with L-cysteine²⁰, have been employed, respectively. For CdSe/ ZnS modified with BSA, itoshowed detection limitoas low as lOnM for Cu²⁺, unfortunately, real sample analysis has not been reported. With the detection limitoof 0.2µg·L^-1, CdSe modified with MESA was used to determine Cu²⁺ in water. The content of copper in vegetable samples was determined using CdSe/ CdS coated with L-cysteine as fluorescence probé. However, the QDs used in these papers were synthesized via organometallic routes which were cost and required special reaction conditions. Moreover, a further process is needed for the QDs used in aqueous solution because as-prepared QDs often prefer to disperse in nonpolar organic solvents. The aim of this work is to synthesize water-soluble CdTe QDs capped with L-cysteine in aqueous solution through a straightforward one-pot process using safe and low-cost inorganic salts as precursors, and to develop a simple method for the detection of Cu²⁺ based on fluorescence quenching effect.

EXPERIMENTAL

Apparatus

The pH value was controlled with a pHs-3C digital pH meter. The fluorescence measurements were conducted on a 960CRT spectrofluorophotometer. Absorption spectra were taken with a UV-2102 spectrometer. FT-IR spectra were recorded on a Nexus-470 spectrometer. The quantum yield (QY) at room temperature was measured using Rhodamine6G (QY=95%) in ethanol as a reference. The determination of copper in samples was achieved by an Óptima 5300DV ICP-OES spectrometer.

Reagents

Cadmium chloride hemidihydrate (99.0%, Damao Chemical Reagent Factory, Tianjin, China), L-Cysteine hydrochloride monohydrate (99.0%, Sinopharm Chemical Reagent Co.Ltd, Shanghai, China), tellurium powder (99.99%, Delan Chemical Plant, Tianjin, China), sodium borohydride (96%, Shanghai JingHua Scientific & Technological Research Institute) and sodium hydroxide (96%, Shanghai Chemical Reagent Co.Ltd) were used as received without any further purification. Standard stock solution of Cu²⁺ (1.0mg·mL^-1) was prepared by dissolving CuCl₂-2FLO (Guanghua Chemical Factory Co.Ltd, Guangdong, China). The standard solution of Cu²⁺ was obtained by serial dilution of lmg-mL^-1 Cu²⁺ to 2mg·L^-1. Na₂HPO₄-KH₂PO₄ buffer was prepared by mixing 1/15mol·L^-1 of Na₂HPO₄ solution and 1/15mol·L^-1 of KH₂PO₄solution to the required pH value.

Doubly deionized water (DDW) was used throughout.

Synthesis of CdTe QDs capped with L-cysteine

The CdTe QDs was synthesized accordingtothe procedure described inthe literature^21,22. Briefly, sodium borohydride of 160mg was transferred to a small flask, then, DDW of 2.0mL was added. After tellurium powder of 255mg was added in the flask, the reaction system was cooled by ice. During the reaction, a small outlet connected to the flask was kept open to reléase the pressure from the resulting hydrogen. After approximately 5h, the black tellurium powder disappeared and sodiumtetraborate white precipítate appeared on the bottom of the flask nevertheless. The resulting NaHTe in clear supernatant was used in the preparation of CdTe QDs.

For the preparation of CdTe capped with L-cysteine, CdCl₂·2 5H₂O of 0.5450g was dissolved in 125 mL water, and L-cysteine hydrochloride monohydrate of 1.0052g was added under stirring, followed by adjusting the pHto 11.2 with NaOH solution of 1mol·L^-1. The solution was placed in a three-necked flask fitted with a septum and valves and was deaerated by N₂ bubbling for 30 min. Under stirring, freshly prepared oxygen-free NaHTe solution of 1.0mL was injected into the flask. After deaerated for another 30 min, the reaction mixture was refluxed at 100°C under open-air conditions for 30min.

Effect of pH on the fluorescence intensity of CdTe QDs

In seven 10.0 mL colorimetric tubes, 0.2mL CdTe QDs of 1.36x10^-5mol·L^-1 were added, respectively. Then, they were diluted to mark with a series of Na₂HPO₄-KH₂PO₄ buffers of pH values. The fluorescence spectra were recorded on a 960CRT spectrofluorophotometer, the slitowidths of both excitation and emission were 10nm.

Samples preparation

The preparation of samples was conducted according to the procedures described elsewhere ²³. Hair and tea were dried in an oven at 100°C to obtain a constant weight. 2.000g hair or tea sample was weighed and transferred into a crucible. After the sample was ashed on hot píate, it was incinerated in Muffle furnace at 620°C for about 4 hours until the sample turned into white color. After the crucible cooled to room temperature, 5.0mL HC1 of 6 mol·L^-1 and 0.2 mL H₂0₂ of 30% were added, respectively. The mixture was evaporated to almost dry, then 5.0mL HC1 of 6 mol·L^-1 was added. After the solid dissolved completely, the pH value was adjusted to 7.0 with NaOH solution of. 1mol·L^-1. Finally, the solution was diluted with DDW to100mL.

Determination of Cu²⁺

For determining the concentration of Cu²⁺, 0.2mL CdTe QDs solution of 1.36x10^-5mol·L^-1, 2.0 mL Na₂HPO₄-KH₂PO₄ buffer solution 1/15mol·L^-1(pH=7.2) and different amounts of Cu²⁺ standard solutions(2.0 mg-17¹) were sequentially added into lO.OmL colorimetric tubes which were shaken thoroughly afterwards. After the tubes were kept at room temperature for 15min, the mixture was diluted to the mark with buffer solution. The fluorescence intensity was measured at λ _EXλ_EM= 365/ 550nm, the slit widths of both excitation and emission were 10nm.

RESULTS AND DISCUSSION

Characterization of CdTe QDs capped with L-cysteine

The CdTe QDs solution, if kept in a refrigerator, was stable for months. Moreover, the quantum yield can reach as high as 12.5%. The absorption and fluorescence spectra of CdTe QDs were shown in Figure 1. A well-resolved absorption maximum of the first electronic transition suggested a sufficient narrow size distribution of the QDs. The diameter and the concentration of the QDs were determined using an empirical formula ²⁴: D = 9.8127 x 10^-7 λ ³ - 1.7147 x 10^-3 λ ² + 1.0064 x λ - 194.8, ε = 10043 x (D)^2,12 and A = εbC. Where D (nm) is the size of a given CdTe sample; λ (nm) is the wavelength of the first excitome absorption peak of the corresponding sample. ε (L·moL^-1·cm^-1) is the extinction coefficient of CdTe QDs; A is the absorbance at the peak position of the first exciton absorption peak; b is the path length (cm) of the radiation beam used for recording the absorption spectra, and C is the molar concentration (mol·L^-1) of the QDs. The result showed that the particle diameter and the concentration of the QDs was approximately 3.0 nm and 1.36 x10^-5mol·L^-1, respectively. The emission peak located at 550nm, and itocould be observed that the fluorescence spectra were symmetric with the FWHM (full width at half maximum) of about 60nm. Being irradiated under ultraviolet light, it emitted green fluorescence.

Figure 2 presented the FT-IR spectra of L-cysteine and CdTe capped with L-cysteine. A broad absorption band around 3400cm^-1 and an absorption band at 2900 cm^-1 were assigned to O-H vibrations of the absorbed H₂0 and C-H vibration in the alkyl chain of the surface modifiers, respectively. A broad absorption band at 1550-1600cm^-1 due to C=O vibration was observed in all cases, and an absorption band at 2560cm^-1 due to S-H vibration disappeared in CdTe with capped L-cysteine. These revealed that L-cysteine was bound to the surface of Cd²⁺ site through Cd-SR bond formation.

Effect of Cu²⁺ on the fluorescence intensity of CdTe QDs

The effect of Cu²⁺ on the fluorescence spectra of QDs has been investigated. As seen in Figure 3, itois evident that the fluorescence intensity of CdTe QDs decreases as the Cu²⁺ concentration increases. The effective quenching of the fluorescence of CdTe QDs in terms of Cu²⁺ ions is attributed to extremely low soluble particles of CuTe formation onto the surface of QDs in place of CdTe. For these structures, electron and hole transfer from the CdTe to the CuTe energy levels is assumed to be much faster than the process of fluorescence generation in the CdTe, the energy of the electrons is released in non-radiative form ¹⁸. As a result, the fluorescence of QDs is quenched efficiently.

Considering the quenching of fluorescence observed in the presence of ultra trace amounts of Cu²⁺ ions, the possibility of developing a sensitive method for determination of Cu²⁺ has been investigated.

Optimum conditions for determination

The fluorescence emission spectra of QDs are sensitive to the system environment. In order to develop a sensitive and rapid spectrophotometric method for the determination of Cu²⁺, the experimental conditions wereimproved by studying the effect of various factors such as pH value , QDs concentration and incubation time.

The influence of pH in a range between 5.96 and 8.34 was studied in order to select the best acidity for the determination of Cu²⁺ with QDs. The intensity reached its maximum at pH=7.2 as shown in Figure 4, however, pH values both lower and higher than 7.2 resulted in a significant decrease in the fluorescent intensity. itocould be rationale that in preparation, cadmium and thiol were excessive, and the pH was adjusted to 11.2, so cadmium thiol complexes were formed in the solution. When the CdTe solution becomes acidic, partial thiols and cadmium ions would be released from the cadmium thiol complexes, the particle surface covered with TGA was increased. Therefore, the trap sites on the CdTe surface would be removed, which dramatically improved the fluorescence intensity ⁿ. When pH value decreased too low, the thiols attached to the QDs detached from the surface, more surface defects formed, resulting in a decrease of the fluorescence intensity. Therefore, to obtain high fluorescence intensity with good precisión, pH=7.2(1/15 mol·L^-1 Na₂HPO₄-KH₂PO₄ buffer solution) was chosen for the further studies.

To investígate the influence of QDs concentration on the sensitivity and linear range of determination, four calibration curves were constructed with the QDs concentrations of 1.36x10^-7, 2.72x10^-7, 3.40x10^-7 and 4.08x10^-7mol·L^-1, respectively. The results in Figure 5 showed that the linear range of calibration curve became wider on the expense of the decrease of sensitivity when the concentration of QDs increased. Therefore, for the consideration of the sensitivity and the linear range of calibration curve, 0.2 mL (2.72 x10^-7mol·L^-1) CdTe QDs solution was adopted.

The effect of reaction times on fluorescence quenching value was also explored. After all reagents had been added, fluorescence measurement was performed every 5 minutes. it was found that the fluorescence quenching value reached the maximum after 15 minutes and kept constant in 60 minutes. Thus, fluorescence intensity was measured after the system was 15 minutes later.

The effect of other metal ions on the fluorescence intensity of the CdTe QDs was examined. As observed, Hg²⁺ and Ag⁺ showed strong quenching for the QDs fluorescence intensity. itocould be derived from great affinity between thiol group and "soft" metal ions. In the presence of Hg²⁺ or Ag⁺, the thiols attached to the QDs were detached from the surface, more surface defects formed, giving rise to a decrease of the fluorescence intensity. The quenching effect of Fe³⁺ on the fluorescence of QDs is attributed to the inner filter effect ¹², which can be eliminated by adding fluoride ions to form the colorless FeF₆^3- complex. When the relative error was less than ± 5%, the tolerance of some coexistence ions for the determination of 40 µg·L^-1 Cu²⁺was examined to evaluate the selectivity of the proposed method. The results showed that 200-fold Ca²⁺, Mg²⁺ and AP+, 100-fold Pb²⁺ , Mn²⁺ and Zn²⁺, 10-fold Co²⁺ and Ni²⁺, 5-fold Fe³⁺ had no interference on the determination. Consequently, this method is applicable for the analysis of Cu²⁺ in biological samples.

Linear equation and detection limit

The calibration curve for determination of Cu²⁺ was presented in Figure 6. it was found that the fluorescence intensity of CdTe QDs quenched by Cu²⁺ in a relationship that was best described by a Stern-Volmer equation: F₀/F = K [Cu²⁺] + 1. Where, K_AV is a Stern-Volmer constant found to be 5.62 x 10^-5L·mol^-1.

The linear regression equation was F₀/F = 0.0088C+1.044, where F₀ and F were the fluorescence intensity of CdTe QDs in the absence and presence of Cu²⁺; C was the concentration of Cu²⁺, the unit was µg·L^-1. The linear range was from 20 to 300µg·L^-1 with a correlation coefficient of 0.998. The limitoof detection (LOD) 9.3µg·L^-1 and limitoof quantification (LOQ) 44.0µg·L^-1 were calculated from 3.52σ_B and 16.67σ_B ²⁵, respectively. The relative standard deviation (RSD) for five replicate determinations was 3.6% for a Cu²⁺ concentration of 40µg·L^-1, suggesting one of the most sensitive methods for the determination of Cu²⁺.

Analytical performance

The developed method has been applied to the determination of copper in real samples of hair and tea. A suitable volume of sample solution was added to a 10.0 mL colorimetric tube, the content of Cu²⁺ was determined according to the proposed technique. The results were presented in Table 1. The contents of copper determined in these samples are in good agreement with those obtained by ICP-AES method.

CONCLUSIONS

CdTe QDs capped with L-cysteine were synthesized in aqueous solution, and a novel method for the determination of Cu²⁺ was developed based on its fluorescence quenching for the QDs. Under the optimum conditions, calibration curve was linear in the range of 20~300µg·L^-1 with the correlation coefficient of 0.998, the detection limit of Cu²⁺ was 9.3µg·L^-1. Furthermore, the feasibility of the method has been proven by the determination of trace copper in hair and tea samples with satisfactory results.

ACKNOWLEDGEMENT

Financial support from the scientific research fund of Guangxi University (No. X081057) is gratefully acknowledged.

 

REFERENCES

1 M. J. Bruchez, M. Moronne, A. P. Alivisatos, et al. Science, 281, 2013, (1998)

2 C. W. Warren, S. M. Nie. Science, 281, 2016, (1998)

3 Z. A. Peng, X. G. Peng. J. Am. Chem. Soc, 123,183, (2001)

4 D. V. Talapin, A L Rogach, H. Weller, et al. Nano Lett, 1, 207, (2001)

5 N. L. Gaponik, D. V. Talapin, H. Weller, et al. J. Phys. Chem B, 106, 7177, (2002)

6 N. Pradhan, D. Goorskey, X. G. Peng, et al. J. Am. Chem. Soc, 127, 17586, (2005)

7 J. K. Jaiswal, H. Mattoussi, J. M. Mauro, et al. Nature biotechnology, 21, 47, (2003)

8 X. H. Gao, Y. Y. Cui, S. M. Nie, et al. Nature biotechnology, 22, 969, (2004)

9 L. Spanhel, M. Haase, A. Henglein, et al. J. Am. Chem. Soc, 109, 5649, (1987)

10 D. Diaz, J. N. Robles, N. Kotov, et al. J. Phys. Chem B, 103, 9859, (1999)

11 M. Y. Gao, S. Kirstein, H. Weller, et al. J. Phys. Chem B, 102, 8360, (1998)

12 Y. F. Chen, Z. Rosenzweig. Anal. Chem, 74, 5132, (2002)

13 M Kerim, A Gattas, M Roger, et al. Chem comm, 38, 2684, (2003)

14 Z. X. Cai, H. Yang, Y. Zhang, X. P. Yan. Analytica Chimica Acta, 559, 234, (2006)

15 J. L. Chen, C. Q. Zhu. Analytica Chimica Acta, 546, 147, (2005)

16 H. M. Wu, J. G. Liang, H. Y. Han. Microchim Acta, 161, 81, (2008)

17 Y. S. Xia, C. Q. Zhu. Talanta, 75, 215, (2008)

18 H. Y. Xie, J. G. Liang, Z. L. Zhang, et al. Spectro chimica Acta Part A, 60, 2527, (2004)

19 T. F. Maria , W. J. Jin, S. M. Alfredo, et al. Analytica Chimica Acta, 549, 20, (2005)

20 Y. H. Zhang, H. S. Zhang, X. F. Guo, et al. Microchemical Journal, 89,142,(2008)

21 H. Zhang, Z. Zhou, B.Yang, M.Y. Gao. J. Phys. Chem. B, 107, 8, (2003)

22 L.D. Klayman, T.S. Griffin. J. Am. Chem. Soc, 95, 197, (1973)

23 G. W. Zhang, H. Zhang, Y. H. Liu. Spectroscopy and Spectral Analysis, 17, 74, (1997)

24 W. W. Yu, L. H. Qu, W. Z. Guo, et al. Chem. Mater, 15, 2854, (2003)

25 L. Currie. Analytica Chimica Acta, 391, 105, (1999)

 

(Received: March 2, 2009 - Accepted: May 29, 2009)