Ultrasensitive and Selective Fluorimetric Detection of Copper Ions

Jul 1, 2014 - (31) Zhang et al. presented a platform for ultrasensitive Cu2+ detection using emission-enhanced 16-mercaptohexadecanoic acid (16-MHA) c...
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Ultrasensitive and Selective Fluorimetric Detection of Copper Ions Using Thiosulfate-Involved Quantum Dots Li-Hua Jin and Chang-Soo Han* School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Korea S Supporting Information *

ABSTRACT: A simple and effective quantum dots (QDs)based sensing method for copper ion (Cu2+) in water is developed with improved selectivity and ultrahigh sensitivity in the presence of thiosulfate. For this, hexadecyl trimethylammonium bromide (CTAB) modified CdSe/ZnS QDs is used as a fluorescent probe. In the absence of thiosulfate, mercury and silver ions show strong interference with Cu2+ ions even though the sensitivity can be obtained within a few nanomolar. By using our method, the lowest detected concentration for Cu2+ is 0.15 nM in the presence of thiosulfate in DI water. Also, it is successfully demonstrated for Cu2+ ion detection in practical application (tap water) down to lowest detection limit, 0.14 nM. This method provides a good potential for copper ions detection with simplicity, rapidity, ultrahigh sensitivity, and excellent selectivity. shell quantum dots were used as the selective fluorescence probe for Cu2+ determination with the limit of detection (LOD) of 3.0 nM in aqueous solution.31 Zhang et al. presented a platform for ultrasensitive Cu2+ detection using emissionenhanced 16-mercaptohexadecanoic acid (16-MHA) capped CdSe quantum dots (QDs) by Ag nanoprisms with a detection limit of 5 nM.4 Also, hybrid dual-emission QDs for Cu2+ ion detection was demonstrated by Yao with a detection limit of 1.1 nM.32 However, according to our result we found that both Ag+ and Hg2+ interfered with Cu2+ ion in QDs-based metal ion detection, and most of the research did not mention the interference of Ag+ and Hg2+ ions toward Cu2+ ions. Here, we reported a simple and effective QD-based fluorimetric sensing method for Cu2+ ions with much improved selectivity and sensitivity by using the thiosulfate. The detection mechanism was discussed using X-ray photoelectron spectroscopy (XPS). On the basis of the parametric experiments, the lowest concentration of 0.14 nM was obtained in real samples (tap water).

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owadays, environmental pollution has become a serious problem because of the wide and long-term harm. Therefore, detection of toxic metals is highly desired as an important issue both in environmental monitoring and clinical research. As a widely used metal, copper is an essential trace element in various biological processes. However, it may exhibit high toxicity and can cause damage to the central nervous system and disorders associated with neurodegenerative diseases (e.g., Wilson’s diseases and Alzheimer’s disease)1−3 under overloading conditions. The daily allowance of copper suggested by the National Research Council ranges from 1.5 to 3.0 mg for adults, 1.5 to 2.5 mg for children, and 0.4 to 0.6 mg for infants.4,5 The normal range for copper ions in ground or surface water is 0.005−30 ppm.6,7 Thus, the identification and measurement of copper ions (Cu2+) in environmental matrix and biological fluids has become increasingly important and been highly demanded. A number of methods for the detection of Cu2+ have been reported, such as DNA enzymes,8−11 colorimetric assay,12−14 electrochemical sensors,15,16 and fluorimetric detection.17−25 Of these, the fluorescence technique has been studied much in recent years, owing to several advantages including improved sensitivity. Semiconductor quantum dots (QDs) with high fluorescent quantum yield and intense and stable fluorescent properties have gained more and more research interests in various fields, and it can offer the highest sensitivity as a QD-based ion sensor for applications in biology, pharmacology, and environmental science due to the highly sensitive response to their surface states.26−29 In recent years, QDs-based heavy metal ion determination have been continuously explored.30,31 For example, thioglycerol or L-cysteine-capped CdS QDs were used for determination of Cu2+ and Zn2+ ions in aqueous samples with detection limits of 0.1 μM for Cu2+ and 0.8 μM for Zn2+, respectively.26 L-Cysteine-coated CdSe/CdS core− © XXXX American Chemical Society

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RESULTS AND DISCUSSION Mechanism of Cu2+ Detection by CTAB-QD in the Presence of Thiosulfate. Detection of Cu2+ by using semiconductor QDs has been greatly studied. The sensing mechanisms included cation exchange, fluorescence resonance energy transfer, etc. There were two kinds of explanation for the cation exchange between Cu2+ ions and QDs: (I) the chemical replacement of surface Cd2+ ions by Cu2+ ions;33,34 (II) Cu2+ reduced to Cu+ by QDs and then copper selenide Received: April 23, 2014 Accepted: July 1, 2014

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Figure 1. (A) Schematic illustration of the sensing mechanism of CTAB-QDs in the absence and presence of thiosulfate for the detection of Cu2+. (B) XPS spectra of Cd(3d), Se(3d), and C(1s) of CTAB-QDs before and after reacting with 5 μM Cu2+ ions. Parts C and D show the high resolution spectra of the Cu(2p) and Ag(3d).

formed on the surface of QDs.35,36 The possible sensing mechanism of Cu2+detection by using CTAB-QDs in the absence and presence of thiosulfate in our research is shown in Figure 1. When CTAB-QDs directly interact with metal ions, the metal ions will replace Cd2+, and this makes the fluorescence of QDs quenching. According to our experimental results (data not shown), we found that Ag+ and Hg2+ ions interfered with the Cu2+ ions during metal ion determination in aqueous solution. When thiosulfate was added in the QDs solution, the thiosulfate first generated a passivation layer on the surface of QDs, then after metal ion addition, the Cu2+ replaced Cd2+ while Ag+ and Hg2+ were resisted outside of the QDs by thiosulfate. All of the Cu2+, Cu+, and Cu0 were likely to be used to replace Cd2+, so CuX was used to indicate these cases in Figure 1a. XPS measurements were performed in an effort to investigate the mechanism of the selective copper ion response of CTAB-QDs. Figure 1b showed XPS spectra of Cd(3d), Se(3d), and C(1s) of CTAB-QDs before and after reacting with 5 μM Cu2+ ions. The high-resolution spectra of Cu(2p) and Ag(3d) are shown in parts c and d of Figure 1, respectively. For Cu(2p), binding energies (BEs) of 932.6 and 952.2 eV which corresponded to the Cu(2p3/2) and Cu(2p1/2) were obtained

(Figure 1c).4,35,36 Even though it is difficult to distinguish the binding energies of Cu+ and Cu0 by XPS (only 0.1−0.2 eV difference), the peaks in the spectra indicated solely Cu+/C0 rather than Cu2+ ions, which have been widely seen in copper(I) selenide and copper(I) sulfide compounds.4 From the XPS spectra of Ag(3d) (Figure 1d), two peaks related to BEs of Ag(3d5/2) and Ag(3d3/2) were centered at 367.5 and 373.6 eV, respectively. These BEs were close to that of Ag0. This confirmed that the Ag+ interacted with QDs and reduced to Ag0 by QDs.37,38 Optimization of Experimental Conditions: Concentration of QDs, Thiosulfate, and Acidity. The comparison spectra for the emission intensity of CTAB-QDs with different concentration (5.6 and 11.2 nM) in the absence and presence of thiosulfate are shown in Figure 2. As we mentioned before, Hg2+ and Ag+ ions disturbed the special of Cu2+ in QDs-based metal ions detection. Therefore, we selected these three metal (Hg2+, Ag+, and Cu2+) ions during this study with a concentration of 5 μM. It is clear that in the absence of thiosulfate, strong interference was obviously observed for all the metal ions for 5.6 nM CTAB-QDs (Figure 2a) and two metal (Ag+ and Cu2+) ions for 11.2 nM CTAB-QDs (Figure B

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Figure 2. Comparison of different concentrations of CTAB-QDs based metal ions (5 μM of Hg2+, Ag+ and Cu2+) determination in the absence (A, C) and presence (B, D) of thiosulfate. Emission spectra of (A) 5.6 nM of CTAB-QDs and (C) 11.2 nM of CTAB-QDs in the determination of metal ions in the absence of thiosulfate and (B) 5.6 nM of CTAB-QDs and (D) 11.2 nM of CTAB-QDs on determination of metal ions in the presence of thiosulfate, respectively. The insets in parts C and D show the color change of QDs for metal ions detection in the absence and presence of thiosulfate, respectively.

Here, the pH was adjusted from 4.8 to 9.6 by using 1 M NaOH solution, and I0 and I were the emission intensity of CTABQDs before and after adjusting pH, respectively. In the absence of Cu2+ ions, the ratio I0/I decreased with increased pH of the solution, and in the presence of Cu2+ ions, I0/I showed small values at 5.5 and 7.2 pH and at 8.5 to 9.6 the change was clearer. It means higher pH was more beneficial for increasing the sensitivity of the detection of Cu2+. However, the QDs intensity decreases much at higher pH (9.6) due to the passive layer of M(OH)2, MO, or Me(OH)3 on the surface of CTABQDs.39 At lower pH, thiosulfate will decompose and form sulfide, sulfite, sulfate, trithionate, tetrathionate, polythionates, and polysulfides, and it is not good for high sensitivity. For our experiment, we selected a pH of 8.5. Selectivity and Sensitivity Study for Cu 2+ Ion Detection in DI Water. To assess the selectivity of QDsthiosulfate system for Cu2+, we examined different metal ions under identical conditions. Figure 3 shows the emission intensity change on adding Ag+, Hg2+, In3+, Ca2+, Pb2+, Zn2+, Co2+, Cu2+, Fe3+, and Cd2+ ions. By using optimal conditions, the large intensity change (I0/I ≈ 75) was caused by Cu2+, indicating that the interference of other metal ions was quite reduced and selectivity was highly improved by thiosulfate. Here, the concentration of metal ions was 75 μM for Zn2+, Pb2+, In3+, Fe3+, Co2+, Cd2+, and Ca2+, 5 μM for Hg2+ and Ag+; and 750 nM for Cu2+ in 10 mM thiosulfate solution at 8.5 pH.

2b), respectively. However, this interference disappeared after adding thiosulfate in the sample solution (Figure 2c,d), and the selective response for Cu2+ was more clear for 11.2 nM QDs than 5.6 nM QDs. So, 11.2 nM of CTAB-QDs was used in the following study. The color photographs of QDs (11.2 nM) for Hg2+, Ag+, and Cu2+ in the absence and presence of thiosulfate were shown in the inset of parts c and d of Figure 2, respectively. The influence of thiosulfate amount was also studied, and the results are shown in Figure S1 (Supporting Information). The concentration of thiosulfate was varied from 0.1 to 200 mM, and the concentration for metal ions except Cu2+, Ag+, and Hg2+ (10 μM) was 250 μM. According to the result, 10 mM was selected to apply in the further study because of the better selectivity in Cu2+ ion determination. Another factor needed to be considered in this work was pH. By keeping the concentration of QDs (11.2 nM) and the thiosulfate (10 mM) while changing the pH value of the sample solution, the influence of the solution acidity on the fluorescence intensity of QDs-thiosulfate system was investigated in the absence and presence of Cu2+ (5 μM). The emission spectra of QDs with pH variation in the absence and presence of Cu2+ are shown in Figures S2 and S3 (Supporting Information), respectively. For easy comparison, the dependence of emission intensity change with the variation of pH was plotted and shown in Figure S4 in the Supporting Information. C

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The color change of QDs for metal ions in the presence of thiosulfate was shown in the inset of Figure 3. The sensitivity of CTAB-QDs for Cu2+ detection was also investigated. Figure 4a shows emission spectra of CTAB-QDs upon the exposure to Cu2+ ions in DI water containing 10 mM thiosulfate and different concentrations (0−600 nM) of Cu2+.

The intensity decrease can be clearly observed with the increase of the Cu2+concentration. For further analysis, plot of emission intensity change (I0/I) as a function of the Cu2+ concentration is shown in Figure 4b, where I0 and I are the emission intensity of QDs in the absence and presence of different concentrations of Cu 2+. A linear relationship was obtained over Cu 2+ concentrations, with a correlation coefficient of R2 = 0.982. The LOD of 0.15 nM was obtained, which was calculated as LOD = 3SR/m, where m is the slope of the calibration curve and SR is the standard deviation of the blank. To determine the relative standard deviation (RSD), measurements were run five times for each concentration. The RSD was 0.58%, and the error bar indicated 1 standard deviation in the measurement. Detection of Cu2+ in Tap Water. To further assess its practicality of the proposed method, the CTAB-QD was used as a ratiometric probe to detect Cu2+ in real tap water samples spiked with different amounts of Cu2+. The water samples with different concentrations of Cu2+ (0−100 nM) were added to the QDs solution in the presence of thiosulfate, and the fluorescence spectra were collected. With the addition of Cu2+ in the water samples, the fluorescence intensity of QDs decreased. The linear correlations of the I0/I with the concentration of Cu2+ are shown in Figure 5. The LODs of 0.14 nM with a correlation coefficient of R2 = 0.985 was obtained. The RSD was 0.32%, and the error bar indicated 1

Figure 4. (A) Emission spectra of CTAB-QDs upon the exposure to different concentrations of Cu2+ in DI water. (B) Plot of emission intensity change (I0/I) as a function of the Cu2+ concentration, where I0 and I are the emission intensity of QDs in the absence and presence of different concentrations of Cu2+. The concentration of Cu2+ was varied from 0 to 600 nM.

Figure 5. (A) Emission spectra of CTAB-QDs upon the exposure to different concentrations of Cu2+ in tap water. (B) Plot of emission intensity change (I0/I) as a function of the Cu2+ concentration, where I0 and I are the emission intensity of QDs in the absence and presence of different concentrations of Cu2+. The concentration of Cu2+ was varied from 0 to 100 nM.

Figure 3. Fluorescence response of CTAB-QDs to various metal ions: 75 μM of Zn2+, Pb2+, In3+, Fe3+, Co2+, Cd2+, and Ca2+; 5 μM of Hg2+ and Ag+; and 750 nM of Cu2+ in 10 mM thiosulfate solution at 8.5 pH. The inset shows the color change of QDs for copper ions detection in the presence of thiosulfate.

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standard deviation in the measurement. This result revealed that our method was potentially applicable for the determination of Cu2+ ions in environmental water samples.

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CONCLUSIONS This work has demonstrated the QDs-based sensing method for Cu2+ ions detection in water with improved selectivity and ultrahigh sensitivity in the presence of thiosulfate. Here, hexadecyl trimethylammonium bromide (CTAB) modified CdSe/ZnS QDs is used as a fluorescent probe. According to the XPS analysis, the sensing mechanism can be understood that parts of Cu2+ are reduced to Cu+/Cu0, and Cux (x=1,2)Se are formed on the surface of QDs, leading to the quenching of the QDs emission. The lowest detected concentrations for Cu2+ achieved are 0.15 nM in the DI water. Also, it is successfully demonstrated for Cu 2+ ion determination in practical application (tap water) with the lowest detection of 0.14 nM.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the experimental section and additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 82-2-3290-3354. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Top Engineering Co. REFERENCES

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