Research Article pubs.acs.org/journal/ascecg
Visual and Label-Free Detection of Cadmium Ions Based on Oscillatory Reaction Tianxiang Wu, Jiao Shan, and Zhanfang Ma* Department of Chemistry, Capital Normal University, No. 105 West Third Ring North Road, Haidian District, Beijing 100048, China S Supporting Information *
ABSTRACT: A method for ultrasensitive visual determination of Cd2+ was developed based on the oscillatory reaction of poly(toluidine blue O)-gold (PTBO−Au). PTBO−Au was fabricated using HAuCl4 as the oxidizing reagent to polymerize toluidine blue O (TBO), a dye utilized as an indicator for colorimetric analysis and detection of Cd2+. Thiosulfate was used to leach Au nanoparticles onto PTBO−Au to form Au+ ions in the presence of (1-hexadecyl)trimethylammonium bromide. After addition of NaBH4, PTBO−Au, Au+, and Cd2+ were reduced to form reduced PTBO−Au (rPTBO−Au) and Au−Cd nanoalloy. The color of the reaction solution changed from blue to red (the color of the Au−Cd nanoalloy). When rPTBO−Au was oxidized by dissolved oxygen, a critical point of color change occurred, and the solution color returned to blue as the reduction rate of PTBO−Au became lower than that of oxidating rPTBO−Au (determined by the concentration of NaBH4). The Au−Cd nanoalloy was found to have minimal catalytic effect on PTBO−Au reduction in the oscillatory reaction, which is indicative of the slowest consumption of NaBH4 and the longest cycle time. Further, the time of the transition from blue → red → blue has positive correlation with the concentration of Cd2+. The color transition can be effectively used as an indicative method to quantitatively detect Cd2+ with the naked eye in the concentration range 1 μM to 1 nM without expensive instrumentation. This highly sensitive and selective method was applied for the determination of Cd2+ in spiked river water samples and obtained satisfactory recoveries, which suggests its potential other environmental applications. KEYWORDS: Oscillatory reaction, Poly(toluidine blue O)-gold, Catalytic performance, Cadmium ion, Visual detection, Au−Cd nanoalloy
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INTRODUCTION
It is well-known that thiosulfate can leach Au nanoparticles (AuNPs) to form Au+,24 which can be further reduced by NaBH4 into AuNPs. Reports have suggested that growing nanoparticles are more efficient than stable colloidal particles as catalysts.25 Toluidine blue O (TBO) can be reduced rapidly by sodium borohydride (NaBH4) to form reduced TBO (rTBO) in the presence of a catalyst; the blue color (TBO) disappears and becomes background color. rTBO (colorless) can be oxidized by dissolved oxygen to be TBO, and the background color will return to blue. This is a so-called oscillatory reaction, and the reaction equation is shown in Figure S1. In this work, we combined TBO with gold nanoparticles using hydrogen tetrachloroaurate hydrate (HAuCl4) as the oxidizing reagent to polymerize TBO and obtain PTBO−Au composites.26 Then, Au+ was obtained by leaching Au24 onto PTBO−Au with Na2S2O3. Because the catalyst can be dramatically changed by slight modification27−29 of the composition (e.g., changing single metal nanoparticles into a bimetallic nanoalloy), we altered the catalytic performance of growing AuNPs through the formation of a nanoalloy in which Au+ and other metal ions
Cadmium is widely used in many fields such as electroplating, agriculture, metallurgy, and so on. However, as a heavy metal ion, Cd2+ is extremely toxic and carcinogenic, and excessive intake of Cd2+ can damage the liver and kidneys and increase the risk of cardiovascular diseases and cancer mortality.1,2 Nowadays, numerous analytical techniques for the detection of Cd2+ have been developed, including atomic absorption spectroscopy,3−5 atomic fluorescence spectrometry,6,7 colorimetric assay,8−11 inductively coupled plasma atomic emission spectroscopy,12 inductively coupled plasma mass spectrometry,13,14 X-ray fluorescence,15 surface enhanced Raman scattering,16 and electrochemistry.17,18 Although these methods offer excellent sensitivity and selectivity, sample pretreatment can be time-consuming, and expensive instruments are commonly involved. While colorimetric methods19−23 for Cd2+ detections have been developed to overcome such disadvantageous, the methods are not sufficient for real-world applications. Further, without UV−vis spectroscopy, these methods are only semiquantitative with the naked eye. Therefore, a simple, sensitive, and visible method without expensive instrumentation is greatly needed to quantitatively detect Cd2+. © 2017 American Chemical Society
Received: February 5, 2017 Revised: April 5, 2017 Published: May 1, 2017 4976
DOI: 10.1021/acssuschemeng.7b00356 ACS Sustainable Chem. Eng. 2017, 5, 4976−4981
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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of Assay for Cd2+
Figure 1. UV−vis spectra of the (A) start point of the cycle time and (B) end point of the cycle time. Au−Cd nanoalloy (including the conditions of existing different concentrations of Cd2+) (C) after leaching and (D) without leaching. Experimental conditions were as follows: 0.05 M Na2S2O3, 0.01 M CTAB, 5.6 mM NaBH4, pH 8.5, and 15 min of leaching time.
quantitatively detect Cd2+, eliminating expensive instrumentation required by other detection methods.
were coreduced by NaBH4. We found that the formation of the Au−Cd nanoalloy dramatically changed the catalytic performance of growing AuNPs, which was the weakest for the reduction of leached PTBO−Au by NaBH4, while the catalytic performance of other nanoalloys (Au + other interference mental ions) were only slightly modified, far less from that which the formation of Au−Cd nanoalloy brings. Accordingly, a simple and novel strategy with high sensitivity and selectivity was designed to detect Cd2+ by combining the oscillatory reaction and changes in the catalytic performance of growing AuNPs. Furthermore, this method allows the naked eye to
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EXPERIMENTAL SECTION
Chemicals. Sodium thiosulfate pentahydrate was purchased from Beijing Chemical Reagents Company (Beijing, China). Mg(NO3)2, Ca(NO3)2, Ba(NO3)2, Cr(NO3)3, Fe(NO3)3, FeCl2, MnCl2, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2, Cd(NO3)2, and Pb(NO3)2 were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Mercury standard solution was purchased from Acros (China). Toluidine blue O was purchased from Sigma-Aldrich 4977
DOI: 10.1021/acssuschemeng.7b00356 ACS Sustainable Chem. Eng. 2017, 5, 4976−4981
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ACS Sustainable Chemistry & Engineering
Figure 2. (A) Color changes of various concentrations of the Cd2+ along with time. (B) Calibration curves of the assay toward Cd2+. The experimental conditions were as follows: 0.05 M Na2S2O3, 0.01 M CTAB, 5.6 mM NaBH4, pH 8.5, and 15 min of leaching time. (China). (1-Hexadecyl)trimethylammonium bromide (CTAB) and hydrogen tetrachloroaurate hydrate were purchased from Alfa Aesar (China). All aqueous solutions were prepared with double distilled water. Apparatus. A JEOL-100CX transmission electron microscope (TEM) was used to characterize the morphology of nanocomposites. High-resolution transmission electron microscope (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray (EDX) mapping characterizations were obtained through a JEOL-2011 electron microscope. X-ray photoelectron spectroscopy (XPS) was obtained from an Escalab 250 X-ray photoelectron spectroscope (Thermo Fisher, United States). Centrifugation was carried out using a refrigerated centrifuge (3−30 K, Sigma). The UV−vis absorption spectra were recorded using a 2550 UV−vis spectrometer (Shimadzu, Japan). The water used in the experiment was purified by an Olst ultrapure K8 apparatus (Olst, Ltd., resistivity 18.2 MΩ cm). Preparation of Poly(TBO)-Au Composites. In the preparation of PTBO−Au, the monomer of TBO was easily polymerized by introducing HAuCl4 as the oxidizing reagent, which was reduced into a noble metal.30 The PTBO−Au composites were synthesized according to the literature with a slight modification.26 Toluidine blue O (0.5 mL, 11.25 mM) was added into 5 mL of water; 0.1 mL of HAuCl4 (4%) was added, and then the mixture was kept under stirring for 12 h. The resulting suspension was centrifuged for 10 min 3 times at the speed of 10 000 rpm. After the resulting solution was centrifuged, the supernatant was removed, and the precipitate was dispersed into 5 mL of water. The obtained suspension was stored at 4 °C. Detection of Cd2+. For detection of Cd2+, 100 μL of Cd2+ with different concentrations, 100 μL of Na2S2O3 (0.5 M), and 100 μL of CTAB (0.1 M) were added in 600 μL of PTBO−Au suspension (42 mM Tris-HCl buffer, pH 8.5). After 15 min, 100 μL of freshly prepared NaBH4 (0.056 M) was added into the mixture. The time of color change (blue → red → blue) was recorded.
AuNPs were dispersed uniformly on PTBO−Au. Thiosulfate was used to leach the Au onto the PTBO−Au to form Au+; the obtained leached PTBO−Au is shown in Figure S2B. After the addition of NaBH4, PTBO−Au, Au+, and Cd2+ were reduced by NaBH4 to form reduced PTBO−Au (rPTBO−Au) and Au−Cd nanoalloy (Figures S2C and 2D, respectively), which were wrapped with CTAB and prompted a color change in the reaction solution from blue to red (the color of Au−Cd nanoalloy). However, rPTBO−Au was oxidized by dissolved oxygen, and a critical point of color change back to blue (Figure 1B) occurred when the reduction rate of PTBO−Au was lower than that of oxidizing rPTBO−Au (determined by the concentration of NaBH4). The time at which the color changed from blue → red → blue was defined as cycle time and is positively correlated with the concentration of Cd2+. In this case, Cd2+ can be measured based on the cycle time. Compared with that in the presence of other metal ions or the blank, the cycle time in the presence of Cd2+ was far longer, indicating that the Au−Cd nanoalloy has a weaker catalytic performance for PTBO−Au reduction by NaBH4 than other nanoalloys. The consumption rate of the NaBH4 in the presence Cd2+ was slower than the rate of other metal ions, further suggesting that the reduction effect of the Au−Cd nanoalloy is the most enduring and results in the longest cycle time. To further verify the mechanism of present method, after the addition of NaBH4 (in the presence of different concentrations of Cd2+), a blue-shift in the wavelength and decrease in the absorbance intensity were observed (Figure 1C) due to the formation of the Au−Cd nanoalloy31,32 (proved by XPS in Figure S4). To prove the need for leaching, after the addition of NaBH4 without leaching (in the presence of different concentrations of Cd2+), no blue-shift of the wavelength was observed (Figure 1D). It should also be noted that the cycle times without leaching were the same for different concentrations of Cd2+. However, the absorbance intensity between the blank and sample with Cd2+ showed little difference. The reason for this may be that a small concentration of PTBO−Au was destructed by NaBH4 and formed a small number of Au− Cd nanoalloys. These results demonstrate that PTBO−Au without leaching cannot be used to detect Cd2+. Optimization of the Experimental Conditions. To improve the limit of detection (LOD), the effects of Na2S2O3, CTAB, NaBH4, pH, and the leaching time on LOD were investigated, as shown in Figures S5−S9. We found that
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RESULTS AND DISCUSSION Sensing Mechanism. The sensing principle is shown in Scheme 1. PTBO−Au was fabricated using HAuCl4 as the oxidizing reagent to polymerize TBO (TEM in Figure S2A), which appeared blue (UV−vis spectrum in Figure 1A). The HAADF-STEM mapping of PTBO−Au is shown in Figure S3, and Figure S3A provides the morphology image of PTBO−Au. The EDX elemental maps of Au, C, N, Cl, and S are shown in Figures S3B, C, D, E, and F, respectively. The colors light blue, blue, green, purple, and red represent the existence of Au, C, N, Cl, and S, respectively, and indicate the successful synthesis of PTBO−Au. Figure S3G was obtained by combining Figures S3B, C, D, E, and F. As shown in Figures S3A, B, and G, 4978
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Figure 3. (A) Color changes for various ions (Hg2+, Fe3+, Ni2+, Mg2+, Co2+, Ba2+, blank, Cu2+, Fe2+, Cr3+, Zn2+, Pb2+, and Cd2+; all concentrations of ions are 1 μM. (B) Differences in cycle time between other metal ions (Hg2+, Fe3+, Ni2+, Mg2+, Co2+, Ba2+, Cu2+, Fe2+, Cr3+, Zn2+, Pb2+, and Cd2+ and the blank; all concentration of ions are 1 μM. (C) Color changes and (D) cycle time for Cd2+ (10 nM) and the binary ion system; panels a−f represent Cd2+ (10 nM) and Cd2+ (10 nM) mixed with other single ions, including Cu2+, Fe2+, Cr3+, Zn2+, and Pb2+ (100 nM of other ions), respectively. (E) Color changes and (F) cycle time for Cd2+ (10 nM) and the mixture system containing Hg2+, Fe3+, Ni2+, Mg2+, Co2+, Ba2+, Cu2+, Fe2+, Cr3+, Zn2+, Pb2+, and Cd2+ (the concentration of each ion was 10 nM). These experiments were performed under conditions of 0.05 M Na2S2O3, 0.01 M CTAB, 5.6 mM NaBH4, pH 8.5, and 15 min of leaching time.
Table 1. Comparison of Present Method with the Other Reported Methods to Detect Cd2+ materials coated pyrolytic graphite electrodes glassy carbon electrode graphene nanosheets electrode CdTe tetraphenylethene bridged cyclodextrin CdTe@CdS AuNPs AuNPs AuNPs PTBO−Au
LOD (nM)
linear range (nM)
expensive instrument needed
method
reference
7.58 160 3.11 0.01 10 6 30 100 50 1.08
21.8−1 × 10 444.5−2224 4.45−890 0.02−600 200−2000 100−2000 60−480 200−1700 500−2000 1−1000
yes yes yes yes yes yes yes yes yes no
electrochemistry electrochemistry electrochemistry fluorescence fluorescence fluorescence colorimetric colorimetric colorimetric colorimetric
34 35 36 37 38 39 20 21 22 this work
8
0.001 to 1 μM with an LOD of 1.08 nM (LOD = 3σ) (Figure 3).33 To determine the selectivity, we investigated the cycle times for various ions (1 μM of all ions), including Hg2+, Fe3+, Ni2+, Mg2+, Co2+, Ba2+, Cu2+, Fe2+, Cr3+, Zn2+, Pb2+, and Cd2+. Figure 3A represents the color change with time. Figure 3B shows the difference in cycle times between other metal ions and the blank. Compared with that of the blank, shorter cycle times were observed for Hg2+, Fe3+, Ni2+, Mg2+, Co2+, and Ba2+, while longer times were observed for Cu2+, Fe2+, Cr3+, Zn2+, Pb2+, and
the LOD was lowest and the difference in cycle times between the blank and 1 nM Cd2+ was at a maximum in a mixture of 0.05 M Na2S2O3, 0.01 M CTAB, and 5.6 mM NaBH4 at pH 8.5 after 15 min of leaching time. Analytical Performance. The LOD of the assay was measured under optimal conditions, where the concentration of Cd2+ was increased from 0 to 1 μM and the cycle time ranged from 12.31 to 29.02 min, as shown in Figure 2A. The calibration curve of the cycle time against Cd2+ was linear from 4979
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Cd2+. The difference (16.71 min) in cycle time between 1 μM Cd2+ and the blank was found to be far longer than the difference for the other ions and blank. This indicates that the Au−Cd nanoalloy displayed a weaker catalytic effect for the reduction reaction than Au and another metal nanoalloy. We also explored binary (Cd2+ + metal ions that produce a positive effect like Cd2+) and mixture systems (Cd2+ mixed with all other metal ions) to confirm the selectivity and applicability of our method. As shown in Figures 3C and D, there was almost no difference between the cycle time of the binary system, which was composed of 10 nM Cd2+ and 100 nM (each) Cu2+, Fe2+, Cr3+, Zn2+, and Pb2+, and that of 10 nM Cd2+ alone. In addition, in Figures 3E and F, the cycle times of 12 metal ions (including 10 nM (each) Hg2+, Fe3+, Ni2+, Mg2+, Co2+, Ba2+, Cu2+, Fe2+, Cr3+, Zn2+, Pb2+, and Cd2+) also displayed minimal difference from that of 10 nM Cd2+ alone. Therefore, these results illustrate that this work shows high selectively of and has potential application in the detection of Cd2+ in environmental samples. Comparisons of the proposed method with others reported20−22,34−39 in literature are listed in Table 1. While our method exhibits a wider linear range and lower LOD than those of the colorimetric method and is comparable with electrochemical and fluorescent methods, it does not require expensive instrumentation to quantitatively detect Cd2+. To further evaluate the practical application, we applied our method to the determination of Cd2+ in spiked river water samples. The standard Cd2+ solution with different concentrations (0.1, 0.05, 0.01, 0.05, and 0.01 μM) was added into the river water samples and detected (Table 2). Satisfactory
added (μM)
found (μM)
recovery (%)
RSD (%)
1 2 3 4 5
0.1 0.05 0.01 0.005 0.001
0.102 0.0487 0.00978 0.0051 0.00101
102 97.4 97.8 102 101
1.79 1.87 1.43 1.21 1.25
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00356. Figure S1, reaction equation of oscillatory reaction; Figures S2−S4, characterizations of PTBO−Au, leached PTBO−Au, and Au−Cd nanoalloy; and Figures S5−S9, optimization of the experimental conditions (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhanfang Ma: 0000-0002-2144-3957 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financed by Grants from the Research Base Construction Projects of Beijing Municipal Education Commission and Natural Science Foundation of Beijing Municipality (Grant 2132008).
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REFERENCES
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recoveries of Cd2+ were obtained from all spiked samples, demonstrating that no significant difference exists between the measured values and the added ones, showing that the present method possessed a potential practical application.
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ASSOCIATED CONTENT
S Supporting Information *
Table 2. Assay Results of Spiked River Water Samples Using the Proposed Method Cd2+ sample no.
Research Article
CONCLUSIONS
In summary, a novel method was developed for the detection of Cd2+ simply by the naked eye based on the cycle time of the oscillatory reaction. This method allows quantitative detection of Cd2+ without expensive instruments, simply requiring a clock. Under the optimized conditions, this method exhibits high sensitivity and selectivity for Cd2+, and satisfactory recoveries were obtained for real river water samples. This approach provides a new potential tool for the identification and quantitation of Cd2+ in environmental samples. However, further work is needed to gain a deeper understanding of the weak catalytic performance of the Cd−Au nanoalloy. 4980
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