Label-Free Colorimetric Detection of Cadmium Ions in Rice Samples

Aug 13, 2014 - ABSTRACT: A simple and label-free colorimetric method for ... in a high-salt environment to detect Cd2+ in water and digested rice samp...
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Label-Free Colorimetric Detection of Cadmium Ions in Rice Samples Using Gold Nanoparticles Yongming Guo,†,‡,⊥ Yi Zhang,†,⊥ Huawu Shao,∥ Zhuo Wang,*,† Xuefei Wang,*,§ and Xingyu Jiang*,† †

Beijing Engineering Research Center for BioNanotechnology, National Center for Nanoscience and Technology, Beijing 100190, China ‡ College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang, Henan Province 473061, China § School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ∥ Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu, Sichuan Province 610041, China S Supporting Information *

ABSTRACT: A simple and label-free colorimetric method for cadmium ions (Cd2+) detection using unmodified gold nanoparticles (AuNPs) is reported. The unmodified AuNPs easily aggregate in a high concentration of NaCl solution, but the presence of glutathione (GSH) can prevent the saltinduced aggregation of AuNPs. When Cd2+ is added to the stable mixture of AuNPs, GSH, and NaCl, Cd2+ can coordinate with 4× GSH as a spherical shaped complex, which decreases the amount of free GSH on the surface of gold nanoparticles to weaken the stability of AuNPs, and AuNPs will easily aggregate in high-salt conditions. On the basis of the mechanism, we design a simple, label-free colorimetric method using AuNPs accompanied by GSH in a high-salt environment to detect Cd2+ in water and digested rice samples.

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sensitivity.18 AuNPs cofunctionalized with 6-mercaptonicotinic acid and L-cysteine can sense Cd2+ with a visible color change. However, Pb2+ and Cu2+ show interference with the detection of Cd2+.19 A new test paper for the colorimetric immunosensing Cd2+ has been constructed by using the Cd2+-ethylenediaminetetraacetic acid-bovine serum albumin-AuNPs conjugate. The test paper can be successfully used to detect Cd2+ in drinking and tap water with almost 100% accuracy. However, the practical application is not included in this paper.22 A new colorimetric method for sensing Cd2+ has been developed based on the aggregation of AuNPs mediated by the Cd2+aptamer and cationic polymer. The detection limit is as low as 4.6 nM.23 However, masking reagents are needed to eliminate the interference from other metal ions. Therefore, it is desirable to develop a simple method for Cd2+ detection that possess the advantages of high sensitivity and good selectivity and especially show good behavior in a real sample analysis. Here, we describe a simple, rapid, and label-free colorimetric method for Cd2+ detection by using unmodified AuNPs. The method does not need any chemical modification and complicated operations, and it possesses good selectivity and sensitivity. Unmodified AuNPs are prone to aggregate in highsalt conditions because enough salt can screen the electrostatic

s an important natural element, cadmium is widely used in many fields, such as industry, agriculture, and so on.1 However, chronic exposure of Cd2+ has caused many serious environmental and health problems, including renal dysfunction, reduced lung capacity, and some cancers.2,3 In 2013, the government test indicated that some rice samples in South China were contaminated with cadmium. In China, the maximum permitted concentration for cadmium in rice has been set at a level of 0.2 mg/kg,4 which is lower than the standard of Codex Alimentarius Commission (CAC), but these events are people alarming. Thus, sensitive sensing cadmium in rice is of considerable significance for food safety and human health. Though many methods for Cd2+ detection have been developed, these methods either are time-consuming or require costly instruments and complicated operation.5−9 To satisfy the requirement of an on-site and quick testing, a facile, simple, and on-site method for monitoring Cd2+ is, therefore, still required. Nowadays, many colorimetric detection methods based on gold nanoparticles (AuNPs) or silver nanoparticles have been well developed because these nanoparticles have extremely high extinction coefficients and strong distance-dependent optical properties.10−17 However, there are very few colorimetric methods for Cd2+ detection, and the selectivity and sensitivity of those detection methods of Cd2+ is not sufficiently good for real-world applications.18−23 For example, a colorimetric method for Cd2+ detection is developed with triazole-ester modified silver nanoparticles. This method suffers from complex organic synthesis, long response time, and poor © XXXX American Chemical Society

Received: July 1, 2014 Accepted: August 13, 2014

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S-2). Meanwhile, the binding energy of Cd-S (208.5 ± 20.9 kJ/ mol) is larger than that of Au-S (120 kJ/mol), which also indicates that the Cd(SG)4 complex is more stable than the AuSG.31,32 The spherical shaped (GS)4Cd complexes have more net negative charge, thus the (GS)4Cd complexes have weaker electrostatic interaction with AuNPs than that of the linear GSM-SG. Because AuNPs are synthesized by citrate reduction of HAuCl4, the surface of AuNPs has negative charges from citrate anions. In the presence of a high concentration of NaCl, the addition of Cd2+ to the AuNPs solutions with GSH induces the aggregation of AuNPs. By contrast, the addition of other metal ions cannot cause the aggregation of AuNPs (Scheme 1). On the basis of this fact, we construct this colorimetric method for Cd2+ detection using unmodified AuNPs.

repulsion between AuNPs to result in the aggregation of AuNPs.24−26 We find that GSH can prevent AuNPs against salt-induced aggregation, whereas other amino acids, including cysteine and homocysteine, do not stabilize AuNPs in high-salt conditions. This observation suggests that GSH can stabilize AuNPs in high-salt conditions. It has been reported that GSH is an efficient detoxification agent of Cd2+, which indicates that GSH can strongly interact with Cd2+. Some paper reported that Cd2+ could interact with GSH to form a spherical shaped (GS)4Cd complex, while other metal ions, including Hg2+ and Pb2+, interact with GSH to form linear GS-M-SG complexes. The spherical shaped (GS)4Cd complex has more net negative charge than that of the linear GS-M-SG, so the (GS)4Cd complex shows weaker interaction with AuNPs and cannot stabilize AuNPs well.27 We calculate the binding energy of GSH and Cd2+/Hg2+ with density functional theory (DFT) calculation, and the data support that the complex of Cd2+ and GSH is a spherical tetra-coordinated structure. Therefore, the presence of Cd2+ in the AuNPs solution with GSH can induce the aggregation of AuNPs in a high-salt condition, while the other metal ions cannot cause the aggregation of AuNPs in the same condition. On the basis of the mechanism, we design a straightforward, label-free colorimetric method for Cd2+ detection using unmodified AuNPs and apply this method to detect Cd2+ in a rice sample for the first time.

Scheme 1. Schematic Representation of the Colorimetric Detection of Cd2+ Using the Unmodified AuNPs

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RESULTS AND DISCUSSION Sensing Mechanism. AuNPs can easily aggregate in highsalt conditions, when there is no any stabilizing agent on the surface of AuNPs. Many reported methods for the detection of some metal ions, small molecules, proteins, and DNA are well developed based on this character.10−17,19−26 GSH can inhibit the salt-induced aggregation of AuNPs, which is attributed to the presence of one sulfhydryl group, two free carboxyl groups, and one amino group of GSH.28 Moreover, the flexible structure of GSH can stabilize AuNPs in the high-salt conditions. As a thiophilic metal ion, Cd2+ can strongly interact with GSH, and many kinds of complexes of GSH with Cd2+ have been reported due to the fact that GSH has eight potential binding sites: two carboxyls, one sulfhydryl, one amino group, and two pairs of carbonyl and amide donors. The sulfhydryl group of GSH always binds Cd2+ preferentially. The interaction of Cd2+ with other groups of GSH depends on the specific conditions. The pH and the ratio of Cd2+/GSH all affect the structure of the complex. Many kinds of complexes of GSH with Cd2+ have been reported up to now, such as Cd(SG)4, GSM-SG, and Cd2(SG)2.29 The Cd(SG)4 complex is dominant in the present system due to the pH effect. It has been found that at pH < 6.5, the interaction of Cd2+ with the glutamyl carboxyl and amino group decreases, indicating that Cd2+ mainly interacts with the sulfhydryl group of GSH at pH 6.0.30 To satisfy the tetrahedral coordination structure, Cd2+ interacts with four GSHs, as a Cd(SG)4 complex in the present system. Other metal ions, including Hg2+ and Pb2+, interact with GSH to form linear GS-M-SG complexes.27 In order to understand the interaction between GSH and Cd2+ or Hg2+, we applied the DFT calculation to obtain the binding energy between GSH and metal ions, including Cd2+ and Hg2+. The binding energy is −420 kcal mol−1 for GSH and Hg2+, while the binding energy of GSH and Cd2+ is −621 kcal mol−1. The binding energy data indicate that the complex of Cd2+ and GSH is more stable than the Hg2+-GSH complex. The models of GSH and Hg2+, Cd2+ are presented in the Supporting Information (Figures S-1 and

Optimization of Detection Conditions. To obtain the optimum conditions of the detection system, we first study the salt effect on the stability of the AuNPs. To get good sensitivity, we choose the final concentration of AuNPs about 2 nM as the detection system.10 With the increase of the concentration of NaCl, the absorbance of AuNPs at 520 nm decreases gradually and the absorbance at longer wavelength (640 nm) correspondingly increases. We use the ratio of the absorbance of AuNPs at 640 and 520 nm (A640/520) to evaluate the degree of AuNPs aggregation. A lower A640/520 indicates that AuNPs disperse well in the solution, and a higher A640/520 is related with the aggregated AuNPs. When the concentration of NaCl is 40 mM, A640/520 is the highest, which indicates that AuNPs aggregate almost completely (Figure S-3 in the Supporting Information). Transmission electron microscopy (TEM) images verify the results (Figure S-4 in the Supporting Information). AuNPs easily aggregate in 40 mM of NaCl solution, and A640/520 does not change greatly after 10 min (Figure S-5 in the Supporting Information). Therefore, 40 mM of NaCl and the incubation time of 10 min are applied to the detection system for the following experiments. To evaluate the effect of various amino acids on the stability of AuNPs in high-salt conditions, we investigate the colorimetric response of these amino acids and some small peptides, including GSH, oxidized glutathione (GSSG), and arginine-glycine-aspartic acid (RGD), at 1 mM in different pH values. At pH 6.0, AuNPs are only stabilized by GSH in the presence of 40 mM of NaCl. However, at pH 7.0, methionine, GSSG and GSH can stabilize AuNPs, and at pH 8.0, AuNPs are stabilized by methionine, lysine, GSSG, and GSH. The results indicate that pH 6.0 is the optimum pH environment of the detection system (Figure S-6 in the Supporting Information). The stabilizing ability of GSH to AuNPs comes from its unique structure. The presence of the sulfhydryl group and its flexible structure may contribute to its stabilizing ability to AuNPs. GSH has also been used to stabilize fluorescent AuNPs, which B

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are stabilized well in phosphate-buffered saline with fetal bovine serum.33 The TEM images of AuNPs in the presence of 1 mM GSH also show that GSH can stabilize AuNPs in aqueous solutions (Figure S-4C in the Supporting Information). These facts verify that GSH can indeed stabilize AuNPs in high-salt conditions. To obtain the best concentration of GSH stabilizing AuNPs, we investigated the colorimetric response of AuNPs in the presence of different concentrations of GSH. With the increase of the concentrations of GSH, the absorbance at 520 nm increases gradually and the absorbance at 640 nm decreases simultaneously. Also the A640/A520 also decreases slowly, and it reaches a plateau when the concentration of GSH is 2.5 μM. The pictures indicate that the color of AuNPs does not change greatly when the concentration of GSH is above 1 μM (Figure 1). TEM images of AuNPs show that AuNPs disperse well in

Figure 2. Selectivity of the detection system. The plot of A640/A520 of AuNPs versus different metal ions at different pH values (the concentration of AuNPs is about 2 nM, 10 mM phosphate sodium buffer, the concentrations of metal ions are 50 μM, the concentration of NaCl is 40 mM, the concentration of GSH is 1 μM, the error bars are obtained based on five samples detection).

complexes. As a result, addition of Cd2+ to the AuNPs solutions with GSH and high concentrations of NaCl can induce the aggregation of AuNPs. Moreover, we also studied the changes of A640/A520 of AuNPs with time and found that Cd2+ would easily cause the complete aggregation of AuNPs within 10 min (Figure S-9 in the Supporting Information). We thus choose 10 min as the incubation time and pH 6.0 as the detection conditions for Cd2+ detection. Sensitivity of the Detection System. We investigated the sensitivity of the detection system. As the increasing of the concentration of Cd2+, the UV−vis absorbance curves show red-shift and broaden gradually, and a peak emerges slowly at longer wavelength (640 nm) (Figure 3A). The ratio of A640/

Figure 1. UV−vis absorption spectra of AuNPs and GSH. (A) UV−vis absorption spectra of AuNPs in the presence of different concentrations of GSH. (B) The plot of A640/A520 of AuNPs versus the concentration of GSH. (C) Photographs of AuNPs in the presence of different concentration of GSH (the concentration of AuNPs is about 2 nM, 10 mM pH 6.0 phosphate sodium buffer, the concentration of NaCl is 40 mM, the error bars are obtained based on five samples detection).

aqueous solution with 1 μM of GSH (Figure S-7A in the Supporting Information). Therefore, 1 μM of GSH can stabilize the unmodified AuNPs with 40 mM of NaCl at pH 6.0. Selectivity of the Detection System. To evaluate the selectivity of our detection system, we first tested the colorimetric response of different metal ions at a concentration of 50 μM in different pH (from 4 to 9) values with GSH (1 μM). At pH 6.0, 7.0, and 8.0, Cd2+ can cause the aggregation of AuNPs with the obvious increasement of A640/A520 value. At pH 6.0, the A640/A520 value is the largest (Figure 2). At pH 4.0, AuNPs is not stable, and the aggregation happens even without any metal ion. At pH 5.0, Cd2+ and Pb2+ both can cause the aggregation of AuNPs. Also AuNPs did not aggregate with Cd2+ and other cations at pH 9.0 (Figure S-8 in the Supporting Information). TEM images of AuNPs also show that AuNPs indeed aggregate after the addition of Cd2+ at pH 6.0 (Figure S7B in the Supporting Information). Therefore, pH 6.0 is the suitable detection environment for this assay. As mentioned above, the selectivity of the detection system is attributed to the different interaction abilities of these metal ions and GSH. For Cd2+ and other metal ions, tetrahedral (GS)4Cd and linear GSM-SG complexes form, respectively. Because tetrahedral (GS)4Cd complexes have more net negative charge than linear GS-M-SG, tetrahedral (GS)4Cd complexes have weaker interaction ability with AuNPs than that of linear GS-M-SG

Figure 3. UV−vis absorption spectra changes of the detection system with Cd2+. (A) UV−vis absorption spectra of AuNPs in the presence of different concentration of Cd2+. (B) The plot of A640/A520 of AuNPs versus the concentration of Cd2+. (C) Photographs of AuNPs in the presence of different concentrations of Cd2+ (the concentration of AuNPs is about 2 nM, 10 mM pH 6.0 phosphate sodium buffer, the concentration of NaCl is 40 mM, the concentration of GSH is 1 μM, the error bars are obtained based on five samples detection).

A520 as a function of the concentration of Cd2+ is plotted in Figure 3B, which increases gradually with the increase of the concentration of Cd2+. It indicates that the AuNPs aggregates with the increase of concentration of Cd2+, and the color change from red to blue with increasing Cd2+ concentration can be observed. The lowest detection concentration with the naked eye is 10 μM (Figure 3C). The detection limit of this sensor system is as low as 5 μM, which is much lower than that C

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of the reported methods based on silver nanoparticles.18 These results indicate that the detection system has potential applications in the detection of Cd2+ in practical samples. Moreover, we also investigated the sensitivity of the detection system at different pH values (from 4 to 9). AuNPs easily aggregated at pH 4.0 even when there is no metal ion. With increasing of the pH value, from 5 to 8, the sensitivity is similar. Also at pH 9.0, AuNPs exhibit almost no color change at 50 μM Cd2+ (Figure S-10 in the Supporting Information). Considering the optimized selectivity, pH 6.0 is the optimum experimental condition for the detection system. Applicability of the Detection System. The rice contaminated with cadmium is reported in several provinces of China in 2013, which draws more attention of people to these events. We apply the method based on the aggregation of AuNPs to detect Cd2+ in rice samples for evaluating the applicability of the detection system. We digested the rice samples, spiked Cd2+ with the digestion, and used the system to detect the rice digestion solution. With the increase of the concentration of the spiked Cd2+, the absorbance band at 520 nm decreases in general, along with the peak broadening and a distinctive red shift, and a broad absorption at 640 nm emerges by degrees (Figure 4A). The ratio of A640/A520 as a function of

complicated operations, and it has fast response rate and good selectivity. The different complex structures of metal ions and GSH make this detection system show good selectivity to Cd2+. The naked-eye detection of Cd2+ is not reported too much, and our method is the first time to use the visible signal readout to analyze cadmium-contaminated rice samples. The result presents this system can monitor Cd2+ qualitatively by eyes, and with the aid of UV−vis absorption spectra, the detection system can screen some samples semiquantitatively. For its simplicity and cost-effectiveness, we believe that the detection system will have some potential applications in the food safety field.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, theoretical calculation, and Figures S-1− S-8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-10-82545575. Fax: +86-10-82545631. E-mail: [email protected]. *Phone: +86-10-88256963. Fax: +86-10-88256092. E-mail: [email protected]. *Phone: +86-10-82545558. Fax: +86-10-82545631. E-mail: [email protected]. Author Contributions ⊥

Y.G. and Y.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (Garnts 21025520, 21105018, 21222502, 91233107, and 91213305), the Ministry of Science and Technology of China (Grant 2012AA022703), Beijing Natural Science Foundation (Grant 2122058), Youth Innovation Promotion Association (CAS), and Research Supported by the CAS/SAFEA International Partnership Program for Creative Research Teams.

Figure 4. Detection of Cd2+ in rice samples. (A) UV−vis absorption spectra of AuNPs in the presence of different concentrations of Cd2+ in the rice digestion. (B) The plot of A640/A520 of AuNPs versus the concentrations of Cd2+ in the rice digestion. (C) Photographs of AuNPs in the presence of different concentrations of Cd2+ in the rice digestion (the concentration of AuNPs is about 2 nM, 10 mM pH 6.0 phosphate sodium buffer, the concentration of NaCl is 40 mM, the concentration of GSH is 1 μM, the rice digestion is diluted to 500 times, the error bars are obtained based on three samples detection).

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REFERENCES

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the concentration of Cd2+ is plotted in Figure 4B, which increases gradually with the increase of the concentration of Cd2+. The color of AuNPs solution changes from red to blue with increasing of the concentration of Cd2+, which is easily recognized by naked eyes. Also the lowest detection concentration with the naked eye is 10 μM (Figure 4C). These results further validate the potential applications of the detection system in the food monitoring. However, many efforts are needed to improve the sensitivity and selectivity of the detection system, such as design of similar structure ligands with some functional groups, which maybe contribute to good recognization of Cd2+.

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CONCLUSIONS In conclusion, we successfully develop a facile, simple label-free colorimetric method for Cd2+ detection using unmodified AuNPs. The method does not need chemical modification and D

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