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A Nanoparticle Autocatalytic Sensor for Agþ and Cu2þ Ions in Aqueous Solution with High Sensitivity and Selectivity and Its Application in Test Paper Xuan Yang and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
bS Supporting Information ABSTRACT:
A novel nanoparticle autocatalytic sensor for the detection of Agþ and Cu2þ has been constructed based on the oxidative ability of Agþ and Cu2þ toward o-phenylenediamine (OPDA). Agþ and Cu2þ can be reduced to zerovalent silver and copper, respectively, and then such zerovalent Ag and Cu species form silver and copper nanoparticles that can catalyze the reaction between OPDA and Agþ and Cu2þ. In the reaction, OPDA is oxidized to 2,3-diaminophenazine (OPDAox), which has a fluorescence emission at 568 nm. Under the optimal conditions, Agþ and Cu2þ can be detected in the concentration ranges from 60 nM to 60 μM and from 2.5 nM to 25 μM, respectively. Through this facile approach, Agþ and Cu2þ can be detected down to 60 nM and 2.5 nM, respectively. In addition, the sensor is utilized for the detection of Agþ and Cu2þ in sewage, and we have obtained very good results that are consistent with those of inductively coupled plasmamass spectroscopy (ICP-MS). Moreover, such a nanoparticle autocatalytic sensor is applied to test paper for the detection of Agþ and Cu2þ with the naked eye. With such test paper, Agþ and Cu2þ could be detected at levels as low as 0.06 nmol and 0.3 nmol, respectively, with detection ranges of 0.0660 nmol for Agþ and 0.360 nmol for Cu2þ, under the irradiation of UV light (365 nm). The test paper could be potentially used in the rapid detection of Agþ and Cu2þ in real samples.
’ INTRODUCTION With ever-increasing industrial sprawl, the possibility for a release of pollutants into the environment increases. Environmental pollution has become one of the most important problems in the world, especially water pollution caused by heavy-metal ions. Many heavy-metal ions can do irreversible and permanent damage to the human body and cause various diseases. Because of the toxicity and the ability to bioaccumulate in organisms,1 heavy-metal ions are particularly dangerous to the entire ecosystem. Thus, the detection of heavy-metal ions has become a hot topic in scientific research, and numerous approaches have been developed for the detection of heavymetal ions such as fluorescence spectra,2 UVvis spectra,3 and electrochemistry.4 Recently, some novel detection methods based on DNA or DNAzyme have been developed for the rapid detection of heavy metal ions selectively.5 However, most of these methods either require sophisticated equipment or need r 2011 American Chemical Society
complicated operations. Therefore, it is necessary to develop a simple, economical, sensitive, and portable sensor for the detection of heavy-metal ions selectively. Recently, metal nanoparticles (NPs) have attracted much attention, because of their unique optical, electronic, and catalytic properties. Utilizing the optical, electronic, and catalytic ability of NPs, a great amount of sensors have been developed for the sensitive detection of heavy-metal ions.6 However, most of these sensors are subjected to complicated NPs synthesis and modification procedures. Autocatalytic reaction is one of the most common phenomena during the synthesis of NPs, and it can be utilized for the detection of heavy-metal ions. However, to the best of our knowledge, there Received: April 4, 2011 Accepted: May 17, 2011 Published: May 17, 2011 5005
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Scheme 1. The Principle of the Nanoparticle Autocatalytic Sensor for Agþ and Cu2þ
are almost no sensors based on NPs for the detection of heavymetal ions making use of the autocatalytic reaction during the synthesis progress of NPs. Herein, we report a novel facile NP autocatalytic sensor for the determination of Agþ and Cu2þ, based on the oxidative ability of Agþ and Cu2þ toward ophenylenediamine (OPDA). In addition, compared to other methods, whose results are shown in Table 3 in the Supporting Information,2,3 our method is more simple and sensitive. We have utilized such sensors for the detection of Agþ and Cu2þ in sewage and have obtained very good results that are consistent with the results of inductively coupled plasmamass spectroscopy (ICP-MS). Ultimately, we have tried to apply such new methods to test paper and have received very good results on the detection of Ag ion and Cu ion just by the color change of the test paper observed by the naked eye. Moreover, compared with other test papers previously reported,7 our test paper is more economical and simpler, because it can be used just like pH test paper.
measurements were made on a Model PW1700 X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) measurements were made on a Thermo ESCALAB 250 X-ray photoelectron spectrometer. Experiment Details. For fluorescence and colorimetric assays: 10 μL of different concentrations of AgNO3 or Cu(Ac)2 were diluted into 960 μL of PBS buffer (0.1 M, pH 7.4), followed by the addition of 30 μL of 6 mM OPDA. The solution then was maintained at 50 °C for 90 min. After the solution was cooled to room temperature, fluorescence and colorimetric assays were operated. For test paper assays: 10 μL of 6 mM OPDA was dripped on the surface of the chromatography paper, and after the solution was evaporated, 10 μL of different concentrations of AgNO3 or Cu(Ac)2 were dripped in the zones of OPDA on the surface of the filter paper. The test paper was placed at an oven under a stable temperature of 50 °C for 90 min. For real sample detection: all the pretreatments of sewage are the same as those of ICP-MS.
’ EXPERIMENTAL SECTION
’ RESULTS AND DISCUSSION As shown in Scheme 1, o-phenylenediamine (OPDA) could be oxidized to 2,3-diaminophenazine (DAP or OPDAox) by silver (Agþ) and copper (Cu2þ) selectively. As the reaction continued, Agþ and Cu2þ were reduced to zerovalent silver and copper and formed silver and copper nanoparticles (NPs), respectively. The thus-formed silver and copper NPs could catalyze the reaction between OPDA and Agþ and Cu2þ. Because of the catalytic ability of silver NPs and copper NPs, such sensors based on OPDA showed good selectivity toward Agþ and Cu2þ. After the addition of chloride anions (Cl) and ethylenediamine tetraacetic acid disodium salt (EDTA 3 Na2 3 H2O), Agþ and Cu2þ were respectively shielded. Thus, OPDA could not be oxidized to fluorescent OPDAox and there was no fluorescence signal output, because OPDAox had a fluorescence emission8 in the visible light region under the irradiation of UV light (365 nm), while OPDA had no fluorescnece emission. Such a nanoparticle autocatalytic system was applied for the detection of Agþ and Cu2þ in sewage, and the test paper was used to detect Agþ and Cu2þ with the naked eye.
Chemicals and Materials. Potassium persulfate (>99%) was purchased from Amresco, Inc. (Solon, OH). All the other chemicals were of analytical-reagent grade and were used as received without further purification. All the solutions were prepared with double-distilled water purified by a Milli-Q system (Millipore, Bedford, MA) and stored at 4 °C. Nitrogen were pumped into the solution of o-phenylenediamine (OPDA) and buffer for 30 min to remove oxygen in double-distilled water. Chromatography paper used in the test paper assays was from Whatman (U.K.). Apparatus. UVvisible absorbance spectra were recorded on a Cary Model 50 scan UVvisNIR spectrophotometer (Varian, Harbor City, CA) at room temperature. Scanning electron microscopy (SEM) measurements were made on a Model S4800 SEM microscope (Hitachi, Japan) at an accelerating voltage of 10.0 kV. Transmission electron microscopy (TEM) measurements were made using a FEI TECNAI G2 TEM microscope (Eindhoven, The Netherlands) operated at an accelerating voltage of 120 kV. X-ray diffraction (XRD)
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Figure 1. Fluorescence emission graph of OPDA with different oxidants.
To confirm the principle of the nanoparticle autocatalytic sensor for Agþ and Cu2þ and determine which materials in the reaction products had a fluorescence emission at 568 nm, five oxidants—such as phosphate balanced solution (PBS), hydrogen peroxide (H2O2), silver nitrate (AgNO3), copper(II) acetate (Cu(AC)2), and potassium persulfate (K2S2O8)—were estimated. As shown in Figure 1, the solution of OPDA with PBS had no fluorescence emission and the fluorescence emission peaks of other solutions appeared at 568 nm, despite the different oxidants, which demonstrated that the fluorescence originated from the oxidation products of OPDA, because the reduction products of H2O2 and K2S2O8 were just water (H2O) and potassium sulfate (K2SO4), which had no fluorescence emission. Therefore, we demonstrated that OPDA could be oxidized to OPDAox, which had a fluorescence emission at 568 nm by Agþ and Cu2þ, and that was the principle of the sensor for the detection of Agþ and Cu2þ. In addition, the demonstration was consistent with preview work.8 To estimate the selectivity of such nanoparticle autocatalytic sensors toward Agþ and Cu2þ, 11 other metal cations were also investigated, as shown in Figure 2. After the addition of OPDA, there were fluorescence signals in the samples that contained Agþ and Cu2þ, but the samples with 11 other metal ions still had no fluorescence, which was shown in Figure 2A. As shown in Figure 2A, there was no fluorescence if the sample contained OPDA without any metal ions. Therefore, the fluorescence signal was neither from the metal ions nor OPDA but, rather, from the products of the reaction between OPDA and Agþ and Cu2þ. Figure 2B showed very good selectivity of such nanoparticle autocatalytic sensors toward Agþ and Cu2þ and the samples that contained Agþ and Cu2þ could be easily distinguished by observing the color change of the solution with the naked eye. Thus, such autocatalytic sensors could be easily utilized for colorimetric detection of Agþ and Cu2þ just using the naked eye. Figure 2C indicates that such a nanoparticle autocatalytic sensor displayed excellent selectivity toward Agþ and Cu2þ. To determine the reasons why OPDA had good selectivity toward Agþ and Cu2þ, the reaction products of OPDA and Agþ and Cu2þ were characterized by TEM analysis. As shown in Figures 3A and 3B, some nanoparticles with a diameter of ∼100 nm formed on a membrane layer. To confirm the components that comprise such nanoparticles, the thus-formed
Figure 2. Investigation of the selectivity of such nanoparticle autocatalytic sensors toward different heavy-metal ions: (A) the fluorescence graph of OPDA with different heavy-metal ions; (B) photograph of OPDA solutions with different heavy-metal ions; and (C) selectivity of the OPDA to different metal ions.
nanoparticles were characterized by energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). As shown in Figure S1C in the Supporting Information, the element that comprised the nanoparticles was silver, which was confirmed by the XRD data shown in Figure S1A in the Supporting Information. As the XPS results show in Figure S1B in the Supporting Information, the silver element that formed the nanoparticles was zerovalent. Figure S1A in the Supporting Information showed the XRD patterns. The XRD pattern of the sample matched well with the standard XRD patterns of silver (JCPDS Powder Diffraction File No. 04-0783). All of the peaks of the patterns of the samples can be readily indexed to face-centered-cubic (fcc) silver (JCPDS Powder Diffraction File No. 04-0783), where the diffraction peaks at 2θ values of 38.24°, 44.42°, 64.44°, and 77.40° can be ascribed to the reflections of the (111), (200), (220), and (311) planes of the 5007
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Figure 3. Transmission electron microscopy (TEM) images of (A, B) silver nanoparticles and (C, D) copper nanoparticles, which were synthesized during the reaction progress of Agþ, Cu2þ, and OPDA.
fcc silver, respectively. No peaks from other phases were detected, indicating the high purity of the products. In addition, based on these characterizations, we assumed that Agþ could be reduced to zerovalent silver by OPDA and form silver nanoparticles, and thus-formed silver nanoparticles could catalyze the reaction of Agþ and OPDA.9 As shown in Figures 3C and 3D, some nanoparticles with a diameter of ∼5 nm formed on a membrane layer also were present. Such nanoparticles were characterized with XPS and EDS, as shown in Figure S2 in the Supporting Information. Moreover, based on these characterizations, we assumed that Cu2þ could be reduced to zerovalent copper by OPDA and form copper nanoparticles, and the thusformed copper nanoparticles could catalyze the reaction of Cu2þ and OPDA.9 Therefore, the autocatalytic phenomena in the reaction between OPDA and Agþ and Cu2þ would be the reason why sensors based on OPDA had good selectivity toward Agþ and Cu2þ. To demonstrate how such a nanoparticle autocatalytic system was used for the identification of Agþ and Cu2þ, fluorescence spectrometry and colorimetric assays were utilized to monitor the fluorescence intensities and the color changes for 16 input modes (see Figure 4). With no input, OPDA could not be oxidized to OPDAox and there were no fluorescence and color changes of the solution, as shown in Figures 4A and 4B. However, if one of the Agþ and Cu2þ was input, OPDA was oxidized to OPDAox, fluorescence emissions at 568 nm were observed, and the color of the samples changed to light yellow. In addition, after Cl or EDTA was added to shield the Agþ or Cu2þ, respectively, there was almost no dissociative Agþ or Cu2þ to oxidize OPDA, no fluorescence was observed, and the solutions were still colorless and transparent, as shown in Figures 4A
and 4B. Considering that there were 16 input modes, with each input mode corresponding to a result, the experimental results are shown in Figures 4A and 4B and the truth table showed the results of each input mode in Figure 4C. The truth table in Figure 4C was consistent with the results shown in Figures 4A and 4B. Figure 5 depicted the fluorescence analysis of different concentrations of Agþ and Cu2þ, using the nanoparticle autocatalytic system as a sensor for Agþ and Cu2þ. As shown in Figure 5A, the fluorescence intensity at 568 nm was enhanced as the Agþ concentration increased. This indicated that OPDA was oxidized to OPDAox, since the fluorescence at 568 nm originated from OPDAox, which is the oxidation product of OPDA. It was found that there was an observable change in the fluorescence spectra upon the addition of 60 nM Agþ. The inset of Figure 5A showed the relationship between the fluorescence intensity at 568 nm and the Agþ concentration. A detection range from 60 nM to 60 μM was achieved, and the detection limit was 60 nM for Agþ. Figure 5B showed that the fluorescence intensity at 568 nm increased as the Cu2þ concentration increased, which indicated that OPDA was oxidized to OPDAox. There was an observable change in the fluorescence spectra upon the addition of 2.5 nM Cu2þ. The inset in Figure 5B shows the relationship between the fluorescence intensity at 568 nm and the Cu2þ concentration. A detection range of 2.5 nM to 25 μM is observed, and a limit of detection for Cu2þ analysis was achieved (2.5 nM). To test the applicability of the nanoparticle autocatalytic sensor for Cu2þ and Agþ detection in real samples, such autocatalytic sensors were applied for the detection of Cu2þ and Agþ in sewage, as shown in Figure 6. Figure 6A showed the 5008
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Figure 4. (A) Fluorescence intensity and (B) photographs of the OPDAox for 16 input modes. (C) Truth table for the 16 input modes. (The serial numbers in the colorimetric assays in panel B correspond to the different input modes.)
fluorescence intensities of different concentrations of Cu2þ, and Figure 6B showed the relative fluorescence intensities at 568 nm with different Cu2þ concentrations. Figure 6C showed the fluorescence intensities of real samples that were diluted with double-distilled water (3 times (volume)). As shown in Table 1 in the Supporting Information, the results showed excellent agreement with ICP-MS in the real sample detection. Figure 6D showed the fluorescence intensities of 6 μM Agþ and a real sample plus 6 μM Agþ. Table 2 in the Supporting Information shows the recovery of Agþ in real sample detection, and the recovery was 98.71%, which showed that the result was coincident with that of ICP-MS. These results showed that our nanoparticle autocatalytic sensor could be used for the detection of Cu2þ and Agþ in real samples. Figure 7 showed the images of the test paper for the detection of Agþ and Cu2þ, based on the nanoparticle autocatalytic sensor. Each zone in the test paper was first immersed in 10 μL of 6 mM OPDA and after the solution evaporated
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Figure 5. Nanoparticle autocatalytic sensor for the detection of Agþ and Cu2þ. (A) Fluorescence spectra for analyzing different concentrations of Agþ (from bottom to top): 0, 0.06, 0.3, 0.6, 3, 6, 30, and 60 μM. (Inset shows the dependence of fluorescence intensity at 568 nm on the concentration of Agþ.) (B) Fluorescence spectra for analyzing different concentrations of Cu2þ (from bottom to top): 0, 2.5 nM, 10 nM, 50 nM, 100 nM, 250 nM, 500 nM, 2.5 μM, 10 μM, and 25 μM. (Inset shows the dependence of the fluorescence intensity at 568 nm on the Cu2þ concentration.)
sufficiently, 10 μL of different concentrations of Agþ and Cu2þ were added into the zones. The test papers were placed in an oven at a stable temperature of 50 °C for 90 min. The color of the test zones with the addition of Agþ and Cu2þ changed to light yellow, and there was fluorescence in the test zones under UV light (365 nm), as shown in Figures 7ad. The detection limits of such test paper for Agþ and Cu2þ were 0.3 and 0.6 nmol, respectively, without the irradiation of UV light (365 nm), as shown in Figure 7a and 7c. Moreover, the detection limit for Agþ and Cu2þ was reduced to 0.06 and 0.3 nmol, respectively, as shown in Figures 7b and 7d, under the irradiation of UV light (365 nm). Compared to other test papers, such test paper for the detection of Agþ and Cu2þ was simpler without the premodification of different concentrations of target on the surface of the paper. Since the test paper was just chromatography paper, such test paper was more economical than other reported test papers.7 5009
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Figure 6. Analytical results for Cu2þ and Agþ in sewage. For Cu2þ: (A) fluorescence intensities of different Cu2þ concentrations, (B) the calibration curve from 0 to 500 nM; and (C) fluorescence intensities of a real sample that was diluted with double-distilled water (3 times (volume)), and (D) fluorescence intensities of 6 μM Agþ and a real sample plus 6 μM Agþ.
spectra with detection ranges from 60 nM to 60 μM and from 2.5 nM to 25 μM, respectively. The detection limit for Agþ and Cu2þ are 60 nM and 2.5 nM, respectively. In addition, the sensor is utilized for the detection of Agþ and Cu2þ in sewage, and we have obtained very good results that are consistent with those of inductively coupled plasmamass spectroscopy (ICP-MS). Moreover, such a nanoparticle autocatalytic sensor is applied to test paper for the detection of Agþ and Cu2þ with the naked eye. With such test paper, Agþ and Cu2þ could be detected at levels as low as 0.06 nmol and 0.3 nmol, respectively, with a detection range from 0.06 to 60 nmol for Agþ and from 0.3 to 60 nmol for Cu2þ, under the irradiation of UV light (365 nm). The test paper would be potentially used in the rapid detection of Agþ and Cu2þ in real samples. Figure 7. Test paper for the detection of (a and b) Agþ and (c and d) Cu2þ. Panels b and d show the images of the test paper under the irradiation of UV light (365 nm).
’ CONCLUSION In conclusion, we have constructed a novel nanoparticle autocatalytic sensor for the detection of Agþ and Cu2þ, utilizing the oxidative ability of Agþ and Cu2þ toward OPDA. Compared to other sensors for the detection of Agþ and Cu2þ,26 our sensor is simpler and more economical. Based on the nanoparticle autocatalytic sensor, Agþ and Cu2þ are detected by utilizing fluorescence
’ ASSOCIATED CONTENT
bS
Supporting Information. XRD, XPS, and EDS characterization, optimization of fluorescence assay, time-dependent photoluminescence spectra, and UVvis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: (þ86) 431-85262003. Fax: (þ86) 431-85689711. E-mail:
[email protected]. 5010
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’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (through Grant Nos. 20735003, 20890020, and 21075120) and 973 Project Nos. 2009CB930100 and 2010CB933600. ’ REFERENCES (1) (a) Chang, L. W. Toxicology of Metals; Lewis Publishers: Boca Raton, FL, 1996. (b) Chapman, P. J.; Long, Z.; Datskos, P. G.; Archibald, R.; Sepaniak, M. J. Anal. Chem. 2007, 79, 7062–7068. (c) El-Safty, S. A.; Ismail, A. A.; Matsunaga, H.; Hanaoka, T.; Mizukami, F. Adv. Funct. Mater. 2008, 18, 1485–1500. (d) Chung, Y. T.; Ling, Y. C.; Yang, C. S.; Sun, Y. C.; Lee, P. L.; Lin, C. Y.; Hong, C. C.; Yang, M. H. Anal. Chem. 2007, 79, 8900–8910. (2) (a) Shang, L.; Dong, S. J. J. Mater. Chem. 2008, 18, 4636–4640. (b) Qin, H. X.; Ren, J. T.; Wang, J. H.; Wang, E. K. Chem. Commun. 2010, 46, 7385–7387. (c) Li, B. L.; Du, Y.; Dong, S. J. Anal. Chim. Acta 2009, 644, 78–82. (3) Yin, B. C.; Ye, B. C.; Tan, W. H.; Wang, H.; Xie, C. C. J. Am. Chem. Soc. 2009, 131, 14624–14625. (4) Andria, S. E.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2010, 5, 1720–1726. (5) (a) Liu, J. W.; Lu, Y. Angew. Chem., Int. Ed. 2007, 46, 7587–7590. (b) Liu, J. W.; Lu, Y. J. Am. Chem. Soc. 2007, 129, 9839–9839. (c) Liu, J. W.; Lu, Y. J. Am. Chem. Soc. 2004, 126, 12298–12305. (d) Liu, J. W.; Lu, Y. Chem. Commun. 2007, 4872–4874. (e) Xiang, Y.; Tong, A. J.; Lu, Y. J. Am. Chem. Soc. 2009, 131, 15352–15357. (6) (a) Hurst, S. J.; Hill, H. D.; Mirkin, C. A. J. Am. Chem. Soc. 2008, 36, 12192–12200. (b) Lee, J. S.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 22, 4093–4096. (c) Lee, J. S.; Mirkin, C. A. Anal. Chem. 2008, 17, 6805–6808. (d) He, S. J.; Li, D.; Zhu, C. F.; Song, S. P.; Wang, L. H.; Long, Y. T.; Fan, C. H. Chem. Commun. 2008, 40, 4885–4887. (e) Freeman, R.; Finder, T.; Willner, I. Angew. Chem., Int. Ed. 2009, 42, 7818–7821. (7) (a) Cheng, C. M.; Martinez, A. W.; Gong, J. L.; Mace, C. R.; Phillips, S. T.; Carrilho, E.; Mirica, K. A.; Whitesides, G. M. Angew. Chem., Int. Ed. 2010, 49, 4771–4774. (b) Li, J. S.; Yao, J. J.; Zhong, W. W. Chem. Commun. 2009, 4962–4964. (c) Zhu, J. B.; Li, T.; Hu, J. M.; Wang, E. K. Anal. Bioanal. Chem. 2010, 397, 2923–2927. (d) Gu, Z.; Zhao, M. X.; Sheng, Y. W.; Bentolila, L. A.; Tang, Y. Anal. Chem. 2011, 83, 2324–2329. (e) Guo, Z. Q.; Zhu, W. H.; Zhu, M. M.; Wu, X. M.; Tian, H. Chem.—Eur. J. 2010, 16, 14424–14432. (8) (a) Kawakubo, S.; Ogihara, K.; Iwatsuki, M. Analyst 1995, 120, 2719–2723. (b) Sun, W.; Jiao, K.; Zhang, S. S. Talanta 2001, 55, 1211–1218. (c) Sivarama, G.; Krishnan, V. R. Indian J. Chem. 1967, 5, 635–&. (9) (a) Nancollas, G. H.; Wefel, J. S. J. Den. Res. 1976, 55, 617–624. (b) Cui, H.; Guo, J. Z.; Li, N.; Liu, L. J. J. Phys. Chem. C 2008, 112, 11319–11323.
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