Colorimetric Assay for Lead Ions Based on the Leaching of Gold

Oct 23, 2009 - Zhiyang Zhang , Zhaopeng Chen , Shasha Wang , Fangbin Cheng , and Lingxin .... Wei-Yu ChenYen-Chun ShiangChi-Lin LiArun Prakash ...
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Anal. Chem. 2009, 81, 9433–9439

Colorimetric Assay for Lead Ions Based on the Leaching of Gold Nanoparticles Yi-You Chen,† Huan-Tsung Chang,‡ Yen-Chun Shiang,‡ Yu-Lun Hung,† Cheng-Kang Chiang,‡ and Chih-Ching Huang*,†,§ Institute of Bioscience and Biotechnology and Center for Marine Bioscience and Biotechnology, National Taiwan Ocean University, Keelung, Taiwan, and Department of Chemistry, National Taiwan University, Taipei, Taiwan A colorimetric, label-free, and nonaggregation-based gold nanoparticles (Au NPs) probe has been developed for the detection of Pb2+ in aqueous solution, based on the fact that Pb2+ ions accelerate the leaching rate of Au NPs by thiosulfate (S2O32-) and 2-mercaptoethanol (2-ME). Au NPs reacted with S2O32- ions in solution to form Au(S2O3)23- complexes on the Au NP surfaces, leading to slight decreases in their surface plasmon resonance (SPR) absorption. Surface-assisted laser desorption/ ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) data reveals the formation of Pb-Au alloys on the surfaces of the Au NPs in the presence of Pb2+ ions and 2-ME. The formation of Pb-Au alloys accelerated the Au NPs rapidly dissolved into solution, leading to dramatic decreases in the SPR absorption. The 2-ME/S2O32--Au NP probe is highly sensitive (LOD ) 0.5 nM) and selective (by at least 1000-fold over other metal ions) toward Pb2+ ions, with a linear detection range (2.5 nM-10 µM) over nearly 4 orders of magnitude. The cost-effective probe allows rapid and simple determination of the concentrations of Pb2+ ions in environmental samples (Montana soil and river), with results showing its great practicality for the detection of lead in real samples. The cyanidation process has been used to leach gold for over 100 years since it was patented in 1888 by MacArthur and the Forrest brothers.1 Despite the long-term use of cyanidation, there is growing interest in the development of noncyanide lixiviant systems, due mainly to the failure of cyanidation to extract gold from so-called difficult-to-treat ores and to environmental and safety concerns.2 Gold forms soluble Au(I)L2 and Au(III)L4 species in solutions containing various oxidants and ligands (L).2,3 Electrochemistry studies performed in various lixiviant systems have revealed that the rate of gold oxidation depends on * To whom correspondence should be addressed. Chih-Ching Huang, Institute of Bioscience and Biotechnology, Center for Marine Bioscience and Biotechnology, National Taiwan Ocean University, 2, Beining Road, Keelung, 20224, Taiwan. Phone: 011-886-2-24622192 ext. 5517. Fax: 011-886-2-2462-2320. E-mail: [email protected]. † Institute of Bioscience and Biotechnology, National Taiwan Ocean University. ‡ National Taiwan University. § Center for Marine Bioscience and Biotechnology, National Taiwan Ocean University. (1) MacArthur, J. S.; Forrest, R.; Forrest, W. W. British Patent No. 14174, 1888. (2) Senanayake, G. Miner. Eng. 2004, 17, 785–801. (3) Senanayake, G. Gold Bull. 2005, 38, 170–179. 10.1021/ac9018268 CCC: $40.75  2009 American Chemical Society Published on Web 10/23/2009

the temperature, pH, and concentrations of the oxidants, ligands, and additives, such as foreign heavy metallic ions and sulfur species.2-6 Thiosulfate (S2O32-) is a leading contender in the search for alternative leachants for the extraction of gold (4Au0 + O2 + 2H2O + 8S2O32- f 4Au(S2O3)23- + 4OH-) from copper-gold, refractory, and carbonaceous ores.3-5 Relative to the cyanidation process, this process is advantageous from the points of view of environmental safety, cost, and simplicity. Several studies have suggested that the nature and concentrations of the ions and reagents are important parameters influencing the dissolution of gold in ammoniacal S2O32- systems.3-5 Heavy metal ion additives, including Cu+/Cu2+, Fe2+/Fe3+, Cr3+/Cr6+, and Co2+/Co3+ redox mediators, have been employed in S2O32leaching systems to increase the recovery of gold.4,5 Other studies have suggested that the formation of a passive layer on gold surfaces, using such metals as Pb, Ag, Hg, Tl, and Bi, has a significant influence during the initial stages of leaching.4-9 Because gold nanoparticles (Au NPs) possess many interesting chemical and physical properties, they have been used for the fabrication of miniaturized optical devices, sensors, and photonic circuits, as well as in medical diagnostics and therapeutics.10-12 One of the most important properties is their strong surface plasmon resonance (SPR) absorption with extremely high extinction coefficients (108-1010 M-1 cm-1) in the visible wavelength range.13 The extinction cross sections of the particles and the wavelengths at which they absorb and scatter light both depend on their size and shape, the dielectric properties (refractive (4) Feng, D.; van Deventer, J. S. J. Hydrometallurgy 2002, 64, 231–246. (5) Grosse, A. C.; Dicinoski, G. W.; Shaw, M. J.; Haddad, P. R. Hydrometallurgy 2003, 69, 1–21. (6) Senanayake, G. Hydrometallurgy 2008, 90, 46–73. (7) Chimenos, J. M.; Segarra, M.; Guzman, L.; Karagueorguieva, A.; Espiell, F. Hydrometallurgy 1997, 44, 269–286. (8) Mclntyre, J. D. E.; Peck, W. F., Jr. J. Electrochem. Soc. 1976, 123, 1800– 1813. (9) Sandenbergh, R. F.; Miller, J. D. Miner. Eng. 2001, 14, 1379–1386. (10) Wilson, R. Chem. Soc. Rev. 2008, 37, 2028–2045. (11) (a) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Chem. Soc. Rev. 2006, 35, 1084–1094. (b) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. Rev. 2008, 108, 494–521. (12) (a) Lal, S.; Grady, N. K.; Kundu, J.; Levin, C. S.; Lassiter, J. B.; Halas, N. J. Chem. Soc. Rev. 2008, 37, 898–911. (b) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547–1562. (13) (a) Kubo, S.; Diaz, A.; Tang, Y.; Mayer, T. S.; Khoo, I. C.; Mallouk, T. E. Nano Lett. 2007, 7, 3418–3423. (b) Wang, H.; Brandl, D. W.; Nordlander, P.; Halas, N. J. Acc. Chem. Res. 2007, 40, 53–62. (c) Myroshnychenko, V.; Rodrı´guez-Ferna´ndez, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marza´n, L. M.; Abajo, F. J. G. Chem. Soc. Rev. 2008, 37, 1792–1805.

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index) of the surrounding medium, and their interactions with neighboring particles.10-13 The SPR frequency of Au NPs changes dramatically upon varying the refractive index of the local environment or the average distance between Au NPs.10-13 Systems based on analyte-induced aggregation of Au NPs have been employed for the colorimetric detection of cells, viruses, nucleic acids, proteins, small molecules, and metal ions.14 Although the absorption cross sections of spherical Au NPs are proportional to the third power of their diameters (R3), there are only a few reports describing label-free optical detection methods based on the size dependence of Au NPs during their leaching processes.15 The monitoring of toxic metal ions in aquatic ecosystems is an important issue because these contaminants can have severe effects on human health and the environment.16 Lead is one of the most toxic metallic pollutants; for example, it is associated with damage to the kidneys, the liver, and the gastrointestinal tract, as well as with neurological damage and decreased hemoglobin production.17,18 The maximum contamination level for lead in drinking water is defined by the U.S. Environmental Protection Agency (EPA) to be 75 nM.16-18 Because of its toxicity, the accurate determination of Pb is critical. Several methods for Pb analysis have been developed in the past decade, including those based on atomic absorption spectrometry, atomic emission spectrometry, inductively coupled plasma mass spectrometry (ICPMS), anodic stripping voltammetry, and reversed-phase high-performance liquid chromatography coupled with UV-vis or fluorescence detection.18 With regard to sensitivity and accuracy, these methods are all efficient tools for Pb determination, but they are time-consuming, expensive, and/or require sophisticated equipment. The past few years have witnessed great progress in the development of optical and electrochemical techniques for the detection of lead ions.19-24 Procedures using chromophores,19 DNAzymes,20 oligonucleotides,21 polymers,22 antibody,23 and functional nanoparticles14c,24 have all been developed for the selective detection of lead ions (Pb2+). Nevertheless, many of these systems have limited practical use because of, for example, poor aqueous solubility, cross-sensitivity toward other (14) (a) Zhao, W.; Brook, M. A.; Li, Y. ChemBioChem 2008, 9, 2363–2371. (b) Medley, C. D.; Smith, J. E.; Tang, Z.; Wu, Y.; Bamrungsap, S.; Tan, W. Anal. Chem. 2008, 80, 1067–1072. (c) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2005, 127, 12677–12683. (d) Darbha, G. K.; Singh, A. K.; Rai, U. S.; Yu, E.; Yu, H.; Ray, P. C. J. Am. Chem. Soc. 2008, 130, 8038–8043. (e) Li, D.; Wieckowska, A.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 3927–3931. (f) Xue, X.; Wang, F.; Liu, X. J. Am. Chem. Soc. 2008, 130, 3244–3245. (g) Huang, C.-C.; Huang, Y.-F.; Cao, Z.; Tan, W.; Chang, H.-T. Anal. Chem. 2005, 77, 5735–5741. (h) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (15) (a) Shang, L.; Dong, S. Anal. Chem. 2009, 81, 1465–1470. (b) Shang, L.; Jin, L.; Dong, S. Chem. Commun. 2009, 3077–3079. (16) (a) Campbell, L.; Dixon, D. G.; Hecky, R. E. J. Toxicol. Environ. Health, Part B 2003, 6, 325–356. (b) Needleman, H. L. Human Lead Exposure; CRC Press: Boca Raton, FL, 1991. (17) Needleman, H. Annu. Rev. Med. 2004, 55, 209–222. (18) (a) Casas, J. S.; Sordo, J. Lead: Chemistry, Analytical Aspects, Environmental Impact and Health Effects; Elsevier: Amsterdam, The Netherlands, 2006. (b) http://www.epa.gov/safewater/contamin ants/index.html (accessed August 2009). (19) (a) Deo, S.; Godwin, H. A. J. Am. Chem. Soc. 2000, 122, 174–175. (b) Zapata, F.; Caballero, A.; Espinosa, A.; Ta´rraga, A.; Molina, P. Org. Lett. 2008, 10, 41–44. (c) Wu, F.-Y.; Bae, S. W.; Hong, J. I. Tetrahedron Lett. 2006, 47, 8851–8854. (d) Kavallieratos, K.; Rosenberg, J. M.; Chen, W.-Z.; Ren, T. J. Am. Chem. Soc. 2005, 127, 6514–6515. (20) (a) Li, J.; Lu, Y. J. Am. Chem. Soc. 2000, 122, 10466–10467. (b) Brown, A. K.; Li, J.; Pavot, C. M.-B.; Lu, Y. Biochemistry 2003, 42, 7152–7161.

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metal ions, matrix interference, high cost, complicated processing, the use of unstable molecules, or poor sensitivity. In response to these shortcomings, a cost-effective and simple detection method is required for rapid detection of lead ions. The colorimetric and size-dependent properties of Au NPs are traits that can be easily exploited using simple chemistries to develop the desired components and subsequent in-field devices. In this study, we monitored changes in the SPR absorptions of Au NPs to investigate the S2O32--leaching system in the presence of 2-mercaptoethanol (2-ME) and Pb2+. We also developed a label-free, rapid, and homogeneous assay, employing Au NPs, S2O32-, and 2-ME, for the highly selective and sensitive detection of Pb2+ ions. EXPERIMENTAL SECTION Chemicals. 2-Mercaptoethanol, sodium thiosulfate, trisodium citrate, and all metallic salts used in this study were purchased from Aldrich (Milwaukee, WI). Sodium tetraborate and hydrogen tetrachloroaurate(III) trihydrate were obtained from Acros (Geel, Belgium). Montana Soil (SRM 2710) was obtained from the National Institute of Standards and Technology (NIST, Maryland). Milli-Q ultrapure water was used in each experiment. The buffer was 50 mM glycine solution (pH 10.0, adjusted with 1.0 N NaOH). Synthesis of 14.2 nm Spherical Au NPs. The 14.2 nm spherical Au NPs were prepared through citrate-mediated reduction of HAuCl4.25 Aqueous 1 mM HAuCl4 (250 mL) was brought to a vigorous boil while stirring in a round-bottom flask fitted with a reflux condenser; 38.8 mM trisodium citrate (25 mL) was added rapidly to the solution, which was heated for another 15 min, during which time its color changed from pale yellow to deep red. The solution was cooled to room temperature with continuous stirring. The sizes of the Au NPs were verified through transmission electron microscope (TEM) analysis (H7100, Hitachi High-Technologies Corporation, Tokyo, Japan); they appeared to be nearly monodisperse, with an average size of 14.2 ± 0.3 nm. A double-beam UV-vis spectrophotometer (Cintra 10e, GBC, Victoria, Australia) was used to measure the absorption of the Au NP solution. The particle concentration of the Au NPs (15 nM) was determined according to Beer’s law using an extinction coefficient of ∼2.43 × 108 M-1 cm-1 at 520 nm for the 14.2 nm Au NPs. Characterization of Au NPs. TEM images of Au NPs were captured on a H7100 TEM (operating at 125 kV). Samples for TEM measurements were prepared by placing 20 µL of the Au NPs solution on a carbon-coated copper grid and then drying at room temperature. The ζ potentials of the Au NPs were measured on a Zetasizer 3000HS analyzer (Malvern Instruments, Malvern, (21) (a) Liu, C.-W.; Huang, C.-C.; Chang, H.-T. Anal. Chem. 2009, 81, 2383– 2387. (b) Babkina, S. S.; Ulakhovich, N. A. Anal. Chem. 2005, 77, 5678– 5685. (22) Geary, C. D.; Zudans, I.; Goponenko, A. V.; Asher, S. A.; Weber, S. G. Anal. Chem. 2005, 77, 185–192. (23) Lin, T.-J.; Chung, M.-F. Sensors 2008, 8, 582–593. (24) (a) Wei, H.; Li, B.; Li, J.; Dong, S.; Wang, E. Nanotechnology 2008, 19, 095501. (b) Li, J.; Lu, Y. J. Am. Chem. Soc. 2003, 125, 6642–6643. (c) Li, J.; Lu, Y. J. Am. Chem. Soc. 2004, 126, 12298–12305. (d) Wang, Z.; Lee, J. H.; Lu, Y. Adv. Mater. 2008, 20, 3263–3267. (25) (a) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55–75. (b) Enu ¨ stu ¨ n, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317–3328. (c) Turkevich, J. Gold Bull. 1985, 18, 86–91. (d) Frens, G. Nat. Phys. Sci. 1973, 241, 20–22.

U.K.). For surface-assisted laser desorption/ionization time-offlight ionization mass spectrometry (SALDI-TOF MS) measurements, we equilibrated aliquots (1.0 mL) of the 2-ME/S2O32--Au NPs (1.5 nM) in the absence and presence of Pb2+ (1.0-4.0 µM) for 60 min at ambient temperature and then subjected the systems to centrifugation at a relative centrifugation force (RCF) of 35 000g for 20 min. Following removal of the supernatants, the precipitates were washed with water. After three centrifugation/washing cycles, the pellets were resuspended in water. A portion of the samples (∼1.0 µL) was cast onto a stainless-steel 96-well MALDI target and dried in air at room temperature prior to SALDI-TOF MS measurements. The samples were irradiated with a 337 nm nitrogen laser at 10 Hz. Ions produced by laser desorption were stabilized energetically during a delayed extraction period of 200 ns and then accelerated through the TOF system in the reflection mode prior to entering the mass analyzer. Positive ions were detected in a range from 1 to 4 kDa. To obtain high resolution and high signal-to-noise (S/N) ratios, the laser fluence was adjusted to slightly higher than the threshold and each mass spectrum was generated by averaging 300 laser pulses. 2-ME/S2O32--Au NP-Based Sensor for Pb2+. For Pb2+ sensing, aliquots (490 µL) of 5 mM glycine-NaOH solution (pH 10.0) solutions containing the Au NPs (1.5 nM), Na2S2O3 (1.0 mM), and Pb2+ ions (0-10 µM) were equilibrated at room temperature for 15 min. 2-ME (50 mM, 10 µL) was added to each of these mixtures, which were then equilibrated through gentle shaking at room temperature for other 1.5 h. The mixtures were then transferred separately into 96-well microtiter plates, and their UV-vis absorption spectra were recorded using an µ-Quant microplate reader (Biotek Instruments, Winooski, VT). In this article, the final concentrations of the species are provided. Analysis of Water and Soil Samples. Acidic digestion of soil samples was performed according to EPA method 305B.26 A soil sample (0.01 g) was weighed in an Erlenmeyer flask, and 10 mL of HNO3 1:1 (v/v) was added. The solution was heated on a hot plate to ∼95 °C without boiling, and this temperature was maintained for 15 min. After cooling to less than 70 °C, 5 mL of concentrated HNO3 was added and the sample was refluxed for 30 min at ∼95 °C without boiling. This step was repeated a second time. The sample was evaporated to ∼5 mL without boiling. After cooling to less than 70 °C, 2 mL of 18 MΩ water was added followed by the slow addition of 10 mL of H2O2 (30%). Care must be taken to ensure that losses do not occur due to excessively vigorous effervescence caused by rapidly adding the strong oxidizer, H2O2. The solution was then heated until effervescence subsided. After cooling to less than 70 °C, 5 mL of concentrated HCl and 10 mL of 18 MΩ water were added and the sample was refluxed for 15 min without boiling. After cooling to room temperature, the sample was filtered and diluted to 100 mL using DI water. The aqueous soil sample (2 mL) was further diluted with DI water (98 mL). Aliquots of the diluted (250 µL) were spiked with standard Pb2+ solutions (10 µL) at concentrations over the range 0-75 nM. The spiked (26) (a) Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, 3rd ed.; U.S. EPA SW-846, U.S. Government Printing Office: Washington, DC, 1996. (b) Lorentzen, E. M. L.; Kingston, H. M. Anal. Chem. 1996, 68, 4316–4320.

Scheme 1. Cartoon Representation of the Sensing Mechanism of the 2-ME/S2O32--Au NP Probe for the Detection of Pb2+ Ions

samples were then diluted to 500 µL with solutions containing the 2-ME/S2O32--Au NP (1.5 nM) probe and 5 mM glycineNaOH (pH 10.0). A water sample collected from a river on the campus of National Taiwan Ocean University was filtered through a 0.2 µm membrane. Aliquots of the river water (250 µL) were spiked with standard Pb2+ solutions (10 µL) at concentrations over the range 0-10 µM. The spiked samples were then diluted to 500 µL with solutions containing the 2-ME/S2O32--Au NP (1.5 nM) probe and 5 mM glycine-NaOH (pH 10.0). The spiked samples were then analyzed separately using ICPMS and the developed sensing technique. RESULTS AND DISCUSSION Sensing Strategy. Scheme 1 outlines the sensing mechanism employed in this study. When the Au NPs reacted with S2O32- ions in solution, Au(S2O3)23- complexes were formed immediately on the Au NP surfaces, leading to slight decreases in their SPR absorption. After adding Pb2+ ions and 2-ME, the Au NPs rapidly dissolved to form Au+-2-ME complexes in solution. As a result, the SPR absorption decreased dramatically, allowing quantitation of the Pb2+ ions in the aqueous solution. To understand the roles that 2-ME and Pb2+ played in accelerating the leaching of Au NPs, the SPR absorption of Au NPs was monitored (Figure 1A). Curve a displays the absorption spectrum of 1.5 nM Au NPs (average diameter, 14.2 ± 0.3 nm, from 100 counts) in 5 mM glycine-NaOH solution (pH 10.0); a strong SPR absorption appears at 520 nm (extinction coefficient ) 2.43 × 108 M-1 cm-1). After addition of 1.0 mM sodium thiosulfate (Na2S2O3) solution, the absorption at 520 nm (curve b) decreased slightly. The transmission electron microscopy (TEM) image in Figure S1 in the Supporting Information reveals that the average particle size of the Au NPs was 14.0 ± 0.2 nm; therefore, the decreased intensity of the SPR absorption was due to both the slightly smaller particles size, changes in the dielectric constant, or slight aggregation of the Au NPs resulting from the adsorption of S2O32- ions. S2O32- is a divalent-type soft ligand that tends to form stable complexes with low-spin d10 Au+ ions {log β[Au(S2O3)23-] ) ∼26}.27 In the S2O32- leaching liquor, dissolved oxygen gas (O2) acted as an oxidant. Commonly, S2O32- ions act as monodentate ligands via the terminal sulfur atom, establishing strong σ bonds with gold ions that are stabilized by pπsdπ back-bonding.27 We used X-ray photoelectron spectroscopy (XPS) to investigate the oxidation states of the surfaces of the Au NPs in the absence and presence of 1.0 mM Na2S2O3 (Figure S2 in the Supporting Information). The binding energy (BE) for the Au 4f7/2 electrons in the Au NPs in the absence of S2O32- was 84.1 eV (Figure (27) Bryce, R. A.; Charnock, J. M.; Pattrick, R. A. D.; Lennie, A. R. J. Phys. Chem. A 2003, 107, 2516–2523.

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Figure 1. (A) UV-vis absorbance spectra and (B) time-course measurement of the values of Ex520 of solutions containing (a) Au NPs, (b) Au NPs and Na2S2O3, (c) Au NPs and 2-ME, (d) Au NPs, Na2S2O3, and 2-ME, (e) Au NPs, Na2S2O3, PbCl2, and 2-ME, and (f) Au NPs and PbCl2. Insets to part A: photographic images of the Au NP solutions. The concentrations of the Au NPs, Na2S2O3, PbCl2, and 2-ME were 1.5 nM, 1.0 mM, 1.0 µM, and 1.0 mM, respectively. Buffer: 5 mM glycine-NaOH solution (pH 10.0).

S2, curve a in the Supporting Information); i.e., within the range from 84.0 (Au) to 85.0 eV [polynuclear Au(I)sligand complex].28 The BE of the Au 4f7/2 electrons is a common signature for Au oxidation states when using the BE (285.3 eV) of the alkyl chain C 1s orbital as an internal reference. The BE of the Au NPs in the presence of S2O32- was 84.8 eV (Figure S2, curve b in the Supporting Information). This BE shift was primarily due to the greater contribution of the S2O32--passivated surfaces (i.e., the higher-BE component) to the Au4f core-level photoemission spectrum.28 We found that the ζ potentials of the Au NPs in the absence and presence of 1.0 mM Na2S2O3 solution were -23.8 and -36.5 mV, respectively. The morenegative ζ potential supports the notion that S2O32- ions were bound to the surfaces of the Au NPs. These results agree with our hypothesis that some Au(S2O3)23- complexes were formed and released into the bulk solution (Figure 1 and Figure S1 in the Supporting Information). The lower leaching rate (curve b in Figure 1B) caused by S2O32- is due to a high activation energy (Ea ) 27.99 kJ/mol) in the absence of a redox mediator, such as Cu+/Cu2+.3-5,29 In comparison, the activation energy of only 15.54 kJ/mol is required to lixiviate metallic gold in an ammonia solution (pH 10.0) containing S2O32- and Cu2+ ions.29 In the absorption spectra (curve d) in Figure 1A, we observe that the Au NPs were leached to a greater extent after we had added 2-ME (1.0 mM) to the (28) Tanaka, A.; Takeda, Y.; Nagasawa, T.; Takahashi, K. Solid State Commun. 2003, 126, 191–196. (29) Tao, J.; Doyle, F. M.; Jin, C.; Peters, E.; Shi, X. Hydrometallurgy Fundamentals, Technology and Innovation, Proceedings of the 4th Milton E. Wadsworth (IV) Internation Symposium on Hydrometallurgy; Society for Mining, Metallurgy and Exploration: Littleton, CO, 1993.

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thiosulfate-treated Au NPs (S2O32--Au NPs) solution. The average size of Au NPs decreased further, from 14.0 ± 0.2 to 13.1 ± 0.3 nm, after their reaction with 2-ME for 2 h (Figure S1 in the Supporting Information). Alkanethiols are strong etching agents because of the high Au-S bond energy (184 kJ/mol).30 We note that 2-ME induced the aggregation of Au NPs that were not pretreated with Na2S2O3 (curve c in Figure 1A; Figure S1 in the Supporting Information). The dispersed Au NPs displayed an extinction band at 520 nm (curve a in Figure 1A); upon aggregation, the signal underwent a red shift with decreased extinction, while the extinction at 650 nm increased (curve c in Figure 1A).14 The negative ζ potential of the Au NPs decreased dramatically to -6.4 mV in the presence of 2-ME as a result of the formation of Au NPs aggregates. In contrast, the δ potential decreased only slightly to -22.3 mV for the S2O32--passivated Au NPs in the presence of 2-ME. It is more difficult for 2-ME to access and bind to the surfaces of the S2O32--passivated Au NPs. Electrospray ionization mass spectrometry (ESI-MS) revealed only the presence of Au(2-ME)2- complexes at m/z 350.98 in the supernatant of the leached solution (Figure S3 in the Supporting Information). The stability constant {log β[Au(2-ME)2-] ) ∼30} for Au(SR)2- species suggests that they are 4 orders of magnitude more stable than Au(S2O3)23- complexes in aqueous solution at 298 K, corresponding to a free energy difference of 5.4 kcal/mol.27,31 Thus, we suspect that the formation of strong Au(2-ME)2- complexes accelerated the leaching of the Au NPs in the S2O32- leaching liquor. We also compared the leaching efficiencies of S2O32--Au NPs in 5 mM glycine-NaOH solution (pH 10.0) containing alkanethiols of various chain lengths [2-ME, 4-mercaptobutanol (4-MB), 6-mercaptohexanol (6-MH), 9-mercapto-1-nonanol (9-MN)] and functionality [2-mercaptoacetic acid (2-MAA) and 3-mercaptopropionic acid (3-MPA)]. Figure S4 in the Supporting Information reveals that the neutral thiols 4-MB, 6-MH, and 9-MN did not induce aggregation of the S2O32--passivated Au NPs and that only 2-ME accelerated the leaching of the S2O32--Au NPs. The shorter alkanethiol chain of 2-ME increased the solubility of the Au-thiolate complexes in aqueous solution; therefore, 2-ME induced stronger etching. In addition, 2-ME acted as both a reducing agent and a complexing agent. We suggest that the redox mediator (RS-SR/RSH) of 2-ME was involved in the thiosulfate leaching system, leading to the enhanced rate of leaching of the S2O32--Au NPs. Evidence of the Formation of Au-Pb Alloys. Several studies have been performed to examine how foreign heavy metal ions affect the dissolution behavior of gold in the thiosulfate and cyanide systems.4-9 Here, we observed that PbCl2 (1.0 µM) accelerated the dissolution of gold in the 2-ME/S2O32--Au NPs system (curve e in Figure 1). The average particle size of the Au NPs decreased further to 10.8 ± 1.3 nm in the presence of Pb2+ (Figure S1 in the Supporting Information). In cyanide and ammoniacal S2O32- systems, Pb2+ reacts with gold to form AuPb2 and AuPb3 alloys or metallic Pb on the gold surface, resulting in a potential drop of the gold and, thus, an acceler(30) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437–463. (31) (a) Renders, P. J.; Seward, T. M. Geochim. Cosmochim. Acta 1989, 53, 245–253. (b) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630–2641. (c) Pyykko ¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412–4456. (d) Pyykko ¨, P. Inorg. Chim. Acta 2005, 358, 4113–4130.

ated rate of dissolution.4,32 We used ICPMS to quantitatively determine the contents of Pb on the Au NPs and leached Au ions from the Au NPs. We estimated that the leaching ratio of Au NPs into the supernatant was 56% and that 570 Pb atoms/Au NP were present in the precipitate. SALDI-TOF MS techniques have been applied recently using Au NPs, aptamer-functionalized Au NPs, and TiO2 NPs to analyze aminothiols, adenosine triphosphate, catechins, and estrogens, respectively.33-36 In this present study, we took advantage of the high reproducibility of SALDI-MS and its simple sample preparation techniques to prove that Pb-Au alloys formed on the surfaces of the Au NPs. In the SALDI-MS method, photoabsorption of Au NPs induces the desorption/ionization of surface atoms. Figure 2A reveals the presence of cationic clusters [Aun]+ (n ) 6-20) that indicate the fragmentation/vaporization of the Au NPs. Figure 2B-D present mass spectra of the 2-ME/S2O32--Au NPs in the presence of various concentrations of Pb2+ ions (1.0-4.0 µM). For example, we assign the peaks at m/z 1772.70-1860.78 (insets to Figure 2B-D) to [Au9-nPbn]+ ions in which n is an integer ranging from 0 to 8. These [Au9-nPbn]+ signals increased in intensity upon increasing the concentration of Pb2+, providing strong evidence for the formation of Aum-nPbn alloys on the Au NPs surfaces. In the control experiment, the UV-vis absorption spectra of Au NPs (1.5 nM) in 5 mM glycine-NaOH solution (pH 10.0) in the absence and presence of PbCl2 (1.0 µM) are similar (curves a and f in Figure 1). In addition, we found no Pb element on the Au NPs in the absence of Na2S2O3 and 2-ME. Effect of pH and O2. Figure 3 displays the effect of pH on the leaching of Au NPs in the 2-ME/S2O32--Au NPs system in the absence and presence of Pb2+ ions (1.0 µM). The rate of Au NP dissolution was maximized at pH 10.0. The effect of the pH was much more pronounced for solutions containing Pb2+ ions. In acidic media, S2O32- ions can break down to form sulfide, sulfite, sulfate, trithionate, tetrathionate, polythionates (SxOy2-), and polysulfides (Sx2-).5,37 Therefore, increasing the pH may increase the stability of S2O32- anions and, consequently, the stability of S2O32--Au NPs. On the other hand, Au(2-ME)2complexes are more stable at values of pH greater than the pKa (8.8-9.1) of 2-ME, thereby increasing the rate of dissolution of Au NPs.31 Retardation of gold dissolution at values of pH greater than 10.0 may be possible because of the formation of passive layers of Pb(OH)2, PbO, Au(OH)3, or Au2O3 on the surface of the Au NPs. Our present results also indicate that the reduction in the rate of leaching of gold in aqueous sulfate solutions was not due to the reduced oxidizing power of the oxygen half-cell (O2 + 2H2O + 4e- f 4OH-) at higher pH values but rather to a change in the nature of the anodic dissolution process. After plotting the values of (Ex0520 s Ex520)/ 0 Ex520 for Au NPs against the pH, we found that Pb2+ ions (32) Deschenes, G.; Lastra, R.; Brown, J. R.; Jin, S.; May, O.; Ghali, E. Miner. Eng. 2000, 13, 1263–1279. (33) (a) Chiu, T.-C.; Huang, L.-S.; Lin, P.-C.; Chen, Y.-C.; Chen, Y.-J.; Lin, C.-C.; Chang, H.-T. Recent Pat. Nanotechnol. 2007, 1, 99–111. (b) Huang, Y.-F.; Chang, H.-T. Anal. Chem. 2006, 78, 1485–1493. (34) Huang, Y.-F.; Chang, H.-T. Anal. Chem. 2007, 79, 4852–4859. (35) Lee, K.-H.; Chiang, C.-K.; Lin, Z.-H.; Chang, H.-T. Rapid Commun. Mass Spectrom. 2007, 21, 2023–2030. (36) Chiu, T.-C.; Chang, L.-C.; Chiang, C.-K.; Chang, H.-T. J. Am. Soc. Mass Spectrom. 2008, 19, 1343–1346. (37) Senanayake, G. Hydrometallurgy 2004, 75, 55–75.

Figure 2. SALDI-MS spectra of solutions containing Au NPs, Na2S2O3 (1.0 mM), and 2-ME (1.0 mM) in the (A) absence and (B-E) presence of (B) 1.0, (C) 2.0 (D) 3.0, and (E) 4.0 µM PbCl2. The peaks at m/z 1181.80, 1378.77, 1575.73, 1772.70, 1969.67, 2166.63, 2363.60, 2560.56, 2757.53, 2954.50, 3151.46, 3348.43, 3545.40, 3742.36, and 3939.33 are assigned to [Au6]+, [Au7]+, [Au8]+, [Au9]+, [Au10]+, [Au11]+, [Au12]+, [Au13]+, [Au14]+, [Au15]+, [Au16]+, [Au17]+, [Au18]+, [Au19]+, and [Au20]+ ions, respectively. The peaks at m/z 1783.71, 1794.72, 1805.73, 1816.74, 1827.75, 1838.76, 1849.77, and 1860.78 are assigned to [Au8Pb]+, [Au7Pb2]+, [Au6Pb3]+, [Au5Pb4]+, [Au4Pb5]+, [Au3Pb6]+, [Au2Pb7]+, and [AuPb8]+ ions, respectively. A total of 300 pulsed laser shots were applied under a laser fluence set at 51.25 µJ. Other conditions were the same as those described in Figure 1.

Figure 3. Effect of the pH (6.0-12.0) on the values of (a, b) Ex520 0 0 and (c) (Ex520 sEx520)/Ex520 of the 2-ME/S2O32--Au NP (1.5 nM) solutions in the (a) absence and (b) presence of 1.0 µM PbCl2. Error bars represent standard deviations from four repeated experiments. Other conditions were the same as those described in Figure 1.

accelerated the dissolution of the Au NPs, which reached a 0 maximum value at pH 10.0 (curve c in Figure 3). Here, Ex520 and Ex520 are the extinction values of the Au NPs in the absence and presence of Pb2+ (1.0 µM), respectively. On the basis of Analytical Chemistry, Vol. 81, No. 22, November 15, 2009

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Figure 4. Effect of the concentration of 2-ME (0-25 mM) on the 0 0 values of (A) Ex520 and (B) (Ex520 s Ex520)/Ex520 of the 2-ME/S2O32-Au NPs (1.5 nM) in the presence of PbCl2 (0-10 µM). Other conditions were the same as those described in Figure 1.

the general equation for gold leaching in S2O32- liquor (4Au0 + O2 + 2H2O + 8 L f 4AuL2 + 4OH-), the addition of O2 favors the reaction proceeding to the right and, thus, dissolution of the Au NPs. Figure S5 in the Supporting Information reveals that dissolution of the 2-ME/S2O32--Au NPs was relatively rapid under an atmosphere of air or under O2-saturated conditions, whereas the rate of dissolution under N2saturated conditions (sparging with 99.9995% N2; O2 < 1 ppm) in the presence of Pb2+ (1.0 µM) was very low (curve b). Oxidation of free S2O32- and 2-ME to form polythionates under excess O2 had a slight negative effect on the rate of gold dissolution. Having such unique phenomena, the leaching of 2-ME/S2O32--Au NP at pH 10 under air conditions was used for sensing Pb2+ in aqueous solutions. Sensitivity and Selectivity for Pb2+. We further investigated the effect of the 2-ME concentration (0-25 mM) on the leaching of Au NPs in the absence and presence of Pb2+ ions (1.0 µM). The rate of dissolution of gold in the 2-ME/S2O32-Au NP solution increased upon increasing the concentration of 2-ME in the absence of Pb2+ ions (Figure 4A). In addition, the rate of leaching of gold increased upon increasing the concentration of Pb2+ ions (0-10 µM) in the presence of a constant concentration of 2-ME (within the range 0.25-25 mM). Although the 2-ME (10 mM)/S2O32--Au NP solution provided the best sensitivity for the detection of Pb2+ ions, the reproducibility was poor (RSD ) ∼5.6%) and the linear range was narrow (2.5-25 nM). When using solutions of S2O32--Au NPs (1.5 nM) containing 2-ME (1.0 mM), we obtained a linear relationship (R2 ) 0.97) between the values 0 0 of (Ex520 s Ex520)/Ex520 of the Au NPs and the logarithm of the concentration of Pb2+ ions over the range from 2.5 nM to 10 µM (Figure 4B). The limit of detection (LOD; S/N ratio ) 3) for Pb2+ ions was ∼0.5 nM. 9438

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Because we found that our 2-ME/S2O32--Au NPs allowed the detection of Pb2+ at concentrations as low as 0.5 nM, we further investigated the changes in extinction at 520 nm of the 2-ME/ S2O32--Au NPs (1.5 nM) in the presence of Na2S2O32- (1.0 mM), 2-ME (1.0 mM), and one the metal ions Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pt2+, Cd2+, Pd2+, Cr3+, Fe3+, Ag+, Au3+, and Hg2+ (10 µM) and Pb2+ (100 nM). As indicated in Figure S6 in the Supporting Information, our system responded selectively toward Pb2+ ions, by a factor of 1000-fold or more relative to the other metal ions. The tolerance concentrations of these metal ions for the sensing of Pb2+ using this approach were at least 10 times higher than the Pb2+ concentration (Figure S7 in the Supporting Information). The concentration of 2-ME (1.0 mM) in the 2-ME/S2O32-Au NP system for selective detection of Pb2+ was much higher than those of the interfering metal ions, e.g., 10 µM in Figure S6 in the Supporting Information and 1.0 µM in Figure S7 in the Supporting Information. We found that 2-ME at a high concentration formed strong Au(2-ME)2- complexes with Au ions, which accelerated the leaching of the Au NPs in the presence of Pb2+. Therefore, the highly selective of 2-ME/ S2O32--Au NPs probe toward Pb2+ against to other metal ions was as a result of formation of specific Pb-Au alloy at high concentrations of Na2S2O32- (1.0 mM) and 2-ME (1.0 mM). Because 2-ME and Hg2+ formed stable complexes in the bulk solution, reduction of Hg2+ was not found, which was supported by the SALDI-MS data (no Hg species were detected). Detection of Pb2+ in Real Samples. To test the practicality of our developed approach, we applied this assay to determine the concentrations of Pb2+ ions in a sample of river water from the National Taiwan Ocean University (NTOU) campus (Figure 5A) and in Montana Soil (SRM 2710; Figure 5B). We obtained a linear correlation (R2 ) 0.96) between the response and the concentration of Pb2+ ions spiked into the river water (diluted 2-fold) over the range from 2.5 nM to 10 µM. Neither our sensing approach nor the ICPMS-based system detected the presence of Pb2+ ions in the river water sample. The LOD (S/N ratio ) 3) of the 2-ME/S2O32--Au NP probe for Pb2+ ions in the complicated matrixes was ∼0.8 nM. In these measurements, the 2-ME/S2O32--Au NP probe provided recoveries of 94-107% of Pb2+ ions. By applying standard addition methods to our new approach and to an ICPMSbased analysis, we determined the concentrations of Pb2+ ions in the Montana Soil sample (certified values 5.53 mg/ g) to be 5.18 (±0.12) and 5.05 (±0.07) mg/g (n ) 5), respectively. The Student’s t-test and F-test value for the correlation between the two methods was 2.09 and 2.94 (the t-test and F-test value is 2.31 and 6.39 at a 95% confidence level, respectively), suggesting that the two methods did not provide significantly different results. Thus, our 2-ME/ S2O32--Au NP system is a practical tool for the determination of Pb2+ ions in environmental samples. CONCLUSIONS We have demonstrated that monitoring the SPR absorption of Au NPs is a useful means of studying the leaching of gold. To understand the basis for 2-ME and Pb2+ accelerating the leaching of Au NPs, we used TEM, XPS, UV-vis absorption spectroscopy, ESI-MS, and ζ potential measurements to

characterize the Au NPs. In addition, we used SALDI-TOF MS to demonstrate formation of Pb-Au alloy on Au NP via detection of the formation of [Aum-nPbn]+ ions in the 2-ME/ S2O32--Au NP samples in the presence of Pb2+ ions. We also developed a colorimetric, label-free, and nonaggregationbased Au NP probe for the detection of Pb2+ ions. Under optimal conditions, the 2-ME/S2O32--Au NP probe was highly sensitive (LOD ) 0.5 nM) and selective (by at least 1000fold over other metal ions) toward Pb2+ ions, with a linear detection range (2.5 nM-10 µM) over nearly 4 orders of magnitude. This approach avoids the need for complicated chemosensors or sophisticated equipment. We validated its practicality through analyses of water and soil samples. This simple, rapid, and cost-effective sensing system holds great potential practicality for the detection of heavy metal ions in real samples. ACKNOWLEDGMENT This study was supported by the National Science Council of Taiwan under Contract 97-2113-M-019-001-MY2. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 5. Analyses of representative (A) river water and (B) soil samples using the 2-ME/S2O32--Au NP (1.5 nM) probe. The samples were spiked with Pb2+ ions at concentrations of (A) 0-10 µM and (B) 0-75 nM. Error bars represent standard deviations from five repeated experiments. Other conditions were the same as those used to obtain Figure 1.

Received for review August 13, 2009. Accepted September 29, 2009. AC9018268

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