Colorimetric Detection of Heavy Metal Ions Using Label-Free Gold

Sep 2, 2010 - degree of alkanethiol-induced aggregation of the Au NPs decreases in the order of 6-mercaptohexanol (6-. MH) ∼ 4-mercaptobutanol (4-MB...
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J. Phys. Chem. C 2010, 114, 16329–16334

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Colorimetric Detection of Heavy Metal Ions Using Label-Free Gold Nanoparticles and Alkanethiols Yu-Lun Hung,† Tung-Ming Hsiung,† Yi-You Chen,† Yu-Fen Huang,*,‡ and Chih-Ching Huang*,†,§ Institute of Bioscience and Biotechnology and Center for Marine BioenVironment and Biotechnology (CMBB), National Taiwan Ocean UniVersity, Keelung, Taiwan, and Department of Biomedical Engineering and EnVironmental Sciences, National Tsing Hua UniVersity, Hsinchu, Taiwan ReceiVed: July 3, 2010; ReVised Manuscript ReceiVed: August 16, 2010

We have developed a simple method for the selective colorimetric detection of aqueous mercuric (Hg2+), silver (Ag+), and lead (Pb2+) ions by using label-free gold nanoparticles (Au NPs) and alkanethiols. The degree of alkanethiol-induced aggregation of the Au NPs decreases in the order of 6-mercaptohexanol (6MH) ∼ 4-mercaptobutanol (4-MB) > 11-mercaptoundecanol (11-MU) > 2-mercaptoethanol (2-ME). The specific and strong interactions of these alkanethiols with Au NPs and heavy metal ions enabled us to develop label-free assays for the sensitive and selective detection of Hg2+ ions using the 4-MB/Au NPs probe, as well as Ag+ and Pb2+ ions using the 2-ME/Au NPs probe. The presence of strong Hg2+-S bonds alleviated the extent of 4-MB-induced aggregation of the Au NPs, resulting in a declining ratio of the extinction coefficients at 650 to 520 nm (Ex650/520, a measure of the molar ratio of the aggregated to the dispersed Au NPs) of the Au NP solution. In contrast, the presence of Ag+, Cu2+, and Pb2+ ions led to a severe aggregation of the Au NPs, mediated by the deposition of these ions on the surfaces of the Au NPs in the 2-ME/Au NPs system. In the presence of masking agents [ethylenediaminetetraacetic acid (EDTA), Na2S], the 2-ME/Au NP-EDTA and 2-ME/Au NP-Na2S sensors permitted the selective detection of Ag+ and Pb2+ ions, respectively, at concentrations down to the nanomolar range. This cost-effective process also allowed the rapid and simple determination of the concentrations of heavy metal ions in real environmental samples (river water and Montana soil). These alkanethiol/Au NP-based sensor probes enabled us to detect three different heavy metal ions, and we feel confident that, because of the simplicity, rapidity, and cost-effectiveness of these analyses, such systems demonstrate great potential for the practical detection of heavy metal ions in real samples. Introduction Contamination of the environment with heavy metal ions has been a major concern throughout the world for several decades.1–5 Mercury (Hg), lead (Pb), and cadmium (Cd), for example, can cause long-term damage to many biological systems, can disrupt biological events at the cellular level, and can cause significant oxidative damage; they are also carcinogens. Because heavy metal ions can cause severe risk to human health and the environment, it is vital to develop methods for detecting them at low concentrations that are normally found in environmental samples. Several methods for the analysis of heavy metal ions have been developed during the past decade including techniques based on atomic absorption spectroscopy (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma mass spectrometry (ICP-MS), and electrochemical sensing platforms.6 With regard to sensitivity and accuracy, all of these methods are efficient tools for heavy metal ion determination; however, they are time-consuming, expensive, and/or require sophisticated * To whom correspondence should be addressed. Chih-Ching Huang, Institute of Bioscience and Biotechnology and Center for Marine Bioscience and Biotechnology (CMBB), National Taiwan Ocean University, Keelung, 20224,Taiwan;tel.:011-886-2-24622192#5517;e-mail:[email protected]. Yu-Fen Huang, Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, 30013, Taiwan; tel.: 011-886-3-5715131 #34212, 80813; e-mail: [email protected]. † Institute of Bioscience and Biotechnology. ‡ Center for Marine Bioenvironment and Biotechnology. § National Tsing Hua University.

equipment. Therefore, the development of a simple and inexpensive method for the analysis of heavy metal ions remains desirable for the real-time monitoring of environmental, biological, and industrial samples. In-field detectors which are easy to operate and are highly sensitive toward heavy metal ions can be regarded as powerful tools for probing global pollution.7 For example, simple colorimetric sensors that can eliminate the need for analytical instruments are attracting much more attention recently.8–10 The colorimetric and size-dependent properties of gold nanoparticles (Au NPs) are traits that can be readily exploited to develop the desired components of potential in-field devices.11–14 Au NPs possess intrinsically strong surface plasmon resonance (SPR) absorptions, with extremely high extinction coefficients (108-1010 M-1 cm-1), in the visible wavelength range. The extinction cross sections of the Au NPs and the wavelengths at which they absorb and scatter light depend on both their sizes and shapes, the dielectric properties (refractive indices) of their surrounding media, and their interactions with neighboring particles.15–17 For example, solutions of smaller individual Au NPs (5-20 nm) appear ruby red, while those of larger particles or aggregates of smaller particles range in color from purple to deep blue. The aggregation of Au NPs can be controlled by employing surface ligands that bind to target analytes in solution; when a specific analyte is introduced, binding occurs through the ligands of multiple Au NPs, thus resulting in noncontrolled aggregation and a subsequent color change of the solution.18–20

10.1021/jp1061573  2010 American Chemical Society Published on Web 09/02/2010

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This color change effect is the result of the coupling of the SPR between particles in close proximity. Systems based on analyteinduced aggregation of Au NPs have been employed for the colorimetric detection of cells, nucleic acids, proteins, and small molecules.21–26 Strategies for developing colorimetric metal ion sensors are usually based on manipulating Au NPs with different sensing elements such as oligonucleotides, DNAzymes, peptides, proteins, and small thiolate ligands.27 The preparation of these colorimetric sensors involves several steps which includes modifying the thiolated biomolecules on the Au NPs and separating the modified Au NPs from the unmodified molecules, a process that is a complex and relatively expensive undertaking. Therefore, much attention has been focused on developing modification-free Au NP-based colorimetric sensors to simplify the detection process.28–31 In our previous study, we developed colorimetric Au NP probes for the detection of Pb2+ in aqueous solution on the basis of Pb2+-induced leaching or aggregation of Au NPs by thiosulfate and alkanethiols.32 In this study, we present a label-free, rapid, and homogeneous assay for the highly selective and sensitive detection of heavy metal ions, particularly Hg2+, Pb2+, and Ag+ ions by using Au NPs and alkanethiols. We also demonstrate the practicality of using this approach for the determination of these metal ions in river water and soil samples. Experimental Section Materials. 2-Mercaptoethanol (2-ME), 4-mercaptobutanol (4MB), 6-mercaptohexanol (6-MH), 11-mercaptoundecanol (11MU), trisodium citrate, ethylenediaminetetraacetic acid (EDTA), and all of the metal salts used in this study were purchased from Aldrich (Milwaukee, WI, USA), except for hydrogen tetrachloroaurate(III) trihydrate, which was purchased from Acros (Geel, Belgium). Montana soil (SRM 2710) was obtained from the National Institute of Standards and Technology (NIST, Maryland, USA). SRM 2710 was originally collected from the upper 10 cm of pasture along Silver Bow Creek in Butte, Montana, USA. This site is contaminated by Cu, Mn, Pb, and Zn from inputs to the creek from the settling ponds of the Anaconda processing plant. Milli-Q ultrapure water was used in each experiment. The buffer used was a 50 mM glycine-NaOH solution (pH 10.0, adjusted with 1.0 N NaOH). Preparation and Characterization of Au NPs. The 14.2 nm spherical Au NPs were prepared by a (4.0 mM) citratemediated reduction of 1.0 mM HAuCl4.33 The sizes of the Au NPs were verified using an H7100 transmission electron microscope (TEM; Hitachi High-Technologies Corporation, Tokyo, Japan); the nanoparticles appeared to be nearly monodisperse with an average size of 14.2 ( 0.3 nm. A Cintra 10e double-beam ultraviolet-visible (UV-vis) spectrophotometer (GBC, Victoria, Australia) was used to measure the absorptions of the Au NP solutions. The particle concentration of the Au NPs (15 nM) was determined by 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.34 The zeta potentials (ξ) of the Au NPs were measured using a Zetasizer 3000HS analyzer (Malvern Instruments, Malvern, UK). The analysis of Au NP surfaces by using surfaceassisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) was carried out according to our previous report.32 Detection of Heavy Metal Ions. A stock solution of metal ions (0.1 M) was prepared in 0.1 M HNO3 and diluted to 0-100 µM in ultrapure water. Aliquots of metal ion solutions (5 µL) were added separately to 5 mM glycine-NaOH buffer (pH 10.0)

Hung et al. solutions containing 0.6 nM Au NPs to give final volumes of 490 µL. After equilibration at ambient temperature for 30 min, an alkanethiol (10 µL) was added to the 490 µL Au NP solutions, and the solutions were equilibrated for another 2 h. For the selective determination of Ag+ and Pb2+, masking agents (1 mM EDTA or 0.5 µM Na2S, respectively) were equilibrated with the metal ion/Au NP solutions (490 µL) prior to the addition of 2-ME (10 µL, 50 mM). The mixtures were transferred separately into 96 well microtiter plates, and their UV-vis absorption spectra were recorded using a µ-Quant microplate reader (Biotek Instruments, Winooski, VT). Herein, only the final concentrations of the species are provided. Analysis of Water and Soil Samples. Acidic digestion of soil samples was performed according to the U.S. Environmental Protection Agency method 305B35 and our previous report.34 The aqueous soil sample (2 mL) was further diluted with DI water (98 mL). Aliquots of the diluted solution (250 µL) were spiked with standard Pb2+ solutions. The spiked samples were then diluted to 500 µL using a solution containing the 2-ME/ Au NPs (0.6 nM) probe, Na2S (0.5 µM), and 5 mM glycineNaOH (pH 10.0). A water sample collected from a river on the campus of National Taiwan Ocean University (NTOU) was filtered through a 0.2 µm membrane. Aliquots of the river water (250 µL) were spiked with standard metal ion solutions (10 µL) of desired concentrations. The spiked samples were then diluted to 500 µL with a solution containing the 4-MB/Au NPs (0.6 nM) or 2-ME/Au NPs (0.6 nM) probe and 5 mM glycine-NaOH (pH 10.0). The spiked samples were then analyzed separately using ICP-MS and the developed sensing technique. Results and Discussion Alkanethiol-Induced Aggregation of Au NPs. Alkanethiols are strongly electron-releasing ligands of high polarizability; they prefer to form complexes with soft Lewis acids such as Au(I) and Au(III).36 When we added the alkanethiols 2-ME, 4-MB, 6-MH, and 11-MU individually to solutions of the Au NPs (0.6 nM) in 5 mM glycine-NaOH buffer (pH 10), these thiols readily accessed the surfaces of the Au NPs, displacing the citrate ions through the formation of stronger Au-S linkages (bond energy: ca. 184 kJ mol-1).37 The negatively surfacecharged density of the Au NPs was then reduced, and aggregation occurred. Except for 2-ME, all of the alkanethiols (>10 µM) induced large degrees of aggregation of the Au NPs (Figure 1). Our experimental results further confirm that the negative ξ potential of the Au NPs decreased dramatically from -24 to -5 mV in the presence of 4-MB (10 µM), leading to the formation of Au NP aggregates. The extinction coefficients at 520 and 650 nm are related to the quantities of the dispersed and aggregated Au NPs, respectively. Therefore, we used the value of Ex650/520, which is the ratio of the extinction coefficients at these two wavelengths, to express the molar ratio of the aggregated to the dispersed Au NPs. Figure 1 reveals that the degree of aggregation of the Au NPs in the presence of the four alkanethiols decreases in the following order: 6-MH ∼ 4-MB > 11-MU > 2-ME. Long chain length increases the hydrophobicity of the alkanethiol, resulting in a larger extent of induced aggregation of the Au NPs. Notably, however, the low solubility and great steric effects of the longest alkane chain (11-MU) used in this study retarded its access to the Au NPs, thereby resulting in less aggregation of the Au NPs (curve d in Figure 1). We also note that high steric branched alkanethiols [3-mercaptohexanol (3-MH)] induced less aggregation of Au NPs than 6-MH (Figure S1 in the Supporting Information, SI). The Ex650/520 curve of the 2-ME-induced aggregation of the Au

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Figure 1. Values of Ex650/520 of Au NPs (0.6 nM) in 5 mM glycineNaOH solutions (pH 10.0) containing the alkanethiols (0-10 mM) (a) 2-ME, (b) 4-MB, (c) 6-MH, and (d) 11-MU. Error bars represent standard deviations from four repeated experiments.

SCHEME 1: Cartoon Representations of the Sensing Mechanism of (A) the 4-MB/Au NP Probe for the Detection of Hg2+ Ion and (B) the 2-ME/Au NP Probe for the Detection for Ag+, Cu2+, and Pb2+ Ions

NPs exhibited a biphasic relationship; the extinction ratio Ex650/520 reached its maximum value at a 2-ME concentration of 10 µM. In addition, by monitoring the value of Ex650/520, each of the 4-MB- and 6-MH-induced aggregations of the Au NPs reached their plateaus at a concentration of 10 µM. From these data, we estimated that the number of alkanethiol molecules required to saturate the particle surface of a single 14.2 nm Au

Figure 2. (A) Selectivity of the 4-MB/Au NPs (0.6 nM) probe toward Hg2+ ions and (B) UV-vis absorption spectra of 4-MB/Au NP (0.6 nM) solutions used as probes for the detection of Hg2+ ions (0-10 µM). Inset to A: photographic images of the Au NP solutions. Inset to 0 B: plot of Ex650/520 versus the Hg2+ concentration. The Ex650/520 and Ex650/520 represent the extinction ratios (Ex650/520) of the 4-MB/Au NPs in the absence and presence of metal ions, respectively. The concentrations of 4-MB and each metal ion in A were 2.0 and 10 µM, respectively. Buffer: 5 mM glycine-NaOH solution (pH 10.0). Other conditions were the same as those described in Figure 1.

NP was approximately 16 700, very close to the value reported for small thiolated molecules.38 Upon increasing the concentration of 2-ME from 10 µM to 10 mM, the degree of aggregation of the Au NPs decreased (curve a in Figure 1), presumably because of the excellent water solubility of the Au(2-ME)2 complexes on the NP surfaces after rapid saturation with the short-chained 2-ME molecules.32 4-MB/Au NPs Probe for Hg2+ Ions. The specific and strong interactions of alkanethiols with both Au NPs and heavy metal ions and metallophilic interactions of metal ions-Au+ enabled us to develop a label-free assay for the selective detection of the heavy metal ions Hg2+, Pb2+, and Ag+ through the competition between the metal ions and Au NPs for their binding toward alkanethiols (Scheme 1). In one of the systems, we employed Au NPs and 4-MB for the selective detection of Hg2+ ions (Scheme 1A). Figure 2A reveals that the addition of 4-MB (20 µM, 0.1 mL) to the Au NP solution (0.67 nM, 0.9 mL) in 5 mM glycine-NaOH buffer (pH 10) changed the color of the solution from red to purple with a dramatic increase in the value of Ex650/520. In the presence of Hg2+ ions, however, 4-MB tends to form Hg2+-S bonds, owing to their stronger affinity toward Hg2+ ions (Kd ) 10-45 M) than that toward Au NPs (Kd ) 10-30 M).39 Therefore, the color and value of Ex650/520 of the Au NP solution incubated with 10 µM Hg2+ remains unchanged following the addition of 2.0 µM 4-MB. In contrast, we observed dramatic colorimetric differences when the other metal ions were added at the same concentration (Figure 2A). As indicated in Figure 2B, the aggregation of the Au NPs decreases upon increasing the concentration of Hg2+ ions from 0 to 10 µM. We obtained a linear relationship (R2 ) 0.96) for the value of Ex650/520 with respect to the concentration of Hg2+ ions over the range 1.0-7.5 µM (inset to Figure 2B). The detection of limit (LOD; S/N ratio )3) for Hg2+ ions was approximately 500 nM. A similar color changing mechanism was proposed recently by Hirayama et al., who found that Hg2+ ions played

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Figure 3. (A) UV-vis absorbance spectra of solutions containing (a) Au NPs, (b) Au NPs and 2-ME, (c) Au NPs, 2-ME, and Ag+ ions, (d) Au NPs, 2-ME, and Pb2+ ions, and (e) Au NPs, 2-ME, and Cu2+ ions. 0 0 (B) Values of (Ex650/520 - Ex650/520 )/Ex650/520 of 2-ME/Au NP solutions after the addition of metal ions. The concentrations of the Au NPs, 2-ME, and metal ions were 0.6 nM, 1.0 mM, and 10 µM, respectively. Other conditions were the same as those described in Figure 2.

an important role in breaking Au-S bonds prior to the aggregation process using triethylene glycol ligand (EG3-SH)capped Au NPs.40 Notably, our new sensing strategy improves the detection sensitivity up to 10-fold and could be used to detect Hg2+ ions in a complex matrix of river water. We obtained a linear correlation (R2 ) 0.95) between the response and the concentration of Hg2+ ions spiked into the sample of river water from the NTOU campus over the range 0-10 µM (Figure S2 in the Supporting Information). Neither our sensing approach nor the ICP-MS-based system detected the presence of Hg2+ ions in this river water sample. In comparison with other colorimetric Au NP-based sensors for Hg2+,41–43 our label-free 4-MB/Au NPs assay is relatively simple, cost-effective, selective, and rapid. 2-ME/Au NPs Probe for Ag+, Cu2+, and Pb2+ Ions. The UV-vis spectra in Figure 3A reveal a slight decrease in the SPR absorption of the Au NP (0.6 nM) solution at 520 nm after the addition of 2-ME (1.0 mM). 2-ME not only appears to be a strong complexing agent but can also be regarded as an etching agent.32a,44 Our previous study also confirmed that the signal of the Au(2-ME)2- complex at m/z 350.98 could be detected from the supernatant of the 2-ME/Au NP solution in the electrospray ionization (ESI) mass spectrum.32 After the incubation of Au NPs with Ag+, Pb2+, or Cu2+ cations, the Au surface was easily passivated by Ag, Pb, or Cu atoms,45–49 respectively, and, as a result, became less accessible toward 2-ME; therefore, irreversible aggregation occurs (Scheme 1B and Figure 3A). Figure 3B displays the colorimetric responses of Au NP solutions (0.6 nM) toward various metal cations (10 µM) in 5 mM glycineNaOH buffer solutions (pH 10.0) after the 1 h incubation, followed by the addition of 2-ME (1.0 mM). With the interaction of 2-ME, the values of Ex650/520 of the Au NP solutions containing Ag+, Pb2+, and Cu2+ ions changed dramatically from their initial value of 0.2 to values of 1.2, 1.5, and 1.4, respectively. In contrast, we observed no aggregation in the presence of the other metal ions. ICP-MS was further used to estimate the amount of metal atoms deposited on each Au NP, showing that 420 Pb, 1750 Ag, and 540 Cu atoms were present

Hung et al.

Figure 4. (A) Selectivity of the 2-ME/Au NPs probe toward Pb2+ ions and (B) UV-vis absorption spectra of 2-ME/Au NP solutions used as probes for the detection of Pb2+ (0-500 nM) in the presence of Na2S (0.5 µM). Inset to B: plot of Ex650/520 versus Pb2+ concentration. Other conditions were the same as those described in Figure 2.

in the respective precipitates. In addition, we took advantage of the high reproducibility of SALDI-MS and its simple sample preparation techniques to prove the formation of Ag-Au, Pb-Au, and Cu-Au alloys on the Au NPs surface. During laser irradiation, the photoenergy absorbed by Au NPs will induce the desorption/ionization of surface atoms. The observation of pure cationic clusters of [Aun]+ (n ) 1-20) indicated that the fragmentation/vaporization of the Au NPs occurred (Figure S3A in the SI). The SALDI mass spectra of the Au NPs which had been subjected to Ag+, Pb2+, and Cu2+ ions (Figure S3B-D in the SI) revealed additional ionic signals derived from the Ag-Au, Pb-Au, and Cu-Au alloys, respectively. However, no signals were observed from other metal atoms on the Au NP surfaces through either the ICP-MS or the SALDI-MS measurements. Selectivity and Sensitivity of 2-ME/Au NP Probes for Ag+ and Pb2+ Ions. To ensure the feasibility of the respective Ag+ and Pb2+ ion detection, we tested the effects of two masking reagentssEDTA and sodium sulfide (Na2S)son the 2-ME/Au NPs sensing system. EDTA is a strong chelator of divalent cations (pKf ) 30-50).50 As indicated in Figure S4A (in the SI), the presence of 1.0 mM EDTA suppressed the interference from Pb2+ and Cu2+, indicating that the 2-ME/Au NPs probe had great selectivity toward Ag+ ions. Specifically, after the addition of 2-ME (1.0 mM), a mixture of Au NPs (6 nM) in 5 mM glycine-NaOH solution (pH 10.0) containing 1 mM EDTA exhibited high selectivity (>100-fold) toward Ag+ over the other metal ions. To improve the selectivity of the 2-ME/Au NPs probe toward Pb2+ ions, we added Na2S as a masking agent by taking advantage of the stronger formation constants of CuS (pKf ) 36.1) and Ag2S (pKf ) 50.1) relative to that of PbS (pKf ) 27.5).50 Gratifyingly, only the presence of Pb2+ ions caused a significant degree of Au NPs aggregation in the presence of 0.5 µM Na2S (Figure 4A), demonstrating that this sensing system is specific to Pb2+ ions. Under the optimal conditions, we investigated the sensitivity of the Au NP sensors toward Ag+ and Pb2+ individually. Figure

Colorimetric Detection of Heavy Metal Ions S4B (Figure 4B) reveals that the SPR absorption red-shifted and broadened when Ag+ (Pb2+) ions were present, with the value of Ex650/520 increasing upon increasing the concentration of Ag+ (Pb2+) ions. When using a solution of 2-ME/Au NPs (0.6 nM) containing EDTA (1.0 mM), we obtained a linear relationship (R2 ) 0.97) between the value of Ex650/520 of the Au NPs and the concentration of Ag+ ions over the range from 100 nM to 5.0 µM (R2 ) 0.97). The LOD (S/N ratio ) 3) for Ag+ ions was approximately 70 nM (Figure S4B in the SI). We also obtained a linear response of the value of Ex650/520 toward the concentration of Pb2+ ions over the range 50-500 nM (R2 ) 0.96) when using the 2-ME/Au NPs probe in the presence of Na2S (0.5 µM). The 2-ME/Au NP-Na2S probe provided an LOD (S/N ratio ) 3) for Pb2+ ions of approximately 45 nM (Figure 4B). Detection of Ag+ and Pb2+ Ions in Real Samples. To validate the practicality of our proposed sensing strategy for the analysis of environmental samples, we used our 2-ME/ Au NPs to determine the concentrations of Ag+ and Pb2+ ions in both river water and soil samples. Here, we obtained linear correlations (R2 ) 0.93-0.95) between the values of Ex650/520 and the concentrations of Ag+ and Pb2+ ions spiked into the river water over the ranges 0.1-7.5 µM (Figure S5A in the SI) and 50-500 nM (Figure S5B in the SI), respectively, by using the 2-ME/Au NP-EDTA and 2-ME/ Au NP-Na2S probes, respectively. Neither our sensing approach nor the ICP-MS-based system detected the presence of Ag+ and Pb2+ ions in the original river water sample. We also observed a good linear relationship (R2 ) 0.95) between the value of Ex650/520 and the concentration of Pb2+ ions spiked into the Montana soil sample (SRM 2710) over the range 50-250 nM (Figure S6 in the SI). Using this new approach, we obtained recoveries of 96-103% for these measurements. By applying standard addition methods to our new approach and to the ICP-MS-based analysis, we determined the concentration of Pb2+ ions in the Montana soil sample (certified value: 5.53 mg g-1) to be 5.48 ( 0.45 and 5.25 ( 0.96 mg g-1 (n ) 5), respectively. The Student’s t-test and F-test values for the correlation between the two methods were 2.09 and 4.55, respectively (the t-test and F-test values are 2.31 and 6.39, respectively, at a 95% confidence level), suggesting that the two methods did not provide significantly different results. Therefore, our 2-ME/Au NPs probe is a practical tool for the determination of Pb2+ ions in environmental samples. Relative to our previous thiosulfate/2-MEAu NP and thiosulfate/4-MB-Au NP sensors and other reports,32,51,52 the main advantage of these present probes is to provide the selective detection of three different heavy metal ions. Conclusion We have developed a new assay for the sensitive and selective detection of Hg2+ ions by using a 4-MB/Au NPs probe and Ag+ and Pb2+ ions by using a 2-ME/Au NPs probe. We found that the alkanethiol-mediated aggregation of Au NPs was highly dependent on the chain length of the alkanethiol. The high strength of Hg2+-S bonds decreased the degree of 4-MB-induced aggregation of the Au NPs, allowing us to determine Hg2+ in terms of the decrease in the value of Ex650/520 of the Au NP solution. In contrast, we observed an increased aggregation of the Au NPs after deposition of Ag+, Cu2+, and Pb2+ ions on the surfaces of the Au NPs in the 2-ME/Au NP system. 2-ME/Au NP-EDTA and 2-ME/Au NP-Na2S sensors allowed the selective detec-

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16333 tion of Ag+ and Pb2+ ions at concentrations as low as 70 and 45 nM, respectively. We validated the practical applicability of these 4-MB/Au NP and 2-ME/Au NP probes through analyses of real soil and water samples. To the best of our knowledge, these systems are the first examples of alkanethiol/Au NP-based sensors for the detection of three different heavy metal ions. These simple, rapid, and costeffective sensing systems demonstrate great potential 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-019-001MY2. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Porter, S. K.; Scheckel, K. G.; Impellitteri, C. A.; Ryan, J. A. Crit. ReV. EnViron. Sci. Technol. 2004, 34, 495–604. (2) Prasad, M. N. V. EnViron. Exp. Bot. 1995, 35, 525–545. (3) Zhang, L.; Wong, M. H. EnViron. Int. 2007, 33, 108–121. (4) Casas, J. S.; Sordo, J. Lead: Chemistry, analytical aspects, enVironmentalimpact and health effects; Elsevier: Amsterdam, the Netherlands, 2006. (5) Fowler, B. A. Toxicol. Appl. Pharmacol. 2009, 238, 294–300. (6) Butler, O. T.; Cook, J. M.; Harrington, C. F.; Hill, S. J.; Rieuwerts, J.; Miles, D. L. J. Anal. Atom. Spectrom. 2007, 22, 187–221. (7) Nolan, E. M.; Lippard, S. J. Chem. ReV. 2008, 108, 3443–3480. (8) Wanga, G.; Wanga, Y.; Chena, L.; Choo, J. Biosens. Bioelectron. 2010, 25, 1759–1768. (9) Wallace, K. J. Supramol. Chem. 2009, 21, 89–102. (10) Moores, A.; Goettmann, F. New J. Chem. 2006, 30, 1121–1132. (11) 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. (12) 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. (13) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547–1562. (14) Wilson, R. Chem. Soc. ReV. 2008, 37, 2028–2045. (15) Kubo, S.; Diaz, A.; Tang, Y.; Mayer, T. S.; Khoo, I. C.; Mallouk, T. E. Nano Lett. 2007, 7, 3418–3423. (16) Wang, H.; Brandl, D. W.; Nordlander, P.; Halas, N. J. Acc. Chem. Res. 2007, 40, 53–62. (17) Myroshnychenko, V.; Rodrı´guez-Ferna´ndez, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marza´n, L. M.; de Abajo, F. J. G. Chem. Soc. ReV. 2008, 37, 1792–1805. (18) Lin, Y.-W.; Liu, C.-W.; Chang, H.-T. Anal. Methods 2009, 1, 14– 24. (19) Radwan, S. H.; Azzazy, H. M. Expert ReV. Mol. Diagn. 2009, 9, 511–524. (20) Zhao, W.; Brook, M. A.; Li, Y. ChemBioChem 2008, 9, 2363– 2371. (21) Aili, D.; Selega˚rd, R.; Baltzer, L.; Enander, K.; Liedberg, B. Small 2009, 5, 2445–2452. (22) Chen, S.-J.; Huang, Y.-F.; Huang, C.-C.; Lee, K.-H.; Lin, Z.-H.; Chang, H.-T. Biosens. Bioelectron. 2008, 23, 1749–1753. (23) Chiu, T.-C.; Huang, C.-C. Sensors 2009, 9, 10356–10388. (24) Huang, C.-C.; Huang, Y.-F.; Cao, Z.; Tan, W.; Chang, H.-T. Anal. Chem. 2005, 77, 5735–5741. (25) Medley, C. D.; Smith, J. E.; Tang, Z.; Wu, Y.; Bamrungsap, S.; Tan, W. Anal. Chem. 2008, 80, 1067–1072. (26) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (27) Wang, Z.; Ma, L. Coord. Chem. ReV. 2009, 253, 1607–1618. (28) Liu, C.-W.; Hsieh, Y.-T.; Huang, C.-C.; Lin, Z.-H.; Chang, H.-T. Chem. Commun. 2008, 2242–2244. (29) Lee, J. H.; Wang, Z.; Liu, J.; Lu, Y. J. Am. Chem. Soc. 2008, 130, 14217–14226. (30) Lou, X.; Xiao, Y.; Wang, Y.; Mao, H.; Zhao, J. ChemBioChem 2009, 10, 1973–1977. (31) Nath, N.; Chilkoti, A. J. Fluoresc. 2004, 14, 377–389. (32) (a) Chen, Y.-Y.; Chang, H.-T.; Shiang, Y.-C.; Hung, Y.-L.; Chiang, C.-K.; Huang, C.-C. Anal. Chem. 2009, 81, 9433–9439. (b) Hung, Y.-L.; Hsiung, T.-M.; Chen, Y.-Y.; Huang, C.-C. Talanta 2010, 82, 516–522. (33) Turkevich, J. Gold Bull. 1985, 18, 86–91. (34) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426.

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