Chemical redox-regulated mesoporous silica-coated gold nanorods for colorimetric probing of Hg²⁺ and S²⁻

Abstract

The past a few years have witnessed the wide use of metallic nanoparticles as ideal reporters for colorimetric detection, which generally involves an analyte-triggered alteration of aggregation degree of applied nanoparticles, and thus the change of colloidal color. However, these aggregation-based colorimetric probe are associated with a number of drawbacks, including poor stability of nanoaggregates, requirement of complicated functionalization and non-linearity of output signals. To address these problems, we herein employ mesoporous silica-coated gold nanorods (MS AuNRs) as novel nanocomposites for non-aggregation-based label-free colorimetric sensing relying on their chemical redox-modulated surface chemistry. In our sensing system, Hg²⁺ ions are reduced to Hg⁰ depositing on the surface of MS AuNPs and result in a great color change of MS AuNRs, while the subsequent introduction of S²⁻ leads to a reverse process owing to the extraction of Hg⁰ by S²⁻. The experimental results for colorimetric sensing of Hg²⁺ and S²⁻ imply considerable sensitivity and specificity, suggesting the high potential of our approach for rapid environmental monitoring and bioanalysis in the future.

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Introduction

Colorimetric detection is an attractive approach in chemical and biological analysis, in which the visual color change in a reaction medium is used to signal recognition events. The colorimetric detection method offers advantages of simplicity and rapidity, along with the additional benefits of cost-effectiveness and no requirement of any sophisticated instrumentation. Of signaling agents, organic dyes,¹ polymers,² enzymes³ and nanoparticles⁴ have been employed for colorimetric assays of different analytes. In particular, gold nanoparticles (AuNPs) are routinely used as ideal colorimetric reporters by virtue of their size-/distance-dependent optical properties^4a and high extinction coefficients.⁵ By taking advantage of the color change arising from interparticle plasmon coupling of AuNPs (red to blue) or dispersion of aggregates (blue to red), AuNPs have enabled the colorimetric detection of DNA,^6a,b proteins,^6c,d metal ions,^6e,f small organic molecules^6g,h and even whole cells⁶ⁱ with considerable sensitivity. Nevertheless, several drawbacks are still associated with these AuNP aggregation-based colorimetric sensors. The poorly stable aggregates usually lead to quick variation of aggregation degree of AuNPs even after reaction equilibrium, making this strategy not advantageous for precise quantitative analysis.⁷ Moreover, the unpredicted aggregation of bare AuNPs limits their future applications in complicated samples.

Recently, mesoporous silica-encapsulated nanoparticles (MS AuNPs) have emerged as novel nanocomposites for bioimaging, drug delivery and catalysis.⁸ Such core–shell configuration not only shows stability and biocompatibility, but also allows the nanocore to be exposed to the surrounding environment through silica nanopores. The large pore size and high surface area further make them appropriate for surface chemical processes. Despite many promising properties for various applications, MS AuNPs have rarely been used as colorimetric probes so far.⁹ Thus, the exploration of these increasingly striking materials for colorimetric detection still remains an attractive goal.

Hg²⁺ and S²⁻ are both widespread pollutants with distinct toxicological profiles. Specifically, pollution by Hg²⁺ and its derivates and the resulting severe risks on the ecosystem and human health are problems of increasing concern. For instance, methylmercury can be accumulated in human bodies through food chains and the exposure to high Hg levels produces potent toxic effects on the brain, kidney and lungs of people of all ages.¹⁰ In addition, S²⁻ primarily originating from industrial and microbial processes easily generates poisonous H₂S gas, and is responsible for the corrosion of many metals (e.g. steel, copper), making it hazardous for human health and industrial processing.¹¹ Accordingly, it is of importance to develop rapid, simple and sensitive techniques to monitor Hg²⁺ and S²⁻ levels in aquatic environments. Herein, we designed a mesoporous silica-coated gold nanorod (MS AuNR)-based platform for label-free colorimetric detection, in which Hg²⁺ and S²⁻ served as the model analytes. We applied AuNRs as the core of the MS AuNPs owing to the fact that AuNRs show higher sensitivity of localized surface plasmon resonance (LSPR) to the local dielectric environment than gold nanospheres.¹² For Hg²⁺ analysis, the MS AuNRs were used to initiate nucleation catalysis to induce efficient Hg⁰ deposition on their surfaces in the presence of ascorbic acid (AA), which generated a great color change of the MS AuNRs for Hg²⁺. In the detection of S²⁻, Hg⁰–MS AuNRs were adopted as the indicators. The interaction among Hg⁰–MS AuNRs, S²⁻ and O₂ led to rapid exfoliation of Hg⁰ from MS AuNR surfaces within seconds, allowing Hg⁰–MS AuNRs to be directly transformed into colorimetric S²⁻ sensors.

Experimental

Chemicals and apparatus

Cetyltrimethylammonium bromide (CTAB) was purchased from Bio Basic Int. Hg(NO₃)₂ was obtained from Sigma. All other chemicals were received from Sinopharm Chemical Reagent Co., Ltd, China. All of the reagents were of analytical grade and used without further purification. Solutions were prepared with deionized water (18.2 MΩ cm specific resistance) purified by a Cascada™ LS Ultrapure water system (Pall Corp., USA). TEM analyses were performed on a JEM-1230 electron microscope (JEOL, Ltd., Japan) operating at 100 kV. Absorption spectra were measured on a Beckman coulter DU-800 UV/visible spectrophotometer (USA).

Synthesis of MS AuNRs

AuNRs were synthesized according to a typical seed-mediated and CTAB surfactant-directed method reported elsewhere.¹³ Briefly, 2.5 mL of CTAB solution (0.20 M) was mixed with 2.5 mL of HAuCl₄ (0.6 mM) upon stirring, where 0.30 mL of fresh, ice-cold NaBH₄ solution (0.01 M) was then added, inducing the change of solution color to brown. The obtained solution was stirred for another 2 min and stored as the seed solution for the next procedure. Then, after mixing 3.75 mL of HAuCl₄ solution (50 mM) and 0.56 mL of AgNO₃ solution (50 mM) with 250 mL of 0.10 M CTAB water solution at room temperature, 3.125 mL of 0.08 M ascorbic acid (AA) was added with gentle stirring. Immediately, the growth solution changed the color from dark yellow to colorless. 0.50 mL of the seed solution was subsequently added to the growth solution at 28 °C. The mixture was stirred for 10 h and the resulting colloid of AuNRs was stored for further use.

Mesoporous silica coating on AuNRs was carried out according to Gorelikov and Matsuura's protocol.¹⁴ First of all, 15 mL of AuNR colloidal solution was centrifuged at 13 000 rpm for 10 min to remove excess CTAB molecules, and collected in 10 mL deionized water. 0.10 mL of NaOH solution (0.10 M) was then mixed with the above AuNR colloid by stirring. Finally, 30 μL of 20% (m/m) tetraethoxysilane (TEOS) in methanol was added three times at 30 min intervals. The mixture was stirred for a further 24 h. After the as-prepared MS AuNRs were subjected to two centrifugation/wash cycles with methanol, they were finally dispersed in 10 mL deionized water. TEM characterization proved that the MS AuNRs we prepared were at an aspect ratio of 2.5 and uniformly capped with disordered mesoporous silica (∼8 nm thick) (see the TEM image in the ESI, Fig. S1†). The concentration of the MS AuNRs colloidal solution was calculated to be approximately 0.68 nM assuming that all gold in the HAuCl₄ was reduced.

Assay process for Hg²⁺ and S²⁻

50 μL of the MS AuNRs and 50 μL of 0.01 M ascorbic acid/NaBH₄ were subsequently dissolved in 350 μL of 50 mM HCl-borate (or glycine-NaOH) buffer solution (pH 8.0), and the mixture was equilibrated at room temperature for 5 min. Then, 50 μL of Hg²⁺ solution was added to the mixture and the absorption measurements were performed after incubation for 15 min. The final concentration of Hg²⁺ ranged from 1 nM to 1 μM and the absorption spectra of the sensing system with each concentration of Hg²⁺ were collected from four independent measurements.

A tap water sample collected from our institute was filtered through a 0.20 μm membrane. Three aliquots of the tap water were then spiked with standard Hg²⁺ solutions to result in different final concentrations (10.0, 55.0, and 375 nM). Both the blank and spiked samples were analyzed by the present colorimetric approach.

The analysis procedure for sulfide samples is described as below. 100 μL MS AuNRs and 50 μL AA at 0.01 M were dissolved in 300 μL 50 mM HCl-borate buffer solution at pH 8.0, and the mixture was equilibrated at room temperature for 5 min. Then 50 μL Hg²⁺ solution was added to give a final concentration of 10 μM. The as-obtained reaction medium was equilibrated for 30 min, and then centrifuged and redispersed in 100 μL deionized water with a sonicator. Finally, 50 μL Hg⁰–MS AuNRs was dissolved in 400 μL HAC-NaAC buffer (pH 4.2) for sulfide analysis. The acidic environment is expected to hamper the interaction between sulfides and other metal ions.

Results and discussions

In this work, we took advantage of MS AuNRs together with AA, a moderate reductant, to construct a colorimetric sensing system (AA–MS AuNRs) for Hg²⁺ in HCl-borate buffer (50 mM, pH 8.0). As shown in Fig. 1A, the introduction of 10 μM Hg²⁺ to the MS AuNR solution containing 1.0 mM AA induced great absorption and color changes (purple to blue green). It was assumed that such a phenomenon resulted from the chemical redox reaction between Hg²⁺ and AA that was accompanied with catalytic deposition of Hg⁰ on the mesoporous nanoshell of the MS AuNRs. Hence, AuNR cores readily formed amalgam with Hg⁰ through silica nanopores and experienced a significant dielectric constant change of the surrounding mediums, creating a distinct color change of the solution of the MS AuNRs. The invisible silica shells of the Hg⁰–MS AuNRs further confirmed our speculation, as displayed in the TEM images in Fig. 1B. We believe that the mesoporous silica nanoshells with large surface area facilitated the nucleation catalysis, and accelerated the colorimetric response of the AuNRs that could be observed within 1 min by the naked eye. Also, the absorption variation of the integrated Hg⁰–MS AuNRs achieved equilibrium within only 12 min (Fig. 1C), which is mainly due to the rapid transformation of Hg²⁺ to Hg⁰ in the presence of excess AA.

Fig. 1 (A) UV-Vis absorption spectra and (B) TEM images of AA–MS AuNRs in 0.05 M HCl-borate buffer solution (pH 8.0) in the absence (a) and presence (b) of Hg²⁺ (10 μM). Inset of part A: Photographic images of MS AuNRs containing AA in the absence (a) and presence (b) of Hg²⁺ (10 μM). (C) Time course measurement of Ex₆₆₈ for MS AuNRs containing AA upon the addition of Hg²⁺ (10 μM). The concentrations of AA and MS AuNRs are 1.0 mM and 68 pM, respectively.

On the basis of the fact that Hg²⁺ could cause an absorption change of the AA–MS AuNRs, we consequently tested whether this sensing system could be utilized for the sensitive colorimetric detection of Hg²⁺. We first explored the effect of pH on the Hg²⁺-induced colorimetric response of the AA–MS AuNR system in HCl-borate buffer. The absorption change of the AA–MS AuNRs in the presence of Hg²⁺ (10 μM) was maximized at pH 8.0 or 9.0 (see Fig. S2†). Next, The UV-Vis absorption spectra of AA–MS AuNRs in HCl-borate buffer solution at pH 8.0 with varying Hg²⁺ concentrations ranging from 1 nM to 1 μM were recorded, as shown in Fig. 2A. The absorption intensity at the peak wavelength of the longitudinal surface plasmon band of the AuNRs was used for Hg²⁺ quantification. Fig. 2B indicates that the relative extinction decrease of AA–MS AuNRs goes up with increasing logarithmic concentration of Hg²⁺ ions, with a linear correlation (R² = 0.98) of 3 orders of magnitude. The limit of detection (LOD) of this sensing system was determined to be 7.9 × 10⁻¹⁰ M at a signal-to-noise ratio of 3, which is 2 orders of magnitude lower than that of detection methods using Hg²⁺ acceptor-labelled AuNPs.¹⁵

Fig. 2 (A) UV-Vis absorption responses of AA–MS AuNRs to the addition of various concentrations of Hg²⁺ ions (0, 1.0, 3.0, 10, 30, 100, 300 and 1000 nM) in 0.05 M HCl-borate buffer solution. (B) Plots of the value of (Ex⁰ − Ex)/Ex⁰ of AA–MS AuNRs as a function of the logarithmic concentration of Hg²⁺. The error bars denote standard derivation from four independent measurements.

Under the optimized conditions at room temperature, we then investigated the detection specificity of the AA–MS AuNR probes towards Hg²⁺ relative to other metal ions. As illustrated in Fig. 3, only Hg²⁺ ions caused a significant absorption decrease of AA–MS AuNRs. Indeed, alkali metal ions (K⁺) and alkaline earth metal ions (Mg²⁺ and Ca²⁺) are inert, and their effects on the LSPR of MS AuNRs can be neglected. Besides, the transition metal ions (Zn²⁺, Co²⁺, Cd²⁺, Pb²⁺ and Cu²⁺) show ignorable reactivity with AA because of their lower oxidation potential. It is worth noting that although Fe³⁺ could be reduced to Fe²⁺ in the presence of AA, there was still no obvious absorption change of this sensing system since no metal precipitation was experienced by MS AuNRs. Moreover, the redox between AA and Ag⁺ was thought to be greatly hampered by the generation of AgCl in the HCl-borate buffer (0.05 M), thus no significant change was observed, likewise. The control experiment indicates that Ag⁺ could induce much greater absorption change of AA–MS AuNRs without Cl⁻ in the sensing system (Fig. 3). Additionally, it has been unveiled in Fig. 3 that Ag⁺ could display notable interference in the HCl-borate buffer in the presence of 1.0 mM NaBH₄, demonstrating that AA is an appropriate reductant for this redox-based approach. Compared to the amalgamation process-based method,¹⁰ the present colorimetric detection shows high specificity and reliability for Hg²⁺.

Fig. 3 Relative absorption decreases (values of (Ex⁰ − Ex)/Ex⁰) of AA (1.0 mM)–MS AuNRs upon the addition of various metal ions (1.0 μM) in 0.05 M HCl-borate (pH 8.0). For the control experiment without Cl⁻ in buffer system, the response to Ag⁺ was recorded in 0.05 M glycine-NaOH buffer solution (pH 8.0). The (Ex⁰ − Ex)/Ex⁰ value of NaBH₄–MS AuNRs to Ag⁺ was measured in 0.05 M borate buffer solution. The final concentration of NaBH₄ was 1.0 mM.

To assess its application potential, we further subjected these MS AuNR-based nanoprobes to detect Hg²⁺ in tap water samples. We found that no Hg²⁺ was detected in the tap water from our institute by the present approach. Then, we applied the standard addition technique for analyzing Hg²⁺ at EPA-desired nanomolar levels¹¹ in tap water. The recoveries of spiked Hg²⁺ ranging from 88 to 126% (see Table S1†) indicate the high potential of this AA–MS AuNR-based colorimetric method for Hg²⁺ quantification in aqueous solution.

Owing to the strong binding effect between Hg²⁺ and S²⁻ (HgS K_sp = 4.0 × 10⁻⁵³), it would be very interesting to investigate the interaction between Hg⁰–MS AuNRs and S²⁻, which may serve for the sensing of S²⁻. Currently, the complexiometric titration and electrochemical determination for S²⁻ have been well-established. However, they are not sensitive and practical.¹⁶ For example, the flow injection and ion chromatography-assisted electrochemical method^16c requires a long time and significant effort for the measurements of S²⁻, and only achieves a detection limit of 0.3 μM. Moreover, loss of S²⁻ through volatilization or oxidation would occur during the time-consuming analytical process. Despite considerable effort, reports on rapid visual detection of S²⁻ are very few, to the best of our knowledge.^11a In order to develop a new way to detect S²⁻, we prepared the Hg⁰–MS AuNRs by incubating Hg²⁺ with AA–MS AuNRs, and purified them for probing S²⁻ by centrifugation. Very interestingly, the color change immediately took place after the introduction of Na₂S solution (10 μM). Meanwhile, the absorption of Hg⁰–MS AuNRs underwent a red shift, as visualized in Fig. 4A, reversing the Hg²⁺-induced absorption peak shift of MS AuNRs. We hypothesized that this intriguing phenomenon resulted from the exfoliation of Hg⁰ that precipitated on the MS AuNRs according to the following reaction:

Hg⁰ + S²⁻ + ½O₂ + 2H⁺ → HgS + H₂O.

Fig. 4 (A) Comparison of the absorption spectra of Hg⁰–MS AuNR after adding 10 μM S²⁻ (red) to MS AuNR (black) and Hg⁰–MS AuNRs system (blue). Insert shows the corresponding photographs of the MS AuNRs. (B) Digital photographs of Hg⁰–MS AuNRs in 0.05 M HAC-NaAC buffer solutions (pH 4.2) upon addition of Na₂S. From right to left: 0, 1.0, 2.5, 5.0, 7.5, 10, 25, 50 μM, and the MS AuNRs solution for comparison. (C) TEM image of Hg⁰–MS AuNRs with the addition of 10 μM S²⁻. The concentration of Hg⁰–MS AuNRs probes is 68 pM.

We then analyzed the absorption change of the Hg⁰–MS AuNR system upon addition of different amounts of Na₂S. With increasing concentration of S²⁻, the color and absorption peak of the resulting colloid was gradually restored to the original state of the MS AuNRs and the color change was observed to saturate at the concentration of 10 μM (Fig. 4B). It is important to note that the absorbance cannot be restored to the original intensity of the MS AuNRs, which is probably attributed to the incomplete extraction of Hg⁰ by S²⁻. Also, the peak wavelength of the Hg⁰–MS AuNRs even shifted to 682 nm by introducing S²⁻ (10 μM), consistent with the aggregation of MS AuNRs displayed in the TEM image (Fig. 4C). We also explored the interference of other anions, such as SO₄²⁻, NO₃⁻, PO₄³⁻, Cl⁻ and C₂O₄²⁻, while no color change or absorption peak shift was observed for the Hg⁰–MS AuNRs (data not shown). Even though currently we are unable to analyze S²⁻ in a quantitative manner, mainly due to the decreased stability of Hg⁰–MS AuNRs compared with MS AuNRs, we still propose the exploitation of this new method for the rapid and cost-effective visual detection of S²⁻ at low micromolar levels.

Conclusions

In summary, we have successfully developed a practical colorimetric detection platform based on the mechanism of chemical redox-mediated inner particle interaction, in which the analyses were readily implemented with absorption or color change. Our method avoids complex labelling/modification, and provides a general approach to the selective detection of heavy metal ions by adding a masking agent to hamper the interferences. Meanwhile, the reversible absorption band shift of MS AuNRs opens a new way for the colorimetric probing of S²⁻, and further foretells a facile control of LSPR of nanoparticles. Given the simplicity and high sensitivity of this colorimetric detection platform, we expect that it can be applied for environmental field analysis in developing areas where resources are limited, and be extended for rapid medical diagnostics in the future.

Acknowledgements

This work was supported by grants from the National Natural Science Foundation of China (No. 20975089), the Department of Science and Technology of Shandong Province (No. 2008GG20005005, BS2009DX006), the Department of Science and Technology of Yantai City of China (2007156), the National Basic Research Program of China (973 Program 2010CB933504) and the 100 Talents Program of the Chinese Academy of Sciences.

References

1. (a) M. N. Stojanovic and D. W. Landry, J. Am. Chem. Soc., 2002, 124, 9678; (b) N. T. Greene and K. D. Shimizu, J. Am. Chem. Soc., 2005, 127, 5695; (c) C. Coll, R. Martínez-Máňez, M. D. Marcos, F. Sancenón and J. Soto, Angew. Chem., Int. Ed., 2007, 46, 1675; (d) W. M. J. Ma, M. P. P. Morais, F. D'Hooge, J. M. H. Van den Elsen, J. P. L. Cox, T. D. James and J. S. Fossey, Chem. Commun., 2009, 532.
2. (a) H.-A. Ho and M. Leclerc, J. Am. Chem. Soc., 2004, 126, 1384; (b) K. Matsubara, M. Watanabe and Y. Takeoka, Angew. Chem., Int. Ed., 2007, 46, 1688; (c) X. Liu, Y. Tang, L. Wang, J. Zhang, S. Song, C. Fan and S. Wang, Adv. Mater., 2007, 19, 1471; (d) X. Hu, J. Huang, W. Zhang, M. Li, C. Tao and G. Li, Adv. Mater., 2008, 20, 4074; (e) Z. Wu, C. Tao, C. Lin, D. Shen and G. Li, Chem.–Eur. J., 2008, 14, 11358.
3. (a) Y. Tang, F. Feng, F. He, S. Wang, Y. Li and D. Zhu, J. Am. Chem. Soc., 2006, 128, 14972; (b) N. Zhang and D. H. Appella, J. Am. Chem. Soc., 2007, 129, 8424; (c) A. W. Martinez, S. T. Phillips, M. J. Butte and George M. Whitesides, Angew. Chem., Int. Ed., 2007, 46, 1318; (d) A. W. Martinez, S. T. Phillips, E. Carrilho, S. W. Thomas III, H. Sindi and G. M. Whitesides, Anal. Chem., 2008, 80, 3699; (e) D. M. Kolpashchikov, J. Am. Chem. Soc., 2008, 130, 2934; (f) Z. Zhu, C. Wu, H. Liu, Y. Zou, X. Zhang, H. Kang, C. J. Yang and W. Tan, Angew. Chem., Int. Ed., 2010, 49, 1052.
4. (a) R. Elghanian, J. J. Storhoff, R. C. Mucic, R. L. Letsinger and C. A. Mirkin, Science, 1999, 283, 661; (b) N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547; (c) R. A. Sperling, P. R. Gil, F. Zhang, M. Zanella and W. J. Parak, Chem. Soc. Rev., 2008, 37, 1896; (d) M. De, P. S. Ghosh and V. M. Rotello, Adv. Mater., 2008, 20, 4225; (e) Z. Wang and Y. Lu, J. Mater. Chem., 2009, 19, 1788; (f) W. Zhao, M. A. Brook and Y. Li, ChemBioChem, 2008, 9, 2363; (g) G. Wang, Y. Wang, L. Chen and J. Choo, Biosens. Bioelectron., 2010, 25, 1859; (h) D. G. Thompson, A. Enright, K. Faulds, W. E. Simith and D. Graham, Anal. Chem., 2008, 80, 2805; (i) H. Wei, C. Chen, B. Han and E. Wang, Anal. Chem., 2008, 80, 7051.
5. (a) C.-C. Huang, Y.-F. Huang, Z. Cao, W. Tan and H.-T. Chang, Anal. Chem., 2005, 77, 5735; (b) S. K. Ghosh and T. Pal, Chem. Rev., 2007, 107, 4797.
6. (a) R. Elghanian, J. J. Storhoff, R. C. Mucic, R. L. Letsinger and C. A. Mirkin, Science, 1997, 277, 1078; (b) R. A. Reynolds, III, C. A. Mirkin and R. L. Letsinger, J. Am. Chem. Soc., 2000, 122, 3795; (c) V. Pavlov, Y. Xiao, B. Shlyahovsky and I. Willner, J. Am. Chem. Soc., 2004, 126, 11768; (d) C.-C. Huang, Y.-F. Huang, Z. Cao, W. Tan and H.-T. Chang, Anal. Chem., 2005, 77, 5735; (e) J. Liu and Y. Lu, J. Am. Chem. Soc., 2003, 125, 6642; (f) L. Wang, X. Liu, X. Hu, S. Song and C. Fan, Chem. Commun., 2006, 3780; (g) J. Liu and Y. Lu, Angew. Chem., Int. Ed., 2006, 45, 90; (h) W. Zhao, W. Chiuman, J. C. F. Lam, S. A. McManus, W. Chen, Y. Cui, R. Pelton, M. A. Brook and Y. Li, J. Am. Chem. Soc., 2008, 130, 3610; (i) C. D. Medley, J. E. Smith, Z. Tang, Y. Wu, S. Bamrungsap and W. Tan, Anal. Chem., 2008, 80, 1067.
7. J. Zhang, L. Wang, D. Pan, S. Song, F. Y. C. Boey, H. Zhang and C. Fan, Small, 2008, 4, 1196.
8. (a) J. Kim, J. E. Lee, J. Lee, J. H. Yu, B. C. Kim, K. An, Y. Hwang, C. H. Shin, J. G. Park and T. Hyeon, J. Am. Chem. Soc., 2006, 128, 688; (b) J. Kim, H. S. Kim, N. Lee, T. Kim, H. Kim, T. Yu, I. C. Song, W. K. Moon and T. Hyeon, Angew. Chem., Int. Ed., 2008, 47, 8438; (c) S. H. Joo, J. Y. Par, C.-K. Tsung, Y. Yamada, P. Yang and G. A. Somorjai, Nat. Mater., 2009, 8, 126.
9. C. Wu and Q.-H. Xu, Langmuir, 2009, 25, 9441.
10. (a) H. H. Harris, I. J. Pickering and G. N. George, Science, 2003, 301, 1203; (b) http://www.epa.gov/mercury/ ; (c) I. Onyido, A. R. Norris and E. Buncel, Chem. Rev., 2004, 104, 5911; (d) P. B. Tchounwou, W. K. Ayensu, N. Ninashvili and D. Sutton, Environ. Toxicol., 2003, 18, 149.
11. (a) D. Jiménez, R. Martinez-Máñez, F. Sancenón, J. V. Ros-Lis, A. Benito and J. Soto, J. Am. Chem. Soc., 2003, 125, 9000; (b) I. G. Casella, M. R. Guascito and E. Desimoni, Anal. Chim. Acta, 2000, 409, 27; (c) J. A. Young, J. Chem. Educ., 2005, 82, 202; (d) D. R. Sigler, J. G. Schroth, Y. Wang and D. Radovic, Weld. Research., 2007, 86, 340; (e) G. Gerasimon, S. Bennett, J. Musser and J. Rinard, Clin. Toxicol., 2007, 45, 420.
12. (a) X. Huang, S. Neretina and M. A. EI-Sayed, Adv. Mater., 2009, 21, 1; (b) H. Chen, X. Kou, Z. Yang, W. Ni and J. Wang, Langmuir, 2008, 24, 5233.
13. B. Nikoobakht and M. A. EI-Sayed, Chem. Mater., 2003, 15, 1957.
14. I. Gorelikov and N. Matsuura, Nano Lett., 2008, 8, 369.
15. (a) C. C. Huang and H. T. Chang, Chem. Commun., 2007, 1215; (b) J. S. Lee, M. S. Han and C. A. Mirkin, Angew. Chem., Int. Ed., 2007, 46, 4093; (c) X. Xue, F. Wang and X. Liu, J. Am. Chem. Soc., 2008, 130, 3244; (d) S. He, D. Li, C. Zhu, S. Song, L. Wang, Y. Long and C. Fan, Chem. Commun., 2008, 4885; (e) C.-W. Liu, Y.-T. Hsieh, C.-C. Huang, Z.-H. Lin and H.-T. Chang, Chem. Commun., 2008, 2242; (f) J. M. Slocik, J. S. Zabinski Jr., D. M. Phillips and R. R. Naik, Small, 2008, 4, 548.
16. (a) P. Kivalo, Anal. Chem., 1955, 27, 1809; (b) E. W. Baumann, Anal. Chem., 1974, 46, 1345; (c) I. G. Casella, M. R. Guascito and E. Desimoni, Anal. Chim. Acta, 2000, 409, 27.

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Footnotes

† Electronic supplementary information (ESI) available: TEM characterization, pH effect and results for Hg²⁺ analysis in real samples. See DOI: 10.1039/c0an00597e

‡ Contributed equally to this work.

§ Present address: Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 001-0021, Japan.

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