Highly Sensitive, Colorimetric Detection of Mercury(II) in Aqueous

Nov 11, 2010 - We provide a highly sensitive and selective assay to detect Hg2+ in aqueous solutions using gold nanoparticles modified with quaternary...
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Anal. Chem. 2010, 82, 9606–9610

Letters to Analytical Chemistry Highly Sensitive, Colorimetric Detection of Mercury(II) in Aqueous Media by Quaternary Ammonium Group-Capped Gold Nanoparticles at Room Temperature Dingbin Liu,†,‡ Weisi Qu,†,‡ Wenwen Chen,†,‡ Wei Zhang,† Zhuo Wang,*,† and Xingyu Jiang*,† CAS Key Lab for Biological Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology, Beijing 100190, China, and Graduate School of Chinese Academy of Sciences, Beijing 100080, China We provide a highly sensitive and selective assay to detect Hg2+ in aqueous solutions using gold nanoparticles modified with quaternary ammonium group-terminated thiols at room temperature. The mechanism is the abstraction of thiols by Hg2+ that led to the aggregation of nanoparticles. With the assistance of solar light irradiation, the detection limit can be as low as 30 nM, which satisfies the guideline concentration of Hg2+ in drinking water set by the WHO. In addition, the dynamic range of detection is wide (3 × 10-8-1 × 10-2 M). This range, to our best knowledge, is the widest one that has been reported so far in gold nanoparticle (AuNP)-based assays for Hg2+. We report a simple method to detect Hg2+ in aqueous media by quaternary ammonium group-capped gold nanoparticles (QA-AuNPs). Hg2+ poses severe threats to both human health and the environment.1 Long-term exposure to high levels of Hg2+-based toxins leads to serious and permanent damage of the central nervous system and other organs.2 Many of the settings required for such assays lack advanced resources, such as electricity. Highly sensitive and selective assays for Hg2+, without resorting to advanced instruments are urgently needed. Researchers have published a number of methods for detecting Hg2+, based on chemical sensors using small organic molecules,3 thin films,4,5 electrochemistry methods,6,7 polymeric materials,8 oligonucleotides,9,10 proteins,11 inductively coupled plasma-atomic emission spectrometry,12 and atomic absorption spectroscopy.13 Most of these methods, however, have limitations with respect to sensitivity and selectivity or require complex instrumentation or at least electricity. In particular, methods that * To whom correspondence should be addressed. E-mail: xingyujiang@ nanoctr.cn (X.J.); [email protected] (Z.W.). Fax: (+86)10-82545631 (X.J. and Z.W.). Phone: (+86)10-82545611 (X.J. and Z.W.). † CAS Key Lab for Biological Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology. ‡ Graduate School of Chinese Academy of Sciences. (1) Bolger, P. M.; Schwetz, B. A. N. Engl. J. Med. 2002, 347, 1735–1736. (2) Clarkson, T. W.; Magos, L.; Myers, G. J. N. Engl. J. Med. 2003, 349, 1731– 1737.

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require no sophisticated starting materials and allow visual readout might be very useful for detecting Hg2+ in resource-poor settings. AuNPs are increasingly employed for a wide spectrum of biological and biomedical applications.14-18 Colorimetric assays based on AuNPs have attracted increasing consideration on account of their unique and size-dependent optical and electronic properties. Recently, DNA-functionalized AuNPs have been widely used as colorimetric sensors for a variety of targets, including metallic ions.19-23 The thymine (T) bases in DNA sequences endow DNA-AuNP assays excellent selectivity for Hg2+ that can interact with T-T mismatches to form T-Hg2+-T complexes. However, most DNA-AuNPs assays rely on accurate control of the detection conditions, such as temperature. In addition, DNA can be costly and difficult to handle. Another kind of (3) Nolan, E. M.; Lippard, S. J. Chem. Rev. 2008, 108, 3443–3480. (4) Brainina, K. Z.; Stozhko, N. Y.; Shalygina, Z. V. J. Anal. Chem. 2002, 57, 945–949. (5) Palomares, E.; Vilar, R.; Durrant, J. R. Chem. Commun. 2004, 362–363. (6) Wang, S. P.; Forzani, E. S.; Tao, N. J. Anal. Chem. 2007, 79, 4427–4432. (7) Liu, S. J.; Nie, H. G.; Jiang, J. H.; Shen, G. L.; Yu, R. Q. Anal. Chem. 2009, 81, 5724–5730. (8) Kim, I. B.; Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 2818–2819. (9) Ono, A.; Togashi, H. Angew. Chem., Int. Ed. 2004, 43, 4300–4302. (10) Chiang, C. K.; Huang, C. C.; Liu, C. W.; Chang, H. T. Anal. Chem. 2008, 80, 3716–3721. (11) Chen, P.; He, C. J. Am. Chem. Soc. 2004, 126, 728–729. (12) Trimble, C. A.; Hoenstine, R. W.; Highley, A. B.; Donoghue, J. F.; Ragland, P. C.; Georesour, M. Geotechnology 1999, 17, 187–197. (13) Kopysc, E.; Pyrzynska, K.; Garbos, S.; Bulska, E. Anal. Sci. 2000, 16, 1309– 1312. (14) Zhou, Y.; Wang, S. X.; Zhang, K.; Jiang, X. Y. Angew. Chem., Int. Ed. 2008, 47, 7454–7456. (15) Medley, C. D.; Smith, J. E.; Tang, Z.; Wu, Y.; Bamrungsap, S.; Tan, W. Anal. Chem. 2008, 80, 1067–1072. (16) De, M.; Ghosh, P. S.; Rotello, V. M. Adv. Mater. 2008, 20, 4225–4241. (17) Jiang, Y.; Zhao, H.; Zhu, N.; Lin, Y.; Mao, L. Angew. Chem., Int. Ed. 2008, 47, 8601–8604. (18) Jiang, Y.; Zhao, H.; Lin, Y.; Zhu, N.; Ma, Y.; Mao, L. Angew. Chem., Int. Ed. 2010, 49, 4800–4804. (19) Lee, J. S.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 4093– 4096. (20) Li, D.; Wieckowska, A.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 3927– 3931. (21) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2003, 125, 6642–6643. (22) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2004, 126, 12298–12305. (23) Xue, X.; Wang, F.; Liu, X. J. Am. Chem. Soc. 2008, 130, 3244–3245. 10.1021/ac1021503  2010 American Chemical Society Published on Web 11/11/2010

Scheme 1. Proposed Mechanism for the Hg2+-Induced Colorimetric Response of QA-AuNPs

colorimetric assay for Hg2+ is based on acid-capped AuNPs.24 The ion-templated chelation between the acid groups on AuNPs and Hg2+ can induce the aggregation of AuNPs. Acid-capped AuNPs are simple but often lack sufficient selectivity for Hg2+, because other kinds of metallic ions such as Cd2+ and Pb2+ can readily interact with acid groups to cause the aggregation of acid-capped AuNPs. To improve the selectivity, it is indispensable to add 2,6-pyridinedicarboxylic acid to mask Cd2+ and Pb2+.25 Tween 20-modified AuNPs have been reported as another simple colorimetric assay for Hg2+ by the reduction of Hg2+ to form Hg-Au alloys, but Ag+ also can be reduced and absorbed on the surface of the AuNPs to cause AuNP aggregate in high-ionic-strength solutions.26 Thus, the abovediscussed AuNPs may not be convenient enough for detecting Hg2+. More details about the AuNPs-based assays for Hg2+ are summarized in Table S1, Supporting Information. The limitations of the classical AuNP-based assays for Hg2+ encourage us to develop a simpler approach for detecting Hg2+ with better sensitivity and selectivity. In this study, we demonstrate that QA-AuNPs can detect Hg2+ in aqueous solutions at room temperature without the addition of any other masking agents. The excellent selectivity of this system can be expected by comparing the stability constant (log Kf) of metallic ions with a model small molecule thiol. We note that the log Kf of Hg(cysteine)2 is ca. 43.5, whereas those of Co2+, Zn2+, Cd2+, Ni2+, Pb2+, and Mg2+ are ca. 16, 18, 17, 19, 12, and 4, respectively.27 Additionally, the log Kf of Hg(SCN)n is ca. 21.8, whereas those of Co2+, Zn2+, Cd2+, Ni2+, Pb2+, Mn2+, Fe2+, Fe3+, Cr3+, Cu2+, and Au+ are ca. 1.72, 2.0, 2.8, 1.76, 1.48, 1.23, 1.31, 4.64, 3.08, 10.4, and 16.98, respectively.28 The log Kf of Hg(SCN)n is the only one that is larger than that of Au(SCN)n. -SCN is similar to -SH that can bind strongly onto the surfaces of AuNPs,29,30 so among the metallic ions, we expect only Hg2+ is capable of removing thiolates chemisorbed on Au surface, thus destabilizing a well-dispersed aqueous solution of AuNPs results in a naked eye-based assay for Hg2+ (Scheme 1). (24) Huang, C. C.; Chang, H. T. Chem. Commun. 2007, 1215–1217. (25) Tan, Z.; Liu, J.; Liu, R.; Yin, Y.; Jiang, G. Chem. Commun. 2009, 7030– 7032. (26) Lin, C. Y.; Yu, C. J.; Lin, Y. H.; Tseng, W. L. Anal. Chem. 2010, 82, 6830– 6837. (27) Bjerrum, J.; Schwarzenbach, G.; Sille´n, L. G. Stability constants of metal-ion complexs, with solubility products of inorganic substances, part I: organic ligands; Chemical Society: London, 1957. (28) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1976; Vol. 4: Inorganic complexes. (29) Dawson, A.; Kamat, P. V. J. Phys. Chem. B 2000, 104, 11842–11846. (30) Thomas, K. G.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 3722–3727.

We used the hydrophilic (11-mercapto-undecyl)-trimethylammonium (MTA) as the QA-terminated thiols in this study because it is water-soluble and positive in charge and acts as a stabilizing agent to disperse AuNPs in aqueous solutions.31-33 MTA capped onto Au surfaces via Au-S bonds to form QA-AuNPs by means of ligand exchange.34-37 Compared with AuNPs modified with ligands terminated in other types of functional groups such as oligo(ethylene glycol),38,39 zwitterion,40,41 and negatively charged groups,42,43 QA group-terminated thiols can stabilize the dispersity of AuNPs only in acidic aqueous solution due to the electrostatic repulsion between the QA cations and positively charged H+. While in basic conditions, the QA-AuNPs aggregate rapidly because the QA cations on surfaces of AuNPs can bind with anions (OH-) via dipole-dipole interactions,44 which decrease the electrostatic repulsion among AuNPs and, thus, cause the aggregation, resulting in the color change from red to blue immediately along with the appearance of the new absorption band at 600 nm (Figure S1, Supporting Information). Therefore, we choose acidic conditions for the following detection experiments. As expected, QA-AuNPs quickly aggregated after the addition of Hg2+ (Scheme 1) in acidic solution. We reasoned that the electrostatic repulsion between the QA cations and the positively charged H+ and Hg2+ can accelerate the displacement reaction of Hg2+ with the thiols chemisorbed on the surfaces of AuNPs, (31) Daniel, W. L.; Han, M. S.; Lee, J. S.; Mirkin, C. A. J. Am. Chem. Soc. 2009, 131, 6362–6363. (32) Cliffel, D. E.; Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2000, 16, 9699–9702. (33) Lagzi, I.; Kowalczyk, B.; Grzybowski, B. A. J. Am. Chem. Soc. 2010, 132, 58–60. (34) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36. (35) Dass, A.; Guo, R.; Tracy, J. B.; Balasubramanian, R.; Douglas, A. D.; Murray, R. W. Langmuir 2008, 24, 310–315. (36) Song, Y.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 7096–7102. (37) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782–3789. (38) Kanaras, A. G.; Kamounah, F. S.; Schaumburg, K.; Kiely, C. J.; Brust, M. Chem. Commun. 2002, 2294–2295. (39) Liu, D. B.; Xie, Y. Y.; Shao, H. W.; Jiang, X. Y. Angew. Chem., Int. Ed. 2009, 48, 4406–4408. (40) Rouhana, L. L.; Jaber, J. A.; Schlenoff, J. B. Langmuir 2007, 23, 12799– 12801. (41) Kim, C. K.; Ghosh, P.; Pagliuca, C.; Zhu, Z. J.; Menichetti, S.; Rotello, V. M. J. Am. Chem. Soc. 2009, 131, 1360–1361. (42) Song, S.; Liang, Z.; Zhang, J.; Wang, L.; Li, G.; Fan, C. Angew. Chem., Int. Ed. 2009, 48, 8670–8674. (43) Cheng, W.; Hartman, M. R.; Smilgies, D. M.; Long, R.; Campolongo, M. J.; Li, R.; Sekar, K.; Hui, C. Y.; Luo, D. Angew. Chem., Int. Ed. 2010, 49, 380–384. (44) Ojea-Jime´nez, I.; Puntes, V. J. Am. Chem. Soc. 2009, 131, 13320–13327.

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making the abstraction of ligands from Au surfaces easy. The lack of sufficient charges or stabilizing agents on Au surfaces would induce the aggregation of AuNPs.45 The red color of AuNPs well-dispersed in aqueous solution was due to a plasmon peak around 520 nm with extremely high extinction coefficients (2.7 × 108 M-1cm-1), >1000 times higher than those of organic dyes.46 In the presence of Hg2+, the color of the solution turned from red to blue immediately because of the red-shifted absorption band with decreased extinction coefficients.47 The extremely high affinity of thiolates toward Hg2+ triggered the breakage of Au-S bonds on surfaces of AuNPs, causing QAterminated thiols to dissociate from Au surfaces.48 Electrospray ionization mass spectrometry (ESI-MS; Figure S2, Supporting Information) of the products confirmed this abstraction mechanism. We compared samples before (Figure S2b, Supporting Information) and after (Figure S2a, Supporting Information) the addition of Hg2+ into the QA-AuNPs solutions, the distinct difference was the prominent peak at m/z ) 245.2 in Figure S2a (Supporting Information), which was attributed to the QAterminated ligands dissociated from surfaces of AuNPs. X-ray photoelectron spectroscopy (XPS) data of QA-AuNPs also confirmed the abstraction mechanism: the ratios between Au/S, Au/ C, and Au/N before and after adding the Hg2+ match our expectations (Table S2, Supporting Information). With the addition of Hg2+, ligands left the surfaces of AuNPs, while the amount of AuNPs was unchanged; hence, the ratios of Au/S, Au/C, and Au/N increased as the amount of ligands decreased on Au surfaces. Additional analysis of the mass and charge on surfaces of AuNPs further confirm a mechanism of the abstraction of thiols from AuNPs. We used thermogravimetric analysis (TGA), which allowed for direct measurement of the weight content of organic layer on AuNPs,34-36 to investigate the amount of ligands before and after the addition of Hg2+. The TGA of well-dispersed QAAuNPs revealed that the ligands on AuNPs accounted for approximately 7.06% of the mass of AuNPs, from which we can calculate the amount of ligands to be about 4500 molecules per AuNP. After adding Hg2+, the composition of ligands on AuNPs was determinated to be 4.12% (Figure S3, Supporting Information). The remaining ligands on the aggregates of QAAuNPs were calculated to be approximately 3000 molecules per AuNP, indicating the loss of 1500 molecules of ligands per AuNP. We used zeta potential measurements to compare the surface charge on AuNPs before and after the addition of Hg2+. Zeta potential correlated with the surface charge and the local environment of AuNPs. The zeta potential of well-dispersed QA-AuNPs was about 26 mV, while that of Hg2+-induced aggregates of QA-AuNPs decreased to be approximately 20 mV (Figure S4, Supporting Information). The decrease of positive charges on QA-AuNPs displayed the Hg2+-induced loss of ligands, which consistently agreed with the results that came from ESI-MS, XPS, and TGA. (45) Zhao, W.; Brook, M. A.; Li, Y. ChemBioChem 2008, 9, 2363–2371. (46) Jin, R.; Wu, G.; Li, Z.; Mirkin, C. A.; Schatz, G. C. J. Am. Chem. Soc. 2003, 125, 1643–1654. (47) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (48) Hirayama, T.; Taki, M.; Kashiwagi, Y.; Nakamoto, M.; Kunishita, A.; Itohd, S.; Yamamotoa, Y. Dalton Trans. 2008, 4705–4507.

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Figure 1. Selectivity of this assay. (a) Color change of the solution in the presence of various representative metallic ions at concentrations of 100 µM. (b) The change of the absorption bands at 520 nm for different metallic ions; (c) TEM images of QA-AuNPs before (c1) and after incubation with Hg2+ (c2).

We confirm that the color change of the solution of QA-AuNPs in the presence of Hg2+ is caused by the aggregation of AuNPs. In the UV-vis spectra of an aqueous solution of QA-AuNPs (1.5 nM, pH 1.0), there is an extinction band at 520 nm (Figure S5, Supporting Information). Upon the addition of 100 µM Hg2+, QA-AuNPs aggregate with a color change of the solution from red to blue within several seconds, while the absorption band red-shifts to about 600 nm with a broad peak. The intensity of the broad absorption band decreases gradually; a precipitate appears after 8 h, and the solution becomes nearly colorless with precipitates at the bottom of the bottle. Next, we investigate the selectivity of this assay for Hg2+ by testing the response of the assay to other environmentally relevant metallic ions, including Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr2+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, and Zn2+ (Figure 1b), each with a concentration of 100 µM. Only Hg2+ causes the aggregation of QA-AuNPs, resulting in a color change from red to blue within several seconds. This selectivity can be visualized with the naked eye (Figure 1a). Figure 1a covers only a small fraction of the results: other samples show similar responses, and all are summarized in Figure 1b. In addition, organic mercury such as methyl mercury cannot induce aggregation. To further confirm that these AuNPs aggregate, we compare the TEM images of QA-AuNPs before and after incubation with Hg2+ (Figure 1c). The selectivity can be guaranteed when the concentrations of other metallic ions reach as high as 10 mM. None of the other ions seem to induce such aggregation as observed under TEM. This observation is also supported by the dynamic light scattering (DLS) data (Figure S6, Supporting Information). The average hydrodynamic diameter of welldispersed QA-AuNPs is 23.9 nm, while that of the Hg2+-induced

Figure 2. Light irradiation-assisted detection of Hg2+. (a) Color change with the increase of concentrations for Hg2+ from left to right. (b) Absorbance response for (a). Inset: A670/A520 vs Hg2+ concentrations. The irradiation time was 30 s; the pH value of the solutions was 1.0.

aggregates of QA-AuNPs increases to be 402.8 nm, congruent with the TEM analyses and UV-vis data. We tested the sensitivity of this assay by finding the lowest detectable concentration of Hg2+. With the naked eye alone, we can detect Hg2+ at a concentration of 1.0 µM. To confirm this result, we measured the changes in the UV-vis spectra to show the 1.0 µM detection limit was authentic (Figure S7, Supporting Information). Since some metal nanoparticles may employ the photothermal effect to absorb photos to break the Au-S bond on Au surface,49,50 we wish to see if light (from a solar light simulator) could further improve the detection limit of this system by accelerating the displacement reaction of QA-AuNPs and Hg2+. Unassisted, when the concentration of Hg2+ is lower than 1.0 µM, no apparent color changes occur. With the assistance of solar light irradiation for 30 s, however, the detection limit can be significantly decreased. With the increase of the concentration of Hg2+, the absorption peak at 520 nm decreases while that at 670 nm increases, along with the color of the solutions changing from red to purple gradually (Figure 2a). The ratio between A670 and A520 (A670/A520) is linear with the Hg2+ concentrations within a range from 0 to 600 nM. The lowest detectable concentration of this assay is 30 nM ()6 ppb; Figure 2b), which is the lowest concentration based on naked-eye readout. This detection limit is particularly attractive because the World Health Organization (WHO) has set a guideline value of the Hg2+ in drinking water (49) Hleb, E. Y.; Lapotko, D. O. Nanotechnology 2008, 19, 355702 (10pp). (50) Peng, Z. Q.; Walther, T.; Kleinermanns, K. Langmuir 2005, 21, 4249– 4253.

to be 0.006 mg/L ()30 nM).51 To the best of our knowledge, our system is the first one that provides sufficient sensitivity for detecting Hg2+ in drinking water without resorting to advanced, complex readout equipment. Researchers found that thiolates on gold surfaces can be photooxidized to sulfonates upon exposure to long-term light irradiation in air.52,53 To confirm the working mechanism of this assay under solar light irradiation still relies on the loss of ligands induced by the addition of Hg2+ other than the formation of sulfonates, we prepared a concentrated AuNP solution to which we added 500 nM Hg2+ before irradiating with solar light (30 s) and identified the products by ESI-MS. We attributed the prominent peak at m/z ) 245.5 in Figure S8a (Supporting Information) to the thiol ligand dissociated from AuNP. In addition, the ESI-MS of MTA that had been irradiated for 30 s also displayed that such a short period of irradiation time was unable to cause the formation of sulfonates (Figure S8b, Supporting Information). We conclude that solar light irradiation has the capacity to accelerate the Hg2+-induced dissociation of ligands from the surfaces of QA-AuNPs, thus causing the aggregation of QA-AuNPs. The selectivity of the system was not influenced by solar light irradiation for 30 s. The selectivity was evaluated by testing the response of the same set of other metallic ions as those discussed in Figure 1. The concentrations of all ions were 500 nM, only Hg2+ caused the color change of the AuNP solution from red to purple (Figure S9, Supporting Information). To further investigate the potential practical application of this colorimetric assay, we tried to detect Hg2+ in simulated polluted samples (by adding Hg2+ into drinking water). Using drinking water to dissolve QA-AuNPs resulting in a red solution, upon the addition of Hg2+ (at a final concentration of 10 µM) into drinking water (pH 1.0), the color of the solution turned from red to blue within several seconds. When 30 nM Hg2+ is added, the guideline value set by the WHO, followed with 30 s of solar light irradiation, the color changed to purple; a result also confirmed by the UV-vis spectra (Figure S10, Supporting Information). In conclusion, we designed a colorimetric method to detect Hg2+ that offered advantages of simplicity, rapidity, high sensitivity, and selectivity compared with many reported AuNPbased approaches (Table S1, Supporting Information). Most of the materials used in this assay are inexpensive and available commercially, making this assay particularly useful for resourcepoor settings. The lowest detectable concentration by the nakedeye was 30 nM, which satisfies the guidelines of drinking water set by the WHO. The dynamic range of detection is wide (3 × 10-8-1 × 10-2 M, Figure S11, Supporting Information). This range, to our knowledge, is the widest one that has been reported so far in AuNP-based assays for Hg2+. Interestingly, biothiols such as cysteine can redisperse the Hg2+-induced aggregates of QA-AuNPs, which can be potentially used to detect biothiols for the diagnosis of related diseases. We will investigate the (51) World Health Organization. Guidelines for drinking-water quality: incorporating 1st and 2nd addenda, Vol. 1, Recommendations, 3rd ed.; World Health Organization: Geneva, 2008. http://www.who.int/water_sanitation_health/ dwq/fulltext.pdf (accessed November 10, 2010). (52) Huang, J. Y.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342–3343. (53) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305–5306.

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detection of biothiols by the Hg2+-induced aggregates of QAAuNPs in future work. We hope that this type of assay will be useful for many settings, including assays based on lab-on-chip format, where highly sensitive assays requiring no advanced instrumentation are highly desired.54-57 ACKNOWLEDGMENT We thank Prof. M. Gao and Dr. Q. Jia (The Institute of Chemistry, CAS) for the measurements of TGA. Financial support (54) Liu, Y. Y.; Yang, D. Y.; Yu, T.; Jiang, X. Y. Electrophoresis 2009, 30, 3269– 3275. (55) Sun, Y.; Liu, Y. Y.; Qu, W. S.; Jiang, X. Y. Anal. Chim. Acta 2009, 650, 98–105. (56) Yang, D. Y.; Niu, X.; Liu, Y. Y.; Wang, Y.; Gu, X.; Song, L. S.; Zhao, R.; Ma, L. Y.; Shao, Y. M.; Jiang, X. Y. Adv. Mater. 2008, 20, 4770–4775. (57) Zhang, W.; Lin, S.; Wang, C.; Hu, J.; Li, C.; Zhuang, Z.; Zhou, Y.; Mathies, R. A.; Yang, C. J. Lab Chip 2009, 9, 3088–3094.

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from the Chinese Academy of Sciences, the National Science Foundation of China (90813032, 20890020), the Ministry of Science and Technology (2009CB30001), the Ministry of Human Resources and Social Security of China, and Corning Inc. is highly acknowledged.

SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review August 16, 2010. Accepted November 8, 2010. AC1021503