Silver Nanoplates: A Highly Sensitive Material toward Inorganic

Mar 5, 2008 - Langmuir , 2008, 24 (8), pp 4300–4309 ..... Suber , G. Campi , A. Pifferi , P. Andreozzi , C. La Mesa , H. Amenitsch , R. Cocco and W...
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Langmuir 2008, 24, 4300-4309

Silver Nanoplates: A Highly Sensitive Material toward Inorganic Anions X. C. Jiang†,‡ and A. B. Yu*,† School of Materials Science and Engineering, UniVersity of New South Wales, Sydney, NSW 2052, Australia, and School of Materials and Metallurgy, Northeastern UniVersity, Shenyang, 110004, P. R. China ReceiVed October 16, 2007. In Final Form: January 16, 2008 This paper demonstrates a simple sensing method to detect inorganic anions by silver nanoplates (edge length of ∼70 nm and thickness of ∼2 nm) in aqueous solution. By this method, the solution system containing silver nanoplates shows a high sensitivity on the order of 1 × 10-6 M in detecting halides, phosphate, and thiocyanate ions in water at room temperature. The sensitivity could be identified by the shift in the surface plasmon resonance (SPR) band in UV-vis spectrum. The selectivity of such a sensing system toward various anions was also studied, and it was found that this sensing system could distinguish individual anions (e.g., Cl-, Br-, I-, H2PO4-, and SCN-) from other anions (e.g., F-, SO42-, CH3COO-, NO3-, and ClO4-) and inorganic cations (e.g., Zn2+, Cd2+, and Cu2+) under the given conditions. The sensing mechanism was also analyzed. It was proposed that the particle surface electron charging, which is mainly determined by the interaction tendency between silver atoms and various inorganic anions in water, is responsible for the shift in the SPR observed. The need for further studies was finally discussed, particularly for systems composed of mixed anions.

1. Introduction Determination of common anions such as fluoride (F-), chloride (Cl-), ortho-phosphate (PO43-), and perchlorate (ClO4-) is a significant component in the characterization of the quality and extent of drinking water. Many attempts have been made in the past to monitor such anions (e.g., Cl- and PO43- ions) in drinking water and industrial effluents by various techniques, including the traditional ion chromatography with mass spectrometer and capillary electrophoresis1,2 and the recently developed sensor methods such as fiber-optic sensing probe, molecular fluorescence probe, and metal-based probes.3 Of the approaches proposed thus far, metal-based probes that can selectively recognize the guest species via a macroscopic physical response have attracted considerable attention in recent years.4 This is particularly true for precious metal nanoparticles (e.g., Au and Ag) due to their intensive surface plasma resonance (SPR) in the ultraviolet-visible (UV-vis) and near-infrared region of electromagnetic wavelength. Such metal nanoparticles meet the selective requirements as colorimetric sensors.5 To date, many studies focus on gold colloids as a major molecular recognition component to detect cations (e.g., Li+, K+, Pb2+, * To whom correspondence should be addressed. Email: a.yu@ unsw.edu.au. Phone: +61-2-93854429. Fax: +61-2-93855956. † University of New South Wales. ‡ Northeastern University. (1) Hedrick, E. J.; Munch, D. J. J. Chromatogr., A 2004, 1039, 83-88. (2) Chen, M.; Cassidy, R. M. J. Chromatogr. 1993, 640, 425-431. (3) (a) Brasuel, M. G.; Miller, T. J.; Kopelman, R.; Philbert, M. A. Analyst 2003, 128, 1262-67. (b) Ma, A.; Rosenzweig, Z. Anal. Chem. 2004, 76, 569575. (c) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486-516. (d) Beer, P. D.; Gale, P. A.; Chen, G. Z. J. Chem. Soc., Dalton Trans. 1999, 18971910. (e) Labande, A.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2002, 124, 178289. (f) Daniel, M.; Ruiz, J.; Nlate, S.; Blais, J.-C.; Astruc, D. J. Am. Chem. Soc. 2003, 125, 2617-28. (4) (a) Lee, K.-S.; El-Sayed M. A. J. Phys. Chem. B 2006, 110, 19220-25. (b) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem. 2000, 1, 18-52. (c) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (d) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098-2101. (5) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549-56.

Cd2+, Cu2+, and Hg2+) through surface modification with specific mediators such as 1,10-phenanthroline derivatives, crown ether (e.g., 15-crown-5), or biomolecules (e.g., DNA and amino acids).6-12 In these studies, however, gold particles have to be assisted by surface modification with selective receptors, and some experimental procedures in operation are complicated. Therefore, there is a need to develop a facile sensing method to detect inorganic ions in aqueous solution. Silver nanoparticles are cost effective in preparation compared to gold ones, and they also exhibit intensive SPR band in the wavelength range of 300-900 nm. To date, many studies concentrate on the optical applications (e.g., chemical and biochemical sensors) of silver nanoparticles.13-17 For example, various shapes of silver nanoparticles (nanorods, nanowires, and (6) Obare, S. O.; Hollowell, R. E.; Murphy, C. J. Langmuir 2002, 18, 1040710. (7) Lin, S. Y.; Liu, S. W.; Lin, C. M.; Chen, C. H. Anal. Chem. 2002, 74, 330-335. (8) Kim, Y. J.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165-167. (9) Liu, J. W.; Lu, Y. J. Am. Chem. Soc. 2003, 125, 6642-43. (10) (a) Gooding, J.; Hibbert, D. B.; Yang, W. Sensors 2001, 1, 75-90. (b) Yang, W.; Gooding, J.; Hibbert, D. B. J. Electroanal. Chem. 2001, 516, 10-16. (c) Yang, W.; Chow, E.; Willett, G. D.; Hibbert, D. B.; Gooding, J. Analyst 2003, 128, 712-718. (d) Chow, E.; Hibbert, D. B.; Gooding, J. Electrochem. Commun. 2005, 7, 101-106. (11) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (b) Storhofff, J. J.; Elghanian, R.; Mirkin, C. A.; Letsinger, R. L. Langmuir 2002, 18, 6666-70. (c) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640-50. (d) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959-64. (12) (a) Hone, D. C.; Haines, A. H.; Russell, D. A. Langmuir 2003, 19, 714144. (b) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516-17. (c) Zhang, F. X.; Han, L.; Israel, L. B.; Daras, J. G.; Maye, M. M.; Ly, N. K.; Zhong, C. J. Analyst 2002, 127, 462-465. (d) Thanh, N. T. K.; Rosenzweig, Z. Anal. Chem. 2002, 74, 1624-28. (13) (a) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (b) Bloemer, M. J.; Buncick, M. C.; Warmack, R. J.; Ferrell. T. L. J. Opt. Soc. Am. B 1988, 5, 2552. (c) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang. P. D. Nano Lett. 2003, 3, 1229-33. (d) Shanmukh, S.; Jones, L.; Driskell, J.; Zhao, Y.-P.; Dluhy, R.; Tripp. R. A. Nano Lett. 2006, 6, 2630-36. (e) Yang, W.-H.; Hulteen, J.; Schatz, G. C.; van Duyne, R. P. J. Chem. Phys. 1996, 104, 4313-23. (f) Lucotti, A.; Giuseppe, Z. Sens. Actuators, B 2007, 121, 356-364. (g) Muniz-Miranda, M. Chem. Phys. Lett. 2001, 340, 437-443.

10.1021/la7032252 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008

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spherical particles) were used for surface enhancement Raman scattering (SERS) measurement that readily reveals the vibrational signature of an analyte.13 Silver nanoparticles deposited in silica film or silica glass substrate were used for surface plasma resonance sensors.14 Individual silver nanoparticles or their arrays were applied for real-time optical sensors based on localized surface plasma resonance (LSPR).15 Coupled planar silver nanoparticle arrays were used for refractive index sensor.16 Silver nanoparticles were also used to evoke intensive chemiluminescence with tris(2,2′-bipyridyl) ruthenium(II) and cerium(IV).17 In addition, Henglein et al.18 and Mulvaney et al.19 demonstrated the effect of the chemisorbed cations or anions on the SPR band of spherical silver nanoparticles and gold nanorods in different systems. However, little literature was reported on the application of silver nanoplates in colorimetric sensing detection of inorganic ions in aqueous solution. In this work, we present a simple but effective method to detect anions (e.g., Cl-, Br-, I-, H2PO4-, and SCN-) by using silver nanoplates in aqueous media. The sensitivity and selectivity toward inorganic anions that are dependent upon the shift in the SPR band are investigated. The characteristics (shape, size, and size distribution) of silver nanoplates are analyzed. To understand the sensing mechanism, the effects of other anions (e.g., NO3-, SO42-, CH3COO-, ClO4-) and their mixtures, cations (e.g., Zn2+, Cd2+, and Cu2+), as well as the surface modifiers (e.g., thiols and cysteine) are also investigated. 2. Experimental work 2.1. Chemicals. Silver nitrate (AgNO3, 99.9%), citric acid (99%), L-ascorbic acid (g99.0%), sodium borohydride (NaBH4, 99%), and sodium bis(2-ethylhexyl)sulfosuccinate (NaAOT, 99%) for synthesis of silver nanoplates were all purchased from Sigma-Aldrich and used as received. This is also the case for chemicals for ionic sensing detection such as sodium fluoride (NaF, g99.0%), sodium chloride (NaCl, g99.5%), sodium bromide (NaBr, g99.0%), sodium iodide (NaI, g99.0%), sodium nitrate (NaNO3, g99.0%), sodium sulfate (Na2SO4, 99.99+%), sodium phosphate dibasic (Na3PO4, 99.95%), sodium thiocyanate (NaSCN, g99.99%), zinc nitrate hexahydrate (Zn(NO3)2‚6H2O, 98%), cadmium nitrate tetrahydrate (Cd(NO3)2‚ 4H2O, 99.999%), copper nitrate (Cu(NO3)2, 99+%), thiols (e.g., 1-dodecanethiol), and amino acids (e.g., cysteine). All the solutions were freshly made for all the experimental procedures in this work. Ultrapure water was used, and all glassware were cleaned with aqua regia and thoroughly rinsed with ultrapure water prior to use. 2.2. Synthesis of Silver Nanoparticles. The synthesis of silver nanoplates was conducted by modifying the procedures described (14) (a) Hashimoto, N.; Hashimoto, T.; Teranishi, T.; Nasu, H.; Kamiya, K. Sens. Actuators, B 2006, 113, 382-388. (b) Hutter, E.; Fendler, J. H.; Roy, D. J. Phys. Chem. B 2001, 105, 11159-68. (15) (a) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471-82. (b) Mcfarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057-62. (c) Haes, A. J.; Chang, L.; Klein, W. L.; Van Duyne, R. P. J. Am. Chem. Soc. 2005, 127, 2264-71. (d) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029-34. (e) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279-85. (f) Katherine, A.; Willets, K. A.; Van Duyne, R. P. Annu. ReV. Phys. Chem. 2007, 58, 267-97. (g) Kelly, K. L.; Coronado, E.; Zhao, L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-77. (h) Miller, M. M.; Lazarides, A. A. J. Phys. Chem. B 2005, 109, 21556-65. (16) Malynych, S.; Chumanov, G. J. Opt. A: Pure Appl. Opt. 2006, 8, S144S147. (17) Gorman, B. A.; Francis, P. S.; Dunstan, D. E.; Barnett, N. W. Chem. Commun. 2007, 395-397. (18) (a) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. 1991, 92, 31-44. (b) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392-96. (c) Henglein, A. Chem. Mater. 1998, 10, 444-450. (d) Linnert, T.; Mulvaney, P.; Henglein A. J. Phys. Chem. 1993, 97, 679-682. (19) (a) Mulvaney, P. Langmuir 1996, 12, 788-800. (b) Mulvaney, P.; Pe´rezJuste, J.; Giersig, M.; Liz-Marza´n, L. M.; Pecharroma´n, C. Plasmonics 2006, 1, 61-66. (c) Pe´rez-Juste, J.; Liz-Marza´n, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P. AdV. Funct. Mater. 2004, 14, 571-579.

in our recent work.20a-c Mainly, three steps were involved. First, 10 mL aqueous AgNO3 (0.01 M) and 10 mL NaAOT (0.02 M) solution were added to a 500 mL conical flask containing ultrapure water (the final volume of the mixture was fixed at 200 mL) followed by stirring to ensure that the mixture was homogeneous. Second, 0.9 mL citric acid (0.10 M) and 0.6 mL L-ascorbic acid (0.10 M) aqueous solution freshly made were injected into the mixed solution containing Ag+ ions and AOT, followed by vigorous stirring to ensure that the mixture was homogeneous. Third, 20.0 µL ice-bathed NaBH4 aqueous solution (0.01 M) was quickly injected into the above mixed solution and stirred. On aging for ∼10 min, the as-prepared silver nanoplates formed, and then the solution was placed in a fridge (temperature set to ∼4 °C) to prevent shape evolution, although the evolution from triangular plates to the intermediates of triangle and disc and to circle discs needs a long time (>48 h) at room temperature (∼25 °C). This is due to the different interaction energies between AOT molecules and silver surfaces, i.e., Ag{111} -460.1, Ag{100} -249.9, and Ag{110} -323.4 kcal/mol, which could lead to the surface atoms migrating among the different planes. The detailed experimental and theoretical investigations of how the silver triangular nanoplates evolved have been discussed in our recent work.20a-c Silver colloids with ∼5 nm diameter were prepared by following the modified procedures described in our recent work.20d All the experimental procedures were carried out at room temperature (∼25 °C). 2.3. Sensing Detection of Anions. Many tests were carried out to optimize the sensing conditions such as addition time, concentration, and stirring rate. In a typical procedure, three steps were involved, for example, in sensing detection of Cl- ions. First, a 10 mL aqueous solution containing silver nanoplates was injected into a 25 mL vial. Here, the concentration of silver nanoplates was estimated to be ∼1 nM based on the size distribution obtained in our recent work.20a-c Second, various volumes (i.e., 4, 8, 12, 16, or 20 µL) of sodium chloride solution (0.01 M) were separately added into 5 vials containing the silver nanoplates followed by vigorous stirring. It was found that all the sensing systems were very stable and no precipitates or flocs were observed. Third, 3 mL reaction solution was separately taken out from the vial and injected into a standard 1 cm quartz cell for the measurement of UV-vis spectrum. All the other measurements involved with the different inorganic ions were conducted by following the above mentioned procedure. Note that the AOT molecules for stabilizing silver nanoplates were not removed in the detection process to avoid possible aggregation of particles. The injection of anions has less effect on the pH value of the sensing system due to the small volumes, and the pH value was kept at ∼5.5. 2.4. Selective Detection of Halides, H2PO4-, and SCN- Ions in a Mixture. The selectivity of silver nanoplates toward halides, H2PO4-, and SCN- ions in a mixture was investigated. All 0.01 M stock solutions containing individual or mixed anions were prepared from their corresponding sodium salts. Briefly, three steps were involved. First, the mixed ions of F-, NO3-, SO42-, CH3COO-, and ClO4- (named as B) were put into a 25 mL vial containing 10 mL of the silver nanoplate solution followed by stirring to ensure the homogeneity. The individual concentration of these ions was fixed at 20 × 10-6 M. Second, 11 mixtures containing B anions and halides, H2PO4-, or SCN- ions were prepared. The mixtures containing individual halides (e.g., Cl-, Br-, and I-) were labeled as S1 (Cl- + B), S2 (Br- + B), and S3 (I- + B), and the concentration of Cl-, Br-, or I- ions was fixed at 8 × 10-6 M in the corresponding mixtures. The mixtures containing two or more halides and H2PO4and SCN- ions were labeled as S4 (Cl- + Br- + B), S5 (Cl- + I- + B), S6 (Br- + I- + B), S7 (Cl- + Br- + I- + B), S8 (H2PO4+ SCN- + B), S9 (Br- + H2PO4- + B), S10 (Br- + SCN- + B), and S11 (Br- + H2PO4- + SCN- + B), in which the individual concentration of Cl-, Br-, I-, H2PO4-, and SCN- ions was fixed at 4 × 10-6 M for comparison in the UV-vis spectra. It was found (20) (a) Jiang, X. C.; Zeng, Q. H.; Yu, A. B. Nanotechnology 2006, 17, 492935. (b) Jiang, X. C.; Zeng, Q. H.; Yu, A. B. Langmuir 2007, 23, 2218-23. (c) Zeng, Q. H.; Jiang, X. C.; Yu, A. B.; Lu, G. Nanotechnology 2007, 18, 035708. (d) Jiang, X. C.; Yu, A. B. J. Nanopart. Res. 2007, in press (online version is available: DOI 10.1007/s11051-007-9281-z).

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Figure 1. UV-vis spectra showing that silver nanoplates can be used for sensing detection of halides in aqueous solution: (A) F-, (B) Cl-, (C) Br-, and (D) I- ions. Note that the unit of ppm stands for 1 × 10-6 M in all the figures. that all the 11 sensing systems were stable and no precipitates or flocs were observed. Finally, a 3 mL mixed solution was taken and injected into a standard 1 cm quartz cell for the measurement of UV-vis spectrum after 3 min stirring. 2.5. Amount Determination of Individual Halide Ions in a Mixture. The content of individual halide ions in a mixture could be estimated by comparison of the shift in the strongest SPR in UV-vis spectra. A set of experiments were carried out, and the Clions were chosen as a representative in this study. In a typical measurement, three steps were involved. First, various anions including F-, NO3-, SO42-, CH3COO-, and ClO4- were injected into a vial containing 10 mL aqueous solution of silver nanoplates, and the concentration of each kind of ions was fixed at 20 × 10-6 M. Then an appropriate volume of NaCl solution (0.01 M) was added to the above mixture, so that the final concentration of Clions was fixed at 4, 8, 12, 16, or 20 × 10-6 M. Finally, a 3 mL mixed solution was taken and injected into a standard 1 cm quartz cell for the measurement of UV-vis spectrum. 2.6. Characterization. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a Philips CM200 instrument, operated at the accelerating voltage of 200 kV. The specimen was prepared by dropping the solution on a copper grid covered with carbon film and allowing to dry naturally in air. UV-vis spectrum was obtained on a CARY 5G UV-vis spectrophotometer (Varian) with 1 cm quartz cell. In each measurement, 3 mL reaction solution was used.

3. Results and Discussion 3.1. Sensitivity in Detecting Anions. The shift in the SPR band of the silver nanoplates is sensitive to the presence of adsorbed substances, particularly for anions. The sensitivity of silver nanoplates toward halides (e.g., F-, Cl-, Br-, and I-) was hence identified by UV-vis spectra in this work. Figure 1 shows that these halides have different optical absorptions against the silver nanoplates. Seen from Figure 1A, there are three distinctive plasma resonances centered at around 769, 450, and 330 nm in the reference sample (curve a), and they could be assigned to in-plane dipolar mode and in-plane and out-of-plane quadrupolar modes of silver nanoplates, respectively. This is consistent with the recent studies on silver nanoprisms and nanodiscs.20-22 When (21) Mie, G. Ann. Phys. 1908, 25, 377-445. (22) (a) Brioude, A.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 23371-77. (b) Schatz, G. C.; Van Duyne, R. P. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths P. R., Eds.; Wiley: New York, 2002.

F- ions were added to the sensing system, the intensive plasma band had a slight damping in the absorption spectra, but it was hard to yield any shift in the SPR bands, even if the concentration of the F- ions was increased to 20 × 10-6 M. This suggested that the silver nanoplates are not significantly sensitive to the Fions under the given conditions. In comparison to F- ions, remarkable shifts were observed in the intensive SPR band toward shorter wavelengths for other halides under the current conditions. Figure 1B shows that the shift in the intensive SPR band is heavily dependent on the concentration of Cl- ions. When the concentration was increased to 4, 8, 12, 16, and 20 × 10-6 M, the intensive SPR band centered at ∼769 nm shifted toward shorter wavelengths to 757, 701, 646, 571, and 546 nm, respectively. A high shifting rate (i.e., ∆λmax ) 15.3 nm/10-6 M) estimated based on the relationship between the concentration of Cl- ions and the shifted wavelengths suggested that the silver nanoplates were highly sensitive to the Cl- ions under the given conditions. In the interim, the profile of the intensive SPR band also changed with the concentration of Cl- ions, but the other two plasma bands centered at ∼450 and ∼330 nm nearly maintained their positions in the sensing process (Figure 1B). This is consistent with the observations made in the suspension containing gold nanorods.19 The addition of BH4- ions could lead to blue-shift in the intensive SPR band of gold rods, and the surface plasmon band would become damped when charging becomes high. The effects are in qualitative agreement with a model in which the gold plasma frequency increases due to an increase in electron density.19 The sensing detection toward Br- and I- ions by silver nanoplates was subsequently investigated. Figure 1C shows that the intensive SPR band centered at 769 nm shifted to shorter wavelengths of 698, 548, and 476 nm corresponding to different concentrations of Br- ions, 4, 8, and 12 × 10-6 M, respectively. When the Br- concentration was increased to 16 × 10-6 M or higher, only one SPR band centered at ∼413 nm was observed in the UV-vis spectra (Figure 1C). On comparison with the Ag0-Cl- system, Br- ions seem to have a stronger impact on the sensing detection of silver nanoplates under similar conditions. The UV-vis spectra taken in detecting I- ions (Figure 1D) indicated that the intensive SPR band centered at 769 nm blueshifted distinctively to 650 and 502 nm when the concentration

SilVer Nanoplates

of I- ions was increased to 4 and 8 × 10-6 M, respectively. Whereas on increasing the concentration of I- ions to 12, 16, and 20 × 10-6 M, only two plasma resonance bands centered at 480 and 409 nm were observed. Moreover, higher concentrations, e.g., 100 × 10-6 M of halide ions have been tested and the corresponding UV-vis spectra were recorded as shown in Figure S1 (Supporting Information). Only a slight change in the absorption intensity but not the position was observed for the Fions, but remarkable shifts in intensive SPR band toward shorter wavelengths were observed for other halide ions, such as ∆λmax ) 269 nm (from 769 to 500 nm) for Cl- ions and ∆λmax ) 359 nm (from 769 to 410 nm) for Br- or I- ions. Such a UV-vis spectrum analysis indicated that the shift in the SPR band is drastically dependent on the concentration and type of anions, i.e., the as-prepared silver nanoplates exhibited high sensitivity toward Cl-, Br-, and I- ions rather than F- ions under the conditions considered. There are some studies on the SPR of silver nanoparticles (e.g., nanorods or their arrays) in the past. For example, rod-type silver nanoprticles exhibit two characteristic SPR bands, namely, the transverse SPR (⊥) and the longitudinal SPR (|), depending on the light polarization direction to the principal axis.13a The peak position of the transverse SPR is similar to that of spherical particles. However, the peak position of the longitudinal SPR depends on the axial ratio (b/a).13b Most of the previous studies focused on the SERS sensing detection of specific target molecules based on their unique vibration signatures, in which the silver nanowires, nanorods, or their arrays served as substrates in SERS enhancement.13 For example, Yang et al.13c reported that Langmuir-Blodgett silver nanowire monolayer served as a substrate for SERS enhancement of thiols, 2,4-dinitrotoluene, and rhodamine 6G molecules. Shanmukh et al.13d described the sensitive detection of respiratory virus molecular signatures using the array of silver nanorods as a SERS substrate. Van Duyne et al.13e reported a SERS study of trans-1,2-bis(4-pyridyl)ethylene that adsorbed onto silver film over nanosphere electrodes. Such a SERS enhancement can be attributed to the increased local optical fields near the silver surface as a result of the excitation of surface plasmon resonances. In addition, Van Duyne et al.15 explored the optical properties of Ag nanoparticles chemically modified with alkanethiol self-assembled monolayers (SAMs) by measuring the localized surface plasmon resonance (LSPR) spectrum using UV-vis extinction spectroscopy, in which the Ag nanoparticles were fabricated using the technique of nanosphere lithography (NSL) and had in-plane widths of 100 nm and out-of-plane heights of 50 nm. The authors suggested that silver nanoparticles of different shapes show different refractive index sensitivities, with rods showing the highest sensitivity followed by triangles and then spheres.15 In the above studies on SPR, the electromagnetic-field-enhanced SERS spectrum requires coupling of the incident radiation to the metal surface, particularly on roughened metallic substrates. In the case of LSPR spectrum, light is required to interact with particles much smaller than the incident wavelength, which leads to a plasmon that oscillates locally around the nanoparticle with a frequency known as the LSPR.15 In the present study, the SPR spectrum is used to identify the sensitivity of silver nanoplates toward inorganic anions in aqueous solution but on substrates composed of silver nanorods, nanowires, or their arrays. The SPR spectrum can offer thermodynamic and realtime kinetic data for chemical adsorption/desorption processes. Our experimental observations suggested that the SPR wavelengthshift sensing method can be used to detect ions of chemical relevance.

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Figure 2. (A) TEM image of the as-prepared silver colloids, and (B) the size distribution of silver colloids.

Spherical silver colloids can also be used to detect inorganic anions (e.g., Cl-) in aqueous solution. Their sensitivity was also tested against that of silver nanoplates. Figure 2 shows the TEM image of the as-prepared silver colloids and their size distribution (5 nm in diameter). As expected, only one single SPR band centered at ∼422 nm was observed (Figure 3). No shifts in the SPR were observed during sensing detection toward F-, Cl-, Br-, and I- ions, even if the concentration of the ions was increased to 120 × 10-6 M. The absorption intensity remained the same except for a slight damping in I- ionic system (Figure 3D). This is consistent with the study on the surface plasmon damping of silver colloids (6 nm mean diameter) by chemisorbed I- ions,18 where the authors demonstrated that the I- ions could lead to a distinctive damping in the plasma band of silver colloids due to possible redox reduction. There is no simple way to account for the intensity of this damping at present, but the damping strength is correlated with the strength of the Ag-anion bond. The effect is not simply due to the reduction in the electron mean free path but the channeling of the surface plasmon energy into excitation modes of the surface metal-adsorbate complex.23 In addition, no eye-perceptible color change was observed in the silver colloidal system, although the halide ion concentration was increased up to 120 × 10-6 M. The results suggested that the silver colloids coated by AOT molecules are not sensitive to halides. So, silver nanoplates are more superior to spherical particles in sensing detection of halides under the conditions considered. 3.2. Selectivity in Detecting Anions. The selective detection toward individual anions such as NO3-, ClO4-, CH3COO-, or SO42- was investigated. Figure 4A shows that no apparent shifts were observed in all the three SPR bands of the silver nanoplates, even if the concentration of each type of anions was increased to 20 × 10-6 M, indicating that these anions could not significantly alter the surface dielectric properties of silver nanoplates under the current conditions. On the contrary, the silver nanoplates exhibited remarkable sensitivity toward Cl- ions, because a distinctive shift (i.e., ∆λmax ) 233 nm) toward shorter wavelengths was observed when the concentration of Cl- ions was fixed at 16 × 10-6 M, i.e., the silver nanoplates can selectively detect some anions under the given conditions. To further confirm the selectivity of silver nanoplates toward individual halides, a set of controlled experiments were carried out in a mixture of various anions. In particular, we investigated three mixtures (S1-S3) containing B anions (i.e., F-, NO3-, ClO4-, CH3COO-, and SO42-, where the individual concentration of the ions was fixed at 20 × 10-6 M) and Cl-, Br-, or I- at a concentration of 8 × 10-6 M. No shifts in the SPR were observed for the mixture without addition of halide ions (e.g., Cl-, Br-, and I-), consistent with the results in Figure 4A. Once the halides were introduced into the mixture, the solution color changed. Their corresponding UV-vis spectra were recorded. Figure 4B (23) (23) Person, B. N. J. Surf. Sci. 1993, 281, 153.

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Figure 3. UV-vis spectra showing that the spherical silver colloids are not significantly sensitive to the halides under similar conditions: (A) F-, (B) Cl-, (C) Br-, and (D) I- ions.

Figure 4. (A), UV-vis spectra showing that the silver nanoplates are sensitive to Cl- ions (16 × 10-6 M) but not to NO3-, ClO4-, CH3COO-, or SO42- ions with an individual concentration of 20 × 10-6 M; (B) the comparison of selectivity in detecting individual ions (Cl-, Br-, or I-) (solid line) with mixed S1, S2, and S3 systems (dash dot line), in which the concentration of Cl-, Br-, or I- ion was fixed at 8 × 10-6 M and the concentration of other ions were separately fixed at 20 × 10-6 M; (C) the variation of the SPR shift (∆λmax) with the concentration of Cl- ions ([Cl-]) in mixture (S1); and (D) the fitted curve showing the relationship between ∆λmax and [Cl-].

shows that the shift in the SPR was mainly caused by addition of halide ions (e.g., Cl-, Br-, and I-) rather than by other above mentioned ions. Compared with those simple systems containing individual halides (Figures 1B-D), the position of the intensive SPR band of silver nanoplates in the mixture shifted a bit toward longer wavelengths. The shifts are ∆λmax ) 23 nm (from 701 to 724 nm) for Cl- ions, ∆λmax ) 30 nm (from 548 to 578 nm) for Br- ions, and ∆λmax ) 32 nm (from 502 to 534 nm) for Iions, suggesting that the sensitivity of silver nanoplates toward halide ions decreased a bit in such mixtures. One possible reason is that the mixed anions affected the distribution of Cl-, Br-, or I- ions in the charged double layer surrounding silver nanoplates and hence altered the surface electron density. This is in agreement with the observations by Mulvaney et al.,19 who discussed the double layer charge on silver colloids and showed that the input of electrons (e.g., anions) can affect the double layer capacity

or the electron density on the colloidal surfaces and hence the SPR in UV-vis spectra. For example, citrate stabilized silver sols typically have a band around 385-390 nm whereas extensively gamma-irradiated silver sols (strong reducing conditions) possess absorption bands at λ < 380 nm.19a The SPR shift in the UV-vis spectrum is heavily dependent upon the concentration of the halide ions (e.g., Cl-, Br-, or I-) as shown in Figure 1(B-D). In turn, the amount of such an individual halide in a mixture could be estimated by measuring the shift in the intensive SPR. The mixture (S1) containing anions (i.e., F-, NO3-, ClO4-, CH3COO-, and SO42-) and Cl- ions was chosen as an example, in which the concentration of the individual anions was fixed at 20 × 10-6 M except for Cl- ions. Figure 4C shows that the intensive SPR shifted toward shorter wavelengths when increasing the concentration of Cl- ions. Such shifts were observed around 9, 54, 119, 158, and 201 nm

SilVer Nanoplates

Figure 5. UV-vis spectra showing that the silver nanoplates are not significantly sensitive to inorganic cations (e.g., Zn2+, Cd2+, and Cu2+) in aqueous solution.

corresponding to Cl- concentrations of 4, 8, 12, 16, and 20 × 10-6 M, respectively. The shift in the SPR band was mainly caused by Cl- ions based on the above experimental observations. Further investigation indicated that the relationship between the SPR shift (∆λmax) and the concentration of Cl- ions ([Cl-]) could be plotted and fitted by a simple linear equation, i.e., ∆λmax ) 12.2 × [Cl-] - 38 (Figure 4D). The slope, 12.2 nm/10-6 M, shows that the sensitivity of silver nanoplates toward Cl- ions is high in the mixture (S1). According to the equation, the amount of halide ions (e.g., Cl-) in the mixture could be estimated by the shift in the SPR (i.e., ∆λmax) under the current conditions. The sensitivity and selectivity of silver nanoplates toward inorganic cations such as Zn2+, Cd2+, and Cu2+ were also investigated. From the UV-vis spectra shown in Figure 5, it was hard to see any shift occurring in the SPR bands when these cations were injected. This suggested that the injection of such cations could not alter the silver surface electron status even if the cationic concentration was increased to 200 × 10-6 M, i.e., the present sensing system could selectively detect some anions (e.g., halides) but not inorganic cations. This is different from the findings of Henglein et al.18 on the colloidal silver nanoparticles, who demonstrated that the chemisorbed metal cations (e.g., Cd2+, Ni2+, or Hg2+) affected the plasma band of silver colloids coated by polymer stabilizers. Those authors suggested that the effect could be interpreted in terms of the donation of electron from the silver colloids to the adsorbed cations in the concentration range of 20-100 × 10-6 M, and different surface stabilizers could also affect the chemisorptions of ions, leading to different drifts in the plasma response of metal nanoparticles. 3.3. Sensing Mechanism. To understand the sensing mechanism, a few perturbations that affect SPR wavelength shifting need to be explored such as refractive index changes to the surroundings, particle geometry (shape and size), and particle surface electron charging.18,19 Electron injection into the metal particle solution can alter the metal plasma frequency, leading to a shift toward shorter wavelengths in the optical absorption. Electrons can be injected by simply adding chemical reducing agents to the colloidal solution or by immersing an electrode to transfer electrons to the particles in solution.18,19 In addition, the optical effects due to electron charging can be greatly enhanced by altering the particle morphology. The surface plasma modes in plate- and rod-shaped particles have higher oscillator strengths and are more sensitive to the surface perturbations than in spheres or colloids.19 It is expected according to the Mie theory21 that a change in the refractive index would lead solely to a shift in the SPR band and would not significantly influence the width of the band. This mechanism cannot completely explain the present experimental observations, in which the injected ions like F-, NO3-, ClO4-, CH3COO-, and SO42- ions did not lead

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to any change in the position of the SPR maximum. This means that the effect of the refractive index of the media surrounding silver particles on the SPR shift is not the main cause under the conditions considered. The remarkable shifts in the SPR band in detection of halide ions (e.g., Cl-, Br-, and I- ions) may result from other factors, e.g., the change in particle geometry or the particle surface electron charging. The impact of particle geometry on the shift in the SPR band was checked by using the TEM technique. The detailed characteristics of the as-prepared silver nanoplates including shape, size, and thickness have been demonstrated in our recent work.20a-c Herein, we focus on the particle shape and size before and after sensing detection of anions. Figure 6 shows the TEM images of silver nanoplates before (Figure 6A) and after sensing detection toward Cl- (Figure 6B), Br- (Figure 6C), and I- ions (Figure 6D), respectively. The concentration of the individual halide ions was fixed at 20 × 10-6 M in the solution mixture. Their corresponding morphology and size distributions were estimated and are shown in Figure 6. The TEM analysis suggests that the geometry of silver nanoplates used in the sensing detection was nearly the same before and after sensing detection, with the difference almost within the error of estimate. Therefore, the shift in the SPR band was not mainly caused by the changes in particle geometry. This is consistent with the observations of Henglein et al.,18 who found that the distinctive damping in the plasma band around 380 nm of silver colloids was caused by the electron transfer and adsorption/desorption on the silver colloids but not by changing the particles themselves, even when the concentration of I- ions was increased to 20 × 10-6 M. The above experimental observations suggested that the effects of the refractive index to the surroundings and the changes in particle geometry were not significant, so other factors (e.g., surface electron charging) may be responsible for the shift in the intensive SPR band. In aqueous solution, metal particles were probably cathodically or anodically charged resulting in electron density changes of up to 10-15%.19 The electron injection can alter the electron status (e.g., dielectric property) on the particle surface and hence the plasma frequency of metal particles, particularly for Au and Ag. However, seen from the UV-vis spectra (Figures 1 and 4), the silver nanoplates were highly sensitive to Cl-, Br-, and I- ions but not to F-, NO3-, ClO4-, CH3COO-, and SO42- ions, albeit they all are negatively charged. Interestingly, the concept of solubility of silver compounds was found to be useful in the explanation of the observed phenomena. The solubility constants (Ksp)24 of silver compounds listed in Table 1 determine thermodynamically the interaction tendency between the surface silver atoms and the injected anions. The difference in the interaction tendency to form silver compounds (e.g., AgCl, AgBr, and AgI) within an Ag-anion bond would result in different surface electron charging or affect the electron mean free path and hence the shift in the plasma frequencies. It was worth to note that the Ksp order of silver compounds in water is F- > Cl- > Br- > I-, particularly the Ksp, AgF is at least 12 orders of magnitude (182 g/100 mL water at 15 °C) larger than others (e.g., Ksp, AgCl ) 1.76 × 10-10, Ksp, -13, and K -17) in the same AgBr ) 5.32 × 10 sp, AgI ) 8.49 × 10 group. The smaller the Ksp, the stronger the interaction tendency between the silver surface atoms and the injected anions, so Fis the weakest among the four. This means that the surface electron charging is readily achieved for Cl-, Br-, and I- ions but not for F- ions under similar conditions. Such a proposed mechanism (24) (24) Handbook of Chemistry & Physics, 64th ed.; CRC Press: Boca Raton, 1983-1984

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Figure 6. TEM images and the corresponding shape/size distributions before (A, A′, and A′′) and after sensing detection toward Cl- (B, B′, and B′′), Br- (C, C′ and C′′), and I- ions (D, D′ and D′′). Table 1. The Solubility of Silver Compounds in Water

a

silver compound

Ksp (conditions)a

AgF AgCl AgBr AgI AgSCN Ag3PO4 AgNO3 AgClO4 Ag(CH3COO) Ag2SO4

182 g/100 mL water (15.5 °C) 1.76 × 10-10 5.32 × 10-13 8.49 × 10-17 1.03 × 10-12 1.05 × 10-16 122 g/100 mL water 5.57 g/100 mL water 1.02 g/100 mL water 1.19 × 10-5

From ref 24.

could also be supported by the anions, such as NO3-, ClO4-, CH3COO-, and SO42-, which could not lead to any significant shifts in the SPR due to their relatively larger solubility (Table 1) than that of the AgX (X ) Cl-, Br-, and I-) in water, i.e., the interaction tendency between the silver atoms and the anions plays a dominant role in the surface electron charging and subsequently affect the shifts in the intensive plasma frequency.

To further verify the role of the interaction tendency played in the surface electron charging, two additional anions (i.e., PO43and SCN-) were investigated in this study. These two inorganic anions were chosen because both of them have a strong interaction tendency in forming silver compounds, and their corresponding sodium salts easily dissolve in water. Seen from Table 1, the solubility constants (Ksp) of Ag3PO4 and AgSCN are 1.05 × 10-16 and 1.03 × 10-12, respectively. Figure 7A shows that the intensive SPR band shifted toward shorter wavelengths from 769 to 765, 654, 568, and 536 nm when the concentration of the initial PO43- ions was increased to 4, 8, 12, and 16 × 10-6 M, respectively. The injection of a small amount of PO43- ions could lead to a slight change in the pH value from 5.5 to 6.5. This is because these PO43- ions can hydrolyze to form HPO42and then H2PO4- ions (weak acid) based on the dissociation constants of phosphoric acid, i.e., pKa1 ) 2.12, pKa2 ) 7.21, and pKa3 ) 12.67 values at 25 °C,24 but no H3PO4 is formed in such a weak acid solution (pH ∼6.5). Therefore, the detected anions in such a system are H2PO4- rather than PO43- ions themselves

SilVer Nanoplates

Figure 7. UV-vis spectra showing that the silver nanoplates can be used for sensing detection of inorganic anions: (A) H2PO4- and (B) SCN- ions.

Figure 8. UV-vis spectra showing that the sensitivity of silver nanoplates toward halide anions (e.g., Cl- ions) is largely reduced when the silver atomic surfaces were modified by molecules: (A) C12SH and (B) cysteine.

under the current conditions. However, the Ksp (1.05 × 10-16) of Ag3PO4 was still used in the explanation of the sensing mechanism because the final form is Ag3PO4 but not Ag2HPO4 or AgH2PO4, no matter what kind of anions (i.e., PO43-, HPO42-, or H2PO4- ions) are used to react with Ag+ ions.24 Figure 7B shows that the silver nanoplates also exhibit high sensitivity in detection of SCN- ions. The intensive SPR band red-shifted from 769 to 781 first and then conversely blue-shifted to 720, 616, and 574 nm, corresponding to the SCN- ions concentration of 4, 8, 12, and 16 × 10-6 M, respectively. Here, a slight shift from 769 to 781 nm occurred in the SPR band when the concentration of SCN- ions was very small (4 × 10-6 M), but increasing the SCN- concentration (g8 × 10-6 M) resulted in a shift toward shorter wavelengths, as shown in Figure 7B. Such shifts in the SPR band suggested that the surface electron density decreases at the initial stage and then the surface electron charging becomes dominant when the content of SCN- ions is

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increased. Detailed investigation on this issue is still in progress. Nonetheless, to confirm the proposed sensing mechanism, as mentioned above, the shape and size distributions of the silver nanoplates were analyzed based on TEM images, as shown in Figure S2 of Supporting Information. It was observed that the nanoplates did not have any obvious changes in shape and size before and after sensing detection of H2PO4- or SCN- ions. These results supported our proposed explanation on sensing mechanism that the interaction tendency between H2PO4- or SCN- ions and silver surface atoms dominated surface electron charging and subsequently resulted in the remarkable shift in SPR band. Further evidence regarding the surface electron charging was supported by adding thiols (e.g., C12SH) or cysteine to modify the silver nanoplates before sensing detection. Such sulfurcontaining molecules can interact with heavy metal ions or atoms (e.g., Au, Ag, Zn, Pb, etc.). Our recent work discussed the functionality of thiols to stabilize silver nanoplates, where the particle geometry could be maintained without change for months at room temperature in aqueous solution.20b-c In this work, we utilized such sulfur-containing molecules to modify silver surfaces for further examining the effect of surface electron charging on the shift in the SPR band. Figure 8 shows that the addition of anions (e.g., Cl-) could not lead to any shifts in the SPR once again when C12SH or cysteine was added into the sensing system. The unshifted SPR bands suggested that the surface modification could hinder the injected Cl- ions adsorbing onto the silver plate surface and hence reduce the effect on the surface electron density. Such a UV-vis analysis could support our proposed explanation in the sensing mechanism, i.e., the surface electron charging dependent on the interaction tendency between silver atoms and anions dominated the shift in the SPR band observed. 3.4. Discrimination of Mixed Anions. The aforementioned results demonstrated that the silver nanoplates show high sensitivity in detecting individual halides, phosphate, and thiocyanate ions. However, a real system such as polluted water or industrial effluents may be much more complicated, because it is commonly composed of mixed inorganic anions. To distinguish precisely the individual ones in such a system is of significant importance. However, this is beyond the scope of the present work. Nevertheless, as an extension of this work, we examined two complicated systems composed of mixed ions to identify the capability of silver nanoplates in discriminating individual ones in a mixture. For the first case, various halides were mixed as described in Section 2.4. Figure 9 shows the UV-vis spectra of silver nanoplates in distinguishing mixed halides in a mixture, in which the concentration was fixed for the individual halide ions at 4 × 10-6 M and for each of the other anions (e.g., NO3-, ClO4-, CH3COO-, and SO42-) at 20 × 10-6. The sensing systems (S4S7) show that only one intensive SPR band was observed in the UV-vis spectra. The remarkable shift in the intensive SPR band was probably caused by a synergetic effect of two or three halides, but it is hard to discriminate clearly between the individual halides in the absorption spectra observed. For example, the intensive SPR band in the S4 system shifted from 770 to 676 nm, which is different from that in either the individual Cl- (centered at ∼760 nm) or the individual Br- (centered at ∼701 nm) in a mixture, i.e., the SPR centered at 676 nm was originated from the synergetic effect of both Cl- and Br- ions but not the individual ones. Similarly, the shifts in the SPR band caused by the synergetic effect of mixed anions were observed in three other systems (S5-S7). The intensive SPR bands centered around 638, 602, and 566 nm are contributed by the synergetic effect from both

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Figure 9. Discrimination of mixed halides in a mixture using UV-vis spectra: (A) S4 (Cl- + Br- + B), (B) S5 (Cl- + I- + B), (C) S6 (Br- + I- + B), and (D) S7 (Cl- + Br- + I- + B), where B ) (F- + NO3- + ClO4- + CH3COO- + SO42-). Note that the individual concentration of halides was fixed at 4 × 10-6 M and the individual concentration of other anions was fixed at 20 × 10-6 M.

Figure 10. Discrimination of mixed H2PO4-, SCN-, and halides (e.g., Br-) in a mixture by UV-vis spectra: (A) S8 (H2PO4- + SCN+ B), (B) S9 (Br- + H2PO4- + B), (C) S10 (Br- + SCN- + B), and (D) S11 (Br- + H2PO4- + SCN- + B). Note that the individual concentration of Br-, H2PO4-, or SCN- was fixed at 4 × 10-6 M and the individual concentration of other anions was fixed at 20 × 10-6 M.

Cl- and I- ions (Figure 9B), both Br- and I- ions (Figure 9C), and mixed Cl-, Br-, and I- anions (Figure 9D), respectively. The role of the individual anions played in the SPR shift could not be clearly distinguished by such a sensing system. For the second one, the capability of such a sensing system in distinguishing mixed phosphate, thiocyanate, and halides (e.g., Br-) in a mixture (e.g., S8-S11) was examined. Figure 10 shows that only one intensive SPR band was observed when detecting the mixed anions in a mixture, in which the concentration was fixed for the individual H2PO4-, SCN-, and Br- ions at 4 × 10-6 M and for each of the other anions (e.g., NO3-, ClO4-, CH3COO-, and SO42-) at 20 × 10-6 . Similar to the observations shown in

Figure 9, the shift in the SPR band was probably caused by the synergetic effect of H2PO4-, SCN-, and halides but not the individual ones. Unfortunately, the extent or the ratios contributed by the individual anions in a mixture is difficult to estimate. For instance, the intensive SPR band of the S8 system shifted from 770 to 743 nm (Figure 10A), which is different from those for the mixtures containing individual H2PO4- (centered at 736 nm) or SCN- ions (centered at 780 nm). Apparently, the SPR centered at 743 nm should be attributed to both anions but not any individual ones. Similarly, the synergetic effect on the SPR shift was observed in the mixed systems (e.g., S9-S11), i.e., 666 nm for both Br- and H2PO4- (Figure 10B), 695 nm for both Br- and

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SCN- (Figure 10C), and 642 nm for the mixed H2PO4-, SCN-, and Br- ions (Figure 10D), respectively. The results suggested that the silver nanoplates could not discriminate quantitatively the mixed H2PO4-, SCN-, and halides in a mixture under the conditions considered. From the above two cases, it was found that the present sensing system, silver nanoplates, may have a limited capability in discriminating among individual anions in a mixture composed of mixed anions such as H2PO4-, SCN-, and halides because of the possible synergetic effect. Further investigation is necessary in order to understand the nature of this effect or the interference among the anions and to develop a sensing system for practical application.

4. Conclusions We have investigated the sensitivity, selectivity, and sensing mechanism of silver nanoplates in aqueous solution through a combined TEM and UV-vis spectral analysis. The findings can be summarized as follows: (i) Silver nanoplates show a high sensitivity on the order of 1 × 10-6 M in detecting individual anions such as halide, phosphate, and thiocyanate ions. The sensitivity could be identified by the shift occurred in the maximum surface plasma resonance (SPR) band in a UV-vis spectrum. The extent of the individual halides (e.g., Cl-, Br-, or I-), phosphate, or thioyanate ions can be estimated based on a simple linear relationship between the concentration and the shifted SPR. (ii) Such a sensing system could selectively discriminate the individual halides, phosphate, or thioyanate ions from others

such as F-, NO3-, ClO4-, CH3COO-, and SO42- ions and inorganic cations (e.g., Zn2+, Cd2+, Cu2+, etc.). However, the selectivity may have a limited capability in distinguishing precisely between the individual halides in a mixture. The synergetic effect or the interference among mixed anions that can affect the selectivity needs further investigation. (iii) The remarkable shift in SPR band correlating to the sensitivity and selectivity of silver nanoplates is dominated by particle surface electron charging but not the refractive index of the surroundings and the changes in particle geometry. The interaction tendency between anions and silver surface atoms appears to be a useful concept to understand the surface electron charging mechanism in this study. The results would be useful for developing silver-plate-based chemical sensors. Acknowledgment. We gratefully acknowledge the financial support of the Australia Research Council (ARC) through the ARC Centers of Excellence for Functional Nanomaterials, Natural Science Foundation of China (NSF50671019), and China Postdoctoral Science Foundation (No. 2005038252). X.J. gratefully thanks Dr. W. Yang (University of NSW) for useful discussion. Supporting Information Available: UV-vis spectra of silver nanoplates with higher concentrations of halide ions; TEM images and the corresponding shape and size distributions of silver nanoplates used to sensing detection of individual H2PO4- and SCN- ions. This material is available free of charge via the Internet at http://pubs.acs.org. LA7032252