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The Effects of Mercury Adsorption on the Optical Response of Size-Selected Gold and Silver Nanoparticles Todd Morris, Hollie Copeland,† Emily McLinden,‡ Scott Wilson, and Greg Szulczewski* The University of Alabama, Department of Chemistry, 6th Avenue Lloyd Hall, Tuscaloosa, Alabama 35487 Received March 6, 2002. In Final Form: June 14, 2002 We have studied the adsorption of Hg atoms onto gold and silver nanoparticles by optical spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis. Mercury adsorption induces a blue shift of the surface plasmon mode of gold and silver nanoparticles. Two trends are observed in the UV-vis spectra. First, silver particles experience a larger blue shift than gold particles (given the same particle size and Hg surface coverage). Second, smaller Au and Ag particles experience a greater blue shift than larger particles. We have calculated the UV-vis absorption spectra using a core/Hg(shell) model and found good agreement between the experimental results and theoretical calculations. The results suggest the basis for a novel colorimetric assay for Hg.
Introduction Many studies have established that chemisorption on a nanoparticle1-5 or thin film6,7 surface can lead to dramatic changes in optical properties. For example, when a submonolayer of lead is deposited on colloidal gold particles, a color change is readily discerned with the naked eye.1 Mercury ions have been radiolytically reduced on Ag8 and Au9 particles and caused a blue shift in the surface plasmon wavelength. We have recently shown that sub-monolayer coverages of Hg can be detected on ∼50 nm gold films by surface plasmon resonance spectroscopy.6 In addition, Butler et al. have shown that the optical reflectivity of thin gold films (∼10-40 nm) changes with Hg adsorption.7 From a scientific perspective, we wanted to understand how particle size influences the optical spectra of silver and gold nanoparticles upon Hg adsorption. From a practical point of view, we wanted to explore if the optical changes could be exploited to develop a fast, sensitive, and inexpensive colorimetric assay for Hg. The optical response of colloidal gold and silver to electromagnetic radiation is well understood.10-13 Colloidal gold and silver particles exhibit characteristic absorption peaks near 520 and 400 nm, respectively, due to the sur* Corresponding author: e-mail,
[email protected]; phone, 205 348-0610; fax, 205 348-9104. † Current address: Southern Ionics, West Point, MS. ‡ 2001 NSF REU Summer Student. (1) Michaelis, M.; Henglein, A.; Mulvaney, P. J. Phys. Chem. 1994, 98, 6212-6215. (2) Henglein, A.; Giersig, M. J. Phys. Chem. 1994, 98, 6931-6935. (3) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679-682. (4) Henglein, A.; Mulvaney, P.; Linnert, T.; Holzwarth, A. J. Phys. Chem. 1992, 96, 2411-2414. (5) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1992, 96, 10419-10424. (6) Morris, T.; Szulczewski, G. Langmuir 2002, 18, 2260-2264. (7) Butler, M. A.; Ricco, A. J.; Baughman, R. J. J. Appl. Phys. 1990, 67, 4320-4326. (8) Katsikas, L.; Gutierrez, M.; Henglein, A. J. Phys. Chem. 1996, 100, 11203-11206. (9) Henglein, A.; Giersig, M. J. Phys. Chem. B 2000, 104, 50565060. (10) Mulvaney, P. Langmuir 1996, 12, 788-800. (11) Optical Properties of Metal Clusters; Kreibig, U., Vollmer, M., Eds.; Springer-Verlag: New York, 1995. (12) Henglein, A. J. Phys. Chem. 1993, 97, 5457-5471. (13) Absorption and Scattering of Light by Small Particles; Bohren, C. F., Huffman, D. R., Eds.; Wiley: New York, 1983.
face plasmon oscillation. Mie was the first to provide a theoretical explanation of the color of colloidal gold.14 Mie solved Maxwell’s equations for the interaction of electromagnetic radiation and a sphere with a diameter much smaller than the wavelength of the exciting radiation. Experimentally it has been observed that the surface plasmon oscillation of gold nanoparticles red shifts slightly from ∼520 to 530 nm as the particle diameter increases from ∼5 to 40 nm. Consequently, there have been modifications of Mie’s theory to account for size effects.15 Unfortunately most colloidal syntheses produce a large distribution of particle sizes. In the middle 1990s several groups reported new synthetic methods to prepare and isolate narrow size distributions of gold nanoparticles.16-18 This was a significant advance in nanoparticle synthesis because sizedependent optical properties could be measured.19 These methods exploit the reduction of a metal salt in the presence of an alkanethiol. The synthesis produces a metal core (usually Au or Ag) capped with a monolayer of alkanethiol bound through a metal-sulfur bond. An attractive property of these materials is the ability to suspend the particles in organic solvents and exchange the alkanethiol ligands.20 In the late 1990s several groups introduced the synthesis of dendrimer-stabilized nanoparticles.21-23 These nanocomposites have many attractive properties including tunable solubility, enhanced stability against aggregation, and a built-in “nanofilter”. (14) Mie, G. Ann. Phys. 1908, 25, 377-445. (15) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678-700. (16) (a) Brust, M.; Walker, M.; Bethell, D.; Schriffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (17) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchinson, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S. Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548. (18) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, M. M.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428-433. (19) (a) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706-3712. (b) Schaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutie´rrez-Wing, C.; Ascensio, J.; JoseYacama´n, M. J. J. Phys. Chem. B 1997, 101, 7885-7891. (20) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36.
10.1021/la020229n CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002
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In this Letter we have prepared different size Au and Ag nanoparticles and exposed them to Hg atoms. The changes in the optical absorption spectra suggest the novel basis for a colorimetric assay for Hg. Experimental Section All purchased reagents (Aldrich) were of the highest purity available and used without further purification. All glassware was thoroughly cleaned with aqua regia and rinsed with filtered, deionized water (i.e., >18 MΩ). Gold nanoparticles with an average diameter of 5 nm were made by reduction of HAuCl4 with NaBH4.24 Gold particles with an average diameter of ∼15 and ∼25 nm were synthesized by reduction of HAuCl4 with sodium citrate.25 Colloidal silver was prepared by reducing AgNO3 with NaBH4 according to the method of Lee and Meisel.26 Amineterminated G4-PAMAM dendrimer-gold nanocomposites were prepared by the methods published by Crooks.21 We used the procedure outlined by Zheng et al. to make hydroxyl-terminated G4-PAMAM dendrimer-silver nanocomposites.27 The average diameter of the colloidal particles was determined from transmission electron microscopy (TEM) images. The average diameter of the dendrimer-stabilized particles was estimated from the peak wavelength of the surface plasmon oscillation. We have set up a simple apparatus to expose the nanoparticles to mercury vapor. Dry nitrogen gas was passed over a few grams of 99.999% Hg held in a two-neck round-bottom flask at room temperature. The mercury-saturated nitrogen gas (∼15 ppm Hg) was introduced at ∼60 mL/s through 10 mL of a colloidal solution. This produces a saturated solution of elemental mercury in water (∼60 ppb).28 A 1 mL aliquot of the solution was removed at various times, and an absorption spectrum was acquired with a Varian Cary 50 UV-vis spectrometer. The colloidal nanoparticles were collected on Nucleopore polycarbonate membranes (30 nm diameter pores) by vacuum filtration for X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) measurements. XPS measurements were made with a Kratos Axis 165 spectrometer using monochromatic Al KR radiation at 1486.7 eV and an 80 eV pass energy. EDS measurements were made with a JEOL model JXA-8600. We used the Shirley background subtraction29 algorithm and atomic sensitivity factors from Kratos to determine the atomic composition from the Au(4f) and Hg(4f) peak areas. We used a Hitachi H-8000 transmission electron microscope operating at 200 keV to image the 5 nm Au particles. A Hitachi H-7000 transmission electron microscope operating at 75 keV was used to image all other particles. About 100 particles were measured to determine the particle size distribution. We have calculated the probability of light absorption by a homogeneous sphere and two concentric spheres (i.e., core/shell structure) in water using a published program.30 The program requires the radii and optical constants of the sphere(s) as a function of wavelength as input. We used the bulk optical constants for Au, Ag, and Hg.31 The extinction coefficients were determined at 1 nm intervals from 300 to 700 nm.
Results and Discussion Figure 1 shows representative UV-vis spectra of gold particles with an average diameter of 5 and 15 nm before and after exposure to mercury. These observations are (21) (a) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877-4878. (b) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256-258. (c) Cchechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364-6369. (22) (a) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355-7356. (b) Balogh, L.; Valluzzi, R.; Laverdure, K. S.; Gido, S. P.; Hagnauer, G. L.; Tomalia, D. A. J. Nanopart. Res. 1999, 1, 353-368. (23) Esumi, K.; Suzuki, A.; Aihara, N.; Ushi, A.; Torigoe, K. Langmuir 1998, 14, 3157-3159. (24) Colloidal Gold: Principles, Methods, and Application, Volume 1; Hayat, M. A., Ed.; Academic Press: San Diego, CA, Chapter 1, 1989. (25) Frens, G. Science 1973, 241, 20-22. (26) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391-3395. (27) Zheng, J.; Stevenson, M. S.; Hikida, R. S.; Van Patten, P. G. J. Phys. Chem. B 2002, 106, 1252-1255. (28) Clever, H. L.; Johnson, S. A.; Derrick, M. E. J. Phys. Chem. Ref. Data 1985, 14, 631-680.
Figure 1. Representative UV-vis absorption spectra of colloidal gold particles before and after Hg exposure. The spectra were recorded every 15 min.
reproducible and have been observed in dozens of experiments. For each particle size there is a systematic blue shift of the plasmon mode and decrease in absorbance at the peak. The color change is obvious to the naked eye for the 5 nm Au particles (the solution changes from red to orange). We conducted several control experiments to verify that the blue shift was due to mercury adsorption on the gold nanoparticles. First, after mercury vapor was bubbled through pure water, no absorption peak at near 520 nm was observed. Second, a test tube containing ∼1 g of Hg and another test tube containing 1 mL of a colloidal gold solution were placed in a sealed jar. After 72 h the surface plasmon peak blue shifted ∼5 nm. The long exposure time was required by the limited mass transfer rate of Hg vapor into the solution. Third, when 0.1 mL of a colloidal gold solution was added to 1 mL of Hg saturated water (∼60 ppb), a 5 nm blue shift was observed. Finally, pure nitrogen gas was bubbled for 1 h through the colloidal solutions that exhibited the maximum blue shift (to expel dissolved Hg from the colloidal solution), and the spectra did not change. We used TEM to examine the particles before and after mercury exposure. There was no statistical difference in the average particle diameter or size distribution (both the TEM images and particle-size histograms are shown in the Supporting Information). We used XPS and EDS to measure the surface and bulk composition of the particles, respectively.The mean free path of Hg(4f) (29) Shirley, D. A. Phys. Rev. B 1972, 5, 4709-4714. (30) Fortran program in Appendix of ref 13. (31) Palik, E. D. Handbook of Optical Constants of Solids II; Academic Press: Boston, MA, 1991.
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Table 1. Atomic Composition of Colloidal Particles after Saturation of Blue Shift in Surface Plasmon Wavelength av particle diameter
atom % Hg by EDSa
calcd ML of Hg (eq 1)
5 nm Au 15 nm Au 25 nm Au 51 nm Ag
6.9 ( 0.6 3.1 ( 0.4 2.6 ( 0.4 1.7 ( 0.4
0.3 0.3 0.5 0.9
a The error bar represents the standard deviation determined from 5 to 7 measurements.
Figure 3. Experimental UV-vis absorption spectra of colloidal Ag nanoparticles (∼51 nm in diameter) before and after saturation of the surface plasmon mode. The calculation is for a 51 nm particle covered by 1 monolayer of Hg.
throughout the particle, then the atomic composition determined by XPS and EDS should be the same within experimental error. Using a core/shell model,9 we can estimate the thickness of the mercury shell (in terms of a monolayer of Hg) using the equation
Figure 2. Calculated UV-vis absorption spectra of Au(core)/ Hg(shell) particles: (a) 0; (b) 0.1; (c) 0.2; (d) 0.3 nm thick Hg shell.
photoelectrons in gold films and Au(4f) photoelectrons has been measured to be ∼1 and 4.1 nm, respectively.32,33 The sampling depth for Au (i.e., approximately three times the mean free path) is about 12 nm. This means that the Hg:Au atomic ratio determined by XPS should remain constant for particles greater than 12 nm if less than 1 monolayer (ML) of Hg is adsorbed on the surface.34 In fact, for all the particles measured in this work, the Hg content with respect to Au (or Ag) was ∼10-15 atomic % Hg. In contrast, the EDS measurements always revealed a smaller Hg content than XPS. The XPS and EDS data (representative spectra are provided in the Supporting Information) indicate that the surfaces of the particles are Hg-rich. If mercury formed a homogeneous alloy (32) Brundle, C. R.; Roberts, M. W. Chem. Phys. Lett. 1973, 18, 380381. (33) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 16701673. (34) We vapor deposited 5, 12, and 25 nm films of Au on Cr-primed Si(100) surfaces. These films were exposed to ∼15 ppm Hg vapor in air for 60 min and analyzed by XPS. Each film contained ∼14% mercury.
[(
MLHg ) rcore/dHg 1 +
VMHg[Hg]
) ]
VMcore[core]
1/3
-1
(1)
where rcore is the radius of the core particle, dHg is the diameter of a mercury atom, VM is molar volume, and [core] and [Hg] are the atomic concentrations measured by EDS. Table 1 shows the EDS data and calculated Hg shell thickness. In all cases we calculate that less than 1 monolayer of Hg is adsorbed on the surface. Consequently, we calculated the absorption spectra using the core/shell model for the Au colloids and show the results in Figure 2. The calculations reproduce the two trends observed in the experimental data. First, the calculations predict a blue shift and decrease in the intensity of the plasmon mode. Second, the calculations predict that the smaller particles produce a greater blue shift than the larger particles for the same Hg shell thickness. We also prepared colloidal Ag particles and exposed them to Hg. The UV-vis spectra for ∼51 nm colloidal Ag particles before and after Hg exposure are shown in Figure 3 along with a calculation of a core/shell structure. An interesting observation is that the higher multiple oscillation35 observed near 360 nm for the Ag particles (35) Chu, L.; Wang, S. J. Opt. Soc. Am. B 1985, 2, 950-955.
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than larger particles. Also, given the same particle size and Hg coverage, Ag particles blue shift more than Au particles. We also comment on the stability of the particles since it will be a practical consideration if a colorimetric assay can be developed. The colloidal particles were stable for at least 2 weeks after exposure to air. In contrast, Henglein and Giersig reported that 46 nm Au particles covered by thick Hg layers (i.e., >2 nm) were unstable when exposed to air. They observed that over the course of hours to days the plasmon mode became characteristic of colloidal gold. They observed a much faster degradation of the adsorbed Hg by adding excess Hg2+ in solution. It is important to remember that Henglein produced Hg atoms in situ by radiolytic reduction of Hg(ClO4)2 in 2-propanol. In contrast, our solutions are exposed directly to Hg atoms. We attribute the enhanced stability of our particles to the absence of an oxidant at high concentration. Furthermore our particles have a sub-monolayer of Hg on the surface, which presumably has not significantly perturbed the double layer necessary to maintain colloidal stability. All the particles we studied were stable in Hg-free water or Hg-saturated water. Conclusions
Figure 4. Representative UV-vis spectra of dendrimerstabilized nanoparticles before and after Hg exposure.
disappears after Hg adsorption. We currently do not fully understand this effect but believe it is related to a change in the polarization of the near surface region (mercury is known to lower the work function of Au and Ag surfaces by ∼0.3 eV36). In Figure 4 we show the UV-vis spectra for the dendrimer-stabilized Au (∼3 nm in diameter) and Ag (∼6 nm in diameter) nanoparticles before and after Hg exposure. The dendrimer-stabilized Au particles blue shifted ∼30 nm after 7 h of mercury exposure and showed an increase in the intensity of the plasmon mode (due to evaporation of the methanol used in the synthesis). The dendrimer-stabilized Ag particles blue shift more than the 51 nm colloidal particles. The results of calculations (shown in Supporting Information) predict that Ag follows the same trend seen for the Au particles, namely, smaller particles blue shift more (for the same coverage of Hg) (36) Bajpai, R. P.; Kita, H.; Azuma, K. Jpn. J. Appl. Phys. 1976, 15, 2083-2086.
The adsorption of Hg on Au and Ag nanoparticles results in a blue shift of the surface plasmon mode. The magnitude of the blue shift is more pronounced for smaller particles. The elemental composition determined from XPS and EDS suggests that mercury resides on the gold surface and does not form an amalgam. Simulations of the UV-vis spectra using a core/Hg(shell) model reproduce the experimental results. Acknowledgment. G.S. thanks the School of Mines and Energy Development at The University of Alabama for support of this work and the National Science Foundation for use of shared instrumentation through Materials Research Science and Engineering Center Grant #DMR-98-09423. We thank Dr. Mike Bersch for performing the EDS measurements, Mark Tomich for writing the Fortran program to calculate the UV-vis spectra, and Dr. Mohammad Shamsuzzoha and Ms. Mi-Kyoung Park for obtaining the TEM images. T.M. thanks The University of Alabama Graduate Council for providing full support in the form of a fellowship. E.M. thanks the National Science Foundation for support through the Research Experience for Undergraduates Grant #CHE-9987889. Supporting Information Available: Representative EDS and XPS spectra of the 15 nm colloidal Au particles and TEM micrographs and particle size distributions. This material is available free of charge via the Internet at http://pubs.acs.org. LA020229N
