Role of Substrate Metal in Gold Nanoparticle Enhanced Surface

B , 2001, 105 (1), pp 8–12 ... The width and the efficiency of SPR for both substrates are affected by the Au particles. .... The Journal of Physica...
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J. Phys. Chem. B 2001, 105, 8-12

Role of Substrate Metal in Gold Nanoparticle Enhanced Surface Plasmon Resonance Imaging E. Hutter,† S. Cha,‡ J-F. Liu,† J. Park,‡ J. Yi,‡ J. H. Fendler,*,†,§ and D. Roy*,|,⊥ Center for AdVanced Materials Processing and Department of Physics, Clarkson UniVersity, Potsdam, New York 13699, and EnVironmental Materials and Process Laboratory, School of Chemical Engineering, Seoul National UniVersity, Seoul 151-744, South Korea ReceiVed: September 29, 2000; In Final Form: NoVember 16, 2000

Using Au nanoparticles, it is possible under certain experimental conditions to considerably enhance the sensitivity of a conventional surface plasmon resonance (SPR) device. In this report, we examine the mechanism of this enhancement and discuss the experimental factors that are crucial to the performance of such a nanoparticle based SPR device. Among these factors, the surface plasmon-supporting metal substrate plays a major role. We demonstrate this by comparing experimental SPR data for Au and Ag substrates. In both cases, ∼25-30 nm diameter Au particles are attached to the substrate metal via a sandwiched monolayer of 1,6-hexanedithiol. The width and the efficiency of SPR for both substrates are affected by the Au particles. However, the particle-induced shift in the SPR angle is observed only for the Au substrate. This observation is explained in terms of competitive effects of propagating surface plasmons in the substrate metal and localized surface plasmons in the Au nanoparticles.

In recent years, the SPR imaging technique has been successfully employed for immunosensing and for thickness measurements of ultrathin (2-20 Å) films.1-3 In a typical SPR device, the sample layer is adsorbed onto a metal film, and the total attenuated reflection (ATR) efficiency, R, of the metal is measured as a function of the incidence angle, θ, of a probe light beam. The behavior of R is determined by the propagating surface plasmon polariton (SPP) modes at the metal/sample interface. In the absence of the sample, the R-θ plot (SPR plot) exhibits a dip at the angle, θp, where the condition for SPR (resonance in SPP oscillations) is satisfied. In the presence of the sample layer(s), θp shifts to a new value, and this shift is analyzed to determine the thickness (amount) and/or the dielectric function (chemical properties) of the sample.4,5 Usually, the width, position and height of the SPR plot, as well as the fractional reflectivity change, (∆R/R)p, at the SPR angle are also modified in the presence of the adsorbed sample. These latter changes can provide additional information about the sample. However, often the sensitivity of the SPR device is limited by small shifts in θp and (∆R/R)p. An effort to overcome the limitations of conventional SPR imaging have been recently reported by Lyon et al.6,7 The discussion of our present communication focuses on the SPR device proposed by these authors. They use a gold nanoparticle-modified device, where an organic self-assembled monolayer (SAM) is sandwiched between a gold film and a layer of colloidal gold particles (in the 10-60 nm diameter range). The experimental sample is adsorbed on top on to the Au nanoparticles. The Au particles enhance the magnitude of ∆θp, and at the same time, introduce an additional change in (∆R/R)p. These particles have their own * Corresponding authors. † Center for Advanced Materials Processing, Clarkson University, Box 5814. ‡ Seoul National University. § E-mail: [email protected]. | Department of Physics, Clarkson University, Box 5820. ⊥ E-mail: [email protected].

localized surface plasmons (LSP) due to collective oscillations of their conduction electrons. It has been suggested that SPP(substrate film)-LSP(Au nanoparticles) interactions are responsible for the above-mentioned enhancement in the sensitivity of the Au nanoparticle based SPR device.6,7 To our knowledge, however, no further explanation of this enhancement mechanism has been reported so far. Therefore, it is difficult at this time to identify the relevant experimental parameters that would be necessary to systematically design such high resolution SPR instruments. The amount of the currently available experimental data is also limited in this regard.6,7 In particular, no experiments have been reported to date investigating the role of the SPPsupporting substrate film. In this communication, we outline certain essential features of the SPP-LSP interactions, and examine the basic working principle of a nanoparticle based SPR device. We also identify the main parameters that affect the performance of such a device. Subsequently, we present preliminary results, demonstrating how the choice of the SPPsupporting metal film can be crucial to the design of such a device. We consider three different multilayer systems (in the Kretschmann configuration)5 labeled as systems (0125), (01235), and (012345). Here, 0 ≡ a BK7 glass slide brought into optical contact with the base of a 90° glass prism by an index matching oil; 1 ≡ a metal layer (Cr, in this case) that binds the SPPsupporting metal film to the prism; 2 ≡ the SPP-supporting metal film; 3 ≡ a self-assembled monolayer (SAM) of 1,6hexanedithiol (HDT); 4 ≡ a layer of nearly spherical, highly monodispersed Au nanoparticles; 5 ≡ air. The geometry of such a system is described with Cartesian coordinates, where the x-axis is along the intersection of the plane of optical incidence and the (0-1) interface, and the z axis is pointed toward the subsequent interfaces. The x-component of the incident photon wave vector is written as kx ) ω ˜ x0 sin θ, where ω ˜ ) ω/c, and ω is the circular frequency of the incident light. Here, c and 0 are the velocity of light and the dielectric function of

10.1021/jp003565q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/09/2000

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J. Phys. Chem. B, Vol. 105, No. 1, 2001 9

the prism (medium 0), respectively. In the mth layer, 0 is replaced by m, and m ) ′m + i′′m; i ) x-1, ′m and ′′m are the real and imaginary parts of the complex dielectric function, m, of the mth layer. The thicknesses of layers 1, 2, and 3 are denoted as d1, d2 and d3, respectively. These finite widths of the layers introduce the phase factors, exp(-2iβm) (with m ) 1-3), in the Fresnel eq for the multilayer system. If the z-component of the photon wave vector in the mth medium is denoted as kmz, then βm ) kmzdm. Let us first consider the (0125) system. In the ATR coupling scheme, kx ) Re(ksp), where ksp is the complex SPP wavenumber, and ksp ) Re[ksp] + i Im[ksp]. Re[ksp] and Im[ksp] are the real and imaginary parts of ksp, respectively. Assuming that exp(-2iβ1) ≈ 1 (for very small d1), and ksp ) k0sp + ∆ksp, and with certain additional assumptions as described in ref 4, it is possible to write the reflectivity of this system in the following Lorentzian form:4,5,8

R(0125) ≈ 1 -

4ΓinΓrad (kx - Γr)2 + (Γi)2

(1)

where Γr ) (Γ* + Γ+), and Γi ) (Γin + Γrad). Labeling the real and imaginary quantities with Re and Im, respectively, Γ+ ) (01) Re[∆k(12) sp ], Γrad ) Im[∆ksp ],

Γ* ) Re[k0sp] + Re[∆k(01) sp ]

(2)

Γin ) Im[k0sp] + Im[∆k(12) sp ]

(3)

Here, Γin is the internal damping of SPR, affected by the 1-2 interface, and Γrad represents an additional damping associated with the 0-1 interface. The term k0sp satisfies the surface plasmon dispersion relation,

k0sp

)ω ˜ [(25)/(2 + 5)]

1/2

(4)

at the interface of two-half spaces, defined by the media 2 and 0 5. ∆k(01) sp is the component of change in ksp due to the presence (12) of the 0-1 interface. ∆ksp is the corresponding change of k0sp (12) due to the presence of the 1-2 interface. Both ∆k(01) sp and ksp are independent of θ, and using Maxwell’s equations, can be expressed in terms of ω ˜ , m (with m ) 0, 1, 2, 5), and d2. Chen and Chen have presented such a calculation for Cs and Cs-O covered Ag surfaces.9 With relatively straightforward modifications, their calculations can be adapted to evaluate the ∆ksp(12) terms considered above (∆k(01) sp and ∆ksp introduced in this R work are analogous to the terms, K and KT, of these authors’ report, respectively). Next, consider the (01235) system. R(01235) would have a form similar to eq 1, where the old Γ-factors would be replaced by a set of new Γ*′, Γ′in, Γ+′, and Γ′rad, that result from the introduction of layer 3. These Γ′-parameters would depend on the old Γ-parameters, as well as on 3 and d3. In other words, (35) 0 new changes, ∆k(23) sp and ∆ksp , in the value of ksp would be introduced by the 2-3 and 3-5 interfaces, and these changes would be included in the new Γ′-parameters. Analytical expressions for these quantities for a four layer system are given in refs 4 and 9, and similar equations for a general N-layer system can be developed by following the derivation presented there.8 Neglecting the possible anisotropy of layer 3, its dielectric function, 3, can be expressed in the effective medium approximation (EMA) as follows10

fs(s - 3) fHDT(3 - HDT) ) s + 23 HDT - 23

(5)

Here, the subscript, “s”, denotes the medium surrounding the adsorbed HDT molecules; fs and fHDT represent the volume fractions of the surrounding material and the adsorbed HDT in layer 3. The dielectric function of HDT (in the presence of an underlying substrate) is denoted as HDT. Now consider the (012345) system. Here again, R(012345) can be expressed in a form similar to eq 1. In this new equation (not shown), the Γ-parameters would be replaced by another new set of Γ-parameters, denoted as Γ*′′, Γ′′in, Γ+′, and Γ′′rad. These latter parameters would be functions of their corresponding Γ′-parameters, as well as of 4 and d4 of the Au particles layer. The optical properties of this layer 4 are relatively more complex than those of the other layers. However, the main effects of this layer can be qualitatively noted by using the above-mentioned EMA. Assuming that the void space in the particles layer is partially hydrated and contains air, the dielectric function, 4, of this layer in the EMA is written as

np + 24 fnp(4 - np) ) v + 24 fv(v - 4)

(6)

where np and v are the effective dielectric functions of the Au nanoparticles and the void space in layer 4, respectively. The volume fractions of the void space and the Au nanoparticles are denoted as fv and fnp, respectively. Assuming that radiation damping of LSP is small, np can be expressed as11

np ) (1 + bAu) - pl

(7)

pl ) [ω02(ω - iγ)]/[ω(ω2 + γ2)]

(8)

where γ characterizes the width of the LSP resonance. In eq 7, bAu, and ω0 are the bulk dielectric function and the bulk plasmon frequency of Au, respectively. The LSP contribution to the dielectric function of the Au particles is denoted as pl. During their localized oscillation, the conduction electrons of these particles collide with the particle-boundary. The term γ arises due to the effects of these collisions. For spherical particles of diameter, d, this γ can be expressed as γ ) Vf[lf-1 + (d/ 2)-1], where lf and Vf are the mean free path and the Fermi velocity of electrons in Au, respectively. Equations 1-8 provide a framework for an over all description of the expected SPR features of the three multilayer systems considered in this work. According to eq 1, R(0125) passes through a minimum when kx ≡ kpx ) Γ* + Γ+. The incidence angle, θ, corresponding to this minimum is written as

˜ x0] θ(SPR) ) θp ) sin-1 [kpx /ω

(9)

The term, (Γin + Γrad), in eq 1 defines the width of the SPR plot. Furthermore, R(θp) ≈ 0, when the condition, Γin ≈ Γrad, is also satisfied in eq 1. The last condition determines the magnitude of (∆R/R)p (efficiency of SPR). These same features of SPR are predicted for the (01235) system. In the latter case; however, both the width and the efficiency of SPR would change due to the new Γ′in and Γ′rad. Similarly, Γ+′ and Γ*′ would change the value of kpx , thus causing a new shift in θp. These factors are determined by the chemical nature, surface coverage and orientation of the SAM (layer 3) used in the SPR device. For the (012345) system, the width and the efficiency of SPR would be controlled by Γ′rad and Γ′in, and θp would be determined

10 J. Phys. Chem. B, Vol. 105, No. 1, 2001 by Γ*′′ and Γ+′′. R(012345) is related to 4, and as shown in eqs 6 and 7, 4 depends on pl. The LSP oscillations in the Au nanoparticles affect this pl, and thus, through the 4 dependence of R(012345), interfere with the SPP of layer 2. For very small particles, γ becomes very large and then, according to eq 8, the LSP contribution (pl) to the dielectric function of the Au particle becomes negligible. Thus, a minimum particle size is necessary to activate any appreciable LSP effects. To reach this threshold, the diameter of the Au particle should be at least of the same order of magnitude as lf (∼10 nm or larger). The LSP resonance in the Au particle occurs when the electric field near the particle becomes very large. For a spherical particle surrounded by air, this LSP resonance condition is met when Re np ≈ -2.11 As indicated in eq 8, this latter condition depends on both the size of the particle (through γ) and the incident photon frequency, ω. In addition, according to eq 6, the value of 4 and hence, the strength of the SPP-LSP interactions depends on the packing density of the Au particles in layer 4. The thickness (d3) of the SAM, separating the metal substrate and the Au particles is also expected to affect these interactions. Previously, a few experiments have been reported that address some of the above-mentioned issues of nanoparticle based SPR imaging. For instance, the effects of changing ω and the particlefilm distance have been studied for a “glass-Al-SiO2-Ag” system.12 The effects of particle size have been studied for a system involving a gold film (on glass), an organic layer and gold nanoparticles.7 Apart from the above-mentioned factors, the intrinsic SPP resonance characteristic of the substrate film plays an important role in determining the strength of the SPP-LSP interactions.12,13 If the SPP resonance of the substrate is strong compared to the LSP-perturbations, then the LSP-induced changes in one or more of the four Γ-parameters can be relatively small. On the other hand, if the SPP oscillations are heavily damped, and/or the LSP oscillations are near resonance, one would expect a strong signature of LSP in the observed SPR plot. No experiments addressing this particular issue have been reported to date. The experiments of our present work focus on this very aspect of the nanoparticle based device. The results of these experiments are presented next. We examine Au and Ag as the SPP-supporting metal in two separate sets of SPR experiments. SPR measurements were carried out on a home-constructed instrument, which is similar to those reported by Tao et al.14,15 and Lyon et al.16 Gold and silver films (50 ( 5 nm) were deposited on chromium coated (∼1 nm) piranha cleaned glass slides in an Edwards Vacuum Evaporator. These chromium-gold coated glass slides were used as the reflection element. The uncoated side of the slide was brought into optical contact with the base of a 90° glass prism (of refractive index 1.5151) by an index-matching oil (of refractive index 1.5180 ( 0.0005; R. P. Cargille Laboratories, Inc.). A p-polarized light from a HeNe laser, at the wavelength of 632.8 nm (Hughes, 3235H-PC, 20 mW) was directed to the base of the prism which was mounted on a stepping motor driven rotator (Oriel), capable of synchronously varying the angle of incidence, θ, and the direction of a large area silicon detector (Newport, 818-SL) with an angle resolution of 0.01°. Each 20 s scan produced a reflectivity vs incident angle curve. HDT (Fluka) and the gold nanoparticles17 were attached to the substrate by self-assembly.18 For each SPP-supporting film, SPR plots are recorded for the three systems, (0125), (01235), and (012345). The size and dispersion of the Au nanoparticles were estimated using an atomic force microscope (AFM). In the

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Figure 1. Reflectivity, R, of a gold film measured as a function of the incidence angle, θ of a probe laser beam in the ATR configuration. The symbols represent the measured reflectivities in air for the Au film (circles), for a layer of HDT deposited on the Au film (triangles), and for a layer of Au nanoparticles deposited on the Au film-HDT system (squares). The solid lines through the circles and the triangles are calculated fits to the experimental data obtained in the absence of the Au particles.

present work, the size distribution is typically centered around the 25-30 nm diameter range. The SPR data for the Au substrate (used as layer 2) are shown in Figure 1. The plot (denoted by the circles) for the system (0125) is collected first. Then the HDT layer is deposited on the same substrate, and the data indicated by the triangles are collected for the (01235) system. The squares represent the data obtained after the Au particles layer is deposited on this sample [system (012345)]. In Figure 1, only minor changes in the values of θp and (∆R/R)p are observed in going from the (0125) system to the (01235) system. The widths of the SPR plots for these two cases are also comparable. Thus, the presence of the HDT layer only weakly affects the SPR response of the Au substrate film. In the presence of the Au particles, however, θp, (∆R/R)p, and the SPR width - all three quantities change drastically. Thus, in view of the theoretical considerations discussed earlier in this report, all the four Γ′′-factors for the (012345) system in Figure 1 are different from their (Γ′- and) Γ-counterparts. This corresponds to a situation where the SPP resonance of the substrate film is damped enough to be affected by the LSP of the Au particles. This is the case of nanoparticle induced enhancement in the SPR device sensitivity. He and co-workers have discussed how the increased sensitivity of the Au nanoparticle-based SPR device can be characterized in terms of measurable quantities.19 It should be noted in this context that the internal sensitivity (figure of merit, Sint) and the detection sensitivity (Sdet) of the SPR device are traditionally defined as: 20,21 S ) (∆Γr/∆n )/Γi, and S int 3 det ) (∆R/∆n3). Here, n3 ) x3 (refractive index of layer 3), and Γr and Γi have been defined in the context of eq 1. Typically, to measure changes in these sensitivity parameters, one would measure changes of x3 in response to exposure to different solutions (medium 5).20,22 However, to apply this method to the Au-nanoparticle based SPR device, it would be necessary to account for possible changes in the interactions between layers 3 and 4 as the surrounding medium 5 is changed. Unless such interactions are properly characterized, it is difficult to measure the increased sensitivity of the nanoparticle based system in a quantitative manner by using the conventional definitions of Sint and Sdet. Using the known dielectric functions of BK7 glass, the Cr film,23 and the Au film,5 and fitting the data for the (0125) system (solid line through the circles in Figure 1) to the standard four-phase model of reflectivity (based on Fresnel equations),

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Figure 2. Reflectivity, R, of a silver film measured as a function of the incidence angle, θ of a probe laser beam in the ATR configuration. The symbols represent the measured reflectivities in air for the Ag film (circles), for a layer of HDT deposited on the Ag film (triangles), and for a layer of Au nanoparticles deposited on the Ag film-HDT system (squares). The solid lines through the circles and the triangles are calculated fits to the experimental data obtained in the absence of the Au particles.

we obtain the unknown quantities, d1 ) 1.3 nm, and d2 ) 43.2 nm. Using these parameters in a five-phase model, and by fitting this model (solid line through the triangles) to the data for the (01235) system, we obtain d3 ) 0.7 nm, ′3 ) 2.33, and ′′3 ) 0. The addition of the Au particles introduces the new unknowns, d4 and 4, where according to eqs 6 and 7, 4 is related to d4 via another unknown, fv. Moreover, due to interactions between the HDT and the Au particles, the orientation and hence, d3 of the HDT layer is expected to change after the deposition of these particles. The interactions between the HDT and the Au particle can also change electron distributions in both,24 further changing 3 and 4 for the particle-deposited system. Thus, the (012345) system is associated with several currently unknown parameters, all of which would become simultaneously variable parameters if the data for this system were subjected to a theoretical fit using the standard six-layer model. Therefore, we do not attempt to fit this particular data set in this report. Further experiments will be necessary to determine all the parameters for a reliable fit in this case. Furthermore, it may also be possible to account for the effects of the Au particles by considering the adsorbed layer of these particles as a transition region of the SAM layer itself.8,9,13 The validity of this latter approach would also require additional experiments where ω can be varied. The SPR data for the Ag substrate (used as layer 2) are shown in Figure 2, where layers 0, 1, 3, 4, and 5 are the same as in Figure 1. The circles, triangles and squares represent systems (0125), (01235), and (012345), respective. As in Figure 1, here also the three sets of data are collected sequentially, when a new layer is added to the initial (0125) system. Note how these plots in Figure 2 are different from those in Figure 1. Unlike the case of Figure 1, both (∆R/R)p and θp in Figure 2 shift considerably with the deposition of HDT on Ag. This indicates that the SPR signal from the Ag film is more sensitive to the HDT than from the Au film. To address the possible reason(s) for this effect, we note that R(01235) (that is, the earlier mentioned Γ′), contains the Γ-parameters, as well as the plasmon wavenumber shift, ∆k(23) sp , where the latter is induced by the 2-3 interface. If ∆k(23) sp is large compared to the Γ-terms, the SPR signal of the (012345) system would be strongly sensitive to interactions between layers 2 and 3. On the other hand, if ∆ k(23) sp is small compared to the Γ-terms, the SPR signal of this system would be relatively less sensitive to interactions between layers 3 and 4. In addition, such a competition between ∆k(23) sp

J. Phys. Chem. B, Vol. 105, No. 1, 2001 11 and Γ would change if medium 2 is changed (from Au to Ag) while using the same material (HDT) for medium 3. Thus, the increased sensitivity of the SPR signal of Ag to HDT is probably a manifestation of such competitive roles of ∆k(23) sp and Γ. In Figure 2, upon the deposition of the Au particles on the HDT layer, ∆R/R drops and the width of the SPR resonance also shows some changes. However, the shift of θp in going from (01235) to (012345) is quite small in Figure 2. This is the most striking difference between Figures 1 and 2. It indicates that under the given set of experimental conditions, and by changing the SPP-supporting substrate from Au to Ag, the enhanced sensitivity of the nanoparticle based SPR device is considerably lost. To clarify this observation, (comparing the plots indicated by the circles in Figures 1 and 2,) we note that the intrinsic SPP resonance width of the Ag substrate is significantly sharper than that of Au. Thus, in comparison with the case of the Au substrate, the less damped SPP modes of Ag experience a weaker perturbation of the LSP of the Au particles. It is also indicated in Figure 2 that among the four characteristic Γ-parameters of the SPR plot for Ag, only Γin and Γrad are measurably affected by the LSP of the Au particles. This is seen in the broadening of the SPR plot, as well as in the decrease of (∆R/R)p as we go from systems (01235) to (012345) in Figure 2. On the other hand, Γ* and Γ+, the factors responsible for changing θp, remain relatively unaffected by the LSP of the Au particles. The solid lines through the SPR data for the (0125) and (01235) systems are theoretical fits. For reasons already mentioned in the discussion of Figure 1, we did not attempt to fit the data for the (012345) case. From the two fits shown in Figure 2, we obtain d1 ) 0.60 nm and d2 ) 77.0 nm. Note that both the orientation and the surface coverage of the HDT layer on Ag [and hence, fs in eq 5] are expected to be different from those on Au. This means that 3 and d3 in this case should be different from their corresponding values for the Au substrate. Accurate determination of these coverage- and thicknessparameters requires further experiments. Here, in the calculations for the (01235) system in Figure 2, we hold d3 in the neighborhood of its value found in the case of Figure 1, and allow 3 to vary over a wider range. Using this method, we obtain d3 ) 3.0 nm, ′3 ) 9.953, and ′′3 ) 0.973. The different values of d3 and 3 found for the Au and Ag substrates are consistent with the above discussion. However, the interplay between 3 and d3 is not fully understood at this time. This issue will be addressed with further experimental results in a future report.8 The above discussion offers an explanation of the over all features of a nanoparticle based SPR device. It has been previously suggested that SPP-LSP interactions play a key role in the performance of such a device.6,7 Here, we have discussed several aspects of this phenomenon. Our experimental results are consistent with the considerations of such SPP-LSP effects. These results also demonstrate the competitive roles of the SPP and LSP in the nanoparticle modified device. With these results, we have shown that the SPP signature of the substrate film is an important factor to be considered in the design of a nanoparticle based SPR device. Acknowledgment. This work was supported by the National Science Foundation. References and Notes (1) Liley, M.; Bouvier, J.; Vogel, H. J. Colloid Interface Sci. 1997, 194, 53-58. (2) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740.

12 J. Phys. Chem. B, Vol. 105, No. 1, 2001 (3) Frutos, A. G.; Corn, R. M. Anal. Chem. News Features 1998, 449A-455A. (4) de Bruijin, H. E.; Altenburg, B. S. F.; Kooyman, R. P. H.; Greve, G. J. Opt. Commun. 1991, 425-432. (5) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and Gratings; Springer-Verlag: New York, 1988. (6) Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70, 5177-5183. (7) Lyon, L. A.; Pen˜a, D. J.; Natan, M. J. J. Phys. Chem. B 1999, 103, 5826-5831. (8) Hutter, E.; Fendler, J. H.; Roy, D. To be published. (9) Chen, W. P.; Chen, J. M. Surf. Sci. 1980, 91, 601-617. (10) Aspnes, D. E. Thin Solid Films 1982, 89, 249-262. (11) Wokaun, A. Mol. Phys. 1985, 56, 1-33. (12) Kume, T.; Nakagawa, N.; Hyashi, S.; Yamamoto, K. Solid State Commun. 1995, 93, 171-175. (13) Holland, W. R.; Hall, D. G. Phys. ReV. B 1983, 27, 7765-7768. (14) Tao, N. J.; Boussaad, S.; Huang, W. L.; Arechabaleta, R. A.; D’Agnese, J. ReV. Sci. Instrum. 1999, 70, 4656-4660 and references therein. (15) Wang, S.; Boussaad, S.; Tao, N. J. Anal. Chem. 2000, 72, 40034008.

Letters (16) Lyon, L. A.; Holliway, W. D.; Natan, M. J. ReV. Sci. Instrum. 1999, 70, 2076-2081 and references therein. (17) Hayat, M. A. Colloidal Gold, Priciples, Methods and Applications; Academic Press: San Diego, 1989 (see also references therein). (18) Winter, C.; Weckenmann, U.; Fischer, R. A.; Kashammer, J.; Scheumann, V.; Mittler, S. Chem. Vap. Deposition 2000, 6, 199. Wolfart, P.; Weiss, J.; Kashammer, C.; Winter, V.; Scheumann, V.; Mittler-Neher, S. Thin Solid Films 1999, 340, 274. Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (19) He, L.; Musick, M. D.; Nicewarner, S. R.; Salinas, F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem. Soc. 2000, 122, 90719077. (20) Johnson, K.; Arwin, H.; Lundstrum, I.; Liedberg, B. ReV. Sci. Instrum. 2000, 71, 3530-3538. (21) Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Langmuir 1998, 14, 5636-5648. (22) Yeatman, E. M. Biosens. Bioelectron. 1996, 11, 636-641. (23) Handbook of Chemistry and Physics, 64th ed.; CRC Press Inc.: Boca Raton, FL, 1983-1984; p E368. (24) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284.