Anal. Chem. 1999, 71, 4554-4558
In Situ Monitoring of the Change in Extinction of Stabilized Nanoscopic Gold Particles in Contact with Aqueous Phenol Solutions Maryann Gluodenis, Christine Manley, and Colby A. Foss, Jr.*
Department of Chemistry, Georgetown University Washington, D.C. 20057
Changes to the surface plasmon resonance band of stable nanoscopic gold particle films were monitored with a UVvis spectrometer as the films come in contact with a host analyte of varying concentrations. The gold particle films were prepared via a sputtering and annealing method and were secured in a flow cell with enough stability to remain intact while analytes flow through the cell. Time scans are performed as pure water is passed over the film with plugs of phenol systematically introduced for quantitation. With increasing host concentration surrounding the film, a characteristic red-shift and increase in intensity is observed as expected from Drude theory and the ClausiusMossotti equation for small metal particle polarizability. The reversible response was evaluated by baseline-peak extinction difference (∆Cext) and integrated peak area. In both methods of analysis, a linear agreement in ∆Cext is found in the 5 mM-0.4 M range. Gold particle films modified with octanethiol exhibit the same linearity in this range. However, the normalized ∆Cext is noticably enhanced. The analytical import of nanoscopic metal particles (e.g., as optical and electron microscopic imaging agents) is wellestablished in the biological sciences1,2 and in chemistry as surface-enhanced Raman and other spectroscopies become more routine.3-5 In the surface-enhanced spectroscopy context, the relevant metal particle properties are those that enhance the absorption or emission processes of the molecular analytes. The linear spectra of the metal particles themselves are a key, but nonetheless secondary analytical concern, as they provide information about metal particle size, shape, and orientation, all of which can influence the efficiency of electromagnetic surface enhancements.6 Recent work by Mirkin has sought to exploit spectral shifts in the plasmon resonance bands of nanoscopic gold particles as indicators of DNA oligomer interactions.7 When free oligomers in solution bind to complimentary strands that are covalently * Corresponding author: (e-mail)
[email protected]; (fax) (202) 687-6209. (1) Beesley, J. E. Proc. R. Microsc. Soc. 1985, 20, 187. (2) Hayat, M. A., Ed. Colloidal Gold Principles, Methods, and Applications; Academic Press: New York, 1989; Vol. 3. (3) Kerker, M., Ed. Selected Papers on Surface Enhanced Raman Scattering; SPIE Optical Engineering Press: Bellingham, WA, 1996. (4) Garrell, R. Anal. Chem. 1989, 61, 401A.
4554 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
bound to gold nanoparticles, numerous gold particles are brought into closer proximity, resulting in a characteristic red-shift of the plasmon resonance band.8 Thus, in this example, the linear spectral properties of the gold particles themselves provide useful information. It is well known that the spectral reflectance properties of continuous metal films are very sensitive to the host environment and surface chemical effects. Indeed, the conventional surface plasmon resonance technique is based on adsorbate or dielectric overlayer-induced changes in reflectance.9 While the extinction spectra of nanoscopic metal particles are also sensitive to surface chemistry and the dielectric properties of their host matrix,10 limited work to date has focused on exploiting such phenomena in detection and quantitation.11 In this paper, we present the results of a very simple experiment which examines the electromagnetic interaction of nanoscopic gold particles with their host environment. Unlike previous studies,7,8,11 our gold nanoparticle films are prepared on glass substrates in such a way that they can be exposed to a solvent flow stream and not undergo significant ablation. In this study, the flow stream causes a temporary change in the dielectric surrounding the gold particles. The change in extinction, which arises from analyte (phenol)-based perturbations of the gold particle plasmon resonance condition, can be conveniently monitored with a UV-visible spectrometer. (5) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (b) Sokolov, K.; Chumanov, G; Cotton, T. M. Anal. Chem. 1998, 70, 3898. (c) Osawa, M.; Ataka, K. Surf. Sci. Lett. 1992, 262, L118. (6) (a) Zeman, E.; Schatz, G. C. J. Phys. Chem. 1987, 91, 634. (b) Creighton, J. A. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. C., Eds.; Plenum: New York, 1982. (7) (a) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (b) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (8) (a) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; SpringerVerlag: Berlin, 1995; Chapter 2. (b) Cohen, R. W.; Cody, G. D.; Coutts, M. D.; Abeles, B. Phys. Rev. B 1973, 8, 3689. (9) (a) Krone, J.; Nelson, R.; Dogruel, D.; Williams, P.; Granzow, R. Anal. Biochem. 1997, 244, 124. (b) Frutos, A.; Corn, R. Anal. Chem. News Features 1998, 70, 449A. (c) Georgiadis, R.; Peterlinz, K. Langmuir 1996, 12, 4731. (10) (a) Papavassiliou, G. C. Prog. Solid State Chem. 1979, 12, 185-271. (b) Chumanov, G.; Sikolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466-9471. (c) Mulvaney, P. In Electrochemistry of Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH Publishers: New York, 1992. (d) Henglein, A.; Gasig, M. J. Phys. Chem. 1994, 98, 6 931. (e) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (f) Ali, A. H.; Foss, C. A., Jr. J. Electrochem. Soc. 1999, (2), 628-636. (11) Englebienne, P. Analyst 1998, 123, 1599. 10.1021/ac990639p CCC: $18.00
© 1999 American Chemical Society Published on Web 09/04/1999
The essential behavior of the gold nanoparticle films employed in this study can be understood in terms of the quasi-static Lorentz expression for particle polarizability R ˜ :12
R ˜)
(
)
V p - o Lj p + κjo
(1)
where V is the particle volume, Lj is the depolarization factor along a given particle axis j, and κj is the screening parameter to Lj via κj ) Lj-1 - 1.12 The factors Lj and κj depend on particle shape, and p and o are the complex dielectric functions of the particle and host medium, respectively. The optical extinction cross section for a single particle is related to R ˜ via12
Cext ) Cabs + Csca
(2)
Cabs ) kIm{R ˜}
(3a)
Csca ) (k4/6π)|R ˜ |2
(3b)
Figure 1. The real (m′) and imaginary (m′′) components of the complex dielectric function of gold (m). ωp ) 1.08 × 1016 s-1, τ ) 1.0 × 10-14 s. The lined arrows at m′ ) -15 and -17 show the effects of alterations to the dielectric constant of the medium surrounding the gold nanoparticles on the plasmon resonance condition.
where13
and
Ignoring reflectance losses from the sample or container, the measured transmittance is related to the extinction cross section via
T ) exp(-NCextd)
(4)
where N is the number density of particles and d is the sample length. For dilute collections of particles, the spectrometer absorbance output A is directly proportional to Cext. All of the preceding expressions are general and apply to all particles, provided that their dimensions are much smaller than the incident wavelength. The special nature of nanoscopic metal particle optics can be seen when one considers the dielectric function of free metal electrons. The familiar Drude expression for the frequency-dependent metal dielectric function, modified to include bound electron polarization modes, is14
˜ m ) ˜ bound -
ωp2 ω(ω + i/τ)
(5)
˜bound is the dielectric constant contribution from bound electrons. ωp is the plasma frequency and is related to the concentration of conduction electrons in the metal. τ is the mean free lifetime of the conduction electrons, and ω is the incident frequency (related to the incident wavelength via ω ) 2πc/λ). (12) (a) van de Hulst, H. C. Light Scattering of Small Particles, Dover: New York, 1981. (b) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley and Sons: New York, 1983. (13) “Im” signifies that we consider the imaginary part of R ˜ only. |R ˜ |2 denotes the square modulus of R ˜. (14) Hummel, R. Electronic Properties of Materials, 2nd ed.; Springer-Verlag: Berlin, 1993.
Figure 2. Simulated spectra for a 9-nm gold particle in different host media. The host refractive indices examined are 1.33, 1.42, 1.51, and 1.60. As the host refractive index increases, a red-shift and increase in intensity are noted.
Figure 1 shows a plot of the real and imaginary parts of ˜m for gold versus λ.15 In the case of gold and certain other metals, the imaginary component is small in the visible region of the spectrum and the real part is negative and monotonically decreasing with λ. Considering the denominator of eq 1 (with ˜p ) ˜m), and furthermore, only the real parts of ˜m and ˜o, it is clear that R ˜ will be very large when m′ ) -κjo′. This is the so-called particle plasmon resonance condition7a and is shown schematically in Figure 1. The plasmon resonance condition allows us to tune the extinction maximum (λmax) by altering particle shape16 or the host dielectric constant.11c Figure 2 shows extinction spectra for nanoscopic gold spheres simulated using eqs 1-3, assuming different host dielectric constants. If the host material is nonabsorbing in the spectral range near the plasmon resonance band, one would expect an increase in o′ leads to a red-shift in the (15) Resolving eq 5 into its real (m′) and imaginary (m′′) parts yields m′ ) ′bound - ωp2/(ω2 + ω/τ) and m′′ ) ′′bound + ωp2/ω(ω2 + ω/τ). These two quantities are also related to the experimental refractive index (n) and absorption coefficient (k) via m′ ) n2 - k2 and m′′ ) 2nk. The data in Figure 1 are taken from: Johnson, P. B.; Christy, R. W. Phys. Rev. B. 1972, 6, 4370. (16) (a) Al-Rawashdeh, N. R.; Sandrock, M. L.; Seugling, C. J.; Foss, C. A., Jr. J. Phys. Chem. B 1998, 102, 361. (b) Foss, C. A., Jr.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (c) Foss, C. A., Jr; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1992, 96, 7497.
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plasmon resonance extinction maximum and an increase in the total extinction. In this study, aqueous solutions flow over stationary solid nanoparticles on a glass substrate. When the solution is pure water, the host refractive index is ∼1.33 (or o′ ) 1.77) over the 400-800 nm spectral range.17 Pure phenol has a refractive index of 1.527 (o′ ) 2.33).17 While we do not consider the pure phenol case experimentally, the addition of phenol to the aqueous medium slightly increases the host refractive index and dielectric constant. The slight increase in o has a measurable effect on the gold particle plasmon resonance spectra. EXPERIMENTAL SECTION Microscope slides (Corning Glass Works, 5.12 cm × 30.8 cm × 0.1 cm) were cut to dimensions (∼4.1 cm × 1.9 cm × 0.1 cm) appropriate to the liquid sample cell (Wilmad demountable flow cell). The slides were etched in 0.68 M hydrofluoric acid (Mallinckrodt) for 10 min, rinsed with water followed by ethanol, and dried under ambient conditions. Caution was taken when handling the corrosive acid. The clean and dried slides were then coated with gold using an Anatech Hummer 10.2 plasma deposition system. The typical coating thickness was 15 nm, as estimated by an internal quartz oscillator. The Au-coated slides were then baked at 450 °C for 20 min in a preheated furnace (Gardsman). Octanethiol-coated gold nanoparticle films were prepared by soaking the annealed Au-coated slides in a 60 mM ethanolic solution of octanethiol (Fluka 97%). Aqueous phenol solutions ranging from 1 mM to 0.4 M were made through serial dilution. The refractive indices of each solution were measured 20 times and averaged prior to use with a temperature controlled Bausch and Lomb refractometer regulated at 25.0 °C. Prior to the solution-contact studies, the plasmon resonance spectra of the Au-coated slides were collected before and after the oven annealing step. These spectra were collected at normal incidence and under p-polarization using a Hitachi U3501 doublebeam UV/vis/NIR spectrometer equipped with a model 210-2130 polarizer attachment. In all spectral measurements, baseline scans were collected with the sample holder and polarizer in the sample beam and no material in the reference beam. Spectra were collected at 1-nm increments with a fixed slit width of 3 nm. For the studies in which the Au nanoparticle films were in contact with liquid, the Au-coated glass slides were incorporated into the Wilmad liquid cell such that they functioned as one window. The other window was a 0.58-cm-thick common doublestrength window glass plate with 0.28-cm-diameter holes drilled to match the injection and exhaust ports on the Wilmad housing. A 1-mm Teflon spacer separated the two windows. The assembled cell volume was 295 mm3. For spectral scans of stationary liquid solutions, the liquid was injected by syringe. In the flow studies, a Milli-Q water (18 MΩ) reservoir (25 L) was supported 1.05 m above the Wilmad cell in the spectrometer. A 2.5-mm-i.d. Tygon tube delivered solution to the cell at ∼120 mL/min. The outlet of a 0.5-L separatory funnel containing the aqueous phenol solutions was connected to the flow stream via a common Teflon stopcock (120° bore 2-mm capillary arms). To avoid backwash problems, the solution level in the separatory funnel was maintained at the same level as the water in the main reservoir. Because of the large (17) Marcus, Y. Ion Solvation; Wiley: New York, 1985.
4556 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
Figure 3. p-Polarized absorbance spectra of a gold film prior to heat annealing. The arrow indicates the direction of the trend upon tilting at angles of 0, 20, 30, and 45°. The inset shows a possible arrangement of gold particles on the slide with a high-volume fraction.
Figure 4. p-Polarized absorbance spectra of a gold film after annealing. The arrow indicates the direction of the trend upon tilting at angles of 0, 20, 30, and 45°. The inset shows a possible schematic of gold particles on the slide with a lower volume fraction, resulting in the observed blue-shift in λmax.
reservoir volume, the solution flow rate did not change appreciably over the course of a typical flow experiment (∼100 s). In the flow studies, the spectrometer monitored the sample beam power at λ ) 620 nm (optical band-pass 3 nm). Data points were collected every 0.10 s. RESULTS AND DISCUSSION Figures 3 and 4 show the plasmon resonance spectra of the Au-coated slides before and after oven annealing under ppolarization. We utilize p-polarized light, where the electric field is parallel to the plane of incidence, because it allows us to detect optical anisotropies arising from either particle shape or interparticle interaction as a function of incidence angle.18 In Figures 3 and 4, the spectra before the annealing step are indicative of a high metal volume fraction Au/air composite film. The broad plasmon resonance band centered at ∼680 nm does not shift as the incidence angle is increased under p-polarization; we interpret this as evidence of an isotropic composite where the metal is present as a dense multilayer collection of spherical particles.18 If the metal particles consisted of islandlike nanostructures or a dense monolayer of spheres, the spectra might be expected to show a blue-shift in the plasmon resonance maximum as the incidence angle increases.18 After heat treatment, the plasmon resonance band blue-shifts considerably; the visual (18) (a) Schmitt, J.; Machtle, P.; Eck, D.; Mohwald, H.; Helm, C. A. Langmuir 1999, 15, 3256-3266. (b) Chumanov, G.; Sokolov, K.; Cotton, T. M. J. Phys. Chem. 1996, 100, 5166-5168.
Table 1. Average Refractive Index Values for Each Phenol Concentration phenol conc (M)
av refractive index
phenol conc (M)
av refractive index
0.001 0.002 0.004 0.005 0.05
1.3337 1.3337 1.3338 1.3338 1.3342
0.1 0.2 0.3 0.4
1.3351 1.3362 1.3375 1.3389
Figure 6. Time scan of alternating water and phenol solutions flowing past an octanethiol-modified gold surface. The arrow indicates a distinct increase in the measured absorbance as the phenol concentration increases from 1 mM to 0.4 M. Figure 5. (A) Absorbance spectra of aqueous phenol solutions (1 mM-0.4 M) in contact with annealed gold particle films. A subtle redshift and increase in intensity are noted with increasing concentrations. (B) A magnified view of the low-energy slope of (a) showing the increase in absorbance associated with an increase in surrounding host refractive index.
appearance of the coated slides changes from blue-green to pink, and the λmax at normal incidence is ∼530 nm (Figure 4). The blueshift and sharpening of the plasmon resonance band is consistent with a decrease in metal volume fraction, which in this case would result from coalescence of small metal particles into larger ones, partial evaporation of some gold, or a combination of both.19 We were not able to determine the dimensions of the gold nanoparticles, either before or after furnace annealing. Scanning electron microscopy (SEM) was successfully employed in a previous study involving flame annealing,20 but the gold structures in that case were larger (∼60 nm). For the current study, the Au particles are too small for SEM analysis or our scanning probe instrument.21 While a knowledge of particle size and size distribution is desirable in terms of rigorous experiment-theory comparison, host-dielectric constant-induced spectral shifts are essentially independent of particle size for gold particles whose dimensions lie in the 2-50 nm range (vide infra). The furnace annealing step is important in two respects. First, host (liquid sample)-induced spectral changes are more readily (19) In our initial attempts to prepare these Au-coated slides, we employed a Fischer burner (T ) 1700° C) to anneal the gold. The spectral consequences are similar to those after furnace annealing, but metal evaporation was more severe. (20) Lu, A. H.; Lu, G. H.; Kessinger, A. M.; Foss, C. A., Jr. J. Phys. Chem. B 1997, 101, 9139. (21) Using a Digital Instruments Nanoscope II AFM with D-scanner, we are unable to distinguish between bare and sputter-coated glass slides.
detectable when the plasmon resonance band is sharp. Second, the heat treatment strengthens the physical bond between the gold and glass. Without heat treatment, the sputtered Au films are easily detached when the slides are exposed to liquid. Even after annealing, some Au particles are more robust than others in terms of mechanical ruggedness. We find that annealed films that show a blue-shift in the plasmon resonance maximum with increasing incidence angle under p-polarization are more robust; this result is consistent with Au particles that are not spherical, but rather somewhat oblate in geometry (see inset of Figure 4). Oblate gold particles would have a greater contact area with the glass than spherical particles, which would in turn, result in greater resistance to detachment. Figure 5A shows the plasmon resonance spectra of furnaceannealed Au particles in contact with pure water and aqueous phenol solutions of different concentrations. The refractive index change over the phenol concentration range considered is small (1.3337 for pure water to 1.3389 for 0.4 M phenol) and leads to only very slight changes in the measured spectra. The measured refractive indices are shown in Table 1. The largest relative changes in extinction occur on the low-energy slope of the plasmon resonance band (Figure 5B). Thus, a probe wavelength of 620 nm was selected for the time scan analysis under solution flow conditions. Figure 6 shows the results of a series of time scan studies where aqueous phenol solutions of various concentrations were injected into the aqueous flow stream past an octanethiol-modified film. When pure water flows through the cell, the extinction at λ ) 620 nm is low. When a phenol sample plug passes over the Au nanoparticles, the extinction increases as the plasmon resonance band is red-shifted slightly. Once the phenol sample plug exits the cell, the extinction returns to its previous value. The extinction Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
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Table 2. Comparison of the Recorded Extinction Response for the Modified and Unmodified Gold Films Determined Using Two Methods conc (M)
∆Cext (×10-4) gold thiol
0.001 0.002 0.004 0.005 0.05 0.1 0.2 0.3 0.4
4.00 1.00 1.10 6.07 9.34 8.74 22.3 45.9 70.6
1.46 2.27 6.22 5.28 31.8 45.2 65.5 80.2 96.1
area (×10-4) gold thiol 22.8 27.8 45.1 238 372 565 865 1140 1710
8.38 6.79 48.2 33.7 200 262 379 819 902
calc (×10-3) gold thiol 6.27 6.27 6.46 6.46 7.14 8.63 10.5 12.7 15.1
7.18 7.18 7.38 7.38 8.16 9.88 12.0 14.6 17.3
response for different phenol solutions is summarized in Table 2. The response is evaluated in two ways. The first is based on the change in extinction according to
∆Cext ) (Cext,plateau - Cext,baseline)
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Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
area
∆Cext
calc
conc (M)
gold
thiol
gold
thiol
gold
thiol
0.001 0.002 0.004 0.005 0.05 0.1 0.2 0.3 0.4
1.00 0.25 0.28 1.52 2.34 2.19 5.58 11.5 17.6
1.00 1.56 4.27 3.63 21.8 31.1 45.0 55.0 66.0
1.00 1.22 1.98 1.05 1.63 2.48 3.79 5.01 7.50
1.00 0.81 5.75 4.03 23.8 31.3 45.2 97.8 108
1.000 1.000 1.030 1.030 1.139 1.376 1.673 2.030 2.410
1.000 1.000 1.027 1.027 1.137 1.377 1.678 2.027 2.411
the data, it can be seen that the thiol modification to the gold film results in a relative increase in response experimentally, while the theoretical values seem unaffected by the extra thiol layer. This difference can be attributed to an enhanced partitioning of the phenol analyte into the region adjacent the gold particles’ modified surface which is not taken into account in the simulation.
(6)
where 〈Cext,plateau〉 is defined over the central 5 s of the 10-s period during which the T-joint valve to the phenol reservoir is open.22 The second response parameter is the integrated area of the extinction change in the peak using the midpoint rule. Both peak height and peak area parameters show a linear correlation with phenol concentration for analyte concentrations in the 5 mM0.4 M range. At concentrations below 5 mM, the response is not linear with analyte concentration. Table 2 also summarizes the optical response of Au nanoparticles whose surfaces have not yet been modified with octanethiol. For this system, both the ∆Cext and A parameters show a decreased response relative to the modified Au nanoparticles. Calculated Cext values for both gold and the thiol-modified films are included showing a slight advantage for the modified films. Table 3 shows the experimental and calculated Cext values for both gold and thiol-modified gold films after normalization. From (22) 〈Cext, plateau〉 is based on an average of 50 data points.
Table 3. Normalized Experimental and Theoretical Data for Modified and Unmodified Gold Films
CONCLUSIONS Stable discontinuous nanoparticle gold films have been prepared which respond to electromagnetic interactions with their host environment. The discontinuity of these films allows for a reversible analysis of surface enhancements using a UV-visible spectrometer, a widely accessible instrument. The stability of the discontinuous films seems to be a direct result of the annealing process. An increase in the relative response for the thiol-modified films may be attributed to enhanced partitioning of the phenol into the coated surface region. Further areas of investigation include the theoretical and experimental analysis of other host analyte systems with different surface-specific effects. ACKNOWLEDGMENT This material is based on work supported in part by the National Science Foundation under Grant DMR 9625151. Received for review June 15, 1999. Accepted July 29, 1999. AC990639P
