J. Phys. Chem. 1996, 100, 3169-3174
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A Microscopic Surface-Enhanced Raman Study of a Single Adsorbate-Covered Colloidal Silver Aggregate B. Vlcˇ kova´ ,*,† X. J. Gu,‡ D. P. Tsai,§ and M. Moskovits*,§ Department of Physical and Macromolecular Chemistry, Charles UniVersity, HlaVoVa 2030, 128 40 Prague 2, Czech Republic, and Department of Chemistry and The Ontario Laser and LightwaVe Research Centre, UniVersity of Toronto, Toronto, Canada M5S 1A1 ReceiVed: July 25, 1995; In Final Form: October 23, 1995X
Surface-enhanced Raman scattering (SERS) spectra have been successfully recorded from individual, selected clusters of colloidal silver particles on which phthalazine, dopamine, or 2,2′-bipyridine was adsorbed. The method involved the controlled deposition of silver colloid aggregates onto a Pyrex microscope cover slide and the selection of an individual cluster by the optical microscope of a micro-Raman spectrometer followed by Raman measurement. SERS spectra of clusters of various size were recorded. With phthalazine, good quality SERS spectra were obtained with 476.2- and 530.9-nm laser excitation. Poor spectra were obtained with 568.2- and 590-nm excitation, probably due to the photofragmentation of the cluster. The wavelength at which photofragmentation occurred was cluster size and structure dependent. Smaller clusters tended to fragment at shorter wavelengths. A micropreparative technique was developed for preparing single adsorbatecovered silver colloid aggregates. Using this technique in conjunction with Raman microscopy, good quality SERS spectra were obtained from 5 pmol of phthalazine and dopamine. SERS spectra of 2,2′-bipyridine deposited out of a dichloromethane solution also were obtained, illustrating the possibility of using this singlecluster technique for recording SERS spectra of water-insoluble adsorbates or of adsorbates dissolved in nonaqueous solvents.
Introduction The elucidation of the relationship between the morphology of a roughened free-electron metal surface and the enhancement of the Raman scattering of a molecule deposited on it has been a focus of interest since 1978, when the electromagnetic mechanism explaining the surface enhancement of Raman scattering was first outlined.1 Considerable progress in this regard has been achieved by the application of the Raman microprobe to surface-enhanced Raman scattering (SERS) spectroscopy.2 Raman microspectroscopy enables one to visualize the morphology of the surface by optical spectroscopy on a scale of the order of the wavelength and larger while simultaneously recording the SERS spectrum of individual structural components of the surface with a resolution of the order of 1-5 µm, the size of the focal spot of the laser.3,4 Up to the present, SERS microspectroscopy has been restricted to studies of disordered metal surfaces (mainly silver) such as roughened electrodes,1,5,6 island films deposited on glass, silvercoated filter paper,3,4,7 solid silver membrane filters,3,4 etc., all of which are prepared prior to the deposition of the adsorbate investigated. The morphology of these surfaces is more or less predetermined by the surface preparation procedure and is not strongly affected by the chemical nature of the deposited adsorbate.7 In contrast, the structure and character of aggregated metal colloids is determined significantly by the kinetics of the aggregation process which, in turn, depends on the adsorbate and the adsorption-desorption processes taking place at the metal surface involving, additionally, the various components †
Charles University. The Ontario Laser and Lightwave Research Centre, University of Toronto. § Department of Chemistry and The Ontario Laser and Lightwave Research Centre, University of Toronto. X Abstract published in AdVance ACS Abstracts, January 15, 1996. ‡
0022-3654/96/20100-3169$12.00/0
of the double layer surrounding the colloidal particles.8,9 The major factors influencing the mechanism of aggregation are the surface potential of the colloid, the chemical nature and charge of the adsorbate and its concentration in the system, the temperature, and the possible presence of competing adsorbates sometimes added intentionally as preaggregating agents.8-11 Clusters of colloidal gold and silver particles that have been aggregated by an adsorbate have been shown to be fractals.8,9 Recently, a theory describing the optical excitations of fractal clusters of colloidal particles has been presented which also considers enhancement by fractal aggregates.12 Among its predictions is the fact that the normal modes of a fractal cluster associated with surface plasmon excitations are spatially localized in regions of the cluster small with respect to the overall size of the aggregate. This leads to the conclusion that many optical properties of the cluster such as its absorption spectrum and its SERS excitation profile become independent of cluster size, that is, essentially approaching universal functions (for a given metal) in the scaling region. The theory also shows that the SERS enhancement of N adsorbate-covered colloidal particles forming a fractal cluster will be greatly enhanced over the equivalent number of adsorbate-covered independent colloid particles (which are themselves already contributing some enhancement). Hence, the SERS signal and its excitation profile will be dominated by the contributions of fractal clusters. A particularly simple, almost parameter-free expression for the SERS excitation profile was determined in ref 12 and shown to compare favorably with previously published experimental data for phthalazine adsorbed on Ag colloid.13 So far, the comparison with experiment has been limited to aqueous Ag colloid/adsorbate systems in which a wide distribution of cluster sizes are probed by the laser and processes such as laser-induced aggregation and cluster fragmentation can © 1996 American Chemical Society
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TABLE 1: Preparation Conditions for Ag Colloid Solutions Ag colloid CI CII CIII
silver nitrate soln vol/ concn/ mL (mol/L) 9.0 7.5 9.0
2.2 × 10-3 4.4 × 10-3 4.4 × 10-3
final Ag concna/ (mol/L)
NaBH4:AgNO3/ mol:mol
2.4 × 10-4 4.0 × 10-4 4.8 × 10-4
4.5 2.7 2.3
a
75 mL of sodium borohydride solution containing 3.5 mg of NaBH4 dissolved in distilled deionized water was used in all of the preparations.
occur.14 In order to investigate the role of cluster size, it is crucial to develop a technique which probes single colloid clusters. In this paper, we take the first step in that direction by describing a simple method for preparing and depositing individual Ag colloidal aggregates on a supporting glass slide under carefully controlled conditions. SERS spectra of selected single aggregates of adsorbate-covered silver particles (adsorbate ) phthalazine, pht; dopamine, dopa; and 2,2′-bipyridine, bpy) measured by Raman microspectroscopy are reported. Additionally, a micropreparative method is described by which single adsorbate-covered colloid aggregates can be produced on a glass substrate and its SERS microspectroscopy recorded. This technique can be used to produce good quality SERS spectra with picomolar levels of sensitivity even for water-insoluble adsorbates. Experimental Section Sample Preparation. Ag colloids were prepared by reducing AgNO3 (Aldrich) with NaBH4 (Merck) using the procedure described in ref 10. Three types of colloids were prepared by varying the silver-to-borohydride ratio (Table 1). Phthalazine and L-dopamine were purchased from Aldrich, 2,2′-bipyridine from Sigma, and dichloromethane (UV spectral grade) from ACS Chemicals. Distilled deionized water was used throughout. Ag colloid/phthalazine and Ag colloid/2,2′-bipyridine samples were prepared by adding 10 µL of a 10-2 M solution of the adsorbate to 2 mL of Ag colloid. The final concentration of adsorbate in the sample was 5 × 10-5 M. Colloidal aggregates were gravitationally deposited from the Ag colloid/adsorbate system on a glass microscope cover slide placed on the bottom of a covered Petri dish. Samples allowed to settle for varying lengths of time were examined by a confocal optical microscope in order to determine the conditions that resulted in well-separated clusters. The deposition rate was affected by the concentration of clusters in the colloidal suspension. This fact was occasionally put to use either by diluting the Ag colloid prior to addition of adsorbate or by adding water to the Ag colloid/adsorbate system. In general, the two procedures do not produce equivalent results. The effects of both procedures were studied with the Ag colloid/ pht systems over a 10-fold dilution range and the resulting deposits investigated by confocal optical microscopy. The effect of varying the deposition time on the morphology of the deposits was also investigated by confocal optical microscopy for deposits formed after 2-10 h of deposition. Well-isolated but abundant aggregates were obtained after 2.5-3.5 h of deposition. Samples consisting of only a single colloidal aggregate were also produced. This was done in one of two ways. In the first, 5 µL of Ag colloid CII was mixed with 5 µL of 10-6 M aqueous solution of pht and/or dopa on the surface of a glass microscope cover slide using appropriate micropipets. The resulting drop of Ag colloid and adsorbate was allowed to dry in air. The second technique also allows one to adsorb water-insoluble molecules onto the Ag colloid surface. A 5-µL drop of Ag
colloid CII was deposited on the surface of a Pyrex microscope cover slide and allowed to dry in air. The dried colloid was then overlaid with a 5-µL drop of a 2 × 10-3 M solution of 2,2′-bipyridine dissolved in CH2Cl2. The colloid was dispersed in the drop of the bpy solution by stirring locally, and the mixture was allowed to dry in air. Instrumentation. SERS spectra of isolated aggregates deposited on pyrex microscope cover slides were measured with a Raman spectrometer based on a Spex 877 Triple spectrometer with CCD (Princeton Instruments) detection. Spectra were excited with a krypton ion laser (Lexel Laser Inc.). Samples were mounted on an XYZ stage, and the desired colloid aggregate was located by micrometric adjustment. Aggregates were imaged in the reflected light mode, and the laser was focused on the desired cluster by means of a Zeiss microscope equipped with a 40× objective. Images of the aggregates were collected with a Sanyo Color CCTV camera and displayed on a Panasonic video monitor. The 476.2-, 530.9-, and 568.2-nm lines of the Kr+ laser were used to excite spectra. Additionally 590-nm excitation was obtained by pumping Rhodamine 6G in a dye laser. Approximately 60 mW of laser radiation was used. The spectra reported were the result of a single 120-s accumulation. A single-crystal silicon surface was used as an external intensity standard. Results and Discussion 1. Deposition of Ag Colloidal Aggregates. The addition of pht or bpy resulted in the aggregation of the colloid. The morphology of the aggregate assemblies deposited on the glass cover slides was found to depend critically on the colloid preparation conditions, the chemical nature of the adsorbate, the order in which the colloid was diluted, the final concentration of the adsorbate in the system, and the deposition time. The last two variables were easily and unambiguously optimized; however, the first three affect the nature of the aggregates in a more complicated and subtle fashion that was not exhaustively investigated in this study. Of the three types of borohydride-reduced Ag colloids (Table 1) that are characterized by differing silver concentrations and varying reducing agent to silver ion concentration ratios, resulting in different surface potentials, only colloids CI and CII yielded reproducible assemblies of well-separated colloid clusters. With CIII colloids, large, connected agglomerates were observed in some cases and well-separated clusters in others in an unpredictable and uncontrollable manner. Dilution of the colloid either prior to or after the addition of the adsorbate slowed the deposition process. Ten-fold dilution was found to be a suitable level. The order of dilution had a marked effect on the ensuing cluster as shown in Figure 1 for colloid CII aggregated with pht. Dilution of the colloid prior to adsorbate addition results in large (15-25-µm) clusters with large cluster-cluster separation (Figure 1A). In contrast, diluting the colloid/adsorbate system produces rather dense deposits of clusters ranging from 1 to 10 µm in size (Figure 1B). The adsorbate used to aggregate the colloid also influences the resultant deposit. By using bpy in place of pht with the CII colloid and diluting after adsorbate addition, one obtained elongated clusters (Figure 1C) with dimensions ranging from 1 to 8 µm along the longest axes and 1 to 4 µm along the axes perpendicular to these. The larger clusters appear to consist of linear chains of two or more smaller aggregates. This aggregate morphology was observed in all systems aggregated with bpy and contrasts what is observed with pht (Figure 1B). 2. SERS Spectra of Single Deposited Clusters of phtCovered Ag Particles. SERS spectra of pht were obtained from
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Figure 2. SERS spectra of the pht-covered 3- × 6-µm silver colloid aggregate (surrounded by a circle in Figure 3A). Excited with (A) 476.2-nm, (B) 530.9-nm, (C) 568.2-nm, and (D) 590-nm laser light.
Figure 1. Confocal optical micrographs (500× magnification) of (A) pht-covered Ag aggregates deposited from the Ag colloid CII which was 10-fold diluted to which was added pht, (B) pht-covered Ag aggregates deposited from a sample of Ag colloid CII/pht which was 10-fold diluted, (C) as in B but with bpy. The bar indicates 50 µm.
a single cluster of pht-covered Ag colloidal particles using micro-Raman and 476.2-, 530.9-, 568.2-, and 590-nm excitation (Figure 2, spectra A-D). The laser was focused on the selected cluster using the Raman microscope in the optical mode. The single 3- × 6-µm colloid cluster from which the spectra in Figure 2 originate is circled in Figure 3A. The spectra (Figure 2), recorded in the order 530.9, 476.2, 568, and 590 nm, show a pronounced dependence on the excitation wavelength. Good
quality SERS spectra were only obtained with the 530.9- and 568.2-nm excitation (Figure 2B, 2C). The band frequencies observed in these two spectra correspond closely to those reported previously for pht adsorbed on aqueous Ag colloid13 as well as for SERS of pht adsorbed on a 60-Å SiO2 film sputtered over a 50-Å silver island film.15 The pht molecule likely adsorbs “standing up” on the surface, i.e., coordinated to a silver atom through the two heterocyclic nitrogen atoms, as was previously suggested.13,15 The assignment of the bands, taken from refs 13 and 15, is, in turn, based on the assignment of the IR and Raman spectra of diazanaphthalenes by Mitchell and co-workers.16 In addition to the a1 modes reported in refs 13 and 15, three other bands attributed to a1 modes are observed at 654, 1164, and 1307 cm-1 (Figure 2B, 2C) which have counterparts in the normal Raman spectrum of pht and are assigned to skeletal distortion, CH bending, and ring stretching modes, respectively.16 Although the SERS spectra (Figure 2B, 2C) are dominated by a1 modes, a band of b1 and two of b2 symmetry are also observed at 498, 479, and 756 cm-1, respectively. These bands are less intense in this spectrum (as compared to the a1 modes) than in the SERS spectrum of pht adsorbed on aqueous Ag colloid excited with 514.5-nm Ar+ laser light. They are, however, not totally absent, as was the case in the SERS spectrum of pht adsorbed on aqueous Ag colloid and excited with 602-nm radiation. The SERS spectra shown in Figure 2B, 2C correspond closely to those obtained for pht adsorbed on a 60-Å sputtered SiO2 film over a 50-Å island film and excited with 514.5-nm light.15 The major difference is the substantially lower intensity of the 1568-cm-1 band (Figure 2B, 2C) in comparison to the ∼1565-cm-1 band reported but not assigned in ref 15. The Raman spectrum obtained with 476.2-nm excitation (Figure 2A) is strikingly different from those of Figure 2B, 2C. Additionally, the intensities of the two broad SERS peaks as well as that of the background are very much more intense than those of the two previously discussed. The same sort of behavior was observed for the SERS spectra of a 3- × 3-µm cluster, which is shown surrounded by a square in Figure 3A.
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Figure 4. SERS spectrum of pht originating from the single aggregate shown in Figure 3B, excited with 568.2-nm laser light.
Figure 3. Micrographs of pht-covered Ag aggregates deposited on a Pyrex slide obtained through the Raman microscope switched to the optical mode (ca. 1000× magnification): (A) aggregates deposited from a 10-fold diluted Ag colloid CII (2 mL)/pht (10 µL of 10-2 M) sample. The clusters selected for SERS measurement are surrounded by a circle, a 3- × 6-µm aggregate, and a square, a 3- × 3-µm aggregate. (B) An approximately 3-µm-diameter cluster produced by the micropreparation technique with 5 µL of Ag colloid CII and 5 µL of 10-6 M aqueous pht.
The spectrum excited with 476.2 nm is likely dominated by photodecomposition products of pht, which includes graphitic carbon known to possess bands at 1580 cm-1, the E2g mode of graphite, and at ∼1355 cm-1 the A1g mode of disordered microcrystalline graphite.17,18 The strong band in spectrum 2A at 1457 cm-1, on the other hand, implies the presence of species other than graphite, which could either be another decomposition product and/or some undecomposed pht. It is unlikely that the carrier of most of the band intensity at 1457 cm-1 is pht since the relative band intensities and widths are so greatly changed from those observed with other excitation wavelengths. However, some undecomposed pht is present on the surface as evidenced by weak bands at 1233, 1306, and 1384 cm-1 belonging to the a1 CH bending and two a1 ring stretching modes, respectively. The possibility that the SERS spectrum observed in Figure 2A is entirely that of pht modified by changes in the relative intensities of the normal and tangential field components seems unlikely. The explanation in terms of photodecomposition is complicated, however, by the fact that the spectra shown in Figure 2B and 2C dominated by the SERS signature of pht were obtained after that shown in Figure 2A. The most probable explanation is that the species observed in Figure 2A are photodecomposition products that are resonant with the blue excitation (as graphitic and other photofragments
tend to be). The extra intensity observed for SERS spectrum 2A is, therefore, likely due to the resonance enhancement of the photoproducts resulting in a relatively more intense Raman spectrum for them as compared to the SERS spectrum of the pht adsorbate. When longer wavelength excitation was used, the photoproducts were no longer resonant, so the spectrum was dominated by the SERS spectrum of the more abundant pht. The SERS spectrum excited by 590-nm light (Figure 2D) contains only three broad bands at approximately 949, 1233, and 1331 cm-1. These bands are observed in all the SERS spectra of pht reported so far (Figure 2 and ref 13 and 15); however, they are normally accompanied by much stronger bands (e.g., at 528, 804, 1384, and 1457 cm-1). Microscopic imaging of the aggregate after recording the SERS spectrum with 590-nm excitation yielded a poorly focused image which, upon refocusing, indicated that the cluster had changed shape to become a much flatter, two-dimensional aggregate. SERS spectra could not be obtained from this shape-modified cluster using the 530.9-nm excitation. The spectrum observed in Figure 2D seems to be associated, therefore, with a change in the structure of the pht-covered silver aggregate, which apparently makes it more prone to SERS excitation by red as opposed to blue light. Our results also suggest that it is 590-nm radiation that produces the structural change in the deposited colloidal aggregate, while radiation of shorter wavelength is not as efficient in bringing this modification about. The smaller, ∼3- × 3-µm cluster, shown surrounded by a square in Figure 3A, behaved a little differently in this regard. For it, the SERS spectra excited with 476.2, 530.9, and 590 nm are nearly identical with those of the larger cluster (Figure 2A,B,D); the spectrum obtained with the 568.2-nm laser excitation is markedly different from that of Figure 2C, resembling more closely that of Figure 2D. This might imply that for the smaller cluster, the laser-induced structural modification occurs even with 568.2-nm irradiation. The general trends of these results were reproduced for several small and large aggregates. Since it is our intention, ultimately, to use this technique to measure the SERS excitation profile of a single colloidal aggregate, the poor spectrum obtained with 590-nm excitation (and also with 568.2-nm excitation for the smaller clusters) was disappointing. Fortunately, we show below that individual colloid clusters produced by micropreparation did not seem to suffer from this drawback and produced good SERS spectra with 568.2-nm excitation (Figure 4). 3. SERS of a Single Adsorbate-Covered Microprepared Aggregate. The fact that good quality SERS spectra are measurable from single adsorbate-covered colloid clusters
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Figure 5. SERS spectrum of dopamine originating from a single dopacovered silver aggregate generated by the micropreparation technique and excited with 530.9-nm laser light.
Figure 6. SERS spectrum of 2,2′-bipyridine originating from a single bpy-covered silver aggregate. The bpy was applied from a dichloromethane solution (530.2-nm excitation).
suggests a new strategy for using SERS in chemical microanalysis whereby only a single colloidal aggregate is prepared at the outset as the substrate for analytes. This contrasts with the procedure described above in which many colloid clusters were prepared but only one was excited at a time. By restricting the cluster synthesis to a single cluster, one also automatically reduces by several orders of magnitude the quantity of analyte required in the analysis. The single pht-covered cluster shown in Figure 3B, prepared using only 5µL of 10-6 M aqueous solution of pht, appears somewhat more compact than those shown in Figure 3A which were prepared by deposition under the influence of gravity. However, the SERS spectrum of pht (Figure 4) obtained from this single aggregate corresponds closely to the spectra of pht in Figure 2B,C. Furthermore, the good quality of the spectrum measured with 568.2-nm excitation (Figure 4) indicates that the microprepared aggregate is not as prone to laser-induced modification at this wavelength as aggregates of approximately the same size prepared by deposition from solution. The reduced degradation tendency of single colloid clusters prepared from microdroplets of colloid and adsorbate solution extends the wavelength range over which one can study the optical properties of individual clusters. The contrasting photodegradation behavior between these clusters and ones deposited out of aqueous solution may be related to cluster geometry. The cluster shown in Figure 3B appears to be much more compact than those formed with an equivalent concentration of pht (5 × 10-5 M) in larger volumes of aqueous colloid. Although our level of magnification is insufficient to decide the issue unequivocally, the compactness of the microprepared cluster suggests that it is characterized by a larger fractal dimension than those of the clusters which settle out of solution. Weitz showed that the fractal dimension of a colloid cluster depends on the rate of aggregation, which in turn depends on the adsorbate concentration.9 Rapid aggregation (high adsorbate concentration) forms fractal clusters with Hausdorff dimensions approximately equal to 1.75, while clusters aggregated more slowly (low adsorbate concentration) are characterized by a much larger fractal dimension. The optical response of clusters depends on their fractality.12 This may, in turn, be reflected in their photofragmentation behavior. The sample size of pht required in the micropreparative technique is 5 pmol, i.e., 650 pg, which is, therefore, also an upper bound to the detection limit. The feasibility of doing SERS microscopy on a single analyte-covered silver cluster was further tested by investigating dopamine (1-(ethylamino)-3,4dihydroxybenzene), a catecholamine neurotransmitter and an
analyte of some bioanalytical importance.19 The SERS spectrum of a single dopa-covered cluster excited at 530.9 nm prepared with 5µL of 10-6 M aqueous solution of dopa is shown in Figure 5. The most prominent SERS bands of dopa (Figure 5) at 1143, 1267, 1343, 1427, and 1497 cm-1 correspond to those reported in the SERS spectrum of dopa adsorbed on a roughened silver electrode.20 The only exception is the position of the 1497cm-1 band which is reported to be at 1479 cm-1 in ref 20. Detectable SERS signals of the 1479-cm-1 band were obtained with samples prepared out of 3 × 10-7 M solutions. The sample size of dopa used in our experiment is 5 pmol, i.e., 785 pg, which is also an upper bound of its detection limit. The detection limit is presumably considerably smaller than this quantity since the SERS spectra observed were of good quality; hence, useful analytical information would still be obtainable from spectra of much smaller samples. The 5 pmol detection limits (650 pg for pht and 785 pg for dopa) required to produce good quality SERS spectra (Figures 4 and 5) are lower than the analogous limits reported for adsorbates in aqueous Ag colloids, 35 ng for p-aminobenzoic acid and 7 ng for 2-aminofluorene,21 and comparable to the SERS detection limits obtained for adsorbates on silver-coated submicrometer spheres deposited on filter paper, which is reported to be 0.2 ng for carbazole, 1.4 ng for 4-aminopyrene, and 0.3 ng for benzoic acid.22 A substantially lower SERS detection limit (1 pg) has been reported for crystal violet in a microsample prepared from a drop of silver colloid evaporated to dryness.23 However, crystal violet benefits from resonance enhancement in addition to its SERS enhancement (i.e., its spectrum is actually surface-enhanced resonance Raman scattering), facilitating its detection. 4. SERS of bpy Adsorbed on a Single Silver Colloid Aggregate out of Dichloromethane. The micropreparative technique also allows the application of SERS microscopy to the detection of SERS from water-insoluble adsorbates or from water-soluble adsorbates dissolved in a nonaqueous solvent. This is normally difficult to accomplish with conventional aqueous colloids. The SERS spectrum of a bpy-covered Ag cluster prepared by aggregating predeposited Ag colloidal particles by adsorption of bpy from a dichloromethane solution is shown in Figure 6. The frequencies of the bpy modes differ from those routinely observed for bpy adsorbed on aqueous Ag colloid.24 However, the positions of the prominent bands (Figure 6) at 1173, 1276, 1323, 1487, 1558, and 1601 cm-1 correspond well to the SERS bands attributed to bpy molecules adsorbed out of
3174 J. Phys. Chem., Vol. 100, No. 8, 1996 dichloromethane onto silver particles, which have been deposited on glass as Ag-bpy films (1169, 1273, 1319, 1489, 1559, 1607 cm-1).25 In conclusion, SERS microspectroscopy of single adsorbatecovered silver aggregates appears to be feasible, opening the door to the detailed investigation of the optical properties of fractal cluster as a function of parameters such as cluster size and fractal dimension. Additionally, it appears to be a promising analytical tool, with detection limits potentially in the subpicomolar range. Acknowledgment. We are grateful to NSERC for financial support. B. V. thanks the Grant Agency of the Czech Republic for financial support (Grant No. 203/93/0106). References and Notes (1) Moskovits, M. J. Chem. Phys. 1978, 69, 4159. (2) Van Duyne, R. P.; Haller, K. L.; Altkorn, R. I. Chem. Phys. Lett. 1986, 126, 190. (3) Sutherland, W. S.; Winefordner, J. D. J. Raman Spectrosc. 1991, 22, 541. (4) Laserna, J. J.; Sutherland, W. S.; Winefordner, J. D. Anal. Chim. Acta 1990, 237, 439. (5) Hembree, D. M., Jr.; Oswald, J. C.; Smyrl, N. R. Appl. Spectrosc. 1987, 41, 267. (6) McGlashen, M. L.; Guhathakurta, U.; Davis, K. L.; Morris, M. D. Appl. Spectrosc. 1991, 45, 543. (7) Sutherland, W. S.; Laserna, J. J.; Winefordner, J. D. Spectrochim. Acta 1991, 47A, 329-337.
Vlcˇkova´ et al. (8) Weitz, D. A.; Oliveria, M. Phys. ReV. Lett. 1984, 52, 1433. (9) Weitz, D. A.; Lin, M. Y.; Sandroff, C. J. Surf. Sci. 1985, 158, 147164. (10) Vlcˇkova´, B.; Mateˇjka, P.; Sˇ imonova´, J.; Pancˇosˇka, P.; C ˇ erma´kova´, K.; Baumruk, V. J. Phys. Chem. 1993, 97, 9719-9729. (11) C ˇ erma´kova´, K.; Sˇ esta´k, O.; Mateˇjka, P.; Baumruk, V.; Vlcˇkova´, B. Collect. Czech. Chem. Commun. 1993, 58, 2682. (12) Stockman, M. I.; Shalaev, V. M.; Moskovits, M.; Botet, R.; George, T. F. Phys. ReV. B 1992, 46, 2821. (13) Suh, J. S.; Moskovits, M. J. Phys. Chem. 1984, 88, 5526. (14) Suh, J. S.; Moskovits, M.; Shakhesemapour, J. J. Phys. Chem. 1993, 97, 1678. (15) Walls, D. J.; Bohn, P. W. J. Phys. Chem. 1990, 94, 2039. (16) Mitchell, R. W.; Glass, R. W.; Merrit, J. A. J. Mol. Spectrosc. 1970, 36, 310. (17) Ishida, H.; Fukuda, H.; Katagiri, G.; Ishitani, A. Appl. Spectrosc. 1986, 40, 322. (18) Okada, K.; Komatsu, S.; Ishigaki, T.; Matsumoto, S. Appl. Phys. Lett. 1992, 60, 959. (19) Seeman, P.; Guan, H.-Ch.; Van Tol, H. H. M. Nature 1993, 365, 441. (20) Lee, N.-S.; Hsieh, Y.-Z.; Paisley, R. F. Morris, M. D. Anal. Chem. 1988, 60, 442. (21) Torres, E. L.; Winefordner, J. D. Anal. Chem. 1987, 59, 16261632. (22) Vo-Dinh, T.; Hiromoto, M. Y. K.; Begun, G. M.; Moody, R. L. Alan. Chem. 1984, 56, 1667. (23) Sheng, R.-S.; Zhu, L.; Morris, M. D. Anal. Chem. 1986, 58, 1116. (24) Kim, M.; Itoh, K. J. Phys. Chem. 1987, 91, 126. (25) Vlcˇkova´, B.; Barnett, S. M.; Kanigan, T.; Butler, I. S. Langmuir 1993, 9, 3234.
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