Preparation and Characterization of Ag Colloid Monolayers

Langmuir 1998, 14, 5695-5701

5695

Preparation and Characterization of Ag Colloid Monolayers Robin M. Bright, Michael D. Musick, and Michael J. Natan* Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802-6300 Received February 4, 1998. In Final Form: June 15, 1998 The process of Ag colloid self-assembly onto organosilane-functionalized glass, carbon, and Au surfaces has been studied using UV-visible spectrophotometry, quartz crystal microgravimetry, and atomic force microscopy. Ag colloids prepared by citrate reduction and by ethylenediaminetetraacetic acid reduction of Ag+ yield different deposition kinetics, primarily a result of differences in particle concentration and monodispersity. The sticking probabilities and equilibrium constants for each type of colloid were measured. Surface-enhanced Raman scattering (SERS) and electrochemical properties of Ag colloid monolayers were compared to bulk Ag. Deposition of Ag colloid onto carbon electrodes improves the heterogeneous rate constant for electron transfer to [Ru(NH3)6]3+, but the modified electrodes still fall short of values obtained for bulk Ag. Likewise, Ag colloid monolayers are less SERS-active than roughened bulk Ag. The effects of particles polydispersity on these phenomena are discussed.

Introduction Covalent and noncovalent self-assembly of colloidal metal nanoparticles to yield macroscopic surfaces is beginning to gain notice.1-5 In many respects, the properties of colloidal Ag and Ausfor which there are many examples of self-assembly6-10sare quite similar, but Ag exhibits superior catalytic activity11 and improved enhancement factors for surface-enhanced Raman scattering (SERS).12 Accordingly, understanding the process of Ag colloid self-assembly with the same level of detail * To whom all correspondence should be addressed. (1) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313-1317. (2) (a) Chumanov, G.; Solkolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466-9471. (b) Chumanov, G.; Sokolov, K.; Cotton, T. M. J. Phys. Chem. 1996, 100, 5166-5168. (c) Fritzsche, W.; Symanzik, J.; Sokolov, K.; Cotton, T. M.; Henderson, E. J. Colloid Interface Sci. 1997, 185, 466-472. (3) (a) Rubin, S.; Bar, G.; Taylor, T. N.; Cutts, R. W.; Zawodzinski Jr., A. J. Vac. Sci. Technol. A 1996, 14, 1870-1877. (b) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski Jr., T. A. Langmuir 1996, 12, 1172-1179. (4) (a) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (b) Emory, S. R.; Nie, S. Anal. Chem. 1997, 69, 2631-2635. (5) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904-13910. (6) (a) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-743. (b) 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, 1629-1632. (7) Bright, R. M.; Walter, D. W.; Musick, M. D.; Jackson, M. A.; Allision, K. J.; Natan, M. J. Langmuir 1996, 12, 810-817. (8) Grabar, K. C.; Allision, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353-2361. (9) (a) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148-1153. (b) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1154-1157. (10) Grabar, K. C.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S.-L.; Natan, M. J. Anal. Chem. 1997, 69, 471-477. (11) (a) Seyedmonir, S. R.; Plischke, J. K.; Vannice, M. A.; Young, H. W. J. Catal. 1990, 123, 534-549. (b) Mao, C.-F.; Vannice, M. A. J. Catal. 1995, 154, 230-244. (12) (a) Brandt, E. S.; Cotton, T. M. In Investigations of Surfaces and InterfacessPart B, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; Vol. IXB. (b) Creighton, J. A. Anal. Proc. 1993, 30, 28-30. (c) Garrell, R. L. Anal. Chem. 1989, 61, 400A411A. (d) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys: Condens. Matter 1992, 4, 1143-1212.

known for Au6-10 is of value. This is particularly true in light of recent reports of SERS from single, immobilized Ag nanoparticles.4 One critical (and complicating) difference between colloidal Au and Ag is that the latter is more difficult to prepare as monodisperse solutions of a prespecified size. Published syntheses of colloidal Ag date back a century,13 and modern syntheses describe a variety of reductants that yield colloid upon mixing with aqueous solutions of Ag+, including citrate,14 ethylenediaminetetraacetic acid (EDTA),15 dye molecules,16 and NaBH4.17 Water/oil emulsions have been used to control the size and dispersity of colloidal Ag prepared by reduction of Ag+ reduction.18 In addition, colloidal Ag particles have been prepared by photochemical means,19 by γ irradiation,20 and by laser ablation of bulk Ag surfaces.21 The latter approach, judging from subsequently published electron microscopy images,2 appears to have produced monodisperse 100nm-diameter Ag nanoparticles, but detailed characterization was not reported. An earlier paper7 described efforts to make reproducible SERS-active substrates by chemical and electrochemical reduction of Ag+ onto a Au colloid monolayers. That work was motivated by the excellent monodispersity of colloidal Au and by success in preparing reproducible, macroscopic colloidal Au substrates.6,8-9 Herein, we focus on direct assembly of colloidal Ag formed by both citrate and EDTA reduction. Using optical spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), and quartz crystal microgravimetry (QCM), we have investigated the kinetics, thermodynamics, and reproduc(13) Carey Lea, M. Am. J. Sci. 1889, 37, 476. (14) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391-3395. (15) Lee, N.-S.; Sheng, R.-s.; Morris, M. D.; Schopfer, L. M. J. Am. Chem. Soc. 1986, 108, 6179-6183. (16) Zhai, X.; Efrima, S. J. Phys. Chem. 1996, 100, 1779-1785. (17) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790-798. (18) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H.-F. J. Colloid Interface Sci. 1992, 148, 80-90. (19) Subramanian, S.; Nedeljkovic, J. M.; Patel, R. C. J. Colloid Interface Sci. 1992, 150, 81-83. (20) Henglein, A. J. Phys. Chem. 1980, 84, 2461-3467. (21) Nedderson, J.; Chumanov, G.; Cotton, T. M. Appl. Spectrosc. 1993, 47, 1959-1964.

S0743-7463(98)00138-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/04/1998

5696 Langmuir, Vol. 14, No. 20, 1998

Bright et al.

ibility of surface formation. In addition, the SERS and electrochemical properties of the resulting surfaces have been investigated. We find that Ag nanoparticles prepared by both EDTA and citrate reduction can be self-assembled onto organosilane-coated glass slides. However, the favorable SERS properties of selected isolated Ag nanoparticles4 do not translate to these ensembles. To the contrary, Ag colloid monolayers derived from both methods of particle synthesis yield weak SERS spectra compared to Ag-clad Au7,22 and to evaporated Ag films on several different substrates.23 Experimental Section Materials. All reagents were used as received unless noted. AgNO3, pyridine, EDTA, sodium molybdate, and Na2SO4 were obtained from Aldrich. (3-Aminopropyl)trimethoxysilane (APTMS) was obtained from United Technologies. Carbon rods and Ag foil for electrodes were obtained from Johnson Matthey. Trisodium citrate dihydrate, imidazole, and KCl were obtained from Sigma. HAuCl4 was obtained from Acros. H2SO4, spectrophotometric grade CH3OH, and HCl were obtained from EM Science. HNO3 was obtained from Baker. H2O2 was obtained from VWR. Cu grids were obtained from Ted Pella. Insulating white and clear epoxy (Epoxy-Patch) were purchased from Dexter Corp. [Ru(NH3)6]Cl3 was obtained from Strem. Polished ATcut quartz crystals (9.0 MHz, surface features ∼0.02 µm in depth)8 with Au electrodes were obtained from International Crystal Manufacturing (Oklahoma City, OK). 0.05-, 0.3-, and 0.1-µm alumina polish was obtained from Buehler. Glass slides were obtained from Fisher Scientific. H2O (18 MΩ) was obtained from a Barnstead Nanopure water purification apparatus. Preparation of Colloidal Ag Nanoparticles. Two Ag colloidal preparations were used throughout these experiments. The first used citrate as a reductant following the procedure of Lee and Meisel.14 The second colloidal Ag solution was generated by EDTA reduction of Ag+.15 It should be noted that if the EDTAbased preparation was allowed to heat too long after the addition of HCl, the colloidal sol became similar to the citrate Ag in appearance (i.e., turbid). In such cases, the colloidal Ag solutions were discarded. The preparation of colloidal Au nanoparticles used in some of the experiments was previously described.6 Methods for analysis of colloidal particle sizes by TEM have been previously published.8,10 Surface Preparation. General procedures for surface assembly have been previously described, as have descriptions of substrate preparation.6-9 Electrodes. Ag foil electrodes were prepared by attaching a copper wire to the back surface of the electrode face with solder and embedding the back face into an epoxy-filled glass tube. Electrochemical connection was made through the copper wire. Ag foil electrodes were polished with alumina prior to use. Carbon rod electrodes were made by incasing the C rod in a glass tube sealed with clear epoxy. Electrochemical connection was made between the C rod and a Cu wire by a drop of mercury. After the initial [Ru(NH3)6]Cl3 scans for each electrode, the electrodes were rinsed well with H2O, derivatized in 1% APTMS for 15 min, rinsed with CH3OH and H2O, and immersed in colloidal solutions overnight. Instrumentation/Data Analysis. SERS spectra were acquired using a Spex 1877 triple monochromator fitted with 1200 grate/mm spectrograph grating and 600 grate/mm filter stage gratings and equipped with a liquid N2-cooled charge coupled device (CCD) detector. Excitation was provide by a rebuilt Spectra-Physics model 164 laser fitted with an Ar+/Kr+ mixed (22) (a) Baker, B. E.; Kline, N. J.; Treado, P. J.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 8721-8722. (b) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. Phys. Chem. 1996, 100, 718-724. (23) (a) Aroca, R.; Martin, F. J. Raman Spectrosc. 1985, 16, 156162. (b) Jennings, C. A.; Kovacs, G. J.; Aroca, R. J. Phys. Chem. 1992, 96, 1340-1343. (c) Roark, S. E.; Rowlen, K. L. Appl. Spectrosc. 1992, 46, 1759-1761. (d) Roark, S. E.; Rowlen, K. L. Chem. Phys. Lett. 1993, 212, 50-56. (e) Roark, S. E.; Semin, D. J.; Rowlen, K. L. Anal. Chem. 1996, 68, 473-480. (f) Douketis, C.; Haslett, T. L.; Wang, Z.; Moskovits, M.; Iannotta, S. Prog. Surf. Sci. 1995, 50, 187-195.

Figure 1. Optical spectra of citrate-derived colloidal Ag a, particle concentration 18.2 pM and EDTA-derived colloidal Ag b, particle concentration 160 pM. gas tube (Exitek). CCD calibration was accomplished by assigning known imidazole peaks at each wavelength change to the spectra acquired. SERS corrections for Raman cross section, excitation frequency, power, and band-pass were done for the wavelength-dependent experiments by running a standard of solid NaMoO4 at each excitation wavelength and normalizing to the Raman intensity above background for the 999-cm-1 peak of NaMoO4. Optical spectra were recorded using an HP8452A diode array ultraviolet-visible spectrophotometer with 2-nm resolution and a 1-s integration time. A Teflon block (one-third sample height) was used to keep samples upright in cuvettes. All electrochemical measurements were carried out using a PAR model 273A potentiostat/galvanostat operated with model 270 software on a Gateway 486 IBM-compatible computer. QCM measurements were carried out using a home-built oscillating circuit regulated by a Heathkit power supply (model IP-18). The frequency change was monitored by an Fluke high-resolution programmable timer/counter (model PM-6680B) equipped with a high-stability oscillator time base. Data were collected via an IEEE interface to an PC computer. All QCM experiments were conducted inside an Faraday cage to reduce electrical interference. AFM Instrumentation. All AFM measurements were made on a Digital Nanoscope IIIa in tapping mode with Digital TESP tips and a large sample stage. Images were captured using Nanoscope IIIa version 4.22r2 computer program. Samples were prepared by rinsing with CH3OH and leaving in air until dry.

Results and Discussion Optical spectra of Ag colloidal particles contain information about the particle concentration (absorbance), particle size (position of λmax), and dispersity of particle size in solution (width of absorbance band). Figure 1 shows the optical spectra of diluted citrate- and EDTA-derived colloidal Ag acquired immediately after synthesis. The citrate colloid (λmax ) 410 nm) has an asymmetric peak with large tail that extends into the red. The EDTA colloid has a symmetric and narrow peak (λmax ) 408 nm) suggesting a smaller average particle size and more monodisperse size range. Confirmation of the particle size and shape was done by TEM analysis of both Ag colloid preparations. Figure 2 compares TEM images of the citrate (top) and EDTAderived Ag colloids (bottom). The citrate Ag has both spherical- and rod-shaped colloidal Ag particles in a very wide range of sizes; the longest dimensions (the major axis of rods) are 15 times the diameter of the smallest spheres. In several additional images of citrate-derived Ag sols (Supporting Information), both the spherical and rod particles are present in each image, but it appears that the rods comprise less than 1% of the total colloidal particles. The EDTA colloid shows only a polydisperse assortment of spherical particles. Additional images of the EDTA colloid show no evidence of rod-shaped particles (Supporting Information). In contrast, TEM images of

Ag Colloid Monolayers

Langmuir, Vol. 14, No. 20, 1998 5697

Figure 3. Histograms showing distribution of particle diameters for two preparations each of citrate- (A, B) and EDTAreduced Ag colloid, (C, D). 9 and b correspond to diameters of the major and minor axes, respectively. Table 1. Sizing Information on Ag Colloidal Particles reductant

batch

major axis × minor axis (nm)a

min (nm)

max (nm)

citrate

1 2 1 2

55 (29) × 38 (19) 22 (21) × 18 (11) 14 (9) × 12 (8) 15 (9) × 13 (8)

11 4.7 3.7 3.0

187 342 55 49

EDTA a

Figure 2. TEM images of (top) citrate-reduced and (bottom) EDTA-reduced Ag colloids. Scale bar equals 100 nm for each image.

colloidal Au particles typically indicate a small distribution in particle sizes.6-10 Histograms showing the number of particles of a given size versus particle size (as derived from TEM image analysis) provide a useful measure of particle size and dispersity. Four such plots are shown in Figure 3. The first batch of citrate-reduced Ag colloid (panel A, 223 particles counted) shows an even distribution of particle diameters between 15 and 60 nm and a number of particles with major axes of >100 nm. The second batch of the citrate Ag colloid (panel B, 641 particles counted) exhibits a greater percentage of spherical particles with smaller average diameter and fewer rod-shaped particles. Both particle size and dispersity of the two batches of citratereduced Ag are drastically different in composition. Panels C and D represent two different preparations of EDTA Ag colloids (221 and 448 particles counted, respectively). All the nanoparticles in the EDTA preparation have diameters less than 60 nm with the greatest percentage less than 20-nm diameter; the batch-to-batch variation in EDTAderived Ag is substantially lower than for the citrate(24) PWHM is defined as twice the peak width from λmax to the wavelength at lower energy for which A ) 0.5Amaximum.

Mean values, with standard deviations in parentheses.

derived Ag. Numerical representation of these data (average major and minor axis diameters, standard deviation, and range of particle sizes) are given in Table 1. The extremely high particle size dispersity in EDTAand citrate-derived colloidal Ag presages significant difficulties for characterization of Ag colloid monolayers by UV-visible spectroscopy, a rapid and simple method that is very useful for Au colloid monolayers. For colloidal Au, the peak width at half-maximum (pwhm)24 has proven to be a good yardstick of particle dispersity; for 12-nmdiameter particles with standard deviations of (1.5 nm, pwhm ) 80 nm.25 As a result, the dispersity of a colloidal Au preparation can be quickly estimated by UV-vis. In contrast, this parameter is of little, if any, value for colloidal Ag: EDTA has a 56-nm pwhm, yet the standard deviations in particle diameters is >50% of the mean (Table 1). Likewise, with a pwhm of 224 nm, little can be said about the size distribution of citrate Ag other than that it is large. Similar arguments obtain for the position of λmax, generally observed to move to longer wavelengths with increasing particle size.20,26-28 While this is true for colloidal Au,29 there is but a 2-nm shift in λmax (which is (25) Brown, K. R.; Walter, D. G.; Natan, M. J., submitted. (26) Heath, J. R. Phys. Rev. B: Condens. Matter 1989, 40, 99829985. (27) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014-1023. (28) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Germany, 1995; Vol. 25, p 532. (29) (a) Frens, G. Nature (London) 1973, 241, 20-22. (b) Goodman, S. L.; Hodges, G. M.; Trejdosiewicz, L. K.; Livingsten, D. C. J. Microsc. 1981, 123, 201-213.

5698 Langmuir, Vol. 14, No. 20, 1998

equal to the instrumental resolution) for the EDTA and citrate Ag colloids, despite a 30-nm difference in mean diameter. Indeed, there is even disagreement in the literature on whether λmax shifts to the red or to the blue with increasing Ag particle size.30-32 Part of this inconsistency likely reflects variation in mean particle from batch to batch. Moreover, work by Martin’s group has shown that colloidal metal rodssclearly observed in Figure 2 and Supporting Information datashave optical properties that differ from spherical particles.33 The net effect is that, for these preparations of colloidal Ag, particle size cannot be deduced from λmax. The final optical property impacted by particle dispersity is the absorbance at λmax, used to calculate extinction coefficients for particles in solution or on surfaces.9 When the assumption of monodispersity is justified, dividing the total absorbance by the particle concentration (calculated by mass balance34) yields a per particle extinction coefficient that for Au agrees well with theory.35 Data in Supporting Information show that Beer’s law holds for suspensions of colloidal Ag: dilution of the as-prepared colloid leads to proportional losses in absorbance for both citrate and EDTA Ag. However, the actual particle concentrations are not known, nor can they be accurately calculated from these data, because of the large spread in particle sizes.36 Extinction coefficients for particles confined to flat, optically transparent substrates can be calculated by a combination of AFM (to count particles) and UV-vis. This approach has been previously validated for colloidal Au,9 although it should be noted that surface and solution extinction coefficients are not identical, owing to slight differences in surrounding dielectric.37 Figure 4 shows optical spectra in H2O of citrate and EDTA Ag colloid monolayers on APTMS/glass and 3 µm × 3 µm AFM images of both surfaces after drying. The images reveal several key aspects about Ag colloidal self-assembly. (i) The polydispersity present in solution is maintained on surfaces; that is, there appears to be no preferential binding by particles of a particular size or shape. (ii) There are fewer particles per unit area than previously reported for 12-nm-diameter colloidal Au immobilized on the same substrate for the same amount of time. (iii) In both images, but especially in that of the citrate Ag-modified surfaces, there is drying-induced particle aggregation, evidenced by the large number of particles in close proximity to one another. This phenomenon has already been analyzed in detail for large-particle Ag colloidal monolayers2c and has been shown to depend on coverage for Au colloid mono(30) Heard, S. M.; Grieser, F.; Barraclough, C. G. J. Colloid Interface Sci. 1983, 93, 545-555. (31) Chernov, S. F.; Zakharov, V. N. J. Mod. Opt. 1989, 36, 15411544. (32) Sa´nchez-Corte´s, S.; Garcı´a-Ramos, J. V.; Morcillo, G.; Tinti, A. J. Colloid Interface Sci. 1995, 175, 358-368. (33) (a) Foss, C. A., Jr.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1992, 96, 7497-7499. (b) Foss, C. A., Jr.; Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1992, 96, 9001-9007. (c) Foss, C. A., Jr.; Hornyak, G. l.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963-2971. (34) The actual quantity of Ag in solution can be easily verified by atomic absorption. (35) (a) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley-Interscience: New York, 1983. (b) Baker, B. E.; Natan, M. J., unpublished results. (36) Note that the extinction coefficient is proportional to the square of the particle radius,35a while the number of particles is inversely proportional to the cube of the radius (i.e., the volume). Thus, if a sphere of radius r is converted to eight spheres of radius r/2, the net extinction is expected to increase by a factor of 2. (37) (a) Mulvaney, P. Langmuir 1996, 12, 788-800. (b) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1 1991, 87, 38813891.

Bright et al.

Figure 4. Optical spectra in H2O of (top) (a) citrate- and (b) EDTA-derived Ag colloids, and AFM images (3 µm × 3 µm) in air of (middle) citrate-reduced and (bottom) EDTA-reduced Ag colloids. Z scale is 100 and 50 nm for citrate and EDTA Ag colloids, respectively.

layers.38 In accord with previous findings citing the increased tendency of large particles to aggregate,39 a noticeably greater fraction of the bigger particles are found in clusters. By counting the number of particles in each image and using a previously described formula,9 the surface extinction coefficient at λmax is calculated to be  ) 1.1 × 1011 M-1 cm-1 for citrate Ag and  ) 1.5 × 1010 M-1 cm-1 for EDTA Ag.40 As expected,41 these values are higher than for Au, explaining the large extinction of surfaces with such low particle coverages. (38) Bright, R. M.; Keating, C. D.; Baker, B. E.; Grabar, K. C.; Smith, P. C.; Natan, M. J., submitted. (39) Frens, G. Kolloid-Z. Z. Polym. 1972, 250, 736-741. (40) Note that these values do not agree with those calculated from solution spectra (using the assumption of monodispersity), highlighting the batch-to-batch variation in properties of colloidal Ag. (41) Barber, P. W.; Chang, R. K., Ed. Optical Effects Associated With Small Particles; World Scientific: Sinapore, 1988.

Ag Colloid Monolayers

Langmuir, Vol. 14, No. 20, 1998 5699

Figure 6. Change in frequency in time upon exposure of an APTMS-dervitized 9-MHz QCM crystal upon exposure to colloidal Ag prepared by citrate reduction and EDTA reduction.

Figure 5. Absorbance at λmax versus immersion time in (top) citrate- and (bottom) EDTA-reduced Ag colloids. Slopes for the best-fit lines to t1/2 are (top) (9, s) 1.85 × 10-3 and (b, - - -) 1.50 × 10-3; and (bottom) (9, s) 1.16 × 10-2; (b, - - -) 9.01 × 10-3; and (1, ‚‚‚) 8.59 × 10-3.

With these very high extinction coefficients, it is quite easy to monitor the kinetics of surface evolution in real time by UV-vis.42 Plots of absorbance at λmax versus time for both citrate and EDTA Ag colloids are shown in Figure 5; each set of symbols within a given panel corresponds to different kinetics experiments carried out with the same batch of colloid [two for citrate (top), three for EDTA (bottom)]. The data are generated by removing an APTMS-derivatized glass slide from colloid at various times, rinsing, and measuring the optical spectra. Three aspects of the data in Figure 5 are noteworthy. First there are significant differences in the rates of surface formation from trial to trial (with the same colloid solution). For monodisperse colloidal Au particles, it has been shown that the rate of particle immobilization is proportional to (time)1/2, and if the particle radius, solution viscosity, and particle concentration are known, accurate fits of A versus t data can be generated, with the particle sticking probability as the only adjustable variable.9 Here, t1/2 best fits are plotted for the various data sets. Although there is a range of particle sizes (and unknown concentrations for each size), the fit lines are qualitatively reasonable. However, it is unclear why different rates would be measured within a given batch of colloidal particles. Moreover, at early times, the measured absorbances are consistently low compared to the best fits, and the data at later times are consistently high. This can be understood by examination of two of the factors controlling absorbance: surface coverage and extinction coefficient. Since the former goes as (radius)-1/2 and the latter goes as the square of the radius,9 larger particles (which diffuse more slowly) contribute more to the absorbance than smaller particles. This explains the lack of quantitative fits to the data and precludes calculation of meaningful diffusion coefficients. The third interesting feature of the data is differences in the rate of surface formation for the EDTA and citrate Ag. For the latter, the absorbance starts to level off after ∼3000-4000 s, while for the former, the beginnings of a (42) Using a detection of only 0.02 absorbance unit, which is almost 1 order of magnitude above the true limit, an extinction coefficient of 1.1 × 1011 M-1 cm-1 translates to a coverage detection limit of ∼1 × 108 cm-2, or 1 particle/µm2.

plateau can be seen after 600-800 s. This difference reflects the concentration of EDTA-Ag particles: assuming monodisperse particles of the mean values shown in Table 1, the EDTA Ag is ∼9 times more concentrated (2.4 nM vs 54.7 pM). An alternative approach to monitoring surface evolution involves measurement of the mass of immobilized nanoparticles using QCM. For colloidal Au, this approach yielded results very similar to those obtained by UV-vis.8 Figure 6 shows deposition of Ag colloid onto an APTMSderivatized, Au-coated quartz crystal for each colloid preparation. Over a 4500-s time range, the frequency change of the citrate Ag-derivatized crystal changed just over 1000 Hz; the EDTA frequency change was 3000 Hz. While the citrate Ag data exhibit a t1/2-like coverage dependence, the EDTA Ag is deposited much faster, and at two different rates, with the crossover coming at ∼900 s; the origin of this phenomenon is not presently understood. Stabilization of the crystal in air before and after deposition of Ag colloidal particles led to mass changes of 10.5 and 17.2 µg/cm2 for citrate Ag and EDTA Ag, respectively. Conversion of QCM-derived mass changes to particle coverages is based on the same approximations elaborated above for UV-vis measurements, but if the mean diameters in Table 1 are used, there is good agreement between AFM and QCM coverages for samples prepared under identical conditions.43 Measurement of the particle coverage as a function of particle concentration in solution provides data regarding the thermodynamics of particle binding. Previous work on colloidal Au has shown that adsorption can be fit by a Frumkin-type44 isotherm, in which coverage is limited by interparticle repulsion.45 In contrast to kinetics experiments, which can be carried out effectively with a single substrate (and a stopwatch), thermodynamics experiments require multiple substrates, with different coverages on each. This poses a problem for colloidal Ag in terms of reproducibility. Figure 7 shows optical spectra of 21 identically prepared APTMS/glass slides that were placed in solutions of citrate Ag (7 slides, A), EDTA Ag (7 slides, B), and 12-nm-diameter Au colloid (7 slides, C) for the same amount of time. Surface-bound citrate Ag coverage varied between absorbances of 0.4 and 1.0 for (43) For citrate Ag, the coverages determined by QCM are nearly identical to the citrate-derived Ag colloid coverage used in the AFM images (3 × 109 versus 2.6 × 109 particles/cm2). The EDTA-derived Ag colloid has a lower particle coverage in the AFM due to a low coverage sample being used (1.77 × 1011 particles/cm2 for QCM versus 4 × 109 particles/cm2 for AFM), because the absorbance of the sample used for the AFM of the EDTA colloid was only 0.1, whereas typical samples have an absorbance closer to 0.2. (44) Gileadi, E., Ed. Electrosorption; Plenum: New York, 1967. (45) Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J. Chem. Ed., in press.

5700 Langmuir, Vol. 14, No. 20, 1998

Figure 7. Optical spectra in H2O of three sets of seven APTMS derivatized slides in citrate-reduced (A), EDTA-reduced (B), and 12-nm-diameter, citrate-reduced Au colloids (C).

the seven slides prepared. Surfaces derivatized with EDTA Ag showed much less scatter in the absorbance for the seven slides, but it still exceeded that seen for colloidal Au. These poor coverages lead to extremely large errors in adsorption isotherms (Supporting Information). For each colloidal Ag concentration (calculated by assuming monodispersity), eight slides were functionalized by immersion for one week (to reach equilibrium). For both citrate Ag and EDTA Ag, the scatter is sufficiently large to preclude extraction of useful thermodynamic data. It is clear that increasing concentrations of citrate Ag lead to increased absorbance, but a shape for the isotherm cannot be clearly defined. Likewise, coverage in the EDTA seems to have reached a plateau by 20-30 pM, but the location of the plateau is impossible to estimate. Fortunately, the extremely high sample-to-sample variability in Ag colloid coverage has minimal impact on electrochemical properties of colloid-modified electrodes: these particle coverages are so far into the total overlap regime46 that even a factor of 10-20 in coverage difference has little bearing on the observed voltammetry. Thus, carbon rods derivatized with either citrate Ag/APTMS or EDTA Ag/APTMS exhibit “macroelectrode-like” voltammograms for 5 mM [Ru(NH3)6]3+/0.1 M Na2SO4 with peakto-peak separations slightly lower than for underivatized carbon rod and slightly higher than for Ag foils (Supporting Information). The SERS behavior of these surfaces is of paramount importance in determining their ultimate utility. Figure 8 shows SERS data at four excitation wavelengths [647.1 (a), 568 (b), 530.9 (c), and 488 nm (d)] for citrate Ag (top panel) and EDTA Ag (bottom panel) on an APTMSderivatized glass slide immersed in a 50 mM pyridine/0.1 M KCl solution. Through use of a standard to normalize for band-pass, laser power at the sample, and laser frequency factors, the relative intensities are proportional to absolute enhancement factors. For the citrate Ag, 568nm excitation was by far the best with 647.1 nm producing the next best intensity. Interestingly, the two lower wavelengths (closer to the surface plasmon band) did not give the highest enhancement and were much lower than (46) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51.

Bright et al.

Figure 8. Normalized SERS spectra of 50 mM pyridine/0.1 M KCl as a function of wavelength for 647.1, 568, 530.9, and 488 nm, 10 s × 5 accumulations (top) citrate and (bottom) EDTAreduced Ag colloid monolayers.

the 568 nm. The EDTA Ag gave similar results except almost no signal was seen from any wavelength except 568 nm. In both panels, the relative enhancement by 568 nm is almost 4 times that for any other wavelength for both surfaces. Moreover, comparison of relative intensities from the top and bottom panels indicate that citrate Ag is roughly 4-fold more enhancing than EDTA Ag. The SERS intensity depends on particle coverage, with increasing coverage (as shown by UV-vis) leading to increased signal (Supporting Information). For citrate, the SERS data seem to track the absorbance nearly linearly. In contrast, there is poor correlation between EDTA Ag optical spectra and SERS intensity, suggesting that the SERS signal may arise from fraction of particles too small to contribute to the observed extinction. These results are interesting in view of single-particle SERS studies on very similar surfaces.4 In that work, intense SERS spectra were seen for molecules adsorbed to certain subsets of Ag nanoparticles (large spheres, rods, and linear aggregates of smaller particles). The weak overall signal observed herein suggests that only a very small fraction of the particles are contributing to the SERS signal. One possible explanation is that interparticle spacing in these samples is too large to achieve significant interparticle electromagnetic coupling, as previously observed with higher coverages of colloidal Au6 and more profoundly with closely spaced Ag-clad Au.22a Alternatively, the lack of SERS signals could derive from the absence of special sitesseither Ag+ or Ag0sthought to contribute to chemical enhancement mechanisms.12 Summary Self-assembly onto organosilane-coated substrates of colloidal Ag nanoparticles derived from citrate reduction and from EDTA reduction was studied using a variety of bulk and nanoscale techniques. As for colloidal Au, selfassembly can be followed easily by UV-vis and quartz crystal microgravimetry, and the resulting 2-D particle arrays can be imaged by atomic force microscopy. However, particle polydispersity precludes calculation of meaningful vaules for optical, kinetic, and thermodynamic parameters. Likewise, polydispersity in both citrate Ag

Ag Colloid Monolayers

and EDTA Ag severely limits the construction of useful, reproducible, SERS-active Ag substrates. This limitation will exist until a method to make large quantities of monodisperse Ag particles becomes available. Until then, the optimal procedure for production of reproducible, SERS-active substrates appears to be the chemical reduction of Ag+ onto a preformed Au colloid monolayer. Acknowledgment. Support from NSF (Grants CHE9256692, CHE-9627338), NIH (Grant GM55312-01), EPA (Grant R825363-01-0), and USDA (Grant 96-35102-3840), is gratefully acknowledged. Instrumentation for scanning probe microscopy experiments was provided by NSF Grant CHE-9626326. Acknowledgment is also made to the

Langmuir, Vol. 14, No. 20, 1998 5701

Electron Microscopy Facility for the Life Sciences in the Biotechnology Institute at The Pennsylvania State University. Supporting Information Available: TEM images of citrate- and EDTA-reduced colloidal Ag, Beer’s law plots for both Ag colloids, absorbance versus colloid concentration plots for both sets of colloidal Ag, and coverage-dependent absorbance and SERS spectra for both batches of colloidal Ag and cyclic voltammetry carbon rods functionalized with both types of particles (6 pages). See any current masthead page for ordering and Internet access instructions. LA980138J