Controlled and Reversible Formation of Nanoparticle Aggregates and

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Langmuir 2000, 16, 6682-6688

Controlled and Reversible Formation of Nanoparticle Aggregates and Films Using Cu2+-Carboxylate Chemistry Allen C. Templeton, Francis P. Zamborini, W. Peter Wuelfing, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received February 28, 2000. In Final Form: May 24, 2000 The controlled and reversible formation of transiently soluble alkanethiolate- and tiopronin-monolayerprotected cluster (MPC) aggregates is demonstrated by using Cu2+-carboxylate chemistry to form clusterCu2+-cluster linkages. The aggregation process and its reversibility were monitored using UV-vis and FTIR spectroscopy. The amount of Cu2+ required to induce aggregation of dissolved tiopronin-MPCs is a strong function of pH, increasing at lower pH. Cu2+-carboxylate chemistry can also be utilized to prepare surface-attached films of tiopronin-MPCs in a manner analogous to alkanethiolate-MPCs, monitoring the film formation using FTIR external reflectance spectroscopy. Experiments exploring the diffusional and capacitive properties of tiopronin-MPCs are also reported.

Introduction Substantial progress1 has been made toward understanding the electronic, optical, chemical, and physical properties of monolayer-protected clusters (MPCs). Although most progress has been made with alkanethiolateMPCs,2 water-soluble monolayer-protected gold clusters, such as the tiopronin-MPCs recently reported3 by our group, are beginning to receive attention. Water-soluble MPCs can be prepared with ease, comparable to their alkanethiolate counterparts, and offer similar handling and characterization features.3 A recent report4 has shown that tiopronin-MPCs can be functionalized with multiple copies of electroactive and fluorescent moieties. Substantial research attention has also been aimed at the preparation of ordered arrays and films of Au nanoparticles.5 Motivations to produce Au nanoparticle surface structures include, among others, their utilization (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 26 and references therein. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (3) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (4) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081-7089. (5) (a) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vexmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (b) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (c) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (d) Giersig, M.; Mulvaney, P. J. Phys. Chem. 1993, 97, 6334. (e) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (f) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (g) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (h) Kiely, C. J.; Fink, J.; Brunst, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (i) Lin, X. M.; Boz, J.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 1999, 11, 198. (j) Zhong, C. J.; Douglas, M.; Zhang, W. X.; Leibowitz, F. L.; Eichelberger, H. H. Chem. Commun. 1999, 1211. (k) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849. (l) Alivastos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr; Schultz, P. G. Nature 1996, 382, 609. (m) Brust, M.; Kiely, C. J.; Bethel, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367. (n) Li, M.; Wong, K. K. W.; Mann, S. Chem. Mater. 1999, 11, 23. (o) Liu, J.; Mendoza, S.; Roma´n, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (p) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (q) Aherne, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 1999, 103, 1821.

in nanoscale electronic device components and as substrates for surface-enhanced Raman spectroscopy (SERS).6 A central problem in building arrays of clusters is achieving the formation of ordered domains and longrange organization across multiple cluster layers. Methods devised for the preparation of surface arrays of Au nanoparticles include R,ω-dithiol linkages,5p spontaneous self-assembly,5g,h and electrophoretic deposition.5d,e In addition to surface-confined Au nanoparticle films, there are reports of the formation of Au nanoparticle aggregates using DNA hybridization,5k,l C60 mediation,5m cyclodextrins,5o and substrate-receptor binding (i.e., biotinstreptavidin) schemes.5n-q Methods for forming aggregates that can be reversibly returned to solution have received little attention by comparison. We have recently developed7 Cu2+- and Zn2+-carboxylate chemistry to prepare monolayer and multilayer films of Au alkanethiolateMPCs; quantized double-layer (QDL) charging could be readily observed in these films. This report extends Cu2+carboxylate linker chemistry to the reversible formation of transiently soluble alkanethiolate- and tiopronin-MPC aggregates and their characterization by FTIR and UVvis spectroscopy. We also report herein further experiments on tiopronin-MPCs that shed additional light on their diffusional and capacitive properties. Experimental Section Chemicals. Tiopronin-MPCs were prepared as described previously.3 Tiopronin-MPCs used for surface film formation had the average composition Au201Tiopronin85 (average core diameter 1.8 nm), and those used in aggregation studies had the average composition Au1289Tiopronin126 (average core diameter 3.1 nm). The formulas are based on a combination of thermogravimetric and transmission electron microscopic data and the assump(6) (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. (b) Musick, M. D.; Pen˜a, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844. (c) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (d) 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. (7) Zamborini, F. P.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514-4515.

10.1021/la000287d CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000

Formation of Nanoparticle Aggregates and Films

tion of an ideal shell structure.3 Alkanethiolate-MPCs were also prepared as detailed elsewhere8 and had the average composition, Au140[CH3(CH2)11S]53. Place-exchange reactions9 of alkanethiolate-MPCs with mercaptoundecanoic acid (abbreviated MUA, Aldrich 98%) led to MUA/ alkanethiolate-MPCs of average composition Au140[CH3(CH2)11S]38[CO2H(CH2)10S]15. House-distilled water was further purified using a Barnstead Nanopure system (g18 MΩ). All other chemicals were reagent grade and used as received. Spectroscopy. UV-vis spectra (200-800 nm, 1 nm resolution) were acquired using an ATI UNICAM UV4 double-beam spectrophotometer using 1 cm quartz cells (Starna). Infrared spectra were collected with a Bio-Rad model 6000 FTIR spectrometer in external reflectance mode for MPC films, or in transmission mode for MPC aggregates pressed into KBr plates. Formation of Tiopronin-MPC Multilayer Films using Cu+2 Linkers (Scheme 1). Multilayer films of tiopronin-MPCs were prepared as follows: (a) a Au metal film was deposited onto a freshly cleaned (1:1 H2O2/H2SO4 etch followed by 2-propanol rinse) glass slide using an Edwards 306 metal film evaporator; (b) an initial thiolate monolayer was formed on the freshly prepared Au metal film by placing it into a ∼1 mM ethanol solution of mercaptoundecanoic acid (MUA) for 48 h; (c) the MUA surface was then rinsed with ethanol and dipped into a ∼5 mM ethanolic Cu(ClO4)2 solution; and (d) the resulting Cu2+-carboxylate film was removed, rinsed with ethanol, dipped into a ∼1 mM basic methanol (pH 9.5) suspension of tiopronin-MPC, and then rinsed successively with Nanopure water and ethanol. The above procedure results in the formation of a layer of tiopronin-MPCs. Repetition of steps c and d leads to the formation of a multilayer film of tiopronin-MPCs held together by Cu2+-carboxylate bonds. Reversible Formation of Alkanethiolate- and Tiopronin-MPC Aggregates using Cu+2 (Scheme 2). A 2 mL aliquot of aqueous (pH 2 or 6.1) 1.8 × 10-5 M tioproninMPC (4.6 × 10-6 M in acids) in a 1 cm quartz cuvette (Starna) was mixed with a fixed quantity of 0.1 mM aqueous Cu(NO3)2 (Table 1). The cuvette was capped and inverted once, and the UV-vis spectrum and surface plasmon band position were measured. Aggregation of individual tiopronin-MPCs was soon evident in the spectrum, and after a few minutes, aggregates began to settle out. Aggregation and precipitation can, to a substantial degree, be suppressed by lowering the pH and adjusting the quantity of Cu2+ added. Aggregated material can be completely redissolved into solution by adding sodium acetate in an amount equivalent to the number of moles of Cu2+ added to the original mixture of tioproninMPC/Cu(NO3)2 or by adding 1-2 drops of concentrated acetic acid. For MUA/alkanethiolate-MPCs, aggregation was induced in ∼1 mM EtOH solutions (only the EtOH-soluble portion of the exchanged MUA/alkanethiolate-MPCs was used) by delivering Cu2+ from a ∼5 mM ethanolic Cu(ClO4)2 solution; these aggregates could also be redissolved using acetic acid. Taylor Dispersion Measurements. The Taylor apparatus consisted of a Rheodyne model 5020 low pressure (8) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-Z.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (9) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175.

Langmuir, Vol. 16, No. 16, 2000 6683 Scheme 1. Cartoon Illustration of Tiopronin-MPC Film Formationa

a Step a: exposure of mercaptoundecanoic acid (MUA) monolayer to Cu2+ solution. Step b: exposure of Cu2+carboxylates to basic methanol suspension of tiopronin-MPC. The above two steps are repeated to construct multilayers of tiopronin-MPC held together by Cu2+-carboxylate bonds

injection valve in-line with a Waters model 1050 HPLC pump, a 30.5 m Teflon column with an inner diameter of 0.3 mm (Supelco), and a Waters model 484 UV-vis detector. All runs were performed at a mobile phase flow rate of 0.1 mL/min at 25 °C. These dimensions satisfy the Taylor criteria, which are outlined in our previous report.10 The UV-vis detector was set to 250 nm to detect the eluted plug of MPC solution. The data were collected on a Waters model 745B data module (strip chart recorder), from which the peak width, W1/2, was measured manually. Rotated-Disk Voltammetry. Rotated disk electrode voltammetry was performed using a MSRX rotator (Pine Instrument Co.) with a 0.15 cm2 Au working electrode, (10) Wuelfing, W. P.; Templeton, A. C.; Hicks, J. F.; Murray, R. W. Anal. Chem. 1999, 71, 4069.

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Scheme 2. Cartoon Illustration of Reversible Aggregation of Tiopronin-MPCs Using Cu2+-Carboxylate Chemistry and Disaggregation by Addition of Sodium Acetate or Acetic Acid

Table 1. Mixing Tiopronin-MPCs with Cu2+ to Induce Aggregation with Corresponding Surface Plasmon Band Position moles moles mole ratio moles tiopronin Cu2+ Cu2+/ surface -CO2H added mole ratio tiopronin- plasmon MPC CO2H (µM)a (µM) Cu2+/MPC pH (µM)a band, nm 2

0.037

6.1 0.037

4.6

4.6

0 28.5 42.8 57 76 95 123 0 0.35 0.59 0.72 0.94

770 1156 1541 2054 2568 3324 10 16 20 25

6 9 12 17 21 27 0.08 0.13 0.16 0.20

507 512 516 528 536 543 554 508 514 520 532 546

a The tiopronin-MPCs used here were of the average composition Au1289Tiopronin126 (average core diameter of 3.1 nm).

which had been previously polished with 0.5 µm diamond paste (Buehler)and then rinsed with water, ethanol, and acetone in succession. Cycling of the potential in 0.5 M H2SO4 was used to verify the cleanliness of the electrode surface prior to each experiment.11 Following this procedure, a monolayer of hexadecanethiol was formed on the electrode surface by placing the freshly cleaned electrode in an ethanol solution ∼1 mM in hexadecanethiol (Aldrich) for 24 h and then washing the surface with ethanol and acetone immediately prior to use. A threecompartment cell with the Au rotated disk electrode in the central compartment and with Pt counter and Ag/ AgCl reference electrodes in the fritted sidearms of the cell was employed. The rotation rate data reported are at (11) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1980; Vol. 9, p 1.

Figure 1. (A) FTIR-ERS spectra of mercaptoundecanoic acid (MUA) monolayer on gold before (- -) and after (s) exposure to ∼5 mM ethanolic Cu2+ for 5 min. (b) FTIR-ERS spectra after five 5 min alternate dip/rinse steps with ∼5 mM ethanolic Cu2+ and a ∼1 mM basic methanol (pH 9.5) suspension of tioproninMPC (- -) and after a final 5 min exposure of the final tioproninMPC multilayer to a ∼5 mM ethanolic Cu2+ solution (s). Characteristic bands associated with the MUA monolayer or tiopronin-MPC are labeled.

100, 400, 800, 1200, 1600, 2000, and 2400 rpm in aqueous (0.7-1.8) × 10-5 M tiopronin-MPC/0.1 M NH4PF6 and at a potential sweep rate of 10 mV/s, generated using a Pine Model AFCBP1 bipotentiostat. Results and Discussion Experiments using Cu2+-carboxylate chemistry to form tiopronin-MPC films are described first, followed by the reversible formation of alkanethiolate- and tiopronin-MPC aggregates. Last, we describe experiments aimed at further elucidating diffusion and capacitance properties of tiopronin-MPCs. Formation of Tiopronin-MPC multilayers with Cu+2 Linkers. Each tiopronin ligand on tiopronin-MPCs has a free terminal -CO2H group. Scheme 1 outlines a procedure for preparing films of tiopronin-MPCs that is analogous to that reported previously7 for MPCs with mixed alkanethiolate/mercaptoundecanoic acid (MUA) monolayers. A MUA monolayer is formed on a clean Au substrate and then metalated by soaking in ethanolic Cu(ClO4)2 (step a). Tiopronin-MPCs are attached to this surface using a basic (pH 9.5) methanol suspension of tiopronin-MPC (step b). Steps a and b can be repeated to attach additional layers of tiopronin-MPCs, and the process can be conveniently followed using FTIR external reflectance spectroscopy (FTIR-ERS) as shown in Figure 1. Figure 1A shows the spectrum for the Au substrate after formation of an MUA monolayer (- -) and after a 5 min exposure to the Cu(ClO4)2 solution (s). Following the latter, the MUA acid band (1714 cm-1) diminishes

Formation of Nanoparticle Aggregates and Films

substantially, and the carboxylate band (1549 cm-1) increases in intensity, reflecting metalation of the surface as Cu2+-carboxylates. Figure 1B shows the FTIR-ERS of a MUA monolayer after five alternating, sequential exposures to the ethanolic Cu2+ solution and the basic (pH 9.5) methanol suspension of tiopronin-MPCs, resulting in the formation of a multilayer tiopronin-MPC film. Following exposures to a tiopronin-MPC solution (- -), bands clearly associated with tiopronin-MPC are observed (amide I, 1635 cm-1, and amide II, 1537 cm-1, labeled). (No amide bands were seen if the dip exposures to Cu2+ solution were omitted.) Following exposures to Cu2+ solutions (s), the intensity of the tiopronin-MPC terminal carboxylic acid bands (1738 cm-1, labeled) decreased, as expected for formation of Cu2+-carboxylates. The concomitant increase in the carboxylate band intensity is less easy to follow, because of its overlap with the tioproninMPC amide I band. The overall spectral absorbance increases with the sequential Cu2+/tiopronin-MPC exposures. These data show that the tiopronin-MPC film formation is mediated by Cu2+-carboxylate chemistry, and that the amount of tiopronin-MPC deposited is a function of the number of Cu2+/tiopronin-MPC dip cycles. Quantification of the amount of cluster deposited in each dipping cycle is an important consideration. UV-vis spectra of MUA/alkanethiolate-MPC films show7 that multilayer formation is possible even from a single Cu2+/ MPC dip cycle, so we ask whether multilayers can also be formed by single Cu2+/tiopronin-MPC dip cycles. The spectra in Figure 1 show that bands characteristic of tiopronin-MPC increase upon each Cu2+/tiopronin-MPC dip cycle. We draw upon previous reports of Gaillard and co-workers12a using FTIR-ERS absorbance data to quantify surface absorption. They reported an extinction coefficient () of 200 cm2/mol for the FTIR-ERS CdO vibration and used the relation A ) Γ, where Γ is surface coverage (mol/cm2), to quantify multilayers of deposited poly(amino acid). Application of this relation and  after five Cu2+/ tiopronin-MPC dips, where the measured absorbance of the CdO stretch is 0.0054, yields a surface coverage of Γ ) 2.7 × 10-5 mol acid/cm2. Taking into account the 85 acid groups per tiopronin-MPC and the five dip cycles, this yields an average of 6.4 × 10-8 mol tiopronin-MPC/cm2 deposited per Cu2+/tiopronin-MPC dip cycle. This is a very large surface attachment yield; for comparison, a monolayer of tiopronin-MPC is expected4 to have a coverage Γ ≈ 1.3 × 10-11 mol/cm2. Of course, CdO orientational effects have been neglected in the above analysis and may contribute to the unexpectedly large experimental surface tiopronin-MPC surface coverage. However, the associated error is not anticipated to be large enough to reduce coverage values to approximate monolayer coverages. Thus, as in the case of MUA/alkanethiolate-MPC film formation using Cu2+, multilayers of tiopronin-MPC are deposited in each Cu2+/tiopronin-MPC dip cycle. As a final note, the positions of the tiopronin-MPC amide I and amide II bands are essentially the same as those reported previously for pressed KBr plate tiopronin-MPC samples.3 Thus, the same mode of hydrogen bonding reported in these samples and discussed in detail elsewhere is observed in tiopronin-MPC films. Reversible Formation of Alkanethiolate- and Tiopronin-MPC Aggregates using Cu+2 Linkers. The preceding Cu2+-carboxylate chemistry also induces the (reversible) formation of transiently soluble tiopronin(12) (a) Auduc-Boyer, N.; Stevenson, I.; Duc, T. M.; Linossier, I.; Gaillard, F. Surf. Interface Anal. 1996, 23, 673. (b) Flach, C. R.; Gericek, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58.

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Figure 2. UV-vis spectra for 4.6 µM (in acid groups) tioproninMPC as a function of Cu2+ addition at pH 2 (bottom to top, µM): 0, 28.5, 42.8, 57, 76, 95, and 123. Inset: Plot of shift of tioproninMPC band position as a function of amount of Cu2+ added. * denotes a band arising from Cu2+ that can be observed in the spectra in the presence of sufficient Cu2+ in solution.

MPC aggregates. Among the reports to date on the controlled formation of Au nanoparticle aggregates,5 there are none on aggregates that can be reversibly disaggregated. The formation and disaggregation of MPC aggregates is cartooned in Scheme 2. Mixing a solution of tiopronin-MPC (or MUA/alkanethiolate-MPC) with a Cu2+ solution initiates aggregation, and in time, the MPC aggregates slowly precipitate from solution. The MPC aggregates can be returned to solution by treating them with a sodium acetate solution or concentrated acetic acid. Formation of solution aggregates can be followed in the UV-vis region by a red shift in the surface plasmon (SP) band toward wavelengths associated with a “collective” band.13 Table 1 lists SP band data for addition of Cu2+ to a tiopronin-MPC solution at pH 2 and 6.1. The surface plasmon band positions were measured immediately after mixing the tiopronin-MPC with a fixed amount of Cu2+ (Figure 2). As the SP band shifts, the substantially broader collective band grows and tails off to the red over several hundred nanometers. Spectra for the aggregation of tiopronin-MPC with Cu2+ at pH 2 and 6.1 are found in Figures 2 and 3, respectively. At pH 2 (Table 1 top and Figure 2), the tiopronin-MPC acid groups are fully protonated so that only with a substantial excess of Cu2+ is the SP band shifted significantly to a collective band, signifying the formation of MPC aggregates. A plot indicating the variation in tiopronin-MPC SP band energy with added Cu2+ is shown in the Figure 2 inset. As Table 1 and Figure 2 (inset) show, only upon addition of a 27-fold molar excess of Cu2+ relative to tiopronin-MPC acid groups (3324-fold molar excess of Cu2+ relative to tiopronin-MPCs), did the collective band maximum match the wavelength of the band maximum observed in a control experiment in which tiopronin-MPC aggregation was forced by suspension in methanol. Addition of much smaller amounts of Cu2+ causes minimal precipitation in a 2-3 h time period, and the band maximum remains close to that typical for fully dissolved (disaggregated) tiopronin-MPC (Table 1, top). It is evident that low pH has the effect of suppressing Cu2+-carboxylate complexation and, thus, tiopronin-MPC aggregation. (13) (a) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; AP: New York, 1969. (b) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (c) Mulvaney, P. Langmuir 1996, 12, 788.

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Figure 3. UV-vis spectra for 4.6 µM (in acid groups) tioproninMPC as a function of Cu2+ addition at pH 6.1 (bottom to top, µM): 0, 0.35, 0.59, 0.72, and 0.94. Inset: Plot of shift of tioproninMPC band position as a function of amount of Cu2+ added.

Templeton et al.

at pH 2 with a 27-fold excess of Cu2+ relative to acid groups) by addition of an amount of sodium acetate equal to the original number of moles of Cu2+ added. Disaggregation is accompanied by dissolution of insoluble material and a shift of the UV-vis cluster SP band from the collective position to that typical for noninteracting Au clusters. Taylor Dispersion Measurements. We previously reported10 the use of the Taylor dispersion method to measure the diffusion coefficients (D) of alkanethiolateand tiopronin-MPCs. The focus of these efforts was the elucidation of the hydrodynamic dimensions of MPCs dissolved in solutionsa topic that had received little previous attention. Dispersion results in the alkanethiolate-MPCs were understandable, but those for tioproninMPCs were problematic in that measured Taylor diffusion coefficient values did not respond systematically to increasing tiopronin-MPC core size. The possibility exists for interaction14 between the ionic tiopronin-MPCs (at pH 6.9, 95% of the tiopronin acid groups are deprotonated) and the surface charges on the glass column used in prior Taylor experiments. In the present experimental studies, the glass column was replaced with a Teflon column, which has a surface layer much less prone to interact with ionic solutes. Briefly, the Taylor dispersion method is based on the Gaussian spreading of a plug of solute (MPC or other) solution injected into a slowly moving solvent stream in an open tubular column. During the course of the experiment, the solute “plug” is spread, or dispersed, by a combination of radial diffusion of the solute in the column and the variation in cross-sectional velocity in an open tubular column. The solute diffusion coefficient is calculated from

D)

Figure 4. Transmission FTIR spectra of aggregated (- -) and nonaggregated (s) tiopronin-MPC pressed into a KBr plate.

At higher solution pH (6.1, Table 1 bottom and Figure 3 inset), where the tiopronin acid groups are ∼85% deprotonated, aggregation is induced by much lower quantities of Cu2+ ion. Table 1 (bottom) and Figure 3 show that a single Cu2+ ion for every five tiopronin-MPC acid groups (or a mole ratio of Cu2+/MPC ) 25) is sufficient to induce extensive aggregation. Stated differently, metalation of ca. 20% of the total MPC carboxylate population suffices to cause aggregation. It is important to note that aggregation can be constrained and, to some extent, controlled by the use of lower pH and smaller quantities of Cu2+, which will allow further analytical studies of dissolved aggregates. A sample of tiopronin-MPC that had been aggregated at pH 2 with a 27-fold excess of Cu2+ was collected on a frit, rinsed with ethanol, and pressed into a KBr plate for transmission IR analysis, the results of which are shown in Figure 4 (- -), along with those of nonaggregated tiopronin-MPC (s). The tiopronin-MPC carboxylic acid band in nonaggregated cluster (1738 cm-1) is entirely absent in the aggregated material. Not only is the mechanism of tiopronin-MPC aggregation shown to be intercluster linking by Cu2+-carboxylate chemistry, but the insoluble aggregates also become entirely metalated even at low pH. An important aspect of cluster aggregation is the ability to reverse aggregation. This can be accomplished on fully aggregated tiopronin-MPCs (such as aggregates prepared

2 r2t ln 2 0.231(r t) ) 3W1/22 W1/22

(1)

where D is the solute diffusion coefficient (cm2/s), r the radius of the open tubular column (cm), t the retention time of the solute in the column (s), and W1/2 the width at half-height of the eluted peak (s). Because the Taylor measurement relies implicitly on the dispersion of the solute during the course of the experiment, any adsorption or interaction of the solute with the walls of the column could skew the results. Taylor measurements for tiopronin-MPCs using a Teflon open tubular column are reported in Table 2. As in our previous10 experiments, the average core diameter (dTEM) was varied on the basis8 of synthetic reaction stoichiometry. The Taylor diffusion coefficient data from the smaller core tiopronin-MPCs (samples 1 and 2) using a Teflon column are in good agreement with those measured10 using a glass column (Table 2). There is a substantial difference between results from the two columns for the larger-core MPCs. The hydrodynamic diameters of the MPCs were calculated using both the “sticking condition” and “slip condition” of the Stokes relationship (see Table 2, footnote e). The sticking model has been generally applied when the diameter of the solute is larger than that of the solvent. Conversely, the slip model is employed when the diameter of the solute is approximately the same as that of the solvent. MPCs are candidates for both approaches because of their unique composition: a central gold core (1.8-5.2 nm, larger than solvent) surrounded by a monolayer shell (14) (a) Wright, S.; Leaist D. G. J. Chem. Soc., Faraday Trans. 1998, 94, 1457. (b) Hao L.; Lu, R. H.; Leaist, D. G. J. Solution Chem. 1997, 26, 113.

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Table 2. Tiopronin-MPC Taylor Dispersion Data in Water Using Teflon Column

tiopronin-MPC samplea 1 2 3 4

3X 1X (1/6)X (1/12)X

Taylor diffusion coeff (10-6 cm2/s)

hydrodynamic diam, dH (nm)e Teflon column

% ligand “detected”f Teflon column

model diameter, dTEM + 2L (nm)b

Teflon columnc

glass columnd

stick cond.

slip cond.

stick cond.

slip cond.

1.8((0.7) + (2 × 0.8) ) 3.4 2.2((1.0) + (2 × 0.8) ) 3.8 3.1((1.2) + (2 × 0.8) ) 4.7 3.9((1.7) + (2 × 0.8) ) 5.5

1.7 ( 0.1 1.3 ( 0.1 1.0 ( 0.2 0.86 ( 0.2

1.7 ( 0.1 1.4 ( 0.2 1.9 ( 0.1 1.5 ( 0.2

2.6 ( 0.1 3.3 ( 0.2 4.4 ( 0.1 5.0 ( 0.1

3.8 ( 0.1 5.0 ( 0.1 6.5 ( 0.2 7.6 ( 0.1

50 69 81 69

125 175 210 230

a Sample coding refers to the mole ratio of tiopronin and AuCl - used in the synthesis of each tiopronin-MPC sample (ref 8). b Diameter 4 modeled as average TEM diameter (dTEM) plus twice the extended tiopronin chain length (2L). c Calculated from experimental Taylor dispersion data using eq 1. d From ref 10. e Calculated from dH ) kT/3πηD (sticking condition) and dH ) kT/2πηD (slip condition), where k is the Boltzmann constant, T is the absolute temperature, η is the solvent viscosity, and D is the Taylor-measured diffusion coefficient in a Teflon column. f Calculated from % chain detected ) 100(dH - dTEM/2L), where dTEM is the TEM-measured core diameter, dH is the hydrodynamic diameter determined from Teflon column Taylor D values using the equations in d above, and 2L is twice the fully extended monolayer ligand length.

(same size as solvent). As in previous work,10 the hydrodynamic diameter (dH) calculated using the slip model (dH ) kT/2πηD) agrees with the monolayer extended-chainpredicted value (dTEM + 2L), whereas the sticking model (dH ) kT/3πηD) gives a smaller value of dH that detects less than the entire monolayer shell. Evaluating the data using the slip condition yields dH values for the smaller tiopronin-MPCs that favorably compare to the summation of the core diameter and the fully extended monolayer chain length. In previous work, the Taylor results from larger-core tiopronin-MPCs (samples 3 and 4) gave physically unreasonable dH values whether the sticking or slip model was employed. Also, the Taylor D values from experiments performed in glass columns did not vary systematically with increasing core size, as expected and as did the Taylor results for alkanethiolate-MPCs. We attribute these problems in the earlier study, in comparison to the more reasonable analysis found in Table 2, as being due to adsorption effects in the glass columns. The larger-core tiopronin-MPCs apparently are more prone to undergo such adsorption. Taylor dispersion methods employing Teflon columns may also prove useful for other MPCs for which adsorption interactions with a glass column might be anticipated, such as other charged-ligand MPCs or alkanethiolate-MPCs in which core charge is not effectively screened (eC10). Double-Layer Capacitance of Tiopronin-MPCs Measured with Rotated-Disk Electrode Voltammetry. Previous studies of alkanethiolate-MPC electrochemistry have revealed that MPC cores exhibit electronic capacitative charging.15 The charging consists of electron transfer to/from the MPC Au core and the associated formation of an ionic space charge around the MPC (a double layer). In the case of polydisperse MPC samples (in which the MPCs have an assortment of cluster capacitance values, CCLU, as might be caused by dispersity in MPC core size), capacitative charging of MPC cluster (15) (a) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663-2668. (b) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5612-5619. (c) Ingram, R. S.; Murray, R. W. Langmuir 1998, 14, 4115-4121. (d) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 20982101. (e) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279-9280. (f) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B 1998, 102, 9898-9907. (g) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703-3711. (h) Pietron, J. J.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 5565-5579.

cores that occurs by their diffusion-controlled transport to a solid electrode/solution interface is manifested as a featureless current-potential envelope.15a-c We employed15a-c steady-state voltammetries (rotated-disk electrode and microelectrode) to detect and quantify cluster double-layer capacitance from the slopes of the currentpotential curves in non-faradaic potential regions. Those measurements gave sub-attofarad (