J. Phys. Chem. B 2001, 105, 8785-8796
8785
Properties of a Ubiquitous 29 kDa Au:SR Cluster Compound† T. Gregory Schaaff,*,‡ Marat N. Shafigullin,§ Joseph T. Khoury,§,| Igor Vezmar,§ and Robert L. Whetten*,§ Chemical and Analytical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Schools of Chemistry and Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: March 27, 2001; In Final Form: June 16, 2001
The broad, intense peak found near 29 kDa in the laser-desorption mass-spectral abundances of various aurothiol (Au:SR) cluster compounds has been used to optimize the preferential formation of the species in that mass range. Recrystallization gives enriched fractions, on the 10 mg scale, that in several cases appear free of species outside that mass range. Elemental analysis and X-ray photoelectron spectra (XPS) confirm the absence of elements other than Au, S, C, and H, while infrared and NMR (1H,13C) spectra are consistent with intact thio groups. The Au 4f7/2 XPS peak is only slightly shifted (∼0.2 eV) and broadened from that of bulk Au(0) metal, and intense optical absorption extends far into the infrared region (0.5 eV), consistent with a metallic Au core. Recrystallized samples of the R ) C4, C6, and C12 materials readily form highly diffractive crystalline films, powders, and single crystals on the scale of 10 µm, consistent with perfect ordering in >100 nm grains, and a negligible amorphous content. Uniformity is quantified through several independent measures: (1) the mass spectrometrically determined core mass of 29.2 ( 2 kDa is invariant to that of the thiol used, indicating an equivalent Au core diameter of 1.68 ( 0.05 nm, assuming bulk density. (2) The powder X-ray diffraction intensities are sensitively fit to a Au core of 1.64 ( 0.03 nm equivalent diameter. (3) The powder patterns index unambiguously to bcc packing, with nearest-neighbor distances of 2.68 ( 0.02 nm (R ) C6) and 3.15 ( 0.02 nm (R ) C12). An fcc packing structure with 2.56 ( 0.04 nm distance (11.9 nm3 volume) is found for R ) C4. A formulation consistent with this mass spectral, diffraction, and average compositional information is Au144-146(SR)50-60, with the structure of the inorganic core being influenced by the type of adsorbate used to produce the entire inorganic/organic assembly.
Introduction
SCHEME 1
Many reasons have been given for the recent interest in the large, metal-rich aurothiol (Au:SR) cluster compounds;1-4 however, from a fundamental perspective, it suffices to mention just one, namely, that they have no small-molecule antecedent, or “precursor”. Specifically, two phases in the binary Au:SR system are well established (Scheme 1): I. Polymeric Au(I)SR compounds feature the linear coordination typical of divalent Au(I) (Scheme 1) and a large band-gap (>5 eV). They form exothermically upon exposure of soluble Au salts to thiols, via Au(III) + 3RSH f p-AuSR + RSSR + 3H+, and decompose thermally in the solid state at temperatures near 200 °C to yield metallic Au(0). Hydrophobic forms are used in so-called liquid bright gold suspensions for decorative and electronic applications (>$100 million/year);5 hydrophilic forms include antirheumatic gold drugs6 and aurothionein proteins.7 The solubility is quite limited except in the case of bulky (tertiary) R groups,8 and total structure determination is rare.9,10 II. Extended surface phases, or adsorbate monolayers (Scheme 1), formed on crystalline-Au substrates. These are of variable †
Part of the special issue “Royce W. Murray Festschrift”. * Corresponding authors. T.G.S.:
[email protected] (e-mail), (865) 574-4878 (phone), (865) 576-8559 (FAX). R.L.W.: robert.whetten@ chemistry.gatech.edu (e-mail), (404) 894-8255 (phone), (404) 894-9958 (FAX). ‡ Oak Ridge National Laboratory. § Georgia Institute of Technology. | Current address: Rowland Institute for Science, 100 Edwin H. Land Blvd., Cambridge, MA 02142.
SCHEME 2
stoichiometry, e.g., 3:1 or 2:1 (surface Au:S), depending on the crystal face exposed and also on the steric requirements of the R groups, which in favorable combinations can be quite densely packed and ordered. These phases form spontaneously upon exposure of Au surfaces to thiol RSH or disulfide RSSR molecules from solution or vapor, hence the colloquial appellation self-assembled monolayers, SAMs.11 The precise headgroup structure, or surface-chemical bond, is of an uncertain, even controversial, character; however, it does seem that the surface Au layer is not necessarily oxidized from its metallic, Au(0) state. In contrast to the abundance of known (I and II) forms, not to mention their utility, small binary compounds and multinuclear [Au(SR)m, Au2(SR)m, ...], are not known. The more recently explored giant Au:SR cluster compounds (Scheme 2)
10.1021/jp011122w CCC: $20.00 © 2001 American Chemical Society Published on Web 08/29/2001
8786 J. Phys. Chem. B, Vol. 105, No. 37, 2001 thus stand in stark contrast to more traditional large transition metal cluster compounds, which exhibit bulklike phenomena in their crystal,12 electronic,13 and charging properties.14,15 These compounds are typically synthesized through a systematic and rational combination of smaller ANLM antecedent (A ) transition metal, L ) ligand) to form the next larger homologue.16 Rather, the Au:SR clusters, which are generally assumed to share the bonding character of II, are more analogous to that of graphitic carbon molecules17 or discrete MoS2 compounds,18 where (i) no molecular precursor existed but (ii) an asymptotic surface or 2D phase is clearly identified (single graphene or MoS2 sheets). Brust et al. developed the first general preparation of the hydrophobic Au:SR clusters,19 and there have since been many refinements of the general preparation scheme.20-25 Laser desorption ionization mass spectrometry (LDI-MS) and X-ray diffraction (XRD) measurements indicated that this preparation yields a mixture of separable cluster compounds26 with inorganic core masses and dimensions corresponding to theoretically predicted structures of exceptional stability.27,28 Then through a combination of LDI-MS and XRD, reaction parameters were refined22 in order to prepare Au:SR’s in the size range at which the electronic properties evolve from bulklike to where they show discrete transitions corresponding to quantized energy levels (1-2 nm).29 The extremely high surface-to-volume ratio of the Au:SR’s makes it possible to investigate the monolayer and interfacial structure of the Au:SR SAM phases using conventional chemical techniques. Spectroscopic analyses such as NMR and FT-IR can be easily employed to investigate the surface chemical bonding and the R group conformations.30-34 In addition, the thermodynamics of the adsorbate monolayer can be studied by differential scanning calorimetry.35,34 Among the many interesting properties, the electrochemical and charging properties of gold clusters has become a subject of particular interest. The electrochemical charging of the metallic core of Au:SR compounds was demonstrated by Hostetler and co-workers.36 The electrochemical charging of a mixture of different sized clusters was indicated by the slope of pre- and post-redox wave of a ferrocene-derivatized thiol attached to the surface of a gold cluster. However, once separated by size, the discrete oxidation and reduction occurrences could be resolved for each of the different cluster compounds.37,38 With these and following studies, Murray and co-workers have provided a quite detailed description of the electrochemical behavior of Au:SR cluster compounds through careful study of many aspects of the charging phenomenon, e.g., thiol chain length,39 charging of clusters tethered to an electrode,40 and the use of other derivatized thiols attached to the gold clusters.41,42 Throughout our separations of the Au:SR cluster compounds, where R is a hydrophobic organic group,43 one specific compound has been produced with high yield and has repeatedly produced the highest quality of data. Selected properties of this special cluster compound have been reported in comparison to larger and smaller structural homologues. A brief history of this cluster compound is as follows: It was first identified26 by laser desorption ionization mass spectrometry (LDI-MS) in 1995 as the smallest separable fraction obtained from the phase-transfer catalysis method19 with dodecanethiol (1:1, Au:RSH reactant). Although it constituted ,1% of the cluster mass, in purified form, it yielded superior mass spectra but relatively poor ordering and low thermochemical stability. On the basis of these preparations, it was used as a source in cluster-beam experiments.44 An analysis of its core structure by X-ray diffraction,
Schaaff et al.
Figure 1. (a) Laser desorption ionization mass spectra from the original, as-prepared, cluster compounds (R ) C6). (b) LDI-MS of the mixture after heating in neat dodecanethiol for 18 h and (c) the LDI-MS of the final separated cluster compound that produces ions in the 29 kDa range.
in conjunction with theoretical models, suggested that it has a compact core of dimensions corresponding to the 29 kDa mass measured by LDI-MS. The Au core structure for these first separated compounds indicated a structure closely related to the bulk cubic-close-packed (fcc) Au lattice structure but slightly strained (2%) in a manner consistent with a pentatwinned morphology.27,28 The main breakthrough making the present report possible came with the development22 of a stoichiometric modification of the two-phase method; the decomposition of the polymer (p-AuSR) enhanced the yield of Au:SR cluster compounds having core mass of 29 kDa yield from 45 kDa) in Figure 1b arise from aggregation in the mass spectrometer and can be suppressed by diluting the compounds in a suitable matrix. Integration of the primary peaks indicates that yields of species corresponding to ions detected in the 27-31 kDa window can well exceed 50% of the total abundance, which is also supported by gravimetric analysis of separated fractions. Similar results have been obtained from the 29 kDa optimized preparations of R ) benzyl, C4, C6, and C18, the last only with elevated reaction temperature. In the case of R ) benzyl, usable mass spectra could not be obtained on the as-prepared samples, and so a thiol-exchange reaction and subsequent cleanup are carried out on a small portion of the sample prior to analysis by LDI-MS. In every case, the peak maximum appears near the same mass, with at most a shift of 1 kDa in position, although the tail toward higher mass can be extended at lower fluences. For convenience, we will refer to the species giving rise to this peak simply as the 29 kDa cluster compounds, although it is to be understood that the actual parent masses (including all intact adsorbates) are higher and variable. In two other cases, it has not been found possible to obtain significant yields of 29 kDa cluster compounds: (i) With R ) phenyl, only heavier species (∼45 kDa and higher) predominate, as indicated first by optical and X-ray scattering properties and confirmed by LDMS analysis after thiol exchange. (ii) With R ) G, where GSH ) glutathione, lighter species, in the 4-14 kDa range, are resolved using a suitable matrix for desorption.23,50 Despite these exceptions, it is our impression, gained from pursuing the reductive decomposition of the Au(I)SR polymer, as well as by analyzing the reaction mixtures obtained by others,51 that the species responsible for the 29 kDa feature are of special importance, for their ease of high-yield preparation and separation, their stability or robust character, and their rather central position within the ∼1.0 to 3+ nm range of c-Au:SR systems that have been investigated. Isolation of 29 kDa Au:SR Compounds by Recrystallization. The 29 kDa rich preparations of c-Au:SR compounds with various R groups have been fractionated by recrystallization from appropriate solvent-nonsolvent pair solutions, resulting in samples that are highly enriched (typically, >80%) with the 29 kDa compounds. The composition of various fractions are analyzed mass-spectrometrically, and the improvement in uniformity of the dominant fractions was also verified in the crystallinity of powder samples (see below). In several cases, we have repeated the recrystallization of the main 29 kDa rich fractions to remove all traces of other peaks from the mass spectrum and to see whether the 29 kDa peak itself could be further narrowed. A LDI-MS result typical of those fractions that were obtained on (C12, C6, C4) is shown in Figure 1c. The width of the main 29 kDa peak has only been slightly narrowed, likely due to the removal of the shoulder at ∼36 kDa.
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8787
Figure 2. Higher-resolution LDI (middle) and MALDI (upper and lower) mass spectra for 29 kDa c-Au:SC12. During the first 25 laser pulses, even more pronounced fragmentation occurs (lower); however, after this initial period, the profile of the mass spectrum shifts to higher mass (higher).
The absence of further improvement with subsequent recrystallization could indicate either that the limit of this method has been reached or that a molecular level of purity has been obtained. At higher resolution (Figure 2, middle curve), the broad peak seen in the low-resolution mass spectra centered at ∼29 kDa consists of ions having spacings of either 197 or 229 amu, consistent with [AuNSM]- cluster ions, i.e., having no hydrocarbon content, a finding entirely consistent with previous work on a 15 kDa c-Au:SC18 compound.52 Attempts have been made to eliminate the large-scale adsorbate loss evident here. Fragmentation free laser desorption mass spectrometry has been achieved for large biopolymers through cocrystallization of the biopolymer with a suitable strongly absorbing small molecule host or “matrix”.53 Figure 2 includes typical results obtained by cocrystallizing a 29 kDa c-Au:SC12 with dihydrobenzoic acid at a 1:1000 molar ratio. At initial exposure, the peak is positioned at 28.3 kDa with a fwhm of ∼1.5 kDa (lower curve). This suggests that the initial irradiation is even more violent than that in the direct LDI-MS. (However, due to the low ion concentration in the laser plume, the aggregation of various fragment ion species likely does not occur, which may cause the shift to lower mass.) Longer exposure (upper curve) produces a peak profile that is highly asymmetric, with an onset almost unchanged at 29 kDa, a peak near 36 kDa, and a tail extending to near 39 kDa. Both the inorganic composition of the fragment ions and the high-mass tail in the low fragmentation MALDI mass spectrum are consistent with the compositional analysis (below). The Au:SR clusters with R ) C18 resisted this method of fractionation due to solubility problems, so large quantities of highly purified clusters were not typically obtained. The mass
8788 J. Phys. Chem. B, Vol. 105, No. 37, 2001 spectrometric analysis of the R ) benzyl cluster compounds has also not been pursued to the same extent as the clusters with straight chain R groups. The recrystallized samples yielded sufficient sample quantities for all the measurements and experiments described below, with the exception of the elemental analysis, for which a 29 kDa rich crude Au:SC6 sample sufficed. The R ) C4 and C6 reactions consistently provided superior yields of the 29 kDa cluster compounds, having the least lower mass size impurities. On the basis of this, we have found that, to generate other 29 kDa cluster compounds with various R groups, the most efficient process involves first preparing large quantities of Au:SC6 cluster compounds. Since the solubility properties of the compounds with C6 and C4 R groups lend well to the recrystallization methods, the 29 kDa cluster compounds are separated from the mixture. Then any desired thiol can be substituted simply by placing the separated 29 kDa c-Au:SC6 compounds in the neat thiol or a concentrated thiol solution.54 Composition of 29 kDa c-Au:SR Compounds. Elemental analysis of the 29 kDa rich preparations of Au:SC6 clusters yields a Au:S ratio of 2.57:1 and gives a composition of S:C:H consistent with the starting-material composition (1:6:13), assuming that the thiol proton (RS-H) has been lost. Several other likely elements (Cl, Br, N, B, Na) sought were absent at detection levels. Separately, XPS spectra supported the elemental analysis fully. In conjunction with the spectroscopic information below, these results are consistent with a cluster compound having a neutral Au(0) core surrounded by dense adsorbate monolayer of SR groups, without counterions. The 1H and 13C NMR spectroscopy was performed on the 29 kDa Au:SR’s (R ) C4, C6, C12). Besides confirming that the R group is intact, 1H NMR (Figure 3) can be used to determine purity of the samples, i.e., the removal of reactants and byproducts. The line widths of the 1H resonances for adsorbate R groups are severely broadened, and thereby weakened, when compared to those of nonadsorbed species in the solution (free thiols and disulfides).32,35 Therefore, complete removal of all byproducts and unreacted starting material could be easily monitored. Despite the severe broadening of the methyl and methylene peaks, the resonances for even the closest (C1, C1) methylene protons could usually be identified. Each assigned peak was integrated, and in all cases, the integration was consistent with the labels as shown in Figure 3. The 13C NMR spectra show superior signal/noise compared to other spectra that have been obtained on solutions containing mixtures of cluster sizes. The solution spectra for Au:SR clusters (R ) C4, C6, C12) are shown in Figure 4. The chemical shifts of the outer carbon groups (from the end position to the C3 position) are very similar to that of an unbound thiol or disulfide. However, at the C1 and C2 positions for R ) C6, a substantial shift downfield is observed when compared to either the thiol or disulfide (see Table 1). The resonances corresponding to the C1 and C2 positions in the Au:SC12 sample were not located; however, integration of the peaks present is consistent with the labels in the figure. With increasing proximity to the Au core, the width of the peak increases until, in the case with R ) C6, the peaks corresponding to positions C1 and C2 are split. For the R ) C4 and C12 cluster compounds, the resonance corresponding to the C1 position was not detected, but the peak for the C2 position was resolved. The solubility of the Au:SC12 compounds in benzene was substantially less than the Au:SC6 compounds, which may have hindered resolving the severely broadened resonances closest to the Au-core. The Au:SC4, on the other hand, was not separated in as large a quantity as that
Schaaff et al.
Figure 3. 1H NMR of 29 kDa c-Au:SR compounds, where R ) C4 (a), C6 (b), and C12 (c). The numbering scheme represents the carbon positions with respect to the s headgroup. As with 13C NMR spectra (Figure 4), the width of the peaks increases as the 1H position approaches the gold core of the compound. The (1) position was resolved in all three cases (unlike previous studies) as an extremely broad peak centered near 4 ppm.
of the Au:SC6 at the time of these experiments; ongoing experiments are pursuing the final resolution of these shortcomings both through ligand exchange and further separations. The resonances corresponding to the C1 and C2 positions for the R ) C6 cluster compounds were identified and interpreted as extremely broad and overlapping doublets, which have integrated intensities equal to that of the peak corresponding to the C6 position. However, it seems that two distinct chemical environments exist for the R ) C6 adsorbate monolayer. Along with the two distinct carbon groups present on the cluster compounds, the broadening of the peaks can easily be seen to be heterogeneous in nature (inset Figure 4b). In the C1 and C2 positions of the Au:SC6 cluster compounds, a large number of reproducible sub-peaks can be seen in the inset. When the peaks corresponding to the C3-C6 positions are expanded along the abscissa, the same behavior is evident (see Supporting Information). The peak width of each of the sub-peaks has the same fwhm as those of the methanol peak (seen at 21.3 ppm) and the peaks due to benzene (not shown). In addition, the magnitude of the spacing between adjacent (sub-)peaks increases with proximity of the carbon group to the Au core, causing the apparent broadening. The infrared spectroscopy of the organic adsorbates is largely consistent with previous studies that have been performed on mixtures of gold nanocrystals made by similar preparations.30 As an example, the FTIR spectrum of the 29 kDa Au:SC6 cluster compound (Figure 5) differs only slightly from those obtained on mixtures. In the spectral region between 3000 and
Ubiquitous 29 kDa Au:SR Cluster Compound
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8789
Figure 5. FT-IR spectrum of 29 kDa c-Au:SC6 is consistent with previous reports in some respects, the missing absorption due to the S-H stretch at 2565 cm-1 and the slight perturbations of certain rocking modes. However, from this separated 29 kDa c-Au:SC6 compound, it was found that a much higher gauche:trans population (at the closest carbon to the surface) was present than that of thio-SAMs on gold surfaces. Figure 4. 13C NMR of 29 kDa c-Au:SR compounds (R ) C4 (a), C6 (b), and C12 (c)) shows the same trend in broadening as the 13C group approaches the gold core of the compound. The typical peak width can be seen in panel b due to residual toluene. The broadening of the peaks is due to a heterogeneous broadening mechanism, as can be clearly seen in the envelope of peaks corresponding to the C1, C2 and C3 positions of the 29 kDa c-Au:SC6 compound (b).
TABLE 1:
C6C5 C4 C3 C2 C1 a
13C
NMR Data for R ) C4 and C6
C4SH/ C4SSC4a
δ (ppm) C4
C6SH/ C6SSC6a
δ (ppm) C6
13.5/13.7 21.6/21.7 36.3/31.4 24.3/38.9
14.5 22.8 38.3 -
14.1/13.9 22.7/22.4 31.5/31.0 28.2/25.9 34.3/29.2 24.7/39.2
14.5 23.4 32.7 30.1 36/38 39/47
From Sadtler Handbook of Carbon-13 Spectra.
2800 cm-1, the peaks due to the symmetric/antisymmetric methyl and methylene C-H stretching modes are found to be identical to those previously reported.30 From the peaks corresponding to the rocking modes of the C-S bond (723 cm-1, V(C-S)T, and 641 cm-1, V(C-S)G), the population of gauche conformers was estimated to be ∼30% by deconvoluting the contribution of the trans conformer, which appears as a shoulder on the peak corresponding to the methylene rocking-twisting mode (inset, Figure 5). This value is distinctly higher than that reported for extended gold surfaces (near zero population for C5 and longer) and slightly higher than from independent studies involving mixtures of Au:SR clusters (∼20% for C5-C12).55 Optical Properties of 29 kDa Compounds. One of the most important questions involving the electronic structure of this new class of giant gold cluster compounds concerns the extent to which the adsorbates perturb the electronic structure of the Au cluster core. X-ray photoelectron spectroscopy (XPS) of the 29 kDa clusters shows that the Au 4f7/2,5/2 doublet (Figure 6) is only slightly shifted (+0.3 eV) and broadened from that of bulk gold, which can be completely accounted for by the effect of
Figure 6. When the X-ray photoelectron spectroscopy (electron count vs binding energy) of a 29 kDa c-Au:SC12 compound (s) is compared to that of bulk (- -) gold, the 4f5/2,7/2 peak is only slightly broadened and shifted.
the charging energy of a small metallic particle. Therefore, it is appropriate to visualize the Au core as a small fragment of bulk gold with a noninvasive (nonperturbing) monolayer that protects the cluster core from chemical change. Consequently, for the 29 kDa cluster compounds with short chains (R ) C4, C6), charge reversibly in solution and vacuum and can be fit to classical predictions for both the double-layer capacitance and double tunnel-junction of a small (1.7 nm) metallic sphere surrounded by a dense, dielectric medium.37 The extremely intense optical absorption spectra of the 29 kDa cluster compounds were found to be independent of the adsorbate used for these studies. The most striking feature in the optical absorption spectrum (Figure 7a) is the complete suppression of the plasmon-type resonance absorption, typical of large (>3 nm) gold clusters. The absorptivity of the 29 kDa cluster compounds increase in a nearly linear fashion from ∼5000 M-1 cm-1 at 0.7 eV to over 1 × 106 M-1 cm-1 at energies approaching 6 eV, reflecting the extremely large optical cross section of a neutral gold compound. When sufficient purity is achieved, a steplike structure is seen in the NIR, visible and near-UV region of the spectra, which is enhanced by taking the first derivative of the spectrum (Figure
8790 J. Phys. Chem. B, Vol. 105, No. 37, 2001
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Figure 8. Optical micrograph (a) and SEM micrograph (b) show the typical crystal that is grown from slow precipitation out of a dilute solvent-nonsolvent interaction.
Figure 7. Optical spectra of 29 kDa c-Au:SR compounds were not dependent on the R group. The extremely high absorptivity in the optical spectrum (a) is typical of a many electron metallic system. The optical spectrum before the ∼1.6 eV onset is provided in the inset. The derivative of the optical spectrum (b) enhances the small reproducible oscillations in the visible and UV region of the spectrum.
7b). The usual interpretation is the following: the intraband transitions (involving sp f sp type orbital transitions) are observed as a weak, but steady, increase in the absorbance in the NIR region ( C6 > C12 . C18, and hence, the ease of separation through recrystallization or fractional crystallization followed the same trend. Laser Desorption Mass Spectrometry (LDI-MS). Films of c-Au:SR’s for LDI-MS were prepared by vacuum-drying 1-2 µL of concentrated solutions in toluene (10-30 mg/mL) on polished, stainless steel rods (1/8 in. diameter). Irradiating the films using various pulsed lasers generated negative ions. Two types of mass spectrometers were constructed to achieve different types of measurements. In both instruments, the ion mass was determined by comparing observed flight times with
Schaaff et al. the flight times (under identical ion extraction conditions) of large protein negative ions, such as insulin (M. W. ) 5733 amu) and cytochrome-C (M. W. ) 15 255 amu), and their aggregates were generated from matrix assisted laser desorption ionization (MALDI) samples. Analysis of separated samples, especially those obtained in small quantities, required maximum ion sensitivity but only minimal resolution; therefore, a small, low-resolution linear time-of-flight (TOF) mass spectrometer was constructed, allowing maximum ion throughput even at extreme mass ranges (∼1 × 106 amu). Typically, the third harmonic of a Q-switched Continuum Nd:YAG laser (355 nm) was used for routine analyses. The incident fluence of each laser pulse from each of the lasers was typically on the order of ∼10 mJ/cm2. The second mass spectrometer was a laser desorption modification of an existing reflectron time-of-flight instrument used in cluster beam experiments.72 This allowed for an increase in mass resolution but still provided adequate mass range for the ions generated from the Au:SR cluster compounds. The Au:SC12 cluster compounds are too insoluble in methanol or water, so alternative solvents were used for preparing matrix diluted samples for matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). A 2,5 dihydroxybenzoic acid (DHB) solution was prepared at 0.070.1 M; however, reagent ethanol was used instead of methanol and water. The cluster compounds were first dissolved at a concentration of 1.3 mg/mL in hexane. Then 0.1 mL of this solution was diluted to 0.5 mL with anhydrous ether. After ensuring complete dissolution, 0.5 mL of the DHB solution was added, and 4 µL of this solution was placed on a probe tip and allowed to dry. Typical MALDI mass spectra were obtained from averaging 20-30 spectra obtained from consecutive laser pulses. Elemental Analysis. Elemental analysis was performed by Galbraith Laboratories, Inc., Knoxville, TN. giving the following mass composition for clusters prepared from R ) C6 thiol: C, 11.26%; H, 2.11%; S, 5.15%; Au, 81.84%. Likely impurities containing Cl, Br, and N were sought but found to be below detection levels. XPS analysis was conducted with a Surface Science Laboratories Inc. SSX-100 X-ray photoelectron spectrometer using a Cu-ΚR X-ray source (1486.6 eV) with a base pressure lower than 1 × 10-8 Torr. A survey scan was first recorded using 149 eV analyzer pass energy and 1000 µm spot size for high sensitivity; then the C 1s, S 2p (3/2, 1/2), and Au 4f (7/2, 5/2) regions were recorded with a 300 µm spot and 50 eV analyzer pass energy for higher resolution. Spectroscopy. 13C and 1H NMR spectra were obtained on a Bruker AMX-400 MHz spectrometer. The NMR samples were prepared as highly concentrated (∼100 mg/mL) solutions in C6D6. The 1H NMR is typically used to determine the purity of the samples, i.e., the complete removal of the starting materials (thiols and TOAB) and reaction byproducts (disulfides). The 13C NMR spectra were obtained at 100 MHz using a 90° pulse with a 4 s repetition time and inverse-gated decoupling to compensate for nuclear Overhauser effect (NOE), allowing for the integration of the adsorbate carbon groups. Typical 13C NMR spectra were generated from ∼15000 sweeps. FTIR. The IR spectra were collected with a Nicolet 520 FTIR, using samples in three forms: (i) as a KBr pellet (∼2 mg c-Au:SR’s in 1 g of dry KBr), (ii) as a neat film on a potassium bromide plate, or (iii) in CCl4 solution. Optical absorption spectra were measured on hexane and toluene solutions at concentrations ranging from 1 to over 100 mg/mL to determine linear scaling of the optical features. When
Ubiquitous 29 kDa Au:SR Cluster Compound measuring absorbance in the 1100 to 200 nm range, a Beckman DU7 spectrophotometer was used with a scan rate of 600 nm/ min and slit width of 5 nm. Then when extending the measurements into the NIR (to 3000 nm), a double-beam PerkinElmer Lambda 19 spectrophotometer (variable slit width) was used. All spectral features including the steplike features in the NIR and visible regions were proportional to concentration and observed on both instruments. Molar absorptivity was calculated using a molecular weight determined by (i) the core mass estimate from mass spectrometry and X-ray diffraction measurements and (ii) the thio content from the Galbraith elemental analysis and SA-XRD. Films, Powders, Monocrystals, and X-Ray Diffraction. Films for X-ray diffraction were prepared by depositing solutions of concentration > 50 mg/mL of Au:SR clusters in toluene or hexane on miscut Si (111) wafers and air-drying. Fine powders are made easily through slow addition of acetone to a toluene solution to ∼20% v/v and allowing the clusters to precipitate. Larger crystals were grown by slowing the crystallization process (described in subsection 3.2.3), starting with a dilute sample in toluene (∼5 mg/mL) and allowing the vapor transfer to occur over weeks, instead of hours. Acknowledgment. The Authors are grateful to our colleagues (A. Wilkinson, P. W. Stephens, S. A. Harfenist, Z. L. Wang, U. Landman, and W. D. Luedtke) who have collaborated and provided advice in many aspects of this research. We would also like to thank Prof. R. Murray’s and Prof. D. Schiffrin’s research groups for providing their Au:SR cluster compounds and their past and present collaborative efforts and discussions. The authors also thank Prof. Dahl’s research group for the coordinates for and advice concerning the large Pd145 cluster compound prior to submission of their recent manuscript. This research has been supported by the National Science Foundation, the Packard Foundation, the Georgia Tech Foundation, and the Division of Chemical Sciences, Geosciences and Bioscience Office of Basic Energy Sciences US Department of Energy, under Contrct DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Batelle, LLC. Supporting Information Available: NMR spectra from the 29 kDa Au:SC6 cluster compound. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wuelfing, W. P.; Zamborini, F. P.; Templeton, A. C.; Wen, X. G.; Yoon, H.; Murray, R. W. Chem. Mater. 2001, 13, 87-95. (2) Pietron, J. J.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 5565-5570. (3) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. J. Mater. Chem. 2000, 10, 79-83. (4) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67-69. (5) Papazian, A. N. Gold Bull. 1982, 15, 3. (6) Kean, W. F.; Hart, L.; Buchanan, W. W. Br. J. Rheumatol. 1997, 36, 560-572. (7) In Metallothioneins; Stillman, M. J., Shaw, C. F., Suzuki, K. T., Eds.; VCH: New York, 1992. (8) Fitch, H. M. Gold Tertiary Mercaptides and Method for the Preparation Thereof. U.S. Patent 2,984,575, 1961. (9) Wojnowski, W.; Becker, B.; Sabmannshause, J.; Peters, E.-M.; Peters, K.; vonSchnering, H. G. Z. Anorg. Allg. Chem. 1994, 620, 14171421. (10) Bonasia, P. J.; Gindelberger, D. E.; Arnold, J. Inorg. Chem. 1993, 32, 5162-5131. (11) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. AdV. Mater. 1996, 8, 719-729. (12) Chini, P. J. Organomet. Chem. 1980, 200, 37-61. (13) Mingos, D. M. P. Chem. Soc. ReV. 1986, 15, 31-61.
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