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FAB Mass Spectrometry of Au25(SR)18 Nanoparticles Amala Dass,† George R. Dubay,‡ Christina A. Fields-Zinna,† and Royce W. Murray*,† Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, and Department of Chemistry, Duke University, Durham, North Carolina 27008 The molecular ion of the nanoparticle Au25(SCH2CH2Ph)18 (A25(SR)18) is observed at 7394 Da in fast atom bombardment (FAB, Xe atoms) ionization mass spectrometry using a 3-nitrobenzyl alcohol matrix. A distinctive pattern of positive fragment ions is evident in the mass interval 5225-7394 Da, where peaks are seen for successive mass losses equivalent to R2S entities. Because the Au25(SCH2CH2Ph)18 nanoparticle structure is crystallographically known to consist of a centered Au13 icosahedral core surrounded by six Au2(SR)3 semirings, the R2S loses are proposed to represent serial rearrangements and decompositions of the semiring structures. Mass losses equivalent to R2S2 and R2 entities also appear at the lower end of this mass interval. The most intense spectral peak, at m/z ) 5246 Da, is assigned to the fragment Au25S10, from which all of the CH2CH2Ph organic units have been cleaved but from which no gold atoms have been lost. A different pattern of fragmentation is observed at lower masses, producing ions corresponding to serial losses of one gold atom and varied numbers of sulfur atoms, which continues down to a Au9S2 fragment. FAB mass spectra of the Au nanoparticle are much easier to interpret than laser desorption/ionization spectra, but they show more extensive fragmentation than do electrospray and low laser pulse intensity MALDI spectra. The loss of R2S fragmentation in FAB is distinctive and unlike that seen in the other ionization modes. The FAB spectrum for the nanoparticle Au25(S(CH2)9CH3)18 is also reported; its fragmentation parallels that for Au25(SCH2CH2Ph)18, implying that this nanoparticle has the same surprising stellated (staples) structure. Molecule-like gold nanoparticles protected by organothiolate ligands have been intensely studied due to their exceptional stability and other interesting properties. Progress has been strongly correlated to improvements in tools used to determine nanoparticle composition and structure. When the organothiolate ligand-protected gold nanoparticle synthesis by Brust et al.1 was reported in 1994, transmission electron microscopy (TEM) was the usual method for characterizing the nanoparticles, labeling the gold nanoparticle by its core diameter. While appropriate and * To whom correspondence should be addressed. E-mail:
[email protected]. † University of North Carolina. ‡ Duke University. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–802. 10.1021/ac801259j CCC: $40.75 2008 American Chemical Society Published on Web 08/16/2008
still the preferred option to characterize metal nanoparticles in the 5-100-nm range, TEM suffers from significant resolution problems below 3 nm. Whetten and co-workers2 in 1996, recognizing that as-prepared gold nanoparticles were actually mixtures of nanoparticles of different sizes, carried out size fractionations and reported results from laser ionization/desorption mass spectrometry (LDI). This started a transition from TEM toward different ways to characterize and label very small gold nanoparticles. Further results were reported3 in 1998 using electrospray ionization (ESI) mass spectrometry on water-soluble glutathioneprotected gold nanoparticles that had been fractionated by gel electrophoresis. Most recently, detailed structural characterizations by X-ray crystallography have been reported4a-c for Au102(SR)44 and Au25(SR)18 nanoparticles (where R ) PhCO2H and CH2CH2Ph, respectively). Concurrently, density functional theory correctly predicted4d the structure of the latter nanoparticle. The crystallography of these nanoparticles revealed4a,b a “staple” motif of short, stellated semiring gold-thiolate chains protecting a central gold core. While the preceding efforts reflect substantial progress, detailed composition and structure information remains lacking for a wide range of small metal nanoparticles, of different sizes and ligand shells. Attention is especially needed to mass spectrometric tactics that produce multicharged Au nanoparticles (which are massive metal objects) and to improve our understanding of the mass spectral behaviors of small metal nanoparticles. The fast atom bombardment (FAB) ionization mode has not previously been reported for small nanoparticles. Here, we explore FAB mass spectrometry (FAB-MS) for the Au25(SCH2CH2Ph)18 nanoparticle and, more briefly, a Au25(S(CH2)9CH3)18 nanoparticle. Thiolate-protected Au25 nanoparticles were first isolated from synthetic nanoparticle mixtures as early as 19975 and again in 20046 but owing in part to the resolution limitations6f of TEM were mislabeled as Au28 and Au38, respectively. The correct designation (2) 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, 5, 428–433. (3) (a) Schaaff, T. G.; Knight, G.; Borkman, R.; Whetten, R. L. J. Phys. Chem. 1998, 102, 10643–10646. (b) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630–2641. (4) (a) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430–433. (b) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754–3755. (c) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2008, 130, 5883–5885. (d) Akola, J.; Walter, M.; Whetten, R. L.; Hakkinen, H.; Gronbeck, H. J. Am. Chem. Soc. 2008, 130, 3756–3757. (5) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M.; Vezmar, I.; Whetten, R. L. Chem. Phys. Lett. 1997, 266, 91–98.
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Figure 1. Abbreviated crystal structure of [TOA+][Au25(SCH2CH2Ph)18-], omitting the Oct4N+ counterion and showingsexcept for three examples of “R” groupssonly the gold and sulfur atoms, with six orthogonal -Au2(SCH2CH2Ph)3- “staples” surrounding the centered icosahedral Au13 core. (Two examples of possible aurophilic bonding in this structure are shown as dashed lines). (Legend: Au ) yellow; S ) orange; R ) CH2CH2Ph ) black).
Au25(SR)18 was made in 2005 by Tsukuda and co-workers, using ESI-MS.7 Subsequent high-resolution ESI-MS,8 ESI-FT-ICR,9 and MALDI-MS10 experiments confirmed the Au25(SR)18 composition and have also delineated the distributions of different ligands on Au25 nanoparticles having mixed protecting layers (SCH2CH2Ph and other organothiolates). The average MW (7394 Da) of the Au25(SCH2CH2Ph)18 nanoparticle (and its fit to expected isotopic distributions) is known with certainty from these experiments. The recently obtained crystal structure of the Au25(SCH2CH2Ph)18 nanoparticle4b contained surprises (as did4a the Au102 nanoparticle), since the organothiolate ligands were not monomerically ligated to a central Au core but were instead bonded in short stellated semirings, or “staples”, of Aux(SR)y chains (Au2(SR)3 in the case of Au25(SCH2CH2Ph)18). The native (reduced) nanoparticle is an anion, and the crystal structure delineated its Oct4N+ counterion. Figure 1 displays an abbreviated structure, showing only the centered Au13 icosadedron protected by six orthogonal Au2(SR)3 semirings (shown without the R groups except for one illustrative semiring). All of the organothiolates are in bridging positions, so the previous picture11 of “monolayer protected clusters”, drawn by analogy to thiolate self-assembled monolayers on planar Au(111) surfaces, requires a footnote that the protecting monolayers may instead commonly or even generally be “polyligands”. Recognition of this more complex protecting layer raises new questions about which aspects of nanoparticle structure influence particular nanoparticle properties, such as their photo(6) (a) Donkers, R.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 1945–1952. (b) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193–6199. (c) Choi, J.-P.; Murray, R. W. J. Am. Chem. Soc. 2006, 128, 10496–10502. (d) Wang, G.; Guo, R.; Kalyuzhny, G.; Choi, J.-P.; Murray, R. W. 2006, 110, 20282-20289. (e) Guo, R.; Song, Y.; Wang, G.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 2752–2757. (f) Nanoparticle preparations are usually mixtures of different sizes, and examining the hundreds of images required to define dominant sizes in the overall population is unfeasible at ultrahigh resolution, while more usual TEM resolution leaves uncertainties at the 1-nm scale. (7) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261– 5270. (8) Tracy, J. B.; Kalyuzhny, G.; Crowe, M. C.; Balasubramanian, R.; Choi, J.P.; Murray, R. W. J. Am. Chem. Soc. 2007, 129, 6706–6707. (9) Tracy, J. B.; Crowe, M. C.; Parker, J. F.; Hampe, O.; Fields-Zinna, C. A.; Dass, A.; Murray, R. W. J. Am. Chem. Soc. 2007, 129, 16209–16215. (10) Dass, A.; Stevenson, A.; Dubay, G. R.; Tracy, J. B.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 5940–5946. (11) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36.
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luminescence, voltammetry, and optical absorbance. In the case of photoluminescence, the near-infrared emission energies are nearly independent of organothiolate-protected nanoparticle sizes, from ∼Au13 to ∼Au201; the emission was accordingly assigned12 as a “surface state” common to these nanoparticles. A tempting inference is that all of these small nanoparticles bear surface “polyligand” structures. Given the information above, the Au25(SCH2CH2Ph)18 nanoparticle was an obvious choice to explore the FAB-MS experiment. The known nanoparticle structure assists interpretation of FAB fragmentation results. Also, the possible presence of the semiring structure on other sizes of nanoparticles lends importance to understanding its fragmentation signatureswhich for Au25 is a heretofore unknown and distinctive loss of masses equivalent to sulfide, R2S. Relative to other ionization modes, the FAB fragmentation peaks are generally more assignable than typical LDI mass spectra, including observation of parent ion peaks in the case of Au25(SR)18 (for R ) CH2CH2Ph and (CH2)9CH3). The FAB spectra, on the other hand, do show considerably more fragmentation than ESI-MS and (threshold laser pulse energy) MALDIMS spectra. The FAB spectra exhibit, in common with MALDIMS spectra,10 a peculiar loss of Au4(SR)4 units. EXPERIMENTAL SECTION Au25(SCH2CH2Ph)18 nanoparticles were prepared as reported previously.10 A detailed synthetic procedure noting a minor modification is found in Supporting Information. Although we know of no toxicities of these materials, they are always handled in small dissolved quantities and when as solids, in hoods. The matrix medium 3-nitrobenzyl alcohol (Aldrich) and solvent methylene chloride (Fisher, 99.9%) were used as received. A film of 3-nitrobenzyl alcohol was applied to the sample target, and droplets of a CH2Cl2 solution of Au25 nanoparticles were applied over it. Fast atom bombardment ionization was performed using Xe accelerated at 3 kV in a JEOL JMS-SX-102 HRMS instrument. The mass scale was calibrated in positive mode using CsI on the probe inserted into the instrument in the scan range of 600-8000 Da. The accelerating voltage was decreased to 3.0 kV, allowing observation of ions up to 8000 Da. (Maximum scan range at 3.0 kV is 8666 Da.) The system tuning is different for the liquid matrixes and the dried CsI so the system is tuned up for each type of sample. The mass accuracy of the instrument is independent of the form of sample on the probe. RESULTS AND DISCUSSION FAB-MS Spectra of Au25(SCH2CH2Ph)18 Nanoparticles. FABMS is known to be a good ionization technique, but with tendencies toward significant fragmentation. The FAB-MS experiment produced a rich spectrum for the Au25 nanoparticle, as shown in the wide mass scan of Figure 2. This figure shows the entire mass range recorded for the spectrum, in which the peak for the parent ion (labeled as $, and known by ESI8,9 and MALDI10 measurements to lie at 7394 Da) is prominent in both positive and negative ion modes. The positive mode spectra offered superior signal intensity and spectral definition and will receive sole attention from here. The matrix used for acquisition of FAB (12) Wang, G.; Huang, T.; Murray, R. W.; Menard, L.; Nuzzo, R. G. J. Am. Chem. Soc. 2005, 127, 812–813.
Figure 2. Positive (green curve) and negative (blue curve) FAB mass spectra of nanoparticle Au25(SCH2CH2Ph)18 using 3-nitrobenzyl alcohol matrix. The $ peak is the parent ion, known to have an average mass of 7394 Da. The text shows that the peaks at masses roughly higher than the black arrow (5500 Da) mainly originate from serial losses of the organic part of the ligands (i.e., CH2CH2Ph), and peaks below 5500 Da depict serial losses of Au and sulfur atoms. (See Figures 3-5 for amplification).
Figure 3. Positive FAB-MS spectrum of Au25(SCH2CH2Ph)18 in 3-nitrobenzyl alcohol matrix, in the high mass range 5225-7500 m/z (expansion of high mass portion of Figure 1). The peaks indicated as $ and * are respectively the parent ion and the parent ion minus a Au4(SR)4 fragment (ref 10). The labels x,y,z denote the numbers of Au atoms, intact SCH2CH2Ph ligands and S atoms, respectively. The repetitive spacing between peaks is generally 32 Da (i.e., sulfur). In particular, note the systematic loss of two CH2CH2Ph and one S atom moieties (e.g., sulfide R2S) from the parent peak at 25,18,0, in the (solid green) peaks at 25,16,1; 25,14,2; 25,12,3; 25,10,4; 25,8,5; 25,6,6; and 25,4,7). Peaks at one (32) Da lower mass than this series are of peaks assigned to alternative fragmentation losses of disulfide (R2S2) and peaks at one (32 Da) higher mass to loss of hydrocarbon (R2). For example, 25,14,1 may be assigned to net loss of one R2S and one R2S2, and 25,6,8 may be assigned to net loss of four R2S and two R2 units. These assignments are of course implications based on mass losses, but their multiple recurrences (see Table S-1, Supporting Information) make the assignments persuasive.
spectra was 3-nitrobenzyl alcohol, which produced much more intense ions than other common choices, i.e., thioglycerol, glycerol, and DTT (3:1 mixture of dithiothreitol and dithioerythritol). We observed no indication of multiply charged ions (i.e., z ) 1), which is consistent with FAB spectra. Expanded views of Figure 2 (Figures 3-5; Supporting Information gives enlarged versions of all) show that there are two discernible, and quite different, fragmentation patterns, above m/z ∼5500 Da and below m/z ∼5500 Da. The mass range above m/z ∼5500 Da (Figure 3) contains a distinctive pattern of FAB fragmentation: a series of fragments characterized by pairwise
Figure 4. Positive FAB-MS spectrum of Au25(SCH2CH2Ph)18 with 3-nitrobenzyl alcohol matrix in the intermediate mass range 3691-5350 m/z (an expansion of portion of Figure 1 that mainly includes loss of Au atoms). The set of related peaks that differ by 32 Da (mass of sulfur atom) is denoted by the same color. Adjacent sets of peaks that differ by one Au atom are alternatively color-coded solid green and orange to differentiate.
Figure 5. Positive FAB-MS spectrum of Au25(SCH2CH2Ph)18 with 3-nitrobenzyl alcohol matrix in the low mass range 1217-3691 m/z (an expansion of portion of Figure 1 that mainly includes loss of the Au atoms).
losses of the organic components of the organothiolate ligands, leaving most of the sulfur atoms and (with the exception of one peak) all of the Au atoms. In the mass range below m/z ∼5500 Da (Figures 4, 5), the pattern observed is similar to that more typical1a of LDI spectraslosses of Au atoms and various numbers of sulfur atoms. Fragmentation of the Ligand Staples. Figure 3 shows an expanded view of the higher mass region of Figure 2, specifically m/z ) 5225-7500. The peak labels x,y,z represent, respectively, the numbers of Au atoms, of remaining intact SCH2CH2Ph ligands, and of S atoms (ligands which by C-S bond cleavage have lost their carbon portions). Thus, the [25,18,0] peak represents the parent cation of the nanoparticle, [Au25(SCH2CH2Ph)18]+, and [25,17,0] is the parent minus the mass of a thiolate radical, i.e., [Au25(SCH2CH2Ph)17]+. This latter peak is the sole clear example of single thiolate radical (•SCH2CH2Ph) dissociation. The distinctive pattern of fragmentations in Figure 3 is found in the series of peaks (colored solid green in Figure 3) [25,16,1], [25,14,2], [25,12,3], [25,10,4], [25,8,5], and [25,6,6], which are assigned as, respectively, [Au25(SCH2CH2Ph)16S1], [Au25(SCH2CH2Ph)14S2], [Au25(SCH2CH2Ph)12S3], [Au25(SCH2CH2Ph)10S4], [Au25(SCH2CH2Ph)8S5], and [Au25(SCH2CH2Ph)6S6]. This series of peaks arises from successive mass losses equivalent to two Analytical Chemistry, Vol. 80, No. 18, September 15, 2008
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phenylethylene units plus a sulfur, i.e., loss of a neutral sulfide, S(CH2CH2Ph)2. The [25,6,6] peak corresponds to loss of six R2S groups, in line with the six semirings of the structure shown in Figure 1. It is well-known that carbon-heteroatom bonds are easily broken during FAB ionization13 and that, generally, in fragmentation, stability of the ejected neutral fragment (e.g., R2S) can steer the dominant fragmentation pattern.14 In the preceding series of fragment ions, the [25,16,1] and [25,14,2] peaks are most intense but their intensity diminishes as more R2S sulfide units have become dissociated. Competing processes for mass losses that are equivalent to disulfide (R2S2) and hydrocarbon (R2) begin to appear. For example, the peak at [25,14,1] (not labeled in Figure 3, adjacent to peak [25,14,2]) is assigned to loss of one R2S and one R2S2 moiety, and the peak at [25,6,8] to loss of four R2S and two R2 moieties. Other assignments of mass losses are found in Table S-1 (see Supporting Information). The general pattern that phenylethylene groups are lost in pairs persists down to the [25,6,6] fragment at 5940 (the left-most solid green peak). This fragmentation and variability among loss of R2S versus R2S2, and R2 mass equivalent units leads to a general pattern of 32 Da mass differences between peaks, which can be seen throughout Figure 3. These assignments, like the above, do not prove dissociation of the mentioned neutral species, but the multiple recurrences of their loss, and their stability in comparison to thiolate radical species, make them plausible estimates. It is also obvious, given the Figure 1 structure, that the above mass loses must involve multiple (or concerted) bond cleavages (i.e., structural rearrangements) in the Au2(SR)3 semirings surrounding the Au13 nanoparticle core. An exception to the above pattern appears at 6057 Da (see * in Figure 3) as the fragment Au21(SR)14 (i.e., [25,14,0]). This ion arises from loss of Au4(SR)4, which is the only prominent ion above 5500 Da in which the nanoparticle clearly dissociates a goldcontaining fragment. This fragment (but not the loss of pairs of -R masses) is also seen in low pulse intensity MALDI-TOF mass spectra10 taken using the matrix trans-2-[3-(4-tert-butylphenyl)-2methyl-2-propenyldidene]malononitrile (DCTB). (The MALDI spectra also show weak peaks for the loses of Au(SR), Au2(SR)2, and Au3(SR)3.) Fragmentation of the Gold Core. The most intense FAB fragment in Figure 2, [25,0,10], lies at 5246 Da and arises from the exhaustive loss of the CH2CH2Ph components of the protecting shell of staples around the Au13 core. The adjacent, nearly equally intense peak [25,0,9] at 5214 Da corresponds to the overall loss of exactly nine R2S sulfide entities. Figure 4 shows the intermediate mass interval of the spectrum, which is dominated by fragmentation of the gold core plus losses of sulfur atoms. The most intense peak, labeled [25,0,10], represents Au25S10 with no ligands attached to it, followed by Au24S9, Au23S9, Au22S9, Au21S8, Au20S8, Au19S7, and Au18S7 peaks. The above labeled peaks and neighbor peaks that differ by S atom content (spaced by 32 Da) are marked by the same color shading. Sets of such alternating (13) (a) Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. Nature 1981, 293, 270–275. (b) Barber, M.; Bordoli, R. S.; Sedgwick, D.; Tyler, A. J. Chem. Soc., Chem. Commun. 1981, 348, 325–327. (c) Surman, D. J.; Vickerman, J. C. J. Chem. Soc., Chem. Commun. 1981, 7, 324–325. (14) McLafferty, F. W. Interpretation of Mass Spectra; W. A. Benjamin, Inc.: New York, 1966.
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Figure 6. FAB-MS of Au25(SCH2CH2Ph)18 and Au25(SC10)18 obtained using 3-nitrobenzene alcohol matrix. For both nanoparticles, the most intense fragment peaks (center of spectra) lie at 5246 and 5278 Da, which correspond in both cases to [25,10,0] and [25,11,0], fragments in which all of the S-C bonds have been cleaved and all 18 organic groups lost. At lower masses, the masses and fragmentation patterns are nearly identical. The spectra at higher masses differ because the organic groups have differing masses, 173 and 137 Da for SC10 and SCH2CH2Ph ligands, respectively, but the pattern of preferred losses of pairs of organic groups (as seen in Figure 3) persists for the Au25(SC10)18 nanoparticle. The parent ion for the Au25(SC10)18 nanoparticle appears at 8046 Da. For the Au25(SC10)18 nanoparticle spectrum, the red arrow labels identify the same peaks that are labeled in Figure 3, namely, [25,18,0], [25,16,1], and [25,14,2]. The * identifies the Au21(SR)14 fragment, which is seen for both nanoparticles.
Figure 7. FAB-MS, MALDI-MS, and ESI-MS of Au25(SCH2CH2Ph)18 obtained respectively using 3-nitrobenzene alcohol matrix, DCTB matrix and in a 30% MeOH/70% CH2Cl2 solvent. The peak indicated by $ is the parent ion, Au25(SCH2CH2Ph)18, and that denoted by * shows a Au21(SCH2CH2Ph)14 fragment.
peaks are denoted by solid green and orange colors. Figure 5 shows the lower mass interval of the FAB spectrum that continues the pattern of Figure 4sfurther fragmentation of the gold core and groups of peaks corresponding to Au17S5, Au16S6, Au15S4, Au14S5, Au13S3, Au12S4, Au11S3, Au10S3, and Au9S2. FAB-MS Spectra of a Au25(S(CH2)9CH3)18 Nanoparticle. Figure 6 shows the mass spectrum of a C10 alkanethiolateprotected Au25 nanoparticle, compared to that of the Au25(SCH2CH2Ph)18 nanoparticle. The parent ion for the Au25(S(CH2)9CH3)18 nanoparticle appears at 8046 Da; its organothiolate ligands have masses of 173 Da (as compared to 137 Da for SCH2CH2Ph). The Au25(S(CH2)9CH3)18 spectrum, although generally less intense, mirrors that of Au25(SCH2CH2Ph)18. There are, again, higher and lower mass intervals within which distinctive patterns of fragmentation occur. Within the higher mass interval for Au25(S(CH2)9CH3)18, fragments assignable as loss of pairs of organic groups (i.e., ((CH2)9CH3)2S) can be seen; these are labeled with
red arrows for the same peaks labeled in Figure 6. The * labels the Au21(SR)14 fragment, which is seen for both nanoparticles. For both nanoparticles, the most intense fragment peaks (center of spectra) lie at 5246 and 5278 Da, which correspond in both cases to [25,0,10] and [25,0,11], respectively. These nanoparticle fragments have lost all of their organic groupings, and accordingly have the same masses, and subsequent losses of Au atoms that are nearly identical to those seen for Au25(SCH2CH2Ph)18. An important implication of the substantial similarities of the FAB-MS fragmentations of the two nanoparticles is that they have the same structural motif, i.e., that the Au25(S(CH2)9CH3)18 nanoparticle has a structure like that seen in Figure 1. FAB versus MALDI versus ESI. Finally, we compare (Figure 7) the FAB mass spectrum of the Au25(SCH2CH2Ph)18 nanoparticle to its MALDI-MS and ESI-MS spectra (reported earlier8-10). The latter ionization modes show very little fragmentation; in the case of MALDI-MS, this required use of a favored matrix and threshold laser pulse intensities. The MALDI spectrum in Figure
7 shows a series of fragments corresponding to losses of Au(SR) units, with loss of Au4(SR)4 being favored. The latter fragment is also seen in the FAB-MS spectrum (see *), but the others were too weak to be discernible. ACKNOWLEDGMENT This research was supported by grants from the National Science Foundation and Office of Naval Research. We thank Matthew C. Crowe for assistance with the ESI-MS data. A.D. and GRD were equal contributors to this study. SUPPORTING INFORMATION AVAILABLE Expanded versions of Figures 1-7; concentration effects on FAB-MS spectra; table of observed and calculated masses and assignments. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review June 20, 2008. Accepted August 1, 2008. AC801259J
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