16004
J. Phys. Chem. C 2010, 114, 16004–16009
MALDI Mass Analysis of 11 kDa Gold Clusters Protected by Octadecanethiolate Ligands†
Risako Tsunoyama,‡ Hironori Tsunoyama,‡ Panvika Pannopard,§ Jumras Limtrakul,§ and
Tatsuya Tsukuda*,‡
Catalysis Research Center, Hokkaido UniVersity, Nishi 10, Kita 21, Sapporo 001-0021, Japan, and Department
of Chemistry and Center of Nanotechnology, Kasetsart UniVersity, Bangkok 10900, Thailand
ReceiVed: February 26, 2010; ReVised Manuscript ReceiVed: July 8, 2010
We previously reported the isolation of octadecanethiolate-protected Au (Au:SC18H37) clusters having a core
mass of ∼11 kDa from the crude Au:SC18H37 samples obtained in the reaction between octadecanethiol
(C18H37SH) and poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Au clusters. This stable Au:SC18H37 compound
was assigned to be Au55(SCnH2n+1)32 by destructive mass spectrometry and thermogravimetry (TG) performed
in the framework of the classical structure model, in which the thiolates are bound to the surface of a magic
Au55 core. In the present study, the molecular formula of the Au:SC18H37 (11 kDa) cluster was reassessed by
non-destructive matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and it was
confirmed that the Au:SC18H37 (11 kDa) cluster is a mixture of Au54(SC18H37)30 and Au55(SC18H37)31. On the
basis of our present understanding of the structures of other stable Au:SR clusters, we proposed that
Au54(SC18H37)30 and Au55(SC18H37)31 have a structural motif comprising a Au37 cluster core that is completely
protected by -SR-[Au(I)-SR-]x oligomers (x ) 1 and 2). The nonformation of the Au:SR (11 kDa) cluster
during the conventional chemical reduction of Au(I)-SR oligomers is ascribed to the kinetics of Au:SR
formation.
Introduction
In 1994, Schiffrin developed a simple method involving the
reduction of Au(I)-SR oligomers1 to prepare Au clusters whose
surfaces were chemically passivated by thiolate monolayers (Au:
SR) (Figure 1A). Shortly after this report, Whetten2 and
Murray3,4 synthesized small, monodispersed Au:SR clusters
(core diameter < 2 nm), whose electronic and optical properties
were dependent on the core size and significantly different from
those of the bulk Au. Whetten isolated a series of stable Au:
SR clusters by treating them as normal chemical entities and
evaluated their core masses to be ∼8, 14, 22, and 29 kDa using
laser desorption ionization mass spectrometry (LDI-MS).5–7
Later, we employed the nondestructive electrospray ionization
MS (ESI-MS) technique and demonstrated that the Au:SR
clusters with well-defined molecular formulas can be isolated.8–10
We also showed that in the Schiffrin method, metastable Au:
SR clusters are produced concomitantly along with highly stable
Au:SR clusters because of kinetic hindrance during the growth
of the Au core;11 further, we showed that the stable Au:SR
clusters can be selectively formed by core etching using thiols.12
Au25(SR)18 is a prototypical system that has been discovered in
such a manner.8,13,14 Now, the size focusing of preformed Au:
SR clusters by thiol etching has become a general protocol for
selective synthesis of magic Au:SR clusters.7,15–18 Recent
experimental efforts have enabled researchers to share a series
of highly stable Au:SR clusters such as Au20(SR)16,19 Au25(SR)18,20–22 Au38(SR)24,15,16 Au44(SR)28,17 Au68(SR)34,23 Au102(SR)44,24 and Au144(SR)60.18,25
†
Part of the special issue “Protected Metallic Clusters, Quantum Wells
and Metallic Nanocrystal Molecules”.
* To whom correspondence should be addressed. E-mail address:
[email protected]. Fax: +81-11-706-9156.
‡
Hokkaido University.
§
Kasetsart University.
As for their geometrical structures, it had long been believed
that the thiolates are bound to the bridged or hollow sites on
the facets of the Au nanocrystals26,27 based on the time-honored
structure model for the self-assembled monolayer (SAM) of
thiolates on an extended Au surface.28,29 Garzón was the first
to point out that thiolate ligation can cause significant structural
distortion of the Au core.30,31 Häkkinen extended this idea to
propose a “divide-and-protect” concept, according to which
thiolates form cyclic tetramers, [Au(I)-SR-]4, on the surface
of the Au core.32 Nobusada proposed a “core-in-cage” model,
in which the Au core is caged by the larger cyclic [Au-SR-]12
oligomers.33 In 2007, Kornberg24 achieved a breakthrough in
the research of Au:SR systems by elucidating the structure of
Au102(SR)44 using single-crystal X-ray diffraction. Subsequently,
Murray20 and Jin21 experimentally determined the structure of
Au25(SR)18, while Häkkinen34 theoretically predicted the structure of this cluster. These reports have dramatically changed
the traditional view of the structure of Au:SR clusters;35,36
Au102(SR)44 and Au25(SR)18 have a common structural motif
and are composed of a highly symmetrical Au core whose
surface is protected by -SR-[Au-SR-] and/or -SR[Au-SR-]2 bidentate ligands. The stabilities of Au102(SR)44 and
Au25(SR)18 have been explained in the theoretical studies
performed by Aikens,21 Li,37 and Han.38 Recent theoretical
studies have predicted that other stable Au:SR clusters such as
Au20(SR)16,39,40 Au38(SR)24,41–43 Au44(SR)28,44 and Au144(SR)6045
have a similar structural motif. The theoretical prediction made
for Au38(SR)2442,43 was confirmed by Jin, who carried out singlecrystal X-ray crystallography studies; the results revealed that
Au38(SR)24 is composed of a face-fused bi-icosahedral Au23 core
capped by three -SR-[Au-SR-] and six -SR-[Au-SR-]2
ligands.46 Electronic structure is another determining factor of
the stability of Au:SR clusters. The high stability of specific
charge states such as [Au25(SR)18]-1 (refs 9 and 20–22),
[Au44(SR)28]-2 (ref 17), [Au68(SR)34]0 (ref 23), and [Au102-
10.1021/jp101741a  2010 American Chemical Society
Published on Web 07/23/2010
MALDI Mass Analysis of 11 kDa Gold Clusters
J. Phys. Chem. C, Vol. 114, No. 38, 2010 16005
Figure 1. Preparation of Au:SR cluster via (A) chemical reduction of the Au(I)-SR oligomer and (B) thiolation of polymer-stabilized Au(0)
clusters.
(SR)44]0 (ref 24) is explained by the electron counting rule
(superatom concept);47 however, the charge state of Au:SR
clusters can be altered depending on the synthesis and storage
conditions and by redox reactions.9,48,49
Another frequently used method for preparing Au:SR clusters
involves replacement of the ligands on the preformed Au(0)
clusters with thiolates.50 The reaction of thiols with Au(0)
clusters that are weakly stabilized by a polymer chain can be
viewed as the SAM formation on the three-dimensional Au
surface (Figure 1B). In 2006, we reported that reaction of
n-alkanethiol CnH2n+1SH (n ) 12 and 18) with small Au clusters
stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) yields stable
Au:SCnH2n+1 clusters with unprecedented Au core masses of
∼11 and ∼26 kDa.51,52 The high stability of these Au:SCnH2n+1
clusters (core mass: 11 and 26 kDa) is mainly governed by the
structure of the Au and S moieties since van der Waals
interactions between the alkyl chains are negligibly small. In
our 2006 study, we tentatively assigned the 11 kDa Au:SCnH2n+1
cluster to Au55(SCnH2n+1)32 on the basis of the results of
destructive LDI-MS and thermogravimetry (TG)51 performed
in the framework of the classical structure model, in which the
thiolates are bound to the surface of the “magic” Au55 core, as
in the case of the well-known Au cluster compound
Au55(PR3)12Cl6.53 The monodentate and bidentate ligation of
thiolates to the Au clusters has been demonstrated by crystallographic studies of Au11(SR)3(PPh3)7 (ref 54) and [Au25(SR)5(PPh3)10Cl2]2+ (ref 55), respectively. The aims of the
present study are as follows: (1) determination of the molecular
formula of the 11 kDa Au:SC18H37 cluster by nondestructive
MS, (2) reinterpretation of the structure of this compound
according to the current understanding, and (3) identification
of the reason for the preferential formation of this compound
in the thiolation reaction.
Experimental Section
an aqueous solution (5 mL) of HAuCl4 (0.05 mmol) and PVP
(0.5 mmol of monomer units) was mixed with an aqueous
solution (5 mL) of NaBH4 (0.25 mmol) and PVP (0.5 mmol)
in a microfluid reactor maintained at 313 K.56 The brown
hydrosol eluted from the mixer was stirred for 1 h at 273 K.
Then, a toluene solution (25 mL) of C18H37SH (0.25 mol) was
placed on top of the Au:PVP hydrosol, and the resulting biphasic
mixture was vigorously stirred at room temperature to obtain a
uniform emulsion. After some time, the aqueous phase became
colorless, and the toluene phase became brown, confirming the
transformation of the protecting layer from PVP to C18H37S.
After 2 h of mixing, the aqueous phase was discarded, and the
organic phase was evacuated to dryness. The crude Au:SC18H37
clusters were washed with ethanol and incubated in neat
C18H37SH (2 mL) at 353 K for 20 h with stirring. The resulting
Au:SC18H37 clusters were washed three times with warm ethanol
and pretreated on a silica column. Then, the product was
fractionated by size using a recycling size-exclusion chromatography (SEC) system (LC-908, Japan Analytical Industry Co.,
Ltd.) equipped with two columns (JAIGEL-W253, Japan
Analytical Industry Co., Ltd.) connected in series.51,52 Toluene
was used as an eluent (flow rate: 3.5 mL/min), and the eluted
clusters were optically detected at 290 nm.
UV-vis/near-IR absorption spectra of the fractionated Au:
SC18H37 clusters in toluene were measured under ambient
conditions by using a spectrophotometer (V670, JASCO). The
absorbance in wavelength units, I(w), were converted to that in
energy units, I(E), by using the relation I(E) ∝ I(w) · w2.57 For
mass analysis, the Au:SC18H37 samples were mixed with trans2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB)15,18,22,23,58,59 in a typical molar ratio of 1:500 and
irradiated by a N2 laser (337 nm). Matrix-assisted laser
desorption/ionization (MALDI) mass spectra were recorded
using a linear time-of-flight (TOF) mass spectrometer (VoyagerDE STR-H, Applied Biosystems) in the positive-ion mode.
n-Alkanethiol CnH2n+1SH (n ) 12, 16, and 18) was used as
the protecting ligand to study the intrinsic stability of the 11
kDa Au:SR clusters. In the present study, we focused on
octadecanethiolates (SC18H37) because the yield of the 11 kDa
Au clusters was highest. The 11 kDa Au:SC18H37 clusters were
synthesized in the following two steps: (1) preparation of Au
clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) and
(2) reaction of the Au:PVP clusters with C18H37SH.51,52 First,
Results and Discussion
1. Synthesis of the 11 kDa Au:SC18H37 Cluster. The sizeexclusion chromatogram of the crude Au:SC18H37 sample
originally showed a single band, which split into two bands
upon continuous recycling. Figure 2A shows the presence of
two bands peaked at 225 and 231 min in the chromatogram
obtained in the sixth cycle. In this cycle, eluent fractions were
16006
J. Phys. Chem. C, Vol. 114, No. 38, 2010
Figure 2. (A) Chromatogram of crude Au:SC18H37 sample in the sixth
cycle. (B) Optical spectra of fractions 1 and 2. The dotted line represents
the baseline of the spectrum of 2.
Figure 3. Positive-ion MALDI mass spectra of fraction 1 recorded at
low (a) and high (b) laser power. Asterisks indicate the parent and
fragment ions of the Au38(SC18H37)24 impurity. Details of the assignment
are provided in Figure S1 (Supporting Information).
collected at 1 min intervals and characterized by optical
spectroscopy. Figure 2B shows the absorption spectra of
fractions 1 and 2. The spectral profile of fraction 1 was very
similar to that reported previously for Au:SC18H37 clusters with
a core mass of ∼11 kDa.51,52 The purity of the present sample
was higher than that of the previous studies51,52 judging from
the well-structured spectral profile. The optical gap was
determined to be ∼0.9 eV.60 The optical spectrum of fraction
2, on the other hand, was similar to that of Au38(SC18H37)24.10,18
Thus, we concluded that the major components of fractions 1
and 2 were Au:SC18H37 clusters (core mass: ∼11 kDa) and
Au38(SC18H37)24, respectively. These clusters showed no sign
of degradation as long as they were stored in powder form in
a refrigerator.
2. Molecular Formula of the 11 kDa Au:SC18H37 Cluster
(Fraction 1). For determining the molecular formula of the 11
kDa Au:SC18H37 cluster in fraction 1, it is essential to suppress
the fragmentation of the clusters so that it is ionized in the intact
form. Figure 3 shows the typical MALDI mass spectra of
fraction 1 recorded at different laser fluences. Spectrum (b),
which was recorded at a high laser power, shows mass peaks
assignable to [AunSm]+, as observed in the conventional LDI
mass analysis. The mass spectral pattern changed significantly
with a decrease in the laser fluence; spectrum (a) is a typical
MALDI mass spectrum recorded at the minimum laser fluence
required for ion detection. The extremely low efficiency for the
Tsunoyama et al.
Figure 4. MALDI mass spectra of Au38(SC18H37)24 (fraction 2)
recorded in the positive-ion mode. Laser power increases in the order
(a) < (b) < (c). The inset in panel (b) shows the splitting of the [Au38Sm]+
peaks.
desorption and ionization of the 11 kDa Au:SC18H37 cluster gave
rise to two problems in the analysis of spectrum (a). The first
problem was the contamination of the spectrum by peaks due
to an impurity. To our surprise, we found that the dominant
mass peaks observed in the m/z range of 11500-14500 Da were
attributed to the impurity Au38(SC18H37)24 (Figure S1, Supporting
Information), whose concentration in fraction 1 was extremely
small, as was evident from the optical spectra shown in Figure
2B. This result indicated that the ionization efficiency for the
11 kDa Au:SC18H37 cluster was considerably lower than that
for the Au38(SC18H37)24. The second problem was the trade-off
between the ion intensity and the degree of laser-induced
fragmentation. We observed a series of weak peaks in the m/z
range of 16000-20000 Da, where peaks due to the parent ions
generated from the intact 11 kDa Au:SC18H37 cluster were
expected to appear. Despite extensive efforts, fragmentation
from the 11 kDa Au:SC18H37 cluster could not be suppressed
even at the minimum laser fluence. Hence, it is important to
distinguish the parent species from the fragment species for the
assignment of the clusters in fraction 1 to a specific molecular
formula.
For accurate assignment of the peaks in spectrum (a) (Figure
3), we first studied the fragmentation pattern of a known cluster
compound Au38(SC18H37)24 present in fraction 2. Figure 4 shows
the MALDI mass spectra of Au38(SC18H37)24 (fraction 2) as a
function of laser power; the laser power increases in the order
(a) < (b) < (c). When the laser power is just above the ion
detection threshold, the peak due to the intact parent ion
[Au38(SC18H37)24]+ is detected along with several fragment
ion peaks (spectrum (a)). The major fragmentation channels
include the loss of Au4(SC18H37)4, Au4(SC18H37)4 + C18H37, and
the SC18H37 ligands (see also Figure 5). Previous studies have
reported the fragmentation of the Au4(SR)4 unit in the MS
analysis of Au38(SC2H4Ph)24 (ref 15) and Au68(SC2H4Ph)34 (ref
23). The preferential loss of Au4(SC18H37)4 can be attributed to
the high stability of Au4(SC18H37)4 in the ring form.61 With an
increase in the laser power, the peak due to the intact parent
ion disappears, while new peaks appear (spectrum (b)). The most
dominant peaks are assigned to gold sulfide clusters [Au38S13-17]+ and [Au34S12-15]+, while the minor peaks observed in
the m/z range of 8000-12000 Da contain several alkyl groups
(Figure S2, Supporting Information). It is important to note that
MALDI Mass Analysis of 11 kDa Gold Clusters
J. Phys. Chem. C, Vol. 114, No. 38, 2010 16007
arrows can be assigned to the parent ions whose molecular
formulas are Au54(SC18H37)30 and Au55(SC18H37)31, respectively.62 These molecular formulas are slightly different from
the actual formula determined on the basis of our original
assignment of Au55(SC18H37)32;51 however, we believe that
Au54(SC18H37)30 and Au55(SC18H37)31 are not the products of the
selective photofragmentation of Au55(SC18H37)32. The assignment
of the two aforementioned parent ions is confirmed by the
MALDI-MS analysis of the 11 kDa Au cluster protected by
C16H33S (ref 63). The assignment is further supported by the
fact that the fragmentation pattern of Au54(SC18H37)30 and
Au55(SC18H37)31 is similar to that of Au38(SC18H37)24. Figure 5
compares the detailed fragmentation patterns obtained for
Au38(SC18H37)24, Au54(SC18H37)30, and Au55(SC18H37)31; the mass
peaks corresponding to Au38(SC18H37)24 and Au55(SC18H37)31 are
aligned vertically for ease of comparison. As can be seen from
the figure, the three clusters show a similar fragmentation
pattern, that is, loss of several SC18H37 ligands, Au4(SC18H37)4,
and Au4(SC18H37)4 + C18H37. Thus, we conclude that the 11
kDa Au:SC18H37 clusters are composed of Au55(SC18H37)31 and
a measurable amount of Au54(SC18H37)30.
3. Structure Model of Au54(SC18H37)30 and Au55(SC18H37)31. The high stability of Au54(SC18H37)30 and Au55(SC18H37)31
can be ascribed to their electronic and geometric structures.
However, it is difficult to explain the reason for this high stability
on the basis of the electronic structures of the clusters alone
since the electronic charge state of the clusters is not clear.
Hence, we focus on the geometric structures of Au54(SC18H37)30
and Au55(SC18H37)31. One may expect the structure of the
interface between the Au(0) cores and the thiolates in
Au54(SC18H37)30 and Au55(SC18H37)31 to be different from the
well-accepted -SR-[Au-SR-]x oligomer structure; this is
because the aforementioned clusters are obtained only by the
thiolation of Au:PVP and not by the reduction of the Au(I)SR oligomer (Figure 1). In our previous study,51 we speculated
that 32 thiolates are bound to the facet of the Au55 core having
an icosahedral or cuboctahedral motif, similar to the case of
Au55(PR3)12Cl6, which has a cuboctahedral Au55 core.53 A recent
computational study performed at the density functional theory
(DFT) level on Au55(SCH3)32 revealed that this cluster has a
local minimum structure in which a deformed icosahedral Au55
core is coordinated with 32 RS ligands via Au-SR single
bonds.64 However, with the Au core-RS shell structure model,
we cannot explain why Au54(SC18H37)30 and Au55(SC18H37)31
prefer the Au54 core and 31 thiolate ligands, respectively.
The following discussions lead us to conclude that Au54(SC18H37)30 and Au55(SC18H37)31 can be categorized as Au:SR
clusters having a -SR-[Au-SR-]x shell around the Au core.
Table 1 lists all possible combinations of the Au core size and
the number of the -SR-[Au-SR-]x (x ) 1 and 2) oligomers.
Let us consider the most reasonable combination among these.
Figure 5. Comparison of the fragmentation patterns of (a) Au38(SC18H37)24 and (b) Au54(SC18H37)30 with Au55(SC18H37)31. Mass peaks
of Au38(SC18H37)24 and Au55(SC18H37)31 are aligned vertically for ease
of comparison.
the number of Au atoms in the larger fragment [Au38Sm]+ is
identical with that of that parent Au38(SC18H37)24, regardless of
the extensive fragmentation in the MALDI process. Further
increase in the laser power results in the formation of a series
of [AunSm]+ ions with ∼30 < n < ∼50 (spectrum (c)); this is
because of the laser-induced fragmentation and aggregation of
the Au38 core. This observation indicates that the MALDI mass
spectra do not provide direct information on the number of Au
atoms when the laser power exceeds a certain critical value.
The characteristic fragmentation of Au38(SC18H37)24 (spectrum
(b) in Figure 4) prompted us to determine the number of Au
atoms present in the clusters in fraction 1 on the basis of the
mass spectra recorded at a high laser power. Spectrum (b) in
Figure 3, which is recorded at a high laser power, shows a mass
peak assignable to [AunSm]+. Further, this spectrum includes
two sets of doublet peaks at n ) 55/51 and 54/50 with a spacing
equal to the mass of the Au4 unit, as in the case of
Au38(SC18H37)24. The doublet peaks at n ) 54 and 50 cannot
be assigned to the fragments of [Au55Sm]+ and [Au51Sm]+,
respectively, because the peaks were observed even when the
laser power was decreased to a considerable extent. These
spectral features lead us to conclude that fraction 1 contains
two AuN(SC18H37)M clusters, one with N ) 54 and the other
with N ) 55. Then, we attempted to assign the peaks in spectrum
(a) shown in Figure 3 by assuming that the number of Au atoms
is either 54 or 55. The mass peaks indicated by blue and red
TABLE 1: Structural Models for Au54(SC18H37)30 and Au55(SC18H37)31
numbers of -SR-[Au-SR-]x oligomersa
Au54(SR)30a
Au55(SR)31a
Au core size
x)1
x)2
x)1
x)2
total numbers of S anchors
outer/inner atoms
in the Au core
39
38
37
36
35
34
15
12
9
6
3
0
0
2
4
6
8
10
14
11
8
5
2
1
3
5
7
9
30
28
26
24
22
20
30/9
28/10
26/11
24/12
22/13
20/14
a
RS represents C18H37S.
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J. Phys. Chem. C, Vol. 114, No. 38, 2010
Tsunoyama et al.
is similar to that considered in the production of the ubiquitous
magic cluster Au38(SR)24; in the Schiffrin method, the direct
yield of Au38(SC18H37)24 is very low.51,52 The yield of Au38(SR)24
can be increased significantly by thiol etching of larger Au:SR
clusters.15,16,51,52,67 The nonformation of Au38(SR)24 and the Au:
SR (11 kDa) cluster during the Schiffrin method is ascribed to
the different kinetics of Au:SR formation.
Summary
Figure 6. Population of Au atoms in the core and -SR-[Au-SR-]x
oligomers in the AuN(SR)M compounds reported thus far. The inset
shows the theoretical42,44,45 and experimentally observed20,21,24,46 geometrical structures of the Au cores.
Previously, we had proposed three empirical rules that govern
the formation of a stable Au:SR cluster10 on the basis of the
structures of Au102(SR)44 (ref 24) and Au25(SR)18.20,21,34 First,
the Au cores should form highly symmetric and stable geometric
structures. Second, all of the surface atoms on the Au core
should bind to both ends of the thiolates of -SR[Au-SR-]x. Third, the number ratio of -SR-[Au-SR-]2 to
-SR-[Au-SR-]1 should decrease with an increase in the core
size, probably because larger Au cores with a smaller degree
of curvature prefer -SR-[Au-SR-]1 over SR-[Au-SR-]2.
Figure 6 shows the population of the Au atoms in the core and
the -SR-[Au-SR-]x layers as a function of the total numbers
of Au atoms determined experimentally and theoretically.15–25,37–46
The populations of Au atoms evolve smoothly across
Au54(SC18H37)30 and Au55(SC18H37)31 if we assume that their
Au core size is 37. According to the empirical rule, the Au37
core should comprise 26 atoms in the outer and inner layer and
11 atoms in the inner layer.65 However, detailed experimental
and theoretical studies should be carried out to reveal the exact
structures of Au54(SC18H37)30 and Au55(SC18H37)31. The
Au54(SC18H37)30 and Au55(SC18H37)31 clusters show a characteristic fragmentation pattern in the MALDI process, similar to
Au:SR clusters having a -SR-[Au-SR-]x (x ) 1 and 2)
interfacial structure, such as Au38(SC18H37)24 (Figure 4),
Au38(SC2H4Ph)24,15 and Au68(SC2H4Ph)34;23 these observations
support the results obtained using the aforementioned structure
model.
How is such an interfacial structure formed during the
thiolation of Au:PVP clusters (Figure 1)? A recent MALDI study
has revealed that the Au:PVP samples prepared under the
conditions employed in the present study contain magic Au
clusters (Au34, Au42, and Au58).66 The present results indicate
that the thiols pull out the Au atoms from the magic Au clusters
to afford a -SR-[Au-SR-]n interface; further, the thiol
molecules etch out Au atoms from the Au clusters so that
Au54(SC18H37)30 and Au55(SC18H37)31 are selectively populated
with the desired Au core and the appropriate number of thiol
ligands.
Finally, we attempt to clarify why Au54(SC18H37)30 and
Au55(SC18H37)31 cannot be produced by the Schiffrin method
(Figure 1A), although they have a structural motif similar to
that of the other members of the Au:SR family. The situation
First, Au:SC18H37 clusters were prepared by a reaction
between C18H37SH and mixtures of PVP-stabilized Aun clusters
(n ) 34, 42, 58). The Au:SC18H37 samples thus prepared were
subjected to recycling SEC to afford a Au:SC18H37 cluster (core
mass: 11 kDa) and Au38(SC18H37)24. On the basis of the MALDI
mass spectra recorded at a low laser power, we concluded that
the 11 kDa Au:SC18H37 cluster is a mixture of Au54(SC18H37)30
and Au55(SC18H37)31. The stability of these clusters could not
be explained by the classical structure model, in which the
thiolates are bound to the Au54 and Au55 cores. Alternatively,
we propose that Au54(SC18H37)30 and Au55(SC18H37)31 are
composed of a Au37 core whose surface is completely protected
by -SR-[Au-SR-]x (x ) 1 and 2) oligomers. The fact that
the 11 kDa Au:SR clusters were formed only via the thiolation
of Au:PVP could be ascribed to the reaction kinetics rather than
to the formation of new Au:SR clusters with different interfacial
structures.
Acknowledgment. This study was financially supported by
a Grant-in-Aid (Grant No. 18064017, Synergy of Elements),
MEXT Japan. This research was conducted under the JSPS
Exchange Program for East Asian Young Researchers (JENESYS 2009). P.P. and J.L. thank the National Science and
Technology Development Agency (NSTDA Chair Professor and
National Nanotechnology Center) and the Thailand Research
Fund for their support. The MALDI-MS analysis was carried
out at the open facility in Hokkaido University.
Supporting Information Available: Mass assignment of the
peaks in Figures 3 and 4 and the structure model of the Au37
core. This material is available free of charge via the Internet
at http://pubs.acs.org.
References and Notes
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Landman, U. AdV. Mater. 1996, 8, 428.
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A.; Chen, A. D.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono,
J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray,
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