SCIENCE ADVANCES | RESEARCH ARTICLE
PHYSICAL SCIENCES
Molecular “surgery” on a 23-gold-atom nanoparticle
Qi Li,1* Tian-Yi Luo,2* Michael G. Taylor,3* Shuxin Wang,1† Xiaofan Zhu,1 Yongbo Song,1
Giannis Mpourmpakis,3 Nathaniel L. Rosi,2 Rongchao Jin1‡
INTRODUCTION
Recent years have witnessed significant research efforts on atomically precise ultrasmall metal nanoparticles (often called nanoclusters)
(1–4). Major advances have been achieved in the synthesis of gold,
silver, and Au/Ag alloy nanoclusters and the identification of aesthetic
structural patterns (5–22). Exploring effective methods to exquisitely
tailor the size, structure, and composition of atomically precise nanoclusters constitutes an ultimate goal in this field, which will offer opportunities to pursue fundamental understanding of the properties of
nanoclusters (23–26) and establish definitive structure-property relationships (1, 5, 27). With precise formulas, molecular purity, and total
structures solved by single-crystal x-ray analysis, it has become possible
to investigate the transformation chemistry of nanoclusters at the atomic
level (28–31), akin to organic transformation chemistry. This atomic-level
nanochemistry may provide new opportunities to discover some unexpected chemical reactions, which so far remain a mystery.
Although different precise synthesis methods have been developed
in the past decade, realizing the atomic-level tailoring of specific sites
in a nanoparticle without changing the structure of other parts (for
example, replacing specific surface motifs and deleting one or two metal
atoms) remains extremely difficult. This “molecular surgery,” which is
highly desirable but is so far the least feasible in nanochemistry, hinders
the fundamental understanding of how different motifs in a nanoparticle contribute to its overall properties.
Here, we report the first case of molecular surgery on the nanocluster
surface via site-specific tailoring of the protecting motif (Fig. 1A), which
leads to the “resection” of two gold atoms from the [Au23(SR)16]−
1
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA.
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.
3
Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA
15261, USA.
*These authors contributed equally to this work.
†Present address: Department of Chemistry, Anhui University Hefei, Anhui
230601, China.
‡Corresponding author. Email: [email protected]
2
Li et al., Sci. Adv. 2017; 3 : e1603193
19 May 2017
(R: cyclo-C6H11) nanocluster and thus the formation of a new
[Au21(SR)12(Ph2PCH2PPh2)2]+ (abbreviated as P–C–P hereafter) nanocluster. Direct transformation did not occur; thus, our method
consists of a two-step metal exchange via the formation of a critical
[Au23−xAgx(SR)16]− (x ~ 1) intermediate. We determined the structures of [Au21(SR)12(P–C–P)2]+ and [Au23−xAgx(SR)16]− (x ~ 1) by
single-crystal x-ray analysis. First-principles calculations further reveal the critical role of the first-step silver doping (step 1) in opening
up the thermodynamically favorable pathway of surface motif exchange
from RS–Au–SR to P–C–P motifs in the second step. Meanwhile, a new
metal-exchange scenario in which an Ag dopant goes to a counterion
is also presented.
RESULTS
The starting material, [Au23(SR)16]− [counterion: tetraoctylammonium
(TOA+)], was synthesized by a previously reported method (32). Singlecrystal x-ray analysis revealed the details of the two-step molecular
surgery process for the transformation of [Au 23(SR) 16]− to the
[Au21(SR)12(P–C–P)2]+ nanocluster (Fig. 1B). In the first step, a
small amount of AgI(SR) at a molar ratio of Au/Ag = 1:0.07 was added
to target-dope the [Au23(SR)16]−, and a silver-doped Au cluster,
[Au23−xAgx(SR)16]− (x ~ 1), was obtained. The dopant Ag was found
to be at two specific positions (centrosymmetric) in [Au23−xAgx(SR)16]−
(Fig. 1B, middle). In the next step, [Au23−xAgx(SR)16]− was transformed
to [Au21(SR)12(P–C–P)2]+ by reacting with a gold(I)-diphosphine
complex, Au2Cl2(P–C–P). It was found that the dopant Ag was reversely
replaced by Au, and meanwhile, the two monomeric RS–Au–SR motifs,
which protect the dopant Ag sites, are also exchanged by the P–C–P
motif from the Au2Cl2(P–C–P) (33) reactant. This simultaneous exchange
of metal and surface motif produced the new [Au21(SR)12(P–C–P)2]+
cluster and a silver-containing counterion, AgCl2−, which was formed
after the dopant Ag was pulled out from the cluster.
Details of the x-ray structures of [Au23(SR)16]−, [Au23−xAgx(SR)16]−,
and [Au21(SR)12(P–C–P)2]+ are shown in Fig. 2. A previous study (32)
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Compared to molecular chemistry, nanochemistry is still far from being capable of tailoring particle structure
and functionality at an atomic level. Numerous effective methodologies that can precisely tailor specific groups
in organic molecules without altering the major carbon bones have been developed, but for nanoparticles, it is
still extremely difficult to realize the atomic-level tailoring of specific sites in a particle without changing the
structure of other parts (for example, replacing specific surface motifs and deleting one or two metal atoms).
This issue severely limits nanochemists from knowing how different motifs in a nanoparticle contribute to its
overall properties. We demonstrate a site-specific “surgery” on the surface motif of an atomically precise 23gold-atom [Au23(SR)16]− nanoparticle by a two-step metal-exchange method, which leads to the “resection” of
two surface gold atoms and the formation of a new 21-gold-atom nanoparticle, [Au21(SR)12(Ph2PCH2PPh2)2]+,
without changing the other parts of the starting nanoparticle structure. This precise surgery of the nanocluster
reveals the different reactivity of the surface motifs and the inner core: the least effect of surface motifs on
optical absorption but a distinct effect on photoluminescence (that is, a 10-fold enhancement of luminescence
after the tailoring). First-principles calculations further reveal the thermodynamically preferred reaction
pathway for the formation of [Au21(SR)12(Ph2PCH2PPh2)2]+. This work constitutes a major step toward the development of atomically precise, versatile nanochemistry for the precise tailoring of the nanocluster structure to
control its properties.
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SCIENCE ADVANCES | RESEARCH ARTICLE
reported on the structure of [Au23(SR)16]−, which has a 15-atom bipyramidal Au core, that is, a 13-atom cuboctahedron plus two extra “hub”
gold atoms (Fig. 2A, in blue) that links the surface-protecting staple
motifs together. The core is protected by two belt-like trimeric Au3(SR)4
and two monomeric Au(SR)2 staple motifs, as well as four simple
bridging SR ligands. The structure of [Au23−xAgx(SR)16]− is shown
in Fig. 2B. The 15-atom bipyramidal Au–Ag core is shown on the left.
There are only two specific sites (centrosymmetric) where Ag can be
found. This is different from previously reported Au/Ag alloy nanoclusters in which the Ag atoms are distributed in many sites (31, 34, 35). The
occupancy of Ag is determined to be 31.5 and 30.0% at position 1 in
the two crystallographically independent clusters, whereas position
2 has a lower occupancy, determined to be 12.7 and 6.4%. The x-ray
crystallography–averaged composition is [Au22.13Ag0.87(SR)16]−, and
the fractional occupancy is caused by the compositional variation of
the Au or Ag atom. One TOA+ counterion was found, although it
shows heavy disorder, indicating that the −1 charge is retained in
[Au23−xAgx(SR)16]− during the silver doping process of [Au23(SR)16]−.
The structure of [Au21(SR)12(P–C–P)2]+ is shown in Fig. 2C. Although
the 15-atom bipyramidal core is retained in [Au21(SR)12(P–C–P)2]+, the
positions of the two hub gold atoms shift. In [Au23(SR)16]−, the two
hub gold atoms (labeled Au-2) are closer to Au-3, with distances of
3.234 and 3.245 Å (Fig. 2A), whereas the distance between Au-1 and
Au-2 is so large (3.462 Å) that no bond is formed. However, in
[Au21(SR)12(P–C–P)2]+, the Au-2 atom is closer to Au-1, with a much
shorter distance of 2.934 Å; therefore, a bond forms. The distance
between Au-2 and Au-3 is 3.425 Å in [Au21(SR)12(P–C–P)2]+, with
Li et al., Sci. Adv. 2017; 3 : e1603193
19 May 2017
no Au2–Au3 bond formed. The average lengths of core Au–Au bonds in
[Au21(SR)12(P–C–P)2]+ and [Au23(SR)16]− are 2.95 and 2.98 Å, respectively.
For the surface Au–Au bonds, the average lengths in [Au21(SR)12(P–C–P)2]+
and [Au23(SR)16]− are 3.08 and 3.16 Å, respectively. The shorter
bond distances in Au21 might enhance the radiative decay of the
photoexcited particles. More significant changes occur on the surface.
Surprisingly, the two monomeric S–Au–S motifs, which originally protect the two dopants Ag-1 and Ag-2 (partial occupancy), are exchanged by the P–C–P motifs from the Au2Cl2(P–C–P) complexes;
in addition, the two doping sites become homogold. The two P atoms
are bonded with the Au-1 and Au-2 with distances of 2.288 and 2.293 Å,
respectively. The motif exchange leads to the loss of two staple gold
atoms and thus gives rise to [Au21(SR)12(P–C–P)2]+.
As shown in Fig. 3A, two phenyl rings are arranged in parallel
through p-p stacking in each P–C–P motif. A silver-containing counterion, AgCl2−, is also identified. The AgCl2− anion has been previously
reported in the Cs complex (36), but in metal nanoclusters, before this
current work the Ag-containing counterion has never been identified.
The Ag–Cl bond length is 2.348 Å, and the AgCl2− shows a nearly linear
configuration (the angle of Cl–Ag–Cl is about 175°), which is similar
to the previously reported metal complex (36). The packing of
[Au21(SR)12(P–C–P)2]+[AgCl2]− in the single crystal is shown in Fig. 3B,
and each unit cell comprises two [Au 21 (SR) 12 (P–C–P) 2 ] + and
[AgCl2]−. The presence of the AgCl2− counterion in a one-to-one ratio
with the cluster indicates a +1 charge of [Au21(SR)12(P–C–P)2]+. In
view of the monovalent thiolate ligand and the neutral phosphine
ligand, the nominal count of gold core free valence electrons (6s1) is
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Fig. 1. Molecular surgery on the atomically precise 23-gold-atom nanocluster by a two-step metal-exchange method: peeling off two parts of the cluster wrapper and
closing the gaps with two P–C–P plasters. (A) Schematic of the molecular surgery on [Au23(SR)16]−; all carbon tails are omitted for clarity. (B) Site-specific surface motif
tailoring with a two-step metal-exchange method. The transformation from [Au23(SR)16]− through [Au23−xAgx(SR)16]− (x ~ 1) to [Au21(SR)12(P–C–P)2]+ is revealed by
single-crystal x-ray analysis. Magenta and blue, Au; gray, Ag; yellow, S; orange, P; green, C; light green, Cl. Other C and all H atoms are omitted for clarity.
SCIENCE ADVANCES | RESEARCH ARTICLE
8e (that is, 21 − 12 − 1 = 8e), which is isoelectronic as [Au23(SR)16]−
and [Au23−xAgx(SR)16]−.
It can be concluded from the above single-crystal analysis that the
new [Au21(SR)12(P–C–P)2]+ cluster holds a very similar atomic structure to the starting [Au23(SR)16]−, except for the site-specific replacement of the surface motif. It will be interesting to study how this
targeted surgery influences the nanocluster’s overall property. The optical absorption spectrum of [Au21(SR)12(P–C–P)2]+ is shown in Fig. 3C,
and is similar to that of [Au23(SR)16]− as well as [Au23−xAgx(SR)16]−
(fig. S1), all with a distinct peak at ~570 nm and a less prominent
one at 460 nm. Surprisingly, the photoluminescence (PL) efficiency of
[Au21(SR)12(P–C–P)2]+ is found to be enhanced ~10 times compared to
that of [Au23(SR)16]− (Fig. 3D). This 10-fold improvement may arise
from the motif exchange–induced change of electronic interaction between the surface motifs and the Au core, leading to enhanced radiative
recombination of the electron-hole pair, as reported in gold and other
nanoparticles (37, 38). These results unambiguously reveal the surface
motifs have little effect on optical absorption but a distinct effect on PL.
Matrix-assisted laser desorption ionization (MALDI) spectra of the
three clusters are also shown in fig. S2, and confirm the composition
of the three clusters. The positive-mode electrospray ionization (ESI)
spectrum of [Au21(SR)12(P–C–P)2]+ is shown in fig. S3, and the 1-Da
spacing of the isotropic peaks confirms the +1 charge of the cluster. The
experimental isotope pattern is consistent with the simulated pattern.
No signals of silver-doped [AgxAu21−x(SR)12(P–C–P)2]+ can be observed in the ESI mass spectrum. Energy-dispersive x-ray spectroscopic
(EDS) measurement under scanning electron microscopy (SEM) was
also conducted on several [Au21(SR)12(P–C–P)2]+[AgCl2]− crystals to
further confirm the elemental ratio of Au/Ag. The average elemental
Li et al., Sci. Adv. 2017; 3 : e1603193
19 May 2017
ratio of Au/Ag from a number of crystals is calculated to be 21:1.04
(table S1), which is very close to 21:1 determined by single-crystal x-ray
analysis. In addition, a 31P nuclear magnetic resonance (31P-NMR)
experiment was also carried out, which showed a doublet peak at ~24/26
parts per million (fig. S4), corresponding to the Au2(P–C–P) environment
in the Au21 cluster. To gain insight into the Au23 transformation to Au21
via the (Au–Ag)23 intermediate, we performed a control experiment
in which the Au2Cl2(P–C–P) complex was directly reacted with
[Au23(SR)16]− under the same conditions, but no [Au21(SR)12(P–C–P)2]+
was obtained. This result suggests the importance of the intermediate
product of [Au23−xAgx(SR)16]− (x ~ 1), which is deemed to serve as a
prerequisite for [Au21(SR)12(P–C–P)2]+. More Au complex/salt [for
example, AuI(SR) and AuCl] have been tested to investigate how the
composition or structure of Au complex/salt influences structural transformation. The control experimental results show that [Au23−xAgx(SR)16]−
(x ~ 1) was retained when reacting with AuI(SR), whereas the reaction
with AuCl led to the conversion of [Au23−xAgx(SR)16]− (x ~ 1) to other
clusters. These results indicate that the Cl− may act as the driving force
to extract the dopant Ag out of the cluster. Meanwhile, the structure of
P–C–P in the Au complex, which perfectly matches the original S–Au–S
motif, is also critical to stabilize the whole structure of the cluster.
On the other hand, structure/size transformation of [Au23(SR)16]− to
[Au25−xAgx(SR)18]− upon heavy silver doping (x ~ 19) was reported in
our previous work (31). Compared to the present work, the different
results (Au23−xAgx with light doping versus Au25−xAgx with heavy doping)
arise from the different amounts of AgI(SR) added in the reaction. The
evolution of ultraviolet-visible (UV-Vis) absorption spectrum from
[Au23(SR)16]− that reacted with increasing amounts of AgI(SR) is shown
in fig. S5, which suggests the transformation from [Au23(SR)16]− through
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Fig. 2. Comparison of the [Au23(SR)16]−, [Au23−xAgx(SR)16]−, and [Au21(SR)12(P–C–P)2]+ structures. (A) Crystal structure of [Au23(SR)16]−. Left: 15-atom Au bipyramidal
core. Right: Au23S16 framework. (B) Crystal structure of [Au23−xAgx(SR)16]−. Left: 15-atom Au–Ag bipyramidal core. Right: Au23−xAgxS16 framework. (C) Crystal structure of
[Au21(SR)12(P–C–P)2]+. Left: 15-atom bipyramidal core. Right: Au21S12(P–C–P)2 framework. Magenta and blue, Au; gray, Ag; yellow, S; orange, P; green, C. Other C and all
H atoms are omitted for clarity. The counterions TOA+ and AgCl2− are also omitted.
SCIENCE ADVANCES | RESEARCH ARTICLE
[Au23−xAgx(SR)16]− to [Au25−xAgx(SR)18]−. The x-ray structure of
Ag-doped [Au25−xAgx(SR)18]− (x ~ 4) is also solved (fig. S6), and the
entire transformation pathway from [Au23(SR)16]− to the heavily
doped [Au25−xAgx(SR)18]− is thus unveiled. As shown in fig. S7, the
[Au23(SR)16]− is first transformed to [Au23−xAgx(SR)16]− (x = 1 to 2),
and then it undergoes a size/structural change to [Au25−xAgx(SR)18]−
(x ~ 4), with the dopant Ag located exclusively on the sites of the
12-atom icosahedral inner shell. Upon heavy doping, the dopant
Ag will also go onto the surface motifs, and the heavily doped
[Au25−xAgx(SR)18]− is accordingly obtained. These results suggest
that the number of dopant Ag atoms is very important for the final
structure of the alloy nanocluster product in this case. The dopinginduced transformation from [Au23(SR)16]− to [Au21(SR)12(P–C–P)2]+
and [Au25−xAgx(SR)18]− is summarized in Fig. 4. Here, a small amount
of AgI(SR) is critical to place the necessary dopant Ag atoms in specific
positions (targeted doping) and retain the structure of the original
[Au23(SR)16]−, which provides the way for the subsequent motif
exchange to obtain [Au21(SR)12(P–C–P)2]+.
To further understand the driving forces in the synthesis of
[Au21(SR)12(P–C–P)2]+ and [Au25−xAgx(SR)18]−, we performed a thermodynamic analysis of elementary growth steps using density functional
theory (DFT) calculations as shown in Fig. 5 (see the Supplementary
Materials for computational details). The growth and doping reaction
steps involving the addition of M1(SR) species (M = Au or Ag) have been
referenced to the energies of the thermodynamically very stable tetramer species [M4(SR)4] that have been both computationally (39) and
Fig. 4. Metal-exchange transformation from [Au23(SR)16]− to [Au21(SR)12(P–C–P)2]+ and [Au25−xAgx(SR)18]−.
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Fig. 3. Single-crystal structure and optical properties of [Au21(SR)12(P–C–P)2]+[AgCl2]−. (A) The counteranion [AgCl2]− and coordination of PPh2CH2PPh2 motifs.
Other carbon tails and all H atoms are removed for clarity. (B) Total structure and arrangement of [Au21(SR)12(P–C–P)2]+[AgCl2]− in a single-crystal unit cell. Magenta, Au;
gray, Ag; yellow, S; orange, P; green, C; light green, Cl; white, H. (C) UV-Vis absorption spectrum of [Au21(SR)12(P–C–P)2]+. (D) PL spectrum of the Au21 (solid line); the PL
efficiency is enhanced ~10 times compared to Au23 (dashed line). Inset shows a photograph of the Au21 sample under 365-nm UV light.
SCIENCE ADVANCES | RESEARCH ARTICLE
transformation barrier and thus enables transformation from Au23 to Au21
when [Au23−xAgx(SR)16]− reacts with Au2Cl2(P–C–P), whereas the reaction
with more AgI(SR) leads to size/structure changes and the formation of
Au25−xAgx. This work offers a promising strategy for molecular surgery on nanoclusters to tailor their structure and functionality.
MATERIALS AND METHODS
experimentally (40) observed in thiolated group XI metal complexes
(this reference selection does not affect the relative thermodynamic
stability of the species and results in accurate reaction energy calculations; see table S4). First, we observe that for the [Au23(SR)16]− and
[Au22Ag(SR)16]− clusters, Ag doping reactions are exothermic and
slightly preferred over growth reactions to form [Au25−xAgx(SR)18]−
nanoclusters. Additionally, we see that the growth of [Au23(SR)16]− to
[Au25(SR)18]− is unfavorable. However, for the [Au21Ag2(SR)16]− cluster,
growth to [Au21Ag4(SR)18]− becomes energetically more preferred
than the doping step to [Au20Ag3(SR)16]−, rationalizing the lack of
observed [Au20Ag3(SR)16]−. This preference in the Ag growth step over
Ag doping is further enhanced in the reaction of [Au20Ag3(SR)16]− to
form [Au20Ag5(SR)18]− over [Au19Ag4(SR)16]−. This demonstrates an
increasing energetic preference for growth to [Au25−xAgx(SR)18]− nanoclusters, in agreement with the observation that [Au23−xAgx(SR)16]−,
with x more than 2, is not formed. Next, we observe a significantly uphill
thermodynamic reaction between Au2Cl2(P–C–P) and [Au23(SR)16]− to
form the [Au21(SR)12(P–C–P)2(AuCl2)2]− cluster (representing motif exchange reactions). However, when Ag atoms are doped into [Au23(SR)16]−
to form [Au21Ag2(SR)16]−, the presence of the higher-energy (2,2) isomer of [Au21Ag2(SR)16]− creates an almost thermoneutral path, enabling
the formation of [Au21(SR)12(P–C–P)2(AgCl2)2]−. Note that all these
theoretical findings are in agreement with the experimental observations, demonstrating that a thermodynamic (free energy) analysis can
capture the growth behavior of these nanoclusters (at least for the
systems of interest).
DISCUSSION
Here, a two-step metal-exchange method has been developed for
site-specific surface motif exchange on [Au23(SR)16]− to form a new
[Au21(SR)12(P–C–P)2]+ nanocluster. Both experimental and DFT calculations indicate that the formation of the [Au23−xAgx(SR)16]− (x = 1 to 2)
intermediate is critical, and the dopant Ag atoms are target-doped at
two specific positions (partial occupancy). Doping lowers the
Li et al., Sci. Adv. 2017; 3 : e1603193
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Synthesis and crystallization
To synthesize [Au23−xAgx(SR)16]− (x = 1 to 2), 20 mg of molecularly
pure [Au23(SR)16]− [made using a previously reported method (32)]
was first dissolved in 3 ml of DCM, and 1.2 mg of AgI(SR) (powder)
(29) was added, with a molar ratio of elemental Au/Ag in reactants of
1:0.07. The reaction was allowed to proceed for ~1 hour and was monitored by MALDI mass and UV-Vis spectroscopy. After [Au23(SR)16]−
was converted to [Au23−xAgx(SR)16]− (judging from MALDI spectra),
the DCM solution containing the [Au23−xAgx(SR)16]− product was
directly transferred to a glass tube, and pentane was diffused into the
solution at room temperature for crystallization of clusters. After
~2 days, crystals of [Au23−xAgx(SR)16]− were obtained on the inner wall
and at the bottom of the glass tube. Notably, this crystallization step also
serves as isolation/purification of the [Au23−xAgx(SR)16]− product, and to
obtain high-quality single crystals, the crystals were redissolved and then
recrystallized for several times. The product yield of [Au23−xAgx(SR)16]−
was ~70% (crystals after the final step).
To synthesize [Au21(SR)12(P–C–P)2]+, 30 mg of accumulated
[Au23−xAgx(SR)16]− crystals was first redissolved in 6 ml of DCM,
and 8.0 mg of the Au2Cl2(P–C–P) complex was added (mole ratio of
reactants = 1:2). The reaction was allowed to proceed at room temperature for ~3 hours, which was also monitored by MALDI mass
and UV-Vis spectroscopy. After [Au23−xAgx(SR)16]− was converted to
[Au21(SR)12(P–C–P)2]+, the DCM solution containing [Au21(SR)12(P–C–P)2]+
clusters was transferred to a glass tube, to which pentane was diffused at
room temperature for crystallization. After 3 to 5 days, black crystals of
[Au21(SR)12(P–C–P)2]+ were observed at the bottom and side wall of
the glass tube and then collected. The redissolving and recrystallization
processes were performed to obtain high-quality [Au21(SR)12(P–C–P)2]+
single crystals. The product yield was ~80% at the final step. Notably,
if [Au23(SR)16]− was used as the reference, then the product yield of
[Au21(SR)12(P–C–P)2]+ would be ~56%.
For the synthesis of [Au25−xAgx(SR)18]− (x ~ 4), 20 mg of molecularpure [Au23(SR)16]− was first dissolved in 3 ml of DCM, and 4 mg of
AgI(SR) (powder) was then added (molar ratio of elemental Au/Ag
in reactants = 1:0.23). The reaction was allowed to proceed overnight at
0°C, and was monitored by MALDI mass and UV-Vis spectroscopy until the [Au23(SR)16]− reactant was converted to [Au25−xAgx(SR)18]−.
Next, the solution containing [Au25−xAgx(SR)18]− was mixed with acetonitrile (DCM/acetonitrile ratio = 3:2) and was allowed to evaporate
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Fig. 5. DFT-calculated free energies (DGrxn) of elementary reaction steps of the
experimentally synthesized pure and Ag-doped Au nanoclusters. Detailed reaction network energetics analysis can be found in table S4. Inset shows the different (thermodynamically stable) doping positions of Ag in the Au15 core of the
[Au23−xAgx(SR)16]− clusters. The different energy levels of the [Au23−xAgx(SR)16]− clusters represent the lowest-energy isomers (based on doping positions of the inset),
which are also analyzed in fig. S8.
Chemicals
Tetrachloroauric(III) acid (HAuCl4·3H2O, 99.99%; Sigma-Aldrich),
cyclohexanethiol (C6H11SH, 99%; Sigma-Aldrich), bis[chlorogold(I)]
bis(diphenylphosphino)methane [Au2Cl2(P–C–P), 97%; Sigma-Aldrich],
sodium borohydride (NaBH4, 99.99%; Sigma-Aldrich), tetraoctylammonium bromide (TOAB, 98%; Fluka), pentane [high-performance liquid
chromatography (HPLC) grade, 99.9%; Sigma-Aldrich], ethanol (HPLC
grade; Sigma-Aldrich), methanol (HPLC grade, 99%; Sigma-Aldrich),
and dichloromethane (DCM) (HPLC grade, 99.9%; Sigma-Aldrich)
were used as received.
SCIENCE ADVANCES | RESEARCH ARTICLE
slowly for ~4 days at 4°C. Black crystals of [Au25−xAgx(SR)18]− were
observed at the bottom and side wall of the glass tube and then collected.
The product yield of [Au25−xAgx(SR)18]− was ~30%.
Characterization
UV-Vis spectra of the clusters (dissolved in CH2Cl2) were acquired on a
Hewlett-Packard Agilent 8453 diode array spectrophotometer at
room temperature. MALDI mass spectrometry was performed on a
PerSeptive Biosystems Voyager DE super-STR time-of-flight (TOF)
mass spectrometer. ESI mass spectra were recorded using a Waters
Q-Tof mass spectrometer equipped with Z-Spray Source. 31P-NMR
spectrum was obtained using Bruker Avance III 400 MHz spectrometers. EDS-SEM was conducted with ZEISS Sigma 500 VP SEM with
Oxford AZtec X-EDS.
Etot ¼ Eelectronic þ ZPE þ RT RT lnðqrot qvib qtrans Þ
where Etot represents the total molar free energy, Eelectronic represents the
electronic energy, ZPE represents the zero-point vibrational energy, R is
the ideal gas constant, T is the temperature, qrot is the rotational
partition function, qvib is the vibrational partition function, and qtrans
is the translational vibrational motion (set to 1 atomic unit to neglect in
this case).
qvib ¼ ∏
i
1
1 exp w eðiÞ=kB T
where e(i) represents the vibrational energy of the ith vibrational state
and kB is Boltzmann’s constant. The factor of w = 0.9914 was used as a
correction to the vibrational energies (47).
qrot ¼
½ð2 p kB TÞ2 A B C0:5
p
where A, B, and C represent the moments of inertia of the molecules and
s represents the symmetry number.
The [Au23(SR)16]− and [Au25(SR)18]− structures were taken from previously published crystallographic information, the R groups of the thiolates
were substituted by methyl groups, and the positive counterions were removed (15, 32). The structure of [Au21(SR)12(P–C–P)4(AgCl2)z]− was
taken from table S3, and the R groups (–C6H11) of the thiolates were
substituted for (–CH3). Note that the BP86 functional has been successfully used on thiolated metal nanocluster systems (48), and R =
methyl group substitution has had little impact on RS–Au bond
strength, as has been previously applied in computational nanoparticle
structural determinations (48). From the optimized [Au23(SCH3)16]−
Li et al., Sci. Adv. 2017; 3 : e1603193
19 May 2017
X-ray analysis
Details of the x-ray crystallographic analysis are provided in the
Supplementary Materials.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/3/5/e1603193/DC1
X-ray Experimentals
fig. S1. UV-Vis absorption spectra of [Au23(SR)16]− and [Au23−xAgx(SR)16]−.
fig. S2. MALDI mass spectra of [Au23(SR)16]− (black), [Au23−xAgx(SR)16]− (green), and [Au21(SR)12(P–C–P)2]+
(gray) sample.
fig. S3. ESI mass spectrum of [Au21(SR)12(P–C–P)2]+.
fig. S4. 31P-NMR spectrum of [Au21(SR)12(P–C–P)2]+.
fig. S5. UV-Vis absorption spectra of samples with increasing mass ratio of AgI(SR) that reacted
with [Au23(SR)16]−.
fig. S6. X-ray crystal structure of [Au25−xAgx(SR)18]− (x ~ 4).
fig. S7. AgI(SR) complex–induced transformation from [Au23(SR)16] − to heavily Ag-doped
[Au25−xAg x(SR) 18 ]− .
fig. S8. DFT-relaxed [Au23−xAgx(SR)18]− (x = 1 to 3) and [Au25−yAgy(SR)18]− (y = 2, 3) nanoclusters
and associated relative electronic energies.
table S1. Atomic percentages of Au and Ag in the [Au21(SR)12(P–C–P)2]+[AgCl2]− obtained by EDS-SEM.
table S2. DFT free energies of reactions of intermediates and possible reaction pathways.
table S3. Crystal data and structure refinement for Au22.13Ag0.87(SR)16− TOA+.
table S4. Crystal data and structure refinement for Au20.51Ag4.49(SR)18− TOA+.
table S5. Crystal data and structure refinement for [Au21(SR)12(P–C–P)2]+[AgCl2]−.
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Acknowledgments
Funding: This work was financially supported by the Air Force Office of Scientific Research
under award no. FA9550-15-1-9999 (FA9550-15-1-0154). M.G.T. acknowledges support from
the NSF Graduate Research Fellowship under grant no. 1247842. Author contributions: Q.L.
and R.J. designed the research; Q.L. performed synthesis and crystallization; M.G.T. and G.M.
performed DFT calculations; and T.-Y.L. and N.L.R conducted x-ray crystallographic analysis.
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Submitted 15 December 2016
Accepted 20 March 2017
Published 19 May 2017
10.1126/sciadv.1603193
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Molecular ''surgery'' on a 23-gold-atom nanoparticle
Qi Li, Tian-Yi Luo, Michael G. Taylor, Shuxin Wang, Xiaofan Zhu, Yongbo Song, Giannis Mpourmpakis, Nathaniel L. Rosi and
Rongchao Jin
Sci Adv 3 (5), e1603193.
DOI: 10.1126/sciadv.1603193
http://advances.sciencemag.org/content/3/5/e1603193
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