Alkyne-Monofunctionalized Gold Nanoparticles as Massive Molecular Building Blocks

Article abstract of DOI:10.1002/ejic.202000273

Using a tripodal, thioether-based ligand comprising a remote protected acetylene, we efficiently stabilize small gold nanoparticles (? ≈ 1.2 nm) which are isolated and purified by chromatography. The 1:1 ligand-to-particle ratio is obtained by comparing t

Full text of DOI:10.1002/ejic.202000273

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Monofunctionalized Au Nanoparticles | Very Important Paper |
Alkyne-Monofunctionalized Gold Nanoparticles as Massive
Molecular Building Blocks
Erich Henrik Peters[a] and Marcel Mayor*[a,b,c]
Abstract: Using a tripodal, thioether-based ligand comprising of “massive molecules”. The “massive molecule” nature of the
a remote protected acetylene, we efficiently stabilize small gold particles is demonstrated by involving them in wet chemical
nanoparticles (Ø ≈ 1.2 nm) which are isolated and purified by coupling protocols like oxidative acetylene coupling providing
chromatography. The 1:1 ligand-to-particle ratio is obtained by gold nanoparticle dimers (34 % isolated yield) or alkyne-azide
comparing the particles' dimensions measured by transmission “click”-chemistry with a suitable triazide, giving trimeric particle
electron microscopy with the weight fraction of the coating architectures (30 % determined by transmission electron mi-
ligand determined by thermogravimetric analysis. The single li- croscopy). The particle stabilization obtained by the coating li-
gand coating of the gold particle guarantees the presence of a gand allows, for the first time, to treat these multi-particle archi-
single masked alkyne per particle. It can be addressed by wet tectures by size exclusion chromatography.
chemical protocols providing the particles with the properties
Introduction
ligand place-exchange reactions on solid support[33,34] or in so-
lution, followed by separation by electrophoresis,[35] on-Au NP
surface polymerization of suitable ligands[36] or exchange of the
entire ligand shell with carefully tuned thiol mixtures on citrate-
stabilized Au NPs.[37] Recently, the scope of sulfur-containing
ligands for Au NPs has been enhanced towards more complex
benzylic thioether-based ligands. The reversible coordinative
binding of the thioether to the Au NP surface and the flexible
adaptive structure of the multidentate coating ligand holds the
promise of efficient and size-selective Au NP passivation. In-
deed, oligomeric,[38–43] tripodal,[44–46] dendrimeric,[46–50] and
even cage-like[51] thioether-based ligands yielded Au NPs with,
in some cases, narrow size-distributions, good yields and re-
markable stability features. We reported earlier about the favor-
able Au NP stabilization features of a family of ligands consist-
ing of a central tetraphenylmethane subunit decorated three-
fold with multidentate oligomeric chains.[52] Particularly appeal-
ing were these ligands with tetraphenylmethane-based benz-
ylic thioether oligomers, as in these cases, a single ligand
turned out to be able to stabilize an entire Au NP. Interestingly,
as only three of the four phenyl units of the central tetraphenyl-
methane are decorated with an oligomer, the molecular design
leaves the fourth phenyl ring upright-standing on the coated
Au NP as ideal site for further chemical functionalization.[53–55]
Colloidal gold has experienced large interest due to its optical
and physical properties.[1–5] Since the pioneering work by Brust
et al.,[6,7] various thiol-based organic ligands have been investi-
gated for the stabilization of gold nanoparticles (Au NPs), owing
to their synthetic tunability and availability, ease of handling,
and favorable stabilization properties. Thiol-stabilized Au NPs
were applied in a plethora of fields including catalysis,[8–10]
sensing and labeling applications,[11–19] medical diagnostics
and therapeutics[20–24] or as building blocks for hybrid materi-
als[25] and molecular electronics.[26–28] For many of these appli-
cations, Au NPs exposing a single functional group, behaving
like “massive molecules” enabling their positioning and integra-
tion by covalent chemistry are desirable.[29–32] Several ap-
proaches resulting in monofunctionalized Au NPs stabilized
with thiol-based ligands have been reported, including single-
[a] E. H. Peters, Prof. Dr. M. Mayor
Departement of Chemistry, University of Basel,
St. Johanns-Ring 19, 4056 Basel, Switzerland
E-mail: marcel.mayor@unibas.ch
https://www.chemie1.unibas.ch/~mayor/index.html
[b] Prof. Dr. M. Mayor
Institute for Nanotechnology (INT), Karlsruhe Institute of Technology (KIT),
P. O. Box 3640, 76021 Karlsruhe, Germany
E-mail: marcel.mayor@kit.edu
[c] Prof. Dr. M. Mayor
Here, we report the successful functionalization of the re-
maining free phenyl ring by an oligo(phenylene-ethynylene)
(OPE) rod exposing a triisopropylsilyl(TIPS)-masked alkyne
group. As sketched in Scheme 1, the ligand 1 exposing the OPE
rod as a functional group keeps the ability to stabilize an entire
Au NP, resulting in monofunctionalized Au NPs which readily
undergo chemical reactions in a single position. The combina-
tion of a reasonably stable Au NP with a reactive functional
group makes these particles addressable by wet chemistry and
Lehn Institute of Functional Materials (LIFM), Sun Yat-Sen University
(SYSU),
Xingang Xi Rd. 135, 510275 Guangzhou, P. R. China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000273.
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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Scheme 1. a) Conceptual overview and synthesis of the Au NP exposing a single functional group (fg). b) Sketch of the ligand-coated Au NP as massive
object exposing a functional group accessible by wet chemistry.
they can be considered as “massive molecules”. The chemical
accessibility of the exposed alkyne was demonstrated by
standard coupling reactions like oxidative acetylene cou-
pling[39,41,48] resulting in “dumbbell”-type dimers and alkyne-
azide “click” chemistry[56,57] providing Au NP trimers thanks to
a suitable triazide linker. Interestingly, the here reported ligand
shell stabilizes the Au NP to an extent enabling to expose these
small nano-architectures consisting of interlinked Au NPs to
size-exclusion chromatography (SEC), which was not the case
for previously reported ligand designs.[39,41,48,49]
Results and Discussion
Ligand Synthesis
Scheme 2. Synthesis of functionalized central linking unit 2. a) 1) BF3Et2O,
CH2Cl2, –10 °C, 45′, 2) tBuONO, CH2Cl2, –10 °C, 45′, 3) I2, KI, –10 °C to r.t., 15 h,
The synthesis of tripodal ligand 1 is shown in Scheme 1. The
three benzylic bromides of precursor 2 were substituted with a
moderate excess of thiol 3 (1.23 equivalents per bromide), using
sodium hydride (NaH) as a base in freshly distilled tetrahydro-
furan (THF). After aqueous work-up, the ligand was isolated in
55 % yield by silica plug filtration and automated gel permea-
tion chromatography (GPC, in chloroform).
The synthesis of the central linking unit was performed in
six steps with an overall yield of 54 % (Scheme 2). Precursor 4
was obtained via Sandmeyer-type reaction applied to 4-tritylan-
iline in good 80 % yield. Subsequent treatment with bromine
gave the tribromide 5 quantitatively. Profiting from the im-
proved reactivity of the iodine substituent in palladium-cata-
lyzed coupling reactions,[58] the OPE-rod exposing the TIPS-pro-
tected alkyne was introduced via Sonogashira cross-coupling
reaction performed at room temperature, providing the OPE-
rod 6 exposing three terminal para-bromophenyls. Threefold
80 %; b) Br2, neat, r.t., 30′, quant.; c) Pd(PPh3)2Cl2, TIPSC≡ C–pC6H4-C≡ CH, CuI,
Et3N, CH2Cl2, r.t., 2 h, 76 %; d) 1) tBuLi, THF, –78 °C, 2 h, 2) DMF, –78 °C to r.t.,
2 h, 88 %; e) 1) NaBH4, MeOH, 0 °C, 2 h, 2) HCl (aq.), 0 °C, r.t., quant.; f) CBr4,
PPh3, CH2Cl2, –10 °C , 1 h quant.
dium borohydride. Finally, in a threefold Appel-reaction, the
benzylic alcohols were converted into the benzylic bromides of
2 quantitatively.
The assembly of the teraphenylmethane building block 3 ex-
posing a benzylic thiol was achieved following a previously
published protocol.[42]
1
All new compounds were characterized by H- and 13C-NMR
spectroscopy and high-resolution mass spectrometry.
Au NP Synthesis
lithium–halogen exchange was achieved with tert-butyl-lithium The Au NPs stabilized by ligand 1 were synthesized following a
in –78 °C cold THF, and quenching with N,N-dimethylformamide previously published protocol[38] based on a variation of the Au
(DMF) gave the trialdehyde 7 in excellent 88 % yield after aque- NP synthesis proposed by Brust et al.[6] In the aqueous phase
ous work-up and column chromatography. The three aldehydes of a biphasic system, one equivalent of gold salt (HAuCl4) for
were quantitatively reduced to benzylic alcohols in 8 with so- each sulfur atom in the ligand, i.e. 6 equivalents, was added.
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The transfer of the gold salt from the aqueous to the organic (TEM), and thermogravimetric analysis (TGA). Finally, the
phase was achieved by addition of tetra-n-butyl ammonium “molecule-like” behavior of Au-1-TIPS was demonstrated by en-
bromide (TOAB, 2 equivalents with respect to HAuCl4, i.e. 12
equivalents) to the organic phase. The ligand was added dis-
solved in CH2Cl2. Nucleation of the Au NPs was induced via
reduction by aqueous NaBH4 (8 equivalents with respect to
HAuCl4, i.e. 48 equivalents). Upon addition of the reducing
agent, an immediate color change of the organic phase from
bright orange to opaque dark brown was observed. After rigor-
ous stirring for 15 min, the organic phase was separated and
concentrated in a N2 stream to prevent possible thermal aggre-
gation of NPs. Precipitation of the Au NPs from the concen-
trated organic solution was achieved by addition of ethanol.
Repeated centrifugation and subsequent SEC provided pure
samples of the Au NPs without remaining traces of ligand mol-
ecules or TOAB. The colloidal gold particles stabilized by ligand
1 are labeled as Au-1-TIPS and were isolated in excellent yields
above 95 %. Au-1-TIPS was the only detectable form of gold
throughout the synthesis and the mass losses were due to the
handling during the purification procedures.
gaging them in wet chemistry coupling protocols producing
small nano-architectures like dimers and trimers which could
be detected by TEM.
The UV/Vis absorption spectrum of Au-1-TIPS (Figure 1c) dis-
plays discrete bands between 300–350 nm, characteristic for
the absorbance of the OPE subunit of the ligand 1. Apart from
these features, the continuous increase towards shorter wave-
length without a surface plasmon band points at Au NPs with
diameters below 2 nm. Only with Au NPs of dimensions below
2 nm, the rate of surface scattering exceeds the rate of bulk
scattering and the electron-donating character of the coordi-
nating sulfur atoms of the coating ligand increases the surface
electron density significantly, reflected in a broadening of the
surface plasmon band.[59] It is noteworthy that very similar opti-
cal behavior was reported for the parent ligand structure with-
out the exposed functional group.[52] The OPE subunit of 1 acts
as an optical label and the very comparable absorption of the
OPE subunit of Au-1-TIPS compared with the free ligand 1 not
only proves the presence of the ligand on the Au NP, but also
supports the hypothetical picture of an exposed and dissolved
OPE rod which does not interfere with the Au NP surface. The
dimensions of the Au NPs were further analyzed by TEM (Fig-
Au NP Analysis and Characterization
The Au NPs were characterized by variable temperature UV/
Vis absorption spectroscopy, transmission electron microscopy
Figure 1. a) TEM micrograph of Au-1-TIPS; b) size-distribution of Au-1-TIPS obtained from TEM micrographs; c) UV/Vis absorption spectrum of Au-1-TIPS in
comparison with ligand 1; d) UV/Vis absorption spectra at selected temperatures, offset for clarity.
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ure 1a–b) and the recorded diameters of 1.21 0.36 nm con-
After the determination of the thermal stability of Au-1-TIPS,
firmed the initial claims derived from the UV/Vis experiments. its potential as precursor in wet-chemical reactions as building
The Au NP size distribution was determined with TEM micro- block for nano-architectures was explored.
graphs comprising more than 6000 NPs, using the threshold
and particle analysis tools from ImageJ.[60]
Au-1-TIPS as Precursor in Coupling Reactions I:
TGA of Au-1-TIPS revealed a mass loss of 19.0 % between
Dimerization
200 and 600 °C which was attributed to the combustion of the
coating organic ligand 1. Assuming a spherical shape, the Au The analytical data of Au-1-TIPS displayed a Au NP coated by a
NP diameter of 1.21 nm determined by TEM allowed to deter- single ligand and, consequently, also only one OPE rod per Au
mine the Au NP's volume (0.93 nm3). With the density of gold NP. Furthermore, the comparison of the UV/Vis spectra of Au-
(19.32 g cm–3), the mass of an average Au NP was determined 1-TIPS and the free ligand 1 suggested an entirely dissolved
to be 1.79 × 10–20 g, and, by multiplication with Avogadro′s and thus wet-chemically addressable OPE subunit exposed at
number (NA: 6.022 × 1023 mol–1), the molecular weight of the the Au NP surface. The OPE rod exposes a TIPS-protected termi-
average uncoated Au NP was determined to be 10792 g mol–1. nal alkyne as masked but reactive functional group which can
The molecular weight of the ligand 1 with the formula be involved e.g. in various coupling protocols. To demonstrate
C
176H188S6Si is 2523.89 g mol–1. The molecular weight of the the engagement of Au-1-TIPS as “massive molecule” in cou-
proposed particle Au-1-TIPS coated by a single ligand is, with pling chemistry, the TIPS-masked alkyne was deprotected prof-
13316 g mol–1, the sum of both. The weight contribution of the iting from the fluoride ions of tetra-n-butylammoniumfluoride
coating ligand 1 fits, with 18.95 %, perfectly the weight loss (TBAF) in CH2Cl2 (Scheme 3). After aqueous work-up, the depro-
recorded during TGA (19.0 %), corroborating the claim that a tected Au NPs were exposed to typical Glaser–Hay oxidative
single ligand 1 is coating and stabilizing an entire Au NP. The acetylene coupling conditions.[61,62] They were re-dissolved in
same issue is examined in Table 1 displaying that the same CH2Cl2 and Cu(I) ions were added together with N,N,N′,N′-tetra-
number of gold atoms per particle is obtained from the TGA methylethane-1,2-diamine (TMEDA) as ligand dissolving the
and the TEM analysis.
copper ion. The reaction was vigorously stirred at ambient air
to guarantee the presence of dissolved oxygen. To avoid aggre-
gation of the coated gold particles, the reaction was kept for 4
hours at room temperature. After aqueous work-up and subse-
quent repetitive separation by SEC using CH2Cl2 and toluene as
eluent, Au NP dimers [(Au-1)2] were isolated in 34 % yield. It is
noteworthy that these dimeric Au NP architectures based on
the ligand 1 were the first larger systems resisting decomposi-
tion on the SEC column, documenting further the superior sta-
bility of Au NP coated by the ligand 1. All our earlier ligand
designs based on benzylic thioethers survived SEC conditions
as monomers, but the resulting interlinked NP architectures dis-
integrated when exposed to the shear forces of SEC.[39,41,48]
The yield of (Au-1)2 was also determined by analyzing TEM
The Au NPs Au-1-TIPS coated with a single ligand exposing
an OPE rod can be regarded as “massive molecules” with a
peripheral TIPS-protected alkyne as functional group. To ex-
plore the stability of this “massive molecule” and thus also the
scope of potential reaction conditions, samples of Au-1-TIPS
were dispersed in ortho-xylene and gradually heated while their
optical features were monitored by UV/Vis spectroscopy. Apply-
ing a heating gradient of 10 °C per hour, the UV/Vis spectra of
the sample remained unchanged up to 105 °C (Figure 1d).
Above this temperature, the samples turned red and the emer-
gence of a plasmon band in the UV/Vis was observed. Upon
extended exposure to temperatures above 105 °C, decoloration
of the sample due to precipitation was observed. The thermal
decomposition of Au-1-TIPS above 105 °C is interpreted as ag- micrographs of the crude reaction mixture. The reaction mix-
glomeration to larger particles, either due to too violent colli- ture was diluted to an extent avoiding the agglomeration of Au
sions for the delicate steric protection provided by coating 1 or NPs on the TEM grid. 53 micrographs (each 340 nm × 260 nm)
due to prior thermal detachment of 1. It is noteworthy that a comprising a total of 2077 Au NPs were analyzed, covering
comparable decomposition temperature was observed for the 4.69 μm2 of the TEM grid. Pairs of Au NPs with a spacing up to
parent ligand structure (110 °C),[52] documenting further the 3.2 nm were considered as (Au-1)2. With 826 of 2077 NPs being
separation between the coating ligand subunit of 1 and the part of a dimer corresponding to a yield of 39.8 %, the TEM
attached OPE rod.
analysis confirmed the yield isolated by SEC.
Table 1. Ligand-to-gold ratio of Au-1-TIPS. The number of gold atoms of an average particle is calculated from the TGA data (top) and from the dimension
of the NP determined by TEM (bottom). m(1): mass loss due to burned ligand 1; M(1): molar mass of 1; n(1): number of mol of ligand 1; mAu: mass of
remaining gold; nAu: number of mol of gold; nAu/n(1): average number of gold atoms per ligand according to the TGA data; ØNP: diameter of an average NP
determined by TEM; VNP: volume of the NP = 4/3πr3; mNP: mass of the NP = VNP·ρAu (with ρAu = 19.32 g cm–3); MAu: molar mass of gold (196.97 g mol–1); nAu
number of mol of gold=mNP/MAu; nAu/NP: average number of gold atoms per NP = nAU·NA (NA = 6.022 × 1023 mol–1).
:
Au-1-TIPS
m(1)
M(1)
n(1)
mAu
nAu
nAu/n(1)
[mg]
[g/mol]
[mol]
[mg]
[mol]
TGA
0.545
2523.9
2.18 × 10–7
2.33
1.18 × 10–5
54.7
ØNP
[nm]
VNP
mNP
[g]
MAu
[g/mol]
nAu
[mol]
nAu/NP
[nm3]
TEM
1.21 0.36
0.928
1.79 × 10–20
196.97
9.1 × 10–23
54.8
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Scheme 3. Nano-architectures obtained by using Au-1-TIPS as starting materials. a) Dimerization by oxidative acetylene coupling. b) Trimerization profiting
from alkyne-azide “click”-chemistry.
Additional structural features of (Au-1)2 were extracted from the stabilized Au NP in Au-1. Interestingly, trimeric structures
TEM micrographs (Figure 2a). The size-distribution of the Au e.g. consisting of “dumbbell”-type dimers formed by oxidative
NPs after the coupling reaction (Figure 2b) resembles, with acetylene coupling with one of its terminal Au NPs engaged in
1.18 0.29 nm, within the limits of the method, the one re- a coupling with the terminal acetylene of a third deprotected
corded for the parent sample Au-1-TIPS. This suggests that, dur- Au-1 were, with only 4 observed cases, too rare to exclude ran-
ing the entire deprotection–homocoupling protocol, the Au NPs dom arrangements on the TEM grid. The Au NP dimers with
remain stable and protected by the coating ligand, corroborat- short spacing are of comparable stability to the longer ones
ing the “massive molecule” behavior of Au-1-TIPS in the reac- and do also not decompose during the SEC process. Also, we
tion sequence.
were not able to distinguish both structures during the SEC
experiment, which was not surprising, considering the working
hypothesis that both structures exclusively differ in the inter-
linking of two Au-1-type subunits.
TEM micrographs of the isolated, “dumbbell-like” Au NP di-
mers (Figure 2a) enabled to analyze the inter-NP distances in
more details. Interestingly, the distribution of the NP spacing
clearly displayed two maxima (Figure 2c). The majority of about
80 % of the recorded Au–NP distances was, with 2.62 0.21 nm,
in the dimensions of the diethynyl-linker expected as product
of the oxidative acetylene coupling, as the distance for the fully
stretched linker estimated by simple MM2 simulation was
Au-1-TIPS as Precursor in Coupling Reactions II:
Trimerization
2.8 nm (distance between both tertiary methane carbon atoms The lack of trimers observed in the homocoupling protocol dis-
of the rod-terminating tetraphenylmethane subunits, see cussed above raised the question if there could be an intrinsic
Scheme 3a). In principle, any shorter distance might be attrib- limitation disqualifying trimerization. As alternative approach to
uted to the projection of the Au NPs of a “dumbbell” dimer not couple three Au NPs to a trimeric nano-architecture, alkyne-
lying flat on the TEM grid. For such an explanation, one would, azide “click”-chemistry has already been applied successfully for
however, rather expect a continuous tailing to shorter distances dendrimer-stabilized Au NPs.[49] Thus, a sample of Au-1-TIPS
and not a discrete maximum. Thus, in similarity to “dumbbell” was deprotected and the liberated alkynes were exposed to a
architectures based on dendrimer-coated Au-NPs,[48] the “click” protocol with the triazide 1,3,5-tris[p-(azidomethyl)phe-
shorter inter-NP distance of 1.64 0.20 nm (recorded for about nyl]benzene (9 in Scheme 3) as threefold central coupling unit.
20 %) most likely points at direct coordinative contacts be- To the deprotected Au-1 dissolved in CH2Cl2, the triazide 9 was
tween an Au NP and the terminal acetylene exposed by the added and, while vigorously stirring, aqueous solutions of cop-
OPE rod of the deprotected Au-1.[63] Such a coordination be- per sulfate and sodium ascorbate as reducing agent were
tween alkyne and Au NP requires access to the Au NP surface, added (Scheme 3b). To avoid aggregation of the coated gold
showing that the ligand 1 is not covering the entire surface of particles, the reaction was kept for 4 hours at room tempera-
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Figure 2. “Dumbbell”-type dimers formed by exposing Au-1-TIPS to a deprotection–homocoupling protocol. a) TEM micrographs of (Au-1)2 (scale bar is valid
for all 8 micrographs). b) Size-distribution of Au NPs after the deprotection–homocoupling protocol. c) Distribution of the interparticle distance in (Au-1)2.
ture. After extraction with CH2Cl2, the composition of the crude meric Au NP architectures could be separated from the mono-
reaction mixture was analyzed by TEM. 42 micrographs (each meric Au NPs, the method failed to isolate pure (Au-1)39
340 × 260 nm) comprising a total of 1587 Au NPs were ana- fractions.
lyzed, covering 3.71 μm2 of the TEM grid. Groups of 3 NPs with
a maximal separation smaller than 4.5 nm were considered as
the trimeric (Au-1)39. The slightly larger accepted radius com-
pared with the homocoupled (Au-1)2 is due to both the larger
TEM micrographs of (Au-1)39 (Figure 3a) enabled the analy-
ses of the interparticle distances (Figure 3c). The rather broad
and unspecific distance distribution reflects the increased struc-
tural variety in the arrangement of the three Au NP in (Au-1)39,
dimensions and the increased flexibility of the particle-interlink-
due to the flexibility of the interlinking scaffold comprising tri-
ing organic structure. From the 1587 NPs, 477 NPs or 30.1 %
azole subunits attached at benzylic positions.
were engaged in a (Au-1)39 structure, 324 NPs or 20.4 % were
While the spatial grouping of Au NPs by TEM analyses of
the reaction mixtures support for both coupling reactions the
formation of the desired oligo-NP nano-architectures, the
method does not comprise any further chemical information.
As example, an interesting chemical issue was the rather mod-
erate yield of the desired oligo-NP objects with 39.8 % for
(Au-1)2 and 30.1 % for (Au-1)39 respectively. Whether these
moderate yields reflect the lack of reactivity of the “massive
molecule” Au-1 in the coupling reaction or, in the step before,
observed as dimers of Au NPs, and 786 NPs or 49.5 % were
recorded as monomers. Whether the dimers are formed due to
only twofold “click”-chemistry-type coupling to 9, or to homo-
coupled (Au-1)2, cannot be distinguished in the TEM micro-
graphs.
Also for the “click”-chemistry protocol, a very comparable
size-distribution (Figure 3b, 1.28 0.30 nm) of the Au NPs after
the coupling reaction was observed, pointing at the integrity
of Au-1 during the applied reaction conditions. In analogy to
the homocoupled (Au-1)2, also (Au-1)39 survived SEC condi- an incomplete deprotection of Au-1-TIPS cannot be distin-
tions. However, while fractions comprising (Au-1)39 and the di- guished by the here reported studies.
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Figure 3. TEM analyses of (Au-1)39. a) TEM micrographs of (Au-1)39 (scale bar is valid for all 8 micrographs). b) Size-distribution of Au NPs after the
deprotection–“click”-coupling protocol. c) Distribution of the interparticle distance in (Au-1)39.
Conclusion
Experimental Section
Materials
In summary, a new ligand for the efficient and controlled syn-
thesis of monofunctionalized Au NPs combining the massive
nature of the particles with the wet chemical addressability of
a molecule and thus behaving as “massive molecules” with su-
perior stability properties is reported. The ligand design is
based on a central tetraphenylmethane subunit unifying three
oligomeric benzyl sulfide chains forming the Au NP-stabilizing
subunit and exposing a masked alkyne on an OPE rod attached
to the fourth phenyl ring of the central unit. Au NPs with narrow
size distribution around 1.2 nm coated by a single ligand are
obtained efficiently and display very good stability features,
withstanding decomposition up to 105 °C in dispersion. The
“massive molecule”-like behavior is displayed by deprotecting
the terminal alkyne of the coated Au NP and engaging it in
coupling protocols providing interlinked dimeric and trimeric
Au NP systems as displayed by TEM analyses. With the here
reported coating ligand, the particles of these nano-architec-
tures are stable enough to resist the shear forces during purifi-
cation by SEC.
All commercially available starting materials were of reagent grade
and used as received, unless stated differently. Absolute THF was
purchased from Acros, stored over 4 Å molecular sieves, pre-dried
with CaH, handled under argon and freshly distilled from sodium
before each use. Pure CH2Cl2 was purchased from J. T. Baker. Pure
toluene was purchased from Acros. Pure DMF was purchased from
Acros Organics. tert-butylmethyl ether (TBME), c-hexane, ethyl acet-
ate (EtOAc), and CH2Cl2 from Biosolve were used for purification
and were of technical grade. Column chromatography was carried
out on SiliaFlash P60 (particle size 40–63 μm) from SiliCycle. SEC for
the purification of Au NPs mono-, di- and trimers was performed
manually using Bio-Rad Bio-Beads S-X1 (operating range 600–14
000 g mol–1) with CH2Cl2 or toluene as eluent. Deuterated solvents
were purchased from Cambridge Isotope Laboratories.
Equipment and Measurements
1H- and 13C-NMR spectra were recorded with a Bruker DPX 400
instrument (1H resonance 400 MHz, 13C resonance 101 MHz) or a
Bruker DRX 500 instrument (1H resonance 500 MHz, 13C resonance
126 MHz) at 298 K. The chemical shifts (δ) are reported in ppm and
are referenced to the residual proton signal of the deuterated sol-
We are currently investigating the catalytic activity of these
ligand-coated Au NPs and explore their scope as labels in pro-
teins and even larger biological systems.
1
vent ([D]chloroform: 7.26 ppm) for H spectra or the carbon of the
solvent ([D]chloroform: 77.1 ppm) for 13C spectra. The coupling con-
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stants (J) are given in Hertz (Hz), the multiplicities are denoted as:
s (singlet), d (duplet), t (triplet), m (multiplet), and br (broad). High-
resolution mass spectra (HRMS) were measured as HR-ESI-ToF-MS
with a Maxis 4G instrument from Bruker or a Bruker UltraFlex II –
MALDI-ToF-MS. For purification of the ligands, a (automated) Shim-
adzu Prominence System was used with SDV preparative columns
from Polymer Standards Service (two Showdex columns in series,
20 mm × 600 mm each, exclusion limit: 30 000 g mol–1) with chloro-
form as eluent. UV/Vis measurements were recorded on a Jasco V-
p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyltris[p-
(formyl)phenyl]methane (7): In a dry, degassed 250 mL flask, 6
(2.10 g, 2.51 mmol, 1 equiv.) was dissolved in 50 mL of freshly dis-
tilled THF and cooled to –78 °C. tBuLi (1.7
M in pentane, 20 mL,
34.1 mmol, 13.5 equiv.) was added to the mixture and stirred for 2
hours. DMF (5 mL, 65.3 mmol, 26 equiv.) was added to the mixture
which was warmed up to r.t. over 2 hours and was thereafter stirred
for 2 more hours. The reaction was cautiously quenched by addition
of water, the organic phase separated, and the aqueous phase ex-
770 spectrophotometer using 117.100F-QS cuvettes from Hellma tracted twice with CH2Cl2. The combined organic phase was dried
Analytics (10 mm light path). TEM was performed on a Philips
CM100 TEM at 80 kV using copper grids (Cu-400HD) from Pacific
Grid Tech. TGA was measured on a Mettler Toledo TGA/SDTA851e
with Na2SO4, and the solvent evaporated. The purification was af-
forded by column chromatography (c-hexane 4:1 EtOAc) to yield 7
as a yellowish solid (1.51 g, 2.20 mmol, 88 %). 1H-NMR (400 MHz,
[D]Chloroform) δ = 10.00 (s, 3H), 7.89–7.77 (m, 6H), 7.51–7.39 (m,
12H), 7.25–7.16 (m, 2H), 1.13 (s, 21H). 13C-NMR (63 MHz, [D]Chloro-
form) δ = 191.43, 151.52, 144.71, 134.75, 131.99, 131.56, 131.40,
131.29, 130.64,129.50, 123.59, 122.76, 121.89, 106.56, 93.08, 90.38,
90.11, 65.96, 18.69, 11.32. HRMS (ESI): m/z = [M + Na]+ calcd. for
C44H41Br3SiNa: 707.2952, found 707.2942.
with a heating rate of 10 °C min–1
.
p-Iodophenyltriphenylmethane (4):[64] To a degassed 250 mL
two-neck flask equipped with rubber septum and thermometer,
BF3OEt2 (3.67 mL, 29.0 mmol, 2 equiv.) was added and cooled to
–10 °C. In a separate flask, 4-tritylaniline (5.01 g, 14.5 mmol, 1 equiv.)
was dissolved in 100 mL of degassed CH2Cl2 and was added drop-
wise via transfer cannula. The mixture was stirred at –10 °C for 45
minutes after which time tBuNO2 (3.00 mL, 25.0 mmol, 1.75 equiv.)
dissolved in 50 mL of degassed CH2Cl2 was added dropwise via
transfer cannula. The mixture was stirred at –10 °C during 45 min-
utes. I2 (4.78 g, 18.9 mmol, 1.3 equiv.) and KI (3.61 g, 21.8 mmol,
1.5 equiv.) were added to the mixture which was thereafter stirred
vigorously during 15 h and allowed to slowly warm up to r.t. Excess
halogen was quenched upon addition of saturated aqueous
Na2S2O3. After phase separation, the aqueous phase was extracted
p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyltri(p-
hydroxymethylphenyl)methane (8): In a 100 mL flask, 7 (1.51 mg,
2.20 mmol, 1 equiv.) was dissolved in 20 mL of MeOH and 20 mL
of THF and cooled to 0 °C. NaBH4 (749 mg, 19.8 mmol, 9 equiv.)
was added to the solution portionwise over 1 hour. The mixture
was allowed to stir for 1 more hour until the reaction was quenched
upon careful addition of aqueous HCl (10 %). The mixture was di-
luted with CH2Cl2 and transferred to a separation funnel. The aque-
ous phase was extracted three times with CH2Cl2, the combined
twice more with CH2Cl2 and dried with Na2SO4. After evaporation organic phase dried with Na2SO4 and the solvent removed in vacuo.
of the volatile in vacuo, compound 4 was purified by column chro-
matography (c-hexane) as a pale solid (5.2 g, 11.6 mmol, 80 %). 1H-
NMR (400 MHz, [D]Chloroform): δ = 7.60–7.55 (m, 2H), 7.25–7.20 (m,
6H), 7.20–7.17 (m, 6H), 7.17–7.15 (m, 3H), 7.00–6.96 (m, 2H).
The crude product was filtered through a silica plug (c-hexane 1:1
EtOAc), to afford 8 as a yellowish solid (1.52 mg, 2.20 mmol, quant.).
1H-NMR (400 MHz, [D]Chloroform): δ = 7.49–7.41 (m, 4H), 7.39–7.34
(m, 2H), 7.17 (s, 14H), 4.51 (s, 6H), 3.02–2.86 (s, 3H), 1.17 (s, 21H).
13C-NMR (63 MHz, [D]Chloroform) δ = 147.23, 145.70, 138.63,
131.98, 131.37, 131.01, 130.94, 130.89, 126.45, 123.33, 123.07,
120.61, 106.69, 92.85, 91.03, 89.34, 64.57, 64.42, 18.71, 11.34. HRMS
(ESI-ToF): m/z = [M + Na]+ calcd. for C47H50NaO3Si: 713.3421, found
713.3420.
p-Iodophenyltri(p-bromophenyl)methane (5):[65] In a 250 mL
flask, 4 (4.91 g, 11.0 mmol, 1 equiv.) was dissolved in Br2 (20 mL,
385 mmol, 35 equiv.) and stirred at r.t. for 30 minutes. The reaction
was diluted with CH2Cl2 and crushed ice was added. The mixture
was further cooled with an ice bath and saturated aqueous Na2S2O3
was slowly added until all the excess bromine was quenched. The
mixture was extracted twice with CH2Cl2, dried with Na2SO4, and
the volatile evaporated in vacuo. The crude was subjected to silica
plug filtration (CH2Cl2) and recrystallized from hot c-hexane to yield
p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyltri[p-(bro-
momethyl)phenyl]methane (2): In a dry, degassed 100 mL flask,
8 (419 mg, 0.606 mmol, 1 equiv.) was dissolved in 10 mL of dry,
degassed CH2Cl2 and cooled to –10 °C. Triphenylphosphine
5 as a pale solid (7.51 g, 11.0 mmol, quant.). 1H-NMR (400 MHz, (965 mg, 3.64 mmol, 2 equiv.) and carbon tetrabromide (1.21 g,
[D]Chloroform): δ = 7.63–7.57 (m, 2H), 7.41–7.37 (m, 6H), 7.04–6.99
(m, 6H), 6.92–6.84 (m, 2H).
3.64 mmol, 2 equiv.) were added portionwise over 20 minutes. The
mixture was allowed to stir 1 more hour and was then quenched
by addition of saturated aqueous NaHCO3. The organic phase was
separated, dried with Na2SO4 and the solvent evaporated. The
crude was subjected to silica plug filtration (CH2Cl2) to afford 2 a
yellowish foam (533 mg, 0.606 mmol, quant.). 1H-NMR (400 MHz,
[D]Chloroform): δ = 7.47–7.38 (m, 6H), 7.34–7.27 (m, 6H), 7.23–7.15
(m, 8H), 4.48 (s, 6H), 1.15 (s, 21H). 13C-NMR (101 MHz, [D]Chloro-
form): δ = 146.46, 146.20, 135.75, 131.98, 131.38, 131.24, 131.05,
130.89, 128.53, 123.39, 123.04, 120.98, 106.66, 92.87, 90.85, 89.50,
64.53, 33.07, 18.71, 11.34. HRMS (MALDI-ToF): m/z = [M]+ calcd. for
C47H47Br3Si: 876.0992, found 876.1003.
p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyltri(p-bromo
phenyl)methane (6): In a dry, degassed 100 mL flask, 5 (1.4 g,
2.05 mmol, 1 equiv.), bis(triphenylphosphine)palladium(II)chloride
(0.24 g, 0.205 mmol, 0.1 equiv., cat.) and CuI (0.195 g, 1.03 mmol,
0.5 equiv., cat.) were dissolved in 40 mL of CH2Cl2 and 10 mL of
triethylamine (71.2 mmol, 35 equiv.). The mixture was bubbled with
Ar for 20 minutes after which time, p-(triisopropylsilylethynyl)-
phenylethyne (1.16 g, 4.1 mmol, 2 equiv.) was added portionwise
while stirring for 2 hours at r.t. The solvent was then evaporated in
vacuo and the crude subjected to column chromatography (c-hex-
ane 10:1 CH2Cl2) to afford 6 as a yellow foam (1.3 g, 1.55 mmol,
76 %). 1H-NMR (400 MHz, [D]Chloroform) δ = 7.47–7.39 (m, 12H),
Ligand 1 (p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyl-
tris{p-[({p-[bis[p-(tert-butyl)phenyl](p-{[(p-methylbenzyl)-
7.17–7.13 (m, 2H), 7.08–7.02 (m, 6H), 1.16 (s, 21H). 13C-NMR thio]methyl}phenyl)methyl]benzyl}thio)methyl]phenyl}meth-
(101 MHz, [D]Chloroform) δ = 145.71, 144.55, 132.45, 132.00, 131.40,
ane): In a dry, degassed 25 mL flask, 2 (28 mg, 0.032 mmol, 1 equiv.)
131.20, 131.09, 130.70, 123.50, 122.93, 121.40, 120.80, 106.64, 92.96, and 3 (74 mg, 0.118 mmol, 3.7 equiv.) were dissolved in 10 mL of
90.62, 89.77, 63.98, 18.72, 11.35. HRMS (MALDI-ToF): m/z = [M]+ freshly distilled THF. In order to start the reaction, NaH (60 % dis-
calcd. for C44H41Br3Si: 834.0522, found 834.0531.
persed in mineral oil, 40 mg, 1.0 mmol, 10 equiv.) was added. The
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EurJIC
European Journal of Inorganic Chemistry
mixture was allowed to stir at room temperature for 15 hours after proximity of non-linked NPs. Interparticle distances were measured
which time, the reaction was quenched upon addition of minimum
amounts of water. The mixture was dried with Na2SO4, and the
solvents evaporated to dryness. The crude was filtered through a
silica plug (CH2Cl2) and subjected to automated GPC (chloroform)
to give an orange oil (44 mg, 0.018 mmol, 55 %). 1H-NMR (400 MHz,
[D]Chloroform): δ = 7.44 (s, 2H), 7.43–7.33 (m, 2H), 7.26–7.20 (m,
12H), 7.19–7.08 (m, 64H), 3.61 (d, J = 5.3 Hz, 12H), 3.59 (d, J = 9.7 Hz,
12H), 2.33 (s, 9H), 1.30 (s, 54H), 1.15 (s, 21H). 13C-NMR (126 MHz,
[D]Chloroform) δ = 148.53, 147.28, 146.00, 145.85, 145.07, 143.67,
136.55, 135.98, 135.60, 135.41, 135.13, 131.97, 131.70, 131.37,
131.33, 131.26, 131.09, 131.01, 130.85, 130.71, 130.66, 130.57,
129.16, 128.90, 128.29, 127.95, 127.93, 124.55, 124.24, 123.28,
120.62, 106.71, 92.75, 64.33, 63.74, 35.55, 35.39, 35.26, 34.34, 31.41,
21.14, 18.71, 11.35. HRMS (MALDI-ToF): m/z = [M + Na]+ calcd. for
manually from the TEM micrographs.
Gold Nanoparticle Trimerization
The Au NP “click” reactions were carried out with 3 mg of Au NPs
(MW 13357 g/mol [one ligand 1 with 55 Au atoms]). The acetylene-
monofunctionalized Au NPs were dispersed in dichloromethane
(200 μL) and TBAF (1
M in THF, 50 μL) was added. The mixture was
left stirring for 1 hour and quenched with water, extracted with
dichloromethane and dried with Na2SO4. After filtration, the solu-
tion was concentrated by vacuum at 30 °C. The dry, deprotected
Au NPs were dissolved in 50 μL dichloromethane. Corresponding to
the amount of azide moieties, 0.33 equiv. 1,3,5-tris[p-(azido-
methyl)phenyl]benzene was dissolved in 50μL CH2Cl2 and added to
the solution. While intensely stirring, CuSO4 (20 mol-% in respect
to Au-1-TIPS dissolved in 20 μL water) and sodium ascorbate
(30 mol-% in respect to Au-1-TIPS dissolved in 20 μL water) were
added. After 4 h reaction time, the mixture was extracted with di-
C176H188S6Si: 2544.2697, found 2544.2725.
Gold Nanoparticle Formation and Purification
Au NP syntheses were carried out on a 30–50 μmol scale with re- chloromethane and dried with Na2SO4 and dried in vacuo to yield
spect to the Au equivalents. Tetrachloroauric acid (6 equiv., where
6 is the number of sulfur atoms in the used ligand) was dissolved
a mixture of Au NP mono-, di- and trimers. The crude was subjected
to four cycles of SEC with toluene as an eluent to remove unreacted
in 2.0 mL of deionized water. A solution of TOAB (12 equiv.) dis- monomers. Di- and trimers were not separated. Highly diluted solu-
solved in 2.0 mL of CH2Cl2 was added and the two-phase mixture
was stirred for 15 minutes after which time, the aqueous phase had
turned colorless and the bright orange color of the organic phase
indicated complete phase transfer of the gold. The ligand (1 equiv.),
dissolved in 2.5 mL of CH2Cl2, was added to the reaction mixture
which was stirred for 15 minutes, allowing the thioether moieties
and the gold to pre-organize their conformation. A freshly prepared
solution of sodium borohydride (48 equiv.) in 2.0 mL of water was
then added quickly to the reaction mixture, reducing the gold spe-
cies and thereby causing an immediate color change to dark brown.
After 15 min stirring, the resulting opaque organic phase was sepa-
rated from the aqueous phase via Pasteur pipette and the aqueous
phase was washed three times with CH2Cl2 and separated in the
same manner. The combined organic fractions were concentrated
to a volume of approximately 0.5 mL in a Falcon tube by constant
bubbling of protection gas to avoid possible thermal decomposi-
tion. 35 mL of ethanol was added in order to precipitate the parti-
cles which were then centrifuged three times at during 25 minutes
at 5 °C and 4000 rpm, discarding the supernatant after every centrif-
ugation step. After redispersion in CH2Cl2, the Au NPs were sepa-
rated from excess ligand via size-exclusion chromatography, and
stored in the freezer dispersed in CH2Cl2 at a concentration of
1 mg mL–1.
tions were used for deposition on the grids to avoid accidental
proximity of non-linked NPs. Interparticle distances and di- and tri-
merization yields were measured manually from the TEM micro-
graphs.
Acknowledgments
The authors thank the Swiss National Science Foundation (Grant
no. 200020_178808) for financial support, Cedric Wobill for pro-
viding the TGA measurements, Dr. Michael Pfeffer and Dr. Heinz
Nadig for measuring ESI-HRMS and the ETH Zürich MoBiAs lab
for measurement of MALDI-HRMS. M. M. acknowledges support
by the 111 project (90002-18011002).
Keywords: Gold · Nanoparticles · Monofunctionalized
particles · Macromolecular ligands · Oligomers · Organic
coating
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