Linear Tetraphenylmethane-Based Thioether Oligomers Stabilising an Entire Gold Nanoparticle by Enwrapping

Article abstract of DOI:10.1002/chem.201504575

The design and synthesis of a novel linear thioether-based ligand subunit with a tetraphenylmethane core used in the stabilisation of gold nanoparticles (AuNPs) are presented. Mono-, tri, penta- and heptamers of the ligand have been synthesised and used to stabilise AuNPs by enwrapping. With the exception of the monomer, all ligands provide reliable long-term stability and redispersibility for the coated nanoparticles in common organic solvents. Despite variation of the oligomer length, all stable particles were of the same size within error tolerance (1.16±0.32 nm for the trimer, 1.15±0.30 nm for the pentamer, 1.17±0.34 nm for the heptamer), as investigated by transmission electron microscopy (TEM). These findings suggest that not only the number of sulfur atoms in the ligand, but also its bulkiness play a crucial role in stabilising the AuNPs. These findings are supported by thermogravimetric analysis (TGA), showing that AuNPs stabilised by the penta- or heptamer are passivated by a single ligand. Thermal stability measurements suggest a correlation between ligand coverage and thermal stability, further supporting these findings.

Full text of DOI:10.1002/chem.201504575

DOI: 10.1002/chem.201504575
Communication
&
Nanoparticle Stabilization
Linear Tetraphenylmethane-Based Thioether Oligomers Stabilising
an Entire Gold Nanoparticle by Enwrapping
Mario Lehmann,[a] Erich Henrik Peters,[a] and Marcel Mayor*[a, b, c]
chemical methods.[6] When using AuNPs as functional subunits,
Abstract: The design and synthesis of a novel linear thio-
the following features should be taken into account: 1) the
ether-based ligand subunit with a tetraphenylmethane
particle’s size and shape, which control its physical properties,
core used in the stabilisation of gold nanoparticles
so, in an ideal sample, should be as uniform as possible; 2) the
(AuNPs) are presented. Mono-, tri, penta- and heptamers
chemical nature, the number and the spatial arrangement of
of the ligand have been synthesised and used to stabilise
functional groups exposed at the particle surface as connect-
AuNPs by enwrapping. With the exception of the mono-
ing points addressed by wet chemistry; 3) the stability of the
mer, all ligands provide reliable long-term stability and re-
particle, which determines the harshness of reaction conditions
dispersibility for the coated nanoparticles in common or-
applicable for their integration/decoration by wet chemistry
ganic solvents. Despite variation of the oligomer length,
methods; 4) the NPs synthetic availability and purity.
all stable particles were of the same size within error toler-
Although there is a plethora of reports on NPs stabilised by
ance (1.16Æ0.32 nm for the trimer, 1.15Æ0.30 nm for the
pentamer, 1.17Æ0.34 nm for the heptamer), as investigat-
ed by transmission electron microscopy (TEM). These find-
ings suggest that not only the number of sulfur atoms in
the ligand, but also its bulkiness play a crucial role in sta-
bilising the AuNPs. These findings are supported by ther-
mogravimetric analysis (TGA), showing that AuNPs stabi-
lised by the penta- or heptamer are passivated by a single
ligand. Thermal stability measurements suggest a correla-
tion between ligand coverage and thermal stability, fur-
ther supporting these findings.
various thiolates,[12] stabilisation of NPs by thioether-based
structures has only been reported on rare occasions. The weak-
ness of the interaction between the sulfur atom of a thioether
moiety and the NPs’ metal surface is particularly appealing, as
it can amount to a considerable contribution by using multi-
dentate oligothioether systems, and it might even allow for
optimization of the arrangement of the coating structure by
reversible ligand–particle interactions. Inspired by the concept,
we explored the potential of linear,[13] as well as dendritic,[14]
multidentate thioether systems as passivating surface coatings
of small AuNPs. The integer ratios between coating ligands
and AuNPs even paved the way to stable, coated particles ex-
posing an integer number of functional groups. In particular,
the use of ligands comprising a central acetylene unit yielded
coated AuNPs with two ethynyl groups exposed on opposite
sides in the case of linear oligomeric thioether ligands,[15,16]
and even in AuNPs with a single ethynyl handle in the case of
the dendritic ligand system.[17,18] Oxidative acetylene homo
coupling protocols enabled the assembly of organic–inorganic
hybrid materials as “pearl necklace”-type arrays in the case of
the bifunctionalised AuNPs[15,16] and as dumbbell-type struc-
tures in the case of monofunctionalised AuNPs.[17] The scope of
wet chemical methods that profit from these AuNPs as artificial
molecules was further widened by applying azide–acetylene
click-reaction protocols to decorate oligoazide linkers with par-
ticles resulting in dumbbell-, trike- or squad-like superstruc-
tures.[18] In all of these thioether ligand structures, the sulfur
Gold nanoparticles (AuNPs) have fascinated mankind ever
since the fourth century A.D.[1] and unambiguously still do
nowadays. Because of their unique multifaceted properties,
AuNPs have been used in numerous fields of research. They
have found application in medical therapeutics,[2[ catalysis,[3]
chemical and biological sensing,[4,5] molecular electronics[6–8]
and recently also as functional subunits of hybrid materials.[9]
AuNPs have also been used as molecule-like building
blocks[10,11] and integrated into larger architectures by wet
[a] M. Lehmann, E. H. Peters, Prof. Dr. M. Mayor
Department of Chemistry, University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: marcel.mayor@unibas.ch
[b] Prof. Dr. M. Mayor
atoms were interlinked by
a 3-tert-butyl-a,a’-meta-xylene
Institute for Nanotechnology (INT)
Karlsruhe Institute of Technology (KIT)
P. O. Box 3640, 76021 Karlsruhe (Germany)
moiety and the importance of having a sterically demanding
ligand shell covering the rather reactive surface of a AuNP
became evident during stabilisation studies with various den-
dritic systems.[14] Although the 2nd generation dendrimer,
which stabilised an entire particle and thereby provided mono-
functionalised AuNPs, was ideally suited for most of the appli-
cations we had in mind, the limited synthetic availability of the
macromolecular ligand handicapped the further exploration of
[c] Prof. Dr. M. Mayor
Lehn Institute of Functional Materials (LIFM)
Sun Yat-Sen University (SYSU), Guangzhou (P. R. China)
Supporting information for this article, including synthetic procedures and
analytical data of the compounds 1–17, details on nanoparticle formation,
purification and analysis (UV/Vis, 1H NMR and thermogravimetric analysis),
is available on the WWW under http://dx.doi.org/10.1002/chem.201504575.
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the system. We thus sought alternative ligand motifs that com-
bine coordinating benzylic thioethers with steric bulk to pro-
tect the coated ligand.
cursors 9–11 were synthesised with slight modification of the
protocol reported by Peterle et al.:[13] mild radical bromination
of compound 8 upon illumination with a halogen lamp with
N-bromosuccinimide (NBS) as bromine source in methyl for-
mate gave the bis-benzylbromide derivative 9. The bis-mercap-
to derivative 11 was obtained by treatment of the bromine
precursor 9 with thiourea in dimethyl sulfoxide (DMSO) during
15 h to form its thiouronium salt, which was precipitated by an
excess of dichloromethane. The precipitate was redissolved in
MeOH and treatment with aqueous sodium hydroxide (1m),
followed by reprotonation with aqueous hydrochloric acid
(1m) provided the desired bis-methylenemercapto tetraphenyl-
methane derivative 11. Continuous degassing of all solvents,
aqueous solutions and reaction mixtures in a steady stream of
argon turned out to be crucial to avoiding polymerisation due
to disulfide formation.
We report herein our new bulky tetraphenylmethane-based
series of ligands and its excellent AuNP-stabilising properties
(Scheme 1). In particular, the obtained linear oligomers not
only provide stable AuNPs with very good processability, but
the pentamer (3) and heptamer (4) are also both able to stabi-
lise an entire particle and thus are interesting lead structures
towards monofunctionalised AuNPs. The molecular design
combines an increased bulkiness of the parent building block
due to the two tert-butyl-decorated phenyl rings and also to
the three-dimensional tetraphenylmethane core structure, with
an increased spacing between both sulfur atoms compared to
the meta-xylene moiety. Whereas the bulkiness is likely to solu-
bilise both the bare ligand and the coated particle, the in-
creased spacing between neighbouring sulfur atoms results in
more isolated contact points on the AuNPs, which should be
reflected in the ligand–particle interactions. Either the in-
creased spacing influences the particle’s size or it alters the ar-
rangement of the ligand at the particle’s surface, which holds
the potential for new packing motifs and ligand/particle ratios.
As final optimisation, the terminal sulfur atoms were masked
by 4-methylbenzyl groups instead of the benzyl groups used
previously, mainly to provide the subunit with an easily detect-
ed NMR signal.
The monomeric parent ligand 1 was isolated in good yield
by column chromatography after treating the bis-mercapto
precursor 11 with a-bromo para-xylene and sodium hydride as
base in THF at room temperature. The bifunctional compound
10, made from the bis-bromine precursor 9 through a SN2 re-
action with trityl mercaptan and sodium hydride as base in
THF, comprises one trityl-masked thiol and a benzylic bromide
as leaving group, and is therefore an ideal building block for
the stepwise assembly of the longer oligomers 2–4. Elongation
of the dithiol derivative 11 on both sides with 10 in THF at
room temperature using sodium hydride as base gave the ter-
minally trityl-protected trimer 12 in a good yield of 88%. De-
protection of 12 by treatment with TFA and SiEt3 in CH2Cl2
gave quantitatively the trimer 13 exposing terminally free
thiols. A similar elongation protocol enabled the transforma-
tion from the trimer 13 to the pentamer 14 and from the free
thiol pentamer 15 to the heptamer 16. With 83% for 14 and
79% for 16, the yields of isolated products decreased slightly,
the longer the oligomers became. Deprotection using the con-
ditions described above for 12 provided the corresponding
free dithiol trimer 13, pentamer 15, and heptamer 17 almost
quantitatively. Subsequent end-capping of the dithiol oligo-
mers was performed under conditions similar to those de-
scribed above for the assembly of 1 and provided the end-
For the synthesis of ligands 1–4 (Scheme 2), the core precur-
sor 8 was first synthesised, following a marginally modified lit-
erature protocol:[19] triphenylmethanol derivative 5 was ob-
tained by a twofold Grignard reaction of the Grignard reagent
obtained from 1-bromo-4-tert-butylbenzene and solid magne-
sium in tetrahydrofuran (THF) and methyl p-toluate. Subse-
quent electrophilic aromatic substitution with aniline in glacial
acetic acid with hydrochloric acid as catalyst led to compound
6. A one-pot Sandmeyer-type reaction delivered compound 7
by substitution of the amine by an iodine atom through pre-
liminary in situ formation of its diazonium salt. Subsequent
methylation with methyllithium in THF yielded compound 8
with the completed carbon skeleton of the target structure in
a yield of approximately 50% over the four steps. The key pre-
Scheme 1. Linear tetraphenylmethane-based thioether oligomers 1–4 and concept of their Au nanoparticle stabilisation by surface coating.
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Scheme 2. Molecular structures and synthesis of ligands 1–4: a) 1) Mg, THF, reflux, 22 h; 2) H2O, sat. NH4Cl, 87%; b) aniline, HCl, AcOH, 1408C, 15 h, 71%;
c) 1) BF3·OEt2, tBuNO2, CH2Cl2, À308C, 3 h; 2) KI, I2, À308C!room temperature, 15 h, 84%; d) MeLi, THF, À608C, 15 h, quant.; e) NBS, azobisisobutyronitrile
(AIBN), methyl formate, hn, reflux, 15 h, 60%; f) TrtSH, NaH, THF, room temperature, 15 h, 48%; g) 1) thiourea, DMSO, 408C, 15 h; 2) CH2Cl2; 3) 1m aq. NaOH,
1m aq. HCl, MeOH, 3 h, 51–93%; h) 4-methylbenzyl bromide, NaH, THF, 5 h, 91%; i) NaH, THF, room temperature, 5 h, 88%; j) SiEt3H, TFA, CH2Cl2, room temper-
ature, 1 h, quant.; k) 4-methylbenzyl bromide, NaH, THF, 15 h, 89%; l) 10, NaH, THF, room temperature, 5 h, 83%; m) SiEt3H, TFA, CH2Cl2, room temperature,
1 h, quant.; n) 4-methylbenzyl bromide, NaH, THF, 15 h, 77%; o) 10, NaH, THF, room temperature, 5 h, 79%; p) SiEt3H, TFA, CH2Cl2, room temperature, 1 h,
quant.; q) 4-methylbenzyl bromide, NaH, THF, 15 h, 72%. Me=Methyl, Bn=benzyl, Trt =trityl.
capped oligomers 2–4 in good yields. All new ligand structures
1–4, as well as their precursors, were fully characterised by H
ganic phase, the biphasic reaction mixture was vigorously
stirred while an aqueous NaBH4 solution was added. After an-
other 10 min stirring at room temperature, the phases were al-
lowed to separate and the intense dark brown-coloured CH2Cl2
phase indicated the presence of AuNPs dissolved in the organ-
ic phase. Addition of excess ethanol caused the precipitation
of the AuNPs, which were separated by centrifugation and
dried under vacuum before being redissolved in CH2Cl2. Inter-
estingly, some of these particles were not only formed almost
quantitatively (in fact the AuNPs were the only detectable
form of gold), but also displayed very promising stabilities, de-
pending on the length of the oligomer used to stabilise the
AuNP.
1
and 13C NMR spectroscopy and MALDI-TOF mass spectrometry.
The small-molecule-type precursors were further analysed by
elemental analysis, whereas the more precious oligomers were
characterised by high-resolution MALDI-TOF mass spectrome-
try instead.
The ability of these oligothioether type structures 1–4 to sta-
bilise AuNPs was analysed by using a similar method to that al-
ready applied successfully for linear and dendritic multidentate
ligand structures (Scheme 1).[13–18] It basically consist of a varia-
tion of the AuNP synthesis reported by Brust et al.[20] in the
presence of the multidentate ligand structure of interest. In
a biphasic water/CH2Cl2 system containing tetra-n-octylammo-
nium bromide (TOAB) as phase-transfer catalyst, equal molar
equivalents of gold and sulfur atoms were dissolved. In other
words, 2 molar equivalents of the gold salt HAuCl4 were used
for the bidentate ligand 1, 4 molar equivalents for the tetra-
dentate ligand 2, and 6 and 8 molar equivalents of the gold
salt were used for the hexa- and octadentate ligands 3 and 4,
respectively. After complete transfer of the gold salt to the or-
With the monomeric ligand 1, no precipitation of gold was
observed during the synthesis suggesting superior AuNP-sta-
bilising properties compared to the meta-xylene motif used
before, which caused precipitation of gold during the particle
synthesis.[13] In spite of this promising behaviour during the
synthesis, the ligand 1 coated AuNPs (Au-1) did not display
suitable stability features agglomerating to larger AuNPs
within hours in solution. The initially intense brown AuNPs so-
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lution turned gradually reddish during workup, indicating the
formation of larger particles. To our delight, the AuNPs Au-2,
Au-3 and Au-4, stabilised by the trimer 2, the pentamer 3 and
the heptamer 4, displayed considerably improved stabilities.
These particles remained stable and soluble over several days
without alteration of the UV spectra, indicating consistent par-
ticle sizes. In their dried state, the particles were even stored
over months without losing their redispersibility. These parti-
cles were easily redispersed in CH2Cl2, allowing for repeated
precipitation (with ethanol), centrifugation and re-dispersion
cycles without loss of material. Furthermore, these particles
were stable enough to allow for purification by gel permeation
chromatography, providing AuNPs samples of excellent purity.
The extensively purified and vacuum-dried coated particles
Figure 2. Top: Representative sections of the TEM micrographs for samples
of the oligomer-stabilised nanoparticles Au-2, Au-3 and Au-4; bottom: size
distribution of the particles observed in the TEM micrographs.
1
Au-2, Au-3 and Au-4 were analysed by UV/Vis and H NMR-
spectroscopy, transmission electron microscopy (TEM) and
thermogravimetric analysis (TGA).
1
The H NMR spectra of the AuNPs Au-2, Au-3 and Au-4 dis-
measuring, resulting in the size distributions displayed at the
bottom of Figure 2. In agreement with the observations made
in the UV/Vis spectra, all three ligands 2–4 stabilise particles
with diameters below 2 nm. The AuNPs obtained are similar
for all three ligands, even with very comparable size distribu-
tions. Average particle sizes of 1.16Æ0.32 nm, 1.15Æ0.30 nm
and 1.17Æ0.34 nm were recorded for Au-2, Au-3 and Au-4, re-
spectively. Thus the particle sizes obtained are comparable to
those reported for particles with meta-xylene interlinked thio-
ether oligomers (ca. 1.1 nm).[13–16]
played broadening of all signals characteristic of particles due
1
to reduced tumbling motion of the H labels in comparison to
the free ligands 2–4 (see the Supporting Information, Figur-
es S1–S3). In spite of the reduced resolution of the H NMR sig-
1
nals, the spectra document clearly the successful separation of
the AuNPs from both the phase transfer catalyst TOAB and the
excess oligomeric ligand due to purification by gel-permeation
chromatography.
The UV/Vis absorption spectra of ligand-stabilised particles
Au-2, Au-3 and Au-4 are very similar (Figure 1). The eye-catch-
The purity of the coated AuNPs makes the results of ther-
mogravimetric analysis (TGA) particularly interesting, allowing
conclusions with respect to the ratios of organic ligands coat-
ing the AuNPs’ surfaces. For all three AuNPs nanoparticles Au-
2, Au-3 and Au-4 the weight loss attributed to the decomposi-
tion of the organic coating starts at about 2008C and levels
out at about 6008C (TGAs are displayed in the Supporting In-
formation). Weight losses of 26.5%, 24.5% and 32.1% were re-
corded for Au-2, Au-3 and Au-4 respectively. As the lost
weight must arise from the coating ligands 2–4, with the re-
maining weight attributed to the Au atoms forming the
AuNPs. As the molecular weights of both the coating ligand
and the Au atoms are known, the data even allow determina-
tion of the number of Au atoms per coating ligand (see the
Supporting Information).
In the case of Au-2, the remaining 73.5% of the weight cor-
responds to 24.14 Au atoms per ligand 2. Assuming a spherical
shape for the AuNP with a diameter of 1.16 nm, obtained as
an average number by TEM analysis, the density of gold allows
the average AuNP’s mass to be calculated and the number of
Au atoms involved to be determined. Applying these calcula-
tions to Au-2, an average number of 48.27 Au atoms per parti-
cle is obtained, corresponding to twice the number of Au
atoms calculated per ligand. We thus conclude that two li-
gands 2 coat each particle, as was already reported for meta-
xylene based linear oligomers[15,16] and first-generation dendrit-
ic ligands.[14] The analysis became yet more interesting for par-
ticles Au-3 and Au-4. For Au-3, TGA gave a ratio of 42.18 Au
atoms per ligand 3, and the TEM-based dimensional analysis of
the particles suggested 47.04 Au atoms per average AuNP. In
Figure 1. Normalised UV/Vis spectra of the oligomer stabilised nanoparticles
Au-2, Au-3 and Au-4 recorded in CH2Cl2. Their individual absorption spectra
are shifted vertically for clarity (offset).
ing absence of a distinct surface plasmon resonance (SPR)
band at around 520 nm indicates AuNPs with diameters small-
er than 2 nm.[21] To gain further information about the particles’
sizes, TEM micrographs were recorded from samples spread
over a carbon network-covered TEM grid. Typical sections of
the TEM micrographs for all particles are displayed in Figure 2
(top; for larger areas, see the Supporting Information). The
gold particles observed in the TEM micrographs were analysed
by using the software ImageJ[22] for particle counting and
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the case of Au-4, 39.49 Au atoms per ligand 4 were calculated
and the average particle was determined to consist of 49.54
Au atoms. In spite of the inaccuracy of these estimations of
the number of Au atoms per ligand and particle, the obtained
numbers clearly suggest that both ligand structures 3 and 4
are able to coat and stabilise an entire AuNP as a single wrap-
ping ligand.
extent, supported by the work of McCaffrey et al., who report-
ed the stabilisation of AuNPs in a rigid organic molecular cage
bearing only three thioether groups as coordination points for
the encapsulated AuNP.[23] We are currently investigating li-
gands exposing functional groups, as well as more elaborate
branched ligand systems based on the herein-reported new
motif.
This 1:1 ratio of coating ligand per particle is particularly ap-
pealing for future development of particles exposing a single
functional group as inorganic/organic hybrid macromolecules
addressable by wet chemical methods. Equally important for
their use as “artificial macromolecules” is their thermal integrity
in suspension, limiting the range of potentially applicable reac-
tion conditions. Therefore, suspensions of the particles Au-2,
Au-3 and Au-4 dispersed in toluene were gradually heated in
steps of 108C and kept at the elevated temperature for one
hour before optical analysis (UV/Vis). For Au-2, a colour change
from brown to bluish accompanied by formation of a black
precipitate was observed above 908C, indicating thermal de-
composition of the ligand-coated Au-2. Similar behaviour at
the same temperature was observed for Au-4, indicating com-
parable thermal stability of this AuNP coated by a single
ligand. For Au-3, slightly lower decomposition temperatures
were recorded, as the alteration in the UV/Vis spectra and pre-
cipitation was already observed above 808C. Our current work-
ing hypothesis is that the observed thermal stabilities might
reflect the number of binding sites (i.e. thioether moieties) per
particle. Consequently, pentamer 3, with 6 thioethers, results in
slightly less stable coatings than two molecules of the trimer 2
or one heptamer 4, whereby, in both cases, 8 thioether units
connect the coating to the particle’s surface. For all three
coated particles Au-2, Au-3 and Au-4, promising thermal sta-
bilities were observed, rendering them increasingly attractive
as future ligand structures for inorganic/organic hybrid build-
ing blocks.
Acknowledgements
We thank the Swiss National Science Foundation (Grant No.
200020 159730) for financial support.
Keywords: hybrid materials · ligand design · nanoparticles ·
oligomers · tetraphenylmethane
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Received: November 13, 2015
Published online on January 15, 2016
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