Multidentate thioether ligands coating gold nanoparticles

Article abstract of DOI:10.1039/b802460j

The design and synthesis of oligomeric ligands based on benzylic thioethers is presented together with their ability to enwrap and stabilize gold nanoparticles with diameters below 2 nm, which become - with increasing length of the oligomer - more monodisperse and stable. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802460j

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Multidentate thioether ligands coating gold nanoparticlesw
Torsten Peterle,a Annika Leifert,b Jan Timper,b Alla Sologubenko,c Ulrich Simon*b
and Marcel Mayor*ad
Received (in Cambridge, UK) 12th February 2008, Accepted 27th March 2008
First published as an Advance Article on the webc 28th May 2008
DOI: 10.1039/b802460j
The design and synthesis of oligomeric ligands based on benzylic
thioethers is presented together with their ability to enwrap and
stabilize gold nanoparticles with diameters below 2 nm, which
become—with increasing length of the oligomer—more mono-
disperse and stable.
lity and monodispersity has been reported for multidentate
ligands comprising more than one thioether unit.7–10 Investi-
gated thioether ligands vary from flexible long chain dialkyl-
sulfides7 to rather rigid dendritic structures containing
thioethers either at the core structure9 or peripherically.10
However, the comparability of these studies is further limited
by various additional functional groups like ethers or acetals,
which may influence their ability to stabilize the nanoparticles.
Here, the synthesis and characterization of thioether oligo-
mers 1–4 as a series of di- to octadentate linear ligands is
reported. Their ability to stabilize nanoparticles and correla-
tions between ligand structure and properties of the formed
nanoparticles are investigated.
Although gold colloids have been used for centuries as colour-
ants,1 the controlled synthesis of monodisperse gold nanopar-
ticles remains a challenge. Well-defined gold nanoparticles,
such as thiolate protected Au25 clusters, have been obtained
only after separation from other cluster sizes2 or by choosing
the reaction conditions carefully. However, the formed nano-
particles depend strongly on slight variations of the reaction
conditions.3 Recently, even the solid state structure of a thiol-
monolayer protected gold nanoparticle has been obtained
after purification of the sample by numerous recrystallization
steps.4 The control over size, shape and surface chemistry of
nanoparticles would allow their integration in devices as stable
building blocks providing size dependent physical properties.
While wet chemistry has already been applied successfully to
organize surface functionalized Au particles in one, two and
three dimensions,5 their integration in electronic devices
remains a challenge. In particular single electron memory
devices oblige one to control the size and the assembly of
particles, as only particles with diameters below 2 nm provide
the required coulomb blockade properties.6
The ligands 1–4 (Scheme 1) are designed to bind to the gold
nanoparticle surface via flexible benzylic sulfides. The tert-
butyl groups meta to the benzylic thioethers are used to
enhance the stability of the resulting gold nanoparticles by
steric repulsion as well as to enhance the solubility and thus
the processability of the large ligands and the coated nano-
particles in common organic solvents.
The monomeric ligand 1 was synthesized by the reaction of
5-tert-butyl-1,3-bis(bromomethyl)benzene11 (6) with benzyl
Here, investigations geared towards controlling the size and
surface chemistry of Au particles by tailor-made multidentate
ligands are presented. A large multidentate structure might
favour well defined particle sizes by enwrapping the whole
particle by an integer number of ligands. Furthermore, the
concept enables to control the number and topology of
functional groups on the particle surface, subjecting the
particle itself to wet chemistry as synthetic building block.
Gold particles have already been stabilized by rather weak
interactions with thioether moieties, whereas increased stabi-
a University of Basel, Department of Chemistry, St. Johanns-Ring 19,
CH-4056 Basel, Switzerland. E-mail: marcel.mayor@unibas.ch;
Fax: +41 61 267 1016; Tel: +41 61 267 1006
b RWTH Aachen University, Institute of Inorganic Chemistry, Julich-
¨
Aachen Research Alliance (JARA), Landoltweg 1, 52074 Aachen,
Germany
Scheme 1 Molecular structures and synthesis of the ligands 1–4.w (a)
NBS, AIBN, CHOOCH3, hv, 3 h, 70%; (b) TrtSH, K2CO3, THF,
reflux, 12 h, 51%; (c) thiourea, DMSO, 12 h, NaOH, 64%; (d) BnSH,
NaH, THF, rt, 1 h, 95%; (e) NaH, THF, rt, 1 h, 96%; (f) Et3SiH,
TFA, DCM, rt, 15 min, 99%; (g) BnCl, NaH, THF, rt, 1 h, 98%; (h) 7,
NaH, THF, rt, 1 h, 96%; (i) Et3SiH, TFA, DCM, rt, 15 min, 99%; (j)
BnCl, NaH, THF, rt, 1 h, 99%; (k) 7, NaH, THF, rt, 1 h, 91%; (l)
Et3SiH, TFA, DCM, rt, 15 min, 99%; (m) BnCl, NaH, THF, rt, 1 h,
86%. Bn = benzyl, Trt = trityl.
c Gemeinschaftslabor fur Elektronenmikroskopie, RWTH Aachen,
Ahornstrasse 55, 52074 Aachen, Germany
¨
d Forschungszentrum Karlsruhe GmbH, Institute for Nanotechnology,
P. O. Box 3640, D-76021 Karlsruhe, Germany
w Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data of the compounds 1–4, 7 and 9–14.
Details on nanoparticle formation, purification and analysis (UV-vis,
1H-NMR and elemental analysis). See DOI: 10.1039/b802460j
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mercaptan and sodium hydride as base, in high yield. The
bifunctional compound 7, comprising one trityl protected thiol
and a bromide leaving group, turned out to be an ideal
building block for the assembly of the higher oligomers 2–4.
Monofunctionalisation of 6 was achieved in good yields by
using the sterically demanding tritylthiol as the sulfur source,
which also allows chromatographic separation of the desired
product from the starting material and the disubstituted
compound. Reaction of the dithiol 812 with the monomer 7
and sodium hydride as base yielded the trityl protected trimer
9, which was deprotected in the presence of trifluoroacetic acid
and triethylsilane to the dithiol 10. Benzyl protection of the
terminal thiols was achieved by the reaction with benzyl
chloride to give the trimeric ligand 2. The pentameric ligand
3 and the heptameric ligand 4 were synthesized from 10, using
similar elongation/deprotection/end capping sequences that
were used to assemble the trimer 2. All precursors and
oligomeric ligands 1–4 have been fully characterized by
1H- and 13C-NMR spectra, MALDI-ToF mass spectrometry
and elemental analysis.
Fig. 1 UV-Vis absorption spectra of gold nanoparticles stabilized by
monomer 1 (directly after synthesis: dashed line; after 24 h in disper-
sion: dotted line) and heptamer 4 (after 1 month in dispersion: straight
line) in dichloromethane.
Gold nanoparticles have been formed in the presence of the
oligomeric thioether ligands 1–4 to investigate the ability of
these ligands to stabilize nanoparticles and to prevent them
from coagulation. The preparation of the nanoparticles was
carried out in a two-phase water/dichloromethane system,
closely following the procedure developed by Brust et al.13
for the synthesis of alkanethiol protected gold nanoparticles.w
The gold(III) precursor tetrachloroauric acid was transferred to
the organic phase by tetra-n-octylammonium bromide
(TOAB). To keep the ratio between the gold(III) precursor
and thioether moieties comparable, the amount of added
ligand was normalized to its number of thioethers. Hence,
equimolar amounts of sulfide units and the gold(III) precursor
were used. The reduction of gold(III) in the presence of the
thioether ligands was then carried out by adding an aqueous
sodium borohydride solution to the two-phase system.
larger particles, as was further corroborated by the increasing
surface plasmon resonance after 24 h in dispersion (dashed
and dotted lines in Fig. 1). However, the dispersions of the
nanoparticles stabilized by the pentameric and heptameric
thioether oligomers 3 and 4 remained unchanged even after
several weeks, as displayed in Fig. 1 for heptamer stabilized
clusters. The UV-vis spectra obtained directly after the synth-
eses of the nanoparticles in the presence of 3 and 4w were
similar to the one shown in Fig. 1 of the heptameric ligand 4
after one month in dispersion. In contrast to that, particles
stabilized by the trimeric ligand 2 slowly coagulate when
stored in dispersion. This manifests in the formation of a fine
gold layer on the glass surface of the storage vessel after a
few days.
Purification of the thioether stabilized nanoparticles was
achieved by precipitation from a dichloromethane dispersion
with ethanol. The precipitated particles were separated from
the liquid phase by centrifugation. After three cycles of this
procedure, the phase-transfer agent TOAB was no longer
observed in the 1H-NMRw spectrum of the nanoparticles.
Even after these purification steps, a considerable yield of
around 80% was determined by elemental analysis for the
particles stabilized by ligand 4. However, the method was only
applicable in the case of particles stabilized with penta- or
heptameric ligands 3 or 4. If shorter oligomers were used to
stabilize the particles, the obtained precipitates were no longer
redispersable in dichloromethane or other organic solvents
such as toluene or THF. The purified nanoparticles were
obtained as waxy solids and are stable for months in the solid
phase and in their dispersed form in dichloromethane. Also,
the UV-vis spectra of the purified nanoparticlesw resemble the
spectrum shown in Fig. 1 even after one month at ambient
conditions. Obviously, the coagulation to particles with dia-
meters larger than 2 nm is strongly suppressed, demonstrating
the considerable stabilization of these particles by the oligo-
meric thioether ligands 3 or 4. These results further suggest
that at least 5–6 thioether units are required for an efficient
stabilization of each Au particle formed.
In the case of the monomer ligand 1, a black precipitate was
formed immediately after addition of the reducing agent to the
two-phase reaction system, suggesting rather limited stabiliza-
tion features of this ligand system. However, the reddish
brown colour of the liquid phase points at the formation of
Au particles. Nevertheless, if the larger oligomers 2, 3 or 4
were present instead of 1, no precipitate was formed during the
reaction. The intense dark coloured organic phases indicated
the formation of gold nanoparticles in all three cases. The
UV-vis absorption spectraw of the dispersions are very similar
to previously reported spectra of alkanethiol capped gold
nanoparticles. The absence of a strong plasmon resonance
band at around 520 nm in the cases of the longer oligomers
2, 3 or 4 points towards gold nanoparticles with a diameter
smaller than 2 nm.14 On the other hand, the presence of such an
absorption band indicates the presence of larger nanoparticles
stabilized by the monomer 1 remaining in the liquid phase.
The stability of the formed nanoparticles increased strongly
with the length of the stabilizing oligomer. The bidentate
monomer 1 stabilized nanoparticles coagulate fast, as was
evidenced by further precipitation of bulk gold and a colour
change to red of the remaining dispersed nanoparticles after a
few hours. The colour change is attributed to the formation of
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these results, we are currently working on dendritic ligands
based on comparable structural motifs.
Support from the Swiss National Science Foundation, the
Gebert Ruf Foundation, and the German Science Foundation
¨
(DFG) is gratefully acknowledged.
Notes and references
z HRSTEM was performed on a FEI Tecnai F20 operating at 200 kV
and a FEI Titan S operating at 300 kV. For TEM studies, particles
were brought on a conventional amorphous carbon TEM grid by
carefully putting a drop of the dichloromethane suspension on a grid
and letting it dry in air. Statistical analysis of the particle size
distributions were carried out by averaging the results of evaluations
from a large number of STEM micrographs (as ones shown in Fig. 2)
taken from different areas of the same TEM specimen. The strong
atomic number contrast employed in a STEM mode reveals the precise
size of the gold core.
1. M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.
2. J. B. Tracy, G. Kalyuzhny, M. C. Crowe, R. Balasubramanian,
J.-P. Choi and R. W. Murray, J. Am. Chem. Soc., 2007, 129,
6706–6707.
3. M. Zhu, E. Lanni, N. Garg, M. E. Bier and R. Jin, J. Am. Chem.
Soc., 2008, 130, 1138–1139.
4. P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and
R. D. Kornberg, Science, 2007, 318, 430–433.
Fig. 2 Representative HRSTEM micrographs of gold nanoparticles
stabilized by pentamer 3 (A) and heptamer 4 before (B) and after
purification (C). The histogram illustrates the different particle size
distributions in B and C.
5. e.g.: (a) C. A. Mirkin, R. L. Letsinger, R. C. Mucic and
J. J. Storhoff, Nature, 1996, 382, 607–609; (b) A. P. Alivisatos,
K. P. Johnsson, X. Peng, T. E. Wilson, C. J. Loweth,
M. P. Bruchez, Jr and P. G. Schultz, Nature, 1996, 382,
609–611; (c) R. P. Andres, J. D. Bielefeld, J. I. Henderson,
D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney
and R. G. Osifchin, Science, 1996, 273, 1690–1693;
(d) C. M. Niemeyer, Angew. Chem., Int. Ed., 2003, 42,
5796–5800; (e) E. Katz and I. Willner, Angew. Chem., Int. Ed.,
2004, 43, 6042–6108; (f) M. Fischler, A. Sologubenko, J. Mayer,
G. Clever, G. Burley, J. Gierlich, T. Carell and U. Simon, Chem.
Commun., 2008, 169–171.
6. G. Schmid and U. Simon, Chem. Commun., 2005, 697–710.
7. X.-M. Li, M. R. de Jong, K. Inoue, S. Shinkai, J. Huskens and
D. N. Reinhoudt, J. Mater. Chem., 2001, 11, 1919–1923;
E. J. Shelley, D. Ryan, S. R. Johnson, M. Couillard,
D. Fitzmaurice, P. D. Nellist, Y. Chen, R. E. Palmer and
J. A. Preece, Langmuir, 2002, 18, 1791–1795.
The control of the particle sizes and particle size distribu-
tions was accomplished by High Resolution Scanning Trans-
mission Electron Microscopy (HRSTEM).z Particles
stabilized by the pentameric ligand 3 show a large size
distribution with diameters ranging from 1 to 5 nm. A much
narrower size distribution is found in the case of the nano-
particles stabilized by the heptamer 4. The histogram in Fig. 2
shows that the main yield of the particle stabilization reaction
is about 1 nm large particles, though the distribution reveals a
tail towards larger sizes up to 2.1 nm. Interestingly, after
removal of the phase transfer agent by precipitation of the
nanoparticles, a different size distribution is found. In addition
to the parent accumulation with diameters around 1 nm a
second one with a maximum at about 1.9 nm was observed.
The results suggest a partial destabilization of the particles
upon removing the TOAB resulting in some coalescence of
small (1 nm) particles. Probably, these larger particles are
stabilized by several ligands 4. However, the defined size
distribution of particles, as well as elemental analysis of the
particlesw are in agreement with the working hypothesis of an
integer low number of ligands enwrapping the particles.
In summary, the synthesis of linear oligomeric multidentate
thioether ligands 1–4 and their ability to stabilize gold nano-
particles was reported. A considerable increase in the stability
and monodispersity of the formed Au particles with the
increasing length of the oligomeric ligands has been observed,
pointing at the potential of these systems in view of controlled
nanoparticle formation and functionalization. Encouraged by
8. X. M. Lin, C. M. Sorensen and K. J. Klabunde, Chem. Mater.,
1999, 11, 198–202; W. M. Pankau, G. von Kiedrowski and
K. Verbist, Chem. Commun., 2001, 519–520; W. M. Pankau,
S. Monninghoff and G. von Kiedrowski, Angew. Chem., Int.
¨
Ed., 2006, 45, 1889–1891; Y. Hosokawa, S. Maki and
T. Nagata, Bull. Chem. Soc. Jpn., 2005, 78, 1773–1782.
9. A. D’Aleo, R. M. Williams, F. Osswald, P. Edamana, U. Hahn,
´
J. van Heyst, F. D. Tichelaar, F. Vogtle and L. De Cola, Adv.
¨
Funct. Mater., 2004, 14, 1167–1177.
10. A. Taubert, U.-M. Wiesler and K. Mullen, J. Mater. Chem., 2003,
¨
13, 1090–1093.
11. E. Weber, H. P. Josel, H. Puff and S. Franken, J. Org. Chem.,
1985, 50, 3125–3132.
12. M. Tashiro and T. Yamato, J. Org. Chem., 1981, 46, 1543–1552;
A. Tsuge, Y. Ueda, T. Araki, T. Moriguchi, K. Sakata, K.
Koya, S. Mataka and M. Tashiro, J. Chem. Res. (S), 1997,
168–169.
13. M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman,
J. Chem. Soc., Chem. Commun., 1994, 801–802.
14. M. M. Alvarez, J. T. Khoury, T. G. Schaaff, M. N. Shafigullin,
I. Vezmar and R. L. Whetten, J. Phys. Chem. B, 1997, 101,
3706–3712.
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