Chemistry on surface-confined molecules: An approach to anchor isolated functional units to surfaces

Article abstract of DOI:10.1021/ja010257c

The synthesis of surface-confined, nanometer-sized dendrimers and Au nanoparticles was performed starting from single Pd^II pincer adsorbate molecules (10) embedded as isolated species into 11-mercapto-1-undecanol and decanethiol self-assembled

Full text of DOI:10.1021/ja010257c

6388
J. Am. Chem. Soc. 2001, 123, 6388-6395
Chemistry on Surface-Confined Molecules: An Approach to Anchor
Isolated Functional Units to Surfaces
Arianna Friggeri,† Henk-Jan van Manen,† Tommaso Auletta,† Xue-Mei Li,†
Szczepan Zapotoczny,‡,§ Holger Scho1nherr,‡,| G. Julius Vancso,‡ Jurriaan Huskens,*,†
Frank C. J. M. van Veggel,*,† and David N. Reinhoudt*,†
Contribution from Supramolecular Chemistry and Technology, Materials Science and
Technology of Polymers, and MESA+ Research Institute, UniVersity of Twente, P.O. Box 217,
7500 AE Enschede, The Netherlands
ReceiVed January 29, 2001
Abstract: The synthesis of surface-confined, nanometer-sized dendrimers and Au nanoparticles was performed
starting from single PdII pincer adsorbate molecules (10) embedded as isolated species into 11-mercapto-1-
undecanol and decanethiol self-assembled monolayers (SAMs) on gold. The coordination of monolayer-protected
Au nanoclusters (MPCs) bearing phosphine moieties at the periphery (13), or dendritic wedges (8) having a
phosphine group at the focal point, to SAMs containing individual PdII pincer molecules was monitored by
tapping mode atomic force microscopy (TM AFM). The individual PdII pincer molecules embedded in the
decanethiol SAM were visualized by their coordination to phosphine MPCs 13; isolated objects with a height
of 3.5 ( 0.7 nm were observed by TM AFM. Reaction of these embedded PdII pincer molecules with the
dendritic wedge 8 yielded individual molecules with a height of 4.3 ( 0.2 nm.
Introduction
tunneling microscopy, date back to 1983.7 In recent years, the
development of optical tweezers has made the manipulation of
single biomolecules possible,8 and techniques such as scanning
near-field optical microscopy (SNOM) and atomic force mi-
croscopy (AFM) can be used to study the physical,9 dynamic,10
and mechanical11 properties of single molecules.
Nowadays, the dimensions of some synthetic molecular and
supramolecular structures, for example dendrimers12 and metal
nanoparticles,13 are in the same range as those of biological
systems, and they have the desired dimensions of nano-electronic
devices. Therefore, the “bottom-up” alternative, which consists
of the controlled construction of complex structures of nano-
meter dimensions starting from the molecular level, is becoming
an increasingly realistic tool for nanofabrication.7
Nanometer-scale fabrication is a requirement for applications
in the fields of, for example, future electronic devices,1 high-
density data storage,2 drug delivery, and analytical chemistry.3
One of the ultimate goals of nanofabrication/nanotechnology
is computation by individual molecules.4 To achieve single-
molecule computation one must rely on the understanding and
thus study of individual molecules. Single-molecule studies
allow one to identify and examine individual members of a
heterogeneous population, while ensemble studies only yield
information on the average properties of a system.5
Single-molecule research began with biomolecules, in par-
ticular DNA, not only because they are the “building blocks of
life” but also because of their large size, which facilitates their
visualization.6 The first imaging attempts, using scanning
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* To whom correspondence should be addressed. Fax: +31 53 4894645.
Telephone: +31 53 4892980. E-mail: smct@ct.utwente.nl.
† Supramolecular Chemistry and Technology, MESA+ Research Institute.
‡ Materials Science and Technology of Polymers, MESA+ Research
Institute.
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Y., Yanagida, T. Single Mol. 2000, 1, 5-13.
§ On leave from Jagiellonian University, Faculty of Chemistry, Ingardena
3, 30-060 Cracow, Poland.
| Current address: Stanford University, Department of Chemical Engi-
neering, 381 North-South Mall, Stanford, CA, 94305, U.S.A.
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10.1021/ja010257c CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/09/2001

Synthesis of Nanometer-Sized Adsorbates
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6389
Chart 1. (a) Metallodendrimer Adsorbate 1;16 (b) Covalent Dendrimer Adsorbate 217
Self-assembled monolayers (SAMs) are useful starting plat-
forms for the development of nanometer-scale devices,14 since
they are highly ordered and have uniform film thicknesses.15
In our group we have developed metallodendrimer adsorbates
such as 1 (Chart 1a) containing a dialkylsulfide moiety for
anchorage to a gold substrate, and we have shown that these
can be inserted into alkanethiol SAMs as single molecules.16
The sulfide moiety was found to be essential for adsorption of
the dendrimers to the gold surface. Furthermore, we have
recently elucidated the insertion mechanism of individual
molecules of the covalent dendrimer adsorbate 2 (Chart 1b) into
11-mercapto-1-undecanol SAMs.17 AFM and electrochemistry
data indicated that the dendrimer insertion is the rate-determining
step of the process.
Further studies on the properties of single dendrimer mol-
ecules have recently been reported by several groups. Crooks
and co-workers18 have measured the height of G4 and G8
polyamidoamine (PAMAM) dendrimers on gold using TM
AFM. Similarly, Tomalia and co-workers19 have measured the
molecular diameter and height of individual G5-G10 PAMAM
dendrimers on mica, using TM AFM. However, AFM images
of individual dendrimers smaller than G5 could not easily be
obtained. De Schryver and co-workers20 have used TM AFM
to measure the height of G4 polyphenylene dendrimer molecules
spin-coated on mica. Good agreement was found between the
observed height and values calculated from molecular dynamics
simulations. We have observed single metallodendrimer mol-
ecules containing a rhodamine B core using the SNOM
technique.10c
Complementary contributions to the bottom-up approach
originate from the use of metal or semiconductor nanoparticles,
which can furthermore act as nanosized model systems for more
complex molecular structures. Monolayer-protected nano-
particles have already shown to be versatile materials in several
fields21 due to their unique optical22 and electronic properties23
related to quantum size effects. Moreover, their straightforward
synthesis allows introduction of a variety of terminal groups in
the organic monolayer.24
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M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. ReV.
1999, 28, 179-185. (c) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.;
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Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem Soc., Chem Commun. 1995,
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Manen, H.-J.; Menozzi, E.; Vancso, G. J.; van Veggel, F. C. J. M.;
Reinhoudt, D. N. Angew. Chem., Int. Ed. 1999, 38, 2248-2251.
(17) Friggeri, A.; Scho¨nherr, H.; van Manen, H.-J.; Huisman, B.-H.;
Vancso, G. J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N
Langmuir 2000, 16, 7757-7763.
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Langmuir 2000, 16, 5613-5616.

6390 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Friggeri et al.
Chart 2. Synthesis of Dendritic Ligands 4 and 8a
Although several publications regarding single-molecule
imaging have appeared in the literature in the past few
years7-10,18-20 and coordination reactions have successfully been
performed on SAMs on gold,25 coordination chemistry on
surface-immobilized, isolated molecules have, to the best of our
knowledge, not been reported. In this article, the synthesis of
isolated nanometer-sized objects starting from single molecules
embedded in SAMs on gold is described. We first describe the
coordination reactions onto isolated PdII pincer adsorbate
molecules in alkanethiol monolayers. Subsequently we show
that individual PdII pincer systems react with MPCs and
dendritic molecules, leading to the spatial confinement of
nanosized features. In both cases we present evidence that the
process is indeed governed by metal-ligand coordination.
a (a) Isonicotinoyl chloride hydrochloride, NEt3, CH2Cl2, rt, 8 H,
62%; (b) KNPhth, MeCN, reflux, 2 h, 85%; (c) H2NNH2‚H2O, EtOH,
THF, reflux, 2 h, 90%; (d) 4-(diphenylphosphino)benzoic acid, DCC,
HOBt, CH2Cl2, rt, 2 h, 85%.
Results and Discussion
Synthesis of Phosphine-Derivatized, Monolayer-Protected
Gold Clusters. Au nanoparticles stabilized by a mixed mono-
layer of decanethiol and 11-mercapto-1-undecanol in a 4/1 ratio
were prepared in a one-pot synthesis following the Brust
method.24a 1H NMR spectroscopy (CDCl3) showed that the
percentage of the two thiols in the monolayer is the same as
that in the solution from which the nanoparticles are formed
(ca. 20 OH moieties per nanoparticle), indicating that the layer
formation is under kinetic control. After purification of the
product, the hydroxyl moieties were coupled with 4-(diphenyl-
phosphino)benzoic acid in the presence of 1-(3-dimethyl-
aminopropyl)-3-ethyl-carbodiimide hydrochloride (EDC) and
4-(dimethylamino)pyridine (DMAP) in CH2Cl2. 1H NMR
spectroscopy proved that ester formation had occurred (disap-
pearance of the CH2OH signal). By etching of the nanoparticles
solution (in CH2Cl2) with an aqueous solution of KI/I2 (0.02
M), followed by reduction of the excess of I2 with Na2S2O3,
the gold core is transformed to an unidentified substrate, and
the thiolates bound to gold are oxidized to disulfides and
desorb.26 The mass spectrum of the etching extract shows peaks
at m/z values of 681.1 (oxidized phosphine/CH3 mixed disul-
fide), 346.1 (CH3/CH3 symmetric disulfide), but not at 378.3,
corresponding to the OH/CH3 mixed disulfide, proving that all
the hydroxyl groups had been converted into ester functionalities
(within the accuracy of the mass spectrometer). Transmission
electron microscopy (TEM) analysis provided an average size
of the gold core of 2.0 ( 0.5 nm.
Synthesis of Adsorbates and Dendrimer Wedges. The
synthesis of dendritic wedges 4 and 8 containing pyridine and
phosphine ligands at their focal points, respectively, is outlined
in Chart 2. Dendritic wedge 4 has hydrolyzable moieties at the
periphery which can be used for further functionalization and
growth of the molecule. The previously reported16 adsorbate 9
(Chart 3) combines a dialkylsulfide moiety with an SCS PdII
pincer group and is therefore the anchoring molecule that is
able to bind both to gold (via the sulfide) and to MPC 13 (Chart
3) and dendritic wedges 4 and 8 (via the PdII pincer).
Adsorbates 11 and 12 (Chart 3) were synthesized in solution
by removal of the inert chloride ligand of 9 with AgBF4,
followed by addition of dendritic wedges 4 and 8, respectively.
In the case of 4, 1H NMR spectroscopy indicated that, in addition
to the pyridine wedge, the sulfide also coordinated to PdII
Chart 3. Structure of Adsorbates 9-12 and Schematic
Representation of Monolayer-Protected Au Nanocluster 13
because the binding strengths of the pyridine ester and the
dialkylsulfide are similar. Two sets of signals were observed
for both the pyridine R protons, the CH2S protons of the pincer
system, and the CH2S protons of the sulfide moiety. Upon
addition of excess pyridine wedge, an increasing amount of
pyridine wedge coordinates to the PdII pincer; however, a
product free of coordinated sulfide could not be obtained.27
Clearly, a ligand stronger than a pyridine ester is needed in order
to avoid coordination of the sulfide to PdII. Therefore, we
employed phosphine 8 as a ligand, which resulted in the
quantitative formation of complex 12. Previously, we have
shown that phosphines are much stronger ligands for the SCS
PdII pincer system than pyridines.28
Spatial Confinement of Au Nanoparticles on SAMs.
Monolayer experiments have been carried out using building
blocks that are able to coordinate to SCS PdII pincer moieties.
The main reasons are the quantitative yield of the ligand
(27) As a control experiment, 1 equiv of the pyridine wedge 2 was added
to 1 equiv of a pincer system containing both a nonfunctional n-butyl chain
instead of a sulfide chain and a labile acetonitrile ligand. Quantitative
exchange of acetonitrile by the pyridine wedge was observed in this case,
(25) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature
1988, 332, 426-429.
(26) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.;
Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W.
J. Am. Chem. Soc. 1998, 120, 4845-4849.
1
as evidenced by H NMR spectroscopy.
(28) van Manen, H.-J.; Nakashima, K.; Shinkai, S.; Kooijman, H.; Spek,
A. L.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Eur. J. Inorg. Chem.
2000, 2533-2540.

Synthesis of Nanometer-Sized Adsorbates
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6391
exchange reactions involved29 and the short reaction times
required.29,30
Spatial confinement of Au nanoparticles on planar gold was
achieved starting from isolated PdII pincer adsorbate molecules
10 embedded into decanethiol SAMs on gold. The chloride of
adsorbate 9 was first exchanged for a pyridine ligand in solution,
and the resulting pincer molecules 10 were inserted into
decanethiol SAMs by exposing the latter to a 1 mM dichloro-
methane solution of 10 for 3 h. This procedure avoids the use
of AgBF4 on a monolayer.31 As expected, TM AFM images of
these layers showed no features due to the small size of the
PdII pincer moiety protruding from the SAM. Subsequently, this
substrate was immersed in a phosphine-functionalized colloid
solution (0.4 mM in dichloromethane) for 10 min at r.t., followed
by rinsing. Tapping mode AFM images acquired in air show
the presence of nanosized features with a height of 3.5 ( 0.7
nm (Figure 1). This value correlates with the average nanopar-
ticle size obtained by TEM, taking into account that the thickness
of the alkyl chains can be measured by AFM, while TEM gives
exclusively information about the metal core.32 The particles
do not form a complete monolayer, but are instead well
separated and randomly distributed over the entire surface. This
behavior is consistent with localized metal-ligand interactions
between the phosphines of the Au nanoparticles and isolated
PdII moieties in the layer.
To rule out aspecific interactions or physisorption processes,
sulfur-containing molecules with varying ω-susbstituents were
used to prepare monolayers both on flat gold and on the
nanoparticles. Two different types of experiments were per-
formed. In one case Au nanoparticles bearing only CH3 and
OH moieties as terminal groups were used and in the other the
pincer sulfide 10 was not inserted into the decanethiol layer;
all the samples were then imaged by TM AFM. The absence
of adsorbed particles on these layers clearly supports the fact
that both the phosphine moiety on the colloidal particles and
the pincer molecules in the layers are necessary for the growth
process, therefore the anchoring only occurs via metal-ligand
interactions.
Spatial Confinement of Dendrimers on SAMs: Optimiza-
tion of Growth Conditions. On the basis of the above-reported
results, we anticipated the same process to occur using dendritic
molecules. The growth of 11 and 12 on gold was carried out
starting from isolated PdII pincer adsorbate molecules 10
embedded into 11-mercapto-1-undecanol and decanethiol SAMs.
These monolayers were subsequently exposed to 0.1 and 0.01
mM dichloromethane solutions of 4 and 8, both for 10 and 60
min, and rinsed afterward with CH2Cl2 as described in the
Experimental Section. In the case of the dendrimer wedge 4,
TM AFM images of the monolayer surface show only a few,
very large features (8-9 nm in height), most likely physisorbed
material. Similar images were obtained when a pure decanethiol
SAM was immersed into a solution of 4 as a reference
experiment. However, TM AFM images of monolayers that
were exposed to dendrimer wedge 8 reveal the presence of
isolated, nanometer-sized features (Figure 2). The different
Figure 1. (a) TM AFM height image (acquired in air) of mixed
monolayers of decanethiol and PdII pincer sulfide 10 after exposure to
a 0.04 mM dichloromethane solution of Au nanoparticle 13 for 10 min;
(b) The height profile corresponds to the line drawn in the AFM image
(z-scale ) 10 nm).
reaction times (10 or 60 min) of the dendritic wedge 8 with the
PdII pincer-containing SAMs had no influence on the number
of observed molecules.
The height and width of the isolated features, as determined
by TM AFM in air, are 4.3 ( 0.2 nm and 15.3 ( 4 nm,
respectively. The height of that part of the adsorbate which
protrudes above the decanethiol monolayer, as obtained from a
computer-generated model, is approximately 3.4 nm. Therefore,
the average height, as determined by TM AFM, is in good
agreement with the calculated value of the single molecules.
The very soft tapping conditions employed in this experiment
render the height value a good representation of the feature
height.33 The width of the dendrimer wedges appears to be much
larger than in reality due to tip convolution.34 The average
number of nanometer-sized features counted in a 500 × 500
nm2 area of the PdII pincer-containing (10) decanethiol SAMs
exposed to dendrimer wedge 8, is 56 ( 6, which is in the same
order of magnitude as we have previously found for direct
insertion of other dendritic adsorbates.16,17
(29) Huck, W. T. S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1213-1215.
(30) (a) Huck, W. T. S.; van Veggel, F. C. J. M.; Kropman, B. L.; Blank,
D. H. A.; Keim, E. G.; Smithers, M. M. A.; Reinhoudt, D. N. J. Am. Chem.
Soc. 1995, 117, 8293-8294. (b) Huck, W. T. S.; van Veggel, F. C. J. M.;
Reinhoudt, D. N. J. Mater. Chem. 1997, 7, 1213-1219.
(31) Deprotection with AgBF4 of the coordinated chloride ligands of 9
inserted in a decanethiol monolayer gave rise to an insoluble residue
(probably AgCl), which remained on the substrate, thereby hindering
scanning force microscopy measurements.
Reference Experiments. To certify that the nanometer-sized
features observed in Figure 2 are single dendritic molecules,
(33) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349-
6353.
(32) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Vogel, R.
Science 1995, 267, 367-370.
(34) Scho¨nherr, H. Ph.D. Thesis, University of Twente, The Netherlands,
1999.

6392 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Friggeri et al.
Figure 3. (a) TM AFM height image (acquired in air) of a decanethiol
SAMs after exposure to a 1 mM dichloromethane solution of dendrimer
adsorbate 12, for 3 h; (b) The height profile corresponds to the line
drawn in the AFM image (z-scale ) 10 nm). The isolated dendrimer
molecules appear slightly elongated due to a tip asymmetry.
Figure 2. (a) TM AFM height image (acquired in air) of PdII pincer-
containing (10) decanethiol SAMs after exposure to a 0.01 mM
dichloromethane solution of dendrimer wedge 8 for 10 min; (b) The
height profile corresponds to the line drawn in the AFM image (z-
scale ) 10 nm).
layer, have reacted with the dendritic wedge molecules, 8,
forming a 1:1 complex in a similar way to the solution reaction.
However, it should be taken into account that some PdII pincer
adsorbates might not be easily accessible to the dendritic wedge
molecules in solution (e.g. if two or more PdII pincer molecules
adsorb close to each other in the SAM), thereby remaining in
the monolayer, unreacted, and undetected. Further studies with
larger molecules are currently being carried out to help clarify
this matter.
Moreover, decanethiol SAMs devoid of PdII pincer molecules
were immersed in a 0.01 mM dichloromethane solution of 8
for 10 min. The absence of nanosized features on these layers
confirmed that the isolated molecules shown in Figure 2 are
not simply physisorbed to the SAM and that the PdII center is
indispensable for the attachment of the dendrimer wedges.
direct insertion of dendritic adsorbate complex 12 was carried
out, which should yield similar layers as the surface reaction
described above. For this purpose, decanethiol SAMs were
immersed in a 1 mM dichloromethane solution of 12, for 3 h,
and subjected to the same rinsing procedure as the layers which
had been reacted with 8. Using the same atomic force
microscopy conditions as for the previous experiments, nanom-
eter-sized features, very similar to those observed after the
single-molecule growth experiments, were observed (Figure 3).
The dendritic adsorbate molecules appear to be slightly elon-
gated probably due to an asymmetry of the tip. The height and
width of these features, as determined by TM AFM, are 4.1 (
0.2 nm and 18.8 ( 4 nm, respectively, which correspond very
well to the dimensions of the molecules synthesized on the
monolayer by reaction of isolated PdII pincer adsorbates, 10,
with dendritic wedge 8.
In addition, the average number of nanometer-sized features
counted in a 500 × 500 nm2 area of the decanethiol monolayer
exposed to a solution of dendrimer adsorbate 12, is 51 ( 9.
Again, this corresponds to the dendrimer coverage obtained by
synthesizing the same dendritic adsorbate on the monolayer.
These data strongly suggest that a significant number of the
PdII pincer adsorbates 10 inserted into the decanethiol mono-
Insertion of adsorbate 10 into SAMs of 11-mercapto-1-
undecanol was hampered by physisorption, as seen in TM AFM
images. These TM AFM results were confirmed by contact angle
measurements (with water) performed on the PdII pincer-
containing layers which had been exposed to 0.01 and 0.1 mM
solutions of 8 (Table 1).
Various attempts to remove the physisorbed material using
different rinsing procedures were made (see Experimental

Synthesis of Nanometer-Sized Adsorbates
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6393
Table 1. Advancing Contact Angles (θa) and Hysteresis Values
(∆θ) of 11-Mercapto-1-undecanol Monolayers Containing PdII
Pincer Sulfide 10 after Exposure to 0.01 mM and 0.1 mM Solutions
of 8
interactions between the layer and the dendritic wedges;
however, this could successfully be removed by rinsing of the
layer.
5 min
θa/∆θ (deg)
30 min
θa/∆θ (deg)
60 min
θa/∆θ (deg)
Conclusions
Surface reactions on isolated molecules inserted into alkane-
thiol SAMs on gold have been performed. Coordination
chemistry has been used to successfully grow isolated molecules
into nanometer-sized adsorbates which can be detected by
atomic force microscopy. This approach as a general strategy
offers the possibility to isolate functional nanometer-sized
objects, as shown by spatial confinement of different objects
such as Au nanoparticles or dendritic structures on self-
assembled monolayers. In principle it can be applied to any
suitably designed nanosized object incorporating specific func-
tions (e.g., semiconductor or magnetic nanoparticles). To render
the “bottom-up” approach a viable option for the current “top-
down” techniques in the manufacturing of nanometer-sized
devices, two of the remaining challenges are currently being
tackled: the exact positioning of nanometer-sized entities and
the optimization of techniques able to detect and study such
entities.
0.01 mM
0.1 mM
36 ( 1/16
43 ( 1/29
27 ( 1/13
41 ( 1/26
27 ( 1/13
40 ( 1/26
Experimental Section
General Procedures. Melting points were determined with a
Reichert melting point apparatus and are uncorrected. NMR spectra
were recorded in CDCl3 on a Varian Unity 300 locked to the deuterated
solvent at 300.1 (1H), 75.5 (13C), and 121.5 (31P) MHz, respectively.
Chemical shifts are given relative to tetramethylsilane (TMS). FAB-
MS spectra were recorded on a Finnigan MAT 90 spectrometer with
m-nitrobenzyl alcohol (NBA) as the matrix. Elemental analyses were
performed using a Carlo Erba EA1106. The presence of solvents in
the analytical samples was confirmed by 1H NMR spectroscopy.
Column chromatography was performed using silica gel (SiO2, E.
Merck, 0.040-0.063 mm, 230-240 mesh). Contact angle (CA)
measurements were carried out on a KRU¨ SS Contact Angle Measuring
System G10. Measurements for a drop of water whose volume was
gradually increased (advancing CA) and then decreased (receding CA)
were repeated on three sites of the same sample. For each receptor
monolayer three samples were measured, and the average values are
reported.
Materials. CH2Cl2 was freshly distilled from CaCl2. Acetone, EtOH,
THF, and CHCl3 were used as received (p.a. from Merck). MeCN (p.a.
from Merck) was stored over molecular sieves (4 Å). Water was purified
by Millipore membrane units. Solvents for colloid preparation were of
p.a. grade (from Merck). HAuCl4‚xH2O (99.99%, from Acros) and
tetraoctylammonium bromide (from Fluka) were used as received. All
other reagents and decanethiol were purchased from Aldrich and used
without further purification. Dendritic wedges Br4-[G-2]-Br (5)35 and
(MeO2C)4-[G-2]-OH (3),36 and 11-mercapto-1-undecanol37 were pre-
pared according to literature procedures.
(MeO2C)4-[G-2]-pyr (4). A solution of isonicotinoyl chloride
hydrochloride (0.46 g, 2.6 mmol) and NEt3 (0.85 mL, 6.1 mmol) in
CH2Cl2 (25 mL) was added dropwise to a solution of dendritic wedge
(MeO2C)4-[G-2]-OH (3) (1.00 g, 1.0 mmol) in CH2Cl2 (75 mL). The
mixture was stirred at rt overnight under an argon atmosphere. After
the addition of 1 M HCl (aqueous, 100 mL), the organic phase was
separated and washed with NaHCO3 (saturated) and brine and
subsequently dried over anhydrous Na2SO4. After evaporation of the
solvent in vacuo, the crude product was purified by column chroma-
tography, using CH2Cl2/acetone 90:10 (v/v) as the eluent, affording a
white solid. Yield 0.69 g (62%).
Figure 4. TM AFM height images of 11-mercapto-1-undecanol (a)
and decanethiol (b) SAMs after immersion into a 0.1 mM dichloro-
methane solution of 8 for 1 h (z-scale ) 5 nm). Both images were
acquired in air.
Section), but they were not successful. AFM images of the alkyl-
terminated SAMs show much less “contamination” than the
corresponding hydroxyl-terminated surfaces, as can be seen from
the results of the reference experiments carried out by immersing
11-mercapto-1-undecanol and decanethiol SAMs into 0.1 mM
dichloromethane solutions of 8 for 1 h (Figure 4).
This shows that hydrogen bonding is the most probable cause
of the physisorbed material observed on the 11-mercapto-1-
undecanol SAMs. Some physisorbed material did remain on
the decanethiol surface, most probably due to hydrophobic
(35) Wooley, K. L.; Hawker, C. J.; Fre´chet, J. M. J. J. Chem. Soc., Perkin
Trans. 1 1991, 1059-1076.
(36) Hawker, C. J.; Wooley, K. L.; Fre´chet, J. M. J. J. Chem. Soc., Perkin
Trans. 1 1993, 1287-1297.
(37) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G.
M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.

6394 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Friggeri et al.
Br4-[G-2]-NPhth (6). A mixture of dendritic bromide 5 (0.40 g,
0.36 mmol), potassium phthalimide (0.08 g, 0.43 mmol), and potassium
carbonate (0.08 g, 0.58 mmol) in MeCN (50 mL) was refluxed under
an argon atmosphere for 3 h. After evaporation of the solvent in vacuo,
the resulting paste was taken up in CH2Cl2 (100 mL), and the solution
was washed with brine and dried over Na2SO4. After removal of the
solvent, the crude product was purified by column chromatography
(eluent: CH2Cl2/hexane 80:20 (v/v)) to afford a white solid. Yield 0.41
g (96%).
Br4-[G-2]-NH2 (7). A solution of dendritic phthalimide 6 (0.14 g,
0.12 mmol) in EtOH (30 mL) and THF (10 mL) was heated to reflux,
and hydrazine monohydrate (0.5 mL) was added. After refluxing for 2
h, the solution was cooled to room temperature and evaporated until
20 mL of solution remained. HCl (1 M, 50 mL) was added, and the
solution was extracted with CH2Cl2 (2 × 50 mL). The organic layer
was washed with NaHCO3 (saturated) and brine and dried over Na2SO4.
After evaporation of the solvent, the dendritic amine 7 was obtained
as a colorless oil. Yield 0.12 g (96%). For characterization, the amine
was further purified by column chromatography (eluent: CH2Cl2/MeOH
95:5 (v/v)).
Br4-[G-2]-phos (8). To a solution of dendritic amine 7 (0.18 g, 0.17
mmol), 4-(diphenylphosphino)benzoic acid (48 mg, 0.16 mmol), and
1-hydroxybenzotriazole hydrate (HOBt, 21 mg, 0.16 mmol) in CHCl3
(50 mL) was added 1,3-dicyclohexylcarbodiimide (DCC, 35 mg, 0.17
mmol). The mixture was stirred at room temperature under an argon
atmosphere for 3 h. The solution was subsequently washed with
NaHCO3 (saturated) and brine and dried over Na2SO4. After evaporation
of the solvent in vacuo the crude product was purified by column
chromatography (eluent: CH2Cl2) to afford the dendritic phosphine
wedge 8 as an off-white solid. Yield 0.18 g (79%).
Sulfide Pincer, Pyridine Complex (10). To a solution of the sulfide
pincer 9 (8.1 mg, 10.1 µmol) in CH2Cl2 (3 mL) were added a few
drops of MeCN. Next, 78 µL (10.1 µmol) of a stock solution of AgBF4
in water (0.130 M) was added, and the resulting mixture was stirred
for 10 min. A white precipitate formed, indicating the formation of
insoluble AgCl. Pyridine (90 µL of a 0.120 M stock solution in CH2Cl2,
10.8 µmol) was subsequently added. After stirring the mixture at rt for
10 min, the mixture was filtered through Hyflo into a 10 mL volumetric
flask. The volumetric flask was filled with CH2Cl2 to 10 mL, providing
a 1 mM solution of the pyridine complex of the sulfide pincer. For
analytical investigation, this process was carried out in deuterated
solvents.
Sulfide Pincer, Pyridine Wedge Complex (11). The sulfide pincer
9 (4.1 mg, 5.1 µmol) was deprotected with AgBF4 in the same manner
as described above, and subsequently (MeO2C)4-[G-2]-pyr (5.5 mg,
5.1 µmol) was added. After filtration of the solution through Hyflo,
the solvents were evaporated in vacuo. 1H NMR spectroscopy indicated
coordination of both the pyridine wedge and the sulfide chain to PdII,
for example δ (ppm) 8.8 and 8.4, respectively, for uncoordinated and
coordinated R-pyridyl protons.
Sulfide Pincer, Phosphine Wedge Complex (12). The sulfide pincer
9 (4.3 mg, 5.3 µmol) was deprotected with AgBF4 in the same manner
as described above, and subsequently dendritic phosphine 8 (7.2 mg,
5.3 µmol) was added. After filtration of the solution through Hyflo,
the solvents were evaporated in vacuo. This provided the product as a
yellow solid in quantitative yield.
Preparation of Mixed Monolayer-Protected Gold Colloids. To a
solution of tetraoctylammonium bromide (320 mg, 0.6 mmol) in toluene
(6 mL) was added a solution of tetrachloroauric acid (200 mg, 0.6
mmol) in water (15 mL). AuCl4- was transferred into the organic phase
as witnessed by decoloration of the aqueous phase while the organic
phase turned orange-brown. The mixture was stirred vigorously for 20
min to ensure complete transfer. Hereafter, a mixture of decanethiol
(83.52 mg, 0.48 mmol) and 11-mercapto-1-undecanol (24.48 mg, 0.12
mmol) in toluene (15 mL) was added to the mixture. After 5 min, a
freshly prepared aqueous solution (8 mL) of NaBH4 (266 mg, 7.2 mmol)
was added dropwise to the reaction mixture. The colloids were formed
instantaneously as indicated by the color change of the solution from
orange to red and then to black-brown. After 4 h, the organic layer
was collected, washed with water, and dried over Na2SO4. The colloidal
solution was then concentrated to 3 mL, and 300 mL of methanol were
added to precipitate the colloids at -20 °C for 4 h. Centrifugation gave
the colloids as a black solid (90 mg, 90% yield). They were repeatedly
1
precipitated from methanol until there was no free ligand present. H
NMR (CDCl3, 300 MHz) δ 3.60 (bs), 1.47 (bs), 0.81 (bs).
Etching of the Colloids. Mixed monolayer-protected gold nano-
particles (20 mg) were dissolved in CH2Cl2 (20 mL) and then washed
with a 0.1 M I2/KI aqueous solution (2 × 30 mL). The excess of I2
was removed by treating the solution with a NaS2O3 solution (2 × 30
mL) and washing with water. The solution was then dried over Na2SO4.
By integration of the 1H NMR signals the amount of the two
components in the MPCs was determined, and the percentage of the
hydroxyl ligand was found to be 27%.
Coupling Reaction on Mixed Monolayer-Protected Gold Colloids
(13). Mixed monolayer-protected gold colloids (15 mg) were dissolved
in degassed CH2Cl2 (10 mL). To this solution was added 4-(di-
phenylphosphino)benzoic acid (0.75 mg, 0.24 µmol), EDC (0.68 mg,
3.5µmol), and DMAP (0.72 mg, 5.9 µmol), and the reaction mixture
was stirred at room temperature for 2 h. The solution was concentrated
to a minimum amount (0.5 mL), and 300 mL of degassed methanol
was added to precipitate the colloids at -20 °C for 4 h. After
centrifugation and washing with methanol, a quantitative yield of the
1
colloids was obtained. H NMR showed broad spectra, and no signal
was observed from 31P NMR spectroscopy. The colloids were then
etched by I2/KI in a similar procedure as described above. The organic
residue was analyzed by FAB mass spectroscopy. Found: m/z: 681.1
([M + H]+; calcd for C40H57O3PS2: 680.3), 346.1 ([M]+, calcd for
C20H42S2: 346.3).
Substrate Preparation. Gold substrates were obtained from Met-
allhandel Schro¨er GmbH (Lienen, Germany). Immediately before use,
the substrates were rinsed with high-purity water (Millipore) and then
flame-annealed with a H2 flame (purity 6) as described previously.38
After the annealing procedure, the substrates were placed in p.a. ethanol
for 10 min39 and then immersed into the adsorbate solution for the
desired time. All adsorbate solutions were prepared just before use.
After each adsorption step the samples were removed from the solutions
and rinsed thoroughly with dichloromethane, ethanol, and water
(Millipore). SAMs of 11-mercapto-1-undecanol and decanethiol were
prepared by immersing the gold substrates in the corresponding 1 mM
ethanol solutions at room temperature for 3 h. Solutions of 8, 12, and
13 were deoxygenated prior to immersion of the monolayers by
bubbling N2 through the solutions for 10 min and were kept under Ar
during the monolayer experiments. After immersion into solutions of
4, 8, 11, 12, or 13, the samples were rinsed by placing them in CH2Cl2
for 3 × 5 min (3 × 20 mL CH2Cl2). Two other rinsing procedures
were attempted: (a) CH2Cl2 (20 mL, 5 min), followed by EtOH (20
mL, 5 min) and H2O (20 mL, 5 min), and (b) 15 min in CH2Cl2 in the
sonicator, but some physisorbed material remained on the layers.
Hydrophilic layers were dried under a nitrogen stream. All experiments
were repeated 3-4 times.
Atomic Force Microscopy. The AFM measurements were carried
out with a NanoScope III multimode AFM (Digital Instruments, Santa
Barbara, CA). Tapping mode AFM scans were performed in air using
silicon cantilevers/tips (Nanosensors, Wetzlar, Germany; cantilever
resonance frequency f0 ) 280-320 kHz). The free amplitude was kept
constant for all experiments, and the amplitude damping (setpoint) ratio
was adjusted to ˜0.90. Prior to the measurements the setup was thermally
equilibrated for several hours to minimize the drift and to ensure a
constant temperature (˜30 °C). The piezo scanner was calibrated in
lateral directions using a grid with repeat distances of 1.0 µm, as well
as self-assembled monolayers of thiols on Au(111) (e.g. octadecane-
thiol, repeat distance 0.51 nm),38 and in z-direction by measuring step
heights of Au(111) (2.9 Å). The number of nanometer-sized fea-
tures and their standard deviations were determined by counting the
features on at least four areas of the same sample, and taking the
average.
(38) Scho¨nherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J.
M.; Reinhoudt, D. N. Langmuir 1999, 15, 5541-5546.
(39) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566-4573.

Synthesis of Nanometer-Sized Adsorbates
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6395
Acknowledgment. Financial support of this research by the
Council for the Chemical Sciences of The Netherlands Or-
ganization for Scientific Research (CW-NWO) is gratefully
acknowledged.
and 31P NMR spectroscopy, FAB mass spectrometry, and
elemental analysis (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
Supporting Information Available: Characterization of
1
compounds 4, 6, 7, 8, 10, and 12 by melting point, H, 13C,
JA010257C