Two-dimensional self-assembly of disulfide functionalized bis-acylurea: A nanosheet template for gold nanoparticle arrays

Article abstract of DOI:10.1039/c0cc00716a

A new functional bis-acylurea molecule allows a two-stage self-organization process. It self-organizes - at first - into 2D nanosheets with disulfide groups at the surface, which act - in the second stage - as a template for gold nanoparticle arrays.

Full text of DOI:10.1039/c0cc00716a

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COMMUNICATION
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Two-dimensional self-assembly of disulfide functionalized bis-acylurea:
a nanosheet template for gold nanoparticle arraysw
Jong-Uk Kim,a Ki-Hyun Kim,b Niko Haberkorn,a Peter J. Roth,a Jong-Chan Lee,b
Patrick Theatoa and Rudolf Zentel*a
Received 1st April 2010, Accepted 19th May 2010
First published as an Advance Article on the web 11th June 2010
DOI: 10.1039/c0cc00716a
A new functional bis-acylurea molecule allows a two-stage
self-organization process. It self-organizes—at first—into 2D
nanosheets with disulfide groups at the surface, which act—in
the second stage—as a template for gold nanoparticle arrays.
applications.11 Therefore, it is of great interest to devise
artificial (synthetic) 2D self-assembled organic systems, which
can serve as host materials for hybridization with inorganic
guest components. Such functional nano-architectures would
broaden the application range and enable more complex
setups on a nano-scale.
Self-assembly of low molecular mass organic molecules has
been a fascinating subject for understanding the nature of
molecular recognition. At the same time, it is a useful strategy
to generate various supramolecular structures, due to its
superiority in controlling the organization at a molecular
level.1 As the organization usually occurs on a nanometre
scale, researchers have made many successful attempts to
utilize self-organization for fabricating nanostructures such
as fibers, vesicles, tubules, helixes, or platelets.1
Recently, we reported a class of bis-acylurea derivatives
showing an unusual self-assembling behavior.12 The derivatives,
BuCnBu, contain bis-acylurea (CO–NH–CO–NH) moieties
separated by a (CH2)n spacer and butyl (Bu) groups at each
end. They self-organize into 2D molecular sheets when
they are gelatinized in solvents. As the self-complementary
hydrogen bonds of these bis-acylureas are exceptionally
strong, the ‘odd–even effect’ of the spacer,13 which is one of
the usual characteristics of most 1D fibrous organogelators, is
not prominent. Even ultrasonication of several minutes
does not destroy the 2D organization of the supramolecular
structures. The resulting 2D nanostructures are several tens of
mm wide, but only some tens of nm high. They are molecularly
flat (height of individual molecule: 2.3 nm) over distances of
200 nm. Therefore, we believed the bis-acylurea derivatives
could serve as robust building blocks for 2D nanosheets
exhibiting various functionalities.
Recently, hybrid inorganic–organic nanoobjects have
attracted growing attention.2 They are constructed by using
self-assembled organic components to coordinate the inorganic
nanophases to produce discrete integrated objects or higher-
order structures. In fact, this two-stage self-assembly and the
resulting hybrids are the basic concepts underlying bio-
inspired (or bio-mimetic) materials chemistry. The most
prominent example for this is biomineralization,3 in which—
at first—lipids self-assemble into membranes with the proper
head groups, which then act as a template for the crystalli-
zation of the inorganic component (mostly CaCO3). The
results are found in bones, sea-shells or the skeletons of
different microorganisms. The resulting inorganic/organic
hybrid structure combines toughness with mechanical hard-
ness in a very superior way. Another system, in which two
types of self-assembly are combined, is liquid crystals gelled
reversibly by gelator molecules, which self-assemble into fiber
or platelet-like structures.4–7
In this communication, we present a new route to gold
nanoparticle arrays on a nanosheet template by a two-stage
self-assembly process. For this we synthesized a new functio-
nalized bis-acylurea SSC5SS with reactive methyl disulfide
moieties at both ends. This bis-acylurea self-assembled—at
first—into 2D nanosheets, which can serve—later on—as a 2D
template for gold nanoparticles.
Two-dimensional (2D) hybrid inorganic–organic materials
have been realized using 2D multimetric proteins such as
bacterial S-layer proteins,8 ferritin,9 and heat-shock proteins,10
due to potential applications in biotemplating as well as their
interesting structures of biomimicry. However, some issues
about the biomolecule-based hybrid structures, such as
low yields, lack of long-range order, narrow operating
conditions (temperature, solvent) limit their uses in various
At the beginning of this work, we anticipated 2D nanosheet
structures of thiol containing bis-acylurea, 2, since thiols are
highly versatile functional groups in nano- and biochemistry
and strongly bind to metal surfaces. The synthesis of 2 was
achieved in two steps; trityl-protected bis-acylurea, 1, was
synthesized through the reaction of Trt-S-mercaptoethylurea
and pimeloyl chloride in the presence of base, and deprotection
a Institute of Organic Chemistry, Johannes Gutenberg University
Mainz, Duesbergweg 10-14, 55099, Mainz, Germany.
E-mail: zentel@uni-mainz.de
of
1 under acidic conditions afforded thiol containing
bis-acylurea 2. However, we could not control the self-
organization of 2, as it had various crystalline structures,
i.e., irregular shapes or leaflets (ESIw). Additionally, upon
heating in solvents, 2 formed a brownish precipitate, probably
b School of Chemical Engineering, Seoul National University,
Shilim-9-Dong, Gwanak-Gu, Seoul 151-744, Korea
w Electronic supplementary information (ESI) available: Experimental
details, microscope images, UV-vis spectra. See DOI: 10.1039/c0cc00716a
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due to an oxidative dimerization of thiols. Hence, an alter-
native functional group was investigated.
manner offers many opportunities in optoelectronics15 and for
biosensors.16 In particular, closely spaced colloidal gold
nanoparticles enhance Raman scattering of absorbed
materials on the surface (SERS).15a Therefore, it is considered
to be of great interest to prepare 2D arrays of nanoparticles
on functionalized substrates. To this goal, many attempts
have been reported on flat substrates like polished silicon
wafers,15a,17 alumina,18 self-assembled layer on graphite,19 or
gold nanosheets,20 whereby the surfaces of these substrates
have sometimes been modified by layer-by-layer21 films to
increase the interaction with the nanoparticle. Generally
it is possible to assemble the nanoparticles on these surfaces
and to vary the coverage up to a dense, but mostly amor-
phous packing. For self-assembled nanostructures, on the
contrary, little work has been done as far as we are
aware. In one report,22 the binding of the nanoparticles to
the edges or inclined steps of organic nanosheets was
described.
A derivative of 2 with disulfide moieties instead of thiols
(SSC5SS) was found to have a 2D morphology regardless of
the disulfide end groups, which are an equally strong ligand
for various metals (e.g. Au) as thiols.14 SSC5SS was derived
from 2 by simply stirring it with an excess amount of S-methyl
methanethiosulfonate in ethanol. The methyl disulfide groups
were stable during the synthesis and upon heating up to 80 1C,
1
as was confirmed by H NMR.
Two-dimensional multilayered structures formed sponta-
neously upon cooling of a hot solution of SSC5SS in ethanol.
Scanning electron microscopy (SEM) and atomic force micro-
scopy (AFM) analyses were used to characterize the supra-
molecular structure (Fig. 1a and 1b). The aspect ratio of the
nanosheets was 20–100, as their widths and thicknesses were
several micrometres and 100–200 nm, respectively. The mean
thickness of each layer was determined to be 2.4 nm by AFM,
which agreed well with the d-spacing (24.0 A) recorded in a
powder X-ray diffraction (XRD) measurement. The 2D
organization of SSC5SS was studied using the XRD pattern
of the wide angle region. SSC5SS has reflections at 4.49 A and
3.51 A, which are also found in the XRD pattern of BuC5Bu
nanosheets (ESIw). Since these two reflections correspond to
the (020) and the (117) planes, respectively, of the BuC5Bu 2D
crystal,12 it can be assumed that the crystalline structure of
SSC5SS is very close to that of BuC5Bu (Fig. 1c).
We succeeded in immobilizing gold nanoparticles (AuNPs,
19 Æ 3 nm) onto SSC5SS nanosheets by adding an aqueous
gold colloid solution into a dispersion of SSC5SS nanosheets
in H2O followed by vigorous shaking. The gold colloid
solution was prepared by the typical citrate reduction method
as reported by others.23 Prior to the immobilization reaction,
the gold colloid solution was adjusted to pH 10 in order to
prevent aggregation between AuNPs. The originally wine-red
colored mixture, (lmax = 520 nm) turned—in contact with
the nanosheet suspension—within hours into bluish purple
(lmax = 650 nm), which indicated the formation of super-
lattices of AuNPs.15 With an increased AuNP concentration,
the absorbance band at 650 nm increased, implying that the
number of attached gold nanoparticles increased. SEM images
of the purified sample showed that AuNPs were attached onto
the surface of SSC5SS nanosheets (Fig. 2a and b, ESIw), and
the number of the tethered AuNPs increased with an increasing
dose of AuNPs. We believe that the AuNPs are bound to
the surface functional groups chemically rather than by
physisorption, as the chemical binding between AuNPs and
disulfide-terminated surfaces was reported in the literature.14b,24
To further investigate the surface chemistry, we performed a
similar experiment except that nanosheets of BuC5Bu
were used. Provided that the binding occurs mainly by physi-
sorption, AuNPs should be adsorbed on the surfaces of
BuC5Bu nanosheets as well. But even after a week, no color
change of the white solids (nanosheets) was observed, implying
that no considerable amount of binding of AuNPs onto the
surfaces of the nanosheets occurred. Because of this control
experiment, we assume that AuNPs are attached to the surface
of the SSC5SS nanosheets by covalent bonding with the
functionality. When an excess amount of AuNPs was added
in order to obtain more densely covered nanosheets, a fully
covered monolayer in combination with multilayers of AuNPs
was observed (ESIw). Presumably, trace amounts of SSC5SS
dissolved in H2O, then interlinked the AuNPs to form the
multilayers. Thus the self-assembled organic nanosheets can
be used for the further assembly of inorganic nanoobjects and
their performance for this task is like those of modified
macroscopic surfaces.15–20 So far, we could not yet achieve
perfect and homogeneous monolayers over larger sheets. Our
We examined SSC5SS as a template for 2D arrays of gold
nanoparticles. Immobilizing gold nanoparticles in a controlled
Fig. 1 Two-dimensional nanosheet structure of SSC5SS. (a) SEM
image. The thicknesses of the nanosheets were determined by SEM
images as shown in the inset. (b) AFM image shown in amplitude
mode. The height profile along the arrow is shown in the inset. The
layer thickness is 2.4 nm, which agrees well with the XRD result
(length of molecule). (c) Proposed supramolecular structure of
bis-acylureas stabilized by biaxial hydrogen bonding. (d) Schematic
illustration for the self-assembly of SSC5SS upon cooling from a hot
homogeneous solution.
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to acknowledge scientific support from Dr I. Lieberwirth and
G. Glaßer in MPI-P for SEM measurement. K.-H. Kim
thanks KOSEF and DAAD for the 2009 Summer Institute
Program.
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next efforts will focus on defining the mechanism of the
binding process and improving the stability.
In a different experiment, we examined the immobilization
of AuNPs onto SSC5SS nanosheets that had been attached to
a glass substrate using a layer-by-layer (LBL) film (Fig. 2c,
ESIw). SSC5SS nanosheets were transferred onto a negatively
charged LBL thin film on a glass substrate, followed by
annealing at 80 1C. After thoroughly rinsing with water, the
glass substrate was immersed into a AuNP colloid solution.
SEM images showed that AuNPs had been immobilized onto
the 2D nanosheets, and only a small portion of AuNPs
were attached to the negatively charged surface (Fig. 2d).
Apparently, discrete islands of AuNP arrays were fabricated
on a substrate, which might be utilized in bioassay devices.
In conclusion, we prepared organic nanosheets having
disulfide functional groups at the surface by self-assembly,
and examined the nanosheets as templates for AuNP arrays in
a second self-assembly process. We hope this report will
stimulate researchers’ interest to fabricate hybrid inorganic–
organic nanostructures using self-organized organic templates
in a biomimetic way.
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This work was supported by the IRTG1404 jointly funded
by DFG and NRF and by DFG (Ze 230/14). The authors wish
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Chem. Commun., 2010, 46, 5343–5345 | 5345