Writing Patterns of Molecules on Molecular Printboards

Article abstract of DOI:10.1002/anie.200352767

Self-assembled monolayers of cyclodextrin receptor molecules on gold or silicon oxide surfaces function as molecular printboards to which molecules with complementary recognition motifs can be transferred selectively by writing or printing. One example demonstrating the application of this methodology is the supramolecular microcontact printing of dendrimers loaded with fluorescent dyes (see confocal microscopy images after loading with rose bengal and fluorescein).

Full text of DOI:10.1002/anie.200352767

Angewandte
Chemie
gold and silicon oxide surfaces. Herein we describe how to
print or write, by microcontact printing (mCP) and dip-pen
nanolithography (DPN), respectively, molecular patterns of
guest-functionalized calixarene molecules, dendritic wedges
labeled by fluorescent groups, and dendrimers on b-CD-
terminated printboards. The binding, as well as the desorption
of the molecules, can be fine-tuned by chemical design, which
allows virtually unlimited flexibility in the chemical functions
that can be employed. These structures can be subsequently
used to direct the adsorption of different materials, for
example, fluorescent dyes.
Microcontact printing has been developed by Whitesides
for the preparation of patterns of molecules on bare surfaces
by, for example, the transfer of thiols to gold substrates in the
contact areas between a soft polymeric stamp and the
substrate.[2,3] This has recently been extended by Mirkin and
co-workers to writing with molecules on such surfaces by
using the DPN approach.[4] Various types of molecules were
deposited onto different substrates by DPN which led to
arrays of, for example, DNA,[5] proteins,[6] and nanoparti-
cles.[7] Registry capabilities have been demonstrated as well,[8]
and a multipen nanoplotter able to produce parallel patterns
with different ink molecules has been developed.[9]
b-CD (1a, Scheme 1) can act as a host for the binding of a
variety of small, organic guest functionalities in water through
hydrophobic interactions. We prepared self-assembled mono-
layers (SAMs) of a b-CD heptathioether adsorbate 1b
(Scheme 1) on gold as described before.[10,11] Such adsorbates
form densely packed, well-ordered SAMs with equivalent
Microcontact Printing
Writing Patterns of Molecules on Molecular
Printboards**
Tommaso Auletta, Barbara Dordi, Alart Mulder,
Andrea Sartori, Steffen Onclin, Christiaan M. Bruinink,
Mµria PØter, Christian A. Nijhuis, Hans Beijleveld,
Holger Schönherr, G. Julius Vancso, Alessandro Casnati,
Rocco Ungaro, Bart Jan Ravoo, Jurriaan Huskens,* and
David N. Reinhoudt*
Bottom-up nanotechnology has to start with the precise
positioning of molecules. For this purpose we are developing
molecular printboards, that is, self-assembled monolayers
(SAMs) of molecules that have specific recognition sites, for
example, molecular cavities, to which molecules can be
anchored through specific and directional supramolecular
interactions.[1] Such molecular printboards are prepared by
the self-assembly of b-cyclodextrin (b-CD) derivatives on
[*] Dr. T. Auletta, A. Mulder, S. Onclin, C. M. Bruinink, Dr. M. PØter,
C. A. Nijhuis, H. Beijleveld, Dr. B. J. Ravoo, Dr. J. Huskens,
Prof. Dr. D. N. Reinhoudt
University of Twente, MESA+ Institute for Nanotechnology
Supramolecular Chemistry and Technology
P.O. Box 217, 7500 AE Enschede (The Netherlands)
Fax: (+31)53-4894645
E-mail: j.huskens@utwente.nl
d.n.reinhoudt@utwente.nl
B. Dordi, Dr. H. Schönherr, Prof. Dr. G. J. Vancso
University of Twente, MESA+ Institute for Nanotechnology
Materials Science and Technology of Polymers
P.O. Box 217, 7500 AE Enschede (The Netherlands)
A. Sartori, Dr. A. Casnati, Prof. Dr. R. Ungaro
Università degli Studi di Parma
Dipartimento di Chimica Organica e Industriale
Parco Area delle Scienze 17A, 43100 Parma (Italy).
[**] The support given by the Netherlands Organization for Scientific
Research (NWO; grant 97041), the Applied Science division of the
NWO and the Ministry of Economic Affairs (STW; TST 4946), and
the MESA+ Institute for Nanotechnology is gratefully acknowl-
edged. Tascon GmbH (Münster, Germany) is gratefully acknowl-
edged for the SIMS measurements.
Scheme 1. Structures of b-cyclodextrin (b-CD, 1a) and b-CD adsor-
bates 1b and 1c for attachment to gold and silicon oxide, respectively;
bis(adamantyl)-functionalized calixarene (2); lissamine-rhodamine B
functionalized dendritic wedge (3); generic second generation PPI
dendrimer with 8 R groups: 4a has 64 Ad moieties (fifth generation)
and 4b has 16 Fc moieties (third generation).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 369 –373
DOI: 10.1002/anie.200352767
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369

Communications
binding sites, the near-hexagonal packing of which
can be visualized by AFM.[11]
We have measured the interaction thermody-
namics of these b-CD-terminated printboards with
several guests, by using a variety of techniques,
including AFM dynamic force spectroscopy, which
allows the detection of individual rupture forces of
host–guest complexes.[12] The binding of small,
univalent guest molecules to these SAMs has been
studied by surface plasmon resonance (SPR)[13] and
electrochemical
impedance
spectroscopies
(EIS).[11] An important finding from these studies
was that the interaction strength of such guest
molecules with b-CDs on the printboards is
identical to the binding strength in solution.[13]
The binding of large dendrimers functionalized
with adamantyl (Ad) groups through multiple
interactions has also been recently discussed.[1]
Multivalency, that is, the use of multiple
interactions which are assumed to act in a stepwise,
Figure 1. AFM friction force images (acquired in air) of patterns obtained by mCP of
the calixarene guest 2 (brighter areas) on b-CD- (top) and OH-terminated SAMs
(bottom): before rinsing, after rinsing with Milli-Q water, or after rinsing with
10 mm aqueous b-CD, respectively (image size 50”50 mm2; friction forces (a.u.)
increase from dark to bright contrast). SIMS-TOF image of a b-CD-terminated SAM
˙
after printing of 2 (image size: 56”56 mm): the bright areas indicate the presence
independent assembly process, is of strong current of the molecular ion peak at m/z=1418.
interest[14] for its implications in the design of new
ligands and inhibitors for proteins because it offers
a tool for fine-tuning overall interaction strengths. It has been,
for example, employed in the development of high-affinity
vancomycin–peptide systems,[15] which has very recently been
extended to the assembly of vancomycin-functionalized
polymers at peptide SAMs.[16] Therefore, the use of molecules
which allow the formation of multiple supramolecular inter-
actions constitutes a tool to tune adsorption and desorption
process parameters at surfaces—thermodynamics and kinet-
ics are in principle related in a straightforward manner to the
number of interactions and to the strength and kinetics[17] of
an individual interaction. Thus, we synthesized the water-
soluble calix[4]arene 2 which contains two Ad moieties as the
guest motif (Scheme 1). The binding of 2 with b-CD in
solution and onto b-CD-terminated SAMs was investigated
by microcalorimetry and SPR, respectively. The binding
behavior with b-CD in water showed the two Ad groups
bound independently[18] to two separate b-CDs, with binding
parameters characteristic of Ad–b-CD interactions.[13] The
binding constant K obtained for this guest with a b-CD
dimer[19] was 1.4 ” 107 mÀ1, based on a 1:1 model.[20] The
interaction with the b-CD-terminated SAMs appeared to be
much stronger (K = 109–1010 mÀ1),[21] which is ascribed to the
high local concentration of hosts at the surface.[1]
Such interaction strengths are already sufficiently high to
apply these molecules in nanofabrication as, for example, in
mCP. We employed, for the first time, supramolecular mCP,
which is the transfer of molecules from a stamp to a surface to
give specific host–guest assemblies on the printboards.
Patterns were visible directly after printing and remained
clearly visible when rinsed with substantial amounts of water
or 50 mm aqueous NaCl (Figure 1).[22] Even prolonged wash-
ing with b-CD in solution, to promote competition between
the host–guest binding in solution and at the printboard, did
not completely remove the patterns. The presence of the guest
molecules in the contacted areas was also confirmed by
secondary ion mass spectrometry (SIMS) imaging, as shown
for an image of the molecular ion peak of the calixarene 2
(Figure 1).
The requirement of specific host–guest interactions for
sustained pattern stability is evident when mCP on the b-CD-
terminated SAMs is compared to mCP on 11-sulfanyl-1-
undecanol SAMs (OH SAMs) on gold (Figure 1). Ink transfer
also occurs on the OH SAMs, but patterns were instantly
removed upon washing with water or 50 mm aqueous NaCl.
We also employed supramolecular DPN to write local
patterns of molecules on the printboard (Figure 2). Silicon
nitride AFM tips were dipped into an aqueous solution of the
calixarene (2) ink and scanned across b-CD- as well as OH-
modified SAMs. The transfer of ink was observed for both
SAMs, but patterns stable to rinsing were only obtained for
the b-CD printboards (Figure 2, left). As a control experi-
ment, a b-CD-terminated SAM was scanned with a bare Si3N4
tip under the same experimental conditions reported above
for the writing experiment. Subsequent AFM friction force
imaging did not show any visible pattern on the surface. This
result proves that the structures of Figure 2 do not arise from
mechanical forces applied by the tip to the SAM, but rather
by the transfer of ink molecules. The resolution that can be
attained by this technique is well below 100 nm, as shown by
the line patterns in Figure 2 (right). It is likely to be a function
of contact time, tip radius, ink concentration, and ink-transfer
mechanism, as in traditional DPN.
To widen the scope of the supramolecular patterning
approaches described above, we also prepared b-CD print-
boards on silicon oxide (1c, see Supporting Information)
because SiO2 permits fluorescence detection of the assem-
blies, whereas quenching occurs for SAMs on gold. Micro-
contact printing of the calixarene guest 2 gave results similar
to the b-CD-terminated SAMs on gold, both in regard to ink
transfer and pattern stability. The printing of fluorescent
dendritic wedge 3 (Scheme 1), which was transferred as the
per-b-CD complex, allowed pattern detection by confocal
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layer of the b-CD printboard
acts as the working electrode.
Sequential cyclic voltammo-
grams of a monolayer of Fc-
modified dendrimer 4b on the
b-CD printboard showed a
decrease in the peak current
that corresponds to a decrease
in the amount of adsorbed
material (see the Supporting
Information). Peak currents
remained constant when the
cyclic voltammograms were
recorded in the presence of
the dendrimer 4b in solution,
which indicates that oxidized
and desorbed dendrimers
Figure 2. Left: AFM friction force images (friction forces (a.u.) increase from dark to bright contrast) show-
ing, from left to right, patterns produced by DPN on a b-CD-terminated SAM (top) and on an OH-termi-
nated SAM (bottom) using 2 as the ink before rinsing (image acquired in water), after rinsing in situ with
Milli-Q water (image acquired in water), and with 50 mm aqueous NaCl (image acquired in aqueous NaCl),
respectively. Right: AFM friction force images in air of arrays of lines with mean widths (Æ standard devia-
tion) of 60Æ20 nm produced by DPN (friction inversion with respect to the mCP experiments is related to
the AFM tip employed in the imaging step, which is the same as in the writing step and thus still covered
with the molecular ink). For DPN, cleaned Si3N4 tips were coated with the calixarene ink by soaking them
for 15 min in a 0.5 mm aqueous solution of 2 and drying, and then scanned twice across a 7”7 mm2 area of
a SAM on gold (scan velocity ca. 21 mmsÀ1, T=258C, relative humidity 47Æ3%). The scan size was then
increased to 30”30 mm2, the scan velocity was set to 100–300 mmsÀ1 and the relative scan angle was
adjusted accordingly. The lines were produced by scanning each line for a 40 s period (scan velocity was
ca. 2 mmsÀ1, T=258C and relative humidity 47Æ3%). The scan size was then increased to 4”4 mm2, and
the scan velocity increased to 30 mmsÀ1. The tip was withdrawn prior to rinsing. The AFM liquid cell was
then flushed with 5 mL of Milli-Q water and new images were recorded. Eventually the cell was flushed with
5 mL of 50 mm aqueous NaCl, before newimages were recorded.
were
constantly
being
replaced by nonoxidized mol-
ecules from solution.
In summary, we have
shown that molecular print-
boards allow for versatile
nanofabrication schemes of
printing and writing that are
applicable to various types of
substrates and molecules.
Their advantages lie in the
tunability of the type and
number of host–guest motifs
and the possibility to erase
patterns. Only
a
limited
number of such supramolec-
microscopy (Figure 3, top). Rinsing the surface with a 10 mm
aqueous phosphate buffer hardly affected the printed pat-
terns, as witnessed in this case by monitoring fluorescence
intensities. Only rinsing with 10 mm b-CD led to a significant
reduction of the fluorescence.[23]
Patterns of a fifth generation Ad-terminated poly(propyl-
ene imine) dendrimer 4a (Scheme 1), functionalized with 64
Ad end groups,[24] on b-CD printboards on silicon oxide were
created by supramolecular mCP. Such molecules behave as
molecular boxes for dyes through electrostatic interactions,
both as the free box[25] and as complexes with b-CD in
aqueous solution.[24] The dendrimers were printed as the per-
b-CD complex. The dendritic boxes were subsequently
loaded with two different fluorescent dyes (either fluorescein
or rose bengal) by immersing the patterned substrates into
aqueous solutions of these dyes (Figure 3, bottom).
We can also use electroactive third generation dendrimers
4b terminated with ferrocene (Fc; Scheme 1) as the ink (as
they also form inclusion complexes with b-CD[26]) to control
the desorption of the host–guest assemblies electrochemi-
cally. The Fc-functionalized dendrimers were adsorbed from
solution onto the b-CD printboards on gold. The known
oxidation of the Fc groups to ferrocenium cations that leads
to complete loss of the Fc–b-CD interaction[26] provides us
with a tool to induce desorption by an external (electro-
chemical) stimulus in an electrochemical setup where the gold
ular interactions per guest molecule are required for the
formation of stable patterns, thus eliminating the need for
very complicated synthesis. Molecular printboards will open
up ways to fabricate arrays of multiple host–guest systems, to
study the formation and dynamics of individual supramolec-
ular assemblies at surfaces, and for the preparation of 3D
supramolecular architectures on substrates.
Experimental Section
The synthesis of the b-CD adsorbate 1b and the preparation of SAMs
on Au substrates were reported previously.[10,11] The synthesis of
adsorbate 2, of lissamine–rhodamine B/poly(propylene imine) den-
dritic wedge 3, and the preparation of monolayers on silicon oxide are
reported in the Supporting Information. The fifth generation
adamantane-terminated poly(propylene imine) dendrimer (G5-PPI-
(Ad)64) 4a, the third generation ferrocene-terminated poly(propylene
imine) dendrimer (G3-PPI-(Fc)16) 4b, and their per-b-CD complexes
were synthesised according to literature procedures.[24,27] Microcon-
tact printed substrates were prepared according to literature proce-
dures.[2]
Calorimetric titrations were performed at 258C using a Microcal
VP-ITC isothermal titration microcalorimeter. Titrations of 2 with
native b-CD or a b-CD dimer[19] were performed by adding aliquots of
a guest solution to the host solution or vice versa. The titrations were
analyzed with a least-squares curve-fitting procedure. Control experi-
ments involved addition of the burette solution to pure water and
addition of water to the cell solution.
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Communications
Electrochemical measurements were performed with an AUTO-
LAB PGSTAT10 in a custom built three-electrode set-up equipped
with a platinum counterelectrode, a mercury sulfate reference
electrode (VMSE = + 0.61VNHE), and a screw cap holding the gold
working electrode (area exposed to the solution = 0.44 cm2).
Received: September 2, 2003 [Z52767]
Published Online: December 9, 2003
Keywords: cyclodextrins · host–guest systems · microcontact
.
printing · nanolithography · self-assembly
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Figure 3. Top: confocal microscopy images after mCP of 3 on b-CD-
terminated SAMs on SiO2. Images shown are after printing, from left
to right: without rinsing; after rinsing with ca. 200 mL of an aqueous
buffer solution, followed by rinsing with water and drying in a stream
of nitrogen; after rinsing with ca. 200 mL of 10 mm b-CD, followed by
rinsing with water and drying in a stream of nitrogen. Dendritic wedge
3 was printed as the per-b-CD complex from a 0.1 mm aqueous solu-
tion. The oxidized stamp was soaked in the wedge solution, blown dry
with nitrogen, and brought into contact with the surface for 1 min
without external pressure. Bottom: Confocal microscopy images of
mCP of dendrimer 4a as the per-b-CD complex on a b-CD-terminated
SAM on SiO2, followed by loading with an anionic dye. The dendrimers
were printed from a 0.1 mm solution in water for 1 min, without exter-
nal pressure, and the patterns were washed with ca. 200 mL of 10 mm
aqueous phosphate buffer (pH 7) and water to remove physisorbed
material. The dendritic boxes were subsequently loaded with rose
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SPR measurements were performed in a two-channel vibrating
mirror angle scan set-up based on the Kretschmann configuration, as
described by Kooyman and co-workers.[28]
SIMS-TOF measurements were performed on an ION-TOF IV
spectrometer (Tascon GmbH Münster, Germany) in which an Au3
gun was employed in a burst alignment mode; target current =
0.02 pA.
XPS measurements were performed on a Physical Electronics
Quantum2000 equipped with a spherical sector analyzer and a
multichannel plate detector (16 detector elements).
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multimode AFM (Digital Instruments, Santa Barbara, CA, USA) in
the contact mode using V-shaped Si3N4 cantilevers (Nanoprobes,
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and thus dissociation rates follow directly from association rates
and thermodynamics.
[18] Microcalorimetry gave an intrinsic binding constant for the Ad–
b-CD interaction of 5.0 ” 104 mÀ1 and a binding enthalpy of
À7.5 kcalmolÀ1 for calixarene guest 2.
Confocal microscopy images were taken on a Carl Zeiss LSM
510. The dendrimer wedge 3 and rose bengal were excited at 543 nm,
while fluorescein was excited at 488 nm. The emitted fluorescence
was collected on a a PMT Hamamatsu R6357 spectrophotometer.
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[20] The stoichiometry (1:1), the K value (1.4 ” 107 mÀ1), and the
binding enthalpy (À16.2 kcalmolÀ1) indicate that both Ad
372
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Chemie
groups of the guest interact with both b-CD cavities of the dimer
to form a 1:1 complex.
[21] Value obtained for a 1:1 model in which the b-CD SAM is
treated as a SAM of dimeric hosts with a concentration of half
the b-CD concentration at the surface.
[22] The contrast in the AFM images does not allow any conclusion
on the amount of ink present on the patterns directly after
printing or after subsequent washing. XPS and EIS measure-
ments showed that: a) three to five monolayers of guest ink are
transferred by mCP onto both OH- and b-CD-terminated SAMs;
b) approximately one monolayer of guests is present after a first
rinsing of the b-CD-terminated SAMs with water; c) submono-
layer coverages remain on the b-CD-terminated SAMs upon
prolonged washing with water and b-CD solutions (See Sup-
porting Information). A monolayer is defined as a layer of the
calixarene guest molecules bound through both Ad groups to
two b-CD cavities at the surface.
[23] In contrast, patterns created on a reference polyethyleneglycol
monolayer (A. Papra, N. Gadegaard, N. B. Larsen, Langmuir
2001, 17, 1457 – 1460) were removed easily by rinsing with an
aqueous buffer solution.
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