Inkless microcontact printing on self-assembled monolayers of Fmoc-protected aminothiols

Article abstract of DOI:10.1021/ja076226k

Here we describe a new microcontact printing technique (μCP), based on a catalytic reaction between an Fmoc-protected self-assembled monolayer (SAM) of aminothiols chemisorbed on a gold surface and an elastomeric stamp bearing covalently attached piperidine. The elastomeric stamp was used to selectively cleave Fmoc groups from the surface of a SAM on gold in the places of conformal contact. The technique permits nanoscale pattern fabrication with edge resolution less than 50 nm, avoiding problems of conventional microcontact printing associated with diffusion of chemical ink , and provides a simple method for the chemical modification of SAMs. Copyright

Full text of DOI:10.1021/ja076226k

Published on Web 10/19/2007
Inkless Microcontact Printing on Self-Assembled Monolayers of
Fmoc-Protected Aminothiols
Alexander A. Shestopalov, Robert L. Clark,* and Eric J. Toone*
Department of Chemistry, Duke UniVersity, Durham, North Carolina 27708-0346, and Pratt School of Engineering,
Duke UniVersity, Durham, North Carolina 27708-0271
Received August 28, 2007; E-mail: eric.toone@duke.edu
Originally developed by Whitesides and co-workers, microcon-
tact printing (µCP) has become the method of choice for micro-
1
and nanoscale fabrication of surfaces and nanoparticles. Despite
its many advantages, several limitations remain, specifically (i)
relatively high level of defects due to distortion and deformation
2
of the elastomeric stamp, (ii) a limited number of substrates and
molecular inks, and (iii) practical limits to feature sizes near 100-
2
00 nm due to diffusion of molecular inks, both diffusive wetting
3
of surfaces and diffusion of volatile inks through the gas phase.
Recent efforts to circumvent these limitations include the replace-
ment of liquid inks with solid analogues, the use of reactive
Figure 1. Preparation of polyurethane acrylate stamps.
4
5
polymers, and the subtraction of inks from flat PDMS stamps in
the patterned area followed by printing of the remaining inks on a
6
different substrate.
In 2003, Reinhoudt et al. obviated the diffusive limitations of
molecular inks through the use of functionalized stamps that transfer
pattern through covalent modification of a preformed surface.7
Specifically, a SAM surface displaying trimethylsilyl (TMS) ethers
was converted to free alcohols during conformal contact with an
oxidized PDMS stamp, although the protocol achieved only 30%
cleavage. A similar study used plasma-oxidized flat PDMS to
promote coupling between amino-terminated SAMs and N-protected
amino acids.8
Figure 2. Experiments with unpatterned stamps.
We recently reported a µCP method that transfers pattern through
the action of a stamp-immobilized biocatalyst on a monolayer of
substrate, a process that does not depend on the transfer of the
high aspect ratio nanopatterns with sub-100 nm features for use
in replica molding.12 Monomer 2, synthesized from isophorone
9
diisocyanate, polyethylene glycol (av. M
propyl acrylate,13 was diluted by 30% with trimethylolpropane
ethoxylate triacrylate (2, av. M 912 g/mol) to reduce viscosity.
w
400 g/mol), and hydroxy-
molecular inks. Briefly, an acrylamide stamp bearing immobilized
exonuclease I (ExoI) was used to catalyze ablation of single-
stranded DNA immobilized on both glass and gold surfaces. This
work demonstrated that catalytic µCP can successfully transfer
patterns with 14 µm features. Here, we report complete transfer of
patterns with sub-micron features using a chemical catalyst bound
to a rigid polymeric stamp.
n
To the mixture were added photoinitiators, and the resulting
prepolymeric solution was polymerized between two glass slides
under UV light to produce control unfunctionalized flat stamp I
(Figure 1). Flat (II) and patterned (III) stamps bearing reactive
piperidine functionalities were prepared through Michael addition
of 2-aminomethyl piperidine (4) (8 v/v %) to prepolymeric mixtures
prior to polymerization. Preceding the printing experiments, stamps
were washed with EtOH for at least 1 h, rinsed with EtOH and
The 9-fluorenylmethoxycarbonyl (Fmoc) amino protecting group
is selectively cleaved under mildly basic nonhydrolytic conditions
using aliphatic amines such as piperidine, morpholine, and
10
8-diazabicyclo[5.4.0]undec-7-ene (DBU). In this embodiment of
inkless µCP, a stamp bearing polymerized piperidin-4-ylmetha-
namine is brought into contact with a gold surface functionalized
with the SAM of an Fmoc-protected aminothiol, promoting catalytic
cleavage of the Fmoc groups. Fmoc-protected SAMs on gold were
formed from (9H-fluoren-9-yl)methyl 11-mercaptoundecylcarbam-
ate (1), prepared in seven steps from 11-aminoundecanoic acid
2
H O, and dried with filtered nitrogen. The shape and size of the
stamp features were identical to those of the corresponding masters,
were unaffected during storage at room temperature, and retained
their integrity even after heating to 70 °C.
Fmoc-protected SAMs on gold (substrate 1) were formed by
immersing clean gold substrates in 1 mM EtOH solution of 1 for
at least 2 h at room temperature. Amino-terminated SAMs (substrate
2) were prepared from these surfaces by deprotection with a 1 M
piperidine solution in DMSO for 45 min at room temperature
(Figure 2).14 Unfunctionalized and reactive featureless stamps were
brought into contact with Fmoc-modified SAMs on gold at 50 °C
and permitted to react for 3 h (substrates 5 and 3). Following the
first application, the reactive stamp was soaked in EtOH for 1 h,
(
Supporting Information).11
A significant limitation to the resolution of catalytic µCP reported
9
previously was the use of acrylamide stamps: while these materials
are easily functionalized, they lack the mechanical rigidity necessary
for high fidelity transfer at short length scales. To alleviate this
limitation, we utilized a polyurethane acrylate polymer, a material
that has previously been used to prepare molds with densely arrayed
13818
9
J. AM. CHEM. SOC. 2007, 129, 13818-13819
10.1021/ja076226k CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. XPS Analysis of Substrates 1-5
shows the efficiency of the catalytic stamp in nanoscale pattern
fabrication, producing patterns consistent across the entire substrate
surface and generating features identical in shape and size to those
of the corresponding silicon-PMMA master. Produced pattern
shows a height difference of approximately 0.65 nm between
deprotected and protected regions, which correlates well with the
size of the fully extended Fmoc group (∼9 Å), and a friction
difference of approximately 14 mV. Printed features demonstrate
edge resolution less than 50 nm, indicating diffusion-free process
(Supporting Information).
water contact
angle
substrate
Au4p
C1s
C1s/Au4p
Fmoc %
1
2
3
4
5
23934.2
32414.7
35545.4
33141.2
24534.9
10562.5
6745.6
7566.8
6859.7
10783.9
5.03
2.37
2.42
2.38
5.01
100
0
1.9
0.4
99.2
78°
65°
64°
64°
77°
2
rinsed with EtOH and H O, dried with filtered nitrogen, and applied
again to fresh Fmoc-protected surfaces to ensure repeatability
In conclusion, we have demonstrated that piperidine-modified
polyurethane acrylate stamps effectively transfer patterns in an
inkless variant of µCP. The technique offers several advantages
over traditional µCP. Most significantly, the approach obviates the
diffusive resolution limitation of µCP and is constrained now only
by the mechanical properties of the stamp material. The use of
polyurethane acrylate polymer, which was recently utilized to make
highly accurate patterned molds with high aspect ratios, eliminates
some of the problems of traditional µCP related to deformation
(substrate 4).
The efficiency of the deprotection by piperidine-modified and
blank stamps was determined by comparison of water contact angles
and ratios of the C1s and Au4p signals in XPS spectra. C1s/Au4p
signal ratio of unreacted surface was used as a reference for 100%
Fmoc-protected sample, whereas C1s/Au4p ratio of Fmoc-protected
sample treated with piperidine solution provided a completely
deprotected reference. On the basis of these values, piperidine-
functionalized stamps effect complete deprotection, while blank
stamp produced no detectable change during the reaction. Evaluation
based on O1s/Au4p ratios yielded similar results (Supporting
Information). These conclusions are further supported by water
contact angle measurements (Table 1).
3
and collapse of the elastomeric stamps prepared from PDMS.
Finally, the method permits rapid subsequent functionalization of
the printed surfaces, providing a route to the patterned SAMs with
a range of chemical and physical properties. We continue to explore
the utility of inkless µCP and will report our results in due course.
The temperature dependence of the deprotection was examined
Supporting Information). At 60 °C, complete deprotection is
Acknowledgment. We acknowledge the financial support of
the NSF (DMI-0600513) and to Matthew S. Johannes and Briana
N. Vogen for technical assistance.
(
achieved in 30 min, while at lower temperatures, significantly longer
reaction times are required. The extent to which steric constraints
of the closely packed substrates affect the rate of reaction is unclear,
and awaits further study.
The goal of this work is to provide a methodology for pattern
transfer that alleviates the diffusive resolution limit of conventional
µCP. To evaluate this capability, stamps III bearing 620 nm lines
separated by 380 nm with aspect ratio of 0.15 were used to
selectively deprotect SAMs of (9H-fluoren-9-yl)methyl 11-mer-
captoundecylcarbamate (1) on gold. An Fmoc-modified substrate
was placed on top of patterned stamps preheated to 50 °C and held
in contact for 3 h (substrate 6). Following reaction, the substrate
Supporting Information Available: Experimental details are
available. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(
1) (a) Rogers, J. A.; Nuzzo, R. G. Mater. Today 2005, 8, 50-56. (b) Xia,
Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575. (c)
Rolland, J. P.; Maynor, B. W.; Euliss, L. E.; Exner, A. E.; Denison, G.
M.; DeSimone, J. M. J. Am. Chem. Soc. 2005, 127, 10096-10100.
(2) Delamarche, E.; Schmid, H.; Michel, B.; Biebuyck, H. AdV. Mater. 1997,
9, 741-746.
(
3) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.;
Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324-3334.
4) (a) Childs, W. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2002, 124, 13583-
2
was washed with EtOH and H O, dried with filtered nitrogen, and
analyzed by contact mode lateral AFM and SEM. Figure 3 clearly
(
1
3596. (b) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J.
Am. Chem. Soc. 2002, 124, 7654-7655.
(
5) (a) Feng, C. L.; Embrechts, A.; Bredebusch, I.; Schnekenburger, J.;
Domschke, W.; Vancso, G. J.; Sch o¨ nherr, H. AdV. Mater. 2007, 19, 286-
2
90. (b) Feng, C. L.; Vancso, G. J.; Schoenherr, H. AdV. Funct. Mater.
2
006, 16, 1306-1312.
(
(
(
(
6) Coyer, S. R.; Garc ´ı a, A. J.; Delamarche, E. Angew. Chem., Int. Ed. 2007,
DOI: 10.1002/anie.200700989.
7) Li, X.-M.; Peter, M.; Huskens, J.; Reinhoudt, D. N. Nano Lett. 2003, 3,
1
449-1453.
8) Sullivan, T. P.; van Poll, M. L.; Dankers, P. Y. W.; Huck, W. T. S. Angew.
Chem., Int. Ed. 2004, 43, 4190-4193.
9) Snyder, P. W.; Johannes, M. S.; Vogen, B. N.; Clark, R. L.; Toone, E. J.
J. Org. Chem. 2007, 72, 7459-7461.
(
10) (a) Carpino, L. A.; Han, G. Y. J. Org. Chem. 1972, 37, 3404-3409. (b)
Carpino, L. A.; Mansour, E. M. E.; Knapczyk, J. J. Org. Chem. 1983,
4
8, 666-669. (c) Gude, M.; Ryf, J.; White, P. D. Lett. Pept. Sci. 2003, 9,
2
03-206. (d) Wade, J. D.; Bedford, J.; Sheppard, R. C.; Tregear, G. W.
Pept. Res. 1991, 4, 194-199.
(
11) (a) Belser, T.; Stohr, M.; Pfaltz, A. J. Am. Chem. Soc. 2005, 127, 8720-
8
731. (b) Yakata, K.; Sato, M.; Fukuoka, Y.; Yamaguchi, T.; Nagira, K.
Application JP 2001-167653 2002363154, Dojindo Laboratories, Japan,
002, 10 pp.
2
(
12) (a) Choi, S.; Yoo, P. J.; Baek, S. J.; Kim, T. W.; Lee, H. H. J. Am. Chem.
Soc. 2004, 126, 7744-7745. (b) Kim, Y. S.; Lee, H. H.; Hammond, P. T.
Nanotechnology 2003, 14, 1140-1144.
(13) Wolf, J.-P.; Schulthess, A.; Steinmann, B.; Hunziker, M. Application EP
94-810110 614122, Ciba-Geigy A.-G., Switzterland, 1994, 22 pp.
(14) Frutos, A. G.; Brockman, J. M.; Corn, R. M. Langmuir 2000, 16,
192-2197.
2
Figure 3. SEM image (bottom right), height (top), and friction (bottom
left) LFM images of patterned substrate 6.
JA076226K
J. AM. CHEM. SOC.
9
VOL. 129, NO. 45, 2007 13819