Sulfonic acid-functionalized gold nanoparticles: A colloid-bound catalyst for soft lithographic application on self-assembled monolayers

Article abstract of DOI:10.1021/ja0271431

In this report, we present a new lithographic approach to prepare patterned surfaces. Self-assembled monolayers (SAMs) of the acid-labile trimethylsilyl ether (TMS-OC₁₁H₂₂S)₂ (TMS adsorbate) was formed on gold. 5-Mercapto-2-benzimidazole sulfonic acid sodium salt (MBS-Na⁺) was used as a ligand for gold nanoparticles. These monolayer-protected gold colloids (MPCs) were transformed into the catalytically active H⁺-form by ion exchange. This colloid-bound catalyst hydrolyzed the TMS adsorbate (TMS-OC₁₁H₂₂S)₂ both in solution and when self-assembled on gold surfaces. Microcontact printing of the active colloid-bound catalyst on the preformed TMS SAM led to the deposition of the colloid onto the SAMs. After the catalyst nanoparticles were rinsed off, a patterned surface was created as shown by AFM.

Full text of DOI:10.1021/ja0271431

Published on Web 03/13/2003
Sulfonic Acid-Functionalized Gold Nanoparticles: A
Colloid-Bound Catalyst for Soft Lithographic Application on
Self-Assembled Monolayers
Xue-Mei Li, Vasile Paraschiv, Jurriaan Huskens,* and David N. Reinhoudt*
Contribution from the Laboratory of Supramolecular Chemistry and Technology, MESA+
Research Institute, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Received June 3, 2002; E-mail: smct@ct.utwente.nl; j.huskens@ct.utwente.nl
Abstract: In this report, we present a new lithographic approach to prepare patterned surfaces.
Self-assembled monolayers (SAMs) of the acid-labile trimethylsilyl ether (TMS-OC11H22S)2 (TMS adsorbate)
was formed on gold. 5-Mercapto-2-benzimidazole sulfonic acid sodium salt (MBS-Na+) was used as a
ligand for gold nanoparticles. These monolayer-protected gold colloids (MPCs) were transformed into the
catalytically active H+-form by ion exchange. This colloid-bound catalyst hydrolyzed the TMS adsorbate
(TMS-OC11H22S)2 both in solution and when self-assembled on gold surfaces. Microcontact printing of the
active colloid-bound catalyst on the preformed TMS SAM led to the deposition of the colloid onto the SAMs.
After the catalyst nanoparticles were rinsed off, a patterned surface was created as shown by AFM.
Introduction
Microcontact printing employs an elastomeric stamp to
transfer ink (alkanethiols) to a substrate via conformal contact.
Self-assembled monolayers (SAMs) on gold are attractive for
the development of sensors,1,2 interfaces,3 and for nanofabrica-
tion.4-6 Recently, such SAMs have been applied for the
generation of well-defined patterns on the micro/nanometer
scale. These patterned SAMs can be used as resists for pattern
transfer7 and as templates for patterning proteins and other
biosystems.8,9
Patterned SAMs can be created by various direct techniques
such as UV-photolithography, e-beam lithography, and by repli-
cation techniques, such as microcontact printing (µCP).10,11
Microcontact printing can be conducted routinely in the lab
without the need of cleanroom equipment, and it is compatible
with a wide range of surface functional groups, including the
structurally complex and fragile groups found in biology and
biochemistry.12 The substrate used for printing is not limited to
flat surfaces, thus rendering this technique a versatile tool for
chemical patterning.
It has been widely used to create patterned SAMs on gold
because alkanethiols give a strong interaction with gold and
form robust SAMs in the contacted areas. The microcontact
printing process depends on various factors to attain reliable
and reproducible results. Stamp materials need to be elastomeric
to allow conformal contact and show little adhesion for easy
release after printing. The most widely used material is poly-
(dimethylsiloxane) (PDMS). Second, long chain alkanethiols
have been used as the ink because they form densely packed,
etch resistant SAMs. Other factors such as printing time and
inking method are also important.13
Microcontact printing relies on ink diffusion from stamp to
substrate, and this imposes limitations on the resolution that
can be attained. Several ink transfer pathways have been
identified which led to the undesired ink transport to the
noncontacted areas, for example, by vapor phase deposition and
ink diffusion along the surface.14 Such processes may be
restricted by the use of heavier inks.15 Macromolecules such as
proteins have also been used as the ink.16 Even palladium
nanoparticles have been transferred, which initiated metallization
of copper as shown by Whitesides et al.17,18 They showed that
patterns were faithfully transferred without diffusion, indicating
that nanoparticles are of potential use as ink for microcontact
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Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324-3334.
(15) Liebau, M.; Huskens, J.; Reinhoudt, D. N. AdV. Funct. Mater. 2001, 11,
147-150.
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Lauffenburger, D. A.; Whitesides, G. M.; Rich, A. Biomaterials 1999, 20,
1213-1220.
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Biomaterials 1999, 20, 2363-2376.
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AdV. Mater. 2000, 12, 1067-1070.
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(11) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker,
D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel,
P.; Stutz, R.; Wolf, H. IBM J. Res. DeV. 2001, 45, 697-719.
(12) Tien, J.; Xia, Y.; Whitesides, G. M. Thin Films 1998, 24, 227-254.
(17) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996,
12, 1375-1380.
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J. AM. CHEM. SOC. 2003, 125, 4279-4284
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A R T I C L E S
Li et al.
MBS-H+-Functionalized Gold Nanoparticles (H+-MPCs). Na+-
MPCs (30 mg) were dissolved in water (10 mL). The pH of the colloid
solution was neutral. This solution was passed over a DOWEX-H+
ion-exchange column. The pH of the solution decreased to 2. Water
was removed by lyophilization, giving a black, hygroscopic solid.
Hydrolysis of TMS Adsorbate by H+-MPCs in Solution. The TMS
adsorbate (10 mg) was dissolved in CDCl3. A catalytic amount of H+-
MPCs (∼1 mg in CD3OD) was then added. A 1H NMR spectrum was
taken directly, which showed that the cleavage was complete. 1H NMR
(CDCl3) δ (ppm): 3.65 (t, 4H, J ) 7.6 Hz), 2.70 (t, 4H, J ) 4.3 Hz),
1.50-1.75 (m, 8H), 1.20-1.40 (m, 28H). MS (FAB-MS) m/z: 407.4
([M + H]+; calcd. for C22H46O2S2: 406.3).
Monolayer Preparation. All glassware used for monolayer prepara-
tion was immersed in piranha (concentrated H2SO4 and 33% aqueous
H2O2 in a 3:1 ratio; warning: Piranha should be handled with caution;
it has been reported to detonate unexpectedly) and rinsed with large
amounts of water (Millipore). Nearly atomically flat gold substrates
were obtained from Holland Biomaterials Group BV (Enschede, The
Netherlands) as a layer of 20 nm gold on titanium (2 nm) on silicon.
Before use, the substrates were treated with oxygen plasma (5 min)
and ethanol (5 min). After being rinsed, the substrates were directly
immersed into the adsorbate solution. Self-assembled monolayers
(SAMs) were prepared by immersing the freshly cleaned gold substrates
in a 1 mM TMS adsorbate solution at room temperature for 14 h. The
SAMs were rinsed by chloroform, ethanol, and water, and dried under
a stream of nitrogen.
printing. Potential problems with printing nanoparticles lie in
the control of the amount and order of materials transferred.
We envisage the use of catalytic nanoparticles to prepare
patterned SAMs, thus avoiding those problems while retaining
the aforementioned advantages of using nanoparticles as the ink.
Here we describe the preparation of sulfonic acid derivatized
monolayer-protected gold nanoparticles (MPCs) and their ap-
plication as a catalyst to hydrolyze a trimethylsilyl ether (TMS)
adsorbate. The reactivity of this colloid-bound catalyst was
investigated in the hydrolysis of this TMS adsorbate both in
solution and when self-assembled on gold. The MPCs were
further used as an ink for microcontact printing on the preformed
TMS SAMs. Local hydrolysis of the TMS adsorbate molecules
is shown to be a novel method for pattern creation.
Experimental Section
Chemicals. All chemicals were used as received, unless otherwise
stated. HAuCl4‚xH2O (99.99%) was purchased from Acros Organics,
tetraoctylammonium bromide from Fluka. DOWEX-H+ ion-exchange
resin was purchased from Janssen Chimica. 11-Mercapto-1-undecanol
(MUD), 5-mercapto-2-benzimidazole sulfonic acid sodium salt (MBS-
Na+), and trimethylsilyl chloride (TMS-Cl) were purchased from
Aldrich. Silver enhancer was obtained from Sigma. Water was purified
by Millipore membrane units. Solvents for colloid preparation were
reagent grade. Solvents used for organic synthesis were purified
according to standard laboratory methods.19
Contact Angle Measurements. Water contact angles were measured
on a Kru¨ss G10 contact angle goniometer, equipped with a CCD
camera. The advancing and receding contact angles were measured
during the growth and shrinkage of a droplet, respectively.
Bis(ω-trimethylsiloxyundecyl)disulfide ((TMS-OC11H22S)2). To
11-mercapto-1-undecanol (1.0 g, 4.89 mmol) in dichloromethane (50
mL) was added iodine (0.5 g, 3.10 mmol). The solution was stirred
overnight. The reaction mixture was washed twice with 1 M aqueous
NaS2O3 and twice with water, and subsequently dried over Na2SO4.
After the solvent was removed, bis(ω-hydroxyundecyl)disulfide was
Microcontact Printing (µCP). PDMS stamps were prepared ac-
cording to a published procedure.20 To allow application of aqueous
ink solutions, they were treated with an O2 plasma for 15 s.17 No
treatment was required when methanolic solutions were used, that is,
a H+-MPC solution. The (pretreated) PDMS stamp was inked with the
gold nanoparticle solution and blown dry under a stream of nitrogen;
this procedure was repeated two or three times. The stamp was then
brought into contact with the SAM substrate for 5 min. After the stamp
was released, the surface was rinsed with large amounts of methanol
and water. Alternatively, the substrates were treated with silver
enhancer,21 which enlarges the colloidal gold by electroless deposition
of metallic silver to give a high contrast. The silver enhancer kit consists
of solution A (silver salt), solution B (initiator), and sodium thiosulfate
(fixer). In brief, solution A and solution B were mixed 1:1 immediately
before use and applied to the SAM with the gold colloidal particles.
After 5-10 min, the substrate was rinsed with water, fixed 2-3 min
in a sodium thiosulfate solution, and rinsed again.
1
obtained in a quantitative yield as a white solid. H NMR (CDCl3) δ
(ppm): 3.65 (t, 4H, J ) 7.6 Hz), 2.70 (t, 4H, J ) 4.3 Hz), 1.50-1.75
(m, 8H), 1.20-1.40 (m, 28H). To a solution of this disulfide (200 mg,
0.50 mmol) in dry THF (50 mL) were added TMSCl (133 mg, 1.25
mmol) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (164 mg, 1.10
mmol). The reaction mixture was stirred overnight under argon. After
the precipitate was filtered off from the reaction mixture, the solution
was washed twice with water and dried over Na2SO4. After the solvent
was removed and dried under vacuum, (TMS-OC11H22S)2 was obtained
1
as an oil (0.24 g, 90%). H NMR (CDCl3) δ (ppm): 3.57 (t, 4H, J )
6.6 Hz), 2.68 (t, 4h, J ) 7.3 Hz), 1.43-1.72 (m, 8H), 1.24-1.37 (m,
28H), 0.11 (s, 18H). MS (FAB-MS) m/z: 550.7 ([M]+; calcd. for
C28H62O2S2Si2: 551.1).
MBS-Na+-Functionalized Gold Nanoparticles (Na+-MPCs). An
aqueous solution (7 mL) of MBS-Na+ (126 mg, 0.50 mmol) was added
to tetrachloroauric acid (100 mg, 0.29 mmol) in water (3 mL). The
reduction was carried out by dropwise addition of freshly prepared
sodium borohydride (130 mg, 3.50 mmol) in water (20 mL) at a rate
of approximately 0.3 mL/min while stirring. The nanoparticles were
formed instantaneously as witnessed by the color change to dark brown.
The solution was stirred at room temperature for 2 h. Na+-MPCs were
lyophilized after filtering. The black solid was suspended in methanol
(150 mL) at -18 °C for 4 h and re-collected by centrifugation, followed
by washing with methanol (3×). To remove water-soluble byproducts,
Na+-MPCs were further purified by dialysis and dried under vacuum,
yielding a black solid (30 mg, 52%). 1H NMR in D2O showed no
free MBS ligands, only broadened resonances for the MPC-attached
MBS. IR: ν ) 3419 (NH), 1625 (CdC, CdN), 1140 (OdSdO), 1087
FT-IR. Infrared spectra were collected by pressing the MPCs into
KBr pellets and using KBr as the background on a Perkin-Elmer FT
IR BX system.
XPS. X-ray spectroscopy was collected from a Quantum 2000
Scanning Esca Microprobe; the Quantum 2000 uses a Quartz crystal
monochromator and a scanning electron source that excites the
aluminum anode to produce a focused X-ray beam. The created
photoelectrons pass through a Spherical Capacitor Energy Analyzer
and are detected with a MultiChannel Detector (16 channels). Surface
survey data were collected followed by high-resolution scans over C1s
(278-298 eV), O1s (525-545 eV), S2s (222-242 eV), and Si2s (145-
165 eV). Peak areas were calculated using the Gaussian fit program.
Relative peak area ratios were calculated according to published
results.22
(OdSdO) cm-1
.
(20) Zhao, X.-M.; Xia, Y.; Whitesides, G. M. J. Mater. Chem. 1997, 7, 1069-
1074.
(19) Perrin, D. D.; Armarego, W. F. L. Purification of Laboratory Chemicals,
(21) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757-
3rd ed.; Pergamon Press: Oxford, 1989.
1760.
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Sulfonic Acid-Functionalized Gold Nanoparticles
A R T I C L E S
Figure 1. A transmission electron microscope (TEM) image and the size histogram of the Na+-MPCs.
Scheme 1
were made in a one-phase system (water) by the reduction of
hydrogen tetrachloroaurate using sodium borohydride in the
presence of MBS-Na+ (Scheme 2). Fast addition of the reducing
agent gave a black precipitate. Soluble Na+-MPCs were obtained
only when the reduction was carried out at an addition rate of
0.3 mL/min or slower. The gold nanoparticles were purified by
precipitation from methanol, giving a gray-black solid. The Na+-
MPCs were water soluble, stable in air, and were dried and
redissolved without change. The IR spectrum of the Na+-MPCs
resembles that of MBS-Na+, which confirmed the presence of
the ligands on the colloids. The absence of a thiol absorption
band in the Na+-MPCs spectrum indicates the absence of free
MBS ligand in the colloids. The presence of the sulfonate
functionality is further evidenced by OdSdO stretching vibra-
TEM. Transmission electron microscopy (TEM) images were
collected on a Philips CM 30 Twin STEM, fitted with Kevex delta
plus X-ray dispersive electron spectroscopy (EDX) and Gatan model
666 PEELs, operating at 300 kV. Samples were prepared by drop-
casting a drop of gold colloid dichloromethane solution onto a 200
mesh copper grid and were left to dry for 10 min.
NMR. NMR spectra were recorded at 25 °C using a Varian Inova
300 spectrometer. 1H NMR chemical shifts (300 MHz) are given relative
to residual CHCl3 (7.25 ppm) as an internal standard. 13C NMR
chemical shifts (75 MHz) are given relative to CDCl3 (77.0 ppm).
FAB-MS. Mass spectra were recorded with a Finnigan MAT 90
spectrometer using NBA/NPOE as a matrix.
1
tions at 1140 and 1086 cm-1. The H NMR spectrum showed
line broadening as compared to MBS-Na+ and again the absence
of free MBS-Na+. UV/vis showed a typical surface plasmon
absorbance of gold (520 nm). The particle size (4.0 ( 1.5 nm)
was determined by TEM (Figure 1).24 A significant number of
particles exhibiting twinning and preferential {111} surface facet
planes are clearly visible, which was also observed by others.25
Exchange to the catalytically active H+ form (H+-MPCs) was
achieved by passing an aqueous solution of the Na+-MPCs over
a DOWEX-H+ ion-exchange resin column (Scheme 2). The pH
value of the colloid solution after treatment was around 2. These
colloids were soluble both in water and in methanol.
AFM. Tapping mode AFM images were acquired in air on a
Nanoscope III instrument.
Results and Discussion
Hydrolysis in Solution. To probe the catalytic activity, H+-
MPCs were tested as a catalyst in solution. The TMS adsorbate
was dissolved in CDCl3. A catalytic amount of H+-MPCs (∼1
Trimethylsilyl (TMS) ethers are widely used as protective
groups for organic synthesis. They are labile to acid or base
hydrolysis, but acid stability is quite dependent on the local
steric environment.23 Bis(ω-trimethylsiloxyundecyl)disulfide
((TMS-OC11H22S)2, TMS adsorbate) was synthesized as shown
in Scheme 1. 11-Mercapto-1-undecanol (MUD) was oxidized
to the disulfide by iodine in a quantitative yield. The disulfide
was then turned into the TMS adsorbate by reaction with
trimethylsilyl chloride (TMS-Cl).
1
mg in CD3OD) was then added. A H NMR spectrum taken
directly after addition showed that the cleavage was complete.
This was confirmed by FAB mass spectroscopy. Thus, it was
concluded that the colloid-bound catalyst on the H+-MPCs is
an active catalyst in the hydrolysis of TMS.
Since the introduction of the two-phase preparation of self-
assembled monolayer-protected gold colloids (MPCs) by Brust
et al.,26 a variety of MPCs with desired functionalities has been
prepared.25,27,28 The ligands on the particles bind in the same
5-Mercapto-2-benzimidazole sulfonic acid sodium salt (MBS-
Na+) was used as a protecting ligand in gold nanoparticle
preparation. The monolayer-protected gold nanoparticles (MPCs)
(24) Only primary particles are counted for the size distribution. The size
distribution does not hamper our application of the nanoparticles as ink
because the particle size is much smaller than the eventually obtained pattern
resolution.
(22) (a) Evans, S. D.; Cooper, S. D.; Johnson, S. R.; Flynn, T. M.; Ulman, A.
Supramol. Sci. 1997, 4, 247-253. (b) Evans, S. D.; Flynn, T. M.; Ulman,
A.; Beamson, G. Surf. Interface Anal. 1996, 24, 187. (c) Nakano, N.; Sato,
T.; Tazaki, M.; Tagaki, M. Langmuir 2000, 16, 2225-2229. (d) Evans, S.
D.; Goppertberarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A.
Langmuir 1991, 7, 2700-2709. (e) Rieley, H.; Kendall, G. K.; Zemicael,
F. W.; Smith, T. L.; Yang, S. H. Langmuir 1998, 14, 5147-5153.
(23) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic Synthesis;
John Wiley & Sons: New York, 1991.
(25) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc.,
Chem. Commun. 1995, 1655-1656.
(26) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem.
Soc., Chem. Commun. 1994, 801-802.
(27) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am.
Chem. Soc. 1998, 120, 1906-1911.
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A R T I C L E S
Li et al.
Scheme 2
Figure 2. Advancing (0) and receding ([) contact angles of a TMS
adsorbate monolayer versus the immersion time in a H+-MPC methanol
solution.
Figure 4. An optical microscopy image of a TMS SAM substrate after
µCP of H+-MPCs followed by silver enhancement (see text).
mercaptopropyl)trimethoxylsilane on gold has been investigated
by Pemberton et al.32 They have used XPS and Raman
spectroscopy to quantify the reaction yield. So far, very little is
known about the hydrolysis of surface-bound silyl ethers.
SAMs were prepared by immersing gold substrates in a 1
mM solution of the TMS adsorbate in chloroform overnight.
The water contact angles of the SAMs were determined to be
75 ( 2° (advancing) and 27 ( 2° (receding).33 Hydrolysis was
carried out by dipping SAM substrates into H+-MPCs in
methanol (0.25 g/L) for varying periods of time and was
monitored by contact angle goniometry and X-ray photoelectron
spectroscopy (XPS).
Grazing incidence angle FT-IR of the TMS SAM showed
CH2 vibration at 2920 cm-1 (νas) and 2854 cm-1 (νs), indicating
a crystalline well-ordered nature of the SAM. Vibrations of
methyl groups on silicon atoms are expected to appear around
1255-1270 cm-1. However, this peak becomes obscured due
to the water absorbance. Thus, it is not reliable to use FT-IR
for studying the hydrolysis process.
Figure 3. Procedure for the selective hydrolysis of a TMS adsorbate SAM
on gold by microcontact printing of H+-MPCs. A PDMS stamp is inked
with a solution of H+-MPCs. The stamp is then brought into contact with
a TMS adsorbate SAM. After release of the stamp from the surface and
rinsing off the nanoparticles, a patterned surface is created.
way as on flat gold surfaces,29 thus serving as a perfect model
for surface reactions at SAMs. Tremel et al.30 used an ω-func-
tionalized alkanethiol as a capping ligand for gold nanoparticles,
and treatment with RuCl3 gave a ring-opening metathesis
catalyst. Frigeri et al.31 prepared N-methylimidazole-function-
alized MPCs and used these as a catalyst for the cleavage of
2,4-dinitrophenyl acetate. In both studies, the colloid-bound
catalysts were applied in solution only.
Contact angle changes are shown in Figure 2. These results
show that the SAMs became more hydrophilic upon prolonged
contact with the gold nanoparticle solution, which is attributed
to the hydrolysis of TMS moieties. The slow hydrolysis rate
with the H+-MPCs in solution is attributed to the good solubility
of the colloids and thus to the little actual contact time between
catalyst and reactive surface groups.
Hydrolysis on a SAM Surface. Surface-surface reactivity
tests were carried out to investigate the applicability of the
catalyst in surface patterning. Hydrolysis of self-assembled (3-
XPS was used to further analyze the hydrolysis of TMS at
the surface. Three layers were compared: an intact TMS layer,
(28) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15,
3782-3789.
(29) Bourg, M. C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562-
6567.
(30) Bartz, M.; Ku¨ther, J.; Seshadri, R.; Tremel, W. Angew. Chem., Int. Ed.
1998, 37, 2466-2468.
(31) Pasquato, L.; Rancan, F.; Scrimin, P.; Mancin, F.; Frigeri, C. Chem.
Commun. 2000, 2253-2254.
(32) Thompson, W. R.; Cai, M.; Ho, M.; Pemberton, J. E. Langmuir 1997, 13,
2291-2302.
(33) The significant hysteresis might be due to the bulkiness of the TMS moiety
and gauche defects as mentioned in ref 32.
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Sulfonic Acid-Functionalized Gold Nanoparticles
A R T I C L E S
Figure 5. Tapping mode AFM images of TMS adsorbate SAMs after µCP of H+ or Na+-MPCs and thorough rinsing: (a) height image (z range 20 nm)
and (b) phase image (z range 40°) after µCP of H+-MPCs (image size: 30 × 30 µM2); (c) height image (z range 20 nm) and (d) phase image (z range 30°)
after µCP of Na+-MPCs (image size: 50 × 50 µM2).
a TMS layer after 30 min of contact with a H+-MPC solution,
and a reference MUD SAM. The concentration of Si in the layer
is indicative of the presence of the TMS moieties. The atomic
concentration of Si on the intact TMS SAM was 5.2 ( 0.6%,
while the reference SAM did not show any Si at all. The TMS
SAM which had been in contact with H+-MPCs for 30 min
showed 3.4 ( 0.9% of silicon, indicating partial hydrolysis of
the SAM, thus confirming the contact angle data.
Microcontact Printing (µCP) of MPCs onto SAMs. µCP
employs ink transfer from an elastomeric stamp to a solid
substrate by conformal contact. Here, we did not transfer an
adsorbate ink, but rather a catalyst which reacts with an already
present, densely packed, reactive SAM. The process is illustrated
in Figure 3. For application of an aqueous solution of Na+-
MPCs, the stamp was made hydrophilic by treatment with an
oxygen plasma for 15 s. For application of H+-MPCs in
methanol, untreated stamps were used. The stamp was inked
with a Na+ or H+-MPC solution, dried under a stream of
nitrogen, and then put into contact with the TMS adsorbate
SAM. After release of the stamp after 5 min,34 the substrate
was either rinsed thoroughly or put in a silver enhancer solution
(Figure 3).
used in which silver ions were reduced by hydroquinone to silver
metal at the surfaces of the gold nanoparticles. Silver enhance-
ment has been used to visualize protein-, antibody-, and DNA-
conjugated gold nanoparticles in histochemical electron mi-
croscopy studies.21 An optical microscopy image (Figure 4)35
confirmed that the colloids (H+-MPCs) had been transferred
faithfully.
Alternatively, the MPCs were rinsed off after printing.
Tapping mode AFM was used to image the surfaces (Figure
5). The substrate which had been in contact with the H+-MPCs
showed replication of the stamp pattern into the SAM. This
pattern was barely visible in the height profile of the image
(Figure 5a). The phase image (Figure 5b), however, showed a
high contrast. We attribute the patterns to alternating areas of
unreacted TMS functionalities and hydrolyzed, thus OH-
terminated, functionalities at the outer face of the SAM, caused
by hydrolysis of the TMS groups by the catalytically active H+-
MPCs in the contacted area. The accurate replication of the
stamp feature sizes indicates that the nanoparticles have not
diffused away from the printed area during the, for normal µCP,
long printing times and that catalysis happened only locally in
the contacted area.
In a control experiment, µCP of the Na+-MPCs was per-
formed on a TMS SAM surface. The particles were transferred
to the TMS SAM according to optical microscopy. After the
The MPCs could be visualized by optical microscope after
microcontact printing on TMS SAM. However, the contrast was
not very high. To facilitate the optical visualization of the printed
nanoparticles on surfaces, a pattern amplification method was
(35) Oxygen plasma treatment created cracks on the stamps as found by: Chua
(34) Shorter contact times do not lead to observable patterns on TMS SAMs.
et al. Appl. Phys. Lett. 2000, 76, 721-723.
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A R T I C L E S
Li et al.
particles were rinsed off, AFM imaging did not show any pattern
formation on the surfaces (Figure 5c and 5d). This proves that
the catalytic activity of the H+-MPCs is essential for pattern
replication.
µCP has been used for the transfer of a catalytic palladium
colloid, which initiated metallization of copper as shown by
Whitesides.17,18 Here we have shown that catalytic nanoparticles
can be transferred by µCP onto a chemically active SAM
surface. This catalyst has selectively and locally catalyzed the
TMS adsorbate, providing patterns on the SAM surface. The
liberated OH functional groups generated in this way may be
utilized for further surface modification.
allowing in principle more accurate pattern transfer than
normally in µCP, which relies on adsorbate diffusion from the
stamp interior to the surface. Scanning force microscopy (SPM)
has proven to be a new tool for pattern creation in the nanometer
range.36-42 We expect to use the present system and apply it to
SPM lithography, using an AFM tip modified with such a
catalyst for writing chemical patterns on reactive SAMs.
Acknowledgment. This research is supported by the Tech-
nology Foundation STW, applied science division of NWO, and
the technology program of the Ministry of Economic Affairs.
Supporting Information Available: XPS and IR results
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Conclusions
Microcontact printing of a catalytically active species is for
the first time performed on a preformed, densely packed, and
chemically active TMS SAM. The catalyst was immobilized
on colloidal gold, which was used as an ink in microcontact
printing. The colloid-bound catalyst proved to hydrolyze the
TMS adsorbate in solution as well as when self-assembled on
gold. By using µCP, the colloid-bound catalyst was transferred
to a TMS adsorbate SAM, which hydrolyzed the TMS adsorbate
in the contacted areas, providing patterns with different func-
tional groups. The advantage of printing nanoparticles with µCP
is that no diffusion occurs upon transfer as shown before,17 thus
JA0271431
(36) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457-466.
(37) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 725-
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(38) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 424-
429.
(39) Gorman, C. B.; Carroll, R. L.; He, Y. F.; Tian, F.; Fuierer, R. Langmuir
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(40) Amro, N. A.; Xu, S.; Liu, G.-Y. Langmuir 2000, 16, 3006-3009.
(41) Wadu-Mesthrige, K.; Song, X.; Amro, N. A.; Liu, G. Y. Langmuir 1999,
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(42) Mu¨ller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; McEuen, P. L.; Schultz, P.
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