Unmasking photolithography: A versatile way to site-selectively pattern gold substrates

Article abstract of DOI:10.1002/anie.201107671

Surface chemistry: A new method for creating complex patterns on gold substrates is reported. Substrates were functionalized with nitroveratryl- protected carboxylic acid and hydroxy-terminated thiol monomers and patterned with a direct-write photolithography system to produce complex functional group gradients. In addition, two amine molecules were sequentially coupled on the substrate under spatial control (see picture). Copyright

Full text of DOI:10.1002/anie.201107671

Angewandte
Chemie
DOI: 10.1002/anie.201107671
Pattern Formation
Unmasking Photolithography: AVersatile Way to Site-Selectively
Pattern Gold Substrates**
Matthew J. Hynes and Joshua A. Maurer*
Patterned substrates with well-defined micro- and nanoscale
features are central to the development of a broad range of
applications and fields, including, but not limited to, micro-
electronics,[1] solar cell development,[2] and biotechnology.[3]
Typically, these applications require the functionalization of
inorganic substrates to meet the specific demands of an
application. While this is classically achieved using photo-
resist, lift-off techniques, and chemical etching, one of the
methods that has emerged for direct conjugation of active
molecules to substrates is thiol-terminated self-assembled
monolayers (SAMs) on gold, silver, copper, palladium, and
platinum substrates.[4] These substrates are especially useful
for biological applications and have been employed in a wide
variety of studies ranging from basic cell biology[5] to
biosensing.[6] SAMs are an ideal platform for direct function-
alization because the monomers bind covalently to substrates
through the thiol “head” group and self-assemble through van
der Waals packing interactions between adjacent long-chain
alkane “tail” groups. This packing orients the terminal
functional group to create a new interface with defined
chemistry. As a result, many techniques have been developed
to pattern SAMs, including soft photolithography,[7–10] photo-
oxidation,[9,11] and dip-pen nanolithography.[12] However, the
development of a single technique to create smooth gradients
of functional groups and for patterning multiple molecules on
a single substrate remains a major challenge in pattern
generation. For example, functional group gradients have
been generated by diffusing two molecules across a sub-
strate[13] or through photolithographic methods, including
gradient photomasks[7] and controlling light exposure.[8,14]
While molecular diffusion produces defined gradients, in its
most basic form, it does not allow for pattern generation.
Patterned gradients can be prepared using microfluidic
devices.[15] However, traditional polydimethylsiloxane devices
are susceptible to monomer leeching and solvent swelling that
can lead to pattern distortion and limits precise molecular
control. While gradient photomasks have previously been
used to produce functional groups on a surface,[16] the
fabrication of high-quality gradient masks is expensive.
Moreover, controlling the overall light exposure to a surface
has produced regions of varying functional group densi-
ties;[7,8,17] however these methods have failed to produce
a continuous gradient. Another major shortfall of all these
methods is the inability to provide a simple method for
patterning multiple molecules on a single substrate. By
utilizing a commercial direct-write grayscale photolithogra-
phy system, we have removed the need for the tradition
photomask which provides us with two distinct advantages.
First, we can produce smooth, complex functional group
gradients on a surface and, second, we are able to pattern
multiple molecules sequentially on the same substrate.
To produce continuous gradients using direct-write photo-
lithography, a glycol-terminated photoprotected carboxylic
acid monomer was synthesized, shown in Scheme 1 attached
to a gold substrate. The nitroveratryl photoprotecting group
was employed for our monomer, because it has sufficient
absorption and reactivity at 325 nm[18] to allow for rapid
photodeprotection by the He–Cd laser in our commercial
direct-write photolithography system. Gradient patterns were
created from 8-bit gray scale bitmap images with black
representing 100% exposure and white representing 0%
exposure (Figure 1A,C). These images were directly read by
the photolithography system and transferred to the photo-
protected SAM using beam scan direct-write photolithogra-
phy. In this mode, the laser power is tightly controlled using
a mirror mounted on a piezoelectric actuator and the surface
is scanned by a beam in one dimension.[18] The second writing
dimension is achieved with a high-resolution linear encoded
motorized stage. After gradient patterns were generated with
the direct-write system, they were imaged using scanning
probe microscopy (SPM).
[*] M. J. Hynes, Prof. J. A. Maurer
Department of Chemistry and
Center for Materials Innovation
Washington University in St. Louis
St. Louis, MO 63130 (USA)
E-mail: maurer@wustl.edu
[**] This work was supported by the National Institute of Mental Health
(grant number 1R01MH085495).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107671.
Scheme 1. Photodeprotection of glycol-terminated photoprotected car-
boxylic acid monomer at 325 nm.
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Figure 1. A and C) 8-bit images patterned by direct-write lithography
(the scale bar is relative to the laser intensity). B) The resulting KFM
image after deprotection. D) The adhesion channel for our patterned
surface using PeakForce QNM.
To image our gradient patterns, we have taken advantage
of the chemical differences that result upon photodeprotec-
tion. One of the most pronounced changes that we would
expect to occur upon photodeprotection is a change in surface
potential. Upon deprotection, we generate highly polar
carboxylic acids in a relatively hydrophobic monolayer back-
ground. As a result, we would expect regions with exposed
carboxylic acids to have a larger surface potential than the
background monolayer. Moreover, the observed surface
potential should be related to the number of free carboxylate
groups. Kelvin probe microscopy (KFM), an SPM technique,
allowed us to directly measure the surface potential. As
shown in Figure 1B, our gradient pattern is clearly visible
using KFM with white representing the relative amount of
carboxylic acid in a particular region, which is consistent with
the image patterned on the surface, Figure 1A. As expected,
the regions of high carboxylate concentration gave a larger
surface potential than the nonpatterned region.
While KFM allows us to clearly visualize our molecular
gradients, it requires the use of a relatively large SPM probes
(20 nm) compared to high-resolution probes (1–2 nm). How-
ever, by utilizing quantitative nanomechanical mapping
(QNM), we can image changes in the mechanical properties
that result upon photodeprotection using high-resolution
SPM probes. Upon photodeprotection, we remove a hydro-
phobic portion of the molecule that alters the mechanical
properties of the underlying structure with the magnitude of
the change being proportional to the amount of carboxylic
acids revealed. Using QNM SPM, we observe changes in
adhesion, dissipation, and deformation (Figure 1D and Fig-
ure S1 in the Supporting Information). In Figure 1D, the
patterned regions show a lower adhesion signal (darker) than
the nonpatterned regions due to a greater adhesion force
between the probe and the nitroveratryl monomer compared
to a free carboxylic acid. The observed images are a result of
Scheme 2. Two-molecule patterning scheme. 1) Pattern circle, 2) acti-
vate carboxylic acid with EDC/HOAt and couple hexaethylene glycol
amine, 3) pattern frame around the circle, and 4) activate carboxylic
acid with EDC/HOAt and couple tetraethylene glycol amine.
both changes in the bulk nanomechanical properties and in
tip-sample interactions that result from protecting group
cleavage.
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Another major advantage of our methodology is the
ability to pattern two molecules on the same surface. To
accomplish this, photolithography was carried out using two
distinct overlaid patterns with each pattern encoding the
spatial distribution of a different amine molecule as shown in
Scheme 2. Briefly, a circle was patterned on the substrate
producing free carboxylic acids, which were subsequently
activated with 1-ethyl-3-(3-dimethylaminopropyl) carbodi-
imid/1-hydroxy-7-azabenzotriazole (EDC/HOAt). Hexaethy-
lene glycol amine was then added to the solution, which
resulted in the molecule being coupled to the surface. After
coupling the first amine, a frame was patterned around the
circle. The newly formed carboxylic acids were activated with
EDC/HOAt and tetraethylene glycol amine was coupled to
the substrate. This multi-molecule pattern was designed to
highlight the flexibility of maskless photolithography, because
production of this image by traditional methods would
require at least two masks and the alignment would be
extremely difficult.
Patterned samples were characterized by imaging matrix-
assisted laser desorption–ionization time of flight mass
spectrometry (MALDI-TOF MS),[19] to ensure that only
site-selective deprotection and coupling had occurred. Imag-
ing was carried out using 100 mm spots spaced 250 mm apart
(center to center) with each spectrum consisting of 20
averaged spectra containing 50 shots. The resulting spectra
were then analyzed for the molecular weights of the coupled
products (1 and 2), the disulfide of the glycol (3), and the
photoprotected monomer, and the corresponding heat maps
were generated (Figure 2). As shown in Figure 2A, a bright
circle was produced when analyzing for 1 which is consistent
with our patterning scheme. A bright frame was produced
when analyzing for 2, Figure 2B. However there is a small
amount of 2 observed in the circle region, which is a result of
steric packing of the acids limiting activated ester formation.
In addition, we observed a loss in signal when analyzing for
the photoprotected monomer as shown in Figure 2C. This
result is expected because the photoprotected monomer is
converted to a coupled product in the patterned regions. The
versatility of this method is shown in Figure 2D, which is an
overlay of the two heat maps generated from the coupled
molecules 1 and 2 showing that two distinct molecules were
coupled to the same substrate in a site-specific manner using
our patterning methodology.
Here we have developed a versatile method for patterning
multiple molecules on a single substrate at defined molecular
densities using direct-write photolithography. Smooth molec-
ular gradients were straightforward to generate using gray-
scale photolithography and could be characterized using SPM
in both KFM and QNM modes. The alignment of two
molecules on a single substrate was implemented using
multilayer photolithography. In conclusion, the methodology
developed here is broadly applicable to the development of
patterned molecular substrates for materials applications and
is especially pertinent to the development of biosensors and
cell-based assays.
Figure 2. A,B,C) Heat maps generated after analysis for molecules 1, 2, and glycol-terminated photoprotected carboxylic acid monomer,
respectively. D) Overlay of A and B. E,F) Representative MALDI-TOF spectrums for the circle and frame region, respectively. G) Molecules analyzed
in the MALDI-TOF spectrum.
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Experimental Section
For detailed synthetic methods and procedures for the surface
preparation, analysis, and characterization see the Supporting Infor-
mation.
Two molecule attachment: Photoreactive SAMs for surface
coupling studies were prepared by soaking a gold slide (50 ꢀ Ti,
100 ꢀ Au) in an ethanolic solution of a 0.25 mm glycol-terminated
photoprotected carboxylic acid monomer and a 0.75 mm hydroxy-
terminated glycol monomer. The substrates were then photodepro-
tected using our direct-write photolithography system according to
the uploaded 8-bit file in beam scan mode. After photodeprotection,
slides were rinsed with ethanol, water, and ethanol, and dried under
a stream of nitrogen. The freshly exposed carboxylate groups were
then activated with 1 mL of 5 mm 1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide (EDC·HCl) in anhydrous CH2Cl2 for 1 min followed by
the addition of 1 mL of 2.5 mm 1-hydroxy-7-aza-benzotriazole
(HOAt) in anhydrous dimethylformamide/dichloromethane (DMF/
CH2Cl2) at a volume ratio of 1:1. The reaction proceeded with shaking
at 200 rpm for 15 min before 1 mL of 1.5 mm hexaethylene glycol
amine in anhydrous DMF was added. The reaction proceeded for an
additional hour. Slides were then removed from the reaction mixture
and rinsed with ethanol, water, and ethanol, and dried under nitrogen.
Three rounds of activation and coupling were carried out before the
process was repeated for tetraethylene glycol.
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Received: October 31, 2011
Revised: January 3, 2012
Published online: January 23, 2012
Keywords: monolayers · photochemistry ·
.
scanning probe microscopy · surface chemistry
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