AFM-induced amine deprotection: Triggering localized bond cleavage by application of tip/substrate voltage bias for the surface self-assembly of nanosized dendritic objects

Article abstract of DOI:10.1021/ja047774q

An α,α-dimethyl-3,5-dimethoxybenzyloxycarbonyl (DDZ)-protected amine monolayer can be selectively deprotected by the application of a voltage bias from a conducting AFM tip to afford localized nanoscale patterns that can be visualized by self-assembly of

Full text of DOI:10.1021/ja047774q

Published on Web 06/16/2004
AFM-Induced Amine Deprotection: Triggering Localized Bond Cleavage by
Application of Tip/Substrate Voltage Bias for the Surface Self-Assembly of
Nanosized Dendritic Objects
Zachary M. Fresco, Itai Suez, Scott A. Backer, and Jean M. J. Fre´chet*
Department of Chemistry, UniVersity of California Berkeley, Berkeley, California, 94720-1460
Received April 17, 2004; E-mail: frechet@cchem.berkeley.edu
Surface modification has been extensively studied because of
its great utility in applications ranging from lithography1 to
bioanalysis.2 Surface functionalization with an amine monolayer
is especially popular because of the high reactivity of amine surfaces
toward both covalent attachment3 and ionic self-assembly.4 Pat-
ternwise fabrication of amine monolayers has been previously
accomplished on the micrometer scale using light,5 and on the
nanometer scale using classical e-beam lithography6-12 or AFM.13
We wish to report the localized chemical activation of a protected
amine surface by the application of a voltage bias between the tip
of an atomic force microscope (AFM) and a silicon substrate. The
latent amine patterns are visualized by the self-assembly of dendritic
carboxylates.
The fact that cleavage of the DDZ group proceeds via ionic,
rather than radical, intermediates was a determining factor in our
choice of this group. We postulated that the intense local electric
field that can be produced by an AFM tip would exert a force on
an already polarized bond. Therefore, an ionic reaction would be
more likely to be triggered by this stimulus than a reaction that
proceeds via radical intermediates, such as the photochemical
deprotection of 2-nitrobenzylcarbamates. Furthermore, in contrast
to 2-nitrobenzylcarbamates, which produce both carbonyl and
nitroso groups, the only byproducts of DDZ cleavage are 3,5-
dimethoxy-R-methylstyrene and carbon dioxide.15 While both
carbonyl and nitroso compounds are prone to recombination with
amines, 3,5-dimethoxy-R-methylstyrene is not reactive.
The R,R-dimethyl-3,5-dimethoxybenzyloxycarbonyl (DDZ) group
has been used as a photocleavable amine-protecting group in peptide
synthesis,14,15 as well as a photobase generator in lithography.16
The commonly accepted mechanism for photochemical deprotection
is heterolytic bond cleavage followed by elimination of the
carbocation to release 3,5-dimethoxy-R-methylstyrene, carbon
dioxide, and a primary amine.16 The structure of our monolayer
precursor 117 and mechanism of photochemical cleavage is shown
in Figure 1.
The protected amine monolayer was prepared by immersing a
freshly cleaned and hydroxylated silicon wafer in a solution of 1
in dry toluene. The monolayer was pattered using an AFM operating
in tapping mode with a conducting tip. Typically, the amplitude
set point was reduced to ca. 10% of the imaging amplitude, and
the tip was translated across the surface (1-10 µm/s) while applying
a +12 V potential to the sample and grounding the tip. To avoid
local anodic oxidation of the underlying substrate,18 all of the
patterning was performed under an inert atmosphere. After pat-
Figure 1. Structure of monolayer precursor (1), dendron (2), dendronized polymer (3), and schematic of surface deposition procedure. (a) Heterolytic bond
cleavage. (b) Release of carbon dioxide and 3,5-dimethoxy-R-methylsytrene to reveal primary amine. (c) Ionic self-assembly of dendritic macromolecules
forming a relief pattern.
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J. AM. CHEM. SOC. 2004, 126, 8374-8375
10.1021/ja047774q CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
rial.20 DSC shows that this polymer undergoes complete and
irreversible cross-linking at 140 °C. Deposition of polymer 3
produced lines approximately 4 nm in height (Figure 2b). The
theoretical length of this polymer (all-trans conformation) is 10 nm.
The production of 4 nm relief features from a patterned monolayer
constitutes a form of “physical amplification”, distinct from the
chemical amplification process well-known in conventional lithog-
raphy.21
In summary, we have achieved patternwise fabrication of a
primary aliphatic amine surface on the nanometer scale and shown
that the amines are active toward ionic self-assembly. The relation-
ship between the size of the macromolecule and the height of the
resulting features is strong evidence that the features are indeed
composed of these molecules. We expect that this process will be
applicable wherever amines can be used to selectively assemble
materials onto a surface. Specifically, the use of amines for
deposition of resist materials for lithography, electroless metaliza-
tion, device assembly, and protein binding can all be applied on
the nanometer length scale. This work potentially provides a method
for sequential patternwise deposition of single molecules and may
be applied to the surface assembly of more complex molecular
machines.
Acknowledgment. Financial support of this work by SRC/
DARPA and NSF (SINAM) is acknowledged.
Figure 2. AFM images and height profiles. (a) Lines made from dendrimer
(2) are 25 nm wide and 1 nm tall. (b) Lines made from dendronized polymer
(3) are 35 nm wide and 4 nm tall.
Supporting Information Available: Experimental procedures and
compound characterization data (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
terning, the substrate was placed in a solution of the appropriate
dendritic macromolecule for 24 h, rinsed thoroughly, and returned
to the AFM for imaging (tapping mode). The width of the amine
lines is affected by the amplitude set point, scan rate, and applied
voltage bias. AFM images of lines formed from dendrimer 2 and
dendronized polymer 3 are shown in Figure 2, parts a and b,
respectively. The larger molecule produces wider features because
of the flexibility of the chain as well as tip convolution effects. An
advantage of this process is that the deprotection and assembly
sequence does not affect the carbamate in the unexposed regions.
Therefore, it should be possible to repeat the process and deposit
several distinct materials onto a surface with nanometer scale
resolution.
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