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Surface Reactions On Demand Electrochemical Control of SAM-Based Reactions.

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DOI: 10.1002/anie.200601502
Tailor-Made Surfaces
Surface Reactions On Demand: Electrochemical
Control of SAM-Based Reactions**
Insung S. Choi* and Young Shik Chi
electrochemistry · microelectrodes · monolayers ·
self-assembly · surface chemistry
elf-assembled monolayers (SAMs)
are well-ordered, single-molecule-thick
structures in two dimensions, which
form spontaneously at interfaces (usually at solid surfaces).[1] SAMs have
been used for controlling physical properties of interfaces, such as wetting,
adhesion, lubrication, and corrosion, as
well as understanding fundamental aspects of interfacial phenomena. In addition, SAMs comprise an excellent platform for generating two-dimensional
micro- and nanostructures. For example,
SAMs are widely utilized for the spatially resolved immobilization of biomolecules (i.e., DNAs, peptides, and
polysaccharides) and cells onto surfaces.
Although useful in these various domains, these applications are based on
the “static” property of SAMs. In other
words, SAMs (especially the head
groups of SAMs) are designed to meet
the criteria of the applications because
the head groups intimately, but “statically”, interact with the outside environment. SAMs execute their predetermined roles once they are formed on
surfaces, these roles are exemplified by
corrosion barriers, etching masks, and
recognition of biological entities.
One of the next challenges in the
study of SAMs was how to induce
surface reactions only when needed
(“surface reactions on demand”). These
surface reactions on demand have sev-
eral implications in surface science: 1)
we can generate dynamic surfaces (in
other words, stimuli-responsive surfaces) in which the physical, chemical, and
biological properties of surfaces are
reversibly tuned at our disposal as we
dynamically tailor the functional groups
that interact with the environments, and
2) we can site-selectively localize chemical reactions at surfaces, leading to
independent addressability of surface
reactions. Two reaction methods would
be obvious candidates for surface reactions on demand: photochemically induced and electrochemically induced
reactions. Photochemical induction can
be combined with photolithographic
techniques. The “static” property of
the SAMs manifests in the fabrication
of well-known DNA microarrays in
which photolabile protecting groups
are site-selectively removed by photolithography. Recently, a light-induced
Wolff rearrangement of diazomethylcarbonyl groups was combined with photolithography to generate micropatterns.[2]
In addition to the static surface, dynamic
changes in the water wettability of
surfaces were demonstrated by the use
of a light-induced, reversible cis–trans
transition of azo groups.[3]
On the other hand, electrochemical
induction has some advantages over
other methods for surface reactions on
demand. It is easily incorporated into
electronic devices because the electrochemically induced surface reaction involves an electron transfer between a
surface (e.g., gold and silicon) and a
reaction site. The electrochemical control of surface reactions at electrodes
could generate independently addressable electrodes because a reaction can
be induced electrochemically on a designated electrode. Furthermore, dynamic control of surface properties could be
achieved easily by electrical potentials
and reversible oxidation–reduction reactions.
Initial attempts to use electrochemistry for SAM-based reactions were at
the early stage in the development of
surface reactions on demand. These
attempts were based on simple desorp-
[*] Prof. Dr. I. S. Choi, Y. S. Chi
Department of Chemistry
Daejeon 305-701 (Korea)
Fax: (+ 82) 42-869-2810
[**] Figure 1 and Scheme 2 b were kindly provided by Prof. Langer and Prof. Kwak.
Figure 1. Electrochemically controlled transition between straight (hydrophilic) and bent (hydrophobic) molecular conformations on gold. MHA: 16-mercaptohexadecanoic acid.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4894 – 4897
tion/adsorption of alkanethiols on gold.
However, the methods were applied to
the construction of individually addressed electrodes.[4] For example, hexadecanethiol was electrochemically desorbed from a gold microelectrode and a
second thiol was adsorbed onto the
electrode.[4a] Electrochemically directed
formation of self-assembled monolayers
(SAMs) is another interesting approach.
Freund, Ferguson, and co-workers selectively modified one electrode in the
close proximity of another by electrochemical oxidation of alkyl thiosulfates
to gold-reacting species, such as alkylsulfide radicals and disulfides.[4b] In
addition, the accelerated formation of
SAMs of alkanethiols was achieved by
using a cathodic polarization.[4c] The
property of SAM-coated surfaces was
also dynamically tuned by electrochemical control. Langer and co-workers
demonstrated that electrical potential
could be used for dynamically controlling conformational transitions of surface-confined alkanethiols. To establish
sufficient spatial freedom for each alkanethiol at the surface, they elegantly
designed a low-density SAM of 16mercaptohexadecanoic acid. They reported that electrochemically controlled
conformational reorientations of singlelayered alkanethiols at the surface were
amplified into a macroscopically detectable change in wettability (Figure 1).[5]
The concepts above are limited to
simple alkanethiols. Although SAMs of
more complicated, but technologically
useful alkanethiols (and other self-assembling molecules) could also be generated, this approach requires a separate
solution-based synthesis of the required
molecules. In principle, the separate
synthesis of self-assembling molecules
in solution gives an opportunity to
introduce virtually any functional group,
but in practice, this approach requires
cumbersome syntheses and shows a
limited compatibility with functional
groups. In other words, practically, it is
not simple to introduce the large, complex molecules and ligands needed for
wider applications. In this respect, it is
preferable to perform surface reactions
after the formation of SAMs, which
would be one of the solid approaches
to surface reactions on demand.
The first demonstration of electrochemically induced, surface reactions on
Angew. Chem. Int. Ed. 2006, 45, 4894 – 4897
demand was Mrksich and co-worker;s
quinone chemistry (Scheme 1).[6] They
formed a SAM that presented hydroquinone (HQ) and studied electrochem-
ical characteristics of the SAM. Cyclic
voltammetry showed that HQ underwent oxidation at 220 mV to give quinone (Q) and reduction at 150 mV,
Scheme 1. a) Electrochemical oxidation of hydroquinone to quinone, and introduction of the
RGD peptide by a Diels–Alder reaction. b) Selective release of the RGD ligand from the
monolayer presenting the O-silyl-hydroquinone by electrochemical oxidation and introduction of
a second ligand by a Diels–Alder reaction.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
and the oxidation–reduction was reversibly controlled by voltages.[6a] The electrochemically generated quinone has
been utilized for immobilizing various
functional molecules, such as small molecules, peptides, and polysaccharides,
through Diels-Alder reactions with cyclopentadiene (Cp)-containing molecules. A salient demonstration would
entail electrochemically controlled
“turning-on” of cell migration at surfaces by a surface reaction on demand for
the attachment of a cell-adhering tripeptide, Arg-Gly-Asp (RGD) (Scheme 1 a).[6b] This report was the first
example of an electroactive substrate
that uses electrical stimulation to modulate the activity of ligands. The same
reaction was subsequently applied to the
pattern generation of two different cell
types.[6c] Mrksich and co-workers also
reported the release of one ligand and
the subsequent immobilization of a
second ligand by surface reactions on
demand (Scheme 1 b).[6d] RGD-containing O-silyl-HQ was oxidized at the
surface. Upon the oxidation of HQ to
Q, the silyl ether was hydrolyzed and the
RGD ligand was selectively released.
After the release of the RGD ligand, the
electrochemically generated Q was coupled with another Cp-containing ligand.
The electrochemical oxidation of
HQ to Q was also used for site-selective
conjugation of biotin onto silicon microand nanoelectrodes.[7] Heath and coworkers demonstrated that the surface
reaction on demand could be used for
differentiating 100-nm wide Si nanowire
electrodes (separated by 300 nm). They,
for the first time, applied the surface
reaction on demand to individual electrodes. Similarly, the array of indium
oxide nanowires was selectively functionalized by the oxidation of HQ to Q
and subsequent bioconjugation.[8]
Kwak and co-workers elegantly utilized the HQ-based, surface reaction on
demand to selectively functionalize individual electrodes (Scheme 2).[9] They
oxidized the HQ group, which was used
as a protecting group for biologically
active biotin, to a hydrolysis-labile Q
group. Through this process, a biologically inactive surface was controllably
transformed to a biologically active surface.
Recently, another ingenious approach to surface reactions on demand
Scheme 2. a) Electrochemical oxidation for the
generation of bioactive biotin. b) An optical
micrograph of the independently addressed,
interdigitated microelectrode array. The color
change resulted from catalytic precipitation of
4-chloro-1-naphthol by horseradish peroxidase
that was site-selectively immobilized to the
was reported. Instead of activating surface-bound reactants, such as HQ, Chidsey, Collman, and co-workers electrochemically activated a catalyst for copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition (Sharpless “click” chemistry) (Figure 2).[10] The click chemistry
between azido and acetylenyl groups,
leading to the formation of 1,2,3-triazoles,[11] has been applied to SAM-based
reactions[12] because the reaction is fast
and proceeds under mild aqueous conditions. To demonstrate individual addressability, they brought an electrode
in contact with a solution containing
copper(II) species, and biased the electrode at 300 mV versus a Ag/AgCl/
saturated NaCl reference electrode
(roughly 300 mV negative of the copper(II/I) standard potential). This potential
ensured that copper(I) was formed at
the electrode surface. The electrode was
coated with a SAM terminating in azido
groups. Therefore, upon the selective
electrochemical induction of copper(I),
acetylenyl-containing compounds could
be coupled with the azido groups at the
electrode through the use of click
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
chemistry. The electrical potential of
+ 250 mV was applied to the other
electrodes for deactivating the catalyst
near the electrodes. By using electroactive ethynylferrocene, they confirmed
a selective coupling at the designated
electrode. This approach is rather simple
and advantageously differentiated from
the activation of reactants in some
aspects. The stoichiometric amount of
electroactive reactants is not necessary
because a catalyst is activated. In the
system designed by Chidsey, Collman,
and co-workers, a single catalyst amplifies the response due to a single electron. In addition, because surface-bound
reactants need not be electroactive,
design flexibility in immobilizing molecules is increased. It is worth noting that
this work is the first report on the
catalytic version of SAM-based, surface
reactions on demand.[13]
In summary, recent advances in the
study of SAMs promise wider applications of SAMs to many areas in science
and technology. Especially SAM-based
surface reactions on demand would find
applications in the construction of dyAngew. Chem. Int. Ed. 2006, 45, 4894 – 4897
Figure 2. Selective functionalization of independently addressed microelectrodes. Copper(II)
was electrochemically reduced to copper(I), and copper(I) catalyzed the click chemistry between
surface-bound azido groups and acetylenyl-containing compounds (ethynylferrocene in the
namic surfaces and multianalyte-sensor
arrays for multiplexing. In addition to
the applications to electrode- and nanowire-based sensors, surface reactions on
demand can also be applied to other
technologically important areas, such as
cantilever-based sensors.[14] Electrochemically induced, surface reactions
on demand are one of the strong candidates for the above-mentioned applications. They are largely insensitive to the
shape and spatial extent of surfaces to
be differentiated. All manner of threedimensional structures should be differentiable as long as they have contact
with a fluid electrolyte. Therefore, individual addressability of surface reactions could be achieved by electrochemical control. One problem that remains
to be solved is how one performs
parallel surface reactions on demand
because the serial processing might be
time-consuming and produce non-negligible wastes.
Angew. Chem. Int. Ed. 2006, 45, 4894 – 4897
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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base, sam, reaction, surface, electrochemically, control, demand
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