Patterning of surfaces by oxidation of amine-containing compounds using scanning electrochemical microscopy

Full text of DOI:10.1002/anie.200903092

Angewandte
Chemie
DOI: 10.1002/anie.200903092
Surface Derivatization
Patterning of Surfaces by Oxidation of Amine-Containing Compounds
Using Scanning Electrochemical Microscopy**
Charles Cougnon,* Janine Mauzeroll,* and Daniel Bꢀlanger*
The microstructuring of surfaces with organic moieties is
important for the development of new analytical tools, but the
production of such surfaces by conventional photolitho-
graphic procedures remains challenging. Scanning electro-
chemical microscopy (SECM) has recently emerged as a
viable patterning alternative because it can be used both for
production and imaging of micro- and nanopatterned surfa-
ces.[1–3] SECM patterning of surfaces can occur either in the
direct mode or by using the tip as a source of reactive
species.[4,5] The first approach relies on the use of the SECM
tip as a microscopic auxiliary electrode, which constrains the
current lines near the sample that acts as a working electrode.
The faradaic current that flows through the tip-to-sample gap
induces an electrochemical reaction, which is restricted to a
localized volume beneath the tip. In the second approach, the
reactive species are locally electrogenerated at the SECM tip.
As the reactive species diffuse to the sample, the species can
be either regenerated after modification[6,7] (feedback mode
SECM) or consumed[8] (tip-generation/sample-collection
mode; TG/SC SECM).
complex organic microstructures because of rapid passivation
of the SECM tip. This problem occurs as the diazonium
Free-radical grafting methods constitute the more flexible
and versatile procedure for direct introduction of organic and
biological moieties in order to achieve the permanent
derivatization of surfaces.[9] The two main strategies consist
of the electrochemical reduction of diazonium salts[10] or
oxidation of amine-containing compounds.[11] Nevertheless,
free-radical grafting is not amenable to writing procedures
since the spontaneous derivatization of the entire surface
exposed renders the direct micropatterning of surfaces
impossible.[12,13] We recently reported a new procedure that
circumvents this problem, and demonstrated the utility of
diazonium salts for surface microstructuring (Figure 1a).[14]
This strategy is ideally suited for the writing of single organic
micropatterns, but is less adapted for the production of
[*] Dr. C. Cougnon
Unitꢀ de Chimie Molꢀculaire et Macromolꢀculaire (UCO2M)
UMR-CNRS 6011, Universitꢀ du Maine
Avenue Olivier Messiaen, 72085 Le Mans (France)
Fax: (+33)2-4383-3754
E-mail: charles.cougnon@univ-lemans.fr
Prof. J. Mauzeroll, Prof. D. Bꢀlanger
Dꢀpartement de Chimie
Universitꢀ du Quꢀbec ꢁ Montrꢀal, C.P. 8888
Succ. Centre-ville, Montrꢀal (Quꢀbec), H3C 3P8 (Canada)
[**] This work was supported by the National Science and Engineering
Research Council of Canada, the Canadian Foundation for Innova-
tion, the University of Le Mans and the CNRS. Juliette, Victor, and
Capucine Cougnon are also gratefully acknowledged.
Figure 1. Reaction systems suitable for SECM patterning of surfaces in
TG/SC mode based on a) reduction of diazonium salts or b) oxidation
of amine-containing compounds. c) Structure of the nitro precursor.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903092.
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7395

Communications
moieties produced in the tip-to-sample gap can be reduced
the electrogenerated amine confined in the thin-layer solution
is oxidized to produce the derivatized surface. Importantly,
previous reports have shown that only aliphatic amines bind
to the surface. The mechanism for surface derivatization by
oxidation of an aromatic amine is beyond the scope of this
work and further studies are required for a better under-
standing of this process. After derivatization by successive
potential sweeps over the range from 0.8 to ꢀ1.2 V, the GC
electrode was investigated by conventional CV after sonica-
tion in methanol for 10 minutes. After exposure to 1m TBAF/
THF solution for 5 minutes in order to deprotect the silyl
group, CV in sodium phosphate solution shows the redox
waves of the catechol functionality (Figure 3). The linear
both at the sample and at the SECM tip. To overcome this
problem, we have devised a simpler procedure based on the
oxidation of amine-containing compounds (Figure 1b). In this
strategy, an amine is generated at the SECM tip by reduction
of a nitro-containing compound (Figure 1c) and the substrate
is locally derivatized by oxidation of the electrogenerated
amine after diffusion in the interelectrode space.
This novel approach avoids tip passivation since the
generation of the amine and the derivatization of the
substrate occur by reduction and oxidation, respectively.
This procedure is expected to be suitable for complex
microstructuring of surfaces and would constitute an electro-
chemical alternative to the standard lithographic procedures.
The feasibility of producing organic micropatterns on
surfaces using the proposed amine-tip-generation/amine-
sample-collection sequence was investigated by thin-layer
cyclic voltammetry (CV). In the thin-layer configuration, the
starting nitro compound and all the electrogenerated species
are confined to a thin solution layer at the working electrode,
thus mimicking the conventional SECM setup. Figure 2 shows
Figure 3. Cyclic voltammograms recorded with a GC electrode derivat-
ized by oxidation of the amine that was electrogenerated in a thin-layer
cell. Conditions: sodium phosphate solution (pH 7.4) and 0.1m KCl.
Cyclic voltammograms were recorded before (c) and after (a)
exposure to 1m TBAF in THF for 5 min. Sweep rate: 50 mVsꢀ1
.
relationship between the anodic and cathodic peak currents
with scan rate over 0.005–0.5 Vsꢀ1 range indicates that the
catechol groups are confined to the electrode surface and can
be designated as a diffusionless system. Hence, with this
procedure, the GC electrode was successfully derivatized by
oxidation of the electrogenerated amine that is trapped
against the GC disk.
Based on the thin-layer CV results, it was expected that
similar surface derivatization with the SECM setup would
occur at short tip-to-sample distance (Figure S1 in the
Supporting Information). As such, the writing of organic
micropatterns with the SECM setup is achieved by generation
of an amine at a 7 mm diameter carbon tip (the ratio RG of the
radii of the insulating sheath over the conducting disc is 10),
which is oxidized at a gold sample after diffusion in the
interelectrode space. In the experimental setup, the SECM tip
was positioned 1 mm away from the substrate using conven-
tional feedback mode in nanopure water containing ferrocene
methanol (1 mm) and KCl (0.1m). After rinsing with nanopure
water and methanol, a solution of the nitro precursor (2 mm)
and LiClO4 (0.1m) in methanol was introduced without
retraction of the SECM tip. The potential of the SECM tip
Figure 2. Thin-layer cyclic voltammograms recorded with a GC elec-
trode. Conditions: methanol, 0.1m LiClO4 and 2 mm nitro precursor.
Potential was scanned from 0.8 to ꢀ1.2 V for the first (c) and fifth
(a) CV cycle. Sweep rate: 10 mVsꢀ1
.
typical thin-layer CV patterns recorded at a glassy carbon
(GC) disk. The cathodic peak located at ꢀ0.8 V versus Ag/
AgCl corresponds to the conversion of the nitro group into
the amino group and the first anodic peak at ꢀ0.3 V recorded
on the reverse potential sweep corresponds to the oxidation
of the arylhydroxylamine confined in the thin-layer solu-
tion.[15] The second anodic wave at 0.5 V, which appears only
when scanning the potential range in which the nitro
reduction occurs, correlates well with the anodic peak
obtained in conventional CV for the solubilized amine-
containing compound. In subsequent cyclic voltammograms,
and in accordance with passivation of the electrode, the
anodic peak current decreases and the peak potential shifts in
the anodic direction.
In light of recent studies that addressed surface derivati-
zation by oxidation of primary amines,[16,17] we postulated that
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Systems, 3 mm diameter; model MF-2012), the counter electrode was
was set to ET = ꢀ0.8 V for 5 seconds and the sample
concomitantly was polarized at ES = 0.8 V. Under these
conditions, the electrogenerated amine is oxidized just
beneath the SECM tip, and produces the local derivatization
of the gold sample. Following the SECM patterning of the
gold surface, the microstructure was imaged with a 10 mm
diameter platinum tip (RG = 5) in a solution of K4Fe(CN)6
(1 mm) and KCl (0.1m) in nanopure water.
a platinum wire and the reference electrode was Ag j AgCl (saturated
KCl). A potentiostat/galvanostat model VMP3 (Bio-Logic) moni-
tored by ECLab software was used for the electrochemical experi-
ments. TG/SC SECM measurements were performed with a 100 nm
ELPro Scan system (HEKA; model PG 340).
Received: June 9, 2009
Published online: August 27, 2009
The SECM image in Figure 4 shows a well-defined
structure, which is consistent with the formation of a 15 mm
wide pattern of organic moieties that passivates the sample.
An SECM line scan provides a better estimation of the lateral
resolution of the deposit (see Figure S2 in the Supporting
Information).
Keywords: amines · electrochemical microscopy ·
electrochemistry · micro-electrochemical patterning · oxidation
.
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Figure 4. SECM image of a micropattern of organic moieties deposited
on a gold sample by oxidation of an amine electrogenerated at the tip.
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Experimental Section
Electrochemical measurements were performed in a three-electrode
cell where the working electrode was a GC electrode (Bioanalytical
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