Electrografting via diazonium chemistry: From multilayer to monolayer using radical scavenger

Article abstract of DOI:10.1021/cm401512c

A simple strategy to avoid the formation of polyaryl layer during the functionalization of carbon surface by diazonium electroreduction is presented. The approach proposes to directly act on the polymerization mechanism by the use of a radical scavenger. The kinetic gap between the surface coupling and the multilayer formation is exploited to prevent the growth of the layer without interfering with the grafting. The well-known 4-nitrobenzenediazonium electrografting was used to demonstrate the possibility of reaching a monolayer surface coverage with an excess of DPPH (2,2-diphenyl-1-picrylhydrazyl). Experimental conditions were varied to validate the efficiency of the grafting limitation and the radical capture was confirmed by isolation of the aryl radical/DPPH coupling product.

Full text of DOI:10.1021/cm401512c

Article
pubs.acs.org/cm
Electrografting via Diazonium Chemistry: From Multilayer to
Monolayer Using Radical Scavenger
Thibaud Menanteau, Eric Levillain, and Tony Breton*
́
MOLTECH-Anjou, Universite d’Angers, UMR CNRS 6200, 2 Boulevard Lavoisier, 49045 Angers, France
S
* Supporting Information
ABSTRACT: A simple strategy to avoid the formation of polyaryl layer during the functionalization of carbon surface by
diazonium electroreduction is presented. The approach proposes to directly act on the polymerization mechanism by the use of a
radical scavenger. The kinetic gap between the surface coupling and the multilayer formation is exploited to prevent the growth
of the layer without interfering with the grafting. The well-known 4-nitrobenzenediazonium electrografting was used to
demonstrate the possibility of reaching a monolayer surface coverage with an excess of DPPH (2,2-diphenyl-1-picrylhydrazyl).
Experimental conditions were varied to validate the efficiency of the grafting limitation and the radical capture was confirmed by
isolation of the aryl radical/DPPH coupling product.
KEYWORDS: Surface functionalization, DPPH, diazonium reduction, radical capture
In this way, the blocking of aryl-reactive positions by bulky
substituents9 and the use of protected diazonium10a−d were
reported to generate very thin films and demonstrate the
validity of such structural approach to act on the grafting
process.
In the present paper, we propose to directly intervene on the
grafting mechanism using a radical scavenger to control, or
prevent, the polymerization of the electrogenerated aryl radicals
on a carbon electrode. Previous works reported the use of
radical scavengers to evidence mechanistic aspects of the
grafting, and they noted the surface modification cannot be
completely suppressed.11a,b We speculated that it should be
possible to exploit the difference of kinetics between the
radical/surface (grafting) and radical/aryl (polymerization)
reactions. Because the reactive radical species are produced at
the electrode−solution interface, the kinetics of the coupling
INTRODUCTION
■
Electrografting of organic structures using diazonium chemistry
is a recognized method for obtaining functionalized surfaces.1
Various conducting materials such as metals2 and carbon3 can
be modified in aprotic or aqueous media, and provide
covalently tethered layers.4 The efficiency of this grafting
process rests upon the high reactivity of the aryl radicals
produced at the electrode−solution interface.5 This reactivity
leads to the generation of polyaryl layers via the radicalar attack
of already grafted aryl species on the surface.6 Consequently,
the method routinely provides disordered organic films having
a thickness varying from 1 to 50 nm.2,7 The lack of control, in
terms of thickness and organization, represents the major
drawback of this elegant and versatile technique. However,
examples of monolayer grafting were reported by careful
control of the consumed charge8a,b or by using viqueous ionic
liquid to minimize the diffusion during the electroreduction.8c,d
Since the past few years, the main approach developed to avoid
the formation of multilayers and obtain controlled organic films
is based on the use of sterically hindered aryldiazonium cations.
Received: May 7, 2013
Revised: June 24, 2013
Published: July 1, 2013
© 2013 American Chemical Society
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Figure 1. First and second CV cycle recorded in CH3CN 0.1 nBu4NPF6 containing 1 mM of 4-nitrobenzenediazonium on a glassy carbon (GC)
electrode at 50 mV/s without DPPH (left) and with 1 mM of DPPH (right).
should be greatly favored compared to the polymerization
mechanism, which involves reactions further from the surface.
The introduction of an appropriate amount of radical scavenger
is expected to prevent the attack of grafted species by
electrogenerated aryl radicals, without interfering with the
direct grafting process.
nBu4NPF6 (0.1 M) was recorded without and with 2,2-
diphenyl-1-picrylhydrazyl (DPPH), used as radical scavenger
(Figure 1). A characteristic reduction behavior (peaks at +0.07
and −0.21 V) was observed in the absence of DPPH. The
strong electrode passivation recorded for the second and
subsequent cycles is consistent with the grafting process.5
When one DPPH equivalent is added, the reduction
corresponding to the anion formation (DPPH → DPPH−) is
observed at −0.06 V, superimposed on the diazonium
reduction peaks. The reversible oxidation of DPPH into
DPPH+ is also observed at +0.48 V (the CV of DPPH in
solution is presented in the Supporting Information, Figure
S1). A weak passivation is visible when repetitive cycles are
recorded, consistent with a thin coverage of the surface.
EXPERIMENTAL SECTION
■
4-Nitrobenzenediazonium tetrafluoroborate, 2,2-diphenyl-1-
picrylhydrazyl, and 4-bromobenzenediazonium tetrafluorobo-
rate were used as received from Aldrich. Glassy carbon
electrodes were obtained from Bioanalytical Systems Inc.
(Model MF-2012; diameter 3 mm). All potentials were
reported versus a Ag/AgCl (sat. KCl) reference electrode in
aqueous medium and Ag/AgNO3 (10 mM) in organic medium.
The glassy carbon electrode surface was cleaned by polishing
with Buehler 1 and 0.05 μm alumina slurry. After each polishing
step the electrode was washed with Nanopure water (18.2
MΩ·cm) by sonication. Prior to and after each electrochemical
derivatization, the electrode was sonicated in acetonitrile for 1
min. Modification of electrodes was as follows: modified glassy
carbon electrodes were prepared at fixed potential or by
recording cyclic voltammetry (CV) at 50 mV/s in deaerated
acetonitrile containing 0.1 M nBu4NPF6 and 1 mM diazonium
salt (20 mL cell). A potentiostat/galvanostat model VSP (from
Bio-Logic) monitored by ECLab software was used for the
electrochemical experiments. Electrochemical impedance spec-
troscopy (EIS) measurements were performed at open circuit
potential in 5 mM Fe(CN)63−/Fe(CN)64−, 0.1 M KCl. The
frequency range was 1 MHz to 2 Hz, using a 20 mV sine wave
amplitude. Electrochemical quartz crystal microbalance
(EQCM) measurements were performed with a QCA922
(Seiko-EG&G, Princeton Applied Research) associated to
carbon-coated quartz crystal (Biologic).
Figure 2 presents frequency−time curves recorded during the
modification of a carbon-coated quartz at a fixed potential of
Figure 2. Frequency variation vs time on a carbon quartz measured by
EQCM for the electrochemical grafting using 4-nitrobenzenediazo-
nium (1 mM) at a fixed potential of −0.5 V without DPPH (gray plot)
and with 1 mM DPPH (black plot). Data are fitted using the Langmuir
model (solid lines).
RESULTS AND DISCUSSION
■
4-Nitrobenzenediazonium is undoubtedly the most studied of
the functionalized diazoniums.4,7,12a−i Its simple structure
allows generation of densely packed film under very mild
potential conditions and the electroresponsive nitro group can
serve as immobilized redox probe to estimate the surface
coverage.7,12h Cyclic voltammograms on vitreous carbon
electrode in the presence of a millimolar solution of 4-
nitrobenzenediazonium tetrafluoroborate in acetonitrile/
−0.5 V using EQCM. The mass increase obtained without
radical scavenger fits with previously reported results on such
coated quartz, with a deposition rate of 0.39 mol·s−1·cm−2 at
the initial stage and an absence of stabilization, traducing a
continuous grafting.13 In the presence of one DPPH equivalent,
the frequency−time curve drastically differs. First, the grafting
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rate is lower (0.16 mol·s−1·cm−2 at initial stage), and second,
the deposited mass reaches a maximum and steady-state value
after 20 s. The calculated surface coverage extracted from this
mass plateau is 5.7 × 10−10 mol·cm−2, which corresponds to a
monolayer equivalent for an aryl structure.14
To further characterize the grafted layer, voltammetric study
of modified electrodes was performed in 0.1 M KOH to obtain
information from the electroresponsive nitrophenyl group.12e
Figure 3 shows cyclic voltammograms of a carbon electrode
roughness of our polished carbon. Furthermore, the peak to
peak separation between oxidation and reduction peaks varies
from 47 mV without DPPH to only 9 mV when DPPH is used.
This evolution, significant of a change in the electron-transfer
rate constant, could be interpreted as a decrease of the
electroactive layer thickness and/or a more favorable
organization of the nitrophenyl group when the modification
is achieved in the presence of DPPH.
A better understanding of the radical scavenger role in the
grafting process can be undertaken by varying the concen-
tration of DPPH in the deposition solution. Voltametric data
presented in Figure 4 show the electrode response for
Figure 3. First CV recorded in aqueous 0.1 M KOH on a modified GC
electrode at 50 mV/s. Grafting was achieved at −0.5 V for 30 s in the
presence of 1 mM 4-nitrobenzenediazonium without (gray line) and
with 1 mM DPPH (black line). Inset: second voltammetric cycle.
Figure 4. First CV cycle recorded in CH3CN 0.1 nBu4PF6 containing
1 mM 4-nitrobenzenediazonium on a GC electrode at 50 mV/s
without DPPH and with various DPPH concentrations.
modified at −0.5 V for 30 s without and with 1 mM DPPH.
During the first scan, the reduction of the Ar−NO2 function
can be visualized at −0.92 V (−0.84 V with DPPH). During the
positive scan, the two-electron reoxidation of Ar−NHOH into
Ar−NO is observed at 0.4 V. It is well-known that, in acidic
conditions, the nitrophenyl group (Ar−NO2) is partially
electroreduced into aminophenyl (Ar−NH2), leading to a
variable composition of Ar−NH2 (six-electron process) and
Ar−NHOH (four-electron process).12a,15 However, under our
alkaline conditions, the ratio between the integrated charges of
the Ar−NO2 reduction peak and Ar−NHOH reoxidation peak
(two electrons to Ar−NO) was close to 1.9 and independent of
the DPPH concentration (see the Supporting Information,
Figure S2). This feature indicates that the electroreduction
stopped at the Ar−NHOH stage, allowing a reoxidation of the
totality of the grafted species during the positive sweep. Under
such conditions, the surface coverage can be calculated from the
reversible Ar−NHOH/Ar−NO electrochemical system identi-
fied on the second cycle (Figure 3, inset).
increasing DPPH concentrations. The two DPPH reversible
redox systems superimposed to the diazonium reduction signal
are clearly observed. In Figure 5 (left), the surface coverage,
determined via the Ar−NHOH/Ar−NO redox system, is
plotted as a function of the DPPH concentration. The
evolution can be divided in two parts. For low concentrations
(<0.5 mM), a rapid decrease of the surface coverage is
observed, from 17 to 7 × 10−10 mol·cm−2. For higher DPPH
concentrations, the coverage reaches a quasi-stable value of 6.5
× 10−10 mol·cm−2. This behavior demonstrates the strong effect
of the radical scavenger on the grafting process but the most
interesting feature is that this effect cannot prevent the coupling
between the surface and the radicalar reactive species, even for
high DPPH concentration. The surface coverage reached is
close to a nitrophenyl monolayer equivalent.
Impedance spectroscopy measurements were achieved in the
presence of Fe(CN)63−/4− to obtain information on the charge-
transfer resistance (RCT) of the deposited layers. Figure 5
(right) shows the variation of the RCT for increasing
concentration of DPPH in the diazonium solution. The curve
looks very similar to the surface coverage one. For electrodes
modified in the presence of low DPPH concentration, the
electronic transfer is completely blocked (see the correspond-
ing voltammetric data in Supporting Information, Figure S3)
and then a rapid decrease of the RCT (from 104 to 103 Ω is
observed. At higher concentration, the RCT tends to reach a
stable value around 420 Ω (150 Ω for a bare electrode). The
passivation behavior observed for modification achieved
without radical scavenger is consistent with reported data for
electrografting of diazonium derivatives in similar conditions.12h
The decrease of the layer resistivity recorded for increasing
Without radical scavenger, a value of 17.5 × 10−10 mol·cm−2
is calculated. This surface coverage corresponds to a multilayer
deposition, in accordance with EQCM measurements. Note
that a higher coverage can be calculated from the gravimetric
data (29 × 10−10 mol·cm−2 for a 30 s deposition time), what
could be interpreted as an incomplete electroactivity of the
nitro species inside the layer, as previously reported.16 With a
millimolar concentration of DPPH in solution, a surface
coverage of 6.7 × 10−10 mol·cm−2 is found. This value is
consistent with a near monolayer and in fair agreement with
EQCM data. The slight difference observed with data obtained
by Downard and co-worker7 on flat pyrolytic photoresist film
(i.e., 2.5
0.5 × 10−10 mol·cm−2) could be assigned to the
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Figure 5. (Left): Nitrophenyl surface coverage on a modified carbon electrode as a function of the DPPH concentration. (Right): Charge-transfer
resistance calculated from EIS measurement in 5 mM Fe(CN)63−/4−, 0.1 M KCl. (Nyquist representations are shown in Supporting Information,
Figure S4).
DPPH concentrations confirms the effect of the radical
scavenger on the organic layer growth. The stabilization at a
very low RCT value (lower than values reported in the
literature12h,i) traduces the presence of a very thin layer when
more than one DPPH equivalent is used. The combination of
this result with the surface coverage variation prove that the
polymerization can be controlled and stopped independently of
the grafting process.
obtained with an excess of scavenger, which corresponds to a
monolayer equivalent. This noteworthy result can be explained
if we consider the coupling of the electrogenerated aryl radical
with the surface much faster than the attack of a grafted phenyl.
Considering the aryl radical/DPPH coupling is more efficient
than the aryl attack, it becomes possible to avoid polymer-
ization reactions without interfering with the grafting.
Confirmation of the nitrophenyl radical capture was obtained
by electrolysis at the grafting potential (−0.5 V) in the presence
of 1.9 mM 4-nitrobenzenediazonium and 3.8 mM DPPH using
a 8 cm2 electrode. Three milligrams of the coupling product
was isolated and identified as 2-[4-(4-nitro)phenyl]-2-phenyl-1-
picrylhydrazine (Scheme 1) (see the Supporting Information,
Grafting conditions were varied to prove the efficiency of the
radical scavenger independently of the grafting potential (Table
1). First, functionalizations were carried out at more anodic
Table 1. Surface Coverage of Modified GC Electrode as a
Function of the Potential Grafting Conditions and DPPH
Concentration
a
Scheme 1. Structure of 2-[4-(4-Nitro)phenyl]-2-phenyl-1-
picrylhydrazine
grafting
conditions
without DPPH,
1 mM DPPH,
2 mM DPPH,
×10−10 mol/cm2
×10−10 mol/cm2
×10−10 mol/cm2
b
E = 0.06 V
9.2
12.5
17
5.81
6.03
6.78
7.40
5.64
5.90
6.37
6.59
b
E = −0.21 V
c
one cycle
c
two cycles
17
a
4-Nitrobenzenediazonium concentration was 1 mM for modification
b
in CH3CN, 0.1 M nBu4NPF6. Fixed potential was applied for 30 s.
c
Figures S5−S7 for characterization data). The scavenging
mechanism is still under investigation but seems to proceed via
a radical attack on one of the positively polarized
diphenylamino end of DPPH, as already evidenced for phenol17
Cyclic voltammograms were recorded between 0.6 and −0.5 V at 50
mV/s.
potentials: −0.21 and 0.06 V, corresponding to the two
reduction peaks of the diazonium. Without DPPH, the grafting
occurs in both cases, and the surface coverage appears
correlated to the fixed potential, as previously reported in the
literature for several aryldiazoniums.4 When DPPH is added,
the coverage strongly decreases and reaches a quasi-constant
value, around 5.8 × 10−10 mol·cm−2. These experiments
confirm the efficiency of the polymerization control by the
DPPH, independently of the chosen potential (before and after
the DPPH reduction peak). Electrografting was also carried out
using cyclic voltammetry (one or two cycles). From a high
surface coverage (17 × 10−10 mol·cm−2), similar to the one
calculated for the modification at −0.5 V, the nitrophenyl group
surface concentration falls to 6.5 × 10−10 mol·cm−2 after DPPH
addition.
18
and NO2 radicals.
CONCLUSION
■
This work proposes a new approach for controlling the growth
of a layer electrografted via the diazonium reduction method.
We show that it is possible to considerably lower the amount of
attached organic species by the use of a radical scavenger in
solution. Moreover, it seems to be possible to easily obtain a
monolayer when the radical scavenger is introduced in excess.
To obtain such control, the electrogenerated aryl radicals are
supposed to undergo coupling according to the following
order: surface ≫ DPPH > grafted phenyl. The very high
reproducibility of electrode modifications (i.e., voltammetric
peak shape, surface coverage, and RCT measurements)
prepared in the presence of radical scavenger opens the way
for the preparation of well-controlled materials via diazonium
Those results demonstrate that the grafting conditions have
almost no influence on the Ar−NO2 surface concentration
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using 4-bromobenzenediazonium was achieved and confirms
the efficiency of radical control independently of the diazonium
substitution.
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ASSOCIATED CONTENT
* Supporting Information
CV recorded in KOH on modified electrodes with various
́
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1997, 13, 6805−6813. (i) Khoshroo, M.; Rostami, A. A. J. Electroanal.
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Chem. 2010, 647, 117−122.
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(13) Breton, T.; Belanger, D. Langmuir 2008, 24, 8711−8718.
DPPH concentrations (Figure S1), CV recorded in the
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presence of DPPH on GC electrode (Figure S2), CV recorded
3−/4−
in Fe(CN)6
on modified electrodes (Figure S3), Nyquist
(15) Yu, S. S. C.; Tan, E. S. Q.; Jane, R. T.; Downard, A. J. Langmuir
2007, 23, 11074−11082.
representations from modified electrodes (Figure S4), MALDI
spectrum of the coupling product (Figure S5), 1H NMR
spectrum of the coupling product (Figure S6), and 13C NMR
spectrum of the coupling product (Figure S7). This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) (a) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109,
8791−8798. (b) Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tony.breton@univ-angers.fr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the “Centre National de la
Recherche Scientifique” (CNRS France), the “Agence
■
́
Nationale de la Recherche” (ANR France), and the “Region
des Pays de la Loire” (France).
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