Boronate-functionalized polypyrrole as a new fluoride sensing material

Article abstract of DOI:10.1039/a905136h

The anodic oxidation of boronate-substituted pyrrole led to a fluoride anion-responsive electroactive polymer film.

Full text of DOI:10.1039/a905136h

Boronate-functionalized polypyrrole as a new fluoride sensing material
M. Nicolas, B. Fabre* and J. Simonet
Laboratoire d’Electrochimie Moléculaire et Macromoléculaire (Unité Mixte de Recherche n°6510 associée au
CNRS), Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France. E-mail: fabre@univ-rennes1.fr
Received (in Cambridge, UK) 25th June 1999, Accepted 8th July 1999
The anodic oxidation of boronate-substituted pyrrole led to
a fluoride anion-responsive electroactive polymer film.
colorless oil in 23% overall yield after purification by column
chromatography (CH2Cl2). This was deprotected electro-
chemically at 22.45 V (vs. 1021 M Ag+/Ag) using pyrene as a
redox mediator to give 1b (yield 58%).†
The modification of the electrochemical response of a system in
the presence of a specific ion or molecule can be considered as
a recognition event. In the case where such a system is deposited
onto an electrode surface, this mode of recognition can be
exploited for the construction of electrochemical sensors. As an
example, numerous recent reports have focused on the sensing
properties towards cations of some polyether or crown ether
functionalized redox-active conjugated polymers.1–7 In con-
trast, anion recognition by this class of polymers derivatized
with a suitable receptor has been much less developed.8,9 We
have tested a novel route to anion-responsive electrodes based
on a conjugated polymer substituted by a boronate group. Here,
the boron-containing moiety was used as a sensing element
owing to the strong interaction of this heteroatom with hard
bases like fluoride giving rise to specific orbital changes from
sp2 to more stable sp3. So, the possible electronic and/or
conformational changes induced by the boron–fluoride binding
could be detected from the modification of the electrochemical
response of the conjugated polymer. Although numerous
boronic acid-based anion and sugar receptors have been
developed by Shinkai and co-workers,10–13 their immobiliza-
tion onto an electrode surface has not been investigated so far.
The incorporation of boron atoms in the backbone of conjugated
polymers has already been described,14,15 but in our case we
chose to attach the boronate group to the conjugated polymer by
a spacer of appropriate length.
The electropolymerization of 1b (1022 M) was carried out in
thoroughly dried MeCN containing 1021 M Bu4NPF6 as the
supporting electrolyte. The doped film was potentiodynam-
ically electroformed with an anodic limit close to the irreversi-
ble oxidation peak potential of monomer, e.g. 0.75 V [Fig. 1(a)].
Its electrochemical response in an hydroorganic medium19
(H2O–MeCN) showed a stable reversible redox system at
20.11 V vs. SCE (DEp = 40 mV at 100 mV s21) corresponding
to the p-doping/undoping process [Fig. 1(b)]. The doping level
deduced from the cyclic voltammetry curve was found to be in
the range 0.20–0.30 depending on the film thickness.
The addition of KF led to the decrease of this system together
with the emergence of a new redox system at 20.37 V, the
magnitude of which increased with F2 concentration. For a film
To achieve this purpose, conjugated polymer films were
electrodeposited in one step onto a platinum (diameter 1 mm)
electrode surface via the anodic oxidation in MeCN of the
functionalized pyrrole. 1-(Phenylsulfonyl)-3-vinylpyrrole was
synthesized in three steps from 1-(phenylsulfonyl)pyrrole using
literature procedures.16,17 Hydroboration with diisopinocam-
pheylborane18 (Ipc2BH) prepared from (+)-a-pinene and bor-
ane–dimethyl sulfide complex gave the diethyl boronate
derivative (Scheme 1). Treatment with pinacol gave 1a as a
Scheme 1 Reagents and conditions: i, Ipc2BH (1.2 equiv.), THF, 240 °C,
then addition of the vinylated compound, room temp., 24 h; ii, acetaldehyde
(10 equiv.), 0° C, 24 h; iii, pinacol (1 equiv.) in THF, room temp., 24 h; iv,
Fig. 1 (a) Successive cyclic voltammograms of 1b (1022 M) in MeCN +
1021 M Bu4NPF6 (final electropolymerization charge: 70 mC cm22). (b)
Electrochemical response of corresponding poly(1b) in H2O–MeCN (1+1
v/v) + 5 3 1021 M LiClO4 in the absence and presence of 2 mM KF.
pyrene (20%), reduction at 22.45 V (vs. Ag+/Ag) in MeCN + 1021
M
Bu4NClO4.
Potential scan rate: 100 mV s21
.
Chem. Commun., 1999, 1881–1882
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boron site, must be now optimized in order to obtain sensitive
detection operating over a large F2 concentration range.
Drs M. Vaultier, J.-F. Pilard and G. Marchand are fully
acknowledged for their help in the synthesis of monomers. The
‘Région de Bretagne’ is also acknowledged for a research grant
to one of the authors (M. N.).
Notes and references
† Selected data for 1b: dH(CDCl3) 1.04 (t, 2 H), 1.15 (s, 12 H), 2.55 (t, 2 H),
6.11 (dd, 1 H), 6.48–6.60 (m, 2 H), 8.01 (s, 1 H); dC 21.60, 25.22, 83.38,
108.66, 114.94, 117.90, 126.74.
1 B. Fabre and J. Simonet, Coord. Chem. Rev., 1998, 180, 1211.
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Fig. 2 Calibration curve for F2 in H2O–MeCN (1+1 v/v) + 5 3 1021
M
LiClO4 obtained with a poly(1b)-coated platinum (8 3 1023 cm2) electrode
(electropolymerization charge: 280 mC cm22). I and Io are the currents
measured at 20.42 V on the voltammetric curves (100 mV s21) in the
presence and absence of KF, respectively.
7 F. Sannicolo, E. Brenna, T. Benincori, G. Zotti, S. Zecchin, G. Schiavon
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electropolymerized using 70 mC cm22, only the second system
was observed for a large F2 concentration, e.g. 2 mM [Fig.
1(b)]. It must be pointed out that the electrochemical response of
poly(1b) was not changed when F2 was replaced by Cl2 or Br2
up to a tested concentration of 20 mM. Moreover the interaction
with F2 was specific for the boronate-containing polymer, as
the voltammetric response of an unfunctionalized polymer film
was found to be unmodified in the presence of this anion. Thus,
all these results are consistent with a selective electrochemical
recognition of F2 by poly(1b). The negative shift of the redox
potential indicates that the F2 bound polymer was easier to
oxidize. Such a result could be explained by a stabilization of
F2 bound to the boron atom by the positive charges along the
oxidized polymer backbone. The effect of the F2 concentration
on the current intensity of the system assigned to the F2 bound
polymer is shown in Fig. 2. As can be seen, an initial linear plot
at low F2 concentrations ( < 0.4 mM) is followed by a plateau
for larger concentrations indicating that all the binding sites in
the film were occupied. The slope and the linearity range of the
first part were found to be dependent on the film thickness. As
a matter of fact, the sensitivity to F2 was higher for thin films,
as already observed for other sensory materials.20
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19 In water, the electroactivity of poly(1b) was weaker and less reversible
than that exhibited in organic electrolyte, probably owing to the
presence of the hydrophobic pinacol moiety.
20 See for example: A. C. Ion, J. C. Moutet, A. Pailleret, A. Popescu, E.
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Finally, this work constitutes a rare example of function-
alized conducting polymer-based selective anion recognition.
Some parameters, such as the film thickness, and the nature and
length of the spacer arm between the monomer unit and the
Communication 9/05136H
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Chem. Commun., 1999, 1881–1882