Materials chemistry approach to anion-sensor design

Article abstract of DOI:10.1016/j.tet.2004.08.060

A new approach to sensing of aqueous phosphate-related anions based on chromogenic conductive polymers is demonstrated. The anion-sensor affinity can be adjusted by externally applied voltage. Introduction of p-doping to the polymers results in augmented anion sensitivity, which is ascribed to the effect of synergy between low-level p-doping in a polythiophene polymer and hydrogen bonding. The chromogenic conductive polymer films display anion-specific changes in color, conductivity, and mass upon increasing concentration of anions. Graphical Abstract.

Full text of DOI:10.1016/j.tet.2004.08.060

Tetrahedron 60 (2004) 11163–11168
Materials chemistry approach to anion-sensor design
a
a
b,c
Pavel Anzenbacher, Jr.,a, Karolina Jursikova, Dmitry Aldakov, Manuel Marquez
*
and Radek Pohla,d
aCenter for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA
bLos Alamos National Laboratory, Chemistry Division, Los Alamos, NM 87545, USA
cKraft Foods, Inc., R&D, The Nanotechnology Lab, 801 Waukegan Road, Glenview, IL 60025, USA
dInstitute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2,
166 10 Prague, Czech Republic
Received 8 February 2004; revised 26 June 2004; accepted 19 August 2004
Available online 28 September 2004
Abstract—A new approach to sensing of aqueous phosphate-related anions based on chromogenic conductive polymers is demonstrated.
The anion-sensor affinity can be adjusted by externally applied voltage. Introduction of p-doping to the polymers results in augmented anion
sensitivity, which is ascribed to the effect of synergy between low-level p-doping in a polythiophene polymer and hydrogen bonding. The
chromogenic conductive polymer films display anion-specific changes in color, conductivity, and mass upon increasing concentration of
anions.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
manner is, by no means, a trivial task, and the few sensors of
this type, despite their elegant design and structures, are
difficult to synthesize and their practical use is limited.1 This
is because the focus on the supramolecular chemistry of
receptors usually results in a high cost of synthesis, decrease
in chemical stability, and a potential loss of the real-time
response due to slow dissociation of the substrate–receptor
complex.
The design of reliable optical sensors for inorganic anions
remains a challenge even after more than a decade of efforts,
and reliable and selective sensors that can potentially be
applied in real-life applications are still relatively rare.1
Very few optical sensors exist that display high selectivity
and reliability, and function in aqueous media without
interference from endogenous substrates. This is because,
compared to isoelectric cations, anions often display high
energy of hydration, tautomerism, and possess low surface-
charge density, features that make the binding of anions less
effective.2 In order for a sensor to be able to compete for an
anion in the presence of water, the corresponding receptors
must display high substrate selectivity and affinity. The
increase in the receptor–anion affinity may be achieved by
incorporating a positive charge in the receptor moieties.
Unfortunately, electrostatic interactions are nondirectional
and, as a result, all anions present in the medium are
attracted to the receptor. It is, therefore, desirable to include
weak directional interactions such as hydrogen bonding to
improve the selectivity in receptor–anion association.2b,3
The synthesis of receptors that comprise both positive
charges and hydrogen bond donors acting in a synergistic
The materials chemistry approach to the anion-sensor
design offers numerous advantages over the classical
supramolecular chemistry approaches, namely due to its
focus on the collective properties of materials that allow
harnessing the potential of materials for self-assembly, self-
organization, self-replication, and inter-component com-
munication. Additionally, the materials bulk properties offer
support and spatial definition to the material components
(e.g., preventing undesired receptor self-association). In
terms of the optical sensor development, the above features
are particularly useful in signal transduction because the
changes in conformation4 and inter-strand/component
association5 generate the optical output signal.
In the design of our sensor material, we decided to take the
material concept even further. We reasoned that the sensor
material could serve both as a receptor and signal transducer
at the same time. Perhaps, a conductive polymer with
incorporated receptors capable of hydrogen bonding, while
being able to be injected with positive charge (undergo an
Keywords: Anion; Sensor; Receptor; Conductive polymer; Chromogenic
material.
* Corresponding author. Tel.: C1-4193722080; fax: C1-4193729809;
e-mail: pavel@bgnet.bgsu.edu
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.08.060

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Figure 1. Schematic representation of the synergy between hydrogen-bonding and coulombic attraction in anion sensor materials based on a p-doped
conductive polymer.
adjustable degree of p-doping), could provide an inexpen-
sive alternative to the complex multi-feature sensors
carrying a positive charge as well as hydrogen-bond donors
(Fig. 1).
polymerization process, we also synthesized monomer 1C
and poly-1C with chloro-substituents blocking the pyrrole
alpha-positions.
The synthesis of monomers 1 and 1C is shown in Figure 3.
Treatment of 3,6-dibromo-1,2-benzenediamine (2)10 with
1,2-di(pyrrole-2-yl)ethane-1,2-dione (3)8a in acetic acid
yielded 5,8-dibromo-2,3-di(1H-2-pyrrolyl)quinoxaline (4).
Compound 4 was then coupled with (2,3-dihydrothieno-
[3,4-b][1,4]dioxin-5-yl)tributylstannane (6)11 to give mono-
mer 1. Monomer 1C was prepared by treatment of 4 with
sulfuryl chloride according to literature,12 providing
dichloro-dibromo intermediate 5. The intermediate 5 was
coupled with 6 using the same procedure for monomer 1.
2. Results and discussion
In order to utilize the synergy between hydrogen bonding
and coulombic interaction, we designed a conductive
polymer that combines both properties: the monomers are
hydrogen-bond donors and can bind anions, and the
resulting polymer can be p-doped, thus possessing a
positively charged backbone, which aids in anion binding.
We decided to utilize the propensity of thiophenes6 and
quinoxalines7 to yield conductive polymers. Additionally,
quinoxaline modified with two pyrrole units (DPQ) is
capable of binding anions via hydrogen bonding while more
or less pronounced color change occurs.8 Because DPQ
alone does not polymerize to yield a stable conductive
polymer, we modified it with two polymerizable ethylene-
dioxythiophene (EDOT)9 units to obtain monomer 1.
Monomer 1 was polymerized to yield poly-1, a sensor
material used in this study (see Fig. 2). In order to ensure
that the pyrrole moieties of DPQ are not involved in the
We have tested the monomers 113 and 1C affinity for anions
in DMSO with various aqueous anions.14 Monomer 1 and,
even more so, 1C show changes in color upon binding of
pyrophosphate2K (PP), fluoride, and, to a lesser extent,
dihydrogenphosphate, and cyanide anions. Analysis of
anion-induced changes in the absorption spectra of 1 and
1C allowed for determination of the respective binding
constants that were found to be in the order of 106 MK1 for
fluoride and pyrophosphate and less than 102 MK1 for
chloride and sulfate. In the case of phosphate anion, the
Figure 2. Schematic representation of the design of sensor materials poly-1 and poly-1C.

P. Anzenbacher, Jr. et al. / Tetrahedron 60 (2004) 11163–11168
11165
Figure 3. Synthetic sequence used to synthesize monomer receptor precursors 1 and 1C.
binding constants calculated for 1 and 1C were significantly
different (!102 vs. 106, respectively). Such an increase in
the affinity for dihydrogenphosphate in receptors com-
prising halogenated pyrroles such as DPQ was previously
described.15
In order to test the anion sensing ability of the polymers,
poly-1 and poly-1C films were titrated at a constant
potential of 0.0 V by anions added as tetrabutylammonium
(TBA) salts (5.0 mM, pHz6.5) in water while vis-NIR
spectra were recorded. As expected, the addition of fluoride,
pyrophosphate, and, in the case of poly-1C, also phosphate
anions into the cell containing the sensor films resulted in
gradual changes in the absorption spectra (Fig. 5). The
respective binding isotherms that show well-defined satu-
ration were obtained, and apparent affinity constants were
calculated.16 The low-level p-doping (at 0.70 V) and a
corresponding positive charge in the polymer resulted in a
dramatic increase of the anion affinity. For example, the
apparent affinity constant (dm3/mol) for poly-1 and
pyrophosphate recorded at 0.70 V was calculated as
KPP(0.7 V)Z260,000 MK1, while the constant recorded at
0.00 V was KPP(0.0 V)Z61,100 MK1. The examples of
such vis-NIR spectroscopic titrations are shown in Figure 5.
The top panel shows poly-1 titrated by aqueous pyrophos-
phate at 0.00 V (KPPZ61,100 MK1) (Fig. 5A), the center
panel shows poly-1 titrated by aqueous pyrophosphate at
0.70 V (KPPZ260,100 MK1) (Fig. 5B). The lower panel
(Fig. 5C) shows the result of a control experiment, where
poly-1 was titrated by aqueous chloride. It is known that the
Encouraged by the anion binding properties of 1 and 1C, we
decided to subject the monomers to oxidative electropoly-
merization to form a film of conductive poly-1 or poly-1C
deposited on an ITO-modified transparent electrode.14
Poly-1 or poly-1C were deposited as green-colored poly-
mers on the ITO surface (Fig. 4). This poly-1 or poly-1C
modified transparent working electrode was rinsed and
immersed in the spectroelectrochemistry cell equipped with
a stirrer, a miniature Ag/AgC reference electrode, and
platinum wire auxiliary electrode. DMSO solution contain-
ing 0.1 M tetrabutylammonium perchlorate (TBAP) was
used as a supporting electrolyte. Spectroelectrochemical
analysis of the poly-1 and poly-1C films allowed for
investigation of the polymer electronic structure and
determination of the polymer band gaps (Fig. 4). The
band gaps for poly-1 and poly-1C were determined as the
onset of p–p* absorption peaks and were found to be 1.39
and 1.36 eV, respectively.
Figure 4. Left panel: Electrochemical polymerization of 1 in 0.1 M TBAP in ACN at 100 mV/s with Ag/AgC used as a reference; Right panel:
Optoelectrochemical analysis of poly-1 at applied potentials (shown) varying with an increment of 0.1 V in ACN.

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P. Anzenbacher, Jr. et al. / Tetrahedron 60 (2004) 11163–11168
DPQ receptors do not bind chloride,8 and, therefore, we
were not surprised that poly-1 does not bind or respond to
chloride regardless of the applied voltage (0.00–0.70 V).
Poly-1C displays generally similar anion binding proper-
ties; however, stronger binding and higher affinity constants
compared to poly-1 are observed, which is not unexpected
given higher acidity of NH-bonds of the chlorinated pyrrole.
Additionally, to confirm that the changes in vis-NIR spectra
are due to the anion binding to poly-1 and poly-1C, we used
the electrochemical quartz crystal microbalance method
(EQCM).17 After the deposition of poly-1 on the EQCM
probe, aqueous solutions of the anions were added. The
addition of anions (pyrophosphate2K, fluoride, dihydrogen-
phosphate, but not chloride) caused a rapid increase in the
mass of the deposited polymer. EQCM titration experiments
resulted in a typical saturation profile, thus confirming that
the changes in vis-NIR spectra are, indeed, a result of anion
binding. In addition, EQCM provides an indispensable tool
for the number of control experiments. The role of water in
the anion sensing studies utilizing hydrogen bonding is
generally considered to be disruptive factor as it competes
for the coordination with anionic analytes.18 However, our
control experiments show that the addition of water does
change neither the mass of the deposited polymer nor its
vis-NIR spectrum. Conversely, the addition of aqueous
anion solutions leads to significant increase in sensor mass
as well as anion-induced change in vis-NIR spectra. From
these observations we conclude that the presence of water
does not significantly affect the binding. We ascribe this to
the effective synergy between the strong coulombic
attraction and the hydrogen bonding. EQCM titrations
offer a great potential for the quantitative characterization of
anion binding process by monitoring the mass accumu-
lation, and the corresponding studies are currently
underway.
In summary, the electropolymerized anion receptors yielded
uniform transparent thin films of conductive polymers
potentially useful for optical sensing of aqueous phosphate-
type anions. We demonstrated that the anion-sensor affinity
may be adjusted by external voltage. We believe that this
increase in anion-sensor affinity is a direct result of synergy
between low-level p-doping in a polythiophene polymer and
receptor–anion hydrogen bonding. The chromogenic con-
ductive polymers poly-1 and poly-1C show anion-specific
changes both in color and conductivity upon increasing
concentration of anions, namely fluoride, pyrophosphate
and, to a lesser extent, also dihydrogenphosphate.
3. Experimental
3.1. General
1H and 13C NMR spectra were recorded using a Varian
Unity 400 (400 MHz) spectrometer. The chemical shifts (d,
ppm) are referenced to a solvent. EI-DIP mass spectra were
recorded using a Shimadzu QP5050A spectrometer.
Absorption spectra were recorded using a Hitachi U-3010
and Hewlett Packard 8453 spectrophotometers. Optically
dilute solutions used for all photophysical experiments were
prepared in spectroscopic grade solvents.
Figure 5. Panel A: Changes in vis-NIR spectra of poly-1 during titration
with aqueous pyrophosphate (pHw6.0) at 0.00 V; Panel B: Titration of
poly-1 by aqueous pyrophosphate at 0.70 V; Panel C: Titration of poly-1 by
aqueous chloride at 0.70 V does not induce observable changes in vis-NIR
spectra.

P. Anzenbacher, Jr. et al. / Tetrahedron 60 (2004) 11163–11168
11167
Electrochemical measurements were performed in DMSO
solutions containing 0.1 M recrystallized tetrabutylammo-
nium perchlorate (TBAP) as a supporting electrolyte. TBAP
was recrystallized from absolute ethanol. A BioAnalytical
Systems Epsilon controller was used to perform the
electropolymerization reaction and the electrochemistry
experiments. A platinum wire auxiliary electrode and
Ag/AgC nonaqueous reference electrode were used for all
measurements. A scan rate of 100 mV/s was typically
employed. Under these experimental conditions the ferro-
cene/ferrocenium couple was determined to be C0.124 V
versus Ag/AgC.
114.13 CH, 118.71 C, 122.59 C, 128.12 C, 133.35 CH,
138.25 C, 145.45 C. EI/MS (70 eV): 451 (100) [MC], 486
(55), 416 (50).
3.1.3. 5,8-Di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-
2,3-di(1H-2-pyrrolyl)quinoxaline (1). 5,8-Dibromo-2,3-
di(1H-2-pyrrolyl)quinoxaline (50 mg, 0.12 mmol) and
2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)tributylstannane11
(258 mg, 0.6 mmol) were dissolved in dry THF (100 ml),
the solution was purged with argon for 15 min, and
PdCl2(PPh3)2 (15 mg, 0.02 mmol) was added at room
temperature under argon atmosphere. The mixture was
stirred at 100 8C under argon atmosphere for 15 h, cooled,
and concentrated on the rotary evaporator. The residue was
purified by column chromatography (DCM–hexane 3:1) to
afford an orange solid (31 mg, 48% yield). Mp O260 8C. 1H
NMR (CDCl3, d) 4.28–4.41 (m, 8H, ethylene); 6.29 (ddd,
2H, JZ2.6, 3.7, 5.4 Hz); 6.57 (s, 2H, EDOT), 7.09 (ddd, 2H,
JZ1.3, 2.5, 3.7 Hz); 7.26 (ddd, 2H, JZ1.3, 2.6, 3.6 Hz);
8.41 (s, 2H, H6); 10.03 (br s, 2H, NH). 13C APT NMR
(CDCl3, d): 65.74 CH2, 65.29 CH2, 102.20, 110.21, 113.48,
113.67, 122.10, 127.49, 127.86, 129.91, 135.90, 140.57,
141.14, 142.05. EI/MS (70 eV): m/z 540 (87) [MC], 269
(100), 267 (70). Anal. Calcd (%) for C28H20N4O4S2$0.5
H2O (549.1): C, 61.19; H, 3.85; N, 10.19; O, 13.10; S,
11.67. Found: C, 61.16; H, 3.71; N, 10.04.
Anion titrations were carried out in DMSO solutions (0.1 M
tetrabutylammonium perchlorate) of the monomer by
adding the aqueous solutions of the following anions as
tetrabutylammonium salts 5.5 mM (for chloride sensing
attempts 180 mM solution was used). Data fitting was
performed using a quadratic equation for 1:1 binding model.
Spectroelectrochemical measurements were carried out in a
custom-built sealed stirred cell using indium–tin-oxide
(ITO) coated glass slide (50!7!0.9 mm3, Rs%100 U) as
a working electrode. Electrochemical quartz crystal micro-
balance (EQCM) experiments were carried out on CHI-430
electrochemical workstation equipped with time-resolved
EQCM from CH Instruments with AT-cut polished bound
100 A CrC1000A gold-coated quartz crystals.
3.1.4. 5,8-Di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-
2,3-di(5-chloro-1H-2-pyrrolyl)quinoxaline (1C). The
title compound was synthesized from 5,8-dibromo-2,3-
di(5-chloro-1H-2-pyrrolyl)quinoxaline according to the
procedure described above for 1. The product was a red
solid (256 mg, 33% yield). Mp O260 8C. 1H NMR (CDCl3,
d) 4.28–4.41 (m, 8H, ethylene); 6.12 (dd, 2H, JZ1.0,
3.7 Hz); 6.59 (s, 2H, EDOT), 7.17 (dd, 2H, JZ1.0, 3.7 Hz);
8.36 (s, 2H, H6); 9.95 (br s, 2H, NH). 13C APT NMR
(DMSO-d6, d): 64.43 CH2, 65.42 CH2, 103.78 CH, 108.24
CH, 112.21 C, 113.30 CH, 117.51 C, 127.51 CH, 128.13 C,
128.59 C, 135.97 C, 141.05 C, 141.81 C, 142.06 C. EI/MS
(70 eV): m/z 608 (100) [MC], 610 (78), 573 (86). HRMS
(ESI) Calcd for C28H19Cl2N4O4S2 (MCHC): m/z 609.0225.
Found: m/z 609.0219.
3.1.1. 5,8-Dibromo-2,3-di(1H-2-pyrrolyl)quinoxaline (4).
3,6-Dibromo-1,2-benzenediamine (326 mg, 1.0 mmol) and
1,2-di(1H-2-pyrrolyl)ethane-1,2-dione (188 mg, 1.0 mmol)
were dissolved in glacial acetic acid (50 ml), and heated to
reflux for 24 h shielded from ambient light. The volume of
the reaction mixture was reduced to 10% in vacuum,
dissolved in chloroform–methanol (9:1, 300 ml), washed
with 1 M NaOH, brine, and dried over anhydrous Na2SO4.
The residue after evaporation was subjected to column
chromatography on silica gel using hexane–dichloro-
methane mixture (1:3, v/v). The product was obtained as
1
yellow powder (210 mg, 50% yield). Mp 229–230 8C. H
NMR (DMSO-d6, d): 6.19 (ddd, 2H, JZ2.5, 3.7, 4.7 Hz),
6.41 (ddd, 2H, JZ1.5, 2.5, 3.7 Hz), 7.09 (ddd, 2H, JZ1.5,
2.5, 4.1 Hz), 7.92 (s, 2H), 11.39 (br s, 2H). 13C APT NMR
(DMSO-d6, d): 109.38 CH, 113.08 CH, 121.96 C, 122.98
CH, 127.75 C, 132.32 CH, 137.58 C, 145.79 C. EI/MS
(70 eV): 418 (100) [MC], 336 (15), 256 (25). Anal. Calcd
(%) for C16H10Br2N2 (418.09): C, 45.96; H, 2.41; N, 13.40.
Found: C, 45.86; H, 2.41; N, 13.22.
3.1.5. Poly-5,8-di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-
yl)2,3-di(1H-2-pyrrolyl)quinoxaline (poly-1). The title
compound was prepared potentiodynamically from
0.5 mM solution of 1 in TBAP/ACN. Monomer oxidation
potential (onset of polymerization current): EmZ0.40 V.
Polymer half-wave oxidation: E1/2,pZ0.70 V. Band gap as
determined from vis-NIR spectroelectrochemistry in the
reduced form: EgZ1.39 eV. Conductivity: sZ54 S/cm.
3.1.2. 5,8-Dibromo-2,3-di(5-chloro-1H-2-pyrrolyl)quin-
oxaline (5). 5,8-Dibromo-2,3-di(1H-2-pyrrolyl)quinoxaline
(4) (100 mg, 0.239 mmol) was dissolved in chloroform
(50 mL) and cooled down to 0 8C. SO2Cl2 (20 mL) was
added to the reaction mixture in chloroform (15 mL). After
5 h the mixture was washed with a saturated aqueous
solution of sodium hydrogencarbonate, the organic layer
was collected, dried, and evaporated in vacuum. The residue
was subjected to column chromatography on silica gel using
hexane–dichloromethane mixture (1:3, v/v) as the eluent.
The product was obtained as a yellow powder (48 mg, 41%
3.1.6. Poly-5,8-di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-
yl)2,3-di(5-chloro-1H-2-pyrrolyl)quinoxaline (poly-1C).
The title compound was prepared potentiodynamically from
0.5 mM solution of 1C in TBAP/ACN. EmZ0.48 V. E1/2,p
Z
0.72 V. EgZ1.36 eV. sZ23 S/cm.
Acknowledgements
1
yield). Mp O260 8C. H NMR (CDCl3, d): 6.14 (dd, 2H,
JZ1.0, 3.7 Hz), 7.18 (dd, 2H, JZ1.2, 3.7 Hz), 7.70 (s, 2H),
This work was financially supported by Kraft Foods, Inc.,
NSF (NER Grant#0304320, SENSOR Grant# 0330267
9.65 (br s, 2H). 13C APT NMR (DMSO-d6, d): 108.40 CH,

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P. Anzenbacher, Jr. et al. / Tetrahedron 60 (2004) 11163–11168
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