Selectivity properties of corrin-doped polypyrrole film

Article abstract of DOI:10.1007/s00706-013-0946-2

We report the synthesis of a new pyrrole-substituted vitamin B₁₂ derivative functioning as a corrin monomer, its electrochemical copolymerization with pyrrole, and the potentiometric behavior of the resultant electrodes based on the corrin-doped polypyrrole film. Electropolymerization of the corrin monomer on a glassy carbon electrode and its redox properties were evaluated by cyclic voltammetry in acetonitrile in the presence of tetrabutylammonium perchlorate. We investigated the effect of various parameters that control the film properties, such as the concentration of the monomer, potential range, scan rate, and number of scans. The optimal conditions for potentiometric measurements were also determined. We found that the electropolymerized corrin film-based electrode exhibited an anion selectivity pattern that favors thiocyanate and deviates from the Hofmeister series. The obtained electrode demonstrates near-Nernstian behavior for a period of at least 6 months. The detection limit of the electrode along with selectivity coefficients toward various anions was also determined.

Full text of DOI:10.1007/s00706-013-0946-2

Monatsh Chem (2013) 144:781–791
DOI 10.1007/s00706-013-0946-2
ORIGINAL PAPER
Selectivity properties of corrin-doped polypyrrole film
¨
Ay c¸ a Demirel Ozel Emre Dikici Leonidas G. Bachas
•
•
Received: 17 November 2012 / Accepted: 5 February 2013 / Published online: 3 April 2013
Ó Springer-Verlag Wien 2013
Abstract We report the synthesis of a new pyrrole-
substituted vitamin B12 derivative functioning as a corrin
monomer, its electrochemical copolymerization with pyr-
role, and the potentiometric behavior of the resultant
electrodes based on the corrin-doped polypyrrole film.
Electropolymerization of the corrin monomer on a glassy
carbon electrode and its redox properties were evaluated by
cyclic voltammetry in acetonitrile in the presence of tet-
rabutylammonium perchlorate. We investigated the effect
of various parameters that control the film properties, such
as the concentration of the monomer, potential range, scan
rate, and number of scans. The optimal conditions for
potentiometric measurements were also determined. We
found that the electropolymerized corrin film-based elec-
trode exhibited an anion selectivity pattern that favors
thiocyanate and deviates from the Hofmeister series. The
obtained electrode demonstrates near-Nernstian behavior
for a period of at least 6 months. The detection limit of the
electrode along with selectivity coefficients toward various
anions was also determined.
Keywords Electropolymerization ꢀ Vitamin B
1
2
derivative ꢀ Cobalt corrin ꢀ Thiocyanate-selective
electrode ꢀ Conducting polymers
Introduction
Doping polymers with ions or molecules has become
popular in the preparation of functional polymeric mate-
rials. These polymeric materials can selectively bind
various chemical species through coordination with rec-
ognition sites within the polymer [1, 2]. Conductive
polymers constitute a special class of doped polymer that
have high electrical conductivity. Such conducting poly-
mers doped with molecules/ions have drawn considerable
attention because of their mechanical/chemical stability,
low cost, and ease of preparation. Electrochemical sensors
based on conducting polymers maintain chemical stability
and allow for control of selectivity that is determined by
the nature of both the monomer and doping ions/molecules
¨
A. D. Ozel
Department of Chemistry, University of Kentucky,
Lexington, KY, USA
¨
A. D. Ozel ꢀ L. G. Bachas (&)
[
3, 4]. Accordingly, ion-selective electrodes (ISEs) have
Department of Chemistry, University of Miami,
Coral Gables, FL, USA
e-mail: bachas@miami.edu
been constructed using conductive polymers, such as
poly(o-phenylenediamine) [5], polyphenol [6], polyaniline
7], and polypyrrole [2, 8–11].
[
Present Address:
¨
The demand for the development of sensitive and
A. D. Ozel (&)
selective ISEs with sufficient stability has been an area of
recent interest. Numerous attempts have been made to
produce electrodes with good response properties and long
lifetimes. Electrochemical polymerization of a suitable
monomer onto an electroactive substrate has proven to be a
powerful way to achieve ISEs with enhanced selectivity
and longer stabilities [12]. The successful implementation
Department of Chemistry, Faculty of Science, Ankara
University, 06100 Tandogan, Ankara, Turkey
e-mail: ademirel@science.ankara.edu.tr
E. Dikici
Department of Biochemistry and Molecular Biology, Miller
School of Medicine, University of Miami, Miami,
FL 33136, USA
123

¨
A. D. Ozel et al.
7
82
of electropolymerized membranes in potentiometric sen-
sors has been recently reviewed [3, 4]. Electropolymerized
membranes offer advantages over electrodes prepared with
conventional polymers such as plasticized poly(vinyl
chloride) (PVC); PVC membrane electrodes can have
shorter lifetimes that have been attributed to the leaching of
the ionophore and other membrane components from the
sensor matrix to the sample solution [13, 14] and occa-
sionally to crystallization of ionophore in the membrane
O
OCH3
O
H C H C
3
3
O
H
NH-R
O
H
H CO
H C
N
N
3
3
CN
N
N
H C
3
Co+
H
OCH3
[
15]. The lack of solubility of electropolymerized films in
H
H CO
3
organic solvents is another advantage that enables the use
of such electrodes in potentiometric analysis in non-aque-
ous media.
H O
CH3
CH3
2
O
H
From the chemical recognition point of view, the design
and synthesis of new ionophores, such as organometallic
compounds, hydrogen-bonding molecules, metallomacro-
cycles including corrin, porphyrin, and phthalocyanine
derivatives that can selectively coordinate anions has been
the focus of much activity [16–18]. Despite the significant
number of publications on electropolymerized phthalo-
cyanine and porphyrin derivatives coordinated with
different metals, only limited attention has been paid to
the electropolymerization of corrins [19, 20], although
hydrophobic derivatives of vitamin B12 have been suc-
cessfully employed in the development of thiocyanate-,
nitrite-, iodide-, and salicylate-selective PVC membrane
electrodes [21–23].
CH3
H
CH3
OCH3
O
OCH3
O
R= CH2 CH2 CH2
N
Fig. 1 Structure of new pyrrole-substituted vitamin B12 derivative
isexpectedtorespond tovarious anionsvia the replacement of
water molecule by anions.
The electrochemical behavior of the new corrin mono-
mer (corrin monomer concentration, C , 10.0 mM) in
M
Considering the ability to electropolymerize metallo-
porphyrin and metallophthalocyanine derivatives bearing
pyrrole substituents as well as the anion coordination
characteristics of corrins, we aimed to synthesize a new
pyrrole-substituted corrin, a,b,d,e,f,g-hexamethyl-c-[3-(1H-
pyrrol-1-yl)propyl]-Coa,Cob-di(cyano-jC)cob(III)yrinate
0.1 M tetrabutylammonium perchlorate (TBAP)/acetoni-
trile solution on a glassy carbon (GC) electrode surface was
investigated by cyclic voltammetry at a scan rate of
-
1
0
.1 V s . The cyclic voltammogram illustrated two
cathodic peaks upon reductive scanning from 0.2 to
1.6 V (Fig. 2). The first cathodic peak can be attributed
-
(
III) to demonstrate the feasibility of combining the
III
II
c
to Co /Co (E -0.79 V); at more negative potentials the
current increased as a result of further conversion of Co(II)
chemical recognition properties of corrins and the con-
trolled pore organization in the design of novel
electrochemical sensors. The resulting electropolymerized
corrin film-coated electrode displayed good selectivity
toward thiocyanate over other anions with a fast response
time and a long lifetime.
p
II
to Co(I) (Co /Co E -1.42 V). The peaks observed are in
I
c
p
good agreement with those discussed by other authors
studying cobalt redox couples [19, 20, 25]. On the reverse
a
potential scan, two oxidation peaks occurred at E -0.56 V
p
a
and E -0.14 V, which can be assigned to the oxidation of
products formed during the reduction of the corrin mono-
p
a
mer. The broad peak observed at E 0.74 V was due to the
oxidation of pyrrole. In the subsequent scan, a broad peak
Results and discussion
p
c
appeared at E *0.60 V, which demonstrates the electro-
Corrins, in which a cyano group occupies one of the axial
ligands and the second ligand is water, have been shown
previously to behave as charged carriers in ISE membranes
p
chemical response of the polymerized pyrrole in the film.
Figure 2 is a representative cyclic voltammogram typically
observed in this study for a low number of scans with a
corrin monomer concentration of 10.0 mM. When the
concentration of the corrin monomer was decreased to
[
23, 24]. Conversely, after the removal of one of the axial
CN ligands from the dicyano derivative, the corre-
-
sponding corrin should behave as a positively charged
carrier in the electropolymerized film on the glassy carbon
electrode. This acetic acid treated compound IV (Fig. 1),
referred to as the corrin monomer throughout this manuscript,
1.0 mM, similar voltammograms were observed with lower
peak currents. Hence, 1.0 mM was deemed to be a suffi-
cient concentration to electropolymerize compound IV on
1
23

Selectivity properties of corrin-doped polypyrrole film
783
in the film according to the following equation reported for
a Nernstian adsorbate layer [26]:
ip
ꢁ
C ¼
0
5
2
ð9:30 ꢂ 10 Þn mA
where C is the number of moles of corrin per unit area,
*
0
i is the peak current with respect to the baseline (average
p
-
5
of three peak currents, i = 1.4 9 10 A) obtained during
p
cyclic voltammetry, n is the number of electrons involved
in the oxidation/reduction (n = 1), m is the scan rate
-
1
(
m = 0.1 V s ), and A represents the surface area of
2
*
the electrode (A = 0.07 cm ). It was found that C =
0
-
.2 9 10 mol/cm , and the total number of moles of
9
2
2
-
10
corrin on the surface of the electrode was 1.5 9 10
mol.
The surface coverage achieved in this study was found to
be comparable to values reported in previous studies
involving various porphyrin and vitamin B12 derivatives
Fig. 2 Cyclic voltammogram of corrin monomer in acetonitrile
-
1
containing 0.1 M TBAP on GC electrode at a scan rate of 0.10 V s
vs. Ag/AgCl, 12 scans). Inset is the overlaid voltammogram of corrin
monomer in 0.1 M TBAP/acetonitrile
(
[
27–29].
Effect of electropolymerization conditions
on potentiometric response
the GC electrode. As it can be seen in Fig. 2, after con-
II
III
tinuous potential cycling the currents assigned to Co /Co
It is well known that the morphology of the polymer film
can change the ion-exchange properties of electropoly-
merized film-based ISEs; these ion-exchange properties
can be controlled by the electropolymerization conditions
[30, 31]. It has been previously demonstrated that these
conditions not only affect the detection limit and slope of
the calibration plots of ISEs, but also other response
characteristics such as selectivity and response time.
Therefore, the conditions used for the copolymerization of
pyrrole with the corrin monomer were varied to determine
their effect on the potentiometric response of the corre-
sponding electrodes.
II
I
and Co /Co decreased dramatically owing to surface
modification via PPy film formation, while the peak current
for polymerized pyrrole increased indicating that the film
deposited on the electrode surface was conductive. The
III
II
II
I
decrease in the Co /Co and Co /Co peak currents also
revealed that access of the corrin monomer from the
solution to the GC electrode surface was being hindered by
steric effects.
In order to improve the electropolymerization capacity
of the pyrrole-substituted corrin monomer, copolymeriza-
tion with pyrrole was performed in solutions containing a
mixture of different concentrations of pyrrole with a fixed
concentration of the corrin monomer at 1.0 mM. Each
time, uniform, insoluble, adherent, and reproducible films
were obtained on the surface of the electrode.
The polymerization rate of the new pyrrole-substituted
corrin was expected to be significantly lower than that of
unsubstituted pyrrole because of steric effects. Numerous
studies in the literature have shown that pyrrole and
substituted pyrrole monomers are readily polymerized
from dilute solutions (e.g., 1.0 mM) [32]. Tables 1, 2, and
3 show the conditions used for the preparation of electrodes
that provided promising response characteristics as thio-
cyanate-selective electrodes with regard to the slope of the
calibration plot and linear working range.
The polymer film-coated electrode was immersed in a
0
.1 M TBAP/acetonitrile solution to obtain evidence of the
successful copolymerization of the pyrrole-substituted
corrin. In the voltammogram, a small peak at more nega-
tive potential than the PPy reduction was observed. To
ensure that the additional peak was not due to the reduction
of polypyrrole, cyclic voltammetry of a PPy film-coated
GC electrode was conducted in the absence of the corrin
monomer in the same electrolyte solution. Because the
cyclic voltammogram of this electrode showed no peak
other than the reduction of PPy, the smaller peak in the
cyclic voltammogram of the original electrode can be
attributed to the doping of corrin units and the reduction
process for their cobalt center. The peak currents for this
reduction were measured to determine the amount of corrin
When the anodic potential was less than 0.55 V, no
polymer film was obtained on the electrode surface. In
order to identify the positive potential limit of the working
potential range, the anodic switching potentials were
changed in the range of 0.75–1.2 V. As the potential
became more positive, the slope of the calibration plots of
the corresponding ISEs were closer to Nernstian, and the
detection limit observed was better. However, when the
anodic switching potential exceeded 1.3 V, the potential
123

¨
A. D. Ozel et al.
7
84
Table 1 Effect of the potential scan range on the response behavior
of the thiocyanate-selective electrode
Table 3 Effect of electropolymerization conditions on the potentio-
metric response behavior of the thiocyanate-selective electrode
Starting Negative
potential/ switching
Positive
switching
Slope/mV/
pSCN ± tS/
Detection
limit/M
C
C
TBAP or
TBAC/M
C
P
/M
Slope/mV/pSCN
± tS/HN
Response
time/s
V
potential/V
potential/V HN (N = 4)
C
TBAP 0.1
1.0
52.7 ± 2.2 (N = 3)
56.9 ± 0.6 (N = 42)
51.1 ± 1.2 (N = 3)
47.7 ± 1.5 (N = 3)
48.9 ± 0.8 (N = 3)
54.1 ± 1.2 (N = 3)
[25
10–25
15
-
5
5
4
4
4
5
4
0
0
0
0
0
0
0
.2
-1.6
-1.6
-1.6
-1.4
-1.5
-1.6
-1.6
1.1
1.2
1.3
1.2
1.2
1.1
1.2
56.5 ± 1.0
58.7 ± 0.9
49.3 ± 1.0
42.8 ± 2.9
51.2 ± 2.6
54.2 ± 2.2
55.3 ± 2.0
4.0 9 10
1.5 9 10
3.1 9 10
4.6 9 10
1.0 9 10
8.0 9 10
1.0 9 10
0
0
0
.5
-
-
-
-
-
-
.2
.2
.2
.2
.1
.01
10
C
C
TBAC 0.1
TBAP 0.5
0.5
0.5
10–15
30–35
Starting potential, 0.2 V; negative switching potential, -1.6 V;
positive switching potential, 1.2 V; C , 1.0 mM; ± tS/HN is the
M
-
1
confidence interval for the mean slope of N replicate measurements
calculated from the standard deviation S; the statistical parameter
t corresponded to a probability level of 95 %. Given data are for the
same electrode; however, at least three replicates of each scan range
were employed for the reproducibility of the electrode preparation
Electropolymerization conditions: scan rate, 0.1 V s ; number of
scans, 100; C , 1.0 mM; C , 0.5 M in 0.1 M TBAP/acetonitrile.
M
P
Given data are for the same electrode; however, at least three repli-
cates of each scan range were employed for the reproducibility of the
electrode preparation
-
4
-5
Table 2 Effect of electropolymerization conditions on the potentio-
metric response behavior of the thiocyanate-selective electrode
the range of 4.6 9 10 –1.5 9 10 M. Moreover, when
the electrodes were prepared with potentials more negative
than -1.6 V, sub-Nernstian slopes and narrower working
ranges were observed. This can be attributed to the dif-
ferent surface modification of the electrodes.
Scan
rate/V s
Number Slope/mV/pSCN
of scans ± tS/HN
Response
time/s
-1
0
0
0
0
0
0
.2
100
100
100
100
50
53.8 ± 1.4 (N = 3)
[15
10–15
10–25
10
According to the literature, the potentiometric anion
response of electrodes could be affected depending on
whether the electrodes are poised at a reduced or an oxi-
dized state [33, 34] after electropolymerization. Therefore,
the electrodes were kept under constant final potentials of
either -1.6 or 1.2 V for a period of 2 min. In our study, a
selective response to thiocyanate was obtained in both
cases; however, the electrodes that were prepared at
.15
.1
55.7 ± 0.4 (N = 6)
56.9 ± 0.6 (N = 42)
43.6 ± 1.8 (N = 5)
.05
.1
52.8 ± 1.5 (N = 3)
5–10
–
.1
200
No stable potentiometric response
Starting potential, 0.2 V; negative switching potential, -1.6 V;
positive switching potential, 1.2 V; C , 1.0 mM; C , 0.5 M in 0.1 M
M
-
P
1
TBAP/acetonitrile; scan rate, 0.1 V s ; ± tS/HN is the confidence
interval for the mean slope of N replicate measurements calculated
from the standard deviation S; the statistical parameter t corresponded
to a probability level of 95 %. Given data are for the same electrode;
however, at least three replicates of each scan range were employed
for the reproducibility of the electrode preparation
-
1.6 V showed a better anion selectivity pattern with the
slope of the calibration plot being closer to Nernstian.
Employing a different number of scans and scan rates
during electropolymerization may also influence the poten-
tiometric response of the ISEs. To determine the optimum
film thickness, electrodes were prepared with a different
number of scans, and their potentiometric response was
compared by means of slopes and detection limits. When the
number of scans was raised to 100, the sensitivity of the
electrodes increased as evidenced by the value of the slope of
the calibration plots. Electrodes prepared with 100 scans also
gave the longest lifetimes. Thicker films obtained with 200
scans gave unstable potentiometric responses, and the films
peeled off after a few measurements because of weak
adherence to the electrode surface. Increasing the scan rate
readings of the resulting electrodes were unstable. This
could be attributed to alteration of the film due to ove-
roxidation of pyrrole. Altering the starting potential to
more positive values did not significantly change the
potential response of the ISEs.
In order to obtain electrode surfaces with good polymer
coverage, the effect of the negative potential limit of the
working range was investigated. Potentials that were more
II
I
negative than the potential at which the Co /Co reduction
takes place were applied as the negative switching potential
up to -1.7 V. ISEs obtained by potential cycling using
different negative switching potentials between -1.45 and
-
1
from 0.05 to 0.2 V s resulted in electrodes with good
response characteristics. However, no further improvement
-
was obtained when the scan rate was greater than 0.1 V s
1
.
-
1.7 V showed important differences in the potentiomet-
ric response. The slopes of the electrodes were between
2.8 and 58.7 mV/pSCN, while the detection limits were in
Consequently, the optimum potential scan rate and number
-
1
of scans were determined to be 0.1 V s and 100, respec-
tively (Table 2).
4
1
23

Selectivity properties of corrin-doped polypyrrole film
785
Effect of monomer concentration
After establishing the optimum working potential range,
we investigated the effect of various concentrations of
pyrrole. A solution containing 0.01 M pyrrole was
employed in the initial experiments. The potential response
was poor resulting in narrow working ranges between
-
2
-4
1
.0 9 10
and 1.0 9 10
M. Therefore, films were
grown in solutions containing a constant corrin monomer
concentration of 1 mM and pyrrole concentration varying
from 0.1 to 1.0 M to achieve better functioning sensor
responses. The results are given in Table 3. The data in
Fig. 3 suggested that different pyrrole to corrin ratios
formed different arrangements in the polymer film by
means of the amount of corrin doped and the polymer
surrounding the corrin ring. Lower slopes in the presence
of 0.01 and 0.1 M pyrrole can be attributed to the control of
the selectivity provided by the variation of film properties
which resulted in different possible potentiometric
response mechanisms. Figure 4 shows the calibration plots
of the polymer film-coated electrodes for comparison
purposes. ISEs with slopes close to the Nernstian value for
thiocyanate were obtained upon increasing pyrrole con-
centration to 0.5 M. Further increase in pyrrole
concentration led to worse detection limits and slopes of
the calibration plots. In conclusion, employing a 0.5 M
pyrrole concentration while maintaining 1 mM corrin
resulted in ISEs with slopes that were closer to Nernstian in
Fig. 4 Effect of pyrrole concentration on the potentiometric response
of the electrodes in 0.1 M 2-(N-morpholino)ethanesulfonic acid
(MES)–NaOH, pH 5.5 (electropolymerization conditions:
0
m
1.0, 0.5, 0.1,
-
.1 V s ; 100 scans in 0.1 M TBAP/acetonitrile; C
1
P
0
.01 M as indicated in the figure). The open circle is the potentio-
metric response of the blank electrode prepared in the absence of
corrin with a pyrrole concentration C of 0.5 M
P
the electroneutrality of the films [35]. Therefore, the size,
shape, and polarity of these anions may influence the
electrical conductivity and the mechanical properties of the
polymer films [36]. We maintained a constant optimum
working potential range, scan rate, number of scans, and
pyrrole concentration to investigate the effect of different
electrolyte solutions and their concentration (0.01–0.5 M,
Table 3). Polymer films were deposited in the presence of
perchlorate and chloride ions as templates. When the
concentration of TBAP was 0.01 M, the electrodes gave
worse slopes, and the response disappeared after a week.
Electrodes prepared by using 0.5 M TBAP showed unsta-
ble responses, and the slope could not be determined.
However, when the electropolymerization was performed in
-
1
-4
the linear range of 1.0 9 10 –1.0 9 10 M and a
-
detection limit of 1.5 9 10 M.
5
Effect of electrolyte and its concentration
During the oxidative and reductive processes, anions from
the electrolyte diffuse into or out of the polymer to ensure
0.1 M TBAP in acetonitrile, a good stable near-Nernstian
response and an improved detection limit were observed.
When using 0.1 M tetrabutylammonium chloride (TBAC) as
the electrolyte, electrodes with worse slopes and selectivity
for thiocyanate over chloride were obtained. The reason for
this phenomenon may be the difference in efficiency of the
electrodeposition, which can be attributed to using a differ-
ent electrolyte anion during electrodeposition.
In the corrin-doped polypyrrole film, the corrin provides
coordination sites for selective anion binding, whereas
pyrrole, which is in excess, determines the morphology of
the polymer around the corrin monomer. During electrop-
olymerization, the electrolyte anions enter and leave the
film to maintain electroneutrality, which in turn results in
Fig. 3 Cyclic voltammograms of corrin-doped polypyrrole prepared
in a 1.0 M, b 0.5 M, c 0.1 M, d 0.01 M pyrrole containing 0.1 M
-1
TBAP/acetonitrile on GC electrode at a scan rate of 0.10 V s (C
.0 mM, vs. Ag/AgCl)
M
1
123

¨
A. D. Ozel et al.
7
86
films of different characteristics. Of the two electrolyte
anions, perchlorate is larger leading to an electropolymer-
ized film in which enough space around the cobalt center is
provided for thiocyanate to diffuse to and coordinate with
the metal center. In the presence of a smaller anion like
chloride, thiocyanate was still the most preferable anion;
however, the slope of the corresponding electrodes was
worse, and the working range was narrower than those
obtained when perchlorate was used as the electrolyte
anion. In short, the proposed mechanism for the observed
selectivity depends on cobalt coordination with the anion
of interest as the axial ligand of the corrin, and this
selectivity can be modified by changing the nature of the
chemical environment that surrounds the corrin ring
[
37, 38]. Additionally, anions like nitrate and perchlorate,
which are known to be weak coordination ligands, do not
interact with the cobalt center [39]. In the case of thiocy-
anate, it is easier for this linear anion to coordinate to the
cobalt center, whereas other strong ligand anions like
nitrite are bulkier and may be sterically hindered from
coordinating directly to the cobalt center.
Fig. 5 Potentiometric response curves of electrodes assembled under
the optimized electropolymerization conditions (C
M P
1.0 mM, C
-
1
0.5 M, m 0.1 V s , 100 scans) in 0.1 M MES–NaOH, pH 5.5; 1
-
SCN , 2 NO
-
- -
, 4 I , 5 NO
-
-
, 6 Br , 7 Cl , 8 salicylate, 9
-
2
, 3 ClO
4
3
-
-
HCO
3
, 10 CH
3
COO
As shown in Table 4, the electropolymerized corrin
film-coated GC electrode gave the best selectivity toward
thiocyanate when the copolymerization was performed in
-
3
1.0 9 10 M NaSCN. Better slopes and working ranges
were obtained when the electrodes were conditioned in
-
2
0
.1 M TBAP/acetonitrile solution containing 1.0 mM cor-
1.0 9 10 M NaSCN for at least 12 h. The electrode
showed no further improvement in response after 12 h of
conditioning. Consequently, the thiocyanate-selective
electrodes were conditioned for at least 12 h before
experiments.
rin monomer and 0.5 M pyrrole by cycling in an optimum
potential range starting at 0.2 V, going cathodically to
-
1.6 V, and then scanning anodically to 1.2 V with a scan
-
1
rate of 0.1 V s . Figure 5 shows the anion selectivity
pattern of this electrode.
The polymer film properties are strongly affected by
solution acidity because of the possible protonation and
deprotonation of the polymer. Changes in the potential
response of polypyrrole film-based electrodes were
Effect of conditioning time and pH on potentiometric
response
observed when the pH was below 3.5 and above 7.5 as a
?
The potentiometric response of the electrodes was tested
result of simultaneous response of the electrodes to H O
3
-
after 2, 12, and 24 h of conditioning in 1.0 9 10 and
2
-
and OH , respectively [40]. Thus, to identify the best
buffer system, calibration plots of the thiocyanate-selective
electrode were obtained by using different buffers (0.01 M)
in the pH range of 4.0–7.0. The starting potentials of the
electrodes shifted to more negative values when the pH
was increased in this range. As a result, significant changes
were observed in the detection limit and slope of the
electrodes. The electrodes presented better response and
linearity at pH 5.0 for the 0.01 M MES–NaOH buffer
solution. An additional study was undertaken by using the
same buffer system to investigate the effect of its con-
centration on the provision of higher buffer capacity. The
electrode had better detection limits with a negligible
change in the slope of the calibration plot in 0.1 M MES–
NaOH. The pH for this buffer was changed in the range of
pot
ꢃ
Table 4 Selectivity coefficients, log KSCN ;j, of the thiocyanate-
selective electrode in comparison with the polypyrrole-coated elec-
trode calculated by the separate solution method
Anion
Corrin film-coated electrode
pot
log K
PPy-coated electrode
pot
log K
ꢃ
ꢃ
SCN ;j
SCN ;j
-
4
ClO
-
-1.08
-1.00
-2.09
-1.06
-2.65
-1.25
-1.63
-0.78
-2.31
-0.70
-0.66
-1.77
-0.22
-2.35
-0.83
-1.26
-0.49
-1.98
I
Salicylate
-
NO
CH
3
-
3
COO
-
Br
-
Cl
5.5–7.0 to optimize the buffer pH. Our findings suggest that
-
NO
2
a pH 5.0–5.5 can be used for optimum response charac-
teristics for thiocyanate. As Fig. 6 shows, the detection
-
3
HCO
1
23

Selectivity properties of corrin-doped polypyrrole film
787
the organic backbone of polypyrrole film with anions. The
corrin-based thiocyanate-selective electrode showed
improved selectivity coefficients compared to the PPy film-
coated electrode as a result of additional anion coordination
to the cobalt center.
Response time and lifetime
The response time of an ISE is an important factor in
determining its analytical utility, and it was evaluated by
measuring the time required to achieve a steady-state
potential (within ± 1 mV). The electrode responded rapidly
after additions of thiocyanate to the sample solution, with
response times ranging from 10 s at concentrations above
-
4
1
.0 9 10 M to 25 s at lower concentrations.
The effect of light on the lifetime of the corrin film-
coated thiocyanate-selective electrode was also evaluated.
The electrode demonstrated a lifetime of a few weeks when
it was exposed to light between measurements because of
the light sensitivity of the film. In contrast, the electrode,
when stored in the dark at 4 °C, presented a potentiometric
response that remained almost the same for at least
Fig. 6 Effect of pH on the potentiometric response of electrodes
assembled under the optimized electropolymerization conditions (C
P
1.0 mM, C 0.5 M, m 0.1 V s , 100 scans) in 0.1 M MES–NaOH
buffer at pH a 7.0, b 6.5, c 6.0, d 5.5
M
-1
6
months.
limit of the electrodes shifted toward a higher concentra-
tion as the pH of the solution increased up to 7.0. As a
result of these studies, all experiments were performed in
In order to compare the lifetimes and response times of
the thiocyanate-selective corrin film-coated GC electrode
with those of conventional PVC membrane electrodes,
different compositions of PVC membrane electrodes were
prepared. A composition of 2 % (w/w) corrin monomer,
64.7 % PVC, 32.5 % 2-nitrophenyl octyl ether (NPOE),
and 0.8 % tridodecylmethylammonium chloride (TDMACl)
was found to be the optimum membrane composition. This
PVC membrane electrode showed a different anion selec-
tivity pattern than the electropolymerized corrin film-
coated electrode because of the different nature of the
membrane. Deviation from the Hofmeister series was
0
.1 M MES–NaOH buffer at an optimum pH of 5.5.
Selectivity coefficients
Selectivity is one of the most important characteristics of
any sensor and demonstrates the relative response for the
primary ion over other ions present in solution. The
selectivity coefficients were determined by the separate
solution method, and the electropolymerized corrin elec-
trode demonstrated a selectivity pattern as follows:
-
-
observed in the order of I & salicylate [ ClO
[
4
-
-
-
-
-
-
-
-
-
-
-
-
-
SCN [ NO [ I [ ClO & NO [ Br [ Cl [
SCN [ Br [ NO & NO2 [ Cl [ HCO & CH3
2
4
3
3
3
-
-
-
salicylate [ HCO3 [ CH COO , which clearly indi-
COO . Among several anions, the PVC membrane elec-
trode gave the best response to iodide with an initial near-
Nernstian slope of 56.3 ± 1.2 mV/pI within the concen-
3
cates deviation from the Hofmeister series (Table 4).
The selectivity of a polypyrrole film-coated electrode
-
1
-6
(
no corrin present) was established for comparison pur-
-
tration range 1.0 9 10 –1.0 9 10 M. Its response time
was close to that of the electropolymerized film-coated
thiocyanate-selective GC electrode, which was 20–30 s.
However, because membrane components can leach out of
the membrane during conditioning and potentiometric
measurement periods, the iodide-selective PVC membrane
electrodes lost their response after 2 weeks. This fact
demonstrates the stability advantage of the developed
thiocyanate-selective polymer film-coated GC electrode.
On the basis of the results in this study, the proposed
thiocyanate-selective polymer film-coated GC electrode
and iodide-selective PVC membrane electrode have com-
parable Nernstian slopes, selectivities, and response times.
-
poses. It showed a selectivity pattern of SCN [ NO
NO2 [I & ClO [Br [Cl [salicylate [HCO
[
[
3
-
-
-
4
-
-
-
3
-
CH COO , which was different from the electropoly-
3
merized corrin electrode while also exhibiting a lower
slope of the calibration plots than the latter. Although the
selectivity coefficients of the polypyrrole electrode were
worse than the corrin electrode, both electrode types
showed preference toward the thiocyanate ion. Therefore,
it may be concluded that the polypyrrole in the copoly-
merized corrin electrode may also play a role in
determining its anion selectivity because of the possibility
of interaction between the charged poly(cationic) sites on
123

¨
A. D. Ozel et al.
7
88
They have a narrower working range and higher detection
limit when compared to other thiocyanate-selective elec-
trodes based on similar organometallic compounds and
polymer films reported in the literature. However, the
corrin-doped polypyrrole film-coated GC electrode has an
improved lifetime over most of the previously reported
thiocyanate-selective polymer film-coated electrodes and
PVC membrane electrodes [14, 15, 33, 41].
Chemicals (Gibbstown, NJ) in high purity, and they were
used without any further purification.
Pyrrole, obtained from Acros Organics, was stored under
nitrogenatmosphere at -20 °C, protected from light and was
distilled prior to use. The solutions were prepared in anhy-
˚
drous acetonitrile, which was stored over 3 A molecular
sieves. Sodium bicarbonate, sodium phosphate dibasic,
sodium acetate, sodium hydroxide, citric acid, and glacial
acetic acid were obtained from EMD Chemicals. Selecto-
phore-grade 2-nitrophenyl octyl ether (NPOE), poly(vinyl
chloride) (PVC), and tridodecylmethylammonium chloride
Conclusions
(TDMACl) were purchased from Fluka (Ronkonkoma, NY).
This work demonstrates that electropolymerized corrin
film-coated anion-selective electrodes can be obtained by
using a new pyrrole-substituted vitamin B12 derivative. The
response characteristics of the new thiocyanate-selective
electrode were controlled by changing the electropoly-
merization conditions, pH of the solution, and conditioning
time. This electrode exhibits a selectivity pattern that
deviates from the Hofmeister series. The majority of the
selectivity coefficients against the anions tested were in
For all aqueous media experiments, deionized water of
18 MX (Mili-Q water purification system; Milipore, Bed-
ford, MA) was used to prepare working solutions.
Instrumentation
Electropolymerization studies were carried out with a
Bioanalytical Systems (BAS) (Bioanalytical Systems, West
Lafayette, IN) BASi CV-50 W voltammetric analyzer and
BAS 100B C3 electrode cell stand. A conventional three-
electrode cell was employed with a glassy carbon electrode
(GC, BAS MF-2012) as the working electrode, while a
platinum wire (BAS MW-1032) and Ag/AgCl (BAS MF-
2052) were used as the counter and reference electrodes,
respectively. The redox potentials were determined using
-
2
the order of 1.0 9 10 . The electrodes exhibited a
near-Nernstian slope of 56.9 ± 0.6 mV/pSCN within
-
the range 1.0 9 10 –1.0 9 10 M, a detection limit of
1
-4
-
.5 9 10 M, and rapid response time of 10–25 s. An
5
1
advantage of this electrode is the extended lifetime of at
least 6 months with no significant deterioration in the
slopes during this period.
?
the ferrocenium/ferrocene redox couple, Fc /Fc, a standard
reference material for nonaqueous media.
For potentiometric measurements, potential differences
between the electropolymerized film-coated GC electrodes
and a double junction Ag/AgCl electrode (Orion, model
Experimental
9
00200, Beverly, MA) were obtained using an eight-
Coa,Cob-di(cyano-jC)cobyrinic acid a,b,d,e,f,g-hexa-
methyl ester [42], 1-(3-aminopropyl)pyrrole [43], and
a,b,d,e,f,g-hexamethyl-c-[3-(1H-pyrrol-1-yl)propyl]-Coa,
Cob-di(cyano-jC)cob(III)yrinate [44] were synthesized
as described earlier with minor modifications. TLC plates
channel SC-2345 interface (SCC-LP01, National Instru-
ments, Austin, TX). A PCI-6036E multifunction I/O data
acquisition device was used to connect the interface to a
computer running LabVIEW 7.0 (National Instruments)
software with a custom-designed program for data acqui-
sition and analysis. The pH studies and buffer preparations
were performed with the aid of a Fisher Scientific Accumet
˚
with a 250-lm layer thickness and 60 A pore size, silica
˚
gel (70–230 mesh, 60 A for column chromatography), the
sodium salts of the anions tested, Amberlite IRA-400(Cl)
ion-exchange resin, tetrabutylammonium perchlorate
915 pH/ion meter, which was equipped with an Orion pH
glass electrode.
(
(
TBAP), TBAC, tetrabutylammonium hydrogen sulfate
TBAS), 2,2,2-trichloro-1,1-dimethylethyl chloroformate,
1
13
H and C nuclear magnetic resonance spectroscopy
was carried out with Varian 200 MHz and Varian
00 MHz (Agilent Technologies, Santa Clara, CA) spec-
triethylamine, anhydrous acetonitrile, and ferrocene (Fc)
were purchased from Sigma-Aldrich (St. Louis, MO).
N-(2-Hydroxyethyl)piperazine-N-(2-ethanesulfonic acid)
4
trometers. High-resolution mass spectroscopy analysis was
performed with a Bruker microTOF-Q (Bruker Daltonics,
Billerica, MA).
(
HEPES), 2-amino-2-(hydroxymethyl)propane-1,3-diol
hydrochloride (Tris–HCl), and 2-(N-morpholino)ethane-
sulfonic acid (MES) for buffer solutions were also
obtained from Sigma-Aldrich. The solvents for all
chemical reactions and chromatography were obtained
from Acros Organics (Geel, Belgium) and EMD
Coa,Cob-di(cyano-jC)cobyrinic acid a,b,d,e,f,g-hexa-
methyl ester (I)
Compound I was prepared according to a published
procedure [42] in 68 % yield. R = 0.44 (CH Cl /MeOH
f
2
2
1
23

Selectivity properties of corrin-doped polypyrrole film
789
1
0:1 with 0.1 % KCN); spectroscopic results closely
achieve a 10 % (v/v) acetic acid solution) was added. The
solvent was evaporated in a rotary evaporator while heating
matched the previously reported results [42, 45–49]. The
3
product was stored in the dark at 4 °C.
at 45 °C. The residue was redissolved in 10 cm of a 10 %
acetic acid–ethanol solution, and the procedure described
above was repeated for a total of three times. After the last
treatment, the color of the compound had changed from
purple to red. This method removes only one of the -CN
ligands from the dicyano derivative resulting in an aquo-
cyano derivative [23].
1
-(3-Aminopropyl)pyrrole (II)
The synthesis of compound II was performed according to
Pablo et al. [43]. Briefly, to a solution containing 4.0 g of
3
freshly distilled pyrrole (0.06 mol) in 33.0 cm of aceto-
nitrile, 9.4 g of powdered sodium hydroxide (0.23 mol)
and 0.80 g of TBAS (2.4 mmol) were added, and the
mixture was stirred at room temperature for 30 min. Then,
Preparation of electropolymerized corrin film-coated
electrode
1
0.0 g of 3-chloropropylamine hydrochloride (0.07 mol)
was added, and the reaction mixture was refluxed for 24 h
in an oil bath. The inorganic solid was filtered off, and the
solvent was removed under reduced pressure to yield a
brown liquid product, 1-(3-aminopropyl)pyrrole. The prod-
uct was purified with silica gel column chromatography using
Before electropolymerization, the surface of the GC elec-
trodes was polished on microcloths to a mirror finish by
using successively finer grades of a slurry made from
alumina powder, deionized water, and suspensions of 1-,
1
:1 ethyl acetate/MeOH containing 0.1 % NH OH (7.40 g,
4
3-, and 15-lm-sized diamond dust. Polished electrodes
9
2 %). R = 0.23 (ethyl acetate/MeOH 1:1 ? 0.1 %
f
were successively sonicated in deionized water, electrolyte
solution, and acetone for 5 min each. Electropolymeriza-
tion of the corrin films was accomplished in anhydrous
acetonitrile containing TBAC or TBAP (CTBAP 0.1 M and
1
NH OH); H NMR spectroscopic results closely matched
4
the previously reported results [50].
a,b,d,e,f,g-Hexamethyl-c-[3-(1H-pyrrole-1-yl)propyl]-
Coa,Cob-di(cyano-jC)cob(III)yrinate
0.5 M; CTBAC 0.1 M). The concentration of the corrin
monomer was kept constant at 1.0 mM, and the concen-
(
III, C H CoN O )
60 81 8 13
tration of pyrrole (C ) was varied between 0.01 and 1.0 M.
P
The cobester I (142 mg, 132 lmol) was dissolved in
1
3
The corrin monomer was copolymerized with pyrrole on
GC electrodes by cycling repetitively between a set of
starting potential (0–0.2 V), negative switching potential
(-1.45 to -1.7 V), positive switching potential (0.55–
0 cm dry CH Cl under Ar, and the solution was cooled
2 2
externally with ice/H O/NaCl to -10 °C. Then, 36 mg
2
2
,2,2-trichloro-1,1-dimethylethyl chloroformate (150 lmol)
3
and 117 mm triethylamine (833 lmol) were added to the
cooled solution. The solution was stirred magnetically for
1.3 V), and a constant final potential at -1.6 V for 2-min
periods. The electrodes were then thoroughly washed with
water. The total number of scans and scan rate were varied
1
1
5 min and warmed to room temperature. Then, 579 mg of
-(3-aminopropyl)pyrrole (4.7 mmol) was added, and the
-
1
between 50–200 repetitions and 0.05–0.20 V s , respec-
tively. All solutions were thoroughly deoxygenated directly
in the electrochemical cell for 20 min by purging with
pure nitrogen before electropolymerization. Electropoly-
merization was also performed under an inert nitrogen
atmosphere. Once the film electrodes were prepared, they
were rinsed thoroughly with water and stored in the dark at
mixture was heated under reflux for 18 h. The product was
purified by column chromatography using 96:4 CH Cl /
2
2
MeOH (0.1 % KCN) as the mobile phase. The product
fraction was shaken with aq. NaHCO (containing KCN),
3
and the organic phase was subsequently dried over
anhydrous Na SO . The solvent was removed by evapo-
2
4
ration at ambient temperature to give the violet-red
4
°C.
product (125.9 mg, 81 %). R = 0.70 (CH Cl /MeOH
f
2
2
9
6:4 ? 0.1 % KCN). Compound III was co-purified with
.4 mol ratio of the starting material, 1-(3-aminopro-
Preparation of PVC membrane electrode based on new
corrin compound
0
pyl)pyrrole (II). ESI-TOF HRMS found 1,154.5229
?
(
M – CN), calculated for C H CoN O 1,180.5255.
60 81 8 13
A PVC membrane electrode based on the new corrin
compound was prepared by using NPOE as plasticizer and
TDMACl as lipophilic additive. Different membrane
compositions in the range of 1–3 % ionophore, 32–33 %
PVC, 67–64 % NPOE, and 60 mol % of TDMACl relative
to the ionophore were used to prepare electrodes to com-
pare with the electropolymerized corrin electrodes with
respect to their lifetimes and selectivities. The PVC
membrane-based ISEs were prepared by dissolving the
Compound III was treated with 10 % (v/v) acetic acid in
ethanol solution to displace one of the coordinated cyanide
ligands with water, and thus generate a positively charged
coordination site on the cobalt center (compound IV,
Fig. 1). This treatment has been successfully employed in
previous syntheses of vitamin B derivatives [33]. A
1
2
3
volume of 10.0 cm of this solution was placed in a round-
bottom flask and concentrated acetic acid (enough to
123

¨
A. D. Ozel et al.
7
90
specific amounts of compound IV (the ionophore),
3
TDMACl, PVC, and NPOE in 3 cm of THF. After com-
Acknowledgments We gratefully acknowledge financial support by
the National Aeronautics and Space Administration (NASA) and the
University of Miami. ADO was funded by the international post-
doctoral research scholarship program of The Scientific and
Technological Research Council of Turkey (TUBITAK).
plete dissolution of all components, the membrane cocktail
solution was poured into a 22-mm-i.d. glass ring located on
a glass slide. The solvent was allowed to evaporate at room
temperature overnight, leaving a PVC membrane with a
thickness of about 150–200 lm. Small disks of each
membrane were punched using a cork borer and mounted
onto commercial Philips IS-561 (Gl a¨ sblaserei M o¨ ller,
Zurich, Switzerland) electrode bodies. A solution of
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