Cu(II) complexes as receptor molecules for development of new chloride sensors

Article abstract of DOI:10.1016/j.electacta.2006.05.023

Highly sensitive and selective chloride polymeric membrane sensors are developed that employ Cu(II) complexes as anion carriers. Optimized membrane sensors showed a near-Nernstian response towards chloride anion over a wide concentration range of 2.5 × 10^-5 to 1.0 × 10^-1 M and have micromolar level detection limits. These sensors have fast response time and work well in the pH range of 4.2-9.6. They exhibited enhanced potentiometric selectivity for chloride over other anions, including more lipophilic anions such as perchlorate, thiocyanate, nitrate, etc. Response characteristics (e.g. detection limit, linear range, response slope and selectivity) of these sensors remain essentially the same over a period of ～5 months, reflecting remarkable stability.

Full text of DOI:10.1016/j.electacta.2006.05.023

Electrochimica Acta 52 (2006) 408–414
Cu(II) complexes as receptor molecules for development
of new chloride sensors
Rakesh Kumar Mahajana,∗, Ravneet Kaura, Sartaj Tabassumb,
Farukh Arjmandb, Suvigya Mathurb
a Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India
b Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India
Received 8 March 2006; received in revised form 8 May 2006; accepted 12 May 2006
Available online 5 July 2006
Abstract
Highly sensitive and selective chloride polymeric membrane sensors are developed that employ Cu(II) complexes as anion carriers. Optimized
membrane sensors showed a near-Nernstian response towards chloride anion over a wide concentration range of 2.5 × 10−5 to 1.0 × 10−1 M and
have micromolar level detection limits. These sensors have fast response time and work well in the pH range of 4.2–9.6. They exhibited enhanced
potentiometric selectivity for chloride over other anions, including more lipophilic anions such as perchlorate, thiocyanate, nitrate, etc. Response
characteristics (e.g. detection limit, linear range, response slope and selectivity) of these sensors remain essentially the same over a period of ∼5
months, reflecting remarkable stability.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Chloride anion; Ion carriers; Cu(II) complexes; Membrane sensors
1. Introduction
applications [13]. Several approaches of reducing interferences
from lipophilic anions have been reported. Chloride sensors
based on neutral carrier type metal complexes such as metal-
loporphyrins [14,15] and mercury organic derivatives [16,17]
have also been developed. However, the non-classical behavior,
chemical instability and insufficient selectivity impeded the use
of these electrodes in direct monitoring of chloride in phys-
iological samples. Recently, a new chloride sensor based on
ruthenium(III) Schiff’s base complex has been reported [18].
The sensor was successfully applied for the determination of
chloride in serum samples.
Since various inorganic anions play fundamental roles in
chemical and biological systems, the development of specific
receptors to monitor the targeted anions of biological and envi-
ronmental interest remains a formidable challenge. Developing
better chloride ionophores is of high interest, mainly because of
the significance of measuring chloride in physiological appli-
cations. An attempt has been made to prepare metal complexes
for use as chloride anion carriers. The design of dinuclear com-
plexes employs a synthetic strategy where different multidentate
ligand systems are coordinated around vacant coordination sites
on the metal ions [19–22]. Much emphasis has been given
to the molecular framework by choosing an appropriate lig-
Carrier-based ion-selective electrodes have become increas-
ingly useful in clinical and physiological measurements [1,2].
The development of anion-selective liquid/polymeric mem-
brane electrodes for the assessment of anion concentrations in
real samples is of increasing interest for clinical, environmen-
tal and industrial applications [3–7]. Various approaches have
been attempted to develop chloride selective electrodes. Anion
exchangers have been employed as electroactive species for
the development of chloride sensors [8,9]. However, these sen-
sors lack selectivity over more lipophilic anions and respond
significantly to more lipophilic anions according to well-
known Hofmeister series [10]. A host of organotin compounds
[11,12], utilized as anion carriers, in chloride selective sen-
sors demonstrate a selectivity pattern that also deviates from
the lipophilicity-based Hofmeister selectivity. These sensors,
due to certain limitations of long response time, non-Nernstian
response and potentiometric signal instability, lacked practical
∗
Corresponding author. Fax: +91 183 2258820.
E-mail address: rakesh chem@yahoo.com (R.K. Mahajan).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.05.023

R.K. Mahajan et al. / Electrochimica Acta 52 (2006) 408–414
409
and system and pendant functional groups around the metal
site. The incorporation of a functionally active ligand system,
its denticity, chelate ring size, donor type and substituents on
or near N-donors influence the selectivity and M2-O2 struc-
ture reactivity [23,24]. In this work, a biologically significant
class of ligand ␤-diketone was chosen primarily as it exhibits
versatile coordination ability and shows bidentate chelate for-
mation [25]. Secondly, ␤-ketonate complexes have proved to be
efficient antitumor agents against various carcinoma [26]. The
dinuclear copper complexes with two metal ions in close prox-
imity have received great attention [27–29] due to their potential
use as bimetallic catalysts modeling the enzyme activity. Schiff
bases are also currently being studied extensively owing to their
biological and anticancer activity [30–34]. They are versatile lig-
andsformanytransitionmetalsandprovidedonoratomssuitable
for the formation of di- and trinuclear complexes. On complexa-
tion, the biological activity including anticancer is considerably
enhanced which has led to metal-based pharmaceuticals [35].
Diazines in aromatic heterocyclic ligands have been utilized in
bringing two copper(II) centers into close proximity to study the
influence of antiferromagnetic intramolecular exchange.
In the present work, Cu(II) complexes are examined as unique
ion carriers in PVC-matrix-based anion sensors. They have been
demonstrated to work as efficient carriers of chloride anion in
the wide working linear range with non-Hofmeister selectivity
pattern indicating better selectivity for chloride over other inor-
ganic and organic anions.
2. Experimental
2.1. Cu(II) complexes
Cu(II) complexes used in this work are shown in Fig. 1. Cu(II)
complex 1 has been synthesized by the method reported in lit-
erature [36], whereas Cu(II) complex 2 has been synthesized
by following method: to a solution of the ligand LH2 (0.5 g,
1 mmol) in dichloromethane (25 cm3) was added CuCl2·2H2O
(0.3 g, 2 mmol) in MeOH in 1:2 molar ratio. The colored product
was isolated by slow evaporation of the resultant mixture, fil-
tered, washedwithhexaneanddriedinvacuo. Purplesolid, yield,
70%; mp 98 ◦C; calculated for C32H30N6O2Cu2Cl2: 58.8% C,
4.1% H, 11.5% N. Found: 58.5% C, 4.1% H, 11.4% N. IR
Fig. 1. Structures of Cu(II) complexes.

410
R.K. Mahajan et al. / Electrochimica Acta 52 (2006) 408–414
(in nujol mull) (cm−1): 1608 (C N of N2O2 donor set), 1007
(N N), 1490 (C N), 1240 (C O), 530 (Cu N), 475 (Cu O),
360 (Cu Cl). UV–vis (in CH2Cl2) (cm−1): 26,310, 18,796.
The measurements of the electrode potentials were done with
an Equip-tronics model EQ-602 potentiometer, and pH mea-
surements were made with an Elico LI-model-120 pH meter.
2.2. Other reagents
3. Results and discussion
Poly(vinyl chloride) (PVC), dioctylsebacate (DOS),
dioctylphthalate (DOP) and dibutylphthalate (DBP) were
purchased from Fluka. Dried tetrahydrofuran (THF) and
hexadecyltrimethylammonium bromide (HTAB) were used as
received from Lancaster. Sodium salts of various interfering
anions obtained from S.D. Fine-Chemicals were of analytical
reagent grade. Doubly distilled de-ionized water was used
throughout.
3.1. Response characteristics of sensors
In the preliminary experiments, the membrane electrodes
were prepared incorporating Cu(II) complex 1 as ion carrier
and DOP as plasticizer. The potentiometric response of these
electrodes was observed for different anions such as fluoride,
chloride, iodide, nitrate, salicylate and thiocyanate anions. The
potential versus logarithm of concentration curves for these
anions so obtained are shown in Fig. 2. It is clear from Fig. 2
that the sensor for chloride anion showed better potentiomet-
ric response as compared to other anions studied. This indicates
that Cu(II) complex 1 is capable of binding chloride anion selec-
tively than other anions. Similar experiments were preformed for
Cu(II) complex 2 and this complex was also found to sense chlo-
ride anion sensitively and selectively over a wide concentration
range as compared to others.
The sensitivities and selectivities obtained for a given ion
carrier depend significantly on the nature of the plasticizer used
in the preparation of membranes. Since it influences the dielec-
tric property of the membrane phase and the mobility of the ion
carrier in the PVC matrix, its selection is one of the most impor-
tant tasks in designing a sensitive and selective anion-selective
sensor. In the present study, membranes were prepared by incor-
porating three different plasticizers (DOP, DOS and DBP). The
membrane compositions and electrode characteristics of these
membranes are given in Table 1. Among all of these (mem-
branes A–F), membrane electrodes C and F incorporating DBP
as plasticizer showed best sensitivity for the sensors in terms of
slope and working linear concentration range, whereas the use
of DOP and DOS in membranes A, D and B, and E, respec-
tively, resulted in a pronounced reduction in the slope of the
2.3. Electrode preparation and potential measurements
General procedure adopted to prepare PVC membrane was
as follows: the appropriate amounts of various membrane ingre-
dients [4.9 mg copper(II) complex 1, 100.3 mg PVC, 2.0 mg
HTAB, 200.4 mg DBP, and 5.1 mg copper(II) complex 2,
100.0 mg PVC, 2.0 mg HTAB, 202.3 mg DBP] were dissolved
in 5 mL of THF. The resulting mixture stirred vigorously and
after removing air bubbles, was transferred into 50 mm petridish.
The solvent was allowed to evaporate off at room tempera-
ture overnight. A flexible, transparent membrane of thickness
∼0.4 mm was obtained. A membrane of suitable size was cut
and fixed to PVC tube with the help of PVC glue and condi-
tioned in 1.0 × 10−2 M NaCl solution for 2 days. Table 1 shows
the optimized composition of various Cl− anion-selective elec-
trode membranes.
All emf measurements were carried out with the following
cell assembly:
Ag–AgCl|3 M KCl|internal reference solution
(1.0 × 10−2 M NaCl)|PVC membrane|test solution|
3 M KCl|Ag–AgCl
Table 1
Membrane compositions and electrode characteristics of various chloride sensors prepared
ISEs
PVCa
HTABa
Complexa
Plasticizera
DOP
Slopes (mV per decade)
Linear range (M)
1
2
DOS
DBP
A
B
C
D
E
F
G
H
I
102.4
100.5
100.3
102.0
100.0
100.0
103.2
100.4
101.0
100.4
100.0
100.2
100.2
2.0
2.0
2.0
2.0
1.9
2.0
–
2.0
1.9
2.0
2.0
3.0
5.0
5.1
5.0
4.9
–
–
–
5.0
10.1
15.2
–
–
5.2
5.0
–
–
–
5.2
4.9
5.1
–
–
–
200.5
–
–
–
–
–
−49.49
−46.52
−56.49
−49.97
−47.21
−55.17
−34.87
−52.28
−50.22
−49.35
−44.62
−41.48
−37.37
2.5 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
2.5 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
2.5 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
5.0 × 10−5–1.0 × 10−1
1.0 × 10−4–1.0 × 10−1
2.5 × 10−5–1.0 × 10−2
5.0 × 10−5–1.0 × 10−2
200.7
–
–
200.4
–
–
208.1
–
–
–
–
–
–
–
–
–
201.9
–
–
–
–
–
–
–
–
202.3
203.9
201.5
203.1
200.5
200.5
205.7
200.6
J
9.9
15.3
–
K
L
M
–
HTAB: hexadecyltrimethylammonium bromide, DOP: dioctylphthalate, DOS: dioctylsebacate, DBP: dibutylphthalate.
a
Compositions (in mg).

R.K. Mahajan et al. / Electrochimica Acta 52 (2006) 408–414
411
carrier 2). Both Cu(II) complexes are expected to act as neutral
carriers for anions. Accordingly, membrane electrodes prepared
without lipophilic cationic additives should show a little anionic
response, while a better anionic response is expected only in the
presence of cationic sites in the membranes [37]. These cationic
additives induce the anion permselectivity of the membrane and
work as the counterion for the chloride complex of ion carri-
ers. It is also well known that the cationic additives (HTAB) can
reduce ohmic resistance [38] and improve the response of anion-
selective electrodes [39,40]. It is clear from Table 1 that the
presence of HTAB in the membrane composition (membrane G
without any additive as compared to membrane C) increases the
sensitivity of the sensors. However, the addition of increasing
amount of HTAB (membranes L and M) decreases the sensi-
tivity of the sensors for chloride anions. The membranes with
the optimum ratio of ion carrier/HTAB: 5 mg/2 mg, revealed
near-Nernstian response to the concentration of chloride
anion.
Fig. 2. Potentiometric responses for representative individual anions obtained
with sensors based on Cu-complex 1.
The effect of concentration of internal reference solution
was also studied. The potentiometric response was recorded at
three different concentrations, i.e., 1.0 × 10−1, 5.0 × 10−2 and
1.0 × 10−2 M NaCl of internal reference solution. No significant
change in potential response of the sensors was observed with
the variation of the concentration of internal reference solution
except for an expected change in the intercept of the curves.
Thus, 1.0 × 10−2 M NaCl solution was found quite appropriate
for smooth functioning of the electrode system.
electrode response. The electrodes C and F based on Cu(II)
complexes 1 and 2 showed near-Nernstian slopes of −56.49
and −55.17 mV per decade, respectively, in the linear con-
centration range of 2.5 × 10−5 to 1.0 × 10−1 M (Fig. 3). The
detection limits of these sensors were found to be 7.586 × 10−6
and 9.772 × 10−6 M for C and F, respectively, with the internal
reference solution of 1.0 × 10−2 M NaCl solution.
Ion-carrier-based electrodes are considerably influenced by
the nature of the ion carrier as well as composition of the mem-
brane. We also investigated the influence of membrane compo-
sitions on the potential response of chloride sensors (Table 1).
Various membranes with varying amounts of additive and ion
carrier were prepared. As is obvious from Table 1, increasing
the amount of ion carrier in the membranes resulted in decreased
sensitivity with relative decrease in slope values of the sensors
(membranes C, H and I for ion carrier 1 and F, J and K for ion
3.2. Effect of pH and response time of sensors
The pH dependence of chloride sensors based on Cu(II)
complexes was examined over a pH range of 1–12 at different
concentrations of chloride anion solutions. Fig. 4 demonstrates
the variation of the electrode potentials of chloride sensors C and
F, with pH at a chloride anion concentration of 1.0 × 10−2 M.
From the plots shown in Fig. 4, it is evident that for every sensor
Fig. 3. Potentiometric responses of chloride sensors C and F based on Cu(II)
complexes 1 and 2, respectively, incorporating DBP as plasticizer.
Fig. 4. Plots showing the variation of electrode potential of chloride sensors
with pH at 1.0 × 10−2 M NaCl solution.

412
R.K. Mahajan et al. / Electrochimica Acta 52 (2006) 408–414
Table 2
Pot
Cl ,X
Selectivity coefficient values, log K
− , of chloride sensors
C
−
Diverse anions
F
−
NO2−
−2.46
−2.45
−2.53
−0.12
−2.50
−3.45
−2.62
−2.58
−2.37
−0.08
+0.31
−2.66
−3.33
−3.47
−2.44
−2.73
−2.57
+0.03
−2.52
−3.26
−3.05
−2.42
−2.35
−0.29
+0.15
−2.65
−3.48
−3.53
NO3
−
ClO4
SCN−
−
HCO3
2−
HPO4
−
H2PO4
Salicylate−
F−
Br−
I−
CH3COO−
2−
2−
SO4
CO3
Fig. 5. Response time curve of chloride sensor F, based on Cu-complex 2 for
step changes in concentration of Cl− anion.
3.3. Selectivity
One of the most important characteristics of any ion-selective
electrode is its relative response to the primary ion over other
ions in solution, which is expressed in terms of potentio-
metric selectivity coefficients. The selectivity coefficients
have been determined using fixed interference method (FIM)
based on the semi-empirical Nicolskii–Eisenman equation.
In this method, the concentration of primary ion, chloride
anion, is varied, whereas the concentration of secondary
interfering ions is kept constant in the test solutions which
the electrode potential remained constant over a wide pH range
of 4.2–9.6, which was taken as the functional pH range of the
sensor. The observed drift at higher pH values could be due to
the simultaneous response of the sensors to chloride and hydrox-
ide ions. Pure solutions of sodium chloride do not need any pH
adjustment, since their pH lies in the functional pH ranges of the
sensors.
The average time required for the chloride sensors to reach
a potential within 1 mV of the final equilibrium value after
successive immersion of a series of NaCl solutions, each hav-
ing a 10-fold difference in concentration, was measured. Fig. 5
shows the potential versus time trace for chloride sensor, F, from
lower (1.0 × 10−4 M) to higher (1.0 × 10−1 M) concentration of
NaCl solutions. The sensor yielded steady state potentials within
20 s, and the electrode potential remained constant for more
than 5 min, after which a very slow divergence was observed.
Similarly response time recorded for chloride sensor, C, was
reasonably short (within 20 s), and the stability of the potentio-
metric signal was quite satisfactory. The sensing behavior of the
membrane sensors remained unchanged when the potential was
recorded either from low to high concentration or vice versa.
The present type of sensors gave reproducible potentials with
the standard deviation of 1 mV and their potentials remained
constant for more than 5 min. In terms of lifetime of the sensors,
these could be used over a period of 5–6 months without any
significant change in the value of slope, working concentration
range and response time.
is 1.0 × 10−2 M concentrations of interfering ions in present
Pot
case. The potentiometric selectivity coefficients (log K
)
Cl−,X−
of chloride sensors for various interfering ions are given in
Table 2. The potentiometric selectivity sequence for sensor C
wasfoundtobeintheorderofI− > Br− > SCN− > F− > NO3
≈
−
NO2− > HCO3− > ClO4− > salicylate > H2PO4− > CH3COO− >
SO42− > HPO42− > CO3
,
whereas that for sensor −
F
>
>
2−
was found as I− > SCN− > Br− > F− > salicylate > NO2
HCO3− > ClO4− > CH3COO− > NO3− > H2PO4− > HPO4
2−
SO42− > CO32−. The non-Hofmeister selectivity sequence so
obtained for both chloride sensors C and F is assumed to be
associated with an interaction between the central metal of
the carrier and chloride ion. Most of the anions were found
to induce negligible disturbance on the normal functioning of
the chloride sensors. Thiocyanate, bromide and iodide anions
were observed to be interfering anions but the selectivity values
calculated for these anions were found to be better than those
reported earlier in literature (Table 3). As can be seen from
Table 2, the selectivity coefficients for these sensors were quite
Table 3
Pot
Cl ,X
Comparison of selectivity coefficient values, logK
− , of proposed chloride sensors with those reported in literature
−
Interfering anion
Sensor C
Sensor F
Ref. [41]
Ref. [42]
Ref. [43]
Ref. [44]
Ref. [45]
SCN−
I−
−0.12
+0.31
−0.08
−2.45
−2.50
+0.03
+0.15
−0.29
−2.73
−2.52
+1.0
+0.5
+0.4
+0.7
–
+3.8
+3.4
+1.4
+2.1
–
+0.6
+0.8
+0.1
−0.9
−1.9
+3.4
+2.4
+0.9
+1.2
–
−0.16
+2.34
+1.04
−2.71
–
Br−
−
NO3
−
HCO3

R.K. Mahajan et al. / Electrochimica Acta 52 (2006) 408–414
413
Table 4
Acknowledgement
Determination of chloride anion concentration in synthetic water samples using
chloride sensor F
One of the authors (Ravneet Kaur) is thankful to Council of
Scientific and Industrial Research (CSIR), New Delhi, India, for
the financial assistance.
Amount of Cl−
added (mM)
Amount of Cl−
recovered (mM)
Recovery (%)
80
50
30
20
10
77.8
47.2
28.7
18.8
9.1
97.2
94.4
95.7
94.0
91.0
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performance of the sensors in the presence of this interfering
anion, various experiments were carried out at 1.0 × 10−2
,
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4. Conclusion
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