1. Full Paper
Ionic Liquids Modify the Performance of Carbon Based
Potentiometric Sensors
Afsaneh Safavi,* Norouz Maleki,* Fatemeh Honarasa, Fariba Tajabadi, Fatemeh Sedaghatpour
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran
*e-mail: safavi@chem.susc.ac.ir
Received: October 31, 2006
Accepted: December 27, 2006
Abstract
A new type of potentiometric sensor based on a recently constructed carbon ionic liquid electrode (CILE) is
described. Two kinds of ionic liquids, i.e., N-octylpyridinium hexafluorophosphate (OPFP) and 1-butyl-3-
methylimidazoluim hexafluorophosphate (BMFP) were tested as binder for construction of the carbon composite
electrode. The characteristics of these electrodes as potentiometric sensors were evaluated and compared with those
of the traditional carbon paste electrode (CPE). The results indicate that potentiometric sensors constructed with
ionic liquid show an increase in performance in terms of Nernstian slope, selectivity, response time, and response
stability compared to CPE.
Keywords: Carbon ionic liquid electrode, Potentiometric sensor
DOI: 10.1002/elan.200603767
1. Introduction
Ion selective electrodes (ISEs) have become popular in
environmental and biological fields because of their inter-
esting abilities such as simple, quick, low cost, accurate and
wide-dynamic-range analysis. The majority of works on
ISEs are based on the use of polymeric membranes as the
suitable material for construction of potentiometric sensors.
But finding a suitable internal solution and selection of
composition to obtain an elastic and stable membrane are
accounted as a main trouble in these kinds of electrodes.
Therefore, in recent years carbon paste electrodes (CPEs)
have attracted attention as ion selective electrodes mainly
due to their advantages over membrane electrodes such as
chemical inertness, robustness, low cost, renewability, stable
response, low ohmic resistance, no need for internal solution
and suitability for a variety of sensing and detection
applications [1 – 9].
The CPE based potentiometric sensors which have been
introduced up to now are mainly based on incorporation of a
selective agent into carbon paste. The carbon paste usually
consists of graphite powder dispersed in a nonconductive
mineral oil. Incorporation of mineral oil gives CPE some
disadvantages. Mineral oil is not component-fixed since it is
involved in various refining of petroleum and processing of
crude oil, and some unaccounted ingredients may engender
unpredictable influences on detection and analysis [10]. In
addition they have mechanical problem, their mechanical
stability is something between membrane electrodes and
solid electrodes.
Ionic liquids (ILs) make a lot of impact on branches of
analytical chemistry [11, 12]. It has been reported that IL
incorporates several advantages in the membrane electro-
des. It was approved that the presence of IL (1-butyl-3-
methylimidazolium hexafluorophosphate) as additive and
ionophore (polyazacycloalkane derivative) at the same time
improved selectivity of sulfate in PVC membrane electrode
[13]. A successful utilization of imidazolium and phospho-
nium based ILs has been reported in various polymer
membranes as both plasticizers and ion responsive media
[14]. Membranes containing IL even promote the character-
ization of all-solid-state Ag/AgCl reference microelectrode
[15]. Although the above reports have been appeared
recently in the literature on the use of IL in potentiometric
sensors, to the best of our knowledge there has been no
report on incorporation of IL in to carbon-based potentio-
metric sensors.
Recently we introduced and characterized the electro-
chemical behavior of a new type of carbon composite
electrode called carbon ionic liquid electrode (CILE), in
which an ionic liquid, N-octylpyridinium hexafluorophos-
phate (OPFP), has been used as a binder [16]. The
voltammetric responses of CILE towards different inorgan-
ic, organic and biomolecules have been reported [16 – 18].
Room-temperature ionic liquids (ILs) are a good choice as
binder in carbon composite electrodes, because of their
interesting properties, such as stability, low vapor pressure,
low toxicity, low melting temperature, high ionic conduc-
tivity and good electrochemical and thermal stability. As we
have demonstrated previously [16], this type of electrode
has lower ohmic resistance than CPE and gives very
reproducible and sensitive voltammetric results. However,
the performance of this type of electrode has not yet been
tested as a potentiometric sensor.
582
Electroanalysis 19, 2007, No. 5, 582 – 586 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim

2. Besides the lower ohmic resistance of CILE with respect
to CPE [16], it is expected that the presence of a hydrophilic
binder in the electrode rather than the hydrophobic, non-
component fixed mineral oil used in traditional CPE, can
significantly alter the response of the electrode as the
potentiometric sensor. The majority of potentiometric
sensors use a selective agent in the paste to increase
selectivity of the sensor. However, when the ionophore
does not form a strong interaction with the ion of interest,
the response of the electrode mainly depends on the relative
partition coefficient of the ion of interest in water and in the
electrode.
Our main goal in this work is to determine how
incorporation of a hydrophilic binder can change the
performance of a carbon paste electrode. For this purpose
we solely investigated the response characteristics of
potentiometric carbon ionic liquid electrodes without
incorporation of any selective reagent, and compared this
type of electrode with an unmodified CPE. There are few
reports on the use of unmodified CPEs as potentiometric
sensors. It has been reported previously that unmodified
CPE can be used for the determination of Agþ
[19].
However, Cu2þ
, Fe3þ
, Cr3þ
, Hg2þ
and Al3þ
severely inter-
fered with this determination. To increase the applicability
of such a sensor Shamsipur et al. used artificial neural
network for simultaneous potentiometric determination of
Agþ
, Hg2þ
and Cu2þ
[20].
In this paper, we report on the successful utilization of
CILE as a potentiometric sensor and will demonstrate that
the use of ionic liquid increases the analytical performance
of the carbon electrodes as potentiometric sensors. The
characteristics of this electrode have been compared with
those of CPE based potentiometric sensors.
2. Experimental
2.1. Reagents and Apparatus
All reagents were of analytical reagent grade. Triply distilled
water was used throughout. Graphite powder (Fluka),
particle size 100 mm and paraffin oil (Merck) were used
for preparation of CPE. Stock solutions (0.1 M and 0.01 M)
of appropriate salts of different ions (all from Merck) were
prepared. Ionic liquids, N-octylpyridinium hexafluorophos-
phate (OPFP) and 1-butyl-3-methylimidazoluim hexafluor-
ophosphate (BMFP) were synthesized as described else-
where [21 – 23]. The electromotive force (EMF) measure-
ments were performed at 25 Æ 28C with Metrohm 691
Digital pH-Meter.
2.2. Procedure
2.2.1. Preparation of Electrodes
The traditional carbon paste electrode (CPE) was prepared
by hand-mixing of the graphite powder and paraffin (70% –
30%). The mixture was mixed until a uniform paste was
obtained. The paste was packed into the cavity of a Teflon
tube. Electrical contact to the carbon paste was established
via a stainless steel handle. Fresh surface was obtained by
applying manual pressure to the piston. The resulting fresh
surface was polished on a paper until the surface had a shiny
appearance. It is important to renew the electrode surface
when the solution changed because residual ion will still be
adsorbed on the surface of the CPE, which will lead to poor
reproducibility.
The carbonionic liquidelectrodes (CILEs) wereprepared
similar to CPE. In these electrodes either OPFP or BMFP
were used instead of paraffin with different percentages.
When using OPFP which is solid at room temperature, as
suggested previously [16], heating the ionic liquid- graphite
mixture to a temperature higher than the melting point of
OPFP (m.p. 658C) is essential.
2.2.2. Emf Measurements
The CPE or CILE was used as an indicator electrode. A
double-junction silver/silver chloride electrode (Metrohm)
was used as a reference electrode. The potential response of
the electrode was investigated by measuring the emf values
of a solution of the desired ion over a concentration range by
adding successive aliquots of known concentrations of the
ion of interest to 25 mL of triply distilled water.
3. Results and Discussion
Preliminary experiments were performed to elucidate the
effect of replacing mineral oil with ionic liquid in the carbon
paste on potentiometric responses. For this purpose the
potentiometric responses of the unmodified CPE and
CILEs towards different cations were studied. Selectivity,
response time, calibration slope, linear range, and response
stability are important parameters in studies of ion selective
electrodes and are going to be discussed herein.
3.1. Electrode Composition
It is known that the sensitivity and linearity for a given
electrode depend significantly on the electrode composi-
tion. For investigation of the electrode composition, seven
electrodes were prepared. The compositions of these
electrodes are shown in Table 1. The potentiometric re-
sponse of each electrode towards different ions was inves-
tigated and the results are compared. The OPFP containing
CILE has a higher mechanical stability than BMFP based
CILE and CPEs.
3.2. Potentiometric Responses
Since no modifier was used in the electrodes studied, the
selectivity sequence is related to the relative cation partition
583Performance of Carbon Based Potentiometric Sensors
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3. coefficients between water and the electrode. Thus, the
response of traditional CPE to some cations is incurred by
paraffin liquid extraction and by the exchange current
caused by some cations on the surface of the carbon.
Paraffin liquid on the graphite is apt to extract compounds
with nonpolar character. The extent of extraction of lip-
ophilic cations is thus expected to be higher in CPE rather
than CILE which contains ionic liquid with much higher
dielectric constant as the binder. Among different cations
tested with CPE (electrode no.1), potentiometric responses
were observed for six cations, namely Agþ
, Hg2þ
, Fe3þ
, Cr3þ
,
Cu2þ
and Al3þ
. In comparison, the electrodes prepared with
ionic liquid respond primarily towards cations such as Agþ
Hg2þ
and Fe3þ
. Tables 2 and 3 show the linear ranges,
calibration slopes and correlation coefficients obtained for
potentiometric responses of different electrodes towards
different ions, respectively. These tables also show that the
electrodes containing ionic liquid show better selectivity. It
is also clear from Table 3, that among all the cations tested,
the response of all the electrodes were approximately
Nernstian towards Agþ
. Potential dependences of electro-
des no.1, 3 and 6 on Agþ
cation are represented in Figure 1.
Thus, in the rest of this study, the responses of different
electrodes are compared towards Agþ
ion. The response
time of all the electrodes were measured. For this purpose,
the average time needed for the electrode to reach 90% of
the potential response after successive immersion in a series
of Agþ
solutions, each having 10 fold differences in
concentration, are reported in Table 4.
Table 1. Electrodes with different composition.
Composition No.
1 2 3 4 5 6 7
Graphite (%) 70 60 70 80 60 70 80
Paraffin (%) 30 – – – – – –
N-Octylpyridinium hexafluorophosphate(%) – 40 30 20 – – –
1-Butyl-3-methylimidazoluim hexafluorophosphate(%) – – – – 40 30 20
Fig. 1. Potential dependence of Agþ
cation for electrodes no.1, 3, and 6.
Table 2. Linear range for different cations using different electrodes.
Electrode
number
Linear range
Agþ
Hg2þ
Fe3þ
Cr3þ
Cu2þ
Al3þ
1 9.8 Â 10À6
– 9.2 Â 10À3
3.1 Â 10À5
– 7.4 Â 10À5
2.0 Â 10À6
– 4.6 Â 10À5
2.7 Â 10À3
– 1.6 Â 10À2
8.0 Â 10À6
–
3.1 Â 10À3
2.6 Â 10À3
–
1.6 Â 10À2
2 1.3 Â 10À5
– 4.5 Â 10À3
1.7 Â 10À5
– 6.4 Â 10À5
1.1 Â 10À4
– 6.2 Â 10À3
2.4 Â 10À3
– 1.2 Â 10À2
– –
3 6.2 Â 10À6
– 1.3 Â 10À2
1.3 Â 10À5
– 6.7 Â 10À5
1.4 Â 10À4
– 1.2 Â 10À2
4.0 Â 10À3
– 1.5 Â 10À2
– –
4 2.0 Â 10À5
– 4.0 Â 10À4
1.9 Â 10À5
– 4.9 Â 10À5
2.1 Â 10À4
– 7.7 Â 10À3
1.3 Â 10À3
– 9.7 Â 10À3
– –
5 3.9 Â 10À5
– 4.9 Â 10À3
2.4 Â 10À5
– 1.8 Â 10À4
4.0 Â 10À4
– 7.7 Â 10À3
– – –
6 2.07 Â 10À5
– 6.33 Â 10À3
6.5 Â 10À5
– 1.6 Â 10À4
1.1 Â 10À4
– 1.0 Â 10À3
– – –
7 1.1 Â 10À5
– 6.4 Â 10À4
3.0 Â 10À5
– 8.6 Â 10À5
5.9 Â 10À4
– 7.2 Â 10À3
– – –
584 Afsaneh Safavi et al.
Electroanalysis 19, 2007, No. 5, 582 – 586 www.electroanalysis.wiley-vch.de 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim

4. As a result of comparing the responses of all electrodes
towards Agþ
, electrode no. 3containing OPFP, graphite with
the ratio of 30:70 displayed the best performance as a
potentiometric sensor.
3.1.2. Stability
The equilibrium potentials remained essentially constant
for about 170 min when CILE was used as the sensor, after
which only a small divergence was observed. In comparison,
the CPE response was constant up to 50 min after which a
steady change in potential reading was observed. The
standard deviations of the potential responses over a period
of 4 h in a 1 mM solution of Agþ
ion were 1.5, 2.0 and 4.3
(n ¼ 25), for electrodes no. 3, 6, and 1, respectively (Fig. 2).
This shows that the presence of ionic liquid as the binder,
imparts better stability in carbon composite electrodes.
3.1.3. Effect of pH
The effects of pH value have been investigated on CPE and
CILEs. The solution was diluted with acetate buffers with
pH values in the range of 4 to 6.2 (a limited range due to
precipitation of Agþ
at higher pH values) and the responses
for Agþ
were recorded. The obtained results are illustrated
in Table 5.
Variation of pH values did not cause serious change in
slope. But the best results were obtained at pH 4.1 for the
three electrodes tested. Comparison between the three
electrodes reveals that CILE with OPFP as the binder show
the best slope at different pH values.
4. Conclusion
This study shows that replacing paraffin with ionic liquid in
carbon paste electrodes, modifies the performance of the
electrode as potentiometric sensor. The electrode contain-
ing 30% – 70% OPFP – graphite offers the best Nerntian
slope, response stability, and response time. The potentio-
metric responses are very stable and the electrodes can be
easily renewed by removing the surface. This electrode
shows the least variation in response with changing pH from
4 to 6 and is thus better suited for work on real samples.
Table 3. Calibration slopes and correlation coefficients for differ-
ent electrodes
No. of electrodes Calibration slope (mV/decade)
Correlation coefficient (R)
Agþ
Hg2þ
Fe3þ
Cr3þ
Cu2þ
Al3þ
1 56.2 138.3 121.5 126.9 29.7 183.0
0.998 0.994 0.995 0.997 0.985 0.994
2 53.0 32.25 81.0 212.4 – –
0.993 0.996 0.997 0.992 – –
3 58.8 50.0 103.8 143.6 – –
0.997 0.999 0.997 0.997 – –
4 55.9 62.8 92.6 143.6 – –
0.984 0.984 0.998 0.980 – –
5 53.2 306.6 104.1 – – –
0.998 0.994 0.999 – – –
6 54.7 53.0 153.4 – – –
0.997 0.982 0.996 – – –
7 62.3 261.0 92.2 – – –
0.998 0.992 0.998 – – –
Table 4. Response times of different electrodes towards Agþ
ion.
Type of electrodes 1 2 3 4 5 6 7
Response time (s) 15 13 8 9 10 8 10
Fig. 2. Potential stability of electrodes no.1, 3, and 6.
Table 5. Calibration slope for different electrodes towards Agþ
in
several pH range
No. of electrodes pH 4.1 pH 5.0 pH 6.2
1 60.3 63.1 63.1
3 58.3 60.5 60.3
6 60.3 63.2 63.7
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5. 5. Acknowledgement
The authors wish to acknowledge the support of this work by
Iranian Ministry of Sciences, Research and Technology.
6. References
[1] J. Jezkova, J. Musibva, K. Vytras, Electroanalysis 1997, 9,
1433.
[2] M. N. Abbas , J. Pharm. Bio. Anal 2003, 31, 819.
[3] M. Shamsipur, S. Ershad, N. Samadi, A. Moghimi, H.
Aghabozorg, J. Solid State Electrochem. 2005, 9, 788.
[4] A. Abbaspour, S. M. M. Moosavi, Talanta 2002, 56, 91.
[5] H. Ibrahim, Anal. Chim. Acta 2005, 545, 158.
[6] M. J.Gismera, D. Hueso, J. R. Procopio, M. T.Sevilla, Anal.
Chim. Acta 2004, 524, 347.
[7] K. Vytras, E. Khaled, J. Jezˇkova´, H. N. A. Hassan„B. N.
Barsoum, Fresenius J. Anal Chem. 2000, 367, 203.
[8] M. H. Mashhadizadeh, A. Mostafavi, H. Allah-Abadi,
I.Sheikhshoai, Sens. Actuators B 2006, 113, 930.
[9] H. Ibrahim, J. Pharm. Biomed. Anal 2005, 38, 624.
[10] H. Liu, P. He, H. Li, C. Sun., L. Shi, Y. Liu, G. Zhu, J. Li,
Electrochem. Commun. 2005, 7, 1357.
[11] L. Jing-fu , J. A. Jonsson, G.b. Jiang, TRAC 2005, 24, 20.
[12] S. Pandey, Anal. Chim. Acta 2006, 556, 38.
[13] C. Coll, R. H. Labrador, R. M. Manez, J. Soto, F. Sancenon,
M. J. Seguí, E.Sanchez, Chem. Commun. 2005, 3033.
[14] N. V. Shvedene, D. V. Chernyshov, M. G. Khrenova, A. A.
Formanovsky, V. E. Baulin, I. V. Pletnev, Electroanalysis
2006, 18, 1416.
[15] R. Maminska, A. Dybko, W.Wroblewski, Sens. Actuators B
2006, 115, 552.
[16] N. Maleki, A. Safavi, F. Tajabadi, Anal. Chem. 2006, 78, 3820.
[17] A. Safavi, N. Maleki, O. Moradlou, F. Tajabadi, Anal.
Biochem. 2006, 359, 224.
[18] A. Safavi, N. Maleki, F. Tajabadi, Analyst 2007, 132, 54.
[19] J. Pei, Q. Yin, J. Zhong, Talanta 1991, 38, 1185.
[20] M. Shamsipur, J. Tashkhourian, B. Hemmatinejad, H. Shar-
ghi, Talanta 2004, 64, 590.
[21] J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willa-
uer, G. A. Broker, R. D. Rogers, Green Chem. 2001, 3, 156.
[22] S. Chun, S. V. Dzyuba, R. A. Bartsch, Anal. Chem. 2001, 73,
3737.
[23] J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E.
Visser, R. D. Rogers, Chem. Commun. 1998, 1765.
586 Afsaneh Safavi et al.
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