America n Journal of Analy tic al Chemistry, 2011, 2, 376-382
doi:10.4236/ajac.2011.23046 Published Online July 2011 (http://www.scirp.org/journal/ajac)
Imprinted Polymer Inclusion Membrane Based
Potenti om et ric S en sor fo r Determinat i on an d
Quantification of Diethyl Chlorophosphate in Natural
Waters
Varada Vishnuvardhan1, Yakka la Kalyan1, Krishn apillai Padmajaku mar i Prathish2,
Battala Gangadhar1, Yadamari Tharakeswar1, Talasila Prasada Rao2, Gurijala Ramakrishna Naidu1*
1Department of Environmental Sciences, Sri Venkateswara University, Tirupati, India
2National Institute for Interdisciplinary Science & Technology, CSIR, Trivandrum, India
Received January 3, 2011; revised Janua ry 26, 2011; accepted May 6, 2011
Abstract
Biomimetic potentiometric sensor for the determination of diethyl chlorophosphate was developed using im-
pr int ed pol ymer inclusion membrane strategy. Semi-covalent i mprinted and non-imprinted polymer particles
were synthesized and found that non-imprinted polymer inclusion membrane was unstable in contrast to im-
printed polymer inclusion membrane in determination and quantification of diethyl chlorophosphate. Im-
printed polymer inclusion membrane based sensor found to be pH dependant with a 5 min equilibrium re-
sponse time at pH = 10.5 and linearly responds to diethyl chlorophosphate in the concentration range of 1 ×
10–9 to 1 × 10–4 a nd 1 × 10–4 to 1 × 10–2 mol·L–1 with a detection limit of 1 × 10–9 mol·L–1 (0.17 ppb). It was
found that diethyl chlorophosphate response was selective against various selected interferents like pinacolyl
methylphosphonate, dimethyl methyl phosphonate, methylphosphonic acid, Phorate and 2, 4-D. The devel-
oped sensor was found to be stable for 3 months and can be reusable more than 30 times without loosing
sensitivity. The developed sensor was successfully applied for the determination of diethyl chlorophosphate
in natural waters.
Keywords: Sensor, Imprinted Polymer Inclusion Membrane, Potentiometry, Diethyl Chlorophosphate,
Natural Waters
1. Introduction
Growing concerns with regard to determine the trace
amounts of chemical warfare agents (CWAs) in envi-
ronment, necessitate to the continuous development of
simple analytical methods that can be employed as long
term monitoring aids, used primarily as alarms. Nerve
agents, in particular, are among the most lethal CWAs.
The uses of nerve agents by terrorist organizations or
even states are significant as they can be readily synthe-
sized by simple chemical reactions and often have an
extremely high toxicity. Highly toxic nerve agents such
as G series agents-(GA) Tabun, GB (Sarin), GD (So-
man), GF and V series agents-VE, VG, VM and VX are
pow- erful inhibitors of acetyl cholinesterase, which is
critical in nerve function [1]. The use of nerve agents in
1988 that killed thousands of Kurdish villagers and 1991
Gulf war further emphasized the threat of chemical war-
fare [2]. Two Sarin gas attacks in Matsumotoa and
Tokyo, Japan in 1994-1995 and events in the United
States in 2001 have confirmed this horrible reality [3].
Due to the lethality of these agents, detection and moni-
tor ing of ner ve age nts a re of prime importance in overall
safety and security of humans, animals and plants.
Therefore, it is necessary to develop detection systems
that are port- able, inexpensive, simple, rapid, selective
and sensitive for analyzing environmental security
threats.
The molecular imprinting technique [4] continues to
be a fascinating field of analytical chemistry offering
strategies for creating molecule-specific recognition ma-
trices with recognition capabilities analogous to those o f

V. V I SHN UVAR D HAN ET AL.
377
biological receptors [5]. The shape, size and p ositions of
the functional groups in the recognition sites generated
are complementary to thos e of the o rigina l anal yte . Thus,
molecularly imprinted polymers (MIPs) rebind their
original analytes in preference to related molecules.
MIPs have considerable potential for applications in the
areas of clinical analysis, medical diagnostics, environ-
mental monitoring and drug delivery. Imprinted polymer
materials possess several other virtues viz. physical and
chemical stability, storage endurance and imprint mem-
ory which is esse ntial for preparation of recognition me-
mbranes in a robust and reusable sensing device. More-
over, MIPs are usually cheaper and more accessible high
affinity rec ognition material s in contrast to many biolo g-
ical entities [6].
Because of the high toxicity of CWAs, less toxic
structural analogs that directly mimic or imitate the ac-
tual CWAs of simulant diethyl chlorophosphate (DCP)
detection is very important task for verification studies
since the corresponding simulant can be used to indicate
the presence of the nerve agents produced or used. There
have been many innovations for the detection of this
species including colorimetric detection methods [7,8]
surface acoustic wave devices [9,10], enzymatic assays
[11], interferometry [12] and fluo re sce nt sensor s [1 3 -15].
However, all are plagued by at least one limitation or
other such as slow response, lack of selectivity, poor
sensitivity, operational complexities or non-portability.
Even though sensitivities are high, most of the fluores-
cent detectors performances have been demonstrated in
non-aqueous media, which makes them unsuitable for
real time analysis, which may require tedious extraction
procedures. Pote ntiometric sensor s, a subgroup of chemi -
cal sensors, are attractive for practical applications, as
they are associated with small size, portability and low
energy consumption and cost compared to other group of
sensors. The development of MIP based sensors with
potentio metric transduct ion do es not require the te mplate
or print molecule to be extracted from the membrane to
create membrane potential and does not have to diffuse
through the membra ne, so that there is no size restriction
on the template molecule, the main achilles heel of
MIP’s until recently [16-18]. Zhou, et al. [19] reported
for the first time MIP based potentiometric sensor for
methylphosphonic acid, an ultimate degradation product
of nerve age nts b y c oupl ing sur face i mp rin ting t ec hnique
with a nanoscale transducer, indium tinoxide. The litera-
ture reveals that the prepared imprinted polymer mem-
branes can be effectively used for the detection of nerve
agents by fabricating them into potentiometric sensors
[19-21]. These sensing devices are essentially based on
use of polymer materials prepared via non-covalent
strategy. However, due to non-persistent nature of nerve
agents, in non-covalent strategy, the decomposition pro-
ducts of nerve agents lead to a variety of binding sites
depending on the nature of decomposition products. The
previous studies [22] had succeeded in construction of
semi-covalent strategy based in-situ monolithic polymer
membrane based sensor for DCP via single pot synthesis.
The extractio n of all possible degrad ation pro ducts o f the
chemical warfare agents using molecularly imprinted
polymers were developed by various research groups
[20,23,24]. The semi-covalent strategy has been suc-
cess fully d emonstra ted in t he pr esent stud y for the fabr i-
cation of imprinted polymer inclusion membrane (IPIM)
based sensor for the detection and quantification of DCP ,
a si mulant of so man pre sent in sp iked na tural wat ers. As
this technology does not require large instrumentation, it
has feasibility for routine monitoring studies for the de-
termination of chemical warfare agents or their simu-
lants, where samplin g is dif ficult and t his met hod ca n be
extensively applied for the determination of trace
amou nts o f c ont a mi na nt s i n e nvir o n ment , p ha rmaceutical
and food processing industries and in biomedical appli-
cations
2. Experimental
2.1. Reagents and Electrochemical Equipment
Diethyl chlorophosphate (DCP), 4-vinyl aniline (VA),
dimethyl methyl phosphonate (DMMP), pinacolyl meth-
ylphosphonate (PMP), methylphosphonic acid (MPA)
were obtained form Aldrich (Milwauke, WI, USA).
Phorate, 2,4-D were obtained from SUPELCO, Penn-
sylvania, USA. 2-hydroxyethyl methacrylate (HEMA),
ethylene glycol dimethacrylate (EGDMA), 2,2’-azobis
isobutyronitrile, 2-nitrophenyl octyl ether (NPOE), di-n-
octyl phthalate (DOP), bis-(2-ethylhexyl) sebacate (BE
HS), tris-(2-eth ylhexyl ) phosp hate (T EHP) a nd high mo-
lecular mass poly (vinyl chlor ide) (PVC) were purchased
from Aldrich (Milwauke, WI, USA). All other chemicals
were of analytical grade reagents. Stock standard solu-
tion of (0.1 mol·L–1) DCP was prepared by dissolving
1.725 g of DCP in 100 mL of deionized water. The solu-
tions of 1.0 × 10–2 to 1.0 × 10–11 mo l·L–1 were prepared
by aqueous dilution of a definite volume from the stock
standard solution. Deionized water was used throughout
the experiment. Potentiometric response characteristics
were studied with an ELICO makes Ion analyzer, Model
No. LI 126 (ELICO, Hyderabad, India).
2.2. Synthesis of Semi-Covalent Imprinted and
Non-Imprinted Po ly mer Parti cles
Diethyl chlorophosphate imprinted particles were syn-

V. V I SHN UVAR D HAN ET AL.
378
thesized as follows, 1 mmol of DCP, 1 mmol of VA to
form covalent bond with DCP thus resulting in covalent
interactions, 8 mmol HEMA, 32 mmol of EGDMA, 0.03
g of AIBN and 5 mL of methanol (porogen) were taken
in a 50 mL round bottom flask. The mixture was purged
with N2 for 5 min. and the flask was sealed under inert
atmosphere. Then it was kept for stirring in an oil bath
maintained at a temperature of 60˚C. Af ter 2 h, the mate-
rial obtained was ground and sieved and the particles
with sizes between 50 - 105 m were collected. The re-
sulting imprinted particles obtained were washed with
metha nol a nd the n leac hed with 0. 1 mol·L–1 HCl for 1 h.
During leaching (hydrolysis) the template DCP is str ip -
ped off as the diethylphosphonic acid. So the cavity
formed in the polymer matrix on leaching presumably
corresponds to that of diethylphosphonic acid. During
the rebinding process, the analyte DCP when added to
the sample solution containing tris buffer will undergo
hydrolysis to form diethylphosphonic acid [25], which is
same a s that o f the leached template. Therefore, it can be
construed that template rebinding takes place by non-
covalent interactions. The covalent imprinting and sub-
sequent rebinding via non-covalent interactions will be
termed as semi-covalent strategy. The non-i mprinted
polymer particles were synthesized, washed and treated
analogous to the imprinted polymer in the absence of
template, i.e. DCP during syn thesis.
2.3. Casting of Semi-Covalent Imprinted and
Non-Imprinted Po ly mer I nclus ion Mem-
branes
The polymer inclusion membranes were cast by the fol-
lowing procedure mentioned below. DCP imprinted po-
lymer particles (90 mg) synthesized via semi-covalent
strategy were dispersed in 0.2 mL of NPOE and were
mixed with tetrahydrofuran (THF) (2.5 mL) solution of
PVC (90 mg). The sol utio ns were ho moge nize d and t hen
poured into a Teflon mould having an internal diameter
of 21 mm. The THF on evaporation at room temperature
results in the formation of imprinted polymer inclusion
membranes of thickness ~0.45 mm. In a similar manner
non-imprinted polymer inclusion membranes were also
casted.
2.4. Sensor Fabrication and EMF Measurement
The imprinted and non-imprinted membranes cast via
inclusion by semi-covalent strategy were glued to one
end of a pyrex glass tube with Araldite. The tube was
then filled with an internal filling solution of 10–3
mol ·L–1 DCP. A schematic diagram of membrane for ma-
tion and fabrication of biomimetic potentiometric sensor
is given in (Figure 1). The sensor was kept in air when
not in use.
Figure 1. A Schematic representation of membrane forma-
tion and f abrication of IPIM ba sed sensor for DCP.
Table 1. Effect of plasticizers on potential response of IPIM
based sensor for each decade.
DCP (m ol·L–1)
Potential response (mV/decade)
NPOE
TEHP
DOP
BEHS
1 × 10–7 to 1 × 10–6
3.0
-
-
-
1 × 10–6 to 1 × 10–5
3.0
-
1.0
-
1 × 10–5 to 1 × 10–4
6.0
2.0
1.5
-
1 × 10–4 to 1 × 10–3
18.0
12.0
3.0
10.0
1 × 10
–3
to 1 × 10
–2
40.0 21.0 21.0 24.0
Sensors were conditioned in 10–5 mol ·L–1 DCP solu-
tion +0.1 mo l·L–1 tris buffer (adjusted to pH 10.5) for 24
h and the n s ti rr ed i n tr is b uf fer for 1/2 h to remo ve b ound
DCP ions after which the membranes would generate
stable potentials. The test solution whose pH was main-
tained at 10.5 after the addition of 5 mL of 1 mo l·L–1 tris
buffer was taken and response of the sensor was exam-
ined by measuring the electromotive force (EMF) of the
following electrochemical cell. Ag-AgCl10–3 mol·L–1
DCP| DCP membrane║test solution| Hg-HgCl2·KCl (sa-
turated). The potential responses of the sample solu-
tions containing different concentrations (1.0 × 10–11 to
1.0 × 10–2 mol·L–1) of DCP in 50 mL of 0.1 mol·L–1 Tris
buffer (pH 10.5) was measured. The EMF was plotted as
a func t ion of DCP concent ration.
2.5. Analysis of Natural Water Samples
The river water or ground water samples (~45 mL) were
adjusted to pH = 10.5 after the addition of 5 mL of 1
mol ·L–1 tris buffer using HCl or NaOH. The samples
were analyzed using the above fabricated IPIM based
potentiometric sensor by following the analytical proce-
dure mentioned in section 2.4.
3. Results and Discussions
The optimal design of the membrane enables the per-

V. V I SHN UVAR D HAN ET AL.
379
formance of the sensor . As it is well known that the elec-
trochemical properties of conventional potentiometric
sensors depends on various features of the membrane
such as nature of plasticizer, nature and amount of sens-
ing material used as described elsewhere [26-28]. In
view of this, the suitability of the membrane casted by
dispersing the imprinted polymer particles in plasticizer
and embedded in PVC were examined for selective rec-
ognization of DCP .
3.1. Influen ce of Plast ic iz er
The response mechanism of the sensor strongly depends
on the mobilit y of electr oacti ve species, thereby reducing
the resistance. Incorporation of suitable plasticizer influ-
ences the working concentration range of potentiometric
sensor by enhancing the mobility of target analytes. In
accordance with that the effect of different plasticizers
on the performance of IPIM based sensor was investi-
gated. Ta ble 1 shows the po tential output of IPIM b ased
sensor with different plasticizers NPOE, TEHP, DOP
and BEHS. From the table, it is evident that membrane
with NPOE alone offer better potential responses in the
entire concentration range of DCP compared to TEHP,
DOP and BEHS based sensors. Also, the magnitude of
potential difference, stability of EMF output and better
precision for NPOE based sensor is higher in each dec-
ade. The plasticizer NPOE having a high dielectric con-
stant of 24.0 has given better response characteristics
than that of DOP (
ε
= 5.0), BEHS (
ε
= 4.0) and
TEHP (
ε
= 4.8). This is in agreement with earlier re-
ports [20,21]. It was also observed that the imprinted
membranes casted without plasticizer were not suitable
for use as recognition membranes as they are brittle.
3.2. Effect o f MIP Particles to PVC Ratio
The ratio of MIP particles to PVC was found to play a
key role in the sensor performance since the weight of
MIP particles determine the number of binding sites
available for selective rebinding of DCP. The effect of
changing the ratio of imprinted polymer particles (pre-
pared by semi-covalent strategy) to PVC was varied in
the ratio 0.5:1, 1:1 and 2:1. The results indicate that 1:1
ratio gave the best results in terms of linear calibration
range, limit of detection, response time and magnitude of
potential change for each decade change of concentration
of DCP (Table 2). In the case of membranes with 0.5:1
ratio, the total number of binding sites available for re-
binding of DCP is relatively lower for the membrane to
respond effectively. On the other hand, during the prep-
aration of membranes with 2:1 ratio, the MIP particles
were dispersed non-uniformly resulting in poor perfor-
mance. Since the potential response is not improved
beyond par ticular limit on inc reasing the amount o f MIP
Table 2. Effect of weight ratio of MIP particles to PVC on
response of IPIM base d sensor.
Weight of MIP
particl es (g) Weight of PVC (g) Weight ratio Working concentration range (mol·L–1)
0.045 0.09 0.5:1 4.5 × 10–5 to 1 × 10–2
0.09 0.09 1:1 1 × 10–9 to 1 × 10–4
and 1 × 10–4 to 1 × 10–2
0.18 0.09 2:1 3.3 × 10–7 to 1 × 10–4
and 1 × 10–4 to 1 × 10–2
Figure 2. Effect of pH on response of the IPIM based sen-
sor.
particles, further studies were restricted with membranes
having 1:1 ratio.
3.3. Effect of pH of Test Solution
The effect of pH of test solution on the performance of
IPIM based sensor response for decade change of DCP
concentration i.e. 10–5 to 10–4 mo l·L–1 was studied by
var yin g pH = 7 to pH = 12 after addition of 5 mL of 1.0
mol ·L–1 tris buffer (Figure 2). The results indicate that
the optimum pH for constant and maximum potential
response over the entire concentration range of DCP (1.0
× 10–9 to 1.0 × 10–2 mo l·L–1) was found to be 10.0 to
11.0. Hence, the pH of the test solution was adjusted to
~10.5 after the addition of 5 mL of 1.0 mol·L–1 tris buf-
fer.
3.4. Sensitivity of the IPIM Based Sensor
The potential responses of the imprinted membrane and
non-imprinted membrane based sensors fabricated under

V. V I SHN UVAR D HAN ET AL.
380
the optimal conditions arrived above was studied and the
results obtained are shown in (Figure 3). It can be no-
ticed from the figure that the plot obtained for the IPIM
based sensor gave a linear calibration curve in the con
Figure 3. Potential response of the IPIM and NIPM based
sensors with respect to DCP concentration.
Figure 4 . Potentiometric resp onse of the IPIM based sensor
to DCP and other selected interferents for each decade
change of concentrati on from 1 × 10–9 to 1 × 10–2 mol L–1.
centration range 1 × 10–9 to 1 × 10–4 and 1 × 10–4 to 1 ×
10–2 mo l·L–1 of DCP. On the other hand, the non-im
printed polymer inclusion membrane (NIPIM) based
sensor gave linear response for DCP in the concentration
range 1 × 10–4 to 1 × 10–2 mol·L–1. It was observed that
the absolute potentials obtained from NIPIM based sen-
sor were unstable which is due to nonspecific binding o f
analyte in contrast to the specific site selective binding in
the case of IPIM based sensor. In addition, the LOD for
IPIM and NIPIM based sensors calculated based on
IUPAC recommendation [29] were found to be 1 × 10–9
mol ·L–1 and 1 × 10–4 mol·L–1, respectively. Whereas the
higher
E∆
value for IPIM based sensor over NIPIM
based sensor in the entire concentration range is attri-
buted to significant imprinting effect. The equilibrium
Table 3. Co mpariso n of experi mental sel ectivi ty coef ficient s
of DCP against various selected interferents using NIPIM
and IPIM based sensors.
Inter ferents
,
pot
AB
K
N IPIM
IPIM
PMP
2.0 × 10–2
3.3 × 10–3
DMMP
1.0 × 10–1
3.3 × 10-–2
MPA
1.0 × 10–1
3.6 × 10–2
Phorate
1.0 × 10–1
2.5 × 10–2
2,4-D
4.0 × 10–2
5.0 × 10–2
,
pot
AB
K
= Potentiometric se lectivity coefficient ; A = DCP; B = Interferent.
response time was found to be 5 min. for the IPIM based
sensors employing particles prepared by semi-covalent
strategy.
3.5. Selectivity of the IPIM Based Sensor
Selectivity refers to the extent of suitability of the de-
veloped IPIM based sensors to determine particular ana-
lyte in mixtures or matrices without interferences from
other components. In environmental applications, the
concentrations of the analytes are quite low and thus,
high selectivity is essential for an effective monitoring.
Hence, the selectivity of the developed IPIM based sen-
sor with various common simulants (PMP and DMMP)
and degradation product (MPA) of CWAs, pesticides
like phorate and 2,4-D which may co-exist in real sam-
ples were tested. The response profiles of DCP and se-
lected coexisting interferents obtained with IPIM based
sensor fabricated with particles prepared by
semi-covale nt strate gy are shown in Fig ure 4. T he high-
er selectivity noticed in the case of IPIM based sensor
can be attributed to the more rigid polymeric structure
leading to more stabilized cavities. Similar imprinting
effec t can also be visualized from Table 3 a nd compares
the selectivity coefficients of DCP over selected interfe-
rents o btained by IPIM based sensor with corresponding
NIPIM based sensor by employing IUPAC method [30]
as described elsewhere.
3.6. Stability and Reus a bil ity
Another important criteria for any sensing device in ad-
dition to sensitivity and selectivity is stability and reusa-
bility. The developed IPIM based sensors prepared by
employing semi-covalent strategy were found to be sta-
ble with deviatio ns less than 0 .5 mV for 1 × 10–4 mol·L–1

V. V I SHN UVAR D HAN ET AL.
381
DCP for 3 months and can be reused for more than 30
times without loosing sensing abilit y.
3.7. Analytical Application to Natu ral Water
Samples
It was succe ss fully ap plied to natural water samples, as it
Table 4. Analysi s of natural water samples.
Sample
Concentrati on of
selected interferents
in mixture spiked to
nat ura l wa ters
(mol·L
–1
)
a
DCP added
(× 10–8
mol· L–1)
DCP fou ndb
(× 10–8
mol· L–1)
Reco very
(%)
Ground
water
-
1.0
1.01 ± 0.09
101
10–8
1.0
0.99 ± 0.11
99
10–7
1.0
0.98 ± 0.16
98
Ri v er
water
-
1.0
1.00 ± 0.03
100
10–8
1.0
0.98 ± 0.14
98
10–7
1.0
1.01 ± 0.09
101
aMixture of PMP, DMMP, MPA, Phorate and 2,4-D; bAverage of three
determinations.
is clear from the selectivity studies that several interfe-
rents co-exist in real samples do not have any deleterious
effect on IPIM based sensor performance. Ground and
river water samples were analyzed by spiking known
amounts of DCP and varying concentrations of interfe-
rent mixtures. The results thus obtained are shown in
Table 4. T he recovery obtained in the range 98% - 101%
shows that the developed IPIM based sensor can reliably
be used for monitoring the natural waters, which, if
found, can alert the authorities for appropriate control
measures.
4. Conclusions
Semi-co va l ent i mprinted and non-imprinted polymer par-
ticles were synthesized and found that non-imprinted
polymer inclusion membrane (NIPIM) was unstable in
contrast to imprinted polymer inclusion membrane
(IPIM) in determination and quantification of DCP. In
addition, the sensor performance of the IPIM based sen-
sor is remarkable with a detection limit of 1 × 10–9
mol ·L–1 (0.17 ppb) compared to corresponding NIPIM
based sensor. The interferents co-exist in real sa mples do
not have any deleterious effect on IPIM based sensor
performance. The recovery studies of DCP from ground
and river waters indicated the possibility of using the
sensor investigated in the present work for nature water
samples. The developed technology found to be a effec-
tive analytical tool as it requires simple sample prepara-
tion procedures and does not require large instrumenta-
tion as it is a miniat ure device that respo nds to a particu-
lar analyte in a selective way and can be effectively used
for the determination of pollutants in the environmental
and biological samples [20,23]. Further studies are in
progress to integrate other transducers with MIP mate-
rials for a chosen template.
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