American Journal of Anal yt ical Chemistry, 2011, 2, 820-831
doi:10.4236/ajac.2011.27094 Published Online November 2011 (http://www.SciRP.org/journal/ajac)
Solid-Contact Perchlorate Sensor with Nanomolar
Detection Limit Based on Cobalt Phthalocyanine
Ionophores Covalently Attached to Polyacrylamide
Mohammad Nooredeen Abbas1*, Abdel Lattief A. Radwan1, Philippe Bühlmann2,
Mahmud A. Abd El Ghaffar3
1Analytic al Laboratory , Department of Applied Organic Chemistry, National Research Centre, Cairo, Egypt
2Department of Chemistry, University of Minnesota, Saint Pau l, United St a t es
3Department of Polymers and Pigments, National Research Centre, Cairo, Egypt
Received July 29, 2011; revised September 3, 2011; accepted Septembe r 15, 2011
Abstract
Novel solid-contact perchlorate sensors based on cobalt phthalocyanine-C-monocarboxylic acid (I), and cobalt
phthalocyanine-C,C,C,C-tetracarboxylic acid (II) as free ionophores and covalently attached to polyacryla-
mide (PAA)—ionophores III and IV, respectively were prepared. The all solid-state sensors were constructed
by the application of a thin film of a polymer cocktail containing a phthalocyanine ionophore and cetyl-
trimethylammonium bromide (CTMAB) as a lipophilic cationic additive onto a gold electrode precoated with
the conducting polymer poly (3,4-ethylenedioxythiophene) (PEDOT) as an ion and electron transducer. The
sensor with 10.3% of ionophore (III) covalently attached to plasticizer-free poly (butyl methacrylate-co-do-
decyl methacrylate) (PBDA) exhibited a good selectivity for perchlorate and discriminated many ions, in-
cluding F–, Cl–, Br–, I–, SCN–, 3
–
N
O, S2– and 2
4
SO

. The covalent attachment of the ionophore to the polymer
resulted in a near-Nernstian anionic slope of –62.3 mV/decade whereas a super-Nernstian slope of –79.9 mV/
decade was obtained for the free ionophore. The sensor covered a linear concentration range of 5 × 10–9 - 1 ×
10–2 mol·L–1 with a lower detection limit (LDL) of 1 × 10–9 mol·L–1 and gave a stable response over a pH
range of 4 - 10.5. The all-solid state sensors were utilized for the selective flow injection potentiometric de-
termination of perchlorate in natural water and human urine samples in the nanomolar concentration range.
Keywords: Ion-Selective Electrode, Solid Contact, Covalent Ionophore Attachment, Perchlorate,
Flow-Injection Analysis, Urine
1. Introduction
Perchlorate () is an environmental contaminant
4
whose occurrence is most clearly linked with its use as
an oxidizer in rocket propellants, fireworks, matches, and
highway safety flares [1]. Through accidental releases
and improper disposal of materials containing its salts,
perchlorate has entered the soil, surface water, and g-
roundwater, and its solubility, mobility, and persistence
characteristics have allowed it to contaminate drinking
water. Perchlorate presents an environmental health risk
to humans as it interferes with iodine uptake by the thy-
roid gland and is associated with the disruption of its
function [2,3]. These reasons have stimulated research
towards the accurate determination of perchlorate ions in
different samples, such as urine and natural water [4-7].
Recently, chemical sensors have been used in many fie-
lds of applications, including clinical diagnosis, biome-
dical analysis and monitoring of environmentally hazar-
dous materials [8-10]. The life span of these sensors and
their lower limit of detection are among their most im-
portant characteristics. Therefore, increasing efforts are
being directed towards the optimum design and operating
conditions of long-living sensors. In particular, covalent
binding of ionophores to polymeric sensing membranes
provides long-term stability by preventing the ionophore
from crystallizing, evaporating [11,12] and leaching into
sample solutions. In addition, it can improve the sensor
–
ClO

821 M. N. ABBAS ET AL.
selectivities and detection limits [13,14]. At the same
time, all-solid-state ion-selective electrodes (ISEs) based
on polymeric membranes doped with electrically neutral
or charged ionophores (carriers) have attracted consid-
erable interest since the invention of the so-called coat-
ed-wire electrode (CWE) more than 30 years ago [15].
Their further development led to ion-selective electrodes
with a solid internal contact (SCISEs) [16,17] character-
ized by a well-defined ion-to-electron transduction proc-
ess between the ionically conducting ion-selective mem-
brane and the electronically conducting substrate. The
notable progress in sensor technology is manifested in
the wide application of ion sensors in environmental
analysis [18]. However, while there a considerable num-
ber of perchlorate ISEs were reported in the literature,
most of them are not sensitive and selective enough to
permit accurate measurements of the low levels of per-
chlorate usually encountered in real samples [19] and
only have a limited lifetime. Therefore, the development
of durable perchlorate sensor with improved sensitivity
and selectivity is still an urgent need.
In this contribution, we report long-living highly selec-
tive and sensitive membrane electrodes for the determina-
tion of subnanomolar amounts of perchlorate in tap water,
ground water and urine. We have utilized the combined use
of covalent ionophore attachment and solid internal con-
tacts for the preparation of these perchlorate selective elec-
trodes. The construction and evaluation of perchlorate
SCISEs based on cobalt phthalocyanine (Co-Pc) covalently
attached to a polyacrylamide polymer backbone is de-
scribed. As the ion-to-electron transducer, the conducting
polymer poly (ethylenedioxy-thiophene) (PEDOT) was
used. It was coated with either ionophore-doped plasticized
poly (vinyl chloride) (PVC) or ionophore-doped plasti-
cizer-free polymethacrylate membranes. At last, it is wor-
thy to note that we have integrated the excellent potenti-
ometric performance of the developed sensors with the
agreed advantages of the flow injection technique with the
high sample throughput and low sample volume.
2. Experimental
2.1. Reagents and Materials
All chemicals used were of analytical reagent grade
unless stated otherwise, and doubly distilled water was
used throughout. o-Nitrophenyl octyl ether (o-NPOE),
bis (2-ethylhexyl) phthalate (DOP), dibutyl sebacate
(DBS), dodecyl methacrylate and tetrahydrofuran (THF)
were purchased from Sigma-Aldrich. Gold wires and
conducting polymer dispersion in water composed of
0.5% poly (3,4-ethylene-dioxythiophene) with 0.8% poly
(styrene sulfonate) (PEDOT/PSS) as dopant were pur-
chased from Sigma-Aldrich. Sodium tetraphenylborate
was obtained from Riedel de Haen, and oleic acid (OA)
and high relative molecular weight PVC from Fluka.
Tetrahydrofuran (THF), methyl methacrylate, n-butyl
methacrylate and the sodium and potassium salts of all
anions were purchased from Merck. Cetylpyridinium
chloride (CPC) was purchased from ACROS, and dibutyl
phthalate (DBP) and cetyltrimethylammonium bromide
(CTMAB) from BDH.
A stock solution of 0.1 mol·L–1 potassium perchlorate
was prepared in water, and 8 working standards in the
range from 10−2 mol·L–1 to 10−9 mol·L–1 (each differing
from the next more concentrated standard by a factor of
10) were freshly prepared by stepwise dilution. Phos-
phate buffer (0.1 mol·L−1) of pH 7.2 was used to adjust
the pH of all sample solutions.
2.2. Instrumentation
All pH measurements were made at 25 ± 1˚C using a
pH/Ion meter Model 692 (Metrohm). A PC-based EMF-
16 high-resolution data logger (Lawson) was used to re-
cord the output signals. A combination pH glass elec-
trode Model Metrohm 6.0202.100 was used for all pH
measurements, and a Metrohm double junction Ag/AgCl
reference electrode Model 6.0726.100 containing 3
mol·L−1 KCl solution in the outer compartment and 3
mol·L−1 KCl solution saturated with AgCl in the inner
compartment was used for all other potentiometric
measurements. Standard solutions were freshly prepared
with doubly distilled water.
2.3. Synthesis of Ionophores
The ionophores cobalt phthalocyanine-C-monocarboxylic
acid (I) and cobalt phthalocyanine-C,C,C,C-tetracarbox
ylic acid (II) and the adducts of phthalocyanine-C-mono
carboxylic acid-PAA (III) and cobalt phthalocyanine-
C,C,C,C-tetracarboxylic acid-PAA (IV), were synthesized
according to the Figure 1 and characterized by elemental
analysis and UV-Vis and IR spectroscopy. In brief, con-
densation of trimellitic anhydride in the presence of urea,
ammonium molybdate and cobalt acetate was used to
form the tetraformamido-phthalocyanine cobalt. By hy-
drolyzing the product in alkaline medium, ionophore II
was obtained. The synthesis of ionophore I was carried
out according to a procedure described by Chen et al.
[20]. By heating a mixture of trimellitic anhydride and
phthalic anhydride at a ratio of 1:7 to 190˚C in the pre-
sence of urea, cobalt acetate, and ammonium molybdate,
a mixture of cobalt 2-formamido-phthalocyanine and
cobalt phthalocyanine was obtained. The cobalt forma-
mido-phthalocyanine thus prepared was hydrolyzed under

M. N. ABBAS ET AL.
822
Figure 1. Synthesis of the free ionophore cobalt phthalocyanine monocarboxylic acid (I) and the ionophore III, covalently at-
tached to polyacrylamide.
alkaline condition, yielding ionophore I. The latter was
isolated from the hydrolysis mixture by separation of the
carboxy derivative from the cobalt phthalocyanine and
the remaining formamido derivative on a silica gel col-
umn, eluting with a dimethyl formamide:acetone mixture
(3:1). While the formamido-substituted and the unsub-
stituted phthalocyanine eluted as a leading band, iono-
phore I was more strongly retained and eluted as a
well-separated second band.
The ionophore-polymer adducts were synthesized by
condensation of the carboxylic group of ionophores I and
II with the NH2 groups of PAA by refluxing in dimethyl
formamide at 150˚C in the presence of polyphosphoric
acid, adopting a previously reported procedure [21,22].
In the IR spectrum of compound I, the peaks at 1577,
1461, 1299, 1036 and 719 cm–1 are those characteristic of
the phthalocyanine ring. The existence of a carboxylic
group on the phthalocyanine ring of I was confirmed by
the peaks at 1726 cm–1 and 1664 cm–1 (C = O stretching)
and 3199 cm–1 (OH stretching). In the spectrum of the
PAA-attached ionophore (compound III), the peaks at
3435 (N-H), 2926 (C-H), 1634 (amide II, NH), and 1404
cm–1 (C-N) belong to the polyacrylamide backbone. The
characteristic phthalocyanine peaks at 1268, 1079 and
635 cm–1 were still present. The peak at 1726 cm–1 and
3199 cm–1 disappeared, indicating reaction of the COOH
with the NH group from the PAA. The UV spectrum of
the cobalt phthalocyanine-C-monocarboxylic acid show-
ed a Q band at 661 nm and a higher B band at 296 nm,
while the spectrum of the polymer-attached ionophore
(III) showed a weaker satellite band at 661 nm with a
shift of the B band to 328 nm, which is in agreement
with results of Chen et al. [23].
2.4. Preparation of Coated Graphite Electrodes
(CGEs)
The proper amounts of the specified ionophore (I, II, III
or IV), PVC and the plasticizer (DOP, DBP, o-NPOE, or
DBS) were placed in a 10 mL vial and mixed thoroughly.
Additives such as oleic acid and/or cetyltrimethylammo-
nium bromide (CTMAB) were added (see Results and
Discussion). Then the mixture was dissolved in 5 mL THF
under magnetic stirring until the thus prepared membrane
cocktail became clear. Some membranes were prepared
using the plasticizer-free copolymer of butyl methacry-
late and dodecyl methacrylate (PBDA) instead of PVC
and plasticizer. A clean and dry carbon rod of 4 cm length

823 M. N. ABBAS ET AL.
and 2 mm diameter was dipped to about 1 cm depth into
the membrane cocktail for 2 s, and then lifted out of the
solution to evaporate the THF, leaving the polymeric
membrane layer coating the carbon rod. That operation
was repeated for 12 - 17 times to give a proper mem-
brane thickness. The rod was fitted into a plastic body
and connected with the membrane-free end to the pH/ion
meter using a copper wire.
2.5. Preparation of Solid-Contact Electrodes
(SCEs) with Gold Contacts
Gold wire (1 cm length and 0.1 mm diameter) was care-
fully washed with 1 mol·L–1 H2SO4, water and acetone,
dried, and attached to a silver wire using conducting sil-
ver-epoxy glue (silver epoxy E10-101, Alfa). The con-
ducting polymer films were cast from an aqueous disper-
sion of PEDOT/PSS containing of FeCl3 onto Au wire.
The thus obtained polymer films possess low water solu-
bility and allowed for stabilized standard electrode po-
tential [24]. Finally, the membrane cocktail was applied
onto the dry PEDOT/PSS polymer layer deposited onto
the gold electrode. The electrodes were conditioned in a
1 × 10–2 mol·L–1 perchlorate solution for at least 48 h
before their first use and kept in such a solution over-
night when not in use. Figure 2 shows a schematic
drawing of the all-solid contact sensor.
2.6. ISE Calibration
The ISEs were calibrated by immersion, along with an
Ag/AgCl/Cl– reference electrode, in a 50-mL beaker
containing 9.0 mL phosphate buffer solution (0.1 mol·L−1)
of pH 7.2. Then aliquots of a standard perchlorate solu-
tion were added successively under continuous stirring to
obtain solutions with a perchlorate concentration ranging
from 1 × 10-10 to 1 × 10–1 mol·L–1, and the potential was
recorded after stabilization to ±0.5 mV within approxi-
mately 1 min. A calibration graph was then constructed
by plotting the recorded potentials as a function of the
logarithm of the perchlorate concentration.
Figure 2. Schematic of the solid contact electrode (SCE).
2.7. Sensor Selectivity
Potentiometric selectivity coefficients ,
p
ot
AB
K
were deter-
mined according to IUPAC guidelines using the separate
solutions method (SSM) [25,26]. Different interfering
anions of a concentration of 1 × 10−3 mol·L−1 in phos-
phate buffer (pH 7.2, 0.1 mol·L−1) were utilized, and se-
lectivity coefficients were obtained using Equation (1).
,1log
log pot BA A
AB A
B
EE zc
KSz






where S is the slope of the calibration curve, cA the con-
centration of perchlorate, and zA and zB are the charges of
perchlorate and the interfering anion, respectively.
2.8. Flow Injection Analysis
Test solutions were injected using an injection valve
Model 5060 (Rheodyne). The carrier buffer solution
was propelled by means of a four channel peristaltic
pump model MCP (Ismatec) through Tygon tubing R-
3603 of 0.25 mm i.d. The all-solid state perchlorate
selective electrodes along with Ag/AgCl/Cl− double jun-
ction reference electrodes were used to detect perchlo-
rate in a home-built micro-flow cell of 250 uL volume.
Figure 3 shows a schematic diagram for the flow injec-
tion set-up.
3. Results and Discussions
3.1. Effect of Ionophores Type, Plasticizer, and
Ionic Sites on Response Slope and
Detection Limit
Phthalocyanines and metal phthalocyanines are well-kno-
wn, readily available pigments that have good chemical,
acid-base, and thermal stabilities. They represent one of
the most studied classes of organic functional materials
[26-28]. In particular, they have been successfully used
as ionophores for potentiometric sensing, which takes
advantage of their selective coordination chemistry and
structural diversity [29,30]. The type of phthalocyanine
ring, the nature of their peripheral substituents, and the
choice of the central metal control the axial ligation of
different anions to these compounds [31,32]. We at-
tached the phthalocyanine ionophores I and II covalently
to polyacrylamide (PAA) through the condensation reac-
tion between the carboxyl groups of the ionophores I and
II and the NH2 groups of PAA, giving ionophores III and
IV, respectively. The four compounds were investigated
as anion sensing ionophores, and preliminary results
showed a strong response towards perchlorate.

M. N. ABBAS ET AL.
824
Figure 3. Schematic diagram for flow injection system for perchlorate-selective sensor.
UV-VIS spectra of 10−4 mol·L−1 cobalt phthalocyanine
monocarboxylic acid in DMSO/water (1:1) in presence
and absence of 10−4 mol·L−1 perchlorate are shown in
Figure 4. As previously shown for other Co(II) phthalo-
cyanines [33], axial ligation of anions leads to a very
significant increase in the intensities of the Q band at 661
nm as well as the smaller bands at 294 and 328 nm.
Moreover, there is a small shift in the band at 250 nm.
All this is consistent with perchlorate binding to the
metal centre of the ionophore.
sites but 3.86% of ionophore I, which is the cobalt phth-
alocyanine-C-monocarboxylic acid not attached to the
polymer, showed a super-Nernstian slope of about –120
mV/decade in the range of 1 × 10−4 to 1 × 10−5 mol ·L−1
(see Figure 5). Ionophore IV was less soluble in the
membrane cocktail than III, with a maximum concentra-
tion of 5.5%, which is about half as much as for III. This
may be related to the formation of a cross-linked polymer
as a result of the reaction of more than one carboxylic
group, while ionophore III was attached to the poly- mer
through only one carboxylic acid group. At a ratio of
3.86%, ionophore IV gave a slightly better detection limit
than ionophore III. On the other hand, it was possible to
increase the ratio of ionophore III in the membrane up to
10.3%, where a lower detection limit than for the other
three ionophores were achieved.
It is well known that the potentiometric sensitivity and
selectivity of a given ionophore depends significantly on
the membrane composition [34-36]. For preliminary ex-
periments, a number of coated graphite electrodes (CGEs)
with PVC membranes containing no ionophore or dif-
ferent concentrations of ionophores I, II, III or IV were
prepared, and their potentiometric response to perchlo-
rate in phosphate buffer of pH 7.2 was evaluated. All
membranes contained DOP as plasticizer, 2.6% CTMAB
to provide for ionic sites, and 2.6% oleic acid as additional
additive. Blank electrodes without ionophores showed no
response at all. Membranes containing no added ionic
Preliminary studies using CGEs to test the effect of the
plasticizer type, ionic sites and oleic acid were also
Figure 5. Effect of the type of and concentration of iono-
phore (in %, wt/wt) on the perchlorate response of coated-
graphite electrodes using ionophores I, III or IV. All mem-
branes contained PVC, were plasticized with DOP, and
contained 2.6% CTMAB and 2.6% OA in addition to the
ionophore.
Figure 4. UV-VIS spectra of 10−4 mol·L−1 cobalt phthalocya-
nine monocarboxylic acid in 1:1 DMSO/water in presence
and absence of 10−4 mol·L−1 potassium perchlorate.

825 M. N. ABBAS ET AL.
Table 1. Effect of membrane composition on the potentiometric response of all solid-contact perchlorate sensors.
No. Composition % A# M.R. OA M.R.Slope mV/dLDL M Linear Range M
1 No ionophore, No PAA, PBDA 100 ― ― ― ― ―
2 No ionophore, PAA 9.3, PBDA 85.5 2.6* 2.6* –32.7 2 × 10–5 8 × 10–5 - 1 × 10–2
3 I 2.52, PAA 6.78, PBDA 85.5 0.48 0.38 –79.94 2 × 10–6 7 × 10–6 - 1 × 10–2
4 III 7.24, PBDA 87.6 0.82 0.64 –61.7 8 × 10–8 2 × 10–7 - 1 × 10-2
5 III 8.3, PBDA 86.5 0.71 0.56 –61.9 1 × 10-8 8 × 10–8 - 1 × 10–2
6 III 9.3, PBDA 85.5 0.64 0.50 –62.3 2 × 10–9 6 × 10–9 - 1 × 10–2
7 III 10.3, PBDA 84.5 0.57 0.45 –63.9 1 × 10–9 5 × 10-9 - 1 × 10–2
8 III 10.3, PVC 28.2, DOP 56.4 0.57 0.45 –63.4 1.5 × 10–9 6 × 10–9 - 1 × 10–2
-A# = CTMAB-M.R. = molar ratio of additive/ionophore. *The membrane contains 2.6% w/w CTMAB + 2.6% w/w OA, and doesn’t
contain ionophore.
performed. It is well known that the nature and concen-
tration of the membrane plasticizer influences both the
dielectric constant of the membrane as well as the mobil-
ity of ions. Among the plasticizers used in this work (o-
NPOE, DBP, DOS, DBS and DOP), DOP gave potenti-
ometric slopes closest to the theoretically expected val-
ues. Therefore, DOP was used as the plasticizer for all
further studies.
The presence of lipophilic cations as ionic sites in ISE
membranes selective for monoanions diminishes the oh-
mic resistance of ISE membranes doped with ionophores,
and the ratio of ionic sites and ionophore can be used to
control the selectivity [37,38]. Moreover, in the case of
electrically neutral ionophores, cationic sites are a neces-
sity for the observation of Nernstian responses. Indeed,
better response characteristics, i.e., Nernstian responses
and lower detection limits were observed when incorpo-
rating an optimum amount of CTMAB in the membrane
of the perchlorate-selective electrode, which in agree-
ment with recent reports [39-41]. Moreover, the detec-
tion limits of the electrodes were slightly improved in the
presence of a small amount of oleic acid (OA), a result
that resembles previous results for anion-selective elec-
trodes [42,43] and may be related to an improvement in
ionophore solubility due to the presence of OA.
3.2. Solid Contact Electrodes with
Gold Contacts
We subsequently applied the optimized membrane com-
position for the fabrication of solid contact electrodes
with gold contacts (SCE-Au). PEDOT is known as one
of the most stable conducting polymers available today
and particularly suitable as a solid contact material for
SCISEs due to its low sensitivity to O2 and CO2 [44].
The conducting polymer films were prepared by casting
of an aqueous dispersion of PEDOT/PSS onto the gold
electrodes. The crosslinked conducting polymer films
has a water solubility and hence stabilize the standard
potential of the sensors [45]. The produced layers of con-
ducting polymer were then coated with a film of plasti-
cizer-free PBDA copolymer, either with or without co-
balt phthalocyanine ionophore.
The absence of an aqueous layer between the solid-
contact and ion-sensitive membrane was confirmed using
the method proposed by Fibbioli et al. [46]. The elec-
trodes were exposed first to samples containing a high
concentration of the interfering ion chlorate, and the po-
tential was then monitored as the sample was changed to
a solution of the primary ion perchlorate. The presence
of an aqueous layer would be indicated by a slow posi-
tive potential drift after changing from the interfering to
the primary ion, resulting from ion exchange between the
sample and the aqueous layer. However, the obtained
results show that there is no such potential drift, indicat-
ing the absence of an aqueous layer between the solid
contact and the ion-selective membrane.
Table 1 shows that blank sensors without ionophores
did not result in any response to perchlorate, and sensors
containing PAA without ionophore exhibited a response
with a low slope of –32.7 mV/decade (electrode #2).
Membrane sensors with 2.52% of ionophore I (not at-
tached to the polymer) showed a super-Nernstian response
with a slope of –79.9 mV/decade (#3) (see Figure 5).
Only the sensor with a membrane containing 2.6%
CTMAB, 2.6% OA, and 10.3% of ionophore III (#7) i.e.
attached ionophore, showed a near-Nernstian slope of
–63.9 mV/decade. Its linear range covered the range from
5 × 10–9 to 1 × 10–2 mol·L–1 and gave a lower detection
limit of (LDL) of 1 × 10–9 mol·L–1, as shown in Figure 6.
Electrodes with membranes with an otherwise identi-
cal composition but prepared with DOP-plasticized PVC
as membrane matrix (#8) exhibited a slope of –63.4
mV/decade, a linear range of 8 × 10–9 - 1 × 10–2
mol·L–1and a LDL of 4 × 10–9, which is rather similar to
that of the plasticizer-free PBDA membranes, the former
being only slightly better.
3.3. Elimination of the Super-Nernstian
Response
Potentiometric responses to monanions with response

M. N. ABBAS ET AL.
826
Figure 6. Potentiometric response of perchlorate SCISEs:
Electrode 1 : 100% BDA (blank 1). Electrode 2: 9.3% PAA,
85.5% BDA (blank 2). Electrode 3: 2.5% ionophore I,
6.78% PAA, 85.5% BDA. Electrode 4: 9.3% ionophore III,
85.5% BDA. Electrode membranes 2 to 4 also all contained
2.6% OA and 2.6% CTMAB.
slopes larger than –59 mV/decade were observed previ-
ously and could be explained in one case with a thermo-
dynamic model by the formation of hydroxide-bridged
metalloporphyrin ionophore dimers. Indeed, the covalent
attachment of the ionophore to a polymer backbone was
shown to prevent ionophore dimerization and eliminated
the super Nernstian response [47,48]. This makes it note-
worthy that in the present work an increasing ratio of the
covalently attached cobalt phthalocyanine in the polymer
matrix started to slightly increase the potentiometric
slope of the sensor again at the highest concentration of
ionophore, as can be seen in Table 1. Because the elec-
trodes were always conditioned in a 1 × 10–2 mol·L–1 per-
chlorate solution before use and stored in the same type
of solution overnight, it appears that here too a thermo-
dynamic explanation may explain this finding. The high-
er ionophore concentrations may permit dimerization of
the ionophore despite the covalent ionophore attachment
to the polymer.
3.4. Effect of PH Value
The effect of the pH on the potential of the perchlorate
electrodes with ionophores III and IV was examined in
the range of perchlorate concentrations from 1 × 10–5 to 1
× 10–1 mol·L–1. The pH was varied by adding HCl or
NaOH. For ionophore IV and all four concentrations
assayed, the electrode potential was independent of pH in
a range of pH 6 - 11. The potentiometric response was
also independent on pH from 4 to 10.5 for ionophore III
for all five perchlorate concentrations assayed. The po-
tential changes observed outside of this pH range are
likely due to the response of the electrode to OH− (pH >
10) and to chloride ions (pH < 4). It is concluded that the
perchlorate selective all solid-state sensor possesses a
very stable response over a wide pH range. A pH of 7.2
adjusted with 0.1 mol·L−1 phosphate buffer was used for
all further studies.
3.5. Potentiometric Selectivity
Potentiometric selectivity coefficients of the proposed
sensors for several anions relative to perchlorate were
determined with the separate solution method (SSM). It
is clear from Table 2 that the electrode possesses good
selectivity for perchlorate over all other studied anions
and that there is a notable improvement in selectivity by
using covalently attached ionophore (III). The observed
selectivity pattern for the proposed sensor is in the se-
quence of > > >I– >SCN– > F– >
Br– > Cl– > –
2 > 3
–
4
ClO
NO
–
3
IO
NO
–
3
BrO

> CH3COO– > 2
4
SO

> S2–
2
3
> CO

>3
4
PO

, which shows a deviation from the
Table 2. Selectivity of the perchlorate SCISE’s.
Interfering
ion Perchlorate SCISE, Log,
p
ot
AB
K
Free
Ionophore I,
9.3%
Free Ionophore
I,*1.86% +
7.44% PAA
Attached Ionophore
III, 9.3% = (1.86%
Co-Pc)
3
ClO

−1.08 −1.13 −1.35
3
IO

−1.12 −1.46 −1.57
3
BrO

−1.21 −1.44 −1.81
I− −1.39 −1.85 −2.01
SCN− −1.10 −1.32 −2.33
F− −2.83 −2.87 −2.50
Br− −2.11 −2.64 −2.52
Cl− −2.31 −2.75 −2.70
–
2
NO −2.12 −2.24 −2.85
3
NO

−2.87 −2.82 −2.99
CH3COO−−3.01 −3.02 −2.99
2
4
SO

−2.67 −2.75 −3.05
S−2 −1.25 −1.77 −3.14
2
3
CO

−3.48 −3.01 −3.55
3
4
PO

−3.78 −3.80 −3.90
24
HPO

−3.88 −3.92 −3.94
2
4
HPO

−3.91 −3.95 −3.98
*The amount of ionophore I and PAA is equivalent to those present in
ionophore III-All membranes contains 2.6% w/w CTMAB + 2.6% w/w OA
III was stable for at least 90 days to ±0.5 mV; longer experiments were not
performed.

M. N. ABBAS ET AL.
827
Table 3. Comparison between the potentiometric performance and the selectivity of the proposed electrode and those for
previously reported perchlorate-selective electrodes.
Interfering ion ≥ 1.0 × 10–2 Ref. Slope mV/decade Linear range (M) LDL (M)
4
.
,
pot
ClO B
K
This work –62.3 5.0 × 10–9 - 1.0 × 10–2 1.0 × 10–9
–
3
ClO, ,
–
3
IO 3 BrO 
15 –60.3 8 .0 × 10–7 - 1.0 × 10–1 5.6 × 10–7
4
IO, SCN−, I−,
3
NO 
50 –60.0 1.0 × 10−6 - 1.0 × 10−1 6.6 × 10−7
2
24 CO, , , , , ,F−, Cl−,
3
HCO 2
4
SO 2
3
SO 
2
27
Cr O
3
BrO –
2
NO
51 –56.8 1.0 × 10−6 - 1.0 × 10−1 8.3 × 10–7
2
3
CO , , , , Cl−, SCN−
2
4
SO –
2
NO 2
24
CO
52 –60.6 1.0 × 10–6 - 1.0 × 10−1 8 × 10–7 SCN−, Br−, CN−, , , , ,
3
ClO2
3
CO 3 NO 2
24
CO2
4
CrO

, I–
53 –51.3 - 55.7 5.2 × 10−6 - 1.0 × 10−1 9.3 × 10−6
2
4
SO , Cl−, , Br−, , CH3COO−,,
3
HCO 3 NO 2
24
CO2
4
HPO

54 –59.3 5.0 × 10–7 - 1.0 × 10–1 2 × 10–7
4
MnO
55 –57.8 8 × 10–6 - 1.6 × 10–1 5 × 10–6 Cl−, SCN−, Br−, ,
2
4
SO 3 NO

, , F−, CN−
–
2
NO
56 –48.3 1.0 × 10−5 - 1.0 × 10−2 5.6 × 10−6
2
4
SO , , Cl−, F−,
2
3
SO 3 HCO

, Br−, ,
3
NO 3 ClO
57 –57 5.0 × 10−6 - 1.0 × 10−2 1.1 × 10−6
2
3
CO , , , , I−, Br−, , Cl−,
,
2
4
SO 2
3
SO 
HCO
3
BrO 
3
3
NO
3
ClO

58 –59 1.0 × 10−5 - 1.0 × 10−2 5.0 × 10−6
SCN−, 2
3
CO

, I–
hydrophobicity-based Hofmeister series: 4 > SCN−
> I− > CN− > 3 > 3 > Br− > 3 > Cl− > F−
(i.e., the series based solely on lipophilicity of anions)
[49]. The selectivities of the proposed sensor along with
other performance characteristics are shown in Table 3
along with data for some previously reported perchlo-
rate-selective electrodes for comparison.
ClO
O ClONOBr
3.6. Response Time and Life Span
Calibrations were carried out by immersing the sensors in
a 1 × 10−2 mol·L−1 perchlorate solution buffered with
phosphate to a pH of 7.2, followed by serial dilution. Car-
rying out the calibration in a reverse way from low to high
concentrations resulted always in the same potentiometric
slope. The sensor was typically kept in 1 × 10−2 mol· L−1
phosphate buffered perchlorate solution of pH 7.2 before
use in the next experiment. However, it was stored dry
when not in use for a long time. The response slope of the
sensor constructed using ionophore III was almost stable
over a period of 90 days and it is expected to retain its
characteristics over longer living time. The response time
of the sensor was found to be ~12 s over concentration
range from 10-7 to 10-2 M. which is reliably fast .
3.7. Optimization of the Parameters of FIP
Method
Flow injection analysis with potentiometric detection
using ISEs has many advantages, including low cost,
simple instrumentation, rapid response, high selectivity
and sensitivity [59,60]. The high reproducibility, low
detection limit and need for only a small sample for
analysis are now well-recognized features of this tech-
nique. Moreover, the high sample throughput is an ap-
preciated advantage of flow injection potentiometry (FI-
P). Therefore, the proposed miniaturized all-solid-state
perchlorate-sensor and a tubular flow cell with phos-
phate buffer as carrier was utilized for FIP determina-
tions of perchlorate. The parameters of the FIP method
were optimized in order to obtain the best signal sensi-
tivity and sampling rate under low dispersion conditions.
The geometry of the homebuilt flow-cell allowed for lim-
ited sample dispersion and, thereby, for optimum sensitivity
and fast response of the sensor. A sample volume of 500
μL and a flow rate of 30 mL/min were found to offer the
best results. Under optimum conditions, the residence
time T was 10 - 15 s (where T is the time span from in-
jection to the appearance of maximum signal), the travel
time t was 5 - 6 s (where t is the period elapsing from
injection to the start of the signal), the return time T’ was
30 - 40 s (where T’ is the period between the appearance
of the maximum signal and the return to the base line),
and the baseline-to-baseline time ΔT was 50 - 60 s (where
ΔT is the interval between the start of the signal and its
return to the baseline). The results prove that the proposed
sensor exhibited a very fast response toward perchlorate,
allowing a sampling rate of about 60 samples per hour at
least. Figure 7 shows triplicate peaks from the proposed

M. N. ABBAS ET AL.
828
Figure 7. Responses of the FIP system obtained under optimized experimental conditions for triplicate injections of in
the range of 1.0 × 10–7 to 1.0 × 10–2 mol·L–1.
–
4
ClO
FIP system obtained under optimal experimental condi-
tions for varying concentrations of in the range of
1.0 × 10–7 to 1.0 × 10–2 mol·L–1.
–
4
ClO
3.8. Analytical Applications
We utilized the new sensor for the determination of per-
chlorate in water and human urine samples. Mineral wa-
ter, tap water and human urine samples were spiked with
various concentrations of perchlorate, and the pH of the
sample solutions was adjusted to pH 7.2 using phosphate
buffer solution, before the samples were analyzed by
direct and flow injection analysis using the all solid-
contact electrodes. As Table 4 shows, quantitative re-
covery was achieved for samples spiked with perchlorate
in a concentration as low as 10 pbb.
4. Conclusions
An all-solid-state perchlorate sensor based on cobalt mo-
nocarboxyphthalocyanine covalently attached to a poly-
acrylamide backbone was successfully prepared. The
covalent attachment of the ionophore to the polymer was
crucial for improving the characteristic performance of
the sensor, resulting in a Nernstian potentiometric re-
sponse by inhibiting dimerization of the free ionophore
molecules. Additionally, the sensor containing the at-
tached ionophore exhibited good selectivity for perchlo-
rate. Finally, the combination of the developed perchlo-
rate SCISE with FIA method allowed for FIP analysis of
Table 4. Determination of perchlorate anions in tap water,
ground water and human urine by direct and flow injection
potentiometric analysis using the proposed all solid-state
electrode.
Sample Perchlorate
added (µg/ml)DP R*, % FIP R*, %
Bottled water5.00 99.9 ± 0.3 99.1 ± 0.2
0.50 97.9 ± 0.2 97.9 ± 0.4
0.01 92.9 ± 0.6 92.7 ± 0.5
Tap water 5.00 99.7 ± 0.1 97.8 ± 0.3
0.50 95.6 ± 0.2 94.9 ± 0.1
0.01 93.1 ± 0.2 92.6 ± 0.2
Urine 5.00 98.1 ± 0.3 96.8 ± 0.2
0.50 92.7 ± 0.4 92.0 ± 0.5
0.01 89.4 ± 0.6 88.8 ± 0.5
DP = direct potentiometry, FIP = flow injection potentiometry. *Recovery,
mean of five determinations.
the hazardous perchlorate ion at very low concentrations,
with the known advantages of the FIA method of low
sample volume and high throughput.
5. Acknowledgements
This work was supported by the Office of International
Science and Engineering (OISE) of the National Science
Foundation (Project 0809328).

829 M. N. ABBAS ET AL.
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