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Electrochimica Acta 111 (2013) 234– 241

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta

ifunctional polymeric membrane ion selective electrodes usinghenylboronic acid as a precursor of anionic sites and fluoride as anffector: A potentiometric sensor for sodium ion and an impedimetricensor for fluoride ion

anlapa Wongsana, Wanlapa Aeungmaitrepiroma, Orawon Chailapakula,ittaya Ngeontaeb, Thawatchai Tuntulania,∗

Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, ThailandMaterials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaenniversity, Khon Kaen 40002, Thailand

r t i c l e i n f o

rticle history:eceived 2 February 2013eceived in revised form 29 July 2013ccepted 15 August 2013vailable online xxx

eywords:ifunctional sensororonic acidluoride effector

a b s t r a c t

A new concept in ion sensing mimicking allosteric membranes in biological systems was pro-posed and demonstrated using fluoride-effector Na+ selective electrodes (Na-ISEs). DOS-plasticizedPVC membranes containing p-tert-butylcalix[4]arene tetraethyl ester (NaX) and 4-[(4-tert-butyl-2-methylphenoxy)methyl]phenyl boronic acid (50 mol% compared to NaX) were found to give the bestcharacteristic of Na-ISEs upon conditioning the membranes in 10−2 M NaF: a response slope of 57.48 mVper decade and detection limit of 7.42 × 10−7 M. The selectivity coefficients (log Kpot

Na,j) for the optimal elec-

trode were −2.29, −2.84, −3.16, −3.42, and −3.01 for K+, Li+, Cs+, Mg2+ and Ca2+, respectively. The optimalelectrodes also gave good reversibility and could be used in a wide range of pH (pH 3.6–9.7). The fabri-cated Na-ISEs were found to have membrane resistances (Rm) varied with concentrations of fluoride ions

on selective electrodempedance spectroscopy

used in membrane conditioning steps. Electrochemical impedance spectra showed a linear relationshipbetween Rm and the logarithm of the concentration range of fluoride ions from 1 × 10−1 to 1 × 10−8 M.Conditioning optimal membranes with various concentrations of Bu4NF gave a non-linear relationship.The proposed membrane electrode, thus, showed permselectivity toward Na+ using F− as an effector.Therefore, the proposed new concept led to the fabrication of a bifunctional sensor: a potentiometricsensor for sodium ion and an impedimetric sensor for fluoride ion.

. Introduction

In biochemistry, allosteric refers to the regulation of an enzymer a protein by binding an effector molecule at the protein’sllosteric site. Effectors can either enhance or decrease the protein’sctivity. There are a number of reports regarding allosteric mem-ranes in biological systems [1,2]. Along with the developmentf supramolecular host-guest chemistry, the design of allostericeceptors is significant because the guest (similar to an effector iniochemistry) can either enhance or decrease the binding or cat-lytic efficiency of the sensor-molecule [3]. Most recently, several

xamples of allosteric supramolecular receptors for metal ions andarious organic guest molecules have been reported [4]. Recently,ur group has reported the heteroditopic ion receptors displaying

∗ Corresponding author. Tel.: +66 2 2187643; fax: +66 2 2187598.E-mail addresses: tthawatc@chula.ac.th, tthawatc@gmail.com (T. Tuntulani).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.08.072

© 2013 Elsevier Ltd. All rights reserved.

allosteric behavior of enhanced binding of anions in the presenceof alkali metal ions [5,6]. In most cases, a metal ion is the allostericeffector which can improve the sensitivity and selectivity of anallosteric sensor.

In mimicking the transport of ions in biological membranes,chemists have used artificial membrane to extract or transportmetal ions into the organic phases in order to detect such ions.Ion selective membranes can be used to detect ions either by mea-suring membrane potentials upon exposing to ions as found in thecase of ion selective electrodes (ISEs). Ion selective PVC membraneelectrodes have been explored extensively and used in detections ofmetal ions and anions in environmental and medical interests [7].In the case of metal ion selective electrodes, the anionic additiveis required in order to improve the permselectivity of the mem-

brane and reduce its electrical resistance. General anionic additivesemployed in PVC membrane electrodes and optodes are derivativesof borate salts such as potassium tetrakis[4-chlorophenyl]borate(KTpClPB).

W. Wongsan et al. / Electrochimica Acta 111 (2013) 234– 241 235

c mem

pppQaaaeewfi4aeaflfei[tcbcw

ti

Scheme 1. Schematic representation of the polymeri

Boronic acids are trivalent boron-containing organic com-ounds, where the sp2-hybridized boron atom possesses a vacant

orbital. Therefore, they can act as Lewis acids and form com-lexes with hard bases like F− and sacharides [8,9]. Recently,in and co-worker have fabricated optode membranes containing

phenylboronic acid functionalized with boron dipyrromethanes a saccharide receptor and fluorescent probe [10]. Wróblewskind colleague have synthesized orgonoboron compounds andmployed them as ionophores for fluoride ions in PVC membranelectrodes [11]. However, the Nernstian slopes can be obtained butith the risk of selectivity. In this paper, we propose a new idea

or recognition of ions mimicking allosteric membranes in biolog-cal systems by using a lypophillic phenylboronic acid derivative,-[(4-tert-butyl-2-methylphenoxy)methyl]phenyl boronic acid, as

precursor of an anionic site in sodium ion selective membranelectrodes (Na-ISEs) and F− as an effector to generate anionic sitess shown in Scheme 1. The reaction of phenylboronic acids withuoride ions leads to the establishment of series of the equilibria

orming a single fluoride adduct PhB(OH)2F−, and from the OH−/F−

xchange based on the protonation of hydroxide groups, provid-ng species of 1:2 and 1:3 stoichiometry (PhB(OH)F2

− and PhBF3−)

12,13]. All 3 species resulted in changes of the hybridization ofhe boron center from sp2 to sp3 and gave negative charged boronomplexes and thus fulfill the property of anionic sites in mem-rane electrodes [7]. Therefore, plasticized PVC membrane Na-ISEsan be obtained using the phenylboronic acid as pre-anionic sites

ith the fluoride effector.

The characteristic of ISE depends largely on ion transfer fromhe interface of sample solution to membrane phase. It was real-zed that ions can be transported through polymeric membranes

brane ion selective electrode employed in this work.

of ISEs when an external voltage or a current was applied. Manyresearchers have found that ion fluxes across ISE membranes canbe analytically useful. Both ion fluxes of primary ions in the direc-tion of the inner solution [14,15] and of the sample solution [16,17]were demonstrated to usefully detect important biological species.On the other hand, it was found that the bulk membrane resistanceof an ISE is mainly related to the membrane’s composition (the typeof ionophore and additives), its thickness, viscosity and the temper-ature [18,19]. Varying the amount of anionic sites (upon membraneconditioning with various concentrations of the fluoride effector)in the membrane phase can, therefore, affect the membrane resis-tance.

We describe herein the fabrication of polymeric membrane Na+

selective electrodes using phenylboronic acid as a precursor of theanionic site (functioned as an allosteric site) and preconditioned inthe solution of fluoride ions (functioned as an effector) using theimplement from charge changes to enhance the good characteris-tic of Na-ISEs and to measure changes in membrane resistance. Theionophore used in this work is p-tert-butylcalix[4]arene tetraethylester (NaX) which was reported to bind Na+ selectively by usingoxygen atoms in the ethyl ester groups to coordinate with Na+ [20].The properties of DOS-plasticized PVC membrane Na-ISEs such asresponse slopes, limits of detection, selectivity coefficients, effectof pH and reversibility of the electrodes are studied. On the otherhand, electrochemical impedance spectroscopy (EIS) was used tomeasure the membrane resistance of the fabricated Na-ISE upon

varying the concentration of fluoride effector in the membraneconditioning step. We, therefore, demonstrate the implications ofthe bifunctional membrane concept in fabricating a potentiometricsensor for sodium ion and an impedimetric sensor for fluoride ion.

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36 W. Wongsan et al. / Electroc

. Experimental

.1. Reagents and materials

4-[(4-tert-Butyl-2-methylphenoxy)methyl]phenylboronic acidas purchased from Sigma–Aldrich. p-tert-Butylcalix[4]arene-

etraacetic acid tetraethyl ester (NaX), high molecular weightoly(vinylchloride) (PVC), bis(2-ethylhexyl)sebacate (DOS), 9-diethylamino)-5-octadecanoylimino-5H-benzo[a]phenoxazinechromoionophore I, ETH 5294) and tetrahydrofuran (THF) wereurchased from Fluka. Sample solutions were prepared fromaNO3 (Carlo Erba), NaF and NaCl (Merck). Buffer solutions wererepared from glycine (Sigma–Aldrich). Standard solutions anduffers were made up in ultrahigh-purity Milli-Q water (Bedford,A, USA).

.2. Determination of the anionic site concentration in PVCembranes

Typically, a membrane was prepared by dissolving 1.79 mg20 mmol kg−1) of NaX, 0.27 mg (10 mmol kg−1) of phenylboroniccid, 0.12 mg (2.5 mmol kg−1) of chromoionophore ETH 5294,9.27 mg of PVC and 58.54 mg of DOS in the total amount of 90 mg.ll components were dissolved in 2.0 mL of THF. The cocktail waspread by pipetting 50 �L of the solution onto a square glass slides22-mm No. 1). After coating, the film membrane was left to dryt room temperature for at least 2 h before use. The transparentnd pinkish purple membranes were obtained indicating the pres-nce of the deprotonated form of ETH 5294. The membranes weremmersed in sample solution containing 10−10–10−1 M of NaF pre-ared in 10−3 M glycine/HCl buffer pH 3.5. A glass slide withoutlm membrane was used as blank for absorbance measurement.bsorption spectra were recorded in the range of 400-800 nmsing a UV–vis spectrophotometer (Varian Cary50). The equilib-ium response of the membrane was reached when the absorptionalue remained constant for 10 min.

.3. Preparation and measurement of Na-ISEs

.3.1. Preparation of Na+ selective bifunctional polymericembranes and electrodes

The membrane solution for Na+-selective electrode was pre-ared by dissolving 20 mmol kg−1 of NaX, phenylboronic acid (0,5, 50, 75 and 100 mol% relative to NaX ionophore and the ratiof PVC:DOS plasticizer (1:2, w/w) in the total amount of 220 mg.ll components were dissolved in 2.5 mL of THF then casting

he membrane cocktail into a glass ring fixed on a glass supporti.d. = 30 mm). After evaporation at room temperature a trans-arent membrane was obtained with membrane thickness of ca.00 �m, and a 7.5 mm-diameter disk was cut to put on an elec-rode body. The membrane containing both NaX and phenylboroniccid was conditioned in 10−2 M NaF overnight before potentiomet-ic measurements while the membrane without boronic acid wasonditioned in 10−2 M NaNO3. The response of the membrane elec-rode toward Na+ was examined by measuring electromotive forceEMF) of the following electrochemical cell:

Ag, AgCl/3 M KCl//1 M LiOAc//sample solution/membrane/innerlling solution/AgCl, Ag.

The reference electrode Ag/AgCl with double junction was usedtype 6.0726.100, Metrohm AG, CH-9010 Herisau, Switzerland)

ith 1 M LiOAc as salt bridge electrolyte. Continuous EMF mea-

urements were carried out with a 16-channel electrode monitorLawson Labs Inc., Malvern, PA 19355, USA). Dynamic responseurves and calibration plots were achieved through the adding step

Acta 111 (2013) 234– 241

of a standard NaNO3 solution into 25 mL of Milli Q water. The solu-tions were magnetically stirred during the EMF measurements.

2.3.2. Selectivity measurementsThe potentiometric selectivity coefficients of the electrode for

primary anion with respect to other interfering anions were deter-mined by the separate solution method (SSM) as recommended bythe IUPAC commission [21]. The electrode responses were mea-sured in two separate solutions, one containing primary cations (i)at the concentration range of 10−8–10−2 M, the other containinginterfering cations (j) at the same concentration range. The lowerdetection limit was also calculated as proposed by IUPAC.

2.3.3. Effect of pH on membrane potentialsThe effect of pH on membrane potentials was examined by mea-

suring the EMF of the optimized Na-ISE as well as the pH of 10−3 Mor 10−2 M NaNO3 solutions simultaneously. The pH of the solutionwas varied from 2 to 12 by using HNO3 and KOH.

2.3.4. Reversibility of the fabricated electrodeThe membrane reversibility was examined by measuring the

EMF of the optimized Na-ISEs in 10−4 M NaNO3. Subsequently, theelectrode was rinsed and immersed in 10−3 M NaNO3. The mea-suring cycle was repeated four times. The experiment was thenrepeated again for the reversibility of another two concentrationsof 10−3 and 10−2 M NaNO3.

2.4. Impedance spectroscopy measurements

A conventional three-electrode cell was used in the EIS studies:Na-ISE as the working electrode, a Ag/AgCl reference electrode anda platinum counter electrode. All experiments were conducted atroom temperature. The membranes were conditioned in the differ-ent concentrations (1 × 10−10–1 × 10−1 M) of NaF overnight using10−2 M NaCl as the inner and outer electrolyte solution. Impedancemeasurements were carried out using a potentiostat/galvanostatinstrument (Autolab PG STAT 30, Eco Chemie B.V., Utrecht, TheNetherlands) monitored by Frequency Response Analyser. Thespectra were recorded within a frequency range 10–0.1 KHz with anAC voltage amplitude of 0.01 V. The impedance data were analyzedusing the fitting program available in the Autolab software.

3. Results and discussion

3.1. Measuring of the anionic site concentration in the polymericmembrane

Gyurcsányi and Lindner reported that in the pres-ence of a pH-sensitive chromoionophore, ETH 5294, theconcentration of the anionic site, potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB), in a DOS-plasticizedpolymeric membrane could be determined by the optical mea-surement [22]. In the same manner, the measurement of F− uptakeinto the membrane containing phenyl boronic acid to form theanionic site is carried out by using UV–vis spectroscopy. We haveprepared a PVC membrane with bis(2-ethylhexyl)sebacate (DOS)plasticizer on a glass slide. The polymeric membrane contains theionophore NaX, phenylboronic acid and chromoionophore (ETH5294). Experiments are carried out in aqueous buffered solutionpH 3.5. The boron atom of phenylboronic acid acts as a selectivereceptor for F− whereas ETH 5294 is selective to H+. The uptakeof F− into the membrane occurs by the co-extraction mechanism

[7] of H+ and F− ions to the membrane phase following by abinding of H+ to ETH 5294 and F− to neutral phenylboronic acidto form a phenylborate complex to maintain charge neutralityin the membrane phase. Therefore, the concentration of the

W. Wongsan et al. / Electrochimica Acta 111 (2013) 234– 241 237

Fg

poo(ra

A

wao

fm

C

a

C

catmtsotFegaupaT1c

amptCm

Fig. 2. (A) Time trace lines and response slopes of Na-ISEs prepared using various

ig. 1. Absorption spectra of the membrane optode measured in 1 × 10−3 Mlycine/HCl buffer solution, pH 3.5 at varying NaF concentrations of 10−8–10−1 M.

rotonated form of ETH 5294 (Cp) is equal to the concentrationf the anionic site (C-site) in the membrane. The optical spectrumf the membrane gives the absorbance values for the protonatedAp) and deprotonated (Ad) forms of the chromoionophore of theespective concentrations, Cp and Cd, which have the relationships shown in Eq. (1):

p = �pbCp and Ad = �dbCd (1)

here C-- p and C-- d are the molar absorption coefficients of proton-ted and deprotonated forms of the chromoionophore and b is theptical path length (membrane thickness).

The anionic site concentration (C-site) can then be calculatedrom the known total concentration of the chromoionophore in the

embrane (Ctot) and the degree of protonation (˛H):

−site = Cp = Ctot˛H = CtotAp/εp

Ap/εp + Ad/εd(2)

Eq. (2) can be simplified by replacing the ratio of the two molarbsorption coefficients (C-- d/C-- p) with f:

−site = Cp = CtotApf

Apf + Ad(3)

The fabricated membrane containing phenylboronic acid, thehromoionophore (ETH 5294) and NaX yielded an absorption bandt 540 nm. Extraction of F− into the polymeric membrane gavehe optical response at 665 nm. In the case of our membrane, the

olar absorption ratio (f) was found to be 0.88. We performedhe measurement by immersing the prepared membrane in NaFolutions (1 × 10−10–1 × 10−1 M). Fig. 1 shows absorption spectraf the membranes in various concentrations of NaF. It was foundhat this particular membrane responded to the concentration of− as low as 1 × 10−8 M. With increasing F− concentration, thextent of protonation of ETH 5294 by proton coextraction becamereater, yielding an increase in absorbance of the protonated formt 665 nm. The anionic site concentration of the membrane uponptaking of F− measured in 1 × 10−8–1 × 10−1 M F− in glycine/HClH 3.5 can be calculated from Eq. (3). The calculated C-site and %nionic site relative to the ionophore are summarized in Table 1.he results show that increasing concentration of fluoride ions from

× 10−8 to 1 × 10−1 M caused small changes in the anionic siteoncentration in the membrane phase.

The prepared membrane is not responsive to other anions suchs H2PO4

−, SO42−, NO2

−, Cl−, Br− and I− (Fig. S1 in the supple-entary data). It is well know that OH− and CN− can react with

henyl boronic acid to give the negative charged adducts similaro F−. However, exposure of our prepared membrane to OH− andN− resulted in white spots in the membranes probably due toembrane decomposition. High concentration of CN− results in

amounts of phenylboronic acid and conditioned with 10−2 M NaF. (B) Time tracelines and response slopes of the optimal Na-ISE membrane conditioned with 10−2,10−5 and 10−8 M NaF.

the formation of OH− in aqueous solution. The effect of pH to thestability of the fabricated membrane is discussed in Section 3.2.3.

3.2. Preparation and characteristics of bifunctional polymericmembrane electrodes

3.2.1. Fabrication of bifunctional Na-ISEs using F− as an effectorand their electrode characteristics

In this work, phenylboronic acid was used as a precursor ofthe anionic site in the membrane electrodes. Amounts of phenyl-boronic acid used to prepare ISE membranes were 25, 50, 75 and100 mol% compared to the ionophore NaX. All membranes wereconditioned in 1 × 10−2 M NaF prior to ISE measurements. The timetrace lines and response slopes of the prepared Na-ISEs are shownin Fig. 2A. The characteristics of the prepared Na-ISEs using variousamounts of phenylboronic acid are shown in Table 2.

Without phenylboronic acid, the response slope of the mem-brane (1) was found to be 48.68 mV per decade. In the presenceof phenyl boronic acid, the response slopes of the membranesimproved to be closer to Nernstian slope (59.2 mV per decade). Theresults in Table 2 show that the best Na-ISE was obtained from

the membrane (3) composed of 50 mol% phenylboronic acid, pos-sessing the best characteristic in membrane potential response anddetection limit. Other membranes (2, 4, 5) responded to Na+, butdid not give Nernstian response slopes. We also performed the ISE

238 W. Wongsan et al. / Electrochimica Acta 111 (2013) 234– 241

Table 1Anionic site concentrations (C-site) and % anionic site relative to the ionophore.

Concentration of F− (M) Ap (665 nm) Ad (540 nm) ˛H C-site (mmol kg−1) % Anionic site relative to the ionophore

10−8 0.015 0.051 0.206 0.514 2.5710−7 0.017 0.052 0.226 0.564 2.8210−6 0.017 0.051 0.234 0.585 2.9210−5 0.018 0.049 0.242 0.606 3.0310−4 0.018 0.048 0.246 0.615 3.0810−3 0.018 0.046 0.254 0.635 3.1810−2 0.022 0.039 0.332 0.829 4.1510−1 0.026 0.038 0.376 0.940 4.70

Table 2Characteristics of Na-ISEs prepared using various amounts of phenylboronic acid.

Membrane number Ionophore (mmol kg−1, wt%) Boronic acid (mmol kg−1, wt%) Slope (mV per decade) Detection limit (M) Rm (M�)

1 20, 1.98 0, 0 48.68 ± 0.48 6.06 × 10−6 3.572 20, 1.98 5, 0.17 55.19 ± 0.60 4.72 × 10−7 2.103 20, 1.98 10, 0.30 57.48 ± 0.37 7.42 × 10−7 1.254 20, 1.98 15, 0.45 55.51 ± 0.51 6.14 × 10−7 2.345 20, 1.98 20, 0.59 54.75 ± 0.41 9.97 × 10−7 2.43

N ass ratio, the slope values results from a linear range 10−5 to 10−2 M. No buffer solutionw aNO3. Membranes No. 2–5 were conditioned in 10−2 M NaF.

m1srw

3

aiimomtia

hot

3

(rsto1

3

r

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Fig. 3. Interference study of the optimal Na-ISE with various concentrations of K+.

ote: All polymeric membranes were prepared using a 1:2 PVC to DOS plasticizer mas employed in ISE measurements. Membrane No.1 was conditioned in 10−2 M N

easurements of the optimal membrane (3) preconditioned with0−8 and 10−5 M NaF. Time trace lines and response slopes arehown in Fig. 2B. The membrane ISEs still responded to Na+, but theesponse slope was slightly worse than the membrane conditionedith 10−2 M NaF.

.2.2. Selectivity of the optimal membraneSelectivity of an ionophore is the most crucial characteristic of

n ISE. The ionophore should have good selectivity over interfer-ng ions in order to avoid the bias response from such interferingons. The selectivity of the ionophore results from the complex for-

ation between the ionophore and an analyte ion. The selectivityf the optimal Na-ISEs (3) was explored using separate solutionethod (SSM) [21] and the result is shown in Table 3. As compared

o the reported Ag-ISEs, the selectivity of our bifunctional Na-ISEs comparable to those of the previously published ones using NaXnd other crown ether related as ionophores [23,24].

From the most interfering ion to our Na-ISE is K+. We, therefore,ave performed the interference study with various concentrationsf K+. The result depicts in Fig. 3 suggests that K+ starts to interferehe detection of Na+ at concentrations higher than 1 × 10−5 M.

.2.3. Effect of pH to the stability of the optimal membraneWe have measured the EMF changes of the optimal membrane

3) upon adding 10−2 and 10−3 M NaNO3 at pH range 2–12. Theesult shown in Fig. 4 reveals that the membrane gave stable EMFignals from pH 3.6 to pH 9.7. The result agrees with the observationhat we found white spots in the membrane when we perform theptode experiment in the presence of higher than 10−5 M OH− and0−2 M CN−, and the membrane decomposes at such concentration.

.2.4. Reversibility of the fabricated Na-ISEThe reversibility of the electrode is another important factor that

epresents the precision of the detection. In the experiment to test

able 3otentiometric selectivity coefficients of recently fabricated Na-ISEs.

Reference Logarithms of selectivity coefficients

K+ Li+ Cs+ Mg2+ Ca2+

This work −2.29 −2.84 −3.16 −3.42 −3.01[23] −2.20 −2.97 −3.04 −4.60 −4.60[24] −2.01 −2.75 −1.82 −4.51 −3.75

Fig. 4. Effect of pH to the EMF changes of the optimal membrane (3).

W. Wongsan et al. / Electrochimica

Fa

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ig. 5. EMF changes of the optimized Na-ISE upon varying concentration of Na+

lternately.

he reversibility of the electrode, the optimize Na-ISEs (precondi-ioned in 10−2 M of NaF) were used to measure the EMF values ofwo different concentrations of Na+ alternately. As seen in Fig. 5,he observed reversibility at ambient temperature was excellent,onsidering from the EMF signals that could be restored at the sameoncentration in every measuring cycle. In addition, Na-ISEs pre-onditioned in 10−8 and 10−5 M concentration of NaF also gaveood reversibility upon measuring EMF changes of two alternateoncentrations of Na+.

.3. Effects of the amount of F− effector toward the membraneesistance

We performed the EIS measurements of the ISE membranesontaining 0, 25, 50, 75 and 100 mol% of phenylboronic acid. A con-entional three-electrode cell was used in the EIS studies using

Na-ISE as the working electrode, a Ag/AgCl reference electrodend a platinum counter electrode. The membranes were condi-ioned in 1 × 10−2 M NaF solution overnight using 10−2 M NaCl ashe inner and outer electrolyte solution. The results in Table 2 indi-ated that the membrane containing 50 mol% of phenylboronic acidmembrane 3) showed the best ISE characteristic and also the leastesistance of the membrane toward the transfer of Na+ into theembrane phase.

−

The aforementioned results suggested that amounts of F effec-or used in the conditioning steps of the bifunctional membranesffected their potentiometric responses. This result signified themportance of lipophilic anionic sites on the membrane phase,

Fig. 6. EIS and inset calibration curve of NaF in PVC membrane electrodes (A

Acta 111 (2013) 234– 241 239

which compensated the charge number of the ion–ligand com-plexes within the membrane as shown in Scheme 1. Kucza andco-workers have performed numerical simulations of steady-statepotentials and impedance spectra of ion-selective membranes elec-trodes (ISEs) with ionic sites (different concentrations of primaryion in the bathing solution) [25]. Grabrielli et al. demonstrated byboth experimental and calculation that K-ISEs containing variousconcentrations of an anionic site, BPh4

−, yielded different mem-brane resistances (Rm) [18,19]. Grabrielli expressed the relationshipbetween membrane parameters and the membrane resistance asshown in Eq. (4):

Rm = RTd

F2(DMLCML + DMCM), (4)

where d, DML, CML, DM and CM represent membrane thickness, dif-fusion coefficient and concentration of metal–ionophore complex(ML) and metal ion (M), respectively. R, T and F are gas constant,temperature and Faraday constant, respectively. If d and T are thesame for every fabricated membrane and for potentiometric mea-surements and the ratio of ionophore/anionic site is greater than1, the ML complex controls the conductivity [18,19]. Therefore, themembrane resistance Rm varies along the concentration of the MLcomplex which equals to the concentration of the anionic site inthe membrane phase (C-site) as shown in Eq. (5).

Rm˛1

C−site(5)

Impedance spectra can be used to determine the bulk mem-brane resistance, Rm, under defined conditions, and to compareRm for different membrane compositions (anionic sites) withthe same contacting electrolytes. According to Eq. (5), increasingconcentration of the anionic site resulted in reduction of the mem-brane resistance. We then used the optimal membrane component(membrane 3) to perform preconditions of membranes in vari-ous concentrations of F− (from 10−1 to 10−10 M) and measuredEIS spectra of each membrane using 10−2 M NaCl as the inner andouter electrolyte solution. The results are displayed as semicircleNyquist plots shown in Fig. 6A. A typical equivalent circuit used forinterpretation of impedance spectra of ion selective membranesis presented in the inset of Fig. 6A. The values of the interfacialparameters (Rsol, Rm and Cdl) could be obtained and calculated. Rm

values of the membranes containing both phenylboronic acid andNaX showed a linear relationship with the logarithm of the concen-tration range of F− between 1 × 10−1 and 1 × 10−8 M, −log [F−] form1 to 8, as illustrated in the inset of Fig. 6A. The membrane pretreatedwith F− lower than 10−8 M gave Rm values higher than 4.0 M�

which were not related linearly to Rm values obtained from themembrane pretreated with 1 × 10−1–1 × 10−8 M of F−. This resultagrees well with results obtained from the optical study whichshowed the response to F− concentration as low as 1 × 10−8 M.

) with phenylboronic acid and NaX (B) with only phenylboronic acid.

240 W. Wongsan et al. / Electrochimica

F1

TambSaria

iTsdstn

cattt(cNfiNdKtcftpd(sl[

3b

i

ig. 7. EIS of the membranes preconditioned in various concentrations of NaF using0−2 M KCl as inner and outer electrolyte solution.

herefore, F− concentration lower than 10−8 was too low to gener-te the anionic site in the membrane composition and cause a highembrane resistance. In addition, we also conditioned ISE mem-

ranes with various concentrations of Bu4NF. Nyquist plots (Fig.2 in the supplementary data) of these membranes did not show

linear relationship between the logarithm of the concentrationange of Bu4NF and the Rm values. This suggests the permselectiv-ty of the fabricated bifunctional membranes toward Na+ using F−

s effector.Membranes contained only phenylboronic acid without NaX

onophore were also prepared and their EIS spectra were measured.he Rm values of the membrane containing only boronic acid gavemall changes of Rm values in a range of 4.14–4.51 M�, when con-itioned with 10−8–10−1 M of NaF, as shown in Fig. 6B. This resultignified the importance of the ionophore in ion transportation inhe membrane. Without NaX, Na+ transfer in the membrane wasot efficient.

Mazloum-Ardakani et al. showed that the dissymmetrical con-entration of the inner solution and outer solution (analyte) hasffected the characteristics of the membrane electrode for detec-ion of sulphate ion [26]. It was found that a low concentration ofhe inner solution (10−7 M of sodium sulfate) gave a lower detec-ion limit. We also varied the concentration of the inner solutionNaCl) of our electrode system. However, the dissymmetrical con-entration of the inner and outer solution gave unfitted semicircleyquist plots (Fig. S3 in the supplementary data). We also per-

ormed the EIS experiments using 10−2 M KCl and t-Bu4NCl asnner and outer electrolyte solutions. In the case of t-Bu4NCl, theyquist plots cannot be obtained (Fig. S4 in the supplementaryata). Fig. 7 shows the Nyquist plots of the membranes using 10−2 MCl as inner and outer electrolyte solutions. It is obviously seen that

he EIS responses have a lesser linear relationship with −log [F−]ompared to that of NaCl electrolyte. Although K+ is a major inter-erence of Na+ for the ionophore NaX, the ionophore still showshe preference for Na+. Thus far, we have demonstrated that thearticular bifunctional membrane ISE can be potentially used toetect fluoride by impedimetry, especially low concentration of F−

down to 10−8 M) in aqueous solution in which other molecularensing methods usually give a detection limit in a micromo-ar range as suffered from high hydration energy of F− in water27,28].

.4. Accounts on the possible response mechanism of the

ifunctional Na-ISE

Since the proposed bifunctional Na-ISEs were designed tonduce a proper characteristic of ISE by varying the degree of

Acta 111 (2013) 234– 241

increasing anionic sites generated by the reaction of a phenyl-boronic acid with fluoride ions in the membrane phase, the reactionof phenylboronic acids with fluoride ions must be considered.Three stepwise equilibria are shown in Eqs. (6)–(8), and stepwiseconstants of the presented equilibria determined potentiometri-cally are log K = 0.6, 6.2 and 6.5, respectively [29].

PhB(OH)2 + F− � PhB(OH)2F− (6)

PhB(OH)2F− + F− + H+ � PhB(OH)F2− + H2O (7)

PhB(OH)F2− + F− + H+ � PhBF3

− + H2O (8)

The 1:1 stoichiometry may predominate in the membrane phasecontacting with lower concentration of fluoride ions. At higher flu-oride concentration, the 1:2 and especially 1:3 species could occur.However, all 1:1, 1:2 and 1:3 species gave the net charge of −1,and the bifunctional membrane contained only 10 mmol kg−1 ofphenylboronic acid, the precursor of the anionic site. Therefore,the negative charge in the membrane from the overall reaction offluoride and boronic acid would increase slightly upon increasingthe concentration of fluoride ions, which implied that the amountof the anionic site in the membrane did not change significantlyupon increasing the concentration of fluoride ions. This agreed verywell with the results obtained from the optical study (Fig. 1 andTable 1). Regarding potentiometric responses, the small change ofthe anionic site concentration affects slightly to the response slopeof the bifunctional Na-ISE (the slope ranging from 54 to 57 mVper decade upon varying the bathing NaF solution from 10−8 to10−2 M) and the reversibility of the Na-ISE. For EIS results, smallchanges in the anionic site concentration result in small changes inthe membrane resistance (Rm) upon varying the bathing fluorideconcentrations from 10−8 to 10−1 M (Eq. (5) and Fig. 6).

The fluoride ISE membrane reported by Wróblewski andco-workers using phenylboronic acid as an ionophore and trido-decylmethylammonium chloride as a cationic site gave a largeEMF change upon increasing concentration of fluoride ions [11].However, the membrane encountered a drawback in the super-Nernstian slopes also causing from the stepwise complexationequilibria of phenylboronic acid and fluoride ions mentioned above.Therefore, our bifunctional membrane may be used as an alter-native way to detect fluoride ions by measuring the membraneresistance upon transferring Na+ into the membrane. However,it should be noted that the bathing or preconditioning process iscarried out for each individual membrane to each anionic site con-centration. Therefore, the membrane for EIS measurements cannotbe reused.

4. Conclusions

In this work, we introduce a new concept of ion recognition inPVC membrane electrodes. Fluoride-effector bifunctional Na-ISEswere demonstrated to detect Na+ with good ISE characteristicswith a response slope of 57.48 mV per decade and detection limitof 7.42 × 10−7 M upon membrane preconditioning with 10−2 MNaF. The optimal electrode gave good selectivity toward Na+ witha potent interference from K+, stable EMF measurements in therange of pH 3.6–9.7 and good reversibility. EIS responses of Na-ISEs were found to depend on membrane resistances (Rm) variedwith concentrations of fluoride ions used in membrane condi-tioning steps. EIS spectra obtained from using 10−2 M NaCl asinner and outer electrolyte solution showed a linear relation-

ship between the logarithm of the concentration of fluoride ions(1 × 10−1–1 × 10−8 M) versus the membrane resistances. Resultsshowed that the proposed bifunctional membrane electrodes pos-sessed permselectivity toward Na+ using F− as an effector. The new

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oncept thus leads to the fabrication of a potentiometric sensor forodium ion and an impedimetric sensor for fluoride ion.

cknowledgements

The authors gratefully acknowledge financial support from thehailand Research Fund (RTA5380003). Wanlapa Wongsan is a MStudent supported by the Development and Promotion of Sciencend Technology Talent Project (DPST).

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.electacta.013.08.072.

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