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Sensors and Actuators B 125 (2007) 453–461

Thiocyanate selective sensor based on tripodal zinc complex for directdetermination of thiocyanate in biological samples

Ashok Kumar Singh ∗, Udai P. Singh, Sameena Mehtab, Vaibhave AggarwalDepartment of Chemistry, Indian Institute of Technology-Roorkee, Roorkee-247 667, India

Received 11 January 2007; received in revised form 23 February 2007; accepted 26 February 2007Available online 12 March 2007

bstract

A potentiometric thiocyanate-selective sensor based on the use of zinc-tris(N-tert-butyl-2-thioimidazolyl)hydroborate complex [Ttt-Bu–Zn]s a neutral carrier for a thiocyanate-selective electrode is reported. Effect of various plasticizers viz., o-nitrophenyloctyl ether (o-NPOE),ioctylphthalate (DOP), dibutylphthalate (DBP), benzylacetate (BA), and anion excluder, hexadecyltrimethylammonium bromide (HTAB),ith Ttt-Bu–Zn complex in poly (vinyl chloride) (PVC) were studied. The best performance was obtained with a membrane composition ofOP:PVC:Ttt-Bu–Zn:HTAB percent ratio (w/w) of 60:33:5:2. The sensor exhibits significantly enhanced selectivity toward thiocyanate ions over

he concentration range 6.3 × 10−7 to 1.0 × 10−2 M with a lower detection limit of 3.16 × 10−7 M and a Nernstian slope of 59.4 ± 1.1 mV decade−1.nfluences of the membrane composition, pH, and possible interfering anions were investigated on the response properties of the electrode. Fastnd stable response, good reproducibility, long-term stability, applicability over a wide pH range (3.5–9.0) are demonstrated. The sensor has a

esponse time of 14 s and can be used for at least 45 days without any considerable divergence in their potential response. The proposed electrodehows fairly good discrimination of thiocyanate from several inorganic and organic anions. It was successfully applied to direct determination ofhiocyanate within physiological fluids and environmental samples.

2007 Elsevier B.V. All rights reserved.

comp

tttetitTshata

eywords: Thiocyanate; Ion-selective electrode; Neutral carrier; Tripodal zinc

. Introduction

Ion-selective sensors have been used for analytical determi-ation of a wide variety of ions since the 1970’s. Ion-selectiveensor’s utility and simplicity has replaced other wet analyticalethods that were often far slower and more cumbersome to

erform. Ionophore plays a key role in the sensitivity of anon-selective electrode (ISE). The creation of cavities and cleftn the ionophore that are complementary to the size and chargef a particular ion can lead to very selective interactions. Onef the most important figures of merit for ISEs is the selectivityowards a specific analyte, which is generally limited by thenteraction of ionophore within the membrane with other ions

n solution. The type ion-ionophore interactions based on theiron-exchange property, or size-exclusion of the ionophore,etermine selectivity of an ion sensor and its proper functioning

∗ Corresponding author.E-mail address: akscyfcy@iitr.ernet.in (A.K. Singh).

mo

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925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2007.02.056

lexes

owards a specific ion. Recently, there has been much focus onhe construction of anion-selective electrodes that function onhe basis of chemical recognition principle [1]. Ion-selectivelectrodes are also of obvious interest because they can helpo translate the chemistry of new substrate-binding systemsnto tools that can be used to recognize selectively variousarget species in the presence of potentially interfering analytes.he demand for ionophores with either new or improvedelectivities in the field of ion-selective electrodes (ISEs) isigh particularly in the area of anion-selective electrodes. Fortruly anion-selective electrode, a strong interaction between

he ionophore and the anion is required in order to complexnion in a selective fashion. Potentiometric response of theembranes doped with these complexes is believed to be based

n the coordination of analyte anion with carrier molecule.Thiocyanate is the end product of detoxification of cyanide

ompounds and excreted in urine, saliva and serum. The deter-ination of SCN− is particularly important in saliva, urine,

nd blood serum because it is considered to be a biomarkern distinguishing smokers from non-smokers. Saliva of non-

4 Actu

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54 A.K. Singh et al. / Sensors and

mokers contains thiocyanate concentrations between 0.5 andmM while in smokers concentrations as high as 6 mM can be

ound. Human plasma levels are 2–3 mg/L in non-smokers and–12 mg/L in smokers [2]. Chronically elevated levels of thio-yanate in body fluids are known to be toxic and its relationo local goiter, vertigo, or unconsciousness has been pointedut [3]. Another important sample where the determinationf the thiocyanate anion is of interest is in water, especiallyastewaters. At low pH values, thiocyanate present in wasteater converts into cyanide ion in the presence of oxidants and

hereby causes profound damage to aquatic life. Thiocyanates administered as drug in the treatment of thyroid conditions4] and to achieve an optimal antibacterial effect of the lac-operoxidase system in milk [5]. Therefore, the determinationf thiocyanate in food, wastewater, and biological samples is ofmportant practical significance. Therefore, precise knowledgef the thiocyanate content in biological fluids and environmentalamples is mandatory.

Zinc-tris(N-tert-butyl-2-thioimidazolyl)hydroborate com-lex is utilized here as neutral carrier for the first time in thereparation of anion-selective electrode. We were interestedo investigate the possibility of use of thioimidazolyl com-lexes as anion carriers in PVC membrane electrodes. Theris(2-thioimidazolyl)hydroborate (Tt) ligands used in thisork have enabled new insights in the field of sulfur-rich

oordination compounds of zinc for potentiometric determina-ion of anions. They are reliable tetrahedral enforcers and theropeller-like arrangement of the three-thioimidazole planesn the TtZn complexes allows for a much better adherence ofhe ZnS3X centers to tetrahedral geometry. It is noteworthyhat the soft ZnS3 units go along very well with hard donorsF, O and N) on the fourth coordination site [6–9]. In thisaper, we describe the construction and evaluation of alasticized PVC membrane thiocyanate-selective electrodeased on a zinc-tris (N-tert-butyl-2-thioimidazolyl)hydroborateomplex.

Recently, large efforts have been made to design thiocyanate-elective membranes for sensor applications. They were basedn various ionophores viz., Zn(II) Schiff base complexes10–12], Zn-phthalocyanine complex [13], Cu(II) bis-[N-(2-ydroxyethyl)salicyladiamino] complex [14], Ni(II) N,N′-thylene-bis-(4-methylsalicylidineiminato) complex [15],i(II)-azamacrocycle complex [16], Ni(II)benzoN4macrocycl-

complex [17], bis(2-mercaptobenzoxazolato) mercury(II) andis(2-pyridinethiolato) mercury(II) complexes [18], Rh(III)omplex [19], Mn(II) N,N′-bis-(4-phenylazosalicylidine)omplex [20], Mn-porphyrin derivative [21], Cu(II)N,N′-bis-benzaldehyde)-gylcine complex [22], Cd-Schiff base complex23], Crown ether [24], bis-bebzoin-semitriethylenetetraamineinuclear copper(II) complex [25], etc. The aim of this paperas to develop a new thiocyanate-selective electrode for theirect determination of thiocyanate in saliva and urine withouthe need for prior separation steps. The goal of the present work

s to devise a highly selective thiocyanate membrane electrodeith good response properties, such as slope and response

imes, using the selective coordination chemistry betweenhiocyanate and Zn(II)-complex.

s(vo

ators B 125 (2007) 453–461

. Experimental

.1. Chemicals

All the reagents for the synthetic portion of this workere purchased from Aldrich and used as received. For mem-rane preparation, high molecular weight polyvinyl chloridePVC), o-nitrophenyloctyl ether (o-NPOE), dioctylphthalateDOP), dibutylphthalate (DBP), benzylacetate (BA), hexade-yltrimethylammonium bromide (HTAB), and tetrahydrofuranTHF) were used as received from Fluka. Reagent gradeodium salts of all anions used were of highest purity avail-ble from SRL (Mumbai, INDIA) and used without anyurther purification except for vacuum drying over P2O5.ris-hydroxymethylaminomethane (TRIS), Glycine (Gly) and-morpholinoethanesulfonic acid (MES) were purchased fromluka. MES-NaOH buffer were prepared by titrating 50 mMolution of the acid form with concentrated sodium hydrox-de to a pH values of 4.00 ± 0.01, 5.00 ± 0.01 and 6.00 ± 0.01.RIS-SO4 buffer was prepared by titrating 50 mM of the basic

orm of TRIS using concentrated solution of H2SO4 to pHalues of greater then 6.00. 0.05 M Gly adjusted for pH lesshen 4 with 0.1 M HCl solution. Triply distilled water was usedor the preparation of all aqueous solutions. The ionophore N-ert-butyl-2-thioimidazole was synthesized by method describedreviously [26] and was identified by spectral data. Urine, salivand serum samples were collected from Hospital (Indian Insti-ute of Technology Roorkee, India) and River water sample fromanga river (Roorkee, India).

.1.1. Synthesis of tris (N-tert-butyl-2-thioimidazolyl)ydroborate ligand K[Ttt-Bu]

The ligand tris (N-tert-butyl-2-thioimidazolyl)hydroborateTtt-Bu) and its zinc(II) complex were prepared by theethod of Vahrenkamp and coworkers [27]. N-tert-butyl-2-

hioimidazole (7.67 g, 49.1 mmol) and finely ground KBH4882 mg, 16.4 mmol) were suspended in 50 mL of toluene andeated to reflux for 4 days. The resulting precipitate was filteredff, washed with toluene, dried in vacuum and recrystallizedrom ethyl acetate/cyclohexane (1:1), yielding 4.91 g (51%) of[Ttt-Bu] as a colorless powder, mp 180 ◦C, which loses ethyl

cetate upon prolonged pumping.Anal Calcd. for C21H34BKN6S3 (516.7): C, 48.77; H, 6.58;

, 16.25; S, 18.58%. Found C, 48.12; H, 6.47; N, 16.07;, 18.42%. IR (KBr): � (BH) at 2556 (w) cm−1. 1H NMRCD3OD): δ 1.79 [s, 27H, t-Bu], 6.10 [d, J = 2.0 Hz, 3H], 6.95d, J = 2.0 Hz, 3H].

.1.2. Synthesis ofris(N-tert-butyl-2-thioimidazolyl)hydroborate zinc(II)omplex Ttt-BuZn-OClO3

A solution of K[Ttt-Bu] (200 mg, 0.39 mmol) in methanol15 mL) was added dropwise with stirring within 20 min to a

olution of Zn(ClO4)2·6H2O (144 mg, 0.39 mmol) in methanol15 mL). The volume of the solution was reduced to 15 mL inacuo, the precipitate was filtered off, and the filtrate evap-rated to dryness. The residue was taken up in 10 mL of

A.K. Singh et al. / Sensors and Actu

Fp

d1d

3N(J

2P

smtdagrwcPipNcmp

viawipttmss

ufi

2

p(c

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3

auaoitawTiimiSCN ion. At the same time, there was no detectable change inthe ionophore UV–vis. spectra for other anions. These changesin UV–vis. spectra evidence the specific interaction between theionophore and SCN−.

ig. 1. Structure of zinc-tris(N-tert-butyl-2-thioimidazolyl)hydroborate com-lex [Ttt-Bu–Zn] ionophore.

ichloromethane, filtered and evaporated to dryness again,93 mg (76%) of Ttt-BuZn-OClO3 remained as a colorless pow-er, mp 248 ◦C (Fig. 1).

Anal. Calcd. for C21H34BClN6O4S3Zn (Mr = 642.4): C,9.26; H, 5.33; N, 13.08; S, 14.98. Found: C, 39.87; H, 5.31;, 12.23; S, 14.40. IR (KBr): � (BH) 2456 (w), � (ClO4) 1105

s) cm−1. 1H NMR (CD3CN): δ 1.73 [s, 27H, t-Bu], 6.97 [d,= 2.2 Hz, 3H], 7.23 [d, J = 2.2 Hz, 3H].

.2. Preparation of normal PVC-membrane and sandwichVC membranes

The PVC-membranes were prepared and assembled into sen-or electrodes using established procedures [28,29]. To prepareembranes 10 mg ionophore (Ttt-Bu–Zn), 4 mg lipophilic addi-

ive salt, 66 mg PVC and 120 mg solvent mediator, mixed andissolved in 2 mL THF. The resulting solution was poured ontoglass ring with an inner diameter of 20 mm resting on a smoothlass plate. THF was allowed to evaporate for 24 h standing atoom temperature. Transparent PVC membranes were obtainedith a thickness of about 0.2 mm. A 15 mm diameter piece was

ut out from the PVC membrane and glued to one end of ayrex glass tube with Araldite. The dried tube was filled with

nternal solution contained 10−2 M NaSCN/50 mM MES buffer,H 5.0. Then, the electrode was conditioned for 24 h in 10−1 MaSCN solution. The ratio of membrane ingredients, time of

ontact and concentration of equilibrating solution were opti-ized so that the membrane develops reproducible and stable

otentials.Sandwich membranes were prepared by pressing two indi-

idual membranes (one without ionophore and the other withonophore) together immediately after blotting them individu-lly, dry with tissue paper. The combined segmented membraneas then rapidly mounted into the electrode body with the

onophore-containing segment facing the sample solution andotential immediately measured. The time required from makinghe membrane sandwich contact until final membrane poten-

ial measurement was kept minimum (∼1 min). The individual

embranes use already equilibrated before using them forandwich membranes. All membrane electrode potential mea-urements were performed at laboratory ambient temperature in

FiS

ators B 125 (2007) 453–461 455

nstirred salt solution (identical to the conditioning and innerlling solution) versus Ag/AgCl reference electrode.

.3. Potential measurement

All potential measurements were preformed at ambient tem-erature using a Century scientific-digital pH/milivoltmeterModel CP 901). The electrochemical system for this electrodean be represented as follows:

Ag/AgCl/KCl (satd.)/test solution/PVC membrane/10−2 MaSCN/Hg/Hg2Cl2.The performance of the electrode was

nvestigated by measuring its potential in sodium thiocyanateolutions prepared in the concentration range (10−2 to 10−8 M)y gradual dilution of stock standard solution 0.01 M of NaSCN,ith triply distilled water. Sample solutions were buffered using0 mM MES buffer, pH 5.0.

. Results and discussion

.1. UV–vis and IR measurements (preliminary tests)

In order to investigate the selective interaction of complexs a potential ion carrier with different anionic species, theltra violet and infrared spectral studies of the complex in thebsence and presence of a number of common anions werebtained in methanol solution. The resulting UV–vis. spectrums shown in Fig. 2. From Fig. 2, it is immediately obvioushat the addition of ionophore (with two absorption maximat 257.06 and 216.12 nm) to an equimolar solution of SCN−ill result in significant shift in position (216.12 → 203.88 nm).his could possibly happen via the replacement of perchlorate

on by thiocyanate ion in Ttt-Bu–Zn complex. However, furtherncrement of the amount of SCN− solution to two equimolar

erely affected the absorption maxima. These results stronglymply that ionophore (Ttt-Bu–Zn) forms 1:1 complex with the

−

ig. 2. UV–vis spectra of ionophore solution (1.0 × 10−4 M) in methanol (A)n the absence (B) in the presence of SCN− ion (1:1) and (C) in the presence ofCN− ion (1:2).

456 A.K. Singh et al. / Sensors and Actu

ra[Stoaiei

3r

snpvacees

ttfwpattgoDss

aostciwmitopwobDwtthe most sensitive anions based on Tt –Zn are also shown inFig. 5 with optimized membrane. As is obvious from Fig. 5,

Fig. 3. Response mechanism of ionophore [Ttt-Bu–Zn] towards SCN.

This was further proved by IR spectrum. It was literatureeported that Zn-OClO3 and Zn-NCS bands in infrared spectrare present in region 1105 cm−1 and 2075 cm−1, respectively27,30]. So the recognition properties of ionophore towardsCN− ion can be studied through IR spectra. It was observed

hat pure ionophore give perchlorate band while on additionf one equivalent NaSCN solution perchlorate band disappearnd a very strong thiocyanate band was observed. This clearlyndicates that Zn-perchlorate complexes are labile and they canxchange perchlorate ion from complex to thiocyanate counteron by solution (Fig. 3).

.2. The effect of membrane composition on potentialesponse of the thiocyanate sensor

The sensitivity and performance of an ionophore (Ttt-Bu–Zn)ignificantly depend on the membrane composition and theature of solvent mediator and additive used. Thus to optimizeotentiometric performance of thiocyanate-selective electrodearious PVC membrane based on Ttt-Bu–Zn were preparednd investigated. The influence of plasticizer type and con-

entration on the characteristics of thiocyanate ion-selectivelectrode was investigated by using four plasticizer with differ-nt polarities including DBP, DOP, o-NPOE and BA (Fig. 4). Ithould be noted that the nature of plasticizer influences both

Fig. 4. Variation of membrane potentials with different plasticizers.

ar

FvC

ators B 125 (2007) 453–461

he dielectric constant of the membrane and the mobility ofhe ionophore and its complex with SCN− [31]. Among theour different plasticizer used, DOP gives the best performanceith regard to working concentration range and slope in thereparation of SCN− ion-selective electrode. It seems that DOPs a low polarity compound provides more appropriate condi-ions for incorporation of highly lipophilic thiocyanate ion intohe membrane. Several membrane compositions were investi-ated by varying the proportions of PVC and DOP. Irrespectivef ionophore concentration the slope was relative larger whenOP/PVC weight ratio was approximately 2.0. It has been

hown that plasticizer PVC ratios of ca. 2 produced maximumensitivity [32].

It has also been demonstrated that the presence of ionicdditives improves potentiometric behavior by reducing thehmic resistance [33] and catalyzing exchange kinetics at theample-membrane interface [34]. The effects of cationic addi-ive concentrations in the membrane were investigated at severalarrier/additive weight ratios. Better response characteristics,.e. Nernstian response and improved selectivity were observedith an optimum ionophore/HTAB weight ratio of approxi-ately 2.5. Sensitivity of electrode response increases with

ncrease in the ionophore content from 2 to 5%, further addi-ion of ionophore will, however, result in diminished responsef the electrode, most probably due to some inhomogeneties andossible saturation of the membrane [35]. Several membraneere prepared with different compositions, based on resultsbtained on the optimization of the membrane composition, theest response was observed with the membrane composed ofOP:PVC:Ttt-Bu–Zn:HTAB percent ratio of 60:33:5:2, and thisas selected for the preparation of polymeric membrane elec-

rode for thiocyanate ion. The potential responses of some oft-Bu

mong different anions tested, SCN− with the most sensitiveesponse seems to be suitably determined with the electrode.

ig. 5. Potential responses of ion-selective electrode based on [Ttt-Bu–Zn] forarious anions (B, SCN−); (C, S2O3

2−); (D, CN−); (E, Sal−); (F, ClO3−); (G,

r2O72−); (H, Cl−); (I, MnO4

−); (J, CO32−).

A.K. Singh et al. / Sensors and Actuators B 125 (2007) 453–461 457

FBs

3

apsoteN6Tppsi1ctopamim

3

twtbf2t

Table 1Characteristics of optimized thiocyanate ion-selective membrane sensor

Properties Value range

Optimized membranecomposition

DOP:PVC:Ttt-Bu–Zn:HTAB percentratio of 60:33:5:2

Electrode type PVC membrane electrodeLinear range 6.3 × 10−7 to 1.0 × 10−2 MConditioning time 24 h in 1.0 × 10−2 M NaSCNSlope 59.4 mV decade−1 of thiocyanate activitypH range 3.5–9.0Standard deviation of slope ±1.1Detection limit 3.16 × 10−7 MLR

w5telieoe(

3

siowrrafter successive immersion of a series of SCN− solution, eachhaving a ten-fold difference in concentration is only 14 s. Theoptimum conditioning time for the membrane electrodes in a

ig. 6. Effect of pH on the different test solution (A = 1.0×10−2 M,= 1.0×10−3 M, C=1.0 × 10−4 M) on the potential response of the SCN− ion-

elective electrode.

.3. Influence of pH on the electrode’s parameters

Using the optimized membrane composition describedbove, tests were conducted to determine pH range of the pro-osed thiocyanate selective electrode. The pH of the sampleolution is an important factor, often influencing the responsef ion-selective electrodes. Thus, the effect of sample solu-ion pH on the thiocyanate response characteristics of preparedlectrodes was investigated in detailed. For this purpose, MES-aOH buffer of pH values of 4.00 ± 0.01, 5.00 ± 0.01 and.00 ± 0.01; 0.05 M Gly/HCl buffer for of pH less then 4;RIS-SO4 buffer of pH values of greater then 6.00 were pre-ared and examined as background electrolyte solutions forotentiometric measurements. The effect of the pH of the testolution on the response of the membrane electrode was exam-ned at three SCN− concentrations (1.0 × 10−2, 1.0 × 10−3 and.0 × 10−4 M). As illustrated in Fig. 6 the potentials remainonstant within a pH range of approximately 3.5–9.0. Varia-ion of the potential at pH <3.5 could be related to protonationf SCN− in the aqueous phase and ionophore in the membranehase [Ttt-Bu–Zn], which results in a loss of its ability to inter-ct with the ionophore. At higher pH >9.0, the potential dropay be due to interference of hydroxide ions. The hydroxide

on will compete with thiocyanate ion for the cationic site in theembrane.

.4. Calibration plot

The optimum equilibration time for the membrane elec-rode in the presence of 1.0 × 10−2 M NaSCN was 24 h, afterhich it would generate stable potentials in contact with SCN−

est solutions. A MES buffer solution pH 5.0 was used as a

ackground electrolyte in the measurements. It was found thator selected membrane (60% DOP, 33% PVC, 5% Ttt-Bu–Zn,% HTAB) potential response was linear for SCN− ions, overhe concentration range 6.3 × 10−7 to 1.0 × 10−2 M (Fig. 4 F

ife time At least 45 daysesponse time 14 s

ith DOP plasticizer and Table 1) with a Nernstian slope9.4 mV decade−1 of SCN− concentration. The limit of detec-ion, defined as the concentration of thiocyanate obtained whenxtrapolating the linear range of calibration curve to the baseine potential is 3.16 × 10−7 M. The reproducibility and stabil-ty of the electrode was evaluated by repeated calibration of thelectrode in sodium thiocyanate solutions. Repeated monitoringf potentials and calibration, using the same electrode, over sev-ral days gave good slope reproducibility; the standard deviationS.D.) of slope was ≤1.2 mVdecade−1, as given in Table 1.

.5. Dynamic response and lifetime

It is well known that the dynamic response time of a sen-or is one of the most important factor for evaluation of anyon-selective electrode. To measure the dynamic response timef the proposed sensor the concentration of the test solutionas successively changed from 1.0 × 10−6 to 1.0 × 10−2 M. The

esulting data depicted in Fig. 7, shows that the time needed toeach a potential with in ±1.0 mV of the final equilibrium value

ig. 7. Dynamic response of the membrane electrode based on [Ttt-Bu–Zn].

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58 A.K. Singh et al. / Sensors and

.01 M NaSCN solution was 24 h after which it generated stableotentials with SCN− ions solutions.

To record the lifetime of the best membrane the potentialsere checked daily, the sensing behavior of the membrane

emained reasonably constant over a period of 45 days.tt-Bu–Zn complex has very low solubility in aqueous solution,hich minimize their leaching from the membrane and contam-

nation of the sample solutions. After life time period swelling inembrane was too high that membrane became mechanicallyeak and leaching of ionophore from membrane to aqueous

olution took place. Therefore, deviation in potential and slightradual decrease in slope occurs. However, it is important tomphasize that they were stored in 0.01 M NaSCN solution whenot in use.

.6. Internal solution effects

In accord with generally adopted ion sensor response formal-sm, the influence of the concentration of internal solution onhe potential response of the polymeric membrane electrode forCN− ion based on Ttt-Bu–Zn was studied. The concentrationas varied from (1.0 × 10−1 to 1.0 × 10−6 M) and the potential

esponse of the electrode was obtained. It was found that the bestesults in terms of slope and working concentration range werebtained with internal solution of activity 1.0 × 10−2 M. Thus,.0 × 10−2 M concentration of the reference solution was quiteppropriate for the smooth functioning of the proposed sensor.

.7. Determination of formation constant

Formation constant of the ion–ionophore complex withinhe membrane phase is a very important parameter that dic-ates the practical selectivity of the sensor. In this method, two

embrane segments are fused together, with only one contain-ng the ionophore, to give a concentration-polarized sandwich

embrane. A membrane potential measurement of this tran-ient condition reveals the ion activity ratio at both interfaces,

hich translates into the apparent binding constants of the

on–ionophore complex [36]. In this method, complex forma-ion constants obtained by neglecting ion pairing. As reported,he membrane potential EM is determined by subtracting the cell

riia

able 2electivity coefficients and formation constant for thiocyanate ion-selective electrode

nterferingons

Formation constant(log βILn)a ± S.D.

Selectivity coefficientlog (Kpot

SCN,B)a ± S.D.Inter

CN− 7.45 ± 0.2 BrO3

lO4− 2.45 ± 0.2 −4.2 ± 0.2 Cl−

2O32− 0.71 ± 0.8 −3.4 ± 0.3 Br−

rO42− 2.45 ± 0.5 −4.8 ± 0.1 I−

N− 4.62 ± 0.4 −3.5 ± 0.2 F−O2

− 1.65 ± 0.5 −4.2 ± 0.2 Sal−O3

− 1.84 ± 0.3 −4.6 ± 0.3 PO43

Ac− 2.15 ± 0.2 −4.9 ± 0.4 H2POr2O7

2− 1.58 ± 1.2 −4.1 ± 0.1 HS−O3

2− 0.84 ± 0.5 −4.7 ± 0.2 MnOO4

2− 1.14 ± 0.8 −4.3 ± 0.4 OH−

a Mean value ± standard deviation (three measurements).

ators B 125 (2007) 453–461

otential for a membrane without ionophore from that for theandwich membrane. The formation constant is then calculatedrom the following equation:

ILn =(

LT − nRT

zI

)−n

exp

(EMzIF

RT

)(1)

here LT is the total concentration of ionophore in the membraneegment, RT is the concentration of lipophilic ionic site additives,is the ion–ionophore complex stoichiometry, and R, T and F are

he gas constant, the absolute temperature, and the Faraday con-tant. The ion I carries a charge of zI. The determined formationonstants (log βILn) for the examined different complexes wereecorded in Table 2. The elapsed time between sandwich fusionnd exposure to electrolyte was typically <1 min. The potentialas recorded as the mean of the last minute of a 5 min measure-ent period in the appropriate salt solution. The potential of such

andwich membranes remains free of diffusion-induced poten-ial drifts for about 20 min. Standard deviations were obtainedased on the measurements of sets of at least three replicateembrane disks that were made from the same parent mem-

rane. A careful analysis of the data in Table 2, reveals thathiocyanate ion has significant anion-binding characteristics.

.8. Determination of selectivity coefficients

Selectivity of solvent polymeric membrane ion-selectivelectrodes (ISEs) are quantitatively related to equilibrium at thenterface between sample and the electrode membrane. Matchedotential method (MPM) was used to determine selectivity ofhe sensor in this paper [37,38]. This method has an advantagef removing limitations imposed by Nicolsky–Eisenman equa-ion while calculating selectivity coefficient by other methods.hese limitations include non-Nernstian behavior of interfer-

ng ion and problem of inequality of charges of primary andnterfering ions.

The quantity used to express the extent of interference is the

atio of primary ion concentration increment to the interferingon concentration that gives same potential change in a constantnitial background of primary ion. The interfering ion would bedded to an identical reference solution until the same poten-

fering ions Formation constant(log βILn)a ± S.D.

Selectivity coefficient(Kpot

SCN,B)a ± S.D.

− 2.28 ± 0.5 −4.8 ± 0.23.28 ± 0.5 −4.4 ± 0.43.56 ± 0.8 −4.3 ± 0.33.05 ± 0.4 −3.6 ± 0.22.08 ± 1.2 −4.2 ± 0.21.28 ± 0.4 −3.6 ± 0.2

− 2.72 ± 0.9 −4.8 ± 0.3

4− 0.72 ± 0.4 −4.6 ± 0.3

3.24 ± 0.2 −3.6 ± 0.1

4− 3.52 ± 0.5 −4.0 ± 0.2

2.12 ± 0.2 −3.8 ± 1.0

A.K

.Singhetal./Sensors

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125(2007)

453–461459

Table 3Comparison of the proposed thiocyanate ion-selective membrane sensor with the reported electrode

Ionophore Membrane composition (%,w/w)

Selectivity coefficient log (KpotSCN,B) Method Linear range

(M)Slope(mV decade−1)

pH range Responsetime (s)

Ref. no.

Zinc–phthalocyaninecomplex

5.1% ionophore, 2 9.2%PVC, 65.7% (NPOE)

Sal−(−0.76), I−(−1.81), ClO4− (−1.92),

Br− (−2.42), Cl− (−2.82), NO3− (−2.89),

NO2− (−3.13), H2PO4

− (−3.61), SO42−

(−4.32)

SSM 1.0 × 10−6 to1.0 × 10−1

58.1 ± 0.5 2.0–7.0 15 [13]

Butane-2,3-dionebis(salicylhydrazonato)zinc(II)

6% ionophore, 32% PVC,60% DOP, 2% TOMAC

ClO4−(−2.8), CO3

2−(−4.0), Cl− (−4.1),MnO4

− (−3.4), Br− (−3.3), NO3− (−3.0),

I− (−3.8), SO42− (−4.1), NO2

− (−3.1),S2O3

2− (−4.2)CN− (−3.5), PO43−(−4.2),

OAc− (−4.2), C2O42− (−4.0), Sala (−3.2),

HPO42− (−4.0), Cr2O7

2−(−3.4), CrO42−

(−3.4), F− (−4.1), ClO3− (−3.2)

FIM 1.0 × 10−6 to1.0 × 10−1

56.5 ± 1.1 3.5–8.5 5–15 [12]

N,N’-bis-(benzaldehyde)-glycine metalliccomplexes

3.0% ionophore, 29.8% PVC,67.2% (NPOE)

ClO4−(−1.8), Sala (−2.1), Br− (−2.7), I−

(−2.4), NO2− (−3.3), NO3

− (−3.7), SO42−

(−4.1), Cl− (−4.3), H2PO42− (−4.5)

MPM 1.0 × 10−7 to9.0 × 10 −1

57.6 At pH 4 6–15 [22]

Crown ether-cetyltrimethylammonium-thiocyanate

5.0% ionophore, 31%PVC,64% DOP

CrO42− (−4.6), Cl− (−4.3), ClO4

− (−3.7),Br− (−4.2), MnO4

− (−4.5), I− (−4.1),PO4

3− (−4.6), SO42− (−3.9), CO3

2−(−3.9), SO3

2− (−3.7), BrO3− (−3.1), CN−

(−4.2), CH3COO− (−3.8), NO2− (−4.1),

C2O42− (−3.7), NO3

− (−3.7), Sala (−3.6)

SSM 1.0 × 10−6 to1.0 × 10−1

57.6 3.8–9.2 ≤ 20 [24]

N,N′-bis-(4-phenylazosalicylidene)o-phenylene diamine

3.8% ionophore„30.5% PVC,65.7% o-NPOE

I− (−1.1), Sali (−1.2), CH3COO− (−3.7),NO2

− (−2.6), NO3− (−3.8), Cl− (−4.2),

ClO4− (−2.2), Br− (−3.0),

SSM 1.0 × 10−6 to7.0 × 10−1

57.3 4.0–6.5 – [20]

2,2′-[Methylenebis(4,1phenyl-enenitrilomethylidyne)]bisphenol Zn(II) complex

5.0% ionophore, 31% PVC,62% DOP

ClO4−(−0.2), Cl− (−2.0), Br− (−1.8),

NO3− (−1.1), I− (−0.4), SO4

2− (−1.7),NO2

− (−2.0), CN− (−2.3), OAc− (−3.2),Sala (−0.2), H2PO4

2− (−3.6), F− (−4.0)

SSM 4.0 × 10−7 to1.0 × 10−2

57.5 – – [10]

(N,N′-Bis-salicylidene-1,2ethylenediamine) cd(II)complex

7.0% ionophore, 30% PVC,60% DBP

Cl− (−2.5), ClO4− (−0.4), Br− (−2.2), I−

(−1.3), PO43− (−3.6), SO4

2− (−2.9),CO3

2− (−2.8), CN− (−1.6), CH3COO−(−4.4), NO2

− (−2.43), C2O42− (−4.4),

NO3− (−2.7), Sala (−2.6)

MPM 1.0 × 10−6 to1.0 × 10 −1

59.1 ± 0.2 1.5–11.0 15 [23]

Nickel(II)-1,4,8,11,15,18,22,25-octabutoxyphthalocyanine

30% PVC, 65%DBP, 3%NOBP, 2% HTAB

CrO42− (−4.6), Cl− (−4.1), ClO4

− (−3.1),Br− (−4.2), I− (−3.4), SO4

2− (−4.0),CO3

2− (−4.0), CN− (−4.2), CH3COO−(−4.5), NO2

− (−4.0), NO3− (−3.4), Sala

(−3.6)

MPM 1.0 × 10−6 to1.0 × 10−1

58.7 ± 0.6 4.3–9.8 <10 s [15]

Zinc-tris(N-tert-butyl-2-thioimidazolyl)hydroboratecomplex

5% ionophore, 33% PVC,60% DOP, 2% HTAB

ClO4−(−4.2), CO3

2−(−4.7), Cl− (−4.4),MnO4

− (−4.0), Br− (−4.3), NO3− (−2.8),

I− (−3.6), SO42− (−4.3), NO2

− (−4.2),S2O3

2− (−3.4), CN− (−3.5), PO43−(−4.8),

OAc− (−4.9), Sala (−3.6), H2PO4− (−4.6),

Cr2O72−(−4.1), CrO4

2− (−4.8), F− (−4.2),ClO3

− (−3.8)

MPM 6.3 × 10−7 to1.0 × 10−2

59.4 ± 1.2 3.5–9.0 14 This work

460 A.K. Singh et al. / Sensors and Actuators B 125 (2007) 453–461

Table 4Determination of thiocyanate in saliva, urine, serum and river water samples

Thiocyanate amount (�g/mL)a Samples

Saliva(smoker)

Saliva(non-smoker)

Urine(smoker)

Urine (non-smoker)

Serum(smoker)

Serum(non-smoker)

Riverwater

P 8 ± 0C 5 ± 0

td

K

w(tiac

iathpTS>=ht[i

rtsfpS

4

tstsalroat

cp

5

o[PiTilhpc

A

Ia

R

roposed thiocyanate sensor 20.2 ± 0.54 5.5 ± 0.40 7.olorimetric method 20.5 ± 0.54 5.6 ± 0.40 7.

a Mean value ± standard deviation (five measurements).

ial change is obtained. The selectivity coefficient KpotSCN,B, is

etermined as:

potSCN,B = �aSCN

aB= a

′SCN − aSCN

aB(2)

here, �a = a′SCN − aSCN, aSCN is the initial primary ion

SCN−) activity and a′SCN the activity of primary ion (SCN−) in

he presence of interfering ion (B) and aB is activity of interfer-ng ion. The activity of SCN− as reference solution was takens 1.0 × 10−2 M in this study. The selectivity coefficients soalculated are shown in Table 2.

The selectivity coefficients clearly indicate that the electrodes selective to thiocyanate over a number of other inorganicnd organic anions. As it is evident from the data in Table 2,he electrode based on Ttt-Bu–Zn(II) complex has relativelyigh selectivity toward SCN− relative to anions such aserchlorate, salicylate, oxalate, and several common anions.he interfering effect of the ions is in the following order:2O3

2− > CN− > Sal− > HS− > I− > OH− > MnO4− > Cr2O7

2−NO2

− > F− = ClO4− > SO4

2− = C2O42− = Br− > Cl− > NO3

−H2PO4

− > CO32− > PO4

3− = BrO3− > OAc−. The reason for

igh selectivity of this electrode to thiocyanate ion is thoughto be due to the possible interaction of the anions with Zn39]. According to hard–soft acid and base (HSAB) theorem,nteraction of SCN− with complex is via nitrogen.

Table 3 lists the linear range, detection limit, slope,esponse time, and selectivity coefficients of some of otherhiocyanate-selective electrodes against proposed thiocyanate-elective electrode for comparative purposes. As can be seenrom the table, the selectivity coefficients obtained for theroposed electrode are superior to those reported for otherCN−-selective electrodes listed in Table 3.

. Analytical application

Urine, saliva, and serum samples containing differenthiocyanate concentrations were collected from smoker and non-moker patient, and same samples were assayed for multipleimes. Samples were treated by MES/NaOH buffer (pH 5.5)olution, while the river water sample was used directly bydjusting pH 5.5 by dilute HCl solution. All samples were ana-yzed in five replicate using the proposed electrode, and the

esults were compared with those obtained by a standard col-rimetric method. The results given in Table 4, show that themounts of thiocyanate ion evaluated with the help of the elec-rode are in good agreement with those obtained by the standard

.61 1.8 ± 0.24 11.4 ± 0.46 2.4 ± 0.54 0.95

.61 1.6 ± 0.24 11.3 ± 0.46 2.5 ± 0.54 0.98

olorimetric method, thereby reflecting the utility of the pro-osed sensors.

. Conclusions

On the basis of these results discussed in this paper, complexf zinc-tris(N-tert-butyl-2-thioimidazolyl)hydroborate complexTtt-Bu–Zn] can be regarded as a carrier for construction of aVC-based membrane ISE for thiocyanate ion. The thiocyanate

on-selective electrode was used for the analytical applications.he proposed electrode has been shown to have good operat-

ng characteristics (sensitivity, stability, response time, detectionimit, and a wide linear range). It is easy to prepare and use. Theigh degree of thiocyanate selectivity by the electrode makes itotentially useful for monitoring concentration levels of thio-yanate in different water and biological samples.

cknowledgement

Ms. Sameena Mehtab is greatful to Council of Scientific andndustrial Research, New Delhi, India for providing financialssistance for this work.

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iographies

shok Kumar Singh is a professor of organic chemistry at Indian Institute ofechnology Roorkee, Roorkee, India. He received his PhD in Organic Chemistryrom Banaras Hindu University, Varanasi, India in 1977. His research work haseen mainly focused on the synthesis of macrocyclic ligand-complexes and theirpplications in analytical chemistry as chemical sensors.

dai Pratap Singh is an associate professor of inorganic chemistry at Indiannstitute of Technology Roorkee, Roorkee, India. He received his PhD in Inr-anic Chemistry from Banaras Hindu University, Varanasi, India. His researchork has been mainly focused on the synthesis of Coordination Chemistry,io-inorganic Chemistry, Enantioselective Catalysis and Heme-Protein.

ameena Mehtab obtained her MSc degree in Chemistry from Indian Institutef Technology, Roorkee, Roorkee in 2004. She is currently working towards herhD at the Indian Institute of Technology Roorkee, Roorkee, India under theupervision of Prof. Ashok Kumar Singh.

aibhave Aggarwal obtained his MSc degree in Chemistry from Indian Institutef Technology, Roorkee, Roorkee in 2004. He is currently working towards hishD at the Indian Institute of Technology Roorkee, Roorkee, India under theupervision of Prof. Udai Pratap Singh.