Materials Science and Engineering C 45 (2014) 109–116

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Materials Science and Engineering C

j ourna l homepage: www.e lsev ie r .com/ locate /msec

A new potentiometric electrode incorporating functionalizedβ-cyclodextrins for diclofenac determination

Joanna LenikDepartment of Analytical Chemistry and Instrumental Analysis, Chemical Faculty, Maria Curie-Skłodowska University, M. Curie-Skłodowska Sq. 3, 20–031 Lublin, Poland

E-mail address: j.lenik@poczta.umcs.lublin.pl.

http://dx.doi.org/10.1016/j.msec.2014.08.0720928-4931/© 2014 Published by Elsevier B.V.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 June 2014Received in revised form 31 July 2014Accepted 30 August 2014Available online 6 September 2014

Keywords:Potentiometric electrodeDiclofenacPharmaceutical analysis

This paper reports the preparation of diclofenac-selective membrane electrodes incorporating β-cyclodextrins:(2-hydroxypropyl)-β-cyclodextrin, heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin, and heptakis(2,3,6-tri-O-benzoyl)-β-cyclodextrin. Several plasticized poly(vinyl chloride) membranes of different compositionswere tested with the best electrode being the one incorporating heptakis(2,3,6-tri-O-benzoyl)-β-cyclodextrin with the membrane plasticized with 2-nitrophenyloctyl ether. The electrode is characterized by anear-Nernstian response slope of −60.0 mV decade−1 over the linear range of 5.0 × 10−5 − 1.0 × 10−2 mol L−1

and a limit of detection of 1.4 × 10−5 mol L−1. The proposed electrode can easily discriminate diclofenac ionsfrom several inorganic and organic interferents and some common drug excipients. The electrode has a responsetime of 10 s and can be used within a pH range of 6.2–8.5 over 10 months without any considerable deterioration.The electrical properties of the membrane electrode were studied by impedance spectroscopy. The notableadvantages of the diclofenac-selective electrode include its high sensitivity, selectivity, cost-effectiveness, andcomfortable application in drug and urine analysis.

© 2014 Published by Elsevier B.V.

1. Introduction

In recent years, there has been a surge in the development of electro-chemical sensors applicable in biomedical analysis and various trendshave been observed in this field. Modifications of electrochemical solidcontact sensors have been widely conducted. A large number of differ-entmaterials, such as carbon nanotubes, metal oxides, conductive poly-mers, and inorganic catalysts, have been used for electrode surfacemodification. In particular, multiwalled carbon nanotubes (MWCNTs)can be applied to promote electron transfer between electroactivespecies and the electrode [1–3].

Another direction of development is the search for new interesting ac-tive substances reacting selectively with a given compound of biomedicalimportance. Of great interest to researchers are host–guest reactions,with the host molecules including ethers, cryptands, calixarenes, andcyclodextrins.

Cyclodextrins (CDs) are compoundsmade of glucopyranose units, themost important among them being those comprising 6, 7, and 8 units,i.e.,α-, β- and γ-CDs. They form a lipophilic inner cavity and a hydrophilicouter surface. CDs are capable of interacting with a large variety of guestmolecules to form inclusion complexes, and so they are used in thepharmaceutical, food, clothing, cosmetic, and chemical industries [4,5],and they also have agricultural applications. In the pharmaceutical indus-try, CDs can be used to increase the solubility and dissolution rate ofpoorly water soluble drugs. Many pharmaceutical products containing

β-CDs are already commercially available, including the following non-steroidal anti-inflammatory drugs (NSAIDs): indomethacin, piroxicam,meloxicam, nimesulide, tiaprofenic acid, anddiclofenac [6]. The formationof inclusion complexes by cyclodextrins can alter some physicochemicalproperties of the guest molecules (e.g., electrochemical properties). CDsare also used in analytical techniques, such as potentiometry [7,8], polar-ography [9], and voltammetry [10,11]. The electroanalytical applicationsof CDs are also based on chiral molecular recognition and selectivepreconcentration of analyte at the electrode.

Potentiometric studies concerning the use of cyclodextrins as iono-phores (in sensor development) are very new and not numerous. Inparticular, lipophilic cyclodextrins (with an alkylated hydroxyl group inpositions 2, 3, and 6 [12]), dihydroxypropyl-β-cyclodextrin [13], andother CDs, triacetyl-β-cyclodextrin [14], α-, β-, and γ-cyclodextrinderivatives and heptakis (2,3,6,-tri-O-methyl)-β-cyclodextrin [15–17],as well as (2-hydroxypropyl)-γ-cyclodextrin [18] can be incorporated inplasticized PVC membrane graphite electrodes as excellent ionophores.

The aim of the presented work was to apply selected β-cyclodextrinderivatives as ionophores in solid-contact plasticized PVC membraneelectrodes selective for diclofenac.

Diclofenac [[2-(2,6-dichlorophenyl)amino]benzeneacetic acid],used in the form of a sodium salt (Fig. 1) [19], is a non-steroidal anti-inflammatory drug belonging to the group of heterocyclic and arylicderivatives of acetic acid, similarly as indomethacin, sulindac, andtolmetin. It is commonly used to reduce fever, pain, stiffness, andswelling. Diclofenac is one of themost active cyclooxygenase inhibitors.It works by inhibiting the production of prostaglandins. Diclofenac has a

110 J. Lenik / Materials Science and Engineering C 45 (2014) 109–116

stronger effect than indomethacin, while it is much less likely to irritatethe digestive tract [20,21]. In pharmaceutical formulations, diclofenac isavailable as a sodium salt. Diclofenac sodium salt crystallizes fromwater; it has a melting point of 283–285 °C and dissolves at 25 °C indeionized water, methanol, acetone, acetonitrile, cyclohexane, HCl,and phosphate buffer [19].

Several analytical methods have been reported in the literature forthe determination of diclofenac in pharmaceutical and biological fluids,including spectrophotometry [22,23] and chromatography [24]. Thesemethods are characterized by long and complicated preparation ofsamples for analysis, involving the use of organic solvents, as well as re-quire expensive apparatus. In recent years, there has been a growingneed for constructing chemical sensors for faster,more economical, pre-cise, and selective pharmaceutical and environmental analysis [25–27].

In the presented work, a novel potentiometric electrode for the de-termination of diclofenac was developed. For this end, modificationsof the membrane composition were studied and a number of cyclodex-trins were applied as new ionophores. The basic analytical parameters(selectivity, response time, pH effect on potential, lifetime) of the con-structed ion-selective electrodeswere determined. A notable advantageof the proposed electrode is its simple and cheap construction. Inaddition, the proposed electrode is very easy to use. Unlike electrodeswith internal solution, it does not have to be stored in a vertical positionand does not require external solution for storage between measure-ments. The electrode may be kept in air and is mechanically robustand maintenance-free. The proposed electrode was satisfactorilyapplied to the determination of diclofenac in real samples, pharmaceu-ticals, and urine samples.

According to the literature data, no investigations have been carriedout to date into the application of β-cyclodextrins as sensing agents in adiclofenac-selective electrode.

2. Experimental

2.1. Reagents and solutions

All chemicals were of analytical–reagent grade. The components ofthe membrane are: tributyl phosphate (TBP) (Merck), bis(2ethylheksyl)sebacate (DOS) (Merck Schuchard), diisobutyl phthalate (DIBP)(Fluka), 2-nitrophenyloctyl ether (o-NPOE) (Fluka), emulsionPVC (Tarwinyl Tarnów, Poland), heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (HSMβCD), heptakis (2,3,6-tri-O-benzoyl)-β-cyclodextrin(HSBβCD), (2-hydroxypropyl)-β-cyclodextrin (HPβCD), and tetraoctylammonium chloride (TOA-Cl) (Fluka). The sodium salts of interferentions are obtained fromFluka. The other reagents are: diclofenac— sodiumsalt (Sigma-Aldrich), Majamil 100 mg (Polfarma, Poland), Olfen 150 mg(Tewa Operations, Poland) tetrahydrofuran (THF) (Merk), and theionic strength stabilizing solution CH3COONa (0.05mol L−1). All aqueoussolutionswere preparedwith deionizedwater of conductivity 0.07 μs/cm(Elix Advantage SystemMili-Q plus Milipore, Austria).

Fig. 1. Diclofenac 2-[(2,6-dichlorophenyl)-amino]benzeneacetic acid.

A stock solution of diclofenac sodium salt was prepared byweighingabout 0.1592 g of reagent into a 50mL volumetric flask and subsequentto the mark with deionized water (adjusted to pH 7.0). The workingcalibration solutions were prepared from it by rigorous dilution withthe same water.

2.2. Potentiometric measurements

Themeasurements of the electromotive force of the systemdiclofenacelectrode - reference electrode (Orion 90-02) - were carried out at 22 ±1 °C using Electrochemistry EMF Interface system (Lawson Labs. Inc.,USA) and IBM PC computer. Before the first measurement the electrodeswere conditioned in 10−2mol L−1 diclofenac solution for 24 h. Before thenext measurements the electrodes were conditioned in 10−2 mol L−1

diclofenac solution for 30 min. EMF potentiometric measurements weremade in themain ion and interfering ions solutions at the concentrationsof 10−6–10−2mol L−1 of pH 7.0 in deionizedwater. During themeasure-ments the solutions were stirred with a mechanical stirrer and thepotential (±0.5 mV) was recorded after stabilization.

A Thermo Orion 81-72 glass electrode C using a MultifunctionalComputer device CX-721 Elmetron (±0.1 mV) (Poland) was used forpH measurements.

2.3. Electrochemical impedance spectroscopy (EIS) measurements

Electrochemical impedance spectroscopy (EIS) measurements wereperformed using an Autolab electrochemical analyzer controlled by FRAsoftware, version 4.9 (Eco Chemie Utrecht, The Netherlands) connectedwith a three-electrode system. The studied electrode was connected asa working electrode, the reference electrode was Ag/AgCl (3 M KCl),and the auxiliary electrode was a platinum wire. The impedance spectrawere recorded at potential 0.2 V with an amplitude 0.1 V over the fre-quency range 5 kHz–1 MHz. The measurements recorded at lower 0.1 Vand higher 0.3 V potentials gave the same impedance plot. On the otherhand the measurements recorded with lower amplitude down to 0.1 Vgave the same impedance plot but with more noise. All experimentswere carried out in solutions of 10−2 M diclofenac (pH 7.0). The EISmeasurements were recorded for the ten month membrane electrode.

2.4. Membrane preparation

The major component of the electrode is a polymeric membranephase which is in direct contact with a silver–silver chloride electrode.This is placed in a cylindrical Teflon container screwed on to the elec-trode frame. The body of the electrode is made of insulating substance.

The membrane phase of the electrode consists of two layers: theinner layer containing plasticized PVC in which the Ag/AgCl electrodeis placed and the outer layer, contacting with the tested solution andcontaining the potential creating substance apart from the inner layercomponents. The internal layer shall be composed of the following in-gredients: 30 wt.% PVC, 70 wt.% of mixed plasticizers: 95 wt.% DIBP,DOS or NPOE, and 5 wt.%. TBP (e.g. addition of tributyl phosphateleads to significant decrease in the resistance). The outer layer(Table 1) consist of: 32.8 wt.% PVC, 65.6 wt.% of mixed plasticizers,1.2% of appropriate cyclodextrin, and 0.4% of tetraoctyl ammoniumchloride. The potential of the internal reference Ag/AgCl electrode isstable and determined by Cl− ions originating from PVC degradationaswell as fromdissolution dissociation of AgCl in TBP [28]. The potentialof these construction electrodes with solid contacts is reproducible andstable in time. The electrodes prepared were stored in air between themeasurements.

2.5. Determination of diclofenac in pharmaceuticals and urine samples

Pharmaceutical formulation was obtained from local drug stores.Samples to determinediclofenacwere prepared: 10 tablets of diclofenac

Table1

Thecompo

sition

ofmem

bran

eph

aseof

prep

ared

ionselectiveelectrod

esan

dtheiran

alytical

parameters.

Mem

bran

eco

mpo

sition

Electrod

enu

mbe

r1

23

45

67

8

HSM

βCD

0.00

30–

0.00

30–

0.00

30–

––

HSB

βCD

–0.00

30–

0.00

30–

0.00

30–

–

HPβ

CD–

––

––

–0.00

30–

DIBP

0.15

580.15

58–

––

––

–

DOS

––

0.15

580.15

58–

––

0.15

79o-NPO

E–

––

–0.15

580.15

580.15

58–

TBP

0.00

820.00

820.00

820.00

820.00

820.00

820.00

820.00

84TO

A-Cl

0.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

10

Electrod

epe

rforman

ceSlop

e[m

V/decad

e]−

59.9

±2.1

−59

.7±

1.8

−55

.1±

5.8

−56

.5±

1.3

−58

.3±

3.0

−60

.0±

3.3

−60

.6±

1.3

−55

.0±

1.7

Line

arityrang

e[m

olL−

1]

10−

4–10

−2

5×

10−

5–10

−2

5×

10−

5–10

−2

5×

10−

5–10

−2

5×

10−

5–10

−2

5×

10−

5–10

−2

5×

10−

5–10

−2

10−

4–10

−2

E0[m

V]

−10

0.3

−10

3.3

−11

2.9

−10

6.0

−16

7.1

10.56

−23

1.4

−12

1.3

Correlationco

efficien

t(r)

0.99

540.99

930.99

970.99

990.99

990.99

830.99

990.99

98LO

D[m

olL−

1]

2.5×

10−

51.0×

10−

51.0×

10−

51.4×

10−

51.4×

10−

51.4×

10−

57.9×

10−

68.5×

10−

5

Resp

onse

time[s]

1510

2025

1210

1620

pHrang

e6.5–

8.5

6.2–

8.0

5.7–

8.5

6.2–

8.5

6.7–

8.5

6.2–

8.5

6.0–

8.5

6.5–

8.5

Life

time[m

onths]

5.5

86.5

99

1010

3

±s(n

=11

).

111J. Lenik / Materials Science and Engineering C 45 (2014) 109–116

100 mg and 150 mg were weighed and then ground in a mortar. Theamounts of the powder corresponding to the active substance wereweighed and dissolved in 50 mL deionized water. The solutions werethen filtered into a 100 mL volumetric flask and then completed to themark with deionized water of pH 7.0 and shaken. The sample solutions(3.14 × 10−4 mol L−1 and 4.71 × 10−4 mol L−1) were prepared by di-luting the stock solution and they were complete with ionic strengthstabilizing solution acetate at pH 7.0. The standard solutions ofdiclofenac were prepared in ionic strength stabilizing solution acetateat pH 7.0.

Urine samples: 2.5 and 5 mL of urine samples were transfered to100 mL volumetric flask. After addition of 10−3 mol L−1 diclofenacsolution, the solution was diluted to the mark with ionic strength stabi-lizing solution acetate at pH 7.0.

Finally, the proposed ion-selective electrode was used to determinediclofenac content in the solutions.

3. Results and discussion

3.1. Membrane composition and electrode performance

Due to the interesting structure of cyclodextrins and the resultingphysical and chemical properties of these compounds, they are appliedas ionophores in ion-selective electrodes. In aqueous solutions, theweak-ly hydrophobic cavity of CDs is occupied by water molecules, and thesecan be readily replaced by less polar molecules, i.e., pharmaceuticals.The formation of inclusion complexes is based on interactions of non-covalent nature (electrostatic interactions, van der Waals forces, π–πinteractions) and on steric effects [29,30]. This is also the case with com-plexes formed with diclofenac, as indicated by means of spectroscopic(UV–vis, steady-state fluorescence, H NMR) and thermodynamic(conductivity) techniques [31,32]. It has been found that complexes areusually formedwith 1:1 stoichiometry. This work evaluated the behaviorof selected lipophilic cyclodextrin derivatives acting as ionophores inplasticized PVC membrane electrodes. The response of electrodes incor-porating heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin, heptakis(2,3,6-tri-O-benzoyl)-β-cyclodextrin, and (2-hydroxypropyl)-β-cyclodextrintowards diclofenacwas determined. Since cyclodextrins are neutral iono-phores, a lipophilic additivewith an opposite sign to the guest compound[[2-(2,6-dichlorophenyl)amino]benzeneacetate] is required,which in thiscase is the tetraoctyl ammonium cation. The incorporation of lipophilicions in the membrane also serves to stabilize charged complexes and en-sure permselectivity and electroneutrality of membranes. Furthermore,ionic sites reduce the electrical resistance of the membranes, allowinggreater ion mobility within the organic layer. To evaluate the potentio-metric characteristics of ion-selective electrodes incorporating othercyclodextrins, three other plasticizers mixed with TBP were tested inthe preparation of the membrane phase. The study results concerningmembrane composition and electrode performance are given in Table 1.

As can be seen, membrane electrodes with β-cyclodextrins (elec-trodes 1–7) afford a sensitivity of about −55 to −60 mV decade−1 inthe same wide linear range (5 × 10−5–10−2 mol L−1) except for elec-trode 1, and the limit of detection is 1.0 × 10−5–7.9 × 10−6 mol L−1.The calibration graphs of electrodes 2, 3, 4, 5, and 7 exhibit a goodcorrelation coefficient (approx. 0.9999–0.9993).

Electrodes 3 and 4, plasticizedwith DOS (dielectric constant ε=4.8)and containing HSBβCD and HSMβCD reveal the lowest sensitivity.Electrodes 5, 6, and 7, with the membrane plasticized with o-NPOE,characterized by the highest polarity (dielectric constant ε = 14), aremore sensitive by 5 mV decade−1 than electrodes 3 and 4. In the caseof the electrode prepared with HPβCD, more stable sensitivity and thelowest limit of detection were obtained. This may be due to the interac-tion of the carboxyl group of the guest with the hydroxyl group of thehost (H⋯O\H). The potentiometric electrodes prepared with HSBβCD(2 and 6) show better performance than electrodes with HSMβCD(1 and 3). In this case, there occur interactions of delocalized electrons

Fig. 2. Calibration curves of two days diclofenac electrodes in the diclofenac sodium saltsolutions at concentrations of 10−6–10−2 mol L−1 at pH 7.0. The limit of detection(LOD) was appointed according to the method, as the ion concentration level at whichthe measured cell voltage deviates 18/z (where z is the ion charge) from the Nernstianresponse curve.

112 J. Lenik / Materials Science and Engineering C 45 (2014) 109–116

of the aromatic rings of diclofenac and the benzoyl group. This in turnmay lead to the formation of complexes of lower energy and to morefavorable guest extraction.

To evaluate the influence of ionophores on response characteristics, anelectrode without an ionic additive was prepared (8). The slope of thiselectrode is sub-Nernstian in the narrow range of 10−2–10−4 mol L−1.Such behavior of the electrode provides evidence for an inward flux ofdiclofenac and for a changed concentration of its ions in the membranephase.

Consequently, the best electrode function for the determination ofdiclofenac was found for electrodes 2, 5, 6, and 7. The calibration curves

Fig. 3.Calibration curves of twomonths diclofenac electrodes in the diclofenac sodium saltsolutions at concentrations of 10−6–10−2 mol L−1 at pH 7.0.

of these electrodes are presented in Figs. 2 and 3 at 2 days and 2 months,respectively.

The presented electrodes exhibit much better calibration parame-ters than electrodes incorporating an ion associate of diclofenac withRhodamine B [33]. Those electrodes exhibited a Nernstian slope over anarrower concentration range (5 × 10−4–5 × 10−2 mol L−1) and adetection limit higher than 1 × 10−5 mol L−1.

3.2. Selectivity

The selectivity coefficient is the most important characteristic ofISEs, providing information about the ability of the sensing membraneto discriminate the primary ion against other ions of the same chargesign [34]. It depends both on standard Gibbs energies of ion transferfrom the aqueous to the membrane phase as well as on the selectivityof ion complexation by the ionophore. It is well known that membranescontaining only a lipophilic salt (without an ionophore) display a selec-tivity pattern known as the Hofmeister series, which is linked to thelipophilicity of ions [35]. In the case of ionophores other than ionexchangers, this series may slightly differ because of the presence ofother chemical bonds and due to interactions of the active center ofthe molecule and counter ions.

In thiswork, the potentiometric selectivity coefficients for diclofenacISEs were determined in relation to some organic and inorganicanions as well as amino acids. Determinations were performed by theSeparate Solution Method (SSM). The potential of a cell comprising adiclofenac-selective electrode and a reference electrode was measuredfor the main and interfering ion solutions in the concentration rangeof 1.0 × 10−6–1.0 × 10−2 mol L−1. The following Eq. (1) was used fordetermining selectivity coefficients:

logKpotDICLO;N ¼ − E2−E1

S

� �− zDICLO

zN−1

� �logcDICLO ð1Þ

where N stands for an interfering ion, E2 — electrode potential in theinterfering ion solution, E1 — electrode potential in diclofenac solution,S — characteristic slope of the main ion, zDICLO — charge of the mainion (zDICLO= 1), zN — charge of interfering ions, and cDICLO = 1mol L−1.

It can be seen from Table 2 that logK values are different for particu-lar ions. The selectivity sequences obtained for the presented electrodesdeviate from the Hofmeister series. For all electrodes, the sequence of

Table 2Selectivity coefficients log KDICLO,N

pot of diclofenac ion selective electrode.

log KDICLO,Npot

1 2 3 4 5 6 7

ClO4− −0.42 −0.51 −0.66 0.50 −0.46 −0.61 −0.33

SCN− −1.60 −1.64 −1.88 −1.79 −1.64 −1.96 −1.77I− −2.43 −2.98 −2.58 −2.43 −2.42 −2.30 −1.96NO3

− −4.01 −5.34 −4.01 −3.89 −3.47 −3.79 −3.13Br− −4.74 −4.59 −4.82 −4.82 −5.24 −5.04 −3.95Cl− −5.90 −5.60 −5.20 −5.20 −5.53 −5.95 −5.74SO4

2− −5.94 −5.72 −5.37 −4.81 −3.69 −5.96 −5.62H2PO4

− −3.19 −3.26 −3.35 −2.99 −2.61 −3.50 −2.70Salicylate −2.51 −2.12 −2.26 −2.02 −2.29 −2.45 −1.74Propionate −6.00 −5.60 −4.91 −4.70 −6.02 −5.85 −5.20Benzoate −5.10 −5.19 −4.82 −4.50 −4.63 −4.88 −4.46Citrate −6.32 −5.71 −5.34 −5.10 −6.17 −5.98 −5.49Acetate −5.81 −5.58 −5.13 −4.46 −5.46 −5.49 −5.59Oxalate −6.10 −6.35 −4.99 −4.91 −5.05 −5.93 −5.24Tartrate −6.28 −5.84 −5.39 −5.07 −6.02 −5.93 −5.63Formate −6.14 −5.74 −5.27 −4.70 −5.72 −5.71 −5.49Glutamic acid −5.60 −6.39 −5.32 −4.77 −5.70 −5.47 −5.09Aspartic acid −5.66 −5.65 −5.44 −4.90 −5.45 −5.61 −5.38Glycine −5.30 −5.37 −4.76 −5.00 −5.03 −5.30 −5.42Glucose −5.66 −5.75 −5.51 −5.14 −5.61 −5.90 −5.81Lactose −5.77 −6.42 −5.73 −5.33 −5.61 −5.84 −5.73Mannit −5.89 −5.85 −5.70 −5.31 −5.68 −6.00 −5.94

113J. Lenik / Materials Science and Engineering C 45 (2014) 109–116

log KDICLO,Npot I− NH2PO4

− NNO3− is very clear. For electrodes 1–7, different

lipophilic patterns were obtained, respectively:

C6H4(OH)COO− N H2PO4− N NO3

− N CH3COO− N Cl−

C6H4(OH)COO− N I− N H2PO4− N Br- N NO3

− N CH3COO− N Cl−

C6H4(OH)COO− N I− N H2PO4− N CH3COO− N Cl−

C6H4(OH)COO− N I− N H2PO4− N NO3

− N SO42- N Br− N Cl−

C6H4(OH)COO− N I− N H2PO4− N NO3

− N SO42− N Br− N CH3COO− N Cl−

I− N C6H4(OH)COO− N H2PO4− N NO3

− N Br− N CH3COO− N Cl−

C6H4(OH)COO− N SCN− N I− N H2PO4− N NO3

− N Br− N CH3COO− N Cl−.

Importantly, small selectivity coefficients were obtained fordiclofenac-selective electrodes 1 and 6 with respect to Cl− and SO4

2−

ions (about 1 × 10−6). Furthermore, it should be noted that electrode6 (HSBβCD, o-NPOE) exhibited the best selectivity values for the inor-ganic anions SCN−, Cl−, SO4

2−, and H2PO4−. Electrode 2 (HSBβCD and

DIBP) yielded better results only for amino acids and pharmaceuticalexcipients, i.e., glycine and glucose. With respect to NO3

− and I− thebest selectivity coefficients were obtained for electrode 2 (4.5 × 10−6

and 1.0 × 10−3, respectively), and with respect to Br− for electrode 5(5.0 × 10−6). Electrodes 1, 3, and 5 (incorporating HSMβCD) exhibitedsimilar selectivity coefficient sequences with respect to organic anions:salicylate N benzoate N acetate ~ oxalate ~ propionate N formate N

citrate ~ tartrate. Electrodes containing HSBβCD, DIBP, and o-NPOEexhibited the highest selectivity values for C2O4

2−. The worst selectivitycoefficient values were found for electrodes 3 and 4, which were plasti-cized with the most lipophilic DOS (log P = 10.9), and for electrode 7incorporating HSβCD.

Fig. 4. Selectivity profiles of presented electrode no. 2 and electrodes reported in theliterature.

The results of this work show that the selectivity of electrodes 2 and6 is the highest. The selectivity coefficients of the proposed electrodeagainst some interferents were compared to the corresponding valuesfor previously reported diclofenac ion-selective electrodes [36–40],mainly those based on ion-pair formation or those incorporatingporphyrin. The results (Fig. 4) show that the selectivity coefficients ofthe proposed diclofenac-selective electrode are in most cases betterthan those of other ion-selective electrodes.

The best selectivity coefficients were obtained for the proposedelectrode with respect to NO3−, Br−, Cl−, SO4

2−, and the followingorganic anions: benzoate, citrate, acetate, oxalate, tartrate, and formate.

3.3. Reversibility of the electrode potential

The reversibility of the electrode response is one of themost impor-tant analytical parameters of ion-selective electrodes. In order toevaluate potential reversibility, selected electrodes were immersedrepetitively in diclofenac sodium salt solutions at two different concen-trations (1× 10−3 and 1 × 10−4mol L−1). The results obtained for two-month-old electrodes are presented in Table 1 and Figs. 5–7. The meanpotential values from all measurements were 186 mV and 242 mVfor electrode 6 and −46 mV and 12 mV for electrode 7 in 1 × 10−3

and 1 × 10−4 mol L−1 diclofenac solutions, respectively. The maximumrelative standard deviation in potential measurements was 2%.

3.4. Response time and potential drift of the diclofenac-selective electrode

In analytical applications, dynamic response time is an importantfactor for selective electrodes. In the case of the proposed electrode,the response time was obtained from the dynamic potential responsecorresponding to diclofenac concentration steps between 1.0 × 10−6

and 1.0 × 10−2 mol L−1. The static response time for the proposed elec-trodeswas between 10 and 25 s throughout the entire linear concentra-tion range. The dynamic response times of all electrodes are given inTable 1.

The potential drift of the examined electrodes was determined byimmersing the electrodes in stirred 5 × 10−5 mol L−1 diclofenacsolution for 24 h. The electrodes were about 2 months old. The resultsfor electrodes 1–7 are presented in Fig. 6.

It can be seen that the most stable potential was achieved forelectrodes 2 and 6, incorporating HSBβCD.

Fig. 5. Reversibility of the response of twomonth electrodes a) no. 5 with HSMβCD, b) no.6 with HSBβCD, and c) no. 7 with HMβCD.

Fig. 6. Potential drift of electrodes in 5 × 10−5 mol L−1 diclofenac solution during day andnight.

114 J. Lenik / Materials Science and Engineering C 45 (2014) 109–116

3.5. pH dependence

The effect of pH on the potential of diclofenac-selective electrodeswas studied in diclofenac sodium salt solution at a concentration of1.0 × 10−4 mol L−1. pH was changed using small volumes of HCl(c = 0.05 mol L−1) added to 20 mL of diclofenac sodium salt solution.Independently, NaOH solution (c= 0.01mol L−1) was added dropwiseto a newdiclofenac sodium salt sample. After each acid or base addition,the solution pH and the electrode potential change were measured.

Table 1 shows a stable potential response for pH in the range of6.5–8.5 in 1.0 × 10−4 mol L−1 solution for the majority of electrodes.The effects of diclofenac solution pH on electrode 2 and 6 are shownin Fig. 7. In an acidic medium, a characteristic jump in potential can beobserved. This can be explained by the acidic form of diclofenac in thetested solution.

Fig. 7. Effect of the pH on the potential response of the diclofenac electrode nos. 2 and 6.

3.6. Lifetime

Lifetimewas tested bymeasuring the slopes of electrodes kept in airat room temperature.Measurements weremade systematically, usuallyevery 7–14 days, in freshly prepared diclofenac sodium salt solutions(pH = 7.0). Lifetime was defined as the period of time up to themoment when the electrodes showed a deviation of ±16% from theNernstian slope [41].

Among the investigated electrodes, those incorporating HSBβCDrevealed the longest lifetime (8–10 months). This may be due to thedifferent construction of the electrodes (solid contact without internalsolution) and different active substances, that is, cyclodextrins with alipophilic cationic additive, which lead to a longer time of elution ofthe diclofenac–β-cyclodextrin complex from the polymer membrane(the electrodes were stored in air). The obtained lifetime results arebetter than those of electrodes based on ion-associate complexes(3 months) [33]. The other electrodes studied in this work revealedthe following lifetime values: HSMβCD electrodes 1 and 3 — 5.5 and6.5 months, respectively, and the HPβCD electrode — 10 months.

3.7. Electrochemical impedance spectroscopy

Impedance spectra were recorded for the diclofenac-selective PVCmembrane electrode 7, which gave one of the best potentiometric re-sponses and a long lifetime (1.2% HPβCD + 0.4% TOA-Cl, 65.6% NPOE+ TBP + 32.8% PVC). The results are presented in Fig. 8, where a singlehigh-frequency semicircle is shown. For the studied electrode, bulkresistance was 945 kΩ and geometric capacitance was 39.3 pF.

3.8. Potentiometric determination of diclofenac in pharmaceuticals andurine samples

Following the experimental results, the new membrane electrode(no. 6) was successfully applied for determining diclofenac sodiumsalt in pure samples (Sigma), samples of pharmaceutical preparations,and urine samples of different concentrations. In the case of diclofenacdetermination in biological samples, drug recovery testing was con-ducted. The obtained results are presented in Table 3. Determinationwas performed by the calibration curve method. Recovery from 6replicate diclofenac measurements was found to be 94.0–100.5%(RSD 2.2–3.4%) for clear solutions, 96.1–98.8% (RSD 2.5–4.5%) for

Fig. 8. Impedance plot for diclofenac electrode recorded at potential 200 mV with anamplitude 100 mV and in the frequency range 5 kHz–1 MHz.

115J. Lenik / Materials Science and Engineering C 45 (2014) 109–116

pharmaceuticals, and 99.0–99.7% (RSD 2.7–3.1%) for urine solutions.The obtained results are characterized by exactness typical of the deter-mination methods employed.

A comparison of diclofenac determination results obtained by theproposed method with those obtained by other reported analyticalmethods is as follows. Theworking range offered by the presented elec-trode (12.6 μg mL−1–2.52 mg mL−1) is broader than that afforded byspectrophotometry (4–36 μgmL−1) [23]. The limits of detection obtain-ed for CE and LC (25.46 μg/mL and 13.93 μg/mL, respectively [42]) arehigher than the limit of detection established for the proposed electrode(1.99 μgmL−1). The accuracy and precision of the potentiometricmeth-od (mean recovery 98.7%, RSD 3.5%) are comparable with those foundfor HPLC and LC methods (101.6% [24] and 99.59 ± 0.21%, respectively[42]) or for other methods (capillary electrophoresis— 103.12 ± 0.90%[42] and spectrophotometry — 98.88% [22]). Other advantages of thepotentiometric method are its low cost and the short time of a singledetermination (~10–20 s). This method does not involve any pretreat-ment (except filtration), complex formation–extraction or centrifuga-tion steps, which are required in other instrumental methods.

In view of the above, it can be concluded that the electrodesconstructed and presented in this work can be successfully applied inpharmaceutical laboratories and other research facilities.

4. Conclusions

The application of β-cyclodextrins as ionophores in the pseudo-liquid membrane phase of diclofenac ion-selective electrodes was pro-posed for the first time. The overall good characteristics including awide linear range (5.0 × 10−5–1 × 10−2 mol L−1), a low limit of detec-tion (1.0 × 10−5–5.0 × 10−5 mol L−1), and a reproducible Nernstianslope were obtained. In particular, the incorporation of heptakis(2,3,6-tri-O-benzoyl)-β-cyclodextrin resulted in excellent electrodes (nos. 2and 6) with a short response time and long lifetime (8–10 months).At the endof their lifetime, these electrodes require only a simple regen-eration of the secondmembrane phase and are ready to resumework asif they were new electrodes.

It is also worth emphasizing the notable advantage of the proposedelectrode, that is, its high selectivity. In the case ofmost interferents its se-lectivity coefficients are much better than those previously reported inthe literature for electrodes based on ion associates. β-cyclodextrins areless affected by organic and inorganic anions than ion exchangers andthey show a preference for aromatic and heterocyclic compounds.Complexes of cyclodextrins with other anions are weaker, and hencethe relatively good selectivity of the proposed electrode for diclofenac.Obviously, the steric hindrance caused by the complicated structure ofthe β-cyclodextrin molecule also influences electrode selectivity fordiclofenac.

Due the simple construction of the presented electrode (withoutinternal solution), it may be kept in air, it does not have to be storedin a vertical position, and it does not require external solution forstorage between measurements. Furthermore, it is cheap and easy toboth prepare and use for determining diclofenac in pharmaceuticalsand urine samples.

Table 3Results obtained in the analysis of diclofenac samples by the proposed ion-selective electrode.

Sample Taken mg L−1 Found mg L−1 n

Pure (Sigma) 31.8 29.9 6100.0 100.5 6150.0 149.8 6

Pharmaceutical 100.0(Majamil)

96.1 8

150.0 (Olfen) 148.3 8Urine 31.8 + 2.5 mL urine 31.7 10

31.8 + 5 mL urine 31.5 10

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