1. Introduction

The ion-selective membrane electrodes are based on selecting an appropriate matter which works like a sensing part. The sensing materials are the main component of the potentiometric ion sensors. They are responsible for forming a type of discrimination in the electrode behavior toward one ion rather than others. There are several types of the sensing material. They might have host-guest structure, which allows the inclusion of the analyzed ionic material. Another type is able to form a complex with the analyte ion. The third type is based on ion exchange reaction with the ion under investigation.

A potential difference will be aroused when the analyte ion can penetrate across the phase boundary between the two phases (analyte solution and internal reference solution) (Figure 1). Accordingly, an electrochemical equilibrium will be formed due to different potentials at both sides of the membrane. The potential difference (E) across the membrane is described by the Nernst equation:

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

$$\mathbf{E} = \mathbf{E}^{\circ} + (\mathbf{RT}/\mathbf{ZF})\ln\mathbf{a} \tag{1}$$

1.1. Voltammetric sensors

Figure 3. γ-CD toroid structure showing spatial arrangement.

(3.2 <sup>10</sup><sup>5</sup> –<sup>1</sup> <sup>10</sup><sup>2</sup> mol L<sup>1</sup>

peridinium hexa-fluoro-phosphate, and cyclodextrin [4].

obtained in this work were 6.2 and 3.5 nM, respectively.

tical compounds.

Voltammetric sensors are an electrochemical sensor based on measurements of the diffusion current at polarization potential. Several examples of voltammetric sensors were recorded in the literature. They were used for the determination of amine-containing organic/pharmaceu-

Komiyama [1] prepared cyclodextrin electrode for the determination of p-nitrophenolate. The electrode was prepared by dipping Pt-wire into cyclodextrin polymer. Stefan et al. [2] used a multidextrin as a sensor for preparing selective membrane for S-captopril. Lenik and Nieszporek [3] constructed a modified glassy carbon ibuprofen electrode.. The electrode was based on multi-walled carbon nanotubes and β-cyclodextrins. It showed wider linear range

applied like a sensor for GC. The determination of some neurotransmitters such as dopamine (DA), epinephrine (EP), norepinephrine (NEP), levodopa (L-DOPA), 3,4-dihydroxy-phenyl acetic acid (DOPAC) and serotonin (ST) was applied by reduced graphene oxide (RGO) composite glassy carbon electrode modified with ionic liquid crystal (ILC), 1-butyl-1-methylpi-

CD-based sensor was applied for the determination of an antibiotic chloramphenicol by Sun et al. [5]. They used a voltammetric sensor on mesoporous carbon @polydopamine and β-CD. It showed a response for chloramphenicol in the range of 5 <sup>10</sup><sup>7</sup> <sup>5</sup> <sup>10</sup><sup>5</sup> M. Li et al. [6] determined o- and p-nitrophenol by using reduced graphene oxide-CD-Chitosan electrochemical sensor. It was based on the host-guest recognition sites on CD and abundant functional groups of Chitosan. It showed linear range for 0.12–0.2 and 4–50 μM for NPH. But for pNPh it was from 0.06 to 0.16 μM and 5 to 40 μM. Yi et al. [7] applied electropolymerization for βCD and L-arginin on the surface of carbon nanotubes @graphene nanoribbons (CNTs@GNRs) for preparing a modified electrode for 2- and 4-aminophenols (AP). A linear response ranging from 25.0 to 1300.0 nM for both 2- and 4-AP and the detection limits (DLs) of 2- and 4-AP

) and lower limit of detection (1.25 <sup>10</sup><sup>5</sup> mol L<sup>1</sup>

β-Cyclodextrin as an Ionophore for Membrane Electrode http://dx.doi.org/10.5772/intechopen.73597 293

). It was

where E� is the standard cell potential, R is the general gas constant, Z is the valency of the analyte ion, F is the Faraday constant, and a is the activity of the analyte ion.

Cyclodextrin is produced from starch by enzymatic conversion. They are formed of a cycle of oligosaccharides. There are three types of cyclodextrin according to the size of the ring. They may be composed of 5-, 6-, or 7-membered ring. They are named like α, β, and γ, respectively (Figure 2).

Typical cyclodextrins contain a number of glucose monomers ranging between 6 and 8. They create a cone shape, which is suitable for the inclusion of different cations (Figure 3).

Many researches were recorded where cyclodextrin was used for preparing sensor membrane electrode for the determination of organic compounds. The studies about the application of CD as an ionophore for sensors of inorganic cations were rarely found. The recorded sensors were either voltammetric sensors or potentiometric sensors.

Figure 1. The response of membrane toward different ions.

Figure 2. Structure formula of α, β, and γ-cyclodextrin.

Figure 3. γ-CD toroid structure showing spatial arrangement.

#### 1.1. Voltammetric sensors

E ¼ E �

analyte ion, F is the Faraday constant, and a is the activity of the analyte ion.

(Figure 2).

292 Cyclodextrin - A Versatile Ingredient

where E� is the standard cell potential, R is the general gas constant, Z is the valency of the

Cyclodextrin is produced from starch by enzymatic conversion. They are formed of a cycle of oligosaccharides. There are three types of cyclodextrin according to the size of the ring. They may be composed of 5-, 6-, or 7-membered ring. They are named like α, β, and γ, respectively

Typical cyclodextrins contain a number of glucose monomers ranging between 6 and 8. They

Many researches were recorded where cyclodextrin was used for preparing sensor membrane electrode for the determination of organic compounds. The studies about the application of CD as an ionophore for sensors of inorganic cations were rarely found. The recorded sensors were

create a cone shape, which is suitable for the inclusion of different cations (Figure 3).

either voltammetric sensors or potentiometric sensors.

Figure 1. The response of membrane toward different ions.

Figure 2. Structure formula of α, β, and γ-cyclodextrin.

þ ð Þ RT=ZF ln a (1)

Voltammetric sensors are an electrochemical sensor based on measurements of the diffusion current at polarization potential. Several examples of voltammetric sensors were recorded in the literature. They were used for the determination of amine-containing organic/pharmaceutical compounds.

Komiyama [1] prepared cyclodextrin electrode for the determination of p-nitrophenolate. The electrode was prepared by dipping Pt-wire into cyclodextrin polymer. Stefan et al. [2] used a multidextrin as a sensor for preparing selective membrane for S-captopril. Lenik and Nieszporek [3] constructed a modified glassy carbon ibuprofen electrode.. The electrode was based on multi-walled carbon nanotubes and β-cyclodextrins. It showed wider linear range (3.2 <sup>10</sup><sup>5</sup> –<sup>1</sup> <sup>10</sup><sup>2</sup> mol L<sup>1</sup> ) and lower limit of detection (1.25 <sup>10</sup><sup>5</sup> mol L<sup>1</sup> ). It was applied like a sensor for GC. The determination of some neurotransmitters such as dopamine (DA), epinephrine (EP), norepinephrine (NEP), levodopa (L-DOPA), 3,4-dihydroxy-phenyl acetic acid (DOPAC) and serotonin (ST) was applied by reduced graphene oxide (RGO) composite glassy carbon electrode modified with ionic liquid crystal (ILC), 1-butyl-1-methylpiperidinium hexa-fluoro-phosphate, and cyclodextrin [4].

CD-based sensor was applied for the determination of an antibiotic chloramphenicol by Sun et al. [5]. They used a voltammetric sensor on mesoporous carbon @polydopamine and β-CD. It showed a response for chloramphenicol in the range of 5 <sup>10</sup><sup>7</sup> <sup>5</sup> <sup>10</sup><sup>5</sup> M. Li et al. [6] determined o- and p-nitrophenol by using reduced graphene oxide-CD-Chitosan electrochemical sensor. It was based on the host-guest recognition sites on CD and abundant functional groups of Chitosan. It showed linear range for 0.12–0.2 and 4–50 μM for NPH. But for pNPh it was from 0.06 to 0.16 μM and 5 to 40 μM. Yi et al. [7] applied electropolymerization for βCD and L-arginin on the surface of carbon nanotubes @graphene nanoribbons (CNTs@GNRs) for preparing a modified electrode for 2- and 4-aminophenols (AP). A linear response ranging from 25.0 to 1300.0 nM for both 2- and 4-AP and the detection limits (DLs) of 2- and 4-AP obtained in this work were 6.2 and 3.5 nM, respectively.

#### 1.2. Potentiometric sensors

They are a type of sensors based on measuring the potential changes with concentration in accordance with the Nernst equation.

Odashima et al. [8] used a long derivative chain of β-cyclodextrin for preparing a new sensory element for the determination of organic amines. Stefan et al. [9] used either α or γ-cyclodextrinbased electrodes for the determination of R-baclofen. The detection limits were 7 <sup>10</sup><sup>9</sup> mol<sup>1</sup> <sup>L</sup><sup>1</sup> for <sup>α</sup>-cyclodextrin-based electrode and 1.44 <sup>10</sup><sup>10</sup> mol<sup>1</sup> <sup>L</sup><sup>1</sup> for <sup>γ</sup>-cyclodextrin-based electrode. (2-hydroxypropyl)-β-cyclodextrin, heptakis(2,3,6-tri-O-methyl)-β-cyclodextrins and heptakis(2,3,6-tri-O-benzoyl)-β-cyclodextrin were applied for preparing diclofenac electrode by Lenik [10]. Staden and Nejem [11] determined L-vesamicol by CD derivative-based potentiometric sensor. The detection limit was of the order of 10<sup>10</sup> M. They showed a linear range between 10<sup>9</sup> and 10<sup>7</sup> M. Lenik and Łyszczek [12] determined naproxen by functionalized β-CD. It is based on inclusion complex of naproxen with <sup>β</sup>-CD-derivative. The linear range was 5.0 <sup>10</sup><sup>5</sup> – 1.0 <sup>10</sup><sup>2</sup> mol L<sup>1</sup> . Staden and Holo [13] determined L-histidine by an entioselective potentiometric membrane electrode based on CD. They applied carbon paste impregnated with α-, β-, 2 hydroxyl-3-trimethylammoniopropyl-β-(as chloride salt) and γ-cyclodextrins (γ-CDs). The recovery of L-his in the presence of D-his was higher than 99.10%. L-proline electrode was constructed based on carbon paste impregnated with α, β, or γ cyclodextrin [14]. The detection limit of the membrane was in the region of 10<sup>10</sup>–10<sup>9</sup> M.

All the aforementioned cyclodextrin applications were examples of sensors for drug analysis. The application of cyclodextrin for sensors for inorganic cations is rarely found. We choose one of these sensors that was sensing for nickel which was constructed by Zareh et al. [15], to study as an example of CD sensors. Two membrane compositions (Table 1) were prepared for the optimization to get the best. Electrode type I comprised DEP, while II comprised NPOE. The behavior of DEP-containing electrodes showed better Nernstian slope value than NOPE-containing membrane electrode. Both electrode types showed the same linear range of 5.01 <sup>10</sup><sup>5</sup> –<sup>10</sup><sup>2</sup> M. Figure 4 shows the calibration graphs for each electrode type.

The effect of inner filling (IF) solution was studied by the application of the electrode type I containing three types of IF-solutions (A, B, and C). They were corresponding to compositions (0.1 M KCl + 0.1 M NiCl2), (0.01 M KCl + 0.01 M NiCl2), and (0.001 M KCl + 0.001 M NiCl2), respectively. The slopes of Ni electrode were 30.9, 28.17, and 29.7 mV/decade, for electrodes with IF A, B, and C, respectively. When A and B IF was applied, the lower linear range reached 5.0 <sup>10</sup><sup>5</sup> M. In the case of IF solution type C, the linear range was not less than 5 <sup>10</sup><sup>4</sup> M. Table 2 summarizes the obtained results.

The detection limit (DL) of an ISE can be defined as the cross point of line segments fitted to linear segments of emf versus log aNi2+ [16]. For membrane types I and II, the values of the

Table 2. Effect of inner filling on the performance of Ni-selective electrode based on β-CDX with DEP plasticizer.

–<sup>5</sup> <sup>10</sup><sup>5</sup> 0.9987

Detection limit,

M

Slope, mV/ Decade

β-Cyclodextrin as an Ionophore for Membrane Electrode http://dx.doi.org/10.5772/intechopen.73597 295

–<sup>5</sup> <sup>10</sup><sup>5</sup> 0.9986

–<sup>5</sup> <sup>10</sup><sup>4</sup> 0.9931

Inner filling solution Slope, mV/Decade Linear range, M R2

NPOE, mg

Figure 4. Calibration graphs for Ni-selective electrodes with membrane contains either (a) DEP or (b) NPOE plasticizers.

I-Membrane 63.6 12.8 126.4 0 6.31 28.76 3.98 x10<sup>5</sup> II-Membrane 60 14 0 121 7.18 23.33 6.3 x 10<sup>5</sup>

% Ionophore

The process of the electrode response is based on a nonpolarized electrochemical equilibrium. There are two equilibrium steps to explain the response mechanism of the proposed Niselective electrode. The first step is the equilibrium between Ni2+ in membrane and solution sites. The second step is the equilibrium of formation of Ni2+-β-CDX into the membrane site.

detection limits were 3.98 <sup>10</sup><sup>5</sup> and 6.3 <sup>10</sup><sup>5</sup> M.

Composition, w/w

PVC, mg

%

A-(10<sup>1</sup> M NiCl2 + 10<sup>1</sup> M KCl) 30.89 10<sup>2</sup>

βCDX, mg

DEP, mg

Table 1. Composition of membrane types I and II used for preparing Ni2+-selective electrode.

B-(10<sup>2</sup> M NiCl2 + 10<sup>2</sup> M KCl) 28.17 10<sup>2</sup>

C-(10<sup>3</sup> M NiCl2 + 10<sup>3</sup> M KCl) 29.68 10<sup>2</sup>

The steps of this mechanism can be represented below:

The response time was estimated according to the IUPAC definition [16]. It is defined as the time between the instant when the Ni electrode was brought into contact with sample solution and the first instant when emf/time slope becomes equal to the limiting value selected on the basis of experimental conditions. Dynamic response was studied for both electrode types containing DEP or NPOE. From Figures 5 and 6, the response times for electrode types I and II were between 2 and 3 s for both types of electrodes for the tested concentrations 10<sup>2</sup> –10<sup>4</sup> M. This fast response for both types will help in the application of both electrode types for real measurements.

1.2. Potentiometric sensors

294 Cyclodextrin - A Versatile Ingredient

1.0 <sup>10</sup><sup>2</sup> mol L<sup>1</sup>

accordance with the Nernst equation.

limit of the membrane was in the region of 10<sup>10</sup>–10<sup>9</sup> M.

Table 2 summarizes the obtained results.

They are a type of sensors based on measuring the potential changes with concentration in

Odashima et al. [8] used a long derivative chain of β-cyclodextrin for preparing a new sensory element for the determination of organic amines. Stefan et al. [9] used either α or γ-cyclodextrinbased electrodes for the determination of R-baclofen. The detection limits were 7 <sup>10</sup><sup>9</sup> mol<sup>1</sup> <sup>L</sup><sup>1</sup> for <sup>α</sup>-cyclodextrin-based electrode and 1.44 <sup>10</sup><sup>10</sup> mol<sup>1</sup> <sup>L</sup><sup>1</sup> for <sup>γ</sup>-cyclodextrin-based electrode. (2-hydroxypropyl)-β-cyclodextrin, heptakis(2,3,6-tri-O-methyl)-β-cyclodextrins and heptakis(2,3,6-tri-O-benzoyl)-β-cyclodextrin were applied for preparing diclofenac electrode by Lenik [10]. Staden and Nejem [11] determined L-vesamicol by CD derivative-based potentiometric sensor. The detection limit was of the order of 10<sup>10</sup> M. They showed a linear range between 10<sup>9</sup> and 10<sup>7</sup> M. Lenik and Łyszczek [12] determined naproxen by functionalized β-CD. It is based on inclusion complex of naproxen with <sup>β</sup>-CD-derivative. The linear range was 5.0 <sup>10</sup><sup>5</sup>

metric membrane electrode based on CD. They applied carbon paste impregnated with α-, β-, 2 hydroxyl-3-trimethylammoniopropyl-β-(as chloride salt) and γ-cyclodextrins (γ-CDs). The recovery of L-his in the presence of D-his was higher than 99.10%. L-proline electrode was constructed based on carbon paste impregnated with α, β, or γ cyclodextrin [14]. The detection

All the aforementioned cyclodextrin applications were examples of sensors for drug analysis. The application of cyclodextrin for sensors for inorganic cations is rarely found. We choose one of these sensors that was sensing for nickel which was constructed by Zareh et al. [15], to study as an example of CD sensors. Two membrane compositions (Table 1) were prepared for the optimization to get the best. Electrode type I comprised DEP, while II comprised NPOE. The behavior of DEP-containing electrodes showed better Nernstian slope value than NOPE-containing membrane electrode. Both electrode types showed the same linear range of 5.01 <sup>10</sup><sup>5</sup> –<sup>10</sup><sup>2</sup> M. Figure 4 shows the calibration graphs for each electrode type.

The effect of inner filling (IF) solution was studied by the application of the electrode type I containing three types of IF-solutions (A, B, and C). They were corresponding to compositions (0.1 M KCl + 0.1 M NiCl2), (0.01 M KCl + 0.01 M NiCl2), and (0.001 M KCl + 0.001 M NiCl2), respectively. The slopes of Ni electrode were 30.9, 28.17, and 29.7 mV/decade, for electrodes with IF A, B, and C, respectively. When A and B IF was applied, the lower linear range reached 5.0 <sup>10</sup><sup>5</sup> M. In the case of IF solution type C, the linear range was not less than 5 <sup>10</sup><sup>4</sup> M.

The response time was estimated according to the IUPAC definition [16]. It is defined as the time between the instant when the Ni electrode was brought into contact with sample solution and the first instant when emf/time slope becomes equal to the limiting value selected on the basis of experimental conditions. Dynamic response was studied for both electrode types containing DEP or NPOE. From Figures 5 and 6, the response times for electrode types I and II were between 2

for both types will help in the application of both electrode types for real measurements.

and 3 s for both types of electrodes for the tested concentrations 10<sup>2</sup>

. Staden and Holo [13] determined L-histidine by an entioselective potentio-

–

–10<sup>4</sup> M. This fast response

Figure 4. Calibration graphs for Ni-selective electrodes with membrane contains either (a) DEP or (b) NPOE plasticizers.


Table 1. Composition of membrane types I and II used for preparing Ni2+-selective electrode.


Table 2. Effect of inner filling on the performance of Ni-selective electrode based on β-CDX with DEP plasticizer.

The detection limit (DL) of an ISE can be defined as the cross point of line segments fitted to linear segments of emf versus log aNi2+ [16]. For membrane types I and II, the values of the detection limits were 3.98 <sup>10</sup><sup>5</sup> and 6.3 <sup>10</sup><sup>5</sup> M.

The process of the electrode response is based on a nonpolarized electrochemical equilibrium. There are two equilibrium steps to explain the response mechanism of the proposed Niselective electrode. The first step is the equilibrium between Ni2+ in membrane and solution sites. The second step is the equilibrium of formation of Ni2+-β-CDX into the membrane site. The steps of this mechanism can be represented below:

2. pH-effect

brane types I and II are shown in Figure 7.

3. Selectivity properties

10�<sup>5</sup>

valent cations (Na<sup>+</sup>

, K<sup>+</sup>

, and NH4

electrodes should be conducted in the absence of these cations.

+

The potential changes versus different pH values for the DEP-membrane electrode type I and type II were tested. In acidic part, the break was observed at pH values 3.8 and 3.2 for type I and II electrodes, respectively. The sensitivity of β-CDX toward the H<sup>+</sup> is the reason for the potential break in acidic medium. The break in acidic medium was due to the inclusion of H<sup>+</sup> into ionophore in the membrane site. Above pH 3.2 and 3.8, the potential was not changed whatever the pH value for types I and II, respectively. This was due to the absence of the H+ ion concentration, which disturbs the equilibrium. Representative curves for nickel with mem-

β-Cyclodextrin as an Ionophore for Membrane Electrode http://dx.doi.org/10.5772/intechopen.73597 297

The selectivity of Ni electrode with different membrane types I and II was calculated according

From the results, it can be reported that most of the tested interferents for Ni-ISE type I showed perfect selectivity. When the electrode type I was used, the values of the selectivity coefficient toward divalent cations were so perfect to consider this electrode selective for Ni++ cation. The

trivalent cations, this type of electrode showed better selectivity coefficient values (of order

type I or type II. Accordingly, it is recommended that measurements with the proposed

). When electrode type II was used, the selectivity coefficient values were higher than that for electrode type I. It showed values of order 10�<sup>2</sup> for trivalent cations and 10�<sup>1</sup> for most of the divalent cations. This can be attributed to that in type II electrode the NPOE has active sites to interact with cationic species which lowers the selectivity toward Ni++ [17]. The tested mono-

) showed interference with the measurements with either

Ni2+, j z+).

. In case of

to the SSM [16]. Table 3 shows the obtained values of the selectivity coefficient (KPot

Figure 7. pH-effect for Ni electrode for 0.001 M Ni2+ solutions for electrodes comprising (a) DEP and (b) NPOE.

recorded values for most of the tested divalent cations were of the order of 10�<sup>3</sup>

Figure 5. Dynamic response for Ni-selective electrodes with membrane type I containing DEP, for different concentrations (a) 0.01 M, (b) 0.001 M, and (c) 0.0001 M Ni2+ solutions.

Figure 6. Dynamic response for Ni-selective electrodes with membrane type II containing NPOE, for different concentrations (a) 0.01 M, (b) 0.001 M, and (c) 0.0001 M Ni2+ solutions.

$$\left[\text{Ni}^{2+}\right]\_{\text{s}} \rightleftharpoons \left[\text{Ni}^{2+}\right]\_{\text{m}}$$

$$\left[\text{Ni}^{2+}\right]\_{\text{m}} + \beta-\text{CDX}\_{\text{m}} \rightleftharpoons \left[\text{Ni}^{2+}-\beta\text{-CDX}\right]^{2+}\_{\text{m}}$$

$$\Delta E = \text{E}\_{\text{m}} - \text{E}\_{\text{s}} = \left(\text{RT/F}\right)\ln\left\{\text{k}\_{\text{i}}\,\text{a}\_{\text{Ni}}\right\}^{2+}\left[1+\text{K}\_{\text{f}}\,\left(\text{carrier}\right)\right]/\text{(sites)}^{2}$$
