Lanthanum(III) potentiometric sensors based on ethyl benzoyl acetate

2.1 Materials

Reagent grade acetyl acetone (AA), β-diketone 1-benzoyl-2-nonanone (Lix 54), ethylacetoacetate (EAA), and ethyl benzoyl acetate (EBA) were supplied from Aldrich as electroactive materials. Lanthanum chloride was supplied from Aldrich. Graphite fine powder (extra pure) was of analytical grade and purchased from Merck. High molecular weight polyvinyl chloride (PVC) powder was supplied from Aldrich, while o-nitrophenyl octyl ether (o-NPOE) was supplied from Fluka. Dioctylphthalate (DOP), tricresyl phosphate (TCP), dibutyl phthalate (DBP) and dioctylsebacate (DOS) were supplied from BDH. Acetone and cyclohexanone were supplied from Fluka (Switzerland).

2.2 Apparatus and emf measurements

Digital Hanna pH/mV meter (model 8417) was used for all potentiometric measurements at 25 °C. Spectrophotometer (U-2001, model 121-0032 Hitachi, Tokyo, Japan) and scanning electron microscope (National Research Center Quanta FEG250) were used and microanalysis was completed using the energy dispersive X-ray analyzer (EDX) (National research center, Egypt). FT-IR spectra were recorded on a Perkin-Elmer 1650 spectrometer (4000–400 cm⁻¹) in KBr pellets at the Microanalytical Center, Cairo University, Egypt. Ag/AgCl reference electrode contains 10% (w/v) potassium chloride.

2.3 Sensors preparation (screen printed and carbon paste)

CPEs were prepared using Teflon holder for carbon paste filling as the electrode body, and the carbon paste was formed by mixing graphite powder (250 mg), a suitable liquid binder (DOP, TCP, DBP, DOS or o-NPOE) (100 μL) and 2.5–12.5 mg of ionophore. This matrix was thoroughly mixed in the mortar and the resulted paste was used to fill the electrode body. A new carbon-paste surface was obtained by pushing gently the stainless-steel screw forward.

SPEs were printed in arrays of six couples consisting of the working electrodes (each 5 × 35 mm) following the procedures previously described (Nour El-Dien et al., 2012; Frag et al., 2011a–c, 2012; Akl et al., 2013; Mohamed et al., 2010). A polyvinyl chloride flexible sheet (0.2 mm) was used as a substrate which was not affected by the curing temperature or the ink solvent and easily cut by scissors. The working electrodes were printed using homemade carbon ink (prepared by mixing 1–15 mg of EBA electroactive material, 450 mg TCP, 1.25 g of polyvinyl chloride (8% w $⧹$ v) and 0.75 g carbon powder). They were printed and cured at 50 °C for 2 h. A layer of an insulator was then placed onto the printed electrodes, leaving a defined rectangular shaped (5 × 5 mm) working area and a similar area (for the electrical contact) on the other side. Fabricated electrodes were stored at 4 °C and used directly in the potentiometric measurements without any preconditioning.

2.4 EMF measurements

The emf measurement with the plasticized SPE and CPEs was carried out with the following cell assemblies: $$\text{Ag}|\text{AgCl}|\text{satd}.\text{KCl}\Vert \text{sample solution}|\text{sensor}(\text{CPE and SPE})\text{.}$$

The detection limit was taken at the point of intersection of the extrapolated linear segments of the calibration curve which was plotted with emf as a function of the negative logarithm of lanthanum ion concentration, and the selectivity of the electrodes is obtained by measuring the selectivity coefficient (K^pot_La,J) using the separate solution method using 0.001 mol L⁻¹ La(III) and interfering ions.

2.5 Determination of La(III) ion in different water samples

Tap water and wastewater samples were collected from different environment filtered and analyzed for La(III) ion by the proposed method. Tap water (sample 1) and wastewater (sample 2) samples were supplied from Sandoub, Mansoura, Dakahliya, Egypt.

The water samples were spiked with known amounts of lanthanum(III) ion, the pH was adjusted with acetate buffer (pH = 4) and they were analyzed by the proposed general procedure.

For spectrophotometric measurements at 430 nm a given amount of alizarin Red S was added to 5 beakers containing 10 ml spiked water samples with acetate buffer solution and the pH of the solution was adjusted to 4.60 with hydrochloric acid or sodium hydroxide and then diluted to 50 ml. This was followed by absorbance measurements. The molecular ratio of lanthanum:alizarin Red S is 1:2 (Kawashima et al., 1961).

2.6 Surface analysis

Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analyzer were used for morphological analysis of the sensors surface at 4000× magnifications for electrodes I and III before and after the interaction with lanthanum (III) ion.

3.1 Electrode composition

3.1.2

3.1.2 Ionophore content

Different amounts of EBA ionophore as electroactive material were used to prepare the modified SPE and CPEs for La(III) ion determination. The potential response is shown in Figs. 1 and 2 for modified SPE and CPEs electrodes, respectively.

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Fig. 1

Effect of ionophore content on potential response of SPE (electrode I) for determination of La(III) in acetate buffer (pH = 4).

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Fig. 2

Effect of ionophore content on potential response of CPE (electrode III) for determination of La(III) in acetate buffer (pH = 4).

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It was found from the figures that the sensors modified with 2.5 mg and 10 mg of EBA ionophore which was the most effective amount gave the best sensitivity with the best Nernstian slope of 21.0, 19.9 and 20.5 mV decade⁻¹, over a wide concentration range from 1.0 × 10⁻⁶ to 1.0 × 10⁻² mol L⁻¹ with a good correlation coefficient and very low detection limit for electrodes I, II and III, respectively. The limits of detection were determined from the intersection of the two extrapolated segments of the calibration plots and were found to be 1.0 × 10⁻⁶ mol L⁻¹.

3.1.4

3.1.4 Surface characterization

In order to characterize the morphology of the CPE and SPE, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyzer were used as shown in Figs. 3–6. All electrodes surfaces showed a similar configuration with randomly directed graphite particles ensemble into an insulator matrix (PVC or plasticizer for SPE and CPE, respectively) where the graphite particle size is longer for CPE than for SPE, so this distribution of the electrode material in case of SPE (microelectrodes) could lead to a highly packed structure. SPE showed a more uniform surface topography rather than CPE. Incorporation of a suitable ionophore in the paste followed by soaking of the sensors in La(III) ion solution led to the complex formation between the La(III) ions and EBA ionophore which was observed in filling the space between graphite surface after the interaction with La(III) ion. Formation of ionophore-La³⁺ complex at electrode surface which was subsequently extracted by the plasticizer into the paste is shown in Figs. 4 and 6. These data are supported by IR spectra as shown in Figs. 7 and 8, where the ligand EBA exhibited medium intensity bands at 1740 and 1685 cm⁻¹ due to υ(C⚌O) of benzoyl and ester groups for electrodes I and III, respectively. After soaking electrodes I and III in La(III) ion solution for 1 h and carrying out IR spectra, a difference in the spectra is observed. The bands due to υ(C⚌O) of benzoyl and ester groups were shifted to lower wave numbers in the spectrum (1729 and 1644 for electrode I and 1741 and 1613 cm⁻¹ for electrode III) suggesting complex formation via coordination of the EBA ionophore to La(III) ion through the C⚌O of benzoyl and C⚌O of ester groups .

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Fig. 3

SEM images for SPE surface, electrode I (a) before and (b) after soaking in 1 × 10⁻³ mol L⁻¹ La(III) ion.

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Fig. 4

EDX of SPE surface, electrode I (a) before and (b) after soaking in 1.0 × 10⁻³ mol L⁻¹ La(III) ion.

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Fig. 5

SEM images for CPE surface, electrode III (a) before and (b) after soaking in 1 × 10⁻³ mol L⁻¹ La(III) ion.

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Fig. 6

EDX of CPE surface, electrode III (a) before and (b) after soaking in 1.0 × 10⁻³ mol L⁻¹ La(III) ion.

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Fig. 7

IR spectra for electrode III (a) before and (b) after soaking in 1.0 × 10⁻³ mol L⁻¹ La(III) ion.

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Fig. 8

IR spectra for electrode I (a) before and (b) after soaking in 1.0 × 10⁻³ mol L⁻¹ La(III) ion.

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3.2 The effect of pH

The pH dependence of the electrode potential was investigated over the pH range of 2.0–10.0 in a 1.0 × 10⁻³ and 1.0 × 10⁻⁵ mol L⁻¹ solution of La(III) ion. The pH was adjusted by using very small volumes of HCl and NaOH solutions. The results indicated that the potential was independent of pH in the range of 4–8, 5–8 and 4–7 for electrodes I, II and III, respectively. Outside this range, the potential changed significantly. The increase of potential below pH 4.0 may be ascribed to the competitive binding of protons to the ligands on the electrode surface rather than La(III) ion. The decrease of potential above pH 8.0 can be accounted for to the formation of hydroxyl complexes of La(III) such as La(OH)⁺², La(OH)₂⁺ and La(OH)₃ which monished its ability to combine with the carrier (Yuan et al., 2013; Shamsipur et al., 2002).

3.3 Response time

The time it takes for the electrode potential of an ion selective electrode to become stable within a range of variation of ±1 mV depends on electrode type and structure, ion type, concentration, and ionic strength. The response time when measuring a high ion concentration after measuring low ion concentrations was relatively short, while a longer response time was obtained for the reverse process. Additionally, at around the minimum measurement limit, the response time was generally relatively long, being of the order of several minutes. Hence, when using the ion electrode method, it was necessary to wait until the potentiometer indicated a stable value before taking a reading (Eric et al., 1997).

One of the most important factors for any ion-selective electrode is response time. The practical response time of the sensor was recorded by observing the potential change over a concentration range from 1.0 × 10⁻⁶ to 1.0 × 10⁻² mol L⁻¹ of La(III). The potentials versus time traces are shown in Fig. 9.

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Fig. 9

Dynamic response time of (a) electrode I, (b) electrode II and (c) electrode III for La(III) in acetate buffer (pH = 4).

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As can be shown in Fig. 9, electrodes reached their equilibrium responses in a very short time of 9, 10 and 13 s over the linear concentration range for electrodes I, II and III, respectively. This may be due to the fast exchange kinetics of complexation–decomplexation of La(III) ion with the ionophore at the paste of the electrodes (Ganjali et al., 2004).

3.4 Effect of temperature

One of the important parameters which has effect on the performance of the sensors is the change of temperature at test solution. The stability of the electrodes was studied within the temperatures 10, 20, 30, 40, 50 and 60 °C. The electrode exhibited good Nernstian behavior in the temperature range (10–50 °C). The slope of electrode did not show a good Nernstian behavior at higher temperature. The standard cell potentials (E°C_ell), were determined at different temperatures from the respective calibration plots as the intercepts of these plots at p La(III) = 0, and were used to determine the isothermal temperature coefficient (dE⁰/dt) of the cell with the aid of the following equation (Khalil and El-Aliem, 2002; Electrochemistry, 1972): $$E_{\text{cell}}^0+E_{\text{reference}}^0=E_{\text{electrode}}^0$$

Plot of E⁰ electrode versus (t–25) gave a straight line. The slope of the line was taken as the isothermal temperature coefficient of the La (III) electrode. It amounted to 0.121 × 10⁻², 0.55 × 10⁻³ and 0.245 × 10⁻² V/°C for electrodes I, II and III, respectively. The small values of (dE⁰/dt)_cell and (dE⁰/dt)_electrode revealed the high thermal stability of the electrodes within the investigated temperature range.

3.5 Selectivity

Clearly, the selectivity was one of the most important characteristics of a sensor. It often determines whether a reliable measurement in the target sample was possible or not. In clinical applications, it was critical where the allowed emf deviation (error) may sometimes not be larger than 0.1 mV for whole blood or serum measurements. Theoretically, selectivity description allowed researchers to identify the key parameters for optimizing the performance of potentiometric sensors, e.g., by adjusting weighing parameters (i.e., absolute membrane concentrations) or choosing different plasticizers or matrices (Eric et al., 1997). Ion selective electrodes have the most important characteristic that is its relative response to other ions present in solution, which expressed in terms of selectivity coefficients (K_A,B^Pot). Different methods can be used for measuring the selectivity coefficients of the electrode such as separate solution method (SSM) Bakker et al. (2000) and matched potential method (MPM) Lindner and Umezawa (2008). In the present study SSM was used which depends on measuring the potential of a cell comprising an ISE and a reference electrode with two separate solutions. One contains the ion of interest A at the activity aA (but no B) and the other contains the interfering ion B at the same activity aB = aA (but no A). In this method, the values of the selectivity coefficient can be derived from the following equation: $$\log {\text{K}}_{\text{A,B}}^{\text{pot}}=[({\text{E}}_{\text{B}}-{\text{E}}_{\text{A}})/\text{S}]+(1-({\text{Z}}_{\text{B}}/{\text{Z}}_{\text{A}}))\log {\text{a}}_{\text{A}}$$ E_A and E_B mean the potentiometric response of a_A and a_B, respectively. a_A is the primary ion activity and a_B is the activity of an interfering ion. The single ion activities were calculated by the extended Debye-Hückel equation (Kamata et al., 1998).

In this work, a_A (1.0 × 10⁻³ mol L⁻¹ La(III) ion) and a_B (1.0 × 10⁻³ mol L⁻¹ interfering ion) were used to measure the selectivity coefficients. S, Nernstian slope, Z_A and Z_B are the charge of the primary and interfering ions, respectively. The selectivity coefficient values are listed in Table 1. The value of selectivity coefficient reached to 1.0 indicated equal response to both primary ion and interfering ions. If the value of selectivity coefficient was smaller than 1.0, it showed that the sensor was selective to the primary ion over the interfering ions. The data given in Table 1 indicate that the electrodes (I and III) have good selectivity toward the lanthanum ion than other metal ions. But for electrode II, it had good selectivity toward primary lanthanum ion except in the presence of Al(III), Ca(II) and Mg(II) ions, and it showed interfering ion with those metal ions.

3.6 Life time

Electrodes (I–III) were tested at different intervals to follow up its life time and reproducibility. It was clear from the figure that modified SPE had the longest life time up to three weeks.

The surface of CPEs was continuously polished using a filter paper during calibration process to obtain new working surface and rinsed carefully with double distilled water to remove the memory effect of electrode. This indicated that the SPE had high mechanical durability and good adherent properties. Longer stability test was also investigated and the electrodes were successfully used for at least 50 consecutive measurements without any preconditioning before use.

3.7 Analytical application

In general, the maximum La(III) ion concentration that ion selective electrodes can measure was found to be 1.0 × 10⁻² mol L⁻¹, and the minimum was 1.0 × 10⁻⁶ mol L⁻¹. The proposed Lanthanum ion-selective electrodes (I-III) were used for potentiometric determination of lanthanum(III) ion in different water samples (waste and tape water). As mentioned above in procedure (2.7) the samples were treated and analyzed. The results obtained were also compared with those from spectrophotometric analysis. The results obtained and listed in Table 2 indicated the successful use of the proposed electrodes (I-III) for determination of lanthanum ion and there was a satisfactory agreement with those obtained by spectrophotometric method.

3.8 Method validation