Nickel sulphate heptahydrate (NiSO₄·7H₂O), 25% ammonia (NH₃), zinc nitrate [Zn(NO₃)₂·6H₂O] ionophore (12-crown-4), sodium tetra phenyl borate (NaTPB), di-n-butyl-phthalate (DBP), tetrahydrofuran (THF) and polyvinyl chloride (PVC) were purchased from Sigma Aldrich Sweden (Stockholm, Sweden). All other chemicals used were of analytical grade.

The honeycomb-like NiO nanostructures were fabricated on nickel foam substrates using NiSO₄·7H₂O and NH₃ as primary chemical reactants. The nickel foam substrates were sonicated in an ultrasonic bath for 15 minutes using an ethanol solution. Then, the nickel foam substrates were cleaned with deionized water and dried in air. Afterwards the substrates were affixed in a Teflon sample holder and vertically dipped into a mixture of 0.1 M NiSO₄·7H₂O and 0.1 M NH₃ solutions prepared in deionized water. The sample solutions were kept in an oven at 90 °C for 7 hours. The role of NH₃ in growth process is to act as a complexing reagent for nickel. When the growth time was completed then the substrates were taken out from the oven and a visible greenish colour type film was visible on the nickel foam, then the sample substrates were washed with deionized water and dried in an oven at 80 °C for 3 hours. After drying, some of the as-prepared nickel foam substrates were annealed in air at 500 °C in order to achieve NiO honeycomb nanostructures.

The functionalization of the honeycomb-like NiO nanostructures with a selective zinc ion ionophore was performed as followed: the ingredients and amounts of each for the preparation of the membrane composition were as follows: 12-crown-4 (40 mg), NaTPB (40 mg), PVC (125 mg), and DBP (100 mg) in tetrahydrofuran (12 mL) [32]. Afterwards the NiO electrodes were dipped into membrane solution for 5 minutes using the direct adsorption method and dried at room temperature for 1 hour then the functionalized electrode were kept in a refrigerator overnight at 4 °C.

The potential measurements were carried out for the range of 0.0005 mM to 100 mM of zinc nitrate concentrations prepared in 1 mM phosphate buffer solution of pH 7.3 at room temperature. The functionalised NiO honeycomb-like nanostructures were used as working electrode and silver-silver chloride (Ag/AgCl as reference electrode. A model 744 pH-meter (Metrohm, Switzerland) was used for the potential measurements and a Keithley 2400 (Tektronix, Beaverton, OR, USA) source meter was used for the response time measurements.

The grown honeycomb-like nanostructures were characterised by FESEM, HRTEM, and XRD techniques. In Figure 1(a–d), low magnification FESEM images of the honeycomb-like nanostructures of NiO are shown, which demonstrated the attachment of NiO thin films on the nickel foam substrate. The honeycomb-like NiO nanostructures were clearly seen at high FESEM magnification and from this it can be seen that the grown NiO nanostructures are highly uniform and dense.

The x-ray diffraction study showed that the nickel oxide honeycomb nanostructures exhibited a face centred cubic phase crystalline structure as shown in Figure 2 and the pattern of XRD is supported by literature data (JCPDS NO. 47-1049).

The transmission electron microscopy study revealed that the NiO honeycomb nanostructures possessed face centred cubic crystallinity with a space lattice of 0.21 nm for the (002) crystal plane of NiO, as shown in Figure 3(a). In the selected area electron diffraction pattern (SAED, Figure 3(b)), it has been observed the results are almost consistent with the XRD patterns; therefore the honeycomb-like nanostructures of NiO are cubic in crystallinity with the presence of rings in the nanostructures described the nanoflakes nature of NiO as shown in Figure 3(c). The crystal size of NiO was calculated by XRD using Scherer's equation for 200 crystal plane and the obtained value for the crystal size is 62 Å.

The potentiometric response of the honeycomb-like functionalised NiO-based zinc ion selective electrode was measured for the zinc ions concentration range of 0.0005 to 100 mM. The sensor electrode detected 0.0005 mM concentration of zinc ion, but it was out of the linear range. After the 0.0005 mM concentration of zinc ions, the proposed sensor has shown a highly linear response for 0.001 mM to 100 mM concentrations of zinc ions, as shown in Figure 4.

A sensitivity of 36 mV/decade for the functionalised NiO nanostructures-based zinc ion sensor was observed, with a regression coefficient of r² = 0.99. This is due to the high surface provided by the honeycomb-like nanostructures for the attachment of the selective zinc ion ionophore and due to shortened diffusion path in the solid phase of the NiO nanostructures which might be responsible for the slightly higher slope value than the theoretical value. These two characteristics of the proposed sensor electrode confirmed its potential applicability for analytical purposes.

In this study, repeatability, reproducibility, and selectivity of the proposed ion selective electrode were examined. The repeatability of the ion selective electrode describes the response of a specific electrode which has been more than once under the same set of conditions. The functionalised NiO-based zinc ion selective electrode was tested for three consecutive days and it showed good repeatability with similar ranges of zinc ion detection, sensitivity and regression coefficient values as shown in Figure 5.

For reproducibility, seven independent ion selective electrodes based on the honey- comb-like nanostructures of NiO were functionalised under the same conditions. All these zinc ion selective electrodes were used in a 0.1 mM solution of zinc nitrate electrolyte. It was observed that the proposed zinc ion selective sensor electrodes demonstrated high reproducibility with less than 5% standard deviation, as shown in Figure 6.

Selectivity of an ion selective sensor electrode is the fundamental parameter among all other parameters for the performance evaluation of an ion selective electrode. The separation solution method, which is recommended by IUPAC [52], was used for the study of the selectivity of the ion selective electrode by determination of the selectivity of coefficient values of both mono and divalent metal cations using 1 mM solution of each interferent. The calculated selectivity coefficient values are given in Table 1. The determined selectivity coefficient values are fairly constant and this study revealed that the proposed ion selective electrode is highly selective towards zinc ion.

The response of an ion selective electrode is also temperature dependent due to changes in the ionic mobility of target ions in solution. In this experiment, the effect of temperature on the response of the proposed electrode was observed from room temperature to 75 °C.

It can be inferred from the Figure 7(a), that the output response was increasing gradually up to 55 °C due to the increase in the mobility of zinc(II) ion, but above 55 °C the response trend was increasing due to possible detachment of ionophore membrane from the surface of the honeycomb-like nanostructures of NiO and also at higher temperature the analyte ions suffered from self-resistance in the solution. Moreover, the proposed zinc ion sensor has shown response time of less than 10 s, as shown in Figure 7(b).

The functionalized NiO nanostructures-based ion selective electrode was used as indicator electrode for the potentiometric titration of 70 mL of 10 mM zinc(II) ions against 100 mM ethylenediaminetetraacetate solution [21] as shown in Figure 7(c). It can be seen from Figure 7(c) that with addition of EDTA the response of ion selective electrode was decreasing due to complex formation among zinc (II) ions and EDTA molecules, but at 7 mL a sharp intersection point was observed, which shows a good stoichiometric relationship for the determination of zinc ion concentration in unknown samples. Table 2 shows a comparison of the proposed zinc ion sensor based on the functionalised NiO honeycomb-like nanostructures with reported zinc ion sensors. The performance of the present zinc ion sensor is better than that of the reported zinc ion sensors due to the three dimensional network of NiO nanostructures and enhanced electrochemical properties of NiO in nanodimensional form.