A New Type of Nitrate Potentiometric Sensor Prepared Using Hybrid Metal Oxide/Metal Nanoparticles

Abstract

In this study, for the first time, ZnO nanoparticles doped with noble metals (Pt, Ag, Au) were employed as a solid contact in nitrate ion-selective electrodes based on a glassy carbon internal electrode, and their performance was described and studied. Nanoparticles were synthesized by pulsed laser ablation in liquid. They were placed as an intermediate layer between the inner electrode and the ion-selective membrane. The impact of nanoparticle type on electrode performance was assessed by analyzing their analytical and electrical parameters using both potentiometry and electrochemical impedance spectroscopy. It was found that the determined properties of hybrid nanoparticles, as well as their effectiveness as a solid contact, depend significantly on the type of metal doping. Doping ZnO nanoparticles with metals increases their electrical capacity and reduces contact angles. The best results were obtained for the electrode modified with platinum-doped zinc oxide nanoparticles, characterized by the largest electric capacitance and the most hydrophobic properties among the hybrid nanoparticles. This electrode has been successfully used for the potentiometric determination of nitrate content in soil.

1. Introduction

Ion-selective electrodes are the most widely used electrochemical devices for selective detection of specific ions in a sample solution [1]. Their operation is based on an ion-selective membrane, which, due to the presence of an active substance (ionophore), enables the generation of an electrochemical signal proportional to the concentration of a main ion in the sample over time [2,3]. They are widely used in chemical analysis [4], medicine [5], the food industry [6,7] and environmental monitoring [8], for example, to study electrolytes in blood or to monitor the levels of ions in water samples [5]. Their advantages include high sensitivity, precision, low price, and relatively simple use compared with other analytical methods, such as UV–Vis spectroscopy [9].

As a result, ion-selective electrodes are among the best analytical tools for environmental measurements and represent an excellent alternative for determining nitrate ions, whose monitoring in water samples is essential [10]. Excess nitrate (NO₃⁻) can lead to numerous problems. Aquatic ecosystems are among the most heavily affected by excess nitrate. This can lead to water eutrophication, reducing oxygen saturation, and the eventual loss of aquatic organisms [11,12]. Problems arising from excess NO₃⁻ are also faced by agricultural environments, where the soil is oversaturated with nitrates. When this occurs, phytotoxic effects may develop, leading to reduced plant growth, inhibition of root development, and disturbances in overall plant development [13]. Humans are also affected by these problems: consuming excessive amounts of water containing more than 50 mg/L of nitrate ions and exposure to these ions is devastating [14]. NO₃⁻ ions can contribute to methemoglobinemia [15], increase the risk of conversion to nitrosamines (potentially carcinogenic compounds), cause diseases of the renal, gastrointestinal, and respiratory systems, and pose a risk to pregnant women [16,17]. Therefore, it is important to use appropriate measuring tools to monitor the levels of these ions in the surface and groundwater. Among ion-selective electrodes, solid contact electrodes (SCISEs) constitute an excellent option. By eliminating the internal solution, they offer simplified measurement and transport. Most importantly, they enable the conduct of studies under field conditions—a significant advantage [2,18]. Resulting from the elimination of the internal electrolyte, SCISEs require additional modification with a material exhibiting ion-to-electron conductivity to ensure improved parameters, i.e., potential stability and reversibility [19].

The key role in ensuring the potential stability and repeatability of SCISE operation is played by the conductive material (solid contact). The intermediate layer is placed between the electrode surface and the ion-selective membrane or as an additional ingredient of the membrane. The presence of such a layer significantly improves charge transfer between the membrane and the internal components of the electrode by providing ionic and electronic conductivity [20]. The electrical parameters of ISEs are particularly important. Improvements in this area contribute to enhancing the potential stability, durability, and precision of ion-selective electrodes in various analytical applications [21].

In previous studies [22], we described the positive change in parameters and properties of ion-selective electrodes following the use of metal oxide nanoparticles (ZnO, Cu₂O, and Fe₂O₃) as solid contacts. The best parameters were determined for electrodes modified with ZnONPs. Recent studies indicate that, in many cases, composite or hybrid materials perform better as a solid contact than their individual components [23,24,25]. Therefore, in our further research, we decided to use ZnONPs doped with various noble metals (Ag, Au, and Pt), which also functions effectively as a solid contact in ISEs [26,27,28]. We expect that, thanks to the high conductivity of these metals, the doped ZnONPs will exhibit more favorable electrical parameters and thus function more effectively as an ion-to-electron transducer between the membrane and the internal electrode. The nanoparticles were obtained as a result of a two-step synthesis combining the fabrication of targets using the pulsed laser deposition (PLD) technique (where thin layers of doping metals have been deposited on a zinc oxide bulk substrate) and, for the production of doped nanoparticles (Pt:ZnO, Ag:ZnO, and Au:ZnO), the pulsed laser ablation in liquid (PLAL) technique, whereby the target was ablated with laser pulses as based on a previously published procedure [29]. The method is quick, simple, and enables the production of high-purity doped nanoparticles, and it is protected by a patent application [30].

The main goal of this article was to obtain sensors with improved parameters (mainly stability and reversibility of the potential) resulting from the enhanced performance of synthesized hybrid nanomaterials as a solid contact. Additional tests were performed on freshly prepared doped nanoparticle solutions according to the preparation procedure, confirming their high repeatability. Then, the focus was on using them for the construction of ion-selective electrodes (ISEs), with the solid contact (doped nanoparticles) placed as an intermediate layer between the glassy carbon electrode (GCE) and the nitrate-sensitive membrane (ISM). Both the intermediate nanomaterial layers applied on the electrodes’ surfaces (contact angle measurements) and the completed sensors (slope, linearity range, detection limit, stability and reversibility of potential, selectivity, sensitivity to redox and light changes, water layer test, electrochemical impedance spectroscopy) were examined. Their practical application for determining nitrates in soil samples is also described. To the best of our knowledge, these are the first ion-selective electrodes developed using such hybrid nanoparticles.

2. Materials and Methods

2.1. Materials

Tetrahydrofuran (THF, Chempur, purity 99.8%), 2-nitrophenyl octyl ether (NPOE, Fluka, Selectophore purity ≥ 99.0%), poly(vinylchloride) high molecular weight (PVC, Sigma Aldrich, St. Louis, MO, USA), and tridodecylmethylammonium nitrate (TDMANO₃, Sigma Aldrich, purity 99.0%) were used to prepare the ion-sensitive membrane. The following high-purity salts (puriss. p.a.) (Fluka) were used during the study: potassium nitrate, sodium nitrite, sodium chloride, sodium bromide, sodium bicarbonate, sodium sulfate, sodium acetate, and sodium perchlorate. Solutions of the individual salts were made by dissolving their weights in distilled deionized water (resistance 18.2 MΩ, Milli-Q plus, Millipore, Burlington, MA, USA). In addition, hydrochloric acid, sodium hydroxide, sodium ferricyanide, and potassium ferricyanide were also used (all puriss. p.a.).

2.2. Apparatus

2.2.1. Doped Nanoparticle Synthesis

Pulse laser ablation is a versatile and effective technique for producing nanoparticles using a high-energy laser pulse to ablate a solid target material. This technique can be applied to a wide range of materials, including metals, oxides, and alloys, making it highly useful for various research and industrial applications. Pulse laser ablation produces ultrapure nanoparticles without the need for chemical precursors, making it a green and environmentally friendly method. The process enables the adjustment of various conditions, including material and liquid types, as well as multiple laser parameters, depending on the type and properties of the desired nanoparticles [31].

The silver, gold, and platinum-doped zinc oxide nanoparticles were synthesized by pulsed laser ablation in liquid of targets prepared by using the pulsed laser deposition technique. The foils of silver (purity 99.99+%, thickness 1 mm, Goodfellow, County Durham, UK), gold (purity 99.95%, thickness 0.25 mm, Goodfellow), and platinum (purity 99.99+%, thickness 0.125 mm, Goodfellow) were used for PLD in vacuum onto the ZnO bulk substrate (purity 99.99%, thickness 3 mm, Goodfellow). Such targets consist of a thin metal film deposited onto the bulk ZnO and, together with pure bulk ZnO as a reference, were used for the synthesis of doped nanoparticles via PLAL. For this purpose, such targets were immersed in a beaker filled with 25 mL of MilliQ water. Upon PLAL, ablated material is in the form of Zn, O, and Ag or Au or Pt atoms and ions, forming a constrained plasma plume. The formation of doped ZnO nanoparticles is described with a dynamical formation mechanism, which includes diffusion growth processes responsible for doping (replacement of Zn ions with noble metal ions as a dopant in the crystal lattice of the final nanoparticle) [32,33]. This synthesis method was previously described in detail by Radičić et al. [29], where the thorough analysis of the obtained doped nanoparticles is provided: synthesized ZnO nanoparticles were doped with noble metal ions and have a hexagonal wurtzite structure, while noble metals do not appear in a metallic form, indicating doping rather than appearance in a metallic nanoparticle form. The atomic fractions estimated based on weighing the doped ZnO target before and after laser ablation of Ag, Au, and Pt in ZnO nanoparticles were 2.32, 0.55, and 0.41%, respectively [29]. Photos showing the equipment and the process of nanoparticle synthesis are shown in Figure 1.

The laser used for both PLD and PLAL is a nanosecond Nd:YAG laser (Quantel, Brilliant) operating at a 1064 nm wavelength, with an output energy of 300 mJ, a pulse duration of 5 ns, and a repetition rate of 5 Hz. The energy delivered to the sample surface was, in both cases, 120 mJ, yielding a fluence of 80 mJ/cm² per laser pulse. In all PLD and PLAL measurements, 2000 laser pulses were applied.

The freshly obtained nanoparticles were examined using the following methods: transmission electron microscopy (TEM, JEOL JEM-1400 Flash, Tokyo, Japan) to study their morphology, UV–Vis spectroscopy (UV-Vis, Cary 5000 UV-Vis-NIR Spectrophotometer, Agilent, Santa Clara, CA, USA) to assess their optical absorption properties, and dynamic light scattering (DLS) and the zeta potential method (both measured by DLS, Zetasizer Ultra, Malvern Panalytical, Malvern, UK) to estimate their size and the stability of their suspensions, respectively.

2.2.2. Electrochemical Measurements

According to the sessile drop method, the measurements of the liquid contact angles were performed using a goniometer (optical contact angle measuring and contour analysis systems of the OCA series, Filderstadt, Germany) at a temperature of 20 °C. A milliQ water drop with a volume of 3 µL was deposited on the surfaces of the electrodes covered with a layer of nanoparticles and a polished one with no deposited NPs.

Potentiometric measurements were carried out using a two-electrode system consisting of an indicator electrode, based on a glassy carbon electrode (tested electrodes), and a silver/silver chloride reference electrode (Metrohm 6.0750.100, Herisau, Switzerland). The electrodes were connected to a 16-channel data acquisition system (Lawson Labs, Malvern, PA, USA), and the data were processed by electromotive force (EMF-16) software. All measurements were performed at room temperature while stirring the solution with a magnetic stirrer.

For impedance measurements, the electrochemical impedance spectroscopy (EIS) method was used in a three-electrode configuration: the tested electrodes were used as working electrodes, a Metrohm carbon rod acted as an auxiliary electrode, and an Ag/AgCl electrode (Metrohm 6.0733.100) was used as a reference electrode. Measurements were carried out in a solution of 0.1 M potassium nitrate in a frequency range from 0.1 Hz to 100 kHz, with open circuit potential (OCP) and a voltage amplitude of 10 mV. An AUTOLAB electrochemical analyzer (Eco Chemie, Utrecht, The Netherlands) working with NOVA software version 2.1 was used for data recording. All measurements were carried out under room temperature conditions.

2.3. Ion-Selective Electrode Preparation

2.3.1. Preparation and Application of Intermediate Layer

Sensor preparation included the polishing of glassy carbon electrodes (0.3 cm diameter) with 5000 grit sandpaper, then with wetted alumina powder (0.3 μm grain diameter). Then they were rinsed thoroughly with distilled water, immersed in an ultrasonic bath, and rinsed again to remove all powder residues. The electrodes were immersed in tetrahydrofuran, an organic solvent, to get rid of the rest of the organic residues, then they were allowed to dry in air. Then, an aqueous suspension of nanoparticles was dripped onto the surface of the electrodes to obtain a uniform layer (10 × 10 μL), each time waiting for the previous layer to dry and previously placing the suspension in an ultrasonic water bath to obtain homogeneity. After the completion of evaporation of the water, the electrodes with the applied layer of nanoparticles were subjected to contact angle tests. In the same way, intermediate layers of solid contact were also prepared, on which a layer of ion-selective membrane was applied in a further stage, in order to create a full construction of correctly functioning electrodes. The control electrodes were prepared by covering the ISEs only with ISM (GCE/ISM).

2.3.2. Ion-Sensitive Membrane

To make the electrodes selective towards nitrate ions, an ion-selective membrane was prepared. This membrane was made by dissolving 0.3 g of a pre-weighed mixture of membrane components in 3 mL of THF. The mixture contained the following components in mass percentages: 62% 2-nitrophenyl octyl ether, 32% poly(vinyl chloride), and 6% tridodecylmethylammonium nitrate. The resulting membrane solution was then homogenized in an ultrasonic bath to ensure equal distribution of the components. After homogenization, the membrane solution was spotted onto the previously prepared electrodes three times, 50 μL each, at 30-minute intervals. After applying a total volume of 150 μL, the electrodes were placed vertically on a stand and left for 24 h to allow complete evaporation of the solvent. After this time, the electrodes were placed in a conditioning solution of 1 × 10⁻³ M of potassium nitrate (KNO₃) for several days to allow the membrane to stabilize and to obtain the appropriate measurement properties. Between measurements, all electrodes were immersed in the same solution stored in a closed, dark place.

2.4. Sample Preparation

Soil samples were prepared according to the following procedure. Soil material was collected from the surface of the ground, mixed thoroughly, then dried at 115 °C for 2 h to remove humidity. After drying, the samples were weighed accurately, and solutions were prepared (15 g of soil per 100 mL of distilled water). The prepared mixtures were placed on a shaker and extracted for 24 h at room temperature to transfer the nitrates from the soil phase to the liquid phase. After extraction, the suspension was filtered, and the obtained filtrate was used for further analysis.

3. Results and Discussion

Detailed studies of the synthesized nanoparticles were performed. The obtained nitrate-sensitive ion-selective electrodes were studied for both potentiometric and electrical parameters. As a preliminary step, the interlayers of nanoparticles were additionally studied to estimate their contact angles, capacitance, and electrical resistance.

3.1. Characteristics of Nanoparticles

The Pt, Ag, and Au-doped ZnONP nanoparticles were synthesized in a two-step process using PLD and PLAL techniques. Detailed studies, including the optical properties, sizes, morphology, structures, and composition, measured using UV–Vis, photoluminescence (PL), X-ray photoelectron (XPS) spectroscopies, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy, were already described in [29]. Additional tests were also performed, described in the paper below, including the zeta potential of the nanoparticles and the contact angle of their layers. In

Table 1. Summary of ZnO, ZnO:Ag, ZnO:Au, and ZnO:Pt nanoparticle properties. Nanoparticle diameters were obtained from SEM images, crystallite sizes from XRD data, dopant atomic fractions from mass measurements, and band gap energies from UV–Vis spectra, as reported by Radičić et al. [29]. Zeta potential values measured in the present study are also presented.

Parameter	ZnONPs	Pt:ZnONPs	Ag:ZnONPs	Au:ZnONPs
Diameter [nm]	51	78	71	73
Crystallite size [nm]	50	24	24	25
Dopant atomic fraction [at. %]	-	0.42 ± 0.10	2.32 ± 0.20	0.55 ± 0.10
Band gap energy [eV]	3.20	3.15	3.06	3.08
Zeta potential [mV]	+37.73	+28.06	+30.02	+32.98

, a summary of the structural, optical, and surface properties of ZnO, ZnO: Ag, ZnO: Au, and ZnO: Pt NPs are presented.

Zeta Potential

Zeta potential, the electrokinetic potential in colloidal systems, representing the electrical potential at the boundary of the electric double layer versus the bulk fluid, was measured for all colloid nanoparticle solutions. It is an important parameter, useful for understanding and predicting the stability of colloidal dispersions, as it indicates the degree of electrostatic repulsion between particles. Aggregation of nanoparticles is an undesirable phenomenon because nanoparticles must remain dispersed to maintain their suspension stability. The aggregation process can lead to sedimentation, reducing the effectiveness of the nanoparticles in their intended applications. Moreover, aggregated nanoparticles often exhibit altered properties, such as reduced surface area and changes in shape, which can negatively impact their functionality, environmental interactions, and biological activity [34,35]. Zeta potential helps in nanoparticle surface charge characterization. High zeta potential values (both positive or negative) suggest strong repulsive forces, which prevent particle aggregation and indicate stable colloidal systems. A zeta potential value of ±30 mV indicates that the suspension of nanoparticles is on the threshold of stability [36,37]. At least such a value of zeta potential is able to ensure electrostatic stability of the suspension. The measured values of the zeta potential were +37.73, +28.06, +30.02, and +32.98 mV, respectively, for ZnONPs, Pt:ZnONPs, Ag:ZnONPs and Au:ZnONPs. As all values were in a range close to 30 mV, it meant that the suspensions were relatively stable. The ZnONP suspension with the highest zeta potential value showed the lowest tendency to aggregate. Doping nanoparticles typically reduces their zeta potential due to modifications in surface charge, electrostatic interactions, surface chemistry, structural changes, and aggregation. These changes collectively reduce the electrostatic repulsion between particles, leading to a lower zeta potential. This is the case for ZnO nanoparticles doped with Yttrium (Y) and Cerium (Ce) [38], or CuO nanoparticles doped with Fe [39], where doping significantly alters their zeta potential, making doped NPs less stable in deionized water compared to undoped NPs. Silver, gold, and platinum doping reduces the zeta potential of ZnO nanoparticles because noble metal ions neutralize surface charges and decrease hydroxyl group density, leading to weaker electrostatic repulsion [40]. It is confirmed in [29] that Zn²⁺ ions are replaced with noble metal ions, with a larger ionic radius than Zn²⁺ ions, which induces lattice distortion and modification of the surface charge state.

3.2. Characteristics of Intermediate Layers

Contact angle measurement is a key aspect in the study of the surface properties of materials. For this purpose, the deposited drop method was used, which consists in placing a drop of liquid (in this case milliQ water) on the surface of the tested material (a layer of nanoparticles deposited on the surface of a glassy carbon electrode), then measuring the angle formed between the tangent to the drop at the point of contact with the surface and the surface itself [41]. Measurements for each type of surface (ZnO, Pt:ZnO, Ag:ZnO, Au:ZnO nanoparticles layers, and polished electrode surface) were performed three times on two electrode specimens (six repetitions for each type of surface). Average contact angles were determined by taking into account the angles on both sides of the drop (adding up to 12 values), and the standard deviation of each was estimated. Example illustrations showing selected measurements for each tested surface are shown in Figure 2.

For the polished GCE surface, the measured contact angle was 67.24 ± 1.26°. For the applied layer of pure ZnO nanoparticles, the contact angle was 134.64 ± 1.23° and, for Pt, Ag, and Au-doped nanoparticles, 98.65 ± 3.79°, 84.56 ± 0.42°, and 91.45 ± 3.57°, respectively. It can be concluded that the most hydrophobic surface is the ZnONP layer, and doped nanoparticles show decreased hydrophobicity, whereby the surface remains hydrophobic in the case of Pt:ZnONPs and Au:ZnONPs (contact angle > 90°). Only Ag:ZnONPs became slightly hydrophilic (contact angle < 90°), but not enough to cause the formation of an undesirable water layer (see 3.3.5 Water Layer Test).

The decrease in water contact angle when ZnO nanoparticles are doped with noble metals is primarily due to changes in chemical composition and surface energy. Noble metal dopants introduce new surface states, modify charge distribution, and increase hydrophilicity, which lowers the contact angle [42,43]. Particle size and morphology can also contribute, but the dominant factor is the altered surface chemistry. These changes enhance the interaction between water molecules and the nanoparticle surface, leading to increased wettability and a lower contact angle. The specific effects depend on the type of dopant and the resulting modifications to the ZnO nanoparticles’ surfaces and electronic structures. Studies have been described in which the doping of ZnONPs with metals such as Mg or Ca can alter the surface energy and hydrophilicity of the nanoparticles. This is because doping can introduce defects or alter the surface chemistry, making the surface more attractive to water molecules. The introduction of dopants can also change the surface morphology of ZnONPs. For example, Ca-doped ZnO nanoparticles exhibit different surface structures compared to undoped ZnO, which can influence wettability [39]. The sizes of our nanoparticles, based on SEM analysis, were 51 nm, 71 nm, 73 nm, and 89 nm for the pure ZnONPs, Ag:ZnONPs, Au:ZnONPs, and Pt:ZnONPs, respectively. The variability in nanoparticle size observed in TEM images is typical for nanoparticles obtained by laser ablation in liquid, which leads to the formation of particles in the range of several to several dozen nanometers. However, individual particles of larger sizes also occur, as confirmed by SEM measurements and the resulting size distributions. The XRD results showed that the average crystallite sizes for doped ZnO (24–25 nm) are significantly smaller than for pure ZnO (approximately 50 nm), which we attribute to the presence of Ag, Au, and Pt ions in the ZnO lattice, which limits crystallite growth [29]. This may also be related to the decrease in the contact angle values. The atomic fractions of dopants were calculated from mass measurements. For this purpose, the weights of the ZnO substrates were measured before and after the PLD and PLAL steps used for the synthesis of doped ZnO NPs, enabling determination of the deposited metal layer mass and the mass of the ablated NPs. The calculated dopant atomic fractions are presented in Table 1, where it is evident that ZnO:Ag NPs contain the highest dopant atomic fraction (2.32 at.%). The lack of characteristic noble metal peaks in the XRD spectra, and the absence of signals corresponding to separate Ag, Au, or Pt particles in the UV–Vis spectra, indicate that the dopant particles were incorporated into the ZnO lattice rather than forming separate nanoparticles. Therefore, the observed changes in properties should result primarily from ZnO doping. Doping can modify the chemical composition and surface states of ZnO nanoparticles. This can affect the interaction between water molecules and the nanoparticle surface. For example, the presence of dopants can create more active sites for water adsorption, thereby reducing the contact angle [44,45]. The electronic properties of ZnO nanoparticles, such as the band gap, can also be influenced by doping. This can affect the surface charge distribution and the interaction with water molecules. For instance, doping with metals like aluminum (Al) can change the band gap and surface charge, potentially leading to a decrease in the water contact angle [46]. In the case of the doped ZnONPs we used, the band gap energy decreased in the order ZnONPs, Pt:ZnONPs, Au:ZnONPs, and Ag:ZnONPs (3.20 ± 0.04 eV, 3.15 ± 0.03 eV, 3.08 ± 0.02 eV, and 3.06 ± 0.02 eV, respectively) [29], confirming that a decrease in band gap energy may lead to increased surface reactivity and potentially higher wettability due to enhanced electronic interactions at the surface.

3.3. Characteristics of Ion-Selective Electrodes

3.3.1. Calibration Curves and Basic Electrode Parameters

Many potentiometric measurements are carried out to characterize ion-selective electrodes. One of the basic research areas is determining the electrode’s response–calibration curves. For this purpose, all the electrodes, with the reference electrode, were placed in a beaker containing 50 mL of distilled water. Then, using the method of standard addition, the characteristics of the electrodes were determined in a concentration range of 1 × 10⁻¹–1 × 10⁻⁷ M (appropriate amounts of KNO₃ with concentrations of 1 M, 0.1 M, 1 × 10⁻² M, and 1 × 10⁻³ were added). The results obtained in the form of the dependence of the electromotive force (EMF) on the negative logarithm of the activity of the main ion (pa_NO3−) are placed in Figure 3. For electrodes GCE/ISM, GCE/ZnO/ISM, GCE/Pt:ZnO/ISM, GCE/Ag:ZnO/ISM, and GCE/Au:ZnO/ISM, among others, the slope (which determines the sensitivity of the electrode), the linearity range (working range), the detection limit, and the E⁰ parameter (calculated based on calibration repeated weekly for 1 month) were determined. The results are placed in

Table 2. Basic results calculated based on the electrodes’ responses.

Sensor	Slope [mV dec−1]	Linearity Range [M]	Detection Limit [µM]	SD from E0 (n = 4) [mV]
GCE/ISM	−57.13	1 × 10−1–1 × 10−5	7.5	11.7
GCE/ZnO/ISM	−57.71	1 × 10−1–1 × 10−5	6.3	4.16
GCE/Pt:ZnO/ISM	−62.52	1 × 10−1–5 × 10−6	3.2	2.12
GCE/Ag:ZnO/ISM	−59.80	1 × 10−1–1 × 10−5	6.2	3.47
GCE/Au:ZnO/ISM	−62.87	1 × 10−1–5 × 10−6	2.5	2.48

. Both the electrode modified with pure zinc oxide nanoparticles and the electrodes modified with hybrid material (ZnO doped with platinum, silver, and gold) showed excellent sensitivity. For the electrodes modified with a hybrid material, the highest sensitivity was obtained, exceeding the theoretical value for the monovalent ion (59.16 mV/dec). For the GCE/Pt:ZnO/ISM and GCE/Au:ZnO/ISM electrodes, linearity ranges wider than the other electrodes by half an order of magnitude were obtained. All of the modified electrodes showed a great E⁰ parameter—an indication of the reproducibility of the electrode potential. The E⁰ value is determined by extrapolating the calibration curve. These values were determined once a week based on the calibrations performed. Then, the average potential value and standard deviation were calculated from the obtained E⁰ values, which allowed the repeatability and stability of the electrode to be assessed over a longer period. The variability of parameter E⁰ determines the frequency of control calibrations during electrode use. The long-term stability of the unmodified electrode was at least three times worse than that of the ZnO- and ZnO:noble-metal-modified electrodes.

3.3.2. Short-Term Stability and Reversibility of the Potential

A drawback of solid contact ion-selective electrodes is the lack of good ion-electron conductivity resulting from the elimination of the internal electrolyte, which acts as a transducer medium. Therefore, in SCISEs, mediation layers are introduced to improve electrical parameters and to upgrade conversion and charge transfer, thus ensuring a stable and reversible potential over time. To determine the stability, the potential was measured for 1.5 h in a KNO₃ solution of 1 × 10⁻³ M (Figure 4). Based on the results, the potential drift in (μV/s) was calculated as the quotient between the initial and final potential difference (∆E) and the time (∆t) (potential drift= ∆E/∆t) (

Table 3. The potential drift for unmodified, ZnO- and ZnO-metal-modified electrodes.

Electrode	GCE/ISM	GCE/ZnO/ISM	GCE/Pt:ZnO/ISM	GCE/Ag:ZnO/ISM	GCE/Au:ZnO/ISM
Potential drift in the first 10 min [µV/s]	1.72	8.13	0.22	2.15	0.25
Potential drift during rest time [µV/s]	5.17	1.25	0.78	1.21	1.91

). All the studied electrodes showed excellent stability, especially those where hybrid materials were used. The GCE/ZnO/ISM electrode initially showed an increased potential drift for about 10 min, which then stabilized, giving us a stable potential over time. The rest of the electrodes in the first 10 minutes of the measurement showed less instability, the best of which was recorded for GCE/Pt:ZnO, with a drift of 0.22 µV/s (almost 37 times better than for GCE/ZnO/ISM and eight times better than for GCE/ISM). For this electrode, the best potential drift over time in the further part of the measurement was also obtained: 0.78 µV/s (six times better than for the unmodified electrode).

On the other hand, the reversibility of the potential was studied by measuring the EMF alternately in 1 × 10⁻³ and 1 × 10⁻⁴ KNO₃ solutions in four series. The relationship representing this parameter in graphical form is shown in Figure 5. The standard deviation values for a given concentration determined from the average potential values in the four measurements are shown in

Table 4. The standard deviation values from mean potential values calculated for 1 × 10⁻³ and 1 × 10⁻⁴ M solution of KNO₃ for all tested electrodes.

SD from Mean Potential Value for Certain NO3− Concentration (n = 3) [mV]	C = 1 × 10−3 [M]	C = 1 × 10−4 [M]
GCE/ISM	11.21	13.56
GCE/ZnO/ISM	8.62	4.95
GCE/Pt:ZnO/ISM	1.60	1.22
GCE/Ag:ZnO/ISM	6.09	1.98
GCE/Au:ZnO/ISM	5.43	3.82

. Based on the results, we can conclude that the best reversibility was characterized by the electrode modified with zinc oxide nanoparticles doped with platinum—the SD values were 1.60 mV and 1.22 mV for 1 × 10⁻⁴ and 1 × 10⁻³ M KNO₃, respectively. The other SCISEs modified with the hybrid material had slightly worse potential reversibility than the best-performing electrode, but they were better than the ZnO-modified electrode and the unmodified electrode.

3.3.3. Selectivity

To evaluate the effect of interfering ions on the potential measured by the electrodes, logarithmic potentiometric selectivity coefficients (K_a,b^pot—a—main ion, b—interfering ion) were determined from the results. According to IUPAC recommendations, these coefficients were determined by the Separate Solution Method (SSM) [47] for anions NO₂⁻, Cl⁻, Br⁻, HCO₃⁻, SO₄²⁻, CH₃COO⁻, F⁻, and ClO₄⁻ (

Table 5. Logarithmic potentiometric selectivity coefficients determined for the tested electrodes.

Interfering Ion	GCE/ZnO/ISM	GCE/Pt:ZnO/ISM	GCE/Ag:ZnO/ISM	GCE/Au:ZnO/ISM
NO2−	−1.76	−2.09	−1.64	−1.93
Cl−	−1.75	−1.89	−1.59	−1.58
Br−	−0.91	−1.01	−0.82	−0.76
HCO3−	−3.15	−3.20	−3.05	−3.08
SO42−	−3.51	−3.56	−3.48	−3.74
CH3COO−	−3.97	−4.15	−3.92	−4.12
F−	−4.47	−4.56	−4.15	−4.66
ClO4−	4.24	3.57	3.36	3.51

). The strongest interfering influence was observed for the ClO₄⁻ ions, which are highly lipophilic and exhibit a high affinity for the polymer membrane based on quaternary ammonium salts. In addition, logK_a,b^pot < −1 values were also recorded for the Br⁻ ion, in the case of GCE/ZnO/ISM, GCE/Ag:ZnO/ISM, and GCE/Au:ZnO/ISM electrodes. Fortunately, the content of both these ions, in samples in which nitrate content is usually determined, is very low and does not cause interference. In other cases, the obtained values of selectivity coefficients were very similar, indicating comparable resistance to interference. Similar selectivity coefficients result from the fact that all electrodes have the same membrane, the composition of which determines the selectivity of the electrode. The slight differences observed for individual electrodes result from differences in their characteristic slope, which also affects the selectivity coefficient value.

3.3.4. Influence of Measurement Conditions on Potential Changeability

By developing the new potentiometric sensors, we aim to implement them in environmental measurements as well as in measurements of real samples. However, to do so, we need to be sure of the introduced modification—in our case, ZnO nanoparticles and a hybrid material consisting of ZnO nanoparticles in combination with Pt, Ag, and Au. One of the measurements was to check the effect of the sample’s acidification level (pH) on the potential values. Appropriate amounts of concentrated acid (HCl) and base (NaOH) were added to obtain the desired pH values. Each electrode worked properly in a range of about 3–10 pH. The next step was to evaluate the sensitivity of the electrodes to a change in redox potential. To achieve this, potential measurements were carried out in solutions of 1 × 10⁻² M KNO₃ enriched with a redox pair of hexacyanoferrate(III) and hexacyanoferrate(II) ([Fe(CN)₆]³⁻ and [Fe(CN)₆]⁴⁻) at different ratios (1:10, 1:5, 1:1:, 5:1, 10:1) (the concentration of Fe²⁺ and Fe³⁺ ions was equal to 1 × 10⁻³ M). The results are shown in Figure 6. As we can see for the electrodes modified with the hybrid material, no redox effect on the potential was observed (a slight deviation from the norm was observed for the GCE/ZnO/ISM electrode).

In addition to the pH of the solution and the presence of redox vapor, the potential can also be affected by light and the presence of gases (CO₂ and O₂). This is mainly relevant for polymeric media, but determining the influence of these factors is equally important for electrodes modified with other compounds. To check the effect of light, alternating potential measurements are carried out in a 1 × 10⁻³ M KNO₃ solution when the sample is illuminated with daylight, then in complete darkness. A graph showing the illumination effect on the EMF is shown in Figure 7. As we can see, the light does not affect the potential values of the tested electrodes (in the case of the GCE/ZnO/ISM electrode, we observe a slight drift of the potential at the beginning of the measurement, which was also noted in the case of the stability measurement). Similar measurements are carried out for the effect of gases on the potential of SCISEs. Here, the EMF is measured alternately in a 1 × 10⁻³ M KNO₃ solution saturated with gases, followed by one saturated with N₂ (nitrogen was used to get rid of oxygen and carbon dioxide from the solution by passing it through the sample solution for about 20 min before measurement). Similarly, in this case, no deviation from the potential value for specific conditions was obtained for all electrodes.

3.3.5. Water Layer Test

A water layer test was performed as proposed by Fibbioli et al. [48]. The potential was first measured in the solution of the main ion (0.1 M NaNO₃), then in the solution of the interferent ion (0.1 M CH₃COONa), after which the solution was changed to one containing NO₃⁻ anions. The presence of an aqueous layer between the membrane elements (at the membrane–interlayer or interlayer–membrane interface) can be inferred from the potential drift. Analyzing the obtained results shown in Figure 8, we can see that no potential drift was observed for any of the tested electrodes, which indicates that the tested solid contact materials show sufficient hydrophobicity and prevent the formation of an aqueous layer. A slight decrease in water repellency for hybrid materials does not affect the formation of a water layer.

3.3.6. Electrochemical Impedance Spectroscopy Measurements

In order to determine the values of the basic electrical parameters of the electrodes, such as membrane resistance and double-layer capacitance, measurements were made using electrochemical impedance spectroscopy. The tests were carried out according to the conditions described in Section 2.2.2. A graphical representation of the results in the form of an impedance spectrum are shown in Figure 9, where it can be seen that all spectra consist of two parts. The first part of the spectrum recorded for high frequencies has the form of a partial semicircle and corresponds to the bulk resistance of the membrane R_b in connection with its geometric capacitance (CPE₁). The second part of the spectrum obtained for medium and high frequencies is related to the charge transfer resistance R_ct and the capacitance of the double layer (CPE ₂) at the interface between the polymeric membrane and the substrate electrode. The obtained experimental data were fitted to the equivalent circuit shown in Figure 9 (in insert) using NOVA 2.1 software, and the determined electrical parameter values are presented in

Table 6. Electrical parameters of each tested electrode determined using equivalent circuit showed in Figure 9. (R_u uncompensated series resistance, R_b bulk resistance, R_ct charge transfer resistance, CPE constant phase element (CPE₁ connected with membrane geometric capacitance; CPE₂ connected with capacitance of the double layer), Y⁰ initial value for the admittance for the CPE element, n—parameter showing to what extent the CPE is the ideal capacitance; if n = 1, then CPE is ideal capacitance, and, when = 0.5, it is Warburg impedance).

Electrical Parameter	Ru [kΩ]	Rb [kΩ]	CPE1 Y0 (n) pF	Rct [MΩ]	CPE2 Y0 (n) µF
GCE/ZnO/ISM	63.0	102	15.6 (0.903)	2.18	0.74 (0.469)
GCE/Pt:ZnO/ISM	46.2	93.7	17.0 (0.931)	0.58	2.74 (0.513)
GCE/Ag:ZnO/ISM	52.6	99.9	15.9 (0.924)	1.10	1.37 (0.655)
GCE/Au:ZnO/ISM	64.9	106	15.0 (0.882)	1.01	1.58 (0.661)

. The data show that the parameters related to the polymer membrane, i.e., membrane resistance R_b and geometric capacitance CPE₁, have similar values for all tested electrodes, which is understandable since the same membrane was used in each case. Significant differences in electrical parameters occur in the case of charge transfer resistance R_ct and double layer capacitance CPE₂, which are directly related to the properties of the intermediate layer material. All electrodes based on hybrid nanoparticles showed lower charge transfer resistance values compared to the GCE/ZnO/ISM electrode. The lowest R_ct value was obtained for the GCE/Pt:ZnO/ISM electrode, which was 0.58 MΩ and almost four times lower than the R_ct value determined for the electrode based on non-doped ZnO nanoparticles. In the case of other electrodes based on hybrid particles, the reduction in R_ct value was almost twofold. The reduction in charge transfer resistance is accompanied by an increase in the capacitance of the double layer, as evidenced by the value and parameter CPE₂. This is confirmed by the capacitance values (C) determined at a specific measurement point (for a given frequency), according to the following formula:

$$C= {\displaystyle \frac{1}{2\mathsf{\pi }f{Z}^{″}}}$$

where: $C$—electrical capacitance, F; $f$—frequency of the signal, Hz; $Z^{″}$—imaginary part of the impedance obtained from the EIS spectrum, Ω.

The calculated value of capacitance for GCE/ZnO/ISM was 5.95 μF, and it was in all cases smaller than for SCISEs modified with the doped ZnO nanoparticles. The largest capacitance of 22.18 μF was obtained for GCE/Pt:ZnO/ISM. It was five times higher than for the electrode modified with pure ZnO nanoparticles, and about three times higher than for SCISEs modified with the other hybrid materials. Electrodes where the hybrid material was enriched with Au or Ag showed capacitance values of 7.25 and 7.67 μF for GCE/Au:ZnO/ISM and GCE/Ag:ZnO/ISM, respectively. These results indicate that hybrid nanoparticles are more effective in facilitating the diffusion processes and charge transport at the interface than undoped ZnO nanoparticles. The EIS measurement results clearly demonstrate that the electrical properties of the solid contact material play a crucial role in stabilizing electrode potential.

3.3.7. Analytical Application of Pt:ZnO-Modified Nitrate-Selective Electrodes to the Determination of Nitrates in Soil

To confirm the analytical usefulness of the proposed Pt:ZnO-modified electrode, it was used for the determination of the nitrate content in the soil samples. The analysis was performed using the multiple standard addition method, applying 0.05 M CH₃COONa as the ionic strength buffer. The linear range was the same as in pure nitrate solutions and the LOD was equal at 6.3 × 10⁻⁵ M. The nitrate content in the soil was 196 ± 8.7 mg/kg. The correct operation of the electrode was confirmed by a recovery study. For this purpose, an analysis of a soil sample enriched with nitrates (40 mg/kg and 200 mg/kg) was performed in the same manner. To do this, soil samples spiked with nitrate were analyzed analogously. The recovery values were calculated by subtracting the initial nitrate content determined for the soil sample (196 mg/kg) from the nitrate content determined for the enriched sample. The recovery values obtained in the range of 97.2–101% confirm the correct operation of the electrode and its suitability for soil analysis.

4. Conclusions

The present work successfully investigated the implementation of zinc oxide nanoparticles doped with noble metals—platinum, silver, and gold—as a solid contact layer in nitrate ion-selective electrodes. A series of physicochemical analyses was carried out to characterize the obtained materials, as well as to evaluate the electrical and analytical properties of electrodes modified with these nanomaterials. The best modification turned out to be the introduction of Pt:ZnO and Au:ZnO, as these ISEs had the widest linearity range of 1 × 10⁻¹–5 × 10⁻⁶ M and the lowest limits of detection at 3.2 and 2.5 µM, respectively. These electrodes showed a higher than theoretical characteristic slope of −62.5 and −62.9 mV dec⁻¹ for GCE/Pt:ZnO/ISM and GCE/Au:ZnO/ISM, respectively. This is not a defect of the electrode, as it does not cause measurement errors. It is important that the slope does not change over time and that this condition is met. The metal:ZnO-modified electrodes showed very good long-term stability (E⁰), as well as short-term stability. While the lowest SD from E⁰ values were recorded for both GCE/Pt:ZnO/ISM and GCE/Au:ZnO/ISM electrodes (they also have similar values of short-term potential drift), the best short-term potential stability was that of the Pt:ZnO-modified ISE. An equally important parameter was the reversibility of the potential, in which the GCE/Pt:ZnO/ISM electrode stood out—its deviations from the average potential values at a given concentration were the smallest among all the tested sensors. To summarize, the potential reversibility and stability were improved due to the application of the hybrid material. Such good results are directly related to the favorable electrical parameters of the solid contact layer, such as low charge transfer resistance and high electrical capacitance, which also reached the best values in the case of Pt:ZnO modification. The electrical properties of the material have a greater impact on its effectiveness as a solid contact than its hydrophobic properties. The application of the Pt:ZnONP layer as a solid contact enables obtaining a capacitance equal to 22.18 μF, which was nearly four times higher than that of ISE, where the pure ZnONPs were applied as SC (5.95 μF). In addition, the value of charge transfer resistance obtained for this electrode (0.58 MΩ) was almost four times lower than that of the electrode based on undoped ZnONPs (2.18 MΩ). All electrodes modified with the hybrid material exhibit better parameters compared to the ZnONP-modified electrode with the most hydrophobic properties, which means that the highest hydrophobicity is not always the most favorable property. In addition, all the tested electrodes exhibited the typical selectivity characteristic of nitrate sensors with a TDMANO₃-based membrane. Importantly for potential environmental applications, the electrodes were not susceptible to interference from the presence of gases, changes in redox potential, or lighting intensity. Each of the developed electrodes successfully passed the aqueous layer test, confirming the effectiveness of using a hydrophobic material as a solid contact layer. Moreover,

Table 7. The comparison of basic potentiometric parameters of SCISEs developed in recent years.

Transducer Media Material	Slope [mV/dec]	Linearity Range [M]	Detection Limit [×10−6 M]	Analytical Application	Reference
MWCNTs/CuONPs	−60.4	1.0 × 10−6–1.0 × 10−1	0.85	Environmental water samples	[49]
MWCNTs-THTDPCl	−57.1	1.0 × 10−6–1.0 × 10−1	0.5	-	[50]
TTF-TCNQ	−58.5	1.0 × 10−5–1.0 × 10−1	1.6	Water samples	[51]
PEDOT:PEG	−55.8	1.1 × 10−6–1.0 × 10−1	1.1	Hydroponics and microalgae culture media	[52]
PAAm-MnO2	−50.6	1.0 × 10−5–1.0 × 10−1	6.3	-	[53]
AuNPs	−50.4	5.3 × 10−5–1.0 × 10−1	5.3	Aqueous samples of fertilizers	[27]
PANINFs-NO3	−57.8	1.0 × 10−6–1.0 × 10−1	1.1	Environmental samples	[54]
ZnONPs:Pt	−62.87	5.0 × 10−6–1.0 × 10−1	3.2	Soil monitoring	This article

THTDPCl—trihexyltetra-decylphosphonium chloride; TTF-TCNQ—etrathiafulvalene (TTF) and teracyanoquinodimethane (TCNQ) composite; PEDOT:PEG—poly(3,4-ethylenedioxythiophene) (PEDOT) and polyethylene glycol (PEG) composite; PAAm-MnO₂—poly(3-octylthiophene) and MoS₂; AuNPs—gold nanoparticles; PANINFs-NO₃—polyaniline nanofibers doped with nitrate ions.

presented a comparison of different ISE parameters. Based on these results, we can conclude that our electrode, compared to the other SCISEs, has high sensitivity, a wide linear range, and a satisfying limit of detection.