Introduction

Nowadays, environmental contamination is one of the serious global issues (Ezzatahmadi et al. 2019). Rapid industrialization and urbanization have increased the environmental concentration of toxic and hazardous pollutants (Qu et al. 2020). The toxicity, mutagenicity, carcinogenicity, persistence, and biological accumulation of chlorophenols in aqueous media have been raising concerns (Dong et al. 2016). 2, 4-Dichlorophenol (2, 4-DCP) is a typical chemical compound of CPs that is widely utilized in pharmaceutical manufacture (Yu et al. 2016), petrochemicals, preservatives, insecticides, and herbicides (Liang et al. 2017). 2, 4-DCP, has gained recognition as a priority pollutant by esteemed environmental agencies in both the European Union and the United States (Yashas et al. 2019). It is crucial to highlight that this compound has negative effects on both human health and the environment (CG et al. 2020). Even though the maximum allowable amount of 2,4-DCP in water is 0.5 ng mL-1, some contaminated regions of water have greater quantities (Nguyen et al. 2020). As a result, sensitive, reliable, quick, easy, and selective analytical techniques for 2, 4-DCP measurement are urgently required.

2, 4-DCP, a chemical molecule of concern, has been quantified using numerous analytical methods. These methods include HPLC, spectrophotometry, GC/MS, capillary electrophoresis, and chemiluminescence (Katowah et al. 2020). However, these methods possess some demerits such as high cost, complex manipulation, incapability of in-situ analysis and low selectivity (Liu et al. 2019). The simplicity, ease of use, quick response time, and sensitivity of electrochemical techniques have made them extremely popular in recent years. CPs are electrochemically active and oxidizable owing to presence of phenolic hydroxyl group. Phenoxy radicals are known to be generated during the oxidation process of CPs. These radicals have the potential to undergo two different pathways: polymerization or further oxidation. In the latter case, o-benzoquinone and p-benzoquinone are formed as byproducts. In electrochemistry, we often encounter a phenomenon known as fouling on the electrode surface. This occurs when the byproducts of a reaction find their way onto the electrode and gradually build up, forming a layer of accumulation. Electrode surfaces are poisoned, measurements are non-reproducible and the sensitivity is decreased. In order to eliminate the negative impacts of fouling, scientists are currently exploring the promising avenue of chemically modifying electrodes (Al-Qasmi et al. 2019). The characteristics of modified electrodes, including their high selectivity, large surface area, and catalytic activity, have made them a very interesting subject of study (Brett et al. 2022). This procedure accelerates the transfer of electrons and increases the reactivity of electrodes with analytes (Alizadeh et al. 2019). The nature of the used modifiers in sensing layer of electrodes affects the performance of modified electrodes (Ilager et al. 2020). However, expensive and complex synthesis method, instability and lack of conductivity and electrocatalytic activity of the reported electrode materials for 2, 4- DCP measurements requires the development of other modifiers.

Numerous exhaustive investigations have been conducted on the electrochemical phenomena associated with the reactions of 2,4-dichlorophenol (2,4-DCP) using various carbon-based electrodes, such as glassy carbon (GC), carbon fibre paper (CFP), and carbon paste electrodes (CPE) (Liang et al. 2017). Among them, carbon paste electrode (CPE) has drawn much attention owing to its exceptional benefits including wide potential window, surface renewability (Antoniazzi et al. 2020), low background currents, biocompatibility, stability, low cost and ease of modification with various modifiers (Alizadeh et al. 2020). Recently, several 2, 4- DCP modified carbon paste electrodes based on cyclodextrin functionalized ionic liquid (Rasdi et al. 2016), metal–organic framework (MOF) (Cui et al. 2018) and fluorohectorite clay (Ozkan et al. 2002) have been reported.

Nanoparticles (NPs) are proper candidates for electrode modification and tracing low concentration of analyte compounds (Gawli et al. 2014) because they have high surface area and active sites (Baig et al. 2019). Many transition metals and their oxide- based NPs widely explored as electrode active materials due to their supreme features, including the high biocompatibility, electrocatalytic properties and low cost in contrast to other materials (Saha et al. 2020). Due to their stability, ubiquity, and low toxicity, manganese oxides have garnered a great deal of interest. However, their poor electrical conductivity can limit their application. In order to solve this shortcoming, researchers have formed a mixed metal oxide spinels (AxB3-xO4) which exhibit better electrochemical performance (Li et al. 2018) due to synergy effect of both metal oxides, high conductivity and presence of multiple oxidation states (Ma et al. 2015). Among spinel structures, remarkable attention has been paid towards the investigation of NixMn3-xO4 due to its good electrical conductivity (Zhang et al. 2019), large surface area, low cost (Dhas et al. 2021), strong absorption ability, high redox active sites, eco-friendly preparation and high stability which expands the application aspects in catalysis, sensors and energy storage (Busacca et al. 2019). However, there are few reports on the use of NixMn3-xO4 in sensing application. Through the strategic utilization of the synergistic effect of magnesium aluminium double hydroxide and nickel manganese oxide, Ahmadi-Kashani et al. (2020) adeptly engineered a glassy carbon electrode modification with the primary objective of quantifying levodopa. The electrode exhibited remarkable repeatability and stability, demonstrating a linear range spanning from 0.1 to 100 nM.

In our knowledge, there is currently no documented utilisation of NixMn3-xO4 in the development of electrochemical sensors specifically designed for detecting 2, 4-dichlorophenol (2,4-DCP). This research focuses on the highly selective and accurate quantitative measurement of 2,4-DCP using NixMn3-xO4/CPE. The NixMn3-xO4 NPs were synthesized by a facile co- precipitation rout. There are many favorable properties of the sensor developed, including high sensitivity, low detection limit, reproducibility, and low overvoltage, which make it highly promising for 2, 4-DCP detection.

Materials and methods

Chemicals and instrumentation

The chemicals used in this study were carefully selected for their high quality and purity, meeting the standards required for accurate analysis and research purposes, thereby eliminating the necessity for subsequent purification. Manganese(II) acetate tetrahydrate (Mn(CH3COO)2.4H2O), Nickel(II) chloride hexahydrate (NiCl2.6H2O), 2,4-dichlorophenol (2,4-DCP), graphite powder, and paraffin oil were procured from Merck (Darmstadt, Germany). Chemicals used in electrolyte solution preparation and interference effect study were all purchased from Merck. In order to prepare the stock solutions, we dissolved precise quantities of 2,4-DCP in double distilled water. Following this, the aforementioned solutions were subjected to dilution using a phosphate buffer solution (0.12 mol L-1, pH 6.0), which was employed as the supporting electrolyte.

Electrochemical studies were performed using NOVA 2.1.3 and an Autolab potentiostat/galvanostat (PGSTAT 302N, Eco Chemie, Netherlands). Three electrodes—modified or bare carbon paste, Ag/AgCl (saturated KCl), and platinum rod—were employed to measure 2,4-DCP at ambient temperature. The pH of the solutions was carefully regulated by employing a Metrohm 710 pH metre.

XRD and FT-IR were carried out by using a PW1730 diffractometer (PHILIPS, Netherlands) and Avatar set-up (Thermo, USA) utilized for characterization of NixMn3-xO4 structure, respectively. MIRA III FE-SEM with energy dispersive X-ray (EDAX) was used to study NixMn3-xO4 nanoparticle morphology. This examination employed Czech-made TESCAN equipment.

Synthesis of NixMn3-xO4 nanoparticles

Nickel manganite nanoparticles was synthesized following a co-precipitation procedure, as previously reported, with certain modifications implemented (Li et al. 2011). By dissolving manganese acetate and nickel chloride to a mole ratio of 2:1, a mixed aqueous solution was prepared. Then, the solution thoroughly mixed by magnetic stirrer and heated to 70 °C for 5 min. Brownish precipitation was created by adding NH3 solution (1M) until the solution pH became about 8.5. Once the product underwent multiple washes using water and ethanol, it was then subjected to a drying process for a duration of 24 h at a temperature of 70 °C. The final product (NixMn3-xO4 nanoparticles) was produced by calcination of resulting sample at 200 °C for 2h.

Preparation of modified and unmodified CPE

Graphite and paraffin oil (binder) were mixed 80/20 (w/w) to prepare the unmodified paste for CPE fabrication. The prepared paste was then inserted into the hole (2.0 mm in diameter, 3.0 mm in depth) at the end of a plastic tube, in conjunction with a copper conductor for facilitating electrical connectivity. The surface of electrode was renewed through polishing it on a soft paper. The modified CPEs were constructed by similar procedure, except that 1–5% wt. of the NixMn3-xO4 nanoparticles were mixed with graphite powder for 15 min, in the first step.

Preparation of water real sample

Several samples of water were collected from two different sources, including tap water from Tehran university, and wastewater from the Arak petrochemical industry and stored at 4 °C. Preceding the analytical procedure, aqueous samples were subjected to filtration employing filter paper with a pore size of 0.45 m. Filtered samples (1 mL) were diluted to 7 mL with PBS (0.12 mol L-1, pH = 8) for electrochemical investigation. To evaluate the suitability of modified electrodes for the analysis of 2, 4-DCP, DPV was performed under optimal conditions.

Results and discussion

Characterization of synthesized NixMn3-xO4 nanoparticles

The determination of the crystallinity and structure of the modifier was conducted through XRD analysis. The X-ray diffraction (XRD) pattern depicting the structure of NixMn3-xO4 is visually presented in Fig. 1a. The diffraction peaks were observed at the 2θ values of 19.8°, 34.43°, and 62.39° can be confidently attributed to the indexing of (1 1 1), (3 1 1), and (4 4 0) diffraction planes, respectively. These findings align with the established JCPDS no. 71–0852. The position of all the diffraction peaks reveals that NixMn3-xO4 sample has the standard single-phase spinel (face centered cubic) with space group Fd-3m. The absence of additional peaks observed in the XRD pattern suggests that the synthesised NixMn3-xO4 compound exhibits a notable degree of purity. The lack of crystallinity and weakness of these diffraction peaks indicate that NixMn3-xO4 nanoparticles display excellent electrochemical behavior because numerous loosely packed atoms are available to undergo redox reactions (Zhang et al. 2019).

Fig. 1
figure 1

a XRD pattern of NixMn3-xO4; b The FT-IR spectrum of NixMn3-xO4 NPs

Full size image

To evaluate the chemical bonding within the compound NixMn3-xO4, FT-IR is conducted within the spectral region from 4000 to 400 cm-1, as illustrated in Fig. 1b. The observed spectral feature at 3444.88 cm-1 can potentially be attributed to the vibration of O–H bending arising from the presence of H2O molecules that have become adsorbed onto the surface of the nanoparticle. Intense bands at 595.12 and 460.48 cm-1 are caused by tetrahedral Ni–O and Ni–Mn–O atom vibrations (Dhas et al. 2021). As shown by the FT-IR study, the synthesized sample contains spinel mixed oxide.

According to FESEM analysis (Fig. 2a), highly dense agglomerated spherical NixMn3-xO4 nanoparticles with sizes ranging from 9.04 to 35.59 nm are formed. It is believed that these densely packed NixMn3-xO4 nanoparticles provide porous surfaces with high specific surface areas that enhance the electrochemical performance of electrodes.

Fig. 2
figure 2

a FE-SEM images of NixMn3-xO4 NPs; b The EDAX spectrum of NixMn3-xO4 NPs

Full size image

A sample powder was analyzed by EDAX using K lines for its elemental composition. Figure 2b illustrates the occurrence of oxygen, manganese, and nickel atoms, with atomic percentages (O:Mn:Ni) of 56.66%, 25.49%, and 17.85%, correspondingly. Based on the elemental analysis, NixMn3-xO4 NPs are pure and in agreement with the XRD results.

Electrochemical characterization of the NixMn3-xO4

2,4-DCP (0.5 mM) was electrochemically studied on bare CPE and NixMn3-xO4-CPE in 0.12 mol L−1 PBS (pH 8.0) at 50 mV/s using CV. Compared to SCE, bare CPE has an reduction peak in 0.74 V (Fig. 3a). Upon conducting a sweep of the identical potential range on the NixMn3-xO4/CPE surface, a noticeable amplification in the oxidation peak current (approximately fourfold compared to the unmodified CPE) and a reduction in potential of up to approximately 0.70 V were seen. The absence of a reduction peak observed throughout the reverse scan suggests that the electrochemical reaction being studied demonstrates irreversibility. The lack of a reduction peak in the reverse scan peak suggests that the electrochemical process exhibits irreversibility. Electrochemical response NiO/CPE and Mn2O3/CPE was also examined through cyclic voltammetry. This investigation aimed to gain additional insights into the electrocatalytic performance of NixMn3-xO4 for the 2,4-DCP oxidation.

Fig. 3
figure 3

a, b Cyclic voltammetric of bare CPE and NixMn3-xO4/CPE in 0.5 mM 2, 4-DCP and 0.12 M PBS (pH: 8); c CV of Mn2O3/CPE in in 0.5 mM 2, 4-DCP and 0.12 M PBS (pH: 8); d CV of NiO/CPE in 0.5 mM 2, 4-DCP and 0.12 M PBS

Full size image

The CVs depicted in Fig. 3b indicate that Mn2O3/CPE exhibits negligible electrocatalytic activity towards 2, 4-DCP. This is evident from the weak and indistinct oxidation signal observed. Nevertheless, an observable oxidation peak related to the redox process of Ni2+/Ni3+ was detected in the Nickel Oxide/Carbon Paste Electrode (NiO/CPE) in close proximity to 2, 4-DCP oxidation peak (Fig. 3c). The NixMn3-xO4/CPE electrocatalytic properties are mostly controlled by Ni2 + /Ni3 + as shown by an increase in oxidation peak current on NiO/CPE. Comparing cyclic voltammograms of NiO and NixMn3-xO4 shows that nickel introduction increased the 2,4-DCP voltammetric signal significantly. It can be calculated from the CVs that the 2, 4-DCP oxidation signal enhancement factor is approximately 4 for NixMn3-xO4/CPE and 1.5 for NiO/CPE (vs. bare CPE). Manganese oxide by itself does not manifest electrocatalytic characteristics. Nevertheless, upon combination with nickel oxide as a bimetallic metal oxide, a remarkable synergistic phenomenon results, thereby developing of electrocatalytic characteristics.

Electrochemical impedance spectroscopic technique was carried out to investigate properties of electrode surface and electrolyte (Alizadeh et al. 2017). In Fig. 6, Nyquist plots are shown for CPE and NixMn3-xO4 CPE in 2, 4-DCP solution (0.1 mM) at 0.01 Hz to 100 kHz with a polarization potential of 0.7.

In the Nyquist plot of bare CPE (Fig. 3d), there is an electron transfer resistance (Rct)-dependent semicircle in the high frequency region. The Nyquist diagram was fitted with the equivalent circuits, which is shown as inset of Fig. 3d, where the Rs is related to solution resistance, CPE1 is a constant phase element and Rct is corresponded to charge transfer resistance.

The diameter of semicircle of the Nyquist plots (Rct) can be utilized to demonstrate the charge transfer rates of the electroactive material on the electrodes (Alizadeh et al. 2017). According to the diagram, charge transfer resistance of the materials is reduced from bare CPE, to NixMn3-xO4/CPE, confirming a better electrochemical performance and faster charge transfer rate of NixMn3-xO4/CPE in comparison with CPE (Fig. 4).

Fig. 4
figure 4

Nyquist plots of the bare CPE and NixMn3-xO4CPE in 2, 4-DCP solution (0.1 mM) in AC frequency region of 0.01 Hz to 100 kHz at polarization potential 0.7

Full size image

Optimization of effective parameters

Figure 5a presents the cyclic voltammetry analysis of 2, 4-DCP on NixMn3-xO4/CPE in PBS at varying pH levels ranging from 5.0 to 10.0. The electrochemical response of 2, 4-DCP was evidently influenced by the pH of the supporting electrolyte. At pH 8.0, the maximum oxidation peak was reached, but it started to decrease with increasing pH (Fig. 5b). Therefore, an optimum pH of 8 was chosen. As pH increased from 5.0 to 10.0, the oxidation peak linearly shifted toward negative values. This observation implies that protons directly participate in the electrochemical oxidation mechanism of 2, 4-DCP at the modified electrode. The experimental data presented in Fig. 5c exhibits a clear and direct relationship between the oxidation peak potentials (Epa) and the corresponding pH values. This correlation is supported by a high coefficient of determination (R2) value of 0.9942, indicating a strong linear association between these variables. The fact that the measured slope of 59.6 mV/pH is so near to the predicted Nernst value of 59 mV/pH shows that proton and electron transfer numbers during electrochemical reactions are equal (Scheme 1) (Ahangari et al. 2021).

Fig. 5
figure 5

a CVs of 1.0 × 10−4 mol L−1 2, 4-DCP on NixMn3-xO4/CPE in 0.12 mol L−1 PBS adjusted at selected pH values at scan rate 50 mV s−1; b variation of oxidation peak current for NixMn3-xO4/CPE with all pH values studied here; c The effect of pH on the oxidation peak potential of 2, 4-DCP at NixMn3-xO4/CPE

Full size image

The sensitivity of NixMn3-xO4/CPE depends on the NixMn3-xO4 quantity, utilized to fabricate the modified CPE. To achieve an optimal amount of NixMn3-xO4, the amount of NixMn3-xO4 in the electrode composition was changed from 1 to 9 mg. A higher NixMn3-xO4 content made 2,4-DCP adsorb more efficiently on the electrode surface, resulting in enhanced anodic current (Fig. 5a). In contrast, the background current increased when NixMn3-xO4 exceeded 3%. To further investigate the electrode's performance, 72% graphite powder, 25% paraffin and 3% modifier were selected (w/w).

A study was conducted to investigate how different amounts of binders in carbon paste electrodes affect the sensitivity of these electrodes in the determination of 2, 4-DCP. The current response exhibited a marked increase when the paraffin amount ranged from 10 to 20%, as shown in Fig. 5b. The 2, 4-DCP signal decreased as the paraffin amount increased. The conductivity of the electrodes may have been boosted by better packing of the electrode components. Due to its insulating property, excessive amounts of paraffin decreased the kinetics of electron transfer reactions. Therefore, 20% paraffin was chosen as the optimal amount.

Study of scan rate

CV studies were done at 30 to 70 mV s−1 in a buffer solution (pH 8.0) containing 1.0 × 10–4 mol L−1 2,4-DCP to study its oxidation processes and kinetics. Figure 6a shows the results obtained. Figure 6b shows that the anodic current peak (Ipa) increases linearly with scan rate. Thus, 2, 4-DCP is oxidized on NixMn3-xO4/CPE surfaces by an adsorption-controlled process. The expression for the linear regression is:

$$ {\text{I}}_{{{\text{pa}}}} = 0.{18}0{\text{5V }} + \, 0.0{77},{\text{ R}}^{{2}} = 0.{999} $$
(1)
Fig. 6
figure 6

a The effect of modifier amount on the oxidation peak current of 2, 4-DCP at NixMn3xO4/CPE; b The effect of binder amount on the 2, 4-DCP signal at NixMn3xO4/CPE

Full size image

The influence of the scan rate on the anodic potentials (Epa) was subsequently utilized to ascertain the kinetic parameters. In accordance with Fig. 6c, the anodic peak potential (Epa) changes with scan rate, indicating an irreversible electrode response. Epa(V) = 0.0167 ln(ʋ) + 0.6453 (R2 = 0.9901) illustrates the relationship between Epa and ln(v) in Fig. 7b. The relationship between EPa and scan rate for irreversible electrochemical reactions can be expressed using Laviron theory as follows (Fekry et al. 2020):

$$ E_{pa} = \, E^{0} - \, RT/ \, \left[ {1 - \alpha nF} \right] \, ln \, \left( {RTk_{s} / \, \left[ {1 - \alpha nF} \right]} \right) \, + \, RT/ \, \left[ {1 - \alpha nF} \right] \, ln() $$
(2)
Fig. 7
figure 7

a CVs of 0.1 mM 2, 4-DCP on NixMn3-xO4/CPE in phosphate buffer (pH 8.0) at various scan rates; b The linear relationship between the anodic peak current and the scan rate; c The relationship between the anodic peak potential and the logarithm of the scan rate

Full size image

In the given relationship, The symbol "n" is used to denote the quantity of electrons that have undergone transfer. The symbol "α" represents the electron transfer coefficient. The symbol "ks" corresponds to the heterogeneous rate constant of the reaction. On the other hand, the symbols "R", "F", and "T" are employed to represent the gas constant, Faraday constant, and absolute temperature, respectively. The estimation of the electron transfer number (n) was derived as 2, utilising the slope (RT/αnF) of the linear equation.

Mechanism of electroxidation of 2, 4-DCP at NixMn3-xO4/CPE was proposed regarding the experimental results described above. The first step herein, is adsorption of 2, 4-DCP at NiOOH formed according to Eqs. 3 and 4, possibly through hydrogen bonding between the hydroxyl group of 2, 4-DCP and NiOOH. Afterwards, 3, 5-dichloro-1, 2-hydroxy benzene is produced through the reaction of 2, 4-DCP with H2O and oxygen and then oxidized directly and indirectly. In indirectly oxidation process, the electron transfer, happened between 3, 5-dichloro-1, 2-hydroxy benzene and Ni3+‏, results in oxidation of 3, 5-dichloro-1, 2-hydroxy benzene and production of Ni2+‏, which can be electrochemically re-oxidized to Ni3+‏. Thus, Mn4+ ions increase NMO electrocatalytic activity for 2, 4-DCP oxidation. In addition to the existence of Mn4 + within NixMn3-xO4, these ions can also be acquired through the conversion of Mn (IӀ, ӀӀӀ) to Mn (ӀV). A proposed mechanism for the oxidation of 2, 4-DCP is outlined in the following steps:

$$ {\text{NiO }} + {\text{ H}}_{{2}} {\text{O}} \leftrightarrow {\text{Ni}}\left( {{\text{OH}}} \right)_{{2}} $$
(3)
$$ {\text{Ni}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ OH}}^{ - } \leftrightarrow {\text{NiOOH }} + {\text{ H}}^{ + } + {\text{ e}}^{ - } $$
(4)
$$ {2},{ 4} - {\text{DCP}}\mathop{\longrightarrow}\limits^{{\text{NiOOH, adsorption}}} {2},{ 4} - {\text{DCP}}_{{{\text{ads}}}} $$
(5)
$$ {2},{ 4} - {\text{DCP}}_{{{\text{ads}}}} \mathop{\longrightarrow}\limits^{{{\text{O}}_{2} {\text{,H}}_{2} {\text{O}}}}\left( {{3},{ 5} - {\text{dichloro}} - {1},{ 2} - {\text{hydroxy benzene}}} \right)_{{{\text{ads}}}} $$
(6)

Indirect oxidation:

$$ \left( {{3},{5} - {\text{dichloro}} - {1},{2} - {\text{hydroxy benzene}}} \right)_{{{\text{ads}}}} + {\text{ NiOOH}} \to \left( {{3},{ 5} - {\text{ dichloro}} - {1},{ 2} - {\text{benzoquinone}}} \right)_{{{\text{ads}}}} + {\text{NiO}} $$
(7)

Direct oxidation:

$$ \left( {{3},{5} - {\text{dichloro}} - {1},{2} - {\text{hydroxy benzene}}} \right)_{{{\text{ads}}}} \to \left( {{3},{ 5} - {\text{ dichloro}} - {1},{ 2} - {\text{benzoquinone}}} \right)_{{{\text{ads}}}} + {\text{ 2H}}^{ + } + {\text{ 2e}}^{ - } $$
(8)

Chronoamperometry study

2,4-DCP was measured chronoamperometrically on a NMO/CPE at 0.70 V. 2,4-DCP concentrations were varied in 0.12 M PBS at a pH of 8.0. This experimental setup is shown in Fig. 7a. Chronoamperometry can be utilized to evaluate the catalytic rate constant, kcat, for the 2, 4-DCP and NixMn3-xO4-CPE reaction, using the method expressed by Galus (Fekry et al. 2020):

$$ I_{Cat} /I_{d} = \, \pi^{1/2} \left( {k_{cat} Ct} \right)^{1/2} $$
(9)

The catalytic anodic current, Icat, is measured at NixMn3-xO4-CPE, while the limited current, Id, is measured in absence of 2,4-DCP. As illustrated in the graph, the slope of the relation between Icat/Id and t1/2 determined Kcat. The rate constant for 2,4-DCP at 1.0 × 10–5 mol L−1 was 1.13 × 104 mol−1 L−2 s−1, according to Fig. 7b. This high kcat value suggests 2,4-DCP interacts quickly with the NMO/CPE surface.

Active surface area of the electrodes

Figure 8a and b display the cyclic voltammetry measurements of NMO/CPE and CPE surface areas in a ferricyanide redox probe solution containing 0.1 mol L−1 KCl. The peak current of a reversible process is determined by the electroactive surface area of the working electrode, as described by the Sevcik-Randles Equation (Alizadeh et al. 2019):

$$ Ip \, = \, 2.69 \, \times \, 10^{5} \times \, n^{3/2} \times \, A \, \times \, D^{1/2} \times \, C \, \times \, \nu^{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} } $$
(10)
Fig. 8
figure 8

a Chronoamperograms at NixMn3-xO4/CPE for increasing concentrations of 2, 4-DCP (up to 1.0 × 10−3 mol L-1); b Plot of Icat/Id versus t1/2

Full size image

Ip is the current peak (A), n is the number of electrons, V is the scan rate (V/s), D is the diffusion coefficient of K3 [Fe(CN)6] (cm2s−1), A is the electrode surface area (cm2), and C is the K3 [Fe(CN)6] molar concentration. The electroactive surfaces of CPE and NixMn3-xO4-CPE were calculated to be 4.04 × 10–4 cm2 and 6.73 × 10–4 cm2, respectively, from the slope of Ip versus the square root of the scan rate (n = 1 and D = 7.6 × 10–6 cm2s-1), indicating that the electroactive surface of the modified electrode was increased by NixMn3-xO4 NPs.

Analytical performance

The study examined the analytical performance of the enhanced electrode under optimal experimental conditions. In the study, the linear range, LQ, LOD, and RSDs were evaluated. Using DPV, a calibration curve was generated using NMO/CPE in 0.12 mol L−1 of PBS (pH 8.0) and 2, 4-DCP concentrations ranging from 0.050 to 500 mol L−1. Figure 9a displays the results. As shown, the concentration of 2, 4-DCP increases the anodic peak current. Figure 9b displays the peak current of 2, 4-DCP oxidation at various concentrations. 0.9987 correlation coefficient demonstrates linearity. LOD and LOQ were calculated using the formula k Sb/m, where k = 10 for LOQ and 3 for LOD, Sb is the standard deviation of the blank solution (n = 5), and m is the slope of the calibration graph for 2, 4-DCP [45]. In this study, the LOQ and LOD were 0.29 mmol L−1 and 89 nmol L−1, respectively. Table 1 compares 2, 4-DCP analytical characteristics at a NMO/CPE to those published in the scientific literature for other electrodes. Comparatively, the modified electrode demonstrated comparable analytical performance to other methods. By performing five measurements with the same standard 2, 4-DCP solution, the repeatability of NixMn3-xO4-CPE was examined. The RSD for five samples of 2, 4-DCP (5.0 mmol L-1) was 3.2%. Five independently modified electrodes were used to determine the repeatability of an electrode by measuring 0.5 mmol L-1 2, 4-DCP. The RSD was calculated to be 3.5%. Based on the experimental results, the proposed modified electrode has satisfactory figures of merit for determining 2, 4-DCP. In addition, the anodic peak current decreased by approximately 1.5% after two weeks, 3.4% after four weeks, and 7.4% after three months. During this period of storage, the NixMn3-xO4/CPE has proved to be stable (Fig. 10).

Fig. 9
figure 9

a, b CV responses of NixMn3-xO4/CPE and CPE in 0.1 mol/L KCl solution containing 1 mmol/L [Fe(CN)6]3−/[Fe(CN)6]4− at various scan rates, respectively; c, d plot of anodic peak current vs. square root of scan rate, respectively

Full size image
Table 1 Comparison of electrochemical performance of NiMn2O4 electrode with literature
Full size table
Fig. 10
figure 10

(a) DP voltammograms of 2, 4-DCP at modified CPE at different concentrations of 2, 4-DCP; b linear relationship of oxidation peak currents vs. concentration of 2, 4-DCP

Full size image

Interference studies

To evaluate the modified electrode's selectivity, many possible interfering chemicals were examined in PBS (pH 8.0) with 10 mol L−1 2, 4-DCP. Fe3+, Cl−, NO2−, Mn2+, Cu2+, and Zn2+ were found to have no notable effect on 2, 4-DCP's anodic current with a 50-fold molar excess. Similarly, tenfold molar excesses of 4-nitrophenol, 2-chlorophenol, and catechol did not have any significant effect (Table 2). Based on the results, the suggested approach is very selective and can detect 2,4-DCP in real samples by voltammetry.

Table 2 Effect of some possible interfering species on the 2, 4-DCP (10 µmol L−1) determination at NixMn3-xO4/CPE under optimum conditions
Full size table

Real sample analysis

NMO/CPE was used to determine the concentration of 2, 4-DCP in samples of contaminated drinking water and oil effluent. 2, 4-DCP was added to pretreated water in a precise amount. Under optimum conditions, the material was evaluated. From 97.3 to 105.5% of 2, 4-DCP was recovered using the quantification method, demonstrating its accuracy. Using HPLC as the standard method, the electrode's reliability was evaluated. Table 3 demonstrates the method's ability to measure 2, 4-DCP in real samples.

Table 3 Determination results of 2, 4-DCP in water samples using NixMn3-xO4/CPE
Full size table

Conclusion

An eco-friendly and straightforward method was used to fabricate a novel electrochemical sensor. The NMO/CPE exhibits a high level of electrocatalytic activity in the oxidation of 2,4-DCP. This can be attributed to the exceptional conductivity and large surface area of NixMn3-xO4. The method described here offers several advantages compared to previously reported electrochemical techniques. These advantages include simplicity, low cost, low detection limit, wide linear range, high selectivity, acceptable stability, and reproducibility. Therefore, the study successfully demonstrated that this approach can be effectively utilized for the precise detection of 2, 4-DCP in water samples. According to this study, NixMn3-xO4 nanoparticles show promise as a novel material for applications in electroanalytical chemistry.