Simultaneous electrochemical detection of Cu, Cd and Mn by nickel ferrite–cobalt ferrite bimetallic nanocomposite

Abstract

The current study focuses on the synthesis of nanomaterials for fabrication of electrochemical sensors for the simultaneous detection of toxic metals, such as Cd, Cu, and Mn. Nickel and cobalt ferrite nanoparticles were synthesized by the sol–gel method and then combined by the green method to obtain a nickel ferrite–cobalt ferrite nanocomposite (NiFe₂O₄·CoFe₂O₄-NC) for electrochemical sensing of Cd, Cu and Mn. The characterization of the synthesized NiFe₂O₄·CoFe₂O₄-NC confirmed its coral-like shape with a highly rough and porous structure, cubic-spinel phase, crystalline size (<23.1 nm), good stability and electronic properties. NiFe₂O₄.CoFe₂O₄-NC/GCE was fabricated and showed good sensitivity due to its large electroactive surface area, fast electron transfer rate, and powerful electrocatalytic activity, which were beneficial for the quantitative determination of Cd, Cu and Mn. The NiFe₂O₄·CoFe₂O₄-NC/GCE showed the limit of detection of 0.083, 0.188 and 0.026 µg L⁻¹ for Cd, Cu and Mn, respectively. The proposed sensor was successfully applied for the analysis of Cd, Cu and Mn in tap water, mango juice, and milk samples, which confirmed its excellent efficiency with RSD < 4.0%.

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1 Introduction

Environmental pollution is increasing every day due to the release of several substances, including toxic metals (TMs), dyes, pesticides, pharmaceuticals, and other chemical residues that contaminate the environment.^1,2 Among them, the study of TMs is quite important for food safety, environmental protection, and human health. The study of TM pollution also holds importance in the field of agriculture and industries for controlling environmental pollution.³ Many TMs, such as cadmium (Cd), copper (Cu), and manganese (Mn), have a specific role in many aspects of the environment and life. However, their excess levels can harm human health and the environment. For example, Cd has several industrial applications, notably in batteries, pigments, and as a coating, but its improper disposal in the environment can damage the lungs, liver, induce osteotoxicity and nephrotoxicity, pulmonary adenocarcinomas, prostatic proliferative lesions, pulmonary adenocarcinomas and disturb the immune system of the body.^4–6 Furthermore, trace levels of Cu play a significant role in the physiological function of the human body. Still, little excess intake of Cu can have unfavorable effects on humans, such as disrupting the intestine, liver and stomach,^7,8 and if swallowed in higher concentrations, can lead to skin cancer, dermal lesions, angiosarcoma, peripheral neuropathy, and vascular disease.⁹ Moreover, Mn plays an important role in the metabolism of carbohydrates, cholesterol, and amino acids, and higher concentrations can cause numerous diseases such as male infertility, neurological disorders, birth disability, and bone defects.¹⁰

Several methods have been reported up to now for detecting these TMs, including complexometric titration,¹¹ chromatography,¹² chemiluminescence,¹³ and spectrophotometry.¹⁴ But the sophisticated procedures, time-intensive nature, and expense of these methods limit their suitability for routine analysis. Fortunately, voltammetry is a simple, fast, and low-cost alternative to other reported methods.^15–17 It stands out due to its ultra-high sensitivity, simultaneous detection of multiple metals, preconcentration capability, and improved selectivity for analysis of TMs. The stripping mode of voltammetry is mainly beneficial for analyzing trace and ultra-trace metals. Despite these benefits, the voltammetric techniques may have limitations such as unstable response in complex matrices, no response for electrochemically inactive species, or thermodynamically predicted to undergo heterogeneous redox. These limitations are subject to the analysis of electrolytes that may be restricted to the aqueous medium and inappropriate potential ranges for targeted analytes.¹⁸ The selection of Cd, Cu, and Mn in the present work is not only due to their environmental and health relevance but also because these metals exhibit sufficiently distinct redox potentials, allowing clear and non-overlapping voltammetric peaks. This inherent electrochemical separation makes them particularly suitable for simultaneous detection using modified electrodes. However, despite its advantages, voltammetric analysis can encounter challenges, such as unstable responses in complex matrices, limited applicability for electrochemically inactive species, and constraints arising from electrolyte composition or limited potential windows.¹⁸

Further, the chemical modification of these electrodes has been demonstrated to be the most effective for trace-level metal measurement due to their sensitivity, selectivity, stability, and versatility. The chemically modified electrodes (CMEs) offer lower detection limits, extended operational lifespans and greater complexity handling. These advantages make CMEs valuable for environmental monitoring, pharmaceutical testing, and industrial quality control applications. For the development of CME, a thin film of desired chemicals is coated onto the electrode surface to provide the required film properties. Various materials are employed as chemical modifiers to enhance the sensitivity of electrodes, such as gold nanoparticles (Au-NPs),¹⁹ sodium ferrite nanoparticles (Na₂Fe₄O₇-NPs),²⁰ magnesium nanoferrites (MgFe₄O₇-NPs),²¹ and copper oxide/tin oxide nanoparticles (CuO/SnO₂-NPs).²² Although several materials such as graphene, carbon nanomaterials, and biopolymers (e.g., chitosan) are commonly used to enhance charge-transfer properties of sensor surfaces, ferrite-based nanocomposites offer additional benefits that justify their use in this work. Ferrite nanoparticles possess tunable structure, high porosity, large surface area, chemical stability, and strong electrocatalytic activity. When combined to form ferrite nanocomposites (FNCs), the synergistic interaction between multiple ferrite phases further enhances electron-transfer kinetics, increases the density of active sites, and improves selective binding toward metal ions. These combined features provide higher sensitivity, better peak resolution, and improved operational stability compared to many single-component modifiers. Therefore, using NiFe₂O₄·CoFe₂O₄ nanocomposite enables superior electrochemical performance for simultaneous detection of Cu, Cd, and Mn at trace levels.^20–22

Ferrite based nanoparticles (FNPs) are ideal and superior for a wide range of applications due to their cost-effectiveness, easy synthesis, high sensitivity, and selectivity. FNPs are emerging nanomaterials that have been broadly employed due to their easy modification, tunable structure, high porosity, and large surface area.²¹ To further enhance the electrochemical performance, these FNPs are combined with various materials to synthesize their composites. The outstanding electrocatalytic performance and high specific surface area of FNCs make them desirable for applications in absorbents, biomedicine, electronic devices, and sensors.²¹ As, ZnFe₂O₄/SPE sensor have applied for excellent electrochemical sensing of paracetamol.²³ Different transition FNPs (MFe₂O₄, M = Cu, Co, Ni, Zn) integrated into a polyaniline/Nafion polymer matrix with immobilized enzyme demonstrated significantly enhance biosensor response compared to bare polymer systems indicating enhanced electron transfer and favorable surface interactions provided by ferrite FNPs.²⁴ The FNPs are attractive for electrochemical sensors and biosensors because of their high surface to volume ratio, chemical stability, tunable surface chemistry, and ability to facilitate adsorption and enrichment of analytes at the electrode interface.^25,26

Thus, the current study was considered to synthesize a nickel–cobalt ferrite nanocomposite (NiFe₂O₄·CoFe₂O₄-NC). The NiFe₂O₄·CoFe₂O₄-NC was thoroughly characterized by advanced analytical techniques to which confirm coral-like shape with a highly rough and porous structure, cubic crystalline structure with particle size of 53.7 nm. The synthesized NiFe₂O₄·CoFe₂O₄-NC was coated on the surface of GCE to fabricate electrochemical sensor (NiFe₂O₄·CoFe₂O₄-NC/GCE) for simultaneous detection of TMs, including Cu, Cd, and Mn in tap water and milk samples. The method showed acceptable performance with excellent stability and reproducibility. In this study, the target analytes were Cu, Cd, and Mn ions, which represent the most stable and soluble oxidation states of copper, cadmium, and manganese in aqueous media under the experimental conditions used.

2 Experimental work

2.1 Chemicals

The analytical grade chemicals were accessed from Fluka (Bush, Switzerland), including the iron nitrate Fe(NO₃)₃·9H₂O, nickel nitrate Ni(NO₃)₂, ethanol (C₂H₅OH), potassium chloride (KCl) and Nafion for experimental work. Phosphoric acid (H₃PO₄), sodium hydroxide (NaOH), boric acid (H₃BO₃), di-sodium hydrogen phosphate (Na₂HPO₄), sodium phosphate monobasic dehydrate (NaH₂PO₄·2H₂O), sodium acetate (CH₃CHOONa), copper chloride (CuCl₂), cadmium chloride (CdCl₂), manganese chloride (MnCl₂), potassium ferricyanide; K₃[Fe(CN)₆], acetic acid(CH₃COOH), hydrochloric acid (HCl) were obtained from Sigma-Aldrich, USA. Deionized (DI) water was obtained from the plant of the Centre of Excellence in Analytical Chemistry, Jamshoro, Pakistan.

2.2 Synthesis of ferrite (NiFe₂O₄ and CoFe₂O₄) nanoparticles

Ferrite nanoparticles (NiFe₂O₄ and CoFe₂O₄) were synthesized with slight modification using sol–gel auto-combustion technique.²⁷ For the preparation of NiFe₂O₄ nanoparticles, appropriate amount of Fe(NO₃)₃·9H₂O and Ni(NO₃)₂ were taken in 500 mL water and ascorbic acid is added as a fuel agent. The pH of the mixture was adjusted between 8 and 11 using ammonium hydroxide. The resulted mixture was heated on an electric hotplate at 250 °C until water is evaporated and fine powder of NiFe₂O₄ is obtained. The obtained product was finely ground and calcined at 900 °C for 5 hours to achieve a well crystallized NiFe₂O₄ phase. The yield of NiFe₂O₄ is obtained 81.64%.

Similarly, CoFe₂O₄ nanoparticles were synthesized using the stoichiometric ratio of Fe(NO₃)₃·9H₂O and Co(NO₃)₂ in a 500 mL beaker. The solution was heated at 250 °C on an electric hotplate to yield a brownish powder, which was then calcined at 800 °C for 5 hours to obtain phase CoFe₂O₄ and the yield of CoFe₂O₄ is obtained 83.19%. The synthesized NiFe₂O₄ and CoFe₂O₄ nanoparticles were used to synthesize NiFe₂O₄·CoFe₂O₄-NC.

2.3 Synthesis of NiFe₂O₄·CoFe₂O₄-NC

The NiFe₂O₄·CoFe₂O₄-NC was synthesized by mixing of NiFe₂O₄ and CoFe₂O₄ in a 2 : 1 ratio respectively. The obtained mixture was thoroughly ground using a mortar and pestle for 15 hours, followed by calcination in a muffle furnace at 900 °C for 5 hours.^27,28 The prepared NiFe₂O₄·CoFe₂O₄-NC was then characterized using advanced spectroscopic techniques and utilized for the fabrication of an electrochemical sensor.

2.4 Modification of NiFe₂O₄·CoFe₂O₄-NC based glassy carbon electrode (NiFe₂O₄·CoFe₂O₄-NC/GCE)

The GCE is polished with 0.05 µm alumina slurry, sonicated in DI water for 10 minutes, then rinsed with ethanol.²⁹ For the fabrication of NiFe₂O₄·CoFe₂O₄-NC/GCE, a suspension was prepared by dispersing 2 mg of NiFe₂O₄·CoFe₂O₄-NC in 2 mL of DI water containing 50 µL of Nafion, and sonicated for 15 min. Subsequently, 10 µL of the prepared suspension was drop cast onto the surface of the GCE and dried overnight at room temperature. This process resulted in the formation of a stable, uniformly immobilized layer of NiFe₂O₄·CoFe₂O₄-NC on the surface of GCE.³⁰

2.5 Electrochemical measurements

NiFe₂O₄·CoFe₂O₄-NC/GCE is employed for the electrochemical determination of Cu, Cd and Mn using differential pulse stripping voltammetry (DPSV). The voltammograms were probed using a cell and three electrodes. Ag/AgCl/KCl (2.0 mol L⁻¹), NiFe₂O₄·CoFe₂O₄-NC/GCE, and a Pt wire were used as the reference, working, and auxiliary electrodes, respectively. During the analysis, the target metal ions were pre-concentrated at a potential of −1.6 V for 2.0 min, followed by anodic stripping within the potential range of −0.8 to 0.0 V. The DPSV measurements were conducted under optimized conditions, with pulse amplitude of 0.05 V, pulse period of 0.5 s, pulse width of 0.05 s, and sampling time of 0.0167 s.

2.6 Instrumentation

The NiFe₂O₄·CoFe₂O₄-NC is characterized using various analytical techniques to determine its structural, morphological and electrochemical properties. X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses were carried out using D-8 and Nano-Scope V instruments (Bruker) to evaluate the crystal structure and particle size distribution. The surface morphology and elemental composition were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) with JEOL JSM-7600F (Japan).

Differential pulse stripping voltammetry (DPSV) was conducted using a CHI820D electrochemical analyzer (Austin, USA), employing a conventional three electrode configuration consisting of an Ag/AgCl reference electrode, platinum (Pt) wire counter electrode, and a NiFe₂O₄·CoFe₂O₄-NC/GCE working electrode. The zeta potential (ZP) and hydrodynamic size of the NiFe₂O₄·CoFe₂O₄-NC is determined using a zeta potential analyzer and a dynamic light scattering (DLS) system (ELSZ-2000).

3 Results and discussion

3.1 Characterization

The XRD technique was used to study the structure, size and crystallinity of synthesized materials, and the results are shown in Fig. 1a. In the XRD spectrum of NiFe₂O₄-NPs, the most powerful diffraction peak at 2Θ = 35.6 and plane spacing d = 2.494 Å corresponds to the (311) plane of NiFe₂O₄-NPs.³¹ According to the standard of JCPDS (Card No. 10-0325), the NiFe₂O₄-NPs can be categorized as spinel and face-centred cubic (fcc) structures.³² No diffraction peaks of other impurities are perceived. The Debye–Scherrer equation was useful in defining the average size of synthesized NiFe₂O₄-NPs and the average size was found to be 11.21 nm.³³ The sharpness of peaks shows a higher degree of crystallinity, and the % crystallinity of NiFe₂O₄-NPs was found to be 77%.²¹ Similarly, the XRD patterns of CoFe₂O₄-NPs show that all the peaks relate to the characteristic peaks of the cubic spinel lattice of CoFe₂O₄ (JCPDS File No. 22-1086), indicating that the samples have a single-phase spinel structure. The Debye–Scherrer equation was applied to determine the average size of synthesized CoFe₂O₄-NPs and the average size was found 7.78 nm.³³ The quickness of peaks shows a higher degree of crystallinity and the % crystallinity of CoFe₂O₄-NPs was found 79%.²¹ Additional, diffraction peaks at 2Θ values of 30.7, 35.9, 37.4, 43.6, 57.6, 63.1 resemble to the crystal planes (220), (311), (222), (400), (422), (511), (440), correspondingly and one additional peak was detected as an impurity may be due to formation of α-Fe₂O₃ at 34.2 which corresponds to 104 (JCPDS No. 33-0664).³⁴ The diffraction peak patterns of NiFe₂O₄·CoFe₂O₄-NC expose that all the peaks correspond to the characteristic peaks of the spinel lattice of NiFe₂O₄·CoFe₂O₄-NC.³⁵ The Debye–Scherrer equation was useful in defining the average size of synthesized NiFe₂O₄·CoFe₂O₄-NC, and the average size was found to be 23.11 nm.³³ The sharpness of peaks shows a higher degree of crystallinity, and the % crystallinity of NiFe₂O₄·CoFe₂O₄-NC was found to be 96%.²¹

Fig. 1 (a) XRD, (b) SEM, and (c) EDX analysis of synthesized NiFe₂O₄·CoFe₂O₄-NC.

Further, the SEM analysis is carried out to study the surface morphology. The SEM results are shown in Fig. 1(b), which demonstrates the porosity of NiFe₂O₄·CoFe₂O₄-NC with a rough and uniform surface.

Moreover, the purity of synthesized NiFe₂O₄·CoFe₂O₄-NC is also studied by analyzing its elemental composition using EDX. The results of elemental composition are provided in Fig. 1 (c), which shows the presence of cobalt (Co), nickel (Ni), iron (Fe) and oxygen (O) at 6.1, 14.4, 29.3 and 50.1%, respectively. The presence of these elements confirms the successful synthesis of the NiFe₂O₄·CoFe₂O₄-NC, and it also confirms the purity of synthesized material.

The surface structure and size distribution of NiFe₂O₄·CoFe₂O₄-NC were considered using the AFM technique. The two-dimensional image of AFM showed that the NiFe₂O₄·CoFe₂O₄-NC had seemed to merge (Fig. 2a). The NiFe₂O₄·CoFe₂O₄-NC present a plane surface that may be understood to point out that it had an identical attachments without significant disturbances.³⁶ Further, the size of the NiFe₂O₄·CoFe₂O₄-NC was also studied from AFM bar graph which showed the size distribution in the range of 1.5 to 36.1 nm with an average particle size of 15.7 nm (Fig. 2b).

Fig. 2 (a) Surface topography and (b) bar graph study of NiFe₂O₄·CoFe₂O₄-NC using AFM, (c) zeta potential, and (d) hydrodynamic size of NiFe₂O₄·CoFe₂O₄-NC.

The ZP is studied to confirm surface charge of NiFe₂O₄·CoFe₂O₄-NC in aqueous solution. Typically, a ZP value >±30 mV shows repulsive forces that prevent the aggregation of the particles. Fig. 2(c) shows that the average ZP of synthesized NiFe₂O₄·CoFe₂O₄-NC is −34.53 ± 9.9 mV, which confirms excellent stability of nanocomposite. Furthermore, the average hydrodynamic size of NiFe₂O₄·CoFe₂O₄-NC is determined to be 32.0 ± 5.43 nm (Fig. 2d).

3.2 Electrochemical studies

The electrochemical performance of both of bare GCE and NiFe₂O₄·CoFe₂O₄-NC/GCE are evaluated by cyclic voltammetry (CV) in the solution [Fe(CN)₆]⁴⁻ (5.0 mM) and KCl (100 mM) as supporting electrolyte (Fig. 3a). The CV results revealed that both electrodes exhibited redox current responses, with the NiFe₂O₄·CoFe₂O₄-NC/GCE showing the highier response. This enhancement is attributed to the larger electroactive surface area of the NiFe₂O₄·CoFe₂O₄-NC/GCE.³³ Therefore, NiFe₂O₄·CoFe₂O₄-NC/GCE is selected for further electrochemical investigations.

Fig. 3 (a) Optimization of sensitivity of bare and NiFe₂O₄·CoFe₂O₄-NC/GCE and (b) scan rate study and (c) linear relationship of current vs. square root of scan rate of NiFe₂O₄·CoFe₂O₄-NC/GCE.

The electron transfer kinetics of NiFe₂O₄·CoFe₂O₄-NC/GCE is further examined through CV at varying scan rates ranging from 10 to 90 mV s⁻¹ in the same electrolyte (Fig. 3b). An increase in the scan rate resulted in a proportional rise in redox peak currents, suggesting that the electrochemical process is diffusion controlled. The higher scan rates reduce reaction time, reducing the diffusion layer and allowing a greater number of electroactive species to reach the electrode surface.^30,37 A linear relationship (with R² > 0.998) was observed between the peak current and the square root of the scan rate (Fig. 3c), confirming that the redox process at the NiFe₂O₄·CoFe₂O₄-NC/GCE is followed diffusion controlled kinetics.

After successful electrochemical characterization of NiFe₂O₄·CoFe₂O₄-NC/GCE. various experimental parameters, including supporting electrolyte, pH, deposition potential, and deposition time, were optimized to enhance electrochemical performance. The effect of different electrolytes Britton Robinson (BR) buffer, phosphate buffer (PB), and acetic acid/sodium acetate (NaAc/HAc; AC) are evaluated (Fig. 4a). Among these, the BR buffer exhibited the highest and most stable current response, therefore, it was selected as the optimal supporting electrolyte for subsequent electrochemical studies.

Fig. 4 Optimization of (a) electrolyte, (b) pH, (c) deposition potential and (d) deposition time of NiFe₂O₄·CoFe₂O₄-NC/GCE for the detection of Cd, Cu and Mn.

The effect of pH on electrochemical detection of Cd, Cu and Mn was examined using BR buffer (0.1 M) within the pH range of 2.0–8.0 (Fig. 4b). The results showed that the peak current increased with rising pH up to 4.0, after which it declined, likely due to the presence of oxygen containing functional groups on the NiFe₂O₄·CoFe₂O₄-NC surface.³⁸ At lower pH levels (pH < 4.0), NiFe₂O₄·CoFe₂O₄-NC become protonated, competing with metal ions for active binding sites and disrupting electrostatic interactions with Cd, Cu, and Mn. Conversely, the decrease in current at higher pH values (pH > 4.0) may be attributed to the hydrolysis of metal ions, reducing their availability for electrochemical interaction.^30,39 Therefore, pH 4.0 was selected as the optimal condition for subsequent analyses. Notably, the Mn²⁺ reduction peak was observed at −0.6 V, which is more negative than values commonly reported for unmodified electrodes. This shift arises from the use of the NiFe₂O₄·CoFe₂O₄-NC/GCE, which affects electron-transfer kinetics and surface adsorption properties. Additionally, the supporting electrolyte composition and pH influence the reduction potential, leading to a more cathodic peak compared to literature values obtained on bare or differently modified electrodes.

The influence of deposition potential on the simultaneous detection of Cd, Cu and Mn was examined within the range of −0.4 to −2.0 V (Fig. 4c). The highest current response for all three metals was obtained at 60 mV, indicating that this potential provided the greatest detection efficiency of the NiFe₂O₄·CoFe₂O₄-NC/GCE. Selecting a low deposition potential is crucial due to the standard reduction potentials of Cd²⁺/Cd (E° = −0.402 V), Cu²⁺/Cu (E° = +0.340 V), and Mn²⁺/Mn (E° = −1.18 V). For Cd²⁺ and Mn²⁺, which possess more negative reduction potentials, operating at −1.6 V ensures effective reduction and pre-concentration without triggering hydrogen evolution that typically occurs at more negative potentials. Although Cu²⁺ has a more positive reduction potential, efficient deposition at −1.6 V is attributed to the catalytic activity of the NiFe₂O₄·CoFe₂O₄-NC/GCE. Thus, applying −1.6 V allows efficient reduction and accumulation of Cd, Cu, and Mn while minimizing interference from hydrogen evolution.⁴⁰ Additionally, this potential enhances selectivity by favoring the reduction of the target metals over other competing ions, ensuring precise and reliable detection.⁴¹ Consequently, −1.6 V was selected as the optimal deposition potential for further electrochemical analyses.

The effect of deposition time was also evaluated in the range of 2–8 min for the simultaneous detection of Cd, Cu, and Mn. The negligible increase in current response is observed with longer deposition times. However, after 8 min, no significant improvement was observed (Fig. 4d). Therefore, a deposition time of 2 min was identified as optimal for subsequent electrochemical measurements.

The performance of the NiFe₂O₄·CoFe₂O₄-NC/GCE is evaluated for the simultaneous detection of Cd, Cu, and Mn over concentration ranges of 0.2–420 µg L⁻¹, 0.5–400 µg L⁻¹, and 0.05–500 µg L⁻¹, respectively, using differential pulse voltammetry (DPV) (Fig. 5a and b). A linear relationship was observed between the peak current response and the concentration of each metal ion. The limits of detection and quantification (LOD and LOQ) are calculated 0.083 µg L⁻¹ and 0.25 µg L⁻¹ for Cd, 0.188 µg L⁻¹ and 0.57 µg L⁻¹ for Cu, and 0.026 µg L⁻¹ and 0.08 µg L⁻¹ for Mn. The LOD and LOQ was calculated by using formulas as “3.3 (Standard deviation of intercept of calibration curve)/Slope” and “10 (Standard deviation of intercept of calibration curve)/Slope”, respectively.^42,43 These results demonstrate that the NiFe₂O₄·CoFe₂O₄-NC/GCE provides effective and simultaneous detection of Cd, Cu, and Mn, making it highly suitable for the analysis of environmental samples.

Fig. 5 (a) Calibration, (b) linear relationship between current and concentration, (c) interference and stability analysis of NiFe₂O₄·CoFe₂O₄-NC/GCE for the detection of Cd, Cu and Mn, and (d) stability of NiFe₂O₄·CoFe₂O₄-NC/GCE for simultaneous electrochemical detection of Cd, Cu, and Mn.

The selectivity of NiFe₂O₄·CoFe₂O₄-NC/GCE is evaluated by introducing eight possible interfering ions (Pb²⁺, Ag⁺, Hg²⁺, and Fe³⁺, Na⁺, Cl⁻, SO₄²⁻, Mg²⁺, Al³⁺, CO₃²⁻, and PO₄³⁻) into the analyte (Cd, Cu and Mn). These interfering ions were present at concentrations 5 times higher than those of the target metals. The results revealed that the presence of these ions had no significant impact on the simultaneous detection of Cd, Cu, and Mn (Fig. 5c). This demonstrates that the NiFe₂O₄·CoFe₂O₄-NC/GCE possesses strong selectivity and can accurately detect these metals in real samples.

The stability NiFe₂O₄·CoFe₂O₄-NC/GCE is further examined through ten repeated DPV measurements taken at different time intervals, which showed consistent current responses with a relative standard deviation (RSD) of 2.84% (Fig. 5d). These results confirm the excellent stability and reusability of the NiFe₂O₄·CoFe₂O₄-NC/GCE for simultaneous electrocatalytic detection of Cd, Cu, and Mn. Furthermore, the reproducibility of fabricated sensor was also studied by synthesizing four new electrodes by identical method. The current responses of all the newly fabricated sensors were very close to each other with RSD of less than 5%. These results indicated that the fabrication method is reproducible and can be successfully applied for the routine analysis TMS.

To validate its practical application, the developed NiFe₂O₄·CoFe₂O₄-NC/GCE based method was applied to tap water, mango juice, and milk samples collected from the local market in Jamshoro, Sindh, Pakistan. Tap water and juice samples were filtered and centrifuged to remove suspended particles. Milk sample are filtered to remove the solid impurities and analyzed by voltammetry for the detection selected metals. The concentrations of Cd, Cu, and Mn are found 6.42, 374, and 182 µg L⁻¹ in tap water, 0.217, 253, and 163 µg L⁻¹ in milk, and 5.09, 14.2, and 3.21 µg L⁻¹ in juice samples respectively. Moreover the standard addition method was employed to verify the accuracy and reliability of the developed approach, and the corresponding results are presented in Table 1. The high recovery values ranging from 92.1% to 98.8% for the simultaneous determination of Cd, Cu, and Mn, with RSD of less than 4%, demonstrate that the developed method is reliable, and suitable for real sample analysis of these metals.

Table 1 Simultaneous determination of Cd, Cu and Mn in real samples by developed electrochemical method

Sample	Analyte	Added (µg L^−1)	Found (µg L^−1)	Recovery (%)	RSD (%)
Tap water	Cd	0.00	6.42	—	1.56
		100	97.1	97.1	1.64
		200	197	98.8	1.52
	Cu	0.00	374	—	2.07
		100	96.4	96.4	1.72
		200	195	97.7	1.46
	Mn	0.00	182	—	1.69
		100	96.8	96.8	1.53
		200	196	97.9	1.44
Milk	Cd	0.00	0.217	—	2.71
		100	92.8	92.8	3.24
		200	184	92.1	2.48
	Cu	0.00	253	—	2.91
		100	96.7	96.7	2.46
		200	192	95.7	1.93
	Mn	0.00	163	—	2.68
		100	96.6	96.6	2.93
		200	192	96.2	2.71
Mango juice	Cd	0.00	5.09	—	1.97
		100	94.9	94.9	3.02
		200	196	98.1	2.76
	Cu	0.00	14.2	—	2.38
		100	95.7	95.7	2.34
		200	197.5	98.8	2.95
	Mn	0.00	3.21	—	3.42
		100	96.9	96.9	2.62
		200	197	98.6	2.19

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The analytical performance of the proposed voltammetric method using NiFe₂O₄·CoFe₂O₄-NC/GCE for detecting Cd, Cu, and Mn was compared with previously reported techniques. The comparison (Table 2) reveals that the NiFe₂O₄·CoFe₂O₄-NC/GCE based method offers a wider concentration range and lower detection limits than those achieved in earlier studies, demonstrating its superior analytical efficiency. Improved electron transfer efficiency reduces the charge transfer resistance during the redox reactions of Cd, Cu, and Mn, thereby enhancing the electrode's catalytic performance. The outstanding electrochemical behavior of NiFe₂O₄·CoFe₂O₄-NC/GCE can also be attributed to the effective synthesis of the nanocomposite. Comparison with previously reported Cd, Cu, and Mn detection methods further validates the reliability and effectiveness of the developed approach.

Table 2 Comparative study of current work with reported methods for the detection of Cd, Cu and Mn^a

Electrodes	Technique	Analyte	Linear range (µg L^−1)	LOD (µg L^−1)	References
a PPE/GCE; 1-phenyl-N-(pyridin-2-ylmethyl)ethanamine based glassy carbon electrode, Bi@BAC/CPE; bismuth/biomass derived activated carbon, PPy/graphene/Au; porous graphene oxide-polypyrrole (pGO/PPy) polymer nanocomposite, AuNPs/SPGE; screen-printed gold electrode with gold nanoparticles, Au SPGE-LT; low temperature gold-cured screen-printed electrode, AuNPs SPCE arrays; gold nanoparticles screen-printed carbon electrode arrays, edge plane pyrolytic graphite electrode; edge plane pyrolytic graphite electrode, MBPCE; modified bentonite porphyrin carbon paste electrode, MBPCE; modified bentonite–porphyrin carbon paste electrode, MCFCNT; metal catalyst free carbon nanotube.
PPE/GCE	SWASV	Cd	2.0–100.0	0.8	44
Bi@BAC/CPE	DPASV	Cd	4.5–446.4	0.7	45
PPy/graphene/Au	SWASV	Cd	8.9–89.3	0.45	46
AuNPs/SPGE	DPASV	Cu	20 – 300	1.6	47
Au SPGE-LT	SWASV	Cu	5–300	1.5	48
AuNPs SPCE arrays	SWASV	Cu	10–150	1.4	49
Edge plane pyrolytic graphite electrode	SWASV	Mn	0.62–174	0.3	50
Modified bentonite–porphyrin carbon paste electrode	SWASV	Mn	0.6–500	0.1	51
Metal catalyst free carbon nanotube (MCFCNT)	SWASV	Mn	0.6–6.7	0.093	52
NiFe2O4·CoFe2O4/GCE	DPV	Cd	0.05–500	0.083	Current study
		Cu	0.2–400	0.188
		Mn	0.5–400	0.026

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4 Conclusion

The NiFe₂O₄·CoFe₂O₄ nanocomposite was employed for the determination of Cd, Cu and Mn based on the modified GCE. Incorporating NiFe₂O₄·CoFe₂O₄-NC onto the GCE significantly enhanced the oxidation peak current, sensitivity, and sharpness while reducing the overpotential for Cd, Cu, and Mn oxidation. These improvements can be attributed to the strong and efficient interaction between the analytes and the metallic sites on the NiFe₂O₄·CoFe₂O₄-NC surface. The large active surface area, excellent adsorption capacity, and selective interaction properties of the nanocomposite contribute to its high detection efficiency. Compared to previously reported sensors, the developed electrode offers several advantages, including an exceptionally low detection limit, broad linear range, high sensitivity, strong stability, excellent repeatability and reproducibility, and a straightforward modification process for detecting Cd, Cu, and Mn in tap water, mango juice, and milk samples.

Ethical statement

The current experimental investigations do not contain any kind of involvement either direct or indirect of human contributors and animals.

Author contributions

Hassan Imran Afridi and Jameel Ahmed Baig; conceptualization, supervision, writing and editing, formal analysis, methodology, and resources. Zahid Ali, Khalil Akhtar, Saima Perveen, and Nadeem Raza; methodology, writing, editing, visualization.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Access to the data assisting with the current research project can be granted upon formal request.

Acknowledgements

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502).

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