Introduction

EZO is a rare-earth perovskite oxide that has attracted increasing attention due to its unusual combination of structural stability, electronic tunability, and optical activity1,2,3,4,5. The zirconium-based framework provides high thermal and chemical stability as well as the ability to host oxygen vacancies, which is advantageous for applications such as solid oxide fuel cells, catalytic converters, and gas sensors6,7,8,9. At the same time, the presence of europium ions introduces distinctive optical and magnetic features; in particular, Eu³⁺ imparts strong red luminescence, making the material useful in photonic devices, phosphors, and biomedical imaging10. In addition, its dielectric and magnetic properties enable potential use in spintronics, memory devices, and multifunctional sensors. This unique convergence of chemical robustness, redox activity, and functional versatility positions EZO as a promising candidate for next-generation technologies in the fields of energy, environment, and advanced electronics.

Biosensor technology offers a promising alternative by enabling fast, sensitive, and portable detection of PA using biologically based recognition systems integrated with physical or chemical transducers11,12,13. Recent advancements in nanotechnology have played a significant role in enhancing biosensor performance14. Nanoparticles (NPs) such as graphene, carbon nanotubes, and metal NPs increase the surface area and conductivity of sensor platforms, allowing for better interaction with biomolecules and improving signal detection at even very low concentrations15,16. Biosensors tailored for PA detection especially electrochemical types can provide real-time results with high specificity and are increasingly being designed for point-of-care diagnostics and environmental monitoring17,18,19. These innovations are driving the development of next-generation sensing tools that are not only efficient and scalable but also crucial in addressing both public health and environmental challenges. Electrochemical sensing platforms, such as the EuZrO₃-modified carbon paste electrode, show great potential in dentistry for the rapid detection of pharmaceutical residues and biomarkers linked to oral and periodontal diseases. By identifying pathogenic metabolites or drug traces in saliva early on, dentists can provide timely diagnoses, create personalized treatment plans, and monitor the effectiveness of treatments, ultimately improving dental care.

Pharmaceutical contaminants, arising from the widespread use and improper disposal of drugs, are increasingly detected in the environment. These residues can pose serious health and ecological risks, including bioaccumulation, endocrine disruption, antimicrobial resistance, and toxicity to aquatic organisms. Among these contaminants, PA is widely used and frequently detected in water bodies and biological fluids, making it an important target for monitoring. The development of sensitive, selective, and real-time detection methods for PA is therefore critical to ensure environmental safety, human health, and effective pharmaceutical quality control. The growing prevalence of pharmaceutical contaminants in the environment, including residues of widely used drugs like PA, poses significant risks to aquatic ecosystems, human health, and water safety. These contaminants can induce bioaccumulation, endocrine disruption, and antimicrobial resistance, even at trace concentrations. PA is one of the most widely prescribed analgesic and antipyretic drugs, recognized for its safety profile at therapeutic doses and over-the-counter availability20,21. It is frequently used in both clinical and household settings for the treatment of fever, headaches, and mild to moderate pain22,23. Despite its benefits, PA has a narrow therapeutic window, and even modest overdoses can result in severe adverse effects24,25. The increasing consumption of PA worldwide has also led to its detection in hospital effluents, wastewater, and surface waters, raising concerns about its persistence in the environment and potential ecological risks26,27,28,29. These factors highlight the necessity of developing sensitive, reliable, and cost-effective analytical approaches for monitoring PA in pharmaceutical formulations, biological fluids, and environmental samples.

The clinical importance of PA detection is further emphasized by its well-documented hepatotoxicity when consumed in excess30,31. After ingestion, most PA undergoes glucuronidation and sulfation, while a small fraction is metabolized by cytochrome P450 enzymes to produce N-acetyl-p-benzoquinone imine (NAPQI), a highly reactive electrophilic metabolite32. Under normal physiological conditions, NAPQI is detoxified by conjugation with glutathione33,34,35. However, in overdose cases, glutathione reserves are rapidly depleted, resulting in NAPQI accumulation36. The metabolite subsequently binds covalently to cellular proteins, leading to oxidative stress, mitochondrial dysfunction, and ultimately hepatocellular necrosis37,38,39. This biochemical pathway underscores the serious health risks associated with PA overdose and highlights the need for rapid, precise, and accessible detection methods to ensure patient safety and effective clinical management.

A variety of analytical methods have been employed for the quantitative determination of PA. High-performance liquid chromatography (hplc) remains the gold standard owing to its high accuracy and reproducibility40. Other approaches, such as UV-visible spectrophotometry, chemiluminescence, and capillary electrophoresis, have also been widely reported41. While these methods provide excellent sensitivity and selectivity, they require costly instrumentation, trained personnel, and complex sample preparation steps42. Moreover, their use in point-of-care diagnostics and on-site monitoring is limited due to poor portability and longer analysis times43,44. These drawbacks make them less suitable for routine screening of PA, particularly in resource-limited laboratories and field applications, thereby creating the demand for alternative detection strategies that are simpler, faster, and more economical.

In this context, electrochemical sensing has emerged as an attractive alternative for PA detection. Electrochemical techniques offer advantages such as rapid response, low operational cost, high sensitivity, and the ability to work with small sample volumes in complex matrices. Among them, cyclic voltammetry (cv), differential pulse voltammetry (dpv), and square wave voltammetry (swv) are particularly effective due to their capacity to resolve overlapping redox signals and provide quantitative information at low analyte concentrations. The performance of electrochemical sensors can be further enhanced by electrode modification with nanostructured materials, which improve electron transfer kinetics and increase surface active sites.

This research presents EZO NPs, synthesized through solution combustion synthesis, as a novel sensing platform for detecting compounds such as PA. The solution combustion technique enables the rapid fabrication of EZO NPs with well-defined crystalline phases and a high surface area, essential for effective sensor performance. X-ray diffraction (XRD) analysis confirmed the successful formation of pure EZO, while scanning electron microscopy (SEM) revealed a uniform, porous morphology that supports efficient analyte interaction. Additionally, energy-dispersive X-ray spectroscopy (EDAX) verified the material’s elemental composition and homogeneity. These structural and morphological features contribute to the excellent catalytic and electrochemical behavior of EZO, facilitating the creation of sensitive, selective, and reliable biosensors for pharmaceutical applications, particularly in the detection of PA for environmental and clinical monitoring.

Materials and methods

Materials

High-purity starting materials were employed to ensure phase-pure EZO formation with minimal impurities. Europium(III) oxide (Eu2O3, 99.9%, Sigma-Aldrich) and zirconium(IV) oxide (ZrO2, 99.9%, Sigma-Aldrich) served as the Eu and Zr sources, respectively. Both oxides were stored in sealed desiccators prior to use to avoid moisture uptake. Due to the propensity of europium to remain in the trivalent state, the reduction to divalent Eu was achieved under carefully controlled reducing conditions. Additional reagents, including high-purity graphite powder (99.9%, Loba Chemie) and silicone oil (viscosity: 350 cSt; specific gravity: 0.97; Sigma-Aldrich), were employed in the preparation of carbon paste electrodes. These materials served as the conductive matrix and binder, respectively, facilitating uniform dispersion of active NPs and stable electrochemical performance.

Synthesis of EZO NPs

EZO NPs were synthesized via a conventional high-temperature solid-state reaction route. Stoichiometric quantities of 3.519 g of Eu2O3 and 1.232 g of ZrO2 (corresponding to a 1:1 molar ratio) were accurately weighed and thoroughly homogenized in an agate mortar and pestle for 2 h. The blended mixture was uniaxially pressed into cylindrical pellets at 5 MPa to promote solid-state diffusion during subsequent heating.

The pellets were placed in high-purity alumina crucibles and subjected to a reducing atmosphere of 97 vol% Ar and 3 vol% H2, flowing at 100 mL/min. The furnace temperature was ramped from room temperature to 1300 °C at a heating rate of 5 °C/min and maintained at this temperature for 10 h. To ensure complete reaction, the pellets were removed from the furnace every 8 h, reground to a fine powder, and re-pelletized before being returned to the furnace for further heating. After the final calcination cycle, the furnace was allowed to cool naturally to room temperature under the same reducing atmosphere. The resulting product was collected and stored in airtight containers for further characterization (Fig. 1a).

Fabrication of bare and EZO modified carbon paste electrodes

The Bare carbon paste electrode (BE1) was fabricated by mixing graphite powder with high-viscosity silicon oil in a 70:30 weight ratio to form a uniform paste. This blending process involved grinding the mixture continuously for about 30 min to ensure homogeneity and achieve the desired consistency. Once the uniform paste was obtained, it was carefully packed into a carbon-filled Teflon tube to serve as the electrode body. To create a smooth and even surface suitable for electrochemical measurements, the outer part of the Teflon tube was polished gently using a soft abrasive paper, followed by thorough cleaning with double-distilled water to remove any impurities or residues.

For the modified electrode EZO-ME1, the synthesized EZO NPs was incorporated by grinding it together with carbon powder and silicon oil, following the same 70:30 ratio and mixing procedure as the bare electrode. This mixture was then packed into a Teflon tube similarly to form the sensing surface. The addition of EZO-ME1 was aimed to enhance the electrode’s electrochemical performance by introducing catalytic properties and increasing surface area, thereby improving sensitivity and selectivity toward the target analyte. The careful polishing and cleaning steps ensured that the modified electrode had a uniform and clean surface for reliable and reproducible measurements (Fig. 1b).

Fig. 1
figure 1

(a) Schematic illustration of the solid-state synthesis process for EuZrO3 NPs. (b) Diagram representing the fabrication of the modified working electrode (EZO-ME1) used in electrochemical studies.

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Characterization

Powder X-ray diffraction (XRD) data were collected on a (Bruker D2 PHASER) diffractometer using Cu Kα radiation (λ = 1.5406 Å), operating at 40 kV and 30 mA. The scans were recorded from 20° to 80° (2θ) with a step size of 0.02°. Structural refinement was conducted using the FULLPROF suite 5.10 to extract lattice constants and confirm the space group45. Polyhedral visualization and bonding analysis were performed in VESTA 3.5.8 using Crystallographic Information Files (CIF) generated from refinement45. Morphological features were investigated by scanning electron microscopy (SEM, ZEISS EVO18), while energy-dispersive X-ray spectroscopy (EDX) was used to assess elemental composition. Figure 1 was created by the authors using open-source images from the Biorender.com image repository (https://biorender.com).

Electrochemical measurements

Electrochemical studies were carried out on a CHI660E workstation using a three-electrode system with BE1 or EZO-ME1 as the working electrode, a platinum wire counter electrode, and an Ag/AgCl (3 M KCl) reference electrode. A 1mM PA stock solution was prepared in phosphate buffer solution (PBS), and dilutions were made before use. Unless stated otherwise, measurements were performed in 0.1 M PBS (pH 7.0). cv was recorded between − 0.2 and + 0.8 V at scan rates of 100–500 mV/s, while SWV were conducted with standard parameters to enhance sensitivity.

Selectivity was examined by recording PA responses in the presence of common interferents such as dopamine (DA). Practical applicability was assessed using commercial PA tablets dissolved in PBS and analyzed by the standard addition method. The high recovery values confirmed that the EZO-ME1 electrode can reliably detect PA in real pharmaceutical samples.

Results and discussion

Structural characterization and phase purity

Figure 2a presents the XRD pattern of the synthesized EZO, which exhibits sharp and well-defined Bragg reflections indicative of a crystalline single-phase material. All prominent diffraction peaks could be indexed to an orthorhombic perovskite structure belonging to the Pnma space group (No. 62), consistent with previous literature on Eu2⁺-containing zirconates46. The absence of any secondary reflections associated with Eu2Zr2O7 pyrochlore, unreacted Eu2O3, or ZrO2 indicates complete reaction and phase-pure synthesis under the employed reducing conditions.

To evaluate the microstructural features, the crystallite size was calculated using the Scherrer Eqs47. ,

$$\:D=\frac{k\lambda\:}{\beta\:cos\theta\:}$$
(1)

where D is the crystallite size, k is the shape factor (taken as 0.9), λ is the X-ray wavelength (1.5406 Å), β is the FWHM of individual peaks in radians, and θ is the Bragg angle. Instead of relying on the most intense peak alone, FWHM values were extracted from several representative diffraction peaks to improve statistical accuracy. Crystallite sizes were calculated independently for each reflection, and the arithmetic mean of these values was determined. The resulting average crystallite size was found to be approximately 23 nm, suggesting nanoscale grain formation with reasonably uniform distribution. Such fine crystallinity is advantageous for enhanced surface reactivity and diffusion kinetics in functional applications, while also supporting the complete phase evolution and solid-state reaction efficiency of the synthesis protocol.

Further quantitative structural analysis was performed using Rietveld refinement, as shown in Fig. 2b. The refinement yielded an excellent fit between experimental and simulated patterns, with low residual values: Rwp = 2.03%, Rp = 1.82%, and goodness-of-fit χ2 = 1.65. The refined lattice parameters were found to be a = 5.8172 Å, b = 8.1805 Å, and c = 5.7833 Å, corresponding to a unit cell volume of 274.27 Å3. These values are in close agreement with previously reported data for EZO synthesized under similar conditions and affirm the successful stabilization of the Eu2⁺ oxidation state within the perovskite framework.

Polyhedral distortion and bonding environment

Detailed crystallographic insights into the local bonding environment and coordination geometry of EZO were derived from Rietveld-refined structural models. The corresponding Crystallographic Information File (CIF), generated from the refinement, was used to construct detailed visualizations using VESTA 3.5.8 software, as presented in Fig. 2c–e. The material adopts an orthorhombic perovskite structure with space group Pnma (No. 62). The refined lattice parameters (a = 5.8172 Å, b = 8.1805 Å, c = 5.7833 Å; unit cell volume, V = 275.21 Å3) closely match prior reports on Eu2+-stabilized zirconates, confirming structural reliability. The polyhedral representations clearly display the long-range periodicity and corner-sharing network of ZrO6 octahedra. Europium ions are located at the 4c Wyckoff positions (x = 0.0190, y = 0.2500, z = 0.4950) and are coordinated by seven oxygen atoms in a distorted capped trigonal prismatic geometry. The Eu-O bond distances vary between 2.79 and 3.13 Å, consistent with the ionic radius of Eu2+ in sevenfold coordination, confirming the stabilization of the divalent oxidation state under the reducing synthesis conditions. Zirconium occupies the 4a sites (0, 0, 0), forming nearly regular octahedral ZrO6 units, with Zr-O bond lengths ranging from 2.07 to 2.12 Å. Overall, the polyhedral analysis reveals a robust and distorted perovskite network with asymmetric coordination geometries that are known to influence the material’s magnetic ground state, dielectric response, and defect-mediated functionalities.

Fig. 2
figure 2

(a) XRD pattern of synthesized EuZrO3 NPs; (b) Rietveld refinement profile fitting of the EuZrO3 XRD pattern; (c) Crystal structure model showing the arrangement of Eu and Zr atoms; (d) Distorted capped trigonal prismatic coordination of Eu2+ ions; (e) Connectivity of ZrO6 tetrahedra in EuZrO3 crystal structure; (g, h) SEM images; (i) EDAX spectrum of EuZrO3 NPs.

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Morphological studies

The SEM micrographs (Fig. 2f–h) reveal a heterogeneous surface morphology consisting of loosely agglomerated particles with dimensions ranging from the submicron to low-micrometer scale. At higher magnifications, the crystallites appear irregularly shaped with pronounced surface roughness and interparticle voids. Such features are characteristic of high-temperature solid-state synthesized perovskites, where grain growth and sintering are restricted in the absence of flux agents or growth modifiers. The presence of surface asperities and open voids indicates a porous microstructure, which is expected to be beneficial for applications requiring enhanced surface reactivity and active site accessibility, such as electrocatalysis, gas sensing, and heterogeneous catalysis. Elemental analysis conducted via EDX (Fig. 2i) confirms the presence of europium, zirconium, and oxygen as the sole constituent elements of the synthesized EuZrO3. The absence of extraneous peaks or detectable contaminants supports the compositional purity of the synthesized EuZrO3 phase, which is further validated by the phase-pure XRD results.

Electrochemical analysis

In the fabrication of carbon paste electrodes, the amount of active material incorporated plays a crucial role in determining the sensor’s performance in Fig. 3. To optimize the electrode composition, weight studies were conducted by varying the quantity of the active component from 2 mg to 10 mg while keeping other parameters constant. At lower weights such as 2 mg, the electrode surface may not have sufficient active sites, resulting in lower sensitivity and weaker electrochemical signals48,49. Conversely, at higher weights above 8 mg, the excess material can lead to poor paste consistency, reduced conductivity, and slower electron transfer, which negatively affects the sensor’s responsiveness and stability.

Figure 3a shows the systematic evaluation, it was observed that 6 mg of active material provided the best balance between surface area, conductivity, and mechanical stability of the carbon paste electrode. At this optimum weight, the electrode exhibited enhanced electrochemical activity, showing stronger and more reproducible signals for the target analyte. The 6 mg composition ensured a uniform and well-dispersed active material within the carbon matrix, facilitating efficient electron transfer and better interaction with analyte molecules as shown in the Fig. 3b. Therefore, 6 mg was identified as the ideal amount for fabricating carbon paste electrodes to achieve maximum sensitivity and reliable sensor performance.

Figure 3c depicts the electrochemical active surface areas of the BE1 and EZO-ME1 were examined using cv in a solution of 0.1 mM potassium ferrocyanide and 0.1 M KCl. The potential was swept from 0 to 0.8 V at different scan rates ranging from 0.05 to 0.5 V/s represented in the Fig. 3d. It was observed that as the scan rate increased, the anodic peak current (Ipa) also increased, while the peak potential shifted slightly. The EZO-ME1 electrode showed a significantly higher peak current compared to BE1, indicating that the modification with EZO NPs improved the electrode’s conductivity and electron transfer capabilities.

The electroactive surface area of each electrode was estimated using the Randles-Ševčík equation50:

$$\:{I}_{p}=(2.69\times\:{10}^{5}){n}^{3/2}A{D}^{1/2}C{v}^{1/2}$$
(2)

where Ip is the peak current (A), n is the number of electrons transferred, A is the electroactive surface area (cm2), D is the diffusion coefficient (cm2 s− 1), C is the concentration (mol cm− 3), and v is the scan rate (V s− 1). Based on this relation, the EZO-ME1 electrode exhibited an electroactive area of 0.052 cm2, which is approximately 40% larger than that of the bare electrode (BE1, 0.032 cm2), as shown in Fig. 3e & S1.

Fig. 3
figure 3

(a) cv curves at different EZO-ME1 loadings show the effect of material weight on current response; (b) bar graph of peak current vs. loading; (c) comparison of BE1 and EZO-ME1 electrodes indicates enhanced activity; (d) linear Ipa vs. scan rate plot for surface area estimation at EZO-ME1; (e) bar graph showing higher current for EZO-ME1 than BE1.

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The increase in surface area is attributed to the incorporation of EZO, which provides a greater number of electroactive sites and improves surface texture, resulting in more efficient electron transfer. Consequently, the modified electrode demonstrates superior charge-transfer characteristics and enhanced electrocatalytic activity, leading to a more sensitive and stable response toward paracetamol detection. The sensitivity values were further normalized to the electroactive surface area to accurately represent the intrinsic performance of the EZO-ME1 sensor.

pH plays a critical role in influencing both the sensitivity and reaction pathways of biomolecules during electrochemical detection, as illustrated in Fig. 4. Figure 4a shows the cv of 1 mM PA at various pH values ranging from 4.0 to 9.0, recorded within a potential window of -0.2 to 0.8 V at a scan rate of 0.1 V s− 1. Analysis of the anodic peak current (Ipa) versus pH Fig. 4b indicates that the maximum current occurs at pH 7.0, which was therefore selected as the optimal pH for all subsequent electrochemical experiments. This optimum corresponds to the deprotonation process, where proton transfer is involved in the redox reaction, leading to a pronounced peak current.

Figure 4c illustrates the linear relationship between anodic peak potential (Epa) and pH, showing a negative shift in Epa as pH increases from 4.0 to 9.051. The slope of the plot is approximately − 0.058 V/pH, closely matching the theoretical Nernstian slope of -0.059 V/pH52, indicating that an equal number of protons and electrons are involved in the electrochemical oxidation of PA. The linear relationship can be expressed as:

$${\text{Epa }}\left( {\text{V}} \right){\text{ }}={\text{ }} - 0.0{\text{58}}0*{\text{pH}}\,+\,0.{\text{412}}$$
(3)

Figure 4d represents the electrochemical response of electrodes toward PA clearly varies depending on their surface modifications. The electrode without PA (WE1) showed the lowest current response, indicating that it has limited catalytic activity and poor electron transfer capabilities for PA detection53. This low response suggests that without any active sensing material or surface treatment, the electrode surface is not effective in facilitating the redox reaction of PA, resulting in minimal sensitivity.

In contrast, EZO-ME1 exhibited the highest current response among the three, due to the presence of EZO NPs that increase the active surface area and improve electron transfer kinetics54. This modification promotes faster charge transfer and greater catalytic efficiency, resulting in a more sensitive and reliable detection of PA compared to WE1.

Fig. 4
figure 4

(a) cv responses at various pH values; (b) bar graph of anodic peak current (Ipa) vs. pH; (c) anodic peak potential (Epa) vs. pH showing linear shift (d) pH sensing comparison between WE1 and EZO-ME1; (e) cv responses at various scan rates; (f) linear plots of anodic (Ipa) and cathodic (Ipc) peak currents vs. scan rate.

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cv was employed to examine the effect of varying scan rates on the electrochemical behavior of the modified EZO-ME1 electrode in a 1mM PA solution at pH 7.0. The scan rates were incrementally increased from 0.05 V/s to 0.5 V/s, as shown in Fig. 4e to assess their impact on PA oxidation55. The results revealed that the oxidation peak current increased progressively with higher scan rates, indicating a direct correlation between scan speed and current response56,57. Additionally, the oxidation peak potential shifted toward more positive values as the scan rate increased, suggesting that the electron transfer process was governed primarily by diffusion rather than surface adsorption. This indicates that the redox reaction of PA at the EZO-ME1 surface was controlled by a mass transport mechanism.

Further analysis showed a strong linear relationship between scan rate and peak current, as depicted in Fig. 4f with a regression coefficient of 0.99. A similarly high linear correlation (R2 = 0.99) was observed between the square root of the scan rate and the corresponding oxidation current in reinforcing the conclusion that the process follows diffusion-controlled kinetics58. These consistent linear responses across different scan rates reflect the excellent electron transfer characteristics of EZO-ME1. This behavior underscores the electrode’s reliability and efficiency for real-time electrochemical sensing of PA, making it a promising candidate for practical sensor applications.

Figure 5 indicates the electrochemical behavior of the EZO-ME1 toward PA detection was investigated using swv and compared with cv and dpv59,60. Among the three techniques, swv exhibited the highest sensitivity and resolution, attributed to its rapid scanning and superior signal-to-noise ratio described. The EZO-ME1 displayed a sharp and distinct oxidation peak for PA in the swv response, signifying enhanced electrocatalytic activity and efficient electron transfer61. This sharp peak, coupled with low background current, reinforces swv advantage for quantitative measurements. While cv and dpv provided valuable insights into the redox characteristics and reversibility of PA oxidation, swv emerged as the most effective approach for precise and sensitive analysis at the modified electrode interface.

The electrochemical response of the EZO-ME1 was evaluated for PA in the concentration range of 0.1 µM to 1 µM using cv. As shown in Fig. 5a, the cv response exhibits a clear increase in the anodic peak current with rising PA concentration. The corresponding linear regression analysis Fig. 5d demonstrated excellent linearity with an R2 value of 0.99, indicating a strong correlation between PA concentration and electrochemical response. The limit of detection (LOD) and limit of quantification (LOQ) were calculated as approximately 4.29 µM and 14.3 µM, respectively. These results highlight the high sensitivity and reliability of the electrode for PA detection using cv.

The performance of the EZO-ME1 for PA detection was further evaluated using dpv. As shown in Fig. 5b, the dpv response displayed a clear increase in the anodic peak current with increasing PA concentration. The corresponding linear regression analysis Fig. 5e exhibited excellent linearity with an R2 value of 0.997, confirming a strong correlation between PA concentration and current response. From the calibration data, LOD and LOQ were calculated to be approximately 1.036 µM and 3.452 µM, respectively. These results demonstrate the high sensitivity, accuracy, and reliability of EZO-ME1 for PA detection using dpv. similarly Fig. 5c indicates the swv response at various concentration ranging from 0.1micromolar to 1micro molar and Fig. 5f exhibited excellent linearity with an R2 value of 0.995, confirming a strong correlation between PA concentration and current response. From the calibration data, LOD and LOQ were calculated to be approximately 0.096 µM and 0.322 µM.

Fig. 5
figure 5

(a) cv responses at various concentrations (0.1–1.0 µM); (b) dpv curves for different PA concentrations (0.1–1.0 µM); (c) swv curves for different PA concentrations (0.1–1.0 µM); (d) linear calibration plot of Ipa versus PA concentration at the EZO-ME1 electrode obtained from cv. (e) Linear calibration plot of Ipa versus PA concentration at the EZO-ME1 electrode obtained from dpv. (f) Linear calibration plot of Ipa versus PA concentration at the EZO-ME1 electrode obtained from swv.

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The effective electrode surface area (ESCA = 0.05 cm2) was used to calculate the area-normalized sensitivity (S) using the formula62:

$$\: S = \frac{{{\text{slope}}\:{\text{of}}\:{\text{the}}\:{\text{calibration}}\:{\text{curve}}\:({\text{ }}\mu {\text{A}}\:\mu {\text{M}} - {\text{1)}}}}{{{\text{electrode}}\:{\text{surface}}\:{\text{area}}\:({\text{cm}}^{{\text{2}}} {\text{)}}}}$$
(4)

where the slope was obtained from linear regression of current versus concentration. The cv showed a slope of 4.95 µA µM− 1, corresponding to an area-normalized sensitivity of 99 µA µM− 1 cm⁻2, with excellent linearity (R2 = 0.99) and a LOD of 4.29 µM, indicating that cv is most suitable for mechanistic investigations rather than trace-level detection. dpv displayed a higher slope of 15.47 µA µM− 1, yielding an area-normalized sensitivity of 309.4 µA µM− 1 cm⁻2, strong linearity (R2 = 0.99), and an LOD of 1.036 µM. swv demonstrated the best overall performance, with a slope of 15.04 µA µM− 1, area-normalized sensitivity of 300.8 µA µM− 1 cm⁻2, R2 = 0.99, and the lowest LOD of 0.096 µM. The superior performance of swv is attributed to its pulsed waveform, which effectively minimizes background charging currents, enhances faradaic response, and facilitates rapid electron transfer at the electrode interface. Overall, the EZO-ME1 electrode exhibited enhanced detection capability compared to recently reported electrodes (Table 1). Among the tested techniques, swv offered the optimal combination of high sensitivity and low LOD, followed by dpv, whereas cv was primarily useful for mechanistic studies.

To further evaluate the sensor’s quantitative capabilities, swv measurements were performed across a PA concentration range of 0.1-1.0 µM (0.0001–0.001 mmol L− 1), as shown in Fig. 6. The anodic peak current increased proportionally with increasing PA concentration (Fig. 6a), confirming a strong linear correlation between analyte concentration and electrochemical response. The corresponding calibration plots (Fig. 6b, c) further demonstrate the high surface activity and excellent electron-transfer properties of the EZO-ME1 NPs. The calibration curve obtained for paracetamol showed an excellent linear relationship, described by the regression equation I (µA) = 15.04x + 0.484 (R2 = 0.995). The slope corresponds to a sensitivity of 15.04 µA mmol− 1 L, and using a standard deviation of 0.0485 µA, the LOD and LOQ were calculated to be 0.096 µM (0.000096 mmol L− 1) and 0.322 µM (0.000322 mmol L− 1), respectively. These low detection limits demonstrate the high sensitivity and analytical efficiency of the EZO-ME1 electrode toward paracetamol detection. These values are lower than those reported for most previously published PA sensors (Table 1)63,64,65,66,67,68,69,70,71,72,73,74. The exceptionally low detection thresholds highlight the superior sensitivity of the EZO-ME1 electrode, underscoring its potential for clinical diagnostics and trace-level biological sensing of PA.

Table 1 Comparative overview of reported electrochemical sensors for Paracetamol detection, highlighting electrode type, synthesis method, electrochemical technique, analytical performance (linear range, LOD, LOQ, Sensitivity), and key advantages and limitations.
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EIS was a valuable technique used to assess the charge transfer resistance and conductivity of electrode materials. In this work, eis measurements were performed on EZO-ME1 using a solution containing 1mM PA as the electrolyte at a sweep rate of 0.1 V/s. The Nyquist plot obtained for EZO-ME1, depicted in Fig. 6d, shows a semicircular feature that reflects the resistance to charge transfer at the electrode electrolyte interface75. Compared to BE1, the EZO-ME1 exhibits a noticeably smaller semicircle, indicating enhanced charge transfer capabilities.

The size of the semicircle in the Nyquist plot correlates with the electrode’s resistance to electron flow; a smaller diameter means lower resistance and improved conductivity. The larger semicircle observed with BE1 suggests greater resistance, likely due to the formation of a surface film that impedes electron movement76. In contrast, the reduced semicircle for EZO-ME1 demonstrates that the EZO NPs significantly improve the electrode’s conductivity and facilitate faster electron transfer. These results confirm that the modified electrode is well-suited for electrochemical sensing applications, highlighting the beneficial impact of the synthesized NPs in enhancing electrode performance.

The Nyquist plots provide insight into the electron transfer kinetics at the electrode interface. The charge-transfer resistance (Rct) represents the resistance to electron transfer during the redox reaction of PA. In our study, the Rct value for the BE1 was 2.07 × 105 Ω, while for the EZO-ME1 it increased slightly to 2.20 × 105 Ω. This slight increase can be attributed to the incorporation of the EZO NPs, which provides a more selective surface for PA oxidation. Overall, the modified electrode demonstrates efficient electron transfer with enhanced sensitivity for PA detection.

Figure 6e, f shows the long-term stability of the EZO-ME1 electrode was investigated by conducting continuous cv measurements at specific intervals: after 5, 10, 15, 20, 25, and 30 cycles. The electrode maintained about 90.09% of its initial anodic peak current even after 25 cycles, highlighting its strong resistance to degradation during repeated redox processes77. This consistent retention of current demonstrates that the electrode’s electrocatalytic properties remain stable over time, with only a slight decrease indicating minimal wear.

Fig. 6
figure 6

(a) swv curves for different PA concentrations; (b) Bar graph representation of the anodic peak current (Ipa) obtained for paracetamol at the EZO-ME1 electrode; (c) Linear calibration plot of Ipa versus PA concentration at the EZO-ME1 electrode obtained from swv; (d) eis plots of BE1 and EZO-ME1 showing charge transfer resistance; (e) stability test over 5 cycles; (f) stability test at 25th cycles.

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Such durability suggests that the EZO-ME1 electrode preserves its structural integrity and effectively resists surface fouling, which is essential for dependable performance in continuous sensing applications78. This stability was particularly important for PA detection, where consistent and long-lasting responsiveness was critical. Overall, these findings establish EZO-ME1 as a reliable material for advanced biosensors and clinical diagnostic devices that require sustained operation and repeatable electrochemical responses.

The simultaneous detection of DA and PA was investigated using cv on both the BE1 and the EZO-ME1 in a solution containing 1 mM DA and 1 mM PA, as shown in Fig. 7a, d. At BE1, the oxidation peaks of DA and PA were poorly resolved and overlapped, indicating limited selectivity and electrocatalytic activity. In contrast, EZO-ME1 produced well-separated and clearly defined peaks for both analytes, along with significantly higher currents, highlighting the improved sensitivity, selectivity, and electrocatalytic efficiency of the modified electrode. These findings demonstrate that EZO-ME1 can effectively enable simultaneous detection of DA and PA with enhanced analytical performance compared to the bare electrode.

The effect of DA concentration on the electrochemical response was investigated using 1 mM PA at a constant concentration and varying DA levels, measured by swv with the -ME1. As shown in the Fig. 7b, e the DA concentration increased, the anodic peak current of DA increased proportionally, while the peak current of PA remained essentially constant, demonstrating the electrode’s ability to selectively monitor DA in the presence of PA. The calibration curve for DA exhibited excellent linearity with an R2 value of 0.99, indicating high sensitivity and reliable quantitative performance.

Similarly, the effect of PA concentration was studied using 1 mM DA at a constant concentration while varying PA levels. The anodic peak current of PA increased linearly with its concentration, whereas the DA peak remained unchanged, confirming that EZO-ME1 can selectively detect PA even in the presence of DA as shown in the Fig. 7c, f. The calibration curve for PA also showed excellent linearity (R2 = 0.99), highlighting the electrode’s high selectivity and sensitivity for simultaneous detection of both analytes.

Fig. 7
figure 7

(a) Simultaneous electrochemical response of DA and PA at BE1 and EZO-ME1 electrodes; (b) Interference study showing the effect of varying DA concentration on the oxidation of constant PA at the electrode; (c) Interference study showing the effect of varying PA concentration on the oxidation of constant DA at the electrode; (d) Comparative anodic peak current response of DA and PA at BE1 and EZO-ME1 electrodes; (e) Linear calibration plot of Ipa versus various concentration of DA at constant PA; (f) Linear calibration plot of Ipa versus various concentration of PA at constant DA.

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Repeatability studies were conducted to assess the consistency and reliability of the EZO-ME1 electrode for PA detection shown in the Fig. 8a, b. The sensors electrochemical response was measured over multiple independent trials under identical experimental conditions using cv. The relative standard deviation (RSD) of the anodic peak currents across these repeated measurements was found to be within 1.9% an acceptable range, indicating excellent repeatability. This low variation confirms that the EZO-ME1 electrode produces stable and reproducible signals, which is essential for practical sensor applications79. The consistent performance across repeated measurements suggests minimal electrode surface fouling, further reinforcing the reliability of the modified electrode in real-time sensing environments. The reproducibility of the fabricated electrochemical sensor was systematically examined to ensure consistent analytical performance in detecting PA. A 1 mM PA solution prepared at pH 7.0 was used as the test analyte shown in Fig. 8c, d. The electrochemical response was recorded using cv, under identical experimental conditions for each measurement. To evaluate reproducibility, the same modified carbon paste electrode was used to perform five consecutive measurements in freshly prepared or well-mixed 1 mM PA solutions to minimize any external variability. For each trial, the oxidation peak current related to PA was carefully recorded. These current values were then analyzed, and the RSD was calculated. The resulting RSD was found to be 1.97%, indicating good reproducibility of the sensor. This low variation confirms that the modified electrode delivers a stable and consistent electrochemical response, demonstrating its suitability and reliability for practical applications such as pharmaceutical analysis and quality control. The stability of the EZO-ME1 electrode toward PA detection was evaluated, as presented in Fig. 8e, f. The electrode’s performance was examined over five successive cycles (5st − 30th ) using cv under identical experimental conditions. As shown in Fig. 8e, the anodic peak current remained almost unchanged throughout the repeated cycles, indicating stable electrochemical behavior. Figure 8f displays a bar chart illustrating the percentage retention from the first to the fifth cycle, which confirms that the electrode experienced only minimal degradation. These findings demonstrate that the EZO-ME1 electrode provides consistent and reliable responses with negligible surface deterioration, confirming its suitability for long-term and real-time sensing applications.

Fig. 8
figure 8

(a) Repeatability study of five electrodes. (b) Reproducibility assessment. (c) Cyclic stability over 5–30 cycles. (d) Bar graph representation of repeatability. (e) Bar graph representation of reproducibility. (f) Bar graph representation of the cyclic stability of the SSE electrode.

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Real sample analysis

To evaluate the practical applicability of the EZO-modified carbon paste electrode for PA detection, real sample analysis was carried out using commercially available PA tablet formulations. Accurately weighed and dissolved tablet samples were pre-analyzed, after which known concentrations of PA (10, 20, and 30 µM) were spiked into the solutions. SWV was employed to measure the electrochemical responses, and the recovery percentages were calculated using the standard addition method. As shown in Table 2, the sensor exhibited excellent recovery values of 102%, 99%, and 100.3% for the respective concentrations. These results indicate minimal interference from tablet excipients and confirm the sensor’s high analytical accuracy and selectivity. The consistent and reliable recovery percentages further highlight the robustness of the EZO-modified electrode, emphasizing its strong potential for practical applications in pharmaceutical quality control and clinical diagnostics.

Table 2 PA recoveries in PA injection using the EZO-modified electrode.
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Conclusion

This study successfully demonstrates the synthesis and application of EZO NPs, fabricated via a solid-state route under reducing atmosphere, and thoroughly characterized through XRD, Rietveld refinement, SEM, EDS, and electrochemical techniques. The EZO-ME1 was engineered to enhance the detection performance of PA, achieving a low detection limit of 0.096 µM and a limit of quantification of 0.323 µM. The electrode exhibited excellent linearity across a clinically relevant PA concentration range (0.1-1 µM), optimized at physiological pH (pH 7.0), with a surface area enhancement from 0.032 cm2 (BE1) to 0.052 cm2 (EZO-ME1). Kinetic analysis revealed diffusion-controlled electrochemical behavior, confirmed through scan rate studies and supported by a near-Nernstian slope of −0.058 V/pH. EIS analysis further validated the electrodes reduced charge transfer resistance, affirming improved conductivity upon EZO integration. Long-term operational stability was retained with 90.09% of the initial response after 25 CV cycles. Repeatability and reproducibility studies revealed RSD values of 1.9% and 1.97%, respectively, confirming excellent electrode consistency. Real tablet analyses returned high recovery rates (98–103%), showcasing the sensor’s accuracy and robustness against matrix interference. These results underline the multifunctional role of EZO as both a catalytic and structural enhancer, paving the way for its integration into next-generation biosensors that are scalable, reliable, and suitable for clinical and environmental applications.