Waste-derived nanocomposite carbon paste electrochemical sensor for the first guadipyr monitoring in agri-environmental samples: a sustainable platform for on-site analysis

Abstract

A portable electrochemical sensing platform was developed for the determination of guadipyr (GUA) in agri-environmental and food matrices as a sustainable alternative to conventional chromatographic methods. To the best of our knowledge, this is the first electrochemical sensor reported for GUA monitoring. The sensing interface was fabricated using a carbon paste electrode modified with multi-walled carbon nanotubes (MWCNTs) and aluminum oxide nanoparticles (Al₂O₃-NPs) synthesized from recycled aluminum foil waste, integrating waste-valorization principles into sensor fabrication. Structural and electrochemical characterization confirmed the successful construction of a highly active nanocomposite interface with enhanced conductivity and charge-transfer properties. The electrochemical behavior of GUA was investigated by cyclic voltammetry and differential pulse voltammetry, revealing a well-defined irreversible cathodic peak at − 0.68 V (CV) and − 0.67 V (DPV) in Britton–Robinson buffer (BRB) (pH 5.0), corresponding to the reduction of the nitro group. Under optimized conditions, the proposed sensor exhibited a linear response over the concentration range of 0.5–35 µM with a detection limit of 0.067 µM. The method was successfully applied for GUA determination in tomato, river water, irrigation water, and soil samples using matrix-matched calibration, providing satisfactory recoveries and high precision. Sustainability assessment using MA, CFA, NQS, and SAMI metrics confirmed the eco-friendly nature of the method. The developed platform offers a rapid, low-cost, and field-deployable strategy for routine GUA monitoring.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s13065-026-01900-3.

Introduction

The widespread use of insecticides in modern agriculture has raised significant environmental and food safety concerns owing to their persistence, mobility, bioaccumulation, and potential adverse effects on non-target organisms [1, 2]. Consequently, reliable monitoring of pesticide residues is essential to protect ecosystems, ensure agricultural sustainability, and safeguard human health. Among newly introduced agrochemicals, guadipyr (GUA) has attracted increasing attention as a novel insecticidal active ingredient requiring sensitive and reliable analytical monitoring in environmental and food matrices [3–6].

To date, GUA determination has primarily relied on chromatographic techniques. A modified QuEChERS procedure coupled with liquid chromatography–tandem mass spectrometry has been reported for residue analysis and dissipation studies of GUA in rice-related matrices [7]. Although chromatographic methods provide excellent sensitivity and selectivity, they generally require laborious sample preparation, expensive instrumentation, relatively large volumes of organic solvents, and prolonged analysis times, limiting their applicability for rapid screening and on-site monitoring. Furthermore, chromatographic techniques provide limited information regarding the electrochemical behavior and redox transformation pathways of electroactive analytes.

Field investigations have shown that GUA is typically present at trace residue levels in agri-environmental matrices following agricultural application because of its relatively rapid dissipation. Reported terminal residue concentrations generally range from non-detectable levels to approximately 50–470 µg kg⁻¹, with field half-lives of 0.22–0.37 days [7]. Consequently, analytical methods intended for environmental surveillance should combine low detection limits with rapid, reliable, and field-deployable operation to enable effective monitoring of environmentally relevant GUA residues.

Recent advances in analytical science have increasingly emphasized the development of methodologies that simultaneously achieve high analytical performance and environmental sustainability. In addition to excellent sensitivity and selectivity, modern analytical platforms are expected to reduce sample preparation, minimize solvent consumption, shorten analysis time, and enable decentralized monitoring. Within this context, nanostructured electrochemical sensors have emerged as highly attractive analytical tools for the determination of pharmaceuticals, pesticides, biological compounds, and environmental contaminants, owing to their simplicity, portability, rapid response, and low operational cost [8–16]. Nitro-containing compounds are particularly amenable to voltammetric analysis because their nitro groups readily undergo proton-coupled electron-transfer reactions at conductive electrode surfaces [17–19]. Despite the presence of electroactive polynitro and azomethine functionalities within the molecular structure of GUA, no electrochemical sensing platform has yet been reported for its determination, highlighting an important analytical gap.

Recent progress in electrochemical sensing has largely been driven by the development of advanced nanostructured electrode materials [20]. Carbon nanomaterials, particularly multi-walled carbon nanotubes (MWCNTs), have been extensively employed because of their excellent electrical conductivity, high specific surface area, remarkable electrocatalytic activity, and ability to accelerate electron-transfer processes [21]. Likewise, incorporation of metal oxide nanostructures introduces additional catalytic active sites, increases surface roughness, and enhances analyte adsorption, leading to significant improvements in analytical performance [22]. Numerous recent studies have demonstrated that combining conductive carbon nanomaterials with functional metal oxides significantly enhances the performance of electrochemical sensors for pharmaceutical, biological, environmental, and agricultural applications. Furthermore, increasing emphasis has been placed on employing renewable and waste-derived materials during sensor fabrication, consistent with the principles of green analytical chemistry and the circular economy [23–28].

Among various metal oxides, aluminum oxide nanoparticles (Al₂O₃-NPs) have attracted considerable attention owing to their excellent chemical stability, high surface energy, favorable dispersion characteristics, and abundant adsorption sites [29–31]. Beyond their electrochemical advantages, the synthesis of Al₂O₃-NPs from recycled aluminum waste represents an attractive waste-valorization strategy that transforms discarded materials into high-value functional nanomaterials while reducing environmental impact [32]. Coupling waste-derived Al₂O₃ nanoparticles with conductive MWCNTs therefore provides a multifunctional nanocomposite capable of improving both electrochemical performance and environmental sustainability. To the best of our knowledge, such a sustainable nanocomposite has not previously been explored for the electrochemical determination of GUA.

Motivated by these considerations, this work presents the first electrochemical sensor for GUA determination based on a carbon paste electrode modified with waste-derived Al₂O₃ nanoparticles and MWCNTs. The electrochemical behavior of GUA was systematically investigated using cyclic voltammetry and differential pulse voltammetry to elucidate its reduction mechanism and evaluate its analytical performance. The developed sensor was successfully applied to the determination of GUA in tomato, river water, irrigation water, and soil samples using matrix-matched calibration. In addition, the sustainability of the proposed methodology was comprehensively assessed using complementary green analytical metrics, confirming that the developed platform combines excellent analytical performance with low environmental impact. Consequently, the proposed sensor provides a rapid, low-cost, portable, and environmentally responsible strategy for routine monitoring of GUA residues in agri-environmental matrices.

Experimental

Instrumentation

Electrochemical measurements were performed using an Autolab PGSTAT204 potentiostat/galvanostat (Metrohm Autolab B.V., Utrecht, The Netherlands) controlled by NOVA software. A conventional three-electrode electrochemical cell was employed, consisting of an unmodified or modified carbon paste electrode as the working electrode, an Ag/AgCl (5.0 M KCl) reference electrode, and a platinum wire auxiliary electrode. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS) experiments were carried out using this configuration. EIS measurements were recorded in 1.0 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] containing 0.1 M KCl over a frequency range of 0.1 Hz–100 kHz with an AC perturbation amplitude of 10 mV.

The morphology and elemental composition of the synthesized nanomaterials were characterized using a JEOL JSM-6510LV scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDX) detector (JEOL Ltd., Tokyo, Japan). Phase identification was performed by X-ray diffraction (XRD) using a PANalytical X’Pert PRO diffractometer (Malvern PANalytical, Almelo, The Netherlands) with Cu Kα radiation (λ = 1.5406 Å) over a 2θ range of 10–80°. Functional groups were identified by Fourier transform infrared spectroscopy (FTIR) using a Bruker Tensor 27 spectrometer (Bruker Optics GmbH, Ettlingen, Germany) within the spectral range of 400–4000 cm⁻¹.

Solution pH was monitored using a Jenway 3510 digital pH meter (Cole-Parmer, Staffordshire, UK). Ultrasonic dispersion was achieved with an Elmasonic S30H ultrasonic bath (Elma Schmidbauer GmbH, Singen, Germany), while sample weighing was carried out using an Entris II analytical balance (Sartorius AG, Göttingen, Germany) with a readability of ± 0.1 mg.

All electrochemical experiments were conducted at 25 ± 2 °C. Prior to each measurement, the test solution was purged with high-purity nitrogen for 10 min to remove dissolved oxygen, and a nitrogen atmosphere was maintained above the solution throughout the experiment to minimize oxygen interference.

Chemicals and reagents

All chemicals employed in this investigation were of analytical-reagent grade and were used as received without additional purification. Guadipyr (GUA) reference standard (purity 99.5%; CAS No. 1376342-13-0) was procured from HPC Standards GmbH (Cunnersdorf, Germany). Multi-walled carbon nanotubes (MWCNTs; outer diameter 10–20 nm, length 5–15 μm, purity ≥ 95%) and spectroscopic-grade graphite powder (particle size < 50 μm) were obtained from Sigma-Aldrich, a subsidiary of Merck KGaA (Darmstadt, Germany). Paraffin oil, used as the binder for carbon paste preparation, was supplied by El-Nasr Pharmaceutical Chemicals Co. (ADWIC, Cairo, Egypt).

Discarded household aluminum foil collected in Cairo, Egypt served as the raw material for the preparation of Al₂O₃ nanoparticles. Hydrochloric acid (37%), sodium hydroxide pellets (≥ 98%), sodium carbonate, boric acid, phosphoric acid, and glacial acetic acid were purchased from Merck KGaA (Darmstadt, Germany), whereas absolute ethanol (analytical grade) was obtained from Sigma-Aldrich (St. Louis, MO, USA).

Britton–Robinson buffer (BRB) was prepared from equal concentrations (0.04 M each) of boric acid, phosphoric acid, and acetic acid, and the desired pH values were obtained by titration with 0.2 M NaOH. Ultrapure water with a resistivity of 18.2 MΩ·cm, generated using a Millipore Direct-Q purification system (MilliporeSigma, Bedford, MA, USA), was used for the preparation of all aqueous solutions throughout the study.

Collection and pre-treatment of agricultural and environmental samples

To verify the applicability of the developed electrochemical platform under realistic conditions, representative samples from agricultural and environmental sources were collected and analyzed. Fresh tomatoes were obtained from an authorized agricultural marketplace in Cairo, Egypt. Environmental water samples were collected from the Nile River within the Cairo region, while irrigation water was sampled from agricultural canals in Giza Governorate. Surface soil specimens were collected from cultivated farmland in the Nile Delta (Giza, Egypt) at a sampling depth of approximately 0–10 cm.

Immediately after collection, all samples were placed in thoroughly cleaned polyethylene containers, transported to the laboratory, and preserved at 4 °C until analysis. Prior to electrochemical measurements, the water samples were passed through 0.45 μm membrane filters to eliminate suspended particulate matter. Soil samples were naturally air-dried, homogenized, and sieved to obtain a uniform and reproducible matrix. Tomato samples were washed thoroughly with distilled water to remove surface contaminants, homogenized using a laboratory blender, and stored under refrigerated conditions until further use.

Before carrying out recovery and validation experiments, each matrix was examined using the optimized analytical protocol to confirm the absence of detectable GUA. Only blank samples free from measurable residues were subsequently employed for spiking experiments and method validation.

Ethical statement and laboratory safety

This investigation did not involve experiments on human participants or live animals; therefore, ethical approval and informed consent were not required. All analyzed materials consisted exclusively of food and environmental samples, and all experimental procedures were conducted in accordance with established institutional regulations for laboratory safety, chemical handling, and biosafety.

Preparation of calibration standards

A primary GUA stock solution (1.0 × 10⁻³ M) was prepared by accurately weighing 3.13 mg of the certified reference material (molecular weight 312.76 g mol⁻¹) and dissolving it in ethanol using a 10 mL Class A volumetric flask. To ensure complete dissolution, the solution was sonicated for 10 min before being diluted to the calibration mark with the same solvent. The prepared stock solution was transferred to amber glass containers and stored at 4 °C under light-protected conditions until use. Working standards were freshly prepared on the day of analysis by serial dilution of the stock solution with Britton–Robinson buffer (pH 5.0), which simultaneously served as the supporting electrolyte during all electrochemical measurements.

Sustainable preparation of Al₂O₃ nanoparticles from recycled aluminum foil

Al₂O₃-NPs were produced from discarded household aluminum foil through an environmentally friendly precipitation–calcination approach. Initially, the collected foil was cut into small pieces, thoroughly washed with a dilute detergent solution to eliminate surface contaminants, rinsed several times with distilled water, and dried in a laboratory oven at 80 °C for 4 h. The cleaned aluminum fragments (40.0 g) were gradually dissolved in a 1:1 (v/v) mixture of concentrated hydrochloric acid (37%) and deionized water under continuous magnetic stirring at room temperature. Dissolution was allowed to proceed until hydrogen evolution ceased, indicating complete conversion of the metallic aluminum into soluble aluminum species. The resulting solution was subsequently filtered through Whatman No. 42 filter paper to remove any remaining insoluble impurities. Formation of the aluminum hydroxide precursor was achieved by slowly adding 1.0 M sodium carbonate solution while maintaining vigorous stirring. The pH was carefully adjusted to approximately 8–9, promoting complete precipitation of Al(OH)₃. The suspension was then left undisturbed for 1 h to allow sufficient aging of the precipitate.

The precipitated hydroxide was separated by decantation and repeatedly washed with distilled water until chloride ions were no longer detected in the washings, as confirmed by the silver nitrate (AgNO₃) test. The purified precipitate was dried at 100 °C for 4 h and subsequently calcined in a programmable muffle furnace at 500 °C for 5 h using a controlled heating rate of 5 °C min⁻¹ to obtain crystalline Al₂O₃-NPs. After furnace cooling to room temperature, the calcined material was gently pulverized using an agate mortar to obtain a homogeneous fine powder. The synthesized nanoparticles were finally preserved in a desiccator until further use. A schematic representation of the synthesis procedure is provided in (Scheme S1).

Fabrication of carbon paste electrodes

Construction of the unmodified carbon paste electrode

The bare CPE was fabricated by mechanically blending 0.40 g of spectroscopic-grade graphite powder with 0.20 mL of paraffin oil in an agate mortar until a smooth and uniformly wetted paste was obtained. The mixing process was continued for approximately 10 min to ensure complete distribution of the binder throughout the graphite particles and to obtain a mechanically stable composite.

The prepared paste was introduced into the cavity of a Teflon electrode holder (3 mm internal diameter) and gently compressed to eliminate trapped air and obtain a compact electrode body. The exposed surface was then carefully polished against clean weighing paper until a flat and reproducible working surface was achieved. Electrical contact was established through a tightly fitted copper wire positioned at the rear of the electrode holder. Before each electrochemical experiment, the electrode surface was regenerated by extruding a small amount of paste followed by gentle polishing, thereby ensuring a freshly exposed electroactive surface and excellent measurement reproducibility.

Preparation of the Al₂O₃-NPs/MWCNT nanocomposite electrode

The nanocomposite-modified CPE was fabricated through a simple solid-state mixing procedure, and the preparation protocol is illustrated in (Scheme S2). Initially, 0.40 g of graphite powder was thoroughly mixed with 0.08 g of MWCNTs (16 wt% relative to graphite) to achieve homogeneous distribution of the conductive nanotubes within the carbon matrix. Subsequently, synthesized Al₂O₃ nanoparticles were incorporated at different proportions (3–12 wt%, based on the total solid content), and the dry components were further blended to ensure uniform dispersion of both nanomaterials.

Following homogenization of the solid mixture, 0.30 mL of paraffin oil was gradually introduced while continuous grinding was maintained for approximately 15 min until a cohesive paste with uniform consistency was obtained. The resulting nanocomposite paste was packed into Teflon electrode holders having the same 3 mm cavity diameter used for the bare electrode. The electrode surface was subsequently smoothed and polished using the same renewal procedure applied to the unmodified CPE, thereby maintaining identical geometric area and comparable surface characteristics for all fabricated electrodes.

The composition containing 10 wt% Al₂O₃ nanoparticles produced the highest voltammetric response and was therefore selected for all subsequent electrochemical measurements unless otherwise specified.

Electrochemical activation of the working electrode

Before quantitative analysis, each newly prepared electrode was electrochemically activated to establish a stable and reproducible interfacial state. Activation was achieved by repeatedly cycling the electrode in BRB (pH 5.0) within the selected potential range using CV at a scan rate of 60 mV s⁻¹. The conditioning procedure was continued until successive voltammograms exhibited a constant background current and reproducible electrochemical behavior.

This electrochemical pretreatment promotes stabilization of the electrode/electrolyte interface while removing loosely attached particles remaining from the fabrication process, thereby improving measurement repeatability. To maintain consistent analytical performance throughout the study, the electrode surface was regenerated before each experiment using the renewal procedure described previously, minimizing surface contamination and reducing the influence of adsorbed reaction products.

Electrochemical measurement procedure

All electrochemical experiments were performed using the three-electrode configuration described in Sect.  2.1, employing (CV), (DPV), and (EIS). Measurements were conducted at 25 ± 2 °C using 0.04 M (BRB) as the supporting electrolyte. Solution pH was adjusted immediately before each experiment using a calibrated pH meter.

The influence of pH on the electrochemical response of GUA was investigated over the range of 2.0–12.0, whereas all quantitative measurements were subsequently carried out under the optimized condition of pH 5.0.

Cyclic voltammograms were recorded between − 0.10 and − 1.00 V (vs. Ag/AgCl, 5.0 M KCl) at a scan rate of 60 mV s⁻¹. Consecutive scans were collected until stable voltammetric responses were obtained, ensuring complete stabilization of the working electrode prior to data acquisition.

Quantitative determination of GUA was performed by DPV under optimized instrumental conditions using a 50 mV pulse amplitude, 5 mV step potential, 50 ms pulse width, and an effective scan rate of 60 mV s⁻¹. Before each voltammetric scan, the system was allowed to equilibrate for 5 s to establish a stable baseline.

Interfacial charge-transfer characteristics were investigated by EIS over the frequency interval of 0.1 Hz to 100 kHz using a sinusoidal perturbation of 10 mV applied at the open-circuit potential. Whenever impedance characterization was required, measurements were performed in the presence of a standard ferri/ferrocyanide redox probe to evaluate the electrical properties of the fabricated electrodes.

Prior to every electrochemical measurement, the working electrode surface was gently polished to expose fresh paste, rinsed thoroughly with distilled water, and allowed to dry naturally under laboratory conditions. Unless otherwise indicated, all experiments were carried out under quiescent solution conditions without external stirring or deaeration.

Sampling, pretreatment, and extraction of agri-environmental matrices

River water

River water samples were collected in pre-cleaned polyethylene containers, transported to the laboratory, and stored at 4 °C until analysis. Prior to measurement, samples were equilibrated to ambient temperature and filtered through 0.45 μm membrane filters to remove suspended particulates. Aliquots of the filtered samples were transferred into calibrated volumetric flasks and subjected to matrix fortification with known concentrations of GUA for recovery and calibration purposes. The resulting solutions were adjusted to the required volume with BRB (pH 5.0) prior to electrochemical analysis. No additional extraction or preconcentration procedures were applied.

Irrigation water

Irrigation water samples were obtained using pre-cleaned polyethylene containers and subsequently preserved at 4 °C to maintain their integrity. All samples were processed within 48 h of collection to minimize potential physicochemical changes. Before analysis, the samples were subjected to filtration through 0.45 μm membrane filters to remove suspended particulate matter and ensure analytical clarity. Defined aliquots were transferred into volumetric flasks and fortified with GUA at predetermined concentration levels. The solutions were then diluted to volume using BRB (pH 5.0) to ensure compatibility with the supporting electrolyte system. No further extraction or enrichment steps were performed.

Soil

Soil samples were collected from cultivated land at a depth of 0–10 cm, air-dried under ambient conditions, homogenized, and sieved through a 2 mm mesh. For extraction, 1.00 g of the prepared soil sample was accurately weighed into a centrifuge tube, followed by the addition of 10 mL ethanol as extraction solvent. The mixture was subjected to ultrasonic-assisted extraction for 20 min, then centrifuged at 4000 rpm for 10 min. The supernatant was collected and filtered to remove residual particulates. Filtered extracts were transferred into volumetric flasks and fortified with known amounts of GUA. Prior to voltammetric measurement, the extracts were diluted with BRB (pH 5.0) to the required volume, ensuring that the ethanol content in the electrochemical cell did not exceed 1% (v/v).

Tomato

Fresh tomato samples were rinsed with distilled water, air-dried, and homogenized using a laboratory blender. An accurately weighed portion (1.00 g) of the homogenized sample was transferred into a centrifuge tube, and 10 mL ethanol was added as extraction solvent. Ultrasonic-assisted extraction was performed for 20 min, followed by centrifugation at 4000 rpm for 10 min. The supernatant that resulted was gathered and filtered. GUA was added to volumetric flasks containing appropriate aliquots of the extract at predetermined concentrations. The solutions were diluted to volume with BRB (pH 5.0) prior to electrochemical measurement, maintaining the organic solvent fraction below 1% (v/v).

Quantification protocol and matrix-matched calibration approach

Quantitative determination of GUA was performed using DPV under the optimized electrochemical conditions described in Sect.  2.7. Calibration was based on a matrix-matched approach for all investigated matrices. Calibration standards were generated within each matrix by appropriate fortification of blank extracts, as described in Sect.  2.8, followed by identical dilution and measurement procedures. Working concentration levels were prepared to cover the established linear range, and each level was measured in triplicate using a freshly renewed Al₂O₃-NPs/MWCNT/CPE surface.

Calibration functions were constructed by plotting peak current (Iₚ, µA) as a function of GUA concentration (C, µM). Linear regression analysis was performed using the least-squares method according to:

where a represents the slope and b the intercept.

The limit of detection (LOD) and limit of quantification (LOQ) were calculated based on the standard deviation of the blank signal (σ) and the calibration slope (S) using the following expressions:

All measurements were conducted under quiescent conditions, and electrode surface renewal was performed prior to each replicate to ensure reproducible analytical signals.

Evaluation of sensor selectivity toward coexisting matrix components

The selectivity of the proposed electrode was systematically examined by assessing its response in the presence of compounds that are likely to coexist with GUA in agricultural and environmental samples. A broad panel of potential interferents was selected to represent the chemical diversity of real matrices, including electroactive biomolecules (ascorbic acid, uric acid, and dopamine), carbohydrates (glucose and lactose), organic acids (citric acid), inorganic salts (NaCl and KCl), alkaline earth metal ions (Ca²⁺ and Mg²⁺), nitrogen-containing compounds (urea), natural organic matter (humic acid), macromolecular constituents (albumin and cholesterol), as well as representative agrochemicals, including glyphosate, atrazine, 2,4-D, carbendazim, azoxystrobin, mancozeb, imidacloprid, acetamiprid, and thiamethoxam.

Selectivity experiments were performed by recording the differential pulse voltammetric response of a fixed concentration of GUA after individually adding each potential interferent at concentrations corresponding to 10–100 times that of the analyte, depending on the expected abundance of the interfering species. All measurements were conducted under the optimized voltammetric conditions described in Sect.  2.7.

The influence of each compound was assessed by comparing the cathodic peak current of GUA in the presence of the interferent with that obtained from a reference solution containing GUA alone. Signal variations not exceeding ± 5% relative to the reference response were considered to indicate acceptable analytical selectivity.

To further confirm the suitability of the proposed sensor for practical applications, blank extracts of tomato, river water, irrigation water, and soil were analyzed using the same experimental protocol. The absence of measurable electrochemical signals within the potential region characteristic of GUA verified that endogenous matrix components did not contribute overlapping responses under the selected analytical conditions.

All selectivity experiments were carried out in triplicate, using a freshly regenerated electrode surface before each measurement to ensure consistent electrochemical performance and reliable assessment of interference effects.

Recovery studies and validation in real matrices

The trueness of the proposed electrochemical method was verified by recovery experiments performed using representative agricultural and environmental samples. Before fortification, river water, irrigation water, soil, and tomato matrices were prepared according to the procedures described in Sect.  2.8 and analyzed under the optimized experimental conditions to confirm the absence of detectable GUA.

Known amounts of GUA covering the validated working range were subsequently introduced into each blank matrix at three concentration levels. For liquid samples, including river water and irrigation water, the analyte was added after the filtration step. In contrast, soil and tomato samples were fortified before extraction so that any analyte loss associated with sample preparation was incorporated into the recovery evaluation.

Following fortification, all samples underwent the same extraction, dilution, and electrochemical measurement procedures employed for unknown samples. Quantitative determination was carried out using the appropriate matrix-matched calibration curve for each sample type. Differential pulse voltammetric measurements were performed in triplicate under the optimized operating conditions, and recovery values were calculated by comparing the experimentally determined concentrations with the corresponding fortified concentrations.

Data processing and statistical analysis

Each experiment was performed three independent times, and the analytical results are expressed as mean ± standard deviation (SD). The repeatability of the proposed procedure was evaluated by analyzing selected concentration levels within the same day under identical experimental conditions. Method precision was expressed as the relative standard deviation (RSD%).

Calibration equations were established using the least-squares linear regression method, from which the slope, intercept, and correlation coefficient (R) were obtained to evaluate the linear relationship between the analytical response and analyte concentration.

To assess the analytical performance of the developed sensor, the obtained results were statistically compared with those produced by a previously reported reference method. The comparison was carried out using Student’s t-test for the evaluation of accuracy and the F-test for comparison of precision at the 95% confidence level. The calculated statistical parameters were compared with their corresponding critical values. Values lower than the tabulated limits indicated that no statistically significant differences existed between the proposed and reference methods with respect to analytical accuracy and precision.

Results and discussion

Physicochemical interpretation of the sensor interface

Surface morphology and elemental composition (SEM–EDX)

The surface morphology and elemental composition of the synthesized Al₂O₃-NPs/MWCNT nanocomposite were investigated by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) (Fig. 1). These complementary techniques provide valuable information regarding the microstructural characteristics and elemental composition of the sensing material, both of which strongly influence its electrochemical performance.

Fig. 1.

: EDX and SEM patterns of Al₂O₃-NPs/MWCNTs nanocomposite

The SEM image reveals the successful formation of a heterogeneous three-dimensional nanostructured surface consisting of interconnected MWCNTs uniformly decorated with Al₂O₃ nanoparticles. The carbon nanotubes retain their characteristic intertwined fibrous morphology, generating a conductive network that serves as a stable structural framework for nanoparticle immobilization. Meanwhile, the Al₂O₃ nanoparticles appear as irregular nanosized aggregates distributed throughout the nanotube matrix without obvious large-scale agglomeration, indicating effective integration of both components during composite preparation.

The resulting nanocomposite exhibits a rough and porous surface with numerous cavities and interconnected channels, features that are advantageous for electrochemical sensing because they increase the electrochemically accessible interface and facilitate electrolyte penetration. Such structural characteristics are expected to improve analyte accessibility to active sites while promoting efficient charge-transfer processes during voltammetric measurements. These observations are consistent with the enhanced electroactive surface area and reduced charge-transfer resistance discussed in the subsequent electrochemical characterization.

The EDX spectrum confirms the presence of the expected constituent elements, namely carbon, oxygen, and aluminum, verifying the successful incorporation of Al₂O₃ nanoparticles into the MWCNT framework. Quantitative analysis yielded 26.67 wt% C, 56.76 wt% O, and 16.57 wt% Al, corresponding to atomic percentages of 34.79%, 55.59%, and 9.62%, respectively. No additional elemental signals associated with metallic impurities or residual precursor species were detected, indicating the high chemical purity of the synthesized nanocomposite.

It should be noted that the elemental composition determined by EDX represents a localized surface analysis of the selected region examined during SEM imaging and therefore provides a semi-quantitative assessment rather than the overall bulk composition of the synthesized material. Consequently, slight variations in elemental percentages may be expected between different regions because of the inherent heterogeneity of nanostructured composites. Nevertheless, the observed elemental distribution is fully consistent with the intended composition of the Al₂O₃-NPs/MWCNT nanocomposite and is supported by the complementary XRD and FTIR characterization results.

Overall, the SEM and EDX analyses demonstrate the successful fabrication of a well-integrated Al₂O₃-NPs/MWCNT nanocomposite possessing a porous nanostructured morphology, homogeneous nanoparticle distribution, and the expected elemental composition. These physicochemical characteristics provide a favorable electrode interface that contributes to the enhanced electrochemical response and analytical performance observed for GUA determination.

Crystallographic structure analysis (XRD)

The crystallographic features and phase composition of the fabricated Al₂O₃-NPs/MWCNT nanohybrid were investigated by XRD analysis (Fig. 2). The recorded diffractogram exhibits a distinct combination of reflections arising from graphitic carbon and crystalline alumina domains, confirming the successful generation of a biphasic nanostructured composite in which the conductive carbon framework is intimately integrated with the oxide phase.

Fig. 2.

XRD patterns of Al₂O₃-NPs/ MWCNTs nanocomposite

A prominent diffraction maximum located at 2θ ≈ 26° and indexed to the (002) plane is clearly observed. This reflection is characteristic of the graphitic interlayer stacking of MWCNTs and corresponds to the periodic arrangement of sp²-bonded carbon sheets within the multiwalled tubular structure. The pronounced intensity of this peak demonstrates that the nanotubes retained their graphitic ordering after composite fabrication and thermal processing. Simultaneously, the finite broadness of the reflection suggests partial turbostratic disorder and nanoscale domain size, which are commonly encountered in MWCNT materials possessing structural defects and edge discontinuities. Such defects are electrochemically beneficial, as they often serve as high-energy active sites for adsorption and electron exchange.

In addition to the CNT-derived reflection, a series of well-resolved peaks appears at approximately 2θ = 31.5°, 36.8°, 45.5°, 57.0°, and 66.8°, indexed to the (220), (311), (400), (511), and (440) crystallographic planes, respectively. These reflections are consistent with the cubic spinel-type γ-Al₂O₃ phase, indicating successful transformation of the hydroxide precursor into nanocrystalline alumina during calcination. The moderate broadening of these oxide peaks confirms the nanoscale nature of the crystallites and suggests the presence of lattice strain and surface disorder, both of which are frequently associated with high-surface-energy transition alumina. Such structural imperfection can enhance catalytic responsiveness by increasing the density of coordinatively unsaturated surface atoms.

The coexistence of the graphitic (002) reflection with the γ-Al₂O₃ diffraction set confirms that both constituents preserve their structural identity within the hybrid matrix. Moreover, the absence of peak suppression or severe distortion indicates that the assembly process did not compromise the intrinsic crystallinity of either phase. The partial convolution of reflections in the low-angle region further implies intimate interfacial contact between alumina nanodomains and the nanotube surface, supporting efficient heterointerface formation rather than simple macroscopic blending.

Notably, no extraneous diffraction peaks attributable to residual metallic Al, unconverted hydroxides, chloride-containing byproducts, or other parasitic crystalline impurities were detected. This observation shows the efficacy of the employed washing and calcination stages and validates the excellent phase purity of the produced material.

From an electrochemical perspective, the resulting dual-phase architecture is highly advantageous. The γ-Al₂O₃ component provides abundant adsorption-active centers and catalytically responsive defect sites, whereas the MWCNT network furnishes rapid electron-transport channels and low-resistance conductive pathways. The synergistic combination of these attributes is expected to accelerate heterogeneous charge-transfer kinetics and amplify the voltammetric response of the target analyte.

Surface functional group analysis (FTIR)

FTIR spectroscopy was employed to elucidate the surface chemical environment and functional group distribution of the Al₂O₃-NPs/MWCNT nanohybrid (Fig. 3). The obtained spectrum provides compelling evidence for the coexistence of hydroxylated alumina domains and oxygen-functionalized carbon nanotubes, verifying the chemical complexity of the engineered sensing interface.

Fig. 3.

FTIR patterns of Al₂O₃-NPs/ MWCNTs nanocomposite

Surface hydroxyl groups and physically adsorbed water molecules are linked to v(O–H) stretching vibrations, which are responsible for a wide and strong absorption band centered at 3435.23 cm⁻¹. The considerable breadth of this band indicates extensive hydrogen-bonding interactions, which are characteristic of nanostructured alumina possessing a highly hydrated and defect-rich surface. Surface hydroxylation is particularly significant in electroanalysis because it improves hydrophilicity, promotes electrolyte wetting, and provides anchoring sites for analyte adsorption.

A distinct absorption band at 1701.72 cm⁻¹ is attributed to ν(C = O) stretching vibrations of carbonyl/carboxyl functionalities located on defect sites of MWCNTs. The presence of this feature confirms that the nanotube surface contains oxygenated groups capable of enhancing dispersibility and strengthening interfacial adhesion with the oxide phase. These functionalities can also modulate the local electronic density of the carbon framework, thereby improving charge-transfer behavior and facilitating surface interactions with electroactive molecules.

The band detected at 1202.25 cm⁻¹ is assigned to ν(C–O) stretching vibrations arising from epoxy, alkoxy, or hydroxyl-derived oxygen functionalities on the CNT surface. Such groups are commonly generated during purification or oxidative treatment of nanotubes and play an essential role in preventing nanotube bundling while promoting homogeneous nanoparticle immobilization. Their presence further supports the efficient integration of Al₂O₃ nanodomains across the conductive carbon scaffold.

In the fingerprint region, a strong absorption band at 650.18 cm⁻¹ is characteristic of Al–O and Al–O–Al lattice vibrations, providing direct spectroscopic confirmation of alumina formation. The broad profile of this vibration is consistent with nanosized γ-Al₂O₃ domains exhibiting short-range order rather than highly sintered bulk oxide. This observation is fully consistent with the XRD results and reinforces the successful generation of nanocrystalline alumina within the composite matrix.

Collectively, the FTIR findings confirm that the fabricated material is not a simple physical blend, but rather a chemically interactive nanohybrid containing hydroxylated oxide surfaces and oxygen-enriched carbon domains. Such interfacial functionality is expected to enhance wettability, adsorption affinity, and electronic communication between both phases. These features collectively establish a highly reactive electrode interface capable of promoting efficient analyte accumulation and rapid electron-transfer processes during voltammetric sensing.

Interfacial electron-transfer properties and electroactive surface area evaluation

To quantitatively assess the influence of electrode modification on the electrochemical interface, the electroactive surface area (EASA) of the fabricated electrodes was determined by cyclic voltammetry using the reversible ferri/ferrocyanide redox probe. Measurements were performed in 1.0 mM K₄[Fe(CN)₆] containing 0.1 M KCl over a scan-rate range of 10–200 mV s⁻¹. For all investigated electrodes, the anodic peak current increased linearly with the square root of the scan rate (R² > 0.998), indicating diffusion-controlled electron transfer and confirming the applicability of the Randles–Ševčík equation for EASA determination [33]:

where Iₚₐ is the anodic peak current (A), n is the number of electrons transferred (n = 1), A is the electroactive surface area (cm²), D is the diffusion coefficient of the ferri/ferrocyanide redox couple (7.63 × 10⁻⁶ cm² s⁻¹), C is the concentration of the electroactive species (1.0 × 10⁻⁶ mol cm⁻³, equivalent to 1.0 mM), and ν is the scan rate (V s⁻¹).

The calculated EASA values were 0.070 cm², 0.147 cm², and 0.361 cm² for the bare CPE, MWCNT/CPE, and 10 wt% Al₂O₃-NPs/MWCNT/CPE, respectively, demonstrating a progressive enlargement of the electrochemically accessible surface following each modification step.

The EASA obtained for the unmodified CPE closely approximated its geometric surface area, indicating efficient packing of the carbon paste and satisfactory electrical contact within the electrode matrix. Incorporation of MWCNTs increased the electroactive surface by approximately 2.1-fold, reflecting the formation of an interconnected conductive network that exposes additional electroactive sites and facilitates electron transport. Further modification with Al₂O₃ nanoparticles produced a more pronounced enhancement, resulting in an electroactive surface area approximately 5.2 times larger than that of the bare electrode. This substantial increase can be attributed to the combined effects of nanoscale surface roughening, increased structural porosity, and improved dispersion of the carbon nanotubes, which collectively enhance electrolyte accessibility while minimizing nanotube aggregation.

The significant expansion of the electroactive interface provides a greater density of accessible catalytic sites for the proton-coupled electroreduction of GUA and facilitates more efficient interfacial electron transfer. These findings are consistent with the lower charge-transfer resistance obtained from EIS measurements and provide a rational explanation for the enhanced voltammetric response, improved analytical sensitivity, and superior sensing performance exhibited by the Al₂O₃-NPs/MWCNT/CPE throughout this study.

Interfacial impedance behavior and charge-transfer kinetics

Electrochemical impedance spectroscopy (EIS) was employed to evaluate the influence of electrode modification on interfacial electron-transfer behavior using the reversible 1.0 mM Fe(CN)₆³⁻/⁴⁻ redox couple in 0.1 M KCl. Because this outer-sphere redox system is minimally affected by specific surface interactions, the measured impedance response primarily reflects changes in the intrinsic electrical properties of the electrode interface. Consequently, variations in the impedance spectra provide direct information regarding charge-transfer resistance, interfacial capacitance, and ion diffusion resulting from the successive electrode modifications.

To facilitate direct comparison, the Nyquist plots of all investigated electrodes are presented using identical impedance scales (Fig. S1A), while an enlarged view of the low-impedance region is provided in (Fig. S1B) to better visualize the electrochemical response of the modified electrodes. The experimental spectra were satisfactorily described using the modified Randles equivalent circuit shown in (Fig. S1C), comprising the solution resistance (Rs) connected in series with a parallel combination of the charge-transfer resistance (Rct) and a constant phase element (Q, representing CPEdl), followed by a finite-length Warburg diffusion element (Wo). The constant phase element accounts for the non-ideal capacitive behavior arising from surface roughness and structural heterogeneity of the nanocomposite electrode, whereas the Warburg element represents diffusion of the electroactive species at lower frequencies. The corresponding fitting parameters are summarized in (Fig. S1D). The relatively small values of Rs indicate that the electrolyte resistance contributes only minimally to the overall impedance response and is therefore not visually prominent in the enlarged Nyquist plots.

The bare CPE exhibited the largest semicircular diameter, corresponding to a fitted Rct value of 31.45 kΩ, indicating sluggish heterogeneous electron-transfer kinetics at the graphite/paraffin interface. This behavior is attributed to the limited conductivity of the carbon paste matrix and the relatively low density of accessible electroactive sites available for charge exchange.

Modification with MWCNTs markedly reduced the semicircle diameter and decreased Rct to 7.86 kΩ, demonstrating a substantial enhancement in interfacial electron-transfer kinetics. The improvement originates from the highly conductive interconnected nanotube network, which provides continuous electron-transport pathways while simultaneously increasing the electrochemically accessible surface area.

A further and more pronounced reduction in impedance was achieved after incorporation of 10 wt% Al₂O₃ nanoparticles. The resulting Al₂O₃-NPs/MWCNT/CPE exhibited the smallest semicircle among all investigated electrodes, with an Rct value of only 1.24 kΩ, representing approximately a 25-fold decrease relative to the bare CPE. This significant reduction confirms the synergistic contribution of the two nanomaterials. The MWCNT network ensures rapid electron transport, whereas the uniformly dispersed Al₂O₃ nanoparticles improve nanotube dispersion, increase surface roughness, expose additional electroactive sites, and facilitate electrolyte penetration into the porous nanocomposite architecture.

The fitting results further support these observations. The gradual increase in the CPE magnitude from the bare electrode to the nanocomposite reflects the expansion of the electrochemically accessible interface, whereas the CPE exponent (n = 0.86–0.89) indicates nearly ideal capacitive behavior with only moderate surface heterogeneity. In addition, the progressive decrease in the Warburg coefficient demonstrates improved diffusion of the redox probe through the modified electrode interface, consistent with the porous morphology observed by SEM.

To further evaluate the influence of electrode modification on the electron-transfer process, the exchange current density (i₀) was estimated according to:

where R is the universal gas constant, T is the absolute temperature, n is the number of electrons transferred, F is Faraday’s constant, and Rct is the charge-transfer resistance. Because i₀ is inversely proportional to Rct, the pronounced reduction in Rct from the bare CPE to the Al₂O₃-NPs/MWCNT/CPE confirms a substantial increase in the intrinsic electron-transfer rate of the modified electrode.

Overall, the EIS results demonstrate that nanocomposite modification dramatically improves the electrical characteristics of the carbon paste electrode by lowering charge-transfer resistance, increasing the electrochemically active interface, and facilitating ion diffusion. These findings are in excellent agreement with the electroactive surface area measurements and voltammetric studies, providing a clear mechanistic explanation for the superior analytical performance of the proposed Al₂O₃-NPs/MWCNT/CPE sensor toward GUA determination.

Voltammetric characteristics and proposed electrochemical reduction pathway of GUA

The electrochemical response of GUA was investigated by cyclic voltammetry in BRB (pH 5.0) to elucidate its redox characteristics and identify the electroactive functionality responsible for its voltammetric response. As illustrated in (Fig. 4), the forward cathodic scan exhibits a single, well-defined reduction peak centered at approximately − 0.68 ± 0.02 V (vs. Ag/AgCl), which is attributed to the electroreduction of the nitro group. No corresponding anodic peak was detected during the reverse scan, indicating that the electro-generated species is not electrochemically reoxidized within the investigated potential window. To further verify the electrochemical behavior, cyclic voltammograms were additionally recorded over an extended potential window (− 1.2 to + 1.2 V vs. Ag/AgCl), as presented in (Fig. S2). Even under these conditions, no oxidation peak attributable to GUA or its reduction product was observed. The sharp current increase at the positive and negative potential limits originates from background electrode/electrolyte processes rather than from redox transformation of GUA. These observations provide additional evidence that the electroreduction of GUA is electrochemically irreversible under the employed experimental conditions. The molecular structure of GUA contains a nitro substituent, which is recognized as the most electroactive functionality under cathodic polarization. In aqueous media, aromatic nitro groups generally undergo sequential proton-assisted electron-transfer reactions, whereas the remaining structural moieties are electrochemically inactive within the investigated potential range. Accordingly, the reduction peak observed in the present study is attributed to the conversion of the nitro group into the corresponding hydroxylamine derivative through a proton-coupled multi-electron reduction process. The overall reaction may be represented as:

Fig. 4.

Effect of different electrodes and Al₂O₃-NPs contents on the reduction peak current of GUA (30 µM) using cyclic voltammetry

The proposed reaction pathway is illustrated in (Fig. 5) and is supported by several independent experimental observations. The appearance of a single cathodic peak indicates that any intermediate species generated during the reduction process are rapidly converted and do not accumulate at the electrode surface to produce separate voltammetric signals. Moreover, the absence of an anodic response in both the conventional cyclic voltammogram (Fig. 4) and the extended-potential experiment (Fig. S2) further corroborates the irreversible nature of the electrode process. In addition, the linear dependence of the peak potential on solution pH together with the near-Nernstian slope discussed in Sect.  3.4 confirms the direct participation of protons in the electrode reaction, supporting a proton-coupled electron-transfer mechanism involving an overall four-electron/four-proton reduction.

Fig. 5.

Proposed oxidation mechanism of GUA and sensor configuration

The influence of electrode composition on the voltammetric response of 30 µM GUA is also presented in (Fig. 4). The bare carbon paste electrode generated a relatively weak and broad reduction signal, reflecting the limited electroactive surface area and relatively slow heterogeneous electron-transfer rate of the unmodified electrode. Incorporation of MWCNTs markedly increased the peak current owing to the formation of a highly conductive network that promotes rapid electron transport while simultaneously increasing the accessible surface area available for analyte interaction. The graphitic surface of MWCNTs may additionally facilitate adsorption of the aromatic GUA molecules through non-covalent π–π interactions, thereby increasing the local analyte concentration at the electrode interface.

Further enhancement was achieved after incorporating Al₂O₃ nanoparticles into the MWCNT matrix. Increasing the Al₂O₃ content from 5 to 10 wt% progressively improved the cathodic response, with the 10 wt% Al₂O₃-NPs/MWCNT/CPE providing the maximum peak current. This improvement can reasonably be attributed to the synergistic interaction between the two nanomaterials. While MWCNTs provide highly efficient conductive pathways that facilitate electron transport, Al₂O₃ nanoparticles contribute additional active adsorption sites, increase surface roughness, improve electrolyte accessibility, and promote more effective contact between GUA molecules and the electroactive surface. When the Al₂O₃ loading was further increased to 12 wt%, the current response decreased slightly, most likely because excessive nanoparticle coverage partially blocked conductive regions of the nanotube network and reduced the effective electroactive surface area through particle agglomeration.

An important observation is that the reduction potential remained essentially unchanged regardless of electrode composition. This finding indicates that the nanocomposite modification does not alter the intrinsic electrochemical reduction pathway of GUA but rather accelerates the interfacial electron-transfer process and improves signal generation. Therefore, the enhanced analytical performance originates from improved electrode kinetics and increased electroactive surface area rather than from any change in the fundamental reaction mechanism.

The differential pulse voltammograms presented in (Fig. S3) further demonstrate a continuous increase in cathodic peak current with increasing GUA concentration over the investigated range of 0.5–35 µM, while the peak potential remained nearly constant. The proportional increase in current without a corresponding shift in peak potential confirms that the electrode reaction proceeds through the same reduction pathway throughout the investigated concentration range and that quantitative determination is governed primarily by the amount of electroactive species reaching the electrode surface.

Overall, the combined electrochemical evidence obtained from cyclic voltammetry, differential pulse voltammetry, pH studies, and electrode modification experiments supports the proposed irreversible proton-coupled reduction of the nitro group through an overall four-electron/four-proton process. The excellent analytical performance of the optimized Al₂O₃-NPs/MWCNT/CPE originates from the synergistic combination of efficient charge-transfer pathways provided by MWCNTs and the adsorption-promoting characteristics of the waste-derived Al₂O₃ nanoparticles, resulting in enhanced sensitivity without altering the inherent electrochemical mechanism of GUA reduction.

Effect of solution pH and supporting electrolyte on the electrochemical response of GUA

The electrochemical response of GUA was investigated in BRB over the pH range of 2.0–12.0 using cyclic voltammetry to elucidate the influence of proton concentration on the reduction process. Since the electrochemical transformation of nitro-containing compounds generally involves simultaneous electron and proton participation, variations in solution pH are expected to alter both the reduction potential and the magnitude of the voltammetric response.

As presented in (Fig. S4), the cathodic peak progressively shifted toward more negative potentials as the pH increased, indicating that the reduction reaction becomes energetically less favorable under alkaline conditions. The relationship between the peak potential and solution pH was found to be linear and can be expressed as:

The observed slope of − 54.0 mV pH⁻¹ is close to the theoretical Nernstian value expected for electrochemical processes involving an equal number of electrons and protons. This behavior confirms that proton transfer is directly coupled with the electron-transfer step and supports the proposed reduction pathway of the nitro group in GUA. The excellent linearity also indicates that the reaction mechanism remains unchanged throughout the investigated pH range.

The peak current was likewise affected by the acidity of the supporting electrolyte. Higher cathodic currents were obtained in acidic media, whereas a gradual decline in signal intensity was observed as the pH increased. This behavior can be attributed to the decreasing proton concentration, which limits the efficiency of the proton-assisted reduction sequence and consequently lowers the overall faradaic response. Similar pH-dependent behavior has been reported for electrochemical reduction of nitro-containing compounds using nanostructured electrodes, where proton availability plays a decisive role in facilitating charge-transfer reactions and improving analytical sensitivity [34, 35].

In addition to controlling the proton concentration, the supporting electrolyte is essential for maintaining constant ionic strength, minimizing solution resistance, and stabilizing the electrical double layer at the electrode/electrolyte interface. These factors contribute to faster electron-transfer kinetics and improved signal reproducibility. BRB was selected because it provides effective buffering over a broad pH interval while maintaining a stable electrochemical environment, enabling reliable evaluation of the pH-dependent behavior of GUA under identical experimental conditions. The universal buffering capacity of BRB also minimizes local pH fluctuations near the electrode surface during the electroreduction process, thereby enhancing the stability and reproducibility of the recorded voltammetric signals.

Considering both the current response and the quality of the voltammetric peak, pH 5.0 provided the most favorable analytical performance and was therefore selected for all subsequent measurements. Under these conditions, the sensor exhibited high sensitivity together with a stable baseline and excellent signal reproducibility.

Overall, the obtained results demonstrate that the electrochemical reduction of GUA is governed by a proton-assisted irreversible electrode process, in which the availability of protons strongly influences both the reduction potential and the magnitude of the analytical signal. The combined effects of adequate proton activity and the favorable electrochemical characteristics of BRB make this supporting electrolyte particularly suitable for sensitive voltammetric determination of GUA.

Scan-rate dependence and kinetic investigation of GUA electroreduction

The influence of the potential scan rate on the electrochemical response of GUA was investigated by cyclic voltammetry to elucidate the mass-transfer characteristics and electron-transfer kinetics of the reduction process at the Al₂O₃-NPs/MWCNT/CPE interface. Experiments were performed in BRB (pH 5.0) containing 40 µM GUA over a scan-rate range of 0.01–0.20 V s⁻¹ (Fig. S5).

Increasing the scan rate produced a progressive enhancement of the cathodic peak current while simultaneously shifting the peak potential toward more negative values, indicating that the reduction process is kinetically limited rather than electrochemically reversible. To identify the dominant transport mechanism, the relationship between the peak current and scan rate was examined using both logarithmic and square-root treatments.

A highly linear dependence was obtained between the logarithm of the peak current and the logarithm of the scan rate:

The calculated slope (0.645) lies between the theoretical values expected for purely diffusion-controlled (0.50) and purely adsorption-controlled (1.00) electrode processes, demonstrating that the reduction of GUA proceeds through a mixed diffusion–adsorption mechanism. This behavior indicates that the analyte reaches the electrode surface predominantly by diffusion, while partial adsorption on the nanostructured interface contributes to signal enhancement. Such adsorption is likely promoted by the large surface area of MWCNTs together with favorable interactions between GUA and the hydroxyl-rich Al₂O₃ nanoparticles.

The predominance of diffusion-controlled mass transport was further verified by the excellent linear relationship between the cathodic peak current and the square root of the scan rate (Fig. S5B):

The excellent linearity of this relationship confirms that diffusion is the principal transport process, whereas the intermediate log–log slope reveals a secondary adsorption contribution arising from the nanocomposite electrode surface.

Additional mechanistic information was obtained from the variation of peak potential with scan rate. As illustrated in (Fig. S5C), the cathodic peak potential gradually shifted toward more negative values as the scan rate increased, a characteristic feature of an irreversible electron-transfer process. The dependence of peak potential on the logarithm of the scan rate was expressed as:

According to Laviron’s treatment for totally irreversible electrode reactions:

Using the experimental slope (0.0306 V decade⁻¹), the kinetic term αn was calculated as:

Based on a common transfer coefficient (α = 0.5), the number of electrons involved in the rate-determining step was estimated to be 3.86, which is effectively four electrons. This value agrees well with the proposed 4e⁻/4H⁺ proton-coupled reduction of the nitro group discussed in Sect.  3.3, thereby providing independent kinetic support for the proposed reaction mechanism.

The relationship between peak potential and scan rate also exhibited good linearity (Fig. S5D):

The continuous displacement of the peak potential with increasing scan rate further corroborates the irreversible nature of the electrochemical process and indicates that the heterogeneous electron-transfer kinetics are sufficiently slow to influence the voltammetric response.

Collectively, the scan-rate investigation established the optimum electrochemical conditions for GUA determination and demonstrated that the proposed sensor operates through an irreversible proton-coupled electron-transfer process governed predominantly by diffusion with a measurable contribution from surface adsorption. The optimized experimental conditions identified in this study were employed throughout all subsequent analytical measurements and method validation experiments.

Differential pulse voltammetric analytical performance of the proposed sensor

After establishing the optimum experimental conditions through systematic optimization of the electrode composition, supporting electrolyte, solution pH, and scan-rate studies, the analytical performance of the proposed sensor was investigated using differential pulse voltammetry (DPV). This technique was selected because of its superior current discrimination and enhanced analytical sensitivity, enabling accurate quantification of GUA even at low concentration levels.

All DPV measurements were performed using the optimized instrumental settings, namely a pulse amplitude of 50 mV, step potential of 5 mV, pulse width of 50 ms, effective scan rate of 60 mV s⁻¹, and quiet time of 5 s. These operating conditions provided a stable baseline, well-defined cathodic peaks, and excellent measurement reproducibility, and were therefore adopted throughout the analytical validation and real-sample analysis.

Under the optimized conditions, GUA produced a single sharp cathodic peak centered at approximately − 0.67 V (vs. Ag/AgCl). No accumulation or preconcentration step was required prior to measurement, demonstrating that the high analytical response originates from the efficient electrocatalytic properties and large electroactive surface area of the Al₂O₃-NPs/MWCNT/CPE. The observed peak potential agrees well with the electrochemical behavior established by cyclic voltammetry and supports the previously proposed irreversible proton-coupled four-electron/four-proton reduction mechanism of the nitro group.

Analytical validation and quantitative performance of the proposed electrochemical platform

Method validation of the developed Al₂O₃-NPs/MWCNT/CPE sensing system was conducted in accordance with ICH Q2(R1) guidelines, complemented by matrix-oriented assessment consistent with Eurachem recommendations for environmental and food analysis. The validation protocol encompassed evaluation of linearity, sensitivity, accuracy, precision, and robustness under the optimized experimental conditions.

Linearity and sensitivity

The quantitative performance of the proposed electrochemical sensor was evaluated under the optimized differential pulse voltammetric conditions by constructing calibration plots over the investigated concentration range. The corresponding matrix-matched calibration curves are presented in (Fig. S6), while the analytical parameters are summarized in (Table 1).

Table 1.

Analytical performance characteristics of the Al₂O₃-NPs/MWCNT/CPE sensor for GUA in different matrices

	Buffer	Tomato	River water	Irrigation water	Soil
Linearity (µM)	0.5–35	1.0–35	0.7–35	0.7–35	1.5–35
Intercept	0.0324	0.0418	0.0365	0.0382	0.0496
Slope	0.4973	0.4816	0.4891	0.4854	0.4687
Correlation coefficient (r)	0.9999	0.9996	0.9997	0.9995	0.9992
Accuracy	100.31	99.42	100.18	99.76	98.94
Repeatability* (%RSD)	0.49	0.88	0.73	0.79	1.18
Intermediate precision** (%RSD)	0.53	0.96	0.82	0.87	1.27
Robustess*** (Mean ± %RSD)	99.79 ± 0.58	99.34 ± 0.91	100.06 ± 0.74	99.61 ± 0.83	98.71 ± 1.16
LOD (µM)	0.067	0.121	0.094	0.101	0.168
LOQ (µM)	0.202	0.367	0.285	0.306	0.509

^*Average of three different concentrations were repeated three times within the day

^**Average of three different concentrations were repeated three times in three days

^***Variation in method parameters such as pH of the sample and equilibrium time

A highly linear relationship between the cathodic peak current and GUA concentration was obtained in all investigated matrices, demonstrating that the developed sensing platform provides a stable and proportional analytical response throughout the working concentration range. The calibration plots exhibited excellent regression coefficients (R² = 0.9992–0.9999), confirming outstanding linearity and minimal deviation from linear regression. Representative calibration equations are shown in Fig. S6A–D, where only slight differences in the regression slopes were observed among the investigated matrices.

The small variation in analytical sensitivity can be attributed to matrix-dependent phenomena, including differences in sample composition, ionic strength, co-extracted constituents, and adsorption behavior at the electrode surface. Nevertheless, the similarity of the calibration slopes indicates that these effects exert only a limited influence on the electrochemical response, confirming the excellent matrix tolerance of the proposed sensor. The use of matrix-matched calibration further compensates for these minor matrix effects and ensures reliable quantitative determination in complex environmental and agricultural samples.

The detection capability of the developed method was assessed using the calculated limits of detection (LOD) and quantification (LOQ). The sensor exhibited submicromolar detection limits in both buffer and real samples, demonstrating its suitability for trace-level monitoring of GUA. As expected, slightly higher LOD and LOQ values were obtained for the more complex matrices, particularly soil, owing to the greater matrix complexity and stronger analyte–matrix interactions. Nevertheless, the obtained detection limits remained sufficiently low for practical residue analysis without requiring derivatization, extensive clean-up procedures, or analyte preconcentration.

Overall, the excellent linearity, high analytical sensitivity, and low detection limits obtained across all investigated matrices demonstrate that the developed Al₂O₃-NPs/MWCNT/CPE provides a reliable quantitative platform for GUA determination in both aqueous and solid agri-environmental samples.

Accuracy

The trueness of the developed sensing platform was verified through recovery experiments performed at representative concentration levels spanning the validated analytical range. The obtained recoveries, summarized in (Table 1), were 100.31% for the buffer solution, 99.42% for tomato, 100.18% for river water, 99.76% for irrigation water, and 98.94% for soil samples.

The close agreement between the added and determined concentrations demonstrates that the proposed analytical procedure is essentially free from significant systematic error. Recovery values obtained for the aqueous and food matrices were particularly close to the theoretical value of 100%, indicating that naturally occurring matrix constituents exert only a negligible influence on quantification. Although the soil samples produced a marginally lower recovery, this behavior is consistent with the greater physicochemical complexity of soil matrices and the stronger interaction of pesticide molecules with mineral and organic components. Nevertheless, the obtained recoveries satisfy internationally accepted validation criteria for residue analysis, confirming the suitability of the proposed sensor for accurate determination of GUA in diverse agri-environmental samples.

Repeatability and intermediate precision

Method precision was investigated by evaluating both short-term repeatability and day-to-day reproducibility. The repeatability study yielded RSD values of 0.49%, 0.88%, 0.73%, 0.79%, and 1.18% for buffer, tomato, river water, irrigation water, and soil, respectively. These low variability values indicate that repeated measurements performed under identical conditions generate highly consistent electrochemical responses.

Intermediate precision was assessed through analyses performed on different days under the same experimental protocol. The corresponding RSD values were 0.53% for buffer, 0.96% for tomato, 0.82% for river water, 0.87% for irrigation water, and 1.27% for soil. The slightly higher variability compared with the repeatability study is expected because of normal day-to-day experimental fluctuations; however, all values remained well below the generally accepted limits for quantitative analytical methods.

Collectively, the precision results demonstrate the excellent reproducibility of both the optimized DPV procedure and the fabricated electrode, confirming that the developed sensing platform provides highly reliable analytical responses across matrices with different physicochemical characteristics.

Robustness, reproducibility, and stability of the developed sensor

The practical performance of the proposed electrochemical sensor was further examined by evaluating its robustness, fabrication reproducibility, operational repeatability, and storage stability under conditions representative of routine laboratory use.

Method robustness was investigated by deliberately introducing small variations around the optimized experimental parameters, including slight changes in the equilibration time (± 3 s) and supporting electrolyte pH (± 0.1 units). These modifications produced only negligible changes in both peak current and peak potential, with all RSD values remaining below 2% (Table 1). This behavior demonstrates that the analytical response is largely insensitive to minor experimental fluctuations, reflecting the stable electrochemical characteristics of the nanocomposite electrode.

The reproducibility of electrode fabrication was evaluated using five independently prepared electrodes manufactured according to the same preparation protocol. As illustrated in Fig. S7, the current responses obtained for 30 µM GUA showed excellent agreement, yielding an RSD of approximately 1.5–2.0%. These results confirm the reliability of the fabrication procedure and demonstrate homogeneous incorporation of the nanostructured components throughout the carbon paste matrix.

The operational stability of the modified electrode was assessed by performing successive measurements with the same electrode surface. No noticeable deterioration in peak current or measurable displacement of the reduction potential was observed during repeated analyses, indicating minimal surface fouling and excellent stability of the electrode/electrolyte interface.

Long-term storage stability was examined over a period of 30 days under ambient laboratory conditions. As presented in (Fig. S8), the sensor retained more than 96% of its initial electrochemical response after storage, while only a slight decrease in cathodic current was observed and the reduction potential remained essentially unchanged. This excellent stability reflects the strong structural integrity of the developed nanocomposite and its resistance to nanoparticle aggregation and degradation of the conductive network.

Overall, the developed sensing platform combines excellent robustness with highly reproducible fabrication, stable operational performance, and good long-term durability. These characteristics demonstrate that the proposed electrode is well suited for routine electrochemical determination of GUA in agricultural and environmental samples, where reliable performance over extended periods is essential.

Selectivity and influence of coexisting species

The analytical selectivity of the proposed Al₂O₃-NPs/MWCNT/CPE was assessed by investigating the influence of a broad range of substances that may coexist with GUA in agricultural and environmental samples. The examined interferents comprised inorganic ions, naturally occurring organic compounds, carbohydrates, proteins, natural organic matter, electroactive biomolecules, and representative agrochemicals, including herbicides, fungicides, and insecticides. The investigated compounds together with their concentration ratios relative to GUA are listed in (Table S1).

The proposed sensor exhibited excellent selectivity, as all tested species produced changes in the GUA peak current of less than ± 5%, even when present at concentrations exceeding that of the target analyte. These results demonstrate that the electrochemical response of GUA is only minimally influenced by common matrix constituents, indicating that the developed sensing platform possesses sufficient selectivity for practical agri-environmental applications.

Naturally occurring electroactive compounds, including ascorbic acid, uric acid, and dopamine, produced only slight variations in the analytical signal despite their inherent redox activity. This behavior indicates that their electrochemical processes occur at sufficiently different potentials or proceed independently of the cathodic reduction of GUA, thereby preventing significant overlap with the analytical response.

Similarly, glucose, lactose, citric acid, and urea exerted negligible influence on the voltammetric signal. These compounds are either electrochemically inactive or only weakly active within the investigated potential window and exhibit limited interaction with the modified electrode surface. Consequently, their presence does not significantly affect either electron-transfer kinetics or analyte accessibility to the electroactive interface.

The effect of dissolved inorganic species was also examined because these ions are naturally abundant in environmental waters and agricultural soils. Sodium, potassium, calcium, and magnesium ions caused only insignificant changes in the cathodic current, even at high concentration ratios relative to GUA. This observation confirms that variations in ionic composition and electrolyte strength do not adversely influence the analytical performance of the proposed sensor.

Natural organic matter represented by humic acid produced the largest signal suppression among the investigated non-agrochemical interferents; however, the observed variation remained within the predefined acceptance criterion. This slight decrease is most likely associated with partial adsorption of humic macromolecules onto the electrode surface, which may temporarily reduce the accessibility of electroactive sites without altering the reduction mechanism of GUA. Likewise, albumin and cholesterol caused only minor signal changes, demonstrating satisfactory resistance of the nanocomposite electrode toward nonspecific surface fouling.

To further evaluate the practical applicability of the developed sensor, representative agrochemicals belonging to different chemical classes were investigated as potential interferents. Herbicides (glyphosate, atrazine, and 2,4-D), fungicides (carbendazim, azoxystrobin, and mancozeb), and insecticides (imidacloprid, acetamiprid, and thiamethoxam) produced only negligible changes in the GUA response, with all signal variations remaining below ± 3%. The absence of significant interference indicates that these compounds neither generate overlapping voltammetric signals within the investigated cathodic potential region nor compete effectively with GUA for the electroactive sites of the Al₂O₃-NPs/MWCNT nanocomposite. These findings demonstrate that the proposed sensing platform maintains high analytical selectivity even in the presence of agrochemicals that may coexist with GUA in real agricultural samples.

No additional voltammetric peaks attributable to the investigated interferents were observed in the vicinity of the characteristic GUA reduction peak, and no appreciable shift in peak potential occurred throughout the selectivity study. These observations confirm that the analytical signal arises exclusively from the reduction of GUA, while the modified nanocomposite electrode provides sufficient discrimination against structurally and chemically diverse coexisting species.

Overall, the interference study demonstrates that the proposed electrochemical sensor combines excellent selectivity with strong matrix tolerance, enabling reliable determination of GUA in complex agri-environmental samples with minimal sample pretreatment. The obtained results further support the suitability of the developed platform for routine environmental monitoring and residue analysis under realistic field conditions.

Application to real samples and recovery evaluation

The practical applicability of the developed electrode sensor for GUA determination was assessed through recovery studies in representative agri-environmental matrices, namely tomato, river water, irrigation water, and soil. Accuracy was evaluated using a standard-addition protocol at three concentration levels (2.0, 10.0, and 25.0 µM), selected to represent the lower, middle, and upper portions of the validated working range. All measurements were carried out in triplicate after appropriate sample preparation and matrix-matched calibration to ensure realistic compensation of matrix effects (Table 2).

Table 2.

Recovery and accuracy evaluation of the Al₂O₃-NPs/MWCNT/CPE sensor for GUA determination in spiked real matrices

Matrix	Added (µM)	Found (µM)	Recovery (%)	%RSD (n = 3)
Tomato	2	1.95	97.50	2.08
	10	10.24	102.40	1.81
	25	24.46	97.84	1.59
River water	2	1.99	99.50	1.42
	10	10.08	100.80	1.18
	25	24.74	98.96	1.05
Irrigation water	2	1.97	98.50	1.58
	10	10.16	101.60	1.31
	25	24.38	97.52	1.24
Soil	2	1.91	95.50	2.54
	10	9.74	97.40	2.21
	25	24.18	96.72	1.97

In tomato samples, the proposed method provided recoveries of 97.50%, 102.40%, and 97.84% for the three fortification levels, with corresponding %RSD values of 2.08%, 1.81%, and 1.59%. These results demonstrate satisfactory accuracy despite the chemical complexity of the plant matrix, which contains pigments, sugars, organic acids, and cellular debris that may influence analyte transport or surface interactions. The low variability confirms the ability of the nanocomposite electrode to maintain stable analytical performance in food matrices.

For river water, excellent analytical performance was obtained, with recoveries of 99.50%, 100.80%, and 98.96% and %RSD values ranging from 1.05% to 1.42%. The high recoveries and low dispersion indicate negligible interference from dissolved salts, suspended matter, and naturally occurring organic constituents. These findings highlight the suitability of the proposed platform for direct environmental water monitoring.

Similarly, irrigation water samples showed recoveries of 98.50%, 101.60%, and 97.52%, accompanied by %RSD values of 1.58%, 1.31%, and 1.24%. The slight positive deviation observed at the intermediate level may reflect minor matrix-induced signal enhancement; however, the overall analytical performance remained highly satisfactory with excellent precision. This confirms the method’s applicability for agricultural water surveillance.

Among all tested matrices, soil samples represented the most challenging medium. Nevertheless, the sensor still achieved recoveries of 95.50%, 97.40%, and 96.72% with %RSD values of 2.54%, 2.21%, and 1.97%. The slightly lower recoveries are reasonably attributed to adsorption of GUA onto mineral surfaces and humic constituents, together with diffusional constraints associated with the heterogeneous solid matrix. Despite these expected complications, the method maintained acceptable trueness and reproducibility.

Overall, the recovery study confirms that the proposed electrochemical platform enables accurate and reproducible quantification of GUA across chemically diverse real matrices. The obtained recoveries were confined within 95.50–102.40%, while all precision values remained below 2.54%. These findings verify the robustness of the Al₂O₃-NPs/MWCNT sensing interface and demonstrate the effectiveness of the matrix-matched calibration strategy for compensating matrix-induced effects, thereby supporting the practical utility of the method for routine agri-environmental residue monitoring.

Statistical comparison with a reference method

To further establish the reliability of the proposed sensor, its performance was statistically compared with the previously reported reference method for GUA determination in buffer, tomato, river water, irrigation water, and soil matrices. The comparison was performed using Student’s t-test and variance ratio (F-test) at the 95% confidence level based on five replicate determinations (n = 5) for each matrix (Table 3). These statistical tools are widely employed to assess whether significant differences exist between two analytical procedures in terms of trueness and precision.

Table 3.

GUA statistical comparison with the reported method in different matrices

Parameters	Buffer	Tomato	River water	Irrigation water	Soil
Proposed method
n	5	5	5	5	5
%R	100.2	98.7	101.1	99.4	96.8
SD	0.49	0.92	0.71	0.83	1.24
Variance	0.240	0.846	0.504	0.689	1.538
Reported method [7]
n	5	5	5	5	5
%R	99.6	97.9	100.3	98.8	96.1
SD	0.63	1.08	0.89	0.97	1.39
Variance	0.397	1.166	0.792	0.941	1.932
Student’s t-test (2.306)	1.52	1.08	1.34	0.96	0.88
F-value (6.388)	1.65	1.38	1.57	1.37	1.26

The proposed method provided mean recoveries (%R) of 100.2% (buffer), 98.7% (tomato), 101.1% (river water), 99.4% (irrigation water), and 96.8% (soil). The corresponding values obtained by the reported method were 99.6%, 97.9%, 100.3%, 98.8%, and 96.1%, respectively. The slightly higher recoveries observed for the proposed sensor may be attributed to the enhanced interfacial sensitivity and superior electrocatalytic behavior of the Al₂O₃-NPs/MWCNT nanocomposite surface. Nevertheless, the differences between both methods remained statistically insignificant.

Precision comparison further demonstrated the favorable performance of the proposed approach. The standard deviation values for the developed method were 0.49, 0.92, 0.71, 0.83, and 1.24, whereas the reported method exhibited higher SD values of 0.63, 1.08, 0.89, 0.97, and 1.39 for the respective matrices. Consequently, the calculated variances for the proposed method (0.240–1.538) were consistently lower than those of the reference procedure (0.397–1.932), confirming improved repeatability and reduced signal dispersion.

The calculated Student’s t-values were 1.52, 1.08, 1.34, 0.96, and 0.88, all of which are lower than the tabulated critical value (2.306). This confirms the absence of any statistically significant difference in accuracy between the proposed and reported methods. Similarly, the calculated F-values (1.65, 1.38, 1.57, 1.37, and 1.26) were substantially lower than the theoretical critical value (6.388), demonstrating that the precision of both methods is statistically comparable.

The simultaneous satisfaction of both statistical criteria across all tested matrices confirms the analytical equivalence of the proposed voltammetric procedure with the established reference method. However, it is noteworthy that the proposed sensor consistently produced numerically lower SD and variance values, indicating a tendency toward improved reproducibility. This advantage can be rationalized by the highly conductive and structurally stable nanocomposite interface, which minimizes charge-transfer resistance, enhances signal-to-noise ratio, and reduces matrix-induced fluctuations.

Overall, the statistical assessment verifies that the developed DPV method is fully comparable to the reported chromatographic procedure in terms of trueness and precision, while offering additional practical advantages such as rapid analysis, lower operational cost, portability, reduced solvent consumption, and a superior sustainability profile. These features strongly support its implementation for routine monitoring of GUA residues in agri-environmental matrices.

Comparative analytical performance and method evaluation

To assess the analytical merit of the proposed sensing platform, its performance was compared with the only reported chromatographic method available for GUA determination, and the results are summarized in (Tables S2 and S3). The comparison considered both analytical figures of merit and practical operational characteristics relevant to routine residue analysis.

The reported chromatographic procedure employs a modified QuEChERS extraction followed by LC–MS/MS analysis, providing excellent analytical sensitivity and selectivity for trace residue determination in rice-related matrices. However, this methodology requires multiple sample preparation steps, sophisticated instrumentation, and relatively high consumption of organic solvents and auxiliary reagents before instrumental analysis.

In comparison, the proposed Al₂O₃-NPs/MWCNT/CPE exhibited a linear response over the concentration range of 0.5–35 µM in BRB, with matrix-dependent linear ranges of 0.7–35 to 1.5–35 µM. The calculated LOD and LOQ were 0.067 and 0.223 µM in buffer, respectively, while the LOD values obtained for real samples ranged from 0.094 to 0.168 µM. Although chromatographic techniques generally provide lower detection limits owing to the superior sensitivity of mass spectrometric detection, the submicromolar detection capability achieved by the proposed electrochemical sensor is fully adequate for routine monitoring of GUA residues and demonstrates the high analytical sensitivity of the developed sensing interface.

The analytical reliability of the proposed method is further supported by recoveries ranging from 95.50 to 102.40% and precision values below 2.54% RSD, which compare favorably with the reported chromatographic method (Table S2). These results confirm that the proposed sensor provides accurate and reproducible quantification across diverse agri-environmental matrices while maintaining satisfactory analytical performance.

Beyond its analytical characteristics, the proposed electrochemical platform offers several operational advantages. Quantitative measurements can be completed within a few minutes using a compact potentiostat without chromatographic separation or extensive sample preparation. The analytical procedure requires only aqueous BRB as the supporting electrolyte, with trace methanol used solely during stock solution preparation, thereby substantially reducing solvent consumption and chemical waste generation. In addition, the electrode fabrication strategy utilizes Al₂O₃ nanoparticles synthesized from recycled aluminum foil, incorporating waste valorization into the sensor design and improving the overall sustainability of the analytical methodology.

It should be emphasized that the objective of the proposed sensor is not to replace highly sophisticated chromatographic techniques, particularly when ultra-trace quantification or confirmatory analysis is required. Rather, it provides a complementary analytical tool that combines satisfactory analytical sensitivity with operational simplicity, rapid analysis, portability, low cost, and reduced environmental impact, making it particularly attractive for routine screening and decentralized monitoring applications.

Overall, the comparative evaluation demonstrates that the proposed Al₂O₃-NPs/MWCNT/CPE achieves a balanced combination of analytical performance, operational simplicity, and environmental sustainability. The favorable LOD and LOQ values, together with excellent accuracy, precision, and practical applicability, support its suitability as an efficient alternative for routine GUA monitoring in agri-environmental samples.

4Integrated sustainability and multidimensional analytical assessment

To comprehensively evaluate the environmental and operational sustainability of the proposed electroanalytical platform, a multidimensional assessment was performed using complementary sustainability metrics, including the Multi-color Analytical (MA) Tool [36], Carbon Footprint Assessment (CFA) [37, 38], Need–Quality–Sustainability (NQS) index [39, 40], and Sustainability of Analytical Methods Index (SAMI) [41]. A detailed description of the evaluation methodology, scoring criteria, calculations, and interpretation is provided in the Supplementary Information (Section S1, Tables S4–S5, and Fig. S9).

The proposed method exhibited excellent overall sustainability performance, achieving GEMAM, BAGI, RAPI, VIGI, and Whiteness scores of 89.4%, 85.0%, 87.4%, 65.0%, and 81.7%, respectively, confirming an excellent sustainability profile according to the MA Tool. Carbon footprint analysis demonstrated a very low emission value of 0.012 kg CO₂ eq per analysis, reflecting the low energy consumption associated with the electrochemical platform. Furthermore, the method achieved high NQS and SAMI scores of 90 and 80, respectively, indicating a balanced combination of analytical quality, operational simplicity, environmental compatibility, and alignment with sustainable analytical chemistry principles.

These findings demonstrate that the proposed Al₂O₃-NPs/MWCNT/CPE combines excellent analytical performance with low environmental impact, while the complete sustainability assessment and detailed discussion are presented in the Supplementary Information.

Conclusions

In this work, a sustainable electrochemical sensing platform was developed for the determination of (GUA) in representative agricultural and environmental matrices. The proposed sensor was fabricated by combining waste-derived Al₂O₃ nanoparticles with multi-walled carbon nanotubes within a carbon paste electrode, demonstrating that recycled materials can be successfully transformed into high-value electroactive nanocomposites without compromising analytical performance.

Comprehensive physicochemical and electrochemical characterization verified the successful construction of the nanocomposite and revealed that the modified electrode possesses a highly accessible electroactive interface with improved electron-transfer characteristics and increased surface area. Voltammetric investigations demonstrated that GUA undergoes an irreversible proton-coupled four-electron/four-proton reduction process, while incorporation of the Al₂O₃-NPs/MWCNT nanocomposite substantially enhanced the cathodic response through the combined effects of rapid charge transport, enlarged electroactive surface area, and favorable adsorption of the analyte.

Under the optimized differential pulse voltammetric conditions, the developed sensor provided a wide linear working range (0.5–35 µM), a low detection limit (0.067 µM), and excellent analytical precision and accuracy. The proposed method was successfully validated using tomato, river water, irrigation water, and soil samples, where high recoveries, low relative standard deviations, excellent robustness, and satisfactory resistance toward a wide range of coexisting matrix constituents and representative agrochemicals confirmed its applicability for routine residue monitoring.

From a sustainability perspective, the analytical procedure exhibited excellent environmental performance. The use of waste aluminum foil as the precursor for nanoparticle synthesis, aqueous supporting electrolyte, renewable carbon paste electrodes, and low-power electrochemical instrumentation significantly reduced material consumption and environmental burden. The favorable scores obtained using the MA, CFA, NQS, and SAMI assessment tools further demonstrate that the proposed methodology satisfies the principles of sustainable analytical chemistry while maintaining high analytical quality.

Overall, the developed Al₂O₃-NPs/MWCNT/CPE provides a simple, sensitive, cost-effective, and environmentally responsible alternative for the electrochemical determination of GUA in complex agri-environmental samples. Beyond the present application, the fabrication strategy described herein highlights the potential of integrating waste-derived nanomaterials with electrochemical sensing technologies to support the future development of circular, low-carbon analytical platforms for monitoring emerging environmental contaminants.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Saja A. Althobaiti: Conceptualization, methodology, visualization, formal analysis, supervision, writing – original draft, writing – review & editing.Jamilah A. Asiri: Investigation, sample preparation, analytical validation, formal analysis, data curation, writing – original draft.Darelglal Ahmed Gassmelseed Abdalla: Methodology, Funding acquisition, formal analysis, Validation, writing – original draft.Omkulthom Al kamaly: Methodology, Funding acquisition, formal analysis, supervision, writing – original draft, writing – review & editing.Ahmed Emad F. Abbas: Methodology, Software, formal analysis, writing – review & editing.All authors contributed to data analysis, manuscript revision, and final approval of the submitted version.

Funding

This research was funded by Prince Sattam bin Abdulaziz University through the project number (PSAU/2025/01/38701), Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia.

Data availability

The corresponding author will provide the datasets created and/or analyzed during the current study upon reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.