Deep eutectic solvent-driven synthesis of La₂NiO₄@rGO Ruddlesden–Popper hybrids for ethyl parathion sensing supported with theoretical studies

Abstract

In this work, Ruddlesden–Popper phase perovskite La₂NiO₄ was synthesized via a hydrothermal method employing a deep eutectic solvent as the reaction medium. Benefiting from its pronounced faradaic activity, La₂NiO₄ served as the primary electroactive component for sensing studies. To further enhance electron transport and increase the density of accessible active sites, a hybrid heterostructure La₂NiO₄@rGO was made through a versatile sonochemical method. This synergistic architecture markedly enhanced the electrochemical charge-transfer kinetics and sensing performance toward ethyl parathion (EP). This La₂NiO₄@rGO-modified glassy carbon electrode delivered a broad linear detection range of 0.05–1625 µM for the electrochemical reduction of EP, with a lower limit of detection of 18 nM. Density functional theory (DFT) calculations provided additional molecular-level insight, revealing an ionization potential of 6.86 eV and an electron affinity of 2.37 eV for EP based on frontier orbital analysis. The La₂NiO₄@rGO/GCE sensor further exhibited excellent sensitivity, stability, selectivity, repeatability, and reproducibility, confirming its suitability for real-sample analysis. Overall, the La₂NiO₄@rGO/GCE platform offers a promising strategy for rapid and reliable EP detection, demonstrating its strong potential for advanced food-safety monitoring applications.

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1. Introduction

The rising global population has intensified food production demands, leading to extensive pesticide use and widespread contamination of soil, water, and food. Organophosphate pesticides (OPs), including ethyl parathion (EP) and methyl parathion (MP), are among the most toxic agrochemicals, widely applied due to their low cost and high efficacy.^1–6 EP is a prominent member of this family, and has emerged as a substitute for dichlorodiphenyltrichloroethane and other chlorinated hydrocarbon pesticides due to its broad-spectrum insecticide activity. Despite its uses as an insecticide in contemporary agricultural practices, it also has a huge influence on chemical warfare agents in military operations and terrorist attacks, and has also been linked to chemical warfare incidents, including the 1994 Matsumoto and 1995 Tokyo subway attacks.^7–12 OP toxicity results from irreversible inhibition of acetylcholinesterase, causing lethal accumulation of acetylcholine.¹³ Consequently, due to their high toxicity, there is a compelling need for rapid detection and screening of EP compounds for environmental protection, including public areas and workplaces, as well as for health protection.

Conventional analytical methods, including HPLC, LC-MS, and spectrophotometry, provide accurate detection but require high cost, complex procedures, and long analysis times.^14–31 Electrochemical sensing offers a faster, economical, and highly sensitive alternative. However, issues such as high overpotentials, poor selectivity, and electrode fouling limit performance, necessitating surface-engineered electrodes.

Nanostructured metal oxides are widely used in electrochemical sensing due to their stability, biocompatibility, and high surface reactivity.^32–35 Among them, layered Ruddlesden–Popper perovskites (A₂BO₄, A = La, B = Ni) have gained interest. La₂NiO₄, a K₂NiF₄-type oxide, exhibits excellent chemical stability, mixed ionic-electronic conductivity, and high oxygen surface-exchange rates, making it promising for sensors and energy storage devices.^36–42 However, particle aggregation restricts its electrochemical efficiency. To overcome this, reduced graphene oxide (rGO) is employed as a conductive platform due to its large surface area and restored π-conjugation. However, the lack of functional groups on rGO hinders the effective attachment of metal or metal oxide nanoparticles. Despite these challenges, functionalized rGO remains a promising support material for various inorganic components due to its better dispersibility in water and ability to interact with functional groups. Hence, integrating La₂NiO₄ with rGO enhances dispersion, active-site accessibility, and electron transfer, improving the sensing performance for real-world applications.

In this work, La₂NiO₄ was synthesized hydrothermally using a deep eutectic solvent (DES) and hybridized with rGO via π–π interactions to form La₂NiO₄@rGO. The nanocomposite was thoroughly characterized and applied to a glassy carbon electrode for EP detection. The La₂NiO₄@rGO/GCE demonstrated a wide detection range, ultralow detection limit, and excellent selectivity, stability, and reproducibility. High recovery values in real food samples confirm its potential for practical environmental and food-safety monitoring.

2. Experimental section

2.1. Chemical reagents

Detailed information about the chemical reagents is provided in SI 1.

2.2. Chemical methods and instruments

Detailed information on chemical methods and instruments is provided in SI 2.

2.3. Preparation of La₂NiO₄

Firstly, the deep eutectic solvent (DES) was prepared at a 1 : 1 molar ratio of choline chloride and fructose (non-toxic reagents). Under magnetic stirring, 2 M citric acid was added to the mixture and stirred for 15 min. Subsequently, 3 M KOH was added to the reaction solution. This prepared DES was then used as the reaction medium for the synthesis of La₂NiO₄ nanoparticles. Initially, an equimolar ratio (0.3 : 0.3) of lanthanum(III) nitrate hexahydrate and nickel(II) nitrate hexahydrate was dissolved in 40 mL of the DES. The overall solution was vigorously stirred for 30 min and then transferred to a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated to 180 °C in a hot-air oven for 12 hours. After completion of the reaction, the solution was cooled down naturally at room temperature. The resulting product was washed repeatedly with deionized water and ethanol to remove residual impurities, then dried for 12 hours at 80 °C in a hot-air oven. Finally, the dried nanoparticles were calcined at 750 °C for 2 hours under an inert atmosphere to obtain the crystalline La₂NiO₄ nanoparticles.

2.4. Preparation of rGO

Firstly, GO was synthesized from graphite using a modified Hummers’ method. The rGO was then derived from GO according to the modification described in previous reports. Briefly, 1.2 g of GO was dispersed in 100 mL of deionized water, followed by the addition of 0.6 g of caffeic acid (CA). The mixture was heated to 90 °C for 24 hours under stirring in an oil bath. After cooling, the resulting suspension was washed with deionized water and ethanol, and the rGO was dried under vacuum at 40 °C.⁴³

2.5. Preparation of the La₂NiO₄@rGO nanocomposite

To produce the hybrid matrix of La₂NiO₄@rGO nanocomposite, a mixture containing 50 mg of rGO sheets and the addition of 100 mg of La₂NiO₄ was dissolved in 40 mL of DI water. This dispersion was kept in an ultrasonic bath for 2 hours under irradiation to ensure the formation of the desired nanocomposite. The resulting La₂NiO₄@rGO nanocomposite solution was centrifuged at 5000 rpm for 20 min. Subsequently, the solution was washed with distilled water several times and then dried at 80 °C overnight for 12 hours.

2.6. Electrode modification

To obtain a working electrode, electrode modification is necessary. To modify the GCE, it was initially polished with 0.5 micron alumina powder on microsuede. This rigorous polishing regimen yielded a mirror-like finish, which was then subsequently washed with distilled water to remove any residual impurities. The polished GCE was subsequently modified with La₂NiO₄@rGO nanocomposite via drop-casting, yielding the La₂NiO₄@rGO modified GCE. Subsequently, other modified electrodes La₂NiO₄/GCE and rGO/GCE were also obtained by the previously described drop-casting method. Scheme 1 shows the sequential synthesis process of La₂NiO₄@rGO in the DES-assisted hydrothermal approach.

Scheme 1 Pictorial depiction of the preparation and application of the La₂NiO₄@rGO modified GCE.

2.7. Preparation of real samples

For the preparation of real specimens, locally available agricultural products such as carrots, cabbage, oranges, and apples were carefully selected for real sample analysis. The samples mentioned above were procured from Binjiang Market in Taipei, Taiwan. First, 10 g of each sample was carefully chopped to ensure uniformity. Subsequently, the finely chopped specimens were soaked in 0.1 M PB solution at pH 7 for 24 hours to facilitate optimal extraction. The samples were then centrifuged for 30 min at 5000 rpm to isolate the desired specimen. Furthermore, the supernatant from each real sample was mixed with a PBS (pH 7) electrolyte solution for further electrochemical analysis with a standard addition calibration approach. To evaluate the recovery efficiency of EP in practical samples, the standard-addition method was employed. In this method, known amounts of EP were introduced into real samples and the recovery percentage (%R) was determined using eqn (1),¹⁹

Here, C_S represents the measured concentration of the spiked sample, C_U corresponds to the concentration in the unspiked sample, and C_A refers to the actual concentration of the analyte added. Using this relationship, the recovery values for real samples were calculated.

3. Results and discussion

3.1. X-ray diffraction (XRD) analysis

The crystal phase structure of the as-synthesized materials was examined by XRD (Fig. 1). Fig. 1a depicts the XRD patterns of the pure rGO, La₂NiO₄, and La₂NiO₄@rGO nanocomposite, respectively. The XRD pattern of La₂NiO₄ exhibits a series of well-defined diffraction peaks at 2θ values of 24.07°, 28.12°, 31.35°, 32.75°, 43.7°, 47.01°, 54.86°, 55.67°, 56.01°, 57.59°, 65.38°, 68.69°, 77.46° and 78.22°, which can be unequivocally indexed to the (101), (004), (103), (110), (114), (200), (116), (204), (107), (213), (206), (220), (303) and (310) lattice plane reflections, respectively. This result is consistent with previous reports. The X-ray diffraction reflections reveal that La₂NiO₄, which is characterized by a distinct set of lattice parameters (a = 3.85 nm, b = 3.85 nm, c = 12.65 nm; a = b ≠ c), yields reflections corresponding to the tetragonal phase with lattice parameters a = 3.85 nm, b = 3.85 nm, c = 12.65 nm (a = b ≠ c). The corresponding interplanar spacing (d-spacing) values are 2.62, 2.26, 1.90, 2.38, 1.34, 1.24, and 1.5 Å, which also provide supplementary confirmation of its tetragonal crystallographic structure. The observed diffraction peaks exhibit a remarkable congruence with the standard X-ray diffraction pattern for La₂NiO₄, crystallizing in a tetragonal lattice structure, as authenticated by the Joint Committee on Powder Diffraction Standards (JCPDS) database, specifically card no. (JCPDS No: 00-011-0557). Thus, the synthesized material reveals a remarkable degree of compositional homogeneity and structural coherence, attesting to its high purity and well-ordered formation. Additionally, the presence of a prominent diffraction peak confirms the irrefutable evidence of the formation of reduced graphene (rGO) at diffraction peak 2θ, which corresponds to the crystal plane (002). Additionally, the diffraction peaks in the La₂NiO₄@rGO nanocomposite match those of the standard La₂NiO₄ and rGO patterns, indicating the successful formation of the nanocomposite.⁴⁴ Thus, for the La₂NiO₄@rGO nanocomposite, the XRD profile confirms the seamless integration of the two distinct phases, La₂NiO₄ and rGO, indicating the successful formation of the architecture.

Fig. 1 XRD phase analysis of the prepared rGO, La₂NiO₄, and La₂NiO₄@rGO (a), crystal structure of La₂NiO₄ (b), FTIR analysis of different produced materials (c), and XPS survey spectrum of La₂NiO₄@rGO (d).

The crystal structure of the as-prepared material is shown in Fig. 1b, and it crystallizes in a tetragonal lattice. In details, the La³⁺ atoms are bonded to the nine-coordinate geometry with the 9 O²⁻ atoms in the crystal plane. The bond length ranges approximately from 2.31 to 2.83 Å. Furthermore, corner-sharing NiO₆ octahedra were observed due to bond formation between the Ni²⁺ atoms and six O²⁻ atoms. In particular, the corner-sharing octahedra were not tilted. In the crystal image, we have noticed that there are two inequivalent O²⁻ sites presented in the crystal lattice. The first O²⁻ site was bonded to four equivalent La³⁺ and two equivalent Ni²⁺ ions, producing a mixture of distorted corner-, edge-, and face-sharing La₂NiO₄ octahedra. And the second O²⁻ site was bonded in the 6-coordinate geometry to five equivalent La³⁺ and one Ni²⁺.

3.2. Fourier transform infrared spectroscopy (FT-IR) analysis

The presence of functional groups in the as-produced materials was analysed by FT-IR spectroscopy and is shown in Fig. 1c. Firstly, the peaks at 502 cm⁻¹ and 642 cm⁻¹ were attributed to the Ni–O and La–O stretching vibrations, respectively. Additionally, strong stretching vibrations of C–O bonds were observed at 1023 cm⁻¹ and 1223 cm⁻¹.^45,46 Moreover, the peak observed at 1741 cm⁻¹ is associated with the C=O stretching vibration. The formation of the desired La₂NiO₄@rGO nanocomposite shows all the peaks observed individually, suggesting that the materials have formed without any additional impurities.

3.3. X-ray photoelectron spectroscopy (XPS) analysis

The XPS analysis was meticulously employed to elucidate the valence states of each elemental constituent, providing high quantitative precision in determining their electronic configurations, thereby underscoring the compositional integrity and homogeneity of the hybrid material. High-resolution XPS spectra revealed the survey spectrum of La₂NiO₄@rGO, identifying the presence of lanthanide (La), nickel (Ni), oxygen (O), and carbon (C) elements, thereby verifying the material's elemental constituents, as shown in Fig. 1d. The deconvoluted La 3d spectra exhibit distinct La 3d_5/2 and La 3d_3/2 peaks, inferring the spin–orbit coupling assigned at binding energies (BE) of 833.31 eV and 850.07 eV, respectively^21,47–49 (Fig. 2a). Additionally, the two corresponding satellite peaks are observed at binding energies of 836.46 eV and 853.450.07 eV, respectively, which are associated with charge transfer, confirming the presence of a characteristic predominant phase of the La³⁺ oxidation state.

Fig. 2 High resolution deconvoluted spectra of La 3d (a), Ni 2p (b), O 1s (c), and C 1s (d).

Notably, the energy separation between these peaks is 16.8 eV, indicating the presence of trivalent oxidation state La in the prepared nanocomposite. The XPS spectrum of Ni was deconvoluted into two main components at approximately 852.8 and 870.8 eV, which are attributed to the Ni 2p_3/2 and Ni 2p_1/2 levels, respectively. Furthermore, the Ni 2p_3/2 peak was split into two components at 852.8 and 854.7 eV, indicating the coexistence of Ni²⁺ and Ni³⁺ oxidation states. The corresponding satellite peaks observed at 861.4 and 864.7 eV further support the presence of mixed Ni²⁺/Ni³⁺ states. Additionally, the Ni 2p_1/2 region was deconvoluted into two peaks at 870.8 and 874.2 eV, which can be assigned to the Ni²⁺ and Ni³⁺ oxidation states, respectively (Fig. 2b). This Ni²⁺/Ni³⁺ redox couple also provides active sites for analyte adsorption, thereby increasing the current response in electrochemical measurements. Furthermore, the deconvolution spectra of O 1s were observed with binding energies at 527.1 eV and 530.3 eV, corresponding to the oxygen lattice present in the La₂NiO₄ and oxygen functionalities in oxidized reduced graphene oxide that adsorbed hydrocarbon moieties, as shown in Fig. 2c. Moreover, the O 1s spectral landscape is attributed to lattice oxygen (O²⁻) and an oxygen vacancy (O_v), thereby offering unique insight into the material's oxygen-related defect phenomena. However, the high-resolution C 1s XPS spectra exhibit two peaks, indicating different carbon bonding environments, as depicted in Fig. 2d. The peak at 283.5 eV is associated with C–C/C=C bonds, typically found in sp³-hybridized carbon networks or extended conjugated systems. The peak situated at 287.3 eV ascribed to the C=C–O bond. Ultimately, the XPS spectral analysis provides clear evidence of the La₂NiO₄@rGO nanocomposite, confirming the accurate characterization of its electronic structure.

3.4. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX) analysis

FESEM was employed to comprehensively investigate the surface morphological features and microstructural characteristics of La₂NiO₄, rGO, and the La₂NiO₄@rGO nanocomposite, providing a detailed understanding of their topographical and textural properties. Fig. 3(a)–(c) presents FESEM images of the as-synthesized La₂NiO₄, revealing an assembly of fractal-inspired nano capsule branches with soft, rounded edges. The morphography images of rGO highlight an ultrathin 2D nanosheet architecture with a characteristic stacked and crumpled surface in different orientations, attributed mainly to the reduction process that transforms GO into rGO with abundant functional surface sites and a large surface area in Fig. 3(d)–(f). As evident from Fig. 3(g)–(i), the strategic integration of La₂NiO₄ and rGO via systematic grafting establishes a synergistic conduit with a high surface area, enriched with abundant functional sites, providing an unobstructed path for electrons during electrochemical sensing, with the intrinsic morphologies of both components maintained throughout the composite formation process.

Fig. 3 FE-SEM micrographs of La₂NiO₄@rGO at different magnifications (a)–(c), La₂NiO₄ (d)–(f), and rGO (g)–(i).

Furthermore, Fig. S1(a) and (b) shows the overall EDX elemental mapping distributions of the La₂NiO₄@rGO nanocomposite, visualizing the coexistence of La, Ni, O, and C. Moreover, Fig. S1(c)–(f) identifies the uniform distribution of La, C, Ni, and O elements in the La₂NiO₄@rGO composite. Meanwhile, EDX spectroscopy reveals the exact presence of La (12.1 wt%), Ni (38.8 wt%), O (22.3 wt%), and C (26.8 wt%) elements, confirming the purity of the synthesized materials (Fig. S1g).

3.5. Transmission electron microscopy (TEM) analysis

To gain deeper insight into the microstructural features of the La₂NiO₄@rGO composite, high-resolution transmission electron microscopy (TEM) was employed. Fig. 4(a)–(d) depicts the TEM images of the as-prepared La₂NiO₄@rGO, supporting the emergence of fractal-inspired nano-capsules with their soft and round edges, which are consistent with the FE-SEM. Interestingly, Fig. 4(a)–(d) shows that the nanostructured capsules are enveloped by a layer of crumpled reduced graphene oxide (rGO) sheets, giving rise to a nanoscale morphology that provides a crucial electrochemical feature. As shown in Fig. 4(c), the La₂NiO₄ particles exhibit a distinctive branched morphology at the 100 nm scale, with a large surface area. The figures mentioned above, Fig. 4(a)–(d), apparently shows a significant diminution in clusters of La₂NiO₄, giving rise to a likely uniform distribution over the reduced graphene oxide matrix. Fig. 4(e) shows the clear lattice fringes with an interplanar d-spacing of approximately 0.23 nm, confirming the crystalline nature of the material. Additionally, a deeper insight into the crystallographic nature of La₂NiO₄@rGO is evident from the concentric bright rings in the SAED pattern shown in Fig. 4f. This analysis reveals a precise correlation with the planes indexed to the (103), (200), and (213), providing a compelling validation of the X-ray diffraction (XRD) data. Thus, despite maintaining individualistic structural integrity, TEM images of the La₂NiO₄@rGO nanocomposite indicate the successful formation of the desired material, with enhanced electron transfer rates and a revived active surface, thereby improving electrocatalytic performance.

Fig. 4 HR-TEM topographical images of La₂NiO₄@rGO (a)–(d). Lattice fringes (e) and the selective area diffraction electron (f) of the prepared La₂NiO₄@rGO.

3.6. Electrochemical investigations

3.6.1. Electrochemical impedance spectroscopy (EIS) analysis. The EIS is used to investigate charge-transfer characteristics at the electrolyte–electrode interface, providing valuable insights into the interfacial properties of the modified electrode. Accordingly, the EIS measurements were conducted with a redox probe of 5 mM [Fe(CN)₆]^3−/4− in the presence of 0.1 M KCl solution. In electrochemical impedance spectroscopy (EIS), the electrode–electrolyte interface properties and the electron charge transmission of the diverse electrodes were evaluated through several components of the Randles equivalent circuit, including double-layer capacitance (C_dl), impedance of Warburg (Z_W), solution resistance (R_s), and charge transfer resistance (R_ct) as depicted in Fig. 5a. The Nyquist plot of EIS reveals the surface characteristics of each electrode, showing that the semicircular and linear regions correspond to kinetically controlled electron-transfer processes at higher frequencies and mass-transfer limitations at lower frequencies, respectively. The Nyquist plot recorded for the bare GCE, rGO/GCE, La₂NiO₄/GCE, and La₂NiO₄@rGO/GCE features characteristic semi-circular arc-like patterns in the high-frequency region as shown in Fig. 5a. The charge transfer resistance (R_ct) values of the modified electrodes, correlating with the semi-circular region of the Nyquist plot, indicate the extent of redox reactions occurring at each electrode–electrolyte interface. Notably, the bare GCE exhibited the largest arc diameter, with an R_ct value of 974 Ω cm², indicating high resistance due to its unmodified surface. Interestingly, the R_ct value for the modified electrode La₂NiO₄@rGO/GCE is 345 Ω cm², with the smallest arc diameter compared to La₂NiO₄/GCE (536 Ω cm²) and rGO/GCE (779 Ω cm²). Thus, the reduced semicircle for La₂NiO₄@rGO/GCE indicates lower charge-transfer resistance, suggesting enhanced electrochemical reactivity and peak response. Interestingly, it also shows that the synthesized nanocomposites exhibited minimal internal resistivity, facilitating effortless electronic and ionic transmission through synergistic interactions. This enables hassle-free ionic transport, underscoring the potential of these nanocomposites for efficient electrochemical applications.

Fig. 5 EIS spectra of different material modified GCEs (a), CV graphs of the bare GCE, rGO/GCE, La₂NiO₄/GCE, and La₂NiO₄@rGO/GCE in 5 mM [Fe(CN)₆]³⁻/⁴⁻ with 0.1 M KCl solution (b), CV profile of the La₂NiO₄@rGO/GCE in 5 mM [Fe(CN)₆]³⁻/⁴⁻ with 0.1 M KCl at different scan rates (c), and corresponding calibration plot for the scan rate studies (d).

3.6.2. Electrocatalytic behavior of different electrodes. Additionally, the electrocatalytically active surface area of the proposed nanocomposites plays a pivotal role in governing interface characteristics, catalytic efficiency, charge-transfer kinetics, and electrical conductivity. Fig. 5b unveils the CV responses of the bare GCE, La₂NiO₄/GCE rGO/GCE, and La₂NiO₄@rGO/GCE electrodes in 5 mM [Fe(CN)₆]³⁻/⁴⁻ in 0.1 M KCl electrolyte at 100 mV s⁻¹ sweep rate. The [Fe(CN)₆]^3−/4− redox probe exhibited a characteristic anodic peak current due to the oxidation of [Fe(CN)₆]⁴⁻ to [Fe(CN)₆]³⁻, while the cathodic peak current manifested in the reverse scan, signifying the reduction process shown in Fig. 5b. This presents a comparative analysis of the CV responses of the bare GCE, rGO/GCE, La₂NiO₄/GCE, and La₂NiO₄@rGO/GCE modified electrodes. The CV response of the bare GCE exhibited redox current values of I_pa = 81.86 µA and I_pc = −62.89 µA at E_pa = 0.38 V and E_pc = 0.10 V, respectively. The rGO/GCE electrode shows redox currents of I_pa = 118.9 µA and I_pc = −106.3 µA at E_pa = 0.31 V and E_pc = 0.18 V, due to the highly conductive nature of the carbonaceous species. The La₂NiO₄/GCE modified electrode demonstrated enhanced electrochemical activity, with increased redox currents of I_pa = 129.2 µA and I_pc = −119.6 µA at E_pa = 0.298 V and E_pc = 0.196 V, respectively, attributed to the layered perovskite structure of La₂NiO₄. Most notably, the La₂NiO₄@rGO/GCE modified electrode demonstrates exceptional electrochemical performance, characterized by the highest peak currents (I_pa = 138.8 µA and I_pc = −124 µA), lowest optimized peak potentials (E_pa = 0.308 V and E_pc 0.183), and a peak separation (ΔE_p) of 0.125 mV. This outstanding performance underscores the synergistic benefits of integrating La₂NiO₄ and rGO, which together yield enhanced active site density, elevated conductivity, improved electron transfer efficiency, and an expanded active surface area.

To investigate the electroactive surface area of La₂NiO₄@rGO/GCE, we evaluated the redox peak responses in the presence of 5 mM [Fe(CN)₆]^3−/4− at various scan rates ranging from 20 to 200 mV s⁻¹. The resulting CV signals are displayed in Fig. 5c. Additionally, linear plots of redox peak responses versus the square root of the scan rate are shown in Fig. 5d. The linear scaling of the peak value with the scan rate results in I_pa = 17.532x + 15.065, with an R² value of 0.9984 for the anodic peak response and I_pc = −14.314x − 19.677, with an R² value of 0.9982 for the cathodic current response. The relation between the active surface and electrochemical activity has been obtained by calculating the electrochemical active surface area (ESA). Based on the voltametric peak current, the Randles–Sevcik equation was used to calculate the ESA values as shown below in eqn (2),

I_p = 2.69 × 10⁵AD^½n^3/2ν^1/2C (2)

where n represents the number of electrons transferred, A denotes the working surface area of the electrode, C represents the analyte concentration, D represents the diffusion coefficient of the analyte (7.6 × 10⁻⁶ cm² s⁻¹), and ν represents the scan rate (mV s⁻¹). Calculations based on this equation revealed that the electrochemically active surface area of La₂NiO₄@rGO/GCE was 0.1176 cm², surpassing that of the bare GCE (0.0685 cm²). Thus, due to the greater number of exposed surface sites, the La₂NiO₄@rGO-modified GCE yields the highest ESA value. This finding suggests that the La₂NiO₄@rGO nanocomposite enhances the redox reaction kinetics more effectively than other electrodes, thereby proposing improved electrochemical performance.

3.7. Electrochemical activity of the prepared electrodes towards EP

3.7.1. Electrochemical activity of the diverse electrode. The electrochemical behavior of EP and the corresponding diverse current responses were systematically recorded at various electrodes, including GCE, La₂NiO₄/GCE, rGO/GCE, and La₂NiO₄@rGO/GCE, using the CV technique. Thus, the aforementioned experiment was performed with N₂-saturated 0.1 M PBS solution (pH = 7) in the presence of 40 µL EP at a scan rate of 100 mV s⁻¹, and the results are summarized in Fig. 6a. Here, the result clearly shows well-defined reduction peaks within the potential window of 0.2–1.0 V, indicating a favourable electrochemical reaction. The recorded CV curves of the bare GCE, rGO/GCE, La₂NiO₄/GCE, and La₂NiO₄@rGO/GCE reveal distinct reduction characteristics. In detail, the bare GCE exhibited a negligible redox peak current (I_pc = −8.1 µA) at −0.69 V, indicative of sluggish electron-transfer kinetics. In contrast, the La₂NiO₄-modified GCE demonstrates an enhanced reduction peak current response (I_pc = −35.99 µA, E_pc = −0.90 V), suggesting that La₂NiO₄ facilitates electron transfer. Additionally, the rGO-modified GCE showed a lower peak current than La₂NiO₄, but the peak potential was well shifted towards a lower potential (I_pc = −28.14 µA, E_pc = −0.781 V), suggesting that it is a more suitable material for incorporation with the as-prepared material. Notably, the La₂NiO₄@rGO/GCE exhibited a well-defined, sharp redox peak current response (I_pc = −37.76 µA, E_pc = −0.84 V) for reduction when it is modified with rGO, which indicates significantly enhanced electrochemical performance. Furthermore, a comparative analysis of the current responses for each electrode configuration (bare GCE, La₂NiO₄/GCE, rGO/GCE, and La₂NiO₄@rGO/GCE) is illustrated in the bar diagram (Fig. 6b). The results demonstrate that the La₂NiO₄@rGO/GCE exhibits superior electrocatalytic activity, facilitating efficient electron mediation for the sensitive detection of EP. This may be due to oxygen vacancies at the lattice sites of the La₂NiO₄@rGO/GCE, which result in a lower reduction peak potential and enhance the peak current.

Fig. 6 CV curves of the bare GCE and different modified electrodes with pH 7 PBS in the presence of EP at 100 mV s⁻¹ (a), corresponding bar chart for the different electrodes' current responses (b), CVs at different loading amounts of La₂NiO₄@rGO/GCE (c) and their corresponding bar chart (d), CV graphs of EP at various pH solutions (e), and corresponding peak current and peak potential against the diverse pH (f).

3.7.2. Influence of the catalyst quantity. The optimized amount of the electrode modifier catalyst plays a vital role in enhancing EP detection. The different CV curves were recorded to determine the optimal catalyst loading for EP detection. Various catalyst amounts ranging from 2.0 to 10 µL mg⁻¹ were investigated in 0.1 M PBS at a scan rate of 100 mV s⁻¹, with a constant 40 µM addition of EP, as shown in Fig. 6c. The voltammograms show that the reduction peak current increases linearly with increasing catalyst loading from 2.0 to 6.0 µL, indicating that greater amounts of La₂NiO₄@rGO nanocomposite on the GCE surface improve the sensitivity for EP detection. However, the reduction peak decreases rapidly with increasing catalyst quantity from 8 to 10 µL, which is attributed to a thicker electrode that restricts electron conduction, resulting in diminished reduction peak current and unsatisfactory electroanalytical performance. Interestingly, optimal EP detection was achieved at a catalyst loading of 6.0 µL, as evidenced by the highest reduction peak current (I_pc = −37.91 µA). This highest current was attributed to enhanced electron conduction between the modified GCE surface and the electrolyte, enabling efficient charge transfer and yielding enhanced sensitivity and selectivity. The corresponding bar diagram illustrating the optimization study is presented in Fig. 6d.

3.7.3. Influence of the pH. The pH of the electrolyte solution exerted a considerable influence on the electrochemical reduction behavior of EP, indicating a pH-dependent redox process. Thus, the influence of pH in the La₂NiO₄@rGO/GCE electrode for the electrochemical detection of EP was systematically recorded using CV. Accordingly, CVs were investigated over a range of pH values in the presence of 40 µM EP in 0.1 M PBS solution at a scan rate of 100 mV s⁻¹. The resulting pH-dependent CV profiles, illustrating the effects of pH on the electrochemical response of EP, are presented in Fig. 6e. The increased reduction peak current was observed with an increase in pH from 3.0 to 11.0, accompanied by a slight shift in potential. To facilitate comparison, a calibration plot was constructed to display the cathodic peak currents across different pH levels (Fig. 6f). This visualization highlights the correlation between pH and current response, providing insights into the electrochemical behavior under varying acidity or alkalinity conditions. So, the pH-dependent investigation revealed that acidic environments (pH 3.0 and 5.0) yielded well-defined peak currents, whereas alkaline conditions (pH 9.0 and 11.0) yielded poorly resolved peaks with reduced electrochemical activity. But, interestingly, a neutral pH (pH 7.0) was found to optimize the electrochemical response, producing sharp, well-defined peaks with enhanced reduction peak currents, which may be due to proton-coupled electron transfer. Fig. 6f shows the calibration plots for different pH levels. The obtained slope values from the calibration plots were used in the Nernst equation,

E_b = −(0.0592m/n) pH + b (3)

Hence, “m” and “n” symbolize the stoichiometric values for protons and electrons. The ratio of “n” to “m” is derived from eqn (3), as provided earlier. Therefore, the findings suggest that the number of electrons transferred in the EP redox mechanism aligns with the number of protons involved in the reaction, both representing an identical count of transferred electrons. However, interestingly, the electrochemical reduction indicates a 1 : 1 stoichiometry between the number of electrons transferred and the number of protons involved, suggesting a concerted electron–proton transfer process.

3.7.4. Effect of various concentrations and scan rates. To investigate charge conduction from the electrode La₂NiO₄@rGO/GCE to the electrolyte, the electrochemical study was assessed by increasing the addition of EP in 0.1 M PBS (pH 7.0) using the CV technique, as illustrated in Fig. 7a. The corresponding CV curves indicate an increase in the reduction peak current without a decrease in peak potential, demonstrating the good accumulation ability of EP on the surface of the La₂NiO₄@rGO/GCE-modified GCE. Furthermore, it has been noted that the successive additions of 20–280 µM of EP resulted in a steady increase in reduction current with a minimal change in peak potential. Additionally, the linear calibration plot of reduction peak current vs. EP concentration is displayed in Fig. 7b, along with a linear regression equation and correlation coefficient: I_pc (µA) = −0.42 [µM − 25.91], R² = 0.993, indicating that the quantity of EP linearly affects the peak current. Thus, the developed La₂NiO₄@rGO/GCE sensor exhibits a notable enhancement in reduction peak current, as evidenced by the increase with rising EP concentrations. This improved conductivity is attributed to the synergistic effect between La₂NiO₄ nanoparticles and rGO, which increases the surface area and the number of catalytically active sites. A more detailed investigation of the concentration-dependent effects was conducted using DPV analysis.

Fig. 7 CV graphs of La₂NiO₄@rGO/GCE with increasing concentration of EP in 0.1 M PB (pH 7.0) (a), linear plot of various concentrations of EP vs. cathodic peak current (b), CVs of La₂NiO₄@rGO/GCE in 0.1 M PB (pH 7.0) with an increment of scan speed from 20–200 mV s⁻¹ (c), and linear plot of cathodic peak current vs. sq. root of scan rate (d).

The electron transport properties and kinetics of La₂NiO₄@rGO/GCE at various scan rates were studied using the CV technique. The impact of scan rate on the La₂NiO₄@rGO/GCE electrode towards EP was investigated in 0.1 M PBS containing 20 µM EP by varying the sweep rate from 20–200 mV s⁻¹ (Fig. 7c). Thus, the detailed analysis of the voltametric curves shows a linear correlation between the cathodic peak current (I_pc) and scan rate (v), with a proportional increase in I_pc observed over the scan rate range of 20–200 mV s⁻¹. A closer inspection reveals two distinct cathodic peaks: a minor peak (R₂) and a dominant peak (R₁). Furthermore, a small peak (O₁) is discernible in the reverse scan and appears insensitive mainly to the presence of analyte molecules. The emergence of this minor peak is likely attributed to modifications at the electrode surface and the formation of electrochemically generated intermediates. Moreover, the progressive negative shift in peak potentials observed during the electro-reduction of EP suggests that the transfer of radicals from the electrode surface influences the reduction process. This phenomenon likely promotes the accumulation of ions near the electrode, thereby contributing to the linear increase in the reduction peak current. However, the aforementioned shift in the cathodic peak potential with increasing scan rate was also influenced by the scan-rate-dependent diffusion layer thickness, which governs the mass transport flux at the electrode surface. Furthermore, the electrochemical reaction mechanism of EP on the La₂NiO₄@rGO/GCE was gleaned from the scan-rate-dependent analysis. To gain further insights, the linear correlation between the cathodic peak current and the square root of the scan rate was also studied (shown in Fig. 7d). The obtained regression, I_pc (µA) = −73.039 v^1/2 [V s]^−1/2 + 0.488; R² = 0.9901.

Thus, the observed correlation rigorously confirms the predominance of a diffusion-controlled process at the modified electrode, where mass transport limitations dictate the reaction kinetics for the electrochemical reduction of EP at the La₂NiO₄@rGO/GCE interface. For a deeper understanding, the mechanisms of the electrochemical reduction of the EP involve a multi-step process. Initially, the EP molecules diffuse from the bulk solution to the electrode surface, where they adsorb onto the La₂NiO₄@rGO interface. This adsorption is facilitated by π–π stacking interactions with the rGO and potential hydrogen bonding with surface functional groups. Their diffusion primarily determines the rate at which EP molecules reach the electrode surface through the solution. Once adsorbed, the nitro (–NO₂) group of EP undergoes electrochemical reduction, involving a four-electron (4e⁻) and four-proton (4H⁺) transfer process, resulting in the formation of N-phenylhydroxylamine. The La₂NiO₄ component, containing Ni(II) species, may undergo redox transitions during this process, enhancing electron transfer and amplifying the reduction of EP, thereby increasing the current response. During the reverse scan in cyclic voltammetry, N-phenylhydroxylamine can be oxidized to a nitroso derivative through a two-electron (2e⁻) and two-proton (2H⁺) transfer process. This sequence of reactions aligns with established electrochemical mechanisms for the reduction of nitroaromatic compounds. Consequently, the addition of EP induces a distinct reduction that is significantly influenced by the kinetics of the electrochemical reactions.⁵⁰ Additionally, the detailed mechanisms of the La₂NiO₄@rGO/GCE electrode towards the detection of EP are shown in Scheme 2.

Scheme 2 The probable mechanism occurring at the electrode–electrolyte interface.

3.7.5. Analytical assessment of the proposed sensor. The differential pulse voltammetry (DPV) technique is widely used for most of the optimization for the detection of EP. However, to investigate low-level EP concentrations, highly sensitive analytical methods, such as DPV, are used. Accordingly, the electrochemical analysis of EP at the La₂NiO₄@rGO-modified electrode was carried out using DPV. Thus, the proposed sensor's analytical features, including limit of detection (LOD), linear range, threshold activities, and sensor performance, were evaluated. The LOD of the La₂NiO₄@rGO modified GCE for EP was determined using the following eqn (4):where s is the slope of the calibration curve and SD is the standard deviation of the blank. To conduct the DPV analysis, an electrochemical cell was assembled with three electrodes containing varying concentrations of EP in 0.1 M PBS solution, ranging from 0.05 to 93 µM (Fig. 8a). It has been observed that upon the addition of EP, a distinct reduction peak was observed at −0.84 V. The reduction peak current increases as the concentration increases from the lower to higher values. However, the absence of a potential shift and the appearance of broader peaks at higher concentrations contribute to the outstanding sensing capabilities of the constructed sensor. Furthermore, Fig. 8b shows the calibration graph between the EP concentration and the reduction peak current I_pc, with a correlation coefficient of R² = 0.991. Based on this equation, the LOD (34 nM) and sensitivity (7.4923 µA µM⁻¹ cm⁻²) of the La₂NiO₄@rGO/GCE were calculated towards the detection of EP.

Fig. 8 DPV of La₂NiO₄@rGO/GCE for EP concentrations of 0.05–93 µM (a), linear plot of EP concentration vs. reduction peak current, i–t amperometry study of La₂NiO₄@rGO/RRDE against EP concentration (c), and corresponding linear plot of EP concentration vs. reduction peak current (d).

3.7.6. Amperometry (i–t) determination of EP. Amperometry (i–t) is a highly sensitive and reliable electrochemical method for detecting analytes using a rotating ring-disc electrode (RRDE), performed under optimized CV conditions. However, the i–t performance exhibits outstanding capabilities in detecting analytes at low concentrations, offering high selectivity, rapid sensitivity, and excellent detection limits. Thus, measurements for the La₂NiO₄@rGO/RRDE sensor were conducted using the i–t technique to detect EP. The tests were carried out in a nitrogen-saturated 0.1 M PBS solution at an optimal potential of 0.84 V and a rotation speed of 1848 rpm. Specifically, the first-rate outstanding electrochemical properties of the modified electrode were demonstrated by its rapid attainment of steady state, accompanied by a swift and sensitive current response to EP additions at a controlled rate. Different concentrations of EP were added to the electrolyte solution, and the resulting increase in cathodic peak responses was recorded (Fig. 8c). However, the results show that the stepwise increase in current is in good agreement with the addition of EP, reaching a steady-state current within 5 seconds. This electrocatalytic process exhibits two distinct linear ranges: the first (0.05–117.4 µM) is described by the equation given aside, I (µA) = 0.044 [EP]/µM + 5.5692, with a correlation coefficient of R² = 0.9905 (Fig. 8d) at lower concentrations. The second linear range (125.75–1625 µM) is observed, with a linear regression equation of I (µA) = 0.006 [EP]/µM + 10.983 and R² = 0.9997 at higher concentrations. The limit of detection (LOD) calculated from the lower concentration range is 18 nM, while the sensitivity of the La₂NiO₄@rGO modified RRDE is 0.374 µA µM⁻¹ cm⁻². Notably, our results demonstrate a very low LOD, a wide linear range, and sound sensitivity for EP detection, comparable to or surpassing those of previously reported modified electrodes (Table 1).

Table 1 The La₂NiO₄@rGO nanohybrid modified GCE in comparison with the previously reported EP sensor

Electrode	Technique	Linear range	LOD	Ref.
a Glassy carbon electrode/chitosan/magnetite (iron(II,III) oxide NP.b 2D molybdenum carbide/screen printed electrode.c Zinc stannate-stannic acid nanoparticle/glassy carbon electrode.d Gold nanoparticle/glassy carbon electrode.e Lanthanum nickel oxide-reduced graphene oxide/glassy carbon electrode.
GCE/CS-Fe2O3^a	DPV	0.34–4.46	0.187 µM	51
MoC/SPCE^b	DPV	0.02–43	0.004 µM	3
Zn2SnO4/SnO2/GCE^c	DPV	0.1–78.4	0.0059 µM	52
Ag NP/GCE^d	LSV	0.0412–7.90	0.0412 µM	53
La2NiO4@rGO/GCE^e	DPV	0.05–93	34 nM	This work
	i–t	0.05–1625	18 nM

---

3.7.7. Anti-interference ability. To prevent EP responses from overlapping with those of other species with similar reduction potentials, selectivity is still a crucial factor. Thus, the selectivity of the proposed sensor was studied by analyzing its response to a range of organic compounds and inorganic ions typically presented in real samples. Fig. S2 represents i–t responses with 5-fold excess addition of interfering analytes in the presence of 50 µM of EP such as (a) fenitrothion, (b) 4-nitrophenol, (c) chlorpyrifos, (d) diuron, (e) carbofuran, (f) Na⁺, (g) K⁺, (h) Zn²⁺, (i) Mg²⁺, (j) dopamine, and (k) catechol. Interestingly, it has been observed that the interfering molecules do not produce any substantial change in the reduction current for EP quantification. This result clearly demonstrates the exceptional selectivity of La₂NiO₄@rGO/RRDE, making it an ideal platform for real-time sensing applications.

3.7.8. Reproducibility, repeatability, and stability studies. Reproducibility is a critical factor that significantly impacts the practical applicability of sensors. To evaluate this aspect, five different La₂NiO₄@rGO-modified GCE electrodes were subjected to a single measurement. This experiment was performed in a N₂-purged PBS electrolyte solution (pH 7.0) containing 40 µM of EP ethyl at a scan rate of 100 mV s⁻¹. The observed results, along with the bar representation graph, are presented in Fig. 9a and b, respectively. Again, to confirm the sensor's repeatability, five consecutive trials were performed using La₂NiO₄@rGO/GCE-modified electrodes (Fig. 9c), and the corresponding current values are summarized in a bar chart in Fig. 9d. Thus, the repeatability of the La₂NiO₄@rGO/GCE was validated by a calculated RSD of approximately 3.61%, indicating high consistency and reliability. Furthermore, to ensure long-term durability, the cycling stability of the La₂NiO₄@rGO/GCE electrode was assessed by monitoring material degradation, electrode fouling, and redox reaction reversibility through CV experiments for 100 cycles towards the detection of EP. The corresponding CV results are demonstrated in Fig. 9e. The noted outcomes after the 100 segments have retained up to 98.4% of the original current response. The findings confirm that the modified electrodes exhibit reliable reproducibility and repeatability towards EP. These findings conclusively demonstrate that La₂NiO₄@rGO/GCE exhibits marked improvements in stability and longevity for EP reduction, rendering it an ideal platform for sustained electrochemical applications.

Fig. 9 CVs of repeatability and their corresponding bar chart diagram (a) and (b), CV graphs and their bar diagram against reduction current (c) and (d), cycling stability (e), and operational stability (f).

However, the operational stability of the sensor above was assessed using the amperometry (i–t) technique in the presence of 50 µM EP. The current response was recorded over an extended operational period of 3000 s, exhibiting minimal signal oscillation and retaining approximately 99.02% of its initial current value (Fig. 9f). This result confirms the excellent operational stability of the La₂NiO₄@rGO nanocomposite.

3.7.9. Real sample analysis. Finally, the presented sensor was scrutinized for real-time monitoring to validate its pragmatic feasibility.^28,53–56 The specimens were collected from the local market in Taiwan and processed as previously reported, including grinding, centrifuging, and distillation. For the analysis of the real specimens, a known volume of EP was spiked into the sample. This spiking step was essential for establishing a controlled environment that allowed evaluation of the sensor's accuracy and precision in measuring EP in conditions that simulate real-life scenarios. The DPV method was employed to detect the EP in food specimens using a La₂NiO₄@rGO/GCE modified electrode submerged in N₂-purged 0.1 M PB 7. Furthermore, the obtained results are displayed in Fig. 10. Additionally, the calibration plots for the obtained values were portrayed in Fig. S3. The obtained results indicating the sensor's recovery efficacy are comprehensively documented in Table 2.

Fig. 10 DPV real-time analysis of carrot juice (a), cabbage (b), apple juice (c), and orange juice (d).

Table 2 Recovery percentage table for the determination of EP in various real-world specimens

Sample	Added	Found	Recovery	±RSD (n = 3)
Cabbage	10	9.961	98.49246	0.039
	20	19.755	92.57801	0.245
	30	29.745	93.25397	0.255
Carrot juice	10	9.827	93.73188	0.173
	20	19.611	89.45799	0.389
	30	29.57	89.78622	0.43
Apple juice	10	9.923	97.536	0.077
	20	19.8848	97.13647	0.1152
	30	29.964	99.2046	0.036
Orange juice	10	9.812	92.7329	0.188
	20	19.589	87.54923	0.411
	30	29.859	96.26984	0.141

---

3.7.10. Quantum chemical calculations. DFT calculations were also performed using Gaussian 16 to gain a deeper understanding of the mechanism of electrochemical reduction of ethyl parathion (EP). Geometry optimization of the EP molecule was performed using the B3LYP functional with the 6-31G(d,p) basis set. The optimized molecular structure of EP has been found, as shown in Fig. 11a. The energy bandgap was calculated as ΔE_g = E_LUMO − E_HOMO = 4.49 eV. In this study, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies are used to approximate the ionization potential (I = –HOMO) and electron affinity (A = –LUMO), which were found to be 6.86 eV and 2.37 eV, respectively (Fig. 11b and c).⁵⁷ Furthermore, to evaluate the hardness (η) and softness of the EP molecule, the obtained HOMO and LUMO energy values were utilized.⁵⁸ The hardness (η) and softness (σ) of the molecule can be expressed as η = ½ (I − A) and σ = 1/2η. The values of η and σ are found to be 2.25 and 0.222, respectively. Where I and A are the ionization potential and electron affinity, respectively.

Fig. 11 Density function theoretically predicted (a) optimized structure of EP, (b) lowest unoccupied molecular orbital (LUMO), (c) highest occupied molecular orbital (HOMO), and (d) electrostatic potential (ESP) distribution. Color codes of the elements: C-gray, N-blue, O-red, H-white, S-yellow.

Furthermore, the calculated LUMO distribution provides crucial insight into the reduction mechanism at the La₂NiO₄@rGO-modified electrode. The electrostatic potential of the optimized EP reveals a surface charge distribution. The electrostatic potential (ESP) mapping of the optimized EP structure displays the charge distribution across its surface (Fig. 11d). This mapping reveals a color shift from blue to red, corresponding to the increasing electron density associated with a negative potential. However, the negative ESP is concentrated predominantly over the nitro group (–NO₂) of EP, showing the presence of electronegative oxygen atoms and their lone electron pairs. The Mulliken atomic charge distribution further supports this localization around the nitro group. Thus, these negatively charged domains exhibit stronger interaction with the La₂NiO₄@rGO electrode surface, thereby facilitating preferential adsorption. This is because the electrode surface is engineered to have positively charged sites that attract the negative domains. Such a strong electrostatic interaction at the modified working electrode surface and the analyte lowers the energy barrier for adsorption, promotes efficient electron transfer, and enhances the reduction of EP at the electrode interface.

4. Conclusion

In this study, a facile DES-assisted hydrothermal method was employed to synthesize La₂NiO₄. Furthermore, the resulting La₂NiO₄@rGO composite was designed as a modified electrode for the electrochemical detection of EP. The integration of rGO sheets with La₂NiO₄ significantly enriched the density of electroactive sites for greater accessibility of EP adsorption, while the strong synergistic interaction between the two components collectively boosted the electrochemical performance of the La₂NiO₄@rGO nanocomposite in EP detection. Thus, the afore-mentioned La₂NiO₄@rGO sensor reveals a wide concentration range (0.02–1625 µM) through i–t studies, and a lower limit of detection (18 nM). Moreover, this work proposes a reliable method for EP monitoring, resulting in high sensitivity, selectivity, stability, and reproducibility. The structural and morphological characteristics of La₂NiO₄, rGO, and La₂NiO₄@rGO were also studied using several techniques, including XRD, FT-IR, XPS, FESEM, and TEM. In particular, the functional groups present and the electrostatic interactions of rGO facilitated the firm anchoring of La₂NiO₄ throughout the characterization and analysis of the La₂NiO₄@rGO nanocomposite. Furthermore, the La₂NiO₄@rGO sensor has been used to evaluate real samples, including carrot juice, apple juice, orange juice, and cabbage, demonstrating excellent recovery of EP (87–99%) and highlighting its strong potential for reliable EP detection. Nevertheless, it is planned to strategically enhance the electrocatalytic efficiency of the La₂NiO₄@rGO nanocomposite to achieve superior sensitivity and reliability in EP detection.

Author contributions

Farhana Yasmin Rahman: writing – review original draft, editing, validation, visualization, software, methodology, formal analysis, data curation, and conceptualization. Surya Rajendran: reviewing, visualization, validation, formal analysis, and conceptualization. Sakthinthan Subramanian: supervision, project administration, and funding acquisition. Mani Govindsamy, Te-Wei Chiu, and Syed Arshad Hussain: validation, resources, software, and conceptualization.

Conflicts of interest

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

Data availability

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5tc04157k.

Data will be made available on request.

Acknowledgements

We thank the Ming Chi University of Technology, Taishan, New Taipei City, Taiwan, and the National Taipei University of Technology, Taipei. This research was funded by the TEEP 2024 and the National Science and Technology Council (NSTC), Taiwan (NSTC 112-2221-E-027-039), (112-2221-E-027-032-), (113-2221-E-027-018-) and (114-2221E-027-071-).

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