Novel electrochemical sensor based on NiZnFe2O4/CPE for measurement of p-coumaric acid in honey

Zeinab Nabhan Khoveis; Elahe Mahmoodi-Khaledi; Hossain-Ali Rafiee-Pour; Mohsen Farhadpour

doi:10.1016/j.arabjc.2023.105472

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02 2023

:17;

105472

doi:

10.1016/j.arabjc.2023.105472

Novel electrochemical sensor based on NiZnFe₂O₄/CPE for measurement of p-coumaric acid in honey

Zeinab Nabhan Khoveis^{^a}, Elahe Mahmoodi-Khaledi^{^a,⁎}, Hossain-Ali Rafiee-Pour^{^a,⁎}, Mohsen Farhadpour^{^b}

aDepartment of Cell and Molecular Biology, Faculty of Chemistry, University of Kashan, Kashan, Iran

bDepartment of Plant Bioproducts, National Institute of Genetic Engineering and Biotechnology, Tehran, Iran

⁎Corresponding authors. e.mahmoodi_kh@kashanu.ac.ir (Elahe Mahmoodi-Khaledi), rafieepour@kashanu.ac.ir (Hossain-Ali Rafiee-Pour)

Received: 2023-5-29, Accepted: 2023-11-21,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

One of the most abundant phenolic acids in honey is p-coumaric acid (pCA), which can be used to identify natural honey from synthetic honey. In this research, a simple, sensitive, and fast method using a modified electrode was developed to determine pCA in honey samples. A carbon paste electrode modified with NiZnFe₂O₄ nanospinels (NiZnFe₂O₄/CPE) was used to measure pCA in aqueous samples using Differential Pulse Voltammetry (DPV). NiZnFe₂O₄ nanospinels were synthesized by the sol-gel method and were characterized by different methods. pCA oxidation peak currents from DPV showed linearity in the range of 1.60–250 μM with a detection limit of 0.48 μM. Repeatability and reproducibility of the developed method and electrode showed a relative standard deviation lower than 0.95%. The developed method based on NiZnFe₂O₄/CPE kept performance after over 16 days. Satisfactory result for selectivity in the presence of some interference species is an advantage of the developed method. Finally, NiZnFe₂O₄/CPE was successfully used for the determination of pCA in two honey samples. The pCA values were calculated from the standard addition method and they were 3.41 × 10^-5 and 1.70 × 10^-5 M, respectively. These results were confirmed by high performance liquid chromatography (HPLC) with UV detector.

Keywords

Carbon paste electrode

P-coumaric acid

NiZnFe2O4 nanospinel

Cyclic voltammetry

Differential pulse voltammetry

Honey

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1 Introduction

p-Coumaric acids (pCA) is a phenolic acid derivative of cinnamic acid with an OH group in its phenyl group at the para position of to the side (Boz, 2015). Tyrosine in microorganisms is responsible for pCA production (Vanholme et al., 2012). The biosynthesis of pCA in plants involves two chemical processes; conversion of phenylalanine into trans-cinnamic acid in the presence of phenylalanine ammonium-lyase (PAL) as a catalyst. In the next step, trans-cinnamic acid is hydroxylated in the para position under the action of the trans-cinnamic 4-hydroxylase enzyme (C4H) (Achnine et al., 2004). pCA has some biological features, including antioxidant (Kiliç and Yeşiloğlu, 2013, Peng et al., 2018), antibacterial (Lou et al., 2012), and anti-inflammatory (Pragasam et al., 2013) properties and is used as a conventional precursor in the production of perfumes and flavors in chemical products. pCA can inhibit free radicals breaking the propagation of free-radical chain reactions.

The hydroxyl group on the benzene ring reacts with reactive oxygen (or nitrogen) species (ROS or RNS) by donating hydrogen atoms and forming a radical form of pCA. This form is more stable than ROS or RNS owing to the interaction of the OH group with π electrons of the benzene ring. In other words, the radical form of pCA is stabilized by displacement and resonance frequency (Bakar et al., 2012, El-gizawy and Hussein, 2017) (Pereira et al., 2009, Hakyemez et al., 2021). It is also a good candidate in the pharmaceutical, cosmetics, food, and chemical industries (Gunia-Krzyżak et al., 2018; Ajel and Al-Nayili, 2022, Albdiry and Al-Nayili, 2022 ). pCA play as a common precursor for the biosynthesis of many derivatives such as flavonoids (Zhao et al., 2015), antocyans (Zha and Koffas, 2017), stilbenes (Li et al., 2015, Albo Hay Allah and Alshamsi, 2022) and phenylpropanoid compounds (Kawaguchi et al., 2017). Since pCA is of great biological importance, its diagnosis in pharmaceutical or pharmaceutical products is essential.

Although pollinator populations are a precious species due to their production of honey and their contribution to pollination services, their population is declining worldwide. In honey bees, a combination of stressors is known to cause colony losses (Mitton et al., 2020, Al-Nayili and Albdiry, 2021, Al-nayili and Rzoqy, 2022 ). Various studies show that pCA supplementation is directly related to the bee’s ability to cope with challenges or stressors. By measuring the amount of pCA in the honey of an area, it can be shown whether the colony population is decreasing or increasing. Also, since honey has both dietary and medicinal uses, the existence or absence of pCA in honey is an indicator to distinguish natural honey from artificial honey.

According to the literature, various techniques used for the analysis of pCA which include high performance liquid chromatography (HPLC) coupled with UV detection (Liu et al., 2006, Jeszka-Skowron et al., 2018, Nicacio et al., 2020, Tian et al., 2021 ), HPLC coupled with diode-array detection (Khanam et al., 2012), HPLC coupled with mass spectrometry detector (Zhang et al., 2019, Gbair and Alshamsi, 2022), reversed-phase HPLC (RP-HPLC) (Karthikeyan et al., 2015), HPLC coupled with pulse asymmetry (Freitas et al., 2018). The electrochemical sensors offer numerous applications in clinical diagnosis, environmental monitoring and food analysis which can offer advantages of low detection limits, a wide linear response range, and good stability and reproducibility. (Manjunatha et al., 2009, Manjunatha et al., 2011, Raril and Manjunatha, 2018, Manjunatha, 2019, Manohar, 2019, Prinith and Manjunatha, 2020, Hareesha et al., 2021a, Hareesha et al., 2021b ). CPE is a group of promising electrochemical or bioelectrochemical sensors with many applications (Monnappa et al., 2019; Prinith et al., 2019). Instability in non-aqueous solution, low ohmic resistance, lipophilicity and heterogeneity are some advantages of CPE. Various modifiers were used to improve the electrochemical performance of this electrode. Spinel ferrite nanomaterials are soft magnetic materials with an AFe₂O₄ formula where A and Fe are metal cations occupying tetrahedral and octahedral sites, respectively (Kale et al., 2018, Bajorek et al., 2019, Ghalkhani et al., 2022a, Ghalkhani et al., 2022b, Al-nayili and Muhammad, 2023, Shang et al., 2023, Tian et al., 2023 ). They showed catalytic and photocatalytic properties in various processes (Kefeni et al., 2017, Abdulhusain et al., 2022, Alwan and Alshamsi, 2022 ). Ferrites are generally fabricated by solid-state reactions between oxides or carbonates at high temperatures. They have advantages in diverse bio-usage and electronic industries (Sugimoto, 1999, Somvanshi et al., 2020). Various techniques have been used for the preparation of spinel ferrite nanomaterials, including sol–gel auto-combustion (Brykała et al., 2015, Bhagwat et al., 2019), coprecipitation (Kim et al., 2003), hydrothermal (Raskar et al., 2019), solvothermal (Zhan et al., 2019, Al-Abidy and Al-Nayili, 2023), sonochemical (Gedanken et al., 2016), etc. NiZnFe₂O₄ is used as a modifier in electrochemical sensors for several reasons. Firstly, it has a high electrical conductivity, which allows for efficient electron transfer between the sensor surface and the analyte. This improves the sensitivity and response time of the sensor (Teymourinia et al., 2021, Ahmad et al., 2022). Secondly, NiZnFe₂O₄ nanostructure has a large surface area, which provides more active sites for the detection of analytes. This increases the sensor's improves its detection limit. Additionally, NiZnFe₂O₄ is chemically stable and resistant to corrosion, making it suitable for long-term use in harsh environments (Tahir et al., 2021, Teymourinia et al., 2023a, Teymourinia et al., 2023b ). It also exhibits good catalytic activity, facilitating the electrochemical reactions involved in sensing. Overall, the use of NiZnFe₂O₄ as a modifier in electrochemical sensors helps enhance their performance by improving conductivity, increasing surface area, providing chemical stability, and promoting catalytic activity (Khan et al., 2021, Alwan and Salem, 2022, Baladi et al., 2023 ).

Various studies have been conducted on the authenticity of honey samples from the perspective of its biological, physical and chemical activities (Huzortey et al., 2022, Mahmoodi-Khaledi et al., 2017).

In this work, an electrochemical sensor based on a carbon paste electrode modified by NiZnFe₂O₄ nanospinels was used for the electrochemical determination of pCA in honey samples. The electrochemical properties of the prepared electrode were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods. pCA was determined by differential pulse voltammetry (DPV) methods. HPLC was used to determine pCA in honey samples.

2 Materials and methods

2.1

2.1 Materials

Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, >97%) and pCA (>99%) were prepared from Sigma Aldrich. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 98%), iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.95%), graphite (>99%), paraffin oil (>99%), potassium hexacyanoferrate (III, 99%), potassium hexacyanoferrate (II, 99%), glucose (>99%), Manganese sulfate (98%-102%), sodium nitrate (99.5%), potassium nitrate (99.9%) and solvents or other chemicals were purchased from Merck company (Germany). All other chemicals and solvents were in analytical grade.

2.2

2.2 Instrument

The used instruments in this work, their models and company manufacture are Autolab Potentiostat – Galvanostat (Metrohm, μIII Auto 71174, Swiss), pH meter (FanAzma Gostar, Iran), Digital balance (PA114C, OHAUS, Swiss), Ultrasonic bath (Eurosonic, Euronda, Italy), Magnetic stirrer with hot plate (MSH BASIC, IKA, Germany), X-ray Diffractometer (X pertpro Cu kα radiation with λ = 0.154 nm, Philips, Netherlands), Scanning Electron Microscope (VP 1450, LEO, Germany), Energy Dispersive X-Ray spectroscopy (VP 1450, LEO, Germany), Incubator INB200, (Memmert, Germany), Furnace (M2l, azar Iran), Vibrating magnetometer (MDKB, Magnetic Kavir Kashan) and Fourier transform infrared (Nicolet IS 50, Thermo Scientific Fisher, USA). Electrochemical measurements were performed with a three-electrode system, including a silver/silver chloride electrode (solution of 0.3 M KCl) as a reference electrode, a platinum electrode rod (Pt) as an auxiliary electrode and a CPE and CPE/NiZnFe₂O₄ as working electrode were used. The 0.5 mM solution prepared from the salt of potassium hexacyanoferrate (III) and potassium hexacyanoferrate (II) was used as an electrolyte. Briton-Robinson (BR) buffer with a concentration of 0.2 M has used pH adjustment of electrolytes.

HPLC-UV analysis was performed using a 1260A Agilent HPLC separation system (USA) equipped with a C₁₈ column (100 mm; 5 μm, Beckman). The analytes were detected in a UV detector that was adjusted at the wavelength of 305 nm. A mixture of methanol and water (85:15, v/v) was delivered at a flow rate of 0.8 mL min⁻¹ into the column as the mobile phase as an isocratic elution mode. Extraction and purification before HPLC analysis were performed by a solid phase extraction method with C18 sorbent. Activation of the cartridge was done by methanol (2 × 5 mL) and water (3 × 5 mL). Then honey sample solution (10% w/v) was passed through the cartridge. After that, the cartridge was washed with 8 mL water to remove polar and sugar impurities. Then, phenolic compounds were extracted using a 5 mL mobile phase. This fraction was rotary evaporated in a vacuum and then redissolved in 100 μL of mobile phase and 20 μL of it was injected into the HPLC device.

2.3

2.3 Preparation of NiZnFe₂O₄ nanospinel

For the synthesis of NiZnFe₂O₄ nano spinel, 2.0 mmol (0.06 M) of iron (III) nitrate nonahydrate, 0.5 mmol (0.01 M) of nickel (II) nitrate hexahydrate and 0.5 mmol (0.01 M) of zinc (II) nitrate hexahydrate with 1:1:4 ratio were dissolved in 30 mL bidistilled water and then 1.3 g citric acid was added to the solution under magnetic stirring. Then, ammonia was added dropwise to the obtained solution until the pH reached 8. The temperature was increased up to 80 °C to remove dissolved water. After forming a gel-like substance, the product was placed in an oven at 100 °C for 12–24 h. Finally, the precipitate was calcinated in a furnace at 800 °C for 2 h (Ali et al., 2015, Yousaf et al., 2020). The schematic of the synthesis of NiZnFe₂O₄ is shown in Fig. 1.

Schematic of the synthesis steps of NiZnFe2O4 nanospinels.

2.4

2.4 Preparation of NiZnFe₂O₄/CPE

10 mg of NiZnFe₂O₄ was weighed and added to 10 mL of deionized water under sonication for 30 min to obtain a homogenized suspention. The resulting suspention was added to 0.5 g of graphite powder and dried at 80 °C. Finally, 0.18 g paraffin oil was added to the mixture and homogenized the product by abrasion. The resultant paste, NiZnFe₂O₄/CPE, was transferred to a 2 mm diameter polyethylene pipe, and electrical contact was established via a copper wire (Gandomi et al., 2020, Sohouli et al., 2021).

2.5

2.5 Characterization of synthesized NiZnFe₂O₄ nanospinels

The synthesized NiZnFe₂O₄ nanospinels were characterized by various techniques; Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and Transmission electron microscopy (TEM) for the study of size and surface morphology, X-ray diffraction spectrum (XRD) and Energy-dispersive X-ray diffraction spectrum (EDX) for study the elemental analysis of the prepared electrode and crystal structure. A vibrating magnetometer was used to investigate the magnetic properties of the synthesized nanospinels. FT-IR spectra were used to confirm the successful preparation of NiZnFe₂O₄ nanospinels.

2.6

2.6 Electrochemical analysis

The catalytic activity of NiZnFe₂O₄/CPE was first studied by CV and electrochemical EIS methods. Subsequently, the electrochemical behavior of the modified electrode was investigated by DPV, chronocometry (CA) and chronocoulometry (CC) techniques.

2.7

2.7 Reproducibility and stability analysis

The reproducibility of CPE and NiZnFe₂O₄/CPE was examined in solutions containing certain amounts of the analyte under optimal conditions by recording the voltammetric responses and calculating the standard deviation. Moreover, the stability of NiZnFe₂O₄/CPE in the presence of a specific amount of analyte under optimal conditions for 16 days was investigated by the DPV technique.

2.8

2.8 The effect of possible interference

For investigation of influence of interfering substances, the DPV voltammograms of the solution containing a certain concentration of analyte and foreign species were recorded separately. Subsequently, DPV voltammograms of a similar solution containing the analyte and interfering species (with different concentration ratios of interfering species to the analyte) were recorded. By following the DPV voltammograms of both species and comparing them with the DPV voltammograms of each species alone, the interference of foreign species with the analyte is determined.

3 Results and discussion

3.1

3.1 Characterization results

X-ray diffraction analysis was performed at an angle ranging from 10° to 80° with a Cu radiation source (λ = 0.154 nm). The recorded XRD patterns of the samples shown in Fig. 2 confirmed the crystal structure of NiZnFe₂O₄ nanospinels. In this Fig., the diffraction peaks at 2θ are equal to 18.51, 31.30, 35.66, 37.26, 25.43, 53.65, 57.57, 75.62, 16.71, 74.21, 75.19 and 79.14 degrees with Miller indices (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 2 2), (5 1 1), (4 4 0) and (6 2 2), respectively, which refers to the Ni-Zn ferrite structure as a spinel phase. The average nanoparticle size calculated by the Scherer equation was 30 nm. FT-IR spectrum of NiZnFe₂O₄ nanospinels is shown in Fig. 3.

FT-IR spectrum of NiZnFe2O4 nanospinels.

In this spectrum, two specific peaks in 407.74 cm⁻¹ and 588.93 cm⁻¹ are attributed to the existence of Zn and Ni, respectively. Since the peak location of the two species, Zn and Ni appear in the same location, the peak that appeared at 588.93 cm⁻¹ is elongated (Mapossa et al., 2020).

The results of surface morphology, chemical composition and elemental mapping of synthesized NiZnFe₂O₄ nanospinels and NiZnFe₂O₄/CPE examined by SEM and TEM are presented in Fig.s 4 and 5. According to the SEM image of NiZnFe₂O₄ nanospinels shown in Fig. 4, fine nanoparticles with homogeneous and porous nanograins are formed.

SEM images of (A) CPE with 50,000 times magnification, (B) CPE with 100,000 times magnification, (C) NiZnFe2O4/CPE with 50,000 times magnification, and (D) NiZnFe2O4/CPE with 100,000 times magnification.

The TEM images for NiZnFe₂O₄ nanospinels (Fig. 5) show uniform dispersion and good porosity. The TEM images of NiZnFe₂O₄/CPE presented in Fig. 5 show the existence of NiZnFe₂O₄ nanospinels on the surface of the modified electrodes. It is observed that the NiZnFe₂O₄ nanospinels are scattered on the surface of CPE, and a 3D structure is formed. The catalytic effect is increased by the enhanced surface area of NiZnFe₂O₄/CPE.

SEM images of (A) NiZnFe2O4 nanospinels with 30,000 magnification, (B) NiZnFe2O4 nanospinels with 100,000 times magnification, and (C) TEM image of NiZnFe2O4 nanospinels.

EDX analysis and quantitative elemental analysis of NiZnFe₂O₄ nanospinels and NiZnFe₂O₄/CPE were presented in Fig. 6. The analysis results confirmed the existence of Fe, Ni, Zn and O elements in NiZnFe₂O₄ nanospinels and Fe, Ni, Zn, C and O elements in NiZnFe₂O₄/CPE as well as their quantitative values (Fig. 6).

EDX spectra and elemental analysis of (A) of NiZnFe2O4 nanospinels, and (B) NiZnFe2O4/CPE.

The magnetic properties of NiZnFe₂O₄ nanospinels prepared at room temperature with an applied field of ± 15000 Oersted (Oe) was studied, and the VSM spectrum of NiZnFe₂O₄ nano-spinel is presented in Fig. 7. Using vibration measurement magnetometer measurements, saturation magnetization (Ms), residual magnetism (Mr) and inhibitory magnet (Hc) were evaluated which included 62.56 emu/g, 15.16 emu/g, and 100 Oe, respectively (Manikandan et al., 2015).

3.2

3.2 Electrochemical analysis

3.2.1

3.2.1 Calculation of the modified electrodes' surface active area

The diameter of the electrode cavity was 0.2 cm, so the geometric surface area of the used electrode was about 0.031 cm². The active surface area value (A, cm²), was calculated by recording cyclic voltammograms (CVs) of NiZnFe₂O₄/CPE in the 0.1 M KCl containing 5 mM [Fe(CN)₆]^3-/4- at different scan rates (α, from 0.01 to 0.40 Vs⁻¹). According to Randles–Sevcik equation modified for irreversible/quasi reversible redox reactions, Eq. (1) (Ghalkhani and Sohouli, 2021, Ghalkhani et al., 2023),

(1)

I_{p} = 2.69 \times 10^{5} n {(α n_{a})}^{1 / 2} A C^{*} {(D ν)}^{1 / 2}

where I_p (µA) relates to the anodic peak current, n is the number of the electron transfer in the oxidation and reduction process of the redox probe ([Fe(CN)₆]^3-/4-, n = 1), α is the transfer coefficient of the electrochemical reaction, C* is the concentration of [Fe(CN)₆]^3-/4- (5 mM), and D is diffusion coefficient of the redox probe (7.6 × 10^-6 cm²s⁻¹). The A value for NiZnFe₂O₄/CPE (0.061 cm²) is higher than the CPE (0.041 cm²).

3.2.2

3.2.2 EIS measurement

For evidence the effect of NiZnFe₂O₄ nanopowders in anodic oxidation of pCA, EIS technique was used. (Sohouli et al., 2022a, Sohouli et al., 2022b). The Nyquist plots of the CPE and NiZnFe₂O₄/CPE in a 0.1 M KCl solution containing 5 mM [Fe(CN)₆]^3-/4- were shown in Fig. 8A. The semicircular zone does have a greater amplitude region, which is attributed to the charge transfer resistance (Rct). The linear zone has a lower frequency area that is linked to the diffusion region. According to the Nyquist curve presented in Fig. 8A, the charge transfer rate increases by modifying the surface of the CPE with NiZnFe₂O₄ nanospinels and a clear difference is observed between the Rct between CPE and NiZnFe₂O₄/CPE and decreases from 4.14 kΩ to 3.42 kΩ after modification. As a result, the bare electrode has a higher Rct value, indicating that it has poor electron transport properties. However, the NiZnFe₂O₄/CPE has lower resistance compared to bare, the Rct values also decreased because of its catalytic properties. In this work the Randles equivalent circuit was used.

(A) Nyquist curves of CPE and NiZnFe2O4/CPE (The inset shows the Randles equivalent circuit to fit the experimental EIS data) and (B) CV voltammograms of CPE and NiZnFe2O4/CPE in BR buffer solution (0.2 M, pH 4.5) in the presence and absence of 99.0 μM pCA.

3.2.3

3.2.3 CV analysis

To investigate the effect of NiZnFe₂O₄ on the catalytic activities of CPE, the cyclic voltammograms of 99.0 μM pCA at the surface of NiZnFe₂O₄/CPE and CPE in BR buffer solution (0.2 M, pH 4.5) was recorded at the scan rate of 100.0 mV/s and potential step of 4.4 mV. As shown in Fig. 8b, the oxidation current of pCA at the surface of the NiZnFe₂O₄/CPE increases by three times that of the CPE. Thus, electrode modification using NiZnFe₂O₄ nanospinels increases the sensitivity of the measurement. As shown in Fig. 8b, the oxidation potential of pCA at the NiZnFe₂O₄/CPE surface has slightly shifted towards negative values compared to the CPE. Also, the oxidation peak current of pCA at the NiZnFe₂O₄/CPE surface is higher than the unmodified electrode. On the basis of the observations, it is clear that addition of NiZnFe₂O₄ exerts a significant catalytic effect on the electrochemical reduction of pCA leading to decrease of overpotential in the process and an enhancement in the peak current is observed (Bounegru, et al. 2021). The better performance of the NiZnFe₂O₄/CPE is due to the nanometer dimensions of the NiZnFe₂O₄NPs and high surface area to volume ratio, resulting in a highly sensitive sensor.

3.2.4

3.2.4 The effect of scan rate

Fig. 9A shows the cyclic voltammograms of 19.0 μM pCA at the surface of NiZnFe₂O₄/CPE at various scan rates. As shown in this figure, the observed current intensities and capacitance currents increase with the increase in potential scan rate.

A) CV voltammograms of 19.0 μM pCA at the surface of NiZnFe2O4/CPE in BR buffer solution (0.2 M, pH 4.5) at various scan rates (0.02, 0.04, 0.08, 0.1, 0.2, 0.3, and 0.4 Vs−1), B) Relationship between anodic peak current and the square root of the scan rate (ν1/2).

It is also observed a linear relationship between peak current and the square root of the scan rate (I vs ν^1/2) in the range of 0.02 – 0.4 Vs⁻¹ (Fig. 9B). In the redox of pCA, there was a linear relationship between peak current and the square root of scan rate in the range of 0.02 – 0.4 Vs⁻¹ for oxidation (Eq. (2):

(2)

I (μ A) = 0.6976 v^{(1 / 2)} {(m V s^{(- 1)})}^{(1 / 2)} - 1.1138, R 2 = 0.998

According to Randles-Sevick equation, the linear relation between I_p and ν^1/2 indicates that the electrochemical oxidation process of pCA at the surface of NiZnFe₂O₄/CPE is diffusion-controlled.

3.2.5

3.2.5 Differential pulse voltammetry

DPV is used for both quantitative chemical analysis and to study the mechanism, kinetics, and thermodynamics of chemical reactions. DPV used as an analytical tool offers advantages when compared to other electrochemical techniques. DPV is very sensitive due to the relatively short pulse time and its differential nature. The short pulse time increases the measured currents, while the differential measurement discriminates against background processes (Thomas,et al., 2013).

3.2.6

3.2.6 The effect of pH

The pH of the buffer used has a direct effect on the electrochemical oxidation of pCA and the performance of NiZnFe₂O₄/CPE (Bard and Faulkner, 2001). Accordingly, the effect of pH (3.5–8.5) on the differential pulse voltammograms of 160.0 μM pCA at the surface of NiZnFe₂O₄/CPE, was studied, and the recorded voltammograms were exhibited in this pH range. As shown in Fig. 10A, the oxidation potential of pCA shifts to more negative potentials with an increase in pH, indicating the proton exchange in the oxidation process of pCA. Based on the maximum current intensity in Fig. 10A, pH = 4.5 was selected as the optimum pH value. The diagrams of anodic peak potential and anodic peak current (I) versus pH are shown in Fig. 10B. As observed in this figure, the oxidation peak potential of pCA shifts to more negative potentials with increasing the pH value, which indicates proton exchange in the oxidation process of pCA. By plotting the oxidation peak potential in terms of pH, a linear relationship between the oxidation potential and the pH is obtained according to the following equation (Fig. 10B):

(3)

E = E p + (\frac{0.0591}{n}) L o g [\frac{(O x) a}{(R e d) b}] - (0.591 \frac{n}{m}) p H

The protons and the exchanged electrons can be obtained by comparing the obtained equation with the Nernst equation (Borland et al., 2013). The slope of the above equation is equal to 60.6 mV (about 59.1 mV), and thus the number of electrons and protons exchanged in the oxidation process for pCA is 1 (Moreira Ferreira, Dutra de Souza, Nome Aguilera, & Cruz Vieira, 2017). The maximum point in Fig. 10B (I versus pH) is related to pH 4.5, which was selected as the optimal pH with the highest current intensity. The proposed mechanism for pCA oxidation at the surface of NiZnFe₂O₄/CPE is presented in Fig. 11.

A) DPV voltammograms of 160.0 μM pCA at the NiZnFe2O4/CPE surface in 0.2 M BR buffer solution at various pH values (3.5, 4.5, 5.5, 6.6, 7.5, and 8.5), B) Maximum peak potential versus pH, and C) peak current versus pH.

Proposed mechanism for electrochemical oxidation process of pCA on the NiZnFe2O4/CPE surface.

3.3

3.3 The effect of step potential

Since the potential step directly affects the electrochemical oxidation (Colmati et al., 2009) of pCA at the surface of NiZnFe₂O₄/CPE, DPV voltammograms were recorded at various step potentials of 1.0, 2.0, 3.0, 4.0, 5.0 and 7.0 mV. Fig. 12A shows the peak current intensities of recorded voltammograms versus potentials. According to Fig. 12B, a step potential of 4 mV with maximum current intensity was selected as the optimal step potential.

(A) DPV voltammograms, of 160.0 μM pCA at the NiZnFe2O4/CPE surface in BR buffer solution (0.2 M, pH 4.5) at step potentials of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0 mV, and (B) Relationship between oxidation peak current versus step potentials.

3.4

3.4 The effect of NiZnFe₂O₄ nanospinels ratio in NiZnFe₂O₄/CPE

Fig. 13A exhibits DPV voltammograms of the pCA (175.0 μM) at the surface of NiZnFe₂O₄/CPE in 0.2 M BR buffer solution, at the step potentials of 4.0 mV for different amounts of NiZnFe₂O₄ (2.0, 4.0, 6.0, 10.0 and 25.0 mg). The curve of peak current intensity versus the amount of NiZnFe₂O₄ nanospinels was shown in Fig. 13B. As shown in both Fig. 13, the maximum peak current recorded for NiZnFe₂O₄/CPE prepared with 10 mg of NiZnFe₂O₄ nanospinels. By increasing the amount of NiZnFe₂O₄ nanospinels from 2.0 to 10.0 mg, the peak current intensity increases and the peak potential shift to more negative potentials. However, further improvement of the CPE surface is not seen when the amount of NiZnFe₂O₄ becomes > 10.0 mg.

(A) DPV voltammograms of 175.0 μM pCA at the NiZnFe2O4/CPE surface in BR buffer solution (0.2 M, pH 4.5) at step potential of 4.0 mV for different amounts of NiZnFe2O4 nanospinels (2.0, 4.0, 6.0, 10.0, and 25.0 mg), and (B) Oxidation peak current intensities versus the amount of NiZnFe2O4 nanospinels,

3.5

3.5 Chronoamprometry and chronocolometry techniques

CA and CC methods were used to evaluate the diffusion coefficient (D) and oxidation of pCA on the surface of NiZnFe₂O₄ /CPE.

The CA method was used for determining the diffusion coefficient (D) of pCA in the 0.2 M BR buffer solution at pH 4.5 and different concentrations of pCA from 13.0 to 82.0 μM at the surface of NiZnFe₂O₄/CPE (Fig. 14A). Catterl equation was used to calculate D value according to the Eq. (4):

(4)

I = n F A D^{1 / 2} π^{- 1 / 2} t^{- 1 / 2} C

(A) Chronoamprograms of pCA with different concentrations (13.0, 33.0, 49.0, 66.0, and 82.0 μM) at the surface of NiZnFe2O4/CPE in BR buffer solution (0.2 M, pH 4.5) at a potential of 0.8 V, and (B) Slope (I vs. t−1/2 curves from A) versus concentrations of pCA.

where in this equation, n is the number of electrons and is equal to 1, A is surface active area of the electrode, and C (mol cm⁻³) is the concentration of the species in the bulk solution, respectively. The D value calculated from the slope of I versus t^−1/2 under the diffusion-controlled condition was 3.2084 × 10⁻⁶ cm²s⁻¹ which indicates the slight diffusion of pCA during oxidation at the electrode surface.

3.6

3.6 Chronocolometry studies

The D value of pCA at the surface of NiZnFe₂O₄/CPE was calculated by the CC method to confirm the data obtained from CA. Fig. 15A shows a chronocologram of pCA solutions (from 13.0 to 160.0 μM) in 0.2 M BR buffer at pH 4.5 and 0.8 V. Total cumulative charge versus t^1/2 and slopes of the diagram of Fig. 15b versus the concentration of pCA are presented in Fig.s 15B and 15C, respectively. The diffusion coefficient calculated from the Catterl equation was 3.8619 × 10⁻⁶ cm²s⁻¹ which suggests the slow diffusion of pCA during oxidation at the surface of NiZnFe₂O₄/CPE. The D value obtained by the two methods, CA and CC, is close to each other.

(A) Chronocolougrams of pCA with different concentrations (13.0, 49.0, 82.0, 99.0, and 160.0 μM) at the surface of NiZnFe2O4/CPE in BR buffer solution (0.2 M, pH 4.5) at a potential of 0.8 V, (B) Cumulative charge diagram versus square root of time, and C) slope (obtained from curves in B) versus pCA concentrations.

3.7

3.7 DPV measurements

Fig. 16A shows the DPV voltammograms of different concentration of pCA in BR buffer solution (0.2 M, pH 4.5) at the surface of NiZnFe₂O₄/CPE under optimal conditions. According to the calibration curve presented in Fig. 16B, there is a linear relationship between the oxidation peak current and pCA concentration in the range of 1.6–250.0 μM. The calibration equation for this concentration range is as Eq. (5):

(5)

I (μ A) = 0.1203 C (μ M) + 0.289 R^{2} = 0.9992

(A) DPV voltammograms of pCA with various concentrations (1.6, 3.3, 6.6, 13.0, 19.0, 33.0, 66.0, 82.0, 99.0, 110.0, 130.0, 140.0, 160.0, 190.0, and 250.0 μM) at the surface of NiZnFe2O4/CPE in BR buffer solution (0.2 M, pH 4.5), and (B) Calibration curve of oxidation peak currents versus pCA concentration.

As this regard, the limit of detection (LOD) of the pCA at the NiZnFe₂O₄/CPE was estimated by using Eq. (6):

(6)

(3 \times S D) / s

The standard deviation (SD) of the buffer current was calculated at the place corresponding to the oxidation peak of pCA and s is the slope of the calibration curve for obtained from the sensitivity measurements. The detection limit was calculated to be 480.0 nM. The NiZnFe₂O₄/CPE exhibited the suitable sensitivity of 0.171 μA cm⁻² mM⁻¹ for pCA sensing. A comparison of the performance of the NiZnFe₂O₄/CPE in this work for pCA measurement was performed with other sensors, and the results are presented in Table 1. As the results show, therefore, the proposed NiZnFe₂O₄/CPE electrode is a good choice for determination of pCA due to appropriate detection limit, high selectivity and use of inexpensive material. In Table 1, the proposed NiZnFe₂O₄/CPE is compared to other electrochemical sensors reported in previous literatures. Accordingly, pCA detection through electrochemical methods has received huge attention over the past few years and introducing a sensitive sensor with simple synthesis procedures, low cost and available modifiers have been remained a great challenge. Compared to other mentioned electrochemical sensors, NiZnFe₂O₄/CPE has provided superior results with detection limit of 0.48 μM and long linear range (1.60–250 μM). Therefore, the proposed NiZnFe₂O₄/CPE electrode is a good choice for determination of pCA due to appropriate detection limit, high selectivity and use of inexpensive material.

Table 1 Comparison of analytical parameters between developed NiZnFe₂O₄/CPE and other electrochemical methods for determination of pCA.

Technique	Electrode	Modifier	linear range (µM)	LOD (µM)	Application	Ref.
SWAd SV^a	CPE	–	0.91–121.91	0.67	Lemon, Watermelon and Mango-pineapple	(Demir, 2019)
LSV^b, SWV^c, DPV	GCE^d	Rd NPs stabilized ImS₃	–	0.60, 0.60 and 0.47	Cellulose matrix	(Moreira Ferreira et al., 2017)
DPV, and SWV	GCE	–	3.0––100	0.083	–	(Janeiro et al., 2007)
SWV	SPE CNF^e	cobalt phthalocyanine and laccase	0.4–6.4	0.483	–	(Bounegru and Apetrei, 2021)
DPV	CPE	NiZnFe₂O₄	1.6–250	0.480	Thyme and Astragalus honey	This work

(a) Square wave adsorptive stripping voltammetry; (b) Linear sweep voltammetry; (c) Square-wave voltammetry (d) Glassy carbon electrode; (e) Screen-printed electrode based on carbon nanofibers.

3.8

3.8 Selectivity of the NiZnFe₂O₄/CPE for pCA detection

For selectivity evaluation of the proposed sensor, the interferences of common interfering substances on the sensor response were investigated by analyzing a solution containing 160.0 μM pCA. The tolerance limit was defined as the concentrations which give an error less than ± 5.0% in the oxidation peak current of pCA (Wang and Chen, 2009). Some common cations and anions such as Na⁺, K⁺, Ca²⁺, Mn²⁺, NO₃²⁻, SO₃^2- were studied for its interference with detection of pCA. The results demonstrate that these ions virtually have no obvious interference to the DPV signals of the targets at the NiZnFe₂O₄/CPE. Some organic compounds contained in honey such as citric acid, malic acid, tartaric acid, lactic acid, gluconic acid and glucose have been considered to have no influence on the signals of pCA with deviations below 5%. These results have been reported in Table 2. Overall, the high selectivity of the NiZnFe₂O₄ modifier for the detection of pCA can be attributed to its chemical affinity, electrochemical properties and surface morphology. These factors work together to enhance the sensitivity and selectivity of the NiZnFe₂O₄/CPE towards pCA, and making it a suitable modifier for pCA detection.

Table 2 The effect of possible interferences in pCA detection with NiZnFe₂O₄/CPE.

Interfering species	Molar ratio of interfering/pCA
Na⁺, K⁺, Ca²⁺, Mn²⁺	200.0
NO₃^2–, SO₄^2-	200.0
Citric acid, malic acid	150.0
Tartaric acid, lactic acid, gluconic acid, glucose	100.0

3.9

3.9 Determination of pCA in honey samples and comparison with HPLC-UV results

To evaluate the ability of NiZnFe₂O₄/CPE to measure pCA in real samples, two honey samples (Thyme and Astragalus) were examined. The standard addition method was used to determine pCA concentration in samples. Fig. 17 shows the DPV of the honey samples. The amount of pCA in two Thyme and Astragalus honey samples was calculated as 1.70 × 10^-5 and 3.41 × 10^-5 M, respectively. Table 3 shows the DPV responses of honey samples. As well, to check the matrix effect and calculate the percentage of relative recovery, the peak current for the equivalent concentration in the buffer sample was compared with the current in the honey sample. The relative recovery of all spiked samples was in the range of 92.5–110.7%. These acceptable results prove that the developed NiZnFe₂O₄/CPE electrode can be used to determine pCA in real honey samples without a matrix effect.

DPV voltammograms of pCA in Thyme and Astragalus honey samples, after spiking various concentrations of pCA (19–40 μM and 8–26 μM, respectively).

Table 3 Analysis of real honey samples and relative recovery results for pCA determination with NiZnFe₂O₄/CPE.

Sample	Spiked (μM)	Determined (μM)	Relative recovery (%)
Thyme Honey	–	1.460	–
	8	8.750	92.49
	11	12.83	102.97
	15	17.44	105.95
	16	19.19	109.91
	20	23.75	110.67
	26	30.06	109.47
Astragalus Honey	–	3.129	–
	19	23.99	108.40
	22	27.92	111.10
	24	29.90	110.58
	33	36.93	102.24
	35	38.06	99.84
	40	39.48	98.79

To check the accuracy of the developed method (NiZnFe₂O₄/CPE) in the analysis of real samples, a comparison with the standard HPLC method was also used. The results of both methods are presented in Table 3. The results showed clearly that there is a good agreement between the two methods based on applying the student’s t-test and there is no significant difference between electrochemical and chromatographic methods at a confidence level of 95%. A typical chromatogram of honey samples is shown in Fig. 18.

The typical HPLC chromatograms of Thyme and Astragalus honey samples (Peak in Rt = 21.7 min related to pCA).

4 Conclusion

In the present study, the electrochemical oxidation of pCA has been studied using electrochemical methods at the surface of the modified electrode (NiZnFe₂O₄/CPE). NiZnFe₂O₄ as a modifier enhanced the electrical conductivity and surface area of the CPE, leads to attain higher oxidation of pCA for NiZnFe₂O₄/CPE than CPE. After optimizing the experimental conditions, the electron transfer coefficient, diffusion coefficient, detection limit, and concentration range obtained for pCA using the NiZnFe₂O₄/CPE were 0.7, 3.2084 × 10⁻⁶ cm² s ⁻¹, 480.0 nM, and 1.6–250.0 μM, respectively. The proposed method is sensitive and selective for pCA determination in honey real samples with satisfactory results.

Acknowledgments

We are indebted to the University of Kashan for their invaluable contributions.

Declaration of Competing 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.

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Zhao S., Jones J.A., Lachance D.M., . Improvement of catechin production in Escherichia coli through combinatorial metabolic engineering. Metab. Eng.. 2015;28:43-53.
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