{Sensitive determination of uric acid at layered zinc hydroxide-sodium dodecyl sulphate-propoxur nanocomposite:} http://dx.doi.org/10.5599/jese.1237 331 J. Electrochem. Sci. Eng. 12(2) (2022) 331-341; http://dx.doi.org/10.5599/jese.1237 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Sensitive determination of uric acid at layered zinc hydroxide- sodium dodecyl sulphate-propoxur nanocomposite Mohamad Hafiz Ahmad Tajudin1,2, Mohamad Syahrizal Ahmad2,3, , Illyas Md Isa2,3, Norhayati Hashim2,3, Anwar Ul-Hamid4, Mohamad Idris Saidin2,3, Rahadian Zainul5 and Suyanta M. Si6 1Faculty of Applied Sciences, Universiti Teknologi MARA, Perak Branch, Tapah Campus, Tapah Road, 35400 Perak, Malaysia 2Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjong Malim, Perak, Malaysia 3Nanotechnology Research Centre, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjong Malim, Perak, Malaysia 4Center for Engineering Research, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia 5Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas Negeri Padang, West Sumatera 25171, Indonesia 6Department of Chemistry Education, Faculty of Mathematics and Natural Science, Yogyakarta State University, Indonesia Corresponding author:  syahrizal@fsmt.upsi.edu.my; Tel.: +60-196266485; Fax: +60-54506000 Received: January 15, 2022; Accepted: February 15, 2022; Published: February 26, 2022 Abstract An electrochemical chemical sensor for the determination of uric acid (UA) with high sensiti- vity and a wide working range was fabricated using the layered zinc hydroxide-sodium dode- cyl sulphate-propoxur (LZH-SDS-PRO) nanocomposite, modified with multiwall carbon nano- tubes (MWCNT). The introduction of LZH-SDS-PRO as a conducting matrix has enhanced the conductivity of MWCNT. The morphology of LZH-SDS-PRO/MWCNT was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), while electrochemical behavior of UA and K3[Fe(CN)6] at LZH-SDS-PRO/MWCNT paste electrode was studied by square wave and cyclic voltammetry, respectively. Under the optimized experi- mental conditions, the electrode established linear plot for UA concentrations 7.0 mol L-1 to 0.7 mmol L-1 (R2 = 0.9920) and LOD was calculated to be 4.28 µmol L-1 (S/N = 3). The fabricated LZH-SDS-PRO/MWCNT electrode was successsfully applied to urine samples, exhibiting excellent stability and reproducibility, which made it worthwhile for analytical applications. Keywords Electrochemical sensor; pharmaceutical sensor; modified MWCNT; layered metal hydroxide; functional nanocomposite; square wave voltammetry http://dx.doi.org/10.5599/jese.1237 http://dx.doi.org/10.5599/jese.1237 http://www.jese-online.org/ mailto:syahrizal@fsmt.upsi.edu.my J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 DETERMINATION OF URIC ACID 332 Introduction 2,6,8-trihydroxypurine, also known as uric acid (UA), is a fundamental electroactive molecule resulting from the metabolism of endogenous purine, which occurs inside the human body [1,2]. Biological responses such as inflammation, vasoconstriction, oxidative stress and endothelial dysfunction can be stimulated by UA. Identifying diseases such as gout, hyperuricemia, kidney stone, type-2 diabetes, renal impairment, and Lesch-Nyan syndrome can be done by quantifying the concentration level of UA in blood and urine [3,4]. Various high precision, accurate and robust methods have been developed for analytical purposes, such as chromatography, fluorescence, electrophoresis, chemiluminescence and spectro- photometry, but all these methods are energy-consuming, high-cost, time-consuming and need complex operating processes [5-11]. Since UA is highly electrochemically active compound, alternative electrochemical methods were developed and characterized by cost-effectiveness, fastness, simplicity and portability for quantitative analysis. Different functional materials have been introduced in developing UA sensors to improve the electron transfer of UA at electrodes, such as Co(II)-based zeolitic imidazolate framework [12], platinum nanoparticles (PtNPs) [13,14], nano resin [15] and ferrocene derivative and core-shell magnetic nanoparticles [16]. Since the first discovery of carbon nanotubes (CNT) in 1991, various materials have been incor- porated into them and attracted widespread attention in the field of electroanalysis [17]. The unique electrochemical properties of CNT in terms of large surface area, excellent electron transfer, fine structure and light-weight make them be good electrodes for various applications [18-20]. In recent studies, multi-walled carbon nanotubes (MWCNT) were more preferable than single-walled CNT (SWCNT) due to rapid electron transfer for different reactions and better conductivity [21]. Besides, metal layered hydroxides have also gained much attention in carbon paste electrode (CPE) fabri- cation, owing to their remarkable capability of anion exchange and also excellent physicochemical properties in terms of low toxicity, chemical inertness and high surface area [22]. In our previous work, abilities of layered zinc hydroxide-ferulate (LZH-F), layered zinc hydroxide-L-phenylalanate (LZH-LP) and layered zinc hydroxide-sodium dodecyl sulphate-isoprocarb (LZH-SDS-ISO) applied as modifiers with MWCNT were demonstrated for the determination of hydroquinone (HQ) [23], acetaminophen (PCM) [24] and dopamine (DOP) [25], respectively. Propoxur or 2-isopropoxyphenyl-N-methylcarbamate, was introduced to the market in 1959 and widely used in the pest control industry. Intercalation of propoxur with layered zinc hydroxide produces a nanocomposite material that has good electron transfer ability and a large surface-to- volume ratio. It has also been reported that this material shows low toxicity, high thermal stability, biocompatibility and the potential for controlled release [26,27]. Based on the listed advantages, in this work, we are presenting for the first time LZH-SDS-PRO material as a mediator used to increase the electrocatalytic activity of the redox reaction important for the determination of UA. Experimental Chemicals and reagents The uric acid stock solution was prepared by dissolving the appropriate amount of UA in 0.1 mol L- 1 NaOH solution. Potassium acetate, chloride salts (Sigma-Aldrich, USA), paraffin oil, copper(II) sulphate, potassium nitrate, potassium iodide, phosphate buffer solution (PBS) (K2HSO4 and KH2SO4) (Merck, Germany), barium chloride, glucose, fructose, sucrose and MWCNT (Timesnano, China) were of analytical grade and used as received. Ultra-pure water was used during the work. M. H. Ahmad Tajudin et al. J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 333 Apparatus FESEM model SU8020 UHR (Hitachi, Japan) and FETEM model JEM2100F (Jeol, Japan) were used for the characterization of surface morphologies of LZH-SDS-PRO/MWCNT and MWCNT. Potentiostat/ galvanostat model Ref 3000 (USA) was used for electrochemical impedance spectroscopy (EIS) measurements, while cyclic voltammetry (CV) and square wave voltammetry (SWV) were conducted using Potentiostat series-G750 (USA). Three-electrode system was used in this study, where LZH-SDS- PRO/MWCNT, a platinum wire and Ag/AgCl electrode MF-2052 (Bioanalytical syst, USA) with fiber junction, acted as the working electrode, counter electrode and reference electrode, respectively. Preparation of LZH-SDS-PRO nanocomposite LZH-SDS was prepared by the addition of 40 mL of 0.5 M of Zn(NO3)2·6H2O and 1.0 M NaOH into a solution of 40 mL of 0.25 M SDS. The pH value was adjusted to 6.5. Then, the slurry was centrifugated and dried in an oven at 70 °C. Intercalation of propoxur into the interlayer of LZH-SDS was done by an ion-exchange method. 0.5 g of LZH-SDS was dissociated in 0.001 M propoxur solution and kept under a magnetic stirrer for 3 hours. The slurry was then aged 24 hours in an oil bath at 70 °C. After that, the slurry was centrifuged and the white solid was dried in an oven [26]. Electrode preparation LZH-SDS-PRO (5 mg), MWCNT (100 mg) and paraffin oil (3 drops) were mixed using mortar and pestle. The homogenized mixture was firmly packed into Teflon tubing (i.d. 2.0 mm and 3 cm long). To establish the electrical contact, one of the ends of the paste was connected to the copper wire, and the other end was smoothened using soft paper. The non-modified MWCNT electrode was prepared with the same method but without LZH-SDS-PRO added. Measurement procedure UA solutions present at desired concentrations in 0.1 mol L-1 PBS (pH 6.4) as the supporting elec- trolyte were used throughout the work to perform voltammetry unless otherwise stated. The UA solutions were deoxygenated with N2 before measurements for about 15 minutes. SWV experi- ments of UA determination were performed between 100 mV to 500 mV, with a frequency of 150 Hz, pulse height of 60.0 mV and step increment of 6.0 mV. CVs taken between -300 to 800 mV at the scan rate of 100 mV s-1 were applied for electrochemical characterization of the fabricated electrode with K3[Fe(CN)6] as a redox probe. For EIS measurements, the frequency range of 10 kHz to 0.1 Hz and amplitude of the alternating voltage of 5.0 mV were used. All experiments were conducted at the ambient temperature of 25 ± 1 oC. Results and discussion Surface morphology The SEM image of LZH-SDS-PRO shown in Figure 1, resembles nanoflower-like particles with a thickness of approximately 1.5 – 3.0 µm. To confirm the formation of LZH-SDS-PRO nanocomposite, EDS analysis was carried out. Different areas were focused during the EDS measurement and the conforming peaks were observed. The LZH-SDS-PRO can be seen in the synthesized composite nanostructure in the EDS spectrum. In the spectrum A, the quantity of C, Zn and O (measured in wt.%) were 40.1, 29.4 and 22.6, respectively, while in spectrum B, the values were 50.3, 29.5 and 17.0 % for C, Zn and O, respectively. http://dx.doi.org/10.5599/jese.1237 J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 DETERMINATION OF URIC ACID 334 Figure 1. SEM image and EDS spectrum of (A) LZH-SDS-PRO and (B) LZH-SDS-PRO/MWCNT TEM analysis was performed to further investigate the morphology of the LZH-SDS-PRO composite. Figure 2 shows TEM images of the composite nanostructure at a low magnification where sharp edges and smooth surface of LZH-SDS-PRO (A) and carbon nanotubes strings (B) were observed, confirming the results from SEM images. Figure 2. TEM image of (A) LZH-SDS-PRO and (B) LZH-SDS-PRO/MWCNT Electrochemical response of K3[Fe(CN)6] at LZH-SDS-PRO/MWCNT paste electrode Figure 3 shows CV voltammograms of 4.0 mmol L-1 K3[Fe(CN)6] contained in 0.1 mol L-1 KCl at the LZH-SDS-PRO/MWCNT and unmodified MWCNT paste electrodes. Figure 3. Cyclic voltammograms of (a) non-modified MWCNT and (b) LZH-SDS-PRO/MWCNT for 4.0 mmol L-1 K3[Fe(CN)6] in 0.1 mol L-1 KCl, at scan rate 100 mV s-1 A B A B M. H. Ahmad Tajudin et al. J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 335 The LZH-SDS-PRO/MWCNT paste electrode showed redox peak current at Ipa = 8.743 µA, Ipc = 7.618 µA, and peak-to-peak separation (∆Ep) = 71.6 mV. Meanwhile, redox peak current of the non-modified MWCNT paste electrode was Ipa = 5.854 µA, Ipc = 5.843 µA, and ∆Ep = 92.7 mV. It is evident from these findings that the introduction of LZH-SDS-PRO as an MWCNT modifier is responsible for improving electron transfer rate, electroactive surface area and the conductivity performance of the modified electrode. The investigation of interfacial redox reaction kinetics of [Fe(CN)6]3-/4- redox probe at LZH-SDS-PRO/ /MWCNT paste electrode was done by using the EIS method. The charge transfer resistance, Rct values were estimated as diameters of semicircles appearing in high-frequency regions of Nyquist plots, where the diffusion process is represented by straight-line plots at lower frequencies [28]. In the inset of Figure 4, the Randles equivalent circuit used for fitting measured impedance spectra is presented. Rct value for bare MWCNT paste electrode was 1.325 kΩ, while Rct for LZH-SDS-PRO/ /MWCNT was 0.245 kΩ. It is clearly observed that LZH-SDS-PRO/MWCNT displayed lower charge transfer resistance within the interfacial layer, as suggested by more than five times smaller diameter of the semicircle. Z’ /  Figure 4. Nyquist plots recorded in the solution of 4.0 mmol L-1 K3[Fe(CN)6] in 0.1 mol L-1 KCl using (a) non-modified MWCNT, and (b) LZH-SDS-PRO/MWCNT paste electrode. Inset: Randles equi- valent electrical circuit used for data fitting The apparent rate constant, kapp of the electron transfer on unmodified MWCNT and LZH-SDS- -PRO/MWCNT paste electrode was 1.17×10-5 and 6.38×10-5 cm s-1, respectively, which was calculated using the eq. (1): kapp = RT/F2 Rct c (1) where T represents temperature, R is the gas constant, c is the concentration of K3[Fe(CN)6] and F is Faraday’s constant. As a consequence of the high specific area and high conductivity, LZH-SDS-PRO/MWCNT electrode effectively promotes the electron transfer process. The effect of scan rate () change on the redox peak currents of 4.0 mmol L-1 K3[Fe(CN)6] contained in 0.1 mol L-1 KCl at LZH-SDS-PRO/MWCNT paste electrode was also studied. As can be observed in Figure 5A, anodic and cathodic peak currents were progressively increased with increasing scan rate from 10 to 300 mV s-1, while their Ep values shifted positively and negatively, respectively, suggesting kinetic limitation in the reaction [29]. In addition, there is also a straight- line relationship between peak current and scan rate () with the linear regression equations, Ipa = 0.0635 + 3.9461 (R2 = 0.9910) and Ipc = -0.0636 - 3.599 (R2 = 0.9903) as shown in Figure 5B. -Z  /  http://dx.doi.org/10.5599/jese.1237 J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 DETERMINATION OF URIC ACID 336 Besides that, the graph of peak currents versus square root of scan rate (1/2) was plotted in Figure 5C, showing a good linear relationship with the following linear equations: Ipa = 1.26791/2 - 1.1281 and Ipc = -1.25661/2 + 1.394. The correlation coefficients obtained were 0.9913 and 0.9927, respecttively. These results revealed that the redox reaction of K3[Fe(CN)6] on LZH-SDS-PRO/MWCNT paste electrode is reversible, i.e. diffusion-controlled [30]. Figure 5. (A) Cyclic voltammograms in the solution of 4.0 mmol L-1 K3[Fe(CN)6] in 0.1 mol L-1 KCl at scan rates of 10, 20, 40, 70, 100, 150, 200 and 300 mV s-1; (B) plot of peak currents versus scan rate; (C) plot of peak currents versus square root of scan rate Electrochemistry of UA on LZH-SDS-PRO/MWCNT paste electrode The square wave voltammetry (SWV) measurements were carried out to compare electroanalytical performance of the non-modified MWCNT and LZH-SDS-PRO/MWCNT paste electrodes. As illustrated in Figure 6, the peak current of UA oxidation at the non-modified MWCNT is observed at 5.613 µA. Figure 6. SW Voltammograms in the solution of 0.1 mmol L-1 UA in 0.1 mol L-1 PBS at pH 6.4 using: (a) non-modified MWCNT and (b) LZH-SDS-PRO/MWCNT M. H. Ahmad Tajudin et al. J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 337 The oxidation peak current of LZH-SDS-PRO/MWCNT, however, is dramatically improved by the factor of 3 to 18.16 µA. Except for the effect of increased surface area, this might be due to the excellent electrical conductivity of LZH-SDS-PRO/MWCNT that can act as an effective electrons promoter during the electrochemical reaction. Hence, the addition of LZH-SDS-PRO into MWCNT has enhanced the electrode performance for the detection of UA. Scheme 1 illustrates the proposed mechanism for the oxidation reaction of UA at the LZH-SDS- -PRO/MWCNT paste electrode. By UA oxidation, imine alcohol is produced from UA by donating two protons and electrons, while two moles of propoxur at the electrode surface accept those protons and electrons to produce o-isopropoxyphenol [31,32]. HN N H N H H N O O O HN N N H N O O O HN N N H H N O O O -2H+, -2e- H2O OH O H N O O +H+, +e- OH O 2 Scheme 1. Probable mechanism of oxidation reaction of UA at LZH-SDS-PRO/MWCNT paste electrode The effect of pH Figure 7 shows how pH values between 6.0 and 8.0 of 0.1 mol L-1 PBS affect the oxidation peak currents of 0.1 mmol L-1 UA at LZH-SDS-PRO/MWCNT paste electrode since PBS was optimized at the pH scale of 6.2 to 8.0 [33]. The oxidation peak current of UA increased with increasing the pH value from 6.0, reached a maximum point at pH 6.4, and then decreased with further pH increasing. Therefore, throughout the work for UA determination, the optimum pH was set at 6.4. It is clearly seen from Figure 7 that the peak potential shifted negatively with pH increase, proving thus the involvement of protons in the oxidation of UA. Figure 7. Plot of oxidation peak current (I) and oxidation potential (E) vs. pH of 0.1 mol L-1 PBS containing 0.1 mmol L-1 UA at LZH-SDS-PRO/MWCNT paste electrode http://dx.doi.org/10.5599/jese.1237 J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 DETERMINATION OF URIC ACID 338 The relationship between pH and peak potential (E) of UA can be expressed as: E = -50.226 pH + + 643.83 (R2 = 0.9921), suggesting the equal number of electrons and protons involved in the electrochemical oxidation of UA at LZH-SDS-PRO/MWCNT. This conclusion is based on the obtained slope of 50.226 mV pH-1, close to the Nernst value of 59 mV pH-1 [34]. Calibration curve and limit of detection In order to study the relationship between the concentration of UA and oxidation peak current (I) on the LZH-SDS-PRO/MWCNT paste electrode, a series of UA solutions was prepared, containing 7.0 µmol L-1 to 0.7 mmol L-1. As shown in Figure 8A, I increased linearly as UA concentrations were increased. The plot of I vs. [UA] showed a linear relationship with the following linear regression equation: I = 0.058 cUA – 1.417 (R2 = 0.9920) and the limit of detection (LOD) was found to be 4.28 µmol L-1. LOD was determined using eq. (2): LOD = 3σ / m (2) where m = slope of the calibration curve and σ = relative standard deviation of its intercept. Figure 8. (A) Linear plot of I vs. cUA and (B) SWVs at different concentrations (7, 10, 30, 70, 100, 300 and 700 µM) of UA in 0.1 mol L-1 PBS (pH 6.4) The high sensitivity of the LZH-SDS-PRO/MWCNT paste electrode compared to those obtained for several other electrodes by different electroanalytical methods is presented in Table 1. The obtained results proved that the suggested sensor can be used for the determination of UA in environmental and even biological analytes. Table 1. Comparison of analytical properties of different fabricated electrodes for the determination of UA Electrode materials Method Linear range of concentration, µmol L-1 LOD, µmol L-1 Ref. DMF / SPCE DPV 100.0 – 500.0 0.19 [35] Poly(Isoniazid) / CPE CV 10.0 – 1000.0 1.173 [36] PEDOT / GCE CV 6.0 – 100.0 7.0 [37] MWCNT-PEDOT / GCE DPV 10.0 – 250.0 10.0 [38] GF / NiCo2O4 SWV 10.0 – 26.0 0.2 [39] MWCNT / GCE SWV 10.0 – 200.0 1.0 [40] LZH-SDS-PRO / MWCNT / CPE SWV 7.0 – 700.0 4.28 This work Selectivity, stability, reproducibility and repeatability of LZH-SDS-PRO/MWCNT paste electrode The selectivity of the LZH-SDS-PRO/MWCNT paste electrode was tested by evaluating differren- ces in the oxidation peak current value of 0.1 mmol L-1 UA in 0.1 mol L-1 PBS (pH 6.4) in the presence of possible coexisting interfering species such as D-glucose, L-fructose, and Na+, Mg2+, Ca2+, Cl-, SO42- M. H. Ahmad Tajudin et al. J. Electrochem. Sci. Eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 339 and NO3- ions. As illustrated in Figure 9, LZH-SDS-PRO/MWCNT paste electrode has the anti- interference ability in the presence of 10- and 50- fold higher concentrations of interfering species with the relative error of less than ± 10 %. The stability of the fabricated paste electrode was recorded towards 0.1 mmol L-1 UA in 0.1 mol L-1 PBS (pH 6.4) within 14 days and the results were retained about 90 % from the initial response, indicating high stability of electrode over a long period. The reproducibility of the fabricated paste electrode was conducted using five individual electrodes prepared by the same procedure. The relative standard deviation (RSD) of these electrodes was 3.19 %. Moreover, the RSD value of 4.73 % obtained after 10 successive measure- ments with similarly fabricated electrodes suggested that LZH-SDS-PRO/MWCNT paste electrode can be used repeatedly for the determination of UA. Figure 9. Interference analysis of 0.1 mmol L-1 UA at LZH-SDS-PRO/MWCNT paste electrode Real samples analysis The validity of the results obtained using the LZH-SDS-PRO/MWCNT paste electrode was studied by determining the concentration of UA in the urine sample via the standard addition method. The urine sample was directly diluted 30 times in 0.1 mol L-1 PBS (pH 6.4) without pre-treatment. Then, known concentrations of UA were spiked into the urine sample to study electrode recoveries. As a result, LZH-SDS-PRO/MWCNT paste electrode exhibited good recoveries, as summarized in Table 2. Table 2. Determination of UA in urine sample using LZH-SDS-PRO/MWCNT paste electrode (n = 3) Sample cUA / µmol L-1 Recovery, % RSD, % Determined Spiked Found Urine 140.3 100 243.8 101.5 3.81 200 337.3 99.1 2.94 300 445.7 101.2 3.72 Conclusions In this experiment, a simple, highly sensitive, and cost-effective sensing material was proposed for the determination of UA with low LOD. 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