Electrochemical Detection of Uric Acid Based on a Carbon Paste Electrode Modified with Ta2O5 Recovered from Ore by a Novel Method

Except for well-known commercial production procedures, this study demonstrates that Ta2O5 particles can be produced. Through a series of steps, highly pure Ta2O5 particles (99.45%) were produced from the raw ore. We have electrochemically detected one of the important nitrogenous compounds present in urine, “uric acid”, by a Ta2O5 particle-modified carbon paste electrode (Ta2O5-MCPE) using cyclic voltammetry. The prepared electrode has shown excellent current sensitivity at a pH of 6.0 phosphate-buffered solution. We have found that 4 mg Ta2O5-MCPE has recorded the highest current sensitivity of 75.75 μA. The oxidation peak current was varied with the uric acid concentration in the range from 1 to 5 mM at 4 mg Ta2O5-MCPE. We have calculated the electrode-active surface area for a bare carbon paste electrode and 4 mg Ta2O5-MCPE using the Randles–Sevcik equation, and the values were found to be 0.0202 and 0.0450 cm2, respectively. On the other hand, the calculated values of limit of detection and limit of quantification were reported as 0.5937 × 10–8 M and 1.9791 × 10–8 M, respectively, for the prepared 4 mg Ta2O5-MCPE. The interfere studies revealed that the variation in the electrochemical signal of uric acid in the presence of different metal ions was found to be less than ±5%.


INTRODUCTION
Uric acid (2,6,8-trihydroxy purine) is a very important heterocyclic component of urine and is mainly produced during the metabolic activity of purine. 1 Regular physiological concentrations of uric acid in blood are 25−80 mg/L and 15− 60 mg/L, respectively, for men and women. 2 Maintaining the accurate level of uric acid in the body is quite difficult due to factors like food and genetic variations. 3Seafood and meat can increase the amount of uric acid in blood. 4The abnormal amount of uric acid in humans can cause severe diseases like gout, hypertension, hyperuricemia, diabetes, heart failure, and Lesch−Nyhan syndrome. 5,6Therefore, frequent monitoring of uric acid in the body with a sensitive and selective method is very important.Techniques like liquid chromatography, 7 titration, 8 capillary electrophoresis, 9 and spectrophotometric 10 are employed in the determination of uric acid.However, the high use of chemicals is just one of the many drawbacks of these methods.− 14 There are many superior properties of carbon paste electrodes (CPEs), which are usually preferred for electrochemical studies: ease of modification with aqueous/ anhydrous matrices, high conductivity and adsorption capacity, low residual current, new repeatability, and wide potential window. 15,16−20 Undoubtedly, the goal of these modifications is to maximize selectivity in order to enhance the electrode performance.In this context, one of these metal oxide types used is tantalum oxide (Ta 2 O 5 ).High chemical and thermal stability and high pH sensitivity of Ta 2 O 5 , a transition metal oxide compound, are among its most significant characteristics. 21,22Additionally, Ta 2 O 5 -based carbon and nitrogen molecules can function as catalysts for reduction processes in polymer electrolyte fuel cells. 23In addition, it is also known that metal-doped Ta 2 O 5 films have good electrochemical reversibility and catalytic performances. 24a 2 O 5 is a crucial substance for the design and development of electrochemical sensors because of all of these characteristics.On the other hand, literature data show that CPE modification with Ta 2 O 5 and its electrochemical application are quite limited.One of them is that the increased surface area of CPE modified with Ta 2 O 5 boosts its electrochemical catalytic activity.A fabricated sensor enabled the determination of flavonoids with repeatable and dependable findings. 25n another, chrysin and baicalein determinations were carried out using electrodes that had been modified with Ta 2 O 5 particles and chitosan.In both specimens, the linear range was found to be 0.08−4.0μM.The produced sensor demonstrated good sensitivity and exceptional stability, with detection limits for chrysin and baicalein measured at 0.03 and 0.05 μM, respectively. 12hen the aforementioned studies are taken into consideration, it becomes evident that Ta 2 O 5 powders employed are analytically pure commercial items.Difference and innovative aspect of the present study is that Ta 2 O 5 powders are obtained from natural ore as a result of some processes and evaluated in the determination of uric acid levels.This scope of this paper, frequent monitoring of uric acid in the body, is very important, and therefore, we have developed a simple, economic, stable, and highly sensitive Ta 2 O 5 -MCPE to determine uric acid.Fabricated electrode has shown excellent selectivity and antifouling characteristics in determining uric acid.

Raw Material and Characterization.
Ore containing Ta 2 O 5 was brought from a mine in the Republic of Congo.The chemical composition of raw ore is given in Table 1.According to the data, the ore structure is highly complex.Ta 2 O 5 is not only the substance present; Nb 2 O 5 , Fe 2 O 3 , SnO 2 , and Al 2 O 3 are also present at significant levels.Therefore, it is obvious that a stepwise procedure is required to produce Ta 2 O 5 of high purity.
2.2.Methods.It is vital to enhance the sample surface area and remove impurities in order to increase leaching efficiency.Consequently, following grinding and sieving, magnetic separation was used to separate magnetic components.Then, the sample was mixed with a NaOH/KOH (99 and 100% purity) mixture, 1.5 times its weight, for 10 min before roasting.Roasting was carried out at 500 °C for 90 min using a chamber oven.The roasted specimen was placed inside the reactor and put through following leaching procedures: liquid/ solid ratio 6−8 mL/g, leaching temperature 80−90 °C, leaching time 60−90 min, stirring speed 600 rpm, and pH in the range of 9 and 10.The extraction efficiency of Ta was found to be 94% under these leaching conditions.Following that, in the solvent extraction procedure (liquid/liquid ratio 1.25 mL/mL, temperature 40−50 °C, leaching time 20−30 min, and stirring speed 200 rpm), pure methyl isobutyl ketone and kerosene were employed.Using an incubator, Ta was stripped under the following circumstances: liquid/liquid ratio of 1.0 mL/mL, temperature of 20 °C, stripping time of 30 min, and stirring speed of 200 rpm.Finally, after chemical precipitation at pH 9, calcination was carried out at 700 °C for 90 min.After all these procedures, the purity of Ta 2 O 5 is 99.45%.The flow diagram of the proposed process for the recovery of Ta 2 O 5 from raw ore is presented in Figure 1.
A significant crystalline peak is not present in the structure of the final product according to X-ray diffraction (XRD) data in Figure 2. In other words, it is understood that the product has an amorphous structure and no particular crystallization.XRD data are consistent with previous studies. 26,27The chemical composition of the finished product was examined using inductively coupled plasma optical emission spectrometry (ICP-OES) as a result of multistep processes (Table 2).Despite having a complicated structure, the ore, according to analysis data, yields a 99.45% (Ta 2 O 5 ) pure final product.As can be seen from Figure 3, 10, 50, and 90% of the produced

Fabrication of Carbon Electrodes.
A bare carbon paste electrode (BCPE) was prepared by hand-grinding the 70% graphite powders and 30% silicone oil in an agate mortar for 30 min to obtain a homogeneous mixture of carbon paste.Similarly, we have also hand-ground the same composition of graphite powders and silicone oil along with different concentrations (2, 4, 6, 8, and 10 mg) of the Ta 2 O 5 nanoparticles individually and separately for 30 min in an agate mortar.For electrochemical studies, we have used Zive SP1 galvanostat/potentiostat, which is a three-electrode system composed of a working electrode (prepared carbon paste electrodes), a platinum counter electrode, and a reference electrode, respectively.The prepared carbon paste was inserted into a 3 mm cavity present in a polymer working electrode connected with a copper wire at the end to study the current response.We have used all the fabricated, different concentrated Ta 2 O 5 -modified carbon paste electrodes and recorded their current response in determining 1 mM uric acid.

Optimizing Electrochemical Detection at Different
Ta 2 O 5 -MCPE.Excellent current sensitivity mainly depends on choosing the correct concentration of the modifier.Therefore, we have recorded current response during electro-oxidation of 1 mM uric acid at a pH of 6 phosphate-buffered solution (PBS) using cyclic voltammetry at different concentrations of the modifier as shown in Figure 5a.We have also plotted anodic peak currents obtained, respectively, for BCPEs 2, 4, 6, 8, and 10 mg Ta 2 O 5 -MCPE as shown in Figure 5b.
Figure 5b depicts that 4 mg Ta 2 O 5 -MCPE has shown a maximum anodic peak current of 75.75 μA compared to BCPE, which has depicted only 34.05 μA of peak current.This proves the sensitivity of fabricated Ta 2 O 5 -MCPE, which can determine uric acid even in low concentrations with excellent sensitivity.The surface area of BCPE and MCPE is comparatively different, and this is the main reason for increased sensitivity of MCPE.To study this in detail, we have calculated the electrode surface area by the Randles−Sevcik equation 28,29 as follows The calculated active surface area for BCPE and 4 mg Ta 2 O 5 -MCPE is found to be 0.0202 and 0.0450 cm 2 , respectively.The increased surface area of MCPE increases the reactive sites along with surface roughness, as a result of which the interaction of electrons between the electrode and analyte increases significantly.Therefore, based on the results, we have used 4 mg Ta 2 O 5 -MCPE for further studies.Figure 5c depicts the cyclic voltammetric curve of 1 mM uric acid for BCPE and 4 mg Ta 2 O 5 -MCPE, respectively.From the figure, we can clearly see the differences in anodic peak current during electro-oxidation of uric acid in PBS of pH 6.
3.1.2.Effect of pH.The investigation of the electrochemical response of the electrode in pH is very crucial to understand the number of electrons and protons involved in the electrode reaction.This also has an effect on the stability of the electrode because few analytes are reactive in acidic pH and few of them in basic pH.Therefore, we have studied the current sensitivity of fabricated 4 mg Ta 2 O 5 -MCPE against different pHs from 6 to 7.6, and their cyclic voltammetry curves are shown in Figure 6a.The anodic peak current of uric acid decreased with an increase in pH from 6 to 7.6, as shown in Figure 6b.Maximum current sensitivity for uric acid was recorded at a pH of 6, and therefore, we have recorded all voltammetric measurements at this pH.We have also plotted a graph of anodic peak potential  at different pHs as shown in Figure 6c.From this plot, it is noted that the anodic peak potential is shifting toward a more negative potential with an increase in pH value from 6 to 7.6.This indicates the participation of protons in the electrode reaction.The graph of variation of the anodic peak potential with pH is linear and follows the equation E p (V) = 0.8021− (0.05832) pH (V/pH) (R 2 = 0.9943), with an excellent linear regression coefficient (R 2 ).−34 Further, we have also calculated the number of protons (m) involved in oxidation and reduction reaction based on a graph of pH versus E p by the below equation where R is the gas constant, T is the temperature in kelvin, n is the number of electrons, and F is the Faraday constant.The number of protons (m) involved in electrode reaction is found to be 1.966, and the value is nearly equal to 2. Therefore, the electrochemical redox reaction of uric acid is taking place with involvement of two electrons and two protons.Figure 7 illustrates the electrochemical oxidation reaction process of uric acid on the surface of Ta 2 O 5 -MCPE.

Effect of Scan Rate.
−38 This study reveals whether the electrochemical reaction is adsorption controlled or diffusion controlled.Hence, we reported cyclic voltammetry response of uric acid at different scan rates from 0.1 to 0.8 V as shown in Figure 8a.
From the figure, it is confirmed that an increase in scan rate increases electro-oxidation of uric acid in a PBS of pH 6 along with a positive shift of potential.To understand the dependence of fabricated MCPE and uric acid molecules in detail, we have plotted a graph of scan rate versus anodic current and also square root of scan rate versus anodic peak current, respectively, depicted in Figure 8b,c.There is a linear increase in anodic peak current in both graphs, indicating the fast and direct electron transfer between uric acid and surface of 4 mg Ta 2 O 5 -MCPE.This confirms strong binding of uric acid molecules on MCPE.We have calculated correlation coefficient for graphs Figure 8b,c to understand the type of electrode reaction involved, and their values were found to be 0.9932 and 0.9528, respectively.This confirms that the type of electrode reaction is diffusion controlled.

Effect of Concentration of Uric Acid.
−42 If the concentration of the analyte is greater, we can expect maximum current sensitivity and vice versa with low concentration.To understand the stability and effectiveness of fabricated MCPE, we have studied the effect of uric acid concentration on electro-oxidation of uric acid at a PBS of pH 6. Figure 9a depicts a cyclic voltammogram of uric acid at different concentrations starting from 1 to 5 mM, respectively.From cyclic voltammogram, it is confirmed that the anodic peak current response is increasing from 180.87 to 273.82 μA. Figure 9b depicts a graph of concentration of uric acid versus anodic peak current, and it confirms a linear increase in anodic peak current with increasing concentration of uric acid.The increased current sensitivity is due to increased molecules of uric acid in the PBS electrolyte, which   in turn increases the interaction of uric acid molecules and electron movement between the electrode surface and electrolyte.Therefore, we recorded the maximum anodic peak current at a higher concentration of 5 mM uric acid with a correlation coefficient of 0.9986.
Limit of detection (LOD) and limit of quantification (LOQ) of 4 mg Ta 2 O 5 -MCPE were determined using the slope of Figure 9b   The calculated values of LOD and LOQ were found to be 289.69× 10− 8 M and 965.65 × 10− 8 M, respectively, for the prepared 4 mg Ta 2 O 5 -MCPE.Table 3 depicts a comparison of the LOD of Ta 2 O 5 -MCPE for UA with other reported electrodes.
3.1.5.Effect of Interferents.Electrochemical determination of 1 mM uric acid at 4 mg Ta 2 O 5 -MCPE was carried out in the presence of few interfering metal ions like Fe 2+ , Fe 3+ , Na + , Mg 2+ , K + , Cu 2+ , glucose, and ascorbic acid (AA) to investigate the influence of these metal ions in determining the uric acid analyte and also to check whether any substantial difference in the current response or shifting of potential.From the experiment, it was confirmed that there is no significant variation in the electrochemical signal of 1 mM uric acid even in the presence of interfere ions.This confirms the stability and selectivity of fabricated electrode Ta 2 O 5 -MCPE to determine uric acid.The variation in electrochemical signal recorded is less than ±5% as shown in Figure 10.
3.1.6.Repeatability, Stability, and Reproducibility of 4 mg Ta 2 O 5 -MCPE.The stability, reproducibility, and repeatability of 4 mg Ta 2 O 5 -MCPE were determined by electro-oxidizing UA in a phosphate-buffered solution of pH of 6 and a scan rate of 0.1 V/s.Both repeatability and reproducibility tests were carried out at least five times each by changing the electrode and electrolyte, respectively.The determined relative standard deviation values for reproducibility and repeatability were found to be 2.95 and 2.1%, respectively.Therefore, the fabricated Ta 2 O 5 -MCPE can be repeated and reproduced without much deviation in the original value of the oxidation peak current during the detection of UA.We have also investigated the stability of the prepared Ta 2 O 5 -MCPE by running 50 cycles for detection of UA in a pH 6 solution.We have recorded the initial and final oxidation peak current during the electro-oxidation of UA to determine the degree of stability of the fabricate electrode.We have obtained stability value 95.47%, and this value confirms that the fabricated Ta 2 O 5 -MCPE has shown outstanding stability during electrochemical determination of UA.

CONCLUSIONS
Ta 2 O 5 production procedures and uses are now crucial because of the growing popularity of Ta 2 O 5 .In the context of this paper, high-purity Ta 2 O 5 powders were made from natural ore using a variety of techniques, including atmospheric leaching, solvent extraction, stripping, chemical precipitation, and calcination.Modified carbon electrodes made from 99.45% pure Ta 2 O 5 particles demonstrated sensitive and selective uric acid measurement capabilities.The reported electrode has depicted excellent current sensitivity at a 4 mg modifier concentration.The calculated electrode-active surface area reveals that the Ta 2 O 5 -MCPE surface area is at least 2 times the surface area of BCPE.This shows the importance of modification to determine the analyte very effectively and efficiently.LOD and LOQ were calculated to be 0.5937 × 10 −8 M and 1.9791 × 10 −8 M, respectively, for the prepared 4 mg Ta 2 O 5 -MCPE.We have also reported that the variation in the electrochemical signal of uric acid in the presence of K, Fe, Cu, and Mg metal ions was found to be less than ±5%.This confirms the selectivity of the electrode in which the interfere metal ions do not influence the electrochemical signal of uric acid.

Figure 1 .
Figure 1.Flowchart of the Ta 2 O 5 recovery process from raw ore.

Figure 2 .
Figure 2. XRD pattern of the final product.

Figure 3 .
Figure 3. Particle size distribution of Ta 2 O 5 from ore.

Figure 5 .
Figure 5. (a) Cyclic voltammogram of 1 mM uric acid at a pH of 6 PBS at different concentrations of the modifier; (b) plot of various concentrations of Ta 2 O 5 nanoparticles in MCPE and their respective anodic peak current; and (c) cyclic voltammogram of 1 mM uric acid at BCPE and 4 mg Ta 2 O 5 -MCPE.

Figure 6 .
Figure 6.(a) Cyclic voltammogram of 4 mg Ta 2 O 5 -MCPE in 1 mM uric acid solution at different pHs with a scan rate of 0.1 V s −1 , graph of pH vs (b) I pa and (c) E pa at 1 mM uric acid.

Figure
Figure Electrochemical processes of Ta 2 O 5 -MCPE for detection of uric acid.

Figure 8 .
Figure 8.(a) Cyclic voltammetry response of uric acid at different scan rates from 0.1 to 0.8 V; (b) plot of I pa vs scan rate, and (c) I pa vs square root of scan rate at a pH of 6 PBS.

Figure 9 .
Figure 9. (a) Cyclic voltammogram curve at different concentrations of uric acid.(b) Plot of I pa versus concentration of uric acid.

Figure 10 .
Figure 10.Plot of interferents versus % of error in electrochemical signal of uric acid.

Table 1 .
Chemical Composition of Raw Ore Used in Experiments

Table 2 .
ICP-OES Analysis Results of the Final Product