*3.5. Optimization of pH of the Supporting Electrolyte. Study of the Reaction Kinetics-Influence of Varying Scan Rates on the Material*

For optimization of the pH of the supporting electrolyte, CV measurements were performed with La(OH)3@MWCNTs/CP electrode containing 10% of the modifier mixed in the carbon paste. Measurements were taken in 0.1 M PB, in the pH range from 2 to 9, at the scan rate of 50 mV/s, and they confirm that the reaction is pH-sensitive. The more alkaline the solution becomes, the oxidation peak potential shifts to lower values, from 0.56 V at pH 2 to 0.24 V at pH 9 (Figure 2A). On the contrary, peak current values increase with the rise in pH, only to gradually drop afterwards, reaching the maximum of 0.45 μA at pH 6. Furthermore, peak resolution is fairly good on lower pH values, ending with pH 6 where the peak is well defined and narrow, unlike the broad peaks at higher pH. The obtained results correlate well with UA's pKa [55] values and electrocatalytic oxidation mechanism (Scheme 2) [55].

**Figure 2.** (**A**) Optimization of pH of the 0.1 M PB supporting electrolyte from pH 2 to pH 9. (**B**) CV in 0.1 M PB pH 6 in the potential range from −0.5 V to 1 V at the scan rates from 2 mV/s to 100 mV/s. (inset B) Linear plot of oxidation peak current vs. scan rate.

**Scheme 2.** Mechanism of electrooxidation of UA.

The reported mechanism can be divided into three main steps—2e-/2H+ deprotonation and the oxidation of uric acid, the hydration of the intermediate diimine and its subsequent decomposition into allantoin and CO2. While this mechanism infers that the reaction is pH-dependent and that alkaline conditions allow the initial deprotonation to proceed, it is less obvious as to why the pH values higher than pH 6 are less favorable. Since pKa values for uric acid are 5.4 and 9.8, we can assume that the elevation of pH above pH 6 leads to further deprotonation of urate or diimine, thus introducing new intermediate species and broadening the oxidation peak, lowering its resolution and decreasing the maximal peak current.

The best performance of the electrochemical system at pH 6, and its similarity with the physiological conditions are the reasons why this pH is used for all further measurements.

The CV measurements in PB pH 6 were conducted with optimized electrode surface parameters at various scan rates (from 2 mV/s to 100 mV/s) to determine the reaction kinetics. The oxidation peak potential and ΔE are constant for all scan rates in the measured range (Figure 2B), while the peak current values (inset Figure 2B) increase linearly with the increase in the scan rate, which is described by the equation I(nA) = 3.2436 v (mV/s) + 0.5213, with the linear regression coefficient R = 0.9921. This means that the UA solution is stable, the electron transfer processes are fast, and the electrode reaction is an adsorption-controlled process. From the equation i\_p = (n<sup>2</sup> F2)/4RT vAΓ \*, we can calculate the surface coverage of the adsorbed species (Γ\*) [56]. The surface coverage values for the scan rates from 2 mV/s to 100 mV/s are 0.11 μmol/cm2, 44.08 nmol/cm2, 22.04 nmol/cm2, 11.02 nmol/cm2, 7.35 nmol/cm2, 5.51 nmol/cm2, 4.41 nmol/cm2, 2.94 nmol/cm2 and 2.20 nmol/cm2. These values indicate that, although the adsorption of the UA on the electrode surface decreases with the increase in the scan rate, it is not negligible and can influence the accuracy of the measurements. To prevent introducing a systematic error in the measurements, the surface of the electrode was renewed before each measurement (the excess paste was squeezed out of the electrode, polished with a clean piece of paper and washed with deionized water). The simple and reproducible restoration of the clean electrode surface was one of the major advantages of using the carbon paste electrode in this study.

Even though the anodic peak does not show potential shift and its current linearly increases with the scan rate, the reduction peak is almost absent in all voltammograms, while the cathodic capacitive current increases with the increase in the scan rate. This indicates that the electrode reaction is characterized by a reversible electron transfer followed by an irreversible chemical reaction [56]. In this case, the irreversible chemical reaction is allantoin formation, and it is incorporated in the electrochemical step.
