*2.3. Reagents and Apparatus*

The crystal structure of La(OH)**3**@MWCNT nanocomposite was examined using powder X-ray diffraction (XRD) data collected using a high-resolution SmartLab® X-ray diffractometer (Rigaku, Japan) with a Cu Kα radiation source. The operating current and voltage were 30 mA and 40 kV, respectively. For the experiments, the dried powders were flattened with a zero-background silicon plate. Diffraction data were collected in the range of 10–60◦ 2θ with a step of 0.03◦ and recording speed of 2◦/min. The average crystallite size was estimated by applying Scherrer's equation on the most intensive diffraction peaks (Ls = κλ/βscos(θB. The morphology and surface properties of nanocomposite used for electrode modification were investigated using a field emission-scanning electron microscope FE-SEM MIRA3 (Tescan, Czech Republic) operating at 20 keV. The magnification of the composites was in the range of 10–100,000 times. The samples were prepared by fixating the conductive tape on a holder, vacuum drying, and spray-coating with gold using a Sputter coater.

All electrochemical measurements were carried out using a CH Instruments analyzer (Austin, TX, USA) driven by voltammetric software CHI (Version 4.03). A threeelectrode system was employed, with a modified carbon paste (CP) working electrode (WE), a Calomel reference electrode (RE), and a Pt wire as the counter electrode (CE). The measurements were conducted in 10 mL of 0.1 phosphate buffer (PB) at pH 6. Electrochemical characterization of working electrodes was conducted using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in 5 mM K**3**[Fe(CN)**6**]/K**4**[Fe(CN)**6**] (1:1) in 0.1 M KCl solution. EIS measurements were conducted in the frequency range from 10 kHz to 10 mHz, at 0.3 V. The used experimental parameters for CV measurements were: potential range from −0.5 V to 1 V and the scan rate of 50 mV/s. The CV measurements were also performed in the 0.1 M PB (pH 6) solution containing 10 μM UA Chronoamperometry (CA) was the detection and quantification method of choice. Measurements were taken in 0.1 M PB at pH 6, with the potential set on 0.3 V while adding an increasing quantity of UA during the period of 600 s. High-performance liquid chromatography (HPLC) for uric acid analysis was conducted (Dionex Ultimate 3000, Thermo Fisher, Waltham, Massachusetts, USA) with photodiode array detection on a Hypersil Gold aQ C18 analytical column (150 mm × 3 mm, 3 μm).
