*3.4. The Material Behavior in the Presence of UA*

To further examine electrodes' compatibility with the chosen analyte, their electrochemical response was measured in the presence of UA. The measurements were conducted in 10 mM uric acid solution in 0.1 M PB pH 6, in the potential range from 0.2 V to 1 V and at the scan rate of 50 mV/s. Cyclic voltammograms of CP, La(OH)3/CP, MWCNTs/CP, and La(OH)3@MWCNTs/CP electrodes show the oxidation peak current values 0.31 μA, 0.28 μA, 0.27 μA, and 0.27 μA, respectively (Figure S3A). Considering only anode peak height, we could assume that electrode modification does not significantly influence the electrodes' performance in the presence of UA. We could also incorrectly conclude that the unmodified CP electrode shows the best properties. However, reflecting on the complete potential area, it is obvious this is not the case. We can see that with the electrode modification, the resolution of the peak improves and residual current decreases. This leads to better peak to peak separation and to a lowering of the detection limit. Furthermore, while the oxidation of the analyte occurs on the same potential on every voltammogram, the electrolysis of the supporting electrolyte shifts to higher potentials as we proceed with the electrode modification, from 0.49 V for CP to 0.67 V for La(OH)3@MWCNTs/CP. This additionally lowers capacitive current, improves peak shape, and enables a stable and reproductive environment for detecting low analyte concentrations.

When compared to an unmodified CP electrode, modified electrodes (MWCNTs/CP, La(OH)3/CP, and La(OH)3@MWCNT/CP) provide a significantly improved voltammetric

peak and higher currents (I) toward UA detection. The CV signals of UA using these modified electrodes were similar, so for the selection of the optimal electrode for further experiments, we used the data obtained from the electrochemical characterization of all electrodes, as previously described. The La(OH)3/CP electrode had the poorest CV response based on these measurements, most likely due to a decrease in electrode active surface area. MWCNTs promoted electron transfer and decreased the interfacial resistance, and better CV response was achieved when MWCNTs/CP were used. The La(OH)3 likely makes the entire complex of MWCNTs and La(OH)3 more porous, which can help improve the response of the modified electrode. All of this led to the conclusion that the synergistic effect of MWCNTs and La(OH)3 improved the modified electrode's electrochemical properties, which will contribute to the sensor's analytical performance for uric acid detection. The electrochemical behavior of La(OH)3@MWCNTs/CP electrodes, with different weight percent of the modifier (from 2% to 10%) was compared using CV (Figure S3B). It is evident from the graph that the material manifests a catalytic effect on the electron shuttle, with the oxidation peak currents being 0.27 μA, 0.36 μA, and 0.45 μA for the electrodes containing 2%, 5%, and 10% of the modifier. Furthermore, the increase in the material contained in the paste was followed by oxidation peak potential shift to lower values, from 0.39 V for 2% to 0.33 V for 10% of the composite material in CP, in addition to a further decrease in the capacitive current. Therefore, the material containing 10% of the composite was determined to be the most suitable for further development of the sensing platform for UA detection in human cells. Additionally, we recorded the CV of three different concentrations of UA, using the electrode with 10% of the composite material in CP (Figure S3C).
