*3.6. Uric Acid Detection*

Amperometric determination was used for UA quantification due to its low detection limit, rapidness, wide linear range, easy signal processing, and the possibility of following the real-time concentration changes in flow systems. The amperometric response of the La(OH)3@MWCNTs/CP electrode, containing 10% of nanomaterial mixed in the CP, toward different standard solutions of the UA, was recorded in 0.1 M PB pH 6, at the fixed potential at 0.3 V (Figure 3A). Successive UA addition to the solution was accompanied by a corresponding current increase and the calibration plot I (nA) against c (μM) was constructed (Figure 3B). The linear response was in the range from 0.67 μM to 121 μM UA with the detection limit, calculated from the plot as 3 S/m (where S is the standard deviation of the blanc and m is the slope of the calibration plot), 64.28 nM, and the analogously calculated limit of quantification (10 S/m) was 0.22 μM. The calibration curve follows the trend expressed by the linear regression equation I (nA) = 2,1582 + 0.4430 c (μM), with the linear regression coefficient R = 0.9969.

**Figure 3.** (**A**) Amperometric response of the La(OH)3@MWCNT modified CP electrode in 0.1 M PB pH 6 towards successive addition of UA standard solution. (**B**) Calibration curve of amperometric current depending on the concentration of UA in the analyte solution, in the range from 0.67 μM to 121 μM.

Five successive measurements of 2 μM UA standard solution were performed, to test the repeatability of the proposed sensing platform for UA determination and monitoring, and the obtained current values were 6.01 nA, 5.71 nA, 5.70 nA, 5.71 nA and 5.42 nA, giving an RSD of 3.65% (Figure S4). The reproducibility was estimated with five different electrodes, which were constructed independently using the proposed procedure (Figure S4). The RSD is 4.13% for the peak current measured in 2 μM UA in PBS (pH 6.0), which demonstrates the reliability of the fabrication procedure. The stability of the modified electrode was also studied using CV. When the electrode was cyclically swept for 30 cycles, the decrease in the initial responses of the modified electrode was 3.6%.

Furthermore, we conducted an extensive literature overview and compared the linear working range, LOD and RSD of our method to the best methods documented in the literature (Table 1). The results suggest that the proposed sensing platform is comparable, if not better than those previously reported. Several UA detection methods using only carbon nanotubes as the electrode material have been reported [21,57]. Although these sensors have adequate linear ranges, they generally have slightly higher detection limits compared to electrode utilizing modified materials, or require complex purification techniques prior analysis. By using MWCNTs decorated with La(OH)3 nanoparticles to modify the CP electrode, a very good sensitivity of the UA detection method was achieved. This is probably due to the joint action of the modifier—the alkaline properties of La(OH)3, which facilitate the binding of UA, as well as the increase in the active surface area and electron transfer efficiency by MWCNTs.


**Table 1.** An overview of recently reported nanomaterial-based electrochemical methods for the determination of UA.
