**4. Conclusions**

This paper developed and tested a sensing platform for UA determination in the biological sample. Knowing that the electrode surface modifications are the key enabler for next-generation chemistries based on the interface reactions, we offered a sensing platform based on the novel composite material, optimized to have the best electrochemical performance. Their compatibility with the UA as the analyte was tested and the calibration was performed in the optimal conditions, showing remarkable sensitivity and a wide dynamic working range. Reproducibility and repeatability tests have shown excellent accuracy and precision of the method as a limiting and key factor for practical application. The disadvantage of this method is its limited selectivity in the presence of the common interfering species in the biological matrix. However, this drawback can be bypassed by good experiment planning and/or the use of chemometrics. In the end, the proposed sensing platform was tested in the human cell cultures exposed to the allergens. In the first series of samples (stressed with the papain, and actinidin allergen), concentrations of UA were too low to perform the direct measurement, so the samples had to be spiked. In spite of that, we were able to determine the UA concentration and to repeat the measurements with the unspiked samples stressed with papain. In this way, we not only proved that the stress induced in the cells can be measured by this method, but clearly distinguished the levels of stress induced in HeLa and Hek 293 cell cultures by the use of three different allergens (papain, and actinidin). Since UA is released when cells suffer from stress, sensing devices that can measure a change in the UA concentration can provide us with plenty of information on both the environmental factors and pathological states that cause stress to the human organism. Furthermore, the simplicity of this method and the device itself

opens up opportunities for its commercial use. Hence, we think that this sensing device, as it is, can potentially find applications in medical research and clinical practice.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/bios12090705/s1. Figure S1. Cyclic voltammograms in 5 mM Fe2+/3+ in 0.1 M KCl and 0.1 M PB pH 6, in the potential range from <sup>−</sup>0.5 V to 1 V at the 50 mV/s scan rate of (A) unmodified CP electrode and electrodes modified with 2% of La(OH)3, MWCNT and La(OH)3@MWCNT B) electrodes modified with 2%, 5% and 10% of La(OH)3@MWCNT. Figure S2. EIS spectra in 5 mM Fe2+/3+ in 0.1 M KCl and 0.1 M PB pH 6 on 0.3 V and in the frequency range from 10 kHz to 10 mHz of (A) unmodified CP electrode and electrodes modified with 2% of La(OH)3, MWCNT and La(OH)3@MWCNT (B) electrodes modified with 2%, 5% and 10% of La(OH)3@MWCNT. Figure S3. (A) CV in 0.1 M PB pH 6 in the presence of 10 μM UA—unmodified CP electrode and electrodes modified with 2% of La(OH)3, MWCNT and La(OH)3@MWCNT (B) CV in 0.1 M PB pH 6 in the presence of 10 μM UA—electrodes modified with 2%, 5% and 10% of La(OH)3@MWCNT (C) CV in 0.1 M PB pH 6 in the presence of 7.5; 10 and 100 μM UA using electrode modified with 10% of La(OH)3@MWCNT. Figure S4. Repeatability and reproducibility studies for proposed sensor. Figure S5. Studying the impact of potential interferents on UA detection (A) CV in 0.1 M PB pH 6, at the potential range from −0.05 V to 1 V at the scan rate 50 mV/s, (B) Histogram—percent of the anodic peak current which arose from the oxidation of UA. Figure S6. (A) CV in the potential range from 0.5 V to 1 V at the scan rate of 50 mV/s in the biological matrix in the presence and absence of UA (B) Amperometric curve of UA in the real sample matrix (inset B) Calibration curve of UA standard solutions in the biological matrix.

**Author Contributions:** Conceptualization, S.K., M.O., M.Z. and V.S.; formal analysis, A.N.; investigation, S.K. and M.O.; writing—original draft preparation, S.K., M.O., M.G.-J. and D.S.; writing—review and editing, S.K., M.O., M.G.-J. and D.S.; supervision, M.G.-J. and D.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Ministry of Education, Science and Technological Development of Republic of Serbia Contract number: 451-03-68/2022-14/200168, and the research was funded by Ministry of Science and Higher Education of the Russian Federation (agreement No. 075-15-2022-1135) and South Ural State University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The study did not report any data.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
