## 4 Conclusions  
A series of voltammetric biosensors based on screen-printed carbon and Prussian blue modified carbon electrodes to which horseradish-extracted peroxidase has been added have been successfully used in the analysis of caffeic acid in nutraceuticalals. UV and IR spectrometric analysis demonstrated the presence of peroxidase in the aqueous extract of horseradish root and its enzymatic activity. By the immobilization of the enzyme by cross-linking on two types of screen-printed electrodes, biosensors were obtained that revealed the redox properties of caffeic acid. Among the biosensors studied, the one based on carbon, Prussian blue, and peroxidase has excellent sensitivity for the detection of caffeic acid when cyclic voltammetry is used as a detection method. Limits of detection and limits of quantification in the \(10^{-6}\) M range have been obtained, making it possible to use the biosensors for the detection of caffeic acid from various products of interest. The electroanalytical method has been successfully applied for the determination of caffeic acid content in food supplement products. The advantages of this method are that the sample can be analyzed directly after dissolution, and the equipment is simple, portable, and highly sensitive. This electroanalytical method can be used as a screening method to determine the caffeic acid content in food, nutraceuticalals, or pharmaceutical products.  
The biosensors based on raw extracts containing the enzyme together with the use of nanomaterials could be a solution for the development of cheap, easy to use, and selective novel systems.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13042526/s1](https://www.mdpi.com/article/10.3390/app13042526/s1)]([https://www.mdpi.com/article/10.3390/app13042526/s1](https://www.mdpi.com/article/10.3390/app13042526/s1)), Figure S1: FTIR spectrum of raw horseradishish root. The background was the air; Figure S2: FTIR spectrum of horseradishish root extract. The background was the ultrapure water; Figure S3: UV spectrum of the horseradishish root extract. Figure S4: Dependence of the absorbance determined at 335 nm towards the adding of different volumes of horseradishish extract in a solution containing 0.3% gallic acid with \(10^{-2}\) M hydrogen peroxide in 0.01 M PBS pH 7. Final volume was 5 mL for all samples.  
Conceptualization, I.M.A.; methodology, I.M.A.; validation, I.M.A. and A.G.T.; formal analysis, A.G.T.; investigation, A.G.T.; resources, I.M.A.; data curation, I.M.A. and A.G.T.; writing--original draft preparation, I.M.A. and A.G.T.; writing--review and editing, I.M.A.; supervision, I.M.A.; project administration, I.M.A.; funding acquisition, I.M.A. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a grant from the Romanian Ministry of Education and Research, CNCS--UEFISCDI, project number PN-III-P4-ID-PCE-2020-0923, within PNCDI III.  
Not applicable.  
The authors confirm that the data supporting the findings of this study are available within the article.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.