## 4 Conclusions  
The results obtained in the current work show that the catalytic activity of the MNPs mainly relies on the exposure of the naked surface of the MNPs to the target substrate. Further modification of the  
Figure 6: (**A**) Effect of the interfering compounds on the analytical response of the GCE/MWCNT/ UN-MNPs using chronoamperometry. (**B**) The obtained current responses (1, 2, 7, and 8) denote injections of H\({}_{2}\)O\({}_{2}\), while 3, 4, 5 and 6 denote NaCl, KCl, glucose and ascorbic acid, respectively. The applied potential was \(-0.3\) V, and the concentration of all the injected agents was 100 \(\upmu\)mol L\({}^{-1}\).  
Figure 7: (**A**) The prepared milk sample analysis using multiple standard additions method with selected GCE/MWCNT/UN-MNP biosensors at \(-0.3\) V. (**B**) The calibrated results of the spiked sample.  
synthesized MNPs impaired the peroxidase-like activity. Furthermore, the developed sensor based on UN-MNPs was successfully applied for the selective and sensitive determination of H\({}_{2}\)O\({}_{2}\) at low potential magnitude, which improved the selectivity of the developed senor. The accurate determination of H\({}_{2}\)O\({}_{2}\) in a milk sample was realized using the developed biosensor. Therefore, it was demonstrated that MNPs can be a promising alternative for the detection of natural enzymes with peroxidase-like activity. Because of the low cost, high stability, and durability of nanozymes, the developed sensor is advantageous compared to the analogous enzyme-based sensors. The developed biosensor can be used for the preliminary and rapid determination of H\({}_{2}\)O\({}_{2}\) in foodstuffs. It can be also applied for the online monitoring of H\({}_{2}\)O\({}_{2}\) in a flow system because of its short response time.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/19/6756/s1](http://www.mdpi.com/2076-3417/10/19/6756/s1)]([http://www.mdpi.com/2076-3417/10/19/6756/s1](http://www.mdpi.com/2076-3417/10/19/6756/s1)), S1-S15: The synthesized different magnetic nanoparticles (MNPs) were furthered characterised by using techniques as described.  
Conceptualization, A.M. and A.M.A.; methodology and synthesis of MNPs, A.M.; validation, A.M., A.M.A.; formal analysis, A.M. and A.M.A.; sample and electrode characterization, P.S. and M.B. and J.K.; authors of the manuscript, A.M. and A.M.A.; FTIR, L.R.; Raman spectra, J.P.; final correction, V.A. and L.R. All authors have read and agreed to the published version of the manuscript.  
This research was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic (MEYS CR) under the Czech Infrastructure for Integrative Structural Biology (CIISB) research infrastructure project LM2015043, which supported the Atomic force microscopy (AFM) measurements at Nanobiotechnology Core Facility, and by Central European Institute of Technology (CEITEC) 2020 (LQ1601). CIISB research infrastructure project LM2018127, funded by MEYS CR, is gratefully acknowledged for the financial support provided for the measurements carried out at the Cryo-Electron Microscopy and Tomography Core Facility.  
The authors declare no conflict of interest.