## 3 Results and Discussion  
### Characterization of MNPs  
The different synthesized unmodified and modified MNPs were characterized by SEM and EDX-SEM techniques (Figure 1). The chemical composition of different MNPs is clearly shown in Figure 1, where the presence of the oxygen in the proximity of the iron confirms the successful synthesis of the iron oxide magnetic particles. Iron and oxygen are present in all the MNPs. The elemental analysis table for the presence of Fe, O, and other element compositions are represented in S1, S2 and S3 in the Supplementary Materials. A silicon composite stub was used for the UN-MNPs and MNPs-CH, whereas an aluminum composite stub was used for the mapping of MNPs-TEOS, respectively. The homogenous dispersion of the selected MNPs on the MWCNT/GCE can be observed in Figure 1D. From the image, the rod-like structure belongs to the MWCNT, whereas drop-cast MNPs can be seen, in the sphere-like structure, to from GCE/MWNT/UN-MNPs. The MNPs were further characterized by using an energy-dispersive X-ray fluorescence (ED-XRF) spectrometer (S4, S5 and S6 in the Supplementary Materials). The characterization of the MNPs by ED-XRF confirms the presence of Fe in all the synthesized particles, while silicon only exists in MNP-TEOS. Moreover, Fourier-transformed infrared spectroscopy (FTIR-ATR) spectra (S7 in the Supplementary Materials), and Raman spectroscopy (S8 in the Supplementary Materials) were done to confirm the modification of MNPs by chitosan and TEOS. The modified particles show the presence of Si-O and O-H in their IR spectra appearing at the same wavenumber as TEOS and chitosan, respectively. Moreover, the Fe-O bond appears in all the synthesized particles. The Fe-O bond also appears in the Raman spectra of the synthesized particles. Transmission electron microscopy (TEM) was done to confirm the particle size and dispersion (S9-S12 in the Supplementary Materials). As can be seen, the UN-MNPs are spheric particles with a uniform size smaller than 50 nm. On the other hand, MNP-CH contains the particles which are far from the spheric shape and have an altered uniform size distribution. MNP-TEOS particles are more aggregated. Furthermore, the size of the different synthesized MNPs was studied by dynamic light scattering (DLS) (S13 in the Supplementary Materials), where all the particles show almost the same average size, even though multi-dispersity in MNP-TEOS is visible. The Fe content in different MNPs was studied by using AAS and ICP-MS, respectively (S14 and S15 in the Supplementary Materials). The Fe density of the UN-MNPs was found to be almost identical to both AAS and ICP-MS, which can indicate that these particles form a stable dispersion. The concentration of MNP-CH is almost the same as UN-MNP. However, the concentration of MNP-TEOS dispersion is several times higher, which might be due to the existence of high aggregation and, consequently, more concentrated dispersion. The different synthesized MNPs were found to be less than 50 nm, as characterized by TEM. Moreover, from SEM, and confirmed by TEM, it was observed that the synthesized MNPs were accumulating and found to be present in the cluster. This clustering was responsible for the larger size of MNPs, as observed in DLS. The working principle of DLS is Brownian motion and this was unable to identify individual nanoparticles from heavy clusters.