## 3 Results  
### Nanoparticles Characterization  
The citrate-stabilized maghemite nanoparticles (SPIONs) (Figure 1A,B), 11.8 \(\pm\) 2.4 nm in size (as determined by transmission electron microscopy (TEM), with N = 100), were encapsulated with a 3-5 nm thick (Figure 1C,D), homogeneous silica shell (SPIONs-SIL), as reported in our previous papers [28; 31]. The size of the nanoparticle clusters encapsulated with approximately 6 nm thick silica shell was 95 \(\pm\) 23 nm (Figure 1E,F), which was determined from the TEM micrographs (>100 clusters counted). The synthesis of the nanoparticle clusters is based on the self-assembly of N = 70 \(\pm\) 14 superparamagnetic iron oxide (maghemite) nanoparticles. The core-shell nature of the SPION clusters, with the closely packed maghemite nanoparticles within the cluster cores and the amorphous silica shell, can be clearly distinguished in the TEM micrographs (Figure 1E,F).  
The XRD of the precipitated nanoparticles showed a single spinel phase (Figure 2A), whereas the Mossbauer spectroscopy confirmed that the SPIONs were composed of maghemite, as reported in our previous work [32]. Both the silica-encapsulated nanoparticles and the silica-encapsulated nanoparticle clusters showed good colloidal stability (Figure 2B), while high magnetic responsiveness was observed only for the silica-coated nanoparticles clusters. Thus, when the suspension was placed near a magnet, the silica-encapsulated nanoparticle clusters rapidly migrated toward the magnet (Figure 2B). Subsequently, as soon as the external magnet was removed, the particles could be dispersed spontaneously by a gentle shake. All nanoparticles exhibited superparamagnetic properties (Figure 2C). The saturation magnetizations (_M_s) of the as-synthesized nanocrystals, silica-encapsulated nanoparticles, and silica-encapsulated nanoparticle clusters were 64.3, 50.3, and 53.8 emu/g, respectively (Figure 2C). While the three samples contained the same type of nanocrystals, the saturation magnetization differences among the samples originated from the content of the mass of silica per gram of nanocomposite. While as-synthesized nanocrystals have no silica shell, the saturation magnetization of the SPION-SILs, compared with that of the MNCs, decreased because SPION-SILs have a larger surface area per gram of nanocomposite and, therefore, they contain a larger portion of non-magnetic (diamagnetic) silica per gram compared with MNCs. The electrophoretic mobility of the silica-encapsulated nanoparticles and silica-encapsulated nanoparticle clusters were measured as the function of the operational pH. The silica surface showed a relatively acidic character because its structure comprised negatively charged hydroxyl groups at pH values above the isoelectric point (IEP) at pH 2.6. The zeta potential curves of the silica-encapsulated nanoparticles and silica-encapsulated nanoparticle clusters reached high negative values of zeta potential at a physiological pH of 7.4 at \(-\)29 mV and \(-\)25 mV, respectively (Figure 2D). The high absolute values of the zeta potential provided strong electrostatic repulsive forces between particles, resulting in good colloidal stability of the suspension in neutral and alkaline conditions [29].  
Figure 1: **(A,B)** Transmission electron micrographs of studied nanoparticles: superparamagnetic iron oxide nanoparticles (SPIONs), (**C,D**) silica-encapsulated individual nanoparticles (SPIONs-SIL), and (**E,F**) silica-encapsulated nanoparticle clusters (MNC).