# Characterization of Magnetic-Silica Magnetic Fluids by Laser Scattering
## 3 Results and Discussion  
### Physical Characteristics of Magnetic Fluid  
Analysis of the SEM and TEM images of a sample obtained from a magnetic fluid diluted 200 times (Figure 2a,b) by drying its droplet on the substrate showed the presence of both fine particles with dimensions of the order of 10 nm and their aggregates with dimensions of up to about 100 nm. XRD and RHEED data (Figure 2c,d) confirmed the presence of only one crystalline phase of magnetite. The obtained results are in good agreement with the earlier data [16; 36; 37] on the magnetite-silica colloidal particles. The particle sizes in the magnetic fluid correspond to a stable colloidal solution with a high zeta potential (at least 30 mV [37]). The large objects observed in Figure 2a,b may be aggregates formed during dilution and further drying of a droplet of magnetic fluid on the substrate. The loss of stability upon dilution is presumably associated with the complex (in comparison with Coulomb repulsion) mechanisms of stabilization of the silica-based colloids [38; 39]. This effect is expected to be weaker upon substantial dilution [40]. The authors investigated the possibility of using the obtained magnetite-silica nanoparticles in medicine as a negative contrast agent for magnetic resonance imaging [13]. For this reason, the stability of colloidal solutions of low concentration (about 0.1 mg/mL) was also studied with a change in ionic strength [37] (since nanoparticles have to be injected intravenously only in an isotonic buffer solution) and when exposed to a constant magnetic field [13; 37]. Despite the achievement of high stability (according to the sedimentation analysis data [13] and zeta potential measurements [37]) in distilled water, an increase in the ionic strength of a solution or a high magnetic field led to a loss of stability within several hours, which limits the possibility of using the obtained nanoparticles in vivo due to the high probability of thrombosis. According to the results obtained with the VSM technique for a liquid sample (Figure 3) the saturation mass magnetization of the undiluted magnetic fluid with a concentration of 30 mg/mL is 2.8 emu/g. As was shown in [23], the saturation mass magnetization of a dried sample is 60 emu/g. The difference between the saturation mass magnetization of a dried sample and the saturation mass magnetization of pure magnetite, presumably, may be due to its partial oxidation to maghemite [41], as well as the presence of a nonmagnetic phase of silicon dioxide in the composition of particles. The resulting sample contains superparamagnetic nanoparticles, since the shape of the magnetization curve is described by the Langevin law [42]. The amb hysteretic curve shows the absence of magnetostatic interaction between individual particles [36] and also points to the dimensions of magnetic particles less than 25 nm [43]. Figure 1: Laser correlation spectrometer. 1—laser radiation source; 2—aspherical focusing lens; 3—input polarizer; 4—cuvette with the fluid studied; 5—output diaphragms; 6—output rotating polarizer; 7—focusing lens; 8—optical fiber; 9—photomultiplier; 10— analog-to-digital converter; 11—computer. Thus, it can be assumed that particle dimensions greater than 50 nm observed in Figure 2 are aggregates. The particle size distribution in magnetic fluid diluted 200 times obtained with conventional DLS is shown in Figure 4. According to these data, scatterers with a radius of about 70 nm make the largest contribution to the scattering, which also points to the presence of aggregates in a magnetic fluid (\(I\propto R^{6}\), where \(I\) is the scattered light intensity and \(R\) is the scatterer size). Due to the fact that the intensity of light scattering at the aggregates is much higher [41], measurement with the Photocor Mini does not allow us to distinguish the signal from individual nanoparticles. The y axis represents the light scattering intensity \(I\) (\(R\)) in relative units, which is proportional to particle concentration. The x axis represents the size R of particles.