Introduction

In recent years, the magnetic nanoparticles have drawn much research attention due to their unique magnetic properties originating from their small size (Lu et al. 2007). Among various magnetic nanoparticles, ferrite nanoparticles have become exceptionally popular for a wide variety of applications such as magnetic recording equipment (Han et al. 1996), magnetic fluids (Lopez et al. 2012), drug delivery (Wang et al. 2011), and hyperthermia treatment (Kim et al. 2010) applications. Ferrites, which adopt cubic spinel structures and have a general formula of [A][B]2O4, where [A] and [B] indicate tetrahedral and octahedral cation sites, respectively, are very important classes of magnetic materials. Their magnetic and electronic properties are determined by their crystal structure and chemical compositions and, in particular, by the location of divalent metal ions and iron between the oxygen ions (Carter and Norton 2007). Among the ferrite nanoparticles, ZnFe2O4 nanoparticles have attracted a great deal of research interest due to their unusual magnetic properties that are different from the bulk ones. Bulk ZnFe2O4 has a normal spinel structure with Zn2+ ions in the A-site and Fe3+ ions in the B-sites (Tung et al. 2002). However, nanocrystalline ZnFe2O4 system always shows up as a mixed spinel in which Zn2+ and Fe3+ ions are distributed over the A- and B-sites and exhibits anomaly in its magnetization (Xu et al. 2011; Deraz and Alarifi 2012).

Due to the potential applications of ferrite nanoparticles, a wide variety synthesis process for the production of nanoparticles have been developed including sol–gel (Gatelyte et al. 2011), ball milling (Chakka et al. 2006), solvothermal (Yanez-Vilar et al. 2009), hydrothermal (Nalbandian et al. 2008), and microemulsion (Moumen and Pileni 1996). Since most of these colloidal nanoparticles are synthesized in organic solvents using hydrophobic capping reagents, they are dispersible only in hydrophobic organic solvents. In addition, the organic capping layers can decompose, and the metal nanoparticles can deform and aggregate above 300 °C temperatures (Joo et al. 2009). However, the possible biological applications of these nanoparticles are greatly restricted because of their poor dispersibility in aqueous solutions. One method for compatibility in biological system and high dispersibility under different solutions is to coat the particles, which means to create a water-dispersible core/shell structure. These core/shell nanoparticles have core made of a material coated with water compatible shell.

Coated nanoparticles have become an active field because of their unique chemical, optical, electronic, magnetic properties, and potential applications in many areas compared to those of non-core/shell nanoparticles (Madrakian et al. 2012; Selvan 2010; Ghiaci et al. 2012). In order to synthesize core/shell nanoparticles, many different chemical methods have been developed such as chemical precipitation (Bayal and Jeevanandam 2012), sol–gel (Wu et al. 2012), microemulsion (Wang et al. 2010), and inverse micelles (Lee et al. 2008). To prevent oxidation and magnetic aggregations, nanoparticles are often coated with organic surfactants (Ma et al. 2012), gold (Ahmad et al. 2012), SiO2 (Coskun et al. 2012), and polymers (Quarta et al. 2012). An inert shell encapsulating the surface of the nanoparticles screens the magnetic dipolar attraction and prevents direct contact between the particles (Lu et al. 2007; Quarta et al. 2012; Coskun et al. 2010). In addition, the core/shell structure enhances the thermal and chemical stability of nanoparticles regarding to the chemically inert shell especially in biological systems. SiO2 is one of the non-toxic, water-dispersible, and biocompatible material to coat core structure of the nanoparticles. SiO2 shell provides a chemically inert surface for magnetic nanoparticles in biological systems and allows to conjugate its surface with various functional groups (Deng et al. 2005; Soundarya and Zhang 2010). Stöber (Kobayashi et al. 2005) and reverse micelle methods (Hagura et al. 2010) are the two main process for SiO2 coating the nanoparticles in the literature.

In this manuscript, we reported a detailed comparative study of oleic acid (OA) and SiO2 coating onto ZnFe2O4 nanoparticles based on the characterization for their possible usage in biomedical research and diagnostics. The synthesis is based on a two-step method: (i) synthesis of OAcapped ZnFe2O4 core using a solution process, and (ii) SiO2 coating on the core by the inverse microemulsion technique. Nanoparticles prepared by this two-step method were characterized by XRD, TEM, VSM, and electron paramagnetic resonance (EPR) techniques.

Experimental

OA-coated ZnFe2O4 nanoparticles were synthesized via chemical route developed by Caruntu et al. (2002, 2004, 2007). Briefly, 10 mmol of FeCl3·6H2O (97 %), 5 mmol of ZnCl2 (97 %), 250 mL of DEG (99 %), and 40 mmol of sodium hydroxide (NaOH) (97 %) were combined with solution into a three neck flask at room temperature under a N2 flow with vigorous stirring. The solution was heated to 230 °C for 2 h, and then 8 mmol of OA (95 %) was added to the solution. The black colored product was separated via centrifugation at 5,000 rpm for 20 min. The sample was washed three times in absolute ethanol and then dried in N2 atmosphere. The average diameter of the ferrite nanoparticles was 5.75 nm with a narrow-size distribution as determined by TEM. The thickness of OA layer is approximately in the size of an OA molecule (1–1.2 nm), as was determined by Murai et al. (2007).

Tetraethylorthosilicate (TEOS) was used to coat the ferrite nanoparticles by a base-catalyzed silica formation water-in-oil microemulsion. The details of the process were reported elsewhere (Lee et al. 2006). In brief, 8 mL of Igepal CO-520 was mixed with cyclohexane (175 mL) under vigorous stirring at room temperature for 20 min. OA-coated ferrite nanoparticles were dispersed in cyclohexane at a concentration of 1 mg/mL and slowly added to the Igepal CO-520/cyclohexane mixture and vortexed for 3 h. An aqueous ammonia solution (28 wt%) was added dropwise and stirred for 5 min. Finally, TEOS was slowly dropped into the mixture. Depending on the desired SiO2 shell thickness, the amount of added TEOS was varied from 0.25 to 2.5 mL. The coating reaction proceeded at 30 °C for 72 h under vigorously stirring before the addition of ethanol to precipitate the nanoparticles. Finally, SiO2-coated nanoparticles were collected by centrifugation and washed at least three times to remove unreacted organic chemicals, and then dried at 30 °C for 6 h.

XRD measurements were performed on a powder sample of OA- and SiO2-coated ZnFe2O4 nanoparticles using an EQUINOX 1000 X-ray diffractometer operating with Co Kα radiation. The X-ray patterns were recorded every 0.030° in the range of 2θ = 5°–100°. The structural analyses and the particle size calculations, depending on strain of the particle, were revealed from XRD patterns.

The size and morphologies of the nanoparticles and the shell thicknesses of the coated nanoparticles were examined using a JEOL JEM-2010F high-resolution transmission electron microscope with high angle annular dark field (HAADF) detector operated at 200 kV. The nanoparticles diameters and the shell thicknesses were determined by statistical averaging using digital micrographs. The structural details of the nanoparticles were demystified using High Resolution TEM (HRTEM).

OA- and SiO2-coated ZnFe2O4 were monitored with differential thermal and thermo gravimetric analysis (DT-TGA) employing TG/DTA 6300 model spectrometer under nitrogen flow at 40 mL/min with α-Al2O3 reference container. The heating rate employed was 10 °C/min in the temperature range of 30–800 °C.

Magnetization measurements were performed by means of the standard zero-field cooled (ZFC)–field-cooled (FC) procedures in the range of 5–300 K with applied fields of 500 Oe using a Quantum Design physical property measurement system (PPMS). The temperature dependence of the magnetization (M–T) and magnetic hysteresis (M–H) loops were also recorded using PPMS. EPR spectra of the prepared samples were recorded at the X band (~9.8 GHz), using a Bruker EMX 131 spectrometer with 100 kHz magnetic field modulation. The EPR thermal study was performed in the temperature range of 120–400 K using a nitrogen gas-flow system.

Results and discussion

Figure 1 shows the XRD patterns of OA-coated and 13.0-nm SiO2-coated ZnFe2O4 nanoparticles, in the 2θ range of 5°–100°. The patterns confirm the formation of single-phase (fcc) spinel structure of the prepared samples. The XRD patterns were compared and indexed using JCPDS file no (22-1012). This analysis revealed the single phase cubic spinel structure of ZnFe2O4 with Fd-3m space group. The XRD pattern of SiO2-coated ZnFe2O4 is also shown in Fig. 1. Besides the highest peak originated from (311) of ferrite in the pattern, the XRD pattern of SiO2-coated ZnFe2O4 nanoparticles presented a broad featureless XRD peak between 20° and 40° diffraction angles corresponding to the amorphous nature of SiO2 shells. The crystallite size of synthesized OA-coated ZnFe2O4 nanoparticles were calculated from the full-width at half-maximum (FWHM) measurement from the prominent X-ray diffraction peaks using Scherrer’s formula (Warren 1996). The average particle size was estimated to be about 5.89 ± 0.33 nm by the most intense (311) peak assuming that strain can be neglected.

Fig. 1
figure 1

XRD patterns of a OA and b 13.0-nm thick SiO2-coated ZnFe2O4 nanoparticles

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Figure 2a shows the TEM image of uniform distribution of ZnFe2O4 nanoparticles, which are surface coated by OA. The particles were found to have a narrow-size distribution with an average size of 5.75 ± 0.75 nm which confirms the particle size obtained from XRD data, using Scherrer’s formula (=5.89 nm). Figure 2b shows typical TEM image of core/shell nanoparticles with an average silica shell thickness of 13.0 nm with standard deviation of 0.68 nm. The other TEM images of core/shell nanoparticles with different SiO2-shell thicknesses are not given here due to space limitations. Figure 2c shows the TEM-HAADF image of SiO2-coated nanoparticles. The nanoparticles have a core/shell structure with the bright contrast corresponds to heavier ferrite nanoparticles and the dark contrast corresponds to light silica shells. As shown in the figure, most particles are spherical and have single core as well as some of the core nanoparticles are clustered together approved by HADDF image. In order to observe the detailed structure of SiO2-coated ZnFe2O4 nanoparticles, HRTEM technique was used. HRTEM studies show that the amorphous SiO2 shells completely cover the magnetic nanoparticles and have a “core/shell” structure with a clearly distinguishable interface. The image clearly shows the single crystallinity of ZnFe2O4 core and the amorphous nature of SiO2 shell (Fig. 2d). The corresponding size distribution histograms and log-normal fitting analysis of OA coated and with a SiO2 shell thickness of 13.0-nm ZnFe2O4 nanoparticles are shown in Fig. 3a, b, respectively. The results related to SiO2 shell thickness depends on the amount of TEOS, and the calculated standard deviations for all samples are given in Table 1. It is seen that both particle size and SiO2 shell thickness of the nanoparticles increase with increasing TEOS content. This result clearly demonstrates that the thickness of SiO2 around the nanoparticles core can be easily controlled by simply varying the initial amount of TEOS (Lee et al. 2006; Hsieh et al. 2009).

Fig. 2
figure 2

a TEM image of OA-coated ZnFe2O4 nanoparticles, b TEM image and, c HAADF image of 13.0-nm thick silica-coated ZnFe2O4 nanoparticles and d an expanded view of a high resolution

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Fig. 3
figure 3

Size distribution (histogram) of ZnFe2O4 nanoparticles from TEM images a OA and b 13.0-nm thick SiO2-coated ZnFe2O4 nanoparticles

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Table 1 The average particle size from XRD and TEM
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The mass losses under continous N2 flow were monitored by TGA/DTG measurements for oleic acid and SiO2 coated nanoparticles. The results are given in Fig. 4a, b. The first mass loss about 3.50 % was observed to occur in the temperature range between 30–200 °C due to evaporation of water, methanol, and ethanol for oleic acid coated nanoparticles (Ayyappan et al. 2008). The next mass loss (approximately 1.5 %) occurs in the temperature range of 236–250 °C due to evaporation temperature of diethylene glycol (DEG) which has an evaporation temperature of 244 °C. The two major mass losses about 5 % and 20 %, which are attributed to loosing second and first order bonded functional groups (COOH) around the nanoparticles were observed to occur at about 290–315 °C and 315–500 °C, respectively. Similar results related with the first and second order mass loss originated from the breaking of bonded oleic acids to the ferrite nanoparticles were also reported in the earlier similar works (Ayyappan et al. 2008; Binder and Weinstabl 2007; Zhao et al. 2006). The most important mass loss occuring in the range of 500–600 °C was correlated to the phase transitions of iron atoms in the lattice ending with release of CO, CO2 and H2O gasses from the sample (Ayyappan et al. 2008).

Fig. 4
figure 4

TGA/DTG curves of ZnFe2O4 nanoparticles coated with a OA and b SiO2

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A mass loss about 5 % in temperature range of 30–200 °C was also observed for SiO2-coated ZnFe2O4 (Fig 4b) due to evaporation of water, methanol (T evaporation = 64.7 °C) and the other chemicals. Evaporations of OA (T evaporation = 360 °C) and Igepal CO-520 (T evaporation = 200–300 °C), which were used during the SiO2 coating are believed to give rise to the mass loss observed in the temperature range of 420–440 °C. The phase transitions of Fe atoms in the ferrite nanoparticles produce a mass loss (about 1.5–2 %) observed in temperature range of 540–550 °C.

The magnetic properties of OA- and SiO2-coated nanoparticles were analyzed using a Quantum Design PPMS with a VSM accessory. Comparative measurements of the hysteresis loops of both OA and 13.0-nm SiO2-coated nanoparticles measured at 300, 50, and 5 K are given in Fig. 5a, b. It is seen that OA- and SiO2-coated ZnFe2O4 nanoparticles exhibit superparamagnetic behavior. The magnetization values of OA and 13.0-nm SiO2-coated ZnFe2O4 nanoparticles were determined to be equal to 63.0 and 1.5 emu/g, respectively, at 300 K as shown in Fig. 5a, b. The magnetizations of SiO2-coated samples are much smaller than that of OA-coated nanoparticles. This originates from the fact that non-magnetic SiO2 shells constitute 80 to 95 % of the mass of the samples. This result is in agreement with the estimation made form TEM observation. However the saturation magnetization OA- and SiO2-coated ZnFe2O4 nanoparticles are bigger than that reported value of ~5 emu/g for bulk ZnFe2O4 (Clark and Evans 1997). This type of anomalously high magnetizations occur when the Fe3+ ions migrate to tetrahedral site giving raise to strong super exchange interaction (Ayyappan et al. 2010; Yao et al. 2007). The coercivities (Hc) were measured to be consistently less than 20 Oe and 40 Oe for OA- and SiO2-coated ZnFe2O4 nanoparticles at 300 K, respectively. Thus, all samples exhibit superparamagnetic behavior. Room temperature magnetic properties determined by VSM method are given in Table 2 for prepared samples. These results indicate that non-magnetic SiO2 coating layer on the surface of magnetic particles produced changes in the magnetism of studied samples (Zhao et al. 2008; Jiang et al. 2003). The only differences between OA- and SiO2-coated ZnFe2O4 nanoparticles are the reduced saturation magnetization and slight increase in the coercivity fields of coated ones (Chang et al. 2010; Sotiriou et al. 2011). The fact that the amount of SiO2 in a sample could not be determined precisely, and it was impossible to make the magnetization specific with respect to the amount of the cores. Consequently, the magnetic hysteresis loops of all samples were similar, indicating the negligible influence of SiO2 coating on the magnetic properties.

Fig. 5
figure 5

Magnetization of a OA and b 13.0-nm thick SiO2-coated ZnFe2O4 nanoparticles versus the magnetic field at 5, 50, and 300 K

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Table 2 Summary of magnetic characteristics of samples: T B, saturation magnetization (M s), and coercivity (H c)
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Figure 6 shows the temperature-dependent ZFC and FC magnetization curves of OA and 13.0-nm SiO2-coated ZnFe2O4 nanoparticles, measured at an applied magnetic field of 500 Oe in the temperature range 5–300 K. The maximum of the ZFC curve is a characteristic of the average blocking temperature (T B) of nanoparticles (Parma et al. 2012). Inspection of the ZFC curves given in Fig. 6 indicates that coating with 13.0-nm SiO2 of the ZnFe2O4 nanoparticles causes a decrease in the T B from 15.9 K to about 15.3 K. These values are in agreement with those reported in literature for SiO2-coated magnetic nanoparticles (Zhang et al. 2012; Bumb et al. 2008). T B values of ZnFe2O4 nanoparticles coated with SiO2 shell of various thicknesses are given in Table 2. It is seen that the maximum T B occurs at a SiO2 coating thickness of about 8 nm. The T B of the nanoparticles decreases with the increase in SiO2 thickness. This effect of SiO2 coating on blocking temperature has been well explained in our previous papers, published elsewhere on coated NiFe2O4 and CoFe2O4 nanoparticles (Coskun et al. 2012, 2010). In brief, it might due to SiO2 shell, which improves spin ordering at the surface, and reduces both magnetic dipole–dipole interaction and the average effective volume of ZnFe2O4 cores (Yao et al. 2007; Chang et al. 2010; Pereira et al. 2010).

Fig. 6
figure 6

Temperature dependence of the ZFC and FC magnetization measured at 500 Oe for OA- and SiO2-coated ZnFe2O4 nanoparticles. The arrows indicate the T B

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EPR spectra of OA- and 13.0-nm SiO2-coated ZnFe2O4 nanoparticles are given in Fig. 7. It is seen that the spectrum of OA-coated nanoparticles consists of two clearly distinguished components: a board asymmetric EPR line (signal 1) centered at the magnetic field of H R = 3,080 G (g = 2.01) with a linewidth of ΔH pp = 780 G, and another asymmetric EPR line (signal 2) centered at the magnetic field of H R = 1,650 G (g = 4.3) with a linewidth of ΔH pp = 410 G. The g-factor value 2.01 calculated for signal 1 from EPR measurement is in good agreement with that reported in the literature for nanostructured-ZnFe2O4 particles (Lin et al. 2006). This signal at g = 2.01 is likely due to Fe3+ ions staying in the octahedral symmetry sites in the spinel structure of the samples (Koksharov et al. 2001; Li et al. 2012). According to the signal assignment reported earlier, the weak signal centered at g = 4.3 is very specific and attributed to the isolated 6S5/2 spin of Fe3+ ions in tetrahedral coordination with a strong rhombic distortion (Li et al. 2012; Hsieh and Lue 2002; Shafi et al. 1997; Noginova et al. 2008) and to an impurity of Fe2O3 at an EPR detectable level in OA-coated ZnFe2O4 sample (Shafi et al. 1998; Cao and Gu 2005). Variations of three EPR parameters with temperature of OA-coated ZnFe2O4 nanoparticles in the range of 120–400 K are given in Fig. 8. These parameters are the amplitude of the line (I PP) (top panel), the resonance field (HR) (middle panel), and the linewidth (∆Hpp) (bottom panel) (see Fig. 8). As it is easily seen studied, parameters depend strongly on temperature. Temperature dependences of the resonance field H R and of the linewidth ∆H pp are very similar to those reported for other nanoparticle systems (Guskos et al. 2012), i.e., ∆H pp increases as the temperature decreases and the resonance field shifts toward the lower magnetic field with decrease in temperature. These variations of the EPR line parameter were considered as a revelation of superparamagnetic regime of the ferrite nanoparticle spin system (Guskos et al. 2010). The amplitude of the line increases with decreasing temperature from 400 K, reaching a maximum value at about 200 K. Below this temperature, the line amplitude falls down with the temperature decrease (Guskos et al. 2012). Such a behavior of the resonance line intensity is a characteristic for the agglomeration of magnetic nanoparticles (Guskos et al. 2007, 2008).

Fig. 7
figure 7

ESR spectra of OA- and SiO2-coated ZnFe2O4 nanoparticles measured at 300 K

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Fig. 8
figure 8

The temperature dependence of the EPR parameters for OA-coated ZnFe2O4 nanoparticles a intensity (I PP), b resonance field (H R), and c linewidth (∆H pp)

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Figure 9 shows EPR spectra of SiO2-coated ZnFe2O4 nanoparticles obtained for different TEOS contents at 300 K. From the line fitting analysis it was concluded that the EPR spectrum of SiO2-coated samples consists of a superposition of two resonance lines, a broad line associated with the particle-core contribution, and a narrow one associated with the particle-shell contribution at the center of the main signal appearing at about g = 2.01 (Koksharov et al. 2001; Berger et al. 2000; Vargas et al. 2008). Such a phenomenon, which is described in earlier works (Noginova et al. 2008, 2007), is typical for superparamagnetic nanoparticles embedded in a non-magnetic matrix (especially in diluted magnetic nanoparticle systems).

Fig. 9
figure 9

300 K EPR spectra of SiO2/ZnFe2O4 nanoparticles coated with different thickness in the presence of different TEOS content

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We also studied the effect of temperature on different spectrum parameters of SiO2-coated ZnFe2O4 nanoparticles by EPR spectroscopy in the temperature range of 120–400 K. Experimental spectra recorded at each temperature were simulated by assuming the existence of two superimposed resonance lines of Gaussian shape with different line characteristics. The results of such a calculation using the data of an experimental EPR spectrum recorded at 300 K for 13.3 nm SiO2-coated ZnFe2O4 nanoparticles is given in Fig. 10. It is seen that a model based on the presence of two resonance lines exhibiting different characteristics describes very well experimental spectra recorded at 300 K. Such a spectrum simulation calculations were performed over the temperature range of 150–400 K, and characteristic parameters of the contributing resonance lines were calculated at each measured temperature. The results are given in Fig. 11a–c for 13-nm SiO2-coated nanoparticles. The temperature dependence data derived in the present work for H R and ΔH pp parameters of the contributing broad resonance line are found to be very similar to those reported previously for other nanoparticles in that H R decreases and ΔH pp increases as the temperature decreases (Noginova et al. 2008, 2007; Parekh et al. 2000). The broadening and the shift of the resonance lines to lower magnetic fields with the decrease in temperature are the typical behaviors of superparamagnetic nanoparticles reported in the literature (Cao and Gu 2005; Vargas et al. 2008). For ideal paramagnetic systems, I pp is expected to be proportional to the paramagnetic susceptibility. This is the case for contributing broad line which exhibits the superparamagnetic-core characteristics (Vargas et al. 2008). As for the narrow line, its intensity and ΔH pp remain almost constant down to 170 K while it is resonance field (H R) increases monotonically with decrease in temperature. Such narrow resonance lines were also observed in core/shell nanostructures indicating the presence of free radicals on silica shell due to the presence of dangling bonds in the silica (Vaidya et al. 2011). Variations with temperature of the I pp, ΔH pp, and H R parameters of the resonance line appearing at g = 3.97 of SiO2-coated nanoparticles were also studied (not given here). It is found that investigated parameters exhibit temperature dependences similar to those obtained for OA-coated nanoparticles.

Fig. 10
figure 10

Experimental and fitted EPR spectra at room temperature for 13.0-nm thick SiO2-coated sample. The spectra are seen to be composed of two signals, viz., one intense and broad signal and one narrow signal

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Fig. 11
figure 11

Variations with temperature of the characteristic parameters of the contributing resonance lines determined from spectrum simulation calculations for SiO2-coated ZnFe2O4 nanoparticles. a intensity (I PP), b linewidth (∆H pp), and c resonance field (H R) (circles board line, squares narrow line)

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Dependences of the EPR signal H R and ΔH pp parameters on TEOS content were also investigated at 300 K for OA and SiO2-coated ZnFe2O4 nanoparticles by taking the board lines into account. The results of this study are given in Fig. 12. ΔH pp parameter of SiO2-coated nanoparticles was observed to be smaller than that obtained for OA-coated nanoparticles. As it seen from Fig. 12 while ΔH pp decreases with the increase in the TEOS content and the line shifts toward higher magnetic fields. H R continues to increase up to 0.5 mL TEOS content and then remains nearly constant in the range of 0.5–2.5 mL TEOS content. ΔH pp decreases significantly (1,000 G) with the increase in TEOS content first, then remains almost constant in the TEOS content range of 0.5–2.5 mL. This sharp decrease in ΔH pp parameter is attributed to the increased non-magnetic silica shell thickness on ZnFe2O4 core and to the decrease in the dipole–dipole interaction. These results obviously demonstrate that EPR parameters of SiO2-coated ZnFe2O4 nanoparticles are strongly influenced by the silica shell thickness (Hsieh et al. 2009).

Fig. 12
figure 12

TEOS contents dependence of the EPR H R and H pp parameters of ZnFe2O4 nanoparticles a resonance field (H R), b linewidth (∆H pp)

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Conclusion

In the present work, the magnetic properties of OA- and SiO2-coated ZnFe2O4 core/shell nanoparticles were investigated using different analysis techniques. The influence of SiO2 shell thickness on the magnetic properties was also analyzed. The results of XRD and TEM analyses proved that the particle size of SiO2-coated ZnFe2O4 nanoparticles ranged from ~22 to 34 nm in diameter with a magnetic core of ~6 nm, which increased with the increase in TEOS content. Magnetic properties of OA- and SiO2-coated nanoparticles were investigated in detail to clarify the effect of silica coating on the T B. SiO2-coated ZnFe2O4 nanoparticles were found to exhibit superparamagnetic behavior in the investigated temperature range. The experimental results showed that due to the existence of a non-magnetic silica layer and good dispersion of ZnFe2O4 core particles in the SiO2 matrix, the saturation magnetization of the silica-coated nanoparticles decreased as TEOS content increased. It is found that at the beginning of SiO2 shells coating of ZnFe2O4 nanoparticles in the range of 7–14 nm cause an increase in the T B. However, after certain coating thickness, due to surface anisotropy and magnetic interparticle interaction, T B starts to decrease. The effect on the T B of decreased magnetic interparticle interactions was also investigated by EPR technique using nanoparticle coated with SiO2 of different thickness. Variations with temperature of the EPR spectrum characteristics of OA- and SiO2-coated ZnFe2O4 nanoparticles were also reported in the present works. The main differences between OA- and SiO2-coated ZnFe2O4 nanoparticles were observed to arise from surface environments and magnetic interparticle interactions.

SiO2 surfaces are chemically stable and biocompatible. Therefore, SiO2-coated ferrite nanoparticles prepared by the technique presented in this work have great potential applications in various biomedical fields, such as targeted drug delivery and magnetic hyperthermia.