Preparation, structure and magnetic properties of synthetic ferrihydrite nanoparticles

Superparamagnetic ferrihydrite powders with average nanoparticle sizes of 2.5 nm produced by the chemical deposition method. Static and dynamic magnetic properties are measured. As a result of ultrasonic treatment in the cavitation regime of suspensions of ferrihydrite powders in a solution of the albumin protein, the Fe ions are reduced to the metallic state. A sol of ferrihydrite nanoparticles is prepared in an aqueous solution of arabinogalactan polysaccharide.


Introduction
Fine powders (powders with particle sizes less than 10 nm) based on oxides, hydroxides, and oxyhydroxides of iron are the object of increased interest at the present time.This circumstance is due to the practical use of powders in such areas as catalysis, biomedicine, etc. [1]. Bulk ferrihydrite is an antiferromagnet. However, as the particle sizes decrease to nanoscale, the magnetic properties of the powder change radically. Nanoparticles of ferrihydrite acquire a constant magnetic moment due to incomplete compensation of the magnetic moments of the sublattices. Another important practical feature of ferrihydrite is its absolute biocompatibility, because this mineral is the core of the ferritin protein complex, which is the main carrier of iron in all higher living organisms. Therefore, ferrihydrite powders can compete with powders of conventional ferro-and ferrimagnets used in various practical applications [2], including for targeted drug delivery, as well as for contrasting in magnetic resonance imaging [3].
A method for obtaining stable sols of ferrihydrite nanoparticles of biogenic origin was developed in [4] (ferrihydrite nanoparticles are formed on the cell surface during the cultivation of Klebsiella oxytoca bacteria). As it turned out, the magnetic properties of biogenic nanoparticles of ferrihydrite can be modified by heat treatment [5], however, it is not possible to produce a stable sol on the basis of annealed powders. As a matrix for the preparation of stable sols based on chemically produced ferrihydrite powders, it is possible to use organic preparations such as water-soluble proteins albumin or natural polysaccharide arabinogalactan.
The aim of this work was the production of ferrihydrite powders and finding ways to preparation of sols based on them. The process of preparation of ferrihydrite-based sols consists in the ultrasonic treatment of suspensions of nanoparticles.

Experimental methods
Synthetic ferrihydrite nanoparticles were prepared chemically [6]. At room temperature and constant stirring, a 1 M alkali solution of NaOH was added to a 0.2 M solution of iron nitrate Fe(NO3)3·9H2O to a neutral pH value. The precipitate formed was collected on a filter. Then, the precipitate was washed and dried at room temperature. The electron microscopy examination was carried out on a Hitachi HT7700 transmission electron microscope. The Mössbauer spectra were measured on an MS-1104Em spectrometer with a 57 Co(Cr) source. The chemical isomer shifts were given with respect to α-Fe. The magnetic measurements were carried out on a vibrating-sample magnetometer [7]. The temperature dependences of the magnetic moment M(T) were measured both in the mode of cooling without a magnetic field (zero field cooling (ZFC)) and in the mode of cooling in an external magnetic field (field cooling (FC)). The magnetic resonance spectra were recorded on a Bruker ELEXSYS 560 spectrometer operating in the X band mode (the characteristic microwave radiation frequency was ~9.4 GHz) at temperatures in the range from 100 to 300 K. Ultrasonic treatment was performed on the Volna UZTA-0.4/22-OM apparatus. The intensity of ultrasonic treatment was over 10 W·cm −2 and the frequency was 22 kHz. The treatment time was 4-24 min. Ultrasonic treatment was carried out in an aqueous medium, in a solution of protein albumin and arabinogalactan. Figure 1 shows the results of transmission electron microscopy of the obtained ferrihydrite nanoparticles. Figure 1 (a) shows the microdiffraction pattern of nanoparticles, which is characteristic of ferrihydrite nanoparticles [8,9]. Two diffuse reflections with interplanar distances d1 = 1.6 Å, d2 = 2.7 Å are observed. The average particle size was D = 2.5 nm ( Fig. 1 (b)).  fig.2b) was carried out using the standard approach for systems of noninteracting antiferromagnetic nanoparticles. In this approach, the magnetic moment of the sample is determined by the SP behavior of individual particles, taking into account their distribution over magnetic moments, and also the • component, which determines the antiferromagnetic contribution to the magnetization. In this case, the M(H) dependences are described by the following expression [7,10,11]: In this expression ( , ) -Langevin function, ( )distribution function of the magnetic moment of the particles , number of particles per unit weight of the sample. The lognormal distribution was used [10]. From the temperature dependence 〈 〉( ), by extrapolation to T = 0 K, values of 〈 〉( = 0) were determined, which amounted to 173 B. The fig.2b shows experimental isotherms of magnetization M(H) as well as the results of the best fit using eq.(1) and partial contributions of the first (superparamagnetic) and the second (antiferromagnetic ·H) terms of eq.(1).  Table 1 shows the values of the particle size D, the blocking temperature TB, the coercive field HC, the saturation magnetization MS and the antiferromagnetic susceptibility AF. For comparison, the table also shows the characteristics of ferrihydrite nanoparticles obtained as a result of cultivation of bacteria [12].  Ferromagnetic resonance spectra of ferrihydrite nanoparticles are symmetrical curves of the Lorentz shape, and the absorption maximum is located near ⁄ value, which for a given frequency is 3350 Oe. For ferrihydrite nanoparticles in the investigated temperature range, the intensity of the FMR signal decreased linearly, indicating that the nanoparticles are in the unlocked, SP state.

Results and discussion
According to the Raikher-Stepanov theory [13,14], in the powders of randomly oriented particles of ferromagnets and ferrites, the width of the absorption line is a nonmonotonic function of temperature: where Δ ( )contribution to the broadening due to superparamagnetism of nanoparticles, Δ ( )contribution to the broadening due to the scatter in the directions of the anisotropy fields of the particles (inhomogeneous broadening), which is decisive at low temperatures. Figure 3 shows the values of the width of the FMR line as a function of temperature and the result of fitting of experimental data using the Raikher-Stepanov theory. The curve shown in Figure 3 is characterized by two adjustable parameters: KV and MV. Table 2 shows the values obtained, as well as KV and MV of nanoparticles of ferrihydrite of biogenic origin. The table also shows the corresponding parameters of ferritin complexes calculated from the temperature curves of FMR [15].   Figure 4 (curve 1) shows the Mossbauer spectrum of chemically prepared ferrihydrite nanoparticles, characterized by a paramagnetic doublet. Table 3  To obtain the nanoparticle-arabinogalactan complexes, 1% sol of the ferrihydrite particles in distilled water was used. A solution of arabinogalactan (1%) in distilled water was poured onto the sol of the particles in a ratio of 1:1.Homogenization was performed by ultrasound for 10 min using an ultrasonic radiator -UltrasonicdisintegratorUD-20 (Techpan, Poland). The resulting complexes were washed from the excess unbound polysaccharide by centrifuging the particle sol at 35,000 g on a centrifuge Avanti® J-E (Beck-man Coulter, USA). The resulting precipitate was collected and rehomogenized in distilled water. The cycle was repeated 3 times. The final result was a stable sol of the complexes in distilled water, which was used for further research. The resulting complex has the property of restoring, that is, the dried sol particles with the addition of water also form a sol. The possibility of making a sol on the basis of synthetic nanoparticles of ferrihydrite in a solution of albumin was also investigated. The fraction of particles transferred to the sol was insignificant.

Conclusions
Ferrihydrite powders are produced by the chemical deposition method. Static and dynamic magnetic properties are measured. A sol of ferrihydrite nanoparticles is made in an aqueous solution of a natural polysaccharide, arabinogalactan.