Nanofluids for Biomedical Applications Using Spherical Iron Oxide Magnetic Nanoparticles Fabricated by High-Power Physical Evaporation

Figure 2 shows the X-ray diffraction (XRD) results of MNP studies  performed using a DISCOVER D8 (Bruker) diffractometer operating with  Cu-Kα radiation (λ = 1.5418 A). Quantitative analysis was performed using  TOPAS-3 software. The average size of coherent diffraction domains  was estimated using the Scherrer approach.¹⁴ Figures 2A and 2B show  the XRD spectrum of MNPs collected from the filter before and after  separation by centrifuging. The typical cell parameter for magnetite was  a = 0.8396 nm. Slightly lower values of the cell parameters (a = 0.8390  and a = 0.8368 nm for MNPs from cyclone and filter before separation,  respectively) indicated that the structure can be described as “defective  spinel.” Although particles smaller than 30 nm predominated, a substantial  number of particles with diameters above the superparamagnetic¹³ limit  were still present. This minor fraction of large particles is responsible  for the ferromagnetism; therefore, the separation of large particles is  a requirement for the creation of nanofluids. Conventionally, this is  performed using a liquid suspension of powder by sedimentation,  which is based on Stokes’s law. Fractionization is the result of the more  rapid sedimentation of the large particles compared to the small ones.  Successful fractioning can be performed only when the particles are  dispersed in liquid and move separately from one another. This necessary  condition of de-aggregation is difficult to maintain. Aggregation  during redispersion in liquid suspension is one of the main problems  in processing air-dry nanoparticles.¹⁵ EEW iron oxide MNPs do not  form stable suspensions in pure water even after ultrasonic treatment, i.e., as-prepared air-dry MNPs are strongly aggregated and cannot be  disassembled by shaking the water molecules. To address this, additional  repulsive forces between MNPs must be introduced to overcome  their mutual interactions. We performed specific studies on the factors  controlling the colloidal stability of EEW MNPs in water suspensions. For  this purpose, sodium citrate was selected as the electrostatic dispersant  and stabilizer. We used zeta-potential measurements to determine the  optimal concentration of electrostatic stabilizer, sodium citrate, and  optimal pH to achieve stable EEW Fe₃O₄-water suspensions.¹⁶  

Next, the magnetic properties of the MNPs were examined. M(H)  magnetization curves and magnetization as a function of temperature in  zero-field-cooling (ZFC) and field-cooling (FC) processes were measured  in the temperature range 5 ≤ T ≤ 300 K. Information about the size and  anisotropy of the EEW Fe₃O₄ MNPs was obtained by the fitting of ZFC  curves and assuming a coherent rotation of the magnetic moments  inside each nanoparticle during the relaxation processes (Langevin  formalism).¹⁷ A good fit was obtained, assuming a log-normal size  distribution for the MNPs such that each nanoparticle has an average  uniaxial anisotropy (K ~ 1.2 × 10⁶ erg/cm³) and no interactions between  particles. The absence of hysteresis of M(H) curves at room temperature  and the shape of the hysteresis loops is typical of superparamagnetic  systems (SPM). In SPM systems, at low temperatures the magnetization  process in an increasing external field is associated with the rotation  of the magnetic moment of each MNP toward the direction of the  magnetic field. This process is energetically costly and results in high  coercivity and remanence. With the increase of temperature, thermal  fluctuations give rise to an SPM state, characterized by zero coercivity and  a non-saturating behavior. 

Laser Target Evaporation Technique

LTE MNPs are prepared using a device that includes a Ytterbium (Yb)  fiber laser with a wavelength of 1.07 μm (Figure 3A). To do this, a 65 mm  diameter, 20 mm thick pellet target is pressed from commercially  available coarse micron-sized iron oxide powder with a specific surface area of 0.5 to 1 m²/g. The pellet target is annealed at 1,000 °C for 1 hr in air  prior to MNP fabrication by mounting it as part of the driving mechanism  inside the evaporation chamber (shown as (1) in Figure 3). Both rotation  and horizontal movement of the pellet were controlled. An optical system with 200 mm focal length and 0.45 mm diameter of the focal spot were  used for focusing the laser beam onto the surface of the target. The driving mechanism provides a 20 cm/s beam scan rate on the target, ensuring uniform wear of the surface. A pulsed laser with a frequency of 5 kHz and pulse duration of 60 μs favors the formation of fine MNPs with narrow particle size distribution. The working gas mixture of N₂ and O₂ is  introduced into the evaporation chamber by a fan. Vapors of iron oxide that are driven out of the focal spot, condenses into MNPs, and are further  carried out by the working gas into the cyclone (shown as (4) in Figure 3)  and the fine filter (shown as (5) in Figure 3). The cyclone collected the coarse fraction of MNPs while the filter collects the fine fraction. TEM  studies (Figures 3B and 4A) confirmed that the LTE MNPs are spherical in  shape. Particle size distribution was found to be log-normal in the case of LTE MNPs with a mean diameter of 9.2 nm and dispersion of 0.368. MNPs  fabricated by physical condensation in the vapor are found to aggregate. The proper choice of LTE parameters avoids the coalescence of the liquid  droplets and prevents formation of non-spherical coarse agglomerates.  However, due to the collision of physical surface forces, solid particles  are inevitably aggregated in the gas phase. As a result, the iron oxide  LTE MNPs contained a fraction of physical aggregates. To address this,  the suspension was deaggregated by exhaustive ultrasonic treatment  to achieve a constant average hydrodynamic diameter, followed by  centrifuging. Two types of de-aggregated suspensions were obtained:  MNPs-I (a water-based suspension without electrostatic stabilizer) and  MNPs-II (a water-based suspension with sodium citrate as an electrostatic  stabilizer, as shown in Figure 4B). The stoichiometric ratio Fe²⁺/Fe³⁺ in the MNPs is determined by redox potentiometric titration with potassium  dichromate using an TitroLine automatic titrator (Schott Instruments). Titration is performed under an argon atmosphere to prevent oxidation of the Fe²⁺ in the air. The lattice constant of the crystalline phase is  substantially smaller than that of stoichiometric magnetite, but larger  than the lattice constant of γ-Fe₂O₃.

While a detailed description of the analysis of LTE MNPs magnetization  behavior can be found elsewhere,¹² it is instructive to examine a  few parameters. For example, the temperature dependence of the  spontaneous magnetization in MNPs, M_s(T), is due to thermal magnon  excitation and can be described by the modified Bloch law:¹⁸ where M(0)  is the zero temperature magnetization, B is the Bloch factor and α=3/2  for the material. The factor α is derived by fitting experimental values;  these are strongly size dependent and approach the bulk limit for large  powders with diameters on the order of hundreds of nm. In contrast to  the bulk, the value of α in MNPs is > 3/2, increasing as the size of the  MNP decreases due to the finite-size effect. Using the best fit for M(T),  the value α = 2.27 } 0.03 is obtained for LTE MNPs. This value is consistent  with the data for ferrite particles obtained by other techniques.¹⁸

For practical purposes, both the saturation magnetization of a ferrofluid  at room temperature as well as the de-aggregation state stability  with respect to application of high magnetic field, are very important  parameters. Figure 5 shows an example of M(H) loop LTE MNPs-I waterbased  suspension with a maximum obtained concentration of 5 wt. %  without an electrostatic stabilizer.

Ferromagnetic resonance (FMR) is a very informative technique for determining the sphericity of powder particles. This is because the field location of the line is strongly affected by deviations from spherical  shapes. Accordingly, using techniques that we have developed over the years,^4,9,12,19 we measured FMR at room temperature and 8.5 GHz for all of our powders, both as-prepared as well as when diluted with non-magnetic talc. A typical signal is shown in Figure 5B. The following features are notable:

• Although rather wide (about 1 kOe) in every case, we observe only  one resonance. This indicates that the grain material is magnetically  uniform and homogeneous.
• For the given frequency, the line is centered at 3.1 } 0.1 kOe. This is to  be expected from a spherical ferromagnetic particle with negligible  magnetocrystalline anisotropy.
• Dilution with talc reduces the linewidth only marginally, indicating  that interparticle magnetic coupling is not the main source of the  large linewidth. It is useful to recall that single crystal Fe₃O₄ films show  FMR linewidths of about 1 kOe.

As a final step, the biocompatibility of fabricated LTE MNPs-I and MNPs-II ferrofluids was tested. Two non-pathogenic types of yeasts were used: Exophiala nigrum (black yeasts) and its mutant strain (red yeasts) were selected for study of cytotoxicity and the possibility of MNPs accumulation in a living system (Figure 6). Two yeast strains were originally isolated from the Baikal and the Schumak river. These yeasts are interesting model systems as they play an important role in keeping equilibrium in the Baikal ecosystem. Liquid and gelatinous agar nutrient media were used for the cultivation of yeasts with controlled quantities of the ferrofluids. Magnetic measurements performed on dry yeast samples revealed more effective MNPs absorption from liquid nutrient than from gelatinized nutrient. Total reflection X-ray fluorescence (TXRF) using a Nanohunter spectrometer by Rigaku analyzed iron concentrations: yeasts grown in liquid medium had higher Fe concentrations (about 1,000 ppm for black yeasts) than yeasts grown in gelatinous agar medium (120 ppm for black yeasts). Cell morphology was studied by optical, scanning (SEM), and transmission electron microscopies (TEM). In all cases for the selected conditions, no significant alterations of cell morphology were observed (Figure 6).

Conclusions and Future Prospects

Ferrofluids possess the unique combination of fluidity and capability to be controlled by the application of an external magnetic field. Biomedical applications demand a large amount of de-aggregated magnetic nanoparticles in the form of water-based ferrofluids. High-power physical evaporation methods (EEW and LTE) are excellent candidates to ensure fabrication of fit-for-purpose ferrofluids. Recently, we have shown the possibility of measuring the concentration dependence of an LTE MNPs water-based suspension with an electrostatic stabilizer using a thin film giant magneto-impedance sensor.²⁰ In another case, the sensitivity of giant magneto-impedance sensitive elements was improved by covering the surface with polymer composite-containing EEP Ni water-based MNPs.²¹ These are interesting directions to follow.

Acknowledgments

This work was supported by CRDF and UB RAS grant 16991 and SAIOTEKREMASEN grants. Selected studies were performed at SGIker services of UPV-EHU. We thank A.I. Medvedev, A.V. Bagazeev, A.M. Murzakaev, S.V. Komogortcev, J.P. Novoselova, N.S. Kulesh, T.P. Denisova and K.V. Zarubina for special support.