Magnetic properties of iron oxide nanoparticles in different organic shells
Magnetite nanoparticles (NPs) in two types of organic shells – dextran and citrate – were synthesized. Comparative studies of magnetic properties of aqueous colloidal solutions of nanoparticles were carried out by the methods of electron magnetic resonance (EMR) and longitudinal nonlinear response to a weak alternating field parallel to a constant field H, with registration of field dependences of phase components ImM2(H), ReM2(H) of the second harmonic of magnetization M2 (NLR-M2) at room temperature. Weak field hysteresis of the M2 response observed at the scanning frequency H Fsc = 10 Hz disappeared with an increase in the relaxation time during scanning to 4 sec (Fsc = 0.25 Hz), which indicates the superparamagnetic (SPM) mode of ensembles of nanoparticles both with dextran and in citrate shells. The obtained NLR-M2 data were processed using a formalism based on the Gilbert–Landau–Lifshitz equation for the stochastic dynamics of SPM particles, which is reduced to the Fokker–Planck (FP) kinetic equation. This approach allows one to determine both magnetic and dynamic parameters of SPM systems and analyze their magnetic state. It was shown that the magnetic centers that give a nonlinear response signal are not individual NPs, but aggregates formed by them in a colloidal suspension. As a result of simultaneous approximation of the ImM2(H), ReM2(H) dependencies by numerical solutions of the FP equation, such magnetic and dynamic parameters of aggregates as the average magnetic moment, the field and energy of magnetic anisotropy, the concentration of magnetic centers, the Neel relaxation time, the magnetization decay constant, etc. were determined. The concentration of NPs was determined by the thiocyanate method. The analysis of the obtained results allowed us to estimate the influence of the organic shell type on the aggregation process. From the EMR data for NPs, the crossover field from the mode of magnetically correlated aggregates of NPs to the mode of independent NPs was estimated. The magnetic state of nanoparticles and their magnetic and dynamic parameters are important for assessing the potential of their application for early diagnostics of tumors (enhancement of the contrast of MRI images) and in therapy using physical methods (radiotherapy – the effect of radiosensitization, and hyperthermia – selective heating) under the condition of targeted delivery of nanoparticles to the tumor.
Authors: Ya. Yu. Marchenko, V. A. Ryzhov, V. V. Deriglazov, A. S. Dmitrieva, A. V. Nikitina, Yu. V. Bogachev
Direction: Physics
Keywords: superparamagnetic nanoparticles, nonlinear response, electronic magnetic resonance
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