Orientational Dynamics of Magnetic Iron Oxide Nanoparticles in a Hydrogel: Observation by Magnetic Linear Dichroism under Oscillating Field

Abstract

For the success of biomedical applications of magnetic iron oxide nanoparticles (MION), such as magnetic hyperthermia and magnetic particle imaging, it is essential to understand the orientational dynamics of MION in a complex fluid under an alternating field. Here, using the magnetic linear dichroism (MLD) measurement, we directly observed the orientational behavior of MION in a hydrogel under a damped oscillating magnetic field (DOMF) of 33 kHz in frequency. Hydrophobically modified ethoxylated urethane (HEUR) is examined as the network polymer because the mesh size of the network is controllable with its concentration. We used two MIONs: a bare MION (MION1) and a MION coated with an amphiphilic polymer (MION2). Where the mesh size of the gel network is larger than the particle’s hydrodynamic diameter, MION1 in the hydrogel rotates in the same manner in a simple solution, although the macroscopic rheological property of the medium is quite different. Meanwhile, the orientational behavior of MION2 is dramatically changed by the addition of HEUR molecules even below the minimum gelation concentration, indicating that MION2 is associated with the flower micelles of HEUR. By analyzing the MLD waveform, the orientational behavior of MION1 in the HEUR gel under a DOMF can be explained with single-mode relaxation, whereas that of MION2 is complicated; a rapid partial rotation near the particle and a whole slow rotation of the particle-flower micelle associate are superimposed. It is hard to distinguish this difference in orientational behaviors from the dynamic magnetization curve because the dominant magnetization reversal process is Néel rotation, the rotation of the magnetic moment in the particle. The MLD measurement is a potential tool for optimizing biomedical techniques utilizing MIONs and for nanorheology or colloid science in a complex matrix such as a hydrogel or cytoplasmic matrix.

Applications of magnetic and electromagnetic forces in micro-analytical systems

“…294 Here, again, the aggregation state of MNPs will severely affect the interpretation of the observation. Optical polarization measurements of the magnetic orientation of MNPs will become a promising method to measure the viscosity of the nano-environment, 232 the stability of MNPs in solution, and the aggregation dynamics of nanoparticles given that the optical transition moment of MNPs is parallel to the easy axis of the magnetic moment of MNP 295 and highly sensitive to the rotational relaxation dynamics. 228 Electromagnetophoretic force was applied for the size separation and adsorption-desorption chromatography of microparticles, 238 but more applications should be investigated.…”

“…In addition, a recent study showed that this method will be useful for measuring the local viscous environment around the MNPs. 232…”

Scaling Perspective on Dynamics of Nanoparticles in Polymers: Length- and Time-Scale Dependent Nanoparticle–Polymer Coupling

“…(1) polyhedral oligomeric silsesquioxanes (POSS) nanoparticles with the diameter d on the order of 1 to 3 nm, 27,40,46 (2) quantum dots (QDs) with 2 nm < d < 10 nm, 36,67 (3) gold nanoparticles with 5 nm < d < 400 nm, 11,12,14,19,34,39,43,49,50,55,56,63,78 (4) magnetic nanoparticles with 5 nm < d < 20 nm, 18,31,37,38,51,52,62,64,70,71,76,80 (5) silica (SiO 2 ) nanoparticles with 5 nm < d < 50 nm, 26,29,30,44,48,57,60,61,74,81 (6) polystyrene nanoparticles with 20 nm < d < 2000 nm, 13,16,17,22,33,35,53,65,66,68,69,72 (7) nanorods with dimensions from (1) nm to (100) nm made of Au, magnetic elements and their dioxides, or titanium dioxide (TiO 2 ). 15,…”

“…The development of nanoparticle chemistry in recent years has enabled the synthesis of nanoparticles with well-controlled size, shape, and surface chemistry, yielding a library of nanoparticles available for investigating fundamental scientific problems and exploring nanoparticle-based applications. Nanoparticles commonly used in the studies of nanoparticle dynamics in a polymer matrix − include polyhedral oligomeric silsesquioxanes (POSS) nanoparticles with the diameter d on the order of 1 to 3 nm, ,, quantum dots (QDs) with 2 nm < d < 10 nm, , gold nanoparticles with 5 nm < d < 400 nm, ,,,,,,,,,,,, magnetic nanoparticles with 5 nm < d < 20 nm, ,…”