Monitoring of Superparamagnetic Particle Sizes in the Langevin Law Regime

Abstract

In this work, aqueous solutions of magnetite nanoparticles (NPs) are studied. Magnetite NPs are very useful in biomedicine for magnetic resonance imaging (MRI), for drug delivery therapy, and also for hyperthermia. In order to predict the NP efficiency in these applications, it is crucial to accurately characterize their size distribution and their magnetization. Magnetometry, through the dependence of NP magnetization on the magnetic induction (MB curve), can provide interesting information on these physical properties. In this work, the extraction of the NP size distribution and magnetization from experimental MB curves of aqueous solutions of magnetite NPs is discussed. The results are compared to TEM and XRD characterizations. It is shown that an expression taking into account the size distribution better fits the results than the commonly used simple Langevin function. The size distributions obtained by magnetometry seem comparable to those obtained by TEM measurements. However, a closer look at the results shows some nonnegligible discrepancies: the size distributions obtained by magnetometry vary with the temperature and are closer to the TEM ones at room temperature. Our study suggests that it could be explained by the nonnegligible anisotropy energy of the NPs at low temperature and the lack of NP Brownian rotation below the freezing point of water. This demonstrates that care must be taken when interpreting the results obtained by magnetometry of magnetite NPs: only the size and size distribution obtained at room temperature should be used.

Bare Iron Oxide Nanoparticles as Drug Delivery Carrier for the Short Cationic Peptide Lasioglossin

“…The maximal reached magnetization is 63.2 emu/g. The modified Langevin fit is not describing the ideal curve shape of a superparamagnetic substance [ 37 ]. The slopes at high magnetic fields indicate the existence of paramagnetic material within the sample.…”

High Temperature and FORC Investigations of Semiconducting/Ferromagnetic Nanocomposites

“…Taking into account the magnetic energy mB and thermal energy kBT the magnetization is proportional to the Langevin function, m being the magnetic moment, B the magnetic field, kB the Boltzmann constant and T the temperature. Using this approach the anisotropy energy is neglected due to the small size of the particles and the assumption of neglecting magnetic interactions (16). Figure 3 shows the magnetization in dependence on the temperature (a) and field dependent magnetization curves measured at different temperatures between 80 and 950 K (b).…”

Inversion of magnetic diameter distribution of magnetic fluids under high and low temperatures

“…Conversely, at temperatures of 280 K, 320 K, and 360 K, the magnetic fluids adopts a liquid state, where both Neel relaxation and Brown relaxation contribute to the magnetization behaviour. It is worth noting that within this temperature range, the applicability of the MMF1-L function remains uncertain and can be further elucidated through the classical scaling law [16] for superparamagnetic particles. The validity of the MMF1-L in this temperature range can be confirmed if all the M/M s -H e /T curves collapse into a single curve.…”

“…Currently, the measurement principles of MNPs size distribution can be categorized as mechanical, optical, and magnetic. Measurement methods based on mechanics and optics include transmission electron microscopes [16], small-angle neutron stutterers [17], and x-ray diffractometers [18]. The particle diameter and morphology of magnetic nanoparticles (MNPs) can be obtained using the transmission electron microscope, however, the transmission electron microscope resolution capability is limited, the edges of the nanoparticles in the transmission electron microscope images are blurred, which leads to the low results of the measured particle diameter.…”