Local spin dynamics of iron oxide magnetic nanoparticles dispersed in different solvents with variable size and shape: A 1H NMR study

Abstract

Colloidal magnetic nanoparticles (MNPs) based on a nearly monodisperse iron oxide core and capped by oleic acid have been used as model systems for investigating the superparamagnetic spin dynamics by means of magnetometry measurements and nuclear magnetic resonance (H NMR) relaxometry. The key magnetic properties (saturation magnetization, coercive field, and frequency dependent "blocking" temperature) of MNPs with different core size (3.5 nm, 8.5 nm, and 17.5 nm), shape (spherical and cubic), and dispersant (hexane and water-based formulation) have been determined. H NMR dispersion profiles obtained by measuring the r (longitudinal) and r (transverse) nuclear relaxivities in the frequency range 0.01-60 MHz confirmed that in all samples the physical mechanisms that drive the nuclear relaxation are the Néel reversal at low temperature and the Curie relaxation at high frequency. The magnetization reversal time at room temperature extracted from the fitting of NMR data falls in the typical range of superparamagnetic systems (10-10 s). Furthermore, from the distance of minimum approach we could conclude that water molecules do not arrive in close vicinity of the magnetic core. Our findings contribute to elucidate the local spin dynamics mechanisms in colloidal superparamagnetic nanoparticles which are useful in biomedical application as, e.g., contrast agents for magnetic resonance imaging.

Optimized PAMAM coated magnetic nanoparticles for simultaneous hyperthermic treatment and contrast enhanced MRI diagnosis

“…3B, solid lines). A similar mismatch has previously been observed 26,27 and attributed to one or more physical mechanisms not included in the model itself. On this respect, it should be observed that the RMG model describes systems in the so called motional averaging regime (in short MAR, corresponding to fast motions) while our systems are out of the limit of the MAR regime.…”

R 1 dispersion contrast at high field with fast field-cycling MRI

“…When comparing samples containing cubic shaped IOMNPS (samples 1-3 in Figure 6a), theoretically, maximum signal intensity should be observed for sample 1 as Tevol ≈ T1. In practice, the dreMR contrast in Figure 8 is altered by an additional T2-weighting due to short T2 of the IOMNPs [19]. Consequently, the samples with the highest concentrations seemingly paradoxically, show less contrast enhancement, though, in reality, their T1 enhancement is maximal.…”

“…In order to validate the implementation of the FFC-MRI hardware setup, the availability of a contrast agent exhibiting strong R1 relaxation dispersion at 3 T is crucial. Recently published iron oxide magnetic nanoparticles [19] were selected for system validation because of a suspected R1 dispersion at 3 T. Such IOMNPs are obtained by a high temperature surfactantassisted chemical route which leads to size-and shape-controlled nanocrystals. IOMNPs with similar average size (nearly 8 nm) and with different shapes (spherical and cubical) were used dispersed in hexane, and will be referred to as S8_hex and C8_hex, respectively.…”

Magnetic resonance relaxation induced by superparamagnetic particles used as contrast agents in magnetic resonance imaging: a theoretical review

“…Unfortunately, those equations do not seem able to match the experimental results for transverse relaxation. Indeed, recent studies have shown that it was not possible to fit longitudinal and transverse NMRDs at the same time—indicating that a relaxation mechanism may miss in the models, at least for the transverse relaxation rate . Moreover, most of the experimental R 2 ‐NMRDs seem to exhibit a monotonously increase of R 2 with B 0 while the theoretical predictions predict a decrease of R 2 before it reaches a plateau.…”

“…While theoretical predictions are available, significant variations of the transverse relaxation are expected to occur at fields so low that it is technically very difficult to measure the signal decay. In the dominant Néel relaxation model, this is given by …”