Waiting time dependence of T2 of protons in water suspensions of iron-oxide nanoparticles: Measurements and simulations

Abstract

The transverse relaxation time T2 of protons in water suspensions of iron-oxide particles increases with the waiting time tw after the sample is inserted in the gap of the spectrometer magnet. Such a T2 increase becomes significant if the particles are aggregated into large clusters, for which field-induced formation of cluster-chains will occur and T2 should increase with increasing the length of chains. T2 increases with tw even for small particles, for which no chain formation may be induced, and for large clusters when tw is too small to form long enough chains. The T2 increase is accompanied by a significant echo-time dependence. All this is experimentally and theoretically studied.

Predicting the Self‐Assembly of Superparamagnetic Colloids under Magnetic Fields

“…In applications in which field‐induced self‐assembly is undesired one has to work in conditions of Γ < 4, otherwise the concentration of the suspension should be carefully controlled to be low enough in order to guarantee that N* is still below 1. On the contrary, assembly of particles ( N* > 1) will be unavoidable for particles with Γ > 15, even in extremely diluted conditions, since the smaller volume fractions reported in experiments are of the order of φ 0 ≈ 10 −6 .…”

“…In other cases, field‐induced assembly is undesired because it introduces an aging or time dependence in properties which need to be stationary for the applications. For example, in the case of superparamagnetic particles employed as contrast agents in magnetic resonance imaging (MRI), the transversal ( T 2 ) relaxation time (which is one of the properties of interest) changes with time if the magnetic interaction between particles induced by the external field is strong enough to give rise to the formation of chains of particles . The formation of linear aggregates also has important consequences in hyperthermia applications of superparamagnetic particles …”

The effect of magnetically induced linear aggregates on proton transverse relaxation rates of aqueous suspensions of polymer coated magnetic nanoparticles

“…7 shows the fractional decrease in R 2 as a function of time. The reduction in R 2 with time for each polymer can be tted with a bi-exponential equation that ts the decay in R 2 over the entire measurement time in contrast to the power law equation of Chen et al, 25,27 which generally ts the data only for short time periods. The magnitude of the fractional reduction in R 2 was observed to decrease as the molecular weight of the stabilizing polymer increased (Fig.…”

“…The suggestion that linear aggregation could play a role in driving a nanoparticle system into the echo-limited regime has been recently proposed 24 to explain experimental observations of polymer encapsulated magnetite clusters showing a consistent reduction in R 2 with time in a magnetic eld. [24][25][26][27] While this reduction in R 2 , and hence MRI contrast, has been ascribed to the formation of linear aggregates, direct evidence for linear aggregation was not provided. Furthermore, modelling of nanoparticle systems and chains 27 predicted R 2 would initially increase, or be stable, before decreasing, which was not consistent with the immediate reduction in R 2 observed experimentally.…”

“…[24][25][26][27] While this reduction in R 2 , and hence MRI contrast, has been ascribed to the formation of linear aggregates, direct evidence for linear aggregation was not provided. Furthermore, modelling of nanoparticle systems and chains 27 predicted R 2 would initially increase, or be stable, before decreasing, which was not consistent with the immediate reduction in R 2 observed experimentally. However, theoretical work by Andreu et al 11 also modelled the interaction of colloids as they formed linear aggregates in a magnetic eld and predicted the time dependence in R 2 observed experimentally.…”