Scaling laws at the nanosize: the effect of particle size and shape on the magnetism and relaxivity of iron oxide nanoparticle contrast agents

Abstract

The magnetic properties of iron oxide nanoparticles govern their relaxivities and efficacy as contrast agents for MRI. These properties are in turn determined by their composition, size and morphology. Herein we present a systematic study of the effect of particle size and shape of magnetite nanocrystals synthesized by thermal decompositions of iron salts on both their magnetism and their longitudinal and transverse relaxivities, r1 and r2, respectively. Faceted nanoparticles demonstrate superior magnetism and relaxivities than spherical nanoparticles of similar size. For faceted nanoparticles, but not for spherical ones, r1 and r2 further increase with increasing particle size up to a size of 18 nm. This observation is in accordance with increasing saturation magnetization for nanoparticles increasing in size up to 12 nm, above which a plateau is observed. The NMRD (Nuclear Magnetic Resonance Dispersion) profiles of MIONs (Magnetic Iron Oxide Nanoparticles) display an increase in longitudinal relaxivity with decreasing magnetic field strength with a plateau below 1 MHz. The transverse relaxivity shows no dependence on the magnetic field strength between 20 MHz and 500 MHz. These observations translate to phantom MR images: in T1-weighted SWIFT (SWeep imaging with Fourier Transform) images MIONs have a positive contrast with little dependence on particle size, whereas in T2-weighted gradient-echo images MIONs create a negative contrast which increases in magnitude with increasing particle size. Altogether, these results will enable the development of particulate MRI contrast agents with enhanced efficacy for biomedical and clinical applications.

Colloidal magnetic nanocrystal clusters: variable length-scale interaction mechanisms, synergetic functionalities and technological advantages

“…MAR, motional averaging regime; SDR, static dephasing regime; ELR, echo-limiting regime. It is worth noting though that deviations to the proposed r 2 universal scaling law [135] have been reported for nanosystems presented with faceted [69] or nanoplate [136] morphology particles, thus, corroborating to the consequences of non-spherical particle shape, a parameter that has not been considered in the above model. In summary, upon the assembly of nanocrystals into clusters, features such as the size, magnetization, and inorganic phase volume fraction would determine the relaxivities ( Figure 20); however, other factors may also play a critical role.…”

“…In summary, upon the assembly of nanocrystals into clusters, features such as the size, magnetization, and inorganic phase volume fraction would determine the relaxivities ( Figure 20); however, other factors may also play a critical role. For example, the size distribution [25], shape/morphology [69,136], and surface characteristics (the thickness of the surfactant layer, r 2~1 /L; its hydrophilicity) [131,137] are a few parameters for which the theoretical predictions are poorly accounting for. Further on, microscopic phenomena related to the nanocluster or the constituent nanocrystal (e.g.…”

“…According to these finding, careful engineering of the shape anisotropy of the nanocrystals is, in turn, shown to be beneficial to magnetic resonance imaging (MRI) relaxivities [65] and magnetic heat generation [66,67] efficacies. Overall, faceted particles are found to exhibit higher relaxivity for dark contrast generation and increased heat dissipation capability compared to the isotropic spherical ones due to their enhanced and cooperative magnetic properties [60,[68][69][70].…”

Hydrophobic Forces Are Relevant to Bacteria-Nanoparticle Interactions: Pseudomonas putida Capture Efficiency by Using Arginine, Cysteine or Oxalate Wrapped Magnetic Nanoparticles

“…An interesting feature observed in Figure 4 is that when NPs wrapped with a small molecule (as oxalate) are replaced by bigger ones (as the amino acids), magnetization increases, although a magnetization decrease was expected (same magnetic core, more non-magnetic organic wrap). Previous work shows that magnetic properties of NPs are strongly determined by NP size, where saturation magnetization increases with increasing crystal size until a size of 12 nm [37]. Magnetization measurements provide a weighted average of all the NPs dispersed in the solution.…”

Graphene oxide-Fe3O4 nanoparticle composite with high transverse proton relaxivity value for magnetic resonance imaging

“…13 One important factor that can significantly affect the proton relaxivity value and has not received much attention is the spatial arrangement of nanoparticles in the infected tissue environment. Controlling the spatial distribution of nanoparticles, both with respect to their placement as isolated particles or particle agglomerates of different sizes in a three dimensional framework and then placement of this framework in the infected tissue environment, is relevant, as an optimal arrangement can significantly increase the proton relaxivity value in two possible ways: (a) by generating very high magnetic field gradients from the collective magnetic behaviour of individual nanoparticles in the framework 14,15 and (b) by optimally trapping the water protons within the three dimensional framework, such that a significant increase in the interaction between the water protons and the superparamagnetic nanoparticles along with a reasonable proton exchange rate is achieved. 16,17 One possible way by which the "spatial distribution" factor can be employed is by using a bio-compatible, hydrophilic and flexible substrate, such as graphene oxide 18 as the framework material on which superparamagnetic nanoparticles can be organized in a controlled manner.…”