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

Abstract

It has been recently reported that for some suspensions of magnetic nanoparticles the transverse proton relaxation rate, R(2), is dependent on the time that the sample is exposed to an applied magnetic field. This time dependence has been linked to the formation of linear aggregates or chains in an applied magnetic field via numerical modeling. It is widely known that chain formation occurs in more concentrated ferrofluids systems and that this has an affect on the ferrofluid properties. In this work we examine the relationships between colloidal stability, the formation of these linear structures, and changes observed in the proton transverse relaxation rate of aqueous suspensions of magnetic particles. A series of iron oxide nanoparticles with varying stabilizing ligand brush lengths were synthesized. These systems were characterized with dynamic light scattering, transmission electron microscopy, dark-field optical microscopy, and proton transverse relaxation rate measurements. The dark field optical microscopy and R(2) measurements were made in similar magnetic fields over the same time scale so as to correlate the reduction of the transverse relaxivity with the formation of linear aggregates. Our results indicate that varying the ligand length has a direct effect on the colloidal arrangement of the system in a magnetic field, producing differences in the rate and size of chain formation, and hence systematic changes in transverse relaxation rates over time. With increasing ligand brush length, attractive inter-particle interactions are reduced, which results in slower aggregate formation and shorter linear aggregate length. These results have implications for the stabilization, characterization and potentially the toxicity of magnetic nanoparticle systems used in biomedical applications.

Magnetic iron oxide nanoparticles: Recent trends in design and synthesis of magnetoresponsive nanosystems

“…This critical temperature for a myristic acid sterically stabilized single core magnetite water based ferrofluid resulted to be 42.5ºC [55], a temperature close to the range of interest for live cell experiments. Phase condensation was found to have a negative influence on the effectiveness of magnetic nanoparticles' use as contrast agents in MRI [56] evidenced by` the influence of the magnetically induced linear chains in PEG coated single core particle dispersions on the proton transverse relaxation rates. It was found that the transverse relaxation rate diminishes with increasing particle chaining, i.e.…”

“…In both single core and multicore systems the magnetically induced condensation process may take minutes before reaching equilibrium [49,56]. It is worth mentioning that spontaneous clustering can also occur in both single core and multicore magnetic particle dispersions when the attractive forces (van der Waals, magnetic dipole-dipole) overcome the repulsive ones (steric, electrostatic) [56].…”

“…Magnetically induced linear aggregates can occur in both single core [55,56] and multicore liquid dispersions [57,58]. The magnetically induced phase condensation in single core liquid dispersions can show critical behavior: below a critical temperature (Tc) the condensation occurs only at field intensities larger than a critical value…”

Aggregation of superparamagnetic iron oxide nanoparticles in dilute aqueous dispersions: Effect of coating by double-hydrophilic block polyelectrolyte

“…In addition to Van der Waals attractive forces among the particles, anisotropic magnetic dipole interactions can play a role in the aggregation process. It was observed by several authors [11][12][13][14], that in applied magnetic field SPION or nanoparticles with encapsulated SPION form one-dimensional chainlike aggregates or self-assembled structures. It has also been reported that in diluted aqueous dispersions of surfactant-coated SPIONs, chainlike aggregates with the fractal dimension D = ~1.2 were formed spontaneously [10,15].…”

Nanomaterials

“…For example, magnetic polymer coated iron oxide NM form linear aggregates in the presence of a magnetic field, the structure of which have been examined by TEM imaging of sections of the aggregates captured in gel and resin-embedded for imaging. 62 Furthermore, scanning electron microscopy (SEM) has been used to investigate the dispersion of TiO 2 NM agglomerates in the presence of bacteria. 63 Analysis by TEM however must avoid using sample preparation involving dropping and drying a solution on a grid (the drop-cast method), since it induces NM agglomeration, negating quantitative sizing.…”