Colloidal assemblies of oriented maghemite nanocrystals and their NMR relaxometric properties

Abstract

1H-NMR relaxometric experiments over an extended frequency range show that ferrimagnetic colloidal nanoclusters exhibit enhanced transverse relaxivity, r2.

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

“…The validity of the proposed models have been verified experimentally by superparamagnetic magnetite nanoclusters (of 13 nm-180 nm diameter) capped with a polyacrylate polymer ( Figure 19B) [25] and by ferrimagnetic maghemite nanoclusters (of 50 and 86 nm diameter) with a similar capping agent ( Figure 19C) [127]. In the former, the r 2 for the nanoclusters in the SDR regime is not strictly size independent, as expected from the theoretical model according to the outer-sphere relaxation theory, while the highest r 2 value observed in the SDR regime is smaller than the theoretical one ( Figure 19B).…”

“…; R the radius of the nanoclusters), for different echo times ranging from 0.1 to 20 ms, as predicted by the outer-sphere relaxation theory [128]. Experimental data for (B) superparamagnetic polyacrylate capped magnetite nanoclusters [25] and (C) for ferrimagnetic maghemite nanoclusters of 50 and 86 nm diameter (red and black symbols, respectively) [127]. MAR, motional averaging regime; SDR, static dephasing regime; ELR, echo-limiting regime.…”

“…The water protons straddling around nanocluster entities are expected to relax even faster than when superparamagnetic individual nanocrystals are present in the surrounding media. Recent studies have shown that the transverse relaxivity, r 2 , necessary for improved "dark contrast" in such systems, is considerably enhanced (approximately three to four times) compared to individual nanocrystals and commercial products, such as the Endorem ® ( Figure 17) [25,31,100,127]. In the case of cluster-like systems of volume V, under an external magnetic field B 0 , r 2 can be estimated by the equation:…”

“…Increased number of particles can be incorporated by tuning the volume fraction of the magnetic material (ϕ) in the nanoclusters. Recently, the synthesis of maghemite nanoclusters with tunable ϕ has been successfully demonstrated [21,127]. Nanoclusters of variable volume fraction can evolve to display superparamagnetism or ferrimagnetism.…”

“…Although the majority of them have been reported to be superparamagnetic at ambient conditions, two ferrimagnetic systems, with high transverse relaxivity and a very good stability in aqueous media, have lately emerged as promising probes for MR imaging. These systems consisted of nanoclusters of ϕ = 60 and 72% capped with a polyacrylate [127] (Figure 19C) or encapsulated in liposomes [132]. Despite their ferrimagnetic response at ambient conditions, the rational choice of their surfactant molecules prevented their agglomeration and raised their potential for applicability.…”