Co_xFe_3–xO₄ Nanocubes for Theranostic Applications: Effect of Cobalt Content and Particle Size

We report a facile synthesis of cube-shaped Co x Fe 3– x O 4 nanocrystals (NCs), which could be finely tuned in terms of NC size (from 15 to 27 nm) and cobalt stoichiometry (from 0.1 to 0.7). These particles exhibited high specific absorption rate (SAR) values, relevant for magnetic hyperthermia, and high relaxivity values, significant for magnetic resonance imaging applications. The peculiarity of these NCs is that already at low frequencies (such as 105 kHz, a working frequency used on human patients), they display SAR values that are three-times as large as those of iron oxide nanocubes of comparable sizes (and which were already considered outstanding). The highest SAR value recorded on the NCs reported here (915 ± 10 W/g (Co+Fe) at 105 kHz and 32 kAm –1 ) refers to particles with cubic shape, 20 ± 2 nm edge size, and Co stoichiometry between 0.6 and 0.7. The highest r 2 value (958 mM –1 s –1 ) was instead recorded on nanocubes with Co stoichiometry around 0.5/0.6 and size of 20 ± 2 nm. Remarkably, only at this specific size and Co stoichiometry were the NCs not perfect cubes but had a slightly concave shape, which together with their core–shell structure and magnetic parameters might account for the higher r 2 values recorded. NCs reported here, with optimized SAR and r 2 values, are promising tools for theranostic applications.