Theranostics Enabled by the Giant Susceptibility of Magnetic Nanoclusters

One of the most important properties of magnetic materials is their susceptibility to external magnetic fields. A large susceptibility translates into a material that can be readily magnetized even under exceptionally low field strengths. In the case of magnetic nanoparticles this parameter dictates how easy it is to image, move, or heat particles from a distance and thus directly influences their performance in diverse technologies. Conventional approaches to forming more field-sensitive nanoparticles alter their composition, resulting in only modest susceptibility gains but increased toxicity. Here we show an alternative approach that leverages the striking size-dependent magnetic susceptibility in clustered particles containing safe and earth-abundant iron oxide nanocrystals. These materials can possess an initial magnetic susceptibility an order of magnitude larger than the bulk iron oxides, and we term these systems “supersusceptible”. We hypothesize that moderate exchange coupling between primary particles allows the clusters to remain superparamagnetic up to a larger volume than isolated nanocrystals, yielding larger susceptibilities. Their extraordinary sensitivity to applied fields makes it possible to apply portable, battery-operated devices to the inductive heating of biological materials as well as inexpensive refrigerator magnets to the capture of live cells containing the particles.