Superparamagnetic iron oxide nanoparticles stabilized by a poly(amidoamine)-rhenium complex as potential theranostic probe

Three-component nanocomposites, constituted by a superparamagnetic iron oxide core coated with a
polymeric surfactant bearing tightly bound Re(CO)3 moieties, were prepared and fully characterized. The
water soluble and biocompatible surfactant was a linear poly(amidoamine) copolymer (PAA), containing
cysteamine pendants in the minority part (ISA23SH), able to coordinate Re(CO)3 fragments. For the
synthesis of the nanocomposites two methods were compared, involving either (i) peptization of bare
magnetite nanoparticles by interaction with the preformed ISA23SH-Re(CO)3 complex, or (ii) “one-pot”
synthesis of iron oxide nanoparticles in the presence of the ISA23SH copolymer, followed by complexation
of Re to the SPIO@ISA23SH nanocomposite. Full characterization by TEM, DLS, TGA, SQUID, and
relaxometry showed that the second method gave better results. The magnetic cores had a roundish
shape, with low dispersion (mean diameter ca. 6 nm) and a tendency to form larger aggregates (detected
both by TEM and DLS), arising from multiple interactions of the polymeric coils. Aggregation did not
affect the stability of the nano-suspension, found to be stable for many months without precipitate
formation. The SPIO@PAA-Re nanoparticles (NPs) showed superparamagnetic behaviour and nuclear
relaxivities similar or superior to commercial MRI contrast agents (CAs), which make them promising as
MRI “negative” CAs. The possibility to encapsulate 186/188Re isotopes (γ and β emitters) gives these novel
NPs the potential to behave as bimodal nanostructures devoted to theranostic applications.