Exploring the design space of SMART RHESINs as a modular nanocapsule platform for functional magnetic nanoparticles.

Magnetic core/shell nanoparticles (CSNPs) are central to emerging imaging and theranostic concepts, yet translating different magnetic payloads into a common, chemically addressable and functionally expandable architecture remains challenging. SMART RHESIN nanocapsules encapsulate different magnetic nanoparticle (NP) systems within a protective phenolic resin shell and permit structural variation and post-synthetic functionalisation. Here we map the design space of SMART RHESIN phenolic resin hollow nanocapsules across 52 reactions varying monomer choice, CSNP type, loading, co-encapsulation, and polymer additives. The platform incorporates magnetic payloads ranging from lab-synthesised magnetite to commercial iron oxides and cubic NPs, while identifying biogenic magnetosomes as a boundary case for the present synthesis conditions, with the resulting nanocapsule diameter (about 100 to 400 nm) and shell thickness (about 10 to 80 nm) varying systematically with the choice of CSNP system and reaction conditions. Magnetic particle spectroscopy (MPS) indicates reduced viscosity-induced phase shifts for selected representative formulations from this expanded compositional set, phantom MPI demonstrates multi-colour imaging that resolves aqueous and immobilised SR29 signals within a single field of view, and surface carboxylates enable post-synthetic modification, illustrated by 18F-radiolabelling for PET/CT. These findings establish SMART RHESINs as a versatile nanocapsule platform for integrating magnetic NPs with additional optical, radiochemical and future targeting functionalities.