Supraparticles with enhanced mechanical and signal stability for identification of objects

Micron-scaled, magnetic supraparticles (SPs) composed of oleic acid-function-alized iron oxide nanoparticles (NPs) are promising candidates for object-embedded taggingand tracing using magnetic particle spectroscopy (MPS). A key challenge for such particulateidentifiers lies in maintaining their defined magnetic signatures during processing andintegration into host materials, since mechanical stress can cause structural alterations leadingto both impaired particle integrity and diminished signal reliability. Herein, we elucidate howthe thermal cross-linking of surface-anchored oleic acid ligands within SPs affects theirstructural and magnetic properties. We show significantly enhanced mechanical robustness�asconfirmed by nanoindentation experiments of individual SPs�and resistance to solvent-drivendisintegration of iron oxide-based SPs. By correlating X-ray scattering (SAXS) and magneticcharacterization (SQUID magnetometry, MPS, and Mössbauer spectroscopy), we reveal theimpact of three-dimensional subnanometer distance changes of NPs on the magneticproperties of SPs and suggest a chemical spacing-based strategy to reduce the possiblyundesired magnetic NP−NP interaction increase. For using SPs as magnetic code-carrying particles, this ensures magnetic signalstability during production and processing within matrices. From a broader perspective, this work reveals fundamental structural andmagnetic insights relevant for producing micrometer-scaled particles with improved mechanical stability and processability of almostany material when using the widely used oleic acid as a ligand.