Supraparticles with Enhanced Mechanical and Signal Stability for Identification of Objects

Micron-scaled, magnetic supraparticles (SPs) composed of oleic acid-functionalized iron oxide nanoparticles (NPs) are promising candidates for object-embedded tagging and tracing using magnetic particle spectroscopy (MPS). A key challenge for such particulate identifiers lies in maintaining their defined magnetic signatures during processing and integration into host materials, since mechanical stress can cause structural alterations leading to both impaired particle integrity and diminished signal reliability. Herein, we elucidate how the thermal cross-linking of surface-anchored oleic acid ligands within SPs affects their structural and magnetic properties. We show significantly enhanced mechanical robustnessas confirmed by nanoindentation experiments of individual SPsand resistance to solvent-driven disintegration of iron oxide-based SPs. By correlating X-ray scattering (SAXS) and magnetic characterization (SQUID magnetometry, MPS, and Mössbauer spectroscopy), we reveal the impact of three-dimensional subnanometer distance changes of NPs on the magnetic properties of SPs and suggest a chemical spacing-based strategy to reduce the possibly undesired magnetic NP-NP interaction increase. For using SPs as magnetic code-carrying particles, this ensures magnetic signal stability during production and processing within matrices. From a broader perspective, this work reveals fundamental structural and magnetic insights relevant for producing micrometer-scaled particles with improved mechanical stability and processability of almost any material when using the widely used oleic acid as a ligand.