Ligand biodegradation-induced surface reconstruction of magnetite nanoparticles: potentially overlooked toxicity

The coating of nanoparticles (NPs) with biodegradable ligands has been considered an efficient way to improve the biocompatibility of NPs and to decrease their biopersistence. However, the role of ligand biodegradation in reshaping the core material and the resulting change in toxicity have not been fully explored. In this study, magnetite (Fe3O4) NPs coated with a high or low loading of hyaluronic acid (termed Fe3O4-HA(H) and Fe3O4-HA(L)) were synthesized. Fe3O4-HA(R) was obtained after the enzymatic degradation of Fe3O4-HA(H) by a hyaluronidase, which had a coating weight similar to that of Fe3O4-HA(L), as verified by thermogravimetric analysis. Interestingly, among the three NPs, Fe3O4-HA(R) induced the highest rate of cellular damage. In particular, 2 times more hydroxyl radicals were detected in cells stressed by Fe3O4-HA(R) than in cells stressed by Fe3O4-HA(H), despite the highest cellular uptake being observed for Fe3O4-HA(H) because of the ligand-receptor interactions between hyaluronic acid and membrane receptors. The structural characterization results revealed that ligand degradation induced notable surface reconstruction of the Fe3O4 core, showing a characteristic Fe3O4@Fe2O3 structure with surface-bound ferrous ions. This unique structure endowed Fe3O4-HA(R) with superior competency in initiating the Fenton reaction and accelerating the Fe(iii)/Fe(ii) cycle. Our results emphasize the importance of tracing the in vivo biotransformation of NPs from all of the different architectural parts, particularly deciphering the interplay among the core, shell, surface ligands, and surrounding ions.