Characterization of fluorescent iron nanoparticles—candidates for multimodal tracking of neuronal transport

Magnetic nanoparticles were coated with either dextran or polyacrylic acid (PAA), and compared as potential traceable carriers for targeted intraneuronal therapeutics. Nanoparticles were fabricated using a chemical reduction method and their number mean diameter, aggregation, surface chemistry, crystal structure and magnetic properties were characterized. The crystalline core of the dextran-coated nanoparticles was Fe3O4, while the PAA-coated sample had an iron core. The dextran-coated iron oxide nanoparticles (DIONs) and PAA-coated iron nanoparticles (PAINs) were both stable and had a similar mean diameter of less than 10 nm. However, morphologically, the PAINs were well dispersed, while the DIONs aggregated. DIONs exhibited the presence of hysteresis and ferromagnetic properties due to aggregation, while PAINs displayed superparamagnetic behavior. Surface chemistry analysis after 2 weeks of being exposed to air indicated that DIONs oxidized to Fe2O3, while PAINs were composed of a metallic Fe core and a mixed oxidation state shell. Based on these analyses, we concluded that PAINs are stronger candidates for examining axonal transport, since they were less prone to aggregation, offered a stronger magnetic signal, and were less oxidized. Neurotoxicity analysis of PAINs revealed that no significant toxicity was observed compared to negative controls for concentrations up to 1 mg/ml, thus further indicating their potential utility for biological applications. Finally, we successfully conjugated PAINs to a fluorophore, rhodamine 110 chloride, through a simple two-step reaction, demonstrating the feasibility of functionalizing PAINs. This study suggests that PAINs should be further evaluated as a candidate technology for intraneuronal diagnostics and therapy