Hydroxyapatite nanocrystals dually doped with fluorescent and paramagnetic labels for bimodal (luminomagnetic) cell imaging

Hydroxyapatite nanocrystals (HAp NCs) were prepared that contain both a fluorescent label (fluorescein) and the paramagnetic label Ho3+ ion. Synthesis was performed by a surfactant-free aqueous method using commonly available precursors and dopants. The resulting HAp NCs display a spindle like hexagonal phase morphology. Their average diameter and length are 10 and 60 nm, respectively. The photoluminescence of the HAp NCs displays substantial variation with concentration of the fluorophore. The co-dopant Ho3+ endows the nanocrystals with paramagnetism and has little effect on the luminescence of the fluorescent label. The surface of the nanocrystals was coated with polyethyleneimine and the resulting particles were covalently conjugated to folic acid in order to enable targeting of the folate receptor that is over expressed in cancer cells. The HAp NCs are biocompatible as proven by an MTT assay that revealed no apparent toxicity in doses as high as 800 μg∙mL−1 after a 48 h incubation period. The HAp NCs were used as a fluorescent bioimaging probe for in vitro imaging of HeLa cells. The characterization of the magnetic properties indicates the paramagnetic nature of the nanocrystals with a magnetization of 7.3118 emu∙g−1, and this property makes the system potentially suited for application as a Ho3+ based contrast agent for T2 magnetic resonance imaging.