Potential theranostic and multimodal iron oxide nanoparticles decorated with rhenium-bipyridine and -phenanthroline...

Two structurally similar nanoparticles were designed for multimodal imaging and possible radiotherapy. The assembly consists of ultrasmall superparamagnetic iron oxide nanoparticles that act as contrast agents for MRI with a luminescent rhenium complex, for optical imaging, attached to the surface. Rhenium has the advantage of being luminescent and carries two radio-isotopes ¹⁸⁶Re and ¹⁸⁸Re making it possible to act as a contrast agent for SPECT (γ) and to be used for radiotherapy (β). The iron oxide nanoparticles were treated with a silane and further functionalized with picolyl. This picolyl was used to capture rhenium(i)(CO)₃-1,10-phenanthroline (ReL₁) or rhenium(i)(CO)₃-2,2'-bipyridine (ReL₂) and forms the final product Fe₃O₄-picolyl-rhenium(i)(CO)₃-1,10-phenanthroline (IO-ReL₁) or Fe₃O₄-silica-picolyl-rhenium(i)(CO)₃-2,2'-bipyridine (IO-ReL₂), respectively. All products were characterized properly (TEM, XRD, NMR, IR and TXRF) and a full investigation of the relaxometric and optical properties was conducted. Although iron oxide nanoparticles suffer from strong Rayleigh scattering, an efficient sensitized luminescence was observed and the orange emission wavelength was found to be 585 nm for IO-ReL₁ and 592 nm for IO-ReL₂ after irradiation at 395 nm. The relaxometric study of these ultrasmall nanoparticles showed very promising results. The r₂ values measured at a magnetic field strength of 60 MHz of the nanoparticles being 92.9 mM^-1 s^-1 and 97.5 mM^-1 s^-1 for IO-ReL₁ and IO-ReL₂, respectively, were at least 1.5 times larger than Sinerem®.