Polydopamine-Coated Manganese-Doped Upconversion Nanoparticles as Potential UCL/MRI Dual-Mode Probes

Lanthanide codoped upconversion luminescence nanoparticles (Ln-doped UCNPs) represent a new generation of biological probes for photoluminescence bioimaging applications. However, the strong absorption of short-wave light below 600 nm by tissues necessitates the development of upconversion nanoparticles capable of emitting red light. UCNPs hold great promise as multimodal contrast agents, which could be a promising alternative in a range of medical imaging techniques. Here, we report the fabrication of hybrid nanoparticles, NaYF4:Yb3+/Er3+/Mn2+@PDA-PEG, exhibiting high luminescent emissions and magnetic relaxivities, as potential nanoprobes for dual-modality imaging combining magnetic resonance imaging (MRI) and upconversion luminescence (UCL) imaging. By doping with Mn2+, UCNPs were endowed with MRI performance and tuned with the red/green light ratio of UCL. The biocompatibility and hydrophilicity of the nanoparticles were improved by polymerizing polydopamine (PDA) on their outer surface and grafting onto a poly(ethylene glycol) (PEG) coating, which could further enhance the red-to-green emission ratio. The results show that the Mn2+-doped UCNP@PDA-PEG displays red luminescence and superior MRI contrast enhancement with the longitudinal relaxivity r1 of up to 14.08 mM–1·s–1, thus showing great potential as dual-mode UCL/MRI bioimaging nanoprobes.