Synthesis and characterization of boron-containing biocompatible polymer-coated Gd2O3 nanoparticles for neutron capture therapy-based theranostics

Towards theranostic agents for the selectable or combined gadolinium and boron neutron capture therapy, 100 nm-scale gadolinium oxide (Gd 2 O 3 ) nanoparticles (NPs) coated with the copolymers of 2-methacryloyloxyethyl phosphorylcholine (MPC) and 5-methacrylamido-1,2-benzoxaborole (MAAmBO) were synthesized. The size control of NPs was carried out by a simple mechanical process of grinding . Furthermore, the direct binding of the copolymer to the Gd 2 O 3 surface was achieved through the phosphate groups of MPC, even though they constitute the zwitterion with the trimethylammonium groups. The resulting polymer coating enhanced the paramagnetic longitudinal relaxivity of Gd 2 O 3 NPs towards the protons of water molecules, contrary to the possible decline of the accessibility and direct coordination of water molecules to the surface Gd(III) sites. The rotational motion of the size-controlled NPs (diameter: 1 ×10 2 nm) is characterized by the correlation time of submillisecond order, which is too slow to modulate the electron-nuclear magnetic dipolar interactions and enhance the relaxivity. The overall enhancement of the relaxivity would result from the modulation of water exchange rate and electronic relaxation rate, probably due to the steric and hydrogen-bond interactions of water molecules with the copolymer. • Proposal of a theranostic agent for boron and gadolinium neutron capture therapy. • Facile size control of gadolinium oxide nanoparticles by a mechanical process. • Direct binding of polymers via phosphorylcholine groups to gadolinium oxides. • Enhanced paramagnetic longitudinal relaxation of water protons by polymer coatings.