Preparation of 3H-ATP-incorporated silica nanoparticles and its diffusive release

The fixation of 3H, which is a radioisotope (RI), in silica nanoparticles (NPs) was carried out. 3H-adenosine triphosphate (3H-ATP), which is a 3H-labeled compound, was incorporated in silica NPs by Stöber's method. The optimum incorporation conditions were determined by following the method for the encapsulation of non-RI ATP molecules in silica NPs. The absorption spectrum of non-RI ATP–silica NPs indicated that approximately 20% of the ATP could be fixed in the silica NPs, and its average concentration was approximately 70 μM. The fixation of 3H in the silica NPs was confirmed by carrying out several ultrafiltrations to remove nonfixed 3H-ATP in the solution. After several ultrafiltrations, the radioactivity of the 3H-ATP–silica NP solution was higher than that of the filtrate. This indicated that 3H-ATP was incorporated in the silica NPs. 3H-ATP was diffusively released from the silica NPs in the solution, the amount of which was determined by measuring the radioactivity of the solution. It was found that the silica NPs are porous and that 3H-ATP localizes near the surface layer. More silica was added to form a dense and thick shell, and as a result, it was found that one-third of the radioactivity remained even after 7 weeks.