Methods of preparing a composition comprising non-ionic, radioactive gold nanoparticles (R-GNPs) are disclosed. The method comprises: a) providing an aqueous composition comprising gold (Au-197) ions in the absence of a template; and b) exposing the aqueous composition in the absence of the template to neutron irradiation to generate the composition comprising the non-ionic R-GNPs. Alternatively, the method comprises: a) providing an aqueous composition comprising gold (Au-197) nanoparticles (GNPs) in the absence of a template; and b) exposing the aqueous composition comprising the GNPs in the absence of the template to neutron irradiation and thereby generating the composition comprising the non-ionic R-GNPs. Compositions that comprises mesoporous silica nanoparticles (MSNs) and non-ionic R-GNPs encapsulated within pores and/or channels and further anchored to the surfaces of the MSNs, and methods of making the same are also disclosed.