Molecular reactions in aqueous solutions are often used as dosimetric probes. A major problem with this approach is that other species such as nanoparticles or radical scavenging chemicals can often interfere with these reactions. The results measured in the presence of nanomaterials and scavengers therefore cannot correctly indicate the true dose based on the calibrated results obtained in solutions free of the interfering species. Storing these molecular probes in nanoreactors can overcome this problem. Here we demonstrate for the first time that it is possible to place common probe molecules inside spherical mesoporous silica shells and seal the pores after impregnation for the purpose of using the soformed nanoreactors as X-ray dose probes. The reactions are isolated from the external environment, while the sealed shells still allow X-rays to freely penetrate through the walls of the nanoreactors. These nanoreactor probes can therefore fiducially report the dose of X-rays, whether the nanoreactors are in solutions, in dry form, or in the presence of scavengers and catalysts in solution.