Effective X-ray-excited metal–organic scintillators were used to create complexes with nanodiamonds in order to obtain optically active nanoplatforms capable of delivering photosensitizer molecules to living tissues in photodynamic therapy procedures. Hydrophilic detonation nanodiamonds with specially modified surface that had a positive potential in aqueous media due to saturation with grafted hydrogen atoms were associated with hydrophobic phosphors based on linear alkylbenzenes with organic modifiers carrying gadolinium atoms controlled by X-ray fluorescence spectrometry. This made it possible to convert X-rays into photons in the wavelength range of 350–550 nm, including the Soret absorption band of the Radachlorin® photosensitizer. Binary and ternary formations, diamond-scintillator and diamond-scintillator-Radachlorin®, were additionally stabilized with polyvinylpyrrolidone. As a result, functional nanostructures were obtained that are stable in aqueous media in the temperature range of 20–50 °C according to small-angle neutron scattering data and optical absorption measurements. As shown by neutron experiments, ensembles of diamond particles in combination with the indicated modifiers form chain-like fractal structures on scales of tens of nanometers. These structures retain the photoluminescent properties of the scintillator and photosensitizer that is confirmed by measurements of the luminescence in prepared colloids upon UV excitation. The colloids exhibited intense secondary radiation in visible and near-IR ranges. The developed functional materials are being tested on biological cells and animals for subsequent applications in X-ray photodynamic therapy as combined converter-photosensitizers.