A phosphor excitable by X-ray and blue-light emits light in the near-infrared (NIR-II, 1000-1700 nanometers) forms nanoparticles less than 200 nanometers diameter. The nanoparticles are tagged by coating with silica, then conjugating with polyethylene glycol (PEG) and tissue-selective compounds such as antibodies, nucleic acid chains, and other ligands. In embodiments, we administer the tagged nanoparticles to a subject, then localize the nanoparticles, and thus antigen-bearing tissues, by irradiating the subject with X-ray or other radiation beams while imaging near infrared light emitted from the subject. The nanoparticles are made by mixing 1-50 micron calcium oxide and germanium oxide powders with dilute nitric acid, adding chromium (III) nitrate at a ratio to germanium between 0.001 and 0.1, adding tartaric acid solution with molar ratio to metal ions between 1-10, and adjusting pH to 0.1-4 with nitric acid, then later heating to form a sol, oven drying, and calcinating the sol.
