Abstract: Provided are sulfur- or heavy atom-containing silica or aluminosilica nanoparticles. A nanoparticle can have at least one dye molecule covalently bonded to the silica or aluminosilica matrix of the silica or aluminosilica nanoparticle, respectively. The nanoparticle may have a size, such as, for example, a longest dimension, of less than 10 nm. The silica or aluminosilica nanoparticle can be used in methods such as, for example, imaging methods and photodynamic therapy methods. The imaging methods can provide sub-diffraction limit resolution.