The present invention provides organically modified silica (ORMOSIL) nanoparticles into which have been incorporated two-photon absorption dye molecules. The two photon absorption dye displays a unique aggregation induced fluorescence enhancement behavior. As a result ORMOSIL nanoparticles with high amounts of the dye can be prepared. These particles can be used for imaging. In one embodiment, the nanoparticles can additionally have incorporated therein a photosensitizer. The photosensitizer can be activated by intraparticle fluorescence resonance energy transfer (FRET) from the dye aggregates resulting in enhanced fluorescence and singlet oxygen generation from photosensitizer under two-photon excitation conditions. Such nanoparticles can be used for photodynamic therapy applications.