The present application provides an organosilica nanoparticle comprising : (a) a photosensitizer for photodynamic therapy covalently incorporated therein; and (b) optionally, at least one agent encapsulated therein, as well as a pharmaceutical composition comprising said organosilica nanoparticle. Also provided herein are said organosilica nanoparticle or pharmaceutical composition for use as a medicament or in the treatment of a disease, disorder, or condition. More specifically, provided is a method for treating a disease, disorder, or condition in a subject using said aid organosilica nanoparticle or pharmaceutical composition.