Embodiments of a composition comprising a photosensitive compound and a polymer nanoparticle are disclosed herein. The composition may further comprise a targeting moiety. In some embodiments, the photosensitive compound is a phthalocyanine or phthalocyanine derivative, such as a naphthalocyanine. Upon irradiation with near infrared light, the composition may be used as a fluorescent imaging agent and/or as a phototherapeutic agent, such as for photodynamic and/or photothermal therapies. In certain embodiments, the composition is used to treat certain cancers.