The present invention relates to self-assembly of a photosensitizer on a nanoparticle. The invention also provides methods for production of functionalized (eg. stabilized) nanoparticles. The nanoparticles may be used in Photodynamic Therapy (PDT). The invention can provide, for example, self-assembled phthalocyanine monolayers (SAMs), wherein the monolayer is formed on a metallic nanoparticle. The term "metallic" as used herein refers to metals, metal oxides and other metal-containing compositions.According to the invention a functionalized nanoparticle comprises: a metallic core; a photosensitizer monolayer chemically bonded to said core, said monolayer containing molecules capable of photo-excitation to produce a reactive oxygen species such as singlet oxygen, from oxygen molecules; and a phase transfer reagent.