Water soluble quantum dots as hydrophilic carriers and two-photon excitedenergy donors in photodynamic therapy

In search of strategies to develop deeply penetrating agents for use in Photodynamic Therapy (PDT),we have devised a Quantum Dot-Rose Bengal conjugate that is effective at producing singlet oxygenupon two-photon irradiation. The CdSe/ZnS Quantum Dot, with its high two photon absorption crosssection, serves as a two-photon absorbing antenna and transfers its excited state energy to the attachedphotosensitiser which engages with molecular oxygen to produce cytotoxic singlet oxygen. Thus, wewere able to excite the photosensitiser indirectly, which has an absorption maximum of 565 nm, withtwo-photon irradiation at 800 nm. Given the tissue penetration depth of 800 nm light is at least fourtimes greater than 565 nm light, this offers the opportunity to access much deeper-seated tumours thanis currently possible with pharmaceutically approved photosensitisers. Furthermore, the attachment ofthe photosensitiser to the hydrophilic quantum dot improved the aqueous solubility of thephotosensitiser by 48 fold, thus overcoming another limitation of currently used photosensitisers, thatof poor aqueous solubility.