Insights into the Mechanism of Quantum Dot-Sensitized Singlet Oxygen Production for Photodynamic Therapy

Semiconductor nanoparticles or quantum dots (QDs) have been proposed as potential vehicles for photodynamic therapy (PDT) since 2003. Some studies using cadmium-based QDs have shown promising results when coupled to molecular photosensitizers. However, the toxicity of such QDs and the low overall efficiency of these hybrids are still problematic. We have coupled two types (sizes) of less-toxic InP/ZnS QDs to the photosensitizer chlorin e6. The spectroscopic properties of these hybrids have been studied in detail. Spectroscopic methods have been applied to elucidate the energy transfer pathways and kinetics and the rate of singlet oxygen production of all components. Additionally, the PDT efficacy of the QD/chlorin e6 hybrids has been assessed against a breast cancer (MDA-MB-231) cell line using a colorimetric 3-(4,5 dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay. We have found that the energy transfer between QDs and the molecular photosensitizer is the rate-determining step for the production of singlet oxygen and that the cell viabilities of the hybrid and free photosensitizer are comparable. These systematic findings show that the energy transfer between QDs and photosensitizers is a "bottleneck", which suggests that a better chemical design of the QD/photosensitizer hybrids in future embodiments is essential.