Singlet Molecular Oxygen Generation via Unexpected Emission Color-Tunable CdSe/ZnS Nanocrystals for Applications in Photodynamic Therapy

It is highly desirable in biomedical sciences to utilize the multifunctional nanoparticles of similar size with tunable emission. Since the optoelectronic properties of quantum dots (QDs) originate from size-dependent quantum confinement effects, we developed an alternate approach to synthesize color-tunable CdSe/ZnS QDs based on interfacial ion exchange (predominantly exchange of Se2- by S2- anions), using 1-dodecanethiol and oleylamine solvent systems as a sensitive parameter. The wide-range color-tunability (490-570 nm) was achieved unexpectedly as a result of interfacial alloying without inducing a significant change in the size (from 4.45 to 4.81 nm) of QDs. The local atomic structure order, chemical composition, and nature of alloying in QDs were unraveled by XAFS data analysis. Owing to the molecular-like sensitization behavior, the QDs were evaluated for singlet molecular oxygen (1O2) efficiency. They were further studied in RAW 264.7 macrophages for biocompatibility, bioimaging, and delivering pathways for use in future photodynamic therapy (PDT). The QDs demonstrated efficient singlet molecular oxygen (1O2) quantum yields (phi QDs) of 14, 12, and 18% for QDs (I), QDs (II), and QDs (III), respectively. The QD-treated cells presented high cell viability above 85% and induced no cell activation. Fluorescence and transmission electron microscopy (TEM) images of cells manifested a considerable amount of QDs in the vicinity of the cell membrane and intracellular regions. The pathway-specific inhibition measurements revealed that the QDs were internalized by cells via energy-dependent endocytosis, predominantly macropinocytosis and other receptor-mediated endocytic pathways, and accumulated them presumably in endosome/lysosomes. This study will open new possibilities for engineering interfacial alloying based tunable emission QDs and pathway-specific delivery of QD-based theranostics into a site of interest for simultaneous bioimaging and PDT.