Ultrasmall Quantum Dots: A Tool for in Vitro and in Vivo Fluorescence Imaging

Fluorescence bioimaging is an increasingly popular approach in biomedical research and diagnosis, where semiconductor nanocrystals or quantum dots (QDs) have proved to be excellent fluorescent labels. The use of ultrasmall QDs in nanoprobes extends the possibilities of bioimaging owing to an enhanced capacity for penetrating through cell membranes. However, the QDs synthesis is accompanied by the rapid growth of nanocrystals in colloidal medium what prevents obtaining sufficiently small QDs prepared by conventional approaches. Here, a one-pot injection technique of QD synthesis in an organic medium, with the reaction terminated at an early crystal growth stage and excess precursors eliminated by gel permeation chromatography, is proposed. This technique yields defect-free cadmium selenide QD cores about 1.5 nm in size emitting at the wavelengths less than 500 nm. Coating of these QDs with epitaxial shells of different compositions ensures a photoluminescence quantum yield approaching 100%. The resultant ultrasmall QDs are promising components of nanoprobes to be used for imaging intracellular and intranuclear events down to the molecular level.