Enhancing the physicochemical and photophysical properties of small (<2.0 nm) CdSe nanoclusters for intracellular imaging applications

Abstract

The chemical properties of surface passivating ligands that surround nanoclusters (NCs) are extremely important for controlling their structural, physicochemical, and photophysical properties. However, to investigate these properties in various environments, it is important to be able to dissolve the ligand-coated NCs in a wide range of polar and nonpolar solvents. We have developed a direct synthesis and purification procedure for poly(ethylene glycol) thiolate-protected ultra-small (<2.0 nm diameter) CdSe NCs, which have such solubility characteristics. These CdSe NCs were characterized by UV-vis absorption, photoluminescence, and X-ray photoelectron spectroscopy (XPS), as well as mass spectrometry. Analyses by XPS confirmed the formation of surface Cd-rich NCs and mass spectrometry confirmed a stoichiometric core with mass of 6.5 kDa. The NCs were synthesized in aqueous medium and purified using a simple solvent extraction method, and the organic phase-extracted CdSe NCs were readily soluble in a wide range of polar and nonpolar organic solvents including acetonitrile, ethanol, chlorobenzene, dichloromethane, and chloroform, as well as in water. The diverse solubility properties of poly(ethylene glycol) thiolate-protected ultra-small CdSe NCs allowed us to perform a post synthetic surface ligand treatment with triphenylphosphine in an organic solvent. The post synthetic treatment enhanced the photophysical properties of these ultrasmall NCs and resulted in an ∼8 fold increase in fluorescence quantum yield. These bright yellow, mixed ligand-coated CdSe NCs were then used for intracellular imaging studies for the first time due to their unique physicochemical properties. We also found that the penetration ability of mixed ligand-coated CdSe NCs through fibroblast cell membranes and the fluorescence properties of the NCs inside the cell were both strongly poly(ethylene glycol) chain length dependent.