Fluorescent semiconductor nanocrystals such as CdSe have a wide range of application within biology as well as energy conversion and transduction. The key criteria for CdSe nanoparticle use in biological systems are their fluorescence quantum yield, size and aqueous stability. The surface chemistry and properties of the ligands adsorbed onto the nanocrystal surface influence all these applications, as well as allowing transfer of the particles to different solvents via specific ligand-solvent interactions.

The ligand-solvent interactions of nanocrystals may be investigated using fluorescence correlation spectroscopy (FCS). FCS is a single molecule technique within which an excitation source is focussed to a diffraction-limited spot. A confocal pinhole eliminates out of plane light resulting in the detection of fluorescence from a volume of around one femtolitre. The fluorescence from a single molecule (or in this case, single nanocrystal) diffusing through the confocal volume is able to be detected via the use of efficient photon detectors.

We report here the investigation of ligand-solvent interactions in a variety of systems. The diffusion of spherical CdSe nanoparticles functionalised with a range of bio-compatible and organic ligands have been probed via FCS, allowing their hydrodynamic radius to be determined. The effect of buffer in aqueous solution on the hydrodynamic radius will be described along with the presence of different functional groups at the ligand tail. The relationship between the hydrodynamic radius and nanocrystal size has been found to be non-linear in organic solvents for particles with small diameters, these results will be presented discussed.

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