Determination of ζ-potential, charge, and number of organic ligands on the surface of water soluble quantum dots by capillary electrophoresis.

The number of charges and/or organic ligands covalently attached to the surface of CdTe quantum dot nanoparticles has been determined from their electrophoretic mobilities measured in capillaries filled with free electrolyte buffers. Three sizes of water soluble CdTe quantum dots with 3-mercaptopropionic and thioglycolic acids as surface ligands were prepared. Their electrophoretic mobilities in different pH and ionic strength values of separation buffers were measured by capillary electrophoresis with laser induced fluorescence detection. The ζ-potentials determined from electrophoretic mobilities using analytical solution of Henry function proposed by Ohshima were in the range from -30 to -100 mV. Charges of QDs were calculated from ζ-potentials. As a result, numbers of organic ligands bonded to QDs surface were determined to be 13, 14, and 15 for the sizes of 3.1, 3.5, and 3.9 nm, respectively. The dissociation constants of organic ligands bonded on QDs surfaces estimated from the dependence of QDs charge on pH of the separation buffer were 7.8 and 7.9 for 3-mercaptopropionic acid and 6.9 for thioglycolic acid.