Capillary electrophoresis method for the characterization and separation of CdSe quantum dots

This paper presents a simple and rapid methodology to separate and characterize free CdSe quantum dots (QDs) in aqueous medium by capillary electrophoresis (CE). First, we describe a controlled derivatization procedure to obtain water-soluble QDs through noncovalent interactions. This derivatization methodology was based on the formation of a complex between the QDs and several types of surfactants to enhance the hydrophilicity and stability of the CdSe QDs. The surfactants used to achieve the surface functionalization were trioctylphosphine oxide/trioctylphosphine (TOPO/TOP) and sodium dodecyl sulfate (SDS). Different CdSe QDs core sizes were synthesized as function of the nanocrystals growing time and then subjected to controlled coating. These free QDs were separated by capillary zone electrophoresis (CZE) based on the differences in the charge-to-mass ratio of the QDs-TOPO/TOP-SDS complexes, and the detection was carried out with UV-vis and laser-induced fluorescence (LIF) techniques obtaining detection limits 5 times lower with CE-LIF. Under the optimal working conditions, four different-sized QDs were successfully separated whose average sizes were 3.1, 3.6, 4.3, and 4.9 nm, and the size distribution was less than 7% for all of them [calculated from the full width at half-maximum (fwhm) of the fluorescence spectra and confirmed by high-resolution transmission electron microscopy (HTEM)]. Therefore, we were able to separate QDs that differ in only 0.5 nm in diameter and 19 nm in fluorescence emission maximum. This corresponds to the better resolution achieved in the analysis of these kinds of nanoparticles. Finally, a correlation between the migration times plus or minus peak width and the core sizes plus or minus size distribution was established.