ARTICLE pubs.acs.org/ac
Capillary Electrophoresis Method for the Characterization and Separation of CdSe Quantum Dots Carolina Carrillo-Carrion,† Yolanda Moliner-Martínez,‡ Bartolome M. Simonet,*,† and Miguel Valcarcel† †
Department of Analytical Chemistry, University of Cordoba, E-14071 Cordoba, Spain ABSTRACT: 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.
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emiconductor nanocrystals, also known as quantum dots (QDs), are fluorescent nanoscale inorganic particles with typical diameters ranging from 1 to 10 nm. Due to their quantum confinement, QDs show unique and fascinating optical properties that are advantageous in the fields of bioanalytical, biomedical, and biophotonic research. Such optical properties include sharp and symmetrical emission spectra, broad excitation wavelengths, long fluorescence lifetimes, large Stokes shifts, high quantum yield (QY), and size-tunable emission wavelengths. Other advantageous properties include good chemical and photostability.1-3 Conventionally, transmission electron microscopy (TEM) is used to measure the sizes of metal particles.4-6 TEM, however, is a time-consuming technique that does not include a separation process by which size-dependent properties can be deduced. In addition, it is difficult to infer an ensemble’s average properties, such as average diameter or shape, based on the limited regions typically examined by TEM. This statistical uncertainty arises partly because of human subjectivity when deciding which areas of the grid to image, as well as alteration of particles during sample preparation or by radiation damage. The use of separation techniques for the characterization of nanoparticles has the advantage of allowing particle size distributions to be measured simultaneously with the physical properties (e.g., absorbance, conductivity) of the particles. In addition, such an analysis can be performed using a small sample volume (e.g., r 2011 American Chemical Society
10 nL) for capillary electrophoresis (CE) over a short analysis time (
