Size Determination of Quantum Dots with Fluorescence Correlation Spectroscopy

Semiconductor quantum dots (QDs) are highly interesting fluorophores for a large variety of spectroscopic applications. Although their fluorescence properties are well investigated, accurate size determination of QDs is still a problem. TEM techniques can image the inorganic core/shell system of QDs, but size determination of polymer coated QDs is difficult. SEC (size exclusion chromatography) compares the QD size only with standard polymers and their sizes, and is therefore not easy to use on nanoparticles. As QDs are fluorescent, single molecule spectroscopy methods such as fluorescence correlation spectroscopy (FCS) can be used to determine QDs diffusion coefficients and hence their hydrodynamic radii. Moreover, this method for size determination requires only very low QD concentrations, which is a mayor advantage compared to other techniques such as dynamic light scattering. Within our contribution we present the size determination of commercially available and self-modified QDs with FCS. The commercial QDs (QD525, QD565, QD605, QD655 and QD705 - purchased from Invitrogen Inc.) have a rather thick polymer shell and are functionalized with streptavidin, biotin or carboxylic groups. The self-modified QDs consist of the same commercial core/shell QDs and are modified with a polymer shell and several bio-functionalization groups. For all nanoparticles the diffusion coefficients were measured by FCS and the hydrodynamic radii were calculated according to the Stokes-Einstein equation. The obtained results are in good agreement with the size information provided by Invitrogen Inc., which demonstrates that FCS is an important technique for QD size determination at very low concentrations.