Determination of the size of water-soluble nanoparticles and quantum dots by field-flow fractionation

Field flow fractionation (FFF) technique is used to determine the size of water-soluble Au, ZnS, ZnS-Mn2+ nanoparticles, and CdSe, CdSe-DNA quantum dots (QDs). The results of the FFF measurements are compared with the particle size analysis using conventional techniques like scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS) studies. Water-soluble gold nanoparticles (AuNPs) stabilized by mercaptosuccinic acid (MSA) as the ligand when analyzed by the SEM and DLS showed evidence of extensive aggregation, preventing an accurate determination of the average particle size. The TEM analyses without staining offered a facile measurement of the nanoparticle core but average particle size determination required analysis of the TEM image using image analysis software. On the other hand the FFF is seemingly a convenient and easy method for the determination of the average particle size of the AuNPs. In case of the ZnS and ZnS-Mn2+ nanoparticles with mercaptopropionic acid (MPA) as the capping agent severe aggregation prevented accurate estimation of particle sizes even by the high resolution TEM (HRTEM), where as the size determination by the FFF was very facile. Analysis of the CdSe-DNA conjugate by the TEM was difficult as the sample got damaged upon exposure to the electron beam. The FFF cross-flow condition is apparently noninvasive and hence the technique was very effective in characterizing the CdSe-DNA QDs. Furthermore, using this simple technique it was possible to fractionate a sample of the AuNPs. The FFF measurement of water-soluble nanoparticles is an excellent complement to characterization of such particles by the conventional tools.