Accurate determination of the size distribution for polydisperse, cationic metallic nanomaterials by asymmetric-flow field flow fractionation

In this work we have developed and validated a methodology for determining the size distribution in polydisperse cationic nanoparticle (NP) samples using asymmetric-flow field flow fractionation (A4F), where known compositional influence of previously used calibrants was mitigated. Both the accurate determination of NP size distributions, in general, and the evaluation of cationic species with A4F have proved to be persistent analytical challenges, especially for strong, optically absorbing metallic NPs. In order to overcome these challenges, highly uniform and monomodal cetyl trimethylammonium bromide (CTAB)-stabilized SeNPs were prepared through a facile synthetic procedure and were further validated as an appropriate calibrant for cationic gold NPs based on their inherent optical properties and (nearly) identical retention behavior. Due to the uniform retention behavior resulting from the NP coating and narrow distribution (σ d ≪ diameter) of the calibrant, the contributions from sample polydispersity and instrumental broadening of the theoretical plate height, H, could be isolated and the size distribution determined. Although this has been demonstrated previously for macromolecules, to our knowledge the size distribution has not been reported and validated for NP analytes. Implementation of the calibrant in a polydisperse CTAB-coated AuNP sample demonstrates the capabilities of the current method and could be easily extended to other systems. Overall, the necessity of robust methods for size distribution determination are currently addressed that incorporate appropriate calibrants when light scattering methods are intractable, providing an alternative method in systems that are difficult with traditional microscopy approaches.