Rationalizing Nanomaterial Sizes Measured by Atomic Force Microscopy, Flow Field-Flow Fractionation, and Dynamic Light Scattering: Sample Preparation, Polydispersity, and Particle Structure

This study aims to rationalize the variability in the measured size of nanomaterials (NMs) by some of the most commonly applied techniques in the field of nano(eco)toxicology and environmental sciences, including atomic force microscopy (AFM), dynamic light scattering (DLS), and flow field-flow fractionation (FlFFF). A validated sample preparation procedure for size evaluation by AFM is presented, along with a quantitative explanation of the variability of measured sizes by FlFFF, AFM, and DLS. The ratio of the z-average hydrodynamic diameter (d DLS) by DLS and the particle height by AFM (d AFM) approaches 1.0 for monodisperse samples and increases with sample polydispersity. A polydispersity index of 0.1 is suggested as a suitable limit above which DLS data can no longer be interpreted accurately. Conversion of the volume particle size distribution (PSD) by FlFFF–UV to the number PSD reduces the differences observed between the sizes measured by FlFFF (d FlFFF) and AFM. The remaining differences in the measured sizes can be attributed to particle structure (sphericity and permeability). The ratio d FlFFF/d AFM approaches 1 for small ion-coated NMs, which can be described as hard spheres, whereas d FlFFF/d AFM deviates from 1 for polymer-coated NMs, indicating that these particles are permeable, nonspherical, or both. These findings improve our understanding of the rather scattered data on NM size measurements reported in the environmental and nano(eco)toxicology literature and provide a tool for comparison of the measured sizes by different techniques.