There can be a large variation in the measured diameter of nanoparticles depending on which method is used. In this work, we have strived to accurately determine the mean particle diameter of 30-40 nm colloidal silica particles by using six different techniques. A quantitative agreement between the particle size distributions was obtained by scanning electron microscopy (SEM), and electrospray-scanning mobility particle sizer (ES-SMPS). However, transmission electron microscopy gave a distribution shifted to smaller sizes. After confirming that the magnification calibration was consistent, this was attributed to sample preparation artifacts. The hydrodynamic diameter, dh , was determined by dynamic light scattering (DLS) both in batch mode, and hyphenated with sedimentation field flow fractionation. Surprisingly the dh were smaller than the SEM, and ES-SMPS diameters. A plausible explanation for the smaller sizes found with DLS is that a permeable gel layer forms on the particle surface. Results from nanoparticle tracking analysis were strongly biased towards larger diameters, most likely because the silica particles provide low refractive index contrast. Calculations confirmed that the sensitivity is, depending on the shape of the laser beam, strongly size dependent for particles with diameters close to the visualization limit.
Keywords: gel layer; method comparison; particle diameter; particle morphology; surface properties; validation.