Homogeneity verification of samples produced via Sandwich freeze-drying

Abstract

Sandwich freeze-drying can provide dispersions of nanoparticles on a substrate with little aggregation, which is convenient for microscope imaging. We examined suspensions of spherical silica nanoparticles deposited on silicon substrates via sandwich freeze-drying. The substrates were imaged through atomic force microscopy (AFM) and the particle size distributions were computed. We corrected the distributions in light of the grid-based discrete nature of AFM. This preparation and measurement-computation scheme enabled us to analyze wide-ranging size distributions. For monomodal distributions with nominal diameters of 50nm and 100nm, the z-average diameters coincided with dynamic light scattering results with an accuracy of 1nm. Two monomodal suspensions were mixed to form a bimodal suspension and the size distribution was deduced. Despite the wide size distribution, we obtained a result consistent with the monomodal data. Furthermore, we observed Ostwald ripening in the bimodal suspension via successive measurements over five years. To check to see if the method produced a good representation of the nanoparticles, we performed a high-level analysis by deducing the actual size distribution of spherical nanoparticles from AFM data.