Nanoscale Hollow Spheres: Microemulsion-Based Synthesis, Structural Characterization and Container-Type Functionality

A wide variety of nanoscale hollow spheres can be obtained via a microemulsion approach. This includes oxides (e.g., ZnO, TiO₂, SnO₂, AlO(OH), La(OH)₃), sulfides (e.g., Cu₂S, CuS) as well as elemental metals (e.g., Ag, Au). All hollow spheres are realized with outer diameters of 10-60 nm, an inner cavity size of 2-30 nm and a wall thickness of 2-15 nm. The microemulsion approach allows modification of the composition of the hollow spheres, fine-tuning their diameter and encapsulation of various ingredients inside the resulting "nanocontainers". This review summarizes the experimental conditions of synthesis and compares them to other methods of preparing hollow spheres. Moreover, the structural characterization and selected properties of the as-prepared hollow spheres are discussed. The latter is especially focused on container-functionalities with the encapsulation of inorganic salts (e.g., KSCN, K₂S₂O₈, KF), biomolecules/bioactive molecules (e.g., phenylalanine, quercetin, nicotinic acid) and fluorescent dyes (e.g., rhodamine, riboflavin) as representative examples.