Ultrasmall particles and nanocomposites: state of the art

Synthesis, properties, and applications of ultrasmall particles (USPs, size/ diameter < 10 nm), so-called ultrasmall nanoparticles, are reviewed. USPs mainly consist of nanoparticles (with or without functionalization) of metallic gold, as well as iron and gadolinium oxides, all of which have important biomedical applications. Several other elemental metals and non-metals, oxides, salts, a few polymers and coordination compounds have also been obtained as USPs. Their main applications include biomedical uses {Magnetic Resonance Imaging (MRI) contract agents, virus inactivation, immunoassay labeling, etc.}, varieties of catalysis, hydrogen sorption, as magnetic devices, ultrasmall transistors, sensors for gases or biological objects, parts of solar cells and batteries, among others. Recent approaches, such as size-focusing methodology, for obtaining USPs and preventing their further agglomeration, are discussed.