Synthesis of upconverting nanomaterials: Designing the composition and the nanostructure.

Lanthanide-doped upconverting nanostructures are promising materials in the generation of imaging agents for modern biomedical applications, in particular in optical diagnostics. Upconversion (UC) is a phenomenon involving optical emission at higher energies than that of the exciting radiation, through sequential absorptions of photons. Lanthanide ions are particularly useful for this process, due to a peculiar energy levels scheme and their relatively long-excited states lifetimes. In the past decades, many hosts and types of lanthanide ions have been chosen to customize the luminescence properties of the nanosystems, tailoring also their structure to fit efficiently the final application. In this context, some reviews have appeared in the literature describing preparation of lanthanide-doped nanocrystals and the several factors (morphology, crystalline phase, size, and components) that are crucial parameters for their electrical, photophysical, magnetic, and colloidal stability properties. The target of this chapter is to briefly illustrate the diverse synthesis of UC nanomaterials, with particular attention to the composition of the host and to the architecture of the nanostructures, tailored to produce efficient luminescent nanomaterials.