Nanoparticles have a very large surface area to volume ratio. Because of this, a material made of nanoparticles can have different properties from the same material in bulk, and these properties lead to useful applications.
Titanium dioxide and zinc oxide nanoparticles are used in sun creams. They absorb harmful ultraviolet light, and they look transparent on the skin because they are so small. Larger particles of the same substances leave a thick white layer.
Silver nanoparticles are used in wound dressings and some fabrics to kill bacteria. Their large surface area to volume ratio exposes many silver atoms. Nanoparticles are also used as catalysts, because a small mass has a large surface area, so less of an expensive metal is needed. In medicine, nanoparticles are small enough to enter cells, so they can carry a drug to where it is needed.