A nanoparticle  is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter.The term is sometimes used for larger particles, up to 500 nm,[citation needed] or fibers and tubes that are less than 100 nm in only two directions.At the lowest range, metal particles smaller than 1 nm are usually called atom clusters instead.A nanoparticle is a small particle that ranges between 1 to 100 nanometres in size. Undetectable by the human eye, nanoparticles can exhibit significantly different physical and chemical properties to their larger material counterparts.

The material properties change as their size approaches the atomic scale. This is due to the surface area to volume ratio increasing, resulting in the material’s surface atoms dominating the material performance. Owing to their very small size, nanoparticles have a very large surface area to volume ratio when compared to bulk material, such as powders, plate and sheet. This feature enables nanoparticles to possess unexpected optical, physical and chemical properties, as they are small enough to confine their electrons and produce quantum effects.

For example, copper is considered a soft material, with bulk copper bending when its atoms cluster at the 50nm scale. Consequently, copper nanoparticles smaller then 50nm are considered a very hard material, with drastically different malleability and ductility performance when compared to bulk copper. The change in size can also affect the melting characteristics; gold nanoparticles melt at much lower temperatures (300 °C for 2.5 nm size) than bulk gold (1064 °C). Moreover, absorption of solar radiation is much higher in materials composed of nanoparticles than in thin films of continuous sheets of material.