"Nanoparticles are nearly-spherical particles of diameter between 1 and 100 nanometers. They consist of thousands of atoms, but are still much too small to be directly visible with a light microscope (here I like to draw a line and put various sizes on it: 0.1 nm = atoms, 1–100 nm = nanoparticles, 200 nm = visible with light microscope). Nanoparticles have some interesting properties different from smaller complexes and bigger particles of the same composition, such as color, magnetism, catalytic activity... The properties of nanoparticles usually highly depend on their size, but unlike for molecules and clusters, not all nanoparticles with the same or similar enough properties have exactly the same number of atoms, but just diameters within some range (e.g. 2–4 nm gold nanoparticles)"