Nanoparticles are generally considered to have at least one dimension less than 100nm.
How low can you go to still have a nanoparticle?
It is a tricky question, and may not be so easily answered. With tools like Dynamic Light Scattering (DLS) covering the whole submicron metrology range, one can definitely see from molecules, even as small as sucrose, easily up to nanoparticles – and there is no clear demarcation line between the two.
An interesting study by Thomas Zeuch (Institute for Physical Chemistry) and Udo Buck (Max Planck Institute for Dynamics and Self-Organization) in Göttingen, Germany, sheds some light on a related question:
How many water molecules do you need to put together to create an ice crystal?
Quite surprisingly their experiments show the onset of a crystal as 275 molecules and state that the crystal is completely formed by the time it reaches 475 molecules.
While their research involved very low temperatures (-180 to -150°C) and the formation of ordered lattice structures one can venture to make an educated guess: The transition zone between “molecule” and “particle” is probably in the range of a few hundred molecules for typical substances. Well, there is still a wide open field for future research…