Based on the reviewed literature, the most frequently reported physicochemical property was the shape, reported for 65 out of the 83 ENMs (78%), followed by the hydrodynamic diameter, primary size and zeta potential at 75%, 70% and 70%, respectively (see Table ). The shapes are distributed as follows: of the ENMs in the RDF 37 are spherical, twenty-one are tube-shaped, five are sheets and the RDF contains also one plate-and one cube-shaped ENM. Surface area (m 2 /g) 16