The invention relates to a process for the preparation of a citrate-coated amorphous calcium phosphate nanoparticle which comprises the following steps: 1) providing a first solution of a salt of calcium and a citrate salt wherein the molar ratio of citrate ion to calcium ion is in the range from 1 to 2 thus obtaining a clear first solution; 2) providing a second solution of a salt capable to give phosphate anion and a carbonate salt; 3) mixing together the first and the second solution at a pH in the range from 8 to 11; 4) precipitating the nanoparticle; and 5) drying the nanoparticle obtained from step 4). Preferably and advantageously the invention provides for the addition of a fluoride compound in step 2) for obtaining a fluorine-doped citrate-coated calcium phosphate nanoparticle or a nanoparticle agglomerate. The nanoparticle/nanoparticle agglomerate of the invention has a peculiar superficial area and a diameter that allow to use it as a biomaterial for dentistry application.