The present invention relates to nanoparticles of formula MLnF4 : RE+3, where M is an alkaline metal, Ln is a lanthanide element or an analog thereof and RE+3 is an ion of rare earth elements or combinations thereof, characterized in that said nanoparticles comprise organic ligands bound to the surface thereof by means of covalent bonds. The invention also relates to a method for preparing luminescent upconversion nanoparticles of the invention, to the use of the luminescent upconversion nanoparticles of the invention as fluorescent markers or photosensitizers and to a luminescent composite material comprising the luminescent upconversion nanoparticles of the invention.