A method for preparing a colloidal suspension of functionalized alkaline earth element fluoride nanoparticles with a dry extract of at least 5% by mass (preferably at least 10%, and more preferably at least 15%, and even more preferably at least 25%), wherein: (a) a solution of a salt of the corresponding alkaline earth element is prepared, (b) a solution of a charge transfer complex of formula is prepared by reaction of an amide of formula with a fluoride ion source, preferably hydrofluoric acid, tetrabutylammonium fluoride, triethylamine trihydrofluoride or fluorosilicic acid, where: Ra and Rb, identical or different, represent, each independently of each other, a (C 1 -C 6) alkyl, (C 3 -C 7)cycloalkyl, phenyl or 5- or 6-membered heterocycloalkyl group, said group being optionally substituted, or else Ra and Rb are bonded to each other and form an alkylene chain, comprising 2 to 6 carbon atoms, optionally substituted, and Rc represents a hydrogen atom or a (C 1 -C 6) alkyl, (C 3 -C 7) cycloalkyl, phenyl or 5- or 6-membered heterocycloalkyl group, said group being optionally substituted, or else Rb and Rc are bonded to each other and form an alkylene chain, comprising 2 to 5 carbon atoms, optionally substituted, (c) the solution of the salt of the corresponding alkaline earth element obtained in step (a) and the solution of the charge transfer complex of formula obtained in step (b) are brought into contact, in a solvent or mixture of solvents, (d) the mixture obtained in step (c) is allowed to react, optionally by heating, so as to obtain a colloidal suspension of alkaline earth element fluoride nanoparticles, (e) optionally, the colloidal suspension of alkaline earth element fluoride nanoparticles is washed, preferably by at least one centrifugation/redispersion cycle in an appropriate solvent until the supernatant is clear and colourless, given than the last solvent of redispersion is selected depending on the solvent for using the colloidal suspension of alkaline earth element fluoride nanoparticles, knowing that steps (a) and (b) can be reversed. (a) a solution of a salt of the corresponding alkaline earth element is prepared, (b) a solution of a charge transfer complex of formula is prepared by reaction of an amide of formula with a fluoride ion source, preferably hydrofluoric acid, tetrabutylammonium fluoride, triethylamine trihydrofluoride or fluorosilicic acid, where: Ra and Rb, identical or different, represent, each independently of each other, a (C 1 -C 6) alkyl, (C 3 -C 7)cycloalkyl, phenyl or 5- or 6-membered heterocycloalkyl group, said group being optionally substituted, or else Ra and Rb are bonded to each other and form an alkylene chain, comprising 2 to 6 carbon atoms, optionally substituted, and Rc represents a hydrogen atom or a (C 1 -C 6) alkyl, (C 3 -C 7) cycloalkyl, phenyl or 5- or 6-membered heterocycloalkyl group, said group being optionally substituted, or else Rb and Rc are bonded to each other and form an alkylene chain, comprising 2 to 5 carbon atoms, optionally substituted, (c) the solution of the salt of the corresponding alkaline earth element obtained in step (a) and the solution of the charge transfer complex of formula obtained in step (b) are brought into contact, in a solvent or mixture of solvents, (d) the mixture obtained in step (c) is allowed to react, optionally by heating, so as to obtain a colloidal suspension of alkaline earth element fluoride nanoparticles, (e) optionally, the colloidal suspension of alkaline earth element fluoride nanoparticles is washed, preferably by at least one centrifugation/redispersion cycle in an appropriate solvent until the supernatant is clear and colourless, given than the last solvent of redispersion is selected depending on the solvent for using the colloidal suspension of alkaline earth element fluoride nanoparticles,