METHOD FOR PREPARING A COLLOIDAL SOLUTION OF NANOPARTICLES OF A MIXED OXIDE OF VANADIUM AND EUROPIUM OR A MIXED OXIDE OF VANADIUM, EUROPIUM AND YTTRIUM

The invention relates to a method for preparing a colloidal solution S2 of nanoparticles of a mixed oxide of formula YxEu1-xVO4 in which x is such that 0 ≤ x < 1, which comprises: a) forming a colloidal solution S1 by reacting, in an aqueous or hydroalcoholic medium, a precursor P1 of vanadium oxide with a precursor P2 of europium oxide if x = 0, or indeed with a precursor P2 of europium oxide and a precursor P3 of yttrium oxide if x ≠ 0, the reaction being obtained by the fractionated addition of P2 or P2 and P3 to a solution comprising P1, or by the fractionated addition of P1 to a solution comprising P2 or P2 and P3; and b) ripening the colloidal solution S1 formed in step a); and which is characterised in that: 1°) step a) comprises a regulation of the pH of the medium in which the precursors of metal oxides react with each other, while said reaction is taking place; and in that 2°) step b) comprises the heating of the colloidal solution S1 in a microwave oven. The invention also relates to the uses of the colloidal solution S2 obtained in this way. Applications: detection, by a fluorescent optical sensor, of target molecules present in a gas or liquid medium, in particular of peroxides such as hydrogen peroxide.