Method for preparing a colloidal solution S2 of nanoparticles of a mixed oxide of formula Y x Eu 1-x VO 4 wherein x is such that 0 ≤ x < 1, comprising: a) the formation of a colloidal solution S1 by reaction, in an aqueous or hydro-alcoholic medium, or a precursor P1 of vanadium oxide with a precursor P2 of europium oxide if x = 0, or else with a precursor P2 of europium oxide and a precursor P3 of yttrium oxide if x ≠ 0, the reaction being obtained by fractionated addition of P2 or of P2 and P3 to a solution comprising P1, or by fractionated addition of P1 to a solution comprising P2 or P2 and P3, the medium having an initial pH value, noted pH i; and b) the ripening of the colloidal solution S1 formed in step a), whereby the colloidal solution S2 is obtained; characterized in that: 1°) in step a): - either pH i is equal to a threshold pH value, noted pH s, selected beforehand and at least equal to 8, in which case the pH of the medium is maintained to pH i for the whole duration of step a); - or pH i is greater than pH s and is reduced by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, or else pH i is less than pH s and is increased by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is left to change until it attains pH s, and then the pH of the medium is maintained at pH s until the end of step a); - or pH i is greater than pH s and is increased or unchanged by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is lowered until it attains pH s, and then the pH of the medium is maintained at pH s until the end of step a); - or further pH i is smaller than pH s and is lowered or unchanged by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is increased until it attains pH s and then the pH of the medium is maintained at pH s until the end of step a); and in that 2°) step b) comprises the heating of the colloidal solution S1 in a microwave oven. a) the formation of a colloidal solution S1 by reaction, in an aqueous or hydro-alcoholic medium, or a precursor P1 of vanadium oxide with a precursor P2 of europium oxide if x = 0, or else with a precursor P2 of europium oxide and a precursor P3 of yttrium oxide if x ≠ 0, the reaction being obtained by fractionated addition of P2 or of P2 and P3 to a solution comprising P1, or by fractionated addition of P1 to a solution comprising P2 or P2 and P3, the medium having an initial pH value, noted pH i; and b) the ripening of the colloidal solution S1 formed in step a), whereby the colloidal solution S2 is obtained; 1°) in step a): - either pH i is equal to a threshold pH value, noted pH s, selected beforehand and at least equal to 8, in which case the pH of the medium is maintained to pH i for the whole duration of step a); - or pH i is greater than pH s and is reduced by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, or else pH i is less than pH s and is increased by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is left to change until it attains pH s, and then the pH of the medium is maintained at pH s until the end of step a); - or pH i is greater than pH s and is increased or unchanged by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is lowered until it attains pH s, and then the pH of the medium is maintained at pH s until the end of step a); - or further pH i is smaller than pH s and is lowered or unchanged by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is increased until it attains pH s and then the pH of the medium is maintained at pH s until the end of step a); and in that - either pH i is equal to a threshold pH value, noted pH s, selected beforehand and at least equal to 8, in which case the pH of the medium is maintained to pH i for the whole duration of step a); - or pH i is greater than pH s and is reduced by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, or else pH i is less than pH s and is increased by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is left to change until it attains pH s, and then the pH of the medium is maintained at pH s until the end of step a); - or pH i is greater than pH s and is increased or unchanged by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is lowered until it attains pH s, and then the pH of the medium is maintained at pH s until the end of step a); - or further pH i is smaller than pH s and is lowered or unchanged by adding P2 or P2 and P3 to the solution comprising P1, or by adding P1 to the solution comprising P2 or P2 and P3, in which case the pH of the medium is increased until it attains pH s and then the pH of the medium is maintained at pH s until the end of step a); and in that 2°) step b) comprises the heating of the colloidal solution S1 in a microwave oven.