The present invention discloses functionalized nanoparticles comprising on the surface a covalently bound radical of formula (1), wherein the nanoparticles are SiO2, Al2O3 or mixed SiO2 and Al2O3 nanoparticles, R1 and R2 are independently of each other hydrogen, nanoparticle surface-O-, or a substituent, n is 1, 2, 3, 4, 5, 6, 7 or 8, and Y is a radical of formula -B1-D1 (2a), wherein B1 is the direct bond or a bridge member, and D1 is a radical of a cationic dye, a radical of a phthalocyanine dye which carries no water-solubilizing group, or a radical selected from a fluorescent dye, or Y is a radical of formula -B2 D2 (2b), wherein B2 is an organic radical comprising at least one group having a negative charge, and D2 is a cationic dye selected from the group consisting of monoazo, disazo, polyazo, methine, azamethine, diphenylmethane, triphenylmethane, triaminotriaryl methane, azine, oxazine, cyanine and anthraquinone dyes.