A method for producing a dispersion comprising composite particles, wherein the composite particles comprise - a core comprising a polymeric material matrix and organically modified magnetic nanoparticles embedded within said matrix, wherein the magnetic nanoparticles have an average particle size from 0.1 to 1000 nm and are organically modified with an organic modifier selected from the group consisting of fatty acids, silanes, acetic acid, polyvinyl amines, sulfonic acid derivatives, polyvinyl alcohols, polyesters, polyethers, polyphosphates, polyurethanes, alkylammonium salts, polymers and copolymers having acidic groups, especially carboxylic groups, optionally modified acrylate block copolymers, optionally modified polyurethanes, polymers and copolymers having fatty acid radicals, optionally modified polyesters, such as acid-functionalized polyesters, polyphosphates, optionally modified polyethers and mixtures thereof, and - a coating comprising photocatalytic nanoparticles around the core having an average particle size from 0.1 to 1000 nm, and wherein the method comprises the following steps: a) providing an organic phase comprising an ethylenically unsaturated monomer, the organically modified magnetic nanoparticles, a co-stabilizer, wherein the co-stabilizer is a compound with molecular weight smaller than 2,000 Da and water solubility measured at 25°C smaller than 1x10 -3 wt/wt %, a hydrophobe, wherein the hydrophobe is a compound with molecular weight higher than 2,000 Da and water solubility measured at 25°C smaller than 1x10 -3 wt/wt %, or a co-stabilizer/hydrophobe combination, and an aqueous phase comprising the photocatalytic nanoparticles, b) mixing the organic phase with the aqueous phase from step (a) and providing energy by mechanical devices or by methods based on the chemical potential of the components to the resulting mixture to produce a dispersion of droplets of the monomer and magnetic nanoparticles, and c) adding a polymerization initiator selected from thermal initiators, redox initiators, photo-initiators and combinations thereof and selected from oil- and water-soluble initiators, wherein the polimerization initiator is added to either the organic phase as defined in the previous step (a), to the aqueous phase as defined in the previous step (a) or to the dispersion of droplets of the monomer and magnetic nanoparticles resulting from step (b) and heating to polymerize the monomer. - a core comprising a polymeric material matrix and organically modified magnetic nanoparticles embedded within said matrix, wherein the magnetic nanoparticles have an average particle size from 0.1 to 1000 nm and are organically modified with an organic modifier selected from the group consisting of fatty acids, silanes, acetic acid, polyvinyl amines, sulfonic acid derivatives, polyvinyl alcohols, polyesters, polyethers, polyphosphates, polyurethanes, alkylammonium salts, polymers and copolymers having acidic groups, especially carboxylic groups, optionally modified acrylate block copolymers, optionally modified polyurethanes, polymers and copolymers having fatty acid radicals, optionally modified polyesters, such as acid-functionalized polyesters, polyphosphates, optionally modified polyethers and mixtures thereof, and - a coating comprising photocatalytic nanoparticles around the core having an average particle size from 0.1 to 1000 nm, and a) providing an organic phase comprising an ethylenically unsaturated monomer, the organically modified magnetic nanoparticles, a co-stabilizer, wherein the co-stabilizer is a compound with molecular weight smaller than 2,000 Da and water solubility measured at 25°C smaller than 1x10 -3 wt/wt %, a hydrophobe, wherein the hydrophobe is a compound with molecular weight higher than 2,000 Da and water solubility measured at 25°C smaller than 1x10 -3 wt/wt %, or a co-stabilizer/hydrophobe combination, and an aqueous phase comprising the photocatalytic nanoparticles, b) mixing the organic phase with the aqueous phase from step (a) and providing energy by mechanical devices or by methods based on the chemical potential of the components to the resulting mixture to produce a dispersion of droplets of the monomer and magnetic nanoparticles, and c) adding a polymerization initiator selected from thermal initiators, redox initiators, photo-initiators and combinations thereof and selected from oil- and water-soluble initiators, wherein the polimerization initiator is added to either the organic phase as defined in the previous step (a), to the aqueous phase as defined in the previous step (a) or to the dispersion of droplets of the monomer and magnetic nanoparticles resulting from step (b) and heating to polymerize the monomer.