A method for the in-situ synthesis of metal oxide nanoparticles with atmospheric pressure plasma, the method including the steps of: a) preparing a dispersion from a metal oxide macropowder in a liquid; b) nebulising the dispersion obtained in step (a) in a nebuliser by injection of an inert carrier gas to form an aerosol; c) providing a tubular system having a dielectric tubular mixing section and a tubular plasma generating section, wherein said tubular system has a side wall (16, 16a, 16b), a bottom in the dielectric tubular mixing section and a top in the tubular plasma generating section, wherein the central axis of tubular system is positioned substantially vertical, wherein the dielectric tubular mixing section comprises a dielectric inlet tube having a first end connected to an opening in the side wall of the dielectric tubular mixing section and a second end connected to the nebuliser, wherein the plasma generating section comprises a dielectric tube extending into the dielectric tubular mixing section up to the opening in the side wall of the dielectric tubular mixing section, and wherein the tubular plasma generating section has an opening in its side wall connected to dielectric tube, wherein the opening is positioned above, in the direction of the central axis of tubular system, opening in the side wall of the dielectric tubular mixing section; d) supplying an inert working gas to dielectric tube of the tubular plasma generating section via opening in the side wall and creating an atmospheric pressure plasma from this inert working gas at the outer end of dielectric tube extending into the dielectric tubular mixing section; e) supplying the aerosol obtained in step (b) to the dielectric tubular mixing section via dielectric inlet tube and opening in the side wall; f) allowing interaction of the aerosol and the plasma in the dielectric tubular mixing section to generate metal oxide nanoparticles characterized by a particle size distribution with a Dv99 of less than 100 nm, preferably with a Dv90 below 10 nm; and g) deposition of said metal oxide nanoparticles through an orifice in the bottom of the dielectric tubular mixing section onto a solid or semi-solid substrate. a) preparing a dispersion from a metal oxide macropowder in a liquid; b) nebulising the dispersion obtained in step (a) in a nebuliser by injection of an inert carrier gas to form an aerosol; c) providing a tubular system having a dielectric tubular mixing section and a tubular plasma generating section, wherein said tubular system has a side wall (16, 16a, 16b), a bottom in the dielectric tubular mixing section and a top in the tubular plasma generating section, wherein the central axis of tubular system is positioned substantially vertical, wherein the dielectric tubular mixing section comprises a dielectric inlet tube having a first end connected to an opening in the side wall of the dielectric tubular mixing section and a second end connected to the nebuliser, wherein the plasma generating section comprises a dielectric tube extending into the dielectric tubular mixing section up to the opening in the side wall of the dielectric tubular mixing section, and wherein the tubular plasma generating section has an opening in its side wall connected to dielectric tube, wherein the opening is positioned above, in the direction of the central axis of tubular system, opening in the side wall of the dielectric tubular mixing section; wherein said tubular system has a side wall (16, 16a, 16b), a bottom in the dielectric tubular mixing section and a top in the tubular plasma generating section, wherein the central axis of tubular system is positioned substantially vertical, wherein the dielectric tubular mixing section comprises a dielectric inlet tube having a first end connected to an opening in the side wall of the dielectric tubular mixing section and a second end connected to the nebuliser, wherein the plasma generating section comprises a dielectric tube extending into the dielectric tubular mixing section up to the opening in the side wall of the dielectric tubular mixing section, and wherein the tubular plasma generating section has an opening in its side wall connected to dielectric tube, wherein the opening is positioned above, in the direction of the central axis of tubular system, opening in the side wall of the dielectric tubular mixing section; d) supplying an inert working gas to dielectric tube of the tubular plasma generating section via opening in the side wall and creating an atmospheric pressure plasma from this inert working gas at the outer end of dielectric tube extending into the dielectric tubular mixing section; e) supplying the aerosol obtained in step (b) to the dielectric tubular mixing section via dielectric inlet tube and opening in the side wall; f) allowing interaction of the aerosol and the plasma in the dielectric tubular mixing section to generate metal oxide nanoparticles characterized by a particle size distribution with a Dv99 of less than 100 nm, preferably with a Dv90 below 10 nm; and g) deposition of said metal oxide nanoparticles through an orifice in the bottom of the dielectric tubular mixing section onto a solid or semi-solid substrate.