The present invention relates to a method for producing a suspension of agglomerates of magnetic alkoxysilane-coated metal nanoparticles, wherein an aqueous suspension of magnetic metal nanoparticles is incubated with alkoxysilane, wherein the incubation is carried out essentially in the absence of an organic solvent. The present invention further relates to suspension of agglomerates of magnetic alkoxysilane-coated metal containing nanoparticles obtainable by the method of the present invention and to a composition comprising agglomerates of magnetic alkoxysilane-coated metal nanoparticles, wherein the agglomerates have an average size of 30 to 450 nm, preferably of 50 to 350 nm and especially of 70 to 300 nm as determined by light scattering.