The invention relates to a process for preparing core-shell particles comprising the steps of (i) providing a dispersion of primary magnetic particles having a mean diameter lower than 200 nm in a solvent; (ii) adding one or more (semi-)metal (oxyhydr)oxide(s) and/or one or more precursor(s) of a (semi-)metal (oxyhydr)oxide to said dispersion; (iii) optionally adding a hydrolysis agent for said one or more precursor(s); (iv) injecting the dispersion in a spray dryer; whereby a (semi-)metal (oxyhydr)oxide shell is formed on the magnetic particles during spray drying. The invention also relates to particles obtainable by said process, to a formulation of said particles in a solvent and to the use of said particles or said formulation for RNA or DNA extraction.