Filing Date: 2003-11-28.-- Priority Data ES P200202751 (2002-11-29)The invention relates to improved magnetic particles which are suitable for the preparation of biosensors consisting of antibodies and antigens immobilised on the aforementioned magnetic particles. The inventive magnetic particles take the form of iron oxide particles measuring between 5 and 30 nm, which are disposed in a basic medium, presenting a silica coating with a thickness of between 30 and 100 nm. Said particles are characterised in that they have been subjected to a heat treatment at between 300 and 800 °C for between 1 and 24 hours. The invention also relates to a method of preparing the aforementioned magnetic particles. The invention furhter relates to the above-mentioned biosensors and to a method of preparing same. Finally, the invention relates to a method of using said biosensors in order to identify and quantify analytes.