Composite iron oxide–SiO₂ materials were prepared by a sol–gel method starting with two types of precursors, tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS), as the SiO₂ source. As the iron source a soluble Fe²⁺ salt, mainly Fe(SO₄)₂·7H₂O, was used, the iron oxides were generated during the sol–gel process. The amorphous gels obtained were thermally treated up to 1000 °C in order to obtain iron oxides with different structures and grain size. The initial gels and the thermally treated samples were characterised by DTA/TGA analysis, DR-UV–VIS and IR-spectroscopy, EPR measurements, transmission electron microscopy (TEM) and BET surface area methods. The matrices obtained from the precursors play a major role in the evolution of the process. In both cases the initial gels are amorphous. In the non-porous matrix obtained by thermal treatment using methyltriethoxysilane (MTEOS), the tendency for crystallisation increases, and the iron oxide particle size is increased.

Core/shell nanoparticles consisting of a Fe₃O₄ nanoparticle core and a mesoporous silica shell (Fe₃O₄/m-silica) were used as a matrix for immobilization of horseradish peroxidase (HRP) enzyme and subsequent design of an amperometric hydrogen peroxide biosensor. HRP enzyme was immobilized on the core/shell nanoparticles through the electrostatic interaction between oppositely charged HRP enzyme and the silica shell at neutral pH. The enzyme–core/shell nanoparticle hybrid material was deposited on screen printed electrodes and further characterized by ultraviolet–visible (UV-vis) spectroscopy and scanning electron microscopy (SEM). This set up was used as a biosensor to detect hydrogen peroxide. The hydrogen peroxide biosensor showed a detection limit of 4.3 × 10⁻⁷ M, at a signal-to-noise ratio of 3, and a sensitivity of 84.4 μA mM⁻¹ cm².