A three-dimensional structure of porous silicon considerably improves the anchorage of sensor-active material such as, for example, enzymes, antibodies, etc., on or in the substrate surface of chemical sensors, in particular silicon-based biosensors. This structure is produced by means of suitable etching which forms pore apertures adapted to the penetrability of the sensor-active material. The pore walls advantageously receive a non-conductive boundary layer which consists of oxides of Si and/or Al or Ta or silicon nitride and are preferably 1 - 100 nm thick. The porous layer is advantageously between 10 nm and 100 νm thick and the pores are preferably in the form of branched ducts whose average diameter is 1 nm - 10 νm and in particular 10 - 1000 nm. The sensor-active material can optionally be distributed in a glass, solid, plastics or polymer membrane.