The invention relates to a sensor for determining a concentration of a substance contained in a fluid-containing matrix, with a measurement chamber (5), a sender (3) for the emission of optical radiation (6) into the measurement chamber, and a receiver (4) for receiving optical radiation which has traversed the measurement chamber, wherein the measurement chamber is designed for bringing into the matrix and is filled with a specific fluid, and the wall (12, 13) is at least sectionally permeable to the diffusion of the substance. A control and evaluation circuit (7) is connected to the sender (3) and controls the sender (3), and is connected to the receiver (4) for selecting the receiver signals and determines, from the receiver signal, a measurement for the concentration of the substance in the matrix.