Abstract: A sensor for quantitatively detecting an analyte in a sample, wherein an optical behavior of at least one dye depending on the analyte is used for the quantitative detection is provided. The at least one dye is contained in a medium. A restrictor is provided in order to mechanically limit volume changes of the medium. Furthermore, an osmolality in the medium is set in such a way that the osmolality is higher than the highest sample osmolality for which the sensor is to be used. The cooperation of the restrictor with the specified high osmolality results in a greatly reduced osmolality cross-sensitivity of the sensor. The restrictor can be embedded into the medium as a membrane, mat, braided material, woven fabric, or mesh. Alternatively, the restrictor can also include a carrier plate having a plurality of recesses, in which the medium is arranged. A corresponding measuring apparatus is also provided.