An apparatus for selectively detecting an anion in a medium and/or selectively measuring the concentration of an anion in a medium. The apparatus comprises at least one sensor and a conductive polymer film and is configured such that dielectric and/or electric property changes of the conductive polymer film can be detected by the sensor when the conductive polymer film is in hydraulic contact with the medium or after it has been in hydraulic contact with the medium. Methods of selectively detecting an anion in a medium and/or selectively determining a concentration of an anion in a medium are also provided.