The present invention relates to a sensor (1) for sensing organic carbon in a liquid (L), comprising: a container (2) having an interior space (20) for receiving the liquid (L), a photodetector (3), and a light source (4) configured to emit ultraviolet light (5) so that the ultraviolet light (5) travels along an optical path (P) through liquid (L) residing in the interior space (20) and is absorbable by carbon bonds of organic molecules in the liquid (L). According to the present invention, the photodetector (3) is configured to detect light in the visible or infrared spectrum, and the sensor (1) comprises a down conversion material portion (22; 22a) arranged in the optical path, wherein the down conversion material portion (22; 22a) is configured to receive incoming ultraviolet light (5) emitted by the light source (4) and to down convert received ultraviolet light (5) and to emit said down converted light (50) in the visible or infrared spectrum so that emitted down converted light (50) impinges on the photodetector (3).