The present invention relates to a chemo-optical sensor unit for transcutaneous measurement of a concentration of a gas, comprising: at least one sensing layer adapted to be irradiated with a predetermined radiation； and at least one gas-permeable layer adjacent to one side of the at least one sensing layer, adapted to pass gas whose concentration is to be measured through the gas-permeable layer towards the sensing layer； wherein said chemo-optical sensor unit is adapted to operate with a contact medium between the gas-permeable layer and the skin, wherein said contact medium comprises a barrier layer which is gas-permeable and impermeable to water and ions； and wherein the chemo-optical sensor unit is adapted to measure an optical response of the at least one sensing layer, whose optical response depends on the concentration of the gas. The present invention also relates to a system comprising such a chemo-optical sensor, as well as to a method for conditioning a chemo-optical sensor unit for measuring a concentration of a gas and a thereby obtainable conditioned sensor.
