Abstract: An optical sensor unit (10) for measuring a concentration of a gas is provided, comprising at least one sensing layer (122) adapted to be irradiated with a predetermined radiation; at least one gas-permeable layer (121) adjacent to one side of the at least one sensing layer (122) and adapted to pass gas which concentration is to be measured through the gas-permeable layer (121) towards the sensing layer (122); a removable protective layer (150) covering at least the gas-permeable layer (121) and adapted to be removed before use of the optical sensor unit (10), wherein the optical sensor unit (10) is adapted to measure an optical response of the at least one sensing layer (122), which optical response depends on the concentration of the gas.