The present invention provides an optical sensor unit (10) for measuring gas concentration, comprising: sensor means (12, 13) and first thermal insulation means (14, 16) at least partially surrounding said sensor means (12, 13). The sensor means (12,13) includes at least one sensing layer (12) adapted to be irradiated with a predetermined radiation (100), and at least one gas-permeable layer (13) adjacent to one side of the at least one sensing layer (12) and adapted to pass gas, which concentration is to measured, through the gas-permeable layer (13) towards the at least one sensing layer (12). The optical sensor unit (10) is adapted to measure an optical response of the at least one sensing layer (12), which optical response depends on the gas concentration.