The present invention relates to a chemo-optical sensor unit for transcutaneous measurement of a concentration of a gas, comprising: at least one gas-permeable sensing layer adapted to be irradiated with a predetermined radiation； and at least a first 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； at least one volatile acid and/or base binding layer in the gas-pathway from the skin to the sensing layer； adapted to pass gas whose concentration is to be measured through the volatile acid and/or base binding layer towards the sensing layer； wherein said chemo-optical sensor unit is adapted to operate with a contact medium between the chemo-optical sensor unit and the skin 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 for patient monitoring and/or ventilation of a patient comprising such a chemo-optical sensor.
