The present invention relates to a gas sensor comprising a sensor layer (100) comprising a plasmonic sensor (102) provided so as to allow, upon illumination with electromagnetic radiation a localized surface plasmon resonance condition, a sensing layer (106) comprising a gas permeable material that, when exposed to a gas, modifies the localized surface plasmon resonance condition, a separating layer (104) arranged in between the sensor layer (100) and the sensing layer (106) such that the plasmonic sensor (102) is separated from the sensing layer (106). A gas sensing system and a method for sensing a presence of a gas is further disclosed.