Gas sensor with a porous layer that detectably affects a surface lattice resonant condition of a nanoparticle array

A gas sensing system comprising:

• an optically transparent gas sensor, the gas sensor comprising a first layer and a second layer superimposed on each other and having an interface between the two layers, the first layer comprising an array of nanoparticles formed along the interface, the nanoparticles being provided so as to allow, upon illumination with electromagnetic radiation, a long range diffractive coupling of surface plasmon resonances resulting in a surface lattice resonance condition, the surface lattice resonance condition having an evanescent field associated therewith, the evanescent field having a decay length, the second layer comprising a material that, responsive to exposure to at least one predetermined gas, detectably affects the surface lattice resonance condition, wherein the second layer material has a porosity of at least at least 10%, and wherein the second layer is at least as thick as the decay length of the evanescent field associated with the surface lattice resonance condition;
• a source of electromagnetic radiation for illuminating the gas sensor;
• and
• a detector positioned such that the gas sensor is between the radiation source and the detector, the detector being configured to detect changes in the electromagnetic radiation transmitted through the gas sensor caused by a change in the surface lattice resonance condition when the second layer is exposed to the predetermined gas.