Fig. 2: Principles of optical gas sensing.

a Gases consist of various groups (c) and display distinct properties, such as refractive index (n), absorption coefficient (A) and absorption spectrum (α(λ)). b Colorimetric sensing mechanism: the gas groups react chemically with dye molecules, resulting in a detectable color change. c Surface plasmon polariton (SPP)-based methods: these techniques exploit the sensitivity of SPP to variations in refractive index (n). d NDIR methods: detection is achieved by measuring the absorption of infrared light by gas molecules. e Spectroscopic methods: for instance, surface-enhanced Raman spectroscopy (SERS) leverages Raman scattering for molecule detection. f Various output signal types corresponding to these mechanisms are also illustrated