Figure 4.

(a) Excitation spectra measured at 570 nm of the (HPTS)/(TOA)₄ in the PPMA formulation exposed to different CO₂ partial pressures. (b) Normalized R (between [0,1]) vs. CO₂ partial pressure of a PPMA/white coating sample, showing a delayed diffusion of CO₂ through the white coating (CO₂ → white), which disappears at temperatures over 40 °C. The fluorescence ratio R measured with the wearable is highly sensitive to changes in CO₂, with our prototypes showing a delayed response with respect to the reference $C{O}_2$ sensor at (c) T = 25 °C, attributed to CO₂ diffusion through the white scattering layer, vanishing when heating up to (d) T = 44 °C. (e) Normalized R vs. CO₂ for the wearable at different temperatures, with the delayed response vanishing at higher temperatures. (f) Time delay (lag) between our prototype’s signal and the reference CO₂ sensor as a function of temperature.