\(λ=2nd\cosθ/m\) (1)
where l is the wavelength of light being reflected, n is the refractive index of the microgel layer, d is the distance between the two Au layers (thickness of the microgel layer), θ is the angle of incident light, and m is the order of the reflected peak (e.g., 1, 2, 3,…). As mentioned above, the Au-Au distance (d) can be tuned by exposing the microgel-based etalons to different stimuli. In the work here, we focused on detecting pH and ionic strength changes, yielding a shift in the wavelengths of light reflected from the etalon, and a concomitant visual color change \cite{carvalho2021probing}.
2.4.1. Device Integration
A 3D printed holder was made to accommodate etalon devices, a camera and a lighting system. The camera and lighting system was connected to the wireless control module and could take pictures of the etalons to monitor its color change as the water changes pH and ionic strength. The holder was attached to the Styrofoam boat body and immersed in water (Figure 11(a)).