Reversible humidity-driven tuning of the light scattering properties of PS:PEG-based porous polymer films: Understanding derived from the cross-sensitivity of a luminescent oxygen sensor

We analyze and explain the reversible humidity-responsive behavior of porous structured luminescent oxygen (O₂) sensing films in terms of O₂ permeability and optical light scattering. The luminescent O₂ sensing films are composed of an O₂-sensitive dye within porous polymer matrices, where the porous structure is formed via phase separation between immiscible polystyrene (PS) and polyethylene glycol (PEG). A comparison of the sensing film with a porous polymeric film without an O₂-sensitive dye by in situ measurement of both the photocurrent signal and diffused reflectance under different relative humidity (RH) levels reveals the origin of the optical variations in the luminescent O₂ sensing film as a function of RH. Here, the hygroscopic PEG surrounded by the PS matrix swells and fills up the pores as the RH increases, which mainly tunes the optical scattering property of the porous polymeric films, and the variation in light scattering of the porous polymeric films is explained by Mie scattering theory. By considering and analyzing the humidity response in luminescent O₂ sensing films in the aspects of fabrication processes and experimental systems as well as the hydration property of the materials, this study can provide fundamental insights into the cross-sensitivity of optical O₂ sensors to humidity. Moreover, the fully reversible humidity-driven tuning of the light scattering properties in porous polymeric films allows various applications, such as dual monitoring of O₂ and RH in wearable sensors, responsive coating or smart windows, that require optical tuning without extra energy.