Carbon dioxide (CO₂) detection is of key importance for pollutant detection as a major contributor towards climate change and extreme weather. As the major biproduct of combustion, CO₂ detectors are already required in many industrial settings to limit their prevalence in the atmosphere. However, current CO₂ detectors often operate in high temperatures, high pressure, and corrosive environments. Today, planar microwave resonator sensors such as split-ring resonators (SRRs) have demonstrated robust, wireless performance providing highly sensitivity, real-time, and accurate responses while operating a simple, adjustable, and low-cost testing methods. Previous works have shown that integrating functional materials with microwave resonators can improve sensitivity, selectivity, and extend the range of potential chemical or optical stimuli.

Within material-based CO₂ detection, titanium dioxide (TiO₂) is well known to strongly interact and reduce CO₂ into different hydrocarbon compounds, and this can be made more efficient by using high-surface area nanostructured TiO₂ in the form of nanotubes, nanowires, or nanorods. TiO₂ nanotubes are also favorable due to their ability to exist as a thin free-standing membrane, separate from the growth substrate, enables easy integration with microwave resonator sensors.

Furthermore, the sensitivity of TiO₂ nanotubes towards CO₂ can be increased through UV-enhancement via photoreductive processes. Effectively, incident UV light is absorbed by the TiO₂, generating charge carriers and free radicals in the material, which more readily react with adsorbed gas. The CO₂ scavenges the excess electrons in the membrane, resulting in enhanced sensitivity and higher resolution.

This work investigates the use of a microwave resonator integrated with TiO₂ nanotube membranes with and without UV illumination for CO₂ sensing. The results will illustrate the benefit of UV-enhanced CO₂ detection using a microwave sensor. The study will offer theoretical, simulation, and experimental analysis for the improved sensing response with this method, and offer a path forward for future work in both resonator design and material functionalization. Experimental details and results will be presented and discussed.