Evaluation of Antifungal and Thermal Comfort Properties of Aqueous Paint-Type Coatings Modified with ZnO Nanoparticles Synthesized by Green Chemistry

Fungal growth and insufficient thermal comfort degrade building durability and indoor quality, especially in humid and high-radiation regions. Zinc oxide (ZnO) stands out for its strong antifungal activity and radiative cooling potential. In this study, a commercial coating was modified with ZnO nanoparticles synthesized via a green chemistry route using Cymbopogon citratus (lemongrass) leaf extract as a reducing agent. Structural and morphological characterization by XRD, SEM, and EDS confirmed the formation of hexagonal wurtzite-phase nanoparticles with hemispherical and ellipsoidal morphologies, presenting average sizes of 50.27 ± 19.84 nm and 128.25 ± 33.43 nm, respectively, and an average crystallite size of 20.32 nm. Antifungal activity, evaluated using the poisoned food technique against Aspergillus niger and Penicillium spp., showed significant growth inhibition, reaching up to 94.63% for A. niger and 72.64% for Penicillium at a concentration of 3 mg/mL after 120 h of incubation. Thermal comfort performance was assessed to direct sunlight, in which coatings modified with 5% w/w ZnO nanoparticles achieved an average internal temperature reduction of 0.6 °C and a maximum reduction of 2.4 °C compared to uncoated surfaces. These results demonstrate that ZnO nanoparticles synthesized through environmentally friendly methods can effectively enhance both antifungal resistance and passive cooling performance.