Thermal Processing as a Key Factor in the Surface Chemistry and Preliminary Antimicrobial Activity of ZnO Nanoparticles Synthesized Using Tradescantia spathacea Sw., Extract

ZnO nanoparticles were synthesized through a bio-assisted route using a hydroalcoholic extract of Tradescantia spathacea Sw., a plant previously reported to exhibit antimicrobial activity. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] was used as the precursor, and the resulting materials were subjected to different thermal treatments (room temperature, 50, 100, 200, and 500 °C) to evaluate changes in their physicochemical and biological properties. The samples were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption–desorption, UV–Vis diffuse reflectance spectroscopy, and zeta-potential measurements. Their interaction with 4-nitrophenol (4-NP) under dark conditions was also evaluated as a molecular probe of the solid-solution interfacial environment. Antimicrobial activity was assessed against bacterial and fungal microorganisms, while anti-virulence activity was evaluated through the inhibition of swarming motility in Proteus mirabilis 884. FT-IR analysis showed that organic species derived from the extract remained in the materials treated between room temperature and 200 °C, whereas these contributions disappeared after treatment at 500 °C. XRD confirmed the formation of the hexagonal wurtzite ZnO structure in all samples, while SEM revealed changes in morphology from agglomerated flake-like structures toward more defined particles. Zeta-potential measurements showed distinct negative electrokinetic behavior among the materials, indicating differences in their interfacial surface environments. In addition, contact with 4-NP modified its protonation equilibrium and produced spectral changes compatible with surface-mediated oxidative transformations under dark conditions. The ZnO materials exhibited antimicrobial activity, with a MIC of 320 μg/mL against P. mirabilis 884, and markedly inhibited its swarming motility, reaching more than 96% inhibition for one sample at 24 and 48 h. Overall, the results indicate that the bio-assisted synthesis modifies the surface properties of ZnO and support a surface-mediated mechanism in which changes in the interfacial microenvironment and oxidative processes may contribute to microbial inhibition.