TiO₂ and CaCO₃ Microparticles Produced in Aqueous Extracts from <em>Satureja montana</em>: Synthesis, Characterization and Preliminary Antimicrobial Test

The possibility of modifying the surface chemistry of materials, synthetizing inorganic particles in natural aqueous extracts of plants (at low temperature, avoiding calcination), opens the doors to undoubtedly very interesting scenarios for innovative functionalization strategies that are increasingly eco-sustainable and rich in interesting chemical-physical and biochemical properties. Among the aerial plant, Satureja montana exhibits interesting antibacterial, antifungal, antimicrobial and antioxidant activities due to the rich volatile and non-volatile compounds (as activators/phase controllers characterized by Gas Chromatography-Mass Spectrometry, GC-MS), contained in the aqueous extracts. For the first time, the aqueous extract of Satureja montana plant was applied for the green synthesis of TiO2 and CaCO3 particles, characterized by X-Ray diffraction (XRD), Raman, Infrared/IR spectroscopies, and Scanning Electron Microscopy-SEM/coupled by microanalysis EDX. Screening through antimicrobial assays under indoor passive sedimentation conditions showed opposite trends for both kind of inorganic particles. TiO₂ Anatase spherical particles (400&amp;lt;&amp;lt;600 nm) increase microbial growth, proportionally to increasing particles concentration (dose-dependent response). Probably, the low surface area (1.20 m2/g), low porosity (pore volume 0.0023 cm3/g; pore size of 560 nm) and smooth spherical morphology, don’t provoke chemical or physical damages to microorganisms, and therefore microbial growth is not inhibited. This occurs in the presence of medium/low concentrations of ROS (oxygenated radical species), which are chemically active in generating oxidative stress and cell death. In fact, the chemically reactive defects found in these TiO2/Anatase particles are predominantly carbonyl in nature, less reactive than carboxylic ones in producing ROS. Instead, S. montana-functionalized CaCO₃ prismatic microparticles (1µm x 1µm x 1µm) exhibit strong and dose-dependent antimicrobial activities, achieving near-complete inhibition at 50 mg mL⁻¹. This could be related to the molecular activators/phase controllers that induced defects, edges, dislocations and chemical active surface functionalities on the prismatic calcite particles, making them capable of inhibiting microbial activity. In particular, the presence of carboxyl groups C=(O)-OH, recorded by FTIR on the prismatic surfaces of CaCO3 MPs/SM particles, could be responsible for the production of ROS, the latter known to be dangerous sentinels of oxidative stress and cell death.