Free-radical scavenging activity properties of ZnO sub-micron particles: size effect and kinetics

ZnO particles were synthesized to evaluate the free radical scavenging activity properties of the particles as a function of the particle size. To obtain different particle diameters, the product was heat-treated at 600 °C for 2, 72, and 168 h to promote particle growth, obtaining a d₅₀ of 132, 275, and 403 nm, respectively. The free radical scavenging activity was estimated by a 2,2 diphenyl-1-picrylhydrazyl (DPPH) assay, under UV irradiation. The kinetics of the scavenging reaction was monitored for the three different sizes and four ZnO particle concentrations at different reaction times; we found increases at lower particles size, and a relatively low ZnO dosage of 200 μg mL⁻¹. The results showed that the reaction probably presented a pseudo-first-order kinetics model, with E_a = 3.2 kcal mol⁻¹. Finally, we calculated the EC50 for the 132-μm particles to be 97.65 μg/mL.