Transmission electron micrographs (TEMs) were recorded on JEOL JAX-840A and JEOL JEM-1230 electron micro-analyzers, respectively. The ZnO-NP samples were scattered in ethanol and then treated ultrasonically to disperse the individual particle over the gold grids (Mahamuni et al. 2019 (link)). However, for encapsulated thyme oil (ETO), the droplets were placed onto a carbon-coated copper grid to form a thin liquid film and were negatively stained by one drop of uranyl acetate. The excess of staining was removed using filter paper then the film was air-dried before the observation (Pecarski et al. 2014 (link)). The Orius 1000 CCD camera was used for image acquisition (GATAN, Warrendale, PA, USA). For measuring zeta potential, the sample of ZnO-NPs or ETO was sonicated for 30–60 min just before assessment. The average diameter was calculated using zpw 388 version 2.14 nicomp software. The size distribution and zeta potential of ZnO-NPs and ETO were measured using a particle size analyzer (Nano-ZS, Malvern Instruments Ltd., UK).