Here we only show the results for the samples synthesized with 30 mL of a solution of 0.1 M zinc nitrate hexahydrate and 0.1 g of gum arabic. The results obtained at other concentrations are shown in Figures S1 and S2 of the Support ing Information. The FESEM micrographs in Figure 1a and b show that the Zno mesocrystals have a twin-crystal form at the end of synthesis (21 h). The thickness and diameter of the Zno mesocrystals were both measured to be 1.0:t 0.2 um. The SEM image in Figure 1 b shows that the top sur face of mesocrystals consists of numerous Zno nanorods aligned in parallel. The tops of the nanorods appear to be mostly flat. Thus, the mesostructure of the Zno twin crystals is denoted twin-brush. The FESEM images of Zno meso- crystals prepared from lower concentrations of zinc nitrate hexahydrate (0.01 and 0.02 M) showed less ordered twin- brush crystals (Supporting Information Figure S1). When we changed the concentration of gum arabic from 0.01 gl30 mL to 0.3 g/30 mL, we also obtained the twin-brush form of Zno (see SEM images in Supporting Information Fig ure S2). Although twin-crystal forms of Zno synthesized in [11-13.17.18] this bio-inspired approaches have long been known, is the first report of twin-brush form.