## 1 Introduction  
Zinc oxide nanoparticle (ZnO NP) is one of the most promising materials and has been widely used in various applications, such as chemical sensors, photovoltaic solar cells, piezoelectric/luminescent devices, textiles, and catalysts [1]. Apart from its wide band gap (3.4 eV) and large exciton binding energy (60 meV), other attractive properties of this low-cost semiconductor include biosafety, antibacterial activity, UV shielding, white appearance, and anti-corrosion. Similar to the majority of traditional inorganic UV absorbers, ZnO NPs possess high surface energy and often agglomerates, which deteriorates the aforementioned performances. However, the development of transparent UV-shielding materials, concerning solid NPs with high dispersion, is required [2, 3]. Properties of these NP-incorporating shielding materials are mainly determined by the types of NP, the particle size, and the morphology. Core-shell structured NPs, especially the type of core/shell, the shell thickness, as well as the difference in the refractive indices (\(\Delta\)n) between the interior and the solid shell, all affect the shielding efficiency [4; 5]. Absorption or transmission measurement can assess the shielding performance of NPs when applied in coating. The advantages of hollow NPs, an air void surrounded by a solid shell possessing lower density, multi-light scattering, and a high surface area, are therefore mentioned [5]. Generally, to lower cohesive forces between ZnO NPs, their surface is modified, mainly via two processes; (i) chemical adsorption of small capping and coupling agents, e.g., 3-(trimethoxysil)-propyl methacrylate (MPS), 3-aminopropyltriethoxysilane (APTES), and oleic acid (OA), and (ii) grafting with polymers, e.g., poly (methacrylic acid) [1; 6; 7]. These surface modifications further allow for the preparation of ZnO polymeric nanoparticles (ZPP), i.e., a polymer particle containing ZnO NPs [8; 9; 10]. As with other inorganic NPs, e.g., magnetic (M) NPs, not only does the content of the NP influence the property of the hybrid material, but so too does their position in the composite polymer particle. The polymer shell of magnetic microsphere is of vital importance for the composite to function in the biological conditions that require low toxicity, biocompatibility, and specific bioactive functionalities. ZnO-polystyrene (PS) core-shell particles, with high stability and good dispersion, provided perfect antibacterial film, which would be potentially useful in the field of coatings [9], whereas Janus and yolk-shell hollow PS-ZnO particles, having quantum dots as the co-seeds with PS, showed stable and strong fluorescence [10]. Due to the high sensitivity of ZnO NPs, as a Pickering stabilizer of the PS core, these ZPPs could react with either acid or base and, hence, could be a good pH adjusting material [11]. Among various processes for fabrication of ZPPs, the miniemulsion polymerization (mini-EP) is one of the most effective, facile, and straightforward ways to produce spherical particles with a readily controllable morphology. The mini-EP process provides the ability to control the particle size via the formation of miniemulsion (regarded as individually acting nanoreactors) and to nucleate all the droplets, containing NPs, of similar size and composition [12]. To achieve the desired architecture, relevant parameters, e.g., type and amount of surface modifiers of ZnO NPs that influenced the size and morphologies of the ZPPs, were focused on. By using ZnO NPs modified with various amounts of OA (OA-ZnO NPs), different morphologies of ZPPs made of PS were obtained. With a larger OA content (47 wt%; determined from thermogravimetric analysis (TGA)), yolk-shell ZPPs was formed because of the more uniform dispersion of OA-ZnO NPs in the St emulsion [10]. Whereas a Janus-like structure was obtained when using OA-ZnO NPs with a lower OA content (11 wt%).