# Fabrication of Partially Etched Polystyrene Nanoparticles
## 1 Introduction  
Polymer nanoparticles (NPs) have gained significant attention in recent years due to their diverse range of applications in electronics [1], photonics [2], sensors [3; 4], and pharmaceuticals [5; 6], owing to their versatility and tunable properties. Depending on their shapes, polymer NPs can be classified into spherical, non-spherical, and mesoporous forms. Among them, non-spherical polymer NPs have shown significant potential in biomedical engineering, offering advantages over their spherical counterparts. Specifically, non-spherical NPs have demonstrated improved cellular uptake [7], enhanced drug delivery [8], and more efficient imaging contrast agents [9]. These features are in part attributed to the unique surface properties of non-spherical NPs, such as their increased surface area and surface roughness, which provide more opportunities for interactions with cells and biomolecules.  
Non-spherical NPs can be further classified into ellipsoidal, dumbbell, rod, disk, cube, and other non-spherical shapes [10; 11]. However, producing non-spherical NPs is challenging as they tend to prefer a spherical shape during synthesis to minimize surface energy. Recent advancements in particle manufacturing technology have led to significant progress in developing various types of non-spherical NPs. For example, ellipsoidal NPs were produced by mechanically elongating spherical polystyrene (PS) precursor particles upon heating them above their glass transition temperature [12; 13]. Dumbbell-shaped PS NPs were obtained by emulsion polymerization of styrene monomers in the presence of shape-directing agents [14; 15]. In addition, preformed mesoporous silica NPs were used as a template for synthesizing polymer NPs of various shapes [16]. Furthermore, microfluidic devices facilitated precise control over reactant flows and the size and shapeof the resulting polymer particles [17; 18]. However, research on partially and/or randomly deformed NPs manufacturing techniques has been limited. These particles can maintain the characteristics of spherical particles while increasing surface area and improving interfacial adsorption capability, which can enhance the stability of Pickering emulsions [19; 20]. Additionally, NPs with random shapes can be utilized as model particles mimicking microplastic particles found in nature for cellular toxicity studies that have caused serious environmental problems [21; 22].  
In this study, we focus on developing a simple protocol to fabricate partially etched PS NPs using emulsion polymerization and chemical etching. The degree of crosslinking was controlled by adjusting the time to add a crosslinking reagent during the polymerization reaction. Then, the obtained partially crosslinked PS NPs were chemically etched to selectively dissolve weakly crosslinked portions. These particles with increased surface area were evaluated by using them as solid surfactants to form Pickering emulsions. Note that Pickering emulsions are stabilized by solid particles adsorbed at a fluid-fluid interface, which are more stable and robust than traditional emulsions stabilized by molecular surfactants [23; 24]. Thus, developing solid surfactants with tailored properties is of great interest for the stabilization of Pickering emulsions. Our approach provides insights into the potential of partially etched NPs as solid surfactants in emulsion stabilization. Furthermore, the ability to control the degree of surface roughness and surface area of the NPs offers new opportunities for developing novel materials with tailored properties for various medical and environmental applications.