# Microsphere Polymers in Molecular Imprinting: Current and Future Perspectives
## 3 Preparation of Molecularly Imprinted Microspheres
### Pickering Emulsion Polymerisation  
Pickering emulsion uses dispersed droplets that are stabilised with solid particles, either oil-in-water or water-in-oil (inverse Pickering emulsion). The use of Pickering emulsions must consider the possibility of coalescence between droplets, so stabilising particles are on the surface of droplets [136; 137]. The Pickering emulsion method has been used in cosmetics preparation, oil purification and wastewater treatment [138]. The inverse Pickering emulsion is used to produce spherical hydrogel particles [79] (see Table 1). The formation of MIMs requires a template, which is the target compound. Porogens are used as solvents according to template solubility. Commonly used porogens include organic compounds such as toluene [87]; SiO\({}_{2}\) is important in the Pickering emulsion method as an emulsion stabiliser [50]. Another ingredient that plays a significant role in the formation of fine polymer emulsions is the surfactant. A surfactant that has been used in several studies is Triton-X [48; 87]. [PERSON] et al. [87] showed that the mass of SiO\({}_{2}\) used can affect the diameter of the particles produced. Polymers with 5 mg SiO\({}_{2}\) resulted in irregular spherical particles, whereas with 15 mg SiO\({}_{2}\), a regular shape was formed but deemulsification still occurred. The use of more than 30 mg of SiO\({}_{2}\) resulted in particles with a narrower size distribution, indicating that the particles had a uniform size. The more SiO\({}_{2}\) added, the smaller the diameter of the particles. Experiments by [PERSON] and [PERSON] [139] showed that unmodified silica can be used to stabilise oil in water (O/W) emulsions due to the hydrophilicity of Si-OH groups, whereas hydrophobic modified silica can be used to stabilise water in oil (W/O) emulsions. Modified silica can be produced by mixing dried silica and toluene, then adding trimethylsilyl chloride slowly at room temperature. The mixture was evaporated and then stored under argon atmosphere [79]. Besides SiO\({}_{2}\), lignin can be used as an emulsion stabiliser. However, lignin is hard to eliminate and decompose. However, the exploration of methods utilising lignin has attracted extensive attention [117]. The formation of microsphere polymers using the Pickering emulsion method can produce particles with a more controlled size. This method can also produce either hydrophilic or hydrophobic polymers [138]. The MIM surface is regular, with a uniform pore size distribution and an impressive selective polymer to Malachite Green (MG), despite its lower adsorption capacity as a result of the smaller specific surface area [48]. Varying the porogen volume also contributes to good MIM production. Based on experiments by [PERSON] et al. [87], using different volumes of toluene as the porogen (1.6, 2.6, 3.6, 4.6 and 5.6 mL) resulted in different polymer properties. MIMs with lower and higher porogen volumes had a lower imprinting factor (IF), whereas polymers produced with 3.6 mL as the porogen volume showed the highest IF value. Polymers produced with a porogen volume less than 3.6 mL had fewer accessible recognition sites for the target molecule. A small amount of emulsifier was also needed to avoid demulsification, since the stability of the oil-water phase is affected by the template molecule. The water phase consisted of the monomer, emulsifier and nanosilica (Figure 5). Sonication treatment was important to disperse SiO\({}_{2}\) into the water. The oil phase consisted of a crosslinker, template molecules, porogen and initiator as the pore forming agent. Sonication was also performed in this step to homogenise the oil phase mixture. The water and oil phases were then mixed together with agitation. The free radical polymerisation of the monomer was done in a water bath at 70 \({}^{\circ}\)C for 16 h triggered by the initiator. The sinking microspheres were cleaned by dipping into acidic solution to remove the silica particles. The last step was template extraction, which was achieved with Soxhlet extraction [48; 87; 138].