The utilization of the selfassembly concept to form nanoreactors originates from molecular self-assembly of different functional molecules into one molecular unit. In Figure ) it is illustrated how self-assembly facilitates the combination of different functional nanoparticles i.e. Au and CdSe nanoparticles into one unit. Apart from creating multiple functionalities, the process of self-assembly can also allow the formation of nano or micro reactor units. A straightforward example of this is the self-assembly of nanoparticles to form Pickering emulsions. In Pickering emulsions, nanoparticles encapsulate an oil phase in a continuous aqueous phase and in that way they can facilitate catalytic reactions in either phase. Such a system can be advantageous in different circumstances. For examples, as shown by Burdyny et al., self-assembled TiO2 nanoparticles forming oil-in-water Pickering emulsions are excellent for continuously running photocatalytic reactors as 100% photocatalyst retention can be achieved with 96% conversion efficiency, in contrast to simple nanoparticle suspensions with complete loss of photocatalyst to the outlet stream, Figure . Such biphasic systems become even more interesting with Janus nanoparticles, that have different functionalities or constitutions on different sides of a single nanoparticle. Cho et al. showed that Janus nanoparticles with a catalytic hemisphere assemble at the reactant-water interfaces to form Pickering emulsions, Figure ). In the self-assembly, the catalytic side decorated with catalytic nanoparticles, Ag or Pd, remains in the oil phase to accelerate a specific reaction. As shown in Figure , the conversion from 4-nitrophenol (indicated by UV-Vis band at 400 nm) to 4-aminophenol (UV-Vis band at 250 nm) inside these microreactors takes place efficiently. It is also shown that by further incorporation of magnetic Fe2O3 nanoparticles into the nanostructure facilitates fast and easy separation of the micro-reactors for product separation. Thus, complex separation and purification processes can be simplified to a great extent. In the same line of developments, Zhang et al. recently proposed a biomimetic Pickering emulsion based microreactor concept inspired by multi-compartmental living cells, Figure ). While hydrophobic silica nanoparticles (60 to 80 nm) are used as the emulsifier, mesoporous silica nanoparticles (80-90 nm) that are encapsulated in an ionic liquid (interior medium) act as the support for the metal complex catalysts or enzymes. The as-obtained biomimetic microreactors are then packed in a fixed-bed reactor for continuous flow cascade catalysis, exhibiting 5-to 420-fold enhancement in catalytic efficiency (CE), 99% ee (enantiomeric excess) as well as durability of 240 h in continuous operation. Similarly, Huang et al. demonstrated Pickering emulsions of self-assembled lead sulfide Janus quantum dots with ligand shells as nanoreactors for photocatalytic reactions. Another example is palladium/graphitic carbon nitride (g-C3N4) stabilized emulsions as microreactor for storing hydrogen from ammonia borane for the use in alkene hydrogenation Copper nanoparticles loaded polymer vesicles have also been demonstrated as environmentally amicable nanoreactors for the synthesis of benzimidazole via a cascade reaction. The vesicles exhibited a uniform size distribution, spherical morphology, excellent stability and efficient Cu loading. As in Pickering emulsions the nanoparticles at the interface are in partial contact with two different media i.e. the dispersed and the solvent medium, functionalization or additional formation on any of these two sides can lead to the formation of Janus nanoparticles. Selfassembly of nanoparticles in Pickering emulsions is thus a convenient strategy for the synthesis of such multi-faceted Janus nanoparticles. Figure . (a) Self-assembled TiO2 nanoparticles forming oil-in-water Pickering emulsions are excellent for high catalyst retention (100%) while continuous operation photocatalytic reactors. Reprinted with permission © 2016 Royal Society of Chemistry (b) Janus nanoparticles with a catalytic hemisphere assemble at the reactant-water interfaces to form Pickering emulsions with the catalytic nanoparticles, Ag or Pd, remaining in the oil phase to accelerate a specific reaction: 4nitrophenol (indicated by UV-Vis band at 400 nm) to 4-aminophenol (UV-Vis band at 250 nm). Incorporation of magnetic Fe2O3 nanoparticles facilitates fast and easy separation of the micro-reactors for product separation. Reprinted with permission © 2018 Royal Society of Chemistry. (c) A biomimetic Pickering emulsion based microreactor concept inspired by multi-compartmental living cells, exhibiting 5-to 420-fold enhancement in catalysis efficiency (CE). © 2022 CC-BY 4.0.