With the pursuit of safe and green processing, organic synthesis catalyzed by enzymes has attracted great concern (Reetz, 2013). Compared with homogeneous systems, non-homogeneous systems are more widely used because of the preferred solubility of substrates in organic solvents and the need for enzymatic activity maintenance in water. Among non-homogeneous systems, the biphasic system is the most classical and well-studied (Adlercreutz, 2013). However, this system is still limited to low catalytic efficiency, ascribed to the inadequate contact between enzymes and substrates (Piradashvili et al., 2016). Although surfactants have been used to improve the solubility of organic substrates in water, they cause severe environmental stress and increase the difficulty of product purification. Pickering emulsion has been well developed to overcome these limitations and is considered a promising alternative (Wu et al., 2011).

Pickering emulsion systems for biocatalysis are stabilized by ultrafine solid particles instead of traditional surfactants (Ramsden, 1904; Pickering, 1907), where the solid particles (containing enzymes) are precisely positioned within this system at the interface of the aqueous or liquid-liquid phases. The contact between the oil and water phases forms a huge specific surface area, substantially enhancing the catalytic efficiency while minimizing the emulsifier requirement. The emulsion is characterized by its unique biphasic environment and separated droplet structure, which allows it to achieve chemical transformations even under challenging conditions. Wang et al. demonstrated the efficiency of the Pickering emulsion in an enzyme-catalyzed reaction, where the specific activity of Candida antarctica lipase B (CALB) encapsulated in the lumen of the polymer vesicles was amplified by 12.4 times compared to pristine CALB in a standard biphasic water/toluene system, underscoring the potential of Pickering emulsions in enhancing interfacial catalytic efficiency (Wang et al., 2012).

To maximize the potential of Pickering emulsions in biocatalysis, the precise selection and modification of materials and the innovative design of catalysts are particularly critical. For example, Sun et al. prepared conjugates by growing polymers on benzaldehyde cleavage enzymes to form Pickering emulsion further, resulting in a significant increase of catalytic efficiency by 270- and 9-fold when compared with the biphasic systems and the traditional Pickering emulsions, respectively (Sun et al., 2018). It illustrated the importance of the rational design of Pickering emulsion, which can contribute to maximizing catalytic efficiency. Furthermore, to overcome the limitations of high cost and demulsifying difficulty, novel Pickering emulsion systems with stimuli-responsiveness are well developed, such as systems responsive under pH (Yang et al., 2022), magnetic (Cheng et al., 2022), light (Li et al., 2021), and thermo (Jiang et al., 2022) stimulus conditions (Ni et al., 2022).

There have been a few reviews about the application of Pickering emulsions in chemical and enzymatic catalysis (Ni et al., 2022; Rodriguez and Binks, 2020). Considering the safe and green properties of biocatalysis, we provide a more detailed review to summarize recent reports about the application of Pickering emulsion in the field of biocatalysis, especially its design principles. We summarize the description, design, characterization, application, and future trends of Pickering emulsions in biocatalysis. The types and formation mechanisms of Pickering emulsion biocatalysis are first described. Then, design principles for Pickering emulsion biocatalytic systems are discussed, including material design, enzyme immobilization, emulsion formation control, and reactor design. Furthermore, detailed methods for system characterization are also discussed. Moreover, stimuli-responsive reaction systems are introduced, as well as the applications of Pickering emulsion in single-step, cascade, and continuous flow reactions. Finally, the challenges and future perspectives are emphasized to facilitate the design and application of Pickering emulsion systems in green biocatalysis.