4.3.1. Key Role of Pore Size

Porous materials are attractive and versatile supports for enzyme stabilization. Once encapsulated, the enzyme should be protected from the environment, while being able to be attached via suitable chemical modification of the walls of the pores. The chemical conditions used for the porous material synthesis, as well as pore size and interconnectivity in the matrix for diffusion of substrate and product in and out, while ensuring that the protein cannot diffuse out are main issues to be considered. Depending on the materials and the synthetic procedure, the diameter of pores can be tuned and adapted to a specific enzyme. We should distinguish here micro, meso and macroporosity that are described by the diameter of pores being, respectively, less than 2 nm, between 2 and 50 nm and more than 50 nm. While mesopores are required for enzyme encapsulation, macroporosity will enable substrate diffusion. Hence, getting a material with hierarchical porosity is highly desirable in most cases. In general, pore size close to enzyme size favors stabilization. Multipoint interactions may be one explanation for such a result [138]. It was also suggested that modification of the water structure in nano-containers can induce higher enzyme stability [23]. Given the average size of enzymes in the range of about 3–10 nm, some microporous materials with controlled pore size and geometry are unfortunately not suitable for enzyme encapsulation. This is the case of silica films presenting vertically aligned pores [139], or of the well-known silica SBA15 porous matrix [140]. However, porous materials, with pore size compatible with enzyme diameter, can be obtained by different strategies [141]. Carbonization of MgO-templated precursors yields a pore size-tunable material (from around 30 to 150 nm) with interconnected mesopores [142]. Increased half-life stability was shown when the pore size was the closest to the hydrodynamic enzyme diameter [143].
