Bio-inspired mini-eggs with pH-responsive membrane for enzyme immobilization

A novel family of bio-inspired mini-eggs with a pH-responsive membrane for enzyme immobilization has been designed and prepared by a simple and mild method combining co-extrusion minifluidics, adsorption and biosilicification. The membrane of the proposed mini-eggs is composed of a Ca-alginate/protamine composite layer encapsulated by a silica surface layer, in which the former provides the pH-responsive switching function driven by electrostatic interactions between Ca-alginate networks and protamine molecules, and the latter offers protection for the former to guarantee the stability of pH-responsive performance. When the ambient pH value is lower than a certain threshold (called critical pH value), the enzymatic reaction in the mini-eggs is inactive due to the low diffusional permeability of substrates and products across the membrane; however, when the pH value is higher than the threshold, the enzymatic reaction in the mini-eggs becomes active due to the high diffusional permeability across the membrane. The pH-responsive switching action of the mini-egg membrane is rapid, reversible and reproducible. The proposed mini-eggs provide a new mode of "smart" or "intelligent" controllable mini-systems for pH-responsive enzymatic reactions, which is highly attractive for enzyme catalytic reaction, cell and tissue culture, sensors, etc.