Abstract:

Bio-stimuli-responsive microspheres, which can encapsulate and release actives in response to physiological triggers, have attracted increasing attention in pharmaceutical, cosmetic, food biotechnology, and agricultural industries. However, most microspheres are based on synthetic polymers and suffer from a lack of biocompatibility due to the residues of harsh organic solvents or crosslinkers used in the synthesis process. Herein, we develop a simple and sustainable method for the construction of proteinaceous microspheres templated from Pickering double emulsions. Specifically, silica nanoparticles with a diameter of 100 nm were synthesized by Stöber method and modified by reacting with dichlorodimethylsilane. The Pickering emulsions are stabilized by hydrophobic silica nanoparticles, while zein protein is dissolved in the middle phase. Subsequent ethanol removal from the emulsion template precipitated the protein skeleton. First, we stained the aqueous ethanol phase with rhodamine B and the oil phase with pyrene to demonstrate the formation of double emulsions by confocal laser scanning microscopy (CLSM). The morphology of microspheres and silica nanoparticles was characterized by scanning electron microscopy (SEM). The obtained microspheres showed high sphericity and uniformity. In addition to acting as particulate stabilizers, the silica nanoparticles could improve the mechanical strength and monodispersity of microspheres. Herein, fluorescein isothiocyanate (FITC)-labeled dextran was chosen as the model active for encapsulation into microspheres. The CLSM images showed that it was uniformly dispersed in the microspheres and had no effect on the structure of the microspheres. Next, we investigated the pH tolerance of the microspheres. Through optical microscope, it was noted that the structure was intact under pH 3–11, and thus, it has a high resistance. Finally, we investigated the bio-stimuli-responsive behavior of microspheres. Zein is rich in sulfur-containing amino acids, which can form intra- and inter-molecular disulfide bonds. Because disulfide bonds can be reduced by glutathione (GSH) and the protein itself has enzymatic hydrolysis characteristics, the proteinaceous microspheres can be triggered release in response to GSH and protease. The release profiles of FITC-dextran from microspheres at different concentrations of GSH and protease were evaluated by fluorescence spectrophotometer. The decomposition behavior of microspheres under certain concentrations of GSH and protease was further verified by CLSM and SEM. To conclude, excellent stability and tunability of emulsion templates render the resulting proteinaceous microspheres with adjustable structures. Meanwhile, the proteinaceous microspheres have high encapsulation efficiency of model actives and have shown excellent bio-stimuli-responsiveness to protease and glutathione.

Key words: Bio-stimuli-responsive, Proteinaceous microsphere, Pickering emulsion, Protease, Glutathione