Stimuli-responsive, protein hydrogels for potential applications in enzymology and drug delivery$$^{\S }$$

Abstract

Enhancing the stability of enzymes for sensing or biocatalysis applications is still an unmet challenge. Ordinary paper is a very attractive support for anchoring enzymes but enzyme attachment to cellulose without surface activation is still another challenge. To make progress toward these goals, we developed a simple method to prepare highly active and stable enzyme-hydrogels within the mesh of the cellulose fibers of paper. A mixture of the desired enzyme, bovine serum albumin (BSA) and arginine were reacted with carbodiimide to form stable hydrogels. A set of critical concentrations $$(\hbox {BSA}([\hbox {BSA}]_{0}) \,{\ge }1~\hbox {mM})$$ , $$[\hbox {carbodiimide}]_{0} \ge 100$$ mM and [amino acid] $$\ge 100$$ mM) were required to form transparent hydrogels. The thermal reversibility of gelation proved that the gels are stabilized by non-covalent bonding interactions between the BSA oligomers that were formed via covalent interactions. Both dynamic light scattering and SDS-PAGE studies, under pre-gelation conditions, support idea that one BSA oligomeric unit contained 40–70 protein molecules. Scanning electron micrographs, thermogravimetry and swelling studies suggest that the formation of water cavities inside the cross-linked gel matrix, where the water mass was 7–8 times higher than that of the protein and the free amino acid used as a linker/spacer. Due to the higher water content and benign gelation conditions, active enzymes could be incorporated into the gel structure during the synthesis. Hydrogels, thus, embedded with glucose oxidase (GOx) and horseradish peroxide (HRP) showed catalytic activity towards glucose, where efficient channeling of hydrogen peroxide from GOx to HRP was observed (70% efficiency in initial rate compared to free enzymes in solution). Moreover, the enzymes retained their activity after pasting the hydrogel onto ordinary paper, which was demonstrated as a glucose sensing platform with a detection limit of 5 mM glucose. Trypsin embedded in the gel showed temperature dependent self-degradation by utilizing optimum protease activity at $$37\,{^{\circ }}\hbox {C}$$ . The temperature-triggered degradation of the gel can be used as a drug delivery vehicle, which was demonstrated using a reporter dye. The hydrogel made of a completely proteinaceous material that releases drugs at body temperature but bound to the matrix at room temperature ( $$25\,{^{\circ }}\hbox {C}$$ ) is useful for noninvasive drug delivery platforms. The biocompatibility and non-thermal synthetic route for the hydrogel makes it a superior material for incorporation of temperature sensitive enzymes, drug molecules or nucleic acids, for a diverse set of applications. Synthesis, characterization and applications of multifunctional, biologically benign, protein-derived hydrogels are reported here. Protein hydrogels are formed by both covalent and noncovalent interactions between amino acid residues of the constituents. Active enzymes embedded inside the gel matrix were utilized for biosensing and drug delivery applications.