Dopamine-Modified Alginate Beads Reinforced by Cross-Linking via Titanium Coordination or Self-Polymerization and Its Application in Enzyme Immobilization

Two novel kinds of dopamine-modified alginate beads were developed by using titanium(IV) coordination or self-polymerization, besides the conventional Ca2+ ion cross-linking. Alginate was modified with dopamine (AlgDA) via EDC/NHS chemistry. Fourier transform infrared spectroscopy (FTIR) and circular dichroism (CD) characterization confirmed that dopamine was covalently attached preferentially to the mannuronic acid residues of alginate; thus, gel-forming ability of the as-prepared AlgDA was well retained. The titanium(IV) coordination-reinforced alginate beads [Ca–Ti(AlgDA)] were prepared by cross-linking with Ti4+ and Ca2+. Covalently cross-linking-reinforced alginate beads (Ca–AlgPDA) were also prepared by self-polymerization of dopamine and cross-linking with Ca2+. The swelling of Ca–AlgPDA and Ca–Ti(AlgDA) were both obviously inhibited, and the mechanical properties were enhanced by 3 times compared to those of Ca–Alg beads. The as-prepared beads were utilized for immobilization of alcohol dehydrogenase (YADH). The immobilization efficiency of Ca–Ti(AlgDA) and Ca–AlgPDA reached up to 100% and 89%, respectively, both notably higher than that of Ca–Alg (67.4%). Stabilities of the immobilized YADH in Ca–AlgPDA and Ca–Ti(AlgDA) toward pH, storage, and recycling were all improved compared with those immobilized in Ca–Alg.