Fig. 4 Catalytic kinetics of HRP-PAA immobilized alginate microfibers with different diameters or alginate contents. (a) Photos of substrate solution after catalysis with microfibers of 50 or 100 μm. (b) Photos of substrate solution after catalysis with microfibers prepared with 0.5% or 1% alginate. (c) Time dependent absorption (470 nm) of substrate solution after catalysis with microfibers of 50 or 100 μm. (d) Time dependent absorption (470 nm) of substrate solution after catalysis with microfibers prepared with 0.5% or 1% alginate.
We then prepared two kinds of microfibers with the same diameter (50 μm) and enzyme content but different alginate contents. The catalytic efficiency of HRP-immobilized microfibers was analyzed by the color observation and absorption at 470 nm (Figs 4b and 4d ). Data showed that microfibers prepared with 1% alginate demonstrated slightly lower catalytic activity those prepared with 0.5% alginate. More alginate in the inner phase of preparation led to more crosslinking and compact internal structure of microfibers, which may hinder the exchange of substrates and products during catalysis. However, the mechanical property of microfibers dramatically decreased with the content of alginate. A higher alginate content is usually required for the fabrication of stable microfibers for reuse.
3.4 Effects of pH and temperature on the catalytic activity of microfibers
The pH dependent catalytic activities of free and immobilized GOX were studied at pH 4-9. Fig. 5a showed that free and immobilized GOX-PAA exhibited optimum pH at 4 and 6, respectively, which is similar to the report in the literature (Todea et al., 2021). The additional interaction between GOX and alginate or PAA, in particular electrostatic interaction is pH dependent. The catalysis-related conformation of GOX-PAA may be altered with pH, resulting in different optimal pH for free and immobilized enzymes (Bedade et al., 2019; Hanefeld et al., 2009).