Fig. 3 Scanning electron microscopy (SEM) images of microfibers. (a) and (b) SEM images of microfibers with the diameter of 50 and 100 µm prepared with alginate only. (c) and (d) SEM images of GOX-PAA immobilized alginate microfibers before catalysis use and after six cycles of utilization. Scale bar = 50 µm.
The diameter and surface topography of microfibers did not change when GOX-PAA was added into alginate phase (Fig. 3c) . After repeating use for 6 cycles, the diameter decreased by ~10 μm but the integrity of microfibers remained (Fig. 3d) . The shrinkage in size may be ascribed to the slight dissociation of enzymes, enzyme-PAA, or alginate and the reorganization of instantaneously crosslinked polymer-protein blends during the catalysis reaction. In general, the microfibers composed of alginate and enzyme-PAA held outstanding stability and reusability for enzyme immobilization.
3.3 Effects of diameter and alginate content on the catalytic activity of microfibers
One of advantages of the microfluidic fabrication of microfibers is the capability of tuning the diameter and the composition of microfibers, which affect the catalytic performance of immobilized enzymes. Here we prepared microfibers with different diameters or alginate contents and tested the catalytic activity at the same enzyme concentration. The catalytic ability of HRP-PAA immobilized microfibers could be demonstrated with the kinetics of the conversion of guaiacol into tetraguaiacol (Felisardo et al., 2020). Figures 4a and 4c showed that enzyme-loaded microfibers with the diameter of 50 μm enabled more rapid increase in the color and the absorption at 470 nm comparing with the thicker ones (100 μm), suggesting more efficient conversion of guaiacol into tetraguaiacol by the catalysis with thinner microfibers at the same amount of HRP. The diffusion barriers of substrates and products decreased with the diameter of microfibers, therefore facilitating the mass exchange between the intra-microfiber and the aqueous phase for thinner microfibers. Moreover, the relative surface area of thinner microfibers is larger that of thicker microfibers, which is beneficial for the catalytic reaction.