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.