Figure 1 . SEM images of
OSMD (A, B, C) and CRL@OSMD (D,
E, F)
Figure 2. A shows the FTIR spectra of the original diatomite,
MD, and OSMD. According to the FTIR diagram of the raw diatomite, the
absorption peaks of 3622.3
cm-1, 3450.0 cm-1, and 1639.7
cm-1 are formed
by the stretching vibration of -OH. The absorption peaks at 1025.2 and
792.4 cm-1 correspond to the stretching vibration of
Si-O-Si; the bending vibration of the Si-O bond forms the absorption
peak at 451.2
cm-1.
Instead, there are some new peaks in the FTIR of modified
diatomite. The absorption peaks
at 2925.5 cm-1, 2850.1 cm-1, and
1463.1 cm-1 in the infrared spectrum of modified
diatomite correspond to the stretching vibration and bending
vibration of the C-H bond in
-C18, which is consistent with the study of Li et
al[35]. The peak at around
1396.5 cm-1 and
1140.0 cm-1 is caused by the vibration of S=O in
-SO3H. The
surface modification of the diatomite with alkyl groups can be
demonstrated by the Si-C
associated peaks appearing at 720.3 cm-1, which is
consistent with the work reported by Xu et al[36]. All the above
results indicated that the diatomite was successfully modified.
The magnetization of OSMD and
CRL@OSMD was measured using a Vibrating Sample Magnetometer (VSM) at
room temperature (Figure 2. B ). The remanent magnetization and
coercivity of OSMD and CRL@OSMD were almost zero, indicating that the
sample materials are soft magnetic materials with superparamagnetic and
outstanding magnetic properties[37]. The specific saturation
magnetization of OSMD and CRL@OSMD were 12.2 emu·g-1and 12.1 emu·g-1, respectively. There is almost no
difference between the two, which indicates that the immobilization
process does not affect the magnetic properties of the support. The
exceptional magnetic response helps to recover the immobilized enzyme
under the application of a magnetic field, providing valuable support
for the reuse of biocatalysts.
In addition, we employed the N2 adsorption-desorption
isotherms to provide evidence for the successful immobilization of CRL
to the OSMD (Figure 2. C ). As shown in Figure 2.D , the
overall pore size of CRL@OSMD was significantly decreased compared to
OSMD which may be attributed to that the CRL immobilized into the pore
of OSMD and thus decreased the pore size of it. Moreover, the results
showed that the Brunauer-Emmett-Teller (BET) surface area of OSMD
decreased from 53.6
m2·g-1to 27.2 m2·g-1 after the
immobilization process, accompanied by a corresponding reduction in pore
volume from 0.227
cm3·g-1to 0.176 cm3·g-1 (Table
S1 ). These findings indicate that a substantial portion of the enzyme
occupies the diatomite channels, thereby achieving the effective
immobilization of CRL on OSMD.
Figure
2 . FTIR spectrum of diatomite before and after modification (A), VSM
plot (B), N2 adsorption and desorption isotherms of
OSMD and CRL@OSMD (C), BJH-plot
(D)
Figure 3 compares the XPS spectra of OSMD, MD, and raw
diatomite. The peaks of 100.1 eV
(Si 2p), 259.2 eV (O 1s), and 150.5 eV (Si 2s) are formed by the
diatomite, and the peak intensity is significantly weakened after
modification, which corroborates the change in the surface groups of the
diatomite. Relatively, the peak intensity of C 1s (281.8 eV) was
significantly enhanced after modification. Despite the weak intensity,
the peak at around 161.0 eV (S
2p) and 225.0 eV (S 2s) is caused by -SO3H. Compared to
raw diatomite, nanomagnetic Fe3O4increased the peaks located at 710.5 eV and 719.9 eV[35].Table 1 shows that the C
peak intensity of modified
diatomite is significantly
enhanced, and the content of C is increased from 15.8% to 66.1%.
However, the intensity of the O1s peak
weakened, and the O content
decreased from 61.8% to 22.6%. It is proved that the coating contains
a large amount of -C18, and -SO3H
successfully modified diatomite. In the high-resolution diagram of Si
2p, the electron absorption peaks at 100.4 eV and 99.8 eV correspond to
the Si-O bond and Si-Si bond in diatomite[38]. Compared with
unmodified diatomite and MD, the absorption peak at 98.9 eV corresponds
to the Si-C, which demonstrates the surface modification of the
diatomite with octadecyl groups[39].