Figure 3 SEM (a-c) and TEM (e-g)
images of MTCFs (a, e),
BSA-MTCFs@SIP (b, f),
MTCFs@SIP@CBMA (c, g); SEM images of MTCFs@NIP@CBMA (d); EDS analysis of
MTCFs@SIP@CBMA: Full spectrum(h), C
element (i), N element (g), O
element (k), Fe(l)
Composition analysis
To confirm the composition of the polymer shell and prove the synthesis
of MTCFs@SIP@CBMA, we systematically characterized the products at all
steps. The FT-IR spectra of MTCFs, BSA-MTCFs@SIP, BSA-MTCFs@SIP@CBMA,
MTCFs@SIP@CBMA, and MTCFs@NIP@CBMA are shown in Figure 4a. Compared with
the infrared curves of MTCFs, the absorption peaks of BSA-MTCFs@SIP,
BSA-MTCFs@SIP@CBMA, MTCFs@SIP@CBMA, and MTCFs@NIP@CBMA at 1460
cm-1 were attributed to the aromatic ring on dopamine,
which proved the successful coating of imprinted polymer.
BSA-MTCFs@SIP@CBMA, MTCFs@SIP@CBMA, and MTCFs@NIP@CBMA emerged
absorption peaks at 1734 cm-1 attributable to -CO-O on
CBMA, indicating that CBMA anti-protein adsorption segments were
successfully grafted on the surface of
BSA-MTCFs@SIP. Furthermore, the
infrared spectra of BSA-MTCFs@SIP and BSA-MTCFs@SIP@CBMA both exhibited
absorption peaks at 2568 cm-1. This band can be
assigned to -SH on BSA, confirming
the successful imprinting of BSA on the surface of fibers. After
elution, the absorption peak of -SH in the infrared spectrum of
MTCFs@SIP@CBMA disappeared. It was basically consistent with the
absorption peak of MTCFs@NIP@CBMA, indicating that BSA was completely
eluted.
The composition of organic and inorganic components in the nanofibers
and the thermal stability of the nanofibers were analyzed by
thermogravimetric analysis. The measured TGA curve is presented in
Figure 4b. It was clear to see that all the samples had no obvious
weight loss before 200℃, indicating superior thermal stability of
nanofibers. As the temperature increased, MTCFs, BSA-MTCFs@SIP, and
BSA-MTCFs@SIP@CBMA lost weight at different rates. MTCFs began to
exhibit significant weight loss at 328 ℃ and was completely degraded at
369 ℃. The weight loss rate was 86.18%, which was caused by the
combustion of carbon under oxygen. The degradation of BSA-MTCFs@SIP
began at 268℃ and was complete at 387 ℃, with a weight loss rate of
89.11%. The reason why BSA-MTCFs@SIP demonstrated lower pyrolysis
temperature and higher weight loss rate than MTCFs was that the coating
of PDA on its surface. Compared with BSA-MTCFs@SIP, BSA-MTCFs@SIP@CBMA
began to degrade at 237 ℃ and completely degraded at 390 ℃. The weight
loss rate was 91.39%. The reincrease in weight loss rate confirmed the
successful grafting of CBMA polymer segments on the surface of
BSA-MTCFs@SIP.
The specific composition of the magnetic nanoparticles loaded in the
tube of carbon nanofiber was analyzed by XRD. As shown in Figure 4c,
three diffraction peaks appeared in MTCFs at diffraction angles 2θ of
44.67°, 65.02° and 82.33°, corresponding to (110), (200) and (211)
crystal planes, respectively. According to the PDF card, the diffraction
peak data was consistent with that of Fe (PDF 87-0721) with structure of
body-centered cubic. Therefore, it could be inferred that the magnetic
nanoparticles present in the carbon nanofiber tube were Fe
nanoparticles. Additionally, the XRD diffraction peak positions of
BSA-MTCFs@SIP and MTCFs@SIP@CBMA are basically consistent with that of
MTFs. This meant that the coating process of PDA and the subsequent
grafting of the CBMA anti-protein adsorption segments had no effect on
the crystal form of Fe.
The magnetic properties of MTCFs, BSA-MTCFs@SIP, and MTCFs@SIP@CBMA were
measured by VSM. The results are shown in Figure 4d. It can be seen that
all the samples possessed magnetic properties. The specific saturation
magnetization of MTFs was 8.29 emu/g, while the specific saturation
magnetization of BSA-MTCFs@SIP and MTCFs@SIP@CBMA were 7.25 emu/g and
6.21 emu/g, respectively. The decrease of magnetic susceptibility was
due to the fact that the magnetic responsiveness of the materials was
weakened to some extent by both the coating of PDA and the grafting of
CBMA polymer chains. Fortunately, this magnetic performance was
sufficient to achieve rapid separation of the samples from the solution.
Thus, the separation efficiency was improved.
Figure 4 FT-IR spectra of products at each stage (a); TGA curves of
MTCFs, BSA-MTCFs@SIP and BSA-MTCFs@SIP@CBMA (b); XRD (c) and VSM (d)
curves of MTCFs, BSA-MTCFs@SIP and MTCFs@SIP@CBMA
Characterization of specific surface area and pore performance
High specific surface area could increase the number of effective
imprinted sites, thus improving the adsorption capacity. Therefore,
nitrogen adsorption apparatus was applied to measure the specific
surface area and pore properties of the materials. The nitrogen
adsorption-desorption curves and pore size distribution curves of the
samples are given in Figure 5. According to the nomenclature of the
International Union of Pure and Applied Chemistry, the adsorption and
desorption curves of MTCFs, BSA-MTCFs@SIP, BSA-MTCFs@SIP@CBMA, and
MTCFs@SIP@CBMA all belonged to type IV. The hysteresis loop type was H4,
indicating the slit-like pores in materials. It was worth noting that
the specific surface area of the fibers decreased gradually as the
procedure of PDA coating and CBMA polymer grafting progressed (see Table
1). This was because part of the pores of the fiber were blocked by the
coating of the polymers. After elution, the specific surface area of
MTCFs@SIP@CBMA increased compared with BSA-MTCFs@SIP@CBMA, confirming
the formation of imprinted sites on the surface of fibers. From the pore
size distribution curve, it was found that the pore size of fibers was
mainly divided into two sections, 4-40 nm and 40-100 nm. The pores in
the range of 4-40 nm were attributed to the small pores on the tube wall
of fibers and the imprinted cavities left by the eluted templates. This
part of the pores allowed BSA molecules to diffuse into the inner cavity
of the tube. The range of 40-100 nm corresponded to the diameter of the
cavity in the tube. The large-scale cavity caused an osmotic pressure
difference between the solution inside and outside the tube, which in
turn gave MTCFs@SIP@CBMA self-driven adsorption performance. The
nitrogen adsorption-desorption curve type and pore size distribution of
MTCFs@NIP@CBMA were consistent with those of MTCFs@SIP@CBMA (see Figure
S1). Nevertheless, due to the absence of imprinting sites on its
surface, the specific surface area (170.53 m2/g) and
pore volume (0.02 cm3/g) of MTCFs@NIP@CBMA were lower
than those of MTCFs@SIP@CBMA.
Figure 5 N2 adsorption-desorption curves and pore size
distribution curves of MTCFs (a, b), BSA-MTCFs@SIP (c, d),
BSA-MTCFs@SIP@CBMA (e, f) and MTCFs@SIP@CBMA (g, h)
Table 1 Specific surface area and pore performance data of samples