Figure 2. FTIR spectra of VG, ALG, VGALG, and VGCB8ALG hydrogels in KBr
solids.
Notably, the peak for (NH2) primary amine
stretching[21] at 3202 cm–1 and
bending at 1634 and 1596 cm–1 (in VG) has evolved
into the amide (CONH) peak at 3329 cm–1 and 1627 and
1574 cm–1, respectively (in the modified ALG).
Moreover, a significant enhancement in the intensity of the CH
stretching (in ALG)[22] at 2927 and 2850
cm–1 was observed upon its covalent modification by
VG or VGCB8. The two FTIR spectra measured for VGALG and VGCB8ALG
hydrogels were similar because encapsulation by CB8 does not usually
affect the vibration of the guest molecule’s
bonds.[24] Only peaks in the range from 1500 to
1400 cm–1 due to the stretching vibration of the
benzene rings of VG were slightly weakened.[24]
In the proton NMR spectra (aromatic region), ALG shows a single broad
resonance peak at δ = 12.30 ppm associated with the -OH group in ALG
(Figure S1 in the Supporting Information). The NMR spectrum for VG shows
five peaks, including one singlet δ = 5.30 ppm (for
-NH2) and four doublets with resonances at δ = 6.0,
6.75, 8.2, and 8.75 ppm.[25–27] A new peak at δ =
12.25 ppm appears when VG is linked to ALG due to forming an amide
linkage in the VGALG complex with a concomitant shift to lower ppm and
broadening of peaks b-e. As expected in similar reports, the peaks b-e
of the new hydrogels were further broadened (almost disappeared) upon
engulfing VG in CB8.[26] As additional evidence
for the non-covalent entrapment of VG by CB8 in DMSO solution, we
measured the UV–Visible absorption spectra of all alginate samples. We
noticed several isosbestic points at 250, 300, and 550 nm (Figure S2 in
the Supporting Information) in the spectra of the VG-modified ALG in the
absence and presence of CB8. The addition of CB8 also shifts the
absorption peaks from 245 and 475 nm to 250 and 490 nm, respectively.
Additionally, the data served us to confirm the covalent conjugation of
VG to ALG by the emergence of new peaks at 245 (250 with CB8), 275, and
475 nm (490 with CB8).
Figure 3 shows the TGA of VG, CB8, ALG, VGALG, and VGCB8ALG in the
temperature range from 35°C to 600°C at a heating rate of 10°C. The
traces provide information about the composition and thermal
decomposition of the new hydrogels VGALG and VGCB8ALG compared to
individual viologen and unmodified alginate
samples.[28] More importantly, the results confirm
the host-guest interaction between VG and CB8 on the ALG platform.