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.