Figure 2 (a) Raman spectra (λex = 532 nm), (b) XPS spectrum, (c) High-resolution N 1s and (d) High-resolution C 1s XPS spectra of EDTA-CDs. (e) Raman spectra (λex = 532 nm), (f) XPS spectrum, (g) High-resolution N 1s and (g) High-resolution C 1s XPS spectra of UPy-CDs.
To elucidate whether UPy was covalently decorated or physical attached to the surface of CDs, thermogravimetric tests were conducted in N2 atmosphere. As shown in the thermogravimetric differential curve (Fig. 3a), EDTA-CDs feature two main weight loss peaks at 225 and 347 oC, corresponding to the decomposition of surface groups and the reorganization of carbon core [36]. For UPy-NCO, four sharp peaks are present at 172, 205, 300, and 460oC. The former two peaks could be ascribed to the cleavage of NCO groups, while the latter two peaks could be attributed to decomposition of UPy skeleton. When physically mixing EDTA-CDs and UPy-NCO together, the thermogravimetric curve of the mixture retains the main processes of components. In contrast, only the decomposition peaks of carbon core and UPy skeleton could be found for UPy-CDs, while the decomposition peaks of surface and -N=C=O groups are absent. Such phenomenon suggested that UPy was covalently decorated to the surface to form structurally robust UPy-CDs.
The chemical structures of CDs were further studied by1H NMR and Fourier transform infrared (FT-IR) spectra. As shown in Fig. 3b, the resonances at 7-8 ppm could be attributed to the proton of the aromatic structures in the carbon core, while the peak at 3-5 ppm could be attributed to the protons adjacent to electron negatively atoms (e.g., O, N). Besides these protons that are belong to the carbon core, olefin and alkyl protons were also found for UPy-CDs at 5-6 and 0.5-3 ppm, respectively [37-39]. Such protons are derived from the surface UPy moieties. Additionally, according to the FT-IR spectra (Fig. 3c), C=C/C=O and C-O/C-N stretching vibrations could be found at 1650 cm-1 and 1380 cm-1, respectively. Hydrophilic groups such as -OH and -NH2- were present at 3400 cm-1 for both EDTA-CDs and UPy-CDs [40]. It is worth noting that molecule 1 exhibit characteristic -C=N=O signal at 2283 cm-1[41], which is absent for UPy-CDs. This phenomenon suggested the complete consumption of isocyanate groups during the surface modification process. On this basis, temperature-dependent FT-IR spectrum was performed to characterize the hydrogen bonding interactions. The region of 1610-1500 cm-1 (Fig. 3d) arising from the C=O stretching and -NH- bending exhibit a substantial decrease in the peak intensity upon heating, accompanied by a slight blue-shift. These phenomena indicate the breakage of multiple hydrogen bonds and the generation of corresponding free groups. This result could also be reflected by the remarkable lowered signals of bonded amine and hydroxyl groups (3300 cm-1) under high temperature.