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.