Figure 2 1H NMR data of beta-CD (a), aldehyde group functionalized beta-CD (b), crosslinked beta-CD nanocage (c) and crosslinked beta-CD nanocage loaded with DOX (d).
The assembly process of the beta-CD nanocage was further examined using FTIR. For beta-CD, the broad peaks at 3300 cm-1 and 1420 cm-1 are assigned to the OH stretching and bending vibrations, respectively. The absorption peak at 2920 cm-1 corresponds to CH stretching vibrations, while the prominent peak at 1020 cm-1 is attributed to the CO stretching vibration of the glucose unit25[]. Upon oxidation of beta-CD, a clear characteristic peak at 1740 cm-1 emerged, corresponding to the C=O stretching vibration of the CHO group2426[, ]. Another prominent peak at 2730 cm-1 is associated with the CH stretching vibrations of the CHO group. Other characteristic peaks remained relatively unchanged compared to pure beta-CD. After crosslinking with cystamine, the characteristic peak of the CHO group weakened, indicating the reaction between the aldehyde group-functionalized beta-CD and cystamine. Additionally, two new absorption peaks appeared at 1650 cm-1 and 570 cm-1, respectively assigned to the C=N stretching vibration and the SS bond, indicating the successful introduction of the acid-sensitive imine and reduction-sensitive disulfide bonds. Following the loading of DOX into the beta-CD nanocage, the DOX signal was observed, including the absorption peak at 3000 cm-1assigned to the CH stretching vibrations of the DOX benzene ring, the absorption peak at 3300-3500 cm-1 assigned to the OH and NH stretching vibrations of DOX, the absorption peak at 1750 cm-1 assigned to the C=O stretching vibration, and the absorption peaks at 1650 cm-1 and 1500 cm-1 assigned to the C=C stretching vibration of the benzene skeleton. The FTIR spectrum of pure DOX is also provided in Figure S2. Some characteristic peaks overlapped between DOX and the beta-CD nanocage, but the loaded DOX signals were distinguishable27[]. Through the combination of FTIR and 1H NMR analysis, we were able to identify the components of the beta-CD nanocage and its derivatives.