3. Results and discussion
3.1 Physical and chemical properties of beta-CD nanocage
An acidity/reduction-sensitive beta-CD nanocage was assembled using cystamine dihydrochloride via a Schiff base reaction. The unique structural unit of the nanocage was the aldehyde group-functionalized beta-CD. To obtain this unit, a mild and efficient oxidation of the OH-6 of beta-CD was carried out at room temperature using IBX, resulting in excellent yields. Figure 2 shows the 1H NMR spectroscopy analysis of the pre- and post-oxidation of beta-CD. The chemical shifts of protons in the glucose units of the primitive beta-CD were observed, such as 5.75 (OH-2), 5.69 (OH-3), 4.83 (H-1), 4.47 (OH-6), 3.63 (H-6a,b), 3.57 (H-3), 3.55 (H-5), 3.34 (H-4), and 3.30 (H-2). After oxidation, new peaks appeared at 9.5 ppm, and the OH-6 peaks weakened at 4.47 ppm, which were attributed to the aldehyde proton peaks24[]. Comparison of the results indicated the successful preparation of aldehyde group-functionalized beta-CD using IBX oxidation. Subsequently, we used cystamine to crosslink the aldehyde group-functionalized beta-CD via an aldehyde-amine reaction. As shown in Figure 2 (c), the aldehyde proton peaks at 9.5 ppm disappeared, indicating the nearly completed reaction between the aldehyde of beta-CD and the amino group of cystamine. The1H NMR spectrum of cystamine is shown in Figure S1. Due to the stacking of multiple glucose units, the proton peaks overlapped rather than appearing as distinguishable peaks in beta-CD. After being loaded with doxorubicin (DOX), the benzene ring peak of DOX (7-8 ppm) could be partially observed, but the majority of DOX proton peaks were shielded by the beta-CD nanocage. Meanwhile, the1H NMR spectrum of DOX is shown in Figure S1.