A pH-/thermo-responsive hydrogel formed from N,N′-dibenzoyl-l-cystine: properties, self-assembly structure and release behavior of SA

In this study, we report a pH-/thermo-responsive hydrogel formed by N,N′-dibenzoyl-l-cystine (DBC). It is difficult to dissolve DBC in water even on heating, and it exhibits no gelation ability. Interestingly, DBC is readily soluble in NaOH solution at room temperature and the self-assembled hydrogels are obtained by adjusting the basic DBC aqueous solution with HCl to achieve a given pH value (<3.5). When NaOH is added to the hydrogel (pH > 9.4), it becomes a sol again. This small-molecule hydrogel is characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, rheological measurement and differential scanning calorimetry. The results indicate that the DBC hydrogel exhibits excellent mechanical properties, thermo-reversibility, and pH-responsive properties. Fortunately, the single crystal of DBC is obtained by volatilizing its acid aqueous solution. It crystallizes in the monoclinic space group P2(1) (Z = 2) with lattice parameters a = 10.8180 (11) Å, b = 9.0405 (9) Å, c = 10.9871 (11) Å and β = 90.798 (3)°. By comparing the X-ray diffraction result of the DBC single crystal with that of its xerogel, the self-assembled structure of DBC in hydrogel has been ascertained. The gelators are self-assembled via strong intermolecular hydrogen bonds linking neighboring amide and carboxyl groups, π–π stacking interactions for aromatic rings, and hydrogen bonds between water molecules. In addition, the release behavior of salicylic acid (SA) molecules from the DBC gel is also investigated taking into account the DBC concentration, phosphate buffer solution (PBS) pH and SA concentration. When the concentrations of DBC and SA are 3.0 g L(−1) and 200 mg L(−1), respectively, the release ratio in PBS (pH = 4.0) reaches 58.02%. The diffusion-controlled mechanism is in accordance with Fickian diffusion control within the given time range.