III complex CMC caused the upshift of the N
(III) complex-CMC caused the upshift of the N1s from 399.8 eV for AMBA-CMC to 400.2 eV for Eu(III) complex-CMC, indicating the co-ordination of N PCI32765 with Eu3+. Also, because of the coordination between europium and oxygen, the O1s moved from 532.7 eV for AMBA-CMC to 532.9 eV for Eu(III) complex-CMC (Fig. 1D).
3.2. Stimuli-responsive behaviors
The ion-response of the CDEAC hydrogel was then tested using a fluorescence spectrum. As shown in Fig. 2A, the obtained Eu(III) com-plex-CMC did not emit the characteristic red fluorescence of Eu3+ in aqueous solution under ultraviolet (UV) excitation. However, interest-ingly, when the Eu(III) complex-CMC was simply mixed with K-DPY-CMC, the formed hydrogel emitted red fluorescence with a lumines-cence lifetime of 1.28 ms under UV excitation (Supplementary Fig. S6). However, fluorescence quenching occurs after the addition of ClO− (Fig. 2B). Subsequent addition of SCN− to the above system led to the recovery of the red fluorescence (Fig. 2C). Furthermore, such a fluor-escence reversible cycle upon alternate additions of ClO− and SCN− could be repeated for at least 5 times (Fig. 2D).
The reversible process of CDEAC hydrogel regulated by ClO−/SCN− was investigated. A certain amount of K-DPY-CMC and Eu(III) complex-CMC are dissolved in water respectively to form a homogeneous solu-tion. However, when these two solutions are mixed, the original homogeneous solution transforms into a gel within 5 s. Within 20 s after mixing, the gel was strong enough to keep its shape after the vial was turned over. In the solution, these gels continue to rearrange into the most stable state of thermodynamics, and volume shrinkage is observed (Fig. 3A). We also found that the addition of ClO− could make the gel become the homogeneous solution, and the gel recovered automatically after the addition of the SCN−. Meanwhile, fluorescence quenching and recovery occur in this hydrogel formation process by subsequent ad-dition of ClO− and SCN− (Fig. 3B).
To elucidate the ion-responsive reversibility of the switching pro-cess of the obtained CMC hydrogel, ClO− was first added to the CDEAC hydrogel, followed by adding SCN−, and the product, washed with ultrapure water, was characterized by elemental mapping. As shown in Supplementary Fig. S7A, the Cl element presented in the obtained CMC upon adding ClO−, indicating that ClO− exchange CMC-DPY to co-ordinate with Eu3+. However, when SCN− was added to the above system, due to an oxidation-reduction reaction between ClO− and SCN−, the Cl element disappeared in the obtained product indicating the CMC-DPY cross-linked with CMC-AMBA-Eu by the coordination between DPY and the europium element (Supplementary Fig. S7B). In addition, ClO− and SCN− mediated crosslinking and cleavage of CDEAC hydrogel was also proved by the fluorescence images
Fig. 4. (A–D) SEM images of (A) Eu(III) complex-CMC/K-DPY-CMC, (B) CDEAC hy-drogel, (C) CDEAC hydrogel dissociated by addition of ClO− in aqueous solution and
(D) CMC-DPY + Eu-AMBA-CMC-ClO- re-covered upon addition of SCN − in aqueous solution. (E) Variation in the storage mod-ulus, G’ (black symbols) and loss modulus, G’’ (red symbols) of the CDEAC hydrogel plotted at different hydrogel concentrations. Each data point is based on at least three measurements. The error bars represent the standard deviation. (F) Variation in G’ (black symbols) and G’’ (red symbols) of the CDEAC hydrogel during 3 cycles between ClO− and SCN− addition. (For interpreta-tion of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(Supplementary Fig. S8). Based on the combination of fluorescence ti-tration, Cl elemental mapping, and fluorescence images as well as our previous study , we give the responsive behavior of CMC shown in Fig. 2E. The morphology of these gels after freeze-drying was investigated using scanning electron microscopy (SEM) in Fig. 4. It can be observed that the individual Eu(III) complex-CMC and K-DPY-CMC skeletons (Fig. 4A) exhibits an unordered crosslinking morphology. After contact between Eu(III) complex-CMC and K-DPY-CMC, the formed CDEAC hydrogel exhibits porous lamellar structures generated from a con-nected network of CMC (Fig. 4B). When the ClO− added to above system, ClO− exchanges K-DPY-CMC to coordinate with Eu3+ of Eu(III) complex-CMC and two kinds of CMCs dissociate, leading to the sponge-like structure damage (Fig. 4C). Whereas after adding SCN−, SCN− reacts with ClO− and two kinds CMCs cross-link, thus the sponge-like structure was recovered (Fig. 4D). Moreover, the morphology of CMCs hydrogels complex depends on the ratio of Eu(III) complex-CMC and K-DPY-CMC (Supplementary Fig. S9). The elemental mapping analysis further demonstrates the existence and homogeneous dispersion of Eu,
The viscoelastic properties (storage modulus G′ and loss modulus G″) as a function of concentration at room temperature are shown in Fig. 4E–F and Supplementary S12. As expected from tensile results, the viscoelastic properties of the hydrogel strongly depend on the CMC concentrations. The values of G′ and G″ increased from 65 ± 6 to