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  • br Experimental All reagents were analytical grade and were

    2022-11-18


    Experimental All reagents were analytical grade and were used without further purification. The morphology was characterized using a field emission scanning electron microscope (Philips XL-30 FESEMFeSEM, ZEISS SUPRA 40VP, Germany). The hydrogel sample used for SEM analysis was freeze dried. All electrochemical measurements were performed using CHI 832 electrochemical workstation with a conventional three-electrode system comprised of platinum wire as auxiliary electrode, a 3M Ag/AgCl electrode as reference and a glassy carbon electrode as working electrode. All experiments were conducted at room temperature. The chitosan‑silver hydrogel was prepared based on our previous report with some modification [13]. In a typical synthesis procedure, an amount of silver nitrite solution (10mM) was added into a 2mL 1wt% chitosan solution (in 1% acetic acid). After vigorous shaking for 30s, 0.1M NaOH was added dropwise until the gelation process was initiated. The pH value of the final chitosan‑silver hydrogel was 4.5. For hydrogel depolymerization, a 0.1mM hydrogen peroxide solution was injected into the prepared chitosan hydrogel. Bath sonication was applied for 15s to accelerate the diffusion process. For the antioxidant screening, different antioxidants (ascorbic acid, uric 1080 6 and luteolin) were first added into the H2O2 solution. Then, the mixture was injected into the hydrogel. After a waiting period, the antioxidant analysis was conducted by cyclic voltammetry (CV) or differential pulse voltammetry (DPV) scanning.
    Result and discussion The addition of a silver nitrite solution into chitosan under suitable pH conditions can trigger a cross-linking reaction between silver ions and the chitosan chains. A supramolecular complex formed in the form of a hydrogel with a very high water content. The hydrogel can be considered a closed electrolytic cell with silver ions and electrolytes. The morphology of the hydrogel is shown in the inset of Fig. 1A. It can be seen that the hydrogel is a 3D, porous nanostructure. The cross-linking between the silver ions and chitosan highly affected the diffusion of silver ions towards the electrode surface. Fig. 1A shows the silver redox behavior difference in acetic acid (pH4.5, curve a) and the hydrogel (curve b). It is pertinent to note that the redox current of the silver in the hydrogel is much lower than that of the one recorded in the acetic acid electrolyte. This difference can be ascribed to the supramolecular cross-linking between the silver ions and chitosan chains, which locks the silver ions and hinders their diffusion. It can also be expected that a lower silver concentration could result in a larger current difference before and after the cross-linking process since the excess of silver ions could result in a higher background current, while insufficient silver ions could lead to the unsuccessful formation of a hydrogel. Thus, we optimized the silver ion concentration in this closed system. As shown in Fig. 1B, the maximum difference with a small standard deviation was obtained at 4mM. Therefore, 4mM of silver ions in the hydrogel was used for fabricating the antioxidant screening platform in this work. The depolymerization of chitosan can be induced by the addition of H2O2. H2O2 can produce hydroperoxide, which is very unstable and decomposes to form a reactive hydroxyl radical (•HO) [14]. The rupture of glucoside bonds caused by hydrogen abstraction between the hydroxyl radical and chitosan polysaccharide chain was the main reason for the depolymerization process [15,16]. The addition of a high concentration of H2O2 can result in a clear liquidation process. Fig. S1 displays the XRD patterns of chitosan hydrogel before, 10min after and 2h after the polymerization. Destroy of crystalline form of chitosan can be observed after the depolymerization. The redox of silver could then be partially restored because the interwoven structure between the chitosan chains and the silver ions was slowly destroyed. The broken low-molecular weight chitosan-linked silver ions are more likely to diffuse to the electrode surface when an electric potential is applied. As shown in Fig. 1A, the CV curve recorded after 5min of depolymerization (curve c) showed a more distinct current rise corresponding to the silver redox potential. Fig. S2 also shows the silver redox of silver could be nearly complete recovered if underwent 1h depolymerization.