Because antioxidants have a strong radical
Because antioxidants have a strong radical scavenging efficiency, a mixture of H2O2 and antioxidants should exhibit a much weaker depolymerization process than H2O2 alone. Therefore, we expected the redox behavior of silver could be affected by the presence of the antioxidant. The schematic diagram of the silver redox in the hydrogel upon addition of H2O2 and the H2O2+ antioxidant is shown in Fig. 2A. To evaluate the analytical performance of the proposed hydrogel sensing platform, ascorbic dig this (AA) was chosen as a model antioxidant. Because the silver oxidation peak is more distinct than its reduction peak, the DPV of silver oxidation has been used as the indicator for evaluating the AA effect. Fig. 2B shows the DPV curves of the silver oxidation after 5min of addition of 0.1mM H2O2 as a function of different concentrations of AA. Each measurement was conducted at a new hydrogel sensing platform from the same stock. The presence of AA considerably affected the peak current. The silver oxidation peak significantly decreased with the presence of 0.04μM of AA. Further increasing the AA concentration could result in a continuous current drop. Fig. 2C plots the current changes against the logarithm of the AA concentration. A linear relationship was clearly observed when the AA concentration was between 0.04μM and 0.36μM. The idea proposed in this work was inspired by a DNA-based antioxidant sensor. The immobilized DNA layers on the electrode can be a target for antioxidant assessment by mimicking damage caused in vivo by reactive oxygen species [11,12]. Instead of using a Fenton solution (containing potassium ferrocyanide and H2O2 in different molar ratios) to induce DNA damage, our proposed methods uses hydroxyl radical-induced chitosan depolymerization as evidence for determining the antioxidant capacity.
We further analysed the efficiency of the proposed hydrogel sensing platform for different antioxidant sensors. Fig. 3 shows the sensing efficiency of AA, uric acid (UA) and luteolin (Lu) using the hydrogel as the screening platform. The antioxidant ion efficiency (%) was calculated using the following expression: Antioxidant efficiency (%)=Ic/Ic0×100, where Ic is the silver oxidation difference after addition of H2O2 with 0.5μM of the antioxidant, and Ic0 is the silver oxidation peak current with only the addition of H2O2. As shown in the figure, the AA presented the highest protective role among all antioxidants.
We also tested the injection of ascorbic acid with 10-folds of some interference species such as glucose and sucrose. These compounds were found to have statistically insignificant effect on the intensity of the peak currents. The reproducibility of the proposed sensing platform was tested using 5 freshly synthesized chitosan hydrogels towards 0.2μM AA. A relative standard deviation (RSD) of 4.7% was obtained. The life-time stability of the chitosan hydrogel was tested once per week towards 0.2μM AA for one month using the same bath of chitosan hydrogel. The current difference decreased by approximately 7.95% after one month. The chitosan hydrogel showed a good reproducibility with an acceptable stability. Because the proposed protocol eliminated the electrode modification-induced variation, we believe this method can be further studied in detail to develop a reliable and quick method for antioxidant screening.
Conclusion In conclusion, a chitosan‑silver ion hydrogel was prepared using a simple method and applied as an electrochemical sensing platform for antioxidant screening. Hydroxyl radical-induced chitosan depolymerization frees silver ions to diffuse to the electrode surface and, consequently, it was been used as a signal to reflect the antioxidant capacity. AA was used as the model antioxidant for evaluating the analytical performance of the proposed hydrogel sensing platform. A linear detection can be observed from 0.04μM to 0.36μM. Moreover, since different antioxidants exhibit different antioxidant capacities, our proposed sensing platform can be used for antioxidant screening.