Figure 2. (a) Digital photo of aqueous suspensions of pure MXene, Ag0.2MXene0.8, Ag0.4MXene0.6 and pure Ag. (b) UV-Vis absorption spectra of pure MXene, Ag0.2MXene0.8, Ag0.4MXene0.6 and pure Ag. (c) Temperature changes of pure MXene, Ag0.2MXene0.8, Ag0.4MXene0.6, pure Ag and PBS buffer solution under irradiation (0.3 W cm-2). (d) IR thermal images of pure MXene, Ag0.2MXene0.8, Ag0.4MXene0.6, pure Ag and PBS buffer solution. (e) Photothermal heating curves of Ag0.2MXene0.8 for five light on/off cycles (0.3 W cm-2). Antibacterial activity againstE. coli (f) and S. aureus (g) with pure MXene, Ag0.2MXene0.8, Ag0.4MXene0.6, and pure Ag in dark or under irradiation (0.3 W cm-2). (h) SEM images of the dead E. coli and S. aureus treated with Ag0.2MXene0.8.
2.3. The photothermal sterilization andself-cleaning property of AM/Ch gel
Since in most cases, there are lots of microbes in seawater or polluted water, the sterilization capacity of AM/Ch gel solar evaporator is quite essential to ensure the production of clean and drinkable water. As such, we first evaluated the antibacterial effect of AM/Ch gel under irradiation of 0.2 W cm-2, as illustrated in Figure 3 a. As presented in Figure 3b–e, after being treated for 15 min, the pure chitosan hydrogel displayed a 9.23% antibacterial efficiency towardsE. coli and a 8.14% antibacterial activity against S. aureus , respectively, which could be explained by inherent antibacterial properties of chitosan, brought by the numerous functional groups including positively charged quaternary ammonium salt groups that might interact with the negatively charged bacteria cell membranes to trap and inactivate pathogens.[35]In contrast, AM/Ch gel exhibited much enhanced bactericidal ability, with a killing rate of 58.34% against E. coli and 35.09% against S. aureus after 15 min treatment. After light irradiation for an extra 15 min, the pure chitosan hydrogel showed inferior antibacterial ability, killing only 74.31% of E. coliand 35.44% of S. aureus . Notably, all the E. coli andS. aureus were apparently inactivated after treatment with AM/Ch gel, which demonstrated its potential for producing bacteria-free clean water. The excellent antibacterial activity of AM/Ch gel was partially ascribed to the effect of PTT arising from MXene. Besides, the constant release of Ag+ ions may contribute to the long-term antimicrobial effectiveness of AM/Ch gel system. Specifically, as shown in Figure 3f, the concentration of Ag+ ions increased from 0 to 0.35 ppm in 15 min and achieved 0.70 ppm in 30 min under irradiation. As the Ag+ ions concentration increased, the antibacterial activity against E. coli and S. aureus both rose to 100% after 30 min irradiation. These results revealed that, with the synergistic effect of the chitosan hydrogel matrix (inherent sterilization) and Ag/MXene nanosheets (photothermal sterilization and Ag+ ions release), the designed AM/Ch gel system exhibited robust antibacterial activity.
Except for the capacity to produce bacteria-free clean water, the self-cleaning antibacterial property of AM/Ch gel is also important for its recyclable and sustainable use, as shown in Figure 3g. To investigate its self-cleaning property, AM/Ch gel was first placed into a PBS buffer solution containing mixed bacteria of E. coli and S. aureus for 24 h (Figure 3h). The supernatant was then diluted for 10,000 times and coated on the Luria-Bertani (LB) medium for 18 h to quantify the number of colonies. The growth of colonies on LB media under various conditions was shown in Figure j-o. The treatment with AM/Ch gel significantly reduced the number of colonies, demonstrating its outstanding self-cleaning capacity. Figure 3i showed that the concentration of Ag+ ions released from the hydrogel reached 15.01 ppm within 24 h, which might account largely for the high antibacterial behavior in the dark. In addition, the self-cleaning property of AM/Ch gel might also be attributed to the chitosan chains with intrinsic bacterial killing capacity originating from the abundant positive amino groups (NH2), which could engage in electrostatic interaction with the negatively charged germs, killing them in the end.