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.