Figure 1. (a) Schematic
illustration of the fabrication process of the AM/Ch gel. SEM image
of
Ag0.2MXene0.8(b) and the corresponding EDS elemental mapping of (c) Ag and (d) Ti in
the Ag0.2MXene0.8 nanocomposites. (e)
TEM image of the Ag0.2MXene0.8 (The
inserted magnified image shows the histogram for size distribution of Ag
nanoparticles). (f) HRTEM image of the
Ag0.2MXene0.8. (g)
XRD patterns of pure MXene,
Ag0.2MXene0.8,
Ag0.4MXene0.6 and pure Ag. (h) Survey
scan XPS spectra of pure MXene and
Ag0.2MXene0.8. (i) Core-level XPS
spectra of Ti element in pure MXene and
Ag0.2MXene0.8. (j) Core-level XPS
spectrum of Ag element in Ag0.2MXene0.8.
(k) SEM image of the freeze-dried AM/Ch gel. (l) Different shapes
kneaded from AM/Ch gel and
Ch gel. (m) XRD patterns
of chitosan power, freeze-dried Ch
gel and AM/Ch gel.
2.2. Photothermal sterilization properties ofAg/MXene
composites
The dispersion of Ag/MXene
nanocomposites with different component ratios in water was shown inFigure 2 a. The optical property investigation showed that
Ag/MXene composites could efficiently absorb the light across the
UV-Vis-IR region, hinting its high
capacity for solar light utilization (Figure 2b). Therefore, their
photo-to-thermal conversion efficiency was evaluated.
Ag/MXene composites aqueous
suspensions (150
μg
mL–1) were
exposed to irradiation of 0.3 W cm-2, and the
real-time temperature was recorded using
an infrared thermal camera. In
Figure 2c and Figure 2d, as the irradiation time grew,
the temperature of Ag/MXene
composites aqueous suspensions
gradually increased. After 15 min
of illumination, the temperature of MXene suspension went up to 55.2 °C
(∆T = 31.1 °C), while the temperature of
Ag0.2/MXene0.8 reached 53.2 °C (∆T =
29.2 °C), demonstrating that the
introduction of Ag brought in no enhancement in photo-to-thermal
conversion due to the inferior photothermal effect of silver
nanoparticles. With the increase of
silver content to 40%, the photothermal performance of the
corresponding suspension decreased drastically, which could be
attributed to the destruction of the lamellar structure of
Ag0.4/MXene0.6(Figure S3a, Supporting
Information). In contrast, the phosphate-buffered saline (PBS) exhibited
negligible temperature change, which further confirmed that the
temperature rise could be attributed to the photo-to-thermal conversion
over MXene-based composites. Furthermore, the photo-induced heating
curves of
Ag0.2/MXene0.8for five cycles under the light on/off cycles demonstrated excellent
photothermal stability (Figure 2e), showing its prospective potential
for long-term photothermal applications.
Inspired
by the outstanding photothermal properties of Ag/MXene
composites, we evaluated their
broad-spectrum antibacterial capability against Gram-positive S.
aureus and Gram-negative E. coli in dark or under light through
plate counting method (Figure S6, Supporting
Information). As shown in
Figure 4f-g,
pure MXene and pure Ag both showed
inferior killing effects against E. coli and S. aureus in
dark. The low antibacterial behaviors of pure Ag nanoparticles may be
attributed to the poor dispersion and heavy agglomeration in
aqueous suspensions
(Figure S3b, Supporting
Information).[32] However, for
the Ag/MXene composites, it could
be clearly observed that the introduction of Ag could significantly
decrease the survival rate of both E. coli and S. aureus ,
which was beneficial for the self-cleaning ability of the materials in
dark.
Given
that the Ag/MXene composites will be used for photothermal purposes, we
further evaluated the bacterial inhibition capacity of these composites
under irradiation (0.3 W cm-2), and the results
indicated that both E. coli and S. aureus growth was
dramatically inhibited and the
survival rate was dramatically
reduced correspondingly. For the MXene-based materials, over 90% of the
bacteria were inactivated under the irradiation
density of 0.3 W
cm-2, indicating the brilliant antibacterial effect of
photothermal therapy (PTT) arising from the photothermal effect of
MXene. Although Ag has been widely used as an antibacterial agent, its
antibacterial effect is relatively low compared with MXene. However,
when Ag and MXene are combined, especially
Ag0.2/MXene0.8, showed much enhanced
antibacterial efficiency compared to pure MXene, indicating the synergy
between PTT and Ag+ ions release in bacterial killing.
Up
to 99.76% of E. coli and 99.41% ofS. aureus were killed within
15 min under irradiation of 0.3 W cm-2.
Additionally, it was evident thatE. coli , as previously reported, was more susceptible to
antibacterial treatments, which was owing to its thinner peptidoglycan
layers of the cell wall in Gram-negative
bacteria.[33,34]As discussed earlier, the
synergistic bactericidal effect could be attributable to increased
permeability of the bacterial cell membrane and sensitivity to heat by
PTT, as well as the constant Ag+ ions release from Ag
nanoparticles which in-situ grew on
Ti3C2Tx lamellar
structures. To further investigate
the antibacterial mechanism, we used SEM to observe how E. coliand S. aureus morphology changed in response to various
treatments. As shown in Figure 2h, both E. coli and S.
aureus, before being treated (control group), showed smooth and intact
cell membranes.
However,
after antibacterial treatment, the cell membrane of both E. coliand S. aureus showed severe wrinkles and damage with the presence
of Ag0.2/MXene0.8, confirming the
remarkable bactericidal activity. Based on the high photothermal effect,
well-maintained 2D structure and superb bactericidal performance,
Ag0.2/MXene0.8 (referred as
Ag/MXene) was utilized to fabricate
photothermal AM/Ch gel for solar steam generation.