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.