Improved Simulated-Daylight Photodynamic Therapy and Possible Mechanism of Ag-Modified TiO2 on Melanoma

Abstract

Simulated-daylight photodynamic therapy (SD-PDT) may be an efficacious strategy for treating melanoma because it can overcome the severe stinging pain, erythema, and edema experienced during conventional PDT. However, the poor daylight response of existing common photosensitizers leads to unsatisfactory anti-tumor therapeutic effects and limits the development of daylight PDT. Hence, in this study, we utilized Ag nanoparticles to adjust the daylight response of TiO₂, acquire efficient photochemical activity, and then enhance the anti-tumor therapeutic effect of SD-PDT on melanoma. The synthesized Ag-doped TiO₂ showed an optimal enhanced effect compared to Ag-core TiO₂. Doping Ag into TiO₂ produced a new shallow acceptor impurity level in the energy band structure, which expanded optical absorption in the range of 400-800 nm, and finally improved the photodamage effect of TiO₂ under SD irradiation. Plasmonic near-field distributions were enhanced due to the high refractive index of TiO₂ at the Ag-TiO₂ interface, and then the amount of light captured by TiO₂ was increased to induce the enhanced SD-PDT effect of Ag-core TiO₂. Hence, Ag could effectively improve the photochemical activity and SD-PDT effect of TiO₂ through the change in the energy band structure. Generally, Ag-doped TiO₂ is a promising photosensitizer agent for treating melanoma via SD-PDT.

Keywords: Ag-core TiO2; Ag-doped TiO2; melanoma; simulated-daylight photodynamic therapy.

Figure 1

Transmission electron microscopy image of Ag-modified TiO₂. (A) Transmission electron microscopy image of TiO₂; (B) Transmission electron microscopy image of Ag-doped TiO₂; (C) Transmission electron microscopy image of Ag-core TiO₂ with a high resolution before the calcination treatment; (D) The crystal lattice structure of TiO₂ in Ag-core TiO₂ observed using transmission electron microscopy image with a high resolution after the calcination treatment; (E) Transmission electron microscopy image of Ag; (F) Transmission electron microscopy image of Ag-core TiO₂ with 5 nm thick TiO₂ shell (sodium bicarbonate at 0.9 mL); (G) Transmission electron microscopy image of Ag-core TiO₂ with 20 nm thick TiO₂ shell (sodium bicarbonate at 1.3 mL); (H) Transmission electron microscopy image of Ag-core TiO₂ with shell of TiO₂ agglomerated (sodium bicarbonate at 1.5 mL).

Figure 2

Properties of Ag-modified TiO₂. (A) UV-vis absorption spectra of Ag-doped TiO₂ compared with commercially available TiO₂ (P25) and the synthesized TiO₂; (B) XRD of Ag-doped TiO₂ compared with P25 and the synthesized TiO₂; (C) UV-vis absorption spectra of Ag-core TiO₂ with the different thickness of the shell; (D) TME-EDS of Ag-core TiO₂.

Figure 3

The photocatalytic degradation of methylene blue (MB) induced by Ag-doped TiO₂ (A) and Ag-core TiO₂ (B).

Figure 4

The cytotoxicity assay and phototoxicity of Ag-modified TiO₂ through a CCK-8 assay; (A) Cell viability without irradiation—the cytotoxicity assay of Ag-modified TiO₂ compared with P25 and the synthesized TiO₂; (B) Cell viability after irradiation by daylight—the phototoxicity assay of Ag-modified TiO₂ compared with P25 and the synthesized TiO₂ with different concentrations of TiO₂; (C) Cell viability after different irradiation dosage—the phototoxicity assay of Ag-modified TiO₂ compared with P25 and the synthesized TiO₂. *, p < 0.05, represents statistically significant difference between P25, the synthesized TiO₂, Ag-core TiO₂, the Ag-doped TiO₂ group, and the control group.

Figure 5

The ROS generation induced by Ag-core TiO₂ and Ag-doped TiO₂ and ROS inhibition using histidine compared with P25 and the synthesized TiO₂; (A) Fluorescence imaging of ROS generation using DCFH-DA probe on A375 cells; (B) Fluorescence intensity assay of ROS generation for DCFH-DA probe on A375 cells. (C): ROS inhibition using 20 mM histidine for 30 min on A375 cells. The bar is 20 μm or SD. *, p < 0.05, represents statistically significant difference between the treated histidine group and the untreated histidine group in P25, the synthesized TiO₂, Ag-core TiO₂, Ag-doped TiO₂, and control.

Figure 6

The crystalline structures of TiO₂ and Ag-doped TiO₂ (A,B) and the band structure of TiO₂ and Ag-doped TiO₂ (C,D) obtained through density functional theory analysis. The abscissa axis is in the indicated Brillouin zone for tetragonal structure of TiO₂. The dashed red lines (energy zero) represent the valence-band maximum. The blue lines represent the minimum band gap at the G point.

Figure 7

The total density of states (DOS) and the partial density of states (PDOS) for the projected states of Ti, O, and Ag corresponding to TiO₂ (A–C) and Ag-doped TiO₂ (E–H), and the calculated optical absorption spectrum of TiO₂ and Ag-TiO₂ (D) obtained through density functional theory analysis.

Figure 8

The simulated absorption spectrum (A) and the near-field enhancement distribution (B) and enhancement intensity (C) of Ag-core TiO₂ with the increasing thickness of the TiO₂ shell (from 5–20 nm).