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MXenes-two-dimensional transition metal carbides-and their precursor MAX phases are considered promising agents for photothermal and photodynamic therapy owing to their absorption in the near-IR region, where biological tissues are most transparent. However, the factors governing the balance between photothermal and photodynamic excitation relaxation channels in MXenes remain poorly understood. The aim of this work is to compare the optical properties and, especially, the optical manifestations of singlet oxygen (¹O₂) generation for three Ti₃C₂Tₓ samples (one single-layer and two multilayer samples of different origin) and the Ti₃AlC₂ and Ti₃SiC₂ MAX phases.
The studied compounds (at a concentration of 2 · 10⁻³ M) were prepared in DMSO and ethanol. Absorption spectra were recorded using a Cary 60 spectrophotometer (Agilent), and luminescence spectra were measured on a Horiba FluoroMax spectrofluorometer; especially, ¹O₂ luminescence (1100-1350 nm) was registered using a liquid-nitrogen-cooled DSS detector.
The absorption spectrum of one of the multilayer Ti₃C₂Tₓ samples contains an intense broad absorption band with maximum at ~775 nm. For the single-layer sample, this band is weakly pronounced, whereas for the second multilayer sample only a monotonic increase in absorption in the IR region is observed. The MAX phases absorb in the UV region (270, 303-305 nm) and exhibit strong emission at ~365 nm (with the additional band at ~470 nm for Ti₃SiC₂). Under 780 nm excitation, the band at ~1270 nm characteristic of ¹O₂ luminescence was recorded for most samples. It is most intense for the multilayer Ti₃C₂Tₓ without a pronounced absorption band at 775 nm, distinct for single-layer Ti₃C₂Tₓ and Ti₃SiC₂, and weak for Ti₃AlC₂. Conversely, the sample with the most intense absorption at 775 nm generates almost no ¹O₂. For single-layer Ti₃C₂Tₓ, the ¹O₂ luminescence intensity is higher in ethanol than in DMSO.
Thus, the efficiency of ¹O₂ generation by MXenes under IR excitation is determined neither by structural features nor by the intensity of IR absorption, but depends on the synthesis route of the material. Surface chemistry and defect structure likely play a decisive role, which should be taken into account when selecting MXenes for photodynamic therapy.
The work was performed with financial support by the grant 2025.07/0400 from the National Research Foundation of Ukraine.