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Tryptanthrin (Try) is a natural indoloquinazoline alkaloid with a broad spectrum of biological activity that is considered a potential photosensitizer for photodynamic therapy. However, the generation of singlet oxygen ¹O₂ by Try in ethanol is inefficient. One way to enhance the photosensitizing ability of organic dyes is to combine them with two-dimensional transition metal dichalcogenides. The aim of this work was to investigate the effect of MoS₂ nanolayers on the optical properties of Try and ¹O₂ generation by Try.
Tryptanthrin (Try), MoS₂ samples, and their mixtures in ethanol were investigated. Absorption spectra were recorded on a Cary 60 spectrophotometer (Agilent), while luminescence and luminescence excitation spectra were measured on a Horiba FluoroMax spectrofluorometer. The efficiency of ¹O₂ generation was evaluated from its luminescence intensity in the near-IR region (1100-1350 nm) using a liquid-nitrogen-cooled DSS detector.
The absorption spectrum of Try contains bands at 252, ~315, ~335, and ~392 nm, while its luminescence spectrum exhibits a broad band peaking near 525 nm as well as a short-wavelength indole UV band at ~345-360 nm (under UV excitation). In the absorption spectra of Try/MoS₂ mixtures, the dye bands are preserved and characteristic long-wavelength absorption bands of MoS₂ (~630 and ~690 nm) appear. Adding MoS₂ causes strong quenching of the visible luminescence of Try: the intensity of the 525 nm band decreases sharply (including relative to the short-wavelength indole band at ~360 nm) even when the dye concentration is doubled. Under 395 nm excitation, the Try solution shows no noticeable ¹O₂ signal. In contrast, for the Try/MoS₂ mixtures, a distinct band at 1265-1270 nm characteristic of ¹O₂ luminescence is recorded, which is significantly more intense than that for MoS₂ samples without tryptanthrin.
The obtained results indicate effective interaction of Try with the MoS₂ surface and possible energy/charge transfer between the components promoting ¹O₂ formation. Try/MoS₂ nanosystems can be considered as promising materials for singlet oxygen photosensitization.
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