| Summary: | In this work, we monitored the fluorescence quantum efficiency (<i>η</i>) and the fluorescence lifetime (<i>τ</i>) of natural dye extracts from the leaves of <i>Tradescantia pallida purpurea.</i> The natural dye was extracted from leaves in aqueous solutions as a function of the potential of hydrogen (pH). The <i>η</i> was determined from conical diffraction (CD) pattern measurements due to thermally-driven self-phase modulation. The fluorescence spectra and time-resolved fluorescence measurements corroborate the CD results, and the average <i>η</i> ≈ 0.28 and <i>τ</i> ≈ 3.1 ns values were obtained in the pH range 3.96–8.02. In addition, the extracted natural dye was tested as a possible colorimetric and/or fluorometric pH indicator in milk.
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