pH-Dependent Fluorescence Spectra, Lifetimes, and Quantum Yields of Firefly-Luciferin Aqueous Solutions Studied by Selective-Excitation Fluorescence Spectroscopy

We investigated the pH-dependent fluorescence spectra, quantum yields, and lifetimes of firefly luciferin in aqueous solutions by varying the pH and excitation wavelength. Green fluorescence peaking at 540 nm and red fluorescence peaking at about 620 nm were detectable in the pH range between 1 and 10. The lifetime of the green fluorescence decreased significantly from about 5 to 0.5 ns with decreasing pH from 10 to 1, while the red fluorescence lifetime was almost constant, about 0.4 ns, in the measurable pH range between 1 and 4. The pH dependence of the green-fluorescence lit Mime had analogous pH-dependence with that of the fluorescence quantum yield. This reflects the contribution of the emission-efficiency variation of green fluorescence due to the pH-sensitivity of a non-radiative decay that competes with a pH-insensitive radiative decay. (C) 2010 The Japan Society of Applied Physics