Time-resolved fluorescence spectroscopy of fulvic acid and fulvic acid complexed with Eu 3+ – a comparative study

Abstract

In this work a systematic characterization of the fluorescence kinetics of fulvic acid by means of a single-photon time correlation technique is reported. The data were extracted from both single decay curves and time-resolved emission spectra and analyzed by the exponential series method. The results strongly suggest the existence of a tri-modal decay time distribution with peak decay time values centred around 0.7, 3 and 10 ns. The relative amplitudes of the decay time distributions were shown to depend on the emission wavelength, indicating a three-species mixture with slightly different emission maxima and widths. As sustained by time-resolved emission spectra the same species of fluorophores are responsible for the fluorescence kinetics behaviour of fulvic acid at all excitation wavelengths used in the experiments. For the first time these techniques and methods were applied in investigating the effects of the Eu 3+ complexation upon fluorescence kinetics of the fulvic acid. Single decay analysis indicates that the peak decay time values are not affected by Eu 3+ addition at all emission and excitation wavelengths used in the experiments. On the contrary, the dependency of relative amplitudes on emission wavelength was greatly reduced in comparison with the uncomplexed fulvic acid. This effect is further evidenced by analysing the barycenter transients of time-resolved emission spectra. Finally, it is also indicated that within the temporal resolution of the experimental set up used the singlet states play no role as donor states in intramolecular energy transfer from the organic fulvic acid ligands to the Eu 3+ energy levels.