Investigation of interactions between lanthanide ions and humic substances using time-resolved luminescence spectroscopy and stopped-flow-fluorescence measurementS

Full text of publication follows: Humic substances (HS) play a major role for the speciation of metal ions in the environment. In our study, the lanthanide ions Eu3+, Tb3+ were used as molecular probes in order to characterize the complexation of metal ions by HS. The aim is to shed more light on the mechanisms and kinetics of the complexation process. HS of different origins as well as model compounds were used in the time-resolved luminescence and stopped-flow measurements. In the experiments, the intrinsic HS fluorescence and the luminescence of the lanthanide ions were evaluated. From the data, information on the complexation kinetics and aggregation processes of HS were obtained. In the stopped-flow experiments, the kinetics of metal ion complexation by HS was monitored in the time range from 1 ms to several minutes using the intrinsic HS fluorescence quenching. It was found that even on this relative short time scale a combination of at least two processes is present, one with a very fast pseudo-first order rate constant k1 >> 102 s-1 and a second process with a rate constant k2 ∼ 10-1 s-1. Using time-resolved luminescence spectroscopy 1) the sensitized luminescence of Tb3+ and Eu3+ and 2) the inter-lanthanide energy transfer between Tb3+ and Nd3+ was used to monitor the conformational changes in HS that are induced upon metal ion complexation. The observed sensitized luminescence of Tb3+ and Eu3+ was dependent on the loading of the HS with metal ions. With increased metal loading, the energy transfer processes involved in the sensitization processes are altered. It is tempting to attribute this change to a conformational alteration of the HS structure induced by the complexed metal ions. Based on the inter-lanthanide energy transfer, an estimate on the distribution of metal binding sites in HS was obtained. (authors)