Study on the Interaction between Rhodamine Dyes and Allura Red Based on Fluorescence Spectra and Its Analytical Application in Soft Drinks

Allura red (AR) is a common food additive. It is of significance to detect AR sensitively and selectively in soft drinks. In this report, fluorescence spectra of allura red-rhodamine dyes systems were studied. In a pH 6.0 Britton-Robinson buffer medium, the fluorescence of rhodamine dyes, such as rhodamine B (RB), butylrhodamine B (BRB) and rhodamine 6G (R6G), can be quenched by AR. Impressively, the emission spectrums of the RB and BRB change slightly upon the addition of AR, but it was clear that the emission of R6G decreased dramatically in the presence of AR. Thus, we have succeeded in planning an improved method for specifically detecting AR on the basis of hydrophobic forces and the electrostatic attraction between R6G and AR. The results show that AR could combine with R6G to form an ion association complex, which causes quenching of the emission intensity of R6G and changes of the UV-visible spectra. In this system, 0.097 - 6.0 mu mol L-1 AR could be simply detected owing to the decreased fluorescence of R6G in soft drinks, with a detection limit of 0.029 mu mol L-1. In addition, we also optimized the reaction conditions and evaluated the effects of some coexisting substances. According to the fluorescence decay time, the UV-visible absorption spectra and the Stern-Volmer plots, the fluorescence quenching of R6G by AR is a static quenching process.