Spectral characterization of micellar-enhanced fluorescence of europium (III)-doxycycline complex and its employment as a sensor for development of a highly sensitive fluorimetric assay for determination of indigo carmine in real syrups.

Indigo carmine (IN-CR) is a synthetic blue dye widely used as a coloring agent in various food and beverage products. It is recognized for its ability to enhance the visual appeal, hue, and consistency of food products. However, recent studies have raised concerns about the potential health risks associated with this substance. Therefore, a highly sensitive analytical tool is required for the trace determination of IN-CR in food products. This study describes the spectral characteristics of micellar-enhanced fluorescence of europium (III)-doxycycline complex and its employment as a sensor for the development of a highly sensitive fluorimetric assay for the trace determination of IN-CR. The complex was formed in an alkaline medium containing hydrogen peroxide and encapsulated into cetyltrimethylammonium bromide micelles. This micellar-encapsulated complex exhibited significantly enhanced fluorescence at a wavelength of 613.5 nm, overlapping with the maximum absorption peak of IN-CR at 610 nm. This inner-filter effect phenomenon resulted in IN-CR's concentration-dependent quenching of the complex's fluorescence. The fluorescence quenching of the complex by inner-filter effect of IN-CR was adopted in the development of a highly sensitive fluorimetric assay for IN-CR. The conditions of the assay were refined, and the optimum procedures were established. The assay was validated for its performance characteristics, and all the validation criteria were met. The assay displayed linearity within the IN-CR concentration range of 100-1250 ng mL, with a limit of detection at 41.2 ng mL. Importantly, the assay exhibited no interference from other substances commonly used as food additives. The recoveries ranged from 99.11 % to 101.57 %, with relative standard deviations ranging from 1.34 % to 1.89 %.The assay was successfully applied to the determination of IN-CR in commercial syrup products, and the recoveries ranged from 93.5 % to 106.32 % with relative standard deviations ≤4.72 %, confirming the reliability of the assay. The proposed assay provides some key advantages, including a novel application of the europium (III)-doxycycline complex as a fluorescence sensor for the determination of IN-CR in real syrups innovative, high sensitivity and selectivity, simplicity of analysis procedures, time-saving of entire analysis protocol. In conclusion, the assay serves as a valuable tool for quantifying IN-CR in food products, facilitating the assessment of both food quality and safety.