A Study of the Mechanisms and Characteristics of Fluorescence Enhancement for the Detection of Formononetin and Ononin

In this work, the origins for the spectral difference between two isoflavones, formononetin (F) and ononin (FG), are revealed via a comparison study of the fluorescence molecular structure. The fluorescence enhancement of FG in hot alkaline conditions is reported for the first time. For F, there is almost no fluorescence under acidic conditions, but when the pH is >4.8, its fluorescence begins to increase due to the deprotonation of 7-OH. Under a pH between 9.3 and 12.0, the anionic form of F produces a strong and stable fluorescence. The fluorescence quantum yield (Yf) of F is measured to be 0.042. FG shows only weak fluorescence in aqueous solutions under a wide range of pH until it is placed in hot alkaline solutions, which is attributed to the cleavage reaction of the gamma-pyrone ring in FG. The Yf of FG is determined to be 0.020. Based on the fluorescence sensitization methods of F and FG, the quantitative analysis and detection of two substances can be realized. The limit of the detections for F and FG are 2.60 ng center dot mL(-1) and 9.30 ng center dot mL(-1), respectively. The linear detection ranges of F and FG are 11.7 similar to 1860 ng center dot mL(-1) and 14.6 similar to 2920 ng center dot mL(-1), respectively. Although the structural relationship between F and FG is glycoside and aglycone, under hot alkaline conditions, the final products after the cleavage and hydrolysis reactions are essentially different. The different fluorescence characteristics between F and FG pave a way for further identification and a quantitative analysis of the corresponding components in Chinese herbal medicine.

Automated summary

The spectral difference between formononetin (F) and ononin (FG), two isoflavones, is explained through a study of their fluorescence molecular structure. The fluorescence enhancement of FG in hot alkaline conditions is reported for the first time. F exhibits fluorescence only at pH >4.8 due to deprotonation of 7-OH, while the anionic form of F produces strong and stable fluorescence between pH 9.3 and 12.0. The fluorescence quantum yields of F and FG are measured to be 0.042 and 0.020, respectively. The differences in fluorescence characteristics between F and FG allow for the identification and quantitative analysis of components in Chinese herbal medicine.