Flavonols as Luminescent Probes of Water Activity in Foods and Pharmaceuticals

Abstract

Flavonols are naturally occurring antioxidants with complex photophysical properties. Their emission sensitivity to polarity and hydrogen bonding ability of the local environment suggests their potential as luminescent probes for water activity. Water activity, aw, is an important parameter for ensuring food safety and quality, as it is a determining factor for microbial growth and biochemical reactions.This study focused on 3-hydroxyflavone (3HF), a synthetic molecule that constitutes the backbone of naturally occuring flavonols. However, the photophysical properties and sensitivity to aw of other flavonoid glycosides were also explored. All flavonols were tested in binary solvent mixtures of different aw.3HF is an excited-state intramolecular proton-transfer probe that exhibits dual fluorescent emission bands corresponding to its normal (λem = 405nm) and photo-induced tautomeric (λem = 525nm) form. The normalized intensity of the photo-induced tautomer was sensitive to aw of binary mixtures, with a marked decrease in intensity at aw > 0.8, likely due to progressive aggregation of 3HF molecules. Additional sensitivity to aw was observed in terms of the location of the emission bands. The difference in wavelength between the normal (N) and tautomer (T) forms decreased monotonically at aw > 0.4 due to a hypsochromic shift of the tautomer band. The relative position of the T∗ towards N∗ band as a function of aw was modelled using a log-logistic function. A critical aw value above which the sensitivity of 3HF significantly increased was estimated based on the model.Quercetin and additional flavonoid glycosides were also responsive to changes in aw; e.g., quercetin's fluorescence intensity decreased at aw > 0.6.Although additional validation of these probes in model food systems is required, the available data support the potential use of flavonols as probes of aw in foods and edible pharmceuticals.