The physical properties of amorphous biomolecules play a critical role in stability of food, pharmaceutical, and biotechnological products. Consequently, they have an impact on formulation design and quality control of amorphous products. Molecular mobility, heterogeneity, and air permeability have been monitored in amorphous materials using Generally Recognized as Safe (GRAS) luminescent probes, i.e., tryptophan and erythrosine B. Given its GRAS status and widespread availability, we hypothesize that riboflavin can be used as a convenient and safe intrinsic luminescent probe to study the physical properties of amorphous films. Riboflavin phosphorescence has only been reported at temperatures below 77K and in solid matrices. The objectives of this study were: 1)to investigate riboflavin phosphorescence sensitivity towards molecular mobility in a disaccharide model system, 2)to characterize the temperature dependence of riboflavin phosphorescence in amorphous films. Sucrose was used as a model system to assess riboflavin's applicability as a phosphorescent probe of the physical state of edible films. Steady-state and time-resolved phosphorescence of riboflavin in amorphous sucrose films were collected over the temperature range from À30 C to 60 C. Emission spectra were fitted with a log-normal function. The performance of a stretched exponential and a multi-exponential function were evaluated to characterize riboflavin lifetimes. The rate constant of non-radiative decay (kTS0), a measure of molecular mobility, was calculated from riboflavin's average lifetime. Measures of the distribution of energetically distinct environments were obtained from bandwidth (G), stretching exponential factor (b), and fractional amplitudes (ai) parameters. The analysis of fitted parameters as a function of temperature revealed two temperature regimes below Tg in sucrose films with a transition at 10 C. Implications of these findings on riboflavin's potential applications as a luminescent probe of physical properties of amorphous biomolecules and edible films are discussed.
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