Riboflavin as a Gras Luminescent Probe of Food and Pharmaceutical Quality

Abstract

Riboflavin fluorescence has been characterized profusely, however little attention has been devoted to riboflavin phosphorescence. We have characterized steady state and time resolved phosphorescence of riboflavin from 77K to 333K in solvent mixtures (water:alcohol, water:glycerol) and amorphous solids (glucose, glucose oligomers, sucrose, dextran) to expand our knowledge on riboflavin photophysical properties and assess its potential use in quality monitoring.When excited at 440nm, the emission spectra of riboflavin exhibited maxima at 515nm (delayed fluorescence) and 620nm (phosphorescence). Deconvolution of the spectra to two individual bands characterized by asymmetric lognormal functions (I(ν)=I0∗exp{-ln(2)(ln[1+2b(ν-νp)/Δ2]/b)} ) facilitated the extraction of parameters and the analysis of their temperature dependence. Time resolved intensity decays were fitted using stretch, multi-exponential and distribution (by MEM) models.Riboflavin exhibited a lifetime of 184ms at 77K in glycerol-water. The lifetime decreased slowly below the solvent mixture's Tg (170K) and abruptly above Tg; e.g., an increase of 20K above Tg reduced the lifetime from 100ms to 10ms. This reduction was associated with molecular motions in the matrix.Molecular mobility was also modulated by changing the composition of the matrix, by using components of different molecular sizes or by adding plasticizers. Riboflavin phosphorescence also exhibited good sensitivity towards molecular mobility changes driven by composition. This suggests its use as a GRAS optical probe for molecular mobility and its potential application to optimize matrix composition in food and pharmaceutical products to enhance stability of micro and bioactive components. Continuous monitoring of riboflavin phosphorescence during heating and cooling cycles revealed differences in the delayed luminescence emission spectra, likely due to a higher rate of irreversible photodegradation of riboflavin at high temperatures. If properly characterized, the thermal dependence of riboflavin photodegradation can potentially be operationalized in sensors for temperature abuse.