Phosphorescence of Erythrosin B as a Robust Probe of Molecular Mobility in Amorphous Solid Sucrose

Abstract

Luminescence from the triplet probe erythrosin B (tetra-iodo fluorescein, Ery B) provides spectroscopic characteristics such as lifetime and emission energy that are sensitive to molecular mobility of the local environment in amorphous solids. This study investigated how variations in the local concentration of Ery B free acid as well as the presence of the dispersing solvent affect the spectroscopic measurements of solid matrix properties (the free acid of Ery B is poorly soluble in water and thus must be introduced via an organic solvent). The emission energy of Ery B from 5 to 100 degrees C in thin films of amorphous sucrose at various probe and solvent (N,N-dimethyl formamide, DMF) concentrations was determined using excitation at 500 nm and emission over the range 520-750 nm. The emission lifetime was determined over the same temperature range using a stretched exponential analysis of intensity decays collected using excitation at 530 nm and emission at 680 nm. Variations in the probe/sucrose mole ratio (concentration) over the range from 0.5 to 10 x 10(-4) and 10-fold variations in the amount of DMF used to disperse the probe did not affect the emission energy, the shape of the emission spectra, or the measured lifetimes of Ery B in amorphous sucrose. These results thus indicate that erythrosin B introduced into amorphous solids can provide a robust measure of the intrinsic mobility of the solid matrix that is relatively insensitive to final probe concentration or presence of residual solvent.

Molecular mobility and dynamic site heterogeneity in amorphous lactose and lactitol from erythrosin B phosphorescence

“…Previous spectroscopic studies have demonstrated that Ery B is dispersed throughout the sugar matrix and does not aggregate at the concentrations used in this study [33,53]. At a dye/sugar mole ratio of ∼ 0.8:10 4 , each probe is on average surrounded by a matrix shell ∼ 10-11 sugar molecules thick.…”

The effect of salts on molecular mobility in amorphous sucrose monitored by erythrosin B phosphorescence

“…Previous work has established that Ery B does not aggregate under these conditions and reports the mobility of the unperturbed sucrose matrix. 13,29 The data on the sucroseNaCl matrix reported here have been published as part of a more extensive study of the effect of NaCl on sucrose matrix mobility; 28 they are included here for the sake of comparison.…”

Photophysical Probes of the Amorphous Solid State of Proteins

“…Data were fit to a stretched exponential, or Kohlrausch-Williams-Watts (KWW) decay model, 44 which has been used to fit the intensity decays of Ery B dispersed in a wide variety of amorphous solid sugars 21,26,32,33,[37][38][39][40] and proteins 23,34-36 :…”

“…20,21 At this state of minimal perturbation, phosphorescence of Ery B emission is sensitive both to the motions of molecules in its immediate environment 19,20 and to the permeability of oxygen. 22,23 From the excited triplet state, a molecule can return to the ground singlet state either by emission of a photon (phosphorescence) or by one of two non-radiative pathways, matrix-induced deactivation or oxygen quenching.…”