Photon Trapping and the Enhancement of Electronic Excitation Energy Transfer Efficiency Caused by Colloidal Crystals

Summary: The efficiency of electronic excitation energy transfer from photoexcited fluorescein to rhodamine 6G is enhanced by 2.26 times in an exhaustively deionized colloidal silica suspension. This enhancement is caused by the photon trapping effects due to the Bragg reflection of colloidal crystals formed in the suspension. The lower enhancement efficiency of 1.41 times observed for rhodamine B as an acceptor is attributed to the inefficient spectral overlap between donor fluorescence and acceptor absorption bands.     

Formation of a charge distribution within ionic gels by immobilized enzyme reaction: experimental observations and mathematical simulations

Thermally responsive cationic gels with immobilized urease, in the shape of a small cylinder with diameter 290 - 640 μm, were prepared via gelation of an aqueous monomer solution containing the enzyme. We used N-isopropylacrylamide and N-vinylimidazole as a thermo-sensitive and a pH-sensitive monomer, respectively. Diameters at different positions of the cylinder were microscopically measured in a cell through which substrate solution (pH 4; 35°C) was passed at a constant flow rate; thus, both substrate concentration and pH at the gel surface were maintained at a constant level. It was found that the gel undergoes a shrinking change due to an enzymatically induced increase in pH within the gel phase. There was a marked position dependence of the shrinking degree; the diameter at the center of the cylinder was smaller than those at the top and at the bottom, but the diameters at the top and bottom were identical with each other. This trend was observed at immobilized enzyme concentrations < 1 mg/mL, even after the establishment of swelling equilibrium. By mathematical simulations with a reaction-diffusion model, these results were understood in connection with a charge distribution which is formed as a result of an enzymatically generated pH gradient within the gel phase.