Direct Excitation Energy Transfer as a Technique for in Situ Measurements of Ion-Exchange Processes in Single Polymer Particles

Ion-exchange processes of a cationic dye (Malachite Green, MG) are studied for individual polymer particles (diameter of 20 or 13 μm) by laser trapping microspectroscopy. When a cation-exchange resin, preadsorbed homogeneously with Rhodamine B (RhB), is soaked in an aqueous MG solution, MG is adsorbed in the surface layer of the particle in the initial stage and then diffuses into the inner volume with time. In the MG-diffused layer, RhB fluorescence is quenched by excitation energy transfer from the excited state of RhB to the ground-state MG, as revealed by fluorescence microspectroscopy of individual resin particles in the dye solution. On the basis of the time dependence of the quenching efficiency, the thickness of the MG-diffused layer at a given soaking time is estimated, and the diffusion coefficient of the dye in the particle is determined to be 8 × 10^-11 cm² S^-1. This method is shown to be very useful for determining the diffusion coefficient of a nonfluorescent ion in individual ion-exchange resin microparticles.