Electro-fluorescence polarization studies of the interaction of fluorescent dyes with clay minerals in suspensions

Aqueous suspensions of three clay minerals (sepiolite, attapulgite and hect orite) that have been tagged with fluorescent dye molecules have been studi ed using electro-fluorescence polarization spectroscopy. This electro-optic technique measures the changes that occur in the fluorescence polarization when the clay carrier molecules are oriented in an applied electric field. Under certain conditions, these measurements allow the geometry of the opt ical transition moments within the tagged dye molecules to be calculated, r elative to the electric field orientation axis of the clay carrier. These g eometries may then be used to infer the mode of binding of the dye on the c lay surface. A range of dye molecules with differing sizes and electronic c haracteristics were chosen in order to probe different possible binding sit es. With cationic dye binding to sepiolite, the binding geometry varies acc ording to molecular size. This relationship indicates that binding within t he channels running the length of the sepiolite rods is predominant for the se dyes. Similar relationships were not observed with either of the clays a ttapulgite or hectorite. (C) 1999 Elsevier Science B.V. All rights reserved .