Comparing organized media types using fluorescence probe characterization methods

Organized media, such as surfactant micelles and cyclodextrins, have been shown to provide increased selectivity and lower detection limits in the spectrofluorimetric analyses of many compounds. Analysis performance is generally improved because the medium shields the analyte from quenchers or because inclusion in the medium increases the efficiency of a requisite interaction, as in the case of energy transfer and room-temperature phosphorescence measurements. The ability of other types of microheterogeneous media, such as phospholipid vesicles, to solubilize non-electrolytes is also well known, but the microenvironmental properties of many of these systems have not been characterized under the conditions likely to be used in spectrofluorimetric analysis. In a step toward evaluating the suitability of a wider range of compounds in spectrofluorimetric applications, several medium properties including the microheterogeneity, partition coefficient, sequestering efficiency and apparent polarity of a typical lipid vesicle, dimyristoyl phosphatidylcholine (DMPC), and surfactant micelle, sodium dodecyl sulfate (SDS), were compared using a single fluorescence probe. The results indicate that DMPC vesicles are less microheterogeneous, less polar and partition hydrophobic molecules more readily than SDS micelles. However, probe molecules were sequestered from interferents less effectively in DMPC than in SDS.