LASER EXCITED FLUORESCENCE FROM ORGANIC DYES RELEASED FROM $LIPOSOMES^{*}$

Abstract

Liposomes are vesicles constituted of phospholipid bilayers dispersed in aqueous media. Owing to their structural similarities to membranes of biological cells, lipid vesicles serve as a model system to study the effects of laser radiation at the molecular level. We have been developing methods for preparing liposomes with dyes either encapsulated in the internal volume (e.g. sulforhodamine) or intercalated in the bilayer membrane (e.g. methylene blue). An important aim of these investigations has been to release efficiently the internal contents of the liposomes by pulsed laser excitation. Excitation by pulsed laser radiation at 532 nm was primarily within the nonfluorescent dimer band of sulforhodamine and resulted in localized heating. A single 8 ns wide pulse at 532 nm caused significant release of liposome contents, being dependent on liposome size, internal dye concentration, and pulse energy density. Methylene blue, which is a membrane-associating dye, was released even more efficiently than sulforohodamine. Time correlated single photon counting measurements were made to gain insight into the distribution of dve molecules in the interior of the liposome and the bilayer. Potential applications of the technique of laser-mediated release of dyes form liposomes range from targeted release of drugs to localized photothermal release photosensitizers accompanied by destruction of tumor tissue.