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The two fundamentally different techniques, fluorescence and thermal lens, can be simultaneously enhanced by normal as well as reversed micelles. The fluorescence is enhanced because the micelles isolate and protect the substrate from quencher molecules whereas the thermal lens enhancement is due to the modification of the thermo-optical properties of the solvent by micelles, that is, increase the temperature coefficient of the index of refraction, dn/dT, and decrease thermal conductivity values. Normal micelles provide the most effective means for the thermal lens enhancement of hydrophobic compounds while the reversed micelles are most suitable for hydrophilic compounds. This is because the normal micellar effects such as providing a nonpolar medium for the analyte and changing the structure of water from the compact, well hydrogen bonded structure to a relatively loose one are the main mechanism for the modification of the thermo-optical properties. On the other hand, reversed micelles improve the thermo-optical properties of the environment around the hydrophilic compounds by solubilizing them as very small water droplets in a better thermo-optical nonpolar solvent.
laser, photothermal effect, fluorescence, micelles, reversed micelles, luminescence spectroscopy
Assistant professor, Marquette University, Milwaukee, WI