Simultaneous enhancement of fluorescence and thermal lensing by reversed micelles

Abstract

ABSTRACT Aerosol-OT (AOT) reversed micelles have been used to simultaneously enhance the fluores­ cence as well as the thermal lens effect in the analysis of solubilized lanthanide ions. The fluorescence is enhanced because the reversed micelles isolate and protect the analyte from quencher molecules while the thermal lens enhancement is due to the modification of the thermal physical properties of the solvent, i.e., increased dn/dT and decreased thermal conductivity values. Compared to that in water, AOT/CCla reversed micelles increased fluorescence and thermal lens enhancement factors of Tb3* by factors of 2 and 38, respec­ tively. The mechanism for the fluorescence and thermal lens enhancement is discussed. 1. INTRODUCTION Fluorescence and thermal lens are often used in trace chemical analysis1 4. The fluo­ rescence technique is based on the measurement of emitted photons of an excited analyte. It is, therefore, particularly well suited for the detection of substances which have high fluorescence quantum yields. Conversely, thermal lens technique is based on the measurement of the heat generated by the nonradiative relaxation of an excited analyte. It has been successfully employed for trace detection of nonf1uorescent substances. Thus, the two techniques are complementary and have been used to determine fluorescent as well as non- fluorescent substances at concentrations as low as 10~11 M2~5.Fluorescence can be enhanced by increasing the radiative processes of a molecule. In particular, isolation of the fluorescence molecule from quenching impurities and/or solvent molecules can be used to achieve this enhancement.The thermal lens technique is based on the temperature rise that is produced in an illu­ minated sample by nonradiative relaxation of the energy absorbed from a laser. Thus, its intensity can be enhanced by improving the nonradiative relaxation processes of the analyte and more significantly the thermal physical properties of the solvent. When a modulated laser beam, which has a Gaussian intensity distribution is used to excite the sample, the heat generated will be strongest at the center of the beam since this is where the beam intensity is greatest. Consequently, a lens-like optical element is formed in the sample due to the temperature gradient between the center of the beam and the bulk sample. The effect of the thermal lens is generally measured as a relative change in the beam center intensity, Albc/Ibc , in the far field. When a sample is located 3l'2 Zc beyond the beam waist, this change is given by:A*bc V