Fluorescence self-absorption effects

Figure 4.13 Fluorescence self-absorption effects for sodium molybdate NajMoO,.

Bright and coworkers investigated pyrene-excimer formation in supercritical fluids from a somewhat different angle using not only steady-state but also time-resolved fluorescence techniques (47,167). They measured fluorescence lifetimes of the pyrene monomer and excimer at a pyrene concentration of 100 p,M in supercritical ethane, CO2, and fluoroform at reduced densities higher than 0.8. Since the kinetics for pyrene-excimer formation was found to be diffusion controlled in ethane and CO2 and less than diffusion controlled in fluoroform, they concluded that there was no evidence for enhanced pyrene-pyrene interactions in supercritical fluids. The less efficient excimer formation in fluoroform was discussed in terms of the influence of solute-solvent clustering on excimer lifetime and stability. Experimentally, their fluorescence measurements were influenced by extreme inner-filter (self-absorption) effects due to the high pyrene concentration in the supercritical fluid solutions (35). 

Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

Emission inner filter effect (self-absorption) The fluorescence photons emitted in the region overlapping the absorption spectrum can be absorbed (radiative energy trans-... 

Quenching At shorter distances, ranging from few nanometers to the physical contact with the metallic structure, a mechanism tends to increase the total decay rate. This effect, which is responsible for fluorescence quenching, is due to the absorption of fluorescence photons in the metallic structure itself (99). Another effect is based on interactions of the fluorophore with free electrons in the metal, wherein the plasmon absorption leads to lower fluorescent emission efficiency (100). Theoretical study asserts that the optimized distance between the excitation source and the fluorophore is around 2-5 nm (99, 101,102). Nanoparticles coated with a thin shell (e.g. silica, 5nm in thickness) and the dye attached to the dielectric shell could overcome quenching effects (84, 103). The quenching effect can also be found in the quantum dot / GNP system (104). It is noted that as the concentration of fluorophore is high, the self-quenching effect should also be considered. (100)... 

The emission intensity of fluorescence depends on the product of the molar extinction coefficient, the optical path length, the solute concentration, the fluorescence quantum yield of the dye, and the excitation source intensity. In dilute solutions, the intensity is linearly proportional to these parameters. When sample absorbance exceeds about 0.05 in a 1-cm path length, the relationship becomes nonlinear and measurements may be distorted by artifacts such as self-absorption and the inner-filter effect. 

Experimental problems associated with the measurement of fluorescence yields of strongly absorbing fluorophores, typified by dyes, have been fully discussed. The effects of both the strong absorption at high concentrations in changing the activated volume and self absorption of emission have been evaluated as a function of exciting wavelength and methods for the accurate determination of yields analyzed. Dyes are very convenient in their various properties for use as model... 

In an article which is critical of many generally accepted molecular fluorescence parameters of aromatic molecules (and by inference the parameters for other systems), Birks emphasizes the precautions necessary to eliminate errors due to self-absorption secondary fluorescence and/or self-quenching.1 The points are made that reliable data for rf and Of are available for only a few compounds, e.g. diphenylanthracene (DPA), perylene, quinine bisulphate, and acridone, and that these provide suitable standards. The value of Of (DPA) is now set at 0.83. The importance of solvent effects on Of and t( of DPA is stressed in a publication which reports Tf for DPA in cyclohexane and benzene.2 The value of 6.95 0.04 ns for benzene solution is in good agreement with the earlier work of Birks and Dyson3 and Ware and Baldwin 4 7 (7.35 0.05 ns). The value obtained for cyclohexane solution, 7.58 0.04 ns, although in poor agreement with earlier results, is probably the most acceptable. The absolute fluorescence quantum yield of quinine bisulphate has also been redetermined (Of = 0.56).8... 

This effect is known variously as self-absorption, thickness effect or overabsorption. An analogous effect in photochemistry is known as saturation of the action spectrum. The first term is somewhat of a misnomer because it suggests that the problem has to do with re-absorption of the fluorescence radiation, by analogy with certain effects in optical spectroscopy. Actually, the absorption of the fluorescence is independent of the incident energy, hence does not contribute to any non-linearity. What is important is that the penetration depth for the incident radiation depends on the quantity one wants to measure. 

Low-dimensional crystals such as epitaxial needles and solution-grown platelets of TPCOs act as a microscale gain medium. The self-cavity and self-waveguiding effects of these crystals result in ASE in the wavelength region of the fluorescence band where the self-absorption loss is minimized. Furthermore, the uniaxial orientation of the TPCO molecules in these low-dimensional crystals promotes the stimulated emission process and enhances the polarized ASE. 

At high analyte concentrations, inner filter effects can occur. These lower fluorescence intensity by self-absorption if the observed wavelength is incorrectly chosen (within the absorption band). To prevent this quenching due to high concentrations, measurements are taken at the lowest possible concentrations (usually < 10 mol/L). A rule of thumb is that ab.sorbance at the excitation wavelength should be < 0.02 to prevent errors > 2.5 %. Under these conditions, fluorescence intensity is proportional to the concentration c of the fluorophor [3] ... 

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