Sensitization of luminescence

The very first spectroscopic instruments, from Newton s prism and pinhole to Frauenhofer s simple spectroscope, were constructed to observe luminescence. Even though the great sensitivity of luminescence detection seemed to promise that luminescence would become an important tool for chemical analysis, the fact is that absorption spectroscopy was the first spectroscopic technique to be widely used. At first glance, this may seem surprising since absorption spectroscopy is inherently less sensitive and had to await the development of more complex instrumentation, especially, electronically amplified detection. 

Table 5.12 shows the main features of luminescence spectroscopy. The much higher sensitivity and specificity of luminescence techniques compared to absorption techniques is an obvious advantage for excitation spectra. In solution studies, pg ml. 1 levels can often be determined, as compared to p,gmL-1 levels in absorption spectroscopy. The greater sensitivity of luminescence techniques stems from the fact that the... 

EXAMPLE 1.5 The sensitivity of luminescence. Consider a photoluminescence experiment in which the excitation source provides a power of 100 ptW at a wavelength of400 nm. The phosphor sample can absorb light at this wavelength and emit light with a quantum efficiency of r] = O.I. Assuming that kg = 10 fii.e., only one-thousandth of the emitted light reaches the detector) and a minimum detectable intensity of l(f photons per second, determine the minimum optical density that can be detected by luminescence. 

The processes involved are stepwise energy transfer, cooperative sensitization of luminescence and cooperative luminescence. As an example of stepwise energy transfer a system containing Er and Yb may be considered. The latter ion absorbs at 970 nm (10,300 cm ) and in phonon assisted an Er ion... 

FDCD measurements, and a basic theoretical formalism for this technique, were first reported by Turner, Tinoco and Maestre in 1974 [5]. In this experiment one uses the selectivity and sensitivity of luminescence measurements to probe the local chiral environment of fluorescent chromophores. The ultimate goal in many applications of FDCD is to relate the observed differential fluorescence signal to the conventional CD measurement. In certain multi-component absorbing systems this procedure may be difficult. This technique is sometimes applied to systems for which CD measurements are impossible or very difficult. FDCD, like CPL and other polarization sensitive techniques, is not immune to troublesome background and noise problems, and these will be discussed in Section 3. The only detailed discussion of the applicability of FDCD measurements, and other characteristics of the technique has been presented by Turner in 1978 [6]. In this chapter we will also list some of the more recent applications of FDCD. 

Biological Systems - The sensitivity of luminescence techniques and the use of extrinsic and intrinsic fluorescence probes have found considerable applications in biological research. Only a selection of such studies have been selected for citation. 

The sensitivity of luminescence spectroscopy allows the examination of a wide concentration range from the ultra-dilute regime through to the bulk phase. Fluorescence spectroscopic techniques would appear ideally suited to... 

Exchange interactions Phonon assisted energy transfer Excitation diffusion Sensitization of luminescence... 

Atone M. S., Dhoble S. J. and Moharil S. V. 1993b. Sensitization of luminescence of CaS04 Dy, Phys. Status SolidiA 135 299-305. 

Fig. 9. Proposed electron-transfer mechanism for the sensitization of luminescence by the excited state of tryptophan (Trp). Figures are energies of the states in eV. Redrawn from (deW. Horrocks et al., 1997).

Following this study, Korovin and coworkers tested cryptands 23b-f which are used in time-resolved luminescent immunoassays (Mathis, 1993) for the sensitization of luminescence (Korovin et ah, 2002b). From the lifetimes determined in both water and deuterated water, one calculates that the hydration number varies from 2 (23b), to 1.5 (23a, 23e), and finally to (23c, 23d, 23f). Quantum yields were not determined, but luminescence intensities relative to the cryptate with 23a (in water, at room temperature) point to cryptands 23c and 23d being the best sensitizers of the Yb luminescence with a seven-fold enhancement, while cryptates with 23e and 23b are only 1.5- to 1.8-times more luminescent. 

Abstract Considerable studies have been made on iridium complexes during the past 10 years, due to their high quantum efficiency, color tenability, and potential applications in various areas. In this chapter, we review the synthesis, structure, and photophysical properties of luminescent Ir complexes, as well as their applications in organic light-emitting diodes (OLEDs), biological labeling, sensitizers of luminescence, and chemosensors. 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two measurable quantities, which are obtained in arbitrary units and related to the circular polarization condition of the luminescence. It is appropriate to consider CPLS spectroscopy as a technique that combines the selectivity of CD with the sensitivity of luminescence. The major limitation associated with CPLS spectroscopy is that it is confined to emissive molecules only. 

The use of lucigenin chemiluminescence as a measure of 02 has been questioned because lucigenin has been shown to be capable of mediating the production of 02 . Nevertheless, perhaps because of the convenience and sensitivity of luminescence methods, the use of lucigenin as a measure of 02 continues. 

Similar time resolved techniques to study a host of transfer phenomena such as sensitization of luminescence, cross relaxation and optical trapping have been developed (Yen 1986b). Time resolved absorption or hole burning experiments are also possible but have been less common. In all of the above, the pulse length... 

FDCD measurements have the potential for providing valuable spectroscopic and structural information for systems in which conventional CD measurements are problematic. Of particular interest are complex systems containing multiple absorption chromophores but only one fluorescent chromophore. It is also possible to use the intrinsically higher sensitivity of luminescence detection to determine... 

The sensitivity of luminescence to many substances that cause harmful effects in animals invites preliminary tests for possible nontoxicity of new chemotherapeutic compounds. Unfortunately, the criterion is not... 

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