Luminescence anisotropy

Sakamoto T, Mahara A, Munaka T, Yamagata K, Iwase R, Yamaoka T, Murakami A (2004) Time-resolved luminescence anisotropy-based detection of immunoglobulin G using longlifetime Ru(II) complex-labeled protein A. Anal Biochem 329 142-144... 

In many luminescence centers the intensity is a function of a specific orientation in relation to the crystallographic directions in the mineral. Even if a center consists of one atom or ion, such luminescence anisotropy may be produced by a compensating impurity or an intrinsic defect. In the case of cubic crystals this fact does not disrupt optical isotropy since anisotropic centers are oriented statistically uniformly over different crystallographic directions. However, in excitation of luminescence by polarized fight the hidden anisotropy may be revealed and the orientation of centers can be determined. 

Ghosh, S., Priyam, A., and Saha, A. 2009. Surface charge tunabdity and size dependent luminescence anisotropy of aqueous synthesized ZnS/dendrimer nanocomposites. J. Nanosci. Nanotechnol. 9 6726-6735. 

Figure 1. Delayed Luminescence Anisotropy (DIA). A) The sample (lined sphere) is excited with polarized light (vector. The emission is observed at 90" through an analyzer set at an angle corresponding to vector %). B) The ground state G is excited to the singlet state S (square) which decays directly (fluorescence f) or indirectly (phosphorescence p or delayed fluorescence by thermal reactivation to S) through the triplet state T. The circled states are long-lived and the hollow line connecting T and G denotes a slow decay process. Non-radiative modes are omitted (see text).

The structural and dynamic properties of polymerized surfactant aggregates such as detergent micelles, vesicles and bilayers have been studied extensively (32). From a biological aspect, it is of interest to determine in which way these structures mimic the properties of natural membranes (33). Most luminescent anisotropy studies of lipid rotation have employed the fluorescence characteristics of incorporated probes (34,35), as the time scales of lipid rotation are usually in the ns regime. However, a recent electron spin study using incorporated phospholipid spin-labels (36), indicated that rotation about the long axis of dimyristoyl-phosphatidylcholine (DMPC) lipids below the phase transition occurrs with time constants of about 60-100 is. Such values lie within the time domain of phosphorescence anisotropy measurements. 

The measurement of the hnear polarization of the light emitted by a sample constitute the subject of the luminescence anisotropy technique that will be discussed in Sect. 6.3. As discussed above for CPL and CD spectroscopies, luminescence anisotropy can be considered as the emission equivalent of the linear dichroism technique. 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

The versatility of luminescence goes beyond intensity-, wavelength- and kinetic-based measurements. Fluorescence polarization (or anisotropy) is an additional parameter still largely unexplored for optical sensing yet widely used in Biochemistry to study the interaction of proteins, the microfluidity of cell membranes and in fluorescence immunoassays. Although only a few optosensors based on luminescence polarization measurements can be found in the literature, elegant devices have recently been reported to measure chemical parameters such as pFI or O2 even with the bare eye41. 

In practice, the emission anisotropy of luminescent molecules in solution is considered to be proportional to the viscosity of the medium, except in the case where structural reasons arise, for instance helix-coil transition, cross-linking in polymer systems etc... (18-21). Therefore, if there is only the effect in viscosity, the mobility of the marker must be higher in the case of the compact complex system which have a very low viscosity. 

The influence of anisotropy of acceptor wavefunction upon tunnelling luminescence kinetics was treated in [104]. The conclusion was drawn that for the static tunnelling luminescence it just results in the redefinition of the (7o parameter. However, we are interested here in the non-steady-state kinetics and shall demonstrate below that, particularly at this stage, anisotropic recombination reveals distinctive behaviour which allows us to identify it. 

Fig. 4.20. The influence of the tunnelling recombination anisotropy upon the non-steady-state luminescence kinetics. Curves 1 and 2 correspond to the defect reorientation switching-on and -off, respectively at the moments given by broken lines [106],...

FIGURE 2. In a heavily Mg-doped sample Glaser and co-workers observed a resonance (gy = 2.080, gi = 2.000) on a band peaked at about 3 eV (as well as on deeper luminescence) [15,16], Kunzer and co-workers looked at a number of p-type samples [22,23]. For their most heavily Mg-doped sample they obtained a resonance (gy = 2.067, gi = 2.022), while for a less highly doped sample, they obtained a somewhat more anisotropic line (g, = 2.084, gi = 1.990). With a Zn-doped layer they obtained a resonance with only very weak anisotropy (gy = 1.997 and gi = 1.992). The Zn acceptor is deeper [24] than the Mg acceptor (-350 meV versus -200 meV) and so can be expected to have a g value more typical of a deep level (i.e. near two and nearly isotropic). Glaser and co-workers have assigned the resonances in the heavily Mg-doped samples to Mg-related acceptors rather than simple on-site Mg acceptors. On the other hand, Kunzer and co-workers have argued that the Mg-related resonance in their more lightly doped film is due to isolated on-site Mg acceptors and that the decreased anisotropy observed in their heavily doped films is due to Mg acceptor pairs. 

Although there always seemed to be theoretical unanimity on the existence of magnetochiral anisotropy, it was not observed experimentally until a few years ago. The most strongly chiral optical transitions reported in the literature are the 5D0 —> 7 Fij2 luminescent transitions in tris(3-trifluoroacetyl- -camphorato) europium(III) complexes (Eu(( )tfc)3). These transitions also have a considerable MCD. Such complexes are therefore likely candidates to show a significant magnetochiral effect. The experiment performed by us to observe MChA measures the difference in luminescence intensity in the directions parallel and antiparallel to B [17,18]. In order to increase sensitivity, the magnetic field is alternated and the intensity difference 1B k is phase-sensitively detected by a... 

In conclusion, we have described our observations of magnetochiral anisotropy in luminescence, absorption, asymmetric photochemistry, and electrical resistiv-... 

Cha et al. (1999) used a variant of FRET called LRET for lanthanide-based fluorescence energy transfer. In this technique (Selvin, 1996) the donor is terbium or europium which, in fact, is luminescent. There are several advantages of this technique over regular FRET. It has been found that terbium emits isotropically, which means that the uncertainty due to the dipole orientation is decreased to a maximum error of 10%. This error can be decreased even further if the anisotropy of the acceptor is also known. The second advantage is that the fluorescence decay has a time constant of about 1.5 ms, making it easily measurable with conventional recording techniques. The third advantage is that the emission of terbium is peaked and one can find fluorophores that emit in between peaks. This means that the fluorescence of the acceptor can be measured with little or no contamination from the donor. In addition, as the acceptor has a fast decay, any measurement of the acceptor fluorescence with decays comparable to the donor will exclude any possible direct... 

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