Probes luminescent indicators

Accordingly, this review is focused on actual sensors, and does not cover the multitude of molecular probes or indicators that have been designed for applications in solution. A notable number of research articles and also reviews on LLCs have been published that term responsive luminescent complexes misleadingly as (molecular) sensor . These cannot be considered in the main part of this overview. Nevertheless, since several of these molecular probes can be incorporated into optical sensors as the receptor part, the next section will summarize a selection of LLC structures that respond to different types of analytes and discuss the basic processes involved. 

All luminescent indicators applied in optical sensors display a more or less strong sensitivity towards temperature. The Boltzmann distribution is one factor for this phenomenon because it governs the populations of the different vibrational levels of the electronic states involved. This evokes the demand for temperature sensitive probes for two purposes (a) measurement or imaging of temperature by means of optical sensors and (b) compensation of temperature effects on optical sensors. 

Figure 1. Some examples of luminescent probes with intramolecular charge transfer (ICT) electronic excited states. The numbers in parenthesis indicate the typical wavelengths of the excitation/emission maximums for each of them in polar media however, introduction of chemical groups in the unsubstituted molecular frame or attachment to a solid support may shift those values.

Chiral ruthenium complexes, with luminescence characteristics indicative of binding modes, and stereoselectivities that may be tuned to the helix topology, may be useful molecular probes in solution for nucleic acid secondary structure36). 

SIMS (61,64,86), microscopy (65), XPS (56), electron microprobe techniques (14,66), electron paramagnetic resonance (EPR) (67) and luminescence experiments (68) have been successfully employed to probe and study V mobility and reactivity on a catalyst surface. TEM, STEM and energy dispersive X-ray emission (EDX) measurements have indicated that V interaction with REY-crystals induced vanadate clusters formation (65). Vanadium was also found capable of reacting with rare-earths outside the zeolite cavities to form LaVQ4... 

C6 glioma cells with the Eu3+ complex (10-500 pM, 45 min and 1 h at 37 °C) provided clear images consistent with the localization of the complex at mitochondria. Incubations of the cells at 4 °C, on the other hand, resulted in no probe uptake by the cells, which indicated that the lanthanide probe entered the cells through some biological activities. Finally, the authors performed luminescence and MR imaging experiment by using cocktail mixtures of Eu3+ and Gd3+ complexes (for example, Eu3+ Gd3+ = 40 60), and successfully obtained nice luminescence and MR images from the same population of C6 cells. 

Most of the applications of FDCD that have been reported have been concerned with the use of this technique as a probe of specific aspects of the chiral environment of biochemical systems. Although, as indicated above, this technique is basically a probe of the molecular ground state, it uses the sensitivity and selectivity of luminescence measurements. FDCD has also been applied to highly scattering and optically dense samples for which polarized absorption measurements are not possible [58,59]. Some of the more recent applications of this technique include its use for on-column detection of chiral molecules in capillary electrophoresis [60], and in a modified phase-modulation spectrofluoremeter [61,62]. The purpose of the latter application is to develop a procedure to determine the distribution of chiral molecules in multicomponent samples [62],... 

The slopes and intercepts of Equations 8 and 9 are quite close, indicating that the molecular probes inhibiting the two superficially different processes are probably operating in the same way at the molecular level. Inhibition of luminescence appears to be different from inhibition of bacterial growth (compare Equation 9 with Equations 27 and 29). In various published (13) and unpublished results linear relationships with slopes of about 0.7 for the inhibition of bacterial growth have been found. 

Quantum Yield Efficiency of fluorescence percentage of incident energy emitted after absorption. The higher the quantum yield, the greater the intensity of the fluorescence, luminescence, or phosphorescence. See Papp, S. and Vanderkooi, J.M., Tryptophan phosphorescence at room temperature as a tool to study protein structure and dynamics, Photochem. Photobiol. 49, 775-784, 1989 Plasek, J. and Sigler, K Slow fluorescent indicators of membrane potential a survey of different approaches to probe response analysis, J. Photochem. Photobiol. 33, 101-124, 1996 Vladimirov, Y.A., Free radicals in primary photobiological processes, Membr. Cell Biol. 12, 645-663, 1998 Maeda, M., New label enzymes for bioluminescent enzyme immunoassay, J. Pharm. Biomed. Anal. 30, 1725-1734, 2003 Imahori, H., Porphyrin-fullerene linked systems as artificial photosynthetic mimics, Org. Biomol. Chem. 2, 1425-1433, 2004 Katerinopoulos, H.E., The coumarin moiety as chromophore of fluorescent ion indicators in biological systems, Curr. Pharm. Des. 10, 3835-3852, 2004. 

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