Luminescent probes classes

MAIN CLASSES OF LUMINESCENT PROBES Natural Fluors... 

Lanthanide-doped nanoparticles represent a promising new class of luminescent probes. This is due to their excellent chemical and optical properties availability of multiple lanthanide ions providing different, well-separated emission wavelengths, size- and shape-independent luminescent properties, large effective Stokes shifts. 

On the other hand, it is also apparent that apart from fundamental spectroscopic, photophysical, and photochemical studies, as well as molecular and electronic structure investigations, the long-lived luminescence of d metal complexes also allows this class of compounds to be used as a luminescent probe and sensory material. For example, studies on the interaction of copper(I) [94] and gold(I) [135] complexes with DNA have been reported recently. The attachment of organic receptor type molecules such as crown ethers [157,177,178] and cyclams [145,169] to these metal centers has also been carried out. The binding of different cations to these complexes has been shown to be specific and selective. Studies along these lines will definitely provide a new generation of luminescent sensory materials. 

Another point stemming out of Table 4.1 is that, ideally, the delay and acquisition times should be adapted to the decay time of the luminescent probe. This is not quite easy with chopper-fitted microscopes since the delay and acquisition times not only depend on the rotation speed of the chopper but, also, on the number of blades. Optimum working conditions would therefore require changing the chopper wheel, depending on the luminescent stain, which is not very practical. Microscopes are therefore usually fitted with chopper wheels adequate for one class of LLBs (e.g. lanthanide polyaminocarboxylates, with lifetimes around 600-700 ps). There are other ways of continuously modifying the time delay, but they also have their drawbacks (see below). 

In addition to the ligands described above, several other classes of compounds have been tested as efficient chromophores for lanthanide luminescent probes. For instance, the cationic chelates [Eu(L32)3] (Scheme 4.12) and their Zn° analogues are effective luminescent markers for the diagnosis of oesophageal squamous carcinoma cells at low concentrations, 0.7-1.6pM [181]. 

An important class of luminescence sensors are those based on the decrease of luminescence intensity and lifetime of the probes as function of analyte concentration. Assume that the probe intensity decays by a single exponential with an unquenched lifetime tq. If quenching occurs only by a dynamic (collisional) mechanism, then the ratio to/t is equal to Fq/F and is described by the classic Stern-Volmer equation... 

Wu, M., Lakowicz, J. R. and Geddes, C. D. (2005). Enhanced lanthanide luminescence using silver nanostructures opportunities for a new class of probes with exceptional spectral characteristics. J. Fluorescence 15 53-59. 

The term probe is normally used as a synonym of label. Thus, in contrast to labels, probes respond to their microenvironment or to a chemical species. Those probes responding to a chemical species such as oxygen, an ion, or to pH are also referred to as indicators. In this article, both terms will be used indistinctly. Different classes of fluorescent labels are available. Low molecular weight dyes include xan-thene (rhodamines, fluoresceins), cyanine, coumar-ins, sulfonated pyrenes, and metal phthalocyanine compounds, while high molecular weight labels include phycobiliproteins and other luminescent proteins. 

Hypotheses to explain this observation are the more hydrophobic character of the nucleus with respect to the cytoplasm but also the formation of some solid phase within the nucleus, due to concentration of the probe in the restricted nuclear region. Actually, it is well loiown that in this class of compounds the luminescence is environmentally sensitive, being blueshifted (rigidochromic effect) [73,74] and often also more brilliant, in rigid phases and apolar media [75-77]. 

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