Of cationic probes

The interactions of various cationic species with polyacids, such as polyCmethacrylic acid), PMA and polyCacrylic acid), PAA have been studied. In particular, the effect of polyacid conformation on the interaction is discussed in detail, and also the nature of the aggregation of PMA and cationic surfactants al)tyltrimethylammonium bromide, C TAB. The effect of the intermediate conformation states of PMA around pH 4-6 is noted, where the photophysical properties of cationic probes bound to PMA dramatically change, effects such as a large enhancement of the fluorescence intensity of Auramine 0, Au 0 at pH 4.5, a blue shift of the luminescence spectra of tris(2,2 -bipyridine)ruthenium(II) complex, Ru(bpy)3 at pH 5, and a great increase of the excimer yield of 1-pyrenebutyltrimethyl ammonium bromide, C PN at pH 6. 

CONCLUSIONS This Study shows that fluorescence probing techniques are useful and powerful tools for investigation of conformational transitions of polyelectrolytes as induced by cationic surfactants, pH or other means. Studies on the interaction of cationic probes with polyelectrolytes provide useful information on the intermediates that lie between A states and B states. It is concluded that the conformational transition induced by pH is a progressive process over several pH units. Studies on the interaction of cationic surfactants with PMA at pH 8 show that the aggregates formed are large loose structures, while the interior of the aggregate has a hydrophobicity that is similar to that of a micelle. 

A sonty of the literature reveals that a bsge number of diverse structures can diel cations. >weva a domi nant use of cation probes is imaging of intrsscelluiar cations. In this case the indicators have to be sensitive to the intracdlular concentrations of cations or anions (T e 19.3). lliese concentrations define tite affinity needed the chelators for the cation and the d ree of discrimination required against other cations. Also, it is desirabieto have... 

Related work is dedicated to compounds with (L)AuC=C-functions attached to crown ether and cryptand-type units, following the idea that the luminescence properties of the chromophores will be influenced by complexation of cations in the polyether groups.87 Scheme 15 presents two examples of the devices probed in these highly successful studies. 

Spectroscopic techniques such as electron spin resonance (ESR) offer the possibility to "probe" the chemical environment of the interlayer regions. With the ESR technique, an appropriate paramagnetic ion or molecule is allowed to penetrate the interlayer, and chemical information is deduced from the ESR spectrum. Transition metal ions, such as Cu2+, and nitroxide radical cations, such as TEMPAMINE (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) have been used as probes in this manner (6-14). Since ESR is a sensitive and non-destructive method, investigations of small quantities of cations on layer silicate clays at various stages... 

While research in the area started with probes for cations, the last years have shown that the dual-fluorophore excimer concept seems especially promising for the development of anion probes. The benefit here is often twofold. On one hand, the observation of both emission bands provides a more stable differential or... 

It is important to notice that a change in lifetime is not a necessary result of a change in fluorescence intensity. For instance, the Ca2+ probe Fluo-3 displays a large increase in intensity on binding Ca2+, but there is no change in lifetime. This is because the Ca-free form of the probe is effectively nonfluorescent, and its emission does not contribute to the lifetime measurement. In order to obtain a change in lifetime, the probe must display detectable emission from both the free and cation-bound forms. Then the lifetime reflects the fraction of the probe complexed with cations. Of course, this consideration does not apply to collisional quenching, when the intensity decay of the entire ensemble of fluorophores is decreased by diffusive encounters with the quencher. 

The discovery of crown ethers and cryptands in the late sixties opened new possibilities of cation recognition with improvement of selectivity, especially for alkali metal ions for which there is a lack of selective chelators. Then, the idea of coupling these ionophores to chromophores or fluorophores, leading to so-called chromoionophores and fluoroionophores, respectively, emerged some years later l9) As only fluorescent probes are considered in this chapter, chromoionophores will not be described. 

Fluorescent signaling receptors of cations are generally classed according to the nature of the cation to be probed, but in this chapter devoted to the principles of design, they will be classed according to the involved photophysical processes. 

The development of fluorescent probes for anion recognition has been very limited so far in comparison with those for cations. Most of the presently available methods of detection of anions based on fluorescence involve quenching, redox reactions, substitution reactions, ternary complex formation(15) and thus cannot be considered as recognition methods. For instance, the fluorescent sensors that are used for the determination of chloride anions in living cells are based on collisional quenching of a dye by halide ions 6-methoxy-iV-(sulfopropyl)quinoliniuni and... 

At the present time, there is a striking contrast between the extensive development of fluorescent probes for cation recognition and the limited number of available probes for anions notwithstanding the great need for the latter. This is due to the difficulty of the design of selective anion receptors progress made in the relevant field of supra-molecular chemistry will certainly lead in the future to new selective fluorescent signaling receptors of anions. 

S. A. Jonker, F. Ariese, and J. W. Verhoeven, Cation complexation with functionalized 9-arylacrid-inium ions Possible applications in the development of cation-selective optical probes, Reel Trav. Chim. Pays-Bas 108, 109-115 (1989). 

When protonation of a probe leads to dramatic effects on the electronic absorption and emission as described above, one can expect that cations other than protons might be capable of inducing similar effects if these ions can be made to bind to the basic atom of the probe molecule. A simple but effective method to do so exists in the formal replacement of the amino substituents ofchromophores by aza crowns. In the resulting chromoionophores the amine nitrogens possess simultaneously an electron-donor... 

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