Polarity-sensitive dye

A novel approach for ion sensing is based on the use of potential-sensitive or polarity-sensitive dyes (PSDs) and was presented first106 in 1987. PSDs are charge dyes and typically located at the interface between a lipophilic sensor phase and a hydrophilic sample phase. The transport of an ion into the lipophilic sensor layer causes the PSD to be displaced from the hydrophilic/hydrophobic interface into the interior of the respective phase (or vice versa), thereby undergoing a significant change in its fluorescence properties107 110. 

The ion sensing scheme based on the use of potential-sensitive or polarity-sensitive dyes (PSDs) was extended to other anions. Both the clinically significant chloride ion124 and the environmentally important nitrate anion125 can be sensed in the desired concentration ranges. Such sensors have the unique advantage of having a virtually pH-insensitive response. 

Huber Ch., Werner T., Krause Ch., Wolfbeis O.S., Novel Chloride-Selective Optode Based on Polymer-Stabilized Emulsions Doped with a Lipophilic Fluorescent Polarity-Sensitive Dye. Analyst 1999 124 1617. 

Potential-sensitive or polarity-sensitive dyes are known to optically respond to changes in their micro-environment such as changes in polarity or lipophilicity. 

Dickinson and Walt (119) recently reported the synthesis and characterization of a small four-member model hbrary L20 using two monomers in different relative amounts. The composition of L20 is represented in Fig. 11.27. The library individuals were polymerized at the end of a fiber-optic sensor, and a solvatochromic dye was entrapped in the matrix during the polymerization. The exposure of such a sensor to vapors causes the swelling of the four polymers, depending on their nature and composition, and a spectral shift of the polarity-sensitive dye. The two effects combined lead to a sensing region-specific temporal change in the fluorescence emission... 

There are two important drawbacks of such an approach (1) a polarity scale based on a particular class of probes, in principle, does not account, for example, sizes of probes, which should strongly effect the interactions (2) betain dyes do not fluoresce, which restrict essentially the field of application of this approach, because in many cases, absorption spectrum could not be measured accurately (small volumes of samples, study of cells, and single molecules spectroscopy). Therefore, polarity-sensitive fluorescent dyes offer distinct advantage in many applications. 

Anaerobic azo dye reduction can be mediated by enzymes, low molecular weight redox mediators, and chemical reduction by biogenic reductants. These reactions can be located either intracellular or extracellular. Reduction of highly polar azo dyes, which cannot pass through the cell membranes, is located outside the cell. Like azo dyes, nicotinamide adenine dinucleotide phosphate, which is believed to be the main source of electrons, also cannot pass through the cell membranes. Azo reductase enzyme, which is oxygen-sensitive and released extracellularly, is found to be responsible for the reduction of azo dyes. 

The sensitivity of betain dyes to solvent polarity is exceptionally high, but unfortunately they are not fluorescent. Yet polarity-sensitive fluorescent dyes offer... 

Polar yellow 5G is prepared by condensing p-chloro-m-sulfophenylhydrazine with acetoacetic ester, and coupling the resulting pyrazolone with diazotized p-aminophenol in acetic acid solution. The alkali sensitive dye which is formed is treated at 70°C. with p-toluenesulfonyl chloride, in the presence of soda and 1 mole of sodium hydroxide, to esterify the hydroxyl group. This esterification makes the dye insensitive to alkali and, at the same time, fast to milling on wool. 

In the second paper, Saunders and Lovell examine a series of more modern red- and green-sensitizing dyes, and present a wide variety of polarization results. Some dyes do align parallel to the [110] intercept, but others align perpendicular to it. Most puzzling, these authors examined the two dyes oriented by Gray et al. and produced the opposite sense of polarization to that found in the earlier study. How is this to be explained ... 

Solvent polarity is one of the most common solvent characteristics that have been used for correlations of rate constants with the nature of the reaction medium. Since relative permittivities and dipole moments did not give satisfactory results, there was a strong need for a microscopic parameter and many attempts have been made to develop empirical solvent parameters that are based on a physical parameter which is sensitive to the solvent polarity. Solvatochromic dyes have been the most successful and particularly Reichardt s t(30) solvent polarity parameter. The latter is based on the transition energy of the longest wavelength solvatochromic absorption band of the betaine dye pyridinium-N-phenoxide (in fact, this is dye number 30 in the first publication ). Its value is given by... 

The sensitizer (dye) must be soluble in the reaction solvent or solvent mixtures, limiting the dye solvent combinations that can be used. Most polymers are soluble in nonpolar solvents, whereas many sensitizers (dyes) are soluble only in polar media. 

Wakai and coworkers ( Wakai et al., 2005) determined the static dielectric constants of 1-alkyl-3-methylimidazolium ionic liquids by microwave dielectric spectroscopy in the megahertz/gigahertz regime. The obtained results classify the ILs as moderately polar solvents. The observed e-values at 298.15 K fall between 15.2 and 8.8 and decrease with increasing chain length of the alkyl residue of the cation. The anion sequence is trifluoromethylsulfonate > tetrafluoroborate tetrafluorophosphate. The results indicate markedly lower polarities than the ones found by spectroscopy with polarity-sensitive solvatochromic dyes ( Wakai et al., 2005). 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

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