Sensors Solvatochromic

Solvatochromism Solvent polarity Analysis, sensors, monitoring of polymers and polymerisation... 

For decades solvatochromism was a phenomenon without any great technical importance in commercial applications, being used largely to characterise solvents or to understand the excitation behaviour of chromogenic solutes, e.g. commercial azo dyes." " However, in the last two decades of the 20th century there was a rapid growth in the use of optical probes and sensors in a range of outlets, and this led to a renewed interest in solvatochromism. 

For example, dissolving the betaine (1.98) or another negative solvatochromic dye in PMM and spin coating the solution to give a film on a sensor tip provides a product suitable for the analysis, in visible absorption or transmission mode, of alcohols and ethers in gasoline. 

The use of fluorescent dyes in biological probes and sensors is covered in some detail in Chapter 3 (section 3.5.6). Because there are marked solvatochromic effects on the luminescent spectra of many fluorophores, this phenomenon is utilised to tune their performance and application in biological and other systems. 

Figure 3.13 Two-step grafting process in order to construct a sensor for the detection of volatile organic compounds based on the well-known solvatochromic fluorescent phenoxazinone dyes. Hydrophobizing of the surface of MCM-41 with silazanes leads to better performance of this sensor.

Figure 16.20 Examples of sensor molecules (a) tetraphenyl porphyrin, solvatochromic Rei-chardt s dye and pH indicator (phenol red) (b) image of the 36-dye matrix before exposure, after exposure, and the resulting difference map and (c) colour change profiles for common beverages. (Reprinted with permission from Zhang and Suslick [90], Suslick et at. [91], and Janzen et al. [147]. American Chemical Society 2006-2007)...

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... 

We have found that different polymeric matrices provide diverse response patterns of polymer-incorporated CdSe nanocrystals to vapors of different polarity. Such work promises to complement existing solvatochromic organic dye sensors with more photostable and reliable sensor materials. The stability of the sensor response pattern from 2.8 and 5.6-nm nanocrystals in PMMA film over more than 10 h of continuous exposure of the film to laser radiation is very attractive for diverse applications where continuous monitoring is needed. 

The use of solvatochromic compounds as polarity sensors in a variety of environments constitutes one of the most important applications of these molecules. Solvent polarity scales were designed that rely on the solvatochromic behavior of one or a set of compounds." ... 

The large number of empirical polarity scales derived from solvatochromic sensors raises the question of the degree of correlation among them. In principle, there is no reason to expect a correlation between any pair of these empirical scales. The solvatochromic behavior of a given compound reflects a sum of specific and nonspecific solute-solvent interactions that vary from probe to probe. Good correlations are to be expected only between scales based on solvatochromic compounds that present a similar response to a range of solvents. As a result of this, the concept of solvent polarity is elusive and the claims of a universal polarity scale based on solvatochromic probes cannot be maintained. Different solvents may assume different polarity values, according to the nature of the scale employed to define them. 

The use of solvatochromic compounds as polarity indicators may be extended to solvent mixtures and micellar systems. In this case, an additional difficulty is introduced in the assessment of systems that are not homogeneous from a microscopic point of view The microenvironment actaally seen by the sensor does not correspond to the bulk characteristics of the medium. In a binary solvent mixture, a solvatochromic probe may be more solvated by one of the components, thus reflecting through its spectrum a solvent composition that may be different from that of the bulk mixture. In micellar systems, the solvatochromic response of a probe reflects the nature of its microenvironment and is dependent on the relative solubility of the sensor in the aqueous or the organic pseudophases, or in the micellar interphase. 

Solvatochromic lumophores, 2-substituted 3-ethynylquinoxalines, were synthesized via a one-pot Stephens-Castro coupling cyclocondensation by Gers and collaborators (Scheme 51) (14JOC3296). Due to their photophysical properties and the presence of the ethynyl group in the final product, which allows integration into more complex structures, the authors envision using these compounds as sensors for protein interactions. 

Fluorophores capable of CHEF and other types of environment-sensitive (or solvatochromic) fluorophores have been utilized in sensors for kinase activity. Chelation-sensitive fluorophores manifest altered fluorescent properties upon chelation of various metal ions, while environment-sensitive fluorophores exhibit altered excitation and emission properties with changing environment, such as solvent polarity (Figure 1.1). 

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