Dye indicators

The first indicator dye which came into use, was litmus, which was isolated from the orseille lichen, Roccela tinctoria. It changes its color from red (acid) to blue (alkaline). 

The systematic investigation of natural colors led to the discovery of several dyes and dye classes which are able to change their color. Thus, it was found that the origin of the color of hydrangea blossoms is an anthocyan dye. 

Interestingly, in nature these dyes are not always and necessarily used as indicators the red color of roses and the blue color of the blue cornflower are based on the same molecule (also an anthocyan), which in vitro reacts as an indicator dye. In the plants however, this dye does not change color the red rose does not change to blue due to the presence of an intrinsic buffer system. 

With the detection and introduction of titration (more general volumetry) into the arsenal of analytical methods, an increasing need arose for means which allowed the end-point of such an analysis to be detected. This need stimulated the search for indicator dyes, and consequently the methods of dye synthesis were also used for this purpose. Asa result, most indicator dyes are now synthetic. 

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Indian red Indican [487-60-5 Indicated reserves Indicator Indicator badges Indicator dye Indicators... 

Amino-4,6-dinitropheno1 is an important intermediate in the manufacture of colorants, especially mordant dyes. It has also been used as an indicator dye in titrations (yellow with acid, red with alkali) and as a reagent for albumin deterrnination. 

Table 9. Use of indicator dyes to detect pH-active substances.

Another major second messenger in cells is calcium ion. Virtually any mammalian cell line can be used to measure transient calcium currents in fluorescence assays when cells are preloaded with an indicator dye that allows monitoring of changes in cytosolic calcium concentration. These responses can be observed in real time, but a characteristic of these responses is that they are transient. This may lead to problems with hemi-equilibria in antagonist studies whereby the maximal responses to agonists may be depressed in the presence of antagonists. These effects are discussed more fully in Chapter 6. 

As an example, suppose a chemist needs to know the hydrogen ion concentration in a solution containing both 0.010 M benzoic acid, QHsCOOH, and 0.030 M sodium benzoate, QHtCOONa. Of course, he could go to the laboratory and proceed to investigate the colors of indicator dyes placed in the solution. However, it is easier to calculate the value of [H+], using the accurate value of Ka listed in Appendix 2. 

As a consequence, it is not possible to devise a single class of anodic inhibitor formulation that satisfies all water treatment requirements. Rather, there several types of formulations, and some formulators may offer each of these products in different strengths, with or without oxygen scavengers and indicator dyes in their blends. As examples, a nitrite/sUicate formulation and a molybdate/nitrite formulation are provided here ... 

The reaction may be followed by measuring the rate of appearance or disappearance of MADH at 340 nm. In some methods a coupled reaction using diaphorase or phenazine methosulfate plus an indicator dye are used. The amount of pyruvate formed can also be measured colorimetrically. With the general avail-... 

Most probably, the first - but non-fiberoptic - sensors for continuous use where those for pH and for oxygen. It has been known for decades that cellulosic paper can be soaked with pH indicator dyes to give pH indicator strips which, however, leached and thus were of the "single-use" type. The respective research and development is not easily traced back since it is not well documented in the public literature. However, in the 1970s, indicator strips became available where they pH indicator dye was covalently linked to the cellulose matrix, usually via vinylsulfonyl groups. These "nonbleeding" test strips allowed a distinctly improved and continuous pH measurement, initially by visual inspection. In the late 1980 s instruments were made available that enabled the color (more precisely the reflectance) of such sensor strips to be quantified and related to pH. Respective instruments are based on the use of LEDs and are small enough to be useful for field tests in that they can be even hand-held. This simple and low cost detection system is still superior to many of the complicated, if not expensive optical pH sensors that have been described in the past 20 years. 

Various polymer matrices can be used as solid support into which pH indicator dye can be immobilized (cellulose, PVA, PVC). R. Makote and M.M. Collinson shows that also organically modified silicate films can be used for stable pH sensors37. 

We can just measure the intrinsic fluorescence of the target analyte or design sensors based on the variation of the fluorescence of an indicator dye, with the determinand concentration. In the latter case, the probe molecule... 

Figure 2. Principles of reversible luminescence sensing using photochemical quenching processes (electron, energy or proton transfer). Dye = luminescent indicator Q = quencher species dotted arrow non-radiative deactivation processes. The luminescence intensity (and excited state lifetime) of the indicator dye decreases in the presence of the quencher. The indicator dye is typically supported onto a polymer material in contact with the sample. The quencher may he the analyte itself or a third partner species that interacts with the analyte (see text).

Fluorescence quenching may be dynamic, if the photochemical process is the result of a collision between the photoexcited indicator dye and the quencher species, or static, when the luminophore and the quencher are preassociated before photoexcitation of the former20. It may be easily demonstrated that dynamic quenching in isotropic 3-D medium obeys the so-called Stem-Volmer equation (2)21 ... 

It may also happen that an association equilibrium exists between the luminescent indicator and the quencher. Non-associated indicator molecules will be quenched by a dynamic process however, the paired indicator dye will be instantaneously deactivated after absorption of light (static quenching). Equation 2 still holds provided static quenching is the only luminescence deactivation mechanism (i.e. no simultaneous dynamic quenching occurs) but, in this case, Ksv equals their association constant (Kas). However, if both mechanisms operate simultaneously (a common situation), the Stem-Volmer equation adopts more complicated forms, depending on the stoichiometry of the fluorophore quencher adduct, the occurrence of different complexes, and their different association constants. For instance, if the adduct has a 1 1 composition (the simplest case), the Stem-Volmer equation is given by equation 3 ... 

Figure 4. Luminescence decay profile of an oxygen indicator dye excited by a short flash of light, in (a) solution and (b) embedded into a gas-permeable film used to fabricate fiber-optic sensors for such species. The logarithmic scale of the Y-axis allows to compare the exponential emission decay in homogeneous solution and the strongly non-exponential profile of the photoexcited dye after immobilization in a polymer matrix.

The luminescence lifetime may also be extracted from phase-sensitive detection of the modulated emission that originates from modulated continuous excitation of the indicator dye (equation 9)2 ... 

Polymer materials are frequently used matrices for the indicator chemistry in optical sensors. This is necessary for several reasons first, the indicator has to be immobilized to an optical waveguide or an optical fibre which is then brought into contact with the analyte solution. If one would pour an aqueous solution of the indicator dye directly into the sample solution, e.g. into a bioreactor, then the whole sample solution would be contaminated. 

Next, the indicator dye needs a solvent to interact with the analyte. Pure crystalline indicator dyes might react at the surface but not all indicator would react due to hindered diffusion. Therefore, the indicator is dissolved in a polymer which allows free diffusion of the analyte to and from the indicator molecule. 

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