Immobilized pH indicator

Harper G.B., Glass-immobilized pH indicators, Anal. Chem. 1975 47 348. 

In this section, the results from a survey of the utility of direct dyes as Immobilized pH Indicators at hydrolyzed cellulose acetate are presented. A goal of this effort was to Identify those direct dyes that could serve as components In the construction of an optical sensor that would exhibit a response over an extended pH... 

Weigl BH, Holobar A, Rodriguez NV, Wolfbeis OS (1993) Chemically and mechanically resistant carbon dioxide optrode based on a covalently immobilized pH indicator. Anal Chim Acta 282 335-343 Werner T, Klimant 1, Wolfbeis OS (1995) Ammonia-sensitive polymer matrix employing immobiUsed indicator ion pairs. Analyst 120 1627-1631... 

A single reagent is used. It forms either a coloured or a luminescing substance in equilibrium with the analyte. Immobilized pH indicator substances are typical examples of this case. 

Actually, most common are pH-sensitive optodes. Immobilized pH indicator reagents act as receptors. Classical indicators as well as fluorescent indicator reagents are utilized. 

Kirkbright G.F., Narayanaswamy R., Welti N.A., Fiber-optic pH probe based on the use of an immobilized colorimetric indicator, Analyst 1984 109 1052. 

Optical sensors rely on optical detection of a chemical species. Two basic operation principles are known for optically sensing chemical species intrinsic optical property of the analyte is utilized for its detection indicator lor label) based sensing is used when the analyte has no intrinsic optical property. For example, pH is measured optically by immobilizing a pH indicator on a solid support and observing changes in the absorption or fluorescence of the indicator as the pH of the sample varies with time1 20. 

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. 

The sol-gel co-immobilization of a non-fluorescent blue indicator bromothymol blue (BTB) with an europium (Ill)-complex intense antenna mediated lanthanide dye represents a new scheme for the fluorescence analysis38. Luminescence spectra of europium (Ill)-complex shown in Figure 12 were found to be independent of pH changes in the range 1-10. Therefore, BTB, a non-fluorescent pH indicator with alkaline absorption maximum close to main europium emission band was added to the sol-gel mixture to shield reversibly the emission of the europium (Ill)-complex at different pH s without quenching of the antenna function. 

Probably the best optical sensors for pH are based on a pH indicator dye covalently immobilized on transparent cellulose membranes (Table 15). The pKa of the dye is 7.34 and colour changes from yellow to purple are observed upon deprotonation. The cellulose membrane exhibits still more than 50% of the initial colouration after two years of storage in distilled water at ambient light. 

An optical sensor for the measurement of carbon dioxide in modified atmosphere packaging (MAP) applications was developed89. It was based on the fluorescent pH indicator l-hydroxypyrene-3,6,8-trisulfonate (HPTS) immobilized in a hydrophobic organically modified (ormosil) matrix. The CO2 sensor was stable over a period of at least 7 months and its output was in excellent agreement with a standard reference method for carbon dioxide analysis. 

There have been many reports in which the immobilization method was covalent binding. In fact, many pH indicators used in above reports own at least one active amino or carboxyl group so that they can be covalently bound relatively easily to a solid substrate [165,166], Kostov et al. had discussed the immobilizing process of Congo red, neutral red and phenol... 

Polymeric pH indicators, phenolphthalein-formaldehyde (PPF) and o-cresolphthalein-formaldehyde (CPF) were synthesized with phenolphthalein and o-cresolphthalein reacted by formaldehyde under alkaline conditions, respectively. They can be immobilized in hydrolyzed cellulose diacetate membranes (HCDA) mainly due to macromolecular entrapment, and can be covalently bound to poly(vinyl alcohol) (PVA) via the considerable newly produced hydroxylmethyl groups [168,169], Phenol red (phenolsulfonphthalein) and its derivatives are commonly used for pH determination. 

Various pH sensors have been built with a fluorescent pH indicator (fluorescein, eosin Y, pyranine, 4-methylumbelliferone, SNARF, carboxy-SNAFL) immobilized at the tip of an optical fiber. The response of a pH sensor corresponds to the titration curve of the indicator, which has a sigmoidal shape with an inflection point for pH = pK , but it should be emphasized that the effective pKa value can be strongly influenced by the physical and chemical properties of the matrix in which the indicator is entrapped (or of the surface on which it is immobilized) without forgetting the dependence on temperature and ionic strength. In solution, the dynamic range is restricted to approximately two pH units, whereas it can be significantly extended (up to four units) when the indicator is immobilized in a microhetero-geneous microenvironment (e.g. a sol-gel matrix). 

Optical sensors for ions use indicators, which exist in two different colors, depending on whether the analyte is bound to them. The use of colored indicators is one of the oldest principles of analytical chemistry, used extensively both in direct analytical spectroscopy and in so-called visual titrations. In their sensing application, the indicator is confined to the surface of the optical sensor or immobilized in the selective layer. In that sense, the oldest and most widespread optical sensor is a pH indicator paper, the litmus paper, which is commonly used for the rapid and convenient semiquantitative estimate of pH of solutions or for endpoint detection in acidobasic titrations. Its hi-tech counterpart is a pH optrode (the name of which is intentionally reminiscent of the pH electrode), which essentially does the same thing (Wolfbeis, 2004). The operation principles and limitations of ion optical sensors are common for all ions. 

We have shown that phytic acid readily hydrolyzes to produce phosphate with a projected lifetime of 100-150 years in the absence of microbiological effects, that actinide-phytate compounds are insoluble, and that europium and uranyl phytates are converted to phosphates within a month at 85 °C. Thorium solubility, on the other hand, is controlled by hydroxide or oxide species. Furthermore, the solubilities of radiotracer europium and uranyl are reduced by phosphate dosing of a simulated groundwater solution, even in the presence of citric acid. In the same systems, neptunium(V) solubility is only affected by 0.01 M phosphate at pH greater than 7. The results of these tracer-scale immobilization experiments indicate that phosphate mineral formation from representative deposits is under thermodynamic control. 

Figure 11.11. Comparison of 2D patterns obtained for bean seed proteins after first-dimension IEF with (a) carrier ampholytes and (b) an immobilized pH gradient. IEF proceeded in the horizontal direction, with pH as indicated across the gels, and SDS-PAGE occurred from top to bottom, with molecular weights indicated in (a).13 [Reprinted, with permission, from A. Gorg, W. Postel and S. Gunther, Electrophoresis 9 (1988) 531-546. The Current State of Two-Dimensional Electrophoresis with Immobilized pH Gradients . 1988 by VCH Verlagsgesellschaft mbH, D-6940 Weinheim.]...

Colorimetric and fluorimetric NH3 sensors contain mixtures of pH indicators having suitable dissociation constants at the tip of the fiber bundle. The measuring solution is separated from this indicator layer by an NH3 gas-permeable membrane covered by an immobilized de-aminating enzyme, e.g. urease (Wolfbeis, 1987 Arnold, 1987). The fluorimetric indication of NADH has been used in optical biosensors for lactate, pyruvate, and ethanol, where the respective dehydrogenase is immobilized at the tip of an optical NADH sensor (Arnold, 1987 Wangsa and Arnold, 1988). 

---