Absorption of phenol red

As the pH increases, the concentration of the base form of phenol red increases, resulting in increased energy transfer from eosin to phenol red and in a diminished fluorescence intensity of eosin. Thus, changes in the absorption of phenol red as a function of pH are detected as a change in the fluorescence signal of eosin. 

Yamamoto, A., et al. 1996. Effectiveness and toxicity screening of various absorption enhancers in the rat small intestine Effects of absorption enhancers on the intestinal absorption of phenol red and the release of protein and phospholipids from the intestinal membrane. J Pharm Pharmacol 48 1285. 

Dodecylmaltoside increased the small intestinal and colonic absorption of phenol red in rats without causing membrane protein and phospholipid release (Yamamoto et al. 1996 Uchiyama et al. 1996). These results, however, are not in agreement with an in vitro study showing a significant protein and phospholipid release from rat jejunum and colon when dodecylmaltoside is applied in the same concentration (Yamamoto et al. 1997). 

The absorption-enhancing effects of protease inhibitors on the intestinal absorption of water-soluble compounds in rats was also examined. Some protease inhibitors may have absorption-enhancing activities in addition to their protease inhibitory actions (e.g., NaGC) [43, 44]. Aprotinin, bacitracin, and soybean trypsin inhibitor (STI) were used as protease inhibitors, while phenol red and fluorescein isothiocyanate (FITC)-labeled dextran with an average molecular weight of 4000 Da (FD-4) were selected as poorly absorbable and stable model compounds. Bacitracin enhanced the absorption of phenol red from the rat small and large intestine in the presence of the protease inhibitors, and similar results were noted for the intestinal absorption of FD-4 co-administered with bacitracin. Thus, bacitracin was seen to have not only a proteolytic inhibitory action but also an absorption-enhancing capability. 

Another experiment carried out by Hill s group was an attempt [206] to detect ejection of from the mitochondrial matrix, as associated with the cytochrome c oxidase proton pump . With a Au OTTLE modified by pre-adsorption of bis(4-pyridyl)disulfide, to reduce cytochrome c, pumping was expected to be observed as a transient change in the light absorption of phenol red— in an essentially unbuffered suspension of rat-hver mitochondria— as the electrode potential was jumped to a reducing level. However, the experiment succeeded only in detecting the alkalinization expected from chemical depletion of H as O2 was reduced. 

Table 7.6 Effect of drugs on the enhancement of absorption of phenol red by surfactants (1 % in normal saline)...

Figure 10.12. Absorption spectra of Phenol Red and emission spectrum of Eosin as potential pH probe based on resonance energy transfer.

Suppose we have a pH indicator like Phenol Red whose absorption spectrum is pH-sensitivewith pKa = 7.6 (Figure 10.12). Phenol Red displays two distinct absorption spectra for protonated form (pH 2.5) and for unprotonated form (pH 10.4). One of the possible donors is an Eosin which displays an emission spectrum that overlaps with the absorption spectra of the protonated and unprotonated forms (acceptors) of Phenol Red (Figure 10.12). The critical distances for energy transfer (R0),(32) calculated from spectral properties of Eosin and Phenol Red, are 28.3 and 52.5 A for protonated and unprotonated forms of Phenol Red, respectively. For randomly distributed acceptors in three dimensions with no diffusion, the donor decay is... 

Figure 15.5 Absorption spectra of phenol red dye in solution at various pH values. (From the article by Seller [25] copyright 1994 IEEE, with permission.)... 

The first FOCS described for pH measurements was based on the absorption variations of Phenol Red [10]. This dye is copolymerized with a monomer of the acrylic type (5-10 pm diameter) [24]. Polystyrene microspheres (20<% with diameter 1 pm) improve the diffusion of light and provide a hydrophilic structure that is advantageous for ion transfer. The ratio (/, ) of the intensities at two wavelengths (558 nm at the peak and 478 nm at the isosbestic point) is related to the concentration and pH by the equation... 

On the day of the assay, gendy aspirate medium from wells and use a multichannel pipette to add 50 pi per well of phenol red free DMEM medium with 0.2% probumin 30 min before imaging. The added medium should be equdibrated to 37 °C in a tissue culture incubator before adding to cells. We use phenol red free DMEM to reduce absorption of emitted bioluminescence. The 0.2% probumin serves as a carrier... 

Fig.2 Relative yield of proton uptake from the medium as function of flash number, derived from absorption changes of phenol red at 559 nm. With hexacyanoferrate III as electron acceptor these absorption changes were only due to proton uptake at photosystem II.

Figure 2. UV/Vis absorption spectra of Phenol Red in a (PVA)/borax/surfactant gel mtwork at different temperatures.

A fiber-optic pH sensor based on fluorescence energy transfer can be construeted by coimmobilizing a pH-sensitive fluorophore and apH-sensitive absorber. For example, eosin (donor) and phenol red (acceptor) were coimmobilized in a polymer on the distal end of a silanized single-core optical fiber. Eosin s emission spectrum overlaps with the absorption of the basic form of phenol red. The concentration of the basic form of phenol red increases with an increase in pH. As a result, energy transfer from eosin to phenol red increases and the fluorescence intensity of eosin decreases. Thus, the pH-dependent absorption change of phenol red can be detected as changes in the fluorescence signal of eosin. 

Optical fiber detectors are usually utilized as ether miniature spectrophotometers or fluorimeters, because of the availability of indicator dyes that can be detected by absorbance or fluorescence methods. Figure 12 shows the absorption spectrum for phenol red, a dye commonly used as pH indicator. By measuring the absorption of solutions of phenol red at two wavelengths, the isobestic point where absorption is virtually independent of pH and at 600 m u where the intensity of absorption is very pH sensitive, one... 

FIGURE 12 Concentrations of chemicals can be measured colorimetric ally. This example shows that pH can be estimated by measuring the absorption of light by a solution of phenol red at two wavelengths. At 550 nm the extinction coefficient of phenol red changes dramatically with pH, while at 470 nm the extinction coefficient is virtually independent of pH. By utilizing the ratio of absorbance at these two wavelengths, an accurate estimate of pH can be obtained. 

The immobilization of phenol red (phenolsulfonphthalein) in microspheres of polyacrylamide and polystyrene produces an optical sensor that is sensitive to 0.01 pH units in the pH range 7.0 to 7.4 [197]. Phenol red exists in two tautomeric forms, one acidic and one basic, which have different absorption spectra. Two light emitting diodes (LED) are used a reference diode, emitting at 810 nm, and a working diode, emitting at 565 nm which corresponds to the wavelength of the maximal absorption of the basic form. This sensor can follow variations in pH between 6.5 and 9.0 [198]. Bromothymol blue has a maximal absorption at 593 nm and can be used instead of phenol red [199]. 

Fluoride is also used as an anionic dopant. An early study demonstrated that F substitutes surface OH species leading to an increase in the degradation of phenol at least three times faster than an undoped sample [56]. Other anions are also effective as dopants in reducing the Ti02 bandgap. Chloride, for example, was shown to give active photocatalysts in the visible range thanks to the red shift in the absorption spectra (Eg = 3 eV) but also increased surface acidity [57]. 

For the purpose of the following discussion, the xenobiotics studied in the dogfish shark were divided into three classes 1) those relatively hydrophilic (Table V) those relatively lipophilic (i.e., solubility in water less than 1 mg/ml, Table VI) and, 3) metal-containing pollutants (Table VII) Most of these data have been previously reported (18-23) using C compound, for assay, with the exception of sodium lauryl sulfate (SLS) ( S), cis-Pt (atomic absorption spectroscopy) and phenol red (spectrophotometry). Unless otherwise stated these data are presented as total radioactivity and the hazards of doing so are recognized (24). 

Sutton SC, Rinaldi MT and Vukovinsky KE (2001) Phenol Red and Comparison of the Gravimetric 14c-Peg-3350 Methods to Determine Water Absorption in the Rat Single-Pass Intestinal Perfusion Model. AAPS Pharm Sci 3 pp 1-5. 

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