PH-sensitive dyes

A less accurate but more colorful way to measure pH uses a universal indicator, which is a mixture of acid-base indicators that shows changes in color at different pH values (Figure 13.5, p. 359). A similar principle is used with pH paper. Strips of this paper are coated with a mixture of pH-sensitive dyes these strips are widely used to test the pH of biological fluids,... 

Petersons pH probe also was modified in order to give a miniature fiber optic sensor potentially suitable for glucose measurements90. Kopelman et al.91 developed a fiber-optic pH nanosensor for physiological measurements using a dual-emission sensitive dye. The performance of a pH sensor was reported92. An unclad fiber was dip-coated with a thin layer of porous cladding within which a pH-sensitive dye was entrapped. The fundamental... 

Ratiometric luminescent probes make a smart use of the excitation wavelength effect on the emission intensity for extended optosensor performance. For example, the fluorescence from 8-hydroxy-l,3,5-pyrenetrisulfonate (HPTS) and other pH-sensitive dyes in water comes only from its (photo)excited basic form, but the absorption spectra of HPTS and PTS (pAa 7.3) differ considerably (Figure 3). 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

To monitor the brain tissue, the fiber optic sensor based on the immobilization of a pH-sensitive dye, seminaphthorhodamine-1 carboxylate (SNARF-1C), was designed66. The dye-doped sol-gel was deposited onto the end of a 12 pm-diameter silica optical fiber and in vivo studies were performed. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

The members of Wolfbeis team constructed an optical sensor for ammonia-based on ion pairing76. They immobilized pH-sensitive dye (bromophenol blue) as an ion pair with cetyltrimethylammonium bromide (CTABr) in a silicone polymer matrix. Bromophenol blue, while contact the ammonia (both in water as well as in gaseous form) changes its color reversibly from yellow to blue. The immobilized dye shows long wave absorption with a good photostability. 

The way of enzyme entrapment has been described79 proposing the application of sol-gel matrices. The optodes of urea sensor were prepared by the sol-gel method and were stored in a refrigerator. As the pH sensitive dye the bromothymol blue was used. Since it is best acting in pH range 6 to 7.6, the pH of sol-gel bulks obtained in the experiment was chosen as pH 6. Before measurements, the optodes were incubated in the temperature 36.6°C. 

Figure 7.12. Typical absorbance spectra of a near-lit pH-sensitive dye in methanol. The absorbance maxima at pH 6.5 (A) is 535 nm and at pH 2 (B) is 709 nm.

If the oxygen sensitive dye is replaced by a pH sensitive dye, optical pH sensors can be produced. Thus miniaturization of these sensors is easy, and multisensing systems can be set up. Different sensor types for biotechnical application are described in the literature (e.g., ethanol and chloride sensors) [23,24]. 

Typically the chromophores used are pH sensitive, i.e. acidochromic or ionochromic (see section 1.4), and the medium is one whose pH varies with temperature. A variety of pH sensitive chromophores are known that are readily synthesised and can be modified to provide a wide range of shades. Application of these pH sensitive dyes in pressure and thermally sensitive papers for digital printing, e.g. fax papers, where the whole object is for the process to be irreversible, will be covered under ionochromism. The main area of application for reversible indirect thermochromic materials is as thermochromic organic pigments, as will be described below (section 1.3.4.1). 

Composite thermochromic pigments consist of three components a pH sensitive dye, a proton donor, which acts as the colour developer, and a hydrophobic, nonvolatile co-solvent. To achieve the desired effect the components are mixed in specific ratios and usually encapsulated to protect the system in subsequent applications. A review of the patent literature on these compositions has been published. ... 

It is possible to create gels that are also thermochromic by incorporating pH sensitive dyes into the gels. " The gel is made by up mixing a 4800 MW polyalkoxy polymer, PO EO ratio of 87 13, with lithium chloride and an aqueous buffer and a pH sensitive dye. Using Bromothymol Blue (see section 1.4.2.1) the gel is coloured... 

Historically, pH sensitive dyes have been extensively used as indicators in acid-base titrations and in simple spot test papers, even leading to a common phrase in our everyday language, when people or topics are described as having passed the litmus test . The use of complexometric titrations for metal ions was a later but widely... 

HPTS is a highly water-soluble, pH-sensitive dye with a pK of 7.5 in aqueous solution [8], When in alkaline medium, pH > 7.5, acid-base equilibrium is totally displaced toward the anion form (3sPyO ) of the dye. The electronic character of 3sPyO remains unchanged after photo-excitation, and corresponds to a singlet-excited state [9], Fluorescence from this state undergoes a fast 0.4 ps Stokes shift and has a maximum at 515 nm and a lifetime of 5.3 0.1 ns [10],... 

Certain pH-sensitive dyes, assembled in microbeads into wells of a silicon wafer (Ping et al. [89], for details see last section) can also be utilized as metal indicators, e.g., for water quality monitoring. Table 2 lists the metal indicators and their associated analytes that were applied for a colorimetric RGB (red-green-blue) imaging. 

The first example employed an immobilization of GOx and a pH-sensitive dye (l-hydroxypyrene-3,6,8-trisulfonate, HPTS) on the tip of an optical fiber.44 As the local pH was reduced by gluconic acid production, measurable changes in fluorescence were observed. Using flow-through measurements with buffer solutions of various strengths, the optrode response time was determined to be 8-12 min, with a detection limit of 1.8-36 mg/dL glucose and response saturation at 36-54 mg/dL. 

A selective membrane for chloride was obtained by spin coating onto glass a PVC membrane containing a novel mercury-containing carborand, a compound which displayed strong Lewis acid activity [ 123]. This membrane,which also contained a pH-sensitive dye to give an optical response, exhibited strong... 

Zoccarato, F., Cavallini, L., and Alexandre, A. (1999). The pH-sensitive dye acridine orange as a tool to monitor exocytosis/endocytosis in synaptosomes. J. Neurochem. 72, 625-633. 

Further functionalization of mesoporous films with the pH-sensitive dye fluorescein was accomplished by Wirnsberger et al.204 The organosilane used for the actual co-condensation reaction was first prepared by reaction of fluorescein isothiocyanate (FITC) with APTES. The possible use of the dye-modified films as pH sensors was investigated by measurement of the fluorescence after excitation with an Argon laser (488 nm) a dramatic change in fluorescence intensity was observed around pH 8 with a response time of a few seconds. 

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