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Issue 7, 2004
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Fluorescent pH sensors with negligible sensitivity to ionic strength

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Optical pH determination has the fundamental disadvantage of measuring a signal that depends on the ionic strength of the sample. The problem originates from the complex relationship between the proton activity and the concentration of the pH-sensitive dye. The effect of ionic strength on the signal depends on the charge of the indicator and its environment, e.g. the immobilisation matrix. We present novel lipophilic fluorescein esters carrying one negative charge. They are embedded in an uncharged, highly proton-permeable hydrogel to give optical pH sensors that show a negligible cross-sensitivity towards ionic strength. The fluorescent dyes differ in their substituents. This variation of substituents results in dissociation constants between 5.5 and 8.5. The indicators were made lipophilic by esterification of the carboxy group with a C18 alkyl chain. Since their spectral properties are quite similar, two indicators may be used in one sensor. This results in an optical pH sensor with a dynamic range that extends from pH 4.5 to 8.

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Publication details

The article was received on 18 Mar 2004, accepted on 30 Apr 2004 and first published on 07 Jun 2004

Article type: Paper
DOI: 10.1039/B404098H
Citation: Analyst, 2004,129, 645-650
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    Fluorescent pH sensors with negligible sensitivity to ionic strength

    B. M. Weidgans, C. Krause, I. Klimant and O. S. Wolfbeis, Analyst, 2004, 129, 645
    DOI: 10.1039/B404098H

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