DEPENDENCE OF THE FLUORESCENCE OF IMMOBILIZED 1-HYDROXYPYRENE-3,6,8-TRISULFONATE ON SOLUTION PH - EXTENSION OF THE RANGE OF APPLICABILITY OF A PH FLUOROSENSOR

1-Hydroxy-3,6,8-pyrenetrisulfonate (HPTS), covalently immobilized on c ellulose attached to a plastic strip, has been used as a fluorescent o ptical pH sensor for the neutral pH region. At near neutral pH, HPTS f luoresces green, i.e., from its conjugate base form, exclusively and i ndependent of whether it was excited in its conjugate base form or aci dic (phenol) form, or both. However, the fluorescence of immobilized H PTS also varies in the low pH region from about pH 3 to below pH 1, in that not only the green emission almost completely disappears, but al so a second (blue) band appears which can be attributed to the fluores cence of the conjugate acid (the phenol). The proton transfer in the l owest excited singlet state which accounts for the variation of the fl uorescence in the low pH region is here evaluated kinetically. The app lication of this sensor to measure pH over a wide range of pH values i s suggested.