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 cellulose attached to a plastic strip, has been used as a fluorescent optical pH sensor for the neutral pH region. At near neutral pH, HPTS fluoresces green, i.e., from its conjugate base form, exclusively and independent of whether it was excited in its conjugate base form or acidic (phenol) form, or both. However, the fluorescence of immobilized HPTS 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 also a second (blue) band appears which can be attributed to the fluorescence of the conjugate acid (the phenol). The proton transfer in the lowest excited singlet state which accounts for the variation of the fluorescence in the low pH region is here evaluated kinetically. The application of this sensor to measure pH over a wide range of pH values is suggested.