A fluorescence sensor for quantifying pH in the range from 6.5 to 8.5

A sensor for quantifying pH values in the physiological range has been prepared by immobilizing the trisodium salt of 8-hydroxyl-1,3,6-pyridine trisulfonic acid (HOPSA) on an anion-exchange membrane. Because electronically excited HOPSA undergoes rapid deprotonation, both acid and base forms of HOPSA lead to fluorescence from the excited state of OPSA−. However, the acid and base forms of HOPSA can be selectively detected by appropriate choice of excitation wavelengths. The ratio of fluorescence intensities resulting from excitation at 470 and 405 nm can be used to quantify pH values between 6 and 9. The ratio is unaffected by variables such as temperature and ionic strength which affect abslute intensities. At coverages below 15 μg cm−2, the ratio varies only slightly with the amount of HOPSA immobilized on the membrane. Membranes treated with HOPSA can be stored for extended periods of time without changing characteristics. However, they undergo slow photodegradation which will limit their useful lifetimes.