Spectral and photophysical studies of benzo[c]xanthene dyes: dual emission pH sensors.

A series of fluorescent, long-wavelength, benzo[c]-xanthene dyes has been characterized for pH measurement in both excitation and emission ratio applications. The two general classes of these indicators are seminaphthofluoresceins (SNAFLs) and seminaphthorhodafluors (SNARFs) which are substituted at the 10-position with oxygen or nitrogen, respectively. These probes show separate emissions from the protonated and deprotonated forms of the fluorophores. The dyes may be excited at 488 or 514 nm with argon ion lasers. Most of the indicators have pKa values between 7.6 and 7.9. Detailed photophysical studies were conducted on the carboxy-SNAFL-1 system and excited-state prototropic reactions were compared to structurally related derivatives, such as the umbelliferones. Membrane permeant esters, such as diacetates and acetoxymethyl esters have also been prepared. The indicators are spectrally well resolved from calcium indicators such as fura-2 and indo-1 and should be suitable for simultaneous determination of pH and Ca2+ transients.