Chemical. CAS 128724-35-6. Formula C29H16O7. MW 476.43. 56-Carboxynaphthofluorescein CNF is a fluorescent NIR emitting pH indicator with red fluorescence at pH 9 Spectral Ex/Em 598/668 nm. The fluorescence shifs to shorter wavelengths Ex/Em 512/567nm at neutral or acidic pH environments. Spectral data lambdaex=512nm lambdaem=567nm acid/neutral lambdaex=598nm lambdaem=668nm basic. CNF shows good sensitivity in an alkaline pH range and it can be exploited in the construction of fiber-optic pH sensors. CNF has been used as an intravesicular pH probe for the colorimetric detection of activity, selectivity and cooperativity of ion channels. CNF has been used to design a luminescent probe for determination of pH, based on a Forster resonance energy transfer FRET system, combining a europium chelate as the donor and CNF as a pH sensitive acceptor. The FRET system enables referenced pH detection in an exceptional broad dynamic range from pH 3 to 9.