An optical ionic-strength sensor based on polyelectrolyte association and fluorescence energy transfer

The optical ionic-strength sensor is based on an indicator phase consisting of an aqueous solution of fluorescein-labelled dextran and polyethyleneimine labelled with Sulforhodamine 101 (Texas Red), confined behind a dialysis membrane. At low ionic strength the polymers associate and the average distance between the fluorescein and Texas Red is short enough for efficient energy transfer to occur. With increasing ionic strength the polymers dissociate and the efficiency of energy transfer decreases. The measured parameter is the ratio of the emission intensity at 520 nm, where fluorescein fluorescence is maximal, to the intensity at 620 nm, where the Texas Red emission is strong. The increase in the intensity ratio as a function of ionic strength is similar but not quite the same for different ions, suggesting that the mechanism of response involves more than a simple ionic strength effect.