On the mechanism of Nile blue and Nile red interactions within the ion-selective membrane

The Nile blue/Nile red system interactions within ion-selective membranes are investigated. A dye pair was introduced to the membrane spontaneously during contact of Nile blue aqueous solution, optionally containing Nile red. Both Nile red and Nile blue are accumulated in the membrane of typical composition, however Nile blue cations incorporation requires interaction with ion-exchanger anions: tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. In the presence of Nile blue the fluorescence of Nile red is suppressed due to Fluorescence Resonant Energy Transfer (FRET) to Nile blue as well as reabsorption due to Inner Filter Effect (IFE). Because of the presence of ionophore in the membrane, Nile blue cations can be released from the membrane when primary cations bind to ionophore, as tested on model example of potassium ions. This results in fluorescence intensity increase of Nile red due to mitigation of above mentioned fluorescence suppression effects. It was observed that a ratiometric fluorimetric response, dependent on time due to diffusion limitations, increases with rising potassium ions concentration within the range from 10^-4.5 M to 10^-2.5 M, which can be explored for sensing purposes. This effect can be also used to study accumulation of preferred ions in the membrane, e.g. to follow its pretreatment. The significant contribution of both mechanisms: FRET and IFE in decrease of observed emission intensity is clearly confirmed by results of fluorescence-lifetime imaging microscopy (FLIM) studies.