Turn-on ratiometric fluorescence sensor film for ammonia based on salicylaldehyde-ionic liquid

Abstract

We developed a novel and highly optical sensitive ammonia sensor film with merits including fluorescence turn-on, ratiometric measurements and large Stocks shift. The film was designed based on the incorporation of pH probe methylimidazolium salicylaldehyde ionic liquid (MeIm-Sal-IL) and a reference pH insensitive dye meso-tetrakis(pentafluorophenyl) porphyrin (TFPP) in commercial polystyrene (PS) matrix. The pH probe provides dual solubility, large Stocks shift and fluorescence turn on in basic medium over other pH fluorophores; that makes it promising candidate for the development of a new ammonia sensor film. All homogeneous components are dissolved in a single film then spread on an optically inert transparent polyester support. PS displays a good permeability for ammonia and inhibits protons penetrations that make the sensor film insensitive to pH changes outside the matrix. As ammonia penetrate the PS matrix it converts the doped pH probe MeIm-Sal-IL from protonated form to deprotonated form inducing large spectroscopic changes in both absorption and fluorescence as turn-on obviously occurs in fluorescence. The new film measures the ammonia in moderate low concentration from 0.5 up to 2.5μM with a low limit of detection (LOD) 0.07μM. The sensor film exhibits high reversibility by cycling between ammonia and water.