A facile method to effectively combine plasmon enhanced fluorescence (PEF) and fluoride-Lewis acid based reactions to detect low concentrations of fluoride in solution

Abstract

Solution-based fluorogenic F⁻ chemodosimeters are popular because they are highly sensitive and easy to use. However the low quantum yield and photostability of organic dyes limit their sensitivity and detection limit. Plasmon enhanced fluorescence (PEF) provides a solution to these challenges and has spurred the birth of highly sensitive fluorescent turn-on sensors; sadly these sensors target mainly bimolecular analytes at the expense of other equally important ones such as fluoride ions. Herein, AgNPs were encapsulated in a silica bead onto which an optically non-responsive silyl ether protected fluorescent isothiocyanate (FITC) group was immobilized. The reaction of F⁻ with the silyl ether group to induce fluorescence and the subsequent amplification of the signal by the AgNPs' Purcell effect was investigated for detection of trace amounts of F⁻ in solution. Fluorescence titration experiments in an AcN/HEPES buffer mixture revealed that F⁻ reacts selectivity and quantitatively with silyl ether groups via the cleavage of the Si–O bond to generate the highly fluorescent FITC molecules whose signal was amplified ∼6 folds by the Purcell effect to levels which helped off-set the effect of signal deactivating reactions. The sensor was thus able to exhibit a linear response at low [F⁻] = 0.1–0.01 μM from which a very low detection limit of ∼11.82 nM was calculated. The sensor maintained its high selectivity and sensitivity towards F⁻ in complex solutions and exhibited a rapid detection time of ∼3 min. The proposed sensor architecture may open the door for a new family of ultra-sensitive F⁻ sensors to be synthesized which are needed considering the ever changing complexity of our food and water streams which could make detection of trace amounts of F⁻ challenging.