A Broad-Range Fluorescence Lifetime pH Sensing Material Based on a Single Organic Fluorophore.

A general drawback for optical based pH sensors is that their response is typically limited to within 2-3 pH units centered around the pKa of the indicator. Fluorescence lifetime (FL) is a particularly compelling basis for highly stable pH sensors since this is an intrinsic property of the indicator molecule. Here we demonstrate that it is possible to broaden the sensing range of FL based sensors significantly by placing the indicator in a support material where the indicator's chemical environment itself changes with pH. For acridine immobilized in amine-modified porous silica, a total FL change of 20 ns in the pH range 2-12 is achieved. A linear pH vs FL relationship is observed with three break points occurring at pH 4, 6 and 9 that are related to the pKa values of the indicator and the silica material. This proves the concept that tuning the fluorophore's chemical environment can broaden the FL pH sensing range, where currently available fluorophores do not cover the full pH range. Graphical Abstract.