Abstract: The present invention relates to a method of measuring the acidity and redox potential of a sample, which method comprises: (i) contacting a compound of formula (A) with the sample: Acc(-L1-)aFluo(-L2-)bRedox ??(A); wherein: Acc is a proton acceptor moiety; Fluo is a fluorophore; Redox is a redox unit; each L1 is a first spacer group which is bonded to Acc and Fluo; each L2 is a second spacer group which is bonded to Fluo and Redox; a is 1 or 2; and b is 1 or 2; and (ii) exposing the sample to a light source, measuring fluorescence and thereby determining the acidity and redox potential of the sample. Compounds and the use of such compounds are also described.