Abstract: The invention provides methods and compositions for measuring ion concentration inside a cell by measuring fluorescence of a compound of the general formula I. In particular embodiments, the measured ion is halide, particularly iodide, the cell contains a functional anion transport protein or channel, the method measures a change in fluorescence as a function of a predetermined condition such as the presence of a predetermined amount of a candidate modulator of ion transport in the cell (e.g. for drug screening) or the expression by the cell of a transgene (e.g. to assess the efficacy of gene therapy).