A characteristic of a liquid is measured by providing a surface having a monolayer of a voltage sensitive chromophore that is covalently bound to the surface. The liquid is brought into contact with the surface and it is irradiated with actinic radiation to measure a first fluorescence emission spectrum. A solution of the voltage sensitive chromophore dissolved in a sample of the liquid is also irradiated with actinic radiation and a second fluorescence emission spectrum is measured. The first and second fluorescence emission spectra are compared to determine the characteristic of the liquid.