Non-covalent fluorophore-labelled ChEIs have significantly lower affinities and specificities towards ChEs because the fluorophore usually does not contribute significant new interactions within the active site. A substantial portion of the fluorophore is free and in equilibrium with the bound inhibitor in the active site. Additionally, it can bind to tissue samples in a non-specific manner. The former is responsible for high background fluorescence and the latter for artefacts in the microscopy imaging.