ABSTRACT
The electronic nature of light absorption allows the use of dyes to indicate physical or chemical changes. In solution chromophores, and fluorophores, are surrounded by solvent molecules. In biology acid-base and metal ion indicators are used to measure protonic equilibria and metal ion concentrations in microenvironments. Generally, indicators used as biological probes have fluorescent properties, to allow their detection at very low concentrations. Whereas acidimetric and metallochromic titrations are reversible by the simple addition or removal of protons or metal ions, redox indicators undergo more drastic changes in structure on being reduced or oxidized. The use of acidic, or basic, substituents in the design of indicator dyes is thus a straightforward concept, with the rider that the system must be conjugated to show the desired effect on pH change. In a metal ion-indicator, charge withdrawal, that is displacement of electron density towards the metal ion, following chelation leads to significant changes in the electron cloud of the indicator molecule.
