text: quinolinium - based Cl indicators are based on the capability of halides to quench the fluorescence of heterocyclic organic compounds with quaternary nitrogen. Fluorescence is quenched by a collision mechanism with a linear Stern–Volmer relationship:
  where:<br>
   is the fluorescence in the absence of halide<br>
   is the fluorescence in the presence of halide<br>
   is the Stern–Volmer quenching constant, which depends on the chloride concentration, . in a linear manner. 
  Thus, quinoline-based indicators are one-wavelength dyes - the signal results from monitoring the fluorescence at a single wavelength. Ratiometric measurement of halide concentration is not possible with quinolinium dyes. The kinetics of collision quenching are diffusion-limited only, and these indicators provide submillisecond time resolution. Quinolinium-based dyes are insensitive to physiological changes in pH, but they are prone to strong bleaching and demand ultraviolet excitation, which is harmful for living organisms. Because quinolinium is not occurring in the cells naturally, cell loading is necessary. However, quinolinium-based dyes arent retained perfectly in the cell and cant be targeted easily to subcellular organelles. Also, they cannot be designed specific to a certain type of cell.
  The most used quinolinium-based Cl indicators are 6-methoxy-1-(3-sulfonatopropyl) quinolinium (SPQ), 6-methoxy-N-ethylquinolium Cl (MEQ), and N-(6-methoxyquinolyl)-acetoethyl ester (MQAE).