Used as a fluorescent probe to measure chloride ion concentration in biological systems. It is particularly useful in monitoring intracellular chloride levels due to its sensitivity and selectivity. The fluorescence of MQAE is quenched by chloride ions, allowing quantification based on changes in emission intensity. Commonly applied in cell imaging and physiological studies involving ion transport, such as in neurons and epithelial cells. Also utilized in high-throughput screening assays for studying chloride channel function and in drug development targeting ion channel disorders.