Probes for Calcium and Other Ions

A product that is widely used, and one that has also grown in importance since the advent of high throughput screening methods, is the AM (acetoxymethyl) ester of the fluoran derivative, Fluo-3 (3.67). This product, Fluo-3AM, does not itself bind Ca but is readily hydrolysed to Fluo-3 by endogenous esterases once the dye is in the cells. 

Fluorophores exist for other physiologically important ions, e.g. K+, Na+, Mg +, etc. (See also section 3.5.9.1.) 

The use of indicator dyes, such as phenolphthalein, methyl orange etc., is an age-old practice in industry (Chapter 1, section 1.4), but measurement of pH within a cell requires a different strategy. Greater sensitivity is required for optical pH measurements inside living cells and this can only be provided by specially adapted fluorescent indicator dyes. It is possible to measure pH with microelectrodes but the 

The fluorescent dye hydroxypyrenetrisulfonic acid (3.27), described in section 3.5.1.6, is used in medical monitoring devices for the pH of blood. Absorption (excitation) bands occur at 405 nm (acid form) and 475 nm (basic form) with emission (fluorescence) at 520 nm. 

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