Applications of Ion Exchange Membranes to Sensing Materials

Though application of ion exchange membranes to ion sensors was reported several decades ago, the membranes do not have selectivity for specific ions and are not used practically except for particular cases such as the measurement of the concentration of hydrofluoric acid (instead of with a glass electrode).300 Medically, membranes with specific crown ethers have been widely used as ion sensors for diagnosis such as the determination of Na+ and K+ in blood and urine, and specific bilayer membranes have been used to determine anions for the same purpose (Chapter 5.3.5).301 

Recently, optodes (optical fiber chemical sensors) have been actively studied due to their inherent characteristics such as immunity to electrical noise, ease of miniaturization and the possibility of real time monitoring and remote sensing. For example, a change in absorption peak with humidity is observed using a Nafion membrane ion exchanged with dye, such as crystal violet305 or tri-phenylcarbinol.306 Similarly, a Nafion membrane loaded with the ruthenium complex [Ru(bpy)2(dhphen)]2 (bpy 2,2 -bipyridine, dhphen 4,7-dihydroxy-1,10-phenanthroline) has been used as an optical pH sensor.307 

Nafion membranes have been used in gas sensors, e.g. (i) to determine CO gas (CO gas sensor) by setting the working electrode and counter electrode on a glass substrate and by covering the electrodes with a Nafion recast membrane because Nafion polymer is relatively permeable to 02, CO and C02 (working electrode  

Many other similar applications have been reported such as the electrochemical determination of electroinactive cationic medicines,313 determination of urea,314 uric acid,315 and application to glucose biosensors to decrease interference of ascorbate, urate, and acetaminophen.316 Enzyme immobilized membranes are also sensing membranes, e.g. urea responsive membranes, poly(carboxylic acid) membranes in which urease is immobilized,317 fructose responsive membranes, and polyion complex membranes in which fructose dehydrogenase is immobilized.318 Such applications will expand further in the future and contribute to human life. 

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