Galvanostatically controlled sensors

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemicaUy controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemicaUy by applying a constant current pulse. 

The experimental setup included a three-electrode electrochemical cell with a liquid contact membrane electrode in which the internal Ag/AgCl electrode acted as a working electrode connected to a potentiostat/galvanostat. The instrument was capable of switching rapidly between potentiostatic and galvanostatic modes [51]. 

Let us consider, for instance, the response mechanism of a polycation-selective galvanostatically controlled sensor. The polymeric membrane is in contact with a NaCl solution. The membrane of the sensor is formulated with a lipophilic salt, for instance, tetradodecylammonium dinonylnaphthalenesulfonate (TDDA-DNNS), which has a relatively high affinity to protamine. Even though protamine is presented in the sample, spontaneous extraction does not take place due to the high lipophilicity of TDDA-DNNS, thus the initial concenttation of protamine or sodium cations in the membrane is close to zero. 

The measurement cycle of the sensor begins with a pulse of cathodic current i that induces a net flux J of cations in the direction of the membrane phase. Assuming for 

In comparison to Eq. (11) if the activity of sodium ions is constant for a current pulse of fixed duration and magnitude the phase boundary potential is a function of protamine concentration in the sample. 

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Detection of Li+ in artificial serum with a voltammetric Li-selective electrode in a flowthrough system was demonstrated [64], Lithium salts such as lithium carbonate have been extensively used for treatment of manic depressive and hyperthyroidism disorders. The therapeutic range of Li concentration is generally accepted to be 0.5-1.5mM in blood serum. The authors used normal pulse voltammetry in which a stripping potential was applied between pulses in order to renew the membrane surface and expel all of the extracted ions from the membrane, similar to galvanostatically controlled potentiometric sensors described above. Unfortunately, the insufficient selectivity... 

A. Shvarev and E. Bakker, Pulsed galvanostatic control of ionophore-based polymeric ion sensors. Anal. Chem. 75, 4541-4550 (2003). 

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