Ion-partitioning membranes as electroactive elements for the development of a novel cation-selective CHEMFET sensor system

1. ABSTRACT The development of a cation-selective sensor based on a new response mechanism is presented in this paper. The sensor is based on a bulk active ion-partitioning membrane as the chemical recognition element while the signal transduction is obtained by a pH sensitive transducer, such as the pH-ISFET. The ion-partitioning membrane is doped with two different ionophores, one selective to the analyte cation and the other to protons. The increase of the analyte cation activity in the sample results in the carrier mediated partition of the analyte into the bulk of the membrane phase. Due to the electroneutrality principle requirements, a simultaneous displacement of protons of equal charge out of the membrane must take place. This proton flux takes place towards both the membrane/sample and membrane/transducer interfaces. The monitoring of the proton flow in or out of the membrane by the pH-sensitive transducer is directly related to the analyte ion activity in the sample. Based on this mechanism, it is shown that there is a large effect of all membrane components, as well as the ionic composition of the sample, on the sensor selectivity, sensitivity and detection limit. It is also shown that the optimization of the primary carrier to the proton ionophore ratio in the membrane and that of the properties of the proton ionophore are vital in obtaining the desired sensor characteristics. It is shown that the use of a plasticizer and a polymer matrix with low dielectric constant favors the cation-protons exchange mechanism stabilizing the observed signal. Finally, the proper adjustment of the sample pH and the use of hydrophilic anionic counteranions in the sample contribute to the improvement of both the sensitivity and the detection limit of the sensor. The analytical characteristics of the CHEMFETs selective to the monovalent potassium ion and the divalent calcium ion with the optimized ion-partitioning membranes are evaluated by the analysis of blood serum samples.