Thin Layer Coulometry with Ionophore Based Ion-Selective Membranes

摘要

We are demonstrating here for the first time a thin layerncoulometric detection mode for ionophore based liquidnion-selective membranes. Coulometry promises to achieventhe design of robust, calibration free sensors that arenespecially attractive for applications where recalibrationnin situ is difficult or undesirable. This readout principlenis here achieved with porous polypropylene tubing dopednwith the membrane material and which contains a chlo-nrinated silver wire in the inner compartment, together withnthe fluidically delivered sample solution. The membranenmaterial consists of the lipophilic plasticizer dodecyln2-nitrophenyl ether, the lipophilic electrolyte ETH 500,nand the calcium ionophore ETH 5234. Importantly andnin contrast to earlier work on voltammetric liquid mem-nbrane electrodes, the membrane also contains a cation-nexchanger salt, KTFPB. This renders the membranenpermselective and allows one to observe open circuitnpotentiometric responses for the device, which is con-nfirmed to follow the expected Nernstian equation. More-nover, as the same cationic species is now potentialndetermining at both interfaces of the membrane, it isnpossible to use rapidly diffusing and/or thin membranensystems where transport processes at the inner and outerninterface of the membrane do not perturb each other ornthe overall composition of the membrane. The tubing isnimmersed in an electrolyte solution where the counter andnworking electrode are placed, and the potentials arenapplied relative to the measured open circuit potentials.nExhaustive current decays are observed in the range ofn10 to 100 µM calcium chloride. The observed charge,ncalculated as integrated currents, is linearly dependentnon concentration and forms the basis for the coulometricnreadout of ion-selective membrane electrodes.