Cystic Fibrosis

Cystic Fibrosis is the commonest lethal hereditary disease amongst caucasians. The determination of elevated levels of ions, notably sodium and chloride in sweat is the standard screening test for cystic fibrosis. Inherent disadvantages of the current method include (1) sampling difficulties, (2) the relatively large sample volumes required and (3) centralised analysis leading to delays in diagnosis. We propose to develop a new real time , in-situ CF diagnostic test in which a planar sodium ion-selective electrode is attached directly to the skin for immediate measurement of the raised sodium levels in sweat.

Ion-selective electrodes.

Ion-selective electrodes are electrochemical sensors that allow the potentiometric determination of the activity of certain ions in the presence of other ions. A major stumbling block impeding their further development has been the requirement for an internal reference electrolyte solution.

This electrolyte stabilises the internal boundary potential of the ion-selective membrane and stabilises the internal reference potential at the internal reference electrode (usually a Ag/AgCl reference). Figure 1 shows a sodium selective ISE (sodium ligand dispersed in PVC). Sodium ions are exchanged at the sample/PVC boundary and the mobile Na-ligand complexes provide charge transfer across the PVC membrane to the internal electrolyte. Na⁺ ions can thus exchange freely at the internal PVC/electrolyte boundary, while the Cl^- ions can exchange at the Ag/AgCl electrode, coupled with an exchange of electrons between the AgCl and Ag phases of the internal reference electrode. The use of an internal electrolyte requires the electrodes be used in an upright position, limits miniaturisation prospects and prevents the development of mass produced planar ion-selective sensors. Hence there is great interest in solid state versions of these sensors.

Solid state ion-selective electrodes.

We at the BEST Centre, are currently developing solid state ion-selective sensors, in which the transfer from ionic to electronic conductivity is provided by a hydrogel contact between the PVC membrane and the internal reference electrode.

Sodium selective electrode design

A polyester substrate was screen printed with silver ink followed by a layer of silver chloride ink, with the Ag film being larger than the AgCl film so that electrical contact could be made with the edge.

After curing, the sensing film was introduced. A layer of NaCl doped hydrogel was deposited on the AgCl portion of the electrode. This was followed by a sodium selective PVC membrane. The electrochemical cell was completed by use of a bare screen printed Ag/AgCl reference electrode.

where E_cell = E_ise. - E_ref.

E_cell = Cell potential.

E_ise = Ion-selective electrode potential.

E_ref = Reference electrode potential.

A series of potentiometric experiments was performed in order to evaluate the electrode performance. A perspex cell was prepared to house the electrodes during experiments. The cell consists of two sections held together by six metal screws. Two central wells were machined in the upper portion of the cell. An ion-selective electrode and a reference electrode were sandwiched between the two halves of the cell such that the sensing surfaces were positioned beneath the wells. The cell has a capacity of approximately 30 ml.

Potentiometric experiments

During the potentiometric experiments the electrodes were interfaced to an Elonex PC-5100/I via a national instruments ATMIO16DL data acquisition card. Data was recorded using a program written in LabVIEW 4.0 (National instruments Ltd., Austin, Texas, USA).

A 20 mM NaCl solution was placed in the experimental cell. An injection was made onto the 20 mM background such that the overall sodium concentration was stepped up to 70 mM. The resulting potential change was monitored. This was repeated until data for five injections were obtained for the electrode.

Ion-selective electrodes respond to changes in analyte activity according to the Nernst equation.

where : E_cell =Steady state potential.

S = Electrode slope (59.2 mV/ decade change in activity ,for a singly charged ion at 25° C).

a_1,a₂ = Initial and final analyte activity

Stepping up the concentration of sodium in the sample as described above should theoretically result in a voltage increase of approximately 59.74 mV. The actual results obtained for one such electrode arrangement are shown in figure 4.

Figure 4

Other experiments have demonstrated, good reproducibility, low noise and drift, fast ion-uptake kinetics and good electrical characteristics.

Applications

These electrodes represent a new generic trend in ion-selective electrodes and will have many novel applications. Functioning solid-state electrodes will be much in demand not least because of their ease of manufacture and suitability for mass production. One possible application for the above sodium sensor would be as a screening test for Cystic Fibrosis.