Metallic contacts, solid state internal references

We now turn to the experimental method of measurement of Em. The potential on the membrane exterior is measured by an Ag AgCl or SCE reference electrode. The interior potential is very difficult to measure through a direct metal contact (only in some solid state and in hybrid sensors, Section 13.10) and one opts for another reference electrode called the internal reference. Thus a typical cell would be... 

In these sensors the technology developed for ISFET construction is used in conventional electrodes. Links between the membrane and internal reference are metallic (ohmic contact), by deposition of the metal on the membrane (solid state membranes), or by deposition of an ion-selective membrane on a metal. This latter is an integrated sensor. 

Figure 2 Basic types of ISE (A) glass electrode (B) electrode with a solid homogeneous or heterogeneous membrane (C) classical liquid membrane electrode (D) electrode without internal solution (all-solid-state electrode) (E) coated-wire electrode. 1, Internal standard solution 2, internal reference electrode (Ag/AgCI) 3, membrane 4, glass or plastic body of the electrode 5, reservoir of the electroactive substance solution 6, solid-state contact and 7, metal wire.

Glass pH electrodes and most other practical ion-selective electrodes (ISEs) are membrane devices. The term electrode is, strictly, a misnomer because the active ion-selective membrane has on one side an analyte (the test solution) and on the other an internal (reference) solution containing an internal reference electrode the internal solution and electrode together provide the electrical contact to the membrane. A solid-state ion-selective electrode is one in which the internal components are replaced by a direct back contact to a metal or semiconductor, i.e., an electronic conductor. This is not a trivial matter, because most ion-selective materials are ionic conductors, and... 

Regarding the reference electrode, the conventional Ag/AgCl/KCl electrode is often adapted to the presence of organic solvents and the different conductivity and ionic mobility in the typical HPLC mobile phases by using a porous ceramic frit for the electric contact or by replacing the KCl internal solution by a low-maintenance gel. Solid-state reference electrodes made of Ag or Pd has also been included in commercial equipment. Finally, auxiliary electrodes are currently made of metal and they are often little more than the stainless steel outlet from the cell. 

A new kind of all solid-state sensors was first reported by Cattrall and Preiser in 1971 in which the internal reference element was in direct contact with the sensing membrane and thus contained no aqueous solution. The first group of such simplified sensors was those of the so-called coated wire construction type (CWE). In this approach, a metal wire was dipped with a solution of PVC in THF containing also a suitable electroactive material. During evaporation of the solvent, a PVC film on the metal wire surface was formed. Although different materials such as platinum, silver or sliver chloride, and aluminum could serve as central conductors, the nature of the wire support had no substantial influence on the electrode performance if it did not react with the membrane components. ... 

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