Reference electrode glucose sensor

In the area of consumer products, amperometric glucose sensors hold high potential. Industrially, process monitors for the manufacture of consumer chemicals are under development. However, replacement of defective reference electrodes, which in a laboratory environment may be trivial, may be prohibitively difficult m vivo or in an industrial process environment. 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

In the previous papers(12,13), we reported on the vessel access type, i.e. tubular type, glucose sensor. It consisted of a glucose electrode system with a GOX enzyme immobilized Nylon membrane and a glucose semipermeable membrane, and a reference oxygen electrode system. The sensor could directly measure up to 700 mg/dl of BGL in an arterial blood stream when it was placed into an external A-V shunt. This sensor, however, has some problems such as thrombus during in vivo testing without heparin and clinical complexity associated with implanting the sensor in a blood stream. 

Figure 4.5 A planar 1.2 cm diameter oxygen-glucose sensor array. The small (125 pm) dots are the oxygen electrodes, the ribbon-like counter electrode is visible, and the Ag reference electrodes can be seen. Four of the oxygen electrodes have circular pockets of glucose oxidase integrated into the overlying membrane.

A planar glucose sensor using an ISFET in place of the reference electrode has been introduced by the Japanese NEC Corporation (Mura-... 

Figure 38. A schematic illustration of the enzymic glucose electrode based on the oxygen membrane sensor (1) a layer of immobilized glucose oxidase (2) semipermeable Teflon membrane (3) reference electrode (4) cathode.

The actual steps involved in the fabrication of the sensors have been described elsewhere (18,19), The amperometric sensor onto which the glucose oxidase and outer membrane are deposited was fabricated using thin/thick film processing techniques. This process resulted in a three-electrode sensor with a 0.1 mm platinum-black working electrode. The sensor also contained a platinum-blacked counter electrode and a Ag/AgCl reference electrode. The preparation, deposition, and characterization of the outer membrane and glucose oxidase layers will be discussed here. 

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