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Interdigitated array (IDA) electrodes are used for the determination of components contained in aqueous solutions with high sensitivity. [1][2][3][4][5][6][7] Redox cycles are generated between the two working electrodes; they are called the generator and the collector, and are placed close to each other. The generator current increases when a suitable potential is applied to the collector (a dual-mode), compared to a single-mode when the collector is electrically open. In a previous study 8 , IDA electrodes were applied to gas detection for the first time. The electrodes were fabricated on a gas-permeable membrane in order to obtain good response to the gases (NO x ) through improved conventional photolithography. It was confirmed that the IDA electrodes are useful in a gas-detection system, as well as in solution systems. Generally, the gap and width of the electrodes should be small in order to generate a large number of redox cycles. However, it was difficult to fabricate fine patterns on the gas-permeable membrane, because the membrane was flexible and its surface was not flat. On the other hand, it is already possible to fabricate fine IDA patterns on a hard and flat substrate and are already being marketed. However, if these electrodes fabricated on substrate are set in an electrolyte solution, they do not come into contact with the membrane. Therefore, it is said that the sensitivity and response speed are poor. Accordingly, we experimented with a new method of applying IDA electrodes to a gas-sensing system. We call it a planar IDA gas sensor. A piece of filter paper containing an electrolyte solution was put on IDA electrodes, which were fabricated on a glass substrate. This gas sensor was very easy to construct. A similar planar sensor had been reported for amperometric sensing of oxygen.9 It was developed as a low-cost and disposable sensor, and applied to glucose sensing. A planar sensor is expected to be more useful in complicated detection systems i.e. using IDA electrodes, which was confirmed after being used in a hydrogen-detection system. ExperimentalPropylene carbonate (PC) was purchased from Nacalai Tesque Inc. Tetraethylammonium perchlorate (TEAP) was prepared from tetraethylammonium bromide and perchloric acid, both of which were obtained from Wako Chemical Co. The precipitate of TEAP was purified by recrystallization from water and stored in an evacuated desiccator after baking at 100˚C. A PC solution containing 0.1 M TEAP was prepared as an electrolyte.A planar IDA gas sensor was prepared based on available IDA electrodes (No. 2047 from BAS). The width and gap of the electrodes were 10 µm and 5 µm, respectively. The valid length of the electrode was 2 mm, and the number of electrodes was 65 pairs. Keywords Interdigitated array electrodes, redox cycles, hydrogen gas sensor Fig. 1 Structures of planar gas sensors combined with IDA electrodes. The IDA electrodes used were fabricated on a glass substrate. The bandwidth of the generator and the collector were 10 µm, and the gap between them wa...

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