The detection of nitrogen oxides contained in air was investigated using their redox cycles. NO and NO2 gases can be detected by means of interdigitated array (IDA) electrodes made of Au that are fabricated on a gas-permeable membrane. The IDA electrodes were applied to a gas detection system for the first time. Although the membrane is soft and flexible, and has pores, the electrodes can be formed successfully on it by photolithography devised in our laboratory. Between the electrodes, redox cycles are produced, and, hence, the gas sensitivities increase. The reactions in this redox cycles are not simple, and it was confirmed that nitrate produced on the generator electrode reacts with NO gas on the collector in the NO detection system. The increase in sensitivity based on the generation of the redox cycle in amperometric detection appears to be improved by decreasing the distance between the electrodes.

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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Surface modification of a p-SiOC film induced by Helium (He) plasma was investigated using various measurements. Differential Fourier transform infrared absorption (FT-IR) spectra and the etch-depth measurements with the immersion in the mixed hydrofluoric acid (HF) indicate that the almost all of Si-CH3 bonds are broken in the modification layer, resulting in carbon-depletion, an SiO2-like composition. The x-ray photoelectron spectroscopy (XPS) measurements at different take-off angles reveal that the modified surface forms a double-layered structure, a thin carbon-rich top layer (about 1 nm thick) on a thick carbon-depletion layer (about 20 nm thick) with the irradiation of He plasma over 300 W. Atomic force microscopy (AFM) observations show that the modified surface is the smoothest in the case of the carbon-depletion surface.