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Al Ga N ∕ Ga N high electron mobility transistors (HEMTs) with an Ag∕AgCl gate exhibit significant changes in channel conductance upon exposing the gate region to various concentrations of chloride (Cl−) ion. The Ag∕AgCl gate electrode, prepared by potentiostatic anodization, changes electrical potential when it encounters Cl− ions. This gate potential changes lead to a change of surface charge in the gate region of the HEMT, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMT channel. These anions create an image positive charge on the Ag gate metal for the required neutrality, thus increasing the drain current of the HEMT. The HEMT source-drain current was highly dependent on Cl− ion concentration. The limit of detection achieved was 1×10−8M using a 20×50μm2 gate sensing area.

We have demonstrated a selective area AgCl anodization process, which can be integrated with the fabrication of AlGaN/GaN high electron mobility transistors ͑HEMTs͒ for chloride ion detection. A limit of detection of chloride ion concentration achieved was 1 ϫ 10 −8 M using a 20 ϫ 50 m anodized Ag/AgCl layer on the HEMT gate sensing area. Unlike the conventional open-circuit potential measurement used for the electrochemical measurement, the drain current of the HEMT was monitored as the output signal in our sensor. The effects of anodization bias voltage and time on the AgCl film properties were investigated. A continuous anodized AgCl film was achieved with the bias voltage of 0.5 or 1 V. However, AgCl films anodized at higher biases ͑5 V͒ were not continuous and a larger grain size was obtained. Energy-dispersive X-ray analysis was used to analyze the composition of the anodized AgCl and showed that slightly chloride deficient films ͑around Ag:Cl = 57:43͒ were obtained.Chlorine is used to kill bacteria and other microbes in drinking water supplies and waste water treatment. Excess chlorine also reacts with organics and forms disinfection by-products such as carcinogenic chloroform, which is harmful to human health. Thus, to ensure the safety of public health, it is very important to accurately and effectively monitor chlorine residues, typically in the form of chloride ion concentration, during the treatment and transport of drinking water. 1,2 In addition, the chloride ion is an essential mineral for humans, and is maintained to a total body chloride balance in body fluids such as serum, blood, urine, exhaled breath condensate, etc., by the kidneys. Variations in the chloride ion concentration in serum may serve as an index of renal diseases, adrenalism, and pneumonia; thus, the measurement of this parameter is clinically important. 3 Several analytical methods like colorimetry, 4 ionselective electrode, 5 activation analysis, 6 X-ray fluorescence spectrometry, 7 and ion chromatography 8,9 have been used for the analysis of chlorine in various samples. However, these methods are not portable and require expensive instrumentation. An accurate and fast determination of the inorganic ion content of various aqueous samples at low detection limits has been of great interest for a long time.Electrochemical sensor methods offer an excellent sensitivity compared to that of fluorescence approaches and in addition offer tunable selectivity and compatibility with microfabrication techniques needed to produce completely integrated microanalytical systems. 10 The open-circuit potential ͑OCP͒ measurement was employed as an electric output signal for the electrochemical detection systems via simple electrodes; the OCP signal can be quantified and transmitted wirelessly. The overall system can be compact and used for remote sensing as well as monitoring. The development of miniaturized electrodes is essential for realizing compact electrochemical sensors. Among different reference electrodes, silver/silver chloride ͑Ag/AgCl͒...

LETTERS TO THE EDITOR a factor of 4 on cooling the wave guide to the temperature of dry ice. The theoretical intensity ratio is about 3.6 to 1. Table II gives the rotational constants calculated from the two J = 2 lines and the 0-+1 line. These constants fit the remaining frequencies with a mean error of about 0.7 Me.Quantitative Stark effect measurements and observations of the quadrupolar hyperfine structure of some of the lines have been made. These data are being analyzed, and a full report will be made at an early date.

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