No abstract

The drain current fluctuation of ungated AlGaN/GaN high electron mobility transistors (HEMTs) measured in different fluids at a drain-source voltage of 0.5 V was investigated. The HEMTs with metal on the gate region showed good current stability in deionized water, while a large fluctuation in drain current was observed for HEMTs without gate metal. The fluctuation in drain current for the HEMTs without gate metal was observed and calculated as standard deviation from a real-time measurement in air, deionized water, ethanol, dimethyl sulfoxide, ethylene glycol, 1,2-butanediol, and glycerol. At room temperature, the fluctuation in drain current for the HEMTs without gate metal was found to be relevant to the dipole moment and the viscosity of the liquids. A liquid with a larger viscosity showed a smaller fluctuation in drain current. The viscosity-dependent fluctuation of the drain current was ascribed to the Brownian motions of the liquid molecules, which induced a variation in the surface dipole of the gate region. This study uncovers the causes of the fluctuation in drain current of HEMTs in fluids. The results show that the AlGaN/GaN HEMTs may be used as sensors to measure the viscosity of liquids within a certain range of viscosity.

No abstract

No abstract

In this study, a cost-effective and highly sensitive cholesterol microsensor, which is consisted of cholesterol oxidase (ChOx), horseradish peroxidase (HRP), and polyaniline (PANI), was developed based on the enzyme-induced conductivity change of PANI with fast response. Hydrogen peroxide is produced via the reaction between cholesterol and ChOx, which was immobilized in a dialysis membrane. The produced hydrogen peroxide can oxidize HRP, which can be reduced by oxidizing PANI, thus resulting in decreased conductivity of the polyaniline thin film. The reduced HRP can be oxidized again by hydrogen peroxide and the cycle of the oxidation/reduction continues until all hydrogen peroxide are reacted, leading to the high sensitivity of the sensor due to the signal contributed from all hydrogen peroxide molecules. Cholesterol was detected near the physiological concentrations ranging from 100 mg/dl to 400 mg/dl with the cholesterol microsensors. The results show linear relation between cholesterol concentration and the conductivity change of the PANI. The microsensor showed no response to cholesterol when the PANI was standalone without cholesterol oxidase immobilized, indicating that the enzymatic reaction is required for cholesterol detection. The simple process of the sensor fabrication allows the sensor to be cost-effective and disposable usage. This electronic cholesterol microsensor is promising for point-of-care health monitoring in cholesterol level with low cost and fast response.