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In this study, conductive polyaniline (PANI) was coated on Si3N4/Si substrates and reacted with hydroxyl radicals created by mixing ferrous ions and hydrogen peroxide in a buffer solution, as known as the Fenton reaction. The hydroxyl radicals reacted with PANI, resulting in the decreased conductivity of PANI. The reduction in the conductivity of PANI was found to be proportional to the concentration of hydroxyl radicals in the buffer solution. The concentration of hydroxyl radicals was calibrated by comparing the fluorescence intensities of the Amplex ultrared molecules oxidized by radicals and hydrogen peroxide via the catalysis of horseradish peroxidase (HRP), respectively. The conductivity change of the PANI shows a linear relationship to the concentration of hydroxyl radicals, ranging from 0.2 μM to 0.8 μM. The conductivity of PANI does not respond to either ferrous ions or hydrogen peroxide alone. Only when both ferrous ions and hydrogen peroxide are present in the solution, the conductivity of PANI decreases, indicating that the conductivity change of PANI is attributed to the existence of hydroxyl radicals. The simple process for the sensor fabrication allows the sensor to be cost-effective and disposable. This electronic sensor can detect one of the most important reactive oxygen species (ROS), the hydroxyl radical, and therefore is promising for studying oxidative stress.

AlGaN/GaN high electron mobility transistors (HEMTs) were used to sense the binding between double stranded DNA (dsDNA) and the severe acute respiratory syndrome coronavirus (SARS-CoV) nucleocapsid protein (N protein). The sensing signals were the drain current change of the HEMTs induced by the protein–dsDNA binding. Binding-site models using surface coverage ratios were utilized to analyze the signals from the HEMT-based sensors to extract the dissociation constants and predict the number of binding sites. Two dissociation constants, K D 1 = 0.0955 nM, K D 2 = 51.23 nM, were obtained by fitting the experimental results into the two-binding-site model. The result shows that this technique is more competitive than isotope-labeling electrophoretic mobility shift assay (EMSA). We demonstrated that AlGaN/GaN HEMTs were highly potential in constructing a semiconductor-based-sensor binding assay to extract the dissociation constants of nucleotide–protein interaction.

In this study, we fabricate an ultra-sensitive hydrogen peroxide sensor by using horseradish peroxidase (HRP)-immobilized conducting polymer, polyaniline (PANI). With the proposed detection mechanism, hydrogen peroxide first oxidizes HRP, which then oxidizes polyaniline, thus resulting in decreased conductivity of the polyaniline thin film. The reduced HRP can be further oxidized by hydrogen peroxide and the cycle of the oxidation/reduction would continue until all hydrogen peroxide are reacted, leading to the high sensitivity of the sensor due to the signal contributed from all hydrogen peroxide molecule. The detection limit of this sensor is only 0.7 nM. The detectable concentration of H2O2 is from 0.7 nM to 1 μM. Beyond 1 μM, the sensor gradually saturates and some H2O2 remains, indicating the inhibition of HRP activity at high concentration of H2O2. There is no response to hydrogen peroxide once the PANI is standalone without HRP immobilized, showing the enzymatic reaction is required in the process of hydrogen peroxide detection. The simple process for the sensor fabrication allows the sensor to be cost-effective and disposable. This electronic hydrogen peroxide sensor is promising in applications for low concentration hydrogen peroxide detections, such as the reactive oxygen species (ROS) in oxidative stress studies.

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-caremore » health monitoring in cholesterol level with low cost and fast response.« less

Hydrogen peroxide is a metabolic by-product and a kind of stable reactive oxygen species (ROS). When the ROS levels increase, it may causes several harmful effects of ROS on the cell structure, such as damage of DNA and protein oxidation, and this is known as oxidative stress. The oxidative stress is also the important clinical indicators of cause of aging, Alzheimer disease, and kidney diseases. Now the common test method of oxidative stress is Enzyme-linked immunosorbent assay (ELISA). ELISA needs color giving dyes, complicated preparation of sample, and optical system, so it is very expensive and inconvenient. In our study, we present a high sensitivity hydrogen peroxide sensor with ultra-low detection limit. The sensor can directly test the sample and only need very small amount of sample. And because of the simple structural design and fabrication, the sensor can be used as a cheap, efficient, and portable sensor system. In the future, the novel hydrogen peroxide sensor has various applications in studying oxidative stress and detecting reactive oxygen species for cells. Furthermore, we can combine the PANI sensor with different enzymes to fabricate other highly sensitive sensors to detect diverse materials, such as glucose, lactic acid and cholesterin. In our study, the PANI layer and gold electrodes were deposited on silicon nitride substrate, and the PANI was applied to fabricate a thin film between two electrodes. Then the PANI layer was sultonated by propane sultone and modified with HRP. During the measurement, the sensor was operated at 100mV and different concentrations of hydrogen peroxide citrate buffer solutions (pH=5.4) were dropped on it. The current change was measured when the hydrogen peroxide reacted with the HRP Immobilized PANI thin film. We tested the hydrogen peroxide solution from 0.1 nm to 1mM. The HRP-modified resistive sensors based on n-alkylated polyaniline(PANI) detect hydrogen peroxide in solution with very high sensitivity, ultra-low limit, and short response time. The sensitivity is higher than that of other sensing methods, such as electrochemical sensors or transistor sensors. The detection limit of PANI sensor is 0.7 nM (Figure 1. a, b). It is three orders smaller than that of other common methods with detection limit around 1 μM. To the best of our knowledge, it is the lowest detection limit that has ever been reported. And We combine the hydrogen peroxide sensor with glucose oxidase to build up the glucose sensor. In summary, the hydrogen peroxide sensor can provide a more exact hydrogen peroxide concentration and quick detection. The simple process for the sensor fabrication also allows the sensor to be cheap, disposable and combinable with other sensors to build up sensing system. This work was partially supported by National Science Council grant (No.99B20495A & 101-2221-E-007-102-MY3) and by the research grant (100N2049E1) at National Tsing Hua University.

Paper is an important material in our daily lives. Paper is cheap, environmentally friendly, light, flexible and widespread all over the world. Paper-based sensors are attractive because they are cheap, portable, accessible everywhere, and easily destroyed after being used. Paper-based sensors have been widely used in food safety control, clinical test, and environmental detection. The fiber structure in paper, which can act like a tunnel for microfluidic liquid transport, plays an important role in paper-based sensors by providing a platform for chemical reaction of analytes. However, conventional paper-based biosensors, which usually involve colorimetry, has the limitation in sensitivity and detection limit. Besides, the quantitative analysis of optical signals usually requires expensive optical detection system. In sharp contrast, most electronic sensors can be fabricated in very low cost and display with a simple device. In our study, we present a novel idea, which turns regular paper into conductive by combining paper and conducting polymer together. By integrating this conducting paper with different sensor technologies, various sensors can be achieved with high sensitivity and accuracy, which can be attributed to the great biocompatibility of the conducting paper. Only small amount of sample is required for testing with these sensors. Because of the simple structural design and fabrication, the conducting paper can be fabricated as a cheap, efficient, and portable electronic biosensor. Previous research has shown the HRP-modified n-alkylated polyaniline (PANI) can detect hydrogen peroxide in buffer solution with a very high sensitivity, ultra-low detection limit, and very short response time. During fabrication, the PANI thin film may sometime suffer from cracks after baking the PANI layer on the silicon nitride substrate due to different thermal expansion coefficients between PANI and silicon nitride. By using a filter paper as the substitute for hard substrate, the mechanical stress of PANI after baking can be largely reduced. Figure 1 shows the schematic of the conducting paper. Propane sultone was doped in PANI. The PANI solution was then dropped on the filter paper and silver electrode is then coated on the back of the paper to contact with the PANI, extending to the front side, making it easy for probing. The current was measured from 0-1V. Figure 2 shows the current-voltage characteristics of the conducting paper. Figure 3(a) shows that the PANI layer is tightly merged with the filter paper Figure 3(b) shows the surface morphology of PANI on the paper. In summary, the conducting paper made of PANi can not only achieve electronic paper-based devices, but can also realized electronic paper-based biosensors which can provide better sensitivity and lower cost, compared to the conventional paper-based sensors. Combining it with different technologies and enzymes, it can be used to detect several kinds of analytes. The portability and flexibility of conducting paper also make it more convenient to use. Because the fabrication process is quite simple and cheap, it can be widely promoted commercially in the future. This work was partially supported by National Science Council grant (No.99B20495A & 101-2221-E-007-102-MY3) and by the research grant (100N2049E1) at National Tsing Hua University.

Hydrogen peroxide attracts a great interest due to its important role in food, pharmaceutical, and clinical applications. Hydrogen peroxide is also a by-product in many enzyme catalytic reactions, such as glucose oxidase, lactate oxidase, cholesterol oxidase, alcohol oxidase, urate oxidase, aldehyde oxidase, and oxalate oxidase, which are implemented to detect glucose, lactic acid, cholesterol, ethanol, urea, formaldehyde, and oxalate, respectively. These biomolecules are significant markers in many biologically metabolic reactions. In our study, we present a cholesterol sensor based on a high sensitivity hydrogen peroxide sensor with ultra-low detection limit. The sensor can directly test the sample and only need very small amount of sample. And because of the simple structural design and fabrication, the sensor can be used as a cheap, efficient, and portable sensor system. The HRP-modified resistive sensors based on n-alkylated polyaniline(PANI) detect hydrogen peroxide in solution with very high sensitivity, ultra-low limit, and short response time. The sensitivity is higher than that of other sensing methods, such as electrochemical sensors or transistor sensors. The detection limit of PANI sensor is 0.7 nM. It is three orders smaller than that of other common methods with detection limit around 1 μM. To the best of our knowledge, it is the lowest detection limit that has ever been reported. And we combine the hydrogen peroxide sensor with cholesterol oxidase to build up the cholesterol sensor. In our study, the PANI layer and gold electrodes were deposited on silicon nitride substrate, and the PANI was applied to fabricate a thin film between two electrodes. Then the PANI layer was sultonated by propane sultone and modified with HRP. After that, the device was combined with dialysis membrane which was modified with cholesterol oxidase. During the measurement, the sensor was operated at 100mV and different concentrations of cholesterol PBS solutions (pH=7.0) were dropped on it. The cholesterol reacted with the cholesterol oxidase and created hydrogen peroxide on the PANI thin film (Figure 1). The current change was measured when the hydrogen peroxide reacted with the HRP immobilized PANI thin film. According to regular cholesterol level in human blood, we tested the cholesterol solution from 100 mg/dl to 400 mg/dl ,and we get a very good linear result (Figure 2). In summary, the cholesterol sensor can provide a more exact cholesterol concentration detection. The simple process for the sensor fabrication also allows the sensor to be cheap, disposable and combinable with other sensors to build up sensing system. This work was partially supported by National Science Council grant (No.99B20495A & 101-2221-E-007-102-MY3) and by the research grant (100N2049E1) at National Tsing Hua University.

Recently, more and more studies indicate that free radical is the cause of various diseases such as aging, cancer, neurodegenerative diseases and cardiovascular diseases because of uncontrolled oxidation of lipids, protein and DNA in biological system. Hydroxyl radical (OH°) is the highest oxidized species in free radicals. So far, most techniques for hydroxyl radical detection are sophisticated and expensive. Therefore, a simple and cheap method for hydroxyl radical detection is very necessary. Polyaniline (PANI) has been used as the active layer for different type of sensors because of its good electronic conductivity. The conductive polyaniline can neutralize free radicals by donating electrons and thereby changing their oxidation state from emeraldine to pernigraniline. In here, we used a simple and cheap polyaniline microchip to detect hydroxyl radical generated from Fenton reaction. The result shows significant current (or conductance) change. Figure 1 (a) and (b) show the schematic of hydroxyl radical sensor and the top-view of the devices, respectively. The sensor consists of two metal electrodes made by 200 Å Ti and 1000 Å Au deposited with an e-beam evaporator on a Si3N4/Si substrate. The length and the width of the Au electrodes are 500 μm and 100 μm, respectively. The gap between the two metal electrodes is 10 μm. 0.3 g polyaniline emeraldine base powder was dissolved in 5 mL dimethyl sulfoxide (DMSO) with stirring for 6 h. The polyaniline solution was then mixed with the same volume of 0.01 M NaOH. Use centrifuge to separate two different phase in the solution and pipet out the upper layer of the solution. Propane sultone was added into the polyaniline solution with the ratio 3:7 and stood for 6~8 hr over 30 degree Celsius. Also use centrifuge to separate two different phase in the solution and pipet out the upper layer of the solution. 2 μL of the remaining solution was coated on a chip and baked for 2.5 hr. The hydroxyl radical was generated from Fenton reaction as shown below: Fe2+ + H2O2 → Fe3+ + OH°+ OH- Figure 2 (a) shows significant current (or conductance) decrease approximately 84 μA of hydroxyl radical sensor after dropping hydrogen peroxide. It indicates that hydroxyl radical reacted with polyaniline and stole electron from polyaniline inducing current decrease. The current of the sensor was measured at a constant bias of 0.1 V at room temperature by using Agilent B1500 parameter analyzer with the polyaniline layer exposed. The interval time between any two measurement points is 1 minute. The final concentration of ferrous ion and hydrogen peroxide are 25 mM and 100 mM, respectively. Both two solutions were prepared in citrate phosphate buffer solution. Figure 2 (b) shows the current of sensor after dropping the hydrogen peroxide without ferrous ion. The current doesn’t change significantly. It indicates that the sensor doesn’t react with the hydrogen peroxide. Therefore, we can verify that the signal in figure 2 (a) is from hydroxyl radical, not from hydrogen peroxide. In summary, we develop a simple and cheap sensor to detect hydroxyl radical. The sensor shows significant conductance change when reacting with hydroxyl radical. We will further test different concentration of hydroxyl radical and confirm the mechanism in the future. This work was partially supported by National Science Council grant (101-2221-E-007-102-MY3) and by the research grant (101N7047E1) at National Tsing Hua University.

Variation of temperature, pressure, and pH values has been demonstrated for affecting the stability of AlGaN/GaN HEMTs sensors. In addition to the factors above mentioned, we found out other factors which are related to the stability of AlGaN/GaN HEMTs sensors. In this study, we found out the current variation of AlGaN/GaN HEMTs without gate metal sensors is inversely proportional to the viscosity of liquids. We also found out that dipole moment of liquids is related to the current variation of AlGaN/GaN HEMTs without gate metal sensors.