Investigation of the Hydroxyl Radical Sensor with Conductance Change of Polyaniline

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.

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