Considerations on the use of Impedance Spectroscopy for the Detection of Virions Trapped in Quadrupolar Microelectrode Arrays
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The impedance response of a quadrupolar microelectrode array was studied over a wide frequency range to determine whether particles captured at the center of the array could be detected impedimetrically. The microelectrode array (denoted as DEP chip) uses dielectrophoretic forces to concentrate particles at its center. Initial results showed that there was a large electrode-silicon-electrode (ESE) capacitance which dominated at high frequencies. This capacitance was reduced by decreasing the electrode area and increasing the insulating layer thickness. These measures however proved fruitless as this capacitance was still significantly greater then the dielectric capacitance of the chip. This ESE capacitance can be eliminated through the use of a glass substrate so that the dielectric response of the chip dominates at higher frequencies. Since the ESE capacitance prevented experimental validation of impedance spectroscopy as a signal transduction method, computer simulations were performed. These simulations indicated that capture with the current DEP chips would not have a significant impact on the impedance of the chip. Decreasing the electrode gap distance and reducing the area of the electrodes, which is recommended for future work, can remedy this. As measureable changes in the dielectric capacitance of the chip are not possible, a reaction scheme was developed to translate the capture of viral particles into a change in medium conductivity. An ELISA type system was proposed where the viral particles would be functionalized with urease. This uease would then be used to degrade non-ionic urea into ionic products thereby increasing the medium conductivity. A model was formulated to predict the conductivity increase expected for low concentrations, and validated using higher concentrations of biotinylated-urease. Urease from commercial sources proved not to be a viable option as it does not possess a high enough activity to produce a significant conductivity change given the low concentrations of viral particles expected after collection. Urease with suitable activity is produced by the organism Ureaplasma urealyticum which has an activity of 180 000 µmol urea catalyzed min-1 mg urease-1. It is not recommended that this method be pursued further due to technical challenges that would be encountered.