Development of a ZnO/SiOsub2/Si High Sensitivity Interleukin-6 Biosensor
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Biosensors offer the opportunity to sense biological material providing valuable information for medical diagnostics and monitoring of pathogens in the environment. Thus the development of high sensitivity, cost effective, real-time and portable biosensors is of primary importance. This thesis presents the development of a ZnO/SiOsub2/Si based CMOS compatible biosensor, for the real-time detection of interleukin-6 (IL-6). In this work, high quality ZnO films were grown on SiOsub2/Si substrates by pulsed laser deposition. A protein immobilization procedure for binding the IL-6 protein to the ZnO active area was developed and the morphology of the bio-molecules was studied using SEM and AFM techniques for the first time. A modified solid-phase Enzyme linked Immunosorbent assay (ELISA) technique was developed to measure the mass of protein bound onto the oxide surface. The study resulted in a mass of 0.364 pg/ml of IL-6 bound onto ZnO for an applied mass of 0.5 ng/ml. A guided shear mode surface acoustic wave (SAW) device in the ZnO/SiOsub2/Si system, with ZnO as the guiding layer on a SiOsub2/Si substrate, was modeled and fabricated. Two devices, operating at 708 MHz (device A) and 1.5 GHz (device B) were developed. The mass sensitivities of these devices were calibrated by applying a known copolymer mass in a window area opened in the SAW devices and by measuring the frequency shift due the application of the mass. The maximum mass sensitivity of devices A and B was 4.162 μm²/pg and 8.687 (μm²/pg) for ZnO guiding layer thickness of 340 nm and 160 nm respectively. A technique to apply IL-6 directly onto the SAW sensor surface was developed. For an applied IL-6 mass ranging from 20ng/ml - 2 μg/ml, applied in a 20x20 μm2 sensing area, the device measures IL-6 masses in the range of 1.2 fg-76.45 fg. A proof-of-concept experiment for the biosensor was setup with normal human serum to detect the presence of IL-6 in trace amounts. The device predicts three times as much IL-6 mass for normal human serum derived from pooled donors under the age of 55 as compared to that from a donor over the age of 55. This is understood to be the result of age related increased IL-6 levels and was independently confirmed through ELISA measurements. The ZnO/SiOsub2/Si sensor system therefore enables highly sensitive mass detection of the IL-6 protein to be realized.