Development of a ZnO/SiOsub2/Si High Sensitivity Interleukin-6 Biosensor

Abstract

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.