Graphene based electrical biosensors for the detection of biomolecules

Abstract

Detection of biomolecules provides analytical information about the component of food, safety of drinking water, quality of environment or even diagnostic information about a patient. It is of great importance to develop biosensors with good reliability, sensitivity, LOD and simple in operation. Through the years of development in nano-materials, most of them have proved themselves to be a great candidate in different areas of biosensing, for example, gold nano-particles in colorimetric biosensing, quantum dots in fluoresce biosensing, silica nano-wire and carbon nano-tube in FET biosensing, etc. Among the list of discovered nano-materials, graphene was only being studied empirically for approximately ten years and its miraculous properties amused scientists and opened a new era of the study in two-dimensional material. In this study, graphene was used as transducer in field-effect transistor biosensor and electrochemical biosensor for detection of various targets. In the fist project, reduced graphene oxide was fabricated into field-effect transistor and functionalized with long capture probe for detection of H5N1 avian influenza virus gene detection. The long capture probe proposed in this study contains two sections, one of the recognized and hybridize with target DNA and the other section remained single stranded to π-π stack with graphene. When compared with the conventional DNA immobilization approach on graphene surface (short capture probe and linker involved covalent immobilization), the proposed long capture probe approach was found to be the most sensitive one. It is hypothesized that long capture probe has a better affinity to graphene after DNA hybridization and it brought target DNA closer to graphene surface, which no linker was required. In the second project, long capture probe similar to the first project was applied in a CVD graphene-based biosensor. Here, a secondary reporter probe with gold nano-particles conjugated with target complementary oligonucleotide was applied. The reporter probe enhanced the sensitivity of the biosensor and the detection limit was as low as 64 fM. More importantly, it has the ability to differentiate single-base mismatch from fully complementary sequence which suggested an excellent specificity. In the final chapter, a reduced graphene oxide-based electrochemical biosensor was developed for sensing botulinum neurotoxin type A. An artificially synthesized recognition probe (SNAP-25-GFP) with cleavage site for botulinum neurotoxin type A was immobilized on the surface of reduced graphene oxide. Initially, the electrode surface was covered making it inaccessible to the redox probe in the standard buffer. When botulinum neurotoxin type A presented in the analyte, the probe on the surface of reduced graphene oxide was removed exposing the electrode, thus recovering the electrochemical signal. This sensing system demonstrated a very good limit of detection against botulinum neurotoxin A with excellent specificity, which only fresh and active botulinum neurotoxin A can be detected. More importantly, this electrochemical-sensing platform was proved to be functioning when botulinum neurotoxin A dispersed in milk, mimicking a real-life sample. The low interference suggested its potential to be applied as an on-site toxin screening platform.