Exosomes are a subpopulation of vesicles that are expelled from all cells and are speculated to have some role in the development of cancerous tissue. The work in this thesis presents the sensitive and specific electrical detection of exosomes in solution using a graphene field effect transistor (gFET) biosensor: non-covalently functionalised using an intermediate linker and conjugated with anti-CD63 antibodies. The intermediate linker chosen was a pyrene based molecule, which pi-stacks to the graphene surface without disruption, or introduction of high defect density, to the graphene sp2 hybridised structure unlike covalent functionalisation. The gFET biosensor could act as an cheap, wafer-scalable alternative to more conventional labelling techniques and the current state-of-the-art optical techniques that use surface plasmon resonance (SPR) on functionalised gold substrates, which have so far detected down to 1000 exosomes/μL.
By exposing only part of the graphene surface to charged species using a microfluidic channel, regions of different doping levels arose in the same graphene film. This manifested as an additional minimum, which shifted to higher gate voltage (Vg) in the transfer characteristic and saturated over 30 minutes when graphene was exposed to exosomes. The saturation was attributed to an equilibrium between exosomes binding and unbinding to anti-CD63 antibodies. As exosomes exhibit negative charge when in phosphate buffered saline (PBS) at pH 7, the accumulation of positive charge in the exposed graphene as they come close to its surface causes a shift to higher Vg by the electrostatic field effect. Therefore the exposed graphene becomes p-doped relative to when exposed only to PBS. The value of the shift after 30 minutes was also concentration dependent with sensitivity down to a concentration of 0.1 μg/mL exosomes (5000 exosomes/μL). By using two negative controls: an isotype control antibody and a non-specific protein target, the gFET sensor was also shown to be specific to CD63 in exosome membranes. Both back-gated and electrolyte top-gated gFETs were used for exosome detection whereby capacitative effects as well as electrostatic effects contributed to the sensing mechanism for the electrolyte top-gated gFET.