PhD ThesisThe current applicability and accuracy of point-of-care devices is limited, with
the need of future technologies to simultaneously target multiple analytes in complex
human samples. Graphene’s discovery has provided a valuable opportunity towards
the development of high performance biosensors. The quality and surface properties
of graphene devices are critical for biosensing applications with a preferred low contact
resistance interface between metal and graphene. However, each graphene
production method currently results in inconsistent properties, quality and defects thus
limiting its application towards mass production. Also, post-production processing,
patterning and conventional lithography-based contact deposition negatively impact
graphene properties due to chemical contamination.
The work of this thesis focuses on the development of fully-functional,
label-free graphene-based biosensors and a proof-of-concept was established for the
detection of prostate specific antigen (PSA) in aqueous solution using graphene
platforms. Extensive work was carried out to characterize different graphene family
nanomaterials in order to understand their potential for biosensing applications. Two
graphene materials, obtained via a laser reduction process, were selected for further
investigations: reduced graphene oxide (rGO) and laser induced graphene from
polyimide (LIG). Electrically conductive, porous and chemically active to an extent,
these materials offer the advantage of simultaneous production and patterning as
capacitive biosensing structures, i.e. interdigitated electrode arrays (IDE). Aiming to
enhance the sensitivity of these biosensors, a novel, radio-frequency (RF) detection
method was investigated and compared with conventional electrochemical impedance
spectroscopy (EIS) on a well-known biocompatible material: gold (standard). It was
shown that the RF detection methods require careful design and testing setup, with
conventional EIS performing better in the given conditions. The method was further
used on rGO and LIG IDE devices for the electrochemical impedance detection of PSA
to assess the feasibility of the graphene based materials as biosensors.
The graphene-based materials were successfully functionalized via the
available carboxylic groups, using the EDC-NHS chemistry. Despite the difficulty of
producing reproducible graphene-based electrodes, highly required for biosensor
development, extensive testing was carried out to understand their feasibility. The
calibration curves obtained via successive PSA addition showed a moderate-to-high
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sensitivity of both rGO and LIG IDE. However, further adsorption and drift testing
underlined some major limitations in the case of LIG, due to its complex morphology
and large porosity. To enable low contact resistance to these biosensors, the
electroless nickel coating process is shown to be compatible with various
graphene-based materials. This was demonstrated by tuning the chemical nickel bath
and method conditions for pristine graphene and rGO for nickel contacts deposition