Laser scribed graphene: fabrication and electrochemical biosensors for neurotransmitters

Abstract

Graphene is an attractive class of carbon nanomaterials due to its extraordinary properties that are a direct consequence of its unique atomic structure. As a new member in the nanocarbon family, graphene has been widely studied from its fundamental science to the practical applications. Graphene's unique electrical, electrochemical and mechanical properties have led to the realization of designs for excellent biosensor and bioelectronics devices. However, manufacturing low-cost graphene-based electronic devices is still difficult to accomplish through a single-step fabrication process. Direct laser scribing has been demonstrated as a facile, inexpensive, solid-state approach for fabricating, patterning and turning the electronic properties of graphene. The advantage of the laser scribed graphene (LSG) method is the integration of material synthesis and shape patterning with smart computer design. In this thesis, the direct laser scribing of graphene has been shown to offer an opportunity for fabricating single-use, on-chip electrochemical sensors (Chapter 3). The method is simple and utilizes direct laser scribing using a light scribe DVD drive. The sp2 carbon structure was formed as the consequence of the photo-thermal reduction. The resulting LSG provides electrodes with outstanding conductivity and electrochemical activity. Addition of a polymeric binder, poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), to the graphene oxide before laser scribing contributes to a significantly improved water-stability of the LSG electrodes, which is of a great importance for biological sensing. The surface morphology and material structure were investigated by SEM and Raman, respectively. The PVDF-HFP/LSG electrodes were found to be very porous, highly conducting and electrochemically active. All these properties contribute favourably to applications such as LSG electrodes becoming a promising choice for future biological sensors. Chapter 4 describes fabrication of graphene by direct laser scribing of a commercially available polyimide sheet, which provides a revolutionary approach for the facile and cost effective creation of graphene. This laser scribing technology is able to quickly manufacture graphene electronics in an extensive size range, from sub-micrometre to several centimetres in size. As a further development, laser scribed graphene was combined with the electrodeposition of poly (3,4-ethylenedioxythiophene) (PEDOT) onto the LSG surface (Chapter 4). This electrode material was successfully used to develop a dopamine (DA) sensor with high selectivity and sensitivity. The PEDOT/LSG electrodes possess larger electrochemical-catalytic surface area and exhibit a 3D mesoporous network. The simultaneous determination of DA in the presence of ascorbic acid (AA) and uric acid (UA) was achieved using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The sensitivity towards DA was measured at 0.220 ± 0.011 μA μM-1 and the detection limit was 0.33 μM, which exceed the values shown by number of other graphene-based DA sensors reported previously. Existing carbon nanomaterials already include remarkable morphologies including graphene nanoribbons, carbon nano walls, vertically aligned carbon nanotubes and laser induced graphene fibers. A new LSG nanostructure was prepared for the first time, which showed a 3D vertical aligned "grass" like morphology (Chapter 5). This LSG grass was applied to the sensing of DA, epinephrine (EP), and norepinephrine (NE), using both CV and DPV. Clear anodic peaks were obtained for the oxidation of DA, EP and NE at LSG grass electrodes, with a good degree of separation despite the structural similarities of these important neurotransmitter molecules. UA and AA were also considered as common interferents, but very good analytical determinations for DA, EP and NE were possible. The sensitivity of LSG grass for DA, NE, and NE was highly improved compared to unmodified LSG. The fabricated LSG grass sensors for DA, EP, and NE detection displayed sensitivities of 0.243, 0.067, and 0.110 μA/μΜ, and the detection limits of 0.4, 1.1, and 1.3 μΜ, respectively. The exceptional performance of the LSG grass for the detection of multiple neurotransmitters points to a promising future for nanocarbon materials with various morphologies in biosensors and bioelectronics.