Graduate Thesis Or Dissertation
 

Graphene Biotransistors, an Exploration of Neurosensing and Biofilm-sensing Applications

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  • Atomically-thin graphene sheets have unprecedented characteristics for biosensing applications. These characteristics include mechanical flexibility and strength, optical transparency, electrical sensitivity and biocompatibility. The primary theme of this dissertation is the characterization and application of graphene field-effect transistors (FETs) in biologically-relevant physiological environments.Understanding the interface that forms between an electrolyte and graphene is critical to understanding biosensing mechanisms. The electronic signals measured by a graphene FET biosensor are typically the result of changes in the layer of dissolved ions near the graphene surface. The coupling between dissolved ions and a conductive surface is described by the electric double layer capacitance, CEDL. This interfacial capacitance has been studied for many years using conductive liquids and bulk metal surfaces. Only recently have careful studies of CEDL focused on the interface between graphene and conductive liquids. We use Hall effect measurements to determine the total capacitance at the interface and the tight-binding-model-based theory to separate the quantum capacitance, CQ, and CEDL. Based on this investigation, we find a CEDL of ~ 0.04 F m2 for biologically relevant fluids and ~ 0.11 F m2 for ionic liquids. Both of these values are lower than the typical value of 0.2 F m2 found with metals. This finding is also significant for choosing ideal fluids for graphene-based super capacitors.We have systematically characterized the noise and sensitivity of graphene FETs in aqueous electrolyte environments. We established the minimum attainable noise in this environment by determining the thermal noise limit. We then investigated charge carrier mobility, which is critical to device sensitivity. We performed Hall bar measurements on electrolyte-gated graphene assuming a Drude model, and find that the room temperature carrier mobility in water-gated, SiO2-supported graphene reaches 7000 cm2 Vs. This value is comparable to the best dry SiO2-supported graphene devices. Our results show that the electrical performance of graphene is robust, even in the presence of dissolved ions that introduce an additional mechanism for Coulomb scattering.We established two novel applications of graphene field-effect transistor biosensor. The first application is in-situ monitoring of the pH inside a living biofilm with fast temporal resolution (∼1 s) over multi-hour time periods. The atomically thin sensor is positioned between the biofilm and a supporting silicon oxide surface, providing noninvasive access to conditions at the base of the biofilm. We determine the transient changes in pH when the biofilm metabolizes substrate molecules and when it is exposed to biocide. The pH resolution is approximately 0.01 pH units when using 1 s time averaging; the sensor drift is approximately 0.01 pH units per hour. Our results demonstrate the potential of this technology to further the study of biofilm metabolism and improve monitoring of biofilm health.The second application for GFET biosensors that we established is wearable sensor patches for single cells. Recent advances in the fields of optics, biochemistry, and nanotechnology have instigated a multidisciplinary effort to understand the neural circuitry of the human brain. The electrodes currently used for in vivo single neuron sensing have not significantly advanced over the past century. The industry standard remains simple, insulated, conductive shafts with small, exposed tips. Graphene-based field-effect transistors are flexible yet strong, biocompatible, and able to locally amplify the electrogenic signals produced by neurons. This combination of material characteristics makes graphene ideal for next-generation biosensing applications.The graphene in our experiments is etched into patterns inspired by the Japanese paper art of kirigami to enable in-plane stretching. The devices are then stretched over cells, isolating the graphene from possible substrate noise while forming a conformal coating over the cell to obtain the optimal signal-to-noise ratio. The flexibility of these devices makes them promising as “wearable” electronics for cells with applications for both in vivo and brain-slice electrophysiological experiments. We present characterization and initial single cell measurements from these devices.
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