In recent years, Gr-FET biosensors have attracted substantial interest, and several studies have reported that lattice imperfections can modulate adsorption, local density of states and device response. However, these reports are largely isolated observations and do not constitute a unified, device-oriented strategy for defect exploitation. Notably, defect-engineered graphene sensors still face several critical challenges. These include the trade-off between defect density and device electrical performance, difficulties in functionalizing defect sites, and high variability among sensors, which hinder their practical application. This thesis aims at taking advantage of the defects by forming graphene edge in sensing of Gr-FET biosensors. By harnessing and engineering intrinsic defects, we aim to enhance sensor sensitivity and stability.
In Chaper 2, to overcome the diffusion barrier of ultralow analyte concentrations to the sensor surface, we developed an edge-functionalized...
Show moreIn recent years, Gr-FET biosensors have attracted substantial interest, and several studies have reported that lattice imperfections can modulate adsorption, local density of states and device response. However, these reports are largely isolated observations and do not constitute a unified, device-oriented strategy for defect exploitation. Notably, defect-engineered graphene sensors still face several critical challenges. These include the trade-off between defect density and device electrical performance, difficulties in functionalizing defect sites, and high variability among sensors, which hinder their practical application. This thesis aims at taking advantage of the defects by forming graphene edge in sensing of Gr-FET biosensors. By harnessing and engineering intrinsic defects, we aim to enhance sensor sensitivity and stability.
In Chaper 2, to overcome the diffusion barrier of ultralow analyte concentrations to the sensor surface, we developed an edge-functionalized graphene nanoribbon field-effect transistor biosensor that integrates edge-localized dielectrophoretic (DEP) preconcentration. Graphene edges were functionalized by the electrochemical grafting of 4-N-Hydroxy succinimide benzene diazonium tetrafluoroborate salts (NHS-Ph-N2+) and were subsequently coupled to amine-terminated DNA probes. The atomically sharp graphene edges, together with the 12 nm HfO2 dielectric layer between the graphene and the back-gate electrode, enables strong edge-localized DEP that concentrate target miRNA at the edge enabling detection of attomole concentrations of miRNA.
In Chapter 3, the fundamental trade-off between defect density and electronic performance in graphene-based biosensors is addressed. Although defect sites serve as highly active sites for molecular adsorption and signal transduction, their low density in pristine graphene limits the number of accessible binding sites and thus constrains sensing sensitivity. Meanwhile, increasing defect density often compromises the intrinsic electrical properties of graphene by introducing carrier scattering centers. A polymer-mediated fracturing technique was therefore developed in which monolayer graphene is embedded in an epoxy matrix and then mechanically split at the midpoint. In this configuration, the graphene surface is fully encapsulated within a millimeter-thick polymer layer, preventing parasitic surface effects while enabling the precise formation of a clean graphene edge. This method isolates edge-specific quantum capacitance, as evidenced by an ∧-shaped capacitance profile that directly confirms the presence of localized defect states. Through potential-controlled diazonium grafting, graphene edges were functionalized with 4-N-Hydroxy succinimide (NHS) benzene diazonium tetrafluoroborate salts, enabling ultra-sensitive miRNA-21 detection at 1 fM with single-base discrimination, a 100-fold enhancement over conventional methods such as vinylsulfonated-polyamines (PA-VS), TIPS-Eth-ArN2+, and N-Heterocyclic Carbene (NHC) functionalization on graphene surface.
Chapter 4 describes a study to improve the density of graphene edge sites while maintaining high electrical performance. In addition, the device-to-device variability in graphene-based sensors by improving the reproducibility of the sensing interface. To achieve this, selectively functionalized graphene edges are used, where defects are naturally abundant, and while the basal plane is protected with a ceramic material. Graphene embedded in ceramic exhibits a quantum capacitance that is proportional to the density of states (DOS) and can therefore serve as the sensing transducer. This so-called edge-enhanced graphene quantum capacitor (eGQC) device allows us to reliably characterize the quantum capacitance (CQ), which is proportional to the density of states specific to the graphene edges. The resulting eGQC biosensor with covalently functionalized graphene edges, demonstrates selective sensitivity towards miRNA and reusability for up to 100 cycles using oxygen plasma treatment without loss in performance. Remarkably, the standard deviation of calculated relative sensing response ΔC/C0 over the 100 times continuously miRNA detection concentrated at 41%.
In Chapter 5, a hybrid device architecture is developed by integrating a monolayer graphene channel with vertically aligned polyaniline (PANI) nanoarrays to combine the advantages of organic electrochemical transistors and high-mobility electronic conductors. In this design, a continuous monolayer graphene serves as a lateral charge transport channel, while the PANI nanoarrays act as the active sensing area. The PANI layer provides a volumetric ion–electron coupling medium, where ion penetration from the electrolyte modulates the local doping state of the polymer through protonation/deprotonation processes. DNA probes were immobilized on the PANI surface, enabling specific target recognition. Upon hybridization with target miRNA, the local electrostatic environment and protonation equilibrium within the PANI are perturbed, resulting in a change in its doping level and conductivity. This conductivity modulation in PANI is electrically coupled to the underlying graphene channel, which functions as a high-mobility readout pathway that transduces small changes in carrier density into measurable variations in drain current. Based on this architecture, a graphene–PANI (Gr–PANI) organic electrochemical transistor biosensor was realized, achieving a detection limit down to 1 fM.
Chapter 6 summarizes the main findings of the thesis: various graphene edge-based biosensing platforms (including graphene nanoribbon, mechanically fractured monolayer graphene, ceramic-embedded reduced graphene oxide, and graphene-bridged polyaniline) were developed to detect miRNA with ultrahigh sensitivity, overcoming diffusion limits and being reusable under some conditions. Chapter 6 then outlines future outlooks, focusing on optimizing edge functionalization characterization, quantifying DNA density on the edge, and the use of the sensor in relevant biological conditions such as PBS and blood plasma.
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