Abstract
Abstract : Two dimensional (2D) materials have provided a new era to biosensors research. Biosensors are functional biodevices which include the integration of biology with electronics. The integration of 2D materials with other nanomaterials has transformed the understanding of the biological and electronics world and has paved a way for the design and fabrication of novel 2D nanointerfaces. The use of 2D nanointerfaces has given great success to biosensors and bioelectronics field which ultimately impacts on biomedical diagnosis and sensing applications. The superior properties of 2D materials such as large surface area, ease of hybridization, good biocompatibility, and high electron transfer properties make them ideal interface materials for the design and fabrication of bioelectronic devices including biosensors. The thesis focused on the fabrication of 2D nanointerfaces by combining two 2D hybrid materials and then nanostructuring with metal nanoparticles for better electron transfer within the interface which is followed by immobilization of enzyme as a bio-recognition element for biosensing purposes. The conjugation of the 2D hybrid nanointerface materials was achieved through the self-assembly technique. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used in the study for characterization of the 2D hybrid nanointerface structures and chronoamperometry studies were employed to investigate the electrobiocatalytic properties of the 2D hybrid nanointerfaces structures. Structural characterization was done by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques for morphological details of 2D hybrid nanointerfaces structures. The fabrication of bioelectrodes was achieved by using the conjugated 2D hybrid nanointerface materials. ix There are three different segments in this research study. All of these different segments involved the use of 2D materials for bioelectronics purposes. The first phase involved the fabrication of smart hierarchically self-assembled 2D electrobiocatalytic interface system based on the combination of gold nanoparticles (AuNPs) doped graphene oxide (GO)-molybdenum disulfide (MoS2) layered nanohybrid, conjugated with poly (N-isopropylacrylamide, PNIPAAm) resulting in GO/AuNPs/MoS2/PNIPAAm interface. The introduction of PNIPAAm improved the stability of the self-assembled GO/AuNPs/MoS2 interface structure. Horseradish peroxidase (HRP) was subsequently immobilized on the GO/AuNPs/MoS2/PNIPAAm interface through electrostatic interactions giving GO/AuNPs/MoS2/PNIPAAm/Peroxidase electrobiocatalytic interface system as a platform for electrobiocatalysis reactions for biosensing applications. Morphological characterization of GO/AuNPs/MoS2/PNIPAAm indicates that this 2D nanointerface structure has a wide surface area for enzyme immobilization due to their flake-like structure. CV showed diffusion-controlled electron transfer properties at the interface. The electrobiocatalytic activity of the nanohybrid interface structure was studied using hydrogen peroxide (H2O2) as a model analyte. The fabricated bioelectrode exhibits a wide linear response to the detection of H2O2 from 1.57 to 11.33 mM, with a detection limit of 3.34 mM (S/N=3) and a capacitance of 8.6 F/cm2. The second phase of the study involved the fabrication of hybrid dual 2D-nanohybrid structure through self-assembly combination AuNPs with hybrid 2D materials consisting of boron nitride (BN) and tungsten disulphide (WS2) as a nanointerface system for electrochemical biosensing. HRP was immobilized on the hybrid dual 2Dnanoparticle systems to form a biointerface. Structural characterization showed high crystallinity in the fabricated structure, while morphological characterization confirmed x the high surface to volume area of the hybrid material and the presence of welldispersed AuNPs. Electrochemical characterization also confirmed that the fabricated HRP/BN/WS2/AuNPs/GC bioelectrode exhibited excellent electron transfer properties at the interface. The electrobiocatalytic activity of the nanohybrid interface structure was studied using H2O2 as a model analyte. The fabricated bioelectrode exhibited a wide linear range from 0.15 mM to 15.01 mM towards detection of H2O2 with a limit of detection of 3.0 mM (S/N = 3) and a sensitivity of 19.16 μA/mM/cm2. Theoretical studies of the BN/Au/WS2(001) nanohybrid structure was carried out using density functional theory (DFT) calculation for confirming the charge transport mobility and conductivity of the fabricated material. DFT calculations combined with the experimental studies showed that the self-assembled combination of the BN/Au/WS2(001) nanocomposite enhances the performance of the fabricated biosensor due to an introduced new electronic state emanating from the N 2p orbital. The third phase of the study involved the synthesis of acetylene sourced graphene (Gr) by chemical vapour deposition (CVD) method. Self-assembly method was used to prepare the 2D nanohybrid interfaces, which consist of Gr, WS2, AuNPs and HRP for fabricating electrochemical biosensor for detection of H2O2. The XRD results revealed that Gr/WS2/AuNPs nanohybrid structure has good crystalline nature. CV and electrochemical impedance spectroscopy results showed that due to the incorporation of AuNPs, the redox properties of Gr/WS2/AuNPs/HRP conjugate 2D hybrid structure improved in comparison to Gr/WS2/HRP. The same trend was observed in the chronoamperometric results. The Gr/WS2/AuNPs/HRP/GCE modified bioelectrode exhibited a good electrobiocatalytic performance towards the detection of H2O2 over a relatively wider linear range (0.40 mM to 23 mM), with a higher xi sensitivity (11.07 μA/mM/cm2) than that of Gr/WS2/HRP/GCE modified bioelectrode (9.23 μA/mM/cm2). The results have shown that electrobiocatalytic reactions can be controlled by modifying the nanohybrid interfaces.
D.Phil. (Chemistry