New Electrochemical Biosensors Based On Three-Dimensional Porous Graphene

Owing to the advantages of high sensitivity, good selectivity, low cost, simple preparation, rapid response and easy miniaturization, electrochemical biosensors show potential application in the fields of environmental monitoring, clinical diagnostics, food security, medicine and so on. Material of the electrode is important to the sensitive detection in biosensing and analysis. Recent research demonstrated that three-dimensional-3D porous electrode can efficiently improve the performance of the sensors. Hence, constructing a new electrochemical system based on 3D porous electrode is of great significance. Graphene, a one-atom-thick two-dimensional ?2D? carbon material, is composed of carbon atoms. The unique and perfect structure, excellent electronic conductivity and mechanical properties render graphene highly promising for a wide range of applications in the field of ultra sensitive detection and supercapacitor. Except 2D graphene materials, graphene with special 3D structure has attracted increasing attention. Using Ni foam as template,3D porous graphene prepared via chemical vapor deposition ?CVD? has a monolithic porous struction. Compared to graphene-based material ?reduced oxide graphene, rGO?, which is prepared via exfoliated graphite oxide, such 3D porous graphene could serve as a novel freestanding and monolithic 3D electrode with extraordinary properties including outstanding rapid charge transferstrong, large surface area (-850 m² g^-1), ultra-high porosity ?99.7%? and lower destiney ?-5 mg cm-3?. However, the 3D graphene is defect-free and highly hydrophobic, which imposes difficulties in its surface modification and biological applications, so it is highly desired to develop facile and versatile methods for the functionalization of 3D graphene to improve its hydrophilcity and modifiability. The thesis studied the fabrication of two kinds of biosensors based on such 3D graphene for glucose and hydrogen peroxide detection. The detailed results were summarized as follows:1. This thesis reports on the biofunctionalization of monolithic and freestanding 3D graphene foam for one-step preparation of reagentless enzymatic biosensors by controllable chitosan ?CS? electrodeposition technology. Using a homogeneous three-component electrodeposition solution containing a ferrocene ?Fc? grafted CS hybrid ?CS-Fc?, glucose oxidase ?GOD?, and single-walled carbon nanotubes ?SWNTs?, a homogeneous biocomposite film of CS-Fc/SWNTs/GOD was immobilized on the surface of 3D graphene foam by one-step electrodeposition. The Fc groups grafted on chitosan can be stably immobilized on the 3D graphene surface and keep their original electrochemical activity. The SWNTs doped into the CS-Fc matrix act as a nanowire to facilitate electron transfer and improve the conductivity of the biocomposite film. Combined with the extraordinary properties of 3D graphene foam including large active surface area, high conductivity, and fast mass transport dynamics, the 3D graphene based enzymatic biosensor?3D G/CS-Fc/SWNTs/GOD? achieved a large linear range (5.0 ?M to 19.8 mM and low detection limit ?1.2?M? for reagentless detection of glucose in the phosphate buffer solution.2. This thesis reports on a simple and versatile method to construct high performance three-dimensional ?3D? graphene-based enzyme biosensor?3D G/CNTs-MB/pDA/HRP?. Monolithic and macroporous 3D graphene foam grown by chemical vapor deposition ?CVD? was used as a freestanding electrode for co-immobilization of horseradish peroxidase ?HRP? and a commonly used redox mediator, methylene blue ?MB?. Carbon nanotubes ?CNTs? were employed as carriers of MB molecules to immobilize them on 3D graphene surface through strong ?-? stacking force. Mussel-inspired biopolymer polydopamine ?pDA? was formed by in-situ polymerization and served as a green linker, which could covalently graft HRP on the surface of 3D graphene/CNTs-MB electrode. In addition, pDA layer could also effectively prevent the leakage of inner electron mediators. Owing to the 3D macroporous architecture, exceptional properties of graphene and surface-bound mediators, the biosensor demonstrated outstanding performance for reageatless detection of H₂O₂ in terms of wide linear range ?0.2 ?M?1.1 mM?, high sensitivity (227.8 ?A mM^-1 cm^-2), low detection limit ?58.0 nM?, and fast response ?reaching 95% of the steady current within 3 s?. The biosensor exhibited high reproducibility and stability.