Preparation Of Graphene Conductive Networks And Their Application In Electrochemical Biosensors

Graphene has drawn extensive attention in the field of conductive materials and electrochemical biosensors with its unique two-dimensional structure,excellent electrical properties and good biocompatibility.However,when assembled into macroscopic networks,graphene and its derivatives have bottlenecks of easy agglomeration,high addition content,and poor dispersibility,restricting its excellent properties.Therefore,the spatial stretching structure of graphene is of great significance to increase the conductivity of graphene networks.Focusing on the construction of stretching graphene networks and their applications in electrochemical biosensors,conductive graphene networks with specific structures were constructed according to their application requirements,and high-performance electrochemical hemoglobin and glucose biosensors were obtained.The main contents in this paper are as follows:(1)Construction of conductive graphene networks.Taking advantage of swelling and contraction effects of polymer hydrogels,amine functionalized graphene(G-NH2)was effectively coated on sodium polyacrylate(PAAS)microspheres to prepare graphene-coated PAAS(PAAS@G-NH2)conductive composite microspheres with high conductivity and low graphene content.The G-NH2 prepared by chemical grafting shows a high integrity and a high electrical conductivity of 502.75 S/cm.Taking advantage of negatively charged molecular chains’ gaps of PAAS,positively charged G-NH2 was coated on the surface and the core of PAAS driving by gradient concentrations of G-NH2 and the electrostatic force between G-NH2 and PAAS.G-NH2 sheets aggregated and assembled during the dehydration process of PAAS,forming a conductive network.The electrical resistance of PAAS@G-NH2 was as low as 34 Ω,which is mainly attributed to the swelling and contraction effects of PAAS,strong electrostatic interactions between G-NH2 and PAAS,and the strong binding force of PAAS molecular chains on G-NH2.(2)Self-assembly of gradient graphene network.Taking advantage of high temperature reduction and self-assembly effects of graphene oxide(GO),we proposed a gradient temperature field to prepare a gradient graphene foam(GGF)with gradient pore structure and gradient functionality.GO underwent different degrees of reduction in a gradient temperature field and formed gradient interaction and gradient pore structure.Micro-CT and optical microscopy images show that GGF possesses gradient pore size.EDS,XPS,FT-IR,Raman and XRD spectra of GGF indicate that the oxygen functional groups and interlayer spacing change linearly with the reduction temperature and reduction degree.Contact angle measurement shows that the surface contact angles of GGF changed gradiently from 84.5° on the top surface to 123° on the bottom surface,indicating gradient functionality of both hydrophilicity and hydrophobicity.(3)Fabrication of graphene-based biosensors for the sensitive detection of hemoglobin.Taking advantage of adsorption and deformation effect of hemoglobin by carboxyl groups of electrochemically reduced GO(ERGO),highly sensitive detection of hemoglobin was achieved by ERGO with a high electron transfer rate.Epoxy and hydroxyl groups of GO on glassy carbon electrodes were selectively removed and carboxyl groups were remained in the electrochemical reduction process,obtaining an ERGO-modified electrode.The ERGO-modified electrode shows a high sensitivity of 22.71 μA·μM-1,a detection limit of 5.29 nM and a long-time stability towards the detection of hemoglobin.This performance is attributed to the adsorption and deformation of hemoglobin on ERGO and the high electrical conductivity of ERGO.(4)Fabrication of graphene-based biosensors for the sensitive detection of glucose.A laminated structure of GO and functionalized graphene(FG)was proposed for detecting glucose.By taking advantage of the high adsorption characteristics of GO to glucose oxidase(GOD)and high conductivity of FG,a large amount of GOD was loaded and highly sensitive detection of glucose was realized.The graphene-laminated electrodes exhibited higher GOD loading of 3.80×10-9 mol·cm-2,and higher detection sensitivity of 46.71 μA·mM-1·cm-2.Such high performance is mainly attributed to the abundant functional groups of GO,high electrical conductivity of FG,and strong interactions between the graphene-laminated structure and electrodes.The constructed GGF was used as electrode materials for detection of glucose.Graphene with abundant oxygen groups at one end of GGF could effectively achieve large GOD loadings and conduct electrochemical reactions with glucose,while graphene with a high reduction degree at the other end could achieve efficient electron transfer.Therefore,high GOD loading and efficient electronic transfer were effectively integrated on GGF,breaking through the barrier of two-phase interface between different functional layers of traditional sensor materials.The glucose sensor based on GGF shows a high sensitivity of 8.43 μA·mM-1 and a linear range of 2-40 mM.The high performance is attributed to the favorable effects of gradient reduction degree and gradient pore size of GGF.By virtue of its high sensitivity,the GGF electrode shows great potential as high-performance biosensors in the field of medical diagnosis.