In recent years, with the development of nanotechnology, a variety of nanoparticles, such as multiwall carbon nanotubes (MWCNTs), graphene sheets (GS) and gold nanoparticles (AuNPs), have been widely used in the fabrication of immunosensors [6,7].Chitosan, containing large numbers of -NH2 and -OH groups, has been widely used as an immobilization matrix for biosensors due to its excellent biocompatibility, nontoxicity and cheapness [8,9]. It is preferable to maintain the high biological activity of the immobilized biomolecules and then enhance the sensitivity of the immunosensor, but the chitosan film is not electrically conductive. MWCNTs have attracted a great deal of interest due to their electrical properties, large specific surface areas, high stabilities and strong adsorption properties [10,11].

Thus, in recent years MWCNTs were introduced in chitosan film to improve its electric conductivity [12].GS, a two-dimensional carbon atom monolayer, has attracted great interest for the fabrication of electrochemical immunosensors due to its high conductivity, high surface-to-volume ratio, high elasticity and good biocompatibility [13,14]. However, the water solubility of GS limits their further application in designing biosensors because GS is hydrophobic and tends to form agglomerates in water [15]. As a result, many researchers have made efforts to increase the solubility of GS. Thus, the water-soluble polymers, such as polyvinylpyrrolidone [16], polypyrrole (PPy) [17], chitosan [18,19] and Nafion [20,21] were used as dispersants to prepare homogeneous GS solutions, while the introduction of these polymers could promote electron transfer well.

Significantly, some scientists have found that graphene-based composite Brefeldin_A materials, such as, gold nanoparticles and 1-pyrenebutyric acid-functionalized grapheme [22], graphene/polyaniline nanocomposite [23], AuNPs/PDDA-G [24], AuNPs decorated graphene (AuNPs-GS) [25] and MWCNTs-GS composites [26] are a useful approach. These graphene-based composite materials have good solubility and biocompatibility, and high electrochemical stability and conductivity, due to the synergistic contribution of two or more functional components. Herein, an effective reduction approach for the fabrication of GS-PEI-Au nanocomposites is demonstrated. PEI, an amino-rich cationic polyelectrolyte, is ingeniously used as both a functional agent for GS and a reducing agent and protecting agent for the formation of Au nanoparticles [27]. Combining the two functions of PEI, we can prepare GS-PEI-Au nanocomposites through in-situ reduction of HAuCl4 by PEI adsorbed on the surface of GS. In addition, in this way we improved the solubility and conductivity of GS efficiently.