Template-directed hierarchical self-assembly of graphene based hybrid structure for electrochemical biosensing.

A template-directed self-assembly approach, using functionalised graphene as a fundamental building block to obtain a hierarchically ordered graphene-enzyme-nanoparticle bioelectrode for electrochemical biosensing, is reported. An anionic surfactant was used to prepare a responsive, functional interface and direct the assembly on the surface of the graphene template. The surfactant molecules altered the electrostatic charges of graphene, thereby providing a convenient template-directed assembly approach to a free-standing planar sheet of sp(2) carbons. Cholesterol oxidase and cholesterol esterase were assembled on the surface of graphene by intermolecular attractive forces while gold nanoparticles are incorporated into the hetero-assembly to enhance the electro-bio-catalytic activity. Hydrogen peroxide and cholesterol were used as two representative analytes to demonstrate the electrochemical sensing performance of the graphene-based hybrid structure. The bioelectrode exhibited a linear response to H2O2 from 0.01 to 14 mM, with a detection limit of 25 nM (S/N=3). The amperometric response with cholesterol had a linear range from 0.05 to 0.35 mM, sensitivity of 3.14 µA/µM/cm(2) and a detection limit of 0.05 µM. The apparent Michaelis-Menten constant (Km(app)) was calculated to be 1.22 mM. This promising approach provides a novel methodology for template-directed bio-self-assembly over planar sp(2) carbons of a graphene sheet and furnishes the basis for fabrication of ultra-sensitive and efficient electrochemical biosensors.