Synthesis of a hydrophilic poly-l-lysine/graphene hybrid through multiple non-covalent interactions for biosensors

Surface modification has been proved to be one of the effective strategies for enhancing the properties of graphene sheets. When a non-covalent modification method is appropriately designed, novel opportunities for better performance of graphene nanosheets can be expected since this strategy can tailor the properties of graphene while its natural structure is retained. This paper introduces a simple route to prepare a highly biocompatible, stable and conductive graphene hybrid modified by poly-l-lysine (PLL) for biosensors using the non-covalent strategy. Results show that PLL adopts a random conformation with the nonpolar parts exposed to outside since its side chains are positively charged under neutral conditions. This conformation allows the strong adhesion of PLL to graphene surface via the hydrophobic interaction between butyl chains of PLL and graphene surface, cation-π interaction of protonated amine groups on PLL with the π electrons in graphene, and electrostatic interaction between the protonated amine groups on PLL and the negatively charged carboxyl groups remaining on graphene. All these interactions make the resultant PLL-G hybrid stable and dispersible in aqueous solutions. The resultant hybrid is then used to construct high performance biosensors. As demonstration, hemoglobin (Hb) carrying negative charges can be easily immobilized on the hybrid via electrostatic interactions with the positively charged lysine side chains of PLL modified on graphene surface, forming the Hb@PLL-G bionanocomposite. The immobilized protein retains its native structure and exhibits reversible direct electrochemistry. The Hb@PLL-G based enzymatic electrochemical biosensor shows excellent catalytic activity toward its substrate hydrogen peroxide. Its electrochemical response shows the linear dependence of hydrogen peroxide concentration in a range between 10 μM and 80 μM with a detection limit of 0.1 μM. The apparent Michaelis-Menten constant is calculated as 0.0753 mM, demonstrating the significant catalytic ability of the protein. The present strategy can be extended to modify other carbon materials and the resultant nanocomposites are promising for construction of biosensors, bioelectronics and biofuel cells.