Solution-gated graphene transistors for chemical and biological sensing applications

Abstract

Various fabrication processes were developed in order to make graphene-based chemical and biological sensors on different substrates. Single-layer graphene is grown by chemical vapor deposition and then transferred to silicon dioxide as well as PolyEthylene Naphthalate (PEN) substrate, where graphene solution-gated field-effect transistors (SGFET) are fabricated. The graphene on SiO₂ and PEN SGFETs exhibit high transconductances of 5 and 1 mS.mm-¹ respectively. They can be used as pH sensors in an aqueous environment with sensitivity at the Dirac point of 22 mV/pH. No significant influence of the nature of the substrate and the amount of residues on top of the graphene surface was found. This paves the way for developing low cost, flexible and transparent graphene sensors on plastic. The functionalization of graphene with glucose oxidase enables to build a graphene glucose sensor. The sensor exhibits reliably a high sensitivity of 15mV/pG (pG=point of glucose concentration) at the Dirac point and the lower detection limit found is 0.1 mM. Then, as the noise is the second crucial parameter along with sensitivity for biosensors, it was characterized in graphene SGFETs. The noise measured at the gate is very good around 20 [mu]V, which is an order of magnitude lower than conventional silicon SGFET. Bilayer sensors were also investigated since they could potentially exhibit lower noise than monolayer devices. A transfer method was designed to stack two monolayer graphene films in order to make a bilayer film. Bilayer devices could also be used as pH sensor with similar sensitivity compared to monolayer devices. However, the noise performance of bilayer devices around 15 [mu]V is slightly better than monolayer devices and bilayer graphene is therefore also a promising candidate for sensing applications. Finally, the commercialization of graphene sensors as well as innovative biosensors is hampered in the US by an ill-adapted FDA regulation. The consequences of this regulation are very negative with an outflow of capital from the US to Europe. Policy recommendations are made to restore the US leadership in the biosensor market, especially the implementation of an adaptive FDA regulation with a limited-launch, living-license process in which the effectiveness requirement is removed.