Graphene, a 2-dimensional material, has received increasing attention due to its unique physicochemical properties (high surface area, excellent conductivity, and high mechanical strength). Field-effect transistor is shown to be a very promising candidate for electrically detecting chemical and biological species. Most of the reports on graphene field-effect transistors show that solution-gated graphene field effect transistors have been used so far. Although the traditional solution-gated graphene field effect transistor has high sensitivity, but the graphene channel is contaminated easily. The stability of the device is reduced so that the device cannot be reused. Only very recently, has the top-gated graphene, which is potentially used for pH sensors, been reported. In the top-gated graphene the dielectrics is deposited at the top of graphene. However, the sensitivity is lower than other sensors. To improve the properties, we design and fabricate a top-gated graphene ion-sensitive field effect transistor by using large-area graphene synthesized by chemical vapor deposition. At the top of graphene, HfO2/Al2O3 thin film is deposited by atomic layer deposition. The Al2O3 film plays a role of sensitive membrane, and the HfO2/Al2O3 thin film protects the graphene from contamination of the solution. After depositing the top-gate, because of the shield of the insulation, the boundary between the graphene and the substrate is not clear. And the Raman spectrum indicates the presence of a defective top layer accompanied by an increase in the Raman D peak. After a series of electrical characterizations, compared with solution-gated graphene field effect transistor which directly contacts the graphene channel with the solution, the top-gated graphene ion-sensitive field effect transistor has a high resistance. This increase relative to uncovered grapheme, is attributed to the participation of the top -orbitals in van der Waals bonds to the insulation. The graphene -orbitals contributing to van der Waals bonds have less overlaps and thus result in reduced conductivity. However the output curves and transfer curves show that the top-gated graphene ion-sensitive field effect transistor has higher signal-to-noise ratio and better stability. In view of the biochemical detection, in this paper we also examine the adsorption of single-stranded DNA. Silane functionalization of metal oxide system is a versatile technique that can be used in DNA microarray and nanotechnology. The DNA immobilization process we have developed contains several steps: silanization (APTES), crosslinker attachment (EDC and NHS), reaction with carboxyl-DNA and removal of non-covalently bound DNA. We characterize the process with carboxyl-quantum dots. We also measure the transfer curves before and after the adsorption of DNA, and demonstrate the effectiveness of the functionalized process and the feasibility that the top-gated graphene ion-sensitive field effect transistor is used as the biosensor.