Graphene has been widely investigated in recent years due to its excellent physical, chemical, mechanical, and electronic properties^[1]. With a nanostructure of one-atom-thick and two-dimensional carbon sheet, graphene exhibits many advantages like large specific surface area, remarkable electrical conductivity, excellent chemical stability, and good biocompatibility^[2]. Therefore, graphene shows potential applications in chemical and biological sensors with high sensitivity, high selectivity, and prompt response time. More recently, graphene-based transistors have also been investigated for sensor applications. With some advantages like low operating voltage(less than 1 V)and solution stability, solution-gated graphene field effect transistors(SGGT)have been developed in bio-sensing, such as cell-based biosensor^[3], DNA sensor^[4], and bacteria sensor^[5]. In this paper, we report a sensitive glucose sensor based on SGGT, in which glucose oxidase(GOx)is immobilized on platinum(Pt)gate electrode. The sensor can detect glucose concentration as low as 1×10^-6 mol/L even without any nanomaterial modification for the gate electrode.

Fig1 shows the schematic diagram of a SGGT fabricated on SiO₂/Si substrate. The device is firstly characterized in PBS solution and bare Pt is selected as gate electrode. It can be found that the device shows very stable performance in PBS solution and the response time is less than 1 s. The transfer characteristic(I_DS-U_G, U_DS=-0.05 V)of the device is shown in Fig.2. We can find that the conductance of the device exhibits ambipolar behavior when the gate voltage is applied within 1 V. The Dirac point of the transfer curve is at around 0.7 V.

Fig.2 Transfer characteristic of the device measured in PBS solution

Then SGGT device with CHIT/GOx/Pt electrode is used for glucose sensing. Fig.3 shows the channel current(I_DS)as functions of time(U_DS=-0.05 V, U_G=0.2 V)when glucose with different concentration is added into PBS solution. We can find that I_DS decreases with the addition of glucose. The detection limit of the device for glucose is 1×10^-6 mol/L. A control experiment is conducted when using Pt gate electrode without immobilization of GOx. It can be found that there is nearly no change for the channel current when glucose with different concentration is added into PBS solution. Therefore, the determination of glucose by our device is attributed to the GOx on Pt gate electrode.

Fig.3 IDS as function of time when glucose with different concentration was added into PBS solution

Here we discuss the mechanism of the glucose sensor behavior based on SGGT. The chemical reactions near the gate electrode are shown as follows:

D-glucose enzyme(GOx) D-glucono-1,5-lactone+H₂O₂

2H₂O₂-2e^- O₂+H₂O

D-glucose is catalyzed by GOx with producing H₂O₂ and D-glucono-1,5-lactone. H₂O₂ is then decomposed at the gate electrode, which leads to the electron transfer on the surface of the gate electrode. As a result, the effective gate voltage applied on the SGGT device changes, leading to the change of the channel current(I_DS)of the device^[6-8]. As SGGT has many advantages like low-cost, easy fabrication, low operating voltage, and good biocompatibility, it will be the excellent candidate for the applications in bio-sensing.

SGGT is exploited for the determination of glucose. The detection limit of the sensor is 1×10^-6 mol/L when Pt gate electrode without any nanomaterial modification is used. The sensor response is attributed to the decomposition of H₂O₂ at Pt gate electrode. The detection limitof the sensor can be also improved by the modification of gate electrode with nanomaterials. Moreover, this type of biosensor shows potential applications in enzymatic sensing.

Acknowledgement: We thank professor Yan Feng from the Hong Kong Polytechnic University for his guidance in this work.