Fabrication, electrical characterization, and detection application of graphene-sheet-based electrical circuits
© Peng and Lei; licensee Springer. 2014
Received: 15 September 2014
Accepted: 3 November 2014
Published: 15 November 2014
The distribution of potential, electric field, and gradient of square of electric field was simulated via a finite element method for dielectrophoresis (DEP) assembly. Then reduced graphene oxide sheets (RGOS)- and graphene oxide sheets (GOS)-based electrical circuits were fabricated via DEP assembly. The mechanically exfoliated graphene sheets (MEGS)-based electrical circuit was also fabricated for comparison. The electrical transport properties of three types of graphene-based electrical circuits were measured. The MEGS-based electrical circuit possesses the best electrical conductivity, and the GOS-based electrical circuit has the poorest electrical conductivity among all three circuits. The three types of electrical circuits were applied for the detection of copper ions (Cu2+). The RGOS-based electrical circuit can detect the Cu2+ when the concentration of Cu2+ was as low as 10 nM in solution. The GOS-based electrical circuit can only detect Cu2+ after chemical reduction. The possible mechanism of electron transfer was proposed for the detection. The facile fabrication method and excellent performance imply the RGOS-based electrical circuit has great potential to be applied to metal ion sensors.
Graphene, a single layer of carbon atoms densely packed into a two-dimensional honeycomb structure, has received worldwide attention due to its extraordinary mechanical, electrical, thermal, and optical properties . The attractive properties of graphene make it to be an ideal material for fundamental research and potential applications, such as electrical circuits , chemical and biological sensors , and composite materials .
Mechanical exfoliation of graphite on the pre-patterned electrodes is a popular method to fabricate high-quality graphene sheet-based electrical circuits. The extremely low throughput and lack of position precision severely limit the controllable fabrication of graphene sheet-based electrical circuits . Reduction of graphene oxide provides an alternative method to produce large quantities of reduced graphene oxide sheets (RGOS) . The ease of material processing, low cost of synthesis, and mechanical flexibility make RGOS become a perfect candidate for the fabrication of electrical circuits. In addition, RGOS exhibit a highly sensitive response to the outer environment. Thus, it is meaningful to explore the electrical properties of the RGOS for application. The RGOS was prepared by chemical reduction of graphene oxide sheets (GOS). Graphene sheets can be randomly deposited on the prefabricated electrodes by drop-coating suspension to fabricate graphene sheet-based electrical circuits. However, this method is not suitable for controllable fabrication of electrical circuits. Thus, directed assembly of graphene sheets at predetermined locations is required.
Dielectrophoresis (DEP) is a powerful technique for controllable fabrication of nanoelectronic devices, in which the DEP force is exerted on a polarizable object by non-uniform electric field . DEP provides a simple, scalable, and low-cost method to position graphene sheets. Vijayaraghavan’s group use DEP for rapid assembly of individual flakes and nanoribbons of few-layer graphene into high-density electronic devices with a high yield . Joung’s group explored high-yield fabrication of RGOS-based field effect transistors (FET). The RGOS in suspension were assembled between prefabricated gold source and drain electrodes via DEP . Burg’s group carried out DEP deposition of few-layer graphene oxides between prefabricated electrodes .
Compared to the one-dimensional sensing materials including nanowire, nanotube, and nanoribbon, the two-dimensional graphene sheets used for detection have unique advantages because of their extremely large specific surface area, homogeneous functionalization, and high charge mobility and carrier concentration. As the surface is directly exposed to the ambient environment, the electrical property of graphene sheets is highly sensitive to the external disturbance. Sudibya’s group reported a FET sensor using protein-functionalized RGOS as the conducting and sensing channel . Chen’s group reported an RGOS-based FET sensor, in which the RGOS was modified with gold nanoparticles . The copper ions (Cu2+) are extremely harmful because of their toxicity that even a trace amount can pose detrimental damage to human health . Thus, it is meaningful to find a facile, rapid, and sensitive method to detect the Cu2+.
In this paper, the RGOS, GOS, and mechanically exfoliated graphene sheets (MEGS)-based electrical circuits were fabricated and characterized, respectively. Then the three types of electrical circuits were applied for the detection of Cu2+. According to the relative change of electrical conductivity, the detection performance of graphene sheet-based electrical circuits for the Cu2+ in aqueous solution was evaluated. A possible mechanism for the detection was proposed.
Fabrication and electrical characterization
Fabrication of RGOS- and GOS-based electrical circuits
RGOS and GOS powder purchased from Nanjing XFNano Materials Technology Company (Nanjing, China) were used for the preparation of suspension. Five milligrams of RGOS powder was dispersed in 100 ml of N,N-dimethylformamide (DMF). The mixture was then ultrasonicated for 4 h. After nature sedimentation for 4 h at room temperature, the supernatant of solution was centrifuged at 1,000 rpm for 30 min to precipitate higher density aggregates. The supernatant in the centrifuge tubes was collected and used for the DEP assembly. The GOS were dispersed in de-ionized (DI) water via ultrasonication . Five milligrams of GO powder was added into 100 ml of DI water. The mixture was then ultrasonicated for 2 h. The supernatant solution was then centrifuged at 1,000 rpm for 30 min. The supernatant in the centrifuge tubes was preserved for the DEP assembly.
Numerical simulation of DEP assembly of graphene sheets
where and are the complex permittivity of the medium and graphene sheets, respectively.
The DEP force was exerted on the graphene sheets by the non-uniform electric field; thereby, the distribution of potential and electric field was simulated. As shown in the inset image of Figure 4a, the potential decreased gradually from anode to cathode along the electric field lines. The generated electric field strength reached the maximum value in the gap regions. The order of magnitude of electric field strength was as high as 106 V/m between two Au electrodes. The electric field strength decreased sharply with the distance from electrodes, and the value was close to zero where the regions are far away from the Au electrodes. When the applied frequency was 3 MHz, the Re[K(ω)] was positive. Then the graphene sheets bear positive DEP force and were attracted to the regions of high electric field strength. This is the reason that the RGOS and GOS deposited preferably into the gap regions between two Au electrodes.
The DEP force exerted on the graphene sheets is directly proportional to ∇E2 according to Equation 2. Therefore, ∇E2 is an estimate of the DEP force in direction and magnitude . Figure 4b shows the simulated maps of ∇E2 distribution in the vicinity of Au electrodes; the arrows indicate the direction of ∇E2. The deepest regions near the sharp corners of electrode pairs denoted the ∇E2 reached the maximum. Then DEP force also reached the maximum accordingly. Under the action of the DEP force, the graphene sheets were attracted to higher ∇E2 regions. Thereby most of graphene sheets were deposited near the sharp corner regions of Au electrodes that the experimental result has observed.
Fabrication of MEGS-based electrical circuit
Electrical transport of the MEGS-based electrical circuit was also measured from −1 to +1 V at room temperature. Figure 5c shows the I-V characteristic of the MEGS-based electrical circuit. The characteristic is not perfectly linear, which presumably arises due to contact barrier at the interface of the thin sheets and Au electrodes. The measured resistance was calculated about 370.4 Ω. The electrical conductivity of the MEGS-based electrical circuit was better than that of the RGOS-based electrical circuit. This can be attributed to surface and structural defects of the RGOS .
Results and discussion
Detection of Cu2+
where G a is the conductance at the ambient condition, G g is the conductance after the addition of solution, and ΔG represents the change of conductance after the addition of solution . With respect to the response time, it is defined as the time taken for the relative conductance change to reach 90% of the next steady-state value from a steady-state value . After detection, the remained solution on the graphene electrical circuit was thoroughly blown off with nitrogen gas. The current returned to the initial value after blowing which meant that the graphene sheet-based electrical circuit is able to be used for electrical detection repeatedly.
In order to characterize the detection performance of the graphene sheet-based electrical circuit for Cu2+ in aqueous solution, we measured its electrical conductance upon the addition of Cu2+ solutions with different concentrations. The applied voltage was fixed at a constant of 1 V. Thereby the measured current was equal to the conductance in value. The detection time was set to 250 s. The Cu2+ solution was dropped on the gap of two Au electrodes and the real-time responsive current of the graphene sheet-based electrical circuit was constantly monitored.
The detection performance of the RGOS-based electrical circuit was stable. Whether dropping DI water or Cu2+ solutions with different concentrations, the current of the RGOS-based electrical circuit decreased immediately and then leveled off until it reached a steady-state value within seconds. Then the steplike current kept the constant value for a long recording time until the end of detection. As the response of the RGOS-based electrical circuit to the analytes was almost instantaneous, the response was super sensitive and the response time was just a few seconds.
The relative conductance change of the RGOS-based electrical circuit reached the maximum value when detecting DI water. As for Cu2+ solutions with different concentrations, the relative conductance change gradually decreases when the concentration of Cu2+ increases. The relative conductance change of the RGOS-based electrical circuit can be ascribed to the combined effect of DI water and Cu2+ on the RGOS when detecting Cu2+ solutions. The effect of Cu2+ on the RGOS-based electrical circuit can be considered to subtract the effect of DI water from Cu2+ solutions with different concentrations . After removing the effect of DI water, the relative conductance increases gradually with the concentration of Cu2+ increasing from 10 nM to 1 mM, as shown in the Figure 6b. There was no doubt that the RGOS-based electrical circuit can be used to detect the Cu2+ of 10 nM.
Once the positively charged Cu2+ are absorbed on the surface of the graphene sheets, the electrical conductivity of graphene sheet-based electrical circuits was changed . This can be explained by the charge transfer between the graphene sheets and Cu2+. The charged Cu2+ and water play the roles in acting as electron donors or acceptors which induce the change of charge carrier concentration in the graphene sheets . The current of RGOS-based electrical circuit decreased immediately upon the addition of DI water. Since water molecules acted as an electron donor, the electrons transferred from the water molecules to the RGOS, which decreased the hole concentration in the RGOS and thereby decreased the current of RGOS-based electrical circuit . When detecting Cu2+ solutions with different concentrations, the water molecules and the Cu2+ were simultaneously absorbed on the surface of the RGOS. In order to counteract the accumulation of positive charges from the Cu2+ ions, the electrons transferred from the RGOS to the Cu2+, which increased the hole concentration in the RGOS and thereby increased the current of the RGOS-based electrical circuit. Thereby, compared with the decrease of conductance when dropping DI water, the addition of Cu2+ solutions increased the conductance of the RGOS-based electrical circuit. The electrical conductivity of our RGOS-based electrical circuit increased with the concentration of Cu2+ increasing from 10 nM to 1 mM.
The distribution of potential, electric field, and gradient of square of electric field of pre-patterned Au electrodes was simulated for DEP. The RGOS- and GOS-based electrical circuits were fabricated via DEP assembly. The MEGS-based electrical circuit was also fabricated for comparison. The three types of circuits have different electrical properties. The MEGS-based electrical circuit possesses the best electrical conductivity, and the GOS-based electrical circuit has the poorest electrical conductivity among the three circuits. For the detection the Cu2+, the RGOS-based electrical circuit can detect the Cu2+ at a concentration as low as 10 nM. The GOS-based electrical circuit can detect the Cu2+ only after chemical reduction. The water molecules and Cu2+ ions absorbed onto the surface of the RGOS play the roles of electron donators and acceptors for the detection. The facile fabrication method and excellent detection performance suggest that the RGOS-based electrical circuit has great potential to be developed into a metal ion sensor.
This work is supported by the CAST project, the Natural Science Foundation of China (Grant No. 51002030), the International cooperation Project in Suzhou (Grant No. SH201117), and the New Century Training Program Foundation for the Talents by the State Education Commission (Grant No. NCET-11-0095).
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