Article Cite This: ACS Sens. XXXX, XXX, XXX−XXX
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Graphene Surface Acoustic Wave Sensor for Simultaneous Detection of Charge and Mass Satoshi Okuda,*,†,‡ Takao Ono,† Yasushi Kanai,† Takashi Ikuta,†,§ Masaaki Shimatani,‡ Shinpei Ogawa,‡ Kenzo Maehashi,†,§ Koichi Inoue,† and Kazuhiko Matsumoto† †
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan § Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan ‡
ABSTRACT: We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO3 substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (IA) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (VEg) in solution. We found that IA is controlled by VEg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the IA−VEg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino groups that have positive charges at pH 4.1. In contrast, the downward shift is thought to be due to a reduction in the amplitude of the propagating SAW because of an increase in the mass loading of microbeads. This mass loading was detected by the SAW sensor. Thus, we have demonstrated that the GSAW sensor is a transducer capable of the simultaneous detection of charge and mass, which indicates that it is an attractive platform for highly sensitive and multifunctional solution sensing. KEYWORDS: graphene, field-effect transistor, surface acoustic wave, acoustoelectric current, solution-gated sensor
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piezoelectric substrate. SAW devices are often used as RF and IF filters.16−20 Biosensing and gas sensing applications of SAW devices have been also investigated.21−26 When target molecules are dropped onto the propagation path of a SAW, the amplitude of the SAW is reduced by the consequent mass loading. Therefore, by combining GFETs and SAW sensors, the simultaneous detection of charge and mass is anticipated. The interaction between SAWs and low-dimensional electron systems (LDESs) has also been examined.27−32 Carriers in LDESs are trapped or transported by SAWs because SAWs are not only mechanical waves but also potential waves, and so the carriers are linked by an electromechanical coupling factor. As previously reported, when graphene and two electrodes are placed in the propagation path of a SAW, the carriers in the graphene are transported by the SAW and current flows between the two electrodes without the application of an external voltage. This current, which is caused
imple, rapid, and highly sensitive solution sensing platforms are required not only for medical diagnosis but also for pharmaceutical chemistry, food security, and environmental monitoring. Carbon nanotubes (CNTs) and graphene-based field-effect transistors (FETs) have attracted attention for the development of highly sensitive, real-time-monitoring solution sensors because of their unique physical and chemical characteristics.1−15 Graphene has the advantages of being more easily scalable and having a higher uniformity of electrical characteristics than CNTs. For the realization of GFETs as sensors in various applications, not only is high sensitivity important, but also usability. Conventional GFET sensors detect only changes in the potential due to changes in the drain current (ID) caused by the charges of target analytes in a fewnanometers-thick electrical double layer in solution. If additional functions, such as the detection of weight or even multiple physical parameters, were added to GFET sensors, they will be more widely employed as solution sensors. For the development of such a highly functional solution sensor, we focused on combining a GFET sensor with a surface acoustic wave (SAW) sensor. A SAW is generated by a high-frequency signal input to an interdigital transducer (IDT) formed on a © XXXX American Chemical Society
Received: November 16, 2017 Accepted: December 28, 2017 Published: December 28, 2017 A
DOI: 10.1021/acssensors.7b00851 ACS Sens. XXXX, XXX, XXX−XXX
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nm laser wavelength. This result indicates that a single layer of graphene was formed on the LiTaO3 substrate. First, electrical characteristics of the GSAW device in buffer solution were investigated using a B1500A Semiconductor Device Analyzer (Keysight). Figure 2a shows the V Eg
by the interaction between the carriers in the graphene and the SAW, is called an acoustoelectric current (IA).33−46 This phenomenon was also observed in graphene nanoribbons46 and single-layer MoS247 formed on a LiNbO3 substrate. Because the SAW device was easily operated wirelessly, SAW-driven wireless and lower power consumption GFET sensors are anticipated. In this research, we aimed to realize graphene SAW (GSAW) sensors, which are a new sensing platform combining the advantages of GFETs and SAW sensors. Figure 1a shows a schematic illustration of the fabricated GSAW solution sensor. First, a single layer of graphene was
Figure 2. (a) IA−VEg curve of a typical graphene SAW device in 10 mM phthalate buffer solution. (b) Real-time measurement of IA during input high-frequency switching for VEg equal to −50, 70, 110, 170, and 250 mV. The vertical lines indicate when the input is switched on and off.
Figure 1. (a) Schematic illustration of a fabricated graphene SAW sensor. (b) Raman spectrum of graphene on a LiTaO3 substrate. The background signal derived from the LiTaO3 substrate has been subtracted.
dependence of IA in phthalate buffer solution at pH 4.1. The input high-frequency signal was applied to one side of IDT through an RF preamplifier. The frequency of the input signal (Fin) was 130.1 MHz and the input power (Pin) was 2 dBm. After passing through VEg = 110 mV, the flow direction of IA switched. This result indicates that the flow direction of IA reflects the major type of carriers in graphene at each gate voltage: that is, at low voltages, points (A) and (B), a hole current flows; at high voltages, points (D) and (E), an electron current flows; and at an intermediate voltage, point (C), almost no current flows because this is a charge-neutral point (CNP). Figure 2b shows the time dependence of IA at VEg equal to −50, 70, 110, 170, and 250 mV. These voltages correspond to points (A)−(E) in Figure 2a, respectively. As soon as the highfrequency input signal was turned on or turned off, IA increased or decreased immediately. From these results, it can be seen that IA flows even in solution and is controlled by the gate voltage. Figure 3a shows the Pin dependence of IA−VEg characteristics. Pin was increased from 0 to 2.0 dBm in 0.5 dBm steps. As Pin was increased, IA also increased. This indicates that the value of IA is controlled by changing the SAW amplitude. Figure 3b shows the Fin dependence of IA−VEg characteristics as Fin was increased from 120 to 140 MHz in 0.1
synthesized on a copper foil by chemical vapor deposition (CVD)48 and was then transferred onto the center of the substrate.49 In this research, 36° Y-cut LiTaO3 substrate (Crystal Base Co., Ltd.) was used to induce shear-horizontal SAWs, which offer some advantages for solution sensing.50−52 Two electrodes were formed on the graphene and two pairs of IDTs were formed simultaneously at both ends of the substrate by photolithography, electron-beam deposition, and the lift-off process. These electrodes consist of 10-nm-thick Ni and 30nm-thick Au. In order to suppress the reflection of the SAW inside the IDT, a double electrode structure was employed.53 The period of the IDT was 32 μm and the width of the aperture was 850 μm. The length of between electrodes formed on graphene was 200 μm and the width of electrode was 5 mm. Silicone rubber was attached around the graphene, and buffer solution was dropped into the cavity. An electrolyte-gate voltage (VEg) was applied from a Ag/AgCl reference electrode. In this way, GFET and SAW devices were formed on the same substrate. Figure 1b shows the Raman spectrum of the graphene transferred onto the LiTaO3 substrate with a 633 B
DOI: 10.1021/acssensors.7b00851 ACS Sens. XXXX, XXX, XXX−XXX
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Figure 3. (a) Pin dependence of IA−VEg characteristics. (b) Threedimensional plot of the Fin dependence of IA−VEg characteristics.
Figure 4. (a) IA−VEg characteristics in solution for the pH range 5.71 to 6.99. (b) pH dependence of VEg for the hole current peak of IA.
MHz steps. This result indicates that an acoustoelectric current flows only around the resonance frequency of the SAW device, which is defined by the period of the IDT, and almost no current flows outside the resonance frequency. Based on the results of Figure 3a and b, we can see that IA is controlled by the input high-frequency signals. Next, the pH response of the GSAW sensor was investigated; the results are shown in Figure 4a. In order to control the pH of the buffer solution, the buffering effect between NaH2PO4 and Na2HPO4 was used. The pH of the solution was changed from 5.71 to 6.99 and the same amount of the buffer solution was used in every measurement. As the pH of the buffer solution increased, the hole current peak shifted to higher values of VEg. This occurs because, as the hydrogen ion content decreases, the hole density of graphene increases, hence the IA−VEg characteristics shift in a positive direction. This is consistent with the shift of the CNP to the left as the hydrogen ion concentration decreases in the conventional GFET sensor.6−8 Figure 4b shows the pH dependence of the value of VEg when hole current is at its maximum value. The slope of the linear fit in Figure 4b is 25.2 mV/pH. From these results, it was confirmed that the GSAW sensor detects electrical charges in solution like conventional GFET sensors. Finally, simultaneous detection of charge and mass was demonstrated. In this experiment, amino-group-modified microbeads (screenCORE-Amine, chemicell GmbH) were used as the analyte. These silica matrix-based microbeads with a diameter of about 1 μm are nonmagnetic. Their zeta potential is about 35 mV at pH 7. The microbeads were dropped into a phthalate buffer solution at pH 4.1. The density of microbeads was increased from 0 mg/mL to 6.95 mg/mL.
Figure 5a shows the microbead detection results using a conventional GFET at a drain voltage (VD) of 20 mV. The CNP of the GFET shifted to the left as the amount of microbeads was increased because the dissociated amino groups applied a positive gate potential to the graphene. Figure 5b shows the results of similar measurements using the GSAW sensor. As the density of microbeads in solution increased, the hole current peak shifted to the lower left. As for the conventional GFET sensor, the left shift of the hole current peak can be explained as being due to the increase of positive charges in the amino groups. In contrast, the downward shift is considered to be the result of a reduction of the amplitude of the SAW because of the increase in mass loading of microbeads onto the propagation path of the SAW. Due to the combination of these two effects, the hole current shifted to the lower left. Therefore, the SAW sensor operates as a transducer which detects charge and mass simultaneously. Figure 5c shows the position of hole current peaks for the same densities of microbeads shown in Figure 5b. Thus, the GSAW sensor measured two physical parameters by measuring only electrical characteristics in solution. Because the position of IA and VEg shifted simultaneously with the increase of target materials, the identification of substances may be achieved by employing GSAW sensors in future work. In conclusion, we have fabricated a GSAW sensor which provides a new sensing platform. By combining GFET and SAW devices, the presence of an acoustoelectric current was revealed, even in solution. It was confirmed that IA was controlled by VEg and the attenuation of the SAW. VEg changes caused by the electrical charges of analytes make hole current peaks shift in the horizontal direction. Moreover, a downward C
DOI: 10.1021/acssensors.7b00851 ACS Sens. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS The authors are grateful for the continuous support of Professor Yasuhide Ohno of Tokusima University. The authors wish to acknowledge financial support from JST CREST Grant Number JPMJCR15F4, Japan, from the Innovative Areas “Molecular Architectonics: Orchestration of Single Molecules for Novel Functions” program (No. 25110007) through the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), from a Management Expense Grant for National University Corporations from MEXT, and from a Grant-in-Aid for Young Scientists B (No. 15K17679) and for Scientific Research B (No. 15H03986) from the Japan Society for the Promotion of Science (JPS).
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Figure 5. (a) IA−VEg characteristics of the GFET at VD = 20 mV. The density of microbeads in solution was increased from 0 mg/mL to 6.95 mg/mL. (b) IA−VEg characteristics of the GSAW sensor with the increasing density of microbeads. (c) Shift of the hole current peak of the GSAW sensor as a result of increasing the density of microbeads.
hole current peak shift was caused by the mass loading of analytes. Due to the combination of these two effects, the GSAW sensor enables simultaneous sensing of both charge and mass loading. This result, therefore, contributes to the development of multifunctional sensors employing low-dimensional materials, which will provide a biosensing platform combining high sensitivity and multifunctionality.
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REFERENCES
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[email protected]. ORCID
Satoshi Okuda: 0000-0002-1574-2870 Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acssensors.7b00851 ACS Sens. XXXX, XXX, XXX−XXX
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