Solution-Gated Graphene Field Effect Transistors Integrated in

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Solution-Gated Graphene Field Effect Transistors Integrated in Microfluidic Systems and Used for Flow Velocity Detection Rong Xiang He,†,‡ Peng Lin,† Zhi Ke Liu,† Hong Wei Zhu,† Xing Zhong Zhao,‡ Helen L. W. Chan,† and Feng Yan*,† †

Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-structure of Ministry of Education, Wuhan University, Wuhan, 430072, China

‡

S Supporting Information *

ABSTRACT: Solution-gated graphene field effect transistors (SGGT) were integrated in microfluidic systems. The transfer characteristics of a SGGT with an Ag/AgCl gate electrode shifted horizontally with the change of the ionic concentration of KCl solution in the microchannel and the relationship can be fitted with the Nernst equation, which was attributed to the change of the potential drop at the Ag/AgCl electrode. Therefore the gate electrode is one important factor for the ion sensitive property of the SGGT. Then SGGTs were used as flow velocity sensors, which were based on measuring the streaming potentials in microfluidic channels. A linear relationship between the shift of the transfer curve of the SGGT and the flow velocity was obtained, indicating that the SGGT is a promising transducer for measuring flow velocity in a microchip. Since the streaming potential is influenced by the three physical quantities, including the flow velocity, the ionic strength of the fluid and the zeta potential of the substrate, the device can be used for sensing any one of the three quantities when the other two were known. It is noteworthy that SGGTs have been used for various types of chemical and biological sensors. Array of the devices integrated in multichannel microchips are expected to find many important applications in the lab-on-a-chip systems in the future. KEYWORDS: Graphene, field effect transistor, sensor, microfluidics, streaming potential, flexible electronics

G

sensor in 2008, which shows the horizontal shift of Dirac point (minimum conductance point of the transfer curve) with decreasing pH for 98 mV/pH, which is even higher than the theoretical maximum given by the Nernst equation.9 After that, many groups reported different sensors based on SGGTs, including ion sensors,10−12 DNA sensor,13 glucose sensor,14 protein sensors,10,15,16 cell-based biosensors,17 cancer biomarker sensor,18 bacteria sensor,19 and so forth. Transistor-based sensors can be miniaturized and are suitable for high throughput sensing.7 With the development of lab-ona-chip technology, the microfluidic systems integrated with transistors have shown great potential applications in the biological, physical, and chemical sensors.20−24 Now that large area and uniform single-layer graphene films have been successfully synthesized with the chemical vapor deposition (CVD) method on copper foils and transferred on various substrates, including flexible ones, an array of uniform graphene transistors and circuits thus can be fabricated by microfabrication techniques.25,26 Therefore graphene transistors can be integrated in microfluidic systems for sensing applications,

raphene exhibits many fascinating physical properties, such as the extremely high carrier mobility and saturation velocity, making it very attractive in various applications, including transparent electrodes, transistors, sensors and so forth.1−4 Since graphene has a two-dimensional (2D) structure and every atom is exposed on its surface, the electrical properties of graphene are very sensitive to the change of charge environment induced by the reaction or adsorption of analyte on the surface and thus graphene is a promising material for highly sensitive chemical and biological sensors.5 It is notable that the study on these areas is still at the early stage and lots of new graphene-based sensors with high performance are expected to be developed. A transistor−based sensor is the combination of a sensor and an amplifier, in which a small potential alternation may induce a pronounced change of channel current.6,7 Graphene transistors were used in various types of sensors, such as gas sensors for the detection of individual gas molecules, DNA sensors, and so forth.8−11 A solution-gated graphene field effect transistor (SGGT) operates in solution, in which the gate voltage is applied via the electrolyte/graphene interfaces, that is, electric double layers (EDL).9,10 It has been found that SGGT exhibits very stable performance, low working voltage (less than 1 V), and high transconductance, which are particularly important in sensing applications.7 Ang et al. reported the first SGGT as pH © 2012 American Chemical Society

Received: November 19, 2011 Revised: February 1, 2012 Published: February 10, 2012 1404

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Figure 1. (a) Schematic diagram of a SGGT integrated in a microfluidic chip. (b) Raman spectrum of single-layer graphene on a glass substrate. The insert shows the photograph of the graphene in the microfluidic chip. Scale bar is 0.5 mm. (c) Transfer characteristics (IDS vs VG, VDS = −0.05 V) of a SGGT in a flexible microchip filled with static 1 mM KCI solution at different bending status. (d−f) The photographs of the flexible microchip at different bending status (d) flat, (e) outcurve, and (f) incurve.

shown in Figure 1b, indicating that the graphene film is predominantly single-layer.25 The channel length of a graphene transistor between source and drain electrodes is 1 mm, while the width of microfluidic channel is 0.8 mm, as shown in the inset of Figure 1b. Therefore, the source and drain electrodes of the SGGTs are packaged very well by the PDMS layer that can decrease the leakage currents among the source, drain and gate electrodes. A SGGT integrated in a flexible microchip with a PET substrate (device 1) was characterized in static 1 mM KCl aqueous solution filled in the microchannel. Figure 1c shows the transfer characteristics of the device (IDS vs VG, VDS = −0.05 V) at three different bending status as shown in Figure 1d−f. The flexible microchip was bent to both sides and the device performance was characterized simultaneously. It is noteworthy that the transfer characteristic was slightly changed when the device was bent to both sides. Besides the changes of carrier mobilities,31 a notable shift of Dirac point was induced by bending, which was probably due to the change of the Fermi level of graphene under stress. Further work is needed to better understand the stress effect. Then a SGGT on a glass substrate (device 2) was characterized in static KCl solutions with different ionic concentrations. Figure 2a shows the transfer characteristics (IDS versus VG) of the device measured at the drain voltage (VDS) of −0.05 V. The transfer curve shifts to higher gate voltage with the decrease of ionic concentration. Figure 2b shows the logarithmic dependence between the shift of the transfer curve ΔVG and the KCl concentration. The slope is about 61.9 mV per decade by least-squares fitting, which is very

which have never been reported before. More importantly, graphene film is flexible and can be used in flexible microfluidic systems that have aroused the great interest of research in some applications recently.23,24 In this Letter, we present the integration of SGGTs in microfluidic systems on glass or flexible substrates. The devices were used as a new type of sensor for detecting flow velocity in the microfluidic channel, which has potential applications in lab-on-a-chip systems. When fluid flows along a channel under an pressure gradient, the counterions inside the EDL will move simultaneously, which induce a convection streaming current and a streaming potential between the two ends of the channel.27,28 So the fluid flow velocity can be determined by measuring streaming potential with a SGGT integrated in the microfluidic channel, which is similar to a solution-gated carbon nanotube field effect transistor reported before.29 Figure 1a shows the schematic diagram of a SGGT integrated in a microchip, where the graphene film is located on the floor and covered by a poly(dimethylsiloxane) (PDMS) channel. Two types of devices were fabricated on glass and flexible poly(ethylene terephthalate) (PET) substrates, respectively. An Ag/AgCl wire was inserted into the microchannel and used as the gate electrode of a SGGT. It is notable that the devices were fabricated by typical photolithography process and the detailed fabrication procedure of the microchip is shown in the Supporting Information (Experimental session and Figure S1, Supporting Information). Large-area single-layer graphene was synthesized on copper foil with the chemical vapor deposition (CVD) method.25,30 The Raman spectrum (488 nm laser source) of the graphene film transferred on a glass substrate is 1405

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Figure 2. (a) Transfer characteristics of a SGGT in microchip measured in KCl solutions with different ionic concentrations. (b) The shift of transfer curve (ΔVG) relative to the one measured in 10 mM KCl solution as a function of the concentration of KCl. VDS = −0.05 V. (c) Potential drops between the graphene and the gate electrode of a SGGT in solution. The gate voltage is applied on two EDLs, including the electrolyte/graphene and electrolyte/gate interfaces. (d) The modulation of gate voltage applied on a SGGT due to the streaming potential Vstr along the microfluidic channel.

not expected. So the ion-sensitive behavior of a SGGT reported before was attributed to the adsorbents such as resist residues or some particles on the surface of graphene.12 In our device, the graphene film was rinsed by deionized water flow for many times and thus the surface is clean enough. To further confirm this conclusion, a SGGT on a glass substrate with an Au gate electrode (device 3) was fabricated as well, which showed little change of Dirac point when the device was characterized in KCl solutions with different ionic concentrations (Figure S2, Supporting Information), indicating that neither the electrolyte/Au nor the electrolyte/graphene interfacial voltage was influenced by the ionic concentration. Therefore the gate electrode plays an important role on the ion-sensitive property of a SGGT. In other words, a reliable ion sensor based on an SGGT can be realized by choosing a suitable gate electrode. It is notable that the ion-sensitive behavior of the SGGT is different from the reported organic thin film transistors, which is mainly due to the reaction between organic semiconductor active layer and the ions in electrolytes.33 Next the SGGTs were characterized in flowing KCl solutions. When a fluid flow drives the counterions in the double layer along the surface of substrate, a counterbalancing electric field will be generated to maintain the steady-state condition of zero net charge current and induce the electrochemical streaming potential (Vstr) that can be characterized by using the SGGTs. As shown in Figure 2d, the gate voltage applied on a SGGT was modulated by the streaming potential in the microfluidic channel. To keep the same effective gate voltage of the transistor, we need to increase (decrease) the gate voltage with the value of the streaming

close to the value given by the Nernst equation used in ion sensitive field effect transistors.32 As shown in Figure 2c, the gate voltage VG was applied on the SGGT across the two interfaces, including electrolyte/graphene and electrolyte/gate, which is similar to the gate voltage applied on a solution-gated organic thin film transistor.33 Therefore the ion-sensitive behavior could be attributed to the change of potential drop across the EDL of either interface. The potential drop across the Ag/AgCl interface is related to the Cl− concentration in the KCl solution and given by (at 25 °C)34 EAg/AgCl = E0 − 0.0591 log ACl−

(1)

where the E0 is a constant, ACl− is the ionic activity of Cl−. In the case of low concentration solution, ACl− is equal to the concentration of Cl−. According to eq 1, the potential drop increases for 59.1 mV when the concentration of Cl− decreases for 1 order of magnitude. As shown in Figure 2c, when the ionic concentration in the electrolyte was decreased, a higher gate voltage (VG′) would be applied to keep the same effective gate voltage of the SGGT and thus the transfer curve of the device would shift to a higher gate voltage. Therefore, the shift of the transfer curve of the SGGT with the decrease of ionic concentration was mainly attributed to the potential change at the surface of Ag/AgCl gate electrode while the ionic concentration had little influence on the potential drop across the electrolyte/graphene interface, which was consistent with the sensing property of a SGGT acting as a pH sensor.11 This result is reasonable since graphene film is very stable in aqueous solutions and chemical reaction between ions and graphene is 1406

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Figure 3. Time-dependent channel current of a SGGT characterized in alternate static and flowing KCl solution with different velocities. VG = 0.15 V, VDS = −0.05 V. (a,b) The flow direction is from graphene to Ag/AgCl gate electrode. (c) The flow direction is from Ag/AgCl gate electrode to graphene. (d) The change of channel current and the shift of effective gate voltage as functions of flow velocity.

Figure 4. The transfer characteristics of SGGTs integrated in microchips on (a) glass or (b) PET substrates for flow velocity sensing. VDS = −0.05 V. The ionic concentrations of KCl in (a,b) were 1 μM. (c) The shift of gate voltage (ΔVG) on different substrates as a function of the flow velocity. (d) The shift of gate voltage per unit flow velocity (α = |ΔVG|/Δν) as a function of the ionic concentration. The red curve is the fitting of the function, α = log(C + λ)/(C + λ), where a = 6.6 mV·μM·mm−1 s, and λ= 2 μM.

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stream. It is notable that the shift of transfer curve was much smaller than that of the device on glass substrate at the same flow velocity. Figure 4c shows the shift of transfer curve (ΔVG) as a function of flow velocity for the two devices on the different substrates. The data follow two lines with the slopes of 2.43 and 0.88 mV mm−1 s (α = |ΔVG|/Δν) for the devices on the glass and PET substrates, respectively. The ratio between the two slopes was about 2.8. According to eq 2, the streaming potential was proportional to the zeta potential (ζ) of the substrate. The zeta potentials of a glass and a PET substrate at 1 mM KCl solution were reported to be about −87 and −30 mV, respectively.38,39 The zeta potential ζ of a substrate has a logarithmic dependence on the ionic concentration C and is approximately given by40

potential (Vstr) when the gate is located downstream (upstream) relative to the graphene active layer. The steaming potential is a function of the flow velocity and other fluidic parameters and given by29,35 ΔVstr =

ε0εrζwhR Δv ηe(C + λ)μ

(2)

where ε0 is the vacuum permittivity, εr is the relative dielectric constant of the electrolyte solution, ζ is the zeta potential of the ionic double layer on the surface of the microchip channel; w, h, and R are the width, height, and flow resistance of the microchannel, respectively; η is the dynamic viscosity of the solution, e is the electron charge, C is the ionic concentration, λ is an offset concentration that arises from the background concentration of ions, μ is the effective ionic mobility, and ν is the flow velocity in the microchannel. From eq 2, when the flow velocity was changed, the streaming potential would be modified consequently. As a result, the effective gate voltage applied on the SGGT would be changed by the flow velocity when a fixed gate voltage was applied. Figure 3a shows the channel current of a SGGT on a glass substrate with Ag/AgCl gate electrode (device 4) measured at fixed gate and drain voltages (VG = 0.15 V, VDS = −0.05 V) in alternate static and flowing 10 μM KCl solution. The gate electrode was located downstream relative to the graphene active layer (flow direction was from the graphene to the gate). The current response of the device exhibited a good reproducibility (RSD < 0.5%, n = 3). The fluctuation of the current characterized in flowing fluid was due to the variation of flow velocity driven by a syringe pump. Figure 3b shows the channel current versus time as the flow velocity stepped up sequentially. The channel current increased with the increase of flow velocity. Then we changed the flow direction and did the same measurement. Figure 3c shows the current change when the gate electrode was located upstream (flow direction is from the gate to the graphene). It is notable that the channel current IDS decreased with the increase of flow velocity. The current changes of the SGGT for either flow direction were shown in Figure 3d. Since the current changes were due to the modulation of the effect gate voltage by the steaming potential, the shift of the effective gate voltage at different flow velocity was calculated according to the current change and shown in Figure 3d as well.36,37 Linear relationships between the shift of the effective gate voltage and the flow velocity for both flow directions can be observed in the figure. The slopes of the linear relationships were 1.39 and −1.37 mV/(mm s−1) when the gate located downstream and upstream, respectively. According to eq 2, the numerical value of the slope is not related to the direction of flow. Therefore the experimental results shown in Figure 3d are reasonable. Another way to characterize the effect of flow velocity on the performance of a SGGT is to measure the transfer curves of the device at different velocities. Figure 4a shows the transfer curves of the SGGT on the glass substrate (device 4) characterized in 1 μM KCl solution with different flow velocities. The Ag/AgCl gate electrode was located downstream. The transfer curve shifts to higher gate voltages with the increase of the flow velocity, which is consistent with the results shown in Figure 3. Similarly, the SGGT on the flexible PET substrate (device 1) was characterized in 1 μM KCl solution with different flow velocities. The Ag/AgCl gate electrode was located down-

ζ = a log(C + λ)

(3)

where a is constant, λ is the background concentration of ions. So we can estimate the ratio of the zeta potential of the glass to that of the PET substrate to be about 2.9 at the same ionic concentration, which is in good agreement with the ratio between the two slopes (α) shown in Figure 4c. So the substrate of the microfluidic channel plays an important role on the streaming potential and the sensitivity of the SGGT for flow velocity sensing. Another factor that may influence the streaming potential is the ionic concentration. The SGGT on the glass substrate (device 4) was characterized in flowing KCl solution with different ionic concentrations, including 1, 10, 100, and 1000 μM (see Supporting Information, Figure S3). The Ag/AgCl gate electrode was located downstream. The transfer characteristics all shifted to higher gate voltage with the increase of the flow velocity. However, the voltage shift per flow velocity (α) decreased significantly as the ionic concentration increased as shown in Figure 4d. According to eq 2, the streaming potential was affected by both the zeta potential (ζ) and ionic concentration C. The zeta potential is given by eq 3. Since ΔVG ≈ ΔVstr as shown in Figure 2d, we can get the following relationship α = |ΔVG| /Δv ∝ a

log(C + λ) (C + λ )

(4)

As shown in Figure 4d, the slope α as a function of ionic concentration (in μM) can be fitted with eq 4 very well and the fitting parameters are a = 6.6 mV·μM·mm−1 s and λ = 2 μM. Therefore, compared with the fluidic sensor based on a carbon nanotube field effect transistor,29 the SGGT shows similar response to the flow velocity in the microfluidic channel while the fabrication and integration of the SGGTs in microchips are much more convenient, which will facilitate the broad applications of the SGGTs in lab-on-a-chip systems as multifunctional sensors. In addition, SGGTs will be useful in biological systems since the molecular biology is often governed by fluidic dynamics at nanoscale.41 Moreover, SGGTs can be used in some medical devices for in situ monitoring the velocities of blood or body fluid in human bodies.42 In conclusion, we integrated SGGTs in microfluidic channels on glass or flexible PET substrates. The SGGT with an Ag/ AgCl gate electrode showed a horizontal shift of the transfer curve with the increase of the ionic concentration in KCl solution while the device with an Au gate electrode was not sensitive to ionic concentration. The shift of the transfer curve of the former one could be fitted with the Nernst equation, 1408

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(12) Heller, I.; Chatoor, S.; Mannik, J.; Zevenbergen, M. A. G.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2010, 132, 17149−17156. (13) Dong, X.; Shi, Y.; Huang, W.; Chen, P.; Li, L. J. Adv. Mater. 2010, 22, 1649−1653. (14) Huang, Y.; Dong, X.; Shi, Y.; Li, C. M.; Li, L. J.; Chen, P. Nanoscale 2010, 2, 1485−1488. (15) Ohno, Y.; Maehashi, K.; Matsumoto, K. Biosens. Bioelectrom. 2010, 26, 1727−1730. (16) Ohno, Y.; Maehashi, K.; Matsumoto, K. J. Am. Chem. Soc. 2010, 132, 18012−18013. (17) Cohen-Karni, T.; Qing, Q.; Li, Q.; Fang, Y.; Lieber, C. M. Nano Lett. 2010, 10, 1098−1102. (18) Myung, S.; Solanki, A.; Kim, C.; Park, J.; Kim, K. S.; Lee, K. B. Adv. Mater. 2011, 23, 2221−2225. (19) Huang, Y.; Dong, X.; Liu, Y.; Li, L. J.; Chen, P. J. Mater. Chem. 2011, 21, 12358−12362. (20) Sharma, S. S. S.; Moniz, A. R. B.; Triantis, I.; Michelakis, K.; Trzebinski, J.; Azarbadegan, A.; Field, B.; Toumazou, C.; Eames, I.; Cass, A. Microfluid. Nanofluid. 2011, 10, 1119−1125. (21) Tey, J. N.; Wijaya, I. P. M; Wei, J.; Rodriguez, I.; Mhaisalkar, S. G. Microfluid. Nanofluid. 2010, 9, 1185−1214. (22) Lin, P.; Yan, F.; Yu, J. J.; Chan, H. L. W.; Yang, M. Adv. Mater. 2010, 22, 3655−3660. (23) Lin, P.; Luo, X. T.; Hsing, I M.; Yan, F. Adv. Mater. 2011, 23, 4035−4040. (24) Wang, Y. N.; Kang, Y.; Xu, D.; Chon, C. H.; Barnett, L.; Kalams, S. A.; Li, D.; Li, D. Lab Chip 2008, 8, 309−315. (25) Li, X.; Cai, W. W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312−1314. (26) Bae, S. K.; Kim, H. K.; Lee, Y. B.; Xu, X. B.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. L.; Kim, Y. J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J. H.; Hong, B. H.; Iijima, S. Nat. Nanotechnol. 2010, 5, 574−578. (27) Heyden, F. H. J.; Stein, D.; Dekker, C. Phys. Rev. Lett. 2005, 95 (116104), 1−4. (28) Chun, M. S.; Lee, T. S.; Choi, N. W. J. Micromech. Microeng. 2005, 15, 710−719. (29) Bourlon, B.; Wong, J.; Miko, C.; Forro, L.; Bockrath, M. Nat. Nanotechnol. 2007, 2, 104−107. (30) Liu, Z. K.; Li, J. H.; Sun, Z. H.; Tai, G. A.; Lau, S. P.; Yan, F. ACS Nano 2012, 6, 810−818. (31) Lee, S. K.; Kim, B. J.; Jang, H.; Yoon, S. C.; Lee, C.; Hong, B. H.; Rogers, J. A.; Cho, J. H.; Ahn, J. H. Nano Lett. 2011, 11, 4642− 4646. (32) Yan, F.; Estrela, P.; Mo, Y.; Migliorato, P.; Maeda, H.; Inoue, S.; Shimoda, T. Appl. Phys. Lett. 2005, 86, 053901. (33) Lin, P.; Yan, F.; Chan, H. L. W. ACS Appl. Mater. Interfaces 2010, 2, 1637−1641. (34) Bard, A. J.; Faulkner, L. R. Electrochemical methods fundamentals and applications, 2nd ed.; John Wiley & Sons: New York, 2001. (35) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: New York, 2001. (36) Tang, H.; Yan, F.; Lin, P.; Xu, J.; Chan, H. L.W. Adv. Funct. Mater. 2011, 21, 2264−2272. (37) Tang, H.; Lin, P.; Chan, H. L. W.; Yan, F. Biosens. Bioelectron. 2011, 26, 4559−4563. (38) Sze, A.; Erickson, D.; Ren, L. Q.; Li, D. Q. J. Colloid Interface Sci. 2003, 261, 402−410. (39) Geismann, C.; Yaroshchuk, A.; Ulbricht, M. Langmuir 2007, 23, 76−83. (40) Kirby, B. J.; Hasselbrink, E. F. Electrophoresis 2004, 25, 187− 202. (41) Eijkel, J. C. T.; van den Berg, A. Microfluid. Nanofluid. 2005, 1, 249−267. (42) Doucette, J. W.; Corl, P. D.; Payne, H. M.; Flynn, A. E.; Goto, M.; Nassi, M.; Segal, J. Circulation 1992, 85, 1899−1911.

which was attributed to the potential change at the Ag/AgCl electrode at different ionic concentrations. Therefore a reliable ion sensor based on SGGTs can be realized by choosing a suitable gate electrode. More importantly, there are enormous possibilities for fabricating other chemical and biological sensors based on SGGTs by using functionalized gate electrodes for specific targets. Then the SGGTs were used as flow velocity sensors in microfluidic channels. The transfer curves of the devices shifted horizontally with the change of flow velocity, which were caused by the change of streaming potential generated by the moving counterions inside the EDL. Since the streaming potential is influenced by the three physical quantities, including the flow velocity, the ionic strength of the fluid, and the zeta potential of the substrate, the device in principle could be used for sensing any one of the three quantities when the other two were known. Therefore SGGTs have potential applications in flexible, multifunctional, and miniaturized sensors integrated in lab-on-a-chip platforms, biological systems, or medical devices.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental session; figures include (1) the schematic diagram of the fabrication of a SGGT integrated with a microchip; (2) ion sensitive property of a SGGT with Au gate electrode; and (3) performance of SGGTs characterized in flowing KCl solution with different ionic concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +852 2766 4054. Fax: +852 2333 7629. E-mail: apafyan@ polyu.edu.hk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Research Grants Council (RGC) of Hong Kong, China (Project Number PolyU5322/10E) and the Hong Kong Polytechnic University (Project Number 1-ZV5K)

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