Characteristics of Diamond Electrolyte Solution-Gate FETs (SGFETs) and Applications to Biosensor

Transcription

1 K.-S. New Diamond Song and and H. Kawarada Frontier Carbon Technology 325 Vol. 15, No MYU Tokyo NDFCT 497 Characteristics of Diamond Electrolyte Solution-Gate FETs (SGFETs) and Applications to Biosensor Kwang-Soup Song 1,2,3 and Hiroshi Kawarada 1,2,3 1 School of Science and Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo , Japan 2 Nanotechnology Research Center, Waseda University, 513 Waseda Tsurumaki-cho, Shinjuku-ku, Tokyo , Japan 3 Consolidated Research Institute for Advanced Science and Medical Care, Waseda University, Waseda Tsurumaki-cho 513, Shinjuku-ku, Tokyo , Japan (Received 27 March 2005; accepted 24 May 2005) Key words: Diamond SGFETs, halogen ions, ph, enzyme, biosensor A diamond electrolyte solution-gate FET (SGFET) for use in electrolyte solutions has been fabricated for the first time. Perfect device characteristics (pinch-off and saturation in I DS -V DS ) have been obtained with bias voltages within the potential window of diamond. The hydrogen-terminated (H-terminated) diamond surface was sensitive to halogen ions at approximately 30 mv/decade. We modified the H-terminated diamond surface with oxygen by treatment with ozone. Partially oxygen-terminated sites were insulating and insensitive to halogen ions. The H-terminated channel surface was modified to be partially amineoxygen-terminated (H-A-O-terminated) to achieve ph sensitivity when irradiated with UV light in an ammonia solution. The ph sensitivity of a diamond surface modified was about 50 mv/ph unit from ph 2 to 10. We immobilized specific enzymes (urease and glucose oxidase) on the modified channel surface. The sensitivities to urea and glucose were approximately 30 mv/decade and 20 mv/decade, respectively. 1. Introduction Recently, diamond has attracted attention for use in chemical sensors and biosensors because of its biocompatibility, (1) wide potential window ( V), (2) low background current and chemical/physical stability. (3,4) These properties enable the selective and quantitative in vivo detection of substrates in low concentration. (5) New synthetic methods * Corresponding author: song@kaw.comm.waseda.ac.jp 325

2 326 New Diamond and Frontier Carbon Technology, Vol. 15, No. 6 (2005) using the redox potential, not yet performed with conventional electrodes, have been attempted using diamond electrodes. Generally, diamond electrodes are made of highly boron-doped p-type polycrystalline diamond in which the surfaces are terminated by hydrogen (6 9) or oxygen. The hydrogen-terminated (H-terminated) diamond surface is stable in an electrolyte solution. Since Bergveld first introduced the ion-selective field-effect transistor (ISFET), (10) it has drawn attention as a novel solid chemical sensor. (11 16) Transistor-type (FET) biosensors are fabricated using semiconductor integrated circuit technology. The main reasons for the upsurge in interest in the FET are its numerous advantages in terms of miniaturization, standardization, mass production, low price and its suitable configuration for simple testing equipment. These advantages may make in vivo and in vitro detection possible. We have fabricated a diamond electrolyte solution-gate FET (SGFET) for use in an electrolyte solution for the first time; this combines both the chemical stability and biocompatibility of a diamond surface in electrolyte solution. (5) An Ag/AgCl reference electrode has been used as a gate electrode in the electrolyte. A schematic drawing of the SGFET is shown in Fig. 1(b) while the typical diamond MESFET is shown in Fig. 1(a). The solution (electrolyte) gate, where the electrolyte is in direct contact with the H-terminated diamond surface, is (a) (b) (c) Fig. 1. Schematic drawings of a diamond FET with a H-terminated diamond surface and a Si ISFET: (a) the typical diamond MESFET; (b) the diamond SGFET using an Ag/AgCl reference electrode as the gate electrode; (c) the Si ISFET, which requires a passivation layer and a sensitive membrane on the channel surface.

3 K.-S. Song and H. Kawarada 327 biased within the potential window. The development of a FET in the biosensor domain is determined not only by the sensitivity of the substrate but also by the optimized structure of the FET biosensor. In an ISFET based on a Si MOSFET, the gate is a SiO 2 /Si interface, a critical part of the Si MOSFET, where penetrating ions easily cause the electric properties of the FET to deteriorate. Moreover, SiO 2 /Si is particularly unstable in electrolyte solutions containing common interfering cations (K +, Na + and Ca 2+ ) or anions. A major concern in the design of an ISFET is therefore the encapsulation, which should prevent the ions from the electrolyte solution to penetrating the channel of the Si ISFET. The problems of Si ISFET using multi-layered gate dielectrics such as Si 3 N 4 /SiO 2 /Si interfaces in Fig. 1(c) are that the limitations in terms of long-term reliability and irreversibility remain significant concerns. Compared with this structure, a diamond SGFET uses the H-terminated surface, which is the origin of the surface stability and the p-type conductivity of the diamond biosensors and chemical sensors. The structure of the diamond SGFET is suitable for biosensors because of the lack of passivation layers and membranes on the channel surface in Fig. 1(b). These approaches allow direct contact between biomolecules and the channel surface, meaning that any change of charge or potential due to biomolecular reactions can be directly transferred to the diamond SGFET, giving rise to high sensitivity and low noise. (17,18) The fabrication process for the diamond SGFET is economical due to its simplicity. Heavy doping of the source and drain, gate oxidation, and the deposition of a passivation layer (Si 3 N 4 ) for the gate oxide and local oxidation of silicon (LOCOS) for isolation required in the fabrication of silicon-based ISFETs (19) are unnecessary. Moreover, the diamond SGFET is based on polycrystalline diamond films, which is now commercially available in diameters of several inches. 2. Experimental Methods Polycrystalline diamond films were synthesized on Si substrates by microwave plasmaassisted chemical vapor deposition (MPCVD). Methane was diluted by hydrogen (CH 4 / (H 2 +CH 4 ) = 1%). A surface p-type conductive layer with a high carrier density (10 13 cm 2 ) was obtained after hydrogen-termination without doping. The sheet resistance of the polycrystalline diamond substrate was kω/sq and its conductivity was constant for several weeks to months. From the current understanding, hydrogen-termination is necessary for surface conductivity but not sufficient because conductivity is sometimes sensitive to the atmosphere when the samples are unloaded from the CVD system. (20 22) The thickness of the polycrystalline diamond was approximately 8 µm and the diamond SGFET (channel length and width are 500 µm and 8 mm, respectively) and was fabricated by the following processes. Gold was evaporated through a metal mask onto the H- terminated diamond surfaces to form source and drain electrodes. The channel area of the FET was then covered with a metal mask (Mo). Next, Ar + ions were implanted through the mask to form an insulating region. Finally, wire was bonded on the source and drain electrodes and the electrodes were covered with epoxy resin to protect them from the electrolyte solution. The channel of the diamond SGFET was directly exposed to the electrolyte solutions, and an Ag/AgCl reference electrode was used as the gate electrode. The SGFET was biased within the potential window of diamond where there was no

4 328 New Diamond and Frontier Carbon Technology, Vol. 15, No. 6 (2005) significant electrolytic current flowing; this was carried out at room temperature. Highly purified water (18.5 MΩ cm 1 ) was used. The H-terminated channel surface was partially oxidized by ozone treatment for 60 min and partially aminated by ultraviolet (UV) irradiation in ammonia solution (28%) for 30 min. The wavelength of the UV light used was nm. Nitrogen gas was introduced before the UV irradiation for 6 min to remove oxygen and other activated gases from the UV chamber. The partially aminated channel surface was treated with glutaraldehyde solution (25%) for 2 h, after which urease dissolved in a phosphate buffer (the concentration of urease was 5 mg/ml) and was dropped onto the channel surface. In the case of a glucose sensor, glucose oxidase (GOD) dissolved in a phosphate buffer (the concentration of GOD was 3 mg/ml) and was dropped onto the channel surface and dried in air for 4 h at ambient temperature. Phosphate buffer solution (5 mm, ph 7.4) was used as a buffer solution and the ph measurement was performed using a digital ph meter. Urea, glucose, urease (from jack bean), glucose oxidase and other chemicals were used without any further purification. 3. Results and Discussions The static characteristics of the diamond SGFET were measured by applying a DC voltage in KOH and HCl solutions. In KOH solutions having a ph from 8 to 13, the pinchoff and saturated current-voltage characteristics were reproducible and stable. Figure 2(a) shows the drain current-drain voltage (I DS -V DS ) characteristics of diamond SGFET in a KOH solution at ph 13. In HCl solutions with a ph from 1 to 5, the diamond SGFET operates as effectively as those in KOH solutions. Meanwhile, the I DS -V DS characteristics of the H- terminated diamond SGFET in HCl solution at ph 1 are shown in Fig. 2(b). These results indicate that the H-terminated diamond SGFET operates stably in both strongly alkaline and acidic solutions. The maximum drain-source (V DS ) and gate-source (V GS ) bias is 1.0 V. The I DS -V DS characteristics are obtained in the potential window of the H-terminated diamond and the drain current is perfectly pinched off. Distinct linear and saturation regions have also been obtained in the I DS -V DS characteristics. The transfer characteristics at V DS = 0.1 V (I DS - V GS characteristics in the sub-threshold region) are shown in Fig. 2(c) where the threshold voltage has been recorded at approximately 0.2 V. The drain current difference between the on- (V GS > 0.4 V) and off-region (V GS < 0 V) covers nearly four orders of magnitude. The I DS -V DS characteristics obtained, however, are much better than those expected for the rough surface of the as-grown polycrystalline diamond. One of the reasons for this is that the surface channel of the rough diamond has been completely covered by the electrolyte and the leakage path between source and drain is thus almost completely shut. (5) The H-terminated diamond surface is sensitive to halogen ions (Cl, Br and I ). (23,24) From the I DS -V GS characteristics, the gate voltages shift with changes in the molar concentration of the halogen ion (ranging from 10 1 M to 10 6 M) by approximately 30 mv/decade as shown in Fig. 3(a). A covalent bond in which electrons are shared unequally is known as a polar covalent bond. One atom acquires a partially negative charge (δ ) while the other acquires a partial positive charge (δ + ). A polar molecule is a dipole, a pair of opposite charges of equal magnitude at a specific distance from each other. The magnitude of the difference in electronegativity reflects the degree of polarity. The electronegativity of carbon (2.5 in

5 K.-S. Song and H. Kawarada 329 (a) (b) (c) Fig. 2. Device characteristics of the diamond SGFET in DC mode. (a) I DS -V DS characteristics in KOH solutions (ph 13). (b) I DS -V DS characteristics in HCl solutions (ph 1). (c) I DS -V GS characteristics of the diamond SGFET in KOH solution (ph 8). Pauling units) is larger than that of a hydrogen atom (2.1), which is obtained at the H C 0.06 interface dipole. In other words, 6% of the area density of the surface hydrogen is subject to a partial positive charge. The positively charged H-terminated surface (δ + ) is directly exposed to the electrolyte solution. Anions (especially Cl and Br ) may easily approach the H-terminated diamond surface since these ions have a higher adsorption energy in an electrolyte solution, (25) as shown in Fig. 3(b). Although the origin of surface p-type conductivity has not been clarified, it is certain that the energy band of the H-terminated diamond surface moves upward to form an accumulation layer. Because of the presence of halogen ions, the upwardly bending energy band of the H-terminated diamond surface is enhanced and the gate voltage shifts in the conductive direction in highly concentrated halogen ion solutions. In diamond SGFET, the ground-to-gate voltage is the sum of the applied reference electrode potential (V G ), the interface potential between the reference electrode and the electrolyte solution ( E REF ), and the Nernst potential based on a specific

6 330 New Diamond and Frontier Carbon Technology, Vol. 15, No. 6 (2005) (a) (b) Fig. 3. Shifts of the gate voltage in KCl, NaCl and KBr solutions before and after ozone treatment on the channel surface for 60 min. (a) The H-terminated diamond surface is sensitive to halogen ions. The partially O-terminated diamond surface is insensitive to halogen ions. (b) Characteristics of hydrogen, oxygen and amine-terminated diamond surfaces in electrolyte solutions. ion sensitivity (a i, the activity of a specific ion) in the solution generated at the interface between the electrolyte and the channel surface (E 0 + (RT ln a i )/nf). These terms add up to give V G E REF + E 0 + RT ln a i /nf, which is the gate voltage of the diamond SGFET in an electrolyte solution. The 30 mv/decade change is insufficient for a Nernstian response. Water molecules adhere to directly exposed channel surfaces. The interactions of the adsorbed water molecules and the ions cause the behavior of the H-terminated diamondsurface-electrolyte system to deviate from the classical Nernstian response. In terms of an application, the Cl sensitive diamond SGFET could be used to test for cystic fibrosis because the H-terminated diamond surface is insensitive to the Na + ion, and Br ions are not present in sweat. Moreover, the lifetime of the Cl sensitive sensor using diamond SGFET will exceed that of other Cl sensors using Cl sensitive membranes based on Si ISFET because the endurance of their membranes is usually shorter than that of the H-terminated diamond surface. To reduce the halogen ion sensitivity on the H-terminated diamond surface, we used an ozone treatment. Ozone is a very active molecule and decomposes to form oxygen radicals. The surface conductive layer changes into a highly resistive one through the partial substitution of hydrogen atoms chemically adsorbed onto the surface by oxygen radicals. We characterized the sheet resistance, sheet carrier density and surface coverage of oxygen on partially oxygen-terminated (H-O-terminated) diamond surfaces as a function of ozone treatment time. (22) Since the oxygen-terminated (O-terminated) diamond surface generally behaves as an insulator, the sheet resistance increases and the drain current decreases

7 K.-S. Song and H. Kawarada 331 following ozone treatment. However the transconductance of diamond SGFET following ozone treatment remains at the same level as that recorded during pretreatment. The sensitivity of halogen ions disappears following ozone exposure for 60 min, as shown in Fig. 3(a). These results indicate that the partially H-O-terminated diamond surface has a relatively low sensitive to halogen ions. The upwardly bending band of the H-terminated diamond surface decreases following ozone treatment. In carbon-oxygen bonding, the electronegativity of oxygen (3.5) is larger than that of carbon (2.5), resulting in a negatively charged (δ ) surface. The difference in the electronegativity of the carbon-oxygen bond is larger than that of the carbon-hydrogen bond and these differences reflect the respective degrees of polarity. The magnitude of the carbon-oxygen bonding dipole (δ + δ ) is larger than the carbon-hydrogen bonding dipole (δ δ + ). When the dipole of the partially H-Oterminated diamond surface becomes neutral, the halogen ions are unattracted to the partially H-O-terminated diamond surface in the electrolyte solutions, as shown in Fig. 3(b). This therefore renders the partially H-O-terminated diamond surface insensitive to halogen ions and the bending band does not increase in highly concentrated halogen ion solutions. We investigate the ph sensitivity of the H-terminated diamond surface using a diamond SGFET. The charge of the H-terminated diamond surface is positive. Protons ([H + ]), which are assumed to be the potential-determining ions, cannot be adsorbed on the H-terminated surface because of the repulsion between the protons and the positively charged H- terminated diamond surface in Fig. 3(b). Protonation or deprotonation (27,28) does not occur on the H-terminated diamond surface; therefore, it is insensitive to ph. We modified the H- terminated diamond surface in ammonia solution to achieve ph sensitivity. UV light (halogen lamp, nm) was irradiated onto the H-terminated diamond surface and X-ray photoelectron spectroscopy (XPS) was used to evaluate the partially aminated diamond substrate. Peaks of nitrogen (N1s) and oxygen (O1s) exist and the surface coverage of nitrogen is 0.08 ML (monolayer). On the basis of these results, we conclude that the H- terminated diamond surface becomes partially amine-oxygen-terminated (H-A-O-terminated). The gate voltage (V DS = 0.1 V) shifts in the conductive direction by approximately 50 mv/ph from ph 2 to ph 10 as shown in Fig. 4(a). These shifts indicate ph sensitivity, which is confirmed in terms of the sub-nernstian response. This ph sensitivity is based on the structure of diamond SGFET, which is caused by surface-electrolyte interactions. The partially H-A-O-terminated diamond surface has three kinds of sites, but we consider only the amine and O-terminated sites (-NH 2, -OH, -O ) to be active, since the H-terminated diamond surface is insensitive to ph. Hole density is directly influenced by electrostatic forces as a function of protonation (-NH 3+, -OH 2+, -OH) and deprotonation (-NH 2, -OH, -O )on a p-type channel surface because the diamond channel surface does not have a membrane or a passivation layer. The charge of the diamond surface, because it adsorbs protons, arises not only in the first carbon layer but also extends underneath. At a low ph, the active sites are protonated and the repulsive electrostatic force exerted on the holes increases inversely proportional to the decreasing hole density on the p-type channel. At a high ph, active sites are deprotonated and electrostatic attractive forces and the hole density increase, as shown in Fig. 4(b). The operation of the urea-sensitive diamond SGFET is based on the biocatalyzed decomposition of urea by urease. From this biocatalyzed decomposition, ammonia and

8 332 New Diamond and Frontier Carbon Technology, Vol. 15, No. 6 (2005) (a) Fig. 4. The diamond SGFET modified channel surface is sensitive to ph. Hole density on the partially H-A-O-terminated diamond surface is influenced by protonation or deprotonation with protons in an electrolyte solution. (a) The partially H-A-O-terminated channel surface obeys the classical sub-nernst response (shifts by approximately 50 mv/ph). (b) In low-ph solution, the concentration of protonated sites increases and the hole density decreases because of repulsive electrostatic forces. In high-ph solution, hole density increases because of decreasing repulsive electrostatic force. (b) carbon dioxide are produced. Although these two decomposition products have opposite effects on the ph at the channel surface, the extent of the dissociation of ammonia (NH 4+ OH ) is higher than that of carbonic acid (HCO 3 ), resulting in a ph increase on the channel surface as shown in Fig. 5. Urease (NH 2 ) 2 CO + 2H 2 O + H + 2NH HCO 3 (1) Although the H-terminated diamond surface changes to become partially H-A-O-terminated with urease immobilized on the channel surface, the characteristics of the diamond SGFET, such as transconductance and drain current, are similar to those of the H-terminated diamond SGFET. The drain current of the diamond SGFET with immobilized urease on the channel surface increases with an increasing urea concentration. The gate voltage (V DS = 0.1 V) shifts in the conductive direction in a highly concentrated urea solution by approximately 30 mv/decade in the linear response range (from 10 6 M to10 2 M), as shown in Fig. 6(a). The shift of the gate voltage saturates at a high urea concentration. The detection limit of a potentiometric enzyme sensor is influenced by the ph of the buffer and its capacity, (29) because, in contrast to direct potentiometric sensors, the analytical signal of enzymatic sensors is the potential difference, rather than the potential itself.

9 K.-S. Song and H. Kawarada 333 Fig. 5. A schematic of the diamond-enzyme SGFET sensor and the biocatalized reactions on the channel surface. (a) (b) Fig. 6. The sensitivity to urea and glucose of the diamond-enzyme SGFET sensor. (a) Diamond SGFET with immobilized urease on the channel surface is sensitive to urea concentration (shifts by approximately 30 mv/decade). (b) In the case of the glucose sensor, the gate voltage shifts by approximately 20 mv/decade.

10 334 New Diamond and Frontier Carbon Technology, Vol. 15, No. 6 (2005) A glucose enzyme sensor is fabricated on the partially H-A-O-terminated diamond surface using the same process as for the urea sensor. Glucose is biocatalytically oxidized by molecular oxygen in the presence of glucose oxidase (GOD). The resulting gluconic acid acidifies the gate interface through proton dissociation, which can be detected by the diamond SGFET. D-glucose + H 2 O + O 2 D-gluconic acid + H + + H 2 O 2 (2) The gate voltage (V DS = 0.1 V) shifts in a highly concentrated glucose solution by approximately 20 mv/decade in the linear response range (from 10 2 M to 10 4 M) as shown in Fig. 6(b), which is less in the conductive direction than the urea sensor. The drain current of the diamond SGFET decreases with an increasing glucose concentration. 4. Conclusions GOD A diamond SGFET has been fabricated using polycrystalline diamond films and operates stably in electrolyte solutions with ph values from 1 to 13. The I DS -V DS characteristics of the diamond SGFET pinch off and saturate in all measured solutions. The H-terminated diamond surface is sensitive to halogen ions and its upwardly bending energy band is enhanced in the halogen ion solutions. The H-terminated diamond surface is modified to an oxygen-terminated surface by the exposure of ozone. The partially H-Oterminated diamond surface shows relatively little sensitivity to halogen ions, and its energy band does not increase in halogen ion solutions because the neutralized diamond surface does not attract the halogen ions. The H-terminated diamond surface is modified into a partially H-A-O-terminated surface, which is sensitive to ph by approximately 50 mv/ph unit. The ph sensitivity depends on the surface coverage of active sites. The coverage amount of individual sites on the diamond surface has not been optimized in this work. Fortunately, it may be easy to control the surface coverage and its density on a diamond surface. In future studies, we will optimize the relationship between the number of active sites and hydrogen to conform with the classical Nernstian response to ph. We have fabricated enzyme sensors using diamond SGFETs. An enzyme (urease or glucose oxidase) has been immobilized on the channel surface, and the biocatalyzed reaction occurring between a substrate and its specific enzyme has caused the ph to change. The diamond enzyme SGFET sensors detect the subsequent change in ph on the channel surface. The urea sensor fabricated on the diamond SGFET has a wide range (10 2 M 10 6 M) of sensitivity. The diamond SGFET biosensors show promise that the rapid characterization of nucleic acid samples in the pharmaceutical and in vivo / in vitro diagnosis of disease will be realized. Acknowledgements This work is supported by a Grant-in-Aid for the Center of Excellence (COE) Research from the Ministry of Education, Culture, Sports, Science and Technology. This work is also supported in part by the Advanced Research Institute for Science and Engineering, Waseda University.

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