* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download An All-Solid-State pH Sensor Employing Fluorine
Survey
Document related concepts
Geophysical MASINT wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Current source wikipedia , lookup
Schmitt trigger wikipedia , lookup
Surge protector wikipedia , lookup
Stray voltage wikipedia , lookup
Alternating current wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Voltage regulator wikipedia , lookup
Voltage optimisation wikipedia , lookup
History of the transistor wikipedia , lookup
Buck converter wikipedia , lookup
Rectiverter wikipedia , lookup
Mains electricity wikipedia , lookup
Transcript
sensors Article An All-Solid-State pH Sensor Employing Fluorine-Terminated Polycrystalline Boron-Doped Diamond as a pH-Insensitive Solution-Gate Field-Effect Transistor Yukihiro Shintani 1,2, *, Mikinori Kobayashi 1 and Hiroshi Kawarada 1,3 1 2 3 * Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan; [email protected] (M.K.); [email protected] (H.K.) R&D Departent, Innovation Center, MK-Hdqrs, Yokogawa Electric Corporation, Japan, 2-9-32 Nakacho, Musashino, Tokyo 180-8750, Japan The Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku, Tokyo 169-0051, Japan Correspondence: [email protected] or [email protected]; Tel.: +81-422-52-5546; Fax: +81-422-52-5928 Academic Editor: W. Rudolf Seitz Received: 28 March 2017; Accepted: 3 May 2017; Published: 5 May 2017 Abstract: A fluorine-terminated polycrystalline boron-doped diamond surface is successfully employed as a pH-insensitive SGFET (solution-gate field-effect transistor) for an all-solid-state pH sensor. The fluorinated polycrystalline boron-doped diamond (BDD) channel possesses a pH-insensitivity of less than 3mV/pH compared with a pH-sensitive oxygenated channel. With differential FET (field-effect transistor) sensing, a sensitivity of 27 mv/pH was obtained in the pH range of 2–10; therefore, it demonstrated excellent performance for an all-solid-state pH sensor with a pH-sensitive oxygen-terminated polycrystalline BDD SGFET and a platinum quasi-reference electrode, respectively. Keywords: fluorine-termination; pH-insensitivity; polycrystalline boron-doped diamond; electrolyte-solution-gate field-effect transistor; all-solid-state pH sensor 1. Introduction After Bergveld [1] first presented an ion-sensitive field-effect transistor (ISFET), various types of ISFET have been presented, i.e., silicon-based ISFETs (Si-ISFETs) with an insulator of tantalum pentaoxide (Ta2 O5 ), with silicon nitride (Si3 N4 ), with aluminum oxide (Al2 O3 ) [2], and with a diamond-like carbon insulator [3]. We recently reported no-gate-insulator electrolyte-solution-gate field-effect transistors (SGFETs) with a single crystal diamond surface channel [4–7], with a polycrystalline diamond surface channel, and with a boron-doped diamond surface channel [8–11]. The ISFET has been a promising candidate for an integrated device to realize a high-response chemical sensor at a low cost. However, ISFET sensors have not been widely used in a practical way, in spite of the advantage of its feasibility with regard to miniaturization. One of the reasons is that a miniaturized ISFET requires a miniaturized stable reference system, which is still a challenging research topic. A silver/silver chloride (Ag/AgCl) gate electrode is commonly applied for pH sensing, but it is a liquid-containing electrode. To realize an all-solid-state sensor, two categories of on-chip reference systems have been studied. One of the approaches is the on-chip fabrication of an Ag/AgCl gate electrode with a gel filled in a hole [12,13], but the leakage of the gel resulted in the contamination of the sample, or in a limited lifetime. Another approach is to use a differential FET sensing technique, which is a combination of an ISFET and a Sensors 2017, 17, 1040; doi:10.3390/s17051040 www.mdpi.com/journal/sensors Sensors 2017, 17, 1040 2 of 7 pH-insensitive FET (REFET: reference field-effect transistor). The first REFET introduced by Matsuo [14] was an identical ISFET that possessed a chemically insensitive layer. Janata’s REFET used an ISFET and a quasi-reference electrode (QRE) with the differential FET sensing technique [14]. Various types of REFETs were reported including buffered hydrogels, silanes, polyacetal, and parylene [15]. However, the realization of a REFET, which possesses enough pH-insensitivity compared with ISFETs with typically 30–60 mV/pH sensitivity, has been a struggle for the differential FET sensing technique that results in an all-solid-state pH sensor. In the REFET/ISFET arrangement, a metal is used for quasi-reference electrode (QRE), placed close to the REFET/ISFET, which formally replaces the conventional reference electrode. In this case, the interface between the QRE and the electrolyte may be thermodynamically uncontrolled, and thus may generate an unstable voltage. Nevertheless, those voltages can be obtained by the REFET/ISFET differential sets as a common voltage mode that does not influence the sensor output voltage of the differential sensing technique. A pH change in the solution is measured by the differential output voltage as the ISFET responds with high sensitivity and the REFET responds with less sensitivity. Various types of REFETs are reported for differential FET sensing, such as a hydrogel as an insensitive membrane and a parylene layer for an ion blocking material [12]. Another approach is to use specific-ion-sensitive polymeric membranes such as poly (vinyl chloride) (PVC), whose pH sensitivity for protons is lower than for other cations. However, PVC is considered not suitable for REFET as it typically shows cation permselectivity. This behavior is a common property of polymeric membranes, meaning that polymer membranes are unsuitable for the REFET layer [12]. Here, in order to overcome the abovementioned problems, we first proposed introducing a fluorine-terminated diamond for a pH-insensitive SGFET, which worked in conjunction with an oxygen-terminated diamond channel employing a pH-sensitive SGFET. 2. Experimental 2.1. SGFET Fabrication A boron-doped polycrystalline diamond SGFET was fabricated by the protocol described in our previous reports [9,10]. To fabricate a boron-doped layer, polycrystalline diamond substrates of 10-mm square, purchased from Materials and Electronics Co. Ltd., were used throughout. Boron-doping diamond layers were deposited using a quartz-type microwave chemical vapor deposition (CVD) reactor. The typical CVD conditions were identical to our previous research [8,9]. Two titanium/gold (typically 20 nm/100 nm) contact pads were deposited for drain and source electrodes on the as-grown hydrogen-terminated BDD surface, and then they were patterned with lift-off photolithography techniques with the help of the same process used in our previous work [8,9]. The contact metal pads were positioned to give a channel length of 5–10 mm and a channel width of 0.3–0.6 mm, and electric wires were connected to the drain and source titanium/gold pads with a conductive adhesive for applying external bias voltage. Then, the drain and source electrodes were encapsulated with nonconductive epoxy resin to protect them from the solution; only the area of the boron-doped diamond surface between the pads was exposed to the buffer solutions. 2.2. Surface Modification of Diamond for pH-Insensitive/pH-Sensitive BDD SGFET To decrease/improve the pH sensitivity of BDD SGFET sensor, the direct-wetted diamond surface, employed as the FET channel, was modified to partially fluorine-terminated BDD and partially oxygen-terminated BDD with the help of the method used in the previous study [8–11,16]. Figure 1a,b illustrate the schematic diagrams of the pH-insensitive fluorine-terminated BDD (C-F BDD) SGFET surface and the pH-sensitive oxygen-terminated BDD (C-O BDD) SGFET surface. The transformation coverage percentage of the C-F group and C-O group from the as-grown C–H diamond surface depends of the surface treatment method and condition. The surface treatment of partial fluorination was conducted using inductively coupled plasma (ICP) with an octafluoropropane (C3 F8 ) gas source at a pressure of 2–5 Pa at 300–600 W for 10–120 s. To improve the pH sensitivity for SGFET, partial Sensors 2017, 17, 1040 Sensors 2017, 17, 1040 3 of 7 3 of 7 irradiation chamber with an conducted oxygen atmosphere, a method that was in our previous work oxidation of in theaBDD surface was by ultraviolet irradiation in aused chamber with an oxygen [8,9]. For the estimation of the coverage of the C-F functional group of the surface, the BDD channel atmosphere, a method that was used in our previous work [8,9]. For the estimation of the coverage of after treatment analyzed by using a ULVAC-Phi Model 3300 X-ray the C-Ffluoridation functional group of thewas surface, the BDD channel after fluoridation treatment wasphotoelectron analyzed by spectrometer. using a ULVAC-Phi Model 3300 X-ray photoelectron spectrometer. Figure 1. Schematic of a pH-insensitive fluorine-terminated boron-doped diamond (BDD) solution-gate Figure 1. Schematic of a pH-insensitive fluorine-terminated boron-doped diamond (BDD) solutionfield-effect transistor (SGFET). (a) pH-insensitive fluorine-terminated BDD (C-F BDD) SGFET; gate field-effect transistor (SGFET). (a) pH-insensitive fluorine-terminated BDD (C-F BDD) SGFET; (b) pH-sensitive oxygen-terminated BDD (C-O BDD) SGFET. (b) pH-sensitive oxygen-terminated BDD (C-O BDD) SGFET. 2.3. Measuring System for Characterization of FET 2.3. Measuring System for Characterization of FET An individual FET measurement was characterized by using the measurement system comprising An individual FET measurement was characterized by using the measurement system Keithley Instruments Model 2400 source-measure units (Keithley Instruments, Cleveland, OH, USA) comprising Keithley Instruments Model 2400 source-measure units (Keithley Instruments, Cleveland, and YOKOGAWA Model GS820 source measure units (Yokogawa, Tokyo, Japan) for the application OH, USA) and YOKOGAWA Model GS820 source measure units (Yokogawa, Tokyo, Japan) for the of drain-source voltage and gate-source voltage with a common-source mode. Data acquisition application of drain-source voltage and gate-source voltage with a common-source mode. Data with processing was performed using Labview 2010 software (National Instruments, Austin, TX, acquisition with processing was performed using Labview 2010 software (National Instruments, USA) to maintain a constant source-drain current by applying a compensating gate-source voltage. Austin, TX, USA) to maintain a constant source-drain current by applying a compensating gateThe gate-source voltage was applied via an Ag/AgCl gate electrode for characterization of the pH source voltage. The gate-source voltage was applied via an Ag/AgCl gate electrode for sensitivity. All BDDofSGFET I-V sensitivity. profiles wereAll obtained at room I-V temperature. For theobtained characterization characterization the pH BDD SGFET profiles were at room oftemperature. differential For FETthe pHcharacterization sensing, a lab-made ISFET pH-mV meter, equipped with a source-follow of differential FET pH sensing, a lab-made ISFET pH-mV meter, circuit suitable the BDD SGFET with P-channel semi-conductivity, was used to semi-conductivity, measure the FET equipped withfor a source-follow circuit suitable for the BDD SGFET with P-channel source-gate voltage corresponding to the solution pH and the differential output of FETs. types was used to measure the FET source-gate voltage corresponding to the solution Three pH and the ofdifferential buffer solution were used for evaluation: a 1 millimolar PBS (phosphate-buffered saline) buffer output of FETs. Three types of buffer solution were used for evaluation: a 1 millimolar solution (pH 7.4), four kinds of pH standard solutions offour 4.01kinds (phthalate (phosphate PBS (phosphate-buffered saline) buffer solution (pH 7.4), of pH buffer), standard6.86 solutions of 4.01 buffer), 9.18 (tetraborate buffer), and 10.01 (carbonate buffer), and a wide-range buffer solution (phthalate buffer), 6.86 (phosphate buffer), 9.18 (tetraborate buffer), and 10.01 (carbonate buffer), and (so-called Carmody pH buffer) whose pH range was adjusted from 2 to 12 by mixing 0.1 molar a wide-range buffer solution (so-called Carmody pH buffer) whose pH range was adjusted from 2 to trisodium phosphate (dodecahydrate), 0.2 molar(dodecahydrate), boric acid, and 0.05 12 by mixing 0.1 molar trisodium phosphate 0.2 molar molar citric boricacid acid,(monohydrate). and 0.05 molar All chemicals used in this study were of reagent-grade and were obtained from Tokyo Kasei, Horiba, citric acid (monohydrate). All chemicals used in this study were of reagent-grade and were obtained Kanto and Wako. from Kagaku, Tokyo Kasei, Horiba, Kanto Kagaku, and Wako. 3. Results and Discussion 3. Results and Discussion 3.1. Characterization of pH-Insensitive C-F BDD SGFET 3.1. Characterization of pH-Insensitive C-F BDD SGFET The specification of the boron-doped diamond film of the BDD SGFET used in this study was Theas specification of study the boron-doped film of the BDD SGFET useddensity, in this study was the same our previous [8]. By AC diamond Hall measurements, the sheet carrier mobility, the same as our previous study [8]. By AC Hall measurements, the sheet carrier density, mobility, 13 2 2 and sheet resistance of the BDD film before oxygen treatment was 3.7 × 10 /cm , 8.6 cm /Vs, 13/cm2, 8.6 cm2/Vs, and 24 and24 sheet resistance of the BDDThe filmFET before oxygen treatment × 10SGFET 2 , respectively. and kΩ/cm characteristics of thewas C-F3.7 BDD were examined. 2 kΩ/cm , respectively. The FET characteristics of the C-F BDD SGFET were examined. For the For the estimation of the coverage of the C-F functional group of the surface, the BDD channel estimation of the coverage of the C-F functional group of the surface, the BDD channel after fluoridation treatment was analyzed by X-ray photoelectron spectrometer. In the case of Cafter 3 F8 fluoridation treatmentofwas X-ray photoelectron spectrometer. In the+case C3F0.30. 8 ICP ICP plasma condition 100 analyzed W for 30 by s, the peak height calculation of F1s/(C1s F1s)ofwas condition of 100 W for thethe peak height calculation F1s/(C1s + F1s) was 0.30. In contrast, Inplasma contrast, the peak height of 30 F1ss,of BDD-SGFET surface of without fluorination treatment was the peak height of F1s of the BDD-SGFET surface without fluorination treatment was below the limit of quantification. For the characterization of the fluorine-terminated polycrystalline boron-doped Sensors 2017, 17, 1040 4 of 7 Sensors 2017, 17, 1040 4 of 7 below the limit of quantification. For the characterization of the fluorine-terminated polycrystalline diamond as a diamond pH-insensitive SGFET, a fluorine-terminated BDD (C-F BDD)BDD SGFET andBDD) an Ag/AgCl boron-doped as a pH-insensitive SGFET, a fluorine-terminated (C-F SGFET gatean electrode were immersed a 1 molar PBS buffer solution (pH 7.4) solution for 10 min for7.4) initialization, and Ag/AgCl gate electrodeinwere immersed in a 1 molar PBS buffer (pH for 10 min andinitialization, then its FET and current–voltage I-V) characteristics were evaluated by theevaluated common-source for then its FET (FET current–voltage (FET I-V) characteristics were by the method in two ways: via the drain-source (Ids)-drain-source voltage (Vds) characteristics and common-source method in two ways: viacurrent the drain-source current (Ids)-drain-source voltage (Vds) via the drain-source current (Ids)-gate-source (Vgs) characteristics (Vgs) (the so-called static characteristics). characteristics and via the drain-source current (Ids)-gate-source characteristics (the so-called Figurecharacteristics). 2a shows the Ids-Vgs characteristics of a C-Fcharacteristics BDD SGFET, of asaevaluated theaselectrolytestatic Figure 2a shows the Ids-Vgs C-F BDD using SGFET, evaluated gate-bias voltage mode with the Vgs varying from −1.0 V to 0.2 V at Vds −0.5 V. The using the electrolyte-gate-bias voltage mode with the Vgs varying from −1.0 V to 0.2 V at VdsIds-Vgs −0.5 V. characteristics exhibit a linear relationship in the Vgs range −0.3from V toca. greater than −1.0 V, The Ids-Vgs characteristics exhibit a linear relationship in thefrom Vgs ca. range −0.3 V to greater which the SGFET sensor. On the basis of of the extrapolation than −is 1.0beneficial V, which for is beneficial for the SGFET sensor. Onthe thegate-bias basis of intercept the gate-bias intercept of the of the drain-source curve incurrent the transfer line), (dash-dash the threshold voltage extrapolation of thecurrent drain-source curvecharacteristics in the transfer(dash-dash characteristics line), the of ca. −0.09 V was in the normally-on mode. The mode. static The characteristics (Ids-Vds threshold voltage of ca.estimated −0.09 V was estimated in the normally-on static characteristics characteristics) of the C-FofBDD SGFET shown inshown Figurein 2b.Figure The Vgs Ag/AgCl (Ids-Vds characteristics) the C-F BDDare SGFET are 2b. applied The Vgsthrough appliedan through an gate electrode variedwas stepwise −1.0 Vfrom to 0 − V 1.0 in steps Forof each Vgs,value the Ids Ag/AgCl gate was electrode variedfrom stepwise V to 0ofV0.1 in V. steps 0.1 value V. Forof each of was obtained as aobtained functionasofathe Vds from −1.0 V from to 0 V. TheVdrain-source current was current pinchedwas off, Vgs, the Ids was function of the Vds −1.0 to 0 V. The drain-source and the distinct and saturation wereregions obtained in obtained the Ids-Vds profiles. pinched off, and linear the distinct linear andregions saturation were in the Ids-Vds profiles. Figure 2.2.Device Deviceproperties properties ofelectrolyte-solution-gate an electrolyte-solution-gate diamond FET (SGFET) with a fluorineFigure of an diamond FET (SGFET) with a fluorine-terminated terminated boron-doped layer PBS in a buffer 1 molar PBS buffer (pH 7.4). (a) Ids-Vgs characteristics boron-doped layer in a 1 molar solution (pH solution 7.4). (a) Ids-Vgs characteristics at Vds = −0.5 at V in the= range 1.0range V to 0.2 V; (b)VIds-Vds at Vgs in the at range 1.0range V to 0ofV.−1.0 V to Vds −0.5 Vof in−the of −1.0 to 0.2 V;characteristics (b) Ids-Vds characteristics Vgs of in − the 0 V. 3.2. pH-Insensitivity of C-F BDD SGFET and pH-Sensitive C-O BDD SGFET 3.2. pH-Insensitivity of C-F BDD SGFET and pH-Sensitive C-O BDD SGFET For the evaluation of the pH-insensitivity of C-F BDD SGFET and the pH-sensitivity of For the evaluation of(C-O the pH-insensitivity of C-F BDD SGFET and the pH-sensitivity of oxygenoxygen-terminated BDD BDD) SGFET, the experimental results of the FET-IV profiles were terminated BDD (C-O BDD) SGFET, the experimental results of the FET-IV profiles were obtained obtained using pH standard solutions such as pH = 4.01 (phthalate buffer), 6.86 (phosphate buffer), using pH standard solutions such as pH buffer) = 4.01 with (phthalate buffer), 6.86 (phosphate buffer), 9.18 9.18 (tetraborate buffer) and 10.01 (carbonate reference to JIS (Japanese Industrial Standard) (tetraborate buffer) and 10.01 (carbonate buffer) with reference to JIS (Japanese Industrial Standard) K0802 and JIS Z 8802. Figure 3 shows the FET-IV characteristics of pH-insensitive C-F BDD SGFET. K0802 and Z 8802. Figure 3 shows the FET-IV characteristics pH-insensitive C-FAg/AgCl BDD SGFET. A family of JIS Ids-Vgs curves were generated by applying a series ofofstep voltages at the gate A family ofThe Ids-Vgs curves wereof generated by applying step at thewas Ag/AgCl gate electrode. pH-insensitivity the C-F BDD SGFET aatseries fixedof Ids andvoltages Vds values calculated electrode. The pH-insensitivity of the C-F BDD SGFET at fixed Ids and Vds values was calculated from the data shown in Figure 3. The obtained results are shown in Figure 4 as a blue box plot with from thebar data shown in Figure 3. The obtained results areThe shown in Figure as a than blue ca. box3plot with an error that indicates the standard deviation (n = 5). sensitivity was4 less mV/pH anIds error that indicates the standard 5). The was less than 3 mV/pH at = −bar 7 µA/mm. The Vgs decreaseddeviation with the (n pH= over thesensitivity entire investigated pH ca. range of pH at Ids = −7 µA/mm. The Vgs decreased with the pH over the entire investigated pH range of pH 4.01 4.01 to pH 10.01. By contrast, the pH-sensitive C-O BDD SGFET was also evaluated. The C-O BDD to pH 10.01. contrast, pH-sensitive C-Othe BDD SGFET also evaluated. The C-O BDD SGFET SGFET used By in this workthe was prepared with help of thewas same process described in our previous used in this work was prepared with the help of the same process described in our previous study study [9]. The sensitivity of the C-O BDD SGFET is also shown in Figure 4 as a red rhombus point. [9]. The sensitivity of the C-O BDD SGFET is also shown in Figure 4 as a red rhombus point. The The sensitivity is 37 mV/pH, which is at least 10 times higher than that of the sensitivity of the C-F sensitivity is and 37 mV/pH, which at least 10astimes higher than that of the sensitivity of the C-F BDD BDD SGFET is enough to beisemployed a pH-sensitive SGFET. SGFET and is enough to be employed as a pH-sensitive SGFET. Sensors 2017, 17, 1040 Sensors Sensors 2017, 2017, 17, 17, 1040 1040 Sensors 2017, 17, 1040 5 of 7 55 of of 77 5 of 7 Figure 3. 3. Vgs-Ids BDD SGFET SGFETwith withthe thecondition conditionof Vds and Figure Vgs-Ids curve pH-insensitive == = 0.5 V and Figure 3. Vgs-Idscurve curveofof ofpH-insensitive pH-insensitive C-F C-F BDD BDD SGFET with the condition ofofVds Vds 0.50.5 VV and IdsIds = − 0.7 µA/mm. Ids == −0.7 −0.7 µA/mm. µA/mm. Figure 3. Vgs-Ids curve of pH-insensitive C-F BDD SGFET with the condition of Vds = 0.5 V and Ids = −0.7 µA/mm. Figure pH-insensitivity of the C-F BDD SGFET SGFET. Figure 4.pH-insensitivity pH-insensitivityof ofthe theC-F C-F BDD BDD SGFET SGFET and and pH-sensitivity ofofthe the C-O BDD SGFET. Figure 4. 4. and pH-sensitivity pH-sensitivityof theC-O C-OBDD BDD SGFET. 3.3. Differential FETpH-insensitivity Sensing, UsingofpH-Insensitive C-F BDD SGFET and of pH-Sensitive C-O BDD SGFET Figure the C-F BDD SGFET and pH-sensitivity the C-O BDD SGFET. 3.3. Differential FET4.Sensing, Using pH-Insensitive C-F BDD SGFET and pH-Sensitive C-O BDD SGFET A device specification of the differential FET using pH-insensitive C-F BDD SGFET and C-O 3.3.device Differential FET Sensing, pH-Insensitive C-Fusing BDD SGFET and pH-Sensitive C-O BDD SGFET A specification of the differential FET pH-insensitive C-Fa source-follow BDD SGFET and C-O BDD SGFET was examined byUsing a lab-made ISFET pH-mV meter equipped with circuit. BDD SGFET was examined by of a lab-made ISFETFET pH-mV meter equipped with a source-follow circuit. Figure 5Aisdevice a schematic diagram of the differential FET measurement using the source-follow specification the differential using pH-insensitive C-F BDD SGFET and circuit. C-O Figure 5 is a schematic diagram of the differential FET measurement using the source-follow circuit. BDD6SGFET by a results lab-made pH-mV meter equipped withaapH-sensitive source-followC-O circuit. Figure showswas the examined experimental of ISFET the all-solid-state pH sensor with BDD Figure 6 shows the experimental of the all-solid-state pHthat sensor with pH-sensitive C-O BDD Figure 5 isa apH-insensitive schematic diagram of theSGFET, differential measurement using theathe source-follow circuit. SGFET and C-Fresults BDD withFET an error bar indicates standard deviation 6ashows the experimental results all-solid-state pH sensor with C-O deviation BDD SGFET and pH-insensitive BDD with an and errorthe bar that indicates thedifferential standard (n =Figure 4). The horizontal axis C-F is the pH SGFET, of of thethe solution, vertical axisaispH-sensitive the output SGFET and a pH-insensitive C-F BDD SGFET, with an error bar that indicates the standard deviation (n voltage. = 4). The horizontal axis is the pH of the solution, and the vertical axis is the differential The output was shifted with linearity corresponding to the pH. A good output responseoutput was (n = 4). The horizontal axis isbetween the pH linearity ofpH the2solution, and R the axis is results the differential output 22 =vertical voltage. The output shifted with to the pH. A good output response obtained with goodwas linearity tocorresponding 10 with 0.99. These suggest that the voltage. The was shifted with corresponding pH. AThese good was combination ofoutput thegood pH-insensitive C-Flinearity BDDpH SGFET, thewith pH-sensitive C-O BDDoutput SGFET,response and platinum was obtained with linearity between 2 to 10 Rto2 the = 0.99. results suggest that the obtained with good linearity between pH 2 to 10 with R2 = 0.99. These results suggest that the QRE was employed as the all-solid-state pHSGFET, sensor. the pH-sensitive C-O BDD SGFET, and platinum combination of the pH-insensitive C-F BDD combination of the pH-insensitive C-F BDD SGFET, the pH-sensitive C-O BDD SGFET, and platinum QRE was employed as the all-solid-state pH sensor. QRE was employed as the all-solid-state pH sensor. Figure Figure 5. 5. Schematic Schematic diagram diagram of of differential differential FET FET measurement measurement using using the the source-follower source-follower circuit. circuit. Figure 5. Schematic diagram of differential FET measurement using the source-follower circuit. Figure 5. Schematic diagram of differential FET measurement using the source-follower circuit. Sensors 2017, 17, 1040 Sensors 2017, 17, 1040 6 of 7 6 of 7 Figure 6. pH sensing of the differential FET. The differential output voltage of the pH-insensitive C- Figure 6. pH sensing of the differential FET. The differential output voltage of the pH-insensitive C-F F BDD SGFET and the pH-sensitive C-O BDD SGFET are plotted. BDD SGFET and the pH-sensitive C-O BDD SGFET are plotted. The short-term stability of the all-solid-state pH sensor was examined with reference to JIS (Japanese Industrial Standard) K0802 and JIS Z 8802. The 24 h stability test using with five sensors wereto JIS The short-term stability of the all-solid-state pH sensor was examined reference delta-pH of 0.096Standard) with variation = 25%, the JIS test standard delta-pH (Japanese Industrial K0802coefficient and JIS ZCV 8802. The where 24 h stability usingoffive sensorsis were belowof0.1. delta-pH 0.096 with variation coefficient CV = 25%, where the JIS standard of delta-pH is below 0.1. 4. Conclusions 4. Conclusions In this study, a method of fabricating a pH-insensitive BDD SGFET by introducing a C-F In this study, a method of fabricating a pH-insensitive BDD SGFET by introducing a C-F diamond diamond surface was proposed. With differential FET sensing, a sensitivity of 27 mv/pH was surface was proposed. With differential FET sensing, a sensitivity of 27 mv/pH was obtained in obtained in the pH range of 2–10; thus, it demonstrated excellent performance of an all-solid-state the pH 2–10; it demonstrated performance of an all-solid-state pH sensor pH range sensorofwith thethus, pH-sensitive C-O BDD excellent SGFET and platinum employed as the pH-sensitive with SGFET the pH-sensitive C-O BDD SGFET and platinum employed as the pH-sensitive SGFET and the and the QRE, respectively. The short-time and long-time drift characteristics and the QRE,interference respectively. The ions/chemicals short-time and drift characteristics and the perspective. interference of other of other willlong-time be examined from a practical application ions/chemicals will be examined from a practical application perspective. Acknowledgments: A part of this study was supported by JST’s (Japan Science and Technology) adaptable and seamless technology transfer program through target-driven R&D (A-STEP). Theand authors also appreciate the and Acknowledgments: A part of this study was supported by JST’s (Japan Science Technology) adaptable assistance of Nanotransfer Technology Research Center target-driven (NTRC) of Waseda University forThe the use of their equipment. seamless technology program through R&D (A-STEP). authors also appreciate the assistance of Nano Technology Research Center (NTRC) of Waseda University for the use of their equipment. Author Contributions: Yukihiro Shintani and Hiroshi Kawarada conceived and designed the C-F BDD SGFET; Author Contributions: Yukihiro Shintani and Hiroshi conceived and designed theShintani C-F BDD SGFET; Yukihiro Shintani and Mikinori Kobayashi evaluatedKawarada the characteristics of SGFETs; Yukihiro and Yukihiro Shintani and Mikinori evaluated the characteristics of SGFETs; Yukihiro Shintani and Hiroshi Hiroshi Kawarada optimizedKobayashi the proposed models; Yukihiro Shintani wrote the paper. Kawarada optimized the proposed models; Yukihiro Shintani wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations Abbreviations ISFET ISFET SGFET SGFETREFET REFETC-O BDD C-O BDD C-F BDD QRE C-F BDD QRE BDD BDD Ids Vgs Ids Vgs Vds PVC Vds PVC ICP CVD ion-sensitive field-effect transistor ion-sensitive transistor solution-gatefield-effect field-effect transistor solution-gate field-effect transistor reference field-effect transistor reference field-effect transistor diamond oxygen-terminated boron-doped oxygen-terminated diamond fluorine-terminated boron-doped boron-doped diamond quasi-reference electrode fluorine-terminated boron-doped diamond boron-doped diamond quasi-reference electrode drain-source current boron-doped diamond gate-source voltage drain-source current drain-source voltage gate-source voltage poly (vinyl chloride) drain-source voltage poly (vinyl chloride) inductively coupled plasma chemical vapor deposition Sensors 2017, 17, 1040 7 of 7 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. Bergveld, P. Development of an ion-sensitive solid-state device for neurophysiological measurements. IEEE Trans. Biomed. Eng. 1970, 17, 70–71. [CrossRef] [PubMed] Chiang, J.L.; Chou, J.C.; Chen, Y.C. Study of the pH-ISFET and EnFET for Biosensor Applications. J. Med. Biol. Eng. 2001, 21, 135–146. Voigt, H.; Schitthelm, F.; Lange, T.; Kullick, T.; Ferretti, R. Diamond-like carbon-gate pH-ISFET. Sens. Actuators B Chem. 1997, 44, 441–445. [CrossRef] Kawarada, H.; Araki, Y.; Sakai, T.; Ogawa, T.; Umezawa, H. Electrolyte-solution- gate FETs using diamond surface for biocompatible ion sensors. Phys. Status Solidi (a) 2001, 185, 79–83. [CrossRef] Edgington, R.; Ruslinda, A.R.; Sato, S.; Ishiyama, Y.; Tsuge, K.; Ono, T.; Kawarada, H.; Jackman, R.B. Boron δ-doped (111) diamond solution gate field-effect transistors. Biosens. Bioelectron. 2012, 33, 152–157. [CrossRef] [PubMed] Edgington, R.; Sato, S.; Ishiyama, Y.; Morris, R.; Jackman, R.B.; Kawarada, H. Growth and electrical characterisation of δ-doped boron layers on (111) diamond surfaces. J. Appl. Phys. 2012, 111, 033710. [CrossRef] Sasaki, Y.; Kawarada, H. Low drift and small hysteresis characteristics of diamond electrolyte-solution-gate FET. J. Phys. D Appl. Phys. 2010, 43, 374020. [CrossRef] Shintani, Y.; Ibori, S.; Igarashi, K.; Naramura, T.; Inaba, M.; Kawarada, H. Polycrystalline boron-doped diamond with an oxygen-terminated surface channel as an electrolyte-solution-gate field-effect transistor for pH sensing. Electrochim. Acta 2016, 212, 10–15. [CrossRef] Shintani, Y.; Ibori, S.; Kawarada, H. Polycrystalline Boron-doped Diamond Solution-gate Field-effect Transistor with a Carboxyl-terminated Channel for DNA. J. Appl. Phys. 2017, submitted. Naramura, T.; Inaba, M.; Mizuno, S.; Igarashi, K.; Kida, E.; Falina, S.; Shintani, Y.; Kawarada, H. Threshold voltage control of electrolyte solution gate field-effect transistor by electrochemical oxidation. Appl. Phys. Lett. 2017, submitted. Shintani, Y.; Kawarada, H. Polycrystalline Boron-doped diamond electrolyte-solution-gate field-effect transistor for an application to the measurement of water percentage in ethanol. Anal. Sci. 2017, submitted. Errachid, A.; Bausells, J.; Jaffrezic-Renault, N. A simple REFET for pH detection in differential mode. Sens. Actuators B Chem. 1999, 60, 43–48. [CrossRef] Smith, R.L.; Scott, D.C. An integrated sensor for electrochemical measurements. IEEE Trans. Biomed. Eng. 1986, 33, 83–90. [CrossRef] [PubMed] Chang, K.; Chang, C.; Chao, K.; Chen, J. Development of FET-type Reference Electrodes for pH-ISFET Applications. J. Electrochem. Soc. 2010, 157, J143–J148. [CrossRef] Salm, E.; Zhong, Y.; Reddy, B.; Guevara, C.; Swaminathan, V.; Liu, Y.; Bashir, R. Electrical Detection of Nucleic Acid Amplification Using an On-chip Quasi-reference electrode and a PVC REFET. Anal. Chem. 2014, 86, 6968–6975. [CrossRef] [PubMed] Ruslinda, A.R.; Ishiyama, Y.; Penmatsa, V.; Ibori, S.; Kawarada, H. Repulsive effects of hydrophobic diamond thin films on biomolecule detection. Appl. Surf. Sci. 2014, 328, 314–318. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).