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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
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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
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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
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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.
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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
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