of 10
Strain-Based Chemiresistive Polymer-Coated Graphene Vapor
Sensors
Annelise C. Thompson,
Kyra S. Lee,
and Nathan S. Lewis
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Supporting Information
ABSTRACT:
Suspended chemiresistive graphene sensors have been
fabricated using well-establishe
d nanofabrication techniques to
generate sensors that are highly sensitive to pyridine and with
excellent discrimination between polar and nonpolar analytes. When
coated with a polymer surface layer and suspended on 3-D patterned
glass electrodes, a hybrid combination of polymer and graphene yields
chemiresistive vapor sensors. Expansion and contraction of the
polymer layer produces strain on the suspended graphene (Gr).
Hence, when organic vapors permeate into the polymer layer, the
high gauge factor of the graphene induces substantial electrical resistive changes as folds and creases are induced in the graphene.
The hybrid suspended polymer/Gr sensor exhibits substantial responses to polar organic vapors, especially pyridine, while also
exhibiting reversibility and increased discrimination between polar and nonpolar analytes compared to previous approaches. This
sensor design also allows for potential tunability in the types of polymers used for the reactive surface layer, allowing for use in a
variety of potential applications.
INTRODUCTION
Arti
fi
cial olfactory electronic systems have attracted interest for
air quality monitoring, medical diagnostics, explosives
detection, and other applications.
1
3
Arrays of cross-reactive,
chemically sensitive resistors provide an especially simple
technological implementat
ion of a vapor detection and
classi
fi
cation device.
4
When exposed to volatile organic vapors,
the analyte permeates and interacts with the sensing material,
producing a change in the dc resistance of the
fi
lm.
4
,
5
Di
ff
erent
vapors can be classi
fi
ed and quanti
fi
ed using pattern
recognition algorithms.
4
6
In medical applications, biomarkers
for diseases found in exhaled human breath, such as increased
levels of acetone, formaldehyde, and pyridine, are of particular
interest for rapid, point-of-care diagnostics.
2
,
7
A variety of
materials have been explored as chemiresistive vapor sensors,
including intrinsically conducting or nonconducting polymers
loaded with conducting material such as carbon black (CB), as
well as capped metallic nanoparticles and other related
systems.
4
,
6
Two-dimensional nanomaterials such as graphene have
shown potential for sensors as well as in various applications
such as microelectronics and energy storage while also
a
ff
ording the opportunity to produce elastic and
fl
exible
architectures.
8
11
In a pristine monolayer, graphene is
composed of sp
2
-hybridized carbon atoms that have
unoccupied p
z
-orbitals oriented perpendicular to the basal
plane. These p
z
-orbitals give rise to the impressive electronic
sensitivity of graphene, which has led to the integration of
graphene in pressure, piezoelectric, and chemical sensors.
Hybrids of graphene with metals, polymers, and combinations
of the two have also been widely explored.
12
Suspension of graphene across a series of columns or
trenches minimizes substrate e
ff
ects on the resulting device
and yields an approach to design sensitive graphene-based
devices. Large chemiresistive responses have been observed
upon exposure to polar analytes when monolayer graphene was
used as a top contact on one-dimensional ZnO rods.
8
When
suspended across a narrow trench, single-crystal graphene
exfoliated from highly ordered pyrolytic graphite yielded
devices with higher mobilities and sensitivities for dopamine
than graphene monolayers on planar,
fl
at substrates.
13
Suspension of the graphene monolayer typically produces
e
ff
ects on the nanoscale and requires time-intensive fabrication
methods such as electron-beam lithography. Additionally, the
resulting devices are delicate, and scale-up is challenging.
Strain responses have also been accessed by supporting or
coating graphene with a polymer and then bending the
subsequent sensor to induce strain in the graphene, detected as
a change in resistivity. With a thick polymer overlayer, the
graphene fully conforms to the polymer, so the graphene
deforms as the shape of the polymer changes through
Received:
January 26, 2022
Accepted:
February 28, 2022
Published:
March 15, 2022
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mechanical or chemical deformation. Graphene pressure
sensors are sensitive to small signals and are a well-developed
area of study.
14
These strain-based graphene devices rely on
changes in the resistance of graphene at the surface produced
by compressive and tensile deformation of the lattice, which
decreases and increases, respectively, electron localization.
Devices are most commonly formed by encasing monolayer
graphene in a stretchable polymer. This strain-based approach
is feasible because the graphene lattice can undergo substantial
deformation without breaking.
15
These polymer-coated
monolayers are used less frequently as chemical sensors, in
part because in these sensors, the graphene is typically
integrated with a polymer that does not readily and reversibly
respond to the presence of a vapor [i.e., polydimethylsiloxane
(PDMS)].
16
However, these strain sensors typically have very
high gauge factors, providing large measurable changes in
resistance to small amounts of deformation, with some devices
reported to have gauge factors of over 200.
17
,
18
Additionally,
the presence of the polymer substantially improves the
robustness of the suspended graphene due to the combined
strength of the materials.
Vapor sensors involving monolayer graphene also often
su
ff
er from a lack of sensitivity because cleaned graphene is
chemically inert and sti
ff
and consequently o
ff
ers limited
tunability in its response and physical properties.
19
Typical
transfer methods involve the use of a polymer overlayer, such
as polymethylmethacrylate (PMMA) and PDMS, to support
the graphene during etching of the supporting metal and
transfer to the target substrate.
20
A number of polymer-free
transfer methods have been developed,
21
but the polymer-
supported transfer is widely used because it is relatively simple
to implement, requires minimal skill during transfer, and can
be scaled relatively easily on the lab scale to accommodate
larger pieces of graphene. However, complete removal of the
polymer overlayer is di
ffi
cult after the polymer-supported
transfer.
22
Extra cleaning steps are required to remove the
polymer, including washes in a suitable organic solvent in
which the supporting polymer is soluble. Frequently, a high-
temperature anneal under a reducing atmosphere, such as
forming gas, is also required. Typically, these methods do not
fully remove the transfer polymer, leaving behind networks of
hardened polymer on the surface of the graphene.
22
,
23
When graphene that has been transferred using a polymer
support is used in a sensor, multiple studies have demonstrated
that the remaining polymeric residue gives rise to di
ff
erent
responses than are observed from a pristine monolayer of
graphene that does not have such a residue. For example, a
pristine monolayer of graphene shows very low speci
fi
city
towards most volatile organic compounds (VOCs).
19
An
advantage of using graphene in a sensor derives from the ability
to modify the graphene using physical or chemical methods, in
conjunction with the inherent sensitivity to impurities and
strain due to the underlying electronic structure of the
graphene. Although chemical and physical modi
fi
cation of
graphene through exfoliation from graphite, reaction with
chemical precursors, or lithographic patterning can lead to
increased sensitivity, these techniques rely on fundamentally
changing the chemical structure of graphene through the
attachment of new functional groups or introduction of defects
to induce a response to a VOC of interest.
24
In contrast, limitations associated with the polymer-
supported transfer and any alteration of the monolayer of
graphene can be minimized by retaining the polymer overlayer
covering the graphene. The speci
fi
city that the polymer lends
to the sensor and ampli
fi
cation of the sensor response by the
strain dependence of the graphene can therefore both be
exploited. Polymer composites have the advantage of better
reproducibility and strength than either the polymer or
graphene alone, o
ff
ering remarkable transferable properties
such as electrochemical reinforcement and stability.
9
These
composites have been used in a variety of sensor device
implementations but rely on integration with oxidized
graphene
fl
akes as opposed to pristine graphene.
9
,
25
,
26
We describe herein a facile fabrication method that allows
partially suspended monolayer graphene to be integrated with
a vapor-responsive polymer, producing a sensor with high
responsivity to pyridine as well as to a variety of other polar
and nonpolar volatile organic analytes. A patterned sensor
substrate was developed to support graphene as a partially
suspended layer above the surface. By suspending the
graphene, the material is allowed to expand and contract in
response to the movement of the polymer overlayer.
Additionally, the device con
fi
guration allows exploitation of
the sensitivity of the physical and chemical properties of 2-D
materials to the introduction of curvature, folds, and creases.
27
Numerous studies have reported the properties that result
from folding and wrinkling of graphene, and the unique
behavior due to the curvature of suspended graphene under
mechanical stimulation.
8
,
17
,
28
,
29
The resulting chemiresistive
sensors can consequently access both the sensitivity of the
graphene monolayer and the speci
fi
c response to organic
vapors of di
ff
erent polymer overlayers.
RESULTS AND DISCUSSION
Several sets of sensors were tested in this work, including 4%
Poly(ethylene-
co
-vinyl acetate) (PEVA) mixed with CB
(PEVA/CB) and 4% PEVA coated on graphene (PEVA/Gr)
either on
fl
at substrates (
fl
at) or on substrates with columns
(col) (see
Methods
for details on fabrication). The response
from the sensors was quanti
fi
ed as the change in resistance
(
Δ
R
max
) with respect to the resistance of the baseline (
R
b
),
where
R
p
is the peak resistance of the sensor
R
RR
R
R
R
100
100
pb
bb
=
*=
Δ
*
(1)
The sensitivity of the sensors (
S
R
) was then quanti
fi
ed as the
slope of the linear least-squares
fi
tof
Δ
R
max
/
R
b
(
Δ
R
) versus
P
/
P
o
.
P
̅
is the mean of the exposure partial pressures relative to
the vapor pressure of the analyte (
P/P
o
),
R
Δ
is the mean of the
Δ
R
max
/
R
b
values,
p
i
is the value of (
P/P
o
) at the
i
th exposure,
and
Δ
R
i
is the
Δ
R
max
/
R
b
value at the respective
p
i
value
S
pP R R
pP
()( )
()
i
n
i
i
i
n
i
R
1
1
2
=
̅
Δ−Δ
̅
=
=
(2)
The sensors developed in this work used a conductive
monolayer of graphene coated with a layer of PEVA deposited
onto a substrate patterned with columns to generate a signal in
the presence of a VOC (
Figure 1
A). As the micrographs in
Figures 1
B,C show, the columns do not allow the graphene to
fully adhere to the surface, leaving a gap between the bottom of
the column and the monolayer of graphene. Less adherence is
expected from 4% PEVA/Gr layers as this combined layer
should be less
fl
exible than graphene alone, increasing the gap
between the columns and the 4% PEVA/Gr layer. Typical
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conductive polymer composite sensors contain CB as a
conductive material. The percent composition of CB
determines the baseline resistance and the optimal sensor
response, so several control sensors were fabricated with CB
and with monolayer graphene. These control sensors included
a monolayer of graphene with no polymer on a
fl
at substrate or
columns, PEVA/CB transferred identically to graphene on a
fl
at substrate or columns, and PEVA/CB sprayed onto a
fl
at
substrate (
Figure 2
). Strikingly, the PEVA/CB composite
sprayed onto the surface of a substrate showed a large negative
response to ethanol and ethyl acetate, in accord with behavior
that has been reported previously for such sensors,
30
whereas
the PEVA/CB composites formed as uniform
fi
lms showed a
positive response to these analytes (
Figure S2
).
31
However, the
PEVA/CB composites transferred identically to graphene in
this work were composed of 4% PEVA as compared to 2%
PEVA deposited by an airbrush because 4% PEVA was the
lowest concentration that was su
ffi
ciently stable for the
graphene transfer. Further optimization of the concentration
of PEVA used to transfer the graphene used in these sensors
was not pursued due to challenges with the transfer process.
PEVA/Gr (col) exhibited the largest sensitivity of all the
sensors for most of the analytes, including pyridine (28
138
ppm), acetone (301
1506 ppm), ethyl acetate (124
618
ppm), and tetrahydrofuran (THF) (213
1066 ppm), except
for toluene and ethanol. For both toluene and ethanol, PEVA/
CB (col) showed the largest sensitivity of the sensors
evaluated.
Figure 3
shows the linearity of responses of the
sensor coated with Gr/PEVA to a single pulse of a range of
concentrations of pyridine vapor (28
138 ppm). Upon
exposure of the sensor to the analyte, the resistance steadily
increased until the analyte was purged from the chamber, at
which point the resistance decreased very slowly and
fl
attened.
The pulse peaks for the Gr/PEVA (col) were slower to
respond and recover, whereas the other controls [bare Gr
(col), 4% PEVA/CB (col), 4% PEVA/CB (
fl
at)] exhibited
much smaller but more rapid and reversible responses (
Figure
S3
). The control sensors with 4% PEVA/CB on
fl
at substrates
or substrates with columns were more reversible than a bare
monolayer of graphene on columns after the analyte exposure.
This behavior suggests that the PEVA polymer overlayer with
graphene not only makes the sensor less
fl
exible and perhaps
sti
ff
er and slower in its recovery but also exhibits a higher
resistance change when strained. In comparison, the remaining
control sensors [bare graphene (
fl
at), 4% PEVA/Gr (
fl
at)]
typically produced lower and noisier sensor responses, likely
due to the strong adhesion of the graphene to the sensor
substrate.
The column height was varied to ascertain the optimal
response of the sensors, and changes to the column height
were expected to change the degree to which the PEVA/Gr
stack adhered to the underlying substrate. For most analytes, a
150 nm pillar height resulted in the highest sensitivity, except
for pyridine and toluene, for which the sensitivity increased
substantially as the column height increased, although the
response of pyridine was an order of magnitude higher than
that of toluene (
Figure 4
). Pyridine and toluene are similar in
chemical structure and size but di
ff
er in polarity, consistent
with the observation that higher columns are required to
obtain an optimal response from both of these VOCs.
Aromatic compounds have noncovalent interactions available
with graphene through
π
π
stacking, so the height depend-
ence also suggests that these analytes bene
fi
t from larger areas
of exposed graphene with which to interact.
In addition to column height, the thickness of the polymer
overlayer was varied to obtain the optimal response for the
sensors to the VOCs evaluated in this work.
Figure 5
shows the
responses for sensors with the polymer overlayer deposited at
speeds between 1000 and 8000 rpm. 75 nm (6k rpm) thick
fi
lms showed the largest sensitivity to most analytes except for
toluene, with the responses decreasing substantially for sensors
having thinner layers of PEVA. A PEVA layer of 320 nm in
thickness exhibited comparable responses, but these layers
Figure 1.
Schematic of the sensor used in this work and scanning
electron micrographs of the underlying monolayer of graphene on the
sensor surface. (A) A monolayer of graphene is transferred to the
sensor body by means of the supportive 4% PEVA overlayer. On
exposure to a VOC, the 4% PEVA overlayer deforms the underlying
monolayer of graphene, producing a detectable change in resistance.
(B) Representative image of a single 150 nm tall column within the
sensing region of a device. Some areas around the column are visible
where the graphene had not completely adhered to the underlying
surface. (C) Closer view of the same column. The monolayer of
graphene is stretched out from the top of the column to the
underlying surface.
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were more di
ffi
cult to fabricate uniformly due to the slower
spin speed involved, leading to the wide range of responses
re
fl
ected by the width of the shown error bars. To maintain
ease of fabrication, high responsivity, and more
fl
exibility in the
polymer overlayer, the thinner 75 nm layer was selected for
further investigation.
The sensors were similarly optimized for the number of
columns on the substrate, as an increase in the number of
columns was expected to increase the overall signal from a
sensor (
Figure 6
). The standard pattern had columns with a 3
μ
m diameter and a pitch of 7
μ
m. The pitch was then varied
between 3 and 120
μ
m, with a constant thickness of polymer
overlayer and size of the transferred Gr/PEVA sheet. As
expected, the sensor response decreased as the number of
columns decreased, likely due to fewer locations where the
strain in the polymer overlayer could e
ff
ectively translate to
strain in the underlying graphene. Interestingly, the response of
the sensors exhibited a plateau at
5
×
10
5
columns, indicating
that further study of di
ff
erent patterns with reduced diameters
and closer spacing could be valuable.
The reproducibility of the responses of the optimized sensor
(PEVA/Gr on a substrate with 150 nm high columns with a 3
μ
m diameter and 7
μ
m pitch) was probed through repeated
measurements of the response to the same concentration of
the analyte.
Figure 7
A shows the response of an optimized
sensor to repeated exposures of pyridine at 28 ppm. After
repeated exposures, the response decreased over time to a
plateau at 60% of the original signal (
Figure 7
B). However,
after extended exposure to background N
2
(g), the sensor
recovered the initially observed full response. The mean of the
fi
rst 10 exposures prior to the extended rest was 0.99% with a
95% con
fi
dence interval (CI) of
±
0.06%, whereas the mean of
the
fi
rst 10 exposures after an extended rest was 1.11% with a
95% CI of
±
0.07%. One-way analysis of the variance yielded
an
F
-value of 7.5417 and a
p
-value of 0.0133, indicating that
Figure 2.
S
R
values for the control sensors versus PEVA/Gr on columns [4% PEVA/Gr (col)] exposed to six di
ff
erent VOCs in a range of
concentration (0.001
P
/
P
o
0.005) at a
fl
ow rate of 3000 mL min
1
under N
2
as the carrier gas. Substrates with columns were all 150 nm in
height. Each value is the average of four vapor sensors per sensor type exposed to 4 di
ff
erent VOC concentrations, where the
S
R
value was
calculated using a linear least-squares
fi
t to determine the slope of
Δ
R
max
/
R
b
versus
P
/
P
o
. For most of the analytes, the
S
R
value was the highest for
4% PEVA/Gr (col). This geometry produced larger relative di
ff
erential resistance changes than analogous PEVA/CB or bare graphene
chemiresistive sensors under nominally similar test conditions.
Figure 3.
Typical sensor responses when exposed to 28, 55, 82, and
138 ppm of pyridine at a
fl
ow rate of 3000 mL min
1
under N
2
as the
carrier gas. The times at which the sensor was exposed to the analyte
and purged with the carrier gas, respectively, are marked on the plot.
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the results before and after 100 consecutive exposures were
statistically distinct. The higher mean after the
fi
rst 100
exposures suggests that the performance of the sensor may
improve over time.
To better understand the ability of these sensors to
distinguish between di
ff
erent analytes, the discrimination
performance was analyzed using principal components analysis
(PCA) (
Figure 8
). The
fi
rst, second, and third projections of
the principal components (PCs) showed that the hybrid
graphene polymer sensor array clearly separated polar from
nonpolar vapors. Additional plots of this data illustrating the
separation between VOCs can be found in the
Supporting
Information
(Figure S8). Overlaps between data clusters were
observed, especially for some groups of the polar aprotic
vapors (group 1: THF, ethyl acetate, and acetone; group 2:
isopropanol, ethanol), although these groups were mutually
discriminated as were dimethylformamide (DMF) and
pyridine. Moreover, pyridine generally exhibited the highest
Figure 4.
(A)
S
R
values for controls of column pillar height for suspended hybrid graphene sensors exposed to six di
ff
erent VOCs at 0.001
P
/
P
o
0.005 at a
fl
ow rate of 3000 mL min
1
under N
2
as the carrier gas. Pyridine characteristically exhibited an increase in response as column height
increased. (B) An expanded ordinate of the responses except for pyridine. For the majority of VOCs, 150 nm column heights produced the largest
response.
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resistive response and produced a unique
fi
ngerprint relative to
its aprotic polar counterparts. This behavior indicates that the
nitrogen-based functional groups may have a speci
fi
ce
ff
ect on
the PEVA/Gr sensors, likely due to interactions with the
underlying graphene layer.
In comparison, previous PCA analysis of the response of an
unmodi
fi
ed graphene sensor showed groupings between
chemically similar compounds and separations between polar
and nonpolar groups.
32
In this work, the PEVA/Gr sensors on
substrates with columns showed equal, if not greater,
separation between polar and nonpolar groups while also
exhibiting unique
fi
ngerprints for compounds like pyridine,
which was not evaluated on the unmodi
fi
ed graphene sensor.
Pyridine is a known component of tobacco and cigarette
smoke and can be used as a biomarker in disease detection.
33
The selective detection of pyridine suggests that this family of
sensor designs could be used as part of a cross-reactive sensor
array for environmental and occupational health sensing from
human breath. Moreover, the optimized sensor in this work
serves more generally as an example of a methodology to target
a speci
fi
c functionality or functional group with otherwise
cross-reactive sensors.
The response of the optimized sensors described herein was
a strong function of the number of columns (
Figure 6
), with
increases in the number of columns leading to a higher
response until a plateau was reached at 5
×
10
5
columns. This
plateau indicates that on this scale, the resistance change in the
graphene is limited by the degree of strain that an analyte can
impose on this con
fi
guration on the PEVA/Gr stack. The
observed response also depended on the thickness of the
polymer overlayer, scaling linearly with the square root of the
fi
lm thickness (
Figure 5
). The optimal thickness was
75 nm,
whereas the optimal spacing was obtained using columns with
a3
μ
m diameter and 7
μ
m pitch. Although the signal degraded
with time, the sensors exhibited highly reproducible responses,
exhibiting complete recovery over repeated exposures (
×
100)
with longer recovery periods. Further work on these sensors,
such as changing the patterning on the underlying substrate or
modifying the polymer overlayer could allow for further tuning
of the performance and could potentially decrease the recovery
time of the sensors.
CONCLUSIONS
Polymer-coated monolayer graphene can be integrated with a
simple patterned electrode to produce a larger chemiresistive
response to organic analyte vapors than a polymer-CB
fi
lm or
bare graphene. The use of hybrid materials can allow for
programmed chemiresistive sensors with tunability in the
various types of polymers that can be coupled with graphene.
The response is controlled by the structure of the underlying
substrate along with the thickness of the polymer overlayer.
The sensor had a long recovery time in successive tests
compared to the control sensors but recovered full
functionality after extended exposure to a background gas.
The sensitivity to pyridine increased as the column height
increased, consistent with expectations for an enhanced ability
of the sensor to expand and contract, resulting in large changes
in the resistance. A hybrid graphene/polymer sensor array
exhibited clear discrimination between polar, nonpolar, aprotic,
and protic vapors with unique
fi
ngerprints for DMF and
Figure 5.
S
R
values of controls for polymer overlayer thickness exposed to 6 di
ff
erent VOCs at 0.001
P
/
P
o
0.005 at a
fl
ow rate of 3000 mL
min
1
under N
2
as the carrier gas. The quoted spin speed in thousands (k) of rpm produced a polymer
fi
lm of thickness: 1 k (320 nm), 2 k (220
nm), 3 k (160 nm), 4 k (130 nm), 5 k (80 nm), 6 k (
75 nm), 7 k (
73 nm), and 8 k (
71 nm), respectively. A spin speed of 6 k (
75 nm
resulting
fi
lm thickness) produced the best response for the majority of the exposures.
S
R
values were larger for pyridine than for the other VOCs
tested.
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pyridine. Di
ff
erent polymer support layers on monolayer
graphene within a larger array of cross-reactive sensors could
allow for a similar sensor design to target other VOCs of
interest.
METHODS
Materials.
CVD-grown monolayer graphene on Cu (Cu/
Gr) was purchased from Advanced Chemical Supplier
Materials (Medford, MA). Grains of graphene from this
source were
50
μ
m in diameter, as reported by the
manufacturer. PEVA (vinyl acetate, 18 wt %) was purchased
from Sigma-Aldrich and used without further puri
fi
cation.
Black Pearls 2000 CB was purchased from Cabot Corporation.
All solvents were reagent grade from VWR and were used
without further puri
fi
cation to generate the vapors tested
herein.
Sensor Fabrication.
The patterned sensor substrates were
prepared in a Class 100 cleanroom. Glass slides were
fi
rst
cleaned with acetone and isopropanol before being baked at
170
°
C to remove any residual solvent. Microposit S1813
photoresist (MicroChem) was spun onto the cleaned slide at
500 rpm for 30 s and then at 4000 rpm for another 60 s,
followed by a 10 s exposure through the appropriate mask to a
425 nm lamp in a contact mask aligner (Suss MicroTech
MA6/BA6). The pattern was developed in MF-319 developer
(MicroChem) for 90 s. Columns of di
ff
erent heights were
grown on the patterned slide by using an e-beam evaporator to
deposit 50
300 nm of SiO
2
(CHA Industries Mark 40). Lift-
o
ff
was completed by sonicating the slides at 60
°
C in Remover
PG (MicroChem) for 45 min. Contacts were formed by
sequentially evaporating 5 nm of Ti, followed by 45 nm of Au,
onto masked glass slides. This process produced two metallic
electrodes that were separated by a 0.3 cm gap.
Solutions of 4 wt % PEVA in toluene were sonicated for 2
4
h until the PEVA was well dispersed. To make the coated
sensors, a strip of Cu covered by a monolayer of graphene
(Cu/Gr) was coated with a supporting layer of PEVA at a
speci
fi
ed rotation rate (1000
8000 rpm) for 60 s. The
resulting stack (Cu/Gr/PEVA) was then cured for 1 min at
150
°
C. Smaller pieces,
1cm
×
3 mm (active area
0.1
0.2
cm
2
), were cut and subsequently etched in an FeCl
3
solution
(Copper etch, Transene) until the Cu disappeared by visual
inspection, generally requiring 1.5 h. This Cu-free piece (Gr/
PEVA) was transferred for 1 h to a bath that contained
18.2
M
Ω
cm resistivity H
2
O before transfer to a second clean H
2
O
bath, in which the sample was immersed for 12 h. After transfer
to a
fi
nal fresh H
2
O bath, the stack was pulled onto the
appropriate sensor substrate and dried using a gentle stream of
N
2
(g).
The sensors used as controls were fabricated using similar
transfer techniques. Solutions of 4 wt % PEVA and 1 wt % CB
were sonicated for 2
4 h until the CB was well-dispersed. The
solution was then spun onto bare Cu and transferred as
described above, or alternatively was applied to the sensors
using an airbrush. Gr without a PEVA coating was transferred
with a supporting layer of 495 K A4 polymethyl methacrylate
(PMMA, MicroChem) spun at 3000 rpm for 60 s. After
transfer, the PMMA was removed by soaking the sensor for 10
min in acetone.
Sensor Measurements.
Sensors were tested using a
custom setup that has been described previously.
4
6
Organic
vapors were generated by sparging N
2
(g) at a
fl
ow rate of 3000
mL min
1
through 45 cm tall bubblers that had been
fi
lled with
Figure 6.
S
R
values of controls for columns/pitch of the substrate at various 6 di
ff
erent VOCs at 0.001
P
/
P
o
0.005 at a
fl
ow rate of 3000 mL
min
1
under N
2
as the carrier gas. The number of columns correlated to the pitch as 4.3 million columns (3.5
μ
m), 2 mil (7.5
μ
m), 1 mil (15
μ
m),
500 k (30
μ
m), 250 k (60
μ
m), and 125 k (120
μ
m), respectively. S
R
values were largest at a 7
μ
m pitch for ethyl acetate, pyridine, and THF.
ACS Omega
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Article
https://doi.org/10.1021/acsomega.2c00543
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2022, 7, 10765
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10771
the appropriate solvents. The analyte concentration was
controlled by adjusting the volumetric mixing ratio of the
saturated analyte stream to the background N
2
(g) stream. The
fl
ow rates of the background and analyte gases, respectively,
were regulated using mass
fl
ow controllers. Each run started
with 700 s of background data collection. Each analyte
exposure consisted of 200 s of a pure background gas, 80 s of
the diluted analyte, and then 200 s of a background gas to
purge the system, all at a
fl
ow rate of 3000 mL min
1
. The
sensors were loaded into a rectangular, 16-slot chamber that
was connected by Te
fl
on tubing to the gas delivery system.
The resistance of each of the sensors in the array was measured
by a Keysight Technologies 34970A data acquisition/switch
unit with a Keysight 34903A 20 Channel Actuator. The
measurement electronics were interfaced with a computer via a
GPIB connection and were controlled with LabVIEW software.
The classi
fi
cation ability of the Gr/PEVA column sensors
was visualized using PCA. PCA was performed on 16
individual hybrid graphene polymer arrays. For all volatile
organic carbon (VOC) vapors, 20 test exposures were
recorded at
P
/
P
o
= 0.0050, where
P
is the partial pressure
and
P
o
is the vapor pressure of the analyte at room
temperature.
Sensor Characterization.
Pro
fi
lometric data of the
polymer overlayers were collected on a Bruker Dektak XT
pro
fi
lometer using a probe with a 2
μ
m tip radius. Atomic force
microscopy (AFM) images of the sensors were collected using
a Bruker Dimension Icon AFM. Raman spectra were collected
with a Renishaw Raman microscope at a wavelength of 532 nm
through an objective with a numerical aperture of 0.75. The
laser power was
3 mW.
Signal Processing.
All data processing was performed
using custom routines in Origin and R. The relative di
ff
erential
resistance change,
Δ
R
max
/
R
b
, was calculated from
R
max
, the
baseline-corrected response maximum upon exposure of the
sensor to the test analyte and the baseline resistance under
inert N
2
,
R
b
. A spline was
fi
tted to the baseline data during the
initial baseline preexposure period, and the values of
Δ
R
max
/
R
b
were calculated by subtracting the values of the spline over the
extrapolated exposure time from the observed resistance
during the length of exposure.
S
R
values were calculated
using linear least-squares
fi
tting of
Δ
R
max
/
R
b
versus analyte
concentration.
Sensor discriminant performance was visualized using PCA.
The normalized data were mean-centered, and the diagonal-
ized data set of the covariance matrix was transformed into sets
of dimensions in terms of PCs. The largest amount of variance
is captured in the
fi
rst PC, while the second PC was orthogonal
to the
fi
rst PC and captured the second-most variance in the
data. The normalized mean-centered data were projected onto
the
fi
rst and second PCs in accord with their respective
coordinate vectors as observed through their corresponding
eigenvalues and eigenvectors.
ASSOCIATED CONTENT
*
s
ı
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsomega.2c00543
.
Conversion of partial pressures to exposure concen-
trations, 4% PEVA solution spin curve,
Δ
R
max
/
R
b
responses from all control sensors, raw response curves
of control sensors to single pulses of pyridine at
P
/
P
o
,
Δ
R
max
/
R
b
responses for sensor optimization, including
thickness, area, and pitch dependence, and Raman
spectrum of graphene on the sensor body used in this
work (
PDF
)
AUTHOR INFORMATION
Corresponding Author
Nathan S. Lewis
Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States;
orcid.org/0000-0001-
5245-0538
; Email:
nslewis@caltech.edu
Authors
Annelise C. Thompson
Division of Chemistry and
Chemical Engineering, California Institute of Technology,
Pasadena, California 91125, United States;
orcid.org/
0000-0003-2414-7050
Figure 7.
(A) Optimized pyridine response at 28 ppm (nitrogen
carrier gas at a
fl
ow rate of 3000 mL min
1
) showing reproducibility
in response to repeated exposure, with 200 s under N
2
between
exposures. (B) Comparison of initial 20 exposures (black) versus
sensor response after being subjected to 100 pyridine exposures at 28
ppm (red) under N
2
as the carrier gas at a
fl
ow rate of 3000 mL min
1
after being allowed to recover for 24 h under N
2
(g).
ACS Omega
http://pubs.acs.org/journal/acsodf
Article
https://doi.org/10.1021/acsomega.2c00543
ACSOmega
2022, 7, 10765
10774
10772