Research Article
Cite This: ACS Appl. Mater. Interfaces 2019, 11, 15044−15050

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Paper-Based Disposable Molecular Sensor Constructed from Oxide
Nanowires, Cellulose Nanoﬁbers, and Pencil-Drawn Electrodes
Hirotaka Koga,*,† Kazuki Nagashima,*,‡ Yintong Huang,§ Guozhu Zhang,‡ Chen Wang,‡
Tsunaki Takahashi,‡ Akihide Inoue,‡ Hong Yan,‡ Masaki Kanai,‡ Yong He,∥ Kojiro Uetani,†
Masaya Nogi,† and Takeshi Yanagida‡

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†

The Institute of Scientiﬁc and Industrial Research and §Graduate School of Engineering, Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka, 567-0047, Japan‡
Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan∥Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China, College of Optoelectronic
Engineering, Chongqing University, Chongqing 400044, China
S Supporting Information*

ABSTRACT: Progress toward the concept of “a trillion sensor universe” requires sensor devices to become more abundant,ubiquitous, and be potentially disposable. Here, we report a paper-based disposable molecular sensor device constructed from ananowire sensor based on common zinc oxide (ZnO), a wood-derived biodegradable cellulose nanoﬁber paper substrate, and a
low-cost graphite electrode. The ZnO nanowire/cellulose nanoﬁber composite structure is embedded in the surface of thecellulose nanoﬁber paper substrate via a two-step papermaking process. This structure provides a mechanically robust andeﬃciently bridged network for the nanowire sensor, while ensuring eﬃcient access to target molecules and allowing reliable
electrical contact with electrodes. The as-fabricated paper sensor device with pencil-drawn graphite electrodes exhibits eﬃcientresistance change-based molecular sensing of NO2 as a model gas. The performance of our device is comparable to that of noble
metal electrodes. Furthermore, we demonstrate cut-and-paste usability and easy disposal of the sensor device with its uniform
in-plane sensing properties. Our strategy oﬀers a disposable molecular sensing platform for use in future sensor network
technologies.
KEYWORDS: Cellulose nanoﬁber, Oxide nanowire, Nanocomposite, NO2 sensor, Paper electronics■

INTRODUCTION
The recent rapid growth of the Internet of Things (IoT) has
generated considerable interest in sensor technologies.1,2
Global usage of sensor devices is estimated to reach morethan one trillion units per year by 2022.3 In particular,
molecular sensing has become a core technology of the IoT
because it oﬀers a powerful tool for generating data to be used
in a wide range of ﬁelds such as healthcare, environmental
monitoring, and industrial safety. 2,4 The collection ofmolecular information by sensors will open a new platform
for generating chemical “big data” within a trillion-sensor
society. To collect accurate molecular information over long
periods, reliable molecular sensing approaches are required.
To eﬀectively detect volatile molecules, various types ofmolecular sensors such as polymer-based chemiresistor
sensors,5 two-dimensional nanosheet sensors,6 and inorganic
nanomaterial-based sensors7 have been demonstrated. Amongthese, semiconducting metal oxides show promise from the© 2019 American Chemical Society

viewpoint of long-term molecular sensing owing to their
thermal/chemical stability in air.8 The sensing mechanism of
conventional metal oxide sensors is mainly based on changes inthe electric potential at grain boundaries upon exposure totarget molecules. However, such mechanisms frequently suﬀer
from deteriorated sensing performances owing to contamination of grain boundaries during long-term use.9,10 Disposable
sensor devices might eﬀectively overcome the problem ofdeteriorating device performance in long-term use. A sensor
component suitable for single- or short-term use could be
easily replaced with a fresh component before it deteriorates.
Such sensor devices are required to not only be disposable but
should also be lightweight, inexpensive, and mechanicallyrobust to handling. These features cannot be achieved in
Received: January 21, 2019
Accepted: April 3, 2019
Published: April 3, 2019
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Research Article

RESULTS AND DISCUSSION
In this study, the sensing material was hydrothermallysynthesized single crystalline ZnO nanowires with widths andlengths of approximately 100 nm and 6 μm, respectively (seeSupporting Information Figure S1).30 We ﬁrst investigated therobust integration of the ZnO nanowires with the substratematerials. When the ZnO nanowires were drop-coated onto a
conventional substrate, that is, a polyethylene terephthalate(PET) ﬁlm, they were easily peeled oﬀ by a ﬁnger touch, evenafter a hot-pressing treatment at 110 °C for 1 h under highpressure (8.5 MPa) (Figure 2a,d). The adhesion of the ZnO

conventional powder-form metal oxide sensors equipped with
precious metal electrodes.11
In this study, we propose a disposable sensor device
constructed from a cellulose nanoﬁber paper substrate, a zincoxide (ZnO) nanowire sensor, and a graphite electrode.
Cellulose nanoﬁber paper is expected to be beneﬁcial for
disposable sensors because it is lightweight, thermally/mechanically robust, and made from abundant and renewablebiomass resources.12,13 In previous studies, various cellulose
nanoﬁber paper-based electronic devices, such as transparent
conductive ﬁlms,14,15 transistors,16,17 antennas,18,19 memory,20,21 generators,22 and electronic paper,23 have beendeveloped. Such paper-based devices show high performance,
paper-like ﬂexibility, and biodegradability.24,25 For sensing
materials, single crystalline nanowires of highly abundant metaloxides (e.g., ZnO and CuO) have shown great promise as
molecular sensors, owing to their large surface/volume ratios
and well-deﬁned crystal planes, wide controllability of chemical
reactivity by altering the metal ions,26−28 and high mechanicalrobustness.29 Graphite is also suitable as a disposable electrodefor molecular sensors because of its low cost and high stability
in air.
From this perspective, the integration of an oxide nanowire
sensor, a cellulose nanoﬁber paper substrate, and a graphite
electrode represents a reasonable approach to disposable
molecular sensor devices. However, conventional strategies,
such as the encapsulation of nanowires or drop casting of
nanowires into/onto cellulose nanoﬁber paper, are unsuitable
for molecular sensors because analytes are not accessible to theoxide nanowires or the nanowires easily peel oﬀ from thesubstrate. One essential requirement is strongly adhered
networks of oxide nanowire sensors on the surfaces of thecellulose nanoﬁber paper substrate. To meet these requirements, here, we tailored robust nanowire/cellulose nanoﬁber
composite networks, which are implanted on the surfaces ofcellulose nanoﬁber paper substrates, by a two-step papermaking process. The as-implanted ZnO nanowires provide
strongly adhesive and eﬃciently bridged network structures on
the substrate surface and enable reliable electrical contact withgraphite electrodes (Figure 1). The as-fabricated paper device

Figure 2. Adhesion performance of the ZnO nanowire/cellulose
nanoﬁber composite networks on a cellulose nanoﬁber paper
substrate. (a−c) Optical and (d,e) FE-SEM images of (a,d) ZnO
nanowires on a PET ﬁlm substrate, (b) ZnO nanowires on a cellulose
nanoﬁber paper substrate, and (c,e) ZnO nanowire/cellulose
nanoﬁber composite networks on a cellulose nanoﬁber paper
substrate after running a ﬁnger over their surfaces. In each case, hotpressing was conducted at 110 °C for 1 h (8.5 MPa) before adhesion
performance testing.

nanowires was improved with the use of the cellulose nanoﬁber
paper substrate (Figure 2b), and the ZnO nanowires adhered
to the cellulose nanoﬁber paper substrate much better than to
the PET ﬁlm substrate. However, a small amount of ZnO
nanowires was removed by a ﬁnger touch. Thus, simply coating
the ZnO nanowires onto the substrate did not result in strong
adhesion. To overcome this problem, we deposited the ZnO
nanowire/cellulose nanoﬁber composite networks onto thecellulose nanoﬁber paper substrate by a two-step papermaking
process as follows (see the Supporting Information Figure S2).
First, an aqueous dispersion of wood cellulose nanoﬁbers withwidths 15−50 nm was ﬁltered through a commercial
membrane ﬁlter to form a wet cellulose nanoﬁber gel. Second,
an aqueous dispersion of ZnO nanowires with cellulose
nanoﬁbers was poured onto the wet nanoﬁber gel, followed
by dewatering under pressure ﬁltration. During the second
process, the wet nanoﬁber gel served as a ﬁlter, retaining
almost all the ZnO nanowires on the surface. Finally, theresulting wet material was thoroughly dried by hot pressing at
110 °C for 1 h (8.5 MPa) and then peeled from the membraneﬁlter to form a sheet of paper. The as-prepared ZnO nanowire/
cellulose nanoﬁber composite networks on the cellulose
nanoﬁber paper, denoted as ZnO nanowire@cellulose nanoﬁber paper, were suﬃciently adhesive to withstand ﬁnger touch(Figure 2c,e). We attribute this stability to the entanglement of
ZnO nanowires and cellulose nanoﬁbers on the surfaces of the

Figure 1. Schematic of the disposable molecular sensor device
consisting of a cellulose nanoﬁber paper substrate, a ZnO nanowire
sensor, and a graphite electrode.

showed eﬀective molecular sensing performance for nitrogenoxide (NO2) as a model gas component. The performance ofour device was comparable to that of Pt electrode-based
systems. The paper-like cut-and-paste usability of the sensor
device with an in-plane uniformity of the sensing property was
also demonstrated, suggesting potential for application as
disposable and ubiquitous molecular sensor elements.
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Information Figure S5). Thus, the ZnO nanowire@cellulose
nanoﬁber paper with a dual-layered structure (Figure 3), which
was prepared by a two-step papermaking approach (FigureS2), provided a robust nanowire-bridging structure over its
entire surface.
The ZnO nanowire@cellulose nanoﬁber paper was used as amolecular sensing platform for NO2 as a model gas
component. When the n-type ZnO semiconductor is exposed
to an oxidizing gas such as NO2, gas molecules adsorb to the
ZnO surface and extract electrons from the conduction band of
ZnO, leading to the formation of an electron-depletion region
on the surface.32 Furthermore, in the case of a ZnO nanowire
sensor, a potential barrier develops at nanowire/nanowire
junctions of the bridging nanowire network, which blocks
electron ﬂow more eﬃciently than surface depletion of a single
nanowire and thus enhances sensitivity (i.e., induces aneﬀective increase in the resistance).31 To evaluate the sensing
performance of the ZnO nanowire@cellulose nanoﬁber paper,
Pt electrodes were ﬁrst sputtered on the surfaces of the paper
to fabricate the paper sensor device (Figure 4a). The paper
sensor showed ohmic behavior in current−voltage characteristics, indicating the formation of good electrical contacts
between the ZnO nanowire sensor networks and Pt electrodes(see the Supporting Information Figure S6). The sensing

cellulose nanoﬁber paper substrate (Figure 3a,b). Atomic
absorption analysis conﬁrmed that the ZnO content in the

Figure 3. Characterization of the ZnO nanowire/cellulose nanoﬁber
composite networks on a cellulose nanoﬁber paper substrate denotedas the ZnO nanowire@cellulose nanoﬁber paper. (a) Optical, (b) topview FE-SEM, (c,d) top-view, and (e,f) cross-section FE-SEM and
EDX mapping images of the ZnO nanowire@cellulose nanoﬁber
paper. Paper size: 35 mm diameter.

ZnO nanowire@cellulose nanoﬁber paper was around 5.5 wt%. Thus, our approach is expected as an eﬀective way toconstruct mechanically robust ZnO nanowire networks on
cellulose nanoﬁber paper substrates.
Formation of a ZnO nanowire network on the substratesurface is also important to ensure eﬃcient molecular sensing
response (i.e., a detectable change in the electrical resistanceupon adsorption/desorption of analytes). The network canoﬀer additional modulation to the sensor resistance owing to
the potential barrier at the nanowire/nanowire junctions aswell as the surfaces of individual nanowires.31 Field-emission
scanning electron microscope (FE-SEM) and energy dispersive
X-ray spectrometry (EDX) images of the surface and a crosssection of the ZnO nanowire@cellulose nanoﬁber paper are
shown in Figure 3c−f. The EDX Zn element mapping images
show that the ZnO nanowires were well-dispersed andimplanted over the whole surface of the cellulose nanoﬁber
paper substrate (see also the Supporting Information Figures
S3 and S4). The cellulose nanoﬁber paper without ZnO
nanowires showed a surface resistance as high as 1014 Ωsquare−1 owing to the high electrical insulation properties of
the cellulose nanoﬁbers.25 The ZnO nanowire@cellulose
nanoﬁber paper exhibited a relatively low surface resistanceof approximately 1.0 × 108 Ω square−1 under atmosphericconditions. These results indicated successful formation ofeﬃciently bridged networks of ZnO nanowires in the cellulose
nanoﬁber paper substrate. When an aqueous mixture of ZnO
nanowires and cellulose nanoﬁbers was fabricated into thepaper by a one-step papermaking process, most of the ZnO
nanowires were buried inside the cellulose nanoﬁber matrix,
leading to diﬃculty in forming a nanowire-bridging structureon the cellulose nanoﬁber paper (see the Supporting

Figure 4. NO2 sensing performances of the paper sensor consisting of
ZnO nanowire@cellulose nanoﬁber paper and Pt electrodes under dry
conditions. (a) Photograph of sensor, (b) resistance versus time on
exposure to 98 ppm NO2 gas at 150 °C, (c) resistance versus timeupon exposure to NO2 gas with concentrations ranging from 3.9 to 98
ppm at 150 °C, (d) relative resistance change upon exposure to NO2
gas with concentrations ranging from 3.9 to 98 ppm at 150 °C for 100s, (e) relative resistance change versus time upon exposure to 98 ppm
NO2 gas at temperatures ranging from 25 to 200 °C, and (f) relative
resistance change upon exposure to 98 ppm NO2 gas for 10 s (red)and the following exposure to air for 10 s (blue) at temperatures
ranging from 25 to 200 °C.
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Figure 5. NO2 sensing performances under dry conditions and paper-like usability of the paper sensor consisting of the ZnO nanowire@cellulose
nanoﬁber paper and the pencil-drawn graphite electrodes. (a) Optical and (b) optical microscope images, (c) resistance versus time upon exposure
to 98 ppm NO2 gas at 150 °C, (d) fabrication of the paper sensor by scissor-cutting and pencil-drawing, and (e) inﬂuence of cutting position of theZnO nanowire@cellulose nanoﬁber paper on the NO2 sensing performance of the paper sensors.

performance of the paper sensor for 98 ppm NO2 at 150 °C isshown in Figure 4b. The electrical resistance of the ZnO
nanowire@cellulose nanoﬁber paper clearly increased upon
exposure to 98 ppm NO2 gas. Then, following exposure to air,
the resistance decreased owing to desorption of NO2 from theZnO nanowires. These results suggested that the ZnO
nanowire-bridging structure on the cellulose nanoﬁber paper
substrate (Figure 3d and Figure S3) enabled NO2 sensing. The
paper sensor provided suﬃcient sensitivity to detect NO2
down to concentrations as low as 3.9 ppm (Figure 4c,d).
The NO2 sensing characteristics of the ZnO nanowire@
cellulose nanoﬁber paper were almost similar up to 30% of
relative humidity (RH 30%), while a further increase of relative
humidity decreased its sensitivity (Figure S7). This is plausibly
because the adsorption of NO2 molecules was suppressed by
the presence of water molecules on the surfaces of ZnO
nanowires. However, the ZnO nanowire@cellulose nanoﬁber
paper still could detect 28 ppm NO2 at RH 70%.
As described above, the adsorption/desorption behaviors of
NO2 onto/from ZnO nanowires govern their sensing performances. As shown in Figure 4e, the NO2 sensing behavior of thepaper sensor depended on the surrounding temperatures,indicating that temperature aﬀected the adsorption/desorptionof NO2 onto/from the ZnO nanowire-bridging structure. The
relative resistance values of the paper sensor after the ﬁrst
exposure to 98 ppm NO2 for 10 s (NO2 introduction) and thefollowing exposure to air for 10 s (NO2 subtraction) at
diﬀerent temperatures are shown in Figure 4f. Upon
introducing NO2, the relative resistance values increased withincreasing temperature, suggesting that NO2 adsorption onto
the ZnO nanowires was promoted as the surrounding
temperature increased. At temperatures below 50 °C, therelative resistance values were greater than 1.0 for both theintroduction of NO2 and its attempted removal under an air
ﬂow, indicating that NO2 hardly desorbed from ZnO
nanowires at low temperatures. These results suggest that
low-temperature use of the paper sensor can improvesensitivity (resistance change) for single-use and long-term
sensing. When the paper sensor was used at relatively hightemperatures exceeding 100 °C, the relative resistance values
were less than 1.0 upon loss of NO2, owing to the promotionof NO2 desorption from the ZnO nanowires. Because both
adsorption and desorption of NO2 onto and from the ZnO
nanowire-bridging structure occurred over 100 °C, such hightemperature use of the paper sensor would be advantageous for
short-term and repeat sensing. Thus, the paper sensor is
expected to be suitable for single use at room temperatures andrepeated use at high temperatures over 100 °C.
Although the ZnO nanowire@cellulose nanoﬁber paper
acted as a molecular sensing platform, the use of precious
metal electrodes (i.e., Pt) prevents easy disposal of the sensor
device. One of the most important properties of electrodes forgas sensors is stability to air. However, inexpensive metals,
such as aluminum, copper, and titanium, easily form an
insulation layer on their surfaces owing to oxidation in air,which limits their applicability as electrodes. Graphite is apromising candidate for disposable electrodes because it isearth-abundant, low-cost, and air-stable. Therefore, weinvestigated graphite as an electrode for the ZnO nanowire@
cellulose nanoﬁber paper. Pencil lead generally contains highpurity graphite. As shown in Figure 5a,b, graphite electrodes,
drawn from commercial pencil lead, were successfully
deposited onto the surfaces of the ZnO nanowire@cellulose
nanoﬁber paper by a drawing process. The success of thepencil drawing can be attributed to both the nanoscale roughsurfaces (Figure 3b) and high mechanical robustness (Figure2c,e) of the ZnO nanowire@cellulose nanoﬁber paper. The
electrical resistance of the pencil-drawn graphite electrodes wasof the order of 104 Ω, which is much lower than that of thesensing layer, that is, ZnO nanowire@cellulose nanoﬁber paper(1.0 × 108 Ω square−1). In addition, the as-prepared paper
sensor exhibited ohmic behavior in its current−voltage
characteristics, suggesting the formation of a reliable electricalcontact between the ZnO nanowire sensor networks and
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Figure 6. Disposability test by burning the paper sensor consisting of the ZnO nanowire@cellulose nanoﬁber paper and the pencil-drawn graphite
electrodes.
provided by Nippon Paper Group, Inc., Tokyo, Japan, as follows. An
aqueous suspension of the pulp (0.5 wt %, 2 L) was treated by a highpressure water-jet system (Star Burst, HJP-25005E, Sugino MachineCo., Ltd., Uozu, Japan). The pulp suspension was ejected from a 0.15
mm diameter nozzle at 245 MPa with 50 cycle repetitions. ZnO
nanowires were hydrothermally synthesized based on a previously
reported method30 as follows. An aqueous solution (100 mL) of the
precursor was prepared at room temperature from zinc nitratehexahydrate (25 mM), hexamethylenetetramine (25 mM), polyethylenimine (2.5 mM), and ammonia (500 mM). Then, the ZnO(thickness: around 75 nm)/Cr (thickness: around 7 nm)/Si wafer
substrate (size: 2.25 × 2.25 cm), which was prepared by successive
sputtering on the Si wafer, was immersed upside down into theprecursor solution in a sealed beaker and maintained at 95 °C for 12h, followed by washing with distilled water and drying. The ZnO
nanowires obtained were peeled from the substrate by ultrasonicationin an aqueous dispersion of cellulose nanoﬁbers for 3 min. Zinc nitratehexahydrate (>99.0% purity), hexamethylenetetramine (>99.0%
purity), and ammonia solution (28 wt %) were purchased from
Wako Pure Chemical Industries, Ltd., Osaka, Japan. Polyethylenimine
solution (50 wt %, average molecular weight: ∼2000) was obtained
from Aldrich, Ltd., Tokyo, Japan. The Si wafer (4″ P(100) 1-10) andpencil (9800, Hardness: 2B) were purchased from SUMCO Corp.,
Tokyo, Japan, and Mitsubishi Pencil Co., Ltd., Ohi, Japan,
respectively. The polyethylene terephthalate (PET) ﬁlms (G2,
thickness: 50 μm, Teijin DuPont Films Japan, Ltd., Tokyo, Japan)
were treated at 150 V and 10 mA for 30 s with a plasma etching
device (AP-T02-L, Sekisui Chemical Co., Ltd., Osaka, Japan) to
improve their wettability before use.
Preparation of the ZnO Nanowire/Cellulose Nanoﬁber
Composite Networks on Cellulose Nanoﬁber Paper. To prepare
the ZnO nanowire/cellulose nanoﬁber composite networks on
cellulose nanoﬁber paper, that is, the ZnO nanowire@cellulose
nanoﬁber paper, with a diameter of 35 mm, an aqueous dispersion ofcellulose nanoﬁbers (0.3 wt %, 20 mL) was ﬁrst dewatered on amembrane ﬁlter (H020A090C, hydrophilic polytetraﬂuoroethylene(PTFE) membrane, 0.1 μm pore diameter, Advantec Toyo Kaisha,Ltd., Tokyo, Japan) under pressure ﬁltration (KST-47, Advantec
Toyo Kaisha, Ltd., Tokyo, Japan). Then, the aqueous mixture ofcellulose nanoﬁbers and ZnO nanowires at a speciﬁc concentration(30 mL) was added and ﬁltered in the same manner. The obtained
wet sheet was covered with a hydrophobic glass plate, treated by hotpressing at 110 °C for 1 h (8.5 MPa), and then peeled from themembrane. As a control, the ZnO nanowire networks on the PET ﬁlm
were also prepared by drop coating as follows. An aqueous dispersionof ZnO nanowires (2 mL) at a speciﬁc concentration was cast onto a
PET ﬁlm (diameter: 35 mm), followed by drying at 50 °C for 2 h andthen hot pressing at 110 °C for 1 h (8.5 MPa).
Preparation and Evaluation of the Paper Sensor. The paper
sensor was constructed by depositing the electrode materials on theZnO nanowire@cellulose nanoﬁber paper. A Pt electrode with athickness of around 200 nm was deposited by a metal mask-assisted
sputtering technique with a sputtering power of 50 W in an Ar 0.3 Pa
atmosphere at room temperature. The graphite electrode wasdeposited by line drawing with a commercial pencil. The amount ofdeposited graphite was controlled by the number of pencil strokes.
Typically, we drew the graphite electrode at least 30 times to ensure

graphite electrodes (see the Supporting Information FigureS8). The paper sensor with pencil-drawn graphite electrodes
showed good NO2 sensing performance, comparable to that of
the Pt electrodes (Figure 5c). It has been reported that ﬂexible
pencil lead composed of graphite/polymer composites can act
as a sensing material for volatile organic compounds owing toabsorption/desorption-induced swelling/recovery of polymer
binders.33 The pencil-drawn graphite used in this study
showed no clear sensitivity toward NO2 gas, indicating thatthe graphite behaved as an electrode rather than a sensing layer(Figure S9). Thus, we constructed a paper sensor device basedon graphite electrodes rather than precious metal electrodes,
while maintaining the molecular sensing performance of thedevice.
Furthermore, the ZnO nanowire@cellulose nanoﬁber paper
was easily shaped, as for normal paper. The paper could be
easily cut with scissors into a desired shape. Then graphite
electrodes were formed on the paper by simply drawing with apencil before application as a NO2 sensor (Figure 5d). As
shown in Figure 5e, these paper sensors showed similar NO2
sensing performance regardless of the area of the cutout. We
attribute this result to the high spatial uniformity of the ZnO
nanowire networks on the cellulose nanoﬁber paper. As shownin Figure 6, the paper sensor could also be disposed of simply
and rapidly, through burning within several seconds. Such cutand-paste usability and easy disposal are features required foran ubiquitous molecular sensor network; our paper sensor islight and could be easily cut into a range of shapes and sizes,
mounted as required for sensing, and disposed after use.
Integration of the heating system into the paper sensor without
sacriﬁcing its ﬂexibility and disposability is a further challenge.
Drawing a graphite layer all over the back surfaces of the ZnO
nanowire@cellulose nanoﬁber paper might be a promisingstrategy; the as-drawn graphite layer would allow joule heatingof the sensing layer.

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CONCLUSIONS
To conclude, we fabricated a paper-based disposable molecularsensor device. A cellulose nanoﬁber paper substrate, ZnO
nanowire sensor, and graphite electrodes were robustly
integrated by two-step papermaking and pencil-drawing
processes. The as-fabricated paper sensor device oﬀeredeﬃcient molecular sensing performance for NO2, cut-andpaste usability, and easy disposability. This strategy can beextended to a variety of oxide nanowires and thus spurdevelopment of disposable molecular sensors to support a
future trillion-sensor society.

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EXPERIMENTAL SECTION

Materials. Cellulose nanoﬁbers with widths of 15−50 nm wereprepared from never-dried softwood dissolving sulﬁte pulp, which was
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electrical conduction of the electrode. The gap size of the electrodes
was carefully controlled to be around 200 μm. The electricalconduction measurements and NO2 sensing measurements of thepaper sensor and the graphite electrode were conducted on atemperature programmable probe station. All measurements were
performed by a two-probe method. For the measurements, thetemperature was modulated in the range of 25−200 °C by a state
heater and the NO2 concentration was varied in the range of 3.9−98
ppm with a mass-ﬂow control system using dry air as a carrier gas. For
NO2 sensing, the dry air (N2 80%, O2 20%) was ﬁrst ﬂown for 100 s,
and then NO2 mixed with Ar was ﬂown for 100 s. This process wascycled ﬁve times, and one another cycle with a varied ﬂow time of 300
s was tested for the reliability of sensing characteristics. During the gas
ﬂow, the ﬂow rate was controlled to be 250 mL min−1. The resistancevalues during the NO2 sensing were readout by applying 1 V between
the contact electrodes. The NO2 sensing characteristics at diﬀerent
relative humidities (dry, 10%, 30%, 50%, 70%) were measured at a
ﬂow rate of 1.4 L min−1 (0.4 L min−1 for NO2 and 1.0 L min−1 forhumidity control).
Analyses. Surface and cross-section observations were performed
using a ﬁeld-emission scanning electron microscope (FE-SEM, JEOL
JSM-7610F) and an optical microscope (Olympus, BXFM-N22MBSD). Compositional image analyses were conducted by energy
dispersive X-ray spectrometry (EDX) ﬁtted to the FE-SEM. To
avoid charging issues that disturb the FE-SEM and EDX analysis, Pt
thin ﬁlms with a thickness of 10 nm were deposited onto the paper
sensor prior to these measurements. The ZnO nanowire content wasdetermined by atomic absorption spectrophotometry using a ZA3300
instrument (Hitachi High-Tech Science Corp., Tokyo, Japan). Sheet
resistance values were measured using a resistivity meter with a ringtype probe (Hiresta-UX, MCP-HT800, Mitsubishi Chemical
Analytech Co., Ltd., Chigasaki, Japan).

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Kojiro Uetani: 0000-0003-3245-6929
Takeshi Yanagida: 0000-0003-4837-5701
Author Contributions

H.K., K.N., M.N., and T.Y. designed this work. H.K. and K.N.
prepared the manuscript. H.K., K.N., H.Y., G.Z., C.W., A.I.,
and Y.H. carried out the experiments. H.K., K.N., T.T., K.U.,
M.K., Y.H., M.N., and T.Y. analyzed the results and discussed
the manuscript during the preparation. All authors discussed
the results and implications and commented on the manuscript
at all stages.
Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS
This work was partially supported by Grants-in-Aid for
Scientiﬁc Research (Grant Nos. 26220908 to M.N., No.
18H02256 to H.K., No.18H01831 to K.N., No.18H05243
to T.Y., and No.17H04927 to T.T.) from the Japan Society for
the Promotion of Science, by the JST-Mirai R&D Program(Grant No. JPMJMI17ED to M.N.) of the Japan Science andTechnology Agency, and by the Cooperative Research
Program “CORE Lab” of Network Joint Research Center for
Materials and Devices: Dynamic Alliance for Open Innovation
Bridging Human, Environment and Materials (H.K.). K.N. was
supported by the MEXT Project of Integrated ResearchConsortium on Chemical Sciences. K.N. acknowledges IMRAJapan Co. Ltd. for ﬁnancial support. T.Y. and K.N. were
supported by CAS-JSPS Joint Research Projects (Grant No.
GJHZ1891). We thank Andrew Jackson, Ph.D., from Edanz
Group (www.edanzediting.com/ac) for editing a draft of thismanuscript.

ASSOCIATED CONTENT* Supporting InformationS■

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.9b01287.
FE-SEM image of the ZnO nanowires hydrothermallysynthesized on the ZnO/Cr/Si wafer substrate;schematic of the preparation of the ZnO nanowire@
cellulose nanoﬁber paper by a two-step papermaking
process; top-view FE-SEM and EDX elemental mapping
images of the ZnO nanowire@cellulose nanoﬁber paper;
cross-section-view FE-SEM and EDX elemental mapping images of the ZnO nanowire@cellulose nanoﬁber
paper; optical and top-view FE-SEM images of the paper
composite prepared from an aqueous mixture of ZnO
nanowires and cellulose nanoﬁbers by a one-step
papermaking process; temperature dependent current
versus voltage characteristics of the ZnO nanowire@
cellulose nanoﬁber paper with Pt electrodes; current
versus voltage characteristics of the ZnO nanowire@
cellulose nanoﬁber paper with pencil-drawn graphite;change in the resistance of the ZnO nanowire@cellulose
nanoﬁber paper and the graphite electrode upon
exposure to 98 ppm NO2 gas at 150 °C (PDF)■

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AUTHOR INFORMATION

Corresponding Authors*E-mail: hkoga@eco.sanken.osaka-u.ac.jp (H.K.)*E-mail: kazu-n@cm.kyushu-u.ac.jp (K.N.)
ORCID

Hirotaka Koga: 0000-0001-6295-1731
Kazuki Nagashima: 0000-0003-0180-816X
Tsunaki Takahashi: 0000-0002-2840-8038
15049

DOI: 10.1021/acsami.9b01287
ACS Appl. Mater. Interfaces 2019, 11, 15044−15050

Research Article

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DOI: 10.1021/acsami.9b01287
ACS Appl. Mater. Interfaces 2019, 11, 15044−15050