SnO2 Nanosheet/Nanoparticle Detector for the Sensing of 1-Nonanal Gas Produced by Lung Cancer

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received: 02 December 2014

accepted: 30 March 2015

Published: 20 May 2015

# SnO2 Nanosheet/Nanoparticle Detector for the Sensing of 1-Nonanal Gas Produced by Lung Cancer

Yoshitake Masuda, Toshio Itoh, Woosuck Shin & Kazumi Kato

A sensor has been developed for detecting 1-nonanal gas present in the breath of lung cancer
patients by combining SnO2 nanosheets with SnO2 nanoparticles and noble metal catalysts. A
significant change in the electrical resistance of this sensor was observed with increasing 1-nonanal
gas concentration; the resistance decreased by a factor of 1.12 within the range of 1 to 10 ppm
at 300 °C. The recovery of the sensor’s resistance after detecting 1-nonanal gas concentrations of
0.055, 0.18, 1, and 9.5 ppm was determined to be 86.1, 84.2, 80.4 and 69.2%, respectively. This high
sensitivity is attributed to the accelerated oxidation of 1-nonanal molecules caused by the (101)
crystal faces of the SnO2 nanosheets and should provide a simple and effective approach to the early
detection of lung cancer.

Early detection is the best defence against lung cancer and is therefore of vital importance in the development of a simple exhaled breath analysis system for lung cancer detection 1–4 . It is known that the breath
of lung cancer patients contains traces of 1-nonanal gas 5, which as a by-product of the destruction of
cell membranes, increases in concentration in relation to the damage caused by smoking 6 . Although this
raises the possibility of creating 1-nonanal gas sensors for on-site monitoring in households for early
detection, the difficulty of oxidizing a large molecule such as 1-nonanal creates a significant hurdle to
the development of highly sensitive detector materials. A potential solution to this lies in the use of SnO2
particulate films, which have been successfully used in sensors for volatile organic compounds 7–11 and
combined with nanocarbon materials 12,13 such as carbon nanotubes and graphene to create electronic
gas sensors capable of functioning at room temperature. Indeed, the authors of this paper have recently
reported a 1-nonanal gas sensor based on SnO214, in which concentration is determined from the change

reported a 1-nonanal gas sensor based on SnO214, in which concentration is determined from the change

in the resistance of the sensor that is caused by 1-nonanal oxidizing to CO2 and H2O in the presence of
a noble metal catalyst, thereby reducing the SnO2. The resulting movement of electrons from aldehyde
groups of 1-nonanol back into the conduction band of SnO215 has the effect of reducing the depth of the

groups of 1-nonanol back into the conduction band of SnO215 has the effect of reducing the depth of the

space-charge region 14, which along with the valence of the Sn, influences the sensitivity and response
of the sensor. Thus, although a limited change in resistance has so far been achieved, greater optimisation of the structure should greatly increase the sensitivity of this detector material. To this end, an
improved SnO2 nanosheet detector for the detection of lung cancer 16 has been developed that combines
SnO2 nanosheets with SnO2 nanoparticles and noble metal catalysts, the sensing properties of which are
herein discussed.

Methods
To create the 1-nonanal gas sensor, a silicon oxide layer 1000 nm in thickness was first thermally formed
on a silicon substrate. Onto this, LaAlO3 layers several tens of nanometres in thickness were formed,

National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami,
Moriyama-ku, Nagoya 463-8560, Japan. Correspondence and requests for materials should be addressed to Y.M.
[(email: masuda-y@aist.go.jp)](mailto:masuda-y@aist.go.jp)

Figure 1. A: Schematic view of the reaction of a 1-nonanal molecule on a sensor consisting of SnO2
nanosheets, SnO2 nanoparticles, and noble metal catalysts. B: Schematic showing the layout of Pt interdigital electrodes used in the sensor. C: FE-SEM images of the surface morphology of SnO2 particulate films
with SnO2 nanosheets. ( a1,a2 ) SnO2 particulate film. ( b1,b2 ) SnO2 particulate film with SnO2 nanosheets
formed after 20 min immersion in SnF2 solution. ( c1,c2 ) SnO2 particulate film with SnO2 nanosheets formed
after 6 h immersion.

followed by 300 nm-thick Pt inter-digital electrodes. To create the particulate SnO2 film containing noble
metal catalysts, Pt, Au and Pd nanoparticles were first mixed with SnO2 particles several tens of nanometres in diameter to a SnO2:Pt:Au:Pd ratio of 97:1:1:1 by weight. These mixed particles were heated at
400 °C for 2 h, and then mixed with ethyl cellulose to form a colloidal solution that was pasted onto the
substrates. After drying at 80 °C for 2 h, these were heated at 500 °C for 2 h to create a 3000 nm-thick
SnO2 particulate film. This was exposed to ultraviolet light for 20 min under vacuum using a low-pressure
mercury lamp (PL16-110, SEN Lights Co.) to create a hydrophilic surface. Finally, it was immersed in an
aqueous solution 17 containing SnF2 (5 mM) at 90 °C for 6 h to create a top layer of SnO2 nanosheets, then
washed with running water and dried by blowing air. Fig. 1A provides a schematic view of the resulting
structure. For comparison, nanosheet layers were also produced by 20 min, which has been demonstrated
in a previous study 18 to produce nanosheets of about 10–20 nm in width, 100 nm in length, and 5 nm in
thickness; increasing to 5–10 nm in thickness and about 100–1600 nm in plane size after 24 h 19 .

A fine parallel structure of comb-shaped Pt electrodes was used to increase sensitivity (Fig. 1B).
The surface morphology of the sensors was observed by field emission scanning electron microscopy
(FE-SEM; JSM-6335FM, JEOL Ltd.), while their cross-sectional structure was observed using transmission electron microscopy (TEM; JEM-2100F, HRPP, 200 kV, JEOL Ltd.). The individual SnO2 nanosheets and SnO2 particles were observed with scanning-TEM-high-angle-annular-dark-field (HAADF),
energy dispersive X-ray spectrometry (EDS, JED-2300T, DRY SD100GV, JEOL Ltd.), and electron
energy-loss spectroscopy (EELS, Quantum ER, Gatan, Inc.). The structure of 1-nonanal was calculated

at the Hartree–Fock/3-21G(*) level using molecular modelling and computational chemistry (Spartan,
Wavefunction, Inc.).

The sensor’s ability to detect 1-nonanal gas was evaluated by first measuring its inter electrode resistance in flowing air over a period of 180 s to give a value hereafter defined as Ra (ohm). The flowing
gas was then changed to 1-nonanal, and after 300 s, the value Rg (ohm) was recorded. Air was then
re-introduced as the flowing gas, and at 600 s, the R600 (ohm) value was recorded. The recovery ratio of
the sensor was then calculated as (R600-Rg)/(Ra-Rg). This process was repeated for 1-nonanal gas concentrations of 0.055, 0.18, 1, and 9.5 ppm.

Results and Discussion
The bare SnO2 particulate film was found through FE-SEM analysis to have a porous surface structure
(Fig. 1C,a1,a2), but showed little change after immersion for 20 min due to the small size of the SnO2
nanosheets (Fig. 1C,b1,b2). After 6 h immersion, however, large SnO2 nanosheets measuring 100 nm
in-plane and about 1–5 nm in thickness could be clearly observed on the particulate films (Fig. 1C,c1,c2).
These created a network between the SnO2 particles, the noble metal particles and the nanosheets themselves. Subsequent TEM observation revealed that the SnO2 nanosheets were directly formed on the
SnO2 particulate film (Fig. 2A), as they were aligned parallel to the sheet surface. The continuous lattice
fringe of the nanosheet also suggests that single crystals were formed. The fast Fourier transform (FFT)
spectrum of the nanosheet marked as area “a” in the TEM image revealed lattice spacings of 0.27 and
0.17 nm (Fig. 2A,a), which were assigned to the (101) and (220) planes of SnO2, respectively. The fact that
the strong 0.17 nm FFT spots were aligned perpendicular to the in-plane direction of the nanosheet suggests that the large flat planes of the nanosheets were formed on the (101) crystal faces of SnO2. The SnO2
particle in area “b” exhibited clear lattice fringes and lattice spacings of 0.34 and 0.17 nm (Fig. 2A,b), the
former being assigned to the (110) plane of SnO2.

To obtain clearer information on the chemical composition rather than crystal structure,
STEM-HAADF was used to observe the shapes and positions of the various sensor components along
with the distribution of Sn, O and Pd (Fig. 2B). The Sn and O concentrations differentiate the SnO2 nanosheets/particles from the Pd catalyst, allowing the ratio of Sn:O:Pd to be estimated at 29.6:69.9:0.5. The Pt
and Au, however, had only very tiny peaks in the EDS spectrum (Fig. 2C), indicating that greater control
over the addition and distribution of these additives is needed to maximise the sensing properties of the
detector. The EELS spectra obtained from the SnO2 nanosheets and SnO2 nanoparticles (Fig. 2D, a or b)
were both similar to the spectrum of SnO2 in the EELS-Atlas database (Fig. 2D,c), indicating that they
contained quadrivalent tin.

The recovery ratio of sensors consisting solely of SnO2 particulate film (PF) was about 31.0% at a
1-nonanal gas concentration of 9.5 ppm and a temperature of 250 °C. After immersion for 20 min or 6 h
in SnF2 solution (NS(20 min)/PF and NS(6 h)/PF), this dropped to 8.4 and 2.6%, respectively. However,
when the temperature was increased to 300 °C, the recovery ratios of PF, NS(20 min)/PF and NS(6 h)/
PF were significantly improved to 64.4, 69.2 and 17.1%, respectively, due to the accelerated removal of
adsorbed 1-nonanal molecules; residual adsorbed 1-nonanal molecules on NS(6 h)/PF being responsible
for its much lower recovery ratio. Significantly, the recovery ratio of NS(20 min)/PF was consistently
higher than that of PF in any lean 1-nonanal gas at 300 °C (Fig. 3B, Table); reaching ratios of 86.1, 84.2
and 80.4% in 1-nonanal gas concentrations of 0.055, 0.18 and 1 ppm, respectively. As expected, the Rg
of the sensors also decreased with increasing 1-nonanal gas concentration (Fig. 3C) due to the reduction
of SnO2; however, in the concentration range of 1 to 10 ppm, a far more drastic decrease was seen in the
films with nanosheets (Fig. 3C,b,c) than in PF (Fig. 3C,a). This indicates that the SnO2 nanosheets have
a much greater ability to oxidize 1-nonanal molecules, which can be attributed to their high surface area
and characteristic crystal faces.

A decrease in Ra with temperature was observed in the case of PF (Fig. 3C,a) and NS(20 min)/PF
(Fig. 3C,b), which can explained by an increase in carrier concentration due to the fact that SnO2 is a
known semiconductor. This carrier concentration was not affected by temperature in 1-nonanal gas,
however, as the reduction of SnO2 has a much greater influence. The Ra of NS(6 h)/PF, on the other hand,
was not affected by temperature (Fig. 3C,c), although its Rg slightly increased. Given that NS (6 h)/PF
contained a greater number of nanosheets than NS(20 min)/PF, it is possible that this may be influenced
by the crystallinity, crystal faces, valence, and composition of the nanosheets, but further investigation
of the charge transport mechanism is needed to fully understand this connection.

As both the Ra and Rg of NS(20 min)/PF at 250 and 300 °C (Fig. 3C,b) were lower than those of PF
at low 1-nonanol concentrations (Fig. 3C,a), it would appear that the nanosheets do provide an effective
electrical conduit. The behaviour of the NS(6 h)/PF is very different though, in that its Ra and Rg at
250 °C are slightly reduced at low 1-nonanol concentrations (Fig. 3C,c), but are increased at 300 °C. This
would seem to suggest that fine cracks form in the nanosheet layer of NS/PF(20 min) at higher temperatures, thereby increasing its resistance. In either event, the drastic increase in Ra/Rg at 1-nonanal gas
concentrations of 1 to 10 ppm clearly demonstrates that the sensitivity of the sensors was successfully
enhanced by SnO2 nanosheets (Fig. 3D,a,b). This improved the slope of the Ra/Rg curve from 0.47 and
0.48 at 250 and 300 °C in the case of PF, to 0.95 and 0.98 with NS(6 h)/PF. Further improvement to
1.15 and 1.12 was achieved in the case of NS(20 min)/PF. The fact that this improvement in sensitivity

Figure 2. A: Cross-sectional TEM image of a SnO2 particulate film with SnO2 nanosheets created by 20 min
immersion in SnF2 solution (NS(20 min)/PF). ( a ) SnO2 nanosheet and its FFT power spectrum. ( b ) SnO2
particle and its FFT power spectrum. B: Cross-sectional STEM-HAADF image of NS(20 min)/PF together
with EDS element maps of Sn, O, and Pd. C: EDS spectrum of the film showing its chemical ratio. D: EELS
spectra of NS(20 min)/PF. ( a ) SnO2 nanosheet in the a-EELS area marked in Fig. 2B. (b) SnO2 nanoparticle
in the b-EELS in Fig. 2B. (c) SnO2 standard from the EELS-Atlas database.

becomes more pronounced at higher 1-nonanal gas concentrations (Fig. 3D) suggests that PF alone is
insufficient to oxidise such a high concentration of gas.

Nanosheets formed on substrates free of tin were subsequently used to obtain a clearer picture of
their specific effects and allow precise X-ray diffraction (XRD) analysis of their structure. The results
showed that although the as-prepared nanosheets were predominantly composed of SnO2, they also
exhibited very weak diffraction peaks for SnO and other related materials. Following heat treatment,
both the crystallinity and crystallite size of SnO2 increased, while the weaker peaks disappeared. Since
the SnO and related materials would presumably have an influence on the charge transport and electrical

Figure 3. A: Response curves of ( a1,a2 ) a particulate film (PF), ( b1,b2 ) a particulate film with nanosheets
produced by 20 min immersion in SnF2 solution (NS(20 min)/PF) and ( c1,c2 ) a particulate film with nanosheets
produced by 6 h immersion (NS(6 h)/PF). Concentration of 1-nonanal gas was 0.055, 0.18, 1 or 9.5 ppm, and this
was evaluated at ( a1,b1,c1 ) 250 °C or ( a2,b2,c2 ) 300 °C. B: Recovery ratio of the different particulate films. C:
Change in resistance as a function of 1-nonanal gas concentration. Ra and Rg of ( a ) PF, ( b ) NS(20 min)/PF and ( c )
NS(6 h)/PF at (blue lines) 250 °C or (red lines) 300 °C. D: Change in sensitivity with 1-nonanal gas concentration
for ( a ) NS(20 min)/PF (solid circles and lines) and PF (open circles and dotted lines), and ( b ) NS(6 h)/PF (solid
circles and lines) and PF (open circles and dotted lines) at (blue lines) 250 °C and (red lines) 300 °C.

conductivity, and therefore the sensing properties, this disappearance warrants further investigation to
clarify the mechanism behind it. The sensing properties of this unique nanosheet-based sensor can be
fully optimised only if there is precise control over these other phases.

The SnO2 nanosheets were found through TEM analysis to have large, flat (101) faces, which is
the second most stable surface of SnO2 after the (110) plane. In contrast, the SnO2 particulate films
tend to have a mixture of several crystal faces, most of which have a much lower gas-sensing ability 20 .
Density-function theory and scanning tunnelling microscopy were therefore performed to clarify the
ease with which the (101) surface reversibly switches with a change in atmosphere. The precise mechanism has been previously described 20, but essentially is based on the premise that the (101) surface is not
flat on an atomic scale, and so the removal of O ions changes it from an SnO2 surface to a conductive
SnO surface. In contrast, the SnO2 (110) surface has no analogous arrangement of ions that satisfies the
Sn 2+ oxidation state, and so requires complex reconstruction. In short, this means that the (101) surface
can more readily switch from stoichiometric insulating Sn 4+ O22− to reduced conductive Sn2+O2−, thus

making it the better suited to improving the sensitivity of gas detection.

Conclusions
A 1-nonanal gas sensor has been successfully developed based on a SnO2 nanostructured detector, in
which a combination of SnO2 nanosheets with (101) crystal faces, SnO2 nanoparticles and noble catalysts
is used to accelerate the oxidation of 1-nonanal molecules. The sensor has been demonstrated to have
a dramatically increased sensitivity at concentrations of 1 to 10 ppm, with the slope of its Ra/Rg curve
reaching 1.12 at 300 °C. Its recovery ratio of 86.1, 84.2, 80.4 and 69.2% at 1-nonanal gas concentrations of
0.055, 0.18, 1 and 9.5 ppm, respectively, also represents a significant improvement over sensors based on
SnO2 nanoparticles alone. This indicates that more precise control over the nanostructure of SnO2-based
sensors can significantly improve their sensitivity, and therefore presents a viable approach to developing
simple and highly sensitive 1-nonanal gas sensors for the early detection of lung cancer.

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Author Contributions
Y.M. wrote the main manuscript text and T.I. performed the experiments for Fig. 3. W.S. and K.K.
contributed to the correction and polishing of the whole manuscript. All authors reviewed the manuscript.

Additional Information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Masuda, Y. _et al_ . SnO2 Nanosheet/Nanoparticle Detector for the Sensing of
1-Nonanal Gas Produced by Lung Cancer. _Sci. Rep._ 5, 10122; doi: 10.1038/srep10122 (2015).

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