Preparation and Gas Sensing Properties of In2O3/Au Nanorods for Detection of Volatile Organic Compounds in Exhaled Breath

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T11472 Analytical Chemistry and Sensors

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Abstract A series of In 2 O 3 /Au nanorods (NRs) were fabricated and characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X–ray diffractometer (XRD) and X–ray photoelectron spectroscopy (XPS). The length to diameter ratios of In 2 O 3 /Au NRs was periodically modulated in the range of 2.9–4.5 through controlling the initial content of indium salt and reaction time. Their gas sensing properties to volatile organic compounds (VOCs) were carefully studied and then applied in exhaled breath detection. The results demonstrate that In 2 O 3 /Au NRs gas sensor can effectively detect acetone at 250 °C and ethanol at 400 °C. The corresponding actual detection limit is as low as 0.1 ppm to acetone and 0.05 ppm to ethanol, respectively. Moreover, by using humidity compensation method, In 2 O 3 /Au NRs gas sensor can clearly distinguish the acetone and ethanol biomarkers in human breath. The main reason of the enhanced gas sensing properties was attributed to the “spillover effects” between Au and In 2 O 3 NRs. The excellent sensing performance indicates that In 2 O 3 /Au NRs is a promising functional material to actual application in monitoring and detecting diabetes and safe driving area in a noninvasive and more accurate way.

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## OPEN

received: 10 October 2014

accepted: 29 April 2015

Published: 01 June 2015

# Preparation and Gas Sensing Properties of In2O3/Au Nanorods for Detection of Volatile Organic Compounds in Exhaled Breath

Ruiqing Xing 1 , Lin Xu 1,2 , Jian Song 1 , Chunyang Zhou 1 , Qingling Li 1 , Dali Liu 1 & Hong Wei
Song 1,2

A series of In2O3/Au nanorods (NRs) were fabricated and characterized by scanning electron
microscope (SEM), transmission electron microscope (TEM), X–ray diffractometer (XRD) and X–ray
photoelectron spectroscopy (XPS). The length to diameter ratios of In2O3/Au NRs was periodically
modulated in the range of 2.9–4.5 through controlling the initial content of indium salt and reaction
time. Their gas sensing properties to volatile organic compounds (VOCs) were carefully studied and
then applied in exhaled breath detection. The results demonstrate that In2O3/Au NRs gas sensor
can effectively detect acetone at 250 °C and ethanol at 400 °C. The corresponding actual detection
limit is as low as 0.1 ppm to acetone and 0.05 ppm to ethanol, respectively. Moreover, by using
humidity compensation method, In2O3/Au NRs gas sensor can clearly distinguish the acetone and
ethanol biomarkers in human breath. The main reason of the enhanced gas sensing properties was
attributed to the “spillover effects” between Au and In2O3 NRs. The excellent sensing performance
indicates that In2O3/Au NRs is a promising functional material to actual application in monitoring and
detecting diabetes and safe driving area in a noninvasive and more accurate way.

Significant interest has been generated in the field of selective detection of specific VOCs which also
called biomarkers in exhaled breath from the metabolism process 1–6 . What’s important, these biomarkers can intuitive indicate some abnormalities of human body, including problems with carbohydrate
digestion and blood ethanol levels 7–9 . For example, the routine method to detect diabetes were usually
monitoring the glucose concentration in patient’s blood; however, diabetes can also be reflected by a
biomarker of gaseous acetone in human breath, and clinical data showed that the exhaled acetone of
diabetes exceeded 1.8 ppm, while for healthy people was only 0.3–0.9 ppm 10–12 . Obviously, direct analyzing the exhaled breath possesses advantages of noninvasiveness, ease of operation, and accuracy. Thus,
despite the wide range of constituents in human breath may interfere the actual analysis and make it
complicated, specific biomarkers are still intensively studied.

Recently, nanomaterials based sensors are considered to be a promising clinical and laboratory diagnostic tool, because its large surface–to–volume ratio, controllable structure, easily tailored chemical
and physical properties, which bring high sensitivity, fast dynamic process, and even the increasing
specificity 13–15 . Among various nanomaterials, metal oxide semiconductor based chemiresistive sensors
are likely to become a portable real–time breath detector, because of the small size, low cost, ease of
operation, and particularly strong correlations with instrumental analysis. Although there is no metal
oxide based sensors being developed for human breath sensing, some related works have been reported,

1State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin
University, Changchun, 130012, People’s Republic of China. 2 The State Key Laboratory of Bioelectronics, Southeast
University, 210096 P. R. China. Correspondence and requests for materials should be addressed to L.X. (email:
[linxu@jlu.edu.cn) or H.W.S. (email: songhw@jlu.edu.cn)](mailto:linxu@jlu.edu.cn)

Righettoni _et al._ fabricated Si doped WO3 sensors for quantitative analysis of acetone in dry and 90%
relative humidity (RH) air samples 16 . Very recently, Shin _et al._ prepared Pt nanoparticles (NPs) functionalized SnO2 fibers through electrospinning which could be a potential material for the detection of
gaseous acetone and toluene, which are the biomarkers for the diabetes and lung cancer, respectively 12 .
Inyawilert _et al._ synthesized In2O3 sensing film by a sparking process which can use to detect ethanol
and acetone 17 . However, the working temperature towards different gas as mentioned above could not
be clearly distinguished. Nevertheless, the performances, including sensitivity, selectivity, and response
time, and else, of conventional gas sensors generally could not satisfy the requirements for breath analysis
because the gaseous biomarkers in exhaled breath were complex in contents and with low concentrations.
Therefore, the reports of sensor focused on the practical detection of biomarkers in expired air or even
in simulated exhaled breath environment were rare. Beyond that, no instance demonstration about dual
mode sensor that can diagnose two different biomarkers towards exhale breath has been reported to the
best of our knowledge.

One–dimensional (1D) metal oxide nanostructures have been identified to be one of the most effective nanoarchitectures for chemiresistive VOCs detection 18–24 . Especially, the dual mode sensing properties can be obtained or optimized in 1D system due to their high gas accessibility and large specific
surface area 23,25,26 . Accordingly, in this paper, we present a facile, inexpensive and one–step method to
fabricate the In2O3/Au NRs through co–precipitation method. By this way, the small Au NPs were effectively embedded in In2O3 NRs and acted as sensitizer. In particular, In2O3/Au gas sensor not only can
detect acetone lower to 10 ppb at 250 °C, but also can detect ethanol lower to 50 ppb at 400 °C. Moreover,
this sensor showed good anti–interference to humidity and could effectively detect gaseous acetone and
ethanol in simulated exhale breath environment. And then, a humidity compensation method was used
to increase the detection accuracy and the clinic test was also conducted.

Results and discussions
Morphological and Structural Characteristics. The morphologies and nanostructures of the precursor and annealing samples are first illuminated. The uniform Au NRs used in this work are as shown
in Fig. 1a, which are ~15 nm in diameter and ~50 nm in length. Figure 1b shows the In2O3/Au precursor sample (S3, atom ratio of Au/In = 1.8%) contained with 0.2 g In(NO3)3 and Au NRs (with reaction
time of 120 min), and the as prepared precursor exhibits uniform NRs structures and small Au NRs are
not observed elsewhere. Well, unlike with In2O3/Au precursor, the pure In2O3 without addition of Au
NRs formed flower–like microspheres instead of NRs (Fig. 1c). As in a hydrothermal environment, urea
played a crucial role in the formation of this flower–like precursor which could hydrolyze and release
OH − and CO32 − 27. The nucleation and growth of In(OH)CO3 thus stemmed from the precipitation of

OH − and CO32 − 27. The nucleation and growth of In(OH)CO3 thus stemmed from the precipitation of

In 3+ by OH − and CO32− ions. Then the small In(OH)CO3 NPs aggregated due to the high surface energy

of the NPs and finally self–assembled to flower like spheres 28 . After addition of Au NRs which was
positively charged because of residual CTAB on its surface, the negatively charged OH − and CO32− ions

positively charged because of residual CTAB on its surface, the negatively charged OH − and CO32− ions

tended to gather around Au NRs, and then induced the heterocoagulation process start from the surface
of Au NRs and finally self–assembled to NR structure, that is to say, Au NRs played a role as template
in forming the In2O3/Au NRs 29 . After annealing, the corresponding morphologies are maintained and
have no significant change except the size shrank (Fig. 1d,e). The In2O3/Au NRs after annealing have the
average diameter of 75 nm and length of 310 nm, that is to say, the length to diameter ratio is about 4.1.
Besides, the color of different samples as mentioned above is very different. As can be clearly seen in
inset of Fig. 1b–e, the color of In2O3/Au precursor is dark green, while the color of pure In2O3 is milk
white to the naked eye, after annealing the color of In2O3/Au NRs is dark red, while the pure In2O3 is
light yellow. This indicated that Au NRs were successfully introduced into In2O3/Au NRs.

Besides, the In2O3/Au NRs samples with different initial amount of In(NO3)3 and reaction time were
also synthesized. As shown in Figure S1, the different initial amount of In(NO3)3 and reaction time have a
little influence on the morphologies. The length to diameter ratios are about 3.6, 3.8, and 4.5, respectively
to 0.1g (Figure S1a, S1, atom ratio of Au/In = 3.6%), 0.15 g (Figure S1b, S2, atom ratio of Au/In = 2.4%),
and 0.3 g (Figure S1c, S4, atom ratio of Au/In = 1.2%) In(NO3)3 at the reaction time of 120 min, which
show a slight increase trend with the increasing of the In(NO3)3 amount. When the reaction time is
different, the length to diameter ratios is changed. As can be seen from Figure S1d, when the reaction
time is 50 min (S5), the length to diameter ratios is about 2.9, which is smaller than the sample with the
reaction time of 120 min. While when the reaction time is 180 min (S6) the length to diameter ratios is
nearly same with 120 min, as can be seen from Figure S1e.

The detail information about the microstructure and morphology of the as–synthesized In2O3/Au
NRs sample was further carried out by TEM and STEM. It can be seen that the In2O3/Au NRs contains a
lot of interspaces, indicating a large surface area as shown in Fig. 2a,b. Moreover, the clear lattice fringes
can be observed in entire HRTEM image revealed that the nanostructure is consisted of nanosize grain
particles (Fig. 2c) and the lattice fringes is d = 0.292 nm, which match well with the crystallographic
planes of cubic In2O3. The corresponding SAED pattern (inset of Fig. 2c) indicated that the nanostructure of the as–synthesized In2O3/Au NRs is polycrystalline, being in agreement with the TEM results.

EDX mappings were carried out to further investigate the specific distribution of In and Au elements
in In2O3/Au NRs samples after annealing as shown in Fig. 2. Homogeneous distribution of In and O elements can be clearly seen, and exhibit the NRs structure. For Au element, the corresponding distribution

Figure 1. ( a ) TEM of Au NRs used in this work . SEM images of ( b ) S3 In2O3/Au NRs precursor sample
and ( c ) pure In2O3 precursor sample, which both have 0.2 g initial In(NO3)3 and obtained at the reaction
time of 120 min, ( d ) S3 In2O3/Au NRs and ( e ) pure In2O3 samples after annealing. The insets in ( b–e ) are
the pictures corresponding to each sample.

is uniform dispersion in In2O3/Au NRs rather than accumulated as small Au NRs structure, that is to
say, Au NRs are melting then uniformly dispersed in the In2O3 NRs after annealing, which is consistent
with our previous study 29 .

The typical XRD patterns of the as–synthesized In2O3/Au NRs as well as In2O3 after annealing were
shown in Fig. 3. Pure cubic phase (JCPDS card no. 06–0416) can be well detected in both In2O3/Au
NRs and In2O3 samples and high intensity of the diffraction peaks in the XRD pattern indicate that the
samples have high crystallinity. Besides, in In2O3/Au NRs samples, the diffraction peaks of Au are not
obvious due to the small doses of added Au NRs.

XPS. In order to obtain a more detailed chemical composition of the as–prepared In2O3/Au NRs,
the XPS spectra were conducted and compared with that of the pure In2O3.The complete spectra of
the samples are shown in Fig. 4a, which confirm the presence of In, O, and C atoms in In2O3/Au NRs
as well as pure In2O3.While, because the content of Au is relatively small, the XPS peaks of Au are not
obvious in the complete spectrum of In2O3 / Au NRs 30 . The detailO1s XPS spectra of In2O3/Au NRs and
pure In2O3 samples are enlarged in Fig. 4b. As is shown, both the O1s XPS spectra of In2O3/Au NRs
and In2O3 samples display three peaks. In detail, the binding energies around 530.3 eV and 531.9 eV are
assigned to the deficient oxygen and the adsorbed OH groups or adsorbed oxygen species, respectively 31 .
Obviously, it can be seen that the content of deficient oxygen in In2O3/Au NRs are much higher than that
of pure In2O3. As is calculated, the corresponding deficient oxygen atomic ratio percentages are 35.1%
and 23.4% in In2O3/Au NRs and pure In2O3, respectively, suggesting the more surface oxygen vacancies
were formed in In2O3/Au NRs. The main O1s XPS peak at 529.7 eV in In2O3/Au NRs corresponds to
the lattice oxygen of crystalline In2O3, and compared to pure In2O3, a chemical shift to higher binding
energy side is observed (529.8 eV in pure In2O3). The same phenomenon can be observed in the characteristic spin–orbit split XPS data of trivalent indium as shown in Fig. 4c. The binding energies of In
3d5/2 and In 3d3/2 are at 444.5 and 452.0 eV, respectively. Compared with the pure In2O3, clearly shift to

Figure 2. ( a ) FETEM, ( b ) STEM, ( c ) HRTEM image, and SAED (inset of c ) of S3 In2O3/Au NRs sample
after annealing . ( e–g ) EDX elemental mapping images of O, In, and Au of S3 In2O3/Au NRs after annealed.

Figure 3. XRD patterns of S3 In2O3/Au NRs as well as pure In2O3.

the high binding energy also can be seen, those shifts may relate to the different local environment due
to the introduced of Au. Moreover, typical XPS peaks of the core level region of Au4f can be observed
in as–prepared In2O3/Au NRs. As can be seen in Fig. 4d, Au is present only in the Au 0 state 32–34 . This
indicates the existing of Au in In2O3/Au NRs, which is consist with above analysis.

The Enhanced Gas Sensing Properties towards VOCs. The gas sensing properties of the In2O3/Au
NRs as well as pure In2O3 were carefully studied and compared, and then exhaled gas environment was
simulated to further evaluate the performance of as prepared sensors towards to practical application.

Figure 4. ( a ) Survey, ( b ) O1s, ( c ) In 3d high resolution XPS spectra of S3 In2O3/Au NRs and pure In2O3.
( d ) Au 4f high resolution XPS spectrum of S3 In2O3/Au NRs

First, the responses of the In2O3/Au NRs with different initial amount of In(NO3)3 and reaction time
compared with pure In2O3 sensors to 50 ppm acetone and ethanol gases as a function of operating temperature are exhibited in Fig. 5. It can be seen that the optimum temperature and effective response of
the gas sensors are both influenced by the atom ratio of Au/In. Note that the different reaction time has
little affluence on the sensing behavior (S5 and S6, response curves are not shown), including working
temperature and response value, due to the similar Au/In and length to diameter ratios value. For acetone gas, all the sensors exhibit a good response to acetone in the lower temperature range. Except S1
gas sensors, the responses first increase to their optimal working temperature and then decreases as the
temperature further increases. The optimal working temperature and corresponding responses for S1–S4
In2O3/Au NRs gas sensors are (215 o C, 44.3), (240 o C, 39.4), (250 o C, 36.2), and (250 o C, 22.7), respectively.
Compared to pure In2O3 gas sensor (250 o C, 16.2), the response improves gradually when the atom ratios
of Au/In increases (from S4 to S1), while the optimal working temperature shows a decreased trend only
when the atom ratio of Au/In > 3.6. For ethanol gas, compared to pure In2O3 gas sensor (400 o C, 12.6),
the optimal working temperature and corresponding responses for S1–S4 In2O3/Au NRs gas sensors are
(250 o C, 38.0), (310 o C, 35.0), (400 o C, 42.1), and (400 o C, 15.1), respectively. As can be seen, the optimal
responses mainly appear in the relatively higher temperature range and the changing trend of working
temperature is similar with that of acetone gas. While the change of response value has only a little difference when the atom ratio of Au/In is larger than 3.6.

Furthermore, the response times and recovery times of different sensors working at corresponding
optimal temperature were also evaluated in Table S1. As is listed, for acetone gas, the response time
and recovery time show decreased trend when the introduced Au amount increases at first (In2O3, S4,
and S3, respectively). However, when the Au amount further increases (S2 and S1), with the decrease
of corresponding optimal working temperature, the response time and recovery time become longer
rapidly. The changing trend of response time and recovery time for the ethanol gas is similar to that
for acetone; however, the changing range is relatively small in the higher temperature range in the case
of ethanol. The influence mechanism of the introduced Au on the sensing performance of the studied
sensors is carefully discussed later in this article. It is worth mentioning that from the view of response,

Figure 5. Dual mode responses of S1–S4 In2O3/Au NRs gas sensors compared with pure In2O3 gas
sensor for 50 ppm acetone and ethanol as a function of operating temperature.

Figure 6. Dynamic responses of S3 In2O3/Au NRs gas sensor to different acetone concentrations
(0.1–2 ppm) at 250 °C and ethanol concentrations (0.05–10 ppm) at 400 °C .

temperature difference, response and recovery time to this two gases, the as studied sensors, especially
the S3 In2O3/Au NRs sensor, have dual mode sensing properties, which can quickly detect acetone gas
at a relatively low working temperature (250 °C in our case) and detect ethanol at a higher temperature
section (400 °C in our case), and the response of S3 In2O3/Au NRs sensor is ~2.2 and 3.3 times higher
than that of pure In2O3 sensor under optimal working temperature, respectively.

Generally, the biomarker gases in exhaled breath are low in concentrations. In order to obtain more
information about the dual mode properties of In2O3/Au NRs sensor toward to exhale breath, detail tests
were conducted focused on a very low gas concentration which can meet the requirement of exhaled
gas detection. First, the dynamic responses to different concentration of acetone at 250 °C and ethanol at
400 °C were investigated. Four response and recovery curves are shown in Fig. 6 corresponding to 0.2,
0.5, 1 and 2 ppm acetone gas and 1, 2, 5 and 10 ppm ethanol gas, respectively. As depicted, the sensors
both exhibit sharply rise when acetone or ethanol gas were injected, while quickly drop to the initial
state when exposed to air, which is consistent with the sensing mechanism of n–type semiconductors.

Figure 7 shows the responses of In2O3/Au NRs and pure In2O3 gas sensors versus different acetone
and ethanol concentrations at 250 °C and 400 °C respectively. As can be seen, the sensors show a good
and wide linear relationship with the growth of the acetone and ethanol concentrations in all the studied
range. Besides, the In2O3/Au NRs sensor exhibits good resolution to low concentration gas. For acetone
detection, the In2O3/Au sensor response is below 2.5 for healthy humans (< 0.9 ppm) and above 3.7 for

Figure 7. S3 In2O3/Au NRs and pure In2O3 gas sensors response curves to different ( a ) acetone
concentrations (0.1–50 ppm) at 250 °C and ( b ) ethanol concentrations (0.05–650 ppm) at 400 °C.The dots are
experimental data and the straight lines are the linear fitting functions.

diabetics (> 1.8 ppm). This 48% response increase may allow reliable diagnosis of diabetic patients by
breath acetone monitoring. For ethanol detection, the large linear range clearly covered the different alcohol levels encountered in breath (130–260 ppm, 260–390 ppm, and 390–650 ppm in breath corresponding
to haziness, slight drunkenness, drunkenness, respectively) after drinking 35 . Furthermore, it can actually
detect as low as 0.1 ppm acetone gas with a response of 1.3 at 250 °C and as low as 0.05 ppm ethanol gas
with a response of 1.4 at 400 °C. These concentrations are much lower than the actual detection limit of
diabetes (1.8 ppm), permissible limit level for driving under the influence of alcohol (78 ppm), and even
detection limit of ethanol selective detection limit for the human sense of smell (6.1 ppm) 12,32 . Moreover,
for acetone detection, the low detection limit of acetone is calculated to be 90 ppb for In2O3/Au NRs sensor when Ra/Rg ≥ 1.2 was used as the criterion for reliable gas sensing, confirm that the as–fabricated gas
sensor is a promising candidate for the detection low concentration of acetone and ethanol biomarkers
in breath with a portable metal oxide detector.

In addition, the longtime stability of the In2O3/Au NRs compared with pure In2O3 gas sensors to
1 ppm acetone and 5 ppm ethanol were measured at 250 °C and 400 °C respectively as shown in Figure
S2. As is depicted, after 30 days, the response of In2O3/Au NRs sensor only decreased about 4% for acetone and 5% for ethanol in comparison to the original values, whereas the response of pure In2O3 sensor
deceased about 12% and 13% to acetone and ethanol, respectively. This demonstrates that the sensor
based on the In2O3/Au possesses excellent long time stability.

The Dynamic Process of the Sensors Towards VOCs. Besides, the response times and recovery
times _vs._ different acetone and ethanol concentration of pure In2O3 and S3 In2O3/Au NRs gas sensors are
listed in Table S2. As is calculated, the average response times and recovery times of S3 In2O3/Au NRs
gas sensor are 10.8 s and 18.4 s for acetone gas and 9 s and 13.4 s for ethanol gas, respectively, which are
systemically shorter than that of pure In2O3 gas sensor (14.8 s and 26.2 s for acetone gas and 9.4 s and
17.8 s for ethanol gas, respectively). This can be attributed to the introduction of the Au NPs which have
catalytic properties. In addition, the dynamic processes of ethanol of both sensors are faster than that of
acetone, this is because the higher the working temperature the faster the reaction of thermodynamics.

Actually, we have measured the response and recovery time constants at various temperatures to acetone and ethanol, respectively. As is known, the response and recovery is a reversible process. No matter
in which process, a certain potential barrier should be overcame to complete the reaction. According to
our previously work 36, the response ( _τres_ ) and recovery time ( _τrec_ ) constants can be written as a function
of temperature according to the well–known thermal activation function:

0) exp

_E_ _res_ / _K_ _B_ - _T_

_E_ _rec_ / _K_ _B_ - _T_

_τ_ _res_ = _τ_ _res_ (0) exp( _∆_

_τ_ _rec_ = _τ_ _rec_ (0) exp( _∆_

= _τ_ _res_ (0) exp( _∆E_ _res_ / _K_ _B_ - _T_ )

_res_ = _τ_ _res_ (0) exp( _∆E_ _res_ / _K_ _B_

_rec_ = _τ_ _rec_ (0) exp( _∆E_ _rec_ / _K_ _B_

0) exp( _∆E_ _rec_ / _K_ _B_ - _T_ ) (1

= _τ_ _rec_ (0) exp( _∆E_ _rec_ / _K_ _B_ - _T_ ) (1)

where _τres_ (0) and _τrec_ (0) are time coefficients, which depend on surface to volume ratio and reaction
mechanism. _KB_ is the Boltzmann’s constant and _T_ is the absolute temperature. Δ _Eres_ and Δ E _rec_ present
the forward and backward reaction barrier heights, respectively, while τ _res_ and τ _rec_ present the forward
and backward reaction time. Figure 8 show the Napierian logarithm form of dynamic time constant as
a function of the reverse of temperature for acetone and ethanol gases, respectively. It can be seen that
the response and recovery time constants both increase linearly with the decrease of temperature. By

Figure 8. Logarithm of the response and recovery time (both in s) constant versus the reverse of
temperature for ( a ) acetone and ( b ) ethanol . The dots are experimental data of S3 In2O3/Au NRs and the
lines are the linear fitting functions.

fitting, Δ E _res_ and Δ E _rec_ are deduced to be 211 and 298 meV for acetone gas, and 251 and 355 meV for
ethanol gas. As is calculated, the forward reaction barrier heights always lower than the backward reaction barrier heights, this indicated that to complete recovery processes need to overcome higher barrier
height which may lead to the recovery times always longer than the response times in our case. Besides,
the reaction barrier heights of ethanol gas are systematically higher than that of acetone, that is to say,
more energy is needed to completed the dynamic processes in ethanol environment, this may have some
relation with why the optimum operation temperature of the sensors to ethanol gas is higher than that
to acetone gas.

Selectivity and Humidity Effect in Simulated and Human Exhaled Gas Environment. The
constituent of people exhale breath gas is complicated, which composed by CO2, O2, some other VOCs
and biomarker gases, and especially high humidity levels (> 80% RH at 1 atm and 25 °C). Thus, the
selectivity and the response of the humidity environment are important in the actual breath gas measurement. Here, selectivity of In2O3/Au NRs sensor were evaluated at 250 °C and 400 °C respectively towards
to some typical biomarkers (toluene to lung cancer, formaldehyde to cardiovascular disease) in exhale
breath and compared with pure In2O3 sensor (acetone gas concentration was 1 ppm and ethanol was
5 ppm) 12,37,38 . As demonstrated in Fig. 9a,b, both two kinds of sensors possess good selectivity to detect
acetone at 250 °C and ethanol gas at 400 °C, especially the In2O3/Au NRs sensor, which shows very low
response or almost insensitive to the other typical interference gases at the same temperature.

The interference of humidity is an important problem that must consider in breath sensing, because
the VOCs signals could be screened by the high humidity levels in breath, and the small fluctuations
in humidity also have big effect on the sensitivity of VOCs 39 . Here, the influence of the high humidity
level was considered in our research. Figure 9c,d shows the curve of Ra/Rwet response where Rwet is the
resistance of sensor at a given RH environment versus RH ambience. Usually, Rwet is interpreted as the
interaction between the surface adsorbed oxygen and the water molecules which induces a decrease in
baseline resistance of the gas sensor. It can be seen that Ra/Rwet value of both In2O3/Au NRs and pure
In2O3 sensors rise slightly in the as studied RH range (25%–94%), In the studied RH range, the Ra/Rwet
value of In2O3/Au sensor only changed from 1.0 to 2.8 at 250 °C and 1.0 to 3.4 at 400 °C compared to
pure In2O3 sensor that changed from 1.0 to 1.6 at 250 °C and 1.0 to 2.7 at 400 °C. The In2O3/Au sensor
shows a little larger affected than pure In2O3 sensor due to the sensitized effect of Au.

Since the as prepared In2O3/Au sensor exhibits good dual mode sensing properties towards to very
low acetone and ethanol concentration as studied above, the response in simulated exhaled gas environment was further investigated and compared, as shown in Fig. 10. In this work, in order to meet the
requirement of the volume fraction of the main components gas is 78% for N2, 17% for O2, 4% for CO2,
and 1% for other gas, the simulated exhaled gas environment was built by mixed ambient atmosphere
with a certain amount of CO2 and different concentration of acetone (250 °C) or ethanol gas (400 °C) in
high RH surroundings, moreover, take into consideration the high humidity and individual difference in
exhaled gas environment, 93.5 ± 1% RH (sim1) and 75.5 ± 1% RH (sim2) conditions were also provided.

As depicted in Fig. 10, compared to the response in atmospheric air (Ra/Rg), the responses in simulated exhaled gas environment (Ra/Rsim) of S3 In2O3/Au NRs sensor shows gradually increased trend
with the increasing of the RH value, because the sensor almost has no response to additional CO2, the

Figure 9. Selective tests of S3 In2O3/Au NRs compared with pure In2O3 gas sensors for ( a ) 1 ppm
toluene, ethanol, and formaldehyde at 250 °C and ( b ) 5 ppm formaldehyde, toluene, and acetone at 400 °C .
Ra/Rwet responses of S3 In2O3/Au NRs and pure In2O3 gas sensors versus RH ambience at ( c ) 250 °C and
( d ) at 400 °C.

Figure 10. S3 In2O3/Au NRs gas sensor response curves to different ( a ) acetone concentrations
(0.2–2 ppm) at 250 °C and ( b ) ethanol concentrations (1–10 ppm) at 400 °C under simulate gas atmosphere .
sim1:93.5 ± 1% RH and sim2:75.5 ± 1% RH. The open circles are the compensated data.

main reason can be attributed to the effect of high RH conditions. Besides, all the Ra/Rsim responses in
simulated exhaled gas environment of In2O3/Au NRs sensor show good linearity on the whole, and the
slops of the three curves of acetone at 250 °C and ethanol at 400 °C are independent of the RH level,
indicating the response to the VOCs is not effected by the background humidity. In this case, a humidity
compensation method could use to increase the accuracy of the sensor response in the environment of
high humidity levels or fluctuations in humidity, which has been proved to be effective 40 . In this method,
the humidity compensation parameter can be extracted by deducting the humidity background value
when there is no target VOCs based on the estimated sensor response in Fig. 10. Note that because the
ambient humidity is 25% RH in our test environment, all the response signals are compensated to 25%
RH level. As can be seen, after applying compensation, the relative effect of humidity can be counteracted
(expressed by the open circle in Fig. 10), and the responses to a specific VOCs concentration are mainly
located at the same place, indicating the effect of humidity variations could be effectively eliminated
through this humidity compensation method in our case.

Furthermore, the clinic test were conducted to test the sensing properties of S3 In2O3/Au NRs sensor
to the VOCs in human breath and the RH compensation method also was applied. For acetone detection,

Figure 11. The compensated clinic test data to ( a ) diabetic and healthy volunteers, ( b ) ethanol response
curves after drinking white wine of three healthy volunteers.

1 diabetic and 6 healthy volunteers participated in the test. As shown in Fig. 11, the response data were
obtained by sampling one time for each healthy volunteer and six times for diabetes volunteer. The
results show that the sensor response is about 8. of diabetic, while 5.1 of healthy person. All the response
values are little higher compared to that in simulation environment; the reason can be attributed to
the complex environment in exhaled breath. For ethanol detection, the corresponding response vales
were obtained by sampling the breath of 3 healthy volunteers who have drink white wine (42% vol) in
amounts calculated to provide 0.15–0.20 g white wine per kg body weight. As exhibited in Fig. 11b, the
volunteers started to drink the white wine at zero point, and we continuous monitoring the variation of
sensor response up to 125 min, in the whole range, the responses showed a downward trend the drink
consumed quickly within 60 min. This results are consistent with the previous study about ethanol concentrations in exhaled breath 41 .

Mechanism of the Gas Sensor. Based on the above analysis of gas sensing characteristic, the sensing
mechanism of both In2O3/Au and In2O3 based sensors could be modeled using the surface–controlled
model 42,43 . Generally, when the In2O3 sensor is exposed to air, it interacts with oxygen by transferring
electrons from the conduction band to the adsorbed oxygen atoms, forming reactive oxygen species (O2ˉ,
O 2 ˉ, and Oˉ). The reaction can be expressed as:

_O_ 2( _gas_ ) → _O_ 2( _adsorbed_ ) (2)

_O_ 2( _adsorbed_ ) + _e_ ˉ → _O_ 2( _adsorbed_ )ˉ (3)

_O_ 2( _adsorbed_ )ˉ + _e_ ˉ → 2 _O_ ( _adsorbed_ )ˉ (4)

_O_ ( _adsorbed_ )ˉ + _e_ ˉ → _O_ ( _adsorbed_ ) 2ˉ (5)

Based on the above reactions, a depletion layer is created due to the consumption of the electron in
the surface region of the In2O3 sensor. This leads to the increase of the resistance in the In2O3 sensor.
When the In2O3 sensor is placed into the reducing gas (acetone or ethanol in our case), the reactive oxygen ions will react with the analysis gas molecules and then the electrons are released back to the In2O3
sensor, resulting in the thinning of the depletion layer and the decrease of the resistance of the In2O3
sensor 44 . The above explanation can be expressed by the following equations:

( _gas_ ) + 3H2O( _gas_ )

CH3COCH3( _gas_ ) + 8O( _adsorbed_ )− → 3CO2( _gas_ ) + 3H2O( _gas_ ) + 8e−

and CH CH OH + 6O - → 2CO + 3H O + 6e−

+ 8O( _adsorbed_ )− → 3CO2( _gas_ ) + 3H2O( _gas_ ) +

( _gas_ ) + 8O( _adsorbed_ ) → 3CO2( _gas_ ) + 3H2O( _gas_ )

3COCH3( _gas_ ) + 8O( _adsorbed_ ) → 3CO2( _gas_ ) + 3H2

( _gas_ ) + 8O( _adsorbed_ )

( _gas_ ) + 3H2O( _gas_ )

and CH3CH2OH( _gas_ ) + 6O( _adsorbed_ )− → 2CO2( _gas_ ) + 3H2O( _gas_ ) + 6e− (6

+ 6O( _adsorbed_ )− → 2CO2( _gas_ ) + 3H2O( _gas_ ) + 6e− (6)

_gas_ ) + 6O( _adsorbed_ ) → 2CO2( _gas_ ) + 3H2O( _gas_ )

3CH2OH( _gas_ ) + 6O( _adsorbed_ ) → 2CO2( _gas_ ) + 3H2

( _gas_ ) + 6O( _adsorbed_ )

Obviously, the sensor performance will be strongly dependent of the amount of ion sorption of oxygen. In general, at low temperature, active sites at the semiconductor surface are rare and the amount
of ion sorption of oxygen is deficient. These can lead to incompletely reaction of analysis gas. So the
response is poor at low temperature. With increasing the working temperature, the ion sorption of

oxygen on the sensor surface quickly increases, and the adsorbed oxygen gives rise to Schottky potential
barriers at grain boundaries, and thus increases the resistance of the sensor surface. When reducing
gases are introduced, the response will increase. While, further increasing the working temperature, the
charge-carrier concentration and the conductivity increase and the Debye length decreases, which leads
to the decrease of response. In fact, the response process of a semiconductor sensor on temperature is
very complex. Its effect factors include the rates of adsorption and desorption of oxygen and reducing
gases, the rate of surface decomposition of reducing gases, the charge carrier concentration, and the
Debye length in the semiconductor 45 . Mutual effect makes different gases have characteristic optimum
oxidation temperatures. Thus, the as studied gas sensors, especially S3 In2O3/Au NRs, exhibits good dual
mode sensing properties both to acetone and ethanol gases in this work.

When Au are introduced, the uniform distributed Au in In2O3/Au NRs could promote the reactive oxygen species as chemical sensitizer and the most probable mechanism leading to improve sensor
response is chemical sensitization via spillover effect 23,46,47 . Ionsorptions of oxygen ions can occur on the
Au NPs surface due to the highly conductive nature and availability of free electrons in gold. Then the as
created activated oxygen species are spilled onto the metal oxides surface and interact with the absorbed
oxygen. This consists with the increasing deficient oxygen atomic ratio percentages after introducing of
Au, as studied in XPS (Fig. 4b). This process has been considered to result in greater and faster reaction
between analytic molecules and adsorbed oxygen in a certain range (as shown in Fig. 5 and Table S1).
Note that the oxygen molecules can briefly diffuse to a catalyst metal NPs before it has an opportunity
to be absorbed 48 . Thus, this “chemical sensitization” mechanism is considered to be the predominant
mechanism responsible for enhancement of the gas sensing properties by adding the Au NPs. Further the
XPS binding energy peak positions also proved the existence of the Au/In2O3 interaction. The shift of the
In 3d peak to the higher binding energy is attributed to the interaction of In2O3 with Au and then leads
to a modification of the surface electronic structure. This brings out the second role of Au as modulator
of the surface electronic structure. So in our studies, the response value and response dynamic process
are gradually increased with the increasing of the amount of Au at first (pure In2O3, S4, and S5). When
Au amount further increases, the dynamic process slows down, this may attributed to the change of
corresponding optimal working temperature.

Generally, at low temperature most semiconductor metal oxides gas sensor materials are insulators
rather than semiconductor, thus the gas sensing sensitivity is very poor. After the introduction of Au,
ionsorption of oxygen ions can occur on the metal NPs surface at a low temperature (even at room temperature) due to the highly conductive nature, availability of free electrons, and chemical sensitization in
Au 48 . That is the reason that the optimal working temperature are gradually decreased by increasing the
amount of the Au (Fig. 5), and this effect is more obvious when the introduced amount of Au is relatively large (atom ratio of Au/In > 3.6% in our case). As is known, the dynamic process slow down with
the decrease of working temperature due to the Eqs (2)–(6) are related to the thermodynamics. In our
case, when the introduced Au amount is low, it has little influence on the optimal working temperature.
When the Au amount further increases, the optimal working temperature decreases, however, because
more ionsorption of oxygen ions can occur on the Au NPs surface, the corresponding response values
can have little change (ethanol gas) or even increasing (acetone gas). However, because of the decrease
of working temperature, the response time and recovery time become longer.

However, despite the semiconductor oxide based gas sensors exhibited many advantage which meet
the criteria of an ideal breath sensor, such as high response to low concentrations of VOCs, fast dynamic
process, low-cost, simple and inexpensive to disposable 13, it is still a long way to apply these sensors on
breath analysis in clinical diagnose. How to avoid the effect of humidity and improve the accuracy of
specific VOCs detection against other interference specie needs further study. Since one disease may
corresponding to more than one biomarkers in exhaled breath 14, following the idea of this work, the
dual mode detection may help to increase the accuracy of VOCs detection in semiconductor oxide based
gas sensors.

Conclusions
In summary, In2O3/Au NRs as well as pure In2O3 were fabricated using a facile co–precipitation technique. It is believed that the initial In(NO3)3 amount and reaction time have little effect on the morphology of In2O3/Au NR and the Au NPs were uniformly dispersed in In2O3/Au NRs. The dual mode gas
sensing properties of different In2O3/Au NRs sensors compared with pure In2O3 were carefully studied,
especially for very low concentration of acetone and ethanol. The results demonstrate that the optimal S3
In2O3/Au NRs sensor can effectively detect acetone at 250 °C and ethanol at 400 °C, which has response
of 2.8 to 1 ppm acetone, and 9.8 to 5 ppm ethanol and low actual detection limit of 0.1 ppm to acetone
and 0.05 ppm to ethanol. Besides, a humidity compensation method is applied to increase the accuracy
of the sensor response in high humidity environment. Moreover, clinic tests were also performed, indicating that the In2O3/Au NRs sensor has the ability to actual detection acetone and ethanol biomarkers
in exhaled breath. The present work suggests that the “spillover effects” of introduced Au NPs increased
the activity oxygen species of In2O3 NRs and endowed In2O3/Au NRs gas sensor with enhanced dual
mode gas sensing properties.

Materials and methods
Synthesis of Au NRs. All the experiment reagents were used as received and without further purification. The Au NRs were prepared following the seed–mediated growth method reported by EI–Sayed and
co–workers 49 . Brief descriptions were as follows: 1) Preparation of the seed solution. 2.5 mL of HAuCl4
(1 mM aqueous) and 7.4 mL of cetyltrimethylammonium bromide (CTAB, 0.1 M aqueous) were mixed
together, followed by the injection of 0.6 mL freshly prepared NaBH4 (10 mM aqueous) solution and
magnetic stirring at 28 °C for 2 h. 2) Preparation of the Au NRs. In a flask, 13.2 mL of CTAB (0.1 M aqueous) was mixed with 12 mL of HAuCl4 (1 mM aqueous), and then 240 μ L of AgNO3 (10 mM aqueous)
and 220 μ L of HCl (2 M aqueous) were added. After mixing, 192 μ L of ascorbic acid (AA, 0.1 M) was
added. While continuously stirring this mixture, 120 μ L of the as–synthesized seed solution was added to
initiate the growth of Au NRs. Then, these NRs were kept at 28 °C for 5 h to ensure full growth. Finally,
after centrifuging at 13 000 rpm for 15 min, the upper liquid containing excess CTAB was removed, and
NRs were left over in 40 mL deionized water.

Preparation of In2O3/Au NRs samples. The In2O3/Au NRs were synthesized through co–precipitation method and annealed. First, various amounts (0.1 g, 0.15 g, 0.2 g and 0.3 g) of In(NO3)3 were dissolved in 10 mL of deionized water which contained 1.8 g urea. After stirring, 20 mL of the as–synthesized
Au NRs aqueous solution as motioned above was added. Then, the mixed solution heated to 80 °C and
kept refluxing for 2 h in the oil bath. After self–cooling down to room temperature, the reaction product
was separated by centrifugation and washing with deionized water for three times, then further dried at
60 °C for 6 h. At this time, the precursor NRs of [Au@In(OH)CO3] was obtained. Finally, the as–synthesized products were annealed in a tube furnace with a rising rate of 2.5 °C/min from room temperature to
600 °C and maintained for 3 h at 600 °C to obtain the In2O3/Au NRs samples. For comparison, pure In2O3
was also synthesized as follows, 0.2 g In(NO3)3 was dissolved in 30 mL of deionized water solution containing 1.8 g urea and no Au NRs were added, and the other processes were just the same with the above.

Characterization. The morphology of the samples was inspected using JEOL JSM–7500F field emission SEM (Japan) with accelerating voltage of 15 kV and gold sputtering on surface. TEM and high
resolution TEM (HRTEM) images were recorded on JEM–2010 transmission electron microscope under
a working voltage of 200 kV. The phase structure of the samples were characterized by XRD; XRD patterns were conducted on Rigaku D/max 2550 using a monochromatized Cu target radiation resource
(λ = 1.5045 Å) and the corresponding lattice constants of the samples were calculated by MDI Jade 5.0
software. XPS were conducted on an ESCAlab250 Analytical XPL Spectrometer with a monochromatic
Al KR source. All the binding energies were referred to the C1s peak at 284.7 eV of the surface adventitious carbon. The fitted peaks in XPS spectra were deconvoluted using the XPS Peak 4.1 software.

Fabrication and measurement of gas sensing properties. The as–synthesized samples were
mixed with ethanol in the weight ratio of 5:1 to form a paste. The paste was coated on a ceramic tube
on which a pair of gold electrodes was previously printed. A small spring–like Ni–Cr alloy was inserted
into the ceramic tube to provide the operating temperature. After the solvent was evaporated, the ceramic
tube with samples thin layer was sintered in an oven for 2 h at 400 °C. After sintering, the gas sensors
were thermal aged with a heating voltage of 5 V at the ageing equipment for 6 days before the first
measurement.

The gas–sensing properties were measured on a WS–30A system (Weisheng Instruments Co.,
Zhengzhou, China) and determined under laboratory conditions (25 ± 1RH%, 26 ± 2 °C).The response
is defined as S = Ra/Rg (Ra and Rg are the resistance for sensors in air and in target gas, respectively.).
The response and recovery times are defined as the time required reaching 80% of the final equilibrium
value, and the details of the measurement are similar to those reported in the literature 50 . To clinic test,
the sampling gas were collected in an air pocket (2L).

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Acknowledgements
This work was supported by NSFC (Grant no. 61204015, 81301289, 61177042), Program for Chang Jiang
Scholars and Innovative Research Team in University (No. IRT13018). The China Postdoctoral Science
Foundation Funded Project (2012M511337 and 2013T60327), the State Key Laboratory of Bioelectronics
of Southeast University, the Jilin Province Natural Science Foundation of China (No. 20140101171JC),
and Graduate Innovation Fund of Jilin University (No.2014060).

Author Contributions
R.Q.X. conducted the most of investigation for the samples and wrote the main paper. L.X. supervised
the project, had given valuable advices on the proceeding of this work, and revised the manuscript.
Q.L.D. and H.W.S. had provided precious suggestions on the selection of test. J.S. C.Y.Z. and Q.L.L.
supported the characterization of the samples. All authors discussed the results and commented on the
manuscript at all stages.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

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

How to cite this article : Xing, R. _et al._ Preparation and Gas Sensing Properties of In2O3/Au Nanorods
for Detection of Volatile Organic Compounds in Exhaled Breath. _Sci. Rep._ 5, 10717; doi: 10.1038/
srep10717 (2015).

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