A sensitive cataluminescence-based sensor using a SrCO3/graphene composite for n-propanol

Article abstract of DOI:10.1039/c5ra06632h

In this paper, we developed a cataluminescence-based sensor using SrCO₃/graphene for sensitive and selective detection of n-propanol. The composite was characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and gas adsorption. The sensor was coupled with a miniature vaporizing device to detect n-propanol in liquid samples. The experimental results revealed that the SrCO₃/graphene sensor exhibited a sensitivity for n-propanol 5.8 times higher than that of pure SrCO₃, indicating that the sensitivity of the SrCO₃/graphene sensor was increased by adding graphene to SrCO₃. The linear range of the sensor was 0.2 to 32 mg L^-1 (r = 0.9987) with a limit of detection of 0.08 mg L^-1. The sensor showed a rapid response of 2 s and a recovery time of 20 s, respectively. The sensor was used to analyze samples spiked with known concentrations of n-propanol. The concentrations of n-propanol in all samples were well quantified with satisfactory recoveries, indicating that the SrCO₃/graphene sensor is a promising candidate for fast, sensitive, selective detection of n-propanol. We also discuss the possible mechanism based on the reaction products.

Full text of DOI:10.1039/c5ra06632h

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A sensitive cataluminescence-based sensor using
SrCO3/graphene composite for n-propanol
Qianchun Zhang*, Feifei Meng, Lin Zha, Xingyi Wang and Guoyi Zhang
Abstract: In this paper, we developed a cataluminescenceꢀbased sensor using
SrCO3/graphene for sensitive and selective detection of nꢀpropanol. The
characterization the composite were performed by Xꢀray diffraction (XRD),
transmission electron microscopy (TEM), fourier transform infrared (FTꢀIR) spectra
and gas adsorption instrument. The sensor was successfully applied to sensing
nꢀpropanol in liquid samples by coupling a miniature vaporizing device with the
sensor cell. The experimental results revealed that the SrCO3/graphene sensor
exhibited about 5.8 times higher sensitivity than that of pure SrCO3, indicating that
sensitivity of SrCO3/graphene sensor for nꢀpropanol detection was enhanced by
adding graphene into SrCO3. The linear range of the fabricated sensor was 0.2 to 32
mg Lꢀ1 (r = 0.9987) with a limit of detection (LOD) of 0.08 mg Lꢀ1. The sensor
showed fast rapid response and recovery times of 2 s and 20 s, respectively. The
sensor was used to analyze samples spiked with known concentrations of nꢀpropanol.
The concentrations of nꢀpropanol in all samples were well quantified with satisfactory
recoveries, thereby indicating the SrCO3/graphene based sensor was a promising
candidate for fast, sensitive and selective detection of nꢀpropanol. The possible
School of Biology and Chemistry, Xingyi Normal University for Nationalities, Xingyi, 562400,
People’s Republic of China
* Corresponding author: Tel: +86ꢀ589ꢀ3296359; Fax: +86ꢀ859ꢀ3527567
Eꢀmail: qianchunzhang@qq.com
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mechanism was also discussed on the basis of the reaction products.
1. Introduction
Graphene with a twoꢀdimensional monolayer of fused sp2 carbon bonds in a
honeycombꢀlike network, has attracted a great deal of scientific interest in the area of
chemical gas sensing, which is mainly attributed to its outstanding mechanical,
electrical, thermal, and optical properties and large specific surface area.1ꢀ4 Graphene
is an ideal choice for decorating with other materials, including metal oxides, metal
sulphides and metal nanoparticles for improved properties.5ꢀ7 It was reported that the
mixing of graphene into other materials not only can enhance the sensitivity and
improve the selectivity of the composite, but also can reduce the operating
temperature.8ꢀ10 For example, Singh et al. reported that the sensors based on ZnO
decorated graphene can sensitively detect the industrial gases such as CO, NH3 and
NO at room temperature.11 Liu et al. have reported that mixing of graphene into
ZnFe2O4 can reduce the operating temperature of the sensors for acetone.12
Cataluminescence (CTL) is an interest luminescent emission produced during the
catalytic oxidation reaction of analyte molecule on the surface of solid catalyst.13ꢀ15
CTL has attracted a great deal of research interest in the development of chemical
sensor for the determination and discrimination of analytes.16ꢀ19 The CTLꢀbased gas
sensor possesses many outstanding advantages such as good selectivity, high
sensitivity, rapid response speed, good reproducibility and simple instrument.20ꢀ22 The
sensing element of CTL sensor is solid catalyst that is inconsumable during the
sensing process, leading to this kind of chemical sensor has a longꢀterm stability.23ꢀ24
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To date, a lot of different kinds of materials have been applied to the development of
CTLꢀbased sensor. Examples include an isoꢀbutanol sensor based on Yꢀdoped
metalꢀorganic frameworkꢀ5,25 an nꢀhexane sensor based on Zeolite,26 a nanosized
ZrO2ꢀbased sensor for propionaldehyde,27 and nanoꢀ3TiO2–2BiVO4ꢀbased sensor for
simultaneous determination of benzene and formaldehyde.28 Lv’ group reported a
facile hydrothermal assisted in situ synthesis route for the preparation of graphene
sheets decorated with SnO2. The SnO2/graphene composite was found to be a highly
efficient material for CTL sensor to propanal.29 This research group also reported a
facile catalystꢀfree atmospheric pressure chemical vapour deposition method for the
growth of hierarchical SnO2 architectures on graphene.7 It was found that the
materials prepared by this method showed enhanced CTL response to methanol and a
morphologyꢀdependent CTL performance.
Nꢀpropanol is a colorless, flammable, fragrant liquid with moderately toxic, and
is ubiquitous in nature. It is an important solvent principally used in printing,
cosmetics, making leather product, and pesticides.30 In addition, nꢀpropanol is the
major higher alcohols found in alcoholic beverages for it is widely used as flavor
volatile in food and beverage manufacturing.31 However, exposure to nꢀpropanol can
irritate the skin causing a rash or burning feeling on contact, and higher exposures can
cause headache, dizziness, confusion, nausea and vomiting, and even liver damage.32
Moreover, nꢀpropanol is a flammable liquid and a dangerous fire hazard that will
cause an explosion when it meets open flame. Recently, increasing concerns have
been focused on the levels of higher alcohols include nꢀpropanol in surrogate alcohol
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(i.e., illicit or homeꢀproduced alcoholic beverages) that might result in an increased
incidence of liver diseases in regions where there is a high consumption of such
beverages.31 Therefore, the development of sensor for nꢀpropanol has wide
application in environmental monitoring and foodstuff control.
In this work, we found that the SrCO3 doped with 12 wt% graphene exhibited
about 5.8 times CTL response to nꢀpropanol than that of pure SrCO3. A CTLꢀbased
sensor using SrCO3/graphene was designed for sensitive and selective detection of
nꢀpropanol. Although graphene has been wildly used in the design of electrochemical
sensors, only a few studies on its applications in CTL sensors were reported.
Moreover, the previous CTLꢀbased sensors were usually used for gaseous sample
analysis. In order to expand of the detectable samples, a miniature vaporizing device
was couple with the sensor cell for analysis of nꢀpropanol in liquid sample. In this
context, this paper was focused on the liquid sample sensing. The performance of the
sensor was evaluated systematically in terms of response and recovery times,
selectivity to nꢀpropanol, and stability. The factors influencing the sensor for the
determination of nꢀpropanol were optimized in detail. The potential application of the
sensor was demonstrated by the determination of nꢀpropanol in spiked samples.
Results showed that the sensor provided a simple, rapid, sensitive way for the
determination of nꢀpropanol.
2. Experimental
2.1Chemicals and materials
All chemicals used in our experiments were of analytical grade. Methanol,
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ethanol, ethyl acetate, nꢀhexane, formaldehyde, carbon tetrachloride, chloroform,
propionaldehyde , acetaldehyde, 2ꢀpropanol, acetone were purchased from Damao
Chemical Reagent Company (Tianjin, China). Nꢀheptane, nꢀoctane and isoꢀ octane
was supported by Fuyu Fine Chemical Co. Lit. (Tianjin, China). Sulfuretted hydrogen
(500 ppm) was purchased from Zhandong Gas limited company (Chongqing, China).
Graphite powder (particle size 300~500 nm) was purchased from Alfa Aesar. SrCO3
was supplied by XinxinYuan Chemical Co., Ltd. (Sichuan, China). Other chemicals
were obtained from Aladdin Chemical Co., Ltd. (Shanghai, China).
2.2 Materials preparation and characterization
Graphene was synthesized by hydrazine reduction of graphene oxide (GO). The
GO was prepared by the oxidation of graphite powder with H2SO4/KMnO4 according
to the modified Hummer’s method,33ꢀ34 which resulted in waterꢀsoluble GO. Then, 0,
3, 12, 24, 30, 36, 45 and 54 mg of GO was dispersed in distilled water under
ultrasonication condition to form colloidal suspensions. Subsequently, 0.3 g of SrCO3
powder was direct added to prepare composite contains different mass percent of
graphene. After 30 min of ultrasonic treatment, hydrazine monohydrate was added
into these solutions, then ultrasonication for 30 min, followed by reflux at 90 ◦C for 6
h to ensure the complete reductionof GO. The solid products were collected by
filtered through 0.45 mm filter and washed with distilled water until its filtrate
became neutral. The precipitate was dried at 80 ◦C for 24 h under vacuum.
The morphologies of the materials were characterized with a transmission
electron microscope (TEM, Tecnai™G2 Spirit) at an accelerating voltage of 120 kV.
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Xꢀray power diffraction (XRD) experiments were carried out in a ThermoꢀVG
Scientific ESCALAB 250 diffractometer. Fourier transform infrared (FTꢀIR) spectra
were measured by Nicolet Avatar 330. Surface area measurements were performed on
an ASAPꢀ2020M gas adsorption instrument (Micromeritics, Atlanta, USA) at 77K.
2.3 CTL sensor fabrication and measurement
Fig. 1 shows the schematic diagram of the fabricated sensor device. 0.1 g of
SrCO3/graphene composite was deposited onto a cylindrical ceramic heater (length=
10 cm, diameter=0.6 cm) to form a catalyst layer. The ceramic heater was linked up
with a voltage controller (Power , Model: TG1783SL3A, Tonggao Electronic Co.,
Ltd., Ningbo, China). The temperature of the ceramic heater can be controlled by
adjusting the output voltage of the voltage controller. The CTL sensor cell was
constructed by inserting the ceramic heater into a homeꢀmade quartz tube (length= 9
cm, inner diameter=1 cm, outer diameter=1.2 cm) with inlet and outlet. A miniature
PTC vaporizing device (power: 60 W, working voltage: 24 V, length of central
cylinder=2.5 cm, diameter of central cylinder=2 cm, length of cylinder joint=1.5 cm,
diameter of cylinder joint =0.6 cm, customized from Dongsheng Electronic Heating
and Appliance Factory, Taizhou, China) was coupled with the sensor cell. The
vaporizing device was linked up with another voltage controller (Power ). In the
present work, 5 µL of liquid sample was injected into the vaporizing device. The
liquid sample was vaporized immediately once it was injected into the vaporizing
device, and then was driven towards the sensor cell by an airstream supplied by an air
pump. A BPCL Ultra Weak Chemiluminescence Analyzer (Biophysics Institute of
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Chinese Academy of Science, China) equipped with a photomultiplier (PMT) was
used to record the CTL intensity. The detecting wavelengths could be selected over
the range of 350–550 nm by changing the optical filters.
The data acquisition time for each signal point was set as 0.5 s, and the voltage
for the photomultiplier tube was 850 V. The data was recorded with a computer and
further processed with OriginPro. The response of the nꢀpropanol sensor is defined as:
S=IꢀN
(1)
Where S is the real response of the sensor, I stands for the recorded CTL
intensity, and N is the background noise. The signal to noise (S/N) at 3 was used to
reflect the limit of detection (LOD) of the sensor.
2.4 Instrumental analysis for mechanism study
In order to explore the possible reaction mechanism behind the nꢀpropanol on the
SrCO3/graphene composite, an Agilent 7890A GCꢀMS equipped with a HPꢀINNO
Wax column (30 m, 0.25 mm inner diameter, and 0.25ꢁm film thickness) were used to
analyze the products from the catalytic oxidation. The exhaust gas from the catalytic
oxidation was collected into a sampling bag, and then 200 µL of the sample was
injected into the instrument.
3. Results and discussion
3.1 Characterization of sensing material
Fig. 2a shows the FTꢀIR spectra of pristine graphite powder, GO, graphene,
SrCO3 and SrCO3/graphene. The spectra reveal that after oxidation of graphite, the
GO contains several functional groups includes OH (3432 cm−1), COOH (1724 cm−1),
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and C=O (1625 cm−1). Upon reduction of graphene oxide to graphene, the C=O band
disappears and new bands at 2988 and 2865 cm−1 arise representing the CꢀH stretch
vibrations of the methylene group. Similar results have been reported by Naebe et
al.35 The band at 1474 of IR spectrum for SrCO3 corresponds to the asymmetric
stretching mode of CꢀO bond. The sharp peaks at and 857 and 702 cm−1 are in plane
2ꢀ
and out plane bending CO3 . The peaks of graphene are almost cannot be observed in
the IR spectrum for SrCO3/graphene, possibly due to less active group of graphene
and its content is relative low in the SrCO3/graphene composite.
Fig. 2b shows the XRD patterns of pristine graphite powder, GO, graphene,
SrCO3 and SrCO3/graphene composite. A sharp intensive peak observed at the (002)
diffraction line (dꢀspacing of 0.35 nm at 26.35°) and a weak (004) diffraction peak at
54.6° is highly specific for the nature of graphite powder. GO shows a diffraction
peak at 2θ of 10.6° corresponding to a dꢀspacing of 0.8 nm. The diffraction peak at
about 10.6° of GO is weaker than that of pristine graphite at about 26.35°, which
confirms the successful oxidation of pristine graphite to be converted to GO.
Compared with the pristine graphite, the (002) peak of graphene is decreased in its
intensity and the distance is enlarged. The XRD pattern of SrCO3 is in good
agreement with orthorhombic SrCO3 (strontianite, JCPDS: 05ꢀ0418). The XRD peak
of SrCO3/graphene composite indicates the intensity of SrCO3 peak is low as
compared to the characteristic peaks of pure SrCO3 because of low content and low
diffraction intensity of SrCO3, while the peaks of GO is disappeared. Xu et al. have
reported that if regular stacks of graphite oxide or graphite are destroyed, for example,
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by exfoliation, then their diffraction peaks become weak or may even disappear.36
Therefore, the disappearance of the peaks of graphene can be attributed to exfoliation
of GO sheet.
Fig. 3 shows TEM images of the graphene, SrCO3 and SrCO3/graphene
composite, respectively. As shown in Fig. 3a, a mixture of few layer graphene with
flakesꢀlike structures is observed to be situated on the top of the copper grid. The
TEM micrograph of SrCO3 shown in Fig. 3b indicates that the SrCO3 is orthogonal
crystal, which is consistent with the result of XRD experiment. It can be seen from the
Fig. 3c that the SrCO3 is decorated successfully with graphene, and is partially
intercalated between two graphene sheets. The specific surface areas of SrCO3,
graphene and SrCO3/graphene are measured by N2 (77.4 K) adsorption and desorption
isotherms. The results show that the specific surface areas of SrCO3, graphene and
SrCO3/graphene are 37, 786 and 104 m2 gꢀ1, respectively. An obvious enhanced
specific surface area of SrCO3/graphene composite was observed compare with pure
SrCO3.
3.2 Optimization
3.2.1 Doping concentration
We found that nꢀpropanol cannot produce CTL response on pure graphene, but
can produce CTL response on SrCO3, the addition of graphene into SrCO3 can
enhance the sensitivity for nꢀpropanol. These results demonstrate that SrCO3 is used
as a catalyst while graphene is used as a sensitizer during the CTL detection of
nꢀpropanol. Fig. 4a shows the CTL responses for nꢀpropanol using SrCO3 and SrCO3
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doped with 12 wt% graphene as sensing materials. The CTL response for nꢀpropanol
on SrCO3/graphene composite is about 5.8ꢀfold stronger than that of pure SrCO3. The
relative standard deviation (RSD) of five times responses for nꢀpropanol on
SrCO3/graphene composite is 3.4%, indicating that the SrCO3/graphene composite is
an optional sensing material with stable reproducibility for the determination of
nꢀpropanol .
Fig. 4b shows the CTL intensity versus SrCO3 with different doping
concentrations of graphene. It can be seen that the CTL intensity increases
monotonically with increasing doping concentrations of graphene before 12 wt%. It is
clear that SrCO3 doped with 12 wt% graphene exhibits the best response, and was
subsequently chosen for further sensing characterizations.
3.2.2 Vaporized and detecting temperature
Fig. 5a shows the CTL signal versus different vaporized temperatures under fixing
detecting temperature of 245°C condition. It can be seen that the optimal vaporized
temperature is 120 °C. We found that the liquid sample containing nꢀpropanol could
not be completely vaporized at low temperature resulted in broad peaks on the
response curves. However, as Fig. 5a shows, the CTL intensity decreases with
increasing of vaporized temperature above 120 °C, possibly due to the nꢀpropanol was
already decomposed in part before reaching the sensor cell.
Fig. 5b shows the CTL intensity, background noise and S/N versus detecting
temperature under fixing vaporized temperature at 120 °C condition. It can be seen
that the CTL signal increases with the increasing of detecting temperature and reaches
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to its maximum at 258 °C. However, the background noise that mainly results from
the thermal radiation of the heated sensing material also increases with increase of
temperature, which leads to S/N decreasing significantly at high temperature
Therefore, the temperature of 245 °C was chosen as the optimum detecting
temperature since the S/N ratio was reaches its maximum at this temperature.
3.2.3 Wavelength and flow rate
Fig. 6a shows the CTL intensity, background noise and S/N versus with
wavelength at a flow rate of 260 mL minꢀ1. Although the strongest emission is
observed at 440 nm, the background noise increases dramatically with the increase of
wavelength, leading to the maximum S/N is observed at 425 nm. Therefore, 425 nm
was selected as the optimum wavelength for the quantitative detection of nꢀpropanol.
Fig. 6b shows the change trend of the CTL intensity versus flow rate at 425 nm.
The result shows that the catalytic oxidation reaction of nꢀpropanol is controlled by
diffusion rate below 260 mL minꢀ1, which is the total reaction rate is controlled by the
rate of the transfer of nꢀpropanol molecular from the gas phase to the catalyst surface,
leading to the CTL intensity is proportional to flow rate. However, the total reaction
rate of nꢀpropanol is limited by the oxidation rate of nꢀpropanol on the catalyst surface
when the flow rate above 260 mL minꢀ1, resulting in CTL intensity is independent of
the flow rate. Therefore, the flow rate of 260 mL min− 1 was chosen as the optimal
flow rate for the subsequent work.
3.3 The performance of the sensor
3.3.1 Response and recovery times
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The response time and recovery time are important indicators for evaluating the
performance of the sensor for rapid detection. The response time is defined as the
time needed to reach the maximum value from the injection, and the recovery time is
defined as the time needed to decay to the background noise from the maximum value.
Fig. 7a shows the response of the sensor to different concentrations of nꢀpropanol
from 6.4 to 16 mg Lꢀ1. It can be seen that the CTL response profiles are similar to
each other. For all three concentrations of nꢀpropanol, the maximum signals were
achieved at around 2 s after injection, indicating a rapid response of the sensor to
different concentrations of nꢀpropanol. We can see that the recovery time increases
slightly with the increasing of the nꢀpropanol concentration, and the recovery time of
the sensor to nꢀpropanol at a concentration of 16 mg Lꢀ1 is about 20 s, which represent
the fast recovery capacity of the sensor.
3.3.2 Selectivity and stability
Selectivity is a very important performance indicator for a sensor, as poor
selectivity prone to the generation of false positives. In order to evaluate the
selectivity of the sensor towards nꢀpropanol, in total of 24 common compounds at
concentration of 10 mg Lꢀ1 except gaseous hydrogen sulfide at 500 ppm were injected
into the sensor. Additionally, 1 mL of liquefied petroleum gas (LPG) mainly consists
of butane and pentane was also injected into the sensor to evaluate to selectivity of the
sensor. As shown in Fig. 7b, although propionaldehyde will produce strong response,
only weak or no responses from other compounds are observed, thus the sensor based
on SrCO3/graphene composite has an acceptable selectivity towards nꢀpropanol. It
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was reported that the size, shape and morphology of the catalyst have great effects on
the CTL performances,16 leading to the same type of catalysts with different
characterizations show different CTL performances for different compounds. For
example, vinyl acetate can induce strong CTL response on nanosized MgO,37 but very
weak CTL response on MgO film.38 SrCO3 also was used as catalyst for CTL
detection of H2S,39 however, only very weak CTL response was observed when H2S
passed through the SrCO3/graphene composite, which possibly also results from the
characterizations of previous reported SrCO3 are different from the SrCO3/graphene
used at the present work.
The sensor was stored at room temperature, and the effect of storage time on the
stability was investigated by measuring the CTL intensity of nꢀpropanol at a
concentration of 10 mg Lꢀ1 per 24 h for one week. We found that the sensor exhibited
longꢀterm stability with a RSD of 6.5% for seven replicate determinations during one
week of periodic operations.
3.5. Analytical figures of merit
Fig. 8 shows the CTL response versus the different concentrations of nꢀpropanol
under the optimized conditions. A linear relationship was found between the CTL
intensity and nꢀpropanol concentration ranging 0.2 to 32 mg Lꢀ1. The linear equation
for the SrCO3/grapheneꢀbased nꢀpropanol sensor was characterized by I = 414.5C +
104.9 (correlation coefficient r = 0.9989), where I is the relative CTL intensity and C
is the concentration of nꢀpropanol. The LOD at an S/N of 3 is 0.08 mg L−1.
3.6 Sample analysis
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In order to evaluate the analytical application of the sensor, samples spiked with
known concentrations of nꢀpropanol were prepared for recovery experiments. Because
nꢀpropanol, benzaldehyde and βꢀphenylethanol are common used as flavouring agents
in food manufacturing; nꢀpropanol are commonly found in alcoholic beverages.
Therefore, solution comprises isopyknic of benzaldehyde (10 mg Lꢀ1) and
βꢀphenylethanol (10 mg Lꢀ1) was selected as sample 1, and Tsingtao beer (alcohol
content: 2.5%) was selected as sample 2. The two kinds of sample were spiked with
nꢀpropanol standard at three levels of 1, 5 and 10 mg Lꢀ1 for recovery experiments. As
shown in Table 1, the recoveries for nꢀpropanol in different samples were 82.4–116.5%
with the RSDs of 5.7–10.4%, respectively, thereby indicating the utility of the sensor
for rapid detection of nꢀpropanol in real sample.
3.7 Possible mechanism
According to widely accepted theory about CL reaction, excited intermediates
would be formed during the catalytic oxidation of nꢀpropanol. The luminescence
emission could be produced when the excited intermediates falling to ground state. In
order to study the mechanism, GC–MS experiments were performed to indentify the
reaction products of the catalytic oxidation of nꢀpropanol on SrCO3 and
SrCO3/graphene composite. As Fig. 9 shows, the catalytic oxidations of nꢀpropanol on
both SrCO3 and SrCO3/graphene composite yield nꢀpropanal, acetaldehyde and
carbon dioxide, indicating that the enhanced CTL intensity does not result from the
new products, nꢀpropanol undergoes the same reaction path on SrCO3 and
SrCO3/graphene. It can be seen that the residual amounts of nꢀpropanol,
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propionaldehyde and acetaldehyde on SrCO3/graphene are less than that of on SrCO3,
while the amounts of carbon dioxide on SrCO3/graphene is higher than that of on
SrCO3, which indicates that efficiency of catalytic oxidation for SrCO3/graphene is
higher than that of pure SrCO3. The experiments show that the specific surface area of
SrCO3/graphene is larger than that of pure SrCO3, which may the possible reason for
the enhanced efficiency of catalytic oxidation for SrCO3/graphene composite.
The formation of propionaldehyde during catalytic oxidation might be thought to
suggest the initial point of attack in nꢀpropanol by oxygen occurs mainly at the αꢀCꢀH
and OꢀH bonds. The formation of copious amount of carbon dioxide suggests that it is
the possible final product. The Fig 7b shows that propionaldehyde can produce strong
CTL signal, and acetaldehyde can produce weak CTL signal on SrCO3/graphene
composite. It was reported that acetaldehyde and carbon dioxide at the excited states
are the important luminous intermediates generated during the CTL reaction
process.40ꢀ41 Therefore, the oxidations of propionaldehyde and acetaldehyde during
the oxidation of nꢀpropanol process are supposed to the mainly responsible for the
CTL emission. According to these results, the possible mechanism of oxidation of
nꢀpropanol could be described as the following reactions: oxygen molecule is
absorbed by the catalyst to form active oxygen molecule (reaction 1);
propionaldehyde then is formed from the reaction of the resulting nꢀpropanol with
oxygen (reaction 2); propionaldehyde is oxidized to produce acetaldehyde and carbon
dioxide, accompanied by strong photoemission (reaction 3); acetaldehyde is oxidized
to produce carbon dioxide, which generates weak photoemission (reaction 4).
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The Cullis et al. reported that the gaseous oxidation of nꢀpropanol shows
considerable similarity to that of ethanol, both of them are to form corresponding
aldehydes at their initial reaction periods. The principal difference then between the
oxidation of 2ꢀpropanol and the corresponding reaction of the primary alcohols is that
ketone replaces aldehyde as the main initial reaction product.42 They stated that the
αꢀCꢀH bond should be more reactive in nꢀpropanol than in ethanol since the
electronꢀrepelling CH3 group is further removed from the point of attack, and the
relative reactivities of ethanol, nꢀpropanol and 2ꢀpropanol toward oxygen is
nꢀpropanol > ethanol > 2ꢀpropanol. Possibly for the different reaction paths between
nꢀpropanol and 2ꢀpropanol, and the higher reactivity of nꢀpropanol results in
nꢀpropanol produces higher CTL signal that that of ethanol and 2ꢀpropanol on
SrCO3/graphene composite (Fig. 7b). However, further work should be done to better
understand the mechanism of CTL reaction nꢀpropanol on SrCO3/graphene
composite.
4. Conclusion
In summary, the SrCO3/graphene composite has been synthesized to fabricated
nꢀpropanol sensor. The effect of the graphene loading concentrations of SrCO3 on
sensing property of nꢀpropanol was investigated. It was concluded that the existence
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of graphene in the composite enhanced the sensitivity of the composite toward
nꢀpropanol, and optimum amount of graphene content at 12 wt% has been found for
the fabrication of nꢀpropanol sensor. By coupling with a miniature vaporizing device,
the sensor was successfully applied to the analysis of nꢀpropanol in liquid samples,
which expanded the range of detectable sample. The detecting conditions of the
sensor include vaporized temperature, detecting temperature, wavelength, and flow
rate for the determination of nꢀpropanol were systematically optimized. The sensor
was successfully applied to the determination of nꢀpropanol in spiked samples. The
recoveries were 82.4–116.5% with the RSDs of 5.7–10.4%, respectively. Finally, the
possible reaction mechanism behind the nꢀpropanol on the SrCO3/graphene composite
was also studied by GCꢀMS experiments. It should be noted that the sensor is not
limit to the determination of nꢀpropanol in liquid samples, it also can be used for rapid
sensing gaseous nꢀpropanol. Therefore, our work provides a selective, sensitive and
convenient method for rapid determination of nꢀpropanol.
Acknowledgements
This study was supported by the Science and Technology Foundation of the Guizhou
Provincial Science and Technology Department (No. LH20147406), the Natural
Science Foundation of the Guizhou Provincial Education Department (No. 2012070),
and the Social Development Foundation of the Science and Technology Bureau of
Qianxinan Prefecture (No. 201430).
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Fig. 1. Schematic diagram of the fabricated sensor device.
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Fig. 2. The FTꢀIR spectra (a) and XRD patterns (b) of different materials. pristine graphite powder
(line 1), GO (line 2), graphene (line 3), SrCO3 (line 4), SrCO3/graphene (line 5).
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Fig.3. The TEM images of graphene (a), SrCO3 (b), and SrCO3/graphene (c).
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Fig. 4. (a) CTL response for nꢀpropanol on pure SrCO3 and SrCO3/graphene composite; (b) the
change trend of CTL intensity for nꢀpropanol versus graphene content. Conditions: detecting
temperature, 245 °C; vaporized temperature, 120 °C ; wavelength, 425 nm; air flow rate, 260 mL
min−1; concentration of nꢀpropanol is 10 mg Lꢀ1. All data points in (b) were run in triplicate.
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Fig. 5. The change trends of CTL intensity for nꢀpropanol versus (a) vaporized temperature and (b)
detecting temperature. Conditions: wavelength, 425 nm; air flow rate, 260 mL min−1;
concentration of nꢀpropanol is 10 mg Lꢀ1. All data points were run in triplicate.
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Fig. 6. The change trends of CTL intensity for nꢀpropanol versus (a) wavelength and (b) flow rate.
Conditions: detecting temperature, 245 °C; vaporized temperature, 120 °C; concentration of
nꢀpropanol is 10 mg Lꢀ1. All data points were run in triplicate.
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Fig. 7. (a) CTL response curves for different concentrations of nꢀpropanol on SrCO3/graphene
composite; (b) sensor response of SrCO3/graphene to different compounds. Conditions: detecting
temperature, 245 °C; vaporized temperature, 120 °C; wavelength, 425 nm; air flow rate, 260 mL
min−1.
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Fig. 8. The calibration curve of nꢀpropanol. Conditions: detecting temperature, 245 °C; vaporized
temperature, 120 °C ; wavelength, 425 nm; air flow rate, 260 mL min−1. All data points were run
in triplicate.
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Fig. 9. GC–MS chromatograms from the catalytic products of nꢀpropanol on SrCO3 (a), and
SrCO3/graphene composite (b). Carbon dioxide (1.95 min); acetaldehyde (2.15 min);
propionaldehyde (2.35 min); nꢀpropanol (4.175 min).
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Table 1 Recoveries and precision determined for n-propanol in different samples (n=5).
Spiked concentration (mg L-1)
Sample
1.0
5.0
10.0
no.
Rea (%)
82.4
RSD (%)
8.3
Re (%)
88.1
RSD (%)
6.6
Re (%)
93.8
RSD (%)
5.7
1
2
116.5
10.4.
107.4
8.8
113.2
6.5
aRe: recovery.