Mesoporous Tungsten Oxides with Crystalline Framework for Highly Sensitive and Selective Detection of Foodborne Pathogens

Article abstract of DOI:10.1021/jacs.7b04221

Foodborne pathogens like Listeria monocytogenes can cause various illnesses and pose a serious threat to public health. They produce species-specific microbial volatile organic compounds, i.e., the biomarkers, making it possible to indirectly measure microbial contamination in foodstuff. Herein, highly ordered mesoporous tungsten oxides with high surface areas and tunable pores have been synthesized and used as sensing materials to achieve an exceptionally sensitive and selective detection of trace Listeria monocytogenes. The mesoporous WO₃-based chemiresistive sensors exhibit a rapid response, superior sensitivity, and highly selective detection of 3-hydroxy-2-butanone. The chemical mechanism study reveals that acetic acid is the main product generated by the surface catalytic reaction of the biomarker molecule over mesoporous WO₃. Furthermore, by using the mesoporous WO₃-based sensors, a rapid bacteria detection was achieved, with a high sensitivity, a linear relationship in a broad range, and a high specificity for Listeria monocytogenes. Such a good gas sensing performance foresees the great potential application of mesoporous WO₃-based sensors for fast and effective detection of microbial contamination for the safety of food, water safety and public health.

Full text of DOI:10.1021/jacs.7b04221

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Article
Mesoporous Tungsten Oxides with Crystalline Framework for
Highly Sensitive and Selective Detection of Foodborne Pathogens
Yongheng Zhu, Yong Zhao, Junhao Ma, Xiaowei Cheng, Jing Xie, Pengcheng Xu, Haiquan Liu, Hongping Liu,
Haijiao Zhang, Minghong Wu, Ahmed A. Elzatahry, Abdulaziz Alghamdi, Yonghui Deng, and Dongyuan Zhao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04221 • Publication Date (Web): 06 Jul 2017
Downloaded from http://pubs.acs.org on July 8, 2017
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Mesoporous Tungsten Oxides with Crystalline Framework for
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ogens
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†
, ‡, ǂ
‡, ǂ
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†
‡
⊥
Yongheng Zhu
, Yong Zhao , Junhao Ma , Xiaowei Cheng , Jing Xie , Pengcheng Xu , Haiquan
‡
‡
#
#
§
ψ
Liu , Hongping Liu , Haijiao Zhang , Minghong Wu , Ahmed A. Elzatahry , Abdulaziz Alghamdi ,
*, †,
⊥
†
Yonghui Deng
, Dongyuan Zhao
†
Department of Chemistry, State Key Laboratory of Molecular Engineering of Polymers, Shanghai Key Laboratory of Moꢀ
lecular Catalysis and Innovative Materials, and iChEM, Fudan University, Shanghai 200433, China
‡
College of Food Science and Technology, and Laboratory of Quality & Safety Risk Assessment for Aquatic Products on
Storage and Preservation (Shanghai), Ministry of Agriculture, Shanghai Ocean University, Shanghai 201306, China
⊥
State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese
Academy of Sciences, Shanghai 200050, China
#
Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University,
Shanghai 200444, China
§
Materials Science and Technology Program, College of Arts and Sciences, Qatar University, PO Box 2713, Doha, Qatar
ψ
Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
KEYWORDS mesoporous materials, tungsten oxide, templating synthesis, listeria monocytogenes, gas sensor
ABSTRACT: Foodborne pathogens like Listeria monocytogenes, can cause various illnesses and pose a serious threat to public
health. They produce speciesꢀspecific microbial volatile organic compounds, i.e. the biomarkers, making it possible to indirectly
measure microbial contamination in foodstuff. Herein, highly ordered mesoporous tungsten oxides with high surface areas and tunꢀ
able pores have been synthesized and used as sensing materials to achieve an exceptionally sensitive and selective detection of trace
Listeria monocytogenes. The mesoporous WO ꢀbased chemiresistive sensors exhibit a rapid response, superior sensitivity, and
3
highly selective detection of 3ꢀhydroxyꢀ2ꢀbutanone. The chemical mechanism study reveals that acetic acid is the main product
generated by the surface catalytic reaction of the biomarker molecule over mesoporous WO . Furthermore, by using the mesopoꢀ
3
rous WO ꢀbased sensors, a rapid bacteria detection was achieved, with a high sensitivity, a linear relationship in a broad range, and
3
a high specificity for Listeria monocytogenes. Such a good gas sensing performance foresees the great potential application of mesꢀ
oporous WO ꢀbased sensors for fast and effective detection of microbial contamination for the safety of food, water safety and pubꢀ
3
lic health.
[
5, 6]
INTRODUCTION
intensive labor and timeꢀconsuming procedures.
In conꢀ
Bacterial foodborne pathogens encompass various illnesses
and continue to threaten public health all over the word.
trast, some newly developed detection techniques, such as the
nucleic acidꢀbased method and the immunoꢀbased test can
achieve significantly decreased detection time as compared
with conventional techniques, they still require complex and
expensive instruments highly trained personnel, and in some
cases, they even give false positive results because they fail to
[
1]
Listeria monocytogenes (LM) is one of the most hazardous
bacteria. It can cause a fatal foodborne illness to pregnant
women, neonates, and the elderly or immunocompromised
[
2, 3]
people.
Additionally, LM can grow over a wide range of
[
7, 8]
temperatures (0ꢀ 45 °C) and at highꢀsalinity solutions. They
have been widely found in environmental mediums (e.g. waꢀ
ters and sediments) and a variety of foods. Many countries
have imposed a zero tolerance policy for LM since it has a low
distinguish dead or alive pathogens.
Therefore, it is of
great interest and importance to develop an easyꢀtoꢀperform,
cheap, effective and convenient detection method for pathoꢀ
genic bacteria in food, water and air which are closely related
to people’s safety and health.
[
4]
infectious dose (< 1000 cells) and high mortality rate.
So
far, various approaches for detection of pathogens have been
proposed. Traditional methods including biochemical tests,
cell culture, are standard modes but suffer the drawbacks of
It is well known that microorganisms including Listeria monoꢀ
cytogenes can produce speciesꢀspecific microbial volatile orꢀ
ganic compounds (MVOCs), which can be characterized as
1
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[
9, 10]
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biomarkers.
In recent years, the detection of MVOCs, i.e.
0.17 cm /g, respectively. The mesoporous WO ꢀbased sensor
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the indicators of microbial contamination, has emerged as a
novel and effective approach to reveal the microbial contamiꢀ
nation, owing to its ability to operate in a noninvasive and
rapid mode without the need for complex and expensive inꢀ
for foodborne pathogens show a rapid response (< 10 s), supeꢀ
rior sensitivity (Rair / Rgas > 50), and highly selective detection
against specific target gases at the subꢀppm level, due to their
good merits of suitable pore size, high surface area and active
siteꢀrich pore walls. Furthermore, by using the mesoporous
[
11 ꢀ 15]
struments and highly trained personnel.
chemiresistive sensors based on semiconducting metal oxides
SMOs) have attracted particular attention due to its adꢀ
In this regard,
WO ꢀbased sensor, a rapid bacteria detection was achieved in
3
ꢀ
1
(
1 min with a high sensitivity of 10 CFU mL , a good linear
relationship in the range of 10 ꢀ 10 CFU mL , and a high
specificity for Listeria monocytogenes in the presence of 10
CFU mL Vibrio parahaemolyticus or Escherichia coli. It
6 ꢀ1
vantages such as lowꢀcost, convenient operation, fast response
and recovery process, and tunable responsivity to target gaseꢀ
ous molecules, making it possible to monitor the microbial
contamination indirectly by measuring the concentration of
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opens up the possibility for the mesoporous WO ꢀbased sensor
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[16]
relevant MVOCs.
to be used as a new platform to conveniently and effectively
detect Listeria monocytogenes in food, water and so on.
However, the MVOCs produced by bacterial foodborne pathꢀ
ogens usually contain thousands of potentially interfering gas
[
17]
species.
It is challenging to adopt SMOsꢀbased sensors to
EXPERIMENTAL SECTION
sensitively and selectively detect trace target gases (subꢀppm
Chemicals and Materials. Chemicals and solvents of analytiꢀ
cal grade were purchased and used as received. Monomethoxꢀ
ypoly (ethylene oxide) (Mw: 5000 g / mol) (designed as PEOꢀ
5000) was purchased from Aldrich. 2ꢀbromoisobutyryl broꢀ
mide and Copper (I) bromide were purchased from Alfa Aesar.
N, N, N’, N’, N’’ꢀPentamethylꢀdiethylenetriamine (PMDETA)
level) in exhaled gases of microorganisms like Listeria monoꢀ
[
18]
cytogenes.
In order to achieve an enhanced gas sensing
performance, it is highly necessary to improve the surfaceꢀ
chemisorbed oxygen species and gas transport kinetics in
[
19, 20]
SMOsꢀbased sensors.
Ordered mesoporous SMOs with
high crystallinity, uniform and wellꢀconnected pores roughly
comparable to the mean free path of gas molecules can meet
was purchased from Acros. Pyridine (AR) and Al O (FCP)
2 3
were purchased from Shanghai Chemical Reagent Co. Ltd.
Tetrahydrofuran (THF), ethanol, styrene, anhydrous ethylether,
[
21 ꢀ 28]
both criteria.
In this unique mesoporous architecture, the
high surface area can provide high density of active surface
sites, and numerous surface reactions can take place between
petroleum ether (30 ꢀ 60 °C), WCl , acetylacetone (AcAc) and
6
WO nanoparticles of AR grade were purchased from Sinoꢀ
3
test gases and adsorbed oxygen species on the sensing layers,
Pharm Chemical Reagent Co. Ltd. All gases were purchased
from Dalian Date Gas Co. Ltd, and their purity was up to
99.5%.
[29, 30]
improving the sensitivity of gas sensor.
Meanwhile, the
large porosity and good pore interconnectivity help to achieve
high sensitivity and selectivity by virtue of the good penetraꢀ
tion of gas molecules dominated by Knudsen diffusion in the
Synthesis of Ordered Mesoporous WO . The amphiphilic
3
PEOꢀbꢀPS diblock copolymers with different molecular
[
31, 32]
mesoporous SMOs matrix.
To date, considerable efforts
weight were prepared by atom transfer radical polymerization
[
37]
have been directed toward synthesizing various ordered mesoꢀ
porous metal oxides through different approaches, such as
(ATRP) method reported previously.
In a typical synthesis
of ordered mesoporous WO , PEO117ꢀbꢀPS186 (100 mg) was
3
[
33]
[34]
[36]
precipitation reactions,
sol–gel process,
spray pyrolysis,
These methods,
firstly dissolved in THF (5.0 g) to form a clear solution in a
[
35]
and chemical vapor deposition (CVD).
glass vial, forming the solution A. 0.25 g of WCl was disꢀ
6
however, usually give rise to oxides with illꢀdefined structure,
poor crystallinity, and low porosity, which is unfavorable for
improved sensing performance.
In this study, highly ordered mesoporous tungsten oxides were
synthesized and used as semiconducting sensing materials to
sensitively and selectively detect of trace target biomarkers in
exhaled Listeria monocytogenes breath. The ordered mesopoꢀ
solved in 0.5 g of ethanol and 0.25 g of acetylacetone (AcAc),
forming the solution B. The above two solutions were then
mixed with stirring at room temperature for 2 h, giving rise to
a yellowꢀgreen homogeneous solution. The obtained solution
was cast onto petri dishes, followed by evaporation of the solꢀ
vent at room temperature for 1 ꢀ 2 h and annealing at 100 °C
for 24 h, and the inorganic ꢀ polymer composite was obtained.
After that, the hybrids film was collected and ground into
rous WO with different pore sizes were synthesized through a
3
ligandꢀassisted solvent evaporation induced coꢀassembly apꢀ
proach, by using diblock copolymer poly(ethylene oxide)ꢀ
blockꢀpolystyrene (PEOꢀbꢀPS) as a template, THF/ethanol
mixture as the solvent, and tungsten chloride as tungsten preꢀ
cursors in the presence of acetylacetonate (AcAc). Notably,
acetylacetone was employed as a coordination agent to retard
the hydrolysis and condensation of tungsten precursor by staꢀ
bilizing the hydrolyzed tungsten hydroxide sol, which makes
the assembly process controllable. After coꢀassembly, pyrolyꢀ
sis treatment of the mesostructural composites in inert atmosꢀ
phere and finally calcination in air, ordered mesoporous tungꢀ
sten oxides with a faceꢀcentered cubic (fcc) structure were
obtained. By changing the hydrophobic PS segment length of
powders, followed by pyrolysis at 350 °C in N for 3 h and
2
ꢀ1
then at 500 °C for 2 h (the ramp rate is 1 °C min ). Finally,
the asꢀmade carbonꢀsupported mesoporous tungsten oxide
samples were further calcined at 500 °C in air for 2 h, and
yellowꢀgreen mesoporous tungsten oxides were obtained.
Through the same synthesis procedure, PEO117ꢀbꢀPS232, and
PEO117ꢀbꢀPS297 copolymers were also used as templates to
synthesize mesoporous WO3.
Gas sensing properties of the gas sensor.
The obtained mesoporous WO materials were grounded with
3
terpineol to form a paste which was coated onto the surface of
ceramic tubes printed with Au electrodes and Pt wires in adꢀ
vance. After sintering at 500 °C for 2 h, a NiꢀCr alloy coil was
inserted into the tube as a heater to control the operating temꢀ
perature of the sensors. To enhance the stability, the asꢀ
fabricated sensors were annealed at 290 °C for one week.
the template, the crystalline mesoporous WO can be readily
3
tailored with tunable pore size of 10.6 ꢀ 15.3 nm, high surface
2
area of 76 ꢀ 136 m /g, and adjustable pore volume of 0.13 ꢀ
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−1
A static test system was employed to measure the gas response
of the sensor. As shown in Figure S1, in measuring the electric
10 10 CFU mL ) were plated on TSA (TSB with 18 g/L
agar added). Colonies were counted after the plates incubated
at 37 °C for 48 ± 2 h. Growth curves were obtained by plotting
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circuit for gas sensor, a load resistor (R ) was connected in the
L
−1
series with it. Herein, R and the circuit voltage (V ) was a
log CFU mL against the exposure time. Three trials with
three replicates per trial were done.
L
c
constant, and the voltage of the load resistor (V ) was recordꢀ
out
ed once in every second. In a typical sensing process, the test
gas such as 3ꢀhydroxyꢀ2ꢀbutanone was injected into a chamber
and diluted with air. The test gas can react with the adsorbed
Measurement and Characterization.
Prior to measurements, all the block copolymer samples were
1
dissolved in the CDCl . H NMR spectra was performed over
3
oxygen species in mesoporous WO sensing layers and release
a DMX 500 MHz spectrometer (Bruker, Germany) with tetꢀ
ramethylsilane as the internal standard (Figure. S2A, B). Gelꢀ
permeation chromatography was operated on an Agilent 1100
GPC system with a refractive index detector (Figure. S2C).
The smallꢀangle Xꢀray scattering (SAXS) patterns were colꢀ
lected on a Nanostar U SAXS system (Bruker, Germany),
using Cu Kα radiation (40 kV, 35 mA). The structural characꢀ
3
free electrons, decreasing the resistance and the voltage of the
sensor, while the Vout was increased. The response of the senꢀ
sor was defined as R /R , where R and R stand for the reꢀ
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a
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a
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sistance of the sensor in air and in the test gas, respectively.
The response time or recovery time is counted as the time
taken by the sensor output to achieve 90% of the total reꢀ
sistance change after injecting or releasing the test gas.
teristics of the mesoporous WO were determined by powder
3
Gas chromatographꢀmass spectrometer (GCꢀMS, Agilent
7890Aꢀ5975C) was used to analyze the gases produced in the
sensing experiment. The GCꢀMS is equipped with a DBꢀ17
MS capillary column (30 m × 0.25 mm × 0.25 mm). The exꢀ
periment was carried out as follows.he GC oven temperature
was first held at 40 ºC for 4 min, then gradually increased to
Xꢀray diffraction (XRD) analysis, using a Bruker D4 Xꢀray
diffractometer (Germany) with Niꢀfiltered Cu Kα radiation (40
kV, 40 mA, 1.54056 Å). Highꢀresolution scanning electron
microscopy (HRꢀSEM, Hitachi S4800) was operated at 1 kV
and 10 ꢁA. Transmission electron microscopy (TEM, JEMꢀ
2100 F) was operated at accelerating voltage of 200 kV. The
nitrogen adsorption was measured at 77 K using a Micromeritꢀ
ics Tristar 3020 analyzer (USA). The specific surface areas
were calculated with the BrunauerꢀEmmettꢀTeller (BET)
ꢀ
1
2
80 °C with a ramping rate of 15 °C min to and finally held
for 16 min. The 0.5 mL sample solution was injected into GC
at the split ratio of 50 : 1, and the flowꢀrate of the He carrier
ꢀ
1
gas is 33 mL min . The quadꢀrupole temperature is 150 °C
and the source temperature is 230 °C. Ionization voltage is 70
eV. The mass spectrometer was acquired in the m/z range of
method, using the adsorption data at P/P = 0.02 ꢀ 0.20. The
0
pore volumes and pore size distributions were derived from
the adsorption branches of isotherms, and the windowꢀsize
was calculated from desorption branch, using the Broekoffꢀde
Boer (BdB) sphere model. A Dilor Lab Ramꢀ1B microscopic
Raman spectrometer (France) was utilized to obtain Raman
spectra of the samples, equipped with a HeꢀNe laser working
at wavelength of 632.8 nm. Xꢀray photoelectron spectroscopy
(XPS, PHIꢀ5000C ESCA) was used to determine the surface
composition of the products, operating with Mg Kα radiation
(hν = 1253.6 eV) or Al Kα radiation (hν = 1486.6 eV). Bindꢀ
ing energy values were charge corrected to the adventitious
carbon (C 1s = 284.6 eV).
2
0 ꢀ 500. MS analysis is performed in electron impact (EI)
mode. The product composition identification was based on
the comparison of MS data using a mass spectral library
(NIST08).
Bacteria cultures
The Listeria monocytogenes (ATCC 7644) cultures were purꢀ
chase from the American Type Culture Collection (ATCC).
And then the bacteria were spread onto the PALCAM Medium
plate for 24 h at 37 °C for activation. Before use, Listeria
monocytogenes inoculum was incubated in 10 ml of tryptic
soy broth and grown overnight (18 to 24 h) at 37 °C in a gyraꢀ
1
tory shaker at 180 r/min. Then the culture was diluted to 10 ,
RESULTS AND DISCUSSION
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0 , 10 , 10 , 10 , 10 CFU mL . The sensor S based on mesꢀ
The labꢀmade amphiphilic PEOꢀbꢀPS copolymers with the
same PEO segment length but different PS chain lengths
(PEO117ꢀbꢀPS186, PEO117ꢀbꢀPS232 and PEO117ꢀbꢀPS297), were
d
oporous WO ꢀPEO117ꢀbꢀPS186 was used to detect the diluted
3
samples with different concentration, after which were incubaꢀ
tion for 2 – 18 h every two hours at 30 °C.
prepared via atom transfer radical polymerization (ATRP)
[38]
The variability in extent of Listeria monocytogenes growth.
The growth variability of Listeria monocytogenes was obꢀ
tained using a method based on turbidity measurements. In a
typical process, 200 µL bacterial suspensions of an initial conꢀ
approach.
By virtue of the supporting effect of carbon resiꢀ
dues from the macromolecular template PEOꢀ b ꢀPS during
carbonization in an inert atmosphere, the resulting carbon speꢀ
cies can prevent the collapse of mesostructured walls during
1
2
3
−1
[39 ꢀ 41]
centration of approximately 10 , 10 , 10 CFU mL were inꢀ
oculated TSB at 37 °C, which were transferred into 100ꢀwell
microtiter plates. The turbidity corresponding optical density
(OD) values was obtained with the automated turbidimetric
system Bioscreen C (Oy Growth Curves Ab Ltd., Raisio, Finꢀ
land), and the absorbance at 580 nm using the wide band filter.
In order to obtain the turbidity growth curves, the OD values
were record every 30 min for up to 18 h. Every experiment is
repeated three times in this process.
highꢀtemperature crystallization of titania and niobia.
Crystalline ordered mesoporous WO materials with different
3
pore size were synthesized through a ligand assisted solvent
evaporation induced coꢀassembly approach by using homeꢀ
made amphiphilic PEOꢀbꢀPS as the template molecules, tungꢀ
sten chloride as a precursor in the presence of acetylacetone
(Figure. 1). The waterꢀinsoluble PEOꢀbꢀPS copolymer was
well dissolved in THF solution, and then the solution was
mixed with an ethanolic solution containing WCl and acetyꢀ
6
Standard plate count method.
lacetone (AcAc), forming the WCl (AcAc) compounds due
6ꢀx
x
To obtain the amount of the alive and active Listeria monoꢀ
cytogenes, the plate counting method was used. The total aerꢀ
to the strong complexation of AcAc with tungsten ions by
coordination bonds and thus can significantly retard their hyꢀ
drolysis. It favors the coꢀassembly of the diblock copolymer
1
obic bacteria were obtained after serially diluted samples (10 ,
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and the inorganic precursors, avoiding phase separation beꢀ Smallꢀangle Xꢀray scattering (SAXS) patterns of the mesoꢀ
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tween them in a much wide range of synthesis conditions even
in the humidity of 15ꢀ85%. As THF evaporates from the preꢀ
cursor solution, amphiphilic PEOꢀbꢀPS copolymers coꢀ
assemble with tungsten species into ordered mesostructured
composites which were further subjected to pyrolysis treatꢀ
porous WO ꢀPEO117ꢀbꢀPS186 after calcination at 500 °C in air
3
to remove the templates, are similar to those of WO ꢀPEO117ꢀ
3
bꢀPS232 and WO ꢀPEO117ꢀbꢀPS297 (Figure. 2a). The wellꢀ
3
resolved four scattering peaks with q ꢀvalues of 0.249, 0.434,
ꢀ
1
0.718, and 0.943 nm , respectively, can be exactly indexed as
the 111, 311, 500 and 533 reflections, corresponding to orꢀ
dered faceꢀcentered cubic (fcc) mesostructure (space group
ment in N at 350 °C and calcination at 500 °C in air to reꢀ
2
move the template, resulting in ordered mesoporous WO maꢀ
3
[
42]
terials. By using PEOꢀbꢀPS copolymers with different molecuꢀ
Fm3m).
Due to the longer PS chain length of PEO117ꢀbꢀ
lar weights, a series of mesoporous WO samples can be obꢀ
PS297, the scattering peaks of the sample WO ꢀPEO117ꢀbꢀPS297
3
3
tained and denoted as WO ꢀPEO ꢀbꢀPS , wherein x and y repꢀ
shift to lower q values compared to the sample WO ꢀPEO117ꢀbꢀ
3
x
y
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
resent the numbers of repeating unit of ethylene oxide and
styrene, respectively.
PS186. The former has a unit cell parameter of 47.3 nm, larger
than that of the latter (43.5 nm), implying a larger building
block formed by PEO117ꢀbꢀPS297
.
Figure 1. Schematic illustration of the formation of ordered mesopoꢀ
rous WO
3
materials via the solvent evaporation induced coꢀassembly
ꢀbꢀPS coꢀ
approach. Step 1. With the evaporation of THF, the PEO
x
y
polymers can coꢀassemble with tungsten species to form spherical
composite micelles with hydrophobic PS core surrounded by a shell
of hybrid PEOꢀAcAcꢀstabilized tungsten species. Step 2. With the
continuous evaporation of THF, the composite spherical micelles
further coꢀassemble into 3ꢀD ordered mesostructure which was fixed
by annealing at 100 °C for 24 h. Step 3. The asꢀmade inorganicꢀ
polymer hybrid was calcined in N
supported mesoporous WO materials. Step 4. Ordered mesoporous
WO materials are finally obtained after removal of the supporting
carbon in the pore channel via calcination in air at 500 °C.
2
at 350 °C, leading to carbonꢀ
3
3
Figure 3. (a) Nitrogen adsorptionꢀdesorption isotherms and (b) the
corresponding pore size distributions of WO ꢀPEO ꢀbꢀPS samples
obtained after carbonization in N at 350 °C and calcination at 500 °C
in air (1, WO ꢀPEO117ꢀbꢀPS297, 2, WO ꢀPEO117ꢀbꢀPS232, 3, WO
PEO117ꢀbꢀPS186). TEM images of taken along (c) [100] and (d) [211]
e) [110] directions. (f) HRTEM image of WO ꢀPEO117ꢀbꢀPS232 after
carbonization in N at 350 °C and calcination at 500 °C in air. The
3
x
y
2
3
3
3
ꢀ
(
3
2
insets in (c), (d), and (e) are the corresponding fast FFT images. Scale
bars, 100 nm (c, d and e), 5nm (f).
The HRꢀSEM images taken along both the surface and cross
section of the samples clearly show that all mesoporous WO3
materials possess a highly ordered mesostructure with a uniꢀ
form pores (Figure. 2b ꢀ e). It implies a spherical packing proꢀ
cess of the composite micelles of PEOꢀbꢀPS/tungsten species
during the coꢀassembly process. TEM images (Figure. 3cꢀe)
Figure 2. (a) SAXS patterns of the WO
WO ꢀPEO ꢀbꢀPS samples obtained after carbonization in N
50 °C and calcination at 500 °C in air (1, WO ꢀPEO117ꢀbꢀPS297, 2,
ꢀPEO117ꢀbꢀPS186). FESEM images of
samples: (b, surface) WO ꢀPEO117ꢀbꢀPS297, (c,
ꢀPEO117ꢀbꢀPS232, and (d, surface) WO
PEO117ꢀbꢀPS186. Scale bars, 500 nm.
3
ꢀPEO
x
ꢀbꢀPS
y
samples of
3
x
y
2
at
3
3
WO
WO
3
ꢀPEO117ꢀbꢀPS232, 3, WO
ꢀPEO ꢀbꢀPS
3
3
x
y
3
surface; e, crossꢀsection) WO
3
3
ꢀ
s
h
o
w
t
h
a
t
t
h
e
o
b
t
a
i
n
e
d
m
e
s
o
p
o
r
o
u
s
W
O
s
a
m
p
l
e
(
W
O
ꢀ
3
3
PEO117ꢀbꢀPS232) has a high degree of periodicity viewed from
4
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Journal of the American Chemical Society
[
100], [211], and [110] directions, respectively. It further conꢀ
spectroscopy (XPS) analysis was performed on the obtained
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
firms the WO mesostructure templated from the copolymer
mesoporous WO (Figure. S5). All peaks can be indexed to W
3
3
PEO117ꢀbꢀPS232 has a highly ordered cubic symmetry (Fm3m).
The uniform and tunable pores with a size of about 12.3 nm
can clearly be observed in the TEM images of the sample
and O elements, indicating the high purity of asꢀsynthesized
sample. The highꢀresolution XPS spectra of W 4f is present in
Figure. S5a. The peaks at binding energies of 35.6 and 37.7
6+
WO ꢀPEO117ꢀbꢀPS198 (Figure. 3c), agreeing well with the N2
eV match well with the reported values for the W oxidation
3
[4]
sorption results. The unit cell parameter is estimated from
TEM images to be about 35.2 nm, which is in good agreement
with that (35.6 nm) from SAXS data. HRꢀTEM images (Figꢀ
ure. 3f) clearly shows the crystal lattice, implying that the pore
wall consists of well crystallized and interconnected WO3
nanoparticles corresponding to the different crystal plane. The
lattice spacing is about 0.365 nm, corresponding to the (200)
planes of monoclinic WO3.
the crystal size of the crystallized and interconnected WO3
nanoparticles corresponding to the (200) planes is about 11.0
nm. While the crystal size of well crystallized and interconꢀ
state. The O 1s peak could be resolved to the two Gaussian
function peaks at 530.7 and 531.3 eV for the mesoporous WO3
(Figure. S5b). The state of O 1s indicates two types of oxygen
2ꢀ
in the surface, i.e. the lattice oxygen (O ) and the adsorbed
ꢀ
ꢀ
oxygen (O and O ). Usually, the lattice oxygen hardly interꢀ
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
acts with the reducing gas and thus cannot contribute to the
generation of the main chargeꢀcarriers in nꢀtype semiconducꢀ
tors; while the adsorbed oxygen is very active and can react
with reducing gases, changing the concentration of main carriꢀ
[44]
As measured from Figure 3f,
[
46]
ers.
All the WO ꢀPEO ꢀbꢀPS samples exhibit typical Raꢀ
3 x y
ꢀ1
man adsorption bands of 273, 327, 715 and 807 cm , which
are similar to those of the commercial tungsten oxide powder
and can be identified as the four strong modes of tungsten
ꢀ1
nected WO nanoparticles corresponding to the (200) planes is
3
about 11.5 nm calculated from the XRD patterns (Figure. S4)
by Scherrer equation, matching well with the HRꢀTEM data.
The structure and bonding of WO ꢀPEO ꢀbꢀPS samples were
characterized by XRD spectroscopy. The XRD pattern disꢀ
plays wellꢀresolved diffraction peaks in the range of 10 ꢀ 60
degree (Figure. S4), matching well with the crystalline monoꢀ
clinic phase of WO with lattice parameters of a = 0.7297, b =
0
No diffraction peaks from other crystalline impurities were
observed, suggesting a pure crystalline phase. The broadening
of the diffraction peaks can be ascribed to the WO nanocrysꢀ
[
47]
oxides (Figure. S6).
The Raman bands at 273 and 327 cm
correspond to the OꢀWꢀO bending modes of bridging oxygen,
3
x
y
ꢀ
1
and the bands at 715 and 807 cm can be attributed to stretchꢀ
ing modes. It is worth noting that, compared with the commerꢀ
cial bulk WO , the Raman adsorption peaks became broader
3
gradually from WO ꢀPEO117ꢀbꢀPS297 to WO ꢀPEO117ꢀbꢀPS232
3
3
3
.7539, c = 0.7688 nm and β = 90.918 (JCPDS No. 43ꢀ1035).
and to WO ꢀPEO117ꢀbꢀPS186 with increasing specific surface
3
areas (Figure. S6bꢀd). It implies a continuous increase of the
density of crystal defects and oxygen vacancies, which is diꢀ
rectly relevant to the catalytic activity and gas sensing properꢀ
3
[45]
[48]
tals in the pore wall.
and the oxidation state of the W species, Xꢀray photoelectron
To determine the exact composition
ty of semiconductors.
Table 1. The texture properties and sensing performance of the obtained WO ꢀPEO ꢀbꢀPS and commercial WO particles
3
x
y
3
c
Samples
BET surface area
Total pore volume
Pore size
Sensitivity
Response/Recovery
(s)
2
3
a
b
(
R /R )
(m /g)
(cm /g)
(nm)
a
g
commercial WO3
6
/
/
6.2
18.0 / 43.0
12.0 / 30.0
WO ꢀPEO117ꢀbꢀPS297
76
0.17
15.3
32.7
46.2
3
WO ꢀPEO117ꢀbꢀPS232
95
0.15
0.13
12.1
10.6
10.0 / 26.0
4.0 / 13.0
3
WO ꢀPEO117ꢀbꢀPS186
136
56.1
3
a
b
The pore sizes were derived from the adsorption branches of the isotherms by using the BJH method. The sensitivity of sensors S , S ,
a
b
S , and S to 3ꢀhydroxyꢀ2ꢀbutanone at 25 ppm, and the S
PEO117ꢀbꢀPS232, and WO ꢀPEO117ꢀbꢀPS186, respectively. The response and recovery of S , S , S , and S to 3ꢀhydroxyꢀ2ꢀbutanone at 5 ppm.
3
a b c d
a
, S
b
c
, S , and S
d
are based on commercial WO
3
particles, WO
3
3
ꢀPEO117ꢀbꢀPS297, WO ꢀ
c
d
c
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Journal of the American Chemical Society
Page 6 of 12
commercial bulk WO particles and mesoporous WO ꢀPEO ꢀ
3
3
x
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
bꢀPS samples were first tested in 5 ppm of 3ꢀhydroxyꢀ2ꢀ
y
butanone at different operating temperatures of 200 ꢀ 400 °C
(Figure. 4a). The response of all sensors exhibited a high
response to target gas at 290 °C. It implies that the equilibrium
density of chemisorbed oxygen ions on WO is maximized at
3
this temperature. Thus, 290 °C was adopted as the working
temperature for subsequent detections. All the fabricated gas
sensors show continuous increasing responses to 3ꢀhydroxyꢀ2ꢀ
butanone in a wide concentration ranging from 0.1 to 25 ppm
(
Figure. 4b). Even at a low concentration of 0.1 ppm, the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
sensitivity can reach 5.3, 8.7 and 13.5 for the Sensor S , S , S ,
b
c
d
respectively, and the response values increase with the gas
concentration. Moreover, these sensors show excellent
reversibility and repeatability as the concentration decreased
from 25 to 0.1 ppm. Notably, the sensitivity was found to
increase dramatically with the specific surface areas of
tungsten oxides (Table 1), and the maximal response was 56.1
at 25 ppm of 3ꢀhydroxyꢀ2ꢀbutanone for Sd based on the
mesoporous WO ꢀPEO117ꢀbꢀPS186. The highest response is
3
2
mainly due to its largest surface area (135 m /g), which
provides the highest density of active surface sites for
numerous surface reactions between test gases and adsorbed
oxygen species on the sensing layers can take place. Moreover,
the response of S is nearly 9 times higher than that of S based
d
a
on the commercial WO . It suggests that the sensitivity can be
3
significantly increased by fabricating ordered mesoporous
structure.
Figure 4. Typical response curve and variations of the sensors base
on different samples (S , S , S , and S based on commercial WO
particles, WO WO and WO
PEO117ꢀbꢀPS186, respectively). (a) The responses vs the operating
temperature to 5 ppm of 3ꢀhydroxyꢀ2ꢀbutanone, (b) the responses vs
3ꢀhydroxyꢀ2ꢀbutanone at with concentrations of 0.1 ꢀ 25 ppm measꢀ
ured at 290 °C
a
b
c
d
3
3
ꢀPEO117ꢀbꢀPS297
,
3
ꢀPEO117ꢀbꢀPS232
,
3
ꢀ
As one of the most interesting and important metal oxides,
WO has attracted much attention because of its excellent gas
3
[
49, 50]
sensitivity.
Mesoporous WO3 materials with a high
porosity have unique advantages of short gas diffusion length,
high carrier mobility, and high specific surface area that
facilitates the adsorption and desorption of analyte molecules.
Previous study has revealed that the main MVOCs produced
by Listeria monocytogenes is 3ꢀhydroxyꢀ2ꢀbutanone (32.2% in
abundance of the MOVs), and its concentration in MVOCs
[
51]
increased with the growth of Listeria monocytogenes.
Therefore, 3ꢀhydroxyꢀ2ꢀbutanone can be considered as a
marker volatile chemical for predicting the cell number of
Listeria monocytogenes. In this study, we systematically
investigated the application possibility of our mesoporous
WO ꢀPEO ꢀbꢀPS materials for indirectly sensing detection of
3
x
y
Listeria monocytogenes by measuring the concentration of 3ꢀ
hydroxyꢀ2ꢀbutanone in their MVOCs. We first fabricated four
series of gas sensors, S , S , S , and S , based on commercial
a
b
c
d
WO , WO ꢀPEO117ꢀbꢀPS297, WO ꢀPEO117ꢀbꢀPS232, and WO3ꢀ
3
3
3
PEO117ꢀbꢀPS186, respectively, to examine their responses
toward 3ꢀhydroxyꢀ2ꢀbutanone.
Figure 5. (a) Response and recovery curve of the sensors base on
different samples
cles, WO ꢀPEO117ꢀbꢀPS297, WO
PS186, respectively) to 5 ppm 3ꢀhydroxyꢀ2ꢀbutanone. (b) Responses of
(
S
a
, S
b
, S
c
, and S
d
based on commercial WO
3
partiꢀ
Since the gas sensing properties of semiconductors usually
[52, 53]
3
3
ꢀPEO117ꢀbꢀPS232, and WO
3
ꢀPEO117ꢀbꢀ
depend on the working temperature,
the sensors based on
6
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Journal of the American Chemical Society
the above four sensors to interfering gases at 50 ppm, 3ꢀhydroxyꢀ2ꢀ
butanone at 5 ppm, respectively.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The response and recovery behaviors are an important paꢀ
rameter for evaluating the performance of the sensing materiꢀ
als. The continuous dynamic electrical response of the sensors
to 5 ppm of 3ꢀhydroxyꢀ2ꢀbutanone is shown in Figure. 5a. The
sensor S based on the sample WO ꢀPEO117ꢀbꢀPS186 exhibits a
d
3
markedly higher response upon exposure to 3ꢀhydroxyꢀ2ꢀ
butanone (response = 4.0 s, recovery = 13.0 s) compared to
those of commercial WO nanoparticles (response = 18.0 s,
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
recovery = 43.0 s), WO ꢀPEO117ꢀbꢀPS297 (response = 12.0 s,
3
recovery = 30.0 s), and WO ꢀPEO117ꢀbꢀPS232 (response = 10.0
3
s, recovery = 26.0 s). As the resistive sensor is based on surꢀ
face effects, the higher specific surface area of the mesoporous
WO ꢀPEO117ꢀbꢀPS186 can provide more adsorption sites for
3
sensing reactions in the solidꢀgas interfaces, resulting in the
highest response (4.0 s) of sensor S upon exposure to the
d
same concentration target gas. On the other hand, since the
WO ꢀPEO117ꢀbꢀPS186 sample has larger pore window size (6.3
3
nm) than that of the mesoporous WO ꢀPEO117ꢀbꢀPS (5.4 nm)
3
232
and WO ꢀPEO117ꢀbꢀPS297 (3.5 nm), gas molecules can more
3
rapidly and readily diffuse within the whole porous structure
in S , resulting in the highest recovery (13.0 s) of sensor S in
d
d
the target gas of the same concentration.
The analysis of speciesꢀspecific microbial volatile organic
compounds (MVOCs) produced by microorganisms requires
not only high sensitivity, fast response and recovery, but also
high selectivity. In order to investigate the selectivity of the
mesoporous WO ꢀbased sensors, we also measured their reꢀ
3
sponse to different gases at 50 ppm, such as carbon monoxide,
ethylene, ethanol and acetone at 290 °C. As shown in Figure.
5b, sensor S based on the sample WO ꢀPEO117ꢀbꢀPS186 exhibꢀ
Figure 6. (a) Schematic diagram of the principle for detection of
Listeria monocytogenes using the sensor S based WO ꢀPEO117ꢀbꢀ
PS186. (b) Typical response of the S to various concentrations of
Listeria monocytogenes after incubated at 30 °C for 18 h. (c) Typical
d
3
d
3
d
ited excellent selectivity to 3ꢀhydroxyꢀ2ꢀbutanone at 5 ppm
and little interference from other gases. More importantly, four
typical gases in exhaled Listeria monocytogenes breath, 2, 3ꢀ
butanedione (11.6 % in abundance of the MOVs), 3ꢀ
methylbutanal (10.1 % in abundance of the MOVs), 2, 5ꢀ
dimethylꢀpyrazine (10.4 % in abundance of the MOVs) and
benzaldehyde (17.6 % in abundance of the MOVs) were also
3
−1
response of the S to 10 CFU mL Listeria monocytogenes which
d
were incubated for 2 ꢀ 18 h at 30 °C. (d) The sensitivity of the S
d
to
1
2
3
various concentrations of Listeria monocytogenes (10 , 10 , 10 CFU
−
1
mL , respectively) measured at an interval of 2 h during 18 h incubaꢀ
tion at 30 °C and the control sample of tryptone soy broth medium
(
TSB). (e)The variability of Listeria monocytogenes growth was obꢀ
[
51]
tained every 30 min for up to 18 h in wells inoculated with 200 µL
selected as interfering gases.
As shown in Figure. 5b, the
1
2
3
bacterial suspensions of an initial concentration of 0, 10 , 10 , 10 ,
response value of the sensor S to 3ꢀhydroxyꢀ2ꢀbutanone at 5
−1
d
CFU mL , using a method based on turbidity measurements.
ppm is at least 3 times higher than that for the four selected
interfering gases even at 50 ppm. More strikingly, the reꢀ
The above exciting results drive us to further explore the posꢀ
sponse value of S to 5 ppm of 3ꢀhydroxyꢀ2ꢀbutanone (32.2%
d
sibility to employ the ordered mesoporous WO based gas
3
in abundance of the MOVs) is more than 10 times higher than
that for benzaldehyde (17.6 % in abundance of the MOVs) at
50 ppm. These results clearly indicate that our sensors have
superior selectivity to 3ꢀhydroxyꢀ2ꢀbutanone in exhaled Lisꢀ
teria monocytogenes breath. The stability study on our mesoꢀ
sensors for indirect monitoring and detection of bacteria. Figꢀ
ure. 6a schematically illustrates the principle for detection of
Listeria monocytogenes using sensor Sd based on WO3ꢀ
PEO117ꢀbꢀPS186. Senor S was firstly exposed to various conꢀ
d
centrations of Listeria monocytogenes (initial concentrations:
porous WO ꢀbased gas sensors revealed that the sensors’ reꢀ
1
6
−1
3
1
6
0 to 10 CFU mL , incubation for 18 h). The results (Figure.
sponse to 5 ppm of 3ꢀhydroxyꢀ2ꢀbutanone show a negligible
change of less than 3% after working continuously for 35 days
b) clearly reveal that the response of sensor S is directly
d
proportional to the concentration of bacteria, showing a much
(
Figure. S7), reflecting a good longꢀterm stability of mesopoꢀ
6
higher sensitivity (R / Rgas = 10.3) upon exposure to 10 CFU
air
rous WO sensors. All the results clearly show that the sensor
−1
1
3
mL bacteria than that (R / Rgas = 4.9) for the case of 10
CFU mL bacteria. Subsequently, similar study was carried
air
S based mesoporous WO ꢀPEO117ꢀbꢀPS186 exhibits excellent
−1
d
3
comprehensive performance with high selectivity and sensitivꢀ
ity to 3ꢀhydroxyꢀ2ꢀbutanone. It has a good stability, quick
response and recovery dynamics, which is important for pracꢀ
tical applications.
out by exposing sensor S in Listeria monocytogenes solution
d
3
−1
(
10 CFU mL ) during incubation for 2 ꢀ 18 h, and the reꢀ
sponse was also measured. After incubation for 2 h, the sensor
shows large voltage signal change, which is due to the increasꢀ
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
ing accumulation of MVOCs concentration caused by the conꢀ
tinuous growth of microbial cells (Figure. 6c).
Furthermore, the variability in extent of microbial growth was
measured by using both chemiresistive sensing method based
Page 8 of 12
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
on sensor S (Figure. 6d) and the conventional turbidity methꢀ
d
od (Figure. 6e). It can be seen that the variability of Listeria
monocytogenes is detected using S even for a low concentraꢀ
d
1
−1
tion of 10 CFU mL after incubation for only 2 h, and the
amount of the alive and active bacteria is 1.61 Log10 CFU/mL
(
Figure. S8). By contrast, the turbidity method failed to detect
3 −1
the variability even for a high concentration of 10 CFU mL
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Listeria monocytogenes after incubation for 8 h, corresponding
to an amount of the alive and active bacteria of 4.93 Log10
CFU/mL (Figure. S8). These results indicate that our sensor Sd
is more sensitive and can detect the bacteria more quickly and
conveniently. More importantly, the data obtained using our
sensor reflect the variability of microbial growth of alive and
active bacteria which are the only source of the biomarker
molecule (i.e. 3ꢀhydroxyꢀ2ꢀbutanone) and should be monitored
and evaluated for the safety of food, water safety and public
health in practical applications. The selectivity of sensors to
bacteria is also very important in detection of foodborne pathꢀ
ogens. Two other kinds of typical bacteria, Vibrio parahaemoꢀ
lyticus and Escherichia coli were also evaluated. As shown in
6
−1
Figure. S9, the response of S upon exposure to 10 CFU mL
d
Vibrio parahaemolyticus or Escherichia coli is much lower
3
−1
than the response for the case of 10 CFU mL Listeria. monꢀ
ocytogenes, suggesting a highly selective recognition for Lisꢀ
teria monocytogenes. All these results clearly indicate that the
Figure 7. (a) The schematic illustration of the 3ꢀhydroxyꢀ2ꢀbutanone
sensing mechanism of the sensors d based on mesoporous WO
PEO117ꢀbꢀPS186 exposure at air and target gasꢀair mixture. (E , vaꢀ
lence band edge; E , conduction band edge; E , Fermi energy). Gas
3
ꢀ
V
mesoporous WO ꢀbased sensors possess excellent sensing
3
C
F
performance in detecting active Listeria monocytogenes with
high sensitivity and precision, good convenience and selectiviꢀ
ty.
ChromatographꢀMass Spectrometry (GCꢀMS) characterization of the
gaseous products generated from the sensing reaction. (b) With mesoꢀ
porous WO
3
inside the furnace at 290 °C, and (c) without mesoporous
WO inside the furnace at 290 °C.
3
The gasꢀsensing mechanism of our sensors based on mesopoꢀ
[
54]
rous WO can be explained by a surfaceꢀdepletion model.
3
To better understand the chemical mechanism of the WO3
sensor for detecting the target gas, we conducted a simulated
experiment to identify the products generated during sensing
process. A gaseous mixture of air and 3ꢀhydroxyꢀ2ꢀbutanone
gas was prepared and reserved in a stainlessꢀsteel cylinder
As illustrated Figure. 7a, when WO ꢀbased sensors are exꢀ
3
posed to air, oxygen molecules can be chemically adsorbed on
the surface of WO to capture electrons from the conduction
3
ꢀ
ꢀ
2ꢀ
band and form adsorbed oxygen species (O , O and O ). At
2
the same time, a thick spaceꢀcharge layer is formed on the
(1000 ppm by volume ratio) to be allowed to pass through a
surface of WO , increasing the potential barriers (marked in
3
tubular furnace with preꢀplaced WO in a ceramics boat at
3
green) with a higher resistance (Figure. 7a). In contrast, when
2
90 °C, and the real sensing circumstance is thus imitated. A
the WO sensors are exposed to 3ꢀhydroxyꢀ2ꢀbutanone, the
3
gasꢀwashing bottle containing ethanol as the solvent was used
to collect the reaction products. The products were analyzed
by GCꢀMS. The results show a product peak assigned to acetic
acid (Figure. S10a) only when the sample mesoporous WO3
was exposed to 3ꢀhydroxyꢀ2ꢀbutanone and air at 290 °C.
While without the presence of WO in the furnace, only 3ꢀ
hydroxyꢀ2ꢀbutanone was detected by GC even at 290 °C. The
results clearly revealed that 3ꢀhydroxyꢀ2ꢀbutanone can be oxiꢀ
target molecules can react with the adsorbed oxygen and reꢀ
lease free electrons, which leads to the decrease of the thickꢀ
ness of the potential barriers (marked in green) and the electriꢀ
cal resistance (Figure. 7a). As shown in Figure. 7a, it can be
clearly seen that large porosity and a high specific surface area
3
of the wellꢀcrystalline ordered mesoporous WO ꢀPEO117ꢀbꢀ
3
PS186 materials can provide numerous active sites for gas adꢀ
sorption, increasing the adsorbed oxygen amount. Meanwhile,
the continuous crystalline framework ensures a fast transport
of charge carriers from surface into bulk, leading to a high
sensitivity. Since the average pore size of the synthesized
dized to acetic acid over WO (Figure. 7b, c). The GCꢀMS
spectra of the products generated by WO materials upon exꢀ
posure to 1000 ppm 3ꢀhydroxyꢀ2ꢀbutanone for 0.5, 1.0, 1.5,
3
3
2
.0 h were also obtained (Figure. S10b). The results show a
mesoporous WO ꢀPEO117ꢀbꢀPS186 is around 10.6 nm, the gas
3
continuous accumulation of acetic acid produced by 3ꢀ
hydroxyꢀ2ꢀbutanone as the reaction proceeds. Based on the
above results, it can be concluded that acetic acid is one imꢀ
portant oxidation product of 3ꢀhydroxyꢀ2ꢀbutanone during
sensing measurement, which is different from the conventional
sensing mechanism where carbon dioxide and water are beꢀ
molecules spread across the sensing layer can be dominated by
[
55, 56]
surface diffusion,
selectivity.
resulting in the high sensitivities and
8
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Journal of the American Chemical Society
lieved to be the only two products of ketone such as acetone or
ation chromatograph (GPC) trace of diblock copolymer PEOꢀ
bꢀPS. Windowꢀsize distribution curves of WO ꢀPEO ꢀbꢀPS
y
[
57, 58]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
nꢀbutanol over WO3.
Herein we attempted to use an elecꢀ
3
x
tronꢀliberate theory to explain this interesting phenomenon
combining with our results. When the sensor is exposed to the
samples obtained after carbonization in N at 350 °C and calꢀ
2
cination at 500 °C in air (1) WO ꢀPEO117ꢀbꢀPS297, (2) WO3ꢀ
3
reducing gases 3ꢀhydroxyꢀ2ꢀbutanone at a high temperature
PEO117ꢀbꢀPS232, (3) WO ꢀPEO117ꢀbꢀPS186. XRD patterns of (a)
3
ꢀ
(
290 °C), the species O is more important than the other oxyꢀ
commercial WO3 particles, and WO ꢀPEO ꢀbꢀPS samples
3 x y
ꢀ
2ꢀ
[59, 60]
gen adsorbates (O , O ),
and they can be readily conꢀ
obtained after carbonization in N at 350 °C and calcination at
2
2
sumed, releasing additional electrons, which in turn decreased
the baseline resistivity by following the reactions displayed
below:
500 °C in air, (b) WO ꢀPEO117ꢀbꢀPS297, (c) WO ꢀPEO117ꢀbꢀ
3
3
PS232, (d) WO ꢀPEO117ꢀbꢀPS186. The highꢀresolution Xꢀray
3
photoelectron spectroscopy (XPS) of WO ꢀPEO117ꢀbꢀPS186
3
ꢀ
CH CHOHCOCH (gas) + 2O (chemisorbed) →
samples obtained after carbonization in N
ther calcination at 500 °C in air. (a) W 4f for WO
PS186, (b) O 1s for WO ꢀPEO117ꢀbꢀPS Raman spectra of (a)
2
at 350 °C and furꢀ
3
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ
2
CH COOH (gas) + 2e
(1)
(2)
ꢀPEO117ꢀbꢀ
3
3
ꢀ
CH COOH(gas) + 4O (chemisorbed)→
3
186.
3
ꢀ
commercial WO3 particles, and WO ꢀPEO ꢀbꢀPS samples
2
CO (gas) + 2H O + 4e
3
x
y
2
2
obtained after carbonization in N at 350 °C and calcination at
From the above typical reactive formulas, for target gases of
the same concentration, the surface reaction of acetic acid gas
can release more electrons than 3ꢀhydroxyꢀ2ꢀbutanone, and
2
5
00 °C in air, (b) WO ꢀPEO117ꢀbꢀPS , (c) WO ꢀPEO117ꢀbꢀ
3 297 3
PS232, (d) WO ꢀPEO117ꢀbꢀPS186. The longꢀterm stability of the
3
sensors base on different samples (S , S , S , and S based on
thus the sensitivity of the sensor S to acetic acid should be
a
b
c
d
d
commercial WO3 particles, WO ꢀPEO117ꢀbꢀPS297
,
WO3ꢀ
higher than the sensor S to 3ꢀhydroxyꢀ2ꢀbutanone; however,
3
d
PEO117ꢀbꢀPS232, and WO ꢀPEO117ꢀbꢀPS186, respectively). The
our results (Figure. S11) indicate that the response of the S to
3
d
growth kinetics of Listeria monocytogenes at different initial
3
ꢀhydroxyꢀ2ꢀbutanone at 50 ppm (sensitivity: Rair / Rgas =
1
2
3
−1
concentration of (a) 10 , (b) 10 , (c) 10 CFU mL using plate
count enumeration. Three trials with three replicates per trial
498.3) is much higher than that for acetic acid at 50 ppm
(sensitivity: Rair / Rgas = 14.5). Therefore, it can be concluded
that, under the sensing conditions, 3ꢀhydroxyꢀ2ꢀbutanone
predominantly follows reactions (1), and acetic acid is the
oxidation product instead of carbon dioxide and water.
were done for each temperature. The sensing response of S
d
6
based on WO ꢀPEO117ꢀbꢀPS
mL Vibrio parahaemolyticus, 10 CFU mL Escherichia
upon exposure to 10 CFU
3
186
−1
6
−1
3
−1
coli and 10 CFU mL Listeria Monocytogenes. A series of
experiments results indicate that only the sample exposed to 3ꢀ
hydroxyꢀ2ꢀbutanone and air at 290 °C. (b) Gas Chromatoꢀ
graphꢀMass Spectrometry (GCꢀMS) characterization of the
gaseous products generated from the sensing reaction at difꢀ
ferent time. Responses of sensor S based on WO ꢀPEO117ꢀbꢀ
CONCLUSIONS
In summary, wellꢀcrystalline ordered mesoporous WO with
3
adjustable pore size of 10.6 ꢀ15.3 nm have been synthesized
via a solvent evaporation induced coꢀassembly approach by
using tailorꢀmade PEOꢀbꢀPS diblock copolymers with differꢀ
ent PS chain lengths as a template, and tungsten chloride as a
precursor in the presence of chelating agent acetylacetonate.
The obtained mesoporous tungsten oxides have high surface
d
3
PS186 to 3ꢀhydroxyꢀ2ꢀbutanone and acetic acid gases at 50 ppm,
respectively.
This material is available free of charge via the Internet at
http://pubs.acs.org.
2
areas (76 ꢀ 136 m /g) and adjustable pore volumes of 0.13 ꢀ
3
0
.17 cm /g. Because of the wellꢀconnected mesoporous strucꢀ
AUTHOR INFORMATION
Corresponding Author
ture with high surface areas and crystalline framework, the
mesoporous WO –based sensors exhibit a rapid response, suꢀ
3
perior sensitivity and highly selective detection against 3ꢀ
hydroxyꢀ2ꢀbutanone, the biomarker for Listeria monocytoꢀ
genes, at a low concentration of subꢀppm level. The simulated
onꢀline gas sensing analysis by GC and GCꢀMS indicated that
the sensing performance is attributed to an unexpected new
*
yhdeng@fudan.edu.cn
Author Contributions
ǂ
ZYH and ZY contribute equally to this study. The manuscript
was written by contributions of all authors.
sensing mechanism of the mesoporous WO chemiresistive
3
sensor. Acetic acid is the product of 3ꢀhydroxyꢀ2ꢀbutanone
Notes
over WO . A rapid, sensitive and selective sensing detection of
The authors declare no competing financial interest.
3
Listeria monocytogenes has been achieved by using the mesoꢀ
porous WO ꢀbased gas sensor. This new sensing concept and
ACKNOWLEDGMENT
This work was supportedthe NSF of China (51372041,
3
method hold a great promise for the development of a novel,
simple, noninvasive and lowꢀcost VOC portable sensor device
for the detection of alive and active foodborne bacteria in clinꢀ
ical and food samples.
5
1422202, 51402049 and 51432004), the “Shu Guang” Project
(13SG02) supported by Shanghai Municipal Education Comꢀ
mission and Shanghai Education Development Foundation,
Shanghai Sci. & Tech. Committee (14JC1400700), Youth
Topꢀnotch Talent Support Program of China, China Postꢀ
doctoral Science Foundation (KLH1615138), Shanghai Nature
Science Foundation of China (14ZR1416600) and the state
key laboratory of Transducer Technology of China (Grant No.
SKT1503). The authors extend their appreciation to the Interꢀ
national Scientific Partnership Program ISPP at King Saud
ASSOCIATED CONTENT
Supporting Information.
The structure of the sensor and working principle of the gas
1
sensing measurement system. H NMR spectra of PEOꢀBr and
the synthesized diblock copolymer PEOꢀbꢀPS. The gel permeꢀ
9
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 12
University for funding this research work through ISPP# 0094.
The authors thank Prof. Guangrong Zhou for assistance in
TEM characterization.
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