Reaction-Based Detection of Chemical Warfare Agent Mimics with Affinity Ionic Liquids

Article abstract of DOI:10.1021/acs.analchem.8b01763

We are demonstrating here the successful development of an ionic liquid device based on affinity ionic liquids AIL 1-6 tailored for chemoselective detection of gaseous chemical warfare agent (CWA) mimics on a quartz crystal microbalance (QCM) as well as on disposable filter papers. In the QCM platform, the negligible vapor pressure of ionic liquids ensures that AILs do not dry out on quartz chips and are used free of leakage during the measurements. This lack of vapor pressure makes these ionic liquid devices highly attractive for gas analysis. In the present study, chemoselective CWA mimic vapor detection by QCM occurs when gas rapidly diffuses into the AIL thin film and accordingly the nucleophilic substitution reaction emerges under the experimental conditions developed. Our initial results give credence to three important conclusions: (i) AILs could be prepared in short synthetic steps and are remarkable in detecting CWA mimics, (ii) the analysis of gaseous CWA mimics on QCM is label-free and chemoselective, and (iii) the detection of CWA mimic vapor by AILs on filter paper discs is colorimetric and portable, and, once used, the paper disc can be disposed. To the best of our knowledge, this is the first report in the literature based on the CWA mimic vapor detection in ionic liquids on a QCM.

Full text of DOI:10.1021/acs.analchem.8b01763

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Technical Note
Reaction-Based Detection of Chemical Warfare
Agent Mimics with Affinity Ionic Liquids
Chien-Yuan Chen, Kun-Hsien Li, and Yen-Ho Chu
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01763 • Publication Date (Web): 25 Jun 2018
Downloaded from http://pubs.acs.org on June 25, 2018
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Graphical Abstract
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Reaction-Based Detection of Chemical Warfare Agent Mimics with Affinity Ionic
Liquids
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Chien-Yuan Chen, Kun-Hsien Li, and Yen-Ho Chu*
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Reaction-Based Detection of Chemical Warfare Agent Mimics with Affinity Ionic
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Chien-Yuan Chen, Kun-Hsien Li, and Yen-Ho Chu*
Department of Chemistry and Biochemistry, National Chung Cheng University, 168
University Road, Minhsiung, Chiayi 62102, Taiwan, Republic of China
* Corresponding author. Tel: 886 52729139; fax: 886 52721040; e-mail:
cheyhc@ccu.edu.tw
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Abstract
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We are demonstrating here the successful development of an ionic liquid device based
on affinity ionic liquids AIL 1-6 tailored for chemoselective detection of gaseous
chemical warfare agent (CWA) mimics on a quartz crystal microbalance (QCM) as
well as on disposable filter papers. In the QCM platform, the negligible vapor
pressure of ionic liquids ensures that AILs do not dry out on quartz chips and are used
free of leakage during the measurements. This lack of vapor pressure makes these
ionic liquid devices highly attractive for gas analysis. In the present study,
chemoselective CWA mimics vapor detection by QCM occurs when gas rapidly
diffuses into the AIL thin film and accordingly the nucleophilic substitution reaction
emerges under the experimental conditions developed. Our initial results give
credence to three important conclusions: (i) AILs could be prepared in short synthetic
steps and are remarkable in detecting CWA mimics, (ii) the analysis of gaseous CWA
mimics on QCM is label-free and chemoselective, and (iii) the detection of CWA
mimics vapor by AILs on filter paper discs is colorimetric and portable, and, once
used, the paper disc can be disposed. To the best of our knowledge, this is the first
report in the literature based upon the CWA mimics vapor detection in ionic liquids
on a QCM.
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This paper reports our development of a system based on ionic liquids1 tailored for
chemoselective detection of chemical warfare agent (CWA) mimics2 by quartz crystal
microbalance (QCM),3 on filter paper discs, and in solutions. In the QCM system, the
detection of gaseous CWA mimics was achieved by nucleophilic substitution reactions
with affinity ionic liquids AIL 1-6 (Figure 1) thin-coated on quartz chips. The AILs
studied in this work are embedded with two unique chemical properties in structures:
a nucleophilic pyridyl group responsible for capturing vapor of CWA mimics and,
upon reactions, a red-shifted color change in azobenzene chromophore.
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Figure 1. Structures of affinity ionic liquids AIL 1-6.
The azobenzene motif likely is one of the most studied and best characterized
chromophore groups available to chemists engaged in molecular assembly, and has
found a myriad of applications in science and technology such as molecular
photoswitches and sensors.4-6 The value of this structural scaffold is widely
acknowledged for its facile access to dense structures of defined geometry with
targeted properties for specific studies. Here, we are reporting our initial development
of AIL 1-6 for use to investigate reaction-based detections of CWA mimics with these
polar ionic liquids.
CWAs are considered the most nefarious and reprehensible manmade chemicals that
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are highly toxic. For exactly that reason, rapid and sensitive methods for rapid and
sensitive detection of CWAs are of critical need. Albeit nonportable and expensive
GC-MS and NMR-based instrumental techniques are methods of choice for
comprehensive detection of CWAs, other sensitive methods such as fluorescence and
colorimetric measurements have also been reported and employed.2 The development
of new sensitive and inexpensive methods for CWA analysis should find applications
in forewarning procedure, practice of adequate protection, site mapping of
contamination, and during and after decontamination.2 To our knowledge, no ionic
liquids embedded with azo dyes, such as our AIL 1-6, for detecting CWA mimics on a
QCM have been reported in literature. Ionic liquids are made up exclusively of ions
and have emerged as versatile solvents for numerous applications in chemistry.1 They
can be further altered, tuned, and controlled by the engineering of cations or anions.
In fact, significant progress has been made in exploring functionalized ionic liquids
through the incorporation of functional groups to equip novel ionic liquids with
tailored properties for targeted applications.7,8 Based specifically on azo dye, this
work reports our concise synthesis of affinity ionic liquids AIL 1-6 and demonstrates
their usefulness in CWA mimics detection by QCM, on portable filter papers discs,
and in solutions.
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EXPERIMENTAL SECTION
Chemical Synthesis of Affinity Ionic Liquids AIL 1-6. Detailed synthetic
procedures and spectral characterization of AIL 1-6 were given in the Supporting
Information. All AIL 1-6 synthesized are viscous liquid at ambient temperature.
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31P and H NMR Analysis of AIL Reactions with CWA Mimics in Solution,
Gaseous QCM Measurements, and Detection of CWA Mimic Vapor on Paper
Discs. Detailed preparation and experimental procedures of AIL reactions with CWA
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mimics were given in the Supporting Information.
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RESULTS AND DISCUSSION
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We commenced our syntheses of affinity ionic liquids AIL 1-6 from commercially
available starting materials, 4-aminopyridine (for affinity ionic liquids AIL 1, 3, 4,
and 6) and aniline (for the control ionic liquids AIL 2 and 5), which were diazotized
first and subsequently treated with substituted anilines or phenols in one pot to
proceed the electrophilic aromatic substitution reactions to afford the azo alcohols or
phenols in 22-73% yields (Schemes 1 and S1). For AIL 4-6, mesylations of the
corresponding azo alcohols with methanesulfonyl chloride followed by a nucleophilic
substitution with 6,7-dihydro-5H-pyrrolo[1,2-a]imidazole, a bicyclic imidazole,
previously developed in our laboratory9 and finally the anion metathesis with lithium
bis(trifluoromethane)sulfonimide (LiNTf2) in water readily afforded the desired ionic
liquids AIL 4-6 (Scheme 1). The detailed synthesis of AIL 1-3 is illustrated in
Scheme S1 (Supporting Information). Our overall syntheses of AIL 1-6 were
straightforward and, in our hands, these AILs could be achieved in short synthetic
steps (4 steps for AIL 5; 5 steps for AIL 1-4, 6). All AILs obtained are viscous liquid
at room temperature. Detailed experimental procedures, spectra and data of AIL 1-6
are summarized in the Supporting Information.
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Scheme 1. Synthesis of AIL 4-6.
Analysis of volatile organic compounds (VOCs) emanating from human body, for
example, is of immense value for noninvasive medical diagnosis in the form of
fingerprints that are disease specific.10-12 VOCs are typically collected and
concentrated using commercial portable gas sampling devices, and then analyzed by
GC and identified by MS. Albeit GC-MS often is the method of choice, no one
analytical method is suitable for measuring all VOCs. To detect CWA mimics using
our AIL-on-QCM technology platform developed in this work, both diethyl
chlorophosphate (DCP) as the nerve agent mimic of G-agents (GA, GB, and GD) and
2-chloroethyl ethyl sulfide (CEES) used as a blister sulfur mustard (HD) mimic were
studied.13 We incorporated reactive azo groups in AILs for reasons that the
azopyridine not only acts as a good nucleophile to undergo substitution reactions
chemoselectively with vapors of CWA mimics quantitatively measured by QCM, but
also can be used to develop colorimetric sensors on test paper discs for rapid
qualitative analysis of CWA mimics. Moreover, it is expected that, in comparison to
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common nonpolar and less polar molecular solvents, polar reactions such as the one
studied in this work should proceed with fast rates in inherently polar ionic liquids,
owing to its likely stabilization of the polar transition states of the reactions as well as
favorable interactions between the Lewis acidic cation of ionic liquids and the P=O
(in DCP) and the Cl (in CEES) group of CWA mimics that ultimately promote the
reactions.
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We first carried out screening by 31P NMR to identify the most effective AILs to
accelerate the nucleophilic substitution reactions with DCP in deuterated acetonitrile
at ambient temperature. 31P NMR readily monitors reactions taking place at
phosphorus center, often permits detection of reaction intermediates involved, and
accordingly can offer insight into mechanism or pathway of reactions studied. The
31
screening results are summarized in Figure 2. These kinetic P NMR measurements
clearly showed that, as expected, more nucleophilic N-linked AILs (t1/2 ~ 33 min for
AIL 4 and t1/2 ~ 48 min for AIL 6) react faster than the O-linked AILs (t1/2 > 234 min
for AIL 1 and t1/2 ~ 180 min for AIL 3) toward DCP (Figure 2A). The control
O-linked AIL 2 and N-linked AIL 5 ionic liquids were completely inert toward DCP
(Figure 2). It was worth noting that, in the presence of an organic base
(diisopropylethylamine, DIEA), methoxy-containing AIL 6 (t1/2 = 6 min) reacts
slightly but noticeably faster than AIL 4 (t1/2 = 13 min) with DCP in CD3CN solution
(Figure 2B). Furthermore, the control liquid AIL 5 was inert toward DCP with or
without DIEA (85% and 96% DCP remaining after 240 min, respectively; Figure 2).
Among all AILs tested, AIL 4 and AIL 6 were found to be two most powerful
affinity ionic liquids to accelerate reactions with DCP in solutions.
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Figure 2. Kinetic measurements of AIL 1-6 (30 mM) reactions with diethyl
chlorophosphate (DCP, 30 mM; P = 4.81 ppm) (A) without and (B) with
diisopropylethylamine (DIEA, 30 mM) in CD3CN solvent at ambient temperature.
Trimethyl phosphate (15 mM; P = 3.67 ppm) was used as an internal standard in
experiments. The progress of the reactions could be readily monitored by the
proton-decoupled 31P NMR (162 MHz).
To understand the nucleophilic acyl substitution reactions of AILs with DCP, we
conducted time course experiments of AIL 1 reaction with DCP in CD3CN measured
by 31P NMR. A less reactive AIL 1 was used for this study simply because it allowed
convenient reaction monitoring. The 31P NMR result unambiguously demonstrated the
formation of tetraethyl pyrophosphate (TEPP) product (Figure S1). We found that,
upon addition of DCP (P = 4.81 ppm) to AIL 1 in CD3CN, a new peak at -12.17 ppm
was obtained with an identical chemical shift to that of the freshly prepared TEPP.14,15
DCP alone did not undergo any detectable hydrolysis when dissolved in CD3CN. This
formation of TEPP from AIL 1 reaction with DCP could be explained by DCP
reaction with its hydrolyzed diethyl phosphoric acid in the presence of adventitious
water likely associated with ionic liquid.15 Scheme S2 shows its proposed mechanism.
Results from both reaction photos and UV-vis spectra further confirmed that the
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affinity ionic liquid AIL 4 dissolved in acetonitrile chemoselectively reacted with
DCP at ambient temperature and the control AIL 5 was not at all reactive (Figure 3).
As shown in Figure 3B, the λmax of AIL 5 remained essentially unaffected upon
mixing with DCP (412 nm vs. 414 nm without and with DCP, respectively), clearly
indicating that the reaction of AIL 5 with DCP was negligible, if any, and the reaction
of AIL 4 toward DCP was chemospecific: a large Δλ = 100 nm red-shift was
observed; that is, 440 nm vs. 540 nm without and with DCP, respectively (Figure 3A).
Reaction photos and corresponding UV-vis spectra for all AIL 1-6 were given in
Figure S2. Among AILs investigated, AIL 6 produced the largest red shift in
wavelength (λmax shifted from 441 nm to 551 nm) upon reaction with DCP in
solutions.
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Figure 3. UV-vis spectra (1 mm path length) of AIL 4 (A, 0.1 mM) and AIL 5 (B,
0.1 mM) (labeled in black), and their reactions (labeled in red) with DCP (0.1 mM) all
in acetonitrile (100 L). Spectra were obtained using a NanoDrop 2000
spectrophotometer (Thermo Scientific, Waltham, MA, USA). Optical photographic
images were for solutions of higher concentration (0.6 mM).
Our gas-phase DCP-AIL 4 reactions were performed at ambient temperatures and
monitored on a commercial 9 MHz QCM device.8 Results shown in Figure 4
unambiguously demonstrate that, using the same concentration (526 ppb) for all
gaseous samples tested, the AIL 4 reacted selectively only with DCP (ΔF = -157 Hz)
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and its analogous, more volatile dimethyl chlorophosphate (DMCP, ΔF = -269 Hz).
Under identical experimental conditions, common VOCs such as methanol, acetone,
ethyl acetate, hexane, and water were totally inert toward AIL 4 thin-coated on QCM
sensor chips. As expected, the control AIL 5 was found completely unreactive with
all gaseous samples studied. The resonance frequency drops (ΔF) obtained in our
QCM measurements from the AIL 4 reactions with gaseous CWA mimics correlate
well with their reactivity (DMCP vs. DCP); that is, the less bulky the dialkyl
chlorophosphate is, the greater the reactivity is and accordingly the larger the ΔF
results. From the Kanazawa-Gordon study on gas QCM in contact with liquid,16 these
frequency drops likely attribute to an overall change of mass, density and viscosity on
AIL 4. Our reaction-based QCM system worked well and was totally unreactive to
common VOCs, indicating that any residual water present in the gas stream would not
interfere with the CWA mimic vapor and, accordingly, the frequency drop in this
continuous flow QCM measurement was not at all due to any nonspecific
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physisorption of gas onto ionic liquid (Figure 4).
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Figure 4. Chemoselective detection of methanol, acetone, ethyl acetate, hexane, water,
DCP, and dimethyl chlorophosphate (DMCP) gases (526 ppb each) all by a 9 MHz
QCM thin-coated with AIL 4 (2.2 nL, 200 nm thickness). The QCM sensorgrams for
gases studied were vertically shifted (20 Hz in between) for clarity. The QCM
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responses of DCP and DMCP gas reactions with the control AIL 5 (2.2 nL, 200 nm
thickness) thin-coated on QCM chip were also displayed (vertically shifted by 20 Hz
in between). Nitrogen was used as the carrier gas with a flow rate of 3 mL/min, and
gaseous samples were injected at 500 s. The resonance frequency drop, ΔF, in Hz is
the QCM response on the quartz chip surface.
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Figure S3 detailed a quantitative study of the highly reactive AIL 4 and the control
AIL 5 reactions with DCP gas by QCM and investigated their effectiveness in this
CWA mimic gas analysis based on nucleophilic acyl substitution reactions (for all
corresponding QCM sensorgrams; see Figure S3A). To our delight, it showed a linear
QCM frequency response within the range of concentrations tested (0−1315 ppb). In
our hands, the AIL 4 is highly sensitive to DCP vapor: for DCP at 5 Hz decrease in
resonance frequency (i.e., ΔF = 5 Hz), the sensitivity of detection by AIL 4 was 20
ppb (5/0.24565; Figure S3B). The QCM responses (ΔF) of the control AIL 5
reactions with DCP gas (0−1315 ppb) were totally negligible.
As we successfully developed AILs for DCP analysis, we next turned our attention to
the detection of 2-chloroethyl ethyl sulfide (CEES) as a blister sulfur mustard (HD)
mimic and investigated AIL 4 to demonstrate its effectiveness in CEES analysis
based on a nucleophilic substitution (SN1) reaction.
1
Results from H NMR analysis, reaction photos, and UV-vis spectra confirmed that
the AIL 4 dissolved in dimethyl sulfoxide chemoselectively reacted with CEES under
heated conditions (Figure 5). Using identical experimental conditions, 1-chlorobutane
1
used as the control substrate was totally inert toward AIL 4. The H NMR results
illustrated in Figure 5B unambiguously demonstrated the formation of AIL 4-CEES
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adduct, showing new well defined NMR peaks with all signals down-field shifted on
the expected reactive azopyridine motif. The connectivity between AIL 4 and CEES
in the adduct structure could be readily validated by nuclear Overhauser effect
measurement (Figure 5A; for nOe spectrum, see Supporting Information). In this
reaction, the weak Lewis acidic imidazolium cation17 of AIL 4 likely activates the
chloro leaving group in CEES to facilitate the formation of a transient but highly
reactive sulfonium ion, immediately followed by a SN1 reaction with the nucleophilic
azopyridine in AIL 4, finally yielding the AIL 4-CEES adduct (Figure 5A). Figure
5C showed that the λmax of AIL 4 remained essentially unaffected upon reaction with
1-chlorobutane (454 nm vs. 458 nm without and with 1-chlorobutane, respectively),
indicating that the reaction of AIL 4 with 1-chlorobutane was negligible, and the
reaction of AIL 4 toward CEES was clearly chemospecific: 454 nm vs. 558 nm
without and with CEES, respectively (Figure 5C).
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Figure 5. Chemoselective detection of 2-chloroethyl ethyl sulfide (CEES) by affinity
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ionic liquid AIL 4. (A) Proposed mechanism of AIL 4 reaction with CEES. (B) H
NMR analysis of AIL 4 (30 mM) reactions with CEES (30 mM) and 1-chlorobutane
o
(30 mM), respectively, at 60 C for 17 h in DMSO-d6 solvent, clearly showing the
formation of AIL 4−CEES adduct. (C) UV-vis spectra (1 mm path length) of AIL 4
(0.3 mM, labeled in black), and their reactions with CEES (0.3 mM, labeled in red)
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and 1-chlorobutane (0.3 mM, labeled in blue) all in DMSO. Spectra were obtained
using a NanoDrop 2000 spectrophotometer. Optical photographic images were
obtained with solutions of higher concentration (0.6 mM).
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Albeit the mustard and its mimic CEES are known to be poor electrophilic substrates
for nucleophilic substitution reactions,18,19 we thought that the weak Lewis acidity of
AIL 4 might potentially promote its reaction with CEES and, accordingly, went ahead
to investigate its reactions with gaseous CEES by QCM at ambient temperatures.
After many trial experiments were tested, including screening of metal triflates and
the study of other activated aliphatic chlorides, the AIL 4 was however found not at
all sensitive enough towards capturing CEES gas measured by QCM at ambient
temperatures. The rate of reaction appeared to be much limited by the rather slow
formation of the cyclic ethylenesulfonium ion, resulting in flowing through without
being reacted with AIL 4 from the continuous flow QCM system.
To illustrate that AILs can be used as a device for rapid analysis of CWA mimics,
both AIL 4 and AIL 5 in methanol were individually soaked with filter paper discs,
dried, placed in sample bottles, and test their ability to capture DCP vapor at room
temperature and CEES vapor under heated condition, respectively. We were pleased
that, as shown in Figure 6, the color of AIL 4 on paper discs was intensified and
readily visualized by naked eyes upon exposure to CWA mimics vapor in minutes.
The control AIL 5 ionic liquid and 1-chlorobutane substrate were totally unreactive.
This initial result demonstrated that AIL 4 loaded on filter paper can be used as a
portable sensor for CWA mimics vapor detection and, potentially, a promising tool
for infield detection of CWAs.
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Figure 6. Photos of AIL 4 and AIL 5 manually printed on ADVANTEC No. 1 filter
paper discs (0.3 cm in diameter) upon exposure to (A) DCP, (B) CEES and
1-chlorobutane (as the control) vapors under daylight. For paper disc preparation and
detailed experimental conditions, see the Supporting Information.
CONCLUSION
In summary, we demonstrated here the successful development of an ionic liquid
device based on AIL 1-6 tailored for chemoselective detection of CWA mimics by
quartz crystal microbalance, on filter paper discs, and in solutions. Since 2010, we
have continuously developed the AIL-on-chip technology platforms for
reaction-based analysis of targeted gases.20,21 In these systems, the negligible vapor
pressure of ionic liquids ensures that thin-coated AILs do not dry out on QCM chips
and are used free of leakage during the measurements. This lack of vapor pressure
makes these AIL devices highly attractive for gas analysis. In this work,
chemoselective CWA mimics vapor detection by QCM occurs when gas rapidly
diffuses into the AIL thin film and accordingly the nucleophilic substitution reaction
emerges under the experimental conditions developed. Our present work on the
reaction-based detection and analysis of CWA mimics by AILs lend credence to three
important conclusions: (i) AILs prepared in short synthetic steps are superior in
detecting CWA mimics in solutions, (ii) the analysis of gaseous CWA mimics on a
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QCM is label-free and chemoselective, and (iii), as a promising infield test kit, the
detection of CWA mimics by AILs on paper discs is colorimetric, portable, and, once
used, the paper disc can be disposed. To the best of our knowledge, this is the first
report in the literature based upon the CWA mimics vapor detection in ionic liquids
on a QCM. Albeit this ionic liquid device for CWAs analysis is promising and
cost-effective, no one analytical method is suitable for measuring all CWAs and our
AILs developed here are still preliminary and with no doubt more experimental work
is needed.
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Supporting Information
Schemes S1-S2; Figures S1-S3; detailed synthetic procedures of AIL 1-6; 1H and 13C
NMR and HRMS spectra and data of AIL 1-6; reactivities of AIL 1-6 toward DCP
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without and with DIEA base in CD3CN measured by P NMR; AIL 4 reaction with
2-chloroethyl ethyl sulfide (CEES) and 1-chlorobutane at 60 oC in DMSO-d6 solvent;
time-course 31P NMR experiments on DCP conversion to TEPP; color visualization of
AIL 4 and AIL 5 upon exposure to CWA vapor; QCM measurements.
ACKNOWLEDGEMENTS
This work was supported in part by a grant (MOST 106-2113-M-194-006-MY3) from
the Ministry of Science and Technology of Taiwan, Republic of China. We thank Dr.
I-Nan Chang and Jerry Lo (ANT Technology, Taipei, Taiwan) for support on QCM
instruments. We also thank the reviewers for constructive comments.
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