Chemoselective gas sensing ionic liquids

Article abstract of DOI:10.1039/b924286d

Based on specific chemical reactions, chemoselective gas detection of aldehydes, ketones and amines in real time was efficiently achieved on QCM chips thin-coated with sensing ionic liquids (SILs). The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b924286d

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Chemoselective gas sensing ionic liquidsw
Ming-Chung Tseng and Yen-Ho Chu*
Received (in Cambridge, UK) 19th November 2009, Accepted 11th February 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/b924286d
Based on specific chemical reactions, chemoselective gas detection of
aldehydes, ketones and amines in real time was efficiently achieved
on QCM chips thin-coated with sensing ionic liquids (SILs).
Among many potential materials, we envisaged that, in
conjunction with QCM, ionic liquids should be promising
candidates as chemoselective sensing agents simply because of
their tunable chemical reactivity, negligible volatility, and
acceptable thermal stability.1 Furthermore, this negligible vapor
pressure up to the thermal decomposition point (B300 1C) of
ionic liquids ensures that the sensor does not ‘‘dry out’’ on QCM
chips and our SILs can be readily used free of leakage and the
loss of loading during the measurements. This lack of vapor
pressure makes these materials highly attractive for gas sensing.
Chemoselective gas detection by QCM thus occurs when gases
rapidly diffuse into the SIL thin film and specific chemical
reactions for selective gases in ionic liquid emerge under
appropriate experimental conditions. Here, we demonstrate the
SILs 1 and 2 as chemoselective agents for detections of aldehyde/
ketone and amine gases, respectively. Aldehydes, ketones and
amines are important VOCs for environmental monitoring as
well as disease diagnosis. The SIL 1 readily reacts with aldehyde/
ketone gas samples to form covalent imine adducts. Based on the
transimination reaction, the SIL 2 should be suited for chemo-
selective detection of amine gases. The synthesis of SILs 1 and 2
was straightforward and is outlined in Scheme S1 (ESIw). In our
hands, the overall isolated yields of SILs 1 and 2 preparations
were high: 64% in four steps for SIL 1 and 48% in five steps for
SIL 2, respectively. Both SILs 1 and 2 are liquid at ambient
This communication reports the use of sensing ionic liquids
(SILs) for chemoselective detection of gases by quartz crystal
microbalance (QCM). Ionic liquids are low-melting molten salts
composed entirely of ions, and many of them are liquid at room
temperature.1 Ionic liquids carry numerous desirable properties
such as wide liquid range, thermal stability, excellent solubility
with many small molecules, attractive recyclability, and negligible
vapor pressure that are well suited for a myriad of innovative
applications, including high-performance lubricants and electro-
lytes for solar cells.1 Moreover, considerable progress has
recently been made to explore functionalized ionic liquids
through incorporation of functional groups as a part of
developing ionic liquids possessing tailored properties.2 Herein,
we present SILs as a new class of chemically selective gas sensing
agents that do not require dilution with solvents or solid
adsorbents, and can be used directly and real-time measured
by QCM without any chemical immobilization on quartz chips.
Gases such as air pollutants (footnote 1, ESIw) and volatile
organic compounds (VOCs) from breath (footnote 2, ESIw)
greatly impact environment as well as human health. Gas
analysis by GC-MS, ELISA, or on polymer composite arrays
has been exploited for commercial practices, including the
analysis of VOCs in food spoilage control, fish freshness assay,
and odor identification (footnote 3, ESIw). Albeit it is highly
sensitive and provides the most detailed information on
sample composition, the GC-MS method requires expensive
instrumentation and skilled analysis, limiting its widespread
applications. Moreover, GC-MS is difficult to use to analyze
the lowest molecular weight VOCs such as formaldehyde and
ammonia.3 While important in sensor development, organic
polymer and mixed metal oxide adsorbent arrays are often
nonspecific and exhibit poor selectivity for detection of gas
samples.4 In addition, when low concentrations of gases are
measured, solid-state sensors are not sensitive enough.5 With
the ever increasing demand of efficient and green procedures
for highly selective gas detection and separation in recent
years, sensing materials with tunable structures and tailored
properties must be developed.
1
temperature. Details such as synthetic procedures, H and 13C
NMR spectra and data of SILs 1 and 2 are summarized in
the ESIw. The chemically stable [b-3C-im][NTf2] previously
developed by us was used here as the control ionic liquid.6
As our first step toward the development of chemoselective
sensors, we aimed to establish an integrated platform for
specific gas detection and, in this study, are reporting the
development of a highly sensitive and, most importantly,
selective SIL-based QCM gas analysis system.7 Because of
its ease of operation, low cost, and response in real time, QCM
is a popular transducer and mass sensor among many other
sensing methods, and has been valuable for detection of VOCs
in the gas phase and of biomolecules in solution.8 The key
feature of this QCM device for chemoselective gas detection is
that measurable frequency characteristics are readily obtained
upon the chemical reaction of a gas analyte onto the bulk of an
applied thin film on the chip surface of the device that
ultimately transduces to generate an analytical signal.
Thin coatings are preferred since viscous losses are minimized
Department of Chemistry and Biochemistry, National Chung Cheng
University, Chia-Yi 621, Taiwan, ROC. E-mail: cheyhc@ccu.edu.tw;
Fax: +886 5 2721040; Tel: +886 5 2428148
w Electronic supplementary information (ESI) available: Schemes
S1–S3; Fig. S1–S3; Table S1; Synthesis, NMR spectra and spectral
data of SILs 1 and 2; experimental procedures of SILs reaction with
gases (21 pages). See DOI: 10.1039/b924286d
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in thin liquid layers upon vapor sorption (footnote 4, ESIw).
Coatings were achieved by depositing a specified amount of
diluted methanol solution containing SILs on the surface of the
quartz chip. After the evaporation of the solvent, the coating was
deposited on the QCM chip. The used SIL layer could easily be
replaced with new SIL by washing with methanol, purging with
air, applying the new SIL in methanol, and finally removing
residual methanol. Dried ambient air was used as carrier gas, and
measurements were performed at room temperature. Scheme S2
illustrates the measurement set-up (ESIw).
aldehyde gas sensing. It is also worth mentioning that,
unlike aldehyde reaction, the injection of formic acid gas
instantaneously resulted in an ultrafast (a ‘‘freefall’’) frequency
drop in QCM response, due exclusively to its rapid acid–base
neutralization reaction taking place on the coated quartz chip
(Fig. S1, ESIw); that is, the shape of QCM response signals is
also diagnostically informative.
The formation of the imine adducts from SIL 1 with
aldehydes was experimentally confirmed by FT-IR, 1H
NMR, and MS spectroscopy. The IR spectrum of the benz-
aldehyde-reacted SIL 1 manifested a new band at 1650 cmÀ1
,
Results in Fig. 1 clearly show that, among seven gases tested,
the SIL 1 thin-coated on QCM chip only reacted chemo-
selectively with model aldehydes (propionaldehyde and
benzaldehyde) and ketone (acetone) gases. Having the same
gas concentration (412 ppb), SIL 1 reacted at similar, if not
totally identical, rates with both aliphatic and aromatic alde-
hydes. Here, benzaldehyde gas tested is a well known fragrant
of peach products. As expected in chemical reactivity, both
aldehydes greatly outperformed ketone gas upon capturing by
SIL 1. Acetone studied in this work has been an important
breath biomarker for disease diagnosis (footnote 2, ESIw). It is
also noted that the irreversible nature of the frequency drops
from QCM measurements of aldehyde and ketone sensing by
SIL 1 indicated its non-equilibrium formation of Schiff bases.
This may be understood by the fact that the use of the water-
immiscible and hydrophobic SILs such as SIL 1 potentially
inhibit the reverse hydrolysis of imine adduct formed and the
water, once produced, likely was readily carried away by the
carrier air. Additionally, results in Fig. 1 unambiguously
demonstrates that our SIL-on-chip system was totally insensitive
to common VOCs such as hexane, ethanol, ethyl acetate, and,
most significantly, moisture; that is, any water present in the
gas stream would not be in any direct competition with
aldehydes (and slow-reacting ketones). These control VOC
experiments clearly suggest that, under our condition, the
viscous losses at QCM surfaces due to physical sorption of
gases were minimal, if any (Fig. 1). Results in Fig. 1 show that
SIL 1 is selective and highly reactive towards capturing
aldehyde, suggesting that SIL 1 is valuable and sensitive for
consistent with an aldimine CQN stretch.9 In addition, this
new band in IR was in excellent agreement of that from the
imine compound separately prepared. Perhaps equally note-
worthy is the virtual absence of free aldehyde CQO band
(1700 cmÀ1) associated with dissolved gas, thereby confirming
that benzaldehyde gas once captured was efficiently reacted
with the SIL 1 to produce the imine adduct. This IR result
unambiguously proved that the irreversible frequency drop in
the continuous flow QCM measurement was not due to the
nonspecific dissolution of aldehyde gas in SIL 1. Both 1H
NMR and EI-MS analyses further supported the formation of
the imine adduct of SIL 1 reaction with propionaldehyde gas:
a characteristic aliphatic imine CHQN signal at 7.83 ppm and
a new correct mass of 192.2 for [C11H18N3]+ ion were
experimentally obtained, respectively.
Our SIL-on-chip system has shown excellent sensitivity
towards chemoselective detection of aldehydes, but marginal
QCM response with the much slow-reacting ketone gases
(Fig. 1). Fig. 2 further provides detailed quantitative studies
of SIL 1 reactions with butyraldehyde and 2-butanone as two
model gases with identical molecular weight. Both gases
showed essentially linear QCM frequency responses within
the range of concentrations tested (Fig. 2B). Under our
experimental condition, at one hertz decrease in resonance
frequency, the sensitivity of detection was 4.5 ppb for butyr-
aldehyde and 148 ppb for 2-butanone, respectively; that is, an
Fig. 1 Chemoselective detection of various gases (412 ppb) using
9 MHz QCM thin-coated with SIL 1 (3.3 nL, 300 nm thickness). Inset
details the QCM response of the reaction of acetone with SIL 1. The
resonance frequency drop, DF, is the QCM response on the quartz
chip surface.
Fig. 2 (A) Chemoselective gas detection of butyraldehyde (0, 7.6,
15.2, 22.8, 38.0 and 53.2 ppb) and 2-butanone (0, 60.8, 91.2, 120.4 and
150.5 ppb) using 9 MHz QCM thin-coated with SIL 1 (3.3 nL, 300 nm
thickness). (B) A plot of DF (Hz) vs. gas concentration (ppb).
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approximately 33-fold higher sensitivity in detection for alde-
hyde than that of ketone gas of the same mass was evident. In
our hands, the smallest aldehyde gas, formaldehyde, could
also react with SIL 1 and be detected by QCM (Fig. S2, ESIw).
Encouraged as we were by the aldehyde/ketone results, we
next turned our attention to the SIL detection of amine gases, as
another important class of VOCs (footnote 2, ESIw), and
investigated the SIL 2 to test, based on the transimination
reaction, its effectiveness in amine gas sensing. Similar to the
imine formation, this transimination reaction is one of the most
fundamental and ubiquitous reactions in organic chemistry and
recently has attracted much attention in, for example, the
folding study of supramolecular assemblies (footnote 5, ESIw).
Initially, we investigated the direct transimination reaction of
propylamine as a model amine gas with only the SIL 2 thin-
coated on the quartz chip and found that, albeit detectable at
low concentration (28.5 ppb), the QCM response was minimal
(B0.5 Hz) and seemly reversible in its signal (Fig. 3A and Table
S1, ESIw). Realizing in literature that Lewis acids notably
facilitated the transimination reactions in conventional
molecular solvents (footnote 5, ESIw), we decided to examine
the same methodology for use in ionic liquid and randomly
selected seven metal triflates available in our laboratory, and
carried out screening to identify the most effective Lewis acid to
accelerate the transimination reaction of SIL 2 with propyl-
amine. To our delight, under our experimental condition
(28.5 ppb propylamine gas and 1 mol% metal triflate dissolved
in SIL 2), Sc(OTf)3 was found to be the most powerful Lewis
acid to catalyze the transimination reaction to produce the
largest and irreversible QCM response (20 Hz) (Fig. 3A and
Table S1, ESIw). The reaction course of this Sc(OTf)3-catalyzed
transimination can be explained on the basis of Scheme S3
(ESIw). The result in Fig. 3A also clearly indicates that, without
SIL 2, only Sc(OTf)3 (1 mol%) dissolved in a non-functionalized
[b-3C-im][NTf2] ionic liquid was totally inert to the amine gas,
further proving that the frequency drop in the continuous flow
QCM measurement was not due to the nonspecific dissolution
of amine gas in SIL 2. Fig. 3B details the quantitative study of
SIL 2 reaction with propylamine gas (0–120 ppb). In our
hands, this Sc(OTf)3-catalyzed, transimination-based SIL
platform is highly selective to amine gas: at DF = À1.0 Hz,
the sensitivity of detection was approximately 2.5 ppb for
propylamine. Most remarkably, our reaction-based SIL-on-chip
system is greatly sensitive to the detection of the smallest molecular
weight amine gas, ammonia (Fig. S3A, ESIw): approximately
3.9 ppb at DF = À1.0 Hz (Fig. S3B, ESIw).
In summary, we demonstrate here the successful use of SILs
thin-coated on QCM chips for effective detection of specific
gases. Our chemoselective gas sensing method by SILs is straight-
forward and avoids many problems (surface area, pore size and
shape, thermal stability, sieving effect, diffusivity, and complex
adsorption behaviors) associated when using porous solid
materials as adsorbents for gas adsorption and separation.10
Furthermore, this SIL approach is cost-effective that the QCM
chip can be readily regenerated by washing away the used SIL
and replacing the new one. Our SIL platform is chemoselective
with fast gas diffusion in ILs, readily applicable to very low
molecular weight gases such as ammonia and, most significantly,
insensitive to moisture. It is also noted that this SIL platform
need not be performed in a temperature-controlled device; that
is, the direct measurements on bench at ambient temperatures
sufficed. There are, to our knowledge, no reports in literature
based upon specific chemical reactions demonstrating chemo-
selective gas sensing in ionic liquid measured by QCM (footnotes 6
and 7, ESIw). This research is our first proof-of-concept
inspection of the promising use of a QCM-based sensor for
reaction-directed detection of gas samples, which is part of a
program aimed at studying environmental air quality control
and examining defective goods. Without doubt, more work is
needed. Lastly, the concept of reaction diversity, likely along
with the prefiltering strategy, can be applied and incorporated
in this research to ensure that a set of candidate SILs on QCM
chips will ultimately represent an ‘‘electronic nose’’.
This work was supported by a multi-year Grant-in-Aid
from the ANT Technology (Taipei, Taiwan) and in part
funded by a three-year grant from the National Science
Council (Taiwan, ROC). We thank reviewers for constructive
comments and valuable suggestions.
Notes and references
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Molecules, 2009, 14, 3780; (b) N. V. Plechkova and K. R. Seddon,
Chem. Soc. Rev., 2008, 37, 123.
2 S.-G. Lee, Chem. Commun., 2006, 1049.
3 A. Amann, P. Spanel and D. Smith, Mini-Rev. Med. Chem., 2007,
7, 115.
4 R. A. Potyrailo and V. M. Mirsky, Chem. Rev., 2008, 108, 770.
5 B. Timmer, W. Olthuis and A. van den Berg, Sens. Actuators, B,
2005, 107, 666.
Fig. 3 (A) Chemoselective detection of propylamine gas (28.5 ppb)
using 9 MHz QCM thin-coated with SIL 2 or [b-3C-im][NTf2]
(10 nL, 909 nm thickness). The sensorgrams of propylamine gas
detection by quartz chips coated with SIL 2 only and [b-3C-im][NTf2]
containing 1 mol% catalyst were shifted 2 and 5 Hz, respectively, for
clarity. (B) A plot of DF (Hz) vs. gas concentration (0, 17.1, 34.2, 51.3,
85.5 and 119.7 ppb).
6 H.-C. Kan, M.-C. Tseng and Y.-H. Chu, Tetrahedron, 2007, 63, 1644.
7 A 9 MHz QCM gas sensing device (the piezo sensor system
PSStwww.anttech.com.tw) was employed in this study.
8 M. A. Cooper and V. T. Singleton, J. Mol. Recognit., 2007, 20, 154.
9 A. F. Sowinski and G. M. Whitesides, J. Org. Chem., 1979, 44,
2369.
10 J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38,
1477.
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