A simple turn-on fluorescent chemosensor for CO2 based on aggregation-induced emission: Application as a CO2 absorbent screening method

Article abstract of DOI:10.1016/j.dyepig.2018.11.031

An α-cyanostilbene-based fluorescent chemosensor, which includes a primary amine, was developed for the quantitative analysis of CO₂ in solution. (Z)-3-(4-(3-Aminopropoxy)phenyl)-2-(4-nitrophenyl)acrylonitrile (1) reacts with CO₂ and enables a turn-on type CO₂ chemosensor with increased fluorescence due to aggregation-induced emission. More importantly, 1 required no additives such as an amine media for detecting CO₂, and the limit of detection was estimated to be ～26 ppm. This system was successfully applied to determine the relative efficiency of CO₂ absorbents dependent on the fluorescence change in 1. The use of this chemosensor as a CO₂ absorbent screening method suggests that 1 may be used to develop a high-throughput screening method for the evaluation of high-performance CO₂ absorbents.

Full text of DOI:10.1016/j.dyepig.2018.11.031

Accepted Manuscript
A simple turn-on fluorescent chemosensor for CO based on aggregation-induced
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emission: Application as a CO absorbent screening method
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Mincheol Jang, Seungyoon Kang, Min Su Han
PII:
S0143-7208(18)32100-4
https://doi.org/10.1016/j.dyepig.2018.11.031
DYPI 7175
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 September 2018
Revised Date: 13 November 2018
Accepted Date: 15 November 2018
Please cite this article as: Jang M, Kang S, Han MS, A simple turn-on fluorescent chemosensor for CO
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based on aggregation-induced emission: Application as a CO absorbent screening method, Dyes and
2
Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.031.
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Graphical Abstract

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A Simple Turn-On Fluorescent Chemosensor for CO Based on Aggregation-Induced
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Emission: Application as a CO Absorbent Screening Method
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Mincheol Jang,‡ Seungyoon Kang,‡ and Min Su Han*
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Department of Chemistry, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu,
Gwangju 61005, Republic of Korea.
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*Corresponding author; Tel.: +82 62 715 2850; Fax: +82 62 715 2866.
E-mail address: happyhan@gist.ac.kr (M. S. Han)
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‡ M. J. and S. K. contributed equally to the work in this paper
Abstract
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An α-cyanostilbene-based fluorescent chemosensor, which includes a primary amine, was developed
for the quantitative analysis of CO2 in solution. (Z)-3-(4-(3-Aminopropoxy)phenyl)-2-(4-
nitrophenyl)acrylonitrile (1) reacts with CO and enables a turn-on type CO chemosensor with increased
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fluorescence due to aggregation-induced emission. More importantly, 1 required no additives such as an amine
media for detecting CO , and the limit of detection was estimated to be ~26 ppm. This system was successfully
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applied to determine the relative efficiency of CO absorbents dependent on the fluorescence change in 1. The
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use of this chemosensor as a CO absorbent screening method suggests that 1 may be used to develop a high-
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throughput screening method for the evaluation of high-performance CO absorbents.
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Key words: CO ; Fluorescent chemosensor; Aggregation-induced emission; CO absorbent screening; High
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throughput screening
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1. Introduction
CO has been of international interest owing to environmental concerns with respect to CO emissions,
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because these have been reported to contribute to the greenhouse effect. This has made the detection of CO an
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important issue [1,2]. Moreover, the detection of CO can provide useful applications for the development of
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absorbents to reduce greenhouse gas emission, because CO detection plays a key role in determining the
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efficiency of CO absorbents [3,4]. However, the development of CO detection methods has been limited by the
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inherent properties of CO , such as its inactiveness [5]. Electrochemical (EC), non-dispersive infrared (NDIR),
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and gas chromatography-mass spectrometry (GC-MS) have been used as major protocols to detect CO , and are
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being continuously improved. Unfortunately, these methods require time-consuming measurements and the use
of advanced and high-cost equipment [6–9].
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As an alternative to these methods, fluorescent chemosensors have been attractive candidates due to
their simplicity, cost-effectiveness, and rapid detection capabilities [10–12]. In particular, several organic
molecule-based fluorescent chemosensors systems for CO detection have been reported, due in part to their
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ease of synthesis, high quantum yields, and biocompatibility [13]. For example, pH indicator-based fluorescent
chemosensors, such as 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) and diketopyrrolopyrrole (DPP), have been
designed through a reversible equilibrium with carbonic acid [14,15]. Other examples are deprotonation-assisted
fluorescent chemosensors which are a combination of fluorescent dyes and a base, including triethylamine and
fluoride ions, respectively [16–18]. Furthermore, various aggregation-induced emission (AIE) fluorescent dyes
in amine media have been successfully applied to the detection of CO through a mechanism where the amine
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media reacts with CO and results in the formation of a viscous carbamate ionic liquid (CIL) [19].
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Chemosensor-based high-throughput screening (HTS) methods have several advantages over
conventional screening methods because these methods are simple, fast, and cost effective. Therefore, these
screening methods have been extensively used in the development of catalysts, drug candidates, and NOx
absorbents [20–23]. Although the aforementioned fluorescent chemosensors can be used as highly sensitive
detectors of CO , they have a limitation when applied to screening the efficiency of CO absorbents with the
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HTS method. Specifically, these chemosensors can require additives (i.e., amine media and bases) or are
sensitive to pH changes. It is also important to reduce external interference, to develop an effective CO2
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absorbent screening method. Conventional CO absorbents currently available in the industry tend to have
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unavoidable chemical influences, such as a change in pH or interference from amine derivatives present in the
system. These influences can make it difficult to effectively evaluate the CO absorbents. Therefore, it is highly
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desirable to develop a new chemosensor for CO absorbent screening.
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α-Cyanostilbene, one of the AIE–based fluorogens, forms aggregates based on a stacking effect due to
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π-π conjugation, resulting in the AIE phenomenon. Compared with that of traditional fluorogens, α-
cyanostilbene has a more intense fluorescence emission which is paired with a good photostability in the
aggregated state, relative to a dilute solution [24]. Also, their photoluminescence property could be adjusted by
varying functional group of terminal aryl groups. Especially, the 4-nitro-4’methoxy-α-cyanostilbene showed a
remarkable difference of fluorescence intensity between solid and solution with a large Stokes shift [25].
Furthermore, α-cyanostilbene can be easily designed as a turn-on type fluorescent chemosensor that can take
advantage of the AIE properties of this molecule, by introducing different functional groups to promote self-
assembly [26]. Herein, the α-cyanostilbene derivative containing a primary amine group (1) is proposed as a
simple turn-on type AIE-based fluorescent chemosensor that does not require the use of additives for the
detection of CO and for the design of a chemosensor-based HTS method for CO absorbent screening. The
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primary amine of 1 reacts with CO to form carbamic acid [27,28], which interacts with other amine molecules
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of 1 to form salt bridges [29]. This electrostatic interaction between the carbamate and ammonium salt induces
self-assembly and results in an enhancement in the fluorescence intensity due to the increase in size of the
aggregates of 1. Furthermore, bubbling N into the solution induces the transformation of the carbamate salt to
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the primary amine and CO [30] which results in a decrease in the fluorescence, through the relaxation of the
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aggregates. Therefore, this reversible turn-on type fluorescent chemosensor is reusable and allows for the
quantitative analysis of CO , measured by the increase in fluorescence emission, without the need for additives,
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as shown in Scheme 1.
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Scheme 1. Schematic illustration of compound 1-based CO sensing
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2. Experimental section
2.1. General
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All the materials were purchased from commercial suppliers (Sigma-Aldrich, Tokyo Chemical
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Industry, Alfa Aesar, and SHINYO) and were used without further purification. H and C NMR spectra were
recorded on a Jeol 400 MHz NMR spectrometer. Mass spectra were obtained using an Agilent ESI-Q/TOF
(Quadrupole/Time-of-flight) mass spectrometer. Melting points were measured using a BUCHI Melting point
M-565 apparatus. Fluorescence spectra were recorded with a SCINCO FS-2 fluorescence spectrometer.
Absorption spectra were recorded with a JASCO V-630 UV-vis spectrometer. Fluorescence lifetime was
measured by an FLS-980 fluorometer (Edinburgh Instruments Ltd.) equipped with time-correlated single photon
counting (TCSPC) instruments (TCC2) and Red PMT (R928P-Hamamatsu Photonics K.K.). In gas experiments,
CO gas flowed through an MKP TSC-200 mass flow controller (MFC), the size of aggregates was measured
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using a Malvern Zetasizer Nano-ZS dynamic light scattering (DLS) instruments, and all the experiments were
performed in solution.
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2.2. Synthesis of chemosensor
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Scheme 2. Synthetic procedures for compound 1
2.2.1. Synthesis of 3
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3 was synthesized according to previously reported procedures [31]. 3-bromopropylamine (2.07 g,
15.00 mmol) and triethylamine (8.36 mL, 60.00 mmol) were dissolved in dichloromethane (50 mL). After
stirring at room temperature for 30 min, di-tert-butyl dicarbonate (3.93 g, 18.00 mmol) was added, and stirred at
room temperature for 48 h. The solvent was removed under reduced pressure to give a white oily mixture. This
mixture was dissolved in ethyl acetate and washed with water and brine. Then, the organic layer was dried with
anhydrous sodium sulfate, and filtered. The organic layer was concentrated under reduced pressure to give a
white oil. The oil was purified by column chromatography (silica gel, hexane:ethyl acetate = 5:1 was used as the
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eluent) to yield 3 (1.22 g, 34%) as a colorless oil. H-NMR (400 MHz, CDCl ) δ 4.73 (s, 1H), 3.43–3.40 (t, J =
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6.0 Hz, 2H), 3.27–3.22 (m, 2H), 2.06–1.99 (m, 2H), 1.42 (s, 9H).
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2.2.2. Synthesis of 2
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2 was synthesized according to previously reported procedures [32]. To a mixture of 4-
hydroxybenzaldehyde (2.00 g, 16.40 mmol) and 4-nitrobenzyl cyanide (2.66 g, 16.40 mmol) in absolute ethanol
(40 mL), was added piperidine (2.43 mL, 24.60 mmol) in portions over 30 min. The reaction mixture was stirred
at room temperature for 6 h, cooled to 0 °C, and filtered. The precipitate was washed with ethanol. The brown
colored solid was dissolved in ethyl acetate and washed with water and brine. The organic layer was dried with
anhydrous sodium sulfate, and filtered. Then, the organic layer was concentrated under reduced pressure to give
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a yellow solid. The solid was recrystallized from ethanol to afford 2 (3.98 g, 91%) as a yellow solid. H-NMR
(400 MHz, DMSO-d ) δ 10.48 (s, 1H), 8.34–8.32 (m, 2H), 8.17 (s, 1H), 8.00–7.93 (m, 4H), 6.97–6.93 (m, 2H);
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mp 202–204 °C.
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2.2.3. Synthesis of 1
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A mixture of (Z)-3-(4-hydroxyphenyl)-2-(4-nitrophenyl)acrylonitrile (0.27 g, 1.00 mmol) and anhydrous
potassium carbonate (0.15 g, 1.10 mmol) in dimethylformamide (5 mL) was stirred at room temperature for 10
min, followed by the addition of 3 (1.19 g, 5.00 mmol). The reaction mixture was then stirred at 70 °C for 20 h,
and cooled to room temperature slowly. The reaction mixture was poured into water and allowed to stand for 6 h.
The precipitate was filtered and dried to give an orange-colored solid. The solid was dissolved in chloroform
and washed with water and brine. The organic layer was dried with anhydrous sodium sulfate, filtered, and
concentrated under reduced pressure to give an orange solid. The solid was purified by column chromatography
(silica gel, chloroform:methanol = 100:1 was used as the eluent) to yield a yellow solid. The material was used
for the next reaction without any further purification. To the material (0.70 g, 1.65 mmol) in ethyl acetate (60
mL) was added hydrogen chloride gas until no further precipitation occurred; then, the reaction mixture was
slowly cooled to 0 °C. The precipitate was filtered and dried to give a yellowish solid. The solid was dissolved
in water (300 mL) and stirred at room temperature for 30 min. Then, a 1 M aqueous solution of sodium
hydroxide was added until the pH reached approximately 8. The yellow precipitate was extracted with
chloroform and washed with water and brine. The organic layer was dried with anhydrous sodium sulfate, and
filtered. Then, the organic layer was concentrated under reduced pressure to give 1 (0.36 g, 68%) as a greenish
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brown solids. H-NMR (400 MHz, CDCl ) δ 8.31–8.28 (m, 2H), 7.95–7.93 (m, 2H), 7.83–7.80 (m, 2H), 7.60 (s,
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1H), 7.03–6.99 (m, 2H), 4.16–4.14 (t, J = 6.0 Hz, 2H), 2.96–2.92 (t, J = 6.6 Hz, 2H), 2.00–1.94 (m, 2H); C-
NMR (100 MHz, CDCl ) δ 161.94, 147.56, 145.15, 141.20, 132.05 (2C), 126.41 (2C), 125.70, 124.41 (2C),
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117.85, 115.19 (2C), 106.12, 66.28, 39.07, 32.82; H-NMR (400 MHz, THF-d ) δ 8.31–8.28 (m, 2H), 8.03–7.99
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(m, 2H), 7.96–7.93 (m, 3H), 7.08–7.04 (m, 2H), 4.18–4.15 (t, J = 6.4 Hz, 2H), 2.84–2.80 (t, J = 6.6 Hz, 2H),
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1.90–1.83 (m, 2H); C-NMR (100 MHz, THF-d ) δ 162.93, 148.35, 145.64, 142.05, 132.55 (2C), 127.02 (2C),
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126.73, 124.72 (2C), 118.14, 115.59 (2C), 106.49, 39.44, 33.85; mp 87–90 °C; ESI-MS m/z calculated for
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C H N O [M+H] 324.1343, found 324.1337.
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2.3. Optimization of solvent polarity
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To obtain the optimized system for AIE phenomenon, the change in solvent polarity was taken with
THF and hexane. All sample solutions (1 mL) were prepared by increasing the hexane to THF volume ratio with
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10% units. The final concentration of 1 in each sample solution was maintained at 1 mM. All the fluorescence
spectra were obtained by a SCINCO FS-2 fluorescence spectrometer after bubbling each sample with CO (80
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mL) and without CO . Excitation wavelength, 365 nm; slit widths, 10/10 nm; emission wavelength, 565 nm.
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2.4. General sampling method
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THF and hexane were degassed by bubbling them with N for 30 min before use. The final
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concentration of the sample solution was 1 mM of 1 in the THF:hexane (1:9, v/v) system (900 µL hexane + 100
µL of 10 mM 1 stock). All the sample solutions were degassed before measurements were taken.
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2.5. Measurement of the size change of the aggregates
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The size change of the aggregates was measured by DLS, following the above general sampling
method. Based on the CO and N titration, the control solution was bubbled with N (10 mL) and the sample
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solution was bubbled with CO (0.09 mL).
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2.6. CO titration
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All the fluorescence spectra for the CO titration of 1 were obtained by a SCINCO FS-2 fluorescence
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spectrometer following the above general sampling method: excitation wavelength, 365 nm; slit widths, 10/10
nm; and emission wavelength, 565 nm. The injected volume of CO gas was changed using an MFC with 0.01
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2.7. Test of reversibility of 1
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The fluorescence spectra of 1 (1 mM, 1 mL) with an amount of CO associated with AIE behavior
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were observed in the THF:hexane (1:9, v/v) system: excitation wavelength, 365 nm; slit widths, 10/10 nm; and
emission wavelength, 565 nm. Based on the CO and N titration, the same sample solution was bubbled with
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CO (0.09 mL) and N (10 mL). The change in the fluorescence spectra upon bubbling each gas another time
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was subsequently measured.
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2.8. CO absorbents screening method based on 1
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The stock solutions were prepared in THF. The final sample solutions (1 mL) were composed of 1 (1
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absorbent (20 mM). The control sample was composed of degassed hexane (900 µL) + stock solution of 1 (20
mM, 50 µL) + degassed THF (50 µL), and the other samples were composed of degassed hexane (900 µL) +
stock solution of 1 (20 mM, 50 µL) + stock solution of the CO absorbent (20 mM, 50 µL). After bubbling CO
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(0.09 mL) through each sample, all the fluorescence spectra of the samples containing CO absorbents were
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normalized relative to the control sample: excitation wavelength, 365 nm; slit widths, 10/10 nm; and emission
wavelength, 565 nm.
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3. Results and discussion
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The change in solvent polarity was first conducted to utilize the designed molecule (1) as a fluorescent
chemosensor for CO . The excellent solubility of 1 in pure THF led to no fluorescence detection, whereas the
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solution of only hexane resulted in poor solubility and excessive aggregates, leading to uneven fluorescence
emission. Therefore, a combination of THF and hexane was required to optimize the solvent polarity for the
introduction of the AIE phenomenon. According to the absorption spectrum in THF (Fig. S1), the excitation
wavelength obtained at 365 nm demonstrated optimal fluorescence intensities. As the hexane to THF ratio
gradually increased, with the concentration of 1 maintained at 1 mM, the normalized fluorescence intensity was
plotted at 565 nm (Fig. 1b). As a result, the optimized THF:hexane (1:9, v/v) system was chosen, as it produced
the maximum emission for CO detection.
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Fig. 1. (a) Normalized fluorescence spectra of 1 (1 mL, 1 mM) versus fraction of hexane in THF after bubbling with CO (80
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mL) and (b) plot of normalized fluorescence intensity of 1 (1 mL, 1 mM) at 565 nm, versus fraction of hexane in THF,
including hexane mixtures bubbled with CO (80 mL, red) and without CO (blue). Excitation wavelength, 365 nm; slit
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To demonstrate the aggregation of 1, the size of the aggregates in the THF:hexane (1:9, v/v) solution
before and after bubbling with CO were measured by DLS. The average particle diameter of the aggregates
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before bubbling with CO was 86.99 nm (Fig. 2a), and that after bubbling with CO was 774.1 nm (Fig. 2b), a
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9-fold increase in size. These DLS data and other observed spectral changes show an increase in aggregation
when the solution was bubbled with CO , resulting in an increase in the fluorescence intensity of 1. Furthermore,
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the NMR study was conducted to elucidate the formation of carbamic acid by bubbled with CO . Each NMR
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spectra of a solution of 1 were measured in THF-d before and after bubbling with an excess CO . The H-NMR
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signal of α-proton of amine of 1 was shifted to downfield including a change of the splitting pattern from a
triplet to a quartet after CO bubbling (Fig. S5). Also the new peak at δ 158.60 was appeared after CO bubbling
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in C-NMR spectrum (Fig. S6). These results are in good agreement with previously reported NMR studies of
amine-based CO absorption [33,34].
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Fig. 2. Size analysis of aggregates in THF:hexane (1:9, v/v) (a) after bubbling with N (10 mL) and (b) after bubbling with
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CO (0.09 mL).
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The changes in the fluorescence spectra were measured as the volume of CO introduced into the
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system was increased (as controlled using an MFC) in a solution of 1. As the amount of CO increased, the
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fluorescence of 1 gradually increased, as shown in Fig. 3a. The obtained fluorescence spectral changes for 1
tended to be proportional to the increase in CO , which reached saturation when about 0.09 mL of CO was
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added to a solution of 1 (Fig. 3b). Based on these results, the limit of detection of 1 was estimated to be ca. 26
ppm, as shown in Fig. S2. These results indicate that the quantitative measurement of CO can be conducted
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through changes in the fluorescence intensity of 1.
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Fig. 3. (a) Normalized fluorescence spectra and (b) plot of normalized fluorescence intensity of 1 (1 mL, 1 mM) at 565 nm,
in THF:hexane (1:9, v/v) with step-up CO bubbling (0.01 mL unit). Excitation wavelength, 365 nm; slit widths, 10/10 nm.
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Since reversible chemosensors for CO are likely to be used in environmental monitoring and irregular
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and dynamic changes of CO , it was very important to demonstrate the reversible behavior of 1. To obtain
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reversible fluorescence spectral changes, an N titration test was preferentially performed at the point where the
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fluorescence intensities reached a maximum, as shown in Fig. S3. The reversible fluorescence changes were
measured when the solution was alternately bubbled with a certain volume of the two gases (CO and N ) that
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saturated the fluorescence intensity of 1. As the volume of bubbled N increased after bubbling CO , the
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fluorescence of 1 decreased due to the release of the aggregates through the transformation of the carbamate salt
to the primary amine and CO , by N . Compared with the original fluorescence intensity, a similar fluorescence
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intensity was observed upon bubbling CO after bubbling N in the second cycle. Therefore, CO induced an
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increase in the fluorescence of 1, whereas N reverted the fluorescence of 1 to its initial state. In multiple cycles,
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the fluorescence changes in 1 showed good reproducibility (Fig. 4).
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Fig. 4. Normalized fluorescence intensity of 1 (1 mL, 1 mM) at 565 nm, in THF:hexane (1:9, v/v) with repeated bubbling of
CO (0.09 mL) and N (10 mL). Excitation wavelength, 365 nm; slit widths, 10/10 nm.
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In terms of contributing to the efforts of reducing the impacts of global warming, the importance of
industrial application for capturing and fixing CO has been emphasized. According to previous research,
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various kinds of CO absorbents have been studied to address these problems [35]. The evaluation of CO
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absorbent efficiency is a key step in the development of CO absorbents. The conventional screening methods
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for absorbents require multistep processes, such as a screening test through a specific apparatus, a vapor–liquid
equilibrium test, and heat of reaction measurements [36]. Thus, it is highly meaningful to develop a selection
method with less limitations than the current methods, in the aspect of screening a large number of absorbent
candidates. When compared with conventional technologies, fluorescent chemosensors-based screening
methods have the advantages of a more economical method and a more rapid analysis. From this perspective,
the CO chemosensor 1 was used to evaluate the efficiency of CO absorbents. The efficiency of the CO
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absorbents was expected to be indirectly measured through the change in fluorescence, based on the relative
CO reactivity between the absorbent and 1. A strong CO absorbent in the sample solution results in a weak
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fluorescence of 1 because most of the CO is captured by the absorbent rather than 1. In the opposite case,
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strong fluorescence emission of 1 suggests that 1 is preferentially capturing most of the CO and exhibiting AIE
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behavior, as shown in Scheme 3.
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Scheme 3. Schematic illustration of CO absorbent screening
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When comparing various CO absorbents, alkanolamine derivatives were chosen to demonstrate the
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proposed screening method, as they are widely used in current industrial technologies for the removal of CO2,
due to their high affinity for CO [4]. Each relative change in fluorescence indicates the ability of the CO
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absorbent to capture CO relative to 1, as shown in Fig. 5. Strong CO absorbents, such as 2-piperidinemethanol
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(PM) and monoethanolamine (MEA), showed little fluorescence change in solution, whereas a relatively weak
CO absorbent induced a strong fluorescence change. According to previous studies, the relative CO absorbing
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capacity of the CO absorbents used in this experiment were as follows: 2-piperidinemethanol (PM) >
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monoethanolamine (MEA) and N-methylethanolamine (MMEA)
>
diethanolamine (DEA) and
dimethylethanolamine (DMEA) > N-methyldiethanolamine (MDEA) > triethanolamine (TEA) [37–40]. The
findings of this study have a good correlation with these previously reported results. Therefore, this screening
method has the potential to be used for a variety of applications to distinguish CO absorbents through relative
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fluorescence changes.
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Fig. 5. Normalized fluorescence intensity of 1 (1 mL, 1 mM) at 565 nm, after bubbling with CO (0.09 mL) in THF:hexane
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(1:9, v/v) using various CO absorbents (1 mM). Excitation wavelength, 365 nm; slit widths, 10/10 nm.
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4. Conclusion
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α-Cyanostilbene with a primary amine as the reactive functional group was developed as an AIE-
based fluorescent chemosensor (1) for CO without the need of additives such as amine media and anions. This
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CO -sensitive turn-on type signaling is based on the AIE phenomenon caused by the increase in size of the
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aggregates after bubbling the solution with CO . In addition, 1 showed excellent reversibility when alternately
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bubbling CO and N . Furthermore, 1 can also be utilized as a method for screening the relative efficiency of
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CO absorbents by comparing the changes in the fluorescence intensities. Thus, this method has the potential to
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be used in a high-throughput screening (HTS) method to effectively select high-performance CO absorbents
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from a large number of candidates.
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Acknowledgements
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This research was supported by GIST Research Institute (GRI) in 2018 and this work was supported
by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-
2017M1A2A2049102, NRF-2017R1A2B4009652, and NRF-2018R1A4A1024963).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at doi ****
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Highlights
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α-Cyanostilbene-based fluorescent chemosensor for CO was developed by using AIE
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Chemosensor enables the detection of CO with no additives, such as amine and anion
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This chemosensor was applied to determine the relative efficiency of CO absorbents
2