A reusable test paper based on a simple salicylaldehyde derivate for the real-time detection of phosgene in gas phase

Article abstract of DOI:10.1016/j.saa.2021.119485

Phosgene is an important organic activity intermediate as well as a poisonous gas. However, the widespread use and abuse of phosphene brings potential risks to public safety. So it is very important to detect phosgene quickly and reliably. Up to now, a lo

Full text of DOI:10.1016/j.saa.2021.119485

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 251 (2021) 119485
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Short Communication
A reusable test paper based on a simple salicylaldehyde derivate for the
real-time detection of phosgene in gas phase
Jinbiao Zhu a, Xinyue Mu a, Shiqing Zhang a, Liqiang Yan a, , Xiongzhi Wu a,
⇑
⇑
a College of Chemistry and Bioengineering, Guilin University of Technology, Guilin, Guangxi 541006, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
The chemosensor has a simple
structure but excellent performance.
The chemosensor showed high
specificity and fast response to
phosgene.
ꢀ
The chemosensor loaded test paper
was successfully used in the
monitoring of phosgene vapor with
cyclic utilization.
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosgene is an important organic activity intermediate as well as a poisonous gas. However, the wide-
spread use and abuse of phosphene brings potential risks to public safety. So it is very important to detect
phosgene quickly and reliably. Up to now, a lot of chemical sensors based on organoluminescent groups
have been reported to monitor phosgene. However, most of them have complex molecular structures and
cannot be recycled during detection. Herein, we developed a simple and effective fluorescent chemosen-
sor using 5-chlorsalicylaldehyde as luminophor and azanol as recognition site. It exhibited significant flu-
orescence enhancement, excellent specificity and sensitivity. More importantly, the reusable test paper
prepared by this chemosensor has been successfully used in the point-of-care testing of gaseous
phosgene.
Received 3 December 2020
Received in revised form 9 January 2021
Accepted 11 January 2021
Available online 16 January 2021
Keywords:
Fluorescence detection
Phosgene
Test paper
Ó 2021 Elsevier B.V. All rights reserved.
Reversible detection
Salicylaldehyde
Hydroxylamine
1. Introduction
phosgene can cause a range of adverse symptoms, such as nausea,
vomiting, dizziness, cough, retrosternal discomfort, wheezing and
Phosgene (COCl2), as an important organic activity intermedi-
ate, has been widely applied in organic synthesis, pesticide, medi-
cine, dyestuff and other chemical products [1–5]. However, this
colorless gas is not easily detected but is highly toxic due to its
strong irritant and corrosive. The carbonyl groups of phosgene
molecules are readily combined with the proteins and enzymes
in lung tissues through acylation reaction, which can interfere with
the normal metabolism of cells, damage cell membranes, alveolar
epithelial cells and capillaries, thus leading to chemical pneumonia
and pulmonary edema [6]. Exposure to low concentration of
shortness of breath, and expectoration of blood [7]. Inhalation of
high concentration of phosgene can cause respiratory failure and
rapid death of the patients [8,9]. As a result, this cheap but highly
toxic reagent was used as a chemical weapon in warfare and was
responsible for many fatalities [10]. What’s more, the widespread
use of phosgene in industrial production brings the big risk of leak-
age, thereby casting a shadow over public safety. Therefore, it is
very important to detect phosgene quickly and reliably.
Some detection techniques, such as electrochemical analysis,
gas chromatography, and Raman spectroscopy have been devel-
oped for detecting phosgene [11–13]. Nevertheless, these test
methods are often time-consuming and require tedious process,
which seriously limits their application in on-the-spot detection
⇑
Corresponding authors.
E-mail addresses: liqiangyan@glut.edu.cn (L. Yan), 2004046@glut.edu.cn (X. Wu).
https://doi.org/10.1016/j.saa.2021.119485
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

J. Zhu, X. Mu, S. Zhang et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 251 (2021) 119485
of phosgene. Fluorescence analysis has been widely concerned
because of its simple operation, visual operation and real time
detection [14–16]. A lot of fluorescent probes and sensors have
been reported and showed excellent detection performances for
the monitor of phosgene [17–34]. However, these probes and sen-
sors still have some shortcomings need to be overcome, for
instance, complex and expensive synthesis, and being not reused
or recycled during detection.
In this paper, we designed and synthesized a simple and effec-
tive fluorescent molecule (chemosensor 1) selecting 5-
chlorsalicylaldehyde as luminophor and azanol as recognition site.
It exhibited significant fluorescence enhancement, excellent speci-
ficity and sensitivity. More importantly, the test papers prepared
by these fluorescent molecules realized the point-of-care testing
of phosgene, and could be reused. Therefore, this chemosensor
solved the perennial problem that the chemical sensor for phos-
gene is not reusable and cannot be detected in real time.
2.4. Preparation of test paper
The ordinary filter paper in the laboratory was cut into round
pieces with a diameter of 1.6 cm. These round strips were soaked
in CH2Cl2 solution of chemosensor (1 mM) for 1 min. Then, they
were taken out for drying. The dry strips were soaked again, and
repeated for 3 times.
10
202.5 mM) of triphosgene in CH2Cl2 were transferred into seven
reagent bottles (5 mL), respectively. After the addition of 10
lL of various concentrations (0, 3.38, 6.75, 33.75, 67.5, 135,
l
L
TEA, the bottle caps with the test pieces inside were immediately
placed to seal the reagent bottles. Different amounts (0, 0.5, 1, 5,
10, 20, 30 ppm) of gaseous phosgene were obtained assuming all
the triphosgene was involved in the reaction.
3. Results and discussion
3.1. Absorption spectra of chemosensor 1 toward phosgene
2. Experimental section
UV–vis spectra of chemosensor 1 (50
lM) with increasing
2.1. Instruments and reagents
triphosgene (0 – 210 M) were firstly measured in CH2Cl2 (con-
l
taining 0.1% TEA, v/v). Chemosensor 1 exhibited two main absorp-
tion bands located at 315 nm and 350 nm, respectively. Along with
the increase of triphosgene content (0, 30, 60, 90, 120, 150, 180,
NMR spectra were acquired on a NMR spectrometer (Bruker,
500 MHz) in DMSO d6 solution using TMS as the internal standard.
HRMS was performed on a LCMS (Shimadzu, 8080). Absorption
spectra were measured on spectrophotometer (Lambda 365). Fluo-
rescence spectra were measured on a fluorescence spectropho-
tometer (Hitachi, F700). All reagents, including triphosgene,
diethyl chlorophosphate (DCP), acetylchloride, dimethylaminozoyl
chloride, trifluoroacetic acid (TFA), SOCl2, 2, 4-dinitrobenzene sul-
fonyl chloride, POCl3, HCl, triethylamine (TEA), oxalyl chloride,
paratoluensulfonyl chloride (PTSC), oxalyl hydrazine, bisacry-
lamide, and various solvents (AR), were purchased from market
suppliers, and were used without additional treatment. In order
to ensure the safety of the experiment, triphosgene was selected
to replace phosgene, and reacted with triethylamine (TEA) to
release phosgene in CH2Cl2 solution.
210
lM), the absorbance of spectral band at 315 nm gradually
trailed off, while absorbance at 350 nm steadily improved
(Fig. 1). The change of UV–vis absorption spectra indicates that
the chemosensor can react with triphosgene in a certain concen-
tration range.
3.2. Sensitivity and linear range of chemosensor 1 toward phosgene
In CH2Cl2 solution (containing 0.1% TEA, v/v), the fluorescence
intensity of chemosensor 1 (50 lM) was very weak, and had a very
low fluorescence quantum efficiency (UF = 1.4%). The fluorescence
signal was barely visible under a 365 nm ultraviolet lamp. How-
ever, the fluorescence emission of chemosensor 1 strengthen grad-
ually along with the titration of a series of triphosgene (0, 15, 30,
45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195, 210, 225,
2.2. Synthetic procedure
240
could be observed under a 365 nm ultraviolet lamp when the con-
centration of triphosgene was 240 M (Fig. 2a Inset). What’s more,
a good linear relationship between the fluorescence intensity of
the chemosensor (50 M) and the concentration of triphosgene
lM) (Fig. 2a). And a strong blue fluorescence (UF = 18.3%)
5-chlorsalicylaldehyde (0.31 g, 2 mmol), hydroxylammonium
chloride (0.14 g, 2 mmol) and Et3N (0.5 mL) were dissolved in abso-
lute EtOH (15 mL), and the mixture was stirred and refluxed for 8 h
(Scheme 1). Then the grey-green transparent crystal (chemosensor
1) could be obtained after volatilizing solvent at room temperature.
Yield, 0.28 g (81.9%). 1H NMR (500 MHz, DMSO d6) d: 11.49 (s, 1H),
10.31 (s, 1H), 8.28 (s, 1H), 7.52 (s, 1H), 7.26–7.24 (d, J = 10 HZ, 1H),
6.93–6.91 (d, J = 10 HZ, 1H). 13C NMR (125 MHz, DMSO d6) d:
155.14, 145.93, 130.42, 126.64, 123.40, 120.69, 118.34. HRMS:
172.1112 ([M+H]+), calculated for C7H6ClNO2: 171.0087.
l
l
2.3. Spectral measurements
The stock solutions of chemosensor 1 and various analytes were
prepared in CH2Cl2 containing 0.1% TEA (v/v), and were diluted to
the required concentration for testing, respectively. All the spectral
tests were performed at room temperature. The excitation and
emission wavelengths were 350 nm and 410 nm, respectively.
The slid width of fluorescence spectra was 2.5 nm/5 nm.
Fig. 1. UV–vis spectra of chemosensor 1 (50
v) along with the addition of triphosgene (0–210
l
M) in CH2Cl2 (containing 0.1% TEA, v/
M).
Scheme 1. Synthesis of chemosensor 1.
l
2

J. Zhu, X. Mu, S. Zhang et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 251 (2021) 119485
Fig. 2. (a) Fluorescence spectra of chemosensor 1 (50
l
M) in CH2Cl2 (containing 0.1% TEA, v/v) along with the addition of various amounts of triphosgene. (b) Linearity
between the fluorescence intensity (410 nm) of chemosensor 1 (50 lM) and concentration of triphosgene (0–240 lM).
was found in a wide concentration range of triphosgene (0 –
240 M). And the linear equation was set up as y = 37.53x + 286.
58 (R2 = 0.9982) (Fig. 2b). The limit of detection (LOD) also could
be calculated as 4.0 nM using LOD = 3 /S, where 3 is SNR, rep-
phosgene did not decrease with the coexistence of other analysis
objects (Fig. 3b). Therefore, the chemosensor has excellent specific
recognition ability to phosgene.
l
r
r
resents RSD (5.1%) of fluorescence intensity of chemosensor 1, and
S is the slope (37.53) of the liner equation. These results indicate
that this chemosensor has turn on fluorescence response to phos-
gene, which can be used to detect phosgene quantitatively in a
wide range of phosgene concentration with a good LOD.
3.4. Response time of chemosensor 1 towards phosgene in solution
The response time of chemosensor 1 towards phosgene was
assessed by the change of fluorescence intensity over time in CH2-
Cl2 solution containing 0.1% TEA, v/v. The maximum fluorescence
intensity of the chemosensor was very weak and remained stable
for 44 min. On the contrary, when the chemosensor was treated
3.3. Selective identification of chemosensor 1 towards phosgene
with 240
lM of triphosgene, the fluorescence intensity at
410 nm gradually increased and reached a stable value after
30 min. Although the reaction process of chemosensor 1 towards
phosgene took 30 min, the difference in fluorescence intensity
before and after addition of phosgene was well differentiated after
a few minutes (Fig. 4).
The selectivity of chemosensor 1 (50
was further investigated in the presence of various analytes
(240 M) including phosgene, triphosgene, SOCl2, POCl3, acetyl
lM) towards phosgene
l
chloride, dimethylaminozoyl chloride, 2,4-Dinitrobenzenesulfonyl
chloride, TFA, HCl, oxalyl chloride, paratoluensulfonyl chloride,
oxalyl hydrazine, bisacrylamide, DCP. Phosgene could significantly
increase the fluorescence intensity of the chemosensor. Triphos-
gene (without TEA) could only cause fluorescence improvement
to a small extent. Nevertheless, none of other analytes could
change the fluorescence intensity of the chemosensor significantly
(Fig. 3a). In addition, this chemosensor was selective to phosgene
with little interference from other analytical objects. The maxi-
mum fluorescence intensity (410 nm) of the chemosensor with
3.5. Detection of gaseous phosgene by test paper
In order to realize the quick detection for phosgene, chemosen-
sor 1 was fixed on the common filter paper to develop a simple and
convenient test paper. The test paper stained with chemosensor 1
displayed faint dark blue fluorescence under 365 nm ultraviolet
light. However, when they were exposed to different concentra-
Fig. 3. (a) Selectivity and (b) specificity of chemosensor 1 (50
lM) to triphosgene (240 lM) and other analytes (240 lM). (1) SOCl2, (2) POCl3, (3) acetyl chloride, (4)
Dimethylaminozoyl chloride, (5) TFA, (6) HCl, (7) 2,4-Dinitrobenzenesulfonyl chloride, (8) Oxalyl chloride, (9) paratoluensulfonyl chloride, (10) Oxalyl hydrazine, (11)
bisacrylamide, (12) DCP.
3

J. Zhu, X. Mu, S. Zhang et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 251 (2021) 119485
Fig. 6. The proposed detection mechanisms of chemosensor 1 towards phosgene in
solution (a) and gas phase (b).
Fig. 4. Change in the fluorescence intensity (410 nm) of chemosensor 1 (50
lM)
after the addition of triphosgene (240 lM).
experimental results, the detection mechanism was suggested. In
solution, the PET process of C@N double bond to benzene ring dis-
sipated the energy of the excited chemosensor molecule, hence
chemosensor 1 showed very weak fluorescence. Under the action
of phosgene, the chemosensor molecule was further transformed
into compound 2 slowly, and the PET process was inhibited
(Fig. 6a). As a result, the fluorescence of the chemosensor signifi-
cantly enhanced. On the test paper, the chemosensor molecule
could quickly form compound 3 with phosgene by the hydrogen-
bonding interaction, which could destroy the PET process within
the chemosensor molecule, thus the fluorescence of chemosensor
1 increased rapidly. However, compound 3 formed by hydrogen
bond was unstable, and quickly became chemosensor 1 in the
absence of phosgene (Fig. 6b). Therefore, the detection of phosgene
by chemosensor 1 in solution had a long equilibrium time and the
fluorescence signal was irreversible, but had a short response time
and could be recycled on the test paper.
tions (0.5, 1, 5, 10, 20, 30 ppm) of phosgene for only a few seconds,
the fluorescence color of these test papers gradually changed to
bright blue, accompanied by a significant increase of fluorescence
(Fig. 5a). More importantly, when these test papers were removed
from the phosgene environment for 1 min, the color and fluores-
cence intensity returned to normal. Furthermore, the test papers
could still detected phosgene visually when they were reused ten
times (Fig. 5b). These results show that the chemosensor can con-
veniently detect phosgene in real time, and can be reused many
times.
3.6. Detection mechanism of chemosensor 1 towards phosgene
The different response rate of chemosensor 1 towards phosgene
in solution and gas prompted us to further explore its detection
mechanism by NMR and HR-MS spectra. After the reaction with
phosgene in solution, the hydroxy hydrogen signal (AC@NAOH)
at 10.31 ppm in 1H NMR spectrum of the chemosensor 1 disap-
peared, and the other signal at 11.49 ppm of hydroxyl hydrogen
(Ar-OH) moved to 10.79 ppm (Fig. S5), suggesting that the hydro-
xyl group (AC@NAOH) was involved in the reaction with phos-
gene. In addition, the molecular peak at 172.1112 ([M+H]+) of
chemosensor 1 in HR-MS disappeared, while a new peak at
369.2004 appeared (Fig. S6). The peak at 369.2004 should be
ascribed to the molecular peak of compound 2. Based on these
4. Conclusions
In brief, a simple chemical sensor was developed through the
Schiff
base
condensation
reaction
between
5-
chlorosalicylaldehyde and hydroxylamine. It has been proven to
be capable of reliably detection of phosgene with good selectivity
and a low LOD of 4 nM in solution. More importantly, this chemical
sensor could be stained on common filter paper to make test strip,
realizing the rapid and reversible detection of gaseous phosgene.
This study provides an idea for the design and development of
reversible phosgene probe.
CRediT authorship contribution statement
Jinbiao Zhu: Investigation, Formal analysis, Methodology.
Xinyue Mu: Investigation, Formal analysis, Methodology. Shiqing
Zhang: Investigation, Formal analysis, Methodology. Liqiang
Yan: Conceptualization, Investigation, Formal analysis, Methodol-
ogy, Writing - original draft, Supervision, Project administration.
Xiongzhi Wu: Conceptualization, Formal analysis, Methodology,
Resources, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Fig. 5. (a) Color change of chemosensor test papers upon exposure to various
amounts of phosgene vapor. (b) A reversible process for detecting phosgene with
test papers.
4

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Acknowledgements
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