A highly sensitive fluorogenic chemodosimeter for rapid visual detection of phosgene

Article abstract of DOI:10.1039/c2cc17411a

A highly sensitive chemodosimeter was identified from a panel of rhodamine derivatives for rapid and visual detection of phosgene with a detection limit of 50 nM triphosgene. Visual detection of gaseous phosgene with chemodosimeter absorbed paper strips was demonstrated.

Full text of DOI:10.1039/c2cc17411a

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COMMUNICATION
A highly sensitive fluorogenic chemodosimeter for rapid visual detection
of phosgenew
Xuanjun Wu, Zhisheng Wu, Yuhui Yang and Shoufa Han*
Received 28th November 2011, Accepted 12th December 2011
DOI: 10.1039/c2cc17411a
A highly sensitive chemodosimeter was identified from a panel of
rhodamine derivatives for rapid and visual detection of phosgene
with a detection limit of 50 nM triphosgene. Visual detection of
gaseous phosgene with chemodosimeter absorbed paper strips
was demonstrated.
proceeded via triphosgene mediated hetero-dimerization of a
fluorescent donor and an acceptor fluorophore.4 The resultant
proximity of the donor–acceptor pair imparts efficient intra-
molecular fluorescence resonance energy transfer (FRET), leading
to enhanced fluorescence emission of the acceptor upon excitation
of the donor dye.4 However, the formation of acceptor–acceptor
or donor–donor homodimers could not be prevented under the
assay conditions which compromised the assay efficiency. In
addition, the detection limits of FRET based assays are often
limited by interference due to overlap of emission of the donor
with that of the acceptor.
Phosgene (COCl2) is a widely used chemical intermediate in
industry to produce pesticides, isocyanate-based polymers,
pharmaceuticals, etc.1 Exposure to phosgene can cause severe
damages to respiratory tracks and lungs, often leading to
individual death.2 As a colorless and highly toxic gas, phosgene
had been used as a chemical warfare agent and is currently a
potential agent for terrorist chemical attack. Hence methods
allowing sensitive and rapid detection of phosgene are highly
desirable to alert both the leakage of phosgene gas in industry
and the purposeful deployment of phosgene.
Rhodamines are widely utilized in cellular imaging and bio-
technology owing to their exceptional photophysical properties.
Rhodamine-(deoxy)lactams, which are prone to analyte mediated
opening of the spiro-(deoxy)lactam to give highly fluorescent and
deep colored rhodamine fluorophores, have been extensively
employed as the fluorogenic platforms for sensing of various
metal ions and reactive chemicals.6 Here we report the visual
and chromo-fluorogenic detection of a phosgene simulant with
N-(rhodamine B)-deoxylactam-ethylenediamine (referred as dRB-
EDA) (Scheme 2), a highly sensitive chemodosimeter identified
from a panel of rhodamine-(deoxy)lactams.
Conventional assays for phosgene, e.g. gas chromatography,
are often limited by poor portability or by the requirement of
sophisticated procedures.3 Optical sensors are advantageous as
they use widely available instruments and offer the possibility of
real-time visual detection of analytes. Several chemosensors,4,5
e.g. analyte mediated aggregation of gold nanoparticles and
chromogenic/fluorogenic reagents, have been developed to
detect phosgene. One elegant strategy, as shown in Scheme 1,
dRB-EDA, which was colorless and nonfluorescent,
instantly turned into red color in dimethylformamide (DMF)
upon addition of triphosgene. Triphosgene, the safer substi-
tuent of phosgene used in organic synthesis, decomposes into
phosgene in the presence of tertiary amines in solution. Hence
it was used as a phosgene simulant in previous research and in
this report.4 To optimize the assay conditions, the color
formation rates of dRB-EDA with triphosgene in a variety
of solvents containing triethylamine (TEA, 3%, v/v) were
monitored by fluorescence emission. Fig. S1 (ESIw) shows that
DMF was the preferred assay media compared to acetonitrile
or aqueous DMF (ESIw). The assay was applicable in aqueous
DMF containing 2% (v/v) of water, suggesting its compatibility
Scheme 1 A fluorescence resonance energy transfer (FRET) based
method for detection of a phosgene simulant.4
Department of Chemical Biology, College of Chemistry and
Chemical Engineering, and the Key Laboratory for Chemical Biology
of Fujian Province Xiamen University, Xiamen, China 361005.
E-mail: shoufa@xmu.edu.cn; Tel: +86-0592-2181728
w Electronic supplementary information (ESI) available: Purification
and characterization of compound 2, fluorescence spectra of the
colored species, assay procedures for triphosgene, and characterization
of the reaction of triphosgene with rhodamine-hydroxamate, rhodamine-
hydrazide and RB-EDA. See DOI: 10.1039/c2cc17411a
Scheme 2 Low-background and chromo-fluorogenic detection of
phosgene with nonfluorescent dRB-EDA.
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Fig. 1 Fluorescence emission spectra of dRB-EDA (1 mg mlÀ1) in
the presence of triphosgene (1, 0.8, 0.6, 0.4, 0.2, 0.1, 0.05 mM, from top
to bottom) in DMF containing TEA (A). The titration curve was
plotted by fluorescence emission intensity at 590 nm vs. analyte
concentrations (B) (lex@560 nm).
Fig. 3 HRMS confirmation of the genesis of compound 2 in the
reaction between dRB-EDA and triphosgene.
with practical applications where the moisture might be present
in the samples. Kinetic analysis showed that the reaction between
dRB-EDA and triphosgene completed immediately in various
assay media (ESIw, Fig. S2) which were independent of the
analyte concentrations.
afford compound 2 as the colored species (Scheme 2; ESIw,
Scheme S2). To probe the underlying mechanism of the chromo-
genic response of dRB-EDA towards triphosgene, the colored
species generated in the assay solution was isolated by silica gel
chromatography. High resolution mass spectrometry (HRMS) of
the colored species revealed a major peak located at 497.2910
(Fig. 3), which was consistent with the theoretical molecular
weight of compound 2 (497.2911). 1H-NMR and 13C-NMR
spectra analysis (ESIw, Fig. S10 and S11) further supported the
formation of compound 2 in the assay system. The fluorescence
spectra of the assay solution were almost identical to those of
rhodamine B (ESIw, Fig. S3 and S4). Taken together, the
experimental data suggest the triphosgene triggered opening of
the intramolecular deoxylactam of dRB-EDA under the assay
conditions (Scheme 2).
To determine the detection limit, the fluorescence emission
and UV-vis absorbance of dRB-EDA in DMF containing
TEA were recorded vs. triphosgene concentration. Fig. 1
shows that the fluorescence emission intensity at 590 nm
increased as a function of analyte concentration. As low as
50 nM of triphosgene was clearly identified under the assay
conditions whereas a detection limit of 50 mM of triphosgene
was reported for a FRET based system.4 The new assay, with
3 orders of magnitude enhancement of sensitivity and low-
background interference, is advantageous for practical appli-
cations where it is imperative to detect 0.1 or 2 ppm of gaseous
phosgene (0.4–8 mg LÀ1), which are the thresholds for the low
permissible exposure limit and the immediate danger to health
and life limit.
We then explored the possible coupling of intermediate 1
with dRB-EDA in our assay (ESIw, Scheme S1). HRMS
analysis of the assay solution containing excess dRB-EDA
and triphosgene revealed the presence of compound 2 together
with dRB-EDA whereas no dimers of dRB-EDA were
detected (ESIw, Fig. S12). Since carbamoyl chlorides readily
react with primary amines to afford ureas,7 the absence of
dimeric dRB-EDA (ESIw, Scheme S1) indicated that intra-
molecular rearrangement of intermediate 1 to compound 2
was highly rapid, which prevented the intermolecular coupling
of intermediate 1 with dRB-EDA.
The UV-vis absorption spectra of the reaction solutions
revealed a new strong absorption band at 560 nm with dose
dependent color intensities (Fig. 2). Compared with the color-
less substrate, the deep colored species generated in the assay is
highly visible, suggesting the potential utility of this assay for
on-spot detection of phosgene with ‘‘naked eyes’’.
With the detection limit established, we went on to
determine the structures formed between triphosgene and
dRB-EDA. Carbamoyl chlorides are readily generated upon
nucleophilic substitution of phosgene with amines.7 We reasoned
that intermediate 1 should be formed when dRB-EDA reacts with
triphosgene under alkaline conditions (Scheme 2). Since
carbamoyl chlorides could be dehydrohalogenated in situ to give
an isocyanate derivative,7 intermediate 1 or its corresponding
isocyanate species will undergo intramolecular cyclization to
To explore the role of the deoxylactam of dRB-EDA in the
chromogenic response for triphosgene, N-(rhodamine B)-
lactam-ethylenediamine (referred as RB-EDA) was tested
for its efficacy to detect triphosgene. RB-EDA differs from
dRB-EDA in that it contains an amide moiety (Fig. 4A).
Visual and spectroscopic analysis showed that no colored
species was produced in the assay solution of RB-EDA with
triphosgene (Fig. 4B), implying that the non-nucleophilic
nature of the lactam impedes the intramolecular cyclization
of RB-EDA (ESIw, Scheme S2). The differential responses of
dRB-EDA and RB-EDA highlighted the crucial role of the
deoxylactam of dRB-EDA in chromo-fluorogenic sensing of
triphosgene.
Rhodamine-hydroxamate and rhodamine-hydrazide were eval-
uated next for their capability to sense triphosgene (Fig. 4A).
Detection of triphosgene was achieved with a limit of 2 mM for
rhodamine-hydroxamate and 20 mM for rhodamine-hydrazide
(ESIw, Fig. S8–S11). HRMS analysis indicated that sensing of
triphosgene with rhodamine-hydroxamate proceeded via a Lossen
rearrangement based mechanism (ESIw, Scheme S3 and Fig. S13),
Fig. 2 UV-vis absorption spectra of dRB-EDA (1 mg mlÀ1) in the
presence of triphosgene (1, 0.8, 0.6, 0.4, 0.2, 0.1, 0.05 mM, from top to
bottom) in DMF containing TEA (A), the inset shows the visual
images of dRB-EDA with (left) or without (right) addition of triphosgene
(1 mM); (B) the titration curve was plotted by absorbance at 560 nm
vs. analyte concentration.
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Fig. 5 Visual detection of gaseous phosgene with dRB-EDA absorbed
paper stripes. The strips were placed in sealed flasks containing phenan-
thridine (A), triphosgene (B) or phenanthridine and triphosgene (C) and
then immediately photographed.
In summary, dRB-EDA, a rhodamine-deoxylactam containing
chemodosimeter, was demonstrated to be sensitive for sensing of
triphosgene with a detection limit of 50 nM. The assay proceeds
via analyte triggered opening of the deoxylactam of dRB-EDA
and afforded deep-colored and highly fluorescent species. Given
the promptness of the color formation, low-background inter-
ference, and high sensitivity of this assay, dRB-EDA is attractive
for on-spot detection of gaseous phosgene with routine instru-
ments or ‘‘naked eyes’’.
Fig. 4 Assay efficiency of selected rhodamine-(deoxy)lactams.
(A) Chemical structures of the rhodamine-(deoxy)lactam based chemodo-
simeters; (B) fluorogenic responses of rhodamine-hydroxamate (in green),
rhodamine-hydrazide (in blue) and RB-EDA (in red) towards triphosgene
(1 mM) in DMF as compared to dRB-EDA (in black); (C) fluorescence
emission spectra of dRB-EDA (red) in DMF upon addition of triphosgene
(1 mM, in black) or formaldehyde (10 mM, in blue) (lex@560 nm).
which is analogous to the reported sensing of diethyl chlorophos-
phate.8 Consistent with the documentation that hydrazide could
react with phosgene to form 1,3,4-oxadiazole,7 HRMS indicated
that the detection of triphosgene with rhodamine-hydrazide was
achieved via phosphorylation triggered opening of the lactam,
leading to a 1,3,4-oxadiazole containing rhodamine derivative
(ESIw, Scheme S4 and Fig. S14). Albeit being able to detect tri-
phosgene with rapid kinetics (ESIw, Fig. S3 and S4), rhodamine-
hydroxamate and rhodamine-hydrazide were much inferior to
dRB-EDA in terms of assay sensitivity.
This work was supported by grants from NSFC (No.
21072162 and 20802060), Natural Science Foundation of
Fujian Province of China (No. 2011J06004), the Fundamental
Research Funds for the Central Universities (No. 2011121020)
and NEFTBS (No. J1030415).
Notes and references
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A potential competitive reaction using dRB-EDA is the
fluorogenic reaction with aldehydes.6c Fig. 4C shows that
dRB-EDA could detect 1 mM of triphosgene with superior
sensitivity and assay kinetics relative to 10 mM of formalde-
hyde under the same assay conditions,6c suggesting its good
selectivity in sensing of gaseous phosgene.
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With the favorable features of dRB-EDA, we further probed
its feasibility for real-time detection of gaseous phosgene in a
portable manner. dRB-EDA was absorbed on paper strips and
silica gel respectively. The testing paper was placed in sealed
chambers where gaseous phosgene was generated in situ from
triphosgene and phenanthridine following a reported procedure.9
The test paper quickly turned into red color upon generation of
gaseous phosgene whereas no color was observed in the control
chambers (Fig. 5). The color change of the strips was clearly
visible in the presence of 0.8 mg LÀ1 of gaseous phosgene, which
is 10-fold more sensitive than the threshold of immediate danger
to health and life limit of gaseous phosgene (ESIw, Fig. S17). In a
separate experiment, the colorless silica gel column containing
dRB-EDA quickly became red in contact with gaseous phosgene
(ESIw, Fig. S18 and S19). These results indicated that visual
detection of gaseous phosgene is applicable with dRB-EDA
formulated with appropriate portable platforms, e.g. paper strips.
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c
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