A FRET approach to phosgene detection

Article abstract of DOI:10.1039/b614725a

A FRET approach towards potential detection of phosgene is presented, which is based on a selective chemical reaction between phosgene (or triphosgene as a simulant) and donor and acceptor fluorophores. The Royal Society of Chemistry.

Full text of DOI:10.1039/b614725a

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A FRET approach to phosgene detection{
Hexiang Zhang and Dmitry M. Rudkevich*
Received (in Austin, TX, USA) 9th October 2006, Accepted 9th December 2006
First published as an Advance Article on the web 12th January 2007
DOI: 10.1039/b614725a
property, commercially available laser dyes7 coumarin 2 and
A FRET approach towards potential detection of phosgene is
presented, which is based on a selective chemical reaction
between phosgene (or triphosgene as a simulant) and donor
and acceptor fluorophores.
coumarin 343 were functionalized with primary amino groups (see
ESI{). Taken separately, thus prepared coumarins 1 and 2
smoothly react with triphosgene at room temperature in CHCl3
in the presence of triethylamine (TEA) with the formation of the
corresponding ureas (see ESI{). Donor coumarin 1 absorbs at l =
343 nm and emits at l = 425 nm in CHCl3. When light with the
latter wavelength is used for excitation of acceptor coumarin 2, it
strongly emits at l = 468 nm (in CHCl3).
Phosgene (C(O)Cl2) is a highly toxic and chemically reactive gas
that has been used as a chemical warfare agent.1 It is a lung irritant
and a very insidious poison, which does not irritate immediately,
even when fatal concentrations are inhaled. Inhalation of phosgene
causes severe lung injury, the full effects appearing several hours
after exposure. Phosgene first came into prominence during World
War I, when it was used, either alone or mixed with chlorine,
against troops. There is a renewed interest in the development of
phosgene sensors due to the increased threat of weapons
deployment by terrorists. Sensing methods for phosgene are
typically electrochemical or based on remote spectroscopic
detection techniques.2 At the same time, optical sensors for this
gas are very rare.3 This is surprising since reactive chromophores
and fluorophores are now actively used for sensing other chemical
agents.4 Such sensors utilize a change of absorption or fluorescence
upon detection and better address the needs of specialists in that
they are simple, fast and reliable. In this communication, we
present our preliminary studies towards the design of potential
phosgene sensors based on fluorescence resonance energy transfer
(FRET).5 It includes a selective chemical reaction between
phosgene and donor and acceptor fluorophores, which bring
them together, within the appropriate Fo¨rster distance (Fig. 1).
Phosgene serves here as a cross-linking agent. When the emission
spectrum of the donor overlaps with the absorption spectrum of
the acceptor, FRET occurs upon irradiation of the donor and
strong emission of the acceptor can be detected. This reports on
the presence of phosgene.
When both coumarins 1 and 2 are mixed together with
triphosgene in the presence of TEA in CHCl3, hybrid urea 3
forms as well, in a statistical yield (Fig. 2). It was isolated on a
preparative scale and fully characterized. In molecule 3, the donor
and acceptor units are covalently linked and positioned within
˚
y20 A of one another. The absorption spectrum shows two strong
bands at lmax = 343 and 438 nm that belong to the coumarin 2 and
coumarin 343 units, respectively. More important however is that
at low concentrations of ¡1025 M, the donor excitation at l =
343 nm leads to the acceptor emission at l = 464 nm. In a control
experiment only utilizing acceptor coumarin 2, no emission in this
region was detected under these conditions.{ This implies a FRET
phenomenon in urea 3.
The phosgene sensing experiment involves the formation of urea
3 and its fluorescence. Coumarins 1 and 2 were mixed in a 1 : 1
ratio at the concentration range between 5 6 1024 and 1022 M in
CHCl3, TEA (y10 equiv.) was added and then triphosgene was
introduced. Addition of TEA prevents protonation of the amino
groups and does not affect the fluorescence. Aliquots were taken
and diluted to 1026 M,{ and the emission was recorded upon
excitation at l = 343 nm. Significant fluorescence enhancement at
l = 464 nm was detected (Fig. 3). This is particularly important
since the acceptor unit alone does not emit under these conditions.
The fluorescence increase is obviously due to the formation of urea
For this project, triphosgene (CCl3OC(O)OCCl3) was used as a
phosgene simulant. It is less toxic and easier to handle. In organic
chemistry, triphosgene has been utilized as a safe phosgene
replacement for making ureas.6 To take advantage of this chemical
Fig. 1 A FRET based approach to the detection of phosgene.
Department of Chemistry & Biochemistry, The University of Texas at
Arlington, Arlington, TX, USA. E-mail: rudkevich@uta.edu;
Fax: 1 817 272 3808; Tel: 1 817 272 5245
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures and spectroscopic details. See DOI: 10.1039/b614725a
Fig. 2 Coumarins 1 and 2 smoothly and rapidly react with phosgene/
triphosgene with the formation of urea 3.
1238 | Chem. Commun., 2007, 1238–1239
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In addition to fluorescence spectroscopy, the response of
compounds 1 and 2 to triphosgene can be conveniently followed
even with the naked eye (Fig. 4). The increased fluorescence
intensity was clearly observed within seconds after the addition of
triphosgene.
In conclusion, a proof-of-principle is presented on how to detect
phosgene utilizing it as a cross-linking agent in a designed FRET
system. This systems is selective, since other gases/agents rarely can
serve for cross-linking." The sensor design is clearly not limited to
coumarins: the dyes do not react with phosgene but rather report
on its presence. In principle, any other FRET acceptors and
donors can be used. Further work will be directed towards
lowering the detection limits.
We thank Prof. Koji Araki for experimental advice. We are also
grateful to Shelley M. Hampe and Dr Vaclav Stastny for
experimental assistance. Financial support is acknowledged from
the Alfred P. Sloan Foundation and The University of Texas at
Arlington. We also thank the NSF for funding the purchase of the
300 MHz NMR spectrometer (CHE-0234811).
Fig. 3 Typical changes in fluorescence emission spectra upon titrations
with triphosgene. Coumarins 1 and 2 were mixed in a 1 : 1 ratio at 1023
M
in CHCl3, TEA (10 equiv.) was added and then triphosgene (0.03 to
5 equiv.) was introduced. Aliquots were taken and diluted to 1026 M, and
the emission was recorded upon excitation at l = 343 nm. The arrows
indicate the fluorescence changes upon increasing the triphosgene
concentration.
Notes and references
{ At 1024 M and higher concentrations, excitation at l = 343 nm leads to
the increased fluorescence of acceptor coumarin 2.
§ The formation of urea 3 under these conditions was confirmed by 1H
NMR spectroscopy; at 1025 M and lower the reaction is extremely slow. In
addition to 3, two other, symmetrical ureas are also formed with two donor
or two acceptor fragments. The acceptor–acceptor urea does not emit
fluorescence upon excitation at l = 343 nm at these concentrations.
" One possible exception could be thionyl chloride (S(O)Cl2). These data
will be reported elsewhere.
1 The US Department of Health and Human Services link: http://
www.bt.cdc.gov/agent/phosgene/basics/facts.asp.
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Fig. 4 The ‘‘naked eye’’ detection of triphosgene. Left: coumarins 1 and
2 in CHCl3 before addition of triphosgene. Right: after addition of
y5 equiv. of triphosgene. The experiments were performed at 1023 M,
and the solutions were then diluted to 1026 M. Both vials are irradiated at
l = 365 nm with a laboratory UV lamp.
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3.§ Simultaneously, the fluorescence from the donor unit at l =
424 nm decreased. This is due to the quenching, indicating that
efficient energy transfer took place from the donor to the acceptor.
The fluorescence changes were clearly seen already upon addition
of as little as y0.1 equiv. of phosgene (recalculated from
0.03 equiv. triphosgene), which places the detection limit for this
particular FRET system at 5 6 1025 M.
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