Fluorescent detection of chemical warfare agents: Functional group specific ratiometric chemosensors

Article abstract of DOI:10.1021/ja029265z

Indicators providing highly sensitive and functional group specific fluorescent response to diisopropyl fluorophosphate (DFP, a nerve gas (G-agent) simulant) are reported. Nonemissive indicator 2 reacts with DFP to give a cyclized compound 2⁺A^- that shows a high emission due to its highly planar and rigid structure. Very weak emission was observed by the addition of HCl. Another indicator based on pyridyl naphthalene exhibits a large shift in its emission spectrum after reaction with DFP, which provides for quantitative ratiometric detection. Copyright

Full text of DOI:10.1021/ja029265z

Published on Web 03/04/2003
Fluorescent Detection of Chemical Warfare Agents: Functional Group
Specific Ratiometric Chemosensors
Shi-Wei Zhang and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received November 7, 2002 ; E-mail: tswager@mit.edu
Scheme 1
Despite several decades of research, there continues to be a need
for new and improved methods for the detection of highly toxic
organophosphonates.1 Highly reactive volatile organophosphonates
such as Tabun (GA), Sarin (GB), and Soman (GD) are powerful
inhibitors of acetylcholinesterase, which is critical in nerve function.
The interruption of this enzyme has rapid and fatal consequences
to mammalian life and is the basis of chemical weapons that are
often referred to as nerve gas.2 Related compounds diisopropyl-
fluorophosphate (DFP) and diethylchlorophosphate (DCP) have
similar reactivity, but they lack the efficacy of typical nerve agents
and hence are good model compounds for the design of indicators.
There have been many innovations for the detection of these species,
including colorimetric detection methods,3 surface acoustic wave
(SAW) devices,4 enzymatic assays,5 and interferometry;6 however,
all are plagued by at least one limitation such as slow responses,
lack of specificity, low sensitivity, operational complexity, or
nonportability.
transition between the electron-rich thienyl group and the electron-
poor pyridinium. Although intermediate phosphate esters of 1 form
rapidly, they did not undergo cyclization, perhaps due to an
A guiding principle of our efforts in nerve gas detection is to
develop methods that respond to the general reactivity that provides
unfavorable transition state structure. Forcing conditions produced
the basis of their toxicity. The mechanism of action of nerve agents
the cyclized product 1+A- (Scheme 1) and the desired red shift
is the reaction with a hydroxy group to form a phosphate ester at
the catalytic site of acetylcholinesterase. Hence, we have sought
with a dramatic increase in fluorescence quantum yield (Table 1).
to produce functional group specific sensors that will transduce the
conversion of a hydroxyl to a leaving group such as a phosphate
ester.7 We report herein the development of a sensitive, fluorescent
ratiometric chemosensor and related compounds for the detection
of nerve agents.
Our indicator designs focused on producing a new bathochromic
absorption and fluorescence in response to the formation of reactive
phosphate esters. To achieve this response, we have focused upon
intramolecular cyclization reactions, which transform flexible
nonplanar weakly conjugated chromophores into rigid planar highly
delocalized systems. In addition to generating the expected spectral
shifts, the transformation from a flexible chromophore to a rigid
extended one will further produce a significant increase in the
emission efficiency by reducing the nonradiative rate.
We began by examining thienylpyridyl and phenylpyridyl
systems, 1 and 2, respectively. As shown in Scheme 1, these
compounds were expected to undergo an intramolecular nucleo-
philic substitution reaction upon exposure to a reactive nerve agent.
The indicators in Scheme 1 were studied as both free alcohols and
silylated variants, with the silicon group potentially offering
specificity for P-F groups found in DFP, Sarin, and Soman. Our
initial interest in 1’s framework was due to the fact that in its
cyclized form, 1+A-, we expected a strong charge-transfer optical
Table 1. Spectral Data in CH2Cl2
quantum
yield (%)
compound
abs. max.a (log ꢀ)
314 (3.88)
em. max.a
1
370
345
385
412
382
465
4.90
<0.01
13.1
39.6
52.5
2
273 (3.89)
3
291 (4.06)
310, 353 (3.71, 3.85)
290 (4.12)
1+A-
2+A-
3+A-
275, 340 (4.16, 3.84)
61.9
a Given in nanometers. b Determined on the purified compounds.
In an effort to accelerate the transduction/cyclization reaction,
we considered that compound 2 with a phenyl ring in place of the
thiophene might provide a more favorable cyclization geometry.
Indeed 2 reacts with both DFP and DCP to give the highly
fluorescent cyclized product 2+A-. As shown in Figure 1, simple
protonation of the pyridine nitrogen in 2 by HCl produces a minimal
fluorescence response. This specificity is due to the fact that the
aromatic rings in 2 undergo geometrical (vibrational-rotational)
changes in the excited state that facilitate nonradiative processes
and dramatically lower the quantum yield. Hence, by design, we
observe a highly efficient fluorescence only after the cyclization
reaction, which eliminates rotation about the phenyl-pyridyl bond.
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C O M M U N I C A T I O N S
measurement that proved to be kinetically well-behaved with a
pseudo first-order rate constant of kobs ) 0.0014 s-1
.
Although 2’s response could be enhanced by amplification
methods developed by our group,8 we considered the cyclization
reaction to be unacceptably slow. To produce an intrinsically more
sensitive indicator, we synthesized 3 that should have superior
reaction kinetics due to the restricted conformation of the naph-
thalene group that favors cyclization and the substitutionally
activated R-aryl phosphate ester intermediate. Compound 3 offered
additional spectroscopic advantages such as longer wavelength
emission and absorption as well as relatively strong fluorescence
in its native state. The latter allows 3 to be used directly in
ratiometric detection schemes. Figure 2 shows responses of 3 in a
cellulose acetate film to DFP, DCP, and HCl. In this thin film, the
emission wavelengths are shifted to higher energy relative to the
solution values (Table 1) and occur at 375 and 438 nm for 3 and
3+A-, respectively, suggesting that the cellulose acetate exhibits
less dielectric relaxation about a more polar excited state. The
reaction rates of 3 are now sufficiently fast that detailed kinetics
were not possible, and the reaction rate in CH2Cl2 is estimated to
be kobs > 0.024 s-1 or at least 17 times faster than 2. This faster
rate is reflected in the rapid responses of thin films to 10 ppm vapors
of DFP (Figure 2, bottom). The lower quantum yields observed in
Figure 2 at 1 s and 5 s are likely due to protonation of the pyridine
and/or the transient intermediacy of the phosphate ester.
Figure 1. Absorption and emission of 2 before and after addition of DFP,
DCP, and HCl (8 × 10-4 M) in dichloromethane (8 × 10-6 M).
In summary, we have developed a highly sensitive chemosensor
for chemical warfare agents. Indicator 3’s functional group specific
nature produces a response not only to chemical warfare agents
but also to other similarly reactive toxic industrial chemicals (e.g.,
SOCl2) that also pose a threat to homeland security.
Acknowledgment. This work was funded by DoE and the Army
Research Office’s Tunable Optical Polymers MURI program.
Supporting Information Available: Data and synthetic prepara-
tions (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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Figure 2. (Top) Emission spectra (thin film) of 3 in its initial state and
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The sensitivity of our sensory scheme is directly related to the
rate of the cyclization reaction with higher rates giving lower
detection limits. Monitoring the reaction in situ by 1H NMR analysis
revealed that the reactions of DFP with 2’s alcohol (1:1 stoichi-
ometry) to form the intermediate phosphate ester are fast relative
to the cyclization reaction. Hence, by reacting 2 (8 × 10-6 M) with
a 100× excess of DFP (8 × 10-4 M), we find that the cyclization
is rate limiting and can be studied under pseudo first-order kinetics.
The emission of 2+A- provided a convenient concentration
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