A molecular probe for the highly selective chromogenic detection of DFP, a mimic of sarin and soman nerve agents

Article abstract of DOI:10.1002/chem.201102241

Lethal-weapon visualization: A novel triarylmethanol derivative, which is able to colorimetrically distinguish diisopropylfluorophosphate (DFP; a mimic of Sarin and Soman) from other nerve-agent simulants, is reported (see picture). This probe was designed to contain two reactive sites: an OH group that provides a suitable reactive group for nerve agents and a tert-butyldimethylsilylether moiety that is able to react with fluoride, which is a specific byproduct of the phosphorylation of OH by DFP.

Full text of DOI:10.1002/chem.201102241

DOI: 10.1002/chem.201102241
A Molecular Probe for the Highly Selective Chromogenic Detection of DFP,
a Mimic of Sarin and Soman Nerve Agents**
Raffll Gotor,[a] Ana M. Costero,*[a] Salvador Gil,[a] Margarita Parra,[a]
Ramón Martínez-MµÇez,*[b, c] and FØlix Sancenón[b, c]
The use of chemical-warfare (CW) agents in terrorism has
proven the need for development of reliable and accurate
methods to detect these lethal compounds.[1] Among CW,
nerve agents are especially dangerous, and the UN classifies
them as weapons of mass destruction. Nerve agents are ca-
pable of interfering with the action of the nervous system
through the inhibition of acetylcholinesterase, resulting in
acetylcholine accumulation in the synaptic junctions; this
hinders muscles from relaxing.[2] From a chemical viewpoint,
nerve agents are organophosphonates with good leaving
groups. Current nerve-agents-monitoring methods are
mainly based on biosensors,[3] ion mobility spectroscopy,[4]
photonic crystals,[5] electrochemistry,[6] microcantilevers,[7]
and optical-fiber arrays[8] and show certain limitations, such
as low portability and complexity. Recently, as an alternative
to these classical methods, the development of fluorogenic
and chromogenic probes has proven useful for the recogni-
tion of these chemicals in solution and gas phase.[9] Chromo-
genic systems are especially appealing because of the use of
widely available instrumentation and detection with the
naked eye. Nevertheless, examples of selective and sensitive
chromofluorogenic probes for detection of these derivatives
are still rare. The chemosensors described in the literature
make use of easily observable and measurable fluorescence
and color changes, and involve photon-induced electron
transfer (PET)-based processes,[10] oximate-containing deriv-
atives,[11] molecularly imprinted polymers,[12] nanoparticles,[13]
carbon nanotubes,[14] cyclization reactions in push–pull chro-
mophores,[15] displacement assays,[16] and organic–inorganic
hybrid materials.[17] In most of these studies nerve-gas simu-
lants, such as diethylcyanophosphate (DCNP) and diisopro-
pylfluorophosphate (DFP), are used. Because these com-
pounds contain the same leaving groups as the real nerve
agents Sarin, Soman, and Tabun, they display similar reac-
tivity, but they lack the rigorous toxicity. Most of these re-
ported chromofluorogenic probes rely on the electrophilic
reactivity of nerve gases with suitable nucleophiles, which,
in many cases, result in an ON–OFF behavior. However, de-
spite the intrinsic interest in the design of such chromo-
fluorogenic probes, the reported examples have certain limi-
tations. In particular, the fact that the reactions are nonspe-
cific and, in general, the reported dye-based probes display
the same optical response to all nerve agents. However, the
development of rapid methods for the individual signaling
of Sarin, Soman, or Tabun may prove important. For in-
stance, even though the emergency response protocol is sim-
ilar for all nerve gases, different toxicities and experimental
evidence that some antidotes are ineffective for certain
nerve agents indicate the importance of distinguishing cer-
tain agents within this family of lethal chemicals.[18] Follow-
ing our interest in the development of chromofluorogenic
probes for nerve agents,[15,17] we report herein a simple col-
orimetric system that is not only able to respond to nerve-
agent mimics, but is also able to distinguish DFP (a mimic
of Sarin and Soman nerve gases) from other simulants and
organophosphates. The signaling protocol involves chromo-
genic probe 1 (Scheme 1). This molecule was designed to
contain two chromo–chemosensing moieties; that is, 1) a nu-
cleophilic hydroxyl group that provides a suitable reactive
site for electrophilic phosphorous atoms, such as those in
nerve agents and 2) a tert-butyldimethylsilylether (TBDMS)
group that is known to react with fluoride,[19] which is a spe-
cific byproduct to be exclusively obtained upon the phos-
phorylation of the OH moieties with DFP. Both reactions,
that is, phosphorylation of the hydroxyl moiety and elimina-
tion of the TBDMS group, are expected to occur with a sig-
nificant color modulation (see below). Derivative 1 was
readily prepared from 4-bromophenol containing a TBDMS
group and Michlerꢀs ketone (4,4’-bis(dimethylamino)benzo-
[a] R. Gotor, Prof. A. M. Costero, Dr. S. Gil, Dr. M. Parra
Centro de Reconocimiento Molecular y Desarrollo Tecnolꢁgico,
Unidad Mixta Universidad Politꢂcnica de Valencia - Universidad
de Valencia
Departamento de Quꢃmica Orgꢄnica
Facultad de Ciencias Quꢃmicas,
Universidad de Valencia. Doctor Moliner 50,
46100 Burjassot, Valencia (Spain)
E-mail: ana.costero@uv.es
[b] Prof. R. Martꢃnez-MꢄÇez, Dr. F. Sancenꢁn
Centro de Reconocimiento Molecular y Desarrollo Tecnolꢁgico,
Unidad Mixta Universidad Politꢂcnica de Valencia - Universidad
de Valencia
Departamento de Quꢃmica, Universidad Politꢂcnica de Valencia
Camino de Vera s/n, 46022, Valencia (Spain)
[c] Prof. R. Martꢃnez-MꢄÇez, Dr. F. Sancenꢁn
CIBER de Bioingenierꢃa, Biomateriales y Nanomedicina (CIBER-
BBN)
[**] DFP=diisopropylfluorophosphate.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102241.
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COMMUNICATION
Figure 1. UV/Vis spectra of chromo-reactant 1 (c=1.0ꢆ10À5 moldmÀ3) in
acetonitrile/water solution only (9:1, v/v; buffered with HCl/borax at
pH 8.0) and in the presence of DCNP and DFP (100 equivalents).
Scheme 1. Chemical structures of nerve-agents Sarin, Soman, and Tabun,
their simulants (DFP and DCNP), potential interfering agents, probe 1,
and acid–base indicator 2.
Figure 2. Photograph of probe
1 ) in acetonitrile/
(1.0ꢆ10À5 moldmÀ3
water solutions (9:1, v/v; buffered with HCl/borax at pH 8.0). From left
to right: 1, 1+DCNP, 1+DFP.
phenone) through the corresponding organolithium deriva-
tive (see the Supporting Information).
As a first step, the reactivity of 1 was tested against
nerve-agent simulants DFP and DCNP in acetonitrile/water
(9:1, v/v) mixtures (c=1.0ꢆ10À5 moldmÀ3) buffered at pH 8
(HCl/borax buffer) to avoid pH changes due to a potential
partial hydrolysis of the simulants. Compound 1 is colorless
and displays a unique band at 266 nm. Upon the addition of
100 equivalents of DCNP, a remarkable change in the spec-
trum was observed after a short time (less than 30 s) with
the appearance of two bands at about 342 and 608 nm
(Figure 1); this resulted in a color modulation from colorless
to bright blue–green (see Figure 2). This shift to higher
wavelengths is consistent with a reaction between the elec-
tron-deficient phosphorus atom in DCNP and the nucleo-
philic hydroxyl moiety in chromo-reactant 1. In the presence
of DCNP, the hydroxyl moiety of 1 undergoes phosphoryla-
tion to yield compound I that quickly results in the forma-
tion of the highly conjugated salt II (green, see Scheme 2).
To assess the mechanism involved in the observed chromo-
genic response, compound II was prepared by following a
different synthetic route, which involved the treatment of an
acetonitrile solution of chromo-reactant 1 with one equiva-
lent of trifluoroacetic acid at 408C for 30 min. The reaction
product was isolated and characterized as derivative II. The
product obtained by following this synthetic route showed
Scheme 2. Chromogenic response mechanism for probe 1 in the presence
of nerve-agent-simulants DCNP or DFP.
In contrast, a very different chromogenic behavior was
observed after addition of 100 equivalents of DFP. On this
occasion, the colorless solution of 1 initially became green,
but then turned to pink after a few seconds due to the ap-
pearance of a band at 557 nm (see Figure 1). In this case
and in mechanistic terms, after completing the first step (to
yield structure I) and the additional dephosphorylation pro-
1
an identical H NMR spectrum to that of the product ob-
tained from the reaction of 1 with DCNP.
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A. M. Costero, R. Martꢃnez-MꢄÇez et al.
cess (to yield compound III), the fluoride anion, obtained as
a byproduct of the reaction, was able to attack the silicon
atom of the TBDMS moiety; this resulted in an SN2 depro-
tection reaction that yielded ketone IV (pink, see Figure 2).
To confirm the mechanism proposed, derivative II was treat-
ed with one equivalent of tetrabutylammonium fluoride to
unequivocally yield ketone IV (see the Supporting Informa-
tion for details). A remarkable detection limit (DL) in ace-
tonitrile/water (9:1, v/v) solutions of 130 ppb for DFP was
determined. The DL for DCNP was 30 ppm under similar
conditions (see the Supporting Information).
In an additional step, the reactivity of 1 towards the orga-
nophosphorous (OP) compounds (c=160 ppm; Scheme 1)
was studied in mixed acetonitrile/water (9:1, v/v; buffered
with HCl/borax at pH 8.0). In all cases, the solutions of 1 re-
mained colorless, which indicated that no reaction occurred
between 1 and these OP derivatives. The acetonitrile/water
(9:1, v/v) solutions of 1 were also tested in the presence of
potential interfering agents that may be present in a civilian
setting, such as malathion, dyfonate, 4,4’-DDD, or 4,4’-DDE
(see Scheme 1), and oxidants, such as O3, H2O2, or peroxides
at a concentration of 1000 ppm. No change in color was
noted in the presence of any of these agents. Moreover
when DCNP or DFP were added to the acetonitrile/water
(9:1 v/v) solutions of 1, containing all these potential inter-
fering agents, color changes, such as those shown in
Figure 2, were observed, indicating that these species do not
interfere with the detection of these nerve-agent mimics. No
changes in color were observed for the acetonitrile/water
solutions of 1 in the presence of petrol or fuel oil.
As we were confident with these results, we decided to
move one step further and we carried out studies with
chromo-reactant 1 to develop test strips for the simple col-
orimetric detection of nerve-agent simulants in solution or
gas phase. To test this possibility, a hydrophobic polyethy-
lene oxide film containing 0.1% of compound 1 and 1% of
Cs2CO3 was prepared as a prototype of the colorimetric
probe.[20] Moreover to overcome possible false positives due
to the potential presence of strong acids in solutions or as
vapors, a polyurethane membrane containing 0.1% of acid–
base indicator 2 (see Scheme 1) was also prepared and in-
cluded in the sensing ensemble (see Figure 3). Dye 2 was se-
lected because it was unable to react with the simulants, yet
it reacts with acids by changing the color from orange to col-
orless upon protonation. In a typical assay, polymers were
either exposed to acetonitrile or acetonitrile/water (9:1 v/v)
solutions of DFP, DCNP, HF, or HCl or placed in a contain-
er containing the nerve-agent simulants or the acids (25 ppm
for DFP, DCNP and 10 ppm for HF, HCl introduced into
the container as aerosols). Similar chromogenic behaviors
were seen in both solution and gas phase; that is, in the
presence of DFP the polyethylene oxide film containing 1
turned pink, whereas in the presence of DCNP, the film
became blue–green. In both cases, the film containing 2 re-
mained orange. Moreover, in the presence of HF and HCl,
the films of 1 became pink and blue–green (as observed
with DFP and DCNP, respectively) yet in this case films of 2
turned colorless due to protonation of the dye (see
Figure 3). This simple ensemble, combining the reactive 1
and the acid–base dye 2, is able to differentiate DFP (a
Sarin and Soman simulant) from DCNP (a Tabun simulant)
and at the same time detect the presence of possible inter-
fering agents, including acids, such as HF.
To complete the study of potential interfering agents in
the gas phase, films containing 1 were included in a flask
with saturated vapors of malathion, dyfonate, 4,4’-DDD,
4,4’-DDE, and OP-1-4 derivatives for at least 24 h. The
same experiments were performed in the presence of gaso-
line and diesel exhaust gases, which might be present in
military settings. No changes in film color were observed in
any of the cases. Additionally it was confirmed that the
films of 1 were still able to react with DFP and DCNP in
the presence of these potential interfering agents, with a
similar response to that shown in Figure 3.
To summarize, we report herein a simple colorimetric
probe based on a triarylmethanol derivative that is able to
colorimetrically detect nerve-agent mimics and, at the same
time, selectively distinguish the presence of the Sarin or
Soman simulants (DFP) in either acetonitrile/water (9:1 v/v)
solutions buffered at pH 8.0 or in gas phase. This probe was
designed to contain two reactive sites: an OH group that
provides a suitable reactive place for nerve agents and a
TBDMS moiety that is able to react with fluoride, which is a
specific byproduct of the phosphorylation of OH by DFP.
This study indicates the possibility of developing similar sys-
tems for the selective detection of certain nerve gases that
make use of the unique reactivity of the specific byproduct
obtained upon the generic phosphorylation reaction induced
by nerve agents. To the best of our knowledge, the system
we report herein is the first probe for selective colorimetric
detection of Sarin and Soman versus other nerve gases, such
as Tabun.
Acknowledgements
We thank the Spanish Government (project MAT2009-14564-C04) and
the Regional Valencian Government (Generalitat Valencia; project
PROMETEO/2009/016 and ACOMP07/080) for support. S.R. is grateful
to the Generalitat Valenciana for the fellowship awarded. SCSIE (Uni-
versidad de Valencia) is gratefully acknowledged for all the equipment
employed.
Figure 3. Polyurethane membranes of probe 1 (bottom) and the acid–
base indicator 2 (top) in absence (blank) and in the presence of DFP
(25 ppb), DCNP (25 ppb), HCl (10 ppm), and HF (10 ppm) vapors (from
left to right).
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Nerve-Agent Detection
COMMUNICATION
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Keywords: chemosensors · chromogenic detection · DFP
recognition · nerve-agent simulants · organophosphonates
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Received: July 21, 2011
Published online: September 16, 2011
Chem. Eur. J. 2011, 17, 11994 – 11997
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11997