Discrimination of nerve gases mimics and other organophosphorous derivatives in gas phase using a colorimetric probe array

Article abstract of DOI:10.1039/c2cc34662a

A colorimetric array for the chromogenic discrimination of organophosphorous derivatives in gas phase has been developed. This journal is The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc34662a

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COMMUNICATION
Discrimination of nerve gases mimics and other organophosphorous
derivatives in gas phase using a colorimetric probe arrayw
Katherine Chulvi,ab Pablo Gavin
˜
a,ab Ana M. Costero,*ab Salvador Gil,ab Margarita Parra,ab
acd
acd
Rau
´
Jose-L. Vivancos
l Gotor,ab Santiago Royo,acd Ramon Martınez-Manez,* Felix Sancenon and
´
˜
´
´
´
ade
´
Received 29th June 2012, Accepted 22nd August 2012
DOI: 10.1039/c2cc34662a
A colorimetric array for the chromogenic discrimination of
organophosphorous derivatives in gas phase has been developed.
compound and in some cases present restrictions such as lack of
specificity.5 In fact, the presence (or absence) of certain colour/
fluorescence changes is not necessarily an indication of the presence
(or absence) of nerve gases and in some cases chromo-fluorogenic
systems show false positives or false negatives. In fact more exact
correlations seem to be necessary in order to use chromogenic
systems for safety applications. Moreover, the development of
rapid methods for the individual signalling of a certain nerve
agent and the unequivocal distinction from other organo-
phosphates or phosphonates may prove important.6 For
instance, even though emergency response protocols are similar
for all ‘‘nerve gases’’, their different toxicity and the experimental
evidence that some antidotes are ineffective for some of them,
indicate the importance of distinguishing certain compounds
within this family of toxic chemicals.7
Nerve agents, used as chemical warfare weapons, are some of the
most toxic known chemical agents. They are hazardous as liquid
and vapour and can cause death within minutes after exposure.1
Nerve agents are organophosphonate derivatives and show certain
similarities with organophosphates used as pesticides. Nerve
agents are highly toxic to mammals due to their capacity to
interfere with the action of the nervous system through the
inhibition of acetylcholinesterase, resulting in acetylcholine
accumulation in the synaptic junctions hindering muscles
relaxation.2 Moreover apart from nerve gases, many organo-
phosphates and organophosphonates act as ‘‘nerve agents’’
(i.e. are inhibitors of the acetylcolinesterase enzyme) and they are
one of the most common causes of poisoning worldwide via
intoxication through inhalation, ingestion and dermal absorption.
Some analytical procedures based on biosensing assays and
physical techniques have been used to detect these compounds.3
However, they show certain limitations in their use typically
involving difficult portability and low selectivity. Among the
techniques that develop easy handling of disposable systems, the
use of chromogenic chemosensors is one of the most promising
since they are versatile, printable and colour modulations can
be easily measured using image capturing systems, or allow the
detection of colour changes by the naked eye.4 In fact few
technologies are as inexpensive as visual imaging. Although
some chromogenic indicators have been described for the
detection of nerve gases they are generally based on a single
A suitable approach to avoid some of these drawbacks when
using single analyte indicators is the possibility of designing
optoelectronic noses, as potent and versatile systems to be
applied in complex systems.8 In fact, optoelectronic noses,
built by an array of dyes capable of offering information
through colour changes, have been applied for the detection
of different volatile compounds (VOCs)9 including odorants,10
volatile amines,11 etc. Based on the above issues, and following
our general interest in developing colorimetric probes in this
field12 we report herein a prospective study of the use of an
array of chromogenic indicators which we have applied to the
detection and differentiation of several organo-phosphates,
phosphonates and nerve agent simulants in gas phase.
Optoelectronic noses usually focus on chemical sensors with
a large cross-reactivity and rely on a full range of intermolecular
interactions. In our case, inspired by previously reported
optoelectronic noses, and based on our own experience a total of
16 dyes (see Scheme 1) were chosen. These include push–pull
chromophores containing active sites such as alcohol, amine,
pyridine groups capable of potentially reacting with the organo-
phosphorous compounds shown in Scheme 2 (i.e. dyes 1, 2, 3, 5, 6,
7, 9, 10, 11, 13, and 14), and some dye derivatives capable of
inducing colour modulations in the presence of their possible
hydrolysis products such as fluoride, cyanide, phosphate and
protons (i.e. dyes 1, 4, 8, 9, 12, 13, 15 and 16).
a
´
Centro de Reconocimiento Molecular y Desarrollo Tecnologico
´
(IDM), Unidad Mixta Universidad Politecnica de
`
Valencia-Universitat de Valencia, Spain
b
´
´
`
Departamento de Quımica Organica, Universitat de Valencia,
Dr Moliner, 50, 46100 Burjassot, Valencia, Spain.
E-mail: ana.costero@uv.es; Fax: +34 963543151;
Tel: +34 963544410
c
d
e
´
Departamento de Quımica, Universitat Politecnica de Valencia
Camino de Vera s/n, 46022, Valencia, Spain
`
`
´
CIBER de Bioingenierıa, Biomateriales y Nanomedicina
(CIBER-BBN), Spain
Departamento de Proyectos de Ingenierıa, Universitat Politecnica de
´
Valencia, Camino de Vera s/n, 46022, Valencia, Spain
`
`
The array was prepared by placing 2 mL of the corres-
ponding dye solution (of ca. 10À4 and 10À2 M depending on
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc34662a
c
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Chem. Commun., 2012, 48, 10105–10107 10105

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array response. The same sensing array in the absence of the
tested gases was employed as a control, which showed minor
colour variations throughout the experiments (see ESIw). A
scanner was used to obtain pictures of the array. Moreover
RGB coordinates were measured from the photographs using
image processing software (Photoshop). Difference maps were
obtained by the difference of red, green and blue (RGB)
coordinates of each sensing dye and the initial values were
measured in the control. The subtraction of the two images
resulted in a difference vector of 3N dimensions where N is the
total number of compounds. Each dimension ranges from
À255 to 255; i.e. an RGB value of (0, 0, 0) is black, whereas a
value of (255, 255, 255) is white. To facilitate visualisation
only, the colour palette of the difference map was enhanced by
expanding the colour range from 0–100 to 0–255; any RGB
change of o4 was treated as background noise and ignored,
while changes of >100 were mapped to be 255. The difference
vector is conveniently visualised in Fig. 3 as a map of the
absolute values of the colour changes. At a glance, Fig. 1
clearly shows the existence of characteristic colour fingerprints
for the studied analytes.
Colour differences were also analysed using PCA, which is a
powerful linear unsupervised pattern recognition procedure,
that was used here as a simple method to project data onto a
two-dimensional plane. Typically, PCA decomposes the
primary data matrix by projecting the multi-dimensional data
set onto a new coordinate base formed by the orthogonal
directions with data maximum variance. The eigenvectors of
the data matrix are called principal components and are not
intercorrelated. The principal components (PCs) are ordered
so that PC1 displays the largest amount of variance, followed
by the next largest, PC2, and so forth. The first principal
component contained only 51.64% of the variance of the data.
The first two components represented 63.29% of total variance,
whereas ten PCs were needed to account for 95% of variance.
This large number of independent dimensions is in agreement
with the use of chemical sensing systems that employ several
forms of interactions between dyes and the organo-phosphate
and phosphonate derivatives. Moreover this increasing
dimensionality also helped to discriminate among highly
related samples (very similar compounds). Fig. 2 shows the
resulting PCA for the 9 sampling organo-phosphate and
phosphonate derivatives (three replicates) when using all the
dyes (16 Â 3 coordinates per dye). Here the PCs score plot uses
two PCs (PC1 and PC3) representing the 60.86% of variance.
Scheme 1 The 16 dyes used in the chromogenic array.
Scheme 2 Organophosphorous derivatives used in this study (diiso-
propylfluorophosphate (DFP), diethylchlorophosphate (DCP),
diethylcyanophosphonate (DCNP), ethyldichlorophosphate (EDCP),
diethyl(1-phenylethyl)phosphonate (DPEP), diethyl(methylthiomethyl)phos-
phonate (DMTMP), dimethylchlorothiophosphate (DCTP), diethyl-
(2-oxopropyl)phosphonate (DOPP)) and the nerve agents Sarin, Soman
and Tabun.
the compound) in a suitable support (silica gel and aluminium
oxide plates or polyurethane and PVC films were tested). The
best results were obtained using silica gel plates (see ESIw). As
model compounds for nerve gases we have used DFP, DCP
and DCNP which are derivatives which display a similar
reactivity to that of nerve agents such as Tabun, Sarin and
Soman, although they lack their acute toxicity. The response
of these colorimetric arrays was tested in the presence of 9
analytes including nerve agent simulants DFP, DCP and
DCNP, phosphates (EDCP and DCTP), phosphonates
(DOPP, DMTMP and DPEP) and HCl (see Scheme 2).
In a typical experiment the corresponding saturated analyte
vapour at 25 1C (see ESIw), together with the chromogenic
array were placed in a closed box for 30 minutes. The experi-
ments were carried out in air with a relative humidity of
60–65%. Three completely independent experiments for each
analyte were carried out to check the reproducibility of the
Fig. 1 Colour difference maps. Maps are generated from the absolute
values of the differences of the red, green and blue values for each dye
before and after equilibration with the corresponding analytes as
saturated vapours at 25 1C.
c
10106 Chem. Commun., 2012, 48, 10105–10107
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agreement between the measured and predicted values was
found suggesting that the array could display sensing of this
hazard at concentrations of some few ppm (see ESIw for
details).
In summary, a 16-member colorimetric probe array by
embedding chromogenic chemosensors in a silica gel plate
has been used for the classification of different phosphorous-
containing gases. The array was based on the use of push–pull
chromophores containing reactive sites (i.e. alcohol, amine,
and pyridine active groups) capable of reacting with ‘‘nerve
agents’’, and reactants capable of inducing colour modulation
in the presence of possible hydrolysis products. The system
allowed classifying the nerve agent simulants DFP, DCP and
DCNP and was also capable of discriminating between
other organo-phosphate, phosphonate derivatives and acidic
vapours. This approach may become important for the
rational design of minimal-size high-resolution arrays for the
detection of target phosphorous-containing toxic volatile
derivatives. As far as we know this is the first example in
which discrimination between different organo-phosphorous-
containing derivatives has been achieved using a chromogenic
array. The paradigm might be general and easily adapted to
the simple colorimetric detection of other toxic vapours.
Fig. 2 PCA score plot of PC1 and PC3 for some products in
Scheme 2 and HCl (3 each) and the trial clustering.
Notes and references
Fig. 3 HCA dendrogram showing the Euclidean distances between
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of the nerve agent simulants DCNP, DFP and DCP was
observed and this was not confused with the presence of acidic
vapours (HCl). Moreover other tested products were gathered
in larger clusters which allowed us to classify the samples into
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10105–10107 10107