Fluorometric, water-based sensors for the detection of nerve gas G mimics DMMP, DCP and DCNP

Article abstract of DOI:10.1039/c1cc15978j

Water-based Zn^II bisterpyridine systems were used as fluorometric sensors for the detection of the nerve gas G mimics DMMP, DCP and DCNP. Analyte concentrations in the range of 10^-7 to 10^-6 M are detectable in solution. Th

Full text of DOI:10.1039/c1cc15978j

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Fluorometric, water-based sensors for the detection of nerve gas G
mimics DMMP, DCP and DCNPw
Andreas Wild,abc Andreas Winter,abc Martin D. Hagerab and Ulrich S. Schubert*abc
Received 26th September 2011, Accepted 14th November 2011
DOI: 10.1039/c1cc15978j
Water-based ZnII bisterpyridine systems were used as fluoro-
metric sensors for the detection of the nerve gas G mimics
DMMP, DCP and DCNP. Analyte concentrations in the range
of 10ꢀ7 to 10ꢀ6 M are detectable in solution. The utilization of a
test stripe additionally allows the detection of organophosphonates
from the gas phase.
PEG side chains, the polymers are water-soluble and the
charged character of the zinc–terpyridine complex enables
straightforward anion detection.27
The water-solubility of the bisterpyridine 7 was achieved via
PEG sidechains. Initially, the 2,5-diiodo-hydroquinone core 3
was synthesized by applying optimized reaction conditions.28
The PEG chains were introduced to the core (3) via Williamson
ether synthesis with a tosylate-functionalized mPEG2000 4.
Subsequent Sonogashira reaction, with two equivalents of
40-(4-ethynylphenyl)-2,20:60,20 0-terpyridine (6), resulted in the
PEG-functionalized bisterpyridine 7 (Scheme 1).
Although the research in the detection of chemical warfare
agents was intensified, in particular after the gas attack on the
Tokyo subway in 1995, there is still a need for new and improved
systems for the detection of ‘‘nerve gases’’. These highly toxic
compounds can inhibit acetylcholinesterase (AChE), a critical
central-nervous enzyme.1–4 Different detection methods have
been investigated, which are generally based on emission (i.e.
lanthanide emission5–8 or fluorescence of organic dyes9–13),
enzymes,14,15 interferometry,16 surface acoustic waves,17,18
electrochemistry,19 mass spectrometry,20,21 chromogenic22–24
and calorimetric methods.25 However, for any practical use the
detection system has to be portable, fast, easy to use, selective
and sensitive. Metallopolymers represent a versatile class of
materials featuring these properties.26
The homogeneity of this bisterpyridine was proven by its
monomodal distributions in size exclusion chromatography
(SEC) and MALDI-TOF mass spectrometry. The PDI values
of 7, calculated by SEC (1.04) and MALDI-TOF MS (1.01) are
still very low. In the MALDI-TOF MS the shift to higher m/z
values is clearly visible (Fig. 1, right). The mass difference directly
correlates with the mass difference of 5 and 7, as expected.
As shown in Scheme 1, the addition of one equivalent of
Zn(OAc)2 to the PEG-functionalized bisterpyridine 7 resulted
in the formation of the linear zinc coordination polymer 8,
whereas the addition of two equivalents of ZnII ions yielded the
corresponding dinuclear complex 9.29 Performing a 1H NMR
titration experiment one can clearly follow the complexation
reaction (Fig. S1, ESIw). The formation of 9 was moreover proven
by MALDI-TOF MS (Fig. 1, right). Due to the PEG chains both
the ligand 7 and the corresponding metal complexes (8, 9) revealed
excellent solubility in common organic solvents and water.
Absorption and emission spectra in water and dichloro-
methane are shown in the ESIw (Fig. S2). Upon complexation
the absorption maximum is red-shifted (Table 1). Apparently,
the hydrophobic conjugated core tends to form aggregates in
The herein presented ZnII bisterpyridine systems (8, 9) reflect
excellent examples of materials for such systems. Due to the
a Laboratory of Organic and Macromolecular Chemistry (IOMC),
Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena,
Germany. E-mail: ulrich.schubert@uni-jena.de;
Fax: +49 3641 948202
b Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University
Jena, Humboldtstr. 10, 07743 Jena, Germany
c Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven,
The Netherlands
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc15978j
Scheme 1 Schematic representation of the synthesis of metallopolymer 8 and dinuclear complex 9.
c
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964 Chem. Commun., 2012, 48, 964–966

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Fig. 3 Emission spectra of (a) 8 and (b) 9 (water, c = 5.8 ꢁ 10ꢀ7 M,
lex = 400 nm) upon addition of DMMP (water, c = 0.116 mM).
Fig. 1 Left: SEC traces of 5 and 7. Right: MALDI-TOF MS spectra
of 5, 7 and 9.
However, after addition of 1 equivalent of sodium hydroxide,
the ZnII–terpyridine polymer partially depolymerized, enabling
a DMMP attack (Fig. S3 and S4, ESIw).32 Subsequent addition
of 10 equivalents DMMP (c = 5.8 ꢁ 10ꢀ6 M) leads to a 5-fold
increased emission intensity at 522 nm, as shown in Fig. 3a.
Since the measurements were performed significantly above
the detection limit, concentrations of 10ꢀ7 M are detectable
(Fig. S5a, ESIw), representing a significant improvement in
comparison to published systems (10ꢀ3 to 10ꢀ4 M).5,23,30
Dissociation constants (Kd) were calculated using the
fluorescence intensities at the beginning (I0), the point-of-
equivalence (Iend) and at a certain concentration of added
analyte [A] (Ix) and fitting to eqn (1) (Fig. S5, ESIw).33–35
Table 1 Selected photophysical data of 7–9
lmaxa/nm
lPlb/nm
Fc
Water
CH2Cl2
Water
CH2Cl2
Water
CH2Cl2
7
8
9
377
383
393
379
393
406
524
590
570
429
439
521
0.17
0.03
0.01
0.79
0.78
0.69
a
b
Longest wavelength absorption maximum. Excited at the longest
c
wavelength absorption maximum. Absolute quantum yield.
aqueous solution via p–p interactions, which explains the
bathochromically shifted emission and the lowered quantum
yield, in particular in polymer 8, compared to dichloro-
methane. This assumption was proven by DLS and TEM
measurements. Complex 9 revealed, due to the two attached
terpyridine zinc(II) complexes, a lower solubility in dichloro-
I = (I0 + Iend[A]/Kd)/(1 + [A]/Kd)
(1)
Applying eqn (1), the dissociation constant was calculated to
be Kd = 3.2 ꢁ 10ꢀ7. This value is in the same order of
magnitude as the one obtained by addition of Na3PO4.27
In analogy to 8, also 9 was applied as sensor material. Here
the accessibility and Lewis acidity of the ZnII ion are much
higher, since it is complexed only weakly by one terpyridine
ligand. For that reason, DMMP attacks the ZnII already
without addition of base and the system exhibits a 7-fold
emission intensity (35 eq., Fig. 3b). Since for the detection of
organophosphonates by 8 the addition of base appeared to be
essential, an easy distinction between organophosphonates
and inorganic phosphates is possible. Thus, only inorganic
phosphates and cyanides can potentially interfere with each
other.27 To also distinguish between these two, the well-known
reaction of cyanide ions with ammonium polysulfide and
subsequent reaction with FeIII ions in acidic media leads to
the characteristic red color of Fe(SCN)3 (Fig. 4a III).
methane and, for that reason, red-shifted emission (lPL
=
521 nm) compared to 7 (lPL = 429 nm) and 8 (lPL = 439 nm).
As shown already in a screening of 15 different common
anions, the ZnII bisterpyridine metallopolymer 8 features a high
affinity to phosphate and cyanide ions in aqueous solution.27
The phosphate coordinates as competitive binder with high
binding constants to the ZnII ion, resulting in a recovery of the
ligand emission. This behavior is visualized by an increased
emission intensity and a shift of the emission wavelength.27
Among chemical warfare reagents, nerve gasses such as
Sarin, Tabun or Soman belong probably to the most dangerous,
since they can be lethal in minutes when inhaled3 or absorbed
through the skin (Fig. 2).2,30 As mimics for Sarin and Tabun
we utilized DMMP, DCP and DCNP, which are commonly
used as analytes for organophosphonate sensing.24,31,32
Moreover to study the detection of organophosphates with
reactive phosphoryl halides, DCP was utilized. Addition of an
aqueous DCP solution (c = 0.116 mM) to 9 (c = 5.8 ꢁ 10ꢀ7 M)
led to an increased emission intensity (10 eq., 5 fold, Fig. S7a,
ESIw). Most likely, initial DCP addition leads to hydrolysis
Fig. 2 Schematic representation of the chemical structure of Sarin,
Tabun, DMMP, DCP and DCNP.
The titration experiments were performed as follows: to a
solution of 8 or 9 in water (c = 5.8 ꢁ 10ꢀ7 M) an aqueous
analyte solution (c = 0.116 mM) was added and the
corresponding emission spectra (lex
= 400 nm) were
measured. Apparently, organophosphonates are not capable
of coordinating to ZnII ions bound in a bisterpyridine
complex, since reactive Lewis acid sites are missing.32 The
addition of 5 equivalents (c = 2.9 ꢁ 10ꢀ6 M) of DMMP did
not result in an obvious optical response (Fig. 3a, blue).
Fig. 4 (a) Test stripes dipped into a 10-5 M solution of 8 and NaOH.
One of both vials was exposed to DMMP atmosphere. (b) After
10 minutes the test stripes were irradiated (lex = 365 nm).
c
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and consequently attack at the ZnII ion, resulting in increased
emission intensity (Fig. S4 and S7, ESIw).32,36 Subsequent
addition of DCP leads, due to the phosphorylation of the
N-atoms, to a decreased emission intensity (Fig. S4, ESIw).24
Most probably, an internal charge transfer (ICT) from the
electron-rich chromophore to the electron-accepting pyridinium
moiety occurs, as indicated also by the red-shift in the UV-Vis
absorption spectrum (Fig. S6, ESIw).4,24,37 Performing an
analogue experiment with 8, the addition of 10 equivalents
DCP caused already decreased emission intensity (Fig. S7b,
ESIw). Since in 8 no weakly bound acetate ions are present,
DCP directly phosphorylates the pyridine of a dissociated ZnII
bisterpyridine complex. To prove this assumption, DCP was
added to 7 and also here instantly the emission intensity
decreased (Fig. S7c, ESIw). To exclude a potential N-protonation
upon DCP hydrolysis, the reaction of 8 with DCP was
accomplished again in a buffered solution at pH = 7.0.
A similar observation could be found, underlining the robustness
of the detection system (Fig. S7d, ESIw). The Tabun mimic DCNP
exhibits similar detection behavior, as proven in the reaction
with 8. However, the cyanide ions formed upon DCNP hydrolysis
complex the ZnII ions, accelerating the phosphorylation of the
N-atoms (Fig. S8, ESIw).
Moreover, a straightforward protocol for the detection of
organophosphonates in the gas phase was established. Two
filter papers were dipped into a 10ꢀ5 M solution of 8 and
NaOH. Both test stripes were placed into a small sample vial
and these were transferred into a larger vial (Fig. 4b). Into one
of these larger vials 5 mL of DMMP were added and the vials
were closed. After 10 minutes, the test stripes were taken out and
irradiated with UV light (lex = 365 nm). The dipstick exposed to
the DMMP atmosphere showed significantly enhanced emission
(Fig. 4c).
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A p-conjugated PEG-functionalized water-soluble bister-
pyridine was synthesized and characterized. The corres-
ponding ZnII coordination polymer 8 and complex 9 were
utilized as fluorometric sensor materials for nerve gas G
mimics. Upon analyte addition (c = 10ꢀ7 to 10ꢀ6 M) the
emission intensity increased by a factor of 5 to 7. The different
reactivity of 8 and 9 towards DMMP and DCP allows a clear
distinction between both. To exclude a positive response
caused by inorganic phosphates or cyanides we set up a test
series to selectively determine them in aqueous solution. The
use of a test stripe, dipped into a mixture of polymer 8 and
NaOH, allows additionally the detection of organophosphonates
from the gas phase.
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Financial support by the Dutch Polymer Institute (DPI,
technology area HTE) is kindly acknowledged. The authors
thank A. Baumgartel for the MALDI-TOF MS measurements.
¨
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