Luminescent heteroleptic Eu(iii) probes for the selective detection of diethyl chlorophosphate as a G-series nerve agent mimic in the vapor phase using solid-state films

Article abstract of DOI:10.1039/d1tc01685g

The extreme toxicity of innocuous organophosphate (OP) nerve agents poses a significant public health risk. Developing an efficient sensing and detection system is crucial to contain and mitigate disasters and strengthen national security. Luminescence sp

Full text of DOI:10.1039/d1tc01685g

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Materials Chemistry C
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PAPER
Luminescent heteroleptic Eu(III) probes for the
selective detection of diethyl chlorophosphate as
a G-series nerve agent mimic in the vapor phase
using solid-state films†
Cite this: J. Mater. Chem. C, 2021,
, 10037
9
a
a
b
a
Zafar Abbas, Usha Yadav, Ray J. Butcher
and Ashis K. Patra
*
The extreme toxicity of innocuous organophosphate (OP) nerve agents poses a significant public health
risk. Developing an efficient sensing and detection system is crucial to contain and mitigate disasters
and strengthen national security. Luminescence spectroscopy offers very sensitive, visually identifiable
color changes to enable a low-cost option for the specific detection of nerve agents. Lanthanide ions
exhibit unique optical properties due to sensitized f - f transitions over organic fluorophores for
developing luminescent optical sensors. Here, we designed two heteroleptic time-resolved luminescent
3 3
Ln(III)-probes, namely, [Eu(o-HPIP)(TTA) ] ([Eu(o-OH)]) and [Eu(p-HPIP)(TTA) ] ([Eu(p-OH)]), for the
selective sensing of the G-series nerve agent simulant diethyl chlorophosphate (DCP). The molecular
identity, speciation, and optical properties of the complexes were evaluated using various
physicochemical and spectroscopic methods in solution. The solid-state structure of Eu(o-OH) shows
2 6
an eight-coordinated {LnN O } square-antiprismatic geometry. The interaction of the Eu(III) probes with
DCP results in selective quenching of the characteristic red luminescence of Eu(III) originating from the
5
7
0 J
D - F (J = 0–4) f - f transitions with a limit of detection (LOD) reaching up to the ppb level for
DCP {LOD: 18 ppb [Eu(o-OH)] and 10 ppb [Eu(p-OH)]}. Selective luminescence quenching is distinctly
perceptible to the naked eye under UV-illumination (l = 365 nm). The phenyl hydroxyl group was
strategically incorporated at the periphery of the HPIP ligand anticipating possible nucleophilic attack at
the P–Cl bond of DCP, thereby perturbing the EnT pathway from the ligand to Eu(III). The NMR titration
3 n
data and the structure of the isolated thermodynamically stable polymeric [Eu(DHP) ] end-product
formed by the reaction of Eu(o-OH) with DCP confirm chemical changes that block the indirect-energy
5
transfer from the o/p-HPIP/TTA antenna to the emissive D
0
state of Eu(III) causing significant quenching
of time-resolved luminescence (TRL) intensity. Vapor phase detection of OP-simulants was attempted
using a Eu(III)-probe immobilized on a low-cost filter paper strip. We observed consistent emission
quenching of filter paper strips upon exposure to DCP vapors. The changes in Eu(III)-TRL intensity avoid
any background autofluorescence and possible photobleaching associated with organic fluorophores.
These absorbent paper strips could be developed as a prototype for the selective on-site detection of
G-series nerve agents.
Received 12th April 2021,
Accepted 28th June 2021
DOI: 10.1039/d1tc01685g
rsc.li/materials-c
Introduction
Human exposure to most toxic chemical warfare agents (CWAs)
or nerve gases remains a critical concern for human health
and national security. Most of the nerve gases are odorless,
a
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016,
Uttar Pradesh, India. E-mail: akpatra@iitk.ac.in
1
b
colorless, tasteless, and harmful below our sensory threshold.
Department of Chemistry, Howard University, Washington, DC 20059, USA
†
Electronic supplementary information (ESI) available: Characterization of the These most nefarious chemicals have very low median lethal
probes (ESI-MS, FT-IR, TGA), crystallographic structure details, absorption,
doses (LD ) for humans. They could be used as weapons of
5
0
luminescence properties, PL responses of the Eu(III) probes with acids or bases,
mass destruction (WMDs) due to their easy production, trans-
portation and vapor-phase (aerosols) deployment without much
technical expertise associated with inadequate monitoring and
1
H-NMR of La(o-OH) with DCP, detection limit (PDF). Crystal structure data for
3 n
Eu(o-OH), La(o-OH) and the [Eu(DHP) ] polymeric product (CIF file). CCDC
2
049309, 2049310 and 2069827. For ESI and crystallographic data in CIF or other
2
electronic format see DOI: 10.1039/d1tc01685g
surveillance. Apparently, the featureless attributes of OP nerve
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relaxation. The disruption of neurotransmission signals results
in respiratory failure and neuromuscular paralysis, and
6
,7
immediate death.
0
The lethal dose (LD ) for G-agents is
50
ꢀ
1
.069–117.9 mg kg , while for much more stable V-agents LD
50
ꢀ
1 8
is only 0.0082–1.402 mg kg
.
The extreme toxicity, easy
accessibility, and deployability of nerve agents make them an
extreme threat to human life and national security. Therefore,
reliable, easy-to-use selective sensing for rapid detection and
monitoring of such lethal CWAs is of great priority and crucial
for national security. Various OP derivatives are typically used as
nerve agent simulants (mimics) and as target analytes in place of
real nerve agents in academic research (Fig. 1). Diethyl chloro-
phosphate (DCP) is used as a target analyte to mimic G-series
9
nerve agents like sarin (GB) and soman (GD). Considering the
Fig. 1 Chemical structures of the G-series nerve agents and their relatively threat of CWAs, several detection methods like GC-MS, ion
non-toxic simulants. Diethyl chlorophosphate (DCP) was selectively
detected in this work.
mobility spectrometry, enzymatic activity-based, surface acoustic
wave (SAW), interferometry, colorimetric and fluorogenic
10–17
methods have been developed.
Monitoring of CWAs by the
agents make their detection even more challenging before change in the color of colorimetric or fluorogenic probes is
inhalation compared to various other explosive materials or relatively convenient, cost-effective, and portable and sometimes
1
8–23
WMDs. The nerve agents are OPs of the general formulation offers qualitative naked eye detection in real-time.
[
Mostly
RO(OQP)(X)(OR )] with X = F, CN, SR (Fig. 1). The substituent in organic chromo-fluorogenic probes are reported for such detection
P–X bond serves as a labile leaving group upon the nucleophilic with a few recent examples of luminescent 4d/5d-block transition
0
00
3,4
phosphorylation reaction. The nerve agents are highly reactive metal complexes. Emissive Ru(II) and Ir(III) complexes were recently
at the active site of cholinergic enzyme-acetylcholinesterase reported for selective detection of OP nerve agent simulants and
(
AChE) that mediates the hydrolysis of the acetylcholine (ACh) pesticides based on phosphorylation of the reactive site in the
5
neurotransmitter. The kinetically and thermodynamically ligands by OP agents, thereby inducing modulation of color or
favorable irreversible phosphorylation reaction with the serine emission intensity.
24,25
Swager and coworkers reported a hi-tech
hydroxyl group in the esteratic site prevents binding of ACh and wireless wearable hazard badge based chemical dosimeter linked
its subsequent hydrolysis (Scheme 1). This inhibition of AChE to the changes in chemiresistance for the remote detection
26
causes accumulation of ACh in the synapses, with continuous of DCP. Some of these organic chromo-fluorogenic probes
activated state of ACh-receptors, thereby blocking the muscle show certain limitations, like a poor S/N ratio, background
Scheme 1 (A) Nucleophilic attack of the free –OH of the serine residue at the active site of acetylcholinesterase (AChE) on the electrophilic P-atom of
nerve agents, followed by removal of the leaving group (X) leading to irreversible inhibition of the enzyme active site. (B) Our design is based on a similar
nucleophilic attack of the free OH group at the periphery of the probes on the P–Cl bond of the DCP nerve agent simulant, resulting in the formation of a
phosphorylated product and perturbation of EnT pathways affecting Eu(III)-luminescence.
1
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autofluorescence, photobleaching, false positive responses, etc. (P–X bond). The strategy is motivated from the irreversible
Luminescent lanthanide probes could effectively bypass these nucleophilic attack of the serine hydroxyl group on the OP
inherent limitations of organic fluorophores due to their unique nerve agents at the active site of AChE (Scheme 1). The use of
excited electronic states derived from highly shielded f-orbitals.
reactive hydroxyl groups in the molecular design of inorganic
The unique luminescence properties of lanthanides origi- and organic optical probes for intended changes in the optical
nate from intraconfigurational f - f transitions linked to their parameters for the detection of nerve agents/simulants has been
1
9,24,25
shielded inner f-orbital electrons exhibiting very well-defined documented.
The thermodynamically stable luminescent
electronic states unperturbed by the ligands. Luminescent Eu(III)-complexes Eu(o-OH) and Eu(p-OH) exhibit highly intense
Ln(III)-probes display characteristic intense sharp line-like red emission in solution and the solid state. The o- and p-hydroxy-
emission spectra with high color purity ranging from the phenyl derivatives of imidazophenanthroline ligands (o-HPIP,
UV-vis to the NIR region. The long-lived (ms–ms) emissive excited p-HPIP) containing reactive –OH groups were intentionally
states of Ln(III) allow highly sensitive time-delayed photolumi- positioned to interact with DCP as the nerve agent mimic. The
nescence measurements to detect an analyte, thus eliminating anionic b-diketonate ligand (TTA) is used to impart thermo-
short-lived background autofluorescence for a higher S/N ratio. dynamic stability and charge neutralization via stronger ionic
Other added advantages are the large ligand-based pseudo- interactions with the hard Eu(III) centre. Both Eu(III)-probes show a
Stokes shift (lex ꢀ lem) which enables measurements over a selective ‘turn-off’ luminescence response towards DCP in the
wide spectral window and the photobleaching resistance of presence of various other competitive interferents. The ‘turn-off’
2
7
Ln(III) luminescence. These desirable features of emissive probes were responsive up to the ppb level for DCP [LOD:
Ln(III) probes make them the superior choice for a specific Eu(o-OH), 18 ppb, Eu(p-OH), 10 ppb]. We can also detect DCP
analyte that alters the optical output by modifying the under- in the nanomolar range in solution by the naked eye. The
lying energy-transfer pathways. However, the 4f - 4f transitions reactions of the Eu(III)-probes with DCP were monitored using
1
are electric dipole (ED) and Laporte forbidden, resulting in very
H-NMR titration studies using the diamagnetic isostructural
] [La(o-OH)] to define a
ꢀ1
ꢀ1
weak absorptions (e B 0.1–10 M cm ), and therefore the control compound [La(o-HPIP)(TTA)
3
direct excitation via absorption of Ln(III) ions is ineffective. probable luminescence quenching pathway. The final thermo-
An energy-absorbing organic chromophore (antenna) was linked dynamically driven reaction product was structurally identified
to Ln(III) ions to circumvent this limitation. The light-harvesting and showed possible hydrolysis reaction and degradation of the
antenna is generally an aromatic or unsaturated organic probe. For practical use, vapor phase detection of the OP
molecule that effectively absorbs the radiation and indirectly simulants was carried out using the Eu(III)-probes immobilized
transfers this energy non-radiatively to populate the emissive on a low-cost filter paper strip showing distinct response and
2
7
excited state (Ln*). The radiative decay of Ln* to the ground selectivity towards DCP.
state results in bright luminescence with all the characteristic
features mentioned above. Designing luminescent Ln(III) probes
is a multi-pronged approach based on the underlying photo-
sensitization pathways and a subtle balance of multiple intricate
radiative and non-radiative EnT pathways. Meade and coworkers
Results and discussion
Synthesis and general aspects
extensively reviewed various design principles of analyte- The hydroxy-phenyl substituted-2-imidazophenanthroline ligands
responsive Ln-optical probes and their underlying hypo- (o-HPIP, p-HPIP) were synthesized using the well-known Debus–
2
8
theses. In a nutshell, the detection or sensing of a specific Radziszewski imidazole formation reaction via direct
analyte by the antenna-conjugated sensitized luminescent Ln(III) condensation of 1,10-phenanthroline-5,6-dione with ortho- and
probes is based on the perturbation of the overall energy transfer para-hydroxybenzaldehydes in glacial AcOH with an excess of
36
4
pathways and thereby alteration of various optical parameters as NH OAc. The rationale behind the o/p-isomer was to get insight
a measuring yardstick. Luminescent lanthanide complexes were into the effect of potential intramolecular hydrogen bonding in
used as OP sensors based on their competitive binding of modulating the triplet-state of the ligands in sensitizing Eu(III)
phosphoryl oxygen (PQO) to hard oxophilic Ln(III) ions and luminescence and the difference in reactivity for the nucleophilic
quenching of non-radiative vibrational energy transfer. Other attack to form phosphorylated ligands. The Eu(o-OH) and
strategies involve phosphorylation at the coordinating site of the Eu(p-OH) complexes were synthesized using a stoichiometric ratio
ꢀ
antenna and its concomitant displacement and deactivation of of 1 : 1 : 3 of Eu(III):o/p-HPIP :TTA in MeOH under reflux for 12 h
2
9–35
the antenna effect.
Towards our continuous efforts to explore the lanthanide
via metal-based template synthesis (Scheme 2).
Analogous diamagnetic [La(o-OH)] was prepared using a
chemistry and utilizing fascinating Ln(III) luminescence for similar modular approach, anticipating identical types of
diverse interdisciplinary applications, we realized that organo- bonding and coordination geometry around Ln(III). The three
phosphates are a judicious and critical choice as analytes for anionic b-diketonate ligands (2-thenoyltrifluoroacetonate
sensing via time-delayed luminescence spectroscopy. Herein, (TTA)) were suitable to form stronger ionic bonds with the
our design involves two luminescent Eu(III) complexes with a hard Ln(III), thereby satisfying coordinative saturation with
free O–H group at the periphery of the antenna available for enhanced
thermodynamic
stability
and
minimizing
the potential phosphorylation reaction with OP nerve agents undesirable fast solvent-exchange reactions in solution. The
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the +3 charge of Ln(III) via ionic bonding. The disposition of
ꢀ
o-HPIP and TTA results in eight-coordinated {LnN
2
O
6
} poly-
hedra around the Ln(III) ions with distorted square-
antiprismatic (SAP) geometry (Fig. 2b). The twist angle between
the two square-planes is 401 for Eu(o-OH) and 411 for La(o-OH)
compared to the ideal value of 451. The peripheral phenyl-
hydroxyl group at the ortho-position of the o-HPIP ligand
is involved in a strong intramolecular [H–Oꢁ ꢁ ꢁN, 1.88 Å in
Eu(o-OH) and 1.87 Å in La(o-OH)] hydrogen bonding inter-
action with the free nitrogen on the imidazole by forming a six-
membered ring. The exposed phenyl-hydroxy group is readily
accessible as a reactive site for nucleophilic attack at positively
polarized organophosphates. The Ln–O bond distances are
Scheme 2 Generalized synthetic scheme of the probes: [Eu(o-OH)], [Eu(p-OH)], 2.34–241 Å in Eu(o-OH) and 2.44–2.53 Å in La(o-OH), tracking
and [La(o-OH)]. (a) H
reflux for 4 h. (c) NaOH in methanol. (d) Methanol, reflux for 12 h.
2
SO
4
, HNO
3
, KBr, 24 h at RT. (b) CH
3
COOH, NH
4
OOCH
3
,
the trend in ionic sizes of Eu(III) and La(III). The shorter Ln–O
bonds from TTA possess more ionic character than the Ln–N
ꢀ
bonds from the neutral o-HPIP ligand. The ORTEP views of the
coordinative saturation (C.N. = 8) ensures their structural complexes are shown in Fig. 2a and c. The unit cell packing
integrity in solution and minimizes luminescence quenching diagrams, refinement parameters, selected bond lengths, and
via vibrational energy transfer (VET) mediated by fast exchange bond angles are shown in Fig. S5, S6 and Tables S1, S2 in the
reactions. The Eu(o-OH) and La(o-OH) complexes were structurally ESI,† respectively.
characterized using single-crystal X-ray diffraction, demonstrating
their anticipated isostructural {LnN O } square-antiprismatic
coordination geometry. The complexes were characterized by
2
6
Photophysical characterization
FT-IR, ESI-MS, electronic absorption, and emission spectroscopy, The electronic absorption and emission spectra of the probes
supporting the formation of discrete molecular species in the solid Eu(o-OH) and Eu(p-OH) were recorded in acetonitrile solution
state and solution. The ESI-MS spectral analysis of Eu(o-OH) reveals
+
ꢀ
[
M–H ] molecular ion peaks with the corresponding matching
isotopic distribution patterns of the Eu(III) cations (Fig. S1 in the
ESI†). In the FT-IR spectra of the complexes, n stretching
CQO
ꢀ
1
vibrations were present at B1600 cm compared to the free
vibration bands in the complexes
were observed at B1539 cm and nC–F bands at B1140 cm
ꢀ
1
TTA at B1650 cm . The n
CQN
ꢀ
1
ꢀ1
.
The nEu–O stretching bands in the complexes were observed at
ꢀ1
B580 cm , which suggests the coordination of the ligands with
Eu(III) ions (Fig. S2 and S3 in the ESI†). Electronic absorption
spectroscopy shows the absorption bands characteristic of the
HPIP and TTA ligands in the complexes at 275, 296 nm, and
335 nm unperturbed by the Ln(III) ion (Fig. 3a). The emission
spectroscopy shows the characteristic f - f transitions of Eu(III)
5
7
with bright red luminescence originating from
D
0
-
F
J
transitions ( J = 0–4) (Fig. 3b). To evaluate the reactivity of the
Eu(III) probes with DCP using NMR spectroscopy, isostructural
diamagnetic [La(o-OH)] was used to avoid the broad spectrum
1
and paramagnetic shifts in H signals induced by paramagnetic
37
Eu(III) complexes. Thermogravimetric analysis shows that the
probes withstand heat up to 250 1C, not losing any significant
weight, indicating their stability in the solid state (Fig. S4 in the
ESI†).
Structural characterization
The discrete isostructural mononuclear Eu(o-OH) and La(o-OH)
complexes crystallized in the triclinic P1% space group. Eu(III)
2 6
Fig. 2 The ORTEP view of Eu(o-OH) (a) and the {EuN O } coordination
polyhedra showing a square-antiprismatic (SAP) geometry (b). The ORTEP
0
and La(III) ions are coordinated to one N,N -donor imidazophe-
0
nanthroline (o-HPIP) and three mono-anionic ancillary O,O -
view of the isostructural diamagnetic La(o-OH) complex (c). Dotted lines
ꢀ
donor 2-thenoyltrifluoroacetonate (TTA ) ligands neutralizing show the intramolecular H-bonding in the HPIP ligand.
1
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(
5 mM) at 298 K. The complexes show three strong absorption ligand-centered absorption band. This indicates that the photo-
ꢀ1
ꢀ1
bands at 275 nm [e/M cm ] = 51 800 [Eu(o-OH)], 53 800 sensitized ligands effectively transfer energy (EnT) to the Eu(III)
ꢀ
1
ꢀ1
5
[
[
Eu(p-OH)]; 296 nm [e/M cm ] = 42 600 [Eu(o-OH)], 50 900 ion to populate the emissive excited D0 state, followed by
Eu(p-OH)] and 336 nm [e/M
ꢀ
1
ꢀ1
7
cm ] = 63 600 [Eu(o-OH)], subsequent relaxation to the ground F (J = 0–4) states resulting
J
4
2
5
3 600 [Eu(p-OH)] resulting from the ligand-centered transi- in bright luminescence (Fig. 3b). The excitation spectra of the
5
7
0 2
tions desirable for effective light-harvesting and sensitization of complexes recorded at 613 nm ( D - F ) overlap with the
Eu(III) (Fig. 3a). The N,N-donor ligands show absorption bands absorption spectra of both ligands, and they do not contain any
in the 270–400 nm range with a major band centered at Eu(III)-centered intrinsic sharp absorption peaks, suggesting very
B280 nm attributed to the intra-ligand (IL) p–p* transitions ineffective direct excitation as shown in Fig. S9 in the ESI.† This
3
8
from the imidazophenanthroline derivatives. The O,O-donor further supports the efficient indirect EnT from the dual
5
b-diketonate ligand TTA has an absorption maximum at antenna that populates the excited D states of the Eu(III) ion.
0
3
9
B330 nm assigned to p–p* transitions of the enol form.
The reversible equilibria and binding of the neutral HPIP
The absorption bands of the complexes could be assigned to diimine ligand in [Eu(HPIP)(TTA)3] complexes containing
ligand-centered absorptions with an extensive overlap and o/p-HPIP and TTA ligands in a 1 : 3 ratio were established by
4
3,44
minor shifts in band maxima. The absorption spectra of quantitative luminescence titration studies.
the coordinated ligands were minimally perturbed upon Eu(TTA) O as a precursor for [Eu(HPIP)(TTA)
coordination to the Eu(III) ion, which has valence shell electrons titrated with o/p-HPIP, and recorded the photoluminescence
in deeply buried f-orbitals, suggesting predominantly ionic spectra as shown in Fig. 4a and b. The inset plot shows the
bonding. An overlay of the absorption spectra of all the ligands enhancement in the Eu(III) emission intensity with increasing
We took
3
ꢁ2H
2
3
] complexes,
(
o/p-HPIP and TTA), Eu(TTA) ꢁ2H O, and the complexes is [o/p-HPIP] until reaching a 1 : 1 molar ratio with Eu(TTA) ꢁ2H O,
3
2
3
2
shown in Fig. S8 in the ESI.†
confirming the formation of the coordinatively saturated
] formulation. After reaching this saturation
The PL spectra of Eu(o-OH) and Eu(p-OH) (5 mM) were [Eu(HPIP)(TTA)
3
recorded at the lowest energy absorption bands (lex
=
level, further addition of [o/p-HPIP] does not cause any apparent
5
7
3
40 nm) showing emission resulting from the
D
0
-
F
0
changes in luminescence intensity. These observations suggest
5
7
5
7
5
0
(
580 nm), D
0
- F
(653 nm), and D
1
(592 nm), D
0
- F
2
(613 nm), D
0
-
that Eu(III) is coordinated to one N,N -donor and three TTA
7
5
7
F
3
0
- F
4
(704 nm) transitions character- ligands and exists as eight-coordinated [Eu(o/p-HPIP)(TTA)
3
]
istics of Eu(III) ions. The induced electric-dipole allowed hyper- in solution. The binding constants (K ) of the ligands [7.82 ꢂ
b
5
7
5
ꢀ1
5
ꢀ1
sensitive D - F transitions at 613 nm, which is the most 10
M
for o-HPIP and 6.59 ꢂ 10
M
for p-HPIP] were
0
2
intense band compared with the others and primarily calculated from the above luminescence titration studies, which
responsible for the bright red luminescence of both complexes. suggest the significant stability of the Eu(o-OH) and Eu(p-OH)
Moreover, it also suggests that a highly polarizable environment complexes in solution. The luminescence decay lifetimes (t) of
4
0
5
7
exists in the complexes around the Eu(III) ion. The intensity the complexes for the
ratio of I7F2/I7F1 [18 for Eu(o-OH) and 17 for Eu(p-OH)] suggests a calculated from their decay curves in acetonitrile. The t values
lack of inversion center of symmetry (C ) around the Eu(III) ion for Eu(o-OH) and Eu(p-OH) were 570 ms and 520 ms, respectively,
0 2
D - F transition (613 nm) were
i
4
1
(
Fig. 3b). The steady-state emission spectrum of Eu(o-OH) compared to the labile Eu(TTA) ꢁ2H O (t B 280 ms). These decay
3
2
shows complete quenching of the ligand-centered emission curves fit well with the mono-exponential equation and confirm
and a very weak broad emission band at B450 nm appears for the presence of [Eu(o-/p-HPIP)(TTA) ] as the predominant
3
Eu(p-OH) assigned to the p-HPIP ligand after excitation at the species existing in solution (Fig. 4c). Further speciation studies
Fig. 3 (a) Electronic absorption spectra of the complexes Eu(o-OH) and Eu(p-OH) (5 mM) in MeCN at 298 K. (b) Excitation spectra of the complexes Eu(o-OH) and
5
7
0 J
Eu(p-OH) (5 mM) recorded at lem = 613 nm and their PL spectra showing the characteristic D - F transitions for Eu(III) at lex = 340 nm. Ex./Em. slit width = 5 nm.
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Fig. 4 (a) Enhancement of the luminescence intensity of Eu(TTA)
3
ꢁ2H
2
O (5 mM) with increasing concentration of o-HPIP. Inset: Changes in relative
5
7
emission intensity for the D
increasing concentration of p-HPIP. Inset: Changes in relative emission intensity for the
Luminescence decay profile of the
0
- F
2
transition with equiv. of o-HPIP added. (b) Enhancement of the luminescence intensity of Eu(TTA)
3
ꢁ2H
transition with equiv. of p-HPIP added. (c)
transition used for lifetime measurements for the complexes Eu(TTA) O, Eu(o-OH) and Eu(p-OH) in
2
O (5 mM) with
5
7
0 2
D - F
5
7
D
0
- F
2
3
ꢁ2H
2
MeCN (5 mM) at 298 K. lex = 340 nm, delay time and gate time = 0.1 ms, total decay time = 10.0 ms, Ex. and Em. slit width = 5 nm.
were carried out on the probes, and their hydration numbers (q) analyte in solution. Before the experiments, solutions of the
were calculated from the luminescence decay time in H
2
O (tH2O
)
probes and analytes were prepared in degassed solvents. The
and D O (tD2O) (Fig. S10 and S11 in the ESI†). The calculated ligand-centered (p–p*) absorption bands of the probe Eu(o-OH)
2
q values were close to zero, which directly supports the absence at 296 and 336 nm display significant changes with increasing
of the inner-sphere coordinated water/solvent molecule to concentration of DCP. A decrease of these bands with the
the Eu(III) ion and achievement of coordinative saturation addition of DCP (0–15 mM) was observed with the concomitant
4
5
(
C.N. = 8). After studying the solution speciation of the probes, appearance of two new bands at 314 and 328 nm with a clear
we proceed to study the interactions of the analytes.
isosbestic point at 322 nm (Fig. 5a). No further changes in the
absorption spectra were noticed after the addition of 3 equiv. of the
OPs. A similar response was observed in the absorption spectra
Detection of the DCP nerve agent mimic in solution
The electronic absorption and photoluminescence spectra of of Eu(p-OH) with the addition of increasing amounts of DCP.
the probes (5 mM) were recorded with gradual addition of the Addition of DCP (0–10 mM) results in the decrease of intensity at
Fig. 5 (a) Changes in the electronic absorption spectra of Eu(o-OH) (5 mM) with increasing concentration of DCP (0–15 mM) in MeCN at RT. (b) Changes
in the PL spectra (lex = 340 nm) of Eu(o-OH) (5 mM) with the addition of increasing amounts of DCP (0–15 mM) observed for the characteristic emission
5
7
0 2
from the hypersensitive band of the Eu(III) ion ( D - F = 613 nm). Inset: Plot showing the quenching of PL intensity and percentage quenching of
emission at 613 nm vs. the equivalents of DCP added. (c) Digital images showing quenching of the bright red-luminescence of Eu(o-OH) (5 mM) with
varying equivalents of DCP (0.2–5.0 equiv.) under 365 nm UV-A light.
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96 and 336 nm and the appearance of a new band at 314 nm titrating with HCl or Et
Paper
2
3
N/NaOH (Fig. S12–S15 in the ESI†).
with an isosbestic point at 324 nm for Eu(p-OH) (Fig. 6a). The Further, upon maintaining the neutral pH, the luminescence
presence of one isosbestic point with concomitant appearance was almost completely regained (Fig. S14 in the ESI†). Such
of new bands in both probes suggests the formation of new reversible changes in emission intensity indicate that the
species with the addition of OPs in solution.
quenching caused by acid or base is reversible in nature due
The selective response of DCP, which is a sarin (GB) simulant to possible displacement of pH-sensitive TTA or protonation
4
6
with a relatively lower toxicity, was evaluated with the and deprotonation of the donor-N of HPIP ligands. Complete
luminescent Eu(III) probes using the time-delayed photo- luminescence quenching is achieved by adding 3.0 equivalents
luminescence intensity originating from the f - f transitions of DCP and is irreversible. From these observations, we
as a sensitive spectroscopic yardstick in solution. Quenching of presume that the quenching of the emission of the probes in
5
7
the intense red emission of Eu(III) ( D - F , J = 0–4) from the our experiments is caused by interaction with DCP and moved
0
J
complexes was observed with the addition of DCP. As shown in on to detailed studies with these probes.
Fig. 5b, significant irreversible quenching of the characteristic
The bright red luminescence quenching of the probe (5 mM)
emission of the Eu(III) in the probe Eu(o-OH) (5 mM) results in solution could be visualized under 365 nm UV-A light with
from the gradual addition of DCP (0–15 mM). The luminescence increasing concentration of DCP (Fig. 5c). The PL spectrum of
was nearly turned off upon the addition of B3 equiv. of DCP, the probe Eu(p-OH) shows similar changes after titration with
and no further changes were noticed upon excess addition of DCP. Quenching of the characteristic emission band was
DCP. This is evident from the inset plot in Fig. 5b, which shows observed, and it continues until ‘turn-off’ luminescence
5
7
the relative quenching of the hypersensitive band ( D - F ) appeared with the addition of B2 equiv. of DCP (Fig. 6b and
0
2
for Eu(III) at 613 nm with a molar equivalent of DCP added. c). The differential luminescence quenching rates for the
Before drawing any conclusion from the above observations, we probes Eu(o-OH) (B3 equiv. of DCP) and Eu(p-OH) (B2 equiv.
examined whether the emission quenching is solely triggered of DCP) in the presence of DCP under identical conditions
by DCP or due to possible hydrolyzed products, considering its suggest that the analyte differentially reacts with the probes in
susceptibility towards such hydrolysis. HCl might be present in solution. The reaction rate is affected by the specific molecular
the DCP solution as an impurity at a minute concentration design of the probe (i.e. position of the –OH group in phenyl
due to its hydrolysis, which might cause such quenching of ring). Both the Eu(III) probes were found to be selective towards
luminescence. To gain insights into this reaction, we recorded the DCP nerve gas simulant when tested in the presence of
the luminescence of Eu(o-OH) (5 mM) in both acidic and basic other OP interferents (e.g., DECP, DMMP, DEPP). Except for
media and observed slow quenching of emission upon going to DECP, which causes little luminescence quenching only at high
either acidic or basic ranges from the neutral condition by concentrations, other interfering OPs do not cause any
Fig. 6 (a) The changes in the electronic absorption spectra of Eu(p-OH) (5 mM) with increasing concentration of DCP (0–10 mM) in MeCN. (b) The
changes in the PL spectra (lex = 340 nm) of Eu(p-OH) (5 mM) with the addition of increasing amounts of DCP (0–10 mM) observed for the characteristic
5
7
0 2
emission from the hypersensitive band of the Eu(III) ion ( D - F = 613 nm). Inset: Plot showing the quenching of PL intensity along with percentage
quenching of emission at 613 nm vs. the equivalents of DCP added. (c) Digital images showing quenching of the bright red luminescence of the probe
Eu(p-OH) (5 mM) with varying equivalents of DCP (0.2–2.0 equiv.) under 365 nm UV-A light.
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Fig. 7 The luminescence response (lex = 340 nm) of the probes (a) Eu(o-OH) and (b) Eu(p-OH) (5 mM) in the presence of various OP reagents. OPs: DCP
(
(
diethyl chlorophosphonate), DECP (diethyl cyanophosphonate), DMMP (dimethyl methylphosphonate), DEPP (diethyl phenylethylphosphonate), TPP
triphenyl phosphonate), and TEA (triethylamine). The digital images showing responses in the red luminescence of (c) Eu(o-OH) and (d) Eu(p-OH) (5 mM)
with DCP and other interferents (DECP, DMMP, DEPP, TPP, and TEA in MeCN visualized under 365 nm UV-A light.
quenching of the luminescence of the Eu(III) probes in similar of the sensitizing antenna or phosphorylation of the sensitizing
experimental conditions (Fig. 7a and b). The digital images of ligand at the Ln(III)-coordination site and subsequent
3
5,47,48
the probe solutions with nerve agent simulants and various OP decomplexation.
interferents under UV-A light (l = 365 nm) show the selective dicarboxylate based Eu(III) probe as a turn-on sensor for DCP
turn-off’ luminescence response for DCP (Fig. 7c and d). We based on minimizing non-radiative vibrational energy-transfer,
We have reported a terpyridine-
‘
5
7
0 2
observed a comparatively higher selectivity of the Eu(o-OH) which demonstrated significant enhancement of the D - F
3
2
probe for DCP, which involves a strong intramolecular hydrogen transition. The calculated limit of detection (LOD) was 18 ppb
bonding between the hydroxyl group and a nitrogen atom of the (110 nM) for Eu(o-OH) and 10 ppb (56 nM) for Eu(p-OH). The
imidazole ring (Fig. 2). The high reactivity of the P–Cl bond with selective detection of G-series nerve agent simulants in the ppb
the nucleophilic o-OH group presumably results in quenching of range is immensely encouraging using highly sensitive TRL as a
the luminescence of Eu(o-OH) selectively. The Eu(p-OH) probe is spectroscopic tool that is devoid of autofluorescence and
not involved in such intramolecular H-bonding and has a readily photobleaching, which is superior to organic fluorophores.
available p-OH group to interact with OPs having reactive P–X Only a few optical probes for nerve agents (or simulants) have
(
X = Cl and CN) bonds. Eu(o-OH) is highly selective to DCP with been reported in the literature for their detection at the ppb
3
5
a more reactive P–Cl bond than the less reactive P–CN bond of level.
DECP, showing anticipated reactivity differences of the OPs with
Herein we attempted to explore the possible mechanism or
the free OH group of the p-HPIP ligand in similar conditions. underlying reactions between the DCP and reactive-OH
1
The Eu(p-OH) probe was found to be more sensitive due to the containing emissive Ln(III) probes using H-NMR spectroscopy.
presence of free p-OH and showed complete luminescence We studied the reactivity of DCP with the diamagnetic analog
6
quenching even with 2 equiv. of DCP, while Eu(o-OH) with an La(o-OH) in DMSO-d instead of the paramagnetic Eu(o-OH)
1
1
o-OH group involved in intramolecular H-bonding shows using H-NMR titration to avoid broadening of the H-NMR
3
7
complete quenching with 3 equiv. of DCP. These results suggest peaks. We noticed two new peaks in the aliphatic region at
that selective and sensitive detection of DCP is effectively 1.20 (–CH ) as a triplet and 3.80 ppm (–CH ) as a quartet
3
2
possible with the tested luminescent Eu(III) probes in solution belonging to the ethyl group of DCP along with upfield and
1
by varying the reactivity of the functional groups. The Eu(o-OH) downfield shifts of the H signals in the aromatic region
probe was found to be highly selective for DCP, while the corresponding to both o-HPIP and TTA antenna chromophores
Eu(p-OH) probe is selective as well more sensitive toward DCP. (Fig. S16–S18 in the ESI†). After addition of one equiv. of DCP, the
A few luminescent Ln(III) probes for nerve agents or their peaks at 13.97 ppm (–OH) and 12.60 ppm (–NH) disappeared,
simulants showing quenching (turn-off) were reported. Such suggesting the possible interaction (i.e., phosphorylation reaction)
‘turn-off’ triggering was observed due to competitive displacement of these groups with the organophosphates (Fig. S18 in the ESI†).
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Paper
Addition of one equivalent of DCP causes only partial quenching Vapor phase detection of DCP in a solid-state film
of luminescence and further addition leads to complete ‘turn-off’
Fluorescence-based solution-phase detection of nerve agents
luminescence. The NMR titration data were not sufficient to
using various optical sensors is well-studied compared to their
explicitly propose the intricate luminescence quenching
vapor phase detection in the solid phase for designing practical
mechanism involving multiple energy transfer steps to sensitize
detection kits. Vapor phase detection is highly desirable for
the Eu(III) ion. Only a very limited reports exist on the ultimate
practical applications considering their volatile nature.
products formed over time upon reactions of the Ln(III) sensors
Apparently, inconspicuous volatile colorless and odorless nerve
and such nerve agent mimics. To unambiguously confirm the
agents could be spread as aerosols. Because of their higher
identity of the final thermodynamically stable product(s), we
density than that of air, they remain near the ground, which
attempted to crystallize the reaction product of the probe
makes them more lethal and their timely detection more
Eu(o-OH) and DCP (5 equiv.) by mixing them in CHCl . The
3
5
7
challenging. Therefore, the on-site detection of these nerve
gases in the vapor phase using simple, selective and sensitive
low-cost solid-state sensors is crucial to mitigate real threat
scenarios for public health. Herein, to test this requirement, we
used a low-cost filter paper strip as a solid-state matrix and
impregnated it with the luminescent probes Eu(o-OH) and
Eu(p-OH). The sensor-loaded air-dried filter paper strips show
structure obtained after a prolonged time by slow evaporation
yields a linear polymeric chain containing Eu(III) ions and diethyl
hydrogen phosphate (DHP) ([Eu(DHP) ] ), a hydrolysis product of
3 n
DCP, as shown in Fig. 8. A single polymeric chain unit contains
two Eu(III) ions bridged by three diethyl phosphate anions (Fig. S7
in the ESI†). Each Eu(III) ion is coordinated to six O-atoms of the
six DHP molecules. Based on the literature and our observations
from differential emission quenching rates of the probes, NMR
titration with DCP and identification of the hydrolyzed Eu-DHP
polymeric product, we could only propose a possible lumines-
cence quenching mechanism. The exact sequence of chemical
reactions and their speciation in solution is difficult to establish
considering the fast substitution kinetics, reactivity and
adventitious hydrolysis of DCP and the ionic bonding nature of
the Eu(III) probes. The proposed mechanistic pathway involves a
possible phosphorylation reaction at the ligand periphery via
nucleophilic attack by the OH group of the o/p-HPIP ligand at
5
7
0 2
bright red luminescence due to the D - F f–f transitions
from Eu(III) ion under 365 nm UV-A irradiation (Fig. 9a). These
Eu(III)-probe loaded filter paper strips were exposed to the
vapors of the organophosphorus nerve agents and various
interfering OPs and TEA (100 mL). Upon exposure to the DCP
vapor inside long scintillation vials inside a well-ventilated
fume-hood, the initial bright red luminescence of the filter
paper strips was significantly quenched for both the tested
probes (Fig. 9 and 10). Other interfering OPs do not show
considerable quenching of the luminescence of the filter paper
the reactive P–Cl bond of DCP, followed by hydrolysis of the strip. This observation confirms that the Eu(III)-probes can
intermediate phosphorylated moiety, which leaves diethyl hydro- selectively detect the presence of DCP vapors in the atmosphere
50–53
gen phosphate (DHP) as a product.
However, we could not using a simple, low-cost, filter-paper based solid-state sensor
completely ignore the in situ hydrolysis of the reactive DCP in the (Fig. 9a and 10a). The luminescence of the immobilized filter
presence of traces of water or moisture to form DHP. DHP is a paper strip was also recorded by varying the concentration of
strong oxoacid and ideal for coordination with the hard Lewis DCP. Noticeable quenching of the bright red luminescence was
acid Eu(III) by removing the sensitizing antenna from the observed even at lower concentrations of DCP (20 mL) and a
coordination sphere (Scheme 3). This reaction yields a thermo- total quenching or turn-off luminescence was observed with
dynamically stable product, [Eu(DHP) ] , as a linear polymeric B80 to 100 mL of DCP under similar conditions as shown in
3
n
chain as shown in Fig. 8 and Fig. S7 (ESI†). The absence of any Fig. 9b and 10b. These observations apparently exhibit the
antenna effect for sensitizing Eu(III) due to changes in the dependency of the photoluminescence of the probe upon
coordination sphere (i.e., displacement/chemical alteration of DCP vapor concentration in the solid-state matrix due to
antenna ligands) causes the quenching of the ’’luminescence of anticipated phosphorylation reactions with the hydroxyl-
35,47–49,53–56
the probes with literature precedence (Scheme 3).
containing PIP-antenna as proposed in the solution phase.
3 n
Fig. 8 (a) The molecular structure of a single unit from the 1D chain polymeric product [Eu(DHP) ] obtained after a prolong reaction time of Eu(o-OH)
with DCP. (b) Octahedral geometry acquired by the Eu(III) ion from coordinating O-atoms of DHP. (c) Schematic chemical drawing of a single unit of the
polymeric product. Diethyl hydrogen phosphate (DHP) is shown at the bottom for clarity.
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Scheme 3 A schematic diagram of the proposed reaction pathway for the interaction of Eu(III) probes with DCP leading to turn-off luminescence. One
of the identified hydrolyzed end-products ([Eu(DHP) ) is shown in Fig. 8.
3 n
]
Fig. 9 (a) Selective response of the luminescence of Eu(o-OH) immobilized on filter paper strips in the presence of DCP vapors and vapors of other
interferents (100 mL) under 365 nm UV-A exposure. (b) Quenching of the luminescence of Eu(o-OH) immobilized on paper strips upon exposure to
increasing DCP concentration (0–100 mL). (c) The real-time response of the luminescence (time scan) of strips upon exposure to DCP vapors (100 mL).
Fig. 10 (a) Selective response of the luminescence of Eu(p-OH) immobilized on filter paper strips in the presence of DCP vapors and vapors of other
interferents (100 mL) under 365 nm UV-exposure. (b) Quenching of the luminescence of Eu(p-OH) immobilized on paper strips upon exposure to
increasing DCP concentration (0–100 mL). (c) Real-time response of the luminescence of strips upon exposure to DCP vapors (100 mL).
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Paper
Both the Eu(o-OH) and Eu(p-OH) probes work similarly towards solution, a neutralized solution of TTA (3 equiv., 0.3 mmol and
the selective detection and concentration-dependent response 66.6 mg) in methanol was added and refluxed for 12 h. The
in the presence of DCP vapor.
solvent was evaporated in vacuum and the solid product was
Finally, we also investigated the response-time of the probe redissolved in chloroform and filtered to remove salt
by measuring the real-time photoluminescence of the probe impurities. The filtrate upon evaporation in vacuum yielded
immobilized on filter paper strips. Upon exposure to DCP the desired complex Eu(o-OH). The compound was crystallized
vapor under 365 nm UV irradiation, gradual quenching of the in chloroform solution. Yield: 81% (92 mg). ESI-MS in CHCl
3
+
ꢀ
luminescence of the filter paper strips with time was observed (m/z): [M–H ] calculated for C43
in both probes. The Eu(o-OH) probe took ca. 15 min for found: 1126.983. UV-Vis in MeCN: (labs/nm (e/M cm ): 273
H
23EuF
9
N
4
O
7
S
3
ꢀ
= 1126.980,
1
ꢀ1
ꢀ1
complete quenching of the luminescence, while it took ca. (29 100), 340 (30 800). FT-IR (KBr/cm ): 3424 (br), 1674 (s),
0 min for the Eu(p-OH) probe on exposure to DCP vapor 1601 (nCQO, s), 1539 (s), 1513 (s), 1485, 1465, 1412, 1357, 1307,
100 mL) (Fig. 9c and 10c). The Eu(p-OH) probe is more sensitive 1248, 1230, 1190, 1141, 1080, 1035, 1032, 934, 859, 788, 768,
and reacts faster towards DCP vapor, presumably due to the 751, 720, 681, 669, 641, 581.
freely available p-OH group for nucleophilic attack, leading to [Eu(p-HPIP)(TTA) ] [Eu(p-OH)]. The synthesis of Eu(p-OH)
1
(
3
complete quenching, while in Eu(o-OH), where the o-OH is was performed following a similar procedure to that used for
involved in intramolecular hydrogen bonding with the complex Eu(o-OH), but the p-HPIP ligand was used in place of
+
+
imidazole-N of HPIP takes a slightly longer time. These studies o-HPIP. Yield: 75% (85 mg). ESI-MS in CHCl
reveal that the tested simple solid-state Eu(III) probes are useful calculated for C43 23EuF = 1126.980, found: 1126.983.
towards rapid selective naked-eye visualization of DCP vapor due UV-Vis in MeCN: (labs/nm (e/M
3
(m/z): [M + Na ]
H
9 4 7 3
N O S
ꢀ1
ꢀ1
cm ): 273 (23 800), 340
(29 300). FT-IR (KBr/cm ): 3476 (br), 1675(m), 1602 (nCQO, s),
ꢀ
1
to complete quenching of their photoluminescence intensity.
1
1
539(s), 1507, 1485, 1454, 1411, 1357, 1306, 1258, 1191, 1079,
062, 1037, 934, 859, 842 789, 768, 721, 682, 642, 582, 518.
[
La(o-HPIP)(TTA)
mentioned above was applied for the synthesis of this complex
also using LaCl O in place of the Eu(III) salt. Yield: 75%
3
] [La(o-OH)]. A procedure similar to that
Experimental section
Synthesis and characterization
3
ꢁ6H
2
ꢀ1
The imidazo-phenanthroline ligands (o-HPIP, p-HPIP) were prepared (85 mg). FT-IR (KBr/cm ): 1597 (nCQO, s), 1534(s), 1502, 1480,
using a modified literature procedure from the condensation of 1,10- 1450, 1410, 1353, 1296, 1245, 1183, 1129, 1059, 1035, 931, 858,
36
phenanthraquinone and o/p-hydroxybenzaldehydes.
840, 785, 749, 716, 679, 639, 577. The compound was
2-(2-Hydroxyphenyl)imidazo[4,5-f][1,10]phenanthroline (o-HPIP). recrystallized from chloroform solution and structurally
A mixture of phenanthroline-5,6-dione (100 mg, 0.48 mmol) characterized by X-ray crystallography.
and ammonium acetate (1.1 g, 14.4 mmol) was dissolved in
General and spectroscopic measurements
glacial acetic acid (3 mL). To this reaction mixture, a solution of
2-hydroxybenzaldehyde (58.6 mg, 0.48 mmol) in 2 mL acetic Fourier transform-infrared (FT-IR) spectra were recorded on a
acid was added and refluxed for 4 h. The reaction was PerkinElmer model 1320 spectrometer with KBr disc in the
ꢀ1
quenched with water (50 mL), and then aqueous ammonia 4000–400 cm
range and the NMR characterization and
solution was added for neutralization, which results in a yellow titration data were recorded on a JEOL 400/500 MHz spectro-
precipitate. The precipitate was collected after filtration, meter in deuterated solvents with reference to TMS at 298 K.
washed with a copious amount of water and then with a small The ESI-MS spectra of the compounds used here were recorded
volume of ethanol, and dried in vacuum to obtain the desired on
a
Q-TOF electrospray ionization mass spectrometer.
), d (ppm): Thermogravimetric analysis data were obtained from a Mettler
3.98 (s, 1H), 12.75 (s, 1H), 9.09 (dd, 2H, J1 = 4.4 Hz, J2 = Toledo Star System under a N atmosphere with a heating rate
1
ligand in 45% yield. H-NMR (500 MHz, DMSO-d
6
1
1
7
7
2
ꢀ
1
.5 Hz), 8.98 (dd, 2H, J1 = 7.8 Hz, J2 = 1.5 Hz), 8.21–8.23 (m, 1H), of 10 1C min . The absorption spectra of all ligands (5 mM) and
.89 (dd, 2H, J1 = 7.8 Hz, J2 = 4.4 Hz), 7.41–7.45 (m, 1H), 7.11– complexes (5 mM) were recorded in MeCN on a Varian V670
.14 (m, 2H). ESI-MS in MeOH: calcd m/z for C H N O as spectrophotometer at 298 K. Responses to the addition of
1
9
13 4
+
[
M + H] = 313.109, found = 313.108.
-(4-Hydroxyphenyl)imidazo[4,5-f][1,10]phenanthroline (p-HPIP). complexes (5 mM) were monitored in acetonitrile. The lumines-
A procedure similar to that for o-HPIP was followed for the cence of both complexes was recorded on an Agilent Cary
aliquots of the OP agents on the absorption bands of the
2
1
synthesis of this ligand. H-NMR (500 MHz, DMSO-d
6
), d (ppm): Eclipse fluorescence spectrophotometer. Fluorescence assay
1
8
7
3.52 (s, 1H), 9.98 (s, 1H), 9.03 (dd, 2H, J1 = 4.4 Hz, J2 = 1.5 Hz), measurements of complexes Eu(o-OH) and Eu(p-OH) (5 mM)
.91 (dd, 2H, J1 = 8.8 Hz, J2 = 1.5 Hz), 8.12 (d, 2H, J = 7.8 Hz), were performed with the addition of aliquots of solutions of
.83 (dd, 2H, J1 = 8.8 Hz, J2 = 4.4 Hz), 6.99 (d, 2H, J = 7.8 Hz). diethylchlorophosphate (DCP) or other OP interferants at an
+
ESI-MS in MeOH: calcd m/z for C H N O as [M + H]
=
excitation wavelength (l ) of 340 nm.
1
9
13
4
ex
3
13.109, found = 313.109.
X-ray crystallography
[Eu(o-HPIP)(TTA)
3
] [Eu(o-OH)]. One equivalent of EuCl
3
ꢁ
6H
2
O (0.1 mmol, 52 mg) was stirred with o-HPIP (1 equiv., The single-crystal X-ray diffraction data of the Eu(o-OH) and
0.1 mmol and 31.2 mg) for 1 h in 10 mL methanol. To this La(o-OH) complexes and the Eu(DHP) polymeric product were
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collected on a Bruker D8 Quest Microfocus X-ray CCD diffracto- (diethyl hydrogen phosphate) with a strong binding affinity to
meter (o scans) with Mo Ka radiation (l = 0.71073 Å) at 100(2) K. Eu(III). Real-time detection of the nerve agent simulant in the
Data frame collections, indexing of reflections, and lattice vapor phase on a solid-state filter paper strip immobilized with
parameter determination were done using the SMART program, the probes was shown, indicating the on-site utility of our
while the integration of the intensity of reflections and scaling system. The highly responsive Eu(III) probes are highly selective
5
8
were performed using the SAINT program.
Absorption and sensitive to minimal concentrations of DCP vapors.
5
9
corrections were done using SADABS and the space group The strategy could be applied to develop a prototype rapid,
determination using SHELXTL programs. Structure solution solid-state sensitive detection platform for nerve gases by
and refinement were done using the full matrix least square integrating the luminescence signal output with a suitable
6
0
2
method against F with SHELXT and SHELXL-2018/3 using the optical detector.
6
1–63
WinGX and Olex2 software systems.
The non-hydrogen
atoms were refined until convergence was reached using
anisotropic thermal parameters and riding model refinement
was applied on all the hydrogen atoms to include them in
their idealized positions. The perspective images of complexes
Eu(o-OH) and La(o-OH) are shown in Fig. 2(a and b), respectively.
Conflicts of interest
There are no conflicts to declare.
The unit cell packing diagrams, crystallographic parameters, and Acknowledgements
significant bond lengths and angles of the complexes and
We thank IIT Kanpur for infrastructure and the Science and
Engineering Research Board (SERB) for financial assistance
[
Eu(DHP) ] are shown in Fig. S5–S7 and Tables S1–S3 in the ESI.†
3 n
(
EMR/2016/000521). Z. A. and U. Y. gratefully acknowledge
Vapor phase measurements of DCP in a solid-state film
the University Grants Commission (UGC) and the Ministry of
Human Resource Development (MHRD) for the PhD research
fellowships.
The paper-based strips were prepared by dipping evenly cut-out
small-sized adsorbent Whatman filter paper into a stock
solution (1 mM) of Eu(o-OH) and Eu(p-OH) in acetonitrile
2
and drying with a stream of N flow. The luminescent probe-
loaded paper strips were hung from the top of a tall glass vial in References
the presence of various organophosphate (OP) vapors of various
1
(a) J. B. Sullivan Jr, G. B. Krieger and R. J. Thomas, Hazardous
materials toxicology: clinical principles of environmental
health, J. Occ. Environ. Med., 1992, 34, 365–371;
concentrations for various times, interferents, or other testing
conditions. The luminescence was captured digitally under a
UV lamp (365 nm).
(b) T. Urabe, K. Takahashi, M. Kitagawa, T. Sato, T. Kondo,
S. Enomoto, M. Kidera and Y. Seto, Development of portable
mass spectrometer with electron cyclotron resonance ion
source for detection of chemical warfare agents in air,
Spectrochim. Acta, Part A, 2014, 120, 437–444;
(c) S. Costanzi, J. H. Machado and M. Mitchell, Nerve agents:
What they are, how they work, how to counter them, ACS
Chem. Neurosci., 2018, 9, 873–885.
Conclusions
We synthesized two red light-emitting luminescent probes, viz.
Eu(o-OH) and Eu(p-OH), motivated by the reactivity of the OP
AChE inhibitors with the nucleophilic OH group of the serine
residue at the active site of AChE. The probe Eu(o-OH) was
structurally characterized, showing square-antiprismatic
geometry and the presence of intramolecular hydrogen
bonding of o-OH and N groups of o-HPIP. Both the probes
exhibit selective and sensitive detection of the sarin (GB) nerve
agent simulant DCP in solution. The Eu(o-OH) probe remains
highly selective for DCP compared to Eu(p-OH), suggesting
reactivity differences of the –OH groups mediated by their
position and H-bonding capability. However, the probe
Eu(p-OH) was more sensitive towards DCP, showing ‘turn-off’
luminescence at a lower concentration of DCP. The limits of
detection of the probes Eu(o-OH) (18 ppb) and Eu(p-OH)
2 F. R. Sidell and J. Borak, Chemical warfare agents: II. Nerve
agents, Ann. Emerg. Med., 1992, 21, 865–871.
3 (a) S. Fan, G. Zhang, G. H. Dennison, N. FitzGerald,
P. L. Burn, I. R. Gentle and P. E. Shaw, Challenges in
fluorescence detection of chemical warfare agent vapors
using solid-state films, Adv. Mater., 2020, 32, 1905785;
(b) S. Bencic-Nagale, T. Sternfeld and D. R. Walt, Microbead
chemical switches: an approach to detection of reactive
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