A FRET-based ratiometric fluorescent and colorimetric probe for the facile detection of organophosphonate nerve agent mimic DCP

Article abstract of DOI:10.1039/c3cc46095a

A FRET ratiometric fluorescent probe enabling a fast and highly sensitive response to OP nerve agent mimic DCP within 1 min and with as low as 0.17 ppm concentration detection limit has been developed. Moreover, the probe exhibits noticeable color changes

Full text of DOI:10.1039/c3cc46095a

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A FRET-based ratiometric fluorescent and colorimetric probe for the
facile detection of organophosphonates nerve agent mimic DCP
[a]
[a]
[a,b]
[a,c]*
Weimin Xuan, Yanting Cao, Jiahong Zhou, * and Wei Wang
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
A FRET ratiometric fluorescent probe enabling a fast and highly
sensitive response to OP nerve agent mimic DCP within 1 min
and with as low as 0.17 ppm concentration detection limit has
been developed. Moreover, the probe exhibits noticeable color
change under UV and even with naked eyes. It is also
demonstrated that it can detect both liquid and gas nerve agents.
those of hydroxyl-based sensors. The phosphorylation reactions
of amine derived fluorescence molecules with OP nerve agents
provide an alternative fertile approach.
1
2
1
0
R
R
OEt
R
OEt
OH
O
OEt
F
N
O
OEt
F
N
DCP
F
N
P
P
(1)
O
n = 1, 2
O
n = 1, 2
n = 1, 2
The
demand
for
facile
detection
of
volatile
non- or weak
fluorescent
highly fluorescent
organophosphonates (OP) nerve agents such as sarin (GB),
soman (GD), and tabun (GA) has been driven by the growing
concern of exposure to these chemical weapons on both the
battlefield and through terrorist attacks. These nerve gases,
potent inhibitors of acetylcholinesterase (AChE) in the central
F: fluorophore
1
5
O
OEt
O
N
O
OEt
P
diethyl chlorophosphate
DCP)
N
P
1
Cl
O
(
O
(2)
O
F
F
non- or weak
5
fluorescent
DCP
2
nervous system, have rapid, severe and even fatal effects on
highly fluorescent
5
2
0
human and mammalian life if inhaled. In addition to the extreme
toxicity, their ready production and easy operation render them
Scheme 1. General chemical reaction-based fluorescent deigsn
strategies for the detection of OP nerve agents.
attractive chemical weapons.
3
Although
a
variety of detection methods have been
Despite these elegant studies, there is still significant room for
improvement. In addition to quick response and high sensitivity,
a truly practically useful fluorescence probe should display a
ratiometric and colorimetric response. Such features can reduce
the background interference and false positive results. Moreover,
a probe enabling to analyze samples using easy-to-use devices is
4
developed for these nerve agents such as photoacoustics, mass
60
5
6
2
3
3
4
4
5
5
0
5
0
5
0
spectrometry, capillary electropherograph, electrical sensors and
7
biosensors, optical sensing (especially fluorescent detection)
offers unrivalled merits of high sensitivity, low-cost (e.g.,
instrument), and operational simplicity. Furthermore, small
molecule-derived fluorescent probes afford the advantages of 65 highly desired for the real time studies. A probe with color
easy production, low cost, high stability and easy storage.
However, the development of such fluorescent probes is of
change by naked eyes is especially appealing. Finally, such a
probe should be able to be employed for the analysis of both
liquid and gaseous nerve agents.
formidable challenge. Only
available. The widely used design principle takes advantage of
a
handful of examples are
8
,9
Toward this end, herein we wish to disclose the first FRET
the high electrophilicity of these OP nerve agents by reacting 70 ratiometric fluorescent probe for the detection of OP-nerve
1
3
with a nucleophile embedded in a fluorophore to lead to the
change of fluorescence properties. A nice approach is to utilize
the reaction of nucleophilic hydroxyl group with OP nerve agents
to form phosphate esters, which subsequent undergo an
agents.
The probe displays several notable features. A
carboxylate driving the formation of highly reactive nucleophilic
phenol anion is exploited for the first time. The handle can react
with electrophilic OP nerve agents within fast response (within 1
intramolecular N-alkylation reaction to generate fluorescent 75 min). A FRET strategy by combination of rhodamine and
1
0
molecules (Scheme 1, eq. 1). Pilato et al reported the first “off-
fluorescein (termed ‘rhodol’) structures is implemented by
incorporation of coumarin as a donor in a ratiometric response
manner. Furthermore, the probe exhibits noticeable color change
under UV and even with naked eyes. Finally, it can detect both
1
0a
on” fluorescent 1,2-enedithiolate complex probe. An improved
organophosphate sensor which can work in an ambient condition
and with faster response was reported by Swager and
1
0b
colleagues.
workers reported
fluorescence sensor with quick response and high sensitivity.
Moreover, these pioneering studies have triggered intensive
Using a similar design strategy, Rebek and co- 80 liquid and gas nerve agents.
a
photoinduced electron transfer (PET)
We envisioned that the acidic phenol could be readily ionized
to form more nucleophilic phenoxide ion, which might be a good
nucleophile for quick conjugation with OP nerve agents. The
phenoxide anion could be generated in situ from its precursor
1
0c
1
0d-h
investigation with the design strategy.
Anslyn et al employed
a more nucleophilic oximate anion instead of hydroxyl group in a 85 quinone (Scheme 3). This could be driven by the formation of
1
1
coumarin based PET sensor (Scheme 1, eq. 2). Impressively,
the reaction rate is enhanced by five orders of magnitude than
lactone from an intramolecular carboxylate ion conjugate
addition. The acylation of the formed phenoxide is trapped by an
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OP nerve agent. On the other hand, in the construction of a FRET
N
em.
sensor, we decided to choose commonly used coumarin as the
fluorescent donor, which emission lies in the excite range of
ex.
FRET
on
O
CO
2
K
1
4
rhodamine/fluorescein with high efficiency. Generally, the
coumarin moiety is tethered though a rigid piperazine linker at
the carboxylate position of fluoresecein/rhodamine. However, in
our design, we proposed to attach the moiety to one of the amino
group in rhodamine. As mentioned above, the reason for this is
that the carboxylate moiety is essential for the formation of the
lactone, which leads to the generation of FRET-based ratiometric
response and nucleophilic phenoxide. Before reacting with an OP
nerve agent, the FRET must be on and the carboxylate is in a free
open state. Nevertheless, after the reaction, the free carboxylate is
converted to a closed spirolactone form to induce ratiometric
response. Furthermore, the new hybrid rhodol fluorophore can
maintain the stable open form prior to reaction with OP nerve
agents.
O
5
0
5
O
P
N
O
N
O
O
O
probe 1
EtO
Cl
OEt
N
DCP
em.
O
FRET
off
ex.
1
O
O
O
N
O
N
O
O
P
EtO
OEt
60
product 2
1
Scheme 2. Design of FRET-based ratiometric fluorescence probes for OP
nerve agents.
The designed probe 1 was synthesized in 6 steps with a total
yield of 11% (see SI). With the probe in hand, we firstly tested its
response to OP nerve agents. Due to its similar reactivity while
much less toxicity, diethyl chlorophosphate (DCP, Scheme 1) has
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
1
0
been generally used as the nerve agent surrogate for studies.
After addition of 100 µM DCP to 5 µM probe 1 in DMF, to our
delight, an immediate color and fluorescent changes were
observed (Figure 1). The largest emission wavelength was shifted
from 536 nm (bright yellow) to 460 nm (blue), while the solution
color was changed from yellow to colorless (Figure 1a). These
shifts are attributed to the lactonization and phosphorylation of
the phenoxide with DCP. The formation of the proposed product
2 was confirmed by mass spectrometer (M + H, 780.2659, see
Figure S6 in SI). It is noted that the phenol moiety in
(a)
(b)
65
Figure 1. (a) To 5 µM probe 1 in DMF was added 100 µM DCP, the
emission spectra was recorded every 30 s over 150 s. The inset picture
showed the color change and the fluorescence change under a UV lamp.
(b) The fluorescence intensities at 460 and 536 nm vary in a time range of
1
50 s. The excitation was set at 410 nm with slit/slit 5/1.5.
1
2b
fluoresceinamine chemosensor is in a neutral form, while in
our case, the functionality is deprotonated. The reaction occurred
within 30 s (Figure 1b). Meanwhile, obvious UV absorption
change was observed (see Figure S2 in SI). The probe 1 exhibited
the typical absorption peaks of both coumarin and a rhodol. Upon
the treatment with DCP, the rhodol absorption disappeared
indicating the formation of spirolactone ring. In the absence of
DCP, the probe emission spectra remained the same (see Figure
S3 in SI). Also to exclude the influence of a possible acid, the
solution after treatment with DCP was further added K CO , and
70
isoemission point was observed at 504 nm. Furthermore and
notably, the probe can detect DCP at as low as 0.17 ppm with
discernible emission ratio change. Besides DCP, other related
phosphates were also tested and some of them can also induce
fluorescence intensity enhancement (Figure S7 in SI).
2
3
no observable change can be seen (see Figure S4 in SI).
Moreover, even in the presence of an acid (e. g., HOAc), DCP is
still capable of inducing significant fluorescence enhancement
(see Figure S5 in SI). However, the carboxylic acid instead of
carboxylate salt of 1 failed to induce fluorescence change because
the weaker nucleophilicity of the COOH could not trigger the
lactonization to form lactone and concurrently generate
phenoxide. These studies demonstrated that the probe 1 displayed
fast response to DCP. The changes can be observed by simple
UV light or even naked eyes.
Next, we investigated the sensitivity of this probe with
different concentrations of DCP in DMF. As shown in Figure 2,
the response of this probe to DCP is concentration-dependant.
When 80 µM DCP was used, the highest fluorescence change
was obtained, and the intensity ratio change (F460/F536) reached
a plateau with an enhancement of 1731 folds. According to the
fluorescence increase at 460 nm or decrease at 536 nm, the FRET
efficiency was calculated to be 96.0% - 98.6%. Meanwhile a clear
75
(a)
(b)
Figure 2. (a) To 5 µM probe 1 in DMF was added 0 - 100 µM DCP, the
emission spectra was recorded in 1 min after addition of DCP, λex = 410
nm. (b) Fluorescence intensity ratio changes (F460/F536) of the probe at
5.0 µM upon addition of various concentrations of DCP.
80
Having demonstrated that the probe 1 can response to a DCP
solution with high sensitivity, we also carried out experiments to
detect DCP in gaseous state. To make the detection experiments
operationally simple and practical, pH paper was used. It was
immersed in 0.1 mg/mL probe solution in MeOH in the presence
of K CO . Then the paper was dried in air and the white filter
85
2
3
paper was changed to slightly pink color (Figure 3a, top row).
The probe-loaded filter paper was put across the top of a vial
containing one drop of DCP for a defined exposure time. As
2
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1
0c
Compared with the reported filter paper-based sensing method,
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3
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difference was observed (see Figure S8 in SI).
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1
0
Figure 3. (a) Filter paper treated with 0.1 mg/mL probe solution in
MeOH in the presence of K CO . (b) Filter paper exposed to DCP vapor
2 3
for 1 min. (c) Filter paper exposed to DCP vapor for 5 min. Pictures at the
bottom was irradiated at 365 nm with a UV lamp. Filter paper was used.
1
5
In conclusion, we have developed the first FRET-based
ratiometric fluorescent probe for the detection of OP nerve agents.
Notably, a new rhodol fluorophore has been designed and
synthesized for the first time. Fast response, high sensitivity and
obvious color change makes it very suitable for real time
detection and field analysis. Furthermore it also enables to detect
DCP vapor with a UV hand lamp or even naked eyes. These
features render this probe a very promising tool to detect nerve
agents in practical settings.
8
9
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Department of Chemistry & Chemical Biology, University of New
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5
05-277-2609; E-mail: wwang@unm.edu
Analysis & Detecting Center, Nanjing Normal University, 1 Wen-Yuan
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†
Electronic Supplementary Information (ESI) available: Experimental
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see DOI: 10.1039/b000000x/
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