Squaramide-based lab-on-a-molecule for the detection of silver ion and nitroaromatic explosives

Article abstract of DOI:10.1039/c5ra18754k

A squaramide based chemosensor SA was developed as a quantitatively operating lab-on-a-molecule, for the detection of silver ions and nitroaromatic explosives in aqueous solution, showing significant emission quenching and absorption enhancement in dual c

Full text of DOI:10.1039/c5ra18754k

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DOI: 10.1039/C5RA18754K
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Squaramide-based lab-on-a-molecule for the detection of
silver ion and nitroaromatic explosives
Bo Shana, Rui Shia, Shaohua Jina, Lijie Lia, Shusen Chena, Yunfei Liua, Qinghai Shua*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
quenching-based chemosensors13, where analyte is binding with
A
squaramide based chemosensor SA was developed as a
producers and attenuates in the light emission14, are believed to
be the most suited for the detection of nitroaromatics because
the electron-deficient nitro compounds are the strong
quantitatively operating lab-on-a-molecule, for the detection of
silver ions and nitroaromatic explosives in aqueous solution,
performing significant emission quenching and absorption
enhancement in dual channel along with distinct solution color
change.
quenchers of electron-rich fluorophores via an electron-transfer
15
mechanism
.
Until now, most of the fluorescence resonance energy
transfer (FRET) based chemosensors usually employ two
photoluminescent elements with an approximate energy band
gap, in which the fluorescent emission of a donor is transferred
to an acceptor and thus activates the fluorescence of the
acceptor. However, the FRET from a photoluminescent donor to
a non-emissive receptor for direct chemodetection of analyte
has thus been rarely explored. In principle, if the absorption
band of an analyte (or its derivatives) overlaps with the emission
band of a fluorescent dye, the resonance energy transfer due to
the polar-polar interaction at a spatial proximity will result in
the quenching of donor fluorescence, which may provide a facile
and sensitive detection for the target analyte.
The field of lab-on-a-molecule1 was proposed by de Silva2 as a
molecular logic gate producing an output signal (generally
fluorescent emission) in response to various chemical input signals,
which has been employed to overcome the boundary of single
analyte detection and to initiate highly selective orthogonal
signaling protocols for multi channel ions sensing in large varieties3.
Dramatic improvements in the detection of nitroaromatic
compounds such as 2, 4, 6-trinitrotoluene (TNT) and its
derivatives picric acid (TNP), 1-methyl-4-nitrobenzene (NT), 1-
methyl-2,4-dinitrobenzene (DNT) have aroused wide public
concern since they are highly explosive and environmentally
deleterious substances4. Similarly, trace amount of toxic metal
ions such as Ag+ can pose serious threats to human health and
environment.
The presently used detections of nitroaromatics are usually
time-consuming with the employment of cumbersome 5 and
expensive gas chromatography coupled to a mass spectrometer6,
ion mobility spectrometry7 and surface acoustic wave method8,
which requires frequent instrument calibration, sophisticated
vapor sampling and preconcentration procedures9. In contrast,
the fluorescence-based chemosensors have been of significant
importance with remarkable advantages over the other newly
On the other hand, given the toxic mental ions such as silver
posing serious threats to human health and environment, there
is a growing demand to develop the up-to-date technologies and
methods for the qualitative and quantitative detection of silver.
Although different analytical methods are developed for the
16
detection of Ag+, namely, quantum-dot-based assay
,
electrochemical17 and fluorescent sensor18, which can detect Ag+
ion with high sensitivity and selectivity but need highly
19
sophisticated expensive instrumentation
, the absorption-
based colorimetric chemosensors are greatly favored due to the
distinct visual color change response for fast and easy detection
by naked-eye20.
developed detection technologies owing to high sensitivity,
10
portability, short response time
, low cost and dual
.
compatibility in solid and solution media11
Herein, we reported the resonance energy transfer-amplifying
fluorescence quenching of dye for the sensitive detection of TNT
and its derivatives21 and competitve sensing of silver ions. The
specific binding of TNT with the amine ligands leads to the
formation of TNT-amine complexes, which strongly suppressed
the fluorescence emission of the proximally positioned marker
dye through the resonance energy transfer from the chosen dye
to the TNT derivatives. The amplifying fluorescence quenching
can sensitively detect the ultratrace TNT in the solution22, and
selectively distinguish different types of nitrocompounds.
Therefore, the development of robust and sensitive platforms
for their real-time analytical detection has attracted
considerable research efforts in recent years12. Fluorescence
a.School of Material Science and Engineering, Beijing Institute of Technology,
100081 Beijing, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Notably, the title probe also performed high competitive Hg2+, and K+ did not cause either significant UV-visVsiepweAcrttricolescOonpliniec
selectivity of Ag+ ion over other metal ions by its outstanding change or solution color change. Therefore, it can be concluded
DOI: 10.1039/C5RA18754K
“turn-on” effect in absorption channel, even in the presence of that SA can recognize Ag+ selectively over other anions not only by
other metal ions23.
naked eyes but also by absorption spectroscopy.
SA+Ag
SA+Ag+X
0.08
b
Experimental
a
0.08
0.06
0.06
Synthetic materials and instrumentation
0.04
0.04
0.02
All the chemicals used for the synthesis were of AR grade from
Sigma-Aldrich unless otherwise stated. All the reagents and solvents
were purchased from commercial sources and used without further
treatment. The aqueous stock solutions of cations were prepared
from their perchlorate salts. The stock solution of squaramide
sensor SA was prepared by dissolving an accurately weighed SA in
DMSO, and then diluted to 510-5 mol/L with buffer solution (pH =
7.24, DMSO: buffer = 90 / 10, v / v, aq. buffer = 0.1 M Tris-ClO4).
The 1H-NMR was recorded on a Varian mercury-plus 400 NMR
spectrometer at room temperature using d6-DMSO as solvent
according to the internal standard. Absorption spectra were
0.02
0.00
0
1 2 3 4 5 6 7 8 9 10
0.00
1
2
3
4
5
6
7
8
9 10
Fig. 1 Absorption of SA in the presence of a) 10 equiv. of cations
(Inset: naked-eye detection of Ag+ ion by probe SA under visible)
and b) 10 equiv. of Ag+ + 10 equiv. of other cations at λ = 422 nm in
0.1 M Tris-ClO4: DMSO = 10 / 90, v / v, pH 7.24. [X– = Ag+ (0), Ba2+ (1),
Zn2+ (2), Ni2+ (3), Ca2+ (4), Mg2+ (5), Pb2+ (6), Cd2+ (7), Co2+ (8), Hg2+
(9), K+ (10)].
performed on
a
TU-1810 ultraviolet spectrophotometer.
In addition, the sensitive recognition and binding mode of SA
with Ag+ was demonstrated by carrying out absorption titration
experiments. With the constant addition of Ag+ from 0-10
equivalents to the solution of SA (510-5 mol/L) in aqueous buffer
(pH = 7.24, DMSO: H2O = 90:10, v/v), the absorption intensity at 422
nm was increased sharply due to the formation of complexes
between probe SA and Ag+.
As a rigorous test, the selective detection of silver ion using SA
was challenged in competition experiments (Fig. 1). In none of the
cases, the presence of other anions interfered substantially. The
good linear relationship between the absorption intensity and
concentration of Ag+ (Fig. 2 inset) allowed for a facile quantification
of Ag+ with a detection limit of 4.6 μM according to the literature
method24.
Fluorescence emission spectra were measure with a Hitachi F-4500
luminescence spectrometer.
Synthesis of 3, 4-bis (phenylamino) cyclobut-3-ene-1, 2-dione (SA)
Typically, to a stirred solution of 3, 4-diethoxycyclobut-3-ene-1, 2-
dione (81.3 μL, 0.55 mmol, 1.0 equiv.) and zinc trifluoromethanesul-
fonate (40 mg, 0.11 mmol, 20 mol %) in toluene / DMSO 19:1 (1 mL)
was added aniline (92.6 μL, 1.15 mmol, 2.1 equiv.). The solution was
heated to 100 ℃ and stirred for 12h. When the solution was cooled
down to room temperature, a white precipitate was observed and
isolated by decanting the solvent. The solid was further washed
with methanol (3 × 5 mL) to acquire white solid (138 mg, 99%
yield)24. 1H NMR (400 MHz, DMSO-d6) δ 7.50 (4H, d, J = 7.3 Hz), 7.38
(4H, app t, J = 7.4 Hz), 7.90 (2H, app t, J = 7.4 Hz); 13C NMR (100 MHz,
DMSO-d6) δ 181.5, 165.6, 138.5, 129.3, 123.2, 118.4; HRMS (EI)
calcd. for C16H12N2O2 264, found m/z 264.
0-10 equiv of Ag+
0.20
0.20
0.15
O
O
O
O
O
O
Zn(CF3SO3)2
toluene/12h
2
0.15
R
=0.995
+
0.10
0.05
HN
NH
NH2
0.10
0.05
0.00
2
4
6
8
10
The proportion of Ag and SA
Scheme 1 The synthetic route for the preparation of SA.
Results and discussion
Sensing of Ag+ by SA
400
600
800
1000
Wavelength/ nm
The colorimetric sensing properties of SA combine with various
Fig. 2 The UV-vis titration of 510-5 mol/L SA with 1-10 equiv
cations were investigated by UV-Vis absorption spectroscopy. In
buffer solution (pH = 7.24, DMSO: H2O = 90:10, v/v), SA showed one addition of Ag+ ions. Inset: The absorption intensity of probe SA at λ
= 422 nm as a function of mole equivalents of Ag+ ions.
characteristic peak at 422 nm (Fig. S1) which was due to the n→π*
electronic transition. Upon the addition of equivalent cations into
the buffered aqueous solution of SA, the dramatic absorption FRET Mechanism between SA and TNT Derivatives
enhancement at 422 nm (Fig. S1) accompanied with the obvious
TNT is almost faint yellow in solution and does not absorb any
visible light. Our work showed that the solution of TNT could
change from faint yellow to deep red by adding organic amines such
as SA. A strong charge transfer interaction, as a predominant
solution color change from yellow to dark red could only be
observed in the case of Ag+, clearly indicating the formation of a
complex species between SA and Ag+. In contrast, the addition of
other cations, such as Ba2+, Zn2+, Ni2+, Ca2+, Mg2+, Pb2+, Cd2+, Co2+,
2 | J. Name., 2012, 00, 1-3
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process, occurring between the electron-deficient aromatic ring of
TNT and the electron-rich amine group of SA, could lead to the
formation of a Meisenheimer complex as zwitterions of TNT- and
RNH2 , which is resonance-stabilizing form due to three electron-
withdraw nitro groups (Scheme 2).
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DOI: 10.1039/C5RA18754K
0.10
0.08
0.06
0.04
0.02
0.00
+
CH3
CH2
O2N
NO2
O2N
NO2
NHR
+
2
NH2R
R =0.999
NO2
NO2
Scheme 2 The interaction between TNT and SA and Meisenheimer
complex formed between TNT and the amine.
0
50
The proportion of TNT and SA
100
The absorbance spectrum of TNT solution shows a strong visible
absorption with λ at 264-600 nm, and has a spectral overlapping
with the fluorescence emission of SA solution at 285-540 nm,
suggesting that the resonance energy transfer from SA to TNT
derivatives may occur when the fluorophores and TNT derivatives
are spatially close enough to each other. As a result, the non-
emissive resonance transfer-based fluorescence quenching of SA
will occur, and the quenching efficiency mainly depends on the
degree of spectral overlapping between the emission of donor and
the absorption of analyte acceptor and on the distance between
these two, as depicted by following equation25,
Fig. 4 UV-vis absorption titration of SA (10 μM) upon addition of
various equiv. of TNT in DMSO buffered solution (pH = 7.24, DMSO:
H2O = 90:10, v/v). Inset: Linear correlation of absorption intensity at
λ = 512 nm versus concentration of TNT.
Fluorescence response of SA and nitroaromatic explosives
As expected, upon the addition of TNT,
a charge-transfer
interaction occurred between the electron-deficient aromatic ring
of TNT and the electron-rich amino group, leading to the formation
of a Meisenheimer complex and exhibiting a strong absorption
band at approximate 338 nm. The well overlap (see Fig. S4)
between the absorption of TNT and the emission band of the
chosen SA fluorophore facilitated the FRET mechanism by
intermolecular polar-polar interaction in the spatial proximity, as a
result the efficient quenching effect could be found on the emission
of the SA donor26.
The probe SA also possesses the ability to react with TNT in a
dose-response manner, which can be utilized for quantification of
the analyte. As is shown in Fig. 5, the emission intensity of SA at 338
nm gradually decreased with the successive addition of TNT.
Similar quenching effect was also found in the case of other
aromatic nitroamines such as NT, DNT and TNP with a relative
weaker degree. Therefore, it can be concluded that SA could
selectively response the aromatic nitroamines through emission
quenching.
ΦT = 1/Q, Q = 1+(R/R0)6
where R is the distance of the two fluorescent groups; R0 is the
distance of the Förster (The critical transfer distance of the donor
and acceptor).
UV/Vis response of SA and nitroaromatic explosives
The UV-vis selective response of SA upon the presence of
nitroamine explosives can be seen from the observed color change
of SA solution from pale-yellow to dark red (Fig. S3). In order to
perform these naked-eye experiments, 100 equivalents of various
nitroamine explosives (1.5×10-1 mol/L, NT, DNT, TNT, TNP) were
added to the solution of SA (5×10-5 mol/L) in buffer solution (pH =
7.24, DMSO: H2O = 90:10, v/v). As show in Fig. 3, the distinct
solution color change was only observed in the presence of TNT
along with the significant absorption intensity enhancement in
visible band centered at 512 nm (Fig. S4).
In addition, the sensitive recognition of SA with TNT was
demonstrated by carrying out absorption titration experiment. With
the progressive addition of TNT from 0-100 equivalents to the
solution of SA (5×10-3 mol/L) in buffer solution (pH = 7.24, DMSO:
H2O = 90:10, v/v), the absorption intensity at 512 nm was increased
gradually. The linear relationships between the absorption intensity
and amount of TNT was established for SA (Fig. 4) that allow for
quantification over a wide range of concentration with a detection
limit of 5.0 μM.
16
b
TNT
0
0.5
1
10
8
TNP
DNT
NT
12
8
a
2
6
5
4
10
20
4
2
0
0
0
4
8
12
16
20
250 300 350 400 450 500 550
Wavelength/nm
The proportion of TNT and SA
Fig. 5 a) PL intensity of SA (10 μM) at λexc = 278 nm with respect to
the concentration of TNT from 0 to 20 equivalents in DMSO
buffered solution (pH = 7.24, DMSO: H2O = 90:10, v/v); b)
Stern−Volmer plot of SA treated with various explosives at different
concentrations at λem = 338 nm.
3
Naked-eye color changes of SA (5×10-3 mol/L) with
To evaluate the quenching efficiency of different kinds of
nitroaromatic analytes, the Stern-Volmer equation is applied
Fig.
subsequent addition of TNT from 0.5 to 100 equiv under visible.
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as follows: (I0/I)-1=KSV[TNT], where I0 is the initial fluorescence
intensity in the absence of analyte, I is the fluorescence
intensity in the presence of [TNT], and KSV is quenching
constant with TNT. By employing the emission intensity data of
SA at λexc = 278 nm into Stern-Volmer equation, it is calculated
that the quenching efficiency of NT, DNT, TNT and TNP is
6.01×104, 6.14×104, 6.30×104 and 6.65×104 M-1, respectively.
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DOI: 10.1039/C5RA18754K
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112, 881.
We have successfully demonstrated a novel lab-on-a-molecule
of nitroaromatic compounds and silver ions by using UV-Vis
and emission channels. In UV-Vis channel, the remarkable
absorption intensity enhancement was observed only in the
presence of silver ions, while in emission channel the selective
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standard deviation of blank measurements, and m is the slope between
intensity versus sample concentration).
compounds such as TNT, DNT, NT and TNP through resonance
energy transfer mechanism by the formation of
a
Meisenheimer complex. To the best of our knowledge, this is
the first time reporting dual channel chemosensor for the
selective detection of aromatic nitroamines and Ag+ ions, even
in the presences of other competing analogues, suggesting
that probe SA could be used as dual channel lab-on-a-molecule
for detecting nitroaromatic explosives and metal ions by
simply achieved probes.
Acknowledgment
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We acknowledge the financial support from the National
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4 | J. Name., 2012, 00, 1-3
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Graphical Abstract
Squaramide-based Lab-on-a-molecule for the detection of
silver ion and nitroaromatic explosives
Bo Shana, Rui Shia, Shaohua Jina, Shusen Chena, Yunfei Liua, Qinghai Shua*
School of Materials Science & Engineering, Beijing Institute of Technology, Beijing, China. Tel: +86-10-68915072; Email:
a
qhshu121@bit.edu.cn
0
0.5
1
0.08
0.06
0.04
0.02
0.00
10
8
2
6
5
4
10
20
O
O
2
Ag+
TNT
0
Absorption
enhancement
Emission
quenching
N
H
N
H
250 300 350 400 450 500 550
Wavelength/nm
0
1 2 3 4 5 6 7 8 9 10
SA
A squaramide based Lab-on-a-molecule performed selective absorption enhancement and
emission quenching towards Ag+ and nitroaromatic explosives, respectively in aqueous solution.