Understanding the effect of an amino group on the selective and ultrafast detection of TNP in water using fluorescent organic probes

Article abstract of DOI:10.1039/c7tc05852g

We have designed and developed three single-molecule fluorescent probes differing in the number of amino groups, namely 5-((4,6-Diamino-1,3,5-triazin-2-yl)amino)isophthalic acid (H₂ATAIA, 1), 5-((4-amino-6-methoxy-1,3,5-triazin-2-yl)amino)isophthalic acid (H₂AMTAIA, 2) and 5-((4,6-dimethoxy-1,3,5-triazin-2-yl)amino)isophthalic acid (H₂DMTAIA, 3), from cheap and readily available starting materials via simple procedures in high yields for demonstrating their application in highly selective and ultrafast sensing of 2,4,6-trinitrophenol (TNP) in water (slurry mode). Probes 1-3 have been characterized by various analytical techniques, such as melting point, FTIR, UV-vis and NMR (¹H and ¹³C) spectroscopy and high resolution mass spectrometry (HRMS). It is quite evident that the effect of an amino group is more prominent compared to a methoxy group towards the selective detection of TNP over other potentially interfering nitro compounds. The detection limit for the diamino derivative was found to be 120 ppb compared to those with one amino or no amino group (0.8 ppm and 1.2 ppm, respectively). We also report the ideal real time detection of TNP through a contact mode or instant spot via paper strips. Spectral overlap, time-resolved fluorescence studies, quantum yield, Stern-Volmer plots, field emission scanning electron microscopy (FESEM) and DFT calculations have been used to establish their mechanism of action. Furthermore, competitive nitro-analyte tests demonstrate that the selectivity for TNP is more in 1 compared to 2 and 3. To the best of our knowledge, we have demonstrated for the first time molecular decoding of TNP based on the dual read-out identification scheme constructed from life-time and quantum yield. These probes have been found to be highly photostable in the presence of acidic TNP as well as recyclable without much loss of sensitivity up to five cycles. These results vividly depict that these are excellent candidates for environmental monitoring.

Full text of DOI:10.1039/c7tc05852g

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RSCPublishing
ARTICLE
Understanding the effect of an amino group on the selective
and ultrafast detection of TNP in water by fluorescent organic
probes†
Cite this: DOI: 10.1039/x0xx00000x
Prasenjit Dasa and Sanjay K. Mandala*
Received xxxxxxxxx
Accepted xxxxxxxxx
We have designed and developed three single-molecule fluorescent probes differing in the number of amino groups.
5-((4,6-diamino-1,3,5-triazin-2-yl)amino)isophthalic acid (H2ATAIA, 1), 5-((4-amino-6-methoxy-1,3,5-triazin-2-
yl)amino)isophthalic acid (H2AMTAIA, 2) and 5-((4,6-dimethoxy-1,3,5-triazin-2-yl)amino)isophthalic acid (H2DMTAIA,
3), from cheap and readily available starting materials via simple procedures in high yields for demonstrating their
application in highly selective and ultrafast sensing of 2,4,6-trinitrophenol (TNP) in water (slurry mode). Probes 1-3
have been characterized by various analytical techniques, such as melting point, FTIR, UV-vis and NMR (1H and 13C)
spectroscopy and high resolution mass spectrometry (HRMS). It is quite evident that the effect of an amino group is
more prominent compared to a methoxy group towards the selective detection of TNP over other potentially
interfering nitro compounds. The detection limit for the diamino derivative was found to be 120 ppb compared to
those with one amino or no amino group (0.8 ppm and 1.2 ppm, respectively). We also report the ideal real time
detection of TNP through contact mode or instant spot via paper strips. Spectral overlap, time-resolved fluorescence
studies, quantum yield, Stern-Volmer plots, field emission scanning electron microscopy (FESEM) and DFT
calculations have been used to establish their mechanism of action. Furthermore, competitive nitro-analyte tests
demonstrate that the selectivity for TNP is more in 1 compared to 2 and 3. To the best on our knowledge, we have
demonstrated for the first time molecular decoding of TNP based on the dual read-out identification scheme
constructed from life-time and quantum yield. These probes have been found to be highly photostable in presence
of acidic TNP as well as recyclable without much loss of sensitivity up to five cycles. These results vividly depict that
these are excellent candidates for environmental monitoring.
DOI: 10.1039/x0xx00000x
www.rsc.org/
it goes into the soil and aquatic system, consequently leading to
serious health and environmental issues.7 Hence, the challenge is to
Introduction
In recent years, efforts put for the development of functional, fast
and reliable chemosensors have opened new avenues for the
detection of harmful explosives using a single-molecule probe.1 The
concerns for national security and environmental protection have
increased a wide interest in developing selective and sensitive
detection of highly explosive nitro-aromatics, such as 2,4,6-
trinitrotoluene (TNT)2 and 2,4,6-trinitrophenol (TNP, also known as
picric acid)3, which are frequently used by terrorists.4 TNP has
higher explosive power compared to TNT as well as other nitro-
aromatic explosives (NAEs).5 TNP and its various salts are the major
ingredients for explosive formulations, such as yellow D and
pentolite, used in bombs, grenades and mine fillings.6 TNP is also
used in dyes, fireworks and leather industries while as a by-product
find a practical solution for selective and sensitive detection of trace
amount of TNP hidden in soil and ground water in presence of other
NAEs due to their inherently high electron affinity.8 Currently,
various detection techniques, such as colored precipitates, surface-
enhanced Raman spectroscopy (SERS), nuclear quadrupole
resonance (NQR), cyclic voltammetry and gas chromatography-
mass spectrometry (GC-MS) etc. are used for the detection of
NAEs.9 However, for real time detection of TNP quickly in places
where security is of utmost importance, for example railway
stations or airports, these methods are not practical for complexity
involved in operation and significant analysis time by trained
professionals. Recently, the cost-effective fluorescence based
technique has been preferred for this purpose due to its high
selectivity and fast detection of a single molecule in both solid and
liquid phases.10-15 However, these studies were carried out in
mostly organic and aqueous organic solvents. Thus, for practical
usage recent efforts have been directed towards the design of
single-molecule fluorescent organic sensors for their solid mediated
sensing of NAEs in water.10c,12b On the other hand, the proposed10c
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J. Name., 2013, 00, 1-3 | 1

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DOI: 10.1039/C7TC05852G
addition of a solution of 1a (175 mg, 1.21 mmol,) in 1,4-dioxane (4
involvement of hydrogen bonding and π-π interactions for the
fluorescence quenching of the sensor (via proton transfer followed
by charge transfer) has not been studied with a series of simple
fluorescent organic sensors differing in number of hydrogen bond
donor/acceptor sites. The design of such new functional probes is
important for budding new tools in sensing of NAEs and
understanding the mechanism of their action.
mL) over a period of 10 min. The resulting mixture was then heated
o
at 100 C for 24 h. Upon adjusting the pH of the reaction mixture
between 2 and 3 using dil. HCl (3 mL), an off-brown solid was
filtered, washed several times with distilled water, and dried to
o
obtain 260 mg of 1 (Yield: 72%). M.P.: 345-347 C. Since it was not
soluble in common organic or inorganic solvents, its sodium salt
(prepared with 2 eqv. of NaOH) was used for NMR experiments in
Thus, we initiated this work to design and develop fluorescent
organic probes, judiciously put in a single triazine ring via lateral D2O. 1H NMR (400 MHz, D2O): δ 8.05 (d, J = 1.2 Hz, 2H, Ar), 7.903 (t,
J = 2.2 Hz, 1H, Ar). 13C NMR (100 MHz, D2O): δ 124.07, 124.3, 136.9,
substitution to have one, two and three major interaction sites for
138.2, 164.3, 166.9, 174.7. HRMS (ESI) m/z calculated for [M+H]+:
TNP (Scheme 1). Two of these probes represent first examples of
292.0797, found 292.0706. Selected FTIR peaks (KBr pellet, cm-1):
organic chemosensors with both NH and NH2 groups. Herein, we
report the synthesis and photophysical properties of the first three
examples of such fluorescent organic probes: 5-((4,6-diamino-1,3,5-
triazin-2-yl)amino)isophthalic acid (H2ATAIA, 1), 5-((4-amino-6-
methoxy-1,3,5-triazin-2-yl)amino)isophthalic acid (H2AMTAIA, 2)
and 5-((4,6-dimethoxy-1,3,5-triazin-2-yl)amino)isophthalic acid
(H2DMTAIA, 3). We demonstrate that the effect of an amino group
is more prominent compared to a methoxy group towards the
highly selective and ultrafast detection of TNP in water.
3345(s, -NH2), 3162(s, -NH), 1693(vs, -COOH), 1616(s), 1581(vs),
1500(s), 1463(m), 1440(m), 1382(s), 1286(m), 1255(m), 905(w),
781(m), 762(m), 697(w), 660(w), 562(w). λmax (solid state,
reflectance): 278 and 323 nm (π-π* and n-π*).
Synthesis of 5-((4-amino-6-methoxy-1,3,5-triazin-2-yl)amino)-
isophthalic acid (H2AMTAIA) (2). Step 1. Synthesis of 2,4-dichloro-
6-methoxy-1,3,5-triazine (2a). In a 10 mL RBF, 4 mL of methanol
was taken and cooled to 0 oC in an ice bath. 270 mg of NaHCO3
(3.24 mmol) and 2 mL water were added with stirring. Then, 300 mg
of cyanuric chloride (1.62 mmol) was added portion wise with
o
stirring at 0 C for 10 min. The reaction mixture was further stirred
at room temperature for 20 min (till the evolution of the gas
stopped). After adding water to the resulting mixture, it was
filtered, washed with water and air-dried to obtain a solid product.
Recrystallization of the solid in CH2Cl2/petroleum ether provided
150 mg of 2a (Yield: 50%) as a white shiny solid. M.P.: 85 oC
Scheme 1 Structures of 1, 2 and 3 labelled with interaction site(s) for
TNP.
Experimental section
(literature value, 83 - 86 oC)16b
.
Step 2. Synthesis of 4-chloro-6-methoxy-1,3,5-triazin-2-amine (2b).
In a 10 mL RBF, 2a (150 mg, 0.837 mmol) dissolved in acetone (3
mL) was poured into 3 mL of ice water to form a very fine
suspension. An ammonium hydroxide solution (0.3 mL 1 N, 1.66
mmol) was added dropwise to the suspension at 0 °C. The mixture
Materials and methods have been described in ESI.†
Synthesis of 5-((4,6-diamino-1,3,5-triazin-2-yl)amino)isoph-
thalic acid (H2ATAIA) (1). Step 1. Synthesis of 6-chloro-1,3,5-
triazine-2,4-diamine (1a). In a 25 mL round-bottomed flask
(RBF), cyanuric chloride (726 mg, 3.95 mmol) dissolved in
acetone (6 mL) was poured into 6 mL of ice water to form a
very fine suspension. An ammonium hydroxide solution (7.9
mL 1 N, 7.82 mmol) was added dropwise to the suspension at
0 °C. The mixture was stirred at 0 °C for 30 min and at 40 oC for
30 min. A white solid was filtered, washed with water (3 x 5
ml) and dried under high vacuum to obtain crude 1a, which
was purified by column chromatography on silica gel by using
ethylacetate/ hexane (2:1) as the eluent to give 500 mg of
o
was stirred at 0 °C for 30 min and 40 C for 30 min. A white sunny
solid was filtered, washed with water (3 x 5 mL) and dried under
high vacuum giving 94 mg of 2b (Yield: 71%). M.P.: 240-243 oC.
Step 3. In a 25 mL RBF, 70 mg of 5-aminoisophthalic acid (0.38
mmol), 32 mg of NaHCO3 (0.38 mmol) and 31 mg of NaOH (0.77
mmol) were dissolved in 4.5 mL H2O. The mixture was further
stirred at room temperature for 30 min, followed by dropwise
addition of 60 mg of 2b (0.36 mmol) in 1,4-dioxane (4 mL) using
o
dropping funnel. The resulting mixture was then heated at 100 C
for 24 h. Upon adjusting the pH of the reaction mixture between 2
and 3 using dilute HCl, an off-brown solid was filtered, washed
several times with distilled water, and dried to obtain 106 mg of 2
(Yield: 93%). It was not soluble in common organic and inorganic
o
pure 1a (Yield 68%, lit: 70%)16a. M.P.: 235 C. HRMS (ESI) m/z
calculated for [M+H]+: 145.0155, found 145.0137. Selected
FTIR peaks (KBr pellet, cm-1): 3489(s), 3342(s), 3187(s), 1681(s),
1634(vs), 1583(s), 1563(s), 1525(s), 1473(m), 1297(s), 992(s),
914(s), 797(s), 637(m), 570(m).
o
solvents. M.P.: 320-323 C. HRMS (ESI) m/z calculated for [M+H]+:
306.0793, found 306.0780. Since it was not soluble in common
organic or inorganic solvents, its sodium salt (prepared with 2 eqv.
of NaOH) was used for NMR experiments in D2O. 1H NMR (400
MHz, D2O): δ 8.01 (d, J = 1.2 Hz, 2H, Ar), 7.9 (t, J = 1.6 Hz, 1H, Ar),
3.7 (s, 3H). 13C NMR (100 MHz, D2O): δ 54.8, 118.9, 119.7, 129.7,
Step 2. In a 25 mL RBF, 220 mg of 5-aminoisophthalic acid (1.25
mmol), 104 mg of NaHCO3 (1.3 mmol) and 90 mg of NaOH (2.3
mmol) were dissolved in 4.5 mL H2O. The mixture was further
stirred at room temperature for 30 min, followed by dropwise
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7TC05852G
140.2, 165.01, 169.2, 171.3, 174.9. Selected FTIR peaks (KBr, cm-1): out under a strict temperature regime. Firstly, two out of three
3297(s, -NH2), 3109(s, -NH), 2915(m), 2615(w), 1700(s, -COOH), chlorines in the triazine ring were appropriately replaced with two
1614(vs), 1556(vs), 1505(s), 1458(s), 1410(s), 1387(s), 1353(s), NH2 groups (for 1), one NH2 and one OMe groups (for 2) and two
1298(s), 1274(s), 1215(m), 1114(s), 1014(w), 957(w), 905(w), OMe groups (for 3) yielding 1a, 2b and 3a, respectively, based on
the protocols shown in Scheme 2. Secondly, sodium salt of 5-
aminoisophthalic acid (prepared in situ from 5-aminoisophthalic
acid and NaOH in water) was reacted with 1a, 2b and 3a in a
water:dioxane mixture (v/v, 1:1) at 100 oC, 100 oC and 70 oC,
respectively. The desired products 1, 2 and 3 were obtained by the
addition of 3 N HCl to the sodium salt in 72%, 93% and 95% yields,
respectively. These are fully characterized by M.P., HRMS, NMR (1H
and 13C), FTIR and UV-vis spectroscopy (Fig. S1-S12, ESI†). As 1-3 are
found to be insoluble in water and common organic solvents, their
1H and 13C NMR spectra have been performed with their sodium
salts in D2O. Thus, the protons for NH2, NH and COOH group of all
783(w), 756(m), 661(m), 604(w), 504(s). λmax (solid state,
reflectance): 285 and 327 nm (π-π* and n-π*).
Synthesis of 5-((4,6-dimethoxy-1,3,5-triazin-2-yl)amino)isophthalic
acid (H2DMTAIA) (3). Step 1. Synthesis of 2-chloro-4,6-dimethoxy-
1,3,5-triazine (3a). In a 10 mL RBF, 4 mL of methanol was taken and
cooled to 0 oC in an ice bath. 270 mg of NaHCO3 (3.24 mmol) and 2
mL water were added with stirring. Then, 300 mg of cyanuric
o
chloride (1.62 mmol) was added portion wise with stirring at 0 C
for 10 min. The reaction mixture was further stirred at room
temperature for 20 min (till the evolution of the gas stopped) and
40 oC for 20 min. After adding water to the resulting mixture, it was
filtered, washed with water and air-dried to obtain a solid product.
Recrystallization of the solid in CH2Cl2/petroleum ether provided
210 mg of 3a (Yield: 75%) as a white shiny solid: M.P.: 73 oC
(literature value, 71-74 oC)16c; Selected FTIR peaks (KBr, cm-1):
3424(br), 3297(s), 3148(s), 3024(m), 2554(w), 1694(s), 1614(vs),
1583(vs), 1557(s), 1485(s), 1410(s), 1385(vs), 1353(s), 1292(m),
1241(m), 1204(m), 1114(s), 1013(w), 929(m), 903(w), 808(s), 760(s),
686(m), 606(w), 511(s).
1
probes was not observed in the H NMR spectra. For 2 (one OCH3
group) and 3 (two OCH3 groups), the methoxy protons appear at
3.75 ppm and 3.67 ppm, respectively, in 1H spectra while the
carbon atoms appear at 54.83 ppm and 54.86 ppm, respectively, in
13C NMR spectra. The carbon atoms of the aromatic rings and
triazine ring for all probes appear at 115-140 ppm and 160-174 ppm
in 13C NMR, respectively. Combining the 13C NMR data, the
formation of 1-3 was confirmed by high resolution mass
spectroscopy (HRMS) exhibiting the expected molecular ion peaks
([M+H]+). The functional groups COOH, NH2 and NH were present in
the range 1693-1700 cm-1 and 3000-3300 cm-1 in the FTIR spectra of
1, 2 and 3. The solid state absorption spectra of 1-3 were recorded
with absorption maximum 278 nm, 285 nm, 273 nm and 323 nm,
327 nm, 323 nm for π-π* and n-π*, respectively.
Step 2. In a 25 mL RBF, 390 mg of 5-aminoisophthalic acid
(2.15 mmol), 180 mg of NaHCO3 (2.15 mmol) and 171 mg of
NaOH (4.3 mmol) were dissolved in 5 mL H2O. The mixture was
further stirred at room temperature for 15 min, followed by
dropwise addition of 3a (375 mg, 2.15 mmol) in 1,4-dioxane (4
mL) over a period of 20 min. The resulting mixture was then
heated at 100 oC for 36 h. Upon adjusting the pH of the
reaction mixture between 2 and 3 using dilute HCl, an off-
white solid was filtered, washed several times with distilled
Selective detection of TNP
The photo-excitation at 285, 300 and 310 nm afforded
a
water, and dried to give 566 mg of
3 (Yield: 95%). 3 was not
fluorescence emission maximum at 371, 435 and 380 nm for finely
ground 1, 2 and 3 (1 mg) dispersed in 2 mL water, respectively.
Based on this observation, fluorescence quenching titration
experiments for 1, 2 and 3 were performed with different NAEs
(TNP, TNT, 2,4-DNP, 4-NP, 2-NP, 2,4-DNT, 2,6-DNT, 4-NT, 2-NT,
DNB and NB) by incrementally adding an aqueous solution of
soluble in common organic and inorganic solvents. M.P.: 249-
251 oC. Since it was not soluble in common organic or
inorganic solvents, its sodium salt (prepared with 2 eqv. of
NaOH) was used for NMR experiments in D2O. 1H NMR (400
MHz, D2O):
Ar), 3.67 (s, 6H,). 13C NMR (100 MHz, D2O):
δ
8.03 (d, J = 2.8 Hz, 2H, Ar), 7.87 (t, J = 2 Hz, 1H,
54.9, 118.9,
δ
each analyte (1 mM, total: 50 ꢀL) to the respective probe (1
119.8, 129.7, 140.2, 164.9, 171.1, 174.7. HRMS (ESI) m/z
calculated for [M+H]+: 321.0790, found 321.0779. Selected
FTIR peaks (KBr, cm-1): 3424(br), 3297(s, -NH), 3148(s),
3024(m), 2554(w), 1694(s, -COOH), 1614(vs), 1583(vs),
1557(s), 1485(s), 1410(s), 1385(vs), 1353(s), 1292(m), 1241(m),
1204(m), 1114(s), 1013(w), 929(m), 903(w), 808(s), 760(s),
686(m), 606(w), 511(s). λmax (solid state, reflectance): 275 and
mg dispersed in 2 mL water) at 25 °C (Fig. 1a and S13-S46,
ESI†); the quenching efficiency of different NAEs was
calculated using the equation: (I0 − I)/I0 x 100%. For TNP, the
quenching% was 92%, 82% and 62% for
1, 2 and 3,
respectively. This was further elaborated by using the Stern-
Volmer (SV) equation:
(I0/I) = KSV[A] + 1; Ksv = Kqτ
323 nm (π-π* and n-π*).
where, I0 and I are the luminescence intensities of the probes,
before and after addition of the NAEs, KSV is the Stern-Volmer
constant (M-1), [A] is the molar concentration of the analyte, Kq is
the quenching rate constant and τ is the excited-state lifetime
Results and discussion
Synthesis and characterization
The non-trivial synthesis of 1-3 from the readily available and cheap without adding quencher. In case of TNP, Ksv and Kq values for 1, 2
cyanuric chloride via nucleophilic substitution reactions was carried and 3 are 1.759 X 105 M-1 and 3.03 X 1013 M-1s-1, 9.873 X 104 M-1 and
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DOI: 10.1039/C7TC05852G
1.32 X 1013 M-1s-1, and 1.646 X 104 M-1 and 5.19 X 1012 M-1s-1, solvents. Thus, it is appropriate to make a comparison with those
respectively (Fig. 1b, Table 1, Tables S1-S3, ESI†). Comparing the Ksv studied in water only. For example, fluorescein-based
a
and Kq values of analytes with a phenolic group (Table S4, ESI†), the chemosensor has a Ksv value of 3.94 X 104 M-1 in water.10d On the
quenching efficiency (TNP >> 2,4-DNP > 4-NP > 2-NP), which other hand, Zr(IV)-based MOFs provide Ksv values of 3.1-5.1 X 105 M-
correlates well with the order of their acidity, follows the strength 1 in water.12b The SV plots of TNP in 1, 2 and 3 show linearity at low
and number of hydrogen bonds between the hydroxyl group and concentration and deviation from linearity at higher concentration;
the amino group(s). Thus, the number of hydrogen bonds from the however, other NAEs show linearity even at higher concentration in
interaction between TNP and 1, 2 and 3 (Fig. 2) reflects in the order their corresponding SV plots (Fig. S47-S76, ESI†). In the S-V plot, the
of Ksv values. This also explains the unprecedented selectivity of linearity at low concentration can be attributed to either static or
TNP over other NAEs without a hydroxyl group to interact with the dynamic quenching of the probe; a deviation from linearity at
amino groups of 1 and 2 (see below for a comparison with TNT). higher concentration bending upward is for self-absorption or an
The Ksv value of 1 is higher than or comparable with other organic- energy transfer process between the probe and TNP.1b,3b The
based probes, polymers and MOF-based sensors in water reported quenching constant of TNP was 176, 12 and 2 times higher than
in the literature (Table S5, ESI†).6b,10 As pointed out earlier, most of that of TNT for 1, 2 and 3, respectively. Thus, these are excellent
the published works for the detection TNP are done in organic candidates toward selective detection of TNP in water.
NH2
N
Cl
NH4OH, Acetone
ice cold H2O, 0-40 o
N
N
N
C
Cl
N
NH2
Cl
N
Cl
1a
NH2
N
NH2
COOH
NH2
N
a) NaOH/NaHCO3
H2O/Dioxane
100 oC, 24 h
COOH
b) dil. HCl
N
+
NH2
N
Cl
N
HOOC
HOOC
N
N
NH2
H
H2ATAIA (1)
OMe
Cl
OMe
NH4OH
Acetone/H2O
MeOH/H2O
NaHCO3
0-RT
30 min
N
N
N
N
N
N
0-40 o
60 min
C
Cl
N
Cl
Cl
N
Cl
Cl
N
NH2
2a
2b
OMe
N
NH2
COOH
OMe
a) NaOH/NaHCO3
H2O/Dioxane
100 oC, 24 h
N
+
NH2
N
N
Cl
N
HOOC
COOH
HOOC
N
N
NH2
b) dil. HCl
H
H2AMTAIA (2)
Cl
OMe
N
MeOH/H2O
NaHCO3
0-40 o
N
N
N
Cl
N
Cl
Cl
N
OMe
C
3a
OMe
N
NH2
COOH
OMe
N
a) NaOH/NaHCO3
H2O/Dioxane
70 oC, 36 h
b) dil.HCl
COOH
N
+
OMe
N
Cl
N
HOOC
HOOC
N
N
OMe
H
H2DMTAIA (3)
Scheme 2 Synthetic routes for 1-3.
Table 1. Photophysical data and DFT calculation results for single molecular probes 1, 2 and 3 in presence of TNP.
Probe
λabs
λemi
Ksv (M-1)
DFT
Average
Lifetime (ns)
5.48
Quantum
Yield (φ)
0.165
Detection
Limit
(nm) (nm)
HOMO (eV)
-5.9977
LUMO (eV)
-1.5866
-2.1328
-2.1069
Eg (eV)
4.41
1
2
3
323
320
325
371
430
380
1.759 X 105
9.873 X 104
1.646 X 104
120 ppb
0.8 ppm
1.2 ppm
-6.4695
4.34
7.47
0.253
-6.5462
4.44
3.17
0.066
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Fig. 1 (a) Effect on the emission spectra of
1, 2 and 3 (left to right) dispersed in water upon incremental addition of TNP. (b) Stern-Volmer plot of
NAEs for 1, 2 and 3 (left to right) in water showing high selectivity for TNP; insets indicate visual change upon UV irradiation at 365 nm.
(a)
(b)
O
O2N
H3C
N
N
O
COOH
O
N
O
H
NO2
O
N
O
N
N
H
O
N
O
O
N
H
HOOC
N
N
H
O
O
H
O
H
H
O
H
COOH
N
NO2
O
N
O
N
O
H
O
N
N
O
O
H
N
HOOC
N
H
N
N
H
O
O
NO2
O
H
O
O
N
O
(c)
N
O
O
O
H3C
N
COOH
O
NO2
NO2
O
H
N
N
N
HOOC
N
N
O
O
H
O
CH3
Fig. 2 Possible prominent noncovalent interactions (H-bonding) with NH2 or OMe groups of the probes and OH group of TNP: (a) Three TNP (-
OH) groups interacting with two –NH2 groups of . (b) Two TNP (-OH) groups interacting with one –NH2 group of . (c) One TNP (-OH) group
the OMe group of 3.
1
2
interacting
with
As shown in Fig.
3 (top), TNP shows significantly higher (Fig. S77-S79, ESI†). These results vividly depict the effect of an
fluorescence quenching than the others in each case. The detection amine group compared to a methoxy group towards the selective
limit of TNP for 1 was found to be 120 ppb which is higher detection of TNP over other potentially interfering NAEs. The
compared to those (0.8 ppm and 1.2 ppm, respectively) for 2 and 3 quenching efficiency of 1, 2 and 3 was further visualized by
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fluorescence microscopy technique, in which TNP is discriminated
by the turn-off response in blue intensity of DAPI region (inset in
Fig. 3 (top)). In addition to this, a handy paper strip method was
used for real time TNP detection. Three different pristine strips
coated with 1, 2 and 3 showed good illumination under UV light.
However, after addition of TNP solution to these, a change in color
and fluorescence quenching of the probes was observed (Fig. 3,
bottom). This is ideal for ultrafast real-time detection of TNP in the
field. This type of demonstration for the selective sensing of TNP by
naked eye is rare.3c,10c
both photo-induced electron transfer and energy transfer are
contributed to the efficient quenching of
presence of TNP.
1
,
2
and
3
in
(a)
(b)
Fig. 3 (Top) Fluorescence quenching (%) of 1, 2 and 3 by different NAEs at 25
°C; (inset) Fluorescence Microscopy images: response of 1, 2 and 3 due to
TNP in the DAPI region. (Bottom) Paper sensor for the visual detection of
TNP by 1, 2 and 3 under UV light (λ = 365 nm).
(c)
Inspired from the better quenching efficiency of TNP, the
competitive behaviour of TNP in presence of other NAEs was
performed. In a typical quenching experiment, after addition
of 50 ꢀL other NAEs, 30 ꢀL of TNP was added (Fig. 4). The
addition of other NAEs show negligible quenching effect,
whereas TNP shows efficient quenching. These competitive
results demonstrated the unprecedented selectivity of TNP,
even in presence of higher concentration of other NAEs.
To further enlighten the aforementioned results, the spectral
overlap between the fluorescence emission spectra of
1, 2 and
3
and absorbance spectra of different NAEs was recorded (Fig.
5 and S80-S82, ESI†). The resonance energy transfer was
enhanced from probe to non-emissive NAEs due to effective
spectral overlap. The higher the energy transfer the greater is
the fluorescence quenching efficiency with the NAEs. It was
observed that the effective overlap took place in case of TNP
Fig. 4 Bar diagrams of the emission intensity percentage of
and (c) before and after the addition of NAEs (50
followed by the addition of 30 L TNP. Green bars represent the
1
(a),
2
(b)
3
ꢀL) separately
ꢀ
suspension of pristine in water, red bars represent the addition of
different NAEs to the suspension of pristine (pristine + NAEs) and
purple bars represent the subsequent addition of TNP to the above
suspension (pristine + NAEs + TNP); from left to right (i) NB, (ii) DNB,
(iii) 2-NT, (iv) 4-NT, (v) 2,6-DNT, (vi) 2,4-DNT, (vii) TNT, (viii) 2-NP, (ix) 4-
NP and (x) 2,4-DNP.
with emission band of 1, 2 and 3, respectively. However, other
NAEs have insignificant spectral overlap suggesting that the
quenching of fluorescence intensity in presence of TNP occurs
by an energy transfer mechanism in all three cases. This
explanation was further supported by the red shift of
fluorescence intensity by 5, 26 and 10 nm during the
incremental addition of TNP to 1, 2 and 3, respectively. Thus,
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To better understand the stability and recyclability of
1, 2
and 3, solid materials were regenerated by centrifugation and
washing for several times with water after every experiment.
Surprisingly, all probes almost recuperate their fluorescence
intensity after five repeated cycles with TNP without much loss
of sensitivity (Fig. S83, ESI†). Also, the FTIR spectra were
recorded after five repeated cycles without any change that
confirm the stability of all probes (Fig. S84-S86).
To gain further knowledge about photo-physical and electronic
properties, computational studies based on density functional
theory (DFT) using the Gaussian 09 program were carried out.17 The
B3LYP/6-31G(d, p) basis set was used for optimizing the structures
(Fig. 7 and Fig. S87, ESI†). Interestingly, due to the interaction with
TNP, 1 (-4.0944 kcal/mol) is much more stabilized than 3 (-0.4203
Fig. 5 Spectral overlap of absorption spectra of different NAEs and kcal/mol) for very strong H-bonding with two NH2 groups compared
emission spectra of 1, 2 and , respectively.
3
to two OMe groups. This is well corroborated with the results from
other studies mentioned above. In case of 1, the hydrogen bond
distances are 1.99 Å (N-H…O between one NH2 and o-nitro of TNP)
and 2.13 Å (N-H…O between second NH2 and OH of TNP). In case of
2, the hydrogen bond distances are 2.19 Å (N-H…O between one
NH2 and OH of TNP). On the other hand, the distance 2.36 Å (O-H…O
between OH of TNP and OCH3) shows a weak H-bonding in 3. Also, a
strong H-bonding is present between the o-nitro group of TNP and
a carboxylic acid group of each probe with a distance (O…O) of 1.93
Å. The HOMO and LUMO energy levels of 1-3 and different NAEs
are shown in Fig. 8 and Table S6. The HOMO of 1-3 represents the
π-systems of the conjugated backbone, the lone pair of amino and
methoxy groups. The π∗-system of conjugated backbone and
triazine moiety contributes to the LUMO. The calculated energy gap
(Eg) listed in Table 1 for 1 (4.41 eV) ≤ 3 (4.44 eV) > 2 (4.34 eV) is fully
consistent with the observed photo-luminescence emission
wavelengths (λemi: 1 ≤ 3 < 2). The descending LUMO energy order of
different NAEs confirms that an electron transfer can easily occur
during the fluorescence quenching process.4,18 However, the LUMO
of TNP is more stabilized due to selective quenching of all probes
and thus the quenching efficiency is not in full accordance with the
LUMO energies of the other NAEs. The LUMO-HOMO orbitals were
redistributed when TNP was H-bonded with the probe and the
electrostatic interaction between the probe and TNP occurred. As a
result, the LUMO energy got stabilized and localized on TNP, which
favours the electron transfer from excited state of the probe to TNP
causing fluorescence quenching of the probe (vide supra, Fig. S87-
S88, ESI†).
We have utilized scanning electron microscopy (SEM) to
better understand the interaction between TNP and the
probes through a comparison of surface morphology between
1
,
2
and
L, 1 mM stock solution, soaking for 2 days). Interestingly, as
shown in Fig. 6, the change in morphology from micro cloud to
3 and TNP incorporated 1, 2 and 3, respectively (50
ꢀ
micro whisker (
1
and
1
@TNP), cloudy to micro walnut (for
2
and 2@TNP) and aggregated petal to micro whisker (
3
and
3
@TNP) was observed. Based on these noteworthy changes, it
is clear that TNP interacts with each probe.
Finally, time-resolved fluorescence experiments were performed
(Table 1, Fig. 9 and Tables S7-S9, ESI†) to shed further light into the
quenching mechanism of TNP based on the concentration-
dependency of average lifetimes of each probe. The fluorescence
lifetime of 1-3 was found to be in the range of 0.34 ns to 9.9 ns as
measured with the help of time-correlated singlet photon counting
(TCSPC) method and well fitted with the tri-exponential equation.
However, the fluorescence lifetimes of 1-3 were in the order 2 > 1 >
3, which is in accordance with their band gap. Generally, any
Fig. 6 FESEM images of
respectively.
1
-
3
before and after addition of TNP, change in the fluorescence lifetime of chemosensors is referred to a
dynamic quenching process due to diffusive collisions, while the
unchanged
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Fig. 7 Energy minimized structures of 1, 2 and 3 in presence of TNP along with their corresponding energy diagrams, respectively.
fluorescence lifetime is attributed to a static quenching process
during which the fluorophore molecules form a non-fluorescent
complex or a “dark state” with the quencher and the unbound
fluorophores display their inherent lifetime.19 The average lifetimes
of 1-3 were evaluated in absence and presence of TNP. The average
lifetime values are: for 1, 5.48 ns (before TNP addition), 5.84 ns
(after 20 ꢀL TNP addition) and 4.78 ns (after 50 ꢀL TNP addition);
for 2, 7.47 ns (before TNP addition), 7.67 ns (after 10 ꢀL TNP
addition), 7.66 ns (after 20 ꢀL TNP addition) and 7.71 ns (after 50 ꢀL
TNP addition); for 3, 3.17 ns (before TNP addition), 2.95 ns (after 10
ꢀL TNP addition), 3.0 ns (after 20 ꢀL TNP addition) and 3.02 ns
(after 50 ꢀL TNP addition). These data demonstrate that both static
and dynamic quenching or photo-induced electron transfer (PET)
take place between TNP and 1 while only static quenching is
observed in case of 2 and 3. This can be very well correlated to
having higher
Fig. 8 HOMO-LUMO energy gap of 1, 2, 3 and different NAEs.
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(a)
Fig. 10 2D recognition map of 1, 2 and 3 in presence of TNP with
respect to lifetime and quantum yield.
(b)
Conclusions
In summary, we reported the synthesis, characterization and
photophysical properties of three fluorescent organic probes
with two NH2 groups (for
1
), one NH2 and one OMe groups (for
). Their use in the highly selective
2
) and two OMe groups (for
3
and ultrafast sensing of TNP in water (slurry mode) was
demonstrated through fluorescence spectroscopy and
fluorescence microscopy as well as by the handy paper strip
method. The detection limit of TNP for
ppb, which is better compared to those (0.8 ppm and 1.2 ppm,
respectively) for and . The mechanism of selective TNP
1 was found to be 120
2
3
sensing for all the probes was explained by SV plot, CNA test,
spectral overlap, FESEM, time resolved experiments and DFT
calculations. Comparing the Stern-Volmer constants for
1, 2
(c)
and 3, it is clear that the quenching efficiency of analytes with
a phenolic group correlates well with the order of their acidity
(TNP >> 2,4-DNP > 4-NP > 2-NP). Both photo-induced electron
transfer and energy transfer contributed to the efficient
quenching of each probe in the presence of TNP. The
molecular decoding of TNP was also demonstrated based on a
2D read-out identification scheme constructed from lifetime
and quantum yield of the probes. This work represents a new
strategy to design efficient but simple, highly selective, cost-
effective and reusable florescent sensors with important
functional groups for selective detection of nitro explosives
and environmentally pollutants. In order to improve sensitivity
and efficacy, further efforts to make numerous derivatives are
underway in our laboratory.
Fig. 9 Fluorescence lifetime decay profiles of 1 (a), 2 (b) and 3 (c)
before and after incremental addition of TNP, respectively.
Acknowledgements
Funding for this work was provided by IISER, Mohali. P. D. is
grateful to the MHRD of India for a research fellowship. The
use of NMR and SEM central facilities and other departmental
facilities at IISER Mohali is acknowledged.
H-bonding interactions in 1 with two NH2 groups compared to one
NH2/one OMe in 2 and two OMe groups in 3. Utilizing the
information gathered for the probes, we could construct a dual-
readout orthogonal identification scheme based on the life-time (τ)
and quantum yield (φ) listed in Table 1 for 1, 2 and 3 in a decoded
map shown in Fig. 10. Thus, each probe relates to one unique 2D
readout (τ, φ). These well-separated readouts give a precise way to
differentiate them through a ratiometric lifetime and quantum yield
dual-read out process.
Conflicts of interest
There are no conflicts to declare.
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VJieowuArrtnicalelONnlainme e
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Table of contents – artwork and synopsis
Three single-molecule fluorescent organic probes differing in the number of amino groups have been developed for demonstrating their
effect on the highly selective and ultrafast sensing of TNP in water.
12 | J. Name., 2012, 00, 1-3
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