PET governed fluorescence "Turn ON" BODIPY probe for selective detection of picric acid

Article abstract of DOI:10.1039/c5ra17932g

The non-fluorescent meso diaminophenyl 1,3,5,7-tetramethyl BODIPY dye has been investigated and employed for picric acid sensing in a constructive way by regenerating its fluorescence through PET hindrance. A strong enhanced emission signal was obtained as a consequence of the electrostatic association between the BODIPY probe and picric acid with specific recognition among other explosive nitroaromatics. The probe shows a 1 : 1 binding stoichiometry with picric acid and a detection limit of up to 0.65 ppb.

Full text of DOI:10.1039/c5ra17932g

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PET governed fluorescence “Turn ON” BODIPY probe for
selective detection of picric acid.
Received 00th January
20xx,
Accepted 00th January
20xx
Yogesh Erande, Santosh Chemate, Ankush More and Nagaiyan Sekar*
The non fluorescent meso diaminophenyl 1, 3, 5, 7-tetramethyl BODIPY dye has been investigated and
employed for picric acid sensing in constructive way by regenerating its fluorescence through PET hindrance.
Strong enhanced emission signal was obtained as consequence of electrostatic association between BODIPY
probe and picric acid with specific recognition among other explosive nitro aromatics. The probe shows 1:1
binding stoichiometry with picric acid and detection limit up to 0.65ppb.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
with it is attributed to either enhancement or quenching of
fluorescence14,15
.
Stoichiometric
binding
over collisional
Now a day’s danger of terrorism with increased use of high
explosives challenges the modern technology for their detection
and screening1. Electron deficient nitro aromatics (NACs)are
preferred explosives and picric acid(PA) is one of them whose
explosive power is equivalent to 105% of trinitrotoluene(TNT)
known as primary explosives2,3. PA is an important chemical and
finds applications in medicinal formulations in the treatment of
malaria, trichinosis, herpes, smallpox, and antiseptics4, in dye
industries and chemical laboratories5,6, in the manufacture of
rocket fuels, fireworks, sensitizers in photographic emulsions, as a
component in matches7–9. Owing to its electron-deficient character
the degradation of PA in bio system is difficult making it as an
environmental contaminant10. As an effect of nitro and phenol
functionalities, PA causes strong irritation to the skin, eye and
potential damage to organs of respiratory system8,9. Therefore
rapid, cheap, sensitive and selective detection of picric acid is of
prime importance. Various analytical techniques such as gas
chromatography, ion mobility spectrometry, Raman spectroscopy,
and fluorescence spectroscopy have been reported for the
detection of NACs explosives11. Further, among various techniques
adopted for detection of NACs, fluorescence signalling is superior
with high sensitivity, specificity, real-time monitoring with short
response time and easy sample preparation12 while other methods
cannot be used in the field due to their high cost, lack of selectivity
and sensitivity13. Analyte sensing by fluorescent probe after binding
phenomenon and less interference from fluctuation of background
fluorescence make fluorescence enhancement method more
preferable than quenching of fluorescence for analyte sensing16,17
.
Metal–organic framework 18–22 and organic or inorganic conjugated
polymers 23,24, are mostly employed as fluorescent sensors for the
detection of poly nitro aromatics particularly picric acid. In the field
of NACs detection application of metal organic framework,
quantum dots mostly contain cadmium and pyrene class of sensors
are challenged by their toxicity18,21,25,26
. Among the available
18,27–
sensors for picric acid detection by quenching of fluorescence
31
32–
are more explored than ‘no quenching fluorescence sensors’
36
.
In the current scenario p‑terphenylbenzimidazolium-based
37
molecular baskets
, zinc-phthalocyanine framework based
38,39
chemosensor with fluorescence quenching
, electron-rich
triphenylamine-based sensors with charge-transfer complex
formation40, electrogenerated thin films of microporous polymer
networks-response to NACs41, glucopyranosyl-1,4-dihydropyridine
chemosensor42
[Cd3(TPT)2(DMF)2]·(H2O)0.5 for NACs detection43
,
metal–organic
framework
,
self-assembled
nanoscopic organic cage for PA detection44 are some strategies
focusing on NACs explosive detection. Most of above the discussed
molecules or strategies are complicated so small molecules as
fluorescence sensor having better option as they are easy to
synthesize and their properties can be easily tuned by structural
modifications for desired sensing specificity. BODIPY dyes are
versatile fluorescent probes comprising various properties such as
high chemical and photo stability , high absorption coefficients and
Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute
of Chemical Technology, Mumbai-400 019, India.
E-mail: n.sekar@ictmumbai.edu.in; nethi.sekar@gmail.com;
Tel: +91 22 3361 2707
sharp fluorescence peaks with
high quantum yields45,46. With
number of possible structural modifications it is easy to design the
BODIPY luminophore for desired spectroscopic and photophysical
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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characteristics as chemosensor45–50. Some sensors with aromatic Computational method
amine showed selectivity and sensitivity for picric acid but they are
synthetically challenging 22,51. Considering the entire dilemma we
have presented the current BODIPY probe as selective turn on
fluorescent chemosensor for picric acid detection among other
NACs. This molecule possesses very weak fluorescence which gets
enhanced as consequence of inhibition of photoinduced electron
transfer (PET) phenomenon. Perpendicular meso diaminophenyl
moiety quenches the emission from BODIPY core through PET
utilising its lone pairs which are now associated with PA by static
interaction so to leave BODIPY core to fluoresce that demonstrate
the fluorescence turn on PA sensing.
All DFT computations were performed using the Gaussian09 (G09)
program52,53
Full geometry optimization of BODIPY-2 and its
.
complex with PA were carried out using the hybrid functional B3LYP
and the Pople’s double zeta basis set 6-31G(d)54,55
.
Synthesis of BODIPY-1 and 2
56
2, 4 Dimethyl pyrrole was prepared by the reported method
.
BODIPY-1 and BODIPY-2 was synthesized with gentle modifications
in reported procedure 57. BODIPY-1 &2 were characterised by 1H
and 13C NMR.
BODIPY-1: 1H NMR (500 MHz, cdcl3) δ (ppm)
9.19 (t, J = 2.1 Hz, 1H), 8.59 (d, J = 2.1 Hz, 2H), 6.07 (s, 2H), 2.59 (s,
6H), 1.37 (s, 6H).
Experimental
BODIPY-1: 13C NMR (126 MHz, cdcl3) δ (ppm)
Fluorescence spectra were recorded on a Varian Carry Ecllips
Spectrofluorometer. UV/Visible spectra were recorded on a Perkin 158.00, 149.01, 141.78, 138.81, 134.50, 130.67, 129.13, 122.59,
Elmer Lambda-25 spectrophotometer using a 10 mm path length
quartz cell. The photophysical study was carried out in HPLC-grade BODIPY-2: 1H NMR (500 MHz, DMSO-d6) δ (ppm)
solvents. NMR spectra were recorded in DMSO-d6 solution on
119.34, 15.38, 14.75.
6.13 (s, 2H), 5.90 (t, J = 2 Hz, 1H), 5.67 (d, J = 2.0 Hz, 2H), 4.95 (s,
the Agilent-500 MHz instruments, and the chemical shifts are 4H), 2.41 (s, 6H), 1.68 (s, 6H).
reported in δ values (ppm) relative to TMS. 3, 5-dinitro benzoyl
BODIPY-2: 13C NMR (126 MHz, DMSO-d6) δ (ppm)
chloride was purchased from Sigma Aldrich. All solvents used for
synthesis were of synthetic grade and purchased from s d Fine
Chemicals, India. Other chemicals/ reagents were purchased from
commercial sources. The reactions were carried out under nitrogen
atmosphere and dichloromethane was dried by treatment with
calcium hydride before use.
154.41, 150.68, 144.70, 143.23, 135.11, 130.92, 121.20, 101.48,
100.21, 14.63, 14.20.
Results and discussion
Determination of apparent association constants (Ka) of PA
binding with sensor
The binding constant was calculated from the emission intensity-
titration plot of 1/ (I-Io) as a function of 1/ [PA] in mole. Io and I are
the fluorescence intensities of BODIPY-2 without and with added
PA. BODIPY-2 with a concentration of 1 × 10 −6mol/L in absolute
ethanol was used for the fluorescence titration studies with a PA.
The PA concentration was varied from 4 to 13 eq. for this titration.
The apparent binding constant for the formation of the complex
was evaluated using the Benesi–Hildebrand plot.
Scheme-1 Synthetic route of BODIPY probe.
For photophysical study absolute ethanol was the most suitable
solvent among others. BODIPY-2 shows strong absorption peak in
absolute ethanol at 499nm and very weak emission at 513nm. The
weak emission is attributed to efficient fluorescence quenching
resulting from photo induced electron transfer phenomenon.
BODIPY having methyl groups at 1 and 7 position keeps the meso
phenyl ring out of the plane of BODIPY core (fig.7a) and thus there
is no conjugation between them but when this meso phenyl ring is
substituted with amino group it impedes the emission. Amino group
on angled meso phenyl ring contributes to the photoinduced
electron transfer which suppresses the emission of the molecule.
The engagement of electron density on amino group with other
group could reset the emission is the idea behind construction of
sensor. Two amino groups were placed on meso phenyl ring meta
to each other with expectation of 1:2 binding stoichiometry with PA
for better value of detection limit and binding constant. We have
screened different nitro aromatics (10eq. each) i.e. picric acid(PA),
Estimation of detection limit
Fluorescence titration of probe (conc. =1 × 10−6mol/L) with PA in
ethanol was carried out by adding aliquots of PA from 1 to 12
equivalents. Standard deviation (SD) was obtained from 10 blank
reading of probe (conc. =1
×
10−6mol/L). From the plot of
fluorescence intensity as a function of the added PA eq., the slope
was obtained. Then the detection limit (DL) was calculated from
equation DL= (3xSD)/slope.
Determination of the binding stoichiometry
Stoichiometry of probe-analyte was determined by continuous
variation method known as Job’s plot. In this experiment total
concentration of probe and analyte ([BODIPY] + [PA]) was kept
constant (30μM) and the ratio was varied for 10 different fractions.
The Job’s plot was obtained by plotting fluorescence intensity (I-Io)
as a function of mole fraction of PA.
p-nitrotoluene
(PNT),
2,4-dinitrotoluene
(DNT),
2,4,6-
trinitrotoluene (TNT), p-nitrobenzioc acid (PNBA), p-nitrophenol
(PNP), 2,4-dinitrophenol (DNP), 2,4,6-trichlorophenol (TCP),
nitrobenzene (NBz) and keto RDX (kRDX) with 5 μM solution of
BODIPY probe which shows high selectivity towards PA among
others with 300 fold enhancement of fluorescence intensity and
2 | J. Name., 2012, 00, 1-3
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quick response (with just gentle shaking in <10 seconds) in absolute other NACs (10 equiv.) with PA (10 equiv., blue bar) in absolute
ethanol. Fig.1 shows the graph for recognition of different NACs ethanol (with black error bar on top of each intensity bar).
with BODIPY probe. The specificity of probe towards PA comes from
different NACs (10 eq. each) in 5μM solution of BODIPY+ 10 eq.PA
in ab. ethanol. This competitive binding experiment shows that
there was no considerable lowering in fluorescence intensity by
added NACs except PNT, DNT and TNT for which average 20% fall
was observed. Fig. 2 shows the histogram of recognition and
interference study. The binding constant derived from the
fluorescence titration data in accordance with Benesi–Hildebrand
plot was found to be Ka=1.91×104M-1 (Fig. 3) .
the efficiency of association between them. Though electron
withdrawing effect of nitro groups plays role in interactions the
other NACs were insensitive to the probe.
Fig.1 Fluorescence responses of BODIPY-2 (5 μM, λex= 499 nm) to
the addition of various NACs (10 equiv.) Inset: Zoom view for
recognition of different NACs.
The high selectivity of BODIPY probe to PA has been attributed to
its specific binding with analyte because of proton transfer from
PA to the amino group in sensor which is in accordance with static
nature of association. This selectivity is further supported on the
basis of pka values of NACs. PA has pka value =0.38 which is far less
than its structurally resembled phenolic and nitroaromatic
competitors such as DNP(4.11), PNP(7.15), PNBA(3.41), TCP( 6.23)
and TNT(11.99). Thus most acidic PA is preferred over all screened
NACs. Excluding large enhancement in fluorescence intensity of
BODIPY sensor with PA other signals were remain almost unaltered
indicating its highest specificity towards PA. The interference of
other NACs for PA sensing by sensor was studied by adding
Fig.3 Benesi–Hildebrand plot of BODIPY-2 with PA in absolute.
ethanol at λex = 499 nm. The binding constant (Ka) = 1.91x104 M-1.
The sensitivity of BODIPY probe towards PA was counted on the
scale of detection limit. From the changes in PA concentration
dependent fluorescence intensity the detection limit was estimated
to be 0.65ppb. For Benesi - Hildebrand plot and detection limit
calculation fluoremetric titration of BODIPY-2 with 1-12 equiv. PA
was performed (Fig. S1, ESI). The Job’s plot confirmed the 1:1
stoichiometry of the complexation between host and guest. The
fluorescence intensity changes more rapidly when the
concentration of PA increased from 0.5 to 1 equiv., implying a 1:1
complexation between BODIPY-2 and PA (Fig. 4).
Fig.2 The histogram of fluorescence response of BODIPY-2 (5uM)
to PA or other NACs (10 equiv. each, red bar) and to the mixture of
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Fig.4 Job’s plot of BODIPY probe with PA according to the of
continuous variation method at λex = 499nm in ab.ethanol.
[BODIPY-2] + [PA ]=30μM.
NO2
O2N
H
O
NO2
OH
It was expected to be 1:2 binding stoichiometry between probe and
PA for larger value of binding constant as BODIPY has two amino
groups on meso phenyl ring which can associate with two
molecules of PA independently but it was 1:1. To have more
understanding of the binding stoichiometry we have carried out
1H NMR titration experiment. 1H NMR spectrum of BODIPY-2 in
H2N
NH2
H2N
NH2
O2N
NO2
PET
X
PET
NO2
N
N
N
N
B
B
F
F
F
F
Strong fluorescence
BDP-2 + PA
Weak fluorescence
BDP-2
Fig.6 Proposed association pattern between BODIPY-2 and PA .
To get further insight into binding nature the DFT computation was
carried out for BODIPY-2, BODIPY-2 + 1eq. PA and BODIPY-2 + 2eq.
PA. Three structures were optimised at B3LYP/6-31g(d) level using
Gaussian -09 software programme. Fig.7 shows the optimised
Guass views of these structures respectively. Fig.7b shows the 1:1
binding in which one of the amino group on meso phenyl ring of
BODIPY-2 get protonated from PA while picrate anion is stabilised
by delocalisation of charge over tri nitro substituted ring . Fig.7c
shows again 1:1 binding while second PA molecule become
dissociated by the virtue of the molecular geometry.
Fig.5 The stacked 1H NMR titration spectra(500MHz) of BODIPY
probe with 0, 1, 2 and 4 equivalent of PA in DMSO-d6
.
DMSO-d6 was recorded with added 1, 2 and 4 equivalent of PA
independently. Fig.5 shows the stacked graph of these titration
experiments. After addition of 1 equivalent of PA to BODIPY-2 (1:1
stoichiometry) the peak c (for two symmetric protons on meso
phenyl ring) and peak e (for 2 and 6 position protons on BODIPY
core) shifted to deshielded region by 0.68ppm and 0.30ppm
respectively. The another peak d (for proton between two amino
groups on phenyl ring) comparatively get more shifted towards
deshielded region by 0.74ppm as electron density from
neighbouring amino group is engaged with added PA. The peak f (at
4.95 for amino protons) in plane BODIPY spectrum was vanished
with added PA. Thus the aromatic region NMR signals were altered
efficiently with added PA. When NMR spectrum was recorded with
2-equivalent of added PA (2:1 stoichiometry) the already altered
signals by 1 equivalent PA were not shifted further considerably and
same for added 4 equivalents also. This observation reveals that
there was possibility of only 1:1 binding stoichiometry.
Fig.7 B3LYP/6-31g(d) Optimised structures with ball-stick model
for BODIPY-2 and its association with 1 and 2 equivalent of PA.
Ball colour – atom: gray-carbon, dark blue- nitrogen, pink-boron,
red-oxygen, sky blue-fluorine, white-hydrogen.
In the light of Job’s plot, 1H NMR titration and DFT optimisation
study it is confirmed that only 1:1 binding is possible between PA
and BODIPY probe. The reason behind this might be explained by
the fact that after protonation of one of the amino group with PA
the meso diamine phenyl ring become electron deficient. This make
it difficult the further association between the remaining amino
group and the second PA molecule because of the molecular
geometry constraint revealed by DFT optimisation. Isodensity plots
of these optimised structures at B3LYP/6-31g(d) level shows that
LUMO level of BODIPY-2 has some charge density on meso diamine
phenyl ring which contributes to PET mechanism(Fig.S3,ESI). In
case of both 1 and 2 equiv. PA associated structures there were no
charge density on meso diaminophenyl ring at LUMO level which
erases any possibility of assistance for PET process from meso
position of BODIPY. Thus when one NH2 group associates with PA
4 | J. Name., 2012, 00, 1-3
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molecule it quaternises (+NH3) and makes the meso phenyl ring Acknowledgements:
electron deficient so that lone pair of another NH2 group is
delocalised on phenyl ring more efficiently than in the case of
One of the authors (Yogesh Erande) is thankful to University Grant
Commission -SAP, India for financial support by way of Junior and
Senior Research Fellowships.
unassociated BODIPY-2. Further electron density from the
remaining free NH2 and the phenyl ring is pulled by electron
deficient complex of +NH3 and picrate anion. These things leave the
still free NH2 group with much less electron density compared to
that in the unassociated BODIPY-2 and hence it neither binds with
another PA molecule nor contributes to PET to limit the binding
stoichiometry as 1:1. On the basis of acidic proton transfer from PA
to probe the DFT and NMR study concludes that the association is
of electrostatic nature which is further supported by salt screening
effect study. The salt screening effect is a tool for determining
electrostatic interaction between sensor and analyte58.
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