Benzimidazole Based ‘Turn on’ Fluorescent Chemodosimeter for Zinc Ions in Mixed Aqueous Medium

Article abstract of DOI:10.1007/s10895-016-1836-x

Benzimidazole based compound 3 is designed and synthesized. The compound 3 is evaluated as fluorogenic sensor for metal ions in mixed aqueous solutions. Among all the metal ions tested, the compound 3 selectively senses Zn²⁺ ions. The imine bond of 3 gets cleaved by Zn²⁺ ions. Thus, 3 behave as ‘turn on’ fluorescent chemodosimeter for Zn²⁺ ions with limit of detction in micromolar range. Furthurmore, we demonstated that 3 can detect Zn²⁺ ions in cells of Allium cepa. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10895-016-1836-x

J Fluoresc
DOI 10.1007/s10895-016-1836-x
ORIGINAL ARTICLE
Benzimidazole Based ‘Turn on’ Fluorescent Chemodosimeter
for Zinc Ions in Mixed Aqueous Medium
Shilpa Sharma1 & Chullikkattil P. Pradeep1 & Abhimanew Dhir1
Received: 13 March 2016 /Accepted: 9 May 2016
#
Springer Science+Business Media New York 2016
Abstract Benzimidazole based compound 3 is designed and
synthesized. The compound 3 is evaluated as fluorogenic sen-
sor for metal ions in mixed aqueous solutions. Among all the
metal ions tested, the compound 3 selectively senses Zn2+ ions.
The imine bond of 3 gets cleaved by Zn2+ ions. Thus, 3 behave
as ‘turn on’ fluorescent chemodosimeter for Zn2+ ions with
limit of detction in micromolar range. Furthurmore, we
demonstated that 3 can detect Zn2+ ions in cells of Allium cepa.
tion has been focused on the development of fluorescent
chemosensors for the selective and efficient detection of these
biologically and chemically important species [7–10]. A fluo-
rescent molecule may show either fluorescence enhancement or
fluorescence quenching on exposure to a metal ion, both these
properties have been successfully utilized for the detection of
metal ions [11–16]. Chemosensors showing fluorescence en-
hancement on addition of analytes are preferred over
chemosensors showing fluorescence quenching. Fluorescence
quenching is unfavorable for a high signal output upon recog-
nition and hampers temporal separation of spectrally similar
complexes with time resolved fluorometry. On the other hand,
chemosensor showing fluorescence enhancement permits a
lower detection limit and high-speed spatial resolution [17, 18].
Recently, Benzimidazole based chromophores have re-
ceived much attention due to their applications in host-
guest system and in linear and nonlinear optics [19, 20].
Bezimidazole derivatives display large Stokes shifts and
high fluorescence quantum yields. These derivatives have
been reported for their application in laser dyes [21, 22],
polymer UV-light stabilizers [23], fluorescent probes for
labelling proteins and excellent photo luminescent mate-
rials [24]. In the present manuscript we designed and syn-
thesized new bezimidazole based compound 3 which be-
haves as ‘turn on’ chemodosimeter for Zn2+ (vide infra)
ions. Chemodosimeter is an irreversible approach of inter-
action of metal ion with the probe [25–29]. We also dem-
onstrated that compound 3 can detect Zn2+ ions in the cells
of Allium cepa. Our research group aims at development of
new fluorescent molecules for sensing of metal ions, anions
and molecules by various approaches [30–33]. We recently
reported sensing of sulfate ions by a composite of metal
organic framework and protein cytochrome c [34]. To the
best of our knowledge benzimidazole based ‘turn on’
chemodosimeter for Zn2+ ions is unprecedented.
.
.
Keywords Turn-on fluorescence sensor Chemodosimeter
.
Zinc sensing Confocal study in Allium cepa root tip cells
Introduction
Zinc is the second most abundant transition element in the
human body [1] required for various activities like cellular
metabolism, gene transcription, protein regulation and for nor-
mal growth and development [2]. It’s increase and decrease in
body can cause severe health problems including prostate
cancer, Alzheimer’s and Parkinson’s diseases [3]. Therefore,
the sensing of Zn2+ ions is highly important and significant.
Fluorescence spectroscopy is one of the highly selective,
sensitive and non-destructive technique and therefore most suit-
able tool to sense metal ion in trace amount [4–6]. Much atten-
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-016-1836-x) contains supplementary material,
which is available to authorized users.
* Abhimanew Dhir
abhimanew@iitmandi.ac.in
1
School of Basic Sciences, Indian Institute of Technology,
Mandi, Himachal Pradesh 175001, India

J Fluoresc
F
F
80
70
60
50
40
30
20
10
MeOH
Reflux
N
N
H2N
N
2+
10.0 equiv. of Zn ions
N
H
OH
N
H
OH
1
O
3
2
Scheme 1 Synthetic route for synthesis of 3
Material and Method
General Information
300
350
400
450
Wavelength (nm)
Fig. 2 Fluorescence emission spectrum of 3 (10 μM) upon addition of
10.0 equiv. of Zn2+ ions in THF: H2O (50:50, v/v) ratio, λex = 250 nm
5-Fluorosalicyladehyde was purchased from Sigma Aldrich.
2-Aminobenzimidazole was purchased from across organics.
All the solvents were of spectroscopic grade and were used
without further purification. HEPES buffer of pH 7.1 was
used. Further, nitrate and chloride salts of Na+, K+, Mg2+,
Al3+, Ca2+, Cr2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Fe3+,
Ag+, Cd2+, Hg2+ were used for spectroscopic studies.
Detection Limit
The detection limit of 3 towards zinc ions was determined
from the following Eq. 1:
Instruments
DL ¼ 3 Â σ=M
ð1Þ
All the fluorescence spectra were recorded on Agilent
Technologies Cary Eclipse fluorescence spectrometer. FT-IR
spectra was recorded on Agilent technologies cary 600 series.
1H and 13C NMR spectra were recorded on JEOL 500 MHz
NMR spectrometer using CDCl3 as solvent and TMS as inter-
nal standard. Data are presented as follows: chemical shift in
ppm (δ), multiplicity (s = singlet, d = doublet, br = broad
singlet, m = multiplet), coupling constant, J (Hz). Mass spec-
tra were recorded on Bruker HD compact instrument. The
fluorescence life time were measured by using LASER diode
(280 nm) from HORIBA instrument. Confocal images were
taken in Nikon eclipse Ti inverted microscope at 60× (oil
merging) using 401 nm laser.
Where M is the standard deviation of the blank solution
detected for 10 times, σ is the slope of the calibration curve.
Quantum Yield
Fluorescence quantum yield of 3 was determined by using 2-
aminopyridine (Φr = 0.60) in 1 N H2SO4 as standard. An
excitation wavelength of 250 nm from a xenon
spectroflurophotometer was used. The quantum yield was
800
700
600
500
400
300
200
100
0
Preparation of HEPES Solution of pH = 7.1
To prepare 1 L HEPES buffer of 7.1 pH add 11.92 g of
HEPES, 11.0 ml of 1 M NaOH and 89.0 ml of 1 M NaCl
and dilute it to 1 L by adding distilled water [35].
-100
Zn2+ Hg2+ Cd2+ Mg2+ Ni2+ Cr2+ Al3+ Fe3+ Co2+ K2+ Ca2+ Mn2+ Na2+ Cu2+ Ag+
Fig. 3 Blue colored bars represent percent fluorescence enhancement of
compound 3 (10 μM) in THF: H2O (50:50, v/v) after addition of 10.0
equiv. of different metal ions. The red colored bars represent the response
’
of Zn2+ ions in presence of other metal ions under similar condition. ‘I0
represent initial fluorescence intensity and ‘I’ represent the final fluores-
Fig. 1 X- ray crystal structure of 3
cence intensity after addition of Zn2+ ions

J Fluoresc
Fig. 4 1H NMR titration of 3
Peak at 10.24 ppm
with 1.0 equiv. of Zn2+ ions
3 +1.0 equiv. of Zn2+
3 +0.75 equiv. of Zn2+
3 +0.50 equiv. of Zn2+
3 +0.25 equiv. of Zn2+
3
Experimental
Synthesis
calculated by using Eq. (2), in which Φ(s) is the radiative
quantum yield of sample, Φr is the radiative quantum yield
of reference, Br and Bs are the (B = 1–10–AL) excitation inten-
sity of the reference and sample, respectively, Dr and Ds the
area of emission for reference and the sample, respectively, nr
and ns are the refractive indices of the reference and sample
solutions respectively.
5-Fluorosalicyladehyde (0.30 g, 2.14 mmol) was dissolved in
10 ml methanol and then an equimolar amount of 2-
aminobenzimidazole (0.28 g, 2.14 mmol) was added to it
and the reaction was kept on reflux for 24 h. The solvent
was removed by the vacuum and the crude was put on
recrystallisation in chloroform: methanol (1:1) mixture. After
Equation 2:
2
Φs ¼ ðBr=BsÞðDs=DrÞðns=nrÞ Φr
ð2Þ
Fig. 5 Mass spectra of 3 in
presence of 10.0 equiv. of Zn2+
ions
[3+H]+
[1+H2O+K]+
[3-H++Zn2++H2O+H]+
[1+H2O+ Na]+
[2]+

J Fluoresc
H
10000
1000
100
O H
IRF
Ligand
F
F
3
3
2+
Ligand
+Zn
N
N
N
N
N
H
OH
N
H
O
Zn2+
3
F
+
O
F
H
H
H
O
-H+
1
2
3
N
N
Life time (ns)
N
N
N
H
N
H
O
O
Fig. 8 Fluorescence lifetime profiles of 3 in THF: H2O (50:50, v/v) in
Zn2+
Zn2+
absence and presence of Zn2+ ions
F
H
F
H
O
O
N
N
NH
NH
N
H
O
N
H
O
Zn2+
Table 1 Fluorescence lifetime data of 3 in presence and absence of
Zn2+
Zn2+ ions
Quantum yield (φ) τ (ns) kr (108 × s−1
)
knr (108 × s−1
)
H+
F
3
0.011
1.23 0.089
1.60 1.68
8.041
4.57
N
3 + Zn2+ 0.27
H2N
N
H
OH
O
1
2
(ArCH), 119.55 (ArCH), 118.65 (ArCH), 118.50 (ArCH),
118.31 (ArCH), 110.84(ArCH); MS-m/z 255.98 (M)+.
Fig. 6 Proposed mechanism of metal assisted hydrolysis of imine bond
1 day yellow color crystals were obtained which were charac-
terized by single crystal X-ray analysis, H NMR spectrum
Cell Imaging
1
and mass spectra. Scheme 1 Yield 77 %, m.p. 220 °C; IR;
Allium Cepa Root Tip Cell Imaging Allium cepa bulbs were
kept in distilled water for 1 day so that new sprouted roots
should come. After 3–4 days the bulb with best root growth
was taken. Then 5–8 mm long tips of roots were taken and
were put onto a 2 watch glass. One set of roots was treated
with compound 3 and another set of roots was treated with
Zn2+ ions and were incubated for 40 mins. Then the latter sets
1
vmax = 1614 cm−1 (C = NH); H NMR (500 MHz, CDCl3):
δ (ppm) = 7.01 (m, 1H, J = 15 Hz, ArH), 7.20 (m, 2H,
J = 25 Hz, ArH), 7.31 (m, 2H, J = 15 Hz, ArH), 7.45 (d, 1H,
J = 5 Hz, ArH), 7.75 (d, 1H, J = 10 Hz, ArH), 9.23 (br, 1H,
NH), 9.55 (s, 1H, C = NH), 12.13 (br, 1H, OH); 13C NMR
(500 MHz, CDCl3): δ (ppm) = 166.39 (C = NH), 157.80
(ArCH), 156.73 (ArCH), 154.83 (ArCH), 152.36 (ArCH),
123.62 (ArCH), 123.28 (ArCH), 122.38 (ArCH), 122.19
90
80
70
60
50
40
30
20
3a
3b
H
NH
F
F
N
O
H
N
H
N
N
O
N
H
O
H
O
0
5
10
15
20
25
30
Zn2+
Zn2+
Time (in minutes)
Fig. 7 Proposed intermediate (3a) & (3b) where metal ion assists to
bring water molecule near to the imine bond of compound 3
Fig. 9 Fluorescence response time of 3 in THF: H2O (50:50, v/v) upon
addition of 10.0 equiv. of Zn2+ ions into it

J Fluoresc
Fig. 10 Confocal images of
Allium cepa root tip cells treated
with (a) 3 (10 μM) in THF: H2O
(50:50, v/v) and (b, c) Zn2+
followed by 3
of cells were incubated with compound 3 for more 40
mins. Then both types of cells were washed with dis-
tilled water several times. The root tips were then ex-
posed to 2–3 mL of acetic acid: ethanol (1:3, v/v) for 5
mins. After that the solution was disposed off with the
help of dropper and the root tips were washed with
water several times again. These roots were then mac-
erated in 1 N HCl at 60 (0 °C) for 2–3 min. to soften
the tissue and later on were washed again with distilled
water for three time. Later on, root tips were placed on
glass slide and covered with cover slips. Then slide
were studied with the help of confocal microscope.
To have an insight into the mechanism for ‘turn-on’ behav-
iour of 3 towards Zn2+ ions, we performed 1H NMR titrations.
We recorded NMR of 3 after consecutive additions of Zn2+
ions. On addition up to 1.0 equiv. of Zn2+ ions, a new peak is
generated at 10.24 ppm which might be due to the proton of
aldehyde group (5-fluorosalicyladehyde in this case) (Fig. 4).
For further confirmation we also performed fluorescence stud-
ies of 2-aminobenzimidazole (2) and 5-fluorosalicyladehyde
(1) under similar conditions. 2-Aminobenzimidazole (2)
showed an emission band at 318 nm similar to the band ex-
hibited by 3 in presence of Zn2+ ions (see ESI, S8) whereas
weak fluorescence emission was observed by 5-
fluorosalicyladehyde (1) (see ESI, S9). These results suggest
the hydrolytic cleavage of imine bond on addition of Zn2+ ions
to the compound 3. To confirm our hypothesis, we further per-
formed mass spectrum analysis. The mass spectroscopic analy-
sis of 3 + Zn2+ shows the presence of number of multiple species
in solution at 133.09 [2]+, 181.07[1 + H2O + Na]+, 197.07 [1 +
H2O + K]+, 256.08 [3 + H]+ which also supports the cleavage of
imine bond of 3 on addition of Zn2+ ions (Fig. 5).
Hydrolytic cleavage of the C = N bond is well documented
in the literature and has been successfully used for the devel-
opment of chemodosimeters for the detection of metal ions
[39–41, 31]. Different mechanisms for the hydrolytic cleavage
of the imine bond have been proposed. One of the proposed
mechanisms for metal catalyzed reactions involves the attach-
ment of the metal ion and water molecule to the imine nitrogen
and imine carbon respectively through partial bonds. In one
other proposed mechanism, the role of the metal is to activate
the attacking H2O molecule bringing it closer to the electro-
philic C = N bond [42, 43, 31].
Results and Discussion
Compound 3 was synthesized in a good yield by condensation
of 5-fluorosalicyladehyde and 2-aminobenzimidazole
(Scheme 1). The structure of 3 was confirmed by X-ray anal-
ysis (Fig. 1)1, 1H NMR and mass spectrum analysis (See ESI,
S2-S6).
The fluorescence spectrum (Φ = 0.011) [36] of 3 (10 μM) in
H2O:THF (50:50 v/v, λex = 250 nm) shows weak emission at
318 nm. To explore the metal sensing ability of 3, we per-
formed its fluorescence titration experiments with 15 different
metal ions including alkali, alkaline and transition metals viz.,
Na+, K+, Mg2+, Al3+, Ca2+, Cr2+, Mn2+, Co2+, Ni2+, Cu2+, Ag+,
Zn2+, Fe3+, Cd2+ and Hg2+. The ‘turn on’ emission (Φ = 0.27)
[36] was observed on addition of only Zn2+ ions (10.0 equiv.)
(Fig. 2). The detection limit [37] with Zn2+ ions was found to
be 1.4 μM (see ESI, S7). From physiological view, selec-
tivity is an important parameter, therefore the change in
the fluorescence was investigated by addition of other
competitive ions. The detection of Zn2+ ions by 3 is
slightly perturbed only by Hg2+ ions, however no sig-
nificant change was observed in presence of any other
metal ion (Fig. 3). More interestingly the compound 3
can selectively sense Zn2+ over Hg2+ and Cd2+ which
are having same d10 configuration [38].
Thus, On the basis of mass spectrum analysis, we propose
that the metal assisted hydrolytic cleavage of imine bond (Fig. 6)
by formation of any of the two intermediates 3a & 3b [mass
spectrum peak at 337.19 (3-H++Zn2+ + H2O + H)+] as shown in
Fig. 7 (see Fig. 5).
We also carried out the fluorescence life time measurement
of 3 in absence and presence of Zn2+ ions. The compound 3
exhibited tri-exponential decay suggesting the presence of
three emitting species τ1, τ2 and τ3 with an average life time
of1.23 ns. The addition of 10.0 equiv. of Zn2+ ions to the 3
results in tri-exponential decay with an average life time of
1 CCDC 1413707

J Fluoresc
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1.60 ns (Fig. 8). Further we calculated radiative as well as non-
radiative rate constant (equation τ−1 = kr + knr) [44]. The
radiative rate constant kr for 3 was observed higher in presence
of Zn2+ ions as compared to in absence of Zn2+ ions (Table 1).
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factor. Therefore we investigated the estimated time for the
reaction of 3 with Zn2+ ions. We recorded fluorescence spectra
of 3 at different time intervals up to 22 min. after addition of
10.0 equiv. of Zn2+ ions. We observed a constant increase in
the emission spectra and after that it reaches a plateau (Fig. 9).
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we performed confocal microscopic studies of 3 with Zn2+
ions in root tip cell of Allium cepa. Two set of cells were taken.
One set of cells was treated with only 3 and other was treated
with Zn2+ ions (40 mins. of incubation) followed by 3. After
incubation of more 40 mins the cell imaging was done by
confocal fluorescence microscope. A strong blue fluorescence
was observed in cells where solution of 3 was added followed
by Zn2+ ions, whereas no fluorescence was observed in cells
with compound 3 (Fig 10). Therefore, 3 is capable of sensing
imaging Zn2+ ions in plant cells.
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To conclude, we have synthesized a new benzimidazole based
compound 3 which can sense Zn2+ ions in mixed aqueous
media. The probe 3 could selectively sense Zn2+ ions in mi-
cromolar range through chemodosimetric approach resulting
in ‘turn on’ fluorescence. It can also detect zinc in the root tip
cells of Allium cepa.
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