Benzimidazole and benzothiazole conjugated Schiff base as fluorescent sensors for Al3+ and Zn2+

Article abstract of DOI:10.1016/j.jphotochem.2019.111947

Two benzimidazole/benzothiazole based azomethines, (E)-2-(1H-benzo[d]imidazol-2-yl)-4-(4-(diethylamino)-2-hydroxybenzylideneamino)phenol (HBZA) and (E)–2-(benzo[d]thiazol–2-yl)–4-(4-(diethylamino)–2-hydroxybenzylideneamino)phenol (HBTA) were designed and

Full text of DOI:10.1016/j.jphotochem.2019.111947

Accepted Manuscript
Title: Benzimidazole and benzothiazole conjugated Schiff
3
+
2+
base as fluorescent sensors for Al and Zn
Authors: G.R. Suman, S.G. Bubbly, S.B. Gudennavar, V.
Gayathri
PII:
S1010-6030(19)30790-7
DOI:
Article Number:
https://doi.org/10.1016/j.jphotochem.2019.111947
111947
Reference:
JPC 111947
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
11 May 2019
17 June 2019
19 June 2019
Please cite this article as: Suman GR, Bubbly SG, Gudennavar SB, Gayathri V,
Benzimidazole and benzothiazole conjugated Schiff base as fluorescent sensors for
3
+
2+
Al and Zn , Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.111947
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Benzimidazole and benzothiazole conjugated Schiff base as
3+
2+
fluorescent sensors for Al and Zn
a
a*
a
b
G. R. Suman , S. G. Bubbly , S. B. Gudennavar and V. Gayathri
a
Department of Physics and Electronics, CHRIST (Deemed to be University), Bengaluru-560 029, Karnataka,
India.
Department of Chemistry, Bangalore University, Central College Campus, Bengaluru-560 001, Karnataka, India.
b
*Correspondence to: S. G. Bubbly (e-mail: bubbly.sg@christuniversity.in)
Graphical abstract
Highlights




Synthesis and photophysical behaviour of benzimidazoles /benzothiazoles.
These fluorophores exhibit both ESIPT and AIE behaviour.
3+
2+
Found to be sensitive and selective to Al and Zn metal ions.
Large Stokes shift and high sensitivity makes them potential candidates for the detection of
3+
2+
Al and Zn .
Abstract
Two benzimidazole/benzothiazole based azomethines, (E)-2-(1H-benzo[d]imidazol-2-yl)-4-
(4-(diethylamino)-2-hydroxybenzylideneamino)phenol (HBZA) and (E)–2-(benzo[d]thiazol–
2
-yl)–4-(4-(diethylamino)–2-hydroxybenzylideneamino)phenol (HBTA) were designed and
synthesised. Investigations of solvatochromic behaviour of these fluorophores in solvents of
varying polarities showed large Stokes shift of 134 - 210 nm. Time resolved Laser induced
fluorescence measurements revealed the excited state dynamics of the fluorophores. Molecules
were also found to be emissive in aggregated state as seen from the aggregation induced
emission studies. Appreciable absorption and emission spectral changes upon co-ordination of
3
+
2+
3+
HBZA with Al /Zn and HBTA with Al , as well as good sensitivity and selectivity,
1

indicated their capability of detecting the two analytes. The binding stoichiometry was
determined using electrospray ionization mass spectrometry (ESI-MS) and the binding
mechanism was studied using density functional theory.
Key words: ESIPT, Schiff, Benzimidazole, AIE
1
. Introduction
Fluorescent organic molecules having the characteristic properties of large Stokes shift, quick
photo-response and appreciable photostability are ideal candidates for the fabrication of
OLEDs, probes for recognition of chemical species, drug delivery applications etc. [1-5].
Understanding the photophysical and photochemical behaviour of such systems can help gain
insight into their potential technological capabilities. The family of 2-(2′-hydroxyphenyl)
benzimidazole, benzoxazole, benzothiazole and benzotriazole are known for their excited state
intramolecular proton transfer (ESIPT) photo-tautomers with large Stokes shift [6-9].
Heterocyclic benzimidazole and benzothiazole derivatives have proved to be very good
acceptors in DᴨA structures due to their electron withdrawing nature [10, 11]. Their
molecular structures can be tailored through conjugation with proton donating moieties for
larger Stokes shift as well as high quantum yield [12, 13]. Fluorescence and self-assembling
properties of benzimidazole/benzothiazole derivatives have been exploited for their
pronounced fluorescence through aggregation induced emission (AIE), a phenomenon having
applications in bioimaging, optoelectronics and chemosensors [14, 15]. Moreover, organic
fluorescent molecules featuring large Stokes shift, quantum yield and binding multiple analytes
with effective sensitivity and selectivity are explored as chemosensors for metal ion sensing
[
16-20].
In recent years, chemosensors for biologically and environmentally important species have
3

gained much attention. Among the various metal ions, aluminium (Al ) is the third most
abundant element in the earth’s crust and because of its diverse applications in our everyday
life, there are myriad ways in which humans are exposed to aluminium. Accumulation of Al
2

in the body can cause anaemia, neural and Alzheimer’s disease [21, 22]. However, zinc (Zn )
is a biologically indispensable trace element playing significant structural, catalytic and
2

regulatory roles in physiological processes [23-25]. Any imbalance in Zn causes various
health issues as the deficiency or excess of it would lead to undergrowth, diarrhoea, Parkinson’s
disease and other neurological disorders like Alzheimer’s, amyotrophic lateral sclerosis and
3+
2+
epilepsy [26, 27]. Hence monitoring Al and Zn is of great concern. A combination of two
or more emission phenomena such as intramolecular charge transfer (ICT) - photoinduced
electron transfer (PET), ICT-ESIPT, ESIPT-AIE can be used to obtain multiple signals to sense
various analytes simultaneously [28]. In the present work, we have investigated the
fluorescence behaviour of two systems with dual ESIPT sites and diethyl amine moiety which
can act as charge transferring unit. We have synthesized (E)-2-(1H-benzo[d]imidazol-2-yl)-4-
(4-(diethylamino)-2-hydroxybenzylideneamino)phenol (HBZA) and (E)-2-(benzo[d]thiazol-
2
-yl)-4-(4-(diethylamino)-2-hydroxybenzylideneamino)phenol (HBTA) and studied their
2

ESIPT-AIE properties. Furthermore, HBZA was capable of detecting Al3 and Zn with
2
3

micromolar sensitivity at two different wavelengths and HBTA could sense Al , over other
metal ions. Computational analysis was carried out using Gaussian 09 software to understand
the sensing mechanism [29].
2. Experimental
2
.1 Materials and methods
All solvents (HPLC grade) were used as procured from Alfa Aesar. Chemicals were purchased
from Sigma Aldrich. UV-Visible absorption and fluorescence emission spectra were recorded
using Shimadzu UV-1800 and Shimadzu RF-5301 PC spectrometers respectively.
Fluorescence emission was recorded at the excitation and emission slit widths of 3 nm.
Absolute quantum yields were measured using calibrated integrating sphere incorporated in
Horiba Jobin Yvon Fluoromax-4 spectrofluorimeter. Time resolved fluorescence decay
measurements were carried out using Horiba Jobin Yvon Fluorocube -01-NL Fluorescence
Life time System. Photons with 10 MHz repetition rate were generated using LASER (D-405L
for HBZA) and LED (DD-340) excitation sources and DAS6 software was used to analyse the
data. Dynamic light scattering measurements for particle size determination were performed
1
on Brookhaven ZetaPALS potential analyser. H NMR spectra were recorded on Bruker
AC400 spectrometer in DMSO-d . LC-MS data were obtained using Shimadzu spectrometer
6
equipped with Ascentis Express C18 (50 mm × 2.1 mm × 2.7 μm) column with mobile phase
A: 0.1% formic acid in water and mobile phase B: ACN. ESI-MS data for understanding
binding stoichiometry was recorded on Agilent technologies LC-1200 series and MS-6130
Quadrapole with C18 (4.6 mm  0.75 mm) column using mobile phase A:0.1% formic acid in
water and mobile phase B: ACN. Theoretical calculations using density functional theory
(DFT) were carried out at B3LYP/6-31G(d,p) levels for C, H, N, O and S atoms and LanL2dz
for metal ions using Gaussian 09 software.
2
.2 Synthesis of 4-amino-2-(1H-benzimidazol-2-yl)phenol (HBZ) and 4-amino-2-
benzothiazol-2-yl)phenol (HBT)
(
HBZ and HBT were synthesized according to the procedure reported by Hein et al. [30]. HBZ
was prepared by heating equimolar ratio of 1,2-phenylenediamine (1.08 g, 0.01 mol) and 5-
amino-2-hydroxybenzoic acid (1.37 g, 0.01 mol) at 160 °C for 8 hours in 3.5 mL
polyphosphoric acid. After the completion of the reaction (as indicated by the TLC analysis
of small aliquot) the reaction mixture was quenched in ice and neutralized using 10% Na
2
CO
3
solution. Excess base was removed by washing with ice-cold water. The precipitate was dried
overnight and recrystallized in methanol to obtain 4-amino-2-(1H-benzimidazol-2-yl)phenol
(HBZ). Similarly, 4-amino-2-(benzothiazol-2-yl)phenol (HBT) was synthesized by using 2-
aminothiophenol.
3

2
.3 Synthesis
hydroxybenzylideneamino) phenol (HBZA) and (E)-2-(benzo[d]thiazol-2-yl)-4-(4-
diethylamino)-2-hydroxybenzylideneamino)phenol (HBTA)
of
(E)-2-(1H-benzo[d]imidazol-2-yl)-4-(4-(diethylamino)-2-
(
HBZA and HBTA were obtained from HBZ and HBT respectively, by adopting the following
synthesis procedure. Progress of reactions were monitored by TLC analysis of reaction
mixtures at regular intervals.
HBZ (100 mg, 0.44 mmol) was refluxed with 4-(diethylamino)-2-hydroxybenzaldehyde (85
mg, 0.44 mmol) in 10 mL methanol for 8 hours. Yellow solid obtained was purified by column
chromatography
to
yield
(E)-2-(1H-benzo[d]imidazol-2-yl)-4-(4-(diethylamino)-2-
1
hydroxybenzylideneamino) phenol (HBZA) (Scheme 1; X=NH). Yield: 150 mg, 81%. H
NMR (SFig. 1a) δ in ppm: 13.21 (s, 1H), 8.75 (s, 1H), 8.10 (d, 2H), 7.69 (s, 1H), 7.48-7.30 (m,
3
1
H), 7.10 (d, 8.8 Hz, 1H), 6.35 (d, 8.8 Hz, 1H), 6.08 (dd, 2.4 Hz, 1H), 3.41 (dd, 7.2 Hz, 4H),
.359 (s, 1H), 1.17 (t, 6H). LC-MS (SFig. 2a): Calc. for C20 :400.2; found: 400.0.
H
15
N
3
O
2
HBT (100 mg, 0.41 mmol) was refluxed with 4-(diethylamino)-2-hydroxybenzaldehyde (79
mg, 0.41 mmol) in 10 mL Tetrahydrofuran (THF) for 8 hours. Reaction mixture was
concentrated on water bath and the product obtained was purified by column chromatography
which yielded green solid of (E)-2-(benzo[d]thiazol-2-yl)-4-(4-(diethylamino)-2-
1
hydroxybenzylideneamino)phenol (HBTA) (Scheme 1; X=S). Yield: 155 mg, 87%. H NMR
(SFig. 1b) δ in ppm: 13.79 (s, 1H), 13.38 (s, 1H), 9.85 (1s, 1H), 8.20 (d, 2H), 7.52-7.30 (m,
4
C
H), 6.88 (d, 8.4 Hz, 1H), 6.01 (dd, 0.8 Hz, 2H), 3.559 (dd, H) LC-MS (SFig. 2b): Calc. for
+
20
H
15
N
3
O
2
[M+H ]: 420.2 Found: 420.2.
Scheme 1. Synthesis of HBZA and HBTA
.4 UV-Vis absorption and fluorescence studies
2

5
Absorption and emission spectra were recorded in solvents (10 M) of varying polarities for
HBZA and HBTA. Fluorescence sensing measurements were performed by dissolving HBZA

4
4
and HBTA in DMF/water (10 M in 9:1 v/v) and THF/water (10 M in 9:1 v/v) mixed

3
solvents respectively. 10 M aqueous metal ion solutions were prepared using nitrate salts of
K, Zn, Sr, Ni, Ba, Al, Pb, and chloride salts of Mn, Mg, Hg, Cu and Cd. pH adjustments for
ion sensing were carried out using buffer solutions of Na
2
CO
3
and H
3
PO . Effect of
4

4
competitive ions on absorption and emission spectra were recorded using 10 M solutions of
fluorophores and 10 M solutions of interfering ions. Analyte solutions (104 M) were
prepared at 7.2 pH in water. Detection limit and binding constants were obtained from
3
4

fluorescence spectral measurements of 10 M HBZA and HBTA in presence of varying
4

5
concentrations, (0 - 30) × 10 M, of analytes. Limit of detection was calculated according to (3/s),
where  the standard deviation of blank sample and s is slope of the fluorescence emission intensity
versus metal ion concentration. Binding constants were determined according to modified Benesi-
Hildebrand equation (Supplementary information) according to the reported procedure [22, 31, 32].
Aggregation induced emission studies were carried out for HBZA in DMF/water and HBTA

3
in THF/water. Stock solutions of HBZA and HBTA were prepared at 10 M concentration
in 5 mL DMF and THF respectively and further diluted by adding varying water fractions (f
w

4
=
(
0 to 90%) to get 10 M solutions. Aggregation was confirmed by dynamic light scattering
DLS) method and average diameter of the particles was determined using Brookhaven
ZetaPALS potential analyser.
3
. Results and discussion
3
.1 UV-visible absorption and fluorescence spectroscopic studies
Absorption spectra of HBZA (Fig. 1a) showed variation in absorption wavelength depending
on the hydrogen bonding ability of the solvent. However, there was not much variation
observed for HBTA, except in THF and dichloromethane (DCM), where the absorption
wavelengths showed a red shift of 31 and 32 nm respectively with respect to methanol (Fig.
2
a). This indicates that HBZA is more sensitive to solvent environment than HBTA, in ground
state. Interestingly fluorescence emission in HBZA (Fig. 1b) has only single peak in all the
solvents. This was also supported by excited state life time measurement in
5

Fig. 1. a) Absorption and b) fluorescence emission spectra of HBZA in various solvents
methanol. Time resolved fluorescence decay measurement of HBZA excited at 405 nm and
monitored at 500 nm showed a single species with life time of 3.47×10− s (77%) (SFig. 3).
HBZA and HBTA in water have also showed similar variations in Stokes shift as exhibited in
polar solvents. Steady state fluorescence spectra in case of HBTA exhibited dual emissions
11
(Fig. 2b) along with a large Stokes shift of 210 nm in THF (STable 1). Another interesting
feature of HBTA is that its emission spectra at 586 nm in THF and DCM have shown
yellowish-orange, faint and bright emissions respectively, suggesting colour tunability of
6

HBTA. Intense emission with large Stokes shift in non-polar solvent like DCM and the large
Stokes shift even in hydrogen bonding solvent like water suggests the ICT- coupled ESIPT
emission characteristics of HBTA. The life time measurements with excitation wavelength of
45 nm also showed dual exponential decay with life time of 1.28×10 11 s (75%) for the peak

3
at 436 nm (SFig. 4a) and 5.99 ns (36%) and 0.11 ns (37%) when monitored at 506 nm (SFig.
b). HBZA showed more quantum yield of 0.4 compared to that of HBTA (0.09). This was
also supported by large non- radiative constant value of HBTA compared to HBZA (STable
). Charge distribution on frontier molecular orbitals (FMOs) calculated using DFT can be
4
2
used to understand the nature of transition from ground to excited state. FMOs such as LUMO
and LUMO+1 level in case of HBZA were formed mainly due to the delocalized charges,
however for HBTA charges are localized on benzothiazole in LUMO and on diethylamino
salicylidene unit in LUMO+1 level. HOMO levels in both HBZA and HBTA were contributed
by phenyl-diamino salicylidene (Figs. 3 and 4). These studies indicate that charge transfer
characteristics are more dominant in HBZA.
3.2 Aggregation induced emission studies
HBZA showed decrease in absorbance (SFig. 5a) and increase in intensity of emission (Fig. 5a)
with successive addition of water to THF. The emission intensity at 506 nm reached a maximum
(
104-fold enhancement) at 70% f
showed similar variation in absorption (SFig. 5b). The fluorescence intensity at 587 nm showed 8-
fold enhancement for 90% f compared to 0% f (Fig. 5b) with negligible red shift (3 nm). Average
particle diameter was determined using ZetaPals analyser for 60 - 80% f solutions of HBZA and
0 - 90% f solutions of HBTA (STable 3). Enhancement in emission in the aggregated state may
w
, with a blue shift of 23 nm with respect to THF. HBTA also
w
w
w
7
w
be attributed to restriction of - stacking due to the twisted structure of HBZA and HBTA
observed from DFT optimization (SFig. 6).
3.3 Absorption and fluorescence sensing of HBZA and HBTA in different metal ions
Optical response of HBZA towards various metal ions showed notable changes in absorption
2

3
wavelength for Zn and Al compared to other ions (Fig. 6a). Binding of analyte with HBZA was
2

3
evident from the 10 nm red shift for Zn (Fig. 6a) and 25 nm blue shift for Al (Fig. 6a) complexes.
In case of fluorescence emission, a blue shift of 46 nm with 19-fold enhancement in intensity at 495
2+
3
nm was observed for HBZA- Zn complex (Fig. 6b). Interaction of Al with HBZA resulted in a
2 nm blue shift in emission with a 7-fold increase in intensity at 479 nm (Fig. 6b). Binding
stoichiometry was obtained based on Job’s plot analysis by using fluorescence emission spectra.
6
2

3+
Individual addition of different mole fractions (0 - 9) of Zn and Al to HBZA showed a binding
ratio of 0.5:1 for Zn : ligand (SFig. 7a) and 2:1 for Al : ligand (SFig. 7b). Binding stoichiometry
2+
3+
2+
was also confirmed by ESI-MS spectral analysis. ESI-MS spectrum of Zn :HBZA showed m/z
+
2+
at 526.7 due to [Zn + HBZA + NO ] , calculated: 526.1, suggesting 1:1 stoichiometry for Zn -
3
3+
HBZA (SFig. 8a). For Al -HBZA, ESI-MS exhibited m/z at 640.1 corresponding to the mass of
+
[
Al
2
+ HBZA + (NO
3
)
2
+ HCOOH + H
2
O] , calculated: 640.13, suggesting 2:1 binding ratio for
3+
Al -HBZA (SFig. 8b).
7

Fig. 2. a) Absorption and b) fluorescence emission spectra of HBTA in various solvents
8

Fig. 3. Frontier molecular orbitals of HBZA
3

HBTA also exhibited absorption and emission spectral response for Al different from all other
ions. Complex formation resulted in 28 nm blue shift in absorbance (Fig. 7a) and a large
hypsochromic shift of 82 nm in fluorescence emission with a 3-fold enhancement in emission
3+
intensity at 503 nm (Fig. 7b). Job’s plot for HBTA showed a binding ratio of 1.5:1 for Al : ligand
+
(
SFig. 7c). ESI-MS spectrum showed ion peak at 638.7 due to [Al
2
+ HBTA + (NO
3
)
2
+ HCOOH] ,
3+
calculated: 639.08, suggesting 2:1 binding stoichiometry for Al :HBTA (SFig. 8c).
Fig. 4. Frontier molecular orbitals of HBTA
The sensing potential of HBZA was studied by titration of 10 M receptor against varying

4
2

3
concentrations of Zn and Al and the limits of detection (LOD) were found to be 0.21 µM (Fig.
8
a) and 0.11 µM (Fig. 8b) respectively. Sensitivity studies on HBTA showed a detection limit of
3

0.87 µM (Fig. 8c) for Al . Metal ion sensing ability for both the fluorophores were very much
3+
2+
below the permissible level of Al and Zn ions in drinking water as specified by WHO [22, 33,
4]. HBZA and HBTA provide better sensitivity for the respective metal ions when compared to
the analogues of these fluorophores [35-37] suggesting the importance of HBZA and HBTA for
3
2
1
2
sensing applications. Binding constants (K ) were found to be 3.42×10 M for Zn -HBZA (Fig.
a
8
2
3
8
2
3
9a), 2.25×10 M for Al -HBZA (Fig. 9b) and 2.15×10 M for Al -HBTA (Fig. 9c).
9

Fig. 5. Fluorescence emission of a) HBZA and b) HBTA in various water fractions (0 - 90%)
1
0

Fig. 6. a) Absorption and b) emission spectra of metal ions with HBZA
3.3.1 Interference effect of competitive metal ions
Effect of competitive ions on the emission from complexes was evaluated by recording emission
2+
3+
spectraof1 equivalent of Zn andAl inHBZAand addinga higherconcentration (10 equivalents)
2

3
of competitive ions. Turn on emissions of HBZA-Zn (Fig. 10) and Al complex (Fig. 11) being
1
1

unaltered in the presence of competitive ions (except for a moderate response from strong quencher
2
+
2
3
3
Cu ), indicated that HBZA can be utilized for sensing Zn and Al . In the case of HBTA-Al ,
none of the competitive ions hampered the emission, indicating HBTA to be highly selective
3

towards Al ions (Fig. 12).
Fig. 7. a) Absorption and b) emission spectra of HBTA in presence of various metal ions
1
2

1
3

2
3
3
Fig. 8. Sensitivity of HBZA towards a) Zn and b) Al and c) HBTA towards Al
.4 Computational studies using Gaussian 09
DFT calculations were carried out to study the sensing mechanism. Conjugation of HBZA and
3
3

HBTA with Al resulted in the reduction in HOMO and LUMO levels, whereas it was
2

increased in case of Zn -HBZA (Table 1). Complex formation was also evident from the
reduced band gap of the complex compared to the ligands. Charge distribution pattern in the
complex in HOMO and LUMO levels were obtained from frontier molecular orbitals analysis.
3

In HBZA-Al complex, nitrogen on the Schiff base acts as the binding site resulting in the
suppression of ESIPT. This was supported by the delocalized charge distribution observed in
2

LUMO level (SFig. 9a). In case of HBZA-Zn , oxygen and nitrogen atoms on salicylidene
2

ring have acted as platforms for chelation of Zn . While the contribution for HOMO level
was from the entire molecule, as observed from the delocalized charges, the LUMO level was
due to charge transfer from the ligand to metal (LMCT) bound to salicylidene unit (SFig. 9b).
3

In HBTA-Al , the charge transfer was observed from the salicylidene ring (HOMO) to the
3

benzothiazole unit (LUMO) (SFig. 9c). Co-ordination of Al with HBZA has manifested in
2

a slightly twisted structure. Zn -HBZA complex exhibited a more planar structure (which
3

supports the red shift observed in absorption wavelength). HBTA-Al complex also exhibited
a slightly twisted conformation compared to HBTA (Table 2).
1
4

1
5

2
3
3
Fig. 9. Benesi-Hildebrand plot for a) HBZA-Zn , b) HBZA-Al and c) HBTA-Al
2
Fig. 10. Selectivity of HBZA for Zn in presence of competitive ions.
3
Fig. 11. Selectivity of HBZA for Al in presence of competitive ions.
1
6

3
Fig. 12. Selectivity of HBTA for Al in presence of competitive ions.
Table 1. Energy level comparison obtained from DFT calculations
Molecule
HOMO (eV) LUMO (eV) Bandgap (eV)
HBZA
5.55
3.09
5.94
4.97
3.0
1.57
1.27
2.96
1.66
1.65
3.98
1.82
2.98
3.31
1.35
HBZA-Al3+
HBZA-Zn2+
HBTA
HBTA-Al3+
Table 2. Dihedral angles (°) obtained using DFT
Compound
Ligand dihedral Dihedral angle with Dihedral angle with
3
+
2+
angle
Al
Zn
HBZA
177.322
173.187
179.530
(C15-N21-C22-C29)*
HBTA
C15-N21-C22-C24)*
Refer scheme 1
177.245
174.737
---
(
*
4. Conclusions
1
7

Fluorophores, HBZA and HBTA, exhibiting large Stokes shift and aggregation induced emission
behaviour were designed and synthesized by reflux reaction from HBZ and HBT. The
solvatochromic interaction was studied to understand sensitivity of probes for various polarities of
the solvents. DFT studies and solvatochromic studies showed that HBZA followed ICT
characteristics whereas ESIPT was more probable in HBTA. The probes were found to exhibit
aggregation induced emission. Metal ions sensing studies suggest that HBZA was able to detect
3

2
3+
Al and Zn . Spectral response of HBTA towards various ions showed turn on emission for Al
with no interaction with any other ion. Both the fluorophores were found to be very much sensitive
towards the metal ions with sensitivity very much lower than that specified by WHO. The probes
were also more selective towards the analytes in presence of competitive ions.
Acknowledgements
The authors would like to thank the anonymous reviewers for their valued comments and
suggestions, which greatly helped in improving the quality of the manuscript.
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