Template-Assisted Regioselective Identification of Metal Ions on Coumarin-Furan Conjugated Chemosensors: AIEE Effect and Photo-Switching pH Indicator by ICT

Article abstract of DOI:10.1002/ejic.202000583

A coumarin-furan conjugated Schiff base chemosensor (R1) of straightforward design showed an excellent aggregation-induced emission enhancement effect in ACN-H₂O mixtures of different ratios, which was confirmed by field emission scanning electron microscopy (FESEM) and supported by Dynamic Light Scattering (DLS). R1 identified three metal ions (Pb²⁺, Hg²⁺ and Cu²⁺) in colorimetric and fluorometric techniques with an intramolecular charge transfer (ICT) off-on mechanism. The space specificity of the metal ions (Hg²⁺ and Cu²⁺) resulted in hydrolysis of -CH=N- functionality of the sensor. The latter also exhibited excellent sensitivity in acidic conditions by blocking ICT and remained stable in neutral to basic pH. Cell viability and microscopic fluorescence experiments revealed that R1 is cytocompatible and may be used for the detection of Pb²⁺, Hg²⁺, and Cu²⁺ ions in living cells.

Full text of DOI:10.1002/ejic.202000583

European Journal of Inorganic Chemistry
Accepted Article
Title: Template-assisted regio-selective identification of metal ions
on coumarin-furan conjugated chemosensors; AIEE effect and
photo-switching pH indicator by ICT
Authors: Ashish Kumar, Sumit Kumar Hira, and Swapan Dey
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000583
Link to VoR: https://doi.org/10.1002/ejic.202000583
01/2020

10.1002/ejic.202000583
European Journal of Inorganic Chemistry
FULL PAPER
Template-assisted regio-selective identification of metal ions on
coumarin-furan conjugated chemosensors; AIEE effect and
photo-switching pH indicator by ICT
Ashish Kumar,[a #] Sumit Kumar Hira,[b] and Swapan Dey*[a]
[a]
Dr. Ashish Kumar and Dr. Swapan Dey
Department of Chemistry,
Indian Institute of Technology (ISM), Dhanbad
Dhanbad, Jharkhand-826004, India.
E-mail: swapan@iitism.ac.in; Fax: +91 326 2296563; Tel: +91 326 2235607.
# Presently working as a Research Associate in the Department of Chemistry
University of Delhi, Delhi, India
[b]
Dr. Sumit Kumar Hira
Department of Zoology
University of Burdwan, Burdwan
Burdwan, West Bengal 713104, India
Supporting information for this article is given via a link at the end of the document.
Abstract: A coumarin-furan conjugated Schiff base chemosensor
(R1) of straightforward design showed an excellent aggregation-
induced emission enhancement effect in ACN-H2O mixtures of
different ratios, which was confirmed by field emission scanning
electron microscopy (FESEM) and supported by Dynamic Light
Scattering (DLS). R1 identified three metal ions (Pb2+, Hg2+ and
Cu2+) in colorimetric and fluorometric techniques with an
intramolecular charge transfer (ICT) off-on mechanism. The space
specificity of the metal ions (Hg2+ and Cu2+) resulted in hydrolysis of -
CH=N- functionality of the sensor. The latter also exhibited excellent
sensitivity in acidic conditions by blocking ICT and remained stable
in neutral to basic pH. Cell viability and microscopic fluorescence
experiments revealed that R1 is cytocompatible and may be used for
the detection of Pb2+, Hg2+, and Cu2+ ions in living cells.
Mercury and Lead are highly toxic heavy metal ions and have
been widespread in water, soil and air by numerous human
activities such as gold production, coal plants etc.10 The shallow
levels of Hg2+ and Pb2+ can create various illnesses like
impairments, anemia, neurological damage, memory loss, nerve
disorders, reduced IQ etc.27,28 Copper is an essential element in
the human body which performs a significant role in various
physiological processes and biological functions.29,30
Results and Discussion
A sensor is presented with a region-selective molecular cavity
(R1) for three specific metal ions (Cu2+, Hg2+, and Pb2+) by
fluorometric and colorimetric assays with a low limit of detections
(0.14 × 10-6 M, 0.032 × 10-6 M and 41 × 10-6 M, respectively) (SI;
Figure S23-S25) and complexation occurred with Cu2+, Hg2+ and
Pb2+ in 1:1, 1:1 and 2:1 stoichiometric, respectively with high
quantum yield (Table 1). In addition to that, it also exhibited
strong AIEE in a mixture of acetonitrile-water (ACN-H2O). A
truncated sensor R2 has also been reported, which has three
binding sites to encapsulate the metal ions.
Introduction
Aggregation-induced emission enhancement (AIEE) is one of
the hottest topics in current research, and it has been broadly
studied because of the promising applications in organic,
supramolecular, medicinal, and material chemistry.1 The AIEE
based molecules are used in real-time applications such as
organic light-emitting diodes,2,3 chemosensors,4-6 fluorescent
bio-probes,7-9 heavy and noxious metal ions sensors,10-12
catalysts, organic molecular transformations13, etc.
Scheme 1. Synthesis of R1 and R2.
In 2001, Tang’s group first discovered the innovative
phenomenon of aggregation-induced emission (AIE), which
attracted much attention and solved many critical problems.14-16
They developed and selected the molecules having
tetraphenylethylene (TPE)17-19 and hexaphenylsilole (HPS)20,
moieties which showed good AIE or AIEE effect. In addition to
this, other molecules having pyrans,21 butadienes,22 and
fulvenes23 were also there to explain aggregation better. Small
fluorescent probes have several advantages in the field of
sensing as they can sense ions and neutral molecules with a low
limit of detection, fast response toward selective analytes and
high quantum yield etc.10,24 Coumarin is one of these,
possessing have highly fluorescent properties and working as a
good sensor.25,26
Reagent and conditions: (i) Ethanol, acetic acid, reflux, 24 h; (ii) Ethanol,
reflux, 24h.
1
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10.1002/ejic.202000583
European Journal of Inorganic Chemistry
FULL PAPER
Herein,
a
new Schiff base chemosensor R1 has been
chromen-3-yl)imino)methyl)furan-2-carbaldehyde]
were
synthesized by the simple condensation reaction of two
equivalents of 3-amino-7-(diethylamino)-2H-chromen-2-one (2)
with furan 2,5-dicarboxylaldehyde (1). On the other side, R2 has
been made by the reaction between two equivalents of aniline
and furan 2,5-dicarboxylaldehyde in a similar fashion (Scheme
1). Both the chemosensors were successfully obtained in good
reproduced (Scheme 2). On the other hand, in the presence of
Pb2+ ion, the absorption of R1 (λmax 486 nm) declined
continuously, and a new broad peak at λmax 572 nm was found.
With the continuous addition of Pb2+, the complex solution
became saturated, and one isosbestic point was observed at
550 nm (Fig. 2c). The DLS analysis of complexes R1 with Cu2+,
Hg2+, and Pb2+ were studied, which showed that the complex
R1:Cu2+ and R1:Hg2+ gives two different sizes, but only one
dimension of molecules in the solution was found in the complex
R1:Pb2+ (SI, Figure S26-S28).
1
yield and well-characterized by H-NMR, 13C-NMR, LC-MS, and
FT-IR. Photophysical studies were carried out in UV-visible and
fluorescence spectrophotometer. The morphological changes of
AIEE properties and the complex of metal ions were recorded by
FESEM. R2 was reported as a supportive chemosensor for the
better and exact explanation of multi-point complexation of R1
with metal ions like Pb2+, Hg2+, and Cu2+. Chemosensors R1 and
R2 have an almost common binding site, however, only R2
responded on Cu2+ and Hg2+ ions.
Photophysical studies
The visible and fluorescence colour changes of R1 (c = 1.8 × 10-
6
M in ACN at pH = 7.0 ± 0.05, HEPES buffer) was investigated
with different metal ions (Al3+, Cr3+, Mn2+, Fe3+, Pb2+, Hg2+, Cu2+,
Co2+, Ni2+, Zn2+, Cd2+, Ag+, c = 2.5 × 10-4 M) in UV-grade
acetonitrile. The orange colour solution of R1 became deep blue,
golden and light orange, respectively adding Pb2+, Hg2+, and
Cu2+ ions. Under long wavelength (365 nm) UV light, non-
fluorescent R1 exhibited fluorescence emission on treating with
Hg2+ and Cu2+ ions. So, R1 showed colorimetric selectivity for
Pb2+, Hg2+ and Cu2+ and fluorometric sensitivity for Hg2+and Cu2+
ions (Fig. 1a and 1b).
Figure 2. UV-visible titration spectra of R1 (c = 1.8 × 10-6 M in ACN at pH =
7.0 ± 0.05, HEPES buffer) with (a) Cu2+ (b) Hg2+ and (c) Pb2+ (ions
,
concentration 2.5 × 10-4 M).
The fluorescence titration studies of R1 were carried out with
Cu2+ and Hg2+, but not with Pb2+ as the earlier did not show any
fluorescence behavior (Fig. 1b). Fig. 3a and 3b represent the
fluorescence titration spectra where the fluorescence emission
intensity of R1 increased sharply on the addition of one
equivalent Cu2+ and Hg2+. A new peak was generated at λem 497
nm with a 5.0 fold intensity enhancement with Hg2+ compare to
Cu2+.
Figure 1. Naked eye detection of different metal ions (c = 2.5 × 10-4 M) by R1
(c = 1.8 × 10-6 M in ACN at pH = 7.0 ± 0.05, HEPES buffer) (a) Visible colour
change (b) fluorescence change were taken under long wavelength (365 nm)
hand-held UV-light.
UV-visible titration of R1 (c = 1.8 × 10-6 M in ACN at pH = 7.0 ±
0.05, HEPES buffer) has been carried out with the same metal
ions (c = 2.5 × 10-4 M) in freshly prepared stock solution. R1
exhibited absorption maxima at λmax 486 nm. However, the peak
intensity and maxima were accordingly changed on the regular
addition of Cu2+, Hg2+, and Pb2+. The addition of Cu2+ decreased
the peak intensity at 486 nm and also seemed to induce a
hypsochromic from 486 nm to 423 nm (Fig. 2a). The
absorbance of R1 itself at 486 nm was getting reduced, and a
new peak at λmax 605 nm appeared with increasing Hg2+ ion
concentration. This happens only upon complexation between
R1 and Hg2+. After some time, that peak at λmax 605 nm was also
disappeared and two new peaks at λmax 325 nm and 462 nm
were developed, which indicated the imine bond hydrolysis and
the compound 2 and 3 [(E)-5-(((7-(diethylamino)-2-oxo-2H-
Figure 3. Fluorescent titration spectra of R1 (c = 1.8 × 10-6 M in ACN at pH =
7.0 ± 0.05, HEPES buffer, λex = 365 nm) with (a) Cu2+ and (b) Hg2+ (c = 2.5 ×
10-4 M); insets: visible colour changes under 365 nm UV light rediation.
A simple conjugation was found in R2 so that the photophysical
properties may be changed upon disturbing conjugation. It has
been prepared as an analog of R1 as they have almost the
2
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10.1002/ejic.202000583
European Journal of Inorganic Chemistry
FULL PAPER
same binding sites. The UV-visible titration of R2 has also been
carried out with different metal ions, and it was found that R2
was sensitive towards Cu2+ and Hg2+ only, but not to Pb2+ (Fig.
4). So this observation proved that both the coumarin moieties of
R1 were directly involved in binding of Pb2+ ion (Scheme 2).
glass slide, and it was dried at 60°C for 5.0 hrs under the oven.
A needle-shaped texture of R1 was observed in pure ACN
solvent (Fig. 6a). The fluorescence intensity was recorded at a
maximum when the H2O content reached 70 % in ACN. In this
condition, the needle texture of R1 disappeared and aggregation
was found (Fig. 6b). The study was continued for the complexes
of R1 with metal ions (Cu2+, Hg2+, and Pb2+). Fig. 6c and Fig. 6d
represented almost the same morphology, a sheet-like texture
upon addition of Cu2+ and Hg2+. However, the addition of Pb2+
produced a spherical morphology and it was totally different as
compared to the complexes R1:Cu2+ and R1:Hg2+ (Fig. 6f).
Figure 6. FESEM images of R1 and its complexes (a) Only R1 in ACN (b) only
R1 in 70% H2O in ACN (c) complex R1:Cu2+ (d) complex R1:Hg2+ (e) complex
R1:Pb2+ (f) high resolution of R1:Pb2+
.
Figure 4. UV-visible titration spectra of R2 (c = 1.8 × 10-6 M in ACN at pH =
7.0 ± 0.05, HEPES buffer) with (a) Cu2+, (b) Hg2+ and (c) Pb2+ (c = 2.5 × 10-4
M).
In previous reports, it was found that the change in fluorescence
properties occurred due to aggregation or disaggregation of the
sensors.15 For a better explanation, DLS analysis was carried
out and found 2.2 nm size of R1 existed in pure ACN. Upon
addition of H2O to ACN, the size of R1 was continuously
increasing and it became maximum upto 157.4 nm (H2O-ACN,
70-30%). The highest fluorescence intensity has been achieved
due to the maximum aggregation. The emission intensity was
further decreased on increasing fw, which may be happened due
to interference of size of aggregated molecules (Fig. 7).32
Aggregation-Induced Emission effect
R1 was initially non-fluorescent, and the fluorescence intensity in
ACN was almost a flat line corresponding to the abscissa. When
the fraction of H2O was increased continuously, the fluorescence
of R1 was turned on, and the emission intensity enhanced up to
70% H2O content in ACN. In the presence of H2O, isomerization
of free rotation of imine double bond (C=N) of coumarin at 3
position was restricted by the aggregation and, accordingly, the
transfer of electrons of diethylamine was blocked,31 which
caused the enhancement of fluorescence. This change was
recorded with the spectrophotometer as well as through the
naked eye under a long wavelength UV light (Fig. 5b).
Figure 7. DLS analysis of R1 (c = 1.8 × 10-6 M) in (a) ACN (b) 70% H2O in
ACN.
Figure 5. AIEE studies of R1 (c = 1.8 × 10-6 M) in different ACN-H2O mixed
solvents (λex = 365 nm); (a) fluorescence spectra (b) naked eye observation
under long-wavelength 365 nm UV light (c) change in fluorescence intensity in
different ratio of H2O-ACN.
Performance of R1 in different pH ranges
The effect of pH on R1 was studied at various pH (1.0 to 13)
using UV-visible and fluorescence spectroscopic techniques
(Fig. 8). No change in absorption (λmax 486 nm) occurred from
neutral to basic pH (7.0 to 13.0 pH). The peak absorbance
decreased with decreasing pH (from 7.0 to 1.0 pH), and it was
nearly insignificant at highly acidic (pH 1.0) conditions. The
orange colour of R1 turned to yellow and then nearly colorless
on decreasing pH (Fig. 8a). The change in fluorescence
The morphology of the sensor and the complex in aggregation
were recorded using FESEM (Fig. 6) and DLS analysis (Fig. 7).
All the composite images of FESEM were taken in the solid-
state. The samples were prepared by the drop cast method on a
3
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10.1002/ejic.202000583
European Journal of Inorganic Chemistry
FULL PAPER
intensity was also investigated in the same manner and it was
found that R1 was non-fluorescent at neutral to basic pH (7.0 to
13) and the fluorescence intensity increased continuously from
neutral to pH 1.0 and consequently one new emission peak at
λmax 497 nm appeared. The change in fluorescence intensity
was also observed through the naked eye after irradiation with a
long-range UV light, looking like a strong aqua blue fluorescence.
The difference in fluorescence properties was due to trapping of
lone pair of the ethylamine nitrogen by a proton and prohibited
ICT.33
δ 9.66 ppm for aldehyde (CHO) and imine (-C=N-) functionality,
respectively.36 The same result was not found on employing
Pb2+ to R1. In mass spectra, two peaks were found at m/z 233
(compound 2+H) and 339 (compound 3+H) on the treatment of
either Cu2+ or Hg2+ to R1. However, the binding stoichiometry of
all complexes was established by Job plot.
Figure 8. Effect of pH (1.0 to 13.0) on chemosensor R1 (c = 1.8 × 10-6 M in
ACN at pH = 7.0 ± 0.05, HEPES buffer) (a) UV-visible spectra (b) fluorescence
spectra (c) visible colour changes and (d) colour change under UV-lamp of
365 nm; and its protonation-deprotonation mechanism.
Scheme 2. The possible mode of binding of Cu2+, Hg2+ and Pb2+ with R1 and
R2.
Possible mode of binding
Quantum Yield (Q)
R1 recognized three metal ions (Cu2+, Hg2+ and Pb2+) separately
with different photophysical properties. When R1 was bonded to
Cu2+ and Hg2+, the orange colour of R1 was rehabilitated into
golden and light yellow, respectively and the property has been
changed to exhibit strong fluorescence, accordingly (Fig. 3).
However, the orange colour R1 was turned to deep blue
instantly on the addition of Pb2+ (Fig. 1). However, chemosensor
R2 recognized Cu2+ and Hg2+, not Pb2+. This was due to the
optimum cavity size created by two imine N-atoms and one O-
atom of the furan ring, where both Cu2+ and Hg2+ could easily be
encapsulated. The size of Pb2+ is significantly larger than other
mentioned metal ions (ionic radius Cu2+, Hg2+ and Pb2+ are 73
pm, 102 pm, and 119 pm, respectively)34 and it was incorporated
in the template created by two lactone O-atoms of the coumarin
moiety. According to UV-vis titration of R1 and R2, the result
suggested that the chemosensor R1 strongly binds Cu2+ and
Hg2+ into its cavity, constructed by the two imine N-atoms and
one O-atom of the furan ring. As a result, a bathochromic
shifting of wavelength from 486 nm, which indicated the metal
ion interacted with the O-atom of coumarin carbonyls.35 But,
after few seconds, the shifted peak was shrunk due to the
breaking of bond.
The quantum yield for different conditions was calculated and
summarised in Table 1 by applying the standard formula (SI,
Page 14).37
Table 1. Quantum yield of R1 with different conditions
Bio-imaging studies
The potential of furan-coumarin conjugated Schiff’s base
chemosensor (R1) for region-selective detection of Pb2+, Hg2+
and Cu2+ ions in living cells was investigated. The MTT assay for
MCF-7 (breast adenocarcinoma) cells was used to determine
the cytocompatibility of R1. In Fig. 9, the cell viability was
greater than 80%, following treatment with the compound for 48
h. This result indicated that R1 was safe and induced low
cytotoxicity even at higher concentration (100 µM).
The possible photophysical changes were recorded on the basis
of the binding mode of metal ions and happened through ICT
(scheme 2). When Cu2+ and Hg2+ bind, the imine bond of R1
became hydrolyzed to produce compound 2 and 3 (Scheme 2).
1
This was confirmed by H-NMR and mass spectroscopy. Upon
hydrolysis, the peak of imine hydrogen at δ 9.49 ppm almost
vanished and two new peaks were generated at δ 9.79 ppm and
4
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10.1002/ejic.202000583
European Journal of Inorganic Chemistry
FULL PAPER
Conclusion
In conclusion, the first template-assisted regio-selective
coumarin-furan conjugated Schiff base chemosensor (R1) has
been reported for the discrimination of highly toxic Hg2+ and Pb2+
ions in a straight forward manner. Moreover, R1 selectively
recognized three metal ions (Pb2+, Hg2+, and Cu2+) by
colorimetric and fluorometric technique with an ICT on-off
mechanism. It may also be used as a pH sensor and showed
intense blue fluorescence in acidic condition due to protonation
of lone pair of diethylamine, which blocked ICT. One analogous
chemosensor, R2 was also reported herewith for a better
explanation of the mode of complexation. R1 showed excellent
AIEE in a different fraction of ACN-H2O. In the FESEM image,
R1 looked like agregrated morphology in ACN-H2O (30:70, v/v)
and exhibited maximum aggregation. This has surprisingly
enhanced fluorescence intensity by 180 fold and could observe
without any difficulty through the naked eye. DLS analysis also
confirmed the aggregation. The sensor was cyto-compatible and
detected Pb2+, Hg2+ and Cu2+ ions in living cells.
Figure 9. MTT assay of MCF-7 cells treated in the presence of R1 (0–100µM)
incubated at 37°C for 48 hrs in the presence or absence of Cu2+, Hg2+ and
Pb2+ metal ions.
Fluorescence imaging analysis was performed for demonstrating
the bio-distribution of the compound in living cells using a Leica
DMi600 fluorescent microscope. MCF-7 cells incubated with R1
only did not produce any detectable fluorescence (Fig. 10A-C).
However, after treatment with Hg2+, a bright green fluorescence
was observed in the MCF-7 cells (Fig. 10 J-L). Overlaying of
fluorescence and bright-field images showed that the
fluorescence signals were localized in the intracellular areas,
indicating the subcellular distribution of Hg2+, besides effective
cell-membrane permeability for R1. The specificity of R1 towards
Hg2+ ions was demonstrated by performing similar experiments
using Cu2+ and Pb2+ ions in the absence of Hg2+ions. Cells were
exposed to various concentrations (from 0 to 100 µM) of Cu2+ or
Pb2+ ions in PBS and images were obtained using a Leica
DMi600 fluorescent microscope after the addition of R1. In
contrast to Hg2+, a competing metal ion like Cu2+ displayed weak
fluorescence at higher concentrations (100 µM tested) (Fig. 10
G-I). In the case of Pb2+, no fluorescence was observed at all.
Experimental Section
Materials and equipment used
All the chemicals and analytical grade solvents were obtained
commercially from Mark and Sigma-Aldrich and used without purification.
Metal perchlorates salts were prepared in the laboratory from the
respective carbonate and nitrate salts. 1H-NMR and 13C-NMR spectra
were recorded with a Bruker 400 MHz and 500 MHz spectrometers using
CDCl3 solvent. Mass spectra were analyzed by ESI source. UV-visible
and fluorescence spectra were recorded with a PerkinElmer Lambda 365
and LS 55 instrument, respectively, using 10 mm path length of the
cuvette. FT-IR spectra were recorded in the range of 4000 to 400 cm-1
using KBr pellets.
Synthesis of 1 and 2.
Compound 124 and 216 were synthesized according to the earlier reported
methods.
Synthesis of chemosensor R1
Furan 2,5-dicarbaldehyde (50 mg, 0.40 mmol) was completely dissolved
in ethanol (20.0 mL) and 3-amino-7-diethylamino coumarin (200 mg, 0.86
mmol) was added, followed by a catalytic amount (4-5 drops) of acetic
acid. The reaction mixture was stirred to complete after 24 h at reflux.
Then the volume of the solvent was reduced. Finally, a red solid
precipitate was obtained by further filtration washing with ethanol (30.0
mL). (75.0 mg, yield: 40.0 %).Melting point: 230°C.
FT-IR (KBr, cm-1): 3412 (H2O), 3141 (=C-H, str.), 2967 (-C-H, str.), 2924
(-C-H, str.), 1704 (C=O, str.), 1617 (C=N, str.), 1515 (C=C, str.).
1H-NMR (400 MHz, CDCl3) of R1: δ 9.49 (s, 2H), 7.839 (s, 2H), 7.34 (d,
J = 8.8 Hz, 2H), 7.04 (s, 2H), 6.61 (d, J = 8.8 Hz, 2H), 6.50 (s, 2H),
3.42(q, J = 7.2 Hz, 8H), 1.21(t, J = 7.2 Hz, 12H).
13C NMR (125 MHz, CDCl3) of R1: δ 158.83, 155.14, 155.03, 150.79,
147.92, 139.62, 129.45, 126.82, 116.93, 109.45, 109.25, 96.98, 44.93,
12.50.
Figure 10. Fluorescence images of MCF-7 cells treated with R1 and Pb2+
Cu2+ and Hg2+. Bright-field image; fluorescence image and overlay image.
,
5
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10.1002/ejic.202000583
European Journal of Inorganic Chemistry
FULL PAPER
ESI-MS (+) of R1: Calculated C32H33N4O5: 553.2451 [M+H]; found:
553.2455 [M+H].
Leuven, Belgium for his valuable suggestion. SKH
acknowledges USIC, the University of Burdwan (for Multiskan™
GO facility), DST-SERB, DST-FIST & DST-PURSE programs for
research facilities.
Synthesis of R2
Furan 2,5-carbaldehyde (50 mg, 0.40 mmol) and aniline (100 µL, excess)
were dissolved in ethanol (20 mL) and refluxed for 12 h at 80°C. After
completion of the reaction, the solvent was reduced and filtered. A yellow
solid product was then washed with ethanol several times (20 mL) to get
the pure product (yield: 80 %, 100.0 mg). Melting point: 138°C.
Keywords: Coumarin-furan • Chemosensor • AIEEE • pH
sensor • Pb2+, Hg2+ and Cu2+.
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Entry for the Table of Contents
The first template-assisted, regio-selective, coumarin-furan conjugated Schiff base chemosensor (R1) has been developed for the
recognition of Pb2+, Hg2+, and Cu2+ by colorimetric and fluorometric techniques with an intramolecular charge transfer (ICT) on-off
mechanism. Non-fluorescence R1 turned into green fluorescent upon addition of Hg2+ and Cu2+ and in deep blue with Pb2+. R1
functioned as a pH sensor and showed excellent AIEE, observed through the naked eye. The needle-shaped sensor exhibited
maximum aggregation when the size was increased by 70 fold (2.2 to 157.4 nm) measured in DLS.
8
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