Synthesis and photophysical investigation of (BTHN) Schiff base as off-on Cd2+ fluorescent chemosensor and its live cell imaging

Article abstract of DOI:10.1016/j.molliq.2021.115407

A Schiff base, 1-[2-(1,3-benzothiazol-2-yl)hydrazinylidene]methyl-naphthalen-2-ol (BTHN) have been synthesized by the condensation of 2-hydrazinobenzothiazole and 2-hydroxy-1-naphthaldehyde. Structure of BTHN has been determined with the help of spectroscopic techniques and elemental analysis. Photophysical parameters of the BTHN were studied in the different solvents of varying polarity. The photophysical parameter were largely affected by the varying polarity and absorption as well as emission spectra showed strong bathochromic shift. Further, interactions of the BTHN dye with metal ions were also explored by spectrofluorimetry and BTHN found to be an excellent off-on chemosensor for Cd²⁺ in DMF-H₂O solution. The molecular coordination complex between BTHN with Cd²⁺ is 1:1, confirmed by Benesi-Hildebrand and Job-plot method. In addition, the BTHN has been effectively employed for the fluorescence imaging of cadmium ions in the living cells.

Full text of DOI:10.1016/j.molliq.2021.115407

Journal of Molecular Liquids 328 (2021) 115407
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and photophysical investigation of (BTHN) Schiff base as off-on
2+
Cd fluorescent chemosensor and its live cell imaging
a,
a
b
c
d
Salman A. Khan ⁎, Qasim Ullah , Abdulraheem S.A. Almalki , Sanjay Kumar , Rami J. Obaid ,
e
f
g
Meshari A. Alsharif , S.Y. Alfaifi , Authar Adil Hashmi
Physical Sciences Section, School of Sciences, Maulana Azad National Urdu University, Hyderabad 500032, Telangana, India
Department of Chemistry, Faculty of Science, Taif University, Taif 210974, Saudi Arabia
a
b
c
d
e
f
Department of Chemistry, Multani Mal Modi College Patiala, 147001, Punjab, India
Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India
g
a r t i c l e i n f o
a b s t r a c t
Article history:
A Schiff base, 1-[2-(1,3-benzothiazol-2-yl)hydrazinylidene]methyl-naphthalen-2-ol (BTHN) have been synthe-
sized by the condensation of 2-hydrazinobenzothiazole and 2-hydroxy-1-naphthaldehyde. Structure of BTHN
has been determined with the help of spectroscopic techniques and elemental analysis. Photophysical parame-
ters of the BTHN were studied in the different solvents of varying polarity. The photophysical parameter were
largely affected by the varying polarity and absorption as well as emission spectra showed strong bathochromic
shift. Further, interactions of the BTHN dye with metal ions were also explored by spectrofluorimetry and BTHN
Received 11 September 2020
Received in revised form 8 January 2021
Accepted 13 January 2021
Available online 21 January 2021
Keywords:
Photophysical
Fluorescence
Azomethine
Cadmium
2+
2
found to be an excellent off-on chemosensor for Cd in DMF-H O solution. The molecular coordination complex
between BTHN with Cd2+ is 1:1, confirmed by Benesi-Hildebrand and Job-plot method. In addition, the BTHN has
been effectively employed for the fluorescence imaging of cadmium ions in the living cells.
© 2021 Published by Elsevier B.V.
Sensor
Living cell
1. Introduction
pair of electrons so it can coordinate with the different metals to make
various metal complexes. Donor-acceptor chromophores (D-π-A) are
Azomethine dye commonly known as molecule containing with
HC=N group and represent by the general formula R -CH=N-R [1].
referred as push-pull chromophores having an electron donating
group such as OH, OR, N-CH linked with electron accepting group or
heterocyclic moiety such as CN, CO, NO , pyridine, pyrazoline and
1
2
3
It is produced by the equimolar ratio of primary amine with ketone or
aldehyde; react to each other under specific conditions, like acidic or
basic catalyst or heat, with instantaneous elimination of the water mol-
ecule [2]. It is generally known as Schiff base on the name of great scien-
tist Hugo Schiff in 1864 [3]. Generally, Schiff bases are solid and
crystalline, which are unconvincingly basic in nature [4]. Importance
of these molecules is mainly due to their structural relationship with
natural biological substances and comparatively simple method of the
synthesis as well as synthetic litheness that permit the propose of suit-
able structure properties [5]. It is most extensively used synthetic or-
ganic molecule. They are used as dyes, pigments [6], catalysts, as
polymer stabilizers and precursor for the synthesis of different hetero-
cyclic compounds such as thiazolidinone, azetidinone, formazans, thia-
zole and arylacetamide [7]. Nitrogen atom of the Schiff base have lone
2
benzothiazole through the pi bond conjugated system [8]. Such type
of chromophores transfers the charge from electron donating group to
electron acceptor group through π-bond conjugations, reduce the en-
ergy gap between HOMO-LOMO and subsequently chromophore
shows red shift in absorption and fluorescence spectrum due to ICT
(D-π-A) [9]. These chromophores have been comprehensively used for
numerous optical applications like organic light emitting diodes [10],
sensor for the investigation of cancer cells and various hazardous
metal ions [11–17], photorefractive polymers. Phtophysical and physi-
cal properties of the (D-π-A) chromophore can be improved by the
modification of donor or acceptor group and increasing the pi-bond
conjugation system between donor and acceptor group [18]. In the re-
cent years, fluorescence chemosensors have been remunerated much
consideration to detect metal ions because of their selective and sensi-
tive character when mixed with metal ion. Such sensor based on fluo-
rescence intensity and wavelength of the emission has numerous
⁎
Corresponding author.
E-mail addresses: sakhan.manuu@gmail.com, sahmad_phd@yahoo.co.in (S.A. Khan).
https://doi.org/10.1016/j.molliq.2021.115407
167-7322/© 2021 Published by Elsevier B.V.
0

S.A. Khan, Q. Ullah, A.S.A. Almalki et al.
Journal of Molecular Liquids 328 (2021) 115407
advantages like simple method and instantaneous response [19]. There-
fore, various effective metal ion sensors have been developed. In this
paper, we have described the synthesis of novel highly florescence
Schiff base as florescent sensor for Cd2 metal ion.
2-hydroxy napthaldehyde (200 mg, 1.10 mmol) and 2-
hydrazinobenzothiazole (190 mg, 1.10 mmol) in EtOH was reflexed
for 3 h. After completion of the reaction, the reaction mixture was
allowed to stand overnight. The solid thus obtained was filtered,
dried in air and recrystallised in chloroform and ethanol (8:2)
+
(Scheme 1) [21].
2. Experiment
Yellow color; yields: 87.93%; FT-IR: 3432 (O-H), 3035 (C-H), 2924
(
(
C-H), 1608 (C=Caromatic), 1578 (C=N), 1135 (C-N), 1HNMR
2.1. Chemicals
600 MHz, DMSO‑d
6
, ppm); 12.08 (s, 1H, C-OH), 11.12 (s, 1H, NH),
1
1
1
0.08 (s, 1H, CHAzomethine), 8.51 (d, 1H, CHAromatic, J = 7.8 Hz), 8.31 (d,
H, CHAromatic, J = 7.2 Hz), 8.20 (d, 1H, CHAromatic, J = 7.2 Hz), 7.89 (d,
H, CHAromatic, J = 7.8 Hz), 7.62 (dd, 1H, CHAromatic, J = 5.4 Hz), 7.36
All the reagents and chemicals have been procured from
Sigma-Aldrich Chemical Pvt. Ltd., and used without additional decon-
tamination. All the solvents used for spectral studies of the BTHN were
of spectroscopic grade. Metal salt used to prepare the stock solution of
(
dd, 1H, CHAromatic, J = 3.6 Hz), 7.28 (d, 1H, CHAromatic, J = 7.2 Hz),
.97 (dd, 1H, CHAromatic, J = 5.6, 5.6 Hz); 13C NMR (125 MHz,
DMSO‑d ): 166.98, 158.12 (C=N), 133.05, 131.76, 130.06, 128.76,
28.13, 126.90, 124.12, 123.52, 122.54, 118.65, 110.15; ESI-MS (m/z):
6
metal ions were CuCl
2
.2H
2
O, SrCl
2
.6H
2
O, SnCl
2
.2H
2
O, CdCl
2 2
.2H O,
CoCl .6H O, CrCl .6H O, MnCl
2
2
3
2
2
.4H O, ZnCl
2
2 3
, and AgNO . 2-Hydrazinyl
6
1
3
benzothiazole was prepared by the reported method as mentioned in
Scheme 1. The reaction progress was monitored by thin layer chroma-
tography (TLC) using silica gel 60 F254 pre-coated aluminum sheets
−
13 3
18 [M-H] ; Anal. Calc. for C18H N OS: C, 67.69, H, 4.10, N, 13.16.
Found C, 67.53, H, 4.03, N, 13.11.
(
Merck). Column chromatography have been employed for the purifica-
tion of reaction products [20].
3
. Result and discussion
2.2. Instruments
3.1. Chemistry
Stuart Scientific Co. Ltd. melting point apparatus was used to deter-
The Schiff base BTHN was produced by the condensation of 2-
mine the melting-point (m.p.) of prepared α, β-unsaturated ketone, in
hydrazinobenzothiazole with 2-hydroxy-1-naphthaldehyde (Scheme1).
The structure of the synthesized heterocyclic Schiff Base (BTHN) was
1
13
open capillary tubes. H NMR (600 MHz) and C NMR (125 MHz) spec-
trum of the BTHN was recorded on Bruker spectrometer in deuterated
DMSO. Perkin-Elmer 100 FT-IR spectrometer was used to record IR
spectra of BTHN. UV–visible and fluorescence spectra of the BTHN in
various solvents were recorded with Shimadzu Uv-16550PC UV/Vis
and Shimadzu RF 5301PC respectively.
1
13
established by FT-IR, H NMR, C NMR, elemental, and mass spectral
−
1
analysis. FT-IR spectrum of BTHN gave absorption band at 1578 cm
−
1
and 1135 cm owing to presence of conjugated C=N and conjug-
ated C–N functional group, respectively and simultaneous disappear-
−
1
2
ance of peak at 3412 cm for NH group of reactant amine indicated
the formation of heterocyclic Schiff Base (BTHN).
1
2
.3. Preparation of 1-(2-benzothiazol-2-yl-hydrazono) methyl naphtha-
In the H NMR spectrum of the BTHN, the characteristic singlet for
lene-2-ol(BTHN)
azomethine at δ12.8 ppm inferred the condensation of aldehyde and
amino group. Ten aromatic protons of the BTHN are in the usual range
13
BTHN was prepared by the condensation reaction of 2-hydroxy
of δ8.46-δ6.97 ppm. BTHN structure was further supported by the
C
napthaldehyde with 2-hydrazinobenzothiazole. Briefly, a mixture of
NMR spectrum and the azomethine carbon appeared at 158.12 ppm
Scheme 1. Synthesis of BTHN
2

S.A. Khan, Q. Ullah, A.S.A. Almalki et al.
Journal of Molecular Liquids 328 (2021) 115407
Table 1
Photophysical parameters and fluorescence quantum yields of BTHN in various solvents.
N
T
Solvent
Δf
E
E
T
(30)
λ
ab(nm)
λ
em(nm)
ε
M
f
μ
12
Δνst
Φ
f
−
1
-1
cm−1
(cm−1)
Kcal mol
Debye
DMSO
DMF
EtOH
MeOH
t-BuOH
PrOH
0.263
0.274
0.288
0.308
0.264
0.280
0.148
0.218
0.305
0.028
0.208
0.0002
0.441
0.404
0.654
0.762
0.506
0.520
0.259
0.472
0.164
0.210
0.006
0.0002
45.1
43.8
51.9
55.4
50.2
49.2
39.1
41.1
45.6
36.0
35.1
31.1
390
389
377
379
381
380
382
383
381
380
379
377
510
507
474
484
486
478
486
488
479
478
474
473
33,950
32,920
31,100
26,730
26,080
22,400
28,850
24,890
26,460
27,200
25,200
21,560
0.81
0.78
0.67
0.61
0.59
0.48
0.64
0.55
0.56
0.57
053
8.11
8.01
7.31
6.99
6.90
6.21
7.19
6.67
6.68
6.77
6.51
5.99
6034
5983
5428
5724
5670
5395
5602
5612
5370
5307
5288
5249
0.32
0.28
0.10
0.23
0.33
0.40
0.09
0.11
0.08
0.07
0.12
0.05
CHCl
3
CH
CH
2
Cl
CN
2
3
Dioxane
THF
n-Heptane
0.45
while other aromatic carbons signals in the range of δ133.05-
δ110.15 ppm. In the mass spectrum (ESI-MS), m/z 318 molecular ion
peak further established the formation of BTHN.
The linear relation between different polarities of the solvents and
energies (E & E ) has been assimilated by the given equations.
a
f
Ea ¼ 76:33–0:010 ꢀ ETð30Þ
Ef ¼ 66:95–0:072 ꢀ ETð30Þ
ð1Þ
ð2Þ
3.2. Physicochemical properties of the BTHN
The UV–visible absorption spectra of the heterocyclic azomethine
derivative, BTHN, were recorded in different protic and aprotic solvents
with varying polarity viz. methanol (MeOH), ethanol (EtOH), dimethyl
sulfoxide (DMSO), dimethyl formamide (DMF), tetrahydrofuran
3.3. Determination of transition dipole-moments and stokes shift
(
THF), acetonitrile (CH
3
CN), chloroform (CHCl
3
), dichloromethane
The physico-chemical parameters, viz. Stokes shift, transition dipole
moment, oscillator strength in the solvents of varying polarity have
been determined for the heterocyclic azomethine derivative, BTHN.
The change in dipole-moment between ground state and excited state
(
CH Cl ), n-Heptane and 1,4-dioxane. Broad absorption band was ob-
2
2
served for the heterocyclic azomethine derivative, BTHN, in different
solvents. As evident from Table 1, with increase in the solvent polarity
(
n-Heptane to DMSO), absorption maxima (λmax) of the BTHN exhib-
e g
(Δμ = μ - μ ) in different solvents for the heterocyclic azomethine
BTHN, was determined.
The various physicochemical parameters of the heterocyclic
azomethine, BTHN, in different solvents could be determined by using
Lipert-Mataga equation [25].
ited a red shift of 13 nm. This behavior may be attributed to intermolec-
ular charge transfer (ICT) during transition from electron rich hydroxy
(
OH) group [22]. The molar absorption coefficient also increased with
increase in polarity of the solvent (Table 1). On the similar line, the
emission spectra showed bathochromic shift of 37 nm with the change
in polarity of the solvent from non-polar (for n-Heptane, λem
2
=
2Δμ
Δvst ¼ vabs−vem ¼
Δf þ Const
ð3Þ
3
4
73 nm) to polar (for DMSO, λem = 510 nm) Fig. 1. The red shift resul-
hca
tant of photoinduced intramolecular charge transfer (PICT) by
intramolcular electron transfer from hydroxy (OH) group to the thiazole
group at excited state [23].
Where, (Δvst) is the Stokes shift, a is the Onsager cavity radius, c is
the velocity of the light, h is the Plank's constant and Δf is the orienta-
tion polarizability.
f
The absorption energy (Ea) and emission energy (E ) of the hetero-
cyclic Schiff base showed a linear relationship with Dimroth empirical
polarity parameters E
Fig. 2 [24].
T
(30) of the different solvents as presented in
8
0
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
75
DMSO
DMF
EtOH
MeOH
PrOh
BuOH
CHCl3
E
E
a
f
7
0
65
60
CH Cl
2
2
CH CN
3
Dioxane
THF
CH3CN
5
5
3
0
35
40
45
50
55
0
4
-1
E (30) kcal mol
T
00
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. Plot of energy of absorption (E
a
) and emission (E
f
) versus E
T
(30) of different
solvents.
Fig. 1. Fluorescence spectra of 1 × 10−5 M of BTHN in various solvents.
3

S.A. Khan, Q. Ullah, A.S.A. Almalki et al.
Journal of Molecular Liquids 328 (2021) 115407
2
+
5
4
3
2
1
00
00
00
00
00
Cd
Dye
2
+
Cu
Sr
Sn
Cd
Co
Cr
2
+
2
+
2
+
2
+
3+
3
+
Mn
2
+
Zn
Ag
1+
0
4
00
450
500
550
600
650
Wavelength (nm)
Fig. 5. Emission spectra of 1.0 × 10 mol dm−5 of BTHN upon addition of 5.0 × 10−5
−
5
M
Cu , Sr , Sn , Cd , Co , Cr3+, Mn2+, Zn and Ag+ in DMF/ water (9:1, v/v).
2
+
2+
2+
2+
2+
2+
Fig. 3. Plot of Δf versus stokes shift.
intaramolecular charge transfer character and photoinduced ICT led to
creation of stronger dipole that resulted into enhanced interaction
with surrounding solvent medium to produce solvent reformation.
The transition dipole moments of the heterocyclic azomethine BTHN
in different solvents have been calculated using following Eq. (6).
The orientation polarizability (Δf) of the solvent is given by the fol-
lowing equation [26].
2
ε−1
Δf ¼ 2
η −1
−
ð4Þ
2
ε þ 1 2η þ 1
where ε is solvent dielectric constant and η is the refractive index of the
solvent.
The linear correlation between orientation polarizabilty (Δf) and
Stokes shift (Δvst) (Fig. 3) indicated that Stokes shift is dependent on
the solvent polarity or polarizabilty.
f
μ2
¼
ð6Þ
−
7
4
:72 ꢀ 10
Emax
Where, f is oscillator strength and Emax is energy of absorption
maximum.
The tabulted transition dipole moments (Table 1) revealed the in-
crease in dipole-moments with increasing solvent polarity.
The oscillator strength of heterocyclic azomethine BTHN in the sol-
vents have been calculated by the following Eq. (7) [29].
Using Lippert-Matga Eq. (3) and dipole moment of the ground (μ
and excited state (μ ), the Onsager cavity radius (a) for the heterocyclic
azomethine BTHN was obtained by the following relation [27,28].
g
)
e
ꢀ
ꢁ
1
=3
3
M
a ¼
ð5Þ
4
Nπd
Z
−
9
f ¼ 4:32 ꢀ 10
εðvÞdv
ð7Þ
Where, M is Molar mass, N is Avigadro number and d is density of
BTHN. The change in dipole moments (Δμ), calculated from the plot of
Stokes shift versus polarizability, for the azomethine BTHN was found
to be 0.23 Debye. This increase in dipole moment indicated the
Where,εismolarabsorptioncoefficientandν
̅
iswavenumber(cm−1).
From Table 1, it is clear that oscillator strength (f) of BTHN is directly
correlated to polarity of solvent.
5
4
3
2
1
00
00
00
00
00
0
2
+
2+
+
2
+
2+
2+
3+
3+
2
+
r
n
g
HN
BT
Cu
n
S
Cd
Cr
A
+
Co
M
Zn
+
N
+
+
N
N
N +
HN + S
BT
BTHN + BTHN +
BTHN +
BTH
BTH
BTH
BTH
BTHN
Fig. 4. Plot of ɸ
f T
versus E (30) of the different solvents.
Fig. 6. Emission of BTHN in presence of different metal ions.
4

S.A. Khan, Q. Ullah, A.S.A. Almalki et al.
Journal of Molecular Liquids 328 (2021) 115407
Fig. 9. Inset: Fluorescence titrations cure of BTHN (1 × 10− M) with Cd2+ in aqueous
solution.
5
2
+
Fig. 7. Competitive experiments in the BTHN + Cd system with interfering metal ion.
[
BTHN] = 1 × 10− M, [Cd
5
2+]
= 5 × 10 M, and [M ] = 5 × 10 M, excited at 370 nm.
−5
n+
−5
proton donor solvent and BTHN molecules led to decrease in fluores-
3
.4. Fluorescence quantum yields of the BTHN in different solvents
cence quantum yield (ɸ
f
).
The fluorescence quantum yield (ɸ
f
) of the heterocyclic azomethine
BTHN was obtained by relative method using Quinine Sulphate as refer-
3.5. Fluorescent chemosensor of BTHN for metal ion
ence dye (ɸ
r
= 0.55), following equation was used to obtaine the ɸ
f
value [30].
The sensing behavior of BTHN with different metal ions i.e.Cu2+,
Co2+, Sn , Sr , Zn , Cd , Cr , Mn and Ag in DMF-H
2+
2+
2+
3+
3+
2+
+
2
O (9:1)
2
I ꢀ Ar ꢀ η
Ir ꢀ Ar ꢀ η
Φ ¼ Φr
ð8Þ
were examined by emission measurements corresponding to excitation
at 380 nm. As shown in the Fig. 5, azomethine BTHN showed weak fluo-
rescence. Upon addition of Cd2 , a substantial augmentation (~9 fold) in
fluorescence intensity is observed with the blue shift from 507 nm to
f
2
r
+
Where, ɸ
f
is fluorescence quantum yield of heterocyclic azomethine
is fluorescence quantum yield of Quinine Sulphate (reference),
is fluorescence intensity of BTHN and reference dye (in DMSO), A &
BTHN, ɸ
r
2+
2+
4
94 nm (Fig. 5). Whereas addition of other metal ion i.e. Cu , Co
,
I & I
r
2+
2+
2+
3+
2+
+
Sn , Sr , Zn , Cr , Mn and Ag to the azomethine BTHN did
r
A is absorbance of BTHN and reference dye (in DMSO) and η is refrac-
not give fluorescence intensity change (Fig. 5) and thus showing excel-
tive indices.
The fluorescence spectra of heterocyclic azomethine BTHN was af-
2+
lent selectivity for Cd . The observed enhancement of the fluorescence
intensity and blue shift attributed to the mutual effect of the dissuasion
of intramolecular charge transfer (ICT) and chelation-enhanced fluores-
cence (CHEF) [32]. In addition, the substantial fluorescence change from
light yellow to reddish orange could be practically observable by the
fected by the solvent polarity as indicated by the changes in ɸ
f
values
(Table 1). This seem to be the result of polarizability, hydrogen bonding
with electron donor and electron acceptor group. The plot of ɸ
f T
versus E
(
30) show that fluorescence quantum yield increased with increase in
naked eye under UV-lamp of 365 nm, thus making the azomethine
solvent polarity (Fig. 4) [31]. But positive solvatokinetic effect in
2+
BTHN selective off-on chemosensor for Cd
.
strongly polar solvents (like methanol, ethanol) adversely affect the ɸ
f
In order to assess the further selectivity of BTHN for Cd2+, competi-
tion experiments of Cd in combination with other metal ion were
unlike less polar solvent. With increase in solvent polarity, interaction
of solvent with BTHN increases and interaction of the hydrogen bonding
2+
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
4
00
450
500
550
600
650
Wavelength (nm)
Fig. 8. Emission spectra of BTHN (1 × 10−5 M) exposed to various concentration of Cd2+ in
DMF: water (9:1, v/v).
Fig. 10. Benesi-Hildebrand plot for BTHN (1 × 10−5 M) at various concentration of Cd2+
ion.
5

S.A. Khan, Q. Ullah, A.S.A. Almalki et al.
Journal of Molecular Liquids 328 (2021) 115407
Stoichiometry ratio and association constant between the BTHN and
Cd2+ metal ion have been investigated using the Benesi-Hildebrand
method and the equation as following [33,34].
1
1
1
1
¼
þ
ꢀ
h
þi
ð9Þ
ο
ο
ꢂ
ꢃ
F −F0 F −F0 Kapp
ο
2
F −F0 Cd
0
As shown in the Fig. 10 for the plot between the 1/(F-F ,
) vs 1/Cd2+
the linearity of the plot indicated that stoichiometry ratio for the coordi-
nation of BTHN and Cd2 is 1:1 and the BTHN-Cd association con-
+
2+
stant was calculated from the fluorescence titration of BTHN with
2+
4
−1
Cd and found to be 1.17 × 10 M
.
2+
Stoichiometry ratio between the BTHN and Cd was further con-
firmed by the Job plot method [35]. The fluorescence intensity was re-
corded by varying the mole fraction of Cd2 as mentioned in the
Fig. 11. Maximum fluorescence emission intensity was observed when
+
Fig. 11. Job's plot, Fluorescence Intensity vs. mole fraction of Cd2+ Job's plot.
2+
2+
the mole fraction of Cd was 0.5, indicating the BTHN and Cd coor-
dination possibility as 1:1. Emission enhancement mechanism of BTHN
in presence of Cd2 may be possibility of the formation of coordination
bond between nitrogen atom of the azomethine and hydroxyl group
+
carried out (Fig. 6). The emission intensity of the BTHN with Cd2+ re-
2
+
2+
main almost unchanged when mixed with the other metal ions (Cu
,
with Cd metal ion.
2
+
2+
2+
2+
3+
2+
+
Co , Sn , Sr , Zn , Cr , Mn and Ag ); indicated the selectivity
To utilize the present sensor for the detection of Cd(II) ions in living
cells, fluorescence imaging experiments were carried out. HeLa cells
were cultured in DMEM medium, the cells were incubated with the
probe BTHN (5 μM) for 30 min. After the incubation, HeLa cells were
washed thrice with the HEPES buffer to remove the extracellular
probe and the cells were examined under the fluorescence microscope.
The probe BTHN treated cells showed very weak fluorescence. Then the
BTHN incubated cells were treated with Cd(II) solution (25 μM) for
10 min (Fig. 12). Then the cells were imaged through the fluorescence
microscope, the intracellular region of the cells showed bright fluores-
cence, suggesting the practical utility of the probe for live cell imaging.
2
+
of the azomethine BTHN for the Cd
over the other metal cations.
Hence, BTHN could be employed as a very selective sensor for Cd2
even in the presence of other metal ions (Fig. 7).
+
To understand the mechanism of the complexation of BTHN with
2
+
Cd ion, fluorescence titration experiments upon stepwise addition
of Cd2 ions (0.1 to 0.9 × 10 M) were conducted and resulted into ap-
+
−4
proximately 9 fold enhancement in emission intensity accompnied by
blue shift (Fig. 8). The shift exhibited good linear response (R2
=
0
.98) as shown in Fig. 9, and BTHN was revealed to be a discriminating
2+
turn on fluorescent chemosensor for Cd metal ion.
Fig. 12. Imaging of Cd(II) ions in Living HeLa cells. (a), (c) Fluorescence images of Probe BTHN treated cells and Bright field images of Probe treated cells respectively. (b), (d) Fluorescence
images of BTHN and BTHN with cadmium ions treated cells respectively.
6

S.A. Khan, Q. Ullah, A.S.A. Almalki et al.
Journal of Molecular Liquids 328 (2021) 115407
[
[
13] S.A. Khan, Multi-step synthesis, photophysical and physicochemical investigation of
The bright field images of the cells clearly suggesting that the cells are
viable throughout the experimental conditions.
novel pyrazoline a heterocyclic D- π -A chromophore as a fluorescent chemosensor
3+
for the detection of Fe metal ion, Salman A. Khan J. Mol. Struct. 1211 (2020)
128084.
14] S.A. Khan, P. Kumar, Photophysical and physicochemical investigation of newly syn-
4. Conclusion
thesized polycyclic pyrazoline-benzothiazole as fluorescence chemosensor for the
2
+
detection of Cu metal ion, Polycycl. Aromat. Compd. (2019)https://doi.org/10.
A simple Schiff base BTHN was produced by the condensation of 2-
1080/10406638.2019.1607411.
hydrazinobenzothiazole and 2-hydroxy-1-naphthaldehyde and the
structure was established. Physicochemical parameters of the BTHN
were investigated in different solvents with varying polarity and
found that polarity of the solvent strongly affected the physicochemical
parameters. In addition, BTHN displayed excellent selectivity as turn-on
[15] O. Sahin, U.O. Ozdemir, N. Seferoğlu, Z.K. Genc, K. Kaya, B. Aydıner, S. Tekin, Z.
Seferoğlu, New platinum (II) and palladium (II) complexes of coumarin-thiazole
Schiff base with a fluorescent chemosensor properties: synthesis, spectroscopic
characterization, X-ray structure determination, in vitro anticancer activity on var-
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16] S.A. Khan, Q. Ullah, H. Parveen, S. Mukhtar, K.A. Alzahrani, M. Asad, Synthesis and
photophysical investigation of novel imidazole derivative an efficient multimodal
chemosensor for Cu(II) and fluoride ions, J. Phtochem Photobiol. A 406 (2021)
113022.
2
+
sensor for Cd
metal ion even in the presence of other competing
metal ions. The molecular coordination complex between BTHN with
2+
Cd was found to be 1:1, confirmed by Benesi-Hildebrand and Job-
plot method. Further, the live cell imaging experiments for the detection
of Cd(II) ions in HeLa cells augment the utility of the probe BTHN for
intracellular imaging of cadmium ions.
[
17] M. Chemchem, I. Yahaya, B. Aydıner, O. Doluca, N. Seferoglu, Z. Seferoglu, Substitu-
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19] P.S. Hariharan, N. Hari, S.P. Anthony, Triphenylamine based new Schiff base ligand:
Declaration of Competing Interest
2
+
3+
solvent dependent selective fluorescence sensing of Mg and Fe ions, Inorg.
Chem. Commun. 48 (2014) 1–4.
None.
[20] A.M. Asiri, N.S.M. Al-Ghamdi, H.D. Cancar, P. Kumar, S.A. Khan, Physicochemical and
Photophysical investigation of newly synthesized carbazole containing pyrazoline-
2
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3+
2+
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Fe & Fe
Acknowledgement
[
21] S.A. Khan, A.Y. Obaid, L.M. Al-Harbi, M.N. Arshad, O. Şahin, C.C. Ersanlı, R.M.A.
Rehman, A.M. Asiri, M.B. Hursthouse, Synthesis, spectroscopic, physicochemical,
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Taif University Researchers Supporting Project number ((TURSP-
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