Simple salicylaldimine-functionalized dipodal bis Schiff base chromogenic and fluorogenic chemosensors for selective and sensitive detection of Al3+ and Cr3+

Article abstract of DOI:10.1016/j.ica.2019.119192

Two salicylaldimine-functionalized dipodal bis Schiff base low cost fluorescent colorimetric chemosensors L1 (bis([4-(2-hydroxy-3-methoxybenzyl-ideneamino)phenyl]ether) and L2 (di[(4-phenylimino)4-diethylsalicyl-aldehyde]ether) have been designed and synt

Full text of DOI:10.1016/j.ica.2019.119192

Journal Pre-proofs
Research paper
Simple salicylaldimine-functionalized dipodal bis Schiff base chromogenic and
3
+
3+
fluorogenic chemosensors for selective and sensitive detection of Al and Cr
Rukhmani Chandra, Amit Kumar Manna, Meman Sahu, Kalyani Rout, Goutam
K. Patra
PII:
S0020-1693(19)31349-0
https://doi.org/10.1016/j.ica.2019.119192
ICA 119192
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
5 September 2019
3 October 2019
3 October 2019
Please cite this article as: R. Chandra, A.K. Manna, M. Sahu, K. Rout, G.K. Patra, Simple salicylaldimine-
functionalized dipodal bis Schiff base chromogenic and fluorogenic chemosensors for selective and sensitive
3
+
3+
detection of Al and Cr , Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119192
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Simple salicylaldimine-functionalized dipodal bis Schiff base
chromogenic and fluorogenic chemosensors for selective and sensitive
3
+
3+
detection of Al and Cr
Rukhmani Chandra, Amit Kumar Manna, Meman Sahu, Kalyani Rout and
Goutam K. Patra*
Department of Chemistry, Guru GhasidasVishwavidyalaya, Bilaspur (C.G)
Abstracts
Two salicylaldimine-functionalized dipodal bis Schiff base low cost fluorescent
colorimetric chemosensors L1 (bis([4-(2-hydroxy-3-methoxybenzyl-ideneamino)phenyl] ether)
and L2 (di[(4-phenylimino) 4-diethylsalicyl-aldehyde] ether) have been designed and synthesized.
3
+
3+
These sensors could simultaneously detect two biologically important metal ions (Al and Cr )
by change in both absorption and fluorescence intensity in CH CN-H O (1/1, v/v) solvent mixture
3
2
3
+
at physiological pH. The jobs plot analyses indicate the 2:2 binding stereochemistry for Al and
3
+
1
Cr ions with both the probes L1 and L2, which were further been confirmed by H-NMR and
5
-1
3+
ESI-MS studies. The binding constant values for L1 were found to be 2.35 × 10 M for Al and
5
-1
3+
5
-1
3+
5
1
.26 × 10 M for Cr and in case of L2 these values are 1.46 × 10 M for Al and 3.0 × 10
-1
3+
3+
-7
3+
-
M for Cr . The detections limits of the sensor L1, for Al (1.73 × 10 M) and Cr (1.12 × 10
7
3+
-7
3+
-7
M) and of the sensor L2, for Al (4.34 × 10 M) and Cr (7.73 × 10 M) are far below than the
limit set by the World Health Organization (WHO) for drinking water. Moreover, visible
3
+
3+
colorimetric test kitsand real samples for rapid detection of Al and Cr could be successively
applied for all practical purposes, indicating their potential use in environmental samples.
Keywords: Salicylaldimine-functionalized bis Schiff base; dipodal ligands; fluorescent sensor;
3
+
3+
colorimetric sensor; cation sensing; Al and Cr sensors.
*Corresponding Author: Tel.: 91 7587312992, E-mail: patra29in@yahoo.co.in

1
. Introduction
The development of sensitive and selective chemosensors, which is capable of recognition
of various important heavy and transition metal ions simultaneously, is an active area of current
research for analytical as well as environmental and biological problem [1-4]. Among the various
types of sensors, fluorescent chemosensors for metal ions have attracted much attention due to
their convenient use and high sensitivity [5-7]. Furthermore, colorimetric methods are also
extremely attractive because they allow naked-eye detection of color change without any use of a
spectroscopic instrument [8-10]. Trivalent cations have important biological properties and are
3
+
directly involved in the cell function where there is a critical control of M (trivalent metal ion)
levels [11,12]. Aluminum is the third most abundant and prevalent element (8.3% by weight) after
oxygen and silicon in the earth’s crust. It has been widely used in many fields, such as water
treatment, food additives, medicines, production of light alloys, electronic and electrical
components of different implements, building materials, various packing items etc. [13-15].
According to the World Health Organization (WHO) report, the average daily human intake of
aluminum is approximately 3-10 mg. However, the unregulated amount of aluminum in human
body leads to malfunction of the central nervous system, Parkinson’s disease, Alzheimer’s disease
and amyotrophic lateral sclerosis [16-18].
Chromium, one of the most common elements in the earth’s crust and seawater, exists as
0
3+
6+
3+
metallic Cr , Cr , and Cr forms. Cr is an essential component in human nutrition whereas the
6
+
3+
Cr is highly toxic. Cr plays important role in metabolism of carbohydrates, fats proteins and
nucleic acids by activating certain enzymes and stabilizing proteins and nucleic acids [19,20].
3
+
Further, the deficiency of Cr in the human body often causes different health disorders including
cardiovascular diseases and diabetes. However, a high concentration of chromium is very harmful
to human health and can also have a negative impact on cellular structures and normal enzymatic
activities [21-22]. The environmental protection agency (EPA) has set the maximum permissible
level of total chromium as 0.1 mg/mL [23-25].
3
+
Due to its paramagnetic nature Cr is the most effective fluorescent quenchers, and the
3
+
lack of a selective receptor system for Cr [26]. In the recent past, there are very few fluorescence
3
+
3+
turn-on sensors for Cr have been reported [27, 28]. In contrast, Al is diamagnetic, whose
binding with receptor often enhance the fluorescence. Due to its poor coordination ability and lack

3
+
of spectroscopy characteristics, only a few Al sensors are available in the literature as compared
to transition metal ions [29-30].
3
+
3+
As a result of the potential impact of Al and Cr on the environment and human health,
it is essential to the development of highly selective and sensitive fluorescent chemosensors, which
are capable of recognizing these metal ions by color and fluorescence responses. The most
commonly employed method used for detection of theses metal ions are atomic absorption and
atomic emission spectrometry, inductively coupled plasma mass spectrometry and electrochemical
methods [31-34] but most of these methods require sophisticated instruments and time consuming
for sample preparation protocols etc. On the contrary, fluorescence chemosensors are the best
choices, due to their high sensitivity, fast response, and inexpensive technique.
Schiff bases with o-substituted aromatic rings have found to be most responsive for
chelation with a wide range of metal ions. The coordination of metal ions through the >C=N
linkage would develop intramolecular charge transfer (ICT) transition or make ligand to metal
charge transfer (LMCT) transition, which could be useful for the visual sensing of the metal ions
[35]. In this regard and in continuation with our earlier research work [36, 37], we report here two
chemosensors bis([4-(2-hydroxy-3-methoxybenzyl-ideneamino)phenyl] ether) (L1) and (di[(4-
phenylimino) 4-diethylsalicyl-aldehyde] ether) (L2) which are 1:2 condensation products of 4-(4-
aminophenoxy) benzenamine with 2-hydroxy-3-methoxybenzaldehyde and 4-(diethylamino)-2-
3
+
3+
hydroxybenzaldehyde respectively for simultaneous detection of Al and Cr ions by naked eye
color change from yellow to colorless and flouorescence enhancement in CH CN-H O (1:1, v/v)
3
2
mixture. The strong selectivity and in-field application possibility of the developed sensors are
verified using it in real sample analysis and paper strips. In addition, the theoretical calculation
supported the experimental spectroscopic data for the sensing mechanism.
2
. Experimental
2
.1. General Information
All of the materials used for synthesis were obtained from Sigma Aldrich and used without
further purification. The analytical grade solvents wereused for the overall experiments and freshly
prepared double deionized water was used for dilution purpose and preparing CH CN-H O (1/1,
3
2
1
v/v) solvent mixture. HNMR spectra were recorded on a Bruker DRX spectrometer operating at
00 MHz in CDCl and chemical shifts were recorded in ppm relative to TMS. UV–Visible spectra
4
3
were recorded on a Shimadzu UV 1800 spectophotometer using 10 mm path length quartz

cuvettes with the wavelength in the range of 200–800 nm. The emission spectra were recorded on
a Perkin Elmer LS-55 Fluorescence Spectrometer. Mass spectra were recorded on a Waters mass
spectrometer using mixed solvent methanol and triple distilled water which was equipped with an
ESI source. The pH measurements carried out using a digital pH meter (Merck). All the chemicals
and metal salts were purchased from Merck. All the anions are of sodium salts and re-crystallized
-5
from water (Millipore) before use. Both the receptors L1 and L2 (1 × 10 M) and metal ions (1 ×
-4
1
0 M) solutions were prepared in CH CN-H O (1/1, v/v) and H O respectively.
3 2 2
2
.2. X-ray data collection and structural determination
X-ray single crystal data were collected using MoKα (λ = 0.7107 Å) radiation on a
BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data
reduction, structure solution/refinement were carried out using the software package of SMART
APEX [38]. The structures were solved by direct methods (SHELXS-97) and standard Fourier
techniques, and refined on F2 using full matrix least squares procedures (SHELXL-97) using the
SHELX-97 package [39] incorporated in WinGX [40]. In most of the cases, non-hydrogen atoms
were treated anisotropically. Hydrogen atoms were fixed geometrically at their calculated
positions following riding atom model. The crystallographic data of L1 has been listed in Table 1.
Structural information of L1 has been deposited at the Cambridge Crystallographic Data Center
(CCDC number 1893984).
Table 1. Crystallographic data and refinement parameter of the probe L1
Empirical formula
C H N O
28 24 2 5
Formula weight
468.49
102.91
Temperature/K
Crystal system
monoclinic
C2/c
Space group
a/Å
b/Å
c/Å
α/°
β/°
15.3630(12)
7.5405(6)
19.6174(15)
90
91.634(3)
90
γ/°

Volume/Å3
2271.6(3)
Z
4
3
ρ g/cm
calc
1.370
μ/mm-1
0.095
F(000)
984.0
Crystal size/mm3
Radiation
0.19 × 0.17 × 0.16
MoKα (λ = 0.71073)
5.306 to 64.54
2
Θ range for data collection/°
Index ranges
-22 ≤ h ≤ 19, -9 ≤ k ≤ 11, -28 ≤ l ≤ 25
15672
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
3896 [R = 0.0819, R
= 0.0724]
sigma
int
3896/0/161
1.043
R = 0.0636, wR = 0.1709
1
2
R = 0.0914, wR = 0.2018
1
2
0.38/-0.31
2
.3. Synthesis of L1
To a 30 mL dehydrated methanol solution of 4-(4-aminophenoxy)benzenamine (0.20 g, 1
mmol), 2-hydroxy-3-methoxy benzaldehyde (0.304 g, 2 mmol, in 10 mL dehydrated methanol)
o
was added. The mixture was refluxed for 6 h at 40 C, maintaining dry conditions. A light yellow
precipitate was obtained. It was filtered off and washed several times with n-hexane and then
finally recrystallized from methanol and dried in air. The solid compound was obtained with 0.374
0
1
g (80%) yield; m.p.115 C. H NMR: (400 MHz, CDCl , δ ppm, TMS): 13.61 (s, 2H, -OH); 8.56(s,
3
2
H, -CH=N); 7.20 (t, 4H, -Ar H); 7.02 (d, 4H, -Ar H); 6.94 (m, 4H, -Ar H); 6.81 (t, 2H, --Ar H)
1
3
and 3.87 (s, 6H, -OCH ) (Fig. S1). C NMR (200 MHz, CDCl , δ ppm, TMS): δ 161.74, 156.12,
3
3
1
51.17, 148.38, 143.59, 123.62, 122.57, 119.62, 119.04, 118.54, 114.52, 56.11 (Fig. S2). ESI-MS:
+
-1
m/z 468.36 (M ) (Fig. S3). FTIR/cm (KBr): 3516(s), 2982 (w), 2870 (m), 1621(vs), 1600 (vs),
510(vs), 1500(vs), 1435(s), 1386(m), 1182(m), 1028 (vs), 971(m), 746 (s), 613 (s) (Fig. S4).
Anal. calcd. for, C H N O : C, 71.71 (71.78%); H, 5.14 (5.16%); N, 6.13 (5.98%).
1
2
8
24
2
5
2
.4. Synthesis of L2

To a dehydrated methanol solution of 4-(4-aminophenoxy)benzenamine (0.20 g, 1 mmol,
5
0 mL), 4-(diethylamino)-2-hydroxybenzaldehyde (0.386 g, 2 mmol, in 10 mL dehydrated
o
methanol) was added. The mixture was refluxed for 6 h at 45 C, maintaining dry conditions. A
light yellow precipitate was obtained. The solid compound was obtained with 0.412 g (75%) yield.;
0
1
m.p. 150 C. H NMR: (400 MHz, CDCl , δ ppm, TMS): 13.62 (s, 2H, -OH) 8.41 (s, 2H, -CH=N);
3
7
.22 (m, 4H, -Ar H); 7.16 (m, 4H, -Ar H); 7.03 (m, 4H, -Ar H); 6.25 (d, 2H, -Ar H); 6.19 (s, 2H,
1
3
-
OH); 3.41 (q, 8H, -CH ) and 1.22 (m, 12H, -CH ) (Fig. S5). C NMR (200 MHz , CDCl , δ ppm,
2 3 3
TMS): δ 163.86, 159.90, 155.18, 151.62, 144.38, 133.59, 121.95, 119.47, 109.01, 103.64, 97.70,
+
-1
4
4.57 and 12.67 (Fig. S6). ESI-MS: m/z 551.21 (MH , 65%) (Fig. S7). FTIR/cm (KBr): 3434
m, -OH), 2970 (s), 2687(w, br) 1708 (s), 1632 (vs, C=N), 1493(s), 1354(vs), 1242(s), 1190,
132(s), 839(s), 786(s) 530(s) (Fig. S8). Anal. calcd. For C H N O : C, 74.29 (74.15%); H, 6.91
(
1
3
4
38
4
3
(6.96%); N, 10.13 (10.17%).
2
.5. UV-Vis titrations
L1 ( 4.70 mg, 0.01 mmol) and L2 (5.52 mg, 0.01 mmol) were dissolved in 10 mL of mixed
-3
solvent CH CN-H O (1/1, v/v) individually to make a solution of 1 × 10 M and 30 µL of this
3
2
solution was diluted with 2.97 mL of solvent mixture to make the final concentration of 10 µM.
Al(NO ) (3.75 mg, 0.01 mmol) was dissolved in triple distilled water (10 mL) and 1.5 - 90 µL of
3
3
3
+
this Al (1 mM) was transferred to each receptor solution L1 and L2 (10 µM). After mixing them
for a few seconds, UV-Vis spectra were obtained at room temperature.
2
.6. Fluorescence titrations
L1 (4.70 mg, 0.01 mmol) and L2 (5.52 mg, 0.01 mmol) were dissolved in 10 mL of mixed
-3
solvent CH CN-H O (1/1, v/v) individually to make a solution of 1×10 M and 30 µL of this
3
2
solution was diluted with 2.97 mL of solvent mixture to make the final concentration of 10 µM.
Al(NO ) (3.75 mg, 0.01 mmol) was dissolved in triple distilled water (10 mL) and 1.5 - 90 µL of
3
3
3
+
this Al (1 mM) was transferred to each receptor solution L1 and L2 (10 µM). After mixing them
for a few seconds, fluorescence spectra were obtained at room temperature.
3
+
The same procedure was followed for Cr using the solvent mixture CH CN-H O (1/1, v/v).
3
2
2
.7. Job plot measurements
3
+
For Al , L1 (4.70 mg, 0.01 mmol) and L2 (5.52 mg, 0.01 mmol) were dissolved in
methanol (10 mL) separately. 100, 90, 80, 70, 60, 50, 40, 30, 20, 10 and 0 µL of the each receptor

L1 and L2 solution were taken individually and transferred to vials. Each vial was diluted with 2.9
mL of mixed solvent. Al(NO ) (0.01 mmol) was dissolved in triple-distilled water (10 mL). 0, 10,
3
3
3
+
2
0, 40, 50, 60, 70, 80, 90 and 100 µL of the Al solution were added to each diluted L1 and L2
solution. Each vial had a total volume of 3 mL. After shaking them for a minute, fluorescence
spectra were obtained at room temperature.
3
+
The same procedure were followed for Cr using CH CN-H O (1/1, v/v).
3
2
2
.8. Competition with other metal ions
3
+
For Al , L {L1 (4.66 mg, 0.01 mmol) and L2 (5.52 mg, 0.01 mmol)} were dissolved in
the afore-mentioned CH CN-H O (1/1, v/v) solvent mixture individually (10 mL) and 30 µL of it
3
2
was diluted to 3 mL with the solvent mixture to make a final concentration of 10 µM. Al(NO3)3
0.01 mmol) and M(NO ) (0.1 mmol) were dissolved in 10 mL of triple-distilled water. 30 µL of
(
3
x
each metal solution (10 mM) was taken and added to 3 mL of the solution of receptor L
3
+
individually (10 µM) to give 10 equiv. of metal ions. Then, 30 µL of Al solution (10 µM) was
added to the mixed solution of each metal ion and L to make 10 equiv. After mixing them for a
few seconds, fluorescence spectra were obtained at room temperature.
3
+
The same procedure was followed for Cr using the solvent mixture CH CN-H O (1/1, v/v).
3
2
2
.9. pH effect test
A series of buffers with pH values ranging from 2 to 11 was prepared using 100 mM
HEPES buffer. After a solution with the desired pH was achieved, receptor L {L1 (4.66 mg, 0.01
mmol) and L2 (5.52 mg, 0.01 mmol)} were dissolved in methanol (10 mL) individually, and then
3
0 µL of this solution (1 mM) was diluted to 3 mL with the above-mentioned buffers to make the
final concentration of 10 µM. Al(NO ) .9H O (37.5 mg, 0.1 mmol) was dissolved in HEPES buffer
3
3
2
3
+
(
10 mL, pH 7.00). 30 µL of the Al solution (10 mM) was transferred to each receptor solution
10 µM) prepared above. After mixing them for a few seconds, UV-Vis spectra were obtained at
(
room temperature.
The same procedure was followed for Cr using the solvent mixture CH CN-H O (1/1, v/v).
3
+
3
2
2
.10. Binding constants using spectroscopic titration data
The binding constants for the formation of the respective complexes were evaluated using
the Benesi-Hildebrand plot (eqn1).
n
1
/ (A - A ) = 1 / {K (A – A )C } + 1 / (A – Ao)
(1)
o
max
o
max

3
+
3+
Ao is the absorbance of L at the absorbance maximum Al (317 nm) and Cr (313) nm for L1
3
+
3+
and Al (357 nm) and Cr (373) nm for L2, A is the observed absorbance at that particular
wavelength in the presence of a certain concentration of the metal ion (C), Amax is the maximum
3
+
absorbance value that was obtained at λ = 317 nm (L1) and 357 nm (L2) for Al and λ = 313 nm
3
+
(
L1) and 373 nm (L2) for Cr during titration with varying metal ion concentration, K is the
-1
binding constant (M ), which was determined from the slope of the linear plot, and C is the
3
+
3+
concentration of the concentration of the Al and Cr ion added during titration studies and n is
the number of metal bound per ligand.
3
+
3+
2
.11. Detection limit for Al and Cr
Thedetectionlimit was calculated based on the UV-Vis titration for Al and Cr . The
3
+
3+
absorbance spectrum of L was measured and the standard deviation of the blank measurement was
achieved. To obtain the slope, the UV-Vis absorbance at 317 nm (L1) and 313 nm (L2) for Al3+
3
+
and 350 nm (L1) and 373 nm (L2) for Cr was plotted as a function of the corresponding metal
ion concentration. The detection limit was calculated with the following equation.
Detection limit = 3σ / K
Where σ is the standard deviation of the blank measurement and K is the slope between the ratio
of the UV-Vis absorbance versus metal ion concentration.
2
.12. Colorimetric test kit
mM solution of L {L1 (4.66 mg, 0.01 mmol) and L2 (5.52 mg, 0.01 mmol)} were
prepared in 10 mL CH CN-H O (1/1, v/v). Test kits were obtained by immersing filter-papers into
1
3
2
+
+
2+
this 1 mM solution and then dried in air to get rid of the solvent. Nitate salts of Na , K , Mg ,
2
+,
3+
2+
2+
2+
2+
2+
2+
3+
2+
2+
3+
Ba Fe , Co , Ni , Cu , Zn , Cd , Ca , Cr , Hg , Pb , Al were dissolved in water (10
mL) to prepare 0.1 mM solution. The above test kits were dipped into the aqueous solution of
different cations solutions and then dried at room temperature to observe the color change.
2
.13. Theoretical calculation methods
The GAUSSIAN-09 Revision C.01 program package was used for all calculations [41].
The gas phase geometries of the compound was fully optimized without any symmetry restrictions
in singlet ground state with the gradient-corrected DFT level coupled with the hybrid exchange-
correlation functional that uses Coulomb-attenuating method B3LYP [42]. Basis set 6-31++G
was found to be suitable for the whole molecule.

3
. Results and discussion
3
.1. Synthesis of L1 and L2
The flexible Schiff-base receptors L1 and L2 were synthesized in single step in more than
5% yield as yellow crystalline solids via condensation reaction of 4-(4-
7
aminophenoxy)benzenamine with 2-hydroxy-3-methoxybenzaldehyde and 4-(diethylamino)-2-
hydroxy benzaldehyde respectively (Scheme 1) following reported procedures [43, 44].
Scheme 1. Synthetic procedures of the receptors L1 and L2.
DFT calculations were performed on the molecule L1 and L2. The geometry optimizations staring
from Gauss-view structures of L1 and L2 lead to global minimum as stationary level. The
geometry optimized structures of the receptors L1 and L2 have been shown in Fig. 1 and
representation of the energy of MOs and contours of selected HOMO and LUMO orbitals of the
receptors have been shown in Figs. S9 and S10. The HOMO to LUMO energy gapsfor L1 and L2
are 7.4068 eV and 5.566 eV respectively.
Fig.1. DFT optimized structures of molecules (a) L1 and (b) L2.

The single crystals of the probe L1 (Bis[4-(2-hydroxy-3-methoxybenzylideneamino)phenyl] ether)
were grown by slow evaporation of the methanol solution at room temperature. L1 was crystallized
as monoclinic C2/c space group. The molecule of L1 lies on a two-fold rotation axis, and each
half exhibits an imine E configuration and an O—H···N hydrogen bond (Fig.2). The dihedral angle
between the two benzene rings attached to the central O atom is 69°, and that between each of
these rings and the other benzene ring in the same half of the molecule is about 24°, which makes
the molecule flexible. Recently Kaabi et al. reported the single crystal structure of L2 [45]. The
two C=N bonds in L2 were in E (trans) configuration. The dihedral angle between the two planes
0
defined by O21 and each of the two phenyl rings is 65.00 . However, in the crystal structure, the
two halves of molecule are identical in geometry. The dihedral angles formed by COO8-O002-
0
COO8a-C007a and COO8a-O002-COO8-C007 are 43.02 . These angles are remarkably larger
0
than those formed by the biphenyl-phenol rings (65.00 ). This is possibly due to the presence of
two intramolecular O-H…N hydrogen–nitrogen interactions. The ligand L2 is having a phenol–
imine tautomer with eight molecules in each unit cell.The bond lengths of C-O (phenol) are 1.358
Å (2) and 1.354Å respectively, which lies in usual ranges [46].
Fig.2. Crystal structure of 1. Thermal ellipsoids are drawn at the 50% probability level.
3
.2.1. Colorimetric and spectral response of L1 and L2 towards various metal ions
+
+
2+
The colorimetric sensing ability of L1 and L2 towards various metal ions(Na , K , Mg ,
2
+
3+
2+
2+
2+
2+
2+
2+
3+
2+
2+
3+
Ba , Fe , Co , Ni , Cu , Zn , Cd , Ca , Cr , Hg , Pb , Al in CH CN-H O (1/1, v/v) was
3
2
3
+
3+
monitored by visual color change. In naked-eye experiment addition of only Al and Cr ions

induced a significant color change from yellow to colorless, whereas the remaining cations cause
no color change at the similar condition (Fig. 3).
L1/L2 Na+
K+ Mg+ Ba2+ Fe3+ Co2+ Al3+ Ni2+ Cu2+ Zn2+ Cd2+ Ca2+ Hg2+ Pb2+ Cr3+
Fig.3. The color changes of L1 and L2 upon addition of various metal ions (3 equiv.) in CH CN-
3
H O (1/1, v/v).
2
The spectroscopic properties of the receptors L1 and L2 towards various metal ions as Na+,
+
2+
2+
2+
3+
2+
2+
2+
2+
2+
2+
3+
2+
2+
3+
K , Mg , Ba , Mn , Fe , Co , Ni , Cu , Zn , Cd , Ca , Cr , Hg , Pb , Al were
investigated in CH CN-H O (1/1, v/v) as shown in Fig.4. Here, the free receptor L1 exhibited three
3
2
absorption bands at 234, 287 and 342 nm and the receptor L2 exhibited three bands at 271, 335
and 436 nm. Among these bands, two bands (234 and 287 nm for L1 and 271 and 335 nm for L2)
were assigned to π-π* transition, while the remaining bands (342 nm for L1 and 436 nm for L2)
were accredited to the n-π* electronic transition that originates from the promotion of non-bonding
electrons of the N atom in the C=N group to an anti-bonding orbitals of receptor L1 and L2. After
the addition of number of different metal ions, the decrease in absorption intensity around 287 nm
3
+
3+
and 342 nm for L1 and 436 nm for L2 were observed only in presence of 2 equiv. of Al and Cr .
The remaining metal ions cannot influence the absorption spectra of both the free receptor L1 and
L2 under similar condition. Also the intensity of the absorption band at 234 nm slightly increase
3
+
3+
and small band around 335 nm of L2 gets significant increased on addition of Al and Cr .

Fig.4. Absorbance spectrum of L1 and L2 (10µM) in CH CN-H O (1/1, v/v)in presence of
3
2
+
+
2+
2+
2+
3+
2+
2+
2+
2+
2+
2+
different metal ions (10µM); Na , K , Mg , Ba , Mn , Fe , Co , Ni , Cu , Zn , Cd , Ca ,
3
+
2+
2+
3+
Cr , Hg , Pb and Al .
Sensitivity was determined from absorption titration experiments in CH CN-H O (1/1, v/v)
3
2
3+
homogeneous mixture. In the titration experiment, upon increasing the concentration of Al and
3
+
Cr ions (0–2 equiv.) separately, the absorbance of L1 at 287 nm and 342 nm gets decreased and
blue shifted with concomitant increase in absorbance at 234 nm, causing an well-defined isosbestic
3
+
3+
points at 272 and 230 nm for Al and Cr ion respectively (Fig. 5). Similarly addition of 0–2
3
+
3+
equiv. analytes (Al and Cr ions) to the L2 solution, the absorbance bands at 436 nm gradually
decreased and blue shifted in each step and ultimately a new peak at 445 nm and 450 nm was
appeared. Also a significant absorption band appeared simultaneously 348 nm and 373 nm with a
3
+
3+
well-defined isosbestic points at 255 and 285 nm for Al and Cr ions respectively (Fig. 6).
Consistent with the absorption change color of the medium was changed from yellow to pale
3
+
3+
yellow in both the chemoreceptors. The metal ions (Al and Cr ) may form tight complex with
both the receptors through imine N and O site which diminish the n-π* electronic transition and
intramolecular charge transfer which causes visible sensing in the ground state.

Fig.5. Absorption spectral changes of L1 (10 µM) in the presence of different concentrations of
3
+
3+
Al and Cr in ranges of 0 - 2 equiv. in CH CN-H O (1/1, v/v).
3
2
L2
L2+
Cr
L2
L2+
Al
3
+
3
+
Fig.6. Absorption spectral changes of L2 (10 µM) in the presence of different concentrations of
3
+
3+
Al and Cr in range of 0-2 equivalent in CH CN-H O (1/1, v/v).
3
2
The binding ability depends on size, charge and electron configuration of the metal ion and
3
+
3+
ligandin addition, the acidic nature of Al is more compared to Cr So, more electron accepting
.
3
+
nature of Al has greater affinity to form metal complex. The experimental data showed that the
3
+
absorption intensities of receptors L1 and L2 after addition of Al at 317 and 423 nm fits linearly
3
+
2
with Al concentration, having a good R value of 0.99756 and 0.99454 respectively. Similarly,
3
+
2
absorption bands at 313 and 448 nm showed linear response with Cr concentration with R value

0
.9989 and 0.98617 respectively for L1 and L2. From this linear fitting, the detection limits for
3
+
3+
-7
-7
Al and Cr were determined to be 1.73 x 10 M and 1.12 x
10 M respectively for L1 and
.34 x 10 M and 7.73 x 10 M respectively for L2 on the basis of 3σ/K, where σ is the standard
deviation of the blank solution and K is the slope of the calibration curve (Fig. S11). These results
-7
-7
4
3
+
3+
suggest that the receptors L1 and L2 are very much sensitive toward Al and Cr . Importantly,
the detection limit of these chemosensors are much lower than the WHO limit and suggests that
L1 and L2 could be an effective sensor for the detection of aluminium and chromium in drinking
water. Assuming 1:1 binding stoichiometry (vide infra) the association constants were calculated
5
-1
5
-1
3+
3+
from the UV-vis titration profiles, as 2.35 × 10 M and 1.26 × 10 M for L1-Al and L1-Cr
3
+
3+
5
-1
5
-1
whereas for L2-Al and L2-Cr were found to be 1.46 × 10 M and 3.00 × 10 M respectively
by a Benesi-Hildebrand plot (Figs.S12 and S13).
3
.2.2. Competitive metal ion titrations
To examine the selectivity of the receptors in complex background of competing species,
the UV- vis absorption spectrum of receptors L1 and L2 in CH CN-H O (1/1, v/v) were
3
2
3
+
3+
investigated in the presence other metal ions. Upon treatment with 2 equiv. of Al and Cr
separately in the presence of equal equiv. of commonly employed interfering species, there was
3
+
3+
no obvious interference for the detection of Al and Cr (Fig.7). From the competitive experiment
we found that, both the metals do not interfere with each other when present in same equiv. and
consistent upto 2 fold higher concentrations.

0
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
.0
0.5
3
3
+
+
3+
3+
L1+Al
L1+Cr
L1+Al +metal ions
L1+Cr +metal ions
0
0
0
0
.4
.3
.2
.1
0.0
0.25
0.20
0.15
0.10
0.05
0.00
3
+
+
3+
L2+Al
L2+Cr
3
0.5
3+
L2+Al +metal ions
L2+Cr +metal ions
0.4
0.3
0.2
0.1
0.0
3
+
3+
Fig.7. Selectivity of L1 and L2 for Al and Cr ions in the presence of other competitive metal
ions in (10µM), the absorption wavelength were centered at 317/313 nm (L1) and 423/448 nm
(
L2), respectively for Al3+ and Cr3+ ions. Black bars represent absorption intensity after the
3
+
3+
addition of Al and Cr ions. Red bars represent the subsequent addition of the appropriate metal
+
+
2+
2+
2+
3+
2+
2+
2+
2+
2+
2+
3+
2+
2+
3+
ion (Na , K , Mg , Ba , Mn , Fe , Co , Ni , Cu , Zn , Cd , Ca , Cr , Hg , Pb , Al )
CH CN-H O (1/1, v/v).
3
2
3
.3. Fluorescence spectroscopic studies of L1 and L2 in presence of various metal ions
We have investigated the emission studies of metal ion binding properties of receptor L1
and L2 towards different metal ions (20 equiv.) in CH CN-H O (1/1, v/v) (Fig.8). The free receptor
3
2
L1 showed a broad band emission spectrum with a maximum peak centered at 530 nm when
3
+
3+
excited at 350 nm. Compared to other metal ions examined, only Al and Cr can cause the blue
3
+
shift of the emission maxima of L1 from 530 to 480 nm (in case of Al ) and 530 to 508 nm (in
3
+
3+
case of Cr ). Titration experiments were also performed with separately additions of Al and
3
+
Cr (0-20 equiv.) to the CH CN-H O (1/1, v/v) solution of L1. In both the cases, a considerable
3
2

3
+
3+
decrease in the main emission peak at 530 nm and upon further additions of Al and Cr ions, a
3
+
new peak generated with a hypsochromic shift (blue shift) centered at 480 nm (for Al ) and 508
3
+
3+
3+
nm (for Cr ) and reached saturation when 20 equiv. of Al and Cr ions were separately added.
Further increasing of Al3+ and Cr3+ did not cause significant fluorescence emission changes
(Fig.9). The blue shift in the fluorescence spectra were due to the change of the ICT properties of
the interacted species between the metal ions and the donor atom of the receptor L1.
Fig.8. Fluorescence spectrum of L1 (excited wavelength 350 nm) and L2 (excited wavelength 430
nm) 10µM in CH CN-H O (1/1, v/v) solution in presence of different metal ions (10µM) such as
3
2
+
+
2+
2+
2+
3+
2+
2+
2+
2+
2+
2+
3+
2+
2+
3+
Na , K , Mg , Ba , Mn , Fe , Co , Ni , Cu , Zn , Cd , Ca , Cr , Hg , Pb , Al .

(
A)
(B)
3
+
Fig.9. Changes in fluorescence spectra of solution L1 (10μM) upon incremental addition Al and
3
+
Cr in CH CN-H O (1/1, v/v).
3
2
3
+
3+
In the absence of Al and Cr , L2 (10 μM) is characterized by very weak emission band
centered at 490 nm (on excitation at 430 nm) due to the excited state intramolecular proton transfer
(ESIPT) from phenolic –OH to azomethaine-N and rotation across the C=N bond (cis/trans
isomerization) [47]. The weak fluorescent nature of L2 may be a consequence of strong inter and
intramolecular hydrogen bonding which may be prevent the availability of the lone pairs over
nitrogen leading to the prevent of PET and the fast inter conversion of the cis and trans forms of
L2. Due to the presence of diethyl amine (DEA) group in the L2, enhanced the hydrogen binding
ability which ultimately affected the availability of the lone pair on the N atom for PET and
provided planarity and rigidity to L2 and finally became responsible for its weak fluorescent
3
+
3+
nature. Upon concomitant additions of Al and Cr (0-20 equiv.) to the CH CN-H O (1/1, v/v)
3
2
L2 solution we obtained fully developed emission bands at 507 nm and 486 nm respectively and
3
+
3+
a stable chelation of L2 with Al and Cr (Fig.10.). The enhancement in the fluorescence were
3
+
3+
assigned due to the binding of Al and Cr with donor atoms of L2, which ultimately led to
prevents of ESIPT and cis-trans isomerization around the C=N bond. The hydroxyl group seems
3
+
to play an important role in the identification of the Al by strengthening the CHEF and hence
prevents both the C=N isomerization and PET in L2, resulting in a strong fluorescence intensity.
Furthermore, upon separate respective additions of other metal ions to the L2, no obvious

fluorescence enhancement could be observed, indicating that this change were specific only for
3
+
3+
3+
3+
Al and Cr . The selectivity of L1 and L2 for Al and Cr has been plotted as a bar graph in
Fig.S14.
(
A)
(
B)
Fig.10. Changes in fluorescence spectra of solution L2(10μM) upon incremental addition (0-20
3
+
3+
equiv.) of Al and Cr in CH CN-H O (1/1, v/v).
3
2
3
+
3+
The quenching of emission intensity of L1 in presence of Al and Cr ions was analyzed
by Stern Volmer analysis [48] using the equation,
I /I = 1+ K [Q]
0
sv
Where I is the emission intensity of the free receptor L1, I is the emission intensity of L1 in
0
3
+
3+
presence of various concentration of analytes (Al and Cr ), K is the Stern Volmer constant and
sv
[
(
(
Q] is the concentration of quencher.From the plot a linear dependency of I /I with analytes
0
3
+
3+
Al and Cr ) concentration were observed with the corresponding regression coefficient value
2
R ) were 0.9871 and 0.9834 respectively, indicates the static quenching mechanism. The values
-1
3+
3+
of K were calculated to 2068.88 and 2782.76 M for Al and Cr respectively from the slope of
sv
these calibration curve (Fig. 11).

3
+
3+
Fig.11. Stern Volmer plot of L1 in the presence of (a) Al and (b) Cr .
.4. Stoichiometry and binding mechanism of sensing of the probes L1 and L2 with metal ions
3
3
+
3+
To investigate the binding characteristics of L1 and L2 towards Al and Cr , Job’s
titration method based on absorbance were performed, in which the absorption intensity reached
to a maximum at the 0.5 molar ratio for the both metal ions, indicated that L1 and L2 bound to
3
+
3+
Al and Cr with either 1:1 or 2:2 ratio (Fig.S15). As in the bis-Schiff base ligands L1 and L2
the effective distance between two arms is very large, thus 1:1 binding not possible. Furthermore,
the 2:2 binding stoichiometry were confirmed through mass spectral studies.In the positive ion
3
+
3+
mass spectrum peaks obtained at m/z =495.25 [2L1+2Al ] and m/z =520.25 [2L1+2Cr ] for L1
3
+
3+
and m/z =575.20 [2L2+2Al ], m/z =602.09[2L2+2Cr ] for L2 (Fig.S16; Z=2 in each case)
confirm 2:2 binding.
3
+
3+
1
For better understanding the binding nature of L1 and L2 with Al , and Cr ions, H-NMR
spectra was recorded with and without these metal ions. Both the receptors L1 and L2 showed
chemical shift value δ at 13.8 ppm and 13.7 ppm due to phenolic protons and 8.6 ppm and 8.4 ppm
3
+
due to azomethine protons respectively. Upon binding with Al , the phenolic protons almost
disappeared because of deprotonation of phenolic groups (Fig. 12). The azomethine proton and the
aromatic protons shift at higher δ values, which might be attributed to ligand to metal charge
3
+
3+
transfer. Overall changes in the chemical shifts of the protons in the L-Al and L-Cr adducts
where L = L1 and L2) clearly reveals that the aldimine nitrogen (-CH=N) and -OH of L (both L1
and L2) taking part in the complexation process. The most probable reason for this observation is
(

the formation of symmetrical dinuclear/dimeric complexes of two ligands with two metal ions
(Scheme 2).
1
3+
3+
3+
3+
Fig.12. partial H NMR spectra of (a) L1, L1- Al , L1- Cr and (b) L2, L2- Al , L2- Cr .
3
+
3+
Scheme 2 Binding mechanism of L1 and L2 with M (M = Al and Cr ).
3
.5. pH effect test
It is well- known that fluorescence sensors based on the electron donor/acceptor ends are
usually hindered by the concentration of protons in the medium for identifications of the metal
ions. Thus, for many practical applications, it is very necessary whether the sensors are active or

not at the suitable pH conditions for successful operation. Therefore, we measured the fluorescence
3
+
3+
intensity of L1 and L2 and their Al and Cr adducts between pH 2 and 13. In acidic media with
a pH range of 1-4, the fluorescence intensity of the L1 at 530 nm beginning to quench gradually.
On the other hand, the fluorescence intensity did not change over a wide pH range of 4-12
(Fig.S17A).While the free receptor L2 are very weekly fluorescent in the pH range of 2-13.
3
+
3+
However, in the presence of 2 equiv. of Al and Cr separately, a significant increase in the
emission intensity of L2 recorded at 507 and 486 nm in the same pH range (4-8) were observed
reaching a maximum at pH 8.0 and then decrease rapidly with a further increase in pH (Fig.S17B).
The fluorescence intensity decrease in the acidic (pH <6) region due to protonation of phenolic
hydroxyl group of L2 and hence decreases the chelation ability of L2. Again the decreasing
emission intensity in basic (pH >9) region has been explained due to ICT [47], which hampered
the complexation.Hence, the emission intensity is stable over this wide range of pH (5-9) and well-
suitable for applications under physiological pH conditions.
3
.6. Reversibility study of the probes L1 and L2
Reversibility is a pre-requisite in developing novel sensor for practical application.
Consequently, we investigated the chemical reversibility of the binding of the probes L1 and L2
3
+
3+
towards Al and Cr by using sodium nitrate (NaNO ). The absorption intensity of the L-M
3
3
+
3+
adduct (Where, L = L1 and L2 and M = Al and Cr ) returning to original level upon addition of
two equiv. of NaNO (Fig. 13) solution along with the color of the solution changed back to the
3
original yellow instantly, indicating the regeneration of the receptor L1 and L2. Again, upon
3
+
3+
addition of Al and Cr ions solution, the absorption intensity of L1 at 341 nm was blue shifted
3
+
3+
to lower wavelength at 317 and 313 nm respectively for Al and Cr ions but in case of L2,
absorbance band at 436 nm was decreased and a new peak at 348 nm and 373 nm appeared
3
+
3+
respectively for Al and Cr ions. This ensures that L1 and L2 to be a sensitive sensors which
can be applied in environmental analysis.
3
+
3+
If Al and Cr both the ions are present in the real sample then the color of the CH CN-
3
3
+
3+
H O (1:1, v/v) solution of L1 and L2 will be yellow. Thus the detection of Cr in presence of Al
2
in real sample is somewhat difficult. This problem can be solved by using the complexing agent
3
+
3+
cupferron, as it precipitated with Al and excess addition has no role towards Cr at that
3
+
3+
3+
condition. Thus no problem arises for detection of Cr from the mixture of Al and Cr .

3
+
3+
Fig.13. Reversible investigation of L1 and L2 (10 μM) for Al and Cr (10 μM) upon the addition
of NaNO (10 μM) in CH CN-H O (1/1, v/v).
3
3
2
3
.7. Application of the chemosensors L1 and L2 in real samples
In order to determine the environmental application of the chemosensors L1 and L2,
3
+
3+
artificial Al and Cr contaminated sample has been prepared. The tap water samples were spiked
3
+
3+
with Al and Cr standard solutions at different concentration levels, and then tested their
concentrations with the proposed method. The percentage of recovery along with standard
deviations of the spiked samples analysed by the probe L1 and L2 give satisfactory results and
good R.S.D. values (Tables 2 and 3).
3
+
3+
Table 2 Determination of Al and Cr ions in tap water samples by the probe L1
Metal ion
Spiked amount
Recovered
%Recovery± SD
(μM)
amount(μM)
(n=3)
Al3+
10
9.42
94.2 ± 2.2
99.0 ± 1.2
2
0
19.80

Cr3+
10
9.23
92.3 ± 1.8
2
0
20.76
103.55 ± 1.3
3
+
3+
Table 3 Determination of Al and Cr ions in tap water samples by the probe L2
Metal ion
Spiked amount
Recovered
%Recovery± SD
(μM)
amount(μM)
(n=3)
Al3+
10
9.45
19.60
9.16
94.5 ± 2.1
98.0 ± 1.3
91.6 ± 1.4
104.0 ± 1.5
2
0
Cr3+
10
2
0
20.80
3
.8. Colorimetric test-kits
To check the practical application of the probes L1 and L2, test kits were prepared by
immersing papers in a CH CN solution of L1 and L2 and then dried in air. These test kits were
3
3
+
3+
used to sense Al and Cr ions among different cations. When the test kits coated with receptors
L1 and L2 were added into different cation solutions, the obvious color change were observed
3
+
3+
only with Al and Cr in CH CN-H O (1/1, v/v) solution which have been shown in Fig.14.
3
2
Hence, the test kits coated with receptors L1 and L2 solution would be suitable for simultaneously
3
+
3+
detection of Al and Cr by showing color changesin the presence of different metal ions.
Fig.14. Photographs of the test kits with L1 and L2 for detecting the Al3+ and Cr3+ ions in
acetonitrile-water solution (1:1, v/v) with other cations.
3
+
3+
3
.9. Comparison of L1 and L2 with other trivalent (Al and Cr ) chemosensors
The probe L1 and L2, have been compared with some other recently reported colorimetric
3
+
3+
trivalent (Al and Cr ) chemosensors (Table 4). Our system has some advantages over the others.
In our method L1 and L2 have been prepared by facile, single step synthetic procedure, using
inexpensive reagents without troublesome and chromatographic purification. They have very

quick response time and wide linear response range. They work in CH CN–H O (1 : 1, v/v). It has
3
2
fairly low detection limits and high association constants.
Table 4 Comparison of L1 and L2 with other colorimetric trivalent chemosensors
Probe
No. of
steps
involved
3
Solvent
LOD
Ka
Ref
3
+
9
Pure
0.3µM(Al ) 6.46x10
0.5µM(Cr ) M (Al )
49
3
+
-2
3+
CH CN
3
4
1
.58x10
-
1
3+
M (Cr )
3
+
3
3
3
CH CN- 23µM(Al )
HEPES
Buffer
8.77x10
50
51
3
3
+
-1
3+
25µM(Cr )
M (Al )
3
5.67x10
-1
3+
(40/60,
M (Cr )
v/v)
3
+
THF-
H O
2
0.38nM(Al ) Not
0.38nM(Cr ) determine-
d
3
+
(8:2)
3
+
4
2
3
H O-
EtOH
0.5µM(Al ) 2.00x10
52
53
2
3
+
-1
3+
0.2µM(Cr ) M (Al )
4
(8:2)
5.50x10
-1
3+
M (Cr )
3
+
4
CH OH- 1.74nM(Al ) 1.00x10
3
3
+
-1
3+
H O
2.36nM(Cr ) M (Al )
2
2
Buffer
2.60x10
-1
3+
(6/4, v/v)
M (Cr )

3
+
4
3
CH OH- 0.34µM(Al ) 8.20x10
54
3
3
+
-2
3+
H O
0.31µM(Cr ) M (Al )
2
4
(1/1, v/v)
6.00x10
-1
3+
M (Cr )
3
+
5
1
1
CH CN- 0.17µM(Al ) 2.35x10
This
3
O
3+
-1
3+
H O
0.12µM(Cr ) M (Al ) wor
2
5
(1/1, v/v)
1.26x10
k
N
-1
3+
L1
N
M (Cr )
OH
HO
OCH3
OCH3
O
3+
5
CH CN- 0.43µM(Al ) 4.34x10
This
3
3
+
-1
3+
H O
0.77µM(Cr ) M (Al ) wor
2
N
N
5
L2
(1/1, v/v)
7.73x10
k
OH
-1
3+
HO
M (Cr )
N
N
4
. Conclusion
In conclusion, we report here two simple salicylaldimine-functionalized dipodal bis Schiff
base fluorescent colorimetric chemosensors L1 and L2 for easy and convenient detectionof Al3+
3
+
and Cr over the other competing biologicallyrelevant metal ions in CH CN-H O (1:1, v/v). These
3
2
3
+
chemosensors exhibited visual color change from yellow to colorless upon addition of Al and
3
+
3+
3+
Cr . The fluorescent intensity of L1 increases upon addition of Al and Cr in CH CN-H O (1:1,
3
2
v/v), whereas in case of L2, fluorescent intensity increases. The binding abilities of the probes L1
3
+
3+
1
and L2 with Al and Cr were established by the combined UV-Vis, Fluorescence, H-NMR and
3
+
-7
3+
-7
ESI-MS methods. The detection limits of L1 for Al (1.73 × 10 M) and Cr (1.12 × 10 M) and
3
+
-7
3+
-7
of the sensor L2, for Al (4.34 × 10 M) and Cr (7.73 × 10 M) are far lower than those set by
3
+
3+
the WHO guidelines, thus these could be effective sensors for the detection of Al and Cr in
drinking water. L1 and L2 could be successfully applied to test kits and real samples for detection
3
+
3+
of Al and Cr .
Acknowledgements
G.K.P would like to thank the Department of Science and Technology (SR/FST/CSI-
64/2014; EMR/2017/001789) and Department of Biotechnology, Government of India, New
2

Delhi for financial support. A.K.M. thanks the CSIR, Government of India for financial support
as a form of fellowship.
Supporting Information Summary
CCDC number 1893984contains the supplementary crystallographicdata for the probe L1.
The data can be obtained free of charge from the Cambridge Crytallographic Data Centre, 12
Union Road, Cambridge CB21EZ, UK; Fax: (+44)1223-336-033; or email:
deposit@ccdc.cam.ac.uk: Supplementary data containing Fig. S1-S17 of this article can be found
online at…………………………..
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