A multifunctional Schiff-base as chromogenic chemosensor for Mn2+ and fluorescent chemosensor for Zn2+ in semi-aqueous environment

Article abstract of DOI:10.1016/j.ica.2019.04.053

In this communication, a new multifunctional Schiff-base (S) obtained by reacting hydrazine with 3,5-dichloro-salicylaldehyde. The sensing ability of S was studied with various metal ions by visual test, UV–Vis and spectrofluorometric techniques in THF:H<

Full text of DOI:10.1016/j.ica.2019.04.053

Accepted Manuscript
Research paper
2
+
A multifunctional Schiff-base as chromogenic chemosensor for Mn and flu-
2
+
orescent chemosensor for Zn in semi-aqueous environment
V. Raju, R. Selva Kumar, Y. Tharakeswar, S.K. Ashok Kumar
PII:
S0020-1693(19)30229-4
https://doi.org/10.1016/j.ica.2019.04.053
ICA 18898
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
17 February 2019
28 April 2019
29 April 2019
Please cite this article as: V. Raju, R. Selva Kumar, Y. Tharakeswar, S.K. Ashok Kumar, A multifunctional Schiff-
2
+
2+
base as chromogenic chemosensor for Mn and fluorescent chemosensor for Zn in semi-aqueous environment,
Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.04.053
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2
+
A multifunctional Schiff-base as chromogenic chemosensor for Mn and
2
+
fluorescent chemosensor for Zn in semi-aqueous environment
a,b
a
b
a
V. Raju , Selva Kumar R , Tharakeswar Y , S.K. Ashok Kumar *
a
Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore-
32014, India, E-mail: ashokkumar.sk@vit.ac.in
6
b
Centre for Applied Science, Sree Vidyanikethan Engineering College, Tirupati, Andhra Pradesh-517102,
India.
Abstract
In this communication, a new multifunctional Schiff-base (S) obtained by reacting hydrazine with
3
,5-dichloro-salicylaldehyde. The sensing ability of S was studied with various metal ions by
visual test, UV-Vis and spectrofluorometric techniques in THF:H O (1:1, v/v) media.
2
Accordingly, visual test was confirmed by changing initial colourless solution of S to yellow upon
2
+
interaction with Mn while for other ions, it did not respond in natural light. However, same
solution composition when it was observed under UV light, it exhibits green fluorescent colour
2
+
2+
2+
with Zn . In order to see more photo physical properties of S-Mn and S-Zn are studied through
by spectrophotometric and spectrofluorometric methods respectively. Spectrophotometrically,
2
+
neat S exhibits a strong absorption band at 365 nm upon interaction with Mn , it exhibits a new
absorption band at 425 nm. Spectroflurometrically, a new emission peak was observed at 500 nm
2
+
upon excitation of S-Zn at 390 nm while other metal ions did not exhibit any emission properties.
2
+
2+
Further, the formation 3:2 binding stoichiometry between S with Mn and Zn were studied by
using Job’s plot and association constant for were estimated by Benesi-Hilderbrand (BH) method.
The detection limit of S for Mn2+ and Zn2+ were found to be 3.6×10 M and 9.2×10 M
−7
−8
respectively, which is much lower than the WHO guidance of their metal ions in drinking water.
2
+
2+
The coordination behaviour of S with Mn and Zn are well supported by NMR and theoretical
calculations.
Keywords: Chemosensor, colorimetric, chromogenic, fluorescent, manganese, zinc
1
. Introduction

The hasty advancement of modern industries and agriculture, contamination of environment by
heavy metals and transition metals is attained more serious concern. Hence, there is an immense
attention for the detection of these heavy metals [1] and transition-metal ions [2]. Manganese is
abundantly available in the environment and our food, which afford an average daily intake of
about 5 mg/kg [3-5]. Manganese is an important trace nutrient in all forms of life with wide variety
of biological functions [6]. It participates as structural component of diverse classes of enzymes
and photosynthetic apparatus [7, 8] and is widely used tool in biological studies. However, an
elevated level of manganese can result in central nervous system (CNS) disorders [9], which can
cause a series of symptoms, such as fatigability, sleep disturbances, muscular pain and hypertonia,
gait changes, hallucinations and mental irritability, finally leading to a Mn-induced Parkinson
2
+
disease called Mn-induced Parkinsonism [10]. The high spin Mn as an outstanding magnetic
resonance imaging (MRI) relaxation agent with an interest as a tool in neurobiological research
2
+
[
11]. Therefore, it is an intense need to develop selective sensors of Mn that can distinguish from
2
+
other metal ions especially Ca [12-14]. Among the various valences exhibited by manganese,
2
+
Mn is quite stable in environmental samples. Thus, many efforts have been devoted to develop
2
+
diverse class of assays for Mn ions with high sensitivity and good accuracy.
Zinc is the most profuse metal ion available in human body plays a crucial role in enzyme
regulation, co-factors in metalloenzyme catalysis, gene expression and neural signal transmission.
Over exposure of zinc can induces apoptosis, epilepsy, ischemia and Alzheimer’s disease [15-18]
while its deficiency can lead to several malfunctions in the body such as diarrhoea, growth
2
+
retardation, impotence and delayed sexual maturation [19-22]. Zn ions do not have unpaired
electrons due to this its aqua complexes are colourless and hence it is difficult to detect zinc ions
2
+
in biological and environmental samples [23]. In this regard, designing of an efficient Zn sensor
is indispensable. For this reason, fluorescent probes are the potent tools to monitor zinc ions
because the selective binding of Zn2+ ions upon chemosensor can causes variation in the
fluorescent properties [24,25].
In last two decades a numerous analytical technique, including X-ray photoelectron spectroscopy
(
XPS), inductively coupled plasma atomic emission and mass spectroscopy (ICP-AES, ICP-MS)
have been widely employed for the detection of various metals even at very low concentration
26]. Unfortunately, these techniques are relatively expensive, complicated sample procedures and
[

time-consuming. As an alternative to these expensive techniques, a number of various
chemosensors have been reported to date [27-32]. Among them, colorimetric and fluorometric
chemosensors are the effective tool for the detection and quantification of metal ions and anions,
as this recognition process offers naked eye detection, high efficiency, high sensitivity, simplicity
and low-cost [33]. In spite of these advantages, colorimetric chemosensors for manganese
detections are very rare [34-36]. Therefore, it is still a great challenge to develop highly selective
and sensitive sensor for the recognition of Mn2+ and Zn2+ ions, workable in physiological
circumstance would be of great interest.
2
. Experimental Section
2
.1. Materials and Instrumentation
The solvents like ethyl alcohol, tetrahydrofuran, ethyl acetate and hexane, and all the metal salts
to be purchased either from Sigma-Aldrich or Merck of high purity and are used without any
further purification. All the spectroscopic and fluorometric experiments were carried at room
temperature.
−
1
The infrared spectrum was recorded on a Nexus FT-IR in the range 4000-400 cm using ATR
1
13
mode. The H and C NMR spectra were recorded with Bruker AM 400 spectrometer with
tetramethylsilane (TMS) as internal standard. The UV-Vis absorbance spectra were recorded on a
Shimadzu UV-2450 spectrophotometer. Fluorescence spectra were recorded on JASCO FP-8200
Spectrofluorometer. The pH was measured with Eutech pH–510.
2
+
2+
The 3D structure of S and its complexes S-Zn and S-Mn were obtained by applying the density
functional theory (DFT) method in the gas phase. All calculations were performed by using the
exchange-correlation functional B3LYP and the basis sets LANL2DZ for Zn and Mn atoms,
whereas 6-311G(d,p) for the remaining C, H, N, O and Cl atoms coded in the computational
programme Gaussian 09W [37]. In vibrational frequency analysis there is no imaginary frequency
2
+
2+
for an optimized geometry of S, S-Mn and S-Zn suggesting these geometries are represented
local minima [38].
2
.2. Synthesis of sensor S

Sensor S was prepared according to previously reported literature procedure [39] using a simple
single step condensation reaction of hydrazine with 3,5-dichloro-2-hydroxybenzaldehyde shown
Scheme-1. In 100 ml round bottom flask, 3,5-dichloro-2-hydroxybenzaldehyde (1mmol, 2.03 g)
was dissolved in 10 ml ethyl alcohol and then hydrazine hydrate (1mmol, 1.78 g in 10 ml ethyl
alcohol) were added drop wise. The mixture was stirred for about 30 min. The precipitate formed
was filtered off and recrystallized with an absolute alcohol.
Cl
Cl
Cl
Cl
Cl
O
EtOH
RT
H N NH2
2
+ 2
HO
N N
OH
HO
Cl
(S)
Scheme 1. Reagents and conditions to synthesize S
1
Yield: 93%; Colour: yellow; melting point: 202-204 °C; H NMR (Fig.1S) (400 MHz, CDCl ), δ
3
1
3
ppm: 7.29 (d, 2H), 7.51 (d, 2H), 8.64(s, 2H), 11.79 (s, 2H); C NMR (Fig.2S) (100 MHz, DMSO-
d6), δ ppm: 120.75, 122.47, 123.83, 130.03, 132.93, 153.81 and 164.07; FT-IR (ATR) (Fig.3S), ν
–
1
cm : 3072 (Ar-C−H), 1624 (C=N), 1562 (C=C). HR-MS m/z (C H Cl N O ) (Fig.4S): 378.0380.
1
4
8
4
2
2
3
. Results and discussion
2
+
2+
The selectivity, sensitivity and binding mode of S toward Mn and Zn were examined through
colorimetric, absorption, emission, time-resolved fluorescence decay, NMR, and DFT studies.
3
.1. Chromogenic sensing of Mn2+
The chromogenic sensing performance of S towards diverse metal ions was studied by observing
the naked-eye visible color change and recording the corresponding UV-Vis spectra. Sensor S (10
μM) showed a distinct color changes from colorless to yellow upon the addition of Mn2+ in
THF:H O (1:1, v/v) media which can be visually detected by naked-eye (Fig. 1a). Under similar
2
environment, no such apparent color changes were observed in the presence of other tested metal
+
2+
3+
2+
2+
2+
2+
3+
2+
3+
ions such as K , Mg , Al , Pb , Ni , Hg , Co , Cr , Zn and Fe . Further, UV-Visible
absorption spectra of S (10 μM) were recorded in the absence and presence of different metal in

THF:H O (1:1, v/v) media. The absorption spectra of sensor S showed a major absorption peak at
2
2
+
3
70 nm which may be due to π→π*. While, the addition of Mn ions to S solution resulted in the
instantaneous appearance of new band at 425 nm, this is due to charge transfer from ligand-to-
metal (LMCT) [40]. No such spectral changes were observed upon the addition of other tested
+
2+
3+
2+
2+
2+
2+
3+
2+
3+
metal ions such as K , Mg , Al , Pb , Ni , Co , Hg , Cr , Zn and Fe ions (Fig. 1b). The
spectra and distinct color change from colorless to yellow may be due to the formation of stable
2
+
2+
S-Mn complex. These interpretations supported the high selectivity of sensor S towards Mn
among the diverse tested metal ions. Further, to confirm the real-time recognition in formation of
2
+
stable complex, time evolution process of sensor S in the presence of Mn was investigated. The
2
+
binding interaction of S-Mn complex was found to be maximum in the span of 8 min (Fig. 5S).
(a)
(b)
Fig.1 Sensing performance of S with different metal ions under (a) colorimetric test (b) UV-Vis
spectral response in THF:H O (1:1, v/v) media.
2

2
+
The binding constant between S and Mn along with the sensitivity of sensor S was determined
2
+
by performing the UV-Vis titration. For the titration, successive incremental amount of Mn was
added to S (10 µM) solution, and the spectra were recorded after each addition. The band at 370
nm is declined gradually and a new absorption band at 425 nm increases with the incremental
2
+
addition of Mn (Fig. 2a) and an isosbestic point at 395 nm is indicating the formation of new
2
+
complex species. The absorption of S with the addition of different concentration of Mn found
to obey the Beer's law by showing a linear response at 425 nm (Fig. 2b). From the calibration
curve, the limit of detection (LOD) was calculated by applying the IUPAC approved equation:
LOD =3σ/slope (where ‘σ’ represents the standard deviation of the blank sample) and it was
estimated down to 0.36µM and LOQ was found to 5.86 µM respectively [41]. Importantly, the
2
+
LOD value of S for Mn is far below the WHO guideline (7.28 μM) in the drinking water [42,
3], suggesting that S could be an influential chemosensor for the detection of manganese in the
drinking water. The binding constant (K ) was calculated using the BH plot [44] drawn by plotting
4
a
2
+
2+
the change in absorbance against 1/ [Mn ]. The magnitude of binding constant for S with Mn
5
-1
3
12
-1
was found to be 8.5×10 M (Fig. 2c). The obtained value is within the range of (10 ~10 M )
those previously reported literature for Mn2+ binding chemosensor [45-47]. The binding
2
+
stoichiometry of complexation of S with Mn was studied using Job’s plot method [48] (Fig. 2d)
2
+
and by plotting the change in absorbance with respective to change in mole fraction of Mn . The
2
+
plot gave maximum absorbance at 0.4 which inferred the formation of S-Mn complex with 3:2
ligand-to-metal ratio.
(b)
(a)

(d)
2
+
Fig.2 (a) UV-visible spectral response of S after addition of Mn in THF:H O (1:1, v/v) media
2
(b) Calibration curve (c) Job’s plot (d) BH plot
2
+
The influence of pH over the absorbance of sensor S and its complex S-Mn were studied within
the pH range of 1 to 11 (Fig. 6S). It has been observed that the colour of sensor S remains
unchanged over the pH variation from 0 to 7.5; whereas its complex colour is unaffected between
the pH ranges of 4-7.5. Further, absorbance and wavelength maximum remain unaltered over this
pH region. The increase in absorbance value starts above pH region 7.5 this could be due to
deprotonation of hydroxyl group of S lead to intramolecular change transfer process. The
absorbance at pH below pH 4 decrease may be due to the protonation of binding site which restrict
2
+
the complex formation. Hence, the working pH range for sensing Mn falls in the range of 4.0 -
.5.
7
2
+
To ensure the dynamic response of the S-Mn interactions, the binding reversibility studies of S-
Mn2+ complex in aqueous media was performed, which is very important for practical
2
+
applications. Owing to the high stability constant of EDTA-Mn complex, it was expected that
2
+
2+
2+
EDTA will uncomplex Mn from the S-Mn complex. Quenching of absorbance of the S-Mn
complex takes place upon addition of 2 eq. of EDTA (Fig. 3a). Further accumulation of Mn2+
resulted in the recovery of absorbance and colour to maximum extent representing that the binding
2
+
of receptor S with Mn is reversible.
(a)(c)
(b)

2
+
2+
Fig. 3 (a) Reversible sensing nature of S-Mn complex with sequential addition of EDTA and Zn (b)
2
+
Selectivity of S with Mn in the presence of other metal ions in THF:H O (1:1, v/v) media
2
The preferential selectivity of sensor S as a visual and chromogenic sensor for the detection of
2
+
Mn ion in the presence of diverse competing metal ions was investigated. For interference studies,
-3
2+
sensor S was treated with 1×10 M of Mn ion solution separately in the presence of similar
concentration of other metal ions, as indicated in (Fig 3b, 7S). The obtained results indicate that
2
+
+
2+
3+
2+
there was no interference observed in the detection of Mn from the tested K , Ca , Al , Pb ,
2
+
2+
2+
3+
2+
3+
Ni , Hg , Co , Cr , Zn and Fe ions. Results show a small deviation on relative error (˂5%)
in absorption maxima in presence of other interfering ions and consequently it is possible to detect
2
+
Mn clearly with naked eye. Besides this, the performance of the developed sensor can be
compared to the reported literatures [59-64] which were presented in Table 1S.
3
.2. Fluorogenic sensing of Zn2+
In order to quantify visual test carried out under UV light, more studies have been conducted using
spectrofluorometric method. Accordingly, the fluorescence spectral pattern of sensor S when
excited at 390 nm in the presence of various metal ions exhibited their non-fluorescent behavior
2
+
and becomes highly fluorescent upon Zn addition. Amongst all the metal ions tested, the S
2
+
exhibits high fluorescence enhancement at λ at 500 nm with Zn . Sensor S displayed maximum
em
2
+
2+
Zn -induced fluorescence enhancement (4-fold), showing its higher affinity towards Zn (Fig.
a). Under a UV lamp, S displayed a color change from colourless to green upon the addition of
4
2
+
Zn , which could easily be recognized by the naked-eye (Fig. 4b). The selective enhancement of
fluorescence response of S with Zn2+ could be caused by the operation of CHEF (chelation
2
+
enhanced fluorescence) mechanism [55, 56] owing to their efficient coordination of Zn with S
over the other metal ions. The obtained fluorescence spectra can be explained on the facts that

sensor S is weakly fluorescent due to the isomerization of the C=N double bond at the excited state
along with the excited-state proton transfer (ESPT) involving the phenolic proton of the substituted
2
+
salicylaldehyde moiety. Upon stable chelation with Zn , the C=N isomerization is inhibited and
thus prevents the ESPT process, leading to the fluorescence enhancement. Further, the
2
+
complexation of Zn with S causes rigidity in the resulting molecule and tends to produce large
chelation enhanced fluorescence (CHEF) response which induces the pronounced fluorescence
enhancement. Further, to confirm the real time detection in formation of stable complex, time
2
+
evolution process of sensor S in the presence of Zn was investigated. The binding interaction of
2
+
S-Zn complex was found to be maximum in the span of 3 min (Fig. 8S). The effect of pH on the
2
+
fluorescence response of S and S-Zn were studied in the pH range 1-11 shown in Fig. 9S. Both
2
+
in S and S-Zn the intensity remains same in the working pH range 4-8. Below pH=4 and above
pH=8, there decrease in emission intensity due to the protonation of binding sites in acidic region
2
+
while in basic region S undergo deprotonation and Zn undergo formation of Zn(OH) .
2
(a)
(b)
Fig. 4 (a) Visible test carried inside UV chamber (b) Fluorescence spectral changes of S with different
meta ions in THF:H O (1:1, v/v, λ =370 nm)
2
ex

2
+
2+
To check sensitivity of S towards Zn , the fluorescence titration of S-Zn was performed. The
2
+
spectral changes of S (10 μM) upon incremental addition of Zn (0-15 μM), a new emission band
2
+
at ~505 nm with increasing in intensity was observed (Fig. 5a). The limit of detection of Zn with
S was calculated according to IUPAC guidelines and the obtained value was 92 nM which is far
below the WHO guidelines of drinking water (76 μM) (Fig. 5b) [57]. The binding constant (Ka)
2
+
5
for Zn with S was calculated by using BH method and the estimated K was found to be 1.66×10
a
−
1
2+
M (Fig. 5c). The binding interaction between S and Zn was found by using Job’s plot analysis
Fig. 5d). The plot shows maximum emission intensity at a mole fraction of 0.4, which corresponds
to the formation of 3:2 ligand-metal binding ratio.
(
(a)
(b)
(d)
(c)
2
+
Fig. 5 (a) Fluorogenic spectral titration of S with Zn (b) Calibration curve (c) BH plot (d) Job’s plot
2
+
To examine the practical utility of S as a selective fluorescence sensor for Zn , the competitive
2
+
+
2+
3+
2+
studies were investigated in the presence of Zn with other metal ions such as K , Ca , Al , Pb ,

2
+
2+
2+
3+
2+
3+
2+
Ni , Hg , Co , Cr , Mn and Fe . When S was interacted with Zn in the presence of the
equal concentration of other metal ions (Fig. 6a, 10S), each solution excited at wavelength 370
nm and measured emission intensity at 505 nm. The obtained data reveals that the coexistent metal
2
+
ions had a small and negligible effect on the emission intensity of the S-Zn .
Development of novel chemosensor for practical applications, binding reversibility is a
2
+
prerequisite. Hence, we studied the reversibility experiment [58] of S with Zn and EDTA (Fig.
2
+
6
b). The addition of EDTA to a solution of S-Zn complex resulted in the quenching of its
emission intensity, which represents the regeneration of the free S. Further accumulation of Zn2+
ion to the solution again, the fluorescence intensity was recovered to original intensity of S-Zn2+
2
+
complex. The sequential addition of Zn and EDTA to S, exhibited the same fluorescence change.
2
+
These results imply that the sensor S could be a reversible sensor for Zn . Besides this, the
performance of the synthesized sensor has been compared with reported works [49-54] which were
summarized in Table 2S.
(a)
(b)
2
+
Fig. 6 (a) Interference study (b) Reversible sensing of S with Zn and EDTA
The fluorescence quantum yield of S and S-Zn2+ were calculated in ACN by employing
comparative William’s method using quinine sulfate as a standard (Fig. 7). The estimated
2
+
fluorescence quantum yields for S and S-Zn was found to be 0.03 and 0.13 respectively. The
2
+
time resolved fluorescence decay analysis of S and S-Zn complex reveals that the fluorescence
lifetime found to be 4.25ns and 4.97ns respectively. Using the fluorescence quantum yield and

lifetime data it is possible to calculate the rate constant for radiative decay process (K ) and non-
r
radiative decay process (K ) [65]. Accordingly, the calculated values of K and K for S are
nr
r
nr
6
-1
7
-1
2+
7
-1
7
.06x10 s and 22.8x10 s respectively. Whereas K and K values of S-Zn are 2.62x10 s
r
nr
7
-1
2+
and 17.5x10 s respectively. The results show that the when S is complexed with Zn not only
increase in the radiative decay process but also there is decrease in the non-radiative decay process.
2
+
This supports the enhancement of fluorescence upon the complexing with Zn ions.
2
+
Fig. 7 Time resolved decay profile of S and S-Zn complex.
3
.3. Binding mechanism
2
+
1
The sensing mechanism of S with Zn was studied by H NMR titration. As seen in Fig. 8, receptor
S alone exhibits four different characteristic peaks at 11.84, 9.07 and 7.73-7.77ppm for -OH, -
2
+
N=CH and aromatic (H and H ) respectively. When S titrated with different equivalence of Zn ,
a
b
hydroxy proton (-OH) starts to disappear and imine (-N=CH) proton shows shielding effect. After
2
+
addition of 1eq. of Zn , -OH protons are completely disappeared and -N=CH and aromatic (H
a
and H ) protons are shifted to 9.00, 7.70 and 7.66 ppm respectively, which confirms the S-
b
2
+
2+
1
Zn complex formation by involvement of -OH, and -N=CH with Zn . The H NMR spectral
2
+
titration of S with Mn2+ could not be studied due to paramagnetic nature of Mn , its complex
2
+
with S-Mn .

1
2+
Fig. 8 H NMR titration of S with Zn in DMSO-d
6
3
.4. Theoretical Studies
2
+
2+
To study insights on the observed photophysical changes of S, S-Mn and S-Zn , the DFT
calculations was carried out using Gaussian 09. The initial geometry of S and their metal
2
+
2+
complexes S-Mn and S-Zn were optimized by applying B3LYP-6-311G(d,p) and B3LYP-6-
11G(d,p)-LANL2DZ effective core potential respectively. In order to avoid the computational
3
time, the initial geometries were preoptimized using ground state semiempirical method. The
energy minimized structures are shown in Fig 9. The sensor S shows planar structure was stabilized
with the presence of intramolecular hydrogen bonding between the phenolic -OH and imine-N
2
+
2+
with the bond length of 1.954 Å. The metal complexes S-Mn and S-Zn were shown octahedral
geometries by involving the coordination of 3N O donor system. Each S could act as N O
2
2
2
2
coordination mode with metal center. The obtained geometry was well matched with the reported
in the literature on similar type of ligand with first tow transition metal complex [66,67].
2
+
2+
Accordingly, the interaction energy (E = Ecomplex-Eanalyte) was calculated for S-Mn and S-Zn
int
as -698 kcal/mol and -655 kcal/mol respectively, which is supporting the experimentally obtained
2
+
5
-1
2+
5
-1
values on binding constant of S-Mn (8.5 x10 M ) is higher than that of S-Zn (1.66 x10 M )

complex. Both change in interaction energy and binding constant values are supports the formation
2
+
2+
S-Mn complex as stable complex than S-Zn complex under similar conditions.
2
+
2+
Fig. 9 DFT Optimized structures of S, S-Mn and S-Zn
2
+
2+
Further, the frontier molecular orbitals analysis of S, S-Mn and S-Zn shown in Fig. 10. The S
exhibits, HOMO and LUMO orbitals are located through the ligand-to-ligand charge transfer with
2
+
a band gap of 3.36eV. In case of S-Mn , the HOMO orbitals is mainly located on throughout the
ligand system and the LUMO orbitals are located on metal center, which supports the possible
charge transfer from ligand-to-metal center (LMCT). This supports the change in color from
2
+
colorless to yellow upon the complexation with Mn further supported with decrease in band gap
2
+
from 3.36 eV to 1.24 eV. In case of S-Zn complex the HOMO and LUMO orbitals are mainly
located only on ligand system, due to this there is charge transfer process takes place upon
2
+
complexation with Zn . However, there is reduction in band gap was also noticed (1.13 eV),
2
+
which supports the formed S-Zn complex may results in inducing enhanced fluorescence.

2
+
2+
Fig. 10 FMO’s of S, S-Mn and S-Zn by DFT/B3LYP method
3
.5 Applications
The potential adequacy and practical applicability of sensor S, was applied for the quantitative
estimation of Mn2+ and Zn in tap water samples. In each experiment individually a known
2+
2
+
2+
quantity of Mn or Zn was added to tap water samples, the recovery of metal ion in spiked water
sample were summarized in Table 3S. The obtained data reveals gives satisfactory recovery and
relative standard deviation (RSD) value for the triplicate tap water sample. Thus, the obtained
2
+
2+
results imply that sensor S can be used for the detection of Mn and Zn in real samples.
4
. Conclusion
We designed and developed a new chemosensor ‘S’ based on salicylaldehyde-hydrazine Schiff
2
+
2+
2+
base for highly sensitive and specific recognition of Mn and Zn . It can distinguish Mn from
other metal ions by color change (colorless to yellow). In addition, S enabled the analysis of Mn2+
with detection limit of 0.36 μM, which is below the guideline of WHO in drinking water.
2
+
Moreover, sensor S can also be employed as a fluorogenic chemosensor for Zn . The addition of

2
+
Zn to S showed drastic fluorescence enhancement through an effect of the CHEF. The binding
2
+
2+
ability of S-Mn and S-Zn were also chemically reversible with EDTA. On the other hand, the
2
+
2+
sensor used particularly detect Mn and Zn selectively in in the presence of other competitive
metal ions. Therefore, these results signify that the sensor S would be a platform for the
development of ‘a single chemosensor for two different analytes’ with dual modes.
5
. Acknowledgment
Authors are thankful to DST-VIT-FIST for NMR, SIF-VIT for GC-MS and Vellore Institute of
Technology, Vellore and Sree Vidyanikethan Engineering College, Tirupati for providing research
facilities.
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1

Graphical Abstract
Highlights




Designed sensor ‘S’ significantly senses Mn2+ by naked-eye colorimetric method and
2
+
detected Zn through fluorescence with high selectivity.
2
+
2+
Exhibited an excellent binding constant and lowest detection limit towards Mn and Zn
ions.
2
+
2+
This sensor serves as reversible chemosensor for Mn and Zn by using a strong chelator
such as EDTA.
1
The binding mechanism were studied by H NMR and DFT Calculations.