Combined experimental and theoretical studies on a phenyl thiadiazole-based novel turn-on fluorescent colorimetric Schiff base chemosensor for the selective and sensitive detection of Al3+

Article abstract of DOI:10.1039/d0nj01954b

Herein, we have presented a phenyl thiadiazole-based Schiff base receptor, 2-[(Z)-(5-phenyl-1,3,4-thiadiazol-2-ylimino)methyl]-6-methoxy-4-nitrophenol (L) for the first time for turn-on fluorescent, colourimetric detection of Al3+ ions. The chemosensor L displayed very quick responses, excellent selectivity and sensitivity towards Al3+ ions in methanol-Tris-HCl buffer medium (10 mM, pH 7.2, and 1?:?1 v/v) with colourimetric and fluorometric detection limit values of 1.43 × 10-7 M and 1.15 × 10-7 M, respectively. The binding stoichiometry was obtained as 2?:?1 from the Job plot analysis and ESI-MS spectra with the association constant value of 1.76 × 102 M-1/2. The experimental results have further been supported by thorough DFT and TD-DFT studies. The chemosensor L can be applied for the formation of binary logical devices, recovery of contaminated water samples and smartphone-based chemical analysis.

Full text of DOI:10.1039/d0nj01954b

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Combined experimental and theoretical studies on Phenyl thiadiazole
based novel turn-on fluorescent colorimetric Schiff base chemosensor
for the selective and sensitive detection of Al3+
View Article Online
DOI: 10.1039/D0NJ01954B
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Amit Kumar Manna,a Shubhamoy Chowdhuryb and Goutam K. Patraa*
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aDepartment of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G),
India
bDepartment of Chemistry, Gour Banga University, Malda, West Bengal 732
103, India
Abstract
Herein, we have presented phenyl thiadiazole based Schiff base receptor, 2-[(Z)-(5-phenyl-1,3,4-
thiadiazol-2-ylimino)methyl]-6-methoxy-4-nitrophenol (L) for the first time for turn-on
fluorescent, colourimetric detection of Al3+ ion. The chemosensor L displayed very quick response,
excellent selectivity and sensitivity towards Al3+ ion in methanol-tris-HCl buffer medium (10 mM,
pH 7.2, 1:1 v/v) with colourimetric and fluorometric detection limit values 1.43 × 10-7 M and
1.15 × 10-7 M, respectively. The binding stoichiometry was obtained as 2:1 from job’s plot analysis
and ESI-MS spectra with the association constant value of 1.76 × 102 M-1/2. The experimental
results have further been supported by thorough DFT and TD-DFT studies. The chemosensor L
can be applied for the formation of binary logical devices, recovery of contaminated water samples
and smartphone-based chemical analysis.
Keywords: Phenyl thiadiazole/ colourimetric sensor/ fluorometric sensor/ Al3+ sensor/ Schiff base/
smartphone/ logic gate.
*Corresponding Author: Tel.: 91 7587312992,E-mail: patra29in@yahoo.co.in

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Introduction
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DOI: 10.1039/D0NJ01954B
In supramolecular chemistry, the analyte sensing by a suitable chemoreceptor primarily depends on
host-guest binding through non-covalent interaction like hydrogen bonding, metal-ligand
coordination, van der Waals interactions etc.1-3 Depending on the nature of the emitted signal; it
was named as electrochemical, potentiometric and optical sensor.4 In the case of optical
chemosensor, two main processes, like selective recognition and signal transformation, take place
during analyte detection. The cavity size, the hardness of the coordinating atom and flexibility of
chelator maintain the selectivity by stabilising the host-guest interaction, where as the guest induced
electronic/energy change of the chromo/fluorophore produces a proper analytical signal.5 Upon
incorporation of selective analytes not always produce measurable optical signal; thus proper
functioning of tailor-made chemosensor is indispensable to generate signal through absorption
and/or emission change during host-guest interaction.6
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Recently the emission-based analyte sensing studies have been performed parallelly with
the absorption-based colourimetric measurements, due to its extraordinary sensitivity. Several non-
radiative deactivation pathways such as photo-induced electron transfer (PET),7-8 imine (>C=N)
isomerisation9-10 or the combination of the both,11 can quench the fluorescence intensity of the free
receptor. In PET process, receptors containing non-bonding electron pair donors, typically N, O
atoms are having HOMO energy in between the HOMO-LUMO energy levels of the excited
fluorophore. Thus, electron transfer occurs from the free receptor side to the excited fluorophore,
i.e. in excited state redox reaction takes place, which can quench the fluorescence. Also, in the
excited state, free imine (>C=N) bond behaves almost single bond character; thus non-radiative
deactivation occurs due to the inter-conversion in both cis and trans isomers via free rotation.
Specific metal ion binding through N, O donor sites and >C=N moiety, restrict these deactivation
processes and thus, emission intensity can be enhanced due to chelation enhancement fluorescence
effects (CHEF).12
Among the metals, aluminium (Al) is the most abundant metal in the earth crust (8.3%) and
mainly presents in its trivalent-ionic form Al3+.13-15 Due to easy availability, it is extensively used in
daily life such as food additives, packing materials, water treatments and for several industrial
purposes.16,17 Generally an average of 3-10 mg/day of this metal is consumed by the human body
through drinking water and foods, but it has a negligible physiological role.18,19 High concentrations
of Al3+ ion is dangerous for living aquatic species like algae, bacteria, fish and plants20 and inhibit
plants growth.21 Abnormal intake of Al3+ ion can cause several adverse effects such as Parkinson
diseases, Alzheimer diseases, amyotrophic lateral sclerosis and so on.22,23 It also acts as a
competitive binder for several essential trace metal ions of similar characteristics like Mg2+, Ca2+

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and Fe3+ in the body; thus may reduce their activity.24,25 Accordingly, for human concern WHO has
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DOI: 10.1039/D0NJ01954B
provided guidelines that the maximum tolerable limit of Al ions in the body is not more than 10
mg/day.26
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Some classical analytical techniques like chromatography,27 inductively coupled plasma
emission spectroscopy (ICP-ES),28 potentiometry29 and atomic absorption spectroscopy (AAS)30
were used earlier for the detection of metal ions like Al3+. Due to major drawbacks in terms of
troublesome sample preparation, selectivity, sensitivity, time-consuming, and requirement of
skilled personnel, chemosensor is used as an alternating method for detection and quantification of
Al3+.31 Recently many optical chemo-receptors have been constructed by some Schiff base host, as
the >C=N backbone with proper substituent provided a suitable platform for metal ion sensing in
aqueous or semi-aqueous medium.32 When the Schiff base chemosensor was constructed for
selective chelation of Al3+ ion, it has great demand because unlike other metal ions, Al3+ sensing
has lack of spectroscopic characteristics like charge transfer process and formation of insoluble
Al(OH)3 due to its high charge density. Thus optical-based chemosensor for precise detection and
quantification of Al3+ ion is still challenging.
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From the above-mentioned facts and in continuation to our current research interest on
searching on fluorescent-colourimetric Schiff base chemosensor,33-35 we have designed and
synthesised 5-phenyl-1,3,4-thiadiazole based novel turn-on fluorescent, colourimetric Schiff base
chemoreceptor L by combining one thiadiazole unit with a salicylaldehyde derivative. During the
synthesis of chemoreceptor L, -OH and -N rich hard basic centre, soft -S moiety along with
borderline imine (-C=N) site has been introduced in the skeleton of L for interaction with metal
ions rapidly. Here, electron-donating methoxy group has been utilised to generate strong ligand
field environment and chromophoric nitro group substituted moiety has been selected to visualise
colour through extra conjugation. As expected, proposed receptor L can properly sense Al3+ ion
both colourimetrically and turn-on fluorometrically in an aqueous medium. The recognition process
was further confirmed by the DFT studies, and it has been applied in real sample analysis,
smartphone-based analysis and in the building of molecular logic gates.
Experimental
Materials and General information
All the required materials used for synthesis were obtained from Sigma-Aldrich and directly used.
The analytical grade solvents used for the overall experiments and freshly prepared double
deionised water was used for dilution purpose and preparing tris HCl buffer (10 mM, pH=7.2)
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solution. The metal ion solutions were prepared from their nitrate salts. H NMR and C NMR

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spectra were recorded on a Bruker DRX spectrometer operating at 400MHz in DMSO-d6 solvent,
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DOI: 10.1039/D0NJ01954B
and chemical shifts were recorded in ppm relative to TMS. Absorption spectra were recorded on a
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Shimadzu UV 1800 spectrophotometer using 10 mm path length quartz cuvettes with the
wavelength in the range of 200-800 nm. High-resolution mass (HRMs) spectra were recorded on a
Waters mass spectrometer using mixed solvent HPLC methanol and triple distilled water. The pH
measurements were done using a digital pH meter (Merck) by adjusting dilute hydrochloric acid
and sodium hydroxide in a buffer solution. Solutions of the receptor L (1×10−5 M) and metal salts
(1×10−4 M) were prepared in methanol-tris-HCl buffer (10 mM, pH 7.2) medium (1:1 v/v) and H2O
respectively.
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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 a CCD area detector. Data collection, data reduction,
structure solution/refinement were carried out using the software package of SMART APEX.36 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
package37 incorporated in WinGX.38 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 5-phenyl-1,3,4-thiadiazol-2-amine (1), the
precursor of L has been listed in Table 1. Structural information of 1 has been deposited at the
Cambridge Crystallographic Data Center (CCDC number 1993595).
Synthesis of 5-phenyl-1,3,4-thiadiazol-2-amine (1)
5-phenyl-1,3,4-thiadiazol-2-amine (1), the precursor of L, has been synthesised by following the
reported procedure39 with some modification. Initially, we prepared the individual hot methanolic
solution of benzaldehyde, 1.06 gm. (10 mmol) in 15 mL and thiosemicarbazide 0.75 gm. (10 mmol)
in 15 mL and then mixed these two solutions. This mixture was refluxed for 2h at 700C.
Thiosemicarbazone was obtained as colourless crystal after slow evaporation of methanol which
was further used in the next step without purification and characterisation. The thiosemicarbazone
and FeCl3 (30 mmol) were dissolved in 15 mL distilled water and heated to 80-90 for 1 hour. A
mixture of citric acid (22 mmol) and sodium citrate (10 mmol) were added to the hot solution and
again stirred for 2h. After cooling the solution, neutralised with 10% aqueous ammonia. The brown
product was filtered out, dried and re-crystallised with methanol. Brown needle-like crystals were
obtained on evaporation of methanol solvent, which were suitable for single crystal XRD analysis.
Yield, 1.362 g, 77%; mp. >2200C. Anal. Calc. for C8H7N3S: C, 54.22; H, 3.98; N, 23.71. Found C,
1
54.37; H, 3.93; N, 23.81%. ESI–MS: m/z 178.04 (1-H+, 100%) (Fig. S1). H NMR (400 MHz,

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DMSO-d6, TMS, J in Hz): δ 7.76-7.74 (2H, NH2, J=6.8), 7.49-7.42 (5H, aromatic protons, J=2; 6.4;
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DOI: 10.1039/D0NJ01954B
7.6; 11.2) (Fig. S2). C NMR (DMSO-d6, δ ppm, TMS): 168.68, 156.85, 131.43, 130.06, 129.61,
126.77 (Fig. S3).
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Synthesis of 2-[(Z)-(5-phenyl-1,3,4-thiadiazol-2-ylimino)methyl]-6-methoxy-4-nitrophenol (L)
0.885 g (5 mmol) of 5-phenyl-1,3,4-thiadiazol-2-amine (1) was dissolved in 20 mL of dehydrated
methanol and to it 0.985 g (5mmol) of 2-hydroxy-3-methoxy-5-nitrobenzaldehyde (5-nitro
o- vanillin) was added dropwise with constant stirring. Then the reaction mixture was refluxed for
12h under dry condition. After the reflux is over, the reaction mixture was kept in air for
evaporation; while yellow solid separated out. Yield, 1.47g, 83 %; mp.>2000C. Anal. Calc. for
C16H12N4O4S : C, 53.93; H, 3.39; N, 15.72. Found C, 53.84; H, 3.46; N, 15.81%. ESI–MS: m/z
357.04 (L+H+, 100%) (Fig. S4). FTIR/cm-1 (KBr): 3430 (-OH) 3160 (-CH), 1606(vs, C=N), 1556
(vs) 1512 (vs, NO2), 1410 (m), 1312 (m), 1332 (s), 1160 (m), 750 (s), 670 (m) (Fig. S5). 1H NMR
(400 MHz, DMSO-d6, TMS, J in Hz): δ 8.11 (1H, J=2.8), 7.92 (1H, J=2.4), 7.75 (2H, J=1.6; 6.4),
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7.49-7.43 (m, 5H, aromatic, J=2.0; 8.4; 7.6; 8.0), 3.99 (s, 3H, OMe) (Fig. S6). C NMR (DMSO-
d6, δ ppm, TMS): 189.81, 168.98, 157.36, 156.85, 149.66, 139.55, 131.41, 130.07, 129.61, 126.77,
122.00, 116.29, 110.59, 57.23 (Fig. S7).
Preparation of Stock solution for photophysical measurement
Receptor L solution was prepared initially at the concentration level of 1 × 10-3 M in 10 mL in
methanol-tris HCl buffer medium (10mM, pH 7.2) solution (1:1 v/v), then diluted to the desired
concentration. Also, the stock solutions of guest ions were prepared separately from their nitrate
salts (except sulphate salt of Mn2+ and Fe2+) at a concentration of 1 x 10-3 M in 10 mL double-
deionised water and further diluted to their desired concentration. After mixing L with each of the
metal ions for a few seconds, absorption and fluorescence spectra were obtained at room
temperature.
Computational details
The GAUSSIAN-09 Revision C.01 program package was used for all calculations.40 The gas-phase
geometries of the molecule L and its Al3+ complex (2L+Al3+) were optimised entirely unrestricted
of symmetry in singlet ground states with B3LYP functional41 and LanL2DZ42 respectively as the
basis set. The electronic spectra of the molecule L and 2L+Al3+ were calculated with the TD-DFT
method, and the solvent effect (in methanol) was simulated using the polarising continuum model
with the integral equation formalism (C-PCM).43,44 The light-induced electron transfer (LIET)
calculation method is a superior to investigate the MLCT/LLCT transitions.45

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Table 1 Crystal data and structure refinement for 1.
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CCDC Number
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
1993595
C8H7N3S
177.23
109.38
monoclinic
P21/c
11.1251(14)
7.5242(7)
11.1940(12)
90
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β/°
γ/°
Volume/Å3
Z
115.098(15)
90
848.55(19)
4
ρcalcg/cm3
μ/mm-1
1.3872
0.324
F(000)
368.6
0.18x0.12x0.10
Mo Kα (λ = 0.71073)
Crystal size/mm3
Radiation
2Θ range for data collection/°4.04 to 52.72
Index ranges
-14 ≤ h ≤ 14, -8 ≤ k ≤ 10, -15 ≤ l ≤ 13
4133
1730 [Rint = 0.0188, Rsigma = 0.0304]
1730/0/110
1.002
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)] R1 = 0.0385, wR2 = 0.0982
Final R indexes [all data] R1 = 0.0548, wR2 = 0.1084
Largest diff. peak/hole / e Å-3 0.32/-0.28
Results and discussion
Synthesis and structure of L
Among the most of the N, O based Schiff base receptor for Al3+ ion, the proposed chemosensor L
has been synthesised through the easy and economical procedure with high yield (over 80%) by 1:1
condensation reaction of 5-phenyl-1,3,4-thiadiazol-2-amine (1) and 5-Nitro-o-vanillin (Scheme 1). 1, the
precursor of L has also been synthesised by FeCl3 induced cyclisation of thiosemicarbazone
(Scheme 1) and its structure was determined by X-ray crystallography. 1 crystallised as monoclinic
P21/c space group. The ORTEP diagram of 1 has been shown in Fig. S8. In 1, the bond lengths and
angles are in the expected ranges (Table S1 and S2). Unfortunately, we didn’t obtain and good-

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quality single crystal for L. The structure of L has been fully characterised by FTIR, H-NMR,
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13C-NMR, ESI-mass spectra and elemental analyses.
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Scheme 1. The synthetic procedure of the probe L.
Generally, bioaccumulation of metal ions was mainly originated through water
contamination; thus, the organic receptor should be highly water-soluble and can detect analytes in
an aqueous medium. Here, the probe L cannot be completely soluble in pure water due to the
presence of hydrophobic aromatic side-chain; thus, methanol can be used to improve water
solubility. Although it is frequently soluble in 1:4 methanol-water binary mixture, the stock
solution of L has been prepared in methanol-tris-HCl buffer medium (10 mM, pH 7.2) solution (1:1
v/v), owing to the stability over 5 days.
UV-Vis Spectroscopic studies of L towards different metal ions
The receptor L contains suitable positioned donor–π conjugation-accepter moiety; thus responsible
for typical intramolecular charge transfer (ICT), which can influence the photophysical behaviour.
In order to check the ICT process, absorption spectra of L were taken in the different solvent of
variable polarity like dimethylsulfoxide (DMSO), acetonitrile, methanol and chloroform, as the
solvent dipole can relax the ICT absorption band46 through polarisation/ interaction with multiple
chromophoric systems. As shown in Table 2, small red-shifted absorption bands of L obtained in
relatively more polar solvent DMSO in comparison to moderately polar solvent CH3CN, CH3OH
(Fig. S9) and less polar CHCl3. However, moderately polar solvent MeOH has been selected for
further studies, as it is an environmentally benign solvent, well water miscibility and easy to handle.

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Table 2 UV-Vis data of L in different solvent.
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Solvent
Methanol
CH3CN
DMSO
Logε

maxL
412 nm
441 nm
453 nm
340 nm
3.58
3.50
3.79
4.05
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CHCl3
Initially, the metal selective response of the proposed chemosensor L has been carried out
through UV-Vis spectral studies at room temperature in methanol-tris-HCl buffer (10 mM, pH 7.2)
solution (1:1 v/v). The absorption maxima of free receptor displayed at 304, 362 and 412 nm
corresponding to the existence of two different chromophoric subunits and overall ICT band, which
imparts its deep yellow colourisation. Upon monitoring the 2 equivalent of some biologically
important metal ions like Na+, K+, Mg2+, Fe3+, Ag+, Cu2+, Mn2+, Co2+, Ni2+, Mn7+, Cr6+, Mo6+ and
Ga3+ to the probe solution, negligible change in absorption spectra was observed, while in the
presence of Zn2+, Cd2+, Hg2+, Pb2+, Fe2+ and Pd2+ the absorption band at 372 nm was enhanced with
a decrease at 301 nm due to interaction with S atom of the heterocyclic ring. Unlikely, the addition
of only Al3+ the absorption band at 412 and 362 nm completely vanished, whereas for Cr3+ these
bands exist partially. Consistently, the deep yellow colour of receptor solution immediately
changed to colourless in case of Al3+ ion, very pale yellow in case of Cr3+ and intense yellow colour
in case of the thiophilic metal ions (Fig. 1a).
Fig. 1. (a) Absorption spectra of L (40 µM) changes in presence of 2 equiv. of different metal ions
in methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v). Inset: corresponding colour change
of the solution; (b) Competitive experiment in presence of L and other metal ions (where 1 = L, 2 =
Na+, 3 = K+, 4 = Mg2+, 5 = Fe3+, 6 = Ni2+, 7 = Co2+, 8 = Cd2+, 9 = Hg2+, 10 = Pb2+, 11 = Cu2+, 12 =
Ag+ , 13 = Zn2+, 14 = Mn2+ 15 = Al3+,16 = Cr3+, 17=Mn7+, 18=Cr6+, 19=Mo6+, 20 =Ga3+, 21=Fe2+
and 22= Pd2+) in methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v).

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3+
Achievement of specific Al ion selectivity of L over other interfering ions is an important
feature to construct a chemosensor. Thus the competitive studies were performed in methanol-tris-
HCl buffer (10 mM, pH 7.2) medium (1:1 v/v) by titrating with all the metal ions systematically
into a fixed L-Al3+ solution. From fig. 1b, Al3+ ion can influence the same absorption change of L,
irrespective the other metal ions present or not. Even in the presence of 5 fold higher concentrations
of above-mentioned competitive ions than Al3+ ion, the optical behaviour of L-Al3+ complex
remains consistent.
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As the receptor L contains acid-base sensitive groups like –OH, -OMe and >C=N, the pH
range on the sensing process has primarily set through absorption measurements. For this purpose,
the ligand solutions were suitably adjusted within pH value ranging from 3 to 12 in a series
maintaining identical concentration using 0.1 M HCl and NaOH solution and then absorption
studies were performed in the presence of Al3+ ion (Fig. 2a). At low pH, multiple coordination sites
were protonated, which can retard Al3+ binding as well as ICT process. At pH above 10, ICT effect
becomes much more predominant in case of L, and there was a chance of insoluble Al(OH)3
formation, which can create hindrance for binding. Combining these two effects, the absorption
intensity around 412 nm of L+Al3+ somewhat increases at a highly basic medium. Based on these
observations, the sensing studies were performed at pH 7.2 using tris-HCl buffer, where the acid-
base sensitive groups cannot be disturbed.
Fig. 2. (a) pH effect of L and L–Al3+complex (b) absorption titration of L with Al3+ ion in
methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v).
In order to get the sensitive response on absorption behaviour of receptor L in the presence
of Al3+ ion binding, titration experiment was conducted in methanol-tris-HCl buffer (1:1 v/v, 10
mM, pH 7.2) solution. With the increase in Al3+ ion concentration in the ligand L solution, both the
absorption bands at 361 and 412 nm decreased gradually with a little enhancement of absorption

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band 301 and at 250 nm (Fig. 2b). Almost 0.6 equivalent of Al3+ ion was enough to reach the
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DOI: 10.1039/D0NJ01954B
saturation of absorption change and the deep yellow colour of the ligand solution transformed to
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colourless. A well-defined isosbestic point was developed at 345 nm, indicating that both the L and
L+Al3+ complex are in dynamical equilibrium. The absorption study and visual colour change
primarily indicate that restriction of ICT process of “nitro ortho-vanillin” moiety upon Al3+ ion
complexation, mainly responsible for colourimetric sensing (Scheme 2, vide infra). In the free
receptor L, the -OH group (ICT donor) present in suitable positioned with electron-withdrawing -
NO2 moiety (ICT acceptor) thus intra-molecular charge transfer (ICT) operate in the ground state
which imparts the absorption band at 412 nm with yellow colourisation. When the Al3+ ion binds
with the imine site and the -OH group, the ICT becomes restricted, which decrease in the
absorption band at 412 nm and thus, the yellow colour of the medium changed to colourless.
From the titration data, the variation of absorption intensity at 412, 361, 304 and 250 nm with
the concentration of Al3+ ion has been shown in the inset of fig. 2b. Among these plots, the
absorption band at 412 nm, changes much more rapidly with a linear relationship up to 18 µM.
The limit of detection (LOD) value was calculated at S/N= 3 to 1.43 × 10-7 M from the linear fitting
of this calibration curve based on the convenient formula 3σ/m. From the calibration curve I412 vs.
[Al3+] the slope ‘m’ and from the absorbance vs. [L] plot, the standard deviation ‘σ’ was obtained.
The calculated value of LOD, is much lower than the threshold value specified by WHO and many
of the previously reported Al3+ ion sensors (Fig. 3a).
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Fig. 3. (a) Colourimetric detection limit of L for Al3+and (b) determination of association constant.
To evaluate the binding stoichiometry, the conventional Jobs continuous method analysis
was used (Fig. S10). It indicates that the receptor L binds with analyte Al3+ ion in a 2:1 fashion
which was primarily assumed from absorption titration and further confirmed by mass

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spectrometry. In the ESI-MS the base peak obtained at m/z = 773.06 corresponds to 2L+Al3++2H2O
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3+
DOI: 10.1039/D0NJ01954B
(Fig. S11). Assuming 2:1 binding interaction, the association constant value for L-Al complex
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was obtained to 1.76 × 102 M-1/2 (Fig. 3b) by the Benesi-Hildebrand (B-H) linear fitting of the
inverse of absorption change vs. square root of inverse Al3+ ion concentration which is highly
comparable with the previously reported 2L+Al3+ complexes.47
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In order to understand the nature of the binding interaction and re-usability of the present
chemosensor L, chemical reversibility was verified by absorption spectra in the presence of 1
equiv. of Na2EDTA in methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v). Here the
absorption band at 412 and 361 nm immediately appeared on addition of Na2EDTA to L-Al3+
adduct due to the generation of free receptor L with a colour change from colourless to yellow.
Further addition of excessive Al3+ ion, these optical changes almost restored. This alternative
conversion of the complexation and de-complexation was almost consistent over 5 cycles with
systematic addition of Al3+ and Na2EDTA with negligible loss of absorption intensity (Fig. 4a).
Thus the chemosensor L is reversible and the acidic Al3+ ion binds through chelation rather than
any acid catalyst hydrolysis or irreversible reaction.
Fig. 4. (a) Reversibility study of L with Al3+ and Na2EDTA in methanol-tris-HCl buffer (10 mM,
pH 7.2) solution (1:1 v/v) (b) Time effect of L with Al3+ ion detection in methanol-tris-HCl buffer
(10 mM, pH 7.2) solution (1:1 v/v) by absorption studies.
The time evolution of the receptor L for colorimetric Al3+ ion sensing was also investigated
through absorption spectral measurements in methanol-tris-HCl buffer (10 mM, pH 7.2) solution
(1:1 v/v) (Fig. 4b). As soon as Al3+ ion was added into the receptor solution, the absorption band
at 412 nm and 362 nm immediately disappeared with a very quick colour change from colourless to
yellow was observed in naked eyes. This colour and absorption changes remain consistent over an

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extended period of time. This indicates that the recognition behaviour gets almost completed within
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DOI: 10.1039/D0NJ01954B
2 minutes and cannot be disturbed up to 12 min, thus can be utilised for monitoring Al ion for
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practical purposes.
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Fluorescence studies
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The selective emissive properties of the chemosensor L towards metal ions, fluorescence
measurements were conducted under identical working media in methanol-tris-HCl buffer (1:1 v/v,
10 mM, pH 7.2) solution. In free receptor L, excitation of phenyl-thiadiazole fluorophore at 310
nm, the moderate emission band at 380 nm was observed. Sequential addition of 2 equiv. of each of
the selected metal ions like Na+, K+, Mg2+, Zn2+, Cd2+, Hg2+, Pb2+, Fe2+, Pd2+, Al3+, Fe3+, Ag+, Cu2+,
Mn2+, Co2+, Ni2+, Mn7+, Cr3+, Cr6+, Mo6+ and Ga3+ to the chemosensor L, only Al3+ ion can enhance
the emission intensity much more prominently, whereas the heavy metal ions such as Cd2+, Hg2+,
Pb2+, Cu2+, Ag+ , Zn2+, Pd2+ etc. quenched the original emission band of L (Fig. 5a). As in the
fluorescence quenching process, it is very difficult to differentiate the low-intensity signal from the
false pulsing response by the background environment or due to precipitation of the receptor; thus,
it is considered that the fluorometric response of the receptor L obtained only in the presence of
Al3+ ion. In the competitive experiments, the Al3+ ion depending emission spectra of L cannot be
disturbed by the presence of other quenching metal ions in equal concentration. Although the 5 fold
higher concentration of interfering ions compare to Al3+ ion, slightly decrease the intensity but the
existence of Al3+ ion was still measurable from emission spectra in the mixed solution (Fig. S12).
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Fig. 5. (a) Fluorescence spectra of L in presence of 2 equiv. different metal ions in methanol-tris-
HCl buffer (10 mM, pH 7.2) solution (1:1 v/v). (b) Fluorescence titration of L with Al3+ in
methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v).
The excitation and the emission spectra of L, as well as L+Al3+ have been measured in
methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v) and shown in Fig. S13. Upon
excitation at 310 nm, the free receptor L showed moderate emission centred at 380 nm due to some

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radiative decay from the excited fluorophore phenyl-thiadiazole. In the chromophore L, the PET
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operates from the –OH site to the excited fluorophore, but its efficiency is very less, as it also
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participates in delocalisation at the same time with the –NO2 group. After PET, the single electron in
the excited fluorophore comes to the singly vacant orbital of the receptor through back electron transfer
(BET) process. In the presence of Al3+ ion, the receptor L forms a tight complex, which diminished
both the PET and >C=N isomerisation processes, and emission intensity is enhanced due to CHEF.
In the titration experiments, when the concentration of Al3+ ion was increased to the fixed solution
of L, emission intensity at 380 nm increased gradually and became saturated when the
concentration of Al3+ reached to 1.2 equiv. (Fig. 5b). In the presence of Al3+ ion, the receptor L
forms a tight complex which diminished both the PET and >C=N isomerisation processes, and
emission intensity is enhanced due to CHEF. Furthermore, the emission intensity change showed a
linear relationship with the Al3+ ion concentration and the detection limit was calculated to 1.15 x
10-7 M from this calibration curve using the same formula 3σ/m (Fig. S14). This value is slightly
differing from the DL obtained in absorption spectra as the difference in sensitivity of both this
spectrophotometer.
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Jobs plot analysis has also been performed through fluorometric measurement. 2:1 mode of
binding of L with the analyte Al3+ ion is also confirmed by this measurement (Fig. S15). In pH
studies, the emission intensity of host-guest complex is maximum within pH 6.0-9.0 (Fig. 16).
Below pH 5.0, H+ ion is a competitor for Al3+ ion binding and above pH 9, there is a chance of
insoluble Al(OH)3 formation. Here also the host-guest complex shows reversible behaviour in
presence of Na2EDTA and retained up to three cycles with negligible loss in fluorescence intensity
(Fig. S17). In the time evolution of the receptor L in presence of 2 equiv. Al3+ ion was also
investigated in methanol-tris-HCl buffer (10 mM, pH 7.2) solution (1:1 v/v). The strong emission
band of L at 380 nm due to Al3+ recognition, was appeared within 2 min and remains almost
unchanged up to 12 min (Fig. 18) emission. This quick fluorometric response time of the receptor L
towards Al3+ ion is very crucial to be a sensitive sensor and can be applied in environmental
analysis.
DFT studies on L and 2L-Al3+ complex (2)
The optimised structure of L and 2 are shown in Fig. 6, and listed their calculated bond distances
and angles in Table S1. The bond lengths C1-N1 and C7-O1 of the optimised structure of L are
1.305 and 1.367 Å, respectively. The calculated bond length of Al1-N1 in 2 is 2.125 Å and found in
the typical range 1.921 to 2.191.48 The Al1-O1 bond length 1.860 Å of coordinated phenolic
oxygen for the optimised structure of 2 is observed in the typical range 1.857-1.885 Å.49 The bite
angle of N1-Al1-O1 is 88.38º for the species 2 falls in the typical range.50 There is good agreement

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between the calculated geometry of the probable structure of 2 species with other reported similar
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structural data in literature.48-50
DOI: 10.1039/D0NJ01954B
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Fig. 6. DFT optimized structures of (a) L and (b) 2L-Al3+ (2).
Molecular orbital description and Electronic spectra
The absorption spectra of L and 2 are simulated in the presence of the solvent model (methanol)
employing the TD-DFT methods with the identical basis set and functional as used in geometry
optimisation calculations. The spin-allowed electronic transitions are in good harmony with our
experimental data (Fig. 7). Calculated spin-allowed singlet-singlet electronic transitions with the
experimentally observed data for L and 2 in water are summarised in Table S2. The selected
transitions of L and 2 having significant oscillator strengths are incorporated and also screen the
transitions only with orbital contributions larger than 5% are taken into account for molecular ions.
A schematic representation of the energy of MOs and contours of selected HOMO and LUMO
orbitals of L and 2 are presented in Figs S19 and 8. The HOMO to LUMO energy gap for L and 2
are 3.380 and 2.881 eV, respectively. The energy transition was calculated at 408.92, and 369.03
nm for L and the transition can be assigned as n-n* and/or n-π*. On the basis of molecular orbital
analysis, other calculated transition of L can be assigned intra ligand charge transfer transitions. In
contrary, the calculated transition (H-1→L+1, H-4→L+1) at 377.94 nm for 2 is assigned as metal
to ligand charge transfer (MLCT) along with some contribution of ligand to ligand charge transfer
(LLCT) and rest of the transitions are assigned as intra ligand charge transfer as well as ligand to 
orbitals of coordinated H2O molecules transitions. The HOMO levels H to H-1 of 2 have partially
metallic character. Thus the calculated transitions are either of metal to ligand charge transfer
(MLCT) or intra ligand charge transfer transitions and ligand to  orbitals of coordinated H2O
transitions. The above-calculated bands for L and 2 are good agreement with the experimentally
measured bands in methanol solution and listed in Table S2.

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Fig. 7. The experimental (blue) and the calculated electronic transition (red) of L (left) and 2
(right).
MO of Receptor L
LUMO
HOMO
L+1
H-1
L+2
H-2
L+3
H-3
MO of Aluminum complex of L (2)
LUMO
L+1
L+2
L+3
HOMO
H-1
H-2
H-3
Fig. 8. Selected HOMOs and LUMOs of L and 2 are shown. Positive values of the orbital contour
are represented in yellow (0.03 au) and negative values in blue (−0.03 au).

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Explanation of fluorescence properties from DFT
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3+
The probe L exhibits a simple approach for the selective detection of Al with the enhancement of
fluorescence intensity because of the CHEF (chelation-enhanced fluorescence) process after
interaction with Al3+. The energy gap between the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of L and 2 are 3.380 and 2.881 eV, respectively. The
complex 2 compared to the probe L are established with the easy electronic transition and get the
additional stability of 2 (Fig. 9). The contours of the electronic distribution in HOMO and LUMO
states of these molecules suggested significant energy difference 0.499 eV between L and 2.
Specifically, LUMO states of 2 in comparison to L revealed that the electrons are more delocalised
insight into 2 molecule than the receptor L in agreement with the barrier of the photo-induced
electron transfer process, which may results in the enhancement of fluorescence through CHEF.51
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Fig. 9. HOMO-LUMO energy levels of L and 2.

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Sensing mechanism
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DOI: 10.1039/D0NJ01954B
In order to get the actual host-guest structure, we have tried to isolate Al3+ complex of L, in the
form of single crystal but failed. Thus the Al3+ binding sites of the receptor L were confirmed by
FT-IR spectra of the host-guest complex. The initial broad band at 3430 cm-1 due to -OH stretching
almost vanished in the L-Al3+ complex (2), along with a small vibrational band appeared around at
632 cm-1 probably due to Al-O stretching. Also non-existence of vibrational band around 545 to
560 cm-1 in 2 indicated that the heterocyclic thiadiazole “N” cannot participate in Al3+ ion binding
(Fig. S20). The DFT and TDDFT studies of 2, starring from were also investigated to establish the
mechanism and the binding sites (vide supra).
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Based on the above observations, the detection of the analyte can be explained by the
selective Al3+ ion capturing and the optical signal change. The receptor L contains electron-rich N
and O centre, which showed strong affinity towards hard metal like Al3+. The phenolic -OH of the
free receptor L participate ICT with chromophoric NO2, which imparts yellow coloured solution in
ground state. Also, the reduction of electron density on -O atom, extends PET from phenolic -OH
to phenyl-thiadiazole fluorophore, is also weak, and the receptor L showed moderate emission.
After the Al3+ ion binding through O and imine site, the colour changes from yellow to colourless
due to restriction of ICT and emission enhancement through CHEF due to retardation of both PET
and imine isomerisation. Here the two heterocyclic N atoms of the probe L cannot participate in
actual analyte binding but can preferably select the small Al3+ ion to sit inside the cavity. Thus from
the absorption and emission titration, Job’s plot, FT-IR, ESI-MS spectra, and DFT-TDDFT studies
the plausible sensing mechanism has been established (Scheme 2).
Scheme 2. Plausible sensing mechanism.

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Application of chemosensor L in real samples
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To explore the Al3+ sensing ability of the present chemosensor L in real water samples, we have
initially tested with laboratory tap water samples by colorimetric and fluorometric methods. The
Al3+ ion concentration of the tap water sample cannot be measured by this method. Thus artificial
Al3+ contaminated tap water samples have been prepared by adding known amounts of standard
Al3+ solution. The prepared samples were analysed by the same UV-visspectrometer and spectro-
fluorimeter. The recovery amounts were calculated using their standard calibration curves (intensity
vs. conc.) with the help of Lambert-Beer law. As shown in Table 3a and 3b, the Al3+ concentration
recovered for each sample is in agreement with the spiked amount with good precision (RSD values
of 0.98% and 0.43% by colorimetric and fluorometric methods, respectively). Thus the receptor L
can be suitable to detect Al3+ ion quantitatively in real water samples.
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Table 3a Determination of Al3+ ion in tap water samples by colorimetric method.
Metal Ion
Spiked amount
Recovered
amount(μM)
10.15
Recovery %
RSD % value
(μM)
10
101.5
99.7
Al3+
0.98
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10
9.97
10.11
101.3
Table 3b Determination of Al3+ ion in tap water samples by fluorometric method.
Metal Ion
Spiked amount
Recovered
amount(μM)
9.83
Recovery %
RSD % value
(μM)
10
98.3
98.9
99.1
Al3+
0.43
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10
9.89
9.91
Molecular logic gate
The absorption band at 412 nm of the free receptor L has completely vanished, and the emission
intensity at 380 nm was enhanced upon screening with Al3+ ion. These optical changes were fully
recovered upon the addition of 1 equiv. of Na2EDTA. Thus the presence of two chemical inputs
Al3+ and Na2EDTA, the observed output signal at 412 nm from the absorption intensity and 380 nm
based on the fluorescence intensity change exactly mimic with an “IMPLICATION” logic gate with
threshold value 0.04 and an “INHIBIT” logic gate with threshold value 8000 respectively. The
constructed truth table has been summarised in Fig. 10.

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Fig. 10. Application of L in building molecular logic gate.
Smartphone-Based Colour Image
The colour change in the presence of Al3+ ions can also be applicable by easy handle able
smartphone-based analysis as it produced more accurate and sensitive results than that observed
visually. Here the different standard solutions of Al3+ ions were prepared in the tap water and added
separately to the standard methanol-tris-HCl buffer (10 mM, pH 7.2) solution of receptor L
(40µM). The representative photographs of each set of the receptor solution with different Al3+ ion
concentration was captured by a smartphone (Fig 11a), and the three primary colour (red, green and
blue) intensity values were determined (Fig 11b) by the pre-installed app RGB grabber
shunamicode. The photograph capturing was carried out in a 40 w LED confined square box to
reduce the external light effect and to keep the fixed distance of the solution from the smartphone.
With the concentration Al3+ ion rising from 0 to 16, the colour of the solution shifted from yellow
to colourless accompanied with the increase of its complementary colour blue much more sharply
than the other two. This linear increase of blue colour value with the analyte concentration can be
used to construct calibration curve (Fig 11c) and the calculated detection limit value 1.36 x 10-7 M
based on the same equation 3σ/m. Within this range, any unknown Al3+ ion concentration (e.g. X)
can be calculated by measuring its blue colour value.

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Fig. 11. (a) Smartphone-based colour image; (b) Plot of red, green and blue colour channel level of
signal images obtained from the smartphone using RGB grabber shunamicode and (c) Plot of blue
colour channel level.
Comparison of L with other Schiff base fluorescent, colourimetric chemosensor
The performance of the fluorescent, colourimetric sensing action of the Schiff base probe L towards
Al3+ was compared with some other reported fluorescent, colourimetric Schiff base chemosensors,
as listed in Table 4. It can be seen from Table 4, compared to the other systems, present system has
a number of attractive analytical features, such as high sensitivity, wide linear range, high
selectivity, lower detection limit, simple operation technology, good solubility, visualised sensitive
and good practical applicability. Moreover, the synthesis of the proposed chemosensor L requires
only two steps and less hazardous reagents and no hazardous by-product is formed.
Table 4. Comparison of L with other reported Schiff base fluorescent, colourimetric chemosensor
Probe
Solvent
EtOH
LOD
8.2 x 10-7 M
Application
No application
reported
Ref.
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EtOH
7.2 x 10-7 M
No application
reported
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DMSO-H2O
(9:1, v/v)
6.0 x 10-7 M
1.9 x 10-7 M
No application
reported
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EtOH
No application
reported

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EtOH-H2O
(9:1, v/v)
3.3 x 10-6 M
No application
reported
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MeCN-H2O
(1:4, v/v)
1.5 x 10-6 M
1.9 x 10-6 M
No application
reported
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H2O
No application
reported
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MeOH-tris-
HCl buffer
(10 mM, pH
7.2, 1:1 v/v)
1.15 x 10-7 M Application in real
water samples,
This
work
building the
molecular logic
gate and
smartphone-based
application.
Conclusions
In summary, we have developed a thiadiazole based Schiff base receptor L in a simple way which
can effectively recognise Al3+ with high selectivity and good sensitivity in the presence of other
metal ions. The LOD of L towards Al3+ was calculated to 1.43 × 10-7 M by colourimetry and
1.15 × 10-7 M by fluorometry. When L exploited with Al3+, the fluorescent emission peak appeared
at 380 nm on excitation at 310 nm. The colour change of the solution was observed directly by the
naked eye from yellow to colourless. The interaction between L and Al3+ was studied by Job’s plot,
ESI-MS spectrometry, DFT and TDDFT calculations. L can detect Al3+ through “off−on−off” type
fluorescent signalling. Moreover, L could operate in a wide range of pH and can be successfully
applied for detection and quantification of Al3+ in environmental samples, smartphone-based
analysis and for building IMPLICATION and INHIBIT type of logic gates.
Acknowledgements
G.K.P would like to thank the Department of Science and Technology (SR/FST/CSI-264/2014 and
EMR/2017/0001789) and Department of Biotechnology, Government of India, New Delhi for
financial support. A.K.M. thanks the CSIR, Government of India, for financial support in the form
of the research fellowship.
Supporting Information Summary
CCDC number 1993595 contains the supplementary crystallographic data for 1 and complex 1. 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:

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Supplementary data containing Fig. S1- S20 and Tables S1-S2 of this article can be found online
View Article Online
DOI: 10.1039/D0NJ01954B
at…………………………..
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Conflicts of interest
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Authors declare no conflicts of interest.
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DOI: 10.1039/D0NJ01954B
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GRAPHICAL ABSTRACT
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Combined experimental and theoretical studies on Phenyl thiadiazole
based novel turn-on fluorescent colorimetric Schiff base chemosensor
for the selective and sensitive detection of Al3+
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Amit Kumar Manna,a Shubhamoy Chowdhuryb and Goutam K. Patraa*
aDepartment of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G),
India
bDepartment of Chemistry, Gour Banga University, Malda, West Bengal 732
103, India
Phenyl thiadiazole based Schiff base receptor (L) has been presented for the first time for turn-on
fluorescent colorimetric detection of Al3+ ion. The chemosensor L displayed very quick
response, excellent selectivity and sensitivity towards Al3+ ion, both by colourimetry and
fluorometry with very low detection limit in aqueous medium. The binding stoichiometry was
obtained as 2:1 from job’s plot analysis, ESI-MS spectra and DFT and TDDFT studies. The
chemosensor L can be applied for the formation of binary logical devices, recovery of
contaminated water samples and smart-phone based chemical analysis.