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DOI: 10.1039/C8RA07041E (Paper) RSC Adv. , 2018, 8, 35946-35958

A dipodal molecular probe for naked eye detection of trivalent cations (Al3+, Fe3+and

Cr3+) in aqueous medium and its applications in real sample analysis and molecular

logic gates†

Rukmani Chandra, Amit Kumar Manna, Kalyani Rout, Jahangir Mondal and Goutam K. Patra
*
Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, CG, India. E-mail:
patra29in@yahoo.co.in ; Tel: +91 7587312992

Received 22nd August 2018 , Accepted 14th October 2018

First published on 23rd October 2018

Abstract
A simple and low cost multifunctional colorimetric receptor L has been designed,
synthesized and characterized by 1H-NMR, IR spectroscopy, ESI-MS spectrometry and

elemental analysis. The chemosensor L can selectively detect three biologically and
environmentally important trivalent metal ions (Al3+, Fe3+and Cr3+) both visually and

spectrophotometrically in CH3CN–H2O (1 : 1, v/v) solution in the presence of other

biologically relevant metal ions. The Job's plot analyses indicate the 2 : 2 binding
stereochemistry for Al3+, Fe3+ and Cr3+ ions with L, which was further confirmed by 1H-NMR
 and ESI-MS studies. The binding constant values were found to be 2.9 × 104 M−1 for Al3+, 1.079

× 105 M−1 for Fe3+ and 1.366 × 105 M−1 for Cr3+ respectively. The detections limits of the sensor
for Al3+ (2.8 × 10−7 M), Fe3+ (1.9 × 10−7 M) and Cr3+ (2.5 × 10−7 M) are far below than the limit set

by the World Health Organization (WHO) for drinking water. Moreover, colorimetric test kits
for rapid detection of Al3+, Fe3+, and Cr3+ could be successively applied for all practical

purposes, indicating its potential use in environmental samples. It has also been used in
building molecular logic gates.

Introduction

In recent years, colorimetric and fluorescent chemosensors have been recognized as powerful tools
for naked eye detection of specific metal ions due to their high selectivity and sensitivity. Among
various metal ions, trivalent cations have been a subject of intense research interest in the modern
sensing arena because of their significant application in environment and biological systems. 1 The
Al3+ ion, existing widely in the environment, can enter the human body through food and water. 2,3
Furthermore, aluminium toxicity is responsible for about 40% of the world's acidic soil. 4–6 On the
other hand, aluminum compounds are not only used in industrial fields such as aluminum-based
pharmaceuticals, food additives, medicines, water purification and production of light alloys but
also used in our daily life as electronic and electrical compounds in different gadgets, building
materials, packaging items etc. 7 High concentrations of Al3+ are harmful to plant growth and also to

the human nervous system which further leads to lots of diseases such as microcytic hypochromic
anemia, encephalopathy, myopathy, Alzheimer's disease, Parkinson's disease, amyotrophic lateral
sclerosis etc. 8,9 As a result of the close relationship between aluminum and human health, the
investigation on Al3+ detection attracts more and more attention. Secondly chromium, one of the

most abundant elements in the earth's crust and sea water, exists as metallic (Cr0), trivalent (Cr3+),
and hexavalent (Cr6+) forms. Cr3+ is an essential element required for the proper functioning of

biological processes. A recent study revealed that soluble Cr3+ between pH 6 to 8 can be found
transiently in significant concentrations and has an undesirable effect on microorganism. 10 It acts
as a glucose tolerance factor in some biochemical processes such as metabolism of carbohydrates,
fats, proteins and nucleic acids by activating certain enzymes, stabilizing proteins and nucleic
acids. 11 Chromium deficiency may influence the metabolism of glucose and lipids and lead to a
variety of diseases, including diabetes and cardiovascular disease. 12 On the other hand, Fe3+ plays

vital role in many physiological and pathological processes including cellular metabolism, enzyme
catalysis and oxygen transport as well as DNA and RNA synthesis. The deficiency of iron in the
human body lead to variety of diseases, including Parkinson's, anemia, diabetes, cancer and liver
damage. 13 Therefore detection of Fe3+ is also required to a large extent.

Although, a number of analytical methods available for detections of Al3+, Fe3+and Cr3+, including
chromatography, accelerator mass spectroscopy (AMS), graphite furnace atomic absorbance
spectrometry (GFAAS), neutron activation analysis (NAA), inductively coupled plasma-atomic
emission spectrometry (ICP-AES), inductively coupled plasma-mass spectrometry (ICP-MS), later
ablation microprobe mass analysis (LAMMA) and electro thermal atomic absorption spectrometry
(ETAAS) 14–18 but most of them require sophisticated instruments and time consuming and
laborious procedures for sample preparation and in some cases expensive too. Consequently,
spectro-photometric methods such as absorption and emission have become popular for sensing
cations due to their high selectivity and sensitivity, low cost and real time monitoring. In addition,
the detection of multiple analytes with a single receptor would be more efficient, which can also
decrease the cost compared to one-to-one analysis method, and therefore would attract more
attention. 19 Therefore, it is an urgent need to develop chemical sensors that are capable of
detecting the presence of Al3+, Fe3+ and Cr3+ ions in environmental and biological samples.

Schiff bases with o-substituted aromatic rings have found to be most responsive for chelation
with transition metal ions. The chelation of transition metal ions to the C N linkage would develop
ICT (intramolecular charge transfer) transition or make LMCT (ligand to metal charge transfer)
transition, which could be useful for the visual sensing of the metal ions. 20 In this regard, 4-
(diethylamino)-2-hydroxybenzaldehyde is a well-known chromophore used in the area of
chemosensors. In continuation with our previous earlier research work, 21 we report here a
chemosensor L, based on the combination of 4-(diethylamino)-2-hydroxybenzaldehyde and 4-(4-
aminobenzyl)benzenamine for simultaneous detection of Al3+, Fe3+and Cr3+ ions by naked eye in

CH3CN–H2O (1 : 1, v/v) mixture. For designing Schiff base we have selected amine part using two
aniline moieties joined through a methylene rotor at their para positions and aldehyde part
containing strong electron donating diethylamino group also in para position to the aldehyde, which
makes it electron rich. L showed obvious color changes from colorless to yellow upon selective
binding with Al3+, Fe3+ and Cr3+ along with noticeable change in absorbance properties.
Experimental section

General information and materials

All of the materials used for synthesis were obtained from Sigma-Aldrich and used without further
purification. All the solvents used were of analytical grade. Freshly de-ionized water was used
throughout the experiment. The stock solutions of metal ions were prepared from their nitrate salts
and the solutions of anions were prepared from their sodium salts. Elemental analysis was carried
out on Elemental Vario MICRO cube analyzer. 1H NMR spectra were recorded on a Bruker DRX
spectrometer operating at 400 MHz in CDCl3 and chemical shi$ were recorded in ppm relative to

TMS. UV-visible spectra were recorded on a Shimadzu UV 1800 spectrophotometer using a 10 mm

path length quartz cuvette with the wavelength in the range of 200–800 nm. 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). Both receptor L (1 × 10−5 M) and metal ions (1 × 10−4 M) solutions were prepared in CH3CN–
H2O (1/1, v/v) and H2O respectively.

Synthesis and characterization of L

The synthetic route of L has been illustrated in Scheme 1 . 4-(4-Aminobenzyl)benzenamine (0.198 g,

1 mmol) was dissolved in 30 mL of dehydrated methanol, and to this, 20 mL methanol solution of 4-
(diethylamino)-2hydroxybenzaldehyde (0.386 g, 2 mmol) was added drop-wise over 10 min. The
reaction mixture was refluxed for 6 h at 70 °C, maintaining dry condition. A yellow precipitate was
filtered and washed several times with n-hexane and then re-crystallized from methanol and dried
in vacuum to obtain the pure yellow solid. Yield: 0.410 g (75%). mp 190 °C; anal. calc. for C35H40N4O2:

C, 76.57 (76.62%); H, 7.38 (7.35%); N, 10.25 (10.21%). ESI-MS: m/z 549.30 (MH+, 90%, Th. value

548.32) (Fig. S1 † ). FTIR/cm−1 (KBr): 33 356(w), 2966(s), 2913(m), 2670(w), 1638(vs), 1568(vs),
1516(vs), 1419(s), 1351(s), 1290(m), 1221(m), 1134(m), 1064(m), 1004(m), 977(m), 882(s), 821(vs),
779(vs), 701(s), 639(s), 568(s), 535(m), 448(w) (Fig. S2 † ).1H NMR (400 MHz, CDCl3, TMS): δ 13.88 (s, 1H,
–OH), 8.40 (s, 1H, –CH N), 7.13–7–20 (m, 5H), 6.24 (d, 1H), 6.19 (s, 1H), 3.99 (s, 1H), 3.39 (m, 4H), 1.21
(t, 6H) (Fig. S3 † ). UV-Vis λmax/nm (ε/dm3 mol−1 cm−1) (CH3CN): 380 (4980).
 Scheme 1 Synthetic procedure of L.

Synthesis of 5-(diethylamino)-2-((phenylimino)methyl)phenol (L′)

This was prepared by following reported procedures. 22,23 Freshly distilled aniline (0.93 g, 10 mmol)
in dry methanol (20 mL) was slowly added to a well stirred solution of 4-
(diethylamino)salicylaldehyde (1.93 g, 10 mmol) in dry methanol (40 mL), and the resulting orange
yellow solution was stirred for a further 3 h. The reaction mixture was concentrated in vacuo, the
residue was purified by flash SiO2 column chromatography eluting with hexane/AcOEt. Orange

yellow crystalline compound were filtered off, washed with cold methanol, then recrystallized from
ethanol to give orange single crystals. Yield, 2.15 g (80%), mp 91–92 °C. Anal. calcd for C17H20N2O: C,

76.09 (76.15%); H, 7.51 (7.47%); N, 10.44 (10.51%).

UV-Vis titration

A stock solution of L (1 × 10−3 M) was prepared by dissolving 5.48 mg in 10 mL of mixed solvent

CH3CN–H2O (1/1, v/v). 30 µL of this solution were then diluted with 2.97 mL of solvent mixture to

make a final concentration of 10 µM. Stock solution of Al3+ was prepared by dissolving Al(NO3)3 (37.5
mg, 0.1 mM) in triple distilled water (10 mL) and 1.5–90 µL of this Al3+ solution (1 mM) was

transferred to each receptor solution L (10 µM). A$er mixing them for a few seconds, UV-Vis spectra
were recorded at room temperature.
The same procedure was followed for Fe3+ and Cr3+ using the solvent mixture CH3CN–H2O (1/1,
v/v).

Job plot measurements

L (5.48 mg, 0.01 mM) was dissolved in 10 mL of mixed solvent CH3CN–H2O (1/1, v/v) to make a

solution conc. of 1 × 10−3 M. From this stock solution, 100, 90, 80, 70, 60, 50, 40, 30, 20, 10 and 0 µL of
the L solution were taken and transferred to each vials. For Al3+, Al(NO3)3 (37.5 mg, 0.01 mM) was

dissolved in triple distilled water. 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 µL of the Al3+ solution

were added to each diluted L solution in such a manner that the sum of ligand and metal ion volume
remains constant (3 mL) and CH3CN–H2O used as solvent. A$er mixing them for a minute, UV-Vis

spectra were taken at room temperature.

The same procedure was followed for both Fe3+ and Cr3+ using the solvent mixture CH3CN–H2O

(1/1, v/v).

Competition with other metal ions

To determine the possible interference from other metal ions and selective binding affinity of chemo
sensor L towards Al3+, Fe3+ and Cr3+, UV-Vis spectra were taken in presence of other analytes. Stock

solution of 1 × 10−3 M was prepared for receptor L. 30 µL of it was diluted to 3 mL with the solvent
mixture to make a final concentration of 10 µM. 10 mM stock solution for various metal ions Co2+,

Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Ca2+, Mn2+, Mg2+, Ga3+, Co3+, Ru3+, Cr6+, Na+, K+, Hg2+, Fe2+, Ba2+ and Pb2+
were prepared in triple distilled water. Each M(NO3)x (0.1 mmol) (where, M = Al3+, Fe3+ and Cr3+) were

dissolved in 10 mL of triple distilled water. A$er that 30 µL of each metal solution was taken from the
stock and added to 3 mL of receptor L (10 µM) to maintain 10 equiv. of mixed solutions. Then, 30 µL
of M solution (10 mM) (where, Al3+, Fe3+ and Cr3+) were added to the mixed solution of each metal ion
and L to make a total of 10 equiv. A$er mixing them for seconds, UV-Vis spectra were obtained at
room temperature.

pH effect test

A series of buffers with pH values ranging from 2 to 11 was prepared using 100 mM HEPES buffer.
Ligand L (5.48 mg, 0.01 mmol) was dissolved in 10 mL of solvent mixture CH3CN–H2O (1/1, v/v) and

when the desired pH was achieved, then 30 µL of this stock receptor solution (1 mM) was diluted to 3
mL with the abovementioned buffers to make the final concentration of 10 µM. 37.5 g Al(NO3)3 was

dissolved in 10 mL HEPES buffer to maintain pH 7. Then 30 µL of this Al3+ solution (10 mM) was
transferred to each receptor solution (10 µM) prepared above. A$er mixing them for a few seconds,
UV-Vis spectra were taken at room temperature.
 The same procedure was followed for both Fe3+ and Cr3+ using the solvent mixture CH3CN–H2O

(1/1, v/v).

Colorimetric test kit

1 mM solution of L was prepared by dissolving 5.48 mg in 10 mL acetonitrile–H2O. Test kits were

obtained by immersing filter-papers into this 1 mM solution and then dried in air to get rid of the
solvent. Nitrate salts of Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Ca2+, Mg2+, Ga3+, Co3+, Ru3+, Cr6+, Na+, K+, Hg2+,
Fe2+, Ba2+, Al3+, Fe3+, Cr3+, Pb2+ and sulphate salt of Mn2+ 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 colour change.

Theoretical calculation methods

The GAUSSIAN-09 Revision C.01 program package was used for all calculations. 24 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. 25 Basis set 6-31++G was found to be
suitable for the whole molecule. LanL2DZ basis sets were implemented for the geometry
optimization of L + M3+ complexes (M = Al, Cr and Fe).

Results and discussion

Synthesis and structure of L

The receptor L was synthesized in one single pot with 75% yield via Schiff base condensation
reaction of 4-(4-aminobenzyl)benzenamine with 4-(diethylamino)-2-hydroxybenzaldehyde ( Scheme
1 ). Then synthesized L was characterized by UV-Vis, FTIR, 1H NMR, elemental analysis, and ESI-mass

spectrometric analysis. Here the two –OH groups are directed in cis fashion and are readily
accessible to coordinate with metal ions.
DFT calculations were performed on the molecule L. The geometry optimizations staring from
Gauss view structure of L lead to a global minimum as stationary level. The optimized structure of
the L has been shown in Fig. 1a and its geometry optimized selected bond length and angles are
tabulated in Table S1. † Fig. 1b presents a schematic representation of the contours of selected
HOMO and LUMO orbitals of the receptor L. The HOMO to LUMO energy gap in L is 3.004 eV. The
probe L in presence of M3+ ions (M = Al, Cr and Fe) form tetra-coordinated complexes, co-ordinating

with salicylaldehyde O and imino N atoms of two ligand molecules in 2 : 2 fashion; which have
further been confirmed by ESI-MS studies (vide infra). Energy minimized structure and frontier
orbital contribution of the L + M3+ complexes (M = Al, Cr and Fe) have been shown in Fig. S4 and S5. †

Fig. 1 (a) Geometry optimized diagram of the molecule L; (b) the contour diagrams of selected
HOMO and LUMO orbitals of L. Positive values of the orbital contour are represented in purple
(0.04 au) and negative values in green (−0.03 au).

UV-Vis studies

The sensing properties of receptor L were initially evaluated by the naked-eye and UV-Vis analysis.
The absorbance spectra of different metal cations were studied in three categories. (i) Metal cations
of the same period in the periodic table of elements such as Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cr3+, Cu2+,
Cr6+, Co3+ and Zn2+; (ii) some hazardous heavy metal ions such as Pb2+, Hg2+, Cd2+, Ag+; and (iii) a few

metal cations in the IA, IIA and IIIA groups such as Na+, K+, Ba2+, Ca2+, Mg2+, Ga3+, and Al3+ in CH3CN–
H2O (1 : 1, v/v) mixture. The receptor L showed small absorption bands at 271 nm due to the

conjugated π–π* transition and an intense band centered at 380 nm due to n–π* transition and intra-
molecular charge transfer respectively. Upon the addition of 1 equiv. of each metal ion to the
receptor only Al3+, Fe3+ and Cr3+ ions showed a red shi$ed band at 422 nm, 422 nm and 416 nm
respectively, while other metal ions exhibited no changes in the absorption spectra relative to the
free receptor ( Fig. 2 ). For Cu2+ ion, slightly red shi$ in the spectral position at 380 nm was observed
with negligible colour change visualized through naked eyes. Consistent with the change of the UV-
Vis spectrum, colour of the solution changed from colourless to orange yellow for Al3+and Fe3+ ions
and colourless to light yellow for Cr3+ ion ( Fig. 3 ). The bar graph, showing relative absorbance

intensity has been shown in Fig. S6. † The colour and spectral change can be explained by
intramolecular charge transfer (ICT) and ligand-to-metal charge-transfer (LMCT) mechanisms. The
red shi$ in the absorption band indicated that the energy gap of ICT band decreases, upon binding
of metal ions to the electron withdrawing imine moieties and showed color in visible region in
comparison to other metal ions. The molar extinction coefficient was calculated to 4.2 × 103, 4.4 × 103
and 3.9 × 103 respectively at wavelength 422 nm, 422 nm and 416 nm for Al3+, Fe3+ and Cr3+, which

were too large to be metal based d–d transitions and thus might be assigned as ligand to metal
charge transfer (LMCT) transitions.

Fig. 2 Absorbance spectral change of the receptor L (10 µM) in the presence of 1 equiv. of various
metal ions in CH3CN–H2O (1/1, v/v) solvent mixture.
 Fig. 3 The colour changes of L (10 µM) upon addition of various metal ions (1 equiv.) in CH3CN–H2O
(1 : 1, v/v) at room temperature.

In order to confirm the ICT property of the chemosensor, the absorbance properties of L were
further studied in different solvents such as dimethyl-sulfoxide, N,N-dimethyl formamide,
acetonitrile, methanol and dichloromethanes it has been reported that the solvent dipole can relax
the ICT excited by polar solvents. 26 As shown in Fig. S7 † and summarized in Table 1 , the
absorption spectra of L featured a marginal red-shi$ of absorption maxima with increasing solvent
polarity, signifying an apparent solvent dependence of the absorption band. The chemosensor L
exhibited solvate-chromism due to occurrence of the ICT transition in receptor L.

Table 1 Absorption properties of L in various solvents

Solvent Wavelength in nm (λmax) Molar extinction coefficient (log ε)

DMSO 387 5.04
DCM 381 5.07
DMF 384 5.06
CH3CN 380 4.98
MeOH 377 4.92

To understand the binding property of L with Al3+, Fe3+ and Cr3+, UV-Vis titration experiments of L

with Al3+, Fe3+ and Cr3+ were carried out. The probe L shows that the absorption band at 380 nm
significantly decreased and absorption intensity at 422 nm, 422 nm and 416 nm monotonically
increased upon gradual addition of Al3+, Fe3+ and Cr3+ respectively up to 1 equiv. In case of Al3+ and

Fe3+, two isosbestic points were observed at 285 and 401 nm, but in case of Cr3+, only one isosbestic
point was observed at 402 nm ( Fig. 4 ) indicating that two products were generated from L upon
binding to Al3+, Fe3+ and only one product was generated from L upon binding with Cr3+. The Job's
plot for the binding between L and Al3+, Fe3+ and Cr3+ exhibited 1 : 1 or 2 : 2 stoichiometry for the L–

Al3+, L–Fe3+ and L–Cr3+complex formation (Fig. S8 † ). In addition, the formation of the 2 : 2 complex
for L–Al3+, L–Fe3+and L–Cr3+ were confirmed by ESI-mass spectrometry (Fig. S9 † ). In the positive-ion

mass spectrum, the m/z peaks at 574.12, 603.42 and 599.12 were assignable to 2L + 2Al3+, 2L + 2Fe3+

and 2L + 2Cr3+complexes respectively. Theoretical m/z values of the proposed complexes, 2L + 2Al3+,
2L + 2Fe3+ and 2L + 2Cr3+ are 574.12, 603.42 and 599.12 respectively.

Fig. 4 Absorption spectra change of L (10 µM) in the presence of different concentrations of (a)
Al3+, (b) Fe3+ and (c) Cr3+ (from 0 to 1 equiv.) at room temperature. Inset: ratiometric calibration

curve A416/A380 as function of M3+ concentration.

Based on the Job plot and ESI-mass spectrometry analysis, a 2 : 2 complex structure of L with
Al3+, Fe3+ and Cr3+was proposed. The binding constants for the formation of the L–Al3+, L–Fe3+ and L–

Cr3+ complex were also calculated on the basis of the change in absorbance at 422 nm, 422 nm and
416 nm respectively by considering 2 : 2 binding stoichiometry. The binding constant (K) determined
by using Bensei–Hildebrand method was found to be 2.9 × 104 M−1 for Al3+, 1.079 × 105 M−1 for Fe3+and
1.366 × 105 M−1 for Cr3+ respectively (Fig. S10 † ). The values are within those (103–107) previously

reported for these trivalent cations. 1,27 The detection limit for the L–Al3+, L–Fe3+ and L–Cr3+
complexes were determined to be 2.8 × 10−7 M, 1.9 × 10−7 M and 2.5 × 10−7 M respectively on the basis

of 3σ/K (Fig. S11 † ). Notably, the detection limit of L is much lower than WHO limit, 1 suggesting that
L could be an effective sensing material for the detection of aluminum and iron in drinking water.
To further evaluate the practical applicability of receptor L as an Al3+, Fe3+ and Cr3+ selectivity
sensor, competitive experiments were performed with different metal ions in CH3CN–H2O (1 : 1, v/v)

mixed solvent ( Fig. 5 , S12 and S13 † respectively). The other tested cations showed relatively
negligible spectral change and color change apart from some dilution effect. In the presence of other
competitive cations, the L + Al, 3 L + Fe3+ and L + Cr3+ complex solutions offer similar absorption
profile. Thus, L could be utilized as a selective colorimetric sensor for Al3+, Fe3+ and Cr3+ in the

presence of other competing metal ions.

Fig. 5 Competitive selectivity of L towards Al3+ in the presence of other metal ions (5 equiv.) (a)

absorbance spectra, (b) bar diagram taking absorption intensity at 422 nm in CH3CN/H2O (1/1, v/v)
solution.

Binding mechanism

For better understanding the binding nature of L with Al3+, Fe3+ and Cr3+ions, 1H-NMR spectra was
recorded with and without these metal ions separately in DMSO–D2O mixture. The free receptor

showed chemical shi$s δ at 13.88 ppm and 8.40 ppm due to phenolic proton and azomethine proton
respectively. Upon binding with Al3+, the chemical shi$ at 13.88 ppm disappeared because of

deprotonation of phenolic groups ( Fig. 6 ). The other proton shi$ at higher δ values attributed to
ligand to metal charge transfer along with intensity of the peak somewhat increased in case of M–L
complex. Overall changes in the chemical shi$s of the protons a$er the addition of 1.5 equiv. of Al3+
ion clearly reveals that the aldimine nitrogen (–CH N) and –OH of L taking part in the complexation
process. The most probable reason for this observation is due to the formation of symmetrical
dinuclear/dimeric complex of two ligands with two metal ions ( Fig. 7 ). Further the ESI-MS of host–
guest complex confirmed the 2 : 2 binding stoichiometry (Fig. S8 † ).

Fig. 6 1H NMR spectra of L in presence and absence of 1.5 equiv. Al3+.

Fig. 7 Binding mechanism of L with M (M = Al3+, Fe3+and Cr3+).

Another indirect approach has also been made to confirm the 2 : 2 binding of L with Al3+, Fe3+ and

Cr3+ ions. The colorimetric investigations were performed using alternative Schiff base (L′), a

monodentate version of L, generated from simple aniline and 4-(diethylamino)-2-

hydroxybenzaldehyde under similar experimental condition. Among the mentioned ions an intense
absorption band around 418 nm of receptor L′ in CH3CN–H2O (1/1, v/v) decreased and a new band at

352 nm arose only in presence of Cu2+ ion with obvious color change from yellow to colorless. The
absorption titration experiment also confirms the color and spectral changes in presence of Cu2+

( Fig. 8 ). At the same time, monodentate imine chemosensor L′ has no influence on the target ions
Al3+, Fe3+ and Cr3+ ( Fig. 9 ). So L′ is a Cu2+ ion selective sensor of detection limit, 4.0 × 10−7 M and

stability constant, 1.35 × 105 M−1 (Fig. S14 and S15 † ). It is to be mentioned here that L′ can also be
used in detection of boronic acid derivatives in living cells. 23 Thus the methylene bridged di-aniline
based receptor L has selective affinity towards Al3+, Fe3+ and Cr3+ and there 1 : 1 binding is difficult

due to large distance of two imine moiety.

Fig. 8 Absorption titration of L′ a$er increasing amount of Cu2+, 0–10 equiv. in CH3CN/H2O (1/1,

v/v) solution.
 Fig. 9 Competitive selectivity of L′ towards Cu2+ in the presence of other metal ions.

pH and time dependence

It is very essential to check whether the sensor is active or not for measuring specific cations in the
physiological pH range. Therefore the absorption intensity of L between pH 2 to 11 in absence and
presence of Al3+, Fe3+ and Cr3+were measured. As shown in Fig. 10 , no obvious change in absorption

intensity of free L was observed between pH 2.0–11. In the presence of 1.0 equivalent Al3+, Fe3+ and
Cr3+, L showed maximum absorption intensity at pH range of 5.5–7.5. The absorption intensity

decreases in the acidic (pH < 5) region due to protonation of imine group of L and therefore reduce
the chelation ability. 28 Again the decreasing absorption intensity in the basic (pH > 8) region has
been explained on the basis of ICT, which hindered the complexation. 29 Hence, the absorption
intensity is stable over the range of pH (4–11) and well-suitable for practical application under
physiological pH conditions.
 Fig. 10 Absorbance of (a) L, L–Al3+, (b) L, L–Fe3+and (c) L, L–Cr3+ at various ranges of pH in CH3CN–

H2O (1 : 1, v/v) at room temperature. Inset: intensity at 422 nm, 422 nm and 416 nm respectively
for L–M3+ (M = Al, Fe and Cr).

The time evolution of the receptor L in the presence of 1.5 equiv. of Al3+, Fe3+ and Cr3+ ions in

CH3CN–H2O (1 : 1 v/v) were also being investigated (Fig. S16 † ). The recognition interaction gets
almost completed just a$er the addition of 1.5 equiv. of these metal ions and the absorbance
intensity remains almost the same up to 10 min. This ensures the receptor L to be a sensitive sensor
which can be applied in environmental analysis.

Reversibility study of the chemosensor L towards Al3+, Fe3+and Cr3+

Reversibility is a prerequisite in developing novel sensor for practical application. Consequently, the
chemical reversibility of the binding of L towards Al3+, Fe3+and Cr3+were investigated by using

different anions and disodium salt of ethylenediaminetetraacetate (Na2EDTA). The absorption

intensities of the L–M complexes (M = Al3+, Fe3+ and Cr3+) returned to original level upon addition of
two equiv. of NaNO3 ( Fig. 11 ) solution along with the color of the solution changed back to the

original colorless instantly, indicating the regeneration of the receptor L. Again, upon addition of
Al3+, Fe3+ and Cr3+ ions solution, the absorption intensity at 380 nm decreased and absorption

intensity around 420 nm increased.

Fig. 11 Absorbance spectra of L in the absence and presence of (a) Al3+/NaNO3, (b) Fe3+/NaNO3 and
(c) and Cr3+/NaNO3, respectively in CH3CN–H2O (1 : 1, v/v).
 Moreover, the receptor L was applied to detect and differentiate the presence of individual Al3+,

Fe3+and Cr3+ ions. For this purpose, an aqueous solution of 5 equiv. of KF was added to L–M adduct
(M = Al3+/Cr3+/Fe3+) solution. The color of the receptor, L solution containing Al3+ and Cr3+ functions

as a reversible system in presence of KF whereas, it did not carry out reversibly in case of L–
Fe3+system ( Fig. 12a ). The same has been reflected in their UV-Vis spectra ( Fig. 12b ). The reversible

color changes of L in the presence of Al3+ and Cr3+can be explained by the higher affinity of Al3+ and

Cr3+ toward F−. Therefore, the KF solution can be used for the selective detection of Fe3+ in the
presence of Al3+ and Cr3+ by L. The Al3+ and Cr3+ can be distinguished by the colour intensity, with L,

Al3+ induces orange yellow colour whereas Cr3+ induces very light yellow colour.

Fig. 12 Reversibility study of L towards Al3+, Fe3+and Cr3+ ions using 5 equiv. of KF (a) through color
change (b) absorbance change.

If Al3+ and Cr3+ both the ions are present in the real sample then the colour of the CH3CN–H2O (1 :
1, v/v) solution of L will be intense yellow. Thus the detection of Cr3+ in presence of Al3+ in real

sample is somewhat difficult. This problem can be solved by using the complexing agent cupferron,
as it precipitated with both Al3+ and Fe3+ in the acidic pH and excess addition has no role towards

Cr3+ at that condition. As the receptor L is also stable in the acidic pH, so no problem arises for

detection of Cr3+ from the mixture of Al3+, Fe3+ and Cr3+.

Molecular logic gate application

The colorimetric “on–off” response of the chemosensor L in the presence and absence of Al3+ and

NO3−, was investigated for the fabrication of a Boolean type logic gate at molecular level. The
colorless solution of L becomes yellow only in the presence of Al3+ which was presented in Fig. 3 ,

however, either NO3− or both Al3+ and NO3− as chemical inputs failed to produce any color change for

ligand L. Again the change in absorbance peak at 422 nm in the absorption spectra of L was also
being observed upon addition of Al3+ and NO3− to develop the molecular logic gate. Initially, the free

receptor L was absorbed strongly at 380 nm in CH3CN–H2O (1 : 1, v/v) mixture but when Al3+ was
added as chemical input, the absorbance peak shi$ed to 422 nm. Again a$er the addition of NO3−

ion to the same solution it formed an un-complex system and the absorption peak of the L came
back to its original position. Another interesting result was observed upon the addition of NO3−

alone, where no band was observed at 422 nm. Therefore, using both the change in color and
absorbance intensity at 422 nm of L with Al3+ and NO3− as chemical inputs which mimics the INHIBIT

type logic gate at molecular level, 30 the logic scheme and truth table can be explained as shown in
Fig. 13 . The presence and absence of chemical inputs was assigned as 1 (on-state) and 0 (off-state)
and the enhanced absorbance of L as 1 (on-state) and the low absorbance as 0 (off-state).

Fig. 13 Logic scheme and the truth table for the proposed INHIBIT type logic gate.

Application of chemosensor L in real samples

In order to determine the environmental application of the chemosensor L, artificial Al3+, Fe3+ and

Cr3+ contaminated different water samples have been prepared. The tap water samples were spiked
with Al3+, Fe3+ and Cr3+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 L give satisfactory results ( Table 2 ).

Table 2 Determination of Al3+, Fe3+ and Cr3+ ions in tap water samples

Metal ion Spiked amount (µM) Recovered amount (µM) % recovery ± SD (n=3)
Al3+ 10 9.34 93.4 ± 2.3
20 19.60 98.0 ± 1.1
Fe3+ 10 10.68 106.8 ± 1.8
20 19.47 97.35 ± 3.1
Cr3+ 10 9.03 90.3 ± 1.5
20 20.69 103.45 ± 1.4

Colorimetric test-kits

To check the practical application of probe, test kits were prepared by immersing papers in a CH3CN
solution of L and then dried in air. These test kits were used to sense Al3+, Fe3+ and Cr3+ ions among

different cations. When the test kits coated with receptor L were added into different cation
solutions, the obvious color change were observed only with Al3+, Fe3+ and Cr3+ in CH3CN–H2O (1 : 1,

v/v) solution which was shown in Fig. 14 . Hence, the test kits coated with receptor L solution would
be suitable for simultaneously detection of Al3+, Fe3+ and Cr3+ by showing colors change in the
presence of different metal ions.

Fig. 14 Photographs of the test kits with L for detecting the Al3+, Fe3+ and Cr3+ ions in acetonitrile–
water solution (1 : 1, v/v) with other cations.

Comparison of L with other colorimetric trivalent (Al3+, Fe3+ and Cr3+) chemosensors
The probe L, has been compared with some other recently reported colorimetric trivalent (Al3+, Fe3+

and Cr3+) chemosensors ( Table 3 ). Our system has some advantages over the others. In our method,
L has been prepared by facile, single step synthetic procedure, using inexpensive reagents without
troublesome and chromatographic purification. It has very quick response time and wide linear
response range. It works in CH3CN–H2O (1 : 1, v/v). It has fairly low detection limits and high

association constants.

Table 3 Comparison of L with other colorimetric trivalent chemosensors

No. of steps
Probe involved Solvent LOD Ka
3 Pure CH3CN 0.3 µM 6.46 × 109 M−2
(Al3+) (Al3+)
0.2 µM 1.26 × 105 M−1
(Fe3+) (Fe3+)
0.5 µM 1.58 × 104 M−1
(Cr3+) (Cr3+)
3 CH3CN–HEPES buffer 23 µM 8.77 × 103 M−1
(40/60, v/v) (Al3+) (Al3+)
20 µM 1.08 × 104 M−1
(Fe3+) (Fe3+)
25 µM 5.67 × 103 M−1
(Cr3+) (Cr3+)
3 THF–H2O (8 : 2) 0.38 nM Not
(Al3+) determined
0.38 nM
(Fe3+)
0.38 nM
(Cr3+)

2 H2O–EtOH (8 : 2) 0.5 µM 2.00 × 104 M−1

(Al3+) (Al3+)
0.2 µM 5.50 × 104 M−1
(Cr3+) (Cr3+)

3 1.74 nM
 CH3OH–H2O buffer (6/4, (Al3+) 1.00 × 104 M−1
v/v) (Al3+)
2.90 nM 1.20 × 102 M−1
(Fe3+) (Fe3+)
2.36 nM 2.60 × 102 M−1
(Cr3+) (Cr3+)
3 CH3OH–H2O (1/1, v/v) 0.34 µM 8.20 × 104 M−2
(Al3+) (Al3+)
0.29 µM 6.70 × 104 M−1
(Fe3+) (Fe3+)
0.31 µM 6.00 × 104 M−1
(Cr3+) (Cr3+)
1 CH3CN–H2O (1/1, v/v) 0.28 µM 2.90 × 104 M−1
(Al3+) (Al3+)
0.19 µM 1.07 × 105 M−1
(Fe3+) (Fe3+)
0.25 µM 1.36 × 105 M−1
(Cr3+) (Cr3+)

Conclusions

In brief, a simple colorimetric chemosensor L was successfully designed and synthesized for
recognition of Al3+, Fe3+ and Cr3+ over competing relevant metal ions in CH3CN–H2O (1 : 1, v/v)

solvent. This colorimetric chemosensor L exhibited a visual color change from colorless to orange
yellow upon addition of Al3+ and Fe3+and colorless to yellow upon addition of Cr3+. The binding

abilities of the chemosensor L with Al3+, Fe3+ and Cr3+ were established by the combined UV-Vis and

ESI-MS method. The detection limit of L for Al3+ (0.28 µM), Fe3+ (0.196 µM) and Cr3+ (0.25 µM) were
much lower than those set by the WHO guidelines. L could be successfully applied to test kits and
real samples for detection of Al3+, Fe3+ and Cr3+ and to build an INHIBIT type molecular logic gate.
The chromophore L has been found to exhibit antibacterial and antifungal activity. Thus the
chromophore L was found to be better addition in the field of chemosensors.

Conflicts of interest
There is no conflict of interest.

Acknowledgements

G. K. P. would like to thank the Department of Science and Technology and Department of
Biotechnology, Government of India, New Delhi for financial support. One of the authors, RC is
highly thankful to University of Grant Commission (UGC), New Delhi, India, for financial support in
the form of research fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra07041e

This journal is © The Royal Society of Chemistry 2018

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