A benzofuran-β-Alaninamide based "turn-on" fluorescent chemosensor for selective recognition of Fe3+ ions

Article abstract of DOI:10.1039/c9nj02655j

A benzofuran-β-Alaninamide based chemosensor, 3-(3-((4-methylbenzyl)amino)propanamido)benzofuran-2-carboxamide (BAA), was designed and synthesized for selective detection of Fe³⁺ ions. The binding ability of BAA towards Fe³⁺ in DMSO/

Full text of DOI:10.1039/c9nj02655j

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Benzofuran-β-alaninamide based “turn-on” fluorescent
chemosensor for selective recognition of Fe3+ ions
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P. Madhua,b,*, P. Sivakumarc and Rajendran Sribaland
aResearch and Development Centre, Bharathiar University, Coimbatore-641 046, TamilNadu, India
bDepartment of Chemistry, Thiruvalluvar Government Arts College, Rasipuram-637 401, TamilNadu, India
cDepartment of Chemistry, Arignar Anna Government Arts College, Namakkal-637 002, TamilNadu, India
dBiochemie Innovations Lab, Tindivanam-604 001, TamilNadu, India
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*Corresponding Author E-mail: madhup.chem@gmail.com
Tel: +91 9944502310
Abstract
A
benzofuran-β-alaninamide
based
chemosensor,
3-(3-((4-methylbenzyl)amino)
propanamido)benzofuran-2-carboxamide (BAA) was designed and synthesized for selective
detection of Fe3+ ions. The binding ability of BAA towards Fe3+ in DMSO/H2O solution (9/1,
v/v) have been studied by UV-vis absorption and fluorescence spectroscopy. Interestingly, the
probe BAA exhibits an excellent “turn-on” fluorescence enhancement at 424 nm with the
excitation wavelength of 290 nm. The quantum yield was determined to be 0.248 for BAA and
0.447 for the iron complex. The limit of detection (LOD) calculated to be 1.3 µM and 0.067 µM
by UV-vis absorption and fluorescence methods respectively. These values are much lower than
that of US, Environmental protection agency guidelines in drinking water (5.37 µM). Job’s plot
measurement evidenced the 2:1 binding stoichiometry for the complex formed between BAA
and Fe3+. Moreover, the binding interaction of BAA towards Fe3+ was confirmed by density
functional theory (DFT) study. Finally, the real sample analysis proved that the probe BAA was
more suitable for the detection of Fe3+.
Keywords: Chemosensor; Fluorescent sensor; benzofuran-β-alaninamide sensor; Fe3+ ion;
DFT study.
Introduction
In recent years, the research on chemosensors for environmentally and physiologically
important analytes have attracted immense interests to chemists, biologists due to their potential

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applications in the field of biological, clinical and environmental systems.1-3 Iron is one of the
most abundant and indispensable transition metal ions in biological systems and plays a vital role
in many biochemical processes such as cellular metabolism, DNA synthesis, transport of oxygen
by heme and cofactor in many enzymatic reactions.4,5 The iron-rich organs in the human body
are liver and spleen. It is also present in bones, medulla, kidneys and intestines in smaller
amounts.6 Although iron is a very essential element for the proper functioning of living cells, the
deficiency and excess may cause series effects in the human body. The deficiency of iron causes
many biological disorders such as anemia, liver damage, cancer and decreased immunity.7,8 At
the same time, the excess of iron produces reactive oxygen species (ROS) by Fenton reaction,
which damages proteins, lipids and nucleic acids.9-14 Moreover, hemochromatosis is
characterized by an overload of iron in the body that causes tissue damage and fibrosis with
irreversible damage to various organs.15,16 Recently researchers revealed that iron accumulation
in the brain has been associated with Parkinson’s disease, Alzheimer’s disease and Huntington’s
chorea.17-24 Thus, an urge to develop a new sensitive method for direct sensing of iron in the
living system and environmental samples are of great scientific interest.
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Numerous analytical techniques such as atomic absorption spectroscopy,25
spectrophotometry,26 inductively coupled plasma mass spectroscopy,27 anodic stripping
voltammetry,28 neutron activation analysis,29 electrothermal atomic absorption spectrometry,30
atomic fluorescence spectrometry,31 X-ray fluorescence spectrometry32 and cold vapour atomic
absorption spectrometry33 have been used for the detection of iron ions, but these methods have
some disadvantages such as being expensive, requirement of sophisticated instruments, limited
sample adaptability and necessity of professional operators.34 Thus, the chemosensors are of a
good choice and alternative technique for the detection of metal ions, which include high
sensitivity, good selectivity, cost-effectiveness, rapidity, easy monitoring the analyte, visual
perception and applicability to the environmental and biological milieus. Although a number of
fluorescence based chemosensors have been developed and reported for various transition
metals,35-37 only a few fluorescent probes were reported for Fe3+ ions. As Fe3+ known to be
paramagnetic nature, most of the reported iron sensors exhibited fluorescence quenching
responses,38-40 only in few cases fluorescence enhancement was observed. Thus, there is an
instant requirement to develop “turn-on” fluorescent chemosensors that are capable of detecting
Fe3+ ions in biological and environmental samples.

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A number of bioactive natural products, as well as pharmaceuticals and polymers, contains
benzofuran moiety.41 Many of these benzofuran derivatives are found to be biologically
active.42-58 Thus, herein we developed a benzofuran-β-alaninamide based chemosensor for
selective detection of Fe3+ ions in the presence of other metal ions. The sensor showed “turn-on”
fluorescence response with high selectivity and sensitivity towards Fe3+ ions in DMSO/H2O
solution (9:1, v/v).
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Experimental
Materials and Instrumentation
All the chemicals and solvents were purchased from Sigma-Aldrich/Merck with
analytical and spectroscopic grade and used as received without further purification. The
progression of all the reactions monitored by TLC using silica gel 60F254 and visualized under
UV 254-366 nm and iodine.
Bruker NMR instrument was used to record 1H NMR (300MHz) and 13C NMR (75MHz)
in CDCl3 and DMSO-D6 as solvents. LCQ Fleet mass spectrometer (Thermo Fisher Instruments
Ltd., US) was used to record Electrospray ionization mass spectra (ESI-MS). FT-IR spectra
were recorded on Thermo Scientific Nicolet iS50 FT-IR Spectrometer. For recording the FT-IR
spectrum of the metal complex, the probe, BBA and Fe3+ solutions were mixed, dried at room
temperature and the powder sample obtained was used. Shimadzu UV-1800 and JASCO V-630
UV-vis spectrophotometers and fluorescence spectrophotometers (Agilent Technologies, US and
JASCO F-8500) were used to record UV absorption and Fluorescence spectra respectively.
Synthesis and characterization
Synthesis of 2-(cyanomethoxy)benzonitrile(2)59
2-hydroxybenzonitrile (0.00839mol) 1 was dissolved in DMF solution and stirred at room
temperature. Then potassium carbonate (0.0125mol) and 2-chloroacetonitrile (0.00839mol) was
added to the reaction mixture. The resulting solution was refluxed to 80◦C for 3h. After
completion of the reaction, the reaction mixture was poured into cold water and filtered. The
obtained solid was washed with traces amount of hexane and then dried to get compound 2.
1
Brown solid. Yield 88%. H NMR (CDCl3, 300 MHz): δ 7.62-7.68 (m, 2H), 7.11-7.23 (m, 2H),
4.94 (s, 2H). 13C NMR (CDCl3, 75 MHz): δ 157.66, 134.72, 134.44, 123.36, 115.47, 114.20,
112.55, 102.96, 53.90.

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Synthesis of 3-aminobenzofuran-2-carboxamide (3) 59
2-(cyanomethoxy)benzonitrile (0.00632 mol) 2 was dissolved in 100mL of ethanol and
KOH (0.00948 mol) was added. The reaction mixture was refluxed to 75◦C for 3 h. Then the
reaction mixture was poured into the crushed ice, filtered and dried. The obtained solid was
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1
dried to afford compound 3. White solid. Yield 68%. H NMR (CDCl3, 300 MHz): δ 7.45 (d, J
= 9.0 Hz, 1H), 7.35-7.40 (m, 2H), 7.26 (d, J = 9.0Hz, 1H), 5.97 (s, 2H), 5.06 (s, 2H). 13C NMR
(CDCl3, 75 MHz): δ 162.91, 151.85, 135.66, 126.82, 125.80, 121.80, 121.02, 119.82, 110.73.
Synthesis of 3-(3-((4-methylbenzyl)amino)propanamido)benzofuran-2-carboxamide (BAA)59
The compound 3 (0.00368mol) was dissolved in acetonitrile. Then triethylamine
(0.00552mol) was added at 0–5◦C. To that solution, 3-chloropropionyl chloride (0.00368mol)
was added drop wise over 5 min. And then 4-methylbenzylamine (0.00368mol) was added at
ambient temperature for 6 h. Then the reaction mixture was diluted with dichloromethane (50
mL), washed with water (2-25 mL) and brine solution (25 mL). The organic layer was dried
over anhydrous sodium sulphate, filtered and evaporated to obtain BAA. White solid. Yield
73%. 1H NMR (CDCl3, 300 MHz): δ 10.45 (s, 1H), 8.34 (d, J = 8.1 Hz, 1H), 7.39-7.45 (m,
2H),7.28-7.30 (m,1H), 7.23 (d, J = 7.8Hz, 2H), 7.12 (d, J = 7.8 Hz, 2H), 7.10 (s, 1H), 5.78 (s,
1H), 3.85 (s, 2H), 3.04 (t, J = 6.0 Hz, 2H), 2.69 (t, J = 6.0 Hz, 2H), 2.32 (s, 3H). 13C NMR
(DMSO-d6, 75 MHz): δ 170.72, 163.01, 153.42, 136.78, 136.70, 132.28, 129.20, 128.27, 128.20,
126.73, 123.19, 122.36, 111.62, 53.44, 44.71, 36.95, 21.14. FT-IR (KBr) cm-1; 3378, 3299,
3169, 2968, 1670, 1600, 859, 748. ESI-MS calculated, 351.1; found 352.3(M+1)+ and 350.2 (M-
1)-. Anal. Calcd for: C20H21N3O3.: C, 68.36; H, 6.02; N, 11.96 %. Found: C, 68.38; H, 6.01; N, 11.94
%.
UV-vis and fluorescence spectral studies
The solvent optimization was performed with increasing ratio of DMSO/water and found
best at 9/1 (Fig. S16 and Fig. S17). The BAA was dissolved in DMSO/H2O solution (9/1, v/v)
and prepared at a concentration of 1 x 10-3 M and kept as a stock solution. 50 µL of this stock
solution was diluted to 2 mL with the respective solvent system to cover final concentration as in
25 µM level. The stock solution of metal ion was prepared at a concentration of 1 x 10-2 M and
10 equiv. of this solution was used for the selectivity experiments. This stock solution was
diluted (10 times) for the sensitivity experiments. For job’s plot analysis 5 x 10-5 M of BAA and

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Fe3+ solutions were used as a separate solution. In competitive experiments, 25 µM
concentration of BAA, 10 equiv. of Fe3+ and 15 equiv. of other metal ions were used. Freshly
prepared solutions of BAA and metal solutions were used for the purpose of spectral
measurements.
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UV-vis spectrophotometer and fluorescence spectrophotometer were used to record the
absorption and fluorescence spectra at room temperature respectively. 25 µM solution of BAA
in DMSO/H2O solution (9/1, v/v) at a pH range of 9.0 was prepared at room temperature and
added to the metal ions. This solution was taken in quartz optical cell and the spectra were
recorded. The emission spectral studies were performed on fluorescence spectrophotometer and
the BAA shows a broad band centered at 376 nm with the excitation wavelength of 290 nm.
Binding constant
The binding constant for (K) the formation of the complex was evaluated by UV-vis and
Fluorescence Spectrophotometer. The binding constant value can be determined by UV-vis
spectrophotometer by using the Benesi-Hildebrand equation stated below,
1
1
1
=
+
]
------- (1)
퐾푎 (퐴푚푎푥 ― 퐴0) [퐹푒3 +
퐴 ― 퐴0
퐴푚푎푥 ― 퐴0
Where, A is the absorbance recorded in the presence of added Fe3+ ions, Ao is the
absorbance of the receptor, BAA in the absence of guest Fe3+, Amax is absorbance in presence of
added [Fe3+]max and K is the binding constant (M-1). The binding constant has been determined
from the slope of the straight line obtained in the plot of 1/(A-Ao) against 1/[ Fe3+]. By the same
manner, the binding constant value can also be determined by fluorescence spectrophotometer by
using modified Benesi-Hildebrand equation stated below,
1
1
1
1
=
+
------ (2)
퐾 [퐹푒3 +
퐼 ― 퐼푚푖푛
퐼푚푎푥 ― 퐼푚푖푛
퐼
] 푚푎푥 ― 퐼푚푖푛
Here, Imin, I and Imax are the emission intensities of BAA, at an intermediate Fe3+ concentration
with BAA, and at a concentration of complete saturation respectively. K is the binding constant
and [Fe3+] is the concentration of Fe3+ions. From the plot of 1/(I-Imin) against [Fe3+]-1, the binding
constant has been determined.
Computational methods
Using Gaussian 09W program, all the energy optimized structure calculations were
carried out. The geometry of the energy optimized structures of BAA and the iron complex in

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the ground state and excited state were performed by using density functional theory (DFT)
method with the B3LYP/6.311++G(d,p) basis set and LANL2DZ level.
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Results and discussion
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Synthesis and structural characterization of BAA
NH2
CN
CN
OH
ClCH2CN
KOH
CONH2
O
K2CO3, DMF
O
CN
EtOH,Ref
3
2
1
(i) ClCH2CH2COCl, (ii) 4-methyl
benzylamine
TEA, ACN
HN
HN
O
O
CONH2
BAA
Scheme 1. Synthetic route of BAA
As shown in Scheme 1, the BAA was successfully synthesized from 2-
hydroxybenzonitrile. Initially, 2-hydroxybenzonitrile reacts with 2-chloroacetonitrile in the
presence of potassium carbonate yielded intermediate 2. The intermediate 3 was obtained from
the internal cyclization of intermediate 2 in the presence of potassium hydroxide. The BAA was
obtained from compound 3 with the coupling of 3-chloropropanoyl chloride followed by the
nucleophilic substitution with 4-methylbenzylamine. The structure of BAA was satisfactorily
characterized by using 1H NMR, 13C NMR, ESI-MS and FT-IR spectroscopic techniques. In 1H
NMR, the appearance of singlet at 10.45 ppm indicates the presence of secondary amide NH.
Similarly, the singlet around 6.4 and 5.7 ppm indicates the presence of primary amide NH2. The
appearance of sharp singlet at 2.32 and 3.84 ppm indicates the presence of methyl protons and

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methylene protons of the p-tolylmethanamine unit. The set of triplets at 2.69 and 3.03 ppm
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indicates the presence of methylene protons of alaninamide moiety. The remaining aromatic
13
protons appeared in their corresponding chemical shifts. The C NMR spectrum also confirms
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the product formation. The peaks around 170.72 and 162.01 ppm indicate the presence of
primary and secondary amide carbonyl units. The appearance of carbon signals 21.12 and 53.44
ppm indicates the presence of tolylmethyl and benzyl methylene units. The peaks at 36.95 ppm
and 44.71 ppm indicate the presence of alaninamide methylene units. The Mass spectrum clearly
showed a molecular ion peak at both positive and negative mode. The FT-IR spectrum gave
some additional information for the compound formation. The stretching vibration at 3169 cm-1
and 2958 cm-1 were appeared due to the presence of aromatic CH units. The band at 3299 cm-1
and 3378 cm-1 indicate the presence of amide NH and amine NH units. The strong absorption
band at 1600 cm-1 and 1670 cm-1 showed the compound contains the amide carbonyl units.
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Selectivity of BAA for Fe3+ detection
Various biologically relevant metal ions were used to investigate the selectivity and
binding nature of the probe BAA. Initially, the synthesized probe BAA (25 µM) in DMSO/H2O
solution (9/1, v/v) shows an absorption maximum at 298 nm in UV-vis spectroscopy. Upon
addition of 10 equiv. of various metal ions such as Zn2+, Cd2+, Hg2+, Pb2+, Cu2+, Ni2+, Co2+, Fe2+,
Fe3+, Mn2+, Mg2+, Ba2+ and Ca2+ no significant changes were observed except Fe3+ ions.
Trivalent metal ions such as Mn3+, Co3+, Cr3+ and Al3+ also does not influences the absorbance of
the probe, BBA (Fig. S10). As shown in Fig.1, the addition of Fe3+ to the probe BAA, the
absorption band at 298 nm gradually increased, indicating that the probe BAA selectively bind
with Fe3+ ion in DMSO/H2O solution (9/1, v/v).
To get more evidence for the selectivity of BAA for metal ions, the fluorescence intensity
was measured upon the addition of 10 equiv. of different metal ions such as Zn2+, Cd2+, Hg2+,
Pb2+, Cu2+, Ni2+, Co2+, Fe2+, Fe3+, Mn2+, Mg2+, Ba2+ and Ca2+ using fluorescence
spectrophotometer under identical conditions (Fig.2). The probe BAA (25 µM) in DMSO/H2O
solution (9/1, v/v) exhibit a broad fluorescence band at 376 nm with the excitation wavelength of
290 nm. Upon addition of Fe3+ ion, the emission intensity of BAA at 376 nm decreased and a
significant fluorescence enhancement at 424 nm emerged quickly, while other metal ions did not
induce any distinct spectral changes in fluorescence spectroscopy. Also, trivalent metal ions

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such as Mn3+, Co3+, Cr3+ and Al3+ did not show any significant changes (Fig. S11). This drastic
fluorescent enhancement revealed that the probe BAA strongly binds with Fe3+ ion. The
quantum yield (Φ) of BAA and the iron complex were found to be 0.248 and 0.447 respectively.
These results suggest that the BAA has superior selectivity towards Fe3+ over other metal ions.
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2.4
2.1
1.8
1.5
1.2
BAA+Fe(III)
0.9
BAA+other metal ions
0.6
0.3
0.0
300
400
500
600
700
Wavelength (nm)
Fig. 1 UV-vis spectra of BAA (25 M) with 10 equiv. of various metal ions in DMSO/H2O
solution (9:1, v/v).

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450
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350
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50
BAA+ Fe3+ ions
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BAA+ other metal ions
0
330
360
390
420
450
480
510
540
Wavelength (nm)
Fig. 2 Fluorescence spectra of BAA (25 M) with 10 equiv. of various metal ions in DMSO/H2O
solution (9:1, v/v).
Sensitivityof BAA for Fe3+ detection
The sensitivity of BAA with Fe3+ ion was interrogated by the absorption spectral titration
in DMSO/H2O solution (9/1, v/v). Upon the addition of Fe3+ ion from 0-250 µM to BAA, a new
increment of absorbance was observed at 338 nm as shown in Fig.3. This study clearly shows
that the BAA has good linearity having a linear coefficient of 0.99. The detection limit of BAA
for Fe3+ was found to be 1.3 µM (Fig. S12). This value is much lower than the previously
reported literature (Table 1). The linear range of chemosensor is found to be in the range of 1-19
x 10-5 M. Using the Benesi-Hildebrandplot,60 the binding constant value (K) of BAA with Fe3+
was calculated to be 0.625 x 102 M-1 (Fig. S13). From the Job’s plot experiment,61-63 the binding
stoichiometry between BAA and Fe3+ was confirmed to be 2:1 (Fig. 4).

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300
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wavelength (nm)
Fig. 3 UV-vis spectra of BAA (25 M) with increasing concentration of the Fe3+ ions in
DMSO/H2O solution (9:1, v/v).
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Molefraction of BAA
Fig. 4 Job’s plot for the stoichiometry determination of BAA and Fe3+ in the complexation

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Table 1
Comparison of the detection limit for Fe3+ between the present study and those previously
reported in the literature by UV-vis titration method.
5
6
7
8
Detection limit
Sensor
Target
References
9
(µM)
23.00
5.38
9.50
1.79
1.30
10
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60
Magnesium oxide nanoparticles
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
66
Coumarin schiff based chemosensor
Aldazine-based chemosensors
67
68
Coumarin schiffbase derivative
69
Benzofuran-β-alaninamide based chemosensor
This study
Further to define the sensitive interaction between the BAA and Fe3+, the fluorescence
spectral titration was performed in DMSO/H2O solution (9/1, v/v) (Fig. 5). Before the addition
of Fe3+ ion, BAA exhibited fluorescence emission at 376 nm attributed to internal charge transfer
(ICT) process. Upon addition of Fe3+ ion from 0-60 µM, the emission intensity at 376 nm was
decreased and a new emission band centered at 424 nm increased with a clear isoemissive point
at 393 nm. This is because the emission intensity at 376 nm was decreased due to the retardation
of the ICT process64 and a new emission band centered at 424 nm increased due to a strong
chelation-enhanced fluorescence (CHEF) effect of Fe3+ with BAA. The limit of detection (LOD)
of BAA for Fe3+ was calculated to be 0.067 µM based on the formula 3σ/slope (Fig. S14), which
is better than the previously reported values (Table 2). Accordingly, this LOD is much lower
than the upper limit of Fe3+ level in drinking water set by the US Environmental protection
agency,65 which is 5.37 µM. The linear range of chemosensor is found to be in the range of 2-6
x 10-5 M. The calculated binding constant value from the modified Benesi-Hildebrand plot was
0.089 x 102 M-1 (Fig. S15).

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55
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58
59
60
Isoemissive point
0
330
360
390
420
450
480
510
540
Wavelength (nm)
Fig. 5 Fluorescence spectra of BAA (25 M) with increasing concentration of Fe3+ ions in
DMSO/H2O solution (9:1, v/v).
Table 2
Comparison of the detection limit for Fe3+ between the present study and those previously
reported in the literature by fluorescence titration method.
Detection limit
Sensor
Target
References
(µM)
2.61
4.00
5.05
2.75
0.067
Pyrene based chemosensor
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
70
Quinoline-functionalised Schiff base
Rhodamine based chemosensor
Carbazole based schiff base
71
72
73
Benzofuran-β-alaninamide based chemosensor
This study
Competitive experiment
To evaluate the practical applicability of the probe BAA as a Fe3+ selective fluorescence
sensor, the competitive experiment was performed in DMSO/H2O solution (9/1, v/v) in the
presence of Fe3+ at 10 equiv. mixed with various metal cations like Zn2+, Cd2+, Hg2+, Pb2+, Cu2+,
Ni2+, Co2+, Fe2+, Fe3+, Mn2+, Mg2+, Ba2+, Ca2+, Mn3+, Co3+, Cr3+ and Al3+ at 15 equiv. and the

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resulting individual solutions are analyzed by using UV-vis and fluorescence spectrophotometer.
As shown in Fig. 6 and Fig. 7, the addition of other metal ions even in higher concentration did
not show any significant spectral changes. This result showed that the BAA has remarkable
selectivity towards Fe3+ and could be used as a selective chemosensor for Fe3+ in the presence of
other metal ions.
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1.4
BAA with metal ions
BAA + Fe(III) with other metal ions
1.2
1.0
0.8
0.6
0.4
0.2
0.0
BAA Fe(III) Zn(II) Cd(II) Hg(II) Pb(II) Cu(II) Ni(II) Co(II) Fe(II) Mn(II) Mg(II) Ba(II) Ca(II) Mn(III)Co(III) Al(III) Cr(III)
Fig. 6 Competitive experiment of BAA with 10 equiv. of Fe3+ ions and 15 equiv. of other metal
ions in UV-vis spectroscopy.

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BAA with metal ions
BAA + Fe(III) with other metal ions
450
400
350
300
250
200
150
100
50
9
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0
BAA Fe(III) Zn(II) Cd(II) Hg(II) Pb(II) Cu(II) Ni(II) Co(II) Fe(II) Mn(II) Mg(II) Ba(II) Ca(II) Mn(III) Co(III) Al(III) Cr(III)
Fig. 7 Competitive experiment of BAA with 10 equiv. of Fe3+ ions and 15 equiv. of other metal
ions in Fluorescence spectroscopy.
Binding reversibility
It was of immense interest to investigate the reversible binding ability of BAA with Fe3+
ion. As shown in Fig. 8, the initial fluorescence intensity of BAA was significantly enhanced
upon the addition of 10 equiv. of Fe3+ ions. EDTA as a strong affinity for Fe3+ was introduced
into the solution containing BAA and Fe3+ in the same concentration as Fe3+, the fluorescence
intensity was decreased immediately to the original state of BAA, due to a strong complex
binding between EDTA and Fe3+. The alternate addition of Fe3+ and EDTA to BAA “ON-OFF-
ON” fluorescence signal was observed even after several cycles. These results suggested that the
sensor BAA could be used as a reversible chemosensor for Fe3+.

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Probe Fe EDTA Fe EDTA Fe EDTA Fe EDTA
Fig. 8 Reversible fluorescence intensity changes after the sequential addition of Fe3+ and EDTA
to BAA
Sensing mechanism of BAA with Fe3+
In addition to elucidating the interaction mechanism between BAA and Fe3+, the H
1
NMR spectral analysis of BAA (3.0 x 10-2 M, 2.0 equiv.) with Fe3+ was studied in DMSO-D6.
As shown in Fig. 9, upon addition of Fe3+ to BAA, the Ha proton signal decreased gradually and
disappeared at eventually with 1.0 equiv. concentration of Fe3+ in H NMR spectra, indicating
1
that Ha attached carboxamide nitrogen atom was coordinated with Fe3+ during the complexation,
whereas Hb and Hc proton signals were decreased by only 50%. This clearly signifies that 2:1
mode of binding between BAA and Fe3+, consequently Hb proton of one molecule and Hc proton
of another molecule they are coordinated with Fe3+. The reason is that the protons Hb and Hc,
they are not environmentally same, i.e. they exist as cis and trans in orientation with respect to –
C=O group. The plausible binding interaction of Fe3+ with BAA was represented in Scheme 2.
Further to examine the sensing mechanism of the [BAA]2-Fe3+, FT-IR spectral analysis
was carried out for BAA and [BAA]2-Fe3+ complex. The FT-IR spectrum of BAA (Fig. S8)
showed strong band at 3299 cm-1 and 3378 cm-1 corresponding to secondary amide NH and
primary amide NH2 units and these bands were shifted in the FT-IR spectrum of [BAA]2-Fe3+
complex (Fig. S9), but sharp absorption band at 1600 cm-1 and 1670 cm-1 corresponding to

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amide carbonyl units remains unaffected during complexation indicating that the amide carbonyl
units does not take part in any interaction with Fe3+. These results suggesting that the BAA
strongly interacted with the Fe3+ ions.
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Fig. 9 1H NMR (DMSO-D6, 400MHz) spectra of BAA (3.0 x 10-2 M, 2.0 equiv.) in the presence
of varying amounts of Fe3+.
Theoretical study
In order to get more insight into the interaction mechanism between BAA and Fe3+, the
energy optimized structure of BAA and [BAA]2-Fe3+ complex were obtained from density
functional theory (DFT) method with B3LYP/6.311++G(d,p) and LANL2DZ level of Gaussian
09 program.

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Fig. 10 DFT analysis of the BAA and [BAA]2-Fe3+ complex (left to right).
As illustrated in Fig. 10, the probe BAA displays more electron density on p-
tolylmethanamine moiety in HOMO. In LUMO, these electron densities were completely
transferred into benzofurancarboxamide moiety attributed to the ICT process. After the addition
of Fe3+ to the probe BAA, the electron density spread over on benzofurancarboxamide moiety
and the iron environment in HOMO, whereas in LUMO the electron density was spreading over
on carboxamide unit of a benzofurancarboxamide moiety, furan unit of benzofuran moiety and
the iron environment. This clearly revealed that on the addition of Fe3+to BAA, the ICT process
was retarded and simultaneously a strong CHEF effect was tuned. Moreover, the energy gap
between HOMO and LUMO of BAA (4.02 eV) is higher than the energy gap between HOMO
and LUMO of [BAA]2-Fe3+ complex (1.62 eV). Alternatively, due to the formation of [BAA]2-
Fe3+ complex, the energy gap between HOMO and LUMO of [BAA]2-Fe3+ was decreased to 2.40
eV. The result suggests that the sensor BAA strongly binds with Fe3+ ion and form stable
[BAA]2-Fe3+ complex as shown in Scheme 2.

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9
O
O
O
O
N
Fe3+
NH
O
O
NH
NH
O
Fe3+
HN
H2N
HN
NH
N
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O
O
Scheme 2. Proposed sensing mechanism of BAA with Fe3+ ion.
Real sample analysis
In order to inspect the capability of the reported chemosensor in environmental samples,
BAA was tested to detect the Fe3+ ion in tab water, drinking water and lake water samples in the
presence of Fe3+ ion as previous experiments in the same concentration. As shown in Table 3,
the drinking water samples showed higher absorbance than regular experimental results. Tab
water showed higher absorbance than regular experimental results but slightly lower than the
drinking water. In case of lake water sample, the absorbance was lower than the regular
experiment. The reason is that it may contain other minerals which may interact with our probe,
BAA. This real sample analysis proved that the BAA evidently detecting the Fe3+ in
environmental water samples.
Table 3
Determination of Fe3+ ion in different environmental water samples
S.No. Environmental
water sample
Concentration of the Fe3+
added (µM)
Recovery
RSD (%)
6
percentage (%)
1
2
3
Drinking water
25
104
Tap water
Lake water
25
25
102
99
6.1
5.9
Conclusion
In summary, herein we developed a benzofuran-β-alaninamide based chemosensor for the
detection of Fe3+ ions. The UV-vis absorption and fluorescence spectral studies showed that the
probe BAA acts as a highly selective and sensitive “turn-on” chemosensor for Fe3+ ions. The

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detection limit of BAA for Fe3+ was determined to be 1.3 µM and 0.067 µM using UV-vis and
fluorescence studies. Further, 2:1 binding mode between BAA and Fe3+ was observed from
Job’s plot measurements. In addition, the DFT calculation confirms the interaction mechanism
between BAA and Fe3+. Finally, the probe BAA has proven to be best suitable in real sample
applications.
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