Urea based dipodal fluorescence receptor for sensing of Fe3+ ion in semi-Aqueous medium

Article abstract of DOI:10.1007/s10895-013-1297-4

Urea based fluorescent chemosensor 1 was synthesized. Receptor 1 shows unique selectivity for the Fe³⁺ion and no such significant response was noticed with other metal ions (Cr³⁺, Mn²⁺, Co ²⁺, Ni²⁺, C

Full text of DOI:10.1007/s10895-013-1297-4

J Fluoresc (2014) 24:27–37
DOI 10.1007/s10895-013-1297-4
SHORT COMMUNICATION
Urea Based Dipodal Fluorescence Receptor for Sensing of Fe3+
Ion in Semi-Aqueous Medium
Umesh Fegade & Hemant Sharma & Sanjay Attarde &
Narinder Singh & Anil Kuwar
Received: 6 July 2013 /Accepted: 2 September 2013 /Published online: 27 September 2013
#
Springer Science+Business Media New York 2013
Abstract Urea based fluorescent chemosensor 1 was synthe-
sized. Receptor 1 shows unique selectivity for the Fe3+ion and
no such significant response was noticed with other metal ions
(Cr3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and
Bi3+) in DMSO/H2O (50:50,v/v) semi-aqueous solution. The
binding features have been established by absorption and
fluorescence spectroscopic methods. The binding constant
(K) values obtained from Benesi-Hildebrand, Scatchard and
Connor plot for receptor 1 is (8.3 0.3) × 103 M−1 and has
good detection limit 0.7μM. The stoichiometry of 1.Fe3+
complex was confirmed by mass spectroscopy and Job’s plot.
the field of supramolecular chemistry in which the molecular
recognition, transport and catalysis phenomena have been
deeply studied with the hope of shedding light on enzymatic
and more complicated biological processes [1–3].
Over the past few decades, noncyclic compounds also have
been utilized as the essential synthetic targets for further applica-
tions in host–guest chemistry [4–6]. Noncyclic compounds con-
taining multiples sites for noncovalent bonding interaction have
gained considerable attention because of their ability of complex-
ation towards broad range of analytes like ionic and/or neutral/
non-ionic molecules [7]. With the plan of synthesis of metallo-
supramolecular complexes which can act as sensors and precur-
sors, we made an attempt to design the ligands having selective
and significant binding abilities towards various metal ions.
Among transition metal ions Fe3+, Co2+ and Zn2+ have con-
siderable biological importance and play significant role in many
biochemical processes. The human body contains four grams of
iron [8]. Most of the iron present in biological systems is tightly
associated with enzymes and specialized transport and storage
proteins and found in hemoglobin, the red pigment in the eryth-
rocytes and rest of stored in ferretin. It plays a vital role in oxygen
transfer processes in DNA and RNA synthesis. The deficiency of
Fe3+ causes anaemia, hemochromatosis, liver damage, diabetes,
Parkinson’s disease and cancer. Accumulation of copper and iron
leads to over production of H2O2 in tissues; causes oxidative
stress and neurodegenerative diseases [9–12].
.
.
Keywords Dipodal receptors Semi-aqueous medium
Binding constant
Introduction
The architecture of noncovalent bonding interactions array in
the construction of host molecules for selective complexation
with guest molecules is on the verge of getting transferred
towards non-cyclic analogues of crown ether, cryptands, cy-
clodextrins and spherends because of their solubility problem.
Now a day, noncyclic receptors have been made to develop
:
U. Fegade A. Kuwar (*)
Herein, we have paid attention towards synthesis and de-
signing of fluorescent sensors that utilize photoinduced electron
transfer (PET) to translate a cation binding event into a fluores-
cence signal. Generally these types of molecules were mostly
used as an anion sensor due to the presence of amide groups but
in the present work, we have reported a fluorescent sensor 1
designed for selective recognition of iron. Coordination sites in
the receptors 1 show that there are two amide groups along with
hydroxyl groups (Scheme 1). The binding ability of receptors 1
with Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+,Zn2+, Cd2+, Hg2+,
School of Chemical Sciences, North Maharashtra University,
Jalgaon 425001, MS, India
e-mail: kuwaras@gmail.com
:
H. Sharma N. Singh (*)
Department of Chemistry, Indian Institute of Technology, Ropar,
Rupanagar 140001, Punjab, India
e-mail: nsingh@iitrpr.ac.in
:
U. Fegade S. Attarde
School of Earth and Environmental Sciences, North Maharashtra
University, Jalgaon 425001, MS, India

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J Fluoresc (2014) 24:27–37
Scheme 1 Synthetic route of
receptor 1 (I: acetone, reflux 1 h)
NH2
I
+
2
HN
NH
OCN
NCO
OH
b
a
O
NH
HN
O
OH
HO
1
Pb2+ and Bi3+ ions in DMSO/H2O (50:50, v/v) medium was
evaluated and binding constant was computed for 1.Fe3+ com-
plex for which fluorescence intensity was distinctly quenched
which lead to recognition of Fe3+ ion. To date, the synthesized
receptor 1 was not reported for any molecular recognition
study.
DMSO/H2O (50:50, v/v) solvent system and absorption spec-
tra on Schimadzu UV-24500 UV-visible spectrophotometer at
room temperature using 1 cm path length quartz cell.
General Procedure for Synthesis of Receptor 1
and its Complexes
Synthesis of Receptor 1′-(4-methylbenzene-1,3-diyl)bis
Experimental
[3-(2-hydroxyphenyl)urea] (1)
Reagents and Apparatus
Receptor 1 was synthesized by reaction of one mole of 2,4-
diisocyanato-1-methylbenzene (1.74 g, 1 mmol) with two
moles of 2-amino phenol (2.18 g, 2 mmol) in acetone
(50 ml) and reflux for 1 h. Receptor 1 was obtained with
quantitative yield and having appearance of white powder.
All commercial grade chemicals and solvents were purchased
from Sigma-Aldrich and used as such.1H and 13C-NMR spec-
tra were recorded on a Varian NMR mercury System 300
spectrometer operating at 300 & 75 MHz in DMSO-d6 re-
spectively. The fluorescence spectra of the receptor 1 with
metal ions (Chloride salts of Cr3+, Mn2+, Fe3+, Co2+, Ni2+,
Cu2+, Zn2+ and nitrate salts of Cd2+, Hg2+, Pb2+, Bi3+ were
used) were recorded on Fluoromax-4 spectrofluorometer in
1
Yield-82 %, Solubility in DMSO, mp≥250°C. H-NMR
(300 MHz, DMSO-d6, ppm) δ=2.47 (s, 3H, Ar-CH3), 7.37–
7.42 (d, J =7.2Hz, 2H, Ar-H), 7.87–8.08 (m, 8H, Ar-H), 8.21
(s, 1H, Ar-H), 9.23 (s, 2H, Ar-OH), 9.86 (s, 4H, NH).13C NMR,
(75 MHz, DMSO, ppm) δ=14.4, 113.9, 115.6, 116.0, 121.3,
2
Host
Host + Fe Ion
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
220
230
240
250
260
270
280
290
300
Wavelength (nm)
Fig. 1 Changes in absorbance spectrum of receptor 1 (0.07 mM) upon the addition of Fe3+ metal ion (0.7 mM) in DMSO/H2O (50:50, v/v) solvent
system (inset represent the change in colour upon addition of various cations)

J Fluoresc (2014) 24:27–37
29
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Fig. 2 Graph of A(host+Fe)/Ahost
vs wavelength (nm) from the
selectivity Fig. 1
Host+ Fe ion/Host
220
230
240
250
260
270
280
290
300
123.7, 126.0, 129.2, 129.9, 132.9, 135.6, 149.2, 154.6. IR
(KBr, cm−1) υ =750, 878, 1,036, 1,538, 1,650, 2,924, 3,268.
MS (ESI): m/z required C21H20N4O4:392.43, found 393.00.
solutions of metal ions were prepared, concentration of
0.7 mM. The metal binding study was performed using 5 ml
volumetric flasks, each contain a stock solution of receptor 1
along with particular amount of different transition metal ion.
Each solution was placed for 20 min to get rid from non-
homogenous error. The titration was performed in 10 ml
volumetric flask with gradual addition of metal ion with
constant interval of time. For whole set of experiment
DMSO/H2O (50:50 v/v) choice as solvent system. The selec-
tivity of receptor 1 towards Fe3+ in the presence of other metal
ions was confirmed through interference study. To perform
this, a set of solutions were prepared each contain a 1 eq. of
receptor 1 and 1 eq. of Fe3+ along with 2 eq. of other
competing ions. The stoichiometry of 1.Fe3+ complex was
confirmed through the job’s plot in which total concentration
of host and guest will remain same but molar variations were
varied. The plot between [HG] and [H] /[H] + [G] illustrate the
stoichiometry of the complexes formed. The concentration of
[HG] was calculated by the equation [HG] = ΔA /Ao x [H].
Synthesis of Receptor 1.Fe3+ Complex. 1 Fe3+ complex was
synthesized by reaction of one mole of receptor 1 (0.78 g,
0.2 mmol) with one moles of FeCl3(0.32 g, 0.2 mmol) in
50 ml of DMSO:MeOH (10:90, v/v) reflux with stirring for
3 h. The precipitation was collect at room temperature
and dried in vacuum. Further, it was washed with water
then ethanol followed by petroleum ether. Yield- 72 %,
IR (KBr, cm−1) υ=742, 870, 1,030, 1,377, 1,597, 2,853,
2,923. MS (ESI): m/z requires C21H18FeN4O4: 446.24,
found 446.88.
General UV-Visible and Fluorescence Spectral Measurements
For recognition studies, a stock solution of receptor 1 was
prepared having a concentration of 0.07 mM. Similarly, stock
-1
y = 0.417x + 0.543
R² = 0.989
1.8
1.6
1.4
1.2
1
-1.2
-1.4
-1.6
-1.8
-5.5
-5.0
-4.5
-4.0
Log G
0.8
0.6
0.4
0.2
0
220
230
240
250
260
270
280
290
300
Wavelength (nm)
Fig. 3 Changes in absorbance spectrum of receptor 1 (0.07 mM) upon
the gradually addition of Fe3+ metal ion (0.7 mM) in DMSO/H2O (50:50,
v/v) solvent system (inset depict the plot between (Ao-A)/(Ao-Amax),
where A0 represents the fluorescence intensity in the absence of guest ion,
A represents the fluorescence intensity in presence of guest ion)

30
J Fluoresc (2014) 24:27–37
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
Fig. 4 Graph of A(host+Fe)/Ahost
vs wavelength (nm) from the
titration graph Fig. 3
220
230
240
250
260
270
280
290
300
Wavelength (nm)
Result and Discussion
Furthermore, fluorescence response of receptor 1 towards
various metal ions (Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Hg2+, Pb2+ and Bi3+) was studied in DMSO/H2O
(50:50, v/v) at room temperature, keeping the solvent ratio
constant throughout the experiment. For the recognition stud-
ies Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+,
Pb2+and Bi3+ chloride salts were used (0.7 mM) in DMSO/
H2O (50:50, v/v) solvent system. The fluorescence spectra of
1 were recorded upon addition of different metal ion as shown
in Figs. 3 and 4. It is noticed that addition of Fe3+ lead to
decrease in fluorescence intensity as depict in Fig. 5. In order
to unleash the mechanism of sensing a titration was performed
between receptor 1 and Fe3+ (Fig. 6). Receptor 1 has shown
selective quenching behavior towards Fe3+ ion. The gradual
addition of Fe3+ ions to a solution of receptor 1 results in a
decrease in the fluorescence intensity at 345 nm upon excite at
289 nm. This decrease in the fluorescence intensity is ascribed
to the participation of amide nitrogens and phenolic oxygens
towards the coordination of Fe3+ ions, which represent the
photoinduced electron transfer (PET) phenomenon and hence
Turn-Off the fluorescence emission [13, 14].
Receptor 1 was synthesized by condensation reaction be-
tween 2,4-diisocyanato-1-methylbenzene and 2-amino phenol
in acetone with stirring and refluxing for 1 h (Scheme 1). A
white color product was obtained in good amount. The syn-
thesized receptor 1 was characterized by melting point, IR,
1H-NMR, 13C-NMR and mass spectroscopic methods. The
spectral investigation gave consistent data with structure of
receptor 1.
In order to check its binding behaviour with different metal
ions, metal binding test of receptor 1 was evaluated on ab-
sorption and emission spectroscopy. Firstly, the absorption
profile of receptor 1 was checked upon addition of metal ions
and the appreciable change in absorbance was observed for
Fe3+ metal ion in DMSO/H2O (50:50 v/v) solvent system as
shown in Fig. 1. The absorbance spectrum of receptor 1 is
broad and has two absorption maxima at 260 and 289. Further,
a titration was performed between receptor 1 and Fe3+ ion
shown in Fig. 2 and to understand the relationship between
absorbance and concentration of Fe3+; calibration plot was
drawn (Fig. 2 inset). The plot represents that with the increase
of concentration of Fe3+, absorbance is also increased.
IR spectra of receptor 1 and 1.Fe+3 complex revealed the
importance of hydroxyl groups in Fe3+ ion binding. The IR of
Host
Fig. 5 Fluorescence spectra
Cr(III)
Mn(II)
Fe(III)
Co(II)
Ni(II)
(λex=289 nm) of receptor 1
(0.07 mM) upon addition of a
fixed amount of metal salts
(0.7 mM) in DMSO /H2O (50:50,
v/v) solvent system; inset shows
3.00E+05
2.50E+05
Cu(II)
Zn(II)
Cd(II)
Hg(II)
Pb(II)
Bi(III)
2.00E+05
change in color of receptor 1
upon addition of Fe3+ in the
1.50E+05
presence of UV light
1.00E+05
5.00E+04
0.00E+00
300
320
340
360
380
400
420
440
Wavelength (nm)

J Fluoresc (2014) 24:27–37
31
3.00E+05
2.50E+05
2.00E+05
1.50E+05
1.00E+05
5.00E+04
0.00E+00
Fig. 6 Fluorescence titration of
receptor 1 (0.07 mM) upon the
addition of Fe3+ salt (0–200 μL)
in DMSO/H2O (50:50, v/v); inset
represent the plot between (Fo-F)/
(Fo-Fmax) and log of
y = 0.728x - 34736
R² = 0.979
-3.E+04
-3.E+04
-3.E+04
-3.E+04
-3.E+04
-3.E+04
concentration of Fe3+
-5.5
-5
-4.5
-4
Log G
300
320
340
360
380
400
420
440
Wavelength(nm)
3.00E+05
2.50E+05
2.00E+05
1.50E+05
1.00E+05
5.00E+04
0.00E+00
Fig. 7 Fluorescence ratiometric
response (I0-I/I0) of receptor 1
(0.07 mM) upon the addition of
metal salt (0.7 mM) in DMSO/
H2O (50:50, v/v) solvent system
Host Fe(III) Cr(III) Mn(II) Co(II) Ni(II) Cu(II) Zn(II) Cd(II) Hg(II) Pb(II) Bi(III)
Metal ion
Fig. 8 Recognition of Fe3+ ion
by receptor 1 in the presence of
other metal ion
Competing Ions
Competing Ion + Fe Ion
3.00E+05
2.50E+05
2.00E+05
1.50E+05
1.00E+05
5.00E+04
0.00E+00
Fe(III) Cr(III) Mn(II) Co(II) Ni(II) Cu(II) Zn(II) Cd(II) Hg(II) Pb(II) Bi(III)
Metal Ions

32
J Fluoresc (2014) 24:27–37
0.00E+00
-2.00E-05
-4.00E-05
-6.00E-05
-8.00E-05
-1.00E-04
-1.20E-04
-1.40E-04
Fig. 9 Benesi-Hildebrand Plot
receptor 1 (adjusted equation: 1/
F-F0=−6E-10+5E-06 1/[G],
R =0.9811) at the K value
8333M−1
0
20000
40000
60000
80000
100000
120000
140000
160000
1/G
0.00E+00
-5.00E+08
-1.00E+09
-1.50E+09
-2.00E+09
-2.50E+09
-3.00E+09
-3.50E+09
Fig. 10 Scatchard Plot for
receptor 1 (adjusted equation:
F-F0/[G]=−8556x+2E+09,
R =0.987) at the K value
8556M−1
-2.00E+05
-1.00E+05
0.00E+00
F-Fo
3.50E+09
Fig. 11 Connor Plot receptor 1
(adjusted equation: y =−8556x-
2E+09, R=0.986) at the K value
8556M−1
3.00E+09
2.50E+09
2.00E+09
1.50E+09
1.00E+09
5.00E+08
0.00E+00
-2.00E+05
-1.00E+05
0.00E+00
F-Fo

J Fluoresc (2014) 24:27–37
33
y = 42811x + 0.4209
R² = 0.9849
Fig. 12 The Stern–Volmer plot
of the titrations of probe 1 with
different concentrations of Fe3+
metal salt
4
3
3
2
2
1
1
0
0.00E+00
1.00E-05
2.00E-05
3.00E-05
4.00E-05
5.00E-05
6.00E-05
7.00E-05
Q(M)
0.00012
0.00010
0.00008
0.00006
0.00004
0.00002
0.00000
Fig. 13 1:1 Stoichiometry of the
host guest relationship realised
from the Job’s plot for receptor
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
[H]/[H]+[G]
Fig. 14 The optimized structure of: a receptor 1, b 1.Fe3+ calculated by using B3LYP/6-31G basis set on Gaussian 09 program; red, pink, yellow and
turquoise spheres refer to O, N, C and H atoms respectively and dotted line represent H-bonds

34
J Fluoresc (2014) 24:27–37
Table 1 represent the optimized values of energy and HOMO-LUMO
gaps calculated by using B3LYP/6-31G basis set
ions. From the interference study, it was confirmed that there
was moderately low interference of other metal ions in detec-
tion of Fe3+ ion (Figs. 7 and 8). The quenching of fluorescence
distinctly selective for Fe3+ ion from other surveyed metal
ions.
Compound
Energy(eV)
EHOMO-LUMO (eV)
Receptor 1
Receptor 1.Fe3+
36241.48
70599.7
4.825
1.006
Normalized response of fluorescence signal to changing
Fe3+ ion concentration for receptor 1 was plotted as shown in
inset of Fig. 6. The fluorescence intensity decrease for recep-
tor 1 with gradually increased amount of Fe3+guest concen-
tration. These results confirmed that the Fe3+ ion binding of
receptor 1 in semi aqueous solution is chemically reversible.
The chemosensor could therefore be used for real time track-
ing of Fe3+ in biological samples. Under optimal conditions,
the detection limit for Fe3+ ion is as low as 0.7 μM [17]. It
suggested that receptor 1 can be use for sensing even in sub-
micromolar range. This high selectivity of receptor 1 in-
creased its application for sensing in biological system. For a
normal human being 20 mg iron is required per day for the
1.Fe+3 complexes (Figs. 15 and 16) shows the absence of
hydroxyl group it means that the deprotonation was take place
during the complexation [15, 16]. The hydroxyl group acts as a
good binding site and plays an effective role in binding of
transition metal ions. To explore the possibility of using receptor
1 as a practical ion-selective fluorophore for Fe3+ ions, compet-
itive experiment was carried out, in which receptor 1 was firstly
mixed with 1 equivalent of Fe3+ ion and 2 equivalents of other
surveyed metal ions. The Fe3+ ion still had shown an excellent
turn-off response for the Fe3+ion in the presence of other metal
45.5
44
42
40
38
36
34
32
30
28
26
878.71
1036.99
998.44
24
%T
22
1105.90
750.39
20
18
16
3268.72
14
1709.37
1306.57
12
10
8
1650.75
1599.54
1377.75
1225.54
1186.25
6
1538.79
1455.77
4
2853.96
2911.44
2
0.6
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450.0
cm-1
Fig. 15 IR of receptor 1

J Fluoresc (2014) 24:27–37
35
production of red blood cells. Till date, only few receptors are
available which have very less detection limit [26]. These
things enhance the scope of receptor 1 as an efficient sensor
of Fe3+.
Using the Benesi-Hildebrand plot [18] (Eq. 1), Scatchard
plot (Eq. 2) [19] and Connor plot [20] (Eq. 3) methodologies
binding constant (K) was calculated.
respectively. Where, F0 represents the fluorescence intensity
in the absence of guest ion, F represents the fluorescence
intensity in presence of guest ion, F∞ represents fluorescence
intensity after titration and [G] represents the concentration of
guest ion. The binding constant (K) value obtained from
Benesi-Hildebrand, Scatchard and Connor plot for receptor
1.Fe3+ ion was in conformance and it was (8.3 0.3) ×
103 M−1.
1=F−F0 ¼ 1=ðF∞−F0ÞK½GŠ þ 1=ðF∞−F0Þ
F−F0=½GŠ ¼ ðF∞−F0ÞK−ðF−F0ÞK
1−F=F0=½FŠ ¼ KðF=F0Þ−αK
ð1Þ
ð2Þ
ð3Þ
The Stern–Volmer Quenching Plot
The quenching can be mathematically expressed by the
Stern–Volmer Eq. (4), which allows us to determine the
type of quenching. If the Stern Volmer plot is linear
then the quenching is of static type rather than the dy-
namic quenching. For the receptor 1.Fe3+ ion the linear stern
volmer plot indicates that static quenching is obtained [21].
Applying the above equations for receptor 1.Fe3+ ion com-
plexation the Benesi-Hildebrand plot, Scatchard plot and
Connor’s plot obtained were illustrated in Figs. 9, 10 and 11
36.6
36
35
34
33
32
3929.99
31
30
29
28
27
26
25
24
1597.71
1377.49
23
%T
22
21
20
19
18
17
16
15
14
13
1462.83
2853.67
12
11
2923.89
10
8.8
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450.0
cm-1
Fig. 16 IR of 1.Fe3+

36
J Fluoresc (2014) 24:27–37
This confirmed the formation of one type of complex between
showed that color of the solution changes from colorless to
brown upon addition of Fe3+ ion. In contrast, no obvious
changes in color were observed upon addition of other cations
as shown in the inset of Fig. 1. Fluorescent color change was
showed in Fig. 5 inset.
receptor 1 and Fe3+ ion.
F0=F ¼¼ 1 þ kqτ0½QŠ ¼ 1 þ Ksv½QŠ
ð4Þ
From the fluorescence spectra, proposed binding model of
receptor 1.Fe3+ ion complexation was depicted using DFT cal-
culations, which were performed to explain the photo physical
properties of these receptors by using Becke’s three parameter-
ized Lee-Yang-Parr (B3LYP) exchange functional with 6-31G
basis sets, on Gaussian-09 programs [24–26]. The optimized
structure of receptor 1 showed that it has non-planar arrangement
of atoms and has three H-bonds between O and H atoms
(Fig. 14). The formation of complex between 1 and Fe3+, lead
to stabilize the system as confirmed from comparing the value of
energies of 1 and 1.Fe3+ (Table 1). It was observed that 1.Fe3+
has more symmetry instead of receptor 1 (Figs. 15 and 16).
Where F0 and F are the fluorescence intensities in the
absence and presence of the quencher, kq is the bimolecular
quenching constant, τ0 is the lifetime of the fluorescence in
the absence of the quencher [Q] is the concentration of the
quencher, and Ksv is the Stern–Volmer quenching constant. In
the presence of a quencher, the fluorescence intensity is re-
duced from F0 to F. The ratio (F0/F) is directly proportional to
the quencher concentration [Q].
Evidently:
Ksv ¼ kq τ0
F0=F ¼ 1 þ Ksv½QŠ
ð5Þ
ð6Þ
According to Eq. (6), a plot of F0/F versus [Q] shows a
linear graph with an intercept of 1 and a slope of Ksv. A typical
plot of F0/F versus Fe3+ concentration is shown in Fig. 12.
To determine the binding stoichiometry of complex 1.Fe3+,
Job’s continuous variation method was used [22]. Job’s plot of
the fluorescence intensity of free receptor 1 and the intensity of
the system with the molar fraction of the host [H]/([H] + [G]) for
a series of solutions, in which the total concentration of host and
guest was kept constant, with the molar fraction of host contin-
uously varying. The results showed the formation of a 1:1 (Host:
Guest) complex (Fig. 13). Using the equation: [G]tot = a/2K(1-
a)2[H]tot + a[H]tot/2, where [G]tot is total concentration of guest,
[H]tot is the total concentration of host, a = (F- F0)/(Fi- F0) with F
being the fluorescent intensity at a particular guest
ionconcentration while F0 and Fi are the intensities at zero and
infinite guest concentrations, respectively. IR spectrum of free
receptor 1 exhibits a broad band at 3,268 cm−1 which can be
assigned for OH group on the ring. On complexation deproton-
ation of OH group take place and OH band was disappeared
[23]. Similarly, the ν(C = O) band was 1,650 cm−1for free
receptor 1, after complexation the shift of ν(C = O) band up
to 1,597 cm−1. Furthermore, we have confirmed these trends by
mass spectroscopic data. MALDI/TOF–MS data showed the
formation 1:1 complex between two deprotonated ligand (recep-
tor 1) and a metal ion [(MS (ESI): m/z requires (Calculated)
C21H18FeN4O4: 446.24, found 446.88]. All attempts were failed
to grow the single crystal of receptor 1.Fe3+ion complex.
Selective recognition of receptor 1 by Fe3+ gave a remarkable
colorimetric and fluorescent color change which can be visual-
ized by the naked eye and under UV irradiation. Visual detection
studies of receptor 1 (0.07 mM) were conducted with cations of
all metals.
Conclusion
In summary, we have designed and developed a selective and
sensitive fluorescent receptor 1 for the detection of Fe3+ in
semi aqueous medium. The addition of Fe3+ ion gave rise to
major fluorescent color change, which can be easily detected
by the naked eye and under UV irradiation. Furthermore,
receptor selectivity and sensitivity were not affected in the
presence of other competing metal ions. We have successfully
detected Fe3+ ion even in semi aqueous media. The 1:1
stoichiometry of the host guest relationship was realized from
the titration curves. The association constant (K) obtained
from Benesi-Hildebrand, Scatchard and Connor plot for re-
ceptor 1.Fe3+ ion complexation was (8.3 0.3)×103 M−1.
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