Condensation Product of 4-Methoxybenzaldehyde and Ethylenediamine: “Off-On” Fluorescent Sensor for Cerium(III)

Article abstract of DOI:10.1007/s10895-018-2298-0

The condensation product (L) of 4-methoxybenzaldehyde and ethylenediamine has been synthesised and characterised. L showed a 21 times enhancement in fluorescence intensity on interaction with Ce³⁺ in CH₃OH at λ_max = 360?nm

Full text of DOI:10.1007/s10895-018-2298-0

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2298-0
ORIGINAL ARTICLE
Condensation Product of 4-Methoxybenzaldehyde
and Ethylenediamine: BOff-On^ Fluorescent Sensor for Cerium(III)
Diganta Kumar Das1 & Bidisha Bharali1 & Swrangsi Goyari1
Received: 2 June 2018 /Accepted: 12 September 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
The condensation product (L) of 4-methoxybenzaldehyde and ethylenediamine has been synthesised and characterised. L
showed a 21 times enhancement in fluorescence intensity on interaction with Ce3+ in CH3OH at λmax = 360 nm when excited
with 270 nm photons. Metal ions K+, Na+, Al3+, Co2+, Hg2+, Cd2+, Ni2+, Zn2+, Mn2+, Mg2+ and Ca2+ do not interfere. The
stoichiometry of binding and the binding constants were determined from spectroscopic data and found to be 1:1 and 104.8
M
respectively. The detection limit was found to be 10–5.2 M. The protonation/de-protonation of water molecules coordinated to
Ce3+ was found to show interesting behaviour on the fluorescence of L:Ce3+.
.
.
.
.
Keywords Cerium(III) 4-methoxybenzaldehyde Fluorescence Sensor Protonation/deprotonation
Introduction
coexisting ions, especially some rare earth’s ions, Fe(III) and
phosphate ions, always confront the direct methods.
Ce3+ is the most widely distributed rare earth elements in the
earth’s crust [1–3]. Cerium finds its applications in important
fields like luminescence, agriculture, catalysis, nuclear energy,
metallurgy, microelectronics, therapeutics, magnetism, glass
and ceramics. The wide-range use of cerium prompts re-
searchers to study environmental and biological effects of ce-
rium [4–6]. Different instrumental methods like X-ray fluores-
cence [7], inductively coupled plasma (ICP-OES) [8, 9] spec-
trophotometry [10], spectrofluorometry [11, 12], ion-selective
electrodes [13, 14] and potentiometric sensors [1] are reported
to determine Ce3+cations. Although these methods have cer-
tain advantages, the disadvantages include - high consumption
of energy and reagent, high costs, disposal of harmful
chemicals in the environment, lack of repeatability etc. In
fluorimetric determination of trace Ce3+ two direct and indi-
rect methods are adopted [12]. It is reported that direct method
is the most sensitive one, for example, interference of
The fluorescent sensing of Ce(III) by Bon-off^ mode based
on graphene quantum dots in aqueous medium is reported
[15]. Another fluorescent Bon-off^ sensor based on
glycinedithiocarbamate-functionalised manganese doped
ZnS quantum dots reported [16]. Green fluorescent carbon
quantum dots have been synthesised and found to detect
Ce(III) by fluorescence quenching in aqueous medium [17].
The above recent reports are based on quantum dots only and
detects Ce(III) by fluorescence quenching.
Schiff bases as fluorescent sensors for different metal ions
have gained recent interest. This is basically due to easy one or
two step synthesis. Few recently reported Schiff’s bases for
metal ions Al3+, Zn2+ and Cu2+include condensation products
of 2-hydroxy-1-napthaldehyde and 2-aminophenol [18], 2-
hydroxyacetophenone and ethylenediamine [19],
Phenylalanine and salicylaldehyde [20], L-alanine and
salicylaldehyde [18], 1-naphthylamine and benzaldehyde
[21], phenylalanine and salicylaldehyde [22], naphthylamine
and benzil [23], naphthylamine and benzil [24], benzil and L-
tryptophan [25], benzil and 2-aminophenol [26], etc.
* Diganta Kumar Das
In this paper we report that the condensation product of 4-
methoxybenzaldehyde and ethylenediamine can detect Ce(III)
ion over a host of other metal ions by fluorescent Boff-on^
mode in 1:1 (v/v) CH3OH:H2O (Scheme 1).
digkdas@yahoo.com; digantakdas@gmail.com
1
Department of Chemistry, Gauhati University,
Guwahati, Assam 781014, India

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O
O
Ce3+
N
N
Scheme 1 Structure of the sensor L
Experimental
4-methoxybenzaldehyde and CDCl3 are from Sigma Aldrich,
ethylenediamine and metal salts were either from Merck or
Loba Chemie. The metal salts except Pb(NO3)2, CaCl2 and
HgCl2 were sulphates. Metal salt solutions (0.01 M) were
prepared in doubly distilled water obtained from quartz double
distillation plant. The FT-IR spectra were recorded in a Perkin
Fig 1 Fluorescence spectra of L in CH3OH at different added
concentration of Ce3+. λex = 270 nm
1
Elmer RXI spectrometer as KBr pellets, H NMR and 13C
final increase in fluorescence intensity was ca. 21 times to that
for L in absence of Ce3+. Similar titration was done with other
metal ions – Al3+, Ca2+, Zn2+, Cd2+, Co2+, Li+, Ni2+, Hg2+,
Pb2+, Mn2+, Mg2+, K+ and Na+ in CH3OH. Figure 2 compares
I/Io values for L in presence of one equivalent of different
metal ions. Here I is the fluorescence intensity of L in pres-
ence of one equivalent of a metal ion while Io is the fluores-
cence intensity of L(in absence of a metal ion) in CH3OH.
From the figure it is clear that the effect of Ce3+ on I/Io value of
L in CH3OH is quite distinguishable from the other metal
ions.
To investigate interference by other metal ions on the inter-
action between L and Ce3+, fluorescence spectra were record-
ed for L in presence of one equivalent of Ce3+ and another
metal ion – K+, Na+, Al3+, Co2+, Hg2+, Cd2+, Ni2+, Zn2+,
Mn2+, Mg2+ and Ca2+. Figure 3 compares, through bars, the
I/Io values where Io is the fluorescence intensity of L and I is
the fluorescence intensity of L in presence of one equivalent
NMR spectra were recorded on a Bruker Ultra Shield
300 MHz spectrophotometer using CDCl3 as solvent. The
fluorescence and UV/Visible spectra were recorded in
HITACHI 2500 and Shimadzu UV 1800 spectrophotometer
respectively using quartz cuvette (1 cm path length).
Synthesis and Characterisation of the Sensor (L)
0.02 mol (2.723 g) of 4-methoxybenzaldehyde and 0.01 mol
(0.601 g) of ethylenediamine were dissolved in 10 mL
C2H5OH in a 50 mL round bottom flask and refluxed for
6 h. The mixture was cooled in room temperature. Light yel-
low colored precipitate was obtained, the solvent was evapo-
rated in a rota evaporator to obtain the product which was
recrystallized from CH3OH. Yield: 80%.
FT-IR (KBr, cm−1):3423 (νO-H), 1602 (νC=N), 1508
(νC=O), 771(νC-H).
1H NMR(CDCl3, δ ppm, TMS):6.862–7.632(m,8H, Ar-
H), 8.188(2H,-CH), 3.151–3.344(6H,-OCH), 3.804–
3.896(4H,-CH2).
13C NMR(CDCl3, δ ppm, TMS):161.91(C=N), 113.84,
129.02, 129.55(Carom), 161.48 (C-O), 55.24(CH3O-),
61.61(CH2).
Results and Discussion
Figure 1 shows the fluorescence spectra of L in CH3OH at
different added concentration of Ce3+ when excited with
270 nm photons. A gradual increase in fluorescence intensity
was observed with a shift in λmax from 320 nm to 360 nm. The
Fig. 2 I/Io values of L in CH3OH in presence of one equivalent of
different metal ions. λex = 270 nm, λmax = 360 nm

J Fluoresc
1
[EDTA2-]
0.8
0.6
0.4
0.2
0
Fig. 3 The I/Io values of L in presence of one equivalent of Ce3+ (red
bars) and L in presence of one equivalents of Ce3+ and another metal ion
(blue bars). λex = 270 nm, λmax = 360 nm
Ce3+ and one equivalent another metal ion. Similar height of
the bars shows that L can detect Ce3+ even in presence of
another metal ion.
Fig. 5 Effect of EDTA2− at different added concentration on the
fluorescence spectra of Ce3+:L. The gradual decrease in fluorescence
intensity proves the reversibility of Ce3+ to L binding
The stoichiometry of binding between L and Ce3+ and
binding constant were determined from the plot of log[(I-
Io)/(I-Iα)] versus log[Ce3+] [15] which has been shown in
Figs. 4 and 5. Here Io is the fluorescence of L in absence
of Ce3+ in the solution, I is the fluorescence of L at a
given added concentration of Ce3+ and Iα is the fluores-
cence of L in presence of one equivalent of Ce3+. The
binding ratio between L and Ce3+has been found to be
1:1 with logβ = 4.8.
Effect of pH on Fluorescence Intensity of L and L:Ce3+
Figure 6 shows the fluorescence intensity of L at different
pH in 1:1 v/v CH3OH:H2O. The fluorescence intensity
was found to decrease from pH 2.0 to 4.0 then remained
fixed till pH 6.0 and thereafter it decreased gradually till
pH 12.0. The pH dependence of fluorescence of Ce:L
complex was quite different (Fig. 7) from the pH depen-
dence of L alone. Here, the fluorescence intensity
remained same up to pH 5.8 and then showed a marginal
increase till pH 6.2. The fluorescence intensity did not
enhance significantly up to pH 8.4, after which it started
increasing till pH 9.6 and then ceases.
The binding of Ce3+to L was found to be reversible. The
reversibility was checked by recording the fluorescence of 1:1
solution of L and Ce3+ as a function of di-sodium salt of
ethylenediaminetetraacetate (Na2EDTA) in CH3OH. The
fluorescence intensity was found to gradually decrease with
increasing EDTA2− concentration. EDTA2− chelates away
Ce3+ from its complex with L.
The detection limit was calculated to be 10–5.4 M from the
plot of (Is-Io) versus log[Ce3+] as per reported procedure [24].
Fig. 4 Plot of log[(I-Io)/(I-Iα)] versus log[Ce3+] for fluorescence spectral
titration of L against Ce3+ in CH3OH, λex = 270 nm, λmax = 360 nm
Fig. 6 Fluorescence intensity of L as a function of pH in 1:1 (v/v)
CH3OH:H2O

J Fluoresc
coordinated complexes are obtained (Scheme 3) -L:Ce3+
(from pH 2.0 to 5.8), L:Ce3+(H2O)2 (from pH 6.2 to 8.4)
and L:Ce3+(OH)2 (from pH 9.6 and above). Because of the
absence, presence of H2O and presence of OH− as coordinat-
ing group the effective positive charge on Ce3+ will be differ-
ent. The effective positive charge will be lowest in case of
L:Ce3+(OH)2 followed by L:Ce3+(H2O)2 and L:Ce3+.The
negatively charged hydroxyl groups lower the formal positive
charge on Ce3+. This lowers the electronegativity of Ce3+ and
enhances the electron delocalisation on L:Ce3+ complex.
Hence an enhancement in fluorescence intensity is observed
from pH 8.4 to pH 9.6. When there is no coordinating OH or
H2O on Ce3+, it has highest electronegativity and lowers the
electron density in L and the lowest fluorescence is observed
at pH 2.0.
Fig. 7 Fluorescence intensity of L:Ce3+ as a function of pH in 1:1 (v/v)
CH3OH:H2O
In case of L, at pH 12 the immine N are deprotonated and
the lone pair of electrons on N atoms will get indulged in PET
process as shown in Scheme 2a. From pH 6.2to 4.0 one out of
two immine N will be protonated snapping the PET process
from one immine N. Thus enhanced fluorescence intensity is
observed. As pH is lowered than 4.0 the other immine N will
also start protonating, snapping the second PET and the fluo-
rescence intensity shows a further increase with lowering pH.
Ce3+ binds to L through the two amine N atoms with two
water molecules coordinated to Ce3+. At pH 8.4 the de-
protonation of coordinated H2O starts and at pH 9.8 the de-
protonation is complete and the hydroxyl groups are coordi-
nated to Ce3+. At pH below 5.8 the protonation of the coordi-
nated H2O molecules start and the H2O molecules are de-
tached from Ce3+. Thus due to pH variation three differently
Conclusion
Condensation product of 4-methoxybenzaldehyde and
ethylenediamine could detect Ce3+ by fluorescence Boff-on^
mode in CH3OH. Metal ions - K+, Na+, Al3+, Co2+, Hg2+,
Cd2+, Ni2+, Zn2+, Mn2+, Mg2+ and Ca2+ do not interfere.
The sensor binds Ce3+ in 1:1 stoichiometry with binding con-
stant 104.8 and detection limit 10–5.2 M. The protonation/de-
protonation of coordinated H2O of Ce3+ influences the fluo-
rescence of the L:Ce3+complex in solution.
Scheme 2 a Both the immine N of L deprotonated with the two PET processes taking place shown by the two arrows, (b) One of the two immine N of L
deprotonated and one PET process taking place shown by the arrows, (c) Both the immine N of L are protonated and no PET process possible
Scheme 3 a At pH = 2.0 neither H2O nor −OH coordinates to Ce3+, (b) At pH 6.2 to 8.4 two H2O coordinates to Ce3+ lowering the positive charge on
Ce3+to some extent, [C] at pH 9.6 and above two −OH coordinates to Ce3+lowering the positive charge on Ce3+significantly

J Fluoresc
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