A naphthalene derived Schiff base as a selective fluorescent probe for Fe2+

Article abstract of DOI:10.1016/j.ica.2015.09.030

A naphthalene pyridine Schiff base 1 as a fluorescent chemosensor was developed for the detection of Fe²⁺ ions in CH₃CN-H₂O solution. Its binding properties toward various other heavy and transition metal ions including Fe³⁺ ions were examined. The sensor showed high selectivity and sensitivity towards Fe²⁺ ions in aqueous media with the lower detection limit of 1.5 × 10^-7 M. The fluorescence "turn-on" recognition process follows the ICT process and C=N isomerisation. In addition, determination of Fe²⁺ in a variety of samples were analyzed which includes commercially available tablets, tomato juice, dark chocolate and tap water.

Full text of DOI:10.1016/j.ica.2015.09.030

Inorganica Chimica Acta 439 (2016) 1–7
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A naphthalene derived Schiff base as a selective fluorescent probe
for Fe2+
S. Santhoshkumar a, K. Velmurugan a, J. Prabhu a, G. Radhakrishnan b, R. Nandhakumar a,
⇑
a Department of Chemistry, Karunya University, Karunya Nagar, Coimbatore 641 114, Tamil Nadu, India
b CSIR – Central Electro Chemical Research Institute, Karaikudi 630 006, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A naphthalene pyridine Schiff base 1 as a fluorescent chemosensor was developed for the detection of
Fe2+ ions in CH3CN–H2O solution. Its binding properties toward various other heavy and transition metal
ions including Fe3+ ions were examined. The sensor showed high selectivity and sensitivity towards
Fe2+ ions in aqueous media with the lower detection limit of 1.5 Â 10À7 M. The fluorescence ‘‘turn-on”
recognition process follows the ICT process and C@N isomerisation. In addition, determination of
Fe2+ in a variety of samples were analyzed which includes commercially available tablets, tomato juice,
dark chocolate and tap water.
Received 17 July 2015
Received in revised form 15 September
2015
Accepted 24 September 2015
Available online 9 October 2015
Keywords:
Ó 2015 Elsevier B.V. All rights reserved.
Schiff base
Chemosensor
Fluorescence
ICT
Fe2+ detection
Water analysis
1. Introduction
Presently, many techniques are available for the detection of
Fe2+ such as colorimetry, atomic absorption spectroscopy, voltam-
The development of fluorescent chemosensors for specific
detection of various transition metal ions is challenging to current
researchers due to the potential applications in medicine as well as
environmental research domains [1–5]. Among them, iron is one of
the most essential metal ions in our living systems. In biological
network, it plays a critical role in living organisms such as oxygen
metabolism, oxygen uptake and electron transfer process. Further-
more, iron present as a cofactor in various enzymatic reactions and
easy redox reactions between Fe2+ and Fe3+ [6–13]. However,
excess or deficiency of Iron leads to serious diseases, such as lipid
peroxidation, DNA fragmentation leads to cell death, liver damage,
kidney failure, anemia, cancer, Alzheimer’s, and Parkinson’s
diseases [14–17]. Higher level of iron exists in the protein-bound
form; labile iron and non-protein bound iron were reported to
involve healthy and diseased states in living organisms [18–20].
Therefore, the demand for rapid detection and discrimination of
Fe2+ from other ions including Fe3+ at lower concentration has
significant applications in environment and biological systems
[21,22].
metry, spectrophotometry, and flow injection [23–27]. Above
methods is very complicated, tedious sample preparation and
required sophisticated instrumentation [28–30]. On the other
hand, fluorescent chemosensors overcome all those problems than
other methods due to their simplicity, low-cost, sensitivity, and
instantaneous response and capable of specific recognition of par-
ticular ions [31–35]. Fluorescent probes for iron is a challenging
one because of its effective fluorescence quenching nature as well
as its different oxidation and spin states. Highly reactive iron exists
as ferrous ion (Fe2+) than ferric ion (Fe3+) due to its better water
solubility, intracellular reductive environment and good binding
affinity [36–40]. Besides, Fe2+ is a potential catalyst for chemical
reactions, producing harmful labile oxygen species [41–43]. In
addition, fluorescent probes for Fe2+ with a turn-on response, are
highly desirable for understanding both the physiological and
pathological roles. Even if, there are many reports are available
for turn-on Fe2+ and Fe3+ probes [44–51] but Fe2+-responsive
turn-on with red shift based charge transfer fluorescent probes
not available to date. Therefore, the development of selective
sensing of Fe2+ and implementing a new method to quantify trace
level of Fe2+-ions in environmental systems is current focus of our
research.
⇑
Corresponding author.
E-mail address: nandhakumar@karunya.edu (R. Nandhakumar).
http://dx.doi.org/10.1016/j.ica.2015.09.030
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

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S. Santhoshkumar et al. / Inorganica Chimica Acta 439 (2016) 1–7
According to supramolecular concepts, naphthalene was chosen
as a fluorophore and pyridine used as an ionophore due to their
excellent photo-physical properties, high fluorescence quantum
yield and good coordination ability [52–54]. Herein, we designed
and synthesized a new class of naphthalene based fluorescent
chemosensor (1) which exhibits selective sensing towards Fe2+
with remarkable fluorescence turn-on with red shifted response
via Intramolecular Charge Transfer (ICT) process in CH3CN–H2O
(1:1 v/v, HEPES = 50 mM, pH = 7.4) solutions.
N
N
CHO
EtOH
RT
N
NH2
1
Scheme 1. Synthesis of receptor 1.
2. Experimental
2.1. Materials and instruments
2.2. Synthesis
All solvents were purchased commercially with reagents grade
quality. 2-aminopyridine and 1-naphthaldehyde were obtained
from Sigma. 1H NMR and 13C NMR spectra were recorded on a
Bruker 300 and 100 MHz spectrometer using CDCl3 and DMSO-d6
solutions with TMS as internal standard. LC–MS was determined
on a LC-MSD-Trap-XCT Plus based on infusion methods. Absorption
spectra were recorded on a Shimadzu UV-240 spectrophotometer.
Fluorescence measurements were performed on a Jasco FP-8200
spectrofluorometer equipped with quartz cuvettes of 1 cm path
length. The excitation and emission slit widths were 5.0 nm. All
absorption and emission spectra were recorded at 24 1 °C. Stock
solutions for analysis were prepared (2 Â 10À3 M for compound 1
(CH3CN/H2O, 1:1 (v/v), HEPES = 50 mM, pH = 7.4) immediately
before the experiments. The solutions of metal ions were prepared
2.2.1. Synthesis of (E)-N-(naphthalene-1-ylmethylene)pyridine-2-
amine (1)
1-Naphthaldehyde (0.5 g, 3.20 mmol) was dissolved in ethanol
(20 mL) in the presence of 2-aminopyridine (0.36 g, 3.84 mmol)
and stirred for 5 h at room temperature. The reaction mixture
was allowed to settle down, and the precipitate formed was fil-
tered and recrystallized from methanol to afford the compound 1
as
a
white colour solid (90% yield). M.p. 232 °C, 1H NMR
(300 MHz, CDCl3): 9.59 (s, 1H), 8.54–8.56 (d, 1H), 8.20–8.24 (d,
1H), 8.08–8.11 (d, 1H), 8.02–8.05 (d, 1H), 7.37–7.98 (m, 5H),
7.18–7.25 (m, 2H) ppm. 13C NMR (100 MHz, DMSO-d6): 109.15,
114.81, 115.74, 118.81, 119.46, 120.19, 123.16, 126.84, 127.18,
127.72, 127.94, 128.86, 135.29, 141.55, 155.67, 166.47 ppm. Anal.
Calc. for C16H12N2: C, 82.73; H, 5.21; N, 12.06. Found: C, 82.15;
H, 5.02; N, 12.10%. LC–MS calcd. for C16H12N2: [M+] 232, found
[M++H]+ 233.
from chloride and nitrate salts of Pb2+, Cu2+, Cd2+, Hg2+, La3+, Zn2+
,
Co2+, Ni2+, Ca2+, Mn2+, Cr3+, Ba2+, Ce3+, Mg2+, Al3+, Fe2+, Fe3+, Ag+,
Na+ and K+. The studies on the binding properties of 1 were carried
out in solution (CH3CN/H2O, 1:1 (v/v), HEPES = 50 mM, pH = 7.4).
The different metal ion solutions (100 equiv.) were prepared by dis-
solving the desired amount of metal salts in solution (CH3CN/H2O,
1:1 (v/v), HEPES = 50 mM, pH = 7.4). The fluorescence titration was
performed with a series of 4 Â 10À6 M solutions of 1 containing
various equivalents of Fe2+-ions. Binding studies were confirmed
by job’s plot and nonlinear curve fitting methods.
3. Results and discussion
3.1. Synthetic design of chemosensor 1
The one step synthesis of the naphthalene based chemosensor 1
is shown in Scheme 1. Briefly, Schiff base reaction between
Fig. 1. Fluorescence spectral changes of 1 (4 Â 10À6 M) CH3CN–H2O solution (1:1 v/v, HEPES = 50 mM, pH = 7.4) in the presence of various metal ions (100 equiv. of each, kex.
= 300 nm).

S. Santhoshkumar et al. / Inorganica Chimica Acta 439 (2016) 1–7
3
Fig. 2. Metal ions competition analysis of 1 (4 Â 10À6 M) in CH3CN–H2O, (1:1 v/v, HEPES = 50 mM, pH = 7.4). The blue bars represent the fluorescence emission of 1 and
100 equiv. of other metal ions. The red bars represent the fluorescence changes that occur upon addition of 100 equiv. of other metal ions to the solution containing 1 and Fe2+
(100 equiv.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Fluorescence titration spectrum of 1 (4 Â 10À6 M) in CH3CN–H2O solution (1:1 v/v, HEPES = 50 mM, pH = 7.4) upon addition of different concentration of Fe2+
(0–200 equiv. of each, kex. = 300 nm).
1-naphthaldehyde and 2-aminopyridine in ethanol affords the
crude 1 which is then purified by the recrystallization in methanol
and characterized by the usual analytical and spectroscopic
techniques (Figs. S1–S3).
ion binding ability of sensor 1 was carried out in CH3CN–H2O (1:1
v/v, HEPES = 50 mM, pH = 7.4) solution. Sensor 1 showed a weak
fluorescence. However, upon addition of 100 equiv. of Fe2+ to a
solution of sensor 1, two major things were observed. Firstly, the
fluorescence enhancement and secondly, a notable red shift from
354 to 370 nm. The normalised spectrum clearly indicates the
16 nm bathochromic shift as shown in the Supporting information
(Fig. S5). Nonetheless, under the same concentration (100 equiv.)
3.2. Evaluation of selectivity
The UV–visible absorption spectrum of 1 in CH3CN/H2O (1:1 (v/v),
HEPES = 50 mM, pH = 7.4) show two intense bands with absorption
maximum at 225 and 300 nm (Fig. S4). Based on the absorption
spectrum of 1, the excitation was fixed at 300 nm for metal
ion binding studies by fluorescence. Screening of the specific metal
of other metal ions such as Pb2+, Cu2+, Cd2+, Hg2+, La3+, Zn2+, Co2+
,
Ni2+, Ca2+, Mn2+, Cr3+, Ba2+, Ce3+, Mg2+, Al3+, Fe3+, Ag+, Na+ and K+
addition to sensor 1, produced some negligible changes in the
fluorescence spectra (Fig. 1). As a result, 1 exhibits a selective

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S. Santhoshkumar et al. / Inorganica Chimica Acta 439 (2016) 1–7
Fig. 4. Job plot of 1 and Fe2+ in CH3CN–H2O solution (1:1 v/v, HEPES = 50 mM, pH = 7.4).
Fig. 5. Benesi–Hildebrand plot of 1-Fe2+ complex (2:1) binding stoichiometry.
detection of Fe2+ when compared to other metal ions by a fluores-
cence turn-on response with notable red shift.
between 6 and 8 pH ranges. However, under identical conditions,
on addition of Fe2+ to 1, the fluorescence intensity gradually
increases as the pH was varied from 0 to 6 and attains a maximum
intensity at neutral condition. These results reveal that the Fe2+
recognition process is free from pH interference under the near
neutral pH conditions. Thus, the physiological pH of 7.4 was
selected as the working condition throughout the following spec-
troscopic experiments. Moreover, changes in fluorescence intensi-
ties of 1 to Fe2+ (CH3CN–H2O, 1;1 v/v, HEPES = 50 mM, pH = 7.4)
3.3. Effect of pH and time response
Sensitivity of 1 and 1 + Fe2+ at different pH values were studied
to explore its photophysical properties in CH3CN–H2O solution (1:1
v/v) (Fig. S6). The results showed that the sensor 1 exhibits a weak
fluorescence over a wide range of pH except a small enhancement

S. Santhoshkumar et al. / Inorganica Chimica Acta 439 (2016) 1–7
5
Fig. 6. Changes in fluorescent spectra of 1 (4 Â 10À6 M) solution (CH3CN–H2O, 1:1 (v/v), HEPES = 50 mM, pH = 7.4) in the presence of Fe2+ & EDTA (100 equiv.).
intensity for the detection of Fe2+ ion (Fig. 2). Hence, this result
proved that the receptor 1 could be used for the selective detection
of Fe2+-ion in real sample sensing applications.
3.4. Stoichiometry and binding mode studies
In order to evaluate the binding ability and limit of detection of
Fe2+ with 1, the fluorescence titrations of 1 were performed by
gradually increasing the concentration of Fe2+
. Fluorescence
Scheme 2. Proposed binding mode of 1 with Fe2+
.
intensity was increased with increasing the concentration Fe2+
(0–200 equiv.) with a notable red shift (Fig. 3). Red shift was
completed on the addition 100 equiv. of Fe2+ and the fluorescent
profile changes was gradually enhanced and almost terminated
upon the addition of 200 equiv. of Fe2+, implying that 1 interacts
with Fe2+ in 2:1 stoichiometry.
against time was performed (Fig. S7). However, sensor 1 with
addition of Fe2+, the fluorescence intensity increased and reached
the maximum level within 2 min and kept steady for 50 min. From
this study, it is clear that the sensor 1 can detect the Fe2+ in a short
period of time of 2 min.
The mode of complexation between 1 and Fe2+ (Host–Guest)
was confirmed by the Job’s plot method [55,56]. Complex emission
intensity was gradually increased with increasing the concentra-
tion of 1 as it reached a maximum mole fraction is 0.7. Afterwards,
the intensity decreased with further addition of 1, which eventu-
ally established a 2:1 (1:Fe2+) binding stoichiometry (Fig. 4).
In addition, the binding stoichiometry between 1 and Fe2+ was
also confirmed by the Benesi–Hildebrand nonlinear curve fitting
method [57,58] (Fig. 5). The association constant was determined
to be Ka 5.02 Â 104 MÀ2 for the 1–Fe2+ complex. The detection limit
Furthermore to check the practical utility of sensor 1, with the
help of the competitive complexation analysis, the possible inter-
ferences by other co-existing metal ions were carried out. The flu-
orescence changes of 1 was measured by 100 equiv. of Fe2+ ions in
the presence of same concentration of other interfering metal ions
such as Na+, K+, Pb2+, Cu2+, Cd2+, Hg2+, La3+, Zn2+, Co2+, Ni2+, Ca2+
,
Mn2+, Cr3+, Ba2+, Ce3+, Mg2+, Al3+, Fe3+ and Ag+. The tested interfer-
ing metal ions showed no significant changes in the emission
Fig. 7. SEM images of (a) 1 only and (b) 1 + FeCl2.

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S. Santhoshkumar et al. / Inorganica Chimica Acta 439 (2016) 1–7
Table 1
Determination of Fe2+ in real samples.
Sample
Amount of Fe2+ present
Fe2+-ion spiked (ppm)
Fe2+-ion found (ppm)
Recovery (%)
in blank ppm (AAS)
(Fluorescence) (Mean S.D.)
Tablet-1
Tablet-2
Tomato juice
Dark chocolate
Tap water-1
Tap water-2
100
50
02.7
06.2
21
0
0
0
0
0
0
100.82 0.03
50.24 0.08
02.52 0.05
06.02 0.03
21.53 0.02
18.87 0.04
100.8
100.4
93.3
97.0
102.5
104.8
18
Table 2
Comparison of reported iron sensors in different methods.
Refs.
Detected elements
LOD
Methods of detections
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[44]
[47]
[48]
[51]
This work
Fe2+
Fe2+
Fe3+
Iron
Iron
Fe3+
Iron
Fe2+
Fe3+
Fe3+
Fe3+
Fe2+
12 nm
120 mmol/L
Fluorescence
Fluorescence
Graphite furnace AAS
Multi-syringe flow injection system
Voltammetry
Colorimetry naked-eye, Hydrogel sensor
Flow injection spectrophotometric method
Fluorescence
Fluorescence
Fluorescence
10À7
M
0.50 mg LÀ1
7.7 Â 10À9
M
0.1 ppm
0.02 l
g LÀ1
3.6 Â 10À6
M
M
–
5
lM
0.1
l
M
Fluorescence
Fluorescence
1.5 Â 10À7
of 1, calculated using the formula 3d/S [59,60], is 1.5 Â 10À7 M,
where d is the standard deviation of the blank signal, and S is the
slope of the linear calibration plot.
3.7. Microscopic studies
Finally, the morphological changes of 1 and 1 + Fe2+ were
analyzed by SEM images, which are shown in Fig. 7. A crystal like
random molecular size morphology was detected in sensor 1.
However, upon addition of Fe2+, the size of the complex molecular
size became smaller and agglomerated due to the formation of 1
+ Fe2+ complex. This result further corroborates to the above
discussions as the sensor 1 selectively detects Fe2+-ions among
all the other metal ions tested.
3.5. Reversibility studies
The recognition and reversibility of a sensor 1 is an important
requirement. Here, we examined the reversibility of the binding
between 1 and Fe2+ in the presence of EDTA (100 equiv.) in
CH3CN–H2O (1:1 (v/v), HEPES = 50 mM, pH = 7.4) solutions. The
addition of EDTA (100 equiv.) to the solution of 1 containing Fe2+
led to the disappearance of the fluorescence signals of 1-Fe2+ and
attained the original intensity of free sensor 1, indicating that the
chelation process is reversible as shown in Fig. 6. The completion
of the reversible process between Fe2+ with EDTA is 2 min.
3.8. Application studies
As a practical applicability, we have examined the feasibility of
1 for the determination of the most abundant ion Fe2+ in different
samples via fluorescence techniques. Six numbers of samples were
analyzed by this method and these include commercially available
tablets, tomato juice, dark chocolate and tap water (Table 1). As a
result, the quantitative recoveries of non-spiked samples were
satisfactory and confirmed with known standards. This indicates
that 1 could potentially be used for the determination of Fe2+-ion
in real samples without any other co-existing metal ion interfer-
ences (Table 2).
3.6. Proposed binding mode of the complex
Based on the above mentioned results, a possible sensing
mechanism for the detection of Fe2+ by the sensor is proposed
and illustrated in Scheme 2. Sensor 1 with weak fluorescence
intensity, exhibits a ‘‘turn-on” response with high fluorescence
intensity on the addition of Fe2+ with a notable red shift. This
may be due to the Intramolecular Charge Transfer (ICT) process
between the naphthalene and pyridine moiety which are
covalently linked through an imine bond (Schiff base). In addition
to the ICT, the increase in the fluorescence intensity after the
addition of Fe2+ to sensor 1 is due to the Chelation enhanced fluo-
rescene which also inhibits the C–N torsional rotation by making
the complex a rigid one which eventually arrests the C@N isomeri-
sation. Fe2+ coordination with the nitrogen of C@N led to the slight
rearrangement of C@N, which influences the conjugation between
C@N and naphthalene. Job’s plot and nonlinear curve fitting
(Benesi–Hildebrand) methods reveals that the possible binding
ratio is 2:1 between 1 and Fe2+ complex. From all of the above data,
it could possibly be deduced that the coordination modes of 1-Fe2+
complex is 2:1(Host:Guest) binding stoichiometry.
4. Conclusion
In conclusion, a naphthalene derived fluorescent chemosensor 1
was developed for the detection of Fe2+ ions in CH3CN–H2O solu-
tion with lower detection limit of 1.5 Â 10À7 M. The sensor 1
specifically recognizes Fe2+ over other heavy and transition metal
ions including Fe3+. The ‘‘turn-on” response followed by a notable
red shift for recognition of Fe2+ by 1 is due to the ICT process and
C@N isomerisation. In future, the sensor 1 could be used as a
potential probe for the detection Fe2+ ions in biological and envi-
ronmental monitoring works.
Acknowledgement
This work is supported by the Fast Track Programme for the
young Scientists of the DST (No. SR/FT/CS-95/2010).

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