Sensing of Fe(III) ion via turn-on fluorescence by fluorescence probes derived from 1-naphthylamine

Article abstract of DOI:10.1016/j.tetlet.2013.09.066

Fluorescence probes NA1 and NA2 derived from 1-naphthylamine (NA) as fluorophore have been synthesized and characterized by different spectroscopic studies. Identification behaviour of these probes towards various metal ions has been investigated. Both th

Full text of DOI:10.1016/j.tetlet.2013.09.066

Tetrahedron Letters 54 (2013) 6460–6463
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Sensing of Fe(III) ion via turn-on fluorescence by fluorescence probes
derived from 1-naphthylamine
⇑
Kaushik Ghosh , Sweety Rathi, Ritu Kushwaha
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2013
Revised 12 September 2013
Accepted 14 September 2013
Available online 21 September 2013
Fluorescence probes NA1 and NA2 derived from 1-naphthylamine (NA) as fluorophore have been synthe-
sized and characterized by different spectroscopic studies. Identification behaviour of these probes
towards various metal ions has been investigated. Both the fluorescent probes are selective as well as
sensitive towards Fe(III) ion. Novel fluorescence probe NA2 afforded turn-on fluorescence behaviour
for Fe(III) ion over other metal ions such as Ca(II), Mg(II), Mn(II), Fe(II), Co(II), Fe(III), Ni(II), Cu(II), Zn(II)
and Hg(II).
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Fluorescence probes
Naphthylamine
Emission
Turn-on fluorescence
Iron is one of the essential elements present in the biosystem
and several metalloenzymes need iron to execute catalytic activi-
ties which are responsible for various physiological processes.
CH3
O
OH
1
N
H
N
Several biological functions depend directly or indirectly on the
proper concentration and oxidation states of iron to maintain the
O
H
2
CH
3
homoeostatic mechanism of biosystem. Disturbances in such type
of balance of iron concentration may create physiological disor-
NA1
NA2
2
,3
ders. For that reason detection and localization of Fe(III) are an
important area of chemical research. Hence design and synthesis
of fluorescence probes which are specific and selective for Fe(III)
are highly demanding. For intracellular and extracellular detection
Figure 1. Structure of NA1 and NA2.
4
Fe(III). Naphthyl-based probes are widely used because of their
sensitivity and simplicity. Hence naphthyl-based probes are used
of labile iron many physical and chemical techniques namely
2
2
atomic absorption spectroscopy (AAS),5 colorimetry,6 spectropho-
2
3a
23b
tometry7 and voltammetry8 have been utilized. However, there
for the detection of several metal ions such as Ca(II),
Al(III),
but reports on selectivity of Fe(III)
are very few and exhibited quenching in fluorescence emission
2
3c
23d
23e
9
Zn(II),
Pb(II)
and Ni(II)
are several limitations of such kind of detection described above.
In this regard fluorimetric methods are highly sensitive and appre-
2
4
spectra. However very few literatures based on enhancement of
ciable. Fe(III) is paramagnetic in nature and hence its detection is
2
5
accompanied by quenching of fluorescence.10 Enhancement or
Fe(III) using naphthyl group are reported. To the best of our
knowledge naphthyl based probes exhibiting turn-on phenomena
are still unexplored.
In this Letter we have synthesized two Schiff’s bases namely
NA1 and NA2 (Fig. 1) derived from 1-naphthylamine. NA1 was syn-
thesized in one step by the condensation of p-vanillin with 1-naph-
thylamine in ethanol at 25 °C for 30 h as reported earlier
(Scheme 1). On the other hand, a new Schiff’s base NA2 was syn-
thesized by the reaction of o-hydroxyacetophenone with 1-naph-
thylamine in methanol (Scheme 2). Both NA1 and NA2 are
characterized by various spectroscopic techniques like IR, UV–vis,
H NMR, C NMR spectral studies and ESI-MS (data are deposited
in the Supporting information Figs. S1–S5).
turn-on fluorescence has tremendously gained importance for
1
1
the design and synthesis of probes which are specific for Fe(III).
12
Various probes such as densyl-based probes, rhodamine-based
1
1
3
5
14
probes,
benzimidazole-based
probes,
imidazole-based
16
probes, naphthyl-based probes, anthracene-appended amino
1
7
1-,8-diquinolylnaphthalene probes,18 quinoline-
acid probes,
2
6
1
9
20
based polymer, polymer based on 9-aminofluorine and N-aza-
crown carbazole probes21 have been used for the detection of
⇑
Corresponding author. Fax: +91 1332 273560.
E-mail addresses: ghoshfcy@iitr.ernet.in (K. Ghosh), sweetyiitr@gmail.com (S.
1
13
Rathi), ritukushwahaiitr@gmail.com (R. Kushwaha).
0
040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.09.066

K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6460–6463
6461
H
O
C
O
NH2
CH3
O
EtOH, 30 h
N
H
CH3
O
H
O
H
p-vanillin
1-naphthylamine
NA1
Scheme 1. Synthesis of NA1.
CH3
NH2
OH
MeOH, 5 days
O
N
OH
CH3
o-hydroxyacetophenone
1-naphthylamine
NA2
Scheme 2. Synthesis of NA2.
The photophysical properties of NA1 and NA2 were examined
in methanolic solution. NA1 exhibits fluorescence intensity
enhancement without any shift in emission wavelength upon addi-
binding stoichiometry between NA1 and Fe(III) ion as proposed
(Fig. 3b). As shown in Figure S7, fluorescence intensity was maxi-
mum at around 5 min after mixing and showed a negligible change
3
1
tion of Fe(III) ions and hence this fluorescence probe is probably
in fluorescence intensity after that.
based on photo electron transfer (PET).2
7,28
The UV–vis spectra of
Fluorescence spectral studies on NA2 were performed in
methanol. NA2 alone displayed a very weak, single fluorescence
emission band at 355 nm when it was excited at 290 nm. In
addition to that, only weak changes of fluorescence intensity
of NA2 were observed on addition of the other metal ions such
as Ca(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Hg(II).
However, unlike the previous observation, we did not get any
enhancement of fluorescence (shown in Fig. 4a). Interestingly,
addition of Fe(III) gave rise to the generation of a new band
near 435 nm which indicates selective on–off signalling behav-
iour of Fe(III) (shown in Fig. 4b). NA2 exhibited fluorescence
intensity enhancement in the new fluorescence band as well
as new emission band upon addition of Fe(III) ions. In the
presence of Fe(III) the fluorescent enhancement efficiency was
ꢀ40-fold greater compared to free NA2. Such type of change
in fluorescence spectra was observed in the literature and the
mechanism was probably due to internal charge transfer
NA1 and NA2 in methanol exhibited typical naphthalene absorp-
2
4b
tion band at 280 nm.
An emission band around 425 nm in meth-
anol was found when excitation of NA1 was done at 279 nm. NA1
was found to be selective and sensitive for Fe(III) only among the
served metal ions like Ca(II), Mg(II), Mn(II), Fe(II), Fe(III), Co(II),
Ni(II), Cu(II), Zn(II) and Hg(II) (Fig. 2a). A bar diagram is also shown
for NA1 with different metal ions (Fig. 2b). The emission band at
around 425 nm showed about eightfold emission intensity only
2
9
for Fe(III).
Fluorescence emission spectra were recorded for NA1 with
increasing concentration of Fe(III) (shown in Fig. S6 in supporting
information). Weak fluorescence near 425 nm was observed in
the absence of Fe(III) ions at kex = 279 nm.30 We found a gradual in-
crease in fluorescence intensity upon addition of Fe(III) ions into
the solution. Figure S6 clearly indicated that the maximum inten-
sity of fluorescence was obtained when concentration of Fe(III)
2
7,28
ions was 50
lM. The sensitivity curve indicated that the probe
(ICT).
The observed fluorescence enhancement may be due
NA1 maintained at a concentration of 50
analysis of micromolar concentrations of Fe(III) ions. Job’s plot
l
M, can be used for the
to the formation of rigid system after binding with Fe(III) ion
3
2
(Fig. 4b).
Considering the structures of NA1 and NA2 we
according to the method of continuous variation afforded 1:1
could easily figure out that the metal binding sites for the
5
5
4
4
3
3
2
2
1
1
5
0
5
0
5
0
5
0
5
0
5
0
(a)
(b)
Fe(III)
3
60 380 400 420 440 460 480 500 520
Wavelength (nm)
Figure 2. (a) Fluorescent emission spectra of NA1 (50
lM) in methanol with 1 equiv of Ca(II), Mg(II), Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Hg(II). (b) Emission
intensity of NA1 at 425 nm after addition of 1 equivalent of selected metal ions in methanol at kex 279 nm.

6
462
K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6460–6463
4
00
50
00
50
00
50
00
(
a) 350
(b)
3
3
2
2
1
1
00
50
00
50
00
5
0 µM
3
2
2
0
µM
1
1
5
0
0
50
0
3
75 400 425 450 475 500 525 550
Wavelength (nm)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Mole fraction of Fe(III)
Figure 3. (a) Fluorescent emission titration spectra of NA1 (50
lM) in the presence of varying concentrations of Fe(III) in MeOH at kex 279 nm. (b) Job’s plot of NA1 (50 lM)
and Fe(III) (50 M), where fluorescent intensity of NA1 at 425 nm was plotted against the mole fraction of Fe(III).
l
(
a) 20
(b) 50
40
Fe(III)
30
10
20
10
0
0
3
00 325 350 375 400 425 450 475 500
Wavelength (nm)
3
00
325
350
375
Wavelength (nm)
Figure 4. (a) Fluorescent emission spectra of NA2 (50
lM) in methanol with 1 equiv of Ca(II), Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Hg(II) in the range of 300–
3
6
80 nm (b) Fluorescent emission spectra of NA2 (50 lM) in methanol with 1 equiv of Ca(II), Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Hg(II) in the range of 300–
00 nm.
two probes are different. In case of NA1 metal ion could sit
near OH and OMe groups where the probe could act as a biden-
tate ligand or near imine nitrogen atom where the probe could
act as a monodentate ligand. On the other hand NA2 probe
could act as a bidentate ligand with phenolato and imine func-
tions. The sensitivity curve indicated that the probe NA2
(b) 120
(
a)
100
100
8
6
4
2
0
0
0
0
0
50µM
8
0
0
6
0
µM
40
20
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3
80 400 420 440 460 480 500 520 540
Mole fraction of Fe(III)
Wavelength (nm)
Figure 5. (a) Fluorescent emission titration spectra of NA2 (50
lM) in the presence of varying concentrations of Fe(III) in MeOH at kex 290 nm. (b) Job’s plot of NA2 (50 lM)
and Fe(III) (50 M), where fluorescent intensity of NA2 at 435 nm was plotted against the mole fraction of Fe(III).
l

K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6460–6463
6463
maintained at a concentration of 50
analysis of micromolar concentrations of Fe(III) ions.
l
M, can be used for the
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Supplementary data
Supplementary data (Supplementary material contains IR, UV–
vis, 1H NMR, 13C NMR spectra, ESI-MS and Fluorescence emission
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