Turn-on selective fluorescent probe for trivalent cations

Article abstract of DOI:10.1016/j.inoche.2013.08.025

A novel fluorescent sensor 1 (1 = 10-(2-(((pyridin-2-yl)methylamino)methyl) phenol)methyl-anthracene) for trivalent cations has been synthesized and characterized. Both UV-vis and fluorescence spectroscopic studies demonstrated that the new receptor 1 was

Full text of DOI:10.1016/j.inoche.2013.08.025

Inorganic Chemistry Communications 36 (2013) 72–76
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Turn-on selective fluorescent probe for trivalent cations
a
a
a
a
b
Hyun Kim a, , Kyung Beom Kim , Eun Joo Song , In Hong Hwang , Jin Young Noh , Pan-Gi Kim ,
⁎
Kwang-Duk Jeong c, Cheal Kim a,
⁎
a
Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea
Department of Forest & Environment Resources, Kyungpook National University, Sangju, 742-711, Republic of Korea
Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, 130-650, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel fluorescent sensor 1 (1 = 10-(2-(((pyridin-2-yl)methylamino)methyl)phenol)methyl-anthracene) for
trivalent cations has been synthesized and characterized. Both UV-vis and fluorescence spectroscopic studies
demonstrated that the new receptor 1 was highly sensitive and selective toward trivalent cations, while there
was no response to monovalent and divalent cations in methanol. Upon binding with trivalent cations, the emis-
sion bands of 1 red-shifted for all the trivalent cations from 411 nm to 421 nm and their fluorescence intensities
were enhanced. In particular, Fe3+ could be obviously discriminated from Fe2+. The binding modes of 1 and the
trivalent cations (M3+ ions) were found to be 1:1 and confirmed by Job plot, 1H NMR titration and ESI-mass
spectrometry analysis.
Received 12 July 2013
Accepted 22 August 2013
Available online 30 August 2013
Keywords:
Trivalent cations
Pearson's principle
Fluorescent probe
PET process
© 2013 Elsevier B.V. All rights reserved.
Cation recognition is an area of growing interest in supramolecular
chemistry [1–7]. The recognition and sensing of cationic analytes have
emerged as a key research theme within the generalized area. In partic-
ular, trivalent cation detection is of significant importance due to its
crucial role in a wide range of environmental and biological processes
[8–13]. For instance, aluminum is found in its ionic form Al3+ in most
environmental and biological tissues [14–17]. Al3+ which widely exists
is considered toxic in biological activities [18,19]. Excessive exposure
of the human body to Al3+ leads to a wide range of diseases, such as
Alzheimer's disease, Parkinson's disease, encephalopathy and osteopo-
rosis [20–25]. The trivalent form of chromium is not only an essential
nutrient for humans, but also plays an important role in the metabolism
of carbohydrates, lipids, proteins and nucleic acids [26]. The deficiency
of chromium is known to lead to a variety of disease, and causes distur-
bances in the glucose levels and lipid metabolism [27–29]. Gallium akin
to aluminum (group 3) is small traces in water, vegetables and fruits.
Gallium compounds can cause throat irritation, difficulty breathing
and chest pain. In addition, its fumes can cause even very serious
conditions such as pulmonary edema and partial paralysis. Indium
also belongs to the group 3 and is known to stimulate the metabolism
[30–32]. Moreover, it is about to be known that indium compounds
damage the heart, kidney and liver [33]. Fe3+ is the most compatible
material in the enzyme-catalyzed reaction due to rapid oxidation-
reduction reaction [34–38]. Also, iron is used as a cofactor of electron
transport system [39]. The imbalance of iron in a human body induces
the occurrence of many diseases [40–42]. For examples, excess existence
of Fe3+ is harmful to DNA and protein of an organism by developing the
radical species and its deficiency leads to anemia, liver and kidney
damages, diabetes, and heart diseases [43,44]. Moreover, it is of great
importance to discriminate Fe3+ from Fe2+ through convenient methods
with a simple probe, because the ferrous/ferric (Fe2+/Fe3+) states are one
of the important redox pairs in biological systems [45].
In view of the significance of trivalent cations, therefore, there is a
great demand to develop selective and sensitive methods for trivalent
cation detection and highly desirable to design the probes that presents
selective detection for trivalent cations. However, developing such sen-
sors with recognition capability to detect multiple specific analytes such
as trivalent cations is a huge challenging task, and surprisingly, only few
are known for detection of simultaneously multiple trivalent cations
[46,47].
To achieve the huge challenging work, we employed Pearson's prin-
ciple that hard acids prefer to bind to hard bases and soft acids prefer to
bind to soft bases [48]. Therefore, we have attached an anthracene
group as a chromophore to a moiety with a phenol group capable of
binding to hard metal ions such as the trivalent cations. Indeed, the
new receptor 1 was highly sensitive and selective toward only trivalent
cations.
Herein, we report the new simple turn-on fluorescent probe 1 which
can detect only trivalent cations in methanol. In contrast, 1 did not
respond to monovalent and divalent cations. Furthermore, 1 could dis-
criminate Fe3+ from Fe2+. The binding modes of 1 and M3+ ions were
also studied by Job plot, 1H NMR titration and ESI-mass spectrometry
analysis.
Our goal was to design a selective trivalent cation sensor by Pearson'
principle. Therefore, the chemosensor 1 was synthesized by reaction
of 9-(chloromethyl)anthracene with 2-(((pyridin-3-yl)methylamino)
methyl)phenol (PAP) acting as a hard base which was made by the
⁎
Corresponding authors. Tel.: +82 2 970 6693; fax: +82 2 973 9149.
E-mail addresses: eagleyun@naver.com (H. Kim), chealkim@seoultech.ac.kr (C. Kim).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.08.025

H. Kim et al. / Inorganic Chemistry Communications 36 (2013) 72–76
73
Cl
Et
NH2
NaBH
4
N
O
H
N
N
MeOH
HO
N
3N, MC
OH
PAP
OH
N
1
Scheme 1. Synthetic route of the chemosensor 1.
condensation of 2-aminomethyl pyridine and 2-hydroxy benzaldehyde
as shown in Scheme 1. The anthracene unit acts as a fluorophore
[40,49–51] and PAP does as a receptor for the hard metal ions such
as the trivalent cations. Its molecular structure was confirmed by ele-
mental analysis, 1H NMR,13C NMR and ESI-Mass.
(Fig. S1). In the case of Al3+, with the increase in concentration of
Al3+ ion, the emission intensity was gradually enhanced with induction
period and reached a maximum at 1.4 equiv of Al3+ (Fig. 2). More
accurate stoichiometry of the Al3+/1 was determined by Job plot
which revealed a 1:1 ratio for 1:Al3+ (Fig. 3). The complexation of
Al3+ with 1 induced deprotonation of the phenolic hydroxyl group of
1 (Scheme 2), which was confirmed by ESI-mass spectrometry analysis
of 1-Al3+ complex (Fig. 4). The [1 + Al]2+ complex was calculated to
be m/z 215.08 and measured to be m/z 214.80. The 1:1 binding stoichi-
ometries of Cr3+, Fe3+, Ga3+ and In3+were also determined by Job plot
(Fig. S2). ESI-mass spectrometry analysis of 1 with Cr3+ and Fe3+
further supported the 1:1 binding stoichiometry (Fig. S3).
To examine the selectivity of 1 toward metal ions, its spectroscopic
properties were measured upon addition of various metal ions such as
Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+
,
Zn2+, Ag+, Cd2+, Hg2+, Pb2+, Ga3+, In3+ and Fe2+in methanol. The
emission spectrum of 1, which was excited at 364 nm, exhibited
an emission maximum at 411 nm with a low quantum yield (Φfree
0.02). The addition of Na+, Mg2+, K+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+
Zn2+, Ag+, Cd2+, Hg2+, Pb2+ and Fe2+ had no effect on the fluores-
cence, whereas the addition of trivalent cations (Al3+, Cr3+, Fe3+
=
,
,
Ga3+ and In3+) showed the prominent enhancement of the fluores-
cence (Φ = 0.79, 0.77, 0.72, 0.73 and 0.75, respectively) with a slight
red shift of the emission maxima from 411 to 421 nm, as shown in
Fig. 1. Such a significant change in emission of 1 in the presence of triva-
lent cations might be attributed to the strong coordination of trivalent
cations to the phenolic hydroxyl group of the PAP chelator moiety of
1 by Pearson' principle [52]. The strong binding of the trivalent cations
to 1 prevents a PET process from the aliphatic nitrogen center to the ex-
cited anthracene, thus resulting in a significant enhancement of fluores-
cence. These results indicate that the probe 1 exhibits highly selective
chemosensor for trivalent cations. To our best knowledge, simultaneous
discrimination of three trivalent cations from the monovalent and diva-
lent cations detection was reported only twice [46,47], and this is the
first report in which the probe can detect simultaneously five trivalent
cations.
In order to understand the binding modes between 1 and the triva-
lent cations, the fluorescence titration of 1 with M3+ was carried out
Fig. 2. Fluorescence spectra of 1 (5.0 × 10−6 M) in the presence of different concentration
of Al3+. Inset: the fluorescence at 421 nm of 1 as a function of the Al3+ concentration.
(λex = 364 nm).
Fig. 1. Fluorescence spectra of 1 (5.0 × 10−6 M) upon addition of 2 equiv of Na+, Mg2+
Al3+, K+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+
Ga3+, In3+ and Fe2+ in methanol (λex = 364 nm).
,
,
Fig. 3. Job plot of 1 with Al3+
.

74
H. Kim et al. / Inorganic Chemistry Communications 36 (2013) 72–76
Al(NO3)3
+ HNO3
N
N
Al
O
OH
N
N
O
O
O N
N
O
O
O
Scheme 2. Proposed structure of a 1:1 complex of 1 and Al3+
.
The binding abilities of the trivalent cations with 1 were determined
In particular, it is worthwhile to mention that Fe3+can be obviously
discriminated from Fe2+. Based on competition experiments, 1 could
be used as a selective fluorescent sensor for trivalent cation detection
in the presence of most competing metal ions such as monovalent and
divalent cations.
from modified Benesi-Hildebrand equations (Fig. S4) and the detection
limits for each M3+ ion were obtained using the method of a signal-to-
background (S/B) ratio (Fig. S5) [53]. These values are shown in
Table 1. All trivalent cations showed similar binding abilities to 1
(Ka = 1.0 × 105– 1.7 × 105) and Fe3+ had the lowest detection limit
(0.6 μM), indicating that 1 detects Fe3+ most sensitively among the
trivalent cations.
In order to further inquire into the spectral characteristics of 1, we
investigated UV-vis absorption changes with respect to each M3+. As
shown in Fig. 6, the UV-vis spectrum of 1 exhibited absorption bands
at 332, 348, 366 and 386 nm. Upon addition of Al3+, the absorption
peaks at 332, 348, 366 and 386 nm decreased obviously, whereas new
prominent bands at 337, 353, 371 and 391 nm were developed with
red shift. The well-defined isosbestic points at 335, 341, 351, 359, 369,
379 and 389 nm represent a clean conversion of 1 into the 1-Al3+
The most important criterion for a selective cation probe is the
ability to detect a specific cation in the existence of other competing
ions, although 1 revealed a remarkable selectivity for trivalent cations
against monovalent and divalent metal ions (Fig. 1). To further examine
the selectivity of 1 for trivalent cations, therefore, we investigated the
fluorescence intensity in the presence of trivalent cations mixed with
monovalent and divalent ones. First of all, the change of fluorescence in-
tensity of 1 was measured in the presence of the trivalent cation Ga3+
mixed with various metal ions (Fig. 5). Compared to the intensity
obtained with the trivalent cation Ga3+, the emission spectra were
complex. The spectral characteristic behaviors of 1 toward Cr3+, Fe3+
,
Ga3+ and In3+ were also conducted by UV-vis spectroscopy, respective-
ly, and similar results were obtained (Fig. S7).
The 1H NMR investigation was performed in CD3OD to understand
the nature of interaction between sensor 1 and Al3+. As shown in
Fig. 7, significant spectral changes were observed. Upon addition of
Al3+ ion to receptor 1, Ha, Hb, Hc and Hd of anthracene protons and He
of pyridyl group showed downfield shifts. The shift of pyridyl moiety
protons suggests N-metal coordination. Hf, Hg and Hh of methylene pro-
tons also showed significant downfield shifts, indicating the strong
coordination of nitrogen atoms with Al3+. In a similar way, all signals
corresponding to aromatic protons shifted. Most of the phenol group
peaks at 6.7–7.2 ppm were shifted to 6.6-7.1 ppm. These obvious
changes of the chemical shifts indicated that 1 could form a stable
complex with Al3+. There was no shifts in the position of proton signals
on further addition of metal ions (N1.0 equiv) which confirms 1:1
complexation between Al3+ and 1. These results are consistent with
the formation of 1:1 ligand-to-metal complex supported by Job plot
and ESI-mass spectrometry analysis.
almost identical in the presence of Na+, Mg2+, Al3+, K+, Ca2+, Cr3+
,
Mn2+, Fe3+, Co2+, Ni2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+, In3+ and
Fe2+, except for Cu2+ that inhibited about 75% of the fluorescence. Nev-
ertheless, it still had a sufficient turn-on ratio for the detection of Ga3+
in the presence of Cu2+. These results indicate that the presence of
monovalent and divalent cations did not disturb the detection of the tri-
valent cation Ga3+ with 1. Again, this might be attributed to the strong
complexation of trivalent cations with 1 by Pearson's principal that hard
acids (M3+) prefer to bind the hard base (the oxygen atom of the phe-
nol group of 1). Competition experiments of Al3+, Cr3+, Fe3+, and In3+
were also carried out in the presence of potentially competitive metal
ions (Fig. S6). Fe3+, In3+ and Al3+ showed similar selective patterns
to that of Ga3+, while less selective property was displayed for Cr3+
.
Furthermore, we examined the selectivity of 1 toward metal ions in
various solvents. Any significant selectivity was not observed, except in
DMSO which similar results were obtained as observed in methanol
(Fig. S8).
In conclusion, we have developed a new simple chemosensor 1 by
the combination of an anthracene group as a chromophore moiety
and a phenol group as a binding moiety toward hard metal ions such
as the trivalent cations. The chemosensor 1 showed excellent turn-on
fluorescence signals with high sensitivity and selectivity in the presence
Table 1
Association constants and detection limits for 1 with trivalent cations.
Cation
Ka
Detection limit / μM
Al3+
Cr3+
Fe3+
Ga3+
In3+
1.4 × 105
1.7 × 105
1.4 × 105
1.0 × 105
1.4 × 105
2.4
1.6
0.6
2.4
2.0
Fig. 4. Positive-ion electrospray ionization mass spectrum of 1 (1.0 × 10−4 M) upon
addition of 1 equiv of Al3+ in methanol.

H. Kim et al. / Inorganic Chemistry Communications 36 (2013) 72–76
75
of trivalent cations (Al3+, Cr3+, Fe3+, Ga3+ and In3+) in methanol,
while monovalent and divalent cations had no effect on the fluores-
cence emission. The optical mechanism of 1 was proposed to be a PET
process from the tertiary aliphatic nitrogen to the excited anthracene.
The fluorescence probe had detection ability in the μM range and
detected Fe3+ most sensitively among the trivalent cations. More
importantly, Fe3+could be obviously discriminated from Fe2+ by 1.
Therefore, 1 could be used as an optical sensor for the trivalent cations.
Future study will focus on enhancing the water solubility of receptor
and its potential applications in biological chemistry.
Acknowledgements
Financial support from Converging Research Center Program through
the National Research Foundation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (2012001725, 2012008875 and
2013 K000334) is gratefully acknowledged. We thank Prof. MiSookSeo
(EwhaWomans University) for ESI-Mass running.
Fig. 5. Fluorescence intensity of 1 induced by various metal cations. Red bar represents
emission intensity of 1 in the presence of 3 equiv of Ga3+. Black bars stand for fluorescence
change that occurs upon additionof 3 equiv of Na+, Mg2+, Al3+, K+, Ca2+, Cr3+
,
Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+, In3+ and Fe2+ in the
presence of Ga3+
.
Appendix A. Supplementary material
Supplementary data (experimental procedures and additional ex-
perimental data) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.inoche.2013.08.025.
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