Fluorescent dye containing phenol-pyridyl for selective detection of aluminum ions

Article abstract of DOI:10.1016/j.dyepig.2012.10.008

A phenol-pyridylimine probe was utilized as an optical sensor to quantify the presence of aluminum ions using a turn-on fluorescence enhancement approach. The high sensitivity was the result of FRET amplification of the receptor subunit fluorescence emission. The complex stability constant (K_s) for the stoichiometric 1:1 complex of the sensor with aluminum ions was obtained by fluorimetric titration. Remarkably, fluorescence output was not significantly affected by other trivalent cations, particularly Ga³⁺ and In³⁺.

Full text of DOI:10.1016/j.dyepig.2012.10.008

Dyes and Pigments 96 (2013) 590e594
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorescent dye containing phenol-pyridyl for selective detection of aluminum
ions
,
a,
Jungha Lee a, Hyun Kim b, Soojin Kim a, Jin Young Noh b, Eun Joo Song b, Cheal Kim b , Jinheung Kim
*
**
a Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea
b Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A phenol-pyridylimine probe was utilized as an optical sensor to quantify the presence of aluminum ions
using a turn-on fluorescence enhancement approach. The high sensitivity was the result of FRET
amplification of the receptor subunit fluorescence emission. The complex stability constant (Ks) for the
stoichiometric 1:1 complex of the sensor with aluminum ions was obtained by fluorimetric titration.
Remarkably, fluorescence output was not significantly affected by other trivalent cations, particularly
Received 23 August 2012
Received in revised form
5 October 2012
Accepted 9 October 2012
Available online 29 October 2012
Ga3þ and In3þ
.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Fluorescent chemosensor
Aluminum ion sensing
Phenol
Pyridyl
Selective binding
Turn-on detection
Aluminum is the third most abundant metallic element (about
8% by weight) on Earth, and its soluble form is highly toxic to plant
growth [1]. A trivalent cation of aluminum is found in most animal
and plant tissues as well as in natural waters everywhere. People
are exposed to aluminum due to its widespread use in food addi-
tives, aluminum-based pharmaceuticals, and cooking utensils. If
a significant load of aluminum ions exceeds the body’s excretory
capacity, the excess is deposited in various tissues, including bone,
muscle, heart, spleen, liver, and brain. Aluminum ion accumulation
in tissues and organs results in dysfunction and toxicity [2]. The
iron binding protein is the main carrier for Al3þ in plasma and Al3þ
can enter the brain and reach the placenta and fetus. Aluminum
Alzheimer’s disease. Some scientists believe that breathing
aluminum dust may cause health problems, as it may cause alu-
minosis, and unusual concentrations of aluminum ions have been
found in the brains of patients with Alzheimer’s disease.
Fluorescent chemosensors have attracted significant interest for
use in medical, medicinal, and environmental research. Until
recently, some fluorescent chemosensors based on Schiff bases [8e
10], triazoles [11,12], triazole-pyridyl [13e15], calixarene [16,17],
and secondary/tertiary amines [18,19], have been developed for
detecting Al3þ. Most fluorescent Al3þ sensors have binding envi-
ronments containing imine nitrogen and/or oxygen which create
a hard-base environment for the hard-acid Al3þ. We have reported
on a fluorescent chemosensor containing a salicylimine moiety for
selective aluminum ion sensing [8]. Here, we developed a new
phenol/pyridyl-based chemosensor that exhibited high selectivity
for Al3þ. This chemosensor provided high sensitivity derived from
the turn-on fluorescence changes upon titration with Al3þ and good
selectivity even relative to gallium and indium ions.
The fluorophore CPAP was synthesized by coupling 2-
hydroxyphenyl-2-pyridylmethylamine and 4-chloro-3-nitroco-
umarin with a 71% yield in acetonitrile (Scheme 1). The precursor
2-hydroxyphenyl-2-pyridylmethylamine was prepared by consec-
utive reduction of the imine compound produced from reacting
2-aminomethylpyridine and 2-hydroxy benzaldehyde in methanol.
concentrations in the brain should be maintained at <2 mg/g [3].
Aluminum has been explored as a possible toxic agent involved in
the development of Alzheimer’s disease, as aluminum ions induce
oxidative stress within brain tissue. Al3þ has been implicated as
a causative factor in Alzheimer’s disease and has been associated
with damage to the central nervous system in humans [4e7].
However, it is still unclear whether aluminum ions play any part in
* Corresponding author.
** Corresponding author. Tel.: þ82 2 3277 4453; fax: þ82 2 3277 3419.
E-mail addresses: chealkim@snut.ac.kr (C. Kim), jinheung@ewha.ac.kr (J. Kim).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.10.008

J. Lee et al. / Dyes and Pigments 96 (2013) 590e594
591
CHO
OH
OH
N
N
NaBH4
NH2
N
H
+
O
N
O
NO2
NEt3
O
O
N
OH
NO2
CPAP
Cl
Scheme 1. Synthetic CPAP procedure.
First, we studied CPAP absorbance changes following treatment
with aluminum ions. CPAP had two absorption bands at 268 and
316 nm (Fig. 1a). Al3þ Treatment caused a progressive decrease in
the 268 and 316 nm bands, and two new absorption bands
appeared at 259 and 381 nm. Absorbance at 316 nm continued to
decrease, and the 381 nm band increased with an increase in Al3þ
concentrations up to 1 equiv (Fig. 1b). Three isosbestic points were
observed at 264, 290, and 345 nm, indicating that only one product
was generated from CPAP upon binding to Al3þ. CPAP had
a quantum yield of 0.095.
420 nm. In contrast, no fluorescence enhancement was observed
after adding other mono-, di-, and tri-valent metal ions including
Naþ, Mg2þ, Al3þ, Kþ, Cr3þ, Ca2þ, Mn2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ
,
Ag2þ, Cd2þ, Hg2þ, Pb2þ, In3þ, and Fe2þ. Only, adding Ga3þ to CPAP
produced some emission but with much reduced intensity. Al3þ
selectivity with 10
mM CPAP was evaluated by testing sensor
response to other metal ions at a concentration of 200
m
M (Fig. 2b).
Only the Al3þ solution produced marked fluorescence enhance-
ment relative to that of control ions. The intensity ratio (IAl/IGa) of
25 with CPAP was remarkably high relative to 2.1 and 3.4 reported
with chemosensors based on salicylimine and naphthol/quinoline,
Fluorescence emission was examined following treatment with
various metal ions in methanol (Fig. 2a). CPAP alone did not show
any significant emission upon excitation at 353 nm. However, Al3þ
treatment resulted in a large increase in intensity at a wavelength of
700
a
Al3+
600
500
400
0.6
a
0.5
0.4
0.3
0.2
0.1
0.0
+
2+
+
3+
L, Na , Mg , K , Cr , Ca
2+
,
300
200
100
0
2+
3+
Mn , Fe , Co2+, Ni2+, Cu2+, Zn2+
,
Ag+, Cd2+, Hg2+, Pb2+, In3+
, Fe
2+
Ga3+
330 360 390 420 450 480 510 540 570 600
Wavelength (nm)
b
700
220
275
330
385
440
495
550
600
500
400
300
200
100
0
Wavelength (nm)
0.40
b
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
2+
3+
3+
2+
+
+
3+
2+
2+
3+
Hg
L
Cu
Ga
2+
Mn
Ag
2+
Na
Mg
Na Mg Al
Al
2+
Co
3+
Fe
3+
Cr
+
+
2+
2+
Pb
K
Zn
Cd
In
Fe
Ca
Ni
L
K Cr Ca Mn Fe Co Ni Cu Zn Ag Cd Hg Pb Ga In Fe
0
1
2
3
4
5
Fig. 2. (a) Fluorescence spectra of CPAP (10
m
M) after adding metal salts (20 equiv) of
Liþ, Naþ, Kþ, Agþ, Mg2þ, Ca2þ, Mn2þ, Fe2þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Cd2þ, Hg2þ, Pb2þ
,
Equiv. of
[
Al3+
]
Cr3þ, Fe3þ, and In3þ in MeOH. The excitation wavelength was 353 nm. (b) Selectivity of
Fig. 1. (a) UVevisible spectral changes in CPAP (40
m
M) after adding aluminum ions
the analysis for metal ions with CPAP. The concentration of all metal ions was 200 mM.
(up to 5 equiv) in methanol. (b) Graph of the absorbance at 316 (-, squares) and
Left to right: CPAP alone, Naþ, Mg2þ, Al3þ, Kþ, Cr3þ, Ca2þ, Mn2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ
Zn2þ, Ag2þ, Cd2þ, Hg2þ, Pb2þ, Ga3þ, In3þ, and Fe2þ. L indicates CPAP.
,
381 nm (C, circles) as a function of Al3þ concentration.

592
J. Lee et al. / Dyes and Pigments 96 (2013) 590e594
respectively [8,9]. In another report, a probe containing pyrene
exhibited an intensity ratio of 4.0 [19].
700
600
500
400
300
200
100
0
When a 10 m
M CPAP solution was titrated with Al3þ, fluorescence
intensityat 420 nm increased steeply upto 10 equivand then showed
almost no significant increase up to a concentration of 40 equiv
(Fig. 3). However, the CPAP chemosensor developed in this study
showed spectral changes from 0 to 10 equiv Al3þ, and Al3þ could be
detected down to 0.5 mM based on the 3a/slope when 10 mM CPAP
was employed. A Job plot obtained based on emission data referred to
1:1 stoichiometric complexation of CPAP with Al3þ (Supporting
Information Fig. S1). According to the above titration fluorimetry,
the stability constant (Ks) of CPAP-Al3þ was calculated by using the
nonlinear least-squares analysis. The Ks value turned out 3.3 ꢀ 104
which was within the range 103w109 of those reported for Al3þ
-
binding chemosensors.9,11,13,15,18 The need of more equiv Al3þ to reach
a saturation point resulted from the low CPAP concentration used in
fluorescence titration experiments relative to UVevis and NMR.
Fig. 4 shows CPAP fluorescence intensity with Al3þ in the pres-
ence of various competing metal ions. Compared to CPAP intensity
obtained with 1 equiv Al3þ, the presence of other metal ions resulted
in no interference for detecting Al3þ, except Fe3þ. No significant
fluorescence signal interference was observed, even in the case of
In3þ and Ga3þ. Thus, CPAP appeared to be a selective fluorescent
sensor for detecting Al3þ in the presence of competing metal ions.
The 1:1 binding stoichiometry of CPAP and Al3þ was examined
by 1H NMR in CD3OD. Adding 1 equiv of Al3þ to a solution of
Fig. 4. Relative fluorescence of CPAP alone and its complexation with Al3þ in the
presence of various metal ions. The CPAP response was included as a control (labeled
as L). Conditions: 10.0
m
M CPAP, 1 equiv metal ions with 1 equiv of Al3þ (El ¼ 353 nm).
the receptor resulted in different peak profiles (Fig. 5). Adduct
formation between Al3þ and CPAP was evident from the dramatic
changes in the chemical shifts for both the phenol and pyridyl
protons of CPAP. The proton signals of the phenol group at 6.6e
7.3 ppm were shifted to 6.5e6.8 ppm. At the same time, the pyr-
idyl moiety proton signals at 7.4e8.5 ppm were somewhat shifted
in a similar region. Coumarin aromatic protons at 7.3e7.6 ppm were
also shifted slightly. The two CPAP methylene protons split up into
two doublets upon binding to Al3þ. Such a splitting of methylene
protons in pyridyl methyl moieties was observed when a chemo-
sensor was coordinated to metal ions [20]. In the CPAP-Al3þ coor-
dination complex, two nitrogens and an oxygen of CPAP could bind
a
700
600
500
400
300
200
100
with Al3þ and the remaining three binding sites (X) of the Al3þ
-
CPAP complex may be occupied by nitrate anions or methanol
(Scheme 2). The coordination of nitrate or perchlorate ions was
proposed when chemosensors were bound to Al3þ with a 1:1
stoichiometry in organic solvents [8,13,15].
In conclusion, a phenol-pyridyl chemosensor is reported as an
optical turn-on fluorescent probe for Al3þ ions. This probe enabled
analysisofAl3þ ions witha sensitivitylimitof0.5 M.Inourassay, Al3þ
m
0
selectively participated in complex formation with the receptor,
which resulted in enhanced fluorescence. The chemosensor reported
here exhibited the highest fluorescence intensity ratio between Al3þ
and Ga3þ, relative to the values obtained in other reports. In further
investigations, this chemosensor will be derivatized to improve
sensitivity and to detect other metal ions in aqueous solution.
330 360 390 420 450 480 510 540 570
Wavelength (nm)
b
700
600
500
400
300
200
100
0
1. Experimental section
1.1. Materials and instrumentation
All solvents and reagents (analytical and spectroscopic grades)
were obtained from SigmaeAldrich (St. Louis, MO, USA) and used as
received. The metal ion solutions were prepared with metal nitrate
salts in methanol. NMR spectra were recorded on a Varian 400
spectrometer (Palo Alto, CA, USA). Chemical shifts (d) are reported
in ppm, relative to tetramethylsilane Si(CH3)4. Absorption spectra
were recorded at 25 ꢁC using a PerkineElmer model Lambda 2S UV/
Vis spectrometer (Waltham, MA, USA). Emission spectra were
recorded on a PerkineElmer LS45 fluorescence spectrometer.
0
10
20
30
40
3+
Equiv. of [Al ]
1.2. Synthesis of CPAP
Fig. 3. (a) Fluorescence spectra of CPAP (10
mM) with different concentrations of
Al3þ in MeOH. (b) Graph of the fluorescence intensity at 420 nm as a function of
2-Aminomethyl pyridine (6 mmol, 624
mL) and 2-hydroxy
Al3þ concentration.
benzaldehyde (5 mmol, 539 L) were dissolved in methanol
m

J. Lee et al. / Dyes and Pigments 96 (2013) 590e594
593
Fig. 5. CPAP 1H NMR spectra with and without Al(NO3)3 in CD3OD: (a) CPAP; (b) CPAP with 1 equiv of Al3þ
.
(15 mL). After 1 h, NaBH4 (5.1 mmol, 0.1929 g) was added, and the
1.4. Job’s plot
reaction solution was cooled in an ice bath. After the solution was
stirred for 2 h, the solvent was removed under reduced pressure to
produce a brown oil. The brown oil was dissolved in methylene
chloride, and the solution was washed twice with water and then
with saturated brine solution. The organic layer was dried over
anhydrous Na2SO4, and the solvent was evaporated under vacuum.
The resulting oil (3.4 mmol, 0.7285 g), 4-chloro-3-nitro coumarin
CPAP (6.06 mg, 0.015 mmol) was dissolved in methanol (3 mL).
24, 21.6, 19.2, 16.8, 14.4, 12, 9.6, 7.2, 4.8, and 2.4 L of the receptor
m
solution were taken and transferred to vials. Each vial was diluted
with methanol to make a total volume of 2.976 mL. Al(NO3)3
(5.63 mg, 0.015 mmol) was dissolved in methanol (3 mL). The
Al(NO3)3 solution were added to each diluted receptor solution.
Each vial had a total volume of 3 mL. After voltexing the vials for
a few minutes, UVevis and fluorescence were taken at room
temperature.
(3 mmol, 0.6768 g), and triethylamine (4 mmol, 560 mL) were dis-
solved in acetonitrile (15 mL). After stirring for 2 h, the solvent was
evaporated. The yellow product was recrystallized from diethyl
ether. Yield : 0.847 g (71%). IR(KBr) : 2990(br), 1721(s), 1598(s),
1546(s), 1514(m), 1488(w), 1404(m), 1278(w), 1153(m), 1099(s),
1050(s), 908(s), 756(s), 607(m), 529(m). 1H NMR (methanol-d4,
1.5. Competiton experiments with other metal ions
400 MHz)
d
: 8.55 (d, 1H), 8.04 (d, 1H), 7.92 (t, 1H), 7.72 (t, 1H), 7.54
M(NO3)3 (M ¼ Al, Cr, Fe, Ga, In, 0.2 mmol), M(NO3)2 (M ¼ Mg, Ca,
Mn, Co, Ni, Cu, Zn, Cd, Hg, Pb, 0.2 mmol) or MNO3 (M ¼ Na, K, Ag,
0.2 mmol) were dissolved in methanol (3 mL), respectively. Each
metal solution (20 mM) was taken and added into 3 mL of receptor
(d, 1H), 7.44 (m, 3H), 7.20 (t, 1H), 7.02 (d, 1H), 6.82 (m, 2H), 4.56 (s,
2H), 4.39 (s, 2H). Anal. calcd for C22H17N3O5 (403.39): C, 65.50; H,
4.25; N, 10.42%. Found: C, 65.23; H, 4.21; N, 10.75%. FAB MS m/z
(Mþ): calcd, 403.39; found, 403.39.
solution (10
mM) prepared above. After mixing them, fluorescence
spectra were taken at room temperature. Then, 30
mL of Al solution
1.3. Fluorescence measurements
(20 mM) was added into the mixed solution of each metal ion and
receptor to 20 equivalents. After mixing them, fluorescene spectra
CPAP(6.06 mg, 0.015mmol)wasdissolved inmethanol(3mL), and
were taken at room temperature. In the same method, 60
mL and
150 L of the stock Al soluton (20 mM) were taken, respectively,
m
6
m
L of the receptor (5 mM)wasdiluted in 2.994 mL methanol to make
the final concentration of 10 M. Then, Al(NO3)3 (75.03 mg, 0.2 mmol
[20 mM]) was added to 3 mL of receptor solution (10 M). After
mixing, fluorescence spectra were obtained at room temperature.
m
and added into the mixed solution of each metal ion and receptor to
make 40 and 100 equivalent, respectively. After mixing them,
fluorescence spectra were taken at room temperature.
m
1.6. UVevis measurements
O
O
O
N
O
Al(NO3)3 (5.63 mg, 0.015 mmol) was dissolved in methanol
m
L of Al3þ solution (20 mM) was transferred to each
Al3+
(3 mL). 0.6e12
receptor solution (40
NO2
NO2
mM) prepared above. After mixing them, UV
N
absorption spectra were taken at room temperature.
N
N
X
OH
O
Acknowledgments
Al3+
X
X
This work is supported by the National Research Foundation of
Korea Grant funded by the Korean Government (2009-0087304,
2010-0009525, 2012008875, 2012001725, SRC Program: 2012-
Scheme 2. Proposed structure of a 1:1 complex of CPAP and Al3þ
.

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J. Lee et al. / Dyes and Pigments 96 (2013) 590e594
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chemosensor based-on naphtholequinoline for selective detection of
aluminum ions. Tetrahedron Lett 2011;52:5581e4.
[10] Sen S, Mukherjee T, Chattopadhyay B, Moirangthem A, Basu A, Marek J, et al.
A water soluble Al3þ selective colorimetric and fluorescent turn-on chemo-
sensor and its application in living cell imaging. Analyst 2012;137:3975e81.
[11] Kim SH, Choi HS, Kim J, Lee SJ, Quang DT, Kim JS. A novel optical/electro-
chemical selective 1,2,3-triazole ring-appended chemosensor for the Al3þ ion.
Org Lett 2010;12:560e3.
0000648), Converging Research Center Program through the
National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (2011K000675), and
BK21 (to J.L. and S.K.).
Appendix A. Supplementary data
[12] Arduini M, Felluga F, Mancin F, Rossi P, Tecilla P, Tonellato U, et al. Aluminum
fluorescence detection with a FRET amplified chemosensor. Chem Commun
2003;34:1606e7.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2012.10.008.
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[14] Maity D, Govindaraju T. Differentially selective sensor with fluorescence
turn-on response to Zn(II) and dual-mode ratiometric response to Al(III) in
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