A highly sensitive fluorescent probe for selective detection of Al3+ cation by switching the solvent from aprotic to protic environment

Article abstract of DOI:10.1080/15421406.2015.1097010

We have designed and synthesized known coumarin-rhodamine (CR1) derivative for sensing the Al³⁺ ion in the presence of aprotic (CH₃CN/H₂O) medium via TBET mechanism. The results of the photo physical studies revealed that the sensing mechanism involves Al³⁺ induced change from non-fluoroscent (spirolactam form) to fluorescent (ring opening form) in CR1 leading to the visible color change. The Job's plot indicated the formation of 2:1 complex between CR1 and Al³⁺ with limit of detection is 1.78 × 10^-8 M. Various spectroscopy techniques and theoretical calculation have been employed to investigate the mechanism of the sensing property of CR1 with Al³⁺.

Full text of DOI:10.1080/15421406.2015.1097010

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
A Highly Sensitive Fluorescent Probe for Selective
Detection of Al3+ Cation by Switching the Solvent
from Aprotic to Protic Environment
Junemin Park, Satheshkumar Angupillai & Young-A Son
To cite this article: Junemin Park, Satheshkumar Angupillai & Young-A Son (2015) A Highly
Sensitive Fluorescent Probe for Selective Detection of Al3+ Cation by Switching the Solvent from
Aprotic to Protic Environment , Molecular Crystals and Liquid Crystals, 622:1, 103-113, DOI:
10.1080/15421406.2015.1097010
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1097010
Published online: 16 Dec 2015.
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Date: 14 January 2016, At: 03:41

Mol. Cryst. Liq. Cryst., Vol. 622: pp. 103–113, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1097010
A Highly Sensitive Fluorescent Probe for Selective
Detection of Al3+ Cation by Switching the Solvent
from Aprotic to Protic Environment
JUNEMIN PARK,† SATHESHKUMAR ANGUPILLAI,†
AND YOUNG-A SON∗
Department of Advanced Organic Materials Engineering, Chungnam National
University, Yuseong-gu, Daejeon, South Korea
We have designed and synthesized known coumarin-rhodamine (CR1) derivative for
sensing the Al3+ ion in the presence of aprotic (CH3CN/H2O) medium via TBET mech-
anism. The results of the photo physical studies revealed that the sensing mechanism
involves Al3+ induced change from non-fluoroscent (spirolactam form) to fluorescent
(ring opening form) in CR1 leading to the visible color change. The Job’s plot indi-
cated the formation of 2:1 complex between CR1 and Al3+ with limit of detection is
1.78 × 10−8 M. Various spectroscopy techniques and theoretical calculation have been
employed to investigate the mechanism of the sensing property of CR1 with Al3+
.
Keywords Rhodamine; Coumarin; TBET; FRET; Energy band gap
Introduction
In very recently organic dye molecules are predominantly used for cation sensing. Among
the various dye molecules, rhodamine and coumarin moieties are most prominently used
as a chemosensors, owing to their high fluorescence quantum yield and emission bands at
the red region of the spectrum. Generally, rhodamine 6G derivatives are non-fluorescent
and colorless, whereas when they coordinate with metal ions, ring-opening of the corre-
sponding spirolactam gives rise to a strong florescence emission and a pink color. However,
small Stokes shifts limit their applications in certain cases [1]. Forster Resonance Energy
Transfer (FRET) is generally the most adopted methodology for addressing this issue. For
FRET to be effective, a substantial spectral overlap for the donor emission and acceptor
absorption bands is required, which sometimes restrict the choice and the design of the
FRET-based probe molecules. While in the case of through-bond energy transfer (TBET)
based chemosensors, does not required the spectral overlap between donor emission and ac-
ceptor absorption band, though it exhibits fast energy transfer rates and large pseudo-Stokes
†These authors contributed equally to this work.
∗Address correspondence to Young-A Son, Department of Advanced Organic Materials Engi-
neering, Chungnam National University, 220 Gung-dong, Daejeon, 305-764, South Korea. E-mail:
yason@cnu.ac.kr
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
[455]/103

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J. Park et al.
shifts. Moreover, the TBET cassettes shows obviously higher energy transfer efficiency than
that of the classical FRET system [4–5]. Despite such flexibility, reports on TBET cassettes
for developing biomarkers and/or fluorescent probes for intracellular imaging applications
are actually rare [6].
Al3+ is the third most abundant metal in the earth’s crust, accounting for approxi-
mately 8% of its mass. Due to the acid rain, the free Al3+ concentration is increasing in
the environment and surface water, which is deadly to growing plants [7]. In addition, it
has been widely used in the many field such as water treatment, food additive, aluminum
based pharmaceuticals, occupational dusts, aluminum containers and cooking tools. Al3+
ions are producing neurotoxic issues to humans, such as Alzheimer’s and Parkinson’s
disease [8]. Therefore, the detection and to controlling of Al3+ levels in the biosphere is
essential. Up to date, numbers of techniques are available for Al3+ estimation [9]. How-
ever, these methods are time-consuming and expensive, as they require tedious sample
pretreatment and sophisticated instrumentation. While in the case of colorimetric and fluo-
rimetric detection methods have several advantages than that of the other methods. Hence,
the design and construction of chemosensors with high selectivity and sensitivity towards
Al3+ have become the focus in numerous studies in the field of supramolecular chemistry.
Until recently, only a few fluorescent chemosensors have been developed for detection of
Al3+ [10–12]. Most fluorescent sensors for Al3+ have good selectivity, but this approach
has several disadvantages including complicated synthetic procedures and poor water
solubility.
In this paper, we report the synthesis and colorimetric/fluorometric properties of con-
formationally constrained coumarin-rhodamine ligand CR1 for selective detection of Al3+
based on TBET mechanism. Recently, Guangjie He et al., [13] have developed same
chemosensor CR1 for Cu2+ sensing in CH3CN medium. The spectroscopic studies re-
vealed that the sensing mechanism of CR1 in CH3CN is due to the existence of FRET
between coumarin and rhodamine. On the contrary, during the course of our work, the
chemosensor CR1 has been selectively sense the Al3+ ion in CH3CN/H2O medium. In
general, the molecule composed by two unit such as donor and acceptor (D-A), the sensing
property of chemosensors have been followed by the two different mechanism such as
FRET/TBET [14]. It is well known that the spirolactam ring opening is solvent assisted in
protic environment (Scheme 1) and also hydrogen bonding between solvent and acceptor
Scheme 1. Solvent facilitated TBET transition on CR1 with Al3+ ion.

Fluorescent Probe for Selective Detection of Al3+ Cation
[457]/105
molecules prevent donor and acceptor fragment from becoming plannar and thus the TBET
process is facilitated [14].
Experimental
Instrument
UV-Vis spectral studies were carried out on Agilent 8453 single beam spectrophotometer.
Steady state fluorescence spectra were obtained on a spectrofluorometer, Shimadzu RF-
5301PC. The excitation and emission slit width (1.5 nm) was kept constant for all of
the experiments. FT-IR spectra were obtained as KBr pellets on an FT-IR spectrometer
(FTS-175C, Bio-laboratories, Cambridge, USA). Nuclear magnetic resonance spectra were
recorded in CDCl3 (Bruker, 1H NMR 600 MHz). Electrospray ionization mass spectrometry
(ESI-MS) spectra were measured on a 4000 Q TRAP [AB SCIEX] and elemental analyses
were performed by EA112 [Thermo Fisher Scientific].
Chemicals
All the reagents and solvents, used for synthesis of CR1, were purchased from
Aldrich (St.Louis, USA) and used without further purification. Rhodamine 6G is pur-
chased from the Sigma-Aldrich Company, 4-diethylamino sacylaldehyde is purchased
from Alfa-Aesar. Various metal ion solutions of LiClO4, NaClO4, CsClO4, Mg(ClO4)2,
Fe(ClO4)2•XH2O, Ni(ClO4)2•6H2O, Cu(ClO4)2•6H2O, AgClO4, Hg(ClO4)2•XH2O,
and Al(ClO4)3•9H2O were purchased from Sigma-Aldrich, and Co(ClO4)2•6H2O,
Zn(ClO4)2•6H2O, Cd(ClO4)2•6H2O and Pb(ClO4)2•3H2O were purchased from Alfa-
Aesar Chemical Reagent Co.
Synthetic Procedures
Compounds CR1 (the product of the reaction of CR with Al3+) was efficiently synthesized
following the synthetic methodology shown in Scheme 2. Rhodamine 6G ethylamine,
and 7-diethylamino-3-carboxylic chloride coumarin, were synthesized according to the
literature methods [15].
Scheme 2. Synthesis of chemosensor CR1.

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J. Park et al.
Figure 1. Visible and fluorescence color change of CR1 in CH3CN/ H2O (1:1) in the presence of
Al3+, Li+, Na+, Cs+, Mg2+, Fe2+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+ and Pb2+
.
Synthesis of Compound CR1
A Et3N (0.42 ml,0.003 mol) was added in to a stirred solution of N-(rhodamine-6G)
lactam-ethylenediamine (1.14 g, 0.0025 mol) in CH2Cl2 (20 mL). A solution of 2 (0.70 g,
0.0025 mol) in CH2Cl2 (60 mL) was added drop wise to the stirred reaction mixture.
The reaction mixture was stirred at RT under nitrogen atmosphere for 24 h. The organic
layer was washed with water (200 mL) and dried over anhydrous Na2SO4 and filtered.
The organic layer was removed under reduced pressure. The crude product was purified
by column chromatography on alumina (ethyl acetate) to afford 1.44 g of yellow solid
1
CR1 in 82% yield. H-NMR (600MHz, CDCl3) : 8.579 (s, 1H), 8.565(t, 1H), 7.945(m,
1H), 7.433(m, 2H), 7.385(d, 1H), 7.035(m, 1H), 6.624(q, 1H), 6.467(d, 1H), 6.324(s,
2H), 6.289(s, 2H), 3.449(q, 4H), 3.397(t, 4H), 3.186(m, 4H), 3.136(t, 2H), 1.592(s, 6H),
1.270(t, 6H), 1.232(t, 6H) 13C-NMR (150MHz, CDCl3) : 169.09, 161.41, 154.79, 149.23,
142.06, 139.07, 129.52, 129.49, 129.47, 129.20, 101.43, 101.19, 97.28, 96.48, 80.09, 46.01,
44.9, 39.28, 24.02, 17.37, 15.03, 13.06 ppm ; ESI-MS: [M]+, 700.5 Anal. Calcd for CR1
C42H45N5O5: C, 72.08; H, 6.48; N, 10.01%. Found : C, 71.97; H, 6.78; N, 9.77%
Results and Discussion
Visual Detection
The colorimetric/fluorimetric sensitivity of CR1 toward various cations such as Li+, Na+,
Cs+, Mg2+, Fe2+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+, Pb2+ and Al3+ in their perchlo-
rate form was monitored visually. As depicted in Fig.1 solutions of CR1 in CH3CN/H2O
(1/1; v/v) turn pale yellow to intense pink color (Top) and the fluorescence change from
blue to yellowish green (Bottom) after the addition of Al3+. However, their color remained
unchanged after the addition of other chosen cations. This is indicate that the selectivity of
CR1 toward Al3+ ion.

Fluorescent Probe for Selective Detection of Al3+ Cation
[459]/107
Figure 2. UV-Vis (a) and fluorescence spectra (b) of CR1 (10 µM) with various metal ion (10 µM)
in CH3CN/H2O (1:1).
Photo Physical Property
To further investigate the selectivity of CR1 to Al3+, we performed the UV-Vis absorbance
and fluorescence spectroscopic techniques. As shown in the figure 2a, the electronic spec-
trum of the free CR1 has been appeared at 301 and 423 nm, which is corresponding to the
spirolactam ring and comurin respectively, [13] the addition of Al3+ ions to the CR1, a
new absorption band centred at 524 nm accompanying a remarkable increasing intensity,
corresponds to the absorption of rhodamine, which induced a clear color change from pale
yellow to pink (Fig. 2a). This confirms that CR1 was induced to the ring-opened structure
from the spirolactam by Al3+ which possessed a long absorption wavelength and high
molar extinction coefficient. While in the case of other chosen competitive metal ions, such
as Li+, Na+, Cs+, Mg2+, Fe2+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+ and Pb2+ does
not produce any significant color or spectral change under identical conditions. Likewise,
in the fluorescence spectra (Fig 2b), a pure CR1 (10 µM, CH3CN:H2O (1:1, v/v)) solution
displayed very weak emission peak at 472 nm. While the addition of Al3+ to CR1 solution,
the new emission peak was observed at 556 nm. In the case of other metal ions, the peak at
556 nm does not appear. These results demonstrated that CR1 has high selectivity toward
Al3+ over other competitive metal ions. As spelt in to the introduction section, our molecule
CR1 has followed the TBET mechanism instead of FRET mechanism. In the case of simple
CH3CN solvent (polar-aprotic), CR1-Cu2+ complexes showed the weak emission peak at
472 and 556 nm due to the presence of FRET between coumarin and rhaodamine moiety
[13]. While in the case of polar-protic solvent (CH3CN/H2O), coumarin shows very weak
emission peak at 472 nm, consequently the new strong peak (8 fold higher than that of apro-
tic solvent) has been appeared in the range of 556 nm due to the spirolactom ring opening
followed by the TBET transition [14]. The result of fluorescence study clearly indicated
that the sensing property of CR1 in CH3CN/H2O medium is mainly due to the prescence
of TBET mechanism. Moreover, it shows slight spectral overlap with the acceptor’s ex-
citation with a negligible FRET process which is favorable for the investigation with the
TBET
To gain more insight into the chemosensing properties and mechanism of CR1 to-
ward Al3+, absorption and fluorescence titration (Fig. 3) with Al3+ were recorded. With
the addition of incremental amounts of Al3+ ions to CR1 in CH3CN/H2O the absorption
peak at 321 nm enhanced gradually accompanying the formation of a new peak at 524 nm

108/[460]
J. Park et al.
Figure 3. Change in UV-Vis (a) and fluorescence spectra (b) for CR1 (10µ M) in CH3CN/H2O upon
the incremental addition of (0–10 µM) of Al3+ ion.
(Fig. 3a). This new band observation is due to the ring opening of spiro lactam unit by
Al3+. The Al3+ binding ability of the CR1 was also evaluated by fluorescence titration
studies at an excitation wavelength of 423 nm and emission wavelength of 450–700 nm,
results given in Figure 3. For the free CR1 the emission peak was exhibits at 472 nm
(Figure 3b), and no characteristic emission peak was appeared at 556 nm. Upon grad-
ual addition of Al3+ ions to the CR1 solution, the donor emission at 472 nm slowly
decreased, and a new emission band with a maximum at 556 nm (rhodamine 6G) ap-
peared and gradually increased. From the fluorescence enhancement binding constant
of the 2CR1-Al3+ complex was calculated as 4.904 × 10−10 using the B-H equation
[16].
The stoichiometry of the CR1 and Al3+ was determined using the UV-
Vis spectral data. As shown in figure 4, a curve with a maximum at near

Fluorescent Probe for Selective Detection of Al3+ Cation
[461]/109
Figure 4. Job’s plot for CR1 with Al3+.
0.7 mole fraction, this is indicated the formation of 2:1 (CR1: Al3+) complex
[17].
Competitive Experiments
To check the practical knack of chemosensor CR1 to Al3+ ion, we have carried out
competitive experiments in the presence of Al3+ ions mixed with other chosen metal. Both
the electronic absorbance and fluorescence response of the 2CR1-Al3+ system remains the
same by comparison with or without the other metal ions (Fig. 5). These findings confirmed
the selectivity and effective interaction of probe CR1 with Al3+. For practical applications,
the detection limit of CR1 was also estimated. The fluorescence titration profile of CR1
(10 µM) with Al3+ demonstrated that the detection limit of Al3+ is 1.17 × 10−8 M which
is far below the WHO acceptable limit (0.05 mg L −1 or 1.85×10−3 M of Al3+) in drinking
water.
Sensing Mechanism
In order to ascertain the complexation between CR1 and Al3+, the FT-IR spectrum of CR1
and 2CR1-Al3+ was recorded (Fig. 6). For free CR1, the peaks at 1683 and 1618 cm−1 are
corresponds to spirolactam ring carbonyl (C O) and amide (NH-C O) stretching vibra-
tions respectively and also two sharp peak appear at 3346 and 2970 cm−1are assigned to the
NH stretching peaks. While in the case of 2CR1-Al3+ complexes, characteristic stretch-
ing frequencies for the spirolactam ring carbonyl, that appeared at 1683 cm−1completely
disappeared, this is confirming the spirolactam ring opening, which led to a new peak at
1605 cm−1 representing the oxygen of the spiro ring attached to the Al3+ion. In addition
with that the amide carbonyl peak was shifted from 1618 to 1567, these results indicate
that the oxygen from the spirolactam and amide carbonyl has involved the complexation

110/[462]
J. Park et al.
Figure 5. Absorption (a) and fluorescence response (b) of CR1 (10 µM) in the presence of Al3+
and other metal ions (1 equiv.) in CH3CN:H2O (1;1, v/v); the black bar correspond to the CR1
with all chosen cation and the red bar represents the CR1 in addition to other metal ions with
Al3+
.
with Al3+. However, broad new peaks at 3373 and 2977 cm−1 provide information re-
garding the formation of a quaternary ammonium salt in the rhodamine xanthene moiety
[18–19].
1H-NMR spectra of CR1 and 2CR1-Al3+ in CDCl3 were recorded to determine
the binding mode and mechanism. The signal at 3.5 ppm (triplet) is corresponds to
the xanthene terminal -NH- proton and two singlet around at 6.2 – 6.3 ppm are due
to the xanthene ring hydrogen (Fig. 7A). While in the case of 2CR1-Al3+, xanthene
NH peak is separated into two sets(Fig. 7B). One set of protons is shifted up field
and the second set of amine protons is shifted downfield. This separation in to two
sets of ( NH ) protons is due to the formation of a quaternary amine salt after the

Fluorescent Probe for Selective Detection of Al3+ Cation
[463]/111
Figure 6. FT-IR spectra of CR1 and 2CR1-Al3+
.
Figure 7. 1H NMR spectra of CR1(A) and 2CR1-Al3+(B) complexes.

112/[464]
J. Park et al.
spirolactam ring is opened by the Al3+ion. Subsequently, in 2CR1-Al3+ complex, the
xanthene protons are shifted to the deshielding area and due to the complexation of
Al3+ the amide NH was appear with down field shift [20–21]. Based on the FT-IR and
1H-NMR studies, we proposed the binding sites on CR1 with Al3+ ion as shown in
Scheme 3.
Scheme 3. Proposed binding mechanism of CR1 with Al3+
.
Conclusion
In summary, CR1 has been selectively sense the Al3+ ion based on TBET mechanism in
CH3CN/H2O medium. We modulated the selectivity of CR1 to Al3+ metal ion by switching
the sensing media. CR1 gives a turn-on colorimetric and fluorometric signal toward Al3+
in CH3CN-H2O. To the best of our knowledge, this is the first example of a chemosensor
based on D-A molecule that can change their selectivity by switching the solvent systems.
Because of the simplicity of the analysis, we believe that the proposed probe may find
application in a variety of real samples.
Acknowledgments
This study was supported by Basic Science Research Program through the National Re-
search Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future
Planning (Grant no. 2014006660).
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