A novel rhodamine-based colorimetric chemodosimeter for the rapid detection of Al3+ in aqueous methanol: Fluorescent 'OFF-ON' mechanism

Article abstract of DOI:10.1016/j.tetlet.2013.04.103

A novel rhodamine-based chemosensor, 3′,6′-bis(diethylamino)-2- ((2,7-dimethoxy-9H-fluoren-9-ylidene)amino)spiro[isoindoline-1,9′-xanthen] -3-one (1) for aluminum ion, was designed and synthesized. Compound 1 is an orange colored, weak fluorescent compound which was synthesized via one-step facile reaction of rhodamine B hydrazide with 2,7-dimethoxy-9H-fluoren-9-one in MeOH. When Al³⁺ salt was added in methanol:water (30:70, v/v) solution then spirolactum ring of 1 was opened. The absorbance and fluorescence of the mixed solution were increased dramatically because of the opening of the lactum ring. Thus, signal transduction occurred via reversible CHEF (chelation-enhanced fluorescence) mechanism in the range 580-584 nm.

Full text of DOI:10.1016/j.tetlet.2013.04.103

Tetrahedron Letters 54 (2013) 3630–3634
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A novel rhodamine-based colorimetric chemodosimeter for the rapid
detection of Al3+ in aqueous methanol: fluorescent ‘OFF–ON’
mechanism
Anamika Dhara a, Atanu Jana a, Saugata Konar a, Sumanta Kumar Ghatak a, Sangita Ray a, Kinsuk Das b,
Anasuya Bandyopadhyay a, Nikhil Guchhait a, Susanta K. Kar
⇑
,a
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b Department of Chemistry, Haldia Government College, Debhog Purba Midnapur 721 657, India
a r t i c l e i n f o
a b s t r a c t
novel rhodamine-based chemosensor, 30,60-bis(diethylamino)-2-((2,7-dimethoxy-9H-fluoren-9-yli-
Article history:
A
dene)amino)spiro[isoindoline-1,90-xanthen]-3-one (1) for aluminum ion, was designed and synthesized.
Compound 1 is an orange colored, weak fluorescent compound which was synthesized via one-step facile
reaction of rhodamine B hydrazide with 2,7-dimethoxy-9H-fluoren-9-one in MeOH. When Al3+ salt was
added in methanol:water (30:70, v/v) solution then spirolactum ring of 1 was opened. The absorbance
and fluorescence of the mixed solution were increased dramatically because of the opening of the lactum
ring. Thus, signal transduction occurred via reversible CHEF (chelation-enhanced fluorescence) mecha-
nism in the range 580–584 nm.
Received 19 January 2013
Revised 23 April 2013
Accepted 28 April 2013
Available online 9 May 2013
Ó 2013 Elsevier Ltd. All rights reserved.
The design and synthesis of new molecules capable of perform-
ing the transduction of fluorescence signal represent a fascinating
area of contemporary research related to the detection and quanti-
fication of transition metal ions in biological systems and in the
environmental remains.1 After oxygen and silicon, aluminum is
the third most abundant (8.3% weight) element in the earth’s crust.
Due to its importance in many biological and industrial fields, alu-
minum has been receiving increasing attention. Aluminum is pres-
ent in its ionic form Al3+ in natural waters and biological tissues.
The solubility of Al minerals at lower pH increases the amount of
available Al3+ which is deadly to growing plants and its ultimate ef-
fect is the environmental acidification.2 According to a WHO report,
the average daily intake of aluminum is approximately 3–10 mg per
day for human beings as it has been widely used in water treat-
ment, as a food additive, aluminum-based pharmaceuticals, and
aluminum containers and cooking utensils. Excessive amounts of
Al3+ can cause a wide range of diseases, such as, Alzheimer’s dis-
ease, osteoporosis and intoxication in hemodialysis patients. Rick-
ets, gastrointestinal problems, anemia, headaches, decreased liver
and kidney function may also be caused by aluminum toxicity.3
Currently, aluminum detection methods such as, graphite furnace
atomic absorption spectrometry and inductively coupled plasma
atomic emission spectrometry are relatively complex, expensive
and time-consuming in practice. Therefore, the detection of Al3+ is
highly demanded because of the potential impact of Al3+ ions on
human health and environment. Now a days, the development of
new fluorescent Al3+ indicators, especially those that exhibit selec-
tive Al3+-amplified emission, is still a challenging job.
Small organic dye molecules have found versatile applications as
biomarkers, fluorescent probes, light emitting materials, solar en-
ergy harvesting materials etc. Among various options, dye mole-
cules derived from rhodamine, boron dipyrromethane and cyanine
moieties are most prominent fluorescent probes because of their
long absorption and emission wavelengths within the visible region
along with large absorption coefficient and high fluorescence quan-
tum yield.4 Rhodamine-based fluorescent chemosensors show turn
on detection to the targeted heavy transition metal (HTM) cations,
such as Pb2+, Cu2+, Hg2+, Fe3+, and Cr3+ 5–8
For example, two rhoda-
.
mine B molecules were linked through a diethylenetriamine spacer
to afford a Fe3+ selective sensor.5a Xiang et al. reported a rhodamine-
based hydrazine bearing a salicylaldehyde binding site as a Cu2+
amplified sensor.5b Yang et al. reported a Hg2+ chemodosimeter
based on the rhodamine–hydrazine framework.5c In addition, Kwon
et al. exploited ethylenediamine to link a rhodamine and a DPA moi-
ety to yield a fluorescent chemosensor for the Pb2+ ion.5d Spirolac-
tum derivatives of rhodamine dye are useful sensing platforms as
the spirolactum ring-opening process leads to a turn on fluorescence
change. Addition of metal cation leads to a spirocyclic ring-opening
which is accompanied by a vivid color change from colorless to pink,
thus enabling detection simple with the ‘bare eye’.
Our group has some interest in the synthesis of chemosensors in
general and Al3+ in particular. Accordingly, herein we report for the
first time a new rhodamine-based spirolactam derivative (1) as a
⇑
Corresponding author. Tel.: +91 33 24322936; fax: +91 33 23519755.
E-mail address: skkar_cu@yahoo.co.in (S.K. Kar).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.103

A. Dhara et al. / Tetrahedron Letters 54 (2013) 3630–3634
3631
chemosensor for Al3+ 9–13
where binding phenomena could be
,
probed through binding-induced changes in an electronic spectral
pattern. To the best of our knowledge, no such example is there in lit-
erature. Further, binding of this metal ion to 1 caused a drastic color
change, which could also be detected by the ‘naked eye’ under UV
light. Interestingly, binding of only Al3+ to 1 caused significant fluo-
rescence enhancement in an aqueous-methanol mixture. In this
mixed solvent medium, the chemosensor 1 demonstrated Al3+-spe-
cific emission enhancement through a 1:1 (1+Al3+) binding mode
with the metal ion. Rhodamine B hydrazide was synthesized follow-
ing a literature procedure and characterized by 1H NMR spectra FT-IR
and mass data.14,5b It was then condensed with 2,7-dimethoxy-9H-
fluoren-9-one in methanol to form 1 in 75% yield (Scheme 1). The
structure of compound 1 was confirmed by its spectroscopic and ana-
lytical data (1H NMR, FT-IR, HRMS, see Supplementary data S1–S3).
Figure 1. UV–vis absorption spectra of chemosensor 1 (70
amounts of Al2(SO4)3ꢃ16H2O (total of 100
MeOH:water = 3:7 (v/v), pH ꢀ 7] at 25 °C.
l
M) with increasing
UV–vis spectrum recorded for 1 (70
buffer [50
M, MeOH:water = 3:7 (v/v), pH ꢀ 7] at 25 °C exhibited
absorption peak at 545 nm. This absorption band was generated
predominantly due to intraligand
p⁄ charge transfer transition.
However, upon gradual addition of Al3+ ions (2.5
M) at a time up
lM) in MeOH using HEPES
l
M) in MeOH using HEPES buffer [50 lM,
l
p
–
l
to 100
lM caused a gradual increase in intensity with concomitant
shift of the band to 560 nm (
e
= 2.3 ꢁ 102 Mꢂ1 cmꢂ1) along with a
color change from colorless to deep pink at this concentration.
Therefore, there was a large enhancement of absorbance. The for-
mation of a new band at 560 nm occurred due to the opening of
the spirolactum ring of the rhodamine moiety. This absorption peak
was expected on account of coordination of 1 with Al3+. This phe-
nomenon illustrated the transformation of free 1 to the Al3+-coordi-
nated species. So the chemosensor 1 can indeed serve as a highly
sensitive ‘naked eye’ indicator for Al3+ (Fig. 1). However, upon the
addition of various other biologically important metal ions, such
as perchlorates of Na+, Mg2+, Pd2+, Hg2+, Ni2+, Co2+, Cd2+, Zn2+, and
Pb2+, up to 100
ties of chemosensor 1. A mild increase of absorbance at 550 nm
was also detected upon addition of the same amount (100 M) of
Cu2+ solution. Upon addition of Fe3+ to 1, a red shift occurred and
absorbance increased at 555 nm (Fig. 2). In case of Fe3+ and Cu2+
lM had no significant effect on absorption proper-
Figure 2. UV–vis spectra of receptor 1 (70
Al3+, Na+, Mg2+, Pd2+, Hg2+, Ni2+, Co2+, Cd2+, Zn2+, and Pb2+ (a total of 100
MeOH using HEPES buffer [50
M, MeOH:water = 3:7 (v/v), pH ꢀ 7] at 25 °C.
l
M) with addition of perchlorate salts of
l
lM) in
l
,
this enhancement in absorbance clearly suggests the formation of
the delocalized xanthane moiety of the rhodamine group associated
with pale pink color change. A bright orange fluorescence was ob-
served only for (1+Al3+) solution, and no fluorescence for other me-
tal cations under UV-light (366 nm) was recorded (Fig. 3). This was
an interesting feature by which we could detect Al3+ exclusively
without any other instrumental technique.
was due to the formation of a (1+Al3+) complex, which resulted
in the selective CHEF effect. The association constant (Ka)15 of 1
with Al3+ was 3 ꢁ 107 Mꢂ1, as obtained by nonlinear least-squares
analysis (Figs. S4 and S6 in the Supplementary data). Al3+ could be
detected at least down to be 2.4 ꢁ 10ꢂ6 M by fluorimetric assay,
indicating that the limit of detection of 1 to Al3+ met the limit for
drinking water. The fluorescence quantum yields16
(Ufs) of com-
The fluorescence property of 1 (15
lM) was investigated in
pound 1 in the free and Al3+-bound state were found to be 0.07
and 0.43, respectively. Stoichiometry for the (1+Al3+) complex with
respect to Al3+ and ligand was evaluated on the basis of the Job’s
plot17 and result confirmed the formation of 1:1 complex (Fig. S5
in the Supplementary data). We also tested the fluorescence re-
MeOH using HEPES buffer [50 M, MeOH:water = 3:7 (v/v),
l
pH ꢀ 7] at 25 °C, (kex = 545 nm) (Fig. 4). Without cations, 1 showed
a very weak fluorescence peak at 563 nm which was probably be-
cause of the trace open ring molecules of 1 in solution state. How-
ever, upon gradual addition of Al3+ (2.5
lM) at a time to a 15 lM
sponse of 1 to other metal ions such as Ag+, Ca2+, Zn2+, Ni2+, Co2+
,
solution of 1 in MeOH at 25 °C (with HEPES buffer, pH ꢀ 7), showed
distinct behavior with quenching of emission intensity at 563 nm
and the appearance of a new peak at 584 nm. This red shift
(ꢀ21 nm) was accompanied by enhanced fluorescence intensity.
A turn-on ratio over 15-fold was triggered with the addition 0–
70 equiv of Al3+. The enhancement of the fluorescence intensity
Mg2+, Na+, and K+ (100
lM) in aqueous-MeOH media at pH ꢀ 7
(Fig. 5 and Fig. S7 in the Supplementary data). Only Fe3+ responded
towards slight increase in fluorescence intensity at 577 nm, while
other metal ions did not show any significant change under iden-
tical conditions. Al3+ has ionic radius of 0.57 Å, whereas that of
Fe3+ is 0.67 Å, as the Fe3+ has greater ionic radius compared to that
O
O
O
N NH2
O
O
O
N
N
O
O
HCl
O
N2H4.H2O
O
N
N
N
O
N
Methanol , Reflux
900C
N
O
N
Pink solid
1
(orange solid)
Scheme 1. Synthesis of chemosensor 1.

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A. Dhara et al. / Tetrahedron Letters 54 (2013) 3630–3634
Figure 5. Fluorescence spectra (excitation at 545 nm) of 1 (15
perchlorate salts of Al3+, Na+, Mg2+, Pd2+, Hg2+, Ni2+, Co2+, Cd2+, Zn2+, and Pb2+ (a total
of 100 M) in MeOH using HEPES buffer [50
M, MeOH:water = 3:7 (v/v), pH ꢀ 7]
at 25 °C.
lM) with addition of
l
l
Figure 3. Photographs of chemosensor
(100 lM) in MeOH using HEPES buffer [50 l
1
(70
M, MeOH:water = 3:7 (v/v), pH ꢀ 7]
at 25 °C under (a) visible light and (b) UV light.
lM) with different metal ions
Figure 6. Fluorescence response of 1 (15
lM) to various cations (100
lM) in MeOH
using HEPES buffer (50
l
M, MeOH: water = 3:7 (v/v), pH ꢀ 7] at 25 °C
(kex = 545 nm). Bars represent the emission intensity ratio F/F0 (F0 = initial fluores-
cence intensity at 584 nm; F = final fluorescence intensity at 584 nm after the
addition of Al3+ ions). The blue bars represent the addition of individual metal ions
while the red bars represent the change in the emission that occurs upon the
Figure 4. Fluorescent titration spectra of 1 (15
[50
different concentrations of Al2(SO4)3ꢃ16H2O. Inset: fluorescence intensity at 584 nm
as a function of Al3+ concentration and showing the fluorescence of 1 and the
addition of Al3+ ions.
lM) in MeOH using HEPES buffer
M, MeOH:water = 3:7 (v/v), pH ꢀ 7] at 25 °C (kex = 545 nm) in the presence of
l
subsequent addition of Al3+ (100
lM) to the above solution.
of Al3+. Therefore, it could not fit well into the sterically crowded
coordination zone of the rhodamine-based Schiff based ligand.
We carried out competitive experiments in the presence of
was also examined. Addition of total (0–100 lM) of Na2EDTA to a
mixture of 1 and Al3+ resulted in diminution of the fluorescence
intensity at 584 nm which signifies the regeneration of spirolac-
tum structure (Fig. S8 in the Supplementary data). Such reversibil-
ity and regeneration were important for the fabrication of devices
to sense the Al3+ ions. As pH dependence of fluorescence is gener-
ally undesirable in biological applications, the effect of pH on fluo-
rescence was also studied in aqueous-methanol. It was found that
100 l l ,
M Al3+ along with 100 M other cations such as Zn2+, Ni2+
Co2+, Mg2+, Na+, Cu2+, Hg2+, Cd2+, Fe3+, and Mn2+. There was no sig-
nificant variation in the fluorescence emission of the solution of
(1+Al3+) when the measurement is carried out with or without
the other metal ions (Fig. 6). However, the addition of same
amount of (100 l
M) Cu2+, the fluorescence intensity of the chemo-
sensor 1 was quenched to a great extent. The fluorescence quench-
ing occurred mainly due to the excitation energy transfer from the
ligand to the metal d-orbital and/or ligand to metal charge transfer
(LMCT) in aqueous-methanol medium.18 Thus, the selectivity ob-
served for Al3+ over other metal ions was remarkably high. This un-
ique selectivity of 1 towards Al3+ could be interpreted in terms of
the smaller ionic radii (0.57 Å) and higher charge density
(r = 4.81) of the Al3+ ion. The smaller radii of the Al3+ ion permit
suitable coordination geometry of the chelating chemosensor 1
and the larger charge density allows a strong coordination ability
between 1 and Al3+ 19 As the metal ions Al3+ binds strongly with
.
the ligand, the possible flexible modes which are responsible for
nonradiative decay are hindered and hence fluorescence intensity
enhancement was observed. Reversible binding of 1 with Al3+
Figure 7. Time-resolved fluorescence decay of 1 (green)+Al3+ (red).

A. Dhara et al. / Tetrahedron Letters 54 (2013) 3630–3634
3633
the fluorescence intensity of free 1 at 584 nm remained unchanged
over a wide pH range 4.5–14 (Fig. S9 in the Supplementary data),
whereas the addition of the Al3+ ion could lead to a remarkable in-
crease of fluorescence, due to efficient complexation between 1
and Al3+ ion. These results indicated that 1 could be used as a selec-
tive fluorescent probe to recognize Al3+ in the presence of various
interfering metal ions.
of kr. This enhancement was attributed to the introduction of Al3+
and the strong complexation occurring with 1, as was evident from
the large binding constant value.
1H NMR titration in DMSO-d6 was carried out in order to clarify
the binding mechanism of Al3+ with receptor 1 (Fig. 8). Addition of
1 equiv of Al3+ resulted no change of any chemical shifts. High-res-
olution mass spectra (HRMS) also provided additional evidence for
the formation of a 1:1 complex between Al3+ ion and 1 (Fig. 9). Fur-
ther it was clearly observed that upon addition of the Al3+ ion, the
carbonyl stretching band of 1 at 1699 cmꢂ1 was changed to a lower
wave number (1685 cmꢂ1). Both 1H NMR, mass and IR results
firmly support that the carbonyl group of spirolactum is involved
in the Al3+ binding, thus inducing a ring-opening of the spirolactum
in 1.
This was further confirmed by time correlated single photon-
counting (TCSPC) studies using a 340 nm LED as an excitation
source. It had been used to examine the excited state behavior of
the free chemosensor 1 and its metal complex in aqueous-MeOH
(Fig. 7). According to the equations20 sꢂ1 = kr + knr and kr = Uf
/s,
the radiative rate constant kr and the total nonradiative rate con-
stant knr of 1 and Al3+-bound species were calculated. The fluores-
cence decay curves for the chemosensor 1 and its metal complex
were fitted by bi-exponential function with acceptable v2 values
and all the data were presented in (Table S1 in the Supplementary
data). The data suggested that the factor that induced fluorescent
enhancement was mainly due to the more than 100 times increase
In conclusion, we have synthesized a unique chromogenic and
fluorogenic Al3+ probe using rhodamine derivative as a fluoro-
phore. The mechanism for rhodamine-based sensors depends on
the Al3+ induced ring-opening reaction of a rhodamine spirolactum
scaffold compared with reported Al3+ fluorescent probe. The pro-
posed probe exhibits the following advantages: a quick, simple
and facile synthesis. Furthermore, the solution of 1 itself a weak
fluorescent and colorless (‘clear blank’); thus greater fluorescence
enhancement and remarkable color change toward Al3+ can be
achieved. Under UV light illumination, one can visually detect even
5 ppm Al3+ in aqueous buffer solution without the aid of any
sophisticated instruments. Thus the chemosensor 1 was able to
serve as a ‘naked eye’ chemodosimeter for Al3+, allowing its revers-
ible detection in the presence of a wide range of environmentally
relevant competing metals ions.
Acknowledgments
A.J. thanks CSIR for award of senior research fellowship. This
work is also supported by the Council of Scientific and Industrial
Research (CSIR), New Delhi, India (Project No. 01(2401)/10/EMR-
II).
Supplementary data
Supplementary data (supplementary figures and table) associ-
ated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2013.04.103.
Figure 8. 1H NMR tritration of 1 and with Al3+ ion (DMSO-d6). (a) 1 only (b) 1 with
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