A highly selective fluorescent sensor for Al3+ based on an aza-18-crown-6 derivative

Article abstract of DOI:10.3184/174751915X14425101743684

A new fluorescent sensor derived from 2,5-diphenyl-furan and aza-18-crown-6 has been synthesised. It can selectively bind Al³⁺ in aqeuous ethanol solution with fluorescence enhancement.

Full text of DOI:10.3184/174751915X14425101743684

JOURNAL OF CHEMICAL RESEARCH 2015
VOL. 39 OCTOBER, 603–605
RESEARCH PAPER 603
A highly selective fluorescent sensor for Al3+ based on an aza-18-crown-6
derivative
Song Jingjinga, Hu Yangb, Zhao Fanga and Hu Shenglia*
aHubei Collaborative Innovation Center for Rare Metal Chemistry, Department of Chemistry and Chemical Engineering, Hubei Normal University, Huangshi
435002, P.R. China
bDepartment of Chemistry and Chemical Engineering, The School of Art and Science of Hubei Normal University, Huangshi 435002, P.R. China
A new fluorescent sensor derived from 2,5-diphenyl-furan and aza-18-crown-6 has been synthesised. It can selectively bind Al3+ in aqeuous
ethanol solution with fluorescence enhancement.
Keywords: sensor, fluorescence enhancement, aza-18-crown-6 derivative, Al3+
Fluorescent chemosensors are widely used as powerful tools to
detect neutral and ionic species owing to their high sensitivity,
selectivity, versatility, and relatively simple handling.1-3Aluminium
is the most abundant metal element in the earth’s crust.4,5 Its
compounds are widely used in water treatment, in food additives,
and in medicines.6,7 Aluminium is not a biologically essential
element, studies show that the unregulated amounts of aluminium
in human body may lead to the malfunction of the central nervous
system, Parkinson’s disease, and Alzheimer’s disease.8,9 Therefore,
it is of great necessity to develop powerful recognition and detection
systems for monitoring aluminium cations in daily life. However,
the detection of Al3+ has always been problematic due to the lack
of spectroscopic characteristics and poor coordination ability
compared to transition metals.10 For this reason, the development
of Al3+ probes is comparatively more difficult than those for other
metal ions. So far, very few Al3+ selective fluorescent sensors have
been reported.11-16 Therefore, the development of new, efficient Al3+
fluorescent chemosensors is still a challenge.
of the UV-Vis absorption spectra and fluorescence properties of a
solution of 1 in EtOH/water, (1×10-5 M, 95:5, v/v solution), caused
by various metal ions (K+, Na+, Mg2+, Hg2+, Cd2+, Fe3+, Al3+, Zn2+,
Co2+, Ni2+, Cu2+, Pb2+, Cr3+, and Mn2+) (as their Cl- or NO3- salts)
are shown in Figs 1 and 2, respectively. The results show that
Al3+ produced significant enhancement in the absorption spectra
and fluorescent emission of 1, the other tested metals only show
relatively insignificant changes. So it can be concluded that 1 has
higher selectivity for recognition of Al3+.
To determine the stoichiometry of compound 1 and Al3+ ion in
the complex, Job’s method18 was employed by using the absorption
changes at 304 nm as a function of molar fraction of Al3+. A
minimum emission was observed at a 0.33 ratio (Fig. 3), indicating
that the Al3+ ion forms a 1:2 complex with the sensing compound 1.
The sensitivity of the fluorescence emission response of 1
towards Al3+ was also examined under the same conditions with
various Al3+ concentrations (Fig. 4). The fluorescence intensity
(λem =378 nm) of 1 was increased continually upon addition of Al3+
with no significant in the position of the emission maxima. Upon
gradual addition of up to 0.5 equiv of Al3+, the solution showed an
approximate 2-fold enhancement in the fluorescence intensity.
Based on the above fluorescence titration of 1 with Al3+, the
association constant Kass was calculated to be 1.56×105 M-2 (error
limits ≤10%) by a Benesi–Hildebrand plot19 (Fig. 5). The limit of
detection of 1 is found to be 4.11×10-7 M, which is calculated using
3δ/S,20 where δ was the standard deviation of the blank signal,
and S was the slope of the linear calibration plot.
We now report a novel and selective fluorescent chemosensor
for Al3+.We designed a PET17 sensor 1, in which an aza-18-crown-6
moiety is linked to a 2, 5-diphenyl-furan fluorophore and have
shown that 1 possesses a highly selective response of fluorescence
enhancement toward Al3+ in aqueous ethanol solution.
Results and discussion
The synthesis of chemosensor 1 is shown in Scheme 1. The IR,
NMR and MS spectra and elemental analysis for the product are in
good agreement with the proposed formulation of 1.
The binding properties of molecule 1 were investigated by UV-
Vis absorption spectra and fluorescence measurement. Changes
To further gauge the selectivity for Al3+ ion compared to other
metal ions, competition experiments of Al3+ ion mixed with
other metal ions were carried out using fluorescence spectra
SCH3
O
O
C
O
HAc, HCl
I2, CuO
DMSO
C
C
C
O
SnCl2
SCH3
4
3
O
2
O
O
O
O
O
Br
O
N
O
O
NH
CH3S
O
33%HBr-HAc
(CH2O)n
CH3S
O
CH3CN, K2CO3
reflux
O
5
1
Scheme 1 The synthesis of sensor 1.
* Correspondent. E-mail: hushengli168@126.com

604 JOURNAL OF CHEMICAL RESEARCH 2015
4500
4000
3500
3000
2500
2000
1500
1000
500
0.7
Al3+
Al3+
0.6
0.5
1 , K+, Na+, Mg2+, Hg2+, Cd2+,
Zn2+, Co2+, Ni2+, Cu2+, Pb2+,
Cr3+, Fe3+and Mn2+
1, K+, Na+, Mg2+, Hg2+, Cd2+,
0.4
0.3
0.2
0.1
0.0
Zn2+, Co2+, Ni2+, Cu2+, Pb2+,
Cr3+, Fe3+and Mn2+
0
350
375
400
425
450
475
500
250
300
350
400
450
500
550
Wavelength nm
Wavelength(nm)
Fig. 1 Absorption spectra of 1 (1.0×10-5 M) in EtOH/water (95:5, v/v)
containing HEPES buffer (10mM, pH= 7.0) solution in the presence of
various metal ions (1×10-5M).
Fig. 2 Fluorescence emission changes of 1 (1×10-5 M) in EtOH/water
(95:5, v/v) containing HEPES buffer (10mM, pH= 7.0) solution in the
presence of 1×10-5 M of various metal ions (excitation at 333 nm).
4500
4000
0.5
0.4
0.3
0.2
0.1
0.0
3500
0.5 eq
Al3+
3000
2500
2000
1500
1000
500
0
0
350
375
400
425
450
475
500
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
X=[1]/([1]+[Al3+])
Wavelength (nm)
Fig. 3 Job’s plot of a 2:1 complex of 1 (1×10-5 M) with Al3+.
Total [1] + [Al3+] = 1×10-5 M.
Fig. 4 Fluorescence emission spectra (excitation at 333 nm) of 1
(1×10-5M) EtOH/water (95:5, v/v) containing HEPES buffer(10mM,
pH=7.0) solution in the presence of Al3+.
1.0 0.96 0.97 0.95 0.96 0.97 0.97 0.940.950.96 0.95 0.99 0.99 0.76
1.0
0.08
y=3.13*10-5x+4.88*10-5
R=0.99545
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
0.8
0.6
0.4
0.2
Kass=1.56*105M-2
0.0
+
+
+
+
+
+
+
2
+
+
+
+
2
+
+
2
2
2
2
2
3
2
2
3
3
+
a
g
i
d
l
o
r
u
n
e
b
g
n
0
500
1000
1500
2000
2500
K
N
N
A
C
C
M
C
F
C
Z
M
P
H
-
-
-
-
+
-
-
+
+
-
+
-
-
+
-
+
+
-
-
-
1/[Al3+]2(10-5M)
l
3
+
+
+
3
+
3
3
+
+
3
3
3
l
3
3
3
3
3
l
3
l
l
l
l
l
l
l
l
l
l
A
A
A
A
A
A
A
A
A
A
A
A
A
Fig. 5 Association constant (Kass) determined with fluorescence data
Fig. 6 Competitive experiments in the 1+Al3+system with
interfering metal ions. [1] = 1×10-5 M, [Al3+] = 1×10-5 M, and
[Mn+] = 1×10-5 M. λex = 333 nm.
using the Benesi–Hildebrand equation.

JOURNAL OF CHEMICAL RESEARCH 2015 605
for 3 h, cooled to room temperature, diluted with 2N HCl (50 mL) and
extracted with EtOAc (3 × 20 mL). The extracts were washed with brine
(2×20 mL) and dried over anhydrous MgSO4. After filtration and rotary
evaporation the residue was purified by flash chromatography with
petroleum as elute to give compound 1 (0.42g, 78%) as an oily solid. IR
(KBr, cm-1): 3439, 2909,1639, 1448, 1354, 1255, 1107, 950, 737. 1H NMR
(300 MHz, CDCl3): δ 8.25–8.23(m, 2H), 8.01–7.99(m, 2H),7.46–7.28
(m, 6H), 3.80(s, 2H);3.65–3.60 (m, 19H), 2.89–2.84(m, 5H), 2.27(s,
3H);13C NMR (75 MHz, CDCl3): δ 151.9, 150.0, 130.6, 130.3, 128.3,
128.2,127.6, 127.4, 126.4, 125.5, 122.8, 117.2, 70.5, 70.4, 70.3, 70.0, 69.5,
53.2, 48.6, 19.1. ESI-MS: m/z 542[M+1]+. Anal. calcd for C30H39NO6S
(541.25): C, 66.52; H, 7.26; N, 2.59; found: C, 66.45; H, 7.23; N, 2.54%.
O
O
O
N
O
O
CH3S
O
1
Al3+
Binding studies
A stock solution of compound 1 was prepared by dissolution in EtOH/
water (95:5, v/v) containing HEPES buffer (10mM, pH=7.0; 1.0×10-5M).
Solutions of metal ions were prepared from Pb(NO3)2 , Al(NO3)3 and the
chlorides of K+, Na+, Mg2+, Hg2+, Cd2+, Fe3+, Cu2+, Zn2+, Co2+, Ni2+, Cr3+, and
Mn2+, respectively, and were dissolved in EtOH (3.0×10-3 M). Fluorescence
titration was performed by placing a solution of compound 1 (3 mL) in a
quartz cell of 1 cm optical path length, and adding different stock solutions
of cations into the quartz cell portionwise using a microsyringe each time.
O
SH3C
O
O
O
O
Al3+
N
O
O
N
O
O
O
O
O
CH3S
Electronic Supplementary information
Scheme 2 Proposed 2:1 binding model of 1 with Al3+.
1H NMR (300 MHz, CDCl3) of and the 13C NMR (CDCl3) of 1
is available through stl.publisher.ingentaconnect.com/content/
stl/jcr/supp-data
and the results are shown in Fig. 6. The fluorescence intensity
of 1 (1×10-5 M) in the presence of 1 equiv of the Al3+ ion was
almost unaffected by the addition of 1 equiv. of competing metal
ions (Cd2+, Co2+, Cr3+, Cu2+,Na+, K+, Mg2+, Mn2+, Pb2+, Zn2+, Hg2+,
Ni2+, and Fe3+). These results suggest that molecule 1 could be
used as a selective fluorescent chemosensor for Al3+.
We thank the Science Technology Foundation for Creative
Research Group of HBDE(2013) and Training Programs of
Innovation and Entrepreneurship for Undergraduates of China
(201313256002) for financial support.
Based on above spectroscopy studies and Job’s plot,
a
Received 1 August 2015;accepted 15 September 2015
Paper 1503519 doi: 10.3184/174751915X14425101743684
Published online: 2 October 2015
sandwich-type complex model seems to be reasonable for the
binding site of 1 with Al3+ (Scheme 2). Free 1 shows a relatively
weak fluorescence emission at 378 nm because of photo-
induced electron transfer (PET) from lone pair electrons on
nitrogen atoms in aza-18-crown-6 moieties quenching emission
of 2,5-diphenylfuran. When Al3+ was added into a solution of 1,
interruption of the PET process occurred and a strong emission
was observed with a peak centred at 378 nm.
In conclusion, a new fluorescent molecule 1 derived from
2,5-diphenylfuran and aza-18-crown-6 has been designed and
synthesised. Its binding properties, investigated by fluorescence
spectroscopy, show that it can selectively bind Al3+ in aqueous
ethanol solution.
References
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1
apparatus and were uncorrected. H NMR spectra were recorded on a
Bruker 300 spectrometer with TMS as internal reference and CDCl3 as
solvent. IR were recorded on a PerkinElmer PE-983 IR spectrometer as
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Synthesis
3-(Methylthio)-2,5-diphenylfuran (4)21 and 3-(methylthio)-4-bromo-
methyl-2,5-diphenyl-furan(5)22 were prepared according to the reported
procedures.
2-((4-((Benzo[d]thiazol-2-ylthio)methyl)-2,5-diphenylfuran-3-yl)
methylthio)benzo[d]-thiazole(1): Compound 5 (0.358 g, 1.0 mmol) and
aza-18-crown-6 (0.263 g, 1.0 mmol) were dissolved in CH3CN (20 mL)
and K2CO3 (0.55 g, 4.0 mol) was added. The resultant mixture was refluxed
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