A simple sensitive ESIPT on-off fluorescent sensor for selective detection of Al3+ in water

Article abstract of DOI:10.1039/c3ra47104g

A highly selective and sensitive fluorescent sensor for Al³⁺ has been developed. The sensor shows great fluorescence turn-on upon binding Al³⁺ in complete water, giving strong blue emission. In addition, the sensor's turn-on exhibits

Full text of DOI:10.1039/c3ra47104g

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A simple sensitive ESIPT on-off fluorescent sensor
for selective detection of Al3+ in water†
Cite this: RSC Adv., 2014, 4, 5845
a
ab
*
Junfeng Wang and Yi Pang
Received 27th November 2013
Accepted 17th December 2013
DOI: 10.1039/c3ra47104g
www.rsc.org/advances
A highly selective and sensitive fluorescent sensor for Al3+ has been sensors, which not only recognize Al3+ ions selectively but also
developed. The sensor shows great fluorescence turn-on upon compete effectively with the strong hydration of Al3+ ion during
binding Al3+ in complete water, giving strong blue emission. In addi- the application in aqueous.
tion, the sensor's turn-on exhibits excellent selectivity to the Al3+
cation, with only a slight interference from Zn2+. These findings excited state intramolecular proton transfer (ESIPT) in the
suggest that the developed Al3+ sensor could be a useful molecular sensor design. Recently, ESIPT has attracted attention from
One option is to integrate the Al3+ binding event with the
probe for practical applications.
both theoretical and experimental viewpoints, because it shows
a uniquely large Stokes' shied uorescence emission (6000–
12 000 cmꢀ1).26 In addition, the ESIPT turn-on or turn-off events
will usually lead to a large change in uorescence wavelength,27
which is of great importance in their practical applications. In
general, the ESIPT process requires a proton donor (–OH, –NH2)
and a proton acceptor (–C]O, –N]) group in close proximity in
order to form the intramolecular hydrogen bond (a necessary
condition for ESIPT).28
Aluminum is the most abundant metal in the Earth's crust and
extensively used in modern life.1 The soluble form of aluminum
(Al3+), however, is highly toxic to plant growth.2 Excessive
aluminum, especially when deposited in the brain even in small
amounts, has also been shown to be toxic to humans, and is
believed to cause neurodementia such as Parkinson's disease,
Alzheimer's disease and dialysis encephalopathy, osteoporosis,
etc.3 For these reasons, the development of Al3+ sensors for its
facile detection is of great importance in both environmental
monitoring and biological assays.
In order to demonstrate the concept of using ESIPT in Al3+
sensing, we decide to explore the synthesis of Schiff base 1. In
the sensor design, the hydroxyl group in 1 forms an intra-
molecular hydrogen bond with the adjacent imine bond (–CH]
N–), which gives ESIPT. The hydroxyl and adjacent “acetohy-
drazide” groups also provide a strong binding cavity to host the
Al3+ cation. As a consequence, the new sensor integrates the
following functions into a single molecule: (a) containing
sufficient polar groups to improve water solubility; (b) including
an amine group for photoinduced electron transfer (PET) effect
to suppress the background signal; and (c) utilizing the Al3+
binding to switch the excited-state intramolecular proton
transfer (ESIPT), thereby inducing a large spectral shi. Herein,
we report the uorescence response of sensor 1, which exhibits
remarkable uorescence turned on (by ꢁ73 fold) upon binding
Al3+ ion. In addition, the Al3+ binding also induced a large
spectral shi (by 40 nm) (Fig. 1), as the cation binding turned
off the ESIPT.
Despite the strong interest, the uorescent detection for Al3+
cations remains to be a challenging problem. Owing to its weak
coordination with ligands and strong hydration ability in
water,4 the detection of Al3+ cation is oen affected by the
existence of interfering metal ions. So far, very few uorescent
chemosensors have been reported for detection of Al3+ with
moderate success to date compared to the transition metal
ions.5–25 The majority of the reported Al3+ sensors, however,
have limitations such as tedious synthetic efforts and/or lack of
practical applicability in aqueous solutions.11 Today, almost all
the reported dyes for Al3+ have been tested in organic solvents or
mixed solvents. In order to enable evaluation of Al3+ ions in
aqueous environments, it is highly desirable to develop new
Chemosensor 1 was synthesized in over 90% yield by simple
coupling of 2-hydroxybenzaldehyde with acetohydrazide (A)
(Scheme 1). Compound 1 could exist in the isomers 1a and 1b,
whose ratio was dependent on the equilibrium in different
solvents (See ESI Fig. S1–S3†). The structure of the major isomer
aDepartment of Chemistry, The University of Akron, Akron, Ohio 44325, USA
bMaurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio
44325, USA. E-mail: yp5@Uakron.edu
† Electronic supplementary information (ESI) available: CCDC 970383. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ra47104g
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Ba2+, Pb2+, Cd2+, Mn2+, Ni2+, Co2+, Cu2+, Fe2+, Zn2+, Cr3+, and
Fe3+ in pure water. As shown in Fig. 1a and c, the addition of 5.0
equiv. of Na+, K+, Ag+, Mg2+, Ca2+, Hg2+, Ba2+, Pb2+, Cd2+, Mn2+
and Cr3+ has no obvious effect on the uorescence emission.
The metal ions Ni2+, Co2+, Cu2+, Fe2+ and Fe3+ quenched the
uorescence. Although sensor 1 responded to Zn2+ cation with a
slight increase in the uorescent intensity, the emission wave-
length of 1-Zn was only shied to 473 nm (around 10 nm blue
shi). Therefore, sensor 1 is highly selective and sensitive for
Al3+ detection, which makes it feasible for biological and envi-
ronmental applications.
The absorption peak of 1 (lmax ¼ 317 nm) was progressively
decreased upon addition of Al3+ (Fig. 2a), which was accompa-
nied with a new band at about 352 nm. The large spectral
bathochromic shi indicated the deprotonation, as a conse-
quence of Al3+-binding to phenol. Observation of the distinct
isosbestic point at 333 nm suggests the complex formation with
one new chemical species. On the basis of Job plot, the complex
was assumed to have a ligand-to-metal ratio of 2 : 1. The
assumption was supported by high resolution mass spectros-
copy (HRMS), which detected m/z 381.1135, corresponding to
Fig. 1 (a) Fluorescent spectra of 1 (20.0 mM) with 5.0 equiv. of various
metal ions in pure water: Na+, K+, Ag+, Mg2+, Ca2+, Hg2+, Ba2+, Pb2+
,
[2(1-H+)+Al3+ +
] (See Fig. 4: the calcd mass for C18H18AlN4O4:
Cd2+, Mn2+, Ni2+, Co2+, Cu2+, Fe2+, Zn2+, Cr3+, Fe3+. (b) Fluorescent
images of 1 in the presence of different cations. (c) Change ratio ((FA
FA0)/FA0) of fluorescence of 1 (20.0 mM) in pure water containing
381.1143). Furthermore, mass spectra detected no Al3+ complex
with 1 : 1 ligand-to-metal ratio from the aqueous solution of 1
and Al3+ (ESI Fig. S5†).
ꢀ
various metal ions (5.0 equiv.): Na+, K+, Ag+, Mg2+, Ca2+, Hg2+, Ba2+
,
Pb2+, Cd2+, Mn2+, Ni2+, Co2+, Cu2+, Fe2+, Zn2+, Cr3+, Fe3+, Al3+
.
1
The cation binding was further examined by H NMR titra-
tion. The free ligand exhibited two imine (–CH]N–) signals at
8.25 and 8.15 ppm, corresponding to the major and minor
isomers 1a and 1b respectively (Fig. 3). Upon addition of Al3+
cation, a broad imine signal was developed at 8.45 ppm,
attributing to the formation of 1-Al3+ complex. The triplet signal
Scheme 1 Synthesis of 1 and its Al3+ complex.
1a was determined by X-ray diffraction (ESI Fig. S14†). In
aqueous, the free ligand 1 gave very weak green uorescence
(the emission lem ¼ 485 nm, f ¼ 0.01), partly attributing to the
PET effect from the amine. As expected, the emission of 1
exhibited a large Stocks' shi in water (Dl ¼ 495 (lem) ꢀ 317
(lmax) z 168 nm), as a consequence of ESIPT process. Upon
addition of Al3+ cation, however, the solution gave bright blue
uorescence, with its quantum efficiency reaching as high as f
¼ 0.73 (Fig. 1a). In addition, the Al3+ binding also shied the
emission signal (around 40 nm shi from the weak green
uorescence to strong blue uorescence), which could be used
for naked eye detection (See Fig. 1a and b). Clearly, two “oxygen”
and one “nitrogen” atoms in sensor 1 provided a strong Al3+
binding (via hard acid–base interaction) to compete with the
Al3+ hydration, thereby allowing the reliable uorescence turn-
on in the aqueous solution.
The uorometric behaviour of 1 was further investigated by
Fig. 2 Titration of 1 (10 mM) in water by addition of different amount
addition of the other metal ions Na+, K+, Ag+, Mg2+, Ca2+, Hg2+, of Al3+
.
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Fig. 5 Fluorescent intensity of Dye 1 (120 nM) upon addition of
different concentration of Al3+. The sharp signal at ꢁ410 nm was from
the scattering of water. The inset at the top right shows that the linear
response of 1 to Al3+ concentration.
Fig. 3 1H NMR of 1 in CD3OD upon addition of various equiv. of Al3+
cation.
There were insignicant changes in the uorescence of “1 +
Al3+” in the presence of other competing metal cations. Except
for Fe3+ and Cu2+, most competing metal ions did not interfere
with detection of Al3+ by 1 in H2O (Fig. 4), indicating that 1 can
be used as a selective chemosensor for the Al3+ cation.
at 7.27 ppm (aromatic Hc) disappeared when ꢁ0.5 equiv. molar
Al3+ was added, indicating that all ligand was consumed to bind
Al3+ cation, forming 1-Al3+ complex. Further addition of Al3+
(0.5–1.0 equiv. molar) did not reveal signicant change. Since
all the ligand was used to bind Al3+, the broad peak at 8.28 ppm
was also attributed to 1-Al3+, in addition to the peak at 8.45
ppm. The two peaks at 8.28 and 8.45 ppm suggested possible
existence of two different isomers in the metal complex, as the
mass spectra detected only 1-Al3+ complex in 2 : 1 ligand-to-
metal ratio.
When using 1 in low concentration (120 nM), the water
scattering signal and the background noise could be brought to
a observable level (Fig. 5) to evaluate the detection limit. Sensor
1 exhibited a good linear response towards Al3+ in the concen-
tration range of 1.0 to 60 nM in water, showing its analytical
value. The detection limit for Al3+ was determined to be as low
as 5.0 ꢂ 10ꢀ11 M (0.5 nM) in pure water (ESI Fig. S15†), which
was dened as the three-fold standard deviation of the uo-
rescence obtained from a blank sample (dye 1 in the absence of
Al3+).29,30a The detection limit of 1 for Al3+ cation was about 3
times lower than the highest detection limit reported in
DMSO,30b representing a signicant advance in sensing Al3+
cation in water.
In order to further evaluate the selectivity of 1 for Al3+
sensing, competition experiments were carried out with other
metal cations. The uorescence response of 1 to Al3+ in pure
water in the presence of 5 equiv. of various cations including
Na+, K+, Ag+, Mg2+, Ca2+, Hg2+, Ba2+, Pb2+, Cd2+, Mn2+, Ni2+, Co2+,
Cu2+, Fe2+, Zn2+, Cr3+ and Fe3+ are given in Fig. 4, respectively.
In an effort to seek the potential biological application,
sensor 1 was applied to zebrash. When zebrash was exposed
to dye 1 and Al3+ in sh tank, bright uorescence was observed
in the sh head and tail (Fig. 6 and ESI Fig. S6†), suggesting that
this molecule could be used in organisms at certain conditions.
Fig. 4 Fluorescence intensity of 1 (20.0 mM) in pure water (column 1),
and sensor 1 in the presence of different metal ion(s) (5.0 equiv.): (2)
Al3+; (3) Al3+ + Na+; (4) Al3+ + K+; (5) Al3+ + Ag+; (6) Al3+ + Mg2+; (7) Al3+
+ Ca2+; (8) Al3+ + Hg2+; (9) Al3+ + Ba2+; (10) Al3+ + Pb2+; (11) Al3+
Cd2+; (12) Al3+ + Mn2+; (13) Al3+ + Ni2+; (14) Al3+ + Co2+; (15) Al3+
+
+
Fig. 6 Fluorescent images of Al3+ in Zebrafish (a) fish exposed to dye 1
only in the fish tank at the concentration of 10 mM for 2 hours and (b)
fish exposed to 10 mM dye 1 and Al3+ for 2 hours.
Zn2+; (16) Al3+ + Cr3+; (17) Al3+ + Fe2+; (18) Al3+ + Cu2+; (19) Al3+ + Fe3+
.
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This work was supported by National Institute of Health (Grant
no: 1R15EB014546-01A1). We also thank the Coleman endow-
ment from the University of Akron for partial support, and
thank Dr Qin Liu at the University of Akron for assistance in the
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