A novel "turn-on" fluorescent chemosensor for the selective detection of Al3+ based on aggregation-induced emission

Article abstract of DOI:10.1039/c1cc15681k

A water-soluble, 'turn-on' fluorescent chemosensor based on aggregation-induced emission (AIE) has been developed. It exhibits rapid response, excellent selectivity, and sensitivity to Al³⁺.

Full text of DOI:10.1039/c1cc15681k

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A novel ‘‘turn-on’’ fluorescent chemosensor for the selective detection of
Al3+ based on aggregation-induced emissionw
Tianyu Han,a Xiao Feng,ab Bin Tong,*a Jianbing Shi,a Long Chen,a Junge Zhic and
Yuping Dong*a
Received 14th September 2011, Accepted 25th October 2011
DOI: 10.1039/c1cc15681k
A water-soluble, ‘turn-on’ fluorescent chemosensor based on
aggregation-induced emission (AIE) has been developed. It exhibits
In our group’s previous research, the experimental result
indicated that only pentaphenylpyrrole among five aryl-substituted
pyrrole derivatives clearly showed an aggregation-induced
emission enhancement (AIEE) phenomenon during aggregation
in THF–water mixtures. Upon comparing the optical properties
and single-crystal structures of these pyrrole derivatives, it is
suggested that the more twisted configuration, which prevented
parallel orientation of conjugated chromophores, combined
with the restricted intramolecular rotation (RIR) effect was
the main cause of the AIEE phenomenon.7
rapid response, excellent selectivity, and sensitivity to Al3+
.
Aluminium (Al), as the most abundant metallic element in the
earth’s crust, was found to be neurotoxic to organisms a long
time ago.1 Al3+ has also been linked to Alzheimer’s disease.2
In 1989, World Health Organization (WHO) listed Al to be
one of the food pollution sources and limited Al concentration
to 200 mg LÀ1 (7.41 mM) in drinking water. As a result of the
close relationship between Al and human health, the investigation
of reliable detection methods for Al becomes more and more
important. To date, some conventional methods with high
sensitivity for Al detection based on atomic absorption or emission
spectroscopy have been developed. These methods, however,
require expensive instruments and intricate sample preparation
processes. It is necessary to find other ways to design new
chemosensors for Al3+. Recently, considerable attention has been
focused on fluorometric and colorimetric online detection methods
for Al3+, known for their simplicity and selectivity.3
Herein, we report the synthesis and properties of sodium
4-(2,5-diphenyl-1H-pyrrol-1-yl) benzoate (TriPP-COONa), a water-
soluble, ‘‘turn-on’’ fluorescein-based sensor that exhibits high
selectivity and sensitivity for Al3+. We introduced carboxylic
salt into an aryl-substituted pyrrole derivative to enhance its
water-solubility. UV spectra of TriPP-COONa show two p–p*
transition bands centered at 253 and 298 nm, which can be
assigned to benzoate moiety and 2,5-diphenylpyrrole moiety,
respectively (Fig. 1).
Since THF is a poor solvent for TriPP-COONa, the molecules
of TriPP-COONa must have aggregated in the aqueous/THF
mixtures with high THF contents. The experimental result
indicates that its fluorescence intensity is toned with THF content
in THF–water mixtures (Fig. 2 and Fig. S1 (ESIw)). Upon photo-
excitation at 326 nm, the dilute aqueous solution of TriPP-
COONa hardly shows any fluorescence (Fig. S1, ESIw). When
THF is continually added into the aqueous solution of TriPP-
COONa while keeping the luminogen concentration at 10 mM,
the photoluminescence (PL) intensity of TriPP-COONa changed
in three stages: almost remain in nullity at ‘‘low’’ THF fraction
content (o40%), obviously increased from 40 to 80%, and then
sharply enhanced when the THF content is over 80% (Fig. 2).
Evidently, the emission of TriPP-COONa is spectacularly boosted
by aggregation; in other words, TriPP-COONa is AIE active.
In our previous work,7 we found that the highly twisted
conformation with large torsion angles between the pyrrole group
and the neighbouring phenyl groups, which further fastened by
aromatic C–HÁÁÁp hydrogen bonds in the aggregation state, is a
key factor to the RIR mechanism.8 Herein, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) of TriPP-COONa were calculated using the
B3LYP/6-31+G** method. The results show that HOMO wave
function is located on the pyrrole ring and two neighboring
Development of luminogens whose aggregates emit more
efficiently than that in solution has aroused much interest in recent
years. Tang’s group in 2001 first reported that the luminescence of
silole molecules is stronger in the aggregate state than that in the
solution state. They coined ‘‘aggregation-induced emission’’ (AIE)
for this unusual phenomenon.4 A variety of luminogens, including
distyrylbenzene, fluorene, pentacene, and pyrene derivatives, were
successively proved to have the same properties.5 Such compounds
were regarded as competitive candidates for practical use as highly
emissive materials. Moreover, the introduction of functional
groups into AIE molecules will favor new development of chemo-
or biosensors for detecting metal cations, biomolecules, organic
vapors, chiral molecules, and explosives.6
a College of Materials Science and Engineering, Beijing Institute of
Technology, 5 South Zhongguancun Street, Beijing, 100081, China.
E-mail: tongbin@bit.edu.cn, chdongyp@bit.edu.cn;
Fax: +86-10-6894-8982
b Department of Materials Molecular Science, Institute for Molecular
Science, National Institutes of Natural Sciences, 5-1 Higashiyama,
Okazaki 444-8787, Japan
c College of Science, Beijing Institute of Technology, Beijing 100081, China
w Electronic supplementary information (ESI) available: Experimental
materials and methods, spectra and SEM image. See DOI: 10.1039/
c1cc15681k
c
416 Chem. Commun., 2012, 48, 416–418
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Fig. 1 Absorption response of TriPP-COOÀ (100 mM) to titration of
Al3+ in THF–water mixtures (25/75 v/v). The [Al3+
total increases from
Fig. 3 Emission spectra of Tripp-COOÀ (100 mM) upon addition of
]
Al3+ in THF–water mixtures (25/75 v/v). The Al3+ concentrations are
0, 1, 2, 3. . .28 mM, from bottom to top. Excitation wavelength:
326 nm. Inset: linear relationship between emission growth ratio and
Al3+ concentration from 0 to 11 mM. Curve–fit equation: (I À I0)/I0 =
À0.46936 + 0.89971[Al3+], R2 = 0.9925.
0 to 23 mM along the direction of the arrow. Inset: linear relationship
between absorbance and Al3+ concentration at 298, 253, and 500 nm,
respectively.
three TriPP-COOÀ anions. While increasing Al3+ concentrations,
both the absorption bands of TriPP-COOÀ at 253 and 298 nm
linearly decreased with no blue- or red-shift, demonstrating
that there is no chelation or intramolecular charge transfer
(ICT) effect in the fluorophore–Al3+ system (Fig. 1). Meanwhile,
an unsophisticated aggregation has occurred in the emission
boosting process, which is evident by the linear increment of
light-scattering tails in the long wavelength region of UV-Vis
spectra as well as the decrement of transmittance (Fig. 1 and
Fig. S4 (ESIw)). The nano-scaled aggregates formed in the titration
process were found to have an average radius of 90 nm and a
disorganized morphology, confirmed by dynamic light scattering
(DLS) analysis (Fig. S5 (ESIw)) and scanning electron microscopy
(SEM) measurement (Fig. S6 (ESIw)). So we can draw a conclusion
that the ‘turn on’ detection for Al3+ is ascribed to the AIE
mechanism, in which the non-radiative decay can be suppressed
during the aggregation process (Scheme 1).
Fig. 2 Plot of fluorescence intensity of TriPP-COONa (10 mM) at
406 nm versus solvent composition of THF/water mixture.
The fluorescence responds very quickly to the titration of Al3+
,
owing to the extremely fast aggregation process. Repeated measure-
ments show that the emission intensity immediately enhanced
nearly up to maximum after the addition of Al3+ (Fig. S7, ESIw),
phenyl rings at 2, 5-positions, while the LUMO is mainly
located on the sodium benzoate moiety. Moreover, HOMO
and LUMO energy levels of TriPP-COONa proved the
existence of two conjugate planes, which provide efficient non-
radiative relaxation pathways for the excited states (Fig. S2,
ESIw). Unlike most of the other chemosensors, the functional
group of TriPP-COONa is binding directly to a fluorophore
without any flexible linkages, resulting in a clearer energy level
transition and a smaller energy loss that leads to a modification
of its AIE property.
providing a potential ‘‘zero-wait’’ detecting method for Al3+
.
Furthermore, the emission is found to be linearly proportional to
the amount of Al3+ in the range of 1–10 mM, which is much lower
As shown in Fig. 3, the fluorescent intensity of 100 mM
TriPP-COOÀ in 25/75 v/v THF–water mixtures was obviously
enhanced with increase of Al3+ concentration from 0 to 20 mM.
The increment rate of PL intensity was slowed down when the
Al3+ concentration was over 20 mM, and became 0 after 28 mM.
It is indicated that each Al3+ cation can couple with almost
Scheme 1 Sensing process based on the AIE mechanism.
Chem. Commun., 2012, 48, 416–418 417
c
This journal is The Royal Society of Chemistry 2012

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Doctoral Program of Higher Education of China (Grant Nos.
20091101110031 and 20101101120029), Excellent Young
Scholars Research Fund of Beijing Institute of Technology
(Grant No. 2009Y0914) for financial support of this work.
Notes and references
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Fig. 4 Maximum fluorescent response of TriPP-COOÀ (100 mM) upon
addition of different metal ions (50 mM) in THF–water mixtures (25/75 v/v),
listed from left to right: (A) blank, (B) Ba2+, (C) K+, (D) Mg2+, (E) Pb2+
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(F) Cu2+, (G) Ce2+, (H) Al3+, (I) Ca2+, (J) Ni2+, (K) Cr2+, (L) Hg2+
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To explore the selectivity of metal ions, the fluorescence
responses of TriPP-COOÀ to various cations are conducted
(Fig. 4). The fluorescence spectra of TriPP-COOÀ with the
existence of other metal ions remain the same as that of free
TriPP-COOÀ except for Pb2+, Zn2+ and Al3+. The PL response
of TriPP-COOÀ to Pb2+ and Zn2+ is, however, much lower than
that of TriPP-COOÀ to Al3+ under the identical conditions.
That is, TriPP-COOÀ shows good selectivity and high sensibility
to Al3+. To further testify single selectivity of TriPP-COOÀ for
Al3+ in practical application, we chose K+, Mg2+, Ca2+
,
Ba2+, and Ni2+ ions as interfering ions, some of which have a
relatively high concentration in biological tissue and drinking
water. As shown in Fig. 4, the experimental results indicate
that fluorescent intensities of TriPP-COOÀ at 460 nm enhanced
by Al3+ are not much affected in the background of the
interfering ions, thus providing a potential application for
biological detection and the water quality monitoring.
In summary, the single selective and high sensitive water-
soluble probe for the rapid ‘‘turn-on’’ detection of Al3+ based
on the AIE mechanism was successfully prepared. We suppose
that the selectivity is probably due to the difference of electro-
static binding ability between metal ions and TriPP-COOÀ as
well as their solubility. Such a detection method based on the
AIE mechanism will open up a new way for the designing of
chemosensors and biosensors. Further studies on the binding
mechanism of TriPP-COOÀ and Al3+ are underway.
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The authors are grateful to the National Natural Scientific
Foundation of China (Grant Nos. 51073026, 51061160500,
20944004, 21074011), the Specialized Research Fund for the
c
418 Chem. Commun., 2012, 48, 416–418
This journal is The Royal Society of Chemistry 2012